RECENT DEVELOPMENTS IN PAPERMAKING CHEMICALS

 

Przemyslaw Pruszynski

Nalco Chemical Company, 1601 West Diehl Rd, Naperville, IL 60563-1198, USA

 

Abstract

This paper presents a review of recent developments in the area of specialty chemicals on the background of changes in paper technology and the overall industry economic landscape.  This paper reviews not only new products introduced to the market, but also mechanistic knowledge, application strategies, control and monitoring, and possible future trends.  It is quite clear that the last several years presented a period of optimization of major technologies in terms of cost, impact on product quality and machine efficiency.  Despite this fact, several important developments were introduced and intellectual foundations for others were built.

 

Introduction

The production of paper, basically the filtration process of a papermaking slurry, requires increasing assistance from a variety of chemical additives.  There are a broad variety of paper and board grades that differ in final quality expectations, furnish composition and papermaking equipment used.  These different grades of paper products rely on a different amount of chemical contributions.  The quest for increased productivity, as well as increasingly demanding final product quality specifications and a variety of environmental and economical pressures resulted in almost universal penetration of chemical applications to all paper and board grades.  Chemical additives used in papermaking can be divided into three groups – general (commodity) and two classes of specialty chemicals – process and functional.  Process chemicals are used to optimize the production process by increasing machine speed, runnability, providing deposit control and reducing steam consumption.  Retention aids, defoamers, fixative agents, biocides and defoamers/antifoam additives are some typical examples of process chemicals.  Functional chemicals directly affect paper quality and paper properties – color, water repellency, strength, printability, etc.  Typical examples of such functional chemicals are dyes, coating binders, strength and sizing additives.

 

The boundary between process and functional chemicals is not very definite as process chemicals may either significantly influence performance of functional chemicals and/or affect sheet properties directly.  Retention aids, for example, significantly affect the expected performance of added fillers, sizing and strength additives, but also have a direct impact on sheet properties by influencing formation and porosity.  Moench and Auhorn [1] presented an outlook to the application of chemical additives in papermaking.  According to these authors, chemical additives amount to 1% of the raw materials. 90% of all chemical additives belong to functional additives (coating binders, sizing, bleaching chemicals, strength additives).  The remaining 10% are process chemicals with retention aids (including fixatives, coagulants, flocculants and microparticles) representing the biggest and most important part. 

 

Authors predict a significant increase of chemical usage (from 3.6 million tonnes in 2000 to above 6 million tonnes in 2020).  This prediction is based on the growth of coating binders and other coating additives (lubricants, thickeners) due to predicted increase of coated grades production.  Predicted increase in internal and surface sizing applications reflected expected need for improved printability and runnability of uncoated, wood-free grades.  Strength additives growth will be results from need to assist the trends of basis weight reduction and filler level increase.  In this paper, I will concentrate in principle on the recent developments within the group of specialty chemicals with the special emphasis paid to wet end applications of process chemicals.

 

Historical background

Specialty chemicals have been introduced to the paper industry in response to the industry's production and quality needs.  A number of significant industrial trends within last 20 years dictated an increased need for the application of specialty chemicals and set the direction of the R&D effort that led to the development of many new technologies.  Some of the most important trends related to changes in papermaking furnish composition result from environmental, economic and quality pressures.

Replacing fiber with inorganic fillers of various natures may be, in some cases, a result of any of the triggers mentioned above – cost, environmental impact and quality goals.  The introduction or increased levels of recycled fiber, higher brightness targets, increased degree of closure of water systems, lower basis weight of produced paper, changes in printing technologies, increasing speed and shear of forming sections, are only few of important industry trends, that determined the increasing importance and complexity of chemical applications.  Most of the above trends have exponentially increased the challenge to suppliers of papermaking chemicals by combining increased performance expectations with creating the chemical environment negatively effecting fundamental mechanisms of operation of chemical additives.  For example, a reduction in the basis weight of paper often requires the addition of mineral filler to compensate for loss of opacity.  This, from the point of view of a chemical supplier, means a need for higher retention at a lower mechanical retention contribution, and a significant need for controlling the strength of the paper.  In another case, the introduction of hydrogen peroxide bleached and highly filled grades of paper created the requirement for high filler retention at a high concentration of detrimental substances, which negatively impacts the performance of retention additives.  Finally, increasing the speed of paper machines and higher shear stress in the forming sections have a negative effect on the level of retention and on the stability of colloidal dispersion of pitch and stickies materials, resulting in a new level of technical challenges for a retention and deposit control program.  We could list many examples that are a good illustration of fascinating and challenging relationships between various paper properties.

