Post vulcanization stabilizer in compounding for improved durability.
Over the years, the rubber industry has developed two compounding approaches to address the changes in crosslink structure during thermal aging. These are the use of high accelerator/low sulfur ratios or sulfur donors, both of which provide a lower sulfur content in the crosslinks found daring vulcanization and thus improved thermal stability.
As with many formulation changes in rubber compounding, there is a compromise that must he made when attempting to improve one performance characteristic. Improving the thermal stability of vulcanized natural rubber compounds by reducing the sulfur content of the crosslink through the use of a more efficient vulcanization system will reduce dynamic performance properties such as fatigue resistance, as indicated in figure 1.
[FIGURE 1 OMITTED]
The challenge became to define a way of improving thermal stability while maintaining performance characteristics. Work initiated at our laboratories to address this issue had led to the development and commercialization of disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS or HTS), a rubber chemical that promotes the formation of flexible hybrid crosslinks (figure 2).
[FIGURE 2 OMITTED]
Disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS) is a crosslinking agent which generates hybrid crosslinks, containing both sulfur and carbon atoms interposing a hexamethylene 1,6 dithiyl group within the polysulfidic crosslinks during vulcanization. The generation of such hybrid crosslinks increases the resistance to changes in crosslink structure and density encountered with overcure, high temperature cures and anaerobic aging. This, in turn, reduces deterioration in physical and dynamic properties associated with reversion.
By reducing the average sulfur chain length at the points of attachment to the polymer backbone, thermal stability is improved. Maintaining a long chain within the crosslink structure provides enhanced flexibility under dynamic conditions. The use of DHTS eliminates the compromise between thermal aging and dynamic properties, as illustrated in table 1.
Figure 3 shows that the DHTS compound exhibits reversion resistance which is similar to the semi-EV system, but with fatigue resistance comparable to that of the control system using conventional sulfur levels.
[FIGURE 3 OMITTED]
The overall reaction product of DHTS in a sulfur cure can be visualized as depicted in figure 2. However, the step-wise sequence of reactions leading to a vulcanizate crosslinked in this manner is not well understood. Moreover, the chemical nature of DHTS does not lend itself well to analysis, and much of the mechanistic evidence gathered to date is of an indirect nature. A substantial amount of work has been done to establish how this post vulcanization stabilizer functions during and after sulfur vulcanization (ref. 1). As a bis-Bunte salt, disodium hexamethylene-1,6-bis-thiosulfate dihydrate suffers from insolubility in non-polar solvents and is very reactive with chemical nucleophiles. The net result is that, once DHTS is incorporated in rubber, any remaining DHTS and its reaction products are not easily detected. Nonetheless, significant chemistry of its reactions with and without rubber additives has been elucidated and is shared here.
Conventional and semi-EV cures of natural rubber compounds containing disodium hexamethylene-l,6-bis-thiosulfate exhibit improved tear resistance, flex fatigue and dynamic properties associated with the retention of a polysulfidic network on aging. These performance characteristics of rubber cured with DHTS suggest the formation of flexible crosslinks which compensate for the normal decrease in sulfur rank of the initial polysulfide network during the anaerobic aging process. This observation for DHTS cures is in accord with the work of others (refs. 2 and 3) who reported similar properties in rubber containing hybrid crosslinks. That work provided the incentive to explore possible chemistries that would also indicate the formation of hybrid crosslinks whenever DHTS is used in rubber.
In 1994, Nordsiek and Wolpers reported (ref. 3) that 1,2-dithiacyclooctane (DTCO), when used in combination with rubber accelerators and low levels of sulfur, effectively crosslinks olefinic polymer chains to give heat stable vulcanizates. Indeed, compelling evidence was presented for a stable modulus at curing temperatures as high as 180[degrees]C. Using differential scanning calorimetry (DSC) followed by HPLC analyses of the residue, we have observed efficient formation of a hybrid crosslinker (1) from DTCO and MBTS, as shown in equation 1.
No evidence of the crosslink precursor (I) was discernable under the same conditions when MBT was substituted for MBTS, and only modest concentrations of I formed in the presence of the sulfenamide, TBBS. Furthermore, when we added DTCO to a conventional sulfenamide cure of NR, we observed little reversion and excellent dynamic properties with low heat build-up. When DHTS (3 phr) was added to the NR compound (table 1 master) and heated in the absence of sulfur and accelerator, significant concentrations of DTCO were generated and measured by GC-MS.
