Ozone attack and tire sidewall protection.

The tire sidewall is the outer surface of the tire between the bead and the tread, and its function is to protect the tire casing against weathering. It provides a physical link between the wheel and the tire tread in transmitting power and braking forces to the tire tread. It thus plays a major role in vehicle handling and suspension. Typically, a sidewall compound contains a blend of natural rubber (NR) and butadiene rubber (BR) with carbon black and many other chemicals to protect the casing from weathering, ozone attack, abrasion, cuts and cracking.

The latest trend is to use radial ply tires in the majority of vehicle segments for better safety, mileage and fuel economy. Sidewall contour has undergone major changes over the years to accommodate increased performance demands arising out of high-tech new generation vehicles. Today, tires are lighter in weight, with a wider tread, lower aspect ratio, etc., to address conflicting performance requirements, like improved traction, rolling resistance, mileage and high speed handling, to name a few. These requirements have put a greater demand on the sidewall compound, as the flex area is concentrated into an even smaller region of the tire.

Even in truck tires, which usually undergo more than one retread in their lifetime, it is important that the sidewall is able to perform adequately for the entire life of the tire. Thus, if the ultimate mileage target for a truck tire of one million miles (ref. 1) is ever to be realized, the tires would be expected to undergo at least tour retreads and would be dependent on the sidewall compound maintaining adequate ozone resistance throughout the entire lifetime of the tire.

The problem of ozone cracking in tires was not recognized until after the introduction of amine antioxidants and accelerators in the 1930s. The reason was that tires wore out before they became cracked by ozone (ref. 2), since the service life expectancy in the 1930s was about 5,000-15,000 km. With the commercial introduction and widespread use of reinforcing type carbon black in tire tread, the mileage has increased significantly and the importance of ozone attack has received prominence. Customers today expect about 80,000 km service life and other non-performance attributes like aesthetics (glossy sidewall appearance), presence of colored strips, etc., which have become parameters in deciding the car tire buying behavior. At the same time, the presence of atmospheric pollutants has also increased many-fold over the years.

Thus, the challenge before technologists is not only to fulfill the stringent performance demands associated with the "need for speed," which has led to incorporation of improved variants of compounding ingredients, but to see it performing in a polluted atmosphere, particularly with increased ozone concentration.

Ozone formation in the atmosphere

Ozone is an activated state of oxygen and is a more powerful oxidizing agent than oxygen. Ozone occurs naturally and is formed in the stratosphere by the action of the sun's UV light on atmospheric oxygen. Molecular oxygen undergoes photolysis to give oxygen atoms, which in the presence of an inert body (M) reacts with more oxygen to give ozone.

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The relatively high concentration of ozone in the stratosphere helps to prevent much of the sun's ultraviolet light from reaching the earth's surface. The maximum ozone concentration is at the altitude of 25 km and is carried into the atmosphere by winds. Thus, ozone concentrations in the terrestrial atmosphere vary between 0-10 pphm, depending on the season and geographic locations. The concentration of ozone in thickly industrialized areas may be higher than the average by a factor of ten3. At this higher concentration, ozone attack is very detrimental to molded rubber products like tires.

Ozone is also formed in the troposphere (10-13 km from the earth's surface), but in this case, since the shorter wavelengths of the sun's spectrum are absorbed by the upper atmosphere, another source of energy is required. Peroxyle radicals, which are intermediate in the auto-oxidation of volatile hydrocarbons (RH) in an industrial atmosphere, oxidize molecular oxygen to ozone in the presence of nitric oxide, which acts as an oxygen transfer catalyst (ref. 4). This is shown as:

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The polluted air is transported by convection currents to the earth's surface and, due to its very high ozone concentration, it is dangerous to human life and damaging to polymers. However, ozone is rapidly deactivated in contact with organic materials, particularly those containing double bonds.

Mechanism of ozone attack

Natural rubber and synthetic elastomers containing active ethylenic unsaturation are rapidly degraded by ozone when they are under stress. Degradation is evidenced by cracks on the surface that are perpendicular to the direction of stress.

The mechanism of ozone attack as described by Criegee and Schroeder (ref. 5) is widely accepted and is represented in figure 1. Ozone reacts with the active ethylenic unsaturation to form a trioxolane structure. This structure decomposes to Zwitterion (1) and a carbonyl compound (2), which can recombine to a normal ozonide (3). The Zwitterion can dimerize to form diperoxide (4) or give a higher peroxide (5). The scheme is shown in figure 1.

