Model vulcanization systems for butyl rubber and halobutyl

Model vulcanization systems for butyl rubber and halobutyl rubber manual

Country name(s)

Exxon™ butyl and halobutyl rubber

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


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.


2. Composition of isobutylene polymers

Butyl rubber (IIR) is the copolymer of isobutylene and a small amount of isoprene, typically in the order of 2%. Figure 1 is a schematic of butyl rubber.

Figure 1Structure of butyl rubber

Chlorobutyl (CIIR) and bromobutyl (BIIR) rubbers are manufactured similarly to butyl rubber, but with an added subsequent halogenation step. Due to the nearly equivalent reactivity ratios of the monomers and low concentration of isoprene, the isoprenyl units in these polymers are randomly distributed along the polymer chain.

Figure 2 illustrates the structures of the functional halogen 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 50%-60%, followed by Structure I representing from 30%-40%. Approximately 5%-15% is Structure III; Structure IV is typically only 1%-3%.

Figure 2Structure of isoprenyl units in bromobutyl rubber

Copolymers of isobutylene and para-methylstyrene have a random structure, again due to the monomer reactivity ratios being nearly equivalent. After polymerization and subsequent bromination some of the para-methylstyrene groups are converted to reactive bromomethylstyryl groups. These saturated backbone polymers contain isobutylene, 1 to 5 mole-% of para-methylstyrene, and 0.5 to 1.3 mole-% of brominated para-methylstyrene (Figure 3)2. Their glass transition or second order transition temperature increases with increasing para-methylstyrene content and is nominally around -57oC. This compares to a nominal -59°C to -60°C for halobutyl polymers.

Figure 3Structure of brominated isobutylene-co-para-methylstyrene, BIMSM 2 Table I illustrates the commercial grades of isobutylene-based

elastomers and their typical compositions that are currently available from ExxonMobil Chemical company.

C

CH3

CHCH2 CH2 C CH

CH2

CH2

Br

CH2

C CH

CH2Br

CH2 CH2 C CH

CH3

CH CH2

Br

Structure IStructure II

Structure III Structure IV

Butyl Rubber (IIR)

