Polymer-Matrix Composites Introduction

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Knowledge should be applied in a safe, responsible and ethical manner not only to benefit us personally but also to improve the lot of the people we live with. It is also a duty to ensure that our surrounding habitat is not endangered. This sometimes requires knowledge of the local culture to help achieve a desirable outcome. Martin Palmer’s presentation on BBC Thought for the Day programme, 17/06/2006, on the subject of the protection of the oceans included:

“To many around the world the environmental movement and its proffered solutions – usually economic – are alien ways of thinking and seeing the world, and can be interpreted as telling people what is best for them whether they like it or not. Let me tell you a story. Dynamite-fishing off the East African coast is a major problem. Environmental organisations have been addressing it for years, from working with Governments, to sending armed boats to threaten those illegally fishing. None of this worked because it had no relationship to the actual lives or values of the local fishermen all of whom are Muslims. What has worked off one island, Misali, is the Qur’an. In the Qur’an, waste of natural resources is denounced as a sin. Once local imams had discovered this, they set about preaching that dynamite fishing was anti- Islamic, non-sustainable and sinful. This ended the dynamite fishing of the Misali fishermen because it made sense to them spiritually.”

The perception/foresight of Canadian scholar, Wilfred Cantwell Smith, is also relevant in this context, particularly these days when there is so much misunderstanding and misrepresentation about peoples of different religions and cultures. Regarding Muslims, Wilfred Cantwell Smith in his book Islam in Modern History (Princeton and London, 1957. p. 304) says, “the Muslim segment of human society can only flourish if Islam is strong and vital, is pure and creative and sound”. Practice of pure, creative and sound Islam by its followers will be for the good of all.

Case for composites

Polymers, which are a source of a wide variety of low-priced raw materials, offer many advantages. These include low specific weight, enhanced stability against corrosion, improved electrical and thermal insulation, ease of shaping and economic mass production, and attractive optical properties, e.g. fibre optics, glazing applications, etc. However, they suffer from some serious shortcomings:

 

  • exhibiting, quite often, poor mechanical stiffness and strength, and poor resistance against heat
  • being sensitive to aging e. change of the physical, chemical and mechanical properties by light, heat, oxygen and moisture
  • being combustible
  • exhibiting large values of coefficient of thermal expansion, which can generate high levels of internal frozen-in

 

Table 1.1 provides guidance concerning limited resistance of polymers to heat. The polymer abbreviations are defined in Table 1.2.

 

Polymers Long-term working temperature,°C
Semi-crystalline Amorphous
PEEK, PPS, PTFE, LCP PES, PSU, PEI, EP 150–250
PET, PBT, POM, PA 66, PA46, PP PC, UP 90–120
HDPE,  LDPE, LLDPE PS, HIPS, PVC, PMMA, ABS, SAN < 90

 

Abbrevations used in Table

ABS acrylonitrile-butadiene-styrene
EP epoxy resin
HDPE high-density polyethylene
HIPS high-impact polystyrene
LCP liquid crystalline polymer
LDPE low-density  polyethylene
LLDPE linear low-density polyethylene
PA 46 polyamide 46
PA 66 polyamide 66
PMIA poly (m-phenylene isophthalamide)
PBT polybutylene  terephthalate
PC polycarbonate
PEEK polyetherether ketone
PI Polyimide (e.g. “Kapton”, “Vespel”)
PI Thermosetting polyimide
BMI bismaleimide
PAI Polyamide imide (e.g.“Torlon”)
PEI polyetherimide
PES polyethersulphone
PET polyethylene terephthalate
PAR polyarylate
PMMA polymethyl  methacrylate
POM polyoxymethylene  (acetal)
PP polypropylene
PPO Polyphenylene oxide and/or poly (2,6-dimethyl-1,4-phenylene oxide)
PPS polyphenylene sulphide
PS polystyrene
PSU polysulphone
PPSS polyphenylene sulfide sulfone
PTFE polytetrafluoroethylene
PVC polyvinyl chloride
SAN styrene-acrylonitrile
UP polyester (unsaturated) resin

 

Some of the shortcomings of polymers as engineering materials, particularly poor strength and stiffness, can be improved by combining them with other materials to form composites. Composite materials are defined as a mixture of two or more relatively homogeneous materials which have been bonded together to produce a material with properties that are superior to the ones exhibited by the individual component materials. This synergistic outcome, obviously, is the driving force for the development of  composites.

