A highly efficient cure system for BIMS.

Low functionality elastomers can present advantages in aging, heat and chemical stability provided they can be properly cured. Butylrubber (IIR) which was introduced in the late 1930s was one of the first low functionality synthetic rubbers, incorporating only about 1-2 mole % of potential cure sites. This was a dramatic decrease in functionality from about 100 mole % for natural rubber (NR) and about 80-100 mole % for GPR and BR, presenting a difficult challenge for curatives development. A similar challenge for efficient curing arose when halobutyl polymers (CIIR and BIIR) were introduced during the 1950s and 1960s. These polymers were also low in cure functionality, at only about 1-1.5 mole %. Although satisfactory cures were found for butyl and halo-butyl polymers, they do not consume all the functionality available for cure. This is evident from a molecular weight between crosslinks consideration since the molecular weight between functional groups at 1.5 mole % is about 3,700 which would result in a very tight cure if all functional groups participated. With the recent introduction of isobutylene-co-p-bromomethylstyrene elastomers (BIMS) in the early 1990s there was an additional step in cure functionality reduction; down to as low as 0.5 mole %. These polymers can be cured more effectively with either zinc oxide or amine cures. BIMS elastomers containing over 1 mole % of functionality can be cured tightly and were recommended for use in rubber blends. Only those polymers with 0.5-1 mole % functionality were recommended for single rubber formulations. However, even with 0.5 mole % of cure functionality there is not complete utilization of cure sites with current cure systems since benzylic bromine is evident by NMR after cure. It is believed that additional property gains can be realized if functionality can be even further decreased provided that more of the cure sites can be utilized. For example, extreme demands for heat stability and for retained elastic properties during high temperature service are required by applications such as cure envelopes, tire curing bladders and high temperature hose. Improved stability requires elimination of reactive sites while improved physical properties require proper networks in the cured rubber. This brought about a search for further improvements in cure efficiency with BIMS elastomers. It was found that a combination of 1,6-hexamethylenebis(sodium thiosulfate) (HTS) with zinc oxide, previously studied for BUMS in rubber blends (ref. 1), can provide a highly efficient cure system for BIMS elastomers when properly optimized. At certain levels of HTS and zinc oxide, this cure system can be very effective with BIMS containing cure functionality down to less than 0.2 mole %, as is demonstrated in this article.

Experimental

Materials

The isobutylene-co-p-bromomethylstyrene elastomers (BIMS) used in this study were prepared by Exxon Chemical Co. Some of the BIMS used are commercially available as Exxpro elastomers and some of the lower bromine containing polymers were prepared for this study. The polymer backbone before bromination contained about 5 wt. %, or about 2.5 mole %, of p-methylstyrene (PMS) comonomer. Copolymers were prepared via cationic polymerization as described by Powers et al (ref. 2). Brominations were carried out, following the process by Powers et al (ref. 3) to the levels indicated in mole % by the polymer designation index. For example, BIMS-0.49 is a BIMS elastomer containing 0.49 mole % of benzylic bromine. Benzylic bromine level was determined by proton NMR. All polymers used in this study had a Mooney viscosity, ML 1+8 (125 [degrees] C), of about 45.

For the cure chemicals, 1,6-hexamethylene-bis(sodium thiosulfate) (HTS) was obtained from Flexsys America under the brand name Duralink HTS. It is obtained as a dihydrate and the CAS registration number is 5719-73-3.

Magnesium aluminum hydroxy carbonate, DHT-4A-2, (DHT) obtained from Kyowa Chemical Industry, was used as a retarder. The CAS registration number for DHT is 11097-59-9,

Procedures

Masterbatch mixing was done in a B size internal mixer. Polymer was added first with all other ingredients added at about 30 seconds. The mixing was stopped when the compound temperature reached 160 [degrees] C. Curatives were added on a cooled two roll mill.

Cure parameters were determined by using a Monsanto ODR2000 oscillating disc rheometer. Cure temperatures were 190 [degrees] C with oscillation at a 1 [degree] arc and a frequency of 1.67 Hz. The rheometer test was conducted for 60 minutes unless otherwise noted. For slow cures the rheometer test was carried out until a plateau MH was observed. Cure parameters are defined in the ASTM D2084 test procedure.

