10-16 CHEMICAL RESISTANCE
10-16 CHEMICAL RESISTANCE
The chemical resistance of polymers depends on the chemical nature and molecular arrangement of monomeric units. The chemical attack on polymers is most often internal, involving softening, swelling, and loss of strength of the material. A general rule stating that like materials attract and unlike materials repel is helpful in predicting the chemical resistance of many polymers. Thus a polymer is more soluble in a solvent of similar chemical structure. Polymers having polar groups such as hydroxyl (OH) and carboxyl (COOH) are usually swollen or even dissolved by polar liquids such as water or alcohols, hut they are resistant to nonpolar solvents such as gasoline, henzene, and carbon tetrachloride.
Polymers with nonpolar groups such as methyl (CH 3 ) and phenyl (C 6 H 5 ) are resistant to polar solvents such as water and ethyl alcohol, but they are usually swollen or dissolved by nonpolar solvents such as gasoline, benzene, and carbon tetrachloride. Furthermore, polymers of more aromatic character are more soluble in aromatic solvents, whereas those of aliphatic character are Polymers with nonpolar groups such as methyl (CH 3 ) and phenyl (C 6 H 5 ) are resistant to polar solvents such as water and ethyl alcohol, but they are usually swollen or dissolved by nonpolar solvents such as gasoline, benzene, and carbon tetrachloride. Furthermore, polymers of more aromatic character are more soluble in aromatic solvents, whereas those of aliphatic character are
As the molecular weight of the polymer increases, its solubility or tendency to swell in a particular solvent decreases. Polymers of high molecular weight usually yield solutions of high viscosities. The symmetry of the molecular structure of the polymer also affects the resistance of polymers toward solvents. More crystalline polymers exhibit higher chemical resistance than do less crystalline polymers having the same chemical character. The higher resistance is due to a denser packing of the chain molecules, which makes the penetration of a solvent or other chemical substance in the material more difficult. The degree of crosslinking greatly affects the solubility of polymers, so that even a slight crosslinking insufficient to cause infusibility may make a polymer insoluble. Less effective is the influence of branching, which usually decreases the rate of dissolution, but it does not render a material completely insoluble. The behavior of a heavily branched polymer is similar to that of a slightly crosslinked one, and ¡lis usually very difficult to distinguish between these two types. Crosslinked polymers do not dissolve but only swell if interaction with solvents occurs at all.
Polymers generally have better resistance to attacks by acids and alkalies than metals do; however, they may contain certain vulnerable polar groups that may make them susceptible to attack. Thus alkalies, especially at higher concentrations or higher temperatures, may saponify the ester groupings in cellulose acetate, polyesters, polyvinyl acetate, and similar polymers. Non- oxidizing acids may also hydrolyze these materials in a similar manner. Polyamides (nylons) and polyurethanes having the linkages —NH ⋅ CO— and —NH ⋅ COO—, respectively, are also susceptible to attack by strong acids and alkalies. The relative resistance depends on the chain length of the polymethylene groups between the linkages. All polyolefins, PVC, ABS polymers, fluorocarbons, polystyrene, and others have excellent resistance to all acids and alkalies.