Particulate-Filled
Polymer
Composites
Second Edition
Editor
R.N. Rothon
Rapra Technology Limited
8
Filled Thermoplastics
Chris DeArmitt and Michael Hancock
8.1 Introduction
8.1.1 Thermoplastics and Typical Applications
Thermoplastics have become an essential part of our everyday lives. Our cars and appliances
contain more and more plastics every year. Even our clothes are often made from synthetic
thermoplastics. They are a very important class of material for many reasons. They combine
good mechanical and electrical properties with low density and high formability. Clearly,
the driving force for their success has been that they can often provide an overall solution
that is less expensive than that achievable with other materials such as glass, wood, metal,
thermosetting polymers or ceramics. Thermoplastics, as implied by their name, are materials
that flow upon heating, and harden when cooled. They can be formed using a wide variety
of techniques, such as injection moulding, thermoforming, blow moulding and rotational
moulding. Injection moulding in particular allows complex shapes, so it is possible to
integrate several smaller parts into one larger part, thus saving on assembly costs. As well
as being easily processed when molten, they also have the potential to be recycled by remelting them to form new articles, or burnt and used to generate electrical energy. New
legislation is being introduced to encourage recycling of used products; this is expected to
favour thermoplastics over other materials, which are not as easy to reprocess.
Thermoplastic demand in Western Europe is 37 x 106 tonnes compared to 10 x 106 tonnes
for thermosetting polymers. A breakdown of thermoplastics by application area is given in
Figure 8.1.
Some of the main properties and applications are given next for the five main thermoplastics,
which together account for 75% of the total thermoplastics market.
• Polyethylene (low density) LDPE, (linear low density) LLDPE: 7.6 x 106
tonnes
Properties: Flexible, translucent, very tough, weatherproof, good chemical
resistance, low water absorption, easily processed by most methods, low cost.
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Figure 8.1 Use of thermoplastics in Western Europe
Applications: Squeeze bottles, toys, carrier bags, high frequency insulation,
chemical tank linings, heavy-duty sacks, general packaging, gas and water pipes.
• Polyethylene (high density) HDPE: 5.0 x 106 tonnes
Properties: Semi-rigid, translucent, weatherproof, good low temperature toughness
(to –60 °C), easy to process by most methods, low cost, good chemical resistance.
Applications: Chemical drums, jerricans, carboys, toys, picnic ware, household and
kitchenware, cable insulation, carrier bags, food wrapping material.
• Polypropylene PP: 7.0 x 106 tonnes
Properties: Semi-rigid, translucent, good chemical resistance, tough, good fatigue
resistance, integral hinge property, steam sterilisable, good heat resistance.
Applications: Sterilisable hospital ware, ropes, car battery cases, chair shells, integral
moulded hinges, packaging films, electrical kettles, car bumpers and interior trim
components, video cassette cases.
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• Polyvinyl chloride PVC: 5.8 x 106 tonnes
Properties: Rigid or flexible, clear or opaque, durable, weatherproof, flame resistant,
good impact strength, good electrical insulation properties, limited low temperature
performance.
Applications: Window frames, drain pipes, sewage and soil pipes, roofing sheets,
cable and wire insulation, floor tiles, hose and pipes, stationary covers, fashion
footwear, leathercloth.
• Polystyrene (general purpose) GPPS: 3.1 x 106 tonnes combined with high
impact polystyrene (HIPS)
Properties: Brittle, rigid, transparent, low shrinkage, low cost, excellent X-ray
resistance, free from odour and taste, easy to process.
Applications: Toys and novelties, rigid packaging, refrigerator trays and boxes, cosmetic
packs and costume jewellery, lighting diffusers, audio cassette and CD cases.
• Polystyrene (high impact) HIPS
Properties: Hard, rigid, translucent, impact strength up to seven times that of GPPS,
other properties similar.
Applications: Yoghurt pots, refrigerator linings, vending cups, bathroom cabinets,
toilet seats and tanks, closures, instrument control knobs.
• Polyesters (thermoplastic) PET: 3.1 x 106 tonnes
Properties: Rigid, clear, extremely tough, good creep and fatigue resistance, wide range
of temperature resistance (–40 °C to 200 °C).
Applications: Carbonated drink bottles, synthetic fibres, video and audio tape,
microwave utensils.
8.1.2 Thermoplastic Composites
One disadvantage of thermoplastics is that they soften appreciably as they are heated. As
this happens, their modulus decreases and they begin to creep (slowly deform over time)
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and at higher temperatures they progressively lose their shape, and then melt. There has
been a great deal of effort spent in trying to overcome this limitation. Filling PP with talc
is an early example of this. In fact, addition of mineral fillers increases the modulus of all
thermoplastics, and increases their heat distortion temperature (HDT) [1].
It is commonly thought that the primary goal of adding fillers is to lower the overall
materials cost of the composite compared to the unfilled polymer. This is, however, rarely
the case. Polyethylene (PE) and PP are the world’s number one and two highest volume
polymers, respectively, and they are also the least expensive per unit volume. Addition of
any common filler, with the exception of calcium carbonate, increases the material cost
of these polymers. Even in cases where the main goal is to decrease cost, the addition of
filler changes nearly every property of the polymer. It is therefore now common to use
the term ‘functional fillers’ to emphasise that the fillers change the polymer, giving many
advantages, and naturally, some disadvantages. Making good composites is all about
knowing how to find a good balance of properties at the lowest cost. In order to make
the right decision, one needs to know about polymers, engineering, fillers and surface
science. That is what makes the study and application of composites so challenging,
fascinating and rewarding. In this chapter, we will discuss the main polymer properties
and how they are influenced by the addition of various common fillers. The global fillers
market is estimated as between 5-10 million tons per year, with over 90% of the filler
going into rubbers, PVC and polyolefins, e.g., PE and PP. PP is the one of the most
commercially important filled polymers [2] and it will therefore be used to illustrate
some of the main points. Similar trends are seen when fillers are compounded into other
semi-crystalline thermoplastics such as PE¸ PVC [3] and the polyamides. The amorphous
polymers such as polystyrene and polycarbonate also respond similarly to filler addition.
Incorporation of fillers into thermoplastics alters all the properties of the material. Some
of the changes will be beneficial and some will be detrimental. It should be noted that
these are not absolutes and the determination of pros and cons is only meaningful to the
proposed end-use of the material. Burditt listed 21 reasons why filler may be added to a
polymer [4]. In addition to those intentional changes, there are a multitude of unintentional
effects that must also be considered. In this chapter the main properties of composites
and how they vary with filler type and level of addition will be discussed.
An essential point to note, is that the properties of the composite depend upon the volume
percentage of filler added [5, 6]. Often in the literature, one sees properties plotted versus
the weight percentage of filler, which is not particularly useful and may even be misleading.
It is more meaningful to plot properties versus the volume percentage of filler [7]. In
many cases, this latter approach gives straight lines, allowing simple, accurate
extrapolation and prediction of properties [8, 9]. The properties of a composite are
usually in between those of the component materials. Several of the properties such as
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density, modulus and yield strength can be predicted by the rule of mixtures, according
to the volume fraction of each component. In other cases, there are more complex
mathematical models that can be used to describe and predict properties as the volume
fraction of the ingredients is varied.
Sometimes, the inclusion of filler can chemically or physically modify the polymer phase
to such as extent that it becomes more difficult to apply simple models. It is important to
realise that these complications may occur, and to look for them when characterising
composites. This will help in the understanding and design of new composite materials.
One example of this behaviour is changes in crystallisation, such as nucleation of crystal
growth [10-14] and changing of the crystal phase of the polymer [15, 16]. Chemical
degradation of the polymer may be catalysed by the filler, or impurities in the filler,
especially transition metals [17, 18]. Another common example is where the filler surface
adsorbs stabilisers and antioxidants, which are then unable to protect the polymer during
processing and during its service life [19-21]. Alternatively, mechanical degradation may
occur when high levels of filler cause unduly high viscosity, thereby inducing chain scission
due to the excessive shear needed to process the material.
In this chapter, an attempt has been made to mention each of the factors that influence
composite design and performance. For a given application, one must identify the key
properties that are important and concentrate on those. It will be seen that there are
many different parameters to consider and that in some cases, optimisation of one
precipitates an inevitable worsening of some other property. There is no one optimal
composite; rather the goal is to seek the best balance of properties through compromise
and an awareness of the entire picture in terms of economics and performance. In cases
where the filler is less expensive than the polymer, then the goal is to increase the filler
loading as much as possible, while still retaining sufficient processability and properties.
Conversely, when the filler is more expensive than the polymer on a volume basis, then
one seeks to identify the minimum filler loading that gives sufficient properties.
8.2 Bulk and Process Related Properties
8.2.1 Specific Gravity or Relative Density
The common fillers used in plastics are minerals (densities from 2.4-2.8 g/cm3), which
give a composite of higher density than that of the unfilled polymer (densities of 0.8-1.9
g/cm3). The density of a composite of known composition can be calculated according to
the linear rule of mixtures (Equation 8.1), where ρc, ρf and ρp are the densities of the
composite, filler and matrix, respectively, and mf is the mass fraction of filler.
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ρc =
ρf ρ p
ρ pm f + ρf (1 − m f )
Equation 8.1 Composite density
Such calculations usually agree well with measured values. A lower than expected density
may be due to inclusions of air resulting from poor mixing or poor wetting of the filler
surface by the polymer. Higher than expected density occurs when the filler nucleates
crystal growth in the polymer. The increased crystallinity increases the density of the
composite because crystals are of higher density than amorphous regions in the polymer.
The expected density of the composite can be calculated using Equation 8.1, assuming
no air inclusions and that the filler does not influence density of the polymer phase by
nucleation of crystal growth for example. The filler loading mf is usually determined by
ashing, namely burning away the polymer from a known mass of composite and weighing
the amount of residual filler. This method is simplest if the filler has enough thermal
stability so that it does not lose mass at the high temperatures needed to burn off common
polymers (≥ 300 °C). For fillers that are somewhat thermally unstable, such as aluminium
trihydrate, it is necessary to correct the ashing result using the mass loss for the filler
alone under the same ashing conditions. Recently ‘ovens’ based on microwave ashing
have been introduced. These operate at low temperatures, removing the polymer while
leaving the filler unaffected. Density can also be determined by measuring the volume of
liquid displaced by a known mass of composite or using a density gradient column.
Increased density is usually undesirable because products must inevitably be transported
to be sold, or installed. This may result in increased transportation costs. Weight increases
are undesirable when the material is to be used to make cars, trucks, trains, aeroplanes
or spacecraft. Recent European legislation on packaging will also penalise by weight.
Decreased density is possible through use of fillers such as wood flour (or fibre), hollow glass
microspheres, hollow polymer microspheres [22] (e.g., Expancel®) or hollow spheres from
fly ash. Low-density thermoplastic composites are useful for products that must float.
8.2.2 Acoustic Properties
It is appropriate to mention acoustic properties here, as they are affected by density.
Adding filler usually increases the density compared to the host polymer, and this is
usually an unwanted side effect. However, it is common to make sound deadening
composites by using high-density fillers such as barium sulfate (BaSO4, 4.5 g/cm3) or
magnetite (Fe3O4, 5.1 g/cm3) [23].
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In order to dampen sound, a material must be able to absorb vibrational energy (sound)
and transform the energy into thermal energy (heat). The loss factor, tan δ, is the parameter
that best describes the sound damping ability of materials [5]. It is the inelastic component
of the material’s response to deformation. The loss factor can be measured by dynamic
mechanical thermal analysis (DMTA), which shows its behaviour with changes in
frequency and temperature. It is common to select a matrix with a high tan δ in the
frequency range to be damped. Then, high-density filler can be added in order to further
improve performance. Mica is also used, as the platy particles cause multiple reflection
of the sound waves so they may be absorbed in the composite instead of passing straight
through [5]. Adding too much filler should be avoided however, because this leads to
particle-particle interactions, and eventually percolation, where a continuous path exists
from particle to particle. Under those conditions, the acoustic energy can pass through
the interparticle contacts, largely avoiding interaction with the matrix.
8.2.3 Melt Viscosity (MFI)
The melt viscosity of polymers is usually measured as the melt flow index (MFI), also
known as melt viscosity index (MVI) and melt flow rate (MFR). A pressure is applied to
force the molten polymer through a hole at a controlled temperature [24]. All three
parameters are set out in standards appropriate to the polymer being measured [24]. The
measured value is in grams of polymer extruded through the hole in a set period of time,
e.g., ten minutes. This of course, is actually the reciprocal of the viscosity, so a high MFI
means low viscosity and vice versa.
There are some caveats when using MFI as a measure of viscosity. Firstly, MFI
measurements are performed in the medium shear rate range [24], whereas polymer
processing is often performed at higher shear rates. Therefore, the MFI may not correspond
well to the flowability of the polymer melt during compounding and processing. A further
point is that raw MFI data should not be used to compare viscosities of polymer melts
containing different filler loadings. This is because adding filler increases the density of
the melt, and therefore the MFI will increase because more mass of polymer melt flows
in a given time, even if the volume of material flowing remains constant. Therefore, the
MFI data must be adjusted by dividing by the density. This gives the volume of material
flowing in unit time and allows fair comparison of samples with differing filler amounts
[8]. A detailed description of filled polymer melt rheology can be found in a book by
Shenoy [24]. Of particular interest is Shenoy’s method for extrapolating MFI data to
give an idea of the expected rheology of the polymer melt at the higher shear rates
encountered during polymer processing. Another good review has been made by Hornsby
[25]. This latter work includes an investigation of the degree of filler dispersion at various
points as it passes through a twin-screw extruder.
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An inevitable consequence of adding filler is that the viscosity of the polymer melt increases
[8, 24-26]. This is exacerbated at high volume fractions of filler, and when the filler
particles are smaller, especially for nano-sized fillers such as carbon black [27]. In fact,
the increase in viscosity often limits the amount of filler in the compound. At low filler
addition levels, this effect is very low and is masked by the high inherent viscosity of the
polymer melt. As the filler addition increases, the viscosity begins to rise more sharply,
until eventually it approaches infinity at some critical filler level, which depends upon
particle size, shape and amount of agglomeration. High melt viscosity is to be avoided,
as it lessens the throughput during extrusion, increasing production costs. It may also
prevent complete filling of the mould, leading to high reject levels.
The viscosity of a very dilute dispersion of rigid spherical particles in a Newtonian fluid
is described by the Einstein equation (Equation 8.2) [26]. Where η is the viscosity of the
dispersion, ηl is the viscosity of the fluid alone, φ is the volume fraction of particles and
kE is the Einstein coefficient, which is 2.5 for spherical particles. kE depends upon both
particle shape and orientation.
η = ηl (1 + kE φ)
Equation 8.2 Einstein equation of dispersion viscosity
Although it is a starting point for understanding the effect of filler on viscosity, the
Einstein equation is not applicable to filled polymer melts. Polymer melts are nonNewtonian, and the filler concentrations are often too high to ignore particle-particle
interactions. A plethora of equations exist for modelling dispersions of particles. However,
the best approach is to make the desired formulation and test it under real conditions,
such as measuring the torque and volume throughput during extrusion, plus mould filling
when injection moulding.
8.2.4 Compounding and Extrusion
8.2.4.1 Introduction
Extruders are used to mix ingredients into thermoplastics [28]. The polymer is fed into a
hopper and is then forced into the barrel, either by gravity, or by mechanical means, such
as a feeder screw. The barrel is heated to melt the polymer and a rotating single or twinscrew arrangement transports the polymer melt down the barrel and out of the die (hole)
at the end. There are usually ports at various points along the barrel to allow for the
introduction of additives such as lubricants, antioxidants, pigments and fillers. These
additives may be added individually, but more commonly they are fed in together as a
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concentrate known as a masterbatch. After it emerges from the die, the molten polymer
string is usually cooled rapidly by running it through a water bath. The polymer is then
dried and lastly, the cooled polymer strand is chopped into small granules using a pelletising
machine. If the material is to be stored before use, then it is common to seal the pellets to
protect them from contamination and water pick-up.
The extrusion step is not particularly costly in comparison with the price of the raw materials,
but the cost is still significant and impacts on the overall economics of the final material. It
is therefore worthwhile to devote effort to optimisation of the extrusion process in terms
of increased throughput (productivity) and decreasing energy consumption and machine
wear. In the author’s opinion, the subject of throughput does not receive the attention it
deserves. There are countless reports of the mechanical properties of thermoplastic
composites but no mention of the extrusion characteristics of the materials. For a meaningful
comparison of different composites, one must consider not only their mechanical and
aesthetic properties, but also the relative economics of extrusion.
8.2.4.2 Volume Throughput
As with all of the other properties of a composite, when considering extruder output one
must think in volume terms, not in terms of weight. If one is making a certain amount of
composite, that material must be of sufficient volume to create a certain number of parts,
each of which has a fixed volume determined by the size of the mould to be filled. Therefore,
the mass of material produced is, in itself, of no interest. There have been various reports
that filler increases extruder throughput. These claims are often erroneous, or at least
greatly exaggerated, because the throughput is given in units of mass per unit time. That is
not a valid way to compare throughput, for the same reasons mentioned previously for
MFI. Namely, that the addition of mineral fillers increases the density of the filled polymer
compared to its unfilled counterpart. Naturally, this elevates the mass throughput of the
extruder, giving a false impression of improved productivity. One possible cause for the
confusion over throughput, may be the way in which extruders are rated by the manufacturer.
Usually, the capacity of the extruder is given in terms of kilograms of polymer extruded per
hour. This value conjures up an image that the maximum throughput of the extruder is
limited to a given mass of polymer, as shown on the side of the machine. In reality, that
value is valid only in the case of a standardised grade of unfilled polymer, and is merely a
convenient means for the extruder manufacturer to show the relative capacities of different
machines. Perhaps in the future, the extruder manufacturers will consider expressing the
maximum throughput in volume terms to avoid confusion.
There are two classes of extruder, single-screw and twin-screw. The primary drawback
of single-screw extruders is that they give poor dispersion of fillers compared to the
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more expensive twin-screw variants. Thus, twin-screw extruders are used almost
exclusively when good filler or pigment dispersion is required. Recently, it has been
suggested that the lack of dispersion associated with single-screw extruders is not an
intrinsic limitation [29]. It is argued that single-screw extruders had not been optimised,
as all of the attention and R&D has been on the more lucrative twin-screw machines.
Furthermore, it is claimed that the newly designed single-screw extruders can now give
sufficient dispersion for filled systems along with advantages such as being simpler, less
expensive, easier to maintain and giving higher volume throughput compared to twinscrew machines. It will be interesting to see whether single-screw extruders do indeed
gain acceptance for the manufacture of thermoplastic composites.
The volume throughput and the quality of filler dispersion are the main parameters to
consider. One might assume that it would be a simple matter to measure the volume
MFI, as described previously, and then correlate that to extruder throughput. However,
it has been shown that the transport of the polymer melt in an extruder is more complicated
than the simple flow used in measuring MFI. In fact, the mechanism is different for melt
transportation through a single-screw compared to a twin-screw machine [28]. The best
approach is to extrude the proposed formulations in the production extruder that will be
used. The volume throughput can be measured as well as the torque on the screw and the
energy input to the motor. However, production extruders can have outputs of well over
one thousand kilos (litres) per hour, which means a lot of filler and polymer is needed to
run a test. Even more problematic is the loss of productivity when a production extruder
is used for testing purposes. Usually, the test material made will not conform to existing
specifications and may have to be scrapped/discarded. More commonly an instrumented
laboratory extruder is used for initial testing. The feeding and screws of the laboratory
machine should be set up to mimic the configuration of the production extruder. When
this is done, one can obtain good correlations between the properties of compounds
made in the laboratory and production material. A more detailed treatment of
compounding is given in Chapter 5.
8.2.4.3 Dispersion
Good dispersion is nearly always beneficial for the properties of a composite so one tries
to optimise the dispersion of filler. The level of dispersion can be measured directly or
indirectly. The most common direct measurement is to perform scanning electron
microscopy (SEM) on a cross-section. It is advisable to use two different magnifications
to examine the filler distribution on a macroscopic and dispersion on a microscopic
scale. An indirect measurement is to measure the unnotched impact strength of the
composites, as that property is sensitive to agglomerates. In a rather insightful study,
Hornsby showed the degree of dispersion of filler as it passed through a twin-screw
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extruder [25]. This was achieved by using a clamshell extruder that could be stopped
and opened so that samples of compound could be removed for analysis. It was found
that the greater part of the dispersion was imparted in the melt zone where the pellets of
polymer are just melting. The high viscosity in that region requires a high energy input
and encourages deagglomeration. Feeding the filler into the unmelted polymer may give
good dispersion, but it also results in higher wear of the extruder so that this approach is
only advisable for soft, surface treated fillers. It is more usual to add the filler when the
polymer is already molten although a multitude of feeding possibilities exist [25].
8.2.4.4 Machine Wear
Machine wear is a concern, partly because it costs money to replace a worn barrel or
screw. The main problem though, is the loss of productivity when the machines must be
shut down for maintenance. Potentially, wear may lead to significant metal contamination
levels with accompanying polymer stability problems. Although machine wear is an issue,
it is not a subject that has received much attention.
There are a few studies that have investigated the effect of filler properties on wear
[17, 30]. One simple method is to extrude through a plate of soft metal and measure
weight loss from the perforated plate at regular intervals. It was concluded that hard,
large irregular particles cause most wear. Surface treatment with a lubricating additive
such as stearic acid helps alleviate wear because the additive forms a protective layer
around the particles.
8.2.5 Thermal Conductivity and Specific Heat Capacity
It is not only the final properties of the composite that matter, processability and
economy of manufacture are also important. If the part can be cooled more quickly,
then money can be saved through improved productivity. Mineral fillers typically have
thermal conductivities in the range 0.02–3 WK-1m-1 [5, 22, 31] that are an order of
magnitude higher than those of polymers [2, 32-34]. The volume specific heat capacity
of mineral fillers (~1900-2000 J litre-1K-1) [5, 22, 31] are very similar to those of polymer
melts (~1500-3000 J litre-1K-1) [2, 33-35]. An equation has been proposed for calculating
the heat capacity of composites based on the composition and the heat capacities of
the components [36]. The result is that the filler speeds heating and cooling of the
filled polymer melt through improved conduction. Therefore, filling a polymer often
allows for reduced cycle times in injection moulding [30] and thermoforming [34]
because the part cools and hardens more quickly, allowing it to be removed from the
mould earlier. In the literature, there have been some contradictory statements about
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the relative specific heat capacities of mineral fillers and polymers. This confusion may
have arisen because the specific heat capacity is usually given in terms of mass with
units of J·kg-1K-1, whereas to compare fairly, these values must be converted into volume
terms, i.e., J·L-1K-1.
There is a niche market for thermoplastics with very high thermal conductivity. These
are marketed for CPU cooling in laptop computers and other high performance
applications. An interesting point to note is that thermal and electrical conductivity actually
benefit from poor filler dispersion. Agglomeration and network formation (percolation)
allows better heat conduction due to the network of particle – particle contacts.
8.2.6 Thermal Expansion
The thermal expansion coefficients (CTE) for polymers (~10 x 10-5 mm mm-1 °C-1) [2,
36, 34] are approximately an order of magnitude higher than those for mineral fillers
(~10 x 10-6 mm mm-1 °C-1) [31] or for metals (~20 x 10-5 mm mm-1 °C-1) [31]. This may
lead to problems in applications where plastics and metals are in contact, as differential
expansion and contraction can cause such parts to warp. It is possible to estimate the
CTE of a composite based on the composition and knowledge of the CTE for each
component [37]. The polymer chains can become oriented during flow of the polymer
melt and this can give rise to a difference in the amount of shrinkage parallel and
perpendicular to the flow direction. Addition of particulate fillers such as calcium
carbonate, silica and talc tend to lessen the amount of polymer chain orientation, and
thereby reduce not only shrinkage, but also shrinkage differentials, with a corresponding
decrease in warpage [22, 34]. In contrast, fibrous fillers and other highly anisotropic
fillers, tend to partially align in the flow direction, leading to an increased shrinkage
differential and a tendency for the part to warp during cooling. Warpage is not easy to
predict, and so it is common to use low aspect ratio fillers for parts where warpage must
be avoided. Another approach is to use a mixture of low aspect ratio filler and fibres
[38], which can ameliorate the high warpage observed when fibres alone are used, whilst
maintaining sufficiently good mechanical properties.
•
•
•
It is often not possible to use the same mould for filled and unfilled polymer because the
change in shrinkage gives parts that are out of specification. On the other hand, adding
filler to a polymer allows its shrinkage to be systematically tuned. This tuning method is
useful for example, if one is attempting to use an existing mould with a different polymer.
Filler may be added to the newly chosen polymer to achieve similar shrinkage to that of
the previously used polymer. Injection moulding tools (moulds) are very expensive and it
is preferable to keep using the same mould rather than purchasing a new one specifically
made to accommodate the new polymer.
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8.2.7 Electrical Properties
The high tonnage, commercially important polymers are, in general, excellent insulators
with resistivities in the range 1012 – 1018 Ω cm [22]. The top three polymers in volume
terms (in descending order) PE, PP and PVC are all used extensively as cable insulation.
There are some intrinsically conductive polymers [39-41] such as polyaniline [42, 43]
polythiophene and polypyrrole [43], but these are relatively expensive, intractable, niche
materials, that must be modified to impart processability [44-46].
•
Most fillers, while having good dielectric properties and resistivities, are in general worse
than a plastic. Also, incorporation of a filler will introduce flaws and interfaces, charged
ions and traces of water (all fine particles adsorb some water from their environment) and
hence reduce electrical properties. Usually, however, because of the good electrical properties
of the plastic, fillers may be used simply as an extender, the composite still giving good
properties. Calcined clay (produced at just above 1000 °C) and other calcined silicates, do
not degrade electrical performance as severely as other fillers, because of low water pickup, and immobilisation of matrix ions. Conversely, metakaolin, because of its highly reactive
surface absorbs ions and thus improves the electrical performance of polymers such as
plasticised PVC and ethylene vinyl acetate (EVA) copolymers [47]. Due to their good
insulation characteristics thermoplastics can suffer from tracking, i.e., the build-up of surface
charge which then discharges across the surface, because that path has less resistance than
passage through the plastic. Filler particles can reduce this problem by acting as a physical
barrier and by distributing the charge before critical build-up occurs [48, 49].
In many cases, it is beneficial to introduce some level of electrical conductivity into a
polymeric material [5, 22]. The highly insulating polymers are susceptible to static build
up. This may be a nuisance, attracting dust to the surface, or it may lead to damage of
sensitive electronic parts, when manufacturing integrated circuits, for example. Even
very low levels of surface conductivity will resolve this problem [6]. Organic based
antistatic additives are available or alternatively conductive fillers such as carbon black,
graphite, or metals may be added [6].
Sometimes, a material of much higher conductivity is required. One example is for
electromagnetic interference (EMI) shielding [22]. This is achieved by adding a sufficient
level of conductive filler. Initially, as one adds more conductive filler, the conductivity does
not rise appreciably, because the conductive particles remain isolated from one another. As
the filler loading is gradually increased, the conductivity suddenly rises sharply, by many
orders of magnitude, approaching the conductivity of the conductive filler itself, and then
levels off [50]. This discontinuity is known as the percolation threshold and it signifies the
volume percentage of filler required to attain a continuous network of interconnected
particles throughout the matrix [22]. Usually, 10-30 volume percent of filler is needed to
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achieve percolation, depending upon particle size, shape and the level of dispersion. Smaller,
more anisotropic particles show lower percolation thresholds [39]. As with thermal
conductivity, good dispersion is detrimental to electrical conductivity [22, 51]. On the
contrary, the aim is to achieve percolation by inducing an agglomerated network of particles.
8.2.8 Barrier Properties
Thermoplastics are widely used as packaging materials due to their low cost, excellent
chemical resistance, good barrier properties and the potential for recycling. In fact, over
35% of all thermoplastic is used in packaging. They are also used in a variety of other
applications where barrier properties are required. These include water and gas pipes, as
well as car petrol tanks.
Permeability of a material to small molecule penetrants, such as oxygen and water,
increases with the solubility of the small molecule in the matrix [52] and with the diffusion
coefficient in that material [53]. Polymers are very sensitive to plasticisation by small
molecules. Thus, the presence of small molecules may greatly increase the diffusion
coefficient. Based on these observations, one can envision ways to decrease permeability
by reducing the solubility and/or the diffusion coefficient.
Molecules can neither dissolve in, nor diffuse through, mineral fillers to any appreciable
extent. Therefore the presence of filler reduces the solubility of the diffusant in the composite
material, and thereby the permeability, in proportion to the volume fraction of filler.
