Model Vulcanization Systems for Butyl Rubber and Halobutyl Rubber Manual

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Exxon™ butyl and halobutyl rubber

Model vulcanization systems for butyl rubber and halobutyl rubber manual

Country name(s) 2 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 3

Abstract

The vulcanization of isobutylene-co-isoprene rubber (IIR), brominated isobutylene-co-isoprene rubber (BIIR), chlorinated isobutylene-co-isoprene rubber (CIIR), and brominated isobutylene-co-para-methylstyrene elastomer (BIMSM) differs from that of general-purpose rubbers (GPR). Butyl rubber has approximately 2% unsaturation in the backbone. Halobutyl rubber (BIIR and CIIR) incorporates the butyl backbone with either bromine or chlorine, which significantly increases the chemical reactivity of the isoprenyl units located in the butyl backbone. Similarly, in BIMSM the bromine atom is bonded to the para-methylstyrene (PMS) group, thus affording the completely saturated polymer backbone a site of chemical reactivity. Utilization of the unique attributes of butyl rubber and halobutyl rubbers with their minimal backbone unsaturation and of BIMSM elastomers with no backbone unsaturation is found in many areas of industry. These properties are excellent vapor impermeation, resistance to heat degradation, and improved chemical resistance as compared to general-purpose rubbers. However, this low amount of reactivity requires special consideration to vulcanize these isobutylene-based polymers. The type of vulcanization system selected is a function of the composite structure in which it is used, and the cured product performance requirements. Therefore, vulcanization systems vary and may include an accelerator package along with resins, zinc oxide, zinc oxide and sulfur, and quinoid systems. This review will discuss the types and selection of appropriate vulcanization systems for isobutylene-based elastomers. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 3

1. Introduction...... 4

2. Composition of isobutylene polymers...... 5

3. General trends in vulcanization chemistry...... 7

4. Overview of the vulcanization of isobutylene-based elastomers...... 10

5. Vulcanization of butyl rubber...... 11

6. Halobutyl rubber...... 14

7. Vulcanization of brominated isobutylene-co-para-methylstyrene (BIMSM)...... 18

8. Model cure system formulary...... 20

9. Summary...... 24

Appendix 1 ...... 25

10. References...... 26 4 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 5

1. Introduction

Isobutylene-based elastomers include butyl rubber, halogenated butyl rubber, star-branched versions of these polymers and brominated isobutylene-co-para-methylstyrene (BIMSM). Due to their impermeability and resistance to heat and oxidation, these polymers find application in tire innerliners, innertubes, curing bladders and envelopes, and other specialty applications where air retention and resistance to heat and oxidation are desired. Butyl rubbers are produced via a cationic polymerization in a methyl chloride diluent at temperatures less than -90°C. These unique attributes and difficult manufacturing requirements place butyl rubbers in the special purpose elastomers category distinct from general- purpose rubbers (GPR) such as polybutadiene rubber (BR), natural rubber (NR), and styrene-butadiene rubbers (SBR) 1. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 5

2. Composition of isobutylene polymers

Butyl rubber (IIR) is the copolymer of isobutylene and a small Copolymers of isobutylene and para-methylstyrene have a amount of isoprene, typically in the order of 2%. Figure 1 is a random structure, again due to the monomer reactivity ratios schematic of butyl rubber. being nearly equivalent. After polymerization and subsequent bromination some of the para-methylstyrene Figure 1 groups are converted to reactive bromomethylstyryl groups. Structure of butyl rubber These saturated backbone polymers contain isobutylene, 1 to 5 mole-% of para-methylstyrene, and 0.5 to 1.3 mole-% of 2 brominated para-methylstyrene (Figure 3) . 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m/w39mrOpy14{pp}repeat}b/pAs fillst}{Asc0xl rOstlWst}b/SP{gs/sf-6 L/sg/setgrayeywxbW gr}b/Bd{[{pp}{[{DS}{DD}{gsg SA40 lt{pp pm/aLlt{Ar1}{gs 3gr xllplxmvgegr-1plW n/eyl m}ie-1xm .6672 L/cw/currentlinewidthbbpxmvOA}{270np 0acdv/m2 cmr}b/x{e np 4dv21rpys/b1sc -2lW xcm5lpLB gis-0.4x 0rad12 smsl1 dv270g}ie3aLwF-10 pxdx s12pal 20 smx}if10xgs pyOB l-0.4sta w m}if/bd1pyrcurvetoppL/sl/setlinewidthn/dxrm0 27d}b/cmlyOBcm5CAx/lW pycY 0.5eaRst}{py grCA p 1lx orOnpey5 s-8 ldp mvlWsmr ZLBbW DA}{1s/ly32os -1xcmexsg oOA}{1800acs27 x/bWm wyo dy sc orpxwm2cwlW2filld/rO{4 x}b/DA{npne{ly mtat0 .135smpxdpxexec}{al2 0 xlLBgsdvn/dywxw ne{bW}{2py cXdvcvro bWgrlx2 currentmatrixpy2 3st}{pypx/cW -11 adp dvgs -.667dpatpdv-1radL/cpt/currentpoint cmex27leys/lx CH2 C CH2-C=CH-CH2- with increasing para-methylstyrene content and is nominally CH (0.8 - 2.5 mole%) 3 around -57oC. This compares to a nominal -59°C to -60°C for Butyl Rubber (IIR) halobutyl polymers. Chlorobutyl (CIIR) and bromobutyl (BIIR) rubbers are manufactured similarly to butyl rubber, but with an added Figure 3 subsequent halogenation step. Due to the nearly equivalent Structure of brominated isobutylene-co-para-methylstyrene, reactivity ratios of the monomers and low concentration of BIMSM 2 isoprene, the isoprenyl units in these polymers are randomly Table I illustrates the commercial grades of isobutylene-based distributed along the polymer chain. CH 3 CH 3 CH 2 C CH 2 CH CH 2 C CH 2 CH Figure 2 illustrates the structures of the functional halogen CH 3 CH 3 containing groups found in bromobutyl rubber2,3. The majority of the isoprenyl units are in the trans-configuration. Structure II, the predominant structure in halobutyl rubber, represents CH 2Br CH 3 currentpoint 192837465 50%-60%, followed by Structure I representing from 30%- 40%. Approximately 5%-15% is Structure III; Structure IV is elastomers and their typical compositions that are currently typically only 1%-3%. available from ExxonMobil Chemical company.

Figure 2 Structure of isoprenyl units in bromobutyl rubber

CH3 CH2

CH2 C CH CH2 CH2 C CH CH2 Br Structure I Structure II

CH2Br CH3

CH2 C CH CH2 CH C CH CH2 Br Structure III Structure IV 6 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 7

Table I Commercial grades of ExxonMobil isobutylene-based elastomers

Elastomer Grade Mooney Viscosity Isoprene para- Halogen Halogen Halogen Typical Applications (ML1+8 at 125°C) (mol%) Methylstyrene (wt%) (mol%)1 (wt%) Butyl 065 32 1.05 Tubes, Baldders (low viscosity) 165 32 1.50 Tubes, Baldders 365 33 2.30 Tubes, Baldders Butyl 068 51 1.15 Tubes, Baldders (medium viscosity) 268 51 1.70 Tubes, Baldders Butyl 269 57 1.70 Tubes, Baldders (high viscosity) Chorobutyl 1066 38 1.95 CI 1.26 Tire Innerliners White Sidewalls Bromobutyl 2222 32 1.70 Br 2.00 Automobile Tires 2235 39 1.70 Br 2.00 Automobile Tires 2255 46 1.70 Br 2.00 Truck Tire Innerliners 2211 32 1.70 Br 2.10 Fast Cure Applications 2244 46 1.70 Br 2.10 Fast Cure Applications Exxpro 3035 45 5.00 Br 0.47 Curing Bladders 3433 35 5.00 Br 0.75 Pharmaceuticals 3745 45 7.50 Br 1.20 Tires, Engineerd Products

