This invention relates to rubber compositions having vibration damping and noise insulating properties, and being suitable for use as engine mounts.
The function of engine mounts is to insulate the transmission of vibration from the engine to the frame and passenger compartment of an automobile. The operating temperature of the engine mount can vary in the range from about 25.degree. C. to about 80.degree. C. Therefore, materials having relatively high damping coefficients in this temperature range are desired for engine mount applications. The materials used for engine mounts should also have, preferably, noise insulating properties. Engine vibration is particularly troublesome in the low frequency region (10-20 Hz) while the high frequency region (above 75 Hz) contributes to the generation of noise in the passenger compartment.
lt is desirable that the damping coefficient of the composition (also referred to as tan delta), which is indicative of the vibration damping character of the composition, be relatively high (i.e. highly absorbing) and uniform at least in the 25.degree.-80.degree. C. operating range, and secondly that the ratio of dynamic to static moduli, which is indicative of the noise transmitting property, should remain relatively low. It is to achieving this balance of properties that this invention is directed.
Various rubber compositions have been proposed for engine mount applications. Most commonly, blends of natural rubber with halogenated rubber, e.g. brominated butyl rubber, have been used. Natural rubber is a good noise transmission insulator but a poor vibration damper. Bromobutyl rubber is, on the other hand, a poor noise insulator while it has good vibration damping qualities. The above blends are, therefore, a compromise between these two properties.
Vibration damping is usually considered to be the primary function of engine mounts and noise insulation the secondary function.