making! PAPER
The e-magazine for the Fibrous Forest Products Sector
Produced by: The Paper Industry Technical Association
Volume 2 / Number 2 / 2016
PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 2, 2016
CONTENTS:
FEATURE ARTICLES: 1. Papermaking : Detailed review of initial wet web strength of paper 2. Towel LCA : Outline of LCA assessment of towel manufacturing in the US 3. Laser Processing : Pulsed laser processing of paper materials for personalisation 4. Waste Treatment : Biological treatment of paper board effluent in India 5. Wood Panel : Measurement of Dynamic Viscoeleasticity of Composite Wood Panel 6. Heat Exchanger : White paper on heat exchangers by Spirax Sarco
7. Solenis : Overview of company culture and objectives 8. Feedback : Top tips on how to improve giving feedback 9. Leadership : coaching and mentoring skills 10. Motivation : Ways to boost employee motivation
SUPPLIERS NEWS SECTION: Products & Services : Extended information on the latest products and services from:
ABB
ExxonMobil
Michael Smith Engineering
Airdale Chemical
Fortress Interlocks GoPlasticPallets
Midland Pallet Trucks
CEA
Pilz
Dichtomatik Dustcontrol
Hyster
Spray Nozzle People
Jarshire ( PITA Corporate Member )
DATA COMPILATION: Installations : Overview of equipment orders and installations since March 2016 Research Articles : Recent peer-reviewed articles from the technical paper press Technical Abstracts : Recent peer-reviewed articles from the general scientific press Events : information on forthcoming national and international events
The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology and the PITA Annual Review , both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating .
Page 1 of 1
Contents
PAPERmaking! FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 2, Number 2, 2016
Initial wet web strength of paper
Jürgen Belle & Jürgen Odermatt Despite much research into and development within the complex area of the initial wet web strength of paper, no complete model has yet been developed to describe this property. This type of paper strength is the most important property to ensure an effective paper machine run. Furthermore, the process of strength development in the sheet forming and pressing portion of the process is the basis of the final paper strength. Recent investigations have focused on the surface interactions of fibres on the molecular level. There have also been several innovative findings about fibre swelling and the impact of swelling on the mechanical characteristics of fibres in wet conditions. Overall, progress obtained using new methodologies enables a deeper understanding of the mechanism of strength development. This review discusses these important areas by examining the literature and the authors’ own work to obtain a better understanding of the strength development in wet paper webs. This paper highlights that the fibre morphology and the dryness of the wet web have the greatest impact on the strength of the wet web. As fibre sources and machinery are fixed, the fibre water gel is one of the easiest factors to adjust via the process water quality and the use of chemical additives.
This article is published with open access at Springerlink.com
The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology and the PITA Annual Review , both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating .
Page 1 of 25
Article 1 – Papermaking & Wet Web Strength
Cellulose (2016) 23:2249–2272 DOI 10.1007/s10570-016-0961-7
REVIEW PAPER
Initial wet web strength of paper
Ju¨rgen Belle . Ju¨rgen Odermatt
Received: 13 August 2015 / Accepted: 10 May 2016 / Published online: 17 May 2016 The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Despite much research into and develop- ment within the complex area of the initial wet web strength of paper, no complete model has yet been developed to describe this property. This type of paper strength is the most important property to ensure an effective paper machine run. Furthermore, the process of strength development in the sheet forming and pressing portion of the process is the basis of the final paper strength. Recent investigations have focused on the surface interactions of fibers on the molecular level. There have also been several innovative findings about fiber swelling and the impact of swelling on the mechanical characteristics of fibers in wet conditions. Overall, progress obtained using new methodologies enables a deeper understanding of the mechanism of strength development. This review discusses these important areas by examining the literature and the authors’ own work to obtain a better understanding of the strength development in wet paper webs. This paper highlights that the fiber morphology and the dryness of the wet web have the greatest impact on the strength of the wet web. As fiber sources and J. Belle ( & ) Department of Paper Technology, Munich University of Applied Sciences, 80335 Munich, Germany e-mail: belle@hm.edu J. Odermatt Fakulta¨t MIN Fachbereich Biologie Zentrum Holzwirtschaft, University Hamburg, Hamburg, Germany
machinery are fixed, the fiber water gel is one of the easiest factors to adjust via the process water quality and the use of chemical additives. Capillary force Dryness Fiber water gel Frictional connection Measurement technique Van der Waals force Keywords
Introduction
There is a clear trend in the paper industry towards lower grammages and higher machine speeds to increase productivity while conserving resources and energy. Especially in packaging papers, there is a trend to decrease base weight from 110 to 130 g/m 2 down to 70 to 90 g/m 2 with machine speed up to 1900 m/min (Guldenberg and Schwarz 2004; Mu¨ller 2010). Most papers are manufactured on machinery with an open draw downstream of the forming and/or pressing section (Berger and Schramm 2011; Erhard and Kretschmer 2007; Schwarz and Bechtel 2003). Thus, the initial wet web strength (IWWS) is the utmost important factor for setting the paper in the first open draw, where the paper web is transferred without external support in the paper machine. As a result, the IWWS is the central parameter that controls the number of breaks at this point of the production process (Clark 1978c; Edvardsson and Uesaka 2009; Guldenberg and Schwarz 2004; Lindqvist et al. 2012;
123
2250
Cellulose (2016) 23:2249–2272
Calculation of IWWS
Miettinen et al. 2009; Salminen 2010), thus determin- ing productivity. Against this background, it is critical for both scientists and practitioners to understand the effects of forces and conditions that can be influenced, leading to increased IWWS. This understanding is a prerequisite to be able to draw the correct conclusions regarding the development of new processes and additives.
