A Review on Heat and Reversion Resistance · PDF fileProgress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003 143 A Review on Heat and Reversion Resistance Compounding

143Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

A Review on Heat and Reversion Resistance Compounding

A Review on Heat and Reversion ResistanceCompounding

R. N. DattaFlexsys BV, Technology Center, Zutphenseweg 10, 7418 AJ Deventer, The

Netherlands. E-mail: [email protected]

Received: 25 September 2002 Accepted: 9 January 2003

ABSTRACTWhen sulfur vulcanized natural rubber compounds are exposed to a thermalageing environment significant change in physical properties and performancecharacteristics are observed. These changes are directly related to modificationsof the original crosslink structure. Decomposition reactions tend to predominateand thus leading to reduction in crosslink density and physical properties asobserved during extended cure and when using higher curing temperatures. Thedecrease in network density is common when vulcanizates are subject to ananaerobic ageing process. However, in the presence of oxygen, the networkdensity is increased with the main chain modifications playing a vital role.

Over the years the rubber industry has developed several compoundingapproaches to address the changes in crosslink structure during thermalageing. This paper gives a review of these compounding approaches. As withmany formulation changes in rubber compounding, there is a compromisethat must be made when attempting to improve one performance characteristic.For example, improving the thermal stability of vulcanized natural rubbercompounds by reducing the sulfur content of the crosslink through the use ofthe more efficient vulcanization systems will reduce dynamic performanceproperty such as fatigue resistance. The challenge is to define a way to improvethermal stability while maintaining dynamic performance characteristics.

1. INTRODUCTION

Truck and off-the-road tires of today are frequently required to operateat high loads and high speeds for extended periods of time. These severeoperating conditions result in greater heat build up than would normallybe encountered under less demanding service conditions. As aconsequence, the excessive running temperature leads to reversion incompound components which may in turn lead to reduced tire durabilityor, in extreme circumstances, to tire failure.

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144 Progress in Rubber, Plastics and Recycling Technology, Vol. 19, No. 3, 2003

R. N. Datta

Much effort has been spent over the years to improve the reversioncharacteristics of tire compounds. An established and, probably, bestknown approach is the use of so-called semi-efficient cure systemscomprising reduced sulfur levels and increased accelerator levels.However, though effective in improving the vulcanizate’s resistance toreversion, this approach is only partially successful since lowering ofsulfur levels negatively influences other desirable properties such as tearand flex/fatigue life.

With the advent of the radial tire, natural rubber (NR) usage has increasedsignificantly. Its tack and high green strength serve to maintain green tireuniformity during building and shaping operations.

For both new and retreaded truck tires, however, the reversion characteristicsof NR have been considered to detract from wear performance, particularlyin the early stages of tire life where the tread surface has been subjected tothe greatest degree of overcure. Accelerated wear in later tire life may alsooccur due to thermal-oxidative degradation of the vulcanizate, particularlycured with conventional systems. The need for chemicals that providereversion resistance is, therefore, of great interest.

This paper briefly reviews approaches to improve reversion resistancethrough the use of antireversion chemicals, with emphasis on their applicabilityin 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, VulcurenKA 9188

h) Some other candidates

Figure 1 illustrates the structural formula of these compounds.

3Datta paper 30/5/03, 8:46 am144



Figure 1 Structures of thiuram (Ia), dithiocarbamate (Ib), dithiophosphate(II), zinc soap activator (III), silane coupling agent (IV), post vulcanizationstabilizer, HTS (V), antireversion agent, Perkalink 900 (VI), BDTCH, Vulcuren9188 (VII)

In addition to the above, the changes in the network structure duringoxidative ageing process are addressed. A novel chemical, N-Cyclohexyl-4,6-Dimethyl-2-Pyrimidine Sulfenamide (CDMPS) is recently introducedto address the changes in the network experienced during serviceenvironments. The structure of CDMPS is shown in Figure 2.

Figure 2 Structure of cyclohexyl-4,6-dimethyl-2-pyrimidine sulfenamide(CDMPS)

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R. N. Datta

2. EXPERIMENTAL

Compound formulations are shown in the respective data tables.Compound mixing was carried out in a similar manner as describedearlier(1).

