JOURNAL OF COLLOID AND INTERFACE SCIENCE 195, 329337 (1997)ARTICLE NO. CS975143

Compounding Fumed Silicas into Polydimethylsiloxane:Bound Rubber and Final Aggregate Size

Mirta I. Aranguren,1 Elsi Mora,2 and Christopher W. Macosko3

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455

Received April 21, 1997; accepted August 19, 1997

silica, which are fused together. These aggregates are as-The properties of mechanically mixed suspensions of fumed sumed to be the primary structure of the filler in the suspen-

silicas in polydimethylsiloxane (PDMS) were studied. The work sions. Agglomerates are clusters of aggregates linked byfocuses on two aspects: ( i ) Adsorption of PDMS onto silicas by physical forces.means of the mechanical mixing of the bulk polymer and untreated Dispersion and attrition of agglomerated fillers during rub-or chemically modied fumed silicas. The silicaPDMS suspen-

ber compounding has been treated previously. In most ofsions were completely dispersed or only swollen by a solvent, de-the literature the mixing times reported are shorter than thosepending on the ller concentration, the silica surface chemistry,used in this paper (2040 min). Rwei et al. show that theand the polymer molecular weight. The bound rubber content ofextent of erosion of the clusters is linear with time at shortthe different preparations was determined by a weight difference

technique and by carbon analysis of the samples and was compared mixing times but changes to an exponential behavior forwith previously reported values in similar systems. ( ii ) The effect long mixing times (3) .of the mechanical mixing on the nal aggregate size of the silica Most of the previous work focused on the adsorption fromand on the polymer molecular weight distribution. One PDMS polymer solutions onto solids (4, 5) . One of the reasons,and silicas of different surface area and surface chemistry were which will be further discussed, is that there are no simpleused. The nal size of the aggregates after a long mixing time with techniques to measure adsorption from the bulk. Indirectthe bulk polymer was approximately the same in all cases. q 1997

measurements of bound rubber are the most common resultsAcademic Press

reported for systems similar to those used here (69). Vial-Key Words: silicaPDMS suspensions; compounding; boundlat et al. proposed that in mechanically induced sorption therubber; aggregation.macromolecules are only partially adsorbed as a conse-quence of the strong entanglements formed with other chainsin the bulk (7) . Thus, mechanically mixed suspensions mustINTRODUCTIONbe distinguished from those prepared from polymer solutionsbecause the interactions generated will be different, and con-The fumed silicapolydimethylsiloxane (PDMS) systemsequently, the rheological behavior will also differ.has not only academic importance in the area of polymer

The present work has a dual purpose:adsorption onto solids but also a practical one: mechanicallyprepared highly filled polymeric suspensions are the starting

(i) To characterize the type of bonds (chemical or physi-materials in the manufacture of silicone rubbers. A detailedcal) formed between polymer and filler by adsorption fromdescription of the type of interactions that occur betweenthe bulk and the variables that affect their strength. Fumedfiller and polymer in these suspensions is still lacking. Thesilicas are attractive to use because it is relatively easy totype and number of adsorption sites and bonds created playmodify their surface. Surface chemical groups in the silica,an important role in the rheology of the uncured material,as well as polymer molecular weight, were the variables ofwhich is related to the material behavior during processingthis study. Both have a strong effect on the amount of boundand to the final properties of the cured rubber (1, 2) .rubber (adsorbed polymer) and, thus, on the complete dis-Fumed silicas are aggregates of spherical particles, ca. 10persion (liquid-like behavior) or the swelling (gel-like be-to 20 nm in diameter, essentially monodisperse for a givenhavior) of the suspensions in a solvent and, finally, on thesuspension rheology (1).

1 Current address: INTEMAFacultad de Ingenier a, Juan B. Justo ( ii ) To discuss the effects of the long mechanical mixing4302, 7600 Mar del Plata, Argentina.

time on the aggregate size of the filler and on the polymer2 Current address: Escuela de IngenierB a QuB mica, Universidad de Costamolecular weight distribution. The structure of the filler hasRica, San Jose, Costa Rica.

3 To whom all correspondence should be addressed. been considered of great importance in the performance of

329 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

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330 ARANGUREN, MORA, AND MACOSKO

TABLE 1 Fumed silicas shown in Table 3 were used to study theMolecular Weights and Polydispersity of the Used effect of mixing on the structure of filler aggregates. These

Polydimethylsiloxanes silicas differ in their initial aggregate structure due to differ-ences in surface area and surface chemical groups.

