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3
3.1
3.2
3.3
3.4
3.5
4
1
1.1
1.2
1.3
Introduction
Background
Objective
Who is the guide aimed at?
2 Constituents and their Properties
2.1
Fibres
2.1.1
Glass fibres
2.1.2
Carbon fibres
2.1.3
Aramid fibres
2.1.4
Polyethylene fibres
2.1.5
Polypropylene fibres
2.1.6
Polyethylene terephthalate (PET) fibres
2.1.7
Natural fibres
2.1.7.1
Bast fibres (flax, hemp, jute, kenaf, ramie)
2.1.7.2
Leaf fibres
2.1.7.3
Seed fibres (Cotton, coir, kapok)
2.1.7.4
Advantages and disadvantages of natural fibres
2.1.7.5
Pre-treatment of natural fibres
2.2
Thermoplastic resins
2.2.1
Acrylonitrile-butadiene-styrene (ABS)
2.2.2
Polyamide (PA)/Nylon
2.2.3
Polybutylene Terephthlate (PBT)
2.2.4
Polycarbonate
2.2.5
Polyetheretherketone (PEEK)
2.2.6
Polyetherimide (PEI)
2.2.7
Polyethersulfone (PES)
2.2.8
Polyethylene
2.2.9
Polyethylene terephthalate (PET)
2.2.10
Polymethylmethacrylate (PMMA)
2.2.11
Polyphenylene sulfide (PPS)
2.2.12
Polypropylene (PP)
2.2.13
Polystyrene
2.2.14
Polytetrafluoroethylene (PTFE)
2.2.15
Polyvinylchloride (PVC)
2.2.16
Polyvinylidenedifluoride (PVDF)
2.2.17
Thermoplastic polyurethane
2.2.18
Thermoplastic biopolymers
2.2.19
Production volumes and prices comparisons
2.3
Fibre-matrix interface
Design
Introduction
Conceptual design
Preliminary design
Detailed design
Prototype manufacture and final production
Processing and Fabrication
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6
6.1
6.2
6.3
6.4
4.1
Introduction
4.2
Product forms
4.2.1
Prepreg
4.2.2
Co-mingled and co-woven fabrics
4.2.3
Glass mat reinforced thermoplastics (GMT)
4.2.4
Long fibre thermoplastic sheet (LFT)
4.2.5
Long and short fibre pellets
4.2.6
Thermoplastic/thermoplastic composites
4.3
Manufacturing processes
4.3.1
Injection moulding
4.3.2
Conjection moulding
4.3.3
Compression moulding
4.3.4
Stamp moulding of glass mat reinforced thermoplastic (GMT)
4.3.5
Hot press techniques
4.3.6
Autoclave processing
4.3.7
Diaphragm forming
4.3.8
Tape/filament winding
4.3.9
Pultrusion
5
5.1
5.2
Joining
Introduction
Hybrid joining
5.3
Welding
5.3.1
Introduction
5.3.2
Processes involving heat generation by mechanical movement
5.3.2.1
Friction processes
5.3.2.2
Ultrasonic welding
5.3.3
Processes using external heat sources
5.3.4
Processes using the electromagnetic spectrum
5.3.4.1
Introduction
5.3.4.2
Laser welding
5.3.4.3
Infrared lamp butt welding
5.3.4.4
Microwave welding
5.3.4.5
Resistive implant welding
5.3.4.6
Induction welding
5.3.4.7
Polymer coated material (PCM) technology
5.3.4.8
Adhesive bonding
5.3.4.9
Mechanical fastening
Non-Destructive Testing (NDT) of Bond Integrity
Introduction
Defect types
NDT techniques
Detection of kissing (weak) bond
Standards/Qualification
References
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41
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45
33
33
33
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1.1
1.3
Background
1.2
There is no widely accepted definition of a composite material. For this best practice guide a composite material is defined as a material system consisting of a mixture or combination of two or more micro- or macro-constituents that differ in form and chemical composition, and which are essentially insoluble in each other. In their most basic form, composite materials are a matrix (polymer, ceramic, metal) with reinforcing agents (fibres, whiskers, particulates).
This Best Practice Guide concentrates on fibre reinforced thermoplastic polymers.
Figure 1 - (a) Glass PPS powder thermoplastic rib for AIRBUS fixed wing leading edge by Stork
Fokker press moulded; (b) Carbon PPS Rib; (c) Press glass/PPS woven mouldings. (Courtesy of
TenCate Advanced Composite)
During the mid 1980s there was a great deal of interest in thermoplastic composite materials.
They combined the high strength-to-weight ratio of thermosetting composites, which were becoming readily accepted by engineers, with an improved damage tolerance and environmental resistance. However, the bulk processing of thermoplastic composites turned out to be more problematic than originally anticipated and therefore processing costs for thermoplastic composites were deemed to be too high and they fell out of favour.
Improvements in the understanding of thermoplastic polymers, the formulation of new polymers and improvements in processing technology have meant that once again, thermoplastic composites (Figure 1) are being considered as an alternative to thermosetting composites and metals. The drive towards recyclable materials and the penalties incurred by manufacturers for non-recyclable products mean that a potentially recyclable material, which includes thermoplastic composite materials, are being considered for many new applications.
Objective
The objective of this best practice guide is to give an overview of thermoplastic composites and their use. Information is given about constituents typically used to make a thermoplastic composite, the design processes followed, processes used to manufacture the composites and quality assurance and standard practices used.
Who is the guide aimed at?
2.1
No background knowledge of composite materials is assumed in this Best Practice Guide. It was written to give engineers who have never encountered them a feel for what constitutes a thermoplastic composite material, how they are made and how they are used in engineering structures.
Fibres
In a fibre reinforced polymer the fibres usually serve as a reinforcement and therefore have to show a high tensile strength and stiffness, whereas the tasks of the matrix are to hold the
(c)
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2.1.1 fibres together, to transmit the shear forces, and to work as coating. The mechanical properties of the fibres determine the stiffness and tensile strength of the composite decisively.
When describing fibres used for thermoplastic composites, it should be recognised that the fibres are not always produced to act as reinforcement. It will be seen in Section 4 of this Best
Practice Guide that the fibres are also sometimes produced as an intermediate step in the processing of a thermoplastic composite.
According to the orientation of the fibres, the materials behaviour of composites can be quasiisotropic (all fibres randomly orientated, no privileged direction of mechanical properties), anisotropic (all fibres orientated in one or more directions with corresponding mechanical properties, or orthotropic (fibres orientated in mainly two directions rectangular to each other showing corresponding materials behaviour).
The selection of suitable fibres is determined by the required values of stiffness and tensile strength of a composite. Further criteria for the choice of suitable reinforcing fibre are:
•
Elongation at failure
•
Thermal stability
•
Adhesion of fibres and matrix
•
Dynamic behaviour
•
Long time behaviour
•
Price and processing costs
Some useful terms to be aware of when discussing fibres or textiles are:
•
A tex is a unit to measure the thickness of thread. Thread of one tex has a mass of one gram per kilometre length. Equals 1/9 denier.
•
The denier system is used in the counting of yarns; designated by the weight in grams of
9000m of yarn.
•
Tenacity is the same as ultimate tensile strength. It is the highest load applied during the course of a tensile test, divided by the original cross-sectional area.
The following sections give details of fibres commonly used in the thermoplastic composites industry.
Glass fibres
Commercial continuous glass fibres are produced by melting the raw materials in a reservoir or tank. The molten glass is extruded through an orifice that is usually 0.793-3.175mm in diameter and then rapidly drawn to diameters ranging from 8-20µm.
Glass filaments are highly abrasive to each other and are therefore coated in an abrasion resistant coating or 'size'. This size may be temporary or it may perform other functions such as acting as a coupling agent to the resin being reinforced.
The properties of glass fibres can be tailored by varying the types and amounts of mineral glasses incorporated. Most fibres are based on silica (SiO
2
) with additions of oxides of calcium, boron, sodium, iron and aluminium. The most common types of glass fibres are:
•
E glass is the most commonly used glass, both in the textiles industry and in composites, where it accounts for 90% of the reinforcement used. E glass is so named (‘e’lectrical glass) because of its high electrical resistivity. These types of glass fibres are made of alumino borosilicates glass with very low sodium oxide content. They are often preferred because of its good chemical durability and high softening point. E glass fibres also have good stiffness, strength and weathering properties. E glass is one of the cheapest fibres, the price ranges from about £1-2/kg.
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2.1.2
•
S glass (magnesium aluminosilicate) is so named (‘s’tructural) because of its high tensile strength. S glass is more expensive than E glass, has a higher Young's modulus and is more temperature resistant. S glass is more difficult to process compared to E glass.
•
R, S and T glass are manufacturers trade names for equivalent fibres having higher tensile strength and modulus than E glass, with better wet strength retention. They were developed to have superior fatigue, temperature and humidity resistance. Developed for aerospace and defence industries, and used in some hard ballistic armour applications, these low production volumes lead to relatively high prices. Depending on the type of R or S glass the price ranges from about £12-20/kg.
•
D glass has very good dielectric characteristics and is used in the manufacture of radomes and high performance printed circuit boards.
•
AR glass stands for alkaline resistant glass. It was specially designed for reinforcing cement. Its high zirconium oxide content gives it excellent resistance to an alkaline environment.
•
C glass (‘c’hemical) is so named because it has improved chemical resistance. It is not used in Europe/USA for structural fibres but is used for veils and surfacing layers. C glass is mainly used in chemical and water pipes and tanks.
•
E-CR glass represents boron-free fibres with a modified structure to enhance their long term acid and short term alkali resistance. Their mechanical properties are similar to Eglass but they have less weight loss based on sulphuric acid exposure. The enhanced corrosion resistance results in an increase in cost.
Some of the properties of some types of glass fibre are shown in the table below.
Table 1 - Properties of glass fibres
Property
Ultimate tensile strength, GPa
Tensile modulus, GPa
Elongation to failure, %
Density, g/cm
3
Carbon fibres
E
Glass
3.4
73
4.8
2.6
D
Glass
2.5
55
4.5
2.1
R
Glass
4.4
86
5.2
2.5
AR
Glass
3.0
73
4.3
2.7
C
Glass
2.5
E-CR
Glass
3.6
72
2.6
S
Glass
4.6
86
5.0
2.5
Carbon fibre is a material consisting of extremely thin fibers about 0.0002–0.0004 inches
(0.005–0.010 mm = 5-10 µm) in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment makes the fiber incredibly strong for its size.
Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric.
Carbon fibres are produced by the controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors, which are already in fibre form. The most common precursor is polyacrylonitrile (PAN), because it gives the best carbon fibre properties, but fibres can also be made from pitch or cellulose. Variation of the graphitisation process produces either high strength (HS) fibres (at ~2600°C) or high modulus ( HM) fibres (at ~3000°C) with other types in between. Once formed, the carbon fibre has a surface treatment applied to improve matrix bonding and chemical sizing which serves to protect it during handling. The result is usually
93–95% carbon. Lower-quality fibre can be manufactured using pitch or rayon as the precursor instead of PAN. Since the first production of carbon fibres, carbon fibres have come down in price significantly. Nowadays, a high strength grade costs around £15-40/kg.
Carbon fibres are usually grouped according to the modulus band in which their properties fall. These bands are commonly referred to as: high strength (HS), intermediate modulus
(IM), high modulus (HM) and ultra high modulus (UHM). The filament diameter of most types is about 5-7mm. Carbon fibre has the highest specific stiffness of any commercially available
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2.1.3
2.1.4 fibre, very high strength in both tension and compression and a high resistance to corrosion, creep and fatigue. Their impact strength, however, is lower than either glass or aramid, with particularly brittle characteristics being exhibited by HM and UHM fibres.
Although fibres made by different manufacturers may be similar, they may not behave identically in all respects. Subtle differences in precursor types and processes can significantly affect the behaviour of the carbon fibre in a composite. Table 2 shows some basic properties for groups of carbon fibres.
Table 2 - Basic properties of carbon fibres
Material type Ultimate tensile strength, GPa Tensile modulus, GPa Density, g/cm
3
Carbon HS
Carbon IM
Carbon HM
Carbon UHM
3.5
5.3
3.5
3.5
<100
200-350
350-450
>450
1.8
1.8
1.8
2.0
Aramid fibres
Aramid, or aromatic polyamide, was the first organic fibre with a tensile modulus high enough to be used as reinforcement in advanced composites. Aramid fibres are better known by trade names such as Kevlar (DuPont) and Twaron (Teijin Twaron). It is produced by spinning a solid fibre from solution.
Aramid has a high degree of alignment of long, straight polymer chains parallel to the fibre axis, which gives higher strength and modulus in the fibre longitudinal direction than the axial direction. Aramid is also fibrillar which means its compressive properties are poor, but is tough and highly damage tolerant. Aramid fibres are yellow in appearance. Typical key properties include low density (around 1.5g/cm
3
), relatively high strength, good impact resistance, good abrasion resistance, good chemical resistance, good resistance to thermal degradation, compressive strength similar to E-glass fibres, some grades of aramid fibres can degrade when exposed to ultraviolet light. Typically the price of the high modulus type ranges from £15 to £25 per kg.
