Comparing and Choosing Composite Materials
Composite materials are broadly defined as those in which a binder is reinforced with a strengthening material. Here we take a look at the pros and cons of the components: the resins and the fibers used to strengthen them.
Most modern composites share a common bond - almost literally. The binding resins - the chemical matrix in which the reinforcing fibers are embedded - are relatively few in number. There are three main recipes: polyester, vinylester and epoxy. Various flavours of each are available, depending on whether they are strengthened with glass, carbon or aramid fibers, and the particular application. For example, high UV (sunlight) tolerance may be chemically engineered using additives.
The presence of volatile organic compounds ('VOC') is of concern both for health reasons and 'greenhouse effect' impact. Modern epoxies are VOC free, but polyester and vinylester compounds have high concentrations of VOC in the form of styrene. This means that fabrication using esters should take place in well ventilated space.
The epoxy compound is formed by mixing two different chemicals which react to form a 'copolymer'. The curing rate is sensitive to temperature and the ratio of the two components, but curing is almost always assured. Some epoxy paste formulations will even cure underwater.
Polyester and vinylester by comparison, cure with the use of a peroxide catalyst (usually known as MEKP). Vinylester is sensitive to temperature, and may not cure at all under certain conditions.
Water resistanceEpoxies are highly water resistant, with vinylesters also showing a high resistance.
Polyester composites absorb water to a significant degree, and when used - say, in boat hulls - osmotic blistering occurs due to a reaction with water (hydrolysis) which results in chemical breakdown. Insoluble pthallic acid crystals damage the GRP laminate and acetic acid is a by-product.
Epoxies are very stable chemically, and offer excellent resistance to chemical attack. Polyesters are moderately resistant at room temperatures to most common chemicals, but vinylesters offer much higher resistance, though falling short of the protection that epoxies afford. The resistance of polyesters and vinylesters falls quickly at higher temperatures. Vinylesters may be used to provide a barrier coating to protect polyester, particularly in the marine environment.
Shrinkage, Strength and Stiffness
Polyesters and vinylesters typically shrink by 7% on curing, but epoxies shrink less than 2% and where dimensional stability is important, then epoxies are much to be preferred.
Shrinkage can introduce stress into a structure, and designers much factor this in. Both for tensile strength and stiffness, polyester is lowest on the scale, with epoxy highest and vinylester just superior to polyester.
This is an important property when using composites. Adhesion has to be strong between the resin and the fiber strengthener. Vinylester is not the best in this respect.
Polyester is by far the cheapest of the three resin systems, much cheaper even than vinylester, weight for weight. Polyester is preferred for boats and bathtubs, but where strength/weight is important and budget less of an issue, then epoxies win - for example in motorsport and aerospace.
There are three main families in use at present: glass fiber, carbon fiber and aramid fiber (more commonly known as Kevlar, a trademark of the DuPont Corporation).
Glass fiber is by far the cheapest and most widely used, and works well with all three resin types, but it is relatively heavy. Carbon fiber is much lighter, as are aramid fibers.
Glass fibers (either in chopped strand or woven cloth form) are most commonly used with a polyester resin, whereas carbon fiber, as a relatively high cost strengthener, is most usually combined with epoxy resins.
A resin has to 'stick' to the fiber strengthener, and it is important to select a resin/fiber combination (particularly with carbon and aramid fibers) so that there is good adhesion and the fibers are properly bonded within the resin.
In general terms, Kevlar mechanical properties are good in strength (double that of glass fibers) but very poor in stiffness, whilst the glass composite is ten times as stiff and half the strength.
Kevlar is very expensive compared to glass, so it is used where higher strength and elongation is needed.
Both aramid composites and GRP are good at handling repeated flexing cycles (such as in a boat hull), but carbon fiber has an unpredictable life when subject to repeated flexing.
GRP requires a considerably 'heavier' construction to achieve the strength of carbon fiber. Aramid fibers offer equivalent strength to fiberglass at a much lower weight, although abrasion resistance is lower.
When choosing a composite, there are many factors to take into account. Many users of advanced composites - for example in the premium boat building industry - will combine all three composites to tailor engineering properties and weight distribution. In fact, we now have structures which are composites of composites.