In addition, the presence of impermeable filler in a polymer forces the diffusant molecule
to travel further around the filler particles. This physical blocking effect is known as
tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous,
path through the material. The degree of tortuosity imposed is dependent upon the
anisotropy and orientation of the filler particles with respect to the direction of diffusion.
For example, platy particles oriented perpendicularly to the diffusion vector will be
particularly effective in retarding diffusion. The permeability of a composite can be
calculated using an equation that allows for the reduction in permeant solubility and for
the tortuosity (Equation 8.3). Where Pc and Pp are the permeability of the composite and
the unfilled polymer, respectively. The terms w and t refer to the width and thickness of
the filler and φp and φf represent the volume fraction of polymer and filler.
φp
Pc
=
Pp 1 + (w / 2t )φ f
Equation 8.3 Composite permeability
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Filled Thermoplastics
As mentioned previously, the addition of filler may also change the amount of crystallinity
in the polymer. As polymer crystals are impermeable even to low molecular weight species,
an increase in crystallinity also results in improved barrier properties, through increased
tortuosity [54]. This effect is expected to be especially prevalent for fillers that induce a
high degree of transcrystallinity.
Dispersion and wetting of the filler can also affect the permeability of the composite. It
has been shown that PE filled with 25 volume percent calcium carbonate was actually
four times more permeable to oxygen compared to the unfilled reference PE. This was
attributed to poor wetting of the filler, so that the diffusant was able to travel unimpeded
along the polymer/filler interface. In contrast, stearic acid coated calcium carbonate at
the same loading resulted in three times lower oxygen permeability than the unfilled PE
[55]. Similarly, Tiburcio and Manson showed that the water vapour permeability of
glass-bead filled phenoxy films decreased sharply as the degree of adhesion between the
filler and the matrix was increased [56].
In some cases, it is desirable to increase the permeability of a polymeric material. One
example is breathable films. For example, calcium carbonate filled PP films are first
made by solvent casting, or extrusion casting or as blown film and subsequently stretched
to delaminate the filler – polymer interface [57]. High filler loadings are used to ensure
interconnecting voids, giving unimpeded diffusion [58].
8.3 Mechanical Properties
8.3.1 Introduction
For any given application, certain mechanical properties will be of more importance than
others. It is therefore, essential to identify and rank the most relevant properties and formulate
or purchase the least expensive composite material that satisfies the requirements. The key
mechanical properties for most applications are modulus (tensile or flexural), yield strength,
impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres
are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere
in between. As shown later, it is not appropriate to define reinforcement in terms of particle
shape, because that definition breaks down with variations in anisotropy and particle size. In
agreement with Ram [59], the definition of reinforcement as the simultaneous improvement
of both modulus and yield strength will be used in this chapter.
Polymer mechanics is a broad subject and the interested reader is directed to specialised
texts [60]. Each of the main properties is described here, along with a consideration of
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Particulate-Filled Polymer Composites
how it changes with filler type and level of addition. The effect of surface treatments
such as dispersants and coupling agents are also mentioned, where applicable. A more
detailed description of surface treatments is given in Chapter 4.
8.3.2 Modulus – Tensile and Flexural
One of the main reasons for adding mineral fillers to thermoplastics is to increase the
modulus (stiffness). Tensile (under tension) modulus is the ratio of stress to strain, at some
low amount of strain, below the elastic limit. Flexural (bending) modulus is also often
measured. The most relevant modulus to measure depends upon the expected deformation
mode of the part that will be made from the material. Often, the flexural modulus and
tensile modulus are rather similar. The exception is the case of anisotropic fillers, which
can become aligned in the flow direction when the test specimens are moulded.
There is a good understanding of how addition of filler affects the modulus of a polymer.
Chow has done an extensive review of the area, and the interested reader can consult
that work for a more detailed description [61]. In fact, the simple rule of mixtures is
fairly accurate at low strain levels (Equation 8.4). Ec, Ep and Ef are the moduli of the
composite, polymer and filler, respectively, and φf is the volume fraction of filler. The
moduli of thermoplastics are in the range 1-3 GPa [1, 32, 35, 34] whereas common
fillers have much higher moduli [5, 22, 31] (calcium carbonate and dolomite ~ 35 GPa,
mica ~ 17.2 GPa and wood flour ~10 GPa).
Ec = (1 − φ f )E p + φE f
Equation 8.4 The dependence of composite modulus on volume fraction of filler
The modulus increases with increasing volume percentage of filler [62]. In most cases,
this relationship is linear for filler concentrations up to approximately 20 volume percent.
Filler orientation strongly affects modulus [63]. This is important because injection
moulded ASTM standard ‘dog-bone’ shaped test specimens give high filler orientation.
The moduli measured on such specimens may be far higher than those attained in a real
injection moulded part where the orientation is usually not optimal. Increasing crystallinity
in the polymer phase can also lead to a higher modulus because the crystal phase is
stiffer than amorphous regions [64, 65]. Various attempts have been made to account
for other factors such as polymer-filler interaction and interparticle interactions. One
such semi-empirical model is known as the Nielsen equation, also known as the LewisNielsen or modified Kerner equation [66, 67].
In these theories, modulus is independent of particle size. However, Heikens [68] has
found that when the polymer is strongly bonded to the plastic composite, modulus is
372
Filled Thermoplastics
affected by the filler particle size. Particle size effects are allowed for to some extent in
the modified Kerner equation by the introduction of an effective filler volume fraction.
The most important filler parameter affecting modulus is its shape. Unfortunately, when
the filler is non-spherical theories become much more complicated and the reader is
advised to refer to Chow’s review [61]. Shape factors can be incorporated in the models
mentioned previously but are only useful when applied to very high aspect ratio materials,
e.g., fibres. There is also an almost insurmountable problem with particulate fillers: the
difficulty and effort to measure aspect ratios of micrometre sized particles. Pukansky
examined the effects of 11 different fillers in polypropylene [69] and concluded that
Young’s modulus is affected by the amount of bonded polymer, which is in turn related
to surface area, and therefore to both particle size and shape. That observation helps to
explain the strong effect that nano-fillers have on the modulus of a composite. Schreiber
and Germain showed that modulus depends on the strength of interaction between the
polymer and the filler surface [62].
To exemplify the effect of fillers on a thermoplastic, PP homopolymer filled with differing
filler types and consequently very different shapes is shown in Figure 8.2. It can be seen
that a linear fit can be used successfully for most of the fillers. The exception is mica,
which deviates from linearity at high filler levels where interparticle interactions become
important. The modulus values also reflect the expected shapes of each of the fillers.
Similar trends are reported for other common polymers [22].
Figure 8.2 The effect of common fillers on the tensile modulus of PP homopolymer
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Particulate-Filled Polymer Composites
In conclusion, it can be stated that the effect of fillers on modulus is relatively well
understood and may be predicted. The other properties are less easy to predict as they
are measured under conditions where the composite is deformed to a greater extent.
This means that other factors such as particle debonding, particle re-orientation and
polymer orientation, must be considered.
8.3.3 Heat Deflection Temperature (HDT)
The HDT is the temperature at which a beam of polymer deflects by a given amount
under a specified load. The HDT is a complex function of the composite’s modulus and
polymer properties such as glass transition temperature (Tg), melting temperature (Tm),
degree of crystallinity and amount of bonded polymer in the filler-polymer interphase.
Examining the effect of fillers on the HDT of PP homopolymer (Figure 8.3) shows that
the trends are similar to those for modulus. There are two common standard conditions
for testing the HDT of PP; one uses a force of 0.46 MPa, whereas the other uses 1.8
MPa. Care must be taken when comparing results for different composites to ensure that
the same test conditions were used.
The greatest enhancement of HDT is seen with semi-crystalline thermoplastics such as
PE, PP, polyamides and PET, with only minor enhancements achieved for filled amorphous
Figure 8.3 The effect of common fillers on the HDT of PP homopolymer
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Filled Thermoplastics
polymers like polystyrene, acrylonitrile-butadiene-styrene (ABS), polycarbonate and
polysulfone [1]. Addition of glass fibre to semi-crystalline polymers gives an HDT
approaching the melting point of the matrix. In the case of amorphous polymers,
incorporation of glass fibre gives an HDT (at 1.8 MPa stress), which is close to the Tg of
the matrix. For this reason, filler is most commonly used in semi-crystalline polymers
where the HDT is improved most because the crystalline regions help transfer stress to
the filler under load [70].
8.3.4 Yield Strength
Yield strength is a measure of the force that a material can withstand before it suffers
macroscopic plastic deformation. For most materials, e.g., metal, it is taken as the
point on the stress-strain curve when the line becomes non-linear (the elastic limit).
However, for plastics, it is taken as the peak of the stress-strain curve, as that is
simpler to measure. In practice, most parts are designed so that they never experience
a force approaching the yield stress because yielding represents failure of the material.
Yield strength is a key property when designing parts. Fillers are often added because
they increase the yield strength of the polymer, this effect is known as reinforcement
if the modulus is also improved [59]. The explanation for reinforcement lies in the
fact that adding filler actually changes the polymer phase. It has been shown that
polymers interact with the filler surface, forming an interphase of adsorbed polymer
[71-74]. The thickness of the interphase can vary widely from system to system.
That is to be expected; for example polar polymers such as polyamides are capable of
strong, specific interactions with groups on the filler surface. In contrast, non-polar
polymers such as PE and PP have weaker interactions with fillers. The apparent
thickness of the interphase also depends strongly upon the measurement method.
Lower values of around 0.004 µm are reported from extraction experiments, whereby
all non-adsorbed polymer is solvent extracted [75, 76]. Values deduced from
mechanical data such as by dynamic mechanical analysis or modulus tend to be much
larger, in the range 0.012 to 1.4 µm [77]. This interphase has mechanical properties
intermediate between those of the polymer and the filler [78, 79], thereby allowing
an increased yield strength for the composite.
Several factors determine the level of reinforcement attained by adding filler. These include,
the volume fraction of filler added, the surface area of the filler (related to particle size),
particle shape, the level of adhesion between the filler and polymer [80], as well as the
thickness and nature of the interphase between the two phases. A linear correlation
between yield strength and heat of crystallisation has also been reported in the case of PP
filled with calcium carbonate [81]. It is well known that spherical fillers give least
375
Particulate-Filled Polymer Composites
Figure 8.4 The effect of common fillers on the yield strength of PP homopolymer
reinforcement, platy fillers are better and fibrous fillers are best of all [5, 30]. Usually, it
is considered that spherical fillers such as calcium carbonate and dolomite do not reinforce
at all, and in fact they usually reduce the yield strength of the material (Figure 8.4).
However, that is not necessarily true, as it has been shown that it is possible to increase
the yield strength of PP by using very fine spherical filler with a mean diameter of 0.01
µm [78, 79]. This improvement must be due to the high surface area of the filler as the
filler is isotropic. The high surface area increases overall polymer-filler adhesion and
thereby improves yield strength.
It is observed that spherical fillers do not reinforce whereas platy fillers like mica may do,
and glass fibres are most effective (Figure 8.4). In this particular example, talc does not
reinforce, probably because the talc grade used did not have sufficient anisotropy. As with
yield strength, the data shown is for PP homopolymer, but similar trends are seen for a
wide range of other thermoplastics [3, 22]. The filler creates an additional complication
especially for injection moulded parts. Namely, during mould filling, the filler distribution
becomes non-homogeneous due to the flow. One consequence is flow lines and weld lines.
These are created when two fronts of molten polymer meet. For an unfilled polymer the
melt can easily mix when two melt fronts meet and so the mechanical properties are normally
unchanged (except for the special case of liquid crystalline polymers). The uneven distribution
of filler at the weld lines creates a weak point, so for example, the measured yield strength
and elongation to break are reduced. This effect is not as great for isotropic fillers but for
more anisotropic fillers the yield strength may be reduced by more than fifty percent. It is
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Filled Thermoplastics
therefore essential to design with this in mind. This can be done by designing parts to avoid
weld lines and by judicious placement of injection points. Additionally, it should be
remembered that the reported mechanical properties for composites are for ideal specimens
with no weld lines, whereas the actual yield strength in the part may be far lower.
8.3.5 Impact Strength (Toughness)
The impact strength of polymers and composites is another key property. In contrast to the
other mechanical properties, it is not possible to predict the impact strength of a
thermoplastic composite. The reason is that there are too many factors to be considered.
One of the major complications is that adding hard filler can change the mode of failure
from ductile to brittle [2, 7], or vice versa for rubbery fillers [7]. The filler may act as a
flaw, if there are large particles or agglomerates [82, 83]. Alternatively, well dispersed,
small particles can improve the impact strength by a crack-pinning mechanism. Another
problem in predicting impact strength of composites is that mechanical properties of
polymers are very dependent upon the rate at which the testing is performed. Most
mechanical data are acquired at low speeds; for example, tensile testing is often performed
at a strain rate of 0.1–10 mm·mm-1min-1. In contrast, impact testing is a very rapid event
(>10 000 mm·mm-1min-1) and the polymer often responds very differently. For example, a
polymer that fails in a ductile way during tensile testing may become brittle under impact
test conditions because when the deformation is fast, the polymer chains have insufficient
time to move and accommodate the deformation. It is well known that the response of
polymers is dependent upon testing rate [60, 84-86]. The WLF (Williams, Landel and
Ferry) equation [87] can be used to account for this effect at temperatures near to the Tg.
The WLF equation predicts that the effective Tg of a polymer changes by about 6.9 °C for
every decade of change in the rate of testing [84]. That means that a polymer which has a
Tg below room temperature under tensile testing conditions, may have a Tg well above
room temperature under impact test conditions. Therefore, the polymer may behave in a
ductile manner, with high strength, under tensile testing but it may be brittle under impact,
giving low impact strength. In cases where the mode of fracture does not change with
testing speed, then it is expected that the energy to break determined by tensile testing (the
area under the stress-strain curve) will correlate with the impact strength.
Similar complications exist when it comes to measuring impact strength. Many methods
are available, the most common of which involve either a pendulum striking the sample
(Izod and Charpy), tensile impact testing, or a falling dart. In general, impact tests do not
correlate well with each other, although it has been shown that there is a correlation between
Izod and Charpy values [88]. Impact tests may be performed on unnotched (as moulded)
samples or pre-notched samples, whereby a well-defined flaw is introduced to ensure that
the sample fails at the desired point. Adding a notch improves the reproducibly noticeably,
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Particulate-Filled Polymer Composites
but it may not be realistic as parts in actual use are not notched, at least not intentionally.
Unnotched testing is best for detecting agglomerates and flaws because the crack initiates
at such imperfections. Notched impact testing is comparatively insensitive to agglomerates
and large particles because the sample fails preferentially at the large, introduced flaw.