Aramid fibres reinforced polymer composites are widely used in ballistic protective applications such as bullets proof vests, motorcycle protective clothing, aircraft bodyparts, etc.
Table 3 - Basic properties of aramid fibres
Material type Ultimate tensile strength, GPa Tensile modulus, GPa Density, g/cm
3
Aramid LM
Aramid HM
3.6
3.1
60
120
1.45
1.45
Aramid UHM 3.4 180
LM: low modulus; HM: high modulus; UHM: ultra-high modulus
Polyethylene fibres
1.47
Polyethylene fibres are available in many forms, such as low density (LDPE), high density
(HDPE), high strength, high modulus (HMS) and ultra high molecular weight polyethylene
(UHMWPE). The properties of the fibre depend on the molecular weight and the density of the polyethylene, the degree of crystallinity and the degree of orientation. Fibres made by gel spinning and subsequent drawing can have up to 85% crystallinity and 95% parallel orientation. These are known as ultra high modulus or high performance polyethylene fibres
(UHMPE or HPPE). These fibres have similar properties to Kevlar
®
, but have a lower density.
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2.1.5
2.1.6
2.1.7
In comparison with E glass, the PE fibre's tensile modulus and ultimate strength are only slightly better and less than that of aramid or carbon. Also, PE fibres are costly, and more importantly, the difficulty in creating a good fibre/matrix bond means that polyethylene fibres are not often used in isolation for composite components.
Polyethylene fibres of various grades are used in aerospace, marine, ballistic protection and many types of rope. Properties of two commonly used UHMWPE fibres, Dyneema
®
(DSM) and Spectra
®
(Honeywell Performance Fibers) are given in Table 3. Caution is advised when interpreting values of modulus and strength for polyethylene fibres. The mechanical behaviour of polyethylene is extremely dependent on strain rate and temperature. At relatively high strain rates, polyethylene compares well with glass, carbon and Kevlar, however, polyethylene creeps and therefore long-term strength properties can be significantly reduced.
Table 4 - Properties of ultra high molecular weight polyethylene fibres
Material type
Dyneema®
Spectra fibre
900®
Ultimate tensile strength, GPa
2.8
2.4
Tensile modulus, GPa
88
66
Elongation to failure, %
3.6
4.1
Density, g/cm
3
0.97
0.97
Polypropylene fibres
Polypropylene thermoplastic fibres are manufactured using the melt spin, spunbond or meltblow processes. The properties of the final product depend on the operating parameters used in each process.
Polypropylene fibres have excellent chemical resistance, low density (0.91g/cm
3
) and a moderate cost and are therefore important fibres in industrial applications. They have a softening point in the region of 150°C and melt bet ween 160 and 170°C. At low temperatures of -70°C, polypropylene fibres retain their excelle nt flexibility. However, the fact that it is difficult to dye and texturise have limited the applications in the conventional textile industry.
Polyethylene terephthalate (PET) fibres
Polyethylene terephthalate is a thermoplastic polymer resin of the polyester family and is often used as synthetic fibers. PET is a hard, stiff, strong, dimensionally stable material. It absorbs little water and has good chemical resistance, except to alkalis which hydrolyse it.
Natural fibres
Natural fibres, as a substitute for glass fibres in composite components, have gained renewed interest in the last decade, especially in the automotive industry. Natural fibres can be subdivided into vegetable, animal and mineral fibres. Mineral fibres are no longer or only in very small amounts applied in new technical developments because of their carcinogenic effect.
All vegetable fibres (e.g. cotton, jute, flax, hemp, etc.) are built up from cellulose, whereas fibres of animal origin consist of proteins (e.g. hair, silk, wool). Vegetable fibres can be classified as bast, leaf, or seed-hair fibres, depending on their origin. In the plant, the bast and leaf fibres lend mechanical support to the plant’s stem or leaf, respectively, e.g. flax, hemp, jute, or ramie. In contrast, seed-hair fibres, such as cotton and milkweed, are attached to the plant’s seeds and aid to wind dispersal.
Fibres like flax, hemp or jute are cheap, have better stiffness’s per unit weight and have a lower impact on the environment. Applications at the moment are mainly limited to non-
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structural parts while research is performed to enable the material to be processed using existing production techniques and to improve consistency of the material.
Cellulose is a natural polymer with high stiffness and strength per weight, and it is the building material of long fibrous cells. These cells can be found in the stem (bast fibres), the leaves
(leaf fibres) or the seeds (seed-hair fibres) of plants.
2.1.7.1 Bast fibres (flax, hemp, jute, kenaf, ramie)
The bast fibre consists of a wood core surrounded by a stem. Within the stem there are a number of fibre bundles, each containing individual fibre cells or filaments. The filaments are made of cellulose and hemicellulose and are bonded together by a matrix, which can be lignin or pectin. The pectin surrounds the bundle and bonds it to the stem. The pectin is removed during the retting process. This enables the separation of the bundles from the rest of the stem (scutching). After the fibre bundles are impregnated with a resin during processing of a composite, the weakest part of the material is the lignin between the individual cells. Especially in the case of flax, a much stronger composite is obtained if the bundles are pre-treated in a way that the cells are separated, by removing the lignin between the cells. Boiling in alkali is one of the methods to separate the individual cells. Flax delivers strong, stiff fibres and it can be grown in moderate climates. The fibres can be spun to fine yarns for textile (linen). Other bast fibres are grown in warmer climates. The most common is jute, which is cheap, has a reasonable strength and is resistant to rot. Some properties of these fibres are given in the table below.
Table 5 - Properties of natural fibres in comparison to E-glass
E-Glass
Flax
Hemp
Jute
Ramie
Coir
Sisal
Abaca
Cotton
2.1.7.2 Leaf fibres
Density
, g/cm
3
2.55
1.4
1.48
1.46
1.5
1.25
1.33
1.5
1.51
Ultimate tensile strength,
GPa *
3.4
0.8-1.5
0.55-0.9
0.4-0.8
0.5
0.22
0.6-0.7
0.98
0.4
Tensile modulus
E, GPa
73
60-80
70
10-30
44
6
38
12
Specific
Modulus
E/
29
26-46
47
7-21
29
5
29
8
Elongation to failure, %
4.5
1.2-1.6
1.6
1.8
2
15-25
2-3
3-10
Moisture absorption,
%
7
8
12
12-17
10
11
8-25
Price/kg,
US $
1.3
0.5-1.5
0.6-1.8
0.35-1.5
1.5-2.5
0.25-0.5
0.6-0.7
1.5-2.5
1.5-2.2
In general, leaf fibres are coarser than the bast fibres. Applications are ropes and coarse textiles. Within the total production of leaf fibres, sisal is the most important. It is obtained from the agave plant. The stiffness is relatively high and it is often employed as binder twines.
The abaca fibre, which is from the banana plant, is durable and resistant to seawater.
2.1.7.3 Seed fibres (Cotton, coir, kapok)
Cotton is the most common seed fibre and is used for textiles all over the world. Other seed fibres are applied in less demanding applications such as stuffing of upholstery. Coir is an exception to this. Coir is the fibre of the coconut husk and is thick and coarse but durable.
Applications are ropes, mating and brushes.
2.1.7.4 Advantages and disadvantages of natural fibres
In general, their moderate mechanical properties prevent natural fibres from being used in high-performance applications, but for many reasons, they can compete with glass fibres.
Advantages of natural fibres are:
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•
Low specific weight – can result in a higher specific strength and stiffness than glass fibres.
•
Renewable source – production requires little energy, carbon dioxide is used while oxygen is released.
•
Producible with low investment – interesting product for low-wage countries.
•
Friendly processing – no wear of tooling, no skin irritation.
•
Thermal recycling possible – glass causes problems in combustion furnaces.
•
Good thermal and acoustic insulating properties.
Disadvantages of natural fibres are:
•
Lower strength properties - particularly impact strength.
•
Variable quality - subject to unpredictable influences such as weather.
•
Moisture absorption - causes swelling of the fibres.
•
Limited maximum processing temperature.
•
Lower durability, although fibre treatments can improve this considerably.
•
Poor fire resistance.
•
Price can fluctuate - depends on harvest results or agricultural politics.
•
Irregular fibre lengths - spinning required to obtain continuous yarns (for weaving or winding).
2.1.7.5 Pre-treatment of natural fibres
Very often, treatments of fibres are required to turn the just harvested fibres into fibres suitable for composite processing. Flax is one of the fibres which have to be treated before being used as reinforcement in the composite. Treatments and also processing play important role, they are critical operations that can influence the final properties of the fibres significantly.
Treatments are also required to increase adhesion between fibres and matrix. Most of the time, proper wetting of the natural fibres is difficult, especially with thermoplastic resins.
Although it has been demonstrated that natural fibres with low environmental impact, can offer sustainable and economical alternatives to synthetic fibres, costs and environmental impacts of these treatments should be carefully considered .
2.2 Thermoplastic resins
Thermoplastic resins can be melted by heating, reshaped and reformed if necessary, and then solidified by cooling. Thermoplastics do not form cross-links like thermosetting materials; therefore they are flexible and reformable. Thermoplastic materials can be either amorphous or semi-crystalline, each with its own set of properties. Amorphous thermoplastics have no order and their long-chain molecules can be totally entwined and randomly arranged as shown in Figure 2a. In semi-crystalline thermoplastics, parts of the long chains are arranged in an orderly fashion as shown in Figure 2b. It is not possible to have 100% crystallinity in plastics.
Amorphous thermoplastics have a temperature range in which they change from a hard and relatively brittle state (at the low end of the range) to a viscous, rubbery state (at the high end of the range). The change between these states is known as the glass transition temperature
(T g
) and it is the temperature that amorphous plastics must be above for polymer chains to be able to flow over each other ie for processing or welding, etc.
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Figure 2 - Chemical structure of thermoplastics: a) Amorphous; b) Semi-crystalline thermoplastics.
Semi-crystalline thermoplastics will also exhibit a T g
, associated with the amorphous regions they contain, but for flow of the polymer chain to occur, the crystalline melting temperature
(T m
) must be reached. T m
is higher than T g
. Therefore, processing or welding can only generally occur above T m
.
The following sections of this report give details of some thermoplastic resins commonly used in the thermoplastic composite industry.
The Figure 3 shows that the most engineering thermoplastics have been introduced industrially in the 1950-1980 period (Herman F Mark, 2004).
Figure 3 - The historical development of synthetic thermoplastic resins (Herman F Mark, 2004).
The reported years indicate the presumed entry in the market. Acronyms are presented in the
2.2.1 appendix.
Acrylonitrile-butadiene-styrene (ABS)
ABS is the name of a family of amorphous thermoplastic polymers, developed +in the late
1940s, that are formed from the polymerisation of the three monomers acrylonitrile, butadiene and styrene. They generally have good impact resistance, although the grades with very high impact resistance tend to have slightly lower strength, rigidity, hardness and heat deflection properties. ABS is slightly hygroscopic and therefore should be dried before processing.
Selected properties of three grades of ABS are given in the table below.
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2.2.2
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, GPa
Notched Izod at room temperature, J/m
Water absorption over 24 hours, %
T m
, °C
T g
, °C
Processing temperature, °C
Polyamide (PA)/Nylon
High impact grade
1.01-1.04
33-43
1.65-2.27
347-534
0.20-0.45 0.20-0.45
Amorphous Amorphous
100-110
204-260
Medium impact grade
1.03-1.06
30-52
2.07-2.76
134-320
105-115
204-260
Heat resistant grade
1.05-1.08
41-52
2.07-2.62
107-347
0.20-0.45
Amorphous
110-125
204-260
The thermoplastic nylons are produced by polymerisation in the molten state. The respective monomers are heated in the presence of various quantities of water at 200-300°C at normal or elevated pressure. Excess water and water formed during the reaction must be evaporated during the process in a controlled manner.
Polyamide or Nylon can cover the following materials:
•
Nylon 6; polycaprolactam.
•
Nylon 6,6; poly(hexamethyleneadipamide).
•
Nylon 6,6/6, Nylon 6/6,6; copolyamides of hexamethylenediamine, adipic acid and caprolactam.
•
Nylon 6,10; poly(hexamethylenesebacamide).
•
Nylon 6,12; poly(hexamethylenedodecanamide).
•
Nylon 11; poly(11-aminoundecanoic acid).
•
Nylon 12; poly(12-aminododecanoic acid).
Of all the nylons, nylon 6,6 possesses the greatest hardness and rigidity and the highest resistance to abrasion and heat deformation. The order of progression of toughness is nylon
6,6, nylon 6,6/6, nylon 6/6,6, nylon 6, nylon 6,10, nylon 11 and nylon 12, with nylon 12 being the toughest. The heat distortion and hardness decrease in the same manner.
