Polymetric Matrix Composites

Polymeric matrix composites (PMC)

Composite materials with a polymer matrix can be divided into several basic groups.

We distinguish composites based on:

  • plastics
  • bitumen
  • asphalts
  • tar

Plastics

Plastics (polymers) are the most widely used group of composite materials. Plastics are materials based on macromolecular compounds (substances), which can be shaped for example by heat or pressure. Macromolecules are generated by combining simple molecules, called monomers into various chains – linear or cyclic. Such macromolecules vary from other organic compounds by relative molecular weight of 10,000 and an average degree of polymerization of 100 (more than 100 monomer individuals are connected). Organic macromolecules arise due to the ability of some elements to form chains (Si, C). The atoms of these elements can be easily combined in a linear chain and with a network or spatial structure they create a macromolecule.

The characteristics of the polymers are determined by the type of bonds that prevail in the structure. Primary bonds between components determine the size of the molecule and its layout. In addition, there are further secondary links in the structure that connect other atoms. These are bindings of a different nature, Van der Waals forces, hydrogen bridges and others. Both types of links, primary and secondary, together determine the chemical and physical properties of the polymer.

Categories of plastics according to the behaviour at higher temperature

  • thermoplastics
  • thermosets
  • elastomers

Thermoplastics

Thermoplastics account for about 90% of polymer composites. These are polymers of macromolecules with linear chains in which the macromolecules are bound by random intermolecular forces. These are very long molecules (macromolecules) formed by repeating

the same structural units (several thousand to millions). This type of molecule is known as a polymer.

Very weak secondary bonds (Van der Waals forces, hydrogen bridges) could break with thermal vibrations, therefore these substances soften from a certain temperature, or may transform at a high temperature, and after cooling they get their original characteristics.

Their basic characteristic is the ability of repeated heat treatment.

Thermosets

Thermosets are densely crosslinked polymers. In the case of thermosets the crosslinking in three directions (3-D structure) is formed by primary bonds.

These substances are solid but brittle until the temperature of their decomposition. The method of crosslinking is referred to as curing (at normal or an elevated temperature). They are non-renewable – an irreversible change by heat exposure

Elastomers

In the case of elastomers, the crosslinking is formed by secondary bonds and is called vulcanization. Elastomers are natural and synthetic types of rubber.

Natural rubber is obtained from the rubber milk (latex) of certain tropical trees (rubber trees). Crude rubber is plastic and has a very low tensile strength. It softens when heated in the air and it melts at about 220 °C. Light, especially UV, causes its aging (photochemical oxidation) – rubber loses its elasticity, it becomes brittle. These inappropriate characteristics are lost to vulcanization – a process of crosslinking a linear polymer (elastomer), in which the action of the vulcanizing agent (S, polysulfides, peroxides …) forms structural changes. This creates a modified product – from plastic rubber there is elastic rubber. The curing process (vulcanization) increases the mechanical strength of the rubber, resistance to temperature changes, the effects of light and various chemicals.

Synthetic rubbers can be vulcanized just naturally and are divided into rubbers for general use and special rubbers. Rubbers for general use have similar properties to natural rubber and are generally used as its substitute. Most important is the polybutadiene. Special rubbers are produced in smaller quantities, and some properties are superior to natural rubber. Gum (rubber) is a vulcanised rubber mixture.

  • Soft rubber (sulphur content up to 3 %) has low hardness and high elasticity. It isused in tires, seals, and the insulation of electrical equipment, for the production of conveyor belts and in the consumer goods industry (toys).
  • Hard rubber (sulphur content of 3-3.6 %, ebonite) has a higher hardness, goodmechanical and electrical properties and excellent chemical resistance. It is used for fittings, tanks and surgical instruments.

As the basic raw material for making elastomers there is rubber, for the preparation of thermoplastics and thermosets essential raw materials are mainly from coal, oil, natural gas and biomass. By the cracking of crude oil there are (also) obtained, among other things, fractions such as ethylene, propylene, aromatic hydrocarbons as benzene, xylene and toluene. Their further treatment is carried out through the processes of polymerization, polyaddition or polycondensation.

 Plastics by production (formation of macromolecules)

Polymerisation

It is a chain polyreaction at which macromolecules arise from a monomer.

