Butyl Acrylate Elastomers Cast from Latex J
J
USE OF ALKYL METHACRYLATE POLYMERS AS FILLERS A report of research in developing a lifelike cover for mechanical hands for amputees
PAUL FRAM, ANDREW J. SZLACHTUN, AND FRED LEONARD Army Prosthetics Research Laboratory, Walter Reed Army Medical Center, Washington, D . C .
T
HE development of synthetic rubbers has progressed along
many divergent paths, yielding elastomers capable of meeting unusual requirements in many special applications. One application of an elastomeric material requiring a n unusual combination of properties is the “cosmetic” glove used to cover a mechanical hand or a n artificial restoration for an amputee. Among the many types of synthetic elastomers, the acrylates possess many of the exceptional properties that are demanded by the stringent technological requirements in this application. The field of acrylate elastomers has been recently reviewed by Riddle ( I % ) , from the standpoint of both monomer synthesis and commercial utilization of the polymers. The preparation and properties of a large series of acrylate elastomers have been extensively investigated by the Eastern Utilization Research Branch, United States Department of Agriculture (6-8, 11). T o effect vulcanization, chlorine-containing monomers have been copolymerized with the acrylate monomer, as described by Mast and Fisher ( 5 ) . Owen ( 1 0 ) and Semegen ( 1 6 ) have also discussed vulcanization methods that yield useful elastomeric compositions. The vulcanized products are reported to have outstanding flex life, good heat resistance, and resistance to oil, having applications as gaskets for automatic transmissions for gasoline engines. Furthermore, because of their low softening points, most of the acrylates are particularly suited t o filmforming applications-for instance, textile processing (9)especially in the form of aqueous dispersions which are prepared readily by emulsion polymerization. The strength of acrylate elastomers was shown ( 2 , 4 ) to be significantly increased by copolymerizing the acrylate monomer with acrylonitrile, and also by loading with finely divided fillers. The choice of filler was limited in a n application like cosmetic gloves by the requiremmt of a transparent to translucent composition. Among the noncarbon tvpe of fillers known t o reinforce acrylate elastomers (%) were amorphous silica and certain poly(alky1 methacrylates). I n the present study a wide variety of poly(alky1 methacrylates) were prepared in latex form and evaluated as fillers in copolymers of butyl acrylate and acrylonitrile. EXPERIMENTAL
The alkyl methacrylate monomers were obtained from the Monomer-Polymer Corp., Leominster, Mass., with the exception of methyl and ethyl methacrylates, which were supplied by the Rohm & Haas Co., Philadelphia, Pa. The monomers were freed of inhibitor by alkaline washings, followed by washings with deionized water until the aqueous phase was neutral t o litmus paper. The washed monomers were dried over Drierite, filtered, and stored a t 5 ” C. until ready for use. The approximate charge formula employed in the preparation of the latices is as follows:
Recipe for Emulsion Polymerization of Alkyl Methacrylate Monomer Ingredient Monomer Water Santomerse D
KC1 KZSZOS NazSeOs. 5H~0
Parts by Weight 100 121-129
1.8-4.0 0.14 0.011 0,011
Two of the latices used in this study were supplied as experimental latices through the courtesy of the Rohm & Haas Co., designated as Rhoplex Mc393, a 48.2% total solids latex of poly(methy1 methacrylate), and Rhoplex Mc387, a 47.5% total solids latex of poly( ethyl methacrylate). The other emulsion polymerizations were carried, in nearly every case, to above 95% conversion. I n a few preparations where a lower degree of conversion was obtained, the latex was stripped under vacuum t o remove unreacted monomer. T o prepare all but the methyl and ethyl methacrylate polymers the “bottle” emulsion polymerization technique was employed. The monomer, potassium chloride, and the aqueous solution of the Santomerse D detergent (Monsanto Chemical Co.) were introduced into a 1-liter screw-cap bottle. After flushing for 2 hours with oxygen-free nitrogen, the persulfate and thiosulfate solutions were added and nitrogen flushing was continued for 15 minutes. The bottles were t h e s quickly sealed by inserting a cork in the neck of the bottle and tightening on the metal cap. The bottle was shaken vigorously, placed in a water bath a t 35” C., and rotated end over end a t 26 r.p.m. for 20 hours. The various poly( alkyl methacrylate) latices were mixed with a stock latex of 90: 10 butyl acrylate-acrylonitrile copolymer to yield a solids-weight ratio of 30 parts of poly( alkyl methacrylate) to 100 parts of 9O:lO butyl acrylate-acrylonitrile. The mixed latex was stripped under vacuum for 30 minutes a t 50’ C. before casting the test film. For improved latex storage stability and castability, the p H was adjusted to a value between 7.5 and 8.5. RESULTS AND DISCUSSION