[FIGURE 1 OMITTED]

Saturated rubbers, for example ethylene propylene rubber (EPR), are apparently free from ozone attack due to the absence of ethylenic double bonds in the backbone, while an unsaturated rubber like chloroprene (CR) is more resistant to ozone due to its double bond deactivation by the chlorine atom.

The reaction of ozone is not limited to unsaturated polymers. It also reacts, albeit more slowly, with polymers containing labile hydrogen. This leads to the formation of free radicals, which cause subsequent thermal-oxidative chain reactions at relatively low temperature (ref. 4).

Ozone cracking occurs only in rubbers subjected to tensile stresses. The severity of cracking increases rapidly if the applied strain is above a small threshold level on the order of 10%. Rubber components used in compression crack only in the regions of the surface where tensile stresses are induced. These cracks cannot penetrate very far because they soon encounter compressive rather than tensile stresses. Thus, ozone cracking is not a serious problem for large rubber components like bridge bearing pads, which are held in compression.

These phenomena can be explained in terms of the known chemistry of ozonolysis, as shown in figure 1. The initial attack of ozone occurs in both stressed and unstressed rubber to give a molozonide, which is unstable and dissociates to give a Zwitterion (1) and a carbonyl compound (2). In the absence of stress, further combination of the two species occurs in a "cage reaction" [with (1) and (2)] to give a stable isoozonide (3). But in the stressed condition, this reaction can not occur due to the separation of chain ends. The Zwitterion subsequently reacts with another identical species with a carbonyl compound elsewhere in the rubber. The second process leads to weakening of the rubber structure and incipient crack formation. If the rubber is subjected to dynamic stresses, the development of small cracks accelerates the process of fatigue, since the stresses at the tip of a growing crack are much greater than on the unaffected surface.

In the case of unstretched rubber, the reaction ceases when all the unsaturation is consumed. When the rubber is stretched, new unsaturation sites are brought to the surface so that ozone continues to react, resulting in the formation and growth of cracks.

Even ozone present in the atmosphere at only a few parts per hundred million readily cleaves carbon-carbon double bonds in an elastomer. The rate of ozone attack depends on the fluid flow rate at the surface and the size of the article involved. The rate of reaction is much slower if the medium carrying ozone is liquid rather than gas (ref. 6).

The effects of ozone attack

Unsaturated rubber exposed to ozone in a strained state quickly develops surface cracks. The rate of crack growth depends on the type of rubber and the ozone concentration. The ozone crack grows at a right angle to the direction of principal strain, and the micro cracks generated at the surface may extend, depending on the stress level and ozone concentration. Thus, it is relatively easy to differentiate between cracking caused by ozone and that resulting from thermal oxidation.

Under cyclic loading, when mechanical crack growth is very slow or absent, ozone attack may be a dominant crack growth process. In a fixed deformation, the growth of the crack is time-dependent, and the rate is proportional to ozone concentration and is mostly independent of tearing energy (ref. 6). So, under a favorable set of cyclic fatigue conditions, these cracks may grow further, leading ultimately to failure.

The crack density increases with increasing strain, while there is a concomitant decrease in the growth. At any given strain, except at the very early stage of exposure, the crack density tends to decrease with time owing to crack coalescence or obsolescence (small cracks being overtaken by larger cracks and ceasing to grow because the necessary energy is no longer available).

Testing of ozone resistance

The ozone resistance of any rubber product can be measured by exposing the sample in ozone at a fixed temperature. The standard ozone concentrations for testing are 25, 50, 100 and 200 pphm, with a [+ or -] 10% tolerance at 23[degrees] or 40[degrees]C. Depending on the requirement, testing in various modes is possible. Interested readers may refer to ASTM methods D1149-99B, D3395-99, D518-99 and D1171-99. However, D1149-99B, commonly known as "bent loop testing," needs special mention. In this method, the bent loop offers different tensile strains ranging from 0-25% in different locations. This method is most popular in different testing/standardization agencies and is commonly employed for tires. A rating of the severity of ozone attack is normally provided by visual inspection and/or by stress relaxation techniques after exposure to ozone atmosphere for different periods at a pre-determined temperature. Prior to testing, all the samples are conditioned at room temperature or 40[degrees]C for 24 hours to facilitate blooming.

The measurement of ozone concentration is rather difficult. The most accurate and experimentally convenient way of determining the amount of ozone in the test chamber is by an instrumental technique involving the principle of attenuation of UV light.

Compounding approaches to prevent ozone attack

A tire sidewall is a typical example where ozone protection is required under both static and dynamic conditions. This is achieved by two techniques:

* By creating physical barriers on the surface (by the application of a mixture of PPDs and waxes); and

* by containing the ozone crack growth in the matrix (by introducing new generation polymers in the sidewall formulation).

