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Probing the viscoelastic properties of brominated isobutylene-co-p-methylstyrene rubber/tackifier blends using a rubber process analyzer.

INTRODUCTION

Brominated isobutylene-co-p-methylstyrene (BIMS) rubber is a fairly new commercial rubber used in a number of industrial applications in both tire and nontire areas. In tire sector, BIMS elastomer has been highly evaluated in variety of tire compounds including inner liner (1), sidewall (2), and treads (3). In nontire applications, BIMS compositions are finding application in mechanical and extruded parts. Hubbard et al. (4) and Hubbard and Briddell (5) have shown that useful adhesives in either solvent or cured tape form can be formulated using BIMS in combinations with copolymers. BIMS rubber has enhanced thermal stability and resistance to oxidative attack because of its saturated backbone and also provides high physical strength, excellent resistance to gas and moisture permeability, chemical inertness, good vibrational damping, and excellent resistance to atmospheric aging.

Because BIMS elastomer has potential applications in the adhesive industry and in tire (inner liner, side wall, and treads), the adhesive tack and the autohesive tack properties of this elastomer are very crucial. Kumar et al. (6) have investigated the tack and green strength of BIMS rubber and its blends with reference to level of bromination, fillers, nature of rubber, and blend ration. In general, the low molecular weight tackifiers are compounded with a variety of elastomers to enhance tack and to prevent tack degradation. It is well known that most of the tackifiers duly modify the viscoelastic property of the elastomer to accomplish good autohesive (7) or adhesive tack (8-10).

In literature, there are many reports, which examine the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. For example, Sheriff et al. (8) have demonstrated the effect of adding low molecular weight resins to natural rubber. Aubrey and Sheriff (9, 10) have examined the relationship between viscoelasticity and peel adhesion of rubber-resin blends. Kraus et al. (11-13) have reported the studies of resins blended with styrene-isoprene-styrene block copolymers. Class and Chu (14-16) have studied the viscoelastic properties of rubber-resin blends based on the resin structure, resin molecular weight, and resin concentration.

Despite these investigations, no report is available on the viscoelastic behavior of BIMS rubber-tackifier blends. Therefore, the effect of tackifiers on the viscoelastic behavior of BIMS rubber is worth investigating. In this article, the viscoelastic properties of BIMS/hydrocarbon resin blends and BIMS/phenolic resin blends have been studied by performing frequency sweep, strain sweep, and stress relaxation studies in a rubber process analyzer (RPA) at different temperatures. Moreover, all the earlier studies on the viscoelastic properties of rubber/tackifier blends have been performed using conventional dynamic mechanical analyzer (8), (9), (14-16). Possibly, the viscoelastic behavior of rubber/tackifier blends has been explored with the help of RPA for the first time in this field.

The RPA is a vulcameter designed to measure the shear moduli G', G", and G * of rubber compounds. It can be operated with strain and frequency sweeps in wide range of strain amplitudes and temperatures (17), (18). Compounds can be studied in the uncured (green compound) as well as in the cured state. Vulcanization can be performed with the same sample used for the green compound analysis, and therefore good insight into several characteristics of the rubber compound is obtained. Furthermore, it is also possible to measure samples several times in row to determine the stress softening and relaxation process (19).

EXPERIMENTAL

Materials

Brominated isobutylene-co-p-methylstyrene (BIMS) (grade: Exxpro [TM] 3035; benzylic bromine of 0.47 [+ or -] 0.05 mol% and 2.0 mol% of p-methylstyrene, Mooney viscosity of 45 [+ or -] 5 at ML1 + 8 125[degrees]C and [M.sub.w] = 450,000) and Escorez[TM] hydrocarbon resin (grade: E1102; softening point of 96-104[degrees]C; Glass transition temperature of 50[degrees]C; and [M.sub.w] = 2400) were supplied by Exxon Mobil Chemical Company, Baytown, TX. Octylphenol-formaldehyde thermoplastic phenolic resin tackifier (grade: SP1068; softening point around 80[degrees]C and glass transition temperature of 35[degrees]C) was supplied by Schenectady International, NY.

Preparation of Rubber-Resin Blends

The mixes were prepared in a Brabender Plasticoder (model, PLE-330, capacity 65 ml) at 130 and 110[degrees]C at 60 rpm for BIMS/hydrocarbon resin and BIMS/phenolic resin blends, respectively. BIMS was taken in the Brabender and sheared for 2 min and then the tackifier was added and mixing was continued for additional 3 min. The neat BIMS rubber was also processed for 4 min under the same condition. The composition of the mixes prepared is reported in Table 1.
TABLE 1. Composition of mixes prepared.

