I
FRED LEONARD, JOSHUA NELSON, and GEORGE BRANDES
U.S. Army Prosthetics Research Laboratory, Walter Reed Army Medical Center, Washington 12,D.C.
Vulcaniza ble Saturated Acrylate Elastomers A saturated acrylate elastomer, suitable for use in cosmetic gloves for amputees, was synthesized. This article discussesthe chemistry of cross linking, describes the bulk mechanical and surface properties of the materials prepared as a function of reinforcement and cross linking, and illustrates a method for enhancing the surface qualities of the cast film by further cross linking at the surface through postdipping techniques D u w r o an investigation of latexdispersed elastomers suitable for use in cosmetic gloves for amputees, copolymers containing acrylic acid esters as their major constituents were under study a t this laboratory. The acrylate elastomers exhibit rubberlike extensibility, excellent chemical resistance, and outstanding resistance to aging (4, 72, 73). They may be cast into films of excellent clarity, permitting tinting to delicate pastel skin tones. A serious drawback, however, has been weak strength. Methods for enhancing strength by reinforcement with isotropic silicas, of refractive index similar to that of the acrylate copolymer (Q),by reinforcement with organic fillers (7, 8) and by copolymerization, have been reported. Composite acrylate structures have been prepared to make the surface more resistant to development to tack under cyclic shear and compressive forces, as well as to enhance stain and solvent resistance (3). T o achieve rubberlike behavior, decrease plastic flow, fill requirements for solvent resistance, and obtain permanence of molecular structure, vulcanization of saturated acrylate elastomers is necessary and has been studied by several investigators. Copolymers of ethyl acrylate and 2-chloroethyl vinyl ether prepared in emulsion have been vulcanized by milling with sulfur, polyamines, and other curing agents common in the rubber industry (74). Because of the requirements in the present application for the copolymers in latex form, and the need for colorless translucent films, usual vulcanization methods were not applicable. Three vulcanization methods of cross
linking were studied : of saturated acrylate elastomers by a free radical mechanism through use of benzoyl peroxide; by reaction of polyamines with the epoxy groups of an acrylic acid ester-glycidyl acrylate copolymer ( 5 ); and, by means of formaldehyde, of terpolymers prepared from acrylic acid esters, methyl methacrylate, and methacrylamide. The cross linking of the terpolymer with formaldehyde was selected for further study. Experimental
Monomer Purification. Butyl acrylate and methyl methacrylate (Rohm & Haas Co.) were washed in separate funnels with a solution containing potassium hydroxide (5% w./v.) and sodium chloride (2070 w./v.) until the washings were colorless, and then with demineralized water until the washings were neutral to litmus. Methacrylamide (MAD) (Rohm & Haas Go.) was used as received. The monomers were stored a t 0' C. until ready for use. Preparation of Terpolymer. The terpolymer was prepared by emulsion polymerization in a 5-liter flash having a center neck fitted with a 55/40 tapered joint and three 24/40 tapered necks arranged to form a T through the center neck. The connections throughout were ground-glass ball joints. The reactor, equipped with center-stirring, thermocouple leads for controlling and recording temperature, and facilities for evacuation and for flushing with nitrogen, was set in an aluminum pot. A schematic diagram of the arrangement of the equipment for nitrogen flushing and evacuation is shown in Figure 1. The vacuum and nitrogen sources were connected to a common manifold through two solenoid valves, which were controlled by a double-pole, three-position toggle switch. In one position, nitrogen was permitted to enter the system; in another position, vacuum was applied to the system; and in the center or neutral position, both the nitrogen and vacuum sources were off. When the toggle switch was set to vacuum, ball check valve 1 was raised from its seat and check valve 3 was tightly seated, permitting a vacuum to be drawn on the polymerization vessel. Two vertical condensers, connected in series, through which ethylene glycol-
Figure 1. This equipment is used for nitrogen flushing, evacuation, and polymerization
water solution (-10' C.) was circulated, served to condense the monomer. A cold trap (dry ice-acetone) precluded contamination of the vacuup pumps with volatile materials which could not be condensed. When the desired vacuum was attained, the toggle switch was positioned to nitrogen, opening the nitrogen solenoid and closing the vacuum solenoid. Nitrogen entered the reactor through check valve 2. From the reactor, the nitrogen had two possible routes: If manual valve A was closed, the nitrogen filled the system to atmospheric pressure and then raised the water level in the standpipe to the desired liquid head (usually 1.5 feet); if A was opened, the system was opened to the atmosphere and the reactor ingredients could be flushed with nitrogen. Details of the cooling and heating system have been reported ( 6 ) . Cooling medium was furnished to the system from a 20-gallon refrigerated tank equipped with a centrifugal pump that maintained a constant head against a second centrifugal pump which ran only when cooling water was required in the VOL. 50, NO. 7
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reactor bath. .4 second outlet was connected to another centrifuga1 pump which furnished cooling water to the condensers continuously. Both cooling streams, from the pot and the condensers, returned through a 2-inch copper pipe to the refrigerated tank. The electrical and control system consisted of two elements: a four-point strip chart Brown recorder, and a fourpoint Celectray temperature controller unit. Both units were connected through junction boxes to detachable thermocouples, which may be placed in desired positions. The recorder permitted straightforward recording of the temperatures transmitted from the bath and reactor thermocouple junctions. The controller was such that above a set, desired temperature in the reactor, a cooling circuit was energized. This circuit actuated the cooling water pump, allowing cooling water to flow to the reactor and opening the solenoid valve between the pump and the reactor, permitting the cooling water to enter the reactor bath. A heat-bucking cold cycle or a two-circuit system could be used, to maintain any desired temperature within h1.5' C. Operation of Equipment. The reactor was assembled and care taken to grease all ground-glass joints carefully. The cold traps were filled with dry ice and then valve A was opened. All ingredients, except the activator and the catalyst, were added to the reactor, and the system was thoroughly flushed with nitrogen. A was then closed and a head of liquid established in the standpipe, indicating a positive pressure in the entire reactor system. At this point, the solenoid valve switch was thrown to vacuum (closing check valves 2 and 3 and opening 1) and the reactor evacuated to the desired vacuum, generally to 20 to 25 mm. of mercury. The control switch was positioned to nitrogen, which caused check valve 1 to close and 2 to open, thereby breaking the vacuum. The
.I 0
$ R.
Figure 2. Specimens were clamped in testing machine and cycled to failure
1 054
evacuation and nitrogen-filling cycle could be repeated as often as desired; four or five cycles were usually sufficient. When the final vacuum had been broken, the nitrogen was left on, catalyst and activator were added, A was opened, and the nitrogen adjusted to give a constant moderate flow through the reactor liquid. A was closed when the reactor had started, as noted from a slight temperature rise on the Brown recorder. Then the nitrogen valve was again adjusted to maintain the standpipe head at some desired level, thus keeping the entire system under positive pressure. Samples to determine conversion of monomer to polymer were taken with a hypodermic through the front taper joint in the usual manner. Over many runs, the condenser system was very effective. The two traps at the end of a run had collected in total about 1 to 1.5 ml. of liquid consisting of approximately 50: 50 monomer and water. Reactor Charge. Table I gives a typical charge for preparing the acrylateamide elastomer at approximately p H 2.5. For runs at higher pH, ammonium hydroxide or sodium hydroxide was used in place of sulfuric acid. Compounding. The 90:7.5 :2.Sbutyl acrylate-me thy1 methacrylate-methacrylamide terpolymer latex dispersion was compounded by adding aqueous emulsions of the compounding ingredients to the terpolymer latex with moderate agitation, using a flat, paddle-type stirrer. The compounding recipe consisted of (parts) : 90: 7.5 : 2 . 5 BA/MMA/MAD ter-
polymer Poly(ethy1 methacrylate) (Mc 387, Rohm & Haas Co.) (approximately 50% solids latex) (PEMA) Formaldehyde (added as formalin)
100
37
1.765
