Improving the strength properties of TMP - Pulp and Paper Canada

REFININGImprovingthe strength propertiesof TMPBy K.B. Miles and I. OmholtAbstract: In order to develop the quality of TMP while limiting the drop in fibre length, the refiningintensity was reduced in the second and third stages during pilot-scale refining trials, by reducingthe rotational speed from 1,200 to 900 rpm. The tear index and, in some cases the TEA indeximproved. Similar bulk and air resistance were reached at higher long-fibre content, indicating thatthe fibres were well developed. Both spruce and pine were investigated.K.B. MILESPapricanPointe-Claire,QCI. OMHOLTPapricanPointe-Claire, QCiomholt@paprican.caHE specific energy consumption associatedwith thermomechanical pulp-Ting is high and has long been a matterof concern. However, the mainpriority is still to maintain the pulpquality. In cases where serious fibre shorteningoccurs, or in cases where the strength propertiesperiodically drop due to wood quality variations,the problem may become the inability to applyenough energy to obtain the required pulp qualitybefore the target CSF value is reached.This paper describes experimental work donein a TMP pilot plant with the objective of showinghow the quality can be improved by increasingthe energy application through reducing theintensity in the second and third refining stage. Itrepresents the continuation of earlier work atPaprican, investigating the potential of refining atreduced rotational speed [1,2].Many results have been published from workdone to explore the effect of varying intensity inthe primary stage. In a few pilot studies, the intensityin the post primary stages was investigatedspecifically by increasing it over conventional levels.This generally led to reduced length weightedaverage fibre length [3], reduced long fibreand shive content [4,5,6] and lower CSF [4,5] ata given energy consumption. Reduced tear indexat a given breaking length has been shown and ithas also been observed that breaking length andburst could suffer, depending to some extent onthe wood specie [4,6].BACKGROUNDRefining intensity is defined as the specific energydelivered per bar impact, and at a given consistencyit is proportional to the square of therotational speed [7]. Fibre development dependsupon the material’s ability to deform and absorbthis energy at impact stresses below the point offibre cutting. Through a high number of impacts,the fibre wall will become fatigued, the outerfibre layers will be loosened and partly peeled off,forming fines and ribbon-like material. This isillustrated in Fig. 1.Based upon the physical relationship equatingwork to the product of pressure and volumechange, the stress developed during compressionof any material is a function of the absorbed energydivided by the deformation. For practical pur-poses, the unstressed initial bulk is indicative ofthe potential for such deformation, and so thestresses acting upon impact are related to the ratioof refining intensity to bulk. As specific energyaccumulates and the fibres become more developedduring the refining process, the uncompressedbulk decreases and the potential fordeformation during the impact is reduced. If theaverage refining intensity is kept on the same levelin all the refining stages, impact stress will thusrise for each stage. Consequently, the risk ofexceeding the fibre wall strength and ultimatelyreducing the fibre length, will increase. It isimportant to keep in mind that the absolute abilityof the material to absorb energy, to deform andto withstand stress without severe fibre shortening,probably also depends on the temperature and onthe inherent wood related fibre properties.The principle of this relationship can be illustratedusing the results from a high-speed impacttester [8]. The apparatus delivers an impact to apulp pad at a speed comparable to that of a refiner.By increasing the air pressure acting on thepiston, impact velocity and energy are increased.As seen in Fig. 2, at a given air pressure, or impactenergy, the stress upon the pulp pad increaseswith decreasing bulk value.The stress on the material at a given bulk maybe reduced by reducing the average refiningintensity according to this relationship. Even ifthe forces in the refiner may act on each fibrefrom many