(0.8 - 2.5 mole%)-CH2 C

3CH

3CH2CH 2-C=CH-CHCH3

currentpoint 192837465

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0 aR aL m 180 aA s 180 aA a arc cp fill}b/OA{np0 cw -2 dv mv aL 0 aR aL m 180 aA s 180 arc 0 cw -2 dv rlineto cp fill}b/SA{aF m lW m/aL x aL 1 aR s m np 0 p mv rad 0 p l gs cm sm st gr}b/CA{aF lW m/aL x aL 1 aR s m 2 dv rad dp m o dp m s dp 0 le{pppp pp}{sqrt at 2 m np rad 0 rad 180 6 -1 r s 180 6 -1 r s arc gs cm sm st gr cpt e at ro}ie}b/AA{np rad 0 rad 180 180 6 -1 r a arc gs cm sm st gr}b/RA{lW m/w x np rad w p mv w w p l rad w n p mv w w n p l w 2 m dp p mv 0 0 pl w 2 m dp n p l st}b/HA{lW m/w x np 0 0 p mv w 2 m dp p l w 2 m w p l rad w p l rad w n p l w 2 m w n p l w 2 m dp n p l cp st}b/Ar1{gs 5 1 r 3 ix 3 ix xl 3 -1 r s 3 1 r e s o o at ro dp m e dp m a sqrt/rad x[{2.25 SA DA}{1.5 SA DA}{1SA DA}{cw 5 m sl 3.375 SA DA}{cw 5 m sl 2.25 SA DA}{cw 5 m sl 1.5 SA DA}{270 CA DA}{180 CA DA}{120 CA DA}{90 CA DA}{3 RA}{3 HA}{1 -1 sc 270 CA DA}{1 -1 sc 180 CA DA}{1 -1 sc 120 CA DA}{1 -1 sc 90 CA DA}{6RA}{6 HA}{dL 2.25 SA DA}{dL 1.5 SA DA}{dL 1 SA DA}{2.25 SA OA}{1.5 SA OA}{1 SA OA}{1 -1 sc 2.25 SA OA}{1 -1 sc 1.5 SA OA}{1 -1 sc 1 SA OA}{270 CA OA}{180 CA OA}{120 CA OA}{90 CA OA}{1 -1 sc 270 CA OA}{1-1 sc 180 CA OA}{1 -1 sc 120 CA OA}{1 -1 sc 90 CA OA}{1 -1 sc 270 AA}{1 -1 sc 180 AA}{1 -1 sc 120 AA}{1 -1 sc 90 AA}]e g exec gr}b/ac{arcto 4{pp}repeat}b/pA 32 d/rO{4 lW m}b/Ac{0 0 px dp m py dp m a sqrt 0 360 arc cm sm gs sg fill grst}b/OrA{py px at ro px dp m py dp m a sqrt dp rev{neg}if sc}b/Ov{OrA 1 0.4 sc 0 0 1 0 360 arc cm sm gs sg fill gr st}b/Asc{OrA 1 27 dv dp sc}b/LB{9 -6 mv 21 -10 27 -8 27 0 cv 27 8 21 10 9 6 cv -3 2 -3 -2 9 -6 cv cp}b/DLB{0 0 mv -4.8 4.8 l-8 8 -9.6 12 -9.6 16.8 cv -9.6 21.6 -8 24.6 -4.8 25.8 cv -1.6 27 1.6 27 4.8 25.8 cv 8 24.6 9.6 21.6 9.6 16.8 cv 9.6 12 8 8 4.8 4.8 cv cp}b/ZLB{LB}b/Ar{dp 39 lt{Ar1}{gs 5 1 r o o xl 3 -1 r e s 3 1 r s e o 0 lt o 0 lt ne/rev xdp 0 lt{1 -1 sc neg}if/py x dp 0 lt{-1 1 sc neg}if/px x np[{py 16 div dup 2 S lt{pp 2 S}if/lp x lp 0 p mv 0 0 p l 0 py p l lp py p l px lp s 0 p mv px 0 p l px py p l px lp s py p l cm sm st}{py 16 div dup 2 S lt{pp 2 S}if/lp x lp 0 p mv 0 0 0 py lp ac0 py 2 dv lp neg o lp ac 0 py 2 dv 0 py lp ac 0 py lp py lp ac px lp s 0 p mv px 0 px py lp ac px py 2 dv px lp a o lp ac px py 2 dv px py lp ac px py px lp s py lp ac cm sm st}{py dp 2 dv py 180 pA s 180 pA a arc st np px py s py 2 dvpy pA dp neg arcn st}{0 0 p mv 0 py p l px py p l px 0 p l cp cm sm st}{px lW 2 dv a lW -2 dv p mv rO dp rlineto px lW 2 dv a rO a py lW 2 dv a rO a p l rO lW -2 dv a py lW 2 dv a rO a p l lW -2 dv py lW 2 dv a p l 0 py p l px py p l px 0 p l cp fill0 0 p mv 0 py p l px py p l px 0 p l cp cm sm st}{0 rO p mv 0 py px py rO ac px py px 0 rO ac px 0 0 0 rO ac 0 0 0 py rO ac cp cm sm st}{rO py p mv rO rO xl 0 py px py rO ac px py px 0 rO ac px 0 0 0 rO ac rO neg dp xl px py 0 py rO accp fill 0 rO p mv 0 py px py rO ac px py px 0 rO ac px 0 0 0 rO ac 0 0 0 py rO ac cp st}{1.0 Ac}{0.5 Ac}{1.0 Ov}{0.5 Ov}{Asc LB gs 1 sg fill gr cm sm st}{Asc LB gs 0.5 sg fill gr cm sm st}{Asc LB gs 0.5 sg fill gr gs cm sm st grnp -1 -1 sc LB gs 1 sg fill gr cm sm st}{Asc LB gs 0.5 sg fill gr gs cm sm st gr np -0.4 -0.4 sc LB gs 1 sg fill gr cm sm st}{Asc LB gs 1 sg fill gr gs cm sm st gr np -0.4 -0.4 dp sc LB gs 0.5 sg fill gr cm sm st}{Asc DLB -1 -1 sc DLB gs 1 sg fill grgs cm sm st gr np 90 ro DLB -1 -1 sc DLB gs 0.5 sg fill gr cm sm st}{Asc gs -1 -1 sc ZLB gs 1 sg fill gr cm sm st gr gs 0.3 1 sc 0 0 12 0 360 arc gs 0.5 sg fill gr cm sm st gr ZLB gs 1 sg fill gr cm sm st}{Asc gs -1 -1 sc ZLB gs 0.5 sgfill gr cm sm st gr gs 0.3 1 sc 0 0 12 0 360 arc gs 1 sg fill gr cm sm st gr ZLB gs 0.5 sg fill gr cm sm st}{0 0 p mv px py p l cm sm st}{gs bW 0 ne{bW}{5 lW m}ie sl 0 0 p mv px py p l cm sm st gr}{gs dL 0 0 p mv px py p l cm sm st gr}{OrA 1 16 dv dp sc0 1 p mv 0 0 1 0 1 ac 8 0 8 -1 1 ac 8 0 16 0 1 ac 16 0 16 1 1 ac cm sm st}]e 39 s g exec gr}ie}b/Cr{0 360 np arc st}b/DS{np p mv p l st}b/DD{gs dL DS gr}b/DB{gs 12 OB bW 0 ne{bW}{2 bd m}ie sl np 0 0 p mv 0 p l st gr}b/ap{e 3 ix ae 2 ix a}b/PT{8 OB 1 sc 0 bd p 0 0 p 3 -1 r s 3 1 r e s e 0 0 p mv 1 0 p l 0 0 p ap mv 1 0 p ap l e n e n 0 0 p ap mv 1 0 p ap l pp pp}b/DT{gs np PT cm sm st gr}b/Bd{[{pp}{[{DS}{DD}{gs 12 OB np bW 0 ne{bW 2 dv/bd x}if dp nH dv dp 3 -1 ro 2 dv s{dp bd p mv bd n p l}for st gr}{gs 12 OB 1 sc np bW 0 ne{bW 2 dv/bd x}if 1 1 nH 1 s{nH dv dp bd m wF m o o p mv n p l}for cm sm st gr}{pp}{DB}{gs 12 OB np 0 lW 2 dv o o n p mv p l bW 0 ne{bW 2 dv}{bd}ie wF m o o p l n p lcp fill gr}{pp}{gs 12 OB/bL x bW 0 ne{bW 2 dv/bd x}if np 0 0 p mv bL bd 4 m dv round 2 o o lt{e}if pp cvi/nSq x bL nSq 2 m dv dp sc nSq{.135 .667 .865 .667 1 0 rcurveto .135 -.667 .865 -.667 1 0 rcurveto}repeat cm sm st gr}]o 1 g 1 s g e 2 4 gi al pp5 -1 r exec}{al pp 8 ix 1 eq{DD}{DS}ie 5 -1 r 2 eq{DB}{DS}ie pp}{2 4 gi al pp DT}]o 0 g g exec}b/CS{p mv p l cw lW cW 2 m a sl sp sl}b/cB{12 OB 0 0 p mv 0 p l cm sm cw bW 0 ne{bW}{bd 2 m}ie cW 2 m a sl sp sl}b/CW{12 OB 1 sc cW lW 2 dva 0 o p mv 0 e n p l bW 0 ne{bW 2 dv}{bd}ie wF m cW a 1 o n p l 1 e p l cp cm sm}b/CB{np[{[{CS}{CS}{cB}{CW}{pp}{cB}{CW}{pp}{cB}]o 1 g 1 s g e 2 4 gi al pp 5 -1 r exec}{al pp p mv p l CS pp pp}{2 4 gi al pp PT cm sm cw cW 2 m sl sp sl}]o0 g 1 s g exec clip}b/Ct{bs rot g bs rot g gs o CB CB 1 setgray clippath fil l 0 setgray Bd gr}b/wD 18 dict d/WI{wx dx ne{wy dy s wx dx s dv/m1 x wy m1 wx m s/b1 x}if lx ex ne{ly ey s lx ex s dv/m2 x ly m2 lx m s/b2 x wx dx ne{b2 b1 s m1 m2 s dv}{wx}iedp m2 m b2 a}{ex n dp m1 m b1 a}ie}b/WW{gs wD begin bs e g 2 4 gi al pp o o xl 4 -1 r 3 -1 r s/wx x s/wy x bs e g 2 4 gi al pp 4 -1 r 3 -1 r s/lx x s/ly x 0 bW 2 dv wF m o o wy wx at mt ro tr/dy x/dx x ly lx at mt ro tr n/ey x n/ex x np wxwy p mv WI p l ex n/ex x ey n/ey x dx n/dx x dy n/dy x lx ly p l WI p l cp fill end gr}b/In{px dx ne{py dy s px dx s dv/m1 x py m1 px m s/b1 x}if lx 0 ne{ly lx dv/m2 x ly ey s m2 lx ex s m s/b2 x px dx ne{b2 b1 s m1 m2 s dv}{px}iedp m2 m b2 a}{ex n dp m1 m b1 a}ie}b/BW{wD begin bs e g/wb x bs e g/bb x wb 4 g/cX x wb 5 g/cY x bb 4 g cX eq bb 5 g cY eq and{bb 2 g bb 3 g}{bb 4 g bb 5 g}ie cY s/ly x cX s/lx x/wx wb 2 g cX s d/wy wb 3 g cY s d 0 bW 2 dv ly lx at mt ro tr/ey x/ex x0 bW 2 dv wF m wy wx at mt ro tr/dy x/dx x 0 lW 2 dv wy wx at mt ro tr wy a/py x wx a/px x gs cX cY xl np px py p mv In p l lx ex s ly ey s p l ex n/ex x ey n/ey x dx n/dx x dy n/dy x wx 2 m px s/px x wy 2 m py s/py x lx ex s ly ey s p lIn p l px py p l cp fill gr end}b/Db{bs{dp type[]type eq{dp 0 g 2 eq{gs dp 1 g 1 eq{dL}if 6 4 gi al pp DS gr}{dp 0 g 3 eq{2 4 gi al pp DT}{pp}ie}ie}{pp}ie}forall}b/I{counttomark dp 1 gt{2 1 rot{-1 r}for}{pp}ie}b/DSt{o/iX x dp/iY x o/cXx dp/cY x np p mv counttomark{bs e g 2 4 gi al pp o cX ne o cY ne or{4 1 r 4 1 r}if pp pp o/cX x dp/cY x o iX eq o iY eq and{pp pp cp}{p l}ie}repeat pp st}b/SP{gs/sf x/lW x/bW x/cW x count 7 ge 5 ix 192837465 eq and{ 5 -1 r pp cpt 7 -1 r s e 7 -1 r s e5 -1 r dv neg e 5 -1 r dv neg e cpt xl sc neg e neg e xl}{xl pp pp}ifelse 1 1 S dv dp sc cm currentmatrix pp lW sl 4.0 setmiterlimit np}b end

