Smmaries of November 2004 peer-reviewed papers.

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COATING

OPERATIONAL ISSUES IN HIGH-SPEED CURTAIN COATING OF PAPER, PART 1: THE PRINCIPLES OF CURTAIN COATING

APPLICATION: This paper reviews the basic principles of curtain coating and discuss origins of coating defects. It provides operational insights pertaining to high-speed application of highly concentrated aqueous paper coatings.

Curtain coating is emerging as a new coating process for specialty and printing paper grades, and for paperboard. The method features a liquid sheet that falls freely before impinging onto the moving paper web to be coated. High impinging velocity allows coating at relatively high web speeds. This non-contact coating method does not put mechanical stresses onto the substrate to be coated, thus minimizing web breaks and providing good contour coating coverage, which offers opportunities for improved productivity and reduced coat weight. An operational stability diagram governs the process, which needs to be determined a priori for optimum operation under specific conditions. In spite of its advantages, the curtain coating process is susceptible to operational defects, such as irregular heel formation, air entrainment, and curtain instabilities. These problems can be avoided by formulating coating specifically for curtain applications and optimizing curtain operating parameters. Successful implementation of curtain coating for highly pigmented aqueous coatings at high speeds requires removal of the thin air layer entering the curtain impingement zone on the paper surface to avoid air intrusion, which disrupts coating transfer. We must also suppress the formation of small air bubbles entrained into the coating (i.e., de-aeration). View this paper online at http://www.tappi.org/index.asp?pid=31094

Nick Triantafillopoulos is with RohmNova LLC, Akron, Ohio, USA; Johan Gron is with Stora Enso, Publication Paper, Kymemlaasko, Finland; Iiro Luostarinen and Petri Paloviita are with Metso Paper Inc., Jarvenpaa, Finland. Email Triantafillopoulos at [e-mail].

DILUTION

CONFINED SHEAR DILUTION OF A NYLON FIBER SUSPENSION

APPLICATION: Findings may be used to better understand the behavior of fiber suspensions in a dilution shear flow, which should eventually lead to more efficient control of thick stock dilution.

What are the fluid dynamics at work in thick stock dilution, and how can we measure and control the process?

We researchers used nylon fibers to isolate some of the aspects of diluting pulp and eliminate some of the complexities involved with natural wood fibers. Synthetic fibers have more uniform optical properties as well as a more uniform fiber length distribution. Nevertheless, nylon fiber suspensions form flocks and networks and tend to flow in ways that resemble wood fiber suspensions.

We introduced a stream of dilution water to a stream of nylon fibers suspended in water in a rectangular test section made of transparent Plexiglas. Using a digital camera and image analysis, we looked at how fiber consistency, fiber length, and stream velocities influence the wake boundary angle of a fiber suspension jet in a dilution shear flow.

The velocity difference between the two streams was found to be the primary factor affecting the boundary angle, increasing the angle from 2[degrees] degrees to as high as 9[degrees] before the onset of recirculation. The boundary angle was found to be negatively correlated to the mass concentration of fiber suspension and fiber length. The boundary angle was reduced at a given velocity difference if the fiber suspension jet flow regime was not fully turbulent. View this paper online at http://www.tappi.org/index.asp?pid=31095

When this research was conducted, Eric Andrew Schmidt was with the Institute of Paper Science and Technology, 500 10th St., NW, Atlanta, GA 30318 Schmidt is now with the Swedish Pulp and Paper Research Institute, Box 5604, SE-114 86 Stockholm, Sweden. Junyong Zhu is with the USDA Forest Products Laboratory, One Gifford Pinchot Dr., Madison, WI 53705. Email Schmidt at [e-mail]

DRY SURFACE TREATMENT

POLYMER COATING OF PAPER USING DRY SURFACE TREATMENT: COATING STRUCTURE AND PERFORMANCE

APPLICATION: Dry surface treatment is a solventless, noncontact process that should let papermakers use materials not practical to use in extrusion and wet-laid coating.

Dry surface treatment (DST) is quite different from conventional polymer coating techniques, such as extrusion coating, solvent-based and water-based coating. In DST, layering, softening, and immobilization of the polymeric coating are accomplished within milliseconds. In DST, the low thermal impact on the polymeric coating could allow papermakers to use materials not practical to use in conventional coating processes.

We studied the effects of polymer thermomechanical properties and fixation conditions on the structure and performance of coatings prepared in a laboratory-scale DST prototype. The governing factors are polymer particle size, thermal conductivity, softening temperature, and melt viscoelasticity.

At low coat weights, the prerequisite for homogeneous layers was fine particle size distribution. At high coat weights, enhancing polymer flow during fixation to obtain a coherent, uniform film was increasingly important. Small particle size, high polymer deformability, and high coat weight were beneficial in sealing the paper substrate. Prolonging the dwell time in the DST nip to enhance polymer flow may not be a promising solution because of the problem of sticking. A better way to reach a balance between polymer characteristics and fixation conditions is through polymer adaptation.

DST offers a potential alternative for manufacturing coated and laminated structures. With further work, it may be possible for the process to succeed on the industrial scale. View this paper online at http://www.tappi.org/index.asp?pid=31096

Kaisa Putkisto and Juha Maijala are with Tampere University of Technology, Inst. of Automation and Control, Paper Machine Automation, P.O. Box 692, FIN-33101 Tampere, Finland. Johan Gron is with Metso Paper Inc., Surface Treatment Technology, Wartsilankatu 100, FIN-04400 Jarvenpaa, Finland. Mikael Rigdahl is with Chalmers University of Technology, Dept. of Materials Science and Engineering, SE-41296 Gothenburg, Sweden. Email Putkisto at [e-mail], Maijala at [e-mail], Gron at [e-mail], or Rigdahl at [e-mail].

BLACK LIQUOR

PREDICTION OF CRYSTAL SPECIES TRANSITION IN AQUEOUS SOLUTIONS OF [Na.sub.2]C[O.sub.3] AND [Na.sub.2]S[O.sub.4] AND KRAFT BLACK LIQUOR

APPLICATION: This model, based on a new fundamental understanding of crystallization during black liquor evaporation, may be useful for resolving soluble scale fouling problems.

Many of the fouling problems that occur in black liquor evaporators operating above 50% total solids content are the result of crystallization of sodium carbonate ([Na.sub.2]C[O.sub.3]) and sodium sulfate ([Na.sub.2]S[O.sub.4]). Recent fundamental studies reveal that the crystals that produce scale are a carbonate-rich double salt referred to as dicarbonate. We used data from Shi (2002) to develop relationships for crystal composition as functions of liquid phase composition, and apparent equilibrium expressions for four characteristic crystallization regions (sulfate, burkeite, dicarbonate, and carbonate). The resulting model can be used to predict the onset of crystallization for a solution of arbitrary composition, the changing concentrations of the sodium salts in solution, and the changing composition of the crystals formed as evaporation proceeds. If the initial solution composition produces crystals of burkeite, the model correctly predicts that continued evaporation of water and precipitation of burkeite will cause a transition of the crystallizing species to dicarbonate. View this paper online at p://www.tappi.org/index.asp?pid=31098

Alfi P. Soemardji and Hans Theliander are with the Department of Chemical Engineering and Environmental Science, Chalmers Institute of Technology, Gothenburg. Sweden; Christopher L. Verrill is with the Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, Georgia, USA; William James Frederick, Jr. is with the School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta. Email Verrill at [e-mail].

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