Synergistic effects from performance chemicals.
Rarely do these additives act independently, so it is important to look at their interactions. The following discussion should give you a few ideas that might lead to a bigger bang for your buck when using performance chemicals.
Before we start, let us review what we mean by performance chemicals and synergy. A performance chemical is any chemical that imparts a special benefit to the user. That essentially applies to all additives, to varying degrees. Synergy is a term used to imply that the sum of two or more components is different than the effects taken individually. Most of the literature discusses synergy in terms of a positive effect, but the effects can also be negative.
COAGULANTS AND RETENTION AIDS
Coagulants (scavengers) are used in broke streams and in other contaminated stock streams to neutralize charge and/or to tie up contaminants. These additives, when applied properly, with proper controls, will almost always lower the pounds per ton requirement for short loop retention aids. The reduction in the high molecular weight retention aid frequently leads to improved formation and density at the same retention level. A negative synergy is possible if the coagulant over-charges the system, causing a desorption and loss of efficiency of short loop additives.
Mills use coagulants or low molecular weight retention aids to improve forming zone drainage and saveall efficiency. These applications will have similar effects as those for broke and stock stream coagulation.
STARCH AND RETENTION AIDS
Wet-end starch, when required, imparts strength to paper. It is also a good source for cationic charge and frequently has a positive impact on retention and drainage. On fine writing and printing papers, I have often seen a reduction in retention aid requirement with the application of wet-end starch, which was needed for internal size retention. With the advent of more highly charged starches, there has been a coordinated effort by some suppliers to use highly charged starches to overcome dirty systems and positively affect strength, retention, and drainage. Starch can over charge a wet end and decrease efficiency.
DISPERSANTS AND RETENTION AIDS
This interaction is almost always negative. Dispersants are used throughout the process to wash out contaminants or to assist biocides in penetrating and killing organisms. In either case dispersion is the opposite of retention, and dispersant users should always be aware of the downstream negative effects on retention aid packages.
One example demonstrating the importance of these synergies and on-line control occurred on a machine with an on-line retention monitor. The technical staff noticed a periodic decrease in retention, followed by a recovery to normal retention. Further investigation showed that the drop corresponded to the intermittent biocide feed. The dispersant in the biocide required for organism killing was counterproductive to the retention aid. The mill switched to a more rapid on-off cycle for the biocide feed and made addition point changes to drop retention variation to an acceptable level.
MICROPARTICULATES ACTING TOGETHER
Microparticulates are derived from silica, bentonite, or synthetic materials. Mills use them in combinations with starch and retention aids to impart a variety of retention, drainage, strength, formation, and runnability benefits. With the consolidation of chemical suppliers, many of these technologies have come under the control of a single supplier. With this access to many technologies, the industry is beginning to see microparticulates used in combination to produce packages that can improve retention and formation at the same time. The combinations may increase drainage while holding formation at the current acceptable level. Formation may be improved without losing retention or drainage. These synergistic systems are in the early stages, but the possibilities to the user are potentially great. The negative here is that the increased cost must be weighed against the benefits gained.
DEAERATORS AND WET END ADDITIVES
Mills most often use defoamers or deaerators to prevent foam and deposits. However, they also have a positive influence on wet-end additives. Additives require a clean and properly charged fiber or fine surface for chemical adsorption and particle attachment. If there is too much air in a system, the fiber or fine surface will contain microbubbles that prevent the attachment of other additives. European mills more carefully control defoamer and deaerator use than do U.S. mills. Consequently, the European mills are experiencing an improvement in chemical cost and machine runnability by properly monitoring and controlling defoamer and deaerator addition.
STARCH AND INTERNAL SIZE
Internal size, either alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD), is difficult to retain when used alone. Cationic starch is one chemical used to improve size retention. In some cases, the starch is added at the mill in an emulsifier. In other cases the starch is added by the size manufacturer as a promoter. Either way, the combination of the two chemicals improves size efficiency and paper strength.
COAGULANTS AND INTERNAL SIZE
Coagulants and scavengers aid in the retention of size particles in two ways. Coagulant added to the internal size increases particle attraction to the fibers and fines. Coagulant can also clean the wet end of contaminants, thus presenting a cleaner fiber or fine surface to which the size particle can attach. The challenge is to maintain the charge balance. Over-charging the system will lead to desorption of additives and particulates.
INTERNAL AND SIZE PRESS SIZE
It is possible to reach a paper size requirement through refining, internal sizing, or surface sizing. The papermaker, after obtaining as much sheet size from refining as possible, may seek further help from chemical additives. The mill can add the sizing chemical at the wet end, the size press, or both. The dual addition to the wet end and the size press frequently improves sizing efficiency, lowers cost, and cuts down on the wet end deposit potential. The success of this approach depends on machine configuration and process parameters, but it is works often enough that it should be considered.
ALUM FOR ALKALINE SYSTEMS
One of the most interesting positive synergistic effects over the past several years has been the use of alum in alkaline papermaking. Alum retention chemistry theory would never suggest that alum could be used effectively at alkaline pH. Nevertheless, it can and does work to improve alkaline sizing and retention. Early on in the alkaline conversion story, alum was found to stabilize the ASA emulsion while providing a more uniform particle size distribution. As this ASA-alum emulsion was applied in papermaking, positive effects were seen in increased retention and machine cleanliness. Alum was then tried by itself and the same positive effects were documented. Now mills commonly use alum in alkaline papermaking for retention improvement and sizing efficiency. The down side is that alum can form deposits at alkaline pH if overused and not properly applied.
