Paper: A Modified Natural Polymer J. Arthur Campbell Harvey Mudd College. Claremont. CA 9171 1 You may be more familiar with paper than with most macromolecular substances. But have you ever looked closely at a piece? Say this one? It's grainy. If you tear off a piece (not too big a piece, please), you see fibers projecting from the edee. " You maveven notice that it is easier to tear in some directions than in others. With closer observation you will find that the individual fihers varv in leneth from about half a centimeter down, and they are ;bout aiundredth as wide. Because they are matted together, almost but not quite a t random angles, they tear differently in different directions. If you continue investigating you will find that if the paper is placed in water and the fibers are floated off, some nonfibrous materials often settle to the bottom. Paper is a highly processed material containing several chemicals, but its main properties are derived from the macromolecular properties of cellulose. Wood, from which most paper is made, is mainly composed of cellulose (45 f 5%, a linear D-glucose polymer of up to 10,000 glucose molecules), lignin (25 f 5%, a 3-D polymer based on ~ r o ~benzene). vl and hemi-cellulose (20% f 5%. a branched; a&rphous, pdlysaccharide). The balance (6.i 4%) consists of nonpolvmeric materials (such as abietic acid. . . a principal constituent of rosin), which are characteristic of the speries and which ofcen interfere with paver making. In the simplest paper-making process, logs or &ks of wood are pressed against a rotating pulpstone that tears the wood apart. A stream of water carries away much of the lignin and nonpolymerics, leaving a watery mixture called pulp. The pulp is spread into thin sheets and the water squeezed out or evaporated to leave dry paper. See Figure 1. Paper produced bv this mechanical nrocess is not verv strone. -. not verv white. and, because of chemical changes in the lignin remaining, i t auicklv darkens and becomes brittle as i t aees. ' ~etier-qualitypapers are made by a Gemical pulping process. Chips of wood are placed in big cooking vessels with chemical solutions and cooked a t high temperature (100-150 "C) and pressure (10-15 atm) to soluhilize the lignin by breaking the phenol ether bonds and sulfonating the lignin monomers, leaving pulpy cellulose and hemi-cellulose fibers. The lignin and nonpolymerics are filtered off andmost of the lignin is then precipitated and burned. No chemical use has yet been found for this abundant product, which binds the cellulose fihers together in wood. Two different processes are commonly used in preparing the pulp-one uses sulfite ions and the other, called the sulfate process, actually uses sulfide ions. The sulfite process producespaper that is almost pure cellulose. There are many
variations, h u t t h e principal ingredients a r e either C a ( H S 0 3 )and ~ HzS03 in the acid sulfite process (producing fine papers), or Mg(HCOa)z, NaHS03, and NaHC03 in the bisulfite process (producing newsprint, for example). Sulfite paper can he made very white (by chemical bleaching of the lignin remaining; with ClzO, a common bleach), but sulfite paper is nut strong. Furthermore, the sulfite process requires many chemicals whose concentrations must be carefully controlled, and can rause s e r i ~ ~ usulfite s water pollution if wi~stesolutions arc discharged f n m the processing plant. Thesulfate ororess uses aoueous NaOH and NaHS (made by reducing N ~ ~ S hence O ~ , ihe name of the process) in the ~ n a to soluhilize most of the lienin and nonoolv. u .l ~ i step . mers. ~ b o u 2W; ; of these materials s o l u b k e the lignin, 6'70 neutralize acetic arid hvdrolvzed from the wood. with the balance reacting with the hemi-cellulose. No blkaching is done. The process produces exceptionally strong paper much used for sacks, cardboard, and packaging. Pulp and paper produced in this way are called kraft, from the German word for "strong". This process, however, may release odorous mercaptans (hydrocarbons containing an -SH group) into the atmosphere. One process for treating sulfide waste is to convert to dilute HzS(g), burn to SOn(g),concentrate by dissolving in H20, oxidize to S04-2, then reduce hack to aqueous sulfide. Some 20% of the oaner industw's investment in recentlv built plants has been ior polluti&-control equipment. hi industry is highlv capital-intensive (about $1.50 of invested capital is needed-for each $1.00 of annualsales), so costs have risen considerably, contributing to recent price increases for books and magazines. The rivers and breezes near paper plants are now cleaner, but there are still problems to solve. The mercaptans (which include the odorous compounds emitted by skunks) can he smelled a t a concentration of 1Dart . oer 100 million. Present techniques that reclaim and recycle the pulping chemicals have lowered sulfur emissions to the air from 25 kc to 2 krr for each 1000 kg of paper produced. The aim is to get diwn below 0.05 ke Der 1000 kc. Because the annual nroduction of paper in the-united ~ t a i e is s about 70 X 109 kg, even such "ideal" plants would dump about 3 million kg of sulfur compounds into the air each year. Further reductions seem imminent as plants incorporate such processes as hastening the pulping with high-pressure oxygen, combining mechanicalpulping with chemical treatments, and forming the paper from fibers fluffed with air rather than with water. Figure 2 shows the chemical structure of cellulose. I t is
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Figure 1. Paper manufacture.
