Лако-красочные материалы — производство

Abrasion and impact resistance of acrylic coatings can be improved with the addition of up to 30% of a water borne polyurethane. The addition of a polyurethane dispersion as a physical blend improves the inherent film strength and film formation of the acrylic latices. Polyurethanes generally have better film forming properties because of their lower MFFT at identical surface hardness.

Alternatively a hybrid system is available. These acrylic polyurethane “copolymers” offer improved stability and compatibility, i. e. less phase separation, increased flexibility and low n-methyl pyrolidone content (0 — 1.5%). These copolymers are essentially adaptations of interpenetrating polymer networks (IPN’s). IPNs are combinations of at least two chemically distinct polymers. The molecular chains of one polymer type are completely entangled with those of the other, though, in theory, no covalent bonds exist between separate polymer types. They are essentially intimate blends of two or more crosslinked polymer networks. Film coalescence is believed to take place via the interdiffusion of polyurethane molecules at the particle boundaries, and MFFTs are in the range 25-30°C, which are far lower than hard acrylic latices. An alternative approach involves chain extension of the polyurethane in the presence of an acrylic resin, or vice versa. This results in a hybrid system with increased solids content, one polymer diluting the other throughout synthesis. These improvements in performance, whilst adequate for some applications, are still relatively minor. What is needed is a successful crosslinking mechanism and the formation of a higher molecular weight network of polymer molecules.

(ii) Minor Additives

Improvements in mar resistance can be found through the use of silicone or wax-based additives. In certain applications where the surface appearance needs to be improved, specific flow agents may prove worthwhile.

Mar and slip additives fall into two categories. Waxes which improve abrasion and blocking resistance and silicone type additives that improve the coatings resistance to mar. Flow and levelling agents are utilised to remove film formation faults such as orange peel or cissing. Wetting agents adjust the surface tension of the coating system, improving the “wettability” of the coating.

Freeze / thaw resistance of the wet paint is usually modified with methyl carbitol. However, this will give a corresponding increase in the open time of the wet film. Although propylene and ethylene glycol are more efficient, both are hygroscopic and will retard film formation and decrease corrosion resistance.

Many of the properties imparted by the additives are interdependent, e. g. the addition of a silicone defoamer will often require a flow agent to counteract cissing. The balance of these minor ingredients is crucial and on many occasions the key to a successful waterborne formulation. Some commonly used additives are given in Table 7-10.