New developments in curing halogen-containing polymers.

Di- and poly-functional mercaptans have been used for many years to crosslink polymers with labile halogens (chlorine or bromine). 2,5-dimercapto-1,3,4-thiadiazole (DMTD) is one such crosslinker that displays relatively fast cures but poor scorch safety. Improved scorch safety can be achieved by derivatizing one or both mercapto groups. Several derivatives of DMTD have been developed in recent years for this purpose. Proper selection of the thiadiazole derivative and associated accelerator is often a key to successful use in a given halogen-containing polymer compound. This article reviews the various thiadiazoles and other additives that crosslink halogen-containing polymers.

Polymers containing labile halogens as the cure site have improved resistance to high temperature aging as compared to hydrocarbon rubbers based on dienes, due to the absence of backbone unsaturation. The lability of the cure site depends on the particular halogen, the ease with which it can be attacked by an approaching nucleophile, and the presence of any activating groups. The lability of the halogen is proportional to its size or, more precisely, to the decreasing electronegativity of the halogen. Bromine is more reactive than chlorine; fluorine is essentially nonreactive. Halogens on primary carbon atoms are more reactive than those on secondary carbons. Allylic unsaturation can make the halogen more labile or reactive. The active cure sites in selected halogen-containing polymers are shown in figure 1.

[Figure 1 ILLUSTRATION OMITTED]

Some polymers, such as halobutyl and polychloroprene, have many possible cure systems, while others have just a few. Thiadiazoles take on a greater importance for polymers such as CPE or polyacrylates, which can be crosslinked by relatively few cure systems. Working in partnership with DuPont Dow Elastomers, Vanderbilt has discovered some significant advantages of a particular thiadiazole derivative in polychloroprene (ref. 1).

Polychloroprene

The structure of polychloroprene can be likened to that of natural rubber, in which a chlorine atom replaces the methyl group in the repeating unit of natural rubber. Like natural rubber, polychloroprene undergoes strain-induced crystallization and both polymers have high strength in unfilled gum compounds. The chlorine atom is responsible for many of the performance features of polychloroprene, including oil resistance, improved heat stability and lessened susceptibility to ozone attack.

Polychloroprene is less reactive, not only to factors in the environment that cause degradation, but also for crosslinking purposes. The curatives for polychloroprene are quite reactive. As shown in figure 1, it is thought that the 1 to 2% of monomer that is incorporated in the polymer by 1,2 addition is the reactive cure site. Once these most labile allylic sites are consumed, the remainder of the polymeric structures can also react, but at a slower rate. This classic "marching modulus" cure profile can have detrimental effects on performance. Because polychloroprene degrades by crosslinking and embrittlement, a curative that contributes to crosslinks during service can shorten useful lifetime at elevated temperature. Compression set can be impaired by the development of crosslinks while the sample is deformed. Slight differences in demold time can yield vulcanizates with varying hardness and modulus.

Vanax 189 is a newly introduced thiadiazole derivative that exhibits an exceptionally flat cure plateau in polychloroprene. The general structure of Vanax 189 is shown in figure 2. The 100% active form is a liquid. For easier handling, the product is also being offered as a 70% active powder. Rheometer cure profiles of the two product forms in a black-filled polychloroprene compound are compared to ethylene thiourea (ETU) in figure 3.

[Figures 2-3 ILLUSTRATION OMITTED]

By proper selection of the amount and activity of the magnesium oxide (MgO) used with V-189, the induction period before crosslinking and cure time can be varied as desired. A medium activity MgO of 40 to 80 [m.sup.2]/g surface area gives the best results. High activity MgO ([is greater than] 100 [m.sup.2]/g s.a.) leads to slower cures and poor compression set. The effect of the MgO dose in a black-filled polychloroprene base compound is shown in table 1. By lowering the MgO dose from the typical 4 pier, faster curing compounds can be obtained with some sacrifice in Mooney scorch and rheometer mold flow (ts2) times.

