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Recycling of silicone rubber waste: effect of ground silicone rubber vulcanizate powder on the properties of silicone rubber.

INTRODUCTION

Disposal of worn-out rubber products and factory wastes is a global problem for both environmental and economic reasons. A potentially attractive method Is to grind vulcanized rubber and use the resultant powder as a compounding ingredient or as a replacement of raw polymer (1-3). Encouraging results have been obtained from the reduction of particle size and surface modification of the rubber powder (4). Ground rubber can be produced by cryogenic grinding, ambient grinding and wet ambient grinding. Rothermeyer discussed the effects of grinding and sieving methods on the particle size and structure of the powders obtained from waste rubber as well as the effects of different powders on the physical properties of the rubber vulcanizates (5). Phadke et al. studied the effects of addition of cryoground rubber (CGR) powder in unfilled and carbon black-filled natural rubber compounds (6-8). Naskar et al. studied the effect of ground rubber tire (GRT) particles of different sizes in a natural rubber (NR) compound (9 ). Luo and Isayev reported development of composites based on ultrasonically devulcanized GRT and polypropylene (10). Adam et al. reported that addition of polybutadiene granulate grafted with ethylacrylate to polyacrylic rubber compound causes fall in the physical properties of the rubber (11). Recently Jacob et al. reported utilization of mechanically ground EPDM rubber vulcanizates as a filler in EPDM compound (12).

While the earlier reports (13-20) deal mostly with the utilization of waste tire rubber, reuse of waste specialty rubbers has received less attention. Isayev et al. studied ultrasonic devulcanization of silicone rubber (21). Ghosh et al. reported the effects of addition of fluororubber vulcanizate powder (FVP) in a fluororubber compound and replacement of virgin rubbers in the blends of silicone rubber and fluororubber based on tetrafluoroethylene/propylene/vinylidene fluoride terpolymer by the corresponding ground rubber vulcanizates on the processing and vulcanizate properties (22, 23). This paper reports the results of studies on the effects of mechanically ground silicone rubber vulcanizate powder, abbreviated as SVP, on the processing and vulcanizate properties of silicone rubber. Different grades of SVP were prepared by varying the heat-aging period of the precursor thick sheets used for grinding. Aging was done to simulate the service or the long storage conditions of the waste rubber vulcanizates.

EXPERIMENTAL

Materials

Silicone rubber, containing 40% precipitated silica filler, with specific gravity of 1.21, was provided by GE Bayer Silicone Pvt. Ltd., Bangalore, India. Dicumyl peroxide (DCP, purity 98% and melting point 39-41[degrees]C) was obtained from Aldrich Chemicals Company, Inc., Milwaukee, WI, USA. n-hexane with specific gravity of 0.66 was provided by s.d.fine-chem. Ltd., Mumbai, India. Silicone rubber vulcanizate powder (SVP) was prepared in the Laboratory. The vulcanizate powder, obtained by grinding of the thick sheets (8.5 mm X 25 mm X 120 mm) of the silicone rubber vulcanizate, aged at 200[degrees]C for 3, 7 and 10 days, are designated as SVP-3d, SVP-7d and SVP-10d respectively.

Preparation of Silicone Rubber Vulcanizate Powder (SVP)

SVP was based on the rubber vulcanizate corresponding to formulation [P.sub.0] (Table 1). Silicone rubber was mixed with 2 phr dicumyl peroxide (DCP) in a Brabender Plasticorder, PLE-330 (Brabender OHG Duisburg, Germany) at 80[degrees]C and at a rotor speed of 60 rpm. First, silicone rubber was sheared in the Plasticorder for 2 min. Then, DCP was added and mixed for another 2 min. The hot material was sheeted out in a two-roll mill. The thick sheets of dimension (8.5 mm X 25 mm) x 120 mm) were prepared by molding at 170[degrees]C for 12 min in a hydraulic press at a pressure of 5 MPa. The samples were then aged in a heat-aging oven at 200[degrees]C for 3, 7 and 10 days. Finally, SVP-3d, SVP-7d and SVP-10d were prepared by grinding the thick silicone rubber sheets over the silicon carbide wheel of diameter 150 mm, rotating at 2900 rpm, in a Bench Grinder Type TG6 (Ralli Wolf Ltd., Mumbai, India). The abraded rubber in the powder form was collected in a specially designed holder placed beneath the grinding whee l.

Determination of Particle Size and Shape

The morphology of SVP was measured by a Scanning Electron Microscope, Hitachi S-4 15, Japan, using gold-coated samples. The particle size distribution and the average particle size of the vulcanizate powders were measured by using light optical microscope (LOM)-Ultrasonic technique. The particles were suspended in hexane and subjected to ultrasonic dispersion and examined under an Olympus BH-2, light optical microscope (LOM) at a magnification of 200x. Images of representative areas were transmitted to an on-line Olympus Cue 2, automated image analysis system (IAS). The individual particles were identified and their respective sizes were measured.

