Complete Guide to Composites, Part 2

A composite consists of a resin and a reinforcement. This week we look at the characteristics of different resins and then next week at the reinforcing fibres.

Resin Requirements

Any resin system for use in a composite material will require the following properties:

1. Good mechanical properties

2. Good adhesive properties

3. Good toughness properties

4. Good resistance to environmental degradation

Mechanical

This diagram shows the stress / strain curve for an ‘ideal’ resin system. The curve for this resin shows high ultimate strength, high stiffness (indicated by the initial gradient) and a high strain to failure. This means that the resin is initially stiff but at the same time will not suffer from brittle failure.

It should also be noted that when a composite is loaded in tension, for the full mechanical properties of the fibre component to be achieved, the resin must be able to deform to at least the same extent as the fibre. This diagram gives the strain to failure for Eglass, S-glass, aramid (Kevlar) and high-strength grade carbon fibres on their own (i.e. not in a composite form). Here it can be seen that, for example, the S-glass fibre, with an elongation to break of 5.3%, will require a resin with an elongation to break of at least this value to achieve maximum tensile properties.

2     Adhesive Properties

High adhesion between resin and reinforcement fibres is necessary for any resin system. This will ensure that the loads are transferred efficiently and will prevent cracking or fibre / resin debonding when stressed.

3     Toughness Properties

Toughness is a measure of a material’s resistance to crack propagation, but in a composite this can be hard to measure accurately. However, the stress / strain curve of the resin system on its own provides some indication of the material’s toughness. Generally the more deformation the resin will accept before failure, the tougher and more crack-resistant the material will be. Conversely, a resin system with a low strain to failure will tend to create a brittle composite, which cracks easily. It is important to match this property to the elongation of the fibre reinforcement.

4     Environmental Properties

Good resistance to the environment, water and other aggressive substances, together with an ability to withstand constant stress cycling, are properties essential to any resin system. These properties are particularly important for use in a marine environment.

Resin Types

The resins that are used in fibre reinforced composites are sometimes referred to as ‘polymers’. All polymers exhibit an important common property in that they are composed of long chain-like molecules consisting of many simple repeating units. Manmade polymers are generally called ‘synthetic resins’ or simply ‘resins’.

Polymers can be classified under two types, ‘thermoplastic’ and ‘thermosetting’, according to the effect of heat on their properties. Thermoplastics, like metals, soften with heating and eventually melt, hardening again with cooling. This process of crossing the softening or melting point on the temperature scale can be repeated as often as desired without any appreciable effect on the material properties in either state. Typical thermoplastics include nylon, polypropylene and ABS, and these can be reinforced, although usually only with short, chopped fibres such as glass.

Thermosetting materials, or ‘thermosets’, are formed from a chemical reaction in situ, where the resin and hardener or resin and catalyst are mixed and then undergo a nonreversible chemical reaction to form a hard, infusible product. Once cured, thermosets will not become liquid again if heated, although above a certain temperature their mechanical properties will change significantly. This temperature is known as the Glass Transition Temperature (Tg), and varies widely according to the particular resin system used, its degree of cure and whether it was mixed correctly. Above the Tg, the molecular structure of the thermoset changes from that of a rigid crystalline polymer to a more flexible, amorphous polymer. This change is reversible on cooling back below the Tg. Above the Tg properties such as resin modulus (stiffness) drop sharply, and as a result the compressive and shear strength of the composite does too. Other properties such as water resistance and colour stability also reduce markedly above the resin’s Tg.

Although there are many different types of resin in use in the composite industry, the majority of structural parts are made with three main types, namely polyester, vinylester and epoxy.

1     Polyester Resins

Polyester resins are the most widely used resin systems, particularly in the marine industry. By far the majority of dinghies, yachts and work-boats built in composites make use of this resin system.

Polyester resins such as these are of the ‘unsaturated’ type. Unsaturated polyester resin is a thermoset, capable of being cured from a liquid or solid state when subject to the right conditions.

Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a monomer which is usually styrene. The addition of styrene in amounts of up to 50% helps to make the resin easier to handle by reducing its viscosity. The styrene also performs the vital function of enabling the resin to cure from a liquid to a solid by ‘cross-linking’ the molecular chains of the polyester, without the evolution of any by-products. These resins can therefore be moulded without the use of pressure and are called ‘contact’ or ‘low pressure’ resins. Polyester resins have a limited storage life as they will set or ‘gel’ on their own over a long period of time. Often small quantities of inhibitor are added during the resin manufacture to slow this gelling action. For use in moulding, a polyester resin requires the addition of several ancillary products.