Note: 1. Benzylic bromine content 2. Butyl, chlorobutyl rubber, bromobutyl rubber, and Exxpro polymers are used in pharmaceutical applications

This paper is an overview of the vulcanization of this group of special purpose elastomers and identifies opportunities for further improvement in final compound mechanical properties and final product performance. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 7

3. General trends in vulcanization chemistry

Five factors influence the development of vulcanization system Table II technology: Nitrosamine limits in pharmaceutical, food utensils4

• Elimination of accelerators that may generate nitrosamines Location Nitrosamine Nitrosables • Accelerators for improved scorch resistance and improved ppb maximum ppb maximum ease of compound processing during final product Australia 20 400 assembly, e.g. a tire Denmark 5 100 • Vulcanization systems suitable for high-temperature curing Netherlands 1 100 with associated benefits in productivity Canada 60 N/A • Improved reversion resistance Switzerland 10 200 • Improvement in cured compound attributes such as mechanical or dynamic properties Germany 10 200 UK 30 100 Nitrosamines USA 60 N/A Nitrosamines are nitrogen-based compounds with attached carbon-containing groups. Using ammonia as a reference, European Union directive 93/11/EEC concerns extractable replacing one of the hydrogen atoms produces a primary N-nitrosamines for certain food applications. Accelerators that amine. Substitution of two or three hydrogen atoms produces are considered to give rise to N-nitrosamines are listed below secondary or tertiary amines, respectively. As shown in (see Appendix 1 for explanation of abbreviations): equation 1, secondary amines found in accelerators such as thiurams, dithiocarbamates, and some sulfenamides can react TMTD, TMTM, TETD, TBTD, DPTT, DTDM, with nitrogen oxides in the air to produce a nitrosamine. ZDMC, ZDEC, ZDBC

R NOx R Tetrabenzylthiuramdisulfide (TBzTD) and zinc NH N N O R Nitrogen dibenzyldithiocarbamate (ZBEC) accelerators, though in some Oxides R Secondary Amine instances might give rise to N-nitrosamines, are considered to Nitrosamine (1) be outside the scope of regulatory attention5. The effect of steric factors on thiuram and dithiocarbamate nitrosamine During the vulcanization process accelerators such as formation has also been studied. Thiurams and tetramethylthiuram disulfide (TMTD) are partly decomposed to dithiocarbamates made from sterically bulky amines such as their parent secondary amines. Reaction with atmospheric diisobutylamine produce orders of magnitude lower levels of nitrogen oxide can lead to N-nitrosamine generation. In nitrosamine than does tetramethylthiuram disulfide (TMTD). At Germany, N-nitrosamine levels in the work place atmosphere the same time, when used as a secondary accelerator for are controlled under factory regulation TRGS 552. This cyclobenzothiazole sulfenamides, tetraisobutylthiuram directive permits a maximum atmospheric concentration of disulfide has better scorch safety, but vulcanizes at the same only 1µg/m3 of regulated N-nitrosamines. The cabin space of rate as TMTD. Uniquely, tetraisobutylthiuram monosulfide automotive vehicles now falls under the regulations that have could act as a retarder as well as a secondary accelerator6. implications for rubber parts in this area, which includes the Accelerators that cannot produce N-nitrosamines since they door and window seals. Automotive vehicle manufactures are based on primary amines or do not contain nitrogen have also placed increasing emphasis on the removal of include: N-nitrosamines from all parts of their vehicles including rubber components such as gaskets, engine mounts, and hoses4. CBS, TBBS, DOTG, DPG, MBT, MBTS, ZMBT, Xanthates Maximum nitrosamine levels are listed in Table II. 8 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 9

Accelerators for isobutylene-based elastomers High temperature curing and reversion resistance A broad range of accelerators can be found in formulas for As cure temperature increases, care must be taken to avoid isobutylene-based elastomers. Some of the accelerators found reversion while achieving the desired mechanical properties of are illustrated in Figure 47. These include the product. This infers no marching modulus and the cyclohexylbenzothiazole sulfenamide (CBS), preservation of a stable crosslink network. Cure systems with mercaptobenzothiazole sulfide (MBTS), tetramethylthiuram primary accelerators such as MBTS, or sulfenamides such as disulfide (TMTD), diphenyl guanidine (DPG), and zinc TBBS, tend to meet such requirements. Depending on the cure dimethyldithiocarbamate (ZDMC). system thiurams, dithiocarbamates, and xanthates tend to have poorer reversion resistance. Figure 4 Structure of selected accelerators7 Two reversion resistors have been of particular note. These are bis(citraconimido-methyl) benzene (BCI-MX) and sodium N N N C S NH C S S C hexamethylene-1,6-bisthiosulfide dehydrate. Both function C S S exclusively as reversion resistors. BCI-MX (Figure 6) is Mercaptobenzothiazole disulfide Cyclohexyl benzothiazole sulfenamide understood to react with the polymer chain to form stable flexible carbon-carbon crosslinks which substitutes for and CH S S CH 3 3 8 N C S S C N replaces sulfur crosslinks that disappear during reversion .

CH3 CH3 NH Tetramethylthiuram disulfide C NH Diphenyl guanidine NH Figure 6 Structure of 1,3-bis(citraconimidomethyl)benzene (BCI-MX) CH3 S S CH3 N C S Zn S C N

CH3 CH3 Zinc dimethyldithiocarbamate C4H9 O O C4H9 S P S Zn S P S

S S C4H9 O O C4H9 C H O C S S C O C H 4 9 4 9 Zinc Dibutylphosphorodithiate Dibutyl xanthogen disulphide

Improved scorch resistance is currently achieved by addition of Alternatively, for resistance to high-temperature reversion, magnesium oxide (MgO). In some instances, with a sodium hexamethylene-1,6-bisthiosulfide dihydrate (Figure 7) sulfenamide-based cure system, N-(cyclohexylthio) may be evaluated since it breaks down and inserts a phthalimide (PVI), whose structure can be found in Figure 5, is hexamethylene-1,6-dithiyl group within a disulfide or used. MBTS can also serve as a pre-vulcanization inhibitor. polysulfide crosslink providing a hybrid crosslink. During Calcuim oxide and magnesium oxide can be used in zinc extended vulcanization periods or accumulated heat history oxide-cured halobutyl rubber. However, they will serve as an due to product service, polysulfidic-hexamethylene crosslinks activator in amine accelerated systems. Calcium stearate and rearrange to produce thermally stable elastic monosulfidic zinc stearate can act as retarders in chlorobutyl rubber crosslinks. At levels up to 2.0 phr, there is little effect on compounds. Unlike their use in general-purpose elastomers, compound induction, scorch times, or other compound benzoic acid and salicylic acid are not effective as scorch mechanical properties. retarders. Figure 7 Figure 5 Structure of PVI Na SO S S SO Na 2 H O 3 CH2 6 3 2 O Hexamethylene-1,6-bis(thiosulfate) C disodium salt, dihydrate N S C The bifunctional silane-coupling agent, bis-(3-triethoxysilylpropyl) O tetrasulfane has dual functions as both a sulfur donor and a N ( c y c l o h e x y l t h i o ) p h t h a l i m i d e reversion resistor9. This ability to maintain the polysulfidic or other stable network throughout the curing process and subsequent product service life while offering reversion Model vulcanization systems for butyl rubber and halobutyl rubber manual - 9

resistance can be beneficial (Figure 8)9. Natural rubber compounds containing such systems are believed to display improved abrasion resistance and wear properties.

