Opportunities in Wet-End Chemistry: Feature Essay, from Jan. 2002
"Paper Sizing with ASA: Why Droplet Size Matters"
Martin A. Hubbe
Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh, NC 27695-8005
Citation (public domain): http://www4.ncsu.edu/~hubbe/new
Have you ever observed ragged edges on the letters of an ink-jet printed document? Such raggedness or "feathering" often can be cured by adding alkenylsuccinic anhydride (ASA) size or by increasing its dosage at the wet end of the paper machine. ASA size is fast-acting and it makes paper resist water without making the surface slippery. These reasons help explain why ASA sizing has become popular for production of office papers. Other applications where ASA seems to be the sizing agent of choice include gypsum-board liner, envelope paper, and just about anything that requires good hold-out of starch at a size press.
Before the ASA size can be distributed onto fiber surfaces in the slurry of pulp, it first has to be emulsified. The emulsion will consist of tiny droplets of ASA oil, surrounded by a protective layer of cationic starch or acrylamide copolymer, in a continuous phase of aqueous solution. And one of the first things that operators will do to make sure that the process is working well is to test the particle size of the emulsions droplets. Depending whom you talk to, you may hear people say that a "good" ASA emulsion has relatively uniform droplets of about 1 micrometer in diameter.
But why does it matter? Why does sizing get less efficient if the particle size is too big? And if small is good, why not even smaller?
One key to understanding the critical nature of ASA particle size is its high reactivity. Evidence suggests that the anhydride ring of ASA opens during the drying of paper, and an ester bond is formed to link the molecule to hydroxyl groups on the cellulose or starch at the fiber surface. This reaction anchors the sizing molecule and orients the hydrophobic alkenyl "tail" of ASA outward from the surface. But ASA also can react with water. That's why an ASA formulation has to be used quickly after it has been prepared. The rate of ASA hydrolysis (reaction with water) increases with decreasing droplet size. Other ways to slow down the decomposition of ASA include lowering the pH, lowering the temperature, and maintaining a high first-pass retention on the paper machine to help minimize the contact time of ASA droplets in the furnish before the paper is dried.
The other key to understanding ASA particle size optimization is the need for good distribution. The vapor pressure of ASA oil is very low, so its rate of evaporation and recondensation is slow, even when heated in the dryer section. Though it is conceivable that ASA droplets might spread over fiber surfaces, recent research with other sizing agents suggests that this happens only to a limited extent. The expected consequence is that excessively large ASA droplets succeed only in "sizing" the fiber surfaces nearest to them, and that the surface coverage is non-uniform. Due to the commercial importance of ASA as a sizing agent it is likely that this question will be an active area of research in the coming years.