Cure systems and antidegradant packages for hose and belt polymers.

Traditionally, sulfur cure systems have been used to vulcanize many hose and belt compounds. The heat resistance of sulfur cure systems can be improved by reducing or eliminating elemental sulfur, which minimizes or eliminates polysulfide crosslinks and provides mainly mono- and di-sulfide crosslinks. For maximum heat resistance and thermal stability, peroxides must be used because they provide carbon-carbon crosslinks. Peroxides require special attention to the additives used in compounding. For example, antidegradant selection is more restricted because many antioxidants interfere with peroxide crosslinking. Furthermore, when using peroxides, it is important to minimize acidic fillers to prevent cationic decomposition of the peroxide. The addition of basic ingredients such as zinc oxide or magnesium oxide, when possible, will generally increase peroxide crosslinking efficiency. Finally, paraffinic oils are preferred, and the use of aromatic oils and solvents should be avoided (ref. 1).

This article will discuss cure systems and antidegradant packages for belt polymers: polychloroprene (CR), alkylated chlorosulfonated polyethylene (ACSM), hydrogenated nitrile-butadiene copolymer (HNBR) and ethylene-alkylene(diene) polymers (EPDM, EOM). Cure systems and antidegradant packages for hose applications will then be discussed.

Belt polymers

Polychloroprene (CR)

For many years, the standard v-belt polymer has been chloroprene copolymerized with sulfur. These polymers can be cured with metal oxides alone, but the addition of an organic accelerator such as a thiourea can speed up the rate of cure. Nevertheless, the polysulfide linkages in the chloroprene-sulfur copolymer limit the high temperature resistance of the vulcanizate. With more aerodynamic automotive styling, which restricts the size of the engine compartment, and increased engine operating temperature for better fuel efficiency, the service limit of the chloroprene-sulfur copolymer is exceeded.

Increased heat resistance can be obtained with chloroprene polymerized with a thiuram modifier. The thiuram-modified polychloroprene requires an organic accelerator for cure and generally does not achieve the excellent flex fatigue of the sulfur copolymer. A typical example as reported by Dowd (ref. 2) is shown in figure 1. By judicious selection of the antidegradant and curative, commercial literature (ref. 3) demonstrates that it is possible to improve both the heat resistance and flex fatigue of thiuram modified polychloroprene.

[Figure 1 ILLUSTRATION OMITTED]

While not generally recognized, it is also possible to crosslink polychloroprene with peroxides to generate the most thermally stable carbon-carbon crosslinks (ref. 4). Peroxide crosslinking requires some modification in the antioxidant system to obtain a high state of cure. A polymerized quinoline (TMQ) is typically used with peroxides because it interferes least with a peroxide cure. However, TMQ can not be used in polychloroprene because it causes scorch and results in poor bin storage stability of the mixed compound. The contour curves in figures 2 to 4 demonstrate the state of cure, aging and flex fatigue obtainable by various levels of two antioxidants in a thiuram-modified polychloroprene cured with dicumyl peroxide. An octylated diphenylamine (ODPA) is used in most polychloroprene compounds. This aromatic amine antioxidant provides good oxidative stability. However, when curing with peroxide, the amount of this antioxidant must be limited because it severely lowers the state of cure, as shown in figure 2. In contrast, the zinc 2-mercaptotoluimidazole (ZMTI) antioxidant synergist does not reduce - but rather increases - the state of cure. Both antioxidants contribute to improved high temperature aging, as indicated by the retention of elongation contours in figure 3. While ODPA exhibits better DeMattia flex fatigue, as shown in figure 4, this result could be due to the lower hardness and modulus it conveys. In contrast, the ZMTI increases both modulus and DeMattia cut growth resistance. Properly compounded, polychloroprene is serviceable to 120 [degrees] C (ref. 5).

[Figures 2-4 ILLUSTRATION OMITTED]

Alkylated chlorosulfonated polyethylene (ACSM)

With increasingly higher under-the-hood temperatures, polymers with fully saturated backbones must be employed. One such candidate is ACSM, which can be crosslinked either through the chlorosulfonyl group with sulfur cure systems or at the hydrocarbon backbone with peroxides. With sulfur cures, 1 phr nickel di-n-butyldithiocarbamate (NBC) is commonly used. For performance at higher temperatures, 1 phr meta-phenylene bismaleimide (MBM) is substituted for the elemental sulfur and the amount of NBC is increased to 3 phr (ref. 6). Peroxides crosslink ACSM through the hydrocarbon backbone, but heat resistance is similar to crosslinking through the chlorosulfonyl groups with a high temperature cure system.