Page developed a specific function for IWWS based on his equation for the tensile strength of a dry sheet by including the effects of fiber length and coarseness (Page 1969, 1993). Shallhorn enhanced this equation by considering the effect of pressing load (Shallhorn 2002). This incorporates the fiber length and coarse- ness as fiber morphology parameters, as well as the surface tension of the solvent water. Shallhorn showed that this function is limited to the large-fiber fraction of softwood kraft pulps. When using this Eq. 1, it is important to bear in mind that fines and short fibers are not considered. But those fiber fractions have a great impact on the IWWS and are widely used in papermaking. In addition, surfactants and web temperature clearly have a significant impact on the surface tension of the water and thus on the IWWS of the paper web. Calculation of IWWS (Page 1993; Shallhorn 2002). T IWWS ¼ 1 Þ T IWWS : initial wet web tensile strength; g : friction coefficient between two wet fibers; c : surface tension of water; L: fiber length; W: fiber width at moisture contents between 20 and 60 %; RBA (dry) : Relatively Bonded Area (proportion of the fiber surface contained within a water meniscus); C: fiber coarseness; t: minor axis of the elliptical cross-section of the fiber. 0 : 37 pgc LW RBA dry Ct ð Þ ð
Definition
The IWWS identifies the tensile energy absorption of a wet paper web during the production process. Gener- ally, the designation ‘‘initial wet’’ spans a dryness level from approximately 10 % during web formation up to approx. 60 % in the first dryer. Until approxi- mately 1960, research papers noted dryness levels of approximately 10 %. These values were measured downstream in the forming section or by laboratory web forming equipment. Today, depending on the construction of the forming section and the fibrous material, dryness levels of 18 % up to a maximum of 25 % are achieved. In this context, it is important to consider the different interactions among interfaces during sheet formation (Fig. 1). Solids are dispersed in water at the headbox and on the wire in stage 1. At the end of the wire and in the pressing section, the water contains both, solids and air, with the air presenting another surface interaction to be considered in stage 2. Stage 3 starts in the press section. From here on, solids and water are in air. Due to these different interfaces and the interactions among solids, water and air, the relationships among these components have a critical impact on paper strength. In accordance with ISO 3781, the wet strength of a specific paper type indicates the strength of a manu- factured paper after remoistening, not to be confused with IWWS. In addition to the IWWS, the dry strength of paper has been extensively studied. As a rule, the term ‘‘dry strength’’ indicates the strength of paper after manufacturing with a dryness level of [ 85%. The wet and dry strength of paper follow different rules and principles compared to the IWWS. This literature review considers the IWWS of paper at dryness levels from approx. 10 to 60 %. This review explains the different behaviors of fibers and additives on paper strength properties, with a focus on the IWWS.
Explanatory levels
To explain the IWWS, results from the reviewed literature is divided by scale and considered system- atically at the molecular level (nanometer range), the fiber morphology (micrometer range) and the paper level (macro range). Figure 2 emphasizes the fact that these levels of explanation overlap. This figure shows that although the individual levels are studied sepa- rately, it is important to consider that they occur concomitantly, interacting with and influencing each other. These interactions are addressed in papers referenced in the individual chapters and in the section ‘‘Integral explanatory models’’. As described in the previous paragraph, the strength properties of paper strongly depend on the dryness
123
Cellulose (2016) 23:2249–2272
2251
Fig. 1 Factors influencing the breaking load of paper according to Brecht and Erfurt (1959a, b)
within a sheet of paper at different dryness levels, such as flexible and smooth fibers that enable contact points between fibers (Brecht and Erfurt 1959a, b; Lyne and Gallay 1954a, b). After many years, various authors have begun working on these correlations again, addressing the factors affecting these forces such as capillary forces, van der Waals forces, and entangle- ment that leads to frictional connections (Hubbe 2006; Tejado and van de Ven 2010; Wa˚gberg 2010; Wa˚gberg and Annergren 1997). It is well known that the dependence of strength on the dryness level is not linear, but over a certain range exponential. In addition, different forces of different magnitudes such as capillary and frictional forces do overlap at specific dryness levels, resulting in a force responsible for the paper strength in the first open draw. Given this background, it is necessary to evaluate the measured strength values in relation to their
Fig. 2 Explanatory levels
level, and the IWWS is not an exception. Even in early papers from the last century, several authors noted that certain conditions are responsible for the strength
123
2252
Cellulose (2016) 23:2249–2272
D G elast . elastic free energy of the gel; D G mix free energy of mixing of the gel components and the swelling medium; D G electr. electrostatic free energy In equilibrium the total free energy is 0 and so the following equation is valid: ð 2 Þ D G elast counteracts the osmotic forces described by D G mix and D G electr. . In the case of papermaking fibers it is determined by the constituents of the fiber wall and the arrangement of the different fiber wall layers. D G mix is defined by the molecules in the network of the gel, molecular cellulose fibrils, and the solvent water. These mobile molecules form a mix with the polymer and the solvent. D G electr. is specified by the charges within the gel that gives rise to the osmotic pressure. The status D G = 0 might be valid before sheet forming and is therefore strongly depending of process water quality and used additives. Dewatering and the corresponding approach of the fibers and fibrils is responsible for interfering the fiber water gel and for the intermediate fiber–fiber bonds (Kibble- white 1973; Wa˚gberg and Annergren 1997). Total free energy in a gel: D G D G ¼ G elast þ G mix þ G electr ¼ 0
respective dryness levels. The web dryness is influ- enced by the type of raw material, its composition, and additive usage during sheet formation, even though all mechanical settings for sheet formation and press operation are kept constant. For this reason, the evaluation of IWWS should be done at constant sheet dryness. The effect of changing sheet dryness should be evaluated separately.
