ExxonTM butyl rubber curing bladder technology manualdocshare04.docshare.tips/files/27078/270783064.pdf · 2019. 9. 30. · butyl rubber which has resulted in its wide use for high

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ExxonExxonTMTM butyl rubber butyl rubber curing bladder technology curing bladder technologymanualmanual



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AbstractAbstract

A curing bladder is an important co A curing bladder is an important component of the tire vulcanizingmponent of the tire vulcanizingpress and the tire press and the tire curing process. The proper selection of butyl curing process. The proper selection of butyl polymers andpolymers andcompounding materials for the bladder formulation is compounding materials for the bladder formulation is essential in ensuringessential in ensuringdurability, required service life, and efficient curing bladder operation in a tiredurability, required service life, and efficient curing bladder operation in a tirefactory. This is due to the superior heat and steam resistance of resin curedfactory. This is due to the superior heat and steam resistance of resin curedbutyl rubber which has resulted in its wide use for high heat resistantbutyl rubber which has resulted in its wide use for high heat resistantapplications. Additionally, butyl rubber has very low applications. Additionally, butyl rubber has very low permeability to gases andpermeability to gases andwater vapor that further enhances the performance of butyl rubber tire curingwater vapor that further enhances the performance of butyl rubber tire curingbladders.bladders.

This manual reviews aspects of curing bladder technology andThis manual reviews aspects of curing bladder technology andpresents some basic guidelines on presents some basic guidelines on compounding, processing, applicationscompounding, processing, applicationstechnology, and trouble shooting of common curing bladder failure technology, and trouble shooting of common curing bladder failure modes.modes.Model bladder compounds with selected properties are also reviewed. TheseModel bladder compounds with selected properties are also reviewed. Thesecompounds provide starting points for additional development work dependingcompounds provide starting points for additional development work dependingon the specific properties that are needed or that allow use in different typeson the specific properties that are needed or that allow use in different typesof tire curing of tire curing presses and factory operating conditionspresses and factory operating conditions..



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Table of ContentsTable of Contents

Introduction………………………………………………………………… 4Introduction………………………………………………………………… 4

Curing Curing Bladder Bladder Performance Performance Requirements…...……………………. Requirements…...…………………….

Compounding Compounding of of Tire Tire Curing Curing Bladders…………………….……….. Bladders…………………….……….. 77

Processing Processing of of Tire Tire Curing Curing Bladder Bladder Compounds………………….... Compounds…………………....

Types of Tire Curing Presses and Operation Sequence of TireTypes of Tire Curing Presses and Operation Sequence of TireCuring Curing Bladders………………………………………………………….. Bladders…………………………………………………………..

Curing Curing Bladder Bladder Design Design ………………………………………………….. …………………………………………………..

Guidelines Guidelines for for Bladder Bladder Maintenance…………..……………………… Maintenance…………..………………………

Special Special Test Test for for Curing Curing Bladder Bladder Compounds………………………. Compounds………………………. . .

New New Technology Technology and and Curing Curing Bladder Bladder Market Market Trends……………… Trends……………… 3333

AppendicesAppendices Appendix 1. Typical blad Appendix 1. Typical bladder failures and corrective guideder failures and corrective guidelines.. lines.. 3636 Appendix 2. Check list of failures a Appendix 2. Check list of failures and corrective guidelines…… nd corrective guidelines…… 4141

References………………………………………………………………….. 42References………………………………………………………………….. 42



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IntroductionIntroduction

Resin cured butyl rubber has carbon-carbon crosslinks which yield heatResin cured butyl rubber has carbon-carbon crosslinks which yield heatstable vulcanizates. The superior heat and steam resistance of stable vulcanizates. The superior heat and steam resistance of resin curedresin curedbutyl rubber has resulted in its wide use for high heat service temperaturebutyl rubber has resulted in its wide use for high heat service temperatureapplications such as tire curing bladders. Additionally, butyl applications such as tire curing bladders. Additionally, butyl rubber has veryrubber has verylow permeability to gases and water low permeability to gases and water vapor providing the required propertiesvapor providing the required propertiesfor butyl tire curing bladders. A curing bladder is an important and essentialfor butyl tire curing bladders. A curing bladder is an important and essentialpart of the part of the tire vulcanizing press. Some examples of curing bladders aretire vulcanizing press. Some examples of curing bladders areshown in Figure 1.shown in Figure 1.

Tire producers are always working to Tire producers are always working to improve the performance ofimprove the performance ofcuring bladders, in order to maximize (i) tire curing efficiency, (ii) curing bladders, in order to maximize (i) tire curing efficiency, (ii) factoryfactoryproductivity, and (iii) durability of productivity, and (iii) durability of bladders. This manual reviews current curingbladders. This manual reviews current curingbladder technology and gives some bladder technology and gives some basic guidelines on compounding,basic guidelines on compounding,processing, technology, and trouble shooting for common tire processing, technology, and trouble shooting for common tire press curingpress curingbladder failure modes.bladder failure modes.

Figure 1Figure 1Tire Curing BladdersTire Curing Bladders



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Figure 1Tire Curing Bladders …cont



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Curing Bladder Performance Requirements

Functions of a Curing Bladder.The curing bladder is a cylindrical bag of specially compounded butyl

rubber containing a poly-methylolphenol resin cure system. This collapsiblebladder is mounted in the lower section of the tire curing press and forms apart of the press and mold assembly. The "green" unvulcanized tire ispositioned over the bladder in the bottom half the mold. When the mold isclosed, pressurized steam, air, hot water, or inert gas (nitrogen) is introducedsystematically (pre- programmed) into the bladder to provide internal heat andpressure for the tire shaping and curing process. The two types of tire curingpresses which require bladders are:

1. Slideback (NRM or similar) type press which requires an AutoForm(Bagwell) bladder.

2. Tiltback (Bag-O-Matic or similar) type press which requires a Bag-O-Matic bladder.

Examples of curing bladders used by the tire industry for tire curing pressesare shown in Figure 1.

