einforce
0
J
e PAUL FRAM, A. J. SZLACHTUN, 31. G. DEFRIES, AND FRED EEOXtRB) A r m y Prosthetics Research Laboratory, Walter Reed A r m y Medical Center, V'crshington J2, D. C .
UNIQUE combination of properties is required of an elasromeric composition suitable for use in the fabrication of gloves t,ing human skin in detail, texture, and color. These cosmetic gloves are morn by aniputees over mechanical hands, and this assembly of mechanical hand and glove represents an effort to restore as completely as possible the amputee's norznal prehensile function and appearance ( 7 ) . Such properties as good resistance to staining and outdoor xveathering, and inertness t o skinare not toodifficult to obtainper se. The achievement of these qualities, however, in conjunction nith optical translucence and adequate mechanical properties such as resistance t,o snagging and tearing, high tensile strengt,h, low modulus of elasticity, high extensibility, and flexibility t o a t least -20' C., presents a difficult materials problem. Furthermore, from an economical fabrication viewpoint, t,he complex shape of a hand requires that the material be in the form of a high solids dispersion suitable for low pressure casting (2, I S ) . Previous investigations (6, 26, 19, 23) have shown that the acrylate elastomers have rubberlike extensibility, excellent chciiiical resistance, and outst'anding aging propert'ies. They can De cast into films of excellent clarity. Because of theirlon. softening point, they are particularly suited to film-forming applications. One of the major uses for acrylate polymers and copolymers has Seen in coating applications, and they are most frequently ueed in the form of aqueous dispersions which may be readily prepared by t,he eniulsion polymerization technique. One of the serious drawbacks of the soft aci tion in cosmetic gloves is their veak strength. To improve the strength by c~opolymerization and by reinforcing fillers, compounded directll- in tmhelatex without impairing the translucency of the cast films, is the major purpose of this investigation. The problem of direct reinforcement of latex to improve the properties of rubber has attracted the attention of many investigat,ors because of its great challenge from both the theoretical and the practical point of view. Le Bras and Piccirii ( f 1 ) reported significant success with t,he technique of direct latex reinforcement. They obtained excellent strengt,h properties for natural rubber vulcanizntes by incorporating a resorcinol-fornislde~i~cle resin. Many attempts to reinforce natural rubber later with noncarbon mineral fillers have been tried, but resulted in decreased tensile strength vith increased loading, rvhile the modulus, hardness, and sonlet,imes tear resistance increased ( 1 , 15, 38, 2 4 ) . Recently, holr-ever, Keilen and associates ( I O ) report,ed good results in reinforcement of GR-Stype rubbers by coprecipitating lignin \vith the elast,onler. By a similar technique, Schmidt (22, added to the GR-S type lactices colloidal stannic oxide, silica, Prussian blue, polystyrene, and casein, and his results indicated that the small particle size of the pigmeiite was of prime importance in elastomer reinforcement,, whereas the chemical nature of t,he pigment appeared to be of secondary importance. The test specimens for his studies were obtained by adding the aqueous dispersion of the filler to the latex, precipitatilig the mixture of rubber and rigid material, and milling t o incorporate t,he added vulcanization ingredients. The mariner in which the final distribution of the reinforcing particles was effected by the precipitation and milling operations may have introduced possible
complications, which were avoided in the technique of the invcstigation described in this paper. I n this st,udy, since it iws not possible to use a milling teclinique, the end product was made directly from the compounded latex. The choice of additives was limited by the following restrictions: 1. The additive should not flocculate the latex a t the time of its incorporation. 2. The agent should mix easily Jvith the elastomer latex to form a uniform, stable dispersion. 3. Addition of an agent should allow the production, by camsting, of uniform, nontacky films. 4. Vulcanizat'ion or cross linking should be possible. 5 . The additive should not appreciably lower the light transmission characteristics of the compounded elastomer. POLYMER PREPARATlON