Nanometer level (molecular scale)
The nanometer level describes the bonding forces such as electrostatic forces e.g. van der Waals forces. At this level, the approximation of the contacts and bonding forces between fibers and fibrils is the decisive factor. The following paragraphs present the forces and conditions affecting this bonding type according to their mechanistic priority. If the distances between the solid particles are sufficiently small, electrostatic and van der Waals forces can develop (Israelachvili 2006b; Pelton 1993; Wa˚gberg et al. 1987). Second, the theory of molecular fibrillation and partial solubility has been described (Campbell 1930, 1933; Casey 1960; Clark 1978a). To achieve the most accurate possible fiber and fibril approach, a high degree of fiber flexibility is required, which is significantly influenced by internal hydrogen bonds (Hubbe 2006; McKenzie 1984). External hydrogen bonds between fibers will only form during drying (Forgacs et al. 1957; Lobben 1976; Robertson 1959; Williams 1983). For this reason, external hydrogen bonds are not studied in detail in this paper.
Van der Waals forces
If fibers are in sufficient close proximity, van der Waals forces will occur between fibers and fibrils (Eriksson 2006; Hubbe 2006; McKenzie 1984; Pelton 2004; Wa˚gberg and Annergren 1997; Williams 1983) . Figure 3 describes different forces on the surface of cellulose I, II and amorphous cellulose that may act also between the fiber and fibril surfaces. However, this may not be applicable for initially wet paper due to the high water content and the greater distances between the single fibers (Linhart 2005). For interactions to occur, the distances between the fibers and/or fibrils must be very small. The distances described in the literature are between 0.15 and 0.35 nm (Gardner et al. 2008; Linhart 2005). Remark- ably, these distances are considerably smaller than the fiber roughness, which ranges between 10 and 10,000 nm (Heinemann et al. 2011). Figure 4 shows an example of an uneven fiber surface of a common never dried softwood fiber.
Fiber water gel on the fiber surface
In 1963, Voyutskii proposed the formation of a hydro- gel on macromolecules in ‘‘Autohesion and Adhesion of High Polymers’’ (Voyutskij 1963b). In additional papers, the bonds in wet sheets were explained based on a gel-like surface of the fiber (de Oliveira et al. 2008; Lindqvist et al. 2013; McKenzie 1984; Myllytie 2009). In principle, the swelling of a gel can be determined by the energies summarized in Eq. 2 (Flory 1953; Katchalsky 1954; Yin et al. 1992):
123
Cellulose (2016) 23:2249–2272
2253
Fig. 3 ‘‘Comparison of the interaction force-distance curves between a cellulose sphere and the cellulose I, II and amorphous cellulose surfaces at pH 3.5 and in 0.1 mM aqueous NaCl solution. The interaction with the cellulose I sample ( red ) is characterized by an electrostatic repulsion, while van der Waals forces and steric interactions predominate with the cellulose II sample ( black ) and amorphous cellulose sample ( grey ), respectively’’ [Reprinted with permission of Eriksson (2006)]
Fig. 5 Fiber–fiber interactions of refined NBSK (SR 30) fibers in a paper sheet at 45 % dryness
fibrils bridge the gap due to interactions between fibers and fibrils.
Electrostatic forces
A share of the strength developed at low dryness levels may be attributed to acid/base interactions (Gardner et al. 2008; Lindstro¨m 1980; Wa˚gberg and Annergren 1997; Williams 1983). As a general rule, carboxyl groups and/or sulfonic acid groups are fixed at lignocellulosic fibers (Sjo¨stro¨m 1989). Therefore, the influence of metallic ions in aqueous solutions must be taken into account. The micro and macro fibrils of the fibers react similarly to polyelectrolytes (Caseri 2009). In water with low conductivity, dispersed fines and fibrils stretch into the surrounding water. High conductivity levels force the fibrils to retract from the surrounding suspension onto the attachment points on the fibers. Both parameters, pH and salt content, have a direct influence on the electrostatic potential and swelling behavior of the fibers. Because of that, these parameters have an indirect effect on the strength formation in both, wet and dry paper (Grignon and Scallan 1980; Nelson and Kalkipsakis 1964a, b; Scallan 1983; Scallan and Grignon 1979). These phenomena can be explained with DLVO theory, named after Derjaguin, Landau, Vervey and Overbeek (Derjaguin 1954; Derjaguin and Landau 1941; Israe- lachvili 2006b; Pelton 1993; Vervey and Overbeek
Fig. 4 Uneven fiber surface of a never dried unrefined Nordic bleached softwood kraft (NBSK) fiber in a paper sheet at 20 % dryness
However, if instead of considering the fiber as a smooth, well-formed unit, its actual shape is consid- ered with numerous micro and macro fibrils, espe- cially if it is refined, the formation of van der Waals forces is quite conceivable. In the middle of the image in Fig. 5, two beaten fibers arranged in parallel are bound via fibril bundles from the S1 layer. These
123
2254
Cellulose (2016) 23:2249–2272
1948). The type and amount of salt added to the solvent water has a negative impact on IWWS, similar to the findings of Grignon for dry paper (Belle et al. 2014a; Grignon and Scallan 1980).
‘‘dangling tails’’ on the fiber surface in Fig. 6 that have a length about 60–80 nm (Neuman 1993).
Internal hydrogen bonds
Internal hydrogen bonds (H-bonds) play a key role with regard to the intermolecular forces within the cellulose. On the one hand the intermolecular H-bonds bind the cellulose chains together and contributes to the lateral strength of the fiber, on the other hand the intramolec- ular H-bonds contributes to the axial stiffness of the cellulose molecules. These bonds are weakened by the adsorption of water and results in swelling of the fibers (Linhart 2005). The correlation between the Young’s modulus of the fibers and their internal hydrogen bonds is described in detail in the literature (Nissan and Batten 1990; Zauscher et al. 1996, 1997). However, these papers mainly focus on correlations to dry paper. Furthermore, the strong influence of water on the fiber– fiber bonds in paper (Hubbe 2006; McKenzie 1984) has led to a thermodynamic examination of fiber–fiber bond formation (Wa˚gberg 2010).
