Лако-красочные материалы — производство

1. Automotive Finishing by Kevin O’Hara LRSC

The major application for hydroxy functional thermosetting acrylic is in the field of topcoats for cars and other vehicles. Today and at the moment all the major car manufacturers around the world use paints based on this class of resin.

As was discussed in the thermoplastic acrylic chapter of this book, topcoats for automotive finishing were developed from nitro-cellulose (NC) based finishes into two main streams:

a) synthetic enamels based on alkyd/amino technology and

b) thermoplastic acrylic enamels

The former type offered much higher application solids and resistance to mechanical damage (e. g. stone chip resistance), and was used exclusively in commercial transport applications. Acrylics gave superior colour, colour retention and exterior durability, particularly in the area of the up and coming metallic or polychromatic finishes.

During the 70’s these two routes came together and most automotive manufacturers adopted thermosetting acrylics for automotive topcoats. With TSA’s it became possible to combine the advantages of both routes, i. e. the high solids, lower solvent levels, and resistance properties of a cross linked system with the superior flow, colour and exterior durability of acrylics.

In the early days, although paint manufacturers flirted with acrylamide chemistry for automotive topcoats, their higher curing temperatures of 150°C and above precluded commercialisation in this end use. Carboxy functional acrylics have inferior weathering performance. Consequently, thermosetting acrylic topcoats are almost exclusively based on hydroxyl functional polymers. These are crosslinked with either alkylated melamine formaldehyde resins or as 2 component (2K) systems, with aliphatic isocyanate adducts. The 2 component systems are an extremely important class used mainly in automotive repair applications and warrant a chapter in their own right (see Chapter V on component isocyanate curing systems).

Typically, an automotive quality acrylic will be formulated with a mixture of monomers selected from styrene (S), methyl methacrylate (MMA), butyl methacrylate (BMA) and butyl acrylate (BA). The hydroxy functionality is introduced via a monomer such as hydroxy ethyl acrylate (HEA) or hydroxy ethyl methacrylate (НЕМА) to give hydroxyl values of between 50 and 120 mgKOH/g of solid resin. In addition, a small quantity (1-2%) of methacrylic or acrylic acid is also used in the polymer to aid pigment wetting, adhesion and to promote the curing reaction.

In the resultant paint, the acrylic polymer is cured with an etherified (normally butylated) melamine formaldehyde resin at temperatures of around 120°C. The cross linking reaction has been shown earlier in Figure 4.

Within the field of automotive thermosetting acrylics, the past 25 years has seen developments on two main fronts:

• the drive to reduce solvent emissions

• a technology change from solid colour paint systems to clear over base (C. O.B.) systems.