Formation of a physical barrier

It is a standard practice in rubber compounding to use antiozonants and/or waxes in the formulation for effective ozone protection during service life. These materials, in due course, diffuse from the interior and build up a continuous layer on the surface.

Para phenylene diamines (PPDs) are effective in reducing ozone cracking in diene rubber. The three classes of PPDs that are available differ in their ability to protect robber articles, depending on the use of the article and the environmental conditions. A brief summary (refs. 5 and 7) is shown in table 1.

The major disadvantage with PPDs is their staining characteristics, which render them unsuitable for light colored applications. Black sidewall surface discoloration by the in situ formation of a thin brown film of ozone/para-phenylenediamine reaction product is unacceptable to many customers. This factor alone has sparked a series of scientific investigations on sidewall compounds with new generation ozone resistant polymers.

Several non-staining types of antiozonants have been in use recently. Tetrahydro-1,3,5-(n)-butyl-(S)-triazinethion and 2,4,6-tris-(N-1,4-dimethyl-pentyl-para-phenylenediamino)-1,3,5 triazine need special mention in protecting general purpose rubber sidewalls from ozone attack. They offer excellent static ozone protection and improved flex life, but their diffusion to the surface is limited due to very high molecular weight. However, they are not as effective in protecting the NR/BR sidewall as is the commercially available paraphenylene diamine antiozonant, like 6PPD.

A number of mechanisms has been proposed to explain the functioning of the antiozonants, and the most popular are:

* Scavenger theory (refs. 8 and 9)--reaction of antiozonants with ozone takes place at a much faster rate than ozone reacts with the double bond from rubber;

* protective film theory (ref. 10)--formation of a protective film from the ozone-antiozonant reaction product; and

* self-healing film theory (ref. 11)--formation of a self-healing film by the reaction of antioxidant with ozonized rubber.

The easiest way to protect the surface is to add waxes in the compound formulation, which bloom and form a "physical" protective layer of saturated hydrocarbons. Wax offers effective protection in applications that are static or have critical strain less than 30%. These films are broken above the critical strain during flexing and stretching, and ozone can channel into the breaks, resulting in the formation of deep cracks and subsequent product failure.

Waxes are classified into paraffin, micro crystalline (amorphous) and intermediate types. The paraffins are pure linear [C.sub.n][H.sub.2n+2] hydrocarbon (n = 18-50). They are obtained from the petroleum refining operation and are separated as specific melting point cuts, for example: 54/56[degrees]C, 56/58[degrees]C, 58/60[degrees]C, etc. Each cut has a Gaussian distribution of isomers. The micro crystallines (MCs) are branched chain aliphatic compounds that contain some aromatic or cycloparaffinic features as a part of their sub-structure. They consist of 34-70 carbon units. Intermediate waxes (ref. 12) are mixtures of both linear and branched chain hydrocarbons, and the ratio of linear to branched (MCs) can vary from 3:1 to 1:3.

Paraffin, being more soluble, provides the best protection at lower temperatures (below 40[degrees]C); whereas MCs perform better at higher temperatures. A common practice is to use a combination of both to ensure protection over a wide range of temperatures. Thus, the ozone protecting waxes commonly available are customized blends of linear and branched chain structures specific to typical applications.

A synergism between waxes and chemical antiozonants has been noticed. It is attributed either to the ability of antiozonants to increase the thickness of the wax bloom, giving a more continuous film, or to the carrier effect of the wax, which helps antiozonants to migrate to the surface.

Theories of wax film formation

The formation of a film is based on the ability of wax to migrate to the surface. The governing factor for migration is its insolubility in the rubber matrix, which is a function of the structure of the wax. The solubility of a wax depends on its molecular weight, which again dictates its migration speed. A higher molecular weight will reduce its solubility, and consequently migration to the surface is favored. However, if the molecular weight is too high, that is, the molecular chain is too long, the coefficient of diffusion reduces to such an extent that it can no longer migrate. On the other hand, too low molecular weight will get completely dissolved, preventing the migration to the surface. Solubility decreases with chain length. The area below a solubility-chain length graph at a given temperature represents the solubility limits of a particular grade, and the area above it is actually available to form a protective layer (ref. 13).

Another important factor is the speed at which a protective film is formed. This depends on:

* Solubility dependent migration: and

* mobility of the molecules.

A wax with lower solubility, due to higher molecular weight, tends to migrate at less concentration, but has a reduced degree of mobility. On the other hand, lower molecular weight species have higher mobility, but, due to higher solubility, need a higher concentration to bloom to the surface. Branched chain waxes are less mobile than straight chain waxes.

