Nitrile rubber-based adhesives

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. 

The elastomers considered in this section have been selected considering the most commonly used in rubber base adhesives natural rubber butyl nibber and polyisobutylenes styrene-butadiene rubber nitrile rubber polychloroprene rubber (neoprene). Typical properties of these rubbers are shown in Table 2. 

Most rubbers used in adhesives are not resistant to oxidation. Because the degree of unsaturation present in the polymer backbone of natural rubber, styrene-butadiene rubber, nitrile rubber and polychloroprene rubber, they can easily react with oxygen. Butyl rubber, however, possesses small degree of unsaturation and is quite resistant to oxidation. The effects of oxidation in rubber base adhesives after some years of service life can be assessed using FTIR spectroscopy. The ratio of the intensities of the absorption bands at 1740 cm" (carbonyl group) and at 2900 cm" (carbon-hydrogen bonds) significantly increases when the elastomer has been oxidized [50]. 

Adhesives recommended include nitrile-epoxies, epoxies, silicones, cyanoacrylates, and rubber-based adhesives. ... 

Suggested adhesives include modified acrylics, epoxies, polyesters, resorcinol-formaldehyde, furane, phenol-formaldehyde, polyvinyl formal-phenolic, polyvinyl butyral, nitrile rubber-phenolic, polyisobutylene rubber, polyurethane rubber, reclaimed rubber, melamine-formaldehyde, epoxy-phenolic, and cyanoacrylates. For maximum adhesion primers should be used. Nitrile-phenoUcs give excellent bonds if cured under pressure at temperatures of 149 C. Lower-strength bonds are obtained with most rubber-based adhesives. 

Further information on rubber-based adhesives is to be found in specialized articles Natural Rubber-Based Adhesives, Nitrile Rubber Adhesives, Polychloroprene Rubber Adhesives Applications and Properties and Polychloroprene Rubber Adhesives Modifiers and Additives. 

Nitrile rubber, also known as nitrile-butadiene rubber (NBR), is a copolymer of acrylonitrile and butadiene (Fig. 1). As a base polymer for Rubber-based adhesives, it provides a number of specialized properties, which supplement those summarized in the article Rubber-based adhesives typical characteristics. NBR adhesives comprise a range of materials that may differ in proportion of comonomer or may be compounded with other resins. NBR adhesives are characterized by high oil and plasticizer resistance, excellent heat resistance and high adhesion to metallic substrates. 

Several elastomers can be used in rubber-based adhesives. The elastomer provides the backbone of the adhesive, so the main performance of the adhesive is provided by the rubber properties. However, several specific properties for application are imparted by adding other ingredients in the formulations. The most common elastomers used in rubber-based adhesives are natural rubber (NR), butyl rubber (BR) and polyisobutylenes, styrene-butadiene rubber (SBR), nitrile rubber (NBR) and polychloroprene rubber Neoprene) (CR). 

Various literature references discuss the use of nitrile rubber/phenolic adhesives for the bonding of metals.Tanaka discussed a two part liquid adhesive system based on solutions of nitrile rubber dissolved in methyl methacrylate. Aluminum plates were bonded together with the resulting adhesive. Kubo reported that the adhesive composition of Table 19 was good for bonding steel plates. 

Zinc is used in multiple applications. It is widely employed for galvanizing steel and iron against rust. It is also used for die casting and forming alloys including brass and bronze. This lustrous blue-white metal is hard/brittle at ambient temperatures. It is malleable at 100—150°C, conducts electricity and heat, is anticorrosive, and has a relatively low melting point (419.5°C). Zinc is the fourth most common metal today. Adhesives recommended include nitrile-epoxies, epoxies, silicones, cyanoacrylates, and rubber-based adhesives [21]. 