Table 1 - effect of MgO dose in polychloroprene

MgO, medium activity, phr           2.00    2.85    4.00

Mooney at 121 [degrees] C

Viscosity, MS                      20.7    20.8    20.4
Scorch, t2, minutes                13.3    22.5    23.1

Rheometer at 160 [degrees] C
ML, N-m                             1.5     1.5     1.5
MH, N-m                             7.6     7.6     7.5
Scorch, ts2, minutes                2.5     4.7     5.3
Cure time, tc90, min.               6.3    11.2    14.3

Physical properties

Cured at 160 [degrees] C, min.        8      13      16
Hardness, Shore A                    60      62      62
200% modulus, MPa                   6.9     7.3     7.5
Tensile strength, MPa              18.7    18.4    18.2
Elongation, %                       500     420     405
Die C tear, kN/m                   43.2    41.3    40.8

Aged properties after 96 hrs.
at 125 [degrees]  C

Hardness, Shore A                    76      77      77
200% modulus, MPa                  13.4    13.4    13.1
Tensile strength, MPa              16.0    16.5    16.0
Elongation, %                       275     270     270

After 22 hrs. at 100 [degrees] C
[degrees] C

Compression set, %                  8.0     9.5    14.3

MgO, medium activity, phr           5.15    6.00

Mooney at 121 [degrees] C

Viscosity, MS                      21.1    20.7

Scorch, t2, minutes                21.5    24.3

Rheometer at 160 [degrees] C

ML, N-m                             1.5     1.5
MH, N-m                             7.7     7.9
Scorch, ts2, minutes                5.1     5.6
Cure time, tc90, min.              16.6    16.1

Physical properties

Cured at 160 [degrees] C, min.     18.5      18
Hardness, Shore A                    63      62
200% modulus, MPa                   8.1     8.5
Tensile strength, MPa              18.6    17.8
Elongation, %                       395     360
Die C tear, kN/m                   39.7    41.7

Aged properties after 96 hrs.
at 125 [degrees] C

Hardness, Shore A                    78      78
200% modulus, MPa                  13.7    13.0
Tensile strength, MPa              16.0    16.2
Elongation, %                       250     275

After 22 hrs. at 100 [degrees] C
Compression set, %                 14.0    17.0

Base formulation: 100 Neoprene W, 58 N762 black, 10 aromatic oil, 2 stearic acid, 2 Agerite Stalite S, MgO as indicated, 5 zinc oxide, 1.5 Vanax 189

Stearic acid acts as a retarder, giving longer scorch and cure times. Should a faster cure with no sacrifice in scorch time be desired, the addition of 0.5 phr tetra-iso-butyl thiuram disulfide can shorten tc9O cure time at 160 [degrees] C by about two minutes, with little or no change in Mooney scorch time.

The flat cure plateau exhibited by V-189 suggests that it is a weak nucleophile, unable to attack the more prevalent vinyl chloride structures in polychloroprene. This is consistent with other studies. Vanderbilt finds V-189 gives no cure with the chlorine attached to either the secondary carbon atoms in CPE or the primary carbon atoms in ECO. The cure of V-189 in polychloroprene is affected by pH; acidic clays give slower cures that only gradually approach a cure plateau. Activators such as a toluimidazole will speed up the cure of V-189 in clay-filled polychloroprene compounds.

Halobutyl rubber

Polyisobutylene copolymers with isoprene have low gas permeability for tire innerliners and high energy absorption for dynamic applications. The diene cure site can then be halogenated with either chlorine or bromine for good covulcanization with diene polymers.

A conventional accelerator system for halobutyl such as MBTS and methyl tuads (TMTD) speeds up the zinc oxide/stearic acid cure. But the state of cure (rheometer maximum torque, ODR-MH) is left relatively unchanged. In contrast, V-189 gives a significantly higher state of cure. It also shows the characteristic good mold flow (ODR-ts2), fast cure rate and flat cure plateau. An example in chlorobutyl is shown in figure 4; one in bromobutyl is shown in figure 5.

[Figures 4-5 ILLUSTRATION OMITTED]

As shown by Baldwin and his coworkers at Exxon (ref. 3), the catalyst for the main crosslinking reaction in halobutyl is not zinc oxide, but rather a zinc (hydroxy) halide. The halide catalyst is not formed until the starting ZnO has dehydrohalogenated some of the cure site in a side reaction. This side reaction produces a conjugated diene structure that is nonreactive with conventional ZnO and accelerator cure systems. As much as 85% of the starting halide can be consumed this side reaction. The conjugated diene produced in the side reaction is very sensitive to oxidation nd therefore impairs the heat resistance of the vulcanizate. The use of V-189 gives significantly improved heat resistance as shown in table 2.

