Ind. Eng. Chem. Prod. Res.
Nylon Nylon Nylon Nylon Nylon Kevlar Kevlar Kevlar Kevlar Kevlar
aging aging beg. teomp, time, C days RH
o,, %
CO,,%
RH,%
125 125 125 125 125 125 125 125 125 125
19.62 18.30 19.80 0.0 19.66 20.49 20.62 20.52 20.62 20.67
0.41 1.15 0.55 8.58 0.61 0.10 0.14 0.15 0.12 0.12
49.0 47.0 36.0
10 50 10 10 50 10 50 50 10 50
110 110 150 150 150 130 130 150 170 170
163
terest. Chemiluminescence shows promise as a nondestructive field testing technique.
Table IV. Gas Analyses
sample
Dev. 1982, 21, 163-170
final
100.0 34.0 0.1 0.1 0.07 0.06 0.09
but preliminary results from the thermal method indicate a possible linear relationship between total chemiluminescence and tensile strength. The loss in tensile strength appears inversely proportional to the total chemiluminescence.
Conclusions Nylon and Kevlar yarns are subject to degradation under high temperature and humidity conditions even without ultraviolet radiation. After 26 weeks of exposure ten times higher than normal, smog causes -60% strength loss in nylon and -25% strength loss in Kevlar yarns. Ozone had little effect on either yarn. Infrared techniques are not suitable for detection of strength loss in our range of in-
Acknowledgment The authors would like to thank L. Orear, Jr., 5814, for tensile testing; T. M. Myers and D. M. Haaland, 5823, for infrared data; F. B. Burns, 5821, for gas analyses; and B. M. Boatmun for designing smog and ozone exposure equipment.
Literature Cited Auerbach, I. “An Estimate of the Loss in Tensile Strength in Keviar 29 Foilowing 20 Years of Storage at Ambient Temperatures”; Internal Memorandum, Sandla National Laboratories, Albuquerque, NM, April 1978. du Pont Textile Fibers Dept. “Characteristics and Use of Kevlar 29 Aramid”, Report No. 375, E. I.du Pont de Nemours & Co., Wllmington. DE. George, G. A.; Riddell, S. 2. J . Mecromol. Sci. Chem. 1980, A 1 4 , 161-171. Hanst, P. I. Chemistry 1978, 51, 8-15. Jelilnek, H. H. G.; Chaudhuri, A. K. J. Pohm. Sci. 1972. IO, 1773-1788. Jeilinek, H. H. G.; Ed. “Aspects of Degradation and Stabilization of Polymers”, Elsevier: New York, 1978: Chapter 9. Kohen, M. I.”Nylon Plastics”; Wlley: New York, 1973. Mendenhal, Q. D. Angew. Chem. fnt. Ed. Engl. 1977, 16, 225-232. Mlkolajewski. E.; Swallow, J. E.;Webb, M. W. J . Appl. Pokm. Sci. 1964, 3, 2067-2093. Pacific Environmental Services, Inc. Progress Report, “Flber Exposures to Synthetic Smog and Ozone”; Pacific Envlronmentai Services, Inc., 1930 14th St., Santa Monica, CA, Sept 1979. U.S.Environmental Sciences Research Lab, “Noxious Trace Gases in the Air, Part 1, Photochemical Smog”; US. Environmental Sclences Research Lab, Research Triangle Park, NC, Feb 1978.
Received for review June 16, 1981 Accepted January 11, 1982
IV. Symposium on Surface and Colloid Science A. S. Kertes, Chairman 4th International Conference on Surface and Colloid Science Jerusalem, Israel, July 198 1
Free Radical Polymerization of Styrene and Methyl Methacrylate in Dispersed Systems and the Effect of Carbon Black Yaacov Almog and Moshe Levy’ Department of Plastics Research, Weizmann Institute of Sclence, Rehovot, Israel
Styrene and methyl methacrylate (MMA) were polymerized in dispersed systems to yield particles in the range of 10 pm diameter. In the case of styrene, there is interference by emulsion polymerization. This could be minimized by using lauroyl peroxide (LP) as initiator and a low concentration of polyvinyl alcohol (PVA) or submicellar concentration of sodium dodecyl sulfate (SDS) as stabillzers. Bridging of submicron emulsion particles onto the large dispersion particles was observed when high molecular weight PVA was used as steric stabilizer. However when low molecular weight PVA, or an electrostatic stabilizer such as SDS, was used, no bridging was observed and the submicron particles could be washed away to yield smooth dispersion particles. MMA did not give any emulsion polymer with LP but showed a strong Trommsdorff effect which resulted in a double peaked molecular weight distribution. Carbon black inhibited the polymerization of styrene but not that of MMA. When stearoyl peroxide was used as initiator the inhibition was minimal due to the bulkiness of the aliphatic side chain of the initiator.
