Microencapsulation of antidegradants.

Antiozonants

The protection of tires and rubber articles from ozone aging is of considerable interest to the industry since reaction of ozone with unsaturated elastomers results in decomposition and chain scission of the polymer, as described in the review by Layer and Lattimer (ref. 1). Ozone is thought to react only with the elastomer carbon-carbon double bonds (ref. 2) on the surface of rubber products to form ozonides (refs. 3 and 4). If stress is also applied to the rubber while it is being exposed to ozone, surface cracking results as the molecular chains of the polymer are broken at the location of the ozonide groups. The new areas exposed as a result of surface cracking are more susceptible to ozone attack leading to accelerated product failure.

Chemicals are added to rubber products to impart effective resistance to ozone aging under both static and dynamic service conditions of the rubber articles. The ideal chemical antiozonants will migrate to the surface of rubber, react directly with ozone and reduce the crack propagation rate. Unfortunately, antiozonants are depleted by surface reactions with ozone (refs. 5-8), and may be removed physically such as by washing or scuffing. There are numerous functional classes of chemicals that exhibit antiozonant activity - trimethyldihydroquinoline, n-substituted ureas, triphenyl phosphine, substituted olefins, triazine triamines, N,N'-disubstituted-para-phenylenediamines, metal dithiocarbamates, n-substituted thioureas, phosphites, lactams and triazine thiones. Many of these antiozonants are commercially available.

The N,N'-disubstituted-para-phenylenediamines have found the greatest acceptance in tires (ref. 9) since they are the most effective class of antiozonants. The para-phenylenediamine antiozonants can be symmetric, such as N,N'-dialkyl- or N,N'-diaryl-substituted, or not symmetric, such as N-alkyl,N'-aryl-disubstituted. In particular, the N-alkyl, N'-phenyl-para-phenylenediamines are the most effective antiozonants, particularly those with alkyl groups containing from five to seven carbon atoms (ref. 5, 9-12). Use of these chemical antiozonants typically shows dramatic increases in the protection of rubber from ozone attack, but only up to a limiting value, since para-phenylenediamines have only a finite solubility in rubber. Anther consideration of para-phenylenediamine use is the resultant surface discoloration that forms upon ozonation (ref. 8) requiring that these antiozonants be used in only limited amounts if tire and rubber product appearance is an important consideration.

The significant increases in tire lifetime brought about primarily by the adoption of the radial tire design and improved rubber compounds, particularly in treads, have prompted a need for longer-lasting antiozonants with the activity of the para-phenylenediamines in order to effectively stabilize the tire carcass throughout its useful service lifetime.

Mechanism of antiozonant action

Several theories of antiozonant protection of rubber have been proposed. They include the scavenger, protective film, relinking and self-healing film theories. The scavenger model requires that the antiozonant diffuse to the rubber surface and react with ozone at a faster rate than ozone can react with the backbone carbon-carbon double bonds of unsaturated elastomers (refs. 5 and 6). The protective film theory has the antiozonant/ozone reaction products forming a thin film on the rubber surface thus protecting the rubber product (refs. 13 and 14). The relinking model has the antiozonant preventing polymer chain scission or causing bond recombination (refs. 15 and 16). The reaction of the antiozonant with the ozonized rubber or zwitterion to form a self-healing film on the rubber surface (ref. 17) is another theory of antiozonant action. Lattimer, Rhee and co-workers (refs. 18 and 19) have used liquid chromatography to isolate and field desorption mass spectrometry to identify reaction products of ozone and films of the N,N'-di-(1-methylheptyl)-para-phenylenediamine (ref. 18) and N-(1,3-dimethylbutyl)-N'-phenyl-para-phenylenediamine (ref. 19) (structure 1, 6PPD) in order to investigate the mechanism of antiozonant action. Use of a high resolution mass analyzer generated exact mass data, and allowed calculation of empirical formulas, deduction of structures of reaction products, and derivation of reaction pathways. They concluded that both the scavenger and protective film mechanisms were operative in the protection of rubber.

Waddell, Evans and coworkers (refs. 8, 20 and 21) have used laser desorption mass spectrometry to directly analyze surfaces of laboratory ozone-aged, carbon black-filled natural rubber compounds and in-service tire sections. They obtained mass spectra on intact molecular ions of 6PPD and reaction products with ozone consistent with both the scavenger and the protective barrier models of antiozonant protection of rubber being operative, in agreement with the conclusions reached by Lattimer, Rhee and coworkers (refs. 18 and 19).

