Mechanical Characterization of Microcellular Foamed PVC/Wood-Flour
Composites
Hong-Ru Lin* and Min-Da Shau
Department of Applied Chemistry
Chia Nan University of Pharmacy and Science
Tainan, Taiwan 71710, R. O. C.
Zen-Wen Chiou
Plastics Industry Development Center
No. 35, 23 Rd., Taichung Industrial Park
Taichung, Taiwan 408, R. O. C.
* To whom correspondence should be addressed.
Tel: 886-6-2664911 Ext. 242
Fax: 886-6-2667319
E-mail: hongrulin@mail.chna.edu.tw
ABSTRACT
A series of wood-flour/PVC composites, either foamed or unfoamed, were prepared
to investigate the effects of wood flour and coupling agent (silane) on the mechanical
properties of the samples. Sorption experiments were carried out on these composites
through the dissolution of CO2 in the composites to prepare the foamed samples.
It was
found that the gas-uptake of the treated wood-flour/PVC composites was lower than that of
the untreated samples, which was different from the previous investigation found by other
workers. Silane played a significant role in improving the adhesion between the wood
flours and polymer matrix; nevertheless, the dynamic mechanical properties of the
unfoamed or foamed composites were not changed by the surface treatment of the wood
flours.
In general, the tensile strength of foamed composites increased with increasing
wood flours content and this benefit can be a plus when the wood flours were silane
treated. The morphology of fracture surfaces between untreated and treated
wood-flour/PVC foamed composites reflects the observation in mechanical properties.
Key words: Microcellular foamed composites, coupling agents, mechanical properties,
reinforcing fibers, silane, and wood-flours.
INTRODUCTION
Foamed plastics are composite materials, which consist of a polymer matrix and a fluid
2
phase. Generally, the fluid phase is a gas. The presence of gas disposed throughout the
mass thus reduces the apparent density of the plastic. The nature of the starting materials, i.e.
thermoplastic, thermosetting plastics or latex suspension affects the details of the expansion
process.
With the removal of chlorofluorocarbons (CFCs) from rigid foam products due to their
environmental impact, the search for environmentally compatible blowing agents becomes
essential. While alternate blowing agents such as HCFC-123 and HCFC-141b can be used
as substitutes, their uncertainty in commercial availability, environmental impact, and cost,
have accelerated the development of halogen free blowing agents [1]. One of the possible
choices is to use inert gases, such as nitrogen, argon, helium, and carbon dioxide as blowing
agents.
Martini described the use of gas as a blowing agent to make foamed plastics in batch
process [2]. This process involves: (1) subjecting the sample to constant pressure at room
temperature for the desired period of time, (2) releasing the pressure, (3) removing the sample
from the pressure chamber and placing it in the foaming oven, and (4) annealing the sample in
the range of the glass transition temperature (Tg) to develop the foam structure. The foam
structure developed from this process is classified as microcellular foam.
The advantage of
this method is that the skin of the product is smooth and the sample is well suited to further
analysis.
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There has been much research on the use of inert gases as blowing agents to produce
microcellular foams [3-13] during the recent years. The effects of the foaming conditions
on the physical and mechanical properties on these foams were studied in great details.
The impact strength, toughness, fatigue life, and thermal stability of these microcellular
foams were superior to the unfoamed polymers [14].
The use of glass fiber and inorganic fillers such as mica, talc, and calcium
carbonate to reinforce the plastic composites has been dominated the market.
However,
these high-density reinforcing fillers are not cost-effective on a volumetric basis.
Recently, because wood flours posses some superior properties, such as strong, lightweight,
recyclable, and cost-effective, to the conventional fillers, they have penetrated slowly into
the market.
investigated.
In this paper, microcellular foaming of PVC/wood-flour composites was
However, the design of plastic/wood-flour composites is confronted with
the incompatibility between these two phases [15-18].
In order to conquer this problem,
silane was used as a coupling agent to enhance the interfacial adhesion between
wood-flours and plastic matrix. Previous workers [14] have shown that the surface
modification of fibers with silane strongly affects the solubility and diffusivity of CO2 in
the composites and the cell morphology of the foamed PVC/cellulosic-fiber composites.
The relationships between processing/structure/property of the PVC/wood-fiber
composites have been studied in great details [14, 19, 20]. However, the effects of the
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coupling agent and wood fibers on the mechanical properties of the foamed composites
have not been studied extensively.
