© 2006 by Taylor & Francis Group, LLC

18 Corrosion Control Through Organic Coatings
Poly(methyl methacrylate) is quite resistant to alkaline saponification; the problem
lies with the polyacrylates [6]. However, acrylic emulsion polymers cannot be
composed solely of methyl methacrylate because the resulting polymer would have
a minimum film formation temperature of over 100°C. Forming a film at room
temperature with methyl methacrylate would require unacceptably high amounts of
external plasticizers or coalescing solvents. For paint formulations, acrylic emulsion
polymers must be copolymerized with acrylate monomers.
Acrylics can be successfully formulated for coating zinc or other potentially alkali
surfaces, if careful attention is given to the types of monomer used for copolymerization.
2.2.2.3 Copolymers
Most acrylic coatings are copolymers, in which two or more acrylic polymers are
blended to make the binder. This practice combines the advantages of each polymer.
Poly(methyl methacrylate), for example, is resistant to saponification, or alkali
breakdown. This makes it a highly desirable polymer for coating zinc substrates or
any surfaces where alkali conditions may arise. Certain other properties of methyl
methacrylate, however, require some modification from a copolymer in order to form
a satisfactory paint. For example, the elongation of pure methyl methacrylate is
undesirably low for both solvent-borne and waterborne coatings (see Table 2.2) [7].
A “softer” acrylate copolymer is therefore used to impart to the binder the necessary
ability to flex and bend. Copolymers of acrylates and methacrylates can give the
binder the desired balance between hardness and flexibility. Among other properties,
acrylates give the coating improved cold crack resistance and adhesion to the substrate,
whereas methacrylates contribute toughness and alkali resistance [3,4,6]. In
waterborne formulations, methyl methacrylate emulsion polymers alone could not
form films at room temperature without high amounts of plasticizers, coalescing
solvents, or both.
Copolymerization is also used to improve solvent and water release in the
wet stage, and resistance to solvents and water absorption in the cured coating.
Styrene is used for hardness and water resistance, and acrylonitrile imparts
solvent resistance [3].
TABLE 2.2
Mechanical Properties of Methyl Methacrylate
and Polyacrylates
Methyl methacrylate Polyacrylates
Tensile strength (psi) 9000 3-1000
Elongation at break 4% 750-2000%
Modified from: Brendley, W.H., Paint and Varnish Production, 63, 19, 1973.
© 2006 by Taylor & Francis Group, LLC