Лако-красочные материалы — производство

Hydroxy functional acrylics are used in general industrial (GI) stoving finishes in two distinct capacities. They are used to impart highly durable finishes for exterior components such as tractor sheet metal and cabs, fenders for earth moving equipment, or as one coat finishes in more wide ranging metal finishing such as office furniture, irons, microwave ovens, toasters, food mixers and blenders.

The formulation of the acrylic will be quite different in the two areas. The overall physical film performance characteristics in both applications will be similar, so the amount of OH functionality will be similar, as will the need to balance the hard and soft monomers in the acrylic backbone. However, the key area of long term exterior durability, ultra violet light resistance means that the hard monomer cannot be styrene, as the aromatic ring causes the early onset of loss of gloss and film cracking. The hard monomer used in durable acrylics is methyl methacrylate.

The commercial pressures in the general metal finishing area mean that the much cheaper styrene monomer must be utilised to keep the resin cost down. The other monomer consideration for the two markets is adhesion to substrates. In the general metal finishing area, substrate pretreatment will often be limited and therefore the acrylic will have to be quite adaptable. The paint formulation alone cannot compensate for a substrate intolerable acrylic! Epoxy functionality is commonly included to improve substrate adhesion, particularly when there is no priming scheme. If the curing schedule allows it, modification of the acrylic/melamine formaldehyde binder with up to 15% epoxy resin will enhance metal adhesion, chemical resistance and film flexibility.

It is advisable to include some carboxy functionality also as the cure reaction between acrylic and melamine resins is acid catalysed. In the more sophisticated industrial markets for acrylics, stoving temperatures are well controlled and adequate cure cycles are achievable. However, in the competitive general metal finishing market, low temperatures and short times are common, so whilst it is possible to externally catalyse using PTSA (para toluene sulphonic acid), this polar material remains in the film after curing to act as a pathway for water. Blistering on exposure to humidity is a common problem in acrylic-melamine systems which have been catalysed by sulphonic acids.