Fibre Deformations Of Softwood And Bagasse Pulps And The Load-Elongation Properties Of The Paper

Stress-strain properties （or load-elongation properties） are one of the important paperproperties. Tensile strength and elongation of paper are defined by the end-point of the sheet’sload-elongation curve. Compared with the tensile strength of paper, the elongation is animportant but underrated functional property of paper. Fibre properties, including fibre length,width, coarseness, fines content and so on, play an important role in the load-elongationproperties of paper. However, the deformations of fibre are of high significance but easilyignored properties among them. In general, fibre deformations are the misalignment of thecellulose crystals in fibre wall relative to the overall fibril direction, which include fibre curl,kinks, dislocations, microcompresssion and so on. Fibre curl and kinks could be easilymeasured by commercial fibre analyzers. Fibre deformations affect the load-bearing ability offibres in the paper and further affect the load-elongation properties of the fibre network. Inpresent study, fibre deformations of bleached kraft softwood and bagasse pulps caused bydifferent kinds of mechanical treatments were inverstigated. The effects of fibre deformationson the fibre strength and load-elongation properties of paper, especially on the tensile strengthand elongation, were also studied.The wing defibrator, E-compactor and conventional Valley beater were applied to treatbleached kraft pine pulp from the first thinning. Each mechanical treatment induced fibredeformations in its own characteristic way. The high consistency （HC） wing defibratorinduced fibre kinks and curl whereas the E-compactor, in addition to fibre cutting, favouredkinks. Low consistency （LC） Valley beating straightened the fibres and released fibredeformations. The biggest changes occurred at the beginning of the treatments. However, ahigh temperature treatment on its own seemed to increase the extent of fibre deformationsconsiderably. The zero-span tensile strength correlated better with the number of kinks thanwith the shape factor （fibre curl）.Some modifications were made on the hydrochloride acid （HCl） method developed byAnder et al, which was used to estimate the degree of fibre defects. Firstly, the arithmetic fibrelength parameter, including the contribution of fines fractions, was used instead of thelength-weighted fibre length in the original method. Secondly, the cleavage index was usedinstead of the cleavage in order to better compare fibres with different lengths. The cleavageindex was calculated by the equation Cleavage index=1/L-1/Lo, where Lois the arithmeticfibre length in mm for the control in water （or for untreated reference fibres）, and L is thearithmetic fibre length in mm for HCl-treated fibres. The hydrochloride acid （HCl） treatment-induced cleavage index increased with the HC wing defibrator treatment butdecreased with the LC beating. The difference between the dry and wet zero-span tensilestrengths, which can be used for estimating the changes in fibre defects, increased with thecleavage index for LC beating but decreased for the HC wing defibrator treatment. Thisimplies that the chemical accessibility of HCl to the fibres and the HCl-induced fibre cuttingdo not necessarily correlate with the mechanical strength of the fibres. The cleavage indexmay not be directly related to the mechanical defects of the fibres but may be more dependenton the chemical conditions on the fibre surface and in the fibre wall.The fibres treated by HC mechanical treatment are usually not appropriate forpaperpaking process and need a further LC beating to improve their properties. For bagassepulp fibres, HC refining caused fibre curl considerably, which were stable in the subsequentPFI refining. For pine pulp fibres, most of fibre deformations induced by HC refining werereversible and could be removed by a subsequent PFI refining or valley beating. Based onthe observation by polarized light microscope, HC wing defibrator treatment caused curl,kinks, dislocations, and microcompressions in the fibres. Among them, the small scaledeformations, such as microcompressions have an important role in the elongation potentialof sheets and they can be preserved in subsequent Valley beating, which tends to straightenthe fibres and release kinks and dislocated zones. Increasing fibre curl does not necessarilylead to improved paper elongation due to the reduced load-bearing ability of curly fibres inthe fibre network.The combined HC wing defibrator treatment and subsequent LC Valleybeating was found to be the best strategy to produce paper with a high level of elongation,maintaining high tensile strength, and good dewatering properties.The elongation of the freely dried and restrained dried paper depends on different factors.In the case of freely dried paper, the shrinkage potential is the dominant factor and there is alinear correlation between the elongation and shrinkage potential of paper. While in the caseof restrained dried paper, the fibre wall morphology and microcompressions have a crucialrole.Shear bond strength of softwood pulp and bagasse pulp fibres was estimated using Pageequation for tensile strength. The value increased in LC beating for softwood pulp from4.0MP to11.1MPa and for bagasse pulp from6.8MPa to16.7MPa. However, shear bondstrength of softwood pulp and bagasse pulp fibres estimated from Shallhorn-Karnis tensilestrength model almost stayed constant during different beating processes. Shear bond strengthwas approximately1.7Mpa for softwood pulp and2.2MPa for bagasse pulp. Nevertheless, theshear bond strength of bagasse pulp was better than that of softwood pulp fibres, which propably due to their charcteristic surface chemistry of the fibres. During different beatingprocesses, the increase of the relative bonded area （RBA） in paper resulted in theimprovement of the tensile strength of paper. The positive effects of HC refining on the RBAand internal bond strength of paper were realized during the sequent LC beating.In this study, a semi-empirical model for paper elongation s=φ（K, T） was developed. Itwas constructed using data from different beating series of bleached softwood kraft pulps. Thehypothesis was that the elongation of paper depends on the extensibility potential of the singlefibres and on the factors related to fibre network structure that initiates the final fracture. Themodeling for the elongation of paper is as follows: s=0.149K^0.2104T^0.4279. Its coeffeicent ofdetermination R²is approximately0.72. The parameter of K （kink index） indicates theextensibility of the fibre and the parameter of T （tensile index） includes all the factors thataffect the initiation of paper fracture, which is reasonable considering tensile index andelongation both as the end-point coordinates of stress-strain curve of paper. Additionally, Tcan be estimated from fibre properties and RBA using Page equation or Shallhorn modeling.The modeling of the elongation of paper was improved when the tensile index wasreplaced by relative bonded area （RBA） since the differences among the specimen mainlyfrom the RBA of paper. The expression is s=0.0529K^0.3646RBA^0.4754. Its coefficient ofdetermination R²is0.75. Preferable model is s=φ（K, RBA） since it is simple and explicit,which is beneficial in practical applications. However, it is worth to note that, this model ofthe elongation of paper is not perfect yet. The possible reason is that kink index or curl indexare not the best parameters for indicating the extensibility potential of fibres. Therefore, thepremise of a better elongation model of paper would be to develop a quantitative method forthe number of microcompressions of fibres, which more directly affect the elongationpotential of single fibres.