Waterborne Epoxy Adhesives

Single-component epoxy adhesive formulations are the largest type of epoxy adhesives sold, with about 55 percent of the consumption, while two-component formulations account for another 44 percent of the volume. Radiation cure formulations represent the remainder of the market. Epoxy adhesives can also take many forms including solids, solvent-free liquids, solvent-borne systems, and waterborne systems. 

Both of these processes are applicable to waterborne epoxy adhesives and coatings, although the emulsification process is generally used with adhesives. 

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. 

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. 

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. 

Epoxy adhesives that are available as waterborne emulsions... 

This section reviews the chemistry behind waterborne epoxy adhesives and the formulation possibilities. The characteristics of epoxy dispersions and the performance properties of cured adhesive films are addressed. The advantages and disadvantages of these adhesive systems are discussed with the focus on determining whether waterborne epoxy systems can replace traditional epoxy adhesives. 

In addition to the excellent performance properties and the reduction of solvent carriers, waterborne epoxy adhesives were found to have processing advantages. They could be easily applied by conventional coating systems (spray, roller, etc.) they were less hazardous to workers due to lower dermatitis potential and inflammability ventilation equipment costs could be reduced and application equipment could be easily cleaned with soap and water. In many applications, these processing advantages became the primary market drivers for waterborne epoxy adhesives as alternatives to more conventional adhesives. 

There are many applications for polymeric waterborne adhesives. These include packaging adhesives, pressure-sensitive tape, coatings for textiles, wood adhesives, and various industrial adhesives and coatings. The potential applications for waterborne epoxy adhesives are more limited due to their lack of tack and pressure-sensitive characteristics and the time it takes for the chemical reaction to complete cure. However, waterborne epoxy systems have found significant markets in niche areas. 

Because of the environmental acceptability and economic attractiveness of waterborne epoxy adhesive, one may confidently predict increased research and development in these areas. New products and applications will continue to develop however, the adhesive for-mulator must be creative in choosing waterborne raw materials and formulating products that meet both regulatory and customer requirements. 

The applications and performance characteristics of waterborne epoxy adhesives can be significantly improved by the incorporation of additives and modifiers into the adhesive formulation. Fillers such as calcium carbonate, talc, and silicas are often used to adjust the viscosity of the liquid adhesive and the thermal expansion, modulus, and strength characteristics of the cured adhesive film. 

Reactive diluents can be used to reduce the modulus and increase the elongation of the cured waterborne epoxy formulations just as they are often used for 100 percent solids and solvent-borne epoxy adhesives. The reactive diluents become codispersed in the formulation with mechanical and chemical stability similar to that of the base epoxy emulsion. Polyglycidyl ether of caster oil, phenyl glycidyl ether, and diglycidyl ether of neophenyl glycol are examples of mono- and difunctional reactive diluents that have been used to improve flexibility and increase the tack-free time of waterborne epoxy adhesives. 

Adhesives commonly used on concrete must be formulated so that they cure well in the presence of moisture. The adhesive should have a viscosity that allows it to penetrate the concrete for mechanical bonding. Epoxy and epoxy-polysulfide adhesives are especially effective adhesives for concrete. The starting formulations given in Table 16.16 are for an ambient cure 100 percent solids epoxy and a waterborne epoxy adhesive. 

The waterborne emulsion is formulated so that the mixed adhesive can be easily cleaned with water, yet it provides good water resistance properties of a conventional epoxy adhesive. Bond strength of these adhesives on new concrete is greater than the strength of the concrete itself. 

Formulation details are then presented in Chapters 11 through 14 for the various possible forms of epoxy adhesive systems room temperature and elevated-temperature curing liquids, pastes, and solids. The more or less unconventional forms of epoxy adhesives are also identified and discussed, since these are now achieving prominence in industry. These include uv and electron beam radiation curable, waterborne systems, and epoxy adhesives capable of curing via the indirect application of heat or energy. 

Chem. Descrip. Mixf. of special polyols, emulsifiers, and polysiloxane Uses Air release agenf and defoamer for general purpose high-solids and industrial coatings (all ds, epoxies, acrylics, waterborne architectural), printing inks, adhesives, latex applies. 

Epoxy Solvent, waterborne, powder Air dry, bake Excellent adhesion, chemical resistance, flexibility, abrasion resistance, hardness Rapid chalking on exterior exposure, poor resistance to oxidizing acids, yellows in clears Moderate, hi Mainimance tainte, automotive primers, appliances, metal products... 

Chem. Descrip. Proprietary multi-hydrophobe blend Uses Defoamer for adhesives, waterborne industrial coatings, epoxy coatings, latex, acrylics, food-contact paper coatings food-pkg. adhesives Regulatory FDA 21CFR 175.105,176.200... 

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

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

Buehner and Atzinger, G.D., Waterborne Epoxy Dispersions in Adhesive Applications. ... 

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. 

Conventional silanes are limited to high-solids and solvent adhesive and sealant applications in which moisture is not encountered until use. However, water-stable epoxy silane promoters have recently been developed to enhance the wet strength of waterborne adhesives and sealants.17... 

Representative data are shown in Fig. 10.5 for aluminum joints bonded with an epoxy film adhesive and a standard chromate-containing primer. Up until recently, standard corrosion-resistant primers contained high levels of solvent, contributing to high levels of volatile organic compounds (VOCs) and chromium compounds, which are considered to be carcinogens. As a result, development programs have been conducted on waterborne adhesive primers that contain low VOC levels and little or no chrome. Data are presented on several of these primers in Tables 10.8 and 10.9. 

Epoxy dispersions also can easily be blended with other waterborne polymers to make modified latex adhesives. The resulting hybrid adhesive produces performance properties and application characteristics that are superior to those of the originating latex system. 

Many of the attributes of solvent-borne epoxy coatings could be carried over to the waterborne epoxy coatings. These same attributes are useful in the application of waterborne epoxies as adhesive systems. They include good adhesion to a variety of substrates such as metals, wood, concrete, glass, ceramics, and many plastics chemical resistance low shrinkage toughness and flexibility and abrasion resistance. 

However, waterborne epoxy systems are not without certain disadvantages, which have limited their application as adhesives. These disadvantages include increased use of energy to evaporate the water and dry the adhesive, lower resistance of the cured film to high-humidity environments, and storage and application limitations due to potential freezing at low temperatures. 

Waterborne epoxy coatings and adhesives have established the building and construction industry as their largest market. Commercial systems have been available for many years. The following characteristics propel their use over conventional alternatives ... 

Several hybrid epoxy emulsions have been commercially prepared. An epoxy emulsion blended with waterborne aliphatic urethanes exhibited peel strength on aluminum of 10 lb/in—1.5 times greater than with the polyurethane itself. The optimum concentration of urethane in the final emulsion was about 50 percent by weight.13 Epoxy-phenolic dispersions have also been developed to provide waterborne adhesive systems with high glass transition temperature and chemical resistance. 

TABLE 14.5 Waterborne Epoxy-Dicyandiamide Adhesive Tensile Shear Data16... 

Surfactants act as wetting agents by lowering the surface tension of the waterborne epoxy. Silanes can be used to increase adhesion to certain substrates and fillers, as shown in Table 14.4, formulation C. Water-compatible thickeners and protective colloids such as polyvinyl alcohol, substituted cellulosics and sugars, and some acrylics improve application properties and offset viscosity decrease seen with water dilution. 

Waterborne epoxy-dicyandiamide adhesive tensile shear data... 

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