Similarly, on heating DHTS in the presence of TBBS and sulfur via the DSC technique, lower quantities of DTCO could be measured. This work corroborates that of Moniotte (ref. 4) and shows that DHTS or its reaction products not only generate DTCO on heating in rubber, but that the cyclic disulfide is also produced more slowly in the presence of sulfur and accelerators. To the extent that MBTS is formed in benzothiazole cure packages, some of the DTCO likely is trapped before it can escape the rubber matrix. Thereby, one mode of action that DHTS can take is to form a simple hybrid crosslinker, I, in situ as shown above in equation 1. As Trivette (ref. 2) teaches, 1 readily inserts elemental sulfur and ultimately inserts hybrid crosslinks between polymer chains. However, this is not believed to be the predominant pathway of DHTS action in a vulcanizate.
On heating DHTS and zinc oxide via DSC, a broad exotherm or gas evolution is observed beginning at 130[degrees]C and ending at 165[degrees]C. In the absence of zinc oxide, only the DHTS waters of hydration are liberated prior to attaining 130[degrees]C under the same DSC conditions. It is believed that the primary reaction between zinc oxide and DHTS in rubber involves the formation of disulfidic oligomers of the type shown here where the ligands can be hydroxide and/or stearic acid:
[[Na[O.sub.3]S[(S[(C[H.sub.2]).sub.6]S).sub.n]].sub.2]Zn. Ligands II
In this fashion, the derived zinc oligomers of the bis bunte salt become soluble in the rubber matrix. When heated in the presence of sulfur and accelerator, complexes like II are believed to serve as precursors to oligomeric polysulfides and 1,2-dithiacyclooctane (DTCO) by interacting with accelerator-zinc complexes (ref. 5) present during and after vulcanization. The sulfur that is introduced in the productive stage of mixing inserts into the disulfide bond, forming a hybrid crosslinking reservoir capable of generating hybrid crosslinks in the presence of activating zinc complexes such as Zn[(MBT).sub.2][Ligand.sub.2]. Thereby, the hybrid crosslinks formed (figure 2) help to retain longer crosslinks during and after vulcanization and subsequently, realize physical properties associated with a polysulfidic network. When heat history causes this crosslink network to mature in the presence of zinc complexes, even though elemental sulfur is extruded from the crosslink, the methylene bridges serve to maintain more stable crosslinks between polymer chains. Under post cure aging conditions, the maturated DHTS network continues to sustain the physical properties normally attributed to a conventional polysulfidic network.
Comparison to other thermal resistant vulcanization systems
It is well known that thermal and thermal-oxidative aging resistance of natural rubber can be improved by the use of efficient vulcanization (EV) systems. EV systems are produced by the use of sulfur donors in place of elemental sulfur or by empolying very high ratios of accelerator or sulfur. The vulcanization systems produce vulcanizates in which a high portion of the crosslinks are monosulfidic and disulfidic with minimal modification of the main chain by sulfurization. When compared with conventional high sulfur and low accelerator natural rubber compounds, efficiently vulcanized compounds exhibit excellent resistance to reversion and oxidative aging. However, these compounds give relative poor dynamic fatigue properties. Efforts made to overcome this deficiency have led to the so-called semi-efficient (semi-EV) vulcanization systems. The semi-EV systems are obtained by the use of intermediate sulfur to accelerator ratios, or by partial replacement of sulfur with a sulfur donor. When compared with the conventional high sulfur and low accelerator systems, the semi-EV systems also provide excellent resistance to thermal and thermal oxidative aging with much improved fatigue properties as compared to the EV systems. This section presents some laboratory data comparing the above four vulcanization systems to a number of semi-EV systems in natural rubber compounds in processing, curing and functional properties before and after aging.
Results and discussion
Thermal resistance (reversion)
Reversion resistance is measured by the percent tensile and elongation retentions on overcure (10 x 190 mins.) at 150[degrees]C (figure 4). As expected, all the cure systems with the low sulfur level of 1.5 phr provide improved reversion resistance as compared to the TBBS control compound with the conventional sulfur level of 2.5 phr. The cure systems based on TBSI (compound 2), DHTS and BCI-MX (compounds 7-9) also can give improved reversion resistance in spite of the fact that these systems use the conventional sulfur level of 2.5 phr. Among the improved reversion resistance cure systems, the following ranking in decreasing order is observed:
[FIGURE 4 OMITTED]
BCI-MX/DHTS > BCI-MX > ZBPD > DHTS > TMTD > semi-EV > DTDM > TBSI > TBBS
Thermal oxidative resistance (hot air aging)
Heat aging is conducted in a hot air oven at 100[degrees]C for 24 and 48 hours (figure 5). Aging resistance is measured by the percent of tensile retention of the test samples after exposure to these aging conditions. Again as expected, the low sulfur compounds provided the better heat aging resistance than the conventional sulfur compounds. There are no significant differences in this oxidative aging comparison among the cure systems within the two different sulfur level groups.
