VULCANIZATION SYSTEMS FOR RUBBER COMPOUNDS BASED ON IIR AND HALOGENATED IIR: AN OVERVIEW
ABSTRACT
The properties of IIR and halogenated IIRs, such as excellent steam and gas impermeability, heat aging stability, and oxygen and ozone resistance and increased chemical resistance, meet the demands of various industrial applications. However, due to the low level of unsaturation of IIR, the vulcanization rate is rather low and the degree of cross-linking is usually insufficient, causing IIR to be minimally co-vulcanized with unsaturated general-purpose rubbers. The low reactivity of IIR requires the consideration of a special composition of curing systems to provide the best possible rate and state of vulcanization. The type of curing system selected must also be a function of the composition of rubber formulations in which the IIR is used, and with respect to the final product's performance requirements. Therefore, the curing systems for IIR differ and can include standard sulfur systems, phenol-formaldehyde resins, or quinones. The incorporation of halogen (chlorine or bromine) atoms into the structure of IIR significantly increases the chemical reactivity, which can be subsequently reflected in a higher curing rate and the possibility to use some additional vulcanization systems such as metal oxides, diamines, or peroxide with co-agent. This study reviews the types and selection of applicable curing systems for IIR and its halogenated derivates.
INTRODUCTION
The copolymer of isobutylene and isoprene, IIR, has been produced industrially since 1942. IIR combines the capability of forming a cross-linked network with an excellent chemical resistance of a saturated polymer. It exhibits very good resistance to ozone and atmospheric conditions and to chemical attack by acids, inorganic salts, and alkalis. It is not resistant to swelling in nonpolar solvents and mineral oils, but it swells only slightly in oxygen-containing compounds, such as alcohols, ketones, esters, animal and vegetable oils, and synthetic hydraulic liquids. The rebound resilience of IIR is low at room temperature. It increases only at temperatures above 50 °C, and at 100 °C it is almost equal to the rebound resilience of SBR. Probably the most valuable property of IIR is its extraordinary impermeability to gases, but other properties include very good elastic properties (glass transition temperature around −70 °C), damping of vibrations, and high coefficient of friction. IIR has a geometrically regular structure enabling it to crystallize spontaneously and also under applied deformation forces. Therefore, IIR vulcanizates show high tensile strength even in the unfilled state (about 20 MPa).1–4 IIR is used for production of bladders, inner liners, curing bags and diaphragms, steam hoses, shock absorbers, electrical insulators, and mechanical goods.5–9 The damping property of IIR is also used for elastic bearings for efficient damping of vibrations, mainly in the resonance region.10
The development of halogenated IIR (halobutyl rubber), namely chlorinated (CIIR) and brominated (BIIR), in the 1960s greatly extended the usefulness of IIR by providing much higher curing rates and enabling co-vulcanization with general-purpose rubbers such as NR, BR, and SBR. These properties allowed development of more durable tubeless tires with the air-retaining inner liners chemically bonded to the body of the tire. In principle, the properties of CIIR and BIIR are similar to those of IIR, that is, outstanding resistance to aging, flex-cracking, low compression set, good tensile and tear strengths, high resistance to chemicals, and impermeability to gases.11,12 The main differences from IIR are their higher reactivity with vulcanization agents, absence of reversion, slightly higher polarity, increased adhesion to metals, and compatibility and possibility to co-vulcanize with unsaturated rubbers.13 BIIR possesses a higher vulcanization rate compared to CIIR and better adhesion to NR and SBR. This is important in inner liners that are in contact with the carcass based on NR/SBR. Tire inner liners are by far the largest application of halobutyl rubber nowadays. Among the other applications are side walls (either CIIR alone or together with