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|PRODUCT DATA of 15: Reinforced plastics, PCBs|
|Material||15: Reinforced plastics, PCBs|
|General Information||Reinforced plastics - defined as a reinforcing material, normally a fibre, in a polymer matrix - can be grouped as those used for:
|Use in Spacecraft||Applications for reinforced plastics instructural andsemi-structural uses include:
|Main Categories||The reinforcement phase in polymer matrix composites can be grouped as:
Discontinuous fibres are also available from the same materials (except boron) for non-structural uses.
Reinforcements are rarely supplied as “raw” fibres (other than to companies making pre-impregnated sheet or tapes and doing winding of filaments). The normal forms are yarns or “tows” (containing specified numbers of filaments) or are woven into fabrics of various styles (e.g. plain and satin); felts and mats (of various types) are also available. Yarns and fabrics containing a mixture of thermoplastic composites, hybrids of carbon or aramid reinforcement combined with a high-performance thermoplastic fibre were commercialized to some extent.
Fibres for a particular resin system normally have a specific surface treatment to ensure good bonding to the matrix. The interface characteristics are crucial to achieve load-transfer between matrix and fibre. Fibres for a particular resin system are normally treated with an appropriate size to ensure good bonding to the matrix.
The polymer-matrix phase is usually a thermosetting resin, mainly: epoxies, cyanate esters, phenolics, bismaleimides, polyimides. See also 18.
For structural applications, the most common resins are epoxies and cyanate esters (of various formulations). Higher temperature applications use polyimide and bismaleimide; specialist requirements (e.g. flame-retardant properties) need other resins (e.g. phenolics). A limited number of high-performance thermoplastics were evaluated and commercialised, but to a much smaller extent than thermoset resins.
Reinforced plastics can be supplied as semi-finished items ready for machining to shape (such as, flat laminates and profiles of various simple shapes, e.g. box sections, angles and tubes).
Structural materials are normally supplied as semi-processed forms, the most common of which is “prepreg”, i.e. reinforcement sheet or tape already impregnated with partially cured resin (B-stage). These materials are specifically designed to be sticky (tack) to aid assembly. Prepregs are supplied on a support (backing-sheet) usually as rolls, but sometimes as flat sheets.
Prepregs are limited-life items and therefore strict control of their transport, storage, shelf and working life (also called “out-life”), and of the working environment shall be applied.
|Processing and Assembly||Except where semi-finished products are bought and machined to shape, the processing methods used are an integral part of producing the actual composite material, i.e. the material and the finished part are created at the same time. Unlike metals, which can be subjected to a number of processes to achieve the finished part, once a composite material is produced there are no opportunities to “rework” it to optimize properties, i.e. the properties are “designed-in” at the processing stage. This is why designing for composites is totally different to that of metals, see ECSS-E-ST-32 and ECSS-E-30-04.
Structural components are produced from “prepreg” sheets (plies) or tapes. In continuous fibre prepregs, all the fibres are aligned in one direction (as denoted on the packaging and on the backing-sheet). Depending on the weave style, the principal fibre direction can be denoted for fabrics.
Tooling materials shall be carefully selected to ensure thermal-expansion matching between the composite and the tool over the processing temperatures. Low CTE materials, such as cast iron, certain other metals, ceramics, graphite and composite material tools are used.Thermoset prepreg processing involves the following:
Composite items can also be produced from an individual resin system (base, hardener, catalyst) and combined with a reinforcing agent during the process. There are several different methods: hand-layup or wet-layup; filament winding; near-net shape processes - such as resin transfer moulding (RTM). Some processes do not allow high-reinforcement contents to be obtained, i.e. the resin content is comparatively high. These processes are not normally used for structural components needing optimized mechanical properties for a low weight.
For electronic PCBs, the basic insulation board uses woven glass-reinforced dielectric material. Types G10, G11, FR4, FR5 and polyimide are preferred. Compressed layers with organic fillers shall be avoided.
|Precautions||Most reinforced plastics are anisotropic in all their properties. Design criteria used shall take this fact into account. It is frequently possible to reduce anisotropy by using multi directional reinforcement, but this is done at the cost of a reduction in overall strength or an increase in weight. Reinforced plastics generally retain internal stresses after moulding. These can be relieved by thermal treatment at sub-zero temperatures.
In high-performance structural composites the fibre selection controls the mechanical performance (strength or stiffness) and the resin selection. The resin and associated cure processes largely determine the environmental resistance, e.g. service-temperature; constraints on dimensional tolerances and durability.
Cure schedules or cycles are carefully studied by means of a preliminary test programme during the design and prototyping stage to ensure full and proper consolidation (sufficient resin flow; that the cure is complete; that no thermal degradation of the resin occurs) in order to obtain a final product with optimum properties.
Thermal-analysis equipment can be used to assist in developing appropriate cure schedules.
The main problems in processing are to ensure as far as possible the absence of voids, to maintain the reinforcement in good mechanical condition (high-strength fibres are quite sensitive to surface defects created by handling), and to achieve a good bonding at the fibre interface (use of coupling agent or pretreatment of the fibres).
Assembly methods are of prime importance. Reinforced plastics are sensitive to stress-raisers created by classical fasteners, and hence adhesive bonding is preferred. For guidelines on structural adhesive bonding see ECSS-E-30-05.
Where mechanical fastening is needed to attach composite parts to other parts of the structure, special fasteners offering a large load-transfer areaare used: inserts (a removable threaded fastener and its fixture -- normally light-alloy – embedded and potted into the panel) are used for assembly of honeycomb panels. For guidelines on the design with inserts see ECSS-E-30-06.
Failure of reinforced plastics occurs frequently at the fibre or matrix interface. This type of failure can be accelerated by some terrestrial environments (e.g. high humidity). Carbon-reinforced resins generally show water absorption or desorption associated with dimensional changes. Low moisture-expansion resin formulations were introduced.
Galvanic coupling is a consideration for carbon-fibre reinforced composites when they are attached to metals or have a coating applied to act either as a moisture barrier, as ATOX protection or for optical properties. In galvanic couples, carbon-fibre composites usually behave as the cathode causing the metal or coating (often a metal) to corrode.
|Hazardous and Precluded||Polyester laminates are not generally suitable for space uses. Some reinforcements appearing in ground electronics, such as cotton and paper, also shall be rejected.
Polyimide or polybenzimide resins are applied to prepregs with the use of a low-volatility solvent, traces of which can stay in the cured item: this sometimes renders them unsuitable. All designs directly translated from classical metal design concepts shall be avoided: designers working with new products shall revise their usual way of thinking.
|Effects of Space environment|
|Some Representative Products||High-performance reinforcing fibres are generally known by their trade names. There are a number of European sources; many products have an American or Japanese origin or link. In addition to the large companies, there are a number of independent weavers, providing fabric reinforcements of various styles. The following list of sources is by no means a comprehensive list of what is available.
See also ECSS-E-30-04 for information on carbon fibre-reinforced plastics (CFRP), aramid fibre-reinforced plastics (ARP) and glass fibre-reinforced plastics (GFRP) and other non-standard materials.
NOTE GFRP is used to denote composites using high-performance glass reinforcements, whereas GRP usually refers to other more “industrial” grades.
PCBs used in space hardware shall be qualified in accordance with ECSS-Q-ST-70-10.