Bladder Technology

ExxonTM butyl rubber curing curing bladder technology manual

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butyl rubber curing bladder technology manual – Ref. B1011-598E98 B1011-598E98

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Abstract

A curing bladder is an important component component of the tire vulcanizing press and the tire curing process. The proper selection of butyl polymers and compounding materials for the bladder formulation is essential in ensuring durability, required service life, and efficient curing bladder operation in a tire factory. This is due to the superior heat and steam resistance of resin cured butyl rubber which has resulted in its wide use for high heat resistant applications. Additionally, butyl rubber has very low permeability to gases and water vapor that further enhances the performance of butyl rubber tire curing bladders. This manual reviews aspects of curing bladder technology and presents some basic guidelines on compounding, processing, applications technology, and trouble shooting of common curing bladder failure modes. Model bladder compounds with selected properties are also reviewed. These compounds provide starting points for additional development work depending on the specific properties that are needed or that allow use in different types of tire curing presses and factory operating conditions. conditions.

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Abstract

A curing bladder is an important component component of the tire vulcanizing press and the tire curing process. The proper selection of butyl polymers and compounding materials for the bladder formulation is essential in ensuring durability, required service life, and efficient curing bladder operation in a tire factory. This is due to the superior heat and steam resistance of resin cured butyl rubber which has resulted in its wide use for high heat resistant applications. Additionally, butyl rubber has very low permeability to gases and water vapor that further enhances the performance of butyl rubber tire curing bladders. This manual reviews aspects of curing bladder technology and presents some basic guidelines on compounding, processing, applications technology, and trouble shooting of common curing bladder failure modes. Model bladder compounds with selected properties are also reviewed. These compounds provide starting points for additional development work depending on the specific properties that are needed or that allow use in different types of tire curing presses and factory operating conditions. conditions.

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Table of Contents

Exxon

Introduction…………………………………………………………………

4

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

6

Compounding of Tire Curing Bladders…………………….……….. 7

..

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

16

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

23

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

27

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

30

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

32

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

33

Appendices Appendix 1. Typical bladder bladder failures and corrective guidelines.. guidelines.. Appendix 2. Check list of failures and and corrective guidelines……

36 41

References…………………………………………………………………..

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Introduction Resin cured butyl rubber has carbon-carbon crosslinks which yield heat stable vulcanizates. The superior heat and steam resistance of resin cured butyl rubber has resulted in its wide use for high heat service temperature applications such as tire curing bladders. Additionally, butyl rubber has very low permeability to gases and water vapor providing the required properties for butyl tire curing bladders. A curing bladder is an important and essential part of the tire vulcanizing press. Some examples of curing bladders are shown in Figure 1. Tire producers are always working to improve the performance of curing bladders, in order to maximize (i) tire curing efficiency, (ii) factory productivity, and (iii) durability of bladders. This manual reviews current curing bladder technology and gives some basic guidelines on compounding, processing, technology, and trouble shooting for common tire press curing bladder failure modes.

Figure 1 Tire Curing Bladders

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Figure 1 Tire Curing Bladders …cont

Exxon

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butyl rubber curing bladder technology manual – Ref. B1011-598E98

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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 collapsible bladder is mounted in the lower section of the tire curing press and forms a part of the press and mold assembly. The "green" unvulcanized tire is positioned over the bladder in the bottom half the mold. When the mold is closed, pressurized steam, air, hot water, or inert gas (nitrogen) is introduced systematically (pre- programmed) into the bladder to provide internal heat and pressure for the tire shaping and curing process. The two types of tire curing presses 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-OMatic bladder. Examples of curing bladders used by the tire industry for tire curing presses are 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 – steam cure cycle. Dome temperatures can reach 190°C ( mold sidewall plates at 180°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 as follows: 1. 2. 3. 4.

Steam High Pressure hot water Cold water flush Drain

Total Cure Time (minutes)

12’00” 30’00” 4’00” 0’30” 46’30”

Given the high pressures and temperatures the bladder undergoes in multiple cure cycles, the basic properties required for curing bladder include the 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. 4. Excellent flex and hot tear resistance. 5. Low tension and compression set that maintains high elongation properties. 6. Impermeability to air, inert gas, and water vapor.

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

Compounding of Tire Curing Bladders Tire curing bladders must withstand continuous exposure to high temperatures from high pressure steam, hot water, or inert gas. This is achieved by using butyl rubber specially compounded with reactive alkyl phenol formaldehyde resin. The selection of compounding ingredients is very important with respect to the bladder life. The primary materials used in a bladder compound are the polymer (butyl rubber), cure activator, carbon black, plasticizer, zinc oxide, and curing resin. Butyl rubber (IIR, Isobutylene isoprene copolymer) is the preferred elastomer for tire curing bladders due to the following properties: 1. 2. 3. 4.

