GB2248625A - Tire cure bladder - Google Patents

Abstract

Expandable rubber for curing tires, and the curing of tires therewith, composed of a specified rubber composition involving butyl rubber with low unsaturation combined with the utilization of stearic acid without castor oil.

Description

TIRE CURE BLADDER Field This invention relates to a tire cure bladder. The invention more particularly relates to a butyl rubber bladder for curing rubber tires.

Background Conventionally, pneumatic rubber vehicle tires are produced by molding and curing green, uncured tires in a molding press in which the green tire is pressed outwardly against a mold surface by means of an inner fluid expandable bladder, often referred to as a tire cure bladder. Tire cure bladders are well known in such art.

By such method, the green tire is shaped against the outer mold surface which generally defines the tire's tread pattern and configuration of its sidewalls. Through application of heat and pressure, the tire is cured.

Generally, the bladder is expanded by internal pressure provided by pressure and fluid such as hot gases, hot water and/or steam which also participates in the transfer of heat for the curing or vulcanization purpose. The tire is then allowed to cool somewhat in the mold, sometimes aided by adding cold water to the internal surface of the bladder. The mold is then opened, the bladder collapsed by removal of at least a portion of its internal fluid pressure and the tire removed from the tire mold. Such use of the tire curing bladder is well known to those having skill in such art.

Tire cure bladders undergo considerable stress during their expansion and contraction during a tire cure cycle. After repetitive tire cure cycles a bladder typically gradually become somewhat distorted from its original shape.

The tire cure bladder is typically composed of butyl rubber.

Butyl rubber is a term used to generally refer to a class of rubber compounds containing a major amount of a C4-C7 isoolefin, such as isobutylene, and a minor amount of a C4-C8 diolefin, such as 1,3-butadiene or isoprene, typically isoprene.

Although butyl rubber can, for example, contain about 95 to about 99.5% isobutylene, and, correspondingly, about 5 to about 0.5% of isoprene, for practical purposes, butyl rubber used for tire cure bladders typically contains about 98.2 to about 99 percent isobutylene and, correspondingly, about 1.8 to about one percent isoprene. Often, butyl rubber is made in substantial accordance with well known techniques such as described, for example, in U.S.

Patent No. 2,356,128 and U.S. Patent No. 3,031,423.

The butyl rubber for the bladders is typically cured with a resin cure system instead of a sulfur or peroxide system. The resin cure system is usually composed of a phenolic resin (the resin), chloroprene and zinc oxide. The phenolic resin is typically a para alkyl substituted phenol dialcohol polymer which is an oil-soluble, heat-reactive resin. The resin may typically be prepared by the reaction of para alkyl substituted phenols and formaldehyde. Often the said alkyl substituent is an octyl radical. Disclosures of such resins and their preparation and use may be found in U.S Patent Nos. 2,734,877; 2,726,224; 2,364,192 and 1,996,069.

Furthermore, the butyl rubber for the tire cure bladders conventionally contains castor oil in addition to the resin cure system.

In the production of the butyl rubber bladders themselves, it is desired to enhance and/or increase the cure rate of the butyl rubber while maintaining the performance of the cured butyl rubber for the curing of tires.

Disclosure and Practice of the Invention In accordance with this invention, a rubber bladder comprises a shaped and vulcanized rubber composition comprised of, based on 100 parts by weight of at least one synthetic rubbery copolymer composed of an average of about 99 to about 99.4 percent of an isoolefin containing from 4 to 7 carbon atoms, preferably isobutylene; and correspondingly, about one to about 0.6 percent isoprene; and, in the absence of castor oil, about 2.2 to about 2.8 parts by weight stearic acid; about 40 to about 60 parts by weight carbon black; and a vulcanization system for said copolymer comprised of polychloroprene, para alkyl substituted phenol dialcohol resin and zinc oxide.

The polychloroprene is sometimes referred to as neoprene.

The said phenol dialcohol resin is of the type resulting from the reaction of a para alkyl substituted phenol with formaldehyde in the presence of an alkali catalyst to form an oil soluble resin. Preferably the alkyl substituent is an octyl radical.

The invention is particularly directed to a rubber bladder for curing rubber tires, which is, as hereinbefore pointed out often referred to as a tire cure bladder.

Thus, the invention is also directed to preparing a pneumatic rubber tire comprising shaping and vulcanizing an unvulcanized pneumatic rubber tire under conditions of heat and pressure by pressing the tire outwardly against a mold surface by an inner bladder of this invention.