With respect to water absorption at 23°C and 50% re lative humidity, nylon 6, nylon 6/6,6, nylon 6,6/6 and nylon 6,6 fall in the range of 3 +/- 0.4%. Nylon 6,10 and 6,12 absorb 1.4 +/-
0.2% under the same conditions. The water absorption of nylon 11 and nylon 12 is even lower. Selected properties of Nylon 6 and Nylon 6,6 are given in the table below.
Table 7 - Properties of Nylon
Property
Density, g/cm
3
Table 6 - Properties of various grades of ABS
Ultimate Tensile Strength, MPa
Tensile Modulus, GPa
Elongation to failure, %
Notched Izod at room temperature, J/m
Dry
Moist
Dry
Moist
Dry
Moist
Dry
Moist
Nylon 6
1.13
80
50
3.0
1.5
50-100
200
53
96
Nylon 6,6
1.14
90
65
3.4
1.7
20
80
37
64
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2.2.3
2.2.4
Water absorption over 24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohol
T m
, °C
T g
, °C
Processing temperature, °C
3+/-0.4
Not resistant
Not resistant
Good resistance
Good resistance
Good resistance
Good resistance
220
60
230-335
2.8+/-0.3
Not resistant
Not resistant
Resistant
Resistant
Resistant
Resistant
255
279-300
The advantages of nylons are that they have good mechanical strength, good toughness at low temperatures, high thermal stability, excellent chemical resistance and good dielectric properties. The main disadvantage of nylons is their water absorption. When water is absorbed a change in properties takes place: strength, tensile modulus, hardness and resistance to creep decline and toughness increases.
Polybutylene Terephthlate (PBT)
PBT is a partially crystalline thermoplastic polyester, which is formed by the polycondensation of 1, 4-butanediol with either terephthalic acid or dimethylterephthalate.
PBT resins offer high strength and rigidity, low moisture absorption, high heat deflection temperatures, excellent electrical properties and chemical resistance, rapid moulding cycles and reproducible mould shrinkage. A fast crystallisation rate enhances mouldability, producing smooth surfaces that can be painted or printed and ultrasonically welded.
PBT should not be used in hot water applications, as severe degradation can occur. Strong bases and oxidising acids should also be avoided. Selected properties of PBT are given in the table below.
Table 8 - Properties of PBT
Property
Density, g/cm
3
Ultimate Tensile Strength, MPa
Value
1.31
55
Tensile Modulus, GPa 2.55
Notched Izod at room temperature, J/m 53
Water absorption over 24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
T m
, °C
T g
, °C
Processing temperature, °C
0.1
Not attacked
Not attacked
Minimally attacked
Badly attacked
225
40
239-260
Polycarbonate
Polycarbonate is one of the toughest, most versatile engineering resins. It is an amorphous resin in which groups of dihydric or polyhydric phenols are linked through carbonate groups.
Polycarbonate has very good impact properties and good optical properties. However, when exposed to high stresses, temperatures and moisture it does undergo stress cracking.
Polycarbonate's hydrolytic sensitivity at high temperatures means that it should be dried to
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2.2.5 less than 0.02% moisture before processing. Some properties of polycarbonate are given in the table below.
Table 9 - Properties of polycarbonate
Property
Density, g/cm
3
Ultimate Tensile Strength, MPa
Tensile Modulus, GPa
Notched Izod at room temperature, J/m
Water absorption over 24 hours, %
T m
, °C
T g
, °C
Processing temperature, °C
Polyetheretherketone (PEEK)
Value
1.2
62
2.379
100-150
123 (0.25 inch)
Amorphous
150
260-332
PEEK was first prepared by ICI in 1977 and is a semi-crystalline thermoplastic with high melting point and glass transition temperature. The degree of crystallisation in PEEK can vary from 40% (slow cooling) to essentially amorphous (quenching). PEEK is generally used in high performance applications as it is tough, strong and rigid, has excellent heat, chemical and water resistance and is inherently non flammable. However, PEEK is notch sensitive, brittle at low temperatures, is degraded by UV light and has a high cost. Selected properties of PEEK are shown in the table below.
Table 10 - Properties of PEEK
Property
Density, g/cm
3
Ultimate Tensile Strength, MPa
Value
1.32
92
Tensile Modulus, GPa
Elongation to failure, %
3.6
50
Notched Izod at room temperature, J/m 83
Water absorption over 24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T m
, °C
T g
, °C
Processing temperature, °C
0.5
Not attacked
Attacked*
Not attacked
Not attacked
Not attacked
Not attacked
335
145
360-400
*Only attacked by concentrated nitric and concentrated sulphuric acids.
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Figure 4 - Boeing landing gear hubcap made out of Victrex PEEK 450GL30. (Courtesy of Victrex)
2.2.6 Polyetherimide (PEI)
PEI is a high performance amorphous thermoplastic based on regular repeating ether and imide linkages. There are several synthetic routes to produce PEIs of a general structure.
PEI has high mechanical strength coupled with high temperature and UV stability. PEI has outstanding electrical properties; it is inherently flame resistant and is resistant to aliphatic hydrocarbons, acids and dilute bases. Some typical properties are shown in the table below.
Table 11 - Properties of PEI
2.2.7
Property
Density, g/cm
3
Ultimate Tensile Strength, MPa
Value
1.27
105
Tensile Modulus, GPa
Elongation to failure, %
2.966
7-8
Notched Izod at room temperature, J/m 53
Water absorption over 24 hours, %
T m
, °C
T g
, °C
Processing temperature, °C
0.25
Amorphous
217
335-375
Polyethersulfone (PES)
PES is a high temperature, amorphous thermoplastic. It is tough, strong and creep resistant at room temperature and retains excellent mechanical and electrical properties at elevated temperatures. It is also non-flammable and resistant to degradation by hydrolysis. However,
PES is notch sensitive, has poor fatigue properties, and undergoes degradation when exposed to UV and certain chemicals. Some typical properties are shown in the table below.
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2.2.8
Table 12 - Properties of PES
Property
Density, g/cm
3
Ultimate Tensile Strength, MPa
Tensile Modulus, GPa
Elongation to failure, %
Value
1.37
84
2.44
40-80
Notched Izod at room temperature, J/m 76-120
Water absorption over 24 hours, % 0.12-1.7
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T m
, °C
T g
, °C
Processing temperature, °C
Not attacked
Attacked
Not attacked
Not attacked
Attacked
Not attacked
Amorphous
230
329-343
Polyethylene
Polyethylenes are thermoplastic resins produced by high and low pressure processes using various catalyst systems. The result is several families of polymers (low density, linear low density and high density) each having very different performance characteristics.
•
Low density polyethylene (LDPE) – LDPE is commonly made by polymerising ethylene at high pressure to form polyethylene molecules. The result is a highly branched long chain thermoplastic polymer with a density between 0.915 - 0.925g/cm
3
. The process can also produce medium density polyethylene (MDPE) which has a density of up to 0.935g/cm
3
.
LDPE has a low melting point with a broad range which makes it good for heat-seal operations. It is low cost, easily processed, but does not have very high stiffness, strength, and temperature resistance or barrier properties.
•
Linear low density polyethylene (LLDPE) – LLDPE is a hard, tough thermoplastic material consisting of a linear backbone with short side-chain branches. It is very versatile, low cost, adaptable to many fabrication techniques, chemically inert and resistant to solvents.
The resin density has a significant effect on the flexibility, permeability, tensile strength and chemical and heat resistance of LLDPE.
•
High density polyethylene (HDPE) – HDPE is partially crystalline, the degree depending on the molecular weight, the amount of comonomer present and the heat treatment used.
The range of crystallinity is normally 50-80%. Compared with LDPE and LLDPE, HDPE provides greater stiffness and rigidity. Advantages of HDPE are as follows: o Good moisture barrier properties. o Good stiffness - adequate for some structural applications. o Good load-bearing properties when creep is factored correctly in the design. o Relatively chemically inert. o Good thermal stability (-40 - 316°C).
Disadvantages of HDPE are: o Relatively high gas transmission rates. o Creep prevents it being considered a true engineering plastic. o Prone to environmental stress cracking. o Undergoes high temperature degradation.
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•
Ultra High Molecular Weight polyethylene (UHMWPE) – UHMWPE is a linear high density polyethylene with a molecular weight in the range of three to six million. It has unique combinations of physical and chemical properties plus other attributes that are quite similar to those of HDPE. Unique characteristics of UHMWPE include: o Highest slurry-abrasion resistance of any thermoplastic. o Exceptional impact resistance at cryogenic temperatures. o Low coefficient of friction. o Self-lubricating properties. o Outstanding stress-cracking resistance. o High resistance to cyclical fatigue failures o Noise and energy attenuation.
Properties of various types of polyethylene are shown in the table below.
Table 13 - Properties of polyethylene
Property
Density, g/cm
3
LDPE
0.915-0.935
Ultimate tensile strength, MPa 6.9-17.2
Tensile modulus, MPa
Elongation to failure, %
138-310
100-700
Water absorption over
24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T
T m g
, °C
, °C
Processing temperature, °C
<0.01
Not attacked
Minimally attacked
Not attacked
Not attacked
Minimally attacked
Not attacked
106-112
-126
200-213
LLDPE
0.910-0.925
14-21
137-186
200-1200
<0.01
Not attacked
Minimally attacked
Not attacked
Not attacked
Not soluble
Not soluble
125
-130
190-275
LMDPE
0.926-0.940
14-24
200-1200
Not attacked
Minimally attacked
Not attacked
Not attacked
Not soluble
Not soluble
125
-130
215-275
HDPE
0.941-0.967
18.6-30.3
100-1000
Not attacked
Minimally attacked
Not attacked
Not attacked
Minimally attacked
Not attacked
130-137
-100
190-275
UHMWPE
0.93
19.9-41.4
110,000
300
Not attacked
Not attacked
Not attacked
Not attacked
Swells slightly
Not attacked
125-138
-160
2.2.9 Polyethylene terephthalate (PET)
PET is the most widely used thermoplastic polyester. A PET homopolymer is formed by reacting one acid, such as terephthalic acid, with one glycol or diol. When part of the acid moiety is replaced by another acid, or a part of the diol is replaced by another diol, a copolymer is formed.
PET has the ability to exist in either an amorphous or a totally crystalline state. The degree of crystallinity can range from 0 to 60%. PET homopolymer and some of the copolymers are crystallisable and will crystallise spontaneously when held for a period of time in a range above their T g
(80°C). PET copolymers are characterised by lower melting points. Some of the copolymers, such as PETG
®
copolyester are completely amorphous. This characteristic allows PET copolymers to be processed at lower temperatures.
PET is a versatile plastic because of its excellent physical properties and because it can be converted into either amorphous or semi-crystalline products. Outstanding dimensional stability can be obtained in PET film by controlling orientation and by heat setting during processing. Few other materials offer such a range of processing and property variables.
Selected properties of PET are given in the table below.
Table 14 - Properties of PET
Property
Density, g/cm
3
Ultimate tensile
PETG copolymer PET (0.76 IV Bottle Polymer)
1.27 1.4
50 Hoop 172
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strength, MPa
Tensile modulus, MPa
Elongation to failure, %
Notched Izod at room temperature, J/m
Water absorption over
24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T m
, °C g
, °C T
Processing temperature, °C
1.724
180
90
0.5
Fair
Poor
Fair
Poor
Fair
Fair
None
81
280-300
2.2.10 Polymethylmethacrylate (PMMA)
Axial 69
Hoop 4.275
Axial 2.206
<0.1
Good
Fair
Fair
Fair
Good
245
80
PMMA is formed by free radical polymersiation of methyl methacrylate in bulk, solution, suspension or solution.
PMMA is a hard, stiff, transparent thermoplastic at room temperature but, as with other amorphous thermoplastics, softens and loses strength above its glass transition temperature.
The properties of PMMA depend predominantly on the molecular weight or grade, of which there are nearly 100. It has limited abrasion and stress cracking resistance; it is vulnerable to attack by some chemicals and has a limited service temperature for load-bearing parts.
Selected properties of PMMA are given in the table below.
Table 15 - Properties of PMMA
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Elongation to failure, %
Notched Izod at room temperature,
J/m
Water absorption over
24 hours, %
T m
, °C
T g
, °C
Processing temperature, °C
2.2.11 Polyphenylene sulfide (PPS)
General purpose
1.17-1.19
48-76
2.24-3.24
Per m of notch
16.2-32.4
0.1-0.4
Amorphous
85-103
243-250
Impact modified
1.11-1.18
55-76
2.42-3.1
2-40
Per m of notch
35-135
0.2-0.6
Amorphous
80-100
PPS is made commercially by the reaction of p-chlorobenzene with sodium sulfide in a polar organic solvent. PPS is a rigid, highly crystalline polymer with a melting point of 285°C. The melt viscosity is very low which makes processing relatively easy. PPS has excellent affinity for glass and mineral fillers. PPS compounds have the highest temperature resistance of any
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of the competitively priced engineering plastics; in addition, PPS is one of the most chemically resistant polymers. Typical properties of PPS are given in the table below.
