The present invention relates to sound-deadening material and more particularly, but not exclusively, to sound-deadening material for use in sound deadening within automobiles.

It will be appreciated that there are many situations in which sound deadening must be performed, either for safety reasons or environmental comfort. Previously, wadding-type packing has been used to attenuate -or dampen sound transmission through it. The wadding in effect absorbs the shock waves of the sound.

More recently, fibreglass has been used as the padding material, particularly in situations such as within automobiles where there are temperature and ignition problems. However, fibreglass is quite expensive and furthermore can be difficult to handle in terms of formation and possible detrimental effect on the health of fabricators.

An attempt to overcome the problems of fibreglass padding has been described in International Patent Application No. PCT/GB94/00772 (British United Shoe Machinery Limited) , in which a method of sound deadening in metallic sheet is described using impregnated fabric material formed from a blend of polyester staple fibres. This method has allowed thinner sheets of sound-deadening material to be used and the material used has the ability to be thermoformed to approximate the associated metal panel.

Unfortunately, the method described in Patent

Application No. PCT/GB94/00772 (British United Shoe Machinery Limited) has been found to be subject to degradation after short-term overheating. For example, if an automobile's engine overheats the associated sound-

deadening material becomes distorted and may require replacement.

Typically, a sound-deadening material suitable for automobiles will absorb up to 10% of incident noise at

250Hz, 40% at 500 Hz, 60% at 1000 Hz and 80% at 2000 Hz.

However, optimisation of performance is a matter of design choice and can be determined by gauge/thickness, density, panel spacing and fibre type.

It is an objective of the present invention to provide a sound-deadening material which can withstand short-term elevated temperatures without degradation.

In accordance with the present invention there is provided a sound-deadening material comprising a non-woven fabric impregnated with a mouldable impregnant composition, the non-woven fabric including at least 10% polyester fusible fibres in association with other polyester fibres, the impregnant composition including by weight % at least 15% carboxylated styrene-butadiene with more than 55% of this co-polymer being styrene, at least 5% carboxylated styrene-butadiene with not more than 50% of this co-polymer being styrene and at least 50% of the impregnant composition comprising a mineral filler.

Preferably, the polyester fusible fibres are bi¬ component fibres such as Grilene K170 or Celnet C922. Furthermore, the non-woven fabric includes at least 20% polyester fusible fibres having an average 5.5 decitex.

The impregnant composition may comprise 20-25% of the co-polymer having greater than 55% styrene, 5-10% of a co¬ polymer having less than 50% styrene and the mineral filler comprising 60% of the impregnant.

The mineral filler may be Speswhite 60% clay. The sound-deadening material may include a fire retardant such

as Amgard CU to a weight percentage between 3% and 9%.

The fibres in the non-woven fabric may have an average decitex in the range 1.5-7 decitex

An embodiment of the present invention will now be described by way of example only.

The formation of non-woven fabrics can be achieved in principle by two methods, namely mechanical needling or hydroentanglement. However, whichever method is used, it is entanglement of the staple fibres which produces the non¬ woven fabric or felt, i.e. the fibres remain in position through entangled association with other fibres.

The needling process involves penetrating a web of fibres with barbed needles to a pre-determined depth such that fibres are pushed through the width of the web and thus strata entanglement results. It will be appreciated the more needle strata entanglement the more compacted is the non-woven fabric and thus a greater density achieved.

It will be understood that polyester fibres by their nature are thermomouldable. However, the actual temperature at which a material made from such polyester fibres becomes quite flaccid may be altered by reinforcement using a suitable impregnant. Furthermore, although falling within the same broad polyester group of fibres, there are a wide range of temperatures at which individual fibre types become flaccid, and also the severity of transformation from substantial rigidity to substantial flaccidity is wide.

In the present invention a fusible polyester fibre is used within the non-woven fabric to provide short term resistance to fabric deformation, the fusible fibres ensuring that any moulded component made from the sound- deadening material can withstand temperatures normally associated with the engine compartment of an automobile and

also, for a predetermined time period, temperatures in excess of those normal in such a compartment without deforming. Thus, short term overheating of the engine would not need replacement of the sound-deadening panel due to deformation.

The non-woven fabric in accordance with the present invention provides a framework upon which an impregnant is held. It is the impregnant which provides most of the thermoformability within the sound-deadening material and is tailored to specific performance requirements. Variation in the relative styrene-butadiene co-polymer composition varies the change in viscosity of the impregnant with temperature, and thus the mouldability and resistance to mouldability of the impregnated sound-deadening material. It will be understood that several base carboxylated styrene-butadiene co-polymers can be used in order to produce the appropriate impregnant composition.

in the present invention it has been found most convenient to use two co-polymers, one having a styrene content greater than 55% and the other co-polymer having less than 50% styrene. By appropriate combination of these two co-polymer compositions their necessary impregnant ratios can be achieved.

It has been found to be practical that the impregnant should have the following ranges of styrene-butadiene and mineral filler: the styrene content of the impregnant should e between 5% and 40%, the butadiene content should be up to

22% and there should be between 50% and 70% mineral filler.

In addition to butadiene, styrene and mineral filler, it is usual to add auxiliary fillers and other performance related components, including fire retardant elements and pigments for colouration. Choice of these auxiliary components is determined by performance requirements and can be ascertained by trial and error if necessary.

As indicated above, the non-woven fabric in effect provides a matrix upon which the impregnant is deposited. Thus, it is essential that the fabric is reasonably well condensed in order to support the impregnant. Typically, the fabric will have a weight in the order of 200 g/m ² and a gauge or thickness of 2 mm. Furthermore, several fibre sizes can be used to ensure appropriate performance by the non-woven fabric. Typically, 20% of the fabric may have a decitex of 5.5, 40% have a decitex of 3.3 and the remaining 40% have a decitex of 1.7.

A typical practical sound-deadening material after impregnation will have a weight 650 g/m ² and a gauge of 1.5 mm.

Moulding of the sound-deadening material is achieved by heating the material on a flat sheet until flaccid and then moulding that flaccid sheet in a chilled mould. The temperature of mould is generally determined by fibre type and dimensions. Furthermore, it will be understood that heating generally will be through surface contact conduction. Therefore, scorching or melting must be avoided so as to ensure good sound deadening. It will be understood that a molten layer of fibres will consolidate as a solid coating of limited acoustic attentuation.

Normally, the sound-deadening material is mounted such that there is about a 20 mm air gap between a layer of the material and the panel. This air gap helps to facilitate sound deadening and is determined by expected noise frequency. The present material has a good sound absorption rate at about 2000Hz with a gauge of about 1.5 mm and a weight of 650g/m ² .