Effect of Metal Oxides as Activator for Sulphur ... of Metal Oxides as Activator for Sulphur Vulcanisation in Various Rubbers ... compound, certain metal oxides repre- ... only metal

ROHSTOFFE UND ANWENDUNGENRAW MATERIALS AND APPLICATIONS

Effect of Metal Oxides asActivator for SulphurVulcanisation in Various Rubbers

Zinc oxide � Sulphur vulcanisation � Acti-vators � Metal oxides � Time-concentra-tion development of reaction products

Two widely different rubbers, viz. EPDMand s-SBR have been selected in thepresent study. The influence of the basi-city, the crystal structure and the abilityto form complexes of various metal oxi-des were studied. It was observed thatneither CdO, PbO, BaO, CaO, MgO andBeO are proper substitutes for ZnO asactivator in thiuram-accelerated vulca-nisation of EPDM, nor do they show asynergistic effect with ZnO. In s-SBRcompounds, however, it is demonstratedthat CaO and MgO can function as acti-vator of cure for sulphur vulcanisation,retaining the curing and physical prop-erties of the rubber vulcanisates.Model Compound Vulcanisation hasbeen used to elucidate the influence ofthe activators MgO and CaO on the vul-canisation mechanism.It can be concluded that depending onthe exact requirements for a specificcompound, certain metal oxides repre-sent an alternative route to reduce thezinc level and therefore to minimise theenvironmental impact.

G. Heideman, J. W. M. Noordermeer,

R. N. Datta, Enschede (The Nether-

lands), B. Van Baarle, Eindhoven

(The Netherlands)

Corresponding author:

J. W. M. Noordermeer

University of Twente

Faculty of Science and Technology

Department of Rubber Technology

P.O. Box 217

NL-7500 AE Enschede

E-mail:

[email protected]

Einfluss von Metalloxiden aufdie Aktivierung der Schwefelvul-kanisation von Kautschuken

Zinkoxid � Schwefel Vernetzung � Ak-tivator � Metall Oxide � Zeit-Konzen-trationsverlauf von Reaktionsproduk-ten

In dieser Studie wurden verschiedeneOxide in zwei unterschiedlichen Poly-meren, EPDM und s-SBR, getestet. DerEinfluss des Sauregrades, der Kristall-struktur und der Fahigkeit Komplexezo formen, wurden untersucht. Eszeigte sich, daß weder CdO, PbO, BaO,CaO, MgO noch BeO als Ersatz fur ZnOin einem Thiuram-Vulkanisationssy-stem in EPDM in Frage kommen. Eskonnte auch kein synergistischer Ef-fekt der Kombination der genanntenOxide mit ZnO festgestellt werden. ImGegensatz dazu konnen CaO undMgO als Ersatz fur ZnO eingesetztwerden, ohne daß das Vulkanisati-onsverhalten und die mechanischenEigenschaften des Vulkanisates be-einflußt werden.Mit Hilfe von Modellcompound-Vul-kanisationen wurde der Einfluß vonMgO und CaO auf den Mechanismusder Vulkanisationsreaktion unter-sucht.In dieser Untersuchung konnte ge-zeigt werden, daß abhangig von denProduktanforderungen alternativeMetalloxide geeignet sein konnen, dieZinkkonzentration in einer Gummi-mischung zu verringern und so dieUmweltbelastung zu reduzieren.

It is generally known that for efficient vul-canisation of rubbers by elemental sulphuror by sulphur donors the presence of a me-tal activator is necessary [1]. Zinc oxide isthe most effective activator for sulphur vul-canisation. A great deal of attention hasbeen paid to the problem of reducingthe zinc content in rubber products. Acompletely zinc-free vulcanisation systembased on sulphur keeps on being intri-guing. There have been a number of inve-stigations comparing different metal oxi-des as vulcanisation activators, mostlywith tetramethylthiuram disulphide(TMTD) in NR, with variable results. It isconcluded that a variety of metal oxidescan accelerate cure, but the degree of ac-celeration varies with the specific metal ionused [2]. In a research by Chapman, it wasfound that of metal oxides other than ZnO,CdO on average appears to be best, follo-wed by lead- and mercury-oxide [3]. Lau-tenschlaeger et al. studied the effects ofdifferent metal oxides in accelerated sul-phur vulcanisation with 2-methyl-2-pente-ne, as a model olefin [4]. The results of acomparison of ZnO, CdO, CaO, and a com-bination of ZnO and aniline also indicatedthat CdO is the most effective oxide, resul-ting in a high yield in monosulphides and arelatively low production of byproducts. Inan investigation of the crosslinking of amodel olefin with sulphur, tetraethylthiu-ramdisulphide (TETD) and various metaloxides, cupric oxide was found to give hig-her yields than zinc oxide, and nickel oxidewas also quite effective [5].Some metal oxides behave synergisticallywith ZnO. Replacement of half of theZnO by an equivalent amount of CdO,PbO, Bi2O3, CaO, HgO or CuO in an effici-ent vulcanisation (EV) system gave highermoduli. Other oxides gave moduli equalto or lower than with ZnO alone. CaOand MgO are apparently not so promisingin conventional sulphur/sulphenamide cu-res, especially not in combination withZnO. It has been reported that CaO andMgO interfere with the efficient activationof ZnO [4].

Where ZnO is commonly used in the vulca-nisation recipe, not much is known aboutthe accelerating properties of other metaloxides, so, a comprehensive study wasconsidered worthwhile. In the first partof this article the sulphur vulcanisationof EPDM and s-SBR rubber with several al-ternative metal oxides as activators is de-scribed. The objective of the second partis to gain additional insight into the me-chanistic details of sulphur vulcanisationwith other metal oxides as activator in or-der to judiciously reduce zinc oxide levels inrubber compounding.

Other metal oxides incomparison with ZnO

It has been reported that the high activityof ZnO can be explained on the basis of thechemistry of complex formation. A prece-ding reaction with stearic acid forms therubber hydrocarbon-soluble zinc stearateand liberates water before the onset ofcrosslinking [6]. Furthermore, ZnO is inmany vulcanisation systems a precursorto zinc-derived accelerators [7]. It has fur-ther been suggested in many different stu-dies that Zn2+-ions form these active com-plexes with accelerators, which are morereactive than the free accelerator [8–10].Complex formation of the zinc ion with dif-ferent accelerators is critical to get efficientcuring.

