Publication: PULP & PAPER MAGAZINE
ISSUE FOCUS: WET END CHEMISTRY
North American Fine Paper Producers Continue Alkaline Paper ConversionBy HAROLD A. GOLDSBERRY and JAMES E. MAHER
Market for alkenyl succinic anhydride (ASA) sizing agent expected grow significantly with forecasted 90% mill conversion by 1995
In 1988, approximately 3.5 million tons of uncoated free-sheet were produced under alkaline conditions in the U.S. This production grew to slightly more than 10 million tons by the end of 1992. Forecasts project that by 1995, about 90% of the uncoated fine paper market will be converted to alkaline.
Alkylketene dimer (AKD) emerged as the dominant internal alkaline sizing agent between 1989 and 1991, while alkenyl succinic anhydride (ASA) growth was relatively modest. However, 1992 may be considered the year ASA sizing came of age. Approximately 1 million tons of U.S. alkaline free-sheet production were switched from AKD to ASA during that year.
This article analyzes the growth in conversions from acid to alkaline papermaking and the changes necessary in the U.S. alkaline size market to handle these conversions. As part of the analysis, the article will discuss recent advancements in ASA application technology to meet the demands for continued alkaline conversions.
ALKALINE SIZE MARKET OVERVIEW.
Figure 1 represents the actual and predicted growth in alkaline free-sheet production from 1988 to 1995. Alkaline fine paper production doubled from about 4.6 million tons (annualized) at the end of 1989 to approximately 9.1 million tons at the end of 1991. Approximately 1.2 million tons of additional U.S. alkaline fine paper came onstream in 1992.
Figure 2 shows the growth trends for AKD and ASA alkaline sizing agents as measured in the tonnage of U.S. fine papers from 1989 to 1992. Paper sized with AKD grew 143% from 1989 (2.7 million tons) to 1991 (6.5 million tons). ASA was selected for fewer major fine paper conversions during this same time and grew in tonnage by 37%, from about 1.9 million tons to 2.6 million tons.
The primary reasons for the AKD growth were the product's historical dominance in alkaline fine paper production in Europe, its "pump and go" convenience as a result of being delivered ready-made to the mill, and the industry belief that ASA technology had not satisfactorily addressed shortcomings identified in the late 1970s and early 1980s.
However, significant events in ASA sizing technology did occur during this period. First, several large, multi-machine mills chose ASA for their alkaline conversions. These mills continue to run basically the same wet end chemistry packages chosen for the alkaline startups. Second, a number of large forms bond producers have made rapid and successful changeovers from AKD to ASA to meet requirements for sheet quality.
In 1992, use of ASA grew considerably--almost 50%, from 2.6 million tons (annualized) to nearly 3.9 million tons. By contrast, AKD use held steady at 6.4 million tons.
ADVANCES IN ASA APPLICATION TECHNOLOGY.
ASA has been available to the paper industry as an internal sizing agent since the mid-1970s. The ability of ASA to deliver relatively high levels of presize press and on-machine sizing has always been of interest to papermakers experimenting with alkaline technology. There were very serious concerns, however, regarding continuous on-machine use of the ASA sizing chemistry.
During the late 1970s, the performance and efficiency levels of paper machines running ASA were severely hampered by system deposit problems. Alkaline papermaking in the U.S. was in its infancy during this period, as was ASA technology. Very little emphasis was placed on the importance of starch in the ASA alkaline sizing system or on the value high retention could bring to the alkaline papermaking process.
ASA emulsification equipment was archaic by current standards. There were no identified standards for controlling the ASA emulsification process or sizing-emulsion quality. ASA sizing emulsion was often stored for long periods prior to addition to the papermaking system. No clear-cut criteria had been established regarding the selection of optimal ASA emulsion addition points in the papermaking system. All of these factors contributed to ASA 's lack of industry acceptance.
The ASA sizing agent is a 100%-active, water-insoluble liquid produced by reacting a mixture of isomerized alpha olefin and maleic anhydride. The anhydride portion of the ASA molecule is very reactive with the hydroxyl groups present on cellulose and starch. This allows ASA to routinely develop 90% to 100% of ultimate sizing ahead of the size press. However, the ASA molecule will also react with the water molecule, leading to hydrolysis of the anhydride group. This undesirable compound is an acid and will readily combine with calcium or magnesium to form a tacky, insoluble precipitate. Analyses revealed that the vast majority of system deposits associated with ASA in the late 1970s and early 1980s were products of ASA hydrolysis.
R.B. Wasser's work in the early 1980s clearly established the parameters governing the rate of ASA hydrolysis. Time, temperature, and pH effects on the rate of hydrolysis were studied and quantified. It became clear that storage of the sizing emulsion was one of the leading contributors to system deposit problems associated with ASA.
When ASA is emulsified in water it makes a very unstable, this is okweakly anionic emulsion. This emulsification approach results in a sizing agent that is poorly retained and that hydrolyzes rapidly. Cationic starch was recognized in the late 1970s and early 1980s as a superior emulsification agent for producing starch.
The most common starches used for the ASA sizing system during this time period were cationic corn products. In practice, however, it was discovered that the cationic charge of these starches, because they were not fully quaternized, dissipated quite rapidly at alkaline pH, thus degrading emulsion stability and sizing efficiency.
High-performance quaternary cationic potato starches were identified in the early 1980s as the products of choice for optimal ASA emulsification. These starches produced stable emulsions that gave significantly higher ASA sizing performance and allowed ASA addition levels to be reduced.