 

The late 1980's and 1990's brought another significant challenge that chemical suppliers to the paper industry had to answer – alkaline conversion of wood-free sheet production, practically completed in 1990's, followed by neutral pH conversion of mechanical grades of paper, that still continues.  This change in pH of paper production has been initiated for a number of economic and quality reasons, but it mainly cleared the way for utilization of calcium carbonate fillers in a variety of forms, either through wet end addition or coating formulations.  This pH change also had other significant consequences as it corresponded to major changes in the chemistry of alum.  Again, major chemical suppliers assisted papermakers with the development of chemicals that could substitute or operate in absence of alum.  These developments related mainly to sizing technology and resulted in the introduction of reactive sizes, based on the alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD) chemistries.

Additionally, cationic dispersions of fortified rosin sizes were developed that could operate at pH levels higher than rosin soaps and did not rely on the aluminum ion precipitation on the wet end to operate.  A large number of low molecular weight, cationic polymers – coagulants or fixatives, were also developed to replace alum in its pitch control role.

 

Recent situation

Compared to the period of an intensive influx of radically new technologies required to match the changing needs of paper industry, the last several years could be characterized as optimization of existing technologies.  This seems to be a natural consequence of the technical and economical landscape of the industry.  There are no major technology changes resembling the scale of alkaline conversions, filler introduction, water consumption reduction and paper machine technology observed in 1990's.  Additionally, the economy of the pulp and paper industry is under significant pressures that require cost reduction measures.  The optimization of existing technologies includes some new products, but it also includes cost reduction, increased attention to final sheet quality, a better understanding of the impact of the chemical environment on the performance of additives and stabilization of machine performance.  All these activities result, also, in a better understanding of fundamental, operational principles of chemical applications, and build knowledge that will be used in the future should new radical technological changes occur.

 

Review of technology: Retention polymers

New developments

Several new chemistries have been developed recently that span a variety of applications, such as fixation of pitch, stickies, coated broke treatment and retention.  HYBRID coagulants were introduced by extending the molecular weight of DADMAC coagulants through copolymerization with acrylamide [2].  This type of coagulants offers a relatively high charge combined with an increased molecular weight that extends their mode of action beyond charge neutralization by mosaic adhesion and possibly some minor bridging flocculation.  Developed originally as single retention additives, HYBRID polymers found recently more applications as fixatives in pitch/stickies control and coated broke applications. These polymers are also used in combination with typical high molecular weight cationic flocculants.  Poly-vinylformamides and the products of their hydrolysis, polyvinylamines, represent another group of newly introduced polymers that may potentially serve the role of flocculants, coagulants and fixatives [3].  The control of degree of hydrolysis provides the unique option of varying the ratio of cationic charge and molecular weight.

 

The performance of HYBRID polymers that carry quarternized ammonium salt as a source of cationic charge, is not affected by the pH of the system.  The primary amine functions in polyvinylamine (PVAm) and primary/secondary/tertiary amine functions found in polyethylene imine (PEI) loose a significant amount of cationic charge during neutral conversion, between pH = 5 to pH = 7 or higher.  The negative impact of increased pH on fixing properties was recently confirmed by Richardson [4, 5] and Pruszynski [6] in the laboratory study and based on the paper machine application.  A loss of 6% of machine efficiency observed during neutral pH conversion with PEI-based fixative program was later recovered when switched to the HYBRID, quarternized product [5].  Gerli and co-authors [7, 8] reported the introduction of Core Shell™ cationic polycrylamide-based flocculants, which, through structural modification during the polymerization process, demonstrate improved activity due to greater chain expansion.  The joint application of HYBRID coagulants with Core Shell flocculants has been recently patented for improved performance and formation properties [9].

 

Understanding influencing factors and reducing their impact

In addition to new polymer developments, a significant amount of attention was paid to better understand the impact of the chemical environment of a paper machine on the performance of retention and drainage programs.  Several papers were published regarding the application of PEO/cofactor in highly contaminated systems and development of new cofactors [10, 11, 12].  Some of the data presented in these papers shows the advantage of a PEO-based program at dosages approaching 10 kg/t of active polymer10.  This dosage is not only significantly higher than the dosage used in practice, but one can expect that the potential over-cationization of the surface put cationic polyacrylamides at a disadvantage.  Pruszynski [13] reported a very comprehensive study of the impact of conductivity and cationic demand on several types retention programs and concluded that all programs can be potentially affected by some of the system contaminants. 