Basic Properties Required for Curing Bladder Application.Three types of tire cure cycles can be found, a steam – high pressure

hot water cure cycle, a steam – inert gas cure process, and a steam – steamcure cycle. Dome temperatures can reach 190°C ( mold sidewall plates at180°C) and the bladder temperatures can reach up to 220°C. For a truck tire,size 14.00R20, a simple steam – hot water cure cycle time could be asfollows:

1. Steam 12’00”2. High Pressure hot water 30’00”3. Cold water flush 4’00”4. Drain 0’30”

Total Cure Time (minutes) 46’30”

Given the high pressures and temperatures the bladder undergoes inmultiple cure cycles, the basic properties required for curing bladder includethe following:

1. A homogeneous, well mixed compound for ease of processing(mixing, extruding, and mold flow).

2. Excellent heat aging resistance.3. Resistance to degradation due to saturated steam or high pressure

hot water, or inert gas.



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Attainment of these properties will enable a curing bladder to achieve anadequate service life, i.e., number of tire cure cycles and sometimes referredto as the pull - point. The pull - point is where the bladder is removed beforefailure; thereby preventing failures during tire cure cycles which can lead tothe loss of tires during production.

Compounding of Tire Curing Bladders

Tire curing bladders must withstand continuous exposure to hightemperatures from high pressure steam, hot water, or inert gas. This isachieved by using butyl rubber specially compounded with reactive alkylphenol formaldehyde resin. The selection of compounding ingredients is veryimportant with respect to the bladder life. The primary materials used in abladder compound are the polymer (butyl rubber), cure activator, carbon black,plasticizer, zinc oxide, and curing resin.

Butyl rubber (IIR, Isobutylene isoprene copolymer) is the preferredelastomer for tire curing bladders due to the following properties:

1. Excellent heat aging resistance,2. Good flex and tear resistance,3. Low tension and compression set, and4. Impermeability to air, inert gases, and water vapor.

Butyl rubbers are typically produced via a cationic polymerization in

methyl chloride at temperatures between –90°C and –100°C. The uniqueproperties and difficult manufacturing conditions place butyl rubbers in thespecial purpose elastomers category, distinct from general-purpose rubberssuch as polybutadiene (BR), natural rubber (NR), and styrene-butadienerubbers (SBR)1,2.

Butyl rubber is a copolymer of isobutylene and approximately 2 mol%isoprene (Figure 2). The length of the isobutylene structural unit (0.270 nm) is67% of that of the 1-4-isoprene structural init (0.405 nm)3. Thestereochemistry of the isobutylene unit results in close packing along thepolymer chain, low free volume fraction, and consequently low permeability.Isoprene is incorporated in a trans-1,4 enchained head-to-tail arrangement toproduce a random, linear copolymer.



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Figure 2Structure of Butyl Rubber 2

(Structure I)

CH2 C

CH3

CH3

CH2 C CH

CH3

CH2

(0.8 to 3

.0 mole %)

Elaborating, using 1H NMR to investigate the stereochemistry of theenchained isoprene, it was found that the majority (94%) of Structure Iisoprene units illustrated in Figure 3 are incorporated in the 1,4 configurationThe term ‘Structure I’, is assigned from the description for isoprenyl unitsfound in chlorobutyl and bromobutyl rubbers.

Figure 3Sterochemistry of Isoprene Incorporation4

CH2 C

CH3

CH CH2CH2 C

CH3

CH

CH2

CH2 CH

C CH

CH2

1,4 Add ition

94%

Structur e I1,2 Add ition

6%3,4 Add ition

0%

Structur e I minor

Structure Iminor consisting of the 1,2- enchained isoprene has also beensuggested, at amounts in the order of 6%4. No 3,4-addition products havebeen reported. Expanding on this work, White and coworkers furthersuggested the absence of a 3,4-addition structure but also suggested thatStructure Iminor had the configuration illustrated in Figure 35. The ratio of 1,4enchained isoprene and the minor isoprenyl derivative was dependent uponpolymerization temperature but was still present, albeit in small amounts, incommercial grades of butyl rubber. White and coworkers have also reportedthat Structure I minor is not associated with end groups, and that the R groupsarise from isobutylene and not from isoprene – isoprene addition (Figure 4).Though the authors indicated no definitive determination of the R- group butevidence was presented showing the R- groups may be longer chains5.



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Figure 4Proposed Branched Structure I minor

5

CH2 C

CH3

CH CH

R

R'

R = Long chain br anch f r om polymer

Cyclic r ing d ue to r earr angement of chain end Long continuing iso butylene chain

For butyl rubber curing bladders, EXXONTM Butyl 268 can be used as astarting point in building a compound formulation. Examples of curing bladdercompound formulations can be found at ‘www.butylrubber.com’6. Table Ifurther illustrates a selection of the commercial grades of butyl rubbercurrently available from ExxonMobil Chemical Company. The table shows therange of polymer viscosities available, and nominal isoprene content 6.

Table IExamples of Commercial Grades of ExxonMobilTM Isobutylene

Elastomers6

Elastomer ExxonMobil GradeIdentification

Mooney Viscosity(ML1+8 @ 125°C)

Isoprene(mol %)

Butyl(low viscosity)

065 32 1.05

Butyl(medium viscosity)

068 51 1.15

268 51 1.70

Regular Butyl GradesVarious grades of butyl rubber can be used in curing bladder

applications depending on the final compound and bladder requirements andtire types being produced. Some selected grades of butyl rubber can befurther described as follows:

1. EXXON™ Butyl 268: This is the most commonly used grade forcuring bladder compounds. It has high Mooney viscosity

(ML1+8@125°C: 51 MU) and medium amount of unsaturation (1.7mole%). Though compounds containing this grade of butyl havegood processing and mechanical properties, and good durability,mold flow properties can be further modified by blending with lowerMooney viscosity grades such as EXXON™Butyl 065.



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Caution is also required as a low state of cure can result in anincrease in bladder growth.

