Лако-красочные материалы — производство

A further increase in the solids content of automotive paints came with the advent of acrylic non aqueous dispersions (N. A.D.’s). These were originally developed by I. C.I. in the late 60’s and early 70’s<4)<5). With this development, akin to water based emulsion technology, it became possible to produce higher molecular weight thermoset acrylic particles finely dispersed in a volatile organic phase which is a non-solvent for the polymer. The NAD polymer is produced by charging the monomers to the reaction vessel containing aliphatic hydrocarbons. As polymerisation of the monomers proceeds, the growing copolymer precipitates out of solution forming 2 phases, one of which contains polymer micelles, a few microns in diameter. Stabilisers are necessary to hold the acrylic polymer in suspension. These thermosetting acrylic NAD’s are hydroxy functional and are formulated into enamels with melamine formaldehyde resins and true solvents for the acrylic polymer. The level and choice of true solvent must be carefully controlled to prevent premature solubilisation of the acrylic polymer causing in-can paint instability problems.

After application of the NAD paint, and during heat up in the oven, the coating quickly loses the volatile aliphatic non-solvent components allowing the active or true solvents to coalesce and solvate the polymer micelles. This solvating process also helps to level the film producing good flow and gloss. As dispersions have limited resistance to the application of high shear, pigment dispersion is normally carried out in conventional solution based acrylics to produce mill bases which are subsequently let down with the NAD and other paint components. The 10-15% higher application solids not only results in a further 30-35% reduction in emission levels over standard thermosetting acrylics, but also provides the possibility of achieving a 50 micron dry film in 2 wet coat passes compared with the 3 passes necessary with solution acrylic systems.

The difference in solids contents of the systems already discussed are summarised in the table below.

Reduction of solvent emissions from a paint shop does not only depend on new resin and paint technologies, but also on the practices within the factory. Installation of solvent collection plants, so that the solvents can be reused or alternatively, incinerated to provide heat is another way to reduce overall emission from the factory.