LIQUID AMMONIA-SOLVENT COMBINATIONS IN WOOD PLASTICIZATION Chemical T’eatments CONRAD SCHUERCH, M A R Y P. B U R D I C K , A N D M I R O S L A V M A H D A L I K Chemistry Department, State C’nicersity College of Forestry, Sjracuse, .Y. Y .
W o o d plasticized b y anhydrous ammonia becomes a deformable material without elastic return and with permanent set. Liquid ammonia i s more effective than gaseous ammonia. Deformation in the absence of ammonia, however, i s always followed b y recovery o f shape if the restraining force i s immediately relaxed. There i s little chemical change in the wood, but physical changes include relaxation of the crystal lattice and hydrogen bonding. Swelling and warping during treatment and shrinkage and checking during drying can b e controlled b y r a p i d even penetration, slow evaporation, and addition of nonhydroxylic a d ditives to the ammonia. High concentrations o f additive sometimes maintain flexibility in the wood after removal of the ammonia or make it susceptible to rapid softening with cold water. Hydroxylic solvents, including water, compete with the cellulose for the ammonia and must be kept to a minimum. Factors influencing the rate o f ammonia penetration b y diffusion and convection are described.
F WOOD
is treated with liquid ammonia, ammonia enters into
I and interacts with the entire kvood substance, swelling and
softening the entire structure: crystalline and amorphous cellulose, hemicellulose. and lignin. The resultant plasticized wood can be bent, molded, formed, or compressed without introducing significant permanent stresses into the structure (5-7). This article summarizes the results of investigations on this method of wood plasticization, defines the changes occurring and the properties of the plasticized wood, and describes certain variations of the process. including the use of liquid ammonia-solvent mixtures. The physical properties of the wood after removal of the ammonia are described in detail in another article ( 4 ) .
treatment, was significantly less flexible than wood treated with liquid. This observation was unexpected because gaseous and liquid ammonia in equilibrium are of equal activity. We assume that manipulating plasticized wood produces heat which is dissipated largely as heat of vaporization. If ammonia is not present as a liquid phase, the hydrogen bonding between wood substance and ammonia is rapidly destroyed by the mechanically generated heat and flexibility disappears \vith the formation of new hydrogen bonds between the macromolecules. If free liquid ammonia is present, it can be evaporated, the ammonia-cellulose association is not destroyed, and flexibility remains. Liquid Ammonia Transport into Wood Samples
Gaseous Treatments
Exploratory tests were made to determine whether gaseous ammonia would produce the same plasticity of wood as liquid ammonia. At atmospheric pressure little or no plasticization occurred, because too little ammonia was absorbed by the wood substance. Wood veneer strips (tongue depressors) were, therefore, placed in a pressure vessel attached directly to a tank of liquid ammonia. Air was exhausted from the vessel and the wood treated to equilibrium with gaseous ammonia a t its own vapor pressure a t ambient temperature. This method of treatment was found to be inferior to liquid treatments a t low temperature for two reasons. First, the time required to reach equilibrium was significantly longer than that for liquid ammonia. We believe that mass transport of gas into the wood was probably rate-determining and that this factor will usually restrict the use of gas-phase impregnations of wood to applications which require only small quantities of chemical evenly distributed through the wood substance. Other observations suggest that the homogeneous penetration of wood by gaseous ammonia is more rapid than by liquid ammonia, even though the amount entering per unit time is much less. T h e second disadvantage of the treatments with gaseous ammonia was that the wood, even after long
T o follow the penetration of chemical into wood samples and the accompanying physical changes? wood cubes 11/2 inches square were treated with liquid ammonia a t dry ice temperatures. Ll’ood samples of this thickness are probably of little interest for wood forming, but were chosen to increase the difficulty of even penetration and to increase the physical strains induced by slvelling pressures on treatment and by shrinkage during evaporation. Three methods of observing penetration were used.