Introduction Various polymers are produced as spherical particles ranging in size from 0.01 pm in emulsion processes to lo00 pm in suspension processes. The middle range of about 10-pm particles is not a mass-produced product but is 0196-4321/82/ 1221-0163$01.25/0
nevertheless used in many specialized applications. As shown in Table I, emulsions are characterized by a very high number of particles per milliliter of monomer and as a consequence a large specific surface area, while suspensions have a low number of particles per milliliter and a
0 1982 American
Chemical Society
164
Ind. Eng. Chem. Prod. Res. Dev., Voi. 21, No. 2, 1982
Table I. Characteristic Parameters of Dispersed Systems no. of sp surface particles/ area, c m 2 / m L m L of diam, wm of monomer monomer
0.01 0.1
6000000 600000
1 10 100
60000 6 000 600
1000
60
2 2
X X
2 X
2
X
10" 10"
emulsion
log
dispersion
lo'*
2 X lo6
2 X lo3
suspension
low surface area. The dispersion polymerization systems are in between (Vanzo, 1972; Almog and Levy, 1980). Because of the large difference in size, very few emulsion particles are formed in suspension systems. However, in dispersions there is considerable interference from emulsion polymerization. This was discussed previously for styrene polymerization ( h o g and Levy, 1980,1981,1982). In the present publication we would like to present additional data and to compare the results of styrene with those of methyl methacrylate (MMA). We would also like to show the effect that reactive fillers, such as carbon black (CB), have on the reaction. Experimental Section Materials. Styrene (Fluka) and methyl methacrylate (MMA) (BDH), were purified by passing through a basic alumina column; 2,2'-azobisisobutyronitrile (AIBN) (Fluka), was recrystallized from methanol; benzoyl peroxide (BP) (BDH), and lauroyl peroxide (LP) (Fluka) were reprecipitated from chloroform by methanol. Polyvinyl alcohol (PVA), Gelvatol 20-60(Monsanto), and sodium dodecyl sulfate (SDS) (BDH) were used without further purification. Synthesis of Stearoyl Peroxide (SP). Ninety-one grams of freshly distilled stearoyl chloride (Merck) was dissolved in 200 mL of diethyl ether and cooled to 3-5 "C; 45 mL of 4 N NaOH was also cooled to 3-5 "C. These two solutions were added dropwise alternately into a cooled solution of 54 mL of 15% HzOz. The whole system was mechanically stirred, the temperature was not allowed to rise above 5-8 "C, and the solution was maintained faintly alkaline throughout. When all the reagents were added the mixture was stirred for an additional hour. The flocculent was filtered off and dried. It was reprecipitated three times from chloroform by the addition of methanol. The melting point was 56-58 "C. The peroxide content was determined iodometrically to be 97% . Polymerization Procedure. The dispersions were prepared in a Waring blender equipped with a polytron head assembly; 500 mL of a given concentration of surfactant and 100 g of monomer in which the initiator was dissolved were poured into the blender. A baffle was inserted to prevent air entrainment and the contents were mixed at 7500 rpm for 1 min. The dispersion was then introduced into a 1-L,three-necked, round-bottomed flask equipped with a Teflon paddle stirrer operated at 100 rpm. A slow stream of nitrogen was passed over the solution. The polymerization was carried out at 75 "C. At the end of the reaction a 40-mL sample of the reaction mixture was added to 200 mL of water, mixed thoroughly, and centrifuged at 1500 rpm for 30 min. The supernatant was decanted and the remaining polymer was washed with water and centrifuged once again. This process was repeated three to six times. The product was then filtered and allowed to dry in air. Pretreatment of Carbon Black (CB). CB was dispersed in the monomer by the following procedure: 7 g of CB was added to 100 g of monomer containing initiator
Elution Volume (counts)
Figure 1. GPC chromatograms showing the molecular weight distribution of suspension and dispersion polymers. Initiator, LP: a, dispersion, 2% SDS; b, dispersion 2.5% P V A c, suspersion 2.5% PVA.