Mobility rises with temperature and so does solubility. So, at a particular temperature, the solubility of a given quantity of wax must be such that some excess amount is available that can move quickly to the surface to form a protective layer. This is, thus, dependent on molecular weight, molecular weight distribution and molecular structure of the wax.

The minimum amount of wax bloom required to protect the surface at a particular temperature is thus an important consideration. This depends on the type of wax used, the temperature of the operation, the concentration and the extent of stretch on the article. The dose that is needed to provide full ozone protection depends on the heterogeneity of the molecular weight present in the wax. This means, when different molecular weights are present, higher doses are necessary.

New polymers for sidewall applications

The use of ozone resistant polymers for sidewall applications has received substantial attention in the last two decades to address the growing demands for glossy black sidewalls. Polymers useful for this application are ethylene propylene diene terpolymers (EPDM), halogenated butyl rubbers (CIIR/ BIIR) and brominated isobutylene-co-para-methyl styrene (BIMS). These rubbers are used in sufficient quantities, along with general-purpose rubbers (GPR) (such as NR, SBR, BR), in sidewall formulations, such that the dispersed domains of these rubbers effectively block the ozone-initiated cracks in the diene rubber matrix.

Blends of GPR with EPDM

To combine the strength of NR with the ozone resistance and co-curing ability of EPDM, their blends came as a logical choice for sidewall applications. It has been shown (ref. 14) that around 40 wt. % of EPDM is essential to provide adequate ozone protection of NR in sidewall applications. While ozone resistance has been achieved relatively easily, the general physical properties of NR/EPDM blends have been less than satisfactory. Interestingly, physical properties of the vulcanizates were much inferior compared to those predicted from the component elastomers (refs. 15 and 16). The inferior physical properties of the NR/EPDM blend vulcanizates were attributed to:

* Lower solubility of many curatives in the EPDM phase;

* poor interaction of EPDM with carbon black; and

* diffusion of curatives towards the faster curing NR phase during mixing and curing (ref. 17).

The results of all these effects leads to a system with a highly crosslinked NR phase and a very poorly crosslinked EPDM phase.

Approaches to improve NR/EPDM blend properties

Several methods (refs. 18-22) were suggested in the recent past with a view to increase cure rate of EPDM and select suitable curatives that have increased reactivity towards EPDM.

Another significant development in this regard is to follow "reactive mixing" (ref. 23). This involves pre-curing the EPDM phase in a masterbatch stage at elevated temperature using sulfur donors like bis-alkylphenoldisulfide (BAPD), dithiodimorpholine (DTDM) and dithiodicaprolactam (DTDC), followed by cross-blending with NR masterbatch and then a final batch with curatives.

A reactive mixing technique (ref. 24) was adopted for modification of EPDM (~11% ENB) with sulfur donors, and an adjustment of carbon black/oil dose was done to match typical NR/BR sidewall properties. It was, however, observed that direct substitution of NR/EPDM sidewall compound in place of NR/BR was not appropriate, as it led to premature failure in high speed rig testing. It was suggested that a modification of the sidewall die design and minor changes in the overall tire design were required to achieve comparable test results in line with tires with a NR/BR blend.

Another important step in this regard is the commercial availability of high molecular weight EPDM grades with a higher unsaturation level. These grades not only reduce the cure mismatch but also substantially improve the adhesion to a carcass compound, the aged fatigue life and the dynamic ozone resistance.

Blends with BIMS

Brominated isobutylene-co-para-methyl styrene (BIMS) was developed to combine the beneficial attributes of halobutyl rubber (excellent fatigue properties and adhesion) and EPDM rubber (excellent ozone, heat and aging resistance) in a single polymer. Additionally, BIMS provides better flex and crack growth resistance than halobutyl rubbers. It was observed that at least 40 parts of BIMS are required to achieve the requisite ozone resistance, and the BIMS phase must be highly dispersed in the general purpose rubber matrix to minimize crack growth. The extent of bromination and the para-methyl-styrene (PMS) co-monomer level have appreciable impact on the ozone resistance, which increases with the increase in the bromination level and PMS content.

It has been reported (refs. 25 and 26) that a passenger tire sidewall containing a tri-blend of NR/BIMS/BR provided improved cured adhesion and tear properties. Tires made with this compound provided comparable DOT testing, improved high speed testing, but with slightly higher rolling resistance as compared to a general-purpose rubber control tire.