A great deal of interest has been shown recently in the blending of rigid resins into elastomers and in the advantages of direct mixing of compounding ingredients in latex form. I n regard to improvement in the properties of an elastomer by means of milling in the rigid resin, Sell ( 1 6 ) has reviewed the use of butadiene copolymers of high styrene content, which are of very limited value as resins themselves, but have gained widespread acceptance as a rubber compounding ingredient. Schmidt ( 1 4 )has found from a study of GR-S vulcanizates containing noncarbon fillers, including organic polymeric types, which were added as aqueous dispersions or latices, that the particle size of the filler IS of prime importance in direct latex mixing of fillers into a n elastomer. Le Bras and Piccini ( 3 ) have shown the advantages of latex reinforcement of rubber, employing thermosetting-type resins as fillers for natural rubber. I n the present study, it was assumed t h a t the particle sizes of the various poly(alky1 methacrylate) latices, added t o a 9O:lO butyl acrylate-acrylonitrile latex, were of the same order of mag-
1209
INDUSTRIAL AND ENGINEERING CHEMISTRY
1210
nitude as the particle size desired for reinforcement. This assumption was based on the similarity of the viscosities of most of the poly( alkyl methacrylate) latices with t h a t of a poly(ethy1 methacrylate) latex which had equivalent total solids and a known average particle size of 2000 A. as determined by electron microscope photographs.
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Figure 1.
Effect of number of carbon atoms on tensile strength
90 :10 butyl acrylate-acrylonitrile
copolymers with 30 p a r t s of PAMA fillers
Refractive Index. In an attempt to produce film compositions with good light-transmitting qualities, fillers were selected which had nearly the same refractive index as the 90: 10 butyl acrylateacrylonitrile elastamer. For 90 :10 butyl acrylate-acrylonitiile copolymer, nn a t 25" C. was equal to 1.468. As seen in Table I, the values for the refractive indices of the various poly(alky1 methacrylate) fillers ranged from 1.470 to 1.510. As much as 30 parts by weight of these fillers gave Y0:lO butyl acrylateacrylonitrile compositions which were translucent to transparent in their light-transmitting property. There was a fairly good correlation between the degree of opacity and the difference between the refractive index of the filler and that of the 90:10 butyl acrylate-acrylonitrile copolymer. As a n extreme example, a filler of rigid styrene-butadiene type resin (Marmix 7345, Rlarbon Corp.) whose refractive index was higher than 1.580 gave white, optically opaque 90:10 butyl acrylate-acrylonitrile compositions with as little as 5 parts by weight of filler. Transition Temperature. I n addition to polymer refractive indices Table I also shows the Vicat softening point, the secondorder transition temperature, T,, and the brittle point, Tb, for some of the poly(alky1 methacrylate) fillers, listed in order of decreasing temperature. The T'icat softening points, reporte 1
Table I. Transition Temperatures and Refractive Index of Poly(allcy1 Methacrylates) Softening Refractive Alkyl Temp.a, Tmb, Tbcl c. c. 0 c. Index Group 1.492 (20' C.)d Methyl 72 90 119 1.506 (20' C.) e 105 Cyclohexyl 81 ' 50 1.483 (20' C.)d '47 Ethyl 67 .. .. 1.477 (20' C.)d Isobutyl .. .. 1.470 (25' C.)f 66 2-Ethylbutyl 55 .. 1.510 (25' C.)f Allyl 30 1.485 (20' C.)d 17 'is n-Butyl ,. R 1 . 4 8 1 (20' C.)d n-Amyl + o .. .. 1 , 4 8 1 (25' C.)Q R 2-Ethoxyethyl R .. 5 1 . 4 8 1 (20' C.) e n-Hexyl ...... R 20 - 70 n-Octyl .... - 70 - 30 R n-Decyl a Vicat softening points ( I ) , R indicating rubbery or too soft to measure a t room temperature. b Transition temperature ( 1 7 ) .