(0.02M). The mixture was stirred at 400 rpm for 20 min at 75 "C for styrene or 5 min at 55 "C for MMA. The resulting mixture contained CB which was well dispersed in the monomer solution, due to grafting of polymer chains on its surface. The initiator concentration was then increased to 0.148 M and the mixture was dispersed in a surfactant solution as described above. Surface Tension Measurements. Thirty milliliters of the reaction mixture was cooled to room temperature (about 28 "C). The surface tension was measured by the ring method on an Instron (testing machine). The ring of platinum-iridium and 6.0 cm circumference was cleaned by flaming. All surface tension readings were corrected to true values (Harkins and Jordan, 1930). Electron Microscopy. Transmission electron microscopy (TEM) was carried out with a Philips EM 300 instrument. Samples were diluted and a drop was spotted onto a parldon/carbon coated copper grid and dried with filter paper. Scanning electron microscopy (SEM) was carried out on a JEOL (JSM-35C) instrument. Speciments were prepared by placing the dry recovered powder on a round cover glass, which was mounted with double-faced adhesive tape on a stud and then coated under vacuum with a thin layer of gold to a depth of about 50 A. Conversion and Molecular Weight Measurements. Samples were taken at various time intervals, dissolved in THF, and analyzed for residual monomer by gas chromatography to determine the conversion. The molecular weights and molecular weight distribution were measured on an HPLC (Aerograph 8500)with p-Styragel columns (Waters Associates). Tetrahydrofuran (THF) was the solvent and the eluent. A W spectrophotometer was used as detector at a wavelength of 254 nm for PS and 235 nm for PMMA. Results a n d Discussion We shall discuss the differences between the polymerization of two monomers, styrene and MMA, in the absence and in the presence of CB. The initiation was carried out by LP, BP, and AIBN. (1) Dispersion Polymerization in t h e Absence of CB. Syrene-LP. A comparsion of suspension polymerization of 500-pm droplets with dispersion polymerization of 10-pm droplets with LP initiation and 2.5% PVA stabilizer is shown in Figure 1. It can be seen that the main peaks in both systems are identical. They both result in polymers having mol wt llOK. However, the dispersion
Ind. Eng. Chem. Prod. Res. Dev.. VoI. 21, No. 2. 1982
165
dlP,
Figure 2. Particle size distribution in dispersion polymerized styrene. Initiator LP, stabilizer 0.5% PVA. Emulsion particles were counted from TEM micrographs and dispersion particles by Coulter
4
B
counter.
