Nitrile rubber adhesives compounding

The complexity of the effects that can occur may be judged from a study of the mould release of nitrile-butadiene rubbers (NBR) (see Nitrile rubber adhesives), where a complex interfacial layer including emulsifier and coagulant residues controlled the release properties. The adhesion was affected by a large number of factors, some associated with the mould surface, some with the rubber compound used and some with the base NBR (Fig. 1). 

Acrylonitrile-butadiene rubber (NBR) (also called nitrile or nitrile butadiene rubber) shows high oils and plasticizer s resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients (see Nitrile rubber adhesives). 

Film adhesives have to be strictly delimitated from adhesive tapes and adhesive strips (Sections 5.6 and 5.7). Mainly blocked two-component reactive adhesives (Section 3.1.4) are used raw materials. For transport and storage (at low temperatures) they are applied to a - nonadhesive - substrate. Prior to processing they are removed and then applied between the adherends (compounding) and cured under pressure and heat (Sections 3.1.4, 3.2.2 and 4.1.2). Special film adhesives (e.g., phenolic resin nitrile rubber) are also activated by suitable solvents. 

First made commercially available in Germany in 1936, this elastomer is officially known as acrylonitrile butadiene, and is usually the product of an emulsion polymerization process that combines the two monomers acrylonitrile and butadiene. However, the polymer can also be made in a solution process, and with a variety of monomers. As a specialty polymer, and even though several commercial brands were available, Buna N (as it was first known) was little used until World War II, when the polymer s unique oil and plasticizer resistance and high heat resistance became very important for transportation products. Nitrile rubber also exhibits exceptional adhesion to metallic surfaces and is compatible with a wide range of additives and compounding ingredients. 

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. 

These adhesives are usually made by blending a nitrile rubber with a phenolic novalac resin, along with other compounding ingredients. Usage in... 

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. 

NBR latices can be also used in adhesive applications. The use of latex has the advantage of avoiding the previous solution of the polymer before application and has favourable environmental treats. Compounding with a resorcinol-formaldehyde solution allows to bond nitrile rubber to cotton or rayon fabric. Nitrile latex can be mixed with PVC latex to give excellent adhesion of polypropylene carpet and plywood backings. Combinations of nitrile latices and styrene-butadiene latices provides good laminating bonds for saturated paper and woven fabrics. 

A typical example of preparation of such rubber adhesive solutions is natural rubber, polychloroprene, styrene-butadiene rubbers, nitrile rubber or others, mixed and compounded in the conventional manner using internal (Banbury) mixers or mills. The unvulcanized rubber mix is then made into a solution (or cement). Dry aromatic solvents such as xylene, toluene or chlorobenzene (900 parts of solvent per hundred of rubber mix) are added and agitated to dissolve the elastomer. Forty parts of MDI are added to the elastomer solution with agitation. The cements are stored at room temperature the NR cement is stable for 7 days, the SBR keeps for 3-4 days and the polychloroprene cement is stable for 3 days. 

Synthetic Rubber. Many different types of synthetic rubber are suitable raw materials for adhesives and sealing compounds. Particularly significant are polychlo-roprene rubber, styrene-butadiene rubber (SBR), nitrile rubber, and polyisobutylene. Unless these rubbers are available as directly soluble types, they have to be degraded by mastication on rolls or in kneaders and solubilized before dissolution. 

Nitrile rubber is used preferably for contact adhesives with improved plasticizer resistance. Polyisobutylene is used in pressure-sensitive adhesives. Butyl rubber is sometimes added to pressure-sensitive and hot-melt adhesives, although it is mainly used in sealing compounds. Epoxy resins and reactive (meth)acrylate adhesives (reactive adhesives) are modified with polychloroprene, butyl, and nitrile rubber. Chlorinated rubber is added in small quantities to contact adhesives and also to rubber-to-metal bonding agents for improving the adhesion properties. 

Polyhydroxymethyl compounds phenol-formaldehyde resins, also in combination with poly(vinyl formal) resins, nitrile rubber or epoxy resins (as adhesive film), urea-fprmaldehyde resins, melamine - formaldehyde resins, resorcinol - formaldehyde resins. Uses wood (plywood manufacture), metals. 

Adhesives for bonding plasticized (flexible) PVC (PVC-P) films are adhesive solutions based on THF (80-90%) and PVC-P (10-20 %). Adhesives based on nitrile rubber or polyurethane in a plasticizer-resistant formulation for the bonding of PVC-P films made from differently formulated compounds are also suitable. 

A medium high acrylonitrile latex can be compounded with a resorcinol-formaldehyde solution to give a good adhesive to bond nitrile rubber stocks to cotton or rayon fabric. This treatment results in adhesion as good or better than that obtained by saturation of the fabric with solvent cement. A typical recipe is shown in Table 9. This adhesive would be cured for 1-5 minutes at 160°C. 

An adhesive consisting of an epoxy resin, a medium high acrylonitrile rubber, and Pb, Cu, Ni, Pd or Co compound fillers was used to bond EPDM vulcanizates. Peel strengths of 22.5 kg/20 mm after a 1 hour at 100 C cure were obtained. Another adhesive composition involving a blend of carboxylated nitrile rubber, epoxy resin and a reactive metal filler has been described for bonding EPDM vulcanizates or EPDM rubber-nitrile or butyl rubber blends. 

Polyethylene and Polypropylene Acceptable bonds have been obtained between treated polyolefin surfaces with polar adhesives, such as epoxies, or solvent cements containing synthetic rubber or phenolic resin. The solvent adhesives are applied to both surfaces and the solvents allowed to evaporate before the parts are joined. Recommended epoxies are the anhydride-cured and amine-cured types. Also suitable is a two-component, polyamide-modified epoxy compound. Other adhesives that provide adequate bond strength to treated polyolefins include styrene-unsatmated polyester and solvent-type nitrile-phenolic (15). 

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. 

In sheet molded compounds, mold release agents (stearates of calcium and zinc) were found in the surface layers of molded materials. Their presence influenced the acid/base character of surface layers. In these two research papers, it was pointed out that the surface layer contained no filler but most of the release agent was residing there. Surface accumulation of additives was reported for nitrile butadiene rubber. The layers of additives formed are reported to affect adhesion. ... 

Interfacial layers were formed on the surfaces of molds used with nitrile butadiene rubber compounds.These layers help to reduce adhesion to the mold and thus more easily remove the molded part. Stearic acid is the major component of this layer which was measured to be 6 nm thick on the surface of a steel mold. The parting occurs within this interlayer but most of the residue remains on the steel. 

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