recent developments in crosslinking of elastomers

RECENT DEVELOPMENTS IN CROSSLINKING OF ELASTOMERS

ABI SANTHOSH APREM

HINDUSTAN LATEX LIMITED, AKKULAM, SREEKARIYAM P.O, TRIVANDRUM, KERALA, INDIA -695017

KURUVILLA JOSEPH

ST. BERCHMANS’ COLLEGE, CHANGANACHERRY, KERALA, INDIA

SABU THOMAS

SCHOOL OF CHEMICAL SCIENCES, MAHATMA GANDHI UNIVERSITY, P.D. HILLS P.O., KOTTAYAM, KERALA, INDIA-686560

ABSTRACT

The recent developments in the vulcanization of elastomers, major types of crosslinking, and mechanism ofcrosslinking have been reviewed. Attention has been made to sum up the accelerated vulcanization. Possible mechanismsby which the reaction is taking place are discussed. The role of accelerators, activators, and fillers has been described.The different types of crosslinks and importance of each type on the specific properties of the resulting vulcanizates, etc.,are discussed. Various aspects of vulcanization like model compound vulcanization, nitrosamine generation, etc., are dis-cussed. The importance of binary accelerators and possible mechanism of their action have been mentioned. The methodof double network formation, which is useful for the improvement in properties, is also described. The different meth-ods for characterization of networks and different methods for estimating crosslink densities are also explained.

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458II. Methods of Vulcanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459

A. Peroxide Vulcanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460B. Resin Crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461C. Silane Crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462D. Metal Oxide Crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463E. Radiation Induced Crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465F. High Temperature Crosslinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466G. Dynamic Crosslinking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466H. Sulfur Vulcanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

III. Accelerators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467IV. Accelerated Sulfur Vulcanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469V. Role of Activators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476VI. Sulfur Donor Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477VII. Influence of Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478VIII. Binary Accelerator Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479IX. Concept of Double Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482X. Characterization of Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483XI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484XII. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

I. INTRODUCTION

The superior properties and low cost of natural rubber accounts for its use in a variety ofapplications. Originally natural rubber was used in the uncured state, but there were drawbacks

458

* Corresponding author. Ph: 91-481-2730003; Fax: 091-481-2731002; email: [email protected]

like softening in warm weather and highly increased rigidity in cold weather.1 This led CharlesGoodyear in United States and Thomas Hancock in England to the discovery of the process ofvulcanization in the years 1839 and 1843, respectively. Hancock was the first to observe that sul-fur alone would vulcanize rubber, whereas Goodyear had actually used the first inorganic accel-erator, lead oxide.

Vulcanization is the key process in the technology of rubbers. It is difficult to come up withan equally old industrial process where the exact chemistry remains unclear. Thus, despite thefact that 160 years have passed since the discovery of vulcanization, the exact mechanism is stillfar from completely understood.

The word ‘vulcanization’ has English roots and was derived from the name of the Greek andRoman God ‘Vulcanus’. Nieuwenhuizen wrote in his review,2 "Vulcanus’ was the ugliest of allthe Gods; this symbolizes the fact that vulcanization is a bad-smelling process. In spite of theugliness of Vulcanus, due to his talents and character he married ‘Venus’ the most beautiful ofall. Indeed we find that vulcanized rubber is a wonderful product, as measured by its versatility,usefulness and broad application".

The use of natural rubber as a material for finished products dates back to the 1830's.Vulcanization is basically the process of introducing crosslinks between hydrocarbon chains.According to the definition of American Society for Testing and Materials (ASTM),‘Vulcanization is a chemical process in which the long chains of the rubber molecules becomecrosslinked by reactions with the vulcanizing agent to form three dimensional structure’ (ASTMD1556). This reaction transforms the soft, weak, plastic-like material into a strong elastic prod-uct. The rubber loses its tackiness, becoming insoluble in solvents and more resistant to heat,light and ageing processes. (Scheme 1)

Sulfur alone was the vulcanizing agent up to the discovery of organic accelerators in theearly part of 20th century. It was quickly realized that the use of accelerators gave improvedproperties and significantly reduced the required cure times. The first accelerators were amine-based compounds, with other classes of accelerators following quickly. Other compounds usedin vulcanization in addition to sulfur and accelerators are zinc oxide and saturated fatty acidssuch as stearic acid. These materials are termed as activators. Rubber formulations can alsoinclude fillers such as silica and carbon black and compounds such as antioxidants.

II. METHODS OF VULCANIZATION

The formation of network structure is one of the essential conditions for generating the elas-tomeric properties. Sulfur vulcanization is the most widely used curing technique for rubbers,preferably for unsaturated ones. The other curing methods include peroxide, resin, moisture, ure-thane, metal oxide and radiation crosslinking.3

Uncrosslinked state Crosslinked state

SCHEME 1. — Elastomer chains in the uncured and cured state.

RECENT DEVELOPMENTS IN CROSSLINKING OF ELASTOMERS 459

Since the 1963 Natural Rubber Producers Research Association (NRPRA) overview on thefundamentals of vulcanization chemistry,4 it has become more and more apparent that, for thepurpose of understanding vulcanization on a molecular level, the different vulcanization systemsshould be treated separately. Even though the different vulcanization systems are broadly simi-lar in reactivity, it can be easily understood that a small change in the reactivity of one chemicalmay have an important influence on a whole series of chemicals. Indeed during vulcanizationdifferent reactions take place at the same time. In fact, rubber technology makes ample use of theknowledge that the different vulcanization systems result in rubber materials having differentproperties and specifications.

A. PEROXIDE VULCANIZATION

A wide variety of peroxides are used to crosslink most type of elastomers. The crosslinksformed by peroxides are purely carbon-carbon linkages. The importance of peroxides is theirability to crosslink saturated elastomers such as low-density polyethylene, ethylene-propylenerubber, silicone rubber etc., which cannot be crosslinked with other types of vulcanizing agents.The advantages and disadvantages of peroxide crosslinking are given in Table I.

TABLE IADVANTAGES AND DISADVANTAGES OF PEROXIDE CROSSLINKING5

Advantages Disadvantages

Simple compounding Expensive crosslinking agent

Good heat ageing resistance Low mechanical strength

Low tension set and strain Higher curing time

No contamination Difficult hot-air cure

Low compression set Poor resistance to flex fatigue

Transparent rubbers possible Need secondary cure of high temperature

In addition to the disadvantages described in Table I, there are several other important limi-tations for peroxide crosslinking of rubbers. One of them is that antioxidants can react with per-oxide generated radicals and can result in reduced crosslinking efficiency.5 Another potentialproblem is that carbon-centered radicals can react with oxygen. This reaction will producehydroperoxides and can lead to tacky, unvulcanized surfaces.

The mechanism for peroxide crosslinking is shown in Scheme 2. The crosslinking reactioninvolves the homolytic decomposition of the peroxide to produce alkoxy radicals followed byhydrogen atom abstraction. Studies with model compounds indicate that the hydrocarbon radi-cals predominantly undergo coupling rather than disproportionation. The coupling reactionforms crosslink between polymer chains. For polydiene elastomers experimental evidence indi-cates that the primary radical formed by peroxide decomposition abstracts a hydrogen atom froma carbon alpha to the double bond. In the case of natural rubber the methyl group is also reactivetowards hydrogen atom abstraction.

460 RUBBER CHEMISTRY AND TECHNOLOGY VOL. 78

SCHEME 2. — Mechanism of peroxide crosslinking (P-H saturated or unsaturated elastomer).3,4

B. RESIN CROSSLINKING

Resin crosslinking was discovered around the 1940's.6 It has been used for curing unsatu-rated rubbers. Resin curing systems are extensively used with butyl rubber for high temperatureapplications. Resin cures are slower than accelerated sulfur cures and high temperatures arerequired; and they can be activated only by zinc oxide and halogen atoms. Low molecular weightresin molecules diffuse into rubber and thereby stiffens the rubber. An example of recipe forbutyl rubbers is given in Table II. In this SP-1055 is the phenolic curative. The phenol-resincrosslinking of butyl rubber is accelerated by adding benzocarbonium ions to the double bond,forming a cyclic coumarone structure or a non-cyclic compound as illustrated in the Scheme 3.7

Crosslinking is considered to proceed via a mechanism such that a coumarone ring is pro-duced by a Diels-Alder (4+2)-cyclo-addition reaction of quinomethine. Compared to butyl rub-ber the resin crosslinking rates of EPDM and nitrile rubber are low. Volintu et al.8 have reportedthe crosslinking of nitrile rubber with p-octyl-phenol formaldehyde resin.