system results in an additional peak with mol wt 3900K. It can also be seen that dispersion systems remains milky after centrifugation. Analysis of the precipitate shows mainly the low molecular weight peak, while that of the latex remaining in the emulsion shows only the high molecular weight peak. Moreover, when a more efficient emulsifier is added, such as SDS, the high molecular weight peak increases to about 30% of the total polymer. Thus, the system of dispersion polymerization of styrene initiated by LP clearly demonstrates the two mechanisms of reaction taking place simultaneously. It should be pointed out that by addition of a water-soluble inhibitor the emulsion can be completely eliminated as it is initiated in the aqueous phase. Another method of prevention of formation of emulsion is by decreasing the amount of PVA or SDS to a minimal value where no latex can be formed. In fact, it is possible to form the dispersion in the presence of 5% PVA by vigorous mixing in the blender, centrifugation and separation of the aqueous solution from the stabilized droplets, and washing the droplets 2-3 times until no free PVA is left in the aqueous phase. On addition of water and m i x i i gently, the droplets disperse again and polymerization can be carried out in the absence of any nonadsorbed PVA. This results in 10-pm particles with very little emulsion. This procedure is feasible due to the fact that PVA is adsorbed irreversibly on the monomer droplets and therefore the washings with water do not desorb the molecular layer of the PVA adsorbed on the droplets. It may he argued that submicron emulsion particles are the result of the formation of submicron droplets in the process of the dispersion in the blender. This should lead to a single curve of size distribution with a maximum a t the mean diameter. However, the experimental ohservation (Figure 2) shows clearly that there are two separate curves for the case of LP with 0.5% PVA. The dispersion curve has a peak a t 10 pm and a range of 1.4 to 18 pm, there are no particles in the range of 1.4 to 0.18 pm, and the emulsion curve has a peak a t 0.14 pm and a range of 0.06 to 0.18 pm. This is a further demonstration of the two processes taking place. The particles were also examined by transmission and scanning electron microscopy and the following observations were made. PVA (mol wt 96000) showed emulsion particles bound to the surface of the dispersion while PVA (mol wt 3000) and SDS showed clear dispersion particles, without any attached submicron particles (Figure 3). This phenomenon of attachment due to the high molecular weight PVA was discussed previously (Almog and Levy, 1981). It is due to the fact that the PVA has many acetate blocks along its chain and they can attach simultanously to more than one particle, resulting in a bridging effect.
P-
I
llhri
Figure 4. Rates of styrene polymerization for LP. BP, and AIBN as initiators and PVA and SDS as stabilizers.
However, when the molecular weight is low the possibility of such bridging taking place becomes negligible and therefore no submicron particles are observed on the surface (Figure 3b). The same is true for SDS, where in all cases smooth dispersion particles were recovered. One can also follow the kinetics of the reaction, and see that the rate increases somewhat with the increase in PVA concentration (Figure 4). Plotting the rate as a function of ( P V A P leads to a straight line with a very small slope (Figure 5). This result is in agreement with the finding that the amount of emulsion formed with LP is very small. In general, initiation in emulsions hy means of organic initiators is determined by the following factors: (1)the
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Id.Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 I
I
"I------ I 1
50130
20
&--
---A-
LP
i
lot
Table 111. Amount of PVA Adsorbed on Styrene and MMAa
monomer
dyn/cm
amount adsorbed, mg/m2
styrene methyl methacrylate
27.6 21.7
4.3 0.7
y,b
water solubility, 6.7 x 10-4 1 . 6 X lo-'
M
a 0.2%PVA, 7500 rpm, 2 min mixing, w/O = 5. monomer.
Pure
Table IV. Polymerization of MMA with LP as Initiator and 0.2%SDS as Surfactant (Temp 60 "C)
% I I 2 3 reaction conver- M , X M, x E,3'5
time, min
sion
NWD
Figure 5. Rate of styrene polymerization in dispersion a~ a function of the 0.6 power of PVA concentration (E,o.6). Table 11. Effect of Critical Micelle Concentration (cmc) on the Emulsion Formation. InitatorLP
cmc, %
used
% emulsion formed
0.26 0.021 0.38
0.2 0.2 0.2 0.2
0 6 0 5
concn (%)
surfactant SDS SCSQ Aerosol MAb Aerosol OTC
0.019
r
Sodium cetyl sulfate. Bis(1-methylamyl) sodium sulfosuccinate. Dioctyl sodium sulfosuccinate.