De Coninck, Aarts, Burhin and Orband (ref. 22) heat aged 6PPD in air, isolated active products by column chromatography, determined their structure using plasma-spray liquid chromatography-mass spectrometry, reincorporated active products in a carbon black filled-natural rubber compound, and tested rubber physical properties. Approximately 30-40% of the activity of 6PPD was obtained with the higher molecular weight reaction products having lower activity. The scavenger and protective film theories of antiozonant action are again supported with the protective film having further antiozonant activity.

Stabilization of rubber

A number of approaches have been used to stabilize rubber to ozone attack. Use of ozone-resistant elastomers such as chlorobutyl rubber (refs. 23-25) and/or ethylene-propylenediene rubber (refs. 23-26) to reduce the relative amount of diene rubber (natural rubber or polybutadiene) is reported to stabilize rubber products. Another approach has been the controlled diffusion of the antiozonant using conventional polymers.

Controlled-diffusion antiozonants

Synthesis of a variety of higher molecular weight N,N'-di-substituted-para-phenylene diamines has been studied in a search for permanence by changing the rate of regenerating the antiozonant surface film by changing the solubility in rubber. Alterations to the structure included (ref. 9): (i) use of N-naphthyl in place of N-phenyl: N-(1,3-dimethylbutyl)-N'-naphthyl-para-phenylenediamine, (ii) use of a carboxylic acid or propyl carboxylate substituent on the central phenylene ring, (iii) use of substituent on the N-phenyl ring: N-cumylphenyl-N'-isopropyl-para-phenylenediamine, and (iv) addition of a third substituent: N-isopropyl-N-phenylthio-N'-phenyl-para-phenylenediamine. All new antiozonants diffused to renew their presence on the surface and significant differences in the rates of surface film regeneration were observed. However, there was insufficient activity in these new antiozonants compared to 6PPD, which still showed the best performance.

Combinations of either N-isopropyl, N'-phenyl-para-phenylenediamine and/or 6PPD with high molecular weight antiozonants prepared by either (i) substitution of an alkyl group on the N'-phenyl ring, and/or (ii) increasing the size of the N-alkyl group were tested in a carbon black-filled natural rubber/butadiene rubber (50/50) compound and in these compounds incorporated as sidewalls of truck tires (ref. 27). Tires were tested until the treads were worn off. Combinations of at least one high molecular weight antiozonant with both of the commercial antiozonants afforded the best results in the truck tires, as measured by both reduced sidewall cracking and reduced surface discoloration.

Rubber-matrix antiozonants

Evans et al (ref. 28) incorporated 6PPD into a rubber matrix which was then used as the stabilization system for tire sidewall compounds. A matrix compound was mixed, cured and ground into particles of about 50 [mu] m in size. The matrix rubber compound consisted of 40 phr of 6PPD antiozonant, 50 phr of carbon black and an elastomer: polyisoprene, polybutadiene, butyl rubber, styrene-butadiene rubber, polychloroprene, ethylene-propylene-diene rubber, polynorbornene and nitrile rubber with various acrylonitrile percentages up to 55%. High pressure liquid chromatography was used to determine the partitioning coefficient of the antiozonant in the cured nitrile matrix compounds versus the cured carbon black-filled polyisoprene/polybutadiene (60/40) rubber to be protected. Nitrile rubber matrices with increasing acrylonitrile concentrations had the highest partitioning coefficients (table 1). Sufficient masterbatched rubber was added to the rubber to be protected to afford 6 phr of 6PPD. Best protection of the rubber from ozone was provided by using a nitrile rubber matrix with increasing acrylonitrile concentrations (table 2). A blend of 3 phr of 6PPD and 3 phr of 6PPD in the rubber matrix afforded the best overall protection.

