Rubber brittle-ductile transition

Impact modifiers for PET are generally elastomeric compounds that increase impact strength and elongation while usually decreasing modulus. An effective way to enhance the impact strength and to induce a brittle/ductile transition of the fracture mode, is by the dispersion of a rubber phase within the PET matrix. The... 

Tbd Temperature of brittle-ductile transition Tg Temperature of glass transition Uch Bond energy of polymer chain Up Potential energy of the rubber particle a Coefficient of thermal expansion 8 Cohesive energy density Volume strain... 

Impact Modifiers. Notched impact strength and ductility can be improved with the incorporation of impact modifiers, which can also lower the brittle-ductile transition temperature and give much improved low temperature toughness. Impact modifiers are rubbers (often olefin copolymers) that are either modified or contain functional groups to make them more compatible with the nylon matrix. Dispersion of the rubber into small (micrometer size) particles is important in order to obtain effective toughening (19). Impact modifiers can be combined with other additives, such as glass fiber and minerals, in order to obtain a particular balance of stiffness and toughness. Modified acrylics, silicones, and polyurethanes have also been proposed as impact modifiers. 

The transition of dispersed particles from the rubbery to glassy state defines the lowest temperature at which the incorporated rubber is able to reduce the matrix jdeld stress and accoimt for significant toughening (280,281). The effect of added rubber usually fades away at temperatures 10-20 K above its which is manifested as a sharp drop in the fracture energy (256). An equation was derived for the brittle-ductile transition temperature as a function of the particle... 

Margolina, A., Wu, S. (1988). Percolation Model for Brittle-Ductile Transition inNy-lon/Rubber Blends. Polymer, 2P(72), 2170-2173. 

This transition in polymer composites containing rubber-dispersed phases is sometimes referred to as the brittle-ductile transition (for obvious reasons). The influence of montmorillonite on this transition through the alteration of the rubber-phase morphology for this TPO is very significant. The tensile modulus for this TPO with 30% rubber loading as a function of montmorillonite content increased from approximately 0.8 GPa with no montmorillonite content to approximately 1.5 GPa for 7% montmorillonite content. It is difficult to decon-volute the contribution of the montmorillonite from the contribution of the rubber-dispersed phase to these mechanical properties. 

It is tempting to relate the temperature at which the ductile-brittle transition takes place to either the glass transition or secondary transitions (Section 5.2.6) occurring within the polymer. In some polymers such as natural rubber or polystyrene Tb and Tg occur at approximately the same temperature. Many other polymers are ductile below the glass transition temperature (i.e. Tb < Tg). In this case it is sometimes possible to relate T to the occurrence of secondary low-temperature relaxations. However, more extensive investigations have shown that there is no general correlation between the brittle-ductile transition and molecular relaxations. This may not be too unexpected since these relaxations are detected at low strains whereas Tb is measured at high strains and depends upon factors such as the presence of notches which do not affect molecular relaxations. 

Figure 4 Impact behavior of nylon 66 toughened with a functionalized ethylene-propylene rubber at different loadings (BD= brittle to ductile transition).

The introduction of rubber particles increases the fracture energy of the networks at room temperature, but also decreases the temperature of the ductile-brittle transition (Van der Sanden and Meijer, 1993). This ductile-brittle transition is strongly dependent on the nature (and Tg) of the rubber-rich phase and the amount of rubber dissolved in the matrix. The lowest ductile-brittle transition is obtained with butadiene-based copolymers (Tg — 80°C), compared with butylacrylate copolymers (Tg —40°C). 

Brittleness is found with semi-crystalline polymers below their glass-rubber transition Tg. An example is PP, which becomes brittle at about T -10 °C. PE retains its ductile nature down to very low temperatures. Other polymers have a Tg of some tens of °C above room temperature, such as polyamides and thermoplastic polyesters. Various mechanisms are responsible for a reasonable impact strength at room temperature for polyamides this is, for instance, the absorption of water also secondary transitions in the glassy region may play a role. 

One of the fundamental clues in rubber-toughened systems is to know if the matrix or the rubbery phase is responsible for the unstable failure of a material. There are indeed two possibilities to explain a ductile-brittle transition ... 

Thus at a given rate the lowest rubber content which can induce ductility and suppress spallation will result in maximum energy absorption under impact. If this yield-spall transition coincides with the ductile-brittle transition which occurs in these tensile tests, the effective strain rate of the onset of spallation could be predicted by tests of this type. This ductile-brittle transition occurs at low effective strain rates for the unmodified material since it is brittle through the range of conditions used in these tests. For the 4% material at the highest effective strain rates achieved in these tests, the load maximum is just beginning to disappear. Thus, if rate-temperature equivalence holds, extension of these test to... 

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