Nevertheless, peroxides provide the flexibility to increase the cure state beyond that allowed when crosslinking through the chlorosulfonyl group. For synchronous timing belts, Pillow (ref. 5) finds the normal failure mode is tooth root failure, which can be a consequence of low tooth modulus or excessive tooth loading. Delgarno and Pillow (ref. 7) report a high modulus at the tooth root can minimize failures due to loss of teeth. A high tenacity reinforcing polyamide can give a significant improvement in belt life (ref. 5). Also, higher crosslink density via higher peroxide levels is another way stiffness and modulus can be improved. Delgarno and Pillow (ref. 7) report that ACSM is suitable in timing belts for service to 135 [degrees] C.

Hydrogenated nitrile-butadiene (HNBR)

HNBR polymers possess excellent heat and oil resistance and can be compounded for good flex fatigue. It is the dominant polymer for synchronous belts in Japan and is gaining use in Europe, according to Hashimoto and coworkers (ref. 8). As previously observed with polychloroprene, the selection of the cure system involves a trade-off between heat aging and flex fatigue. Peroxides provide the best heat resistance, but low sulfur or sulfurless cures generally have better flex fatigue. The work of Upadhyay (ref. 9) in this area is shown in figure 5. With peroxide crosslinking that also polymerizes zinc diacrylate to form an in-situ filler, Brown (ref. 10) reports that HNBR can provide excellent toughness and high dynamic modulus at elevated temperature. Recchio and Bradford (ref. 11) have shown that meta-phenylene bismaleimide (MBM) coagent will increase low-strain modulus (MDR-MH) at elevated temperature. As previously mentioned, increased low-strain modulus may be beneficial in preventing tooth loss in synchronous timing belts.

[Figure 5 ILLUSTRATION OMITTED]

As is typical of peroxide cures in many polymers, the selection of the antioxidant system in HNBR is important for optimum high temperature performance. Generally, a combination of a primary, alkoxy radical trap and an antioxidant synergist such as zinc 2-mercaptotoluimidazole (ZMTI) is used. Ferradino (ref. 12), in previously unpublished Vanderbilt work, measured the elongation retention of HNBR containing one of three different radical traps both with and without ZMTI synergist, as shown in figure 6. The addition of ZMTI significantly improves the aging of an otherwise unprotected HNBR compound. However, the best results are obtained with a combination of ZMTI and a radical trap antioxidant. In the presence of ZMTI, the aging differences among the primary, radical trap antioxidants tested are relatively minor.

[Figure 6 ILLUSTRATION OMITTED]

Cure speed and high temperature dynamic properties can be affected by the choice of peroxide and coagent. Figure 7 excerpts the work of Recchio and Bradford (ref. 11). The high performance peroxide, Bis-40HP, can increase scorch time with minimal sacrifice in cure time. The optional use of meta-phenylene bismaleimide (MBM) coagent can provide shorter cure times with lower loss factor (tangent delta) at elevated temperature. Peroxide cured HNBR is suitable for service at 150 [degrees] C (ref. 7).

[Figure 7 ILLUSTRATION OMITTED]

Ethylene-alkylene-(diene) polymer (EPDM and EOM)

It has been determined that, in several belt applications, casual oil and grease contact does not cause belt failure. This allows the use of hydrocarbon polymers such as EPDM as a material of construction for belts. In addition to being relatively economical, EPDM can be compounded to provide excellent heat resistance and flex fatigue. For maximum heat resistance, a peroxide cure is required, and flex fatigue can be increased by the addition of the antioxidant synergist ZMTI.

