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BLADDER CARE IN SERVICE Samir Majumdar
* , GR Reddy
EXEL RUBBER LIMITED, HYDERABAD
** 1. INTRODUCTION
With on going liberalization process there has been a drastic change in the Indian Economy and there is sign of positive growth of both Agricultural and Industrial production. Tyre industry growth in any country is driven from growth on the transportation. India’s transportation growth is basically driven from agricultural growth. Historic trend of agricultural growth in India is 7 - 8% wh ereas, for excep tional year is 2003 where the tyre industry, it is 5-6% and the exceptional tyre industry has already shown to more than 10%. The average growth of vital truck segment has been recorded to above 12%. Average monthly production in 2003 is Truck – 0.9m, PC – 0.773m, LCV – 0.383m. Scooter tyre growth rate has been 9% with 0.7m of tyres per month and MC growth rate by 7% with average production of 1.3m tyres per month. As on today, the major tyre players are; Apollo Tyres Ltd, MRF Ltd., JK Tyres Ltd., Birla Tyres and Good Year India Ltd. Together they produce 65 K tons of materials and 4.0 4. 0 million of tyres per month. Experts predict that number of players in Indian market will shrink like what has happened world wide yet growth of Indian Tyre Industry is evident because of following: Economical growth of 8-9 % ( 8.4% by the end of 2003) , one of the fastest in the world. Continued thrust on the investment plan by Govt. of India.
Forex reserves of more than $ 100 billion A huge population of above 1.0 billion Average income of lower middle class and middle class people are increasing due to economical growth on the investment policy by Government of India. Constraints of Indian Railways to met the growing demand for passenger and goods transport.
* For all correspondences.
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2** Manufacturer of Two/Three Wheeler Tyres, Inner Tubes of all sizes, Curing Airbags and Bladders, Splice Jaws and Turn up Bladders for Tyre building machines and other moulded rubber items .
Road transport is the major link for essential goods to the rural masses and transportation of farm produce to the cities. Development of golden quadrilateral and North-South & East – West Corridors National Highways. More and more capacity utilization by tyre industries. Continued thrust of exports by all tyre industries (25 – 30% growth) Easy financing for vehicle purchase. The year 2003 was a boom period for Indian tyre industries. It is expected that the industry growth of 2003-2004 would be between 1112%.Appeares that Apollo tyre will be leading in the market share in the above financial year. The market share of the major players during April-Oct. 2003 (ATMA) are: TYRE CO. SHARE(%) Apollo
26
JK 24
MRF 18
CEAT 16
BIRLA 10
GOODYEAR 5
OTHERS 1
TOTAL 100%
Tyre became one of the crucial component from bicycle to airplanes for
transport of goods and people particularly on the on set of The World War I (1914-1918). The World War II (1939-45), however, further accelerated making special type of tyres due to different requirement of vehicle in war. Page 3
3The story of the invention of the pneumatic tyre by R. W. Thomson ( 1846) and subsequent reinvention by J. B. Dunlop ( 1888) is well known. The word “Tyre” is practically derived from the word “attire” , meaning a protective covering. “ Tire ” spelled by Americans is closed to “attire”. Curing Bladder is the essential component of tyre manufacturing process. All pneumatic tyres are cured with curing bladder be it with OTR, TB, LCV, PC, MC or BC. There are various reasons on the causes of poor bladder life but one important cause could be due to poor processing of bladder compound and poor handling of bladder on the shop floor. The dispersion level of bladder compound varies from 50 % to > 98% in different tyre industries. High level of dispersion of the ingredients also improves the bladder life. It is the art of processing technology how one achieves the dispersion level greater than 90%. In major cases for better bladder life , bladder care in service is required to give more attention than modifying compounding and / or processing. Particularly in developing countries , bladder care in service plays a very significant role for the life of the bladder. Infact, the major failure of bladder in tyre industries of developing countries is due to the adverse conditions on which the bladder has been exposed to. We would like to discuss here on bladder care in service with respect to : The Product Properties Ingredients Compounding Manufacturing Average life Bladder care in service Summary
2. THE PRODUCT
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4A green tyre is cured with compression moulding in a tyre curing press. Out side tyre i.e. tread and side wall portion is heated by mould which is normally heated with steam. The inside tyre i.e. the bead and inner liner portion will be heated through bladder. In fact, a bladder is the heat transformation device, which is heated either by steam, hot water or by both or by heated nitrogen. A bladder can be stressed up to 25-30% inside the curing press and should be capable to hold 18-35 kg/cm² pressure to give high degree of tyre material consolidation. A good bladder should, therefore, give the following performances : Best air/steam/water impermeability Good heat transfer Capable of retaining high pressure Capable to give high degree of tyre material consolidation High degree of flexibility to take care of cyclic changes in stress under water/nitrogen/steam pressure as well as deformation under vacuum. High degree of resistance to growth, which can cause buckle problem and subsequent scrap of the tyre High degree resistance to Ozone/Oxygen/Heat degradation 3. PROPERTIES Based on the above performance requirement of the curing bladder, the following elastomeric and vulcanizate properties have been suggested by experts : Butyl rubber with high level of impermeability and low level of unsaturation . Average hardness 60 – 70 Shore A % Elongation > 600 Tensile strength > 140 kg/cm². Tension set 12% max. to take care of bladder growth in service. A Carbon - Carbon bond cross link system , capable of highest heat resistance. 4. INGREDIENTS
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5Recommended compound formulation is Butyl- 95.0 - 100.0 / Neoprene WRT -5.0 – 7.0 / HAF black - 50.0 – 60.0 / Castor oil - 5.0 – 7.0 / Stearic Acid – 1.0 / ZnO - 5.0 and Curing Resin SP 1045 – 5.0 10.0 Phr. Each raw materials given above has distinct functions to achieve the desired properties of curing bladder. Criterions for a good bladder compounds are : Should mix well (dispersion). Can be processed satisfactorily (scorch safety). Should have good flow so that moulds satisfactorily. Should have good physical properties. Should exhibit good heat, steam ageing and ozone properties. Should have lowest possible tension set but highest possible elongation. Functions of ingredient are discussed below: 4.1. Butyl Rubber It is a copolymer of isobutylene and isoprene rubber.It has very high level of gas/steam/moisture impermeability. Imparts very high heat resistance/ageing resistance when cross-linked with Methylol Phenol Resin. Normal dose is either 100.0 phr or 95.0 phr. If better flow properties are required for a specific reason eg. Injection molding , low unsaturation / low mooney grades are used or blended. 4.2. N-330 It is a reinforcing filler and it helps in maintaining the mooney for processability and vulcanizate properties after cure. Normal dose in the industry is 50.0 phr. Increasing black levels result in greater embrittlement upon air aging, but shows better resistance to reversion . The 50 parts level appears to be a good compromise for the bladder application. In general, 50 phr loading is very common in tyre industries, in a few cases there are uses between 55-60 phr loading . In some cases, high structure HAF (N347) is also used in bladder stocks at a level of 50 - 60 phr which gives a good balance of properties. Other alternative carbon blacks are FEF (N550) and ISAF(N220) combination . The HAF ( N330) and ISAF (N220) black
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6combination also exhibit the best balance of properties for the bladder application. In general, the smallest particle black (SAF / N110) shows the best resistance to reversion in steam ,while in resistance to air aging, it was somewhat poorer than the other blacks.
Acetylene Black,SAF(N110) and ISAF(N220) black are generally hard to get good dispersion . Acetylene black is reported to give better heat transfer (can save at least 10 % of curing time ) than any other conventional black but it has not yet been popularized due its problem to have inadequate dispersion in bladder compound and eventually it reduces the bladder life considerably. 4.3. Castor Oil It is a processing aid. It helps in the dispersion of black. This is an oil of vegetable origin and chemically it is Ricinoleic Acid ( bp 226.8 °C). Its high boiling point helps to retain elongation at elevated temperature. Acid medium helps to activate cross-linking system. Castor oil is used almost exclusively for bladder compounds but a high viscosity low volatility paraffinic oil or oleic acid oil (bp 285.6°C) can also be used as a replacement. These oils can act not only as a plasticizer for the compound but also as an release agent between the bladder and the t yre liner. The moisture content and the purity of these processing oils (Oleic acid or high viscosity paraffinic oil or castor oil ) will play a vital role in the dispersion and ageing properties of the compound. The normal dose is 5 phr and such a low dose of these oils results in stocks with a considerably higher modulus and retension of the physical properties.However, the oil dose can vary from 5.0 to 7.0 phr depending on the prossability requirement. 4.4. Neoprene It is an activator for the cross linking system of methylol p henol resin and butyl rubber.There is no significant differences in original and aged physical properties with GN or WRT catgories of neoprene. However, WRT is more stable, less crystalline,better dispersible than GN grade so WRT is often used. Dosages can vary from 5 to 7 phr in the compounding formulation. Page 7
7The resin cure of the butyl bladder compound is a Friedel - Crafts type of reaction, via a metal halide, normally ZnX 2
. When using a standared phenolic resin the activator is usally a chlorinated organic chemical or polymer. The only exceptions to this is brominated resin which is also used as the curative for butyl and this contains bromine in the polymer. In order to achieve good results of activator it is better to use neoprene fresh (not older than 2 yrs. from date of manufacture) and neoprene should be milled on two roll
mill before putting in to banbury for better dispersion. Besides neoprene, which has been widely accepted in tyre industry as the best activator, other activators are ; Chlorobutyl, Chlorowax, Chloro Sulphonated Polyethylene etc.The above activators have much theoretical importance, practically , neoprene is used all over,though, it is a costly polymer. 4.5. Stearic Acid It is not absolutely necessary in bladder compound formulation , yet, with comparatively low level of processing oil, stearic acid helps in carbon black dispersion. Acid medium also helps in the activation of cross-linking. It also helps in releasing from mould. Only 1.0 ph r is sufficient. 4.6. Zinc Oxide This is an activator. In fact zinc oxide helps in the formation of metal halide with neoprene which helps in cross link formation. Normal dose is 5.0 phr but at least 2.0 phr is necessary for best retention of physical properties. 4.7. Methylol Phenol Resin Almost all bladder stocks in the world are cured by a standared Alkyl Phenol Folmaldehyde resin containing a methylol group. Although most bladder stocks are cured by neoprene activated Methylol Phenol Resin, it is possible to use a brominated resin as well , such as, Schenectady SP 1055, which is self activating and can eliminate the use of neoprene as the activator. However, it is to be noted that shelf life of any halogenated organic compound is very important for its activity. Page 8
8Simply replaced with brominated resin, it may result in p rocessing difficulties due to a Scorch problem. In such cases, a lower concentrarion of brominated resin or blend of brominated / non brominated resin should be considered. Resin level should also be adjusted when a different level of unsaturation in polymer is used. Alkyl Phenol Folmaldehyde resin is the vulcanizing chemical, which gives carbon-carbon cross-link with butyl rubber via methylol phen ol group. Resin levels above 10 phr offer no advantage in heat resistance. Best property retention after aging is exhibited at the 5.0 phr level, which is the borderline for adequate cure. However, between 5 and 10 phr constitute the best compromise for the bladder application in tyre industry. In most cases it is used between 7-8 phr. 5. COMPOUNDING