Butyl phenolic resin is a typical tackifier for solvent-borne polychloroprene adhesives. For these adhesives, rosin esters and coumarone-indene resins can also be used. For nitrile rubber adhesives, hydrogenated rosins and coumarone-indene resins can be used. For particular applications of both polychloroprene and nitrile rubber adhesives, chlorinated rubber can be added. Styrene-butadiene rubber adhesives use rosins, coumarone-indene, pinene-based resins and other aromatic resins. 

Cellular Cellulose Acetate Conventional adhesives recommended include polyurethanes, synthetic resins, thermoplastics, resorcinol-formaldehyde, nitrile-phenolic, and rubber-based materials (8). 

OTHER COMMENTS used as a flame retarder and plasticizer in chlorinated rubber, nitrile rubber and resins used in coatings and adhesives based on plasticized cellulose esters used as a gasoline additive to control pre-ignition useful as a solvent in the extraction of phenol fi om gas-plant effluents and coke-oven wastewaters use as a synthetic lubricant, a waterproofing agent, a primary component of adhesives, and as an intermediate in the synthesis of pharmaceuticals. 

One can rationalize a need for small rubber inclusions in some of the newer approaches to waterborne and high solids epoxy coating systems. Water-thinned epoxy coating compositions are described (48) where the two-component system consists of a nitrile rubber modified epoxy resin in the epoxide component and a styrene/ butadiene/methylmethacrylate latex modifier for an emulsion-based polyamide hardener component. Showing improved adhesion, impact and water resistance, the paint has good wetting characteristics and can be formulated to a high solids content at low viscosity. 

Thermoplastic rubber block copolymers, with completely new adhesive performance, were developed in 1965 [21]. The first commercial product was Shell Chemical s Kraton 101, of styrene polybutadiene-styrene composition. This development led to the carboxy-terminated nitrile (CTBN) rubber modifiers used to flexibilize epoxy and other brittle resin adhesives in the late 1960s. Today, the thermoplastic rubber block copolymer adhesives are used in hot melt-, solvent- and water-based adhesives, and as hot melt- and solvent-based sealants. Major applications are as pressure-sensitive adhesives, construction adhesives and sealants, and general assembly adhesives. 

Nitrile polymers used for the manufacture of adhesives generally contain 25% or more acrylonitrile, but in the base polymer the acrylonitrile content can vary from 15% to 50%. Increasing the acrylonitrile content improves the oil and plasticizer resistance and increases the polarity of the compound. However, higher levels of acrylonitrile also increase the hardness and modulus of the polymer, reducing the elasticity of the resulting polymer. Nitrile rubber can be produced by a cold (5°C) or hot (25-50°C) process, with most adhesive polymers produced by the hot process which induces more chain branching. Nitriles can be combined with other monomers in solution polymerization which increases functionality and improves compatibility with other reactive resins like acrylics, epoxies, and polyurethanes. 

Occasionally, elastomers may be included in solvent-based formulations. Poly(vinyl butyral) improves adhesion to metal, as does nitrile rubber, while natural and synthetic rubbers may be incorporated to improve flexibility. 

Flexible circuit boards consist primarily of polyimide-based carriers. The problem of bonding the copper foil on the polyimide carrier has not yet been solved satisfactorily. Due especially to their low bonding strength at elevated temperatures, the production of such materials is very limited. Nevertheless, adhesives for copper-polyimide systems were developed, where one-component epoxy resins (e.g., epoxy-polyester mixtures) and reactive hot melts (e.g., phenolic resin-nitrile rubbers) reached importance. 

Phenolic based adhesives are outstanding in their wetting characteristics and strength. In the unmodified form, however, they tend to be brittle. To overcome this problem, phenolic based adhesives usually contain a rubber modifier to increase the crack resistance of the adhesive. The first modified phenolic contained polyvinyl butyral and was known as the "Redux" adhesive. Many modern phenolic adhesives contain nitrile rubbers (copolymers of acrylonitrile and butadiene) as the modifiers. An example of a phenolic based film adhesive is AF-30 which is listed in Table 1 along with its physical properties. This film... 

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