At the request of a reader who is not a papermaker, some of the terms used in the March issue have been defined and their relevance to topics covered in this newsletter explained. Research reports or reference publications that form the basis of some of the statements or definitions are cited.
Alkylketene dimer sizing - A type of internal size that works in an alkaline pH range. Invented in the early 1950s, it made possible for the first time the manufacture of alkaline sized paper under normal commercial conditions. It is typically prepared from a stearic acid and added to the pulp slurry in the form of an emulsion. It does not spread evenly over the fiber surfaces until it reaches the dryer section, where it gets "ironed in" to the web. Alkylketene dimer size (AKD) is used in all sorts of paper and board, including milk cartons and paper cups, as well as in archival paper. It works in the pH range 6-9. If calcium carbonate is used as a filler, a small amount of alum can be used as a retention and drainage aid without significantly affecting the permanence of the paper--but little if any research has been done on this particular issue.
Aquapel - Trade name of the first AKD put on the market. It was used in 1959 in the permanent/durable text paper developed by William J. Barrow.
buffer - 1) A solution whose pH changes only slightly on the addition of acid or alkali. Paper saturated with solution and then dried will show the same pH as the buffer solution. 2) An alkaline reserve in paper, usually a calcium carbonate filler which maintains the pH in the neutral or alkaline range by reacting with acidic gases from the environment or from the deterioration of the paper itself.
dusting - Shedding of particles of filler, fiber or coatings from paper during finishing, converting, printing or use in office machines. This can interfere with the operation. William Bureau, speaking of its effect on the printing operation, says, "Dusting, sometimes called powdering, is the accumulation of very small, visible white particles on the blanket due to the release of fillers from uncoated paper or coating pigment particles from coated papers." He distinguishes it from milking, piling and whitening. It can happen with acidic papers as well as alkaline papers.
effluent - Outflow of water from a pulp or paper mill. Government regulations restrict the impact of effluents on the receiving waters with respect to solids, oxygen demand, toxicity and color. Mills control these factors by re-using or recycling water used for the various processes and by treating it in their own plants before discharge.
filler - A material, typically clay or calcium carbonate, added to the furnish of paper mainly to increase the smoothness of the paper surface and make it a better printing surface. Fillers usually improve brightness and opacity too, and help prevent strike-through of ink, but at levels above 5% they interfere with fiber bonding and weaken the paper (Clark, p. 771). Early fillers (late 18th century, early 19th century) included gypsum, white lead, zinc sulfide, barium sulfate, and even finely ground deep blue glass (called smalts). Today clay and titanium dioxide are the fillers usually used with acid furnish, and calcium carbonate (sometimes with clay or titanium dioxide) with alkaline furnish. Higher filler levels are possible with alkaline paper because the fiber is stronger, but retention is sometimes a problem. European mills, which have been making alkaline paper longer than American mills, are able to achieve filler levels of 30% by weight, and levels of 50% are predicted for the future. Papermakers generally want to include as much calcium carbonate as possible if they are using it, because it lowers the cost of materials used in manufacturing the paper..
fine paper - Paper used for printing, writing, and cultural purposes. Smook defines it as "white, uncoated printing and writing grades which contain no more than 25% mechanical pulp in the furnish" (p. 295). Most furnishes, he says, are wood-free, but he does not indicate whether this only means only mechanical pulp or none at all. It should be noted that terminology is not standardized in the paper industry. Each mill, in a sense, speaks its own dialect. A similar situation is found in hospitals and binderies.
fines - Very small fibers and fiber fragments which tend to pass through the wire with the water, but which can be held in the sheet by the use of retention aids (q.v.).
flexography - Letterpress printing using rubber relief plates on various paper and board surfaces; formerly called aniline printing. Now the ink may contain a dye, a pigment or both. Paper printed by this process cannot be deinked easily for recycling (Fuchs, p. 11).
1) floc; 2) flocculation - 1) An aggregation of suspended particles. In paper, these can be fines or filler or size particles drawn together by a retention aid and thus kept from being lost through the wire along with the draining water. 2) The drawing together and clumping of small suspended particles.
freesheet - Woodfree, groundwood-free. 1) Paper free of mechanical wood pulp (API). 2) Paper containing no rare than 10% mechanical wood pulp (common usage).
furnish - The mixture of pulp, chemicals and other materials from which a particular paper is made.
groundwood - mechanical wood pulp (stone groundwood, refined mechanical pulp, thermomechanical pulp), having a yield of over 90% and thus containing a great deal of lignin. By extension the term sometimes refers to chemi-mechanical and semi-chemi-mechanical pulp, which have yields of 50-90%
hemicelluloses - 1) Cellulose having a DP (degree of polymerization) of 150 or less [i.e., smaller molecules with fewer "building blocks"]. A collective term for beta and gamma cellulose; cellulose that is soluble in hot alkali (Hawley). 2) A number of shorter-chain polysaccharides. Hemicellulose (along with degraded cellulose) is further conveniently categorized... according to DP:
beta cellulose - DP 15-90
gamma cellulose - DP less than 15
By contrast to cellulose, which is a polymer only of glucose, the hemicelluloses are polymers of five different sugars: glucose, mannose, galactose, xylose and arabinose (condensed from Smook, p. 5).