 

Conformational changes at higher conductivity levels, loss of polymer extension and direct polymer consumption due to the interaction with anionic trash material at a higher cationic demand level are the principle sources of performance losses for cationic polyacrylamides.  Changing adsorption characteristics of the PEO polymer or cofactor to the surface of furnish elements, and varying the interaction between the PEO polymer and the cofactor could be the reason for chemical sensitivity of PEO based systems observed.  Polverari [14], Tay [15] and Englezos [16] recognized the need to improve PEO program performance by augmenting it with a polyacrylate polymer, bentonite and coagulant, respectively.  In any case, the market share of the PEO based retention program remains in contrast with the amount of research attention that it receives, possibly due to the well-known shear and chemical sensitivity and its negative impact on sheet formation.

 

Improved water chemistry management

In general, papermakers recognized the detrimental effects of anionic impurities on the process and final sheet qualities and significantly improved control over their influx to the paper machine systems.  To a large extent, it was a result of a consistent research effort on generation, characterization, mechanisms and methods of removal, and the impact of anionic trash on process and paper quality.  In many locations this knowledge brought improvements in the stock preparation area, mainly to the optimization of bleaching, often supported by the introduction of effective water block between the machine and pulp mill.  This process resulted in significant improvements in water chemistry that made the ability to just flocculate the less important criterion of program selection.  Instead, the impact on final sheet quality, resistance to shear, suitability for closed loop control, ease of application and cost contribute to the final decision during the selection of a retention program.

 

New product screening techniques

This trend has created a need to introduce evaluation techniques that would reveal additional aspects of retention polymer activity on the development stage and in the process of program selection.  The application of focused beam reflectance measurement (FBRM) [17, 18, 19] allows one to measure the flocculation of each individual step of multi-component programs, and gives insight related to floc shear resistance and reversibility.

 

Mechanistic understanding of retention polymers

In addition to measurement techniques, the last few years brought a number of fundamental papers that increased our mechanistic understanding of the operation of retention polymers and retention programs.  Wagberg [20] and Nanko [21] discuss polymer adsorption to the fiber; Blanco [19] and Hube [22] discuss de-flocculation and floc reversibility; Stenius [23] reviews the impact of polyelectrolote adsorption on the paper properties; and finally, Eklund [24] and Dunham [25] cover the impact of chemical factors on the flocculation of wood fibers.

 

Translated to practice, better understanding of the kinetics of polymer adsorption will be a key factor in the future in the design of feeding strategies and optimizing mixing for maximum performance and minimum impact on sheet quality.  An area of research that seems to not receive an adequate level of attention and could potentially significantly improve the performance of retention additives, applies to maximizing the uniformity of polymer treatment of entire stock.  This issue would require superimposing the dynamics of mixing with the kinetics of polymer adsorption in order to maximize furnish contact with the polymer within the time frame of its adsorption.

 

Microparticle programs

A significant amount of research and development efforts continues to be invested into better understanding the mode of action, the impact on sheet properties and the development of new microparticle programs.  Several novel microparticle programs were introduced to the market in recent years:

Ultra POSITEK® [8] – program based on the application of the flocculant and the colloidal borosilicate nonosize particle.

Mosaic [26] – this array of microparticles consists of four products – synthetic inorganic anionic material, inorganic cationic boehmite and two cationic emulsion polyacrylamides.

Polyflex - anionic polymeric microparticle [27, 28].

Cationic microparticles including organic [29], inorganic materials and cationized montmorillonite clays [30].

 

Despite new developments, bentonite, colloidal silica and colloidal borosilicate-based microparticle programs clearly dominate the market.  Based on the differences in chemical composition, size and charge density, it is quite difficult to assume that their mode of action may be even remotely similar.  It is quite likely that bentonite acts as a rigid, inorganic bridging flocculation enhancer rather than a microparticle.  This may well explain reported differences in sheet properties for papers produced with bentonite, when compared with a colloidal borosilicate microparticle program [31].  Paper produced with UltraPOSITEK borosilicate material showed better formation, lower porosity and lower roughness when compared with paper produced with a bentonite-based retention program.

 

Possible future trends

Future developments in the area of retention and drainage programs will most likely concentrate on reducing the impact on formation, increased control over required sheet structure and increased robustness to varying chemical environments.  Reducing conformational rigidity of polymers and increasing specificity of interactions with treated surfaces (cellulose binding domains containing polymers) have already started to be explored [32, 33].