Sample Designation BIMS Hydrocarbon Phenolic
 no. rubber resin (phr) resin (phr)

 1. B 100 0
 2. BE10 100 10
 3. BE20 100 20
 4. BE30 100 30
 5. BSP3 100 3
 6. BSP10 100 10
 7. BSP20 100 20
 8. BSP30 100 30


Dynamic Mechanical Analysis

Temperature Ramp Test. Temperature ramp test was carried out in a dynamic mechanical analyzer (DMA) (DMAQ800 from TA instruments), in the temperature range of -100[degrees]C to 100[degrees]C for pristine BIMS and BIMS/tackifier blends at a constant frequency of 1 Hz and at a constant strain of 0.1%. Because tackifiers are low molecular weight species, determination of glass transition temperature ([T.sub.g]) via tan [delta] peak requires special test geometry because tension mode geometry will not be able to resolve the tan [delta] peak of the tackifiers. Therefore, glass fiber cloth was impregnated with the tackifier powder at 110[degrees]C to a uniform thickness following a procedure given by Tobing and Klein [20]. Then, the temperature sweep was carried in DMA in the temperature range of 0 to 100[degrees]C for both the tackifiers at a constant frequency of 1 Hz and at a constant strain of 0.1%. Glass fiber cloth was chosen because it only exhibited glassy behavior throughout the test temperature (0-100[degrees]C) range.

Surface Morphology Study

Scanning Electron Microscope Studies. The dispersion of tackifiers in the rubber matrix was examined by JEOL, JSM 5800 scanning electron microscope (SEM) after sputter coating the samples with gold.

Rubber Process Analyzer Measurements

RPA (RPA 2000, Alpha Technologies) was used to measure the viscoelastic properties of raw BIMS rubber and rubber-resin mixtures. For experiments at 40 and 75[degrees]C, the samples were first heated to 100[degrees]C and then cooled to the respective low temperatures. This operation filled the cavity and gave the uniform and reproducible results. For experiments at other temperatures (at 100 and 150[degrees]C, the samples were directly allowed to fill the cavity at 100 and 150[degrees]C. Three different types of tests-strain sweep, frequency sweep, and stress relaxation were programmed in RPA. RPA is capable to a temperature accuracy of [+ or -]0.1 [degrees] C and can be operated with strain and frequency sweeps in wide ranges of strain amplitude and temperature. In this work, the test conditions for all the experiments performed in RPA were well within the instrument operating limits. All the results were found to be repeatable, and the maximum experimental error involved in the measurements was within [+ or -]1%.

Strain Sweep. Strain sweep was carried at 40 and 100[degrees]C at 6 cpm (cycle per minute) frequency. Strain applied for testing in the RPA was split into two major groups: low strains, which commonly correspond to the linear viscoelastic regime. Shear storage modulus (G') was recorded at each strain.

Frequency Sweep. In frequency sweep, strain and temperature were kept constant, whereas the frequency was varied in a preprogrammed way. Frequency sweep was carried at four selected temperatures: 40, 75, 100, and 150 [degrees] C. In all the cases the applied strain was 7% (selected from the linear viscoelastic region of the strain sweep curves) Shear storage modulus (G') and complex viscosity (n*) were recorded at each frequency.

Stress Relaxation. stress relaxation tests were carried at 40 and 100[degrees]C. In the stress relaxation tests, the preheat time was set as 60 sec and the applied strain was 5 degree arc (70% strain: selected from the linear viscoelastic region of the strain sweep curves). Stress relaxation decay time (the time period starting at the end of the applied deformation pulse to the end of the stress relaxation test) was 120 sec.