Figure 8 EC cure system rheometer profile9 10 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 11

4. Overview of the vulcanization of isobutylene-based elastomers

In butyl rubber, the hydrogen atoms positioned alpha (a) to the Table III carbon-carbon double bond of the isoprenyl unit permits Carbon-halogen bond energies and bond lengths10 vulcanization into a crosslinked network with sulfur and organic accelerators. The low degree of unsaturation has Bond Energy Length traditionally required the use of ultra-fast accelerators, such as J/mol nm thiurams or thiocarbamates in order to cure under acceptable -C — C- 346 0.154 conditions of time and temperature. Phenolic resins, -C — H 413 0.109 bisazidoformates, and quinone derivatives can also be -C — F 452 0.138 employed. Vulcanization introduces a chemical crosslink -C — CI 327 0.177 approximately every 250 carbon atoms along the polymer -C — Br 209 0.194 chain producing a covalently bonded molecular network. Polysulfidic crosslinks have limited stability at elevated The bromine-carbon bond in bromobutyl rubber has a lower temperature and in many instances can rearrange to form disassociation energy than does the chlorine-carbon bond in monosulfidic and disulfidic crosslinks This rearrangement chlorobutyl rubber (Table III). Epoxidized soybean oil (ESBO) results in permanent set of vulcanizates exposed to high can be added to suppress isomerization reactions and HBr temperatures for long periods of time. Resin cure systems such formation. Butylated hydroxytoluene (BHT) can be used to as alkyl phenol-formaldehyde derivatives can provide carbon- 1,2 stabilize the polymer by serving as an antioxidant. Calcium carbon crosslinks resulting in heat-stable vulcanizates . stearate is added to CIIR, BIIR, and BIMSM as an acid trap to diminish dehydrohalogenation catalyzed by HCl or HBr. Halobutyl rubber can be crosslinked by some of the same Calcium stearate can also act as a cure retarder. curatives used for butyl rubber such as zinc oxide, bismaleimides and dithiols. Halogens are good leaving groups in nucleophilic Brominated isobutylene-co-para-methylstyrene (BIMSM) substitution reactions due to the low carbon-halogen bond crosslinking involves the formation of carbon-carbon bonds, energy (Table III). When zinc oxide is used to crosslink halobutyl generally through Friedel-Crafts alkylation chemistry. rubber, carbon-carbon bonds are formed through Diamines, phenolic resins and thiosulfates can also be used to dehydrohalogenation which results in a compound with a very crosslink BIMSM elastomers. The stability of these carbon- stable crosslink system affording a compound having good carbon bonds combined with the chemically saturated retention of aged properties and low compression set. backbone of BIMSM yields excellent resistance to heat and oxidative aging, and to ozone attack. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 11

5. Vulcanization of butyl rubber

The major applications utilizing butyl rubber are tire phenol formaldehyde resin. Curing resins such as a heat innertubes, curing bladders, pharmaceutical closures, and a reactive octylphenol formaldehyde resin, which contains variety of engineered products such as mounts, sheeting, methylol groups, can be used3. Table IV contains a model automotive flaps, and damping pads. These applications formula illustrating the resin cure system. For a faster cure, require good compound aging resistance, oxidative resistance addition of a halogen donor such as polychloroprene or and heat stability. Depending on the application of the final stannous chloride is required13. In this instance, product, one of the following three cure systems have been polychloroprene has not been considered as part of the total used11, 12. compound polymer content.

• Quinone dioxime Table IV • Resin cure Model curing bladder formulation13 • Thiuram (i.e. TMTD) accelerated Material phr Quinone cure systems Butyl 268 100.0 The crosslinking of butyl rubber with para-quinone dioxime Polychloroprene 5.0 (QDO) or para-quinone dioxime dibenzoate (DBQDO) proceeds Carbon black N330 50.0 through an oxidation step that forms the active crosslinking agent para-dinitrosobenzene. The use of metal oxides (such as Castor oil 5.0 PbO2, Pb3O4, MnO2) or MBTS as an oxidizing agent, increases Zinc oxide 5.0 the vulcanization rate allowing use in room temperature cures Octylphenol formaldehyde resin 10.0 such as in cements3. Figure 9 is a schematic illustrating the reaction of a dioxime in the vulcanization of butyl rubber. The A more reactive resin cure system not requiring an activator is dioxime vulcanization system can be used for dry rubber obtained when a portion of the methylol groups are replaced applications. One example is electrical insulation systems, which by bromine. An example of this resin is brominated octylphenol contain butyl rubber for improved ozone resistance. formaldehyde resin3.

Figure 9 Figure 10 is a schematic illustrating the reaction of a curing 3 Dioxime cure of butyl rubber resin (heat reactive octylphenol formaldehyde) in the vulcanization of butyl rubber. Following the elimination of N O H N O P b O 2 water in the reaction sequence, the exomethylene group and O carbonyl oxygen react with an isoprenyl unit in butyl rubber to

P b 3 O 4 form a chroman ring. Chromonone ring structures are very N O H N O stable and are frequently found in natural product biosynthesis. C H 3 C H 3 C H 2 C C H C H 2 C H 2 C C H C H Figure 10 N O H Resin curing of butyl rubber13

OH OH OH O

HO CH2 R' CH2 OH HO CH2 R' CH2 heat N O H - H2O D i o x i m i n e c u r e o f b u t y l r u b b e r C H 2 C C H C H R R R R C H 3 CH2 CH3 Formation of a CH C Chroman ring H CH Resin cure systems OH O

Phenol formaldehyde curing resins are classified as resols, i.e. CH2 CH3 HO CH2 R' CH2 C three-dimensional resin systems forming crosslink networks OH O CH CH2 R R which can serve as reinforcing resins versus novolac resins HOCH2 R' CH3 which are linear. Resin curing of butyl rubber is a function of CH2 CH2 CH3 C C the reactivity of the phenolmethylol groups in the reactive R R CH O O CH CH CH3 R' 2 CH2 C CH CH2

Crosslinked Butyl Network Evolution R R 12 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 13

Thiuram accelerated vulcanization Figure 12 Although thiuram and dithiocarbamate accelerators form Thiurams as sulfur donors14 nitrosamines, a number of published formulas use these S S CH3 S S CH3 CH3 CH3 +2 -2 +2 - - components as vulcanization agents. For completeness, we Zn ...O N C S S C N Zn O C N + S S C N CH CH CH3 3 include the mechanism by which this class of materials is 3 CH3 thought to function14; however, again we recommend the use of alternative non-nitrosamine-generating accelerators. S S S S S Thiuram accelerated systems participate in the vulcanization CH3 CH3 CH3 CH3 CH3 N C S S C N N C SSS C N S C N process through formation of a zinc complex as shown in CH3 CH3 CH3 CH3 CH3 S figure 11. The sulfur rich complexes are formed by insertion of CH3 -S S C N the sulfur into the zinc dimethyldithiocarbamate molecule, CH3 which in turn is an active sulfurating complex. Rubber bounded S S CH3 S S CH3 CH3 CH3 intermediates are subsequently produced followed by N C SSS C N N C S Sx S C N etc. CH3 CH3 CH3 CH3 crosslinking to form polysulfidic crosslinks. These polysulfidic Sulfurating Compex crosslinks evolve into monosulfide and disulfidic crosslinks. Peroxides and butyl rubber Figure 11 Polyisobutylene and butyl rubber undergo degradation rather 14 Possible mechanism of thiuram accelerated vulcanization than crosslinking when treated with peroxides. It is believed that scission occurs via a radical substitution mechanism C H3 S S CH3 S S CH CH3 N C Zn C N 3 (Figure 13). S S N C Sx Zn S C N CH3 CH3 CH3 CH3 S S Figure 13 S6 CH3 Peroxide degradation of polyisobutylene14 S Zn S Sy C N