30 KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 1-2/2005

The Zn2+-ion functions as a Lewis acid inthe complex formation and the cleaved ac-celerator as a Lewis base. The basicity of ametal ion is dependent on its size and thecharge, according to equation 1 [11].

cation acidity ¼ k � Z2

rð1Þ

where:k ¼ constantZ ¼ charge of the cationr ¼ radius of the ion

Since for Ca2+, Zn2+ and Mg2+ the chargesare equal, the size of the cation is the mainfactor that determines the difference inacidity between them. In Table 1, the sizesare given. From Table 1 can be concludedthat the order of acidity from strongestacid to the weakest is Mg2+> Zn2+>Ca2+.Duchacek et al. tested the effects of cop-per-, mercury-, nickel-, zinc-, cadmium-, in-dium-, magnesium-, and calcium-stearateson the course of CBS-accelerated sulphurvulcanisation of NR [12]. Based on resultsand in accordance with Irving’s and Wil-liams’ finding about the order of stabilityof metal complexes [13], the following or-der for the stability of metal complexeswith accelerator ligands was presented:Cu2+, Hg2+ > Ni2+, Zn2+, Cd2+, In2+ >

Mg2+, Ca2+. They concluded that nickel,zinc, cadmium, and indium have a definiteability to form complexes, but they formsignificantly less strong coordinationbonds, than, e.g. copper and mercury.This is why zinc and cadmium form activesulphurating vulcanisation species. Ma-gnesium and calcium have only a slighttendency to form complexes. This fact eli-minates the possibility of formation of anactive sulphur-containing complex.Generally, it is stated that increasing the pHleads to activation of the vulcanisation.Krebs [14] postulated that the acceleratingproperties of amines are related to theirbase strength. The alkaline character ofmagnesium and calcium stearates is thereason for the acceleration of sulphur vul-canisation of rubber in an analogous wayas in the cases of currently used basic ac-celerators. The greater basicity of calciumin comparison with that of magnesium isin good accordance with the larger vulca-nisation rate found for calcium containingrubber compounds [12].In some of the vulcanisation mechanisms itis assumed that ZnO is distributed in theform of crystal particles in rubber mixes.Molecules of accelerators, sulphur and fat-ty acids diffuse through the rubber matrix

and are adsorbed on the ZnO-surface withthe formation of intermediate complexes.Nieuwenhuizen [7] in his thesis proposed amechanism in which the ZnO surface func-tions both as a reactant and as a catalyticreaction template, activating and bringingtogether reactants. The most importantparameter here is the crystal structure ofthe ZnO. The most common crystal struc-ture for ZnO is the zincite (wurtzite) struc-ture. Zincite has a hexagonal structure,Fig. 1, with each atom surrounded by a te-trahedron of atoms of the opposite type.The structure consists of a continuous net-work of interconnected tetrahedra. Theonly metal oxide isostructural with ZnOis BeO.In addition to its role as an activator for sul-phur vulcanisation, there is also evidencethat the inclusion of ZnO in a compoundreduces heat build-up and improves tyreabrasion resistance. ZnO acts as a “heatsink”, which accepts frictional energywithout a large increase in internal tempe-rature. It has also been found that ZnO im-proves the heat resistance of the vulcanisa-tes and their resistance to the action of dy-namic loading [15]. The high thermal con-ductivity of ZnO helps to dissipate localheat concentrations that might otherwiseaffect the properties of rubber. Therefore,reduction of ZnO or complete eliminationmight influence these properties as well,although it is anticipated that other metaloxides also possess high thermal conducti-vities.

Experimental section

Materials. – Solution butadiene-styrenerubber (Buna VSL 2525-0 M) was obtainedfrom Bayer GmbH, Germany. Buna VSL2525-0 M contains 25 wt % of 1,2-vinyl-butadiene and 25 wt % of styrene; it hasa Mooney viscosity, ML (1þ4) @ 100 8Cof 54. Ethylidene norbornene (ENB)-con-taining EPDM rubber (Keltan 4802) wasobtained from DSM Elastomers B.V., theNetherlands. Keltan 4802 contains52 wt % of ethylene units and 4.3 wt %of ENB; it has a relatively narrow molecularweight distribution and a typical Mooneyviscosity, ML (1þ4) @ 125 8C of 77. As fil-lers were used: carbon black N-375 HAF,N-550 FEF and N-762 SRF (Cabot B.V.).Aromatic oil (Enerflex 75) was obtainedfrom BP Oil Europe, paraffinic oil (Sunpar2280) from Sun Petroleum Products Co.,and stearic acid from J.T. Baker. Commer-cially available ZnO Red Seal (Grillo GmbH)and the different metal oxides (Aldrich)were used as received. Sulphur (J.T. Baker),N-tert-butyl-2-benzothiazolesulphenami-de (Santocure TBBS, Flexsys B.V.), 2-Mer-captobenzothiazole (Perkacit MBT, FlexsysB.V.), and Tetramethylthiuram-disulphide(Perkacit TMTD, Flexsys B.V.), were alsocommercial grades and used as such.The materials used in the model com-pound experiments are listed in Table 2.Rubber mixing. – EPDM and s-SBR ma-sterbatches were prepared in an internalmixer (� 50 kg) in order to get a homoge-neous mixture and minimise the influenceof mixing conditions. The vulcanisation sy-stems including the various metal oxidesinvestigated were added in a separate ope-ration, on a two roll mill at � 50 8C. Thecompounds were sheeted off at a thick-ness of approximately 2 mm which wasconvenient for the subsequent preparationof test specimens. The compositions of theEPDM and s-SBR compounds with the dif-ferent metal oxides are given in Tables 3and 4, respectively.Curing. – The cure characteristics of thedifferent compounds were measured at160 8C with a Rubber Process AnalyserRPA2000 (Alpha Technologies), a type ofmoving die rheometer. The optimal vulca-nisation time (t90) and scorch time (t02) ofthe compounds were determined. Thecompounds were cured in a Wickert labo-ratory press WLP 1600/5*4/3 at 160 8Cand 100 bar, according to the t90 of thespecific compounds.Characterisation. – Tensile tests were car-ried out on dumb-bell shaped specimens

Tab. 1. Radii of Ca2+, Zn2+ and Mg2+

Cation Radius(10ÿ12 m)

Ca2+ 94

Zn2+ 74

Mg2+ 65

Tab. 2. Materials used for Model Com-pound Vulcanisation

Material Source

Squalene Merck

2,3-dimethyl-2-butene (TME) Aldrich

TBBS Flexsys B.V.

