This invention is concerned with moulding compositions which contain nylon, e.g. nylon 6 or nylon 66, and is especially concerned with such compositions which contain magnesium hydroxide as a flame retardant.

Nylon is widely used for moulding components. Nylon 6 is produced from caprolactam with the polymerised product containing about 90% of nylon and about 10% of low molecular weight material such as the monomer. Before such a polymerised product is used for moulding, it is washed to remove the monomer. The unwashed product is known as "unwashed nylon" or as "high monomer nylon". In some moulding applications, it is desirable that the moulded component exhibits flame retardant characteristics. Such applications include electrical lamp-holders, and switch housings.

Magnesium hydroxide is a flame retardant which has been suggested for use with nylon. This material, when subjected to a flame, decomposes endothermically with the liberation of water. Nylon containing magnesium hydroxide has been found to be difficult to mould because of its high melt viscosity and, furthermore, it is found that the mouldings have obvious surface defects which render them unacceptable commercially. Such surface defects manifest themselves by the appearance of white patches on the surface of the moulding. Various attempts have been made to overcome this problem but without achieving complete success. The most successful previous attempt known to the applicants involves the addition of organic sulphonamide

(see GB 2,237,573A) but even this has not achieved total success.

It is an object of the present invention to provide a nylon moulding composition containing magnesium hydroxide as a flame retardant which produces mouldings which are substantially free of surface defects of the type referred to.

The invention provides a moulding composition containing nylon and magnesium hydroxide as a flame- retardant, characterised in that the composition also contains sufficient caprolacta to reduce the formation of surface defects in the moulding.

Mouldings made from a composition according to the invention are found to be substantially free of surface defects. Furthermore, such compositions are also found to be much easier to mould.

As nylon is prepared from caprolactam, the caprolactam can be provided by the use of unwashed nylon in the composition, possibly with the addition of further caprolactam. Alternatively, if the nylon has been washed, the caprolactam is added. The caprolactam may be present in up to 10% by weight e.g. between 1% and 5%.

A composition according to the invention may contain up to 60% by weight of magnesium hydroxide e.g. between 45% and 65%. Alternatively, if a lower flame retardant standard is acceptable, the composition may contain 20-45% by weight of magnesium hydroxide. The composition may also contain reinforcing fibres, e.g. glass fibres, which may be present in up to 35% by weight depending on the amount of magnesium hydroxide which is present.

There now follows a detailed description of two moulding compositions which are illustrative of the invention.

The first illustrative composition contained 37.01% by weight of unwashed nylon which contained about 10% of caprolactam and about 2.5% of water. The composition also contained 0.30% by weight of calcium stearate (as a mould lubricate), 9.82% of glass fibre as a reinforcement, and 52.87% of magnesium hydroxide as a flame retardant. The constituents were mixed together in a blender and fed into a compounder through the main hopper. The compounder was a Baker Perkins 2030 twin screw extruder operating with a barrel temperature of 260°C (feed region at 180°C) , a feed rate of 9 to 12 kg/hour, and a screw speed of 250 RPM. The material was found to be easy to extrude and the extruded strand did not break.

Test rods were injection moulded from the first illustrative moulding composition using a Battenfeld BA 750 CD Plus injection moulding machine. The barrel temperature was 280°C and the mould temperature 115°C. The test rods were easy to mould and were found to have a very good surface finish with hardly any defects.

The second illustrative composition contained 34.98% by weight of the same unwashed nylon as the first illustrative composition. It also contained 0.30% of calcium stearate, 14.75% of glass fibres, and 49.97% of magnesium hydroxide. It was mixed, compounded and injection moulded into test rods in the same way as the first illustrative composition. The mouldability and surface finish were substantially the same as for the first illustrative composition.

The second illustrative composition was moulded into lamp-holders and the surface finish was found to be

excellent. These lamp-holders also passed the Underwriters Laboratory Inc. flammability test (UL 94 test) with a rating of Vo at 1.5mm and had good impact strength.

For comparison purposes, a moulding composition falling outside the invention was mixed, compounded and injection moulded into test rods under the same conditions as the first and the second illustrative compositions. This composition contained 35.88% by weight of washed nylon 6, 0.35% calcium stearate, 10% glass fibre and 53.82% magnesium hydroxide. The material was difficult to extrude as strand breakage occurred, and was difficult to injection mould taking 10% longer. The test bar surfaces were covered in white marks.