|Material||17: Thermoplastics, tapes, MLI|
|General Information||This chapter covers thermoplastic materials used in films, non-adhesive tapes and foils, plus either unreinforced or reinforced “bulk” materials |
|Use in Spacecraft||Plastic films appear in:
Thermoplastics, either plain or reinforced, find multiple uses in spacecraft, including:
- electronic circuitry as insulation, dielectrics and bases for printed wiring;
- multi-layer insulations (MLI)usedfor thermal-control purposes: basic components;
- inflatable and erectile devices: e.g. "structural" applications;
- flexible second-surface mirrors (solar reflectors).
- electrical insulators,
- small mechanical parts,
- lacing and tie devices,
- sleeves and tubing.
|Main Categories||The main film-forming polymers used are: polyolefins, polyester, fluorinated plastics, polyimides, polycarbonates and acetals. Composite laminated films are commercially available. Uncoloured films are transparent or translucent white to yellow, but dyed and pigmented grades exist in any shade. Classical plastic additives are used in films: plasticisers, antioxidants, antistatic agents.
Film surfaces can be modified by chemical treatment and by metallization. The latter use mainly vacuum-deposited aluminium, silver, gold or copper.
Films are sold in rolls or sheets. Thickness varies from a few micrometres upwards. Thicknesses of less than 5 µm to7µm are generally difficult to procure in large quantities.
Commercial thermoplastics are extremely numerous. Most of them can find some space use, for example, polyamides, acetal, polyolefins, polycarbonate, acrylics, polystyrene, fluorinated resins and polyphenylene oxide. Some are hard and brittle, others are tough; some are flexible and soft. Pure products vary from transparently clear to translucent white or light yellow, but most of them can be dyed or pigmented. Fillers are sometimes used as well as other additives such as antioxidants, plasticizers, UVstabilizers and processing aids. Reinforced thermo-plastics based on glass fibres or chopped carbon fibres are commercially available.
Many types of thermoplastics appear as textile items. Shrinkable plastics exist on the market, as well as foamed plastics.
High-performance thermoplastics, with continuous fibre-reinforcement, were promoted for structural applications,
|Processing and Assembly|
- Films can be cut to size and tailored to intricate shapes. Attachment is made by glueing, sewing or welding (heat sealing, ultrasonic welding), though not all methods are applicable to any one type of film; for example, plain polyester or polyimide cannot be heat-sealed, but some laminated composites can.
- Operations such as,moulding, extrusion and textile processing are generally done by specialized firms,andaerospace users aremainly concernedwith semi-finished or finished items. Most plastics can be machined and assembled by classical techniques; adhesive bonding is one of the most versatile; welding is sometimes possible. The processing of reinforced thermoplastics is very similar to that of light metals.
|Precautions||Thermoplastics soften at rather low temperatures (from about 80 ºC for polystyrene to more than 300 ºC for polytetrafluoroethylene - PTFE). This should be kept in mind during processing. Thermoplastics are sometimes quite sensitive to chemicals or solvents: tables of chemical resistance should be consulted, particularly when devising cleaning methods.
Films are more or less fragile with respect to tearing, cutting, puncturing or folding, particularly in thin gauges. Anisotropy is frequent, the properties in one direction (the extrusion direction) being quite different from those in the perpendicular direction; this shall be considered in the design. The dimensional stability of plastic films in severe environments is not very good. They can be stabilized by a suitable thermal treatment. Static charges can develop on most plastic films (unless they are specially treated or metallized).
- Sensitivity to chemicals and solvents is similar to that of the base plastic, but attack is rather rapid, owing to high surface or volume ratio. Metallized films are sensitive to abrasion, since the metal layer is extremely thin. Cleaning is not recommended and contamination shall therefore be avoided. Electrical grounding of metallized films is difficult; contacts are very sensitive to corrosion in the terrestrial environment. Most plastic films are flammable. Absorption of water by some plastic films can drastically change their electrical properties.
- The dimensional stability of many thermoplastics is inferior to that of conventionalmetals:many fluorinated resins have a tendency to creep under load; polyamide plastics absorb water in normal atmospheres and shrink under dry conditions. Tough plastics can retain internal stresses after machining or forming operations, and this renders some stress-relieving thermal-treatment necessary (polycarbonate, acetal). Thermal conductivity of plastics is low; this shall be taken into account in the design and during processing. Most current plastics are flammable, but some exceptions exist (fluorinated), and self-extinguishing grades of conventional types can be found. Filled thermoplastics are generally more stable thermally and mechanically than plain grades. Further improvement is given by reinforcement, which permits the design of small, precise mechanical parts.
|Hazardous and Precluded|
- Additives commonly used in plastics can be detrimental in space applications; particularly plasticizers that have a tendency to evaporate in space vacuum. Many commercial films contain volatile additives (plasticizers and antistatic agents) and shall not be used in space.