  • The principle – one type of monomer plus an initiator (for chaining) – forms polymer and heat (warmth).
  • Description – polymerization is such reaction, when no component (hardener, initiator, etc.) enters the macromolecules. The initiator only causes a reaction in hardening and it does not particularly affect the resulting properties of the final product. A small addition of certain substances (the accelerator) can speed up the whole process. To activate polymerisation we can use the temperature, light, or radiation however, it is not possible in construction engineering. It is an exothermic reaction.
  • Plastics obtained by polymerisation ethylene – polyethylene PE isobutylene – PIB, vinyl chloride – PVC styrene – PS, vinyl acetate – PVAC

Polycondensation

It is a polyreaction at which (in addition to the macromolecule) a simple low-molecular compound also forms (H2O, HCl, CH3 OH …).

  • Principle – by linking monomer 1, monomer 2 and a catalytic converter, polymer, water and heat are formed
  • Description – polycondensation is a polyreaction which generates in addition to macromolecular compounds an even low-molecular compound
  • Plastics obtained by polycondensation dicarboxylic acid + diamine produces polyamides dicarboxylic acid + dialcohol produces polyesters formaldehyde + phenol produces phenol-formaldehyde resin Similar or the same for polyester, phenolic and furan resins

Polyaddition

It is a polyreaction method generating macromolecules by gradually adding at least two different compounds associated with the site exchange of hydrogen atoms in functional groups without creating degradation products.

  • Principle – by linking monomer 1, monomer 2 and a catalytic converter, polymer and heat are formed
  • Description – the addition of the hardener forms macromolecules by connecting gradually all components on the functional groups
  • Plastics obtained polyaddition

diepoxide + diamine produces epoxy resin

diisocryl + dialcohol produces polyisocryl (polyurethanes)

The five most important plastics which account for around 60 % of the total production of plastics are: PE (low and high pressure, bags and bottles), PP, PVC and PS.

Thermoplastics are prepared by polymerization and polycondensation. Thermosets are prepared by polyaddition.

Plastics by feedstock (raw material)

Semi-synthetic plastics

Semi-synthetic plastics are produced by the chemical or physical transformation of natural polymers. Starting materials for their preparation are natural rubber, cellulose, casein, etc. Natural rubber is a product of the rubber latex milk and is the starting material for the production of rubber mixture. Cellulose is a structural (building) element of plant tissues (spruce, poplar). Casein (curd) is a protein from milk, the feedstock (starting material) for the production of adhesives.

Fully synthetic plastics

These are plastics manufactured by the polymerization synthesis of low-molecular weight organic compounds. Low-molecular organic weight compounds are derived from crude oil, natural gas and coal. Production of plastic from those materials is done by using chemical polyreaction such as polymerisation, polycondensation and polyaddition.

The properties of plastics

All properties are here again placed in the context of other materials that are used to prepare the composites.

  • The benefits of plastics
  • low weight (0.8 to 2.2 g.cm3), density (1600-2000 kg.m3) compared to steel (7800 kg.m3) or aluminium (2700 kg.m3), it means they are lightweight (foamed/cellular/expanded plastics may be up to 0.01 g.cm3, i.e. density, which is naturally not present at all)
  • advantageous mechanical properties that can be modified as required to obtain substances that are hard, soft, porous, abrasion resistant, elastic and more
    • insulation properties (low thermal conductivity – 300 times lower than aluminum), thermal insulation, electrical insulation (not current) and sound insulation properties
  • easy workability, formability
    • good tolerance of different fillers, can hold a large amount of filler
    • good chemical resistance
    • the possibility of colouring, high durability/lifetime and easy maintenance
  • Disadvantages of plastics
  • low resistance to high temperatures (thermoplastics can be used up to 100 °C, thermosets withstand slightly more)
  • high coefficient of volume expansion (3 to 15 times larger than metals, and 20 times greater than ceramics)
  • low strength that decreases with the temperature and low modulus of elasticity (tensile modulus)
  • easily flammable (low ignition point, the production of dense smoke, toxicity), electrostatic charging
  • ease reaction with atmospheric oxygen which causes aging and aging due to solar radiation
  • swelling with water and associated change of volume

Not all plastics are the same (have the same properties to the same extent) it is only a general overview.

