Sample preparationManufacturing process of rubber sheets is started from latex. Thefresh latex containing about 35–40% of dry rubber content (DRC) wascollected from farmers (Step 1). It was diluted with water to achievethe DRC of about 15–18% and 2% wt./vol. of formic acid was added to coagulate in the coagulation tanks (Step 2). These tank have channels forseparators to give the thick slabs of the rubber sheets. The mixture wasstirred to mix well and all bubbles were removed by surface wiping(Step 3). These bubbles may cause quality degradation of the rubbersheets. Then separators were inserted in the channels and left to formlumps of coagulum for 3–4 h (Step 4). The hardened rubber slabs withmoisture content of about 60% were then removed from the coagulationtank, and squeezed to form thin rubber sheets (Step 5). The thickness ofrubber sheets (3–4 mm) was controlled with the constant spacing ofmilling and it was cross checked with a vernier caliper after regular interval of time. Finally, the rubber sheets were placed in the mixed-modeand indirect solar dryer for drying (Step 6–7). The completemanufacturing process of air dried rubber sheets is shown in Fig. 5.Steps 1 to 5 in Fig. 5 were carried out at Saikao Cooperative, located inthe Muang district, Songkhla province, Thailand and step 6–7 at Princeof Songkla, Hat Yai district, Songkhla province, Thailand.