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis

FT-IR analysis was done by using Nicolet Nexus FT-IR spectrometer at a resolution of 4 [cm.sup.-1] over the range 4000-400 [cm.sup.-1] in the Attenuated Total Reflection (ATR) mode. Standard software (Omnic ESP, version 5.1) was used for data acquisition and analysis.

X-ray Photoelectron Spectroscopy (XPS) Study

The XPS study of silicone rubber and its vulcanizate powder was done using a VG Scientific ESCA Lab MKII Spectrometer, employing an excitation radiation of Mg K[alpha] (1250 eV). The working pressure in the spectrometer was 2 X [10.sup.-9] torr. All the spectra were referenced to the [Si.sub.2p] peak of silicon (24), which was assigned a value of 102.4 eV. Survey scans were collected from 0 to 1200 eV. The XPS data were fitted using a Gaussian-Lorentzian function. The different functional groups on the surfaces were estimated from respective areas of assumed Gaussian-Lorentzian curves using the following equation (25):

[C.sub.A] = ([I.sub.A]/[S.sub.A])/[SIGMA] [I.sub.i]/[S.sub.i] (1)

where [C.sub.A] is the relative concentration of element A; [I.sub.A] and [I.sub.l] are the peak areas and S denotes the sensitivity factor.

Mixing of SVP With Silicone Rubber

The formulations are given in Table 1. The compounds were prepared by using the Brabender Plasticorder, at the same temperature and rotor speed as described under preparation of SVP. After the rubber was sheared in the Plasticorder for 2 min at 80[degrees]C, SVP was added and mixed for 2 min, followed by mixing with DCP for another 2 min. The mixes were then sheeted out In a two-roll mill.

Measurement of Mooney Viscosity and Mooney Scorch Time

Mooney viscosity, ([ML.sub.[1+4]), and scorch time were determined at 10000 by using a Negretti Mooney Viscometer, Mark III (Negretti Automation Ltd., Buckinghamsblre, UK) as per ASTM D 1646 (1997). Mooney scorch time ([t.sub.5] represents the time for 5 Mooney units rise above the minimum torque.

Measurements of Rheological Properties

The rheological properties were measured with the help of a Monsanto Processabiity Tester (MPT, No 83077), which is a microprocessor-based automated and programmable capillary rheometer. The measurements were carried out using a capillary having a length to diameter ratio of 30:1 and the barrel diameter was 19 mm and Its length was 100 mm. The melt was allowed to enter into the round capillary die having multiple entry angles of 45[degrees] and 60[degrees], which minimize the pressure drop. Under the conditions, the Bagley correction (26) can be assumed to be negligible and the apparent shear stress can be taken as equal to true shear stress. The pre-heat time used for each sample was 5 min for uniform temperature attainment. The rate of shear variation was performed by auto programming, by changing the speed of the plunger after pre-selected time interval. The extrusion studies were carried out at three different temperatures (viz. 90, 100 and 110[degrees]C) and at four different shear rates (viz. 919.5, 122 6, 1839 and 2145.5 [s.sup.-1]).

The flow behavior index (n) and consistency index (k) were calculated by using the appropriate Power Law model (27),

[sigma] = k [[gamma].sup.n] (2)

where [sigma] and [gamma] are shear stress and shear rate respectively.

After leaving the MPT capillary, the extrudate was allowed to pass a scanning laser device, which measured its average diameter, as a percentage of the capillary diameter, providing information on the die swell:

% Die swell = 100 X ([d.sub.e] - [[degrees].sub.c])/[d.sub.c] (3)

where [d.sub.e] and [d.sub.c] are the diameter of extrudate and capillary respectively. The swelling index is defined as [d.sub.e]/[d.sub.c]. Maturing time was about 30 seconds.

Extrudate surface morphology was examined under a Scanning Electron Microscope (JEOL JSM 5800), using gold-coated samples.

Measurement of Curing Characteristics

The cure behavior of the samples was determined at 150, 160 and 170[degrees]C, using a Monsanto Moving Die Rheometer (model MDR 2000). The kinetics of the crosslinking reaction was studied from the changes in rheometric torque with time. The rate constant (k) for the first order reaction can be determined by the following equation (28, 29),

In(M.[alpha]] - [M.sub.t]) = - kt - In([M.sub.[alpha]] - [M.sub.o]) (4)

where [M.sub.t], [M.sub.o] and [M.sub.[alpha]] are the torque at time t, the torque at zero time and the maximum torque respectively. For cure curves showing marching modulus, [M.sub.[alpha]] was taken as the torque when the rise in torque is less than one unit in 5 mm; at this stage it is assumed that the reaction has almost come to an end. From the linear plot of In([M.sub.[alpha]] - [M.sub.t]) versus time (t), the rate constant (k) of the first order crosslinkung reaction was determined. The activation energy for the vulcanization reaction was calculated by using the Arrhenius equation.

Molding

For physical testing, thin sheets of approximately 2 mm thickness were prepared by molding the samples in a hydraulic press at 170[degrees]C for 2 min at a pressure of 5 MPa. After molding, the samples were post-cured at 200[degrees]C for 24 h.