TABLE IIRECIPE FOR VULCANIZATION BY RESINS

Ingredients Amount (phr)Butyl rubber 100

ZnO 5

Stearic acid 1

Resin SP-1055 12

Typical vulcanization conditions

Temperature (°C) 182

Time (min) 80


C. SILANE CROSSLINKING

This type of crosslinking is also called moisture crosslinking. Alkoxysilane compounds areused as crosslinking agents together with water. The silanes carry functional groups, which arelikely to react with rubbers.


SCHEME 3. — Mechanism of resin crosslinking.7

SCHEME 4. — Mechanism of silane crosslinking.9

The moisture crosslinking of polyethylene using vinyl silane is shown in Scheme 4. Thecrosslinking reaction includes two steps: (i) the reaction of a silane compound with a polymer(grafting); and (ii) the condensation of the silanol groups produced by hydrolysis of the alkoxysi-lyl groups. This process was used for EPR for the cable industry.10 The major advantage of thistype of cure is the applicability at relatively low temperature.

D. METAL OXIDE CROSSLINKING

Chloroprene rubbers are generally vulcanized by the action of metal oxides.11,12 The primarycrosslinking agent is zinc oxide, which is used along with magnesium oxide. Lead oxides aresometime used when low water absorption is required. The reaction is thought to involve thevinyl group of the elastomer, which is the result of 1, 2 polymerization. A typical recipe for metaloxide crosslinking is given in Table III. Two routes have been proposed for the curing. Onerequires the incorporation of zinc atoms into the crosslink [Scheme 5]; the other leads to ethercrosslinks [Scheme 6].



SCHEME 5. — Mechanism of crosslinking in 1,2-poly chloroprene involving incorporation of Zn atom.11

SCHEME 6. — Mechanism showing crosslinking in 1,2-poly chloroprene through the formation of ether crosslinks.12

TABLE IIIMETAL OXIDE SYSTEMS FOR CHLOROPRENE RUBBER

Ingredients Amount (phr)Chloroprene rubber 100 100 100

ZnO 5 5 5

MgO 4 4

Calcium stearate 5.5

Stearic acid 1

TMTM 1

DOTG 1

ETU 0.5 0.5

Sulfur 1

Vulcanization conditions

Temperature (°C) 153 153 153

Time (min) 15 15 15

E. RADIATION INDUCED CROSSLINKING

The most recent type of crosslinking is the radiation-induced crosslinking. This includeselectron beam crosslinking, photo-crosslinking, microwave crosslinking, ultrasonic crosslinkingetc. Radiation induced crosslinking is a physically induced chemical reaction, which is easier andpreferable for continuous curing technologies, and thus it has some potential for the future. Theradiation dose required for rubber differs. Radiation crosslinking of different rubbers are report-ed.13-15 Upon irradiation free radicals are formed in rubber molecules. The free radicals can com-bine to form crosslinks as in the case with peroxide crosslinking. In radiation crosslinking, rub-ber is pressed at 100-200 °C for 5-10 minutes. It is then allowed to cool under pressure and thenexposed to radiation. The radiation dose required for proper crosslinking is 90 M rad for NR, 17M rad for NBR, 40 M rad for CR, 10 M rad for silicone rubber and 10 M rad for CFM. Themechanism is shown in Scheme 7. The use of sensitizers can reduce the required dose and radi-ation time. Halogen compounds, nitrous oxide, sulfur monochloride, and bases like amine,ammonia etc are used as sensitizers.16 Radiation vulcanization for NR latex, SBR and function-al monomers have been reported.17

SCHEME 7. — Mechanism of radiation vulcanization using sensitizer.18


Radiolysis of NR

RH → R• + H

Radiolysis of water

H2O → H• + OH•

Hydrogen abstraction

RH + OH• → R• + H2O

Homopolymerisation

nM → P•

Graft polymerisation

R• + nM → RP•

Chain transfer

P• + RH → P + R•

RP• + RH → RP + R•

R• + P → RP•

M• + RP → RP

Ternination

R• + R• → R—R

RP• + R• → RP—R

F. HIGH TEMPERATURE CROSSLINKING

This type of crosslinking is also called high velocity crosslinking and is carried out over atemperature range of 170-230 °C. Usually tires are cured routinely at temperatures this high.However it is often associated with reversion. Chen and coworkers19 have shown that the phe-nomenon of reversion seems to appear when two competing reactions occur during vulcaniza-tion. These two reactions are crosslinking and desulfuration. The process is associated with theformation of a trans-methine structure (Figure 1) by the desulfuration reaction. Morrison andPorter20 confirmed that the observed reduction in vulcanizate properties is caused by two reac-tions proceeding in parallel, i.e. desulfurization and decomposition. Loo21 has demonstrated thatas the cure temperature rises, the crosslink density drops and the degree of reversion increases.Temperature is thus a major factor than the duration of vulcanization in determining the degreeof crosslinking. Loss of properties at elevated temperatures could be avoided by two ways: (i)optimization of the accelerator/sulfur ratio; and (ii) use of an accelerator, which is less sensitiveto increased temperatures.

G. DYNAMIC VULCANIZATION

Dynamic vulcanization is the process of vulcanizing the elastomer during the melt-mixingprocess, with a non-vulcanizable molten thermoplastic.22 One of the most interesting applica-tions of accelerated sulfur vulcanization is in the preparation of thermoplastic elastomers bydynamic vulcanization.

In this process, small rubber droplets are vulcanized to give vulcanized rubber particles withstable domain morphology, in which the rubber particles are dispersed in the molten thermo-plastic polymer to allow the blend to be fabricated into finished products in thermoplastic pro-cessing equipment. The diameters of the elastomer particles are reported to be in the 1-2 µmrange. Various blends of EPDM with polypropylene were dynamically vulcanized with acceler-ated vulcanizing systems consisting of sulfur, zinc oxide, stearic acid, tetramethyl thiuram disul-fide and benzothiazyl disulfide.23 Peroxides are also used for dynamic vulcanization.

Recently, new vulcanizing agents were introduced for dynamic crosslinking of elastomerblends.24 They were found to be efficient in crosslinking and impart good mechanical propertiesto the resulting vulcanizates.

H. SULFUR VULCANIZATION

Rubber vulcanization by sulfur, without any accelerators takes several hours and is no longerof commercial importance. With the use of accelerators, optimum curing can now be achieved inperiods as short as 2-5 min. Sulfur vulcanization can be divided into two categories; unacceler-ated and accelerated sulfur vulcanization. Unaccelerated formulations typically consist of sulfur,zinc oxide and a fatty acid while accelerated formulations include an accelerator in the system.Unaccelerated sulfur vulcanization, which is also referred to as ‘sulfur only’ vulcanization, is the


FIG. 1. — Structure of trans-methine.

oldest form of vulcanization practices. Sulfur only vulcanization chemistry involves many reac-tions that either do not occur, or occur to much lesser extent than in accelerated systems. It shouldbe noted that un-accelerated sulfur systems are no longer of commercial significance.

III. ACCELERATORS

The discovery of acceleration of vulcanization using organic compounds in 190625 is nextin importance to the discovery of vulcanization itself. In the early days, vulcanization took sev-eral hours to get completed. Today it has been reduced to a few minutes. Accelerators reducedthe vulcanization time drastically. The temperature can be reduced and in some cases vulcaniza-tion can be done at room temperature. Furthermore the proportion of sulfur is reduced from 8-10 parts to 1-3 parts. The proportion of accelerator is usually 1 part. The lowering of sulfur givesmuch better oxidation resistance and some accelerators are good antioxidants.

The first accelerators were inorganic compounds. Among the basic salts and metals oxides,which are or have been widely used as accelerators, litharge, lime magnesium oxide and zincoxide are the most important ones. Organic vulcanization accelerators were first utilized in therubber industry in the beginning of the 20th century.25-31 Oenslager26 in 1906 introduced theorganic bases aniline and thiocarbanilide as accelerators in rubber compounds to improve thequality of low-grade rubber and to accelerate the rate of vulcanization. Thus organic bases werethe first vulcanization accelerators of rubber.32-34 Other accelerators such as diphenyl guanidine(DPG) diorthotolyl guanidine35-36 (DOTG) and hexamethylene tetramine (HEXA) came in to usearound 1910 and were a great relief for the people in the rubber industry because of their rela-tive non-toxic character compared to the former ones.