Reverse notched testing such as Izod E is especially suitable for composites. This sample is
struck on the opposite side to the notch. In this way the stress is concentrated opposite the
notch but the introduced notch is not the site where crack growth occurs. The notch
encourages reproducibility by localising the stress on the opposite side of the sample, but
the crack initiation occurs at some imperfection in the composite and so the method is still
sensitive to agglomerates.
In the final analysis though, the lab scale impact tests are of limited value. They may be
used to compare materials qualitatively, but the ultimate test is to make the part or
prototype and perform impact tests on that, in a manner that closely simulates the way
in which the part is to be handled during manufacture, or used.
There are many reports describing the effect of fillers on the impact strength of polymers.
These are however, hard to compare as they often use different polymers, test methods
and filler particle size distributions. It is therefore difficult to make any general
comments. One study showed that increased filler anisotropy (aspect ratio) resulted in
reduced falling weight impact strength [89]. That is particularly interesting because
increased filler anisotropy is known to improve modulus. This is therefore an excellent
illustration that one must be prepared to sacrifice some property for the sake of
improving another, more important one. There has been some success in creating high
modulus composites with good impact strength. This was done using ternary composites
where a dispersed, stiff filler is encapsulated in a rubbery layer dispersed in the matrix
polymer [90]. It can be stated that fillers can either decrease or increase the impact
strength compared to the unfilled polymer. A common technique for increasing impact
strength is to disperse a soft, or rubbery, filler in a harder polymer to increase its
impact resistance. This method is used to make HIPS and to improve the impact
resistance of PP, especially at low temperatures [91].
Hard fillers on the other hand may either increase or decrease impact strength [92].
For example, fine calcium carbonate or talc can increase the impact strength of PP
homopolymer by more than a factor of two [93, 94]. This only holds if there is good
dispersion to avoid agglomerates, if there are no large particles, above about 10-20 µm
in diameter, the calcium carbonate is stearate coated and the particle size distribution
has been optimised [94]. There seems to be a definite interaction or synergy between
particle size and coating level, and the highest impact strengths are found with calcium
carbonates with 1% stearate coating and a mean particle size 1-2 µm, depending on
the type of polymer and the nature of the impact test (se Figure 8.5).
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Filled Thermoplastics
(a)
(b)
Figure 8.5
(a) Effect of stearate coating level on a ultrafine calcium carbonate (d50: 0.8 µm) on
the properties of polypropylene
(b) Effect of particle size of stearate coated calcium carbonate on the properties
of polypropylene
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Particulate-Filled Polymer Composites
In the case of PP copolymer and impact modified PP, the effect of filler is very different;
impact strength is lowered significantly by hard particulate fillers. This is because the
filler interacts with the soft or rubbery component and nullifies its ability to adsorb
energy during impact.
In conclusion, it seems that soft rubbery fillers improve impact strength by helping to
dissipate the energy of impact. Hard fillers decrease the impact strength of ductile
materials, which already have high impact strength. On the other hand, well dispersed,
hard particles of the correct particle size may improve the impact strength of brittle
materials like PP homopolymer or polystyrene [22] by promoting crazing.
8.4 Effects of Filler on the Polymer Phase
8.4.1 Introduction
To understand and predict the properties of composites, it is necessary to realise that
adding filler may affect the polymer phase, both chemically and physically. Chemical
changes may occur if the filler, or impurities on the filler surface, catalyse degradation of
the polymer. Alternatively, various physical changes may result from the incorporation
of filler. Some fillers nucleate crystal growth in certain polymers, which in turn influences
manufacturing and mechanical properties.
In the literature, the possibility that the filler may have altered the polymer phase is
rarely considered. It is important to realise that the polymer may be altered by the filler,
especially in the case of semi-crystalline polymers [95] like PE, PP, PVC, PET and the
polyamides. Therefore, these effects are mentioned here to allow a better understanding
of the factors affecting the performance of a composite.
8.4.2 Nucleation
It is widely recognised that fillers may affect the crystallisation of polymers [96]. The
filler may increase the rate of cooling, as mentioned previously, and may thereby affect
crystallisation. Some polymers have more that one type of crystal phase, which occur
preferentially when cooling within a certain temperature range [97, 98]. These crystal
types have different mechanical properties [99, 100] and therefore, the relative amount
of each phase will influence to the properties of the thermoplastic composite.
In other instances, the filler may nucleate crystal growth. This is often beneficial, as it
causes the material to harden more rapidly on cooling, giving the possibility of faster
production. In fact, it is common practice to add a small amount of fine talc to nucleate
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Filled Thermoplastics
Figure 8.6 The effect of some common fillers on the impact strength
of PP homopolymer
crystallisation of PP, especially in thin-walled parts. Interestingly, dolomite is also rather
effective, and calcium carbonate and mica have a slight effect on crystallisation onset
temperature (Figure 8.6). In some cases surface treatment of the filler affected nucleation
and in other cases no effect was seen.
As well as the crystallisation onset temperature, the peak of the crystallisation endotherm
may also be used as a measure of nucleating effectiveness. The two methods show the
same qualitative trends.
Another important parameter is the proportion of crystallinity in the composite. The
crystalline phase has a higher modulus than the amorphous phase, and it has been reported
that the yield strength is linearly proportional to the heat of crystallisation [81]. Clearly,
the mechanical properties of the composite are influenced by the degree of crystallinity.
For a given PP copolymer grade, the degree of crystallinity was 43 weight%, whereas
this was 45-48 weight% when 60 weight% of magnesium hydroxide, dolomite or talc
was added. When measuring the degree of crystallinity one must correct for the amount
of filler added. For example adding 50 volume % filler will reduce the apparent crystallinity
[as measured by differential scanning calorimetry (DSC)] by 50%. The explanation is
quite simply that 50% of the polymer has been removed. It is quite common that this
dilution effect is not accounted for, or it is not stated whether it has been corrected for.
This leads to some confusion in the literature.
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Particulate-Filled Polymer Composites
Unfortunately, it is not possible to predict whether a particular filler type will nucleate
crystal growth. For some time it was thought that high energy surfaces nucleated, but this
has since been proven incorrect [95]. Instead, Hobbs has shown that surface microtopology
influences nucleating ability [101]. In that work, Hobbs made a replica of a nucleating
surface using a non-nucleating material, to produce a surface that was strongly nucleating.
It can be concluded that any thorough study of filled polymers must also consider the
possibility of nucleation and changes in the degree of crystallinity.
8.4.3 Transcrystallinity
Transcrystallinity may occur when a polymer is cooled in contact with a highly nucleating
filler or surface [95, 102]. Usually, polymer crystals are spherulitic, growing out radially
from the nucleation site [2, 85, 86]. In contrast, when crystals are nucleated very close
together, they impinge on each other almost immediately, and are forced to grow in one
direction, away from the nucleating surface [95]. For a polymer where only one crystal
form occurs, it has been shown that the microstructure of the transcrystalline phase is
the same as that for the spherulites [103]. The transcrystalline layer is typically 10-30
µm thick, which means that for higher filler loadings, the whole matrix may be composed
of transcrystalline material [95]. Transcrystallinity has been reported in several systems
including glass [104], PET [105, 106] and some types of carbon fibre in PP [107], as well
as Kevlar fibres in Nylon 6,6 and even in Nylon-PP blends.
Sheets of transcrystalline PP have been prepared by cooling PP from the melt in between
PET (Melinex®) sheets and the properties studied [108]. This rather elegant work by Fowkes
and Hardwick showed that the transcrystalline sheet had a much higher Young’s modulus
(1.09 GPa) compared to the control PP sample that was quenched to give a fine spherulitic
sheet (0.67 GPa). The tensile yield strength of the transcrystalline material was also higher
at 25.0 MPa, compared to 18.6 MPa for the spherulitic sheet. Some of the properties were
much worse compared to normal, spherulitic PP. For example, the transcrystalline sheet
showed just 4% elongation to break and an energy to failure of just 28.0 kJm-2, compared
to >300% and 48.5 kJm-2, respectively, for the fine spherulitic analogue.
Clearly then, the filler can greatly influence the type and level of crystallinity, leading to
profound changes in the properties of the resultant composite material.
8.4.4 Interphase
Aside from changes in crystallinity, there is another way in which the presence of filler may
alter the host polymer. It has been shown that polymer adsorbs onto the filler and that this
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Filled Thermoplastics
adsorbed material has different properties compared to those of the bulk matrix [68, 71,
109, 110]. Pukansky and Fekete have written a review of the importance of the interphase
in thermoplastics [9]. The relevance of the interphase to adhesion at planar interfaces, and
in composites, has been discussed by Berg [109]. As expected, the thickness of the interphase
varies depending upon the extent of interaction between the polymer and filler. It has been
shown that the thickness of the interphase is proportional to the reversible work of adhesion
[8]. Reported thicknesses are usually in the range 0.004–0.15 µm, depending upon the
polymer filler combination and the method used to estimate the thickness [9, 22]. It is
anticipated that the interphase thickness should be influenced by the ubiquitous van der
Waals forces plus any specific chemical interactions such as Lewis acid-base or hydrogen
bonding or covalent bonding [22]. Therefore it should be affected by surface treatment of
the filler. The degree to which the interphase affects the properties of the composite should
also therefore depend on the total surface area of the filler [22] and is therefore especially
important for nano-composites.
8.5 Surface Science Aspects
8.5.1 Introduction
The importance of surface interactions in composites is widely recognised, and yet, it is
probably the one that leads to most confusion. This is partly because it is difficult to
characterise the interphase, but also because the composites field is interdisciplinary,
requiring not only surface science and adhesion science but also an understanding of
polymer science. This section will briefly mention some of the relevant points, but surface
and colloid science is a wide area and specialist texts should be consulted if more detail
is required [111-114]. Pugh has reviewed the wetting and dispersion of ceramic powders
in liquids, which is highly informative and relevant to the case of mineral particles dispersed
in a polymer [115]. The interfacial interactions in particulate filled composites have been
reviewed by Pukánsky and Fekete [9]. Most recently, Berg has written an excellent review
on the subject of wetting and adhesion [109].
8.5.2 Surface Energy and Surface Tension
Surface energy must be introduced in order to be able to understand the forces that
drive wetting and adhesion. If one imagines an atom or molecule located in the bulk
of a material, the forces acting upon that entity are symmetrical. Each unit is attracted
to its neighbours equally. If one then cleaves the material, then the forces acting on
the units (atoms or molecules) at the newly formed surface are no longer symmetrical.
Those surface units are still attracted towards their neighbours, but they lack attractive
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Particulate-Filled Polymer Composites
interaction in the direction of the new interface. Because these units at the interface
are in a new environment, energetically speaking, there must be a change in the total
free energy. This change in energy relative to the bulk is termed the surface free
energy, with units of mJm-2. For liquids, the net force towards the bulk manifests
itself as a tension at the interface. This gives rise to the concept of surface tension in
liquids, with units of mNm-1. There are various old and new units of surface energy
and surface tension, but fortuitously, it turns out that they are numerically equivalent,
i.e., 1 mNm-1 = 1 mJm-2 = 1 dyne cm-1 = 1 erg cm-2. So, in summary, the surface
energy is the energy required per unit area to form a new surface of that material in
a vacuum. The symbol for surface tension and surface energy is γ, where γ S, γ L and γ SL
refer to the surface energies of the solid, the liquid and the interfacial surface energy
between the solid and the liquid, respectively.
•
•
The surface tension of liquids is easily measured by a wide variety of methods, whereas
it is much more difficult to measure the surface energy of solids. A comprehensive overview
covering methods applicable for liquids and solids is presented by Adamson [111].
8.5.3 Wetting and Spreading
When the filler is mixed with the polymer melt, it is important that the polymer wets the
filler. It is desirable for the polymer to penetrate in between the particles, displacing the air
and giving intimate contact between the polymer and filler surface. This will prevent trapped
air bubbles and aid adhesion between the filler and polymer. Wetting, and more specifically,
immersional wetting, is the term used to describe this process [112, 115]. The driving force
for wetting to occur is related to the surface energies of the wetting fluid and the solid [8,
109, 111-115]. The surface energy of solids is usually determined using static contact
angle measurements. This is a useful method, although the results are affected by surface
roughness, drop size and contaminants [116]. Furthermore, contact angle measurements
are only valid when the liquid does not penetrate into the solid under analysis [116], which
is of particular importance for polymers, as they are often swollen by liquids.
Theoretically, under equilibrium conditions, a low surface energy liquid, such as a polymer
melt, should completely wet the high energy mineral filler surface [112, 115]. However,
this view is overly simplistic, and does not apply in reality for several reasons. One
reason is that high energy surfaces attract contaminants and are usually covered in a
layer of hydrocarbon material spontaneously adsorbed from the atmosphere [112]. This
means that the ‘high energy’ surface no longer behaves as such. Another, more fundamental
problem is that polymer processing is a dynamic process and so kinetic aspects of wetting
are more important than thermodynamics. Dynamic wetting has been studied as it is of
great industrial importance. It can be shown that the low surface energy and the high
384
Filled Thermoplastics
viscosity of the polymer melt both disfavour wetting of the filler [111, 112]. Dynamic
wetting in composites is an area that deserves further study in order to understand and
optimise wetting under real processing conditions. Some studies have been performed, in
particular for fibre reinforced composites [117].
The spreading coefficient represents the thermodynamic driving force for the liquid to
spread onto and wet the solid (Equation 8.5). For spontaneous spreading to occur, the
spreading coefficient must be positive, implying a contact angle of zero.
S L / S = γ S − (γ L + γ SL )
Equation 8.5 The spreading coefficient
8.5.4 Adhesion
The degree of adhesion between the filler and polymer is expected to influence the mechanical
properties of the composite. This can be predicted theoretically, although it is much more
difficult to prove experimentally. It is a major challenge to change the adhesion between
the polymer and the filler without invalidating the experiment by unintentionally altering
other parameters [109] such as filler dispersion level or polymer crystallinity.
Even so, many workers have tried to correlate the calculated reversible work of adhesion
with the mechanical properties of composites. Studies on planar surfaces have shown
that measured adhesive bond strengths are, at best, only one tenth of the calculated
value based on van der Waals interactions alone. It might therefore be assumed that the
reversible work of adhesion would have rather limited utility as an indicator of adhesion
at the interface. Despite that, it has been shown numerous times that the reversible work
of adhesion often does correlate rather well with the mechanical properties of composites,
in particular yield strength [8, 109]. Berg addressed the question of whether it was
worthwhile to use concepts such as reversible work of adhesion to predict and tune
adhesion [109]. The conclusion was that the approach is reasonably effective already
and will improve in the future.
