Natural rubber is one of the most important economic crops in Thailand with total growing area of about 35,500 square kilometers .
There are about 1 million households or more than 6 million people involve in the natural rubber production.
This leads Thailand to be one of the main natural rubber exporters in global market.
In 2009, Thailand produced 3.16 million tons of natural rubber, of which about 86 % or about 2.73 million tons were exported.
That was more than 11, 200 million US dollars of Thailand’s exports earnings.
The main constituent of the natural rubber latex is water (Figure 5.1). It has only about 30-40% rubber particles, which can be separated from water by adding diluted formic acid. Normally, the natural rubber has poor mechanical and thermal stability. It becomes sticky and soft at high temperature, while at low temperature it is hard and brittle. Thus, it is not yet a durable material for manufacturing the usable products. Vulcanization process is needed to convert the natural rubber into more durable material. This process modifies the rubber molecular structure by forming three-dimensional cross-link network between individual rubber polymer chains. After the vulcanization, the long-chain molecules are prevented from moving independently so that the vulcanized rubber exhibits better resilient and thermal stability when applying to force or heat. Nowadays the vulcanization process popularly used in rubber industry is performed by the addition of sulfur at high temperature to induce the cross-linking process between the long polyisoprene chains. Comparison of the molecular structures and some physical properties between the unvulcanized and vulcanized natural rubber with sulfur is shown in Figure 5.2.
The vulcanization of rubber with sulfur has the limitation that the curing can occur only with the double bond molecules. This is because the sulfur atoms can form cross-linking between the polymer chains only at the double bond positions. In normal vulcanization process, it also requires chemical accelerator and activator substances to accelerate the cross-linking process. Some activator substance has nitrosamine, which can be a cause of cancer. Furthermore, the adding chemical substances can cause some other problems such as turbid rubber due to free sulfur atoms, the blooming effects and the non-uniform distribution of the chemicals in rubber latex. The turbidity of the vulcanized rubber is an important property of various rubber products such as baby pacifier, condom and Foley catheter. There are several countries, e.g., USA, Germany, Australia and Thailand, have safety regulation for the baby pacifier that the contamination of nitrosamine cannot exceed 10 parts per billion (ppb).
Figure 5.3 (Left) Diagram of the RF Linac system for stimulating cross-linking in rubber and (right) a picture of the accelerator currently under construction .
 Agricultural Information Center, Office of Agricultural Economics, Ministry of Agriculture and Cooperatives, 23 June 2014.
 From http://www.slideshare.net/cikgushaifulkisas/chapter-12-rubbers
 From http://wps.prenhall.com/wps/media/objects/3312/3391650/blb1202.html
 “Radiation Crosslinking by BGS”, From http://en.bgs.eu/wp-content/uploads/2015/05/BGS_radiation_crosslinking_en.pdf
 H. Chirinos and A. Lugao, “Radiation Vulcanization of Natural Rubber Latex Sensitized with Commercial Gases”, From https://www.ipen.br/biblioteca/2002/inac/08856.PDF
 Tywais, “Magnetron Linac 100 Hz 1 MW”, YouTube, 12 July 2016, From https://www.youtube.com/watch?v=GYEY7qL2ZfA
Principal Investigator : Assistant Professor Dr. Chitrlada Thongbai 1)
Collaborators : Asst. Prof. Dr. Sakhorn Rimjaem1), Dr. Jatuporn Saisut1), Asst. Prof. Dr. Piyarat Nimmanpipug2), Dr. Pasaree Laokijcharoen3), Mr. Michael Rhodes4)
Affiliated Institutes : 1) Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, 2) Department of Chemistry, Faculty of Science, Chiang Mai University, 3) National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 4) Thailand Center of Excellence in Physics