Novel Wet End Technology Gives Mills Operational Flexibility

Papermaking operations covering several grades achieved benefits from improved strength to increased speed while reducing the need for more expensive furnish

March 2008
By Michael O’Byrne

True papermaking is more art than science. No other industry produces such a wide variety of products from the same basic starting material: wood fibers. Hundreds of different paper grades are produced, each with unique end properties that must be achieved. Therefore, the papermaker must skillfully manipulate all available resources and variables to consistently make a good product. This challenge is compounded if the paper machine is making numerous grades of varying weights and specifications. Papermakers also operate under limitations, such as furnish quality and undesirable wet end chemical interactions that can prevent them from improving quality, increasing productivity and developing new grades.

Fiber type and source are perhaps the most limiting factors to achieving sheet quality and machine performance targets. As the use of recycled fiber in the global papermaking industry grows, maintaining furnish quality becomes increasingly challenging. Recovered paper fibers now account for approximately 70% of the industry’s raw material needs.

Old corrugated containers (OCC) make up more than half of all recovered fiber tonnage, and are used to make nearly 50% of the corrugated and folding cartons found on store shelves today. As these fibers repeatedly cycle through the processes of papermaking, converting, end use and repulping, papermakers typically have to add more recycled furnish to the sheet to meet strength targets at nominal basis weight. This is not an attractive economic option due to the volatile recycle furnish market.

The paper machine wet end is one of the most complex and influential areas in the papermaking process. In a well-balanced wet end, additives impart sheet functional properties, improve machine performance, and increase operational flexibility. A chemically unbalanced wet end, often typified by overuse of additives, competing ions and performance variability, can severely limit sheet quality and productivity.

Although it is more easily said than done, improving and optimizing the wet end additive scheme can boost machine performance, expand operational flexibility and improve sheet quality, Based on these concurrent needs, Hercules has introduced a unique single-chemical additive called Hercobond® 6000 paper performance technology that is designed to provide step changes in the areas of strength, drainage and retention; thereby increasing operational flexibility. This article provides an overview of this technology, describes potential benefits for the papermaker and includes several industry case histories.

A New Class of Cationic Polymers

Hercobond® 6000 paper performance technology represents a new class of water-soluble, highly-charged cationic polymers. The performance of these polymers, which are synthesized from a pure monomer and then subsequently hydrolyzed, can be tailored to address the demands of different paper grades and wet end chemical systems. A highly flexible polymerization process allows for selecting a wide spectrum of molar mass and charge densities in the final product, which makes these products very versatile for use in papermaking. These products impart significant increases in dry strength properties; increase drainage and retention dramatically; and are highly effective in fixing anions, hydrophobes and sizing agents into the sheet.

Since this new technology is extremely cationic compared with traditional wet end additives, many opportunities exist to reduce and optimize other cationic additives such as starch, flocculants and coagulants, wet and dry strength resins and pitch and stickies fixatives. As the overall retention and cleanliness of the wet end improves, other additives such as size and defoamer can be optimized.

Figure 1 depicts various properties provided by traditional wet end additives compared with this new technology.

Figure 1 - Relative comparison of additive performance

Strength Performance

Consistently achieving good sheet strength is perhaps the most important and the most difficult to attain aspect of papermaking. Historical methods to improve strength included both mechanical and chemical means. When stock refining, formation improvement and wet pressing optimization are not enough to reach target strength levels, papermakers often turn to strength agents. Since the introduction of starch in the 1950s, the portfolio of dry strength additives has grown and evolved to meet higher sheet strength targets. An important advance made in the late 1990s was the introduction of glyoxylated polyacrylamides (GPAMs), cationic resins that proved particularly valuable at increasing compressive strength in board grades.

These technologies have their drawbacks: starch requires on-site cooking and high volumes while GPAM performs best in a narrow pH range, can interfere with sizing, and has a short shelf life.

Alternatively, Hercobond technology is based on chain-type macromolecule polymers that contain primary amino groups, which form strong hydrogen bonds with carboxyl groups on the fiber surface. The relatively flexible molecule backbone allows more effective bridging between fibrils. The result is a stronger inter-fiber network that contains significantly more bonding area than untreated paper. This stronger network is also more resistant to rupture under wet conditions. Paper treated with this new technology has greater temporary and permanent wet strength.

This new technology positively affects an extensive range of dry strength tests, including internal bond, ring crush, burst and tensile. In particular, significant strength improvements have been realized in grades containing high amounts of recycle furnish. The ability to improve strength properties across a wide range of grades and test requirements also brings the ancillary benefits of reducing refining; producing a lower basis weight sheet to meet strength targets; running a higher percentage of recycle fiber in the sheet; and increasing machine speed.

Drainage and Retention Performance

Increasing the level of drainage and dewatering is a common objective because many papermakers desire increased production, reduced energy costs and improved formation. Traditional microparticle and micropolymeric drainage aids allow increased machine speeds but drainage performance is often compromised by the need to maintain formation and sheet strength. For example, increasing refining levels and adding basis weight to the sheet help strength but hurt drainage. With more refining, fibers collapse and fines increase, which cause drainage to suffer and speed to decline.

