Waterborne acrylic resin dispersions

The commercially available acrylic dispersions are broadly of two types acrylic latex and water-reducible acrylics. 

Acrylic latexes are high MW acrylic (co)polymer particles dispersed in water. Synthetic latexes are prepared by a radical polymerization mechanism using an emulsion polymerization technique. The emulsion polymerization is carried out in water using monomer(s), surfactant (emulsifier) and water-soluble initiator. In a typical manufacturing process, an initiator and a separate emulsion of monomer(s) in water are slowly added to a reaction vessel containing water and emulsifier, at a predetermined rate. Polymerization of monomers occurs within tiny pockets formed by aggregation of emulsifier molecules, called micelles, resulting in formation 

Commercial latexes are supplied both as thermoplastic and thermosetting types. Thermoplastic acrylic (and vinyl) latexes are very commonly used as binders for architectural paints and some specialty coatings. Such coatings are required to dry under ambient conditions without any chemical cross-linking and hence are designed to precise T and minimum film formation temperature. 

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Heating oil storage premises must be equipped with a collection trap so that any heating oil leaking from the tank cannot contaminate the soil. The interior of these premises must be painted with an officially approved coating material that is not dissolved or penetrated by heating oil. The coating must also cover cracks in the substrate. Multilayer systems based on waterborne acrylic resin dispersions are suitable for this purpose. 

Acrylic resins are generally well characterized by Py-GC/MS without the need for any derivatization reaction. However, in waterborne polymer dispersions it is common to have minor amounts of acrylic and/or methacrylic acid monomers added in the copolymerization to help the stability of the final latex. These monomers can also appear in the pyrolysis products, and it has been shown that with on-line derivatization they can be more efficiently revealed [85]. 

A second generation of phenolic dispersions, patented by J. S. Fry (33). involved the post dispersion of phenolic resins in a mixture of water and water-miscible solvents. To conform with air pollution regulations, the solvent was held to 20 volume %, or less, of the volatiles. A heat-reactive phenolic resin dispersion (34) and a phenolic-epoxy codispersion have become commercially available based on the above technology. Supplied at 40-45% solids, these products, which have a small particle size (0.75-1.0 ym), are better film formers than the earlier dispersions. Used alone or in blends with other waterborne materials, corrosion-resistant baking coatings may be formulated for coil coating primers, dip primers, spray primer-surfacers, and chemically resistant one-coat systems. Products of this type are also tackifiers for acrylic latexes, and such systems have been employed as contact, heat seal, and laminating adhesives for diverse substrates. 

Although two-pack epoxy primers and polyurethane intermediate coats have high solids contents, they still contain significant amounts (20-30 wt%) of organic solvents. In polyurethane topcoats, the VOC is even higher. Anticorrosive, waterborne primers based on aqueous dispersions of two-pack epoxy resins and one-pack acrylic resins have been developed to decrease solvent emission. Waterborne, one-pack acrylic topcoats are also used. All of these waterborne paints contain 2 - 5 % organic cosolvents that are required for film formation. 

Epoxy resins have been made water reducible by the use of acid functional acrylic resins. They find widespread application as internal lacquers for DWI beer and beverage cans (see Chapter 7- Waterborne Applications). They are often made in solution in butyl glycol and butanol and are then neutralised and dispersed into water under agitation. A full discussion of this technology and all of its variants is given in the Epoxy volume of this series of books. A brief outline of some of the chemistry will be given here. 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acrylic (52). The acrylic—vinyl monomers are grafted onto preformed epoxy resins in the presence of a free-radical initiator grafting occurs mainly at the methylene group of the aliphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, solid epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino—formaldehyde resin. 

Chemical modification of the epoxy resin includes either attaching hydrophilic groups to the epoxy resin or attaching the epoxy resin to hydrophilic polymers. This is most often done by grafting. For example, one of the largest volume uses for waterborne epoxy is the coating of metal cans. In this application the epoxy resin is rendered water-dispersible by the grafting of the epoxy resins to acrylic polymer. 

For many years, paints used to coat cans contained considerable amounts of volatile organic solvents. Waterborne can coatings were developed to reduce solvent emissions and are used worldwide. Binders used in waterborne can coatings are modified epoxy resins (see Section 2.10). Acidic acrylate chains are grafted onto an epoxy molecule. After partially neutralizing with amines, the resins can be dispersed in water. 

EcoSynthetix markets EcoSphere biolatex binder dispersions as a replacement for petroleum-based styrene butadiene latex. EcoSphere biolatex binders are based on starch derived from crops such as com, potatoes, and tapioca. Although the product was originally developed for the paper coating industry, it can also be applied in the textile coating industry. EcoSynthetix also produces EcoMer , a biobased building block to synthesize waterborne sugar-acrylic adhesives and resins. 

The use of polymeric blend composites for corrosion protection of AA 2024-T3 has been reported, including composites formed by incorporating water-soluble conducting polymers (either polymethox-yaniline sulfonic acid or poly(4-(3-pyrrole) )butane sulfonate) into various binders (a cross-linked polyvinyl alcohol, a waterborne epoxy, a modified water-dispersible polyester, and a UV-curable urethane acrylate binder) [149]. The preparation of epoxy and polyaniline composite coatings has been described, using either nanodispersed EB particles [91] or EB that was first dissolved in selected amine hardeners before adding the epoxy resin [98]. Even with very low EB loadings, these workers reported enhanced corrosion protection for steel. 

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