Since the potential for hydrolysis of the ASA emulsion exists even with the best emulsification technique, it has become standard practice to feed the fresh ASA sizing emulsion directly to the paper machine from the emulsifier discharge rather than from an emulsion holding tank. A better understanding of how to select addition points for the ASA sizing emulsion further minimized the potential for buildup of hydrolyzed ASA in the papermaking system.
The high reactivity of ASA makes it essential that it be emulsified at the mill site. Early ASA emulsification equipment had only manual means for controlling ASA and starch flows. Great strides have been made in automating ASA emulsification equipment, making it far more user friendly. Today's ASA emulsification system is a self-contained package with complete mill computer interface for operation and process control. Flows are controlled with magnetic flowmeters and automatic control valves. Unit startup, adjustment, and shutdown procedures are now routinely carried out via mill distributive control and/or computer systems. Automatic control of the proper starch-to-size ratio has further ensured consistency in ASA emulsion quality and sizing efficiency.
Paper producers, along with their chemical and equipment suppliers, have all recognized that maintaining high levels of fines, filler, and sizing emulsion retention plays a critical role in achieving maximum efficiency in an alkaline papermaking system. Additive performance is improved, additive addition levels are minimized, and the papermaking system runs cleaner. Maintaining high retention values has become an industry axiom for successful alkaline papermaking, regardless of the sizing agent.
Retention aid and application technology have made significant advancements since the mid-1980s. Improvements in conventional polyacrylamide products and the development of microparticle systems have made high fines and filler retention levels routine in the alkaline papermaking system. Equipment and forming fabric suppliers have also advanced the mechanical means for supporting this achievement.
There is a much clearer understanding today of the relationship between ASA emulsion particle size and sizing efficiency. The combination of modern, reliable ASA emulsification units and a better understanding of the factors affecting emulsion efficiency have produced significant improvements in ASA emulsification techniques. These relatively recent advances have led to consistent reductions in the ASA addition levels required to meet mill sizing targets. Average ASA dosage has been reduced by 25% to 30% in U.S. alkaline fine paper production during the past two years. It is now quite common for mills to be running at 2-lb/ton or less of ASA to complete the sizing task.
Have paper machine deposits completely vanished with these ASA advancements? An acid rosin system will have deposits if run out of control. AKD deposit problems will occur if the alkaline system is not run in control. The same is true for the ASA sizing system. When it is applied using the best current technology, ASA is allowing papermakers to run machines efficiently and with high productivity. The horror stories of mills having to hydro-blast stock lines due to hydrolyzed ASA deposits are a thing of the past. Mills running efficiently under the acid process (boiling out machines every four to eight weeks) have maintained productivity and boilout frequency with the ASA sizing system. Mills report excellent machine performance with the ASA sizing system.
Today, ASA is being commercially applied on a variety of twin-wire formers and straight fourdrinier machines, running at speeds up to 3,500 fpm. ASA is delivering proven performance on machines equipped with various flooded-nip, metering-blade, and gate-roll size presses. ASA-produced grades include various copier, offset, computer forms bond, carbonless forms, envelope, opaque, ink jet, coating basestock, and specialty grades. These grades are being made successfully with the ASA sizing agent at sheet ash levels approaching 22%.
Evolution of ASA technology has led to growing paper industry acceptance. ASA offers several benefits--higher sheet quality, lower production costs, and higher productivity--to mills that are already running under alkaline conditions or considering a conversion.
Sheet quality: ASA is emerging as the alkaline sizing agent of choice for grades sensitive to surface slip. This has proved especially true for forms bond grades.
No doubt, there are some inherent differences between an acid and alkaline sheet. However, it has been clearly proven on multiple occasions that alkaline forms bond-sized using ASA exhibit superior converting characteristics on the newest and widest high-speed converting presses. It has been noted that certain other slip-sensitive specialty grades, such as adding machine tape, convert much more effectively with an ASA alkaline sheet.
Production costs: Paper mills are realizing significant sizing-cost reduction with the ASA sizing system. Mills that have switched from AKD to ASA are saving $2.00 to $4.00/ton in sizing costs. These savings have come from two major areas--straight ASA for AKD replacement and reduced surface size requirements.
Machine productivity: Due to ASA's high rate of reactivity, 90% to 100% of ultimate sizing is routinely developed ahead of the size press. This is achieved at sizepress moisture levels equal to those run under acid conditions. It is quite common for the sheet to be dried to 1% or lower to achieve appreciable sizing with AKD ahead of the size press. This is often significantly lower than normally required for good profile control. ASA is run routinely at 3% to 3.5% size-press entry moisture, with good profile stability and size-press solution pickup control.
Machines limited in the main dryer section may benefit greatly from running ASA. Mills that have substituted ASA for AKD have used the ability to increase sheet moisture entering the size press to achieve 7% to 10% production gains.
Machines running flooded-nip size presses have also achieved production gains with ASA. High presize press sizing has resulted in reduced steam demand in the after dryers and increased machine speed. The high presize press sizing has also allowed papermakers to increase machine speed with more confidence that runability through the size press can be better controlled. Production gains of 6% to 7% have been realized commercially.
1. R.B. Wasser, The Reactivity of Alkenyl Succinic Anhydride, 1985 TAPPI Alkaline Papermaking Seminar Notes.
2. S.N. Jenkins, ASA--Friend or Foe, 1993 PIRA Conference Proceedings.
Mr. Goldsberry is technology specialistõalkaline size, and Mr. Maher is project manager, Cytec Industries, West Paterson, N.J.
Copyright 2000 Paperloop Inc. All rights reserved.