Elastomer-modified epoxy preparation

Elastomer-modified epoxy resin systems with more complexity to their preparation scheme have been demonstrated. Two examples suffice. Shelley and Clarke (9 ) instruct that a vulcanization procedure can be successfully employed to Improve elevated temperature properties in the cured resin mass. This step occurs subsequent to the esterification regime. It can be practiced with impunity at low rubber contents (7.5-10%) without gelation or Indeed very much viscosity Increase. Peroxides appear to be preferred over sulfur/sulfur donor systems. Table VII displays an example of this procedure with a solid DGEBA resin. 

In a contribution from B. F. Goodrich, Drake and Siebert extensively review the journal and patent literature since 1975 on reactive butadiene/acrylonitrile liquid and solid elastomers used in formulating epoxy structural adhesives. Areas reviewed include the preparation of elastomer-modified epoxy resins, the characterization of rubber-toughened epoxy resins, fracture mechanics and adhesive formulation and testing. 

Carboxylic nitrile liquid and solid elastomers are used to prepare elastomer-modified epoxy liquid and solid resins when it is desirable to have the modifier in the epoxy portion of the system. This is effected through alkyl-hydroxy esterification reactions. This is covered in the literature for uncatalyzed liquid resins (9), for tert-amine catalyzed systems (10), for tert-phosphine catalyzed systems (11), for low molecular weight solid resins advanced from the liquid state (12) and for liquid and solid resins where an additional rubber vulcanization step is carried out in addition to the alkylhydroxy ester adducting step (13). Such adduct preparations offer formulation stability with a wide range of hardener types amines, anhydrides, catalytic, Lewis acids/bases. 

The control resin network used in this study was a diglycidyl ether-based epoxy resin crosslinked with a cycloaliphatic diamine. Cooligomeric modifiers were prepared having varying percentages of TFP and DP siloxane and aminoethylpiperazine end groups. Both siloxane and ATBN and CTBN elastomers were used as epoxy modifiers, the latter two having been included to facilitate direct comparisons between modifiers in similarly prepared networks. 

Table IV shows a variation on this with mixed diacids of varying pKa value employed. Tables V and VI give recipes for the preparation of elastomer-modified solid DGEBA epoxy resins either using a one-step method (direct modification) or a two-step method (advancement to higher molecular weight with bisphenol A after completing the carboxyl/epoxy reaction).

Another chosen example stems from the perceived benefits of combining solid and liquid carboxylic nitrile elastomers in the same modification scheme. This is particularly useful when a degree of tack is required in the system. Table VIII exhibits one approach for preparing such an elastomer-modified solid DGEBA resin. In this instance a resin (epoxide equivalent weight of 650) was prepared by advancing two available liquid epoxy resin adducts — one utilizing a liquid nitrile, the other a solid nitrile elastomer. 

Epoxies have been toughened with elastomer-dispersed phases with a decrease in modulus. Work by Balakrishnan et al. [35] utilized organo-montmorillonite as a dispersed phase in elastomer-toughened epoxies to recover this lost modulus. The montmorillonite was modified with octadecyl ammonium ion. The elastomer-dispersed phase in the epoxy was prepared by free radical polymerization of acrylic monomer within the epoxy. The acrylic elastomer-dispersed phase had epoxy functionality provided through the utilization of glycidyl methacrylate as a comonomer. 

Synthetic resins, such as phenoHc and cresyUc resins (see Phenolic resins), are the most commonly used friction material binders, and are usually modified with drying oils, elastomer, cardanol [37330-39-5] an epoxy, phosphoms- or boron-based compounds, or even combinations of two. They ate prepared by the addition of the appropriate phenol and formaldehyde [50-00-0] in the presence of an acidic or basic catalyst. Polymerization takes place at elevated temperatures. Other resin systems are based on elastomers (see Elastomers, synthetic), drying oils, or combinations of the above or other polymers. 

Three additional LARC-13/ATS formulations were prepared using a 50 50 combination of elastomers having repeat units of 105 and 10. These resins containing a combination of two elastomers were made at three elastomer concentrations of 8, 15 and 25% w/w solids. Bimodal formulations were of particular interest because of a previous study on epoxies resulting in the enhancement of properties when the resins were modified using bimodally distributed elastomer particles (10). A significant drop in the Brookfield viscosity of these resins was observed as the elastomer content was increased from 8 to 25%. This may be due to an offset in the monomer stoichiometry due to elastomer impurity or the presence of small amounts of water in the elastomers. 

Elastomer-epoxies are prepared mostly by nitrile rubber addition as the elastomeric component and are usually called modified or toughened epoxy. The bond strengths of elastomer-epoxies are lower in comparison to those of nylon epoxies. Their durability with respect to moisture resistance is better, but not as good as those of vinyl phenolics or nitrile-phenolics. A wide application is in films and tapes. Elastomer-epoxies typically cure at low pressures and temperatures, and over short cure periods, by adding a catalyst to the adhesive formulation. 

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