High strength paper from high yield pulps by means of hot-pressing

Abstract

<jats:title>Abstract</jats:title><jats:p>The hypothesis is that it should be possible to modify papermaking conditions in line with the softening properties of high yield pulp fibres and achieve similar strength properties to conventional chemical pulp based paper. We therefore investigated the rheological and physical properties of high yield pulp based papers during hot-pressing. Our results confirm that increased temperature combined with sufficient pressure enables permanent densification by softening of lignin, producing very high tensile strength. This treatment also significantly improved the wet tensile strength in comparison to bleached kraft pulp without using wet strength agents. The high yield pulps used here were spruce based thermomechanical pulp, chemi-thermomechanical pulp, and high temperature chemi-thermomechanical pulp, and birch-aspen based neutral sulphite semi chemical pulp, with spruce-pine based bleached kraft pulp as reference. Rapid Köhten sheets of <jats:inline-formula id="j_npprj-2019-0087_ineq_001_w2aab3b7d360b1b6b1aab1c15b1b1Aa"><jats:alternatives><jats:tex-math>150\hspace{0.1667em}\text{g}/{\text{m}^{2}}</jats:tex-math></jats:alternatives></jats:inline-formula> and 50 % dryness were hot-pressed in a cylinder-press at 20–200 °C, 7 MPa, and 1 m/min. The mechanical properties showed great improvements in these high yield pulp papers, with tensile index increased to 75 kNm/kg and compression strength index to 45 kNm/kg; levels close to and better than bleached kraft. Wet strength increased to 16 Nm/g compared to 5 Nm/g for bleached kraft.</jats:p>

The impact of molded pulp product process on the mechanical properties of molded Bleached Chemi-Thermo-Mechanical Pulp

“…Hunt et al [26] studied the influence of pressure on cellulose fibers properties and observed that a drying in a hot press strongly limits the material's shrinkage as well as reduces the product's thickness and water content. Joelsson et al [27] observed that a higher processing temperature and a higher pressure increase the material's density, thus resulting in a lower pores size.…”

“…A higher mould B temperature improves the MPP's Young's Modulus for both initial and after water sorption. Several studies [23,24,27] observed that a higher processing temperature allows the tensile index to be improved as it allows to have a lower WTR and swelling capacity [21,24,37]. Moreover, an even higher temperature used decreases the mechanical properties as given by the results in the drying time of moulds C and D as well as the moulds C and D temperature for the sorption analysis.…”

The impact of molded pulp product process on the mechanical properties of molded BCTMP

“…Hunt et al [26] studied the influence of pressure on cellulose fibers properties and observed that a drying in a hot press strongly limits the material's shrinkage as well as reduces the product's thickness and water content. Joelsson et al [27] observed that a higher processing temperature and a higher pressure increase the material's density, thus resulting in a lower pores size. Reducing the pores size limits the water uptake and fiber swelling while increasing the cellulose's degree of crystallinity.…”

“…A higher mould B temperature improves the MPP's Young's Modulus for both initial and after water sorption. Several studies [23,24,27] observed that a higher processing temperature allows the tensile index to be improved as it allows to have a lower WTR and swelling capacity [21,24,30].…”

“…Several Studies [24,28,31] observed that a high drying temperature reduces the paper breaking length and tensile strength. However, when pressure is applied in combination with high temperature, the cellulose fibers show a different behavior with a higher Young's Modulus for a longer pressing time on moulds C and D. It has been shown [23,27,32] that a high pressure and high temperature drying improve the mechanical properties with higher tensile strength as the average pore size is reduced and the density improved. It was also observed that the crystallinity is improved with high pressure and high temperature, which is translated by a lower water uptake and higher mechanical properties.…”