In this paper, microcellular foaming or unfoaming of PVC/wood-flour composites
were prepared.
Three different types of mixture were formed under the same processing
conditions: (1) PVC alone, (2) untreated wood-flour/PVC composites, and (3) treated
wood-flour /PVC composites. To prepare the foamed composites, sorption experiments
were carried out on these composites through the dissolution of CO2 in the samples.
Then, the gas-saturated composites were microcellular foamed. The effects of the wood
flour and coupling agent (silane) on the mechanical properties of the foamed or unfoamed
composites were investigated.
EXPERIMENTAL
Materials
The materials used and the suppliers as well as the sample formulations are listed in
Table I.
Pretreatment of wood flour
The wood flours were pre-dried in the turbine mixer for 5 minutes at 150℃ to remove
the moisture before being treated with silane.
The pre-dried wood flours were then poured
into a high-speed mixer (Hechell Mixer) equipped with high shear type of blades. The speed
was set at 350 rpm. When the temperature of the mixer reached 80℃, silane was gradually
5
added into the wood flours.
Aminosilane was used as a coupling agent, which can help in
increasing the compatibility between PVC and wood flours.
The mixture was thoroughly mixed
to insure a good dispersion of silane into the flours. The treated wood flours were then cooled
down to 25~ 30℃ in a cooling blender for later use.
Compounding
The PVC resin and other ingredients (as listed in Table I) were first thoroughly mixed in
the high-speed mixer (modified Henchell powder Compounding mixer, Long-Cheng
Machinery Work Co. Ltd., Taiwan, 1000 rpm, 120 ℃, 5 mins.).
The mixture was then
cooled down to 25~30℃ in a cooling blender (Long-Cheng Machinery Work Co. Ltd., Taiwan)
to prepare the control samples (PVC alone).
For the preparation of PVC/wood-flour
composites, different weighting ratios of the treated and untreated wood flours were gradually
added into the PVC system in the high-speed mixer, respectively. Then, the mixture was
cooled down to the room temperature in the cooling blender.
Injection molding
The powder mixture was extruded and cut into cylindrical pellets using a single screw
extruder (Jang-Chang Machinery Co. Ltd., Taiwan, Processing temperatures: Zone 1: 130 ℃,
Zone 2: 140 ℃, Zone 3: 145 ℃, Adaptor: 145 ℃, Die: 147 ℃, 60 rpm, DC motor driven,
Screw diameter: 55 mm, L/D = 28, Compression ratio = 2.8).
In order to increase the
thermal stability of the materials during processing, processing aid (compatibilizer) was added
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into the powder mixture.
Internal and external lubricants were also added to the powder
mixture to reduce the friction between molecular chains in polymer and the friction between
metal and polymer in the extruder during the extrusion process, respectively.
Sample
plates (30010015 mm3) were prepared via an injection molding machine (Batten Field,
BA-750, CD plus, 75 tons, 3.2 oz, Processing temperatures: Zone 1: 140 ℃, Zone 2: 140 ℃,
Zone 3: 140 ℃, Nozzle: 150 ℃, Injection pressure: 60 Bar, Injection speed: 50 % of max.
speed, Cooling temperature: 75 ℃).
Foamed samples preparation
The PVC/wood-flour composite sheets were cut into specimens (10 × 14 cm2) along the
same direction relative to the sheet to reduce the effect of anisotropy. The specimens (either
PVC alone, silane treated, or untreated PVC/wood-flour composites) were subjected to a
constant pressure of CO2 in the pressure chamber at room temperature for the desired period
of time. The gas diffused into the polymer matrix and was dissolved during the saturation.
The pressure in the chamber was released after the desired period of time. The saturated
specimens to be foamed were immersed in a heated glycerol bath for 30 seconds.
In order to
reduce the gas desorption, the saturated specimens must be immersed in the heated glycerin
bath within 5 minutes after removing from pressure chamber. The bath temperature was set
at 150℃ for the foam structure development.
The foamed specimens were then quenched
with water at room temperature.
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The unfoamed and foamed specimens were desorbed at atmospheric pressure and room
temperature for at least 150 hours before conducting mechanical testings and other analysis.
Density measurement
The principal method used to measure density of specimens was based on the ratio of
measured weight to measured volume. For the tensile specimens, volume was determined
by submerging each specimen in water with a known volume of aluminum plate attached to
overcome buoyancy forces and measuring the volume of water displaced.
Since the cells
produced in these samples have diameters in the range of 0.1 to 10 m and since only a small
total area is exposed at the edge surface, it was believed that no water would be absorbed by
the samples in this procedure.
This was confirmed by a series of tests in which
specimens were first weighed, dipped into the water, dried by wiping and then reweighed.
Morphology
Scanning electron microscopy (SEM, Topcon SM-300, variable high/low vacuum
operation, Japan) was employed to view the internal configuration of the gas bubbles, wood
flour, and polymer matrix. Sample cross sections were prepared by sharp bending after
chilling the material in liquid nitrogen. The cleaved surfaces were shadowed by gold and
observed in the SEM.
Mechanical testing
All the specimens were machined according to ASTM standard D638, type IV with a
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gage width of 0.6 cm and a gage length of 2.54 cm. They were pulled on a machine
(Shimadzu, AGS-100D), using compression grips, at a rate of 2 cm/min at room temperature.
The mechanical properties were taken from the average of ten specimens.
Dynamic mechanical thermal analysis (DMTA)
Dynamic mechanical thermal analysis (DMTA, DMA 7e, Perkin-Elmer) was conducted
to determine the dynamic storage modulus (E’) and damping factor (tan ) of the specimens.
This same test was used to determine the glass transition temperature (Tg) of the specimens.
The specimens were cut to a width of 1 cm and a length of 0.25 cm.
The specimens were
hold at 25℃ for 3 minutes and then heated from 25℃ to 160℃ at 5℃/min.
The frequency
used was 1 sec-1.
RESULTS AND DISCUSSION
Effects of wood flour and surface treatment on the gas-uptake
In order to study the effects of wood flour and surface treatment on the feasibility of
producing microcellular foams of PVC/wood-flour composites, neat PVC, untreated, and
treated wood-flour/PVC composites were exposed to various saturation pressures. Figure
1 compares the percentage of gas-uptake between these three specimens at various
saturation pressures. Both treated and untreated wood-flour composites contained 30 phr
of wood flours. All the specimens were saturated for 5 hours. After that, the saturated
pressure was released and the weight gain of the specimens was measured.
It was found
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that the percentage of gas-uptake increased with increasing saturation pressure in all three
types of sample. At low saturation pressures, the PVC/wood-flour composites remained
solid in nature, thus the solubility of CO2 inside the composites was relatively small.
Accordingly, the difference in gas-uptake was not obvious at low saturation pressures.
But it shows significant difference at a high pressure of 5.5 MPa. The same phenomenon
was found with other wood flour contents. At a pressure of 5.5 MPa, the percentage of
gas-uptake of neat PVC was 14 % and 40 % greater than that of untreated and treated
wood-flour composites, respectively.
It was also found that neat PVC had the highest
percentage of gas-uptake among the others. This indicates that the addition of wood
flours into the PVC matrix decreased the solubility of CO2 in the composites.
PVC/wood-flour composites can be viewed as a mixture of two phases, amorphous and
crystalline. PVC matrix belongs to amorphous region while wood flours act like
crystallite. The amorphous region absorbs the gas while the crystallite rejects it [14].
From Figure 1, it shows clearly that the solubility of CO2 in the PVC/wood-flour
composites was smaller than that of neat PVC.
This is caused by the addition of wood
flours thus decreasing the volume of amorphous material available for diffusion,
obstructing the movement of CO2 molecules, and increasing the average length of the
paths CO2 has to travel.
As a result, the diffusion of CO2 in the composites decreased
with decreasing the polymer mass fraction.
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For the composite with untreated wood-flours, the percentage of gas-uptake, in
general, was greater than those treated wood-flour composites. This result is contrary to
the previous investigation found by other workers [14, 19-20]. It is known that the
addition of untreated wood flours into the polymer matrix, due to the poor adhesion
between wood flours and PVC matrix, may provide a channel through which the gas can
escape from the polymer matrix to the environment right after the release of saturated
pressure and before the weight gain measurement.
However, the channeling effect
depends on the length of the fibers, i.e. the longer the fiber, the more pronounced
channeling effect it is, as found by other workers [14, 19-20].
