Лако-красочные материалы — производство

Waterborne resin systems can be divided roughly into two classes — water soluble and water insoluble. A true water soluble resin is a solution of a single polymer molecule solubilised on its own, and completely surrounded by water. The water insoluble types are emulsions, these consist of a particle (or micelle — see Chapter II for full definition) consisting of a number of polymer molecules solubilised by a surface covering of hydrophilic molecules. Consider the continuous reduction in particle size from an emulsion particle to a single molecule. There is a zone of colloidal dispersion where the particle size is so small that it is invisible to the eye and the dispersion is clear. These colloidal dispersions are sometimes referred to as a hydrosol.

In between insoluble and soluble waterborne systems are dispersions of water reducible resins. These are insoluble resins with sufficient acid or amine functionality so that when neutralised (either fully or partially) the resin can be dispersed in water and remain as a stable dispersion. Many people consider emulsions to be dispersions. So they are. As a general rule, the term ‘dispersion’ will refer to solubilised dispersed systems. ‘Emulsion’ will be used to refer to insoluble particles dispersed in water with, in most cases, monomers having been polymerised in the water. Emulsions are sometimes referred to as latices.

In actual practice very few polymers form true solutions and so this chapter will cover polymer dispersions and emulsions. Many resins are only soluble in a solvent/water mixture, others are dispersions of a water soluble resin in an emulsion, e. g. water soluble alkyd dispersed in an acrylic emulsion.

Although there is a continuous progression of properties from water soluble resins, through colloidal dispersions, to aqueous emulsions, it is possible to tabulate the typical properties of each as in Table 7-1.

An organic polymer is only soluble in water if it contains polar groups. Also, a low molecular weight helps to make a polymer more soluble. Therefore, thermoplastic water-soluble polymers are of limited use in coatings, because the hydrophilic groups would make the film water-sensitive and the low molecular weight would detract from the durability and flexibility of the film. These disadvantages are overcome in thermosetting polymers by crosslinking the film. But even then, the film may still have poor water resistance, if the polymer is truely water soluble, rather than water reducible. The departure of the neutralising agent from the film induces some degree of water insolubility and resistance, which is further enhanced during crosslinking, especially for water reducible resins.