Excellent heat aging resistance, Good flex and tear resistance, Low tension and compression set, and 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 unique properties and difficult manufacturing conditions place butyl rubbers in the special purpose elastomers category, distinct from general-purpose rubbers such as polybutadiene (BR), natural rubber (NR), and styrene-butadiene rubbers (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) is 67% of that of the 1-4-isoprene structural init (0.405 nm) 3. The stereochemistry of the isobutylene unit results in close packing along the polymer chain, low free volume fraction, and consequently low permeability. Isoprene is incorporated in a trans-1,4 enchained head-to-tail arrangement to produce a random, linear copolymer.

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Figure 2 Structure of Butyl Rubber 2 (Structure I) CH3

CH2

C

CH3

CH2

C

CH

CH2

. . mo e l %) (0 8 to 3 0

CH3

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

Figure 3 Sterochemistry of Isoprene Incorporation4 CH3

CH3

CH2

C

CH

CH2

1,4 Add ition 94% Structur e I

CH2

C

CH2

CH

C

CH

CH2

CH2

1,2 Add ition 6%

CH3

3,4 Add ition 0% Structur e I minor

Structure Iminor consisting of the 1,2- enchained isoprene has also been suggested, at amounts in the order of 6% 4. No 3,4-addition products have been reported. Expanding on this work, White and coworkers further suggested the absence of a 3,4-addition structure but also suggested that Structure Iminor had the configuration illustrated in Figure 3 5. The ratio of 1,4 enchained isoprene and the minor isoprenyl derivative was dependent upon polymerization temperature but was still present, albeit in small amounts, in commercial grades of butyl rubber. White and coworkers have also reported that Structure I minor is not associated with end groups, and that the R groups arise from isobutylene and not from isoprene – isoprene addition (Figure 4). Though the authors indicated no definitive determination of the R- group but evidence was presented showing the R- groups may be longer chains 5. Thus, though the presence of a Structure I minor is accepted, its precise configuration remains to be further clarified, the primary difficulty being the very low concentration of such structures.

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Figure 4 Proposed Branched Structure I

minor

5

CH3

CH2

C

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, EXXON TM Butyl 268 can be used as a starting point in building a compound formulation. Examples of curing bladder compound formulations can be found at ‘www.butylrubber.com’ 6. Table I further illustrates a selection of the commercial grades of butyl rubber currently available from ExxonMobil Chemical Company. The table shows the range of polymer viscosities available, and nominal isoprene content 6. Table I Examples of Commercial Grades of ExxonMobilTM Isobutylene Elastomers6 Elastomer Butyl (low viscosity) Butyl (medium viscosity)

ExxonMobil Grade Identification 065

Mooney Viscosity (ML1+8 @ 125°C) 32

Isoprene (mol %) 1.05

068 268

51 51

1.15 1.70

Regular Butyl Grades Various grades of butyl rubber can be used in curing bladder applications depending on the final compound and bladder requirements and tire types being produced. Some selected grades of butyl rubber can be further described as follows: 1. EXXON™ Butyl 268: This is the most commonly used grade for curing bladder compounds. It has high Mooney viscosity (ML1+8@125°C: 51 MU) and medium amount of unsaturation (1.7 mole%). Though compounds containing this grade of butyl have good processing and mechanical properties, and good durability, mold flow properties can be further modified by blending with lower Mooney viscosity grades such as EXXON™Butyl 065. 2. EXXON™ Butyl 068: It has the same Mooney viscosity as EXXON™ Butyl 268 but with lower unsaturation (1.15 mole%). It can be blended with EXXON™ Butyl 268 to improve heat aging. Resin level should be adjusted to obtain an optimum state of cure.

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Caution is also required as a low state of cure can result in an increase in bladder growth. 3. EXXON™ Butyl 065: It has low unsaturation (1.05 mole %) and is a low Mooney viscosity (ML1+8@125 °C of 32 MU) grade. It is used in blends with EXXON™ Butyl 268 in large size “Off The Road” (OTR) tire curing bladders for improved mold flow. OTR bladders must sustain very long cure times at tire curing temperature. The term “mole percent unsaturation” refers to the number of isoprene molecules in 100 monomer molecules in the polymer. Thus, a one – mole percent 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 polymers 6.

Activators The 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 butyl require resin cure activation by adding halogen containing materials such as SnCl2 or halogen-containing elastomers such as polychloroprene. A more reactive resin cure system requiring no external activator is obtained if some of the hydroxyl groups of the methylol group are replaced by bromine. An example of commonly used commercial resin is a brominated alkyl phenol formaldehyde resin. Typical grades of polychloroprene used as resin cure activators are shown in Table II.

Carbon Black Generally, high structure carbon black ISAF or HAF (e.g. N330) which give a good balance of properties, are used in bladder compounds at levels of 50 to 60 phr. Other alternative types of carbon black are the GPF grades which show improved air aging, though ISAF grades have better steam aging properties. 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 rubber compound. Generally, a lower loading of carbon black ( e.g. 35 phr) gives better air aging and higher loading of carbon black (e.g. 65 phr) gives better steam aging. In the website www.butylrubber.com, carbon black N220 (50 phr), and N660 (60 phr) are used in the model formulations 6. An example of a curing bladder compound with properties is illustrated in Table III.

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Table II Polychloroprene Grades Grade Neoprene™ GN: Neoprene™ W (1) : Neoprene™ GNA : Neoprene™ TW : Neoprene™ TRT : Neoprene™ WRT :

Properties Shorter aging and scorch resistance. Good processing and most commonly used. Tends to show low scorch resistance. Easy processing, good mechanical properties. Crystalline resistant and good processing . Crystalline resistant, but requires higher organic accelerators levels.

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

Plasticizers Castor oil (5 phr) is the most commonly used plasticizer for bladder compounds due to its low volatility at high temperature. Castor oil reduces the tendency for a marching modulus in resin cured butyl rubber bladder compounds. Additionally it gives lower unaged modulus and good steam aging. If castor oil is not available, then oleic acid (5 phr) could be used. Compounds containing either castor oil or oleic acid have better release properties between the bladder and tire inner liner. These compounds also show better retention in aging properties due to the high boiling point and lower volatility of castor oil. Alternatively if castor oil is not available paraffinic process oils (e.g. Flexon TM 876) could be used though caution is required. Curing bladder compound properties obtained using paraffin oil in place of castor oil have been tabulated in the model formulary on www.butylrubber.com.

Zinc Oxide Zinc 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 III Model Formulary for Curing Bladder With Exxon™ Butyl 268 6 Material

Units

Amount

PHR (1)

100.00

Polychloroprene (W Type)

PHR

5.00

Carbon Black, N220

PHR

50.00

Castor Oil

PHR

5.00

Stearic Acid

PHR

1.00

Zinc Oxide

PHR

5.00

Heat Reactive Octylphenyl Formaldehyde Resin

PHR

10.00

Test Method Based on

Units and Conditions

176.00 Typical Values (2)

Mooney Viscosity ML1+4

ASTM

MU@100ºC

80.8

Mooney Viscosity ML1+8

D1646

MU@100ºC

80.6

Seconds

3.1

EXXON™ Butyl Grade 268

Total PHR Properties

MRI Mooney Scorch Minimum Viscosity

ASTM

MU@125ºC

62.2

Time to 1 pt rise

D1646

Minutes

7.8

Minutes

26.6

Time to 5 pt rise MDR Rheometer

180 °C; Arc 0.5°

ML (Minimum torque)

ASTM

dN.m

2.8

MH (Maximum torque)

D5289

dN.m

10.7

dN.m

7.8

Tc2 (time to 2 torque unit increase)

Minutes

7.0

Tc10 (time to 10% torque increase)

Minutes

3.5


Minutes

12.1


Minutes

24.6

ASTM

MPa

12.5

D412

%

860

MPa

2.5

MH-ML (” T)

Tensile Strength Elongation 300 % Modulus Hardness, Shore A

ASTM D2240

Shore A

47

Tear Strength (Die-B)

ASTM D624

KN/m

34.9

Note: 1. PHR: Parts per Hundred Rubber. 2. Values given are typical and should not be interpreted as a specification.

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Antioxidant Generally, antioxidants are not effective in improving heat resistance of resin cured butyl compounds. Some of the antioxidants (e.g. amines) could significantly retard the cure rate of regular butyl rubber compounds, sulfur based vulcanization systems, and resin curing system. Process Aids Depending 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 as organosilicones and calcium fatty acid soaps suitable for curing bladder compounds7.

Reactive Phenol Formaldehyde Resin as Curing Agent Resin cured butyl rubber compounds may display better r esistance to detrimental effects of exposure to high temperatures when compared with sulfur cured butyl rubber vulcanizates, which tend t o soften during prolonged exposure at elevated temperatures (e.g., 150 0C to 2000C). The resin cure mechanism in butyl rubber is based on the reaction of the methylol groups in the phenol-formaldehyde resin with allylic hydrogen in butyl, usually with a Lewis acid catalyst, to yield carbon-carbon crosslinks that are thermally stable2. Tire curing bladders are cured by alkylphenol formaldehyde derivatives containing methylol groups. Examples of tire curing bladder compounds, using heat reactive octylphenol formaldehyde curing resins, are given in the website www.butylrubber.com6 under Formulary section and in Table III. Commercially, there are several suppliers manufacturing reactive phenol formaldehyde resins (octylphenol formaldehyde resin). Reactive bromomethylated alkylphenol formaldehyde resins are also used. While using brominated reactive resin, the bladder compound does not require an external source of halogen such as polychloroprene. However when using such resins, compound tack can increase resulting in t he need to conduct additional factory compound process development. The simplified chemical structure of vulcanizing resin is given in Figure 5 and its possible cross-linking structures are given in Figure 6 (Van der Meer) and Figure 7 (Greth) 8,9. The vulcanizing resin is a chain of phenolic rings connected by methylene groups as illustrated in Figure 5. The terminal methylol groups (CH2OH) are the points at which the resin molecule crosslinks with the butyl polymer molecule. Van der Meer has postulated that the -OH from the methylol group combines with hydrogen on the carbon atom to double bond, establishing a carbon-carbon linking between the resin and butyl molecules at that point as shown in Figure 6. A vulcanizate crosslink is formed when this takes place at both ends of the resin molecule.

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Figure 5 Simplified Structure of a Vulcanization Resin

OH

H2 COH

OH

OH

CH2

R

CH2

R

CH2 OH

R

n

Figure 6 Crosslinking through - hydrogen8 OH

HC

H 3C

H 2C

OH

OH

CH2

CH2

CH2

CH

C

CH2

CH3

CH CH

R

R

R

n

CH2

CH2

It has also been suggested that the cure mechanism involves both the phenolic and methylol hydroxyls in a substitution reaction across t he double bond, resulting in a chroman structure as shown in Figure 7. Figure 7 Crosslinking Through Chroman Structure Formation 8,9 CH2

H 2C

CH2

O

OH

O

CH3

CH2 CH3

CH2

R

CH2

R

R

n

In summary, at this point the accepted mechanism for the resin crosslinking mechanism is illustrated in Figure 8. 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.

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Figure 8 Resin Curing of Butyl Rubber 10 OH HO

OH

CH2

OH

R'

CH2

OH heat

HO

O

CH2

R'

CH2

- H2O R

R

R

R

CH2 Formation of a Chroman ring

OH HOCH 2

CH3 C

O

HO

CH2

O R'

CH2

R

R

CH2 R

R

CH

CH3 C

C

CH2

O

CH3 CH2

CH

CH CH2

R'

C

H OH

CH2

CH3 CH

O

CH CH2

R' CH

CH3 C

CH2

Crosslinked Butyl Network Evolution R

R

Model Formulary for Tire Curing Bladders Reference should be made to the web site, www.butylrubber.com for model compounds which provide starting points for additional development work. A simplified summary of a model compound is shown in Table III. This includes processing properties and physical properties.

Processing of Curing Bladder Compounds The service condition of the tire curing bladder is unique in that it is subjected to severe conditions of heat, pressure and flexing. Therefore, good dispersion of the ingredients especially carbon black, zinc oxide, polychloroprene and curing resin is very important to achieve an adequate service life. Good dispersion of the compound ingredients enhances physical properties and allows better retention of physical properties.

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Processing of Tire Curing Bladder Compounds The service conditions of the tire curing bladder are unique. It is subjected to severe conditions of heat, pressure and flexing. Therefore, good dispersion of the ingredients, especially carbon black, zinc oxide, polychloroprene and curing resin, is very important to achieve adequate service life. A good dispersion of the compounding ingredients can improve physical properties and also retention of physical properties. The manufacturing process flowchart for a tire curing bladder is illustrated in Figure 9. Briefly, the major processing steps are: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Masterbatch mixing, Straining, Final batch mixing, Extrusion of slugs/blanks, Cutting of slugs/blanks, Blank splicing, Vulcanization (compression molding or injection molding), Post-cure or conditioning, and, Storage of bladders. Figure 9 A Flow Chart for Curing Bladder Production

Not every step in this comprehensive schematic of curing bladder production may be necessary. For example, use of high quality butyl rubber and compounding materials may eliminate the need for straining. Modern cold feed extruders can eliminate the need for warm-up mills. However, strainers on the extruder die can represent good practice and may help in obtaining a more uniform extrudate.

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Guidelines for Mixing Butyl rubber is primarily a saturated polyisobutylene copolymer with 1.05 to 2.30 mol% of isoprene. It is important t o avoid contamination of unsaturated elastomers such as natural rubber, SBR and polybutadiene rubber with butyl rubber. Due to the difference in the state of cure between butyl and other unsaturated elastomers, contamination could lead to loss in compound physical properties. Bale cutters, internal mixers, two roll open mills, strainers and extruders which are used to process butyl rubber and other unsaturated rubbers should be cleaned thoroughly with a clean-out compound. Depending on specific factory processes, it is sometimes suggested that butyl rubber ibe pre-masticate for 45 seconds in an internal mixer or open roll mill. Alternatively preheating for 24 hours at 50ºC will help. An internal bale temperature of 45ºC to 50ºC, or pre-mastication of butyl rubber, facilitates the easy incorporation of compounding ingredients. Butyl rubber compound mixing is done in two stages in the internal mixer (Banbury mixer); the first stage, non-productive, or masterbatch contains all the ingredients except the curatives. It is also sometimes suggested to separately pre-masticate polychloroprene rubber to improve homogeneity of bladder mix compound. The 2 nd stage is the final or productive step where the vulcanization system is added.

