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The further development of softwood-based products will help to increase the competitiveness of the Finnish forest industry and could generate new opportunities and revenue for the northern hemisphere forest industry in general. This thesis examined the thermal stability of refined softwood kraft pulp as raw material for high-density paper for food packaging applications. A thermal treatment device that allows for the accurate combination of high temperature (175–300 °C) with a variable water vapor content (1 – 98 v/v-%) in the atmosphere is introduced for thermal stability studies of kraft pulp. The performance of the device was evaluated with different paper sheets by monitoring the changes in their chemical, optical and strength properties such as intrinsic viscosity, brightness, and burst strength, respectively. The results demonstrated that the innovative thermal treatment device offered excellent performance within the targeted ranges of temperature and water vapor atmospheres. The effects of enzyme pretreatment and refining on the thermal stability of paper were also studied. The enzymatic pre-treatment impaired the chemical and physical properties of the paper, such as the degree of polymerization of cellulose and tensile strength, which in turn also resulted in a reduction of the thermal strength stability of the paper. Residual lignin in pulp was found to counteract the hydrolytic thermal degradation of paper. This was demonstrated by the systematic analysis of ultraviolet resonance Raman spectra and by the determination of the light absorption coefficient. Furthermore, the role of residual lignin in pulp on the thermal stability of refined paper was studied with unbleached and oxygen delignified pulp sheets. Results showed that the residual lignin in pulp is able to attenuate the thermal degradation of pulp polysaccharides as demonstrated by the degree of polymerization of cellulose results. In order to understand the effect of lignin in pulp refining on the thermal stability of paper, bleached pulp was pretreated with kraft lignin - separated from black liquor - prior to refining. When compared to the reference pulp and pulp treated with antioxidant (ascorbic acid), the kraft lignin impregnated pulp improved the burst strength stability of the refined pulp sheets after the hot humid thermal treatment (225 °C and water vapor atmosphere of 75 v/v-%). This work is a significant step towards the optimization of paper making processes that influence the thermal stability of papers and provides important insights into how the thermal stability of kraft pulp in papers can be improved.; Havupuupohjaisten tuotteiden kehitys parantaa suomalaisen metsäteollisuuden kilpailukykyä ja tuottaa uusia mahdollisuuksia ja liikevaihtoa pohjoisen pallonpuoliskon metsäteollisuudelle ylipäänsä. Tämä työ tarkasteli jauhetun havusulfaattisellun lämmönkestoa raaka-aineena elintarvikepakkauksiin tarkoitetussa tiivispaperissa. Sulfaattisellun lämmönkestotutkimuksiin valittiin uuni, jonka on mahdollista saavuttaa korkea lämpötila… Advisors/Committee Members: Vuorinen, Tapani, Prof., Aalto University, Department of Forest Products Technology, Finland (advisor).