3. EXXON™ Butyl 065: It has low unsaturation (1.05 mole %) and is

low Mooney viscosity (ML1+8@125°C of 32 MU) grade. It is used inblends with EXXON™ Butyl 268 in large size “Off The Road” (OTR)tire curing bladders for improved mold flow. OTR bladders mustsustain very long cure times at tire curing temperature.

The term “mole percent unsaturation” refers to the number of isoprenemolecules in 100 monomer molecules in the polymer. Thus, a one – molepercent unsaturation butyl would contain one molecule of isoprene and ninety –nine molecules of isobutylene.

Further reference, if required, should be made to the web site,www.butylrubber.com, for additional information on these polymers6.

ActivatorsThe crosslinking of butyl rubber is dependent on the reactivity of the

phenol-methylol groups of reactive alkyl phenol-formaldehyde resins(octylphenol formaldehyde resin). The low levels of unsaturation of butylrequire resin cure activation by adding halogen containing materials such asSnCl2 or halogen-containing elastomers such as polychloroprene. A morereactive resin cure system requiring no external activator is obtained if someof the hydroxyl groups of the methylol group are replaced by bromine. Anexample of commonly used commercial resin is a brominated alkyl phenolformaldehyde resin.

Typical grades of polychloroprene used as resin cure activators areshown in Table II.

Carbon BlackGenerally, high structure carbon black ISAF or HAF (e.g. N330) which

give a good balance of properties, are used in bladder compounds at levelsof 50 to 60 phr. Other alternative types of carbon black are the GPF gradeswhich show improved air aging, though ISAF grades have better steam agingproperties. Acetylene black compounds in combination with, for example,N330 have good thermal conductivity which may reduce tire curing time.However, acetylene black may be difficult to disperse in the butyl rubbercompound. Generally, a lower loading of carbon black (e.g. 35 phr) givesbetter air aging and higher loading of carbon black (e.g. 65 phr) gives bettersteam aging. In the website www.butylrubber.com, carbon black N220 (50phr), and N660 (60 phr) are used in the model formulations6. An example of acuring bladder compound with properties is illustrated in Table III.



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Table IIPolychloroprene Grades

Grade Properties

Neoprene™ GN: Shorter aging and scorch resistance.Neoprene™ W (1) : Good processing and most commonly used.Neoprene™ GNA : Tends to show low scorch resistance.Neoprene™ TW : Easy processing, good mechanical properties.Neoprene™ TRT : Crystalline resistant and good processing .Neoprene™ WRT : Crystalline resistant, but requires higher

organic accelerators levels.

Note: 1. Polychloroprene (W Type) is cited in the website www.butylrubber.com underformulary section.

PlasticizersCastor oil (5 phr) is the most commonly used plasticizer for bladder

compounds due to its low volatility at high temperature. Castor oil reduces thetendency for a marching modulus in resin cured butyl rubber bladdercompounds. Additionally it gives lower unaged modulus and good steamaging. If castor oil is not available, then oleic acid (5 phr) could be used.Compounds containing either castor oil or oleic acid have better releaseproperties between the bladder and tire inner liner. These compounds alsoshow better retention in aging properties due to the high boiling point andlower volatility of castor oil. Alternatively if castor oil is not available paraffinicprocess oils (e.g. FlexonTM 876) could be used though caution is required.Curing bladder compound properties obtained using paraffin oil in place ofcastor oil have been tabulated in the model formulary onwww.butylrubber.com.

Zinc OxideZinc oxide, typically at 5 phr, is added to form zinc halide that then acts

as the catalyst for the vulcanization of resin cured butyl rubber compounds.Good dispersion of the ZnO is critical for improved tire curing bladder life.



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Table IIIModel Formulary for Curing Bladder With Exxon™ Butyl 2686

Material Units Amount

EXXON™ Butyl Grade 268 PHR (1) 100.00

Polychloroprene (W Type) PHR

Carbon Black, N220 PHR 50.00

Castor Oil PHR

Stearic Acid PHR

Zinc Oxide PHR

Heat Reactive Octylphenyl Formaldehyde Resin PHR

Total PHR 176.00

Properties

Test Method

Based on Units and Conditions

Typical Values

Mooney Viscosity ML1+4 ASTM MU@100ºC

Mooney Viscosity ML1+8 D1646 MU@100ºC

MRI Seconds

Mooney Scorch

Minimum Viscosity ASTM MU@125ºC

Time to 1 pt rise D1646 Minutes

Time to 5 pt rise Minutes

MDR Rheometer 180 °C; Arc 0.5°

ML (Minimum torque) ASTM dN.m

MH (Maximum torque) D5289 dN.m

MH-ML (” T) dN.m

Tc2 (time to 2 torque unit increase) Minutes

Tc10 (time to 10% torque increase) Minutes



Tensile Strength ASTM MPa

Elongation D412 %

300 % Modulus MPa 2.5

Hardness, Shore A ASTM D2240 Shore A

Tear Strength (Die-B) ASTM D624 KN/m

Note:



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AntioxidantGenerally, antioxidants are not effective in improving heat resistance of

resin cured butyl compounds. Some of the antioxidants (e.g. amines) couldsignificantly retard the cure rate of regular butyl rubber compounds, sulfurbased vulcanization systems, and resin curing system.

Process AidsDepending on the equipment, resin cure bladder compounds may be

difficult to mix and process. To facilitate good dispersion and flow properties,it may be necessary to use process aids such as organosilicone compounds.There are several commercially available process aids such asorganosilicones and calcium fatty acid soaps suitable for curing bladdercompounds7.

Reactive Phenol Formaldehyde Resin as Curing AgentResin cured butyl rubber compounds may display better resistance to

detrimental effects of exposure to high temperatures when compared withsulfur cured butyl rubber vulcanizates, which tend to soften during prolongedexposure at elevated temperatures (e.g., 1500C to 2000C).

