Flexibility versus plasticity

The effect of the gas on mechanical properties depends on the paradox of closed- versus open-cell construction. In a closed-cell foam, mechanical deformation compresses the gas, so the gas contributes to rigidity and strength of the plastic product. In an open-cell foam, on the other hand, the fluidity of air permits it to rush out when the foam is deformed and to rush back when the deformation is released therefore, the foam contributes to the softness, flexibility, and resilience of the plastic product. 

The different effects of mutations on the native versus the promiscuous functions are particularly striking in view of the fact that many of these mutations are found within the active site, typically at the wall and perimeter. Structural and thermodynamic insights into the effects of these generalist mutations are needed before any definite statements could be made. Yet it seems probable that the plasticity of these residues lies in the fact that they are not part of the protein s scaffold, or of the catalytic machinery of the enzyme. The mutated residues are typically located on surface loops that exhibit high conformational flexibility and comprise the substrate binding part of the active As discussed in Section 8.03.6.3, there exist fundamental... 

Thermal Properties. Addition of plasticizer shdes the modulus-versus-tempera-ture curve horizontally toward lower and lower temperatures, in proportion to the amount of plasticizer added. This lowers the stiffening temperature of flexible plastics. It also lowers the softening temperature of rigid plastics, which limits maximum use temperature but improves melt processability. 

Most adhesive tapes are composed of a flexible backing (paper, plastic, cloth, metal foil, etc.) to which a pressure-sensitive adhesive has been applied to one side (both sides for double-sided tapes). Pressure-sensitive adhesives typically consist of a rubbery material with a modifying tactifier that may be applied to the tape by a solvent system, hot melt, or by other means. One would expect such materials to be sensitive to the mode of stress (tensile versus shear) in the region where debonding occurs. Furthermore, since tacky rubbers of the type used in pressure-sensitive adhesive are viscoelastic, one would anticipate material properties to be time- and rate-dependent. Are these expectations consistent with the observations from your simple peel test ... 

A quantitative measure of relative plasticizing efficiency is developed by determining the plasticizer concentration (in phr) required to meet a specific hardness versus some reference standard. For most applications, the reference standard chosen for PVC plasticizers is DOP. From the ratio of the DINP phr that is required to achieve a defined hardness to the DOP phr required for the same hardness, one develops a quantitative substitution factor (SF) for DINP versus DOP in providing equal hardness properties to the flexible PVC at room conditions. For example, to achieve at Durometer A = 80, 56.2 phr of DINP is required versus 52.9 phr DOP. Thus, the SF calculated for DINP is SF = 56.2 phr DINP/52.9 phr DOP, or 1.06. This means that for a given hardness, 6 percent additional DINP is required to give the same hardness as that obtained with DOP. While this example targets 80 Durometer A hardness, one may use this calculation at any specified hardness, or other property, required for the given end product of flexible PVC. 

Duncan established the capability of the LSF test using the Haake instrument. She demonstrated not only that DOP consistently fails to meet the LSF requirements as defined by Ford, but its use contributes to wide variations in LSF values this is probably due to co-distillation of low-molecular-weight nonplasticizer additives in the flexible PVC parts. Table 7.19 summarizes the current LSF requirements and the impact of plasticizer type versus the required test conditions. 

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