The first criterion for good adhesion is intimate contact, that is good wetting of the
surface. Wetting is a necessary, but not sufficient, condition for good adhesion [109]. In
addition, one should seek to maximise the work of adhesion. The simplest case is to
consider the work of adhesion to be attributable solely to non polar (London) forces
between the materials. This is indeed the case when at least the adhesive (polymer melt)
or adherand (filler surface) is non-polar. So, for thermoplastics such as PE, PP and
polytetrafluoroethylene (PTFE) this simple case should apply.
385
Particulate-Filled Polymer Composites
The work of adhesion is given by Equation 8.6. This implies that the work of adhesion
can be maximised for a given adherand by increasing the surface energy of the adhesive.
However, we also know that wetting is required and so the surface tension of the adhesive
must not exceed that of the adherand. So, the ideal case is when the surface tension of
the polymer melt is as high as possible, but without exceeding that of the filler surface,
so that spontaneous spreading still occurs. This has been verified experimentally for a
range of polymeric adhesives on a set of surfaces with controlled surface energy [109].
WA = 2 γ Sγ L
Equation 8.6 Work of adhesion (WA) for the non-polar case
Later, it was realised that polar interactions could also play a role in wetting and
adhesion. The surface energy was divided into non-polar and polar components and
new equations were then proposed to allow for the possibility of polar interactions
across the interface (Equation 8.7). From that equation it can be expected that maximal
adhesion will occur when the ratio of polar to non-polar surface energy is the same for
the adhesive and the adherand.
WA = WAd + WAp = 2 γ dS γ Ld + 2 γ Spγ Lp
Equation 8.7 Work of adhesion for the polar case
Wu proposed another equation using an harmonic mean approach to estimate the nonpolar and polar interactions across the interface [8, 109, 116]. This approach gives the
same criterion for maximising adhesion.
Folkes went a step further by arguing the importance of Lewis acid and base interactions.
This leads to a new elaboration of the equation for calculating the work of adhesion
(Equation 8.8).
WA = WAd + WAp + WAAB
Equation 8.8 Work of adhesion in Lewis acid-base terms
The premise is that each component can only interact with its corresponding component
in the other material. Intuitively, it seems reasonable that this should be the case, although
again, it is not clear whether the geometric or harmonic mean, or some other method, is
best for estimating the magnitude of the Lewis acid-base interactions across the interface.
Fowkes’ proposal has been widely accepted and verified experimentally. For example
386
Filled Thermoplastics
Sinicki and Berg varied the Lewis acid-base interactions systematically and found a
correlation between adhesion measured by peel testing and the calculated thermodynamic
work of adhesion [118].
Schreiber and Germain used plasma treatment to alter the Lewis acid-base balance of
three fillers [62]. These were then compounded into two different matrix polymers. Firstly,
polyethylene (LLDPE), which is completely apolar and cannot form Lewis acid-base
interactions. The other polymer was poly(ethylene-vinyl acetate) [EVA], which is based
on polyethylene, but contains some (28 mole%) acetate groups, which they found to be
Lewis acidic (although acetate groups are also reported to be Lewis basic [109]). They
then measured tensile modulus and elongation at break to assess the affect of adhesion
on mechanical performance of the composites. In LLDPE, a fluoropolymer (PTFE-like)
coating on the filler gave lowest modulus and highest elongation to break, attributed to
poor adhesion. An ammonia plasma was used to give a polar, Lewis basic surface. For
this combination, the modulus was lower and the elongation to break higher than for
the untreated filler. This was consistent with reduced adhesion compared to the untreated
filler, but better than for the fluoropolymer coated filler. The modulus and adhesion
were maximised when the filler was plasma treated with methane to make the surface
non-polar like the polymer.
When the matrix polymer was changed to EVA, the results were quite different. In that
case, the highest modulus and least elongation to break were recorded for the ammonia
plasma treated filler. This was interpreted as being due to enhanced adhesion from the
bonding between the Lewis acidic groups in the polymer and the Lewis basic sites
introduced on the filler surface through ammonia treatment.
8.5.5 Dispersion and Agglomeration
Good dispersion of fillers and pigments is a prerequisite for good mechanical and
aesthetic properties. For example, multiple studies have shown that filler agglomerates
act as stress concentrators, reducing tensile and impact strength [82]. Similarly, good
dispersion results in a smoother surface, with a concomitant increase in specular gloss
[119]. Shenoy [24] and Hornsby [25] have each given detailed descriptions of the factors
influencing dispersion of particulates in polymers. In the case of pigments, good
dispersion results in a higher tinting strength so that less pigment is needed to attain a
given level of pigmentation. The principal exceptions are thermal and electrical
conductivity, which are improved by particle agglomeration to form a percolated
(continuous) network [22].
The ubiquitous van der Waals forces ensure that particles attract each other, thus favouring
agglomeration. The magnitude of this attractive force can be calculated if the Hamaker
387
Particulate-Filled Polymer Composites
constants for the disperse phase (filler particles in this instance) and the dispersion medium,
i.e., the polymer melt are known. The Hamaker constants for several inorganic materials,
including fillers such as calcium carbonate, muscovite mica, silica and titania have been
calculated by Bergström and co-workers using Lifshitz theory [120, 121]. In a subsequent
study, it was possible to directly measure the van der Waals forces between materials using
atomic force microscopy (AFM). The results were found to be in good agreement with the
magnitude of the attractive force predicted from the calculated Hamaker constants [122].
Once the particles have agglomerated, the surfaces come into intimate contact and other,
short-range forces can become important. Thus, the force required to separate the particles
will depend on the van der Waals force plus any specific surface interactions that have
occurred such as water bridging, hydrogen bonding or Lewis acid-base interactions. The
polymer melt competes to interact with the surface so that the overall energetics depend
on whether it is more favourable for the filler surface to interact with another particle, or
with the polymer phase.
In order to separate the particles, mechanical energy must be added [25]. Then, once
separated, the particles must be kept from reagglomerating. This can be achieved by
reducing the attractive forces between the particles (by adding a dispersant), or by
increasing the affinity of the polymer phase for the filler surface (by adding some polymer
modified to interact with the filler). These two approaches are well understood and are
described in texts on surface and colloid science [111-115]. A brief description of
dispersants and coupling agents is given below, but is dealt with more thoroughly in
Chapter 4. A theoretical model has been developed to help in optimising dispersion in
twin-screw extruders [123].
8.5.6 Surface Treatments – Dispersants and Coupling Agents
Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson
studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It
was found that each one had a different surface chemistry and they were all amphoteric
(possessing both Lewis acid and basic sites). Another point was that trace impurities of
iron on the filler surface, presumably picked up during processing, significantly changed
the surface chemistry of silica.
In unpublished work, DeArmitt and Breese used a novel rheological method to characterise
a wide variety of mineral fillers. The method was especially attractive in that it allowed
investigation of the surface chemistry of the fillers, while at the same time showing which
types of organic probe molecules were most effective as dispersants. This work confirmed
the findings of Ernstsson and Larsson [125, 126]. Each mineral displayed a unique affinity
for the probe molecules and each was appreciably amphoteric, adsorbing Lewis acid and
388
Filled Thermoplastics
Lewis basic probes. This shows that the dispersant should be chosen to optimise its
affinity for the filler or pigment.
It is important to ensure that the chosen surface treatment agent does in fact bind to the
filler. If the additive does not bond to the surface then it cannot fulfil its function. Excess
additive is, at best, a waste of money, but in some cases it may have worse consequences
such as destabilising the polymer. For example, it has been shown that calcium stearate,
a common dispersant, can destabilise polyolefins and cause yellowing, by interacting
with the antioxidant [127].
8.5.6.1 Dispersants
As mentioned, dispersant design and mode of action are well understood by surface
and colloid scientists [43, 112-115]. In that field, the term dispersant refers to any
additive that reduces the interparticle interactions, thereby encouraging dispersion of
the particles. This is achievable via a number of mechanisms using low molecular weight,
oligomeric, or polymeric additives [128]. Steric stabilisation is most relevant to mineral
fillers in polymers because it is the main way to achieve colloidal stability in low polarity
solvents. This stabilisation mechanism operates by strong adsorption of a layer of
organic additive that physically prevents close interparticle approach.
Stearic acid and metal stearates are widely used as dispersants, especially in cases where
high filler loadings are required. Examples are polyolefins filled with aluminium hydroxide
or magnesium hydroxide where 60 weight percent of filler or more may be needed to
achieve sufficient flame retardancy [129, 130]. Of course the correct level of addition
depends upon the amount of filler surface to be covered, and therefore upon the amount
of filler, and its specific surface area. Excess additive is to be avoided as it can seriously
destabilise some polymers and give yellowing problems [127].
8.5.6.2 Coupling Agents
Dispersants need only adhere to the filler, and help reduce particle – particle interactions.
Whereas for coupling agents, as implied by the name, the additive must be bi-functional,
adhering both to the filler and to the polymer. It is therefore found that the best choice of
coupling agent varies depending on the filler and polymer. In some instances, the socalled ‘coupling agent’ may not couple to either phase, or may only adhere to the filler.
In the latter case it may act solely as a dispersant, rather than as a coupling agent. It is
therefore important to note that an additive marketed as a coupling agent will only
provide coupling for certain combinations of filler and polymer.
389
Particulate-Filled Polymer Composites
Filled Thermoplastics
It is often assumed that coupling (adhesion) is good, but that is not always the case. For
example, good coupling is undesirable during extrusion, because it would dramatically
increase the torque (and energy) needed for extrusion. This would reduce extruder
throughput and that would result in a significantly more expensive material. Coupling is
advantageous when high yield strength is required, but it is often detrimental to the
elongation at break. Impact strength is more difficult to predict, it may increase or decrease
depending on the filler – polymer combination. The effect of coupling agents on modulus
is less clear. The equation for describing modulus has no term for adhesion between filler
and polymer, so coupling agents should not affect modulus (Equation 8.4). However,
there are reports that they do affect modulus. This may be due to increased orientation
of the filler or due to the way that the modulus was determined. The modulus should be
determined at low stress, in the linear part of the stress-strain curve, where filler debonding
has not yet occurred, and should therefore be insensitive to adhesion.
8.6 Aesthetics
8.6.1 Introduction
Often thermoplastic composites are used in applications where the consumer will see the
part [131]. In those instances, it is vital to consider the aesthetic aspects of the material
as well as the mechanical, electrical and other properties. Fillers affect the surface finish,
colour and scratch-resistance of the composite and these factors should be optimised to
give a marketable product.
8.6.2 Colour/Pigmentation
Pigments may be either organic or inorganic particles that are added to give colour to a
plastic, as opposed to dyes, which dissolve in the plastic. Pigments can be considered as a
special class of filler, and they influence the polymer in much the same way as any other
filler would. So, for example, they can lower the thermal stability of the polymer matrix
[18], adsorb antioxidant and nucleate crystallisation of the polymer. They also affect the
mechanical properties in the same way as any other filler, but usually pigments are used at
concentrations that are too low to significantly affect modulus, yield strength or HDT. It is
essential to disperse pigments thoroughly in order to achieve the maximum tinting strength,
and to avoid agglomerates, which would lower the impact strength, as discussed previously.
As dry powders, several common fillers appear white, because they scatter light strongly,
and it might therefore be supposed that they would make good pigments for polymers.
This is usually not the case however, because the amount of scattering is determined by
the difference in refractive index between the particulate and continuous phases. This
390
Filled Thermoplastics
refractive index difference is small for common fillers and thermoplastics and so scattering
is limited. In fact, in some cases the filler is invisible in the polymer because the refractive
indices match very closely. An example is glass beads in PVC. Calcium carbonate is
available in high whiteness and gives a mild pigmentary effect, but if high whiteness is
required, then a pigment of higher refractive index, such as titanium dioxide, must be
used. Titanium dioxide and carbon black are used to protect against UV radiation as
they scatter and adsorb it, respectively. It should be noted that commercial titanium
dioxide is always has an inorganic coating, e.g., aluminosilicate, plus an organic additive
such as a polyol or silicone [132]. This is necessary because naked titania can oxidatively
degrade polymers when exposed to UV light.
8.6.3 Surface Finish and Gloss
Surface finish is important to the end-user. It can be altered to be glossy or matte, as
fashion dictates, and similarly, textures can be used to convey the right feel when handling
the product. The surface may also be tuned for more functional reasons. For example,
smoother surfaces are generally more hygienic and easier to clean, whereas a rough
surface can prevent blocking (self adhesion) of films or increase the coefficient of friction.
It is well known that fillers can affect the surface finish of the host polymer [133]. Coarse
particles tend to give a rough surface finish, whereas fine particles decrease roughness
with a concomitant increase in gloss [119]. Surface treated filler can give better gloss by
aiding dispersion. For a given filler type the gloss is steadily reduced as more filler is
added. However factors such as the mould surface and processing parameters can have
a very large effect on the final gloss. For low shear situations like gas-assisted injection
moulding even 10 weight% of filler may decrease the gloss to unacceptable levels. For a
normal injection moulded part the gloss may still be high at filler levels as high as
40 weight%. At low filler loadings it is possible to retain much of the gloss of the unfilled
polymer, but above a certain filler level (depending on filler type, size and treatment),
there is a sharp reduction in gloss. Special surface treatment of the filler has been shown
to decrease scratch visibility.
8.6.4 Scratch and Abrasion Resistance
Scratch and abrasion resistance is very important, especially in the home appliance
and automotive industries [134]. The manufacturer may be forced to use a more
expensive polymer or an unfilled polymer in order to maximise scratch resistance.
There are two different aspects of scratch resistance. One is the actual size of the
physical scratch, but the more important aspect is usually the visibility of the scratch,
because that is what the user sees. Often filler increases the visibility of scratches. A
391
Particulate-Filled Polymer Composites
commercially important example is talc-filled polypropylene where the talc particles
become exposed by scratching, giving an undesirable white mark. Other fillers suffer
the same problem and a great deal of work has been devoted to lessening the scratch
depth and visibility of thermoplastic composites.
