Stress relaxation chemical

Hayes, M.J. and Lauren, M.D. (1994) Chemical stress relaxation of polyglycolic acid suture. Journal of Applied Biomaterials, 5, 215-220. 

The choice of exposure times is complicated when more than one reaction is taking place as the rates will be different. Generally, it is best if equilibrium absorption is reached relatively quickly in comparison to chemical changes and its effects treated separately from the subsequent chemical changes. This is not unlike the situation for physical and chemical stress relaxation. 

Figure 5-17. Chemical stress relaxation of Natural Rubber cured with dicumyl peroxide. [Data from Y. Takahashi and A. V. Tobolsky, Technical report No. 125, Office of Naval Research, N00014-67-A-0151-0011 (1970).]...

In a few special cases, remarkable quantitative agreement between independently measured chemical degradation reactions and chemical stress relaxation has been achieved. However, in most cases quantitative interpretation is confounded by other effects such as many parallel reactions,... 

It is always a difficult task to select material appropriate for inclusion and exclusion in an introductory text of modest size and cost. Because of the discussion of the topics mentioned above and a somewhat expanded treatment of the phenomenology of viscoelasticity, it was felt appropriate to eliminate the chapter on chemical stress relaxation. In its place, a discussion of this topic has... 

Fig. 166. Kinetics of chemical stress relaxation (la, 2a, 3a, 4a) and the accumulation of residual deformation (1-4) in rubbers prepared on the basis of methyl-vinylpyridine rubber MVP-15. 1 and la) Rubber A, vulcanizing group sulfur + altax 2 and 2a) rubber C, vulcanizing agent benzotrichloride 3 and 3a) rubber based on SKB 4 and 4a) rubber B, vulcanizing group sulfur + altax and bonzotrichloride 5) rubber D, vulcanizing agent tetramethylthiuram disulfide.

Fig. 167. Influence of antioxidants on the kinetics of chemical stress relaxation of rubber based on MVP-15, containing the vulcanizing group sulfur + altax (rubber A). 1) Rubber A 2) rubber A + acetoneanil (3-method -hydroquinoline, polymerized) 3) rubber A + DPPD (di-phenyl-p-phenylenediamine) 4) rubber A + BLE (condensation product of acetone and diphenyl) 5) rubber A + MB (mercaptobenzimidazole) 6) rubber A + oxynezone.

At temperatures below 150°C, the activation energies correspond to physical relaxation, related to displacement of segments of the molecular chains. At higher temperatures, chemical stress relaxation predominates. Hence, the activation energies cited are typical of chemical processes. 

The investigations we conducted of chemical stress relaxation of vul-canizates differing both in the nature of the transverse bonds and in the reactivity of the polymer have made it possible to give an unambiguous answer to the disputed questions [92]. It was shown that two competing tendencies are observed in the process of aging of vulcanizates oxida-... 

The literature contains only the work of Tobolslgr [93] on the influence of carbon blacks on the process of chemical stress relaxation he arrives at the conclusion that carbon blacks do not influence the rate of chemical stress relaxation. However, the available data on the influence of carbon blacks on the rate of oxidation of cured rubbers force us to treat Tobolsky s data with caution. 

In the case of thiuram vulcanizates, stress relaxation, as was indicated above, is due to the oxidative destruction of the polymer chains. Hence we might assume that the presence of carbon black in thiuram cured rubbers produces an increase in the rate of chemical stress relaxation. 

The influence of fillers on the rate of chemical stress relaxation has been investigated on thiuram natural rubber vulcanizates. 

In the case of vulcanizates with polysulfide bonds, when the process of stress relaxation is due to the decomposition of thermally unstable transverse bonds, and not to thermooxidative destruction of the polymer chain, the difference in the rates of chemical stress relaxation of the filled and unfilled cured rubbers is comparatively small (Fig. 187). 

Chemical Stress Relaxation As indicated in Section 10.1.1, stress relaxation can be caused by either physical or chemical phenomena. Examples will be given of each. 

Table 10.1 Chemical stress relaxation times for silicone rubber (8)...

APPENDIX 10.1 ENERGY OF ACTIVATION FROM CHEMICAL STRESS RELAXATION TIMES... 

The effects of chemical stress relaxation (sometimes referred to as chemorheo-logy) depend on whether the network scissions occur at the original cross-links or are randomly disposed at all points along the network strands." " They also depend on whether the broken bonds join again to leave the same number of network... 

If, on the other hand, cross-links are forming and dissociating while the measurements are in progress, phenomena similar to chemical stress relaxation (Section D of Chapter 14) may be encountered. The stress may drop nearly to zero in a stress relaxation process, whereas the shear modulus as measured in a moderately rapid... 

Tobolsky AV, Takahashi Y, Naganuma S (1972) Effect of additional cross-linking of ctmtin-uous chemical stress relaxation of cis-polybutadiene. Polym J 3(l) 60-66... 

Gillen KT (1988) Effect of cross-links which occur during continuous chemical stress-relaxation. Macromolecules 21(2) 442-446... 

Figure 8.3 shows the stress-decay curve of EP07P at 100 °C. The rate constant of chemical stress relaxation that is proportional to the molecular chain scission is expressed by Eq. (8.1) [5] ... 

The effect of pre-irradiation on thermal aging of EP07P is shown in Fig. 8.6. This figure shows the stress-decay curves of EP07P which was irradiated up to 50 kGy. The decay curves are straight line tc was not observed on the curves when sample was pre-irradiated. The chemical stress relaxation curves of the sample which was irradiated up to 96 kGy showed the same decay curves as that irradiated up to 50 Gy [8]. 

Fig. 8.4 Chemical stress relaxation of nonirradiated EP07P at various temperatures...

Figure 8.19 shows the relationship between the induction period of weight change of EP07P and the time tc which was obtained from chemical stress relaxation curve of the elastomer. The temperature range was between 90 to 130 °C. Both values had the same period of time as shown in this figure. 

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