Stress strain and tear properties of the vulcanizates were measured usinga Zwick universal testing machine (model 1445) in accordance with ISO37 and ISO 34 respectively. Heat build up was measured on a GoodrichFlexometer according to the method described in ASTM D623. Fatigueto Failure properties were measured on a Monsanto Fatigue to Failuretester at 23°C and 1.67Hz with 0-100% extension.

The volume fraction (Vr) in the swollen vulcanizate was determined byequilibrium swelling in toluene using the method reported by Ellis andWelding(2). The Vr value so obtained was converted to the Mooney-Rivlinelastic constant (C1) and finally to the concentration of chemical crosslinksby using equations described in the literature(3,4). The proportions ofmono, di, polysulfidic and carbon-carbon crosslinks were determinedusing the method reported elsewhere(1).

3. RESULTS AND DISCUSSION

3.1 Protection against reversion in anaerobic ageingenvironments

A high number of chemicals are available to address reversion concernsof compounds facing anaerobic ageing conditions, such as overcure orexposure to heat under limited supply of air. The structures of thesechemicals are shown in Figure 1 and their functions with respect toperformance characteristics are described in this section.

3.1.1 Thiuram accelerators

Thiurams such as tetramethylthiuram disulfide (TMTD), tetrabenzylthiuramdisulfide (TBzTD) etc. and dithiocarbamates such as zincdimethyldithiocarbamates (ZDMC) and zinc dibenzyldithiocarbamates(ZBEC) (Figure 1, 1a and 1b, where M=Zn) give an improvement inreversion resistance when used together with sulfenamide accelerators.This improvement is clearly demonstrated in Table 1.

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The restriction for the choice of TMTD, ZDMC and similar candidates isimpaired due to the N-Nitrosamine issue(5). In addition, although thecombination of a sulfenamide and a thiuram/dithiocarbamate (TMTD/ZDMC) shows improved reversion resistance, these systems suffer fromadverse effect on scorch and vulcanizate dynamic properties(6).

Taking into consideration all aspects of compounding and industrialhygiene, TBzTD or ZBEC are the better choice to target reversionresistance compounding with thiurams or dithiocarbamates. Recently,Datta et.al reported(7) that small amount (0.1-0.2phr) of TBzTD iscapable of improving reversion resistance without significantly affectingprocessing as well as dynamic properties. The improvement in reversionresistance is due to the presence of a high proportion of mono-sulfidecrosslinks in sulfenamide/thiuram binary combinations (Table 2).

The improved reversion resistance of the sulfenamide/thiuram curesystem is due to formation of zinc accelerator complexes, I and II

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R. N. Datta

respectively for TBzTD and TMTD (Figure 3). The solubility in thesecomplexes in rubber compounds determines the reactivity and consequentlydesulfuration of initially formed polysulfidic crosslinks(7). This results in anetwork containing a higher concentration of monosulfidic crosslinks.

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Figure 3 Complex formed in sulfenamide-Thiuram/Zinc oxide acceleration

3Datta paper 30/5/03, 8:46 am148



3.1.2 Dithiophosphate accelerators

Dithiophosphates such as Zinc dibutyl dithiophosphate (ZBPD, II, Figure 1)give an improvement in reversion resistance when used together withsulfenamide accelerators. This is clearly demonstrated by the cure datalisted in Table 3. The improvement in reversion resistance is due to thegreater proportion of mono and di sulfidic crosslinks formed with thesulfenamide/ZBPD combination - Table 4.

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R. N. Datta

Complex (I) (Figure 4), formed during sulfenamide/dithiophosphatecuring reactions, is responsible for the increased level of mono- and di-sulfidic crosslinks. It reacts with the initially formed polysulfidic crosslinksthereby reducing their sulfur rank(8).

The following advantages and disadvantages have been found forvulcanizates 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

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3.1.3 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 matrixthan the zinc salt of stearic acid(10). Their mode of action is to react withinitially formed polysulfidic crosslinks thereby reducing their sulfur rank.The effect of a zinc salt activator on compound cure characteristics, tensileproperties and network structure is summarized in Tables 5 and 6.

