Opportunities in Wet-End Chemistry: Feature Essay, from Fall 1999
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
A papermaker asks, "What sizing strategy should we be using on this machine?" There are many possible answers. Perhaps the best answer is, "It depends."
Who needs sizing, anyway? Customers do. Sizing specifications are designed, first and foremost, to ensure that paper meets the needs of people who use it. Take, for instance, cupstock. Rapid penetration of beverage fluid through the paper tends to defeat the purpose of the cup. Even when liquid packaging has a barrier coating or laminate film, it still makes sense to make the fibers hydrophobic, i.e. "water-hating" to minimize wicking at the seams.
It's important to keep in mind that sizing specifications need to be reviewed from time to time. Paper gets put to new uses. Consider the case of xerographic copy paper. The word "xerographic" starts with the Greek word for "dry." That means it doesn't have to resist water penetration, right? Wrong! A lot of xerographic copy paper gets used for ink-jet printing. Wet-end treatment of the paper with internal sizes can reduce the amount of visible feathering of the aqueous ink-jet fluid and increase the density of the printed images.
Sometimes a change in how the paper is made means that old sizing specifications get out of date. Assume, for instance, that Paper Machine "A" used to run at a wet-end pH of 5.3 with rosin size and alum. Then a decision was made to produce the same grade with calcium carbonate as the filler. The presence of calcium carbonate typically implies a pH value above 7, favoring a switch from rosin sizing to "alkaline sizing." The best-established alkaline sizing agents are ASA (alkenylsuccinic anhydride) and AKD (alkylketene dimer). In a future issue we can consider the choice between the two of them. But let's assume that the mill personnel chose AKD and successfully matched all of the same sizing targets. The mechanism of action of AKD is a bit different from rosin, so it's worth asking in each case whether the sizing test specification, or the test method, needs to be updated. Too much AKD just raises costs and runs the risk of slipperiness or adhesion problems. Too little sizing might cause problems too.
A second group of people who "need" hydrophobic sizing agents are the papermakers themselves. Consider the difficulties of trying to run an unsized sheet through a puddle-type size press. A "soggy" sheet coming out of the size press usually means a high frequency of web breaks. But let's assume it doesn't break. Extensive re-wetting of paper at a size press or coating station has the potential to make the sheet dimensionally unstable. The resulting cockling and uneven elongation of the sheet can adversely affect performance of the sheet in converting and printing operations. Finally, excessive penetration of size press starch into the sheet can increase the load on the after-section dryers - slowing production in some paper machine systems.
How much size is needed? And what type of size is needed? It depends.