Atomic Force Microscopy (AFM) Studies

The specimens for AFM studies were prepared by cryomicrotoming in a Reichert-Jung Ultracut Ultramicrotome, using a glass knife (using LKB Bromma 7800 Knife Maker), after freezing the specimens below its glass transition temperature using liquid nitrogen. Average sample thickness was 20 [mu]m. The AEM measurements were carried out in air at ambient conditions (2500) using Dimension 3000 Atomic Force Microscopy, manufactured by Digital Instruments, Santa Barbara, California. Topographic images were recorded simultaneously in the tapping mode. Scanning was done using etched silicone tips (TESP probe), each with a nominal tip radius of curvature of 5-10 nm and spring constant In the range of 20-100 N/in. The cantilever was oscillated at its resonance frequency, which ranged between 200 and 400 kHz. The setpoint ratio of the cantilever, which governs the tapping force, was between 0.8 and 0.9 for all the scans. All the images were recorded using free-oscillation amplitude of 140[+ or -]10 rim. The general chara cteristics of the probes are: Cantilever length, 125 [mu]m; Cantilever configuration, single beam; Reflective coating, uncoated; Sidewall angles, 17[degrees] side, 25[degrees] front and 10[degrees] back. All the images contained 256 data points. Scanning was done In the Z scale of 5 [mu]m and In majority of the scans, the scan area was maintained 30 [mu]m X 30 [mu]m. The images obtained after scanning the surfaces were analyzed using Nanoscope software.

Measurement of Physical Properties

The stress-strain properties were measured according to ASTM D412-98 specification using dumb-bell test pieces in a Zwick Universal Testing Machine (UTM), model 1445. The tear strength was determined according to ASTM D624-98 using unnicked 90[degrees]-angle test pieces in the Zwick UTM. Average of five measurements was taken for strength values. The hardness was determined as per ASTM D2240 (1997) and expressed in Shore A units. The tension set at l000% elongation was determined as per ASTM D412 (1997) in the Zwick UTM. Hysteresis loss was determined up to the second cycle by stretching dumb-bell test pieces to a strain level of 100% in the Zwick UTM.

The compression stress-relaxation was measured by using a Wallace Shawbury Compression Stress Relaxometer MK.3 by taking disc test pieces having 12 mm thickness and 20 mm in diameter. The stress was measured at zero time ([F.sub.o]) and after 168 h ([F.sub.t]) and the relaxation over this period was calculated by using the following expression (30):

Compression stress-relaxation

(%) = ([F.sub.0] - [F.sub.t])/[F.sub.0] X 100 (5)

Solvent Swelling Studies

Solvent swelling was done using n-hexane at 25[degrees]C for 48 h and the volume swell of the vulcanizate was measured.

Dynamic Mechanical Thermal Analyses

Dynamic mechanical thermal analyses were carried out in a Dynamic Mechanical Thermal Analyzer (DMTA. MK-II), Polymer Laboratory, U.K. The testing was performed in bending mode at a frequency of 1 Hz and a strain amplitude of 64 [mu]m (peak to peak displacement) over a temperature range of 120[degrees]C to + 100[degrees]C and at a heating rate of 2[degrees]C/min. Data were collected and analyzed by a Compaq computer.

Aging Studies

Aging studies were carried out for 48 hrs at 200[degrees]C in a Toyoseiki multicell aging oven by using dumbbell-shaped tensile specimens. Prior to testing, they were kept at 2500 for 24 h.

RESULTS AND DISCUSSION

Section I: Characterization of Model Waste Silicone Rubber

The physical properties such as hardness and solvent swelling of P-SVP samples (that is, thick rubber sheets used for making SVP) (Table 2) show that the effect of heat aging (200[degrees]C/10 days) on hardness and percent volume swelling is insignificant. Table 2 also reports the results of heat aging of [P.sub.0] (that is, thin silicone rubber sheets of 2 mm thickness used for measurements of physical properties) of same formulation as in P-SVP. It is evident that P-SVP and thin rubber sheets ([P.sub.0]) show similar hardness and solvent swelling, which can provide a relative estimate of the extent of crosslinking. The results are not unexpected because thick P-SVP sheet was cured for 12 mm; while thin [P.sub.0] sheet was cured for 2 min.

Size and Shape of SVP

SEM photomicrograph of the ground silicone rubber vulcanizate powder is shown in Fig. 1[alpha]. Chain-like structures are formed as a result of particle agglomeration. The particles are of irregular shapes. The particle size distribution of SVP Is given in Fig. 1b. It can be seen that the particle size distribution is broad ranging from 2 [mu]m to 110 [micro]m with the average particle size of 33 [mu]m.

FT-IR Spectroscopic Studies

The FT-IR spectra of silicone rubber vulcanizate and SVP are shown in Fig. 2. The spectral analyses reveal that there are no changes in the structure of the silicone rubber on heat-aging and subsequent mechanical grinding of the silicone rubber vulcanizate into its powder (SVP). Silicone rubber vulcanizate and SVP exhibit the characteristic absorbance of the silicone polymer and silica ([SiO.sub.2]), used as the filler in the commercial silicone rubber, in addition to the broad and weak absorbance at 1700--1500 [cm.sup.-1], which may be due to the presence of small amount of vinyl groups and carbonyl groups likely to be present in the silicone rubber backbone.

