Лако-красочные материалы — производство

The OH functional acrylics in the GI metal finishing area are used mostly as one coat finishes for products intended for interior use or for products with a limited life expectancy. The coatings achieved are hard, glossy, scuff resistant but with a tendency to detach from the substrate after a blow etc. This is because acrylics do not easily wet a substrate like an alkyd, which has very low surface energy because of its long fatty acid chains. This section of the GI finishing market often cuts comers on pretreatment of metals, making the acrylic more difficult to formulate with effective adhesion.

Nevertheless, it is possible to formulate an effective coating using a fairly high styrene content acrylic (for cost effectiveness), with some COOH functionality for low bake capability and epoxy groups for adhesion. Alternatively, epoxy can be added at the paint formulation stage. Crosslinking is by butylated melamine resins that require slightly higher temperatures, but are cheaper.

A typical general purpose white gloss stoving acrylic for this market is as follows :

FORMULATION 4-7:

GENERAL PURPOSE WHITE GLOSS STOVING ACRYLIC

Items 1-3 are firstly high speed dispersed and then passed over a horizontal bead mill to achieve ‘0 microns’ grind size. Items 4-8 are added as let down and viscosity is adjusted with item 9 to 60 seconds BSB4 flow cup.

The stoving cycle is 30 minutes at 140° C. The pigment to binder ratio is 1/lto give high opacity. Application is by conventional spray, thinning to 25 seconds BSB4 cup with 1/1 xylene/butyl acetate.

Synocryl 823S,55 %nvc in xylene/n-butanol (3:1) from Cray Valley Ltd, is a hydroxyl functional TSA low temperature stoving resin which has good compatibility with a wide range of amino resins. It also offers good adhesion to untreated aluminium. Beetle 688, a butylated MF resin (BIP Ltd), has relatively low viscosity.

The butanol is included to give a 12 month shelf life by stabilising the condensation reaction between the hydroxylic acrylic and the butylated melamine. The presence of the leaving alcohol, butanol, in the paint moves the equilibrium position to discourage the condensation.

The DC200/100 acts as a surface tension modifier.

The di-iso-octyl (di-ethyl hexy) phthalate is a plasticiser to mitigate against the natural brittleness of the high styrene content acrylic.

The butyl diglycol is there to encourage levelling during the early stages of stoving.

A starting formulation with epoxy incorporation to improve adhesion and corrosion resistance is given below:

FORMULATION 4-8:

GENERAL INDUSTRIAL ACRYLIC STOVING ENAMEL

Viscosity at 25°C Pigment/binder ratio Total solids content Vehicle solids content Curing Schedule

^ See selection below (2) Beetle 688 BIP Ltd.

® Shell Chemicals ^ Dow Chemicals

Hydroxy acrylics are also used as one coat finishes on phosphate pretreated steel in the office furniture industry. The gloss level required here is typically around the 50% level on a 60° head and this helps to cheapen the steel surface preparation needed, thus reducing costs. Film performance requirements are primarily scuff and mar resistance, so again high styrene content acrylics can be used. Application is usually by conventional spray, but quite a few high speed electrostatic bell lines are in operation in Europe for this industry. The following formulation is applied by the latter method :

FORMULATION 4-9:

ACRYLIC STOVING FINISH FOR OFFICE FURNITURE

Items 1-8 are firstly high speed dispersed and then passed through a horizontal bead mill to a 15 micron grind size. Items 9-13 are added as let down. Item 14 is used to adjust the resistivity and 15 to adjust the viscosity to 60 seconds BSB4 flow cup. This gives a gloss level of 55% with 60° Head. The stoving cycle is 20 minutes at 140° C.

The epoxy is used to enhance the adhesion to indifferently pretreated steel. The DC200/100 assists greatly in minimising film defects and encouraging substrate wetting.

The solvent mix must be very slow evaporating, because of the electrostatic bell application. The bell atomises the paint extremely well and if very slow evaporating solvent, such as Solvesso 150, were not used, the paint would be ‘dry’ as it reached the substrate, no wetting would occur and the resultant film after stoving would look like an under stoved powder!

The diacetone alcohol is sometimes sufficient to adjust resistivity on its own, without Catafor. This can be an advantage, as the Catafor, unlike the diacetone alcohol, will remain in the film after stoving. Thus a pathway to moisture will exist leading to possible film failure in humid conditions.

An effect finish often created using stoving acrylics is the textured finish. This is done using polypropylene beads in an acrylic stoving paint. The acrylic is particularly suitable for this effect as it is sufficiently tough to cope with the presence of the beads without giving a soft, easily marred film. A typical formulation is given below:

FORMULATION 4-10:
ACRYLIC STOVING TEXTURED FINISH

Items 1-4 are high speed dispersed and passed through a horizontal bead mill to

‘0 microns’ grind size. Items 5-7 are stirred in and the viscosity is adjusted with item 8.

A typical stoving cycle is 20 minutes at 140°C. Application is by conventional spray after thinning to 25 seconds BSB4 flow cup with xylene.

The wax gives extra mar resistance, which is the chief concern with textured finishes. Thermosetting acrylics recommended for general industrial applications include :

The usage of TSA systems for plastic substrates is limited because of the temperature instability of many plastics and the aggressive solvents used in the component resins (see chapter on thermoplastics).

Hence the application of these systems is limited to thermosetting substrates and, if an acid catalysed acrylic melamine coating is formulated, thermoplastics such as PPO or PA.

It will have been noticed that the volume solids of the various formulations given for hydroxy functional stoving acrylics are quite low. This leads to problems with the VOC content. Acrylic resins can be formulated at a higher solids content, usually by lowering the molecular weights. The technology is so far underdeveloped and the lower molecular weight acrylics are not yet showing good enough film performance characteristics to compete with the alternative methods of obtaining low VOC content stoving coatings, namely powder coatings.

However, where stoving is not the preferred route to achieve film performance, for example when the substrate cannot be heated up to high temperatures as with most thermoplastic substrates, or where very high metal film thicknesses are involved then powders become unsuitable. Here, two component acrylic/isocyanate technology is more appropriate.