solubility of the initiator in the water phase and the distribution of the solute between the water and the organic phase; (2) the ability of the free radicals formed in pairs in the water phase to diffuse away from each other before penetrating into an emulsion nucleus; and (3) the compatibility of the initiator with the polymer which is formed. These three factors make LP the preferable initiator for minimizing emulsion. Another important factor is the surfactant concentration in the water phase. Working with submicellar concentrations of surfactants drastically reduces the water nucleated polymer. The reason is that at these concentrations, there is no enhanced solubilization of monomer and initiator in the water phase. This factor is very pronounced with electrostatic stabilizers and LP as catalyst as seen in Table 11. It should be noted that the value of the cmc depends on the chain length of the surfactant. Short chain length surfactants have high cmc values, low affinity to the dispersed phase, and consequently higher rates of desorption on heating. For a steric stabilizer such as PVA, especially with block copolymer structure, the cmc is not defined sharply because of the ability of formation of unimolecular micelles, at very low concentrations, and converting into polymolecular micelles at higher concentrations (Tuzar and Kratochvil, 1976). As a matter of fact, with such stabilizers there is an emulsion formation even at very low concentrations. MMA-LP. If we now compare the results with those obtained with MMA we can see a somewhat different picture. I t is known that MMA is more hydrophilic and adsorbs less PVA on its surface (Almog and Levy, 1982) (Table 111). One would expect therefore a higher amount of emulsion under similar conditions. However, it was observed that when the polymerization of dispersed MMA is carried out with PVA or SDS as stabilizers and LP as catalyst, no emulsion is formed.
I
I
I
I
I
I
1
1
10090
-
80
-
70
-
C
.-0
r
60-
3
0
8
5040 -
30-
2010-
t (mtn) Figure 6. Rate of MMA polymerization in dispersion. Initiator, LP; stabilizer, 0.2% SDS.
On centrifugation, all the polymer precipitates and very little turbidity remains in the supernatant. Moreover, the particles observed under the electron microscope show no sign of submicron particles. On studying the kinetics of the reaction we can see that MMA polymerizes much faster than styrene and the kinetic curve shows a very strong Trommsdorff effect after 50% conversion (Figure 6). This differs from what was observed in styrene where the reaction proceeds at constant rate up to high conversions. The Trommsdorff effect is also seen from the GPC of the reaction mixture (Figure 7 , Table IV). Up to 47% conversion there is only one peak of low molecular weight. This is followed by a sharp increase in rate accompanied by an increase in the molecular weight and resulting in a double peak distribution. This distribution is not the result of the formation of emulsion particles, as no emulsion is formed in this case, and the molecular weight
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 167
I
b
1
M MA- t olue ne no excess WA 0,
u C
0
n
$
n
?
a
a
\
b.
Y.
v.
v
.;. I 8
6 Elution
E lution volume (counts)
Volume (counts)
Figure 7. a, GPC chromatograms showing the molecular weight distribution of dispersion polymerized MMA. Initiator, LP; stabilizer, 0.2%SDS (A) 30% conversion; (B) 51% conversion; (C) 99% conversion; (D)recovered powder. b, Polymerization in the presence of 50% toluene-polymerization product.
distribution of the final reaction mixture and of the washed powder are identical. On addition of 50% toluene to the MMA prior to dispersing it in water, the Trommsdorff effect disappears and at 0.2% SDS a single peak results (Figure 7b). This is due to the fact that the viscosity does not reach the critical range of autoacceleration. The explanation for the lack of formation of emulsion in MMA is probably connected with the following observation. Dispersed MMA droplets adsorb only 0.7 mg/m2 of PVA on their surface (Table 111). This is much lower than the value of 4.3 mg/m2 obtained for styrene and would thus favor the formation of emulsion. However, on dissolving LP or hexadecane in the MMA prior to dispersion the amount of PVA adsorbed increased to 1.4 mg/m2. It means that hexadecane as well as LP changes the characteristics of the surface. It was argued (Ugelstad et al., 1976, 1980) that when an insoluble component is present in the monomer droplets, it becomes the rate-determining factor in controlling the rate of diffusion from the droplets. This may be a partial explanation in this case. However, the same effect is much less pronounced with styrene. It may be that because styrene itself is more hydrophobic, the presence of hexadecane will not change considerably its surface characteristics and its affinity to the surfactant. Styrene-BP. When the polymerization reaction was carried out with B P and styrene under conditions quite similar to those of LP and styrene, the reaction mixture showed a considerable amount of submicron emulsion which could be separated by centrifugation. However on analysis by GPC a single curve was observed. On reducing the amount of PVA in the water phase only a slight shift to lower molecular weights was noticed (Figure 8). When the reaction was carried out with 2% SDS, a kinked curve could by seen (Figure 9). By centrifugation, separation of the 10-pm dispersion particles was achieved. The submicron particles left in the supernatant showed some change in molecular weights but not as significant as with LP. The molecular weight of the 10-pm particles was 96K while that of the emulsion particles was 210K.
