Waterborne epoxy

Waterborne epoxy dispersions have been employed effectively for many years in the coatings market, primarily for the surface protection of concrete and metals. These products were developed in response to environmental regulations to reduce solvent levels in coatings. Since these... 

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

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. 

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. 

TABLE 4.7 Typical Effect of Dilution and Shear on Viscosity of a Waterborne Epoxy Resin9... 

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. ... 

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. 

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. 

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. 

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. 

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. 

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. 

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 ... 

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. 

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

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. 

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. 

Some waterborne epoxy systems may contain a proportion of water-miscible cosolvent to aid in film coalescence. Its presence may allow the formulator greater latitude to control properties such as stability, drying, and particularly rheology and still meet VOC levels required by pollution legislation. 

Epoxy modified polymer latex systems offer improved handling performance and moisture and chemical strength advantages over unmodified formulations. The wide range of latex polymers and the range of waterborne epoxy dispersions offer the formulator a wide latitude in performance characteristics required by specific applications. 

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. 

Waterborne epoxy-dicyandiamide adhesive tensile shear data... 

Waterborne epoxy formulations have been used quite extensively in some nuclear facilities with good success. They have not yet met the requirements of Class 1 testing, but perform well in certain areas such as radwaste handling areas. 

Vancote 2K [Vanderbilt], TM for a waterborne epoxy fast dry shop primer. 

Levulinic acid can be converted by known processes to the solvents, tetrahydrofuran and 2-methylfuran, to succinic acid, to 5-aminolevulinic acid for weed control (see Chap. 11) and to diphenolic acid. Diphenolic acid may be useful in waterborne epoxy coatings and other resins. If a suitable acid catalyst that can be left in the resin can be found, it may be possible to convert diphenolic acid, by self-acylation, to an analogue of phenol-formaldehyde resins, thus eliminating the use of the carcinogenic... 

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