Providing Moisture-Resistant Epoxy Adhesives

Resistance of adhesive joints to moisture can be improved either by preventing water from reaching the interface or by improving the durability of the interface itself. Several methods of minimizing degradation are possible. 

Selection of a base polymer having a low water permeability and diffusion coefficient 

Chemically modifying the adhesive or sealant to reduce water permeation 

Incorporating inert fillers into the adhesive to lower the volume that can be influenced by moisture 

Coating the exposed edges of the joint with very low-permeability resins 

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Polyamide cured DGEBA epoxies provide improved flexibility, moisture resistance, and adhesion over aliphatic amines alone. However, polyamide cured epoxies are generally inferior in thermal resistance and shear strength due to the reduction in crosslink density. Polyamide cured epoxies lose structural strength rapidly with increasing temperatures and... 

Epoxy-urethane adhesives provide properties when cured that are similar to those of epoxynylon adhesives except they offer a major improvement in moisture resistance. Isocyanate monomers and prepolymer react with the hydroxyl groups on epoxy resins to give tough,... 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

One material that has been used to replace asbestos in certain applications is wollas-tonite or calcium metasilicate. This is also a fibrous mineral filler but with a lower aspect ratio than asbestos. Surface-treated versions are available to improve adhesion to the epoxy matrix. It can be applied at relatively high loading levels to provide for high strength and improvements in moisture resistance.25... 

There have been many investigations to determine the best chemical structures to provide for resistance to moisture and hydrolysis. Attempts have been made to synthesize epoxy adhesives with improved water resistance by replacing some hydrogen atoms by fluorine.50 However, the cost of processing of such materials has restricted commercial development. For electronic sealants, it is highly desirable to keep moisture from penetrating into critical areas. Hydrophobic polymers have been developed to accomplish this task. They are siloxyimides, fluorosilicones, fluoroacrylics, phenylated silicone, and silastyrene. 

Glass is efficiently bonded with UV-aerobic acrylics, and replaces epoxies, anaerobics, or heat-fusing techniques. A manufacturer of pressed glass stemware required an adhesive that would resist repeated exposures to detergent and water while providing a faster cure rate than an epoxy bonding process. A transparent UV acrylic was developed that cured a 2-mil gap completely in 2.5 seconds under a 250-watt mercury lamp. It did not discolor on aging and was moisture resistant (Fig. 2). 

Epoxy adhesives are also easy to modify for special purposes. For example, they can be filled with carbon, silver, or gold to provide electrical conductivity. Other additives can enhance thermal conductivity, while maintaining electrical insulation. Additional performance properties of epoxy-based adhesives that can be modified include impact resistance, shrinkage, glass transition temperature, high-temperature strength, surface specific adhesion characteristics, and chemical or moisture resistance. 

Nylon-epoxy is typically used in film and tape adhesive applications and has maximum service temperature of 138°C, compared with 177°C for unmodified epoxies. The addition of nylon provides increased flexibility and peel strength compared to unmodified epoxies. Nylon-epoxy is a tough material with excellent tensile lap shear strength and good fatigue and impact resistance. However, the addition of nylon results in deterioration in creep resistance and poor peel strength at low temperatures. Furthermore, the hydrophilic nature of nylon imparts poor moisture resistance in both uncured and cured forms of the resin. Consequently, nylon-epoxies are not as durable as elastomer-epoxies or other thermoplastic modified epoxies. 

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