Dithiocarbamates and xanthates were widely used as accelerators. Because of their instabil-ity, fast reactivity and poor processing safety they are generally used in low temperature pro-cessing and curing and in accelerator combinations. In 1925, Sebrell et al.37-39 and Bruni et al.40

discovered independently that 2-mercapto benzothiazole, its homologues, its disulphides41, 42 andits metal salts are very effective accelerators, which yields vulcanizates of improved physicalproperties. 2, 2' dithiobenzothiazole (MBTS) was developed to give greater scorch safety at theuse of fine furnace blacks, delayed action accelerators like sulphenamide type (e.g. Benzothiazylderivative of sulphenamide) were developed for long processing safety and satisfactory curerates. Attempts to modify the extraordinary fast ammonium dithiocarbamates resulted in the zincdialkyl dithiocarbamates. Further efforts to substitute the mercaptan sulfur in dithiocarbamateresulted in tetra alkyl thiuram disulphides like tetra methyl thiuram disulfide (TMTD), tetramethyl thiuram mono sulphides (TMTM) and thiuram tetra disulphides.43-45 They are used asultra accelerators and vulcanizing agents themselves. All of these products are currently used ina large scale as accelerators. Thus the discovery of various types of accelerators which differ intheir effects on the rate of scorching, ageing, etc., of rubber compounds and on the structure ofthe resultant vulcanizates which determines the ultimate property have revolutionized the rubberindustry. Based on the nature of curing and chemical structure accelerators are classified into sev-eral groups. The generalized classification is shown in Table IV.


TABLE IVPRINCIPAL CLASSES OF ACCELERATORS USED IN THE SULFUR VULCANIZATION OF ELASTOMERS


IV. ACCELERATED SULFUR VULCANIZATION

Accelerated sulfur formulations are the most common vulcanization systems used in com-mercial and industrial applications. For this reason, a large amount of work is going on in thefundamental and applied aspects of accelerated sulfur vulcanization. The study of acceleratedsulfur vulcanization suffers from the problems like the ability of sulfur to undergo both radicaland ionic reactions, the intractable nature of cured vulcanizates, how the accelerators and acti-vators interact and how these interactions affect the vulcanization mechanism etc.

A major contributing factor to the disagreement over mechanisms is the possible reactionsthat sulfur and accelerators undergo. Sulfur occurs naturally as an eight membered ring. This ringis capable of both homolytic cleavage to form radicals or heterolytic cleavage to form ions.46

Also, the precise interaction of the accelerator and sulfur in the vulcanization process has notbeen clearly elucidated. It is known that accelerator complexes are found, but the action of actu-al sulfurating species has not been determined. This fact and the possible reactions of sulfur andaccelerators have made it impractical to eliminate the radical or polar mechanism. Severalresearchers have concluded that both radical and polar mechanisms are operative,47-53 and thatthe precise nature is dependent on the formulation.

The scorch delay or induction period is where the majority of accelerator chemistry occurs.It should be noted that the scorch delay varies widely between accelerators. In thiuram acceler-ated systems there is little induction time whereas in MBT accelerated system there is a shortinduction time. In sulphenamides there is a long scorch delay. During the scorch delay variousaccelerator complexes are formed. Early workers have proposed54 a variety of conclusions to theexact nature and role of these complexes. Typical cure curves for samples cured with differentaccelerator systems are given in Figure 2.

While it is clear that specific complexes do form, it is still open for debate whether thesecomplexes are the active sulfurating species or whether an activated intermediate is formed.Accelerated sulfur vulcanization has been found to consume the accelerator in the system at a


FIG. 2. — Typical cure curve with different accelerator systems:A) Dithiocarbamates; B) Thiurams; C) Thiazoles; D) Sulphenamides.

rate far greater than the rate of crosslinking.55 This has led to the proposal that accelerated vul-canization proceeds through an intermediate.56,57 Subsequent research has provided strong evi-dence for the existence of this species.52,58 Coran58 in his work, isolated a compound and identi-fied it as a complex consisting of the zinc salt of the accelerator stabilized by interaction withstearic acid. He modelled the mechanistic scheme for the formation of this complex and subse-quent crosslinking/sulfurization as:

where A = accelerator; B = intermediate; B* = active intermediate (sulfurating agent); Vu =crosslink; and α, β = constants to adjust stoichiometry.

Research by Banerjee and co-workers52,54,59,60 led to the conclusions that both polar and rad-ical mechanisms are operative during vulcanization. Their work indicated that several commonaccelerators including MBT and TMTD were capable of undergoing both polar and radical reac-tions.

The proposed radical mechanism of accelerated sulfur vulcanization is shown in Scheme 10.The intermediate cleaves to form persulfuryl radicals, which then abstract protons. The rubberradical reacts with another intermediate to form a rubber-bound intermediate. Two rubber boundintermediates then form the actual cross-link. Maturity of the network occurs through sulfurexchange reactions. Isomerization, which is widely observed in vulcanization,61 occurs throughalkyl radical.

SCHEME 10. — Radical mechanism of accelerated sulfur vulcanization.62

The proposed polar mechanism is shown in Scheme 11. The key step is a concerted reactionof a ring structure leading to the formation of the rubber bound intermediate on the sulfurcrosslink. Isomerization occurs by loss of the rubber bound intermediate as an ion.

SCHEME 11. — Proposed polar mechanism of accelerated sulfur vulcanization.62

A B B Vu

A B B

→ → →

+ →

∗

∗

α

β


There are varying opinions on the concentration of the rubber bound intermediate at anygiven time in the reaction process. The exact nature of accelerated sulfur vulcanization is still atopic of much debate. Recent advances in instrumental techniques have been able to providemore information about the resulting structures, but the actual chemistry involved is still not suf-ficiently clear. It is probable that both free radical and polar mechanism are operative and thatthe exact nature of the vulcanization process will vary between different curing systems.62

The chemical structure, molecular weight and conformation of the elastomers affect the effi-ciency of sulfur vulcanization and the physical properties.63 The sulfur vulcanization with unsat-urated rubbers occurs through complicated radical mechanism in terms of mono, di or polysul-phidic bridges and sulfur containing intra-cyclization with the polymer molecules. The crosslinkdensity and distribution affect the physical properties and the stability on ageing and are depend-ent on accelerator type, ratio of accelerator to sulfur, reaction temperature and time.64 Variousstructures formed in sulfur vulcanized natural rubber is shown in Scheme 12.

SCHEME 12. — Generalized structures in sulfur vulcanized natural rubber.65

As the rate of vulcanization is directly related to productivity, it is desirable to increase therate by raising temperature. However at higher temperature the effectiveness of sulfur cross-linksis lower with a sacrifice of the physical properties caused by the dissociation of sulfur bonds andrubber chains. Thus the reactivity was controlled by additional amounts of sulfur and accelera-tor instead of raising temperature. The increase of sulfur addition leads to increase in crosslinkdensity and the proportion of polysulfidic linkage, causing a decrease in the stability on ageing.With the increase of accelerators the effectiveness of sulfur vulcanization having mono and disul-fidic linkages is improved to give stability on ageing.66 The sulfur level determines the overallextension of sulfurization, while the accelerator concentration determines the efficiency of theformulation.67

Layer68 used the unique approach of ‘vulcanizate recurring’ to probe the role of sulfur andaccelerators in the curing and reversion process. He concluded that sulfur determines the overallamount of reaction, but the accelerator determines the length of sulfur chains. He proposed thatthe key step in the curing reaction was the sulfur exchange reaction.68,69

Model compound vulcanization (MCV) is the vulcanization of a low molecular weightmodel for the rubber polymer. It is a prime research technique applied to gain information about


the chemistry of vulcanization. The compound selected for MCV should contain at least oneallylic hydrogen, since crosslink formation involves the substitution of allylic hydrogen for a sul-fur link.2

The objective of an MCV study is to obtain information regarding the chemistry of therepeating rubber unit, to elucidate the structure of the crosslinked products that are produced dur-ing vulcanization and to study the reactions of curatives in a rubber like environment. Ultimatelythis knowledge would furnish information about the mechanism of the vulcanization reactions.70-

72 The chemical probe work has allowed characterization and quantification of the number ofmono, di and poly sulphidic crosslinks. The model compound work has been useful in providinginformation for mechanistic studies by allowing comparison of products predicted by a mecha-nism to the products obtained from model compound work.73

A typical cure curve obtained from a rheometer is shown in the Figure 3. The initial portionof the curve is called the induction or scorch period where much of the accelerator chemistry isinvolved. As time goes on torque increases and attains a maximum value. This point indicates themaximum torque. The time required to attain 90% of the maximum torque is termed as the opti-mum cure time (t90) where as the time required to attain 10% of the maximum torque is termedas the scorch time (t10). After attaining a maximum torque the crosslinks formed will be short-ened or destroyed. This behavior is different for different compounds and the type of curing.