CH3 S Polyisobutylene based polymer CH CH CH3 S Zn S CH3 3 3 N C S Sy C N CH3 CH3 CH3 CH3 C CH2 C CH2 CH3 CH3 CH C CH C CH C CH C CH R H 2 2 2 2 2 CH3 CH3 CH3 CH3 CH3 CH3

O Polymer Chain S Sx S CH3 C CH3 CH3 CH3

Zn S CH3 CH2 C CH2 O C S Sy C N CH3 CH3 CH3 R H New Terminal Chain End Dicumyl peroxide

In the absence of sulfur, TMTD can function as a sulfur donor Other vulcanization systems for butyl rubber (Figure 12). Increasing the rate of cure without a significant loss Due to the inherent low reactivity of the nearly saturated butyl of other properties can be obtained with the addition of thiurams backbone, there are instances when a secondary accelerator is or thioureas and zinc oxide at around 5 phr. Substitution of 16 required in the vulcanization system. Table V contains nitrosamine generating thiurams such as TMTD with MBTS or examples of an amylphenol disulfide polymer secondary higher molecular weight thiurams such as TBzTD or accelerator (Vultac series) that can be used. dithiocarbamates such as ZBEC can be explored to eliminate the future use of these accelerators. TBzTD is a high molecular weight accelerator and functions as a sulfur donor. Due to its molecular weight and stearic hindrance these accelerators can have improved scorch resistance hence improving compound processing. When used in combination with sulfenamides such as TBBS, the nitrosamine concentration becomes negligible15. The high molecular weight of the secondary amine, dibenzylamine (DBA), produced by TBzTD is not as volatile as those generated from lower molecular weight thiurams such as TMTD, TMTM, or TBTD. DBA has a boiling point of 300°C and in the presence of oxygen may decompose below this temperature. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 13

Table V16 Vultac series of accelerators

Commercial Chemical Name & Function Formula Sulfur Softening Name Wt% Point (°C)

Vultac 2 Amylphenol disulfide polymer Sulfur donor. Phenol, 4(1,1-dimethyl (C11H16O.Cl2S2)x 23 55 propyl)-polymer with sulfur chloride

Vultac 3 Amylphenol disulfide polymer Sulfur donor. Phenol, 4(1,1-dimethyl (C11H16O.Cl2S2)x 28 85 propyl)-polymer with sulfur chloride

Vultac 5 Amylphenol disulfide polymer Sulfur donor. (C11H16O.Cl2S2)x 18 85

Vultac 7 Amylphenol disulfide polymer Sulfur donor. Phenol, 4(1,1-dimethyl (C11H16O.Cl2S2)x 30 120 propyl)-polymer with sulfur chloride

Vultac 710 Amylphenol disulfide polymer Sulfur donor. Phenol, 4(1,1-dimethyl (C11H16O.Cl2S2)x 27 85 propyl)-polymer with sulfur chloride plus 10% stearic acid

The general structure and crosslink formed by polymeric amylphenol disulfide accelerators is believed to be illustrated in Figure 14. Though these accelerators contain sulfur, the disulfidic group may not be readily assessable due to steric hindrance of the phenol groups. The sulfur linkage is predominantly a disulfide, but trisulfide and tetrasulfide crosslinks have also been reported.

Figure 1416,17,18

CH3 CH2 CH CH C OH OH S S S S CH3 CH3 CH2 C OH CH3 CH3 C CH3 CH3 C CH3 S CH2 CH2 CH2 CH CH C CAmylH3 Disulfide PolymerCH3 n CH3 Schematic of Phenol Sulfide Crosslink

Table VI shows that even though the apparent activation Table VI energies of the amylphenol disulfide polymer accelerators can Vulcanization Systems Containing Vultac Accelerators be higher than that of TMTD, the rheometer delta torque, DT, MDR Rheometer Apparent or state of cure which relates to tensile strength, elongation, Accelerator ΔT (dNm) at Activation Energy, Form and modulus is higher particularly for Vultac 3. Note that these 160°C Ea (KJ/mol) accelerators improve interfacial bonding in some cure system TMTD 4.64 81.77 - blends19. For example, use of amylphenol disulfide polymers in a NR/NBR blend has been investigated. It is suggested that to Vultac 2 4.79 79.52 Glass have good activity the system may require use of up to 4.5 phr Vultac 3 7.23 99.96 Glass of Vultac 320. Although the mechanism of amylphenol disulfide Vultac 5 6.66 100.97 Powder polymer activity in vulcanization remains uncertain, it has been shown that there was a lower degree of preferential Vultac 7 7.23 92.84 Pellets vulcanization in polymer blends when amylphenol disulfide Vultac 710 7.22 96.41 Pellets polymer replaced free sulfur. It has been concluded that the amylphenol disulfide polymer moiety played a role in increasing the solubility of the vulcanization intermediates in natural rubber and that a monosulfidic crosslink is created resulting in better reversion resistance. 14 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 15

6. Halobutyl rubber

Commercially, chlorobutyl rubber (CIIR) and bromobutyl Amines rubber (BIIR) are the most important derivatives of butyl • Zinc free compounds may be required in, for example, rubber. Their predominant application is in tire innerliners with pharmaceutical applications. Curing of bromobutyl rubber the performance of the modern radial tire being impacted by with diamines such as hexamethylene diamine carbamate the amount of halobutyl rubber used in the innerliner may be one solution. compound. Typical performance requirements of the tire Peroxide cure systems innerliner are: • Halobutyl rubber can be crosslinked with peroxide cure systems. A dicumyl peroxide with a coagent, such as • Air retention bismaleimide, yields a low compression set, high heat • Adhesion to tire carcass compound resistant vulcanizate3. Butyl rubber, however, will • Resistance to fatigue and cracking depolymerize in the presence of peroxide. • Heat resistance • High tensile and tear strength Metal oxide curing Metal oxides such as zinc oxide will react with the halogen in Chlorobutyl rubber has an added application in white sidewall chlorobutyl and bromobutyl polymers. The most active compounds. When blended with NR, SBR or BR, halobutyl halogen is in the allylic position. Zinc stearate or zinc octoate is rubber may be used in tread compounds. The addition of required as a solubilizing agent and the mechanism is believed isobutylene-based polymers increases the dynamic loss to be a nucleophilic substitution. Structure II (Figure 2) is the modulus leading to improvements in wet and dry traction kinetically favored bromination product in bromobutyl rubber. performance, braking performance, and vehicle stopping However, at curing temperatures, rearrangement leads to an distance. increase in the amount of Structure III. Structure III then dominates the equilibrium state that is achieved21. Figure 15 Vulcanization of halobutyl rubber presents a possible scheme for the role of zinc oxide in The primary difference between chlorobutyl rubber and crosslinking. In the presence of magnesium oxide, the halogen bromobutyl rubber is due to the higher reactivity of the on the polymer can react with ZnBr2 and reform ZnO and carbon-bromine bond compared to that of carbon-chlorine MgBr2 as seen in the vulcanization of polychloroprene. bond. The carbon-bromine bond has the lowest energy and the longest length (Table III). Bromobutyl rubber has faster Figure 15 cure rates and a shorter scorch or cure induction period (with Zinc oxide in the vulcanization of bromobutyl rubber 21,22,23 productivity benefits), requires lower levels of curatives, and enables one to use a wider range of accelerators than could be 3 2 3 possible for chlorobutyl rubber. A number of cure systems are CH3 CH2Br CH3 CH CH ZnOBr CH 2 2 2 CH2 C CH2 C CH CH2 C CH C CH C CH CH C suitable for halobutyl rubber and halobutyl rubber/general- 3 3 CH3 CH3 CH CH purpose rubber (GPR) blends. Cure system selection will be a function of service conditions and the end use of the rubber CH3 CH3 3 3 product. Typical cure systems for halobutyl rubbers include: CH2 C CH2 C CH CH2 C CH CH 2 2 2 CH3 CH2ZnOBr CH3 CH C CH C CH CH C CH3 CH2 CH3 2 Metal oxides CH3 CH2Br CH3 CH3 CH CH3 • Zinc oxide will react with the halogen in polymers such as CH2 C CH2 C CH CH2 C CH2 C CH2 C CH CH2 C CH3 CH3 CH3 CH3 halobutyl rubber and polychloroprene. Magnesium oxide is + 2 effective as a retarder. ZnBr Sulfur cure systems This scheme was initially proposed based on model compound • Sulfur cure systems can be used in tire innerliners where a studies such as 2,2,4,8,8-pentamethyl-4-nonene. The final high state of cure, fatigue resistance, high tensile and tear crosslink structure is thought to be as illustrated in Figure 16. strength, and adhesion to GPR in components adjacent to There would be less steric hindrance around the methylene the innerliner is important. A typical tire innerliner sulfur group than at the -carbon rendering this interpretation more cure system may include sulfur (0.5-1.0 phr), zinc oxide likely. Lowering sulfur or thiuram levels can control the greater (1.0-3.0 phr), and MBTS (1.0-1.5 phr). reactivity of bromobutyl rubber. Mooney scorch and induction periods can also be extended by: Model vulcanization systems for butyl rubber and halobutyl rubber manual - 15