Table 1 - antiozonant partitioning coefficients
Rubber matrix                    Matrix vs. rubber
                                 stock partitioning
                                     coefficient
Polyisoprene                            0.8
Polybutadiene                           1.1
Butyl rubber                            0.2
Styrene butadiene rubber                1.1
Polychloroprene                         0.9
Ethylene propylene diene rubber         0.6
Polynorbornene                          1.7
Nitrile rubber
18% ACN                                 2.2
21% ACN                                 2.7
24% ACN                                 2.9
32% ACN                                 3.5
45% ACN                                 4.0
55% ACN                                 4.4
Table 2 - effect of matrix
antiozonant on rubber
lifetime
Rubber matrix      Ozone
                   lifetime
                    (days)
Free 6PPD control      21
Polybutadiene          20
Polynorbornene         22
Nitrile rubber
18% ACN                22
21% ACN                24
24% ACN                24
32% ACN                27
45% ACN                29
55% ACN                31

Silica-filled rubber compounds

Use of a reinforcing precipitated silica to extend the effectiveness of the antiozonant in a carbon black-reinforced natural rubber/butadiene rubber (50/50) sidewall compound has been reported by Waddell et al (refs. 29 and 30). Statistically designed compounding experiments varying the N-330 carbon black, silica and 6PPD antiozonant levels showed that moderate levels of silica use in place of some of the carbon black significantly improved the durability properties: Ozone cracking was reduced, tear strength was increased, crack growth was decreased and compound hysteresis was reduced. In addition, the surface discoloration of rubber could be visibly reduced by using a formulation consisting of 9 phr of precipitated silica in place of 8 phr of N-330 carbon black, and 2 phr of polymerized 1,2- dihydro-2,2,4-trimethylquinoline antioxidant in place of 1 phr of 6PPD antiozonant, (table 3).

Table 3 - physical properties of sidewall compound
using silica and persistent antioxidant
                      Carbon    Black/        Black/
                        black   silica       silica/
                      control            antioxidant
Minimum torque, dNm       2.7      3.1           3.1
Maximum torque, dNm      17.2     15.7          15.9
Scorch (T[S.sub.2), min   6.1      6.2           6.9
Cure ([T.sub.50), min     8.0      8.2           8.8
Cure ([T.sub.90), min    11.7     12.6          12.9
Break strength, MPa      23.2     23.1          23.2
Elongation @ break, %     638      691           737
Modulus @ 20%, MPa       0.62     0.63          0.62
Modulus @ 100%, MPa       1.5      1.3           1.3
Modulus @ 300%, MPa       8.1      6.2           5.7
Tear, N/mm               19.0     22.6          23.0
Ozone rating,
12 days aging             8.7      7.7           7.3
Absorbance, 400nm,
7 days aging           0.55     0.42            0.38

Microcapsules

Arshady (refs. 31-33) has made a comprehensive review of microcapsules. Microcapsules are generally defined as particles, usually spherical, in the size range of about 50 nm to 2 mm composed of an excipient polymer matrix (wall or coat) and an incipient active component, referred to as a core substance. Polymer spheres are empty microcapsules called microspheres, but unfortunately the terms are often used synonymously. Microcapsules of nanometer diameter are usually referred to as nanocapsules to emphasize their small size.

Microspheres were first reported in 1953 by Green and Schleicher (refs. 34 and 35) who produced microencapsulated dyes for the manufacture of carbonless copying paper. During the last twenty years, microencapsulation has become an increasingly popular technology, particularly in the pharmaceutical industry. Several thousand patents and publications appear annually. A variety of products and processes in daily use apply microencapsulation technology, including carbonless copying paper, liquid crystals, fire extinguishing compounds, adhesives, perfumes and fragrances, animal feedstocks, washing powders, medicinal products, artificial kidneys, insecticides, algicides, cosmetics and fertilizers. The primary advantages of using microencapsulation technology are:

* Separation of incompatible components;

* Increasing stability/protection from environment;

* Conversion of liquids to free-flowing solids;

* Masking odor, activity, etc.;

* Protection of immediate environment;

* Targeting of drugs;

* Controlled release of active compounds.

Microcapsules are manufactured from a wide range of monomeric and polymeric starting materials and by a variety of polymerization techniques (table 4).

Table 4 - microencapsulation techniques from
monomers and polymers
Microencapsulation techniques from monomers
Suspension polymerization       Emulsion polymerization
Dispersion polymerization       Precipitation polymerization
Suspension polycondensation     Dispersion polycondensation
Precipitation polycondensation  Interfacial polycondensation
Microencapsulation techniques from polymers
Suspension crosslinking         Coacervation/phase separation
Solvent evaporation/extraction  Polymer precipitation
Polymer chelation               Polymer gelation
Polymer melt solidification

Thus various combinations of starting materials and manufacturing technology can be selected to produce microcapsules having a wide variety of compositional and morphological characteristics. Generally the choice of the manufacturing technology depends largely upon the nature of the starting material used (table 5).

