Abstract
Using foam as a carrier fluid in papermaking gives interesting new opportunities. Foam as a more viscous fluid than water is expected to behave differently in a dynamic process. This study presents results obtained under dynamic forming conditions in a semi-pilot scale research environment. Effects of process configurations and running conditions on increased forming speed, web properties, and difference between water-laid and foam-laid processes are shown. The studies were carried out using a water-laid former and the same environment modified for foam forming. In order to achieve increased forming speed, the open headbox was replaced with a closed headbox, and the former geometry was updated. The process foam was boosted with an additional foam pulper. The foam pulper was used as a machine chest for improving the dispersion of fibers into the foam. A much broader tensile strength ratio range (~3 to 8) was achieved with foam forming than with water-laid forming. Foam-laid paper had a broader pore size distribution and higher mean pore size. Formation and the formation spectra of foam-laid sheets were more uniform, leading to improvements in the properties of the fiber network.
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Foam Forming under Dynamic Conditions
Jani Lehmonen,a,* Elias Retulainen,b Marko Kraft,c Jouni Paltakari,d and Karita Kinnunen-Raudaskoski e
Using foam as a carrier fluid in papermaking gives interesting new opportunities. Foam as a more viscous fluid than water is expected to behave differently in a dynamic process. This study presents results obtained under dynamic forming conditions in a semi-pilot scale research environment. Effects of process configurations and running conditions on increased forming speed, web properties, and difference between water-laid and foam-laid processes are shown. The studies were carried out using a water-laid former and the same environment modified for foam forming. In order to achieve increased forming speed, the open headbox was replaced with a closed headbox, and the former geometry was updated. The process foam was boosted with an additional foam pulper. The foam pulper was used as a machine chest for improving the dispersion of fibers into the foam. A much broader tensile strength ratio range (~3 to 8) was achieved with foam forming than with water-laid forming. Foam-laid paper had a broader pore size distribution and higher mean pore size. Formation and the formation spectra of foam-laid sheets were more uniform, leading to improvements in the properties of the fiber network.
Keywords: Aqueous foam; Foam process; Formation; Forming; Sheet properties
Contact information: a: VTT Technical Research Centre of Finland Ltd, P.O. Box 1603, FI-40101 Jyväskylä, Finland; b: VTT Technical Research Centre of Finland Ltd, P.O. Box 1603, FI-40101 Jyväskylä, Finland; c: VTT Technical Research Centre of Finland Ltd, P.O. Box 1603, FI-40101 Jyväskylä, Finland; d: Aalto University, Department of Bioproducts and Biosystems, FI-02150 Espoo, Finland; e: Paptic Ltd, Tekniikantie 2D, FI-02150, Espoo; *Corresponding author: jani.lehmonen@vtt.fi
INTRODUCTION
Foam, which has a viscosity much higher than water, enables new opportunities for the paper and board industry. In foam-laid forming technology in paper and board making, an aqueous foam of small spherical air bubbles is used as a process fluid and flowing medium instead of water (Smith and Punton 1975). The foam-laid process can be accomplished using different methods. In this study, a method combining tank generation of foam and a closed headbox was used. The technology is used especially in the non-woven industry (Hanson 1977), which uses long man-made fibers, mineral fibers, and natural fibers. Processing long fibers is challenging in the water process and, according to Kerekes et al. (1985), increasing fiber length and furnish consistency greatly increase the tendency to form flocs, resulting in non-uniformity of the web. The foam-based process enables the application of long fibers with appropriate formation (Radvan and Gatward 1972; Lehmonen et al. 2013; Kinnunen-Raudaskoski 2017).
Koponen et al. (2016) have reported good formation uniformity from trials with bleached hardwood and softwood kraft pulps and long fibers (6 mm TENCEL Lyocell fibers). The highest mass fraction of Lyocell fibers was 20%. According to that study, web formation with longer fibers improved with decreasing foam density. Kinnunen-Raudaskoski (2017) has verified the uniform sheet structure using foam forming trials with a 70/30% share of unrefined softwood pulp and 6 mm and 12 mm polypropene (PP) fibers on a pilot paper machine modified for foam technology using a foam density of 370 kg/m3. The formation was compared with water-formed samples of refined softwood and hardwood pulps, which were tested before the paper machine modification. The forming consistencies were 0.26% in water forming and 0.61% in foam forming.