XPS Studies

The deconvoluted peaks of [Si.sub.2p], [C.sub.1S] and [O.sub.1S] excitation of silicone rubber and SVP are similar and are shown in Fig. 3. Figure 3a displays two peaks (designated as [Si.sub.A] and [Si.sub.B]) of [Si.sub.2p] atom in both silicone rubber and SVP: the major peak ([Si.sub.A]) having binding energy of 102.4 eV can be attributed to that of poly(dimethyl sioxane) (24) and the smaller peak ([Si.sub.B]) of binding energy of 103.8--103.74 eV Is believed to be that of silica. It is observed that the C15 spectrum of silicone rubber and SVP Is resolved into two components ([C.sub.A] and [C.sub.B]) as shown in Fig. 3b. The [C.sub.A] component having binding energy of 284.8 eV is assigned to that of [CH.sub.3] in poly(dimethyl siloxane) which is found in higher concentration and the smaller component ([C.sub.B]) of binding energy of 286.4--286.1 eV is presumably caused by the oxygen containing group (i.e., C-O). The oxygen spectrum ([O.sub.1S]) is deconvoluted into two peaks viz. [O.sub.A] and [O.sub.B] (Fig. 3c). The major peak ([O.sub.A]) of binding energy of 532.4 eV can be attributed to that one attached to Si-atom in poly (dimethyl siloxane) and silica. The smaller peak ([O.sub.B]) at binding energy of 533.7-533.8 eV is caused by the presence of C-O group in silicone rubber and SVP. The relative concentrations of different elements determined by measuring the areas under the resolved peaks are summarized in Table 3. The total atomic concentration of Si, C and 0 atoms remains almost same on the silicone rubber and SVP surface.

As reported below, the physical properties of the silicone rubber vulcanizate containing SVP are independent of the aging period of the precursor thick sheets (P-SVP) (Table 2). The results are in agreement with observations based on both FT-IR spectroscopic and XPS studies.

Section II. Effect of SVP on the Properties of Silicone Rubber Vulcanizate

Mixing Behavior

SVP can be incorporated into silicone rubber up to a loading of 60 phr without any mixing problem. Attempts to mix higher loadings of SVP failed due to processing difficulties.

Mooney Viscosity Measurements

The results of Mooney viscosity measurements are given in Table 4. Mooney viscosity increases exponentially with increase in SVP loading. Figure 4 shows the effect of SVP on the relative Mooney viscosity. The variation of Mooney viscosity with SVP concentration follows the expression:

[M.sub.f]/[M.sub.g] = 1 + 1.27 c + 4.91 [c.sup.2] (6)

where [M.sub.f] and [M.sub.g] stand for the Mooney viscosity of SVP-filled silicone rubber compound and the unfilled compound (that is, without SVP) respectively and c is the volume fraction of SVP in the filled compound. It Is inferred that the highly aggregated structure of fine SVP is responsible for the sharp increase in Mooney viscosity at higher loadings. Mooney scorch time increases with increase in SVP loading, presumably due to the distribution of a part of DCP into SVP from the virgin rubber matrix during mixing, thereby lowering the concentration of DCP in the virgin silicone rubber. This is also manifested in the curing characteristics discussed later. Gibala, Thomas and Hamed prepared ambiently ground carbon black filled styrene-butadiene rubber vulcanizate, which was then used as an additive to the virgin compound (31). They observed that during vulcanization sulfur migrates from the rubber matrix into the ground rubber particles.

Studies of Rheological Properties

Figure 5 shows the representative log-log plots of shear viscosity versus shear rate at 90[degrees]C. It is observed that silicone rubber compound follows the Power Law model (Eq 2) up to a loading of 45 phr of SVP. At higher loading of SVP (that is, 60 phr) the model is not followed. With increasing SVP loading, shear viscosity of the silicone rubber increases at shear rates below 1226 [s.sup.-1] but at higher shear rates (that is, > 1226 [s.sup.-1]) the shear viscosity of highly filled silicone rubber (that is, 60 phr of SVP) is less as compared to the silicone rubber compound with lower phr of SVP. The shear viscosity of the highly filled silicone rubber is less than that of the unfilled silicone rubber at a shear rate of 2145.5 [s.sup.-1]. This indicates less interaction between silicone rubber and SVP at higher loading of SVP. The n and k values are summarized in Table 5. The pseudoplasticity (which is characterized by n value) of silicone rubber does not change significantly with temperature and SVP lo ading at [less than or equal to]45 phr. As expected, the k value of silicone rubber increases with SVP loading.