Plastics are materials that due to weather conditions, sooner or later age (get old) and thus lose its favourable mechanical properties. Aging can be slowed down by using various stabilizers (antioxidants, softeners, pigments). Well-resistant to the weather are for example various types of formaldehyde resins, on the contrary with little resistance to the weather (weathering) there are PA, PS, PE and PVC.

The oldest manufactured plastic is Bakelite (1909) – formaldehyde resin reinforced with wood floor.

Composites with a polymeric matrix and particulate composites

Particulate composites

  • particles in elastomers
  • particles in thermoplastics
  • particles in thermosets

Fibrous composites

  • fibers in thermoplastics
  • fibers in thermosets

Combined composites

  • Particulate composites with a polymeric matrix

Polymeric matrices have low strength (themselves) and have low heat resistance, in particular. Adding a mixture of dispersion particles to a polymeric matrix increases the modulus of elasticity, thermal (dimensional) stability, reduces shrinkage and improves other properties.

To reinforce the material we mostly use powders of micronized minerals, particularly kaolin, talc, mica, limestone. To improve the sliding properties and abrasion resistance we use for example particles of bronze with a graphite or molybdenum sulphide. To achieve electrical conductivity metal powders (Al, Cu, and Ag) of high concentration are added. To increase stiffness particles of larger dimension (1 mm and more) are used.

Particles in elastomers

Elastomers are generally very homogeneous, therefore we can use very small particles to depressively solidify the elastomers.

A specific example are conveyor belts or tires, where we use from 10 to 20 % of silica powder, talc or carbon black for the base rubber. It often also combines several kinds of dispersion.

Particles in thermoplastics

For thermoplastics there are usually used larger particles with a diameter of 10 mm. They reduce the shrinkage (or final shrinkage), increase the toughness, suppress the viscoelastic behaviour, increase the attenuation and suppress vibrations

Typical examples:

Polystyrene and cellulose particles (sawdust) – anti-vibration material

Polyethylene and 70-80 % of lead powder – protection against radiation (X-ray, gamma-ray) etc.

Particles in thermosets

For thermosets there are also used larger particles with a diameter of at least 10 mm. The effects are: an increase of firmness, moderation of crack growth, an increase of electrical insulation – quartz/silica sand, reduction of flammability – hydrated Al2O3 powder.

Fibrous composites with a polymeric matrix

Fibrous (Fiber) composites with a polymeric matrix are the most common and most traditional composite materials. Reinforcement fibers are used mainly in thermoplastics and in thermosets as well. (Thermoplastics as well as thermosets are mainly used for reinforcing fibers) The most frequently used fibers are glass, carbon, boron and others. The reinforcement is usually made in the form of continuous fibers, nonwoven fabrics, mats, etc.

As thermoset matrices the most commonly used are epoxy, formaldehyde, polyurethane and other resins. Carbon, aramid and glass fibers are preferably used with epoxy resins for laminates.

The thermoplastic matrices are the most common type. A low-cost representative is polypropylene. More expensive and chemically and thermally more resistant are polyamides.

In the case of composites with short fibers, again, thermoplastic matrices are used in the first place and followed by thermoset matrices as well. As reinforcement there are used predominantly chopped glass fibers, but also carbon and other types.

Glass fibers can also be used in the form non-woven fabrics, mats or cloths. Mixtures of polymers with a short fiber provide composite materials with isotropic properties and enable preferable production methods such as injection moulding or extrusion to take place.

  • Laminates – laminated materials

Laminates (laminar composites) are formed by combining multiple layers of polymer and reinforcement. They differ from the composites with a fibrous filler in that the reinforcement is not loose in the matrix, but the individual fibers are spun together in the form of fabrics or mats.

Products from a laminate are manufactured by layering fabrics or mats in order to achieve the desired strength and rigidity of a product. The individual layers are saturated with bitumen, which is then cured. They withstand a bending load well.

The most common laminates:

Fibreglass – glass reinforcement and for example phenol-formaldehyde resin

Formica laminate – tempered paper

Hardened fabrics – (textgumoid)

Fibers for polymeric composites

The most commonly used fibers for polymeric composites are glass, carbon, aramid fibers; boron fibers due to the high price are tended only for special applications in aviation.