Hull (1981, p. 3) outlines that in fibre reinforced plastics, fibres and plastics with some excellent physical and mechanical properties, are combined to give a material with new and superior properties. Fibres have very high strength and elastic modulus but this is only developed in very fine fibres, with diameter in the range 7–15 µm, and they are usually very brittle. Plastics may be ductile or brittle but they usually have considerable resistance to chemical environments. By combining fibres and resin a bulk material is produced with strength and stiffness close to that of the fibres and with the chemical resistance of plastic. In addition, it is possible to achieve some resistance to crack propagation and an ability to absorb energy during deformation.

History of the development of  composites

The history of composites is covered in various sources, including Strong (2006) and Palucka & Bernadette- Vincent (2002). Composites date back to the 1500 BC when early Egyptians and Mesopotamian settlers used a mixture of mud and straw to create strong and durable buildings. Straw provided reinforcement to ancient composite products including pottery and  boats.

The subsequent recorded use of natural fibres include paper making. The first ‘paper’ was invented in ancient China sometime around 200 BC. However, the forerunner of modern paper was also first made in China from rags and plant fibres in 105 AD. The development of paper increasingly made it into a composite material. First, the Chinese used starch as a size for paper as early as 768 to reduce surface porosity and fuzzing with the goal of allowing inks and paints to remain on the surface of the paper and to dry there, rather than be absorbed into the paper. Mineral fillers were also added to improve gloss, whiteness, ink reception and weight. Arabian and Turkestan rag papers dating from the 8th century contained large quantities of talc, chalk or gypsum (hydrated calcium sulphate, CaSO4, 2H2O).

In 1200, the Mongols invented the composite archery bow, using a combination of wood/bamboo, bone/ cattle horn, cattle tendons and animal glue or pine resin wrapped in birch bark or silk. These extremely powerful and extremely accurate bows were the main weapon of Genghis Khan’s military might, and the most powerful weapon on earth until the invention of gunpowder.

The onset of modern composite materials began with the development of synthetic polymers, particularly those of thermosetting resins, such as phenolics and polyesters, and further strides were made with the advent of high performance fibres:

1850s   Plywood: put into commercial production by John Henry Belter, a German emigré to the US. 1900s  Reinforced rubber tyres.

1907 Leo Baekeland produced phenol-formaldehyde, the first truly synthetic plastic, Bakelite. Cast with pigments to resemble onyx, jade, marble and amber it has come to be known as phenolic resin.

1928 Otto Rohm in Germany stuck two sheets of glass together using an acrylic ester and accidentally discovered safety glass, and production of some articles began in 1933.

1933 Melamine formaldehyde resins were developed through the 1930s and 1940s in companies such as American Cyanamid, Ciba and Henkel.

1935    Owens Corning introduced the first glass fibre. 1936    Unsaturated polyesters were patented.

1938    Epoxy resin was discovered by Pierre Castan, a chemist in Switzerland. 1940           Low pressure allyl polyester resins were developed.

1940s The earliest applications for glass-fibre reinforced plastics (GFRP) products were in the marine industry. Fibreglass continues to be a major component of boats and ships today.

1942 The U.S. Navy replaced all the electrical terminal boards on their ships with fibreglass-melamine or asbestos-melamine composite boards with improved electrical insulation properties. Many other composite improvements were developed during WWII including some innovative manufacturing methods such as prepreg production and filament  winding.