Figure 6 represents the variation of extrudate die swell with loading of SVP in silicone rubber at different shear rates at 90[degrees]C. With increasing SVP addition the proportion of elastic component increases, which results in higher die swell of the SVP-filled silicone rubber. A similar observation was made while studying the replacement of virgin silicone rubber by SVP in silicone rubber/fluororubber blend (32). Die swell increases with increasing shear rate, which indicates that the experimental shear rate (that is, 919.5 [s.sup.-1] to 2145.5 [s.sup.-1]) is below the critical shear rate. In the earlier communication (32) it has been shown that the critical shear rate of virgin silicone rubber is > 2145.5 [s.sup.-1]. The effect of extrusion temperature on die swell is insignificant.

Figure 7 shows that the addition of SVP into silicone rubber makes the extrudate surface more rough. The degree of roughness increases with increasing loading of SVP in silicone rubber. This observation is similar to that observed in the case of silicone rubber replacement by SVP in silicone rubber/fluororubber blend (32).

Cure Characteristics

Typical Monsanto rheographs are shown in Fig. 8. The effect of SVP loading on the curing characteristics of silicone rubber compound at 170[degrees]C is summarized in Table 4. The minimum torque increases with SVP loading. However, the maximum torque and the rate of cure decrease, presumably because of the lowering of concentration DCP in the silicone rubber compound containing SVP, as was discussed under Mooney viscosity measurements. The activation energy for curing (in kJ/mole) for the different compounds are as follows: [P.sub.0], 108; [P.sub.15], 119; [P.sub.30], 115; [P.sub.45], 138; and [P.sub.60], 133. It is evident that activation energy for curing increases on incorporation of SVP in the silicone rubber compound, presumably due to distribution of DCP in both virgin silicone rubber matrix and SVP. SVP was prepared from silicone rubber compound containing 2 phr DCP. While studying the effects of DCP loading on curing behavior of silicone rubber, it was observed that the maximum torque in the Monsanto rheograph did not increase beyond 2 phr of DCP loading (rheographs not shown). Also, measurement of physical properties indicated no change in properties of the vulcanizates beyond 2 phr of DCP concentration. On the contrary, migration of a part of DCP to SVP causes lowering of the DCP concentration in the virgin silicone rubber matrix and thus both the rate of curing and maximum torque decrease with increasing SVP loading. Monsanto rheographs also show that the SVP increases the minimum torque, as is observed in the case of fillers. The relative decrease in the maximum rheometric torque due to addition of SVP can be expressed as:

Z = ([DELTA][L.sub.gum] - [DELTA][L.sub.filled])/[DELTA][L.sub.gum] (7)

where [DELTA]L refers to the difference in maximum and minimum torques. When Z is plotted against the volume fraction of SVP (that is, c), a straight line with a slope of 0.66 ([Z.sub.F]) is obtained (Fig. 9) according to the following equation:

Z = 0.66c (8)

It is interesting to note that Z values at three different curing temperatures yield the same straight line. Hence [Z.sub.F] is believed to be the characteristic of SVP. [Z.sub.F] in the present case is similar to the Wolf-parameter ([[alpha].sub.F]), which is, however, based on the relative increase of rheometric torque as is observed in the case of reinforcing fillers (33).

Mechanical Loss Spectra

Plots of tan [delta] and log E' versus temperature (Fig. 10) show that silicone rubber exhibits two transitions in the temperature range of -120[degrees] to + 100[degrees]C. The peak at -98[degrees]C is due to the glass-rubber transition ([T.sub.g]) and the peak at -37[degrees]C is due to the cold crystallization ([T.sub.m]). Earlier it has been reported that silicone rubber has a [T.sub.g] at -92[degrees]C and [T.sub.m] at -20[degrees]C (34). It is seen that the addition of SVP to silicone rubber causes insignificant changes in the transition temperatures (Table 6). With increasing SVP loading in silicone rubber there is marginal decrease in tan [delta] both at [T.sub.g] and [T.sub.m] The changes of storage modulus with SVP loading are insignificant.

Atomic Force Microscopy Studies

As reported by Ghosh et al. (35), the surface of the unfilled silicone rubber vulcanizate contains a large number of tiny hills and corresponding valleys. The surface plot, given in Fig. 11a, shows that silicone rubber surface is having a granular morphology with small granules occupying the entire surface.

Addition of 60 phr of SVP to the silicone rubber compound causes some changes in the granular surface morphology of the original silicone rubber vulcanizate. Comparison of the surface plots of the original silicone rubber vulcanizate (Fig. 11 a) with the compound containing vulcanizate powder (Fig. 11 b) reveals that on incorporation of the vulcanizate powder, the uniformity in the surface morphology of the original silicone rubber compound, which consists of granules having similar dimensions, is lost. Incorporation of the vulcanizate powder causes formation of granules of bigger size with non-uniform dimension. The surface plot also shows that the dispersion of the vulcanizate powder is not completely homogeneous and also at some points there are agglomerations of the particles, whereby granules of higher dimensions are obtained.