563 172 99 290 40 80 36 20 chemdict begin SP /bs[[1 1 10540 6500 11000 6500][1 1 11000 6500 11000 7020][2 0 11000 7020 10380 7380 11000 7123 10470 7431 ][1 1 10380 7380 10382 8097][2 0 10382 8097 11004 8454 10471 8045 11004 8351 ][1 1 11004 8454 11624 8094][1 1 7420 6500 6400 6500][2 0 11624 8094 11622 7377 11534 8043 11533 7429 ][1 1 11000 7020 11622 7377][1 1 11004 8454 11004 9020][1 1 8760 6500 8420 6500][1 1 10540 6500 9660 6500][1 9 8760 6500 9660 6500][1 1 3640 6500 4600 6500][1 1 4600 6280 4600 5960][1 1 4600 6500 4600 7080][1 1 4880 6500 5260 6500][1 1 5940 6500 6400 6500][1 1 7960 6500 8420 6500][1 1 8420 6280 8420 5960][1 1 8420 6500 8420 7080][1 1 6400 6500 6400 7020][2 0 6400 7020 5780 7380 6400 7123 5870 7431 ][1 1 5780 7380 5782 8097][2 0 5782 8097 6404 8454 5871 8045 6404 8351 ][1 1 6404 8454 7024 8094][2 0 7024 8094 7022 7377 6934 8043 6933 7429 ][1 1 6400 7020 7022 7377][1 1 6404 8454 6404 9020][1 1 11440 6480 11820 6480][1 9 11820 6480 13180 6480][1 1 3380 6500 3640 6500][1 9 3380 6500 2000 6500]]d [0 11 I 1 2 3 4 5 7 8 11000 7020 DSt [6 I 17 5940 6500 DSt [9 I 11004 9020 DSt [10 I 18 7960 6500 DSt [13 31 I 15 4600 7080 DSt [14 I 4600 5960 DSt [16 I 5260 6500 DSt [19 I 8420 5960 DSt [20 I 8420 7080 DSt [21 I 22 23 24 25 26 27 6400 7020 DSt [28 I 6404 9020 DSt [29 I 11820 6480 DSt 12 30 32 3{bs e g Bd}repeat Db gr end

CH2 CHCH2

CH3

CH2 CCH3

CH3

CCH2 CH

CH2Br

CH3

CH3


Table ICommercial grades of ExxonMobil isobutylene-based elastomers

Elastomer Grade Mooney Viscosity(ML1+8 at 125°C)

Isoprene(mol%)

para-Methylstyrene(wt%)

Halogen Halogen(wt%)

Halogen(mol%)1

Typical Applications

Butyl(low viscosity)

065165365

323233

1.051.502.30

Tubes, BalddersTubes, BalddersTubes, Baldders

Butyl(medium viscosity)

068268

5151

1.151.70

Tubes, BalddersTubes, Baldders

Butyl(high viscosity)

269 57 1.70 Tubes, Baldders

Chorobutyl 1066 38 1.95 CI 1.26 Tire InnerlinersWhite Sidewalls

Bromobutyl 22222235225522112244

3239463246

1.701.701.701.701.70

BrBrBrBrBr

2.002.002.002.102.10

Automobile TiresAutomobile TiresTruck Tire InnerlinersFast Cure ApplicationsFast Cure Applications

Exxpro 303534333745

453545

5.005.007.50

BrBrBr

0.470.751.20

Curing BladdersPharmaceuticalsTires, 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.


3. General trends in vulcanization chemistry

Five factors influence the development of vulcanization system technology:

• Elimination of accelerators that may generate nitrosamines• Accelerators for improved scorch resistance and improved

ease of compound processing during final product assembly, e.g. a tire

• Vulcanization systems suitable for high-temperature curing with associated benefits in productivity

• Improved reversion resistance• Improvement in cured compound attributes such as

mechanical or dynamic properties

NitrosaminesNitrosamines are nitrogen-based compounds with attached carbon-containing groups. Using ammonia as a reference, replacing one of the hydrogen atoms produces a primary amine. Substitution of two or three hydrogen atoms produces secondary or tertiary amines, respectively. As shown in equation 1, secondary amines found in accelerators such as thiurams, dithiocarbamates, and some sulfenamides can react with nitrogen oxides in the air to produce a nitrosamine.

(1)

During the vulcanization process accelerators such as tetramethylthiuram disulfide (TMTD) are partly decomposed to their parent secondary amines. Reaction with atmospheric nitrogen oxide can lead to N-nitrosamine generation. In Germany, N-nitrosamine levels in the work place atmosphere are controlled under factory regulation TRGS 552. This directive permits a maximum atmospheric concentration of only 1µg/m3 of regulated N-nitrosamines. The cabin space of automotive vehicles now falls under the regulations that have implications for rubber parts in this area, which includes the door and window seals. Automotive vehicle manufactures have also placed increasing emphasis on the removal of N-nitrosamines from all parts of their vehicles including rubber components such as gaskets, engine mounts, and hoses4. Maximum nitrosamine levels are listed in Table II.

Table IINitrosamine limits in pharmaceutical, food utensils4

Location Nitrosamine ppb maximum

Nitrosablesppb maximum

Australia 20 400

Denmark 5 100

Netherlands 1 100

Canada 60 N/A

Switzerland 10 200

Germany 10 200

UK 30 100

USA 60 N/A

European Union directive 93/11/EEC concerns extractable N-nitrosamines for certain food applications. Accelerators that are considered to give rise to N-nitrosamines are listed below (see Appendix 1 for explanation of abbreviations):

TMTD, TMTM, TETD, TBTD, DPTT, DTDM, ZDMC, ZDEC, ZDBC

Tetrabenzylthiuramdisulfide (TBzTD) and zinc dibenzyldithiocarbamate (ZBEC) accelerators, though in some instances might give rise to N-nitrosamines, are considered to be outside the scope of regulatory attention5. The effect of steric factors on thiuram and dithiocarbamate nitrosamine formation has also been studied. Thiurams and dithiocarbamates made from sterically bulky amines such as diisobutylamine produce orders of magnitude lower levels of nitrosamine than does tetramethylthiuram disulfide (TMTD). At the same time, when used as a secondary accelerator for cyclobenzothiazole sulfenamides, tetraisobutylthiuram disulfide has better scorch safety, but vulcanizes at the same rate as TMTD. Uniquely, tetraisobutylthiuram monosulfide could act as a retarder as well as a secondary accelerator6. Accelerators that cannot produce N-nitrosamines since they are based on primary amines or do not contain nitrogen include:

CBS, TBBS, DOTG, DPG, MBT, MBTS, ZMBT, Xanthates

NH

R

RNOx

NR

N OR

Secondary Amine

Nitrogen Oxides

Nitrosamine


Accelerators for isobutylene-based elastomers A broad range of accelerators can be found in formulas for isobutylene-based elastomers. Some of the accelerators found are illustrated in Figure 47. These include cyclohexylbenzothiazole sulfenamide (CBS), mercaptobenzothiazole sulfide (MBTS), tetramethylthiuram disulfide (TMTD), diphenyl guanidine (DPG), and zinc dimethyldithiocarbamate (ZDMC).

