Epoxy elastomeric modifiers

The use of elastomeric modifiers for toughening thermoset resias generally results ia lowering the glass transition temperature, modulus, and strength of the modified system. More recendy, ductile engineering thermoplastics and functional thermoplastic oligomers have been used as modifiers for epoxy matrix resias and other thermosets (12). 

The addition of the polysulfide resin acts as both an elastomeric modifier and a diluent for the epoxy resin. The low viscosity of LP-3, for example, can drastically reduce the viscosity of the overall formulation. This provides greater ease of mixing and application and the ability to be applied and sprayed without a solvent. 

The lower molecular weight LP-3, LP-33 and ZL-1400 C polymers are used as elastomeric modifiers, particularly for epoxy coatings and adhesives. Owing to their low molecular weight these polymers have the lowest viscosities of commercially available LPs. This low viscosity is particularly beneficial under cold conditions. Even at 4°C the polymers have viscosities which are considerably lower than those associated with standard unmodified bisphenol A [diglycidyl ether of bisphenol A (DGEBA)] epoxy resins at 25°C. 

The use of elastomeric or flexibilizing modifiers occurred and grew with epoxy resins first. Various aspects of toughened epoxy adhesives have been covered in reviews by the present authors (2,3), where the elastomeric modifiers have essentially been carboxylic, liquid and solid butadiene/acrylonitrile polymers. There has not been a systematic review, however, of these and other reactive liquid polybutadiene/acrylonitriles in the burgeoning areas of acrylic, anaerobic and radiation-curable systems. Thus, this paper s intent. 

In rubber toughening, an elastomeric modifier is incorporated into the epoxy matrix as a second phase (as discussed in Chapter 4), which is why improvement in toughness is achieved without a significant sacrifice of the and mechanical properties. The toughening agents reported so far can be classified as commercial, other rubber-based, acrylate-based and hyperbranched polymer-based modifiers. 

By the authors [26] the novolac-epoxy networks was modified with reactive epoxy functional polysiloxane oligomers containing various levels of polar nitrile pendent groups was investtigated. It was shown that the using with high content of polysiloxanes as an elastomeric modifiers component in epoxy-novolac networks exhibited micro phase separation. 

Elastomeric modifiers are used to increase the peel strength (toughness) of epoxy resin adhesives. The most commonly used elastomeric materials are functionally terminated polybutadiene resins made by the B.F. Goodrich Company, Chemical Group under the trade name of Hycar Reactive Liquid Polymers. Initially, carboxyl-terminated butadiene acrylonitrile (CTBN) resins were introduced the carboxyl terminated materials are usually adducted with the epoxy resin to improve compatibility and to increase the toughness. 

To assess the effect of elastomer degradation on composite performance, additional composites were fabricated with the same 121°C cure epoxy without any addition of the elastomer (211. The expansion behavior of the modified epoxy composite was similar to the toughened material. For electron doses less than 10 rads the CTE of the toughened and untoughened composites were essentially the same which suggests that the epoxy matrix and not the elastomeric component controls the thermal expansion behavior. 

In summary of these points, it is seen that the isolation of particles from the epoxy matrix, the effective volume fraction of the elastomeric phase, and strength of the interface interact to control modulus. The morphology which a particular siloxane modifier promotes determines the contribution of any or all of these three factors to the modulus of the modified resin. 

The second noteworthy morphological feature is presented in Fig. 12b. This micrograph depicts the pre-crack front of 15-1500-70F, which had a value significantly above that of the control, as shown in Fig. 11 a. The holes may be examples of the dilatation effect observed in CTBN-modified epoxies l9,22> in which rubber particles dilate in mutually perpendicular directions under the application of a triaxial stress and then collapse into spherical cavities following fracture. Dilatation requires a mismatch in coefficients of thermal expansion of resin and rubber 11. This effect will therefore be most striking when the elastomeric phase is homogeneous, as is apparently the case here. 

Secondary ingredients in epoxy adhesives include reactive diluents to adjust viscosity mineral fillers to lower cost, adjust viscosity, or modify the coefficient of thermal expansion and fibrous fillers to improve thixotropy and cohesive strength. Epoxy resins are often modified with other resins to enhance certain properties that are necessary for the application. Often these modifications take the form of additions of elastomeric resins to improve toughness or peel strength. 

The properties of thermosetting and thermoplastic resin systems are continually improved to meet increasing performance requirements of end users. One way to enhance material properties is to incorporate nano-modifiers, based on elastomeric silicone particles, which are optionally grafted with other (acrylic) polymers to control dispersibility, viscosity, and other parameters. As an example, epoxy resin formulations have been modified with silicone nanospheres to improve low-stress behavior. Table 1 shows the outstanding fracture toughness improvement of silicone coreshell nanospheres, even at very low particle loading levels. 

One approach to addition of elastomeric tougheners is to select a modifier which is initially soluble in the resin (22) but precipitates as small rubbery spheres as curing progresses. However, this procedure has always resulted in depression of thermal mechanical properties. Alternatively, an immiscible material can be dispersed in the resin by use of physical and chemical interactions of the solvents and catalysts so that a fine dispersion of rubber particles is produced in the epoxy resin prior to cure. It is also possible to produce a stable dispersion by a reactive blending process without the use of a catalyst or solvents. 

Elastomeric adhesives are prime candidates for polyurethanes, and polyurethane elastomer adhesives are particularly recommended. Other suitable adhesives include epoxies, modified epoxies, polyamide-epoxy, neoprene, and resorcinol-formaldehyde. The latter offers excellent adhesion, but is somewhat brittle and can fail at relatively low loads. "... 

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