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R e f r ~ E t ~ ~ ~ - i n d e x ' ; e p o by r t e dCrawford (1). e Refractive index reported by Polaroid Corp. PB28 '53 (13). I Refractive index of powder by oil immersion techn'Fque. d
B
Refractive index of film employing Abbe refractometer.
.
Vol. 47, No. 6
chiefly by Crawford ( I ) , were determined as the temperature in degrees centigrade a t which a needle with a flat face 1 sq. mm. in area, loaded v i t h 1 kg., penetrated a specimen surface to a depth of 1 mm. while the temperature was raised a t a rate of 50' C. per hour. The second-order transition temperature data were from Wiley and Brauer ( I 7 ) , and the brittle point data were reported by Rehberg and Fisher ( 1 1 ) . The isomeric variations within the alcohol group have a marked influence on the transition temperatures of the polymers. branching of the alcohol raising the temperature. For instance, poly(isobuty1 methacrylate) has a softening point about 37" higher than poly(n-butyl methacrylate). Branching of the hexyl alcohol group, as in 2-ethylbutyl, has an even more pronounced effect, poly( 2-ethylbutyl methacrylate) having a softening point about 45' higher than poly(hexy1 methacrylate). Physical Properties. To show the effect on the physical properties of 90: 10 butyl acrylate-acrylontrile, test films were cast from a mixed latex to which was added the calculated amount of the poly(alliy1 methacrylate) latex to yield a film with 30 parts of filler. Films suitable for test purposes were obtained in 10 of the 12 experiments; adding poly(ally1 methacrylate) and poly(%decyl methacrylate) latices t o the 90:10 butyl acrylate-acrylonitrile latex resulted in cast films which were too thin and tacky for processing into test specimens. I n Table I1 are listed the tensile properties of the various filled unvulcanized compositions in order of decreaEing softening point of the filler. The results of tensile tests mere averaged for five dumbbell specimens cut with die C. Tear resistance was determined by averaging three tests on notched specimens cut with a Graves die.
Table 11.
iMechanica1 Properties of 90: 10 Copolymer Containing 30 Parts of Fillers
Alkyl Tensile ElongaTensile 300% Group Softening Strength tion a t a t Break Modulus, In Temn.a, Lh./Sq.' Break, Lb./Sq.' Lb./Sq PAMA c. Inch % Inch Inch Methyl 119 1130 840 10,600 110 Cyclohexyl 105 600 820 5,520 114 Ethyl 81 1050 660 8,000 312 Isobutyl 67 950 580 6,460 196 2-Ethylbutyl 66 530 820 4.360 188 Allyl 55 Latex gave unsuitable film for test n-Butyl 30 600 820 5,520 156 n-Amyl R 370 840 3,480 89 2-Ethoxyethyl R 296 970 3,460 54 n-Hexyl R 24 1000 264 3 n-Octyi R 14 960 148 4 n-Decyl R Latex gave unsuitable film for test a Vicat softening point temperatures reported by Crawford ( 1 ) .
As noted in Table 11, the list of poly(alky1 methacrylate) fillers included polymers with straight or normal alcohol groups including methyl, ethyl, butyl, amyl, hexyl, and octyl, as well as 2-ethoxyethyl. The poly(alky1 methacrylate) fillers with alcohol groups which were isomeric variations of some of the straight alcohol groups were the polymers of isobutyl, 2-ethylhutyl, and cyclohexyl methacrylate. The plots of tensile strength based on the original cross section and tensile strength calculated a t break, assuming constant volume during stretch, are shown in Figures 1 and 2, respectively, as a function of the number of carbon atoms in the alcohol group of the filler. The curve for the compositions containing fillers with straight alcohol groups showed a fairly rapid decrease in tensile strength with a n increase in the length of the alcohol group. The use of poly(hexy1) or poly(octy1 methacrylates) as fillers gave Teak, gummy compositions. The curve for the fillers with branched alcohol groups was displaced above the curve for the fillers with the straight alcohol groups. This was observed in both plots (Figures 1 and 2) of the tensile strength data. The data for films with poly( cyclohexyl methacrylate) filler agreed rather well with the curve for the polymers with branched alcohol groups. To illustrate the effect of the type and size of the alcohol group
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1955
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of the poly(alky1 methacrylate) filler on the 3ooy0 modulus, the data were plotted t o give the curves shown in Figure 3. The modulus was higher for ethyl t h a n for the methyl homolog, and then decreased as the number of carbon atoms in t h e alkyl group increased in the range from two t o eight carbon atoms. I n this range, fillers with the branched alkyl groups yielded a slower rate of decrease in the modulus, probably as a result of their inherently more rigid nature. T h e significantly lower moduli found for films filled with either poly(methy1) or poly(cyclohexy1 methacrylate) may be explained b y considering the relationship between their softening points and the maximum temperature t o which the films were subjected.