Depending on the accelerator/sulfur ratio desulfurization or decomposition may occur. Anoutline of the mechanism is shown in Scheme 13. Depending on the type and nature of vulcan-ization the cure curves exhibit different nature. After attaining the maximum torque, it mayincrease further (marching), or decrease (reversion), or levels off (Figure 4).


FIG. 3. — Typical rheograph showing different stages on curing.

SCHEME 13. — Generalized mechanism of vulcanization.20

The choice of the accelerator in the process of sulfur vulcanization determines the kind ofnetwork structure and consequently, leads to the specific material properties.74-75 The chosenaccelerator affects the cure rate and scorch safety, as well as the numerosity and the averagelength of the formed crosslinks. Both the number and the length of the crosslinks have an influ-ence on physical properties of rubber. Monosulphidic crosslinks are regarded as being unable toexchange, rearrange or break to relieve mechanical stresses without cleaving main chains.Polysulphidic crosslinks on the other hand are able to rearrange under stress, through breakingand reforming and this is partly associated with high mechanical strength. Also, changes duringservice are determined in considerable measure by the strength of the crosslinks. Shorter monoand disulfide crosslinks contain thermally stronger C-S-C and C-S-S-C bonds, which are associ-ated with superior thermal and oxidative stabilities. Studies have been undertaken on networkcharacterization of natural rubber vulcanizates.76

Based on the amount of accelerator and sulfur present, the vulcanization systems are classi-fied into conventional (CV), semi-efficient (semi-EV) and efficient (EV). The amount of accel-erator and sulfur present in these systems are given in Table V.


TABLE V

COMPOSITION OF CONVENTIONAL, SEMI EV AND EV CURE SYSTEMS88

Type of system Sulfur (phr) Accelerator (phr) A/S ratioConventional 2.0-3.5 1.2--0.4 0.1-0.6

Semi EV 1.0-1.7 2.5-1.2 0.7-2.5

EV 0.4-0.8 5.0-2.0 2.5-12

The properties of the resultant vulcanizates are given in Table VI. As seen from the mecha-nism of vulcanization given by earlier researchers4,78 the active sulfurating agent is the zincperthio mercaptide complex. The concentration of this complex determines the type of crosslinksformed. As the CV system has got greater amount of sulfur compared to the accelerator the pos-sibility of forming more polysulphidic linkages is higher for CV system. As the concentration ofaccelerator increases two reactions viz., desulfurization or decomposition may occur.Desulfurization results in mono and disulphidic linkages while decomposition leads to cyclic sul-phides, conjugated dienes, cyclic sulphides etc. When the temperature is increased the polysul-phidic linkages break into mono and di sulphidic linkages. This explains the reversion shown athigher temperatures. This leads to low strength and modulus. Properties like compression set andthermal stability are better for EV systems. This is primarily due to the lower amount of poly-sulphidic linkage in the EV system.


FIG. 4. — Rheograph showing different behavior in modulus development.

TABLE VIVULCANIZATES STRUCTURE & PROPERTIES OF THE DIFFERENT SULFUR CURING SYSTEMS88

Properties CV Semi EV EVPoly and disulphidic crosslinks (%) 95 50 20

Monosulphidic crosslinks (%) 5 50 80

Cyclic sulphide concentration High Medium Low

Low temperature crystallization resistance High Medium Low

Heat ageing resistance Low Medium High

Reversion resistance Low Medium High

Flex fatigue resistance High Medium Low

Compression set 22 hours at 70 °C (%) 30 20 10

The general nature and amount of crosslinks present in an efficient vulcanizing system andconventional system is given in Figures 5 and 6, respectively. It is seen that the amount of mono-sulphidic linkages is a maximum in the efficient system and it increases initially and decreaseswith cure time. At the same time, the amount of polysulphidic and disulphidic linkages decreas-es with time. The final network formed after the desulfurization and decomposition reactions willbe highly crosslinked with mainly monosulphidic bonds and there will be relatively few modifi-cations of the cyclic sulphide or conjugated triene type.4 Such a network is termed efficientlycrosslinked.


FIG. 5. — Features of NR vulcanizate produced by an efficient crosslinking system.4


On the other hand desulfurization process proceeds slowly as in the case of the compounddepicted in Figure 6. Here the amount of polysulphidic linkages is higher. There will be oppor-tunities for thermal decomposition, leading to reversion or loss of crosslinks and to networkscontaining modifications. Further the crosslinks, which do survive, will be di or polysulphidicand hence will be liable to further decomposition. These networks are said to be inefficientlycrosslinked.

V. ROLE OF ACTIVATORS

Fatty acids are generally regarded as indispensable activators in conjunction with zinc oxide.The function of fatty acid activators such as stearic acid, is to solubilize the zinc oxide, a sec-ondary effect is an increase in the amount of zinc sulphide produced. The zinc salts of fatty acids,which are a type of surfactant, also solubilize insoluble accelerators to form the actual catalyst.A general scheme of accelerated sulfur vulcanization that demonstrates the role of zinc oxide, inconjunction with fatty acids is shown in Scheme 15.78

SCHEME 15. — Role of ZnO, fatty acid and activator in accelerated sulfur vulcanization.77

X= Accelerator residue; L = Ligand (basic nitrogen or zinc carboxylate).

FIG. 6. — Features of NR vulcanizate produced by a conventional crosslinking system.4

Duchacek79 noted that increasing the zinc oxide concentration increased the rate of extent ofcrosslinking up to a certain zinc oxide concentration; this concentration is believed to be the min-imum level of zinc oxide needed to completely convert the accelerator to the zinc-accelerator-sulfur complex. He also noted that the optimum zinc oxide content to minimize reversion wasslightly greater than this minimum. Coran58 also noted that the induction time had a dependenceon the zinc oxide concentration in excess of that required for formation of the accelerator-zinccomplex. This suggests that zinc exerts an influence beyond inclusion in the accelerator com-plexes.

Recently, research work on safe accelerators has gained a lot of interest.80-82 Some of theaccelerators are reported to be unsafe due to the formation of carcinogenic compounds such asnitrosamines. It is reported that accelerators derived from secondary amines are usually evolvingnitrosamines.80 Avoidance of unfavorable conditions, elimination of secondary amine containingaccelerators and elimination of formed nitrosamines are the various possibilities of reducingnitrosamine formation.81 Nitrosamine can be generated during the processing stage of rubber.The faster the amine is produced from cure or the earlier it is produced in the process cycle, thegreater the opportunity for nitrosamine formation.82 This makes the synthesis of safe accelera-tors a field of great interest.

VI. SULFUR DONOR SYSTEMS

Apart from using free sulfur as a crosslinking agent other materials, which could donate sul-fur to the system, are used for curing. Tetramethyl thiuram disulfide is found to act as a crosslink-ing agent in the absence of sulfur. It is not known whether the reaction involves the intermediateformation of elemental sulfur or whether the disulfide itself is the active agent. It appears thatunder cure conditions and in the presence of zinc oxide, two thirds of the thiuram disulfide invari-ably appears eventually as the zinc dithiocarbamate.

Zinc Oxide3 R2N.CS.S.S.C.S.NR2→ 2 R2N.CS.S.Zn.S.CS. NR2 + ?

Rubber

The network structures of sulfurless systems are similar to those obtained from acceleratedsulfur systems discussed in the preceding section. It has been suggested that thiuram disulphidesfunction during cure by decomposition to yield active sulfur and ZDC, which together constitutean accelerated sulfur system. In the past the opinion has been that TMTD vulcanizes via theintermediate formation of TMTM and active sulfur83 and later the reverse had been suggested84

ie., TMTM vulcanizes in the presence of sulfur via the intermediate formation of TMTD.Another approach for sulfur donor vulcanization was the use of model compounds. This

study by Gregg and Lattimer85 used the cyclic tetramer shown in Figure 7 to model cis-polybu-tadiene. Their conclusions were similar to previous findings and they elaborate about an ionicmechanism involved in the process.


