Improvements in Toughness

A variety of toughening agents have been used to modify epoxy adhesives to improve peel strength and fracture toughness without significantly affecting other properties of the epoxy base resin. Generally these modifiers can be classified into three types  

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The rubber and the polystyrene should not be compatible. If they are there will be molecular mixing and no improvement in toughness. 

Glass-fibre-filled grades of these toughened polymers are also available but these do not show the same improvement in toughness over normal glass-fibre-filled nylons. 

Block copolymers of polycarbonates and silicone polymers have also been commercially marketed (e.g. Makrolons KU 1-1198 and KU 1-1207). These block copolymers show a marked increase in toughness at low temperatures coupled with reduced notch sensitivity. (They show little improvement in toughness at normal ambient temperatures.)... 

An example of this improvement in toughness can be demonstrated by the addition of Vamac B-124, an ethylene/methyl acrylate copolymer from DuPont, to ethyl cyanoacrylate [24-26]. Three model instant adhesive formulations, a control without any polymeric additive (A), a formulation with poly(methyl methacrylate) (PMMA) (B), and a formulation with Vamac B-124 (C), are shown in Table 4. The formulation with PMMA, a thermoplastic which is added to modify viscosity, was included to determine if the addition of any polymer, not only rubbers, could improve the toughness properties of an alkyl cyanoacrylate instant adhesive. To demonstrate an improvement in toughness, the three formulations were tested for impact strength, 180° peel strength, and lapshear adhesive strength on steel specimens, before and after thermal exposure at 121°C. 

Y. Li and H. Shimizu, Improvement in toughness of poly(l-lactide) (PLLA) through reactive blending with acrylonitrile-butadiene-styrene copolymer (ABS) Morphology and properties, Eur. Polym. J., 45 (3) 738-746, March 2009. 

It was found that dramatic improvements in toughness, strength, creep strength and thermoresistance could be achieved by incorporating nano-SiC dispersion in a microcrystalline matrix. Subsequently, similar improvements were found in other nanocomposites [1],... 

From the table It appears that the Izod Impact strength Improves as the Interfacial adhesion increases. This Is also concluded In a recent publication about nylon 6.6 compounded with polyethylene-g-maleic anhydride polymer (15). Addition of more rubber In system 3 (30 %) affords hardly any addltonal Improvement In toughness, but the flexural modulus, which In ABA block copolymers Is already lower than In a dispersed rubber system, Is more than halved. 

While thermoplastic additives bring about improvement in toughness, it is usually at the expense of increased dielectric properties which is undesirable for certain critical applications as described earlier. A method to introduce toughness and reduce Dk is based on microporous cyanurates prepared via chemically induced phase separation using a solvent such as cyclohexane. The solvent is removed to generate microvoids by heating the matrix in the vicinity of its Tg. Polycyanurates with significantly less density and Dk were thus prepared [279, 280]. 

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