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R. N. Datta

As shown in Table 6, the zinc salt activator produces a networkcontaining a higher proportion of monosulfidic crosslinks. The networkdegradation is accelerated by the presence of additional complex asshown in Figure 5.

However, whilst vulcanizate reversion resistance is improved, flex/fatiguecharacteristics may be hampered (Table 5).

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3.1.4 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 reversionresistance by acting as a sulfur donor, the so called equilibrium cure concept.

3Datta paper 30/5/03, 8:46 am152



The effect of the silane-coupling agent on cure characteristics, compoundviscosity and vulcanizate tensile properties are presented in Table 7. It isclear that TESPT decreases cure rate (t90-t2) but improves processabilitythrough compound viscosity reduction. The increase in modulus at thet90 cure condition can, in part, be attributed to the increase in crosslinkdensity (Table 8), a result of its sulfur donor activity. The coupling agentalso leads to a network containing a higher proportion of monosulfidic

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R. N. Datta

crosslinks thereby providing improved reversion resistance. The crosslinkdensity data also indicate that TESPT is capable of maintaining a higherlevel of crosslink density on overcure compared to the control compound.

3.1.5 Post-vulcanization stabilizer (Duralink® HTS)

Duralink® HTS (Figure 1, V) is the tradename for hexamethylenebisthiosulphate disodium salt dihydrate. During vulcanization it produceshybrid crosslinks of the structure indicated in Figure 6. These hybridcrosslinks incorporate a flexible hexamethylene group between the sulfuratoms attached to the polymer backbone. The sulfur/accelerator levelsand ratio, as in the normal sulfur vulcanization process will dictate thelength of the sulfidic units.

During overcure or during service, desulfurization of the polysulfidiccrosslink structure would occur as is normally observed during the

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3Datta paper 30/5/03, 8:46 am154



reversion process. However, the structure of the hybrid crosslink is suchthat desulfurization leads to a monosulfidic/hexamethylene crosslink thatretains the flexibility of a polysulfidic crosslink but possessing the thermalstability of a monosulfidic crosslink – Figure 7.

This built in flexibility provides compounds containing Duralink® HTSwith 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 fatiguelife before and following ageing.

Duralink® HTS is also effective in maintaining adhesion to brass-coatedsteelcord(12) .

Figure 6 Hybrid crosslinks formed by Duralink® HTS

Figure 7 Effect of reversion on final crosslink structure

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R. N. Datta

3.1.6 Antireversion agent, Perkalink® 900

The antireversion agent 1,3-bis(citraconimidomethyl)benzene, Perkalink®

900, Figure 1 (VI), is extremely effective at counteracting reversion. Itreacts by a unique crosslink compensation mechanism whereby sulfidiccrosslinks, destroyed during the course of reversion, are replaced bycrosslinks based on the Perkalink®900 molecule, Figure 8. In doing so thecrosslink density of vulcanizates can be maintained if overcure occursduring the vulcanization process or on ageing as encountered during tireservice. As a result, mechanical properties are stabilized thereby improvingtire performance.

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3Datta paper 30/5/03, 8:46 am156



The crosslink compensation mechanism occurs only at the onset ofreversion as a result of the formation of conjugated dienes and trienesalong the polymer backbone(13). The antireversion agent reacts with theseconjugated structures via a Diels-Alder addition reaction(14). In doing soPerkalink®900 does not affect initial cure characteristics: scorch time,cure rate and t90. Furthermore, it can be added to compounds with noadditional change in formulation or modification of mixing and processingconditions.

Heat generated in truck tires under demanding service conditions will leadto reversion of compounds within the tire. The reversion is self-perpetuatingsince it lowers the modulus of the compounds which in turn increases therate of heat build up. Ultimately the integrity of the tire suffers, resultingin reduced tire durability.

The incorporation of Perkalink®900 in truck tread compounds willprevent reversion during service thereby reducing the rate of heat buildup. This will aid in maintaining the integrity of the tire ensuring improveddurability. Laboratory and tire test results are presented belowdemonstrating the beneficial effect in an NR/BR based truck tirecompound.

Compound formulations are shown in Table 10. Pekalink®900 was addedat a level of 0.5 phr, as recommended for a semi-efficient cure system.

The cure characteristics of both compounds are shown in Table 11.