1943 At the Wright-Patterson Air Force Base in 1943, exploratory projects were launched to build structural aircraft parts from composite materials. This resulted in the first plane with a GFRP fuselage being flown at the base a year later (Palucka & Bernadette-Vincent, 2002).

1948    Introduction of sheet moulding compound (SMC) and dough moulding compound   (DMC).

1940/1950s     Development of innovative manufacturing methods, including pultrusion, vacuum bag moulding, and large-scale filament winding.

1956 Cincinnati Developmental laboratories added asbestos fibre to a phenolic resin for use as a possible re-entry nosecone material (the heat generated during re-entry of a spacecraft into the Earth’s atmosphere could exceed 1500°C). Scientists also began looking at metal matrix composites (MMCs) for a solution.

1958 Roger Bacon of Union Carbide developed high-performance carbon fibres using rayon as the starting material. The resulting fibres contained only about 20% carbon and had low strength and stiffness properties.

1960    High-strength and high modulus S-glass and boron fibres were developed.

1961 Akio Shindo of the Government Industrial Research Institute in Osaka, Japan made high strength carbon/graphite fibers that contained about 55% carbon using polyacrylonitrile (PAN) as the precursor, replacing the rayon and pitch precursors used previously.

1963 William Watt et al. of the Royal Aircraft Establishment in Farnborough, England invented a still higher-modulus fibre from PAN. The carbon fibres were rapidly put into commercial production by companies such as Rolls-Royce.

1964 Stephanie Louise Kwolek of DuPont developed Kevlar fibre from polyaramide (an aromatic polyamide).

1970/1980s The composites industry began to mature, developing better plastic resins and improved reinforcing fibres, however, composites made with expensive fibres had to find civil applications when space and military demands declined. Sectors such as sports and leisure, transportation and construction industries became increasingly important markets.

1978 The development of the first fully filament wound aircraft fuselage, the Beech Starship, by Larry Ashton, an engineer at Hercules.

1979 Dyneema fibre was invented by DSM (the Netherlands) and has been in commercial production since 1990 at a plant in Heerlen, the Netherlands and Toyobo Co. in Japan. It is produced by means of a gel-spinning process and its properties combine extreme strength with incredible softness, and have been successfully used in bullet-resistant products (vests and panels, including those used in the doors of aeroplane cockpits), ropes, fishing nets, cut-resistant gloves, sails, sailing ropes and fishing lines. In the United States, Honeywell developed a chemically identical fibre of brand name Spectra. 

1990s Miniaturisation has led to mixing organic and inorganic components at the molecular scale and to nanocomposite materials.

2000s   Smart materials and intelligent  structures.

Henry Ford exhibited his prototype car made from hemp and flax fibre reinforced resin composite body panels in 1941. The body consisted of fourteen plastic panels fixed to a welded tubular frame (instead of the customary parallel I-beam frame).The panels and frame each weighed about 250 pounds. The total weight of the automobile was 2,300 pounds, roughly two-thirds the weight of a steel model of comparable size.

Classification of composites

A Classification of Composites is listed in the text box below:

  1. Polymer-polymer blends;
  2. Particulates/whiskers/flakes filled materials – a continuous matrix phase and a dispersion of filler phase;
  3. Fibre reinforced composites: Classification based on fibre phase:
    1. continuous fibres, (b) discontinuous fibres (chopped or short fibres). Classification based on matrix type:
  4. polymer-matrix composites (PMCs) or fibre-reinforced plastics (FRPs),
  5. metal-matrix composites (MMCs),
  6. ceramic-matrix composites (CMCs);
  7. Sandwich composites with solid skins (e.g. PMC laminate) and foam (e.g. PU, Rohacell) and/or honeycomb (nomex, aluminum) cores;
  8. Nanocomposites;
  9. Foams; 7. Ceramic-metal mixtures (i.e. cermets); 8. Timber; 9. Concrete; 10.
Henry Ford (1940) demonstrated the strength of a natural fiber reinforced plastic deck lid.

