Лако-красочные материалы — производство

The most important parameters governing the formulation of a successful coating, i. e. suitable for the application for which it was specified are:

1. Resin Selection

Clearly the variety of uses to which thermoplastic acrylic resins (TPA) are put, has meant that many polymers have been formulated to meet the demands of particular applications. In most instances, where TPA’s are used, they are the major, or even sole binder component. A good understanding of acrylic polymer technology is therefore essential for optimum resin choice.

By definition, thermoplastic acrylic resins are film formers in their own right, and their film properties are governed by monomer selection, molecular weight and molecular weight distribution.

As we have seen earlier, the choice and level of monomers used in the backbone, control basic film characteristics such as hardness, chemical resistance, solubility, flexibility and exterior durability. These will be discussed below under individual application headings. In general, however, the polymers are formulated to give glass transition temperatures (Tg) of between 30 and 100°C+ with the harder, less flexible copolymers at the top end of the range, and the coatings with greater flexibility at the lower end.

Molecular weight and molecular weight distribution also affect film properties, as well as solubility, solution viscosity and application solids. Typically, solid TPA resins are formulated to a molecular weight range between 30,000 and 200,000. The shorter the chain length for a given monomer composition, the more brittle the film, but the easier the product is to handle, and the higher the solids content at application viscosity. On the other hand, the higher the molecular weight, the higher the viscosity.

Special processing techniques (e. g. polymerisation under pressure) can be used to produce polymers with a narrower molecular weight distribution, thus optimising film performance for a given solids/viscosity relationship. This is of particular importance today where, for environmental reasons, reducing the amount of volatile organic compounds (VOC’s) levels is one of the most important factors affecting and directing future developments of organic coating systems.

The major monomers used in TPA’s are:

Methyl methacrylate (MMA) which, in addition to exterior durability, contributes hardness, tensile strength, good colour and colour stability to the coating.

Ethyl acrylate (EA) which contributes flexibility and elasticity to the coating film.

Butyl acrylate (BA) offers similar properties, but due to the lower Tg of the homopolymer, lower percentages can achieve similar results. BA is more suited for use in exterior applications due to its superior durability performance compared with EA.

2-Ethyl hexyl acrylate (2-EHA) is highly elastic and tacky as a homopolymer and imparts these characteristics to coatings. Normally it is only used in small percentages in resins for coating applications.

Butyl methacrylate (BMA) offers better solvent solubility compared with methyl methacrylate, and due to its homopolymer Tg of approximately 20°C, it is much more flexible and extensible.

Isobutylmethacrylate (IBMA) produces polymers which dissolve in mild solvents. This monomer also imparts good water resistance to its copolymer.

Acrylic acid (AA) contributes increased polarity to the polymer, thus affecting solubility. Small levels of acrylic acid modify the surface wetting and substrate adhesion of the coating. High levels give alkali soluble polymers for temporary coatings and polishes.

Methacrylic acid (MAA) contributes similar properties to acrylic acid, but has a significantly higher Tg (200°C compared with 105°C for acrylic acid), and is therefore much harder and more brittle.

Other, non acrylic monomers used in formulating thermoplastic acrylic resins include:

Styrene (S) which contributes hardness, chemical resistance and economy to the base acrylic, but at the expense of coating film colour stability.

Vinyl toluene (VT) is combined with acrylic monomers such as butyl acrylate to produce polymers soluble in aliphatic hydrocarbons. These are used mainly in masonry coating applications (see below).

In general methacrylates, with the extra methyl group on the alpha carbon, give harder, higher tensile strength and more rigid copolymers than the acrylates. Flexibility and hardness can be predicted from the molecular weights and Tg’s.