Лако-красочные материалы — производство

Acrylic systems are used extensively in the coating of plastic substrates and encompass three technologies

• Thermoplastic acrylics

• Two-component isocyanate cured acrylics.

• Thermosetting acrylics.

The choice of a system depends upon the resistance of the polymer to both temperature and organic solvents and the final requirements of the specifier. The latter two technologies are discussed in other chapters.

Applications of modified TPA’s include:-

• Lacquers for PVC leathercloth.

• Housings for electronic equipment (TV, video, etc.).

• Metallising overlacquers.

• Automotive/aircraft interior coatings.

• Radio frequency and electromagnetic interference (RFI/EMI) shielding.

• UV protection and scratch resistance for polycarbonate glazing

• plasticiser migration resistant coatings for PVC

Acrylic lacquers are used both as basecoats and topcoats in vacuum metallising applications on a number of plastic substrates, including polyester, nylon, acrylic, polyethylene, polystyrene and PVC. The function of the basecoat is to provide a good key between the substrate and the vaporised aluminium metal and to fill the imperfections in the substrate, thus enhancing the brightness and reflectivity of the deposited metal. The basecoat must also have extremely low volatility, since volatile by-products released under vacuum would lead to a poor quality deposition. The fast lacquer drying properties of acrylics and the non volatile inert nature of the polymer film make them an ideal choice for this application.

The deposited aluminium is only a micron or so in thickness, and requires protection from mechanical and chemical attack. The excellent optical properties, chemical resistance and toughness of thermoplastic acrylics make them also an ideal choice for the topcoat.

Coating thicknesses can be as low as 1-2 microns for a barrier application e. g. PVC leather cloth. The coating, based on hard, high molecular weight TPA’s blended with vinyl solution resins (60-85% of the vinyl resin), prevents plasticiser exuding from the PVC and helps to impart slip to the leather cloth. At the other end of the spectrum thicknesses in the order of 40-60 microns are necessary for RFI/EMI shielding6 applications to achieve the correct conductivity in the nickel acrylic coating, making the elimination of film stress, reduction of shrinkage, of paramount importance to the formulator.

Pigmented lacquers or colour coats applied to plastic mouldings such as TV cabinets, computer housing etc., improve the appearance of the moulding and increase the resistance to marring, reduce dirt pick up and make the articles easier to clean. Pigment volume concentrations may be as high as 25% or as low as 10% for a given application. If the prevention of light transmission is the customers specification (e. g. back lit automotive or aircraft instrumentation), then pigment volume concentrations as high as 30% may be utilised with the right choice of acrylic resin.

If coatings are to be applied by spray techniques some molecular weight restrictions need to be observed, especially with methyl methacrylate homopolymers. Molecular weights in the range 55,000 to 105,000 give a good balance between application, avoidance of cobwebbing and the durability of the formed film7.

Modifiers often added to plastic coatings :

• Plasticisers — Since flexibility is often the key to producing a durable plastic coating this is an extremely important modifier. The preferred plasticiser is butyl benzyl phthalate, though for certain applications polymeries or sucrose acetate isobutyrate (SAIB) will offer additional advantages e. g., reduced migration. [6]

tendency to cobweb. It operates as a thickener, increasing the viscosity of TPA solutions thereby allowing the reduction of the solids on application.

• Cellulose nitrate — High nitrogen content grades can be blended with TPA’s to fulfil a similar function to CAB and will offer some improvements in durability and chemical resistance.

• Alkyds — These can be used to improve the gloss and appearance of TPA’s for general purpose coatings. The increase in solid content will enable thicker films to be applied, but the introduction of an aromatic hydrocarbon will create problems on substrates such as HIPS.

• Vinyls — These polymers are often used as an additive to improve the alcohol resistance of TPA’s for interior applications e. g. in cosmetic resistance metallising topcoats. Medium to low molecular weight vinyl chloride / vinyl acetate copolymers offer improved compatibility.