Table 16 - Properties of PPS
Property
Ultimate tensile strength, MPa
Elongation to failure, %
Notched Izod at room temperature, J/m
T m
, °C
T g
, °C
Processing temperature, °C
Value
64-77
3
20
285-295
88
309-350
The crystallinity of the moulded PPS part plays a vital role in the final physical and mechanical properties of the part. At temperatures above the T g
the polymer chains have enough energy to crystallise. At temperatures below the T g
the matrix is frozen in the amorphous state. Cold moulded amorphous parts possess slightly better room temperature properties while sacrificing high temperature properties. Hot moulded parts have better high temperature properties and have a smoother, glossier surface.
Figure 5 - Cetex carbon PPS press moulded for Airbus keel beam rib. (Courtesy of TenCate
Advanced Composites)
2.2.12 Polypropylene (PP)
The term polypropylene covers an enormous range of products with properties and characteristics dependent on:
•
The type of polymer; homopolymer, random or block polymer.
•
The molecular weight and molecular weight distribution.
•
The morphology and crystalline structure.
•
Additives.
•
Fillers and reinforcement.
•
Fabrication technique.
The many different grades of polypropylene allow properties to be tailored to specific applications, however, some properties that are usually considered inherent advantages of polypropylene are:
•
High melting point (relative to most volume plastics).
•
Good stiffness/toughness balance.
•
Wide range of grades.
•
Excellent dielectric properties.
•
Low cost.
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Some of the disadvantages of polypropylene are:
•
It is flammable.
•
Low temperature brittleness.
•
Has only moderate stiffness.
•
Difficult to print, paint or reliably coat with adhesive.
•
Low UV resistance.
•
Low melt strength.
Selected, typical properties for polypropylene are shown in the table below.
Table 17 - Properties of polypropylene
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Elongation to failure, %
Notched Izod at room temperature, J/m
Water absorption over 24 hours, %
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T m
, °C
T g
, °C
Processing temperature, °C
2.2.13 Polystyrene
Value
0.90-1.24
19.7-80.0
0.5-7.6
3-887
0.16-no break
0.01-0.1
Excellent
Varies with acid
Excellent
Good
Nonpolar swells; polar excellent
Excellent
130-168
-20
202-252
Polymerisation of styrene yields a clear, colourless, general purpose polystyrene (GPPS) thermoplastic that is hard and stiff. Rubber (diene and other types) is added to provide the extensibility, toughness and impact resistance needed in certain applications. These latter styrenic plastics are known as high impact polystyrene (HIPS) or sometimes rubber modified polystyrene (RMPS).
GPPS is a hard, crystal-clear, amorphous solid at room temperature, however, polystyrene softens as it approaches its T g
. Above its T g
, polystyrene behaves under stress as a viscous fluid; well above its T g
polystyrene is a readily mouldable plastic. Rubber modified HIPS grades are opaque and are significantly improved in their toughness and impact resistance.
Properties of GPPS and HIPS are given in the table below.
Another commercial form of polystyrene is a semi-crystalline thermoplastic called syndiotactic polystyrene (sPS). It is made using a proprietary metallocene catalyst system and sold by
Dow under the name QUESTRA
®
. SPS provides good heat and chemical resistance, good mechanical and electrical properties, low moisture sensitivity and is easily processed.
Some advantages of polystyrene are:
•
Low density.
•
Low cost per unit weight.
•
Clear transparency.
•
It is stiff and creep is not a major concern.
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•
Hydrophobic.
•
Very good chemical resistance.
•
Good electric insulator and high dielectric strength.
Some of its disadvantages are:
•
The T g
of polystyrene is only just above the boiling point of water – generally use of polystyrene is restricted to temperatures below 80-90°C.
•
Undergoes UV degradation.
•
Burns when exposed to flame.
Table 18 - Properties of polystyrene
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Elongation to failure, %
GPPS
1.04-1.05
32.4-56.5
3.1-3.28
1.2-3.6
HIPS
1.05
16.0-41.3
1.65-2.55
1.0-1.5
QUESTRA
1.05
42
3.5
1.0
Notched Izod at room temperature, J/m 13.3-24.0
Water absorption over 24 hours, % 0.01-0.03
48.1-219.0
0.05-0.07
10
0.01
Amorphous Amorphous 270 T m
, °C
T g
, °C
Processing temperature, °C
74-110
200-218
93-105 100
®
2.2.14 Polytetrafluoroethylene (PTFE)
PTFE is made by a free radical initiated polymerisation of the gaseous monomer tetrafluoroethylene in an aqueous medium under pressure. Although the basic raw materials are relatively inexpensive and readily available, the need for extreme resistance to corrosion and other factors result in a polymer that is expensive.
PTFE is a semi-crystalline plastic that does not absorb visible or UV electromagnetic radiation and it has good chemical resistance and dielectric properties. Probably the best known property of PTFE is it has very low surface tension which makes it a good release agent, ie substances do not stick readily to it.
The rheology properties of PTFE are so different from usual thermoplastic materials that common techniques of melt processing are not feasible. A series of techniques have been developed that are unique to the PTFE industry. Typical properties of PTFE are given in the table below.
Table 19 - Properties of PTFE
Property
Density g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Elongation to yield, %
Value
2.2
20.67
413
40
Notched Izod at room temperature, J/m No break
Water absorption over 24 hours, % <0.05
327 T m
, °C
T g
, °C
Resistant to:
-100
Acid
Alkali
Solvents
Alcohol
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2.2.15 Polyvinylchloride (PVC)
There are three separate and distinct vinyl technologies: flexible, rigid and plastisol. The additives used in each technology may be different or may be used for a different purpose, eg in each technology, a lubrication system uses different types and amounts of materials. All three technologies are thermoplastic and have similar characteristics such as heat sensitivity and the ability to be made clear, opaque and in a wide range of colours. Plastisol PVC, however, is not considered to be suitable as a composite matrix material because it has low modulus and generally high voidage.
•
Flexible PVC – The major advantage of flexible PVC is that it is a formulated plastic and therefore can be made to meet a broad spectrum of requirements. It is the only plastic that can be fabricated by every known plastics process. Other advantages include: o Good cost performance. o Tough. o Very good outdoor weathering resistance. o Excellent electrical properties. o Can be made flame retardant. o Good surface appearance – can be made glossy or dull.
•
Disadvantages include: o Heat sensitivity. o Poor resistance to ketones and chlorinated hydrocarbons.
•
Rigid PVC – Rigid PVC is a formulated and compounded product with outstanding properties. It is very versatile as there are many different compounds. It is made from low cost feed stock and is over 50% chlorine, which means availability and pricing are not wholly related to oil or gas. Advantages of rigid PVC include: o Low cost. o High strength. o Low water absorption. o Good outdoor weathering. o Non-flammable. o Good stiffness. o Excellent electrical properties. o Good surface appearance.
•
Disadvantages include: o Processing difficulty caused by polymer stability. o Low heat deflection temperature. o Poor creep properties at elevated temperatures.
Properties of PVC are shown in the table below.
Table 20 - Properties of PVC
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Elongation to failure, %
Notched Izod at room temperature, J/m
Water absorption over 24 hours,
%
Rigid
1.32-1.58
41-52
40-80
21-1174
0.04-4.0
Flexible
1.18-1.7
5.5-26.2
4.8-12.4
150-450
Varies
Varies
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Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
T m
, °C
T g
, °C
Processing temperature, °C
2.2.16 Polyvinylidenedifluoride (PVDF)
Not attacked
Slightly attacked
Not attacked
Slightly attacked
Resists
175
81
199-21
Not attacked
Not attacked
Not attacked
Not attacked
Ketones, chlorinated hydrocarbons attack
Minimally attacked
Varies
Varies
Varies
PVDF is a hard, tough thermoplastic fluoropolymer patented by Pennwall Corp. It has the lowest melting point of any of the commercial fluoropolymers. The upper use temperature of
PVDF is limited to approximately 150°C, however, PV DF retains its very good stiffness and toughness properties well within its useful range. PVDF is one of the cheapest of the melt processable fluoropolymers and exhibits the same excellent chemical resistance common to fluoropolymers. PVDF has also been found to possess some very unusual piezoelectric and pyroelectric properties. Some of the properties of PVDF are shown in the table below.
Table 21 - Properties of PVDF
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Value
1.77
46
1.6
Elongation to failure, % 15
Notched Izod at room temperature, J/m 340
Water absorption over 24 hours, % 0.05
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Solvents
Alcohols
Not attacked
Not attacked
Not attacked
Not attacked
Not attacked
Not attacked
T m
, °C
T g
, °C
Processing temperature, °C
2.2.17 Thermoplastic polyurethane
170
-40
220-240
Polyurethanes are synthesised using a step-growth polymerisation process in which chain length increases steadily as the reaction progresses. The polymer product is unique; it has the properties of a thermoset elastomer without being cross-linked. Strong intramolecular forces (eg hydrogen bonds, Van der Waals, etc) and intramolecular entanglement of chains contribute to the virtually cross-linked state. On heating, the action of these forces disappears, permitting the polymer to be processed by standard methods. Some properties of thermoplastic polyurethanes are shown in Table 24.
Advantages of thermoplastic polyurethanes include:
•
Excellent abrasion and tear resistance.
•
High tensile strength.
•
Excellent load bearing capacity (high compressive strength).
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•
Very good resistance to non-polar solvents, fuels and oils.
•
Good optical and electrical properties.
Disadvantages of thermoplastic polyurethanes include:
•
Maximum service temperature of 93°C.
•
Hygroscopic.
Table 22 - Properties of thermoplastic polyurethanes
Property
Density, g/cm
3
Ultimate tensile strength, MPa
Tensile modulus, MPa
Value
1.11-1.28
10-76
2-10
Elongation to failure, % 110-1000
Water absorption over 24 hours, % 0.3
Chemical resistance
Weak acid
Strong acid
Weak alkali
Strong alkali
Stable
Medium
Stable
Medium
Solvents
Alcohols
T m
, °C
T g
, °C
Processing temperature, °C
No effect-attacked
No effect
Varies
-51
Varies
Having described typical constituents of thermoplastic composites and their properties, it is now important to discuss what happens when these are brought together. An interface is formed between the two materials, which is often considered by researchers to be so important that it is described as a constituent in its own right.
2.2.18 Thermoplastic biopolymers
Polymers of natural origin, e.g. starch or cellulose have to be modified physically or chemically to be suitable for the processing as thermoplastic resins, e.g. the structure of starch can be made thermoplastic using adjuvants, e.g. glycerol and water. A frequently used option to improve the properties is to add copolymers which can even be of petrochemical origin. In addition, other physical, mechanical, chemical and thermal properties of the biopolymers can be influenced by these modifications.
Thermoplastic biopolymers have already started being used for specific applications but there is still need for developments before being able to apply these materials to a wider range of products. Frequently, polymers from renewable resources do not sufficiently fulfil the requirements to be used as matrix in biocomposites. This deficit is based on historical development since these polymers have originally been designed for the packaging sector. In particular, they show either too high values of elongation at failure, or their rheological behaviour is a strong restriction for the application in biocomposites.
2.2.19 Production volumes and prices comparisons
Roughly, commodities are priced at US$0.5–1/kg, engineering polymers in the range of
US$1–5/kg, and high performance polymers the range of US$5–50/kg. The current prices fluctuate following market conditions and can be found as a price range, for most materials, in technical journals like Plastics Technology. In Figure 3, the prices of engineering thermoplastics are reported as a function of annual production volume, confirming, with a few exceptions, the inverse relationship between the two parameters. The price is reported in
21 TWI Ltd

U.S. cents per volume unit, more significant than the corresponding price per mass unit
(Herman F Mark, 2004).
Figure 6 - Production volumes and prices for volume units for thermoplastics (Herman F Mark,
2004). The dashed line represents an arbitrary border between engineering and high performance thermoplastics. In some cases, reinforced resins have been considered i.e. PPS:
10% glass fibres (GF); PSU, PA11: 30% GF; PAS: 40% GF. Acronyms are in appendix.
2.3 Fibre-matrix interface
The structure and properties of the fibre-matrix interface play a major role in determining the mechanical and physical properties of a composite material. Stresses acting on the matrix are transmitted to the fibre across the interface, so there needs to be a bond between fibre and matrix to use the full properties of the fibre. The strength of this bond can determine the properties of the composite itself. A weak bond produces a tough composite as energy can be absorbed by mechanisms such as fibre pullout. A strong bond between the fibres and matrix can produce a brittle composite.