Attempts to correlate the mechanical properties of polymers to their scratch-resistance
have been largely unsuccessful. This is partly because the response of polymers to
deformation is very dependent on the speed of testing. The mechanical properties of polymers
and composites are usually determined at much lower rates than those encountered during
scratching. Evans and Fogel used the WLF equation to correct tensile testing results and
were then able to correlate the energy to break to the scratch resistance [135]. Another
complication is that there is a wide variety of scratch-testing methods. These vary from
scratching the surface using pencils of varying hardness (B, HB, H, 2H, etc.), through
scratching with hard styli whilst incrementally increasing the load, (e.g., Erichsen pen), all
the way to fully instrumented methods that examine the scratch profile and its dependence
on load [136, 137]. Many polymeric materials display some recovery over time and this
too must be allowed for if the true scratch performance is to be established [138]. Krupicka
has reviewed methods and factors affecting scratch performance of organic coatings [138].
One can say that scratch resistance may be improved by one, or some combination of three
methods. Lowering the coefficient of friction may make it more difficult to scratch the surface.
This can be achieved for example by adding a lubricant that migrates to the surface [136,
137]. Another method is to increase the hardness of the surface, although this is only effective
if the scratching medium is not too hard. For example, this method might protect against
scratching by another polymer, but not against sand, grit or metal objects. By far the best
method is to make the surface elastic using a soft, rubbery coating for example. One prime
example is in the plastic flooring industry, where the flooring is usually coated in a polyurethane
layer optimised to give scratch-resistance, the desired gloss level and the right level of friction.
8.7 Stabilisation and Recycleability
8.7.1 Introduction
Contrary to popular public perception, polymers are not very stable and are readily
attacked by atmospheric oxygen, heat and UV light [132, 139, 140]. Most of the high
volume polymers require additives that stabilise them during processing at high
temperatures, during their service life, and during any subsequent recycling operations.
In fact, polypropylene is so unstable that all commercial grades must be stabilised [18]
and the same is true of PVC. Once properly stabilised, these polymers can have a useful
service life of tens or even hundreds of years.
392
Filled Thermoplastics
Fillers often affect the stability of polymers via a variety of mechanisms. Although this is
recognised, at least to some extent, it has not been studied as thoroughly as the stability
of unfilled polymers. As the stability and recycleability can be critical issues, hopefully
this subject will receive more attention in the future.
Thermoplastics are usually processed in the molten state, at temperatures in the range
150-350 °C depending on the melting point and viscosity of the polymer. There are
many standard stabilisation packages on the market, often containing a process stabiliser
and a long-term stabiliser. Most of the stabilisers are synthetic, although recently, a natural
hindered phenol, α-tocopherol (vitamin E) was found to be effective in polyolefins [141143] and has been commercialised. For further information, the reader can consult books
explicitly dedicated to stabilisation of polymers [132, 144].
During service, most polymers experience mean temperatures in the range 20-40 °C,
whereas peak temperatures may be much higher for short periods. The mechanical
properties of polymers depend upon molecular weight, and it takes relatively little
degradation to seriously impair mechanical performance. The degradation may result in
crosslinking or chain scission depending on the chemistry of the polymer and the
conditions the polymer is exposed to.
By far the most important stabilisers are the hindered phenols, which are used in a wide
range of polymers including the polyolefins, (e.g., PE and PP), polyamides, polycarbonate
and PET. These stabilisers are effective both during processing at high temperature and
for long-term use under ambient conditions. For increased effectiveness, they are usually
combined with other stabilisers to attain an optimised combination of stabilisation and
other properties such as discoloration. Often, the antioxidant is physically lost, primarily
by extraction or volatilisation, rather than by chemical consumption [145]. The trend is
therefore to use higher molecular mass antioxidants [132, 139, 146].
Fillers may affect the stability of polymers via a number of mechanisms. The two most
important ones are discussed here. Those are the catalysis of degradation by the filler’s
surface and the indirect lowering of stability that occurs when the filler surface adsorbs,
and thereby deactivates, the antioxidants.
8.7.2 The Effect of Filler Chemistry and Impurities on Stability
It is well documented that transition metals such as chromium, copper, iron and vanadium
can catalyse the degradation of polymers [132, 139, 144]. These metals promote the
decomposition of hydroperoxides, which are important in the degradation mechanism
of most polymers.
393
Particulate-Filled Polymer Composites
Most common fillers are not based on these elements, but they may be present as impurities
in the filler, or they may be picked up by the filler during processing operations such as
milling. Talc and mica commonly contain traces of iron. In fact, talc grades with low
iron content command a price premium over less pure grades because it is assumed that
iron content correlates to polymer stability. In fact, that assumption is completely
erroneous. For example, iron oxide (Fe2O3) is used as a pigment, but it does not cause
polymer degradation because it is not the total iron concentration that is important, but
more the chemical form of the iron, or other metal [5]. It would be more accurate to
measure the actual destabilisation caused by each talc grade and then adjust the price of
the talc accordingly. However, this has not been done, probably because it would be
rather labour intensive and therefore expensive. There is a fast, inexpensive alternative
and that to measure the effect of filler on the stability (oxidation induction time) of a low
molecular weight model liquid. For example, instead of compounding the filler into PE
or PP, one can mix in a small amount of filler into squalane and measure the effect on
stability [147]. It has been shown that squalane degrades by the same mechanism as PP
[148]. This approach requires only very small samples and the procedure can be automated
so it is very rapid to perform. Other model liquids may be used to simulate other polymers.
8.7.3 The Effect of Antioxidant Adsorption on Stability
Antioxidants often contain functional groups that are capable of interaction with
the filler surface. This can result in antioxidant adsorption depending upon the surface
chemistry of the filler and the type of antioxidant. Once adsorbed, the antioxidant
becomes ineffective because it is unable to diffuse to, and react with, the radicals
that cause polymer degradation. The amount of deactivated antioxidant can be
significant, and the usual response in industry is to add more antioxidant to attain
the required level of stability. However, that approach raises the cost of the compound
significantly. Another commercial approach is to use an epoxy additive that
preferentially adsorbs onto the filler surface, physically blocking antioxidant
adsorption. That helps to reduce cost, but the epoxy additive is itself still a relatively
expensive chemical.
DeArmitt, Breese and Lamèthe [149] studied the propensity of calcium carbonate to
adsorb Irganox 1010 using squalane as a model liquid to simulate PP (Figure 8.7).
Oxidation induction time (OIT) is a well-accepted method for measuring antioxidant
concentration [145]. First a calibration curve of Irganox 1010 in squalane was made.
Then increasing amounts of Irganox 1010 were added to a 20 weight percent
dispersion of calcium carbonate in squalane. This was mixed and left for some time
to allow the antioxidant to adsorb. The dispersion was then centrifuged to give a
clear supernatant solution, which was analysed by OIT to determine the residual
394
Filled Thermoplastics
Figure 8.7 Effect of fillers on the nucleation of PP
Figure 8.8 The effect of calcium carbonate on the stability of squalane
antioxidant concentration in the squalane. The results showed that the Irganox 1010
was completely ineffective until enough had been added to saturate the filler surface.
The 20 weight percent dispersion of calcium carbonate (specific surface area 5 m2g-1)
395
Particulate-Filled Polymer Composites
adsorbed and deactivated 300 ppm of Irganox 1010. The same results were found
when the calcium carbonate was not removed by centrifugation. The OIT
measurements were performed at 190 °C, so the antioxidant must have been strongly
bound to the filler, otherwise it would have desorbed and raised the OIT of the
squalane. Interestingly, calcium carbonate surface treated with stearic acid did not
adsorb any antioxidant. Stearic acid treated calcium carbonate is more expensive
than untreated grades, but the cost differential is largely compensated for by the
reduction in antioxidant required. If this were more widely recognised, it might
promote the use of surface treated calcium carbonate.
8.7.4 Recycleability
Thermoplastics may be recycled in a variety of ways such as mechanical recycling (collection,
sorting, and reprocessing), burning to give energy, or biological recycling [150, 151]. There
is a public perception that synthetic polymers are less friendly to the environment than
natural polymers such as cellulose, poly(lactic acid) and poly(hydroxyalkanoates). That
view is not supported by the facts. Life cycle analysis reveals a very different picture,
favouring the synthetic polymers, especially polyolefins [150].
Although thermoplastics and thermoplastic composites are potentially easy and
economical to recycle, in practice there are some impediments to the implementation of
widespread recycling. The main one is that the used materials must be collected, separated
and cleaned economically. This is feasible in some instances but often it is not. In general,
polymers are immiscible with one another, and, if melt processed as a mixture, the result
is phase separation to give domains of one polymer in the other. This morphology leads
to rather poor mechanical properties. Therefore, there are efforts to find better separation
techniques in order to avoid the problem or to use compatibilisers [152] that lower the
interfacial tension, improve the adhesion of the two phases, and encourage smaller
domains of the disperse phase.
8.8 Uses of Filled Thermoplastics
8.8.1 Uses of Fillers
As shown in Table 8.1 more than 80% of the filler used in thermoplastic is based on
calcium carbonate minerals. Most is used in PVC, with major sectors being cables, flooring,
hose, plastisols, pipe, profiles and fittings. The main reason for this is firstly due to the
fact that PVC has to be compounded in order for it to be used. The incorporation of
stabilisers is an essential prerequisite for its successful use and therefore fillers can be
396
Fillers European
consumption
Filled Thermoplastics
Table 8.1 Estimated European consumption (in kilotonnes) of fillers
in plastics, 1998
Calcium carbonate
PVC
PP
PE
Thermoset
ETP
Total
850
75
50
100
1
1076
10
1
131
6
15
Talc
Calcined kaolin
120
7
Mica
Wollastonite
Kaolin
5
Total
862
small
2
small
Small
4
4
1
199
52
111
8
6
12
1236
ETP: engineering thermoplastics
included without increasing processing costs substantially. Secondly, PVC is compatible
with both organic and inorganic substances so that changes in properties due to the filler
can be more readily controlled than the other thermoplastics.
Not withstanding the previous statement, the use of fillers in polypropylene, polyethylene
and polyamide is of considerable commercial importance and seeing greatest growth
and most research and development. The main fillers used are talc, ground calcium
carbonate and calcined kaolin.
The bulk of the filler comprises low-cost products for which price is the main specification
requirement but in all applications, certain criteria are important. The filler should not
greatly worsen the colour either directly by increasing light absorption (K) or indirectly by
reducing heat stability or by degrading other additives in the plastic. However, in several
applications the functionalities – shape, size, size distribution and coating, are very important.
In unplasticised PVC (uPVC), extrusions and mouldings, PP mouldings, PP film and PE
film, the particle size, size distribution and surface coating play an important role in
determining processing, mechanical and aesthetic properties.
Talc is used principally in PP, and to a lesser extent polyamide and PE, to give rigidity, a
consequence of its (usually) high aspect ratio. In PP it is also used as a nucleating agent.
Calcined kaolin is used in film as an anti-blocking agent, in thermal barrier agricultural
film, and to allow for carbon dioxide laser printing. Aminosilane treated calcined clay is
used principally in polyamide to give rigidity, toughness with low anisotropy. Wollastonite
is also used in polyamide to give rigidity.
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Particulate-Filled Polymer Composites
8.8.2 Fillers in PVC
8.8.2.1 Introduction
PVC is produced by the free-radical polymerisation of vinyl chloride in suspension, emulsion,
solution and mass, with the first two being the most important. Its fundamental unit is:
CH2
CH
Cl
and commercial polymers have molecular weights between 50,000 and 120,000. They
have approximately 5% crystallinity. All PVC polymers are unstable to heat and light
with hydrogen chloride being evolved in an ‘unzipping’ mechanism. The resulting polymer
chains are highly coloured, rigid and infusible. Stabilisers must be added to the PVC for
it to be processed and used satisfactorily. These are added in a compounding operation
at which time other additives can be added with little cost penalty. Density, refractive
index, Tg, melt viscosity and other properties are dependent on the additives used.
PVC is produced as a powder containing irregular grains with diameters between 65 and
170 µm (for suspension grades, other types are different). These grains are quite complex
structures with a ‘strawberry’ looking surface, because they are composites with very small
domains of 10-30 nm diameter which have agglomerated to form ‘spherical’ primary
particles of 0.2–1.5 µm in diameter. The surface of the PVC grain (except for the mass
polymer) is a skin of surfactants, polymerisation aids and other additives. During processing
lubricants, processing aids and plasticisers penetrate this structure aiding melting and
homogenisation. This process is known as gelation or fusion and fillers affect it significantly.
The level of fusion determines many of the properties of the final PVC article.
As a consequence of its complex chemistry and formulations, PVC is used in a very wide
range of applications, making it the plastic with the third largest tonnage. However, the
last few years have seen a very strong movement against PVC as a material because of its
chlorine content and the possibility that, during its production, converting and disposal,
chlorinated organic compounds known as dioxins may be formed. Possible formation of
dioxins is also of concern during burning and disposal. As a consequence of this converters
of PVC have made determined efforts to replace it with non-halogenated polymers. This
has led to some changes in the uses of fillers. There are also moves occurring to replace
the most commonly used lead-based stabilisers with organic and heavy metal free
stabilisers because of concerns over its toxicity. These changes impose more stringent
requirements on the purity of the filler. One of the most obvious changes is in colour
because lead stabilisers give much greater opacity than the alternatives.
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Filled Thermoplastics
Table 8.2 Estimated use of calcium carbonate in thermoplastics in Europe
Application
Grade of calcium carbonate
kilo tonnes
PVC Cables
Medium and fine coated and
uncoated
250
uPVC Extrusions
Ultrafine and fine coated
110
uPVC Windows and Profiles
Ultrafine coated
80
PVC Plastisols
Precipitated, ultrafine coated, fine
and coarse
25
PVC Flooring
Ultrafine coated, fine and very fine
177
PVC Flexibles (general)
Fine, coated and uncoated
133
PE Compound and
Masterbatch
100
PP Compounds
70
8.8.1.1 Fillers in Plasticised PVC
Calcium carbonate is used in virtually all plasticised (flexible) PVC applications as an
extender as seen in Table 8.2.
The effects that the filler has on mechanical properties, colour and stability, are similar
to those reported in Section 8.8.1.2 for cables, and will not be discussed in detail, although
the formulations are far more variable.
8.8.1.2 Cable Coverings
Most of the calcium carbonate used in plasticised PVC cable (insulation, sheathing and
filling) in Europe is fine, good-quality chalk whiting often with a stearate coating, although
where non-lead stabilisers are being used white marble based products are being used
because of higher colour needs. In North America, Italy, Spain and other countries where
relatively pure ground limestone or marble is abundant, then these are already being
used. The use of stearate is not necessary as it can be added as extra lubricant in the
formulation. In this case allowance must be made for the reaction that will occur between
stearic acid and calcium carbonate. Even so, many users prefer a stearate coated filler as
the coating improves the powder flow and general handling of the filler.
