Waterborne Epoxy Resins

Uses Emulsifier for epoxy resins, waterborne industrial coatings Properties Dens. 9.1 Ib/ l vise. 12,000 cps HLB 18 nonionic 65% act., 65% solids, 35% water Use Level 3-5% solids on epoxy resin Hydropalal 88 [Cognis/Coatings Inks Cognis KGaA/Coatings]... 

Waterborne Costing s. These coatings utilize either Hquid or soHd epoxy resins that have been modified to allow their use with water. They are usually in the form of emulsions, suspensions, dispersions, or water-dilutable resins that can be heat- or RT-cured. They are appHed by convenient methods such as roUer-coating, dipping, spray, or electro deposition. 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acryHc (52). The acryhc-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 aHphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, soHd 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. 

Epoxy resins are commercially available as either liquids or solids. The liquids are available as (1) solvent-free resins, ranging in viscosity from waterlike liquids to crystalline solids (2) waterborne emulsions and (3) solvent-borne solutions. Generally, the higher the molecular weight of the epoxy resin molecule, the higher the viscosity or melting point. 

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. 

The emulsification method is primarily used for waterborne epoxy adhesive systems and is the focus of this section. The epoxy resin is made water-dispersible by partitioning the epoxy resin within a micelle, effectively separating the resin from the water. This emulsification can be achieved by a suitable surfactant. 

There are several epoxy resin chemistries that are appropriate for waterborne adhesives. These can be broadly classified into three types ... 

The base epoxy resin can be either liquid or solid. As molecular weight increases, the epoxy equivalent weight and the number of hydroxyl groups available for reaction increase. Waterborne epoxy adhesives provide excellent adhesion to metals and other high-energy substrates. Modified waterborne epoxy adhesives can also provide good adhesion to substrates such as vinyl and flexible plastic film. Characteristics of these epoxy dispersions are summarized in Table 4.6. 

Buehner, R. W., and Atzinger, G. D., Waterborne Epoxy Dispersions in Adhesive Applications, Epoxy Resin Formulators Conference, San Francisco, February 20-22, 1991. 

Water is also sometimes used as a solvent for water-soluble resins. In the case of epoxy resins, water is generally used to disperse epoxy particles in an emulsion. These waterborne epoxy adhesives are discussed in Chaps. 4 and 14. 

The solvent industry has made significant strides in developing newer grades and blends of solvents for a variety of applications. Safety solvents are being developed that are low in volatility (vapor pressure), low in toxicity, and biodegradable. However, these newer solvents are finding commercial acceptance mainly as cleaning solvents rather than as a dilution medium for epoxy resins. For adhesives, rather than replace the solvent, the trend has been to develop waterborne emulsions. 

Bauer, R. S., Formulating Weatherable Epoxy Resin for Maximum Performance, Waterborne and Higher Solids Coating Symposium, New Orleans, 1982. 

Both processes are applicable to waterborne epoxy adhesives and coatings, although the emulsification process is generally used with adhesives. Preparation of epoxy resin emulsions is covered in Chap. 4. 

EPI-REZ 3519-W-50 560-660 5,000-15,000 Waterborne dispersion of a CTBN modified epoxy resin Resolution... 

One can rationalize a need for small rubber inclusions in some of the newer approaches to waterborne and high solids epoxy coating systems. Water-thinned epoxy coating compositions are described (48) where the two-component system consists of a nitrile rubber modified epoxy resin in the epoxide component and a styrene/ butadiene/methylmethacrylate latex modifier for an emulsion-based polyamide hardener component. Showing improved adhesion, impact and water resistance, the paint has good wetting characteristics and can be formulated to a high solids content at low viscosity. 

COOH HOOC xAr COOH branched, low molecular mass, carboxy-functional 1000- 5000 triglycidylisocya-nurate, epoxy resins, melamine resins powder coatings, waterborne paints... 

Waterborne systems based on resol-epoxy resin precondensates are already at an advanced development stage. Carboxyl groups are introduced into the preformed resols and made water soluble by salt formation with amines. These systems offer a significant saving in solvent compared with that of conventional high-solvent, high-viscosity products. 

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. 

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. 

Pigmented, two-pack, waterborne epoxy resin coatings (garages) or one-pack waterborne acrylic resin emulsion paints are used for areas that receive less wear (e.g., cellars). Wooden parquet floors are coated with one- or two-pack polyurethane varnishes that can be applied by spraying or brushing. Acrylate-based waterborne parquet varnishes are also used because they are environmentally friendly. 

---