Nitrile epoxy adhesives

Epoxy-nitrile Nitrile-epoxy adhesives are composed of solid epoxy resin modified with carboxyl-terminated butadiene nitrile (CTBN) copolymer. The CBTN is introduced into die epoxy resin at elevated temperatures. The modification provides toughness and high peel strength without sacrificing heat and chemical resistance. The film adhesives are widely used in the aerospace industry in the construction of jetliners. 

As already mentioned, the weakest component of an aluminum joint is the oxide itself rather than the adhesive/oxide interface. The use of coupling agents would not be expected to provide an enhancement in durability under these circumstances. However, Brockmann has found that priming aluminum with 2,4,5,7-tetrahydroxyflavonene can substantially improve the durability of joints prepared with phenolic or nitrile-epoxy adhesives. This compound may be acting as a hydration inhibitor rather than as a coupling agent. 

Table 26. Low Temperature Curing Nitrile Epoxy Adhesive.

The most common aerospace adhesives include nitrile epoxy systems, epoxy polyamides, epoxy phenolics, and various unmodified epoxies [18]. Polyimides... 

Corrosion-inhibiting primers based on this technology have been in continuous service since they were first utilized with nitrile epoxies in the late 1960s. These inhibitors function by passivating the aluminum. In this process, water permeating the adhesive bondline carries a certain amount of inhibitor to the oxide surface. 

Primer Epoxy vs. Nitrile-Modified Epoxy. The compatibility of the epoxy-polyamide primer with the nitrile-modified epoxy adhesive facsimile and the aluminum oxide surface was also evaluated by the wedge test, since earlier tests using the primer as the adhesive had failed immediately. As shown in Fig. 8, the addition of the primer directly to the prepared... 

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

Adsorbed NTMP exhibits a pH-dependent surface coverage on anodized aluminum, which Includes a region characterized by a multilayer of hydrogen-bonded phosphonate molecules. These thick layers are weak and fall to provide good bond durability in a humid environment. NTMP monolayers are protective against hydration and are compatible with a nitrile-modified epoxy adhesive, but not with an epoxy-polyamide primer topcoat. 

The early aerospace adhesives were primarily based on epoxy resin chemistry. However, unique applications requiring high temperatures and fatigue resistance have forced the development of epoxy-phenolic, epoxy-nitrile, epoxy-nylon, and epoxy-vinyl adhesives specifically for this industry. The aerospace industry has led in the development and utilization of these epoxy-hybrid adhesives. 

TABLE 7.3 Comparison of Bond Strengths of Epoxy Adhesive Formulations with and without Nitrile Rubber Addition1... 

Test temperature, °C Nitrile-modified epoxy adhesive Unmodified epoxy adhesives ... 

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... 

Exposure time, months Epoxy-nylon adhesive Nitrile-phenolic adhesive ... 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

Epoxy-nitrile adhesives have also been developed for the purpose of toughening conventional epoxy adhesives. These early adhesives were made from homogeneous blends of nitrile elastomer and epoxy resin. They are not similar to the modern-day CTBN toughened epoxy adhesive where the nitrile phase consists of discrete particles. An advantage of the... 

Certain adhesive systems are more resistant to interfacial degradation by moist environments than are other adhesives. Table 15.16 illustrates that a nitrile-phenolic adhesive does not succumb to failure through the mechanism of preferential displacement at the interface. Failures occurred cohesively within the adhesive even when tested after 24 months of immersion in water. A nylon-epoxy adhesive bond, however, degraded rapidly under the same conditioning owing to its permeability and preferential displacement by moisture. 

Nylon-epoxy adhesive, psi Nitrile/phenolic adhesive, psi... 

All of the commercial epoxy adhesives presented in App. B bond well to aluminum and to a wide variety of other materials. Sell22 has ranked a number of aluminum adhesives in order of decreasing durability as follows nitrile-phenolics, high-temperature epoxies, elevated-temperature curing epoxies, elevated-temperature curing rubber-modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes. 

Epoxy and nitrile-phenolic adhesives have been used to bond polyolefin plastics after plasma surface preparation. Shear strengths in excess of 3000 psi have been reported on... 

Various adhesives can be used to bond polyphenylene oxide to itself or to other substrates. Parts must be prepared by sanding or by chromic acid etching at elevated temperature. Methyl alcohol is a suitable solvent for surface cleaning. The prime adhesive candidates are epoxies, modified epoxies, nitrile phenolics, and polyurethanes. Epoxy adhesive will provide tensile shear strength on abraded polyphenylene oxide substrates of 600 to 1300 psi and 1300 to 2200 psi on etched (chromic acid) substrates.71... 

