Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1994-12-01
1996-05-28
Dudash, Diana
Coating processes
With post-treatment of coating or coating material
Heating or drying
427409, 427421, 427373, 521 76, 521 78, 521 94, 521 98, C08J 922, C08J 932, B05D 714, B05D 102
Patent
active
055209610
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a new coating composition based on vinyl-chloride-polymer plastisol and containing thermoplastic microspheres as filler. More specifically, the invention concerns a coating composition which is especially suitable as an undercoating for cars. The composition contains expandable thermoplastic microspheres having a polymer shell which is substantially halogen-free and is based on nitrile-containing monomers and halogen-free monomers. The invention also relates to a method for spray-coating, as well as the use of the coating composition.
To provide protection against rust and impacts from stones, it is known to treat the chassis and other exposed car components with coating compositions. Conventional coating compositions usually are bituminous compounds or plastisol-based coating compositions. As is also known, polyvinylchloride (PVC) plastisols of improved stone resistance and reduced weight can be obtained by using thermoplastic microspheres as filler in the plastisol composition (see, for instance, DE 40 18 022 and JP 85-281981).
When employed as coating compositions for cars, plastisols are gelatinised in the furnaces used for hardening or drying car paint. Usually, a car body is subjected to several painting and drying operations.
When used as an undercoating for cars, the PVC plastisol is applied by airless spraying. This method effectively spreads the high-viscosity plastisol, but requires very high pressures. Thus, pressures of up to 200 bar are not uncommon. In addition, the spraying systems are dimensioned to keep the high pressure constant for fairly long periods of time, e.g. 200 bar for 3 h. It has, however, been found that expanded low-density microspheres, which are used in the above patent specifications, do not retain this density when subjected to high pressure over long periods of time. To improve the pressure resistance of the microsphere-filled plastisol, it has been suggested to use unexpanded microspheres. These are caused to expand when the plastisol is gelatinised, which results in a so-called foamed PVC plastisol of homogeneous cellular structure, exhibiting the aimed-at low weight and improved resistance to stone impacts. It has however been found that the microspheres collapse when the foamed plastisol is heated to normal gelatinisation temperatures (100.degree.-180.degree. C.). The conventional microspheres used in the plastisol are thus unable to withstand the high temperatures. As a result, the expanding agents of the spheres are released, resulting in a plastisol surface having large pores and being full of large blisters. Also, the collapsed spheres impart high density to the plastisol, thus ruining the advantage of lightness.
As defined by the appended claims, the present invention relates to a coating composition containing vinyl-chloride-polymer-based plastisol and plastisol thermoplastic microspheres. The expandable thermoplastic microspheres have a polymer shell which is substantially halogen-free and comprises a copolymer of halogen-free monomers and, in an amount of at least about 80% by weight, nitrile-containing monomers. This composition is especially suitable for use as an undercoating for cars, but may successfully be used as protective coating also for other surfaces.
It has surprisingly been found that the problems mentioned above can be solved by using a particular type of microspheres in the plastisol. Thus, the coating composition according to the invention is able to withstand high pressures for a long period of time, and can safely be heated to high gelatinisation temperatures for a relatively long period of time. The microspheres expand, but remain intact without collapsing. This results in a PVC coating having a dense, fine surface and very low density. Densities below 1.0 g/cm.sup.3 can be obtained without difficulties. This is as good as the use of expanded spheres, but there is no problem of low pressure resistance as in expanded spheres.
The thermoplastic microspheres used in the plastisol-based composition according to
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Lysell Lennart
Malmbom Lars
Casco Nobel AB
Dudash Diana
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