Comparison of the surface profile of the unfilled and filled compounds (Figs. 12a-b) also shows that addition of the vulcanizate powder causes changes in the height and width of the granules from the mean plane, though the increase in width is more significant. The increase in the width of the granules may be due to the filling up of the valleys by means of the vulcanizate powder. The agglomeration of the particles at some portions of the compound is also visible in the right hand side of the surface profile in Fig. 12b. The histograms derived from the distribution of the projections of the granules from the mean plane (vertical distance) and width of the granules (horizontal distance) also support the above observation. The histogram derived from the distribution of the vertical distance of the granules shows that in the unfilled compound the height distribution varies from 0.11 to 0.33 p.m whereas incorporation of vulcanizate powder causes the range to increase (from 0.05 to 0.80 [mu]m (Fig. 13a). The s mall percentage of granules, projecting from the mean plane up to a distance 0.80 [mu]m, is due to the agglomeration of the vulcanizate powders. However, the majority of the granules project only up to a distance 0.33 [mu]m, the height that is comparable to that of the unfilled compound, though the number of granules having 0.33 [mu]m is less in the case of SVP filled compound. Significant changes are noticed in the horizontal distance. Figure 13b shows that appreciable amount of granules of higher width (that is, width up to 1.43 [mu]m) are formed with the incorporation of SVP whereas in the case of the unfilled compound, the maximum width of the particle is 0.95 [mu]m only. The histograms in Fig. 13b also shows that in the case of both unfilled and filled compounds, granules with horizontal distance 0.48 [mu]m occupy the maximum surface.

The power spectral density analysis of the unfilled and filled compounds shows distinct changes in the power spectra due to the incorporation of SVP. Application of power spectral density analysis of AFM in the calculation of spatial distribution of particles on the surface is described elsewhere (35-37). Comparison of power spectral density analysis of the unfilled (Fig. 14a) and SVP filled vulcanizate (Fig. 14b) shows that in the unfilled silicone rubber, the smaller features on the surface contribute more to the surface roughness, while in the case of SVP filled rubber, the higher features contribute more to the surface roughness. At the same time, a considerable percentage of the smaller features can also be seen on the surface. The power spectral density analysis of the SVP filled vulcanizate (Fig. 14b) also shows that the smaller features, which appear on the right-band side of the power spectra, are closely packed, whereas the higher features, which appear on the left-hand side of the power spectra, a re distributed over the surface with considerable spacing between the particles.

Physical Properties

The physical properties of the unfilled and SVP-filled silicone rubber vulcanizates are summarized in Table 7. It is observed that the modulus, tensile strength, tear strength and elongation at break decrease with increase in SVP loading. It is also evident that even at a loading of 60 phr of SVP, the tensile and tear strengths drop only by 20% and modulus drops by 15%. However, the hardness and tension set show only a marginal change with the addition of SVP. The hysteresis loss of silicone rubber compound does not change appreciably on incorporation of SVP. Incorporation of SVP has no effect on the percent compression stress relaxation. The percent volume swell (Table 7), which is inversely proportional to the crosslinking density of the polymer, remains almost constant up to an SVP loading of 60 phr. The effect of varying aging period of the precursor thick sheets (used in making SVP) on the physical properties of the corresponding SVP-filled silicone rubber vulcanizate is summarized in Table 8. It is fou nd that there are no significant changes in physical properties of the silicone rubber vulcanizate on incorporation of different grades of SVP.

Earlier workers reported that the addition of rubber powder (rubber crumb), even at low concentrations, to virgin rubber, generally results in a substantial decrease in the physical properties (7, 13, 38). For instance, it has been shown that the addition of only 10% rubber crumb (particle size, 425-600 [mu]m) to a virgin rubber compound leads to 15% reduction in the tensile strength (39). In the present case, however, the fall in properties is much less, presumably because of the small particle size of the SVP and its uniform mixing with the virgin polymer.

The effects of air aging on the physical properties are also shown in Table 7. In general, aging of silicone rubber vulcanizates causes an increase in the modulus and drop in the tensile strength and elongation at break. On incorporation of SVP even at 60 phr level, the fall in properties on aging remains almost unaltered.

CONCLUSIONS

1. The ambiently ground silicone rubber vulcanizate powder (SVP) exists in highly aggregated state. The particle size distribution is broad ranging from 2 [micro]m to 110 [micro]m with an average particle size of 33 [micro]m.

XPS and IR-Spectroscopy studies reveal that there are no chemical changes on the surface of the silicone rubber on mechanical grinding of thick rubber sheets aged at different conditions.

2. Incorporation of SVP into virgin silicone rubber increases the Mooney viscosity, Mooney scorch time, shear viscosity and the activation energy for viscous flow of the silicone rubber.

3. Addition of SVP Into silicone rubber decreases the cure rate and increases the activation energy of curing. DMTA spectra of silicone rubber do not change in the presence of SVP.

4. AFM studies show that incorporation of SVP into silicone rubber causes some changes in the surface morphology of the vulcanizate, though granules having dimension corresponding to that of silicone rubber vulcanizate are visible on the surface of the SVP-filled vulcanizate as well.

5. The addition of SVP even at a loading of 60 phr causes only 20% drop in tensile and tear strengths and 15% drop in modulus. Furthermore, the changes in hardness, hysteresis loss, tension set, and stress relaxation are marginal. The percent changes in modulus and tensile strength of silicone rubber on aging at 200[degrees]C for 48 hrs remain almost same on incorporation of 60 phr of SVP.