EPDM is substituted for part of the unsaturated rubbers, which improves the heat resistance to flex-cracking and weathering) and inner tubes for truck tires for heavy-duty service.14,15 Other applications include steam hoses, conveyor belts, adhesives and cements, reservoir linings, and underlays for rails and bridges; pharmaceutical applications include stoppers, construction sealants, and other mechanical goods.16,17 The compatibility with other rubbers (IIR, EPDM, NR, BR, SBR, CR, chlorosulfonated polyethylene rubber) is used to give general-purpose rubber some properties of IIR or, in contrast, for modifying the properties of halobutyl rubber by another rubber.10
IIR
manufacturing and structure
IIR is produced by solution cationic copolymerization of isobutylene and a small amount of isoprene in methyl chloride solution in the presence of Friedel–Crafts catalysts (AlCl3 or BF3) at very low temperatures (usually below −90 °C). Polymerization is strongly exothermic, and even at low temperatures the reaction proceeds rapidly, leading to the formation of a high molecular weight product with a relatively wide molecular weight distribution. At high temperatures, low molecular weight rubbers are formed. Cationic polymerization gives rise to linear macromolecules in which isobutylene units are connected head to tail. The isoprene units are distributed statistically randomly in macromolecular chains mostly in trans-1,4-positions; the proportion of 1,2- and 3,4-addition is negligible.2,18–21 The structural units of IIR are illustrated in Figure 1. The amount of isoprene units determines the level of unsaturation of IIR and is expressed as a molar percentage. At 2% unsaturation there are two monomeric isoprene units per 98 monomer units of isobutylene. The amount of isoprene in the commercial IIR types moves from 0.6 to 3 mol.%, so the degree of unsaturation is very low. Isoprene units have almost no influence on the properties of IIR, and double bonds are used mainly for cross-linking. Almost all double bonds terminate during the vulcanization course, and incipient vulcanizates have a saturated character. With the low amount of double bonds is connected a low rate of vulcanization; therefore, they can only hardly be co-vulcanized with other unsaturated rubbers. With an increasing amount of double bonds, the vulcanization rate increases, but the properties of the vulcanizates only slightly deteriorate.10,11,22
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
vulcanization
The concentration of double bonds in IIR is about 30–50 times lower than that in NR, BR, and SBR.2 Vulcanization of IIR with systems similar to those applied for general-purpose rubbers would be very slow. Therefore, faster and more efficient systems must be used for its vulcanization, and the vulcanization temperatures are also higher. The curing systems suitable for cross-linking of IIR-based rubber compounds include traditional sulfur curing systems, quinones, and phenol-formaldehyde resins.
Sulfur Vulcanization Systems
Sulfur curing systems with a lower amount of sulfur and a higher amount of accelerators, thus called efficient (EV) or semi-EV systems, are preferred for vulcanization.10,23,24 An adequate rate and degree of vulcanization can be achieved only by use of rapid effect accelerators. These include fast accelerators (thiazoles, sulfenamides), very fast accelerators (thiurams), ultra-accelerators (dithiocarbamates), and their combinations, or systems based on sulfur donors. A list of the applied accelerators from each group is presented in Table I.
It is well known that sulfur vulcanization of rubber compounds is an intricate process and that the chemistry of accelerated sulfur vulcanization is very complex and still not clearly understood. In general, it is supposed that sulfur vulcanization of elastomers runs in three stages. In the first stage, the interaction of the components of the curing system leads to the formation of transition complexes, which together with rubber form the active cross-linking agent. The second stage is characterized by formation of a primary vulcanizate network with a dominance of polysulfidic cross-links. During the third stage, this network is restructured as a consequence