Recommended compound formulation is already discussed. Compounding formulation for curing bladder will be similar in all industries except for the changes in the modulus and hardness that may be required by individual tyre industries due to processing differences. Yet, scorchyness, mould flow and heat degradation are the common problem in curing bladder. We will discuss here on : Flow improver Scorch retarder Improve on heat resistance. 5.1 Flow Improver Good flow properties and good knitting properties of the stock are important criteria to assure the sufficient level of life . These properties are important especially for large size bladders e.g. TB, OTR bladder and injection molding. Typical approaches are : Increase the level of process oil Blend with low Mooney viscosity butyl Add processing aids like STRUKTOL WB 16 /RUBID 16 or STRUKTOL WS 180 ( about 2 Phr is effectve to use in the compounds).
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95.2 Scorch Retarder Typical scorch retarders for butyl and neoprene based compounds are : Magnesium stearate 0.25 - 0.5 phr Reduce resin level Evaluate resins with low methylol content In tyre industry, however, the later two are extensively practiced. Best property retention after aging is exhibited at the 5.0 phr level, which is the borderline for adequate cure. 7-8 phr is very popular in the industry , incase of scorchy problem, it can be tried anywhere between 5-7 phr. Methylol content in most cases varies from 8-12 % and this is the specifications by most of the bladder producing units.Incase of scorchy, specification of 7-10 %methylol content could be well managed. In most cases the scorchy problem is due to improper straining of compound stock. Bladder compound is comparatively harder than inner tube compound and has to be processed differently. During
straining , compound is required to be soft and adequate cuts and folds are to be provided on the two roll mill before feeding for the straining. Straining meshes should be changed at desired number of batches. 5.3. Heat Resistance Resin cured butyl compounds already have good resistance to heat and ozone/oxygen attack and the compound formulation is quite international and has little variation. However, there is still some room to improve on the heat resistance and ozone attack like : Use Butyl with low unsaturation level. Blend with low unsaturation butyl. No process oil in the compounding formulation. Anitoxidants.
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10 When the contents in the central mechanism system include dissolved oxygen beyond certain limit , there is no significant difference reported with lower unsaturation level of butyl. It is known that the retention of physical properties after ageing with or without antioxidants are almost identical . No antioxidants are recommended, therefore, for curing bladder. However , 1.0 phr of crystalline wax helps not only in filler dispersion but also on retaining glossy surface of bladder.Some time rough surface of bladder complaint could be taken care of by addition of wax. 6. MANUFACTURING Compound formulation of bladder compound will be similar in all tyre industries. It is the state-of-art on the processing technology that gives the consistency and higher life expectancy of the product. Processing of bladder is different in different industries.Bottom line is how one gets better dispersion at lowest possible cost and how one gets the best product at lowest possible cost.Finally the produ ct will be judged by the number of heats have been given by the produ ct, higher the heats lower is the cost per kg of tyre production. A flow sheet diagram of the bladder manufacturing is shown below : Master batch Straining Final Batch Straining (mixing in banbury) (mixing in banbury) Splice
Blank Splicing Cooling Extrusion of blanks Jamming Pre-warming Curing Post curing Post curing at room Page 11
11 Of Blank @ 200°C @ 150°C(2-4 hrs.) temperature for (upto 210 °C) (15-30 days) Transport Storage 7. AVERAGE LIFE It is always a debatable issue on the average life of bladder. This is because different tyre industry uses different condition to use of bladder. bladder life also varies on the type of moulding like compression vs. injection moulding. It is reported that bladder with injection moulding provides better life . The reason is obvious because of the fact that bladder gauge can be controlled precisely with injection moulding and a bladder with thin gauge , cure cycle could be reduced to the extent of 10- 20 % and the flex life increases to many folds. Present trend of bladder manufacturing is thinner gauge at the center such that it gives better flexibility in that region as well as better heat transfer. The average life of different bladder can be summarized below: BC--------------- 600 – 1800 PC--------------- 300 – 800 TB----------------100 – 500 OTR-------------- 15 – 50 The most Important thing is how this cures are spread. For example, 400 cures can be spread over 5 to 620 cures which is considered to be bad but if it is spread over 350 to 450 which could be a better
product. 8. BLADDER CARE IN SERVICE Page 12
12 There are several methods to improve the performance of bladder like : change in formulation change in the mixing change in blank preparation change in design of bladder change in severity of use change in tyre curing temperature etc. Bladder care in service tells us how bladder should be handled during its service ,that is, after it has been fixed with the press. Precisely it tells about to control oxygen level in the central mechanism system and care to be taken during its service. But we would also discuss here on uniformity in mixing and care to be taken during curing, post curing and storage. 8.1 Uniformity in mixing The dispersion level of bladder compound varies from 50 % to > 98% in different tyre industries. High level of dispersion of the ingredients also improves the bladder life. It is the art of processing technology how one achieves the dispersion level greater than 90% with out altering the compounding ingredients. In most cases butyl compound mixing is done in two stages. First stage or master batch contains all ingredients except the curatives and in the second stage or final batch, only curatives are added . In some cases both neoprene and resins together or zinc oxide and resin together will be added in the final stage. Use of mother batches of neoprene , zinc oxide and resins are also popular to have good dispersion in the compound and can be tried both in banbury as well as in open mill mixing. Mother batches of zinc oxide, neoprene and curing resin may be made with proportionate quantity of butyl, carbon black and process oil such that the actual batch contains the desired level of butyl, process oil, carbon black and ingredients in mother batches.
