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 Wood 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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An alternative interpretation is that the S2 layer consolidates upon ammonia evaporation and the microfibrils are more closely packed. LOSS of intermicrofibril hemicellulose or lignin is not likely and we must assume that the S2 wall becomes denser if shrinkage is caused by a consolidation mechanism. Such a result is difficult to visualize for ammonia, which relaxes hydrogen bonding and plasticizes the structure, then rapidly evaporates, giving little time for an ordered structure to form even if ordering forces were present. Furthermore, a more consolidated S2 cell wall layer should result in increasing the dynamic modulus and decreasing internal friction, and the observed results are to the contrary. Thus, the first described mechanism is believed to be responsible for the shrinkage observed following treatment with liquid ammonia. Shrinkage can be effectively reduced by adding solvents to the treating ammonia. T o determine the effects of solvents, vacuum-dried (at 50' C.) wood cubes, 1 cm. on each side, were treated with liquid ammonia and ammonia plus solvents for varying treatment times by refluxing a t -30' C. The cubes were then dried a t room conditions for 12 hours and finally vacuumdried a t 50' C. for an additional 12 hours. Solvents used were dimethylsulfoxide (DMSO), tetrahydrofuran (THF), dimethyl ether (DME), butylamine (BA), tetraethylenepentamine (TEPA). and Carbowax-400 ((2-400). The solvents may inhibit the physical attack of the ammonia on the cell structure, bulk the swollen structure, or both. T o illustrate the former, Figure 1 shows the volumetric shrinkage of yellow birch based on untreated volume as a function of the concentration of volatile T H F in the treating solution. Apparently the T H F solvent retards diffusion of ammonia into the lattice structures, because the x-ray patterns a t the 6-hour treating time are cellulose I11 and a t 2 hours are cellulose I. Two-hour treatment results in cellulose I11 x-ray patterns for impregnation with ammonia alone. Similar patterns were found for TEPA, DME, and DMSO. The effect of both inhibition and bulking in shrinkage is illustrated by (2-400 shown in Figure 2. Hygroscopic Properties
Relaxation of the lattice structures and the associated rearrangement of polysaccharide molecules result in a more disordered structure following removal of the treating ammonia. Even though hydrogen bonding between the wood molecules is re-established upon ammonia evaporation, the increased disorder makes new sites available for water adsorption. T h e added sites were estimated by determining the heat of wetting for treated and untreated material. Six kinds of wood were ground and each kind was separated into three replications each for no treatment and ammonia treatment. The total heat of wetting (oven-dry to fiber saturation) was measured for each sample by calorimetric methods (Table I). T h e per cent increase in the heat of wetting may
Spruce Aspen Birch Oak
106
l&EC
Untreated 17.3 17.7 19.9 16.8 18.2 16.9
"3-
treated 20.5 21.2 22.5 19.8 21.6 18.9
W
I
6 HRS
I
a
1 0
10 20 30 40 THF CONCENTRATION. PER CENT
50
Figure 1. Shrinkage in vacuum-dry volume of birch wood resulting from treatment with ammonia and tetra hydrofuran Observed cellulose x-ray patterns given b y Roman numerals
w
/*I
4o 1
- L
-12 TREATING TIMES 0 2 HRS 0 6 HRS
-16
h-201
1
0
IO
C-400
30 40 50 CONCENTRATION, PER CENT
20
Figure 2. Shrinkage in vacuum-dry volume o f birch wood resulting from treatment with ammonia and Carbowax-400 Observed cellulose x - r a y patterns given b y Roman numerals
be interpreted to a good approximation as the per cent increase of water-accessible sites and consequently as a measure of decreased molecular order. O n the basis that ammonia treatment results in relatively more hygroscopic and disordered wood structure, it is not surprising that wood so treated has less dimensional stability with moisture gain. The volume increases 5.1% from oven-dry conditions to equilibrium with 70% relative humidity and 76' F. for untreated wood and increases 8.6y0 for treated wood. Volatile solvents when added to the treating ammonia have a negligible effect on the subsequent dimensional stability (Figure 3). Because of its bulking action, (2-400 materially improves the stability of treated wood, which becomes more stable than untreated wood a t the highest '2-400 concentration tested (Figure 4). I n contrast, wood treated with ammonia plus DMSO has larger volumetric swelling upon moisture regain than wood treated with ammonia alone (Figure 5): because DMSO is a strong hydrogen bonder and is itself hygroscopic. Mechanical Properties
Heaf of Wetting Heat of Wetting, CaI./G. Oven-Dry Wood
Table 1.
Wood Douglas fir Pine
I
%
Increase
18.5 19.8 13.1 17.8 18.7 11.8
PRODUCT RESEARCH A N D DEVELOPMENT
Mechanically, ammonia-treated wood is more flexible and its elastic parameters are more dependent upon the duration or rate of stress application. The effect of ammonia treatment upon the vibration properties for birch wood was evaluated a t approximately 200 C.P.S. for fixed-free end conditions. T h e dynamic modulus, E,, and internal friction, tan 6, were measured for each of the specimens (machined to 6 inches long by 0.5 inch wide by 0.125 inch thick) in the vacuum-dry state before and after ammonia treatment.
w
VOLUME
IO
VOLUME
SWELLING
SWELLING
TREATING TIMES 2 HRS 0 6 HRS
.. ADDITIVE CONCENTRATION ,PER CENT
Figure 3. Volumetric change o f ammonia- and volatile solvent-treated birch wood with moisture gain for ambient conditions from oven-dry to 70% RH and
L/i*rREATED
WOOD
A
l 6 HRS
3 4
B
76" F.
IO 20 30 40 SOLUTION CONCENTRATION ,PER CENT
.O DMSO
> VOLUME
I
6
-
4
UNTREATED WOOD
W (3
z a I
2
SWELLING
TREATING TIMES 2 HRS 0
6 HRS
V
9i
b
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20
IO C-400
SOLUTLON CONC.
40
50
IO 20 30 40 DMSO SOLUTION CONCENTRATION ,PER CENT
0
PER CENT
L
i W 0
RESIDUAL
[r
0-
C-400
Figure
IN WOOD
5. Stability o f treated wood
Upper. Volumetric chonge o f ammonia- and dimethylsulfoxide-treated birch wood with moisture gain for ambient conditions from oven-dry to 7070 RH and 76' F. lower. Weight concentration o f residual dimethylsulfoxide in wood based on oven-dry weight o f untreated wood
40
2.2XIdl
n
0
IO 20 30 40 C - 4 0 0 SOLUTION CONC. PER CENT
Figure 4.
DYNAMIC
50
MODULUS
Stability of treated wood
Upper. Volumetric chonge o f ammonia- and Carbowax-400treated birch wood with moisture gain for ambient conditions from oven-dry to 7 0 7 0 R H and 76' F. lower. Weight concentration o f residual Corbowax-400 in wood based on oven-dry weight o f untreated wood
\Vood specimens of the size used essentially reach treatment equilibrium in about 2 hours of refluxing. T h e treatment results in approximately an 1870 loss in the dynamic modulus and a gain in internal friction of about 1007, (Figure 6), reflecting the more flexible and mobile state of the wood structure. l'olatile nonhydroxylic solvents added to the treating ammonia reduce the changes in both dynamic properties through dilution of the ammonia activity. For example, Figure 7 shows the dynamic modulus and internal friction for \\ ood treated with ammonia plus various concentrations of THF. The retarded development of cellulose I11 structures apparently (:aIses the difference between the curves for the two treating times.
1
0
2
4 6 8 IO TREATMENT T I M E , HOURS
z ,012
INTERNAL
12
14
16
12
14
16
FRICTION
cQ8 .O'"l/0
0
2
4
8
6
TREATMENT TIME
IO
, HOURS
Figure 6. Dynamic modulus and internal friction for vacuumdry birch wood in flexural vibration as a function of time of liquid ammonia treatment VOL.
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107
2.2XlO' 2.1
I
0
DYNAMIC MODULUS
IO THF
20 30 40 CONCENTRATION, PER CENT
50
INTERNAL FRICTION
I
0
IO 20 30 40 THF CONCENTRATION, PER CENT
50
Figure 7. Flexural dynamic modulus and internal friction for vacuum-dry birch wood previously treated with ammonia and tetrahydrofuran Observed cellulose x - r a y patterns given b y Roman numerals
2.2 1 IO6 ' HR
An interesting paradox appears to exist when the volume swelling upon moisture regain (Figure 3) is compared with the dynamic properties (Figure 7) for wood treated with ammonia plus a volatile solvent. T h e dynamic experiments show that as the volatile concentration is increased the wood structure is left in more ordered configurations. However, the results for moisture regain imply that increasing volatile concentration does not decrease water accessibility (which is related in part to extensive molecular order). Evidently order and hydrogen bond density are increased by increasing the volatile concentration, but these are of such a local nature that they are not resistant to relaxation by water. If such is the case, the dynamic properties for wood treated with ammonia plus a volatile solvent should be especially sensitive to initial moisture regain. If a nonvolatile material is added, the effect on the dynamic properties is more complicated. The dynamic modulus passes through a minimum and the internal friction through a maximum with increasing concentration of the nonvolatile in the treating solution, as shown in Figure 8 for C-400 and to a lesser extent in Figure 9 for DMSO. The results are interpreted to reflect the action of two competing mechanisms : reduction in re-establishment of hydrogen bonding by the bulking of the nonvolatile, and dilution of ammonia activity. The first mechanism would reduce the dynamic modulus and increase internal friction and is most effective a t low concentrations of the nonvolatile, where ammonia activity is not greatly reduced. T h e second is effective a t high additive concentrations. Also the heavy bulking a t high concentrations is expected to restrict the freedom of molecular movement physically (Figures 2 and 4) and in this respect compensates for the reduction in hydrogen bonding. The ultimate strength of treated basswood was tested for longitudinal tensile and compressive stresses. Tension speci2.1x10 2 .o
2.0 1.9
DYNAMIC
I
MODULUS
DYNAMIC MOOULUS
HR I.9
I
I 0
0
10
30
20
40
IO 20 30 40 DMSO CONCENTRATION , PER CENT
,029
50
INTERNAL FRICTION
,020
0
1.111 16 HR 6 HR
Ill
,014
L. 2 HR
%A
z
2 .012
INTERNAL
,010 I
.008 Q
= I 3
FRICTION
6 HR * 2 HR
I
,010
""3
.oo 10 C-400
20 30 40 CONCENTRATION PER CENT
50
0
IO
20
30
40
DMSO CONCENTRATION ,PER CENT
Figure 8. Flexural dynamic modulus and internal friction for vacuum-dry birch wood previously treated with arnrnonia and Carbowax-400
Figure 9. Flexural dynamic modulus and internal friction for vacuum-dry birch wood previously treated with arnmonia and dimethylsulfoxide
Observed cellulose x - r a y patterns given b y Roman numerals
Observed cellulose x-ray patterns given b y Roman numerals
108
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
mens were 10 inches long by inch thick by 1.5 inches wide with the width reduced to inch over a 2-inch gage length. The compression specimens were 2 cm. square in cross section by 6 inches long. Tension specimens were treated for 24 hours and the compression for 48 hours. Each condition was replicated eight to ten times. The specimens were stressed to failure at a constant rate of strain of 0.001 per minute with the wood in the vacuum-dry condition. The results show that the three treatments given-ammonia, ammonia plus 30% T H F , and ammonia plus 25% DMSO-all increase the tensile strength from that of untreated wood, hut that the compressive strength is reduced by the latter two treatments. The increases in tensile strengths relative to those for the untreated specimens shown in Table I1 are essentially a reflection of the shrinkage of stressed area caused by treatment. T h e tensile load a t failure does not vary significantly. The relative compression strengths decreased, except for the specimens treated with ammonia only because of substantial decreases in loads a t failure. Failure in compression is initiated by buckline of the cell walls. T h e more flexible and disordered wall structure resulting from treatment is expected to reduce resistance to buckling. Failure in tension is a fracture process and is not expected to be as strongly influenced by the treatments. The high strength in both compression and tension for specimens treated with ammonia only is probably caused by the large area shrinkage and extensive re-establishment of hydrogen bonding (although T H F is volatile, a few per cent by weight remained in the samples). In the ammonia-swollen condition the compression strength is expected to be reduced substantially more than the tensile strength. I n fact, the loss of compression strength probably is responsible far the ease of molding ammonia-plasticized wood. Micros copic examination of specimens repeatedly flexed while plastici zed reveals a high density of minute compression failures in the Ei2 layers of the cell walls, which appear as local bucklings of the Inicrofihrils (Figure IO). These small failures can effectively 5ibsorb lame deformations while the wood is dasticized withou t changing the external appearance ofwaod and enhance flexibiliity of the material after ammonia evaporation. Y
Electric:a1 Properties
COUIITLJ" A r r N o i O 01"
Figure 10. Compression buckling of microfibrils in S2 layer of secondary wall of birch wood caused b y repeated flexing in ammonia-plasticized condition MogniRcotion 30,OOOX, reduced for reproduction
u)
z
uLL 5
5W 'x 0
2
2
T h e dielectric constant and direct current electrical resisterP AptprminpA
- 4 trSslterl .."-- Iln+rert.-rl ...._-.--
fnr hirrh rlrnnA 1-11
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M e c t r i c constant was measured 1.c. resistance was measured a t a B per inch. Electrical measuremens when untreated and conditionea to several moisture contents. T h e specimens were then treated, the ammonia was removed, and the electrical measurements were repeated a t several moisture contents. As expected, the dielectric constant for treated wood is higher
-.
.
4
6
10
8
I
MOISTURE CONTENT, PER CENT - .
Figure I I. Dielectric constant of untreated ond ammoniotrea
.
. _. -.. __. _-..
.__.-.....,
Figu _,.. ",,,, V l r Y ",," ammonia-treated birch wood os a function of moisture content VOL. 5
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109
of the ivood resulting from the volumetric shrinkage upon ammonia removal. Of course, a t a given temperature and relative humidity the treated wood attains a higher equilibrium moisture content, \vhich further increases the dielectric constant. But \Then comparison is made a t the same moisture content, the dielectric constant is proportional to bvood density just as has been found for various unmodified woods (4, 5). Direct current resistivity of treated Wood decreases substantially relative to untreated wood a t the same moisture content (Figure 12). Charge conduction through wood is by ions (2). Because of the volumetric shrinkage caused by ammonia treatment, a decrease in resistivity is to be expected on the basis of a higher ionic density, other things being equal. O n this basis the resistivity should decrease by only 20 %, but the observed decrease is by more than one order of magnitude. Treatment Lvith anhydrous liquid ammonia a t a pressure of 1 atm. can yield ammonium salts from free carboxylic acid groups ( 7 ) . Small concentrations of such salts are probably responsible for the decrease in resistivity observed. Other mechanisms ma) be acting as well-for example, ammonolysis of esters occurs to produce acid amides ( 5 ) . However, ammonia in liquid phase a t room temperature existed in the
wood specimens a very short time under our treatment conditions and the mechanism probably contributes little to the conductivity. Ac knowledgrnent
The assistance of Conrad Schuerch, Mary P. Burdick, Miroslav Mahdalik, and Richard Elliott and financial support by the Sational Science Foundation are gratefully acknowledged. literature Cited
(1) Bjorkvist, K. J., Jorgensen, L., .4cta Chern. Scand. 5 , 978 (1951 ). ( 2 ) Murphy, E. J., J . Phys. Chem. 33, 509-32 (1929). (3) Schuerch, C., Burdick, M. P., Mahdalik, M., IND.EXG.CHEM. PROD.RES.DEVELOP. 5,101 (1966). ( 4 ) Skaar. C., State University College of Forestry, Syracuse, N. Y., Tech. Publ. 69 (1948). ( 5 ) TVong. P. Y.:Balker, H. I., Purvis, C. B., Can. J . Chem. 42, 2434-9 (1964). ( 6 ) Yavorsky, J. M., State University College of Forestry, Syracuse, K. Y.,Tech. Publ. 7 3 (1951). RECEIVED for review October 13, 1965 ACCEPTED January 24, 1966 Division of Cellulose, \Vood and Fiber Chemistry, 150th Meeting, ACS, Atlantic City, N. J., September 1965.
VALERIC ACID ESTERS OF CELLULOSE J. W. M E N C H , BRAZELTON FULKERSON, AND G. D. H l A T T Cellulose Technology Division, Eastman Kodak Co., Rochester, .V. Y.
Cellulose valerate as well as the mixed cellulose acetate, propionate, and butyrate valerate esters have been prepared economically b y the conventional acid-anhydride-sulfuric acid catalyst procedure. Cellulose propionate valerate containing about 8y0 propionyl exhibits properties similar to those of cellulose trivalerate. This ester, prepared from water-activated, propionic acid-dehydrated cellulose, was selected as representative o f this class of esters for a study o f properties. The properties of cellulose propionate valerate are more like those of the higher fatty acid esters o f cellulose than those of the lower (C, to C,) esters. It shows a low melting point, high moisture resistance, good heat stability, and compatibility with a wide variety of resins and plasticizers, forms flexible compositions with several waxes, and has good adhesion to glass, metal, and paper. The ester i s soluble in a wider variety o f organic solvents than any other aliphatic ester of cellulose.
c
esters of aliphatic acids above butyric have never been manufactured in quantity, either because of the cost of the acids or because they must be prepared by uneconomical methods. However, their low melting point and high water resistance should prove desirable for some uses (8). Cellulose caprate is prepared and sold as a cement for lenses and optical systems (4, 73, 74) and it, as well as some of the other higher fatty acid esters, has been proposed for use in adhesives and for heat sealing (7). The chloroacetic anhydride method (7) used for the preparation of cellulose caprate is not practical for large scale production and has limited the potential uses of this and the other higher cellulose esters to those in which cost is not a n important factor. Thus, a different method of preparing these esters or other similar esters that could be prepared more economically is needed. Valeric acid has become available a t reasonable cost and a practical method of making its cellulose esters has been developed (5). The esters containing high proportions of valeryl have properties more like those of the higher fatty acid esters than those of the lower (C, to C , ) acids. This paper describes ELLULOSE
110
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
some of the properties of these esters, with special emphasis on cellulose propionate valerate containing about 8% of propionyl. Preparation
Contrary to statements in the literature (771, valeric acid esters of cellulose are readily prepared by conventional esterification procedures using water-activated cellulose. valeric anhydride, and sulfuric acid catalyst (6).
Cellulose Valerate. One part of cotton linters was activated by soaking in distilled Lvater for 16 hours and, after centrifuging, the water was displaced with methylene chloride or valeric acid. The dehydrated cellulose was then placed in a Werner-Pfleiderer type of mixer with 4.5 parts of valeric anhydride and the esterification conducted a t 80' to 100' F. using from 0.02 to 0.08 part of sulfuric acid catalyst. The viscosity of the product is regulated by the reaction time, temperature, and amount of catalyst used. \\'hen a clear, grainfree solution was obtained, a 5070 excess of magnesium carbonate was added over that required to neutralize the sulfuric acid used and the reaction mass was raised to 245' to 250' F. for 3 hours. This replaced combined sulfate with valeryl and
