Features Research & Innovation
Modern Calendering and Specialty Chemicals -from a supplier’s perspective

These advances in calendering technology and coinciding demands on quality and production costs of supercalendered grades created new requirements for process stability, formation, sheet structure and overall system cleanliness. Supplying a consistent and optimally structured sheet to the calender may permit the papermaker to improve results, particularly with the most aggressive calendering processes.

Calendering, especially the modern supercalendering process, is not able to mask sheet quality deficiencies. Contrarily, it makes them even more apparent in final sheet quality and printing performance.

Although papermaking equipment and furnish play a dominant role in defining sheet quality, a chemical supplier who understands and focuses on specific sheet requirements will significantly contribute to final results. With this in mind, specialty chemicals suppliers have had to focus beyond traditional performance indicators such as retention values, white water consistency, drainage, etc. The new focus should be to better understand, within the existing limitations of furnish and papermaking equipment, how to assist the papermaker in consistently delivering uniform, properly structured in ZD sheet, with an ideal surface topography and surface strength so that the maximum quality and runnability benefits from the calendering process can be achieved.

To better understand optimum requirements for modern calendering, let us briefly review the essence of the calendering process. Calendering is a simple process of paper surface modification through replication of the surface of the smooth roll applied under high pressures. In the process of calendering, the sheet undergoes compaction and a subsequent decrease in sheet thickness (caliper) or bulk. The relationship between bulk reduction and surface properties development indicates the importance of initial bulk of sheet delivered to the calender. In order to at least partially decouple bulk reduction from surface properties development, temperature and/or moisture gradient calendering technologies have been introduced. Since the temperature and moisture content increase the effect of calendering, heating or wetting preferentially the web surface allows for the opportunity to work more effectively on the surface with a lower impact on overall bulk.

Formation and initial surface topography characteristics of uncalendered sheet are of primary importance, as the ultimate goal of calendering is the uniform development of high gloss and surface smoothness. Additionally, even a minimal amount of deposits formed on the calender roll will lead to quality issues through their replication on the sheet surface or regular non-uniformity pattern caused by barring.

Formation

Formation of the paper is to a large extent determined by the furnish composition and papermaking equipment. The type of the former, amount of shear in the forming section, applied jet to wire ratio and the selection of forming wires influence formation significantly. Specific aspects related to general retention program development, application and control as related to formation are discussed below.

Retention program

Nalco’s survey of SCA machines in North America and Europe indicated that most of them use a relatively simple dual polymer program based on coagulant and cationic flocculant. This could be an indication that extra drainage offered by widely available microparticle programs may not best suit the unique needs of SC papers, as they may set the sheet too early, potentially leading to worse formation and excessive porosity and roughness. With advances made in the bleaching area and reduced levels of cationic demand and conductivities, this observation may be valid even more now than it was several years ago. One of the surveyed mills using microparticle program in the relatively “clean” wet chemistry experienced formation, porosity, print mottle and piling issues.

It is our experience that use of beta formation measurement better reflects formation relevant to the calendering process. A random survey of 41 European and North American SCA paper samples indicated large variation of beta formation values between 0.37 and 0.57g ^–0.5/m, with an average value of 0.43g ^–0.5/m.

Target white water

White water consistency seems to be a most important driver of formation when other factors (furnish, equipment) remain constant. It was observed in one mill system that a 30% increase in dosage of cationic flocculant did not result in beta formation change as far as target white water consistency remained unchanged.

Excessive free drainage

Drainage elements in the forming section act as de-flocculation devices and therefore improve formation. These improvements in formation are possible only until the sheet reaches a certain solids content. If the sheet drains too fast in the early stages of formation, its original flocculated structure is frozen and carried all the way to the calender and reel.

Polymer inversion

Since application post-screen should be pursued to ensure optimal chemical efficacy, the flocculant solution (either inverted latex or dissolved dry product) has to be absolutely gel-free and properly filtered.

Mixing and feeding

Uniform distribution of polymer through the cross section of approach pipe, within the limited time available for mixing (determined by adsorption kinetics), determines uniformity of the furnish treatment. Application of Computational Fluid Dynamics has allowed for determining ideal flow velocity (volume and delivery stubs diameter) required for optimum mixing of the retention aid into approach pipe. Novel mixing technology such as Nalco’s PARETO Wet-End Optimization Technology combines the appli- cation of process water (savings on fresh water and energy) with optimum feeding conditions.

