A Review on Heat and Reversion Resistance Compounding

Transcription

1 A Review on Heat and Reversion Resistance Compounding A Review on Heat and Reversion Resistance Compounding R. N. Datta Flexsys BV, Technology Center, Zutphenseweg 10, 7418 AJ Deventer, The Netherlands. Rabin.n.datta@Flexsys.com Received: 2 September 2002 Accepted: 9 January 2003 ABSTRACT When sulfur vulcanized natural rubber compounds are exposed to a thermal ageing environment significant change in physical properties and performance characteristics are observed. These changes are directly related to modifications of the original crosslink structure. Decomposition reactions tend to predominate and thus leading to reduction in crosslink density and physical properties as observed during extended cure and when using higher curing temperatures. The decrease in network density is common when vulcanizates are subject to an anaerobic ageing process. However, in the presence of oxygen, the network density is increased with the main chain modifications playing a vital role. Over the years the rubber industry has developed several compounding approaches to address the changes in crosslink structure during thermal ageing. This paper gives a review of these compounding approaches. As with many formulation changes in rubber compounding, there is a compromise that must be made when attempting to improve one performance characteristic. For example, improving the thermal stability of vulcanized natural rubber compounds by reducing the sulfur content of the crosslink through the use of the more efficient vulcanization systems will reduce dynamic performance property such as fatigue resistance. The challenge is to define a way to improve thermal stability while maintaining dynamic performance characteristics. 1. INTRODUCTION Truck and off-the-road tires of today are frequently required to operate at high loads and high speeds for extended periods of time. These severe operating conditions result in greater heat build up than would normally be encountered under less demanding service conditions. As a consequence, the excessive running temperature leads to reversion in compound components which may in turn lead to reduced tire durability or, in extreme circumstances, to tire failure. Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

2 R. N. Datta Much effort has been spent over the years to improve the reversion characteristics of tire compounds. An established and, probably, best known approach is the use of so-called semi-efficient cure systems comprising reduced sulfur levels and increased accelerator levels. However, though effective in improving the vulcanizate s resistance to reversion, this approach is only partially successful since lowering of sulfur levels negatively influences other desirable properties such as tear and flex/fatigue life. With the advent of the radial tire, natural rubber (NR) usage has increased significantly. Its tack and high green strength serve to maintain green tire uniformity during building and shaping operations. For both new and retreaded truck tires, however, the reversion characteristics of NR have been considered to detract from wear performance, particularly in the early stages of tire life where the tread surface has been subjected to the greatest degree of overcure. Accelerated wear in later tire life may also occur due to thermal-oxidative degradation of the vulcanizate, particularly cured with conventional systems. The need for chemicals that provide reversion resistance is, therefore, of great interest. This paper briefly reviews approaches to improve reversion resistance through the use of antireversion chemicals, with emphasis on their applicability in truck tire tread compounds. These antireversion chemicals are: a) Thiuram accelerators b) Dithiophosphate accelerators (Vocol ZBPD) c) Zinc soap activators (Struktol A73) d) Silane coupling agent, TESPT e) Post vulcanization stabilizer, Duralink HTS f) Antireversion agent, Perkalink 900 g) 1,6-bis (N,N-dibenzylthiocarbamoyldithio) hexane, BDTCH, Vulcuren KA 9188 h) Some other candidates Figure 1 illustrates the structural formula of these compounds. 144 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

3 A Review on Heat and Reversion Resistance Compounding Figure 1 Structures of thiuram (Ia), dithiocarbamate (Ib), dithiophosphate (II), zinc soap activator (III), silane coupling agent (IV), post vulcanization stabilizer, HTS (V), antireversion agent, Perkalink 900 (VI), BDTCH, Vulcuren 9188 (VII) In addition to the above, the changes in the network structure during oxidative ageing process are addressed. A novel chemical, N-Cyclohexyl- 4,6-Dimethyl-2-Pyrimidine Sulfenamide (CDMPS) is recently introduced to address the changes in the network experienced during service environments. The structure of CDMPS is shown in Figure 2. Figure 2 Structure of cyclohexyl-4,6-dimethyl-2-pyrimidine sulfenamide (CDMPS) Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