Sulphur Merck

ZnO pure Merck

MgO Aldrich

CaO Aldrich

Stearic Acid J.T. Baker

KGK Kautschuk Gummi Kunststoffe 58. Jahrgang, Nr. 1-2/2005 31

(Type 2) according to ISO 37. Ageing of thetest specimens was carried out in a venti-lated oven in the presence of air at 100 8Cfor 3 days according to ISO 188. Compres-sion set (CS) tests were performed at 23 8Cand 100 8C for 72 hours according to DIN53517. Hardness of the samples was mea-sured with a Zwick Hardness-meter ShoreA Type, according to DIN 53505.Swelling measurements were performed inorder to obtain information about thecrosslink density. The unextracted filled s-SBR samples were swollen until constantweight in toluene at room temperature.The EPDM samples were swollen in deca-hydronaphthalene at room temperature aswell. The crosslink density was calculatedaccording to the Flory-Rehner equation

[16, 17]. Although this equation as suchis only valid for non-filled systems, thedata obtained with these measurementsdo yet give an indication of the relativecrosslink densities. The Flory-Huggins pa-rameter v for s-SBR-toluene networkswas taken from literature: 0.21. For swol-len EPDM-decahydronaphthalene net-works the v-parameter was calculatedvia the relationship v ¼ 0.121 þ 0.278 *v2, reported by Dikland [19], with v2 ¼ po-lymer network volume fraction at equilibri-um swelling.Model compound vulcanisation. – Threemetal oxides were selected as activator,viz. ZnO, MgO and CaO, and investigatedwith model compound vulcanisation intwo different model compounds: squalene

and TME. The model compound vulcanisa-tion experiments were carried out in aglass ampoule. The reaction mixture wasweighed into this ampoule. In order to mi-nimise the influence of oxygen in the reac-tions, the ampoule was fluxed with nitro-gen and sealed off. The reaction was per-formed in a preheated thermostatic oilbath at 140 8C for a fixed time. A magneticstirrer was added to the mixture to provideadequate stirring during the reaction. Af-ter a definite period of time the reactionwas arrested by taking the ampoule outof the oil bath and by immersion in liquidnitrogen. After cooling, the ampoule wascovered with aluminium foil to avoid anUV influence, and stored in a refrigerator.The compositions of the squalene and TME

Tab. 3. Composition of the EPDM compounds (phr) with different metal oxides

Compound 1 2 3 4 5 6 7 8 9 10

EPDM (Keltan 4802) 100 100 100 100 100 100 100 100 100 100

Carbon Black (N550 FEF) 70 70 70 70 70 70 70 70 70 70

Carbon Black (N762 SRF) 40 40 40 40 40 40 40 40 40 40

Paraf. Oil (Sunpar 2280) 70 70 70 70 70 70 70 70 70 70

Stearic Acid 1 1 1 1 1 1 1 1 1 1

ZnO 3 – – 3 3 – – – – –

MgO – 3 – 3 – – – – – –

CaO – – 3 – 3 – – – – –

BaO – – – – – 6 – – – –

PbO – – – – – – 8 – – –

CdO – – – – – – – 4.5 – –

Cu(II)O – – – – – – – – 5 –

BeO – – – – – – – – – 1

Accelerator (TMTD) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Sec. Accelerator (MBT) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Sulphur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

Tab. 4. Composition of the s-SBR compounds (phr) with different metal oxides

Compound 1 2 3 4 5 6 7

s-SBR (Buna VSL 2525) 100 100 100 100 100 100 100

Carbon Black (N375 HAF) 50 50 50 50 50 50 50

Arom. Oil (Enerflexâ 75) 5 5 5 5 5 5 5

Stearic Acid 2 2 2 2 2 2 2

ZnO 3 – – – – – –

MgO – 3 – – – – –

CaO – – 3 – – – –

BaO – – – 6 – – –

Cu(II)O – – – – 3 – –

BeO – – – – – 1 3

Accelerator (TBBS) 1.5 1.5 1.5 1.5 1.5 1.5 1.5

Sulphur 1.75 1.75 1.75 1.75 1.75 1.75 1.75


reaction mixtures are listed in Tables 5 and6, respectively.Analysis of the reaction products. – A smallportion of the filtered sample (about0.03 g) was weighed and dissolved in2.5 ml of acetonitrile, containing an inter-nal standard: dibutyldisulphide. The re-sponse factors for all the initial compo-nents in reference to the internal standardwere measured with the HPLC at a wave-length of 254 nm. The internal standardwas added after the vulcanisation as an ex-tra component for quantitative analysis.Before injecting in the HPLC, the samplewas filtered twice over a 45 lm porous fil-ter. About 20 ll of this diluted sample wasinjected onto the HPLC-column for analysisaccording to the conditions described inTable 7. The areas of the different peaksin the chromatogram were determinedand converted into concentrations viathe measured response factors for the se-veral components. Reaction conversions asa function of reaction time were calculatedby dividing the concentration of the com-

ponent by the initial concentration and ex-pressed in percentages.

Results

To obtain additional insight in the exactrole of the activator in the vulcanisationmechanisms and to find routes to reducethe amount of ZnO in rubber compounds,experiments with metal oxides other thanZnO have been performed, evaluating vul-canisation behaviour and physical proper-ties. Irrespective of the main goal of thepresent project, to reduce the amount ofZnO for its environmental impact as a hea-vy metal, in the current article not only themore “eco-friendly” metal oxides, viz.MgO and CaO, but also other heavy metaloxides like CdO, CuO and PbO are investi-gated.