325Ka 118Kb 17Kb 88Kc Figures 1a and 1b show micrographs of one of the silicasobtained by transmission electron microscopy (TEM) andMn 146,000 66,400 8,900 22,600scanning electron microscopy (SEM), respectively. In theMw 325,000 118,000 16,500 87,600

Mw/Mn 2.2 1.8 1.9 3.9 first one, the size and shape of the individual particles can beobserved as well as the internal structure of the aggregates. In

a Measured in our laboratories, Waters GPC Model 150-C ALC/GPC the second one, the clusters observed are identified with the(1% solution in THF).aggregates (primary structure in the suspensions) (13).b Measured in Dow Corning Laboratories (Midland, MI), PDMS calibra-

tion was used.c A mixture of 118K and 17K, 70:30 wt% respectively; Mn and Mw I. PDMS ADSORPTION ONTO FUMED SILICA

calculated.

Methods: Silica Surface Treatmentthese materials after curing (1012). Thus, variation of An untreated fumed silica and chemical modifications ofstructure due to mixing (compounding) was investigated. it were used (Table 2).

Diffuse reflectance Fourier transformed infrared (DRIFT)and methyl red adsorption techniques (14, 15a) showed thatMATERIALSthe surface modification was complete in the cases of Modi-fied Silica 1 (MS1) and Modified Silica 2 (MS2), but itThe characteristics of the polydimethylsiloxanes used are

shown in Table 1. PDMS 325K was purchased from Petrarch was only partial in the case of Aerosil R972 (commercialmodification of Aerosil 130). In the last case, DRIFTSystems (Bristol, PA) and is a methyl-terminated linear

polymer. PDMS 118K and 17K (supplied by Dow Corning, showed a very small peak at 3750 cm01 ( isolated silanols) ,and methyl red adsorption indicated that about 90% of theMidland, MI) are vinyl-terminated linear polymers. It was

previously shown that vinyl groups are only important in original silanols, present in Aerosil 130, were reacted.Elemental carbon analysis of the silicas are shown in Ta-the reacted materials but do not affect the behavior of the

suspensions (13). ble 4. The data were used to calculate the fraction of reactedsites on the starting silica, assuming the same reactivity forTo study the type and strength of the fillerpolymer inter-

actions, fumed silicas shown in Table 2 were used. These the hexamethyldisilazane (HMDS) and the tetramethyldivi-nyldisilazane. The value calculated for the modification withsilicas have similar structure and surface area, but different

chemical treatments. They were obtained from an initial HMDS (MS1) was 2.43 sites/nm2, which is in excellentagreement with the value 2.45 OH/nm2 reported by Iler asbatch of Aerosil 130 (Degussa Corp., Teterboro, NJ). Two

different modified silicas (MS) were obtained by reaction the maximum coverage of an amorphous hydroxylated silicaby HMDS (15b). The other calculated coverages were 2.28under mild conditions with disilazanes (1) . Aerosil R972

was also included in the study. This is a commercial modifi- sites/nm2 for Aerosil R972 and 2.19 sites/nm2 for MS2. Thelower coverage in these silicas is due to partial treatmentcation of Aerosil 130 obtained by treatment with dimethyl-

dichlorosilane. and to the bulkier-than-methyl vinyl group, respectively.

TABLE 2Fumed Silicas Used in the Adsorption Study

Aerosil 130 Modified Silica 1(A130) (MS1) Modified Silica 2 (MS2)

BET surface (m2/g)a 133 112 114Bulk density (g/cm3) 0.05Size average particle (nm) 16 16b 16b% Hydroxyl treatedb 0 100 100Type of treatment No treatment Hexamethyl disilazane Tetramethyldivinyldisilazane (67 wt%) /

hexamethyldisilazane (33 wt%)

Note. Aerosil R972 was also used in this study (see Table 3).a Measured by Micromeritics, Inc. (Norcross, GA).b Measured in our laboratory by methyl red adsorption.