different directions simultaneously,our hypothesis is that the stress level would still bereduced. This would potentially lead to a higherlong fibre content at equal bulk, as shown in Fig.3, where handsheet bulk has been taken to representthe bulk of the material in the refiner.From the foregoing it is likely that when higherquality is needed, application of the additionalenergy could be facilitated by reducing theaverage refining intensity in the post primarystages of the process. This was the approachexplored in the series of pilot plant trialsdescribed below.EXPERIMENTALUnless otherwise specified, the pulps were producedfrom black spruce on Paprican’s TMP pilotplant. The primary refiner is a pressurizedAndritz 22-1CP single disc refiner, preceded by an46 ❘❘❘ 105:5 (2004) T 123 Pulp & Paper Canada

REFININGFIG. 1. Example of well-developed TMP fibres produced inthe pilot plant (black spruce, low refining intensity, totalspecific energy 7,088 kWh/odt applied in four stages).FIG. 2. The stress on the pulp pad during high-speedimpact testing increases with stepwise increments in theair pressure acting on the piston. The stress level at a givenair pressure increases with increasing specific energyconsumption and decreasing uncompressed bulk of thepulp. The basis weight of the pad was 300 g/m 2 and theconsistency 30%. The pulp was produced from blackspruce at Paprican’s TMP pilot plant.FIG. 3. As the refining intensity is reduced by reducing therefiner rotational speed from 1,500 to 900 rpm, the longfibre content will be maintained at a higher level as thebulk is reduced by increasing the refining energy.FIG. 4. As the rotational speed of the refiner was reduced,the specific energy consumption at a given freenessincreased. The effect became particularly evident at low CSF.inclined preheater fed, in turn, by a plugscrew. Chip steaming conditions wereeither 250 kPa for 100 seconds or 400 kPafor 20 seconds. All pressures are given asgauge pressure. In these trials a relativelyconventional level of refining intensitywas maintained in the primary refiner byoperating it at a rotational speed of 1,800rpm and discharge consistency of 25-30%.The feed rate was in the range of 2.28-3.74odt/d. Generally, about 1,000 kWh/odtwere applied in this stage.Several additional levels of specificenergy were applied in the secondaryrefiner, comprised of an atmospheric doubledisc Bauer 400 operated at a target dischargeconsistency of 25% and feed rate of3.62 odt/d. In this unit a conventional levelof refining intensity was obtained byoperating at its normal rotational speed of1,200 rpm. Low intensity refining wasachieved by reducing the speed to 900rpm. Lower intensity could also beachieved by increasing the consistency, butthis involves more steam production at agiven specific energy. At the long residencetime resulting from the combinationof a substantially lower refining intensityand high specific energy, this approachleads to increased interference betweensteam and pulp flow. Speed reduction is,therefore, a more appropriate choice forlowering the intensity under these circumstances.In addition, higher consistency isnot an option for large commercial refinersalready running at near maximumpractical levels of consistency. In one ofthe trials described here, a relatively highlevel of secondary intensity was obtainedby operating at 1,500 rpm. When extremelyhigh energy levels were required, thisrefiner was also employed to do a thirdstage at the same refining intensity as thesecond. The plate patterns used wereD17C002 and 36104 (NiHard) for theAndritz and the Bauer refiner respectively.The pulp was screened on a Somervillescreen (0.15-mm slots) before testing.The tests were done according to PAPTACstandard methods. The length weightedaverage fibre length was measured onunscreened pulp using the Fibre QualityAnalyser.RESULTSVarying Secondary Refining Intensity:Beginning with primary pulp in which thechips had been steamed for 100 secondsat 250 kPa, some over-all assessment of theeffects of varying secondary refiningintensity were obtained by operating thesecond stage refiner at speeds of 900,1,200 and 1,500 rpm. The relationshipPulp & Paper Canada T 124 105:5 (2004) ❘❘❘ 47

