Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives such as surfactants. The polymeric binder combines with the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermoplastic or thermosetting binders based on coreactive components.
There is continual demand for lower VOC (volatile organic content) coatings in the can industry. For two piece drawn and ironed (D&I) aluminum beer/beverage cans the majority of commercially useful water based coatings have a VOC of about 3.6 lb/gal (minus water). Environmental regulations now and in the future require 3.1 lb/gal. or lower. It is well known in the can industry that satisfactory metal coverage becomes increasingly more difficult as VOC is reduced to 3.1 lb./gal and lower. While there are some commercially available wetting aids or surfactants which do allow some improvement in coverage at lower VOC's drawbacks occur in that some cause too much water sensitivity and others have to be used at too high a level to be effective and thereby become potentially more harmful for extractables, flavor, water sensitivity, etc. For example, commercially available FDA surfactants, such as sulfosuccinates in particular, provide some improvement in wetting/coverage when added to about 3.1 VOC water based coatings, but are not sufficiently effective until added at levels of about 0.5% based on solids (the coating being at about 18-22% solids at application). At this level, the surfactant has a significant influence on film properties, particularly 150.degree. F.-200.degree. F. water resistant, and can limit the range of curing (bake) cycles. The can industry varies in its bake schedules, with some can plants requiring bakes as short as 30 seconds at 360.degree. F. While commercially available polymeric surfactants (Additol XW-395 from Hoechst, GCA Chemicals Resiflow series) have in general been found to be more effective than anionic and nonionic monomeric surfactants, they exhibit limited usefulness because unclean can surfaces become more difficult to cover due to variations in can cleanliness, treatments, alloys, etc. An increase in additive either does not offer further improvement in wetting or causes an undesirable increase in blistering of the cured coating. Coordinating improved wetting while maintaining blister resistance is difficult to achieve.
It now has been found that certain carboxyl functional acrylic terpolymer polymeric surfactants provide the desired wetting properties to the coating at very low additive levels, desirably below about 0.1i% based on binder polymer solids. Accordingly, the ability to lower the solvent content of water based epoxy-acrylic can coatings and still maintain satisfactory spray coverage is enhanced by adding very low levels of a polymeric surfactant. The polymeric surfactant preferably is an acrylic terpolymer comprising an acrylic composition of 2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid, where the terpolymer has a relatively high acid number and a controlled molecular weight range. The polymeric surfactant effectively lowers the VOC and further improves coverage over variable interior can surfaces. Coatings based on epoxy-acrylic binders and containing very minor amounts of terpolymer surfactant can be spray applied to aluminum beer and beverage can interior surfaces and result in cured continuous films having negligible metal exposure or blistering in critical bend areas. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.