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|PRODUCT DATA of 18: Thermoset plastics|
|Material||18: Thermoset plastics|
|General Information||Synthetic polymers are formed by addition or condensation polymerization. The length of the polymer chains, usually measured by molecular weight, has a very significant effect on the performance properties and a profound effect on processibility.|
|Use in Spacecraft||Thermosetting resins can be used without any reinforcement as bulk plastics or foams.
Fibre-reinforced plastics normally use a thermosetting matrix to support the fibres and allow load-transfer. These can be structural or semi-structural parts. A further use for composites is as electronic circuit board substrate materials.
|Main Categories||Polymer resins used are mainly epoxies, cyanates, phenolics, polyesters, bismaleimides, polyimides, silicones, diallylphthalate and diphenyloxide. Polyimides are really thermoplastic ladder polymers, but are included here by analogy|
|Processing and Assembly||With the exception of one-part resin systems, the component parts of the thermosetting polymers (base, hardener, catalyst) shall be accurately measured and thoroughly mixed. Mixed resins have a limited “pot life” and shall be used before the viscosity increases during cure. Debubbling processes are used to remove air bubbles introduced during mixing or pouring (except resin types for foams).
The cure process temperature depends on the formulation (base polymer type, modifying agents used, one-part or two-part systems - hardener and catalyst used), e.g. epoxies (RT; 50 ºC to 150 ºC); phenolics (150 ºC); polyimides (250 ºC).
|Precautions||The curing schedule shall be carefully studied by means of a preliminary test programme. (Thermal-analysis equipment shall be used for these tests).
Exothermic reactions occur during curing that can raise the temperature of the resin excessively and degrade the polymer and its resultant characteristics. The amount of resinmixed at any one time (pot size/volume) shall be defined precisely and can limit the production of parts with thick sections.
Thermosetting resins, especially unfilledmaterials, are prone to shrinkage during cure. This shall be taken into account during the design stage, for example, final dimensions of specified component; sharp features that increase residual stresses and cause cracking; and combining thin and thick sections.
Parts produced from resins are normally cast into moulds. The thermal expansion characteristics of the resin and mould need consideration. Mould surfaces are normally pre-treated with “mould-release” agents to aid removal of the finished part. The choice of mould-release agent is all important to prevent potential contamination problems.
A major problem in processing is to ensure that the finished part is, as far as possible, free of voids.
|Hazardous and Precluded||Polyester resins are not generally suitable for space uses.
Polyimide or polybenzimide resins containing low-volatility solvents (to ensure flow) can retain traces of them in the cured item which subsequently outgas in vacuum: this can render them unsuitable.
|Effects of Space environment||Before using thermosetting plastics, a full evaluation of the effects of the service conditions shall be performed. In general, they are quite stable under space conditions provided that selection criteria were fully assessed.
|Some Representative Products||There are many large manufacturers on the European market, some of them having a link with the USA. There are also small firmsmaking commercial resins.
Some names can be cited, but the following list is far from complete: