Cellulosic materials are highly sensitive towards environmental changes such as temperature and especially towards humidity. Besides morphological changes like swelling and/or shrinking, the mechanical properties of pulp fibres and fibre to fibre joints change as well. The current study sets to elucidate the changes and the extent to which elevated or decreased relative humidity (RH) influences the load bearing capacity of individual hardwood and softwood fibres and joints. The device used for the measurements is a micro bond tester with a custombuilt humidity chamber in which the humidity can be controlled in a range from 25 to 95% RH. Individual fibres and joints have been tested at 30, 50 and 80% RH after equilibration time of 120 min. The breaking load of individual softwood fibres and joints displayed a maximum breaking load at 50% RH, with the values showing a decreasing trend towards higher or lower RH. In case of hardwoods, no change in the breaking load of either individual fibres or fibre to fibre joints has been observed at different RH.
In the present study hardwood fibre strength and fibre to fibre joint strength measurements using bleached, industrial eucalyptus kraft pulp have been performed. The device used for the measurements was a micro bond tester developed at Graz University of Technology. Results were compared to those obtained in previous studies dealing with both, hardwood and softwood. The mean force to break individual hardwood joints (1:82 AE 0:48 mN) was found to be 72 % smaller than that of unbleached softwood joints and is mainly governed by the bonded area of the joints. Furthermore, the optically bonded area (OBA) of hardwood fibre to fibre joints was investigated. OBA of hardwood joints is about 81 % smaller than those of softwood and the reduction is attributed to smaller width and lower collapsibility of the fibres. Compared to softwood, the force per unit OBA of hardwood is 1.72 times higher (5:32 AE 1:46 MPa). This difference is believed to be due to the well known size effect. Single fibre tensile testing resulted in a mean breaking load of 38:81 AE 16:31 mN and show a 76 % reduction when compared to softwood. The reduction is attributed to the smaller cross sectional area and the effect of bleaching.
Tensile tests of individual bleached kraft pulp fibers and paper sheets of industrial origin were conducted in order to investigate the effect of refining and small-scale fiber deformations on the extensibility of fibers and paper. The shape of the tensile curves of most of the fibers was concave upward (i.e., increasing slope) and consisted of two or three phases suggesting that the fibrillar structure and disordered regions in the fiber wall were straightened out during straining. Only a few of the individual BSKP fiber tensile curves were apparently linear. Elongation of the individual kraft fibers varied from 8 to 32% and the average elongation was not increased by high consistency refining. Tensile test results of laboratory sheets made of the same BSKP pulp suggested that the fiber bonding not only governs paper strength, but also is highly relevant for the elongation of fiber networks. The key conclusion related to this investigation and freely dried sheets was that the increased network elongation and strength after refining is mainly due to increased inter-fiber bonding and a higher shrinkage tendency of the fiber network and not due to the increased elongation or strength of individual fibers.
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