Several metal oxides as activatorin EPDM rubber compounds

Several metal oxides were investigated asactivator in the EPDM masterbatch accor-

ding to Table 3. Fig. 2 shows the cure cha-racteristics for the EPDM compounds withoxides from the group IIa of Mendeleyev’speriodic table as activator, viz. MgO, CaOand BaO. Compared to ZnO the scorchtime increases, the rate and amount ofcure decrease drastically. The presence ofthese oxides resulted in a poorly developednetwork, comparable with the resultswithout any activator. The results of swel-ling experiments with different metal oxi-des are presented in Fig. 3. A higher swel-ling percentage corresponding to a lowercrosslink density was found for the com-pounds with MgO and CaO, which is in ac-cordance with the torque measurements.A combination of metal oxides as activatorsystem has also been tested in EPDM to ex-plore synergistic effects. As mentioned inthe introduction, it has been reportedthat CaO and MgO interfere with the effi-

Fig. 1. Zincite (ZnO) structure, isostructuralwith wurtzite; * ¼ Zn, * ¼ O

Tab. 5. Squalene Model compound systems

No activator ZnO as activator MgO as activator CaO as activator

phr mmole phr mmole phr mmole phr mmole

Squalene 100 1.217 100 1.217 100 1.217 100 1.217

CBS 1.2 0.023 1.2 0.023 1.2 0.023 1.2 0.023

Sulphur 2 0.039 2 0.039 2 0.039 2 0.039

Activator – – 5 0.307 5 0.620 5 0.448

Stearic Acid 2 0.035 2 0.035 2 0.035 2 0.035

Tab. 6. TME Model compound systems

No activator ZnO as activator MgO as activator CaO as activator

phr mmole phr mmole phr mmole phr mmole

TME 100 5.941 100 5.941 100 5.941 100 5.941

TBBS 1.5 0.025 1.5 0.025 1.5 0.025 1.5 0.025

Sulphur 1.75 0.034 1.75 0.034 1.75 0.034 1.75 0.034

Activator – – 5 0.307 5 0.620 5 0.448

Stearic Acid 2 0.035 2 0.035 2 0.035 2 0.035

Tab. 7. HPLC conditions

Column Nucleosil 100-5 C18 HD (reverse phase)

Length of column 250 mm

Internal diameter of the column 4.6 mm

Mobile phase 97 Acetonitrile : 3 Water (vol%)

Flow rate 1 ml/min

Temperature 23 8C

Detector UV

Wavelength 254 nm

Injected Volume 20 ll


cient activation by ZnO. The cure characte-ristics of the compounds with ZnO/MgOand ZnO/CaO as activator combinationsare represented in Fig. 4 and the corre-sponding results of the swelling experi-ments in Fig. 3. The cure characteristicsof the compounds with ZnO/MgO andZnO/CaO as activator combinations indi-cate that only CaO slightly interferes in anegative way with the efficient activationby ZnO. The final states of cure and cross-link densities, as determined with the swel-ling experiments, are comparable with thereference compound with only ZnO.

The physical properties of the EPDM com-pounds with MgO, CaO and the combina-tions of these metal oxides are presentedin Table 8. In accordance with the decrea-sed extent of crosslinking, higher valuesfor the elongation at break and compres-sion set as well as lower tensile strengthare observed in the compounds withMgO and CaO as activator. Examinationof the data of the compounds with activa-tor combinations reveals that neither theaddition of MgO nor of CaO to the stan-dard system with ZnO causes any majordifference in vulcanisate properties, except

for the compression set measured at100 8C. The compression set at elevatedtemperature is sometimes considered toprovide a first indication about the thermalstability or ageing behaviour of the vulca-nisates. The CS at 100 8C is in both com-pounds considerably higher. On the otherhand, the values for elongation at breakafter 168 hours ageing at 100 8C areonly slightly higher as compared to theZnO reference system, suggesting a similarhigh thermal stability, thereby contradic-ting the thermal behaviour indicated bythe data of the compression set at

Fig. 2. Cure characteristics of EPDM compounds with different metaloxides as activator

Fig. 3. Swelling and crosslink density of EPDM compounds with diffe-rent metal oxides as activator

Fig. 4. Cure characteristics of EPDM compounds with ZnO in combina-tion with MgO and CaO as activator

Fig. 5. Cure characteristics of EPDM compounds with other metal oxides


100 8C. So, no conclusions can be drawnfrom these data.Fig. 5 summarises the cure behaviour ofEPDM compounds with other (heavy) me-tal oxides, like PbO, CdO, Cu(II)O, andBeO. The compound with PbO displays ashort scorch time and a substantially lowerdelta torque value compared with ZnO.CdO, which is a member of the zinc group,results in a lower rate of cure and a extentof cure comparable with PbO, albeit with amarching modulus. The latter contradictsearlier statements in literature, that CdOis a good activator for sulphur vulcanisati-on. Frenkel [20] hypothesised that CdO isan effective activator for mercaptobenzo-thiazole-accelerated vulcanisation, butnot an activator for thiuram-acceleratedvulcanisation, which is in accordancewith the reduced effectivity of CdO inthe current and earlier reported experi-mental results on thiuram/mercaptobenzo-

thiazole-accelerated (TMTD/MBT) vulcani-sation. The extent of crosslinking withCu(II)O takes an intermediate position be-tween CdO and ZnO concerning the rateand extent of crosslinking. Disadvantageof BeO is the extreme high toxicity by inha-lation and ingestion, but it was tested an-yway to check whether the wurtzite struc-ture is the governing factor regarding theactivity of ZnO. The compound with BeOas activator resulted in a considerably lo-wer crosslink density than the referencecompound, similar to the compound with-out activator. This provides evidence thatthe crystal structure of the metal oxide isnot the main factor determining the activi-ty. In view of the eco-toxicity of the majo-rity of these metal oxides, further testing ofthe compounds was not considered wort-hwhile and not pursued in the present con-text.