These products are generally:

1     Catalyst

2     Accelerator

3     Additives

o Thixotropic Pigment

o Filler

o Chemical/fire resistance

A manufacturer may supply the resin in its basic form or with any of the above additives already included. Resins can be formulated to the moulder’s requirements ready simply for the addition of the catalyst prior to moulding. As has been mentioned, given enough time an unsaturated polyester resin will set by itself. This rate of polymerisation is too slow for practical purposes and therefore catalysts and accelerators are used to achieve the polymerisation of the resin within a practical time period. Catalysts are added to the resin system shortly before use to initiate the polymerisation.

The catalyst does not take part in the chemical reaction but simply activates the process. An accelerator is added to the catalysed resin to enable the reaction to proceed at a cold workshop temperature and/or at a greater rate. Since in the absence of a catalyst, accelerators have little influence on the resin, they are sometimes added to the resin by the polyester manufacturer to create a ‘pre-accelerated’ resin.

Great care is needed in the preparation of the resin mix prior to moulding. The resin and any additives must be carefully stirred to disperse all the components evenly before the catalyst is added. This stirring must be thorough and careful as any air introduced into the resin mix affects the quality of the final moulding. This is especially so when laminating with layers of reinforcing materials as air bubbles can be formed within the resultant laminate which can weaken the structure. It is also important to add the accelerator and catalyst in carefully measured amounts to control the polymerisation reaction to give the best material properties. Too much catalyst will cause too rapid a gelation time, whereas too little catalyst will result in under-cure.

Colouring of the resin mix can be carried out with pigments. The choice of a suitable pigment material, even though only added at about 3% resin weight, must be carefully considered as it is easy to affect the curing reaction and degrade the final laminate by use of unsuitable pigments.

Filler materials are used extensively with polyester resins for a variety of reasons including:

1     To reduce the cost of the moulding

2     To facilitate the moulding process

3     To impart specific properties to the moulding

Fillers are often added in quantities up to 50% of the resin weight although such addition levels will affect the flexural and tensile strength of the laminate. The use of fillers can be beneficial in the laminating or casting of thick components where otherwise considerable exothermic heating can occur. Addition of certain fillers can also contribute to increasing the fire-resistance of the laminate.

2     Vinylester Resins

Vinylester resins are similar in their molecular structure to polyesters, but differ primarily in the location of their reactive sites, these being positioned only at the ends of the molecular chains. As the whole length of the molecular chain is available to absorb shock loadings this makes vinylester resins tougher and more resilient than polyesters. The vinylester molecule also features fewer ester groups. These ester groups are susceptible to water degradation by hydrolysis which means that vinylesters exhibit better resistance to water and many other chemicals than their polyester counterparts, and are frequently found in applications such as pipelines and chemical storage tanks.

3     Epoxy Resins

The large family of epoxy resins represent some of the highest performance resins of those available at this time. Epoxies generally out-perform most other resin types in terms of mechanical properties and resistance to environmental degradation, which leads to their almost exclusive use in aircraft components. As a laminating resin their increased adhesive properties and resistance to water degradation make these resins ideal for use in applications such as boat building. Here epoxies are widely used as a primary construction material for high-performance boats or as a secondary application to sheath a hull or replace water-degraded polyester resins and gel coats.

Usually identifiable by their characteristic amber or brown colouring, epoxy resins have a number of useful properties. Both the liquid resin and the curing agents form low viscosity easily processed systems. Epoxy resins are easily and quickly cured at any temperature from 5 degrees C to 150 degrees C, depending on the choice of curing agent.

One of the most advantageous properties of epoxies is their low shrinkage during cure which minimises fabric ‘print-through’ and internal stresses. High adhesive strength and high mechanical properties are also enhanced by high electrical insulation and good chemical resistance.

Gelation, Curing and Post-Curing

On addition of the catalyst or hardener, a resin will begin to become more viscous until it reaches a state when it is no longer a liquid and has lost its ability to flow. This is the ‘gel point’. The resin will continue to harden after it has gelled, until, at some time later, it has obtained its full hardness and properties. This reaction itself is accompanied by the generation of exothermic heat, which, in turn, speeds the reaction. The whole process is known as the ‘curing’ of the resin.