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Bladder compound can also be mixed in single stage. Master batch is
mixed in banbury which is dumped at 180 0
C + and then add mother batch of resin and neoprene on the dump mill after applying 2-3 cuts on mill. In this case straining can be done only once. In order to get better dispersion and higher dump temperature, the use of banbury volume is very important. In case of master batch, the normal batch weight is 10-20% higher than that used for GPR based compound. For butyl based compound, it is better to use banbury fill factor between 0.85 to 1.0 for better dispersion of carbon black and zinc oxide in butyl rubber. The dump temperature of master is kept 180 0
C +. For final batch , the batch weight is normally 80% of the master in the same banbury and the dump temperature is maintained between 100 – 110 0
C. Banbury batch weight is calculated in the following manner : Babury batch weight (kg) = Banbury volume (liter) x Fill Factor x Comd.Sp.Gr. One may remember that the banbury volume indicated by manufacturer is the total volume of water having specific gravity 1.0 with ram down and discharge door in closed conditions. A banbury, having 50 liter of capacity, fill factor of 1.0 and compound specific gravity of 1.12 should have 56 kg of batch weight (50 x 1.12 x 1.0 = 56). For a very new banbury, it is desired to keep fill factor around 0.85 because very high batch weight has always adverse effect on the dust seals and as a result, optimum batch weight is always determined practically around the calculated batch weight which should provide adequate mixing in desired mixing time and should provide adequate dump temperature without affecting dust seals. It is desired that the starting temperature of master and final batch are kept around 80 0
C. In fact, adequate mixing time and dump temperature will depend on: Type of banbury (2 wing/4 wing chamber and Tangent system / Interlocking system). Banbury volume loading. Banbury starting temperature.
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14 Black : Oil ratio in the Master Batch. Sequence of loading . Dump temperature. Cleanliness and good housekeeping of the entire bladder mixing area is very important. 8.1.1. Some important parameters in mixing Due to difference in co-cure characters, butyl rubber can not be blended with GPR . During mixing of carbon black with butyl rubber, enough time or shearing force is to be provided for better dispersion because butyl has comparatively lower affinity towards carbon black then GPR. Combination of different types of carbon black are good provided dispersion is also equally good. Different type of carbon black will have different affinity for butyl rubber due to the difference in surface area . Carbon black with higher surface area will provide better green strength but difficult to get dispersed in butyl rubber. If dispersion of above black is inadequate, this can create problems as porous extrusions , lamination and blister in vulcanizate. Mineral fillers are not recommended in bladder compound formulation as this may cause die plating problem. Down time, scrap and rejections often increase due to the use of mineral filler. The key requirement for processing aids is to improve upon the processability with minimum effect on the vulcanizates. Use process oil as minimum as possible. Butyl rubber in bladder compound should always be premasticated either in banbury or in open mill mixing. This increases the acceptance of ingredients . In case of difficulty in premastication, butyl is recommended to keep at hot house (6080
0
C ). In hot house FIFO system should be followed. Page 15
15 8.2. Moulding / Vulcanization It is suggested to warm up the blank for 2 4 hours @ 80 0
C in oven
which has air circulating inside to prevent local hot spots especially for large bladders which have large cross sectional volume and need good mould flow. It is better to have different sizes of oven for different sizes of slugs. Prior to moulding, mould should be completely free from the rubber flash from previous molding. As a general guideline, bladders should be cured at a higher temperature than they are expected to encounter during the tyre cure. Advised to use highest possible mould pressure to ensure good flow on inside the mould. Avoid using work away in bladder compound. Blend of work away is the potential source of lamination. In case work away is more, use in extruder without blending with okay stock . Avoid using different workaway together because different work away will have different heat history and will tend to behave differently in bladder mould which tends to give lamination in the vulcanizate. Compound with higher heat history in the processing will tend to degrade faster in use. Maintain cure temperature at least to 190ºC. At higher curing temperature (210ºC) more stable cross - link is reported. A little over cure in bladder is always better than a little under cure. Mould should be closed as fast as possible. Maintain mould closing pressure in such a way that the rubber flow is as fast as possible. It is better that mould squeezing time is kept minimum at 2 minutes before the mould is closed finally for curing. It is desired that rubber flow takes simultaneously both on the top as well as on the bottom. Watch for proper venting of moulds to avoid cold spot. It is suggested to warm up the blank for 2 4 hours @ 80 0
C in oven which has air circulating inside to prevent local hot spots especially for large bladders which have large cross sectional volume and need good mould flow. It is better to have different sizes of oven for different sizes of slugs. Prior to molding, mold should be completely free from the rubber flash from previous molding. As a general guideline, bladders should be cured at a higher temperature than they are expected to encounter during the tire cure. Page 16
