Ultrasonically treated isoprene rubber.

Synthetic isoprene rubber (IR) is mostly utilized in the tire industry in combination with or instead of natural rubber (NR) (ref. 1). It is the artificial equivalent of NR, since they both share the same basic repeat unit--cis 1, 4-isoprene. The most critical difference between these two rubbers is that NR consists almost exclusively of cis 1,4-isoprene units (~99%) and a small portion of non-rubber components such as protein, carbohydrates, amino acid, fatty acid and other substances (ref. 2); while IR contains a lower content of cis 1,4-isoprene units than NR and it is a 100% pure chemical product. Different polymerization methods lead to IRs with different cis 1,4-isoprene contents (ref. 3). These differences in concentration contribute to the large difference in the rate and the degree of crystallization and mechanical properties. Compared with its natural counterpart, IR is inferior in mechanical strength, anti-aging and crystallization. However, it exceeds NR in the consistency of product, uniformity of cure rate, better processing (mixing, extrusion, molding and calendering) and purity. Particularly, IR does not undergo storage hardening (ref. 2). Products made from NR or IR are less likely than most other elastomers to fail from excessive heat buildup or fatigue when exposed to severe dynamic conditions. This has secured the place of NR or IR as the preferred sidewall elastomer in radial tires (ref. 4).

The degradation of polymers in solutions due to prolonged ultrasonic treatment has a long history. As early as 1933, Szalay (ref. 5) described how ultrasonic waves depolymerized starch, gum Arabic and gelatin, as measured by a reduction in viscosity. Schmid and Rommel (refs. 6-8) also investigated the decrease in viscosity in synthetic polymers such as poly (acrylic acid), poly (vinyl acetate) and nitrocellulose. Besides the observation of the reduction in viscosity, they found that the depolymerization was initially rapid, but soon slowed down and eventually ceased when a minimum molecular weight was reached. The chain cleavage due to the ultrasound exposure has several aspects that differentiate it from the thermal or photochemical processes, and these may be regarded as the characteristic of the ultrasound method. It proceeds faster at high molecular weights and slows down until, at some limiting value, the degradation ceases. Additionally, unique to ultrasonic degradation is the preferable cleavage of the polymer chain near the center of the macromolecules (refs. 9 and 10). The mechanism of ultrasonic depolymerization is now thought to be closely linked to the cavitation phenomenon. The cavitation can produce sufficiently high local pressure and temperature (refs. 11 and 12) to induce homolytic breakage of the macromolecular chains to form macroradicals. The evidence of the presence of these radicals due to ultrasonic exposure was obtained by carrying out the sonication in the presence of unsaturated, polymerizable monomers by the trapping of the radicals using radical scavengers such as DPPH ([alpha],[alpha]'diphenyl picryl hydrazyl).

High power ultrasound has found wide applications through out polymer chemistry. For instance, the radicals produced during sonication have been shown to initiate polymerization in a second unsaturated monomer within a wide range of choices. It has been used in the production of copolymers from two or more different monomers (ref. 13), and in copolymerization during melt processing of immiscible blends (ref. 14). In addition to the degradation and copolymerization, ultrasound can also be applied to the devulcanization of various types of rubbers such as ground rubber tire (GRT) (refs. 15 and 16), natural rubber (refs. 17 and 18), silicone rubber (refs. 19 and 20), styrene butadiene robber (SBR) (ref. 21) and polyurethane rubber (PU) (ref. 22). This technique provides a rapid breakage of the three-dimensional rubber network within the time of several seconds, and it is a continuous process without the involvement of any chemicals. The rubber vulcanizates treated with high power ultrasound are soft, moldable, and can be reshaped and revulcanized similarly to the virgin rubbers. These advantages make the ultrasound technique unique and attractive to the rubber recycling industry. Undoubtedly, the application of ultrasound is a safer process, since it avoids the high temperatures and pressures of thermal methods.

