Blends of brominated EPDM and NR.

Blending two or more rubbers is a useful method to improve certain properties not inherent in a single rubber. By incorporating a suitable amount of EPDM into a diene rubber, for example, significant improvement in heat and ozone resistance was obtained (refs. 1-6). Unfortunately, other physical properties of the blend are often poorer than those of either rubber alone (ref. 7), which suggests that the properties are much related to the miscibility and co-curing behavior of the blended rubbers.

NR/EPDM blend has been found to be immiscible (ref. 8). The absence of comparable cure rate has been highlighted to be the cause of poor tensile strength and high hysteresis in an EPDM/diene rubber blend (ref. 9). Apparently, the EPDM phase was not well cured due to its lower unsaturation than diene rubbers and the diene rubber cured much faster than EPDM resulting in an incompatible blend. Other studies have described that in blend of two rubbers differing in polarity, typical accelerators of sulfur vulcanization migrate to the more polar rubber (refs. 1, 10-11). As a result, the less polar rubber in the blend was undercured.

To improve these properties, co-curing or establishing equivalent cure rate is desirable and a few methods have been reported (refs. 9, 12-16). Grafting of accelerators onto EPDM has been reported to provide excellent cure between EPDM and NR (ref. 9). Modified halogenated EPM has been found to have good cure compatibility with highly unsaturated rubbers (refs. 13 and 16). The enhancement of cure rate could also be obtained by providing reactive sites onto EPDM so that crosslinking other than sulfur vulcanization can occur (ref. 14). Carboxylated EPDM and NR blend has improved compatibility and mechanical properties (ref. 15).

We preliminarily have found that by brominating EPDM at 2.4% by weight, the resulting blends with natural rubber exhibit good overall properties (ref. 17). In this article, we describe the curing behavior and the vulcanizates properties of EPDM/NR and brominated EPDM(BEPDM)/NR blends. The adhesive strength of the NR/BEPDM blend to NR substrate was studied, and the NR/BEPDM blend was investigated as a passenger car tire side wall to demonstrate an example of the applications of this blend.

Materials

The rubbers used in this study are natural rubber (SMR-L) and a high ENB content EPDM. Bromination of the EPDM was carried out in a three liter flask, in which was placed 1,200 ml of carbon tetrachloride and 60 g of EPDM. After stirring for one day at room temperature, 2 vol % solution of bromine in carbon tetrachloride was added. Then, the mixture was stirred again for one hour at room temperature. The brominated EPDM was coagulated in methanol, dried and reprecipitated from toluene/methanol to remove traces of carbon tetrachloride and bromine, and finally freeze-dried in a vacuum oven for three days. The dried BEPDM was stored in a dark place at room temperature until ready to be used.

Compounding, curing and measurements

The mixing of the rubbers and curatives was carried out on an open two roll (six inches) mill at room temperature. The mixes were cured by compression molding for 20 min. at 150 [degrees] C. The actual batch weights of the ingredients for the stocks are given in table 1. The curing behavior of the samples was determined with a Curelastometer-II at 150 [degrees] C.

[TABULAR DATA OMITTED]

Standard ASTM dumbell specimens were used to obtain the stress-strain curves at a strain rate of 100 mm/min. at room temperature on a tensile tester. Rupture surfaces of the dumbbell specimens after tensile test were examined by a scanning electron microscope (SEM). Rubber to rubber adhesion force is evaluated from a peeling test. Samples for the peeling test were prepared by sheeting out each rubber (width, 6 cm; length, 13.5 cm; thickness, 5 mm) against natural rubber and cured in a press for 20 min. at 150 [degrees] C. Strips of 2 cm wide were then cut from the pads. The strips were pulled apart in a 180 [degrees] peel test at a rate of 50 mm/min. at room temperature.

The thermal properties of the vulcanizates were measured by differential scanning calorimetry (DSC) and thermogravimetry (TG). The dynamic viscoelastic properties were obtained using a Rheospectoler at a frequency of 10 Hz and a heating rate of 2 [degrees] C/min. The testing of the tire was conducted according to normal tire testing procedure. The tests include the determinations of spring constant, rolling resistance, cornering force and mes cut properties.

Characterization of BEPDM

Under the reaction conditions described, the degree of bromination was found to be 2.4% by weight. The results by elemental analysis of reprecipitated samples were: Bromine content (wt. %) of repetition #1 - 2.56; #2 - 2.43; #3 - 2.43. The bromine content detected became almost constant after two or three times reprecipitation. The mechanism of bromination is beyond the scope of this article but it is assumed that the bromination occurs at the double bonds.

