Лако-красочные материалы — производство

A typical TPA paint formulation for automotive applications is given below <3).

FORMULATION 3-1:

WHITE AUTOMOTIVE THERMOPLASTIC GLOSS TOPCOAT

This formulation contains a blend of acrylic polymer with acid functionality (for pigment dispersion), with a high molecular weight, methacrylic thermoplastic polymer for film performance including hardness, toughness and chemical resistance. Paraloid A21LV from Rohm + Haas, an MMA polymer of Tg 105°C in toluene, MEK and butanol is a suitable TPA for OEM applications. Polar, polymeric dispersants (hyper-dispersants) enhance the dispersion and stabilise the millbase, improving the rheology, gloss, transparency and strength of the pigment. The effectiveness also depends on pigment-additive interactions.

The coating comprises 60% by weight of the total binder. The remaining 40% of the binder is split equally between a CAB ester, for flow/reflow in the oven, and a low molecular weight plasticiser. CAB also aids pigment suspension without reduction of colour and gloss. Solvent balance is achieved by using strong, fast evaporating ketone solvents and aromatic hydrocarbons for improved dissolution and viscosities with a slower glycol ether ester such as Dowanol PM acetate, to help flow in the oven. Strong solvents such as ketones, esters and aromatic hydrocarbons can dissolve a small percentage of the pigments, causing a ‘bleeding’ into the undercoats.

Pigments are chosen for colour strength, cleanliness, brightness and durability, and must have tolerance to the solvents. Typically, the pigment/binder ratio of a white automotive coating would be 0.6/1 w/w with a pigment volume concentration (PVC) of 16%. This is considerably lower than other white gloss paints, and has been selected to maximise gloss and exterior durability of the final coating. The paints are made in bead mills or ball mills. Where high transparency is required from difficult pigments, ‘chip’ dispersions are made, which are then blended with other ingredients in a conventional mixing vessel. However, with increasing pressure on reduced costs, ‘chip’ technology or ‘chipping’ is finding less and less usage.

For OEM automotive applications, universally, paint is spray applied, and subsequently force dried in ovens at 130-150°C. High molecular weight polymers are generally very viscous and require large dilution to spray viscosity. During the drying cycle the thermoplastic polymer matrix (acrylic and cellulose ester) undergoes a thermal reflow, removing surface imperfections such as “orange peel”.

The major disadvantage of thermoplastic acrylics in automotive lacquers, is low application solids and therefore high VOC (volatile organic compound) levels. The trend for many years has been to work towards reducing VOC levels. This has been partially achieved by higher solids formulations and new application technology such as high volume/low-pressure spray equipment. However, the ever tightening emission requirements mean that the use of TPA’s in OEM automotive finishing has today almost disappeared, essentially being confined to the smaller specialist car manufacturers. Higher product performance expectations have led to the conversion to thermosetting coatings with superior film properties.

(ii) Thermoplastic Acrylic Refinish Topcoat Lacquers

With certain adjustments to the formulation, a TPA refinishing paint is very similar to its OEM counterpart. In particular, if the refinish lacquer is to be dried at ambient temperatures as opposed to within ovens at 130°C+, thermal reflow will not take place. Full gloss must be achieved by other means. A combination of changes can be made, namely:

a) The use of a narrower molecular weight distribution polymer. The absence of the high molecular weight species assists flow.

b) The incorporation of a lower molecular weight, more soluble plasticiser such as butyl benzyl phthalate or dioctylphthalate. The higher volatility is not important since no force drying is taking place.

c) The use of a high level of slow boiling solvents to improve flow.

d) Final sanding and polishing of the dry film.

Starting point formulations are given below for white and metallic refinish coatings. For metallic coatings, high transparency is required to produce good two-tone effect (flip-flop or travel effect). For opaque shades, maximum opacity provides good hiding, usually at 45 to 60 p, m films.

FORMULATION 3-2:

WHITE THERMOPLASTIC REFINISH COATING