RESULTS AND DISCUSSION

Evaluation of Compatibility Between BIMS Rubber and Tackifiers by DMA

The DMA studies have been performed to understand the compatibility between the BIMS rubber and the tackifier blends. Figure 1 a gives the tan [delta] and E' plots against temperature for representative samples (The results of other samples are omitted for clarity). For the near BIMS rubber, the tan [delta] peak appears at -24[degrees]C. For the hydrocarbon resin tackifier, the tan [delta] peak temperature appears at 77 [degrees] C (shown as inset of Fig. 1a). With the incorporation of the hydrocarbon resin tackifier (i.e. BE10 and BE30), the tan [delta] peak temperature of the BIMS rubber shifts toward higher temperature, and the storage modulus (E') of the neat BIMS rubber further decreases in the plateau region. Furthermore, there is no evidence of another peak in the tan [delta] curve, suggesting the good compatibility between the hydrocarbon resin and the BIMS rubber at all proportions. Commonly, tackifier that has good compatibility with rubber will cause an increase in [T sub g], broadening of the transition region, and a decrease in the plateau modulus [14]. Here, the rubber-resin blends exhibit the similar behavior. The increase in [T sub g] indicates that addition of the tackifier results in a reduction in free volume. This also indicates that good mixing has taken place between BIMS rubber and the hydrocarbon resin. The broadening of the transition region indicates the presence of the new relaxation processes associated with the constraints imposed on the molecular motions of the elasto-mers by the addition of the low molecular weight tackifier. The depression of storage modulus in the plateau zone corroborates the increase in viscous flow property of BIMS rubber by the addition of the hydrocarbon resin.

[FIGURE 1 OMITTED]

In the case of BIMS/phenolic resin blends, the plots of Log E' and tan [delta] vs. temperature, presented as Fig. 1b, do not show the expected shift of the temperature at which tan [delta] reaches a peak, nor these display the depression of the storage modulus in the plateau, when the results are compared with the neat BIMS rubber. For the phenolic resin tackifier, the tan [delta] peak temperature appears at 58[degrees]C (shown as inset of Fig. 1b). Also, for the samples BSP10 and BSP30, a high temperature tan [delta] peak is apparent at ~70 [degrees] C, suggesting the phase separation of the phenolic resin in the BIMS rubber matrix. At resin concentration of 30 phr, a second transition can be seen in the storage modulus curve at about 70 [degree] C. This confirms the presence of two phase morphology.

The DMA studies (temperature sweep studies) of the BIMS/hydrocarbon resin blends prove good compatibility between the blend components, whereas the SEM photo-micrograph of sample BE10 show some white resin particles on the rubber surface (Fig. 2a). This observation is very similar to the results reported earlier for the compatible SBR/terpene tackifier blend (21). It is well known that tackifier/rubber blend, which is compatible in bulk undergo tackifier migration to surface region (22), (23). In our earlier communication, we have calculated the ratio of molecular weight (M sub w)/entanglement molecular weight (M sub e) of near BIMS rubber and BIMS/hydrocarbon resin blend (which is in the range of 135 to 40, respectively) and shown why the tackifiers will migrate to a small extent on the surface (24). Despite the fact that the hydrocarbon resin particles are phase-separated on the BIMS rubber surface, the dispersed hydrocarbon resin particles have not been identified as a distinct separate phase in the DMA studies of the BIMS/hydrocarbon resin blend (Fig. 1a). On the other hand, SEM photomicrograph of the sample BSP3 shows bulk phase separation of the phenolic resin on the BIMS rubber surface (Fig. 2b). This suggests the existence of the two distinct phases, which is also clearly recognized in the DMA studies of the BIMS/phenolic resin blend (Fig. 1b).

[FIGURE 2 OMITTED]

Strain Sweep Studies of Neat BIMS Rubber and BIMS/Tackifier Blends by RPA

The viscoelastic properties of near BIMS rubber and BIMS/tackifier blends have been studied using strain sweep measurements at 40 and 100[degrees]C. The two different temperatures have been chosen because 40[degrees]C is well below the softening point of the tackifiers, whereas 100[degrees]C is above the tackifiers' softening point. In the strain sweep measurements, at lower strains (approximately between 0.98% and 100%), it is evident that there is a region where a shear storage modulus (G') is independent of shear strain or the so-called linear viscoelastic region (LVE) for both neat BIMS rubber and BIMS/tackifier blends (discussed in the next section). However, at higher strains (> 100%) the LVE region disappears in all the samples.

Figure 3a and b shows the plot of Log (G') versus Strain (%) for the samples B, BE10, BE30, BSP10, and BSP30 at 40[degrees]C. In Fig. 3a in the LVE region, the [G'] values of the sample BE10 is lower than the G' values of the samples B and BSP10. The results of the samples BE20 and BSP20 (not shown here) are similar to the results of the samples BE10 and BSP10. In fig. 3b, the sample BE30 shows the lowest [G'] values, whereas the sample BSP30 records the highest [G'] values in the LVE region. But at higher strain rates (> 100%), the [G'] values of the sample BSP30 fall rapidly and suddenly and are comparable with the G' values of the samples B and BE30.