Sample molding was done at 190 [degrees] C for a time of t90 + 2 minutes. Sample aging was done in an air circulating oven at 177 [degrees] C for 48 hours. Tensile and tear properties were tested by following ASTM tests D412 and D624 with minor modifications. Tests were carried out with Monsanto T10 and Instron 4204 equipment at room temperature and at 150 [degrees] C. Tension set at a given elongation is the residual elongation set after exposing a sample to one cycle of strain at the stated elongation. A tensile bar is elongated to the stated elongation between two original marks and kept at that strain for 20 minutes. Tension set is the residual elongation after removing the strain and relaxing the sample for 20 minutes. Tension set was measured after 300% strain for unaged samples and after 200% strain for aged samples and for all HTS cured compounds (aged and unaged) at 150 [degrees] C, unless otherwise noted in the tables.

Results and discussion

Based on previous work by Ignatz-Hoover and Newman (ref. 1) it was recognized that HTS may exhibit some desirable cure properties for BIMS, primarily in blends with other rubbers. In order to test this concept in single rubber applications a study was conducted with a simple cure system of HTS/zinc oxide (ZnO)/stearic acid (HSt) and a retarder (DHT). A base bladder formulation previously developed for BIMS-0.49 was utilized for this study as shown in table 1. BIMS-0.49, containing 0.49 mole % of benzylic bromine is the lowest bromine containing commercial BIMS. This polymer was generally recommended for single rubber applications where excellent heat and aging resistance are required such as for curing bladders. BIMS-0.49 based bladders show significant aging advantages over butyl based bladders. However, longer bladder life can potentially be realized if tension set can be improved. In addition, a high consumption of cure functionality should further help in reducing bladder to molded article adhesion during cure. The HTS cure properties were therefore compared to the recommended resin cured IIR and BIMS-0.49 bladder formulations.

Table 1 - bladder formulations and properties for
commercial polymers

Bladder compound         Butyl          BIMS-0.49     BIMS-0.49
                         control        control       HTS cure

IIR                        100            -             -
BIMS-0.49                  -              100           100
N330 black                 50             55            55
Castor oil                 5              7             -
Mineral oil                -              -             7
Paraffin wax               5              2             2
Cure pkg.
Neoprene W                 5              -             -
ZnO                        5              3             2
SP1045 resin               10             7             -
Stearic Acid               -              0.5           -
MBTS                       -              1.2           -
DHT                        -              1.1           0.25
Sulfur                     -              0.75          -
HTS                                                     1.6
Rheometer data
MH-ML, dNm                 24             15            26
ts2, min.                  2.8            3.1           3.1
t90, min.                  37             7.2           22
Room temp. properties,
 unaged/aged
100% modulus, MPa          2.1/4.5        1.8/3.3       3.3/3.8
Tensile set, %             14.6/10.4      14/7.9        12.6/14.6
Elongation, %              630/330        710/390       260/280
150[degrees]C properties   6.3/18.8       6.3/18.8      -/-
 unaged/aged
100% modulus, MPa          1.0/2.8        0.5/1.7       -
Tensile strenght, MPa      3.2/6.0        3.8/3.1       -
Elongation, %              340/260        570/250       -
Tension set, %             31/broke       69/63         -

As a first step, the HTS cure package was optimized for a stable and a high cure state by studying rheometer cure characteristics of statistically designed experiments varying the cure package ingredients in a masterbatch shown in table 1 for HTS cure. Castor oil which is common in bladder formulations was not used in this study since it is believed to participate in the cure. For the optimization it was assumed that a high cure state given by a high rheometer modulus (MH-ML) will contribute to higher elasticity needed for reduced tension set as well as to the lowering of unreacted benzylic bromine. A stable cure for this optimization was defined as having a constant rheometer modulus (MH), assuming that such a behavior will help in providing long term high temperature properties. It was found that stearic acid caused a slight reversion. HTS alone, on the other hand, did not produce any significant cure in BIMS. It was concluded that a small molar excess of HTS over benzylic bromine, counting two equivalents in HTS for each benzylic bromine, with a much larger excess of ZnO, are required to obtain the optimum rheometer cure characteristics.