REFININGFIG. 6. The long fibre content at a given energy level correlateswell with the refining intensity. The error bars representthe 95% confidence interval of the regression linesused for the interpolation.FIG. 5a and 5b. The long fibre content and the averagelength weighted fibre length were maintained at a higherlevel when the rotational speed of the refiner was reduced.FIG. 7. The amount of specific energy necessary to reach agiven freeness is related to the long fibre content. Thetrend is supported by data from mill TMP and RMP madein Paprican’s pilot plant, all made from black spruce.FIG. 8. Reducing the refiner rotational speed in the secondand third stages resulted in higher long fibre content at agiven CSF. All pairs of runs were done with different batchesof chips.between freeness and specific energy is shown in Figure 4. It isclear that the reduced intensity resulting from lower secondaryrotational speed brings the pulp to a given freeness with morespecific energy in it as intended. At 1,500 rpm the highest energytarget had to be reduced because of plate clearance limitations.If, as intended, the stresses on the material have beenreduced by using lower intensity, it should aid in the preservationof fibre length as more energy is applied. The plots of longfibre content against specific energy in Figure 5a and 5b demon-FIG. 9. Reducing the refiner rotational speed in the secondand third stages resulted in higher long fibre content at agiven specific energy consumption. The first-stage samplefor each batch of chips, except for the second batch, isshown as the point at the lowest energy level.48 ❘❘❘ 105:5 (2004) T 125 Pulp & Paper Canada

REFININGFIG. 10. The tear index was higher at low intensity. Theeffect became evident in particular at high levels of burst.FIG. 11. In order to form a sheet with the same bulk withhigher long fibre content, the fibres have to be flexible andconformable. This was accomplished by the use of higherspecific energy.FIG. 12. The same air resistance could be reached at higherlong fibre content after low-intensity refining, indicatingthat the fibres were well developed.FIG. 13. There was no statistically significant difference inlight scattering coefficient between the two intensities.strate that this is the case.It can also be shown, as in Fig. 6, thatthe long fibre content at a given specificenergy is highly dependent on the refiningintensity used in the second stage. Therefining intensity is calculated accordingto [9].Another relevant observation to bemade from Figs 5a and b, is that the actualdifference in fibre length and long fibrecontent between the different intensities,will depend on which energy level is usedfor the comparison.As previously discussed, the fatiguework required in order to flexibilize thelong fibres and improve their bondingability, demands specific energy. This isquite evident in Fig. 7, which shows thatthe specific energy needed to achieve afixed freeness is well related to theamount of long fibre contained by thepulp at that freeness.Adding the points obtained from thisinvestigation to data from pilot plant RMPand three commercial TMP installations,produces a fairly well defined trend.High Quality, Low Freeness Pulps: Thepotential benefits of using lower intensityto produce high quality, low freeness thermomechanicalpulps were also investigated.In this case, a higher preheating temperaturewas used. To do this,conventional primary pulp made fromchips that had been presteamed for 20seconds at 400 kPa was put through twoadditional refining stages. One run wasalso done at 500 kPa at a primary energyconsumption of 569 kWh/odt. The secondand third stages were done at a rotationalspeed of 1,200 rpm for the referencepulp and 900 rpm for thelow-intensity pulp.Refining at 900 rpm yielded substantiallymore long fibre on the basis of eitherfreeness or specific energy, as shown inFigs 8 and 9 respectively show. All pairs ofruns were done with different batches ofchips, which may explain the slight variationin response at 400 kPa. The change inlength weighted average fibre length generallyfollowed the same trend.The general effect for the black spruceruns seems to be that the difference inlong fibre content for the two refinerspeeds increased as the energy increased,confirming that the refining intensitybecomes more critical as lower levels ofbulk are