Chemical additive application on dryer cylinders and fabrics offers an alternative to conventional mechanical control methods

Passivation Technique Reduces Dryer Section Deposits, Dusting

For every papermaker, a balanced wet end chemistry is as equally important a concern as the mechanical aspects of Papermaking. Press fabric conditioners and press roll release products are commonly used to improve the functionality of the press section. However, no effective method exists to improve the dryer section using applied chemistry.

Most of the production and quality limitations of a dryer section related to chemistry are caused by deposition of contaminants in this section. Such deposits can take many different forms, ranging from a thin invisible film on dryer cylinders that causes sheet picking, fiber rising, and dusting to a much thicker, visible deposit on dryer fabrics or dryer cylinders. Deposits of this nature reduce moisture removal. If the deposit is located on the dryer cylinder, it acts as a thermal insulator. If the deposit is located on the dryer fabric, it impairs the moisture removal. Reduced productivity and/or an uneven moisture profile are the result of such deposits.

A number of industry trends have increased the potential for deposition in the dryer section, thereby making it more difficult to maintain high machine productivity and paper quality:

• Closing the water system increases the accumulation of contaminants in the system. Such accumulation may lead to increased deposition. Chemical methods used to prevent the accumulation of contaminants in the wet end rely on retaining contaminants with the paper. The increased load of contaminants in the paper increases the potential for deposition and dusting in the dryer section.

• Use of recycled paper is often associated with increased problems in the dryer section. Many components used in converting paper inherently increase deposition when they are recycled, and the fiber in recycled paper is weaker and contains more fines fragments. Increased deposition and increased dusting in the dryer section are often the consequences of introducing recycled paper into the Papermaking process.

• Functional additives may cause adhesion and deposition in the dryer section. Latex, used in the coating process, can cause deposition in the dryer section when it is recycled as coated broke. Dry strength additives, applied before sheet formation, are natural adhesives that increase sheet adhesion in the dryer section, thereby increasing the tendency for picking, fiber rising, and dusting.

The first part of the dryer section, in particular, is subject to deposit formation, fiber rising, and dusting. The temperature difference between the sheet and the hot cylinder surface promotes deposition and sheet picking. Also, the high moisture of the sheet in the early part of the dryer section promotes deposition. In the later part of the dryer section, when the sheet temperature is higher and the moisture is lower, there is a significantly lower tendency for transfer of the contaminant from the sheet to a dryer cylinder or a dryer fabric.

CONVENTIONAL CONTROL METHODS. A number of conventional methods for controlling deposition and dusting in the dryer section are currently available. These include permanently modifying the surface of the dryer cylinder and fabric, mechanical removal rather than prevention, continuous chemical methods, and reducing productivity.

Methods are available to modify the surface of the dryer cylinder and the dryer fabric so that equilibrium is shifted away from the transfer of the contaminant from the fiber to the surface. This has been applied in different ways:

• Teflon coating of dryer cylinders has been evaluated and is being used, although to a very limited extent. Teflon coating significantly reduces the deposition on dryer cylinders. However, it also reduces the heat transfer, as this is an insulating material; it is sensitive to scratches; and it quickly deteriorates. Chromium plating of dryer cylinders is another method used to prevent deposition on this surface.

• Dryer fabric has been developed with surface properties that reduce deposition. A popular combination is a blend of polyester and polyvinylidene fluoride. Practical experience with this combination has shown that it can reduce the buildup of deposits. However, no fabric is available that permanently prevents these buildups.

Methods are available based on removal rather than prevention. Such methods are all mechanical:

• Doctor blades are used to remove deposits building up on dryer cylinders.

• High-pressure showers have been developed and are used to remove deposits building up on dryer fabrics.

Continuous chemical methods are used to reduce contaminant related problems in the dryer section. However, all such methods are applied in the wet end before sheet formation. These methods include the use of:

• Cationic polymers applied in an attempt to bind the contaminants more firmly to the fibers, reducing the possibility that the contaminants will be transferred from the fiber to dryer surfaces.

• Strength additives applied before sheet formation. This method is based on the theory that increased sheet cohesion will reduce fiber rising and dusting.

• Dispersants applied to stabilize the colloidal contaminants.

Finally, the paper industry often makes compromises by reducing productivity in order to overcome dryer section deposition and dusting. The following practices are not uncommon:

• Using the first series of dryer cylinders only to warm the paper gradually, thereby minimizing sheet picking and deposit buildup.

• Running with fabric tension below optimum to minimize dryer fabric deposition, reduces the contact between the sheet and the hot dryer cylinder, which reduces the rate of moisture removal.

• Removing the bottom fabric or removing both the top and bottom fabrics. This reduces the amount of dust forming in the dryer section, and consequently reduces the number of breaks.

General experience is that these methods can be effective, but that they do not provide the ultimate solution to the problem of dryer section deposition and dusting. Therefore, even with all the above methods combined, a need still exists for a more effective program.

Hercules Incorporated is currently developing a technology that combines equipment, chemistry, and application expertise to improve the functionality of dryer operations. The technology not only reduces deposition and dusting in the dryer section, but also provides opportunities to improve the Papermaking process. Further improvements to this continuous chemical method are being made at a fast pace. By integrating this program with other programs it will be possible to establish a millwide program for total chemistry management.