The acrylate polymers and copolymers were prepared by emulsion polymerization a t low temperature as previously described ( 2 2 ) . The required ingredients, calculated t,o yield a theoretical content of 55%, mere placed in a &liter, three-neck, round-bottom flask, fitted with a reflux condenser, stirrer, thermocouple, and nitrogen inlet tubes. The flask v a s placed in an air-agitated mat,er bath for temperature control. The reaction mixture was flushed wit,h nitrogen and the aqueous solutions of the persulfa,te catalyst and thiosulfate activator mere added. The polymerization was allowed t o proceed to a conversion above 9Oy0 and the acrylate latex was rented a t 60' C. t o remove the unreacted monomers. Some of t,he preliminary polyrnerizationa were pcrformed a t a reaction temperat'ure of 0" C . t80control the reaction. IIoxvever, in subsequent experiments, by raising the reaction temperature to 25' C. and at the same time lorn-ering the pH to a value of about 2.5, it was possible t'o polymerize with a fourfold rate increase without undue difficulty in temperature cont'rol. No significant effeci on the mechanical properties of the compounded films could be observed as a result of the higher reaction temperature. Some of t,he acrylate polymers 'ryere analyzed for nitrogen content by the Kjeldahl method. To purify the polymer for analyeis, it was first isolated by coagulating a portion of the lat'ex in a 10% aqueous calcium chloride solution. After Soxhlet, extraction with a 3: 1 by volume mixture of water and met'hanol, the polymer mas dried for 24 hours a t GO" C. under a vacuum of 30 inches of iner cury , REIYFORCING AGENTS
One material studied as a reinforcing agent Ir-as a finely divided silica product with approximately the same index of ree copolymers. This was supplied Iraction as that of the under t,he trade name o -C by the Monmnto Chemical Co. :is a 15% aqueous dip: (14). I n earlier experiments the silica was employed as received from the ninnui'acturcr, while in later x o r k , to ohtairi a higher solids compounded latex, the silica dispersion 71-as concentrated before use by evaporation t o a concentration of about 35'34 solids. Aft,er evaporation, the dispersion \vas subjected t o the action of a high speed stirrer in older to break up arid disperse whatever silica agglomerates had formed during concentration. According t o elcct,ron microscope photographs, the silica product consist'ed of sphcricai particles \Those size varied from 400 to 1000 A. .1 phas ' pe was t,o observe the silica con ter thr attnent, a ~ i dit as deter t silica
1992
INDUSTRIAL AND ENGXNEERING CHEMISTRY
September 1954
CONTENT OF HIGHCOYVERSION TABLEI. ACRYLONITRILE EMULSION POLYMERIZED BUTYLACRYLATE COPOLYMERS Acrylonitrile (Based on Total Monomer), Yo Sample No.
218 219 220 217 221 222 223 224
Initi,al reaction mixture
2.5 5.0
7.5 10.0
12.5 15.0 17.5 20.0
High, conversion polymer
Deviation5
2.3 4.2 6.5 8.3 11.4 14.4 13.9 15.7
-0.2 -0.8 -1.0 -1.7 -1.1 -0.6 -3.6 -4.3
1993
The low temperature behavior of the materials was generally determined in the range from 0' to -50" C. by means of the Tinius Olsen stiffness t,ester according to ASTM Method D 747-50, modified as described by Blevins and DeFries ( 3 ) . T o obtain specimens for this test, a sheet about 70 mils thick was cast from the latex in a flat artificial stone mold. The stiffness in flexure was measured by placing t,he test machine in an insulated low t,emperature cabinet, which was fitted with canvas covered ports to enable the operator t o manipulate the test specimen. The
Difference between theoretical and analytical values for acrylonitrile content. @
7- UCRYL0N:TRiLE ANALYZED
agglomerates were essentially destroyed and an excellent dispersion of discrete silica spheres exieted. The poly(ethy1 met'hacrylate) (9), employed as a reinforcing agent, was prepared by the emulsion polymerization technique, using a recipe similar to that for the acrylate latex preparation. A more detailed study of the effect of polymerization variables on the latex properties for both the alkyl acrylate and alkyl methacrylate latices is in progress. z v)
IMIXING, CASTING, AND TESTING
w IL
