Nitrile-rubber inclusions

Small nitrile-rubber inclusions in epoxy resin electrical en-capsulants have been examined in both amine (29-31) and acid (32) epoxy cures, in filled and unfilled systems. The value of rubber inclusion in a boron trlfluorlde/amine complex epoxy cure has also been demonstrated (33), where elevated-temperature, high-humldlty testing showed electrical properties retention to be better than a comparable system cured with dodecenylsucclnic anhydride. Rubber benefits low-temperature properties specifically and thermocycling in general. It affects high temperature insulation properties negatively therefore, the amount of rubber incorporated must be judiciously chosen. 

Although the nitrile rubbers employed normally contain about 35% acrylonitrile the inclusion of nitrile rubber with a higher butadiene content will increase the toughness at low temperatures. For example, whereas the typical blend cited above has an impact strength of only 0.9 ft Ibf in notch at 0°F, a blend of 70 parts styrene-acrylonitrile, 30 parts of nitrile rubber (35% acrylonitrile) and 10 parts nitrile rubber (26% acrylonitrile) will have an impact value of 4.5 ftlbfin notch at that temperature. ... 

The ductility of GRT-polyethylene blends drastically decreases at ground rubber concentration in excess of 5%. The inclusion of hnely ground nitrile rubber from waste printing rollers into polyvinyl chloride (PVC) caused an increase in the impact properties of the thermoplastic matrix [76]. Addition of rubber powder that is physically modihed by ultrasonic treatment leads to PP-waste ethylene-propylene-diene monomer (EPDM) powder blends with improved morphology and mechanical properties [77]. 

The use of rubbers (particularly epoxy-terminated butadiene nitrile, ETBN, rubber or carboxy-termi-nated butadiene acrylonitrile, CTBN, rubber) to toughen thermoset polymers is perhaps the most widely explored method and has been applied with some measure of success in epoxy resins. Phase separation of the second rubbery phase occurs during cure and its incorporation in the epoxy matrix can significantly enhance the fracture toughness of the thermoset. Although the rubber has a low shear modulus, its bulk modulus is comparable to the value measured for the epoxy, ensuring that the rubber inclusions introduced... 

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. 

Comparisons of sulfur donor cures with peroxide cures in nitrile rubber are shown in Table 13.7. Each cure system is shown both with and without an antioxidant package. The peroxide cure is inherently more thermally stable than the sulfur donor cure, which provides less change in physical properties during long-term aging and better (lower) compression set. In each cure system, the inclusion of a good antioxidant package can further improve the heat and compression set performance. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

It is these solid carboxylic nitrile elastomers which began to show utility in the modification of epoxy resins. Processing needs for solid elastomer Inclusion, particularly in liquid epoxy resins, have not always been advantageous. Associated problems include gel, viscosity threshold limitations and achieving desired rubber levels in excess of 5-6 phr. Sometimes processing must be carried out in selected solvents, not always a desirable or tolerable step. 

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