After compounding, the latex was stripped for 1 hour at 25' C. at 10 mm. of mercury. This resulted in a loss of approximately 5% by weight of formaldehyde. After stripping, acidic latex had a pH of 3.0, whereas basic latex showed a pH of 9.0 to 7.5. Casting. Test sheets, 33/4 inches in internal diameter and 12 inches long, were cast inside cylindrical stone molds made from Hydrocal A-11 (U. S. Gypsum Co.). Wall thickness of the molds was approximately 3 / ~inch. The films were cast by pouring gauzestrained compounded latex slowly down the side of the mold at a rate regulated so that no portion of the mold once wetted was allowed to dry. After a predetermined dwell time, usually about 60 minutes, the latex was poured from the mold. The deposited film was dried in situ for about 4 hours at 50' C., then removed from the mold. The cylindrical
INDUSTRIAL AND ENGINEERING CHEMISTRY
castings were slit parallel to the major axis and the bottom was cut out to form flat sheets. The films were then heated at looo C. for 1 hour in a circulating-air oven. Posttreatment Procedure. The cured film was dipped in a 46.5Y0 solution of formaldehyde in methanol (Methyl Formcel, Celanese Go.) for 15 minutes. The film was air-dried for 15 minutes, oven-dried at 50" C. for 30 minutes, and again heated at 100 C. for 1 hour. Conditioning of Samples. Prior to testing, all samples were conditioned for 1 week at 77" F. and 50% R.H. Formaldehyde Determination in Latex. A latex sample (25 ml.) was diluted to 200 ml. in a 500-ml. Erlenmeyer flask and adjusted to pH 7 with 1070 sodium hydroxide, and 20% sodium sulfite solution (25 ml.) was added. After 5 minutes of standing, the sodium hydroxide liberated according to the reaction NalSOs
+ CHzO + N 2 0 NaOH
-+
+ HOCH2S03Na
was titrated with 0.5Y hydrochloric acid using thymophthalein indicator, or potentiometrically (76). Swelling Index and Solubility. Swelling index was determined by immersing an accurately weighed sample of approximately 0.5 gram of compounded elastomer in 100 ml. of methyl ethyl ketone for 48 hours. The gel was separated from the solvent and padded gently with hard filter paper before being placed in a tared weighing bottle. The swollen weight was measured. the sample dried, and the dry weight of the nonvolatile material in the gel determined. The swelling index is defined as the ratio of the swollen gel weight to corrected dry weight of nonvolatiles in the swollen gel. Solubility is defined as the per cent soluble material in methyl ethyl ketone. Test Methods. Room temperature stress-strain properties were determined according to ASTM D 624-54 and 412SlT, using a table model Instron tensile testing machine. Low temperature flexibility was determined with a Tinius Olsen stiff ness-in-flexure tester, according to ASTM D 747-50, modified (2). Tensile Fatigue. Specimens of the dimensions shown in Figure 2 were
Table I. Charge Formula Ingredients Parts 90 Butyl acrylate 7.5 Methyl methacrylate 2.5 Methacrylamide 155 Water demineralized 2.0 Santomerse S 0.344 Potassium chloride 0.024 Potassium persulfate (catalyst) 0.0177 Sodium thiosulfate (activator) 0.0625 Sulfuric acid Reaction temperature 25' C. pH 2.5.
ACRYLATE ELASTOMERS clamped between rhe jaws of the Instron testing machine and cycled to failure at 80% of the ultimate break stress. The break stress was initially determined on a standard dumbbell specimen. The cycling rate was 20 inches per minute.
“i
-4
Results and Discussion The conversion curves for the emulsion terpolymerization of butyl acrylate, methyl methacrylate, and methacrylamide in the presence of sulfuric acid, sodium hydroxide, and ammonium hydroxide, and substitution of ammonium persulfate for potassium persulfate are illustrated in Figure 3. The conversion rates for polymerizations were approximately 11% per hour; but when the run was made in the presence of ammonium hydroxide, the rate was approximately 30% per hour. The reason for the enhanced rate in the presence of ammonium hydroxide is not known. That it is not due to the effect of initial p H is shown by the fact that polymerization in the presence of sodium hydroxide at initial pH levels of approximately 11.4 gave the same rate as polymerization in the presence of sulfuric acid at pH 2.5. Sodium hydroxide causes rather rapid hydrolysis of acrylic acid esters (77), and although the pH of the latex is initially high when adjusted with sodium hydroxide, it falls off rapidly to almost neutral pH values (Table 11). With ammonium hydroxide, on the other hand, apparently little or no hydrolysisof the acrylic acid esters occurs, and there is only a small change in pH during the reaction. The rate differences observed may be attributable to the effect of pH during polymerization. For acidic and essentially neutral runs (pH adjusted with sulfuric acid and sodium hydroxide, respectively), the rate is slower than in alkaline runs in which the pH is adjusted with ammonium hydroxide. In runs in which the pH was initially adjusted with sodium hydroxide. 1-butanol, formed as a result of the hydrolysis of butyl acrylate, might act as a retarder. In the run in which the pH was adjusted with ammonium hydroxide, and carried out in the presence of butanol equivalent to that present in a run adjusted wiih sodium hydroxide, the rate was equivalent to the sodium