4 R. N. Datta 2. EXPERIMENTAL Compound formulations are shown in the respective data tables. Compound mixing was carried out in a similar manner as described earlier (1). Stress strain and tear properties of the vulcanizates were measured using a Zwick universal testing machine (model 144) in accordance with ISO 37 and ISO 34 respectively. Heat build up was measured on a Goodrich Flexometer according to the method described in ASTM D623. Fatigue to Failure properties were measured on a Monsanto Fatigue to Failure tester at 23 C and 1.67Hz with 0-100% extension. The volume fraction (Vr) in the swollen vulcanizate was determined by equilibrium swelling in toluene using the method reported by Ellis and Welding (2). The Vr value so obtained was converted to the Mooney-Rivlin elastic constant (C 1 ) and finally to the concentration of chemical crosslinks by using equations described in the literature (3,4). The proportions of mono, di, polysulfidic and carbon-carbon crosslinks were determined using the method reported elsewhere (1). 3. RESULTS AND DISCUSSION 3.1 Protection against reversion in anaerobic ageing environments A high number of chemicals are available to address reversion concerns of compounds facing anaerobic ageing conditions, such as overcure or exposure to heat under limited supply of air. The structures of these chemicals are shown in Figure 1 and their functions with respect to performance characteristics are described in this section Thiuram accelerators Thiurams such as tetramethylthiuram disulfide (TMTD), tetrabenzylthiuram disulfide (TBzTD) etc. and dithiocarbamates such as zinc dimethyldithiocarbamates (ZDMC) and zinc dibenzyldithiocarbamates (ZBEC) (Figure 1, 1a and 1b, where M=Zn) give an improvement in reversion resistance when used together with sulfenamide accelerators. This improvement is clearly demonstrated in Table Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

5 A Review on Heat and Reversion Resistance Compounding Table 1 Curatives, phr Sulfur CBS TMTD TBzTD M ooney scorch, 121 C, t, min R heometer, 10 C M L, Nm M H -M, Nm L ts2, min t90, min R eversion, % (10 C/60 ) Formulations: NR,100; N-330, 0; Aromatic oil, 3; Zinc oxide, ; S tearic acid, 2; Santoflex 6PPD, 2 % Reversion = (R R t)/( R ) x 10 * max max max-min The restriction for the choice of TMTD, ZDMC and similar candidates is impaired due to the N-Nitrosamine issue (). In addition, although the combination of a sulfenamide and a thiuram/dithiocarbamate (TMTD/ ZDMC) shows improved reversion resistance, these systems suffer from adverse effect on scorch and vulcanizate dynamic properties (6). Taking into consideration all aspects of compounding and industrial hygiene, TBzTD or ZBEC are the better choice to target reversion resistance compounding with thiurams or dithiocarbamates. Recently, Datta et.al reported (7) that small amount ( phr) of TBzTD is capable of improving reversion resistance without significantly affecting processing as well as dynamic properties. The improvement in reversion resistance is due to the presence of a high proportion of mono-sulfide crosslinks in sulfenamide/thiuram binary combinations (Table 2). The improved reversion resistance of the sulfenamide/thiuram cure system is due to formation of zinc accelerator complexes, I and II Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

6 R. N. Datta Table 2 Vulcanizate structure effect of thiurams, TMTD and TBzTD Mixes C ure temp., C Cure time, min Crosslink density* Total % polysulfide % disulfide % monosulfide * Crosslink densities expressed in gram mole/gram rubber x 10 respectively for TBzTD and TMTD (Figure 3). The solubility in these complexes in rubber compounds determines the reactivity and consequently desulfuration of initially formed polysulfidic crosslinks (7). This results in a network containing a higher concentration of monosulfidic crosslinks. Figure 3 Complex formed in sulfenamide-thiuram/zinc oxide acceleration 148 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