Overall, close inspection of the data revealsthat in EPDM rubber none of the chosenmetal oxides is as active as ZnO. In the pre-sence of CaO, MgO, BaO, BeO, and PbO,CdO, Cu(II)O the cure characteristics are in-ferior to the ZnO system and in some caseseven perform the same as in an experimentwithout any activator, indicating that thereis less influence of these metal oxides onthe reaction. See for example Fig. 2 forthe effect of CaO and MgO relative toZnO, and Fig. 5 for the effect of CdO.Though the results indicate that depen-dent on the intended applications, thereexists the potential to replace ZnO by othermetal oxides, like e.g. Cu(II)O, as activatorin EPDM compounds, it does not consti-tute a very promising and realistic solutionfor the ZnO problem from an environmen-tal point of view. It might appear that thesealternative metal oxides exert (un)foreseenenvironmental implications even more pro-blematic than the zinc-based systems.

Several metal oxides as activatorin s-SBR rubber compounds

The effect of metal oxides was investigatedin s-SBR rubber as well, according to therecipes in Table 4. A TBBS-accelerated vul-canisation system is used in the s-SBR com-pounds. Fig. 6 shows, that the scorch timesfor the s-SBR compounds with MgO, CaOand BaO are comparable with the referen-ce compound with ZnO, but the rates ofcure lower and mutually comparable.The extent of crosslinking (MH-ML) of thecompounds is slightly lower, in particularfor the BaO-containing compound. Yet,in contrast with ZnO at longer vulcanisati-on times no decrease or reversion is obser-ved.Swelling percentages and crosslink densi-ties of the s-SBR compounds are depictedin Fig. 7. The swelling experiments also re-veal an increase in swelling percentage ordecrease in crosslink density for use ofMgO, CaO and in particular BaO, relativeto ZnO. While in EPDM a considerably lo-wer torque level was observed for the com-pounds with these metal oxides, in s-SBR amore positive effect was found: a grosslycomparable torque level and correspon-dingly a comparable crosslink density.The physical properties of the s-SBR com-pounds with different metal oxides and thecombinations of activators are presented inTable 9. It is evident from the data shownin Table 9, that MgO as well as CaO lead tovulcanisate properties, before as well as af-ter ageing, comparable with the ZnO refe-

Tab. 8. Properties of EPDM compounds with different metal oxides

Compound 1 2 3 4 5

Tensile strength (MPa) 13.3 3.0 4.4 12.2 13.3

Elongation at Break (%) 450 681 776 418 475

Tear strength (N/mm) 37 18 26 38 36

CS 72h/23 8C (%) 6 28 25 4 7

CS 72h/100 8C (%) 37 99 95 64 61

After 168 Hours / 100 8C ageing:

Tensile strength (MPa) 11.6 11.9 13.2 11.9 12.1

Elongation at Break (%) 180 449 466 232 220

Tab. 9. Properties of s-SBR compounds with different metal oxides

Compound 1 2 3

Hardness (Shore A) 67 67 69

M25 (MPa) 1.2 1.2 1.3

M50 (MPa) 1.8 1.8 1.9

M100 (MPa) 3.4 3.3 3.4

M300 (MPa) 17.7 17.1 –

Tensile strength (MPa) 21.3 19.4 16.8

Elongation at Break (%) 345 329 296

Tear strength (N/mm) 37 34 34

CS 72h/23 8C (%) 6 8 7

CS 72h/100 8C (%) 35 35 37

After 168 Hours / 100 8C ageing:

M25 (MPa) 1.7 1.6 1.6

M50 (MPa) 2.8 2.6 2.6

M100 (MPa) 6.2 5.0 4.9

M300 (MPa) – – –

Tensile strength (MPa) 16.6 15.6 13.1

Elongation at Break (%) 203 235 211


rence compound, and therefore can beconsidered as good activators for sulphurvulcanisation of s-SBR rubber.Despite the high toxicity of BeO, the effici-ency of BeO as activator of sulphur curewas investigated in s-SBR compounds aswell, mainly to study the influence of thecrystal structure of the metal oxides. Thecure characteristics of compounds with dif-ferent levels of BeO as activator are given inFig. 8. In the presence of BeO the results donot differ very much from the vulcanisateswithout activator, indicating that BeO isagain not active as an activator in sulphurvulcanisation. It indicates again, that the

wurtzite structure is not the governing fac-tor for the activity of ZnO. In contrast withthe observations in EPDM compounds,Cu(II)O demonstrated hardly any activatinginfluence in the s-SBR compounds.Overall, substitution of ZnO Red Seal in s-SBR compounds by MgO or CaO seems tobe possible without large effects on thecure and physical properties, while the ad-dition of Cu(II)O as activator leads to infe-rior cure characteristics. These findingscontrast with the results in EPDM and im-ply that the activating influence of the me-tal oxides on the reactions in a benzothia-zole- vs. a thiuram/mercaptobenzothia-

zole-accelerated vulcanisation system isdifferent and less dominant. On the otherhand, the difference in nature and reacti-vity of the rubbers can also (partially) causethe difference in activation induced by thevarious metal oxides.

Model compound vulcanisation

In the previous section it was demonstra-ted, that the curing and physical propertiesof EPDM and s-SBR compounds with othermetal oxides as activator, in most cases de-viate considerably from the ones obtainedwith ZnO. A more detailed knowledge of

Fig. 6. Cure characteristics of s-SBR compounds with different metal oxi-des as activator

Fig. 7. Swelling and crosslink density of s-SBR compounds with differentmetal oxides as activator

Fig. 8. BeO and Cu(II)O as cure activator in s-SBR compounds Fig. 9. Decomposition of CBS in squalene as a function of reaction time,with ZnO, MgO, CaO as activator


the effect of the activator on the reactionsand mechanisms during the vulcanisationprocess can be obtained by studying thereactions of the curatives in a rubber-likemodel environment. In this section the re-sults of Model Compound Vulcanisationexperiments with four different metal oxi-des, viz. none, ZnO, MgO and CaO, in twodifferent model compounds, viz. squaleneand TME are described. The poly-unsatura-ted compound squalene was used becauseof its similarity to the real rubber structure,viz. the presence of more than one doublebond and relatively less end-groups. Squa-lene was used to specifically study the first

stage in the vulcanisation process: curati-ves development. The mono-unsaturatedmodel 2,3-dimethyl-2-butene (TME) wasselected to study three different stagesin the vulcanisation process as a functionof reaction time: curatives development,crosslink precursor formation, and thecrosslinked products development.The reactions were done as described inthe experimental section, at 140 8C inthe presence of a vulcanisation system.Since the reactions were performed in inertatmosphere, the amount of products dueto oxidation could be suppressed. Thereaction products were analysed with the

aid of HPLC at room temperature therebypreventing thermal decomposition of theproducts.

