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|PRODUCT DATA of 14: Potting, sealants, foams|
|Material||14: Potting, sealants, foams|
|General Information||The generic name "potting compounds" applies to all types of products which can be applied in bulk fluid form and subsequently become relatively rigid; including sealants.|
|Use in Spacecraft||Potting compounds and sealants have various uses in spacecraft: electrical and mechanical insulation, damping, sealing and thermal coupling. They appear in practically all subsystems.|
|Main Categories||Three main chemical groups of polymers predominate: epoxies, silicones and polyurethanes. These exist as hardened potting compounds, sealants, foams, and syntatic foams (containing micro-balloons). Pigments, fillers and dyes are often added to these organic materials.
All these products vary from soft and elastic to hard and rigid. Specific gravity is from 0,1 for some foams to more than 2 for bulk-filled resins. Syntactic foams are between 0,5 and 0,8. Before the hardening (curing) process, potting compoundsare liquids, pastes or even powders. Some cure by atmospheric exposure (one-part systems), others by the addition of a catalyst (two-part systems).
|Processing and Assembly||The assembly to be sealed or potted is first cleaned. Sometimesa surface treatment (for example etching of PTFE parts) or a primer application is necessary. Two-part potting compounds need mixing in the specified proportions. Application is by pouring into open or closed moulds, caulking or smearing.
To avoid the formation of bubbles, it is frequently necessary to pour small quantities, de-gas under low vacuum (a few Pa), pour again, and so on.
True foam products foam and rise in place (the “debubbling” operation is not applicable).When closedmoulds are used, the quantity of initial product shall be carefully calculated to produce the specified final volume and density. Moulds equipped with overflow vents are also used. The hardening of potting compounds shall be done under a certain cure schedule,which can be as simple as a few hours exposure at room temperature. Sometimes a simple heat treatment is necessary. In other cases, post-curing under vacuum is required. When curing is by atmospheric moisture, the curing time depends on the accessibility of air; thick samples need a longer time. Foams are cured simultaneously with the rising process, and the rate of both should be matched to produce the specified density, implying good control of the curing-temperature profile. When fillers are used, they shall be carefully dried and shall be kept dry until and during mixing.
|Precautions||Many potting compounds create quite high temperatures and pressures during curing, and damage to potted components can occur unless some countermeasure is taken: use of low-temperature, long-duration cure profiles, use of either a flexibilizer as a component of the potting or of a pre-coat on the device, and special design aimed at limiting stresses induced by curing were employed. Pre-coating the device can be essential to obtain good adhesion to all components. When different potting materials or coatings are used successively, it is necessary to ensure that they are “compatible”: some components of one mixture can have a deleterious effect on the curing of the other (the same effect can be found with atmospheric components: water vapour, for example).
Some catalysts used in potting compounds and sealants have corrosive effects on metals (for example dibutyl-tin-dilaurate on copper).Most of them are in one way or another harmful to man and safety precautions are necessary. The need for correct debubbling procedures is stressed above. The viscosity of the mixture to be poured should be low enough to permit a good flow in intricate devices. Most potting compounds and sealants have a limited shelf life and pot life depending on the conditions of storage and use.
Except when special fillers are used, potting compounds and sealants have a rather low thermal conductivity and overheating of enclosed parts can occur in powered devices.
|Hazardous and Precluded||The present trend in space systems is to avoid potting as far as possible and to use conformal coatings in preference. This leads to weight savings and ease of repair, but diminishes the protection against mechanical stresses.
Most of the flexible potting compounds and sealants outgas too much to be useful in space. Polysulphide potting compounds are not stable enough under space environment. Products that shrink severely or are highly exothermic during curing shall be avoided. “Open cell” foams shall not be used, since they do not protect the potted items against corrosion in the atmosphere.
|Effects of Space environment|
|Some Representative Products||Some epoxies, polyurethanes and silicones can be considered for potting as well as conformal coating or adhesion.
The following materials, for which data sheets are included, can be considered: