Effect of different sulfur content in Natural Rubber mixtures on their thermo-mechanical and surface properties
Abstract
In this study, a field experiment was conducted to find out the effect of different levels of sulfur on natural rubbers mixtures. Vulcanization is a chemical process for converting natural rubber or related polymers into more durable materials by heating them with sulfur other equivalent curatives with accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains. Vulcanized materials are less sticky and have superior mechanical properties. The results indicated that the application of sulfur -as a vulcanasing agent-had significant effect on mechanical (Shore A hardness test), thermal (DSC calorimetry) and surface-optical properties (SEM microscopy) of mixtures.
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Introduction
By far the most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic polyolefins. Even with natural rubber, large amounts of sulfur, as well as high temperatures and long heating periods are necessary and one obtains to an unsatisfactory crosslinking efficiency with unsatisfactory strength and aging properties. Only with vulcanization accelerators can the quality corresponding to today's level of technology be achieved. The multiplicity of vulcanization effects demanded cannot be achieved with one universal substance; a large number of diverse additives, comprising the "cure package," are necessary. The combined cure package in a typical rubber compound consists of sulfur together with an assortment of compounds that modify the kinetics of crosslinking and stabilize the final product. In this study the following sulfur quantities were used per samples: S1=3,80 g; S2=7,6 g; S3=11,40 g; S4=15,20 g; S5=19 g at pressure 220 bar and temperature 145°C and t=10 minutes.
Conclusion
The Shore A hardness test of the rubber samples containing different Sulphur showed was a significant difference between Sample 1 and Sample 5. (Fig. 6.). The results demonstrate the influence of sulphur and MBTS during the different stages of the vulcanization. With sulfur present, the crosslinked product distribution is influenced as well.
NR-based master batches, even when similarly compounded, exhibit different cure behavior and cure properties depending on the cure system used, the duration and temperature of cure. The results of DSC tests show that the difference between the samples (Fig.8.). DSC (differential scanning calorimetry), is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment.
Differential scanning calorimetry can be used to measure a number of characteristic properties of a sample. Using this technique it is possible to observe fusion and crystallization events as well as glass transition temperatures Tg. DSC can also be used to study oxidation, as well as other chemical reactions. In this case we checked the process of vulcanization.
Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time (from +60°C to +180°C). The cross-linking of polymer molecules that occurs in the curing process is exothermic, resulting in a positive peak in the DSC curve that usually appears soon after the glass transition. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned. We collected the results below.
The aim of the SEM investigation was the examination of blending of raw caoutchouc and rubber mixtures (base polymer with additives) in 50x and 1000x magnification. It is evident, that besides the surface impurities, Sample 3. and Sample 5. generated a lot of sulfur aggregation from the vulcanising agent used based on the recipe.