Starch penetration in surface sizing at various parameters - pilot and production machine experience

This study investigated various surface sizing parameters, such as linear load and speed (nip dwell time), and such starch properties as solids content, starch viscosity and temperature. Various strength properties as well as starch penetration and pick up were measured. The results of this study showed that linear loads as high as 100 kN/m and doubling the nip dwell time by varying the running speed had less effect on starch penetration than expected, especially when low viscosity starch was used. However, many strength properties did have some positive correlation with increased linear load and dwell time. The increased linear load and dwell time was noticed to increase strength properties mainly due to differences in starch pick up instead of differences in starch penetration. In a production trial, where higher viscosity starch (i.e. 70 mPas) was used, it was noticed that starch penetration increases when linear load is increased up to 60 kN/m. However, starch penetration does not improve further when linear load is increased above 60 kN/m. The results in this study then suggest that relatively high viscosity of the starch promotes the effect of linear load to the penetration at low linear loads up to a certain limit, whereas low viscosity starch (i.e. 20 mPas) penetrates similarly almost regardless of the linear load. According to the finding of this study, there are no arguments to dimension the metered size press unit above linear loads of 60 kN/m With increased linear load, starch pick up increases through more complete film transfer. Then, most strength properties (Huygen, CMT and Burst) improve as more starch is going to the sheet. This observation confirms earlier pilot results. This study was able to narrow the possible explanations for differences in strength properties down to increased starch pickup through more complete film transfer, whereas the measured penetration was not considerably affected by linear load and nip dwell time, especially when relatively low viscosity starch is used. Such differences in starch pickup, on the other hand, can in most cases be offset simply by selecting a grooved rod of increased volumetric application. The roll diameter selection and design of the MSP unit may then be based on mechanical engineering requirements at moderate nip loadings. This study once again confirmed that starch penetration can be adjusted through starch viscosity. It was noticed, however, that viscosity differences produced by tailoring the rheological properties or application temperature of starch, or differences in the solids content of starch, need to be considerable to have a significant impact on penetration.