Masterbatch Mixing Masterbatch mixing of tire curing bladder compounds may be carried out in an internal mixer. In order to improve dispersion and to prevent trapped air, it is suggested that the masterbatch weight be increased by 10% to 20% compared to natural rubber or emulsion SBR compounds at an equivalent specific gravity. There are varieties of loading sequence practiced in the industry as shown in Table IV and Table V. It is suggested that the starting temperature of an internal mixer for masterbatch mixing be set at 75ºC to 80ºC.

Straining & Cooling Upon discharge from the internal mixer it is suggested that the masterbatch or first stage mixed compound be immediately passed through a strainer pack of 20 to 30 mesh or 30 to 40 mesh. This will eliminate the need for re-warming the stock for straining. The strained stock then can be stored on a tray, which has been lightly dusted with talc.

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Table IV Mixing Procedure for a Model Curing Bladder Compound First Stage Masterbatch Mixing in an Internal Mixer Time / Dump Temperature Operation Sequence 0 Minute Add butyl rubber and polychloroprene (if included in the formulation) 3/4 Minute

Add carbon black, oil, other compounding additives.

170ºC to 180ºC

Dump.

Final Productive Batch or Second Stage Mixing Finalization can be carried out in the internal mixer or two roll open mill. For final or productive batch mixing in internal mixer, it is suggested that the batch weight starting point setting be 80% of the mixer volume, and then optimized for the specific equipment. Table V Mixing Procedure for a Model Curing Bladder Compound Second Stage or Productive Mixing in an Internal Mixer Time & Dump Temperature Operation Sequence 0 Minute Add half masterbatch, zinc oxide and powered curing resin, then remaining half of masterbatch 100ºC to 110ºC

Dump

Three Stage Mixing (Masterbatch and Final Batch) Three stage mix cycles are suggested if a lower Mooney viscosity compound is required. This may be needed to improve mold flow properties such as when injection molding bladders (Table VI). Suggested Precautions when Mixing Suggestions include the following: 1. The mixing equipment is cleaned before and after mixing butyl rubber to avoid 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 . 4. If zinc oxide dispersion is a concern, then the addition of polychloroprene and zinc oxide could be reversed. 5. If brominated alkyl phenol formaldehyde resin is used, it is suggested that zinc oxide be added with the carbon black in the masterbatch.

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6. It is suggested that the curing resin be added with the last 1/3 of the masterbatch and folded into the compound stock on the conveyor belt of internal mixer feed hopper. 7. After dumping it is suggested that the batch be cooled immediately.

Table VI Mixing Procedure for a Model Curing Bladder Compound

Time & Dump Temperature 1st Stage / Masterbatch

Three Stage Mixing Operation Sequence

0 Minute 1 Minute 130ºC 160ºC to 170ºC 2nd Stage

Add butyl rubber, polychloroprene Add black, oil, stearic acid Cleaning Dump

0 Minute 130ºC 3rd Stage / Final Batch

Master batch, zinc oxide Dump

0 Minute 100ºC

2nd stage batch and powdered resin Dump

Guidelines for Mill Feeding for Hot Extrusion Some 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 size of 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 extruded blank especially for hot feed extrusion processes. Some additional suggestions and points are listed as follows: 1. Two warm-up mills are suggested to ensure optimum rolling blank to minimize porosity. 2. Force fed extruders fitted with powered feed rolls are suggested so as to minimize trapped air.

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3. Maintain a full extruder barrel and maximum back-pressure to help prevent porosity. 4. Generally, die head temperature can be set at around 110ºC to 130ºC. Compound temperature will also depend on compound Mooney viscosity and the extruder head pressure. 5. While extruding, good water circulation in the screw (typical temperature of the barrel and screw can be in the order of 55ºC to 65ºC) should be maintained, with the screw cooler than the barrel. 6. It is suggested that the extruded stock temperature is maintained at less than 120ºC and internal slug temperature is between 95ºC to 120ºC. 7. The speed of feed conveyor, extruder rpm, take-off conveyor and cooling line speed should be synchronized to prevent pulling and drawdown of slugs dimensions. 8. Cool down the extruded blank with a chilled water bath or chilled water spray. The extruded dried blanks should be stored on clean trays and away from airborne dust and foreign materials.

Guidelines for Cold Feed Extrusion Some suggestions include: 1. Use a force feed roller for extruder feeding. Do not fold feed strips 2. 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 reduce 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 speed should be synchronized to prevent pulling of slugs and draw-down of slug dimensions. 8. Cool down the extruded blank with a chilled water bath or chilled water spray .

