Лако-красочные материалы — производство

ACE finishes are commonly formulated using acrylic urethanes. The reasons are as outlined above. In particular, only an ambient cure coating technology is suitable on such large metal components.

Historically, air drying alkyds were used in this business, but the drive towards more durable and tougher coatings has led to the switch to polyurethanes.

The monomer choice in an OH acrylic polymer for polyurethane formulations conforms to those in the stoving area. Durability outweighs cost, thus the styrene content is low. Hydroxyl content is increased to ensure that the ОН/ NCO reaction is favoured over reaction of NCO with moisture in the air. Polyurethanes are naturally tough coatings due to the ability of the urethane groups to hydrogen bond. Deformation will break these links, but they will reform enhancing the coating’s resilience.

The usual application method in the ACE business is airless spray, sometimes electrostatic. This is because airless spray gives good transfer efficiency and high production throughput with fewer applicators than in conventional spray. The majority of tractor ‘skids’ (chassis) are painted almost fully built, with engines and hydraulics in place. The assembly is flash primed with an anti corrosive two pack epoxy primer. Elevated temperatures are not desirable. After skid painting on the production line, pre-painted sheet metal parts are assembled onto the finished skid, thus speed of cure is very important. To accommodate these requirements, a highly catalysed system is used, with a very short pot life (time taken for the viscosity of the mixed acrylic base and curing agent to double in viscosity and become unsprayable). Hence the airless spray equipment is of the mix at the head type, with twin feeds from an acrylic base reservoir and isocyanate curing agent. A typical formulation is outlined below:

FORMULATION 5-9:

CHARCOAL GREY TWO COMPONENT ACE FINISH — ACRYLIC BASE COMPONENT

Mix the Synocryl 873SD and the range of pigments together with the hyperdispersant and the xylene and high speed disperse. The Synocryl 873SD is then added with further xylene, butyl acetate and Aerosil 200® fumed silica. At this stage the mix is bead milled to ‘0’ microns on a grinding gauge. It is then let down before its resistivity is adjusted to 500K with diacetone alcohol. Viscosity is adjusted by the addition of xylene and acetone.

® Synocryl 873SD Acrylic Disperbyk 130 Hyperdispersant Tiona 472 ТІО2 Aerosil 200 Fumed Silica Acronal 700L Acrylic BYK 310

Desmodur N75 Isocyanate Solvesso 100

The base to curing agent mixing ratio is 2/1. The mixed volume solids are 29% and the mixed components are ready for application without further thinning by airless electrostatic spray.

The choice of the acrylic resin Synocryl 873SD is quite significant in this formulation. Acrylics are not easy to apply by airless spray. They are not easy to atomise by airless spray and as the relatively large droplets progress towards the substrate, they flatten and trap air. This air is carried into the paint film. As it struggles to the surface to leave the film, the large acrylic polymer ’skins’ over to trap it as bubbles in the surface. In a stoving situation there is a second opportunity for this air to escape in the reflow phase before curing commences in the stoving oven. This cannot happen in ambient cure, so other means must be found. For a little understood reason, Synocryl 873SD releases air better than most acrylics, whilst retaining the usual desirable acrylic features, such as good durability, hard, tough, fast cure.

The Disperbyk 130 is used to control the tendency to flood and float of a white and black pigment combination.

The Aerosil 200 prevents pigment settlement on storage and helps control sagging of the film during solvent flash off. As the solvent balance is quite slow it also encourages levelling and air release.

The Acronal 700L, a low molecular weight acrylic resin, together with Byk 310 are used to promote levelling by lowering the surface tension. This is required in acrylic formulations, which have intrinsically higher surface tensions than alkyds.

The dibutyl tin dilaurate is a catalyst for the OH/NCO reaction, and is sufficiently non volatile to remain in the film during solvent flash off. This is a double edged sword as the DBTL will also encourage the degradation of the coating in the presence of UV light and moisture. Hence there is a finite extent to which an acrylic/isocyanate formulation can be accelerated without compromising durability.

The ethoxy propyl acetate is the slow ‘tail’ solvent.

The use of diacetone alcohol (DAA) as a resistivity modifier is unusual as it is normal to be very wary of using alcohols in isocyanate formulations. But in this particular formulation, which is rich in tin catalyst and is a mix at the head formulation, the residence time of the alcohol and isocyanate in the same space is very short and the DAA flashes off quickly. The undesirable use of a quaternary amine salt to modify resistivity can thus be avoided.

This formulation has very good Florida durability figures, with a gloss retention of over 60% after 2 years continuous exposure.