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was the permanent set, measured 10 minutes after break-the 150" C. treated sample had 37.6y0 set, while the 100' C. treated sample showed only 9.4% set. It was concldded from these data t h a t the reinforcing effect of a n organic polymer t h a t is thermoplastic in nature should be interpreted on the basis of the form in which it exists in t h e matrix of the elastomer molecules. I n accordance with the history of the sample treatment, the softening point of the filler appeared t o be related t o its effect on the physical properties of elastomer. Low Temperature Behavior. The stiffness in flexure modulus data a t temperatures ranging from 0' to -30" C. for the various poly(alky1 methacrylate)-filled compositions of 90: 10 butyl acrylate-acr ylonitrile) copolymer are shown in Table I11 and Figure 4. From these d a t a it was concluded t h a t the temperature at which the composition became significantly stiffer, probably similar t o t h a t of the second-order transition temperature, was governed mainly by the low temperature behavior of the base elastomer. Examination of the stiffness moduli measured a t -20" C. revealed a decrease in modulus with an increase in the length of the alcohol group of the filler. The methyl and cyclohexyl methacrylate polymers behaved i n a n anomalous fashion compared t o the other fillers, possibly because of their existence as discrete particles in t h e matrix of elastomer molecules. Latex and Solvent-Cast Films. The data shoun in Table I V were obtained t o compare the mechanical properties of films cast from latex with those of films cast fiom a solution of the base elastomer and the polymeric filler, T h e 90 : 10 butyl acrylate-acrylonitrile copolymer xvas coagulated from t h e latex, dried, and then dissolved in methyl ethyl ketone. Solutions of about 7% solids content were prepared containing varying parts of poly(ethy1 methacrylate) (PEhfA). Films were cast on glass plates fitted with raised edges. After removal from the glass surface, the films were dried under vacuum a t 50' C. for 20 hours, and then heated at 100' C. for 1 hour in a circulating-air oven. T h e tensile strength d a t a were plotted t o compare latex-cast films with solution-cast films. The plot of ultimate tensile strength is shown in Figure 5, while the plot of tensile strength
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The films were i n each case heated at 100" C. for 1 hour as a final treatment t o ensure fusion of the elastomer particles laid down from the latex. Hence, it was possible for particles of fillers with the higher softening points t o exist in a discrete form in the matrix of elastomer molecules. T h e particles of fillers with softening points lower than 100" C. would then be considered either entirely or partially fused, depending on the time of treatment. Evidence in substantiation of this view was obtained bv heating a poly(methy1 methacrylate)-filled sample a t 150" C. for 1 hour. T h e 3oOy0 modulus was changed from 110 t o 530 pounds per square inch, while the tensile strength remained virtually unchanged at 1130 pounds per square inch, and the elongation was lowered somewhat from 840 t o 710%. Another significant effect
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 111. Stiffness in Flexure at Low Temperatures of 90: 10 Copolymers Containing 30 Parts of Fillers Alkyl Group in PAMB
Temperature,
- 30
None Methyl Cyclohexyl Ethyl Isobutyl 2-Ethylbut~ Allyl n-Butyl n-Amyl
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Vol. 47, No. 6
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Figure 6. Effect of poly(ethy1 methacrylate) content on tensile strength at break
definite conclusions were drawn from a comparison of the data on tear resistance. I n regard to the 300% modulus, fairly equivalent results were obtained for both methods of casting. Attempts to improve the mechanical properties of the filled 9O:lO butyl acrylate-acrylonitrile compositions by means of cross linking have been unsuccessful. A low degree of vulcanization was achieved by employing benzoyl peroxide as the cross linking ingredient. Insolubilization of the 9O:lO butyl acrylateacrylonitrile copolymer was effected by this means, but no significant changes in strength or low temperature properties were obtained.