of modifications of the cross-links (polysulfidic cross-links are transferred into di- and monosulfidic cross-links) and macromolecules of rubber (isomerization, dehydrogenation, cyclization), and the final spatial network of vulcanizate is formed.25,26 The cross-linking of IIR proceeds via isoprene structural units with the formation of sulfidic cross-links with lower sulfur atoms in the sulfur bridges (mainly mono- and disulfidic cross-links), and the sulfuration is almost entirely led by substitution of allylic hydrogen atoms (Scheme 1).2,27 Scheme 2 outlines a possible reaction mechanism of accelerated cross-linking of IIR with tetramethylthiuram disulfide (TMTD).28 Thiuram accelerated systems participate in the vulcanization process through formation of a zinc accelerator complex. The sulfur-rich complex is formed by insertion of sulfur into the transition molecule, thus forming an active sulfurating agent. Rubber bonded intermediates are subsequently produced followed by cross-linking to form polysulfidic cross-links. In the later stage of vulcanization, polysulfidic cross-links evolve into mono- and disulfic cross-links. Thiurams are very often used in combination with other accelerators, mainly with sulfenamides and thiazoles. In the absence of sulfur, TMTD can act as a sulfur donor. An increase in the curing rate without significant loss of other properties can be achieved by use of thiurams and ZnO at about 5 parts per hundred of rubber (phr).29 A disadvantage of thiurams is the formation of secondary amines during vulcanization, which can be potential sources of harmful N-nitrosamines. However, thiurams with sterically hindered substituents (e.g., benzyl- and piperydil-) do not show these undesirable effects. Hence, for example, the possible solution is substitution of thiurams TMTD and tetraethylthiuram disulfide (TETD) by higher molecular weight thiurams such as tetrabenzylthiuram disulfide (TBzTD), or by dithiocarbamates such as zinc diethyldithiocarbamate (ZDEC). TBzTD is a high molecular weight accelerator and acts as a sulfur donor as well. Due to its high molecular weight and steric hindrance, it is able to improve scorch safety, thereby improving compound processing. When used in combination with sulfenamides such as N-tert-butyl-2-benzothiazole sulfenamide (TBBS), the concentration of N-nitrosamines becomes negligible.30,31 The high molecular weight secondary amine dibenzylamine, a decomposition product released from TBzTD, is not as volatile as the products produced from the lower molecular weight thiurams TMTD and TETD.
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
The sulfur-cured vulcanizates possess high values of tensile strength and tear strength; they also show good elastic and dynamic behavior, good abrasion resistance, and resistance to dynamic fatigue.32,33
The main disadvantages of sulfur-cured vulcanizates are low curing rate, low degree of cross-linking, low heat aging stability, low thermo-oxidative aging resistance, and high compression set. Compounds cured with sulfur systems exhibit the tendency of reversion, which is connected with decomposition of previously formed cross-links. It has been reported that the addition of oxides with oxidative properties, such as PbO2, MnO2, and CaO in 5–10 phr, suppresses the tendency of reversion and leads to enhanced heat resistance.10
In the EV vulcanization systems, predominantly used for cross-linking of IIR-based compounds, a low level of sulfur is combined with a rather high amount of accelerator. Therefore, the final vulcanizates efficiently use sulfur to form networks in which the cross-links are mainly monosulfidic, and they exhibit a low degree of main-chain modifications. Therefore, they exhibit the highest resistance to increased temperatures and thermo-oxidative aging and the best resistance to compression set and reversion.34–36 Vulcanizates with a dominance of monosulfidic cross-links show worse initial physical-mechanical properties and lower elongation and dynamic characteristics, but over time their properties deteriorate more slowly in comparison with the conventional type vulcanizates that are characterized by the network structure formed mainly from polysulfidic cross-links.