b Figure 3. Conversion rates for polymerization varied from 1 1 to 30% per hour
,’ I/
0
(NH40H A D D E D )
A
(H,SO,
ADDED1
t o P H 10.3 to p H 2.5
[ N o O H A D D E D ) t o p H 11.4 ( F I N A L pH 7.3)
X
TIME
hydroxide-adjusted run and slower than an ammonium hydroxide run in the absence of butanol. Vulcanization Reaction. The terpolymer of butyl acrylate, methyl methacrylate, and methacrylamide contains amide groups which may react with formaldehyde to effect cross linking of polymer chains. The mechanism for the reaction of methacrylamide with formaldehyde under alkaline and acid conditions may be represented as (7, 10, 75, 77):
0
I1
IN
+I
B u t a n o l ) l o pH 10.0
HOURS
with 1 mole of formaldehyde under acidic conditions to form methylene-bismethacrylamide. As m e t h y 1e n e - b i s m e thacrylamide forms as a result of the reaction between methacrylamide and formaldehyde in acidic aqueous systems under very moderate conditions, it seemed that the reaction might occur in the compounded acidic terpolymer latex under storage conditions, and films cast from the latex after different time intervals therefore might vary in cross-linking and mechanical properties. To test this hypothesis, the acidic latices compounded with excess formaldehyde-e.g., 2 moles of formaldehyde per basal mole of methacrylamide-and 37 parts of poly(ethy1 methacrylate) per 100 parts of latex solids were further evaluated. Films were cast and cured, and the stressstrain properties were measured. The data summarized in Figure 4 show that ultimate tensile strength and elongation decrease as a function of latex aging and tensile stress increases at 100 and 300y0 elongation (100 and 300% “modulus”), indicating that the films have decreased
Alkaline medium CHz=C-C--NHz I
(NH, OH
base + CHz0 +
I/ CHz=C-C-NH-CH20H
AH s Upon conversion to a neutral or acidic condition and application of heat, water is removed, and a methylene diamide is formed : 0
0
neutral or
0
0
+ HzO
-NH-cH~-NH-C-C=CH~ II
ii
CH+-
AH3
Acid medium Table II.
Effect of Alkali Type on pH
Although initially high, pH falls off rapidly to almost neutral values
Alkali NHaOH
NaOH
Initial PH
Final PH
10.4 10.3 10.3 10.2 11.4 11.5
9.5 9.4 9.6 9.4 7.8 7.5
0
2CHz=C-C-NHz I1 I
0
0
+ CHzO + H + CHz=C-C-NH-CHz-NH-C-C=CH I1 I/
2
+ Hz0
I
bHs & I3 bH s According to these mechanisms, 1 in flexibility and become “short.” The mole of methacrylamide reacts with 1 tear strength does not seem to vary in any mole of formaldehyde under alkaline regular manner. Particularly indicaconditions to form a methylol derivative, tive of the reaction between formaldeand 2 moles of methacrylamide react hyde and the latex-dispersed terpolymer VOL. 5 0 , NO. 7
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H
N
CI 5
/.
,
~x-++-~,l-i-I-l+-txx
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Too
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40
Figure
hyde should add to the latex-dispersed terpolymer under alkaline conditions to form the methylol derivative and no cross linking should occur in the latex under storage conditions. In contrast to the results obtained under acid conditions and in agreement with theory, tests on films cast from room temperature-stored alkaline latices indicated that tensile strength, ultimate elongation, modulus, and swelling index remained essentially constant. This indicates that little, if any, cross linking occurs in latex stored under alkaline conditions. For reproducible mechanical properties of latex-cast films in this system,
I 60
101 P I R 1 B OF R U B B L R S O L I D S
INDUSTRIAL AND ENGINEERING CHEMISTRY
40
50
the latex should be made alkaline prior to compounding and maintained on the alkaline side under storage conditions. Reinforcement. Because colorless, translucent films were required for this study, reinforcing fillers were limited to colorless materials ofrefractive indexof the same order of magnitude as the rubberlike terpolymer. Use of amorphous silica as a reinforcing agent produced whitening under stress ( 9 ) and a polymeric filler, poly(ethy1 methacrylate), was tried instead. The whitening effect under stress was minimized with the latter as filler, and the effect of poly(ethyl methacrylate) concentration on tensile strength modulus and elongation is shown in Figure 6. A maximum tensile strength was reached at approximately 37 parts per 100 parts of rubber solids, and this filler concentration was