Retention, typically a desired phenomenon, also affects drainage and strength. By fixing fines, fillers and anionic trash onto larger fibers, retention aids generally provide a more porous mat structure that facilitates faster drainage and provides a degree of deposit control. But retention aids can also negatively affect formation and strength through over-flocculation. Additionally, a floccy sheet is more difficult to dewater farther down the table. For this reason, traditional retention aids are not used to provide drainage. Holding more fines in the sheet can initially help tensile and crush tests; however, with too much retention sheet strength suffers because filler and fines particles can act as debonders.

This new technology combines the features of very high charge density with medium molecular weight yielding strong drainage benefits and high retention capability. This unique combination allows papermakers to run at higher machine speeds without sacrificing retention or formation. Since the molecule provides direct strength, strength loss at higher production rates is less of a concern. The use of this new technology in dryer-limited heavyweight grades has demonstrated speed increases of more than 150 ft/min and typical first pass retention values increased by 5-15 percentage points.

Expanding Operational Flexibility

The critical interactive balance of strength, drainage, and retention can sometimes provide a very small operating box in which to improve quality or manage productivity increases. Once the parameters of furnish quality, refining, dewatering and chemical feed rates have been manipulated to their fullest capacity, there is little else that can be done to increase quality or speed until the dynamics of this balance is changed. For example, a paper machine may be able to increase strength through refining but is limited by drainage or dryer steam capacity, Figure 2. The inability to dewater a highly refined furnish prevents the machine from making stronger paper and running at higher production rates. Consequently, the papermaker’s ability to manage the machine is limited.

Figure 2 - Drainage limitations frequently inhibit paper machine flexibility

This new technology, with its ability to directly improve strength, drainage, and retention effectively expands the operating box, providing a higher level of operational flexibility. Figure 3 shows that the machine is no longer drainage-limited and can reach its full strength potential.

Figure 3 - Hercobond 6000 expands the operating box by increasing strength, drainage, and retention

Table 1 shows how this new technology is helping papermaking operations across several grade segments achieve benefits in strength, furnish savings, runnability and production.

Wet End Simplification

The value of running a lean and efficient wet end, and the resulting beneficial effects on paper machine system operation, cannot be underestimated. Reducing unneeded additives and the associated collateral damage can make a tremendous effect on paper machine efficiency and the production of quality product. Understanding the purpose of each additive is easy; figuring out what they are actually doing to the sheet and to each other is much more complex.

Since this new technology delivers multiple benefits with a single product and is highly cationic, it has very successfully allowed mills to dramatically optimize wet end chemistry. Documented benefits include direct replacement of up to 15 lb/ton of wet end cationic starch, elimination of cationic coagulants and anionic drainage aids, and 75% reduction in high molecular weight retention aids. Wet strength resin and size usage have also been positively affected with reductions of up to 25% in highly-sized wet strength grades. These extensive chemical reductions combined with improved additive retention also result in a cleaner wet end that is less prone to machine deposition problems.

Case Histories

The following case histories, taken from commercial applications in different grade segments, demonstrate the ability of Hercobond 6000 to provide a higher degree of operational flexibility through improved strength, drainage and retention.

Recycled linerboard: A US mill producing 100% recycled linerboard wanted to increase production and to reduce basis weight without losing strength as measured by ring crush. This mill was using a cationic dry strength resin in combination with internal starch to meet strength targets. Hercobond 6000 replaced both strength additives while increasing machine speed 60 fpm and reducing basis weight 0.5 lb. Ring crush improved by more than 3% over baseline conditions, Figure 4. The removal of starch in this closed system allowed the mill to significantly optimize biocide feed rates, which substantially reduced the program cost.

Figure 4 - Hercobond 6000 enables significant ring crush improvement in 100% recycled linerboard

Uncoated groundwood, SC-A: A Canadian mill producing supercalendered grades (SC-A) was using up to 35% kraft fiber in the sheet to meet tensile strength requirements. Starch was also being used but was causing wet end upsets during grade changes. In addition, low sheet surface strength was causing printability issues in the pressroom. Converting to Hercobond 6000 allowed the mill to completely eliminate the starch program and to reduce the percentage of kraft fiber in the sheet by 3% to 4%, representing a significant cost savings to the mill; increased tensile strength by 4%; and improved surface strength as measured by IGT pick resistance, resulting in reduced linting and highly improved ink snap, Figure 5.

Figure 5 - Hercobond 6000 provides improved strength tests and reduced kraft use for an SC-A producer

Recycled towel: A Canadian producer of 100% recycled industrial towel and napkin grades wanted to increase production to meet demand. Loss of tensile strength properties and wide swings in retention performance prevented speeding up the machine. The drainage that resulted from implementing a Hercobond 6000 program allowed the machine to speed up an average of 100 ft/min without losing tensile strength.

First pass retention increased 20 points, from 70 to 90. The standard deviation associated with the swings in retention performance was reduced from 15% to less than 5%. The incumbent cationic dry strength resin program was discontinued and wet strength usage decreased by 30%, reducing overall chemical treatment cost significantly, Figure 6.

Figure 6 - First pass retention improves for a 100% recycled towel with Hercobond 6000

Michael O’Byrne is product launch manager, Hercules Paper Technologies and Ventures