%h'en compounding was desired, the additive in the form of an aqueous dispersion was slowly poured (with mild agit,ation) 50 into the acrylate latex. For improved stability, the pH of the latex was adjusted with dilute aqueous ammonia. Optimum -50 -*e .30 -20 .IO TEMPERATURE, 'C. pH from the point of view of shelf life and castability was found to be in the range of 7 to 9. By this technique it was possible to Figure 1. Low Temperature Behavior of obtain a shelf life of over ti months. Butyl Acrylate-.4crylonitrile Copolymers The latex w a s d e a e r a t e d under vacuum before films were cast in cylindrical artificial TABLE 11. L O W TEMPERATURE BEHAVIOR O F .%CRYLATE ELASTOMERS stone molds ( 1 2 ) . The deposit of the desired film thickness Reactor MonoSilica (about 30 mils) usually remer Ratio Loading, Stiffness in Flexure, Lb./Sq. Inch Sample EA/AN 0 ; Parts quired a casting time of 1hour, NO. BA/AS b y Wt.a -50' C. -40' C. -30' C. -20' C. -10' C. 0' C. after a-hich the residual latex was drained from the mold. E t h y l Acrylate-Acrylonitrile Copolymer The cast film was partially 188 100/0 Sone ... 13,800 200 8 dried in the mold a t 50" C. 247 10 ... 152:OOO 140;OOO 23,000 11 800 248 23,500 20 ... ... ... 1,300 GOO for several hours and then removed in the form of a 254 97.5/2,5 40 ... 340,000 210,000 140,000 8,500 4,300 cylinder, closed a t one end. 255 9515 10 ,.. 170,000 170,000 87,000 8,600 4,200 The cylinder was cut open and 257 30 ... 350,000 280,000 145,000 16,000 3,000 the flat sheet was dried and heated a t 100' C. for another 260 220,000 92.517.5 20 ... ... 240,000 27,000 2,300 hour. 3 90jlO Sone . . . , . . 250,000 1,000 196,000 20,000 The mechanical properties 223 30 ... ... 300,000 220,000 60,000 8,000 a t room tempcrature, includ224 40 ... ... 370,000 250,000 85,000 11,500 ing the ultimate tensile 226 85115 None ... ... 300,000 260,000 73,000 4,700 strength, elongation a t break, and 300% modulus (tensile Butyl Acrylate-Acrylonitrile Copolyiner stress a t 300% elongation) 3,700 220 . . .b 190 10010 Xone ...b . . .b ... were determined according to 20 12,000 . , . b 281 800 . b ..,b ... 40 14,500 ..,b 1,800 . .b . . 282 ... ASTM M e t h o d D 41249T, usine: the Scott L-6 tensile 218 97.5/2.5 ?;one 18,500 500 150 65 75 ... testiig machine. The rate of 285 20 18,200 270 1,600 450 320 ,.. 287 40 33,000 6,200 4.800 .. 2,900 2,700 jaw separation was 20 inches per minute. Die C was emSone 53,000 4,900 220 80 219 95/5 70 . . ployed to cut the tensile speci20 59,000 300 4,200 680 360 ... 289 120,000 40 11,500 4,500 ... 29 1 2,700 3,300 mens. A minimum of five specimens was tested and an Sone 17,500 450 85 220 97.5/2,5 70 ... average value was calculated, 27,000 3,200 20 140;000 620 ... 1,000 293 40 190,000 43,000 7,000 3,400 2,900 ... 295 employing the results of those specimens which broke with217 90110 None 60,000 G,5OO 180 120 ... lab: 000 56,000 ti, 100 ... 297 20 920 560 out visible flaws ( 3 ) . Tear 81-2-4 35 14,000 1,500 850 2,000 strength w a s m e a s u r e d o n 180;000 75 1000 26,000 5,000 4,200 299 40 45 ... ,.. 20,000 2,000 81-2B notched specimens, which were 4,300 2,500 50 ... ... 43,000 4.800 9.500 81-2C 6.900 cut with a Graves die. Prior to testing, all specimens were Parts/100 parts by weight of acrylate elastomer. b Too low t o record accurately. conditioned 1 week a t 50% relative humidity and 25' C. Q
INDUSTRIAL AND ENGINEERING CHEMISTRY
1994
Vol. 46, No. 9
ACRYLONITRILE CONTEST
The analyses of t8he butyl acrylate copolymers (Table I ) showed, as expected, that the high conversion niaterial had essentially the samc coniposition as the initial monomer misture. Some differences were found r h e n the initial monomer misture was richer than 15% in acrylonitrile. I n this higher range the resuhing copolymer, according to the analysis, contained somewhat less acrylonitrile than the iriitiitl monomer mixture. The solubility characteristics of the emulsion polymerized copolynicrd in methyl ethyl ketone, as well as unpublished data obtained in this laboratory for the analysis of low conversion, bulk polyn~rrized EAi/.AS copol>,mers, indicated that copolyineriaat,ion occurred and that, a physical mixture of homopolymers m s not, formed. The mechanical properties of the unreinforced eopo1yrner.s were in many instances difficult t o measure because of the tackiness and weakness of the cast film@. 119th an increase in acrylonitrile content, the copolymer material hecame easier to cast and improved in strength properties. I n addition to the mechanical properties a t room t,emperature, the low teiiiperarure flesibilit,y of the copolymers was determined. Since it was expected that subsequent reinforcement by filler loading would
I .50'
I
.40
I
-30
I
I
-13
-20
TENPERAXRE,
\I
'C.
