Mid Sweden University

The effect of increasing the pulp yield by the addition of sodium borohydride (NaBH4) or polysulfide (PS) in softwood kraft cooking, i.e. enhancing the retention of glucomannan, on the physical properties of low-grammage handsheets was studied. In addition to the yield improvement, an increase in tensile index was observed, especially at lower degrees of beating. These higher yield pulps showed an increase in pore volume, indicating an increased degree of swelling of the fibres. Presumably, the increased flexibility of the fibres affects the bonding strength and leads to the higher tensile index observed.

Previous paper (Rahman et al. 2017) showed that the yield of softwood kraft pulp increased by the addition of either polysulfide or sodium borohydride because of higher hemicellulose retention. An increase in hemicellulose content can make dewatering more difficult as WRV of the pulp increases, but instead, an overall increase in pulp yield could improve dewatering as a sheet of a certain weight will contain fewer fibers, giving a more open sheet structure. It was therefore of interest to measure the dewatering properties of low grammage handsheets (20 g / m2) under conditions mimicking the tissue paper machine dewatering processes, and sheet strength properties, WRV, ° SR and fiber dimensions were also studied. The results showed that the positive influence of overall yield increase dominated the negative influence of an increase in hemicellulose content on the dewatering properties, particularly at lower refining energy levels. Furthermore, higher yield and higher hemicellulose content pulps had a higher tensile index at the same dryness. A given tensile index was achieved with less refining energy. The results indicate that increased yield and hemicellulose content by modifying the kraft pulping process will result in a pulp with a potential to improve tissue paper quality.

Minimizing the fiber property distribution would have the potential to improve the pulp properties and the process efficiency of chemimechanical pulp. To achieve this, it is essential to improve the level of knowledge of how evenly distributed the sulfonate concentration is between the individual chemimechanical pulp fibers. Due to the variation in quality between pulpwood and sawmill chips, as well as the on-chip screening method, it is difficult to develop an impregnation system that ensures the even distribution of sodium sulfite (Na2SO3) impregnation liquid. It is, therefore, crucial to measure the distribution of sulfonate groups within wood chips and fibers on a microscale. Typically, the degree of unevenness, i.e., the amount of fiber sulfonation and softening prior to defibration, is unknown on a microlevel due to excessively robust or complex processing methods. The degree of sulfonation at the fiber level can be determined by measuring the distribution of elemental sulfur and counterions of sulfonate groups, such as sodium or calcium. A miniaturized energy-dispersive X-ray fluorescence (ED-XRF) method has been developed to address this issue, enabling the analysis of sulfur distributions. It is effective enough to be applied to industrial laboratories for further development, i.e., improved image resolution and measurement time. 

To improve the competitive advantages of pulp fibre based materials such as tissue and packaging products over the fossil-based products, it is of key importance to improve the knowledge of the selectivity of the cooking process. There is also demand to expand the fundamental scientific understanding of pulp and paper manufacturing systems because of growing demand for replacing plastics. However, it is challenged to improve the selectivity of the cooking process by optimizing unit operation such as impregnation, cooking and refining. For pulp production based on chips regardless of chemical (kraft or sulphite) or high-yield (chemimechanical or semi-chemical pulp) pulping process, the efficiency of the impregnation is always crucial. To improve impregnation uniformity, we need to study how even distribution of lignin releases down to fibre level via easily impregnated wood chips. It can be achieved, using classic measures such as; equalized hydroxide ion concentration, increased initial sulphide ion concentration, low sodium ion concentration and low boiling temperature combined with the oxidative and reductive environment to understand how the chemicals quickly enter and distribute in the chips. However, we have studied the uniformity of impregnation at fibre level by the possibility of accurately measuring S and Na content by collimating the X-ray beam into a ~200 µm spot in diameter using energy-dispersive X-ray fluorescence (ED-XRF) spectrometry. In addition, we have also studied improved impregnation by selective cooking systems for sulphate pulp in oxidative (polysulfide) and reductive (sodium borohydride, NaBH4) environments. Our aim is to develop standard measurement methods to improve the smoothness of fibre properties for tissue and packaging products to reach the sustainable development goal (SDG) stated by the UN at target 9.5 “Enhance research and upgrade industrial technologies”.