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SynopsisThe thermal oxidation of natural rubbers from Hevea brasiliensis (pale crepe and smoked sheet) and Partheniurn argentaturn (Guayule) were studied with differential scanning calorimetry (DSC) using the dynamic method. The atmosphere was oxygen a t 110 ml/min and the temperature range was 393"-473"K. The decomposition reactions showed a pronounced exothermic emission with shoulders on the high-temperature side. The kinetic parameters were computed by using three different mathematical approaches: the heat evolution treatment of Borchardt and Daniels, the diffusion-controlled method by Jander, and the heating rate method developed by Kissinger and Ozawa. The best agreement between these methods was achieved assuming first order in the Borchardt and Daniels method and three-dimensional diffusion with the Jander model. The mass influence was analyzed observing that good agreement is obtained working with sample weight in the order of 3-4 mg. Under such conditions the activation energies (E,) were 16-17,18-19, and 23-24 kcal/mole for the Borchardt-Daniels, Kissinger-Ozawa, and Jander models, respectively.

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This paper presents the results obtained from an experiment which was carried out in order to identify a possible correlation between the presence of static charge and the loss of hydrophobicity on partially and highly hydrophobic surfaces (glass and RTV silicone rubber coating, respectively). Glass and RTV silicone rubber coating samples were initially exposed to corona impingement, and subsequent measurements of charge decay and hydrophobicity recovery (contact angle) were taken. Data obtained from such measurements clearly show that as surface charge decay takes place, hydrophobicity is correspondingly recovered. This observation is valid not only for RTV silicone rubber samples, but also for glass samples. In the silicone rubber compound, recovery could be attributed to a mechanism of migration of silicone oil from the bulk of the polymer to the surface, but in glass samples that mechanism can be totally excluded as the likely responsible agent for hydrophobicity recovery. Therefore, a mechanism different from that of silicone oil migration should be determining the trends observed. When a comparison between both calculated and real surface charge decay times was made, a fairly strong agreement was found. As an additional proof for supporting the hypothesis here presented, when the surface and volume resistance characteristics of the original RTV silicone rubber compound were modified and after exposure to corona impingement, its hydrophobic properties remained unaltered.

An experimental study on the effect of gel and nonrubber constituents on the extrusion behavior of guayule rubber (GR) has been carried out. At 140°C, gel increases the viscosity at low shear rates. When GR is properly stabilized, no peptizing effect is imparted by the resin. However, the resin acts as a moderate plasticizer as compared to paraffinic oil. At 120°C, the effect of gel is noticeable at high extrusion rates, increasing the viscosity, suggesting an increase in the tendency for the occurrence of flow-induced crystallization.