Figure 4Structure of selected accelerators7

Improved scorch resistance is currently achieved by addition of magnesium oxide (MgO). In some instances, with a sulfenamide-based cure system, N-(cyclohexylthio) phthalimide (PVI), whose structure can be found in Figure 5, is used. MBTS can also serve as a pre-vulcanization inhibitor. Calcuim oxide and magnesium oxide can be used in zinc oxide-cured halobutyl rubber. However, they will serve as an activator in amine accelerated systems. Calcium stearate and zinc stearate can act as retarders in chlorobutyl rubber compounds. Unlike their use in general-purpose elastomers, benzoic acid and salicylic acid are not effective as scorch retarders.

Figure 5Structure of PVI

High temperature curing and reversion resistanceAs cure temperature increases, care must be taken to avoid reversion while achieving the desired mechanical properties of the product. This infers no marching modulus and the preservation of a stable crosslink network. Cure systems with primary accelerators such as MBTS, or sulfenamides such as TBBS, tend to meet such requirements. Depending on the cure system thiurams, dithiocarbamates, and xanthates tend to have poorer reversion resistance.

Two reversion resistors have been of particular note. These are bis(citraconimido-methyl) benzene (BCI-MX) and sodium hexamethylene-1,6-bisthiosulfide dehydrate. Both function exclusively as reversion resistors. BCI-MX (Figure 6) is understood to react with the polymer chain to form stable flexible carbon-carbon crosslinks which substitutes for and replaces sulfur crosslinks that disappear during reversion8.

Figure 6Structure of 1,3-bis(citraconimidomethyl)benzene (BCI-MX)

Alternatively, for resistance to high-temperature reversion, sodium hexamethylene-1,6-bisthiosulfide dihydrate (Figure 7) may be evaluated since it breaks down and inserts a hexamethylene-1,6-dithiyl group within a disulfide or polysulfide crosslink providing a hybrid crosslink. During extended vulcanization periods or accumulated heat history due to product service, polysulfidic-hexamethylene crosslinks rearrange to produce thermally stable elastic monosulfidic crosslinks. At levels up to 2.0 phr, there is little effect on compound induction, scorch times, or other compound mechanical properties.

Figure 7

The bifunctional silane-coupling agent, bis-(3-triethoxysilylpropyl) tetrasulfane has dual functions as both a sulfur donor and a reversion resistor9. This ability to maintain the polysulfidic or other stable network throughout the curing process and subsequent product service life while offering reversion

CC

NS NH

SC

NS S

NC

S

CH3

NCH3

C S

S

S C

S

NCH3

CH3 NHC NH

NH

CH3

NCH3

C S

S

Zn S C N

S CH3

CH3

C4H9 O C S

S

S C O

S

C4H9

Zn SS PP S

O

O

S

O

O

C4H9

C4H9

C4H9

C4H9

Mercaptobenzothiazole disulfide

Tetramethylthiuram disulfide Diphenyl guanidine

Zinc DibutylphosphorodithiateDibutyl xanthogen disulphide

Zinc dimethyldithiocarbamate

Cyclohexyl benzothiazole sulfenamide

S N C

C O

O

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

CH2 6

SSSO3 SO3 NaNa 2 H2O

Hexamethylene-1,6-bis(thiosulfate)disodium salt, dihydrate


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

Figure 8EC cure system rheometer profile9


4. Overview of the vulcanization of isobutylene-based elastomers

In butyl rubber, the hydrogen atoms positioned alpha (a) to the carbon-carbon double bond of the isoprenyl unit permits vulcanization into a crosslinked network with sulfur and organic accelerators. The low degree of unsaturation has traditionally required the use of ultra-fast accelerators, such as thiurams or thiocarbamates in order to cure under acceptable conditions of time and temperature. Phenolic resins, bisazidoformates, and quinone derivatives can also be employed. Vulcanization introduces a chemical crosslink approximately every 250 carbon atoms along the polymer chain producing a covalently bonded molecular network. Polysulfidic crosslinks have limited stability at elevated temperature and in many instances can rearrange to form monosulfidic and disulfidic crosslinks This rearrangement results in permanent set of vulcanizates exposed to high temperatures for long periods of time. Resin cure systems such as alkyl phenol-formaldehyde derivatives can provide carbon-carbon crosslinks resulting in heat-stable vulcanizates1,2.

Halobutyl rubber can be crosslinked by some of the same curatives used for butyl rubber such as zinc oxide, bismaleimides and dithiols. Halogens are good leaving groups in nucleophilic substitution reactions due to the low carbon-halogen bond energy (Table III). When zinc oxide is used to crosslink halobutyl rubber, carbon-carbon bonds are formed through dehydrohalogenation which results in a compound with a very stable crosslink system affording a compound having good retention of aged properties and low compression set.

Table IIICarbon-halogen bond energies and bond lengths10

Bond Energy J/mol

Lengthnm

-C — C- 346 0.154

-C — H 413 0.109

-C — F 452 0.138

-C — CI 327 0.177

-C — Br 209 0.194

The bromine-carbon bond in bromobutyl rubber has a lower disassociation energy than does the chlorine-carbon bond in chlorobutyl rubber (Table III). Epoxidized soybean oil (ESBO) can be added to suppress isomerization reactions and HBr formation. Butylated hydroxytoluene (BHT) can be used to stabilize the polymer by serving as an antioxidant. Calcium stearate is added to CIIR, BIIR, and BIMSM as an acid trap to diminish dehydrohalogenation catalyzed by HCl or HBr. Calcium stearate can also act as a cure retarder.

Brominated isobutylene-co-para-methylstyrene (BIMSM) crosslinking involves the formation of carbon-carbon bonds, generally through Friedel-Crafts alkylation chemistry. Diamines, phenolic resins and thiosulfates can also be used to crosslink BIMSM elastomers. The stability of these carbon-carbon bonds combined with the chemically saturated backbone of BIMSM yields excellent resistance to heat and oxidative aging, and to ozone attack.


5. Vulcanization of butyl rubber

The major applications utilizing butyl rubber are tire innertubes, curing bladders, pharmaceutical closures, and a variety of engineered products such as mounts, sheeting, automotive flaps, and damping pads. These applications require good compound aging resistance, oxidative resistance and heat stability. Depending on the application of the final product, one of the following three cure systems have been used11, 12.