Recently, ultrasonic treatment was applied to several unfilled gum rubbers, such as ethylene propylene diene monomer (EPDM) rubber (ref. 23), butadiene rubber (BR) (ref. 24) and butyl rubber (ref. 25). Their structure and properties after the treatment were investigated, and the results showed that different elastomers had different responses alter ultrasound exposure. For example, ultrasonic treatment of BR and EPDM led to measurable gel formation, with the gel fraction dependent on the ultrasonic amplitude. In contrast, the treatment of butyl rubber did not produce any gel (ref. 25).

In this article, the effect of high power ultrasound on the structure and properties of IR gum is investigated and compared with that of virgin IR gum. Finally, this research explores a new way of controlling the structure and properties of elastomers and improving their processability via high power ultrasonic treatment.

Experimental

Materials

IR (Natsyn 2200, cis 1,4-isoprene: 98.0%) was obtained from Goodyear Tire and Rubber with the number-average molecular weight Mn = 810,000 and the weight-average molecular weight Mw = 2,490,000 (provided by Goodyear and measured by thermal field flow fractionation--ThFFF). The compounding recipe included 100 phr rubber, 2 phr sulfur, 1 phr N-cyclohexylbenzothiazole-2-sulphenamide (CBS), 5 phr zinc oxide (ZnO) and 1 phr stearic acid. Except for IR, all the other ingredients were obtained from Akrochem. The benzene used for extraction experiments and the tetrahydrofuran (THF) used for GPC were both obtained from Aldrich Chemical.

Ultrasonic reactor

The ultrasonic treatment coaxial reactor, consisting of a 38.1 mm single screw rubber extruder with a L/D = 11 and a coaxial ultrasonic die attachment, was used in the experiment. A cone-tipped ultrasound horn of 76.2 mm diameter is mounted coaxially to the extruder die to provide a uniform gap in the treatment zone. The schematic drawing of the reactor was given in reference 15. The barrel has three temperature zones equipped with electrical heaters and fans. The ultrasound unit is composed of a 3.0 kW ultrasonic power supply, an acoustic converter, a 1:1 booster and a water-cooled horn. The horn vibrates longitudinally with a frequency of 20 kHz and varying amplitudes ranging from 5 to 10 microns. A flush-mounted thermocouple and a pressure gauge are inserted into the barrel to measure the temperature and the pressure of the rubber at the entrance of the die. The rubber strips were fed into the extruder by a conveyor belt feeder with an adjustable output, passed through the barrel and pushed into the gap between the horn and the die plate, where the rubber was subjected to the longitudinal (compressive) waves perpendicular to the flow direction. Starve feeding to the barrel was applied so that the flow rate of the rubber strips was controlled by the feeding rate. The ultrasonic energy consumed during the experiment was measured by a wattmeter (2,000 bdc 20:3.3, Branson Ultrasonic) attached to the ultrasound unit.

Preparation of the samples

Virgin IR gum strips were conveyed into the reactor for ultrasonic treatment. The barrel temperature and the gap clearance were set at 120[degrees]C and 2.54 mm (0.1 inch), respectively. The amplitudes of the ultrasonic waves were 5, 7.5 and 10 [micro]m. The screw speed and the flow rate were 17 rpm and 0.63 g/s, respectively. The virgin and the treated gum samples were individually compounded with curatives in a two-roll mill (Dependable Rubber Machinery) at room temperature. The nip of the rolls was kept at 2 mm. The total mixing time was 10 minutes. The compounds were then compression molded into slabs (200 mm x 200 mm x 2 mm) at a temperature of 160[degrees]C and at a pressure of 13.8 MPa with a compression-molding press (model 12-12-2T, Wabash Metal Products). The cure time was taken as the time corresponding to the maximum torque measured by an advanced polymer analyzer (APA 2000. Alpha Technologies). The vulcanized samples were used for mechanical testing.

Characterization

Gel fraction of the vulcanized samples was measured by washing out the sol part of these rubbers in a Soxhlet extraction apparatus for 24 hours using benzene as a solvent. Crosslink density of the vulcanized samples was measured by the swelling technique and calculated using the Flory-Rehner equation (ref. 26). The interaction parameter X = 0.42 for IR-benzene system (ref. 27) was used for calculations.

