and Surface Chemistry – Part 1- Fiber Surface and Wet End Chemistry

Pedro Fardim

Instituto de Química, Universidade Estadual de Campinas, SP, Brazil Present Address: Process Chemistry Group, Åbo Akademi University, Turku/Åbo, Finland

Abstract Paper manufacturing and converting involves a multitude of surface chemical interactions. Beating, stock preparation, coating and converting processes are steps where surface phenomena play an important role for product consolidation and performance. Interactions between fibers and colloidal particles, polymers, minerals and, are fundamental in and most of them are complex and not very well understood. Thus, many paper formulations are designed according to an empirical trial-and-error approach. This work is a critical literature review of some of the interactions present in papermaking, coating, and . In the first part, the current models for fiber surfaces and their interactions in the wet end are discussed. Limitations concerning the fiber surface definition and the available models based on direct observation, DLVO theory, and a combination of qualitative and quantitative approaches are highlighted. Complexity factors such as surface composition heterogeneity, contamination from manufacturing processes, and surface mobility are suggested to be considered. Surface interactions present in the wet end, with focus on reversible and irreversible flocculation as well as on the flocculation mechanisms concerning fibers, polymers and micro-particles are also discussed. The role of the fiber surface chemical composition on flocculation and flock size and the influence from the chemical medium in the is reviewed. Papermaking is suggested as being a fiber surface modification process once polymers and other chemicals are used to change the surface energy and chemical composition. Modification by formation of esters or attachment of aluminum abietate onto surface anionic groups is believed to occur during . Modification which affects the fiber-fiber bond with the aim of improving the wet and dry strength properties is also discussed. Accessibility of anionic groups in fibers to the chemicals during the short time scale of papermaking process as well as competition for the same charged sites are still not clear and its implications are reviewed. Comments are made on the limitations regarding the present process monitoring using measurements with no specificity. A combination of different sorption and chemical microscopy methods is suggested as a suitable approach to clarify surface interactions. Why does a paper formulation need such a number of additives? Where are they attached on the fiber? Questions of this nature are challenges not only in the scientific point of view, but also to market perspectives, demanding critical research work. Another idea presented in this review is the application of surface engineering to pulp fibers. Surface engineering is a concept traditionally applied to development of coating for metals and its main goal is the modification of material surfaces in order to achieve desired properties. This concept has a good potential for eucalyptus based pulp and paper, especially for improving strength and absorbing properties. The strategic application of surface chemistry in the can also be a useful tool, not only for patents and product development, but also to improve troubleshooting and customer support.

Key words: Papermaking, pulp fibre, surface chemistry, flocculation, surface engineering. 1. An overview of paper machine and papermaking

Paper is believed to be invented in 105 B.C. by Ts’ai Lun, a member of the Chinese Empire. Since that time until 1798, when the first paper machine was built, all paper produced over the world was hand made. The first paper machine was invented in France by Nicholas Louis Robert and was improved in England Figure 1. Schematics of a Fourdrinier paper by Bryan Donkin and John Gamble. machine. (a) Head box; (b) Forming; (c) Press; (d) Drying; (e) Surface treatment The work of improvement was (optional); (f) Reel (www.abo.fi/fak/ktf/pap, supported by Fourdrinier brothers and 2001). then the paper machine was named The production flow in a Fourdrinier. usually starts with pulp refining and subsequent addition of non-fibrous A paper machine is usually divided in components such as fillers, , different sections, i.e., head box, retention and sizing aids, optical forming, press, drying, surface brighteners and other, then dilution is treatment, and reel (Fig. 1). The wet made with before or inside the end part of the machine is that head box. During this sequence there composed by the head box, forming is a flocculation process where fibers, and press sections and it is named minerals, colloidal particles and like that because it requires a high dissolved substances are aggregated in amount of water to operate. The part macroscopic flocks. The flow goes composed by press and reel is usually from the head box to the forming named dry. Other paper machine section where a significant amount of types than the Fourdrinier were water is removed from the flock before developed using different forming the sheet reaches the press, drying and section designs. Paper machines can reel. The surface treatment section have different size and width, being can be on-line or off-line. Surface from laboratory pilot scale to large sizing, coating and calendaring are industrial installations. The treatments often used. production speed of a paper machine can vary from a few to thousands 2. Pulp fiber and definition of meters per minute. surface

Pulp fibers are cells extracted from plants. The extraction of pulp fibers from wood can be done using chemical, mechanical, thermo- and chemi-mechanical processes. Pulp is

2 composed by different types of cells which are dependent upon the wood used as raw material. The term “fiber” is related to libriform cells in hardwoods and tracheids in softwoods. Other cells are present in minor proportion and are not considered fibers, e. g., vessel elements in hardwoods and parenchyma cells in softwoods and hardwoods.

