Natural rubber crosslink distribution

Figure 10.4 Mesh size distributions in a natural rubber sample filled with carbon black and containing a gradient of crosslinking agent. Data were obtained with n-heptane.

The y-irradiation of natural rubber (Malaysian type) in the presence of nanotubes does not modified the accumulation of gel content, which reaches an upper limit (98 %) at about 25 kGy, but mechanical properties are influenced by the nanofiller loading [103]. The uniform crosslinking in homogenous distribution of CNT confirms the high quality of radiation processed NR. 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

Purified natural rubber (PNR) is of interest because of its potentially lower toxicological effects than whole natural rubber (WNR) due particularly to the reduction in protein content. Improved dynamic mechanical properties have also been reported. Comparison of both gum and filled compounds, vulcanised using conventional cure systems (CV) and efficient vulcanisation systems (EV) prepared from PNR and WNR indicate that generally properties of the PNR are poorer than WNR. The exception is in the flex cracking resistance and the heat build up in the filled samples where PNR shows an improvement. Using the EV cure system on filled PNR gives properties almost comparable to WNR. A study of the distribution of the types of sulphur crosslink in both pNR and WNR vulcanisates indicates a more uniform distribution of monosulphidic (S), disulphidic (S2) and polysulphidic (Sx) crosslinks in the PNR samples (38.7/25/36.4 in PNR compared to 64.1/29.7/6.6 in WNR respectively). This is believed to be the reason for the better dynamic properties of PNR vulcanisates. 7 refs THAILAND... 

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