System stability

1. Every retention program displays some degree of sensitivity to system water chemistry. Mechanical pulp-based paper grades are normally produced in highly integrated systems – from wood (chips or logs) to finished paper. Full integration and only limited separation between individual production stages, leads to a high potential for instability of wet end conditions. Bleaching processes are the major source of system instability. Identifying and addressing sources of variability in every mill system should be a continuous effort. An example of such effort was described earlier [T. Tomney, P. Pruszynski, J. R. Armstrong, and R. Hurley, Pulp Paper Can., 99(8): 66 (1998)]. All possible efforts have to be made to fully optimize bleaching processes, either reducing or oxidative, to reduce and stabilize the amount of chemicals used.

2. A proprietary enzymatic technology has recently been introduced by Nalco as a very effective treatment of anionic trash materials, collectively measured as cationic demand, formed in hydrogen peroxide bleached mechanical furnishes [P. Pruszynski, M. Quinn, B. Kamlin, L. Sherman, J. W. Shing and S. Govoni, APPITA Conference, Gold Coast, 2007, 335.

3. Closing retention program on the control loop with on-line white water consistency equipment is recommended only if a certain level of intrinsic stability is achieved. Targeting stable white water consistency in the system with excessive variability will result in large retention aid dosage swings and corresponding wet web quality variation affecting both runnability (presses) and sheet calenderability.

4. Specific Filler Retention -a strategy of increasing small particles retention without excessive fibre-fibre flocculation has been attempted in the past, mostly with the focus on filler retention level. An example of nonflocculative filler pre-treatment with low dosage of low molecular weight coagulant was described earlier [T. Tomney, P. Pruszynski, J. R. Armstrong, and R. Hurley, Pulp Paper Can., 99(8): 66 (19
98).

Sheet properties

Chinga [G. Chinga, JPPS, 30(8), 222 (2004)] summarized factors determining gloss level and light interactions with the paper surface. The author concludes that surface microstructure affected by calendering and affecting gloss is not detectable by air-leakage instruments. The author demonstrated that image analysis methods applied to surface topographical images are much better predictors of gloss development than surface roughness measurement. Correlation coefficients (r2) for determination of gloss using various surface descriptors reach up to 0.95 for PV and MFO and only 0.4 for PPS.

It becomes quite important that the microstructure of the paper, both calendered and uncalendered, is periodically evaluated using proper techniques. Nalco applied a variety of such techniques, including Low Angle Light Microscopy, Scanning Electron Microscopy (SEM) and Scanning Confocal Laser Microscopy (SCLM).

Filler distribution

Fillers and fines represent an excellent substrate for printing. Both gloss development and good printability require the presence of these particles on the surface of the paper. Incorporation of up to 35% of filler in the sheet without negatively impacting formation, low sheet two-sidedness and high surface filler content (U profile) are clearly challenging goals, but should be attempted with any SC retention program application. Traditional focus on white water consistency control may not be quite enough to achieve these goals.

Odell [M. H. Odell, Appita Journal, 53 (50, 371 (2000)] reviewed factors impacting sheet structure in terms of its two-sidedness and z-directional filler distribution. It is quite clear that filler and fines distribution follows a dewatering pattern in the forming section. With this in mind, it is not surprising that a fourdrinier former, with all of the dewatering occurring through the bottom side of the sheet, shows strong two-sidedness in terms of fillers, fines, smoothness and also high sheet anisotropy (variation in fibre orientation) between the bottom and top of the sheet. It is not unexpected to see paper produced on a fourdrinier former with 2-3 times higher filler content on the top compared to the bottom of the sheet.

Hybrid formers allow for dewatering to occur from both the bottom and through the top of the web, and after fine-tuning of the drainage patpg terns, these formers may provide quite symmetrical filler distribution in the z-direction, although typically the surface of the sheet may be washed of the fillers. Gap formers, specifically roll blade formers, may be tuned to a completely symmetrical drainage split. Varying pressure in the forming nip, wrap angle and vacuums, allows for modifying sheet structure in terms of fillers and fines distribution. Depleting the middle of the sheet of highly bonding fines fraction may lead to sheet delamination problems. The importance of forming wire selection on sheet two-sidedness can’t be overstated.

The degree of possible modification in the sheet structure is dependent not only on available equipment, but also furnish type and further correction is possible for more porous TMP than for denser SGW pulps.

Analysis of sheet two-sidedness and z-directional filler distribution should be a part of the routine examination for glossy, highly calendered paper products. A concerted effort on optimizing sheet structure for maximum calendering effects, despite the complexity of this process, should always be attempted and this effort will be rewarded with continuous improvements in sheet quality. In addition to a proprietary filler distribution technique, Nalco applies SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy), to quickly assess the amount of filler on both sides of the paper. Figures 1 and 2 represent examples of EDS and a sheet splitting analysis for top former and gap former machines. These two graphs are consistent in documenting the ability of a well-optimized top former to match or exceed symmetry in filler distribution provided by gap formers.

Calender deposits

SCA grades utilize significant amounts of mechanical pulps as a part of the furnish mix. It is quite natural for these grades that pitch control has been an important and ongoing concern.

Development of modern calenders made them increasingly sensitive to any deposits on calender stack rolls. These deposits lead to product quality deterioration, barring production losses due to downtime needed for cleaning, and a reduced lifetime of expensive calender rolls.

Pitch problems on the wet end of the paper machine are primarily related to unretained pitch that agglomerates and deposits under the destabilizing impact of high shear, temperature and/or pH shock. Pitch problems beyond the press section of the paper machine are, in contrary, related to this part of the pitch that has been retained and removed from the paper machine wet end within the sheet. There are a number of processes that take place when sheet travels through the dryer section that change pitch form, composition and distribution through within the sheet structure. Lack of differentiation between wet and dry end pitch control and application of the wet end pitch control tool may lead to defending the wet end of paper machine on expense of dry end pitch problems.

Pitch control

Purging -Every opportunity should be explored to purge pitch out of the system. Extractives mass balance and sheet extractives content should be measured regularly. Fixation as a control strategy should be performed with a defined cap of extractives level in the sheet as established based on a given machine experience. Based on the author’s experience, exceeding 0.6% sheet extractives content (Dichloromethane) may be detrimental to calendering. Reduced fixative dosage and increased system purging could also be considered when this level is reached.

Uniform distribution of pitch through z-direction of the sheet, or preferential retention in the middle of the sheet, may prevent it from arriving to the surface where it could contact the surface of the roll. The effectiveness of this approach could be determined through knowing the kinetics of pitch migration in various stages of its journey to the reel. Some data on AKD spreading rates exist [14] and suggest that this process may be slow enough that hopes for such kinetic limitation may be justified.

Pitch detackification -Organic or inorganic detackifiers have always been recommended as an augmentation of the fixation approach for wet end pitch control. Pitch immobilization through chemical reaction (anchoring, sintering) seems to be a very promising avenue to explore. A combination of fixation that provides a good initial z-directional distribution with pitch immobilization could further slow down its arrival to the surface and reduce its detrimental impact on dry end operations. Applications of alum or PAC combined with the application of aluminum chemistry and enzymatic treatment with Lipase could lead to increased content of acid fraction suitable for reaction with aluminum chemistry.

Pitch immobilization with inorganic absorbers such as talc and bentonite are already widely applied for pitch control. Some applications of zeolites and diatomaceous earth were also reported.

Conclusion

Calendering, a process that critically impacts final sheet properties of supercalendered grades, strongly relies on properties of uncalendered sheet. A producer of supercalendered grades will benefit from cooperation with suppliers, including specialty chemicals that understand, focus on and are capable of monitoring the specific technical requirements of these rades.

Przem Pruszynski is a Global Principal Consultant for the Graphic Papers SBU of Nalco Company in Naperville, IL. He holds a PhD in organic chemistry and has over 30 years of experience in the pulp and paper industry with a particular emphasis on wet-en
d chemistry and mechanical pulping. In addition, he holds several papermaking patents and travels globally visiting mills, providing consulting expertise.