Volume 35, Issue 1

This paper is a review of the development of the mechanical pulping process with focus on refiner-based processes. The intention is to provide an overview of the trends and the major advances in the development of the mechanical pulping (MP) process. The focus is on the development of the entire MP process, rather than the refiner as such. However, when discussing the MP process development, it is inevitable to consider the development of the refiner unit operation briefly. Processes for printing papers based on softwood is mainly discussed, but board processes are discussed briefly as well.

This study aimed to investigate the oxalate formation mechanism during chlorine dioxide ( ClO 2 {\mathrm{ClO}_{2}}) bleaching of bamboo kraft pulp, and thus explore favourable ClO 2 {\mathrm{ClO}_{2}} bleaching conditions to better control oxalate formation. The amount of oxalate formed varied linearly with ClO 2 {\mathrm{ClO}_{2}} dosage within the whole research range, while it rose exponentially within the first 90 mins of pulp bleaching. Then the actual bleaching process was simulated by reacting ClO 2 {\mathrm{ClO}_{2}} with three representative lignin model compounds and comparatively studied. The rule of oxalate formation in the simulated reaction system was identical to that in pulp bleaching by ClO 2 {\mathrm{ClO}_{2}}, except for oxalate production by veratraldehyde with prolonged reaction time. Under identical conditions, vanillin formed the highest amount of oxalate, while veratraldehyde formed the least. Furthermore, the amount of oxalate formed increased by 19.59 mg/kg when the kappa number of the delignified pulp was reduced by one unit. Considering the satisfactory pulp brightness and decreased oxalate formation, the recommended conditions for ClO 2 {\mathrm{ClO}_{2}} bleaching of bamboo kraft pulp were a ClO 2 {\mathrm{ClO}_{2}} dosage of 4 %, 60 °C and 70 mins.

Groundwood pulping is a process that employs large machines, making them difficult to use in research. Lab scale grinders exist, but even though they are smaller, the sizes of the grinding stones or segments make them cumbersome to exchange and tailor. This study presents a method and an apparatus for investigating the detailed mechanisms and the energy requirements behind the fibre separation process. A well-defined grinding tool was used at three different temperatures to demonstrate that the equipment can differentiate levels of energy consumption and defibration rates, confirming the well-known fact that a higher temperature facilitates defibration. It is also shown how the equipment can be used to study the influence of grinding parameters, exemplified by the effect of temperature on the way fibres are separated and the character of the produced fibres. A key feature of the equipment is the use and evaluation of small grinding surfaces, more readily designed, produced, evaluated and studied. This reduces both the cost and time necessary for testing and evaluating. At the same time, a technique to produce well defined grinding surfaces was employed, which is necessary for repeatability and robust testing, not achievable with traditional grinding stones.

In this work, the American Old Corrugated Containers (OCC) pulp was screened using pressure screen to investigate the factors affecting the fractionation performance. Three screen cylinders with different geometries, aperture size/contour heights, volumetric reject ratios, and aperture velocities were used. The results showed that the passing ratio of fibres increased with the increase of contour height, volumetric reject ratio, and aperture velocity for all the three screen cylinders. Moreover, among the three cylinders investigated, the 0.81 mm hole cylinder achieved the highest fractionation efficiency with a volumetric reject ratio of 0.1 followed by 0.15 and 0.2 mm slot screens. Freeness change of the accept side of the screen was similar to passage ratio, which shows an increasing trend with the increasing aperture velocity and volumetric reject ratio. However, the reject freeness showed decreasing trends with the increasing aperture velocity V s {V_{s}} and volumetric reject ratio R v {R_{v}}.

This study aimed to fully utilize Ganoderma lucidum (called lingzhi in China, reishi in Japan) residues as raw materials and a sizing agent in the papermaking industry to alleviate their environmental pollution and the shortage of papermaking resources. The refined Ganoderma lucidum residues can be used to replace 30 % of the plant pulps. Moreover, the prepared papers had better physical properties than pure plant fiber papers. After polysaccharide extracting, the tear index and folding endurance increased by 26 % and 34 %, respectively. In addition, the Ganoderma lucidum residues-based resin as a sizing agent achieved a similar effect of alkyl ketene dimer (AKD). The Cobb index reduced from 84.1 g m − 2 84.1\hspace{0.1667em}\text{g}\hspace{0.1667em}{\text{m}^{-2}} to 72.5 g m − 2 72.5\hspace{0.1667em}\text{g}\hspace{0.1667em}{\text{m}^{-2}}.

The medium-consistency (MC) pump is the key unit in modern pulp and paper industry, especially in medium-consistency bleaching plant. Use of MC pump improves the efficiency of bleaching sequences and reduces the pollution load of bleaching effluents. In this work, the component of MC pump, e. g. turbulence generators with three typical structures were studied to evaluate the effects of turbulence generator structure on the performance of MC pump. An automatic control system was built to control operation condition and to collect data online. Head and efficiency were calculated to evaluate the differences among three turbulence generators. The results showed that the linear-type, twist-type and screw-type turbulence generators exhibited similar performance (pump head and efficiency) of MC pump at high rotation speed (2000 rpm to 2900 rpm) compared to the well-known performance of MC pump at rotation speed lower than 2000 rpm which are significant different for the three turbulence generators.