@---SOLVENT A-LATEX
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SUMMARY I
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Effect of poly(ethy1 methacrylate) content on tensile strength
calculated on the basis of the cross-sectional area a t break, assuming constant volume, is shown in Figure 6. I n both plots, the latex-cast film produced higher tensiles in the range from 20 to 50 parts of filler. At higher poly(ethy1 methacrylate) content, the solution-cast films appeared stronger. As predicted, the tensile strength of the solution-cast films increased a t a fairly uniform rate with increasing amounts of the rigid polymer. The curve for the latex-cast films, on the other hand, passed through a maximum a t about 40 parts of poly(ethylmethacrylate), reached a minimum a t 60 parts poly(ethy1 methacrylate), and then proceeded to rise again. It is not clear how much significance can be given to the poorer tear resistance (Table IV) of the films cast from solution compared to those cast from latex. The thickness of the solvent-cast films was considerably less than that of the latex-cast films, and because of this variation no
Table IV. PEMA
Content, Parts by Wt.
n 10 20 30 40 50 60 70
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ACKNOWLEDGMENT
This study was performed as part of a research project of The Office of the Surgeon General, Department of the Army, in connection with the government amputee research program in cooperation with the Advisory Committee on Artificial Limbs,
Properties of Poly(ethy1 Methacrylate)-Filled 90: 10 Films Cast by Solution and Latex Techniquesa Tensile Strength, Lb./Sq. Inch
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Tensile a t Break, Lb./Sq. Inch
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2500+
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A series of alkyl methacrylate polymers, prepared with varying length and structure of the alcohol group, was incorporated by latex mixing in a 90:lO butyl acrylate-acrylontrile elastomer to determine reinforcing quality. I n the compositions containing 30 parts by weight of filler based on 100 parts of elastomer, tensile strength was not only a function of the number of carbon atoms in the alcohol group of the filler but also depended on the structure of the alcohol group. The fillers of higher softening point gave the best mechanical properties. The mechanical properties were significantly influenced by changing the form in which the reinforcing fillers existed in the elastomer structure. The transition of the filler from a discrete form t o a fused or solid solution form resulted in a filled elastomer with a significantly higher modulus.
2780 7650 9000 9150 5780 5250
300V0 Modulus, Lb./Sq. Inch S L, 45 25 60 60 110 75 140 220 370 390 135b 680 850 1756 260 b 108Ob 800b
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Tear Resistance, Lb./Inch
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INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1955
(8) &last, W. C., Smith, L. T., and Fisher, C . H., IND.ENG.CHEM., 37, 365 (1945). (9) Nuessle, A. C., and Kine, B. B., Ibid., 45, 1287 (1953). (IO) Owen, H. P., Rubber A g e , 66, 544 (1950). (11) Rehberg, C. E., and Fisher, C. H., IND.ENG.CHEM.,40, 1429 (1948). (12) Riddle, E. H., Chem. Eng. News, 31, 2854 (1953). (13) Schildknecht, C. E. "Vinyl and Related Polymers," p. 229, Wiley, New York, 1952 (14) Schmidt, E., IND.ENG.CHEM.,43, 679 (1951). (15) Sell, H. S., I n d i a Rubber W o r l d , 129,498 (1954). (16) Semegen, S. T., Rubber Age, 71, 57 (1952). (17) Wiley, R. H., and Brauer, G. M., J. P o l y m e r Sci., 3, 647 (1948).
National Research Council. T h e authors wish t o express gratitude t o M. G. DeFries. W. S. Wright, and A. L. Best for their efforts in obtaining the test results. LITERATURE CITED
(1) Crawford, J. W. C., J . Soc. Chem. I n d . , 68, 201 (1948). (2) Fram, P., Szlachtun, A. J., De Fries, M. G., and Leonard, F., IND.ENG.CHEM.,46, 1992 (1954). (3) LeBras, J., and Piccini, I., I b i d . , 43,381 (1951). (4) Leonard, F., Cort, I., and Blevins, T. E., I b i d . , 43,2500 (1951). (5) Mast, W. C . , and Fisher, C. H., Ibid., 40, 107 (1948). (6) I b i d . , 41,790 (1949). (7) Mast, W. c., Rehberg, C . E., and Fisher, c. H. (to U. S. GOVernment), U. S. Patent 2,449,612(Sept. 21, 1948).