Figure 7. Stressstrain curves are not affected b y formaldehyde concen t r a tion
SO
30
5. Swelling index decreases with aging
50
Figure 6. With PEMA filler, maximum tensile strength was reached at about 3 7 parts of rubber solids
1056
PO
AGE O F L A T E X ( D A Y S )
Figure 4. Ultimate tensile strength and elongation decrease as a function of latex aging
are the swelling index data (Figure 5). The swelling index decreases as a function of latex aging, indicating that the terpolymer chains are becoming more highly cross-linked and forming a tighter structure. The per cent solubility remains essentially constant, although slightly lower than theoretical. Theoretical solubility is based on the assumption that all the poly(ethy1 methacrylate) filler compounded into the terpolymer latex remains soluble. These results may indicate partial involvement of the poly(ethy1 methacrylate) in the crosslinking reaction. Under alkaline conditions, formaldehyde reacts with methacrylamide to form methylol derivatives. Further condensation between methylol derivatives apparently does not occur unless the medium is made neutral or acid and heat is applied. Presumably, formalde-
::
0 0
lo
I 40
I
20
X
% ELONGATION
ACRYLATE EL A S T O M ER S used in the subsequent experiments. Effect of Formaldehyde. The effect of the concentration of formaldehyde on the stress-strain properties of the acrylate-amide film is shown in Figure 7. The curves were obtained on films cast immediately after compounding from acidic latices containing from 0 to 4 moles of formaldehyde (an %fold excess) per basal mole of methacrylamide. The closeness of the curves indicates that these stress-strain curves obtained on the Instron tensile testing machine at the rate of 20 inches per minute are essentially insensitive to formaldehyde concentration. It was postulated from these data that 20 inches per minute was probably much too fast to permit the polymer chains to relax to anywhere near equilibrium stress-strain values, and differences in properties between the differently crosslinked materials were not discernible. Consequently, the stress-strain curves were determined at a strain rate of 0.2 inch per minute, 100th of the original speed (Figure 8). When measurements were made at the slower rate of strain, the effect of formaldehyde on the stiffness of the elastomers could be notedas the amount of formaldehyde increased from 0 to 2 moles per mole, the stressstrain curves shifted closer to the stress axis, with a slight difference between the curves for 2 and 4 moles d formaldehyde per mole of methacrylamide. These data are in agreement with the results for tensile fatigue a t 80% of break load, swelling index, and solubility shown in Figure 9. For no posttreatment the number of cycles to break increases with increase in formaldehyde content up to 2 moles of formaldehyde, after which further increase has little or no effect. The swelling index (swollen weight. to unswollen weight) decreases with increase in formaldehyde, also up to 2 moles of formaldehyde. A further increase, again, has little effect. The same general pattern is observable for solubility data. Low-Temperature Properties Stiffness-in-flexure as a function of temperature curves for films containing varying concentrations of cross-linking agent are shown in Figure 10. The low-temperature properties determined in this manner seem to be unaffected by the concentration of forrnald$hyde. There is only a fourfold increase in stiffness from room temperature to -30' C., which is unusual for elastomers based on acrylic acid esters. Posttreatment. Although the range of bulk properties obtainable in this, system appeared adequate for the intended application, to enhance such surface properties as stain and solvent resistance, cast films were postdipped in a methanolic solution of formaldehyde
1400
1300 1200
1100 1000 900 800 Y
Y)
100
P
600
Y Y) K
100
!il
400
300 200
100 0 100
200
300
400
7.
100 300 ELONGATION
700
800
SO0
IO00
1100
Figure 8. With increase in formaldehyde concentration, stress-strain curves a t 0.2 inch/minute shifted closer to stress axis
Figure 9. Number o f cycles to break increases, and swelling index and solubility decrease with increase in formaldehyde concentration up to 2 moles of formaldehyde
and further cured by subsequent heating. I t was hoped by such treatment to prepare a composite film consisting essentially of two components-a thin, outer layer of a highly cross-linked
terpolymer, and the bulk of the film consisting of a lightly cross-linked material having the desired mechanical properties. In effect, it was desired to achieve a surface-hardened film. The VOL. 50,
NO. 7
JULY 1958
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4
15000
,.
5X Y u.