Plasticizer migration from the vinyl part into the adhesive bond line can degrade the strength of the joint. Adhesives must be tested for their ability to resist the plasticizer. PVC can be made with a variety of plasticizers. An adhesive suitable for a certain flexible PVC formulation may not be compatible with a PVC from another supplier. Nitrile rubber adhesives have been found to be very resistant to plasticizers and are often the preferred adhesive for flexible PVC films. However, certain epoxy adhesive formulations have also been found to provide excellent adhesion to flexible PVC substrates. Several such starting formulations are presented in Table 16.7. A comparison of the performance of several classes of adhesive when bonding PVC to itself and to various other materials is given in Table 16.15. 

Structural adhesives that are commonly used for composites are supplied in two basic forms semisolid B-stage film and thixotropic pastes. The film adhesives are cast or extruded onto carrier fabrics or films and partially cured to a semisolid. They can easily be handled, cut, and applied to the joint area. There is no need for mixing, metering, or dispensing of liquid components. In use, these adhesive systems are activated by heat and pressure. The semisolid B-stage film liquefies briefly on application of heat and then cures to an insoluble state. Epoxy, polyimides, epoxy-nylons, epoxy-phenolic, and nitrile-phenolic adhesives are available as B-stage film. 

Adhesives with filleting properties are required for honeycomb cores. A modified phenolic is often used with aluminum honeycomb for high strength, while a neoprene- or nitrile-based organic solvent type of adhesive is often used with impregnated paper honeycomb. Epoxy adhesives are also commonly used in the fabrication of honeycomb sandwich panels. 

Obviously, adhesive performance was closely associated with compatibility between the PPU/ETBN blend and epoxy matrix as well as between PPU and ETBN. As in CTBN-modified epoxy adhesives (Figure 6), the nitrile content of CTBN played an important role. When PPU containing a polypropylene oxide) segment with Mn 2000 g/mol was blended with ETBN, both lap shear strength and T-peel strength increased with increasing nitrile content. The best results were obtained with ETBN derived from CTBN containing 26 wt% acrylonitrile. 

Fig. 13.7 The TEM image of the nanophase-separated structural epoxy adhesive containing nitrile rubber/phenolic polyurethane liquid rubber blends (cure with dicyandiamide at 180°C).

The nitrile-epoxy structural adhesive systems have moved from an aircraft orientation to include industrial, automotive and electronics areas. These include a host of film adhesive products (21, 22) and thixotropic pastes (23-26). Representative adhesive models have also been studied from a fracture toughness polnt-of-view permitting comparisons of bulk fracture to that of fracture in the adhesive joint (27, 28). 

The precedence for nitrile-epoxy alloys in other epoxy applications suggests that similar compositions in coatings should enhance film properties associated with flexibility, impact resistance and adhesion to metals. The testing scheme employed emphasizes these property features. 

According to one author, epoxy-phenolic adhesives for high-temperature applications were developed during World War II at Forest Product Laboratories in Madison, Wisconsin, and nitrile phenolic adhesives shortly after World War II [9, pp. 153, 156]. A patent for epoxy resins was applied for in Germany in 1934, and the inventor disclosed that it could be hardened with equivalent amounts of amines, diamines, or polyamines and that it showed strong adhesion [14, p. 8]. Epoxy resins are believed to have been commercialized in the United States first by the former Jones Dabney Company sometime after 1942. 

Structural adhesives are not usually applied directly to the prepared metal. Rather, a primer is applied first. The primers are usually of the same generic chemical composition as the adhesive being used over them (i.e., epoxy primer to epoxy adhesive). The primers may or may not contain a corrosion inhibiting pigment and may or may not have to be cured before application of the adhesive. EC-1593 is a 350 F (121 C) curing nitrile-phenolic primer while EC-3917 is a chromated epoxy-phenolic primer curing... 

Durability (63,64) of bonded assemblies continues to receive the prime attention it deserves in the performance regime of structural adhesive testing. And aging studies of selected nitrile-epoxy structural adhesive models are being reported (65,66). 

C.E.M. Morris, etal., "Short-Term Aging Effects in a Nitrile-Epoxy Structural Adhesive," J. Macromol. Sci.-Chem, A 17 (2), 227-242, 1982. 

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