 

Special application strategies

In addition to the development of new retention products, significant improvement in the machine performance cost of application and final sheet properties may be achieved by better application of existing products.  Special applications that allow one to target specific fraction of furnish or boost synergy between additives, have been applied with great results.  Non-flocculative pre-treatment of filler with coagulant allows significantly improved retention of the filler with lower high molecular polymer dosage [34], resulting in increased economics and improved formation, all without measurable impact on sheet properties.  Proposed by Swerin [35], “site blocking mechanism” may explain the significant improvement of performance of cationic flocculant when added in pre-mixed form with the low molecular weight coagulant or HYBRID material when compared with sequential addition at the same dosage.  An example of such a benefit results in about 50% reduction of flocculant dosage in the mill producing telephone directory grade with PCC as filler.

 

Control and Monitoring

Advances in development of reliable online sensors resulted in clear visualization of the impact that instability has on machine performance and sheet quality.  It became clear that closing white water consistency on the closed loop control with flocculant for the systems that are intrinsically unstable might bring more problems than benefits.  This search for stable operation created the need for stability projects that included audits, benchmarking, identification and correcting sources of instability.  Nalco developed a consistent and structured approach that offers stability on the wet end, known as the GENIOUS program.

 

As far as monitoring and control techniques are concerned, white water consistency and charge measurement are most widely discussed.  There are a number of mills that operate with white water online consistency control.  Despite a number of publications, there are few that use online charge measurement for closed loop control purposes.  Some recent examples of such applications can be found in following papers [36, 37].  It is also quite possible that, at least in the systems with low content of soluble anionic trash, knowledge about the surface charge in form of ζ-potential could be of great value as discussed by Lindstrom [38].  One of the best examples of utilization of control and monitoring equipment and strategy to stabilize wet end o paper machine was described by Gill [39].

 

Sizing

The purpose of a sizing application is to inhibit penetration of liquids into the internal structure of the paper.  This is done in order to make the final products more resistant to water, control the application of other products (coating, surface sizing) and to control spreads of inks.  Sizing additives may be added through the wet end (internal sizing) or applied to the surface of the paper (surface sizing).  There is a definitive trend in the industry toward surface sizing applications that is mostly related to significant improvements in size press technology such as speed sizers, which offer good runnability and allow for good economics.

There was not a major change in the offered sizing products.  The most important are still rosin sizes, AKD, ASA used for internal sizing and styrene-maleic anhydride (SMA), polyurethanes and starches used mainly as surface sizes.

 

ASA, AKD

There were no major developments within the typical internal size products.  Recent developments are related, mainly, to change within the hydrophobic backbone of ASA – trends toward higher molecular weight hydrocarbon chain and AKD – introduction of unsaturated hydrocarbon chain (alkenyl ketene dimer).  This last modification resulted in the change of the physical form of the product from wax to liquid, and allowed reducing the slipperiness problem related to classical AKD process.  In recent years there has been a trend toward replacing of AKD with ASA in several grades.  Gypsum wallboard producers benefited from ASA sizing, although there have been some attempts to replace it with cationic dispersed rosin size due to easier handling and reduced deposit concerns.

 

It has to be stated that the biggest improvement in the ASA application, which allowed for increasing importance of this product, was improved quality and improved application knowledge—allowing depositfree application.  Sizing reversion and fugitive sizing, issues that were so dominant in mid 1990's, are of much less interest in recent literature.  There is still a significant interest in explanation of the mechanism of AKD operation [40], dynamics of AKD retention [41], improved stability of AKD emulsions [42], and optimizing emulsification starch selection to improve ASA sizing [43].

 

Starches

Potato starch has several unique properties, including high molecular weight, high phosphate content and high viscosity potential.  These allow it to often outperform derivatives of corn, waxy maize, tapioca and others.  Several modifications of starches are available for use, including cationic starches, hydroxyalkyl starches, oxidized starches and starch acetates.  Starch can be applied internally for fiber-fiber bond development, through surface application and in coating formulations.  Ethoxylated, or oxidized starches, are the main components of size press formulations.  Their main advantage is low cost and the ability to be applied in aqueous media.  The main drawback is their low affinity for fiber, accumulation in the mill white water and resulting contribution to BOD loading of an effluent.  Cationic starches overcome this drawback but do not offer water resistance.  Therefore, for most applications expensive synthetic surface sizes have to be combined with starch to impart hydrophobicity to the paper.  Noncationic, linear film forming polymers, such as CMC (carboxymethyl cellulose), hydroxyethylcellulose, sodium alginate and polyvinyl alcohol, are also blended with starch.  In a very recent paper, surface sizing with chitosan and chitosan blends was described [44].