• Increasing the MBTS level (since MBTS can also serve as a Table VII retarder) Vulcanization retardation with magnesium oxide (MgO) 3, 6 • Replacing phenolic resins such as octylphenol tackifying 1 2 3 4 Compound resins with hydrocarbon resins (phr) (phr) (phr) (phr) • Lowering the stearic acid level or carboxylic acid content TMTD 1.00 1.00 1.00 1.00 • Avoiding amine type antioxidants • Adding magnesium oxide (retarder in bromobutyl rubber) MgO 0.00 0.25 0.50 0.00

• Adding polyethylene glycol since the ether functionality can Mooney Scorch at 126.5°C serve as an efficient retarder for bromobutyl rubber. t-5 5.0 15.0 30.0 8.0

Figure 16 Cured at 153.0°C, 45 minutes Possible mechanism for crosslinking of halobutyl rubber24 Tensile Strenght (MPa) 17.9 18.8 16.6 17.8

X Elongation (%) 315 335 380 320 ZnX3

+ 300% Modulus (MPa) 17.2 16.4 12.8 16.3

Formulation (phr): chlorobutyl 100, antioxidant 2246 1.0, HAF 50, stearic acid 1.0, ZnO 5.0, cure system as above.

MBTS accelerated curing Accelerators tend to be based on one of five structures shown in Figure 1714, a thiazole, thiocarbamyl, alkoxythio carbonyl, dialkylthio phosphoryl, or a diamino-2,4,6-triazinyl.

Figure 17 X Fundamental structures of organic accelerators14 Magnesium oxide dual effects on halobutyl rubber systems S S S NR2 Table VII illustrates the effect of magnesium oxide (MgO) on N R2N N C RO C (RO)2 P cure rate for a TMTD/zinc oxide cure system6. Addition of 0.25 RR' N C S thiocarbamyl alkoxythio dialkylthio N N phr of MgO increases the Mooney scorch time by ten minutes Thiazole carbonyl phosphoryl diamino-2,4,6- triazinyl without significantly effecting chlorobutyl rubber compound properties such as tensile strength. If the level of magnesium As a thiazole, MBTS has a dual function in the vulcanization oxide is increased to 0.5 phr, there will be a detrimental effect reaction. First is as a retarder and then as an accelerator in on cure rate, with magnesium oxide acting as a retarder. As a sulfur crosslink formation where MBTS will react with ZnO to retarder, magnesium oxide is effective with all vulcanization form a sulfurating agent. This intermediate can then react with systems except amine-based cures. In that case magnesium the polymer to form crosslinks (Figure 18)14. oxide will react with chlorine formed during crosslinking preventing the appropriate vulcanization reaction with the Figure 18 curing agent. Illustration of sulfurating agent and crosslink formation14

R R R N NR3

N N N N C S Zn S C C S S 8 Zn S C S S S S

NR3 S S N R R R S6

+ N Zn S N BtS Sx + Sy Bt Zn S C S + Sy C + H RSxSBt BtSSx Zn SySBt RSx ZnSx SBt RBtSSy SBt S S R H Polymer Chain

Zn S N + + S Sy C RSSxR ZnS HSxBt S R H 16 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 17

Although the final state of cure or crosslink density may be Figure 20 low, in the absence of zinc oxide MBTS may vulcanize rubber MBTS (phr) and MDR rheometer at 180°C, 0.5° arc as shown in Figure 1914. Depletion of MBTS due to the initial reaction with bromine could account for the delayed response 8 observed. Again, at higher levels MBTS can serve as a retarder. 7 6 5 Figure 19 4 3 MBTS accelerated crosslinking14 2 1 0 Delta Torque (dNm) N 8 1 tc90 (minutes) N S N N 2 C S S C S S Sx 3 tc10 (minutes) C C 4 S S S S 5

C H 3 H 1 2 3 4 5 CH2 C C CH2

1 2 tc10 (minutes) 0.68 1.04 0.99 0.96 0.95 3 H CH 4 2 3 CH C C 1 tc90 (minutes) 3.02 7.38 4.33 3.31 3.05 2 H CH CH 2 CH Delta Torque (dNm) 4.04 4.78 4.58 4.16 3.92 C CH3 N S N 3 N

C S Sx C 4 CH Sx S C S S CH2 S The proposed retarding mechanism of MBTS is illustrated in 2 CH CH3 Figure 21. Depletion of MBTS due to an initial reaction with C C CH H bromine would account for the delayed response observed. C C H + Sx S MBT CH Sx S N C CH2 S S Figure 21 Retarding action of MBTS Table VIII illustrates the effect of MBTS concentration on rheometer induction time, cure state, and rate of vulcanization. H N N N C S S C C S Table VIII S S S Retarding effect of MBTS 2 1 2 3 4 5 Compound (phr) (phr) (phr) (phr) (phr) 3 2 3 CH3 CH2 CH3 CH CH CH 2 2 2 Zinc oxide 2.0 2.0 2.0 2.0 2.0 CH2 C CH2 C CH CH2 C CH C CH C CH CH C 3 3 CH3 Br CH3 CH CH Sulfur 0.5 0.5 0.50 0.50 0.50 S C H + MBTS 0.0 0.5 1.0 1.2 2.0 N N S HBr C S S Rheometer (MDR), 180°C, 0.5° arc

Mh - Ml (dNm) 4.04 4.78 4.58 4.16 3.92 Other halobutyl rubber cure systems • Alternatives to TMTD accelerated curing in halobutyl tc10 (minutes) 0.68 1.04 0.99 0.96 0.95 rubber: Nitrosamines result when curing with TMTD and its tc90 (minutes) 3.02 7.38 4.33 3.31 3.05 use is not recommended. However, tetrabenzylthiuram Rate (dNm/min) 1.83 2.34 2.38 2.62 2.84 disulfide (TBzTD) or its zinc derivative, zinc dibenzyldithiocarbamate (ZBEC) with a sulfenamide Figure 20 shows that the use of 0.5 phr of MBTS increases the accelerator such as CBS can readily match the compound rheometer induction time (tc10 at 180°C) from 0.68 minutes properties of TMTD. Magnesium oxide may be added to to about one minute. The rheometer tc90 cure times increased assist in controlling scorch and cure rate. to 7.3 minutes then fell to 3.1 minutes as MBTS increased from • Vultac, amylphenol disulfide accelerators: the Vultac group 0.5 phr to 2.0 phr. This further illustrates the dual role for MBTS of sulfur donors can be used in chlorobutyl rubber-based as a retarder and as primary accelerator in the halobutyl rubber tire compounds where good adhesion to natural rubber or vulcanization process. NR/SBR blends is required. These compounds tend to show high cure states, which can be controlled with MBTS Model vulcanization systems for butyl rubber and halobutyl rubber manual - 17

and MgO. However, magnesium oxide use can also increase tack to calender rolls and increase mold fouling. With bromobutyl rubber, the use of amylphenol disulfide polymer results in reduction in scorch times thereby limiting their application. • Diamines and thioureas: diamines and thioureas are very effective accelerators in curing of halobutyl rubbers. • Peroxides: bromobutyl rubber may be crosslinked with peroxides. For optimal cure, a co-agent is required such as phenyl bismaleimide (HVA-2). Typically, 1.0 to 2.0 phr of dicumyl peroxide and 0.5 to 1.5 phr of HVA-2 will provide an adequate state of cure for both carbon black and clay filled bromobutyl rubber compounds. 18 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 19

