Лако-красочные материалы — производство

Hydrophobic groups are polymerised into the backbone of the polymer chain so that they are pendant to the backbone along with the carboxyl.

This class of thickeners is described as ‘associative’ since the hydrophobic groups associate with emulsion particles present, through adsorption onto the particle surface. In addition, they aggregate together in the water phase in a similar way to that in which surfactants form micelles, as shown in Figure 2-10.

The formation of micelles results in a three dimensional transient network in which the latex particles serve as the branch points and the thickener molecules act as associative crosslinks. These crosslinks resist the stretching that must occur when shear is applied to the system. In practice, this means improved brush drag properties. The transient nature of the crosslinks, due to the dynamic equilibrium of the micelle which is continually forming, breaking and re-forming allows the system to flow under low shear resulting in excellent levelling.

Because this type of thickener relies on the micelle effect, a critical micelle concentration must be achieved before full thickening is achieved. At normal paint viscosities, this critical micelle concentration will have been passed and the optimum associative action and benefit will be available.

These associative effects and parameters offer the paint formulator an improved thickening efficiency and highly desirable rheological behaviour. Under high shear conditions, when paint is being applied by brush or roller, the micellar structure exhibits a strong tendency to resist disruption, giving good brush drag characteristics and preventing the natural tendency of the paint under shear to break up into small droplets, known as spatter.

Conversely, in low shear conditions when the paint film is levelling, the state of dynamic equilibrium which exists in the micelles results in a wet paint film with good flow out characteristics. This helps to eliminate brush marks or roller ribbing as the paint film dries.

The chemistry available to design these hydrophobically modified acrylics is quite extensive.

For example, the chain length of the hydrophobe may be varied, the degree of branching, the degree of ethoxylation thus changing the HLB value and, of course, the concentration of the hydrophobe in the polymer.

Clearly, it is possible to produce materials which are more optimised for a given latex, for use in a given range of paints. As raw materials in the modem paint industry are being rationalised to reduce large inventories, this approach has proved to be of commercial interest.