Composites and Reinforced Plastics
The design of products from composite materials is complex, because of the many variables involved. To further understand this, it is necessary to consider some of the details and we hope the following brief overview and discussion of the approach to some typical industrial applications will answer some questions about the choice of materials and processes. Some photographs of various applications are shown in the Photo Gallery and further photographs will be added in due course.
The Polymer Matrix
A thermosetting plastic matrix may be chosen from a variety of polymers including unsaturated polyesters, vinyl esters, epoxies, phenolics, furanes and others. Within each of these types, there are a wide variety of choices, where mechanical strength and its retention at elevated temperatures, elongation under stress, resistance to chemical environments including weather and ultraviolet radiation may be factors, which dictate the correct choice. There are also applications, where electrical insulating, conductive or dielectric properties are important. In general the more that is expected from a polymer, the higher will be the cost of the raw material, so that the correct choice will affect the economic viability and marketability of the end product.
The reinforcements under consideration are mostly in the form of fibres. The fibres may chosen from a range of materials of which glass is the most common choice for most industrial applications, but aramids, carbon and other fibres are found in an increasing number of products ranging from car bodies to aeroplane bodies and sports equipment where their high strength and low weight can justify the higher costs. Glass fibres are available in various type of glass, including A-glass (rarely used for reinforcement), E-glass, C-glass, S-glass and ECR-glass. The variations in composition are to made in order to produce higher strength or stiffness fibres or better resistance to chemicals. The fibres may be short and randomly distributed e.g. as in chopped-strand mat, or continuous untwisted filaments such as in rovings, which may be wound onto rotating mandrels to produce round or cylindrical products. Continuous fibres may assembled into unidirectional sheets or be woven (woven rovings). With continuous-filament reinforcement the orientation of each layer is important and is a critical factor in processes such as filament winding. Glass-fibre cloths can also be produced from glass yarns (twisted fibres) in a variety of weaves, which may be designed for increased drapability and with properties adjusted to produce different strengths in the warp and weft directions. Various thicknesses and weights are possible. Glass fibre cloth is a relatively expensive reinforcement and more commonly used with epoxy resins.
The process will involve a mould or mandrel, to allow the item to be produced to the required dimensions. Its construction will depend on whether the part is one-off or to be made in small numbers or large production runs. In some processes, e.g. press moulding, the interior and exterior surfaces are moulded.
The type of polymer chosen will dictate whether the process can be carried out at room temperature or whether the mould must be heated and/or heat applied to the part to result in a proper cure. Such considerations are the important when choosing the polymer. As an illustration, we may compare polyesters with epoxies. In contact-moulding (hand lay-up) applications, polyester resins can be found with a wide range of properties which enable choices to be made to suit the application.
Consider an application such as a tank simply used to store water at ambient temperatures compared to a tank containing highly aggressive chemicals at elevated temperatures, perhaps with externally imposed loads. Both can use polyester type resins: the water tank using an inexpensive orthophthalic polyester and the chemical tank an expensive vinyl ester able to produce higher strength laminates, which are also able to withstand higher temperatures and more aggressive chemical conditions. Both resins do have some things in common: the esters are solutions in liquid styrene monomer with similar physical characteristics such as viscosity and both can be cured at ambient temperatures, using very small amounts of promoters and initiators (often incorrectly termed catalysts). Heat may be required to post-cure at elevated temperatures, in some cases. Most unsaturated polyester-type resins look and smell alike, because of the styrene monomer and can be handled similarly in processing. The difference is in the polyester or vinyl ester molecule dissolved in the monomer.
If we look at bisphenol-epichlorohydrin-based epoxy resins, they are all very similar chemically and only differ slightly in viscosity. However, the curing of epoxy resins varies widely depending on the properties required. Some of the curing agents used may be in amounts that even exceed the amount of resin and must be calculated stoichiometrically. Others such as aliphatic amines require only a few percent by weight and will produce a cured resin in less than five minutes, as used in some adhesives. Aromatic amines, used in larger amounts, require long curing schedules at 150 °C. Another system might use a small amount of catalyst, which has very long shelf life at room temperature after mixing, but will cure when press moulded in a few minutes at 200 °C. One rule generally applies: the heat distortion temperature, which gives an indication of the final part’s ability to withstand elevated temperatures, depends upon the temperature reached during the exotherm or in the moulding process. Thus room-temperature cured epoxies lose their strength above room temperature, whereas those cured at 150 °C will retain their strength at this higher temperature. Epoxies are generally known for superior strength especially in adhesion, but the generally higher cost or processing requirements will usually prevent their use in large tanks as mentioned above. They are ideally suited to applications such as smaller diameter composite pipes used for high pressures.