Thermoplastic Materials

In their normal state, thermoplastic materials are hard, but become soft and pliable when heated (the Greek word thermo means heat). When softened, thermoplastic materials can be moulded and shaped, and retain their new shape when cooled. Unless their heat limit is exceeded, this process can be repeated many times without damaging the material.

Two types of transparent thermoplastic materials are used for aircraft windshields and side windows, and are usually referred to as cellulose acetate and acrylic.

Older aircraft used cellulose acetate plastic because of its transparency and light-weight. Cellulose acetate can be identified by its slight yellowish tint (especially when aged), and by the fact that a scrap of it will burn with a sputtering flame and give off black smoke. It will also react and soften, upon contact with some materials, such as acetone. A disadvantage of cellulose acetate is its tendency to shrink and discolour with time, which has led to it being phased out almost completely.

Acrylic plastics are identified by such trade names as Perspex (UK) and Plexiglas (USA). It is stiffer than cellulose acetate, more transparent and practically colourless. Acrylic burns with a clear flame and gives off a fairly pleasant odour. If applied, acetone will cause white marks but will leave the material as hard as it previously was.

Use of Thermoplastics

Normally, thermoplastics are used where there are no unusual temperature changes and the majority of all plastics production is thermoplastics. It includes:

• Acetate: Widely used for tool handles and electrical goods.
• Poly-Ethylene: Commonly known as polythene. Its uses include flexible tubing, cable insulation and packaging.
• Poly-Propylene: Stronger, harder and more rigid than polythene. They are used for such items as high-pressure air piping.
• Poly-Vinyl-Chloride: Commonly known as PVC. Varying degrees of rigidity/flexibility are achievable by varying the amount of plasticiser used. Rigid, moulded sections or piping can be produced and also flexible electric cable insulation.
• Polystyrene: Can be produced in rigid form, but is more familiar in the expanded form, when it is useful for thermal insulation, buoyancy or shock-resistant packaging.
• Acrylics: These are particularly useful where light transmission is necessary. Perspex and Plexiglas belong to this family. They have excellent light transmission properties and are also resistant to splintering. There is a tendency for some fine craze-cracking to develop if exposed for long periods to ultra violet light. These transparent plastics may be solid or laminated. When laminated, two or more layers are bonded together with a clear adhesive and in this form, they are more shatter-resistant and are ideally suited to pressurised aircraft windows. An even stronger and more shatterproof transparent plastic can be achieved by stretching the acrylic in both directions before final shaping. These improved properties result from the stretching operation causing a preferential alignment of the long-chain molecules. Extreme care should be taken when handling acrylics, as they are easily scratched. The acrylics are supplied with a paper or rubberised film, which should not be removed until required for further use. If dirty, they should be cleaned with cold or soapy water. Care should also be taken when using solvents in the vicinity of acrylics. Some solvents or their vapours may cause crazing of the material. Reference to the appropriate manuals or manufacturers’ specification sheets are essential.
• Poly-Carbonates: These have similar uses to the acrylics, but are more temperature-resistant and also have superior impact strength. They are also more expensive.
• Nylon: Belongs to the polyamide family and is an extremely useful and versatile material. It is strong, tough and also has low friction properties. It can be used as a fibre or produced as a moulding. Popular uses include textiles, furnishings, ropes, tyre reinforcement, bushes, pulleys, gears and lightweight mouldings such as brackets, handles etc.
• Poly-Tetra-Fluoro-Ethylene: Commonly known as ‘PTFE’, it is similar to nylon in appearance but is denser, whiter and much more expensive. It has a wax-like surface, which results in very low friction properties that make it suitable for bushes and gears. It also has a high temperature capability (over 300ºC) and is extensively used as a non-stick coating e.g. Teflon. PTFE tape is often used as a thread sealant for oxygen pipe threads and as backing rings for hydraulic seals.