Fiber surface
The hypothesis of ‘‘dissolved fiber surfaces’’ was developed in the middle of the twentieth century. This hypothesis assumes that the surfaces of the cellulose fibers partially solute in water and diffuse into each other during sheet formation (Campbell 1930; Casey 1960; Clark 1978a). This approach was later expanded with the explanation that the reduced end groups of the cellulose form a kind of molecular fibrillations that are solvated or partially soluted in water. As a result, the molecular fibrillation rise up, leading to improved availability for bond formation (Clark 1978a). The assumption is that the wetted fiber is surrounded by water and the fibers and fibrils approaching each other during dewatering to such an extent that at first van der Walls bonds and with further drying hydrogen bonds can form. In this regard, Clarks’ theory emphasizes the high bonding capacity of hemicelluloses. For materi- als with a high percentage of hemicelluloses, there is a relatively high proportion of short molecules, which are more active in bond formation than large molecules when present in an easily accessible, upright state (Clark 1978b). McKenzie developed an additional model based on the adhesion between two plasticized surfaces in respect to Voyutskij’s theory about autohesion and adhesion for high polymers (McKenzie 1984; Voyutskij 1963a). It is assumed that in the intermediate area of two fibers in a plastic state, the micro and macro fibrils are close enough to form molecular alignments (Pelton et al. 2000). Neumans surface force measurements are consistent to these hypothesis and lead to the schematic representation of
Measurement techniques
Various technologies have been used to characterize surfaces and measure surface forces at the nanometer level, such as atomic force microscopy (AFM) (Gustafsson et al. 2003; Huang et al. 2009; Koljonen et al. 2003; Leporatti et al. 2005; Paananen 2007; Stenius and Koljonen 2008) and scanning electron microscopy (SEM) (Belle et al. 2015a, 2016; Heine- mann et al. 2011; Pye et al. 1965; Tejado and van de Ven 2010; Washburn and Buchanan 1964). These results must be evaluated taking into account the fact that the AFM is in contact with the scanned surface. As a result, AFM can disturb the sensitive fiber surface. In contrast, the SEM has limited resolution when imag- ing wet samples due to the vacuum required for
Fig. 6 Outline of Neuman’s dangling tail model (Neuman 1993); (Reprinted with permission of The Pulp and Paper Fundamental Research Society)
123
Cellulose (2016) 23:2249–2272
2255
operation. Sample preparation by freeze-drying and freeze fracturing can remedy this problem for certain resolutions (Belle et al. 2015a, 2016; Pye et al. 1965; Washburn and Buchanan 1964). Thomson used the fluorescence resonance energy transfer method to examine cellulose surfaces (Thomson 2007), which in future may be an additional option for the determina- tion of phenomena occurring on the fiber surface. These methods enable the visualization of even the smallest changes on fiber surfaces. Whether and to what extent the nanometer scale fiber surfaces have a direct and verifiable influence on the IWWS will only become evident when the two other size ranges are considered, the micrometer and macro scale because of the strong interactions among all three levels.
h: height of a liquid column; c : surface tension liquid- fiber surface; h : contact angle of water with fiber surface; q : density of liquid; g: gravity; r: radius of capillary The formula shows that as ‘‘r’’ decreases, the capillarity increases. In practice, this relationship can be simplified as displayed in Eq. 4, highlighting the fact that capillary forces are mainly controlled by the thickness of the water film (Lyne and Gallay 1954a, b). Simplified calculation of capillary forces F C 4 Þ F C : capillary force; D: water film thickness between two fibers 1 D ð The applicability of this theory can be observed during sheet forming. As during the dewatering process the gross of the sheet volume is removed in terms of water, the distances between the fibers decrease, resulting in lower water film thickness, and thus in increasing capillary forces. The resulting capillary force increases. The idealistic model rep- resentation of fibers as two cylinders that approach each other during the dewatering process states that the greater the amount of water removed, the closer the fibers come to each other, increasing capillary forces and holding the fibers together (Wa˚gberg and Annergren 1997). Lyne and Gallay showed this in trials with glass fibers (Lyne and Gallay 1954b). However, this model assumes rigid, smooth bodies, and therefore is only a rough approximation of the true phenomenon (Wa˚gberg 2010). This is because fibers have a certain morphology, are flexible, present in various deformed or swollen states, and are very coarse, especially in wet conditions (Belle et al. 2015a; Feiler et al. 2007; Heinemann et al. 2011). Calculations based on the capillary theory showed lower values than one order of magnitude compared to measured values (Miettinen et al. 2007; Tejado and van de Ven 2010). This shows that besides the capillary force other forces interact and contribute to the IWWS. It is proposed that especially the con- formability of the fibers in the network leads to frictional connection that contributes significantly to the IWWS.
Micrometer level (fiber morphology)
At the micrometer level, processes between fibers, fillers and additives are studied more closely. First, the interaction between fibers and water is explained by capillary forces and the processes of swelling, gel formation in the proximity of fibers, and hornification. Subsequently, the influence of the fiber characteristics on the IWWS is discussed, including the surface roughness and the complex fiber morphology com- prising fiber fractures, fibrils and fines particles. This discussion includes both, the beating and blending of fibrous materials and the measurement techniques used to assess the fiber characteristics.