Adhesion between fibres and a matrix can be attributed to five main mechanisms:
•
Adsorption and wetting. Wetting properties depend on the relative surface energies of the fibres and resin.
•
Interdiffusion. If the fibres are polymeric, or pre-treated with a polymeric coating, the polymeric molecules can become entwined.
•
Electrostatic attraction. Attractive forces occur between two surfaces with localised opposing charges.
•
Chemical bonding. Chemical bonds can be formed between the matrix and the fibre or a coating on the fibre.
•
Mechanical adhesion. Some bonding may occur purely by the mechanical interlocking of the fibre and matrix.
Coatings, sometimes known as sizing’s or coupling agents, are often applied to fibres to protect them from dirt, water pickup and damage during processing. These coatings are also quite often tailored to promote adhesion between the fibre and the matrix. The coating used will be dependent on the fibre, resin and interfacial strength required. There is often quite fierce competition between fibre manufacturers to produce the best coatings and therefore many of the treatments used are proprietary.
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3.1
In semi-crystalline thermoplastic composites it has been shown that inclusion of reinforcing fibres can promote the nucleation of crystals at the interface, thus possibly influencing the bond strength and the general reinforcing effect of the fibres.
Introduction
With composites, more than any other material, there is an important interrelationship between shape design, material selection and manufacturing selection. The ability to place oriented reinforcement specifically in required areas means that sections of a part which need extra strength or stiffness do not necessarily have to be designed thicker to provide the appropriate properties - as would occur with isotropic materials such as metals. This means that parts which were conventionally designed and produced in alternative materials quite often have to be totally redesigned when the decision is taken to make them out of a composite in order to make full use of the advantageous properties of composite materials.
Figure 7 - Design flowchart.
The Figure 7 shows a typical design plan for designing with thermoplastic composite materials. The following sections give a description of important considerations within each stage of the design process.
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3.2
3.3
Conceptual design
One of the first things that must be done in the design of a part is to define the functional requirements it must fulfil and other factors that may need to be considered. Some examples
(but by no means all) of these are:
•
Geometry.
•
Loading.
•
Specific mechanical properties.
•
Weight.
•
Temperature limitations.
•
Cost.
•
Electrical properties.
•
Chemical resistance.
•
Durability.
•
Optical properties.
•
Joining requirements.
•
Safety/handling requirements
•
Storage requirements.
•
Time to manufacture.
•
Recyclability.
In many cases, the shape and structure of the part to be produced is fixed by the need for it to fit into a surrounding structure. However, when this is not the case it is often best practice to completely ignore any previous design with an alternative material and design the best shape to give the appropriate localised properties at each point in the design with the materials chosen.
A critical factor that should be considered is joining the part to other parts and whether the material from which the part is to be made can be adhesively bonded or welded easily or whether mechanical joining will be necessary. Attachment points may need to be included in the design of the component. It has also quite often been found that structures originally made from alternative materials, with several parts joined together, can be fabricated as one large continuous structure when made from composite materials, thereby negating the need for joining.
All of these factors must be traded off against each other as some may be mutually exclusive and/or place restrictions on other factors. Eventually, the concept should be sufficiently well defined to allow preliminary design to commence.
Preliminary design
Once the concept has been defined, the next step is the preliminary design. This starts to define factors such as materials to be used, the structural configuration, the manufacturing process, and therefore the tooling required, and the all-important cost estimate.
These factors are all interrelated and, quite often, decisions made at the conceptual stage in relation to one factor will have implications in all factors. This can restrict the choices in all areas and therefore make the job less daunting.
With any material, the job of designing has never been easy; but as the number of materials available increases it becomes even more difficult. With many thousands of plastics obtainable - containing a variety of fillers and stabilisers - and the numerous different fibres - in various lengths, grades and formats - the huge range of possible composites available to the designer can be quite astounding.
The trick when looking for the right combination is to take the list of requirements for the part, and any other relevant factors, and assign each a level of importance. For example, product
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3.4 cost is usually more important in automotive applications, and weight is normally given higher consideration in aerospace applications. By working down the list of factors, starting with the most important first, and rejecting materials to which the factors do not apply, a shortlist of material combinations can be arrived at, from which a final choice can be more easily made.
The structural configuration may well have been defined, to a certain point, within the conceptual design, and choice of materials (and therefore their mechanical properties) will help to determine the structure if it has to be designed to withstand certain loads. If composite materials are being used to replace another material, care should be taken to completely redesign the structure, taking into account the mechanical properties of composites.
The major considerations when selecting a manufacturing process are:
•
The materials to be used. Certain materials are limited in terms of their processability; so, if the processing methods they can tolerate are not feasible for some other reason, an alternative material must be selected.
•
Production rate required. Some manufacturing methods cannot produce components at high rates and are, therefore, inappropriate for a part with a high volume market.
•
Cost. Most consumer markets are cost-sensitive and cannot afford higher production costs. Factors in manufacture that influence cost include tooling, labour, raw materials, process cycle time and assembly time.
•
Performance/surface finish. Some processes inherently produce better compaction (and therefore better mechanical properties) and some provide better surface finishes than others. The appropriate process should be chosen for the part.
•
Size and shape. Some processes are limited in terms of the size or complexity of the part that can be produced.
Once all these factors have been considered and a design has been identified which satisfies all the important parameters at a preliminary level, the next step is to move to the detailed design phase.
Detailed design
Stress analysis is an important task in the design of any part. It provides a quantitative measure of the loading a part will experience and can therefore be used to identify poor design practices before a part is put into service. It should be used once the preliminary design has identified the structural configuration, materials to be used and loading to be applied to the part.
Stress analysis procedures for composite materials can be relatively complex. The tailorability of composites means that the material properties of a part may alter dramatically within a very short space. Finite element analysis (FEA) software allows the part to be modelled, split into tiny segments, and different material properties to be assigned to each segment. Loads can then be virtually applied to the part and the resultant stresses on every segment are calculated. As computing power increases, the speed at which even the most complex parts can be analysed is increasing.
Analysis techniques such as FEA can be extremely useful but it should be remembered that the output is only as accurate as the input. In order for the exact stress state on the finished part to be calculated, the model must be exactly the same as the finished part and be under exactly the same loading conditions. In real life, however, factors such as variations in material due to processing, or variation in method of loading, may well lead to differences between the simulated and true case. FEA should, therefore, always be regarded as an approximation to the real case.
Throughout the design process, routine testing of material properties and part properties should be conducted in parallel with FEA analysis. This will validate or invalidate materials,
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3.5
4.1
4.2
4.2.1 design or manufacturing processes. Multiple tests should be performed for each variable and statistical analysis performed to check the variability of results.
Other aspects which should be considered at this stage are the quality assurance plans for ensuring that each part is manufactured to the required specifications, and the cost estimate of the entire production process.
Prototype manufacture and final production
Once all design decisions have been made, several prototype parts should be manufactured and tested to ensure they fulfil all the relevant criteria and check there are no problems. If the design procedure has been performed correctly, very few, if any, changes should be required.
However, mistakes do happen and, as Figure 7 shows, design corrections may have to be made. Producing prototypes allows for these mistakes to be corrected without the cost of scrapping production parts.
Once prototypes consistently satisfy all criteria, the production phase can begin.
Introduction
Both the raw material product form and the required part configuration usually dictate the fabrication process used. The material form can either be the basic constituents with impregnation occurring during the processing step, or the material may be pre-impregnated.
Pre-impregnated forms lend themselves to the rapid, automated manufacturing processes often perceived to be the key to thermoplastic composite competitiveness. However, these product forms can be stiff and have restricted drapability, which limits the products that can be moulded.
In contrast, using the basic constituent materials means that the fabric will be relatively easily drapable, but the processing requires much longer times at temperatures above the melting point of the resin. The melt viscosity of thermoplastics is high and therefore processes must be carefully planned with certain time, temperature and pressure requirements to ensure that full fibre wet-out occurs.
Some commercially available pre-impregnated product forms are described next, followed by a description of some standard processing techniques for thermoplastic composites.
Product forms
•
Prepreg.
•
Co-mingled and co-weaved fabrics.
•
Glass mat reinforced thermoplastics.
•
Long fibre thermoplastic sheet.
•
Long and short fibre pellets.
•
Thermoplastic/thermoplastic composites.
Prepreg
During the mid 1980s there was great interest in thermoplastic composites and their potential.
Large companies such as ICI invested a lot of effort into producing prepregs from high performance thermoplastics such as Polyetheretherketone (PEEK), polyimide and
Polyetherimide (PEI).
Difficulties in processing, along with other problems, lead to interest dying out in thermoplastic prepregs; but with recyclability high on the agenda, interest in thermoplastic
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4.2.2
4.2.3
4.2.4 prepregs is resurgent. There are now various companies offering thermoplastic prepregs, an example of which is Bond Laminates which sells TEPEX
®
.
TEPEX
®
is sold as a continuously reinforced thermoplastic sheet consisting of any of the following: carbon, aramid, and glass fibres with polyamide (PA), polyphenylene sulphide
(PPS), polybutylene-terephthalate (PBT), thermoplastic polyurethane (TPU), and others.
TEPEX
®
is manufactured using a continuous, double-belt laminator.
Co-mingled and co-woven fabrics
In these materials, fibres are spun from the thermoplastic material and either co-mingled with the reinforcing fibres, or, both types of fibre are interweaved to produce a fabric. These composite fabrics have primarily been used in advanced composites where the material may be layed-up in a reasonable approximation of the finished part as they are more pliable than melt-impregnated thermoplastic prepregs. Another advantage of these fabrics is that high molecular weight polymers can be used to produce the thermoplastic fibres, whereas in the melt impregnation process, low molecular weight polymers tend to be favoured.
Co-mingling is accomplished by spreading the reinforcing fibre filaments apart and intermingling thermoplastic fibres among the reinforcing fibres. Co-weaving involves weaving reinforcing fibres with thermoplastic fibres to produce a bicomponent system.
Another option is to wrap thermoplastic fibres around the reinforcing yarn. Co-weaving is less expensive than co-mingling but co-mingling produces fabrics that drape better and have more uniform distributions of the two fibres.
An example of a commercially available co-mingled fabric is Twintex
®
from Saint-Gobain
Vetrotex. This comes in the form of co-mingled glass/polypropylene and glass/ polyethylene terephthalate (PET).
Glass mat reinforced thermoplastics (GMT)
GMT sheet is made by melting extruded thermoplastic films against a continuous glass filament mat which is compressed under heat to form a polymer-based sandwich.
Polypropylene is normally used but thermoplastic polyurethane and other matrices are now available (eg PET and PBT). The mat may either be continuous, long (25-50mm) or chopped glass fibres.
GMT is light (30% less than sheet moulding compound, SMC), has a short process time and gives wide scope for automation. Glass fibre content can be up to 43%, and GMT has an indefinite shelf life and recycling is possible. However, the surface of GMT must be covered, or it must be used where it is not visible, as the glass can be seen on the surface.
Processing of GMT is wasteful of energy as the components must be reheated to be moulded to shape. GMT is an established technology and is currently being used by numerous automobile manufacturers to produce parts such as bumpers, instrument panels and noise shields.
Long fibre thermoplastic sheet (LFT)
This product was developed by Appryl Composites, based on a polypropylene matrix. It is not unlike GMT but uses long fibres (10mm+) instead of glass mat. LFT may be manufactured online, eliminating the need for a semi-finished product that has to be reheated for moulding.
Class A surfaces (critical exposed/painted applications where cosmetic surface imperfections are objectionable) may be produced. LFT is also less expensive than GMT, and proprietary additives can be used to stop the fibre migration to the surface.
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4.2.5
4.2.6
4.3
4.3.1
Long and short fibre pellets
Pre-impregnated pellets, granulates and chips are available with both long and short fibre reinforcement. They can be produced using wire-coating, cross-head extrusion or pultrusion techniques. Twintex
®
, as mentioned above, is also available in this form.
Thermoplastic/thermoplastic composites
There is a new class of thermoplastic composite coming onto the market, driven by the need to produce fully recyclable composites. These composites comprise thermoplastic fibres surrounded by the same thermoplastic resin. Two examples of these composites are CURV
®
, produced by BP, and PURE
®
, produced by Lankhorst/Indutech. These products are both polypropylene fibre with a polypropylene matrix, but this is not the only polymer being considered for production of such composites.
These products are produced either by heating techniques for selectively melting only the surface of the fibres, or by co-extrusion of a copolymer onto the surface of the fibre that has a slightly lower melt point to that of the fibre so that the outer copolymer melts before the fibre and forms the matrix.
These composites are generally available in tape, fabric or sheet form, and are thermoformable, lightweight, have good mechanical properties and are totally recyclable.
Manufacturing processes
Injection moulding
Injection moulding is the predominant process for producing thermoplastic parts in finished form. It is estimated that 25% of all thermoplastic resins are used for injection moulding. The same process is used for injection moulding of thermoplastic composites, except that the resin contains fibre reinforcement.
