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Four types of extracted solvents i.e. mixtures of chloroform:acetone, chloroform:methanol, cyclohexane:acetone and cyclohexane:methanol, in the volume ratio of 4:1 were used to dissolve the natural rubber. Two grades of natural rubber i.e. air dry sheet (ADS) and ribbed smoked sheet No.3 (RSS3) were investigated. The rubber solution was purified by separated the rubber out with methanol. The non-rubber solution was then quantitative analyzed to verify 2,2-diphenyl-1-picryhydrazyl (DPPH) radical scavenging activity using Oxicount-Antioxidant kit test and also calculated the term of half maximal effective concentration (EC50). The plasticity retention index (PRI) was also investigated in order to compare the ageing properties of the rubber and their antioxidant activity. Furthermore, various rubbers i.e. ADS, RSS3 and Standard Thai Rubber 20 (STR20) were collected during early period of yearly tapping seasons to analyze the DPPH radical scavenging activity and EC50. It was found that the mixture of cyclohexane and methanol showed the highest DPPH radical scavenging activity and lowest EC50. RSS3 showed higher DPPH radical scavenging activity than that of ADS which the higher antioxidant activity also displayed the higher PRI values. It also found that during the early period of yearly tapping seasons, DPPH radical scavenging activity of RSS3 was found higher than that of ADS and STR20, respectively when extracted by mixture of cyclohexane and methanol.

Fresh field natural rubber was coagulated by acetic acid, soaked in water at room temperature (WRT) or 70°C (W70) for 1 hr, and then dried in an oven at 40°C. Non-soaked natural rubber samples (NoW) served as a control. Two grades of natural rubber, namely air-dry sheet (ADS) and ribbed smoked sheet No.3 (RSS3) derived from the same latex, were also investigated. All dry rubber samples were characterized with Lovibond colorimeter according to ASTM D3157, as well as with a HunterLab spectrophotometer. Furthermore, all the dry rubber samples were dissolved in a chloroform:methanol mixture (4:1 v:v). The rubber was then precipitated out of the solution with methanol, and the remaining solution was quantitatively analyzed for total phenolic content (TPC). The plasticity retention index (PRI) was determined for all the dried rubber samples according to ASTM D3194. It was found that WRT, W70 and ADS were similar in lightness L*, while RSS3 had the lowest L*. W70 had the lowest redness a*, which increased in the order WRT, NoW, RSS3 and ADS. W70 also had the lowest yellowness b*, which increased in the order RSS3, NoW and WRT and ADS. Moreover, TPC was the lowest for the W70 sample, increasing in the order ADS, WRT, NoW and RSS3. The PRI was highest for W70, and decreased in the order WRT, RSS3, NoW and ADS. All of the PRI values observed were comparatively high relative to blocked standard Thai rubber 20 (STR20).