For the mechanical properties of paper, tensile testing has been widely used. Among the tensile properties, the tensile stiffness has been used to determine the softness of low-density paper. The lower tensile stiffness, the greater softness of paper. Because the elastic region may not be clearly defined in a load-elongation curve, it is suggested to use the tensile modulus which is defined as the slope between the two points in the curve. The two points which provide the best correlation with subjective softness evaluation should be selected. Low-density paper has a much lower tensile strength, but much larger elongation at the break. It undergoes a continuous structural change during mechanical testing. The degree of the structural change should depend on tensile conditions such as the sample size, the gauge length, and the rate of elongation. For low-density paper, the tensile modulus and the tensile strength should be independent of each other. The structure efficiency factor (SEF) is defined as a ratio of the tensile strength to the tensile modulus and it may be used a guideline in developing superior low-density paper products.

The concentration of fibers and fillers in the pulp suspension is an important parameter in the monitoring process. This paper proposes a versatile optical measurement system to estimate the concentration of a solids mixture in water. The geometry used in a multi-spectrophotometer (MSM) enables the controlled observation of transmission, and forward scattering light from the suspension in the UV-visible spectral range. We have developed the new fibers mixing system which gives a homogenous distribution of the fines and fillers making it possible to increase the reproducibility and accuracy of the measurement. The data analysis is based on the Beer-Lambert law and CIELAB color space equations. The results show that the proposed method is accurate for measuring the fines and filler concentrations in multicomponent suspensions.

Surface characterization is important and has many applications in the paper industry. Surface characterization requires both surface roughness and surface friction. The relationship between the two has not been fully established for paper and paperboard. It has been a common practice that only the average property and the standard deviation with the coefficient of variation (COV) are reported for surface roughness and friction measurements. This practice, however, provides few information on surface structure and can lead to wrong judgments because two samples having the same average and the COV can have different physical properties. To avoid such mistake, a new surface characterization method has been developed. To this end, surface roughness- and friction-profiles have been obtained using a latest version of Kawabata surface tester (Model: KES-SESRU, Kato Tech, Kyoto Japan). This new version uses the same stylus for both measuring surface roughness and friction under the same operating conditions. It was found that a correlation between the surface roughness and surface friction was very low. This indicates that they should be independent of each other. Therefore, both should be determined for surface characterization.

Increasing the filler content of sheet tends to decrease filler retention and paper strength properties. To overcome this problem and make better use of fillers, development of new methods on filler modification has never been stopped. In this study, filler modification was carried out by sequentially adding an anionic polyacrylamide, a cationic starch and a cationic polyacrylamide. It is believed that in this process, multiple polyelectrolyte complexes are formed which can not only encapsulate filler particles but also preflocculate the particles. The results showed that, compared to the single preflocculation treatment, the sequential encapsulation and preflocculation (SEP) treatment brought significantly larger particle size and higher surface charge potential of the filler, thus higher filler retention was achieved. When the modified fillers were used for papermaking and paper ash contents were controlled at the same level, the SEP modification was better in improving the tensile index, internal bond strength and tearing index of paper than the single preflocculation method, in addition, it maintained better paper formation, caused insignificant change on opacity of paper. It is believed that this newly developed SEP method is worthy of being applied to industrial scales in making various grades of filled paper.

The contribution of structural changes of softwood bleached kraft pulp (SBKP) fibers and partly dried SBKP handsheets to their water retention values (WRVs) was studied. Two factors related to mesopores and macropores, which were determined for super critical point (SCP)-dried SBKP sheet samples, should participate in the WRVs determined for wet SBKP sheet samples. The mesopores are formed during pulping/bleaching to remove lignin and a part of hemicelluloses that are present originally in wood cell walls. The combined volumes of mesopores and macropores (totally < ∼0.6 g g −1 ) in SBKP fibers after SCP drying were significantly lower than the entire WRV (∼1.6 g g −1 ) of the original never-dried SBKP. We hypothesize that external fibrils of SBKP fibers are formed during pulping, bleaching, washing, pressure-screening, and pressing processes at high and low solid contents in water under high shear forces. The WRV can be used as an indicator to evaluate such structural changes of external fibrils in pulp fibers and paper sheets during drying/wetting in papermaking process as well as in use under various conditions. In contrast, changes in mesopore and macropore volumes determined for SCP-dried sheets cannot properly detect such structural changes of external fibrils.