Another technique for analysis of sulfurless vulcanization has been the use of DSC coupledwith chemical probe/equilibrium swelling.86 The results of this work indicated that zinc oxidedramatically increases the efficiency of vulcanization, but is not required for sulfurization. Theadvantage of sulfurless cure using TMTD is that the vulcanizates have low sulfur content andexcellent ageing properties. Furthermore the curing is more efficient than the normal efficientvulcanization (EV).

VII. INFLUENCE OF FILLERS

Reinforcing and non-reinforcing fillers are used in rubbers. Fillers are known to influencecrosslinking reaction during vulcanization. Channel blacks retard cure when compared to fur-nace blacks, some silicas when compared to silicates and hard clay when compared to whiting.This behavior depends on particle size. In most cases the cause of this retardation is due to thegreater or lesser acidity of the filler, which influences the kinetics of the crosslinking reaction.Fillers contribute to crosslinking by adsorption of the molecules on their surface and appear tobe capable of modifying the course and efficiency of vulcanization reactions either directly orthrough adsorption of curatives. Studies by Kraus87 have shown that regardless of the nature ofthe apparent filler contribution it is desirable to have the 'filler effect' included in the ‘degree ofcure’.

Carbon black contains not only carbon atoms but also a number of other elements such ashydrogen, oxygen, nitrogen and sulfur.88 These heteroatoms belong to the carboxyl, lactones,phenols and quinones present on the surface of the black.89 The physical and chemical interac-tions between the black and the other ingredients can influence the chemistry of curing and rein-forcement due to the fillers. Porter reported90 that the crosslink density of a black reinforced vul-canization system increased about 25% compared to the corresponding unfilled one. Carbonblack also increases the rate of vulcanization and improves the reversion resistance.91 The degreeof improvement depends largely on the size of the carbon black particles. Structurally the blackfilled vulcanizate is found to contain fewer polysulphidic linkages than the corresponding


FIG. 7. — Complex formed in the presence of sulfur donors.

unfilled vulcanizate.92 However, the structural changes appearing will be small when comparedwith the large effects of the fillers on the physical-mechanical properties of the vulcanizates.Black filled vulcanizates, generally have a faster rate of cure and better reversion resistance.93

This has been related to the formation of trans-methine structure in the vulcanizates in the rever-sion process.

In the presence of carbon black trans-methine content is decreased. This may be attributedto two reasons: (i) polymer-filler interaction; or (ii) the improved crosslinking efficiency. Thetrans-methine structure of the main chain modification is formed from the thermal instability ofthe polysulphidic linkages through a desulfurization process.94 The amount of trans-methine for-mation is a function of the initial concentrations of polysulphidic crosslinks. If the polymer-fillerinteraction exerts a binding effect on the double bond migration, some polysulphidic linkages,which would otherwise break up through desulfurization in the unfilled systems, may survive inthe black filled vulcanizates. Thus, in the reversion stage, the filled sample, in general would beexpected to contain more polysulphidic linkages than the unfilled one. As the polysulphidic link-ages have greater thermal stability in the black filled sample, the trans-methine formation isreduced and the reversion resistance is improved.93

VIII. BINARY ACCELERATOR SYSTEMS

The use of binary accelerator system is an aspect of vulcanization that has been generatingincreased interest in recent years. Due to the increased complexity of these systems only recent-ly have papers begun to probe the intricacies of binary system.

A binary accelerator system refers to the use of two accelerators in a given formulation.Several authors have studied the synergistic behavior of these systems.95-100 Technically the useof binary accelerators is quite old. Many amines such as diphenyl guanidine (DPG) are used incombination with other accelerators such as MBT or sulphenamides to activate the vulcanizationreaction. However, present day binary accelerators usually consist of benzothiazole and thiocar-bamate derivatives. Additionally MBT and sulphenamides are often added to thiuram systems toincrease the scorch delay of these systems. There have been several approaches analyzing thevulcanization behavior of binary accelerators. It is believed that the improved properties of bina-ry systems resulted from the formation of combination accelerator complexes.

One way to illustrate this effect is to measure mechanical or physical properties as a func-tion of accelerator ratio and correlate the optimum property with the complex formation asshown in Figure 8.100 The effect of binary accelerator combination in natural rubber and butylrubber, on the modulus is given in the Figure 8.


In both the rubbers synergism in modulus is observed for the accelerator combinations. It isobserved from the figure that for the same accelerators synergism is observed at different com-positions. It means that optimum concentration (complex formation) is attained at different com-position of accelerator for different rubbers.

Today, binary accelerators are widely used in industry. They are becoming increasingly pop-ular due to the fact that such mixed systems (i) permit vulcanization to be carried out at lowertemperature in lesser time, (ii) produce vulcanizate with superior physical and mechanical prop-erties compared to those of a stock cured with a single accelerator.

Though a great deal of work has been done on elucidating the mechanism of vulcanizationby single accelerators, little attention has been paid so far to the chemistry of vulcanization withbinary accelerator systems. Dogadkin and co-workers101 and Skinner and Watson102,103 investi-gated a number of popular accelerator combinations and found mutual activation with many ofthem.

Depending on the experimental results obtained in the vulcanization with various combina-tions of most generally used accelerators, Dogadkin and co-workers104 classified the variousbinary accelerators into the following three different groups.


FIG. 8. — Effect of binary accelerator systems in mechanical properties of vulcanizates OTOS- N-oxydiethylenethio-carbamyl-N’oxydiethylene-2-benzothiazole sulphenamide, OBTS- N-oxydiethylene-2-benzothiazole sulphenamide.

1. Systems with synergistically active accelerators.This group consists of disulphides or mercaptans with nitrogen containing organic basesand disulphides, e.g. MBTS TMTD/MBT with MBS/CBS, etc.

2. Systems in which the mutual activity of the pair does not exceed the activity of the mostactive accelerator used.This group consists of sulphenamides with nitrogen containing organic bases.

3. Systems with an additive action of the accelerators.They include sulphenamides in combination with TMTD and those containing combi-nation of accelerators belonging to the same class.

Based on these observations, Dogadkin and coworkers104 suggested that in the initial stageof vulcanization, the accelerators interact with one another to form an active complex, whichthen disintegrate with the formation of active free radicals responsible for initiating the interac-tion of rubber with sulfur. Though the above classification of binary accelerator systems is veryuseful, the mechanism suggested cannot explain all the facts exhibited by the mixed acceleratorsystems. The schematic representation of the action of binary accelerator proposed by Dogadkinis shown in Scheme 16. But it could not fully explain all the observed properties shown by bina-ry accelerator vulcanization. Studies are going on in search of more details about the mechanismof acceleration performed by binary systems.

SCHEME 16. — Schematic representation of the action of binary accelerators.

The synergistic activity of two or more different accelerators arise from the interaction of theaccelerators to form new intermediate compounds which again actively take part in the vulcan-ization reaction leading to enhancement of crosslink density and the rate of vulcanization reac-tion.105 Further investigations106-115 with binary accelerator systems have provided more valuableinformation regarding the mechanism of their action.

Moore et al.116 made investigations on the TMTD-TU binary accelerator system. A novelmechanism was suggested (Scheme 17) to explain the synergistic activity. This theory recognizesthe importance of the polysulphidic intermediates (A) formed during the vulcanization process,which subsequently react with the rubber chain to yield further intermediates (B). These inter-mediates finally react to yield sulfurated crosslinks.

(1)

(2)

(3)

SCHEME 17. — Mechanism of the action of TMTD-Thiourea binary system.116

In the above mechanism, X represents (CH3) 2 N-C = S) and RH is the rubber hydrocarbon.The crosslinking reaction shown in Step 3 follows from the products of Step 2 which itself

RH + RS X RS R XS Zn H Om-1ZnO

m-2⎯ →⎯⎯ + ( ) +1

2 2 2

RH + XS X RS X XS Zn H OmZnO

m-1⎯ →⎯⎯ + ( ) +1

2 2 2

m XSSX XS Xm

XO Zn XS ZnZnOm−( ) ⎯ →⎯⎯ +

− ( ) + ( )12

2 2 2


requires the thiuram polysulphides (A) produced in reaction Step 1. Hence, it follows that anyacceleration of the latter must also lead to a corresponding increase in the overall vulcanizationrate.