The shaping and curing of the rubber composition can be accomplished by conventional means in a suitable mold at a somewhat elevated temperature, for example, in the range of about 1900C to about 235 C.

Conventionally cure temperatures have heretofore been used up to about 210 C.

It is important to appreciate, as hereinbefore pointed out, that conventional vulcanization systems for butyl rubber based tire cure bladders are composed of polychloroprene rubber (sometimes referred to as neoprene), alkyl substituted phenol dialcohol resin and zinc oxide. It is sometimes referred to as a resin cure system since it is not based on sulfur or peroxide. Typically such vulcanization system, based on 100 parts by weight butyl rubber, is from about 3 to about 5 parts by weight (phr) polychloroprene rubber, about 8 to about 10 phr alkyl substituted phenol dialcohol resin and from about 3 to about 7 phr of zinc oxide.

Castor oil has heretofore been conventionally used in a butyl rubber composition for tire cure bladders, typically in an amount of about 3 to about 6 phr.

Three important aspects of this invention are that it has been observed that a combination of the elimination of the castor oil and the addition of stearic acid in the compounding of the butyl rubber bladder, together with the use of exceptionally low unsaturation level (low isoprene content) of the butyl rubber itself, the cure rate of the butyl rubber in the preparation of the bladder itself is increased, thereby allowing for a shorter production time of the bladder and, moreover, the performance of the bladder in a tire vulcanization operation is enhanced.

For example, it has been observed that the cure time for the preparation of the butyl rubber bladder can be reduced from a conventional period of about 12 to about 15 minutes to a period of about 7 to about 9 minutes, dependent somewhat on the bladder size and its wall and bead thicknesses, at a cure temperature of about 2100C where castor oil was eliminated and stearic acid was used in the butyl rubber composition.

Thus, economies of a shorter cure time for preparation of the bladder are achievable.

Moreover, it has been observed that the vulcanized tire cure bladder of this invention more fully retained its rubbery properties over repetitive tire cure cycles even though it was cured for a relatively shorter cure time.

While castor oil had been eliminated and stearic acid added to the butyl rubber formulation, it is not considered that stearic acid replaces the castor oil because the castor oil and stearic acid present somewhat different effects for the rubber composition.

In a tire vulcanization operation, a tire cure bladder utilizing stearic acid in its composition, and without castor oil in its formulation was observed to provide substantially more cure cycles before bladder failure. Thus, the rubber bladder maintained its rubber properties over extended tire cure cycles.

The compounded butyl rubber itself demonstrated a relatively flat torque versus time curve according to ASTM test D2084 (at a cure temperature of 210 C) without appreciable cure reversion, primarily due to the elimination of castor oil, and also without an appreciable cure marching modulus, (an increase of crosslink density with time). Indeed, a cure marching modulus normally would have been expected with the elimination of castor oil in the resin cure system, (exhibiting an attendant relatively uncontrollable increase in crosslink density with time during the preparation of the bladder rubber). It also demonstrated a tension set value of less than 20 percent after a period (2 days) of ageing at about 1770C according to ASTM test D412 Sect. 11.2.The retention of the good tension set value upon ageing of the prepared bladder rubber was an unexpected benefit.

This is considered significant and a substantial improvement over such cure bladders utilizing castor oil and without stearic acid. Retention of a low tension set after ageing indicates a retention of elastomeric, or rubbery properties.

It is also significant that a butyl rubber with the lower unsaturation content is used which provides improved ageing and fatigue resistance for the rubber while demonstrating the aforesaid lower tension set value which means it has a more efficient and more permanent crosslinked (cured) structure.

Thus, it has been discovered that if stearic acid is mixed with the uncured butyl rubber (and without castor oil) where the butyl rubber contains a relatively low carbon-to-carbon unsaturation in combination with a phenolic derivative curing agent, particularly an alkyl substituted phenol dialcohol resin as a reaction product of a para alkyl substituted phenol with formaldehyde in the presence of alkali catalyst, particularly where the alkyl radical is an octyl radical, either used alone or in combination with a vulcanization activator (polychloroprene) for curing the butyl rubber, (it is preferred that the activator is present) a special improvement is observed involved in extending the useful life of a tire cure bladder in a tire curing operation over repetitive cure cycles.

This aspect of extending bladder cure life is important because it is apparently a result of the utilization of this invention to enable a retention of rubbery properties after extended tire cure cycles not heretofore experienced by the inventor in a tire cure bladder.

As hereinbefore pointed out, an important aspect of this invention is the utilization of a butyl rubber with a very low (0.6 to one percent) isoprene content and, thus, a very high isobutylene content and, therefore, a relatively low unsaturation content. This is considered to be a departure from conventional practice of using a butyl rubber with higher isoprene content. The butyl rubber with lower isoprene content is used for the bladders of this invention because the advantage of attendant age and fatigue resistance from lowest unsaturation can be utilized while maintaining the crosslink efficiency with the stearic acid.

It is contemplated that various para alkyl substituted phenol dialcohol resins may be used in the curing of butyl rubber for the butyl bladders and a resin formed by the reaction of a para alkyl substituted phenol with formaldehyde is preferred.

For example, the dimethylol para alkyl substituted phenol resins, particularly 2,6-dimethylol-4-hydrocarbon substituted phenol resins as their condensation polymers; 2,6-dimethylol-4-chloro or aryl substituted phenol resins as their condensation polymers; 2,6-di(acyloxymethyl)-4-hydrocarbyl phenols; 2,4,6- tris(acyloxymethyl) phenyl alkanoethes; 2,6di(alkoxymethyl)-4-hydrocarbyl phenols; the reaction product of formaldehyde and 3,4,5-trialkyl phenol; bis (3,alkyl-5-methylol-6-hydroxy phenol) methane; and similar phenolic derivative are contemplated. The 2,6-dimethylol-4-octylphenol resin is preferred where the said 4-octyl phenol is reacted with formaldehyde (with an excess of formaldehyde) to form the resin. A reference may be made to U.S. Patent No. 2,726,224 for examples of such preparations.

Additional references may be made to U.S. Patent Nos. 1,996,069; 2,364,192; 2,734,039; 2,734,877; 2,749,323 and 2,727,874 for further descriptive information. Rubber Age, 83 101 (1958) may also be reviewed.

A methylol content in the resin of about 7-9.5 percent is preferred. This is considered a relatively low methylol content for the resin.

Further improvement in the useful life of butyl rubber bladders might be obtainable when chlorinated compounds are used in conjunction with such phenolic resins, including heavy metal halides, chlorosulfonated polyethylene, polychloroprene and chlorinated paraffin wax. Polychloroprene is preferred.

Although the mechanism of the invention might not be fully understood, it is believed that the stearic acid acts to provide a degree of acidity for the resin cure system which perhaps tends to retard or prevent a cure reversion. Also efficiency of crosslinking of the butyl rubber is realized.

Deletion of the castor oil is also important because, basically castor oil is a considered as a cure retarder which may promote loss of rubber properties on use in tire cure cycles (e.g. increase in tension set values).

It is to be understood that the butyl rubber for the tire cure bladder may be compounded with various compounding ingredients such as, for example, the resin cure components (polychloroprene, and a resin as a reaction product of formaldehyde and a para alkyl substituted phenol, preferably a 2,6-dimethylol-4-alkyl phenol resin and zinc oxide which are to be used in conventional amounts). Such compounding of butyl rubber is well known to those having skill in such art.

The tire cure bladder surface is subject to a degree of friction due to movement against the tire surface during the molding and demolding of the tire occasioned by the tire cure cycle. Accordingly, a lubricant is usually applied to the bladder and/or tire innerliner surface prior to the cure cycle.

The following examples further demonstrate the invention and are not intended to be limited. The parts and percentages are by weight unless otherwise indicated.

EXAMPLE I Butyl rubber was compounded with appropriate compounding ingredients and shaped and cured to form a tire cure bladder.

More specifically, butyl compound (A) was prepared as a control and butyl rubber compound (B) was prepared in a similar fashion to butyl rubber (A) but included oleic acid in its compounding as well of a blend of butyl rubbers. Butyl rubbers C and D are experimental with Exp. D being the butyl rubber compound as used in this invention.

The compounding of the butyl rubber is more clearly shown in the following Table 1.

Table 1 Table 1 Control Butyl Butyl Butyl Butyl Rubber Rubber Rubber Rubber Component Exp.A Exp. BExp. C Exp. D Butyl Rubbery1 (1.6%) 45 45 0 0 Butyl Rubber2 3 Polychloroprene- 5 5 5 5 Stearic Acid 0 0 0 2.5 Oleic Acid 0 6 0 0 Carbon Black4 55 55 45 45 Polymeric phenol dialcohol5 9 9 9 9 Zinc Oxide 5 5 5 5 Castor Oil 6 0 0 0 Wax, Microcrystalline 5 5 0 0 1. Butyl rubber obtainable as Butyl 268 from the Exxon Chemical Americas and having about a 1.6 percent isoprene content in the isobutylene/isoprene copolymer.

