Phenolic-Nitrile Blends

Type Typical examples % Alcohol % Acetate % Acetal Wt. avg. M.W. 

Common additives are zinc oxide, carbon black, iron oxide, magnesium oxide, and sulfur. Adhesives have high peel strength and impact resistance and are at times marketed as two-component systems. 

A cure cycle of 15-30 min at 300-500°F and 50-200 psi pressure gives the best results. A short-term exposure to temperatures as high as 600°F can be tolerated, and the adhesive is suitable for continuous use to 250°F. Durability studies have been reported by Minford and Bethune. Superior fatigue resistance and complete resistance to outdoor exposure in the jungles of Panama has been shown by Bodner and Wegman for nitrile-phenolic-bonded aluminum specimens. 

Recently another type of nitrile rubber, cold rubber, such as Hycar 1041 and 1051 (Goodrich), has seen increased usage. 

The major disadvantages of the nitrile-phenolics are the high cure pressures and temperatures needed, whereas, as already mentioned, the current trend is towards cures at low (250°F) temperatures and pressures. The largest uses of this type of adhesive are for the assembly of brake shoes and clutch disks. 

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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. 

Blending of rubber has been tried out for toughening phenolic resin. Phenolic-nitrile rubber (NBR) was developed in the early 1950s, which has found wide applications [242] such as structural adhesives, O-rings, and gaskets. However, it was reported that when NBR is mixed with resole and cnred, they phase separate into undesirably large domains with low adhesion between the phases [243]. A discussed earlier, for an effective toughening effect the domain size and interfacial adhesion are important. 

Addition of p-cresol formaldehyde (PCF) into phenolic/NBR blends resulted in rednction in the domain size of the dispersed phase and improvement in mechanical properties [244]. PCF resin has an intermediate polarity compared with NBR and resole and can react faster with NBR. Therefore, PCF molecules are likely to be concentrated at the phenolic/NBR interface and act as an external compatibilising agents [245]. Thus compatibility and chemical bonding between NBR and phenolic resin is improved, leading to the enhancement in properties. The other materials used as toughening agents of phenolic resin include elastomers such as natural rubber and nitrile rubber [246, 247], reactive liquid polymers [248] and thermoplastics such as polysulfone, polyamide, polyethylene oxide [249, 250]. 

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. 

To improve low-temperature impact performance of PP, it has been blended with elastomers, viz. TPU or nitrile-butadiene rubber (NBR). Addition of wfTPU) < 15 wt.% improved impact and other mechanical properties. Significant improvements of the tensile strength, as well as the tensile strain and strength at break, were obtained by adding to PP 2 wt% of di-methylol-p-octyl phenol, than blending it with NBR. 

When preparing nitrile rubber/phenolic adhesives, the higher the nitrile content of the rubber, the greater its compatibility with phenolic resins. High acrylonitrile content rubbers thus produce adhesives with superior bonding and film properties, although lower acrylonitrile content rubbers will produce adhesives with better low temperature properties. Table 17 illustrates the effect of acrylonitrile content on the tensile properties of nitrile rubber/phenolic resin blends. ... 

Table 17. Physical Properties of Nitrile Rubber and Phenolic Resin Blends".

During the past four decades phenolic resins have become of increased significance in rubber compounding. For example, the resin based on cashew nut shell liquid, which contains phenolic bodies such as anacardic acid (Figure 23.23), may, when blended with hexamine, be incorporated into nitrile rubber (butadiene-acrylonitrile rubber). 

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. 

Applications of NBR adhesives can be divided in three groups (1) solely nitrile rubber (2) nitrile rubber/phenolic blends (3) nitrile rubber/epoxy blends. 

Coran and Patel [74] investigated the reactive com-patibilization of PP-NBR and HDPE-NBR blends using phenolic modified polyolefin, maleic anhydride modified polyolefin, and amine terminated nitrile rubber as reactive components. Dynamic vulcanization was also inves-... 

A variety of polymers, both thermosets as well as thermoplastics, can be blended and coreacted with epoxy resins to provide for a specific set of desired properties. The most common of these are nitrile, phenolic, nylon, poly sulfide, and polyurethane resins. At high levels of additions these additives result in hybrid or alloyed systems with epoxy resins rather than just modifiers. They differ from reactive diluents in that they are higher-molecular weight-materials, are used at higher concentrations, and generally have less deleterious effect on the cured properties of the epoxy resin. 

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. 

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).

Fig. 13.8 The T-peel resistance as a function of the crosshead speed for nitrile rubber/phenolic polyurethane liquid rubber blend toughened epoxy (upper curve) and conventional nitrile rubber-toughened epoxy structural adhesives (lower curve).

Excellent elastomeric NBR-nylon compositions have also been prepared by dynamic vulcanization during the melt-mixing of intimate blends of NBR with various nylons. In this case, the effect of curatives was complicated by the fact that some nitrile rubbers tend to self-cure at temperatures of mixing. Sulfur, phenolic, maleimide, or peroxide curatives can be used. The thermoplastic elastomeric compositions prepared by the dynamic vulcanization of nylon-NBR blends are highly resistant to hot oil. As in the case of the EPDM-polyolefin blends, increases in the amount of rubber in the composition reduce stiffness but increase resistance to permanent set. 

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

Of the common modified phenolic adhesives, the nitrile-phenolic blend has the best resistance to elevated temperatures. Nitrile-phenolics have high shear strengths up to 121-177 C, and their strength retention on aging at these temperatures is very good. These materials are available in solvent solutions and unsupported and supported films. 

Nitrile rubber/phenolic resin produces one of the most durable and toughest elastomeric materials developed in the adhesive industry. Further, its resistance to water as to organic solvents is excellent. For these reasons, the nitrile rubber/phenolic resin laminates are used in printed circuit board manufacturing, to bond metallic substrates between themselves (aluminium, steel) and to bond rubber to magnesium. Films of nitrile rubber/phenolic blends have also been used in the aircraft industry for bonding metal-to-metal surfaces in both plain and honeycomb sandwich constructions. 

The first completely synthetic resin, based on phenol and formaldehyde, was invented by Baekeland and others in the first decade of this century. " While the phenol-formaldehyde resins yielded excellent adhesives then as now, blends with carboxylic rubber, epoxy resins, neoprene, nitrile rubber, and other polymers Improved adhesive properties for various purposes. 

In somewhat more recent work, it was shown that phenolic resins may be blended with nitrile rubber to bond a wide variety of construction materials, including wood, iron, glass, cardboard, and a number of synthetic polymers (see Chapter 17 in Reference 103, and References 112-114). 

Most phenolic adhesives are based on resoles, novolacs (or resorcinol-modified novolacs), and phenolic alloys. The last mentioned class includes the important blends of phenolic resins with epoxies, vinyl, polyamide, and nitrile rubber. Phenolic-based adhesives perform well on various types of surfaces the more common substrates are shown in Table III. 

Adhesives. Contact adhesives are blends of rubber, phenolic resin, and additives supplied in solvent or aqueous dispersion form they are typically applied to both surfaces to be joined (98). Evaporation of the solvent leaves an adhesive film that forms a strong, peel-resistant bond. Contact adhesives are used widely in the furniture and construction industries and also in the automotive and footwear industries. The phenolic resins promote adhesion and act as tackifiers, usually at a concentration of 20-40%. In solvent-based contact adhesives, neoprene is preferred, whereas nitrile is used in specialty applications. The type and grade of phenolic resin selected control tack time, bond strength, and durability. 

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. 

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