[FIGURE 5 OMITTED]
Flexometer heat build-up data
Overall, the semi-EV cure systems based on TMTD and ZBPD give the lowest heat generation data (as indicated by the longer blowout time), followed by the BCI-MX and DHTS/BCI-MX conventional sulfur cures, which also exhibit excellent resistance to heat generation. These conventional sulfur cures outperform both the semi-EV cures based on the hi/lo and the DTDM systems.
The best fatigue resistance cure system is the DHTS-based conventional cure, followed by the DHTS/BCI-MX system (figure 6). Next in line are the conventional sulfur cures based on TBBS, TBSI and BCI-MX. As expected, the semi-EV cure systems exhibit low fatigue properties. Among these systems, the hi/lo and DTDM cures give better fatigue properties than the TMTD and ZBPD systems.
[FIGURE 6 OMITTED]
Trouser tear data show that the DTDM semi-EV cure system give the best tear resistance properties especially under overcure environment. This is followed by the the two HTS based compounds. All the other cure systems exhibit similar tear properties.
Both the hi/lo and DTDM semi-EV cure systems give long scorch time (Mooney scorch time t5) with fast rheometer cure rates as compared to the control cure system TBBS (figures 7 and 8). There is a slight reduction in scorch satiety with the DHTS-based cure systems. Although giving very fast cure, the TMTD and ZBPD semi-EV cures also give the shortest scorch times.
[FIGURES 7-8 OMITTED]
There is no significant difference in tensile and elongation among the cure systems evaluated. Although the semi-EV based cures tend to exhibit slightly higher 300% modulus.
In summary, the proper selection of semi-efficient vulcanization systems has the potential of providing a good balance of properties which indicates that the choice of a particular system will depend on a number of factors which can include processing, curing, properties retention on aging, cost and other performance properties. Table 3 compares these various vulcanization systems.
Effect of disodium hexamethylene-1,6-bis-thiosulfate (DHTS) on rubber to brassed steel adhesion
As reported in an earlier publication (ref. 1), DHTS is a bonding promoter for natural rubber to brass-coated steel cord adhesion. It has been reported that polysulfides are involved in bonding with the copper subsulfide layer on the brass surface of the wire cord. During service, the polysulfidic crosslinks degrade progressively and the sulfur liberated contributes to a further sulfidification of the copper subsulfide. Thus, bigger and more brittle copper sulfide crystals are formed, thereby weakening the rubber-brass layer bond. During such a process, the zinc can also be sulfidized, further weakening the bond strength. In the presence of DHTS, hybrid crosslinks are formed in the vicinity of the rubber to metal interface. Because these hybrid crosslinks maintain their polysulfidicity longer than the classical polysulfidic crosslinks, the bonds formed between the rubber and the brass layer retain their strength longer during service.
A second benefit of using DHTS is worth noting. With the classical formulation, it is difficult to maintain the best retention of the bond strength during both steam and saline solution aging conditions. Generally, an optimized formulation provides the bond retention during only one of these two aging conditions. The use of DHTS provides the best maintenance of the rubber-brass bond during both steam and saline solution aging tests as shown in table 4.
Table 5 shows that sulfur loadings may be reduced in the presence of DHTS, without loss of adhesion, thus improving the compound stability.
Disodium hexamethylene-1,6-bis-thiosulfate dihydrate (DHTS) is a crosslinking agent which generates hybrid crosslinks containing both sulfur and carbon atoms. DHTS interposes a hexamethylene 1,6-dithiyl group within the polysulfidic crosslinks during vulcanization. The generation of such hybrid crosslinks increases the resistance to anaerobic aging of the rubber vulcanizates and improves the adhesive strength between rubber and brass-coated steel wire. Optimization of DHTS based cure systems can be achieved through statistically designed experiments, giving the compounder flexibility to select the vulcanization system which best fits its performance requirements.
