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Curing of drying oil in wood fibre webs by gamma-irradiation Paszner, Laszlo

Abstract

A technique is described for improving wood fibre web strength by copolymerization of a hydrocarbon drying oil (CTLA Polymer) within hand-sheets made from pure and admixed forms of high yield chemical (sulfite and sulfate) and groundwood fibres. Oil polymerization can be initiated successfully by prolonged thermal treatment at 145°C, as well as by the novel method of low dosage gamma-ray irradiation. The treatment strengthens inter-fibre bonding. Evidence for copolymerization was sought through assessment of tensile and related strength data on variously treated standard handsheets. Effectiveness of the polymerization on pulp types investigated as initiated by heat and low dosage irradiation is strongly related to surface colloidal and basic physical and mechanical properties of the fibres. Maximum strength parameters were obtained with oil saturated sulfite and groundwood handsheets following polymerization by thermal treatment. Copolymerization efficiency by irradiation, on the other hand, is estimated as 73% for sulfite, 65% for groundwood and 50% for kraft handsheets. Effectiveness of oil polymerization by irradiation was found to be proportional to dosages up to 10⁵rad. As anticipated, limit of strength improvement was not set entirely by intrinsic fibre strength, but was also influenced by surface colloidal properties of the fibres. Blends made from chemical-groundwood fibre mixtures behaved more or less proportional to individual pulp strengths and extent of blending. Copolymerized sulfite-groundwood blends at equal proportions approximated the tensile strength of similar kraft papers suggesting better economy with kraft fibres when papers of limited strength are produced. All other strength parameters (elastic modulus, tensile strain and tensile energy absorption) followed similar trends, or if different were of aid in explaining certain phenomena characteristic to treated fibre webs. The presence of graft copolymer could not be conclusively proven, since both the copolymer and oil homopolymer are insoluble in the usual organic solvents. A dual character of the oil polymer was demonstrated with glass filterpaper and highly purified commercial cellulose fibres. Effectiveness of oil polymerization in highly lignified mechanical pulps was greatly depressed by mild sodium chlorite treatment, although the strength and bonding capacity of such fibres increased considerably. This was taken as direct evidence for the possibility of a lignin-CTLA Polymer copolymer system. Site of chemical reaction has not been described, although means for assessment of such information are suggested. The study proves the usefulness of gamma irradiation as energy source for the copolyerization of drying oils in handsheets as replacement for the prolonged thermal treatment hitherto used by the hardboard industry. Added advantages of irradiation processed copolymerized papers are less discoloration, especially with high lignin content pulps, and lower elastic modulus, a factor clearly objectionable for some paper uses.

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