The speed of cure is controlled by the amount of accelerator in a polyester or vinylester resin and by varying the type, not the quantity, of hardener in an epoxy resin. Generally polyester resins produce a more severe exotherm and a faster development of initial mechanical properties than epoxies of a similar working time. With both resin types, however, it is possible to accelerate the cure by the application of heat, so that the higher the temperature the faster the final hardening will occur. This can be most useful when the cure would otherwise take several hours or even days at room temperature. A quick rule of thumb for the accelerating effect of heat on a resin is that a 10 degrees C increase in temperature will roughly double the reaction rate. Therefore if a resin gels in a laminate in 25 minutes at 20 degrees C it will gel in about 12 minutes at 30 degrees C, providing that no extra exotherm occurs.

Curing at elevated temperatures has the added advantage that it actually increases the end mechanical properties of the material, and many resin systems will not reach their ultimate mechanical properties unless the resin is given this ‘postcure’. The postcure involves increasing the laminate temperature after the initial room temperature cure, which increases the amount of crosslinking of the molecules that can take place. To some degree this postcure will occur naturally at warm room temperatures, but higher properties and shorter postcure times will be obtained if elevated temperatures are used. This is particularly true of the material’s softening point or Glass Transition Temperature (Tg), which, up to a point, increases with increasing postcure temperature.

Comparison of Resin Properties

The choice of a resin system for use in any component depends on a number of its characteristics, with the following probably being the most important for most composite structures:

Adhesive properties

Mechanical properties

Micro-cracking resistance

Fatigue resistance

Degradation from water ingress

Adhesive Properties

It has already been discussed how the adhesive properties of the resin system are important in realising the full mechanical properties of a composite. The adhesion of the resin matrix to the fibre reinforcement or to a core material in a sandwich construction are important. Polyester resins generally have the lowest adhesive properties of the three systems described here. Vinylester resin shows improved adhesive properties over polyester but epoxy systems offer the best performance of all, and are therefore frequently found in many high-strength adhesives. As epoxies cure with low shrinkage, the various surface contacts set up between the liquid resin and the adherends are not disturbed during the cure. The adhesive properties of epoxy are especially useful in the construction of honeycomb-cored laminates where the small bonding surface area means that maximum adhesion is required. The strength of the bond between resin and fibre is not solely dependent on the adhesive properties of the resin system but is also affected by the surface coating on the reinforcement fibres.

Mechanical Properties

Two important mechanical properties of any resin system are its tensile strength and stiffness. These diagrams show results for tests carried out on commercially available polyester, vinylester and epoxy resin systems cured at 20 degrees C and 80 degrees C.

After a cure period of seven days at room temperature, it can be seen that a typical epoxy will have higher properties than a typical polyester and vinylester for both strength and stiffness. The beneficial effect of a post cure at 80 degrees C for five hours can also be seen.

Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Polyester and vinylesters can show shrinkage of up to 8%. The typical shrinkage of an epoxy is only around 2%. The absence of shrinkage is, in part, responsible for the improved mechanical properties of epoxies over polyester, as shrinkage is associated with built-in stresses that can weaken the material. Furthermore, shrinkage through the thickness of a laminate leads to ‘print-through’ of the pattern of the reinforcing fibres, a cosmetic defect that is difficult and expensive to eliminate.

Micro-Cracking

The strength of a laminate is usually thought of in terms of how much load it can withstand before it suffers complete failure. This ultimate or breaking strength is the point it which the resin exhibits catastrophic breakdown and the fibre reinforcements break.

However, before this ultimate strength is achieved, the laminate will reach a stress level where the resin will begin to crack away from those fibre reinforcements not aligned with the applied load, and these cracks will spread through the resin matrix. This is known as ‘transverse micro-cracking’ and, although the laminate has not completely failed at this point, the breakdown process has commenced. Consequently, engineers who want a long-lasting structure must ensure that their laminates do not exceed this point under regular service loads.

The strain that a laminate can reach before microcracking depends strongly on the toughness and adhesive properties of the resin system. For brittle resin systems, such as most polyesters, this point occurs a long way before laminate failure, and so severely limits the strains to which such laminates can be subjected. As an example, recent tests have shown that for a polyester/glass woven roving laminate, micro-cracking typically occurs at about 0.2% strain with ultimate failure not occurring until 2.0% strain. This equates to a usable strength of only 10% of the ultimate strength.

