Guide to Glass Reinforced Plastics
GRP (glass reinforced plastic) is a composite of tough resilient, durable plastic resin, and glass fibres of remarkable strength. The resin is a thick, treacly substance which when activated by an appropriate catalyst, sets to a hard but brittle solid. It can be used alone for small castings, or with a variety of fillers, but, when reinforced with glass fibres, becomes a material of exceptional strength and versatility.
The Resins most commonly used in GRP are unsaturated polyesters dissolved in styrene. The polyesters are produced by reacting various organic acids (usually phthalic or maleic anhydrides) with an alcohol such as propylene glycol or ethylene glycol. Depending on the particular alcohol or acid used, various types of resin can be produced-indeed, since a wide variety of both alcohols and acids are available, it is possible to have polyesters tailor-made to specific requirements.
The Polyester/styrene solution sets to a hard, rigid substance, a co-polymer of polyester and styrene, by the cross linking of molecules (i.e. polymerisation). The hardening process is commonly referred to as ‘curing’ Partial cross-linking occurs spontaneously, thereby limiting the storage life of the resins, but, for the process to take place quickly and completely it has to be activated by two additives. One is a catalyst, which triggers the process and the other is an accelerator, which- as the name implies-speeds it up. Typically, the catalyst is an organic peroxide (e.g. methyl ethyl ketone peroxide) and the accelerator is normally cobalt naphanate. These two substances, if mixed directly together will react violently , even explosively. It is therefore , essential to ad the accelerator to the resin first ( stirring in thoroughly) and then mix the catalyst. In practice, many resins are supplied ‘pre-accelerated’- with the accelerator added by the manufacturer- so that only catalyst is needed to activate the curing process.
Internal heat (’exotherm’)is generated within the resin during curing, and can reach relatively high temperatures about 170oC is typical.
The amount of heat generated normally varies according the quantity of catalyst used, the volume of resin and other factors such as the presence of fillers and reactivity of the resin.
The curing of polyester resins takes place at room temperature, preferably about 18°C-20°C, and this is one of the great advantages of the material. Most other plastics require heat or pressure (or both) so that their use in any manufacturing process demands expensive and complex machinery. They are therefore only economically viable for producing high volume runs of relatively small articles. As polyester resins can be cast at room temperatures with the simplest equipment , they are practicable for low-volume production, and even for one-off articles with virtually no size limit. Although the cured resin is very hard, it is also quite brittle, which could prevent its use for large articles but this problem is obviated by the use if reinforcements. A large number of materials can be used to reinforce the resin, but in practice, the one offering the best combination of strength, versatility and economy is glassfibre.
Specification of A typical cured Polyester resin, Without Reinforcement
Specific Gravity: 1.28
Tensile Strength: 55 MN/M2
Compressive Strength: 140 MN/m2
Youngs Modulus: 3.5 Gn/M2
Elongation at Break: 2%
Specific Heat: 2.3kJ/kg0C
Thermal Conductivity: 0.3 W/moC
Coefficient of linear expansion: 100 x 10 -6/oC
Water absorption: (24 hr at 20oC): 0.15%
Voltage breakdown (0.2mm sample): 22kV/mm
The Glass used commonly for GRP is a calcium-alumina borosilicate with an alkali content of less than one per cent. It is commonly known as ‘E’ type glass, since it was originally developed for use in electrical insulation systems.
Glassfibres are produced by running molten glass from a direct melt furnace into a platinum alloy bushing containing a large number of small holes, from each of which a glass filament is drawn. Filaments for commercial use are normally between 9 and 15 microns in diameter. The filaments are “dressed” with an emulsion before being gathered into fibres. The fibres are remarkably strong-the tensile strength being particularly high. They also exhibit good chemical and moisture resistance, have excellent electrical properties, are not subject to biological attack and are non-combustible with a melting point around 1500oC-all excellent qualities in a plastic reinforcement.
The fibres can be used in a variety of ways-chopped into short lengths(“chopped strands”); gathered together into loosely bound ropes (“rovings”); woven into a variety of fabrics, produced from yarn made by twisting and doubling continuous strands. In the UK, the most widely used glassfibre material is chopped strand mat, which consists of glass strands chopped together in short lengths (approx. 50mm) and held together in mat form by a polyvinyl acetate or polyester binder. The mat is available in a range of weights, from 225gm2 to 1200gm2, and is a useful general purpose reinforcement.
Provided the glass reinforcement has been thoroughly impregnated with resin, the result, after curing, is a cohesive completely integrated matrix of resin and fibres. The matrix can have a surprising range of properties, depending on the type of glass material and the formulation of the resin. In general, the GRP laminate will display excellent tensile and compressive strength, acceptable thermal conductivity, a low coefficient of linear expansion, reasonable chemical resistance and good dielectric properties. Compared to other materials of equivalent strength, it will be light durable, moisture-resistant, non-rusting and economic.
This information is, to the best of our knowledge, true and accurate. Recommendations are made without warranty or guarantee. Users are advised to make their own tests to determine the suitability of specific materials or methods