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Table 1

Formulations of Mixes.

 Mix Symbol

Ingredients [P.sub.0] [P.sub.15] [P.sub.30] [P.sub.45]

Silicone rubber 100 100 100 100
SVP 0 15 30 45
DCP 2 2 2 2

 Mix Symbol

Ingredients [P.sub.60]

Silicone rubber 100
SVP 60
DCP 2

Table 2

Physical Properties of [P.sub.0] and P-SVP.

 [P.sub.0]

Properties [P.sub.0]-1d [P.sub.0]-2d [P.sub.0]-3d

Tensile
strength (MPa) 7.9 7.9 7.1

Ultimate
elongation (%) 212 188 175

Modulus at 100%
elongation (2) (%) 4.1 4.3 4.3

Tear strength
(kN/m) 24.1 21.3 22.4

Tension set at
100% elongation (%) 4 4 4

Hysteresis loss at
1st cycle (J/[m.sup.2], X
 [10.sup.-6]) 0.060 0.060 0.059

Hardness
(Shore A) 74 76 76

Volume
swelling (%) 168 158 150

 [P.sub.0] P=SVP

Properties [P.sub.0]-4d P.SVP-3d P-SVP-7d P-SVP-10d

Tensile
strength (MPa) 7.1 -- -- --

Ultimate
elongation (%) 179 -- -- --

Modulus at 100%
elongation (2) (%) 4.3 -- -- --

Tear strength
(kN/m) 23.0 -- -- --

Tension set at
100% elongation (%) 6 -- -- --

Hysteresis loss at
1st cycle (J/[m.sup.2], X
 [10.sup.-6]) 0.060 -- -- --

Hardness
(Shore A) 77 76 76 77

Volume
swelling (%) 166 158 164 167

* This is same as stress at 100% elongation.

Table 3

X-Ray Photoelectron Spectroscopic Characterization of Silicone Rubber
and SVP Surfaces.

 Binding energy (eV) Relative atomic
 percentage of elements

Core level and
 deconvoluted
 spectra Silicone rubber SVP Silicone rubber

[Si.sub.2P]
 [Si.sub.A] 102.4 102.4 24.5
 [Si.sub.B] 103.8 103.7 4.4

[C.sub.1S]
 [C.sub.A] 284.8 284.8 43.5
 [C.sub.B] 286.4 286.1 5.4

[O.sub.1S]
 [O.sub.A] 532.4 532.4 18.5
 [O.sub.B] 533.7 533.8 3.8

 Relative atomic
 percentage of
 elements

Core level and
 deconvoluted
 spectra SVP

[Si.sub.2P]
 [Si.sub.A] 22.6
 [Si.sub.B] 6.4

[C.sub.1S]
 [C.sub.A] 40.4
 [C.sub.B] 5.0

[O.sub.1S]
 [O.sub.A] 22.6
 [O.sub.B] 2.9

Table 4

Results of Mooney Viscosity and Moving Die Rheometric Studies of
Silicone Rubber Vulcanizates Containing SVP.

 Mooney viscosity Monsanto rheometry (MDR
 measurements at 100[degrees]C 2000) at 170[degrees]C

 Mooney viscosity Mooney scorch Minimum Maximum
Mix Symbol ([ML.sub.1 + 4]) time (min) time (N.m) torque (N.m)

[P.sub.0] 29 76 8 203
[P.sub.15] 40 80 13 189
[P.sub.30] 46 87 16 177
[P.sub.45] 61 90 21 173
[P.sub.60] 70 95 27 169


 Monsanto rheometry (MDR 2000) at
 170[degrees]C

 Rate constant of Optimum
Mix Symbol curing ([min.sup.-1]) cure time (min)

[P.sub.0] 1.79 0.83
[P.sub.15] 1.64 0.98
[P.sub.30] 1.55 0.97
[P.sub.45] 1.50 1.02
[P.sub.60] 1.25 1.13

Table 5

Flow Behavior Index (n) and Consistency Index (k).

Mix Symbol n k x [10.sup.-4] (Pa)

[P.sub.0] 0.35 1.55
[P.sub.15] 0.35 1.58
[P.sub.30] 0.35 1.66
[P.sub.45] 0.33 2.04
[P.sub.60] 0.13 8.71

Table 6

Results of DMTA Studies.

 Tan [delta] at T.sub.m]
Mix Symbol [T.sub.g] ([degrees]C) [T.sub.g] ([degrees]C)

[P.sub.0] -98 0.084 -37
[P.sub.30] -98 0.079 -37
[P.sub.45] -99 0.077 -37

 Tan [delta] at
Mix Symbol [T.sub.m]

[P.sub.0] 0.161
[P.sub.30] 0.163
[P.sub.45] 0.154

Table 7

Effect of SVP on Physical Properties of Silicone Rubber Vulcanizate (a).

