Volume 33, Issue 1

Separate refining of pulp fractions before remixing has sometimes been compared to whole pulp refining, with no definitive answer about the interest of doing so. In this work, we present a general method to help decide whether fractions refining can be profitable or not. Recommendations for fractions refining strategies are given, depending on the evolution of pulp properties as a function of applied energy. In the tested case (bleach kraft pulp mix (75 % eucalyptus/25 % radiata pine) fractionated on a 2-stage hydrocyclone process), optimised fractions refining provided a gain of about 40 kWh/T compared to whole pulp mix refining, when targeting a tensile index of 50 N.m/g.

The efficiency and effects of enzymatic deinking using xylanase and lipase mixed with amylase (XLA) for deinking secondary fibers were investigated for mixed office waste (MOW), photocopy prints (PHOT) and old newspapers (ONP). The optical and physical properties of deinked pulps were evaluated. Changes in fiber morphology, crystallinity index and functional groups were also monitored. The results show that for non-impact prints (MOW and PHOT), a maximum deinking efficiency of 19.9 % and 13.9 % were obtained with enzymatic deinking (XLA), respectively. For ONP, conventional alkaline demonstrated the highest efficiency of 12.20 %. XLA treatment significantly improved the tensile index and tear index of MOW and PHOT but caused a slightly reduction for ONP, relative to respective blank. It was found that kink index and curl index were significantly decreased, caused a 55.6 % reduction in the kink index for PHOT and a 35.8 % reduction in the curl index for ONP. FT-IR observation show that the relative intensity of the alkaline/enzymatic-deinked pulp has changed differently according to the type of wastepaper. These results indicate the synergistic effects of xylanase, lipase and amylase making enzymatic treatment a promising method that can substitute alkaline deinking for non-impact prints.

In this study, the effects of low consistency refining (LCR) energy and intensity on mechanical pulp properties have been studied for three different types of reject pulps (softwood TMP, softwood CTMP and hardwood CTMP), which were refined at varying intensity. Resulting pulp properties have been compared with high consistency refining (HCR) of the same reject pulps. For all furnish types, it was shown that LCR can develop pulp properties matching those developed through HCR with significantly less energy. The resulting pulp properties were found to be affected not only by refining intensity and energy, but also by initial fibre morphology. Pilot LCR trials demonstrated that high freeness reject pulp is initially insensitive to refining intensity as specific energy is applied. This enables the first stage of LCR to be carried out at a higher specific energy and intensity, which can reduce the number of stages of LCR required to reach a target quality. This work shows that low intensity LCR is capable of achieving the same tensile index as HCR pulp at a target freeness of 200 ml CSF.

Internal variables in (C)TMP-refining processes (e. g. temperature, consistency, fiber residence time, backward flowing steam and forces acting upon the chips and pulp) are defined as physical states obtained in different parts of the refining zones. In short, they differ from the traditional external variables (e. g. dilution water feed rate and specific energy) which are not obtained from measurements or physical models of the process in the refining zone. The internal variables are the backbone of physical models and are used in this paper as soft sensors for advanced process control. Besides the shape of the temperature profile, the position of the maximum temperature and the shape of the consistency profiles inside the refining zones are important to control as they are directly linked to the development of different pulp and handsheet properties. To illustrate the capability when using combined modeling and control strategies, the concept is applied on a full scale CD82 refiner. This means both temperature and consistency control in the flat zone and the conical zone, together with an overall estimator for fiber development. Finally, in this paper it is shown that the production can be increased significantly without changing the refiner motor load nor violating the pulp property specification.

Consistency profiles in the refining zones of twin-disc refiners have always been cumbersome to estimate with good accuracy. To overcome such challenges, this paper shows that it is vital to measure temperature profiles between the refining discs to estimate uneven chip/pulp feed distribution. It is also shown that the plate clearance, measured by plate gap measurement devices, is changed dramatically when changing the amount of dilution water to the refining zones asymmetrically. At the same time, the inlet temperature will change as well while the maximum temperature is rather stable. This makes the maximum temperature a good candidate for use when estimating the split of the pulp mass flow rates to the refining zones. This also opens for a new consistency control concept for each refining zone. The findings in this paper have been validated in a commercial TMP production line with two serially linked twin-disc refiners, and it is shown that the pulp and handsheet property variations, in terms of mean fiber length and tensile index, between the refining zones can be reduced considerably when running the refiners with similar consistency in each refining zone.

