Weathering

The selection of colour can significantly affect the overall weatherability of the panels. In general, the stronger colours, such as oranges and reds, have a tendency to fade under the effect of ultraviolet light; the ultraviolet light will cause the surface of the material to chalk. Architects often specify these colours in ignorance of the problems associated with some pigments, and all too often the manufacturers will supply the colours as requested.The current fashion for oranges, browns and reds has exaggerated this difficulty, which is not confined to GRP claddings.

In general, colouring of the GRP panels is best done by pigment in the resin or gel coat, and not as an applied finish. However; Blaga (1978) claims that good resistance to the effects of moisture- and/or temperature-induced stress fatigue is achieved by coating the GRP sheets with a lacquer based on ultraviolet-stabilized acrylic resin:

The acrylic acting protects the glass-resin interface against the effects of stress fatigue and the underlying matrix against the action of UV light. This type of coating may be particularly useful for fire-resistant GRP sheets, which are especially susceptible to breakdown in outdoor exposure. Similarly, GRP sheets protected with a UV-stabilized (in-plant laminated) PVF surfacing film (0.025 mm thick) have remarkable resistance to the effect of weathering.

The difficulty may be to achieve a permanent bond between the laminate and this polyvinyl fluoride film (Tedlar).

No completely satisfactory method of assessing the weathering resistance of plastics materials in service by laboratory simulation has yet been found. Weatherometer testing and xenon arc testing are popular methods, but these can, at most, give only an identification of performance in use. After I 000 hours' exposure to the xenon test, any change of colour should be moderate and uniform.

Blue GRP panels with red fascias were used at the Water Research Centre, Swindon. They have suffered from some colour fading (Fig. 3.8).

Colour fastness tends to be uniform within any one batch of panels, but can vary from batch to batch. One method of anticipating differences in colour

3.8 Blue GRP panels with red fascias used at the Water Research Centre, Swindon.

fastness between panels Is to use a striped effect, as at the Olivetti factory, Haslemere (architect: James Stirling) (Fig. 3.1 0).The colour of the outer skin can also increase any possibility of delamination of sandwich panels due to temperature build-up on the surface. The darker the colour and the higher the temperature, the greater the risk of delamination. Interior use of GRP is usually successful.

In theory, minor blemishes in GRP can be made good on site, provided there are correct weather conditions, and a highly skilled labour force is available. In practice, such conditions hardly ever exist, and, as yet, methods of curing on site are not available.Thus normally the panels are returned to the factory for repair To prevent accidental damage on site, it is important to protect the panels during assembly. Panels for the Herman Miller factory, for example, were protected with a plastic coating while being installed (Fig. 3.1 I).

Surface scratching, which can be caused by something as simple as a window cleaner's ladder that has no padded protection, produces unsightly defects.

3.9 Ventilated top-hat stiffener to prevent distortion during manufacture.

The client should be warned that if weather is allowed to get to the back of a panel, the uncoated side will deteriorate. Where panels such as balustrades are exposed to the weather on both sides it is necessary to gel coat both inner and outer surfaces. Condensation on the back face of a panel that has no gel-coat protection on its inner skin can also lead to premature breakdown of the laminate if the glass fibres are exposed at the surface.

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