The main type has a high purity usually 94-98% CaCO3, good whiteness 80-95 ISO, a
mean particle size of 2-3 µm and low levels of coarse particles 1-10% above 10 µm. The
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Particulate-Filled Polymer Composites
Figure 8.9 The effects of calcium carbonate fillers in plasticised PVC
Table 8.3 PVC Cable Sheathing Formulation
Compound
phr
PVC (suspension grade K67)
100
Di-2-ethylhexylphthalate
50
Tribasic lead sulfate
5
Coated calcium carbonate
100
properties of the calcium carbonate have little effect on the mechanical and electrical
properties of a PVC compound. This is shown in Figure 8.9 for the cable-sheathing
compound given in Table 8.3, in which the calcium carbonate filler is based on chalk
whiting that has been ground to different particle size distributions, and the compounds
were extruded as a flat strip. Gloss values show a significant particle size effect increasing
markedly with finer fillers. Many high-gloss cable covers are produced using ultrafine
fillers. Particle size also effects stress whitening and scratch marking.
As a generalisation, it may be said that insulation compounds will be filled with 40-70
parts per hundred resin (phr) and sheathing with 20-100 phr; filler loading, plasticiser
level and lubricants are used to control properties of the cable covering. In some
applications, such as high temperature resistant or high voltage compounds the electrical
properties obtained using calcium carbonate as filler are not good enough. In these cases
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Filled Thermoplastics
metakaolinite (calcined clay produced at around 700 °C) is used at between 5 and 15
phr, although loadings as high as 20 phr are sometimes encountered.
8.8.1.3 Floor Tiles and Homogeneous Flooring
Coarse calcium carbonate with a broad particle size distribution, based on chalk, limestone
or marble, or dolomite with an average particle size of approximately 15 µm, is the main
filler used in PVC floor tiles. Price is the main specification but control of colour and of
levels of coarse particles is needed. It is used as an extender at 200-450 phr. To give extra
dimensional stability, reduced water pick-up, and green or hot strength during calendering
and extrusion of the carpet high aspect ratio platy particles are used with the calcium
carbonate. Stabilisation systems have to be modified to allow for the extra reactivity of
the silicate surface.
In homogeneous flooring all types of calcium carbonate are used but finer products are
more common at loadings between 25 and 250 phr. Sometimes stearate coated grades
are used. The biggest developments in PVC flooring are, unfortunately, its replacement
with other types of floor coverings such as wood and carpet due to fashion, or the
replacement of PVC with other polymers such as polypropylene and polyethylene. These
can be heavily filled but because the developments are new and ongoing little can be said
concerning the filler requirements.
8.8.1.4 Wall Coverings/Leather Cloth/Spread Coatings/Calendered Sheet
Fine calcium carbonates (average particle size 2-5 µm) is used as an extender at loadings
between 25 and 50 phr; precipitated calcium carbonate is sometimes used as a rheological
control, although whatever filler is used there will be a significant effect on rheology,
which is of great importance in these applications. Filler dispersion is also important.
8.8.1.5 Hose and Profiles
Most grades of calcium carbonate are used, except very coarse, (i.e., above 10 µm average
particle size), as extenders at levels of 20-50 phr.
8.8.1.6 Footwear
Precipitated and 1-3 µm grades are used for rheological control in rotational moulded
products and as extenders in injection moulded products.
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Particulate-Filled Polymer Composites
8.8.1.7 Plastisol/Sealants
Mostly precipitated grades are used as rheological control additives in combination
with medium (2-5 µm) or fine (1 µm) grades as extenders, the latter at levels of
40-200 phr.
8.8.2 Uses of Fillers in Unplasticised PVC
8.8.2.1 Introduction
As with plasticised PVC applications, most uPVC uses natural calcium carbonate as
a filler. Loadings, however, are usually lower, in the range 3-30 phr. Low levels of
impact modifier and processing aids are frequently used with the higher loadings to
achieve desired mechanical and processing properties. Often non-specification
products will use higher loadings with levels up to 100 phr being encountered,
especially in pipe in China, Indonesia and the Indian sub-continent.
Another quite significant difference with plasticised products is that in uPVC the
filler affects processing and end-properties significantly. Usually fine and ultrafine
products with top cuts of 10 µm and average particle sizes of 1-2 µm are used, but in
lower specification products 3 µm grades are being used. The filler affects processing
by affecting fusion of the PVC particles. This is a consequence of the interaction and
reaction of the lubricants used in the compound with the surface of the calcium
carbonate changing the balance of internal and external lubrication. This, of course,
depends on the surface coating used on the filler and the filler’s surface area [153].
The speed and level of fusion or gelation play significant roles on efficiency of
processing, and in determining mechanical properties, especially impact strength [154].
The particle size of the filler, especially levels of coarse particles, also affects tensile
and impact strengths either positively by the fine particles acting as stress diffusers
or crack stoppers or adversely with the coarse particles acting as ‘flaws’ or stress
concentrators.
Filler loading has a significant effect on processing and on properties, especially on
impact strength, which can reach a maximum often higher than the unfilled. The
optimum loading is determined by particle size, coating levels and by other additives
in the formulation. For coated ground calcium carbonates with a top cut of 10 µm
and a d50 of about 0.8 µm, maxima have been found at between 15 and 20 phr for
lead stabilised compositions without any impact modifier [153], with acrylic modifier
[153] chlorinated PE [155], and with tin stabiliser and styrenics [156].
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Filled Thermoplastics
8.8.2.2 Pipes, Conduit and Fittings
The fillers used are mostly 1-3 µm grades based on chalk in Europe, limestone and
marble in the rest of the world principally to reduce costs. The finer grades also act as
impact modifiers and processing aids and are preferred to coarser grades in more
demanding applications, and in decorative areas where gloss becomes important. In
pressure pipes and fittings levels are usually between 1 and 5 phr; in rainwater goods,
sewage, soil and agricultural drainage levels are 8-20 phr; and in conduit and ducting
levels are 3-40 phr. In non-specification pipes, loadings can be as high as 100 phr but
most national and international specifications are now including limit values for fillers,
either directly or indirectly by specifying the maximum specific gravity permissible.
This differs from application to application but typically will be around 1.46 g/cm3.
This equates to a stabiliser plus filler content of around 20 phr.
8.8.2.3 Window and Other Profiles
Technical properties supplied by the filler are as described previously for pipes,
although in most 4-5 phr TiO2 is added to give whiteness, light and some heat stability.
Stearate-coated ultrafine (0.7-0.8 µm) produced from chalk, white limestone and
white marbles are most widely used. Growth in the whiter fillers is being spurred by
changes in aesthetic requirements; by the move from lead stabilisers to calcium-zinc
or tin-based which do not give the same levels of opacity as lead composites; and by
developments in coloured profiles.
Filler levels for window and other building profiles have been creeping higher in the
last few years from 3-5 phr to 5-15 phr with the limiting factors being cold impact
strength, extrudate gloss and corner weld-strength. In various non-critical applications
such as blinds and roller blinds coarser grades (1-3 µm) are used with levels up to 75
phr being encountered. In these cases, small amounts of plasticisers (7.5 phr) are
added to aid processing and properties.
8.8.2.4 Film
The market for uPVC film is diminishing under environmental pressure and
consequently the use of particulate fillers is also diminishing. Most of the film is
transparent for food packaging, display, blister packs and so on, and kaolins and
other silicate minerals are used as antiblocking additives, without detriment to colour
and transparency of the film.
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Particulate-Filled Polymer Composites
8.8.3 Uses of Fillers in Polypropylene
8.8.3.1 Introduction
Polypropylene has the general formula:
CH2
CH
n
CH3
with n being about 2000. It is a linear polymer, essentially a hydrocarbon with many
similarities to PE, being chemically inert, flexible, tough and having fairly low softening
and melting points. The presence of the methyl groups, however, introduces several
significant differences. Tacticity is introduced due to the various spatial arrangements of
the methyl groups that are possible. Commercial polymers are 90-95% isotactic; that is
the methyl groups occur on one side of the polymer chains. This introduces some
crystallinity (approximately 50%), and higher softening points. However, the methyl
groups also induce greater susceptibility to oxidation and chemical attack (usually at the
hydrogen atom β to the methyl group). It is the lightest common plastic with a specific
gravity of about 0.9. Ethylene can be polymerised at levels of 4-15% with propylene,
either randomly or as blocks to give copolymers that are more flexible and tougher than
the homopolymer. Alternatively the polypropylene may be compounded with ethylenepropylene rubber to give copolymers as a physical mixture or rubber-modified-grades
(depending on the level of the rubber). All suffer from oxidative instability and are always
stabilised in service. Particulate fillers and coupled glass fibres are used in all these polymers
for many applications to increase rigidity and heat distortion.
8.8.3.2 The Uses of Filled Polypropylene
As mentioned previously, the principle reason why fillers are used in PP is to increase
rigidity, especially at higher than ambient operating temperatures. The rigidity or modulus
of a composite is affected by the modulus of the inorganic component, its loading and by
its aspect ratio, that is the ratio of its length (or largest dimension of a particle) to its
thickness (or smallest dimension). The modulus of any inorganic filler is very much
higher than that of a plastic, and thus differences between types of fillers are not so
important in determining modulus as the other two factors. As volume loading of filler
increases, the modulus of a composite increases almost linearly at low to moderate filler
levels. The higher the aspect ratio the higher the modulus. Thus, where rigidity of the
final product is the most important single parameter, high-aspect-ratio fillers such as talc
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Filled Thermoplastics
or glass fibre are preferred. Mica and wollastonite are available with high aspect ratios
but their main use is in North America where their cost-performance ratio is advantageous;
in Europe, talc is most widely used.
Original developments in filled PP were compounds with much higher stiffness than the
unfilled, especially at higher than ambient temperatures, but which would be low cost to
match the styrenics, especially ABS. Talc, being widely available, low cost and usually
having a high aspect ratio is the preferred filler type, and all polypropylene compounders
have several talc-filled grades available. Typical uses are in: automotive components
such as air-filter covers, timing chain covers, heater boxes, and battery box tops, domestic
appliances, such as washing machine soap dispensers, and in some disposable food
packaging such as skeletal fruit packages. New initiatives on recycling, particularly in
the automotive industry, are tending to limit polymer types used. Polypropylene is strongly
favoured and this is helping drive the market for filled grades.
Crudely, the talcs that are used can be divided into five types: three based on ‘pure’ talc
(less than 10% impurities) with top cuts of 300 BS mesh, 20 and 10 µm; and two based
on less pure minerals with top cuts of 300 BS mesh and 20 µm. Particle size has no effect
per se on rigidity but, depending on the method of processing, finer types may have
higher aspect ratios and therefore will give higher rigidities. Particle size does affect
composite impact and tensile strength with smaller particle size products giving higher
strengths [157], although the results are not unambiguous because methods of producing
fine talcs also produce higher aspect ratio particles [158].
Virtually all applications for talc-filled PP are those that do not require toughness or
high strains because the rigidity imparted by talc is accompanied by brittleness. The
toughness can be improved by changing the base polymer to a copolymer with ethylene
as comonomer, by incorporating ethylene-propylene rubber, or by changing the mineral
to stearate coated calcium carbonate. All methods, however, reduce the rigidity of the
composite compared with the talc filled equivalent. As discussed in [94], particle size
and coating of the calcium carbonate affect impact strength and toughness, while other
properties are not affected greatly. Loading of the coated calcium carbonate also affects
properties. Rigidity increases, and tensile strength decreases virtually linearly with loading
but impact strength can go through a maximum at between 20 and 40 wt%.
This high level of toughness coupled with a rigidity which is higher than the unfilled,
good flow in mouldings, good light and temperature stability means that calcium carbonate
filled PP is regarded as a separate material, with major uses in injection moulded garden
furniture, automotive components, food packaging, fibres, tapes and blown oriented PP
(BOPP) packaging film. A very rapidly growing market for fine coated calcium carbonate
is in breathable films, in which micropores form around the calcium carbonate particles
during film orientation (mostly oriented PP). In fibres and tapes, the stearate-coated
405
Particulate-Filled Polymer Composites
calcium carbonate is used at around 8 wt% and acts as a delustering agent and also
reduces fibrillation. Another large and growing market is in BOPP film used in packaging.
Loadings up to 70 wt% are used in the central layer of a three-layer film, produced by
co-extrusion, with the outer layers unfilled.
Mineral filled polypropylene often replaces ABS, polyamide and other engineering plastics,
although their mechanical, processing and optical properties are different. However, by
experimenting with the large number of permutations and combinations of type of filler,
type of polypropylene and rubber toughening agent, satisfactory matching of cost-property
performance is achievable. Reductions in cycle time, shrinkage, sink marking and improved
noise reduction are among the benefits resulting from mineral filling, but detrimental
effects have usually been observed in the aesthetic properties. Gloss is normally reduced
but the surface can be tailored by choice of filler. For example, a high gloss moulding can
be obtained by using a blend of talc and calcium carbonate [159].
Colour is also changed by the mineral filler: pure talc gives translucent, almost colourless
compounds while high quality calcium carbonate gives a white colour with some opacity.
Increasing levels of impurities in both minerals causes colour degradation. Colour will
also be caused by any instability introduced into the polypropylene by the mineral.
One problem from which all particulate filled plastics, and in particular filled PP, suffer
is that of scratch marking and marring; as the plastic is scratched, light scattering occurs
at exposed filler particles. This has been overcome to some extent by choosing the correct
filler and by modifying the filler plastic interface. Calcined talc is being used in automotive
compounds for its improved resistance to scratching. Filled PP sheet is also being used
extensively as thermoformed packaging materials because the filler confers good sheet
extrudability and thermoforming characteristics [160].
8.8.4 Uses of Fillers in Polyethylene
8.8.4.1 Introduction
Polyethylene –(–CH2–CH2–)–n is essentially a high-molecular-weight paraffin, which as a
consequence, is inert to most chemicals, flexible with low softening and melting points.
Three types are now commercialised: LDPE, produced by polymerisation over a freeradical source at high pressures; HDPE, produced by polymerisation over Ziegler catalysts
(complex catalysts based on metallic co-ordination compounds); and LLDPE, produced
by a variety of techniques designed to give chains with limited short-chain branching
and incorporating low levels of other olefins – butene, hexene and octene. All grades are
semi-crystalline with levels of about 60% for low density, and up to 90% for high density,
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Filled Thermoplastics
but these can be lowered by branching. There is much argument about the Tg with values
from –20 °C to –130 °C being reported. They have softening points from about 77 °C to
124 °C. The main applications for filled polyethylene, film and bags, blow moulding and
electrical insulation dictate the required properties and limit the potential for fillers.