 Mix Number

Properties [P.sub.0] [P.sub.15] [P.sub.30]

Modulus at 100% 4.1 4.0 3.9
elongation (b)(MPa) (+18) (+15) (+11)

Tensile strength 7.9 6.8 6.6
(MPa) (-21) (-10) (-10)

Elongation at break 212 205 190
(%) (-28) (-32) (-31)

Tear strength (kN/m) 24.1 21.6 21.0

Hystereis loss (J/[m.sup.2],
X [10.sup.-8])
1st cycle 0.060 0.061 0.055
2nd cycle 0.019 0.018 0.016

Hardness (Shore A) 74 74 72

Tension set at 100%
elongation (%) 4 4 5

Compression stress
relaxation (%) 23 22 23

Volume swelling (%) 168 160 158

 Mix Number

Properties [P.sub.45] [P.sub.60]

Modulus at 100% 3.5 3.5
elongation (b)(MPa) (+14) (+16)

Tensile strength 6.4 6.3
(MPa) (-8) (-21)

Elongation at break 186 179
(%) (-39) (-37)

Tear strength (kN/m) 20.9 19.1

Hystereis loss (J/[m.sup.2],
X [10.sup.-8])
1st cycle 0.062 0.054
2nd cycle 0.017 0.018

Hardness (Shore A) 72 71

Tension set at 100%
elongation (%) 6 6

Compression stress
relaxation (%) 22 21

Volume swelling (%) 166 161

(a)Values in the parentheses correspond to the percent drop (-) or
increase (+) in tensile properties of silicone rubber on aging for 48
hrs at 200[degrees]C.

(b)This is same as stress at 100% elongation.

Table 8

Effect of Aging Period of P-SVP on the Physical Properties of the
Corresponding SVP-filled Silicone Rubber Vulcanizate.

 Mix Symbol

Physical Properties [P.sub.60] (SVP-3d)

Tensile strength (MPa) 6.3
Ultimate elongation (%) 179
Modulus at 100% elongation (a) (MPa 3.5
Tear strength (kN/m) 19.1
Tension set at 100% elongation (%) 6
Hysteresis loss (J/[m.sup.2], X
 [10.sup.-6])
 1st Cycle 0.060
 2nd Cycle 0.017
Hardness (Shore A) 71
Volume swelling (%) 161

 Mix Symbol

Physical Properties [P.sub.60] (SVP-7d)

Tensile strength (MPa) 6.7
Ultimate elongation (%) 177
Modulus at 100% elongation (a) (MPa 3.7
Tear strength (kN/m) 18.2
Tension set at 100% elongation (%) 6
Hysteresis loss (J/[m.sup.2], X
 [10.sup.-6])
 1st Cycle 0.063
 2nd Cycle 0.017
Hardness (Shore A) 72
Volume swelling (%) 174

 Mix Symbol

Physical Properties [P.sub.60] (SVP-10d)

Tensile strength (MPa) 6.2
Ultimate elongation (%) 174
Modulus at 100% elongation (a) (MPa 3.6
Tear strength (kN/m) 18.6
Tension set at 100% elongation (%) 4
Hysteresis loss (J/[m.sup.2], X
 [10.sup.-6])
 1st Cycle 0.061
 2nd Cycle 0.017
Hardness (Shore A) 71
Volume swelling (%) 166

(a) This is same as stress at 100% elongation.


ACKNOWELDGMENT

The authors wish to express their sincere gratitude to GE Bayer Silicone Pvt. Ltd., Bangalore, India, for providing the silicone rubber. The authors are also thankful to Dr. P. Sadhukhan of Bridgestone/Firestone, Inc., 1200 Firestone Parkway. Akron. OH 44317-0001. for his kind assistance in conducting the LOM-Ultrasonic experiment.

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ABBREVIATED NAMES/SYMBOLS

DCP: Dicumyl peroxide

SVP: Silicone rubber vulcanizate powder

SVP-3d: Silicone rubber vulcanizate powder (precursor sheet aged for 3 days at 200[degrees]C)

SVP-7d: Silicone rubber vulcanizate powder (precursor sheet aged for 7 days at 200[degrees]C)

SVP-10d: Silicone rubber vulcanizate powder (precursor sheet aged for 10 days at 200[degrees]C)

P-SVP: Precursor of SVP

[P.sub.60] (SVP-3d): Silicone rubber filled with 60 phr of SVP-3d.

[P.sub.60] (SVP-7d): Silicone rubber filled with 60 phr of SVP-7d

[P.sub.60] (SVP-10d): Silicone rubber filled with 60 phr of SVP-10d

XPS: X-ray photoelectron spectroscopy

FT-IR: Fourier-transform infrared spectroscopy

SEM: Scanning electron microscope

AFM: Atomic force microscopy

DMTA: Dynamic mechanical thermal analysis

S.K. De *

* Corresponding author. E-mail: skde@rtc.iitkgp.ernet.in
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Author:Ghosh, Arun; Rajeev, R.S.; Bhattacharya, A.K.; Bhowmick, A.K.; De, S.K.
Publication:Polymer Engineering and Science
Date:Feb 1, 2003
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