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The impact of sulfonation and hot-pressing of low-energy high temperature chemi-thermomecanical pulp
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.ORCID iD: 0000-0002-5813-0188
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering.
MoRe Research Örnsköldsvik AB.
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2022 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 76, no 5, p. 463-472Article in journal (Refereed) Published
Abstract [en]

Hot-pressing high yield pulp-based paper, well above softening temperature of lignin, increases paper density and paper strength. It has been investigated whether improved paper strength can be achieved and if lower pressing temperatures can be used in combination with increased sulfonation of HTCTMP (high temperature chemi-thermomechanical pulp).Moist paper sheets from low-energy Norway Spruce HTCTMP were hot-pressed up to 270°C. Sulfite charges from 25 to 120 kg/bdt were used during impregnation, preheating, and refining at 180°C with an electric energy demand of 370–500 kWh/bdt to a shive content of 1%. The pulps were mixed with 20% bleached unrefined kraft pulp to ensure that the sheet formation would not be hampered by the coarseness of the pulps. A tensile index of 70 kNm/kg was reached with highest sulfite dosage at only 150°C in pressing temperature which can be compared to 60 kNm/kg for the corresponding market CTMP. To obtain high wet strength, the highest temperature was required, while the sulfite charge was found to be of minor importance. This study has shown that it is possible to obtain strong and wet-stable paper products from HTCTMP, having a yield of 94-96% and a low energy demand at reduced pressing temperature.

Place, publisher, year, edition, pages
Walter de Gruyter, 2022. Vol. 76, no 5, p. 463-472
Keywords [en]
hot-pressing, Chemi-thermomechanical pulp, refining energy, high strength, sulfite
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:miun:diva-43721DOI: 10.1515/hf-2021-0109ISI: 000754039900001Scopus ID: 2-s2.0-85124830809OAI: oai:DiVA.org:miun-43721DiVA, id: diva2:1611825
Available from: 2022-08-24 Created: 2021-11-16 Last updated: 2023-10-20Bibliographically approved
In thesis
1. The influence of Pulp Type and Hot-pressing Conditions on Paper Strength Development
Open this publication in new window or tab >>The influence of Pulp Type and Hot-pressing Conditions on Paper Strength Development
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The hot-pressing technology has proven to have the potential for manufacturing of strong, wet stable materials based on eco-friendly renewable and recyclable lignocellulose. The purpose of this work was to study how the pulp characteristics and the hot-pressing conditions affect the dry and wet strength properties of paper. Two different devices for hot-pressing were used. One using felted nip and a heated cylinder with a temperature limit at 200°C and one new design using a hard nip and an IR-heated steel belt with a temperature limit of 300°C.

The results showed that dry strength can increase up to 150% for high yield pulp (HYP) based sheets at pressing temperatures well above the softening temperature of lignin. The maximum dry tensile strength obtained was 70 kNm/kg at 200°C pressing temperature and the corresponding value for a lignin-rich kraft pulp was about 130 kNm/kg, an increase of 30%. For all lignin-rich pulps the dry strength increased linearly with density up to 200°C whereafter it levelled off and was reduced.

The wet tensile strength for paper based on HYP increase from 2 to 28 kNm/kg and for paper based on unbleached kraft pulp from 5 up to 60 kNm/kg in the temperature interval 20-270°C. The increase in wet strength independently of pulp grade seemed to be exponential to the pressing temperature with the steepest slope above 150°C. For unbleached kraft pulp a lignin content of minimum 7% seemed to be necessary for improved wet strength but 12% gave the highest value within the studied interval. In HYPs the lignin content is 25-28% depending on the pulping process but the level of wet strength was lower which is probably related to the lower density and lower dry strength compared to unbleached kraft pulps.

Dry strength of lignin-rich paper is enhanced by improved fibre-fibre contact that can be improved by compression at high temperature, well above softening temperature (Tg) of moist lignin, native or chemically modified. It is known that sulfonation of lignin lowers the Tg in moist conditions. It was observed that at 150°C temperature the dry strength increased by 15% to a level of 71 kNm/kg for the high sulfonated pulp compared to the lower sulfonated pulp that had a dry strength of 60 kNm/kg at the same density. The level of wet strength was however not found to be affected by the sulfonation.

Paper strength is to a large extent related to pulp fibre morphology and fines content. In this work studied these aspects where briefly studied with respect to hot-pressing and the results indicate that the relative influence of fibre morphology seems to be reduced with increasing pressing temperature. Hot-pressed sheets based on a coarse fines free fibre fraction showed 100% dry strength increase and wet strength increase up to 20 kNm/kg. The dry and wet strength were however also shown to be favoured by the presence of fines fraction.

Wet strength development as a function of temperature was fitted to an Arrhenius type of equation and activation energies were found to be similar for very different pulp grades provided that the lignin content is above 7%. This could indicate that the process(es) giving wet strength were similar.

It was found that the ratio wet:dry strength was about 35-60% for all lignin containing pulp grades. A rule of thumb for an efficient wet strength resin is that the wet: dry strength ratios are 15%. This means that it should be possible to manufacture wet-strong paper from lignin-rich pulps by means of hot-pressing without using wet strength chemicals. The concern regarding repulpability of such material led to an initial test to disintegrate this paper showing that re-pulping under vigorous mixing at room temperature is possible.

The connection between dry and wet strength, high pressing temperature, and lignin content of pulp fibres is suggested to be related to some redistribution mechanisms of surface lignin between adjacent fibres. The improved wet strength and water resistance could be due to intermixing of lignin polymers across the interface between contacting fibre surfaces, or it could be lignin sufficient to cover the fibre-fibre bonds and/or chemical modifications, but these remain open questions.

Place, publisher, year, edition, pages
Mid Sweden University, 2021. p. 92
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 352
Keywords
Hot pressing, lignin, HYP, high strength, wet strength, softening temperature, kraft pulp, lignocellulose
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-43707 (URN)978-91-89341-28-9 (ISBN)
Public defence
2021-12-15, Fälldinsalen N109, Holmgatan 10, Sundsvall, 09:00 (English)
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Available from: 2021-11-16 Created: 2021-11-16 Last updated: 2023-10-20Bibliographically approved

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Joelsson, TovePersson, ErikPettersson, GunillaEngstrand, Per

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