Vulcanization Systems Based on Quinones
IIR can be efficiently cured with p-quinone dioxime and its derivate p,p′-dibenzoylquinone dioxime, which has better processing safety.23,37–40 The optimal course of vulcanization is achieved in the presence of suitable oxidizing agents, mainly dibenzothiazyl disulfide (MBTS), or metal oxides such as PbO2, Pb3O4, and MnO2. It is supposed that these additives cause the oxidation of quinones into nitroso derivates (p-dinitrosobenzene) that then react by a free radical mechanism with double bonds and allylic hydrogens in isoprene structural units of IIR, leading to the cross-link formation between rubber chains segments (Scheme 3).2,41 Vulcanization takes place faster and at lower temperatures than that with sulfur curing systems. A higher degree of cross-linking is achieved, and the cross-links have higher thermal stability. A typical composition of rubber compounds can be formulated as follows: IIR, 100 phr; ZnO, 5 phr; p-quinone dioxime, 2 phr; and MBTS, 4 phr, fillers are added in the usuall amount.10,42 If PbO2 is used as an oxidizing agent, vulcanization is very fast and can take place at room temperature.43 This is used for the cross-linking of liquid IIR and for adhesives. Although initial tensile strength and rebound resilience of vulcanizates cured with quinones are lower in comparison with sulfur-cured compounds, vulcanizates based on quinones generally show good physical-mechanical and electrical properties, good heat aging stability, and good oxygen and ozone resistance. Quinone vulcanization is used in electrical insulation systems to provide maximum resistance to ozone and absorpotion of water.43–45
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
Vulcanization Systems Based on Phenol-Formaldehyde Resins
Phenol-formaldehyde resins and their halogenated derivates (chlorinated or brominated phenol-formaldehyde resins) have been reported to be favorable reagents for vulcanization and cross-linking of IIR-based rubber compounds.46–48 The characteristic feature of resins used as curing agents is the presence of –CH2–X groups in the ortho-position to an –OH group (where X is an –OH group, or halogen atom substituent, mainly Br or Cl) and an alkyl group in the para-position to an –OH group. Curing resins are mostly solid substances containing about 6–8% of methylene groups and soften at temperatures reaching 65–85 °C. They are incorporated into rubber formulations in a concentration scale ranging from 5 to 12 phr.2,49 The rate and state of the cure can be efficiently enhanced by addition of acidic substances (Lewis acids such as SnCl2 and FeCl3) or halogenated type rubbers (chlorosulfonated polyethylene rubber, CR, CIIR, and BIIR).50 It is supposed that acidic substances can react with terminal –OH groups of resins, leading to the increase of their reactivity. ZnO, typically at 5 phr, is also added to the rubber formulations for faster and more efficient cross-linking. In the presence of CR (5 phr) and ZnO (5 phr), ZnCl2 is formed, acting as an activator. The reaction mechanisms of resin cross-linking have been the subject of ongoing research. Two mechanisms have been proposed for resin curing of IIR. One proposal involves an ortho-methylene quinone intermediate that abstracts allylic hydrogens and reacts with the double bond in the isoprene structural unit via the formation of a six-membered ring ‘ene' intermediate. This is followed by formation of a second ortho-methylene quinone intermediate with another isoprene of an adjacent rubber chain to generate a cross-link. In that case, the cross-linking of rubber chains proceeds via methylene bridges (Scheme 4).41 The second mechanism suggests dissociation of the double bonds in isoprene units, with cross-linking proceeding via chromanone rings (Scheme 5).2 Although this has now become the most likely mechanism, the six-membered ‘ene' reaction mechanism may also play a role in cross-link formation, but with a lower yield.51–53
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
Resin-cured vulcanizates exhibit good heat aging stability and good resistance to oxygen, ozone, and reversion.37,44,54 Phenol-formaldehyde resins can also function as processing additives in rubber formulations; they enhance treatability and tackiness of raw compounds and immixture of powdery ingredients. The use of halogenated resins, mainly brominated resins, results in faster cure rate with still adequate scorch safety, and it does not require the addition of any other activators. The vulcanization temperature can also be lowered. The cross-link chemistry is supposed to be similar to that reported for the non-halogenated resins. Typical systems have the following composition: IIR, 100 phr; ZnO, 5 phr; phenol-formaldehyde resin, 10 phr; and CR, 5 phr or IIR, 100 phr; ZnO, 5 phr; brominated resin, 10 phr.10,55