7 A Review on Heat and Reversion Resistance Compounding Dithiophosphate accelerators Dithiophosphates such as Zinc dibutyl dithiophosphate (ZBPD, II, Figure 1) give an improvement in reversion resistance when used together with sulfenamide accelerators. This is clearly demonstrated by the cure data listed in Table 3. The improvement in reversion resistance is due to the greater proportion of mono and di sulfidic crosslinks formed with the sulfenamide/zbpd combination - Table 4. Table 3 Vulcanizate characteristics Curatives, phr 04 0 M ooney scorch, 13 C R heometer, 144 C Sulfur TBBS ZBPD T, min t2,min t90,min t90-t2,min R eversion, % (160 C/30 ) * Formulations: NR,100; N-330, 0; Naphthenic oil, ; Zinc oxide, ; S tearic acid, 2; Santoflex 6PPD, 2 % Reversion = (R R /( R ) x 10 * - ) 0 max max+ t max-min Table 4 Vulcanizate structure effect of dithiophosphate, ZBPD 04 0 C ure temp., C Cure time, min Crosslink density* Total % Polysulfide % Disulfide % Monosulfide * Crosslink densities expressed in gram mole/gram rubber x 10 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

8 R. N. Datta Complex (I) (Figure 4), formed during sulfenamide/dithiophosphate curing reactions, is responsible for the increased level of mono- and disulfidic crosslinks. It reacts with the initially formed polysulfidic crosslinks thereby reducing their sulfur rank (8). The following advantages and disadvantages have been found for vulcanizates cured with a sulfenamide/dithiophosphate combination (9). Advantages: - Excellent reversion resistance - Improved cure rate - Reduced heat build up and dynamic set Disadvantages: - Reduced scorch safety - Decline in initial fatigue properties - Decreased adhesion of rubber to brass coated steel Figure 4 Complex formed in sulfenamide-dithiophosphate acceleration 10 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

9 A Review on Heat and Reversion Resistance Compounding Zinc soap activators These are mixtures of selected aliphatic and aromatic carboxylic acids (III, Figure 1). Studies suggest that they are more soluble in a rubber matrix than the zinc salt of stearic acid (10). Their mode of action is to react with initially formed polysulfidic crosslinks thereby reducing their sulfur rank. The effect of a zinc salt activator on compound cure characteristics, tensile properties and network structure is summarized in Tables and 6. Table Cure characteristics and vulcanizate properties Struktol A 73, phr 0 3 Stearic acid, phr 2 0 R heometer, 10 C Delta S, Nm t2, min t90,min Cure rate,t90-t2, min R eversion, % (10 C/2h) C ure 10 C/t90 Modulus100%, MPa Modulus 300%, MPa Tensile strength, MPa Elongation at break, % 40 Fatigue to Failure, Kc (0-100% extension) C ure 10 C/2h Modulus100%, MPa Modulus 300%, MPa Tensile strength, MPa Elongation at break, % Formulations: NR, 100; N-37, 4; Zinc oxide, ; Peptizer, 0.3; Wax, 1.4; Santoflex PVI, 0.; Santoflex 6PPD, 1.2; Flectol TMQ, 1.; Santocure TBBS, 1.4; Sulfur, 1.4 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

10 R. N. Datta Table 6 Vulcanizate structure effect of Struktol A 73 Crosslink density* C ure temp., C Cure time, min Total % Polysulfide 0 40 % Di sulfide 1 12 % Monosulfide 3 48 * Crosslink densities expressed in gram mole/gram rubber x 10 As shown in Table 6, the zinc salt activator produces a network containing a higher proportion of monosulfidic crosslinks. The network degradation is accelerated by the presence of additional complex as shown in Figure. However, whilst vulcanizate reversion resistance is improved, flex/fatigue characteristics may be hampered (Table ). Figure Complex formed from Zinc salt activator Silane coupling agent It has been reported (11) that the silane coupling agent TESPT (IV, Figure 1), in addition to its silica coupling role, is capable of providing reversion resistance by acting as a sulfur donor, the so called equilibrium cure concept. 12 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