In this study, the fiber
length of the wood flours is about 0.15 mm comparing to that of 0.3-1.0 mm in other
studies [14, 19-20]. Thus, the channeling effect may not be appropriate to explain the
contrary result found in this study. The main reason may be attributed to many voids and
pockets containing inside the composites due to the insufficiently interfacial adhesion
between the polymer matrix and wood flours, which served as a sink for the gas-uptake,
thus resulting in the increase in percentage of gas-uptake by comparing to the
silane-treated samples.
For silane-treated samples, there are less voids and pockets found
inside the composites due to good adhesion between the two phases. The evidence of
voids and pockets inside the untreated PVC/wood-flour composite was clearly shown in
Figure 2 (a) as compared to the silane-treated composites (Figure 2 (b)).
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Effects of surface treatment on the properties of foamed composites
In order to solve the incompatibility problem between the PVC matrix and wood
flours, silane was used as a coupling agent to enhance the interfacial adhesion between the
two phases. Figures 3 (a) to (c) compare the density, tensile strength, and elongation at
break between untreated and treated wood-flour/PVC foamed composites at various
saturation pressures. The wood flour content for both types of sample was 30 phr,
respectively.
All the specimens were saturated for 5 hours then dipped into the glycerol
bath (150℃) for 30 seconds to develop the foam structure. Since larger cells were
developed at higher saturation pressures, the densities for both types of sample decreased
with increasing saturation pressure, as shown in Figure 3 (a). During the foaming, the
cells inside the polymer matrix tend to grow as a result of the diffusion of gas from the
polymer matrix, and the cell growth continues until all the gases are completely depleted
from the polymer melt.
At such a long foaming time (30 seconds), all the gases have
enough time to diffuse into the cells, as a result, the complete foam structure development
can be achieved for either untreated or treated samples. Thus, if both untreated and
treated samples contained same amount of gases, there should be not much difference in
foam density between them. At lower saturation pressures, there is not much difference
in gas-uptake between these two samples (as shown in Figure 1); as a result, there is not
much variation in foam density as shown in Figure 3 (a). At higher saturation pressure
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(5.5 MPa), although the gas-uptake of untreated sample was higher than that of treated one
(as shown in Figure 1), the foam density of untreated sample was larger than that of treated
one. This may be attributed to the further escaping of gases from voids and pockets to
the environment after the weight gain measurement and prior to the foaming treatment [3,
19, 20]. Thus, the amount of gases of untreated sample available for foam structure
development decreased, as a result, the foam density of the untreated sample increased.
Smaller cells were developed at lower saturation pressures and these cells were better
able to distribute tensile force than larger ones, because smaller cells are usually associated
with lower gas volume fraction [3]. Accordingly, it was found that the tensile strength
decreased with increasing saturation pressure in both types of sample, as shown in Figure 3
(b). The performance in strength of wood-flour/PVC composites is generally lower due
to the natural incompatibility between these two phases. The wood flour and PVC are
hydrophilic and hydrophobic in nature, respectively [19, 20]. The interface between
these two phases is weak and the dispersion of flours into the polymer matrix is poor due
to the strong flour-flour interactions.
In this study, aminosilane was used as a coupling
agent to disrupt the flour-flour interactions and join two phases together by a chemical
bonding. This chemical bonding was formed through acid–base pair interactions between
PVC and silane-treated wood flours [21]. As shown in Figure 3 (b), the tensile strength
of silane treated wood-flour/PVC foamed composite was slightly greater than those
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untreated wood-flour/PVC composites at saturation pressures of 2.5 and 5.5 MPa. At
saturation pressures of 3.5 and 4.5 MPa, the tensile strength of silane treated
wood-flour/PVC foamed composites was 9.1 % and 8.3 % greater than those untreated
wood-flour/PVC composites, respectively.
These results reflect that this coupling agent
enhanced the interfacial adhesion between wood flours and PVC matrix.
Figure 3 (c) compares the percent elongation at break between untreated and treated
wood-flour/PVC foamed composites at various saturation pressures.
In general, the
percent elongation at break increased with increasing saturation pressure in both types of
sample. Although cracks may propagate rapidly in homogeneous polymers, they are
dispersed by the existence of larger gas bubbles as saturation pressure increased, thus
increasing the percent elongation at break [3].
In Figure 3 (c), it shows obviously that the
percent elongation at break for untreated foamed samples was greater than that of treated
ones at each saturation pressure. The difference is caused by the different mode of
fracture between untreated and treated samples.
