Wood Cellulose as a Chemical Feedstock for the Cellulose Esters Industry Jim D. Wilson and J. Kelvin Hamilton Rayonier Research Center, ITT Rayonier Inc., Shelton, WA 98584 Cellulose is a polymeric carbohydrate material and is the basic structural component of the cell walls of trees and other higher plants. I t is the world's most abundant organic compound and serves as a chemical feedstock for several commercially important polymer industries. One of the more important of these industries is based on esterification of cellulose with a short-chain a l i ~ h a t i acid: c namelv. ". acetic. propionic, or n-butyric. The res;lting thermoplastic cellu: lose esters are characterized bv excentional claritv and -good mechanical performance. They are found in a myriad of specialty consumer products, some of which are photographic films, fibers, cigarette filters, membranes, and molded articles. Historically, cellulose esters are the oldest manmade thermoplastics, going back to the 1920's. Being plantderived, cellulose is a renewable resource. Cellulose Structure Cellulose is a linear polymer of 8-D-glucopyranose units linked through 8-(l-4)-glycosidic bonds (Fig. 1). From the fieure. - . one can see that there are three hvdroxvl erouns available on each monomer unit for esterification. As a result of the B confieuration of the elvcosidic hond. cellulose molecules are rigii and straight with a strong tendency to form intra- and intermolecular hydrogen bonds. As a consequence, the molecules form linear aggregates that are strong and insoluble in most solvents, notably water. The importance of the conformation of cellulose is clear when its physical and chemical properties are compared to the amylose fraction of starch, which is also a linear polymer of (1-4)-Dglucopyanose (1). However, the (1-4) hond in amylose has the a configuration, which leads to a flexible molecule with a great affinity for water. An important property of cellulose, especially when used as a feedstock, is the average length of the molecular chain. This length is commonly expressed in terms of the number of glucose units forming the chain; i.e., the degree of polymerization (DP). Since cellulose is generally a polydisperse system, DP represents average chain length. In the natural state. the DP of cellulose is about 15,000 in cottun and 10,000 in wood 121. As natural cellulose is convertrd to a fecd.;tuck, and then into a cellulose ester, the 1)P of the molecular chain is reduced in controlled steps to as low as 200 in the final product.
-"
fied as gymnosperms, or softwoods, such as pine, spruce, western hemlock, etc. Althoueh " containine other elements. softwood is hasicallv composed of long narrow fihers cemented together into a solid mass with an oraanic substance known as lienin. The fibers are aligned within the tree stem in the longitudinal direction. Wood fibers are the "skeletons" of once living cells. Except in the thin growth area under the tree's bark, living cvtovlasm that was once present in the center of the cell hssdriid up leaving a hollow core, or lumen, surrounded hs rhe r r m a i n of rhe cellulose-rich cell u,all. The indi\,idual fibers are 3-5 mm long and have a width about 11100 of the length. Small apertures called pits are scattered throughout the cell wall, allowing for the passage of liquid between fihers, and thus throughout the wood. Figure 2 shows a microscopic cross section of wood; the relatively thick cell wall, the hollow cell lumen, and the manner in which the fihers are bonded toeether are all evident. The complete details of the wood cell-wall construction are vet to he ascertained. but it is clear that the lone cellulose moiecules group together in parallel to form m~croscopic
-
,
,
1
CWii
CMOH
Q
0 k..
H
OH
In
,I
Cellulose is e linear .Dolvmer A kev to the . of o-anhvdroalucose. . propen es of ce lulose is the .i con1 g.raloon ol the (1-4).glucosid c bond oelween the D-g ucose unols
Fioure
1.
phySlCal
Wood Unnurified cotton tvnicallv" contains about 94% cellulose and is an obvious chemical feedstock source. However, cotton is normallvmorevaluableas a textile fiber. somost of the cellulose u s e i as a feedstock comes from wood, which is about 50% cellulose. Understandine how cellulose is recovered from this raw material requires some basic knowledge of the structure and chemistrv of wood (3). For the sake of simplicity, we will deal here only with wood from trees classi-
".
Presented at the American Chemical Society Spring National Meeting. Seattle, WA, March 1983. in the Symposium on "Contemporary Chemistry in the Pulp and Wood Industry" sponsored by the Division of Chemical Education. Contribution No. 238 from the Rayonier Research Center of ITT Rayonier Inc.