The resin cure mechanism in butyl rubber is based on the reaction ofthe methylol groups in the phenol-formaldehyde resin with allylic hydrogen inbutyl, usually with a Lewis acid catalyst, to yield carbon-carbon crosslinks thatare thermally stable2. Tire curing bladders are cured by alkylphenolformaldehyde derivatives containing methylol groups. Examples of tire curingbladder compounds, using heat reactive octylphenol formaldehyde curingresins, are given in the website www.butylrubber.com6 under Formularysection and in Table III. Commercially, there are several suppliersmanufacturing reactive phenol formaldehyde resins (octylphenolformaldehyde resin).

Reactive bromomethylated alkylphenol formaldehyde resins are alsoused. While using brominated reactive resin, the bladder compound does notrequire an external source of halogen such as polychloroprene. Howeverwhen using such resins, compound tack can increase resulting in the need toconduct additional factory compound process development.

The simplified chemical structure of vulcanizing resin is given in Figure5 and its possible cross-linking structures are given in Figure 6 (Van derMeer) and Figure 7 (Greth) 8,9.

The vulcanizing resin is a chain of phenolic rings connected bymethylene groups as illustrated in Figure 5. The terminal methylol groups (-CH2OH) are the points at which the resin molecule crosslinks with the butyl



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Figure 5Simplified Structure of a Vulcanization Resin

CH2H2COH

OH

CH2

OH

R R

OH

R

CH2OH

n

Figure 6

Crosslinking through - hydrogen8

CH2HC

OH

CH2

OH

R R

OH

R

CH

n

CH2C

CH

CH

CH3H3C

CH2

CH2

H2C CH2

It has also been suggested that the cure mechanism involves both thephenolic and methylol hydroxyls in a substitution reaction across the doublebond, resulting in a chroman structure as shown in Figure 7.

Figure 7Crosslinking Through Chroman Structure Formation8,9

CH2 CH2

OH

R

O

R

nR

O

CH2 CH2

CH3 CH3

H2C CH2

In summary, at this point the accepted mechanism for the resincrosslinking mechanism is illustrated in Figure 8. Following the elimination ofwater in the reaction sequence, the exomethylene group and carbonyl oxygen



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Figure 8Resin Curing of Butyl Rubber 10

Model Formulary for Tire Curing BladdersReference should be made to the web site, www.butylrubber.com for

model compounds which provide starting points for additional developmentwork. A simplified summary of a model compound is shown in Table III. Thisincludes processing properties and physical properties.

Processing of Curing Bladder CompoundsThe service condition of the tire curing bladder is unique in that it is

subjected to severe conditions of heat, pressure and flexing. Therefore, gooddispersion of the ingredients especially carbon black, zinc oxide,polychloroprene and curing resin is very important to achieve an adequateservice life. Good dispersion of the compound ingredients enhances physicproperties and allows better retention of physical properties.

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 C

CH3

CH

CH2

H

OH

R

HOCH2 R'

R

CHC

O

CH2

CH3CH2

- H2O

heat

Formation of aChroman ring

R

R'

R

CHC

O

CH2

CH3CH2

OC

CH

CH3CH2

Crosslinked Butyl Network Evolution

CH CH2CCH2

CH3



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Processing of Tire Curing Bladder Compounds

The service conditions of the tire curing bladder are unique. It issubjected to severe conditions of heat, pressure and flexing. Therefore, gooddispersion of the ingredients, especially carbon black, zinc oxide,polychloroprene and curing resin, is very important to achieve adequateservice life. A good dispersion of the compounding ingredients can improvephysical properties and also retention of physical properties.

The manufacturing process flowchart for a tire curing bladder isillustrated in Figure 9. Briefly, the major processing steps are:

1. Masterbatch mixing,2. Straining,3. Final batch mixing,4. Extrusion of slugs/blanks,5. Cutting of slugs/blanks,6. Blank splicing,7. Vulcanization (compression molding or injection molding),8. Post-cure or conditioning, and,9. Storage of bladders.

Figure 9A Flow Chart for Curing Bladder Production

Not every step in this comprehensive schematic of curing bladder production



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Guidelines for MixingButyl rubber is primarily a saturated polyisobutylene copolymer with

1.05 to 2.30 mol% of isoprene. It is important to avoid contamination ofunsaturated elastomers such as natural rubber, SBR and polybutadienerubber with butyl rubber. Due to the difference in the state of cure betweenbutyl and other unsaturated elastomers, contamination could lead to loss incompound physical properties. Bale cutters, internal mixers, two roll openmills, strainers and extruders which are used to process butyl rubber andother unsaturated rubbers should be cleaned thoroughly with a clean-outcompound.

Depending on specific factory processes, it is sometimes suggestedthat butyl rubber ibe pre-masticate for 45 seconds in an internal mixer oropen roll mill. Alternatively preheating for 24 hours at 50ºC will help. Aninternal bale temperature of 45ºC to 50ºC, or pre-mastication of butyl rubber,facilitates the easy incorporation of compounding ingredients. Butyl rubbercompound mixing is done in two stages in the internal mixer (Banbury mixer);the first stage, non-productive, or masterbatch contains all the ingredientsexcept the curatives.It is also sometimes suggested to separately pre-masticate polychloroprenerubber to improve homogeneity of bladder mix compound. The 2nd stage isthe final or productive step where the vulcanization system is added.

Masterbatch MixingMasterbatch mixing of tire curing bladder compounds may be carried

out in an internal mixer. In order to improve dispersion and to prevent trappedair, it is suggested that the masterbatch weight be increased by 10% to 20%compared to natural rubber or emulsion SBR compounds at an equivalentspecific gravity.

There are varieties of loading sequence practiced in the industry asshown in Table IV and Table V. It is suggested that the starting temperatureof an internal mixer for masterbatch mixing be set at 75ºC to 80ºC.

Straining & CoolingUpon discharge from the internal mixer it is suggested that the

masterbatch or first stage mixed compound be immediately passed through strainer pack of 20 to 30 mesh or 30 to 40 mesh. This will eliminate the needfor re-warming the stock for straining. The strained stock then can be storedon a tray, which has been lightly dusted with talc.