Conclusion

The ozone concentration in the atmosphere is on the rise. Although government agencies and environmental groups are more alert to containing pollution of the environment than in the past, it will require more focused and conscious efforts from all quarters to protect from the harmful effects of ozone concentration.

The combined effects of higher pollution level and improved performance needs of tires will generate stringent static and dynamic protection demands in the coming days.

The effective protection of tire sidewalls from ozone attack can be achieved through a cost-effective wax/antiozonant route or a relatively costlier new generation polymer route. The choice in this regard, however, is left to customers.

Table 1--formation of a physical barrier

Chemical type            Example     Characteristics

N-aryl-N'-alkyl para-     IPPD,      Very active
phenylene diamine          6PPD      Low molecular weights are volitile
                                     Activity decreases with alkyl
                                       group containg greater than C8
                                     Dynamic and intermittent dynamic
                                       applications
N-N'-dialkyl para-        77PD,      Highly staining and volatile
phenylene diamine          88PD      Very active
                                     Susceptible to oxygen attack
                                     Mostly static, less for dynamic or
                                       intermittent dynamic applica-
                                       tions
N-N'-diaryl para-         DPPD,      Slow migration rate
phenylene diamine        Wingstay    Least volatile
                           -100      Little staining
                                     Moderately active
                                     Good for dynamic, but poor for
                                       static applications

References

(1.) Fusco, J.V. and Young, D.G., "Isobutylene based polymers in tires--status and future trend," ACS Rubber Division meeting, Oct. 1990.

(2.) Encyclopaedia of Polymer Science & Engineering, v. 14, p. 759, eds. Mark, Bikales, Overberger and Merges.

(3.) Murrey, R.W., Polymer Stabilization., ed. W.L. Hawkins, Wiley Interscience, NY (1972).

(4.) Polymer degradation and stabilization, by N. Grassie and G. Scott, Cambridge University Press (1985), ch. 7, p. 194.

(5.) Criegee. R. and Schroeder. G., Ber. Dtsch. Chem. Ges. 93,689 (1960).

(6.) Lake, G.J. and Thomas, A.G., ch. 5, pp. 117-120, Engineering with Rubber, ed. Gent, A.N., Hanser (2001).

(7.) Encyclopaedia of Polymer Science & Engineering v. 14, p. 707, eds. Mark, Bikales, Overberger and Merges.

(8.) Cox, W.L., Rubber Chem. Technol. 32, 364 (1959).

(9.) Layer, R.W., Rubber Chem. Technol. 39, 1,584 (1966).

(10.) Erickson, E.R., Berntsen, R.A., Hill, E.L. and Kucy, P., Rubber Chem. Technol. 32, 1,059 (1959).

(11.) Loan, L.D, Murray, R.W. and Story, P.R., J. Int. Rubber Technol. 2, 73 (1968).

(12.) Jowett, F., Rubber World, 24, May (1983).

(13.) Rhein Chemic Technical note on wax bloom.

(14.) Von Hellens, C. W., paper no. 40, ACS Rubber Div. Meeting, Oct. (1989)

(15.) Sutton, M.S., Rubber World, 149 (5), 62-68 (1964).

(16.) Corish, P.R., Rubber Chem. Technol. 40 (2), 324-340, 1967.

(17.) Gardiner, J.B., Rubber Chem. Technol. 41 (1), 1,312-1328, 1968.

(18.) Morrissey, R.T., Rubber Chem. Technol. 44 (4), 1,025-1,042, 1971.

(19.) Baranwal, K.C. and Son, P.N. Rubber Chem. Technol. 47 (1), 88-99, 1974.

(20.) Hashimoto, K., Miura, M., Takagi, S. and Okamoto, H-, International Polymer Science & Technology, 3, T84-T-87.

(21.) Coran, A.Y., U.S. patent 4,687,810, 18 August, 1987, Monsanto Chemical, St. Louis.

(22.) Mastromatteo, R.P., Mitchell, J.M. and Brett, Jr. T.J., Rubber Chem. Technol. 44 (4), 1,065-1,079 (1971).

(23.) Cook, S., ch. 14, p. 169, Blends of Natural Rubber, eds. A.J. Tinker and K.P. Jones, Chapman & Hall (1998).

(24.) Cook, S., ch. 14, p. 184, Blends of Natural Rubber, eds. A.J. Tinker and K.P. Jones, Chapman & Hall (1998).

(25.) Flower, D.D., Fusco, J.V. and Tracey, D.S., Rubber World, 209 (6), 32 (1994).

(26.) Flower, D.D., Fusco, J. V. and Tracer, D.S., paper no. 50, presented at ACS meeting, Oct. (1993).