16 8.3. Post Curing Post curing of bladder is a must. In the Ist stage it is done in autoclave for 30' at 190ºC or for 2-4 hrs. @ 150ºC, depending on the thickness of bladder. This will be followed by room temperature post
curing (2nd stage) for 30 days. Post curing helps to get better stabilization of the bladder cross link system and eventually offers better bladder life . 8.4. Bladder Storage As a general rule, bladders that are stored in the warehouse (open and out of UV light.) approximately 2-4 weeks after curing, exhibit better life than those used immediately after curing. This is due to crosslink stabilization and stress relaxation at room temperature for a period of 15-30 days. Post curing not only stabilizes the cross-link but also helps to sweating water as well, which is eliminated during cross-link formations. Bladder should not be stored in folded condition. Oxidative cracks generates on the stressed region when the bladder is folded. Bladder should be stored in dark , away from sun light / UV light. 8.5. Oxidative degradation Softening or becoming sticky is one of the indication of the degradation of inside bladder due to oxygen attack. The degraded and the sticky material could be degraded polymer of butyl. One of the most common method to take care of this problem is to control the dissolved oxygen by chemical method. Many cases have been reported to have bladder failure due to oxidative degredation. In order to prevent this , deoxidation of hot water is important. Typical ingrediants are Sodium Sulphite (Na 2
SO 3
.7 H 2
O) 100 ppm or 20-30 ppm of Hydrazine hydrate N 2
H 4
H 2
O (31.2%). Free oxygen content in hot water should be below 150 pphm. It is better that the free oxygen content is monitored in the laboratory on periodic checks. Page 17
17 Oxygen content may also go up if o-rings in the bladder is worn out. EPDM based (heat resistance) O – rings are reported to have
prolonged service. Central mechanism should not contain copper or brass metal as it accelerates the rate of oxidation in the medium. Bladder should not be stored in folded condition. Oxidative cracks generates on the stressed region when the bladder is folded. Bladder should be stored in dark , away from sun light (UV light). All the above points give the informations on the source of the oxidative degradation. However, degradation problem could be minimized by controlling oxygen level in the central mechanism system. 8.6. Oxygen level in central mechanism Central post mechanism is a system on which bladder is fixed to a central metal post, both top ring and bottom ring are clamped. Desired stacking height and stretching height are maintained. Before actual curing starts the bladder is subject to warm up with steam and should undergo certain numbers of cyclic shaping and deshaping. Central mechanism is subject to contain steam / hot water / cold water / gas / (often called as internal) and vacuum in a cyclic sequence. 8.6.1. Internal Curing medium is often called as internal. Hot water is the curing media for tyre cure and the major cause for inside bladder degradation is due to dissolved oxygen in the system. In order to prevent this, deoxidation of hot water is important. Treatment of hot water can be done with chemicals like oxygen scavenger or with contact heater (Deaerator). It is advisable that the free oxygen content is monitored in the laboratory on periodic checks. Nitrogen gas curing has been proven to be the best against oxidative degradation. Typical curing mediums used in tyre industries are: Page 18
18 Steam/Hot water with chemical oxygen scavenger. Steam/Hot water with contact heater (Deaerator). Steam/Hot inert gas. The quality of hot water (degrees of dissolved oxygen) affects degradation of inside of the bladder and we can see three phases of technological developments in this area. 8.6.1.1. Steam / Hot water with chemical oxygen scavenger : Before chemical treatment for deoxydation, desalting of water is important. The use of chelate compound, Disodium salt of
ethylendiamine – tetra-acetic acid or EDTA can approximately eliminate 90% But ion exchangers are the best technology now being practiced all over, which can take care > 99% desalinity. Little conservative way of elimination of dissolved oxygen from hot water is the use of certain special chemicals such as, oxygen scavenger and typical usages are Sodium sulphite (Na 2
SO 3
.7H 2
O) 100 ppm or Hydrazine Hydrate N 2
H 4
.H 2
O (31.2%), 20-30 ppm. Free oxygen content in hot water should be below 150 pphm. This procedure is however, not powerful / effective and inconvenient to control, so , very often this system resulted in severe deterioration of inside of bladder (very sticky and grown up unevenly). Major problem in this system is in most cases the dissolved oxygen content is not analyzed properly and systematically. Laboratory personnel should be made responsible to monitor and control the dissolved oxygen level in hot water. 8.6.1.2. Steam / Hot water with contact heater (Deaerator) : This is a very active type to eliminate dissolved oxygen by Deaerator physically and it is quite effective to remove dissolved oxygen although it needs investment. In many cases the deaeration equipment is not there or unfortunately it is used incorrectly. The contact heater removes oxygen and free carbon dioxide from hot water so as to protect piping, steel tanks and other equipments from corrosion. Other notable effect is to prolong the bladder life by Page 19
19 preventing degradation of bladder compound which contacts hot water. Advantages experienced are : Markedly reduce the inside deterioration of bladder and improve the life , Easy maintenance of hot water supply pipe lines (no corrosion of
inside of steal pipes).