The dynamic rheological behavior of the virgin and ultrasonically treated IRs was investigated using the APA 2000 at the strain amplitude of 4.2% (0.3[degrees]), a frequency sweep ranging from 0.02 to 209 rad/s and varying temperatures of 60, 90 and 120[degrees]C. A bi-conical rotor with an angle of 7.16 and a diameter of 41.26 mm was used.

An Instron T-5567 tensile testing machine was used to measure the stress-strain characteristics of the vulcanizates. Dumbell-shaped specimens (ASTM D-412 Type C) were punched out of the compression molded sheets and tested at room temperature with an extension rate of 500 mm/min.

The molecular weight and its distribution of the virgin and ultrasonically treated IRs at 0, 5, 7.5 and 10 [micro]m were characterized by GPC with a Waters 410 differential refractometer, a Waters 486 tunable absorbance detector, and a Waters 510 HPLC pump. Tetrahydrofuran (THF) solvent was run at room temperature, and conventional calibration was used against the polystyrene standards.

Thermogravimetric analysis (TGA) was performed with a Hi-Res TGA 2050 analyzer (TA Instruments) at a heating rate of 20[degrees]C/min. from room temperature to 600[degrees]C under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was carried out with a DuPont 2100 (TA Instruments) at a heating rate of 10[degrees]C/min. under a nitrogen atmosphere from -100 to 100[degrees]C. The TGA and DSC experiments were carried out on the virgin gum, ultrasonically treated IRs and their respective vulcanizates. Tg readings in the DSC were taken by the inflection point method.

Results and discussion

Die pressure and power consumption

The extrusion of IR gum strips was carried out using the coaxial ultrasonic reactor at 120[degrees]C, a gap of 2.54 mm and varying amplitudes. During this process, the die pressure and power consumption were recorded. Figure 1 shows the die pressure and power consumption as a function of the ultrasonic amplitude. The power consumption increases with the ultrasonic amplitude dramatically. The increase of power consumption is due to an increase of the strain and stress amplitude causing the degradation of the material. This result was supported by the molecular weight and theological measurements shown below. On the other hand, die pressure drops with increasing ultrasonic amplitude. When no ultrasound is applied, the die pressure is as high as 6.2 MPa. However, upon ultrasound exposure, the die pressure is significantly reduced. With the increase of ultrasonic amplitude, the die pressure decreases further, showing linear dependence on the amplitude. This was previously explained as the combined effect of the frictional reduction between the rubber and the die wall and the degradation taking place as rubber entered the ultrasonic treatment zone (ref. 23).

[FIGURE 1 OMITTED]

Curing, gel fraction and crosslink density

After ultrasonic treatment at different amplitudes, the virgin and treated IR gum samples were individually compounded with the same curing recipe. The cure kinetics measurements were carried out on the APA 2000 at 160[degrees]C using a strain amplitude of 4.2%. As shown in figure 2, the initial torques of the treated IRs before the start of crosslinking are lower than that of the virgin IR. Also with the increase of ultrasonic amplitude, the initial torque decreases. This again reveals the rubber degradation in the ultrasonic extrusion. The induction time of the ultrasonically treated IRs is shorter than that of the virgin IR; however, the dependence of induction time on the amplitude is not straightforward. This is in contrast to the behavior of BR, where the induction time of the ultrasonically treated BR was shorter than that of the virgin BR, but decreased with increasing amplitude (ref. 24). It was also found in figure 2 that all the treated samples, as well as the virgin IR, show a reversion at the later stage of cure. This is probably due to the irreversible destruction of the polysulfidic crosslinks (refs. 28 and 29). It seems that at higher amplitude, the reversion is more severe and the final torque is lower. In addition, it is noticed that the maximum torque of the sample treated at 5 [micro]m is slightly higher than that of the sample passing through the extruder without the imposition of the ultrasound (0 [micro]m).