Thiourea and its derivatives have been used as accelerators in dry rubber and lattices. In theworks of Mathew et al.110-115 the accelerating activity of several thiourea derivatives were com-pared. Dithiobiurets of varying nucleophilicity were synthesized and compared. Mini et al.115 hasused amidino thiourea as a binary accelerator. In these studies attempts have been made to opti-mize the amount of the new accelerator based on the physical properties of the resulting vulcan-izates. The nucleophilic nature of thiourea and its derivatives is suggested as responsible for thecure activating nature of those binary accelerators. Binary accelerators are expected to improvethe cure rate by improving the crosslinking in rubber. This could be responsible for the improve-ment in mechanical properties.

In our studies we have investigated the accelerating effect of 1-phenyl- 2, 4- dithiobiuret(DTB) as a binary accelerator with sulphenamides in Natural Rubber.117-119 From our studies wefound that an optimum concentration exists in the binary accelerator combination. Excellentmechanical properties are shown at this concentration. Based on the observed cure activation andenhancement in properties we have proposed a mechanistic pathway for the action of this bina-ry accelerator combination in NR, SBR and their blends.120-122 In the proposed pathway thenucleophilic character of DTB facilitates the breakage of Zn-S bond of the zincperthiomercap-tide complex.

SCHEME 18. — Proposed mechanism showing the curing in presence of DTB.120


IX. CONCEPT OF DOUBLE NETWORKS

The modulus and strength of materials with flexible chain polymers can be increased by ori-entation and crystallization of polymer chains. In plastics and fibers, molecular chain orientationis maintained after processing as the chains are frozen by glassy state or crystallization. On theother hand, in rubbers, orientation produced during simple processing will decay after process-ing.

Double network rubber refers to an elastomer that has been crosslinked twice, the secondtime while in a deformed state. The deformation employed is simple tension, (it doesn’t have tobe) resulting in a rubber whose final length exceeds the length after the initial, isotropiccrosslinking. The ratio of these two lengths, referred to as residual strain, does not uniquelydefine a double network elastomer. A given residual strain can be achieved with different com-binations of strain and crosslinking apportionment between the two networks.123

Double network rubbers offer a route to obtain superior mechanical properties, in that theymay allow circumvention of the usual compromise between stiffness and strength. Santangeloand Roland123 have shown that gum natural rubber double network crystallizes at lower strainand can have enhanced tensile strength and fatigue life compared to conventional single net-works. Preparing double networks or composite networks could produce rubber vulcanizateswith permanent chain orientation.

Double networks can be viewed as interpenetrating networks in which the same chain seg-ments belong to both networks and more importantly the component networks are oriented. It isthis orientation that gives rise to enhancement and anisotropy in properties.124-126 The expecta-tion, borne out by experiment is that the modulus of a double network rubber will differ from themodulus of the compounding isotropic elastomer. At higher residual strains, the equilibriummodulus is higher. Mott and Roland126 have studied mechanical and optical behavior of double-networked rubbers.

The network formed when a rubber is crosslinked will have an equilibrium set of chain con-figurations. These configurations are usually associated with a macroscopic state of zero strain,since this corresponds to a condition of zero stress.127

When the rubber is cured away from elastic equilibrium, configurations prevailing duringcrosslinking will shift the subsequent equilibrium state away from zero strain.128,129 However,when the crosslinking reaction takes place in a deformed state, an anisotropic crosslink networkwill be produced.130

The properties of elastomeric network depend not only on the density of junctions but alsoon the distribution and orientation of the chains when the junctions are formed. Double networkscan also arise spontaneously via chain scission131 and via strain-induced crystallization132 or inthe presence of reinforcing fillers.133,134 The strength of oriented networks is found to depend onthe extent of orientation and crosslinking. The strain-induced crystallization of natural rubberimparts the superior strength to it. This is accomplished by the crystallization induced due to ori-entation. Studies have to be made in order to find whether any effect of crosslinking would helpto improve the strength of double networks.

X. CHARACTERIZATION OF NETWORKS

Different methods are available for characterization of rubber networks. The importantmethods are: (i) swelling method; (ii) freezing point depression method; (iii) stress-strainmethod; and (iiii) NMR measurements.

Swelling of vulcanizates in solvents has been proved to be an effective method to determinethe chemical crosslink density. Flory-Rehner theory135 is used for the determination of molecu-lar weight between crosslinks. Flory Rehner equation can be applied to the results of swelling of


rubber in suitable solvents.

-[ln(1-Vr) + Vr + χ Vr2] = [ρVs (Vr)

1/3]/ Mc (4)

where Vr is the equilibrium volume fraction of rubber in swollen gel, ρ the density of rubber, χan interaction constant characteristic of rubber and swelling liquid and Vs the molar volume ofthe swelling liquid and Mc is the molecular weight between crosslinks.136 Interpretation ofcrosslink density with this equation requires the interaction parameter. This parameter is influ-enced by factors which affect chain configuration. Treatment of the vulcanizates with differentthiols137 cleave the mono, di and polysulphidic crosslinks preferentially. This helps to find therelative proportion of crosslinks. It has long been known that an anomalous freezing pointdepression of the swelling solvent in swollen gels was observed and that the magnitude of thedepression was closely related to the degree of crosslinking and or the structure of swollengels.138 The stress-strain behavior of rubber like networks is influenced by contributions of con-straints caused by the uncrossability of the network chains. Using theoretical results based on thetube approach to polymer melts and networks,139 a well-founded separation of crosslink and con-straint contributions to the stress-strain behavior can be achieved and a reliable method for theevaluation of the network parameters can be developed. The Mooney equation140 is used for cal-culating the crosslink density from stress-strain data. It was then modified by Rivlin andSaunders.141

σ = [ρRTA0]/Mc {λ-1/λ2} (5)

where 'σ' is the force to extend a sample of cross-sectional area 'A0' to extension ratio 'λ',(ratioof extended to original length of the sample specimen) 'ρ' is the density, 'R' is the gas constantand 'T' is the absolute temperature.

A recent promising approach to direct analysis of the crosslink structure and distribution insulfur vulcanizates was the application of solid-state C13 NMR spectroscopy.61,142-147 In a workon unaccelerated sulfur vulcanization, a number of signals were also assigned to monosulphidiccrosslinks on the basis of calculated chemical shifts of proposed structures.148 Chemical shiftsdifferences between disulphidic and trisulphidic crosslinks are of the order of 0.1 ppm.Crosslinking results in broadening of the peaks. Recent studies149,150 have shown that C13 high-resolution solid state NMR spectroscopy is a valuable method to analyze the formation of poly-sulphidic and monosulphidic crosslinks at various sites on the monomer unit in NR.

XI. CONCLUSIONS

A detailed survey on the crosslinking in elastomers has been made in this review. The dif-ferent methods of crosslinking have been thoroughly reviewed giving special attention to accel-erated sulfur vulcanization. Mechanisms put forward to explain the vulcanization process arediscussed. The role of activators and the effect of fillers are discussed. The property of vulcan-izates depends mainly on the number and type of crosslinks present in it. The different types ofcrosslinks formed during the vulcanization process and their behavior with respect to the amountof accelerator and sulfur present are discussed. Binary accelerators are widely used in industriesso as to obtain better end-use properties and to reduce the time required for vulcanization. Themechanism of some binary accelerator systems found in literature is discussed. Formation ofdouble networks in elastomers helps to preserve the orientation of hydrocarbon chains. Thismethod could revolutionalize the elastomer technology since this method offers better strengthand modulus to the resulting vulcanizates. Various methods available for characterization of net-works formed and estimation of crosslink density are also discussed.


XII. REFERENCES

1W. Hoffman, "Vulcanization and Vulcanization Agents," MacLean, London, England. (1967).2P. J. Nieuwenhuizen, J. Reedijk, M. Vanduin, W.J Mc Gill, RUBBER CHEM. TECHNOL. 70, 368 (1997).3M. Akiba, A.S. Hashim, Prog. Polym. Sci. 22, 475 (1997).4L. Bateman, G. Moore, M. Porter, B. Saville, The Chemistry and Physics of Rubber like Substances,” L. Bateman. Ed.