8.8.4.2 The Uses of Filled Polyethylene
A considerable amount of calcium carbonate is used in LDPE, LLDPE and HDPE. Medium
particle-size grades (with d50 of between 2 and 3 µm) are used in masterbatch either
alone or to extend pigments, principally white TiO2 and carbon black. These in fact
dominate the masterbatch market, with film and bags being the final destination for
most. In natural masterbatch, the filler level will usually be 70-75 wt%; in pigmented
products it is used to dilute the prime pigment by amounts dictated by the requirements
(opacity, colour and gloss) of the end application. Mostly dilution at 10-20 wt% is used
but some masterbatches can be formulated with 10% prime pigment and 60 wt% calcium
carbonate. Levels of course are also dictated by any other additives (stabilisers, lubricants,
etc.), in the masterbatch.
Cost is naturally very important, but the filler does play an important role in several film
properties [161]. For example, in LLDPE extruded film, ground calcium carbonate
improves efficiency by both increasing the cooling rate of the bubble and the level of
fusion; it improves printability; primary pigment dispersion can be improved; it reduces
the coefficient of friction by increasing the surface hardness of the film; and it acts as an
anti-blocking agent.
Other minerals are used to give specific properties. Medium and coarse china clays (average
particle size 2 and 5 µm) are used at 5-10 wt% to reduce stretch, to reduce slip, act as
anti-blocking additives and to give thermal barrier properties (see below). A variety of
minerals are used to anti-block all types of film (although HDPE film is not so prone to
blocking because it is much stiffer, so anti-blocking needs are less). Talc, silica (natural
and synthetic), aluminosilicates, zeolites and calcined kaolin are all used at levels of 0.11.0 wt%, the loading depending on the ‘stickiness’ of the film and the temperatures at
which it may be used. The surface chemistry, shape, purity and refractive index of the
mineral determine the blocking, friction, clarity, haze and colour of the film. A major use
for calcined clay is in agricultural film. Because of its very strong absorption bands in the
far infrared (IR), it renders the plastic opaque to heat (and consequently more like a
flexible glass) [162]. Other minerals absorb IR radiation in lesser amounts but are still
used by some film producers. All of course are added via masterbatch, which will also
include stabilisers and lubricants the levels of which being dictated by the intended service
length of the agricultural film.
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Particulate-Filled Polymer Composites
Linear low-density polyethylene is principally used in film and bags, with similar criteria
applying as in LDPE. There is some interest in using it (because of its toughness) in
engineering and automotive applications, but its stiffness has to be increased by using
fillers. In recent years there has been a rapidly growing and very large market for ground
calcium carbonates in LLDPE in the production of microporous films, where holes have
been incorporated into a plastic sheet or film through de-bonding of the polymer from
filler particles dispersed in the matrix. The most common plastic used is LLDPE (often in
blends) and the most common filler is coated ground calcium carbonate with average
particle size 1-3 µm. The de-bonding is achieved by stretching and orienting the film.
Loading levels of the calcium carbonate are around 55-60 wt%, and the particle size and
efficiency of coating are key to producing film with controlled pore size. The principal
uses are in the production of controlled atmosphere or breathable films and in white
opaque films. The former has various uses – hygiene, medical and industrial garments,
building membranes, house wrap but the biggest single use is in the production of babies’
nappies. The latter includes food packaging, labels and paper replacement.
High-density polyethylene finds its major outlets in blow-moulded bottles and containers,
film and carrier bags and pipes (the last often used with LDPE and the blends are known
as medium density polyethylene (MDPE)). Although fillers have been and are still being
looked at periodically in blow mouldings, processing imposes severe restrictions before
costs and product properties are considered. Film and bags will have some ground calcium
carbonate included via masterbatch as an extender for the prime pigment. The main pipe
sectors are for gas and water transportation, and end-users have imposed very strict
regulations on additives and properties; potential use of fillers is very low. There has
been some interest in using HDPE in ducting, competing against uPVC, and a filler, at
high loadings, will be essential to achieve the required stiffness and cost balance, if this
replacement is to be successful.
8.8.5 The Use of Fillers in Polyamides
8.8.5.1 Introduction
Nylon is the generic name for the family of polyamides (PA) with PA6 and PA6, 6 being
the most common. They are named after the chemical group:
O
H
C
N
formed during the condensation polymerisation which occurs when an organic acid is
heated with an organic amine. The numbers which always occur with the name Nylon or
408
Filled Thermoplastics
polyamide refer to the types of acid or amine used in the production. The regular spacing
of the amide groups means that the polymers crystallise with a high intermolecular
attraction leading to high-strength polymers with high melting points. Levels of
crystallinity depend on thermal history and can vary from 15% to 50%. Nylon is fairly
hygroscopic and its Tg (temperature below which the polymer become brittle) and
mechanical properties depend on the amount of absorbed water.
8.8.5.2 Properties of Filled Nylon
All polyamides are tough, rigid plastics with high heat distortion temperatures but
they are thermoplastic, that is, they exhibit plastic flow (high creep) and soften at
elevated temperatures. Mostly they are used unfilled but there is a significant application
sector in which higher rigidities and higher heat distortion temperatures are required
than can be achieved from unfilled polyamide. Glass fibre (with a size or a silane
coating) fills most requirements giving very high rigidity, etc. Other mechanical
properties of a properly coupled glass-fibre filled Nylon are also good. Some 200,000
tonnes per year of glass-filled Nylon is now used, and it is regarded as a separate
engineering material. However these fibre filled composites suffer from anisotropy
due to fibre orientation during processing; that is, the properties of the composite vary
depending on the direction in which they are measured. Fibre orientation also causes
uneven shrinkage that can lead to unpredictable warpage (bending and distorting),
which is not acceptable in products that require good dimensional stability. Anisotropy
and warpage are proportional to the aspect ratio of the filler used and can be reduced
to zero by using glass spheres [163]. Calcined kaolins give good, low, anisotropy and
when treated with a bifunctional aminosilane, give very good mechanical properties
and tend to dominate the European and Pacific filled Nylon market. Both particle size
and coating level affect impact strengths. Pre-treatment gives much better properties
than when calcined clay and silane are added separately to the compounding operation
[164]. Some talc and wollastonite is also used giving high rigidity (although this depends
on the grade and its aspect ratio), although impact properties are worse. In the case of
talc, this has been related to its inability to couple through silanes with the polyamide.
Calcined talc is being developed to improve the interaction [154].
8.8.5.3 Uses of Filled Nylon
Mineral-filled Nylon is widely used: in automotive applications, such as wheel discs,
headlamps, water pumps, air inlets and grills; in electrical engineering; in electronics;
appliances and consumer goods [165]. Significant recent developments have seen its use
in the production of automotive engine covers. The mineral, as mentioned previously, is
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Particulate-Filled Polymer Composites
frequently used in combination with glass fibre. Loading levels have, in the automotive
industry, dropped to around 20 wt% under pressures to reduce vehicle weight but in
some cases are still as high as 40 wt%.
8.8.6 Polybutylene Terephthalate
Polybutylene terephthalate (PBT) is a fairly tough, rigid thermoplastic which has a
very good gloss in mouldings. It is less susceptible than Nylon to moisture when moulded
but otherwise most of its mechanical properties are not so good. Many glass-fibrefilled compounds are available from specialty compounders and polymer producers
and some mineral filled grades are also widely used. Talc, aminosilane-treated calcined
clay, ultrafine calcium carbonate, ultrafine china clays, glass beads and glass flakes
have all been encountered. Benefits from the particulate fillers have been reported to
be rigidity with good dimensional stability, high impact strength and exceptional surface
finish [166].
8.8.7 Polyethylene Terephthalate
One of the most important uses of PET is in producing many types of film and tapes.
In these a variety of speciality fine grades of china clays, calcined clays, calcium
carbonates (natural and precipitated), synthetic silicas and silicates are used at levels
of about 0.1 wt% as anti-blocking agents; they may also nucleate crystallite formation.
Fillers seem to be precluded from the largest single market for PET – bottles, not
only because of the difficulties in successfully blow moulding filled products in general,
but also their application properties such as optical clarity and burst strength will be
adversely affected by fillers. Many bottles are now being recycled into a variety of
plastic and fibre applications; fibres and particulate fillers are being looked at to
extend the range of applications. The high HDT and softening temperature of PET
have recently led to developments in its use in food ‘cook-in’ applications. However,
it’s HDT is not quite high enough for conventional oven use and fillers are being
looked at to improve this.
8.8.7.1 Polystyrene, High-Impact Polystyrene and ABS
The styrenics are a family of low to medium price, rigid, easily processed plastics with
good gloss and optical properties. ABS and HIPS are more tough, due to the acrylonitrile
rubber content, which has either been incorporated into the plastic or copolymerised
with the styrene monomer, respectively. Fillers have been looked at by several producers
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Filled Thermoplastics
and compounders but do not give good enough properties to allow them to be used in
any significant amounts, although products containing kaolin, calcium carbonate and
talc have been reported. These may have the filler incorporated as a diluent in colour
masterbatches used in the plastic. Some very old patents cover the incorporation of china
clays and ultrafine calcium carbonates into the rubber before compounding this into the
polystyrene but again no noticeable commercial success has been achieved. Talc is used
in expanded polystyrene as a nucleating agent.
8.8.7.2 Polyphenylene Oxide (or Ether)
Polyphenylene oxide is an engineering plastic that is commonly blended with polystyrene,
with the blend having significantly better properties than the separate plastics. Modified
polyphenylene oxide has been available now for approximately 30 years and it’s uses are
widespread including automotive components, business machines, domestic appliances,
wire covers and water treatment. Other blends with PBT and PA are in the market place
offering different property balances. These composites (and other polymer blends) are
being trialed for automotive body parts, and particulate fillers are needed to give the
correct coefficient of thermal expansion. This type of filler is also needed to give rigidity
without affecting impact properties. Very fine kaolins (with mean equivalent spherical
diameter (esd) of about 0.3 µm) with and without silane coupling agent give good impact
strength [167].
8.8.7.3 Polyphenylene Sulfide (PPS)
Polyphenylene sulfide is a high temperature, stiff, fire-resistant, chemically resistant plastic,
which has a very low melt viscosity and accepts fillers and reinforcing agents very well.
Talc, china clay, dolomite, quartz and glass fibre filled grades are all available but volumes
are very small and the situation changes very rapidly.
8.8.7.4 Polyformaldehyde or Polyoxymethylene Polymers
These plastics are characterised by high stiffness, good impact resistance, high HDT,
good chemical resistance and have the best fatigue resistance of any plastic. Glass fibreand mineral-filled grades are used to increase stiffness, hardness and resistance to
distortion. The type of filler that can be used is important as the polymers are badly
depolymerised by acids, the unzipping of the polymer chain giving gaseous formaldehyde.
Minerals with acid surfaces, such as china and calcined clays, can cause this
depolymerisation. Calcium carbonate reduces some mechanical properties but one calcium
carbonate filled grade is claimed to have better abrasion resistance.
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Particulate-Filled Polymer Composites
8.8.7.5 Others
The properties of particulate fillers in a number of other thermoplastics are being investigated
by plastics producers and academic institutions but usually with a low priority rating. This
activity has been growing less and less in recent years as companies, in particular, have
been reducing the staffing levels in the research and development departments.
8.9 Conclusions
Thermoplastic composites are all around us and their use is increasing every year. The
reason for this is that thermoplastics have an excellent combination of cost and
performance. The performance can often be further enhanced by addition of fillers while
maintaining a favourable cost. The recycleability of thermoplastic composites is an
advantage compared to rubbers and thermosetting polymers because the latter two types
cannot be melted and reshaped. This favours the continued growth of the thermoplastics
and their composites at the expense of other polymeric materials.
To understand and optimise composites, one must have an overview of all the different
economic, chemical, surface and physical aspects. Furthermore, one must have a clear
goal, and be able to correctly prioritise the properties of most import for the intended
application. The best composite is the one that makes the best compromise between the
multitude of properties, at the lowest cost.
The use of fillers has been increasing incrementally for many years and that trend is
expected to continue as the use of traditional fillers is optimised and as new nano-fillers
eventually become economically attractive.
What can we expect in the future? In the near future, composites must be designed with
re-use in mind. That means proper stabilisation so that the polymer can be recycled with
sufficient retention of mechanical and aesthetic properties. There will be an increased
tendency to use fewer, standard materials to reduce cost and to reduce the need for
extensive separation of materials for recycling. It can also be anticipated that products
will be designed for easy disassembly.
It seems probable that surface treated filler will become more popular. Although the
treatment adds cost it gives many advantages and when these are summed, the overall
cost and performance of the material may be better for the surface treated type. For
example, stearic acid coated calcium carbonate in PP homopolymer gives higher extruder
throughput, better gloss, better impact strength and improved stability (because it prevents
antioxidant from adsorbing onto the filler and becoming inactive). Any one of these
412
Filled Thermoplastics
benefits may not justify the extra cost of pre-treated filler, but taken together they give a
very attractive combination of price and performance.
Another trend for the future may be the increased use of single-screw extruders to make
composites. At the moment twin-screw extruders are used almost exclusively, because
they are able to achieve better dispersion. However, recent developments have improved
mixing in single-screw extruders. Therefore, it may become common to use single-screw
extruders because they are cheaper, easier to maintain and give higher throughput. Again,
surface treatment of the filler also helps here to give good filler dispersion even for a
single-screw extruder.
Progress in polymer composites has been held back because it is expensive and timeconsuming to prepare multiple formulations and then perform thorough mechanical testing.
It is possible to save time and money by screening new fillers, antioxidants, dispersants
and coupling agents in a model liquid instead of the polymer. The screening can then be
followed up by full testing, using the polymeric matrix. Hopefully, this method will be
used to help develop new filler grades and surface treatments, more quickly and cheaply.
Acknowledgements
We would like to thank several people for their assistance in writing this chapter. Firstly,
the editor Professor Rothon who has been a great help in making suggestions, proofreading and for general discussions. Professor John Berg (who deserves a special thanks
for being such a help with the section on adhesion), Professor Ulf Gedde, Professor
Aubrey Jenkins, Kevin Breese, Massimo Sanità, Carlo Tomaselli, Roy Goodman, Chris
Paynter, Richard Day, Anna Kron and Werner Posch are all warmly thanked for making
significant contributions by reading draft versions and for making valuable comments
that improved the quality of the chapter.
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