[FIGURE 2 OMITTED]

After vulcanization of the ultrasonically treated IR samples, gel traction and crosslink density were evaluated and the results are shown in figure 3. It is worthwhile to mention that the extraction experiments showed no measurable amount of gel detected in the virgin and ultrasonically treated IR gums. This finding is similar to the behavior of butyl rubber (ref. 25), but different from the observation made on EPDM (ref. 23) and BR (ref. 24), where the gel was produced after their exposure to the ultrasound. The gel fraction of the vulcanizates prepared from the ultrasonically treated IR was slightly lower than that of the virgin IR vulcanizate. This suggests that even though degradation occurs, molecular weight of the ultrasonically treated rubber is still very high, such that a significant amount of double bonds survive, allowing the treated samples to form dense three-dimensional networks. However, these gels are different from the gel of the virgin rubber vulcanizate. They exhibit lower crosslink densities than that of the virgin rubber vulcanizate, as shown in figure 3.

[FIGURE 3 OMITTED]

Molecular characteristics of ultrasonically treated IR gums The molecular characteristics of the ultrasonically treated IR gums were determined by GPC. The results are shown in figure 4. The number ([M.sub.n]) and weight ([M.sub.w]) average molecular weight of the virgin IR is 982,000 and 1,998,000, respectively. These values are somewhat lower than those supplied by the manufacturer and measured by the ThFFF method. In figure 4a, curves of the molecular weight distribution of the ultrasonically treated gum rubbers are shifted to the lower molecular weight compared to that of the virgin IR. It is also found that low molecular weight tails are generated at various amplitudes. With the increase of amplitude, the molecular weight of the tails shifts to a lower value. This indicates the degradation of the rubber main chain upon ultrasonic treatment. It was also observed that the sample passing through the extruder without exposure of ultrasound (0 [micro]m) also shows degradation due to the mastication effect of IR occurring in the two-roll mill and in the extruder. This is in agreement with the earlier report on mastication in a two-roll mill (ref. 30). Furthermore, both the number and weight average molecular weight decrease with increasing amplitude as shown in figure 4b. However, the number average molecular weight decreases more significantly than the weight average value. Accordingly, the polydispersity ([M.sub.w]/[M.sub.n]) increases substantially with increasing ultrasonic amplitude. Obviously, it is caused by the low molecular weight tails generated. Therefore, the ultrasonic treatment of IR rubbers can be beneficial and suggested as a possible means of improving their processability.

[FIGURE 4 OMITTED]

Rheological properties

The rheological properties of the virgin, and ultrasonicany treated, rubber gums and their vulcanizates were evaluated by an APA 2000 at 120[degrees]C at a strain amplitude of 4.2%. The complex viscosity of the gums and vulcanizates is plotted as a function of frequency in figure 5. It is clear that the viscosity of the vulcanizates is substantially higher than that of the gums (untreated or treated) within the whole range of frequency. This is quite reasonable and it is due to the formation of a three-dimensional network hindering the flow of the material. For the treated gums, it is found that the dynamic viscosity decreases with the increase of ultrasonic amplitude. This is another proof of degradation in addition to the reduction of molecular weight. The dynamic viscosity of all the vulcanizates did not show any measurable differences. This is in contrast to the findings in butyl rubber (ref. 25). Different behaviors in the dynamic viscosity of the vulcanizates from the ultrasonically treated IR and butyl rubber are due to the fact that IR contains a significant amount of the double bonds needed for vulcanization, while butyl rubber has a very little amount of the double bonds after the treatment.

