Mid Sweden University

Abstract

High Yield Pulp (HYP), i.e. TMP, SGW or CTMP, is normally used in printing papers (News, SC and LWC) or in a middle layer on cardboard i.e. in products that either have high demands on printability and runnability in fast printing presses or contribute to high bulk in cardboards in order to minimize pulp consumption at a certain sheet stiffness. Tensile strength as a function of density is significantly higher for HYP compared to chemical pulps such as sulphate and sulphite pulps. However, chemical pulp is mainly used in packaging materials that require very high tensile strength, while at the same time allowing the density of the paper to be high. By utilizing the softening properties of high-yielding lignin-rich fibres by hot-pressing technology, it is possible to significantly increase sheet density and thereby strength closer to the level of chemical pulps. Furthermore, due to the presence of high levels of lignin, it was shown that considerably higher wet strength can be achieved compared to chemical pulp without the addition of strengthening agents. The study focuses on the softening of stiff and lignin-rich fibres in papers based on HYP with sufficiently high moisture contents, when hot-pressing at temperature levels significantly above the softening temperature (Tg) of lignin Hot-pressing increases the density of the sheet which increases the contact surface between the fibres in the paper structure. The high pressing temperature can be said to induce a viscous flow of lignin, which also increases the potential for fibre-fibre bonding. It is conceivable that covalent bonds are obtained via condensation reactions and partly that interdiffusion between the lignin in the fibre walls can be obtained as they come close enough to each other during the hot-pressing. The research also shows that hot-pressing greatly improves properties in the form of dry and wet strength as well as hydrophobicity for HYP and for lignin-rich kraft paper. The first part of the study shows the effect of hot-pressing on strength properties of paper sheets based on CTMP, HT-CTMP, TMP, NSSC, SCP_al and NBSK. The second part includes a study on how and to what extent different amounts of residual lignin in the pulp contribute to the dry and wet strength of the sheets of paper during hot-pressing as a function of increasing temperature. To demonstrate this, pilot scale cooking of unbleached pulp to various lignin levels was carried out. In all experiments in parts one and two, laboratory sheets with a surface weight of 150 g/m² and a dry content of 50% were made with a Rapid Köthen (RK) sheet former, after which the sheets were hot-pressed in a cylinder press at temperatures up to 200°C, constant high pressure of about 7 MPa, nip pressure dwell time of 1.5 sec and production speed 1 m/min. The third part includes a study on the optimization of variables in a new design of a dynamic cylinder press for hot-pressing technology. This design is based on previous research at Mid Sweden University combined with key knowledge of steel band technology within IPCO AB. The new pilot machine is based on heating of a steel belt with infrared heat (IR) up to 300°C, a maximum line load of 15 kN/m in two press nips and a dwell time of 23-240 ms in the nip depending on the production speed which is up to 5 m/min. The experiments in part three were based on RK paper sheets with 100 g/m² and approximately 63% dry content made by HT-CTMP. The results confirm that hot-pressed HYP-based paper sheets enable permanent densification by softening lignin, which provides a very high dry tensile strength and a remarkable improvement in wet tensile strength compared to bleached kraft pulp without the need for wet strength agents. A tensile index of 75 kNm/kg, compression strength index (SCT) of 45 kNm/kg and wet tensile strength index of 16 kNm/kg were obtained, which can be compared with the corresponding values for bleached kraft pulp based paper sheets of 85 kNm/kg, 35 kNm/kg and 5 kNm/kg respectively, all with the same density after hot-pressing at 200°C. The NSSC reached the highest tensile strength index of 92 kNm/kg. The study with the unbleached kraft pulps showed that the lignin content had a significant effect on both the dry and wet tensile strength indices. The pulps showed a linear relationship between wet strength and lignin content. The increase in lignin content from 0% to 12% improved the dry tensile index by 20% and SCT by 35% and gave a very significant increase in the wet strength index from 3 to 23 kNm/kg after hot-pressing. All lignin-rich paper samples exhibit good wet stability for at least 24 hours and an improved surface hydrophobicity by increasing the pressing temperature and lignin content.