Table 5 - nature of polymeric material
Polyamide                  Interfacial polycondensation
Poly(alkyl cyanoacrylate)  Emulsion polymerization
Albumin, polysaccharide    Suspension crosslinking
Gelatin, gum arabic,       Coacervation
 ethylcellulose
Biodegradable polyester    Solvent evaporation/
 Cellulose acetate         Solvent extraction
Synthetic polymers         Solvent evaporation/
 Polystyrene               Solvent extraction
 Polycarbonate
 Polystyrene-methyl methacrylate
 Polyester
 Silicone

Microcapsules and rubber

The controlled release of active compounds by either sustaining their release or delaying their release is a potentially significant technology to the rubber industry. Norbury, Chang and Zeller (ref. 36) have encapsulated a variety of oils ([C.sub.12] - [C.sub.15] alcohol benzoates, octyl hydroxystearate and jojoba bean, castor, vegetable and mineral oils) and styrene-isoprene-styrene block copolymer (Kraton D1107) in ureaformaldehyde. These microcapsules were used as a non-abrasive skin cleaning cream. Takao, Hirosi and Hirosi (ref. 37) improved the long-term storage stability of uncured organopolysiloxane compounds by encapsulating the organic peroxide hydrosilylation catalyst in a thermoplastic silicone resin having a glass transition temperature of 65 [degrees] C and a softening point of 85 [degrees] C. Upon curing, the microcapsules released the peroxide to the rubber. Cured particles exhibited excellent mold release and the absence of scorch, mold staining and surface tackiness. Condo, Kosowski and Plochocki (ref. 38) developed microencapsulated-chemical stress sensors by encapsulating dyes absorbing in the visible region of the spectrum. They were used as monitors of the shear stress during the extrusion of plastic bonded explosives since the microcapsules would burst at a high shear stress, releasing the colored dye.

Microencapsulated antiozonants

Benko, Evans and coworkers (refs. 39 and 40) applied microencapsulation technology to rubber antiozonants using N-(1,3-dimethylbutyl)-N'-phenyl-para-phenylenediame (6PPD) to stabilize a model tire black sidewall compound (table 6).

Table 6 - rubber compound formula
Compound ingredient     Parts per hundred
                        rubber weight (phr)
Natural rubber                  40.0
Polybutadiene rubber            60.0
ISAF carbon black               50.0
Naphthenic process oil          10.0
Fatty acid                       2.0
Wax                              1.0
Zinc oxide                       3.0
Sulfur                           1.75
Accelerator                      1.0
Antiozonant                 As specified

Considerations for the delayed release antiozonant system using microcapsules include: (i) the wall material must be nonreactive with the 6PPD core material, (ii) the wall must provide an adequate barrier against premature release of the 6PPD core, (iii) the wall material must not interact with other components of the rubber compound host material, (iv) the release of 6PPD must be provided under preselected conditions, namely in-service tire and rubber product use, and (v) the capsule must not interfere with the function of the host tire or rubber product.

Preparation of microencapsulated 6PPD

Microencapsulated 6PPD antiozonant was prepared using: (i) solvent evaporation, a spray drying technique, (ii) meltable dispersion, and/or (iii) coacervation, a phase separation method. In the spray drying technique the antiozonant and polymer wall material were introduced into the top of the drying chamber through an atomizing device. A high-volume, high-velocity flow of air was introduced around the atomizer where the solvent was flash-evaporated from the small droplets. The air stream with the dry microcapsules was passed into a cyclone separator where they were separated from the exhaust air stream and were collected (ref. 40).

The meltable dispersion technique involved heating while constantly agitating the prospective antiozonant core material and a meltable wall polymeric material in a carrier fluid to which both are insoluble above the melting point of the wall polymeric material. The droplets of core antiozonant material were coated with the wall material which hardened upon cooling, usually in the spray drying apparatus.

The phase separation technique involved the addition of a non-solvent containing the core antiozonant material to a solution of a wall material which generated droplets of the core material followed by precipitation of the wall polymer around the droplet (ref. 40).