• Quinone dioxime • Resin cure • Thiuram (i.e. TMTD) accelerated

Quinone cure systemsThe crosslinking of butyl rubber with para-quinone dioxime (QDO) or para-quinone dioxime dibenzoate (DBQDO) proceeds through an oxidation step that forms the active crosslinking agent para-dinitrosobenzene. The use of metal oxides (such as PbO2, Pb3O4, MnO2) or MBTS as an oxidizing agent, increases the vulcanization rate allowing use in room temperature cures such as in cements3. Figure 9 is a schematic illustrating the reaction of a dioxime in the vulcanization of butyl rubber. The dioxime vulcanization system can be used for dry rubber applications. One example is electrical insulation systems, which contain butyl rubber for improved ozone resistance.

Figure 9Dioxime cure of butyl rubber3

Resin cure systemsPhenol formaldehyde curing resins are classified as resols, i.e. three-dimensional resin systems forming crosslink networks which can serve as reinforcing resins versus novolac resins which are linear. Resin curing of butyl rubber is a function of the reactivity of the phenolmethylol groups in the reactive

phenol formaldehyde resin. Curing resins such as a heat reactive octylphenol formaldehyde resin, which contains methylol groups, can be used3. Table IV contains a model formula illustrating the resin cure system. For a faster cure, addition of a halogen donor such as polychloroprene or stannous chloride is required13. In this instance, polychloroprene has not been considered as part of the total compound polymer content.

Table IVModel curing bladder formulation13

Material phr

Butyl 268 100.0

Polychloroprene 5.0

Carbon black N330 50.0

Castor oil 5.0

Zinc oxide 5.0

Octylphenol formaldehyde resin 10.0

A more reactive resin cure system not requiring an activator is obtained when a portion of the methylol groups are replaced by bromine. An example of this resin is brominated octylphenol formaldehyde resin3.

Figure 10 is a schematic illustrating the reaction of a curing resin (heat reactive octylphenol formaldehyde) in the vulcanization of butyl rubber. Following the elimination of water in the reaction sequence, the exomethylene group and carbonyl oxygen react with an isoprenyl unit in butyl rubber to form a chroman ring. Chromonone ring structures are very stable and are frequently found in natural product biosynthesis.

Figure 10Resin curing of butyl rubber13

N

N

O H

O H

O

N

N

O

O

P b O 2

P b 3 O 4

N O H

N O H

C H

C H

C H

C H

C

C

C H 3

C H 3

C H 2

C H 2

C C H C H 3

C H 2 C H 2

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

N

N

O H

O H

O

N

N

O

O

P b O 2

P b 3 O 4

N O H

N O H

C H

C H

C H

C H

C

C

C H 3

C H 3

C H 2

C H 2

C C H C H 3

C H 2 C H 2

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

OH

R

CH2 R'HO

OH

R

CH2 OH

OH

R

CH2 R'HO

O

R

CH2

OH

R

CH2 R'HO

O

R

CH2

CH CCH3

CH

CH2

H

OH

R

HOCH2 R'

R

CHC

O

CH2

CH3CH2

- H2O heat

Formation of a Chroman ring

R

R'

R

CHC

O

CH2

CH3CH2

OC

CH

CH3CH2

Crosslinked Butyl Network Evolution

CH CH2CCH2

CH3


Thiuram accelerated vulcanizationAlthough thiuram and dithiocarbamate accelerators form nitrosamines, a number of published formulas use these components as vulcanization agents. For completeness, we include the mechanism by which this class of materials is thought to function14; however, again we recommend the use of alternative non-nitrosamine-generating accelerators. Thiuram accelerated systems participate in the vulcanization process through formation of a zinc complex as shown in figure 11. The sulfur rich complexes are formed by insertion of the sulfur into the zinc dimethyldithiocarbamate molecule, which in turn is an active sulfurating complex. Rubber bounded intermediates are subsequently produced followed by crosslinking to form polysulfidic crosslinks. These polysulfidic crosslinks evolve into monosulfide and disulfidic crosslinks.

Figure 11Possible mechanism of thiuram accelerated vulcanization14

In the absence of sulfur, TMTD can function as a sulfur donor (Figure 12). Increasing the rate of cure without a significant loss of other properties can be obtained with the addition of thiurams or thioureas and zinc oxide at around 5 phr. Substitution of nitrosamine generating thiurams such as TMTD with MBTS or higher molecular weight thiurams such as TBzTD or 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.

Figure 12Thiurams as sulfur donors14

Peroxides and butyl rubberPolyisobutylene and butyl rubber undergo degradation rather than crosslinking when treated with peroxides. It is believed that scission occurs via a radical substitution mechanism (Figure 13).

Figure 13Peroxide degradation of polyisobutylene14

Other vulcanization systems for butyl rubber Due to the inherent low reactivity of the nearly saturated butyl backbone, there are instances when a secondary accelerator is required in the vulcanization system. Table V16 contains examples of an amylphenol disulfide polymer secondary accelerator (Vultac series) that can be used.

R H

Zn SSyS

Polymer Chain

C NCH3

CH3

ZnS

S S

SCC NN

CH3

CH3

CH3

CH3

S S

S6

Zn SSx C N

S

C

S

NCH3

CH3

CH3

CH3

C

S

NCH3

CH3R H

Zn SSySC

S

NCH3

CH3

S Zn S Sy C NCH3

CH3

S Sx S

CH3

NCH3

C S

S

S C

S

NCH3

CH3

Zn+2...O-2 Zn+2 O- C

S

NCH3

CH3

+ -S S C

S

NCH3

CH3

CH3

NCH3

C S

S

S C

S

NCH3

CH3

-S S C

S

NCH3

CH3

SSS CC

S S

NNCH3

CH3

CH3

CH3

S C

S

NCH3

CH3

SSS CC

S S

NNCH3

CH3

CH3

CH3

CH3

NCH3

C S

S

Sx S C

S

NCH3

CH3

etc.

Sulfurating Compex

CH2 CC

CH3

CH3

CH2 C CH2

CH3

CH3

CCH2

CH3

CH3

CH3

CH3

CH2

O

C CH3CH3

Polyisobutylene based polymer

Dicumyl peroxide

CH2 C

CH3

CH3

CH2 O C

CH3

CH3New Terminal Chain End

C

CH3

CH3

CH2 C

CH3

CH3

CH2


Table V16

Vultac series of accelerators

Commercial Name

Chemical Name & Function Formula SulfurWt%

SofteningPoint (°C)

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

(C11H16O.Cl2S2)x 23 55


(C11H16O.Cl2S2)x 28 85

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


(C11H16O.Cl2S2)x 30 120

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

(C11H16O.Cl2S2)x 27 85

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

Table VI shows that even though the apparent activation energies of the amylphenol disulfide polymer accelerators can be higher than that of TMTD, the rheometer delta torque, DT, or state of cure which relates to tensile strength, elongation, and modulus is higher particularly for Vultac 3. Note that these accelerators improve interfacial bonding in some cure system blends19. For example, use of amylphenol disulfide polymers in a NR/NBR blend has been investigated. It is suggested that to have good activity the system may require use of up to 4.5 phr of Vultac 320. Although the mechanism of amylphenol disulfide polymer activity in vulcanization remains uncertain, it has been shown that there was a lower degree of preferential vulcanization in polymer blends when amylphenol disulfide 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.

