Plasticizer critical concentration

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

In practice, much of the literature still reports notched Izod numbers, but these can be deceptive, especially for materials close to a ductile-brittle transition. Better indicators of real toughness are obtained when samples of different thickness are considered. As one adds more rubber to a plastic, a critical concentration is often found where the toughness increases dramatically. In some cases, e.g., polymers that are capable of shear yielding, this correlates better with the interparticle distance or "ligament thickness" than with any other variable. 

In order to increase the conductivity from polymers, a variety of cmiductive fillers like carbon black and graphite is available for use in plastics. Because of the structure of carbon black, which contains many pores with small mass, conductive paths in the polymer can be obtained with low critical concentration. Graphite as a filler has a laminated structure and similar electrical characteristics as carbon black. 

Figure 10.18 shows that impact strength of plasticized PVC increases with increase in the amoimt of the plasticizer. The impact improvement is not gradnal bnt requires a certain critical concentration of the plasticizer. This critical concentration of the plasticizer depends on properties of pair polymer/plasticizer For example in plasticization of polyamide-11 ductile-brittle transition occurred at 4-6% of one plasticizer and in vicinity of 15% of another plasticizer. 

Two rates of the tensile strength decrease are seen from Figure 11.16. The slow initial decrease is caused by gradual plastidzation and the sudden decrease occurs when some critical concentration of plasticizer is attained. Above the critical plasticizer concentration, the clusters become sufficiently separated to destroy the reinforcing ionic stmc-ture. 

Effect of two plasticizers (DBP - dibutyl phthalate and DD - a mixture of dioxane derivatives) on surface stress has been compared (Figure 11.41). Surface stress is useful in understanding the effect of plasticizer on the critical concentration of micelle formation and the potential stability of water dispersion of stabilizer. 

Knowledge of the critical concentration of the micelle formation is useful in calculation of the surface area occupied by one plasticizer molecule ... 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

No one steel exceeds the tensile modulus of mild steel. Therefore, in applications in which rigidity is a limiting factor for design (e.g., for storage tanks and distillation columns), high-strength steels have no advantage over mild steel. Stress concentrations in mild steel structures are relieved by plastic flow and are not as critical in other, less-ductile steels. 

Nondestructive testing (NDT), 15 747, 748 active, 17 415-416 passive, 17 416, 425 on plastics, 19 588-589 Nonelectrolytes, critical micelle concentration and, 24 122-123 Nonemulsion paints, organic titanium compounds in, 25 121-122... 

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