Capillary forces
Besides drainage pressure and suction in the wire section the capillary forces are acting for the fiber and fibril approach, and as a result are a major factor affecting the IWWS in the early stages of paper dewatering (Campbell 1933; Israelachvili 2006a; Kendall 2001b; Page 1993; Persson et al. 2013; Rance 1980; Schubert 1982; van de Ven 2008; Williams 1983). The capillarity describes the properties of liquids in narrow spaces. Equation 3 shows the formula for the capillarity: Capillarity
2 c cos h q gr
h ¼
ð
3 Þ
123
2256
Cellulose (2016) 23:2249–2272
Fiber swelling
Reeves 1991; Weise 1998; Young 1986). The process of hornification has been divided by Weise (1998) into two different phenomena, called wet and dry hornifi- cation. The wet hornification process is defined in a range of 40–70 % dryness and describes the removal of free water from fiber lumen and fiber surface without or with pressing of the wet web. This causes cell wall collapse and pore closure. Thanks to morphological restrains of the fiber cell wall this process stops at a certain point. This kind of hornifi- cation is mostly reversible if the fibers are rewetted. Further drying of the wet web results in dry hornifi- cation (Bawden and Kibblewhite 1997; De Ruvo and Htun 1981; Laivins and Scallan 1993; Weise 1998), the type of hornification this paper refers to. Depend- ing on the drying temperature this hornification is only partly reversible by mechanical energy or chemical use. The reaction of hydroxyl and carboxyl groups to H-bonds and/or lactone bridge formation in closed pores and collapsed fibers is still under discussion (Fernandes Diniz et al. 2004; Lindstro¨m 1986; Lind- stro¨m and Carlsson 1982; Wang 2006). The degree of hornification can be measured as the reduction in the water retention value (WRV) (Jayme 1944, 1958). Jayme suggested to calculate the degree of hornification in Eq. 6: Degree of hornification according to Jayme (1944) Hornification ¼ ð 6 Þ WRV 0 : initial WRV; WRV 1 : WRV of hornificated sample WRV 0 WRV 1 WRV 0 100 % ½ Hornification as a result of drying leads to better dewatering, decreased fiber flexibility and stiffer fibers as well as to a decrease in the dry strength of paper (Lindstro¨m and Carlsson 1982; Ro¨der and Sixta 2004; Weise 1998). Experiments comparing never-dried with dried unbleached, unbeaten spruce sulfite pulp show that hornification leads to a decrease in strength at any dryness level (Brecht and Erfurt 1959b). Own studies with commercial bleached softwood sulfite pulp demonstrate that hornification has a significant nega- tive impact on the strength values at dryness levels greater than 25 % (Belle et al. 2014a). Below 25 % dryness there is a slightly positive effect. The differ- ences in the results can be explained by the higher accuracy of the DIN 54514 method (see also ‘‘Macro level—Measurement techniques’’).
The ‘‘fiber saturation point’’ (FSP) has been used to characterize the water adsorption and swelling prop- erties (Christensen and Giertz 1965; Scallan and Tigerstro¨m 1992; Scallan 1977; Stone and Scallan 1967; Tejado and van de Ven 2010; van de Ven 2008). An additional method of determining the swelling state is to measure the water retention value (Ho¨pner et al. 1955; Thode et al. 1960; Zellcheming 1957). Scallan and Tigerstro¨m used the van’t Hoff equation to calculate the bulk modulus of the fiber wall shown in Eq. 5 supposing ‘‘that the hydrogen form of the charged groups of the fibers can be taken as a reference state’’ (Wa˚gberg and Annergren 1997). Bulk modulus ‘‘K’’ of the fiber wall (Scallan and Tigerstro¨m 1992)
RT n V V V 0 ð Þ V C þ V 0 ð Þ
K ¼
ð 5 Þ
In this equation the RT n V
is the osmotic pressure in
the fiber wall. R: gas constant; T: absolute temperature; n: mole of charged groups per kg oven dried pulp; V: Volume of water in the swollen fiber wall; V 0 : Volume of water in the fiber wall when the charged groups are in their hydrogen form; V C : specific volume of the material in the fiber wall assuming the density to be 1.5 9 10 3 kg/m 3 . The swelling contributes to the fibers’ flexibility, resulting in considerably better entanglement (Barzyk et al. 1997; Brecht 1947; Brecht and Erfurt 1959b; Erhard et al. 2010; Linhart 2005; Lyne and Gallay 1954a, b; Scallan 1983; Weise et al. 1998). The degree of beating enhances or lessens this effect (Brecht and Erfurt 1959b; Kibblewhite 1973; Lindqvist et al. 2011; Salminen 2010). All authors cited above conclude that proper swelling increases the dry strength of the paper. This means that conditions such as beating, alkaline pH and low conductivity contribute to a strong dry paper. There has been done only few research in case of IWWS.
Hornification
Drying and pressing introduces an irreversible change in lignocellulosic fibers resulting in a loss of swelling ability of fibers that is also named hornification (Fernandes Diniz et al. 2004; Po¨nni et al. 2012;
123
Cellulose (2016) 23:2249–2272
2257
In experiments about the effect of freezing on pulp properties, Kibblewhite determined that the IWWS decreases as the drying rate increases (Kibblewhite 1980). Similar to other research work, this study argues that the increased fiber rigidity/hornification leads to a reduction in bonding intensity. Adapting the sample preparation process enables the investigation of fiber surface effects and the hornification, charac- terized by comparing the fiber collapse of conven- tionally dried samples with freeze-dried samples by means of SEM imaging (Belle et al. 2015a). This study shows that in comparison to conventional drying, freeze-drying leads to significantly reduced fiber, fibril and surface hornification. Dependent on dryness, hornification affects the IWWS in several ways. Below 25 % dryness, horni- ficated fibers are stiffer with a lower young-modulus (Scallan and Tigerstro¨m 1992), higher friction coef- ficients and flatter shapes. This leads to denser sheets with better conditions for capillary forces and fric- tional connection. From 25 % dryness onwards a non- hornificated fiber is needed for better IWWS. Higher young-modulus and a flexible fiber results in better conformability and more initial contact points as well as a better entanglement.
