Cooling tensile modulus

In theory, the flexural modulus and the tensile modulus should be the same. In practice this is only approximately so, because plastics materials are seldom (if ever) isotropic throughout the thickness. DifTerenlial cooling rates, variations in extrusion or injection rates, changes in flow patterns, etc., all contribute to nonuniform properties throughout the thickness. When coupled with the nonuniform stresses already mentioned, it is hardly surprising that inconsistencies with the tensile test arise. 

Fig. 15 Thermomechanical properties of SMPU/POSS nanocomposites, (a) Storage tensile modulus and tanS depending on temperature for SMPU with different POSS/polyol ratios (i) 0, (ii) 0.98, (iii) 1.67, and (iv) 2.63. (b) Cyclic thermomechanical tests for SMPU/POSS (POSS/polyol = 2.623). Three cycles are shown (solid line) first cycle, (broken line) second cycle, and (dotted line) third cycle. The asterisk marks the beginning of the cycle and the arrows denote the various stages, specifically (1) deformation, (2) cooling/fixing, (3) unloading, and (4) recovery. Reprinted with permission from [117], Copyright 2008, American Chemical Society...

The residence time study showed the most significant evidence of degradation, with a 4.7% decrease in tensile strength, and a 29% increase in tensile modulus at 7.6 minutes of residence. This evidence emphasizes the importance of proper injection molding machine sizing. An oversized barrel molding thick, slow cooling parts could easily surpass 7.6 minutes of residence time. 

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

For the calculation of the thermal shock-induced stresses, we consider the plate shown in Fig. 15.1 with Young s modulus E, Poisson s ratio v, and coefficient of thermal expansion (CTE) a, initially held at temperature /j. If the top and bottom surfaces of the plate come into sudden contact with a medium of lower temperature T they will cool and try to contract. However, the inner part of the plate initially remains at a higher temperature, which hinders the contraction of the outer surfaces, giving rise to tensile surface stresses balanced by a distribution of compressive stresses at the interior. By contrast, if the surfaces come into contact with a medium of higher temperature Tm, they will try to expand. As the interior will be at a lower temperature, it will constrain the expansion of the surfaces, thus giving rise to compressive surface stresses balanced by a distribution of tensile stresses at the interior. 

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