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

Sammanfattning

Högutbytesmassa (HYP), d.v.s. SGW, TMP eller CTMP, används normalt i tryckpapper (News, SC och LWC) eller i mittskikt i kartong dvs i produkter som har höga krav på tryckbarhet och körbarhet i snabba tryckpressar eller för att bidra till hög bulk i kartong så att man därmed kan minimera förbrukningen av massa för att nå en viss arkstyvhet. Dragstyrka som funktion av densitet är väsentligt högre för HYP jämfört med kemiska massor som sulfat- och sulfitmassor. Däremot används främst kemiska massor i förpackningsmaterial som kräver mycket hög dragstryka, där man samtidigt tillåter att papperets densitet får vara hög. Genom att utnyttja mjukningsegenskaperna hos högutbytesmassors ligninrika fibrer genom varmpressningsteknik kan man väsentligt öka arkdensitet och därigenom styrka till i nivå med kemiska massors. Vidare visas att man tack vara närvaro av höga halter lignin kan nå väsentligt högre våtstyrka jämfört med kemisk massa utan tillsats av styrkehöjande kemikalier. Studien fokuserar på mjukgörning av styva och ligninrika fibrer vid varmpressning vid temperaturnivåer väsentligt över mjukningstemperaturen (Tg) för lignin av HYP-baserat papper med tillräckligt hög fukthalt. Varmpressning ökar arkens densitet, vilket ökar kontaktytan mellan fibrerna i pappersstrukturen. Den höga pressnings-temperaturen kan sägas inducera ett visköst flöde av lignin, vilket då ökar möjligheten att få starkare för fiber-fiber-bindning. Man kan dels tänka sig att kovalenta bindningar erhålls via kondensationsreaktioner och dels att man kan erhålla interdiffusion mellan ligninet i fiberväggarna då de kommer tillräckligt nära varandra vid varmpressningen. Forskningen visar också att varmpressning högst väsentligt förbättrar egenskaper i form av torr- och våtstyrka samt hydrofobicitet för både HYP-baserat och ligninrikt kraftpapper. Den första delen av studien visar effekten av varmpressning på styrke-egenskaper hos pappersark baserade på CTMP, HT-CTMP, TMP, NSSC, SCPal och NBSK. Den andra delen inkluderar en studie om hur och i vilken utsträckning olika mängder av kvarvarande lignin i massa bidrar till pappersarkens torrstyrka och våtstyrka vid varmpressning som funktion av ökande temperatur. För att demonstrera detta tillverkades oblekt kraftmassa till olika ligninhalter i pilotskala. I alla experiment i del ett och två i avhandlingen tillverkades laboratorieark med ytvikten 150 g/m² och torrhalten 50% i en Rapid Köthen (RK) arkformare varefter arken varmpressades i en cylinderpress vid temperaturer upp till 200°C och konstant högt tryck på cirka 7 MPa i ett pressnyp med uppehållstiden 1,5 s i pressnypet  vid maskinhastigheten 1 m/min. Den tredje delen i avhandlingen inkluderar en studie om optimering av variabler i en ny design av en dynamisk cylinderpress för varmpressteknik. Den nya designen baseras på tidigare forskning vid Mittuniversitetet kombinerat med nyckelkunskap om stålbandstekniker inom IPCO AB. Den nya pilotmaskinen är baserad på att ett stålbälte uppvärms med infraröd värme (IR) upp till 300°C, en linjelast upp till 15 kN/m i två pressnyp vardera, med variabel presstid 23-240 ms i pressnypet beroende av maskinhastigheter upp till 5 m/min. Experimenten i del tre i avhandlingen baserades på RK-pappersark tillverkade av HT-CTMP med ytvikten 100 g/m² och torrhalten ca 63%. Resultaten bekräftar att varmpressande HYP-baserade pappersark möjliggör permanent densifiering genom mjukning av lignin, vilket ger en mycket hög torr dragstyrka och en anmärkningsvärd förbättring av våt dragstyrka jämfört med blekt kraftmassa utan att våtstyrkemedel behöver användas. Ett dragindex på 75 kNm/kg, kompressionsstyrkeindex (SCT) på 45 kNm/kg och våtstyrkeindex på 16 kNm/kg erhölls vilket kan jämföras med motsvarande värden för pappersarken från blekt kraftmassa på 85 kNm/kg, 35 kNm/kg respektive 5 kNm/kg, alla med samma densitet efter varmpressning vid 200°C. Lövvedsbaserad NSSC nådde det högsta dragstyrkeindexet på hela 92 kNm/kg. Studien med de oblekta kraftmassorna visade att lignininnehållet hade en signifikant effekt på både torr- och våtstyrkeindex. Kraftmassorna uppvisade ett linjärt samband mellan våtstyrka och lignininnehåll. Ökningen i ligninhalten från 0% till 12% förbättrade dragindexet med 20% och SCT med 35% och gav en mycket signifikant ökning i våtstyrkeindex från 3 till 23 kNm/kg efter varmpressningen. Alla ligninrika pappersprover uppvisar god våtstyrkestabilitet under minst 24 timmar och en förbättrad ythydrofobicitet genom ökning av presstrycktemperaturen och lignininnehållet.

Optimering av den nya stålbandsbaserade cylinderpressen visade att högt pressnyptryck och två pressnyp hade stor effekt på densitet och styrka. Emedan hög temperatur, långt över Tg för lignin, och lång presstid var viktigare för att nå hög våtstyrka. Det högsta värdet på våtstyrkeindex, 27 kNm/kg, uppnåddes när presstemperaturen var 290°C, pressnyptrycket cirka 8 MPa, presstiden i pressnypet 40 ms och tiden i kontakt med stålbältet 23,5 sek. Det noterades också att ingen delaminering inträffade i dessa tester.

För att erhålla både hög torr- och våtstyrka är det viktigt med högt lignininnehåll, hög temperatur, högt nyptryck och tillräckligt lång presstid.

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.