Squalene model compoundvulcanisation

The composition of the squalene samplesas a function of reaction time was determi-ned. For every reaction time a separateHPLC chromatogram was obtained. Theconcentrations of the curatives can be cal-culated via the internal standard and theresponse factors, and can be plotted asa function of reaction time. Fig. 9 depicts

Fig. 10. Sulphur concentration in squalene as a function of reactiontime, without and with ZnO, MgO and CaO as activator

Fig. 11. MBT concentration in squalene as a function of reaction time,without and with ZnO, MgO and CaO as activator

Fig. 12. General scheme for sulphur vulcanisation [9] Fig. 13. Decomposition of TBBS in TME as a function of reaction time,without and with ZnO, MgO and CaO as activator


the concentration profile of the acceleratorCBS for the four formulations. It can clearlybe seen in Fig. 9, that the various activatorsinfluence the breakdown of CBS differen-tly. Without activator no decomposition ofthe accelerator CBS is observed in the first20 minutes. MgO as activator causes a veryfast decomposition of CBS: within 5 minu-tes all the accelerator is consumed. It is alsoevident from Fig. 9 that CaO hardly hasany influence on the decomposition ofCBS. ZnO takes an intermediate position:a slower decomposition than MgO but fa-ster than CaO. The results are in agree-ment with the earlier results obtained byGarreta [21], who performed similar mea-surements with ZnO, MgO, CaO and CdOas activators in squalene. Similar trendswere observed: a very fast decay of CBSin the system with MgO, slightly slowerwith ZnO and hardly any influence ofCaO on the decomposition of the accele-rator.A very fast decay is observed in the sulphurconcentration in the first five to ten minu-tes for all systems, Fig. 10. In this stage thesulphur is incorporated in the acceleratorduring the first 10 minutes to form polysul-phidic species. The sulphur content in thesystems with ZnO and MgO shows similarincorporation behaviour, while with CaOthe profile is comparable with the non-ac-tivated system. In the presence of CaO therate of formation of active sulphuratingspecies is lower, as indicated by the slowerdecomposition of the accelerator in Fig. 9,and therefore a slower decay of sulphur isobserved.

MBT is formed during the course of thereaction as decomposition product of theaccelerator and as a side product fromthe transformation of a crosslink precursorinto a crosslink. Since MBT is not presentinitially, the amounts are related to the ma-ximum amount that can be formed if allCBS would transform into MBT. A relativeamount of 0.5 suggests a 50 % conversionof the CBS into MBT. Although in the HPLCsystem used, MBT and MBTS have thesame retention time, it is assumed thatthe peak at the retention time of 2 minutesin the HPLC chromatogram contains onlyMBT and that the amount of MBTS is ne-gligible. This assumption is justified by se-veral researches [21, 22]. The concentrati-on profile of MBT for the several metal oxi-des is shown in Fig. 11.Only when ZnO is present in the reactionmixture, the MBTconcentration remains ata low level, contrary to the case where noactivator has been added: MBT tends toaccumulate at longer reaction times. Thiscan be explained by the formation of acomplex between the zinc ions andMBT: ZnMBT. ZnMBT precipitates in thereaction mixture and is therefore difficultto analyse with HPLC. MgO leads to higheramounts of MBT at shorter reaction times.Presumably, less complex formation be-tween the magnesium ions and MBT oc-curs, which corroborates the propositionthat magnesium has only a slight tendencyto form complexes. With CaO present, theMBT concentration follows again grosslythe same profile as the non-activated sy-stem.

Overall, several steps in accelerated sul-phur vulcanisation, generally accepted totake the course as visualised in Fig. 12[23], could be investigated in the presenceof metal oxides. The rate of accelerator de-composition is found to be strongly depen-dent on the metal oxide used, Fig. 9. Anactive accelerating complex is formedwhich interacts with sulphur to generatethe active sulphurating agent: a decreaseof the sulphur content is observed,Fig. 10. A part of the complex forms a (po-ly)sulphidic crosslink precursor, which is,for this particular model system, difficultto analyse with this HPLC-UV setup. De-composition of the accelerator and trans-formation of the precursor into a crosslinkyields a side product: MBT. Only in pre-sence of ZnO, complexation between themetal ion and MBT is observed. In the pre-sence of MgO, at a reaction time of 10 mi-nutes the relative amount of MBT is about25 %, which is not exceeded within a reac-tion time of 60 minutes. It is suggestedthat MBT partly remains in a complexwith the metal ion. CaO is apparently un-able to remain bonded to the MBT. The ac-celerator decomposition and the MBT for-mation proceeds similar for CaO and in theabsence of activator.

TME model compoundvulcanisation

To study the formation and decompositionof crosslink intermediates and crosslinkingreaction in detail, additional experiments

Fig. 14. Sulphur concentration in TME as a function of reaction time,without and with ZnO, MgO and CaO as activator