Ethylene-octene copolymer (EOM) has even better high temperature resistance than EPDM, according to Kotz and Grant (ref. 13) (table 1). Note particularly the aged elongation data. This polymer contains no unsaturation and so must be cured with peroxides. Palys and coworkers (ref. 14) have shown dramatic improvements in crosslinking efficiency with certain high performance peroxides. Figure 8 gives one example. The "HP" or high performance peroxide formulation gives improved scorch times without sacrifice in cure times. In EOM, the dose of the HP peroxide can be cut by one-third and still provide the same state of cure as the regular dicumyl peroxide. If the scorch time or mold flow time of regular DCP is satisfactory, the molding temperature of the HP grade can be increased to shorten cure time and improve productivity by giving more cured parts per hour.

Table 1 - effect of antioxidants in peroxide-cured EPDM and EOM

Antioxidant              None    ADPA     None    ADPA
                                 & ZMTI           & ZMTI

EPDM                      100      100      --      --
EOM                        --       --     100     100
N-650 carbon black         50       50      50      50
Paraffinic oil             30       30      30      30
ADPA antioxidant           --        1      --       1
ZMTI antioxidant           --        2      --       2
TAlC DLC coagent            2        2       2       2
Bis-40 peroxide            10       10      10      10

Mooney at 121 [degrees] C
ML 1+4, Mooney units       24       24      36      37

Rheometer at 180 [degrees] C
ts2, minutes              0.6      0.8     0.6     0.7
tc90, minutes             5.6      6.5     4.1     4.8
tc95, minutes             7.7      8.4     5.8     6.6

Press cured to tc95 + 3 minutes at 180 [degrees] C
100% modulus, MPa         5.0      4.0     3.4     3.7
Tensile strength, MPa    16.0     17.1    13.8    14.0
Elongation, %             330      265     330     360
Hardness, Durometer A      69       71      65      71

Aged 70 hours at 175 [degrees] C
Tensile strength, MPa       2       16       3      14
Elongation, %              15      220      70     390
Hardness, Durometer A      80       75      72      74

Aged 168 hours at 175 [degrees] C
Tensile strength, MPa       5        3       4       9
Elongation, %               2       30       2     215
Hardness, Durometer A      91       83      88      79

Compression set after 70 hours at 175 [degrees] C
Set, %                     11       17      11      16

[Figure 8 ILLUSTRATION OMITTED]

Hose polymers

Fuel hose

In the past, the standard construction for fuel hose in air-aspirated engines with carburetors has been a nitrile rubber (NBR) tube with a polychloroprene (CR) cover. Today, fuel injection systems dominate in Japan and Europe and are increasingly gaining use in the U.S. For these systems, the fuel hose is usually made with a fluoroelastomer (FKM) tube or liner to resist gasoline/alcohol mixtures and survive the peroxidized gasoline formed by fuel recirculation in injection systems. FKM also has lower fuel permeability than predecessor polymers, according to Foster and Capriotti (ref. 15), which assists compliance with stricter emission regulations such as the 1995 California SHED test (ref. 8). Adhering the FKM liner in large diameter hose has been a challenge. Peroxide crosslinking systems with zinc methacrylate coagents are helpful. Table 2 compares DBPH peroxide to its high performance DBPH-HP version in a FKM GF-205NP base compound. The high performance peroxide gives longer time before crosslinking begins, which allows higher cure temperatures for higher productivity.

Table 2 - comparison of peroxides in fluoroelastomer

Peroxide                           DBPH   DBPH-HP

FKM GF-205NP                       100       100
N-330 carbon black                  30        30
TAlC coagent                         3         3
DBPH peroxide                        3        --
DBPH-HP peroxide                    --         3
MDR at 176.7 [degrees] C
ML, dN-m                           0.1       0.1
MH, dN-m                           4.2       4.6
ts2, minutes                       0.4       0.9
tc90, minutes                      1.1       3.1
MDR at 195 [degrees] C
ML, dN-m                            --       0.1
MH, dN-m                            --       4.3
ts2, minutes                        --       0.4
tc90, minutes                       --       0.8
Press cured at 176. 7 [degrees] C
100% modulus, MPa                  6.7       6.3
Tensile strength, MPa             14.9      13.9
Elongation, %                      310       275
Hardness, durometer A               69        70
Compression set after 70
 hours at 200 [degrees] C
Set, %                            26.5      27.6

The cover stock nowadays can be either chlorinated polyethylene (CPE) or an alloy of polyvinyl chloride (PVC) an d NBR. CPE can be cured with either peroxides or thiadiazoles. Two thiadiazoles are compared in table 3 (ref. 16). The original thiadiazole, A-ECO, has a very fast rate of cure, but the mixed compound has poor bin storage stability, sometimes requiring shipment in refrigerated trailers. A newer thiadiazole, A-829, is slower curing, but the mixed compounds have much improved storage stability, and the physical properties of the vulcanizate are very good.