Figure 2. Low Temperature Behavior o f Butyl Acr>late and Ethyl Acrylate Copolymers with Acrjlonitrile
specimens were allowed to remain in the cabinet while the temperature was lowered t o either -40" or -50" C. After the temperature was stable for about 20 minutes, the specimens were placed in the testing machine and their stiffness in flexure modulus was measured. The temperature in the cabinet was iaised to the next higher point, and this operation was repeated until the study of the desired range of temperature xas completed. The whole procedure took about 6 hours, during which time the specimens remained inside the low temperature cabinet.
For conrenience in designating the polymers the follon ing abbreviated forms are used in the text and tables: BA/AN
EA/$?; PE MA PNNA
Butyl aci ylate-acrylonitrile copolymei Ethyl acrylate-acrylonitrile copolymer Polyethyl methacrylate Polymethyl methaci ylate
111. PROPERTIES O F E T H Y L *kCRYLATE-a4CRYLoKITRIl,E TABLE COPOLYMERS as FUSCTIOSO F SILICA LOADIXG StLrnI)k SO.
Reactor Monomer Ratio,
EA/AS
Silica Loading, Parts by FTt.a
95.0/5.0
20
Tenaile ElongaStrength, tion a t (Lb.181~. Break,
Inch)
yo
750 1050 1080
730 720 650
10 20 30 40
790 1050
700
1240 1450
640 620
10 20 30 40
900
710
300% Tear l l o d u l u j , RezistLb./Sq. ance. Inch Lh./Incii
247
248 214 230 2.51 252 233 224
236
30 40
257 25s
2 59 260 2iil 262
92.5/7.5
217 218 223 221;
90.0/10.0
920 1210 1390
660
630 630
590
a Par%/I00 parts by weight of acrylate elastomer
150 230
200
72 9:3 110
130 200 320 440
61 74 99 123
80 180 270 360
33 68 90 112
b A G R Y L O Y I T R L E IN POLYMER
Figure 3. Variation of Tensilc Strength with Acrylonitrile Content Silica loading 20 partsI100 parts by w t copolymer
xigniiicantly increase t,he stiffness, the seiection of a suitable copolymer t,o be used in a composition for cosmetic gloves was arbit'rarily restricted to those unreinforccd copolymers that eshibited a stiffness in flexure modulus at, -20" C. of less than 1000 pounds per square inch. The stiffness in flexure moduli at various low teniperat,ures for unreinforced and silica 1o:drd acr!-late-acrylonit,rile copolymers are given in Table 11. As illust,rat,edin Figure 1, the stiffness in flexure modulus a t IOU temperature for unreinforced butyl acrylate copolymers inrremed rapidly wibh increased acrylonit,rile cont,ent. The same effert wa,s noted for t8heE A / A S copolymers. The mechanical properties a t room temperature, however, of the Eh/AK copolyiiim in their unreinforced form were better than the corresponding butyl acrylate copolymers (Tables 111 and IV). At low tcmperatures the ethyl acrylate copolymers were considerably stiffer (Table IT). The order of magnitude of this difference in flesibilit,y at low temperature is shovn graphicaily in Figure 2. To illustrate the improvement in strengt'h properties as a result of using ethyl acrylate in place of but,yl acrylate in the copolymer, the tensile strengths for the two series are plotted as a function of the acrylonitrile content, for films which contained 20 parts by weight of silica (Figure 3).
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1954
LOO00
20000
10000
1995
-
I 60
50
Figure 4.