Peroxide Vulcanization Systems
The use of organic peroxides for cross-linking of IIR is undesirable, because peroxide-derived radicals formed during vulcanization cause the degradation of rubber chains instead of their cross-linking (Scheme 6).45,56,57
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
HALOGENATED IIR
manufacturing and structure
Halogenated butyl rubber (halobutyl rubber) is produced by chemical modification of IIR. Chlorination is used to produce CIIR, whereas bromination is used to generate BIIR. The reactions are performed in alkane solution (pentane or hexane), mostly in the dark, at a temperature range of 40–60 °C. A relatively rapid electrophilic substitution reaction takes place on the isoprene unit of IIR (chlorination proceeds about five times faster than bromination).58–61 The most common forms of halogenated isoprene units are presented in Figure 2. Structure I, the predominant structure in halobutyl rubbers, represents 80–90%, followed by structure II, representing from 10 to 20%. The other forms of halogenated isoprene units can occur only rarely (<2%).62,63
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
vulcanization
In principle, halobutyl rubbers can be cross-linked with the same curatives used for IIR, but the vulcanization proceeds faster and with a higher degree of cross-linking of the final vulcanizates.23,37,58,64–67 With a higher curing rate, they can be much more easily co-vulcanized with unsaturated rubbers. The increased reactivity of halobutyl rubbers can be attributed to the carbon–halogen bonds; thus, the chemistry of cross-linking of CIIR and BIIR is different. Halogens are good leaving groups in nucleophilic substitution reactions due to their low carbon–halogen bond energy (Table II). Halogen atoms can also participate in reactions during vulcanization. The main difference between CIIR and BIIR is the lower dissociation energy and thus higher reactivity of the carbon–bromine bond compared to that of carbon–chlorine.16,59,68 BIIR-based rubber compounds however exhibit a higher curing rate that can be associated with lower scorch safety. A lower amount of curing agents is required to achieve an optimal degree of cross-linking, with the possibility to use a broader range of accelerators compared to CIIR.45
Vulcanization Systems Based on Metal Oxides
ZnO and MgO can be used for cross-linking of CIIR and BIIR.38,60,67–73 Cross-linking of CIIR with metal oxides is similar to that proposed for CR. ZnO alone or in combination with stearic acid cures CIIR, resulting in the formation of ZnCl2.3,74,75 The vulcanization is relatively low, but it can be activated by addition of TMTD or substituted thiureas, leading to the formation of more stable cross-links.44 Vulcanization in the presence of ZnO/TMTD is very fast; therefore, MgO and MBTS can be added to control scorch safety (they function as retarders). The recommended amounts are as follows: ZnO, 3 phr; TMTD, 1 phr; MBTS, 1–2 phr; and MgO, 0.25 phr. MgO has a notable retarding effect and can significantly affect not only the vulcanization rate but also the cross-link density of vulcanizates; therefore, it is desirable to use it only in minimal amounts necessary for the achievement of an adequate scorch. In rubber formulations of CIIR with other elastomers, the curing system must be selected carefully to balance the cure properties of individual elastomers. In the case of inner liners and side walls, adhesion to the adjacent tire body plies must be taken into consideration.10
The reactivity of BIIR as well as the vulcanization rate of BIIR-based rubber compounds is higher than that based on CIIR, but similar rules are applied.68 However, a direct transfer of the CIIR curing systems to BIIR might lead to a decrease in scorch time. Structure I (Fig. 2) is the kinetically favored bromination product in BIIR. However, at vulcanization temperatures, rearrangement leads to an increase in the amount of structure II. Structure II then dominates the equilibrium state that is achieved.45 However, it is supposed that both structural units can take part in cross-linking. Scheme 7 illustrates a possible reaction mechanism of BIIR cross-linking with ZnO.70 This scheme was primarily suggested based on model low molecular compound studies such as halogenated 2,2,4,8,8-penthamethyl-4-nonene.76,77 Although the C–C cross-links generated by this reaction pathway are thermally