Epoxy adhesives tensile shear strength

FIGURE 3.9 Plot of tensile shear strength of an aluminum joint bonded with an elevated-temperature curing epoxy adhesive as a function of test temperature. 

FIGURE 7.1 The effect of temperature on the tensile shear strength of modified epoxy-phenolic compared to other hybrid adhesives (substrate is aluminum).4... 

Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

TABLE 7.6 Effect of Humidity and Water Immersion on the Tensile Shear Strength of Epoxy-Nylon and Nitrile-Phenolic Adhesives 10... 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

FIGURE 9.9 The effect of mineral extenders on the tensile shear strength of epoxy adhesive (the substrate is aluminum).29... 

With most room temperature curing epoxy adhesives, a significant drop in tensile shear strength occurs between 80 and 120°C. The exact temperature where the drop in strength occurs will depend very much on the specific curing agent used, the fillers or modifiers... 

As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile shear strength that is dependent on the type of filler and concentration. Table 11.10 shows the effect of filler loading on strength of a simple general-purpose, room temperature curing epoxy adhesive composed of liquid DGEBA epoxy mixed with 10 pph of a tertiary amine. 

A fast-curing polymercaptan-epoxy adhesive formulation is shown in Table 11.11. The gel time for the mixed adhesive is about 8 min at room temperature. Tensile shear strength on aluminum is reported to be 2270 psi at 23°C. Note that the different color pigments used in parts A (carbon black) and B (titanium dioxide) are to assist by visually indicating when the two components are well mixed. 

Figures 11.3 and 11.4 show the tensile shear strength development at 23 and -18°C, respectively, of mercaptan cured epoxy formulations relative to accelerated amine and ami-doamine curing systems. The practical cure times at -18°C illustrate why mercaptan cured epoxy adhesive systems are popular in the building and construction industries where the adhesive can be applied at cold, outdoor temperatures.

FIGURE 11.5 Effect of a flexibilized epoxy on tensile shear and peel strength of epoxy adhesive formulation.19... 

Typical properties of several epoxy-polysulfide adhesives are shown in Table 11.18. Room temperature tensile shear strength ranges from about 500 to 3000 psi, depending on the substrate and the adhesive formulation. Generally, tensile strength properties reach a maximum at about 10 to 30 percent polysulfide concentration. Peel strength can be as high as 20 lb/in. 

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. 

A typical dicyandiamide cured epoxy adhesive formulation is described in Table 12.7. These formulations are unaccelerated so that they have a shelf life of greater than 6 months at room temperatures. The adhesives will cure in 30 to 90 min at 149 to 177°C. Tensile shear strength on aluminum is in the range of 2500 to 3000 psi. 

A general-purpose two-component adhesive that will provide high tensile shear strength up to 150°C is described in Table 12.11. The base epoxy resin in this formulation is a mixture of an epoxy novolac and a liquid DGEBA epoxy resin. 

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... 

TABLE 12.13 Tensile Shear Strength of Epoxy Adhesive Cured with PMDA17... 

Table 12.14 shows a comparison of dicyandiamide cured epoxy adhesives formulated with and without a CTBN adduct. When compared to the control epoxy, the toughened formulation exhibits significantly higher peel strength and moderately higher tensile shear strength. CTBN modified epoxy adhesives are generally one-part systems, cured with dicyandiamide at elevated temperature. 

Recently, Acsion Industries and UCB Chemicals have undertaken a development program of cationic initiated epoxy adhesive for aluminum-to-aluminum, aluminum-to-composite, and composite-to-composite bonding. This has led to a series of bonding adhesives that have tensile shear strengths of 13 to 17 MPa on fiberglass-to-fiberglass substrates and 35 to 52 MPa on aluminum-to-aluminum substrates.11... 

TABLE 15.9 Tensile Shear Strength of Pyromellitic Dianhydride Cured Epoxy Adhesives at Elevated Temperatures22... 

Fillers were found to significantly increase the room temperature tensile shear strength of BTDA cured epoxy adhesives. However, other modifiers such as phenolic or nylon resin did not significantly improve the peel strength, flexibility, or temperature resistance of the unmodified formulation. 

Next to the epoxy-nylons and epoxy-urethanes, the epoxy-polysulfide adhesives show the greatest lap shear tensile strength at temperatures below 0°C. Bonded etched steel substrates show a tensile shear strength of 2900 psi at room temperature, and this increases to 3400 psi when the temperature is reduced to -156°C. Epoxy-nylon and epoxy-polysulfide are the only adhesives that show an increase in strength as the temperature is significantly reduced. 

The good low-temperature properties of epoxy-polysulfide adhesives are one reason why they have found significant use in the building and construction industry. Another reason is their excellent flexibility and ability to absorb stress and move with the thermal expansion and contraction of the substrates. However, in these applications the adhesive is generally not formulated for high tensile shear strength but rather for optimum elongation. 

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