Antiozonant activity

A baseline was established for the function of the 6PPD antiozonant in the natural rubber/butadiene rubber compound by varying the levels of 6PPD in the cured compounds, which were tested at 40 [degrees] C by exposure to 50 pphm ozone. After equilibrating for four hours at 40 [degrees] C, the rubber surface concentration of 6PPD was measured. Up to the solubility limit of 6PPD in the rubber blend, a continuous surface film is formed which protected the rubber from ozone attack. Above this solubility limit, 6PPD bloomed to the rubber surface (figure 1). Compounds were continually flexed in the presence of ozone until failure occurred, and increased lifetimes were noted up to about a 4-5 phr level of 6PPD. There was no evidence that further increases in 6PPD concentrations up to 12 phr were beneficial to the rubber compound lifetime (figure 2). The 6PPD that bloomed to the rubber surface was thought consumed by ozone without protecting the rubber, which was verified by the extraction of the failed rubber samples with refluxing chloroform followed by high pressure liquid chromatographic (HPLC) analysis of the extract. The level of 6PPD remaining in the sample was similar for all samples at failure, regardless of the original concentration (refs. 39 and 40).

Wall material selection

A study was conducted to determine the solubility of various encapsulating polymers in molten 6PPD antiozonant. For each polymer evaluated as a capsule wall candidate, two grams of polymer were placed with ten grams of 6PPD in a beaker and examined after heating to 50 [degrees] C and after cooling to ambient temperature. Table 7 (refs. 39 and 40) is a summary.

[Tabular Data Omitted]

Polycarbonate, styrene-butadiene, chlorinated rubber and cellulose acetate were selected as wall materials which could be used with solvent evaporation technology (spray-drying) for formation of microcapsules because of the possibility of using a common solvent system. Nylon microcapsules were prepared using the dispersion polycondensation method in the spray-drying apparatus. Polyester microcapsules were prepared by coacervation technology using the phase separation method in alcohol/hydrocarbon solvents. Polycarbonate,

styrene-butadiene and nylon resins failed to yield a product which could be recovered as microcapsules, largely due to sheeting on the walls of spray dryer. Cellulose acetate and chlorinated rubber provided microcapsules which were used for further study. Polyester also provided capsules (table 8) (refs. 39 and 40).

[Tabular Data Omitted]

The microcapsules formed from cellulose acetate, chlorinated rubber and polyester were tested for extractability of 6PPD from the particles by placing microcapsules containing 0.1 gram of 6PPD in a volumetric flask with 100 mL of a 70/30 water/acetonitrile mixture. After 24 hours, an aliquot of the supernatant was removed and the percent 6PPD in the liquid was compared, via HPLC analysis, to a calibration curve generated by dissolving 0.01-0.2 grams of 6PPD in 100 mL of a 70/30 water/acetonitrile mixture (figure 3). The samples were then decanted and washed, yielding microcapsules which were placed in 100 mL of a water/acetonitrile solution and allowed to stand for an additional 24 hours. This procedure was repeated at 5, 10, 20, 30 and 45 days. Chlorinated rubber delayed the release of the 6PPD antiozonant into the solution for about 10 days, polyester did not release significant 6PPD even after 45 days, while cellulose acetate appeared to provide the desired slow, continuous release of the active 6PPD up to 45 days (figure 4). At the end of the 45-day period, the microcapsules were dissolved in a solution of toluene and dichloromethane to measure the remaining 6PPD level. Chlorinated rubber was t <10%; cellulose acetate 21% and polyester 92% (refs. 39 and 40).

Experiments using cellulose acetate with number average molecular weights of 30,000 to 60,000 and viscosity ratings of 1.14 to 22.8 Pa.s (ref. 41) showed no significant effects on release rates. Eastman Kodak cellulose acetate 398-10, number average molecular weight of 40,000, exhibited the highest recovery of microcapsules having less than 50 microns diameter from the spray-drying process.