[FIGURE 3 OMITTED]

The lowest values of G' (in the LVE) for the samples BE10 and BE30 are due to the effect of hydrocarbon resin tackifier in the BIMS rubber matrix. This also augments the good viscous flow behavior of BIMS/hydrocarbon resin blends. Because the hydrocarbon resin is found to be compatible with the BIMS rubber even at higher proportions and plasticizes the rubber, the rubber-resin mixture will produce a uniform homogeneous mixture, which will enhance the viscous flow property of the blend. However, in the case of BIMS/phenolic resin blends, the viscous flow property of sample BSP10 is marginally better that that of B. But at higher loading of the resin, the phenolic resin does not contribute in enhancing the viscous flow property. Instead, it raises the modulus in the LVE region. Phenolic resin is found to be incompatible with BIMS rubber even at very low concentrations (10 phr) (25). Hence, the rubber-resin mixture will nor form a uniform homogenous mixture at higher resin loading. The resin molecules will not be absorbed on the rubber surface. As a result, the interaction between the resin and the resin will be higher compared with the rubber and the resin, which will result in the formation of the stiff resin aggregates (resin-resin networks). Moreover, the phenolic resin as filler itself can show strong filler networking due to its polar character and the ability to from hydrogen bonds (26). Generally, in carbon black or silica filled rubber samples, the degree of breakdown of the filler-filler networks is usually known as Payne effect (19). As can be seen in Fig. 3b, a phenomenon similar to Payne effect (break down of resin-resin network) is significant for the sample BSP30 when compared with the sample BE30. It appears that the phenolic resin-phenolic resin network of the sample BSP30 breaks in the range of 40 to 50% strain (highlighted portion of Fig. 3b), which is not in the case of the sample BE30. This suggests the aggregation of the phenolic resin in the BIMS rubber matrix at higher resin concentration.

Figure 4a and b shows the plot of Log (G') verus Strain (%) for the samples B, BE10, BE30, BSP10, and BSP30 at 100[degrees]C. In Fig. 4a, in the LVE region, the [G'] values of the resin loaded samples (BE10 and BSP10) are lower than the [G'] values of the sample B. Similar observations are made when the resin content is increased from 10 phr to 20 phr and 30 phr (Fig. 4b). The results of the samples BE20 and BSP20 are omitted in the figure for the sake of clarity. However, at higher strains, the fall in modulus is rapid and sudden for the sample BSPIO and BSP30 when compared with the other samples. There is also difference in the [G'] values for the various samples in the higher strain region.

[FIGURE 4 OMITTED]

At 100[degrees]C test temperature (temperature above the softening point of the phenolic resin), the phenolic resin will exist in the liquid state. This can reduce the phenolic resin-phenolic resin interaction strongly. Hence, the [G'] values of the incompatible BIMS/phenolic resin mixtures are almost similar to or lower than the G' values of the compatible BIMS/hydrocarbon resin mixtures in the LVE region. However, in the nonlinear viscoelastic region, the G' values of the BIMS/phenolic resin mixtures fall rapidly and suddenly relative to the G' values of the neat BIMS rubber and the BIMS/hydrocarbon resin blends. The softening point of the phenolic resin tackifier (75 - 85[degrees]C) is lower than the softening point of the hydrocarbon resin tackifier (96 - 104[degrees]C). Therefore, at 100[degrees]C; phenolic resin will be less viscous than the hydrocarbon resin tackifier, which will result in good drop of the modules at higher applied strain values.