7. Vulcanization of brominated isobutylene-co-para-methylstyrene (BIMSM)

BIMSM elastomers display many properties superior to those ZnO + Stearic Acid > Zn(stearate)2 + Zn(stearate)(OH) + H2O (2) found in general-purpose rubbers or other isobutylene-based polymers. For example, BIMSM shows reduced impermeability, Generation of a Lewis acid results and is believed to be the increased heat stability, increased ozone resistance, increased step controlling the induction period or compound scorch oxidative stability and potentially higher tack levels when time. There is also an increase in the concentrations of Zn(OH) compared to halobutyl rubber25. Figure 22 shows a typical Br, Zn(Br)(St), and ZnBr2 (equation 3). schematic of a BIMSM crosslink.

Zn(St)2 Figure 22 Zn(OH)(St) Schematic of BIMSM crosslink + ZnBr2 + ZnBrSt + ZnBr(OH) St = Stearic Acid (3)

CH2Br CH2St CH3 CH3 A third step involves formation of crosslink networks through CH2 C CH2 CH CH2 C CH2 CH alkylation between benzyl bromide groups and formation of 3 CH3 ZnO CH hydrogen bromide. Hydrogen bromide continues to react with Stearic acid zinc oxide to form additional amounts of Lewis acids (equation R 4)26. CH2 CH2Br

2 R = -‐‑ CH Br CH3 -‐‑ CH3 CH CH2 C CH2 3 Unlike bromobutyl rubber, BIMSM will display poor cureCH rates and lower cure states with use of only ZnO. However, in many instances, the use of 1.0 phr of ZnO and 2.0 phr of stearic acid This is manifested as an increase in cure state or increase in in a carbon black filled BIMSM compound is sufficient to obtain rheometer torque. The concluding or forth stage in the curing a reasonable cure state. Zinc stearate will also cure BIMSM and process is the maturation or plateau phase of the cure profile. reversion may be minimized by addition of around 1.0 phr of Table IX illustrates the effect of zinc oxide concentration on ZnO. Therefore, BIMSM cure systems that will produce a rheometer induction time, rate of vulcanization, and rheometer vulcanizate with adequate scorch and reversion resistance and maximum torque, as a measure of cure state. Some general good mechanical properties will contain a thiazole such as inferences are: MBTS, sulfur, ZnO and stearic acid. Cure systems where zinc is absent and peroxide systems will not be effective for BIMSM. • Reducing the ZnO concentration to a level below 1.0 phr would result in an excess of benzylic bromide Due to the absence of carbon-carbon double bonds in the • High benzylic bromide has a detrimental effect on aging BIMSM backbone and the presence of the reactive benzylic • Excess stearic acid level results in increases in permeability bromide, the vulcanization chemistry differs from that of other • A molar ratio of ZnO to bromine of 0.9 is preferred isobutylene-based elastomers. Crosslinking of BIMSM involves formation of carbon-carbon bonds via a Friedel-Crafts alkylation reaction catalyzed by zinc bromide. Stearic acid functions as an accelerator of BIMSM vulcanization2,25. Zinc stearate formed as a product of the reaction between zinc oxide and stearic acid, can react with BIMSM and displace benzylic bromine. In the absence of stearic acid cure rates are low.

Several distinct steps in the vulcanization of BIMSM have been identified with zinc oxide26. In a classical curing process, the initial step is formation of zinc salts of stearic acid (equation 2). Model vulcanization systems for butyl rubber and halobutyl rubber manual - 19

Table IX Effect of zinc oxide level on vulcanization of BIMSM 26 Compound 1 2 3 4 5 6

ZnO 0.25 0.50 1.00 2.00 3.00 4.00

Stearic Acid 2.00 2.00 2.00 2.00 2.00 2.00

Induction Time (ODR Rheometer at 160°C)

t2 (minutes) 13.97 11.30 9.77 8.22 6.15 5.85

Cure Rate 3.09 5.85 9.83 11.18 11.26 9.12 (dNm/min)

Mh (dNm) 27.70 43.25 48.22 49.85 49.75 49.80

Table X shows that stearic acid has a significant effect on induction times and cure rates. Although zinc stearate can vulcanize BIMSM, such compounds tend to display short scorch times. In the absence of zinc oxide, hydrogen bromide, which is a strong acid and is formed in the alkylation reaction, can react with zinc stearate to form the more stable ZnBr2.

Table X Effect of stearic acid on curing of BIMSM 26 Compound 1 2 3 4 5 6

Stearic Acid 0.5 1.0 2.0 3.0 4.0 5.0

ZnO 1.0 1.0 1.0 1.0 1.0 1.0

Induction Time (ODR Rheometer at 160°C)

t2 (minutes) 60.9 31.7 9.5 7.1 6.2 5.2

Cure Rate 1.6 3.4 9.8 9.5 7.3 4.4 (dNm/min)

Mh (dNm) 52.2 54.2 49.2 47.2 52.5 29.1

Stearic acid is then reformed as shown in equation 5. (5)

Z n B r 2 Z n B r S t + H B r R Z n ( O H ) B r R = St, OH, Br, Ph -‐-‐ C H 2 C H 2 B r

An increase in stearic acid levels can cause lubricating effects with a drop in apparent viscosity and compound stiffness as measured using an oscillating disc rheometer. However, reversion may also occur due to the increase in bromine released via a free radical cleavage from the polymer chain. This may be adjusted by increasing the zinc oxide level in the compound. 20 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 21

8. Model cure system formulary

Isobutylene-based polymers will employ a variety of cure Halobutyl rubber systems depending on the requirements of the final cured Table XII displays a set of model cure systems for halobutyl product. For example, tire innerliners require good resistance elastomers. These include a resin, sulfur-accelerator based to fatigue, tear strength, and adhesion to adjacent system for a bromobutyl rubber radial tire innerliner, and a low components in the tire. Therefore, sulfur-based systems with temperature cure system possibly suitable for pipe seals in their co-curing ability, adequate fatigue and flex life are inaccessible locations. Again, these cure systems may provide preferred for liner compounds. Curing bladders, which a starting point for further development. operate in high temperature environments and undergo many cycles, must have stable crosslinks and not be susceptible to reversion or other oxidative processes. In such instances, resin based cure systems are preferred.

Butyl rubber Table XI illustrates quinone, resin, and sulfur-accelerator based model cure systems for butyl rubber. These cure systems may provide a starting point for further development.