The observed glass transition temperature (Tg) as determined by DSC was - 45 [degrees] C both for EPDM and 2.4% BEPDM. This result suggests that the bromination of EPDM hardly affected the segmental motion of the rubber, because the degree of modification was low enough. Figure 1 shows E' and tan [delta] for the EPDM and BEPDM vulcanizates. Both are very much the same showing a glass transition temperature (Tg) at 38 [degrees] C and only a slight difference at the rubbery plateau region. Apparently, bromination at this low level did not affect the Tg and the viscoelastic behaviors of the rubber. However, from the TG curves in figure 2, the BEPDM showed a shift to a higher temperature at the 5% weight loss, indicating improved heat resistance of the brominated EPDM. The figure also suggests that the NR/BEPDM blend has a better heat resistance than the NR/EPDM blend

Properties of EPDM and BEPDM NR blends

Dynamic mechanical properties

The dynamic mechanical properties of the NR/EPDM and NR/BEPDM blends at 50:50 ratio are shown in figure 3. For either blends, E' was almost the same, showing a decrease of 103 dyn/[cm.sup.2] from -60 [degrees] C to -30 [degrees] C. The tan [delta] for NR/BEPDM blend is only shifted slightly to higher temperatures, showing a peak at -55 [degrees] C. The broadening of the tan [delta] peaks in the blends compared to the unblended rubbers is due to the relatively low glass-transition temperature of NR. Similar to the observation on the unblended EPDM, the present bromination did not have much influence on the dynamic mechanical properties of the blended rubber, which clearly indicates that the miscibility of NR and EPDM is not affected by the bromination.

Tensile properties and adhesive strength

The results of the tensile property measurement for recipes 1-10 are represented in bar charts shown in figures 4 and 5. Table 3 shows the tensile properties of recipes 9 and 10. It should be noted that the curatives for recipes 1-10 are typical and suitable for EPDM while those of recipes 9 and 10 are typical and suitable for NR. Therefore, by analyzing both type of recipes, the effect of the bromination on the tensile properties of NR/BEPDM blends could really be identified. ever, in the case of 9 and 10, carbon black is included as reinforcement. From the data obtained for recipes 1-10, it can be seen that NR/BEPDM exhibited considerably higher tensile strength (Tb) and elongation at break (Eb) than NR/EPDM at the 50:50, 25/75 and 0:100 compositions. For the carbon black reinforced recipes, considerable improvement in modulus and Eb a significantly higher Tb are observed on the NR/BEPDM blend. Since the dynamic mechanical analysis has indicated that the degree of miscibility is not affected by the bromination, the reason for the higher tensile strength of the NR/BEPDM is thought to be due to the migration of curatives from NR to the more polar BEPDM resulting in a much better co-curing of the NR/ BEPDM blend. A significant difference in curing behavior between EPDM and NR/EPDM blends and their brominated versions would be an indication that better co-curing is taking place resulting in improved tensile properties (see discussion on curing behavior).

The SEM photographs in figure 6 show that the rupture surfaces of the NR/BEPDM are much rougher suggesting better co-curing in accord with the above mentioned assumption. This assumption is also supported by and consistent with the results of the peeling test which are presented in figures 7 and 8. BEPDM and NR/BEPDM blend showed a much higher adhesive strength to NR substrate than EPDM and NR/EPDM blend. From the figures, it can be seen that the bromination of EPDM increases the adhesive strength of the rubber to NR substrate by about two times. This better adhesion is very advantageous when preparing rubber composites containing NR.

Curing behaviors

Figure 9 shows the curing behavior of EPDM and BEPDM. BEPDM has a significantly shorter induction cure time than EPDM. Apparently, the bromine played a role in the curing of BEPDM. The curing behavior of the blends studied are shown in figures 10 and 11. For the NR/EPDM blends, as expected, the induction cure time increases with increased EPDM amount. As for the NR/BEPDM, the increases of BEPDM amount has an opposite effect. The curing reaction may be described by the equation

dQ/dt=[k(Q[infinity]-Q).sup.n] (1)

where Q is torque, t is cure time, k is rate constant, and n is the order of the reaction. Q[infinity] is determined by extrapolating the dQ/dt vs. Q plot. Integration of equation (1) gives

In (Q[infinity]-Q)=kt+1nQ[infinity] (2)

for a first-order reaction and

[(Q[infinity]-Q).sup.-1]=kt-[Q[infinity].sup.-1 (3)

for a second-order reaction. In our case, the plot of [(Q[infinity]-Q).sup.-1] vs. t yielded a straight line and the plot according to equation (2) did not give a straight line, suggesting a second-order reaction. k's for four different temperatures (150, 160, 170 and 180 [degrees] C) were determined according to the Arrhenius equation

k=A exp(-Ea/RT) (4)

where A is frequency factor, Ea is activation energy, R is gas constant and T is absolute temperature. The curing activation energy of EPDM and BEPDM were calculated to be 145.3 kJ/mol and 58.4 kJ/mol, respectively. It is noticeable that the energy barrier for BEPDM vulcanization is much smaller than that for EPDM.