Table 2 - cure systems in bromobutyl

                                      1.0 Stearic acid
                                         0.5 Sulfur
                              1.0        1.5 Altax         2.5
                            Stearic     0.25 methyl       vanax
                             acid         tuads            189

Rheometer at
 160 [degrees] C

ML, N-m                       1.5          1.3             1.5
MH, N-m                       3.3          3.8             8.1
Scorch, ts2, minutes          4.4          2.3             2.1
Cure time,tc90, minutes       8.5          4.5             3.8

Mooney at
 121 [degrees] C

Viscosity, ML                  51           46              52
Scorch, t5, minutes          58.5         15.5             8.3

Original physical
 properties

200% modulus, MPa             3.6          2.6             7.4
Tensile strength, MPa        10.5         11.5            10.6
Elongation, %                 505          760             340
Hardness, Shore A              50           52              60

Properties aged 1 week at
 121 [degrees] C

Tensile retained, %           110           90             108
Elongation retained, %         69           75              94
Hardness, points change        +8           +7              +4

Aged 70 hours at
 121 [degrees] C

Compression set, %             44           62              14

Base formula: 100 Exxon Bromobutyl 2244 (BIIR), 55 N550 black, 10 Sunpar 2280, 3.0 Zinc oxide, accelerator(s) as indicated.

Polisobutylene with brominated p-methylstyrene cure site

To further improve the heat resistance of polyisobutylene polymers, Exxon has introduced a brominated p-methylstyrene cure site in Exxpro. This avoids the temperature limitations inherent in a cure site that contains unsaturation, as is the case with all halobutyl polymers. Cure systems are still being developed for this new polymer.

The polysulfide DMTD derivative OCD-139 will speed up the basic zinc stearate cure of Exxpro (ref. 2). The general structure of OCD-139 is shown in figure 6.

[Figure 6 ILLUSTRATION OMITTED]

OCD-139 retards the sulfur cure of diene polymers. This behavior is helpful in blends of Exxpro and diene polymers because the dienes are significantly faster curing. Some accelerators, e.g. diphenylguanidine, in blends of Exxpro and dienes can contribute to bin storage problems. In a cooperative development program with Exxon, Vanderbilt discovered that thiurams such as tetra-iso-butyl thiuram disulfide can increase the cure rate of Exxpro/diene blends without the mixed stock exhibiting bin storage problems. The test data are shown in table 3.

Table 3 - accelerators in Exxpro/diene blends cured with OCD-139

Rheometer at 170 [degrees] C     Vanax DPG   Isobutyl tuads

ML, N-m                            11.5          10.0
MH, N-m                              58            68
Scorch, ts2, minutes                1.6           2.0
Cure time, tc90, min.               4.8           5.6
Mooney at 121 [degrees] C
 - original

Viscosity, ML                      30.5          32.5
Scorch, t5, minutes                29.5          26.3

Mooney at 121 [degrees] C
 - after 1 week at
 38 [degrees] C

Viscosity, ML                      63.5          38.5
Scorch, t5, minutes                26.2          30.1

Change in viscosity, ML points      +33            +6

Base formula: 40 Exxpro 90-10, 50 BR 1207, 10 SMR-20 (NR), 50 N660 black, 12 Fiexon 641 oil, 5 Escorez 1102 resin, 0.5 stearic acid, 1.0 zinc oxide, 1.75 Altax (2-mercaptobenzothiazoledisulfide), 2.0 sulfur, 0.75 OCD-139, 0.5 accererator (as indicated).

Sulfads can be used to cure compounds based on 100% Exxpro with aged properties that are superior to OCD-139. The confirming test data are shown in table 4.

Table 4 - cure systems in Exxpro

                                   No
Rheometer at 160 [degrees] C   accelerator   Sulfads   OCD-139

MH, N-m                             6.4         7.9      6.9
Scorch, ts2, minutes                3.0         1.0      2.7
Cure time, tc90, min.              28.0        18.0     20.5

Mooney at 121 [degrees] C

Viscosity, ML                        80          78       76
Scorch, t5, minutes                 8.2         5.3      7.9

Original physical properties

300% modulus, MPa                   3.6        10.1     10.1
Tensile strength, MPa               4.6        11.4     12.1
Elongation, %                       550         385      480
Hardness, Shore A                    60          60       61

Properties aged 1 week at
 150 [degrees] C

Tensile retained, %                 267          95      114
Elongation retained, %               20          65       41
Hardness, points change             +14          -1       +9

Base: formula: 100 Exxpro 90-10, 60 N660 black, 0.5 zinc oxide, 1 zinc stearate.