MacLean and Sons Ltd. London. (1963) p. 449.5H. Kaneko, Polymer Friends 11, 672 (1980).6J. I. Guneen and E. H. Former, J. Chem. Soc. 472 (1943).7M. C. Kirkhan, Prog. Rubber Technol. 41, 61 (1978).8T. Volintu, E. Bugaru, P. Bujenita, Rev. Roum. Chim. 34(4), 1101 (1989).9H. G.Scott (to Dow Corning Group Corp.) Brit. Patent 1, 286,460, Aug 23, 1972.10G. A. Harpel, D. H. Walrod, RUBBER CHEM. TECHNOL. 46, 1007 (1973).11S. Cartasegna, RUBBER CHEM. TECHNOL. 59, 722 (1986).12W. Hoffman, Kautschuk-Handbuch, Bostrom S, Ed. 4, Verlag Berlin Union, Stuttgart (1961) p. 324.13A. G. Stevenson, "Vulcanization of Elastomers," G. Alliger and I. J. Sjothun, Eds., Rheinhold, New York, (1964) p. 27.14K. Makuuchi, Y. Tsuda, Nippon Gomu Kyokaishi 61, 478 (1988).15H. A. Yoossef, H. A. A. Aziz, F. Yoshii, K. Makuuchi, A. A. El. Milig, Angew. Macromol. Chem. 218, 11 (1994).16I. R. Gelling, C. Metherell, J. Nat. Rubber Res. 8(1), 37 (1993).17G. G. A. Bohm, J. O. Tveekrem, RUBBER CHEM. TECHNOL. 55, 575 (1982).18K. Makkuchi, Y. Tsuda, Nippon Gomu Kyokaishi 61, 471 (1988).19C. H. Chen, J. L. Koenig, E. A. Collins, RUBBER CHEM. TECHNOL. 54, 734 (1981).20N. J. Morrison, M. Porter. RUBBER CHEM. TECHNOL. 57, 63 (1984).21C. T. Loo, Polymer 15, 729 (1974).22G. Holden Legge, and H. E. Schroeder (eds). "Thermoplastic Elastomers-Comprehensive Review," Carl Hanser,

Munich, 1987.23Abdou Sabet, Puydak and Rader. RUBBER CHEM. TECHNOL. 69, 476 (1996).24M. A. L. Manchado, J. M. Kenny, RUBBER CHEM. TECHNOL. 74, 198 (2001).25B. Dogadkin, O. Uchenie, 1947, The Science of Rubber–Mashgiz 1938, (Physics and Chemistry of Rubber –

Goshkhimizdat.26G. Oenslager, India Eng. Chem. 25(2), 232 (1933), India Rubber J. 85(7), 11 (1933).27F. Jones, India Rubber J. 113(2), 12 (1947), Caoutch. Gutta porcha. No. 355, 1651 (1933).28B. K. Leff, Chemistry of Rubber-Nauchnoe Khimiko-technicheskoe izd. Vsekhimprom Vsnkh SSSR (1930).29C. Goodyear, Gum Elastic-New Haven 2, 112 (1885).30A. Yavich, Kauch. I. Rezina No. 2, 9 (1940).31Chronology of More Important Events in the History of Rubber, India Rubber World 90(4), 41 (1934), 90(6), 43 (1934).32W. Geer, Ind. Eng. Chem. 14, 372 (1922).33W. Geer, C. Bedford, Ind. Eng. Chem. 17, 393 (1925). 34A. Luttringer, Caoutchouc et Gutta purcha No. 354, 16481 (1933), 355, 16512 (1933). 35M. L. Weiss, (to Dovan Chemical Co.) US Patent. 1411231 (March 28,1922).36W. Scott, (to E. I. DuPont de Nemours & Co.) US Patent 1721057 (July 6, 1929).37L. B. Sebrell, C. E. Board, Ing. Eng. Chem. 5, 1009 (1923).38W. Bedford, L. B. Sebrell, Ind. Eng. Chem. 13, 1034 (1921).39L. B. Serbrell, C. W. Bedford. (to Goodyear Tire & Rubber Co.) US Patent 1544687 (July 7, 1925).40G. Bruni, E. Romani, India Rubber J. 62, 18 (1921), India Rubber World 67, 20 (1922), Giorn. Chimi. Ind. Appl. 3, 351

(1921).


41L. B. Serbrell, C. E. Board, J. Chem Am. Soc. 45, 2390 (1923).42J. Teppema, L. B. Sebrell, J. Chem Am. Soc. 49, 1748 (1927).43G. Bruni, Russian patent 380774 (Nov. 11, 1923) It. Prior 15.3.1919.44B. E. Lorentz (to R. T. Vanderbilt) US 1413172 (April 18, 1922).45S. M. Cadwell (to Naugatuck Chem. Co.) US 1440962 (Jan 2, 1923). 46W. Scheele, RUBBER CHEM. TECHNOL. 34, 1306 (1961).47W. Prior, Mechanisms of Sulfur Reactions, Mcgraw-Hill, New York (1962).48F. W. H. Kurger, W. J. McGill, J. Appl. Polym. Sci. 42, 2643 (1991).49F. W. H. Kurger, W. J. Mc Gill, J. Appl. Polym. Sci. 42, 2661(1991).50F. W. H. Kurger, W. J. Mc Gill, J. Appl. Polym. Sci. 42, 2669 (1991).51S. Luyt, , J. Appl. Polym. Sci., Appl. Polym. Symp. 48, 449 (1991)52K. Ghosh, C. K. Das, S. Banerjee, J. Polym. Sci., Polym. Chem. Ed. 15, 2773 (1977).53E. Morita, E. J. Young, RUBBER CHEM. TECHNOL. 36, 844 (1963).54P. K. Bandyopadhyay, S. Banerjee, Angew Makromol Chem. 64, 59 (1977).55P. Simon, A. Kuchma, J. Thermal Anal. Calorim. 56, 3, 1107 (1999).56A. B. Sullivan, C. J. Hann, G. H. Kuhls, RUBBER CHEM. TECHNOL. 71, 352 (1999).57R. H.Campbell, R. W. Wise, RUBBER CHEM. TECHNOL. 37, 635, (1964).58A. Y. Coran, RUBBER CHEM. TECHNOL. 37, 689 (1964).59C. K. Das, S. Banerjee, J. Polym. Sci. (Polym. Chem.) 16, 2971 (1978).60P. K. Bandyopadhyay, S. J. Banerjee, J. Appl. Polym. Sci. 23, 185 (1979).61A. M. Zaper, J. L. Koenig, RUBBER CHEM. TECHNOL. 60, 252 (1987).62P. J. Nieuwenhuizen, J. M. van Veen, J. G. Haasnoot, J. Reedijk, RUBBER CHEM. TECHNOL. 72, 43 (1999).63R. M. Seyger, R. Hulst, J. P. P. van Duynhoven, R. Winters, L. J. Vander Does, J. W. M. Noordermeer, A. Bantjes,

Macromolecules 32, 22 (1999).64S. S. Choi, J. Anal. Appl. Pyrolysis 52(1), 105 (1999).65Y. Thanaka, RUBBER CHEM. TECHNOL. 64, 325 (1991).66L. Ibarra, J. Appl. Polym. Sci. 73(6), 927 (1999).67M. R. Krejsa, J. L. Koenig, RUBBER CHEM. TECHNOL. 65, 427 (1992).68R. Layer, RUBBER CHEM. TECHNOL. 65, 211 (1992).69R. Layer, RUBBER CHEM. TECHNOL. 65, 822 (1992).70J. L. Koenig, Acc. Chem. Res. 32(1), 1 (1999).71J. Nieuwenheuizen, J. Reedjik, J. G. Haasnot, Kautsch Gummi Kunstst. 53(3), 144 (2000).72P. Vershoot, J. G. Haasnoot, J. Reedjik, M. vanDvin, J. Put, RUBBER CHEM. TECHNOL. 68, 563 (1995).73P. J. Nieuwenheuizen, J. Reedjik, J. G. Haasnot, RUBBER CHEM. TECHNOL. 72, 15 (1999).74A. Y. Coran, in “Encyclopedia of Polymer Science and Engineering,” Vol. 17, Ed. H. F. Mark, N. M Bikales, O. G.

