It has long been known that interposing mass between a sound source and the area to be kept quiet is an effective means for attaining sound deadening. A sheet of lead is thin, flexible, often highly effective, but costly. The challenge, then, is to attain dense, thin, flexible sheet which can be interposed between a source of noise and the area to be quieted.
Sheets of thermoplastics or of rubber-like materials have long been used as sound-deadening means. To make the sheets flexible, dense, strong, and inexpensive has posed a challenge to compounders for many years. For some uses, such as automobile carpet underlayment, it is desirable that the sound-deadening sheet is also formable.
Sound-deadening sheets made from composites of filled thermoplastic compositions are well known for use in such automotive applications. The thermoplastic compositions typically comprise one or more polymer, a filler, and optionally a plasticizer. A number of patents have been granted for such highly filled thermoplastic compositions as represented by U.S. Pat. Nos. 4,191,798; 4,222,924; 4,263,196; 4,379,190; 4,403,007; 4,430,468; 4,434,258; 4,438,228; 6,472,042; and 6,787,593. Different systems of polymers and fillers with and without plasticizers have been proposed. For example, the above-mentioned patents disclose the use of ethylene interpolymers such as ethylene/vinyl ester, ethylene/unsaturated mono- or di-carboxylic acids, esters of unsaturated acids, metallocene catalyzed ethylene-alpha olefin interpolymers, etc. Blends of such ethylene interpolymers with other elastomers and polymers have also been proposed.
Notwithstanding the proposal of many different polymer based compositions for use in sound-deadening composites, ethylene vinyl acetate has been widely used on a commercial basis. For sheet or flat applications, such materials demonstrate adequate modulus while meeting a number of balanced properties such as impact strength, tensile, elongation, flex modulus and specific gravity. However, where the sound-deadening composition or composite is used in certain applications, such as flooring applications, where formability is a key requirement, ethyl vinyl acetate based compositions lack sufficient heat performance and stiffness requirements. Particularly useful compositions comprising an olefin block copolymer and a filler which demonstrate good elasticity and melt strength which are ideally suited for thermoforming articles demonstrating improved noise, vibration, and harshness properties are disclosed in U.S. Pat. No. 7,741,397, which is incorporated by reference herein in its entirety. However, these compositions may not meet more demanding heat performance and stiffness requirements for injection molding applications, which may, for example, result in binding or inadequate release from injection molds. It would be desirable to have a composition or composite having both heat performance and stiffness such that sound-deadening applications, especially into sound-deadening articles for use in vehicles, may be proficiently produced by an injection molding process.