Sealers

The history of sealers is longer than that of the motor car. Mastic,bitumen compounds and putties of various kinds have been used since the invention of the horseless carriage. It is likely that early coach builders used putties of some kind – possibly paint fillers – to bridge joints in various applications on motor bodies,but it is generally conceded that the first use of specialized sealers on a large scale was in the early 1920s when,in America,the pressed steel body became popular. In this country it was 1927 before one of the first truly effective sealers was introduced. It was known as Dum Dum and is still in use. It was a modified roof sealer,and proved to have many applications in body production. It was not until the late 1930s,when all-steel bodies and unitary construction became a common feature of mass produced cars,that more thought was given to the points that required the use of sealing compounds,and to the nature of these products. Amongst the first developed was interweld sealing compound,primarily to prevent corrosion.
Since then,particularly in the post-war years,there have been remarkable developments,probably accelerated by criticisms from overseas markets that British cars were susceptible to dust and water entry. Companies specializing in the manufacture of mastic compounds have developed a range of materials which are now used not only for welding and for general putty application but also for floor pans,drip rails,body joints,exterior trim and many other points,leading to well sealed car bodies equal to any produced elsewhere in the world (Figure 4.2,Table 4.10).

The term ‘sealer’covers a wide variety of materi-
als used in the motor industry for sealing against water and dust,from products which remain virtually mastic throughout their life to others which harden up but still retain some measure of elasticity.
They range from mixtures of inert fillers and semidrying oils to heat curing plastisols which may be applied in a thin paste form as an interweld sealer oras extruded beads. Sealing compounds can be categorized into the following general groups:oil- based compositions,rubberbased compositions and synthetic-resin-based compositions.

The choice of each of the types will be dependent on the site for application,on the eventual conditions of exposure and often on price. These categories can be subdivided further into the various physical forms in which they can be made available,which include mastic putties for hand application,extruded sections for placing in precise locations,gun grade compositions which have the advantage of speed and economy of application,and pouring and spraying grades. The properties of sealers will obviously vary according to their type and to their application.
Thus preformed strip or putty sealers must adhere to the surfaces to which they are applied,and must not harden or crumble in service. Glazing sealers must be capable of being readily applied from a gun,with the ability to harden off on the surface, but must remain mastic in the assembly so that they are capable of maintaining a leak-tight joint what- ever deflection the body undergoes. Heat gelling sealers must be capable of being readily applied by extrusion or possibly by spraying,and then must set up when cured but still retain a degree of flexibility.
As a result of soaring energy costs together with the need for car aerodynamic design,direct glazing of windscreens and fixed body glass was introduced and an adhesive was required to bond glass windscreens to the metal aperture. The material used is polyurethane adhesive,sealant. It possesses a combination of adhesion,sealant and gap filling qualities; it is a one-component adhesive and sealing compound of permanent elasticity. This dual-pur- pose material is based on a special moisture cured polyurethane with an accelerated setting time. The curing time is dependent upon the humidity levels prevailing,as well as the temperature. For example, at 20°C with a relative humidity level of 65 per cent,a 6mm diameter bead will be tack free within 1 hour and fully cured in 24 hours.
Table 4.11 indicates the uses of various sealant materials.