Table VIVulcanization Systems Containing Vultac Accelerators

AcceleratorMDR RheometerΔT (dNm) at 160°C

Apparent Activation Energy, Ea (KJ/mol)

Form

TMTD 4.64 81.77 -

Vultac 2 4.79 79.52 Glass

Vultac 3 7.23 99.96 Glass

Vultac 5 6.66 100.97 Powder

Vultac 7 7.23 92.84 Pellets

Vultac 710 7.22 96.41 Pellets

OH

S

C

S

CH2

CH3 CH3

CH3

OH

S

CCH2

CH3

CH3

CH3

n

S

S

CH

CH

CCH2

CH3

CH3

CCH

C

CH3

CH

CH3CH2

CH3

OH

CH2

Schematic of Phenol Sulfide CrosslinkAmyl Disulfide Polymer


6. Halobutyl rubber

Commercially, chlorobutyl rubber (CIIR) and bromobutyl rubber (BIIR) are the most important derivatives of butyl rubber. Their predominant application is in tire innerliners with the performance of the modern radial tire being impacted by the amount of halobutyl rubber used in the innerliner compound. Typical performance requirements of the tire innerliner are:

• Air retention• Adhesion to tire carcass compound• Resistance to fatigue and cracking• Heat resistance• High tensile and tear strength

Chlorobutyl rubber has an added application in white sidewall compounds. When blended with NR, SBR or BR, halobutyl rubber may be used in tread compounds. The addition of isobutylene-based polymers increases the dynamic loss modulus leading to improvements in wet and dry traction performance, braking performance, and vehicle stopping distance.

Vulcanization of halobutyl rubberThe primary difference between chlorobutyl rubber and bromobutyl rubber is due to the higher reactivity of the carbon-bromine bond compared to that of carbon-chlorine bond. The carbon-bromine bond has the lowest energy and the longest length (Table III). Bromobutyl rubber has faster cure rates and a shorter scorch or cure induction period (with productivity benefits), requires lower levels of curatives, and enables one to use a wider range of accelerators than could be possible for chlorobutyl rubber. A number of cure systems are suitable for halobutyl rubber and halobutyl rubber/general-purpose rubber (GPR) blends. Cure system selection will be a function of service conditions and the end use of the rubber product. Typical cure systems for halobutyl rubbers include:

Metal oxides• Zinc oxide will react with the halogen in polymers such as

halobutyl rubber and polychloroprene. Magnesium oxide is effective as a retarder.

Sulfur cure systems• Sulfur cure systems can be used in tire innerliners where a

high state of cure, fatigue resistance, high tensile and tear strength, and adhesion to GPR in components adjacent to the innerliner is important. A typical tire innerliner sulfur cure system may include sulfur (0.5-1.0 phr), zinc oxide (1.0-3.0 phr), and MBTS (1.0-1.5 phr).

Amines• Zinc free compounds may be required in, for example,

pharmaceutical applications. Curing of bromobutyl rubber with diamines such as hexamethylene diamine carbamate may be one solution.

Peroxide cure systems• Halobutyl rubber can be crosslinked with peroxide cure

systems. A dicumyl peroxide with a coagent, such as bismaleimide, yields a low compression set, high heat resistant vulcanizate3. Butyl rubber, however, will depolymerize in the presence of peroxide.

Metal oxide curingMetal oxides such as zinc oxide will react with the halogen in chlorobutyl and bromobutyl polymers. The most active halogen is in the allylic position. Zinc stearate or zinc octoate is required as a solubilizing agent and the mechanism is believed to be a nucleophilic substitution. Structure II (Figure 2) is the kinetically favored bromination product in bromobutyl rubber. However, at curing temperatures, rearrangement leads to an increase in the amount of Structure III. Structure III then dominates the equilibrium state that is achieved21. Figure 15 presents a possible scheme for the role of zinc oxide in crosslinking. In the presence of magnesium oxide, the halogen on the polymer can react with ZnBr2 and reform ZnO and MgBr2 as seen in the vulcanization of polychloroprene.

Figure 15Zinc oxide in the vulcanization of bromobutyl rubber 21,22,23

This scheme was initially proposed based on model compound studies such as 2,2,4,8,8-pentamethyl-4-nonene. The final crosslink structure is thought to be as illustrated in Figure 16. There would be less steric hindrance around the methylene group than at the -carbon rendering this interpretation more likely. Lowering sulfur or thiuram levels can control the greater reactivity of bromobutyl rubber. Mooney scorch and induction periods can also be extended by:

C

CH2Br

CHCH2 CH2C

CH3

CH3

CH2 C

CH3

CH3

CCH2C

CH3

CH3

CH2 CH CH2 C

CH3

CH3

CH2ZnOBr

C CHCH2 CH2C

CH3

CH3

CH2 C

CH3

CH3CH2CH2

C CHCH2 CH2C

CH3

CH3

CH2 C

CH3

CH3+ ZnBr2

C

CH2Br

CHCH2 CH2C

CH3

CH3

CH2 C

CH3

CH3

CCH2C

CH3

CH3

CH2 CH CH2 C

CH3

CH3CH2ZnOBr


• Increasing the MBTS level (since MBTS can also serve as a retarder)

• Replacing phenolic resins such as octylphenol tackifying resins with hydrocarbon resins

• Lowering the stearic acid level or carboxylic acid content• Avoiding amine type antioxidants• Adding magnesium oxide (retarder in bromobutyl rubber)• Adding polyethylene glycol since the ether functionality can

serve as an efficient retarder for bromobutyl rubber.

Figure 16Possible mechanism for crosslinking of halobutyl rubber24

Magnesium oxide dual effects on halobutyl rubber systems Table VII illustrates the effect of magnesium oxide (MgO) on cure rate for a TMTD/zinc oxide cure system6. Addition of 0.25 phr of MgO increases the Mooney scorch time by ten minutes without significantly effecting chlorobutyl rubber compound properties such as tensile strength. If the level of magnesium oxide is increased to 0.5 phr, there will be a detrimental effect on cure rate, with magnesium oxide acting as a retarder. As a retarder, magnesium oxide is effective with all vulcanization systems except amine-based cures. In that case magnesium oxide will react with chlorine formed during crosslinking preventing the appropriate vulcanization reaction with the curing agent.

Table VIIVulcanization retardation with magnesium oxide (MgO) 3, 6

Compound 1(phr)

2(phr)

3(phr)

4(phr)

TMTD 1.00 1.00 1.00 1.00

MgO 0.00 0.25 0.50 0.00

Mooney Scorch at 126.5°C

t-5 5.0 15.0 30.0 8.0

Cured at 153.0°C, 45 minutes

Tensile Strenght (MPa) 17.9 18.8 16.6 17.8

Elongation (%) 315 335 380 320

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 curingAccelerators 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 17Fundamental structures of organic accelerators14

As a thiazole, MBTS has a dual function in the vulcanization reaction. First is as a retarder and then as an accelerator in sulfur crosslink formation where MBTS will react with ZnO to form a sulfurating agent. This intermediate can then react with the polymer to form crosslinks (Figure 18)14.