Figure 2. Microscopic view of a cube of sugar pine wwd showing all three The relatively thick cell wail of t h e fibers, the hollow cell lumen, and SU~~BCBS. the manner in which fibers are bonded together are all evident. (Fmm Gray and Parham (4, reprinted with permission.) Volume 63
Number 1 January 1986
49
threads called microfibrils. The cell wall in turn is a complex system of multiple layers formed by a network of microfibril windings as schematically drawn in Figure 3 (4). Molecular alignment in some areas of the microfibrils are highly ordered to form crvstalline reeions. Other areas are disordered " or amorphous. These aspects of cell-wall construction are illustrated in Fiaure 4. As will he seen. the crvstalline repions of the microfib& play an important roll when it com'ks to converting cellulose to its esters. Chemically, softwood is composed of three major components and an extractive fraction: Cellulose Hemicellulose Lignin Extractives
42 f 2% 27 i 2% 28 f 3% 1-5%
Hemicelluloses, like cellulose, are polysaccharides, but with several important differences (5,6). The molecular building units are mainly D-glucose, D-mannose, D-galactose, D-xylose, and L-arabinose, rather than exclusively Dglucose as in cellulose. Uronic acids also occur (i.e., 4-0methyl-D-glucuronic acid). Hemicelluloses are branched and the chain length is much shorter than cellulose, the DP being a few hundred rather than several thousand. An important property of hemicelluloses from the standpoint of this paper is that they are relatively easily hydrolyzed to water soluble fragments. Their location in wood is mainly in the cell wall where they reside in the amorphous state. Lignin is an aromatic polymer (Fig. 5) composed of suhstituted phenylpropane units which are linked primarily through ether bonds; carbon-to-carbon bonds are next in promGence (7). I t is generally assumed that lignin exists in wood as a three-dimensional structure of verv hieh molecular weight. Since it is difficult, if not imposiihle so far, to isolate lignin from wood without degradation, estimates of molecular weight as present in wood are as yet questionable. Lignin concentration is highest in the area between wood fibers, the middle lamella region, where it acts to bold the fibers together. However, the greatest amount, some 70% of
Figure 3. Schematic of what is widely w i d e r e d to be (at least in principle) the general wall architecture af normal wood fibers: ML, middle lamella: P. Primary wall: SllS2IS3, layers of the secondary wall. (Adapted from Dunning (4, printed with permission.)
50
Journal of Chemical Education
Figure 4. Diagrammatic representation of cellulose in the wood fiber wall. A, transversellongitudinal view of several wood fibers. B, portion of a fiber 52 layer, shown here to consist of macrofibrils-aggregated microfibrils (whitetwith interceding noncellulose materials (black). C, pation of a ma. crafibril, shown here to consist of aggregates of micmfibrils, which are in turn Composed of aggregates of cellulose moiecules. D. cross section of a single microfibril. E, region of a microfibril where the utllulose chains exhibit a high degree of order. F, organization of cellulose molecules into a series of "unit cells." Showing the basic repeating unit of cellulose-the cellabiose unit. G, a cellobiose unit-two adjacent glucose residues connected by en oxygen atom. (Adapted from Esau (10).printed wlth permission.)
Figure 5. Prominent structural elements of soliwood lignin as depicted by Adler (7).