Composites enable the generation of a variety of materials: from low cost plastics (by the addition of low-cost fillers to polymers) to expensive high performance engineering materials, such as continuous carbon-fibre reinforced epoxy resins. Composites increasingly successfully compete with and replace conventional materials for various applications, particularly, in leisure/sports, engineering, transportation and construction sectors. Some of the attractive features/properties of composites are highlighted in the examples given below.

Weight reduction using composites has created a huge market demand in automotive, industrial, aerospace and other industries. Particularly, in the commercial airline industry due to the high cost of aviation fuel and environmental legislation, aircraft manufacturers are now competing based upon their aircraft’s fuel efficiency.

The demand for energy-efficient and low-maintenance vehicles has spurred composites use in advanced automobile, truck, bus, and train products. Production parts include everything from small linkage assemblies to very large exterior structural panels. Glass fibre (GF) composite materials are less costly than carbon fibre (CF) alternatives and are, ideally suited for road and rail transportation applications because they are light weight, strong, stiff, and provide good protection from the elements. They can be moulded in to any size and shape, and enable visually attractive finishes. Furthermore, GF composite is resistant to most acids, bases, oxidizing agents and metal salts, making it suitable for corrosion resistant applications.

On the marine side, the consumer use of fibreglass PMCs in low- to high-end boats is the norm. Military ships have seen several applications of PMCs, primarily topside structures and minesweepers. Carbon- fibre composites are used in high-performance engine-powered, sail-powered, and human-powered racing boats. Military armoured vehicles have also benefited greatly from the application of PMCs – they offer ballistic protection of their occupants in addition to light weight.

A potentially huge market exists for composite materials in the upgrading of infrastructure needs. For example, 31% of the highway bridges in the United States are categorized as structurally deficient. To address this, many activities are underway at national, state, and local levels to use composites to repair and, in some cases, replace deficient bridges. Some all-fibreglass bridges, e.g. Butler County, Ohio, are fully instrumented to detect structural performance loss.

Low CTE, in addition to low weight, is a major advantage of PMCs in the production of satellite structures.

The high-temperature (using resins such as polyimides and bismaleimide) PMCs are used in many engine applications for both air and space vehicles.

High specific mechanical properties coupled with dielectric characteristics that render the material radar transparent are required for radomes and antennas: glass or aramid fibre composites meet the requirement but not electrically conducting reinforcements, such as carbon or boron.

Fatigue life is critical in dynamic applications: rotor blades for helicopters such as Lynx and Sea King consist of PMC continuous fibre composite solid skins with honeycomb/foam core for low weight and superior stiffness and improved fatigue life.

Composite sandwich structures, increasingly based on carbon fibre, meet light weight, high stiffness and strength and durability requirements of the latest wind turbines are designed with rotors up to 110 m in diameter and are capable of generating up to 5 MW of power.

Composites enable the achievement of direction-specific properties. This is successfully exploited in applications such as bicycle frames. Appropriate carbon fibre orientation/placement allows designing for lateral stiffness, torsional stiffness, vertical compliance, toughness and shock-dampening properties in the manufacture of frames. Such frames exhibit maximum strength-to-weight ratio: bicycle frames can be made with carbon-fibre epoxy prepregs that weigh just over one kilogram, but are incredibly strong. Furthermore the material is durable: exhibiting good resistance to failure under fatigue and impact conditions and to corrosion or attack by the elements, and it can also lend itself to attractive finishing.

Composite materials have numerous advantages for medical and security applications. CF is X-ray transparent, strong, stiff and lightweight, which is ideal for making panels, covers, support structures and beds for radiology, security or inspection equipment. Other medical uses exploit desirable mechanical properties of PMCs, such as orthopaedic devices as was demonstrated in the 2012 London Olympics by 400 m runner Oscar Pistorious (the first double amputee athlete to compete in the able-bodied Olympics) with his famous CF-composite prosthetic legs and, hence, he is popularly known as the blade runner.