Figure 8. GPC chromatograms showing the molecular weight distribution of dispersion polymerized styrene. Initiator, B P stabilizer, PVA. 1
1
I
Elution volume (counts) Figure 9. GPC chromatograms showing the molecular weight distribution of dispersion polymerized styrene. Initiator, B P stabilizer, 2% SDS. Curves showing the total reaction product and the precipitate separated from the emulsion by centrifugation.
That BP gives a low molecular weight emulsion polymer was reported in the literature (Breitenbach and Edelhauser, 1961). The claim is that it is due to the chain transfer to the catalyst. The effect is more pronounced in the emulsion than in the dispersion, due to the very small volume of the polymer loci. The kinetics of the reaction with BP also showed increase in rate with (PVA)0.6as expected for a superposition of two reaction rates (Figure 5). MMA-BP. The polymerization of MMA initiated by BP gave considerable amount of emulsion as evidenced by the turbidity after centrifugation and also by the GPC
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, 02
20
60
40 O/O
80
100
Conversion
Figure 11. Variation of the surface tension of the aqueous phase with the progress of polymerization: (1) MMA/BP/O.2% SDS; (2) MMA/BP/O.2% SDS in the presence of hexadecane; (3) styrene/ LP 0.2% SDS.
1
I
1
Elution Volume (CWntS) Figure 10. a, GPC chromatograms showing the molecular weight distribution of dispersion polymerized MMA. Initiator, BP; stabilizer, 0.2% SDS: (1) total reaction product; (2) emulsion product; (3) recovered powder. b, Polymerization in the presence of hexadecane and toluene-total reaction product.
Table V. Polymerization of MMA with BP as Initiator and 0.2% SDS as Stabilizer initipolymerization M, X M, X ator component 10-3 10-3 MWD BP BP BP AIBN AIBN AIBN
polymerization product recovered powder emulsion polymerization product recovered powder emulsion
170
1280
7.53
107 680
475 1160
4.44 1.70
135
1050
7.77
98 590
360 1062
3.67 1.80
analysis (Figure loa). I t can be seen that the emulsion peak gives a higher molecular weight polymer (Table V) (unlike the case of styrene-BP). The dispersion curve shows a double-peaked curve due to the Trommsdorff effect. Addition of 50% toluene, from the beginning of the polymerization, prevents the Trommsdorff effect and results in a unimodal dispersion peak separated from the emulsion peak (Figure lob). On addition of hexadecane, a water insoluble component, to the monomer, the amount of emulsion decreased considerably (Figure lob) as in the case of MMA-LP. Measurement of the surface tension during the polymerization (Figure 11) showed that the surface tension increased toward the end of the reaction due to depletion of the surfactant by the emulsion particles. However, when hexadecane was used the final surface tension value was much lower. This is consistent with the fact that less emulsion was formed. Styrene-AIBN. AIBN is much more soluble in water than BP or LP. Moreover, the probability of single radicals escaping from the surrounding water cage is very high, due to its high polarity (the dipole moment of the RCN is about 4 D)(Roberts and Caserio, 1961). As a result, its efficiency in emulsion polymerization is much higher than
4
6 8 E Iu t ion volume(counts)
Figure 12. GPC chromatograms showing the molecular weight distribution of dispersion polymerized styrene. Initator, AIBN; stabilizers, PVA and SDS. that of BP and LP. Therefore, one can expect larger emulsion to dispersion ratios for similar conditions. This can indeed be observed in the case of styrene, where a nonsymmetrical GPC curve was obtained (Figure 12). In the case of 5% PVA it is mainly an emulsion product, while in the case of no-excess PVA it is mainly a dispersion. However, in the latter case the system is unstable and agglomeration results before complete conversion can be achieved. The bridging of emulsion particles to the dispersion particles is very drastic in this case as there are a lot of submicron particles and there is a depletion of PVA so that the effect is more noticeable. Washing of the
Ind. Eng. Chem. prod. Res. Dev., Vol. 21. No. 2. 1982
160
Elukon Volume (counts)
Figure 11. Optical
micruscope mirrr,Eraphs of C'H diaperred in In! nfWr Erafllng c,f 1's !magnlfiCMion
8l)lene. IAl h f O r e @ling; 2511 X 7 , .