Wall to core ratio optimization

A matrix of solvent system, solvent to polymer ratio and wall to core ratio for cellulose acetate microcapsules in the spray-drying system was evaluated to determine the optimum combination. Samples with wall to core ratios of 1/1 and 3/1, which were spray-dried from a mixture of 4/1 acetone/methylene chloride and a solvent/polymer ratio of 18/1, were selected. Material to provide 6 phr of 6PPD was added to the rubber compound via Brabender mixing in the initial stage or mill mixing in the final stage and the compounds cured. The cure rate of the compounds was accelerated by the addition of 6PPD as expected (ref. 42). When the microcapsules were added in the initial mixing stage, the cure rate also increased with increasing 6PPD level. However, when the microcapsules were added in the final mix, the cure rate was unaffected by the addition of 6PPD. Microscopic examination of the rubber compounds confirmed that the microcapsules did not survive the harsher environment of initial mixing, but were largely intact after addition in the final stage. Samples from the rubbers containing the microcapsules added in the final mix were cut and exposed to 50 pphm ozone in dynamic mode with samples removed every five days for HPLC analysis of the surface concentration of 6PPD. The release rate was significantly lower than that for the 6PPD added directly and appeared to be a function of wall to core ratio (figure 5). The surface of rubber samples with 6PPD in 1/1 wall to core microcapsules showed significant cracking after 20 days and were completely failed after 25 days. The rubber compounded with 3/1 wall to core ratio microcapsules showed cracking as early as 10 days, but the samples were not failed at the end of 30 days. To determine if the increased lifetime was due to late release of the 6PPD antiozonant into the rubber matrix or stress relief at the compound surface (ref. 43) an additional sample was prepared containing 2.5 phr of 6PPD added directly and 3.5 phr of 6PPD added as a 3/1 microcapsule. This rubber sample showed no significant cracking until after 25 days and had not completely failed after 30 days of exposure to ozone (table 9, refs. 39 and 40).

Table 9 - sample surface appearance(*)
Sample                    Days exposure to ozone
                          5   10  15  20  25  30
6 phr added directly      0   0   0   2   5   -
6 phr as 1/1 capsules     0   0   0   2   5   -
6 phr as 3/1 capsules     0   1   1   2   2   5
2.5 phr added directly +
3.5 phr as 3/1 capsules   0   0   0   0   2   3
(*) Scale: 0 = cracking, 1 = very slight cracking, 2 = slight
cracking, 3 = some large cracks, 4 = many large cracks,
5 = sample failed

To further test the effect of wall to core ratio, microcapsules were prepared with wall to core ratios of from 1/1 to 4/1 and tested in rubber. The 1/1 ratio microcapsules provided only a slight delay to the release. However, the samples with 2/1 and 3/1 ratios showed progressively longer delays in release when measured by HPLC (figure 6), and provided antiozonant protection to the surface of the rubber compound for significantly longer than that provided by adding 6PPD directly to the compound (figure 7). No additional benefit was found by using 4/1 wall to core ratio microcapsules.

Rubber compound testing

Rubber compounds were mixed with 6PPD and tested for exposure to ambient ozone with continuous flexing to 25% extension and exposure in a rooftop test. Increasing the 6PPD level from 4 to 6 phr as a direct compounding ingredient had no significant effect on the lifetime of the sample, while the samples compounded with 3.5 phr of 6PPD as 3/1 wall to core ratio microcapsules and 2.5 phr of 6PPD added directly for a total of 6 phr of 6PPD in the rubber sample more than doubled the lifetime of the rubber compound (table 10) (refs. 39 and 40).

Table 10 - rooftop ozone exposure sample
appearance
6PPD level and type                Months exposure
                               2   4   6   8   10  12
6 phr, added directly          0   1   4   5   -   -
2.5 phr added directly +
3.5 phr as 3/1 ratio capsules  0   0   0   0   2   4

Summary

The protection of rubber from ozone attack can be accomplished by a variety of methods. Microencapsulation of antiozonants has been demonstrated to be a valuable new technology that significantly extends the useful lifetime of rubber articles that are continually exposed to ozone, even while the rubber samples are being continually flexed. The delayed release to the rubber compound of the N-(1,3-dimethylbutyl)-N'-phenyl-para-phenylenediamine antiozonant by the wall of the microcapsule prevents its early consumption by environmental factors, such as reaction with ozone or water leaching. Microcapsules provided a reservoir of the antiozonant, which is made available to the rubber surface to protect the article at later stages in the product life cycle. Use of cellulose acetate as the wall material provided the best balance of antiozonant release properties and yield of usable microcapsules less than 50 microns in diameter. When added to rubber compounds and tested by accelerated exposure to ozone, significant increases in the crack-free lifetime of the rubber article were achieved. These advantages were translated into extended lifetimes for tire sidewall compounds when tested in continuous exposure to atmospheric ozone.

References

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