Figure 18Illustration of sulfurating agent and crosslink formation14

ZnX3

X

+

X

SC

NRR' N C

SRO C

S(RO)2 P

SN

N N

NR2R2N

Thiazolethiocarbamyl alkoxythio

carbonyldialkylthiophosphoryl

diamino-2,4,6- triazinyl

ZnS SN

CSS

CN

S S

S6

NR3

NR3

NCS

N

RR R

N

RR R

8S Zn SSSC

N

NCS

R H

Zn SSyS

Polymer Chain

RSSxR + ZnS + HSxBt

RSxSBt + BtSSx Zn SySBt RSx ZnSx SBt + BtSSy SBt

NCSS

CN

R H

Zn SSyS

BtS SxR

SyH

Bt Zn S+ +


Although the final state of cure or crosslink density may be low, in the absence of zinc oxide MBTS may vulcanize rubber as shown in Figure 1914. Depletion of MBTS due to the initial reaction with bromine could account for the delayed response observed. Again, at higher levels MBTS can serve as a retarder.

Figure 19MBTS accelerated crosslinking14

Table VIII illustrates the effect of MBTS concentration on rheometer induction time, cure state, and rate of vulcanization.

Table VIIIRetarding effect of MBTS

Compound 1(phr)

2(phr)

3(phr)

4(phr)

5(phr)

Zinc oxide 2.0 2.0 2.0 2.0 2.0

Sulfur 0.5 0.5 0.50 0.50 0.50

MBTS 0.0 0.5 1.0 1.2 2.0

Rheometer (MDR), 180°C, 0.5° arc

Mh - Ml (dNm) 4.04 4.78 4.58 4.16 3.92

tc10 (minutes) 0.68 1.04 0.99 0.96 0.95

tc90 (minutes) 3.02 7.38 4.33 3.31 3.05

Rate (dNm/min) 1.83 2.34 2.38 2.62 2.84

Figure 20 shows that the use of 0.5 phr of MBTS increases the rheometer induction time (tc10 at 180°C) from 0.68 minutes to about one minute. The rheometer tc90 cure times increased to 7.3 minutes then fell to 3.1 minutes as MBTS increased from 0.5 phr to 2.0 phr. This further illustrates the dual role for MBTS as a retarder and as primary accelerator in the halobutyl rubber vulcanization process.

Figure 20MBTS (phr) and MDR rheometer at 180°C, 0.5° arc

The proposed retarding mechanism of MBTS is illustrated in Figure 21. Depletion of MBTS due to an initial reaction with bromine would account for the delayed response observed.

Figure 21Retarding action of MBTS

Other halobutyl rubber cure systems• Alternatives to TMTD accelerated curing in halobutyl

rubber: Nitrosamines result when curing with TMTD and its use is not recommended. However, tetrabenzylthiuram disulfide (TBzTD) or its zinc derivative, zinc dibenzyldithiocarbamate (ZBEC) with a sulfenamide accelerator such as CBS can readily match the compound properties of TMTD. Magnesium oxide may be added to assist in controlling scorch and cure rate.

• Vultac, amylphenol disulfide accelerators: the Vultac group of sulfur donors can be used in chlorobutyl rubber-based tire compounds where good adhesion to natural rubber or NR/SBR blends is required. These compounds tend to show high cure states, which can be controlled with MBTS

SC

NS S

NCS S

CN

SN

CS

S Sx

CCH2

CH3

C

H

CH2

C CCH

CH3

H

H

CH2

SC

NSSSx

NCS

CH

CH2

CCH

CH3

CH2

Sx SN

CS

NCS

SS

C

CH2

CCH

CH2

Sx

C CHHC

H

CH3

Sx S + MBT

S8

1 2

3 41

2

3 4

1 2 3 4 5tc10 (minutes)

tc90 (minutes)Delta Torque (dNm)0

12345678

tc10 (minutes) 0.68 1.04 0.99 0.96 0.95

tc90 (minutes) 3.02 7.38 4.33 3.31 3.05

Delta Torque (dNm) 4.04 4.78 4.58 4.16 3.92

1 2 3 4 5

C

CH2CHCH2 CH2Br

C

CH3

CH3

CH2 C

CH3

CH3

SC

NS S

NCS S

CN

S H

2

SC

NS H

C

CH2CHCH2 CH2C

CH3

CH3

CH2 C

CH3

CH3

SC

N

S

HBr+


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.


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

BIMSM elastomers display many properties superior to those found in general-purpose rubbers or other isobutylene-based polymers. For example, BIMSM shows reduced impermeability, increased heat stability, increased ozone resistance, increased oxidative stability and potentially higher tack levels when compared to halobutyl rubber25. Figure 22 shows a typical schematic of a BIMSM crosslink.

Figure 22Schematic of BIMSM crosslink

Unlike bromobutyl rubber, BIMSM will display poor cure 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 in a carbon black filled BIMSM compound is sufficient to obtain a reasonable cure state. Zinc stearate will also cure BIMSM and reversion may be minimized by addition of around 1.0 phr of ZnO. Therefore, BIMSM cure systems that will produce a vulcanizate with adequate scorch and reversion resistance and good mechanical properties will contain a thiazole such as MBTS, sulfur, ZnO and stearic acid. Cure systems where zinc is absent and peroxide systems will not be effective for BIMSM.

Due to the absence of carbon-carbon double bonds in the BIMSM backbone and the presence of the reactive benzylic bromide, the vulcanization chemistry differs from that of other 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).

ZnO + Stearic Acid > Zn(stearate)2 + Zn(stearate)(OH) + H2O (2)

Generation of a Lewis acid results and is believed to be the step controlling the induction period or compound scorch time. There is also an increase in the concentrations of Zn(OH)Br, Zn(Br)(St), and ZnBr2 (equation 3).

(3)

A third step involves formation of crosslink networks through alkylation between benzyl bromide groups and formation of hydrogen bromide. Hydrogen bromide continues to react with zinc oxide to form additional amounts of Lewis acids (equation 4)26.

This is manifested as an increase in cure state or increase in rheometer torque. The concluding or forth stage in the curing process is the maturation or plateau phase of the cure profile. Table IX illustrates the effect of zinc oxide concentration on rheometer induction time, rate of vulcanization, and rheometer maximum torque, as a measure of cure state. Some general inferences are:

• Reducing the ZnO concentration to a level below 1.0 phr would result in an excess of benzylic bromide

• High benzylic bromide has a detrimental effect on aging • Excess stearic acid level results in increases in permeability• A molar ratio of ZnO to bromine of 0.9 is preferred

C

CH3CH2

CH3

CH2 CH

CH2Br

CH CH2 CCH3

CH2CH3

R

C

CH3CH2

CH3

CH2 CH

CH2

R = -‐‑ CH2Br -‐‑ CH3

ZnO

Stearic acid

CH2Br CH2St

Zn(St)2

Zn(OH)(St)

St = Stearic AcidZnBr2 + + ZnBrSt + ZnBr(OH)


Table IXEffect 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 (dNm/min) 3.09 5.85 9.83 11.18 11.26 9.12

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 XEffect 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 (dNm/min) 1.6 3.4 9.8 9.5 7.3 4.4

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)

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.