[FIGURE 5 OMITTED]

Figure 6 shows the loss angle as a function of frequency for the virgin and ultrasonically treated IR gums, along with their vulcanizates. The loss angle increases as the ultrasonic amplitude increases for the treated gums, indicating that alter the treatment the material exhibits more viscous dissipation. However, the loss angle of the vulcanizates is too small to differentiate among the various vulcanizates. Figure 7 shows the storage modulus as a function of frequency for the virgin and ultrasonically treated IR gums and their vulcanizates at 120[degrees]C. Moduli of the vulcanizates are substantially higher than those of the gums. The plateau region is observed for all of the vulcanizates, indicating complete cure has occurred for all the treated samples. A tendency to the plateau modulus is also observed for the virgin IR. The storage modulus for the gums is dependent on the ultrasonic amplitude, with higher amplitude corresponding to lower modulus. This also indicates the degradation of rubber main chain. However, the storage modulus for the vulcanizates is independent of the ultrasonic amplitude. This suggests that only a slight degradation of the rubber main chain has actually happened.

[FIGURES 6-7 OMITTED]

In order to investigate whether or not branching takes place during the ultrasonic treatment of IR gums, the complex viscosity was determined at three different temperatures for the virgin and ultrasonically treated IR gums. The modified Cross model (ref. 31) was applied to lit the viscosity frequency curve. The following two equations were used for this fitting:

[[eta].sup.*] = [[eta].sup.*.sub.0](T)/1 + [[[[eta].sup.*.sub.0](T)[omega]/[tau]].sup.l-n] (1)

[[eta].sup.*.sub.0](T) = Aexp([T.sub.b]/T]) (2)

with A, [T.sub.b], n and [tau] being the fitting parameters, [[eta].sup.*.sub.0](T) is the zero-frequency viscosity function based on the Arrhenius type of temperature dependence. Figure 8 shows the experimental data (symbols) and the fitted curves for the virgin and the 10 [micro]m IR only. The four fitting parameters (A, Tb, [tau] and n) found by the least-square method for the virgin and treated rubber gums are tabulated in table 1. It is well known that branched polymers usually have a higher activation energy E than the correspondent linear polymers (ref. 32). In terms of [T.sub.b], the value of activation energy is E = [T.sub.b]xR, with R being the gas constant. Table 1 shows that, except for the rubber treated at the amplitude of 5 [micro]m, there is no significant difference in the Tb values among the other treated gums and the virgin IR. Therefore, it is concluded that it is possible that branching occurred only for the sample treated at 5 [micro]m, while the other samples were only subjected to the degradation without any other structural changes (branching or crosslinking) during the ultrasonic treatment.

[FIGURE 8 OMITTED]

Mechanical properties

The stress-strain curves for the vulcanizates of virgin and ultrasonically treated IR at different ultrasonic amplitudes are shown in figure 9. Even though the degradation occurs during the ultrasonic treatment, it is clear that vulcanizates of the treated IR gums, similar to the vulcanizate of the virgin rubber, show a high extent of strain-induced crystallization. This again indicates the destruction of the rubber main chain is minor. It is observed that for the sample treated at the amplitude of 5 [micro]m, the stress-strain slope of the curve is slightly higher than that of the untreated sample just passed through the extruder (0 [micro]m).

[FIGURE 9 OMITTED]

Figure 10 shows the mechanical properties as a function of ultrasonic amplitude for the vulcanizates of virgin and ultrasonically treated IR. Generally, tensile strength, elongation at break and modulus of the vulcanizates decrease with the increase of ultrasonic amplitudes: with more reduction in the sample treated at the amplitude of 10 [micro]m. The reduction is probably due to the degradation of rubber main chain. However, the elongation at break of the treated samples is always higher than that of the virgin vulcanizate.

[FIGURE 10 OMITTED]

Thermal properties

The thermal stability of the untreated and ultrasonically treated IR gums and their vulcanizates was evaluated by the TGA under nitrogen atmosphere. As shown in figure 11, generally there are no noticeable differences of TGA curves for treated and untreated gums, as well as among their different vulcanizates. However, the difference is only evident between the gum rubbers and the vulcanizates: The gums degrade at a slightly lower temperature compared with the vulcanizates. This is reasonable because the vulcanization results in better thermal stability, due to the crosslinking and the reduction of unsaturation in the rubber chain. At the final temperature, the gum rubbers show almost zero weight residues indicating the thermal degradation is complete (100%). The vulcanizates show about 5% weight residue. This exactly corresponds to the amount of ZnO used in the curing recipe.