Frequency Sweep Studies of BIMS/Tackifier Blends by RPA

Figure 5a shows the double logarithmic plot of variation in shear storage modulus (G') and complex viscosity ([eta]*) with frequency for neat BIMS rubber (B) and BIMS-hydrocarbon resin blends (BE10, BE20 and BE30) at 40[degrees]C and 7% strain. At lower frequency, the addition of hydrocarbon resin causes a reduction in both the shear modulus (G') and the complex viscosity ([eta]*) of the rubber-resin mixtures relative to those of the neat BIMS rubber. At the same time, the [G'] values of the samples BE30 and BE20 exceeds those of the neat BIMS rubber in the frequency range of 1.2 Hz and 1.5 Hz (shorter time), respectively. But the storage modulus values of the sample, BE10, do not exceed those of the next BIMS rubber in the same frequency range. However, on extrapolating the storage modulus curves of the samples B and BE10 toward higher frequency range (to the frequencies range which cannot be operated in RPA at 7% applied strain), it can be seen that the [G'] values of the sampel BE10 also tend to exceed over the G' value of the neat BIMS rubber at around 1.7 Hz. Aubrey and Sheriff (8), (9) have observed a similar trend in the frequency sweep master curves (obtained from dynamic mechanial analyzer) for the blends of compatible natural rubber with pentaerythritol ester of hydrogenerated rosin tackifier and poly ([beta]-pi-nene) tackifier.

[FIGURE 5 OMITTED]

To understand the viscoelastic behavior of BIMS/hydrocarbon resin mixtures at higher temperatures, the frequency sweep tests were conducted at 75, 100, and 150[degrees]C also. In all the cases, [G'] and [[eta]*] values of the resin filled samples are lower than those of the unfilled ones at all the measured frequencies (not shown here). Sample B shows the highest G' and [[eta]*] values. This behavior is perhaps due to the softening of the hydrocarbon resin in the BIMS rubber matrix at higher temperatures (softening point of the hydrocarbon resin tackifier is between 96 and 104[degrees]C), which leads to the drastic reduction in the viscosity and the shear modulus of the rubber-resin mixtures.

Figure 5b and c shows the double logarithmic plot of variation in storage modulus (G') and complex viscosity ([eta]*) with frequency for neat BIMS rubber (B) and BIMS-phenolic resin blends (BSP10, BSP20, and BSP30) at 40 and 75[degrees]C, respectively. At 40[degrees]C (Fig. 5b), [G'] and [[eta]*] values of the resin loaded samples are higher than those of the neat BIMS rubber at all the measured frequencies contrary to what was observed for compatible BIMS/hydrocarbon resin blends. The [G'] and [[eta]*] values of the resin loaded samples increase with the increase in loading of the phenolic resin. This behavior can be attributed to the mild reinforcing action of the incompatible phenolic resin in the BIMS rubber matrix at 40[degrees]C. Also, any incompatible tackifier in uncured elastomer will remain in the highly phase-separated stage at low temperature (well below the softening point of the tackifier), and the hard resin will fucntion as reinforcement up to the softening point of the tackifier (27). However, above the softening point, the incompatible tackifier will soften, and the reinforcing effect of the incompatible tackifier will disappear.

In line with this, at 5[degrees]C (temperature close to the softening point of the incompitable phenolic resin), in the low frequency range, the G' and [[eta]*] values of resin loaded samples are lower than those of the neat BIMS rubber. At a higher rate (shorter time), the G' values of the samples BSP30 and BSP20 exceed those of the neat BIMS rubber above a frequency range of 0.6 Hz (fig. 5c). To understand the viscoelastic behavior of BIMS/phenolic resing mixtures at higher temperatures, the frequency swept tests were carried at 100 and 150[degrees]C also. In all the cases, [G'] and [[eta]*] values of resin filled samples are lower than those of the unfilled ones at all the measured frequencies (not shown here). [G'] and [[eta]*] values decrease with the increase in loading of the resin. This behavior is perhaps due to the softening of the phenlic resin in the BIMS rubber matrix at 100 and 150[degrees]C, which leads to the reduction in the viscosity and the shear modulus of the rubber-resin mixtures due to plasticization.

Cole-Cole diagrams are extensively utilized to investigate the structure of polymers and blends (28), (29). It is also well-known that the representation of dynamic modulus data (G" vs. G') in a Cole-Cole plot with linear scale axes gives information about the compatibility in multiphase blends. It is assumed that when a blend is miscible or compatible, the corresponding Cole-Cole plot gives an almost semicricular curve/arc and if the system is immiscible, it will result in a modified semicircle. In this work, Cole-Cole plots have been constructed by plotting dynamic loss modulus (G") against dynamic storage modulus (G') with linear scale axes for net at BIMS rubber and BIMS/tackifier blends at different temperatures.