Table 1 - effect of DHTS on thermal aging and fatigue of natural rubber Sulfur, phr 2.5 2.5 1.20 TBBS, phr 0.6 0.6 1.75 DHTS -- 2.0 -- Retained 300% modulus (%) 10 x t' (90) @ 144[degrees]C 75.0 91.0 105.0 5 x t' (90) @ 181[degrees]C 56.0 68.0 69.0 t' (90) @ 181[degrees]C 73.0 89.0 91.0 vs. t' (90) @ 140[degrees]C Fatigue-to-failure (KC) @ 100% strain t' (90) @ 144[degrees]C 197.0 241.0 123.0 10 x t' (90) @ 144[degrees]C 127.0 169.0 90.0 Table 1 master Phr SMR 5 100 N-330 black 50 Aromatic oil 5 Zinc oxide 5 Stearic acid 2 6 PPD 2 Table 2 - cure system comparison study in natural rubber * Compound 1 2 3 4 5 Cure system I.D. Conv. TBSI Hi/Lo TMTD ZBPD Sulfur 2.5 2.5 1.5 1.5 1.5 TBBS 0.6 -- 1.5 0.7 1.0 TBSI -- 0.6 -- -- -- TMTD -- -- -- 0.5 -- ZBPD -- -- -- -- 1.25 DTDM -- -- -- -- -- DHTS -- -- -- -- -- BCI-MX -- -- -- -- -- Compound 6 7 8 9 Cure system I.D. DTDM DHTS BCI- DHTS/ MX BCI-MX Sulfur 1.5 2.5 2.5 2.5 TBBS 0.8 0.6 0.6 0.6 TBSI -- -- -- -- TMTD -- -- -- -- ZBPD -- -- -- -- DTDM 0.6 -- -- -- DHTS -- 2.0 -- 2.5 BCI-MX -- -- 0.75 1.0 * Table 1 master Table 3 - comparison of natural rubber vulcanization systems Cure system Semi- Conven- Conven- EV tional tional plus DHTS Processing/curing properties Scorch safety 100 -/=/+ - Rheometer cure rate 100 -/=/+ - Reversion resistance Extended cure time 100 - + High temperature cure 100 - + Performance properties Oxidative aging resistance 100 - = Heat build-up resistance 100 - + Fatigue resistance 100 + ++ Tear resistance 100 + + Relative cost comparison 100 - -/=/+ Cure system Conven- Conventional tional plus plus DHTS/BCI- BCI-MX MX Processing/curing properties Scorch safety = = Rheometer cure rate = = Reversion resistance Extended cure time ++ +++ High temperature cure ++ +++ Performance properties Oxidative aging resistance = = Heat build-up resistance ++ +++ Fatigue resistance = + Tear resistance -/= + Relative cost comparison + ++ Table 4 - DHTS vulcanization system in rubber to brassed steel adhesion (effect of DHTS loading) Insoluble sulfur (80%) 5.0 5.0 5.0 DCBS 1.75 1.75 1.75 DHTS 0 3 6 Wire adhesion test (ASTM 2229) Unaged (N) 476 (9) * 494 (9) 478 (9) Steam, 8 hrs./120[degrees]C (N) 466 (8) 417 (8) 407 (8) Salt (5%), 48 hrs./90[degrees]C (N) 184 (0) 363 (4) 467 (5) Aged tensile properties (48 hrs. @ 100[degrees]C) % Tensile retained 79 78 76 % Elongation retained 48 46 49 * Numbers in parentheses indicate wire coverage with (0) = no coverage and (10) = full coverage. Test compound Phr SMR 5L 100 N-339 black 55 Aromatic oil 3 Zinc oxide 8 Stearic acid 0.5 6 PPD 2.0 TMQ 1.0 Cobalt borate 0.1 Table 5 - DHTS vulcanization system in rubber to brassed steel adhesion (effect of reduced sulfur loading) Insoluble sulfur (80%) 5.0 3.27 6.73 DCBS 1.75 0.84 0.84 DHTS 0 1.27 1.27 Wire adhesion test (ASTM 2229) Unaged (N) 476 (9) 522 (9) 494 (9) Steam, 8 hrs./120[degrees]C (N) 466 (8) 439 (8) 470 (7) Salt (5%), 48 hrs./90[degrees]C (N) 184 (0) 253 (2) 143 (2) Aged tensile properties (48 hrs. @ 100[degrees]C) % Tensile retained 79 88 62 % Elongation retained 48 71 39
(1.) Anthoine, G., Lynch, E.R., Mauer, D.E. and Moniotte, P.G., "A new concept to stabilize cured NR properties during thermal aging and improve adhesion to brass," Rubber Division Meeting, 1985.
(2.) C.D. Trivette (to Monsanto Co.), U.S. Pat. 3,869,435 (1975).
(3.) K.H. Nordsiek and J. Wolpers, KGK Kautschuk Gummi Kunststoffe 47, 319 (1994).
(4.) PG. Moniotte, Internal Monsanto Report, September 21, 1992.
(5.) J.I. Cunneen and R.M. Russell, Rubber Chemistry, and Technology, 43, 1,215 (1970).
Byron H. To and Otto W. Maender, Flexsys America and Gilbert Anthoine, Flexsys N.V.
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|Comment:||Post vulcanization stabilizer in compounding for improved durability.|
|Date:||Nov 1, 2002|
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