14000
lP000
11000 l0000
5
9000
t:Y
9000
z
Figure 10. Low-temperature properties are unaffected b y formaldehyde concentration
13000
7000
;6 0 0 0
5000
4000 3000
PO00 1000
0 -e0
-30
-20
-10
0
+ IO
TEMPERATURE’C
4 Figure 11. As a result of posttreatment, stress-strain properties are shifted toward stress axis
ment was set up in which films were heated an additional hour. Negligible effects were obtained on the mechanical properties, swelling index. and solubility. In another control experiment the posttreatment procedure was duplicated precisely, except that the film was dipped in methanol instead of Methyl Formcel. This treatment shifted the stress-strain curve toward the stress axis. but had no effect on the swelling index and solubility. The amount of stiffening due to postdipping in methanol alone was less than that when Methyl Formcel was used. The control experiments indicated that the Methyl Formcel was largely responsible for the effects on mechanical properties, solubility, and swelling index. Of the two methods of cross-linking the polymer chains, posttreatment appears to be more effective. Acknowledgment
The authors express their gratitude to Glenn H. Mansell and Guy -4. Hanks for obtaining some of the data presented. Literature Cited
American Cyanamid Co., “Acrylamide,‘’ New Product Bull. 28 (June 1952). Blevins, T. B., DeFries, M. G.
v1
ASTM Bull. 173, 5960 (1951).
u W)
DeFries. M. G.. Schneider. L., Fram; P., Leonard, F., J . Polymei Sci. 20, 267 (1956). Dietz, T. J., Hanson, 3. E., Ru66er
c c
u)
Age 68, 699 (1955).
Erickson, J. C . (to American Cyanamid Co.), U. s. Patent 2,580,901 (Jan. 1, 1952). Fram, P., Stewart, G. T., Szlachtun, .4.J., IND.E N G . CHEM. 47, 1000 % ELONGATION
compromises in mechanical properties imposed by such type composite films have been described ( 3 ) . Such posttreatment enhances stain and solvent resistance and changes the mechanical properties of the film. The changes in the stress-strain properties are shown in Figure 11, where, as a result of the posttreatment, the stressstrain curves for all formaldehyde concentrations were shifted toward the stress axis. Compositions containing low, as well as zero, concentrations of cross-linking agent are affected more by posttreatment than those containing higher concentrations. The magnitude of the effect is as expected from the swelling index and solubility data-e.g., the higher the swelling index and solubility indicative of lower initial cross linking, the greater the effect. The effect of posttreatment on swelling index and solubility is shown in Figure 9. Posttreatment levels out the effect of initial formaldehyde concentration on both these parameters, and as a result the solubility and swelling index is in-
1058
sensitive to initial formaldehyde concentration. Particularly dramatic is the effect of posttreatment on cyclic tensile fatigue properties. The number of cycles to failure for compositions containing zero concentration of formaldehyde initially is 10 cycles; after treatment, 179 cycles. For a film containing an initial concentration of 0.5 mole of formaldehyde per basal mole of methacrylamide, the number of cycles to failure is 24, compared to 112 after posttreatment. For films containing concentrations of 2 and 4 moles, the structure of the film is initially sufficiently tight, so that posttreatment does not enhance resistance to cyclic tensile fatigue. The posttreatment involved immersion in Methyl Formcel of a film previously heated to 100’ C. After immersion, the film was dried and heated for a n additional hour. As the effects of the posttreatment on the film’s mechanical properties, swelling index, and solubility might be due simply to the additional heat or curing cycles, a control experi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1955). (7) Fram, P., Szlachtun, A. J., DeFries, M, G., Leonard, F., Zbid.,46, 1992 (1954). Fram, P., Szlachtun, A. J., Leonard, F., Zbid., 47, 1209 (1955). Leonard, F., Cort, I., Blevins, T. B., Zbid.,43, 2500 (1951). Lundberg, L. A., U. S. Patent 2,475,846 (July 12, 1949). Mast, W. C., Fisher, C. H., IND. END.CHEM. 41,790 (1949). Nuessle, A. C., Kine, B. B., Ibid., 45, 1287 (1953). Riddle, E. H., Chem. Eng. News 31, 2854 (1953). Riddle, E. H., ‘:Monomeric Acrylic Esters,” p. 128, Reinhold, New York, ‘1952. (15) Rohm & Haas Co., “Methacrylamide,” SP-32 (August 1955). (16) Walker, , J. F., “Formaldehyde,” 2nd ed., Reinhold, New York, 1953. (17) Zbid.,pp. 290-306.
RECEIVED for review August 22, 1957 ACCEPTED January 18, 1958 Division of Rubber Chemistry, ACS, Montreal, Canada, 1957. Part of a research project of the Office of the Surgeqn General, Department of the Army, in connection with the government amputee research program in cooperation with the Advisory Committee on Artificial Limbs, National Research Council.
