Acrylic Resins (see also Synthetic Resins). The polyacrylic resins have been
recently developed. Neher has outlined the history of the work on this class
of compounds and he credits their industrial development to Otto Rohm of
Darmstadt. Chemically, they are closely related to the vinyl resins (see Vinyl
Resins), for they have a CEU = CH—group in common. Although solid polymers
can be made from acrylic acid, CH2 = CH-COOH, and from methacrylic acid,
CH2:C(CH3)COOH, it has been found that the esters of these acids lend them-
selves better to the formation of useful resins. Most useful is that made by the
polymerization of methyl methacrylate, CH2:C(CH3)COOCHs, often referred to
as methacrylate resin.
Methyl methacrylate monomer is a volatile liquid of low viscosity which
boils at 100.3° C. Polymerization is autocatalytic and is easily effected by
light, heat, and oxygen. The polymer is a hard, strong resin which has the
clarity of glass. It is a linear polymer and is thermoplastic, although its softening
temperature is high (12,5° C.)- Now it is used chiefly as a plastic for clear or light-
colored, molded articles. For these it is more suitable than polyvinyl acetate,
because it is harder, is less rubbery, and has little cold flow. It can be worked
well mechanically. The solid resin is so clear that printed matter can be read
through masses of it several inches thick with perfect visibility. It is insoluble
in water, alcohols, and petroleum hydrocarbons (Anonymous, * Methacrylate
Resins/ p. 1163), and is soluble in esters, in ketones, in aromatic and in chlorinated
hydrocarbons. Lacquers and protective coatings may be made by dissolving the
clear resin in these solvents singly or in combination. In general, the solubility
is lower than that of polyvinyl acetate. The acrylic resins are characterized by
their strong adhesion to most surfaces, and advantage may be taken of their
thermoplastic properties to effect good adhesion. Ultra-violet transmissibility and
stability to light are high. The refractive index is 1.482 to 1.521* Polymerized
methyl methacrylate is supplied as a molding powder and in made-up forms
under the trade name, 'Lucite.*
In addition to methyl methacrylate, other methacrylic ester polymers are
available, including ethyl, w-propyl, isobutyl, and 7z-butyl. These have become
commercially important as materials for protective coatings and lacquers. Strain,
Kennelly and Dittmar supply data on their physical properties, solubilities, and
compatibilities with resins and plastics. As the molecular weight of the esterified
alcohol radical increases, the polymers become softer and more plastic. Film-
forming and adhesive properties, as well as solubility and compatibility, also
change markedly along the series from methyl esters to the higher esters. The
higher esters become increasingly more miscible with aliphatic type solvents, the
butyl and isobutyl esters being soluble in petroleum solvents. Strain presents
data which show wide variations in viscosities of methyl methacrylate polymers
made from different solvents in the same concentrations. Toluene gives lower
viscosity for the polymer than any other single solvent tested.