Chlorinated polyethylene

Chlorinated polyethylene (CPE) possesses good heat and solvent resistance suitable for many hose, tube and duct applications. Traditionally, either a peroxide or a monobenzoyl thiadiazole derivative (Echo from Hercules Inc.) have been used to crosslink CPE. Peroxides develop a tacky surface if exposed to air, as can occur in extrusion applications. The traditional thiadiazole cure suffers from poor bin storage stability and erratic cure performance.

The poor bin storage stability of thiadiazole-cured CPE can be traced to the aldehyde-amine accelerator. The variable cure performance of the monobenzoyl thiadiazole derivative is probably inherent in the structure of the thiadiazole. It is necessary to hydrolyze the monobenzoyl derivative in order to liberate the active crosslinking agent, DMTD, as shown in figure 7. Changes in ambient humidity may be sufficient to significantly alter the cure rate of CPE compounds containing this crosslinker.

[Figure 7 ILLUSTRATION OMITTED]

A comparison of the traditional aldehyde-amine accelerator (V-808) with a new, chemically stable amine (V-882B) is show in table 5 for several thiadiazole crosslinkers. For the aldehyde-amine accelerator, the mixed stocks increase from 20 to over 100 Mooney points in two weeks at moderately elevated storage temperature. The increase in viscosity observed depends on the particular thiadiazole used. With V-882B, the in-crease in viscosity is only 10 Mooney points after two weeks at a temperature approximating that of a hot warehouse or trailer in summertime.

Table 5. Comparison of thiadiazoles and accelerators in CPE

Accelerator                  0.8 phr V-808        1.25 phr V-882B
Thiadiazole             Echo     V-829   V-882A   V-829    V-882A

Mooney at 121 [degrees] C - original

Viscosity, ML               48      49.5    49.5      46.5    50
Scorch, t5, minutes         22      39      31.5      11     7.5

Mooney at
 121 [degrees] C
 - after 2
 weeks at
 38 [degrees] C

Viscosity, ML           150+      69      75       56.5     60
Points change           +100     +19.5   +25.5    +10.0    +10.0

Rheometer at 171
 [degrees] C

ML, N-m                    0.8     0.9     0.9      0.8      1.0
MH, N-m                    3.9     6.2     5.2     11.2     11.0
Scorch, ts2, minutes       1.2     2.5     1.9      1.6      1.2
Cure time, tc90, min       3.0    13.6    11.4     16.8     11.0
Physical properties

Hardness, Shore A           78      82      78       75       74
200%modulus, MPa           8.2     9.1     7.9      8.8      7.7
Tensile strength, MPa     14.4    15.1    14.4     21.1     16.8
Elongation, %              450     390     430      420      400

Aged properties after
14 days at 121
[degrees] C

Hardness, Shore A           83      87      84       80       78
Tensile strength, MPa     14.4    14.5    12.4     17.1     15.6
Elongation, %              375             160      340      310

After 70 hrs. at 100
[degrees] C

Compression set, %         32      22      33       14       16

Base formula: 100 Tyrin CMO136, 10 Maglite D, 50 N774 black; 30 aromatic oil, amine accelerator (as indicated), 2.5 thiadiazole (as indicated).

As compared to the traditional Echo thiadiazole, the thiadiazole derivatives introduced exhibit significantly improved bin storage stability, particularly with the more stable amine, V-882B. Both V-829 and V-882A are relatively slow curing. Nevertheless, the chemistry of these thiadiazole derivatives does not depend on hydrolysis for cure activity, which implies more uniform properties from batch to batch and day to day.

The cured properties of V-829 and V-882A are very similar, as are the aged properties. Compression set with either thiadiazole is outstanding when using V-882B as the accelerator. Until recently, the use of V-829 was prohibited by the patent under which Echo was developed (ref. 4). With the expiration of this patent in various countries around the world, the cost of crosslinking CPE and other chlorine-containing polymers with a thiadiazole such as V-829 can be significantly reduced.

Epichlorohydrin

This polymer, especially the 1:1 copolymer with ethylene oxide, has exceptionally good low temperature serviceability, particularly among solvent-resistant elastomers. Its combination of oil and low temperature properties make it suitable for arctic service gaskets, hose and belts. Many of the same curatives used with CPE are also used in epichlorohydrin.