40
c
TEMPERATURE> C
v,
500
Low Temperature Behavior
30 20
of Acrylate Elastomers Reactor lMonomer Ratio
Loading, Parts/ 100 Parts by Wt. Copolymer None PEMA PEMA Silica Silica None
eo0
20
40 20 40
REISFORCEMENT
Figure 5. Low Temperature Behavior of Polyethyl Methacrylate Filled Butyl AcrylateAcrylonitrile Copolymers BA/AN ratio 90/10
h a d i n g i n parts/100 parts by wt. copolymer
SILICA.The effect of silica loading on the properties of ethyl aciylate-acrylonitrile copolymers is reported in Table 111. T o obtain this series of samples the desired amount of Syton-C ( 1 4 ) was added to the various EA/AN copolymer latices with mild agitation. To determine the uniforinity and stability of these dispersions, five successive films were cast from one latex dispersion containing 50 parts by weight of silica based on the EA/AN copolymer content. The analytical results for silica showed that during repeated castings very little change occurred in the composition of the latex. The silica content of the films decreased very slightly, from a value of 33.34 to 32.94%, while the total dry solids of the latex decreased from ~t concentration of 31.7 to 31.2%. I n another series of determinations, after 11 castings from a 90: 10 BA/AN copolymer latex with a silica Ioading of 6.5 and 30 parts of PERIA, no significant change was observed in the latex total solids, the per cent silica in the caet films, or the physical properties of the cast films. From these results, as well as electron micrographs, it was concluded that a uniform dispersion of silica particles existed in the compounded latex and that films cast from the compounded latex consisted of a matrix of elastomer and reinforcing particles in very nearly the same ratio as the parent latex. The mechanical properties of the various ethyl acrylate copolymers were definitely improved by the addition of silica. I n a manner similar to that previously reported for 90: 10 EA/.4K copolymer ( 1 2 ) with an increase in silica loading, the tensile strength, 300% nioddus, and tear resistance increased while the elongation of break decreased only slightly. Table I11 also shows the effect of acrylonitrile content on mechanical properties of elastomers based on EA/AN copolymer. For instance, a copolymer containing 10% acrylonitrile (sample KO.224, based on 9 O : l O EBIAN, compared with No. 250 based on 1OO:O EA/AW) showed a threefold increase in the tensile strength, a threefold increase in the 300% modulus, and a twofold increase in the tear resistance. The measurements of stiffness in flexure a t low temperature for some of the silica-loaded ethyl acrylate elastomers are listed in Table 11. The addition of 10% acrylonitrile to the reaction
mixture yielded approximately a 100-fold increase in the - 10' C. stiffness modulus of the unreinforced materials. Further increase in modulus resulted when the base elastomer was reinforced with silica. For example, the 90: 10 EAjAN copolymer loaded with 40 parts of silica exhibited a t a stiffness in flexure modulus a t -10" C. that was four times the modulus observed for the unreinforced copolymer. I n seeking t o improve the low temperature flexibility while retaining the desirable chemical char-
Figure 6. Low Temperature Behavior o f Polymethyl Methacrylate Filled Butyl Acrylate-Acrylonitrile Copolymers BA/AN ratio 90/10 Loading i n parts/100 parts by wt. copolymer
INDUSTRIAL AND ENGINEERING CHEMISTRY
1996
TABLE
IFr. PROPERTIES O F BUTYLXCRYLATE--kCRYLOXITRILE COPOLYMERS -4s F L X C T I O Y O F SILICA LOADISG Reactor blonomer Ratio,
Sample
KO.
BA/AIi
285 28':
97.5/2.5
289 29 I
95/5
203 291 2!35 296 297 298 81-2
Tensile Silica Loading, Strength, Lb.,Sq. Parts by T V t lncl? 20
.b
R
300% Tear Modulue, ReiibtLb./Sq. ance, Inch Lb.,'Incli d
iiio
90
h L
230
120;
100"
t
20 30 40
230 360 500
il00 970 830
60 80 140
30 LO GO
10 20 30
200 350 430
950 820 750
60 90 120
RR
710 ...
760
son
720 670
20 30 40 XO 90 IO0 103
10
i70
40
02.5/7.5
90/1n
0
h
.,b
__
with t'he except,ion of the dioxane T-ihich was technical gradr obtained from Carbide & Carbon Chemicals Co. EXPERIhf E 3 T A L IIIETMIOD 9
Water-Spray Extraction. The vater-spray extraction W-%R performed with apparatus and methods similar t,o those used by Bartell and Niederhauser (4)and DenekaR and associates (8). In t'his process, fine drops of the aqueous phase are sprayed into the crude oil sample, affording a large crude oil-water interface. The drops flow slowly downward through a tortuous glass colunin gradually forming a honeycomb emulsion with the crude oil. It has been observed that crude oils low in intcrfa,cial activity and was content often do not form an emulsion under these conditions. I n a successful extraction t,he emulsion accumulates i n a bulb a t the bottom of t>hecolumn: the crude oil, occluded in the emulsion, ia released to flow upward. AIost of the aqucous phase separates spontaneously from t'he honeycomb and flows out of the bottom of the bulb. Vlien the bulb has filled with the partly coalesced emulsion, it is removed and the emulsion collected. Then the bulb is replaced and the extraction continued until no more emulsion forms (9).