At 75[degrees]C, the Cole-Cole plots of the neat BIMS rubber and BIMNs/hydrocarbon resn blend (Fig. 6a) give a semicircular curve and the Cole-Cole plot of the neat BIMS rubber depresses and shifts left with the increase in loading of the hydrocarbon resin tackifier. On the other hand, the Cole-Cole plots of the sample BSP10 (given as inset of Fig. 6a) at 75[degrees]C shows a semicircular curve pattern with its height lower than the height of the neat BIMS rubber. Furthermore, the Cole-Cole plot of the neat BIMS rubber is shifted left by the addition of the 10 phr phenolic resin tackifier. With the increase in the phenolic resin concentration (BSP20) and BSP30), the semicircular pattern of the curves disappears and the height of the Cole-Cole plot increased with respect to the samples B and BSP10. This suggests the existensce of the hymogeneity/viscouse flow behavior in the BIMS/phenolic resing blend only at 10 phr of the phenolic resing tackifier concentration. However, at 150[degrees]C, the Cole-Cole plots of the neat BIMS rubber and BIMS/tackifier blends (Fig. 6b) show a perfect semicircular pattern, and the Cole-Cole plot of the neat BIMS rubber depresses and shifts left with the increase in loading of the tackifiers. These suggest the existence of good homogeniety/viscous flow behavior in both the blends at 150[degrees]C.

[FIGURE 6 OMITTED]

Because hydrocarbon resin exhibits compatibility with BMS rubber, the Cole-Cole plots of the BIMS/ hydrocarbon resin blend show good semicircular curves even at 75[degrees]C. On the other hand, the Cole-Cole plots of the incompatible BIMS/phenolic resin blend show good semicircular curves only at 150[degrees]C. This suggests the enhancement of the level of compatibility and plasticization between the BIMS rubber and phenolic resin tackifier only at 150[degrees]C (at temperature above the phenolic resing tackifier softening point).

Generally, the diameter of the semi-circular Cole-Cole plot can be related to the plateau modulus of the system (29). Any compatible tackifier will act as a diluent in the rubbery plateau zone and will cause a reductioning the plateau modulus of the base polumer by increasing the viscous flow behavior of the blend (14-16). Here, at 75[degrees]C (Fig. 6a), the blend containing hydrocarbon resin tackifier exhibits smaller semi-circle diameters than those of the neat BIMS rubber and BIMS/phenolic resin blend, indicating a substantial decrease in the plateau modulus of the neat BIMS rubber by the addition of the hydrocarbon resin tackifier. This clearly confirms the good viscous flow behavior of the compatible BIMS/hydrocarbon resin blend. But at 150[degrees]C (Fig. 6b), the semicircular diameter of the both the blends are almost similar and lower than that of the neat BIMS rubber. This is perhaps due to the increase in the viscous flow behavior of the BIMS/tackifier blends at the temperature far above the softening point of the tackifiers.

Rheological Behavior of Neat BIMS Rubber and BIMS/Tackifier Blends

The complex viscosity at various frequencies was recorded for the neat BIMS rubber and the BIMS/tackifier blends at three different temperatures (40, 100, and 150[degrees]C). The complex viscosity of the neat BIMS rubber and BIMS/tackifiter blends decreases with increasing frequency (which is a measure of shear rate), showing the pseudo-plastic nature of the systems studied at all the test temperatures. Both the neat BIMS rubber and BIMS/tackifier blends follow the power law model (shear dependency) model.

[[eta]*] = k[([omega]).sup.n - 1] (1)

The power law coefficients, k (consistency index) and n (flow behavior index) for different compositions at different temperatures obtained from the linear fit of log-log plot of complex viscosity versus frequency are given in Table 2. The variations between the estimated standard deviation (SD) values (between the experimental points and the best fit line) from the linear fit of log-log plot of complex viscosity versus frequency for the entire system lie well within [+ or -]1%. At 40[degrees]C, the incompatible BIMS/phenolic resing blend compositions show the highest k values, and the k value increases with an increase in loading of the phenolic resin. The compatible BIMS/hydrocarbon resin blend compositions display the lowest k values, and the k value decreases with the increase in loading of the hydrocarbon resin. These clearly confirm the increase in the resistance to flow of the BIMS rubber molecules by the addition of the phenolic resin tackifier and the decrease in the resistance to flow of the BIMS rubber molecules by the addition of the hydrocarbon resin tackifier.

However, with the increase in temperature to 100 and 150[degrees]C, the k value of all the compositions continuously decreases. This is quite obvious because at high temperatures (above the softening point of the tackifiers), the tackifiers will start to soften and will drastically reduce the viscosity of the base polymer irrespective of its compatibility with the base polymer.

