Sound Deadening

This invention relates to the sound-deadening of articles such as covers for internal combustion engines.

We have already proposed a constrained layer sound deadening treatment for engines in which a thin layer of a nitrile rubber-based adhesive is sprayed on to at least part of the surface of the engine, and a layer of lead or lead alloy is sprayed on to the rubber.

A particular advantage of this method of sound deadening is that it involves the use of spraying techniques that are conventional and well-established and are suitable for automatic production line treatment. However, the use of lead presents a health hazard, and nitrile rubber has a tendency to age and flake when sprayed with metal.

According to the present invention therefore, we propose to provide a sound deadening coating on an article by applying a polysulphide, silicone or polyurethane rubber layer to the surface of the article and applying a layer of zinc, aluminium or ferrous metal or their alloys to said rubber layer.

Polysulphide, silicone and polyurethane rubbers have good acoustical and physical properties, and chemical resistance to fuels and oils, and are not prone to flaking. Polysulphide rubber is the preferred material and a suitable example is PR6422A manufactured and marketed by "Berger Elastomers." A suitable silicone rubber is an R.T.V. (Room Temperature Vulcanising) silicone rubber.

Zinc, aluminium and ferrous metal and their alloys are all safe metals to handle and are less costly than lead. Zinc is the preferred material because it bonds well on the rubber layer, particularly, polysulphide rubber. When using aluminium and ferrous metals, these may be applied directly to the rubber, or may be applied to a thin layer of zinc that is first applied to the rubber. The ferrous metals include steel.

The thickness of the layers of rubber and metal can be selected in a known manner to give maximum attenuation of surface vibration and hence maximum sound deadening. For example, in the case of cast aluminium and pressed steel engine components treated with a coating of polysulphide rubber and zinc, the optimum thickness of the polysulphide rubber is 0.5 to 1.0 mm and the optimum thickness of the zinc is 3.0 to 4.0 mm for the cast aluminium components and 1.5 to 2.0 mm for the pressed steel components.

The rubber may be allowed to cure fully before applying the metal layer to it, the rate of curing being controlled by the ambient temperature. Alternatively, in order to speed up the treatment process and possibly to improve the bonding between the rubber and metal, the metal may be applied to the rubber when the latter is only partly cured.

Preferably, the rubber and metal layers are sprayed on. The layers may be sprayed by operators using hand-held tools or by automatic machines.

In a typical example, the timing cover of an engine is treated according to the engine by spraying with a polysulphide rubber layer and then a zinc metal layer.