Лако-красочные материалы — производство

In the early 80’s in Europe, tractor and construction equipment sheet metal was either pretreated with iron phosphate or merely degreased, primed with an alkyd flash primer and then finished in a stoving alkyd/amino system. Application of paint was and remains, predominantly electrostatic airless spray.

The performance characteristics of the paint finish were good resistance to fuels and lubricants, reasonable humidity resistance and poor corrosion resistance. Exterior durability required was ’good gloss resistance after 9 months exposure in Florida’.

The industry has always followed the automotive finishing industry, at a respectful distance due to the capital costs involved. Hence the first improvements were in corrosion resistance by the use of anodic electrodeposited primers. This had no requirements for a change in topcoat technology, but in the middle and late 80s, competitive pressures in the overcapacity tractor market in Europe led to the introduction of cathodic electropaint primers. The cathodic electropaint gave a greater increase in corrosion resistance, but it is impossible to get good intercoat adhesion between cathodic electropaint primers and alkyd finishes. Fortunately, acrylics can be formulated to give excellent adhesion to cathodically deposited primers.

Therefore, stoving acrylics began to be used on tractor sheet metal lines, giving a paint system that has excellent corrosion resistance and very good exterior durability. The acrylic resin has little or no styrene content and is a balance of hard monomers, such as methyl methacrylate, and flexible monomers, such as butyl acrylate. The flexibility requirements of tractor sheet metal finishing are higher than automotive applications as a tractor or construction equipment machine has a much tougher environment with no attention given to the paint work in service.

All paint companies large enough to be involved in either automotive or high quality industrial finishing will be designing and producing their own high performance stoving acrylics, but the major resin manufacturers also have an extensive range of products. These resins are listed in the automotive finishing section.

Today the specification for exterior durability in this industry is 24 months Florida exposure, with a minimum of 60% gloss retention. The flexibility required is the BS3400 mandrel bend with no cracking allowed. Resistance to fuels and lubricants remains.

The cure system for these acrylics is the partially methylated melamine resins. Hexamethoxy methyl melamine, the fully methylated melamine resin, reacts at too high a temperature for the production line requirements in high volume tractor production. The partially methylated melamines can be formulated into acrylic systems to give a cure cycle of 20 minutes at 120° C. The amount of melamine resin used in the formulation is not related to the number of OH groups on the acrylic, but to the degree of hardness and flexibility required. Reactions that take place on stoving an acrylic/melamine system are the crosslinking condensation between methylol and hydroxyl and the self condensation of the melamine resin (see the section on reaction mechanisms).The self condensed melamine gives a very hard but brittle film, so it is important to balance the two processes. This is achieved by balancing the acrylic melamine content. Practical ratios that have been shown to be effective usually lie between 70/30 and 80/20, solid acrylic to solid melamine resin.

The highest quality pigments are required for this industry to complement the durable acrylic resin system. This is more so today than ever, as the use of lead chromate pigments is rapidly being discontinued. The construction equipment industry has been particularly affected by the move away from lead chrome pigments as these pigments were an excellent inexpensive route to bright red, orange and yellow colours, predominant in the industry.

A typical formulation for a stoving acrylic tractor sheet metal finish is given below:

FORMULATION 4-6:

BLUE TRACTOR SHEET METAL ACRYLIC STOVING FINISH

Items 1 -4 are mixed by high speed dispersion. Items 5-7 are added and the base is passed over an horizontal bead mill, preferably of the double barrelled Eiger type. Grind size should be ‘0 microns’ on the grind gauge. Items 8-14 are added as let down and the viscosity is adjusted to 60 seconds BSB4 flow cup, giving a volume solid of 35.7 %.

The stoving schedule is 30 minutes at 130°C. Application is by electrostatic airless spray.

Beetle 3748 is a partially methylated melamine resin, 72 %nvc in isobutanol. Because of the partial methylation, tolerance for aromatic hydrocarbons is limited. Methylated melamine resins generally have a wider range of compatibility than butylated and isobutylated melamines.

The barytes (blanc fixe) are present to aid both hardness and atomization in the airless spray. Bentone gel prevents pigment settlement and helps paint hold up during solvent flash off. Triethylamine is there to give the paint a 12 month shelf life as the acrylic resin with an acid value of around 16 will react with the melamine slowly at room temperature, unless the acid groups are blocked with the amine. The amine volatilises off during stoving. The very small amount of Dow Coming DC200/100 polydimethyl siloxane fluid has a surprisingly large effect, giving the paint tolerance to film defects such as flooding and pigment separation during application. It also improves the mar resistance of the film. There are several BYK Chemie additives, amongst others, that would also do the job.

The pine oil is there as a very slow evaporating diluent, it will not come off in the flash off stages. Acrylics are very difficult to apply by airless application as they are such large polymers (compared to alkyds, which are easy to apply by airless). They are difficult to atomise and as solvent evaporates off during flash off, levelling is insufficient to achieve the excellent final appearance required after stoving. The desire for ever lower temperatures in the cure cycle also means that the acrylic will not flow out well unless it is plasticised in this phase of the cure process. It must also be remembered that the crosslinking mechanism means that the fugitive group is methanol, which must be able to leave the film when it is still sufficiently open rather than be trapped as bubbles, giving a rough final film. The pine oil allows the paint to reflow in the initial stages of stoving, helping the volatiles to escape and then leaves itself, giving a final film with minimum orange peel and no surface imperfections.

The diacetone alcohol and Catafor (quaternary amine) ensure that the paint is sufficiently conductive to pick up the electrostatic charge required for good ‘wrap’ as mentioned previously.

The major European resin manufacturers for high performance stoving acrylics are Cray Valley Ltd., BASF and Akzo-Nobel.