Nitrile-phenolic adhesive formulation

Supported films of nitrile phenolic adhesives are formulated with excess phenolic resin which gives a more brittle polymer with resin on its surface. This surface excess obviates the need for priming the metal substrate with nitrile-phenolic before bonding. The brittleness of the adhesive is offset by the presence of fabric as the carrier. Solvent based nitrile phenolics contain an excess of nitrile rubber. Materials which are used as the support comprise cotton nylon polyester glass tissue paper. The support may be either woven or non-woven. 

Nitrile-phenolics are formulated as two-part adhesives dissolved in ketonic, chlorinated or, most frequently, in blended solvents. Some one-part materials are available which do not contain vulcanizing ingredients for the nitrile rubber but rely on a novolac resin with the addition of hexamine to introduce some cross-linking into the rubber. 

Epoxies are not the only resins used in aerospace bonding. A common formulation is a nitrile/phenolic film adhesive. A typical formulation may consist of the following (pbw) ... 

Nitrile-phenolic and nylon-epoxy adhesives are also available as solvent solutions as well as in film form. The nitrile-phenolic film is cured at 150-260°C for 15-30 minutes with bonding pressures from 0.12 to 1.8 MPa. The liquid alloy is dried at 80°C and cured for 15-30 minutes at 90°C and contact at 0.7 MPa of pressure. The nylon-epoxy paste is cured for 3 days at 20°C to 1 day at 150°C under bonding pressure from 0.11 to 0.32 MPa. Cure temperatures for some formulations can be increased to 200°C with corresponding reduction in cure time (4 hours). No volatiles are released during cure, so large areas can be bonded without venting. " ... 

The durability of the bonded joints was greatly influenced by the nature of the adhesive the best performers in all climates were epoxy-novolak and nitrile-phenolic formulations. A tropical, hot-wet climate was the most damaging to bonded stmctures and the combination of high humidity and applied stress was particularly deleterious. During exposure to natural environments, the failure mode of aluminium joints was found to change gradually from wholly cohesive, within the adhesive, to include increasing amounts of interfacial failure (see Stress distribution mode of failure). 

Polymer blends are often used in adhesive formulations where properties associated with rigid polymers (high temperature resistance, chemical resistance, etc.) must be obtained along with properties associated with tough, elastic polymers (impact strength, high peel strength, etc.). Examples of these adhesive systems are nylon-epoxy, phenolic-nitrile, epoxy-polysulfide, epoxy-nitrile, and epoxyurethane. 

Elastomer epoxies generally contain nitrile rubber as the elastomeric component. This system is also referred to as a modified or toughened epoxy. One of the applications of widest use is in films and tapes. Elastomer epoxies cure at low pressures and low temperatures over a short time interval. This is achieved by adding a catalyst to the adhesive formulation. Bond strengths of elastomer epoxies are lower than those of nylon epoxies. However, the major advantage of elastomer epoxies is their sub-zero peel strengths, which do not decrease as fast as those of nylon epoxies. In addition, the moisture resistance of elastomer epoxies is better than that of nylon epoxies but not as good as that of vinyl-phenolics or nitrile-phenolics. Limitations to the use of elastomer epoxies include poor water immersion resistance and poor properties when exposed to marine conditions. 

The phenolic family of adhesives are very diverse in their formulations and uses. Some are filled or modified with other polymers (vinyl, nitrile or epoxy). The composition depends largely on the intended use, e.g. for temperature and chemical resistance, or for water-proof wood bonding (phenol-resorcinol-formaldehyde). The majority are heat-curing although some wood-bonding adhesives can be cured at room temperature (RT). The comparison table includes Typical Use to differentiate between the types of adhesives. Phenolic adhesives are generally poorly represented by mechanical property data. 

Structural applications of rubber base adhesives were also obtained using rubber-thermosetting resin blends, which provided high strength and low creep. The most common formulations contain phenolic resins and polychloroprene or nitrile rubber, and always need vulcanization. 

Nitrile rubber is compatible with phenol-formaldehyde resins, resorcinol-formaldehyde resins, vinyl chloride resins, alkyd resins, coumarone-indene resins, chlorinated rubber, epoxies and other resins, forming compositions which can be cured providing excellent adhesives of high strength, high oil resistance and high resilience. On the other hand, NBR adhesives are compatible with polar adherends such as fibres, textiles, paper and wood. Specific formulations of NBR adhesives can be found in [12]. 

Nitrile rubber/phenolic resin blends. Blends of equal parts by weight of a nitrile rubber and a phenolic resin in methyl ethyl ketone (at a 20-30 wt% total solids content) is suitable for many adhesive purposes. The more phenolic resin in the formulation, the greater the bond strength and brittleness of the NBR adhesive [67]. Table 10 shows the effect of phenolic resin on nitrile rubber properties. On the other hand, the higher the acrylonitrile content in the rubber. 

Epoxies, nitriles, and phenolics show relatively low diffusion rates and are less susceptible to moisture attack than most other polymers. As a result, these materials are often used in adhesive and sealant formulations where resistance to moisture is essential. 

In adhesives, because of their brittle nature, phenolics are generally formulated in add mixture with other polymers. They are used as the primary cross-linking agents for nitrile rubbers, poly(vinyl formal) and butyral resins, and epoxy resins. 

Structural Adhesives. Films of nitrile rubber/phenolic resins (i.e. tapes) have been employed in the aircraft industry for bonding metal to metal surfaces in both plain and honeycomb sandwich constructions. Several Wright Air Development Center reports " describe in detail the preparation of a nitrile rubber/phenolic resin tape, the metal surface preparation methods, and the methods of evaluating structural adhesives of this type. A formulation for a high-temperature liquid phenolic resin adhesive suitable for structural fabrication is given in Table 21. ... 

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