Specific strength and stiffness coupled with damping ability are reasons for composites to continue to be popularly employed in sports and recreation products: golf clubs, bicycles, snowboards, water skis, tennis racquets, hockey sticks, etc. Composite tennis racquets and golf clubs began to replace wooden racquets and steel club shafts in the late seventies and changed everything, in spite of increased prices. The lighter weight and higher strength of CF/graphite enabled tennis racquets with tighter strings to be swung at higher speeds and, hence, greatly increasing the speed of the tennis ball. The increased stiffness of a golf club shaft transferred more of the energy of the swing to the golf ball, making it go further. In addition to these performance-related improvements, there were also reductions in sports injuries such as wrist strain and tennis elbow due to damping ability/shock absorbance capacity of these composite sports equipment. At present, nearly all tennis racquets are of composite construction.

Some of these desirable features of composite materials are also demonstrated for an ordinary engineering part in a case study (Kurcz et al. 2004) that proposes the replacement of steel with fibre-reinforced thermoplastic. The component studied is a spare-wheel well (SWW) for a vehicle. SWW components are required to pass tests that evaluate impact performance after a crash and resistance to noise-vibration- harshness (NVH), hot and cold climates, flammability, common automotive chemicals, and long-term heat aging. Additional tests include drivability over rough roads and various standard mechanical tests conducted on complete parts for impact strength, tensile strength and elongation behaviour. The benefits of using composites are indicated to be:

  • reduced weight and systems
  • smaller package space required for stowing the tyre (because the steel parts are generally not easy to radius as steeply as those in plastics).
  • better sound and mechanical vibration damping compared with steel for a quieter
  • the ability to tailor stiffness based on fibre lay-up
  • the ability to mould-in hand grips, pockets to stow tools, and other functionality at no additional
  • lower tooling costs – especially attractive for lower build
  • reduced assembly-line space and cost via eliminating some secondary-finishing
  • no corrosion

Issues in terms of heat resistance and flammability have also been indicated.

The study has considered glass-mat thermoplastic (GMT), long-fiber thermoplastic (LFT) and sheet- moulding compound (SMC): GMT and LFT are thermoplastic materials that can be melt-reprocessed, whereas SMC is a thermoset. LFT is processed by injection moulding whereas GMT is press formed, thermoformed or compression moulded and SMC is compression moulded, which influences the rate and volume of production – an important factor in the automotive industry.

Kurcz et al. have summarised that thermoplastic components facilitate recycling both in-plant scrap and postconsumer components – an important feature in Europe where all vehicle components must be able to be recycled. GMT and LFT have polypropylene matrices, offering lower specific gravity than polyester- based SMC, for lighter weight parts at comparable wall thicknesses. Additionally, SMC is known to be brittle, so is not well suited for applications subject to impact. SMC is also characterized by relatively long cycle times, on average 2–3 min for a part like a spare-wheel well.

Another important difference between these technologies is that GMT materials are able to achieve higher stiffness, impact, and strength values than LFT owing to greater preservation of glass fibre length after moulding. For the grades used in SWW applications, GMT tends to maintain fibre lengths of 30–50 mm vs. 5–20 mm for LFT after moulding. GMTs as-moulded cost tends to be lower than that of LFTs.

The study supports GMT composite technology for this application, concluding that it offers the same types of benefits as SMC – lower weight, lower systems costs, lower tooling costs, and design flexibility – while also providing faster cycle times, lighter weight parts, and avoiding brittle-failure problems.

In most applications, specific strength (strength / specific gravity) and specific stiffness (stiffness / specific gravity) becomes an important factor and as can be seen in Figures 1.2 and 1.3 and Table 1.3 composites, particularly continuous-fibre composites, are much superior in this respect compared with other materials.

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