A comparison of two thiadiazoles is shown in figure 8 for a typical hose compound. Neither curative approaches a plateau when cured at 160 [degrees] C. The initial and aged properties are similar for both thiadiazoles. This polymer would probably be a good candidate for accelerator development.

[Figure 8 ILLUSTRATION OMITTED]

Polyacrylate

Polyacrylates with either a vinylchloroacetate or a vinylchloroether cure site are used in under-the-hood applications such as shaft seals and lip seals. Cure systems for these polymers have evolved over the years from fugitive amines to corrosive amine salts, then to soap/sulfur and, more recently, to fast curing multifunctional mercapto compounds.

A comparison of mercapto compounds is shown in table 6 for a polyacrylate with a vinylchloroacetate cure site. Figure 9 shows the rheometer cure curves. The trithiocyanuric acid control gives a very tight cure, with good compression set after post cure but relatively low initial elongation. The low elongation can cause some assembly rejects as the seal is inserted onto the shaft. The difunctional thiadiazoles, either DMTD or V-829, give greater initial elongation with modestly impaired compression set. Based on similar cure speeds, it is possible that a blend of di- and tri-functional curatives could provide a balance of initial elongation and compression set.

Table 6 - curative in polyacrylate with vinylchloroacetate
cure site

                            Trithiocyanuric   Vanchem   Vanax
                                 acid           DMTD     829

Trithiocyanuric acid              0.75          --        --
Vantard PVI                       0.1           --        --
Vanchem DMTD                      --             1        --
Vanax 829                         --            --         1

Mooney at 121 [degrees] C

Scorch, t5, minutes                29             8        34
Viscosity, ML                      41            44        40

Cured 17 minutes at 171
  [degrees] C

100% modulus, MPa                 4.3           5.1       4.1
Tensile strength, MPa            10.3          11.2      10.1
Elongation, %                     220           300       340
Hardness, Shore A                  60            68        65

Post cured 4 hours at 150
[degrees] C

100% modulus, MPa                 6.4           6.8       5.7
Tensile strength, MPa            12.3          12.5      12.0
Elongation, %                     190           230       260
Hardness Shore A                   76            75        72

Compression set after 70
hours at 150 [degrees] C

Press cured only, % set            40            46        56
Post cured, % set                  23            34        44

Base formula: 100 European R, 3 Vanfre AP-2, 1 stearic acid, 50 N-550 FEF black, 1.5 butyl zimate.

[Figure 9 ILLUSTRATION OMITTED]

Similar cures are achieved with the same thiadiazole curatives in polyacrylates containing a vinylchloroether cure site. In this polymer, using a thiuram disulfide (butyl tuads) as the accelerator gives a shorter cure time with longer scorch time than does the zinc dithiocarbamate (butyl zimate) used with the vinylchloroacetate cure site polymer.

Summary

2,5-Dimercapto-1,3,4-thiadiazole and its derivatives can be used to crosslink a wide variety of halogen-containing polymers. Successful use of the thiadiazole in a given polymer depends on the particular derivative and any associated accelerator.

Acknowledgements

"New developments in curing halogen-containing polymers" is based on a paper given at the May, 1998 Rubber Division meeting.

"Anisotropy in thermoplastic elastomers" is based on a paper given at the October, 1997 TPE RETEC meeting.

"Tackifying rubber compositions" is based on a paper given at the May, 1997 Rubber Division meeting.

"Compositions of isoprene and halogenated EPDM rubbers" is based on a paper given at the October, 1997 International Rubber Conference.

References

[1.] Robert F. Ohm and Tommy C. Taylor, Rubber World, p. 33 (March, 1997) and Gummi Fasern Kunststoffe, p. 62 (Jan., 1997) (German), based on Paper No. 45 presented at the ACS Rubber Division meeting (May 1996).

[2.] Irwin J. Gardner, Robert F. Ohm, Douglas D. Flowers and James V. Fusco, Paper No. 86 presented at the ACS Rubber Division meeting (Oct. 1993).

[3.] F.P. Baldwin, et. al., Rubber & Plastics Age, Vol. 42, p. 500 (1961).

[4.] John R. Richwine, U.S. patent no. 4,128,510 (to Hercules) and foreign equivalents.