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331COMPOUNDING FUMED SILICAS INTO PDMS

TABLE 3Fumed Silicas Used to Study the Effect of Mechanical Mixing

Aerosil R972 Aerosil VT345 Cab-o-sil TS720

BET surface (m2/g) 108a 188b 100 { 20Bulk density (g/cm3) 0.05 0.041c 0.0320.048Size average particle (nm) 16 12% Hydroxyl treated 70 (90d)Base product Aerosil 130 Aerosil 200Type of treatment Dimethyldichlorosilane Vinyltriethoxysilane Organosilicones

a Measured by Micromeritics, Inc. (Norcross, GA).b Provided by Degussa (Teterboro, NJ) for the specific batch in use.c Measured in our laboratory in noncompacted samples.d Measured in our laboratory by methyl red adsorption.

Master batch suspensions were prepared in a Haake Rheo- to 15 mm and many small ones. All the blobs were fragileand broke very easily during drying or handling.mix 600 with a chamber capacity of 6070 cm3 using sigma

blades at 35 rpm. Further dilutions were made in the sameequipment. One of the samples (Aerosil R972 and 88K) used Bound Rubberfor the mixing study was also used in adsorption studies.

Bound rubber is defined as the percentage of the polymerthat cannot be extracted from an uncured filled rubber usingResults and Discussiona good solvent, usually, at room temperature. The amountDispersion and Swellingof bound rubber is a function of the interactions betweenpolymer and filler, the surface area of the filler, the molecularAfter mixing the suspensions were placed in chloroform.

Use of MS1 and MS2 resulted in suspensions that could be weight of the polymer, the solvent used in the extraction,the time and temperature of the extraction and the concentra-completely dispersed in chloroform at any concentration of

the silicas in PDMS. Suspensions prepared with untreated tion of filler in the composite.The composites used for this test were all prepared withsilicas did not disperse completely in chloroform. Translu-

cent blobs, such as those seen during the first stages of silicas from Table 2. The parameters varied were the silicasurface treatment, silica concentration and polymer molecu-chemical gelation, were visible to the unaided eye. At 10

phr, the blobs were not larger than 1 mm and very fragile. lar weight. The proportion of filler used ranged from 4 to15% by volume (10 to 40 phr) . Chloroform was the solventThe amount and size of these large blobs increased with

silica concentration. 325K / Aerosil 130 (20 parts of silica used for the PDMS extractions.Samples of 2 g of uncured silicone rubber were immersedper hundred parts of polymer, phr) , on the other hand,

swelled in chloroform, maintaining its shape. After it was in 20 ml of chloroform in 50 ml capped centrifuge tubes.The samples were kept in contact with the solvent for onedried in air, it recovered its original size and retained the

appearance of a silicone rubber (white translucent) . The week and the solvent was renewed twice during that period.The solids were separated from the supernatant by centrifu-piece behaved as a cured rubber and could be reswollen

without breakage. However, it could be permanently de- gation at 13,000 rpm for 45 min.Another measurement was done on the samples containingformed under pressure.

Cohen-Addad et al. (16) found a similar behavior for Aerosil 130, by solvent extracting the polymer from a Sox-hlet thimble immersed in chloroform for one week with threesilicasilicone suspensions. At high concentrations of silica,

these materials showed a tendency to swell and not to be solvent renewals.The amount of adsorbed polymer is calculated as the dif-dispersed, which he interpreted as an increased chainbridge

formation between the aggregates. ference between the weight of the recovered solids and thecalculated amount of filler in the suspension.The behavior of suspensions made with partially treated

silicas was between those of untreated and completely Finally, carbon elemental analysis was performed on thesesolids. The percentage of carbon in these samples and thosetreated silicas. The suspension 118KR972 (20 phr) in chlo-

roform produced fragile blobs of 5 to 10 mm. After eliminat- obtained from the original silica allowed us to calculate howmuch PDMS was adsorbed on the silica (the calculationing the supernatant, the material was left to dry in air, which

caused the complete breakage of the sample, leaving a granu- considered only dimethyl units) .Table 5 shows percentage of adsorbed polymer (boundlated white powder of dry appearance. A sample of 0.95 g

of 118KR972 (30 phr) in solvent gave a large blob of 10 rubber) at three different silica concentrations. Results are

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332 ARANGUREN, MORA, AND MACOSKO

FIG. 1. (a) TEM photograph of the internal structure of fumed silica clusters before compounding (Cab-o-sil TS720). (b) SEM photograph ofaggregates of fumed silica before compounding (Cab-o-sil TS720).