1. Phenolphthalein was dissolved in liquid ammonia and, after treatment of the wood with the solution and evaporation of the liquid ammonia, the wood was split along the grain. The pattern of penetration was observed by wetting the fresh surface lightly with water and exposing it to fumes from a concentrated solution of ammonium hydroxide. The characteristic red color showed the depth of penetration of the dyestuff. 2. A small splinter of wood was separated from a particular site in the block, and its x-ray diffraction pattern \vas determined ( 7 ) . Wood into which ammonia had fully penetrated reverted to the characteristic cellulose I11 structure, unpenetrated wood retained the cellulose I structure, and frequently intermediate cases were found in \vhich both lattice structures were observed. 3. The standard semimicro Kjeldahl nitrogen determination was used. VOL. 5
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Typically untreated birch, pine, and maple had a nitrogen content of about 0.1 to 0.14%. Treatment a t -30' C. for up to 6 hours gave a residual nitrogen content of around 0.4y0, although one sample of beech wood gave values around 0.9 to 1.0%. Birch on 48-hour treatment a t -30' C. contained 1.4470 residual nitrogen, and this appears to be near the maximum for other species as well. (These values were obtained on samples pre-evacuated and filled with carbon dioxide before immersion.) Comparison of the three methods showed agreement between the Kjeldahl and x-ray data, but frequently areas of wood into which ammonia had penetrated contained no phenolphthalein. T h e phenolphthalein method was, therefore, used as an indication of liquid penetration by convection and the other techniques were used to demonstrate penetration by a combination of convection and diffusion. Factors influencing the rate of flow of liquid ammonia into wood blocks were investigated by immersing 1'/,-inch cubes of hard pine in ammonia containing some phenolphthalein, then splitting and testing them with water and ammonia vapor. Single blocks were removed after 2, 4, and 6 hours. A second group of three blocks was evacuated, the air was replaced with carbon dioxide, and they were then treated similarly. The third group of three was evacuated, filled with carbon dioxide, kept a t dry ice temperature overnight, and then treated like the other two sets. The pattern of phenolphthalein penetration showed that liquid flow was fastest in the last group, about SOYo complete after 2 hours and essentially complete after 4 hours (Figure I). IVithout cooling but with carbon dioxide, 6 hours was required for 8070 penetration of the wood block by phenolphthalein, and in the presence of air the penetration of the wood block was somewhat slower. An outer section of the three blocks treated for 6 hours under the three separate conditions showed no penetration by phenolphthalein. This section clearly corresponded to a portion-presumably heartwoodfrom which all liquid flow was excluded. Movement of liquid into the wood was clearly shown to be very largely longitudinal -i.e., with the grain-as is general. Because of the variability of wood substance, the actual times determined in these experiments cannot be considered generally applicable to the species. I t is, nevertheless, clear that the flow of liquid ammonia into wood is delayed by the presence of a n inert gas (compare results with air and carbon dioxide). by temperatures of the wood higher than the boiling point of liquid ammonia (at the pressure employed), and by the presence of impervious sections of the wood structure. T o demonstrate the presence of ammonia in areas of the wood unpenetrated by phenolphthalein, 1'/2 X 11/* X 2 inch blocks of white pine, hard pine, and beech wood were evacuated and treated with carbon dioxide a t ambient temperature. (The 2-inch dimension was parallel to the grain.) They were then immersed in liquid ammonia and treated for 2 hours. The following data obtained from material from the center of the block demonstrated the presence of ammonia throughout the structure in regions which were not reached by phenolphthalein. Wood Sfiecies
\Vhite pine Hard pine Beech
Nitrogen Content, 7 0
Cellulose Lattice
0.36
I and I11 111 I and I11
0.43 0.47
By stopping treatment a t a n appropriate stage it was possible to obtain in smaller samples of hard pine, soft maple, and beech wood heartwood containing cellulose I and sapwood containing cellulose 111. Since in this way we measure a rate of 102
I & E C PRODUCT RESEARCH AND DEVELOPMENT
2 HOURS
4 HOURS
6 HOURS
DIRECTLY IMMERSED FROM AIR
IMMERSED FROM CARBON DIOXIDE
// ///
E4
PRECOOLED IN CARBON DIOXIDE
Figure 1 . Penetration of phenolphthalein in liquid ammonia into hard pine blocks
penetration which has occurred by a combination of convection and diffusion, this result is expected and represents incomplete penetration. Physical Changes on Treatment
On treatment with liquid ammonia all species of wood tested show a volumetric swelling. Since the liquid ammonia enters for the most part longitudinally through the end grain, the wood samples take on a concave shape initially, Jvhich will usually revert to a symmetrical slightly swollen form on lengthy treatment. Thin (1/8 inch) stock will often tend to warp during treatment because of swelling stresses, and occasionally by relaxation of stresses introduced in the \vood during growth. IYith only slight physical restraints this \varping can be controlled and on complete treatment tends to disappear. Rapid penetration minimizes the effect and it can usually be ignored, O n evaporation of the liquid ammonia, shrinkage occurs roughly 10% in the tangential and radial directions and 27, in the longitudinal, or about 20% volumetrically (Table I). More detailed data are presented by Pentoney ( 4 ) . Evaporation occurs more rapidly a t the ends of blocks or boards and the wood samples frequently take on a convex or bulging shape during drying. There is some tendency toward excessive shrinkage a t the ends of samples. Distortion during drling is minimized by a slow rate of evaporation and when necessary by mild restraints. A great variety of hardwood cut veneers (1 X 4 x 40 inches) were treated with liquid ammonia and formed into a variety of shapes, designs, statuettes, etc. I n no case was checking recognized as a problem. However, when blocks of white pine (1 X 5 X 5 inches) and a variety of hard and soft woods (11/* X I1/z X 2 inches) were treated. checking occurred frequently. (Longest dimensions were with the grain.) A controlled rate of evaporation in thick stock minimized checking but we were not always able to eliminate it entirely in this way. Checking was controlled by diluting the ammonia with selected solvents. Although minor differences will probably be observed in the response of various species to ammonia treatment, their similarity is more remarkable. All are plasticized by liquid ammonia and can be bent and formed. I n general, lowdensity species with thin cell walls are more subject to comprpssion failure on bending, but this difficulty appears minimized with liquid ammonia. For example, 3/lo-inch thick balsa wood could be bent into a S/8-inch radius of curvature with only slight regularly spaced creases on the compression side. IVe were unable to duplicate this with steam bending or with gaseous ammonia under pressure. Presumably the even plasticization of the liquid ammonia and the slight stiffening effect of the low temperature were advantageous.
Water and Other Additives
LVhen liquid ammonia is used in conjunction with other solvents, differences are observed in the flexibility, x-ray diagram, Fveight retention, and shrinkage and checking. T h e most common diluent of ammonia is water, since under ordinary conditions it amounts to 6 to 10% of wood weight. Its influence on plasticization by ammonia is, therefore, of significance. If air-dried wood is treated with a n excess of liquid ammonia, the actual moisture content of the interior wood substance cannot be accurately defined. Therefore, to determine the influence of moisture content on the flexibility of plasticized wood, oven-dried veneer strips were treated with excess liquid ammonia containing known amounts of water. When the content of water in ammonia was much above lo%, a substantial decrease in wood flexibility was noted and failures became noticeably more frequent on flexing. Since this moisture content could easily be reached in the interior of air-dried wood samples treated Lvith liquid ammonia, prior drying is recommended. Loeb and Segal (2) have shown that ethylamine does not enter the cellulose crystal lattice if the moisture content of the amine is above 10%. Presumably we are observing a similar phenomenon in the increased stiffness of wood when wet ammonia is used. If glycerol is added in dry ammonia, flexibility is notably lessened in about the same range of concentrations. By contrast, if dimethyl ether, diethyl ether, dimethylsulfoxide, Carbowax, or tetrahydrofuran is used as additive, much larger quantities can be used without noticeably decreasing wood flexibility and its tendency to remain in the formed positions. For example, veneer strips were bent in mixtures of 5070 dimethyl ether and 507, ammonia with results comparable to those reported by Schuerch (5). T h c explanation of this difference in behavior is evident. The second series of solvents listed act only as proton acceptors. They, therefore, do not compete with the cellulose for ammonia and act only as diluents of the active solvent. Dimethylsulfoxide as a strong hydrogen bonder may in part even assist the softening process. ,4substantial fraction of substances of this general type will usually be permissible. Solvents with acidic or proton-donating character-i.e, water, alcohols, acids, phenols, mercaptans-not only dilute the ammonia but also compete for the available ammonia, and reduce the ammonia activity still further by ionization. As a result, much smaller amounts can be tolerated. This generalization will be of significance whenever liquid ammonia is used in mixture with other substances for special purposes. Although the liquid ammonia treatment of wood is a very mild treatment chemically, physically it is drastic. There is little permanent stress introduced in the bending of liquid ammonia-plasticized wood, but swelling forces in polymersolvent systems can be very high and, considering the difficulties of homogeneous penetration, the relaxation of lattice structures, and the differential pressures that can result, it is not surprising that distortion of shape can occur during treatment. Similarly with significant volumetric changes on drying, checking is to be expecttd, especially at rapid rates of evaporation. To investigate the control of checking, 1*/2 11/2 2 inch blocks of birch, red oak, hard and white pines, soft maple, beech, and poplar were evacuated and filled with carbon dioxide. The blocks a t room temperature were immersed in liquid ammonia and liquid ammonia-solvent mixtures with solvent concentrations up to 25% of the solution. T h e solvents used were dimethylsulfoxide, polyethylene glycol (Carbowax-400), and tetrahydrofuran. T h e penetration of
x
x
the blocks was complete in about 7 hours. T h e volumetric shrinkage in pure liquid ammonia was near 20y0; in red oak and probably poplar, somewhat higher. Dilution of the liquid ammonia with solvents resulted in less volumetric shrinkage, and the nonvolatile solvents dimethylsulfoxide and Carbowax completely inhibited shrinkage a t around 25% concentration. T h e volatile solvent tetrahydrofuran required larger concentrations to produce the same effect. Checking was more readily avoided and only about 5 to 107, of nonvolatile additives in the solution were required for its elimination. Again, tetrahydrofuran required larger concentrations to be effective (Table I).
Table 1.
Shrinkage and Checking of Blocks on Drying from Ammonia-Solvent Mixtures"
Species
IVhite pine
Soluent, 7cConcn. in NH3b
DMSO
5
Treatment Time,
Hr . 7
Volumetric Shrinkage, 7 0
Checking
22
None
10
Soft maple
DMSO
Beech
DMSO
Poplar
DMSO
Hard pine
THF
Soft maple
THF
Beech
THF
Poplar
THF
Birch
None
Red oak
None
15 25 5 10 15 25 5 10 15 25 5 10 15 25 5 15 5 15 5 15 5 15
17 10 3
7
20 12 11
None
L
7
11
7
None
9
7
7 7 , 7
2 3 5 7 2
3 0 or negative 22 None 18 20 None 18 15 Severe 12 Slight 27 Slight 19 h'one 5 None 9 13 20
4 6
Hard pine
Birch
7 2
None
10 17 19 19 None
4 6 7 2 3
None
25 None 25 25 5 25 7 Hard pine C-400 5 3 12 None 15 3 9 25 3 None 5 6 10 15 6 7 25 6 None a Blocks ( P/? X P / 2 X 2 inches) jilled w i t h carbon dioxide at room temperature, immersed i n solvent combinations at - 78", then removed and placed in a D e w a r j l a s k f o r 24 hr., air-dried f o r 72 hr., then measured for C-400
volumetric shrinkage. Volumetric shrinkage is based on untreated dimensions. Complete penetration probably required 7 hr. Checking w a s more severe on more rafiid drying f r o m pure ammonia. Concentration calculated g . solvent as g. solvent f ml. h7H3.
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A large number of yellow birch reeds x X 6 inches) were prepared for mechanical testing by treatment with a variety of solvent-liquid ammonia combinations. The solvents included glycerol, ethanolamine, dodecyl alcohol, polyethylene glycol (Carbowax-400), tetraethylenepentamine, butyl-, hexyl-, and dodecylamines, tetrahydrofuran, diethyl ether, and dimethylsulfoxide. The results of mechanical tests are reported by Pentoney ( 4 ) . O n several of these samples x-ray patterns were obtained (Table 11). Usually within a given series the cellulose I11 pattern was observed on samples which were flexible while wet. In solutions with concentrations of ammonia insufficient to cause much flexibility, the xray pattern was usually cellulose I or I and 111, showing incomplete penetration of the crystal lattice. However, in a few cases flexible samples had cellulose I crystal lattice structures. We assume that in these particular samples ammonia-cellulose crystal lattice had not yet formed but that sufficient relaxation of the more disordered regions probably occurred to permit relative movement of groups of ordered macromolecules. This may correspond to movement of microfibrils past each other, while the crystalline regions of the microfibril remain unpenetrated by ammonia. Perhaps other interpretations are possible as well.
(]I8
Miscellaneous Observations
In a few experiments we subjected the ammonia-treated wood to temperatures of 220' C. in the presence and absence of glycerol, but we did not observe the formation of cellulose IV.
Additive,
%
None Glycerol 7 13 43 13 followed by 48
21.6 Tetraethylenepentamine -5 -1 5 20yG Butylamine 20Yc Hexylamine 3y0 Butylamine 5YG Hexylamine Tetrahydrofuran 11 27 52 75 Diethyl ether
Acrylonitrile, vinylidene chloride, and methyl methacrylate dissolve in liquid ammonia to an extent of a t least 257, and styrene in smaller quantity. The solutions can be introduced into wood; i n situ polymerization can then be induced by radiation (3) or presumably by catalysts (8) (not peroxides !). In our case we allowed the ammonia to evaporate and then polymerized the monomer by cobalt-60 irradiation. \\'eight increases up to 1270 were obtained but the properties of the products were not investigated. Vinylidene chloride, 25y0 in ammonia solution, caused considerable cell wall collapse in thin (1,',6-inch) stock and shrinkage of the wood was considerably greater than with ammonia alone. We have investigated the treatment of wood with a number of hydrogen-bonding solvents without ammonia in order to determine whether it is possible to obtain the same degree of flexibility produced by ammonia or ammonia-solvent combinations. T h e solvents have included lower aliphatic amines and dimethylsulfoxide. Dimethylsulfoxide causes greatly increased flexibility, and wood saturated with it can be bent in the cold about as readily as wood heated to 60' to 80' in \vater. However, failures are frequent, perhaps because of excessive plasticization of the lignin. The wood still exhibits immediate elastic recovery and if it is clamped, leached with Jvater, and dried, the wood is not stable to hot water but reverts nearly to its original form with little permanent set. In our experience ammonia is unique in its ability to introduce permanent set into tvood and to eliminate recovery. A wide range of properties can be produced in ammonia-
Table II. Treatment of Yellow Birch Wood with liquid Ammonia-Solvent Combinations" Treatment A v . 7cW t . Increase T i m e at Lattice Remarks of 3 Samples -78' C., Hr. Cellulose I occasionally, all flexible I11 2-6 1 . 9 == ! 0.7b
2 3 3 1 2 3
4.8 6.6 15.5 17.4
I I
4.0
I
2 2 2 2
3.2 5.8 9.1 12.9 5.8; 10.7; 4.8; 5.7; 4.0 3.2
I
+ I11
Rigid Slightly flexible; 220' heat after treatments did not produce Cellulose IV Flexible
I11 111
111
I 7.8 13.7 6.2 6.9
3.7 3.6 3.6 3.2
+ I11
Part of Carbowax insoluble Flexible
111 111 I I I I11 I I11
Probably incomplete penetration Probably incomplete penetration
I I
Flexible Less flexible Slightly flexible Too rigid to flex
++
111 1.9 Slight layering of soln., samples slightly flexible I11 1.8 23 Layered soh. I 2.0 47 Layered soln., samples too rigid to flex 2.2 73 Dimethylsulfoxide i e m p . -23 O ; flexible 111 6.4 2 13 Temp. -16"; less flexible 111 I 12.8 2 31 Temp. - 15 not all DMSO in s o h . 14.7 2 58 0 Straight grained yellow birch reeds ( 1 / 8 X l / 2 X 6 inches) immersed i n liquid ammonia solvent combinations at -78" C. (excrpt f o r dimethylsulfoxide mixtures). Percentages giaen i n column 7 based on weight of total solution. Flexibility refers to wood containing ammonia, shortly after remooal from bath. Average weight increases obtained on ammonia-treated wood are substantially Only minor differences observed between air-dried and ooen-dried samples. 1.9 us. 0.5%. This difference may represent occluded ammonia lost on grinding and drying wood for higher than nitrogen analyses on same samples, &&ally analysis. 9
+
O;
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l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
treated wood by changing the degree of molecular order in the structure. For example, )\.hen 1/,6-inch birch veneer strips Lvere treated with ammonia solutions containing 25% dimethylsulfoxide and then allowed to dry, the veneer remained flexible enough to be bent into a diameter of curvature of 1 inch. O n release of the retaining force the veneer slowly straightened out to its original position. The rate of return was much slower than that of similar stock treated Lvith dimethylsulfoxide alone. l’eneer treated first lvith ammonia and then placed in glycerol until the ammonia evaporated behaved like that treated with mixtures of dimethylsulfoxide and ammonia. Veneer saturated in ammonia, immediately leached xvith water: hardened on drying but reabsorbed tvater rapidly and took on a leathery quality in the moist state. Veneer treated \vith mixtures of ammonia and methyl methacrylate, allowed to dry ivithout polymerization or monomer retention, also became leathery on moistening with Ivater. Aging altered the quality of some of these softer products. I t is thus apparently possible to modify w.ood so that it can later be rapidly softened by moistening \vith water, then bent into moderately curved forms and allolved to reharden on drying. Since dimensional stability to moisture changes is also favored by high concentrations of additives, further research on the effect of combinations of liquid ammonia and additives appears justified. Repeated flexing of plasticized wood also results in less rigidity of the product after drying.
Conclusions
The use of liquid ammonia alone and in conjunction with various additives and physical treatments permits one to obtain materials lvith a wide range of properties. Some of these properties should permit lvood to be more Lvidely useful for manufacturing, structural, and decorative purposes. Acknowledgment
LVe gratefully acknoLvledge the assistance of J. A. Meyer, R . H. Marchessault, and R . LV. Pentoney, and financial support of the National Science Foundation. literature Cited (1) Howsmon, J. .A,> Sisson, I V . A., in “Cellulose and Cellulose Derivatives,” E. Ott, H. M. Spurlin, and M. It‘. Grafflin, Interscience Publishers, New York, 1954. ( 2 ) Loeb, I,,,Segal, L.:Textile Research J . 25, 516 (1955). (3) Meyer, J. .I.,Forest Products J . 15, 362 (1965). (4) Pentoney, R. E.: IND.Eric. CHEM.PROD.RES.DEVELOP. 5,
105 (1966). ( 5 ) Schuerch, C., Forest Products J . 14, 377 (1964). ( 6 ) Schuerch, C., Znd. Ens. Chem. 5 5 , 39 (1963). ( 7 ) Schuerch, C. (to Research Corp.), U. S. Patent Application Serial No. 337,439 (1964). ( 8 ) Siau, J. F., Meyer, J . A , Skaar, C., Forest Products J . 15, 426 (1965). RECEIVED for review October 13, 1965 ACCEPTED January 24, 1966 Division of Cellulose, IYood, and Fiber Chemistry, 150th Meeting, ACS, Atlantic City, E. J., September 1965.
LIQUID A M M O N I A - S O L V E N T COMBINATIONS IN WOOD PLASTICIZATION Properties
of Treated
Wood R I C H A R D E. PENTONEY W o o d Products Engineering Department, State Uniuersity College of Forestry, Syracuse, N . Y.
W o o d treated with anhydrous ammonia and dried i s denser and more flexible than dry untreated wood. Tensile strength i s increased but compressive strength i s decreased b y treatment and the dielectric constant increases proportionally with the increase of density, The treated wood i s more hygroscopic and swells more than untreated wood. Density increases caused b y treatment can b e reduced b y adding solvents to the ammonia. HE physical properties of wood are permanently altered by Ttreatment with and removal of liquid ammonia. Modifications result from physical changes caused by interactions of liquid ammonia with the woody cell walls (3). This article summarizes the mechanical, electrical, and hygroscopic properties observed for wood following evaporation of the treating ammonia.
Volumetric Shrinkage
Treatment with ammonia of small specimens (I-cm. cubes) of dry wood, folloived by evaporation under room conditions and subsequent oven drying, results in volumetric shrinkage of approximately 2070 relative to the volume of untreated dry wood. Shrinkage observed for small wood specimens may be termed “normal,” in contrast with the often larger shrinkage
which includes cell collapse observed for large specimens. Normal shrinkage is believed to result from the coiling tendency of the linear polysaccharides which are released by the ammonia from imperfect lattice restraints. Because most of the volumetric shrinkage is caused by contractions in the tangential and radial directions, the coiling probably is most pronounced in the primary, SI, and S3 cell rvall layers, which have microfibrils oriented nearly circumferentially around the cells. Thus, a reduction in the mean end-to-end distance of the molecules would decrease cell diameter. Longitudinal shrinkage is smaller, possibly because the S2 cell \Tall layer, with microfibrils oriented nearly parallel to the cell axis, is more crystalline and insufficient relaxation of lattice forces takes place to permit a substantial amount of molecular rearrangement. T h e secondary cell wall is usually composed of a n outer layer, S1, a middle layer, S2, and a n inner layer, S3. VOL. 5
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