Opportunities in Wet-End Chemistry: Feature Essay, from September 2002
"A Cure for Stickies?"
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
Your health can benefit from good nutrition, vitamins, exercise, and a positive outlook. But sometimes you get sick anyway. So it is with stickies. For the sake of discussion, let's define stickies as tacky materials that come from recycled fiber sources and end up either as spots in the paper or, more likely, as deposits in felts and other transfer surfaces in the press section and dry end of a paper machine.
The first defense against stickies is to bar the door. That means either avoiding recycled office papers and magazines altogether or carefully selecting and sorting the wastepaper. Manual sorting of wastepaper is a thankless job, requiring a lot of effort for a modest return. It's the kind of job that one would like to turn over to robots, if that were possible. Work at N.C. State University, sponsored by the Department of Energy, is helping to make that possibility a reality. Results to date have demonstrated the feasibility of separating various kinds of paper based on spectral analyses. The question is, can we teach those same robots to cut out the labels from magazines, one of the key sources of elastomers in wastepaper pulp?
|The papermaker's next defense against stickies is to remove them by screening. This is a good strategy, but the figure shows an inherent limitation. That is, stickies can deform. Depending on the aqueous environment, sometimes the string-like sticky particles fold over on themselves and "ball up" again after passing through a set of pressure screens.|
Let's suppose that sticky particles have been broken up by agitation and by the action of screens. Sometimes these very small sticky particles are called "micro-stickies." In such cases one can't expect to achieve much by screening, even with gentle flows and narrow slots. It is reasonable then to consider froth floatation, taking advantage of the water-hating character of typical tackifying resins. Bubbles introduced into the stock will tend to attach themselves to the stickies and other resinous materials, allowing them to be skimmed from the surface in a more concentrated form.
An astute reader will have noticed by now that most of the defenses against stickies already mentioned are best implemented in a pulping operation, well before the stock comes to the paper machine wet end. Hydrocyclones (cleaners) are not especially effective against stickies, since their density is usually too close to that of water. The final pressure screens in the approach flow to a paper machine headbox may remove the largest of the sticky particles. After that, the papermaker has only a few more cards to play - detackify the stickies, retain them on the fibers, spray chemicals onto fabrics to prevent deposition of the stickies, use doctor blades, and periodically clean out the whole wet-end system to remove deposits.
Let's go back and consider what is meant by "detackify." To detackify something is to change its surface, so that it no longer sticks to things. Talc has been used effectively by papermakers for generations in their struggles with wood pitch, and now it is being used to tame stickies by coating their surfaces. Suppliers of papermaking chemicals have come up with a variety of polymeric additives that can do the same thing. Mill trials are needed to find out what formulation is the best match for a given process environment and a given diet of tacky materials in the furnish.
Finally, a word about "barrier chemicals." The idea here is to spray a dilute solution of chemical onto either a forming fabric, a press fabric, or possibly another transfer surface in the press section. As mentioned already, typical elastomeric materials are generally water-hating, so the most effective strategies use either cationic polymers or various surfactants to render the surfaces water-loving. Though the films produced by such chemicals are often extremely thin and invisible, sometimes the films become deeply stained, especially if dyes are being used. No matter. Would you prefer a paper machine with orange clothing, or one with low operating efficiency and defects caused by stickies?
There's no cure for stickies. But
there are many things that one can do to feel better about them.