Transition temperature, toughening mechanism

The improvement of damage resistance and tolerance in interlaminar fracture and under impact loading for the toughened matrix composites is at the expense of other important mechanical properties, such as inferior stiffness and hot/wet compressive strength (Evans and Masters, 1987). These trade offs appear to be associated with the reduction in matrix modulus and glass transition temperature (Jordan et al., 1989). 

The toughening mechanism of elastomer modified epoxy systems is different from that of flexibilized epoxy systems and can be used in combination with them. Flexibilized epoxy systems reduce mechanical damage by a reduction in modulus or plasticization of the adhesive. This allows stress to be relieved through distortion of the adhesive, but it also generally results in a lowering of the adhesive s glass transition temperature with an accompanying reduction in heat and chemical resistance. 

Microwave curing has been found to increase the glass transition temperature of epoxies and in some instances improve the mechanical properties. It is also possible to control the morphology of toughened epoxies more closely with microwaves.35... 

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

As a result of the toughening mechanism, the transition temperature, Tbt, at which the fracture type changes from brittle to tough shifts to lower values (from 60 °C for unmodified PA6 to about -30 °C). The dependence of TBX on rubber-particle content, particle diameter, and type of rubber has been extensively studied by Gaymans, Borggreve, and co-workers (6, 31, 33, 34). 

This classification is valid for a given temperature. With variation in temperature there can be a transition from one mechanism to another. For instance, decreasing the temperature of toughened PP shifts a shear yielding mech-... 

Physical—1. The glass transition temperature of PnBA occurs between -40 C and -50°C. It has a relatively high mechanical damping peak which Is helpful for the toughening mechanism of DGEBA/PnBA composites. 

The glass transition temperature of the PnBA composite cured using the two step procedure was only about 3 C lower than the 3f control without rubber, Figure 11. This shows that the mechanism of rubber toughening was not simple plasticization. Interestingly, the 3-transltion of the composite appeared to be slightly higher than the Epon 828-(DMHDA) control. 

Williams G, Watts DC (1970) Non-symmetrical dielectric relaxation behavior arising from a simple empirical decay function. Trans Faraday Soc 66 80-85 Williams ML, Landel RR, Ferry ID (1955) The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. J Am Chem Soc 77 3701-3707 Wood LA (1958) Glass transition temperatures of copolymers. J Polym Sci 28 319-330 Wu SH (1985) Phase structure and adhesion in polymer blends a criterion for rubber toughening. Polymer 26 1855-1863... 

M.2.2A Toughening Mechanism and Ductile-Brittle (D-B) Transition Temperature... 

---