Durable papers should exhibit high mechanical strength and good soiling resistance. In this study, cellulose nanofibrils (CNF) was investigated as an additive to the polyvinyl alcohol (PVA) impregnation solution to improve the durability of paper. The impregnation suspensions were prepared by adding CNF to PVA solution in various ratios, and were used to impregnate a base paper. The PVA-CNF suspensions exhibited good dispersion stability; however, the low shear viscosity increased as its CNF content increased. The pickup weights of the papers impregnated with the PVA-CNF suspensions were found to be lower than those of the control sample, which were impregnated with the pure PVA solution only. Although the strength of the paper was not significantly improved by the addition of CNF, when the paper was impregnated with a PVA-CNF suspension containing 5 % CNF content, the product exhibited similar strength to the papers impregnated with only a 3 % PVA solution despite its lower pickup weight. The mechanical strength of the impregnated paper was affected by the pickup weight and penetration of the impregnating agent. The addition of a wet strengthening agent to the mixed suspensions was seen to enhance the wet soiling resistance of the papers.

This paper discusses about the scatter of the intensity of vibration signals of paper prints and analyses their mechanical parameters applying the theory of covariance functions. It is an important practical problem, before starting printing process of colour prints, expecting the correct position of fixed raster points, to adjust the paper sheet tension between printing machine sections. The results of measuring the intensity of vibration signals at the fixed points were presented on a time scale in the form of arrays (matrices). The estimates of cross-covariance functions between digital arrays result in measuring the intensity of vibrations, and the estimates of auto-covariance functions of single arrays were calculated upon changing the quantization interval on the time scale. Application of normed auto-covariance and cross-covariance functions enables reduction of preprinting experimental measurements, which saves time (what is actual for industry). Tension force depends on the mechanical properties of the paper sheet and print. These characteristics depend on paper type, layers of printing colors and positioning of the coverage. In the calculation, the software Matlab 7 in batch statement environment was applied.

In this study we have evaluated the suitability of laboratory testing methods to predict inkjet printing results. We have developed and used testing liquids that are spanning the operational window of industrial High Speed Inkjet (HSI) printers while still covering the maximum possible range of viscosity and surface tension. First we correlated liquid penetration measured with ultrasound (ULP) and direct absorption (ASA) to print through from HSI prints. The best correlation ( R 2 ≈ 0.7{R^{2}}\approx 0.7) was found for the sized paper. For papers with increasing liquid penetration speed we found a decreasing ability of both testing methods to predict print through, for the strong absorbing paper the correlation drops to R 2 ≈ 0.2{R^{2}}\approx 0.2. Second we correlated contact angle and drop diameter to the dot area from HSI prints. Contact angle turned out to be a better predictor for printed dot area than drop diameter. Evaluating the change in contact angle over time we found the highest correlation to the dot area in the print when measuring the contact angle as soon as possible, in our case 1 ms after deposition of the drop on the paper. We also compared contact angle with microliter drops to picoliter drops, which are in the size scale of the actual inkjet droplet. To our great surprise correlations for microliter drops were equal or better than for picoliter drops, particularly for highly absorbing papers. Thus in order to predict dot spreading on paper our results suggest to measure the contact angle with microliter drops. Overall we found that, using laboratory testing methods, print through and dot spreading for HSI printing can be quite well predicted for slow absorbing papers but not very well for fast absorbing papers.

This study is the first, to the best of our knowledge, where waste napkin paper was successfully valorized to low-density (27.2 mg cm −3 ) cellulose aerogels for oil sorption material. Two simple methods with different gel coagulators, ethanol and sulfuric acid, were used for preparation of the aerogel. Conditions for the alkaline treatment of the raw material and the pre-freezing temperature in the lyophilization process were optimized. It was found that the water and oil sorption capacities of the aerogels were not significantly affected by alkaline treatment, while they could be adjusted by changing the pre-freezing temperature. Although the produced aerogels were initially amphiphilic, hydrophobic surfaces were obtained by vapor deposition of methyltrimethoxysilane (MTMS) and these materials possessed high sorption capacities, up to 32.24 cm 3  g −1 (28.56 g g −1 ) for pump oil and 26.77 cm 3  g −1 (39.59 g g −1 ) for chloroform. This was comparable to aerogels prepared from fresh cellulosic materials via the sol-gel method, as their sorption capacities varied in the range of 14–45 g g −1 .

Coating wastewater from paper mills has been characterized by a high chemical oxygen demand (COD), high total suspended solids (TSS) and a high degree of color. In this paper, a sequencing batch biofilm reactor (SBBR) and an anaerobic-oxic (AO) system that consisted of a hydrolytic acidification reactor and an activated sludge reactor (HAASR) were implemented to treat coagulated coating wastewater from a paper mill. The COD, color and TSS removal efficiencies of the SBBR were 87.7 %, 33.5 %, and 41.4 %, and those of the HAASR were 85.3 %, 22.5 % and 19.4 %, respectively. It is proposed that more recalcitrant compounds were removed by the SBBR than by the HAASR. The microbial communities of the SBBR sludge and the HAASR sludge were evaluated and indicated that the SBBR sludge had lower microbial richness and diversity than the HAASR sludge. Moreover, Proteobacteria and Bacteroidetes were the dominant bacteria in both the HAASR and SBBR sludges.