 

Starch has also been the culprit for the development of special, highly cationic materials used for strength, surface strength and anionic trash control [45].  The application of starches for strength development in the presence of anionic, dissolved and colloidal substances, requires pre-treatment of furnish with a fixative (organic or inorganic) to allow starch adsorption to the fiber [46].  Lack of such pre-treatment will also result in the formation of water holding, anionic trash-starch complexes and that may negatively affect drainage.

 

Polymeric sizes

Major products in this group include Styrene Maleic Anhydride (SMA), Styrene Acrylic Emulsion (SAE), Styrene Acrylic Acid (SAA) and Polyurethane (PUR).  In last few years, Ethylene Acrylic Acid (EAA) surface size was introduced.  Some of these polymeric sizes have been prepared in the cationic emulsion form and were described in the literature as wet-end soft sizes suitable for mechanical grades applications [47].

 

Strength additives

A recent paper by Wagberg [48] reviews the application and mechanisms of various wet and dry strength additives.  Although no new strength agents were introduced recently, there are several promising developments that could soon change this picture.  Polyvinyl amines (PVAm) are very promising wet strengthening agents [3], and the complexes formed with CMC were studies by Pelton [49].  Highly substituting with primary amine functions starch [42] has been reported to increase wet web strength.  Earlier work with chitosan [50] showed a strengthening impact on the wet web strength of mechanical furnishes.

 

Two interesting applications have been recently reported that apply to the use of uncooked, granular starches in recycled board applications.  In both these cases, chemical pre-treatment of a cold starch slurry significantly increased its retention and incorporation into the web, consequently yielding higher strength benefit and lower mill water BOD loading.  Szymanski [51] described pre-treatment with bentonite microparticle and Pruszynski [52] teaches a variety of pretreating strategies of slurry starch in the presence of a small amount of papermaking slurry that is then added to the rest of the approaching stock.

 

Enzymes

The last several years showed an increase in activity in the area of enzyme applications in papermaking.  Some of these applications have been around for quite some time and will not be discussed further.  Examples of such applications are enzymatic treatments for increased delignification, increase of freeness, improving beatability of fibers and pitch control. Two interesting applications were introduced in recent years.  First follows earlier work by Thornton [53] that described polygalacturonic acids as a source of about 50% of anionic trash generated during hydrogen peroxide bleaching of mechanical pulps.  Thornton in elegant way proved his findings by demonstrating cationic demand reduction on the application of pectinases.  This class of enzymes is capable of catalyzing depolimerization of polygalacturonic acids to simple sugars that no longer contribute to cationic demand of the system.

 

Reid [54] reported the first industrial application of pectinases.  He reported that with a residence time of 15 minutes, in temperatures and pH prevalent in paper mills, the application of pectinase reduced cationic demand by about 50%.  Parallel Britt Jar experiments showed higher responsiveness of retention polymers and no negative effect on strength properties of treated pulp.  The first full scale trial in the mill confirmed these results.  The second interesting application of enzymes is the use of esterase for stickies control [55, 56].  Esterase catalyses the reaction of hydrolysis of the esterf bond and is, for example, capable of converting polyvinyl acatate into polyvinyl alcohol and acetic acid, both significantly potentially troublesome.

 

Other

Interesting technology of gentle debonding materials was introduced to the market that allows to boost opacity and therefore provide benefit of decreased TiO2 dosage or, such important in the case of recycled furnish, increased of stiffness and bulk.  Jokinen [57] reported new developments were of Optical Brighteners that allow to overcome the “greening” effect observed even at low dosages for most popular group of tetrasulfo OBA’s. New group of OBA descibed includes stilbenic OBA (based on stilbene) and distyrylbiphenyl products.

 

Conclusion

Despite the apparent lack of breakthrough technologies, the result of a technically sound market with cost reduction goals, optimization of existing technologies, and an increased focus on product quality and economics, makes our industry well prepared, technically, to face future challenges.

 

Core Shell and UltraPOSITEK are trademarks of Nalco Company.

 

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Originally presented at Wood, Pulp and Paper Conference, Bratislava, Slovakia, 2003, Keynote Speaker.

Reproduced by permission of NALCO ITALIANA S.P.A. VIALE DELL'ESPERANTO 71 • 00144 ROMA, ITALIA