1213
ACCEPTED January 15, 1955, RECEIVED for review September 22, 1964. Presented before the Division of Polymer Chemistry at the 126th Meeting of the AMERICAN CHEMICAL SOCIETY, New York. K. Y.,1964.
Alkyl Amates as Plasticizers of Elastomers J
ARTHUR WILLIAM CAMPBELL Research Department, Commercial Solvents Corp., Terre Haute, Ind.
T
HE development of various elastomeric substances during World War I1 was noteworthy in its primary objective, but
was accompanied by severe shortages in effective compounding ingredients. One of the weak spots was plasticizers, and this report covers t h e exploratory work done on one group of compounds in a n attempt t o relieve t h a t shortage. The use of t h e esters of various acids, chiefly organic acids, as plasticizers of elastomers is not new. The use of imides, amides, and diamides was discussed by Campbell and Tryon (1). To combine the ester and amide groups in one molecule invoked a n old but little known type of compound, t h e amate ( 4 ) . Because two functional groups involving carboxyl are present in the amic acids, t h e starting point is a dibasic acid such a s phthalic acid. By suitable treatments one carboxyl is esterified and the other converted t o a n amide, resulting in a compound of the following structure. 0 0-4-0-R W-C-Y-Rz
e,
R,
The amates described in this paper have been prepared by various methods.
T h e product was distilled in vacuum through a six-bulb jacketed Snyder column; 78 grams were distilled u p t o 80" C. a t 100 microns. T h e second fraction boiling between 80" C. at 100 microns arid 130" C. a t 50 microns weighed 166 grams and amounted t o a yield of 60.5%. Analysis. C,,4H360aN. Molec- . ular weight, 266. Nitrogen calculated, 5.2601,; found, 5.53%. T h e compounds listed in Table I are new compositions of matter here characterized for the first time. TESTING PROCEDURE
Four elastomers were used in the evaluation of the various alkyl amates. Natural rubber (Hevea), a blend of four plantation crudes obtained from The B. F. Goodrich Co. Poly(viny1 chloride), Geon 101, Goodrich Chemical Co. Butadiene-acrylonitrile, Hycar OR-15, Goodrich Chemical Co. Butadiene-styrene, GRS, Rubber Reserve Code 1500 The formulation used in each case appears in the tabulation of t h e results. The mixing procedure followed the methods generally specified by t h e manufacturer for t h a t particular elastomer. T h e press cures were made according t o ASTM D 1541. T h e tensile test employed a Scott compensating head tensile machine and followed ASTM D 142-41, at a room temperature of 82' 2" F. T h e load, tensile, elongation, rebound, and hardness tests were all run at this temperature. T h e sheeted
PREPARATION OF AMATES *
n-Butyl N,N-Di-n-butyl Phthalamate. Phthalic anhydride (148 grams), butanol (74 grams), and benzene (50 ml.) were heated a t reflux until the anhydride had dissolved. Di-nbutylamine (129 grams) was added and this mixture was heated at reflux using a Dean and Stark separator t o remove the water formed, When the theoretical amount of water had been eliminated, the product was distilled through a simple still. Low boilers were removed in aspirator vacuum. T h e main fraction distilled at 145" t o 152" C. a t 75- t o 80-micron pressure. Yield was 195 grams, 58.5%. Analysis. C Z O H ~ J O ~MolecN. ular weight 333. Nitrogen calculated, 4.20%; found, 4.443%. n-Butyl N, N-di-n-butyl Oxamate. Di-n-butyl oxalate (202 grams) and di-n-butylamine (129 grams) were charged into a 500-ml. flask under a 2 4 b u l b jacketed Snyder column. T h e pot heat was adjusted t o provide a moderate reflux while the column was adjusted to remove 1-butanol. As the reaction started slowly, a n excess of 25y0 of t h e amine was added. After about 1 week of heating, t h e theoretical amount of alcohol had been removed.
I
SUBSTITUTED AMATES
. . . have both ester and amide functions . . . satisfactorily plasticize natural and synthetic rubber and PVC
Co m merci a I deve I o p ment depends on availability of low-cost
1
secondary amines