Table XI Model cure systems for butyl rubber 1,2 Sulfur Cure System Quinone Resin Resin Accelerator

Units phr phr phr phr

Electrical Curing Curing Application Cable Tube Bladder Bladder Cover

ZnO 5.0 5.0 5.0 5.0

MgO 2.0 - - -

Stearic Acid - - 1.0 2.0

Sulfur - - - 2.0

MBTS - - - 0.5

TMTD - - - 1.0

Polychloroprene - 5.0 - -

Resin (a) - 10.0 - -

Resin (b) - - 12.0 -

Benzoquinone 2.0 - - - Dioxime

Total phr 9.0 20.0 18.0 10.5

Note: Resin (a) is a heat reactive octylphenol-formaldehyde resin Resin (b) is a brominated octylphenol-formaldehyde heat reactive resin Model vulcanization systems for butyl rubber and halobutyl rubber manual - 21

Table XII Model cure systems for halobutyl rubber 2,3 Sulfur Sulfur Zinc Oxide & Room Cure System Peroxide3 Accelerator2 Sulfenamide2 Resin3 Temperature Cure3

Units phr phr phr phr

Application Innerliner Polymer Blends Pharmaceutical Sheeting Steam Hose

ZnO 3.0 2.0 3.0 5.0 -

ZnCl2 - - - 2.0 -

Stearic Acid 1.0 1.0 1.0 - -

Sulfur 0.5 1.0 - - -

MBTS 1.5 - - - -

TBBS - 1.5 - - -

Resin (a) - - 2.0 - -

Stannous Chloride (SnCl2) - - - 2.0 -

Dicumyl peroxide (DiCup) - - - - 2.0

HVA-2 - - - - 1.0

Total phr 6.0 5.5 6.0 9.0 3.0

Note: Resin (a) is a heat reactive octylphenol-formaldehyde resin

BIMSM elastomers Table XIII presents a set of five cure systems for BIMSM.

Table XIII Model cure systems for BIMSM elastomer2 Ultra Fast Sulfur Cure System Metal Oxide Accelerator Resin Amine Accelerator System

Units phr phr phr phr

Application Damping Device Innerliner Engineerd Products Curing Bladder Engineerd Products

ZnO 2.0 1.0 1.0 1.0 1.0

Zinc Stearate 3.0 - - - -

Stearic Acid - 2.0 2.0 2.0 2.0

Sulfur - 1.0 - 1.5 -

MBTS - 2.0 - 1.5 -

ZDEDC - - 1.0 - -

Triethylene Glycol - - 2.0 1.0 -

Resin (a) - - - 5.0 -

DPPD - - - - 0.5

Total phr 5.0 6.0 6.0 12.0 3.5

Note: Resin (a) is a heat reactive octylphenol-formaldehyde resin 22 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 23

Table XIV illustrates a selection of five model compound formulas one each for butyl rubber, chlorobutyl rubber, bromobutyl rubber, and two for BIMSM elastomers. These model compound formulas illustrate typical properties obtained by utilizing experimental sulfur-accelerator vulcanization systems.

Table XIV Effect of cure system type on fundamental properties of isobutylene based elastomers 1 2 3 4 5 Model BIMSM with Model Butyl Model Chlorobutyl Model Bromobutyl BIMSM Compound Zinc Stearate Compound Compound compound

Master Batch (phr) (phr) (phr) (phr) (phr)

BIMSM 100.0 100.0 - - -

Butyl 268 - - 100.0 - -

CIIR 1066 - - - 100.0 -

BIIR 2222 - - - - 100.0

Carbon Black N660 60.0 60.0 70.0 60.0 60.0

Naphthenic Oil 8.0 8.0 - 8.0 8.0

Paraffinic Oil - - 25. - -

Aromatic & Aliphatic 7.0 7.0 - 7.0 7.0 Hydrocarbon Resin Blend

Phenolic Tackifiying Resin 4.0 4.0 - 4.0 4.0

Hydrocarbon Resin - - 3.0 - -

Stearic Acid 1.0 1.0 1.0 1.0 1.0

MgO - - - 0.2 0.2

Final Batch (phr) (phr) (phr) (phr) (phr)

Sulfur 0.5 0.5 2.0 0.5 0.5

MBTS 1.5 1.5 2.0 1.5 1.2

Zinc Oxide 1.0 - 5.0 1.0 1.0

Zinc Stearate - 2.0 - - -

Zinc dibutylphosphorofi- - - 2.0 - - thiate (ZBPD)

Total (phr) 183.0 184.0 210.0 183.2 182.9

Typical Results (phr) (phr) (phr) (phr) (phr)

Mooney viscosity (ML1+4 69.0 65.0 45.0 55.0 56.0 at 100°C)

Rheometer (MDR) at 160°C, 0.5° arc

Delta torque (dNm) 7.3 2.3 7.3 3.1 3.5

tc10 minutes 2.5 4.4 - 0.9 2.2

tc90 minutes 6.1 4.4 25.0 3.3 12.8

Cure Rate Index 27.8 33.3 4.0 41.7 9.4

Tensile Strenght (MPa) 10.6 9.1 11.1 9.2 9.6

Elongation (%) 565 817 665 868 837 Model vulcanization systems for butyl rubber and halobutyl rubber manual - 23

300% Modulus (MPa) 10.6 9.1 11.1 9.2 9.6

Hardness (Shore A) 61 58 49 51 47

Tear Strenght (KN/m) 58.0 60.0 39.0 56.0 54.0

ExxonMovil Chemical Company Data

Isobutylene-based elastomers are highly saturated and therefore have traditionally required ultra-fast accelerators such as thiurams (TMTD) and dithiocarbamates (ZDMC) for effective cure. Though a variety of replacements for accelerators such as TMTD and ZDMC used in general- purpose rubbers have been evaluated, no direct replacement candidates have emerged. Regardless of the cure system being used in a composite such as a tire, vulcanization rates between adjacent components at the cure temperature of the tire must be similar. Figure 23 illustrates two incompatible cure rates and while the individual components have good mechanical properties, the composite could separate since good interfacial adhesion is not created during cure.

Figure 23 Cure rate compatibility 27

Interface Creation Compound B t90 (time to 90% cure)

Compound A t90 (time to 90% cure)

Compound B t10 (time to 10% cure) Rheometer Torque Rheometer

Compound A t10 (time to 10% cure)

Induction Time Time (minutes)

In conclusion, a new cure system must display the following requirements:

• Adequate induction times • High temperature reversion resistance (i.e., temperatures above 160°C) • Allow adequate adhesion between adjacent components in a composite product

The model cure systems, which provide starting points in new development programs, when used in experimental compounds should be evaluated against the mission profile for which the end-product is intended. 24 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 25

9. Summary

The cure chemistry of isobutylene-based elastomers appears more complex than that for general-purpose rubbers. The backbone of isobutylene-based elastomers are highly saturated, thus ultra-fast accelerators are required for efficient vulcanization. Many of those accelerators are nitrosamine generators, and therefore their use is not recommended. Cure systems typically used for isobutylene-based elastomers tend to be based on

• Resins cures • Sulfur based accelerated curing • Use of metal oxides such as zinc oxide • Dioxime based systems

The selection of a specific system is a function of the polymer type and the specific in-service requirements of the final cured product. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 25

APPENDIX 1 Recognized industry abbreviations for accelerators Abbreviations or commercial Chemical Name Function Descriptions

CBS N-Cyclohexyl-2-benzothiazolesulfenamide Accelerator

BCI-MX 1,3-Bis (citraconimidomethyl) benzene Rversion Resistance

CTP, PVI N-(Cyclohexylthio) phthalmide Retarder

DBQDO Dibenzyl-p-quinone dioxime Quinone Cure

DCBS Dicyclohexylbenzothiazole sulfenamide Accelerator

DETU Diethylthiourea Accelerator

DBTU Dibutylthiourea Accelerator

DOTG Di-o-tolylguandine Accelerator

DPG Diphenyl guanidine Accelerator

DPPD N,N’-Diphenyl-p-phenylenediamine Anti-Oxidant

DPTU N,N’-Diphenylthiourea Accelerator

DTDM 4,4-Dithiodimorpholine Vulcanizing Agent

ETU Ethylenethiourea Accelerator

HTS Hexamethylene-1,6-bis (thiosulfate) Disodium Salt, Dihydrate Reversion Resistance

