ปัจจัยของวัตถุดิบยางก้อนถ้วยต่อการเกิดจุดขาวและสมบัติของยางแท่งเกรด 20

Abstract

White spots caused by contaminants in the Standard Thai Rubber (STR) grade STR 20 are an essential quality problem with this grade. Therefore, the present work aimed to study the factors affecting white spots in STR 20, starting from raw rubber coagula. In preliminary studies, the drying conditions of STR 20 were investigated by comparing oven drying in factory and in laboratory. Suitable conditions for drying rubber in the laboratory were determined based on the properties of STR 20 with and without white spots. Moreover, the coagulation technique (type and amount of acid used), tapping season, addition of non-rubber serum into the fresh latex, and contents of lipids, nitrogen and ash in STR 20, were also studied for their effects on rubber properties. It was found that, in the laboratory, oven drying at 125°C for 3 hr gave the best rubber properties. The samples with white spots contaminants had more volatile matter (VM), ash and carbon, and higher plasticity retention index (PRI) but lower viscosity than samples without white spots. Natural coagulation, or coagulation with wood vinegar or biological acid, gave less white spots contaminants than coagulation with formic acid, which gave more VM. Regarding the tapping period, it was found that fresh latex from the early tapping season showed the most white spots. This might be correlated to non- rubber component contents in the fresh latex, such as nitrogen, lipids and ash. Moreover, adding non-rubber serum into fresh latex increased white spots, lipids and nitrogen in the rubber. To reduce white spots and VM of STR 20, maturation of the cup lumps for more than 15 days or immersing them in sodium hydroxide and sodium metabisulfite solution at 1-2% (wt/wt) for 1 hr before use successfully reduced the white spots and VM, with only small effects otherwise on the final properties of raw rubber.