Fig. 15. MBT concentration in TME as a function of reaction time, with-out and with ZnO, MgO and CaO as activator


with MgO and CaO as activator were per-formed in TME-model systems. The samemetal oxides as in the squalene systemwere studied in TME MCV: none, ZnO,MgO and CaO. Fig. 13 and 14 show theconcentration profiles of the initial vulcani-sation ingredients, TBBS and sulphur, re-spectively, for samples without and withZnO, MgO and CaO. ZnO has hardly anyinfluence on the decomposition of TBBS.In the initial stage, a more or less similarprofile is observed in the non-activated sy-stem and the system with MgO, whereasafter 15 minutes a faster decay of the ac-celerator is observed in the latter system.CaO, on the other hand, seems to imposea slightly delaying effect on the acceleratorconversion.In the TME samples a fast decay of sulphurin the first five minutes is observed, Fig. 14,comparable with the results obtained insqualene. It is evident from this figure,that the amount of sulphur remains hig-hest in the non-activated and CaO-contai-ning systems. The sulphur concentrationprofile with MgO present in the system ta-kes an intermediate position, as observedin the squalene samples.Fig. 15 represents the MBT-developmentas a function of reaction time. Except forthe MgO-containing sample, the formati-on of MBT starts after a reaction time ofapprox. 30 minutes. In contrast with theresults in squalene samples, a relativeamount of 1 for the MBTcontent is not rea-ched in any of the samples, though in allcases all TBBS has reacted after 40 minu-tes. As suggested before, the MBT proba-

bly forms a complex with metal ions andprecipitates in the reaction mixture or ispresent as crosslink precursor, bonded tothe allylic position of the model moleculevia a polysulphidic bridge (rubber-Sy-acc).The formation of this intermediate com-pound can be followed as a function ofreaction time. The crosslink precursor de-velopments for the systems with differentmetal oxides as an activator are shown inFig. 16.Fig. 16 indicates no significant differencesin rate of crosslink precursor formation inthe four systems, which is consistent withthe decomposition profile of TBBS as sum-marised in Fig. 13. In the non-activated sy-stem a maximum concentration of cross-link precursor is observed at approx. 30 mi-nutes reaction time. In the samples withZnO present, the breakdown of the cross-link precursor is delayed to some extent. Inthe presence of MgO crosslink precursorsare formed slightly faster than with theother metal oxides. This coincides withthe faster breakdown of the acceleratorTBBS caused by MgO as shown inFig. 13. At a reaction time of 20 minutesa maximum is reached and after 30 minu-tes the precursor is fully transformedagain. With CaO the formation of crosslinkprecursors is delayed. During the first 10minutes of reaction hardly any precursorsare formed, neither any TBBS consumed.However, the formation of crosslink pre-cursors occurs at a pace comparable tothe other metal oxides. A maximum con-centration of the crosslink precursor is ob-served at approx. 50 minutes. CaO appa-

rently delays the formation as well asbreakdown of precursors.Various mechanisms for the conversion ofcrosslink precursors into crosslinks havebeen proposed. The reaction of the inter-mediate crosslink precursor with anotherintermediate moiety or through the reacti-on with polymer chains leads to the forma-tion of polysulphidic crosslinks. For the sy-stem with ZnO as an activator, the concen-tration profiles of TBBS, crosslink precursorand crosslinked products are summarisedin Fig. 17. The symbols used in Fig. 17 cor-respond with the symbols depicted in thegeneral vulcanisation scheme, Fig. 18. Inaccordance with the concept that thecrosslink precursors convert into crosslinks,the formation of crosslinks begins slightlybefore the concentration of crosslink pre-cursor decreases.Fig. 19–22 show the development in timeof three of the reaction products, viz. S3, S4

and S5 TME-crosslinks, for the systems withdifferent metal oxides, measured withHPLC. The different (poly)sulphidic cross-linked products cannot be compared abso-lutely while the response factors of theseproducts were not all determined, therefo-re only the trends between the different sy-stems can be compared. The peak areas ofthe crosslinked products for the sampleswithout activator, with ZnO, MgO andCaO as activators are plotted versus reac-tion time in Figs. 19, 20, 21 and 22, respec-tively.The peak at 17 minutes corresponds to thelongest sulphur bridge, S5, while the peaksat 12 and 10 minutes represent the S4- and

Fig. 16. Crosslink precursor: TME-MBT, concentration as a function ofreaction time, without and with ZnO, MgO and CaO as activator

Fig. 17. TBBS, Crosslink precursor, and crosslinked products concentrati-on as a function of reaction time with ZnO


S3-TME-crosslinks, respectively, in accor-dance with the linear correlation betweenthe sulphur rank and the logarithm of theretention time reported by Hann et al. [24].It is evident from these graphs that the dis-tribution of the crosslinked products isstrongly affected by the applied metal oxi-des. It is noted that in the presence of MgOthe formation of crosslinked products in-itiates at considerably shorter reactiontimes, which is in accordance with earlierobservations on the decomposition ofthe accelerator and formation and break-down of the crosslink precursor.The final step of the scheme in Fig. 18 iscrosslink shortening. To observe this parti-

cular phenomenon, the crosslinked pro-ducts development for the different metaloxides as a function of reaction timeshould be examined. Crosslink shorteningis most clearly observed in the non-activa-ted system: Fig. 19. The longer crosslinksdecrease while the shortest sulphur bridgeincreases. In all systems the amount of theshortest crosslinks increases with reactiontime.

Discussion

Comparison and evaluation ofsqualene and TME MCV results

The rate of breakdown of the acceleratorin the squalene model compound seems todepend strongly on the activator used. Theorder of the activators from the fastest tothe slowest breakdown is: MgO > ZnO >

CaO.The differences between the rate of de-composition of the accelerator in the pre-sence of metal oxides in TME is not as clearas in squalene. A similar trend is observedin TME as compared to squalene: in thepresence of MgO the accelerator decom-poses the fastest and in presence of CaOthe slowest and even delayed. At shortreaction times, however, considerablysmaller differences between the activatorsare observed. Apparently, the acceleratorTBBS used in the TME model compoundsis more reactive, thereby decreasing thedependence on the activator.The order of reactivity towards the accele-rator is similar to the order of Lewis acidityof the cations. This corroborates the sug-

gested mechanism of complex formationbetween the cations and the acceleratorparts [8–10].It has appeared that the sulphur concen-tration profiles in the two model systemsare influenced by the presence of metaloxides. The order of sulphur levels at shortreaction times observed in squalene, start-ing with the highest sulphur level, is: CaO� MgO ¼ ZnO, whereas the order obser-ved in TME is: CaO�> MgO> ZnO. WhenCaO is present, the sulphur concentrationprofile is comparable with the non-activa-ted system. It can be concluded that in thepresence of CaO hardly any sulphur inser-tion in the active accelerator complex oc-curs. On the other hand, in presence ofMgO, sulphur is consumed much fastercompared to the systems with ZnO pre-sent. Presumably, active sulphurating spe-cies are formed via sulphur insertion in theactive accelerator complex.The formation of the crosslink precursor, asstudied with TME model compounds, pro-ceeded fastest for MgO and slowest forCaO with respect to reaction time. Thebreakdown of the crosslink precursor asa function of reaction time follows a similartrend: MgO > ZnO > CaO. As mentionedbefore, it corresponds with the order of theLewis acidities. It is an indication that theactivators play a role in detaching the ac-celerator part (MBT) of the crosslink precur-sor via complex formation.In conformance with the suggested reacti-on mechanisms, the concentration ofcrosslink precursors starts to decreasewhen the crosslinked products are formed.