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Journal of Chemical Education
Figure 2. Chemical structure of cellulose.
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classified as a ~ o l v m e of r the suear " elucose because manv glucose units join together to make cellulose. The resulting linkage is called an ether linkage. Humans are unable to cleave (digest) this ether linkage if they eat cellulose, but herbivores (animals that thrive on cellulose) either svnthesize the appropriate enzyme themselves or have in their diaestive tracts bacteria that can cleave the ether linkaae and produce glucose. Cellulose is the most abundant readily renewable carbon compound on earth. There would he avery great increase in available food carbohydrate if we could find some direct way to convert wood, straw, and other cellulose matter to glucose for human consumption. The cellulose molecules form strong hydrogen bonds to one another. The fibers of the paper used in the JOURNAL are, in fact, a huild-up of cellulose molecules bonded in this manner. Because there can he as many as five hydrogen bonds per glucose unit, cellulose is stiff and does not stretch. The paper does, however, ahsorb water, swells, and even disintegrates into smaller molecular units if immersed in water long enough. Thus, pure cellulosic paper does not weather well. It is attacked by water, oxidized by air, and digested by bacteria, as you can see by the rapid disinteaation offncbl tissues, whkh are a ~ m o s ; ~ u rce~llulose. e onthe other hand, multiwall bags used for shipping and storing are extremely resistant to wnthering: they can he immersed in wnter or stored outside for more than a year withour serious deterioration. Thc differences between these two uroductn lie in the nature of the additives used to treat the basic cellulosic fiber mat, as well as in the length of the fibers and the degree to which they are matted. If pure cellulose pulp is used, the fiber mat formed when the water is drained and evaporated away is bondedtogether only by the polar, van der Waals, and hydrogen honds that can form between the fibers. Water easily breaks the fibers apart. Addition of white pigments like clay or titanium dioxide fills in the pores, produces a hard white surface for writing, and gives more binding area to hold the fibers together. Resins, water-resistant compounds, and special surface treatments (including surface laminates) provide packaging and covering materials ranging from ice cream and milk cartons through multiwall sacks to roofing paper. The cellulose provides the fundamental structure and most of the hulk and strength of paper. The additives (about 10%of the weight of the paper) provide the specific properties needed for special jobs. The chemical problem is to combine the additives and the cellulose to achieve optimum service a t minimum cost. About one-third of all cut timber now goes into paper. As costs rise, so does interest in recycling paper. Some of this interest is based on the increasingly severe waste-disposal problem (though paper can always be burned efficiently), some on concern for disappearing forests (though all the large paper companies run constant-growth forests that are really tree farms), and some on the general desire to reduce waste. About 20% of paper products (such as glassine, parchment. mease-oroof and roofine.. Daoer. . . . and wallboard) are not r&ily r&ycleable because of their specialized addiDaoer ~ r o d u c t are s recvcled. tives. T o d w onls ahout 20% of . . . . a decrease'frok previous years, primarily because of the increased labor cost of collectina the paper for recvclina . - as compared to the cost of obtaini~igmaierials for virgin pulp. Most wwns in the Ilnlted States used to have door-ro-door junkmen who paid householders for their waste materials,
Figure 3. The paper cycle
including paper. But no more. The cost of reversing the second law's tendency toward disorder increases each year. The second law of thermodynamics describes two more problems encountered in recycling paper (Fig. 3). Some additives (such as printing ink) are strongly bonded and hard to remove, so thev accumulate more in each cvcle. The recvcled paper gets giayer and grayer as the nont;leachahle content increases. Furthermore, the cellulose fihers ret smaller with each processing and less capable of forming strong hydrogen honds, so the strength of the paper decreases in each cycle. In spite of these difficulties, interest in recycling will continue. What we need are (1)an efficient and inexpensive procedure for collecting used paper, and (2) initial additives that are readily removed in the recycling process. Recycling should be integrated in the initial design of the process of paper makina. The cost of usina new DUID versus 25% recv;led pulp fora plant p r o d u c i n g i ~tons ~ of packaging per day presently favors new pulp. Even if recvclina . - does not save money, it can minimize the load on resources and on waste-disposal systems. Trash, the "raw material" for recycling, is growing a t 8% per year. The annual United States level is about 25 billion cans, 50 billion bottles, 8 billion kg of plastic, 10 million television sets, and 70 billion kg of paper (more than 700 pounds of paper per person in the US.). Good sources of further details are encylopedias, especially the scientific encylopedias, under the titles: paper, pulp, cellulose,' and lignin.
' Wilson, J. D.; Hamilton, J. K. J. Chem. Educ. 1986, 63.49.
Volume 63 Number 5
May 1986
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