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PRODUCT DATA of 15: Reinforced plastics, PCBs
Material15: Reinforced plastics, PCBs
General InformationReinforced plastics - defined as a reinforcing material, normally a fibre, in a polymer matrix - can be grouped as those used for:
  • structural applications;
  • electronic uses.
The reinforced plastics within each group have very different mechanical and physical properties dictated by the fibre reinforcement (material and form), the reinforcement content and orientation and the polymer matrix used to support the reinforcement fibres.
Use in SpacecraftApplications for reinforced plastics instructural andsemi-structural uses include:
  • honeycomb facings,
  • antennas, trays,
  • structural members,
  • fairings,
  • spacecraft skin,
  • solar cell substrate.
The substrate materials of electronic printed-circuit boards are made from reinforced plastics; equipment housing can be composite rather than metal.
Main CategoriesThe reinforcement phase in polymer matrix composites can be grouped as:
  • long, continuous fibres, unidirectional or woven,
  • short (discontinuous) fibres, sometimes "chopped" to a specific length or asfelts and mats; or
  • powders and other forms of fillers;
Other forms of reinforcement, such as whiskers and metal wires, are normally used in composites with a metal matrix phase (see also aluminium-, magnesium-based alloys in subclause A.8). Natural materials (cotton and paper)
  • used for some composites for electronic laminates - are unacceptable for space applications.
Common commercial materials for continuous reinforcing fibres used in structural applications are:
  • Carbon - grouped by their dominating mechanical properties: ultra-high modulus (UHM), high modulus (HM), intermediate modulus (IM), high NOTE Some overlap exists between categories, especially for IM grades which are often selected for strength and strain, rather than stiffness.
  • Aromatic polyamide fibres (aramid).
  • Glass - high-performance grades.
  • Boron (to a lesser extent) – these have a larger cross-section than the other fibres (normally known as "filaments"). There are two types: boron deposited onto thin tungsten wires, or onto a carbon fibre substrate.
These fibre-types offer the high-strength and high-modulus properties necessary for structural applications. Glass fibres are usually used for their electrical characteristics, e.g. dielectric, rather than mechanical performance alone. Carbon fibres are conductive, whereas aramid fibres are not. Other polymer-based fibres were proposed, but were not generally evaluated for space.

Discontinuous fibres are also available from the same materials (except boron) for non-structural uses.
NOTE: The use of asbestos is discontinued because of its carcinogenic nature.

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.
NOTE: Polyimides are really thermoplastic ladder polymers, but are included here.

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 AssemblyExcept 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:
  • Tool preparation: e.g. cleaning and applying any materials to release the finished composite from the tooling.
  • Cutting prepreg to size. Removing the backing-sheet.
    NOTE Backing sheets are present only as a handling aid; they are normally plastics films chosen for their “ease-of-removal” characteristics. Their complete removal is critical – any backing-sheet left in consolidated composites means that the plies do not bond together and the properties are seriously impaired.
  • Laying up of the plies in the correct order and in the correct direction: by placing cut plies on top of one another (manually or automatically). Tapes are sometimes “wound” around formers (tools) or wrapped around an existing part (overwrapping).Some designs use a mixture of continuous fibre plies and fabric plies; others can use different types of fabrics.
    NOTE The fibre direction dictates the final mechanical performance of the final composite material or part.
  • An interim consolidation: used during the lay-up stage, by applying a pressure to remove air trapped between plies. For thick sections a vacuum-assisted debulking process can be utilized.
  • Preparation for curing: depending on the process used, other materials are applied to the lay-up (e.g. release films, vacuum-bagging consumables). Peel plies are “disposable layers” that are used on areas of the lay-up that need protection from contamination during processing, or areas to be adhesive bonded or have a coating applied.
  • Curing: needs heat and pressure applied for a specific time (resin-dependent and, normally, to a defined cure schedule for the part (heating and cooling rates; hold-at-temperature called “dwell time”; when pressure is applied and released).
    NOTE Thermoplastic composites are not cured, but they are consolidated under a temperature or pressure cycle. The other process steps are appropriate. After producing the composite, reheating and forming processes can be used to shape the laminate (e.g. press- and vacuum-moulding techniques).

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.

PrecautionsMost 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 PrecludedPolyester 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
  • Thermosetting plastics are in general quite stable under space conditions if the comments already made are borne in mind when they are selected.
  • Vacuum can lead to outgassing. This does not generally degrade the properties of the polymer, but can raise corona or contamination problems in the vicinity.
  • Radiation at levels existing in space is unimportant. In fact, there are some structural reasons for using reinforced organic materials to replace metals where Bremsstrahlung is a problem, i.e. around sensitive electronics.
  • Thermal effects are most noticeable, especially problems raised by the thermal anisotropy of most reinforced plastics (expansion varies with the direction). Microcracks are formed in thermal cycling which could jeopardise long-term properties. The temperature range within which reinforced plastics can be used is similar to that for adhesives of the same chemical nature
  • Atomic oxygen etches classical reinforced plastics and can cause damage to thin structures. Since resin is generally etched more quickly than fibres, fibre fragments can be released and contaminate the environment.
Some Representative ProductsHigh-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.

Carbon fibres:

  • Akzo Fortafil - USA,
  • Amoco - USA,
  • Enka AG (Akzo) - Europe,
  • Hercules - USA,
  • Mitsubishi Chemical Corp. - Japan,
  • R.K. Carbon Fibres - Europe,
  • Sigri GmbH - Europe,
  • Soficar SA (Toray Industries Inc.) - Europe,
  • Tenax - Europe,
  • Toho Rayon Co. Ltd - Japan,
  • Toray Industries Inc. - Japan,
  • Zoltek - USA.

Aramid fibres:
  • Primary sources:
    • Kevlar - DuPont de Nemours (USA and Europe),
    • Twaron - Akzo Fibers and Polymers Div. (Enka AG) - Europe.
  • Others:
    • SAPEM --Anglo-Soviet Materials Ltd. - Russia,
    • Schappe SA - France,
    • Teijin Ltd. - Japan,
    • Toray Industries Inc. - Japan.
Boron fibres:
  • Composites Incorporated - USA,
  • Textron Speciality Materials - USA,
Glass fibres (high-performance grades S-, R-, D-glass and TE-grade):
  • Owens Corning - USA and Europe,
  • Nitto Boseki Co. Ltd. - Japan,
  • Vetrotex St. Gobain - France.
Thermosetting prepreg materials (various fibre reinforcements or resin combinations). Main suppliers of aerospace materials with product ranges available in Europe, include: ACG - UK, AIK - Germany, Bryte - USA, Cytec Engineered Materials - USA, FiberCote --USA, Hexcel - USA, Structil - France, YLA - USA. See also 18: class for resins Thermoplastics for fibre-reinforced plastics:
  • PEEK (polyetheretherketone): Victrex - UK,
  • PES (polyethersulphone): Victrex - UK,
  • UDEL (polysulfone): Union Carbide - USA,
  • ULTEM (polyetherimide): General Electric -USA.
See also 17: class.

PCBs used in space hardware shall be qualified in accordance with ECSS-Q-ST-70-10.
NOTE A list of qualified manufacturers is maintained by the QT Division, ESTEC.