Injection moulding is a process that forces a measured amount of liquid, fibre-filled resin into heated mould cavities. Fibre-filled thermoplastics, in the form of pellets, flow from a feed hopper into a heated compression cylinder. Most injection moulding machines have a reciprocating screw-type barrel that heats the material before injection.
Other systems (see Figure 8) use a plunger which forces the stock around a heated mandrel.
The cylinder temperature increases to the melt temperature of the resin as the material flows towards the nozzle. The plunger forces a controlled amount of material through the injection moulding nozzle into the closed mould. When the viscous liquid is injected into the tightly clamped mould cavity, it forms a part of the desired shape. The mould is cooled to well below the melt temperature of the resin by water running through cooling channels. After the parts cool, the mould halves are opened and the parts are ejected. In most cases, the parts require no further finishing other than trimming off sprues and runners.
Figure 8 - Simple schematic of a plunger-type injection moulding system.
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4.3.2 Conjection moulding
Conjection moulding is a process where two or more materials are simultaneously injected into the mould. One material would be used for the core and another for the skin. The concept of conjection moulding is shown in Figure 9.
4.3.3
Figure 9 - Concept of conjection moulding.
Advantages of the injection moulding process are:
•
Allows production of complex shapes, with or without inserts and core materials, in one shot.
•
Part repeatability is much better than with any other moulding process, with dimensional tolerances typically held to ± 0.06mm.
•
High volume production, with process times ranging from 20 to 60 seconds; can be completely automated.
•
Parts produced can be low in cost due to high production rates and low labour costs
•
Small (5g) to large parts (85kg) can be produced.
•
Produces net-shape or near net-shape parts; eliminates finishing operations such as sanding or trimming, and surface finish is good.
•
Very low material wastage - runners, gates and scrap are all recyclable.
Disadvantages of the injection moulding process are:
•
Acquisition of an injection moulding machine requires significant capital investment; and lack of expertise can result in high start-up and initial running costs.
•
Unsuitable for producing low-volume parts due to high tooling costs.
•
Requires relatively long lead-time due to mould design and manufacture, process design, etc.
•
Unsuitable for producing prototype parts.
•
Many process variables - can make it difficult to produce correct quality parts immediately.
Compression moulding
Compression moulding of thermoplastic composites is very similar to the compression moulding of thermosetting composites, the major difference being the material used.
In thermoplastic compression moulding, the material, in the form of pellets or chopped rods, is placed inside the mould which is located in a hydraulic press. Pressure and heat are applied and the material flows to fill the mould cavity.
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4.3.4
4.3.5
4.3.6
In general, compression mould costs are low due to simplicity of design. Fibre lengths are not shortened and there is usually no associated fibre orientation during processing. However, cycle times are long in comparison to injection moulding, and complex shapes are not as easily moulded.
Stamp moulding of glass mat reinforced thermoplastic (GMT)
GMT is used for making high-volume parts, and stamp moulding of GMT is the only manufacturing technique used in widespread applications for making thermoplastic structural parts. Generally, stamp moulding is exactly the same process as compression moulding except that the material is usually preheated using an infrared source or hot air before being placed in the mould which is water cooled. Preheating of the material significantly reduces moulding times.
Advantages of GMT stamp moulding are:
•
One of the fastest techniques for making composite structural parts.
•
Inherent high productivity means less tooling and labour.
Disadvantages of GMT stamp moulding are:
•
High capital investment to set up process.
•
Limited to high volume production.
•
Typical fibre volume fraction is only 20-30% because of high viscosity of resin.
•
Surface finish of parts is of an intermediate nature.
Hot press techniques
This process is also called compression moulding of thermoplastic prepregs or the matched die technique. Unlike GMT, the prepregs used in this process are typically unidirectional, and fibre volume fraction is greater than 60%. In this technique, the heat for processing is applied by placing the mould between two accurately machined, heated platens made from a thermally stable material such as cast iron.
The process is primarily used for making simple shapes such as flat plates and test coupons.
The main advantages are:
•
High fibre volume fraction can be achieved.
•
Small to large parts can be manufactured.
•
The process has never gained much commercial importance; some of the disadvantages are:
•
Limited to flat plates (essentially).
•
Thick structures not easily produced.
•
Can be difficult to produce parts free of distortion and warping.
Autoclave processing
Autoclave processing of thermoplastic composites is similar to autoclave processing of thermoset composites.
Thermoplastic prepregs are laid down on a tool in the desired sequence and spot welded to make sure they do not move relative to each other. The assembly is then vacuum bagged and placed inside the autoclave. The process cycle is performed and the part is removed.
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4.3.7
4.3.8
Autoclave processing is probably the major manufacturing technique for fabricating thermoplastic composite parts in the aerospace industry. It has the following advantages:
•
Fabricates structural components with high fibre volume fraction.
•
Allows production of any fibre orientation.
•
Autoclave equipment is the same as standard thermoset prepreg processing equipment.
•
Suitable for making prototype parts.
•
Tool design is simple.
•
Prevents distortion of parts during processing.
Autoclave processing has the following disadvantages:
•
Lack of tack and drapability mean that lay-up of thermoplastic prepreg is labour intensive compared with thermoset prepreg.
•
High capital investment if autoclave must be purchased.
•
Higher temperature and pressures required for processing of thermoplastic composites compared with thermoset composites.
Diaphragm forming
Diaphragm forming is a process unique to thermoplastic composites - it was not adapted from thermoset technology.
The material to be processed, which is usually in the form of prepreg or co-mingled fabrics, is laid up and spot-welded to prevent movement. This preform is placed between two flexible diaphragms, between which a vacuum is pulled, heated close to the melt temperature of the thermoplastic and positioned over the mould. Vacuum ports are incorporated into the mould's female surface so that a vacuum can be created between the lower diaphragm and the female mould, pulling the preform into the female tool. Pressure is applied above the top diaphragm to form the composite sheet. As the forming pressure is applied, deformation of the diaphragms creates biaxial tension in the composite preform which prevents wrinkling.
Because the mould is not heated, the composite sheet cools and solidifies as it comes into contact with the mould.
Diaphragm forming has the following advantages:
•
Parts have excellent structural properties as they contain continuous fibres.
•
Reasonably complex parts with uniform thickness can be produced.
•
Reasonably high production rates.
•
Diaphragm forming has the following limitations:
•
Limited to parts with constant thickness.
•
Maintaining fibre orientation can be difficult as fibres have a certain amount of freedom to move between the diaphragms.
Tape/filament winding
Thermoplastic tape or filament winding is similar to filament winding of thermoset composites.
However, it also has some differences. The material, in the form of thermoplastic prepreg tape or co-mingled fibres, is wound onto the mandrel (the mould for the part to be produced) as with the thermoset process, but unlike the thermoset process, heat and pressure are applied at the point of contact of the roller and the mandrel for melting and consolidation of the thermoplastic.
The most common prepreg tapes are carbon/polyetheretherketone (PEEK), carbon/nylon and carbon/polyphenylene sulphide (PPS), but reinforcing fibres such as glass and aramid and matrices such as polyetherimide (PEI), polypropylene and polymethylmethacrylate (PMMA)
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4.3.9 have also been used for tape winding. Researchers have produced prototype parts to demonstrate the feasibility of thermoplastic tape winding, but the process has not been widely used for commercial applications. Tape winding can be used for tubular structures such as bicycle frames and launch tubes.
The advantages of thermoplastic tape winding are:
•
Cleaner production method than thermoset filament winding.
•
Concave surfaces as well as nongeodesic winding attainable because tape consolidates where it's laid down.
•
Thick and large structures can be formed in one shot (with thermoset filament winding, exothermic reactions or residual stress generation can often prevent this).
•
Offers the ability to post-form the structure.
•
No concern over styrene emission during the process.
•
No secondary processing (eg oven curing) required.
Some disadvantages of tape winding are:
•
Process is complicated because it requires a localised heat source and a consolidation roller.
•
Requires high capital investment.
•
Can be difficult to achieve well consolidated parts.
•
Quality of products inferior to those produced using filament winding (during winding, voids and porosity are formed at the intersection of consolidated tapes).
•
Raw material costs can be very high.
Pultrusion
This process is similar to pultrusion of thermoset composites.
Co-mingled fibres or prepreg are pulled through a heated die and compacted to get the final product. Because of the high viscosity of thermoplastic resins, processing can be difficult, and high pulling forces are required.
In pultrusion of thermoplastic composites, the die temperature is generally higher than with thermoset composites, and the thermoplastic profiles must exit the die at temperatures below the forming temperature to avoid distortion, which is not necessary with thermoset composites. The thermoplastic process also provides a surface finish of inferior quality to that produced using the thermoset pultrusion method. On a commercial basis, rods, square and circular tubes, angles, strips channels, rectangular bars and other simple shapes have been produced using this process.
Some advantages of the thermoplastic pultrusion process are:
•
Can process a wide variety of resins (thermoset pultrusion is limited to polyester and vinylester).
•
Parts are reformable and relatively easy to repair.
•
Environmentally friendly and does not emit styrene.
•
Parts can be recycled.
Some disadvantages of the thermoplastic pultrusion process are:
•
High heat and pressure required to ensure consolidation.
•
Surface finish inferior to thermoset pultruded parts.
•
Low flow rates due to high viscosity, difficult to produce complex shapes.
•
Cost of raw materials higher than for thermoset pultrusion.
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5.1
Introduction
Composite materials offer significant benefits over conventional engineering materials, including high specific strength and stiffness and increased corrosion resistance. The selection and successful implementation of a specific joining technique to maintain these benefits throughout a structure depend on the application requirements (including load, geometry, operating environment) and the matrix being used.
Using incorrect joining techniques for a given joint can lead to the strength of the joint being compromised due to material removal (stress concentration) and fibre damage in mechanically fastened joints, or poor surface preparation and debonding in adhesively bonded joints.
There are four basic methods applicable to joining of thermoplastic composites:
•
Welding techniques – the process of using energy to soften or melt a specific region of the component in which a joint is formed upon cooling.
•
Adhesive bonding – the process of joining two components by surface attachment using an adhesive.
•
Mechanical fastening – the process of joining two components by using a fastener (bolt or similar) or some other type of mechanical interlocking.
•
Hybrid joining – a method that combines the benefits of two or more methods of joining.
Hybrid joining 5.2
5.3
Hybrid joining is the combination of two or more joining techniques to produce joints with properties additional to those obtained from a single technique.
Welding
5.3.1 Introduction
5.3.2
For all thermoplastic welding techniques, three main process parameters are involved: heat, pressure and time. If two pieces of polymeric material, such as an uncrosslinked natural rubber, are brought together under sufficient pressure to guarantee intimate contact at the interface, a joint or weld will be formed between them after a few seconds. The strength of the joint will be based upon the time that the materials remain in intimate contact under pressure. Molecular chains within the natural rubber are said to diffuse across the interface, and strength develops at the joint due to the bridging of these chains, ie tack occurs. At room temperature, the natural rubber is above its glass transition temperature, T g
, and thus the molecular chains are mobile and free to move and interdiffuse.
In the case of a thermoplastic material, for example acrylic, the T g
is much higher than room temperature and therefore heat as well as pressure is required to effect the chain diffusion and hence the welding process. Time allows sufficient chain interdiffusion to ensure that weld strength develops, with temperature dictating the rate of interdiffusion. Additional to these three process parameters, the physical parameter, molecular weight, will also dictate the rate of interdiffusion and ability of the polymer to weld.
Processes involving heat generation by mechanical movement
5.3.2.1 Friction processes
When two surfaces are brought into contact and rubbed together under pressure, there is generally a temperature increase at the interface. The thermal welding of thermoplastics may be accomplished most efficiently by the generation of heat at the joint interface and for this reason welding techniques employing mechanical movement have found many commercial applications.
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Welding of thermoplastic parts by frictional heating is usually achieved by three main processes: linear friction (also known as vibration) welding, rotary friction welding and orbital welding. Each process finds its main composite applications where short, discontinuous fibres form the basis of the material. However, linear friction welding has also been demonstrated on continuous fibre reinforced PEEK (polyetheretherketone) (APC-2).
Linear friction welding of thermoplastics involves the rubbing together of the weld interface in a linear reciprocating motion. Typically, the frequency of vibration would be 100-200Hz at an amplitude of 1-4mm. Component design needs to incorporate one plane that allows for vibration movement during the welding process. Therefore, components with two or three directions of curvature would not lend themselves to being welded by this process.
The process is not considered suitable for welding dissimilar materials because the interlayer placed at the interface to promote welding would be disrupted by the vibrational movement.
5.3.2.2 Ultrasonic welding
5.3.3
Ultrasonic welding is a process that uses high frequency (20-40kHz) mechanical vibrations to heat and melt the thermoplastic being welded (Figure 10). Successful welding relies upon careful selection of tooling, welding horn, machine design, welding parameters and joint design. This process has predominately found applications in the assembly of mass produced injection moulded plastic goods where the short cycle times (typically 1-2 seconds) are ideal for high component throughput.
