Лако-красочные материалы — производство

As a general rule, the higher the molecular weight, Tg or viscosity of the emulsion resin, then the higher the MFFT. Increasing the MFFT gives better physical and chemical properties, provided a coherent film is produced. Reducing the MFFT, reduces properties. In order to overcome this dilemma, solvents are used, which effectively reduce the viscosity of the particles as the water evaporates and allows the particles to flow together into a uniform film. The correct choice of coalescing solvent is critical to the successful performance of the final film.

Ideally, strong solvents are used to reduce the viscosity of the emulsion. However, this can dissolve the emulsion in the water/solvent blend, and large viscosity increases and poor emulsion stability can occur. If the solvent is too weak, the emulsion will either not coalesce at all or will only partially coalesce. This will lead to poor final film properties as holes will be left between the particles where they have not flowed completely into one another. The evaporation rate of the solvent is also important. The solvent is required to stay longer than water so that the emulsion can flow out after the water has gone. If the solvent evaporates too fast, it will not coalesce the emulsion properly. If it is too slow, then the emulsion will coalesce correctly, but the film will stay tacky, as the solvent keeps the film liquid or soft.

When film formation takes place under conditions of low or normal relative humidity, water evaporates faster than the coalescing solvent. Partial coalescence takes place at the surface of the coating, thereby hindering further solvent release, and allows the particles to soften and continue to form a film of high integrity. This extended retention of coalescence lengthens the tack-free time and retards early development of hardness and water resistance, but provides the optimum final film properties.

Under conditions of high humidity, the evaporation rate of water is significantly reduced preventing surface coalescence from taking place at an early stage. Since the solvent does not have to diffuse through the thickness of a partially dried film, convection readily removes it, leaving the water behind. As a consequence, the polymer particles return to their unsoftened state and will now merge to form an adequately integrated film. The final coating may exhibit satisfactory levels of flexibility, hardness and gloss but the ultimate corrosion resistance and durability will be impaired.

The selection of low volatility or water insoluble coalescents improves the situation, but always at the expense of slower development of film properties and early product handling. Plasticisers can also be used, but they will affect the ultimate hardness of the final film. Most industrial coating operations require a fast through-put and cannot allow the time for extended drying periods. To overcome this, a blend of high and low volatility coalescents are often employed. The solvents used can vary quite dramatically in evaporation rate and solvency, from relatively fast ether alcohols, such as butoxy propanol, to slow ‘plasticisers’, such as propylene glycol. It is also possible to utilise the solvency properties of plasticisers such as DBP (dibutyl phthalate) to help as a coalescent solvent as well as a plasticiser, if required. The highly volatile solvents are quickly removed from the coating and give early product handling, whilst the slower types are retained to assist adequate film formation.

Industrial users can make allowances for humidity variations by force drying the coating. This reduces the relative humidity and often subjects the film to a temperature above its MFFT.

The types and levels of coalescents used are dependent upon drying conditions and the nature of the formulation and will have to be determined through experimentation.

A higher MFFT will require more and/or stronger solvents than a lower MFFT. If the material is force dried with infra red or hot air, then less or weaker solvents can be used than if the coating is air dried. There is also the possibility of blending two or more emulsions to reduce the effective MFFT so that less or weaker solvents can be used.

The ambient conditions during the application of an acrylic latice have a major influence*8* on the film forming process. Water based systems, unlike solvent based ones, are very susceptible to the relative humidity during film formulation at ambient temperatures. At the higher temperatures encountered during stoving this problem is decreased. The evaporation rate of many organic solvents is not affected by changes in humidity, whereas the evaporation rate of water may change by a factor of twenty or more at differing humidity conditions. At a humidity of 75% in an 80:20 water/ethylene glycol butyl ether blend, the last component to leave the film is water, which results in limited coalescence of the coating. This is easily rectified with the addition of a slower coalescing solvent, but the formulator must consider the end use and the application conditions in developing the new coating.

Care must also be taken during low humidity conditions. The film will partially coalesce too quickly, and this results in faults in the film formation stage. The coalescing solvent must diffuse through the partially formed film.

There are some subtle differences between co-solvents and coalescing solvents. The latter are essential for film formation to occur. They must swell or partially dissolve the surface of the emulsion particle, thereby enabling the particles to fuse. A co-solvent enhances or induces solubility of a resin in another solvent system. The other solvent is water in waterborne systems. Without the presence of co-solvents solubility may well be reduced. In some instances, co-solvents can also act as coalescing solvents.