Table 3 - comparison of thiadiazoles in chlorinated polyethylene (CPE)

Curative                                 A-ECO   A-829

CPE                                        100     100
Magnesium oxide                             10      10
N-774 carbon black                          50      50
Aromatic oil                                30      30
Butyraldehyde-aniline reaction product     0.8     0.8
A-ECO                                      2.5      --
A-829                                       --     2.5
Mooney at 121 [degrees] C-Original
ML t5, minutes                              22      39
Viscosity, ML                               48    49.5
Mooney at 121 [degrees] C after 2 weeks
 at 38 [degrees] C
Viscosity, ML                             150+      69
Rheometer at 171 [degrees] C
ML, N-m                                    0.8     0.9
MH, N-m                                    3.9     6.2
ts2, minutes                               1.2     2.5
tc90, minutes                              3.0    13.6
Press cured at 171 [degrees] C
200% modulus, MPa                          8.2     9.1
Tensile strength, MPa                     14.4    15.1
Elongation, %                              450     390
Hardness, durometer A                       78      82
Aged 336 hours at 121 [degrees] C
Tensile strength, MPa                     14.4    14.5
Elongation, %                              375     310
Hardness, Durometer A                       83      87
Compression set after 70 hours at
 100 [degrees] C
Set, %                                      32      22

A standard sulfur cure of NBR is shown in table 4 (ref. 17). In an actual hose compound, 45 to 50 phr of PVC would be fluxed in during mixing, along with an appropriate stabilizer for the PVC. A newer, low nitrosamine offset to the traditional thiuram monosulfide (TMTM) is shown in the second recipe. The sterically-hindered tetra-isobutyl thiuram disulfide (TiBTD) that emits less nitrosamines also gives comparable cure properties to TMTM. This replacement of TMTM with TiBTD gives higher rheometer maximum torque and modulus, along with lower compression set, properties that are beneficial to the retention of hose couplings in service.

Table 4 - comparison of sulfur curatives in nitrile-butadiene rubber (BNR)

Curative                              Standard           Low
                                                  nitrosamine

NBR                                       100           100
N-550/N990 carbon blacks                35/65         35/65
Processing aid                            2.5           2.5
Dioctyl phthalate                          10            10
Zinc oxide/stearic acid                   5/1           5/1
ODPA/ZMTI antioxidants                    1/1           1/1
Sulfur                                   0.25          0.25
CBS                                         2             2
TMTM                                        3            --
TiBTD                                      --             3
Mooney at 121 [degrees] C
ML t5, minutes                             31            26
Viscosity, ML                            52.5          53.5
Rheometer at 166 [degrees] C
ML, N-m                                   1.1           1.2
MH, N-m                                   5.3           7.6
ts2, minutes                              3.0           2.5
tc90, minutes                             7.2           7.0
Press cured to tc90 + 2 minutes at
 166 [degrees] C
200% modulus, MPa                         4.9           7.4
Tensile strength, MPa                     9.8          12.5
Elongation, %                             480           410
Hardness, durometer A                      72            70
Aged 70 hours at 125 [degrees] C
Tensile strength, MPa                    14.4          15.0
Elongation, %                             290           220
Hardness, durometer A                      80            78
Compression set after 70 hours at
 125 [degrees] C
Set, %                                     58            40

Power steering hose

For many years, an NBR tube and CR cover were used in the power steering pressure hose and return lines. Increasing under-the-hood service temperatures requires a change to fully saturated polymers. Today, chlorosulfonated polyethylene (CSM) is used as the tube stock for power steering hose, with a CSM or CPE cover. CPE curatives were presented in the preceding section and in table 3. The cure systems and antioxidants for CSM can be essentially the same as those previously discussed for ACSM in the belt section. On a practical basis, peroxides are generally used for high modulus and coupling retention.