Surface roughness of the fibers
The surface roughness of the fibers is given by the type of wood and is modulated to a significant degree by the pulping process (Fengel and Wegener 1989). Addi- tionally, the fiber surface roughness is specifically influenced by pulp beating and the associated gener- ation of fiber fragments and suspended fibrils. The surface roughness of the individual fibers and the associated fiber–fiber adhesion play an important role in regard to the strength in the wet paper web because they affect the capillary forces during dewatering (Alince et al. 2006; Hubbe 2006; Thomson 2007). Fiber pore sizes also play an important role (Erhard et al. 2010; Scallan and Tigerstro¨m 1992; Scallan 1977). However, only a few values for fiber surface structure and roughness were available. Additional values can now be generated by the AFM method published in 2011 describing the scanning of fiber surfaces, including the calculation of the surface roughness and fibril angle of fibers shown in Fig. 7 (Heinemann et al. 2011).
Fig. 7 ‘‘AFM phase images and corresponding fibril angle and surface roughness parameters for each individual image representing the inner (S2) and outer (ML/P) fiber wall layers … The images are 3 l m 9 3 l m’’. [Reprinted with permission of Heinemann et al. (2011)]
S q : RSM (root-mean-squared) roughness; S ku : kurtosis; S sk : skewness; REF: Reference; OX: chem- ical treatment of the sample with buffered oxalic acid
123
2258
Cellulose (2016) 23:2249–2272
Robertson 1959). In general, the beating of fibers is a non-specific process during which both the fiber morphology and the particle size distribution are modified in many respects (Banavath et al. 2011; Laitinen et al. 2014). Beating is partly able to reverse hornification and results in better fiber swelling, even with virgin fibers, with all the positive effects on IWWS. The mechanical energy put into the process affects the internal and external fibrillation and enables more contact points for van der Waals bonds in the wet state and more H-bonds in dry state. The change in fiber morphology can be measured as fiber length, coarseness, fines and some other values. For calculation the IWWS as shown by Page (1993) and Shallhorn (2002) only fiber length and coarseness are significant values. In summary, beating has a strong influence on the fibrous material blend and the fiber morphology, which in turn have a decisive impact on the IWWS.
dihydrate; AC: chemical treatment of the sample with hydrochloric acid SEM studies by the author show a very large irregularity of the fiber surface of industrially manu- factured pulp, caused by fragments of the S1 and S2 fiber wall (Belle et al. 2015a).
Fiber morphology
The fiber morphology is related to the fiber structure. These are primarily given by the wood and pulping type, as well as by beating. In addition to the fiber length, additional fiber characteristics can be calculated by means of image analysis procedures. Generally, when describing fibrous suspensions, the following terminol- ogy is used: fiber length, fiber width, kink, curl, coarseness, fibrils, fines and broken fibers (Page et al. 1985; Saren et al. 2013; Weihs and Wa¨tzig 2007). Studies of unbleached pine sulfate pulp have shown that an increase in fiber length contributes to the IWWS (Seth 1995). This paper further demonstrates that an increase in fiber coarseness will decrease the IWWS. Another investigation on various recovered paper fibers about the influence of beating show that the strength index can be increased from 2.5 to 3.3 Nm/g at a dryness level of 50 % (Klein 2007). After the fiber analysis, Klein attributes the increase in strength to the larger specific surface of the fiber material after the beating process. Thus, a certain extent of beating can increase the IWWS due to the higher portion of fines and the creation of additional surface area on the fibers. The curl determines the maximum elongation of the paper up to breakage, and can therefore be a direct indicator of the stability of the paper web in the first open draw (Brecht and Erfurt 1959a; Rance 1954). The more pronounced the curl is, the more the fibers can be elongated under tensile load before the fiber structure breaks (Seth et al. 1984; Tejado and van de Ven 2010).
Fibrils and fines
Fines are in general defined as particle that passes the 100 or 200 mesh wire of e.g. a Bauer McNett fiber classifier and consist of cellulose, hemicelluloses, lignin and extractives (Retulainen et al. 1999). As a rule 0.2 mm is the size that is considered as fines in optical fiber analyzers. Several scientist, amongst others, worked on the topic of using or generating fibrils and fines to enhance the IWWS of paper (Brecht and Erfurt 1959b; Corson and Lobben 1980; Lindqvist 2013; Myllytie et al. 2009; Pye et al. 1965; Retulainen and Salminen 2009; Salminen 2010; Washburn and Buchanan 1964). Fibrils and fibrillar fines are sup- posed to increase the IWWS by better conformability that leads to improved frictional connection. Salminen (2010) reported that wet web tensile is dependent on quality and amount of fines. An ‘‘addition of heavily refined kraft pulp with a high amount of fines to wood containing paper grades could increase the residual tension of wet web significantly … ’’ (Salminen 2010). Lindqvist worked with a harsh and a gently refining strategies (Lindqvist 2013). Pulp that was gently refined generated fines without decreasing the fiber length. The resulting fibers were more flexible and lead to increased wet tensile strength and residual tension. It is concluded that the internal and external fibrillation are critical for the residual tension. The movement of the fibrils and fines during sheet forming
Beating
Many publications have studied the development of paper strength by beating, but only a small number of these studies have focused on the influence of beating on the IWWS (Belle et al. 2016; Bhardwaj et al. 2000; Brecht and Erfurt 1959a; da Silva et al. 1982; Gurnagul and Seth 1997; Kibblewhite and Brookes 1975; Koskenhely et al. 2011; Lindqvist et al. 2011;
123
Cellulose (2016) 23:2249–2272
2259
These trials show, that the addition of up to 70 % of a beaten pulp fraction results in higher IWWS values. Similar experiments show that a specific blend of refined softwood and ground wood pulp results in a higher IWWS compared with the individual compo- nents (Schwarz and Bechtel 2003). This results show that different fiber components have complementary properties that are needed for a good IWWS. The studies reviewed here demonstrate that in each indi- vidual case, an optimal fibrous material blend can be found based on chemical and physical principles.