ALUM AND ROSIN SIZE
The positive effect of alum and rosin forming an alum-rosin complex to impart internal size at acid pH has been known for many years. It is appropriate to include it here as a positive synergistic effect between two very different chemicals. As with any combination, care must be used in the proper ratios of the additives, method of addition, and points of addition or the result can be deposits and poor efficiency.
TALC FOR PITCH CONTROL
Pulp mills frequently use talc to control pitch and other stickies. What is often missed is that the talc also acts as a filler in the paper being produced. Although talc may not be as bright or as opaque as ground or precipitated calcium carbonate (GCC and PCC), it is still an acceptable filler for fiber replacement. Therefore, it is possible to use talc as a deposit control additive and gain its benefit as a filler for fiber replacement. In my experience, when used at the normal pulp mill pitch control level of 10-15 lbs/ton, there are no negative effects to the papermaking process.
PRECIPITATED AND GROUND CALCIUM CARBONATE
These two carbonates have competed for many years, but there are many instances where they are used together to impart positive synergies. Each particle has its positives and negatives that can be balanced to reach specific papermaking goals. By picking and choosing the positives from each technology, papermakers can optimize cost and sheet runnability, physical characteristics, and optical properties.
TITANIUM DIOXIDE EXTENDERS
Titanium dioxide (Ti[O.sub.2]) is a valuable pigment used by the papermaker in the wet end and on coaters to impart opacity and brightness. Because Ti[O.sub.2] is expensive, a wide range of extender pigments are marketed for use with Ti[O.sub.2] to help achieve the required optical properties at a reduced cost. Usually these extenders are more easily retained than Ti[O.sub.2] because they have lower dispersant requirements. Papermakers should pay attention to the dispersant level used in any of these pigments because of the potential negative effect on machine retention.
COLORANTS AND COLORANTS
This may seem a strange heading but in fact, colorants affect each other both positively and negatively. Dyes and whiteners are added in combination to achieve target shade, hue, intensity, brightness, opacity, etc. When papermakers manage colorants correctly, they can reach the desired targets and there are few problems. This is a truly synergistic combination because it is rarely possible to hit targets with only one or even two dyes.
These dyes can also compete with each other. The order in which they are applied can be significant; be careful not to add them randomly. Some of the dyes are cationic and others are anionic. Be careful not to add such dissimilar dyes too closely to each other or they may interact before adsorption onto the fibers. Even when the dyes are all anionic, they can still form hydrogen bonds with one another and should always be added separately to promote fiber adsorption.
COLORANTS AND OTHER ADDITIVES
There are both positive and negative interactions between dyes and other wet-end additives. Dye fixatives and coagulants are used to fix anionic dyes and thus are positive in combination. Anionic dyes and pigment dyes add a great deal of negative charge to the wet end, negatively affecting the wet end cationic additives. Even when an anionic dye is properly laid down on the fiber surface, the increased negative charge will place an additional burden on the typical cationic retention aid or coagulant.
DEBONDERS AND DEFOAMERS
Debonders are often used in tissue and towel manufacturing to impart softness, bulk, and absorbency. These chemicals are almost always surface-active agents and cause increased foam potential. Increased defoamer and deaerator use may be needed. The negative is an increase in chemical cost, while the positive is being able to satisfy a customer's need for bulk, softness, and absorbency.
The examples in this article represent many of the possible synergistic effects between papermaking additives, both positive and negative. There are likely many more. The important point is that pulp and paper mills have powerful allies in their performance additives. These additives--used alone or in combination--satisfy specific quality and cost needs. Papermakers should always consider additives' potential interactions and apply them in ways that optimize the value equation for that particular use.
WHAT YOU WILL LEARN
* How synergy from performance chemicals can be positive or negative.
* Problems generated from the interaction of dispersants and retention aids.
* How specific chemicals work in the "real world" of the mill.
* To access the following documents, go to www.tappi.org and type the Product Code in the search engine. Articles from Solutions! are available free to TAPPI and PIMA members.
* "A marriage for performance sake," by Kasy King, Solutions! April 2003 TAPPI Product Code: 03APRSO29.
* "Wet end management: Paring down, pairing up," by Jan Bottiglieri, Solutions!, December 2002. TAPPI Product Code: 02DECSO26.
* "Commodity, performance, or specialty chemicals?," by Kasy King, Solutions!, August 2002. TAPPI Product Code: 02AUGSO42.
ABOUT THE AUTHOR:
C.A. "Kasy" King is principal of Papermaking Process Consulting LLC, Appleton, Wisconsin, USA, and a member of the TAPPI Editorial Board. King is also secretary of the PIMA Technology Resource Management Group, which plans and implements programs for the PIMA International Management Conference each June. He can be reached at +1 920 991-9102, or by email at email@example.com.
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|Title Annotation:||Paper Chemistry|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Apr 1, 2004|
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