11 A Review on Heat and Reversion Resistance Compounding The effect of the silane-coupling agent on cure characteristics, compound viscosity and vulcanizate tensile properties are presented in Table 7. It is clear that TESPT decreases cure rate (t90-t2) but improves processability through compound viscosity reduction. The increase in modulus at the t90 cure condition can, in part, be attributed to the increase in crosslink density (Table 8), a result of its sulfur donor activity. The coupling agent also leads to a network containing a higher proportion of monosulfidic Table 7 Cure characteristics and vulcanizate properties R heometer, 10 C TESPT, phr 0 3 Delta S, Nm ML, Nm t2, min t90, min Cure rate, t90-t2, min M ooney scorch, t, 121 C M ooney viscosity, ML(1+4), 100 C, MU 94 8 R eversion, % (10 C/4h) C ure: 10 C/t90 C ure: 10 C/4h Modulus100%, MPa Modulus 300%, MPa Tensile strength, MPa Elongation at break, % 80 0 Modulus100%, MPa Modulus 300%, MPa Tensile strength, MPa Elongation at break, % Formulations: NR, 100; N-220, 40; Silica, 20; Zinc oxide, ; Stearic acid, 2; Aromatic oil, 3; Resin Coumaron, 3; Wax, 1.0; Flectol TMQ, 1.; Santoflex 6PPD, 2.; Santocure CBS, 1.4; Perkacit DPG,2; Sulfur, 1.4 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

12 R. N. Datta Table 8 Vulcanizate structure- effect of TESPT Crosslink type* Cure temp. 10 C time Total t h Polysulfide t h Disulfide t h Monosulfide t h * Crosslink densities expressed in gram mole/gram rubber x 10 crosslinks thereby providing improved reversion resistance. The crosslink density data also indicate that TESPT is capable of maintaining a higher level of crosslink density on overcure compared to the control compound Post-vulcanization stabilizer (Duralink HTS) Duralink HTS (Figure 1, V) is the tradename for hexamethylene bisthiosulphate disodium salt dihydrate. During vulcanization it produces hybrid crosslinks of the structure indicated in Figure 6. These hybrid crosslinks incorporate a flexible hexamethylene group between the sulfur atoms attached to the polymer backbone. The sulfur/accelerator levels and ratio, as in the normal sulfur vulcanization process will dictate the length of the sulfidic units. During overcure or during service, desulfurization of the polysulfidic crosslink structure would occur as is normally observed during the 14 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

13 A Review on Heat and Reversion Resistance Compounding reversion process. However, the structure of the hybrid crosslink is such that desulfurization leads to a monosulfidic/hexamethylene crosslink that retains the flexibility of a polysulfidic crosslink but possessing the thermal stability of a monosulfidic crosslink Figure 7. This built in flexibility provides compounds containing Duralink HTS with good retention of flex/fatigue properties. The effect of Duralink HTS in a typical NR stock is shown in Table 9. The data clearly show that HTS improves reversion resistance and fatigue life before and following ageing. Duralink HTS is also effective in maintaining adhesion to brass-coated steelcord (12). Figure 6 Hybrid crosslinks formed by Duralink HTS Figure 7 Effect of reversion on final crosslink structure Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

14 R. N. Datta T able 9 Improved reversion resistance with Duralink HTS Stocks Sulfur Santocure TBBS Duralink HTS R heometer reversion at 181 C % reversion in 60 min 3 23 Modulus reversion Modulus 300%, Mpa t 90 at 144 C x t90 at 144 C t 90 at 181 C Fatigue to Failure, Kc (100% extension) Unaged A ged (2d/100 C) 37 1 Heat build up, T at 100 C t 90 + at 144 C B low out time, min at 100 C t 90 + at 144 C Formulation: NR, 100; N-330, 0; Zinc oxide,.0; Stearic acid, 2.0; A romatic oil,.0; Santoflex 6PPD, Antireversion agent, Perkalink 900 The antireversion agent 1,3-bis(citraconimidomethyl)benzene, Perkalink 900, Figure 1 (VI), is extremely effective at counteracting reversion. It reacts by a unique crosslink compensation mechanism whereby sulfidic crosslinks, destroyed during the course of reversion, are replaced by crosslinks based on the Perkalink 900 molecule, Figure 8. In doing so the crosslink density of vulcanizates can be maintained if overcure occurs during the vulcanization process or on ageing as encountered during tire service. As a result, mechanical properties are stabilized thereby improving tire performance. 16 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