Dissolved oxygen is generally controlled at below than 0.1 ppm. 8.6.1..3. Steam / Hot inert gas (Nitrogen). This is relatively new technology which gives lot of significant advantages. Field tests with Nitrogen have been done by several companies which have supported the expected benefits of:
Increased bladder life (50%)
Reduced cycle time
Reduced operating cost (energy saving) Increases component adhesion due to higher consolidation with higher pressure To get maximum benefits , high purity Nitrogen gas is needed and perfect sealing at all connections (pipe lines, bladder clamping etc) are required. 8.6.2. O- ring / diaphragm of central mechanism Oxygen content may also go up if o-rings or the sealing in the bladder is of poor quality or is worn out. EPDM based (heat resistance) O – rings are reported to give prolonged service. A periodic replacement always gives better results. 8.6.3. Metal contamination in hot water / steam Central post mechanism should not contain copper or brass metal as it accelerates the rate of oxidation in the medium. 8.7. Quality of green tyre
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20 This is particularly related to the quality of inner paint and the application of it. Water based inner paint, compounded with mica, gives better resistance to oxidative degradation on outer surface. Uniform application of inner paint helps to prevent flow of curatives from green tyre to the bladder. Higher curative absorption / adsorption also increases the ageing of bladder. 8.8. Service condition The first thumb rule is to maintain FIFO system. Bladders have been cured, post cured in autoclave and also post cured in room temperature is now ready to be fixed on the press. Very old bladder is subject to oxidative degradation and may not give adequate life . Bladder should not be stored and transported in folded condition.
Oxidative cracks generates on the stressed region when the bladder is folded. Bladder should be stored in dark , away from sun light / UV light. It is better to maintain the continuity of curing of bladder. Once bladder is mounted on the press, should be removed only when the bladder has given full (adequate) life . A bladder discontinued from curing is subject to give rough surface or oxidative degradation on storage. Very high vacuum of bladder is not necessary, only around 150 mm w/g is good enough but should not go beyond 200 mm w/g. Vacuum should not cause to touch the bladder to central post. High vacuum causes faster crack generation on the bladder. Assembling of the vulcanized bladder is very important. Care should be taken specially for fixing the clamp area uniformly and avoid giving too much stress to one area. Fitting should be such that it does not produce unnecessary stress on the bladder. Maintain proper stretching height and stacking height of bladder in use in the press. It is known fact on the bladder related problems during seasonal changes and it is severe, particularly, in winter. Unlike developed countries, tyre industries in developing countries are not operating Page 21
21 under air condition. Seasonal related problems in air conditioned plant are much less. Most of our Indian tyre industries are without air conditioning arrangement and eventually we find problems due to seasonal changes. Normally, in case of compound processing the control of mooney viscosity and the control of scorch plays a vital role to cope up with the process during seasonal changes. When a bladder is cured and exposed to winter for a longer time, it may show stretch related problem in service. It is desired that a bladder after assembled and fixed in the press should undergo at least 2-3 hours / 2-3 cure cycles pre-warming sessions before it is subjected to curing. The pre-warming time could be less in summer time. Pre-warming includes heating with steam/dome and giving at least 10 times cyclic shaping and deshaping in the press before initiation of curing of tyres. This will enable the bladder to sustain stress and strain in actual condition with minimum problems. Note that sufficient screw holes are there for assembly inorder to avoid slippage under stretch. The screw holes are to be separated from one
another uniformly. Better to use pneumatic screw-tight device for tightning clamp rings with torque wrench. Use hoist for bladder assembly and assembled bladder should not be kept in a folded condition on the floor. Use fork lift to handle bladder in the shop floor. Correct bladder - tyre fitting is important to maximize the bladder life . Use right size of bladder so that the overall stretch is not more than 25%. It is not necessary that same bladder should be used for different aspect ratio tyre or different sectional width of tyre though the rim dia of the tyre may be the same. Bladder service condition is very important to have higher life .High purity Nitrogen gas cure has shown improvement on bladder life . Very high vacuum of bladder is not necessary, only around 150 mm w/g is good enough but should not go beyond 200 mm w/g.High vacuum causes faster crack generation on the bladder. Page 22
22 Water based inner paint based compounded with mica gives better resistance to oxidative degradation on outer surface. Uniform application of inner paint helps to prevent flow of curatives from green tyre to the bladder. Higher curative absorption / adsorption increases the ageing of bladder. 8.10. Assessment of bladder life Bladder is one of the parts of tyre curing presses and service condition has wide variation by plants (curing medium for bladders), type of bladders ( open end type or closed end type) and curing press itself (variation of fitting to the press e.g. clamp design and fastening), therefore it is very necessary to record the bladder life with statistically meaningful manner wihich needs following clarifications. Kind of curing medium. Type of bladders (BOM, Bagwell or Rolling-in types). Bladder size. Tire size. Machine number. Heats (bladders life ). Failure modes with clarification e.g. Broken at bead clamping (upper or lower). Delamination ( which area). Blistering ( localized area or evenly).
