2009:036 CIV MASTER'S THESIS Fibre based models for predicting tensile strength of paper Ann Axelsson Luleå University of Technology MSc Programmes in Engineering Wood Engineering Department of Skellefteå Campus Division of Wood Science and Technology 2009:036 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--09/036--SE Preface This study was carried out at SCA R&D Centre in Sundsvall, Sweden and the purpose was to enhance the understanding of the connection between the wood fibres and the tensile strength of paper. I would like to thank everyone that has helped and guided me through the project, my supervisors at SCA R&D Centre, Bo Westerlind and Peter Sandström, my supervisor at Luleå University of Technology in Skellefteå, Olov Karlsson. Another helpful person I would like to thank is professor Per Gradin at Mid Sweden University. Last but not least, I also give a special thank to the people who works at the Pulp and Paper testing group at SCA without whom this study would not have been possible. Ann Axelsson Skellefteå December 2008 I Abstract This degree project concerning evaluation of strength potential in paper made by mechanical pulp based on fibre properties has been carried out at SCA R&D Centre in Sundsvall. In the mechanical pulping process, the energy consumption and the strength properties of the paper depend on the raw material. To evolve the process towards lower energy consumption and an improved end product, a greater understanding of the connection between the wood fibres and the paper strength is needed. In this project, the fibre properties, such as dimensions and strength properties have been characterized and the tensile strength of the final paper has been modelled by modifying the Page and the Shear-lag model as well as with multivariate analysis. The methodology of this study was to collect data from mechanical pulps with a wide range of freeness levels. The fibres were characterized by their dimensions length, width and cell wall thickness, but also properties like curl, kink, fibrillation, slenderness ratio and collapsibility. The strength of fibres as well as the interfibre bonds were measured. Model building followed the measurements and the final models were tested using an independent set of pulps. It was found that the Page model for predicting tensile strength index could be improved by expanding the expression for fibre strength to incorporate Z-parameter and curl index and by expanding the interfibre bond strength expression to incorporate fibrillation index and curl index. The best model to use is Shear-lag whether in its original form or by simplifying it to resemble three serial connected resistors consisting of fibre strength, interfibre bond strength and fibre deformation. II Content 1 Introduction ............................................................................................................ 3 2 Background and theory......................................................................................... 3 2.1 Mechanical pulping............................................................................................ 3 2.2 Bleaching........................................................................................................... 5 2.3 Paper production ............................................................................................... 5 2.4 Pulp properties .................................................................................................. 6 2.4.1 Fibre properties........................................................................................... 6 2.4.2 Fines ........................................................................................................... 9 2.5 Paper formation and strength ............................................................................ 9 2.5.1 Interfibre bonds........................................................................................... 9 2.5.2 Activation .................................................................................................. 10 2.5.3 Densification of paper structure ................................................................ 11 2.6 Paper strength models .................................................................................... 12 2.6.1 Page ......................................................................................................... 12 2.6.2 Shear-lag .................................................................................................. 14 2.6.3 Previous model adaption .......................................................................... 16 2.7 Data analyses.................................................................................................. 17 2.7.1 Linear regression ...................................................................................... 17 2.7.2 Multivariate data analysis.......................................................................... 17 3. Materials and methods ....................................................................................... 19 3.1 Pulps ............................................................................................................... 19 3.1.1 Measurement of fibre properties ............................................................... 19 3.1.2 Water retention value, WRV ..................................................................... 19 3.1.3 Canadian standard freeness, CSF............................................................ 19 3.2 Paper making and testing................................................................................ 19 3.2.1 Test sheets ............................................................................................... 19 3.2.2 Mechanical testing .................................................................................... 19 3.3 Models............................................................................................................. 20 3.3.1 Pre-processing of raw data ....................................................................... 20 3.3.2 Multivariate analyse of data ...................................................................... 21 3.3.3 Modification of existing models ................................................................. 23 3.3.4 Validation .................................................................................................. 26 1 4 Results.................................................................................................................. 26 5 Discussion............................................................................................................ 27 6 Conclusion ........................................................................................................... 29 6.1 Future work ..................................................................................................... 29 7 References............................................................................................................ 31 Appendix A. Pulps .................................................................................................. 32 Appendix B. Abbreviations and symbols ............................................................. 35 2 1 Introduction In the production of paper, wood fibres retain most of their original structure. The tree species and individuals determine the range of fibre structure and dimensions modified by the pulping process and therefore the properties of the final paper product. For a fibre network to arise, bonding between fibres is of great importance. These bonds sprung from the tendency of cellulosic fibres to bond to each other during drying from polar liquids like water. Although fibres and bonds are very important, paper cannot be seen as a collection of fibres attached to one another, since the properties of both fibres and bonds are affected by the structure they form. In other words, the mechanical properties of a fibre dried individually differ from a fibre cut from a paper sheet. When the paper dries, local internal stresses develop and act within and between fibres changing the properties of both fibres and bonds. The internal stresses depend on the anisotropic shrinkage potential of the fibres and on the external stresses applied on the paper during drying. However, macroscopically the net stress is zero. When it comes too mechanical pulping, it is an energy consuming process that results in paper with good optical properties, but with not so good strength properties. To decrease the energy consumption during the pulping process and to increase the strength of the final paper product the understanding of the mechanisms behind the formation of fibre networks has to increase. The aim for this project carried out at SCA R&D Centre in Sundsvall was to develop a model to predict the tensile strength for paper based on fibre geometry and material properties to enable future development of the mechanical pulping processes. 2 Background and theory The goal of the pulping process is to liberate the fibres from the wood matrix and to make them suitable for papermaking. The dominating pulping processes are mechanical or chemical. In mechanical pulping the yield ranges from 90% to nearly 100%, since only easily dissolved carbohydrates and extractives are lost. Unfortunately it requires a lot of external energy. For chemical pulps the yield is lower because nearly half of the raw material is dissolved in the process, on the other hand, the degradation products is commonly used for