For the paper and board industry, foam-laid forming technology enables new production-scale possibilities to achieve, for example, higher forming consistency without at the same time adversely affecting the formation uniformity (Kidner 1974; Smith and Punton 1975; Lehmonen et al. 2013; Kinnunen-Raudaskoski 2017). The foam-laid process leads to a more porous and bulky structure that influences the strength properties of the sheet (Smith et al. 1974; Punton 1975). However, based on published results, this strength loss can be compensated for by wet pressing of the web or by increasing the refining level of the furnish (Radvan and Gatward 1972; Lehmonen et. al 2019b). Strength properties of foam-formed papers can be modified using micro/nano fibrillated cellulose (Kinnunen et al. 2013; Kinnunen and Hjelt 2016) or polyvinyl alcohol (Kinnunen and Hjelt 2017) as strength additives. In general, the effects of foam forming on paper properties can be explained by the physical effects caused by foam bubbles and partly due to the chemical effects brought about by the applied surfactant (Lehmonen et al. 2019a).
Al-Qararah et al. (2015) have studied the link between fiber network structure and foam properties by comparing pore structure with measured bubble size distribution. Those authors have concluded that foam-formed sheets have larger pores than water-formed sheets, and the pore size distribution is more strongly affected by fiber type than by small changes in bubble size distribution.
The potentiality to expand the raw material base and range of product properties with foam technology enables possibilities to generate new paper and board products. This could lead to the renewal of the current paper and board industry through new and novel product applications.
The research reported here is a continuation of earlier semi-pilot and laboratory-scale studies (Lehmonen et al. 2013, 2019a), which showed certain differences in the viscosity and flow characteristics when foam is used instead of water as a carrier fluid for fibers. These differences are also expected to reflect in the dynamics of water removal, subsequent dry solids content, fiber orientation, structure, and mechanical properties of the web. These are important factors when aiming at upscaling the technology for increased forming speed, improved dewatering, better foam stability, and foam recovery in the forming section.
EXPERIMENTAL
Materials
Forming studies were performed using bleached chemical softwood pulp at two refining levels. The Shopper Riegler values were 18 and 23, the average fiber length of the pulps was 2.29 mm, and the fiber coarseness was 167 and 175 µg/m, respectively. The fines content (fraction <0.20 mm) was 6.7% for the lower refining level and 7.4% for the higher refining level. Tap water was used as the process water. The process foam was produced by tank generation, using sodium dodecyl sulfate (SDS) as a surfactant. The average process temperature was ~27 °C for the foam process. In the tank generation, the air led to the foam is at ambient temperature, and so it partly decreases the overall process temperature. When comparing the foam-laid process to normal water-laid forming process, the process temperature is higher (typically around 50 °C) in the water-laid process. Notice that, increased process temperature in the water-laid process decreases viscosity, leading, for example, to improved dewatering properties.
Methods
Process configurations
The main principle of the foam-laid forming process is that, instead of water, the raw materials are mixed with process foam in a pulper. In this study, two process configurations were compared for foam. The first process configuration (Fig. 1a.) was further modified in order to increase forming speed, improve dewatering, and to attain better foam stability and foam recovery in the forming section.
Fig. 1. Schematic diagrams of foam-laid process research environment: a) Open slice-jet forming geometry and foam circulation; b) closed slice-jet based forming geometry and foam circulation
The latter, modified configuration was also used when making water-laid reference points. A schematic of the research environments before and after the modification is presented in Fig. 1. In this study, the average process foam density in the approach system was ~330 kg/m3 (~33% water in foam), i.e. two thirds of the process foam was composed of air.
