Opportunities in Wet-End Chemistry: Feature Essay, from June 2000
"Stickies, Pitch, and Secondary Fiber - A Chemist's View"
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
Question: Why do paper machine crews often speak in hushed tones when using secondary fiber?
Answer: Because at the moment there are no serious problems with pitch or stickies. But luck can change in a moment. It is the nature of secondary fiber to contain both pitch-like and sticky materials. And it is the nature of pitch and stickies to deposit, usually choosing an inconvenient shift (yours).
Let's review some of the reasons why pitch is tacky, stickies are sticky, and why both of them tend to deposit.
Wood pitch consists of a mixture of resin acids, fatty acids, natural oily materials, and anything else in the furnish that has a water-hating nature. We say "resin acids," not "resin acid," because we are dealing with complex mixtures of different isomers and different molecules from the wood. Many of these isomers, if obtained in pure form, would be solids under papermaking conditions. At the very least, they would have sharp melting points. Others, especially those that contain double carbon=carbon bonds are more likely to be liquids or tacky solids in their pure form. A common example of this is seen when comparing unsaturated fats (vegetable oil) and the corresponding saturated fats (hydrogenated margarine).
The trouble is, when we mix several different pitch-like materials together, it gets hard to predict the end result. In general, materials are most tacky when they are in an intermediate range between "liquid" and "solid." Immersion of pitch-like material in water tends to lower its effective melting point, compared to dry pitch. Heating up the mixture also tends to move things closer toward the liquid state. Conversely, increasing molecular weights or oxidative cross-linking reactions tend to make pitch-like materials more like solids.
Just when we think we may be beginning to understand wood pitch, that's about when we realize that there's more to the story. An analysis of pitch deposits often will show the presence of such synthetic materials as latex binders (from coated broke), ethylene-bis-stearamide (a defoamer often used in pulp mills), and byproducts of sizing agents (actually added at the wet end). Each of these types of substance has what it takes to enjoy like in a pitch deposit - that is - water-hating character. And each of these substances has the added ability to affect the tackiness of the resulting mixture. And Murphy's law states that somewhere in the paper machine system, usually an inconvenient location, the local temperature will produce a maximum in tackiness and deposition.
Papermakers know that pitch problems can be influenced by changes in wet-end chemistry. Add enough talc, or other detackifying agent, and the problems get less serious. Especially troublesome problems can occur as a result of pH changes. For example, exposure of alkaline waste paper to low pH conditions tends to liberate a high concentration of calcium ions from the dissolving CaCO3. The calcium ions are then free to complex with the soap forms fatty acids and resin acids, creating tacky precipitates. Usually it is impractical to eliminate pH changes in a system, but it can pay to follow such strategies as (a) keeping the changes gradual through the process, and (b) at any given location the pH (and other things such as temperature) ought to be constant. Papermakers also know that pitch problems are likely to be worse if first-pass-retention is very low, there is a lot of slime in the system, or there is a lot of foam. Your friendly chemical supplier can provide you with suitable additives to deal with such issues.
Stickies get their name from their bad habit. But many of them started out life as coatings on tape or pressure-sensitive labels. In such applications their sticky nature was called "good." Unfortunately, such materials also have a great ability to bunch together into globs and strings having little affinity for the fiber surfaces.
Stickies really ought to be removed from pulp before it gets to the paper machine system. But that's hard to do. Work at N.C. State University has shown that stickies can "extrude" themselves through the slots of fine screens, even if those slots would appear to be too small. Some stickies can be removed by flotation, but this approach becomes less effective if the stickies are covered with hydrophilic materials such as cationic polyelectrolytes. Surfactants that have the effect of making the surface of stickies more compatible with water, and this may interfere with their removal by flotation. Further work at N.C. State is aimed at an understanding of generation and removal of micro-stickies.
A further problem with stickies,
from a chemist's viewpoint, is that they tend to contain polymeric materials.
A common component of pressure-sensitive labels is ethylene vinyl-acetate (EVA),
and this stuff is much harder to dissolve than wood pitch. Continuous and "boil-out"
cleaning programs can be optimized to meet most needs, and doctor blades can
be used to minimize deposition in the drier system, at least if the composition
of the deposit remains constant. And when you here a wet-end chemist mention
"boil-out" as a way to solve a recurring problem, than that's a good
clue that the problem is indeed serious.