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Table IVMixing Procedure for a Model Curing Bladder Compound

First Stage Masterbatch Mixing in an Internal MixerTime / Dump Temperature Operation Sequence0 Minute

3/4 Minute

170ºC to 180ºC

Add butyl rubber and polychloroprene(if included in the formulation)

Add carbon black, oil, othercompounding additives.

Dump.

Final Productive Batch or Second Stage Mixing Finalization can be carried out in the internal mixer or two roll open mill. Forfinal or productive batch mixing in internal mixer, it is suggested that thebatch weight starting point setting be 80% of the mixer volume, and thenoptimized for the specific equipment.

Table VMixing Procedure for a Model Curing Bladder Compound

Second Stage or Productive Mixing in an Internal MixerTime & Dump Temperature Operation Sequence0 Minute

100ºC to 110ºC

Add half masterbatch, zinc oxide andpowered curing resin, then remaininghalf of masterbatch

Dump

Three Stage Mixing (Masterbatch and Final Batch)Three stage mix cycles are suggested if a lower Mooney viscosity compoundis required. This may be needed to improve mold flow properties such aswhen injection molding bladders (Table VI).

Suggested Precautions when MixingSuggestions include the following:

1. The mixing equipment is cleaned before and after mixing butyl rubber toavoid contamination.

2. Carbon black should be added to the mixer before the oil.3. If carbon black blends are used, it is suggested that the high structure

carbon black be added first .



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6. It is suggested that the curing resin be added with the last 1/3 of themasterbatch and folded into the compound stock on the conveyor belt ofinternal mixer feed hopper.

7. After dumping it is suggested that the batch be cooled immediately.

Table VIMixing Procedure for a Model Curing Bladder Compound

Three Stage MixingTime & Dump Temperature Operation Sequence1st Stage / Masterbatch

0 Minute1 Minute130ºC160ºC to 170ºC

Add butyl rubber, polychloroprene Add black, oil, stearic acidCleaningDump

2nd Stage

0 Minute130ºC

Master batch, zinc oxideDump

3rd Stage / Final Batch

0 Minute100ºC

2nd stage batch and powdered resinDump

Guidelines for Mill Feeding for Hot ExtrusionSome guidelines and suggestions include:

1. Auto feeding (no manual feeding) is suggested.2. It is suggested batches be blended at the mill before feeding to the

extruder.3. Avoid addition of reworked compound.4. Control the mill nip setting, especially the feed mill (for large size slugs,

feed width has to be adjusted)5. Maintain a consistent rolling pencil bank on the feed mill and maintain

constant feed strip dimensions (thickness and width) for the particular sizeof slug.

6. Strip feeding temperature should be kept at 85ºC to 95ºC.Guidelines for Extrusion of Blank Slugs by Hot Feed Extrusion

Both hot feed and cold feed extrusion are used for blank extrusion.However special care should be taken to minimize trapped air in the extrudedblank especially for hot feed extrusion processes. Some additionalsuggestions and points are listed as follows:



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3. Maintain a full extruder barrel and maximum back-pressure to helpprevent porosity.

4. Generally, die head temperature can be set at around 110ºC to 130ºC.Compound temperature will also depend on compound Mooneyviscosity and the extruder head pressure.

5. While extruding, good water circulation in the screw (typicaltemperature of the barrel and screw can be in the order of 55ºC to65ºC) should be maintained, with the screw cooler than the barrel.

6. It is suggested that the extruded stock temperature is maintained atless than 120ºC and internal slug temperature is between 95ºC to120ºC.

7. The speed of feed conveyor, extruder rpm, take-off conveyor andcooling line speed should be synchronized to prevent pulling and draw-down of slugs dimensions.

8. Cool down the extruded blank with a chilled water bath or chilled waterspray.

The extruded dried blanks should be stored on clean trays and awayfrom airborne dust and foreign materials.

Guidelines for Cold Feed ExtrusionSome suggestions include:

1. Use a force feed roller for extruder feeding. Do not fold feed strips2. Keep the extruder barrel filled with the compound to avoid porosity.3. Vented cold feed extruders may reduce the chance of porosity.4. Vacuum type cold feed extruders with degassing capability can redu

porosity significantly.5. Optimize extruder screw speeds (rpm) for a particular slug size.6. Suggested extruder temperatures for barrel and screw are 55ºC and

45ºC respectively, and for the die head it is between 110ºC to 120ºC.7. Like hot feed extrusion, it is suggested that the speed of the feed

conveyor, extruder rpm, take-off conveyor, and cooling line speedshould be synchronized to prevent pulling of slugs and draw-down ofslug dimensions.

8. Cool down the extruded blank with a chilled water bath or chilled waterspray .

The Advantages of Cold Feed Over Hot Feed ExtruderThese include:

1. Two roll mills are not required, resulting in floor space savings andlower capital cost.

2. Less staff.3. Longer length (L) of screw over smaller screw diameter (D), L/D ratio



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Secondary Processing Suggested Improvement GuidelinesMold flow, scorch resistance, and heat degradation are three major processand product concerns encountered with tire curing bladders.

1. Guidelines on Flow Improvement: Good compound flow and knittingproperties are important criteria to ensure improved properties during themolding process and to improve bladder life. These properties areparticularly important for larger bladders and for injection molding ofbladders. Typical approaches in tuning compounds can include:

i. Increase plasticizer (process oil).ii. Blending with low Mooney viscosity butyl rubber such as

EXXONTM Butyl 065 (75phr) with EXXONTM Butyl 268 (25 phr).iii. Use about 2phr of process aids such as calcium acid soap or

organosilicone compound.

2. Guidelines on Scorch Retarder: For tire curing bladder compounds basedon EXXONTM Butyl 268 and W type polychloroprene, scorch time can beadjusted by:

i. Addition of magnesium stearate (0.25 to 0.5 phr).ii. Reduction of the level of reactive alkyl phenol formaldehyde

resin (curing resin).iii. Use a curing resin with a lower methylol content.

3. Improvements in Heat Resistance: Resin cured butyl rubber compoundsposses good heat resistance as illustrated in the model compoundFormulary on the website, www.butylrubber.com. By blending lowunsaturation, low Mooney viscosity polymer and eliminating process oil,heat resistance can be enhanced.

