CTBN-epoxy adduct

One of the disadvantages of CTBN epoxy adhesives has been their high viscosity, which limits formulation options. Recently new adducts, such as EPON 58003 and RSM-2577 from Resolution Performance Products, have been introduced that have significantly lower viscosities.28 In addition, lower concentrations of these new CTBN epoxy adducts are generally required to achieve equivalent adhesive performance. New lower-viscosity CTBN adducts have also resulted in formulations where a greater concentration of cost-reducing filler, such as calcium carbonate, can be used. 

Modified polyfunctional aliphatic amines, amidoamines and fatty polyamides are covered for both CTBN/epoxy adducts or ATBN hardener systems (42,44). Examples of systems based on selected epoxy-anhydride (45), an amine-ether (46) and Lewis acid/amine complexes (47) augment this information base. Dicyandiamide (1-cyano guanidine) containing systems alone (48) and with melamine (49) and various proprietary accelerators (50-53) are shown to serve as latent, one-component film and paste adhesives with an excellent balance of adhesive properties. 

Siebert and Riew [1781 first described the chemistry of rubber particle formation in an admixed model involving CTBN, a DGEBA (diglycidyl ether of bis-phenol A) liquid epoxy resin, and a selective catalyst. They proposed that the composition of the rubber particles in the dispersed phase critically depended on the epoxy—CTBN-epoxy adduct being formed in situ and then further chain-extended and crosslinked with additional resin. 

Later, McPherson (39) demonstrated that the Inclusion of a CTBN/liquid epoxy adduct enhanced both the flexibility and adhesion of a dimer acid flexibllized epoxy/brominated epoxy mixture when cured with a combination of chlorendic and nadic-methyl anhydrides. With an epoxy-glass laminate as substrate, such mixtures were coated at 0.5-2.0 mils thickness, cured, sensitized and copper plated (1.5 mils) by an electroless process. 90 peel testing (2 inches/mln. rate, 25 C) gave 14.0 pli peel force where 5.0 pli is considered normal. 

In general, the elastomer must be prereacted (adducted) with the epoxy for the toughening effect to take place. Adducts reduce the likelihood of early phase separation and maintain the solubility of the elastomer in the uncured resin system. For CTBN the reaction is carried out at high temperatures (150 to 160°C) and usually in the presence of a catalyst, such as tris-dimethylamino phenol or piperidine. The resulting epoxy-CTBN adducts are available from several suppliers, and they can be easily formulated into epoxy adhesives. 

Carboxy-terminated curative, such as CTBN, provides excellent toughening in part due to its miscibility in many epoxy resins. Phase separation during cure is required to obtain toughening, and generally the phase separation requires an elevated-temperature cure. However, by prereacting the CTBN with a portion of the epoxy to obtain an adduct, a room temperature curing toughened epoxy is possible. Adduction reduces the likelihood of early phase separation and maintains the solubility of the elastomer in the uncured resin system. 

Table 12.14 shows a comparison of dicyandiamide cured epoxy adhesives formulated with and without a CTBN adduct. When compared to the control epoxy, the toughened formulation exhibits significantly higher peel strength and moderately higher tensile shear strength. CTBN modified epoxy adhesives are generally one-part systems, cured with dicyandiamide at elevated temperature. 

EPON 58901 195-210 1,000-5,000 at 52°C CTBN elastomer epoxy functional adduct with Resolution... 

ADDUCTS OF LIQUID EPOXY, CTBN AND DIMER ACID... 

Pg. 4 - Para 2. CTBN as free dibasic acid does deteriorate all amine, amidoamine or polyamide cured epoxy adhesives if it exists as free acid. This is why the overwhelming majority of instances adducts CTBN and epoxy first. No free acid. 

This adduct is formulated with the epoxy resin during adhesive manufacture. Amine-terminated RLPs (ATBNs) are used as hardeners in two-part epoxy systems. These are prepared by reacting a CTBN with N-aminoethylpiperazine. 

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. 

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