Rubber-Toughened Formulations

Polyamides containing amino groups are also excellent flexibilizers for epoxies since they co-react with the epoxy resin and introduce their inherent thermoplastic properties. 

A second approach to reducing modulus and flexibilizing an adhesive is to add a plasticizer to the formulation. Since most plasticizers are of relatively low-molecular weight and do not co-react with the resin, there is a risk of separation, leaching, or outgassing during thermal or thermal-vacuum environments. 

In general, flexibilizers improve the low-temperature bond strengths at -50°C or lower by reducing stresses, but often at the expense of reducing high-temperature strengths. 

---

The addition of Vamac B-124 to ethyl cyanoacrylate has a more pronounced effect on peel strength, both at ambient temperature and after thermal exposure. After 24 h at ambient temperature, the peel strength of the rubber-toughened formulation is almost 40% greater than the control formulation A without rubber. After heating the test specimens for 2 h at 121°C, the peel strength of formulation A, is almost non-existent, while that of C has increased significantly, as seen in Fig. 7. 

To balance some of the drawbacks produced by the rubber toughening of thermosets, inorganic fillers that increase modulus and yield stress can be added to generate hybrid composites. Inorganic fillers such as glass beads, alumina, or silica - with high values of modulus and strength - are frequently included in thermoset formulations. 

By far, the most used additives are fillers, pigments, and rubber tougheners for transforming polystyrene into useful tangible products for the consumers. Fillers are used to lower the ingredient cost of the final formulations. Pigments and other colorants are used to offer variety of color polystyrene. For improved impact strength, rubbers are used. Flame retardants are used to allow polystyrene to be used indoor. 

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. 

Rubber toughened epoxy formulations do offer excellent fracture properties [76-81]. 

Plate 16 The brass barrel and aluminium outer of this door handle are bonded using a rubber toughened single-part, heat-cured epoxide. This particular grade will flow just like solder prior to solidifying as it cures. This is in direct contrast to the material used in Plate 19 which is formulated so as to maintain a very high viscosity - specifically to prevent unwanted flow. Material Permabond ESP108. 

Therefore, substituted ureas may be used as latent initiators for anionic homopolymerization of epoxy resins. Pearce and Morris [73] reported the use of 1,1-pentamethylene-3-phenyl urea, prepared by the reaction of phenyl isocyanate with piperidine in dry benzene, to cure a tetraglycidyl methylenedianiline resin toughened with carboxyl-terminated butyl rubber. These formulations showed an excellent stability at 23°C and led to a high Tg product when cured at 170°C. 

Cyanoacrylate adhesive can be made from different acrylate monomers such as methyl, ethyl, butyl, isopropyl, and so on. These molecules differ in size and adhesives and exhibit different physical properties. Methyls are the smallest molecule and seem to work best on metal and rubber parts while ethyls work best on plastic parts. Many modifications can be made to the monomers to alter or improve their properties as adhesives. They can be toughened with rubber or formulated to have low odor, resistance to thermal cycling, or less sensitivity to surface conditions which tend to stabilize the adhesive and slow down the cure. ... 

Liquid reactive rubbers were also used for UP and vinyl ester formulations (Suspene et al., 1993 Siebert et al., 1996). Increases in fracture energy and fatigue- crack resistance were reported for some systems, although no significant improvements were observed for some other systems. These different behaviors are probably related to the heterogeneous structure of the matrix (Chapter 7). Toughening mechanisms in three-phase systems are not yet well established. 

The use of liquid reactive rubbers to toughen thermosets leads to a decrease of both T — g and the Young s modulus (see Fig. 13.7). Thermoplastics can be added to these formulations to improve thermal and mechanical properties. 

Polystyrene may be extruded to produce film and other transparent articles with high gloss finish but in commercial practice the formulations toughened with rubber are used to a greater extent (extruded transparent film is toughened by stretching and orientation while it is still hot from the die). 

---