The thickness of the fiber cell wall can vary between 3-15 µm depending on Figure 2. Cell wall structure of a wood fiber. the wood species. The cell wall is Middle lamella (M), primary cell wall (P), formed by the primary and secondary transition lamella (S1 layer), main layer of the secondary wall (S2 layer), tertiary wall or walls and the latter is divided in S1, tertiary layer of secondary wall (S3 layer) S2 and S3 layers (Fig. 2). The primary (Rydholm, 1965). wall is close to the middle lamella while the S3 layer is near the fiber Research and studies on the lumen. Each layer is made of macro- characteristics and interactions of fibrils which are built from micro- external fiber surfaces when present fibrils. Cellulose macromolecules in aqueous and papermaking media disposed on amorphous and have been performed based on crystalline regions build the micro- models. Pelton (1993) did an fibrils (Larsson et al, 1997). A interesting review and has classified monolayer of cellulose macromolecule the models for external surfaces as has a thickness of 0.6 nm (Sugiyama qualitative and quantitative ones. He et al, 1991). has also proposed a combination between them to be more adequate. Pulp fiber has a tubular shape with The current models are discussed in internal and external surfaces. The the next section. internal surface makes an interface with the lumen and the external 2.1 Qualitative models for the surface makes an interface with the fiber external surface middle lamella (Kolseth and Ruvo, 1986). The fiber cell wall is porous Qualitative models are based on and the pores also contribute to the direct observation of the pulp fibers internal surface. During chemical and no physical or mathematical pulping and bleaching a large approach is applied. According to the fraction of the primary cell wal is qualitative models, fiber surfaces show removed with . Thus, the roughness at nanometer scale. external surface of the bleached pulp Experiments performed using Atomic fiber is believed to be the secondary Force Microscopy (AFM) has also fiber wall (Laine and Stenius, 1996; indicated that lignin can be unevenly Treimanis, 1996). In case of distributed on the unbleached fiber mechanical, thermo- and chemi- surfaces, with regions of exposed and mechanical pulps the external surface covered fibrils (Lorenzoni, 1998). is mainly formed by fractions of AFM image of a eucalyptus pulp middle lamella and primary cell wall.

3 refined in a PFI mill also showed 2.2 Quantitative model and DLVO stratification (Fig. 3). colloidal stability theory

The quantitative model assumes that the fiber surfaces are flat and hold electrical charge in aqueous medium. The Deryagin-Landau-Verwey- Overbeek (DLVO) theory (Overbeek, 1970) is the fundament of the quantitative model and it based on estimation of energies of London-van Figure 3. AFM image of a kraft unbleached der Waals (attraction) and double- eucalyptus pulp, 24, refined layer overlapping (repulsion). at 3000 revolutions PFI. Image presented in topographic mode, bi-dimensional (left) and tri-dimensional (right). Roughness and A sum of attractive (Va) and repulsive stratification observed at nanometer scale (Vr) potentials can be represented by (Fardim, 1999). a total energy interaction curve (Fig.4). According to this curve, Limitations concerning formulation of particles at short and long distances models based on direct observation have its attraction favored while are mainly related to the susceptibility particles situated at intermediary of the fiber to its external medium or distances tend to face repulsion. changes on interfaces. When fibers Concentration of electrolytes can are in contact with air or in high affect the maximum of the total vacuum conditions such as used in energy interaction curve. High Scanning Electron Microscopy (SEM), electrolyte concentration reduces the there is a reduction in the number of maximum potential and deposition is pores and deformations in favored if particles have opposite comparison with the situation when surface charge. This deposition can be fibers are in water, according to reversible, if it occurs at the point of thermoporosimetry (Maloney and secondary minimum in the curve or Paulapuro, 1999). Surface specific irreversible, if it occurs at the primary area is also drastically reduced when minimum (Fig. 4). the pulp is dried (Herrington and Midmore, 1984a). This reduction can be irreversible and a phenomenon known as hornification may take place (Wise, 1998). Surface area can be defined in a simple way as the interfacial area between two different phases and if the pulp fiber is compared with a sponge, the surface area means the regions accessible to water or air and thus molecules or present in these components. Figure 4. Total interaction energy curve showing attractive London-van der Waals and electrostatic repulsive potentials (Pelton, 1993).