The initially optimized HTS cure is shown in table 1 and its resulting properties are compared to those of the current recommended bladder formulations. The data shown in table 1 indicate that the HTS cure system resulted in very stable properties. The HTS cured compound showed only a little change in physicals after aging while the control compounds showed evidence of additional cure and a breakdown at 150 [degrees] C. However, the HTS cure of table 1 is believed to be too tight for bladder applications. The elongation was too low to even measure a 300% tension set and the modulus was extremely high. Solid state NMR tests of the cured HTS compound showed an essentially complete consumption of the benzylic bromine functionality. It was evident, therefore, that the HTS cure system was highly efficient and the lowest commercially available benzylic bromine level of 0.49 mole % was too high for this cure system.

To further test the HTS cure potential, lower benzylic bromine containing BIMS elastomers were prepared with resulting properties shown in table 2. It was found that at 0.33 and 0.38 mole % benzylic bromine elongation was increased and 100% modulus was lowered as compared to the HTS cured BIMS-0.49 of table 1. However, elongation was still much lower and modulus somewhat higher than the controls of table 1; but tension set was better, indicating good promise in even lower functional levels. It was also shown that a 70/30 mixture of BIMS-0.49 and its precursor backbone polymer (IMS), resulting in an average level of 0.34 mole % benzylic bromine, does not produce a desirable product. The polymer blend properties shown in table 2 are low in cure modulus and defensive in elongation and tension set, although they are surprisingly high in tensile modulus. This also suggested that polymer blends, especially when functional levels have different extremes, should be avoided. Benzylic level was therefore further reduced, down to 0.17 mole %, with resulting properties shown in table 3. It can be seen that at 0.17 mole % the cure characteristics as evidenced by cure modulus are still respectable, but cure time and elastic properties, as demonstrated by tension set, are defensive. Stearic acid addition to the cure package can accelerate the cure but it damages high temperature tension set, although it is similar to the controls in table 1. Table 3 shows that at 0.24 mole % benzylic bromine, less than half the level of commercially available BIMS, properties are promising. Good tension set and good high temperature properties can be obtained. A smaller variation in physical properties with temperature and aging are also evident for the HTS cured BIMS-0.24 compound. However, additional optimization may be required for improving elongation.

Table 2 - properties for bladder formulations
at lower BIMS functional levels

Benzylic br, mole %     0.33           0.38            0.34
                                                     (average)
BIMS-0.33               100            -                -
BIMS-0.38               -              100              -
BIMS-0.49               -              -                70
IMS                     -              -                30
N330 black              55             55               55
Mineral oil             7              7                7
Paraffin wax            2              2                2
Cure pkg.
ZnO                     2              2                2
HTS                     1.6            1.6              1.6
DHT                     0.25           0.25             0.25
Rheometer data
MH-ML, dNM              26             21               5.9
ts2, min.               3.3            2.7              5.9
t90, min.               23             18               31
Room temp. properties
 unaged/aged
100% modulus, MPa       2.4/2.0        2.5/2.5       3.3/2.9
Tensile strength, MPa   11.1/8.6       12.8/11.8     9.8/9.5
Elongation, %           340/430        360/410       240/290
300% tension set, %     6.25/13.5      3.4/7.5       -/-
Table 3 - properties for formulations at 0.24 and
0.17 male % functionality

Benzylic br, mole %          0.24          0.17        0.17
                                                       (with HSt)
BIMS-0.24                    100           -           -
BIMS-0.17                    -             100         100
N330 black                   55            55          55
Mineral oil                  7             7           7
Paraffin wax                 2             2           2
Cure pkg.
ZnO                          2             2           3
HTS                          1.6           1.6         1.5
HSt                          -             -           0.6
Rheometer data(*)
MH-ML, dNm                   20            15          12
ts2, min.                    3.3           6.8         2.5
t90, min.                    23            120         16
Room temp. properties
 unaged/aged
100% modulus, MPa            1.8/1.7       1.2/NA      1.2/NA
Tensile strenght, MPa        16/13.6       12.5/NA     12.4/NA
Elongation, %                410/510       560/NA      560/NA
300% tension set, %          1.3/3.1       18.8/NA     8/NA
150[degrees]C properties
 umaged/aged
100% modulus, MPa            1.7/1.2       1.0/NA      0.7/NA
Tensile strenght, MPa        6/6.2         5.8/NA      4.1/NA
Elongation, %                240/390       320/NA      380/NA
200% tension set, %          16/22         24/NA       40/NA

The high cure efficiency of the HTS cure system is evident in table 4, where rheometer data for a variety of cure systems which are recommended for BIMS are compared to an HTS cure. A bladder type formulation was used for this comparison based on BIMS-0.23 and N234 black. Compounds 7 and 8 use the currently recommended bladder cure. The HTS cure (compound #1) is the only cure in table 4 with an acceptable cure state, although it is the slowest one. All other cures show a very low state of cure and some show a slight reversion as well.