Optimization of the new steel belt based press machine showed that high nip pressure and two press nips had a great effect on density and strength. Whereas high temperature, well above Tg of lignin, and long pressing time were more important to achieve high wet strength. The highest wet strength index value, 27 kNm/kg, was reached when the pressing temperature was 290°C, the nip pressure about 8 MPa, the pressing time in the press nip 40 ms and the dwell time in contact with the steel belt 23.5 sec. It was also noted that no delamination occurred in these tests.

In order to obtain both high dry and wet strength, it is important to have high lignin content, high temperature, high nip pressure and sufficiently long pressing time

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.

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.

The research presented here show ways to improve wet strength by means of hot-pressing without strength additives when using lignin containing pulps as unbleached softwood chemical pulps (NSK) and lignin rich softwood chemithermomechanical pulps (CTMP). NSK (Northern Softwood kraft) laboratory scale produced pulps of 3 different levels of natural lignin (or kappa number) and two commercial pulps, NBSK (Northern Bleached Softwood kraft) and CTMP, were compared evaluating dry- and wet-strength properties. Staining methods and light microscope were used to study cross sections of paper sheets. The CTMP fibers collapse to an increasing degree with pressing temperature whereas NBSK/NSK do not change. The microscopy methods show the distribution of lignin within the paper structure. Sheets made from NSK show a significant increase in wet strength from 4kNm/kg to 23kNm/kg, when increasing temperature from 20°C to 200°C. CTMP show corresponding increase from 2kNm/kg to 16kNm/kg. No increase in dry strength or in density can be observed in case of NBSK/NSK, while the CTMP show an increase of 53% and 100% respectively. The SCT values show an increase up to 35% for lignin-rich NSK based paper sheets when hot-pressing. 

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 150g/m2 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.

A successful way to increase the strength properties for pulps based on lignin-rich fibres is to compress the fibre structure at high temperature by means of hot-pressing technology. The fundamental knowledge of how the fi-bre morphology influences the mechanical properties when a paper sheet is hot-pressed is still scarce. Paper sheets based on thermomechanical pulp (TMP) produced with single disc and double disc refiners were compared. The effect of degree of refining was studied as well as the effect of fibre shapes by fractionating pulp with hydrocyclones. Additionally, the effect of fines was studied. All pulps were produced at the Holmen Bra-viken Mill, Norrköping, Sweden with Norway Spruce (Picea abies) as raw material. Handsheets (100 g/m2) with 62% ± 3 dryness were hot-pressed at temperatures up to 260°C at a pressure around 8MPa. The hot-press-ing increased both dry and wet strength for all pulps studied. This was true even for pulps with low fines con-tent and low refining energy. Even thick-walled fibres normally giving lower strength showed an increase of 100% when hot-pressed. In summary, hot-pressing technology can make it possible to use different TMPs to produce strong packaging materials for use in dry and wet conditions.

The dry strength properties of hot pressed moist paper improved as stiff high-yield pulp (HYP) fibers soften and the sheet density increased. Very high wet strength was also achieved without adding strengthening agents. This research focuses on a new hot-pressing methodology based on a steel belt-based pilot cylinder press with infrared heating. The heated steel belt transports the moist paper into the cylinder nip with two adjacent steel rollers with adjustable nip pressure. The temperature ranges up to 300°C, maximum speed is 5 m/min, maximum pulling force from the steel belt is 70 kN and the line load in the two press nips is 15 kN/m each. High peak pressures are possible due to the hard press nip between steel rolls and steel belt, allowing a good heat transfer to the paper. The long dwell time allows strained drying of the paper which results to high density and high wet strength. Paper samples from high-yield pulps were tested at different nip pressures, temperatures and machine speeds while the dry content was kept constant at about 63%. High nip pressure showed the largest effect on densification and dry strength. While high temperature and long dwell time seem to be most important in achieving high wet strength. 

The dry strength properties of hot-pressed moist paper improved as stiff high-yield pulp fibers soften and the sheet density increased. Very high wet strength was also achieved without adding strengthening agents. This research focuses on a new hot-pressing methodology based on a steel belt-based pilot cylinder press with infrared heating. The heated steel belt transports the moist paper into the cylinder nip with two adjacent steel rollers with adjustable nip pressure. The temperature ranges up to 300 °C, maximum speed is 5 m/min, maximum pulling force from the steel belt is 70 kN and the line load in the two press nips is 15 kN/m each. High peak pressures are possible due to the hard press nip between steel rolls and steel belt, allowing a good heat transfer to the paper. The long dwell time allows strained drying of the paper which results to high density and high wet strength. Paper samples from high-yield pulps were tested at different nip pressures, temperatures and machine speeds while the dry content was kept constant at about 63%. High nip pressure showed the largest effect on densification and dry strength. While high temperature and long dwell time seem to be most important in achieving high wet strength.

Broader use of bio-based fibres in packaging becomes possible when the mechanical properties of fibre materials exceed those of conventional paperboard. Hot-pressing provides an efficient method to improve both the wet and dry strength of lignin-containing paper webs. Here we study varied pressing conditions for webs formed with thermomechanical pulp (TMP). The results are compared against similar data for a wide range of other fibre types. In addition to standard strength and structural measurements, we characterise the induced structural changes with X-ray microtomography and scanning electron microscopy. The wet strength generally increases monotonously up to a very high pressing temperature of 270 °C. The stronger bonding of wet fibres can be explained by the inter-diffusion of lignin macromolecules with an activation energy around 26 kJ mol−1 after lignin softening. The associated exponential acceleration of diffusion with temperature dominates over other factors such as process dynamics or final material density in setting wet strength. The optimum pressing temperature for dry strength is generally lower, around 200 °C, beyond which hemicellulose degradation begins. By varying the solids content prior to hot-pressing for the TMP sheets, the highest wet strength is achieved for the completely dry web, while no strong correlation was observed for the dry strength. 

Broader use of bio-based fibres in packaging becomes possible when the mechanical properties of fibre materials exceed those of conventional paperboard. Hot-pressing provides an efficient method to improve both the wet and dry strength of lignin-containing paper webs. Here we study varied pressing conditions for webs formed with thermomechanical pulp (TMP). The results are compared against similar data for a wide range of other fibre types. In addition to standard strength and structural measurements, we characterise the induced structural changes with X-ray microtomography and scanning electron microscopy. The wet strength generally increases monotonously up to a very high pressing temperature of 270 ◦C. The stronger bonding of wet fibres can be explained by the inter-diffusion of lignin macromolecules with an activation energy around 26 kJ mol−1 after lignin softening. The associated exponential acceleration of diffusion with temperature dominates over other factors such as process dynamics or final material density in setting wet strength. The optimum pressing temperature for dry strength is generally lower, around 200 ◦C, beyond which hemicellulose degradation begins. By varying the solids content prior to hot-pressing for the TMP sheets, the highest wet strength is achieved for the completely dry web, while no strong correlation was observed for the dry strength.

This work is focused on fundamental aspects of the densification of paper sheets during hot-pressing under conditions where the lignin in the fibre walls is softened. In this study light microscope and scanning electron microscope (SEM) techniques were used to reveal the mechanisms in the fibre network structure within the paper sheets that arises due to densification and the impact of lignin. UV and staining methods and spectrometric observations of the ultrastructure of cross section of paper sheets and fibre surfaces will highlight the changes that occur in the fibre structures. This study improves the understanding of how fibres collapse and how internal fibre-fibre bonds in lignin-rich mechanical pulp affect the physical properties of the final paper sheet. To demonstrate this, paper sheets from five different pulps containing different concentration of natural lignin were produced. Handsheets of 150 g/m2 were prepared in a Rapid Köthen (RK) laboratory sheet former, where the sheets were press-dried at 100 kPa and ca 90oC to a dry content of 45-50% d.c. After 24 hours in room temperature the hand sheets were hot-pressed in a temperature interval from 20 – 200oC at a constant pressure in a cylinder-press at a speed of 1 m/min. The results show that remarkable improvements on paper sheets, based lignin-rich pulps, can be achieved in terms of increased tensile index (up to 85 kNm/kg), compression strength, SCT, (up to 38 kNm/kg) and wet strength (up to 10 kNm/kg), which depends on the densification of the fibre structure at high temperature and pressure in the load nip. It is concluded that this to a major extent is related to that the lignin rich fibres are compressed at high enough temperature to both softened and develop tacky surfaces so that the fibres are locked into their positions within the highly densified sheets. The SEM evaluation shows how the surface structure get dense at pressing at 200oC for the CTMP based paper sheets. The light microscopy studies of the sheet cross sections reveal how the fibres collapse in the case of CTMP based sheets while fibres from bleached kraft pulp based sheets are quite well collapsed already at room temperature.