Fig. 18. General vulcanisation scheme (z < y)

Fig. 19. Crosslinked products (CP) concentration in TME samples withoutactivator

Fig. 20. Crosslinked products (CP) concentration in TME samples withZnO as activator


The initially formed polysulphidic bridgestend to reduce the sulphur chain lengthby releasing sulphur and the formationof shorter crosslinks. Apparently, the ap-plied metal oxide also affects the finalcrosslink distribution in the model com-pound samples. With MgO present inthe system, the formation of crosslinkedproducts initiates at shorter reaction timesand the dominating product appears to bethe shortest crosslink, whereas in the ZnO-containing sample the longer crosslinks areformed to a higher extent. The overall pro-duct yield in presence of MgO, however, isconsiderably lower as compared to theZnO-containing sample.

Comparison of the effect of metaloxides as activator in MCV studiesand rubber compounds

It is to be noted that the latter results applyto model compounds, i.e. squalene andTME, and that these findings may not ne-cessarily be extrapolated to real rubberssuch as EPDM and s-SBR. However, the ge-neral trend of the reactions is unlikely to beinfluenced by the particular allylic structu-res of the various rubbers involved.The crosslinked products as studied withMCV can be evaluated by summing allthe peaks of the various sulphur chains,i.e. area of S3, þ area of S4 etc. Althoughaddition of peak areas is not totally correct,

it provides a good idea of the crosslink de-velopment. Fig. 23 shows in a combinedgraph the crosslink development obtainedwith the TME model compounds and therheograms of s-SBR compounds for sy-stems with ZnO, MgO and CaO.At first sight, the MCV results are not at allin agreement with the s-SBR compounds,probably due to many (fundamental) diffe-rences between the real rubber and modelsystems. After 60 minutes of MCV the to-tal peak area of the crosslinks in the samplewith ZnO, is still increasing and reaches ahigher final level than the samples withMgO and CaO, which is in good agree-ment with the higher torque level observedin the s-SBR compound with ZnO present.The effect of MgO on the onset of cross-linked products formation found in MCVexperiments, is not observed however inthe cure characteristics of s-SBR com-pounds. The lower final crosslink densityand difference in crosslink distribution inthe presence of CaO in particular, is alsoreflected in the physical properties presen-ted in Table 9.

Conclusions

To explore possibilities to reduce ZnO levelsin rubber compounds, this article describesa comprehensive study on the substitutionof the conventional activator ZnO by othermetal oxides. Both, real rubber and modelcompound systems are employed to inve-stigate the effect of these metal oxides onthe different stages of the vulcanisationprocess. It has appeared that CdO, PbO,

Fig. 21. Crosslinked products (CP) concentration in TME samples withMgO as activator

Fig. 22. Crosslinked products (CP) concentration in TME samples withCaO as activator

Fig. 23. Comparisoncure-torque of s-SBRcompounds with to-tal crosslinked pro-ducts in MCV of TMEas a function ofreaction time


BaO, CaO, MgO and BeO do not performas substitutes for ZnO as activator in thiu-ram-accelerated vulcanisation of EPDMneither show MgO and CaO any synergi-stic effect with ZnO. Interestingly, the re-sults clearly demonstrate that in s-SBRcompounds CaO and MgO are suitableas activator, as they show almost compara-ble physical properties, albeit with a slight-ly lower cure rate and state of cure. The ad-dition of Cu(II)O as activator, however,leads to inferior cure characteristics, whichcontrasts with the results in EPDM. Onemay cautiously conclude that the activat-ing influence of the metal oxides on thereactions in the benzothiazole-acceleratedvulcanisation system is different and lessdominant, albeit with the notion thatthe tested rubbers were also rather diffe-rent in nature and reactivity.The second part of the study described inthis article elucidates the effect of the me-tal oxides on the different steps in the ben-zothiazole-accelerated vulcanisation pro-cess. In contrast to the squalene experi-ments, in the TME experiments the activa-tor hardly had any influence on the severalreactions rate constants. The role of themetal oxide, usually described as a catalystfor the vulcanisation, in particular for thedecomposition of the accelerator, appa-rently also depends on the type of accele-rator and the type of model olefin. Overall,the effectiveness of the metal oxides ap-pears to be determined by its ability toform complexes with the accelerator-moie-ties.

Acknowledgement

The Ministry of Economic Affairs in the Ne-therlands financially supports this researchproject in the Innovative Research Program(IOP) Heavy Metals / Environmental Tech-nology (Senter). An industrial consortiumof DSM Elastomers B.V., Flexsys B.V., Vre-destein Tires B.V., Hertel B.V., HelvoetB.V., TNO Industrial Technology, is grateful-ly acknowledged for additional support.The authors would also like to thank Ms.Wilma Dierkes and Mr. Jan van Duijlfrom the University of Twente for their sug-gestions and co-operation. They are alsograteful to Wilco Wennekes for perfor-ming the Model Compound Vulcanisationstudies.

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The authors

G. Heideman, J.W.M. Noordermeer, R.N. Datta, En-schede (The Netherlands); B. van Baarle, Eindhoven(The Netherlands)


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Effect of Metal Oxides as Activator for Sulphur ... of Metal Oxides as Activator for Sulphur Vulcanisation in Various Rubbers ... compound, certain metal oxides repre- ... only metal