Table VI. Roperties of Carbon Black Studied in This
Work (Taken from Catalogs of t h e Respective Companies) -___ ~
sp
, !
!/ _. Figure
13. GPC chromatograms showing the molecular weight distribution of dispersion polymerized styrene before and after cent r i i a t i o n and w & I ~ Initiator, ~ s . A I B N stabilizers: (A) P V A (B) SDS; (C) SEM micrograph of 13A recovered powder; (D) SEM micrograph of 13B recovered powder.
particles, followed by centrifugation, decreases somewhat the amount of the bridged submicron particles hut does not eliminate it completely as is seen by GPC analysis (Figure 13A) and the SEM micrograph (Figure 13C). On the other hand, when SDS is used as surfactant the emulsion particles can he washed off completely (Figure 13B and D). Kinetically AIBN is much more susceptible to the concentration of PVA as evidenced from the rates measured at different PVA concentrations and the extrapolation to zero PVA giving the rate in the dispersion proper (Figures 4 and 5). MMA-AIBN. In hfMA, initiation by A B N results also in a mixture of emulsion and dispersion (Table V). The GPC curve of the recovered dispersion polymer is double-peaked due to the pronounced Trommsdorff effect. The rate itself shows an autoacceleration as in the case of LP and the reaction is over in 35 min. As in the case of BP the amount of emulsion can he reduced by adding hexadecane to the monomer. The effect of bridging is seen here as in the case of styrene. (2) Dispersion Polymerization in the Presence of Carbon Black. Various fillers are added to polymers in order to modify their properties. Normally, these are blended into the polymer melts in the process of extrusion or mixed in the presence of solvents followed by spray drying. It is also possible to incorporate the additives in the monomer during the polymerization itself. For this purpose, two important conditions must be fulfilled; first, the additive must be compatible with the monomer so that it can he evenly dispersed in the monomer, and second, the additive should not interfere with the polymerization. When CB is mixed with the monomer it does not disperse well and agglomerated CB particles can he seen under the optical microscope (Figure 14a). This problem
av
part. area. (ham. m'.X nm surf.
filler
FW 200 Mogul L Raven 4 2 0 lamp Black 101
--
manufacturer
ivw
uegusw
cha""(.l
160
Cabot
furnsw furnace lampl,lnck
138
CITCO Ikgussa
2H
13 24 62
21
95
can be solved by grafting of styrene or MMA on the CR. The procedure WRS described in the experimental section. About 3-5% polymer are grafted under these ronditions. Most of the grafting is the result of termination of the growing polymer radicals on active sites in the CH. The fact that these chains are now an integral part of the particles enables them to act as bound steric stabilizers and maintain a stahle homogenous dispersion (Figwe 14bJ. The inhibition problem is very much dependent on the nature of the CB. As can be seen from Table VI, the blacks are formed by different processes and have different diameters and specific surface areas. Their inhibition effect on the dispersion polymerization of styrene with LP as initiator is shown in Figure 15. I t can he seen that the inhihition effect increases with the surface area. In fact, lampblack has a negligihle effect on the rate of polymerization. It is a very unreactive CB and it is also impossible to Faft styrene ontu it. The other hlacks were tested after standard grafting procedure as specified in the experimental section. The effect of CB on different initiators ran he seen in Figure 16 for styrene and Figure 17 for MMA. For MMA no inhibition is observed, o n the contrary thrre is some acceleratian. Nevertheless, there is grafting of P.MMA on the CB so that there ix some termination of growing polymer chains on the artive sites of the CR. It srems that these active sites can act both as initiatiirs and as terminators (Donnet and Henrich, 19601hecniisr MMA is such a reactive monomer. In the case of styrene the situation is quite different. LP is inhihited considerably. 'I'ht. depee of inhihition ran be reduced by repeawl grafting of PS on CH. In other words, when enough active sites react the CB heromes inert. SP is much le% inhihited than LP. T h i s may hr an indication that the aliphatic chain in SP ( C , u is ~ too h l k y to penetrate into the micropores of the C H particles and can only
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
LP~0.2%SDS/6O'C
I-
Conlrol
AISN/O.2YSCG/ 75.C
-
a control
Figure 17. Effect of CB (Raven 420,7%) on the polymerization of MMA. Initiator BP, LP, and AIBN stabilizer, 0.2% SDS.
