Nitrile rubber adhesive shear strength

Since the stress distribution across the bonded area is not uniform and depends on joint geometry, the failure load of one specimen cannot be used to predict the failure load of another specimen with different joint geometry. The results of a particular shear test pertain only to joints that are exact duplicates. To characterize overlap joints more closely, the ratio of overlap length to adherend thickness l/t can be plotted against shear strength. A set of l/t curves for aluminum bonded with a nitrile-rubber adhesive is shown in Fig. 7.11. 

Table 4.8 Adhesive shear strengths (nitrile rubber)...

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

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. 

Nitrile- phenolic Solvent solutions, unsupported and supported film Heat and pressure Excellent shear strength good peel strength superior to vinyl and neoprene—phenolics good adhesion Metals, plastics, glass, rubber Med... 

The thermosetting films are employed for metal-to-metal bonding in aircraft, as well as less demanding appliance, electrical, and automotive requirements. These adhesives are hybrids designed to provide an optimum combination of shear strength, elongation, and heat resistance. Nitrile-phenolic and epoxy-nitrile are the leaders, followed by vinyl butyral-phe-nolic, epoxy-nylon, and epoxy-phenolic. The nitrile rubber in the epoxy-nitrile is a carboxyl-terminated butadiene-nitrile polymer which reacts with the epoxy at the elevated cure temperature. 

Cohesive failure was found to be the predominant mode of failure for each rubber compound containing Saret 633 (Figure 8.7). Therefore, it would be expected that as the Saret 633 concentration is increased, the rubber compound would become stronger due to additional crosslinking, which would result in an increase in adhesive strength at the interface between rubber and substrate. This proved to be the case and is shown in Figure 8.8 for EPDM bonded to untreated steel. As the Saret 633 concentration was increased from 0 to 20 phr, the shear adhesion increased from approximately 0.55 MPa for the control to over 11.0 MPa. Cohesive failure was the predominant mode of failure at each concentration. Similar performance was observed for other rubbers, such as nitrile, natural, polybutadiene, silicone and hydrogenated nitrile. 

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