15 A Review on Heat and Reversion Resistance Compounding Figure 8 Mechanism of Perkalink 900 reaction The crosslink compensation mechanism occurs only at the onset of reversion as a result of the formation of conjugated dienes and trienes along the polymer backbone (13). The antireversion agent reacts with these conjugated structures via a Diels-Alder addition reaction (14). In doing so Perkalink 900 does not affect initial cure characteristics: scorch time, cure rate and t90. Furthermore, it can be added to compounds with no additional change in formulation or modification of mixing and processing conditions. Heat generated in truck tires under demanding service conditions will lead to reversion of compounds within the tire. The reversion is self-perpetuating since it lowers the modulus of the compounds which in turn increases the rate of heat build up. Ultimately the integrity of the tire suffers, resulting in reduced tire durability. The incorporation of Perkalink 900 in truck tread compounds will prevent reversion during service thereby reducing the rate of heat build up. This will aid in maintaining the integrity of the tire ensuring improved durability. Laboratory and tire test results are presented below demonstrating the beneficial effect in an NR/BR based truck tire compound. Compound formulations are shown in Table 10. Pekalink 900 was added at a level of 0. phr, as recommended for a semi-efficient cure system. The cure characteristics of both compounds are shown in Table 11. Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

16 R. N. Datta Table 10 Compound formulations I ngredients 12 (Control ) 13 (Pk900 ) NR SMR CV BR Buna CB N-37 Zinc oxide Stearic acid 2 2 Aromatic oil 8 8 S antoflex 6PPD 2 2 S antocure CBS Sulfur P erkalink T able 11 Cure characteristics at 143 C 12 (control) 13 (Pk 900) Rheometer MDR 2000 E Torque max, Nm Torque min, Nm ts2, min t90, min Torque at 60 min, Nm It is interesting to note that Perkalink 900 does not affect initial cure characteristics (1). The control compound shows a drop in torque on extended cure (60 minutes) despite the semi-efficient cure system, whilst the test compound shows no drop in torque level, indicative of the antireversion action provided by the Perkalink 900. The effect of Perkalink 900 on vulcanizate physical properties cured at 143 C, a typical truck tire curing temperature, is shown in Table 12. Data are listed for optimum cure, t90, and overcure, 60 minutes. 18 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

17 A Review on Heat and Reversion Resistance Compounding T able 12 Physical properties of vulcanizates cured at 143 C for t90 and 60 minutes 12 (control) 13 (Pk 900) Modulus 300%, MPa, t Tensile strength, MPa, t Elongation at break, %, t Tear strength, kn/m, t Abrasion loss, DIN, mm, t * Fatigue to Failure, kc t * 0-100% extension The antireversion action provided by the Perkalink 900 is demonstrated by the modulus retention on overcure. Improved abrasion resistance is also seen for the compound containing the antireversion agent. Fatigue properties are not hampered. The most striking feature provided by Perkalink 900 is the reduction in heat built up as measured by the Goodrich Flexometer test, Figure 9. Even at the extended cure of 60 minutes, the sample containing Perkalink 900 did not show any increase in heat build up compared to the sample cured for the shorter period of 2 x t90. The control compound on the other hand experienced blow-out failure following a test time of 40 minutes. This significant reduction in heat build up provided by Perkalink 900 should be reflected in improved truck tire performance. Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

18 R. N. Datta Figure 9 Heat build up at 100ºC after 1hour for samples cured at 2 x t90 and 60 minutes (BO Blow Out). BO Temperature rise, C ,6 bis (N,N-dibenzylthiocarbamoyldithio)hexane, BDTCH, Vulcuren KA 9188 BDTCH (Figure 1) is based on dibenzylthiocarbamoyl leaving group with dithiohexane as a crosslinker. Huls AG patented (16,17) this substance in early nineties. It is recently introduced (18) to the rubber industry as an effective antireversion agent. Based on structural similarity (Figure 10) a comparison has been made between BDTCH with TBzTD and with additional amount of HTS, the moiety present in BDTCH. The compound formulation is presented in Table 13. The cure data of the mixes are shown in Table 14. Figure 10 Structures of TBzTD and BDTCH 160 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