4 The retention of colloidal particles by pulp fibers to build flocks is the base of papermaking. According to the DLVO theory, pulp fibers should have a high capacity of particle retention which could be observed in Britt Jar experiments. This prediction is also supported by the low Zeta potential values, within a range of -1.6 to -14 mV for kraft pulps (Herrington and Figure 5. Illustration of the Pelton’s model Midmore, 1984b). However, low (1993). Fiber surface is covered with a polymer bearing electrical charges and retention values were observed for affecting adhesion of colloidal particles due to TiO2 particles and pulp fibers, being steric stabilization. up to 10 % and not improved by electrolyte addiction. This retention 2.4 The fiber surface complexity level is considered very low and it may indicate that the DLVO theory is The approach of a polymeric layer on not suitable to describe the fiber the surface as it was proposed in the surfaces (Pelton, 1993). Pelton’s model requires revision. Recent research using AFM (Okamoto 2.3 Pelton´s Model and Meshitsuka, 1999; Simola et al, 2000), enzymatic hydrolysis and CZE A combination between the (Sjöberg et al, 1999), MALDI-ToF-SIMS quantitative and qualitative models (Linqvist and Dahlman, 1998; Jacobs was suggested by Pelton (1993) which et al, 1999), contact angle (Fardim consider a cover layer of hydrated and Durán, 2000a), XPS (Laine et al, polymer bearing terminal electrical 1994; Westermark, 1999; Fardim and charges on the surface in a similar Durán, 2001) and ToF-SIMS (Fardim way as it was previously proposed by and Durán, 2000b; Fardim et al, Clark (1978). According to this 2001) has shown that the fiber model, the deposition of particles on surfaces have different components the surface is inhibited by steric bearing different hydrophilic stabilization due to effect of a character and ionizable functional hydrated polymeric layer (Fig. 5). groups. These components can be Adhesion between two surfaces can fragments of lignin, hemicelluloses, only be achieved if there is cellulose and extractives located on compression of polymeric cover layers, the surface and originated during but this is not thermodynamically pulping and bleaching or simply favored. Presence of electric charge on deposited (Buchert et al, 1996). the surface layer can also enhance the Contaminants which are present on water solubility and contribute to the closed circuits can also sorb onto the steric stabilization. Surface roughness surface. Other parameter to be is another parameter which affects the considered is the surface mobility or adhesion between fiber surfaces and changes in the chemical composition different particles due to irregular of the surface monolayer according to structure and absence of flat regions. the medium which makes interface with the fiber. Extractives which have hydrophobic character can migrate to internal regions and when the fiber is

5 dried they can return to the surface spontaneous process if there is a again. The complexity factors reduction in the system total free discussed here indicate that the fiber energy. external surfaces have a Considering a system of colloidal heterogeneous character not particles separated firstly at infinite considered in anyone of the current distance and then at H distance, models. makes possible to formulate a general equation for the total free energy (∆G) 3. Surface chemistry and paper involved in the flocculation process as machine wet end following:

The paper machine wet end is ∆G = ∆GAtr (van der Waals) + ∆GRep composed by stock preparation, head (short distance) + ∆GRep (electrostatic) box, formation and press sections. The Rep purpose of the press section is to + ∆G (steric) + ∆G (other effects) remove the excess of water and the [1.1] surface phenomena involved are limited to changes in surface tension All parameters on the Equation 1.1 during paper drying. take part on flocculation, but in case of irreversible flocculation there is a The formation of macroscopic flocks predominance of the parameter composed by polymers, colloidal concerning the van de Waals particles, , and pulp fibers is the attractive forces. When a particle main goal of the wet end during approaches a flock there is a paper production. Flocks must have reduction in the translational enough strength to resist to the movement of the aggregate system and also on its degree of freedom. Thus, turbulent flow in the head box and Agr also the shear forces in the forming the entropy (∆S ) is reduced and the wire. A multitude of surface free energy increases according the phenomena are involved in the wet Equation 1.2. end and are direct related with desired paper characteristics. ∆GAgr = -T[∆SAgr] [1.2] 3.1 Reversible and irreversible flocculation Usually it is assumed that irreversible flocculation is dominated by free Flocculation is an aggregation process energy and the contribution form where the particle identity is entropy is not significant (Everett, maintained and which can classified 1989). The energy barrier to reach the as reversible and irreversible. primary minimum at the DLVO total Reversible flocculation is believed to energy curve is overwhelmed in the occur at inter-particle distances close irreversible flocculation resulting in a to the secondary minimum on the spontaneous process. This effect is DLVO total energy curve (Fig. 4) while obtained by addition of electrolytes irreversible flocculation may occur at which contribute to the compression the primary minimum. Reversible of the Stern-Gouy electrical double flocculation is also termed layer and reduces the repulsion coagulation according some authors. between particles (Mangelsdorf and Irreversible flocculation is a White, 1998).

6 The addition of electrolytes is not amount needed to cover the entire enough for flocculation of pulp fibers particle surface (Lindström, 1989). and mineral particles in papermaking due to reasons already discussed in the section 2. The repulsive potential is then reduced by addition of polymers or polyelectrolytes which hold electric charges with different polarity as those present on the particle and fiber surfaces.

3.2 Retention aids and flocculation mechanisms Figure 6. Bridging flocculation mechanism. Polymers or polyelectrolytes used to Different segments of the polymeric chain improve flocculation in papermaking bind the particles and an aggregate is formed. are usually termed retention aids. Cationic used in stock Network flocculation can happen if preparation is also believed to the polymer concentration is very low, contribute to retention as well as on being adsorbed on the surface of a dry strength. Drainage and retention few particles and thus transform then are important parameters in the paper onto small electric dipoles with production and requires that the flock extremes of opposite charges. Under formed has not only strength, but also this condition, particles are oriented that the water on its structure needs to in a way that mutual electrostatic be easily removed. attraction is achieved (Fig. 7).

Retention is also affected by mechanical interlacing of flocks in the formation wire. High-molar mass polymers used as retention aids can increase the colloidal attraction forces and induce flocculation by different mechanisms, i.e., bridging, network, depletion, and bridging combined with micro-particles.

Bridging flocculation involves Figure 7. Network flocculation mechanism. adsorption of terminal and Polymers used in low concentrations are intermediary regions of the polymeric adsorbed onto the particle surfaces forming electric dipoles and generating inter-particle chain on the particle surface forming attraction. an aggregate fixed by the polymeric chain (Fig. 6). High polymer Flocculation by depletion mechanism concentrations can hinder the bridge occurs if a weakly or no adsorbed formation between particles due to the polymer is present when two particles tendency to cover their surfaces. It is approach each other. The polymer is believed that the optimum polymer excluded from an inter-particle region concentration to achieve flocculation of a volume V, area A and separation by this mechanism is half of the H, causing a negative adsorption

7 according to the Equation 1.3, where Research on flocculation dynamics is c is the polymer concentration traditionally performed in (Everett, 1989). When the particle experimental scale using turbidity separation H decreases, the negative and dynamic drainage or Britt jar adsorption increases resulting in an measurements. However, more attractive force (Fig.8). sophisticate techniques such as Scanning Tunneling Microscopy have Γ = -cV been applied and information about [1.3] flock size and resistance to shear forces may be obtained (Gerli and Micro-particle systems using either Clémencon, 1999). silicates or bentonite combined with high molar mass polymers are used in 3.3 Fiber surfaces and flocculation flocculation by bridging-micro- particle mechanism. The bridging is Flock resistance is affected by different similar as described previously, but in types of forces: normal forces between this case the system highly flocculated fibers, forces due fiber flexibility and after polymer addition is submitted to elasticity, friction forces, and colloidal shear and the macro-flocks are attractive forces. (Kerekes et al, 1985). broken into micro-flocks. At this point, There are different statements micro-particles are added and the regarding which factor is macro-flock is rebuilt, however, it has predominant on flock formation and higher resistance to turbulence and resistance. In one side there are shear than an ordinary macro-flock hypotheses of domination of formed only using polymer. It is hydrodynamic effects (Beghello, believed that the micro-particles 1998) while in another side there interact via electrical charge hypotheses that chemical effects and attraction with the polymer segments colloidal attraction dominate attached to the particles and (Wågberg, 1987). There are also some reflocculation is caused by bridging common points between the different between polymer chains connected to hypotheses. The common points are the micro-particles (Swerin et al, the effects of consistence, fiber length, 1995). fiber-fiber contact, turbulence and shear forces on flocculation. However, the main discordances are related to the effect of the medium, i.e., electrolyte concentration and pH.