[FIGURE 11 OMITTED]

Figure 12 shows the DSC curves of the virgin and ultrasonically treated IR gums and their vulcanizates at low temperatures (-90 to -10[degrees]C) to determine the glass transition temperature. From the curves of gums, no significant differences in Tg are observed for both untreated and treated IRs. Also, there are no observable differences in Tg for the various vulcanizates. The only detectable difference is that the Tg values of vulcanizates are 3-4[degrees]C higher than those of gums. The reason is that the chain mobility is reduced, due to the formation of a three-dimensional network, and higher temperature is necessary for the chain segment motion. The Tg values are listed in table 2. The Tg behavior of the ultrasonically treated IR gums is similar to that of butyl rubber, but ill contrast to that of EPDM. There were substantial Tg differences among the treated EPDM gums, namely 4 to 6[degrees]C. The EPDM gum treated at 5 [micro]m amplitude showed higher Tg value compared to the virgin untreated gums (ref. 23). This was due to the measurable amount of gel formed during ultrasound exposure. In addition, the Tg value decreased with the increase of amplitude for EPDM. The increased mobility was explained by the generation of shorter chains at higher ultrasonic amplitude.

[FIGURE 12 OMITTED]

Conclusions

Ultrasonic treatment alters the structure and properties of 1R gum and the change is highly amplitude dependent. The reduction of die pressure with the increase of ultrasonic amplitude causing the increase of power consumption indicates the degradation of IR gum. This is supported by the measurement of molecular weight, dynamic and mechanical properties. In particular, the molecular weight slightly decreases as the amplitude increases. Ultrasound treatment creates low molecular weight tails, which significantly broaden the molecular weight distribution. The complex viscosity, as well as the storage modulus of the treated rubber gums decreases as the amplitude increases. The mechanical properties (tensile strength, modulus, elongation at break) of the treated rubber vulcanizates decrease with the increasing amplitude. However, the elongation at break of the treated IR vulcanizates is higher than that of the virgin vulcanizate. The fitting of the complex viscosity--frequency curves according to the modified Cross model indirectly indicates possible branching of IR gum treated at the ultrasonic amplitude of 5 [micro]m. The cure kinetics of the treated rubber gums are similar to the virgin IR and show reversion. Because of the degradation, the initial and maximum torque of cure curves reduces with increasing amplitude. The vulcanization creates a comparable amount of gel, but a significantly lower crosslink density for the treated rubber gums compared with the virgin rubber. However, the thermal stability and glass transition temperature of the untreated and treated IR gums, as well as their various vulcanizates, show no significant differences. The difference in Tg, as well as the thermal stability, only occurs between the gums and the vulcanizates.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

(1.) F.M. Herman, ed., "Encyclopedia of chemical technology, polyisoprene," vol. 8, 3rd ed., Wiley, New York, NY, pp. 582-592 (1978).

(2.) A. Subramaniam, "Natural rubber," in Tire Vanderbilt Rubber Handbook, 13th ed., Chapter 2, R.F. Ohm, ed., R.T. Vanderbilt Company, Norwalk, CT (1990).

(3.) K.S. Loganathan, "Rubber engineering," Chapter 10, Indian Rubber Institute, McGraw-Hill, New York (2000).

(4.) P.A. Ciullo and N. Hewitt, "The rubber formulary," Noyes Publications/William Andrew Publishing, LLC (1999).

(5.) A. Szalay, Z. Phys. Chem., A164, 234 (1933).

(6.) G. Schmid and O. Rommel, Z. Phys. Chem., 185A, 97 (1939).

(7.) G. Schmid and O. Rommel, Z. Electrochem., 45, 659 (1939).

(8.) G. Schmid, Z. Phys. Chem., 186A, 113 (1940).

(9.) B.M.E. Vander Hoff and C.E. Gall, J. Macromol. Sci., Chem., All, 9, 1,739 (1977).