4. Polymer Blends: When low unsaturation polymers are used in largeproportions, then caution has to be taken for possible bladder growthduring service. Examples of blends which can be used include:

i. Blend EXXONTM Butyl 268 and EXXONTM Butyl 068 withprocess oil.

ii. Blend EXXONTM Butyl 268 (25 phr) and EXXONTM Butyl 065 orEXXONTM Butyl 165 (75 phr) without process oil.

Guidelines for Slug HandlingSome suggestions include:

• Keep the slugs on clean trays.

• Do not use lubricant when cutting the slugs.

• Slugs should be rectangular in cross section for easier molding.

• Keep the slug weight more than the final trimmed weight of bladder by10% to 15%.



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• Spliced slug must fit the mold ledge with no overlap or extra pieces.

• Slug ends should not to be heated before splicing.

• If slugs have to be preheated before curing, then it is suggested that acirculating air oven (not electrical lamps) is used for uniform heating.Generally, slugs can be heated 1 to 4 hours at 80ºC

• After splicing, wrap the spliced area with polyethylene film to avoidcontamination by air borne dust.

Guidelines for Bladder Vulcanizing (Compression Molding)Some suggestions and guidelines include:

• Keep molds clean, free from flash, and free from plugged vents.

• Mold release agent should be used as little as possible. It is preferred toavoid mold release agents if possible.

• Badder curing temperature should always be set higher than thetemperature the bladder is expected to encounter during tire curingservice life.

• Cure temperatures for a bladder should be in the range of 190ºC to 210ºC.Higher temperature curing can allow more stable cross links and state ofcure.

• Cure time and temperature should be set to obtain optimum tension setand compression set properties.

• Close the press as rapidly as possible and reach pre-programmedhydraulic pressure within 30 seconds.

• Keep the core straight and tight.

• Mold halves should be aligned properly.

• Adequate rib venting and patio venting should be provided whiledesigning the mold to allow venting trapped air during the moldingprocess.

Guidelines for Post Curing of BladdersPost curing suggestions include:

• Generally, post curing of bladders helps to get better stabilization ofcrosslinking and state of cure that eventually offers improved bladder life(higher number of heats). Post curing can be done in an autoclave for 2 to4 hours at 140ºC to 160ºC.

• If post curing is not done, then it is suggested to over cure the bladder forbetter crosslinking stability.

Guidelines for Storage of BladdersBladders should be stored in the warehouse (open and free from ozone

and ultra violet lights) for about 2 weeks after curing. This storage period



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Types of Tire Curing Presses andOperation Sequence of Tire Curing Bladders

An example of a typical operation sequence for an AutoForm tire curingbladder is shown in Figures 10 and 11. An illustration of a typical operationsequences for a Bag-O- Matic tire curing bladder is shown in Figures 12 and13.

Figure 10Structure of a Bladder Press



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Figure 11Sequence of Operation for Curing MPT and AFV Bladders



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Figure 12Operation of a Bladder Press



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Figure 13Sequence for Operation of BOM Bladder press



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Curing Bladder Design

Since the bladder is mechanically stretched and folded at temperaturesup to 200°C with each cure cycle, avoidance of stress concentrations isimportant. The most effective approach from the design perspective is toreduce the gauge of the bladder wall. In injection molding processes, thebladder wall thicknesses can be reduced to 4.0 to 5.0 mm for passenger tireapplications.

Bladder Sidewall ThicknessThe primary purpose of the bladder is to prevent steam from leaking

into the innerliner or tire casing. Since heat is transferred through the bladder,there is a requirement to make the bladder wall as thin as possible, whilemaintaining necessary mechanical properties. If the bladder is too thin it willrupture during service. If it is too thick it will reduce heat transfer and theclamp area could be damaged.

Surface Design on the BladderThe bladder surface design will be a function of several factors:

1. A manufacturer may use the design as trade mark or as a uniquecompany characteristic.

2. The design may be chosen to permit ease of venting trapped airbetween the green tire and the bladder as the mold closes.

3. Improve the uniformity of the tire innerliner surface appearance.

Frequently the chosen bladder design will meet these parameters. Someventing is necessary and typically runs from the crown area to the bead region.Venting channels, if required, should be molded into the bladder surface and itis important to ensure that vent markings are clean, have no flash, and aresuch that no foreign material can get trapped on the bladder surface. Thiswould cause a weak spot and upon subsequent flexing the bladder could fail,particularly if the mold flash or other material is covered, over-cured, orcovered with a layer of mold release. Bladder vent marks can affect innertubedurability so for tube type tires vent markings it is suggested that the curingbladder be designed taking this into consideration. For tubeless tire - bladdercombinations, good venting is more important.

Bladder Venting and EtchingThe design of etching is chosen in order to give venting of air

entrapped between the "green" unvulcanized tire and the bladder during thepress / mold closing operation (Figure 14). Generally venting runs from the



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Figure 14Types of Venting

Examples of Slideback and Tiltback presses are shown in Figures 15and 16. Figure 17 shows a cured tire being removed from the curing press.The tire curing bladder collapses so as to pull the wall away from the tire.Then the tire is raised over the retracted bladder, dropped onto a cooling railfor several minutes before being transferred to final inspection.



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Figure 15Slide-back AutoForm Style Curing Press

Figure 16Tilt-Back Bag-O-Matic Style Curing Press



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Figure 17Tire Removal from the Curing Press Showing the

Collapsed Tire Curing Bladder

Guidelines on Bladder Maintenance

During service, bladders are exposed to severe conditions such ashigh heat (steam or hot water), gas pressure, and excessive flexing underpressure. Bladder service life is a function of a variety of factors and the rootcauses of potential failures requires detailed statistical analysis.

Bladder Life During ServiceBladder life can be improved by taking proper care during service. Forexample:

1. After curing, bladders should be stored in appropriate conditions asdiscussed earlier.

2. Bladders should be protected from ozone and UV light if stored forextended periods.

3. The clamping rings should to be tightened to 10 to 15% compressionand the compression should be uniform around the bladder.

