High Performance, Heat Cured Adhesives and Sealants in Body Construction 920314
Adhesives and sealants used in the construction of automobile body shells are not only required to meet the stresses and strains encountered in every day use, but also to maintain structural integrity during a collision, in which the energy absorbing, controlled collapse of the motor body is designed to reduce deceleration, thereby protecting the passengers. Further demands are the needs to enhance the life of the vehicle by improved corrosion resistance, and reduce transmitted engine and road noise giving a smoother and quieter ride using materials that are safe and will not cause harm to the environment. This paper describes the use of vulcanizing rubber based adhesives and sealants and “supertough” epoxy adhesives, developed in partnership with the automotive industry to meet these demands.
On the one hand, there are many different joints in today's automobiles presenting a broad spectrum of performance demands that require varied solutions, whilst on the other hand vehicle manufacturers are opposed to the needless proliferation of adhesive and sealant types, and would prefer to have a small number of materials capable of fulfilling all their needs. There is an increasing demand for truly structural adhesives capable of supporting continued stress without creep under load. Many joints are subjected to occasional or transient stresses where resistance to creep is less crucial; such joints include the bonding of reinforcements to hood, roof and trunk panels where sufficient compliance to damp out vibrational noise, combined with a sufficiently high modulus to give adequate component rigidity, is paramount. Finally there are joints where there is negligible load bearing capacity and the primary requirement of the adhesive or sealant is to prevent the passage of air or water or to keep metal components together.
In the controlled collapse situation the adhesive may be subject to impact cleavage; to design materials for this eventuality it would be useful to know the speed of the impact to be withstood which would be expected to be sensibly defined by the conditions that a human occupant of the vehicle, properly restrained by a seat belt, is capable of surviving. This is not a simple matter to determine (1) since the damage sustained by individuals will vary from person to person and is a function both of the deceleration (g force) experienced and the duration of deceleration. Both of these factors will be determined by the speed of the vehicle prior to impact and the length of the crumple zone. In addition, as speed increases, the complexity of the accident will be likely to be greater, which, no doubt, explains why both regulators and automotive manufacturers have concentrated firstly on low speed (8 to 50 km/h) collisions. This speed range corresponds to about 2 to 13 m/s and impact testing of bonded joints has been conducted at the bottom end of this range. Virtually all the adhesives and sealants used in body construction are heat curing materials designed to cure as the vehicle body traverses the paint shop. In recent years these have included structural adhesives, though, except in some of the experimental vehicles, few of the applications are truly structural and are more likely to be the bonding of hem flanges or other joints where, the adhesive's ability to improve vehicle stiffness is exploited. Stiffness will be considered in more depth later. The bulk of adhesive applications are, at most, semi-structural but are nevertheless very important. Nearly all the applications are concerned with stiffening and reinforcing the body, in many cases where spotwelds cannot be used. Some typical applications are illustrated in figure 1.