Cured elastomer-modified epoxies

Figure 2. Morphology of Cured Elastomer Modified Epoxies.

Cured Elastomer Modified Epoxies," SPE AMTEC, 5/1974, San Francisco, CA. 

Many of the primers developed during the past 20 years contain corrosion inhibitors. These primers have a second function to further improve hydrolytic stability at the adhesive to metal interface and prevent corrosion during exposure to salt spray. Table 1 shows the difference in strength retention of 2024-T3 alclad aluminum lap shear coupons exposed to salt spray. Both sets of coupons were bonded with a 250°F cure elastomer modified epoxy adhesive. One set of coupons was primed with BR 123 primer (a non-corrosion inhibiting primer) while the other set was primed with... 

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. 

Lewis and co-workers (42) developed improved powder coatings with nitrile rubber-modification of an appropriate epoxy base (solid resin admixture) cured with an imidazoline-accelerated modified phenolic type hardener. Model coatings ground to 55 pm particle size, electrostatically applied to metals, cured 10 170°C, gave excellent therraocycling results as well as retained resistance to solvent attack. Elastomer-modified epoxy powder coatings have been covered extensively by Gelbel, Romanchick and Sohn in Chapter 5 of this volume. 

The amine-reactive nitrile liquids are useful in formulating a modified hardener package through admixture with an amine, amidoamine or fatty poly-amide curing agent of choice (14). And novel elastomer-modified epoxy resin hardeners giving excellent reported adhesive properties have begun to appear in the marketplace (15). 

Oh B S, Kim H S and Ma P (1996) The effect of rubber on stress-whitening in DDS-cured epoxies, in Toughened Plastics IT. Novel approaches in science and engineering, Advances in Chemistry Series 252 (Eds. Riew CK and Kinloch AJ) American Chemical Society, Washington, DC, pp.111-118. Lee H and Neville K (1967) Handbook of epoxy resins, McGraw-Hill, New York, pp. 17-11. Pearson R A and Yee A F (1989) Toughening mechanisms in elastomer-modified epoxies, J Mater Sci 24 2571-2580. 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

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. 

Consequent to documentation surrounding methods of employing reactive nitrile elastomers to modify epoxy resins is a growing body of literature which serves to characterize and elucidate these systems. Such topics as morphology in the cured and uncured state, transitions from toughening to flexlbilization, viscoelastic effects, equilibrium physical properties and phase structure are available to the investigator (12-17). 

Adhesives recommended are neoprene and urethane elastomers, epoxies, and modified epoxies, phenolic-polyvinyl butyral, nitiile-phenohc, polyester, cyanoacrylates, resorcinol-formaldehyde, phenolics, polyacrylates, modified acrylics, PVC, and urea-formaldehyde. Phenolic adhesives give good results, but require higher cure temperatures and are less water-resistant than resorcinol-based adhesives. 

Nylon-epoxy is typically used in film and tape adhesive applications and has maximum service temperature of 138°C, compared with 177°C for unmodified epoxies. The addition of nylon provides increased flexibility and peel strength compared to unmodified epoxies. Nylon-epoxy is a tough material with excellent tensile lap shear strength and good fatigue and impact resistance. However, the addition of nylon results in deterioration in creep resistance and poor peel strength at low temperatures. Furthermore, the hydrophilic nature of nylon imparts poor moisture resistance in both uncured and cured forms of the resin. Consequently, nylon-epoxies are not as durable as elastomer-epoxies or other thermoplastic modified epoxies. 

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