  • Fibers in thermoplastics

As the matrix is used: polyamide, polyethylene, polypropylene and polycarbonate. The optimum amount of fibers is about 40-50 %.

For glass fibers in the thermoplastic the critical fiber length is about 0.2 mm. This allows preferably to produce granules of thermoplastics, where the individual granules already contain glass fibers of a sufficient length. As for thermoset composites, the properties are highly anisotropic and depend on the loading specifications.

  • Fibers in thermosets

The classic and the oldest type of mass-produced composites are polymer and glass fiber. Around 90 % of all thermoset composites contain glass fibers. Typically used fibers are continuous or at least long.

The optimum amount of fibers is 60-75 %, a higher amount in the composite has a larger number of pores.

Basic matrices that are used are:

Polyester – is the most common matrix because polyester resins generally have goodmechanical, electrical and chemical properties. Polyesters are suitable in a slightly alkaline medium and excellent in a mildly acidic medium.

Polyvinyl esters have good resistance to an acidic and alkaline medium, particularly at high temperatures. PVE profiles with glass fibers have good thermal and electrical insulating properties.

Epoxy – epoxies have excellent mechanical and electrical properties and arecommonly used with carbon or glass fibers. (They also have) good electrical insulation properties in a wide range of temperatures, considerable resistance to water, alkali solutions and acids and some solvents. Phenol – phenolic resins are used for the requirement of high fire resistance, high heatresistance, have low smoke generation and limit flames during combustion, specifically, on various designs of electrical appliances.

So-called Pyrrons – belong to the best thermoset composites, have a heat resistance up to 550 °C, tensile strength 76 MPa, and E 5 GPa.

  • Combined composites

It is often not only one single composite material, but the component is composed of several different materials being appropriately combined. An example is the construction of modern fibreglass skis or tire construction. It also often combines several kinds of dispersion.

The production of fiber composites with a polymeric matrix

The properties of the composite are significantly affected by the production process, by the properties of the fibers and their surface treatment.

  • The manual laying (using a fabric)

It is the most traditional method of production, where the glass reinforcement is placed into a mould and subsequently saturated with a binder. The desired thickness is gradually built up from layers (lamination), then the binder is cured and the segment (the workpiece) is removed from the mould. It is the least demanding technology. The fabric provides (ensures) approximately the same orientation of all the fibers and at the same time allows to partially suppress anisotropy, because the fibers are in the two main directions. The fibers are generally continuous. There are various manual or semi-automatic production methods. Specifically, there is for example the manual production of embedding the fabric in the form and over rolling, semi-automatic production by embedding the fabric in a form and over pressing, semi-automatic production by embedding the fabric in the form using overpressure or automated production by embedding the fabric in the form using a vacuum.

  • Winding (using continuous fibers)

It is a technology based on a continuous winding of a fiber bundle or otherwise modified reinforcement to the circular form. The fibers are wound either already moistened with resin, or are moistened after winding. The method of winding continuous fibers onto an appropriate form and the simultaneous or following pouring of polymer is mainly used for hollow, rotary symmetrical parts. It is thus possible to produce even very large parts. Example: winding the wind turbine tower with a height of 55 m and a diameter of 8 m, it is a composite with 50 % of the graphite fibers in an epoxy matrix, the resulting composite has a density of 1.7 g.cm3, tensile stress of 1150 MPa and E 115 GPa.

For the production of various profiles of plastic composites with continuous fibers there has been developed a production method called pultrusion. The continuous fibers are pulled through a forming nozzle (or slot) in which they are simultaneously saturated with a polymer. It is possible to use both thermoplastics and thermosets. The method is fully automatic and very productive.

  • Drawing (fiber drawing)
  • Technology based on drawing fiber bundles, mats and fabrics in a resin bath, where the reinforcement is saturated. Then the (saturated) reinforcement is formed into a desired shape and cured.
  • The injection and blowing method (using discontinuous fibers)

It is used for preparation of composites from granular material which already contains the fibers. You can use slightly modified equipment for the injection, blowing or pressing of conventional plastics. The method can be used for thermoplastics and thermosets as well. The plastic melt has during the processing (e.g. injection) usually relatively high viscosity. This causes the individual fibers to orient in the direction of the melt flow, therefore it is possible to achieve a partial orientation of the fibers of the product.

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