Лако-красочные материалы — производство

A C atom in the backbone chain of an acrylic polymer may have different substituent side groups. Since the structure of the groups on either side may differ, the “chain” C atom itself will be asymmetric.

Thus, in acrylic polymers the group — CHX — may exist in one of two forms of steric isomer.

D form

Not»: Bonds in the plane of the paper are shown as———-

while bonds at right angles to the plane of the paper ere depicted as llllllf to represent a projection ‘out of tha paper and as to represent e

projection ‘into’ the paper.

R,

Figure 1 -57

The regularity of the polymer will be shown by the distribution of the D and L configurations. Polymers with highly regular distributions are referred to as tactic isomers, while those with random distribution are said to be atactic isomers.

Tactic isomers of acrylic polymers can be further classified on the basis of the repetition of the configuration of the asymmetric C atom. Where repetition is consecutive, the isomer is said to be isotactic. Where the configuration of successive asymmetric C atoms alternates, the polymer is said to be syndiotactic.

The properties of tactic polymers show a marked difference from those of an atactic polymer and this fact is used in the formulation of polymers for surface coatings. The presence of symmetry has a profound effect on the ability of the polymer to crystallise which in turn affects the physical properties such as softening point and chemical properties such as solubility.

The reduced solubility of the more stereo regular polymers limits their use in surface coatings prepared from solutions where the non-volatile content is normally very low at application viscosity. Thus, it is normal to carry out polymerisations under conditions which will favour atactic polymers and minimise the propagation of stereo regular structures.

The difference between the properties of stereo regular and atactic polymers is illustrated by the comparison for polymethyl methacrylate, shown below: