When prehydrolyzed pine wood is cooked in a second stage with a kraft liquor containing borohydride, the retention of glucomannan and of xylan increases. This is also fully in accord with the present results because the pine xylan ought to have been modified as a result of prehydrolysis, with almost complete loss of arabinose, making the prehydrolyzed pine xylan rather similar to the birch xylan.
(4) Machek G., Richards, G. N., J . Chem. s o d . 1957, P. 4500. ainter, T. J., Can. J . Chem. 37, 497 (1959). i i u d i e r , A. J., Eberhard, L., Bull. sot. Chirn. 1960, p. 2074. (7) Timell, T. E., Svensk 65, 435 (1962). (8) Whistler, R. L., BeMiller, J. N., Advan. Carbohydrate Chem. 13,289 (1958). (9) Whistler, R. L., Corbett, W. M., J . A m . Chem. Soc. 77, 3822 (1955). (10) Whistler, R. L., Richards, G. N., Zbid., 80, 4888 (1958).
Literature Cited
RECEIVED for review September 4, 1964 ACCEPTEDFebruary 19, 1965
(1) Xurell, R., Hartler, N., Persson, G., Acta Chern. Scand. 17, 545 (1 967). \ - ~ - - I -
(2) Croon: I., Enstrom, B., Tappi 44, 870 (1961). ( 3 ) Klemer, A., Lukowski, H., Zerhusen, F., Cham. Bzr. 96, 1515 (1963).
Division of Cellulose, Fiber, and Wood Chemistry; 148th Meeting, ACS, Chicago, Ill., September 1964.
END O F SYMPOSIUM
PAD- BA KE CAR BOXY M ET HY LAT IO N
OF COTTON TEXTILE MATERIALS ROBERT M. R E I N H A R D T AND T E R R E N C E W . FENNER' Southern Regional Research Laboratory, K e w Orleans, La.
Cotton textile materials can be carboxymethylated b y a new process in which the cotton is impregnated with an aqueous solution containing sodium chloroacetate and sodium hydroxide, padded or centrifuged to remove excess solution, and baked to etherify the cellulose. The effects of processing variables and the properties of the products have been determined. Cotton fabric has been carboxymethylated by this new process on a semipilot plant scale. The treatment appears more suitable for use in continuous processing than the present conventional wet method for carboxymethylating cotton fabric. Standard textile finishing equipment may be utilized. A single solution containing both reagents necessary for reaction i s employed. Much lower concentrations of sodium hydroxide can be used than the 40 to 50% solutions required in the conventional two-stage carboxymethylation process. Other a-halocarboxylic acid salts also were used to prepare alpha-substituted carboxymethylated cotton fabrics.
c
modification of cotton to impart new and improved properties with retention of the important fibrous nature of the starting material has long been a research objective of this laboratory. Many chemically modified cottons have been developed as a result of this research (7-9). Carboxymethylated cotton, a product which has been the subject of several previous publications (4, 5. 10-731, is now commercially produced for a captive use and on a commission basis. Cotton which has been carboxymethylated to a degree of substitution of 0.2 or less has properties considerably changed from those of the untreated material: a built-in starched effect, increased moisture regain, \rater absorbency, water permeability, changed dyeing characteristics, increased resistance to soiling from aqueous dispersions. greater ease of soil removal, cation exchange properties. high water swellability, and a greater receptivity to further chemical treatment than unmodified cotton. Products with degrees of substitution cf 0.3 or more disintegrate in water or dilute alkali solution. Textiles of this type are useful whenever a temporary, easily removable member is needed. More than 40,000,000 pounds per year of nontextile sodium carboxymethylcellulose (cellulose gum or CMC) are being produced in this country. Cellulose pulp is impregnated with sodium hydroxide to form alkali cellulose, which then reacts with chloroacetic acid or sodium chloroacetate (2). Sumerous variations of this technique have been described ( 6 ) . The over-all reaction may be written: 1
82
Cell-OH
HEMICAL
Present Address, U. S. Customs Laboratory, Yew Orleans, La. I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
Cell-ONa
Lf
KaOH
+ ClCHzCOONa
+ Cell-ONa
+
+ H2O
Cell-OCHzCOONa
+ NaCl
which is an example of the classical Williamson synthesis for the preparation of ethers. Recent mechanism studies on the reactions of a-halocarboxylic acids in aqueous alkaline solutions suggest that nucleophilic attack may not occur in one step, but that highly reactive a-lactones may be formed as transient intermediates (3).
No
ClCHzC -+
CHzC=O
\0 - v 0
+ C10
CHzCyO
\/ 0
+ -OH
+ HOCH2C
// \
0-
Such transient intermediates may also be involved in the carboxymethylation of cellulose. Fibrous cotton textile materials have been carboxymethylated by variations of the methods used on pulp. The most s u c c a f u l method 14) has been that in which cloth is padded with aqueous chloroacetic acid and then treated with a concentrated solution of sodium hydroxide in a wet, two-stage batch process. Reversing the order of application of the
reagents produces little reaction, presumably because of the insolubility of sodium chloroacetate in strong sodium hydroxide solution, which precludes penetration of the fiber by the carboxymethylating agent. A greater demand for carboxymethylated cotton would undoubtedly result if its production could be simplified. One problem has been the necessity of using concentrated sodium hydroxide solutions (40 to 50%) to obtain economic reaction efficiencies. T h e use of 30y0 solution greatly diminishes the amount of reaction obtained. Lower concentrations produce so little substitution as to be impractical. This makes finishing in the plant difficult, since the sodium hydroxide solution, which is gradually diluted in processing, must be fortified with solid sodium hydroxide, a reagent so highly deliquescent that it is troublesome to handle as a solid. Solutions of sodium hydroxide cannot be used for fortification because the concentrations employed are so close to the upper limit of reagent solubility. An interesting feature of this present conventional process is the effect of temperature. There is a reaction maximum between 70' and 80' C., further increase in temperature producing markedly less etherification (4). Thus, higher temperatures cannot compensate for the use of less concentrated alkali in the two-stage wet treatment. I n 1956, Walecka disclosed a method for carboxymethylating wood pulp in nonaqueous media utilizing only about 0.3% excess sodium hydroxide (14). This nonaqueous method was adapted for use on cotton textile materials (72) and provided the link for the development of a new carboxymethylation process. I n nonaqueous carboxymethylation, the cotton fiber, impregnated with a n alcoholic solution of chloroacetic acid, is treated in a mixed alcohol solution containing sufficient sodium hydroxide to convert chloroacetic acid to the sodium salt and provide a 0.3?', excess of the alkali. Reaction takes place a t the reflux temperature in the absence or practical absence of water. I t was reasoned that a one-step treatment might be possible without need of the nonaqueous system if chloroacetic acid or sodium chloroacetate and sodium hydroxide could be applied to cotton and made to react with the cellulose a t elevated temperatures while drying. Because pad-dry-cure finishing is already widely used for the production of wrinkle resistance in cotton by etherification crosslinking, a similar technique was investigated for carboxymethylation. This led to the development of a new and simplified process for the carboxymethylation of cotton fabric, yarn, and fiber. This paper presents details of the new process.
tenter frame equipped with high capacity, gas-fired heaters and blowers. T h e carboxyl content of treated samples was determined by a back-titration method (72). From the carboxyl content, the average number of substituent groups per anhydroglucose unit (DS) was calculated using the following equation:
where R is the molecular weight of the ether substituent minus 58 for carboxymethyl, 72 for a-methylcarboxy1-e.g., methyl, etc. Textile tests were performed in an atmosphere of 70" F. and 65% relative humidity using standard procedures of the American Society for Testing Materials ( I ) . Water of imbibition was determined by the weight of water retained by a 100-gram sample after centrifugation over porous metal plates for 45 minutes a t 1000 G. Experimental and Results
Treatment Solution. Since aqueous alkaline solutions o a-halocarboxylic acids are readily hydrolyzed to a-hydroxy derivatives, special care must be exercised in their preparation and use. This may be illustrated by details of the preparation of aqueous sodium chloroacetate-sodium hydroxide solutions. Best results have been obtained when cold sodium hydroxide is poured, with stirring and cooling, into concentrated chloroacetic acid solutions or onto the crystalline acid. Amounts of each reagent are calculated to give the desired concentrations in the treatment solution. Sodium chloroacetate can be utilized instead of the acid, but dissolves more slowly. Figure 1 is a plot of the concentrations of sodium chloroacetate and sodium hydroxide which are compatible when solutions are prepared as described above. Solutions with concentrations below the line are readily prepared ; concentrations above the line result in precipitation. Adequate cooling must be provided, as considerable heat is released when the acid and strong alkali are mixed. T h e temperature of the solution must not be allowed to rise much above room temperature or sodium glycolate, which is ineffective as an etherifying agent for cellulose, is produced. T h e treatment solution should be used immediately after its preparation, as the concentration of sodium chloroacetate is
Materials and Methods
T h e fabric used in this study was a n 80 X 80 cotton printcloth, 3.3 ounces per sq. yard, which had been desized, causticboiled, and bleached. Aqueous solutions containing the carboxymethylating agent and alkali were used for the treatments. Concentrations are reported as per cent by weight of the treatment solution. Reagents were used as obtained from commercial sources without further purification. I n laboratory experiments, fabric samplm were immersed in the treatment solution and passed through pad rolls to impregnate the fibers, remove excess, and adjust pickup of solution to the desired amount (about 100%). Samples were mounted on pin frames and baked in a n electrically heated circulating-air oven. Time and temperature of treatments are given in descriptions of the experiments. After treatment, samples were thoroughly washed and dried. Carboxymethylation also was carried out o n a semipilot plant scale in which the padded fabric was heat-treated on a
SODIUM CHLOROACETATE, V. Figure 1. Limit of compatibility of sodium chloroace tate and sodium hydroxide in water at about 26" C. Concentrations represented b y area above curve result in precipitation
VOL. 4
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83
gradually diminished by hydrolysis on standing. If not used immediately, the treatment solution should be refrigerated to retard hydrolysis. T h e effect of refrigeration of the treatment solution is illustrated in Figure 2, in which the relative efficiencies (E/E,) of solutions stored a t 2' C. and a t 26' C. (room temperature) are compared. In this figure, E/E, was calculated from the carboxyl contents of fabrics treated after various times as compared with the carboxyl content obtained when the solution was used immediately after preparation. T h e rapid loss in effectiveness of solution stored a t room temperature can be contrasted with that of the refrigerated solution, which still retained about 7570 relative efficiency after 96 hours of storage. Higher storage temperatures make the effectiveness of treatment solutions even more short-lived. Concentration of Sodium Chloroacetate. Samples of printcloth were impregnated with solutions containing various concentrations of sodium chloroacetate (1.2 to 37.070) and 5% sodium hydroxide, baked a t 140' C. for 10 minutes, washed, and dried. Results of the treatments are shown in Table I. Strengths of all the carboxymethylated samples were about 85% of the untreated. Etherification up to DS 0.18 was achieved, which is equivalent to that obtained in the conventional wet carboxymethylation process when much higher concentrations of sodium hydroxide are used. Increasing the concentration of sodium chloroacetate above 12.3Y0 only slightly increased substitution. Higher substitutions could be obtained by retreatment-for example, retreating a sample of DS 0.14 by impregnating with
STORED AT e* C.
0.8
24.7% sodium chloroacetate-5% sodium hydroxide solution and baking a t 140' C . for 10 minutes produced DS 0.25. A control sample treated with 5% sodium hydroxide by the pad-bake method had a carboxyl content of only 0.1070. This indicated that little oxidation of the cotton takes place under the treatment conditions, and that the carboxyl contents of the samples are a result of the carboxymethyl substituents. Impregnating cotton with a solution containing 24.7y0 sodium chloroacetate and 57, sodium hydroxide and allowing the sample to stand a t room temperature for 10 minutes introduced only 0.317, C O O H (DS 0.01). This corresponds roughly to a conventional carboxymethylation treatment using dilute sodium hydroxide solution. Concentration of Sodium Hydroxide. The effects of increasing the sodium hydroxide concentration M hile holding that of sodium chloroacetate constant are shown in Table 11. The effectiveness of treatments using solutions containing less than 5y0 sodium hydroxide is striking. The degrees of substitution obtained in this and other series have indicated a peak etherification effectiveness, with higher sodium hydroxide concentrations producing somewhat less reaction. This phenomenon may be connected with reagent hydrolysis. Because of the moderate concentrations of alkali used, carboxymethylated cottons from the pad-bake process can be washed quickly and easily. Souring with dilute acid further shortens the time necessary for washing. Temperature of Treatment. Cotton printcloth padded to about 1007, wet pickup of 24.7% sodium chloroacetate-57, sodium hydroxide solution was divided into several portions which were heat-treated for 10 minutes a t various temperatures ranging from 25 ' to 200 ' C. The degree of substitution obtained as a function of temperature is plotted in Figure 3. The extent of reaction rises rapidly with increase of tempera-
Effect of Change of Concentration of Sodium Chloroacetate on Treatmenta Fabric Properties c B r k . str. yc Sodium /o Chloroacetate COOH DS ( W ) ,16. 0 0.10 ... 47.4 0.51 1.2 0.02 42.2 6.2 2.10 0.08 41.7 12.3 3.66 0.14 42.5 18.5 3.96 0.15 41 . O 24.7 3.71 0.14 42.0 30.8 3.71 0.14 42.2 37.0 4.77 0.18 42.6 Untreated ... ... 48.9 a Samples of cotton printcloth were immersed i n solutions confaining the indicated concentrations of sodium chloroacetate and 5Yo sodium hydroxide, padded to remoce excess ( 100-1 70% p i c k u p ) , baked at 140' C. for 10 minutes, washed, and dried. Table 1.
20
0
40
I
l
60
l
I
I
I
80
100
TREATMENT SOLUTION AGE, hr. Figure 2. Effect of storage conditions on relative efficiencies of treatment solutions in pad-bake carboxymethylation process ..
2
0
5 c
k2
-4
0.15-
Table II.
s -3
0.10-
0
0 W
a
i 0 0
LL
I
0.05t
-2
1
/
/
0
0 20 40 60 80 lOd 120 140 160 180 200
TEMPERATURE, DEG. C.
Figure 3. Effect of baking temperature on carboxymethylation of cotton impregnated with 24.7% sodium chloroacetate-570 sodium hydroxide solution 84
Effect of Change of Sodium Hydroxide Concentration on Treatment" Fabric Properties yc Sodium c" B r k . str. Moisture Hydroxide CO6H DS ( W ) ,16. regain, 0 0.76 0.03 45.8 6.8 1 1.75 0.07 40.6 7 9
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
... ... 48.9 6.5 Samples of cotton printcloth were immersed i n solutions containing 24.77, sodium chloroacetate and the indicated concentration of sodium hydroxide, padded to remoue excess ( 106- 7 10% p i c k u p ) , baked at 140' C. f o r 70 minutes, washed, and dried. Untreated
a
ture to about 160' C., above which slightly lower degrees of substitution were noted. Strengths of samples treated below 160" C. were about 84 to 317, of that of the untreated. Above this temperature, strengths lvere only slightly less; samples retained 76 to 81% of the original breaking strength. I t was not unfeasible to dry samples a t a low temperature and subsequently bake a t a higher temperature. A swatch of fabric impregnated with the treatment solution was dried a t 60" C. for 7 minutes, and then treated a t 160" C. for 10
Table 111.
Properties of Cotton Fabric Carboxymethylated in a Semipilot Plant Scale Treatment AlkaliTreated CarboxyC'ntreated Control Property mcthjlated 0.00 0.03 Carboxyl content, 7, 3.03 Fabric weight, oz./ 3.3 3.3 sq. yd. 3.5
Thread count, yarns /inch 88 x 73 (LV X F) Thickness, inch 0.0080 Breaking strength, lb. LVarp 40.6 Filling 41.8 Elongation at break, yG IVarp 8.0 Filling 29.3 Tearinq strength, g. IVarp 1072 Filling 944 Flat abrasion, cycles 123 Air permeability, cu. ft./min,/sq. ft. 310 Stiffness (bending moment), inch-lb. \\-arp 0.0014 Filling 0.0006 Moisture regain, 7G 8.1 JVater of imbibition: 52.6
88 x 72 0.0073
88 X 75 0.0073
43.0 44.9
40.3 41.2
7.1 31.4
10.9 27.0
110
1458 1136 110
395
340
1176 1004
0.0015 0.0005
6.5
...
0.0011
0.0006 6.5 34.0
Table IV. Carboxymethylation of Cotton Using Solutions Containing Various Alkalies and Chloroacetate Salts 70 B r k . Str., Treatment Solutione COOH DS Lb.
28.1 yG Potassium chloroacetate5yGpotassium hydroxide 24.7Yc Sodium chloroacetate5yb sodium hydroxide 24.7YGSodium chloroacetate5cosodium carbonate 24.7yc Sodium chloroacetate5VCammonium hydroxide 23.65; .Ammonium chloroacetate--5yo ammonium hydroxide Untreated
2.60
0.10
40.7
3.71
0.14
42.0
1.05
0.04
42.5
0.69
0.03
44.6
0.33
0.01
42.5 49.8
...
...
a Cotton printcloth Padded to 100% pickup of indicated solution, baked at 140' C. for 10 minutes, washed, and dried.
Table V.
minutes. T h e degree of substitution was 0.09, the same as that of the sample of Figure 3 treated a t 60" C. Apparently this degree of reaction occurred as a result of the 60" C. drying step, with no additional reaction resulting from the subsequent treatment a t 160' C. Time of Treatment. The effect of varying the time of treatment a t 140 O C. from 2.5 to 30 minutes was investigated with samples of cotton impregnated with 24.77, sodium chloroacetate-57, sodium hydroxide solution. Baking for 2.5 minutes produced as much etherification (DS 0.14) as longer times. Strengths of samples were about 85y0 of that of the untreated over this entire range of time. Treatment on Sempiilot Plant Scale. Fifty yards of 88 X 75 white cotton printcloth were padded to approximately 100% wet pickup of 24.7% sodium chloroacetate-57, sodium hydroxide solution and baked on an enclosed tenter frame a t 140" C., while held a t original width and length. The residence time in the heated chamber was 6 minuIes. The fabric was then washed, and dried a t original dimensions. Another length of fabric was similarly treated as a control, using 5% sodium hydroxide solution. Properties of the carboxymerhylated fabric, the alkalitreated control, and the untreated cotton are given in Table 111. T h e carboxymethylated cotton, DS 0.1 1, was slightly heavier and thicker than the untreated and the control. As a consequence, stiffness was slightly increased and air permeability decreased. Breaking strength and elongation of the etherified cotton were about the same as of untreated. Tearing strengths in the warp and filling directions were about 26 and 17% below those of the untreated, but equal to the tearing strengths of the alkali-treated control. Moisture regain and water of imbibition of the carboxymethylated fabrics were considerably increased. Use of O t h e r Alkalies. A comparison is given in Table ITof the carboxymethylation of cotton by the pad-bake process using treatment solutions containing various alkalies and chloroacetate salts. Equimolar quantities of carboxymethylating agent were employed in each treatment. Treatments using the strong alkalies resulted in markedly greater substitution than those employing weak alkalies. The sodium salt-sodium hydroxide solution produced the greatest reaction and the potassium salt-potassium hydroxide treatment also yielded a relatively high degree of substitution. T h e molar concentration of potassium hydroxide \L as less than that of the sodium hydroxide, since the solutions were prepared on a weight basis. The use of ammonium hydroxide with sodium chloroacetate produced no more reaction than when sodium chloroacetate alone was used (compare with Table 11). The ammonium chloroacetate-ammonium hydroxide treatment was notably ineffective. Alpha-Substituted Carboxymethylated Cottons. Four a-substituted carboxymethylated cottons were prepared using
Alpha-Substituted Carboxymethylated Cottons Prepared by Treatment with Alpha-Halocarboxylate Salts
Treatment Solutiona
24.1 yc Sodium a-chloropropionate5 5 sodium hydroxide
Ether$ed Cotton
70
Cell-OCH (CH3) COOH (a-methylcarboxymethylatedcotton)
COOH 3.28
23.653 Sodium a-chlorobutyrate5yc sodium hydroxide 23. 6YGSodium a-chloroisobutyrate5 yGsodium hydroxide 33.6% Sodium a-bromovalerate5 yo Sodium hydroxide
(1
Cell-OCH ( C H ~ C H PCOOH ) 1.66 (a-ethylcarboxymethylated cotton) Cell-OC (CH3)2 COOH 0.40 (a,a-dimethylcarboxymethylatedcotton) Cell-OCH ( C H ~ C H ~ C HCOOH S) 0.29 (a-propylcarboxymethylated cotton) Cotton printcloth padded to about 100% pickup of indicated solution, baked a t 140' C. for 10 minutes, washed, and dried.
VOL. 4
NO. 2
DS 0.12
0.06 0.02 0.01
JUNE 1965
85
1
.
other a-halocarboxylate salts in the pad-bake process (Table V): a-methyl- ; a-ethyl- ; a,a-dimethyl- ; and a-propylcarboxymethylated cottons. As would be expected, the amount of etherification is decreased as the size and number of substituents on the a-carbon are increased. Summary and Conclusions
A new process for the carboxymethylation of cotton textile materials has been developed. Fabric is padded with an aqueous solution containing sodium chloroacetate and sodium hydroxide and then baked to bring about etherification of the cellulose. Degrees of substitution u p to about 0.2 can be obtained readily. Retreatment affords a means of producing higher substitutions. T h e extent of carboxymethylation realized is a function of the concentrations of sodium chloroacetate and sodium hydroxide employed and the temperature of the baking treatment. Etherification of the cellulose increases with temperature to about 160’ C., above which substitutions are slightly lower. -4treatment time of about 2.5 minutes is sufficient, but longer times apparently have no adverse effects. Relatively low concentrations of sodium hydroxide can be utilized. Other strong alkalies may be used, but reaction is much poorer when weak alkalies are employed. The compatibility of various concentrations of sodium chloroacetate and sodium hydroxide was determined. If not used immediately, treatment solutions should be refrigerated to retard hydrolysis of the carboxymethylating agent. Efficiency of etherification is decreased by hydrolysis of the agent. Carboxymethylation by the pad-bake process was successfully demonstrated on a semipilot plant scale with the baking
step carried out on a tenter frame equipped with heaters and blowers. T h e treatment appears suitable for adaptation to a continuous plant scale process using standard textile finishing equipment. Other a-halocarboxylate salts were used to etherify cotton. Alpha-substituted carboxymethylated cottons prepared by the pad-bake process include a-methyl- ; a-ethyl- ; cu,cr-dimethyl-; and a-propylcarboxymethylated cotton. Literature Cited
(1) Am. SOC.Testing Materials, Philadelphia, Committee D-I 3, “ASTM Standards on Textile Materials,” 32nd ed., 1961. (2) Baird, G. S., Speicher, J. K., in “Water-Soluble Resins,” R. L. Davidson and M. Sittig, eds., pp. 69-87, Reinhold, New York, 1962. (3) Cram, D. J., Hammond, G. S.,“Organic Chemistry,” 2nd ed., p. 232, McGraw-Hill, New York, 1964. (4) D a d , G. C., Reinhardt, R. M., Reid, J. D., Textile Res. J . 2 2 , 787-92 (1952). (5) Ibid., 23, 719-26 (1953). (6) Dhariyal, C. D., Timokhim, I. M., Finkel’shtein, M. Z., Z h . Priklad. Khim. 35, 429-40 (1962). (7) Fisher, C. H., Textile Res. J . 2 5 , 1-11 (1955). (8) Fisher: C. H., Perkerson, F. S.: Ibid., 2 8 , 769-78 (1958). (9) Goheen, G. E., T a p p i 41, 737-42 (1958). (10) Reid, J. D., D a d , G. C., Textile Res. J . 1 7 , 554-61 (1947). (11) Ibid., 1 8 , 551-6 (1948). (12) Reinhardt, R. M., Fenner, T. I V . , Reid: J. D., Ibid., 2 7 , 871-8 . _ (1957). . (13) Rei4har-d;; R. M., Reid, J. D., Fenner, T. W., Mayne, R. Y., Ibid., 2 9 , 802-10 (1959). (14) TValecka, J. A.: T a p p i 39, 458-63 (1956).
RECEIVED for review October 28, 1964 ACCEPTED January 25, 1965 Division of Cellulose, Wood, and Fiber Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
REDUCTIQN OF PQLYMERIC FRICTION BY MINOR CONCENTRATIONS OF PARTIALLY FLUORINATED COMPOUNDS R . C . B O W E R S , N . L. J A R V I S , A N D W . A . Z I S M A N Chemistry Division,U. S. Naval Research Laboratory, Washington, D.C.
A surface chemical approach to the reduction of boundary friction in solid polymers i s described. Dry frictional properties of several classes of polymeric solids have been reduced significantly b y the addition of small proportions of a suitably designed surface-active compound. Appropriate fluorocarbon dsrivatives have been prepared and found effective in poly(methy1methacrylate), poly(viny1 chloride), and several poly(viny1idene chloride) copolymers. These addition agents are effective in both polymer films prepared b y evaporation from a solvent and thick disks prepared from the melt. The low-energy surfaces formed by the fluorinated additives may also be self-healing-that is, any surface-active molecule lost from the film may be replaced b y the diffusion of additional material to the interface. The decrease in friction caused b y the addition agent i s accompanied b y an increase in the equilibrium contact angle of each of several liquids on the polymer surface. The small proportion of addition agent used caused only a small decrease in the hardness of the polymers, Various promising classes of applications are outlined.
FRICTION between dry, clean solids is strongly dependent
upon both the surface and bulk properties. In general, loirering the free surface energy of a solid, y o , or the critical surface tension of wetting, yo decreases the specific adhesion and wettability and also lowers the coefficient of sliding friction. This last relationship was established during investigations (2, 3) of the friction and wetting of polyethylene and a series of 86
I&EC
PRODUCT RESEARCH A N D DEVELOPMENT
halogenated derivatives of polyethylene. Progressive replacement of hydrogen atoms by fluorine atoms decreased both the coefficient of friction and the critical surface tension of wetting; replacement of hydrogen atoms by chlorine atoms increased these properties. Despite their high melting points, remarkable chemical stabilities, and low coefficients of friction, fully fluorinated ethyl-