The fracture surfaces of the foamed
composites were viewed by scanning electron microscope to compare the difference
between treated and untreated specimens. For the treated wood-flour/PVC foamed
composite, the improved adhesion between wood flours and matrix causes fiber breakage
when the samples were fractured [14] (shown in Figure 4 (a)). However, the insufficient
adhesion between untreated wood flours and PVC matrix causes fibers pull-out when the
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samples were fractured (shown in Figure 4(b)).
It requires a higher energy for fracture
and causes the composites to exhibit a higher ductility compared to the treated samples
[14]. Another possible reason is because the large bubbles or pockets created inside the
untreated foamed composites, as shown in Figure 5, may impede the crack propagation
thus increasing the percent elongation at break [3].
Effects of saturation pressure on the properties of silane treated foamed composites
A series of silane treated wood-flour/PVC foamed composites were prepared to
investigate the effects of saturation pressure on the density and mechanical properties of these
materials.
The results were listed in Table II.
Samples A to D were silane treated
wood-flour/PVC composites with 5 phr of wood flour and saturation pressure varied from 2.5
to 5.5 MPa. Samples E to H and I to L were all silane treated wood-flour/PVC composites
with 15 and 20 phr of wood flours and saturation pressure varied from 2.5 to 5.5 MPa,
respectively.
All the specimens were saturated for 5 hours. For the specimens with the
same composition of wood flour, it was found that density, tensile strength, and young’s
modulus of the foamed composites, except the percent elongation at break, had the trend of
decreasing with increasing the saturation pressure.
It was expected since more cell structure
was developed at higher saturation pressure thus reducing the density and tensile strength of
the foamed composites. Both density and tensile strength of sample J were slightly lower
than those of sample K, this unexpected result may be caused by the experimental error. The
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material itself was softer at this high saturation pressure, as a result, the stiffness declined.
The values of elongation at break listed in Table II supposedly should increase with increasing
saturation pressure for each composite with fixed wood flour content, due to larger bubbles
were created at higher saturation pressure, which can help the dispersion of propagation
cracks. However, they do not show any trend with saturation pressure for any composite
with fixed wood flour content. This untoward result may be caused by the experimental
error, which needs further verified.
Dynamic mechanical properties of the wood-flour/PVC composites
The dynamic mechanical properties of neat PVC, untreated/treated wood-flour/PVC
unfoamed composites, and untreated/treated wood-flour/PVC foamed composites were
listed in Table III. A typical DMTA thermogram of silane treated wood flour (30 phr)
unfoamed composite was shown in Figure 6.
Both damping factor and storage modulus
of this specimen and others were determined at the Tg’s.
For untreated/treated
wood-flour/PVC unfoamed composites (specimens A-30 and A-30S), the glass transition
temperatures (Tg) were slightly greater than that of neat PVC (specimen A).
The
increased Tg may result from the friction between PVC and wood flours [22, 23]. But
once they were foamed (specimens A-30-F and A-30S-F), the glass transition temperatures
went back to those of their parent material (neat PVC). Accordingly, the increase of Tg
by the addition of wood flours was compensated through the foaming process.
The
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damping factors (tan ) of the untreated/treated foamed composites were greater than those
of neat PVC and untreated /treated unfoamed composites since most of the energy was
dissipated through the cell bubbles. The storage moduli of the untreated/treated foamed
composites were less than those of neat PVC and untreated /treated unfoamed composites
since the amount of energy stored as potential energy decreased as cell bubbles produced
through the forming process.
From Table III, it also indicated that the effect of surface
modification on the glass transition temperature, damping factor, and storage modulus is
nil for either unfoamed or foamed composites.
Effect of wood flour content and surface modification on the mechanical properties of
unfoamed and foamed composites
The effects of wood flour contents and their surface modification on the mechanical
properties of unfoamed and foamed composites were studied. The wood flour contents
varied from 5 to 30 phr. All the specimens were saturated with a pressure of 5.5 MPa for
5 hours. For the untreated and unfoamed composites, the tensile strength and percent
elongation at break decreased with increasing wood flour content as shown in Figure 7.
The polymer mass fraction (PVC) decreased when wood flour content increased, which
results in the decline of tensile strength of the composites, since the tensile strength of
PVC itself is greater than that of wood flours [14]. However, the brittleness of the
unfoamed composites increased with increasing wood flour content, thus reducing the
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percent elongation at break.