The process conditions and formed web properties can be affected by headbox and former parameters, and therefore the major change was related to the forming geometry. The open slice jet based forming geometry, used in our earlier studies, was modified to a closed jet, as shown in Fig. 2. A tapered slice channel forming geometry was used with a 2 mm slice height.
Fig. 2. Open slice-jet and closed slice-jet forming geometries; a) Open slice-jet forming geometry and b) closed slice-jet forming geometry.
By increasing the amount of suction vacuum boxes from six to seven, dewatering conditions were also improved in the forming section. After modification, the first three vacuum boxes were located under the headbox and the former lid, and the additional vacuum box was located just after the slice opening. All of the vacuum boxes could be individually controlled and adjusted without affecting the vacuum levels of the other vacuum boxes. This kind of adjustable vacuum system leads to better process control and foam recovery in the forming section. A removal of the remaining visible foam due to the additional vacuum box is seen as a sharp line on the moving web.
Fig. 3. Closed headbox and former
Fig. 4. Sharp foam line after the closed headbox
After the foam line, the amount of process foam in the web is very small and basically, the suction boxes suck air through the web. This air flow is not led to the foam generator and so it doesn’t cause any extra air to the process. An example of the sharp foam line is shown in Fig. 4. This headbox solution prevents extra air being led to the process and thus enables a more stable process. More detailed diagrams of the closed headbox and former are shown in Fig. 3.
The vacuum profile used is shown in Fig. 5. Most of the process foam was suctioned and recovered under the closed headbox and the slice opening (suction boxes 1 to 4). When the viscous foam had been removed from the formed fiber network, the vacuum levels were dropped to lower levels.
Fig. 5. Vacuum profiles in the forming section. Error bars show the standard deviation for 24 measurements.
The process foam was recirculated within the flow loop. Another process improvement was the installation of a foam pulper (Fig.1b), which was used as a machine chest. The foam pulper enhanced the mixing phase and conditions of the process foam and fibers. Foam produced in the foam generation tank and the dispersed web together with part of the foam in the pulper were pumped into this foam pulper (Fig. 1b).
A degassing centrifugal pump was used as a feed pump in the approach line. This pump type enables pumping higher amounts of dispersed air-containing foams and suspensions.
When producing process foam, the surface tension of the flowing medium must be modified utilizing different kinds of surface active agents that boost the dispersion of air into the flowing medium and process fluid (Nicolaysen and Borgin 1954; Riddel and Jenkins 1976).
In this study, anionic sodium dodecyl sulfate supplied by Sigma-Aldrich (Darmstadt, Germany) was used as a surface active agent. The dosing system for the surface active agent used for foam generation and control of the quality of the new and recovered process foam was the same as in earlier studies (Lehmonen et al. 2013). The air content of the foam correlates with conductivity measurements (Kruglyakov 1999; Weaire and Hutzler 1999), and this special feature could be utilized, for example, in planning the control system of the foam forming process at production scale. In the present study, the air content of the process foam was followed online based on the conductivity measurement characterized in the foam generator.
In the case of foam, the relevant definition for forming consistency is to calculate based on the dry weight of fibers per total weight of the sample, because the volume is dependent on the air content of the fiber foam suspension and it can be varied over a wide range. The forming consistency was ~0.80% by weight for water-laid paper and 1.0% by weight for foam-laid paper.
The target grammage in the trials was 80 g/m2, and the target foam density was 300 to 330 kg/m3 (measured at atmospheric pressure in the foam generator). Jet-to-wire ratio series were run from 0.6 to 2.5 in foam forming and, correspondingly, from 0.9 to 1.25 in water forming.
Water-formed paper was produced as the reference in the same research environment by changing the closed headbox to an open slice jet headbox and by changing the foam recovery system to a conventional wire pit system. The width of the web was 160 mm for foam-laid papers and 140 mm for water-laid papers.
Raw material characterization
The pulp properties were characterized using the fiber analyzer Fibre-Master (Lorentzen & Wettre, Stockholm). The Shopper Riegler value was determined according to ISO standard 5267-1 (1998).