As the ultimate strength of a laminate in tension is governed by the strength of the fibres, these resin micro-cracks do not immediately reduce the ultimate properties of the laminate. However, in an environment such as water or moist air, the micro-cracked laminate will absorb considerably more water than an uncracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fibre sizing agents, loss of stiffness and, with time, an eventual drop in ultimate properties.

Increased resin/fibre adhesion is generally derived from both the resin’s chemistry and its compatibility with the chemical surface treatments applied to fibres. Here the well-known adhesive properties of epoxy help laminates achieve higher microcracking strains. As has been mentioned previously, resin toughness can be hard to measure, but is broadly indicated by its ultimate strain to failure. A comparison between various resin systems is shown in this diagram.

Fatigue Resistance

Generally composites show excellent fatigue resistance when compared with most metals. However, since fatigue failure tends to result from the gradual accumulation of small amounts of damage, the fatigue behaviour of any composite will be influenced by the toughness of the resin, its resistance to microcracking, and the quantity of voids and other defects which occur during manufacture. As a result, epoxy based laminates tend to show very good fatigue resistance when compared with both polyester and vinylester, this being one of the main reasons for their use in aircraft structures.

Degradation from Water Ingress

An important property of any resin, particularly in a marine environment, is its ability to withstand degradation from water ingress. All resins will absorb some moisture, adding to a laminate’s weight, but what is more significant is how the absorbed water affects the resin and resin/fibre bond in a laminate, leading to a gradual and long-term loss in mechanical properties. Both polyester and vinylester resins are prone to water degradation - a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion in water for a period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

This diagram demonstrates the effects of water on an epoxy and polyester woven glass laminate, which have been subjected to a water soak at 100 degrees C. This elevated temperature soaking gives accelerated degradation properties for the immersed laminate.

Resin Comparison Summary

	Advantages	Disadvantages
Polyesters	- Easy to use - Lowest cost	- Only moderate mechanical properties - High styrene emissions in open moulds - High cure shrinkage Limited range of working time
Vinylesters	- Very high chemical/environmental resistance - Higher mechanical properties than polyesters	- Postcure generally required for highest properties - High styrene content - Higher cost than polyesters - High cure shrinkage
Expoxies	- High mechanical and thermal properties - High water resistance - Long working times available - Temperature resistance can be up to 140 degrees C wet / 220 degrees C dry - Low cure shrinkage	- More expensive than vinylesters - Critical mixing - Corrosive handling

Other Resin Systems used in Composites Besides polyesters, vinylesters and epoxies, there are a number of other specialised resin systems that are used where their unique properties are required: 1     Phenolics Primarily used where high fire-resistance is required, phenolics also retain their properties well at elevated temperatures. For room-temperature curing materials, corrosive acids are used which leads to unpleasant handling. The condensation nature of their curing process tends to lead to the inclusion of many voids and surface defects, and the resins tend to be brittle and do not have high mechanical properties. 2     Cyanate Esters Primarily used in the aerospace industry. The material’s excellent dielectric properties make it very suitable for use with low dielectric fibres such as quartz for the manufacture of radomes. The material also has temperature stability up to around 200°C wet. Typical costs: £40/kg. GTC-1a-1098 - 21 3     Silicones Synthetic resin using silicon as the backbone rather than the carbon of organic polymers. Good fire-resistant properties, and able to withstand elevated temperatures. High temperature cures needed. Used in missile applications. Typical costs: >£15/ kg. 4     Polyurethanes High toughness materials, sometimes hybridised with other resins, due to relatively low laminate mechanical properties in compression. Uses harmful isocyanates as curing agent. 5     Bismaleimides (BMI) Primarily used in aircraft composites where operation at higher temperatures (230 degrees C wet/250 degrees C dry) is required. e.g. engine inlets, high speed aircraft flight surfaces. 6     Polyimides Used where operation at higher temperatures than bismaleimides can stand is required (use up to 250 degrees C wet/300 degrees C dry). Typical applications include missile and aero-engine components. Extremely expensive resin which uses toxic raw materials in its manufacture. Polyimides also tend to be hard to process due to their condensation reaction emitting water during cure, and are relatively brittle when cured. PMR15 and LaRC160 are two of the most commonly used polyimides for composites.

Next week we’ll look at the different reinforcing fibres