Overberger and G. Menges, John Wiley, NewYork, 1990, p. 683.75J. Jelencic, M. Bravar, N. Biga, N. Wolf, Kautsch Gummi Kunstst. 41, 555. (1988).76J. Travas-Sedjic, J. Jelencic, M. Bravar, Z. Frobe, Eur. Polym. J. 32(12), 1395 (1996).77P. Quirk, Prog. Rubber Plast. Technol. 4(1), 31 (1988).78D. Barnard, et al., Encycl. Polym. Sci. Technol. 12, 178 (1970).79V. J. Duchacek, J. Appl. Polym. Sci. 22, 227 (1978).80H. Schuster, F. Nabhol, M. Gmunder, Kautsch Gummi Kunstst. 43, 95 (1990).81G. Willoughby, K. W. Scott. RUBBER CHEM. TECHNOL. 71, 766 (1997)82R. W. Layer, D. W. Chasar, RUBBER CHEM. TECHNOL. 67, 299 (1994).83W. L. Craig, A. E. Davison, A. Juve, J. Polym. Sci. 6, 177 (1951).


84W. Scheele, G. Bielstein, Kautsch. Gumm. Kunstst. 8, 25 (1955).85R. Gregg, R. Lattimer, RUBBER CHEM. TECHNOL. 57, 1056, (1984)86W. H. Kruger, W.J. McGill, J. Appl. Polym. Sci. 45, 563 (1992).87G. Kraus, J. Appl. Polym. Sci. 7, 861(1963).88R. C. Bansal, J. B. Donnet, F. Stoeckli, “Active carbon,” Marcel Dekker, Inc., New York, (1988).89J. B. Donnet, A. Vidal, Adv. Polym. Sci. 76, 103 (1986).90M. Porter, Kautsch. Gummi Kunstst. 22, 419(1969).91R. Mokhopadhyay, S. K. De, RUBBER CHEM. TECHNOL. 52, 263 (1979).92M. L. Studebaker, Kautsch. Gummi 14, 714 (1961).93H Chen, J. L. Koenig, J .R. Shelton, E. A. Collins, RUBBER CHEM. TECHNOL. 55, 103 (1982).94H Chen, J. L. Koenig, J .R. Shelton, E. A. Collins, RUBBER CHEM. TECHNOL. 54, 734 (1981).95C. K. Das, R. N. Datta, D. K. Basu, RUBBER CHEM. TECHNOL. 61, 760 (1983)96R. W. Layer, RUBBER CHEM. TECHNOL. 62, 124 (1989).97R. N. Datta, C. K. Das, D. K. Basu, Kautsch Gummi Kunstst. 39, 1090 (1986).98B. Adhikari, D. Pal, D. K. Basu, A. K. Chaudhuri, RUBBER CHEM. TECHNOL. 56, 827 (1983).99R. P. Mathur, A. Mitra, P. K. Ghoslal, C. K. Das, Kautsch Gummi Kunst. 36, 1067 (1983).100R. W. Layer, RUBBER CHEM. TECHNOL. 60, 89 (1987).101B. A. Dogadkin, V. A. Jhershnev, RUBBER CHEM. TECHNOL. 33, 401 (1960).102D. Skinner, A. A. Watson, Rubber Age 99(11), 76, (1960).103D. Skinner, A. A. Watson, RUBBER CHEM. TECHNOL. 42, 404 (1969).104B. A. Dogadkin, B. Feldshtein, M. Shkurina, V. A. Dobromyslova, Akkad Nauk. Sssr 92, 61 (1963).105L. Pysklo, J. Rouinski, Polimery-Tworzywa Wielkoczasteczowe 33, 452 (1988).106S. C. Debnath, S. K. Mandal, D. K. Basu, J. Appl. Polym. Sci. 57, 555 (1995).107S. C. Debnath, D. K. Basu, J. Appl. Polym. Sci. 52, 597 (1994).108S. C. Debnath, D. K. Basu, Kautsch Gummi Kunstst. 45, 934 (1992).109J. Hann, A. B. Sullivan, B. C. Host, G. H. Kuhls, Jr., RUBBER CHEM. TECHNOL. 67, 77 (1993).110A. P. Kuriakose, G. Mathew, Indian J. Technol. 26, 344 (1988).111G. Mathew, P. V. Pillai, A. P. Kuriakose, RUBBER CHEM. TECHNOL. 65, 277 (1992).112G. Mathew, B. Kuriakose, A. P. Kuriakose, Kautsch. Gummi Kunstst. 45, 490 (1992).113G. Mathew, N. M. Mathew, A. P. Kuriakose, Polym. Plast. Technol. Eng. 32(5), 439 (1993).114G. Mathew, A. P. Kuriakose, J. Appl. Polym. Sci. 49, 2009 (1993).115V. T. E. Mini, C. Mathew, A. P. Kuriakose, D. J. Francis, J. Mater. Sci. 3, 2049 (995).116G. Moore, B. Saville, A. A. Watson, RUBBER CHEM. TECHNOL. 34, 795 (1961).117A. S. Aprem, G. Mathew, G. Mathew, K. Joseph, S. Thomas, Kautsch. Gummi Kunstst. 59, 576 (1999).118A. S. Aprem, G. Mathew, K. Joseph, S. Thomas, J. Rubber Res. 4(1), 44 (2001).119A. S. Aprem, R. Laxminarayanan, K. Joseph, S. Thomas, J. Appl. Polym. Sci. 87(14), 2193, (2003).120A. S. Aprem, K. Joseph, N. M. Barkoula, J. Karger Kocsis and S. Thomas, Plasr. Rubber Compos. 32(2), 1, (2003).121A. S. Aprem, K. Joseph, N. M. Barkoula, J. Karger Kocsis and S. Thomas, J. Elast. Plast. 32(1), 29, (2003).122A. S. Aprem, K. Joseph, T. Mathew, V. Altstaedt and S. Thomas, Eur. Polym. J. 39(5), 1451, (2003).123P. G. Santangelo, C. M. Roland, RUBBER CHEM. TECHNOL. 68, 124 (1995).124G. R. Hamed, M.Y. Huang, RUBBER CHEM. TECHNOL. 71, 846 (1988).125S. Kaang, C. Nah, Polymer 39, 2209 (1998).126P. H. Mott, C. M. Roland, Macromolecules 33, 4132 (2000).127C. M. Roland, M. L. Warzel, RUBBER CHEM. TECHNOL. 63, 285 (1990).128R. L. Ullman, Macromolecules 19, 1748 (1986).


129L. G. Baxandall, S. F. Edwards, Macromolecules 21, 1763 (1988).130P. G. Santangelo, C. M. Roland, RUBBER CHEM. TECHNOL. 67, 359 (1994).131K. T. Gillen, Macromolecules 21, 442 (1988).132R. A. M. Hikmet, J. Lub, P. M. Vanderbrink, Macromolecules 25, 4194 (1992).133C. M. Roland, K. L. Peng, RUBBER CHEM. TECHNOL. 64, 790 (1991).134F. Reichert, D. Goritz, E. J. Duschl, Polymer 34, 1216 (1993).135P. J Flory, J. J. Rehner, J. Chem. Phys. 11, 5120 (1943).136B. Saville, A. A. Watson, RUBBER CHEM. TECHNOL. 40, 100 (1967).137D. S. Campbell, J. Appl. Polym. Sci. B13, 120 (1969).138W. Kuhn, H. Mayer, Angew Chem. 68, 345 (1956).139G. Heinrich, E. Straube, G. Helmis, Adv. Polym. Sci. 85, 33 (1985).140M. Mooney, J. Appl. Phys. 11, 582 (1940).141R. S. Rivlin, D. W. Saunders, Philos. Trans. R. Soc. A 243, 251 (1951).142M. Andreis, J. Liu, J. L. Koenig, J. Polym. Sci. Polym. Phys. 2, 1389 (1989).143M. R. Krejsa, J. L. Koenig, RUBBER CHEM. TECHNOL. 64, 40 (1991).144S. R. Smith, J. L. Koenig, RUBBER CHEM. TECHNOL. 65, 176 (1992).145H. R. Hirst, Paper No. 69, Rubber Division Meeting, Detroit; MI (October, 1991).146W. Gronski, H. Hasenhindl, B. Freund, S. Wolff, Kautsch. Gummi. Kunstst. 44, 119 (1991).147W. Gronski, U. Hoffman, G. Simon, A. Wutzler, E. Straube, RUBBER CHEM. TECHNOL. 65, 63 (1992).148D. D. Parker, J. L. Koenig, J. Appl. Polym. Sci. 70, 1371 (1988).149W. Gronzki, U. Hoffmann, G. Simon, A. Wutzler, E. Straube, RUBBER CHEM. TECHNOL. 65, 63 (1992).150J. Y. Buzare, G. Silly, J. Emery, G. Boccacio, E. Rouault, Eur. Polym. J. 37, 85 (2001).

[ Received October 2003, revised June 2005 ]


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