DRYER SECTION PASSIVATION. Two different types of surfaces can benefit from passivation in the dryer section—dryer cylinders and dryer fabrics. Dryer section passivation, for which patents have been filed, is currently being developed by Hercules based on a license from Maintech Co., Ltd. In a single system, this technology integrates specially designed application equipment, a continuous application of chemical additives on dryer cylinders and dryer fabrics, and application expertise.

Applying chemistry in the dryer section is fundamentally different than applying chemical additives in the wet end. For this reason, specially designed equipment has been developed for the application of products in the dryer section. Figure 1 shows the equipment used to apply the product to the dryer cylinder. This equipment comprises an atomizing nozzle that traverses the full width of the dryer cylinder. Other application equipment is also available.

FIGURE 1: A typical application arrangement might include four application points used to passivate the dryer section. In the photo inset, one method of applying the passivation additive comprises an atomizing nozzle that traverses the full width of the dryer cylinder.

An effective program must be able to prevent adhesion and deposit formation on all surfaces the sheet comes in contact with up to the point when there is no longer any tendency for contaminants to transfer from sheet to surface. The application of dryer section passivation follows simple application principles. Each fabric that has a tendency for deposition should be treated separately. Dryer cylinders are typically treated by applying the product directly to the first heated cylinder on each side of the paper. This application passifies the treated cylinder, as well as subsequent cylinders, on the same side of the paper.

The paper itself transfers the product to the cylinders subsequent to the cylinder that was treated. Figure 1 shows a typical application arrangement. In this arrangement, four application points are used to passivate the dryer section.

PASSIVATION MECHANISM. Deposition in the dryer section is a dynamic process that progressively transfers contaminants between the fiber and the surface. Figure 2 illustrates the bi-directional transfer of contaminants.

FIGURE 2: An illustration shows how contaminants are progressively transferred between the fiber and the dryer cylinder surface.

The nature of the surface onto which the contaminant may transfer is highly significant. Surface roughness and energy are two key parameters. Increased roughness of the surface favors transfer of contaminants from the sheet to the surface. Applying doctoring mechanically removes the deposit but roughens the dryer cylinder surface. The more effectively this is done, the rougher the surface becomes, as illustrated in Figure 3.

FIGURE 3: As the dryer cylinder is doctored, the cylinder surface becomes rougher, thus increasing the chance of increased contamination.

Lowering the surface energy, the other key parameter, shifts the equilibrium away from the transfer of the contaminant from the sheet to the surface. This is why Teflon and other materials with a low surface energy have less tendency for deposition.

The dryer section passivation program minimizes deposition by reducing surface energy and roughness. First, the program provides lubrication to the wear between the doctor blade and the cylinder. Over time, this lubricated wear provides a much smoother cylinder surface, which is one reason why the program is effective. Figure 4 illustrates the effect on the cylinder of lubrication as the doctor blade removes deposits. With lubrication, the doctor blade cuts away smaller pieces of the cylinder, which leaves a smoother cylinder surface, and therefore generates less dust.

FIGURE 4: The passivation program provides lubrication for the doctor blade and the cylinder, which means a smoother cylinder over time.

Secondly, the program provides a protective coating on the cylinder. The thickness of this coating, which is only a few micrometers, is sufficient to change the properties of the surface by reducing the surface energy. However, the coating is too thin to have any negative impact on the heat transfer from the cylinder to the wet sheet.

For this reason, the appearance of a passivated dryer cylinder is totally different from an unpassivated cylinder. A passivated dryer cylinder appears shiny, like a black mirror, unlike the typical mat appearance of an untreated cylinder. The heat transfer of the modified surface is improved because deposition of thermally insulating material has been eliminated or reduced.

PRACTICAL RESULTS. The application of dryer section passivation has produced some impressive results, as illustrated in the following three examples.

Example 1: Demonstration of effect. A portable demonstration unit can be used to illustrate the effectiveness of the program. With this example, a one-meter wide section of the dryer cylinder is treated, with a considerable visual difference in appearance.

Example 2: Reduction in breaks and dusting. The program was established on a wastepaper-based corrugated medium machine that was experiencing considerable dusting. Aggregation of dust bundles on the machine caused breaks in the dryer section. A dryer section passivation product was applied directly onto the first and second dryer cylinders. The program significantly reduced the number of breaks per month and the amount of dust generated per eight-hour shift. Figure 5 shows the data collected on breakage and dust before and after application.

FIGURE 5: The charted results of one passivation program application shows a decline in the frequency of sheet breaks and a reduction in the amount of dust.

As indicated in Figure 5, a direct correlation exists between dust formation and breaks on this machine. A 70% reduction in dust formation yielded a corresponding reduction in breaks.

Example 3: Fabric application reduces deposition and spots in the paper. A linerboard machine (1,500 fpm, 18 ft wide) had a serious problem with defects in the paper. The defects were caused by deposits that formed on the dryer fabric and transferred to the dryer fabric rolls, particularly the stretch roll. Over time, the buildup formed thick deposits. Eventually, pieces of the deposits transferred back to the sheet and formed visible spots. This forced the mill to stop frequently to clean fabric and rolls. Application of dryer section passivation on the first top fabric kept it clean and drastically reduced deposition. The customer was so pleased that they extended it to cover the top fabric in the second section, also.

THORD HASSLER is business development manager, Hercules Incorporated, Pulp and Paper Div., Wilmington, Del., and HIROSHI SEKIYA is international business division director, Maintech Co., Ltd., International Business Div., Tokyo, Japan.