MBS 2-(4-Morpholinothio- benzothiazole Accelerator

MBT Mercaptobenzothiazole Accelerator

MBTS Mercaptobenzothiazole sulfenamide Accelerator

QDO p-Quinone dioxime Quinone Cure

TBBS tert-Butyl-2-benzothiazole sulfenamide Accelerator

TBSI N-t-Butyl-2-benzothiazole sulfenamide Accelerator

TBzTD Tetrabenzylthiuram disulfide Accelerator

TMTD Tetramethylthiuram disulfide Accelerator

TMTM Tetramethylthiuram monosulfide Accelerator

Vultac 2 Amylphenol disulfide polymer (23% Sulfur) Sulfur Donor

Vultac 3 Amylphenol disulfide polymer (28% Sulfur) Sulfur Donor

Vultac 5 Amylphenol disulfide polymer (18% Sulfur) Sulfur Donor

Vultac 7 Amylphenol disulfide polymer (30% Sulfur) Sulfur Donor

Vultac 710 Amylphenol disulfide polymer (27% Sulfur, 10% HSt) Sulfur Donor

ZDMC Zinc dimethyldithiocarbamate Accelerator

ZDEC Zinc diethyldithiocarbamate Accelerator

ZBDC Zinc dibutyldithiocarbamate Accelerator

ZBEC Zinc dibenzyldithiocarbamate Accelerator

ZIX Zinc isopropyl xanthate Accelerator (low temp.)

ZBPD Zinc dibutylphosphorodithioate Accelerator

ExxonMobil Chemical Company Data 26 - Model vulcanization systems for butyl rubber and halobutyl rubber manual Model vulcanization systems for butyl rubber and halobutyl rubber manual - 27

10. References

1. EN Kresge, RH Schatz, HC Wang. Isobutylene Polymers. 16. Atofina Chemicals. Technical Bulletin. ‘Vultac Rubber Encyclopedia of Polymer Science and Engineering. Volume Chemicals’. 1996. nd 8. 2 edition. P 423 – 448. John Wiley and Sons, Inc 1987. 17. DD Flowers, JV Fusco, DS Tracey. Advancements in new 2. WH Waddell, AH Tsou. Butyl Rubber. In ‘Rubber Com- tire sidewalls with a new isobutylene based copolymer. pounding, Chemistry and Applications’. Ed MB Rodgers. Rubber World, March. 1994 Marcel Dekker, Inc. New York, 2004. 18. Atofina Product Literature. Vultac 3 in Chlorinated Butyl 3. JV Fusco, P Hous. Butyl and Halobutyl Rubbers. In Rubber Rubber Innerliners rd Technology, 3 Edition. Editor M Morton. Van Nostrand 19. RL Zapp. Rubber Chemistry and Technol. Vol 46(2) Page Reinhold, 1987. 251-274. 1973 4. Technical Bulletin. Nitrosamine Solutions. Akrochem 20. A Tinker. Crosslink Distribution and Interfacial Adhesion in Corporation. 2003. Vulcanized Blends of Natural Rubber and Nitrile (NBR). 5. T Kuhlmann. A new thiuram acceleration without danger- Rubber Chemistry and Technol. Vol 63, Page 503 – 515, ous and volatile nitrosamines. Kautschuk Gummi Kunstst- 1990 offe, Volume 42, P 878-9. 1989. 21. JC Scott, GDF White, DJ Thom, RA Whitney, W Hopkins. J 6. RW Layer, DW Chasar. Minimizing Nitrosamines Using Polymer Science. Part A, Polymer Chemistry. Vol 41. P Sterically Hindered Thiuram Disulfides/Dithiocarbamates. 1915-1926. 2003 Rubber Chem & Technol. Vol 67(2), P 299-313 22. I Kuntz, RL Zapp, RJ Panchrov. The Chemistry of the Zinc 7. WH Waddell, MB Rodgers. Rubber Compounding. In Oxide Cure of Halobutyl. Rubber Chemistry and Technol. Kirk-Othmer Encyclopedia of Chemical Technology, 5th Vol 57(4) P813 - 825. 1984. Edition. 2004 23. AY Coran. Vulcanization. In Science and Technology of nd 8. RN Datta, AJ deHoog, JH Wilbrink, WF Verhelst, MS Rubber. 2 Edition. Ed. JE Mark, B Erman, FR Eirich, Ivany. Perkalink 900: Reversion Resistant Tire Compound- Academic Press. 1994. ing Without Compromise. International Tire and Exhibition 24. R Vukov. Zinc oxide crosslinking chemistry of halobutyl Conference paper 18A. 1994 elastomers, A model compound approach. Rubber 9. WH Waddell, MB Rodgers. The Science of Rubber Chemistry and Technology, Vol 57(2). Page 284 – 290. Compounding. In Science and Technology of Rubber, 3rd 1984 Edition. John Wiley and Son, NY. 2005. (in press) 25. AT Tsou, WH Waddell. Isobutylene-based Elastomers. th 10. JA Dean. Lange’s Handbook of Chemistry. 14 Edition. Presented at a Meeting of The American Chemical Society McGraw-Hill. New York. 1992 Rubber Division, Grand Rapids, MI. 2004 11. JJ Higgins, FC Jagisch, NE Stucker. Butyl Rubber and 26. D Xie, HC Wang, MW Johnston. FTIR Spectroscopic Polyisobutylene. In Handbook of Pressure Sensitive Studies on Curing of Brominated Poly(Isobutylene-co- Adhesive Technology, 2nd Edition. Editor D. Satas. Van 4-Methylestyrene). Presented at a Meeting of the Ameri- Nostrand Reinhold. 1989. can Chemical Society Rubber Division, San Francisco. 12. B Struck. Tackifying, Curing, and Reinforcing Resins. In 2003. Rubber Technology Compounding and Testing for 27. W.H. Waddell, M.B. Rodgers. Tire Applications of Elasto- Performance. Edited by J.S. Dick, Hanser, Cincinnati 2001. mers 2. Casing 13. JV Fusco, P Hous. Butyl and Halobutyl Rubbers. In the Presented at a Meeting of the American Chemical Society, Vanderbilt Rubber Handbook. 13th Edition. RT Vanderbilt Rubber Division, Grand Rapids, MI, 2004. Company. Norwalk CT. 1990. 14. L Batman, CG Moore, M Porter, B Saville. Chemistry of Vulcanization. In Chemistry and Physics of Rubber Like Substances. (Chapter 15, p465). Maclaren Press, London. 1963. 15. MP Ferrandino, JA Sanders, SW Hong. Tetrabenzylthiuram disulfide: A Secondary Accelerator for Stable Crosslink Systems in Tire Applications. Presented at a meeting of the American Chemical Society Rubber Division, Philadelphia. 1995. Model vulcanization systems for butyl rubber and halobutyl rubber manual - 27 ©2017 ExxonMobil. ExxonMobil, the ExxonMobil logo, the interlocking “X” device and other product or service names used herein are trademarks of ExxonMobil, unless indicated otherwise. This document may not be distributed, displayed, copied or altered without ExxonMobil’s prior written authorization. To the extent ExxonMobil authorizes distributing, displaying and/or copying of this document, the user may do so only if the document is unaltered and complete, including all of its headers, footers, disclaimers and other information. You may not copy this document to or reproduce it in whole or in part on a website. ExxonMobil does not guarantee the typical (or other) values. Any data included herein is based upon analysis of representative samples and not the actual product shipped. 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