4. It is suggested that the internal curing media (steam, inert gas) containless than 150 pphm oxygen and be free from metallic compounds,especially copper.

5. Control of bladder stretching to within 65% circumferentially and 20%



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7. If the correct bladder size is not available, a smaller size bladder ispreferred. Selection of larger size bladders should be avoided toprevent any buckles as the bladder inflates inside the green tire.

8. Periodic surface lubricant coating of the bladder or use of a green tirelubricant is beneficial to extend bladder life.

Statistical Quality Assessment for Bladder LifeBladder life can be analyzed statistically by recording the following variables:

1. Type of curing medium.2. Type of bladder (Bag-O-Matic, Autoform, Toroidal).3. Bladder size.4. Tire size.5. Press / machine number.6. Number of heats (bladder life).7. Failure type and possible causes of failure.8. Examine the defects and determine corrective actions.9. Implement corrective actions.

Typical Examples of Bladder Failure Modes A number of failure modes can be found for tire curing bladders. Forillustrative purposes, some examples would include:

1. Delamination (rough surface or smooth surface).2. Surface hardening.3. Broken at bead clamping (upper or lower).4. Blistering / Porosity (localized area or evenly).5. Inside softening (localized area or evenly).6. Bladder surface cracks (evenly or at corners of vent grooves).7. Tearing of bladder.8. Splice opening.9. Bladder growth.10. Small hard lumps in bladder.

Details on some examples of bladder failures with possible reasons andcorrective guidelines are presented in Appendix 1 and 2.

Pre-Pull Policy of Tire Curing Bladder to Save Tire ScrapBased on the statistical failure data analysis, tire producers can set the

time limit of pre-pull policy (example, after 500 heats or cure cycles). Thisreduces the possibility of tire casing losses due to bladder failure. This is alsoeconomical for the tire producer as it prevents an unscheduled stoppage oftire production and saves scrapping of tires due to a failed tire cure pressbladder.



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Special Tests for Curing Bladder Compounds

Since curing bladder service is under high heat and high pressureconditions, several laboratory tests have been designed for curing bladders.These are laboratory hot milling, hot elongation, hot tear, and hot tension set.

Hot Elongation (190°C)Physical testing at room temperature can give some useful information,

but in many cases the properties of a compound can change when measuredat the temperature used in service. This test uses a normal stress-straintensile tester with an electrically heated oven. The sample is subjected to atemperature of 190°C in the oven for 5 minutes and then mounted in the jawsof the tensile tester. After mounting of the sample in the oven chamber, thetemperature of the oven is allowed to stabilize at 190ºC before carrying outthe test. It is only necessary to measure the hot elongation at 190ºC, as thissimulates the condition of inflation and deflation of the bladder in service .It issuggested that hot elongation at break is in the range of 400% to 800%.

Hot TearThis is an important parameter to predict the durability of the bladder.

Measurement at 190ºC temperature is suggested.

Hot Tension SetHot tension set is another important parameter to assess compound

quality. Test samples are elongated 300% for one hour at 100ºC and thenrelaxed for 24 hours. Tension set property is then measured. The tension setshould be less than 10%.

Other Testing GuidelinesIn addition to the specific tests highlighted above, the tire curing bladdercompounder will want to study other material properties such as tensilestrength, tear strength, and hardness. The Formulary on the web site,www.butylrubber.com6, provides a set of typical properties that can provide asguideline for the compounder. Compound aged property retention isimportant for the bladder to achieve an adequate service life.



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New Technology and TireCuring Bladder Market Trend

All tire manufacturers want longer bladder service lives. In addition,service conditions are becoming more severe. For example, some trendsinclude:

• Tire curing temperatures are increasing and in some instances internaltemperatures may exceed 200ºC.

• Tire cure cycles under 10 minutes can be found for passenger tires.

• Expected service life may be significantly over 500 cure cycles or heats forpassenger tire curing bladders.

• Use of steam and inert gas (nitrogen) as cure heating media are the preferredmeans of tire curing. These curing media may enable an increase in blaand allow some further reduction of cycle times.

These ongoing demands for greater productivity also influence how curing bladdersare produced.

New Technology TrendsWith regard to new technology trends in curing bladders, curing bladder

production using injection molding methods is becoming more popular because ofprecision of gauge control. Figure 18 provides an illustration of both horizontal andvertically configured injection molding machines.

Figure 18Bladder Injection Molding Machines

Vertical Bladder InjectionBladder Injection Molding



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The bladder design is changing, providing less gauge in the center, whichgives more flexibility and also allows rapid and more uniform heat transfer. Sucdesign helps to cure the tire tread region faster (which can be the thickest part of tire), but it also shortens the cure cycle, thus saving energy.

When injection molding curing bladders, the screw, barrel, and extruderhead dwell times should be as short as possible. It is suggested that filling ofthe injection screw should not to be longer than 15 seconds prior to injectionoperation. Key requirements for injection of butyl rubber compounds are:

• Compounds should have good flow properties, which can be achieved bblending low Mooney viscosity butyl rubber with high Mooney viscositybutyl rubber (e.g. EXXONTM Butyl 065 with EXXONTM Butyl 268).

• Scorch times at injection molding temperatures such as 180ºC to 200ºCmust be adequate since the compound travel distance and transferring ofthe compound flowing to the mold takes longer time than the otherapplications.

• The compound from the check valve, injection cylinder, and nozzle shouhave no porosity and should have a provision for evacuation of air from closed mold.

As in any other extrusion operation, the extruder on the injection moldingmachine should be maintained and inspected for items such as screwtolerances and barrel condition to ensure no dead zones in the barrel and noair is being trapped in the compound . Examples of molds are illustrated inFigure 19.

Figure 19Injection Bladder Mold



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Advantages of Injection Molding over Compression MoldingInjection molding can provide:

• Reduced compound waste.

• Good bladder gauge uniformity.