For silane treated and unfoamed wood-flour/PVC composites, the tensile strength and
percent elongation at break (shown in Figure 8) followed the same trend as those of
untreated and unfoamed samples. By comparing Figure 7 with Figure 8, it was found that
the tensile strength for treated and untreated specimens fell in the same magnitude at each
specific composition of wood flours. The same result was found with the percent
elongation at break.
It is interesting to know that the surface modification of the wood
flours did not play a significant role in improving the mechanical properties of the
unfoamed specimens. From Figures 7 and 8, it shows clearly that the tensile strength and
percent elongation at break of the PVC/wood-flour composites were lower than their
control sample (unfoamed PVC). The amount of decrease in tensile strength of either
untreated or treated unfoamed composites increased by increasing the amount of
incorporated wood flours as compared to that of unfoamed PVC. For example, at 30 phr
of wood flour, the tensile strength of either untreated or treated unfoamed composites was
about 19 % lower than that of unfoamed PVC. The decrease in percent elongation at
break was more pronounced. For instance, at 30 phr of wood flour, the percent
elongation at break of either untreated or treated unfoamed composites was about 67 %
lower than that of unfoamed PVC. This means the ductile mode of failure of PVC matrix
was sacrificed by the incorporation of brittle wood flours, which was agree well with the
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results reported by other investigators [19, 20]. From the above results, it concludes that
wood flours did not benefit the mechanical properties of the untreated and treated
unfoamed composites.
Figure 9 shows the mechanical properties of silane treated wood-flour/PVC foamed
composites as a function of wood flour content.
The addition of wood flours would
interfere with the foam structure development, thus the development of foam structure
decreased with increasing wood flour content [20].
It is noted that, in general, the tensile
strength of the silane treated wood-flour foamed composites was greater than that of the
control sample (foamed PVC) at each composition of wood flour (except at 5 phr). From
Figure 9, it indicated that when the content of wood flour was 30 phr, the tensile strength
of the silane treated wood-flour foamed composite was significantly (about 85 %) greater
than that of the control sample. This implies that the addition of silane treated wood
flours to the foamed composites can reinforce their tensile strength.
However, the
elongation at break of the silane treated wood-flour foamed composites was lower than
that of the control sample (foamed PVC) at each composition of wood flour (except at 5
phr). Although surface treatment of the flours by silane created a cohesive interaction
between flours and polymer matrix, which in turn increased the brittleness of the
composites by adding brittle wood flours into the PVC matrix [20]. For example, at 30
phr of wood flour, the elongation at break of the treated foamed composite was 31 % lower
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than that of foamed PVC.
The mechanical properties of untreated wood-flour/PVC foamed composites with
variations of wood flour content were shown in Figure 10. The tensile strength, in
general, increased with increasing wood flour contents (except at 5 and 10 phr), which
followed the same trend of treated and foamed composites as shown in Figure 9.
However, the amount of increase in tensile strength of the untreated and foamed
composites comparing to the control one (foamed PVC) was not as high as the silane
treated and foamed composites. For example, at 30 phr of wood flours, the tensile
strength of the untreated and foamed composite was only 42 % greater than that of control
sample comparing to about 85 % of the silane treated one. This further indicated that
silane played a significant role in improving the adhesion between the wood flours and
polymer matrix of the foamed composites. Unlikely in the case of unfoamed composites,
the wood flours treated by silane benefits the tensile strength of the foamed composites,
especially at composition of 30 phr. The percent elongation at break, however, followed
the opposite trend to the treated and foamed composites. For the untreated specimens,
the addition of brittle wood flours increased the brittleness of the foamed composites.
This effect, however, may be compensated with the ductile nature of the untreated foamed
composites due to voids and pockets created inside the composites were accompanied with
the addition of untreated wood flours.
Thus, as shown in Figure 10, the values of
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percent elongation at break of the untreated and foamed composites at low content of
wood flours (5 to 15 phr) were close to their control sample (foamed PVC).
However,
when the content of wood flours was up to 30 phr, the percent elongation at break of the
untreated and foamed composite was about 19 % higher than that of foamed PVC.
This
increase may be due to the fracture mode was dominated by lots of large voids and pockets,
which were inside the untreated and foamed composite when higher content of untreated
wood flours were added into the specimens.
