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PRODUCT DATA of 18: Thermoset plastics
Material18: Thermoset plastics
General InformationSynthetic 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 SpacecraftThermosetting 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 CategoriesPolymer 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 AssemblyWith 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).
PrecautionsThe 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 PrecludedPolyester 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 environmentBefore 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.
  • Vacuum can lead to outgassing. This does not generally degrade the properties of the plastic, but can raise corona or contamination problems in its vicinity.
  • Radiation at levels existing in space is unimportant.
  • Thermal expansion can be quite large in unreinforced plastics. Cracks are formed in thermal cycling which could jeopardize long-term properties.
  • Atomic oxygen etches thermosetting plastics. Fragments can be released which contaminate the environment.
Some Representative ProductsThere 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:
  • Epoxy resins:
    • Araldite, Vantico, Switzerland,
    • Bakelite - Germany,
    • S Epikote (or Epon), Shell, The Netherlands (Shell, USA).

  • Phenolic, melamine and silicones:
    • Chemical and Insulating Ltd., UK (Hitco, USA),
    • <
    • Dynamit-Nobel, Germany,
    • Kuhlmann, France (Wyandotte, USA),
    • M.A.S., Italy (Synthane, USA).

  • Polyimide: Rhône-Poulenc, France (trade name Kerimid).
PCBs used in space hardware shall be qualified in accordance with ECSS-Q-ST-70-10. A list of qualified manufacturers is maintained by TOS-QT Division, ESTEC. For other uses, the following materials can be considered:
  • Araldite CT205,
  • Cycon C 69/MH--S,
  • Epikote 828,
  • Rexolite 1422