approached. Thus, as shown inthe plot of Fig. 10, the energy needed toincrease burst can be applied with betterretention of tear strength at lower intensity.This was particularly evident in the trialdone at 500 kPa.Again, in addition to containing morelong fibre, it can be seen from Figs 11 and12 that pulps refined at low intensity cangive similar bulk and air resistance aspulps refined at conventional intensity.This indicates that these fibres have beenwell developed.The shive content was slightly higherafter low intensity refining. The differenceswere in the range of 0.1-0.4 percentagepoints Somerville shives at a CSFlevel of 200 mL.Compared at a given CSF there was nostatistically significant difference in lightscattering coefficient between the twointensities, as shown in Fig. 13.Low intensity refining gave higher TEAindex at a given CSF in some cases, butPulp & Paper Canada T 126 105:5 (2004) ❘❘❘ 49

REFININGFIG. 14. Low intensity refining gave higher TEA index at agiven CSF in some cases.FIG. 15a. Low intensity refining gave a higher long fibrecontent at a given freeness. The first-stage sample isshown as the point at the highest freeness level.FIG. 15b. The jack pine used in this trial had low tracheidlength. The R28 fraction (R14 + 14/28) gave a more completeassessment of the differences in long fibre contentdue to the reduction in refining intensity.FIG. 16. As observed for spruce, pine had a higher longfibre content at a given bulk after low-intensity refining.not always, as illustrated in Fig. 14.Application to Other Species: Initial studieshave also been carried out on woodspecies that traditionally provide TMP ofpoor quality. In this case, the structure ofthe fibres themselves or other factors maylimit the deformation available to controlrefining stress at conventional levels ofrefining intensity. Jack pine and southernpine (loblolly pine) are representative ofthis class of raw materials. After conventionalprimary refining with chippresteaming for 20 seconds at 400 kPathey were given second and third stagerefining at either 1,200 rpm for reference,or at 900 rpm for low intensity. Thesepresteaming conditions were chosen inorder to provide a reference for the trialswith black spruce.Lower refining intensity improved thelong fibre content at a given specific energyfor both jack pine and southern pineand, as shown in Figs 15 and 16, resultedin considerably more long fibre at a givenfreeness or bulk. Other properties alsoresponded in a generally similar mannerto those observed for black spruce. Thejack pine used in this trial had relativelyshort wood tracheid length, which is a possibleexplanation for the low over-all levelof the long fibre content in the TMP. Measuredon kraft pulp cooked from thechips used in the refining trials, thelength weighted average fibre length forthe jack pine was 1.89 mm, compared with2.94 mm for the southern pine. A level of2.30 - 2.45 mm was measured for blackspruce using the same method. Accordingto data shown by Gullichsen and Paulapuro(10), jack pine normally has tracheidsalmost as long as black spruce.DISCUSSIONPulp linting is often a concern regardingthe quality of TMP. Previous work hasshown that linting can be reduced byincreasing the specific energy consumption[11]. At a given specific energy, therefiner rotational speed and the refiningconsistency did not seem to affect the riskof linting measured as the linting propensityindex (PLPI) [2,6,12].Another important concern for highvalue paper grades is fibre rising caused bymoistening of the paper surface duringcoating and offset printing. Fibre rising isrelated to reduced gloss and increased surfaceroughness. It may increase withincreased long fibre content, and seems toinvolve particularly thick walled fibres.However, increased fibre bonding abilitythrough refining will counteract this effect[13,14]. In an earlier study, both increasingthe intensity in the first stage andincreasing the specific energy consumptionproduced high long fibre quality,which reduced the changes in gloss androughness on application of water. In thecase of increased intensity, the fraction oflong fibres was however somewhatreduced [15]. The advantage of lower postprimary intensity is its ability to extendgreatly the energy input when needed.CONCLUSIONLower rotational speed was used toreduce the refining intensity in the postprimary stages of TMP. This resulted in:• More long fibres compared at either agiven freeness, at a given specific energy,or at a given bulk; and50 ❘❘❘ 105:5 (2004) T 127 Pulp & Paper Canada