This process requires a projection at the interface to direct the ultrasonic energy and initiate the material melt flow. This type of joint geometry is hard to achieve in a continuous fibre thermoplastic composite fabrication, making the process difficult to use for welding these materials. The only possibility to be considered would be use of a surface roughening treatment on the materials to be joined to direct the energy for welding.
As with the other processes involving mechanical movement, there is also the potential for fibre damage at the interface. Although the size of the horn used in ultrasonic welding is limited to around 250mm, it may be possible to consider multiple ultrasonic welding systems to cover larger components.
Figure 10 - Principle of the ultrasonic welding. (Courtesy of TWI)
Processes using external heat sources
Thermoplastics generally have low coefficients of thermal conductivity, particularly when compared with metals. This means that heat is generally applied to the joint area prior to
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5.3.4 joining. In the case of one of the components of the joint being metal, an external heat source can be used to heat the joint area quickly. There are several welding techniques where heated metal plates or heated gas have served as the heat source, however the only technique truly effective for thermoplastic composites is hot plate welding.
Hot plate welding, sometimes called heated tool welding, is probably the simplest welding technique for thermoplastics. The parts to be welded are held in fixtures that press them against a heated tool. The process cannot, therefore, be applied to pre-assembled parts.
Heating takes place in two stages; the heated tool melts the thermoplastic surfaces and material is displaced against the tool so that a smooth surface is obtained. Mechanical stops in the equipment, or a reduction in pressure, prevent further displacement but the parts continue to be heated by the tool until they are softened some distance away from it. The fixtures then open, the heated tool is withdrawn, and the fixtures force the parts together.
Mechanical stops in the equipment or a pressure controller regulate the displacement of the parts during this stage of the welding process. The parts are held together until the weld is cool. The surface of the hot plate is sometimes coated with PTFE to prevent the adherence of molten thermoplastic. It is important that the temperature across the surface of the hot plate does not vary by more than approximately ±5°C in or der to produce welds of consistent quality. Weld pressure may be applied pneumatically or hydraulically, depending on the size of the joint area and the equipment configuration.
Processes using the electromagnetic spectrum
5.3.4.1 Introduction
There are a number of techniques that directly employ electric or magnetic fields to affect the heating process. These include resistive implant, induction welding, dielectric welding, microwave, infrared and laser welding.
5.3.4.2 Laser welding
Laser welding of thermoplastics involves the generation of an intense beam of radiation usually in the infrared or near visible regions of the electromagnetic spectrum, and the use of this beam for melting thermoplastic in a joint. Lasers were first used for cutting thermoplastics and it is only relatively recently that they have been considered for welding on an industrial scale.
CO
2
lasers can be used to weld films of thermoplastic in lap joint configuration very effectively. Welding speeds can be many tens of metres per minute, making the process ideal for high volume production in areas such as the packaging industry. By careful control of the laser beam profile it is also possible to make a weld and cut at the same time in an operation called cut and seal.
Welds in thermoplastics of over 1mm thickness can be achieved using shorter wavelength
(Nd:YAG and diode) lasers. With these lasers, through transmission laser welding can be used, where the energy from the beam is transmitted through one plastic component and absorbed in the other component or in an infrared absorbent dye placed at the interface.
Absorption causes the thermoplastic to heat, melt and form the weld. This technology, however, would only by suitable if the laser beam energy is not strongly absorbed in the upper component of the joint. The presence of reinforcing fibre in a polymer composite can also limit the passage of light due to increased scattering. This may limit the thickness of a composite that can be welded.
5.3.4.3 Infrared lamp butt welding
Infrared welding has been demonstrated for the welding of thermoplastic composites. In this process, a pair of infrared lamps is used to heat the joint surfaces directly, in a similar manner to hot plate welding. Once the surfaces are heated, the lamps are removed and the surfaces brought together to make a weld.
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The benefit of infrared welding is that it reduces the risk of weld contamination from charred particles of polymer. Unlike hot plate welding, it is a non-contact heating method with no possibility of material sticking to the heat source. Pre-assembly of parts prior to welding is not possible.
5.3.4.4 Microwave welding
Microwave welding is a relatively new technology that utilises a microwave-susceptible implant at the joint line. As yet, it is not used commercially for welding plastics. As the component being welded is subjected to microwave energy, the implant heats and melts the surrounding plastic material to form a weld on cooling. Implants could be carbon-impregnated tape, ferrite-loaded material or conducting polymer. The microwave cavity can be constructed to accommodate the part and allow application of pressure through the cavity wall.
Metals will themselves arc in the presence of microwave energy, so it is not recommended that this process be used for welding metal/polymer combinations. Glass or Kevlar reinforced thermoplastics can be microwave welded. The process may be suitable for thermoplastics to themselves or to thermosets using a suitable interlayer.
5.3.4.5 Resistive implant welding
Resistive implant welding involves trapping an electrically conductive insert between the two parts to be joined and then resistively heating the insert by passing a high electric current through it. The inserts are frequently made of metal (usually nickel/chromium) in wire, braid or mesh form, or unidirectional carbon fibre. The electric current is generally DC or low frequency AC. In the braid form, inserts are typically interwoven with monofilaments of the polymer being welded. As the implant heats due to resistive losses, the surrounding thermoplastic softens and, with the aid of some applied pressure, melts causing wetting at the interface between the parts. The principle is illustrated in Figure 11.
Figure 11 - Principle of the resistance welding.
Resistive implant welding is a simple technique which can be applied to almost any thermoplastic, one exception being where heavy carbon loading or a non-insulated metal component causes current leakage from the joint area.
Resistive implant welding has been developed for the joining of carbon fibre composite materials, where the implant was a single ply of the carbon fibre reinforced composite material. In this case, an electric current was passed down the carbon fibres, which heated due to ohmic losses.
5.3.4.6 Induction welding
Induction welding strictly describes welding techniques where heating is generated by an induction field. The two most commonly encountered mechanisms by which heat can be generated by an induction field are eddy current heating and heating due to hysteresis losses.
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Induction welding is similar to resistive implant welding in that an implant is generally required. Since most unfilled thermoplastics can be described as good insulators, an electrically conducting implant must be present at the joint line.
In induction welding by eddy currents, a work coil connected to a high frequency power supply is placed in close proximity to the joint. As electric current at high frequency passes through the work coil, a dynamic magnetic field is generated whose flux links the implant.
Electric currents are induced in the implant and when these are sufficiently high to heat the conducting material, the surrounding thermoplastic parts soften and melt. Pressure applied to the joint aids wetting of the molten thermoplastics. The implant, typically a wire mesh, remains in the joint at the end of the weld cycle. The implant has, therefore, a potentially detrimental effect on the weld's structural properties. The mesh may be a variety of sizes, with a typical thickness of around 0.5mm.
If there is a conductive component to the joint, this can be heated directly without the use of an implant.
Induction welding has been successfully used to weld APC-2.
5.3.4.7 Polymer coated material (PCM) technology
PCM is a novel technique developed by TWI for welding dissimilar materials.
Previous work at TWI on welding dissimilar materials has shown that the joint interface plays an important role in producing a strong joint. The PCM technique involves surface pretreatment of substrates using one or a combination of several methods: solvent degrease, grit blasting, etching and anodising. The pre-treated surfaces are then coated with a thermoplastic (Figure 12). Most of the work done to date has been with solutions of polymers.
In general, an intermediate film is also used at the joint interface and, in its simplest form, an interlayer may be used without coating the surfaces to be joined.
Figure 12 - Polymer coated titanium plate welded to long carbon fibres - PEEK. (Courtesy of TWI)
Current research is evaluating the use of specialised solutions of chemically modified materials, for example co-polymers, produced by grafting 'sticky' chemical groups onto the backbone of the polymer structure.
After a coating from solution has been applied, the coated substrates are allowed to 'dry' by evaporation. The two polymer coated substrates are then joined together using a conventional welding technique. Work to date has favoured induction, resistive implant and ultrasonic welding.
5.3.4.8 Adhesive bonding
The surface of composite materials usually consists of a layer of polymer resin and it is to this layer that the adhesive is applied. Therefore, the surface preparations or procedures that are
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applicable to thermoplastic polymers are also be applicable to the bonding of thermoplastic composites.
In adhesive applications it is desirable to maximise the wettability of a substrate, or the wetting properties of the adhesive being applied. Better wetting characteristics ensure uniform spreading of the adhesive, resulting in optimised bonding. It can be shown that the amount of spreading that occurs depends on the relative surface energies of the substrate and the adhesive. For wetting to occur, the surface free energy of the substrate must be higher than that of the adhesive. The problem with thermoplastic materials is that they tend to have very low surface free energies. Typical values for plastics are given in Table 1; these, when compared with aluminium and copper, which have values in excess of 8x10
-3
and
1.1x10
-2
N/cm respectively, are seen to be very low.
Table 23 - Surface free energies of common polymers.
Polymer Surface free energy, x10
-5
N/cm
ABS
Epoxy
Nylon 6
Nylon 6,6
Nylon 12
Polyethylene oxide
PBT
Polycarbonate
Polyethylene 31
Polyethylene terephthlate 43
Polymethylmethacrylate 39
Polyphenyl sulfide
Polypropylene
Polystyrene
Polytetrafluoroethylene
Polyurethane
38
29
33
18.5
29
42
<36
42
42.5
44
43
32
42
Polyvinylacetate
Polyvinylalcohol
Polyvinylchloride
Silicon rubber
37
37
39
22
Surface treatment methods may be used to improve the bond strength achieved with these polymers.
An investigation into the effect of different surface treatments on the bond strength of various thermoplastic composites showed that cleaning and abrading the surface prior to using hot melt adhesive gave adequate values of adhesive fracture energy. However, when using epoxy or acrylic-based adhesives, corona discharge had to be used on the composite surface to obtain high adhesive failure energies.
In response to the problems associated with adhesive bonding of low surface energy plastics, many adhesive manufacturers have developed adhesives specifically for these plastics. Dow produces a two-part 'Low energy substrate adhesive' (LESA), which it claims can bond plastics such as polypropylene and PTFE without any surface treatment, and 3M makes the same claims about its product DP-8005. There are also various pressure-sensitive adhesives, such as those sold by Nbond Adhesive International and VisionMark, which also claim to provide adequate bonding of low surface energy plastics.
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5.3.4.9 Mechanical fastening
Mechanical joining of composite materials is an example of technology transfer from the fastening techniques used for isotropic materials where a wealth of experience and understanding exists. However, care must be taken when designing a composite joint.
Factors such as the anisotropic structure and failure mechanisms, the elastic behaviour and the stress concentrations caused by holes mean that more care must be taken in joint design than with isotropic materials.
Hart-Smith has given excellent description of efficient bolted composite joints. He highlights the fact that the net-section stress concentration, shown in Figure 2, is not eliminated in composite joints by gross yielding, as can occur with relatively ductile materials such as metals. Therefore, the traditional approach for bolt holes in metallic aircraft structures is inappropriate for composite materials.
Figure 13 - Bearing and hoop stress distribution at loaded bolt hole in elastic isotropic plate.
Advantages of mechanical joints in composite structures include:
•
Repeated assembly and disassembly for repairs and maintenance without destroying parent material.
•
Easy inspection and quality control.
•
Little or no surface preparation required.
Disadvantages of mechanical joints in composite structures include:
•
Joint fastenings add weight, reducing weight-saving potential of composite.
•
Presence of holes creates stress concentrations. Stress relief does not occur because composites are elastic to failure.
•
Risk of galvanic corrosion due to presence of dissimilar materials.
•
Fibre discontinuities at location of drilled holes. Fibres exposed to environment.
Figure 14 illustrates modes of failure in mechanically fastened joints in composite materials.
Tension, shear and bearing failures are observed in mechanical joints, but composites give two additional modes: cleavage and pull-out.
Failure mechanisms of composites are complex and varied and are dependent on many different factors. A wide range of design variables, some of which are considered in the next section, need to be understood and considered when designing a joint in order to avoid unwanted failures.
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Figure 14 - Modes of failure for mechanically fastened composite joints.
The following parameters should be taken into consideration when designing a mechanical joint for a composite material: each parameter can affect the failure mode and/or strength of the joint.
Figure 15 - Types of joint.
Choice of fastener may be restricted by the need to keep the efficiency of the joint, in terms of strength-to-weight ratio, to an acceptable level. Types of fastener typically used include screws, rivets and bolts.
Self-tapping screws are simple, inexpensive and only require access from one side of the joint. Thread-stripping is a common problem, especially when frequent de-mounting is required.
Rivets are suitable for joining laminates up to 3mm thick. There are many different types of rivet including variations such as hollow and countersunk. The pressure required to install rivets is not readily controllable and can lead to damage of the composite, reducing the loadbearing capacity of the joint.
