Epoxy adhesives shear properties

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength. Future chapters discuss how the epoxy adhesive formulator can merge these properties. 

Epoxy adhesives have cohesive properties that significantly depend on the crosslink density. For example, torsional butt shear strength of epoxy aluminum joints has been noticed to decrease as Mc increased.20 However, flexibility and toughness are increased with increasing Mc. Figure 3.12 shows the general physical relationship between Mc and the physical state of epoxy resins. 

The degree of tensile strength improvement is often in the 50 to 100 percent range. The effect of various fillers and loading ratios on the strength properties of epoxy adhesive formulation is indicated in Fig. 9.9. The effect of different fillers loaded at a constant 100 pph is indicated in Table 9.12 for shear strength on phenolic laminate and aluminum substrates. 

Although most epoxy adhesives have good weather resistance, optimum properties are generally achieved when the adhesive has a combination of good water resistance and thermal shock resistance. Figure 11.6 illustrates the retention of tensile shear strength of copper and aluminum strips bonded with an amidoamine cured epoxy after 2 years of weathering in a temperate climate. 

PMDA or trimellitic anhydride has also been shown to provide epoxy adhesive formulations with high-temperature properties. Table 12.13 shows the elevated-temperature tensile shear strength of an epoxy adhesive cured with 4 pph of PMDA. Another specialized formulation employing PMDA was found to provide high shear strength when tested at 260°C even after aging 1000 h at 260°C.15... 

Composite adhesives are being used for a growing number of applications outside the automotive industry as well. For example, Dow Formulated Systems recently developed a foam core system for wind blades that is bonded by epoxy adhesives. The new system offers long-term dynamic behavior and shear strength properties that deliver the excellent mechanical strength and fatigue resistance necessary to achieve blade durability. 

Adhesives based solely on epoxy resins and amine curing agents give cross-linked materials that have high modulus but are very brittle and have no resistance to peel forces. Epoxy resins became much more useful when it was found that rubber flexibilizers could be added to increase crack resistance. Further, it was found that under appropriate reaction conditions, these rubbers could act to "toughen" the epoxy system. That is, addition of rubber flexibilizers tends to reduce the Tg and shear properties of an epoxy. When an epoxy is "toughened" little or no change occurs in the Tg and shear properties while a substantial increase in... 

Various types of polymeric modifiers have been researched as possible candidates to impart impact resistance of epoxy resins. Sometimes, properties other than impact resistance are to be modified. For example, for adhesive applications, the main focus is usually on improving shear and peel strength. In these cases, epoxies blended with elastomeric nitrile rubbers (Hycar CTBN, B.F. Goodrich), phenolics, nylons (soluble types such as DuPont Zytel-61), and pol5nirethanes are commonly used. For impact modification, there are several approaches, as discussed below. 

Epoxy adhesives such as Huntman s Araldite AW 134 with HY 994 hardener (cured for 15 min at 120°C) and Araldite AV 1566 GB (cured for 1 h at 230°C) give the best results with this engineering resin. Other adhesives that can be used are cyanoacrylate (Loctite 414 with AC primer), anaerobics (Loctite 638 with N primer), and silicone sealant (Loctite Superflex). The highest lap-shear strength was obtained with Araldite AW 134. This adhesive has balanced properties, good resistance to mechanical shock, thermal resistance to 100°C, and reasonable stability in the presence of aliphatic and aromatic solvents. Some solvents, particularly chlorinated hydrocarbons, will cause deterioration of the bond [30]. 

---