Nichols and Pett (ref. 18) report that CSM power steering hose can be deteriorated by zinc dithiophosphates that are present in older power steering fluids. This result might be expected, given the CSM manufacturer's warning that zinc will deteriorate compounds based on CSM (ref. 19). The power steering fluid becomes less aggressive towards CSM as the fluid ages, presumably due to oxidation of the zinc dithiophosphate.

Zinc dithiophosphate is also an accelerator of sulfur vulcanization. Because CSM can be crosslinked either through the chlorosulfonyl group with sulfur cures or at the hydrocarbon backbone with peroxides, it is possible that peroxides make CSM less susceptible to attack by brake fluids that contain zinc dithiophosphates.

Air conditioner hose

Air conditioner hose can be made with any of several polymers. The most basic hose construction is CSM or peroxide cured CPE. With the change from dichlorodifluoromethane (CFC-12) to the less ozone-depleting tetrafluoroethane (HFC-134a), a broader variety of polymers is suitable for use. Concurrent with this change in refrigerant, the compressor lubricant has changed from mineral oil to polyalkylene glycol, and the operating temperature of the system has increased.

Harmsworth (ref. 19) has recently suggested that halobutyl rubber is a suitable candidate for air conditioner hose. Halobutyl rubbers possess outstanding resistance to permeation, thus minimizing the loss of the ozone-depleting refrigerant. One general requirement of hose is a high modulus in order to provide good retention of the metal couplings. Work at Vanderbilt (refs. 21 and 22) shows a significant increase in hardness and modulus through the use of a 2,5-bis (alkylether alkylthio)-1,3,4-thiadiazole crosslinker (A-189) (table 5). As shown, this thiadiazole curative also improves high temperature aging. The higher state of cure and better aging presumably result from the minimization of the dehydrohalogenation side reaction that occurs with conventional accelerators for the zinc oxide cure system. Baldwin and coworkers (ref. 23) provide information on this undesirable side reaction. The low compression set of this new cure system should also contribute to good coupling retention of the hose.

Table 5 - comparison of curatives in bromobutyl rubber

Curative                             MBTS/   A-189
                                      TMTD

XIIR                                   100     100
N-550 carbon black                      55      55
Paraffinic oil                          10      10
Zinc oxide                               3       3
Stearic acid                             1      --
MBTS                                   1.5      --
TMTD                                  0.25      --
Sulfur                                 0.5      --
A-189                                   --     2.5
Mooney at 121 [degrees] C
ML t5, minutes                        15.5     8.3
Viscosity, ML                           46      52
Rheometer at 160 [degrees] C
ML, N-m                                1.3     1.5
MH, N-m                                3.8     8.1
ts2, minutes                           2.3     2.1
tc90, minutes                         11.5     3.8
Press cured to tc90 + 2 minutes at
 160 [degrees] C
200% modulus, MPa                      2.6     7.4
Tensile strength, MPa                 11.5    10.6
Elongation, %                          760     340
Hardness, durometer A                   52      60
Aged 168 hours at 121 [degrees] C
Tensile strength, MPa                 10.4    11.5
Elongation, %                          570     320
Hardness, durometer A                   62      64
Compression set after 70 hours at
 100 [degrees] C
Set, %                                  62      14

Brake hose

Brake hose can be based on either CR or peroxide crosslinked EPDM. The CR should be crystallization resistant for the best low temperature properties, and also contain gel for low nerve and good extrusion characteristics. EPDM will survive longer in higher temperature operating environments. For maximum retention of coupling force, the selection of antioxidant for the peroxide cure is important. Conventional radical trap antioxidants can lower the hardness and modulus of peroxide cured EPDM. In contrast, the inclusion of ZMTI antioxidant synergist actually increases hardness and modulus (ref. 24).

