Proposed Method for Classifying Natural Rubber for Use in Pressure Sensitive Adhesive Tapes

Natural rubber is used as an important raw material by the pressure sensitive adhesives industry. However, the normal quality variation is relatively high compared to synthetic polymer alternatives. This paper discusses measurements of the magnitude and nature of this natural rubber quality variation using a special dynamic mechanical rheological tester. Also the paper discusses some of the causes of this variation and the new ASTM Standard (D7050) that describes how effective sorting can be implemented to improve product consistency Introduction More natural rubber is consumed annually than any single synthetic elastomer; yet the quality of this natural product is influenced by many special factors not associated with synthetic rubber production. For example, the Hevea tree clone from which the rubber latex is obtained and the age of the tree can affect natural rubber quality as well as the soil and climate conditions. Natural rubber molecular weight distribution can be bimodal or unimodal. Figure 1 shows three types of natural rubber distributions. Figure 1. Type A distribution is a skewed unimodal distribution. Type B is bimodal with the height of the lower MW peak at half or less of the higher MW peak while Type C distribution is distinctly bimodal with the height for the lower molecular weight peak almost equal to the height of the peak for the high molecular weight portion. Bimodal is reported as the most common distribution for latex. Also the magnitude of the molecular weight distribution (MWD) can vary greatly. The MW / MN ratio is reported to vary