Rubber

The value of rubber lies in the fact that it can be readily moulded or extruded to any desired shape, and its elastic quality makes it capable of filling unavoidable and irregular gaps and clearances. It is an ideal material in door shuts and as the gasket for window glass,and in both instances it provides the means for excluding dust and water,although with windscreens and back-lights additional use has gen- erally to be made of a sealing material. Rubber specifications have been built on the basis of the properties of material which has given satisfaction.
One major difficulty has been to ensure and measure resistance to weathering; rubber is subject to oxidation by ozone in the atmosphere,and this results in cracking. In addition to natural rubber,a variety of types of synthetic rubbers are used by the motor industry; these vary in price and characteristics,and all are more expensive than natural rubber. For complete ozone resistance,it is necessary to use either butyl or neoprene rubber; both satisfy atmospheric and ozone ageing tests. Butyl rubber,however,is ‘dead’to handle and contains no wax,and so whilst neoprene is costly its use is essential for some parts.

Sponge sealing rubbers can be provided with built-in ozone resistance by giving them a live skin of neoprene,and a further way of providing ozone resistance is to coat the rubber components with Hypalon; more recently,continuously extruded neoprene sponge has been adopted. Apart from weather resistance,the important requirement of door and boot lid seals is that their compression characteristics should ensure that they are capable of accommodating wide variations in clearance, without giving undue resistance to door closing.
Various types of foam rubber have been evolved to suit the different parts of the car seating,and the designer’s choice of material is governed by cost, comfort,durability,the type of base,the type of car,and whether it is a cushion or a squab,a rear or front seat. When considering the foams available,it is apparent that the number of permutations is large. The types of foam available today fall into seven broad categories. These are:

Moulded latex foam
Low-grade fabricated polyether
Fabricated polyether
Moulded polyether
Fabricated polyester
Polyvinyl chloride foam
Reconstituted polyether.

Latex foam today utilizes a mixture of natural and synthetic latexes to obtain the best qualities of both. After being stabilized with ammonia,natural latex is shipped in liquid form to this country from
Malaysia,Indonesia and other rubber producing countries. Synthetic latex,styrene butadiene rubber, is made as a byproduct of the oil cracker plants. Polyether foam can now be made in different grades, and the physical properties of the best grades approach those of latex foam. As a general guide, service life and physical properties improve as the density increases for any given hardness. As the cost is proportional to weight,it follows that the higher-performance foams are more costly.
Flexible polyurethane foam seats are replacing heavy and complicated padded metal spring structures. Moulded seats simplify assembly,reduce weight and give good long-term performance. Amajor innovation here has been the cold cure systems. These produce foams of superb quality, particularly in terms of strength,comfort and longterm ageing. The systems are particularly suitable forthe newer seat technologies such as dual hardness, where the wings are firm to give lateral support, leaving the seat pad softer and more comfortable.