Resins

Natural resins are obtained from the exudations from certain trees and other plants and as clear, translucent, yellow (amber), brown, solid, or semisolid agents, they are used in inks, lacquers, linoleum, varnishes and, of course, plastics.

While the words plastics and resins are often used synonymously, they are in fact, quite different in that plastics refers to the material in the finished items while resins are the raw materials which may be found in the form of flakes, pellets, powder or a syrup.

Resins may be used alone to form plastics. But usually, additives are employed with them, to assist in the moulding characteristics or to enhance the properties of the finished product.

The resin may be thickened and given more ‘body’ by the addition of inert fillers, which may be used to fill gaps and voids in the structure. Typical fillers are micro-balloons, cotton and glass flock and aerosil (fumed silica).

Reinforcing agents, plasticizers, stabilisers, colorants, flame-retardants, smoke suppressants and processing aids, such as lubricants and coupling agents are among the other additives used with resins.

Resins have little strength in themselves and are generally used to impregnate linen, paper, and cloths made up from various synthetic fibres. For many years, aircraft control cable pulleys have been made from thermosetting resins, reinforced with layers of linen cloth. These pulleys are cured in a mould at high temperature and have high strength without causing wear to the control cables.

When layers of paper are impregnated with a thermosetting resin, such as phenol-formaldehyde or urea-formaldehyde, they can be moulded into flat sheets or other shapes. Once hardened, the material makes an exceptional electrical insulator and can be found in use as terminal strips and printed circuit boards.

Polyester Resin

Polyester resin can be extruded into fine filaments and woven into fabric (like nylon) or cast into shape and it is also useful as a heat-resistant lacquer.

For example, glass fibres and mat have great strength for their weight but lack rigidity. So, to convert glass fibre into a useful structural material, it is impregnated with polyester resin and moulded into a desired form.

Polyesters cure by chemical action and so differ from materials, which cure by the evaporation of an oil or solvent. As polyester is thick and unmanageable, a styrene monomer is added to make it thinner and easier to work.

If left alone, the mixture of polyester and styrene will eventually cure into a solid mass. So inhibitors are added to delay this curing process and to improve shelf life.

A catalyst has to be used when the inhibitors are no longer wanted. The curing process is to be started and an accelerator will appreciably shorten the curing time of the resin depending on the temperature and mass of the resin.

The actual cure of polyester resin occurs when a chemical reaction between the catalyst and accelerator generates heat within the resin. This exothermic reaction can be seen when a thick layer cures more rapidly than a thin layer.

Thixotropic Agents

The heat generated by the chemical reaction, can make the material less viscous and cause it to ‘run’ (particularly if it is on a vertical surface). To overcome this problem, a thixotropic agent is added to the resin after mixing, to increase its viscosity. The increased viscosity allows the resin to remain in place no matter where it may be used.

Epoxy Resin

Another type of resin that can be used in place of polyester in laminated structures is epoxy resin. Epoxy resin has a low percentage of shrinkage, high strength for its weight and the ability to adhere to a wide range of materials. Unlike polyester resins that require a catalyst, epoxy resins require a hardener or curing agent without recourse to heating. There is also a difference in the mixing ratios between polyester and epoxy resins. For polyester resin, the ratio is 64:1 (resin to catalyst) whilst for epoxy resin, the ratio is 4:1 (resin to hardener).