Rubber-based Toughening Agents

Apart from carboxyl and amine-terminated nitrile rubbers, the rubbers having other end groups like amine [12-14], mercaptan [15, 16] hydroxyl [17] and epoxy [18, 

19] have also been examined. Riew and co-workers [20] showed that ATBN could be added to the amine curatives which could then be mixed with the epoxy resin. Unfortunately, ATBNs are not universally soluble in amine curatives. Among the reactive ended nitrile rubbers, CTBN gives the best performance. The better adhesive strength of the CTBN-modified epoxy in comparison with epoxy system modified with NBR with other reactive groups can be attributed to the better adhesion of CTBN-modified epoxy with the substrate due to the presence of carboxyl groups. 

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Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

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. 

The most common preformed rubber particles used as a toughening agent for epoxy polymers are the so-called structured, core-shell latex particles (Figure 4). These particles typically have a polybutadiene-based core and an acrylate-based shell. Such additives can be purchased as powders from Rohm and Haas or Elf-Atochem and can be purchased as epoxy concentrates from the Dow Chemical Company. The key parameter for these modifiers is the composition of the shell polymer, since the shell chemistry plays a crucial role in the overall blend morphology. It should be noted that it is possible to obtain commercial core-shell latex particles with reactive groups in the shell for improved dispersion of the rubber particles. 

Adhesives based solely on epoxy resins and amine curing agents give cross-linked materials that have high modulus but are very brittle and have no resistance to peel forces. Epoxy resins became much more useful when it was found that rubber flexibilizers could be added to increase crack resistance. Further, it was found that under appropriate reaction conditions, these rubbers could act to "toughen" the epoxy system. That is, addition of rubber flexibilizers tends to reduce the Tg and shear properties of an epoxy. When an epoxy is "toughened" little or no change occurs in the Tg and shear properties while a substantial increase in... 

FRACTURE ENERGIES FOR EPOXIDE RESINS BASED ON BIS-PHENOL A (wiTH VARIOUS CURING AGENTS AND RUBBER TOUGHENING)... 

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