Detection of the onset of fiber cutting is beneficial in low consistency refining as it may prevent reduction of average fiber length, optimize fiber quality improvements by operating at gaps just wider than the critical gap, avoid decreasing the strength properties of paper, and increase energy efficiency. The objective of this study is to understand the effect of pulp furnish on measured bar forces and, more specifically, on the detection of fiber cutting. Bar forces, i. e. forces applied to pulp fibers by the refiner bars, are measured with a custom-designed piezoelectric force sensor. Trials were conducted with an AIKAWA 16-in. single-disc refiner using hemlock/balsam softwood thermomechanical pulp, SPF softwood thermomechanical pulp, northern bleached softwood kraft pulp, and aspen hardwood thermomechanical pulp at 3.0 to 3.5 % consistency at rotational speeds of 1200 and 1400 rpm. The power of the time domain signal of the measured forces is introduced as an indicator of the onset of fiber cutting. Our results show that this new fiber cutting metric is a sensitive and reliable metric for determination of fibre cutting for a range of pulp furnishes. The study suggests that the refiner force sensor has potential to be exploited for in-process detection of fiber cutting.

The study explores how changes in process variables, residence time and pulp consistency in refining influence the pulp properties. The equipment utilized in this study was a conical disc chip refiner (RGP82CD) producing thermomechanical pulp (TMP). The focus was on the ratio between tensile index and specific energy consumption. Pulp properties were measured for composite pulp samples taken from the refiner blow line. Residence times and pulp consistencies were estimated by use of the extended entropy model. This showed that the CD-refiner, with the flat and conical refining zone, has a process performance similar to that of a two-stage refiner set-up, and that the consistency in both refining zones is of high importance. Comparing different periods revealed that even if the values of measured blow line consistency are similar, significant differences in the estimated consistency in the flat zone can prevail. Therefore, only monitoring blow line consistency is not enough. Specifically, it was found that the pulp consistency after the flat zone could be very high, considerably higher than in the blow line, and this could have negative effects on tensile index and fibre length.

This paper is part two of a study on a CD 82 TMP chip refiner where relations between changes in the process conditions and changes in the properties of the produced pulp are investigated. Focus is on the ratio between tensile index and specific energy consumption when results from five tests are compared. Pulp properties were measured for composite pulp samples taken from the refiner blow line. Residence times and pulp consistencies were estimated by use of the extended entropy model. Clearly, an increase in specific energy does not necessarily implicate an increase in strength properties of the pulp produced. It is of high importance to have access to information about the refining zone conditions when searching for an optimal operation point in terms of the ratio between tensile index and specific energy. In these tests, this ratio had a maximum at about 55 % measured blow line consistency. Unfavourable operating conditions were identified at high pulp consistencies, especially after the FZ, where pulp consistencies well above 70 % were observed. The estimated residence time for each refining zone responded differently when applying changes in production rate, plate gaps and dilution water flow rates. In conclusion, the results associated with estimated pulp consistencies where easier to interpret compared with results for residence times, implying that additional tests are required for the latter variable. In addition to tensile index, pulp properties like freeness, Somerville shives and light scattering coefficient were included in the analysis.

Paper surface hydrophobization, also referred to as .surface sizing., is often necessary for printing and packaging purposes. Typically, hydrophobic polymeric nanoparticles, in combination with starch are applied on the paper surface at the dry-end of the paper machine. In the surface sizing process, the nature of the nanoparticles, starch type, starch concentration and ratio between starch and nanoparticles, paper quality, ionic strength, and application as well as drying temperature are parameters that influence the result. The aim of this work was to systematically evaluate these parameters in order to create knowledge to be used for optimization of the process. Laboratory scale surface sizing trials were performed and the results from the trials showed that cationic particles gave superior performance compared to anionic particles. Both the starch type and the concentration of the oxidized starch had an impact on the performance. The effect of the ionic strength was found to depend on the particle charge: for cationic particles, the addition of salt was detrimental while for the anionic particles it was beneficial. An increase of the application or the drying temperature was found to enhance the performance up to a temperature around the glass transition temperature of the polymer.

In order to improve its printability the effects of pigment sizing on printability and coating structure of decorative base paper were investigated. Silica, styrene-acrylic emulsion (SAE) and cationic starch were used to prepare the pigment sizing solution. After been pigment sized, the physical performances, printing properties and coating structure of the paper were determined. Pigment sizing had a more obvious effect on the physical properties, printability and coating structure. The tensile strength was increased by 26.99 %, the print-surf roughness was decreased by 17.98 %, the water absorption height was increased by 8.51 %. The color density was improved, the percentage of dots loss decreased by 21.33 %, the roundness of dots increased by 43.33 % and thus better printing effect was achieved. The water receptivity and absorption of decorative base paper could be improved by pigment sizing under the appropriate binder-to-pigment ratio. SEM and three-dimensional image showed that silica/cationic starch/SAE had filled the pores of paper and formed an uniform coating structure. Considering the improvement of the physical properties, printability and coating structure, the best ratio of binder-to-pigment was identified as 70:30.