drops to about 3200. Since very high D P cellulose is unsuitable for a feedstock, the decrease in D P is desirable. but must he carefully controlled to a rather narrow range through selection of pulping conditions. At the end of pulping, the contents of the reaction vessel are violently disgorged using pressure. Since most of the lignin is solubilized during pulping, there is little to hold the fibers together, and the physical force accompanying vessel emptying is sufficient to break the delignified wood into a mass of individual fibers called pulp. Solubilized material is washed from the pulp and recovered to be used either as fuel or to produce a variety of saleable products. Another common ~ u l.o i ". n eprocess used for ureoarine cellulose feedstock empioys two stages: First the wood chips are treated with a mildlv acidic aaueous solution to hvdrolvze and thus solubilize the hemiceilulose. Next comes "an alkaline delignification stage using an aaueous solution of caustic and sodium sulfide. This secondatage is the same kraft (German for strond process used to make cellulose . pulping . . .. for paper. A two-stage process is necessary because kraft pulping alone does not remove enough of the hemicelluloses to give the required level of cellulose purity. Reaction conditions for "prehydrolyzed kraft" pulping appear in Table 1. Although most of the lignin, hemicellulose, and extractives are removed during pulping, further processing is needed to reach the levels of ~urificationreouired of most cellulose feedstocks. As with the purification of most materials, thr last trares of impurities ure the hardest tu remove. Final cellulose purification, known as bleaching, requires a sequence of chemical treatments designed to remove residual lignin and hemicellulose without undue degradation or loss of cellulose. Some common treatment chemicals are cblorine, sodium hydroxide, sodium hypochlorite, and chlorine dioxide. The chemicals are dissolved in water and the resulting solutions slurried with the pulp for varying periods of time and temperatures. Only treatment with hot sodium hydroxide is performed under pressure. Pulp is normally washed well with water after each treatment. The sequence of treatments are grouped together under the term bleaching because the end result is a product of high whiteness. A typical bleach sequence is summarized in Table 2. The drop in cellulose D P across the bleaching stages
the total, resides within the cell wall (8). Materials like resins and ~olvsaccharidesthat can be extracted from wood with water o; neutral organic solvents are grouped together under the term extractives. Cellulose Feedstock Manufacture Cellulose is recovered from wood through a series of chemical reactions taking place under aqueous conditions, which degrade and soluhilize the lignin, hemicellulose, and extractives. The cellulose itself is never solubilized. nor is the physical nature of the f i l w destroyed, thus the end result is a hirhl\, fibrous ~ r o d u cesientiallv t free of noncellu"purified . losic wood material. The manufacturing process is similar to that used to make cellulose fiber for paper but is more exacting because of the high purity required of the final product. After harvesting the tree and removing the foliage, limbs, and hark, wood is reduced to chips 2-4 mm thick and 15-25 mm long that can he readily penetrated by the chemical solution used in the first, or pulping, stage of purification. Wood chips are loaded into a large pressure vessel that can be four or more stories high and 6 m in diameter and hold 100 tonnes of chips. The remaining space within the vessel is then filled with the pulping solution. In the commercial processgiven the most attention here, the pulping solution is a nearly saturated water solution of sulfur dioxide (60-80 -eA) . containine a small amount (-0.2 M of a hisulfite salt (ammonium, calcium, magnesium, or sodium). Consequentlv, the orocess is called acid sulfite . ~ u .l ~ .. i nReaction a. conditims are summarized in Tat~le1. Some 95'4 of the lirnin is degraded and soluhilized during arid sulfite pulping, mainly thiough cleavageoio-aryl bunds rr,ith intrud~~rtion ut highlv sulfonir acid groups . . hsdruphilir . (9). Polysaccharides are attacked and depoly~eri&d through acidic cleavage of glycosidic bonds resulting in solubilization of 90% of the hemicellulose. The acidic pulping solution also attacks cellulose. However, the initial D P is high and the glucosidic honds in cellulose are relatively stable to acid hydrolysis, as well as being somewhat physically protected within the crystalline regions. Thus, soluhilization of cellulose is held to a minimum, although the cellulose D P
-
. .
~
~~
~
Table 1. Pulping Condltlons
Methad Acid Sulfite Prehydrolyzed Kran Prehydmlysis Stage Kran Stage
Time at
Softwood
Max. Temp.
Max. Temp.
Pulp Yield
(OC)
(hr)
(%I
pH Range
Cation
Aetive Species
1-2
NHlt, Nat Cast or Mg2+
HSOC, H+ SO2
125-145
3-7
Na+
H+ SH-. OH-
160-175 155-175
0.5-3 1-3
45-50 35-40
3-4 13-14
Table 2. Typlcal Sulflte-PulpBleach Sequence Charge Purpe
Bleach Stage
Lignin anack and
chlorination (GI?)
Solubiiization Lignin and hemiceliulose ~~I~bilization Lignin solubilization. ~ e l l ~ lDP o ~reduction. e and brightening Brightening
Temp.
Time
pH
(OC)
(min)
3
2
20
15
Chemical
Pulp
(%)*
(%)a
6C
caustic exhaelion (NaOH) hypochlorite (NaOH
10
14
12
135
60
lr
12
10
40
90
chlorine dioxide
O.Sc
10
75
180
+ Clz)
4.5
Volume 63 Number 1
January 1986
51
Table 3. Comparison of Western Hemlock Wood and Chemlcal Cellulose Wwd
Cellulo~eDP Compo~ltion( % ) High DP cellulose LOW DP cellulo~e
Hemicellulo~e Llgnin
EXfractive~ Color
Chemical Cellulose
10,000
2,600
40
95 2 3
~
... 28 28 4
tan-brown