The variation in the flow behavior index, n, shows the pseudo-plastic behavior of the neat BIMS rubber and BIMS/tackifier blends at all the temperatures. At 40[degrees]C, the n value of neat BIMS gradually increases with the increase in loading of the hydrocarbon resin tackifier. This again confirms the good viscous flow behavior of the compatible BIMS/hydrocarbon resin blend. However, at 40[degrees]c the n values of the incompatible BIMS/phenolic resin blend are very close to the n value of the neat BIMS rubber. This suggests the poor viscous flow behavior of the BIMS/phenolic resin blend. However, at higher temperatures, the n value of all the compositions increases quite well, which suggests the reduction in the resistance to flow at higher temperatures.
TABLE 2. Flow behavior index, n and consistency index, k for neat BIMS
rubber and BIMS/tackfier blends.

 n (Flow behavior index)

Sample Sample 40[degrees]C 100[degrees]C 150[degrees]C
 no. designation

 1. B 0.08 0.15 0.27
 2. BE10 0.10 0.16 0.29
 3. BE20 0.15 0.18 0.30
 4. BE30 0.18 0.19 0.33
 5. BSP10 0.08 0.18 0.29
 6. BSP20 0.09 0.19 0.32
 7. BSP30 0.09 0.21 0.39

 k X [10.sup.3] (Consistency index (Pa [s.sup.n])

Sample Sample 40[degrees]C 100[degrees]C 150[degrees]C
 no. designation

 1. B 34.0 25.9 13.9
 2. BE10 29.8 19.5 11.1
 3. BE20 27.9 18.2 8.5
 4. BE30 27.3 15.4 6.3
 5. BSP10 46.6 21.2 11.6
 6. BSP20 50.2 15.9 8.1
 7. BSP30 73.8 13.4 4.0


The activation enegies for flow ([E.sub.f]) of the neat BIMS rubber and the BIMS/tackifier blends have been measured from the slope of the natural logarithmic plot of complex viscosit ([eta]*) at different frequencies against the reciprocal of the absolute temperature (T) according to the Arrhenius equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where, A is Arrhenius constant and R is the universal gas constant. The activation energy values of B, BE30, and BSP30 at four different frequencies are plotted in Fig.7. The activation energy values of all the samples decrease with the increase in the frequency. The activation energy of BSP30 is higher than that of B and BE30 at all the frequencies. But BE30 shows activation energy very close to the activation energy of the neat BIMS rubber at all the frequencies. Because hydrocarbon resin exhibits very good compatibility with BIMS rubber, the activation energy of the BIMS/hydrocarbon resin blend appears similar to the neat BIMS rubber. Kamagata et al. (30) have reported a similar result for the compatible natural rubber-pentaerythritol ester tackifier blend. The highest activation energy of the incompatible BIMS/phenolic resing blend can be justified by the mild reinforcing action of the phenolic resin tackifier in the BIMS rubber matrix.

[FIGURE 7 OMITTED]

Stress Relaxation Studies

The stress relaxation experiments provide information on the dynamics of the relaxation processes. Generally, the RPA stress relaxation curve has two zones, one is the plateau zone and another is terminal zone. This plateau results because more time is required for the elastomers entanglement network to disentagle. When enough time is passed, the rubber molecule chains start to disentangle with longer chain requiring a longer time to disentangle. As more and more chains relax, the connection of unrelaxed chains becomes less and it promotes a further drop in modulus in what is called as terminal zone (18).

Here, experiments were conducted at a single value of strain amplitude (70%), and the corresponding time dependent relaxation modulus, G (t), was measured. Fig. 8a-c is the log-log plot of stress relaxation decay versus time for the samples B, BE10, BE20, BE30, BSP10, BSP20, and BSP30 at 40[degrees]C.

[FIGURE 8 OMITTED]

The plateau zone is evident for the neat BIMS rubber and the BIMS/hydrocarbon resin blend (Fig. 8a-c). On the other hand, for BIMS/phenolic resin blend, the plateau zone is seen only for the sample BSP10 (Fig. 8a). At 20 phr and 30phr of phenolic resin concentration, the rubbery plateau almost disappears (Fig. 8b and c). Because the viscosity of teh BIMS/hydrocrbon resin blend is lower than the viscosity of its filled and unfilled counter-part, the stress relaxation decay in the terminal zone is much faster for the BIMS/hydrocarbon resin blend when compared with those of the neat BIMS rubber and the BIMS/phenolic resin blend. Here, the compatible hydrocarbon resin tackifier acts as a diluent and makes the entaglement weaker, which results in the faster stress relaxation decay in the terminal zone.