I 1
I
2
I
I
I
I
3 L 5 6 t (hr.1 Figure 15. Inhibition effects of different CB's on the rate of polymerization of styrene. Initiator, LP; stabilizer, 0.5% PVA, 7% CB: (A) control; (B) lampblack; (C) Raven 420; (D) Mogul L; (E) FW 200.
T n e ii.r
Figure 16. Initiator effect of CB (Raven 420, 7%) on the polymerization of styrene. Initiator, BP, LP, SP, and AIBN; stabilizer, 0.5% PVA.
react with active sites on the surface. It should be noticed that even with SP some emulsion is produced. However, by addition of low concentration of a water soluble inhibitor such as NaN02, the emulsion is inhibited completely and a clear supernatant is obtained after centrifugation of the reaction product. In all cases where emulsion was formed, it was white and did not contain CB particles in it. The major inhibition effect is observed with BP. With this peroxide it is not even possible to graft PS onto the
CB and LP has to be used instead. It was also shown that BP is decomposed by CB even a t room temperature. In the case of initiation by AIBN, no inhibition is observed. It seems that the polar free radicals formed from AIBN react faster with styrene than with the active sites on the carbon. Solution polymerization in the presence of CB was studied and discussed extensively (Ohkita et al., 1978, 1975). They claim that polystyrene radicals attack readily the active sites on CB while polymethyl methacrylate radicals do not. This is in accordance with our results. As carbon blacks are very complex and contain a variety of condensed aromatic structures as well as quinonoid and phenolic molecules, it is reasonable that the overall behavior in a polymerization system will differ according t~ the experimental conditions. In the case of dispersion polymerization there is an advantage that the reaction can be studied up to high conversions under conditions similar to bulk polymerization. Literature Cited Almog, Y.; Levy, M. J . folym. Scl. polym. Chem. Ed. 1980, 18, 1. Almog, Y.; Levy, M. J . folym. Scl. f o k m . Chem. Ed. 1981, 19, 115. Almog, Y.; Levy, M. J . Po&". Chem. Ed. 1982, in press. Azad, A. R. M.; Ugelstad, J.; Pltch, R. M.; Hansen, F. K. ACS Symp. Ser. 1978, No. 24. 1. Breitenbach, J. W.; Edelhauser, H. Mskromol. Chem. 1981, 4 4 , 196. Donnet, K. B.; Henrlch, 0. J . folym. Sci. 1080, 46. 277. Harkins, W. D.; Jordan, H. F. J . Am. Chem. Soc.1930, 52. 1751. Ohkita, K.; Tsubokawa, N.; Saltoh, E.; Noda, M.; Tukashlna, N. Carbon 1978. 76, 41. Ohkka, K.;Tsubokawa, N.; Sitoh, E. Carbon 1075, 13, 443. Roberts, J. D.; Caserb, M. C. "Basic Prlnclples of Organic Chemistry"; W.A. Benjamln: New York, 1965; Chapter 19. Tuzar, 2.;Kretochvil, P. A&. colkld Intdace Sei. 1978, 6 , 201. Ugelstad, J.; Mork, P. C.; Kaggerud, K. H.;EIHngsen, T.; Berge, A. A&. Colbkf Interface Sci. 1080, 73,101. Vanzo, E. J . Appl. Polym. Sci. 1072, 16, 1867.
Receiued for review September 24, 1981 Accepted January 5, 1982 This paper was presented at the 4th International Conference on Surface and Colloid Science, Jerusalem, July 1981.