Inside softening ( localized area or evenly). Bladder surface cracks ( everywhere or at corners of vent grooves). Growth ( localized area or evenly ). Statistically meaningful data will give easier focusing to pin-point the areas to take countermeasures. For example , if one gets frequent problem on delamination, it is to be taken care in the processing only. Let us see how one should systematically proceed on the delamination part. Delamination is the separation between two layers within the bladder gauge. This happens mainly due to four factors : Page 23
23 Contamination Air trapment Work away Improper mould flow in the mould Delamination occur if mica, dust or talc gets on to the surface of the blank. Before moulding , the mould should be maintained clean with out adding any mould lubricant . Chromium plated mould gives better results. It is also caused by air entrapment. Care should be taken minimum air trapment during mixing and in milling as already discussed. It is better to use higher size extruder 8.0” or 10.0” for larger size (20.0 “ & > 20.0 ”) . Avoid using work away in bladder compound. Blend of work away is the potential source of lamination. In case work away is more, use in extruder without blending with okay stock . Avoid using different workaway together because different workaway will have different heat history and will tend to behave (flow) differently in bladder mould which tends to give lamination in the vulcanizate. Compound with higher heat history in the processing will tend to degrade faster in use. Mould should be closed as fast as possible. Maintain mould closing pressure in such a way that the rubber flow is as fast as possible. It is desired that rubber flow takes simultaneously both on the top as well as on the bottom. It is better that mould squeezing time is kept minimum at 2 minutes before the mould is closed finally for curing. For bladder or in tyre or in any rubber industries all problems could be taken care if right logic or the right reasoning is cultivated in a systematic manner. It is definitely a team work . Most important part is gathering right information at right time.
9. SUMMARY
Particularly in developing countries , bladder care in service plays a very significant role for the life of the bladder. Infact, the major failure of bladder in tyre industries of developing
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24 countries is due to the adverse conditions on which the bladder has been exposed to. Recommended compound formulation is Butyl- 95.0 - 100.0 / Neoprene WRT -5.0 – 7.0 / HAF black - 50.0 – 60.0 / Castor oil - 5.0 – 7.0 / Stearic Acid – 1.0 / ZnO - 5.0 and Curing Resin SP 1045 – 5.0 - 10.0 Phr. Compound formulation of bladder compound will be similar in all tyre industries. It is the state-of-art on the processing technology that gives the consistency and higher life expectancy of the product. Processing of bladder is different in different industries. Bottom line is how one gets better dispersion at lowest possible cost and how one gets the best product at lowest possible cost. Finally the product will be judged by the number of heats have been given by the product, higher the heats lower is the cost per kg. of tyre production. Before moulding, blanks are spliced and prewarmed at 80ºC for 2-4 hrs. depending on the thickness. Prewarm blanks helps in good flow inside the mould. Splicing of blanks can also be done online after extrusion. For cutting blank , cold knife is used and is cut in a single length having average weight close to 10 - 15% higher than the weight of the cured bladder. Before putting the blank in the mould, ensure that the mould is clean, free from lubricants, if any, or flash from p revious cure. Use highest possible mould pressure to ensure good flow inside the mould. Delamination occur if mica, dust or talc gets on to the surface of the blank.
Page 25
25 Before moulding , the mould should be maintained clean with out adding any mould lubricant . Chromium plated mould gives
better results. Delamination is also caused by air entrapment. Care should be taken minimum air trapment during mixing and in milling . It is better to use higher size extruder 8.0” or 10.0” for larger size (20.0 “ & > 20.0 ”) of bladder. Avoid using work away in bladder compound. Blend of work away is the potential source of lamination. In case work away is more, use in extruder without blending with okay stock . Avoid using different work away together because different work away will have different heat history and will tend to behave differently in bladder mould which tends to give lamination in the vulcanizate. Compound with higher heat history in the processing will show resistance to flow in the mould and tend to degrade faster in use. Maintain cure temperature at least to 190ºC. At higher curing temperature (210ºC) more stable cross - link is reported. A little over cure in bladder is always better than a little under cure. Mould should be closed as fast as possible. Maintain mould closing pressure in such a way that the rubber flow is as fast as possible. It is better that mould squeezing time is kept minimum at 2 minutes before the mould is closed finally for curing. It is desired that rubber flow takes simultaneously both on the top as well as on the bottom. Post curing of bladder is better. In the Ist stage it is done in autoclave for 30' at 190ºC or for 2-4 hrs. @ 150ºC, depending on the thickness of bladder, followed by (2 nd stage) storage at room temperature for a period of 15-30 days. Post curing stabilizes the cross-link and also helps to sweating water, which is eliminated during cross-link formations.
