Fire resistance

In the UK a Class 0 resistant laminate to BS 476 is required for claddings on buildings above I 5 m high, or Class 2 or 3, within I m of the boundary. In order to provide this degree of fire resistance, it has been usual to improve the fire retardancy of polyester resins by:

- the addition of fillers;

- the use of additives, such as antimony oxide plus chlorinated paraffin, which can be mixed up with the resin;

- building in certain groups in the chemical structure of the resin, for example dibromone-opentyl glycol, hexachloro-endomethylene,tetra-chlorophthalic acid, chlorostyrene.

It has been found, however; that the weathering qualities of the laminate produced in these ways are reduced in positions exposed to ultraviolet rays, and even white will yellow with age.

An additional way is to coat the internal surface of the GRP structure with an intumescent coating. Intumescent polyesters are now available, which contain compounds designed to give a carbonaceous foam on application of a flame. However; all methods used so far in trying to improve the fire retardancy of polyester resins have considerably lowered the GRP's resistance to breakdown (loss of gloss, colour change) when exposed to the outdoor environment. Tests have shown that on one common type of fire-retardant GRP sheet, using tetrachlorophthalic acid polyester as an inhibitor; the surface deterioration was 2.5-3.0 times faster than that of sheets based on general-purpose/conventional polyester resins.

Another solution to the problem of providing the required fire rating has been to apply a two-pack polyurethane surface coating in order to protect the

Mondial House
3.7 Mondial House, London, demolished 2006 (architects: Hubbard Ford & Partners).

laminate from the effects of ultraviolet light: for example, An-o-clad by Anmac Ltd.This uses a modified gel-coat system, which in itself adds to the fire characteristics of the panel and is then coated with the two-pack polyurethane to give good weatherability. This system, when tested to BS 476 Part 7 for a Class 1 test, had 'nil' spread of flame on the test samples. Good resistance to the breakdown of the gel coat is also achieved by coating GRP panels with a lacquer based on an ultraviolet-stabilized acrylic resin.

A more usual solution is either to position the building away from the boundary or to ask for a relaxation to the Class 0 fire requirement. In this case a general-purpose isophthalic gel coat can be used, which itself has no fire-retardant characteristics, but which protects the laminating resin containing the fire-retardant additive from ultraviolet light. But, of course, it does not get a Class 0 fire rating.When applying for such a waiver it may be necessary for tests to be carried out that stimulate in-service conditions on full panels, including the appropriate jointing technique used.The configuration and design of the joints must significantly affect the performance of the panels during a fire.The panels for Mondial House (Fig. 3.7), designed by Peter Hodge Associates, and manufactured by Anmac Ltd and Brensal Plastics Ltd, were tested by Yarsley Laboratories. Two sets of fire tests were performed, the first on tested coupons in accordance with BS 476, the second on two complete panels with a joint, set up in the attitude to be used in service. A large gas flame with heat output of I 90 W/min was set up beneath the bottom of the panels. The results of the test showed that there was no uncontrolled spread of flame over the surface, and even after an hour's exposure the panel still retained its form.

This project was probably the largest structure erected using GRP cladding in the UK, and the inclusion of mini-rib finish to these panels resulted in them still maintaining their appearance 25 years after installation.

Thus the two main problems associated with GRP that of weathering (especially as a result of ultraviolet degradation) and fire resistance, tend to work against each other. The addition of fillers and pigments to reach some sort of fire resistance acceptability through tests laid down in the Building Regulations tends to weaken the material's capacity to resist weathering, and a darkening of the surface pigments may take place.

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