Polyelectrolyte multilayering (PEM) treatment is regarded as one of the surface modification techniques to increase paper strength. In this study, the strengthening effect of PEM treatment of highly filled paper was investigated. PEM treatment was performed on both fiber and filler with cationic starch and anionic polyacrylamide systems. Both approaches provided an improvement in the strength of filled paper. However, the strengthening effect of PEM treatment on fibers became weaker as filler content increased, whilst the PEM treatment on fillers was more effective in improving the strength of the highly filled paper. This was because PEM treatment on fillers improved bonding strength between fiber and filler as well as bonding strength between fillers. This was confirmed by the evaluation of specific debonding factor between fiber and filler, and the measurement of the breaking strength of dense film composed of fillers.

Paper-based materials are viscous materials, the time-dependent behavior of which depends strongly on moisture content. Particularly the creep of paperboard containers under compressive forces is greatly affected by changes in the relative humidity. In the present paper, we examine the creep behavior of paperboard under cyclic humidity conditions using the finite element method. Especially the shape and rate of the through-thickness moisture content gradient on moisture accelerated creep are studied. An isotropic hygro-viscoelastic constitutive law is used for paperboard. The results of the simulations are compared with experiments. It is concluded that the through-thickness moisture gradients have a great impact on the moisture accelerated creep of paperboard. Furthermore, the results show that depending on the direction of external load the through-thickness moisture content gradient may increase or decrease creep rate.

At present, the tissue paper manufacturing is mostly based on the dry crepe technology. During the last decade, the manufacturers have introduced new tissue machines concepts that increase the softness, bulk, and absorption capacity. Such machines produce a strong regular three-dimensional (3D) structure to the sheet before the Yankee cylinder. At present, the quality of the 3D structure is not evaluated, or it is evaluated only subjectively at the mill. This is mostly because of the difficulties to separate reliably the regular 3D pattern from other variations. This paper introduces a frequency analysis based method which separates the surface profile variances in tissue paper to the creping, to the regular 3D pattern and to the residual variation. The 3D surface profiles and their variances were determined online with the photometric stereo method. We show that the introduced analysis method evaluates the variance portions reliably and the results are consistent with the visual perception of the 3D surfaces. In one particular product, the regular 3D pattern explains 74 % of total surface variance; the creping explains 10 % and residual variations 16 %. Furthermore, the creping and residual variances are quite stable over time whereas the variance of the regular 3D pattern fluctuates significantly.

The main objective of the current study was to demonstrate that it is possible to enhance strength properties of sheets from spruce HT-CTMP and CTMP furnishes up to the same level as is common on sheets from softwood kraft pulps by changing conditions in papermaking. To achieve that, sheets of spruce HT-CTMP and CTMP were consolidated at densities close to that of the reference bleach kraft pulp by pressing at press nip temperatures well above the tack and softening temperatures of lignin. On sheets from spruce CTMP (CSF 420 ml), where the fibers were surface treated with cationic starch, it was possible to reach tensile index at the same level as on sheets from the untreated reference kraft pulp. The compression strength (SCT) of CTMP and HT-CTMP sheets, which were achieved at the highest press nip temperature (200 °C) in the study, was equal to or higher than that of the reference kraft pulp sheets. The results show that there is a great yet unexploited potential in papermaking from spruce HT-CTMP and CTMP furnishes, which could be utilized in manufacturing of products where very high requirements upon strength is demanded.

Print mottle is a serious and yet common print defect in offset printing. An imbalance between the feed of fountain solution and the ability of the paper substrate to absorb and transport this water away from the surface can cause moisture/water interference problems. In the study presented here, we have investigated the uniformity of aqueous absorption and coating structure of pilot-coated papers with different types and dosages of dispersants and linked this to print mottle and uncovered areas (UCA). In earlier studies, the print quality of these papers indicated that a moderate addition of excess dispersant caused ink refusal, ink-lift-off (ink-surface adhesion failure) and water-interference mottle when printing at elevated fountain feed. In the present study, we have shown that a majority of the samples with uneven water/moisture absorption and an uneven burn-out reflectance tended to have more severe printing problems related to surface-moisture/water.An aqueous staining technique was used to characterise the absorption non-uniformities. This method has been developed previously with focus on absorption of flexographic water-based inks but can clearly give relevant information also for offset printing, when it comes to moisture/water interference mottle.