19 A Review on Heat and Reversion Resistance Compounding Table 13 Compound formulation Ingredients 14 1 (+TBzTD) 16 (+BDTCH) 17 (TBzTD+HTS) NR SMR CV N Zinc oxide Stearic acid Aromatic oil Santoflex 6PPD Santocure CBS Sulfur Perkacit TBzTD Vulcuren, BDTCH Duralink HTS T able 14 Cure data of the mixes obtained at 10 C Properties 14 1 (+TBzTD) 16 (+BDTCH) 17 (TBzTD+HTS) Delta Torque, Nm ts2, min t90, min Reversion, % ( 10 C/1h) It is interesting to note that both TBzTD and BDTCH behave similar with respect to torque development and cure time reduction. This is typical for any sulfenamide cure accelerated by thiurams or dithiocarbamates. The only difference observed between TBzTD and BDTCH is on the scorch time. TBzTD is marginally worse than the mix containing BDTCH. This is expected based on the chemistry of BDTCH. As shown in Figure 11, BDTCH cleaves to form TBzTD in situ and this actually is the effective Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

20 R. N. Datta Figure 11 Formation of TBzTD from BDTCH accelerator providing the boosting characteristics with no effect on scorch data. The formation of TBzTD from BDTCH is shown in Figure 11. The formation of TBzTD from BDTCH suggests that the vulcanization chemistry of BDTCH and TBzTD is similar, thus providing identical cure data as shown in Table 14. Addition of Duralink HTS (as it contains S-(CH 2 ) 6 -S. ) is further explored (Mix 17). The cure data as presented in Table 14 suggests that the mixes 17 and 16 behave similar except the scorch time (ts2). The effect of TBzTD and BDTCH on the vulcanizates physical properties at 10 C is shown in Table 1. Data are listed for optimum cure, t90, and overcure, 60 minutes. The antireversion actions provided by TBzTD and BDTCH is demonstrated by modulus retention on overcure. Better abrasion resistance, maintenance of tear and flex properties on overcure are the consequences of heat stability offered by the network formed. The network density and the distribution of the crosslink types were measured and the data are tabulated in Table 16. Comparing the crosslink density and distribution of crosslink types in different compounds, it is clear that both TBzTD and BDTCH have similar network structure and hence the properties are expected to be comparable. It can be therefore concluded that the functionality, -S-C(=S)-N((CH 2 )-Ph) 2, plays the vital role and molecule derived from such a leaving group will provide heat and reversion resistance features. 162 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

21 A Review on Heat and Reversion Resistance Compounding T able 1 Physical properties of vulcanizates cured at 10 C for t90 and 60 minutes Properties Cure State, min Hardness, IRHD t Modulus, 300%, Mpa Tensile strength, Mpa Elongation at break, % Tear strength, kn/m Abrasion loss, 3 DIN, mm Fatigue to Failure*, kc * measured at constant energy t t t t t t T able 16 Crosslink density (Cure: 10 C/t90) Crosslink density* Total XL Poly-S Di-S Mono-S * Crosslink density expressed in gram mole/gram RH x 10 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

22 R. N. Datta Miscellaneous candidates Although the previously mentioned substances are main material in the rubber industry, but there are some candidates cited in the literature to be effective antireversion agent. They are summarized in this section. Multifunctional acrylates have been published (19) to be effective antireversion agent. The structures are shown in Figure 12. Maleimides are reported (20,21) to be effective crosslinkers in sulfur as well sulfur free vulcanization. Recently, Vucht a.s introduced (22), 2,2-dithiobis (phenylmaleimido) (FMI), Figure 13, and claimed this as a chemical that increases the resistance against prevulcanization. The maleimides are good crosslinker but consumed during vulcanization and are not available during the reversion regime, i.e., during overcure or in service environments (23). Figure 12 Structures of acrylates Figure 13 Structure of FMI 164 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

23 A Review on Heat and Reversion Resistance Compounding 3.2 Protection against degradation in oxidative ageing environments One of the burning demands in the rubber industry has been to reduce the changes in the network during the service life of rubber components. In a broad sense, this is also considered as degradation (reversion) of network but under oxidative environments. The changes in the network under aerobic conditions are different in nature than experienced under anaerobic conditions. In a true sense, the rubber compounds experience hardening effects in the presence of oxygen and heat. The process of hardening or increase in modulus or increase in crosslink density is as a result of oxidation of sulfidic crosslinks, liberation of active sulfur and consequently re-crosslinking. The process in schematically shown in Figure 14. It is well known that the degradation of polysulfides is catalyzed by the presence of accelerators (Figure 1). The liberated active sulfur is responsible for introduction of additional crosslinks and hence increases in modulus. Figure 14 Reaction of monosulfides with oxygen/heat forming additional crosslinks Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