The Advantages of Cold Feed Over Hot Feed Extruder These include: 1. Two roll mills are not required, resulting in floor space savings and lower capital cost. 2. Less staff. 3. Longer length (L) of screw over smaller screw diameter (D), L/D ratio of the cold feed extruder can result in a more uniform viscosity of the compound. 4. Capability of handling high and low Mooney viscosity compounds. 5. In cold feed extrusion, it may be easier to control stock temperature and slug dimensions.

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Secondary Processing Suggested Improvement Guidelines Mold flow, scorch resistance, and heat degradation are three major process and product concerns encountered with tire curing bladders. 1. Guidelines on Flow Improvement: Good compound flow and knitting properties are important criteria to ensure improved properties during the molding process and to improve bladder life. These properties are particularly important for larger bladders and for injection molding of bladders. 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 based on EXXONTM Butyl 268 and W type polychloroprene, scorch time can be adjusted 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 compounds posses good heat resistance as illustrated in t he model compound Formulary on the website, www.butylrubber.com. By blending low unsaturation, low Mooney viscosity polymer and eliminating process oil, heat resistance can be enhanced. 4. Polymer Blends: When low unsaturation polymers are used in large proportions, then caution has to be taken for possible bladder growth during service. Examples of blends which can be used include: i. Blend EXXONTM Butyl 268 and EXXONTM Butyl 068 with process oil. ii. Blend EXXONTM Butyl 268 (25 phr) and EXXON TM Butyl 065 or EXXONTM Butyl 165 (75 phr) without process oil.

Guidelines for Slug Handling Some 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 by • 10% to 15%.

Guidelines for Slug Splicing Slugs should be skived cut and ends pressed together as a scarf joint, • ensuring that there is no air entrapment.

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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 a circulating 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 avoid contamination 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 to • avoid mold release agents if possible. Badder curing temperature should always be set higher than the • temperature the bladder is expected to encounter during tire curing service 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 of cure. Cure time and temperature should be set to obtain optimum tension set • and compression set properties. Close the press as rapidly as possible and reach pre-programmed • hydraulic 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 while • designing the mold to allow venting trapped air during the molding process.

Guidelines for Post Curing of Bladders Post curing suggestions include: Generally, post curing of bladders helps to get better stabilization of • crosslinking and state of cure that eventually offers improved bladder life (higher number of heats). Post curing can be done in an autoclave for 2 to 4 hours at 140ºC to 160ºC. If post curing is not done, then it is suggested to over cure the bladder for • better crosslinking stability.

Guidelines for Storage of Bladders Bladders should be stored in the warehouse (open and free from ozone and ultra violet lights) for about 2 weeks after curing. This storage period helps to stabilize crosslinks and relaxes stress. Generally bladders, aft er storage, exhibit better life (more number of heats) than those bladders used immediately after curing.

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Types of Tire Curing Presses and Operation Sequence of Tire Curing Bladders An example of a typical operation sequence for an AutoForm tire curing bladder is shown in Figures 10 and 11. An illustration of a typical operation sequences for a Bag-O- Matic tire curing bladder is shown in Figures 12 and 13. Figure 10 Structure of a Bladder Press

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Figure 11 Sequence of Operation for Curing MPT and AFV Bladders

4

3

2

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Figure 12 Operation of a Bladder Press

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Figure 13 Sequence for Operation of BOM Bladder press

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Curing Bladder Design Since the bladder is mechanically stretched and folded at temperatures up to 200°C with each cure cycle, avoidance of stress concentrations is important. The most effective approach from the design perspective is to reduce the gauge of the bladder wall. In injection molding processes, the bladder wall thicknesses can be reduced to 4.0 to 5.0 mm for passenger tire applications.

Bladder Sidewall Thickness The 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, while maintaining necessary mechanical properties. If the bladder is too thin it will rupture during service. If it is too thick it will reduce heat transfer and the clamp area could be damaged.

Surface Design on the Bladder The bladder surface design will be a function of several factors: 1. A manufacturer may use the design as trade mark or as a unique company characteristic. 2. The design may be chosen to permit ease of venting trapped air between 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. Some venting 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 it is important to ensure that vent markings are clean, have no flash, and are such that no foreign material can get trapped on the bladder surface. This would cause a weak spot and upon subsequent flexing the bladder could fail, particularly if the mold flash or other material is covered, over-cured, or covered with a layer of mold release. Bladder vent marks can affect innertube durability so for tube type tires vent markings it is suggested that the curing bladder be designed taking this into consideration. For tubeless tire - bladder combinations, good venting is more important.

Bladder Venting and Etching The design of etching is chosen in order to give venting of air entrapped between the "green" unvulcanized tire and the bladder during the press / mold closing operation (Figure 14). Generally venting runs from the crown area of the bladder to the bead area. Venting channels are preferably molded into the bladder surface. In addition, irregularities such as stamping information on the bladder wall can create flaws in the tire innerliner which can then be a site for crack initiation. Any markings or emblems should therefore designed to prevent any later product performance concerns.