Measurement techniques
The sheet strength comprises the strength values of the individual components of the sheet as well as the strength of the interactions between those compo- nents. A variety of methods are available to measure the strengths of individual fibers and of the interac- tions between fibers. One method involves sample preparation with drops of resin to fix the fibers (Groom et al. 2002; Yu et al. 2010) followed by measurement with special equipment. However, this method is very time consuming. A variety of other approaches have been developed to investigate fiber properties, includ- ing the ‘‘Zero Span Method’’ (Burgert et al. 2003; Derbyshire et al. 1996; Futo 1969; Malhaire et al. 2009; Michon et al. 1994; Saketi and Kallio 2011; Saketi et al. 2013). Until now, these measurements have mainly been used in wood research. Meanwhile, the latest measurement techniques enable fibers to be pulled from the fiber network to determine the strength with which it is anchored to the sheet structure (Saketi and Kallio 2011). Another possibility to get a deeper look inside the paper structure is the X-ray syn- chrotron radiation tomography (Latil et al. 2010; Marulier et al. 2012, 2015; Wernersson et al. 2014). This technique enables researcher to get a 3D-image from the fiber network that might help in the future to get more valid information about fiber contact points and their bonding mechanism.
Fig. 8 Fibrils between two unrefined fibers at 20 % dryness
is determined by the amount of water in the proximity of the fiber and by the elastic modulus that is in the range of 2–15 MPa for fibers (Myllytie 2009; Nilsson et al. 2001; Scallan and Tigerstro¨m 1992). The SEM image performed by the authors in Fig. 8 shows the space between two fibers that is bridged by fibrils (Belle et al. 2015a). The conductivity of the solvent water was below 1 l S/cm, with the result that the fibrils of the fibers were able to move freely in the surrounding water, making contact and bonding with the next fibrils. To facilitate the scientific study of the influence of individual fibrous material components on strength development, a selective approach of using enzymes to reduce the presence of micro and macro fibrils could be useful (Teeri 1997).
Fibrous material blends
The blending of fibrous material components is another possible approach to increase the IWWS. Several authors have shown that a fiber component blend of long fiber, short fiber, flour and mucilaginous substances from groundwood pulp production mainly leads to a higher IWWS compared to the values of the individual components (Back and Andersson 1993; Brecht and Erfurt 1961; Brecht and Klemm 1952). Towards the end of the 1950s, trials have shown (Brecht and Erfurt 1959a) that blending different softwood pulp fractions has an impact on IWWS.
Macro level (sheet level)
The macro level analysis below addresses a number of aspects. First, an overview of the optimization options including chemical additives and adjustments in the forming and press section of a paper machine will be
123
2260
Cellulose (2016) 23:2249–2272
presented. This is followed by an overview of computer simulations to predict IWWS. This section concludes with a summary of the measurement techniques to determine the strength of wet paper webs.
CMC is also an additive that is used for strength improvement. Even for wet web it improves the strength in a range of 20–25 % at 50 % dryness (Klein 2007; Myllytie 2009). This might be explained with the more uniform paper by reduced flocculation tendency that results in stronger wet webs (Linhart et al. 1987; Nazhad et al. 2000). Even a very expensive resource like chitosan is an object of research to improve IWWS. Different researchers got strength improvements [ 30% at dryness between 30 and 55 % (Hamzeh et al. 2013; Klein 2007; Laleg and Pikulik 1992, 1993b; Myllytie 2009; Pikulik 1997; Salminen 2010). The structural similarity with the 1,4-glycosidic bonds are responsi- ble for the adsorption capacity of Chitosan onto Cellulose (Klein 2007). Especially for mechanical pulp the strength improvement is attributed to crosslinking ‘‘the fibrous network via imino bonds formed between the primary amino groups of the polymer and the aldehyde groups present in mechan- ical pulp’’ (Pikulik 1997). Guar gum has also been used to improve the IWWS (Myllytie 2009; Weigl et al. 2004). Weigl et al. (2004) were able to improve the strength of about 20 % at a dryness of 30 % with 0,5 % dosage of cationic guar to a pulp for LWC paper production. The effect of guar is attributed to the hydrophilic character, the cationically and uniform charge of the polymer. Galactoglucomannan (GGM) as a by-product of softwood pulping was recently tested by Lindqvist in 2013. She was able to increase the strength with 3,5 % GGM of about 10 % at 45 % dryness. She used bleached kraft pulp for the laboratory sheets. The GGM is supposed to disperse the fibrils on the fiber that they are more outstretched. In this way, they are able to interact with other fibrils and develop the contact points for web strength (Lindqvist 2013; Lindqvist et al. 2013). Besides these renewable additives, there exist several conventional chemicals that are used in paper making. One kind of these are surface active chem- icals. They are mainly used for stabilizing the paper making process but they influence also the paper strength. For an optimal dosage it has been found that the dewatering of the sheet was improved and the dryness and thereby the IWWS at same process conditions increased (Lindqvist 2013; Retulainen and Salminen 2009). By using oleic acid and defoamer a slight decrease of IWWS was observed (Retulainen
Chemical additives
Many paper additives derived from renewable and conventional resources have been developed and tested in recent years. The most popular additive for strength enhancement is starch. The cationic starch manly used for dry strength improvement is not able to increase IWWS of paper, it works quite contrary. This results in a decreased residual tension of about 10 to 15 % at 50 % dryness (Hamzeh et al. 2013; Laleg et al. 1991; Retulainen and Salminen 2009; Salminen 2010). Figure 9 shows this strength loss described in a paper from Retulainen and Salminen 2009. The decrease is explained by steric and electrosteric repulsion that reduces the friction force between the fibers. As Fig. 9 also shows, with cationic aldehyde starch there is a strength improvement possible at dryness above 45 %. Laleg and Pikulik came some years before to a similar result (Laleg and Pikulik 1991, 1993a). In contrast to the cationic starch, the cationic aldehyde starch is proposed to form a kind of cross linking in the fiber network ‘‘with hemiacetal bonds formed between the aldehyde groups of starch and the hydroxyl groups of cellulose’’ (Pikulik 1997).