• Reduced cure times in bladder production.

• Improved bladder service life.

• Absence of splice related defects in injection molded bladders.

• Bladders with no flash.



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Appendix 1Typical Bladder Failures and Corrective Guidelines

Bladder failure can occur at any time during its service and in manyinstances it can be difficult to assess the true cause. Failure of a curingbladder in service often results from many different factors. A useful guide tobladder life is the ‘average number of cures’ before failure and the ‘type of tfailure'. It is also beneficial to record any statistical data such as failure modesfor each supplier or manufacturer of bladders. This guideline describes typesof bladder failures and possible methods to be considered in correct them.

1. Curing Bladder Compound Mixing



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2. Blank Preparation and Molding



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2. Blank Preparation and Molding….cont.



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3. Curing Bladders in Service



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3. Curing Bladders in Service….cont.



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Appendix 2

Check List of Failures, Possible Reasons, and Comments(The common failures that occur during operation)

Failure Possible reason Comment

Delamination(rough surface)

• GPR contamination • Ensure mixers / extrudes are free from GPR

Delamination(smooth surface)

• Scorching• Lubricant / solvent

contamination

• Reduce polymer unsaturation.• Reduce resin level• Reduce methylol content of resin• Improve black dispersion to redu

temps• Do not use solvent cements in the process

SurfaceHardening

• Curative migration from tire • Use surface coating for bladder

• Consider use of brominated resin

Vent cracking • Bladder too small

• Poor ageing properties

• Improve hot elongation

• Reduce resin level• Reduce polymer unsaturation

Bladder growth • Tension set too high • Increase crosslink density

• Increase cure time or cure temperature• Increase resin level

Softening insideBladder

• Oxidation• Metal contamination (Cu, Ni,

Mn, Co)

• Install contact heater• Oxygen in steam or water (max 150 pphm)• Check for presence of brass ferrules in the

bladder which is used for bladder raising /lowering mechanism.

Porosity • Check mixer/mill operatingconditions

• Straining /Extrusion tempstoo high

• Porosity caused by high extrusion temp.often causes scorching

Small hard lumps inbladder

• Cured chloroprene

• Hard lumps in carbon black• First stage masterbatch should be

Tearing • Low hot elongation • Improve hot elongation• Reduce resin level

• Reduce polymer unsaturation

Splice opening • Poor consolidation

• Lubricant contamination• Solvent contamination

• Do not use solvents or lubricants whensplicing the blank

Poor surface on newbladder

• Dirty mold • Clean mold regularly• Unplug vents

Surface folds • Bladder too big • Check if the bladder is too big; caused bygrowth during service or wrong selection ofbladder or wrong selection of the size.



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References

1. RM Thomas and WJ Sparks. Butyl Rubber. In Synthetic Rubber. Ed GSWhitby. John Wiley and Sons, Inc., New York. 1954.

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

3. MF Tse, KO McElrath, HC Wang. Relating De Mattia Cut Growth to NetworkStructure of Crosslinked Elastomers. Polymer Eng & Science. Vol 42 (6), P 1210-1219, 2002.

4. DM Cheng, IJ Gardner, HC Wang, CB Frederick, AH Dekmezian.Spectroscopic Studies of the Structure of Butyl and Bromobutyl Rubbers.Rubber Chemistry and Technology. Vol 63 (2). P265 – 275. 1990.

5. JL White, TD Shaffer, CJ Ruff, JP Cross. Incorporation of Isoprene inIsobutylene / Isoprene Copolymers: NMR Identification of Branching inButyl Rubber. Macromolecules. Vol 28. P 3290-3300. 1995

6. www.butylrubber.com.7. Technical Bulletin. Rubber Processing Additives. Struktol Corporation. 20068. WC Smith. The Vulcanization of Butyl, Chlorobutyl rubber, and Bromobutyl

rubber. In “Vulcanization of Elastomers”. Ed G Alliger & IJ Sjothun.Reinhold Publishing Corp, NY. 1964.

9. Butyl and Halobutyl Rubbers by J.V. Fusco and P. Hous, ExxonChemical Company- Published in The Vanderbilt RubberHandbook, Thirteenth Edition, 1990.

10. S Solis, MB Rodgers, N Tambe, BB Sharma, WH Waddell. A Review ofthe Vulcanization of Isobutylene-Based Elastomers. Presented at ameeting of the American Chemical Society Rubber Division, San AntonioTX. 2005

©2014 Exxon Mobil Corporation. While the information is accurate to the best of our knowledge and beliefas of the date compiled, it is limited to the information as specified. No representation or warranty,expressed or implied, is made regarding the information, or its completeness, merchantability, or fitness for aparticular use. The user is solely responsible for all determinations regarding use and we disclaim for any loss or damage that may occur from the use of this information To the extent the user is entitled todisclose and distribute this document, the user may forward, distribute, and/or photocopy this copyrighteddocument only if unaltered and complete, including all of its headers, footers, disclaimers, and otherinformation. You may not copy this document to a Web site. ExxonMobil does not guarantee the typical other) values. Analysis may be performed on representative samples and not the actual product shipped.The information in this document relates only to the named products or materials when not in combinationwith any other product or materials. We do not represent, warrant, or otherwise guarantee, expressly orimpliedly, the merchantability, fitness for a particular purpose, suitability, accuracy, reliability, orcompleteness of this information or the products, materials, or processes described. The user iresponsible for all determinations regarding any use of material or product and any process in its territoriesof interest. ExxonMobil makes no representations or warranties against patent infringement or non-infringement of the intellectual property rights of any third party. Likewise ExxonMobil does not grant anylicense, express or implied, under any patents or patent applications owned by ExxonMobil to make, use,sell, offer for sale or import any product based upon this formulation. We expressly disclaim liability for anyloss, damage, or injury directly or indirectly suffered or incurred as a result of or related to anyone using or

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ExxonTM butyl rubber curing bladder technology manualdocshare04.docshare.tips/files/27078/270783064.pdf · 2019. 9. 30. · butyl rubber which has resulted in its wide use for high