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Figure 14 Types of Venting

Examples of Slideback and Tiltback presses are shown in Figures 15 and 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 rail for several minutes before being transferred to final inspection.

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Figure 15 Slide-back AutoForm Style Curing Press

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

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Figure 17 Tire Removal from the Curing Press Showing the Collapsed Tire Curing Bladder

Guidelines on Bladder Maintenance During service, bladders are exposed to severe conditions such as high heat (steam or hot water), gas pressure, and excessive flexing under pressure. Bladder service life is a function of a variety of factors and the root causes of potential failures requires detailed statistical analysis.

Bladder Life During Service Bladder life can be improved by taking proper care during service. For example: 1. After curing, bladders should be stored in appropriate conditions as discussed earlier. 2. Bladders should be protected from ozone and UV light if stored for extended periods. 3. The clamping rings should to be tightened to 10 to 15% compression and the compression should be uniform around the bladder. 4. It is suggested that the internal curing media (steam, inert gas) contain less than 150 pphm oxygen and be free from metallic compounds, especially copper. 5. Control of bladder stretching to within 65% circumferentially and 20% laterally is suggested. 6. The correct size of bladder and tire fitment are important to maximize bladder life.

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7. If the correct bladder size is not available, a smaller size bladder is preferred. Selection of larger size bladders should be avoided to prevent any buckles as the bladder inflates inside t he green tire. 8. Periodic surface lubricant coating of the bladder or use of a green tire lubricant is beneficial to extend bladder life.

Statistical Quality Assessment for Bladder Life Bladder 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. For illustrative 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 and corrective guidelines are presented in Appendix 1 and 2.

Pre-Pull Policy of Tire Curing Bladder to Save Tire Scrap Based on the statistical failure data analysis, tire producers can set the time limit of pre-pull policy (example, after 500 heats or cure cycles). This reduces the possibility of tire casing losses due to bladder failure. This is also economical for the tire producer as it prevents an unscheduled stoppage of tire production and saves scrapping of tires due to a failed tire cure press bladder.

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Special Tests for Curing Bladder Compounds Since curing bladder service is under high heat and high pressure conditions, 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 measured at the temperature used in service. This test uses a normal stress-strain tensile tester with an electrically heated oven. The sample is subjected to a temperature of 190°C in the oven for 5 minutes and then mounted in the jaws of the tensile tester. After mounting of the sample in the oven chamber, the temperature of the oven is allowed to stabilize at 190ºC before carrying out the test. It is only necessary to measure the hot elongation at 190ºC, as this simulates the condition of inflation and deflation of the bladder in service .It is suggested that hot elongation at break is in the range of 400% to 800%.

Hot Tear This is an important parameter to predict the durability of the bladder. Measurement at 190ºC temperature is suggested.

Hot Tension Set Hot tension set is another important parameter to assess compound quality. Test samples are elongated 300% for one hour at 100ºC and then relaxed for 24 hours. Tension set property is then measured. The tension set should be less than 10%.

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

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New Technology and Tire Curing Bladder Market Trend All tire manufacturers want longer bladder service lives. In addition, service conditions are becoming more severe. For example, some trends include: Tire curing temperatures are increasing and in some instances internal • temperatures 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 for • passenger tire curing bladders. Use of steam and inert gas (nitrogen) as cure heating media are the preferred • means of tire curing. These curing media may enable an increase in bladder life and allow some further reduction of cycle times. These ongoing demands for greater productivity also influence how curing bladders are produced.

New Technology Trends With regard to new technology trends in curing bladders, curing bladder production using injection molding methods is becoming more popular because of precision of gauge control. Figure 18 provides an illustration of both horizontal and vertically configured injection molding machines.

Figure 18 Bladder Injection Molding Machines

Bladder Injection Molding Machine

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Vertical Bladder Injection Molding Machine

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The bladder design is changing, providing less gauge in the center, which gives more flexibility and also allows rapid and more uniform heat transfer. Such a design helps to cure the tire tread region faster (which can be the thickest part of the tire), but it also shortens the cure cycle, thus saving energy. When injection molding curing bladders, the screw, barrel, and extruder head dwell times should be as short as possible. It is suggested that filling of the injection screw should not to be longer than 15 seconds prior to injection operation. Key requirements for injection of butyl rubber compounds are: •

•

•

Compounds should have good flow properties, which can be achieved by blending low Mooney viscosity butyl rubber with high Mooney viscosity butyl rubber (e.g. EXXONTM Butyl 065 with EXXONTM Butyl 268). Scorch times at injection molding temperatures such as 180ºC to 200ºC must be adequate since the compound travel distance and transferring of the compound flowing to the mold takes longer time than the other applications. The compound from the check valve, injection cylinder, and nozzle should have no porosity and should have a provision f or evacuation of air from the closed mold.