Table 4 - cure comparison for BIMS-0.23 in a
bladder formulation

Compound No.          1        2        3        4       5

Cure Ingredients
ZnO                   3        1        2        1       1
HTS                   1.5      -        -        -       -
HSt                   0.25     2        -        2       2
Zinc stearate         -        1        3        -       -
ZDEDC                 -        -        -        1.5     -
TMTD                  -        -        -        -       0.2
DPTTS (sulfads)       -        -        -        -       -
MBTS                  -        -        -        -       -
DHT                   -        -        -        -       -
Sulfur                -        -        -        -       -
Resin SP1045          -        -        -        -       -
Rheometer data
MH-ML, dNm            17.2     6.5      5.9      4.5     9.6
ts2, min.             3.2      1.9      2        1.3     1.5
t90, min.             29.8     5.7      6        3.3     6.8
Reversion             -        slight   slight   -       slight

Compound No.          6        7        8

Cure Ingredients      1        3        3
ZnO                   -        -        -
HTS                   2        0.5      0.5
HSt                   -        -        -
Zinc stearate         -        -        -
ZDEDC                 -        -        -
TMTD                  1        -        -
DPTTS (sulfads)       -        1.2      1.2
MBTS                  -        1.1
DHT                   -        0.75     0.75
Sulfur                -        7        7
Resin SP1045
Rheometer data
MH-ML, dNm            8.6      3.3      6.1
ts2, min.             1.4      7        2.6
t90, min.             4.4      11.3     5
Reversion             slight   slight   slight

Masterbatch: BIMS-0.23 - 100, 234 black - 55, mineral oil - 5, paraffin wax - 5

It should be pointed out that the chemical nature of the HTS cure, as well as the nature of the network, is not clear at this point. The current understanding is that for simple ZnO cure in BIMS (ZnO and HSt), a linkage is formed from one benzylic bromine into another p-bromomethylstyrene group on the polymer, by alkylating the ring at the mete position. If the HTS cure requires two p-bromomethylstyrene groups to form a crosslink, then the molecular weight between crosslinks would be very high for the low functionality BIMS polymers described here. For example, the average molecular weight between p-bromomethylstyrene groups in BIMS containing 0.2 mole % of benzylic bromine is about 28,000. This would be the molecular weight between crosslinks if all the benzylic bromine groups are eliminated with the HTS cure as was determined by solid state NMR. Such a high MW between crosslinks is considered to form too loose a network to produce any useful rubber properties. But this article demonstrates that an HTS cure can produce reasonable properties in BIMS even at 0.17 mole % benzylic bromine. In addition, swell measurements on HTS cured BIMS-0.23 bladder formulations were comparable to swell in commercial bladders using polar and non-polar solvents. This suggest a rather complex crosslink network for the HTS cure which is not yet understood. Both the network and the chemical nature of the HTS cure are currently under study.

Conclusions

It was shown that HTS in combination with ZnO can provide highly efficient cures in BIMS. Very low functional levels, down to 0.17 mole % benzylic bromine, show promising cure properties with HTS and ZnO for high temperature applications such as curing bladders. High cure states with excellent elasticity and high temperature stability can be achieved. However, additional optimization of BIMS benzylic bromine level and HTS cure ingredients is required for improved elongation needed in high temperature applications such as curing bladders.

In optimizing the HTS cure it was found that a slight stoichiometric excess of HTS over benzylic bromine is desirable with a larger molar excess of ZnO.

The chemical nature and network structure resulting from the HTS cure are not understood and are under study.

References

(1.) F: Ignatz-Hoover and N.F. Newman, paper No. 110 presented at a meeting of the Rubber Division, American Chemical Society, Cleveland, Ohio, October 17-20, 1995.

(2.) K.W. Powers and H.C. Wang (to Exxon Co.), U.S. 5,426,167 (June 20, 1995) and U.S. 5,430,118 (July 4, 1995).

(3.) K.W. Powers and H.C. Wang (to Exxon Co.), U. 5,162,445 (November 10, 1992).