REFINING• Better tear index at a given burst and insome cases improved TEA at a given CSF.It was also observed that the specific energyrequired to reach a given freeness, forblack spruce, is related to the amount of longfibre contained by the pulp at that freeness.This approach might be useful whenmore specific energy consumption isneeded, for example when refining lowquality wood, when attempting to improvethe strength properties at a given CSF orwhen aiming for low CSF levels while preservinga high fibre length. The implicationscould, for example, be increased useof less desirable wood species in newsprintor reduced content of kraft pulp in higherquality paper.ACKNOWLEDGEMENTSThe contribution of Michael Stacey andDerek Dranfield in carrying out the trials,organizing the results and preparing diagramsis gratefully acknowledged. Theauthors also wish to thank Reza Amiri foruseful discussions.LITERATURE1. US Patent no. 6,336,602 B1 (2002).2. MILES, K.B., MAY, W.D., KARNIS, A., Refiningintensity, energy consumption, and pulp quality intwo-stage chip refining, Tappi J. 74 (3): 221-230(1991).3. KURE, K.-A., DAHLQUIST, G., HELLE, T., Morphologicalcharacteristics of TMP fibres as affected bythe rotational speed of the refiner, Nord. Pulp Pap. Res.J. 14 (2): 105-110 (1999).4. MILES, K.B., KARNIS, A., The response of mechanicaland chemical pulps to refining, Tappi J., 74 (1):157-164 (1991).5. CHAPMAN, D.L.T., ALLAN, R.S., Some basic considerationsin groundwood rejects refining, Pulp PaperCan. 71 (8): 67-72 (1970).6. SINKEY, J., Mechanical pulp properties and distributionsas affected by refining conditions, Internationalsymposium on: Fundamental concepts of refining,Institute of paper chemistry, Appleton, WI, Sept.16-18 (1980).7. MILES, K.B., MAY, W.D., The flow of pulp in chiprefiners, J. Pulp Pap. Sci. 16 (2): J63-J72 (1990).8. AMIRI, R., HOFMANN, R., Dynamic compressibilityof papermaking pulps, Paperi ja Puu, 85 (2): 100-106(2003).9. MILES, K.B., A simplified method for calculatingthe residence time and the refining intensity in a chiprefiner, Paperi ja Puu, 73 (9): 852-857 (1991).10. GULLICHSEN, J., PAULAPURO, H., ed. PapermakingScience and Technology. Book 6A. Chemical Pulping.Helsinki: Fapet Oy (1999).11. WOOD, J.R., KARNIS, A., Towards a lint-freenewsprint sheet, Paperi ja Puu, 59 (10): 660-674(1977).12. WOOD, J.R., KARNIS, A., Linting propensity ofmechanical pulps, Pulp Paper Can. 93 (7): T191-T198(1992).13. HALLAMAA, T., HEIKKURINEN, A., Effect offibre properties on sheet surface roughening, Proc.,International Mechanical Pulping Conference, SPCI,Stockholm, Sweden, 361-363 (1997).14. HOC, M., Fibre rising in papers containingmechanical pulp, Tappi J. 72 (4): 165-169 (1989).15. AMIRI, R., STEPIEN,G., WOOD, J.R., TMP processconditions to produce pulps for mechanicalprinting papers - Effects on surface properties, Proc.1996 Pulping Conference, Atlanta: TAPPI Press, Book1: 265-278 (1996).Résumé: Afin d’améliorer la qualité de la PTM tout en limitant la réduction de la longueur dela fibre, nous avons réduit l’intensité du raffinage aux deuxième et troisième stades lors d’essaispilotes sur le raffinage, en diminuant la vitesse de rotation de 1200 à 900 trs/min. L’indice dedéchirement et, dans certains cas, l’indice TEA se sont aussi améliorés. Une résistance au passagede l’air et un bouffant similaires ont été atteints à une teneur plus élevée en fibres longues, ce quiindique que les fibres étaient bien développées. Les essais ont porté sur l’épinette et le pin.Reference: MILES, K.B., OMHOLT, I. Improving the strength properties of TMP. Pulp & PaperCanada 105(5): T123-128 (May, 2004). Paper presented at the 2003 Intl. Mechanical Pulping Conferencein Québec, QC, on June 2 to 5, 2003. Not to be reproduced without permission of PAP-TAC. Manuscript received on September 26, 2003. Revised manuscript approved for publicationby the Review Panel on November 21, 2003.Keywords: THERMOMECHANICAL PULPING, MECHANICAL PROPERTIES, BEATINGDEGREE, ROTATION, VELOCITY.Pulp & Paper Canada T 128 105:5 (2004) ❘❘❘ 51