stable, those vulcanizates exhibit poor wear resistance and dynamic properties. When MgO is present, the halogen atom in the rubber chain can react with ZnBr2 and reform ZnO and MgBr2. The ZnO vulcanization of BIIR is relatively less efficient. The efficiency can be increased with addition of amine-based compounds, for example, amine-based antioxidants, or with the incorporation of bis-dienophiles such as m-phenylene-bis-maleimide that forms cross-links with conjugated diene butyl through Diels–Alder-type reactions.78,79 BIIR-based compounds can also be cured with ZnO in combination with a sulfur donor (TMTD) or reactive phenol-formaldehyde resin to obtain very good heat aging resistance. The rubber compounds can be formulated as follows: ZnO, 3 phr; TMTD, 0.2–1 phr; and MBTS, 0.5–1.25 phr (to improve scorch safety, acts as retarder) or ZnO, 3 phr and phenol-formaldehyde resin, 0.6–7 phr.10,13
Citation: Rubber Chemistry and Technology 91, 1; 10.5254/rct-18-82609
Sulfur Vulcanization Systems
Sulfur vulcanization systems are used in tire inner liners where a high state of cure, fatigue resistance, tensile and tear strengths, and adhesion to general-purpose rubbers in components adjacent to the inner liner are important.59,80 In sulfur curing systems for halogenated IIRs, the combination of sulfur, ZnO, and MBTS is very often used. MBTS has a dual function in the vulcanization reaction. First, it can act as a retarder, then as an accelerator in sulfur cross-link formation where MBTS reacts with ZnO to form an active sulfurating agent. A typical tire inner liner sulfur cure system can be composed of sulfur, 0.5–1 phr; ZnO, 1–3 phr; and MBTS, 1–1.5 phr.13,44,68,78 In addition to MBTS, thiurams (TMTD, TETD, and TBzTD) as very fast accelerators and dithiocarbamates (zinc dimethyldithiocarbamate [ZDMC] and ZDEC) as ultra-accelerators are also used.24,81 MgO can also take part in sulfur curing systems for CIIR- and BIIR-based compounds to control scorch safety. The introduction of elemental sulfur into the vulcanization systems gives a lower heat resistance, but it provides a simple method of controlling the modulus, and the cure systems can also be used for compounds of halobutyl rubber with NR and NR/EPDM.7,13 The curing system can be formulated as follows: sulfur, 0.5–1 phr; ZnO, 3 phr; MBTS, 0.75–1.5 phr; and TMTD, 0.1–0.3 phr (for compounds with NR, inner liners, and side walls).10,45
Other Vulcanization Systems
Vulcanization of halobutyl rubbers with reactive phenol-formaldehyde resins does not require any addition of promotor or catalyst.82–85 In the presence of ZnO, resin at an amount of about 4–5 phr is sufficient.13,68
If zinc-free compounds are required, for example in pharmaceutical applications, cross-linking of halobutyl rubbers can be performed in the presence of diamines such as hexamethylene diamine carbamate.13,65 Thioureas and multifunctional amines are also very effective curing agents for halobutyl rubbers.38,67,85
Peroxide curing systems can be applied for BIIR-based rubber compounds to generate vulcanizates with high heat aging resistance and low compression set.44,72 For optimal cure, a co-agent such as m-phenylene-bis-maleimide is required. Typically, dicumyl peroxide (1–2 phr) and m-phenylene-bis-maleimide (0.5–1.5 phr) can provide an adequate state of cure for both carbon black and clay-filled, BIIR-based rubber formulations.45
Table III gives an overview of vulcanization systems applied for cross-linking of butyl and halobutyl rubbers, along with the main characteristics and potential applications of the cured rubber compounds.
CONCLUSION
Due to the low amount of double bonds, the chemistry of cross-linking of butyl-based rubbers is different from that of highly unsaturated rubbers. The potential curing systems for IIR and halobutyl rubbers include sulfur systems, phenol-formaldehyde resins, quinones, metal oxides, diamines, and peroxide systems. Sometimes, it is desirable to use a combination of curing systems. The selection of appropriate curing systems depends on the composition of the rubber formulations with butyl or halobutyl rubber and on the final product's operational properties.
Structural units of IIR: (A) isobutylene; (B) trans-1,4-isoprene.
Sulfur cross-linking of IIR.2
Thiuram-accelerated cross-linking of IIR.28
p-Quinone dioxime cross-linking of IIR.41
Phenol-formaldehyde resin cross-linking of IIR via methylene bridges.41
Phenol-formaldehyde resin cross-linking of IIR via chromanone rings.2
Degradation of IIR in the presence of peroxides.56
Halogenated isoprene structural units in halobutyl rubbers.
ZnO cross-linking of BIIR.70
Contributor Notes