Fiber surface composition is recognized as an important effect on flocculation by the different hypotheses. Both statements considered that the higher concentration of anionic groups on Figure 8. Flocculation by depletion the surface makes the fiber more mechanism. A weakly or no adsorbed sensitive to paper machine medium polymer is excluded from an inter-particle and flocculation is reduced. Flock size region. is also affected by surface composition and small flocks are observed when

8 higher concentration of anionic Chemical interference in sizing groups is present on the surface reactions using alkenyl succinic (Beghello, 1998). The bleached pulp anhydride (ASA) can be attributed to fiber with lowest total concentration of dicarboxylic acids formed due to the anionic groups is the pine ECF while ASA hydrolysis in water. Alkyl ketene the highest concentration is observed dimmer (AKD) can also have for eucalyptus ECF (Fardim et al, hydrolysis in water at high 2001). However, effects of formation temperatures and the formation of of a surface layer gel on flocculation ketones interferes on the distribution and particle retention for pulps with of sizing aids on the fiber surface different amounts of anionic groups (Roberts, 1992). are presently been investigated. Type pH Componen Cellulose 3.4 Sizing, and dry and wet of range ts derivativ sizing es strength aids formed Acidic 3.5- Abietic acid Alumium Flocculation is not the unique 5.5 derivatives abitate phenomenon which involves surface and attached interactions in the wet end (Isogai et or on fiber aluminum anionic al, 1997; Berg, 1993). During stock polycloride groups preparation or ahead of the head box, Neutra 7.0- Alkyl ketene components are added to modify the l or 9.0 dimmer fiber surface free energy. These Alkalin (AKD) addictives are compounds with e Cellulose Alkenyl ester hydrophobic character and holding succinic terminal groups which can attach to anhydride the fiber surface via electrostatic (ASA) interaction or chemical reaction. The Table 1. Different sizing process in acidic, latter can yield cellulose ester neutral and alkaline media (Roberts, 1992) derivatives. Modification of fiber surfaces in Modification of fiber surfaces with the papermaking is not limited to sizing, purpose of reduce the water usually other additives such as dry penetration into the paper structure is and aids are employed termed sizing. This process is classified and the interaction fiber-fiber is as alkaline, neutral or acidic sizing, affected. The latter additives are depending on the pH in the reaction believed to modify the fiber surface medium (Tab.1). Additives used in composition by insertion of functional sizing modify both surface free energy groups which hinder the formation and surface chemical composition bonds between fibers and (Ozaki and Sawatari, 1997; Carlsson, water molecules with concomitant 1996). A homogenous sizing aid increase in fiber-fiber bonds. As a distribution on the fiber is dependent consequence the fibrous network has upon the absence of restrictive effects a higher strength in wet and dry and availability of superficial groups. media. Components usually applied Chemical interfering and for this purpose are anionic and morphological parameters such as cationic polyacrylamides, polyamines excessive roughness are the main and polyamides. Imines, urea and restrictive effects. melamine formaldehyde are also

9 employed specifically for wet strength practically unknown. Exchange purposes (Proverb, 1999). between components with different affinity to anionic sites during the 4. Interactions in the paper different stages of papermaking as machine, theory and reality well.