also shown as weight of adsorbed polymer per unit weight Table 6 shows the effect of polymer molecular weight andof filler. For a given silica and PDMS, and assuming perfect silica surface chemistry on the bound rubber. The results ob-wetting, the amount of polymer adsorbed per unit weight of tained by the weighing and the carbon analysis techniques aresilica is independent of the concentration of filler. The situa- in very good agreement, except for the composites made fromtion should be different at high silica concentration due to MS1 and MS2, which could be explained by the following:incomplete wetting of the silica surface and due to a larger

( i) incomplete washing of the samples used for carbonnumber of shared chains (PDMS adsorbed onto more thananalysis and the impossibility of repeating the measurementone aggregate) . In both cases, the amount of adsorbed poly-

mer would be comparatively less. for the lack of material, or

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333COMPOUNDING FUMED SILICAS INTO PDMS

TABLE 4 TABLE 6Elemental Analysis of Silicas Variation of the Amount of Adsorbed PDMS with Varying

Molecular Weight and Silica Surface TreatmentSilica %C %H

PDMSads /silica (wt/wt)Aerosil 130 0.16 traceAerosil R972a 0.98 0.27 17K 88K 118K 325KModified silica 1b 1.63 0.49Modified silica 2 1.61 0.45 Mw 16,500 87,600 118,000 325,000

Aerosil 130 0.196 0.653 1.610a Two methyl groups per reacted site. 0.26 0.410.63 1.61b Three methyl groups per reacted site. Aerosil R972 0.055 0.283 0.216 0.871

0.064 0.26 0.22 0.84Modified Silica 1 0.151a

0( ii ) incomplete sedimentation of the solids in theModified Silica 2 0.167a weighing technique. Agglomeration is very low in these 0

samples according to the observations reported in the Disper-sion and Swelling section. Consequently, centrifugation was Note. Numbers in regular font were calculated from the results of carbon

analysis of the silica before and after the polymer adsorption. Numbers innot strong enough to produce complete sedimentation anditalics were obtained with a weighing technique (see text).some silica was lost in the supernatant. The solids recovered

a There was incomplete washing of the free polymer. Numbers are in-were always 1020% less than the calculated initial amount.cluded to show the trend.

Results from any of the two techniques indicated that thebound rubber was very low in those samples.

shows the spectrum for Aerosil 130 with adsorbed 88K. TheTable 6 shows that for a given silica the amount of poly-spectrum of the silica with bound rubber shows a doublemer attached to it increases with increasing molecular weightpeak at 29602970 cm01 which corresponds to the methyland that at a given molecular weight the amount adsorbedgroups of the polymer, and the disappearance of the peakincreases with the increasing silanol concentration in theassigned to isolated silanols at 3750 cm01 . The adsorptionsilica. These observations are coincident with a strong ad-could be physical or chemical. However chemical reactionssorption of siloxanes onto silicas by hydrogen bonding be-between PDMS and silica surfaces have been reported totween the SiOSi of the polymer backbone and the isolatedoccur at or above 1507C (20). Since adsorption was carriedsilanols of the silica surface (1720). Figure 2 shows similarout at room temperatures, chemical bonding was ruled out.results (reduced to silica unit area for easier comparison)

Measurements of layer thicknesses of polymers adsorbedobtained by Cohen-Addad et al. (19) for suspensions madeonto solids are usually done using ellipsometry or hydrody-with untreated fumed silica. In both cases, the amount ofnamic methods (4) , but these measurements require flat sur-adsorbed polymer increases with the PDMS molecular

weight. The amount of adsorbed PDMS measured in thiswork is higher than that reported by these researchers, butthe trends are similar. The partially modified silica (AerosilR972) shows a similar correlation, but the adsorbed polymeris comparatively less. Other researchers have also reportedthat the amount of adsorbed PDMS decreased with increaseddegree of silica treatment (7, 9, 18).

DRIFT of the solids recovered in the bound rubber test(silica / adsorbed PDMS) was also performed. Figure 3

TABLE 5Bound Rubber vs Silica Concentration (Aerosil R972 / 88K)

Silicaconcentration

(phr) PDMSadsPDMStot

1 100 PDMSadssilica FIG. 2. Weight of polydimethylsiloxane adsorbed per unit of BET area

of the silica. Data are shown as follows: filled circles, adsorption ontountreated silica (Aerosil 130); filled squares, adsorption onto partially20 5.3 { 0.1 0.26 { 0.01treated silica (Aerosil R972). Lines are included to show the trend. Open30 6.1 { 1.6 0.20 { 0.05circles are data from Ref. 19 and correspond to untreated silicaPDMS40 11.8 { 3.0 0.29 { 0.08suspensions at two different concentrations.