Figure 8 Mechanism of Perkalink® 900 reaction

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R. N. Datta

It is interesting to note that Perkalink®900 does not affect initial curecharacteristics(15). The control compound shows a drop in torque onextended cure (60 minutes) despite the semi-efficient cure system, whilstthe test compound shows no drop in torque level, indicative of theantireversion action provided by the Perkalink®900.

The effect of Perkalink®900 on vulcanizate physical properties cured at143°C, a typical truck tire curing temperature, is shown in Table 12. Dataare listed for optimum cure, t90, and overcure, 60 minutes.

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The antireversion action provided by the Perkalink® 900 is demonstratedby the modulus retention on overcure. Improved abrasion resistance isalso seen for the compound containing the antireversion agent. Fatigueproperties are not hampered.

The most striking feature provided by Perkalink® 900 is the reduction inheat built up as measured by the Goodrich Flexometer test, Figure 9.

Even at the extended cure of 60 minutes, the sample containingPerkalink® 900 did not show any increase in heat build up compared tothe sample cured for the shorter period of 2 x t90. The control compoundon the other hand experienced blow-out failure following a test time of40 minutes. This significant reduction in heat build up provided byPerkalink®900 should be reflected in improved truck tire performance.

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R. N. Datta

3.1.7 1,6 –bis (N,N-dibenzylthiocarbamoyldithio)hexane,BDTCH, Vulcuren KA 9188

BDTCH (Figure 1) is based on dibenzylthiocarbamoyl leaving group withdithiohexane as a crosslinker. Huls AG patented(16,17) this substance inearly nineties. It is recently introduced(18) to the rubber industry as aneffective antireversion agent. Based on structural similarity (Figure 10) acomparison has been made between BDTCH with TBzTD and withadditional 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 9 Heat build up at 100ºC after 1hour for samples cured at 2 x t90 and60 minutes (BO – Blow Out).

Tem

pera

ture

ris

e, °

C

BO

Figure 10 Structures of TBzTD and BDTCH

3Datta paper 30/5/03, 8:46 am160



It is interesting to note that both TBzTD and BDTCH behave similar withrespect to torque development and cure time reduction. This is typical forany sulfenamide cure accelerated by thiurams or dithiocarbamates. Theonly difference observed between TBzTD and BDTCH is on the scorchtime. TBzTD is marginally worse than the mix containing BDTCH. Thisis 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

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R. N. Datta

accelerator providing the boosting characteristics with no effect on scorchdata. The formation of TBzTD from BDTCH is shown in Figure 11.

The formation of TBzTD from BDTCH suggests that the vulcanizationchemistry of BDTCH and TBzTD is similar, thus providing identical curedata as shown in Table 14. Addition of Duralink HTS (as it containsS-(CH2)6-S

.) is further explored (Mix 17). The cure data as presented inTable 14 suggests that the mixes 17 and 16 behave similar except thescorch time (ts2).

The effect of TBzTD and BDTCH on the vulcanizates physical propertiesat 150°C is shown in Table 15. Data are listed for optimum cure, t90,and overcure, 60 minutes.

The antireversion actions provided by TBzTD and BDTCH is demonstratedby modulus retention on overcure. Better abrasion resistance, maintenanceof tear and flex properties on overcure are the consequences of heatstability offered by the network formed. The network density and thedistribution of the crosslink types were measured and the data aretabulated in Table 16.

Comparing the crosslink density and distribution of crosslink types indifferent compounds, it is clear that both TBzTD and BDTCH havesimilar network structure and hence the properties are expected to becomparable. It can be therefore concluded that the functionality,-S-C(=S)-N((CH2)-Ph)2, plays the vital role and molecule derived fromsuch a leaving group will provide heat and reversion resistance features.

Figure 11 Formation of TBzTD from BDTCH

3Datta paper 30/5/03, 8:46 am162



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S-yloP 93.2 03.2 22.2 02.2

S-iD 07.0 56.0 18.0 58.0

S-onoM 14.2 67.2 07.2 07.2

01xHRmarg/elommargnidesserpxeytisnedknilssorC* 5

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R. N. Datta

3.1.8 Miscellaneous candidates

Although the previously mentioned substances are main material in therubber industry, but there are some candidates cited in the literature to beeffective antireversion agent. They are summarized in this section.