Paper sampling
The formed paper web was sampled after the forming section without wet pressing, using a sampling apparatus or through simultaneous, gentle wet pressing using a roll nip. Afterwards, unpressed paper samples were wet pressed at wet pressure levels of 50 kPa, 150 kPa, and 350 kPa according to ISO standard 5269:1 (2005). The unpressed and gentle pressed samples were dried with a cylinder drying device (Kodak rotary drum dryer), and the wet pressed paper samples were dried between a plate and a cloth in a standard air-conditioned room at RH 50% and 23 °C.
Paper testing
All paper samples were stored and analyzed in a standard air-conditioned room at RH 50% and 23 °C. The grammage of the paper samples was determined according to ISO standard 536 (2012), while the thickness of the samples was determined according to ISO standard 534 (2005), and density and bulk were based on the measured values of the grammage and sheet thickness.
The tensile strength properties of 10 parallel paper samples were measured using a Lloyd tensile tester in accordance with ISO standard 5270 (2012) in both the MD and CD direction. In order to equalize the effect of fiber orientation, geometrical average values were calculated (Htun and Fellers 1982). The z-directional tensile strength was measured according to ISO standard 15754 (2009). The scattering coefficient of sheets was measured with a Minolta Spectrophotometer CM-3610d according to ISO standard 9416 (2009).
Formation and fiber orientation
Based on the β-formation measurement with Carbon-14 as the radiation force, a storage phosphor screen (SPS) was exposed to β-radiation through the paper sample. This was done in order to obtain the radiation absorption map. Thereafter, the screen was scanned with a Fuji BAS-1800 II SPS reader. The measured values were converted into a grammage map. The size of the scanned area was 100 mm x 100 mm, and the scanning resolution was 100 µm. Following this, the resolution was transformed to the Ambertech resolution (Ø 1 mm) and the specific formation value was calculated (Lehmonen et al. 2013).
Tensile strength ratio (MD/CD) was used for estimating the fiber orientation anisotropy of the restrained dried sample sheets.
Bubble size and pore size distribution
The average bubble size of the process foam in different locations was characterized using a method developed by Lappalainen and Lehmonen (2012). The Sauter mean radius r[3,2] was used for characterizing the bubble size of the process foam.
Based on the article (Lappalainen and Lehmonen 2012) bubble size distributions (BSDs) were characterized using the bubble mean radius r[1,0] and the area-weighted mean radius, i.e. the Sauter mean radius, r[3,2] discovered by the German scientist Sauter (1926):
, (1)
where ri is the radius of the bubble i (µm) and n is the total number of bubbles. This is a known method, and several authors have used Sauter mean radius or diameter when describing BSD distribution (Engelsen et al. 2002; Phongikaroon et al. 2006; Junker 2006; Moruzzi and Reali 2010).
The Sauter mean bubble size was measured using the samples from the process. The pure process foam containing samples were taken from the foam generator and headbox.
Pore size distribution
The pore size distribution of the papers was defined with mercury porosimetry measurement using the method by Koivula et al. (2011).
RESULTS AND DISCUSSION
Effect of Process Configuration on the Foam-Laid Process
Forming speed and foam recovery
The changes in the process configuration made it possible to increase the running speed of the process. The targeted speed enhancement of the process modification was achieved. In the present study, the average forming speed was increased from about 100 m/min up to 300 m/min with the improved foam-laid process. The installation of a higher capacity headbox feed pump and an increase in the foam removal capacity of the forming section were applied when targeting higher forming speed and foam recovery. In an earlier study (Lehmonen et al. 2013), the maximum forming speed was 125 m/min, being restricted by stable dewatering and stability of the foam, and formation of the web.
The updated forming geometry enhanced the control of foam recovery. The stability of the process foam was better, as additional air leakage to the vacuum system was prevented. Air leakage decreases the capability of the vacuum system to handle the foam flow and also forms frothy foam. In addition, the closed headbox forming geometry resulted in improvements in dewatering conditions together with an updated vacuum system.