As in any other extrusion operation, the extruder on the injection molding machine should be maintained and inspected for items such as screw tolerances and barrel condition to ensure no dead zones in the barrel and no air is being trapped in the compound . Examples of molds are illustrated in Figure 19.

Figure 19 Injection Bladder Mold

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Advantages of Injection Molding over Compression Molding Injection molding can provide: • • • • • •

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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 1 Typical Bladder Failures and Corrective Guidelines Bladder failure can occur at any time during its service and in many instances it can be difficult to assess the true cause. Failure of a curing bladder in service often results from many different factors. A useful guide to bladder life is the ‘average number of cures’ before failure and the ‘type of t he failure'. It is also beneficial to record any statistical data such as failure modes for each supplier or manufacturer of bladders. This guideline describes types of 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 Delamination (rough surface) Delamination (smooth surface)

•

• •

Possible reason GPR contamination Scorching Lubricant / solvent contamination

•

• • • •

•

Surface Hardening Vent cracking

•

Curative migration from tire

• •

• •

Bladder too small Poor ageing properties

• • •

Bladder growth

•

Tension set too high

• • •

Softening inside Bladder

Porosity

• •

•

•

Small hard lumps in bladder Tearing

• • •

• •

Poor surface on new bladder Surface folds

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•

Check mixer/mill operating conditions Straining /Extrusion temps too high

•

Porosity caused by high extrusion temp. often causes scorching

Cured chloroprene Hard lumps in carbon black Low hot elongation

•

First stage masterbatch should be strained

•

Improve hot elongation Reduce resin level Reduce polymer unsaturation Do not use solvents or lubricants when splicing the blank

• •

Poor consolidation Lubricant contamination Solvent contamination Dirty mold

•

• •

•

Use surface coating for bladder Consider use of brominated resin Improve hot elongation Reduce resin level Reduce polymer unsaturation Increase crosslink density Increase cure time or cure temperature Increase resin level 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.

•

• •

Reduce polymer unsaturation. Reduce resin level Reduce methylol content of resin Improve black dispersion to reduce strainin temps Do not use solvent cements in the process

Oxidation Metal contamination (Cu, Ni, Mn, Co)

•

Splice opening

Comment Ensure mixers / extrudes are free from GPR

Bladder too big


•

Clean mold regularly Unplug vents Check if the bladder is too big; caused by growth during service or wrong selection of bladder or wrong selection of the size.

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References 1. RM Thomas and WJ Sparks. Butyl Rubber. In Synthetic Rubber. Ed GS Whitby. 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 Network Structure of Crosslinked Elastomers. Polymer Eng & Science. Vol 42 (6), P 12101219, 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 in Isobutylene / Isoprene Copolymers: NMR Identification of Branching in Butyl Rubber. Macromolecules. Vol 28. P 3290-3300. 1995 6. www.butylrubber.com. 7. Technical Bulletin. Rubber Processing Additives. Struktol Corporation. 2006 8. 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, Exxon Chemical Company- Published in The Vanderbilt Rubber Handbook, Thirteenth Edition, 1990. 10. S Solis, MB Rodgers, N Tambe, BB Sharma, WH Waddell. A Review of the Vulcanization of Isobutylene-Based Elastomers. Presented at a meeting of the American Chemical Society Rubber Division, San Antonio TX. 2005 ©2014 Exxon Mobil Corporation. While the information is accurate to the best of our knowledge and belief as 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 a particular use. The user is solely responsible for all determinations regarding use and we disclaim liability for any loss or damage that may occur from the use of this information To the extent the user is entitled to disclose and distribute this document, the user may forward, distribute, and/or photocopy this copyrighted document only if unaltered and complete, including all of its headers, footers, disclaimers, and other information. You may not copy this document to a Web site. ExxonMobil does not guarantee the typical (or 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 combination with any other product or materials. We do not represent, warrant, or otherwise guarantee, expressly or impliedly, the merchantability, fitness for a particular purpose, suitability, accuracy, reliability, or completeness of this information or the products, materials, or processes described. The user is solely responsible for all determinations regarding any use of material or product and any process in its territories of interest. ExxonMobil makes no representations or warranties against patent infringement or noninfringement of the intellectual property rights of any third party. Likewise ExxonMobil does not grant any license, 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 any loss, damage, or injury directly or indirectly suffered or incurred as a result of or related to anyone using or relying on any of the information in this document. There is no endorsement of any product or process, and we expressly disclaim any contrary implication. The terms “we”, “our”, “ExxonMobil Chemical”, or “ExxonMobil” are used for convenience, and may include any one or more of ExxonMobil Chemical Company, Exxon Mobil Corporation, or any affiliates they directly or indirectly steward. ExxonMobil, the ExxonMobil Logo, the “Interlocking X” Device and all other product names used herein are trademarks of ExxonMobil unless indicated otherwise.

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Bladder Technology

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