Method Development For Evaluating The Bonded Area Of Interfiber Bonds Of High Yield PULP

In recent years, high-quality high yield pulp (HYP) has been found increasing applications in many value-added paper grades to replace bleached hardwood kraft pulp, but one of main HYP drawbacks is its weak interfiber bonding strength, which limits its further application in value-added products. There is limited work done for evaluating HYP interfiber bonding properties, and a widely accepted system could not be found so far. Therefore, it’s of interest to develop a system for evaluating the HYP interfiber bonding ability for the papermakers and researchers. As to one of the important indices for interfiber bonding properties of HYPs, that’s great to evaluate the HYP interfiber bonding area.Firstly, different Poplar P-RC Alkaline Peroxide Mechanical Pulp (APMP) fiber fractions were used as raw materials in this research. Some important parameters were measured, including the fiber characteristics (fiber fraction ratio, fiber length, fiber width, and coarseness), carboxyl group content, Zeta potential, and the lignin distribution in fibers. The results show that along with the screen mesh increased from 30-mesh to 200-mesh, the fiber length, width, coarseness, and the perimeter of the fiber fractions decreased; while the carboxyl group content and the absolute value of Zeta potential were increased; the contents of Klason lignin and total lignin increased, while the surface lignin coverage decreased and the lignin distributed more uniformly.Secondly, the method of the extrapolating Page bonding strength was deduced and improved to calculate the relative bonding area (RBA). The conventional Page strength equation was transformed into a new equation by replacing some parameters, then the linear relationship between the Page bonding strength index (B) and light scattering coefficient (S) were found. Using the properities of handsheets made from different fiber fractions under a series of wet press pressures, the total light scattering coefficient for completely unbonded fibers in a sheet (So) of each fiber fraction could be calculated. Combined with the light scattering coefficient method, the RBA of each fiber fractions was calculated. The results show that the higher RBA could be obtained when the fiber fraction was shorter or smaller. The same trend was observed for the shear bond strength per unit area (b). Wet pressing was used to improve the bonding ability; higher RBA could be obtained through the higher wet pressing pressure, but it had no effect on the b value.Thirdly, the R30 and P30/R50 fiber fractions were used to make the interfiber bonding models. The confocal laser scanning microscopy (CLSM) was applied to do the optical sections for the interfiber models without damaging them, and then the bonded area of HYP interfibers could be determined. Physical cutting and scanning electron microscope (SEM) were used to verify the CLSM measurement. The results show that the actual contact area and the actual bonded area (A2) of the interfiber bonding models of R30 fiber fraction was greater than those of P30/R50 fiber fraction; but there was an adverse trend for the relative contact rate (R) and the relative bonded area (RBA’). In order to verify the accuracy of the CLSM method, the technologies combined the conventional physics slice, the ordinary optical microscope (LM) and SEM were used to measure the wall thickness and the fiber length of interfiber bonded area. The results show that the average cell wall thickness measured by conventional physics slice was 2.964 um, and the fiber length of R30 fiber fraction bonded area was 10.78 μm; while those evaluated by CLSM was 3.097 μm and 11.73 μm, respectively, showing that the CLSM method has high accuracy. Similarly, using the "optical sectioning" technology of CLSM to measure the actual interfiber bonded area of HYP fibers also has high accuracy and credibility.Finally, the relationship between the fiber characteristics of four poplar P-RC APMP fiber fractions and their interfiber bonding ability was developed. It was found that the higher carboxyl group contents the fibers have, the higher b can be obtained. While the lignin on the fiber surfaces could hinder the hydrogen bonding between fibers. The higher surface lignin coverage on fibers, the less RBA and b could be obtained in handsheets. In a word, the increase of carboxyl group content had a positive effect on improving the interfiber bonding properties, while the higher lignin coverage on the fiber surface will reduce these properties.