Multifunctional acrylates have been published(19) to be effective antireversionagent. The structures are shown in Figure 12.

Maleimides are reported(20,21) to be effective crosslinkers in sulfur as wellsulfur free vulcanization. Recently, Vucht a.s introduced(22), 2,2-dithiobis(phenylmaleimido) (FMI), Figure 13, and claimed this as a chemical thatincreases the resistance against prevulcanization.

The maleimides are good crosslinker but consumed during vulcanizationand are not available during the reversion regime, i.e., during overcureor in service environments(23).

Figure 12 Structures of acrylates

Figure 13 Structure of FMI

3Datta paper 30/5/03, 8:46 am164



3.2 Protection against degradation in oxidative ageingenvironments

One of the burning demands in the rubber industry has been to reducethe changes in the network during the service life of rubber components.In a broad sense, this is also considered as degradation (reversion) ofnetwork but under oxidative environments. The changes in the networkunder aerobic conditions are different in nature than experienced underanaerobic conditions. In a true sense, the rubber compounds experiencehardening effects in the presence of oxygen and heat.

The process of hardening or increase in modulus or increase in crosslinkdensity is as a result of oxidation of sulfidic crosslinks, liberation of activesulfur and consequently re-crosslinking. The process in schematicallyshown in Figure 14.

It is well known that the degradation of polysulfides is catalyzed by thepresence of accelerators (Figure 15). The liberated active sulfur isresponsible for introduction of additional crosslinks and hence increasesin modulus.

Figure 14 Reaction of monosulfides with oxygen/heat forming additionalcrosslinks

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R. N. Datta

In order to reduce the unwanted hardening effects during oxidativeageing, the trap or removal of accelerator fragments is desirable. Over theyears, focus was on the development of an accelerator, which becomesinactive after the vulcanization and thereby decreases the kinetics ofdegradation of polysulfides. The research culminated in a developmentof a novel accelerator, N-Cyclohexyl-4,6-Dimethyl-2-pyrimidineSulfenamide(24).

CDMPS has been tested in several compounds where the effect onmodulus increase during oxidative ageing is monitored. Some typical dataare shown in Table 17.

Figure 15 Reaction of polysulfides with oxygen/heat forming additionalcrosslinks

3Datta paper 30/5/03, 8:46 am166



It is clear from the data that the changes in hardness and modulus duringoxidative ageing are considerably reduced. The resistance to hardnessincrease positively influences the dynamic properties as shown in Table 17.

In order to understand the changes in the network during oxidative ageingin the presence of CDMPS, some network studies were carried out. Dataare tabulated in Table 18.

The increase in crosslink density upon oxidative ageing is minimised inthe presence of CDMPS. The level of zinc sulfide is higher indicating thatpart of sulfur kicked out during the ageing process is trapped by zinc aszinc sulfide. The mechanism of the process is proposed as shown inFigure 16.

lanoitnevnocrevoSPMDCfoegatnavdafoyrammuS71elbaTrotarelecca

daerTregnessaPRB-RBS

)yrammuS(

061;oehR °C lortnoC SPMDC

mN,SatleD 41.1 51.1 lortnoC SPMDC

nim,2st 01 21 SBC 3.1 0

nim,09t 3.4 5.4 SPMDC 0 3.1

TBM 0 5.0

061:eruC ° 09t/C rufluS 2 2

)*gniegaretfasegnahC( 001/d3 °C

DHRI,ssendraH 11+ 6+

%,)%002Mnodesab(*SM 022+ 851+

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)ssertstnatsnoC(%,xelF 53- 01-

02-,atledtnegnaT ° %,C 02- 3-

:noitalumroF001x)002MdeganU/002MdegA(=%,SM

3Datta paper 30/5/03, 8:46 am167


R. N. Datta

061:eruC(ytisnedknilssorC81elbaT ° )09t/C

01x*seitisned-LX 5 lortnoC SPMDC

deganU

latoT 97.3 28.3

S-yloP 59.1 80.2

S-iD 35.0 65.0

S-onoM 13.1 81.1

01xSnZ 2 055.0 045.0

001/d3,degA °C

latoT 51.5 03.4

S-yloP 46.0 50.1

S-iD 51.0 03.0

S-onoM 63.4 59.2

SnZ ♣ 01x 2 785.0 727.0

nobracordyhrebburmarg/elommargnidesserpxeytisnedknilssorC*♣ nobracordyhrebburmarg/margnidesserpxeruflusedifluscniZ