- Polyvinylchloride (PVC), cellulose and acetates are not stable enough under vacuum and shall not be used (particularly in electrical insulation). The same is true of polyvinylacetate and butyrate. Polyamide films absorb water in normal atmospheres and desorb it in vacuum with dimensional changes and are therefore of limited use. Many polyamides are dangerous because they absorb water and shrink under vacuum; they should be excluded.
|Effects of Space environment||Physico-chemical degradation of plastic films is similar to that of bulk plastics, but the overall effects can be different owing to the particular aspects of films: thinness, need for flexibility, frequent need for stable optical properties.
- Vacuum tends to extract additives from plastics, the consequence of which is a degradation of the properties that were stabilized by the additives (increase in rigidity and fragility when a plasticiser is lost, for example). Plastic films tend to stiffen as a result. There is also a great risk of contamination by evolved products, which are generally quite high-boiling-point chemicals. The exposed surface areas of plastics films are often large, consequently contamination dangers are high.
Polyimides, TFE, FEP and polyterephthalates are generally safe in this respect. Multi-layer systems shall be properly vented to eliminate internal overpressure; these tend to accumulate large amounts of contaminants during handling and shall be baked under vacuum before integration into a spacecraft. In general, “pure” plastics, with the exception of PVC, polyamides, polyvinyl acetates and butyrates, are fairly safe to use, but it is difficult to assess this “purity”, since manufacturers tend to “improve” their products by adding chemicals. In addition it frequently happens that that processing aids or miscellaneous impurities stay absorbed in commercial plastics.The electrical insulation properties of these plastics, which tend to absorb water, are improved by the drying action of a vacuum.
- Radiation: Both UV and particle, can modify plastic materials. The result is frequently discoloration accompanied by evolution of gas and hardening. Some fluorinated plastics are rather sensitive to particle radiation (PTFE is limited to 1Mrad) and shall not be used in such a way that it is fully exposed to space. However, a minimal amount of shielding reduces doses to acceptable levels.Other plastics are far more resistant and are not significantly modified by particle fluxes encountered in space, particularly the filled or reinforced grades. UV damage is generally limited to a very thin surface layer and can be disregarded when optical properties are not a concern. Radiation is quite damaging for thin polymer films exposed to the total space environment. The primary effects are generally deformation, embrittlement and discoloration, which in turn affect the mechanical integrity and the thermal equilibrium of the devices concerned. TFE is very sensitive to particle radiation; polyterephthalates are damaged by solar UV. The best choice is FEP or polyimides (the latter being normally yellow). Radiation effects are frequently increased by impurities and oxidation consecutive to processing.
- Temperature: High temperatures soften thermoplastics and degrade polymer films.The lowthermal conductivity of “bulk” thermoplastics makes it difficult to eliminate heat except when a suitable filler is present (metal powder for example). Most plastics harden significantly and become brittle at temperatures lower than their “glass-transition temperature”. Fluorinated polymers and polyimides can be used over awide range of temperatures from cryogenic to more than 200 ºC. Thermal cycling can be damaging to some metalized films where tiny metal flakes can loosen and contaminate the vicinity.
- Atomic oxygen attacks thermoplastics and affects polymer films with a carbon/ hydrogen skeleton. Protection layers such as SiOx or ITO can be applied in most cases. FEP is sensitive to the combination of ATOX and UV light.
|Some Representative Products||It is impossible to cite all the trade names in this enormous domain. Big European chemical firms are engaged in producing most of the thermoplastics that can be used in aerospace vehicles, including:
BASF, Bayer, Huels, Dynamit-Nobel and Hoechst in Germany;
The following materials can be considered (see annex B for data sheets):
- ICI in UK;
- Aquitaine-Organico, Kuhlmann, Rhone-Poulenc in France;
- Montecatini-Edson in Italy.
Also the following film materials (see annex B for data sheets):
- Hostaform C9020,
- Makrolon GV30,
- Super Gude Space PT,
- Thermofit RT850.
- Kapton H,
- Makrofol N,
- Sheldahl G401500,
- Sheldahl G400900,
- Sheldahl G410620.