Dry strength additives for a competitive edge

By Dan Denowski, Global Market Development Manager - Paper, Buckman

BRUSSELS, Oct. 14, 2009 (RISI) - A large segment of multi-ply board production is manufactured on machines in excess of 50 years old. These board manufacturers have succeeded by finding niches where they can be competitive in the marketplace. Some of the grades manufactured include coated and folding box board, corrugating medium, gypsum board liner, chip board, core board, and tube stock. The fiber furnish is typically 100% recycled fiber from many sources. Increased emphasis on recycling in both the public and private sectors continues to provide the fiber source for many board mills. As fiber reuse rates increase, the overall quality of the fiber begins to deteriorate as individual fibers get recycled multiple times.

Meanwhile, at the mill level, significant attention and effort have resulted in highly efficient reuse of water and fiber, reducing effluent treatment costs while driving higher fiber yield in many systems. Reduction or in some cases, elimination, of effluent essentially eliminated a purge stream for both contaminated water and fines. As a result, whitewater systems are becoming a more challenging environment for traditional papermaking chemistries. Higher whitewater dissolved solids and conductivity negatively impact the performance of many papermaking additives.

Figure 1 - Mutek charge: micro-equivalents/liter

The case in point

Combined, higher recycling rates and efforts to reduce mill effluent have resulted in a gradual loss in strength of the board produced. One multi-ply board machine struggled to meet tensile specifications on standard grades. In addition, the mill manufactures a specialty high strength board representing about 8% of its total production. In both grades, to maintain tensile within the required minimum specification, it increases basis weight. Increasing basis weight results in several areas of added manufacturing cost:

• Higher basis weight makes the board more difficult to dry, requiring more steam while reducing machine speed.
• Increased fiber use further stresses an already overloaded stock preparation system, which reduces the system's ability to remove contaminants. As a result, more contaminants from the incoming recycled fiber reach the paper machine, leading to an increase in sheet defects and web breaks.
• Since the mill sells its board based on area and not on weight, the added basis weight required to maintain tensile within specification drives up overall fiber costs significantly.

The mill ran an evaluation of Bubond 408, a new, proprietary high-solids cationic dry strength resin. Higher active solids than traditionally possible without sacrificing shelf life result in reduced freight and logistics issues delivering product to the mill. Bubond 408 is also significantly more cationic than traditional glyoxalated polyacrylamide dry strength products. This additional charge is beneficial in mills with low to no effluent flow and high levels of whitewater reuse, making the dry strength resin more effective.

Figure 2 - Standard grade: tensile

Figure 3 - Standard grade: basis weight

The dry strength resin was added to the suction side of the fan pump for each ply at a rate of 1 dry kg/ton on the mill's standard grade. Within two hours, personnel noted a significant increase in tensile. After two consecutive high tensile tests, the mill began to methodically make small reductions to the basis weight throughout the rest of the first day. As expected, between the effects of the dry strength resin and the reduced basis weight, steam demand in the dryer section also decreased by 7%. By the end of the first day running on the standard grade, the basis weight was reduced by 6% while the tensile remained within specification and continued through the entire run.

During the evaluation, the mill changed to the specialty high strength grade, maintaining the Bubond 408 dosage at 1 dry kg/ton. Normally, without dry strength, to maintain tensile specification on this grade, it increases basis weight as much as 11% above the 230 g/m² target. With Bubond 408 in the system, the basis weight was actually running 2% below target while the tensile remained in specification. Fiber savings exceeded the cost of the dry strength by a factor of 2.5.

Figure 4 - High strength grade: tensile

Figure 5 - High strength grade: basis weight

Benefits to the papermaker

In this case, the mill chose to reduce steam use rather than speed up the machine as drying rates improved. However, another option could be to maintain steam use while increasing machine speed. Different market situations dictate different strategies for capitalizing on the benefits of an effective dry strength program. While this business case emphasizes basis weight and steam usage, there are many other potential sources of return:

• Reduced furnish cost by substituting lower cost, lower strength fiber sources
• Improved capability by enabling a mill to make new grades with strength levels not attainable without a dry strength program
• Reduce or eliminate wet end starch, which has the potential to reduce effluent BOD, reduce maintenance and operating costs for the starch system, and improve paper machine clothing life and cleanliness
• Reduced refining energy input, leading to electrical savings as well as either faster machine speed or less steam use
• Improved paper machine runnability as a result of higher web strength
• Reduction of other paper machine additives.

It is important to recognize that an effective dry strength program is simply a tool for the papermaker. The dry strength program increases the flexibility and helps to expand the "operating window" for a given paper machine and grade. In doing so, it offers a number of ways to deliver significant financial return beyond the cost of the chemistry.

Pulp & Paper International is FREE to qualified subscribers. Click here to find out more.