Effect of Forming Methods on the Web
Formation
Based on the research of Kerekes et al. (1985) and Martinez et al. (2001), the tendency of the fiber slurry to form flocs and cause a non-uniform web structure is noticeably increased with increasing furnish consistency and fiber length. In this study, the influence of refining level was studied for water forming and foam forming. Specific beta-formation and formation spectra were used in evaluating small-scale basis weight variation and how the variation was distributed along different wavelengths. Results for water-laid and foam-laid papers are shown in Fig. 6 and 7.
Fig. 6. Specific β-formation as a function of Shopper Riegler value when closed slice jet forming geometry was used for foam-laid papers and open slice jet forming geometry for water-laid papers
In a water-laid flowing medium, the rheological behavior of fiber suspensions results in fibers flocs, leading to a non-uniform fiber network (Kerekes 2006). This kind of fiber behavior in a water-laid flowing medium induces a deterioration in formation. The formation of foam-laid paper was distinctly better than that of water-laid papers, which is in line with the earlier study (Lehmonen et al. 2013). Generally, refining improves formation in water-laid forming, which can be seen as a slightly better formation result (Fig. 6), but did not have any effect on foam-laid forming in this study. This means that in foam-laid papers the refining level could be decreased without weakening the formation.
Fig. 7. Effect of forming method and refining degree on the formation spectrum (variance) of basis weight at different wavelengths when closed slice jet forming geometry for used for foam-laid papers and open slice jet forming geometry for water-laid papers
In addition to improved formation, the formation spectra of foam-laid sheets were better in all wavelengths, and especially large improvements were obtained in large-scale variance for both refining levels (Fig. 7). Furthermore, when comparing the formation spectra between the open and closed foam forming geometries, the formation spectra (Fig. 8) for the closed geometry were even better in all wavelengths. The enhancements in process scale, especially mixing conditions and forming geometry, led to improved formation. According to the earlier findings for formation, the variance behavior of the foam-laid sheets was smaller compared to water-laid sheets as a function of wavelength because foam bubbles between the fibers effectively prevent the formation of fiber flocs in a fiber network (Lehmonen et al. 2013; Lehmonen et al. 2019a). According to Kerekes (2006), fibers have a tendency to form fiber flocs in water-laid flowing medium based fiber networks. The benefits of foam bubbles can be seen across the whole spectrum area, but especially in the wavelength area of 4 to 16 mm.
Density
The density of the formed sheets as a function of wet pressing level is presented in Fig. 9. Density is an essential structural property and one of the most important main properties of paper. Density correlates well with the number of contacts between fibers; hence density is strongly related to the porosity and strength of paper. As seen from Fig. 9, compared to water-formed paper at the same grammage level and wet pressing pressure, the foam-formed sheets had consistently higher density than water formed-paper. This was probably due to the more even formation of the foam-formed paper.
Fig. 8. Formation spectrum (variance) of the papers produced by water-laid and open foam-laid forming geometries (a). Formation spectrum with the small-scale variance of the papers produced using open and closed slice jet based forming geometries (b)
Fig. 9. Density of the dry sheets as a function of wet pressing pressure. Error bars show the standard deviation.
Effect of jet-to-wire ratio on fiber orientation and formation
Tensile strength ratio was used to estimate the fiber orientation and β-formation to estimate the structural small-scale homogeneity of the web. The tensile strength ratio and specific β-formation as a function of jet-to-wire ratio are presented in Fig. 10. When varying the jet-to-wire ratio, the average flow rate into the headbox was kept constant at 7.2 l/s ± 0.1 l/s, and the forming speed was varied from 76 to 313 m/min depending on the drag or rush conditions. The resulting papers had, on average, basis weight 80.2 g/m2 ± 4.2 g/m2, thickness 159.6 µm ± 10.1 µm, density 503.1 kg/m3 ± 11.3 kg/m3, and bulk 2.0 cm3/g ± 0.1 cm3/g. The lowest tensile strength ratio was achieved at a jet-to-wire speed difference of close to zero. A broad tensile strength ratio range from ~3 to ~8 was achieved for the closed headbox.