On the other hand, the stress relaxation decay is very slow for the incompatible BIMS/phenolic resin mixtures (in the terminal zone). The stress relaxation decay is further delayed with the increase in loading of the phenolic resin concentration. This behavior can be rationalized by the explanation given earlier in the section on the strain sweep of the BIMS/phenolic resin blend that the phase separated phenolic resin particles will act as an extra topological constraint for the entangled rubber molecule to disentangle. This can increase the time required for the entangled rubber networks to disentangle in the plateau zone and will finally delay the stress relaxation decay in the terminal zone also.

However, at 100[degrees]C, in the terminal zone, the stress relaxation decay of the compatible BIMS/hydrocarbon resin blend and the incompatible BIMS/phenolic resin blend are similar and marginally faster than the stress relaxation decay of the neat BIMS rubber (not shown here). This is perhaps due to the plasticizing effect of the tackifiers in the BIMS rubber matrix at 100[degrees]C.

CONCLUSIONS

The viscoelastic properties of BIMS rubber/tackifier blends have been investigated using DMA and RPA. BIMS rubber and hydrocarbon resin show good compatibility with each other. On the other hand, BIMS/phenolic resin mixture exhibits very limited compatibility with each other. The SEM photomicrograph of the compatible BIMS/hydro-carbon resin blend shows some controlled migration of the hydrocarbon resin tackifier to the rubber surface. On the other hand, the SEM photograph of the incompatible BIMS/phenolic resin blend shows excess phase separation of phenolic resin on the rubber surface. The strain sweep studies of BIMS/tackifier blends explain the good viscous flow behavior of BIMS/hydrocarbon resin mixtures at 40 and 100[degrees]C. On the other hand, strain sweep tests of incompatible BIMS/phenolic resin blends show the formation of resin--resin network at 40[degrees]C. In the frequency sweep studies, the compatible hydrocarbon resin tackifier tunes the viscoelastic behavior of the BIMS rubber by reducing the modulus at the lower frequency and by increasing the modulus at the higher frequencies at 40[degrees]C. However, the incompatible phenolic resin tackifier shows this type of behavior only at 75[degrees]C. This mode of action of tackinfier resins seems to primarily depend on the compatibility level of the tackifier resins with BIMS rubber at a particular test temperature. Cole-Cole plots at temperatures below the softening point of the tackifiers elucidate good compatibility and good viscouse flow behavior of the BIMS/hydrocarbon resin blend relative to that of the neat BIMS rubber and BIMS/phenolic resin blend. Cole-Cole plots at temperature above the softening point of the tackifiers suggest the existence of good viscous flow behavior and plasticization in both the blends in comparison with the neat BIMS rubber. In the stress relaxation studies, at 40[degrees]C, the compatible BIMS/hydrocar-bon resin blends show faster stress relaxation decay in the terminal zone owing to the reduction of the rubbery entanglement by the addition of the hydrocarbon resin tackifier. But, very slow stress relaxation decay in the terminal zone is observed for incompatible BIMS/phenolic resin tackifier blend due to the aggregated resin particles acting as a constraint for the rubber molecules to disentangle. At 100[degrees]C, due to the plasticizing effect of the tackifiers, the stress relaxation decay for both the blends are very similar and faster than that of the neat BIMS rubber.

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K. Dinesh Kumar, (1) Sanjiv Gupta, (2) B.B. Sharma, (2) Andy H. Tsou, (3) Anil K. Bhowmick (1)

(1) Rubber Technology Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

(2) ExxonMobil Company India Private Limited, Bangalore, Karnataka 560066, India

(3) Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, New Jersey 08801

Correspondence to: Anil K. Bhowmick; e-mail: anilkb@rtc.iitkgp.ernet.in

Contract grant sponsors: ExxonMobil Chemical, USA; ExxonMobil Chemical, India.

DOI 10.1002/pen.21195

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Kumar, K. Dinesh; Gupta, Sanjiv; Sharma, B.B.; Tsou, Andy H.; Bhowmick, Anil K.
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:1USA
Date:Dec 1, 2008
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