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334 ARANGUREN, MORA, AND MACOSKO

FIG. 3. DRIFT spectra of an untreated silica (Aerosil 130, dashed line) , and the same silica with adsorbed 88K (continuous line) .

faces or regular shape particles respectively and, therefore, of the chain. Assuming that the longest chains are preferen-are inapplicable in this case. However, an estimation of the tially adsorbed, the size of the coil (2

r 2g) can be estimated.

thicknesses of the adsorbed layers can be obtained if one Results obtained using the weight average molecular weightassumes that: of the PDMS are reported in Table 8. A chain probably has

more than one segment adsorbed onto the silica, so that the(i) All the silica surface can be reached by the polymer.layer thickness should be smaller than the size of the polymer(ii ) The density of the adsorbed polymer is equal to thecoil, and it should be closer to this last value at high adsorp-density of the bulk polymer.tion (untreated silicaPDMS suspensions) . This was ob-

This simplified model leads to the following expression: served in the results of Table 7.The polymer adsorption has an irreversible character,

since the chains are not desorbed in the presence of purethickness (nm) mass of adsorbed polymer (g)mass of silica (g) solvent at room temperature. However, it has been proposed

that in these systems the adsorbed chains could be desorbed(pulled loose) (22) or slip at the filler surface (23) to release1 1

rpolym(cm3/g) 1 1

Sp(g/m2) 1 103,

externally applied stresses. These observations do not contra-dict each other. The unifying idea, explained by de Gennes

where rpolym is the density of the bulk polymer and Sp is theBET area of the silica.

TABLE 7Results obtained with the above equation are reported inThicknesses of the Layers of Adsorbed Polymer (nm)Table 7.

The sizes of the polymer coils can be calculated using the 17K 88K 118K 323Kradii of gyration of the chains. For PDMS chains (21),

Aerosil 130 1.52 5.05 12.452.01 3.174.87 12.45

Aerosil R972 0.52 2.70 2.06 8.30r 2g (nm)

r 2

6 16 [0.073

M] ,

0.61 2.48 2.10 8.00Modified Silica 1 1.49

0where M is the molecular weight of the polymer, r 2 is theModified Silica 2 1.62

end-to-end distance of the coils, r 2g is the radius of gyration 0of the coils, and 0.073 is an experimental constant that takes

Note. Fonts have the same meaning as in Table 6.into account the size of the backbone bonds and the rigidity

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335COMPOUNDING FUMED SILICAS INTO PDMS

in the case of adsorption from solution, is called saturationcondition (5) . To keep the coverage constant and equal tothat of saturation, the chains can be extracted from the solidsurface only if other chains are adsorbed to replace them.This concept can explain some of the observations made onmechanically mixed compounds:

( i) The existence of bound rubber, i.e., the impossibilityof extracting all the polymer present in the composite bysolvent extraction.

( ii ) The relaxation of stresses in filled melts and rubbersthat can occur through interchange between adsorbedstretched chains and nonadsorbed chains, at least when theadsorption is mainly physical.

II. EFFECT OF MIXING ON THE SILICA STRUCTURE

FIG. 4. Aggregate size distribution before (from 49 aggregates) andFumed silicas reported in Table 3 were used in this study.after compounding (from 76 aggregates) of Aerosil R972 with 88K.Suspensions were prepared at Dow Corning in a Baker Per-

kins mixer of 1 liter capacity using helical rollers (35 rpm).The suspension of Aerosil VT345 was prepared in a kitchen

on an SEM stub, the solvent was evaporated, and the samplemixer for bread dough (Montgomery Ward, Chicago, IL).

was coated with AuPd. No reduction in the size of theThe compounding of master batches (40 phr) was done byclusters was observed by using more diluted suspensions or

steps, as follows: longer times in the ultrasonic bath. To observe the silica(i) Three quarters of the polymer was initially loaded in aggregates after the compounding, the free polymer was

the mixer, and the silica added incrementally. After each extracted in chloroform. The sample was then prepared asaddition the sample was mixed for about 20 min. The proce- described above.dure was continued until all the silica was added and dis- The shape of the aggregates was digitized from the micro-persed. graphs using a Hi-Pad digitizing tablet, Houston Instruments.