Figure 16 Formation of zinc sulfide and trapping of accelerator’s rest

3Datta paper 30/5/03, 8:46 am168



CDMPS has also been tested in high sulfur skim, NR/BR tread as well asS-SBR silica treads. The advantages are well confirmed in all applications.

4. SUMMARY

The mode of action of several rubber chemicals that provide improvedantireversion characteristics has been discussed. Whilst some of thesechemicals function through the formation of a crosslink network containinga higher proportion of thermally stable monosulfidic crosslinks, bothDuralink® HTS and Perkalink® 900 function via differing, novelmechanisms.

Duralink HTS functions through the formation of hybrid crosslinks thatprovide improved flex/fatigue properties. The principle of Perkalink®

900 is: crosslink compensation at the onset of reversion leading to theformation of thermally stable C-C crosslinks. These provide reduced heatbuild up under dynamic conditions.

A new accelerator is developed, which restricts the modulus and hardnessincrease during the oxidative ageing process. Data suggests that theaccelerator is capable of maintaining the initially formed crosslinksthereby keeping changes in compound properties at a minimum.

Combining the possibilities as outlined above, it is now possible tooptimise a compound having improved heat stability both under anaerobicand aerobic environments.

5. 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, 571 (1964)

3. P. J. Flory and J. Rehner, J. Chem. Phys. 11, 521 (1943)

4. L. Mullins, J. Appl. Polym. Sci., 2, 1 (1959)

5. 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)

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R. N. Datta

7. R. N. Datta, B. Oude Egbrink, F. Ingham and T. Mori, KauschukGummi Kunstsoffe, 54, 612 (2001)

8. L. H. Davis, A. B. Sullivan and A. Y Coran, Rubber Chem. Technol,60, 125 (1987)

9. R. N. Datta, Unpublished results

10. J. Vander Kooi and J. Sherritt, Paper presented at the 144th meetingof the Rubber Division, ACS, Orlando, Florida, Oct. 26-29, 1993.

11. S. Wolff, Kautschuk Gummi Kunststoffe, 32, 760 (1979)

12. R. N. Datta and F. A. A. Ingham, Kautschuk Gummi Kunststoffe, 52,322 (1999)

13. N. J. Morrison and M. Porter, Rubber Chem. Technol, 57, 63 (1984)

14. R. N. Datta, A. H. M. Schotman, A. J. M. Weber, F. G. H. Van Wijk,P. J. C. Van Haeren, J. W. Hofstraat, A. G. Talma and A. G. V. D.Bovenkamp-Bouwman, Rubber Chem. Technol, 70, 129 (1997)

15. R. N. Datta, J. H. Wilbrink and F. A. A. Ingham, Indian Rubber J.,8, 52 (1994)

16. G. Horpel, K.-H, Nordsiek, and D. Zerpner, Huls AG, EP 0432 405A2, 15.10.1990

17. G. Horpel, K.-H, Nordsiek, and D. Zerpner, Huls AG, EP 0432 417A2, 22.10.1990

18. W. Obrecht, and W. Jeske, EP 1083 200 A2, 14.03.2001

19. E. J. Black, M. L. Kralevich, and J. E. Varner, Rubber Chem.Technol., 73, 114 (2000)

20. P. Kovacic, and R. W. Hein, Rubber Chem. Technol., 35, 528(1962)

21. P. Kovacic and R. W. Hein, Rubber Chem. Technol., 35, 520 (1962)

22. K. Vucht, Paper presented in the Slovakian Rubber Conference,Puchov, 2000

23. R. N. Datta, A. G. Talma, and A. H. M. Schotman, Rubber Chem.Technol., 71, 1073 (1998)

24. H. J. Lin, R. N. Datta, and O. Maender, WO 01/70870 A2, 27-09-2001

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