Bolted joints provide a strong, efficient method of joining composites. They are also easily removed for inspection or maintenance of the structure.
The constituents of the composite must be specifically chosen to withstand the stress state created by the mechanical joint to be used. Factors such as fibre type, volume fraction and form (eg woven, chopped, etc) and resin type will all affect the properties of the composite and can determine whether the joint holds or fails.
In composite laminates, where orientation of the fibres can be varied in each layer, failure mode is highly dependent on the percentage of fibres in each direction and also upon the
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6.1 stacking sequence of the layers. Laminates with a high percentage of fibres in the 0° direction will tend to fail in a tensile mode, whereas laminates with a high percentage of 45° plies will fail via shear.
The geometry of a joint will determine its strength. Due to their elastic nature, composites cannot deform plastically, and the resulting stress concentrations that build up will be dependent on the geometry of the joint. The net tensile strength of the joint will be dependent on the width of the joint, and the shear strength of the joint will be dependent on the end distance (both are also dependent on ply orientation, as previously mentioned).
Hole size affects the joint's gross tensile strength but it has little influence on the net tensile strength or shear strength of glass and carbon fibre reinforced laminates. The net bearing strength of carbon fibre laminates is similarly unaffected by hole size, however, glass fibre laminates can suffer instability when the ratio of hole diameter to laminate thickness is greater than three.
Where the joint is multi-holed, the way in which fasteners are grouped should be considered as there may be interaction effects due to the proximity of the holes. Joint geometry is generally chosen such that potential tensile or shear failures occur simultaneously and at a mean stress as close as possible to the bearing failure stress. On this basis it is generally agreed that when these requirements are met, plus those of spacing to allow installation, the effect of interaction is low. It is also the case that multi-row bolted joints offer no significant strength increase for the same joint geometry.
Holes in composites to accept fasteners are made using conventional drilling methods, but poor technique can cause defects in the composite. When drilling laminated composites, support material, such as wood, should be placed as a backing to prevent delamination of the composites rear plies. Care should also be taken to prevent the material getting too hot during drilling as this can lead to degradation of properties. Other defects commonly found include oversized holes and tearing or chipping of the surface plies.
The degree of clamping force applied in mechanical fastening has a major effect on the bearing strength of the joint; for example, increased tightening torque in a bolted joint increases the bearing strength. However, excessive over-tightening of bolts may cause damage to the surface of the laminate, which means that lateral constraint must be kept to the minimum necessary to develop adequate bearing strength. It should also be recognised that bolt clamping may decrease with time due to the viscoelastic behaviour of the resin. This behaviour is more pronounced with thermoplastic matrix composites than with thermosetting composites and is also increased by temperature rise and moisture uptake.
Introduction
Adhesive bonding of composites is used in a wide variety of industries: from consumer goods packaging, through to aerospace.
While there is a large number of non-destructive testing (NDT) techniques that may be used to detect the variety of defects that might be found within a bonded structure, due to the enormous scope covered by the term 'adhesively bonded joint' and the variety of defects that may be found within a joint, no single technique can be specified as the industry standard.
It is also recognised that inspection of a bonded joint using one, or even two of the techniques available cannot guarantee detection of every defect within the bond-line.
However, when used as part of a systematically managed and controlled operation - from
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6.2 design of the joint, through to final assembly - it can reduce the possibility of poor joints being produced.
This knowledge summary aims to provide information on what defects may be found, what detection techniques are available and which techniques are best suited to the detection of each defect.
Defect types
Types of defect typically found in an adhesive joint are shown in Figure 16.
Figure 16 - Typical defects within adhesively bonded joints.
A – Disbonds occur as a result of failure of the interface between the adhesive and the adherend, or the surface treatment (e.g. primers) applied to the adherend. In the case of a zero-volume disbond (B in Figure 16) the two surfaces are in intimate contact but no mechanical or chemical bonding exists at the interface, which means that they are particularly difficult to detect using NDT methods.
B – Zero-volume disbond is also called weak bond or kissing bond. The interface is bonded but bond strength is not assured.
C – Poor cure results in poor cohesive strength. It can arise from incomplete mixing of the adhesive, inadequate control of temperature, light or other forms of energy to activate the cure, or pressure variations during the cure cycle.
D and E – Porosity and voids are due to volatile substances within the adhesive (such as water vapour), entrapped air or insufficient application of adhesive.
F – Cracking within the adhesive may happen as a result of incorrect cure, brittleness of the resin after cure, or it may be due to filler loading in the adhesive formulation. High strength adhesives and those without toughening agents tend to be brittle and are, therefore, susceptible to cracking when shock loaded or exposed to temperature cycling.
G – Delamination within the adherend, while not actually a failure of the bonded joint itself should be noted as it may obscure other damage within the joint when using certain NDT techniques. This type of failure may also occur in cases where the interfacial strength at the adhesive/adherend regions is stronger than the fibre/matrix interface in the composite adherend.
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6.3 NDT techniques
Some common techniques used for the NDT of composite joints are briefly described below.
•
Manual methods such as the 'coin-tap' rely on the fact that when bonded composites are tapped, areas containing delaminations or disbonds produce a duller sound than good areas. This technique is used as a quick and easy method to get a feel for bond integrity.
•
Ultrasonics techniques, of which there are many variants, are the most commonly used method for inspection of adhesively bonded joints. Ultrasonic waves are propagated through the structure and the amount of energy transmitted or reflected depends on the properties of the materials and any defects within the structure. One of the limitations of ultrasonics is that some composite materials are highly attenuative of ultrasound and therefore the thickness of material that can be inspected may be limited. Most of these techniques also require a coupling agent such as water or a gel between the part to be inspected and the transducer.
•
Low frequency vibration techniques are less sensitive than ultrasound but are still recognised as an effective way of detecting disbonds in bonded joints.
•
Shearographic/Holographic inspection uses interference of laser light to create threedimensional images of the joint before and while being stressed. Comparison of these two images can be used to detect the presence of sub-surface defects. The most common use of this technique is to detect disbonds in adhesively bonded honeycomb panels.
•
Radiography is of limited use in the inspection of adhesively bonded composite joints. Xray adsorption of composite materials is very low and disbonds are generally oriented in a plane perpendicular to the X-ray beam, which makes them difficult to detect. Since damage is generally well contained within the joint, it is not easy to introduce X-ray opaque penetrant into defects. The use of X-ray opaque adhesives is a possibility.
•
Thermography techniques can be passive or active. In passive thermography, an external heat source is applied and the surface temperature monitored either on the same side as the heat source to detect surface defects, or on the opposite side to the heat source to detect defects deeper within the material. Active thermography involves subjecting the joint to a cyclic stress or resonant vibration. This is particularly good at detecting disbonds between skin and honeycomb, as the disbonded area resonates, generates heat and is detected.
•
Dielectric measurement may be performed when electrically conductive composites are being joined. An electromagnetic wave can be generated in the dielectric (adhesive) layer that travels along the length of the bonded joint and back to the start point. Differences in the initial and returning wave can be used to investigate the state of cure of the adhesive, locate voids and find areas of poor interface between the adhesive and adherend.
Other NDT techniques for adhesive joints are summarised below. These are less used than the conventional methods described above, however, each technique can provide satisfactory results if used in the correct way for the specific item under test.
•
Beta-ray Backscatter – Beta rays (stream of electrons) are directed at the specimen's surface layer. A detector measures the rate and intensity of electron backscatter. Gamma rays or X-rays can also be used for this type of testing. This technique is often used to measure paint thickness and examine plastic coatings and composite laminates.
•
Radioactive tracer – A low activity, radioactive tracer is injected into certain materials or equipment. The tracer's presence and migration are then monitored by radiographic film, probes or ionising counters. This technique is used to monitor wear, corrosion, and crack propagation.
•
Electrochemical impedance spectroscopy (EIS) – The impedance spectra change shape depending on the type of flaw detected in the specimen. It can be used to detect corrosion, flaws and the presence of moisture in an adhesive bond.
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6.4
Detection of kissing (weak) bond
Assurance of bond integrity is particularly important for critical components/structures and there is a considerable amount of ongoing research into the reliable detection of defects by non-destructive methods.
Whilst normal disbonds can be detected easily using the technologies outlined above, weak bonds and zero-volume disbonds have proved extremely elusive. This is partially because it is difficult to simulate a kissing bond defect specimen. A commercial product called
ElectRelease
®
that can be 'unzipped' by applying an electric field can possibly be used. So far, several applications of ultrasonic methods are short listed for further evaluation and optimisation. These are: nonlinear harmonic generation, wept-frequency ultrasonic spectroscopy, oblique incidence ultrasound, shear-wave resonance, and guided waves.
These techniques are briefly explained below.
•
Ultrasonic Spectroscopy – Swept-frequency ultrasonic spectroscopy, using a long toneburst 'chirp' involves the capture of multiple superimposed reflections through the structure and determining the frequency content. The ratio of the amplitude of the two peaks (a good part and a defective part) can be used to detect and measure changes in ultrasonic properties across the joint, with the potential to assess the relative strength of the bond.
•
Harmonic Imaging (Nonlinear Ultrasonics) – It is assumed that the kissing bond contact interface is 'nonlinear'. Such nonlinear coupling would be expected to convert incident ultrasonic energy into harmonics of the excitation frequency. This harmonic generation is detected and analysed.
•
Oblique Incidence Ultrasound – Though it is difficult to implement the oblique-incidence method, it is expected to be more sensitive to interfacial properties, which may help in the examination of kissing bonds.
•
Shear-wave Resonance – The presence of adhesive causes the shear-wave resonance to change in frequency, as well as being damped. The level of daming and shift in resonant frequency could possibly be used to determine the modulus of the adhesive layer.
•
Guided waves – It is already known that guided waves can detect a significant change in the stiffness of the interface, however, it is still unclear how to determine the level of weak bonds or kissing bonds.
Development of international Standards occurs within the International Standards
Organisation (ISO) and the Comité Européen de Normalisation (CEN). CEN is developing a new series of Standards in support of the European Single Market and EC Directives.
Technical committees are working at most levels, from major installations to material specification and testing.
For composite materials, there is initially a need to harmonise and then validate the many existing versions of established test methods. In the longer term, research on test methods as supported by the UK Department of Trade and Industry and that undertaken under the
VAMAS (Versailles Agreement on Advanced Materials and Standards) international prestandards programme, aims to reduce the timescale for standardisation). Validation of test methods, which due to the legal position of CEN Standards within the single market is essential, must be considered at an early stage. The UK, through organisations such as the
British Plastics Federation and the National Physical Laboratory, is taking a major role in research, harmonisation, drafting and validation of international Standards.
NPL provides a comprehensive list of ISO Standards for composite testing, some of which are relevant for thermoplastic composites.
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For the UK, BS EN ISO Standards replace the widely used informal recommendations from the Composite Research Advisory Group (CRAG).
Another body commonly recognised within the composite materials community is the ASTM
(formerly known as the American Society for Testing and Materials). Founded in 1898, ASTM
International is a not-for-profit organisation that provides a global forum for the development and publication of voluntary consensus Standards for materials, products, systems, and services. Over 30,000 individuals from 100 nations are members of ASTM International. This membership is drawn from producers, users, consumers, and representatives of government and academia. In over 130 varied industry areas, ASTM Standards serve as the basis for manufacturing, procurement, and regulatory activities. ASTM International provides
Standards that are accepted and used in research and development, product testing, quality systems, and commercial transactions around the globe.
Herman F Mark, 2004, Encyclopedia of Polymer Science and Technology, John Wiley &
Sons, Inc, Vol.2 PP307-328
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C-containing polymers
COP Cyclic olefin copolymers
Ethylene/tetracyclododecene
COC Ethylene/norbornene sPS Syndiotactic polystyrene
O-containing polymers
POM Acetal resins (polyoxymethylene)
PET Poly(ethylene terephthalate)
PBT Poly(butylene terephthalate)
PEN Poly(ethylene naphthalate)
PAR Polyarylates
LCP Liquid crystal line polymers
PPE Poly(phenylene ether)
PC Polycarbonates
Aliphatic polyketones PK
Poly(ether ketones)
PEEK
PEK
PEKK
PEKEKK
PMMA Acrylic resins
Sulfur-Containing Polymers
PPS Poly(phenylene sulfide)
PSU Polysulfones
PES Poly(ether sulfone)
PAS Poly(aryl sulfone)
N-containing polymers
Styrene copolymers
ABS
SAN
SMA
Polyamides, Plastics
PA6,6
PA6,10
PA6,12
PA4,6
PA6
PA11
PA12
ArPA Polyamides, Aromatic
PI Polyimides
PAI Polyamide imide
PPA Polyphthalamides
PEI Polyetherimide
F-containing polymers
PTFE Poly(tetrafluoroethylene)
ETFE Ethylene–tetrafluoroethylene
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