C H 2 B r

Z n B r 2 Z n B r S t Z n ( O H ) B r

C H 2 R

+ H B r R = St, OH, Br, Ph -‐-‐


8. Model cure system formulary

Isobutylene-based polymers will employ a variety of cure systems depending on the requirements of the final cured product. For example, tire innerliners require good resistance to fatigue, tear strength, and adhesion to adjacent components in the tire. Therefore, sulfur-based systems with their co-curing ability, adequate fatigue and flex life are preferred for liner compounds. Curing bladders, which 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 rubberTable 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 XIModel cure systems for butyl rubber 1,2

Cure System Quinone Resin Resin SulfurAccelerator

Units phr phr phr phr

ApplicationElectrical Cable Cover

Curing Bladder

Curing Bladder Tube

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 Dioxime 2.0 - - -

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

Halobutyl rubberTable XII displays a set of model cure systems for halobutyl elastomers. These include a resin, sulfur-accelerator based system for a bromobutyl rubber radial tire innerliner, and a low temperature cure system possibly suitable for pipe seals in inaccessible locations. Again, these cure systems may provide a starting point for further development.


Table XIIModel cure systems for halobutyl rubber 2,3

Cure System SulfurAccelerator2

SulfurSulfenamide2

Zinc Oxide & Resin3

Room Temperature Cure3 Peroxide3


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 elastomersTable XIII presents a set of five cure systems for BIMSM.

Table XIII Model cure systems for BIMSM elastomer2

Cure System Metal Oxide SulfurAccelerator

Ultra Fast Accelerator System

Resin Amine


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


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 XIVEffect of cure system type on fundamental properties of isobutylene based elastomers

1Model BIMSM Compound

2BIMSM withZinc Stearate

3Model Butyl Compound

4Model Chlorobutyl Compound

5Model Bromobutyl 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 Hydrocarbon Resin Blend 7.0 7.0 - 7.0 7.0

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-thiate (ZBPD) - - 2.0 - -

Total (phr) 183.0 184.0 210.0 183.2 182.9

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

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

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


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 23Cure rate compatibility 27

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.

Compound A t90

(time to 90% cure)

Time (minutes)Induction Time

InterfaceCreation

Compound B t10

(time to 10% cure)

Compound B t90

(time to 90% cure)

Compound A t10

(time to 10% cure)

Rhe

omet

er T

orqu

e


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.


APPENDIX 1Recognized industry abbreviations for accelerators

Abbreviations or commercial Descriptions

Chemical Name Function

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 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


10. References

1. EN Kresge, RH Schatz, HC Wang. Isobutylene Polymers. Encyclopedia of Polymer Science and Engineering. Volume 8. 2nd edition. P 423 – 448. John Wiley and Sons, Inc 1987.

2. WH Waddell, AH Tsou. Butyl Rubber. In ‘Rubber Com-pounding, Chemistry and Applications’. Ed MB Rodgers. Marcel Dekker, Inc. New York, 2004.

3. JV Fusco, P Hous. Butyl and Halobutyl Rubbers. In Rubber Technology, 3rd Edition. Editor M Morton. Van Nostrand Reinhold, 1987.

4. Technical Bulletin. Nitrosamine Solutions. Akrochem Corporation. 2003.

5. T Kuhlmann. A new thiuram acceleration without danger-ous and volatile nitrosamines. Kautschuk Gummi Kunstst-offe, Volume 42, P 878-9. 1989.

6. RW Layer, DW Chasar. Minimizing Nitrosamines Using Sterically Hindered Thiuram Disulfides/Dithiocarbamates. Rubber Chem & Technol. Vol 67(2), P 299-313

7. WH Waddell, MB Rodgers. Rubber Compounding. In Kirk-Othmer Encyclopedia of Chemical Technology, 5th Edition. 2004

8. RN Datta, AJ deHoog, JH Wilbrink, WF Verhelst, MS Ivany. Perkalink 900: Reversion Resistant Tire Compound-ing Without Compromise. International Tire and Exhibition Conference paper 18A. 1994

9. WH Waddell, MB Rodgers. The Science of Rubber Compounding. In Science and Technology of Rubber, 3rd Edition. John Wiley and Son, NY. 2005. (in press)

10. JA Dean. Lange’s Handbook of Chemistry. 14th Edition. McGraw-Hill. New York. 1992

11. JJ Higgins, FC Jagisch, NE Stucker. Butyl Rubber and Polyisobutylene. In Handbook of Pressure Sensitive Adhesive Technology, 2nd Edition. Editor D. Satas. Van Nostrand Reinhold. 1989.

12. B Struck. Tackifying, Curing, and Reinforcing Resins. In Rubber Technology Compounding and Testing for Performance. Edited by J.S. Dick, Hanser, Cincinnati 2001.

13. JV Fusco, P Hous. Butyl and Halobutyl Rubbers. In the Vanderbilt Rubber Handbook. 13th Edition. RT Vanderbilt 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.

16. Atofina Chemicals. Technical Bulletin. ‘Vultac Rubber Chemicals’. 1996.

17. DD Flowers, JV Fusco, DS Tracey. Advancements in new tire sidewalls with a new isobutylene based copolymer. Rubber World, March. 1994

18. Atofina Product Literature. Vultac 3 in Chlorinated Butyl Rubber Innerliners

19. RL Zapp. Rubber Chemistry and Technol. Vol 46(2) Page 251-274. 1973

20. A Tinker. Crosslink Distribution and Interfacial Adhesion in Vulcanized Blends of Natural Rubber and Nitrile (NBR). Rubber Chemistry and Technol. Vol 63, Page 503 – 515, 1990

21. JC Scott, GDF White, DJ Thom, RA Whitney, W Hopkins. J Polymer Science. Part A, Polymer Chemistry. Vol 41. P 1915-1926. 2003

22. I Kuntz, RL Zapp, RJ Panchrov. The Chemistry of the Zinc Oxide Cure of Halobutyl. Rubber Chemistry and Technol. Vol 57(4) P813 - 825. 1984.

23. AY Coran. Vulcanization. In Science and Technology of Rubber. 2nd Edition. Ed. JE Mark, B Erman, FR Eirich, Academic Press. 1994.

24. R Vukov. Zinc oxide crosslinking chemistry of halobutyl elastomers, A model compound approach. Rubber Chemistry and Technology, Vol 57(2). Page 284 – 290. 1984

25. AT Tsou, WH Waddell. Isobutylene-based Elastomers. Presented at a Meeting of The American Chemical Society Rubber Division, Grand Rapids, MI. 2004

26. D Xie, HC Wang, MW Johnston. FTIR Spectroscopic Studies on Curing of Brominated Poly(Isobutylene-co-4-Methylestyrene). Presented at a Meeting of the Ameri-can Chemical Society Rubber Division, San Francisco. 2003.

27. W.H. Waddell, M.B. Rodgers. Tire Applications of Elasto-mers 2. Casing Presented at a Meeting of the American Chemical Society, Rubber Division, Grand Rapids, MI, 2004.


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Model vulcanization systems for butyl rubber and halobutyl