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USE OF KAPOK PULP TO IMPROVE PROPERTIES OF RECYCLED PAPER 240 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 recycled fibers is to mix the recycled pulp with the virgin pulp. Softwood pulp can be a good candidate for this purpose since paper made from softwood pulp is generally stronger than paper made from hardwood pulp due to the higher fiber length of softwood fiber as compared to hardwood fiber. However, Thailand as one of the tropical countries usually has no softwood pulp and need to import from abroad. So, finding a new fiber source in Thailand to do this task for softwood fiber is incredibly fascinating. Kapok fiber abundantly found in Thailand might be used as a substitute of the softwood fiber to improve the strength of recycled pulp due to its remarkably high fiber strength. In general, kapok fiber is hollow with a thin fiber wall and large lumen [1, 2]. It has the external diameter of 16.5±2.4 m, the lumen diameter of 14.5±2.4 m and the fiber length of 25±5 mm [3]. For chemi - cal composition, kapok fiber contains 35% cellulose, 21.5% lignin, 22% xylan and 13% acetyl groups on a weight basis [4]. Though kapok fiber has high tensile strength but it is very brittle. As such, kapok fiber is rarely used in textile industry but it is used as stuffing for pillow and mat - tress instead due to its light weight. Taking the advantages of its hydrophobicity, kapok fiber is also used as stuffing for life jacket as well. Also, the papermaking potential of kapok fibers was investigated by Chaiarrekij et al. [5, 6]. It was found that kapok fiber can be a potential fiber source for papermaking in Thailand. Paper made from kapok pulp has high tensile and burst strengths and due to its moderately hydrophobic surface, kapok paper might be potentially used as a packaging paper. This research was thus aimed to study the effects of kapok pulp on improving properties of paper made from recycled pulp and then compare them with the results obtained from using softwood pulp instead of kapok pulp. If kapok pulp seems to provide the competitive results, then this might provide a good opportunity to increase the use of recycled paper in Thailand. Experimental: Preparation of kapok pulp Since kapok fibers were so difficult to be dampened, kapok fibers were first soaked in 5% (w/v) sodium hydroxide solution for 2 weeks. Then, they were cut into small pieces around 0.0-1.0 cm long. These kapok fibers were then pulped at 120°C for 2 husing 20% of (w/v) sodium hydroxide based on O.D. (oven dried) fiber weight. Kapok pulp was then washed and refined twice using a disc refiner with the disk gap of 10/1000 inch. Then, the refined kapok pulp was disintegrated at 80°C for 5 min to remove latency [7].