The relatively low energy barrier and short cure induction time are indicative of a better co-curing of the NR/BEPDM blends. It is apparent that bromination has made the rubber more suitable for sulfur curing recipes. The findings in this study are consistent with a study on the sulfur cure of brominated EPM (ref. 16). In the latter, it was reported that the cure time of the rubber and the heat resistance of the vulcanizates were both found improved. The improvement was attributed to the role of bromine in the formation of C-C and C-O-C bonds during the vulcanization process. In our case, the role of bromine is to be further investigated.

NR/BEPDM blend as tire side wall

Recipe 10 in table 1 was tested against NR/BR blend using the same recipe with respect to blending ratio and the amount of curatives and reinforcement. The NR/BR based recipe is actually a typical commercial recipe for a tire side wall used in the tire industry. The two compounds were used as a tire side wall to fabricate the tires which were subjected to a tire performance evaluation. The size of the tire used for testing was 135R12S. Table 2 shows that while the hardness and modulus are similar, the NR/BEPDM based recipe has higher tensile strength and elongation at break. It should be noted that the tensile properties of recipe 10 (table 1) shown in table 3 are not quite the same as those of the same recipe shown in table 2. This is because the results of table 2 are obtained from the measurement of test pieces obtained from the tire which has been subjected to varying conditions and temperatures during the tire fabrication process. The results of the fatigue to failure test for the two vulcanizates are shown in figure 12, which indicates that the tested compound has a better fatigue resistance than the NR/BR blend. Table 4 shows the results of the tire performance evaluation. The results of the rolling resistance, spring constants and cornering force tests were very comparable. From the mes cut test after 61 hours at 81 km/h, no crack growth was observed for both the commercial and tested formulation. These results indicated that NR/BEPDM blend is suitable to be used as a passenger car tire side wall.

Table 2 - physical properties of NR/EPDM and
NR/BEPDM blends

                          NR/EPDM   NR/BEPDM

50% mod. (MPa)              0.9       1.0
100% mod. (MPa)             1.7       1.8
200% mod. (MPa)             3.6       4.4
300% mod. (MPa)             6.3       7.9
Tensile strength (MPa)     11.3      16.9
Elongation (%)              445       490
Table 3 - physical properties of tire side wall

                          Commercial   Test
                          (NR/BR)      (NR/BEPDM)

Hardness (Shore-A)           56        55
100% mod. (MPa)             1.7       1.4
200% mod. (MPa)             3.3       3.0
300% mod. (MPa)             5.9       5.5
Tensile strength (MPa)     15.3      17.1
Elongation (%)              550       660
Table 4 - tan [delta] and performance evaluation test results of
tire side wall

                              Commercial        Test
                              (NB/BR)           (NR/BEPDM)

Loss tangent (tan [delta])
 vs temp.                     0.1129 - 0.1420   0.1172 - 0.1351
 (0 - 60 [degrees] C
Rolling resistance (N)        25.5 - 56.9       25.9 - 56.6
 (speed 10 - 140 km/h)
Spring constants
 vertical (kgf/mm)            13.4              13.8
 lateral (kgf/mm)             5.6               5.7
 distorsion (kgf.cm/deg)      222               221
Cornering force (kgf)
 at 1 [degree] angle
 (40 - 100% kg load)          17.0 - 38.0       24.5 - 37.5
 at 4 [degrees] angle
 (40 - 100% kg load)          91.0 -144.5       87.0 - 147.5
Mess cut [test.sup.a]         zero crack        zero crack
                               growth            growth

a) Test conditions: Initial cut dimension; width 5 mm, depth 1.5
mm. Air impression
1.8 kg/[cm.sup.2], speed 81 km/hr. Running distance 5000.13 km
(61.44 hrs.)

Conclusions

In this study, it was observed that the bromination of EPDM has resulted in NR/BEPDM blends with better heat resistance and tensile properties when cured using typical sulfur curing recipes. These improvement could be attributed to a significant change of curing behavior observed on the BEPDM and NR/BEPDM blends. Since the miscibility of the NR/BEPDM blends was not affected by the bromination, the results suggest that the positive improvement in curing behavior was brought about by the migration of curatives from NR to the more polar BEPDM resulting in a better co-curing of the blend.

References

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