Plastics
The term “plastic” is used to describe the moulded form of a synthetic (i.e. man-made) resin.
These resins are composed of large, chain-like molecules known as polymers, which also occur naturally as, for example, cellulose, protein, and rubber. Most synthetic resins are made from chemicals derived from oil and it is these man-made polymers which are used to produce what are commonly known as “plastics”.
Plastics in their various forms have existed since the late 19th century, but most of the materials referred to as plastics today have been developed during the past 50 years.
A large variety of plastics is now available, and they exist in various physical forms. They can be bulk solid materials, rigid or flexible foams, or in the form of sheet or film. Most fall into one of two categories; thermoplastic or thermoset.
Thermoplastics can be formed and re-formed by the use of heat
(e.g. polyethylene, PVC, etc.).
Thermosets, on the other hand, harden by a chemical reaction, generating heat when they are formed and cannot be melted or re-formed (e.g. unsaturated polyesters, epoxies, vinyl esters etc.).
The Nature of Reinforced Plastics
Reinforced Plastic is the generic term used to describe specific plastic materials reinforced with high strength fibres. Since their development, these materials have been commonly known by names such as “Fibreglass” and GRP (Glass Reinforced Plastic).
Though GRP is still the most used term, the development and utilisation of fibres other than glass makes FRP (Fibre Reinforced Plastic) a more accurate and comprehensive description. Within the reinforced plastics industry itself, “Composite” is the term felt to best describe this light, durable and astonishingly tough constructional material.
Composites can be fabricated into a wide spectrum of products, from the purely decorative to complex, engineered structures. They may be translucent, opaque, or coloured, thick or thin, flat or shaped and there is virtually no limit on size.
Composites can be found in most areas of daily life, in the form of roof sheeting, tanks, pipes, vehicle bodies, buildings, boats, etc.
To produce a composite item, two basic components are required, these being a synthetic resin and a strong fibre. The resin, which could be a polyester, epoxy, or vinyl ester, is normally supplied as a viscous liquid, which sets to a hard solid when suitably activated. The fibre may be glass, carbon, polyaramid, or a combination of some or all of these.
What makes composites unique is the fact that the material of construction and the end product are produced simultaneously. Using a suitable mould, layers of fibre are impregnated with activated resin until the required thickness is achieved. When complete, the moulding is removed and the mould can then be re-used to produce more, identical items.
Resins
Several resin types are employed in the manufacture of composite products. All of these resins are thermosets, but they differ in their chemical make-up, thus exhibiting diverse properties. This means that
Manufacturers can choose resins which enable them to tailor their products to meet specific requirements.
This handbook is concerned mainly with polyester resins, but other types such as vinyl ester, epoxy, DCPD, phenolic and hybrid systems.
Polyesters
Polyester resins are unsaturated polyesters.
The raw materials used for the manufacture of unsaturated polyester resins are oil based and to produce a polyester of this type, three basic chemical components are generally required: –
A: saturated acid (e.g. phthalic anhydride)
B: unsaturated acid (e.g. maleic anhydride)
C: dihydric alcohol (e.g. propylene glycol)
With the application of heat, these chemicals combine to form a resin which is a viscous liquid when hot, but a brittle solid when cold. The term “polyester” is derived from the link between A or B with C, which is termed an “ester” link.
Whilst it is still hot, the resin is dissolved in a monomer which is usually styrene though others can be, and are, used. The monomer performs the vital function of enabling the resin to cure from a liquid to a solid, by crosslinking the molecular chains of the polyester.
No by-products are evolved during this process, which means the resins can be moulded without the use of pressure. They are therefore known as contact or low-pressure moulding resins.
With the addition of styrene and in the presence of a catalyst and accelerator, the styrene cross-links the chains to form a highly complex three-dimensional network.
At this stage, the polyester resin is said to be cured. It is now a chemically resistant and (usually) hard solid. The cross-linking, or curing, process is called polymerisation and is a non-reversible chemical reaction.
Once cured, the resin will continue to ‘mature’, during which time the moulding will acquire its full properties. This process, which can take several weeks to complete at room temperature, can be accelerated by post curing the moulding at elevated temperatures. Polyester resins with differing characteristics and properties are produced by manipulating the types and amounts of raw materials used in their manufac