( ii ) The rest of the polymer was added and mixing was From the numerical analysis of the data (software developedcontinued until the sample became homogeneous by visual by the Biology Sciences Department of the University ofinspection. Minnesota) , the diameter of the sphere with the same pro-

jected area was obtained. About 50 to 100 aggregates wereThe complete process took 3 h. No processing aids wereused for the calculations.

used. PDMS 88K was used in these preparations. The comparison of the aggregate size before and aftercompounding shows that the distribution is narrowed andResults and Discussionshifted toward smaller sizes. Figure 4 shows the size distri-

Aggregate Size Distribution bution curve for Aerosil R972. The curve presents a tailtoward large size clusters. The aggregates recovered fromSEM was used to study the effect of the compoundingthe silicaPDMS suspensions have smoother shapes than

on the size of the aggregates. This technique gives enoughthe originally branched aggregates. This observation is in

resolution to determine the size of the aggregates but doesagreement with a mechanism in which the breakage of the

not allow their internal structure to be observed. Suspensionsclusters occurs at weak joints, and, thus, small fragments

of silica in chloroform (0.025% by weight) were preparedoriginate from larger clusters (3) .

and mixed in an ultrasonic bath for about 45 min to destroy Aerosil VT345 is a partially treated silica with a relativelyagglomeration (24). A drop of the suspension was placed large hydroxylated area as measured by methyl red adsorp-

tion and DRIFT. In the dry state, there exist clusters largerthan 1 mm, which were considered to be agglomerates (sec-TABLE 8ondary structure) , probably formed by hydrogen bonding.Size of PDMS Coils of Different Molecular WeightsThese clusters were not included in the distribution. In spite

17K 118K 325K of this difference with respect to the other silicas, the averagesize of the aggregates after mixing is very similar for all the

Mw 16,500 118,000 325,000 samples (Fig. 5) . This is surprising when one considers the2

r2g (nm) 3.2 8.4 13.9

many differences between the silicas and specially the fact

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336 ARANGUREN, MORA, AND MACOSKO

Thus, a rupture process (abrupt splitting of a cluster intosmaller fragments) may be responsible for the breakage ofagglomerates into aggregates during the first stages of mix-ing, but the ultimate dispersion of the silica would be dueto the erosion of the aggregates (primary structure) . It isproposed that the similar strength of these aggregates (cova-lently bonded SiO2 spheres) is the most important factor indetermining the final size of the clusters for long mixingtimes.

Compounding introduces some changes in the silica struc-ture, and one might expect that some changes also occur tothe polymer. Adsorption onto solids has been reported (8)to lead to the fractionation of the polymer, which changes

FIG. 5. Aggregate size distribution after compounding for different the composition of the matrix. GPC curves obtained fromfumed silicas with 88K. the original polymer and the extracted (nonadsorbed) poly-

mer were compared, for the suspension 88KR972 (20phr) .There was a small difference between the two curves in thethat Aerosil VT345 was compounded in a different type of

mixer. The use of long mixing times probably makes it region of high molecular weights. This can be explained ifno fractionation occurs, but also if the amount of adsorbedpossible to reach a limiting aggregate size, but the nature of

such a process is still unknown. The results may differ for polymer is small (Table 6). Actually, about 5% of the poly-mer was adsorbed in this preparation, which could be respon-short mixing cycles. The disappearance of the agglomeration

in Aerosil VT345 also indicates that the silica is well wetted sible for the small difference observed at high molecularweights.by the polymer.

Manas-Zloczower et al. (25) studied the rupture of carbonblack agglomerates due to the stresses generated by the hy- SUMMARYdrodynamic forces generated by the flow field during com-

Analysis of the bound rubber results presented in thispounding. They reported that cluster break up is a processwork shows the following:that continues until an ultimate particle size is reached,

which can no longer be reduced by hydrodynamic forces. The weighing technique and the elemental carbon anal-Later, Rwei et al. treated the erosion of carbon black in ysis show good agreement with each other. CentrifugationSBR (3). They define erosion, a breakage mechanism forat high rpm is a good method to separate the solids (silicadispersion of the carbon black, as the continuous detachmentand adsorbed polymer) from the supernatant (solvent and

of small fragments from the outer shell of a cluster. They free polymer) . The exceptions to this observation are thereported that at large extents of erosion (long compounding

suspensions prepared from completely treated silicas, intime) the change of cluster size tends exponentially to zero.