Fig. 9 The effect of two different starches on residual tension of wet web at 2 % strain (Retulainen and Salminen 2009); (Reprinted with permission of The Pulp and Paper Fundamental Research Society)
123
Cellulose (2016) 23:2249–2272
2261
and Salminen 2009). These chemicals might interfere the fiber water gel and this results in less contact points between the fibrils. Another research group developed vinylformamide copolymers to increase the IWWS (Esser et al. 2008; Gels et al. 2012). The presented results are from paper machines and it is proposed that less breaks in the first open draw are a reference for better IWWS. Various cationic polyacylamides are used to improve dry strength of paper. Despite good results in dry paper, a decrease in IWWS is observed (Alince et al. 2006). The explanation for these phenomena is the steric and electrostatic repulsion of fully poly- acrylamide coated fibers. This results in increased distances between fibers with less friction force in the wet web. If the dosage is ‘‘below full coverage, fiber flocculation occurs resulting in weak spots in the wet sheet’’ (Alince et al. 2006). A selective addition of different additives is a good option for improving IWWS. Salminen added a cationic polyacrylamide (C-PAM) to the short fiber fraction and a cationic starch to the long fiber before mixing them. He observed good results in dewatering and IWWS. The positive effects are explained by the prevented flocculation of the long fibers due to adding the C-PAM to the short fibers including better retention of fines and better sheet formation. In addition it is supposed that ‘‘selective addition of chemicals generated pulp with both cationic and anionic surfaces, thus leading to a greater quantity of molecular level interactions’’ (Salminen 2010). Sev- eral other authors got also good results with selective addition of anionic and cationic polyacrylamides (Sutman 2011), microfibrillated cellulose together with anionic and cationic polyethyleneimine (Szeif- fova and Alince 2003), as well as chitosan, cationic starch and polyvinyl alcohol (Hamzeh et al. 2013). Furthermore the spraying technique in the wet end of the paper machine is rediscovered in several recent publications (Oksanen et al. 2011, 2012; Retulainen and Salminen 2009; Salminen 2010). This technology has the advantage that there are almost no interactions between water contaminants and the used additives as well as a very good chemical retention on the web. The dryness of the web is usually [ 45 % and good results were achieved with CMC (Salminen 2010), Chitosan (Salminen 2010), Guar (Oksanen et al. 2011),
Xyloglucan (Oksanen et al. 2011, 2012) and polyvinyl alcohol (Retulainen and Salminen 2009; Salminen 2010). One disadvantage might be the contamination of the felts in the press section with residues of these additives. In general, while chemical additives can improve the IWWS, their effects are minor compared to those of fiber morphology and dryness. Additionally, most of these additives are hydrophilic (Pelton 2004), with the result that the achievable dryness under given process conditions will be lower, thus limiting the IWWS.
Sheet forming
Studies performed on paper machine forming sections usually emphasize the dewatering speed, retention and web formation. In addition to studies that describe the influence of beating on dewatering and strength (Berger and Schramm 2011; da Silva et al. 1982; Kibblewhite 1973; Lindqvist 2013; Lindqvist et al. 2011, 2012; Lindstro¨m and Kolman 1982; Pikulik 1997), there is an extensive body of research papers on dewatering chemicals and the use of mineral fillers (Alince et al. 2006; de Oliveira et al. 2009; Esser et al. 2008; Ga¨rdlund et al. 2003; Hua et al. 2011; Lindqvist et al. 2009; Pikulik 2000; Sutman 2011; Szeiffova and Alince 2003).
Fig. 10 Sheet structure of unbeaten softwood pulp at SR 12 and 20 % dryness
123
Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 24 Page 25 Page 26 Page 27 Page 28 Page 29 Page 30 Page 31 Page 32 Page 33 Page 34 Page 35 Page 36 Page 37 Page 38 Page 39 Page 40 Page 41 Page 42 Page 43 Page 44 Page 45 Page 46 Page 47 Page 48 Page 49 Page 50 Page 51 Page 52 Page 53 Page 54 Page 55 Page 56 Page 57 Page 58 Page 59 Page 60 Page 61 Page 62 Page 63 Page 64 Page 65 Page 66 Page 67 Page 68 Page 69 Page 70 Page 71 Page 72 Page 73 Page 74 Page 75 Page 76 Page 77 Page 78 Page 79 Page 80 Page 81 Page 82 Page 83 Page 84 Page 85 Page 86 Page 87 Page 88 Page 89 Page 90 Page 91 Page 92 Page 93 Page 94 Page 95 Page 96 Page 97 Page 98 Page 99 Page 100 Page 101 Page 102 Page 103 Page 104 Page 105 Page 106 Page 107 Page 108 Page 109 Page 110 Page 111 Page 112 Page 113 Page 114 Page 115 Page 116 Page 117 Page 118 Page 119 Page 120 Page 121 Page 122 Page 123 Page 124 Page 125 Page 126 Page 127 Page 128 Page 129 Page 130 Page 131 Page 132 Page 133 Page 134 Page 135 Page 136 Page 137 Page 138 Page 139 Page 140 Page 141 Page 142 Page 143 Page 144 Page 145 Page 146 Page 147 Page 148 Page 149 Page 150 Page 151 Page 152 Page 153 Page 154Made with FlippingBook Digital Proposal Creator