From Figures 7-10, it was found that, in general, the tensile strength of foamed
composites was greater than that of the control sample (foamed PVC) at each wood flour
composition (except at 5 phr for silane-treated composite as well as 5 and 10 phr for
untreated composites), but not in the case of unfoamed composites.
The reason behind
this is because for foamed specimens, the addition of wood flours would interfere with the
foam structure development during foaming [20]. For neat PVC foam, since the matrix
did not incorporate with any wood flours, the foam structure can be fully developed and
the tensile strength of foamed PVC was deteriorated, accordingly. The degree of foam
structure development decreased with the amount of wood flours added (up to 30 phr),
which in turn, increased the tensile strength of either untreated or treated foamed
specimens.
Observation from Figures 7-10, it is interesting to find that, in general, silane can
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improve the tensile strength of foamed composites but not of unfoamed composites. For
example, at 30 phr wood flour foamed composites, the tensile strength of silane treated
composite is about 35 % greater than that of untreated one.
But for the unfoamed
composites at the same wood flour content, the tensile strength of silane treated composite
is about 10 % less than that of untreated one. The reason behind this may result from gas
saturation process when the foamed composites were prepared. During gas saturation,
the composite retains in an elastic and rubbery state rather than in a glassy state, which can
help enhancing the interfacial bonding between silane treated wood flours and PVC matrix
of foamed composites [3].
It is also noted that although the differences in tensile properties (strength and
elongation) between silane treated and untreated 30 phr wood flour foamed composites are
evident (as compared Figures 9 and 10), the dynamic mechanical properties (e.g., storage
modulus) do not show much difference between these two materials as indicated in Table
III.
This is because the storage modulus of foamed materials is characterized as the
amount of energy stored as potential energy during the dynamic loading, which is mainly
governed by the cell size of the foamed materials and not by the interfacial bonding
between wood flours and PVC matrix [3, 22]. The cell size of either silane treated or
untreated 30 phr wood flour foamed composite should falls almost in the same range,
accordingly, their storage modulus does not vary a lot.
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CONCLUSIONS
From the above investigation, the following conclusions can be drawn:
1.
The higher gas-uptake of untreated wood-flour/PVC composites than that of treated
samples may be due to the many voids and pockets containing inside the composites
served as a sink for gas-uptake.
2.
For foamed composites, the addition of wood flours, in general, benefits the tensile
strength of the foamed composites, especially at composition of 30 phr. This benefit
can be a plus when the wood flours were silane treated.
3.
The effect of surface modification on the dynamic mechanical properties for either
unfoamed or foamed wood-flour/PVC composites is small.
4.
The morphology of fracture surfaces between untreated and treated wood-flour/PVC
foamed composites reflects the observation in mechanical properties.
ACKNOWLEDGMENTS
The authors are grateful to the National Science Council of Republic of China for the
financial support (NSC 87-2216-E-041-004).
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Captions of figures
Figure 1. The difference in gas-uptake between neat PVC and untreated/treated wood-flour
(WF)/PVC composites.
Figure 2. Micrograph of (a)untreated wood-flour/PVC composite showing voids and pockets
inside the composite and (b) silane-treated wood-flour/PVC composite with less
voids and pockets inside the composite.
Figure 3. Comparison in (a) density, (b) tensile strength, and (c) percent elongation at break
between untreated and treated wood-flour (WF)/PVC foamed composites with
variation of saturation pressures.
Figure 4. The difference in morphology of fracture surface between (a) treated
wood-flour/PVC foamed composite and (b) untreated wood-flour/PVC foamed
composite.
Figure 5. Micrograph of untreated wood-flour/PVC foamed composite showing large
bubbles or pockets created inside the composite.
Figure 6. Typical DMTA thermogram of silane-treated wood flour (30 phr) unfoamed
composite.
Figure 7. Comparison in tensile strength and percent elongation at break between untreated
and unfoamed wood-flour/PVC composites with variation of wood flour content.
Figure 8. Comparison in tensile strength and percent elongation at break between silane
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treated and unfoamed wood-flour/PVC composites with variation of wood flour
content.
Figure 9. Comparison in tensile strength and percent elongation at break between silane
treated and foamed wood-flour/PVC composites with variation of wood flour
content.
Figure 10. Comparison in tensile strength and percent elongation at break between untreated
and foamed wood-flour/PVC composites with variation of wood flour content.
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