24 R. N. Datta Figure 1 Reaction of polysulfides with oxygen/heat forming additional crosslinks In order to reduce the unwanted hardening effects during oxidative ageing, the trap or removal of accelerator fragments is desirable. Over the years, focus was on the development of an accelerator, which becomes inactive after the vulcanization and thereby decreases the kinetics of degradation of polysulfides. The research culminated in a development of a novel accelerator, N-Cyclohexyl-4,6-Dimethyl-2-pyrimidine Sulfenamide (24). CDMPS has been tested in several compounds where the effect on modulus increase during oxidative ageing is monitored. Some typical data are shown in Table Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

25 A Review on Heat and Reversion Resistance Compounding Table 17 Summary of advantage of CDMPS over conventional accelerator SBR-BR Passenger Tread (Summary) R heo; 160 C Control CDMPS Delta S, Nm Control CDMPS ts2, min CBS t90, min CDMPS C ure: 160 C/t90 ( Changes after ageing*) 3 d/100 C Hardness, IRHD MS* (based on M200%), % Elongation drop,% Flex, % (Constant stress) T angent delta, -20 C, % Formulation: MS, %= (Aged M200/Unaged M200) x 100 MBT 0 0. Sulfur 2 2 It is clear from the data that the changes in hardness and modulus during oxidative ageing are considerably reduced. The resistance to hardness increase positively influences the dynamic properties as shown in Table 17. In order to understand the changes in the network during oxidative ageing in the presence of CDMPS, some network studies were carried out. Data are tabulated in Table 18. The increase in crosslink density upon oxidative ageing is minimised in the presence of CDMPS. The level of zinc sulfide is higher indicating that part of sulfur kicked out during the ageing process is trapped by zinc as zinc sulfide. The mechanism of the process is proposed as shown in Figure 16. Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

26 R. N. Datta T able 18 Crosslink density (Cure: 160 C/t90) XL-densities* x 10 Control Unaged CDMPS Total Poly-S Di-S Mono-S ZnS x A ged, 3d/100 C * Total Poly-S Di-S Mono-S ZnS 2 x Crosslink density expressed in gram mole/gram rubber hydrocarbon Zinc sulfide sulfur expressed in gram/gram rubber hydrocarbon Figure 16 Formation of zinc sulfide and trapping of accelerator s rest 168 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

27 A Review on Heat and Reversion Resistance Compounding CDMPS has also been tested in high sulfur skim, NR/BR tread as well as S-SBR silica treads. The advantages are well confirmed in all applications. 4. SUMMARY The mode of action of several rubber chemicals that provide improved antireversion characteristics has been discussed. Whilst some of these chemicals function through the formation of a crosslink network containing a higher proportion of thermally stable monosulfidic crosslinks, both Duralink HTS and Perkalink 900 function via differing, novel mechanisms. Duralink HTS functions through the formation of hybrid crosslinks that provide improved flex/fatigue properties. The principle of Perkalink 900 is: crosslink compensation at the onset of reversion leading to the formation of thermally stable C-C crosslinks. These provide reduced heat build up under dynamic conditions. A new accelerator is developed, which restricts the modulus and hardness increase during the oxidative ageing process. Data suggests that the accelerator is capable of maintaining the initially formed crosslinks thereby keeping changes in compound properties at a minimum. Combining the possibilities as outlined above, it is now possible to optimise a compound having improved heat stability both under anaerobic and aerobic environments.. REFERENCES 1. A. H. M. Schotman, P.J.C. Van Haeren, A. J. M. Weber, F. G. H. van Wijk, J. W. Hofstraat, A. G. Talma, A. Steenbergen, R. N. Datta, Rubber Chem. Technol., 69, 727 (1996) 2. B. Ellis and G. N. Welding, Rubber Chem. Technol, 37, 71 (1964) 3. P. J. Flory and J. Rehner, J. Chem. Phys. 11, 21 (1943) 4. L. Mullins, J. Appl. Polym. Sci., 2, 1 (199). H. Lohwasser, Kauschuk Gummi Kunstsoffe, 42, 22 (1989) 6. R. M. Russell, T. D. Skinner, and A. A. Watson, Rubber Chem. Technol., 42, 418 (1969) Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3,

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