which there is no agglomeration.These authors also report that there exists a critical stress The amount of polymer adsorbed is independent of thebelow which no erosion occurs. The extent of the attritionsilica concentration as long as the silica surface is completely

was correlated by means of a parameter proportional to thewetted and there is not a high number of bridging chains

viscosity and shear rate and inversely proportional to a con- (high silica concentration).stant that scales with the particleparticle interaction force Untreated silicas (high OH concentration) adsorb largerand the volume fraction of solids within the cluster.

amounts of polymer than treated silicas. The PDMS chainsHydrodynamic forces are a function of the polymer molec- have a strong affinity for the surface silanols that are lostular weight (viscosity) , geometry of the mixer, and turning during surface treatment.rate of the blades. The polymer molecular weight was kept Polymer chains are strongly adsorbed onto the silicaconstant in this study, and the mixing conditions were the through more than one segment. Some chains can form brid-same in all cases, except for the suspensions prepared with ges between different aggregates, which explains the incom-Aerosil VT345. However, the final size of the aggregates plete dispersion of some samples in chloroform and thewas the same for all the samples, including this silica. Aero-

swelling of others.sil VT345 was initially more agglomerated, due to largerextent of hydrogen bonding between the aggregates (higher It was also shown that there is breakage of the silica

aggregates during mixing. The aggregates in silicas of differ-concentration of surface silanols) . However, since the diam-eter of the fused SiO2 spheres that form aggregates is about ent surface areas and states of agglomeration showed approx-

imately the same final size after a long mixing time (3 h)the same as that of the other silicas (Table 3), the aggregatestructure should be similar in all samples. with the same polymer. We advance the idea that a limit

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337COMPOUNDING FUMED SILICAS INTO PDMS

6. Voet, A., J. Polym. Sci., Macromol. Rev. 15, 327 (1980).size is reached, which in this case would be mostly related7. Viallat, A., Cohen-Addad, J. P., and Pouchelon, A., Polymer 27, 843to the structure and strength of the aggregate cores and not so (1986).

much on their branched outer structure or surface chemistry. 8. Meissner, B., J. Appl. Polym. Sci. 18, 2483 (1974).More detailed study is needed to explain the phenomenon. 9. Dannenberg, E. M., Rubber Chem. Technol. 59, 512 (1986).

10. Polmanteer, K. E., and Lentz, C. W., Rubber Chem Technol. 48, 795In such study, the silica characteristics should be varied one(1975).at a time and the effect of the polymer molecular weight

11. Medalia, A. I., Rubber Chem Technol. 45, 1171 (1972).(MW ) and of the molecular weight distribution (MWD) on 12. Medalia, A. I., Rubber Chem Technol. 47, 411 (1974).the final aggregate size should also be addressed. 13. Aranguren, Mirta I., Ph.D. Thesis, University of Minnesota, MN, 1990.

14. Shapiro, I., and Kolthoff, I. M., J. Am. Chem. Soc. 72, 776 (1950).15a.Iler, R. K., Colloid Chemistry of Silicas and Silicates, p. 473. CornellACKNOWLEDGMENTS Univ. Press, Ithaca, NY, 1955.15b.Iler, R. K., Colloid Chemistry of Silicas and Silicates, p. 700. Cornell

The authors thank the Office of Naval Research, Grant N00014-88-K- Univ. Press, Ithaca, NY, 1955.0366, and Dow Corning (Midland, MI) for the grant and for providing the 16. Cohen-Addad, J. P., Viallat, A., and Huchot, P., Macromolecules 20,materials used for this work. Thanks are also due to Degussa and Cabot 2146 (1987).for supplying the fumed silicas. Dr. John Saams suggestions as well as 17. Vondracek, P., and Schatz, M., J. App. Polym. Sci. 21, 3211 (1977).the valuable discussions with him were very much appreciated. M.I.A. also 18. Brebner, K. I., Chahal, R. S., and St. Pierre, L. E., Polymer 21, 533thanks CONICET (Argentina) for the fellowship that supported her stay (1980).in U.S.A. 19. Cohen-Addad, J. P., Roby, C., and Sauviat, M., Polymer 26, 1231

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