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26 The thumb rule of bladder curing is a little over cure of bladder is always better than a little under cure. A little under cure bladder is subject to faster oxidative degradation. As a general guideline, bladders should be cured at a higher temperature than they are expected to encounter during the tyre cure. During moulding, it is advised to use highest possible mould
pressure to ensure good flow inside the mould. Mould should be closed as fast as possible. It is better that mould squeezing time is kept minimum at 2 minutes before the mould is closed finally for curing. It is desired that rubber flow takes simultaneously both on the top as well as on the bottom.Watch for proper venting of moulds to avoid cold spot. Bladder should not be stored and transported in folded condition.Oxidative cracks generates on the stressed region when the bladder is folded. Bladder should be stored in dark , away from Sun light / UV light. Before chemical treatment for deoxydation, desalting of wa ter is important. The use of chelate compound, Disodium salt of ethylendiamine – tetra-acetic acid or EDTA can approximately eliminate 90%. Ion exchangers is the latest technology and is now being practiced all over, which can take care > 99% desalinity. Special chemicals are known as oxygen scavenger such as , Sodium sulphite (Na
2
SO 3
.7H 2
O) 100 ppm or Hydrazine Hydrate N
2
H 4
.H 2
O (31.2%), 20-30 ppm. Free oxygen content in hot water should be below 150 pphm.
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27 Hot water with contact heater (Deaerator) is much more effective to eliminate dissolved oxygen than chemical treatment. In most cases the dissolved oxygen content is not analysed properly and systematically. Laboratory personnel should be made responsible to monitor and control the dissolved oxygen level in hot water.
Assembling to the vulcanized bladder is very important . Care should be taken specially for fixing the clamp area uniformly and avoid giving too much stress to one area. Note that sufficient screw holes are there for assembly inorder to avoid slippage under stretch. The screw holes are to be separated from one another uniformly. Better to use pneumatic screw-tight device for tightning clamp rings with torque wrench. Use hoist for bladder assembly and assembled bladder should not be kept in a folded condition on the floor. Use fork lift to handle bladder in the shop floor. Correct bladder / tyre fitting is important to maximize the bladder life . Use right size of bladder so that the overall stretch is not more than 25 %. It is not necessary that same bladder should be used for different aspect ratio tyre or different sectional width of tyre though the rim dia of the tyre may be the same. Bladder service condition is very important to have higher life.High purity Nitrogen gas cure has shown improvement on bladder life . Very high vacuum of bladder is not necessary, only around 150 mm w/g is good enough but should not go beyond 200 mm w/g.High vacuum causes faster crack generation on the bladder..
Maintain proper stretching height and stacking height of bladder in use in the press. Page 28
28 Before loading the tyre, proper warm up of bladder is important.Warm up includes (a) putting steam inside for one/two cure cycle (b) at least 10 times cyclic shaping and deshaping. Maintain FIFO system.Very old bladder may not give adequate life. It is better to maintain the continuity of curing of bladder. A bladder discontinued for curing is subject to give rough surface or oxidative degradation on storage. Water based inner paint based compounded with mica gives better resistance to oxidative degradation on outer surface. Uniform application of inner paint helps to prevent flow of
curatives from green tyre to the bladder.Higher curative absorption / adsorption increases the ageing of bladder. Cleanliness and good housekeeping of the entire bladder processing area is very important. 11. REFERENCES 1. Isobutylene Based Polymers for curing Members, H. Nagano, Exxon Chemical International - Asia, Polymer Technology Centre Tsurumi, Yokohoma, Japan. 2. Bag-o-Matic Bladder Compounding And Processing Guidelines, C. Moore, Essochem Europe Inc., Machelan, Belgium. 3. Curing Members, S. Kamano, Exxon Chemical Japan Ltd., Elastomers Department, Tsurumi, Yokohama. 4. More Uniform Bladder Life With Resin Dispersion, A.H. Johansson, PhD, Rhein Chemic, USA, in Tire Technology International (84 - 92), 1995.
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29 5. Understanding Mixing Part I, Samir Majumdar, Rubber Chem Review (17-23), March, 1996. 6. Understanding Mixing Part II, Samir Majumdar, Rubber Chem review(17-26), April, 1996 7. Isobuthylene Based Elastomers : World Wide Status and Future Outlook, D.H. Price, J.P. Cross, H.C. Wang and S.Majumdar, presented in International Rubber conference , Chennai, Dec 7 9, 1998. 8. Rubber Processing Mineral Oil, Dr. Samir Majumdar, Presented at Rubber Processing Oil Seminar, SriLanka, August 24, 1999. 9. Curing Bladder, S. Majumdar, Shanghai Butyl Seminar, Shanghai Regal Estate Hotel, Shanghai , Aug 27 - Sept. 03, 2000.