“I think that also we should be more Other important factor in a paper careful in expressing too confidently machine is interaction time scale, i.e., our opinions about phenomena at a few seconds. Laboratory occurring in the papermaking experiments have shown that in spite processes…in most cases, we still have of the flocculation occur in scales of hypotheses only –which can be very less than a second, the diffusion of a helpful in our daily work, but at the polyelectrolyte through the cell wall same time it can be basically very can take up to 30 minutes in uncertain and only render the chemical pulps, being up to 120 mystery more mysterious” (Stén, minutes for mechanical pulps. This 2000). In fact, concepts of can be an indication that just a interactions involving portion of anionic groups present in macromolecules are still limited not pulp are accessible to -exchange only for papermaking but also for during papermaking. polymer chemistry, bioactive substances, catalysis, lubrication and Anionic group accessibility is microelectronics. However, it is matter dependent upon the medium which of time, budget and interest to clarify the fiber is inserted, its carboxyl group them in a scientific basis. counter-ions and consequently degree of cell wall swelling (Sjöstrom, 1989). Paper machine has a universe of Probably, interactions concerning interactions and a great part of them surface groups are favored due to the are still unknown. A system higher accessibility during the containing fibers and a second papermaking dynamics, but this can component, i.e., filler, polyelectrolyte also increase the possibility of and starch can be studied in competition and or ion-exchange on laboratory using a medium with a the fiber anionic sites. The amount of known ionic strength and a surface anionic groups in ECF pulp theoretical treatment considering the fibers is about 20 % of the total adsorption kinetics can be applied in anionic groups (Laine et al, 1996). spite of some limitations. However, in This value is around 10 µmol/g in the case of a paper machine the system is external surface layer of 10 nm depth. composed by fiber and a great The distribution of these groups as number of additives in conditions well as its interactions with where consistence and ionic strength paperchemicals and in fiber-fiber can vary in a significant way. bonds is still unknown. Another factor of complexity is the presence of organic and inorganic Presently, the routine determinations cations as paper chemicals and also of conductivity, turbidity and cationic in the paper machine white water. demand in papermaking are limited The existence of competition between parameters to interpret the the different cationic components to phenomena here discussed, because the anionic sites on the fiber is they are unspecific parameters.

10 Determination of total organic as well as to fulfill the needs of in the white water can give an different types of paper. This strategy indirect quantification of dissolved can add economic value to the substances and colloidal particles, but products, make possible a positive it is also a parameter not specific. distinction in international markets Application of chemical microscopy and generate patents. The effects of techniques (Hagenhoff, 2000, Brine, pulping and refining processes of 1993) combined with adsorption eucalyptus pulp fibers on the surface methods (Wågberg and Ödberg, chemical composition and its relation 1989) seems to be a suitable approach with physicochemical and absorption to evaluate which components are properties has been already reported really attached onto the fibers. (Fardim, 1999).

5. Surface engineering, concept 6. Conclusion and strategies for eucalyptus pulp fiber Concepts of surface chemistry are practically involved in all stages of papermaking. The papermachine wet Surface engineering is the application end section has a multitude of surface of surface modification strategies to interactions with the aim of formation design new functional properties of a of flocks with defined resistance to the material. This field involves hydrodynamic forces and shear professionals of different areas, being present in process conditions. Fiber usually applied to metal coating, but surface have experimental it was already suggested for pulp characteristics which are not in fibers (Tze and Gardner, 1999). accordance to the current models proposed. Recent research has been Papermaking is a fiber surface showing that the fiber surfaces have a modification process according to heterogeneous character and several aspects discussed here, complexity factors not considered in however; it is usually not recognized current models. Knowledge about like that. The lack in this interactions between pulp fiber and understanding has a consequence of paperchemicals are based on opportunity losses regarding the hypotheses and models limited to two development of new pulp and paper or three components, requiring an products. The knowledge of approach more oriented to realistic papermaking interactions in a situations. The concept of surface molecular level is a great challenge engineering usually applied for metal with attractive perspectives not only coating can also be applied to fiber for scientific interest but also for surfaces with a good potential of market purposes. Reduction of the technological improvement for number of raw materials used in eucalyptus pulp and paper. paper formulations and a new level of competitiveness will be achieved with 7. References this knowledge. Beghello, L. (1998): “The tendency of fibres to Surface modification of eucalypt pulp build flocs”. Doctoral Thesis, Åbo Akademi fibers can be used to improve strength, University, Turku/Åbo. printability and absorption properties

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