(10.) B.M.E. Van der Hoff and P.A.R. Glynn, J. Macromol. Sci., Chem., A8, 2, 429 (1974).

(11.) B.E. Noltingk and E.A. Neppiras, Proc. Phys. Soc. London, B64, 1032 (1951).

(12.) A. Henglein, Ultrasonics, 25, 1, 6 (1987).

(13.) M.A. Bradley, S.W. Prescott, H.A.S. Schoonbrood, K. Landfester and F. Grieser, Macromolecules, 38, 15, 6,346 (2005).

(14.) A.I. Isayev and C.K. Hong, Polym Eng. & Sci, 43, 91 (2003).

(15.) A.I. Isayev, J. Uteri and A. Tuchachinsky, Rubber Chem. Technol., 68, 267 (1995).

(16.) J. Yun, J.S. Oh and A.I. Isayev, Rubber Chem. Technol., 74, 317 (2001).

(17.) M. Tapale and A.I. Isayev, J. Appl. Polwn. Sci., 70, 2007 (1998).

(18.) C.K. Hong and A.I. Isayev, J. Appl. Polym. Sci., 79, 2,340 (2001).

(19.) B. Diao, A.I. Isayev and V.Y. Levin, Rubber Chem. Technol., 72, 152 (1999).

(20.) S.E. Shim and A.I. Isayev, Rubber Chem. Technol., 74, 303 (2001).

(21.) V.Y. Levin, S.H. Kim and A.I. Isayev, Rubber Chem. and Technol., 70, 120 (1997).

(22.) S. Ghose and A.I. Isayev, J. Appl. Polym. Sci., 88, 980 (2003)

(23.) J. Yun and A.I. Isayev, Soc. Plast. Eng. Annual Tech. Conf. 2002, 48, 3,212 (2002).

(24.) J.S. Oh, S. Ghose and A.I. Isayev, J. Polym. Sci., Part B: Polym. Phys., 41, 2,959 (2003).

(25.) W. Feng and A.I. Isayev, J. Polym. Sci., Part B: Polym. Phys., 43, 334 (2005).

(26.) P.J. Flory and J. Rehner, Jr., J. Chem. Phys., 11, 512 (1943).

(27.) A. Barton, "CRC handbook of polymer-solvent interaction parameters and solubility parameters," CRC: Boston, Chapter 7 (1990).

(28.) W. Scheele and G. Bielstein, Rubber Chem. Technol., 29, 48 (1956).

(29.) U. Shankar, Rubber Chem. Technol., 25, 241 (1952).

(30.) N. Sombatsompop and C. Kumnuantip, J. Appl. Polym. Sci., 87, 1,723 (2003).

(31.) M.M. Cross, Rheol. Acta., 18, 609 (1979).

(32.) G. V. Vinogradov and A. Y. Malkin, "Rheology of polymers: Viscoelasticity and flow of polymers," Mir: Moscow, 1980.

by Avraam I. Isayev and Ximei Sun, University of Akron 

Table 1--fitting parameters for the
modified Cross model for the virgin
and ultrasonically treated IR

Gum rubbers            A        n      Tb    [tau]
                  (Pa-s)              (K)     (Pa)

Virgin IR         693.72   0.1363   3,094   89,901
IR 0 [micro]m     269.29   0.1610   3,208   71,378
IR 5 [micro]m      14.55   0.1617   4,253   67,905
IR 7.5 [micro]m   171.56   0.1976   3,109   46,533
IR 10 [micro]m    119.50   0.2025   3,192   40,395

Table 2--Tg ([degrees]C) of the virgin and
ultrasonically treated IR gums and
their vulcanizates

Gum rubbers       Uncured   Cured

Virgin IR           -62.2   -59.0
IR 0 [micro]m       -63.0   -58.5
IR 5 [micro]m       -62.3   -58.0
IR 7.5 [micro]m     -63.9   -58.7
IR 10 [micro]m      -63.9   -59.3