The contribution of hemicelluloses to development of initial strength in paper is well recognized. It is related to fibrillation and bonding. But hemicellulose-rich pulps have a strong tendency to discolor during thermal aging. This tendency can be countered with borohydride treatment (condensed from Feller, p. 301-3C4).
hydrolysis - Literally, "splitting apart with water." Hydrolysis is a reaction that takes place during deterioration of cellulose, sizing compounds, and other organic and inorganic materials. It is facilitated by heat, enzymes, catalysts and low pH, and consists of the formation of new substances from parts of the original molecule when the OH from the water joins up with one part of the molecule and the H joins up with the other part.
lignin - A three-dimensional amorphous polymer with a variable structure so complex that a definitive formula for it has never been written. It makes up 17-32% of wood, , rounding the cellulose fibers and providing the stiffness that enables trees to stand upright. It is chemically stable in wood, but becomes unstable when the wood is broken down to paper. It is removed in pulping with the use of hot chemicals followed by bleaching. Fully bleached pulp contains practically no lignin (Clark, p. 113).
Paper that contains large amounts of bleached lignin, such as newsprint, deteriorates principally by oxidation, which is speeded up by light and high humidity. The lignin reverts to its natural brown color as it oxidizes. Although alkalinity also darkens the pulp noticeably, addition of calcium carbonate to the papermaking furnish will make high-yield papers chemically more stable, slowing down subsequent darkening and degradation considerably.
retention aids - Compounds added to the furnish to keep fines, fillers, size, pigments and other components from draining through the wire along with the water. This is especially important in mills that are recycling as much water as possible (closing up their white water system) but are using a size that must be used while it is fresh. In alkaline systems, retention aids :include cationic and anionic polyacrylamides, cationic starch and colloidal silica, and alum.
size - An additive that makes the fiber surfaces hydrophobic and keeps the paper from acting like blotting paper when you write or print on it. Early papermakers dipped their handmade sheets into vats of gelatin to size them, but productive capacity of the paper machine called for an internal size that could be added directly to the slurry. For 150 years, rosin was the only practical internal size available, and it had to be used with alum to make it stick to the fiber. Since alum is acidic, this made the paper short-lived. Today a growing variety of sizing compounds and systems makes it possible to size paper at any pH from 4 to 10.
Printing and writing papers are moderately sized or "slack-sized." Paper cups and milk cartons are "hard-sized." Paper tissue and towels are not sized at all.
Surface sizing is done at the size press, using starch to even out the surface of the paper and thus slow down the penetration of liquids. It also lays down the fibers on the surface to keep them from causing trouble during printing operations. The size press consists of two drums or rollers between which the paper passes as the size solution is applied to both sides.
stoichiometry - The branch of chemistry that quantifies the substances in a chemical reaction. This is a stoichiometric equation:
H2SO4 + 2nacl=2HCl + Na2SO4
American Paper Institute. The Dictionary of Paper, 4th ed. API, New York, 1980.
J. S. Arney and A. J. Jacobs, "A Study of the Relative Importance of Oxidation by Atmospheric Oxygen in the Aging Chemistry of Paper." American Institute for Conservation of Historic and Artistic Works, Preprints, 1978.
William H. Bureau, "Coming to Terms." Graphic Arts Monthly Sept. 1988, P. 152-3. (His column on "Paper")
James d'A. Clark. Pulp Technology and Treatment for Paper 2nd ed. Miller Freeman, San Francisco, 1985.
Robert L. Feller, Sang B. Lee, and Mary Curran. Three Fundamental Aspects of Cellulose Deterioration. Issued as a supplement to Art and Archaeology Technical Abstracts 22 (1), 1985. 79 pp. Three annotated bibliographies with introductory sections that amount to critical review articles.
Boris Fuchs, "The Recycling-Deinking-Flexo Complex," Newsp. Techn., May 1988, p. 6-11. A paper given at the IFRA Symposium.
Hawley's Condensed Chemical Dictionary, 10th ed. Van Nostrand Reinhold, 1981.
G. A. Smook. Handbook for Pulp & Paper Technologists. M. J. Kocurek, Tech. Editor. TAPPI & CPPA, cl982.