Radiator hose

Coolant or radiator hose compounds are based almost exclusively on EPDM. Both sulfur and peroxide cure systems are used. The latter can be useful in preventing the treeing phenomenon that results in premature failure of hoses coupled to electrically insulated, plastic fittings. Keller (ref. 25) shows that a peroxide cure gives lower electrical conductivity and coolant absorption as compared to a sulfur cure. The absorption of coolant is associated with the development of dendritic structures, the formation of cracks, the filling of cracks by coolant and the premature failure of the hose. Keller finds that other factors, e.g., the type of reinforcing fabric, lower carbon black levels and the optional use of hydrophobic clay fillers, can minimize coolant absorption and prevent the formation of dendritic structures under a small (6 or 12 volt) applied electrical potential. Nevertheless, a major determinant of this type of failure is the cure system. Table 6 from Keller's work shows the effect of four different sulfur cure systems as compared to a peroxide cure on electrical conductivity and coolant absorption. The peroxide cure is significantly better than any of the sulfur cures, despite widely varying crosslink network structures and crosslink densities

Table 6 - relationship of cure system on crosslink network and coolant absorption of EPDM

Cure system                            A      B      C

Sulfur                                  2    0.2    1.5
MBTS                                    1     --     --
TMTD                                  0.8    1.5      3
ZDBC                                  0.8    1.5     --
TMTM                                   --    1.5     --
DTDM                                   --      2     --
Copper salt of MBT                     --     --    0.5
ZDMC                                   --     --     --
MBM                                    --     --     --
DCP-40 peroxide                        --     --     --
Rheometer data
MH-ML, dN-m                          83.6   64.0   78.2
Press cured properties
100% modulus, MPa                     5.7    4.3    5.4
Coolant immersion test conditions:
14 days at 80[degrees]C and 6 volt
applied potential
Current, mA                          0.60    0.5    0.5
Weight increase, %                   10.7   17.1   12.0

Cure system                            D      E

Sulfur                               0.75     --
MBTS                                   --     --
TMTD                                  2.5     --
ZDBC                                  2.5     --
TMTM                                   --     --
DTDM                                  1.7     --
Copper salt of MBT                     --     --
ZDMC                                  2.5     --
MBM                                    --      1
DCP-40 peroxide                        --     10
Rheometer data
MH-ML, dN-m                          73.5   31.1
Press cured properties
100% modulus, MPa                     5.2    4.7
Coolant immersion test conditions:
14 days at 80[degrees]C and 6 volt
applied potential
Current, mA                           0.6    0.1
Weight increase, %                   14.5    2.3

The expected advantage of peroxide crosslinking in the retention of elongation after one week at 150 [degrees] C and in compression set after 70 hours at 150 [degrees] C is shown in figure 9, as derived from Keller's data. The ZDBC, ZDMC, TMTD and DTDM cure system in this study is one of the best sulfur cure systems for compression set and aging. The exceptionally low compression set of the peroxide cure should contribute to good coupling retention.

[Figure 9 ILLUSTRATION OMITTED]

Recently, Vroomen and coworkers (ref. 26) discussed some of the factors that must be considered when selecting peroxide or sulfur cures for EPDM hose compounds. They confirm the work of Keller and others on the superiority of peroxide cures over sulfur cures to prevent electrochemical corrosion. And they verify the typical benefit of peroxide cures for superior heat stability (figure 10) as reproduced from their work. Some of the weak points they report for peroxide cures are: a) restriction on the ingredients as we have previously discussed, b) poor hot tear strength during assembly or disassembly on the mandrel, c) bloom with peroxides that generate high molecular weight decomposition products and d) sticky surfaces upon exposure to oxygen in the air during vulcanization.

[Figure 10 ILLUSTRATION OMITTED]

Summary

In most polymers, heat resistance is improved by going from a sulfur cure to a sulfur donor cure. For maximum heat resistance, a peroxide cure is required. Newer peroxide formulations are being introduced which offer improved processing and enhanced physical properties.

The antidegradant system requires careful consideration when using peroxide cures. The use of a minimal level of primary antioxidant with an antioxidant synergist often gives the best results.

Several examples of cure systems and antidegradant packages in hose and belt polymers have been presented. In belt applications, the data indicate that the traditional fall-off in flex fatigue with peroxides can be overcome by antioxidant optimization. In hose applications, peroxides provide the expected benefits of improved high temperature aging and better compression set.

Acknowledgements

"Cure systems and antidegradant packages for hose and belt polymers" is based on a paper given at the April, 1999 Rubber Division meeting.

"Optimization of the production of EPDM sponge rubber seals for automotive use" is based on a paper given at the April, 1999 Rubber Division meeting.

"Solving the problem of bond failures in swaged cylindrical tubeforms" is based on a paper given at the September, 1999 Rubber Division meeting.

References

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