2. Butyl rubber obtainable as Butyl 077 from the Exxon Chemical Americas and having about a 0.8 percent isoprene content.

3. Obtainable as Neoprene TRT from E. I. Du Pont de Nemours & Company, Inc.

4. An acetylene black, although a blend of acetylene and HAF blacks were used for Exp. A and B.

5. Obtainable as Resin SP-1044 from Schenectady Chemicals, Inc., a reaction product of 4-octyl phenol with formaldehyde.

The compounded butyl rubber was shaped in the form of a tire cure bladder and cured at a temperature of about 2100C for a period of about 15 minutes.

The bladder rubber compounds had the properties shown in the following Table 2.

Table 2 Control Properties Exp A Exp B EXP C Exp D Cure Time (210 C) 15 min 9 min 10 min 8 min Tension Set (%)1 22 25 11 12 Tension Set (%)2 29 32 18 12 Tension Set (%)3 48 46 20 12 Elongation4 750 830 700 670 Elongation5 500 530 470 450 1. Original Tension Set Value (before oven ageing) 1. Original Tension Set Value (before oven ageing) 2. Tension set after 2. (ASTM set after hot air oven ageing one day at 3. Tension set after 3. Tension set after hot air oven ageing two days at 177 C (ASTM D410) 4. Elongation (%), original 5. Elongation (%) after hot air oven ageing one day at 1770C 6.Elongation (%) after hot air oven ageing two days at 1770C Thus, these rubber properties show that the bladder rubber made of the formulation (Exp D) of this invention was observed to have improved retention of rubbery properties (good recovery or low tension set) upon ageing than the more conventional control rubber bladder (Exp A). This was achieved with a substantially reduced cure time.

EXAMPLE II Pneumatic rubber tires of size P155/80R13 were built and prepared for curing.

A multiplicity of such tires were cured with sequential bladder cure cycles using the bladders of the rubbers shown and defined in Example I herein with the following results shown in Table 3. Each uncured tire contained a silicone-based lubricant coating on its inner surface to provide release of the tire from the bladder after cure. Examples A-D refer to tire cure bladders of rubber of the type shown as corresponding A-D of Example I.

Table 3 Control Exp. A Exp. B Exp. CExp. D Cure Timel 15 9 10 8 at 210 C (mien)15 910 8 No ofBladders in ofTire 22 9 33 10 No (average)of Tire Cure2 Cycles (average) Ranking According to Tire Defective Rate 3 3 2 1 1. For preparation of the bladders.

2. Average sequential tire cure cycles per bladder before failure of the bladder.

3. Average bladders ranked on a scale of 1 to 3 with 1 representing lowest tire defective rate.

These examples demonstrate that faster curing of the bladder is possible to create a bladder with substantially greater service life through repetitive cure cycles for the bladder of this invention producing tires at a relatively low defective rate. The low tension set property of the bladder rubber is believed to be an important factor in these results.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

Claims (7)

WHAT IS CLAIMED IS:

1. An expandable rubber bladder comprised of a shaped and vulcanized rubber composition which comprises, based on 100 parts by weight of at least one synthetic rubbery copolymer composed of an average of about 99 to about 99.4 percent of an isoolefin containing from 4 to 7 carbon atoms and, correspondingly, about one to about 0.6 percent isoprene; and, in the absence of castor oil, about 2.2 to about 2.8 parts by weight stearic acid; about 40 to about 60 parts by weight carbon black; and a vulcanization system for said copolymer comprised of polychloroprene, alkyl substituted phenol dialcohol resin and zinc oxide.

2. The rubber bladder of claim 1 where the said isoolefin is isobutylene.

3. The rubber bladder of claim 2 where said alkyl substituent of said resin is an octyl radical.

4. The rubber bladder of claim 2 where said alkyl substituted phenol dialcohol resin is the reaction product of para alkylphenol and formaldehyde.

5. The rubber bladder of claim 4 where said polymeric alkyl substituted phenol dialcohol resin is the reaction product of para octyl phenol and formaldehyde.

6. The rubber bladder of claim 5 as a tire cure bladder.

7. A method of preparing a pneumatic rubber tire which comprises shaping and vulcanizing an unvulcanized pneumatic rubber tire under conditions of heat and pressure by pressing the tire outwardly against a mold surface by an inner bladder of claim 6.