However, some softwood fiber, which is generally longer than hardwood fiber, is necessary to maintain the strength of the sheet during production.23
In a typical operation, pulpwood, once debarked, enters a chipper which chips it into uniformly sized chips.24 Next, digesters cook the wood chips in a chemical solution, which separates the cellulose fibers from lignin and other non-cellulosic substances.25 The resulting wood pulp is then washed,
bleached, and refined in preparation for papermaking operations. CFS paper is typically made on conventional fourdrinier paper machines.26
A highly diluted
solution of wood pulp is pumped through the machine’s headbox27 and onto the wire. Water drains by gravity through the wire and/or by suction from the top as the wire advances, forming a web or sheet on the wire. At the end of the wire, the web is picked off the wire by revolving nylon felts, which deliver it to the press section. The press section consists of as many as four sets of closely spaced steel rollers which press water out of the web as it passes through the nip between each set of rollers. Upon exiting the press, the web of paper, which is now able to support itself, enters the dryer section.28 The steam- heated cylinders of the dryer remove the remaining moisture from the paper as it laps over and under
successive cylinders. At this stage, the paper is ready to be coated and, if necessary, calendered.29
may be installed in line with the paper machine (i.e., on-machine) or completely separate from the paper machine (i.e., off-machine). If on-machine equipment is used, the paper enters the coating equipment as it exits the dryer section. If not, the paper is wound onto large reels as it comes out of the dryers on the paper machine and is subsequently delivered to off-machine coaters. In either case, the essential elements of the coating and calendering processes are the same. The principle component of the coating is often kaolin clay, but other elements such as different clays, calcium carbonate, titanium dioxide, latex, starches, dyes, lubricants, thickners, plastic pigments, cast release agents, rheological control agents, pH control agents, optical brightners, and biocides may be included.30 Coatings are mixed in coating preparation equipment in a mill’s coating “kitchen” and pumped directly to the appropriate paper machine or off-machine coater.
Industry & Trade Summary – Wood Pulp and Waste Paper, USITC Publication 3490, 2002, p. 4. ***.
25 The term, “kraft,” denotes the chemical process by which the wood fiber is pulped in a solution of caustic soda and sodium sulfide. Because the kraft (a.k.a. sulfate) process produces a very strong pulp, it is the most important chemical pulping process. It is noted for its high quality and strength and is a primary component of many grades of paper.
26 Named for the French man who helped popularize the design, all Fourdriniers have a continuous loop of bronze mesh screen, the “wire.” Typically, the wire is oriented horizontally and looped around rollers at both ends. As the wire revolves, a diluted solution of pulp is spread across the surface of the wire at one end. Water drains through the wire as it advances, thereby forming the sheet. In this fashion, a continuous sheet of paper can be formed.
27 The head box extends across the wire and delivers the pulp to the wire through many small openings, orifices, or nozzles.
28 Conventional dryers consist of a number of steam-heated cylinders (30 to 60 inches in diameter) arranged in two or more tiers. The wet paper typically passes over and under successive cylinders.
29 A calender is a set or “stack” of hardened rolls typically resting one on the other in a vertical stack. Paper is passed between some or all of the rolls to increase the smoothness and gloss of its surface. The Dictionary of Paper, 4th ed. s.v. “calender.”
30 A c t u a l c o a t i n g f o r m u l a t i o n s m a y b e c l o s e l y g u a r d e d p r o p r i e t a r y t r a d e s e c r e t s . * * * , * * * r e s p o n s e t o t h producers’ questionnaire, p. 7, and *** response, p. 6. e