Review of antiozonants.
Many people confuse the effects of oxygen and ozone. A common misperception of the neophyte is that once a rubber compound is well protected against oxidation, it will resist ozone as well. The incorrect converse is also heard; that the use of antiozonants eliminates the need for antioxidants. To demonstrate that these are two different processes, table 1 compares the effects of ozone and oxygen on rubber vulcanizates. [TABULAR DATA 1 OMITTED]
Ozone degradation can be observed as a surface discoloration. If the article is stretched while exposed, cracks will develop and grow perpendicular to the applied strain. The whitish surface discoloration is known as frosting and is most likely to occur in summertime when ozone concentrations are highest. Frosting can be differentiated from bloom by heating the rubber article. A chemical bloom will redissolve in the warm rubber; frosting is permanent. While frosting can be of cosmetic concern, the prevention of ozone cracking is more important for a long useful life of the rubber article.
In contrast, oxidation causes a hardening or softening, depending on the polymeric structure. In oxidation, vulcanizate changes occur uniformly throughout the rubber article; surface effects due to oxygen are usually only seen upon exposure to sunlight. Ozone attack occurs only at the surface.
Other differences between ozone and oxygen degradation can be noted by the effect of changes in vulcanizate structure or test conditions.
Ozone has no effect under ordinary circumstances on saturated polymers. Only diene-based polymers with main chain unsaturation are attacked by ozone. In oxidation, unsaturated polymers are more reactive than saturated polymers, but all carbon backbone polymers will react with oxygen.
The cure system has almost no effect on ozone resistance. In natural rubber (NR) and SBR, Braden and Gent have reported small protective effects of peroxide and sulfur-donor cure systems (ref. 1). The slight benefits these cure systems provide are eliminated by extraction, which removes oleic acid in the case of peroxides and curative bloom in the case of sulfur donor cures. In a few cases, peroxide curing may be detrimental to ozone resistance if it creates unsaturation in an otherwise fully-saturated polymer, as reported recently by Davis (ref. 2). The oxidative stability of different cure systems varies widely; increasing in the order sulfur < sulfur donor < peroxide.
Temperature has little effect on the rate at which ozone reacts with double bonds. Ozone is not stable at higher temperatures, and testing at very high temperature (over 100 [degrees] C) shows less rapid crack growth due to lower ozone concentrations, according to Gent and Hirakawa (ref. 3). Also, below 0 [degree] C ozone usually has little effect since rubbers lose segmental mobility and double bonds below the surface don't become exposed. Crack propagation by ozone can show a large increase in rate between 0 and 100 [degrees] C due to temperature-induced changes in polymer viscoelasticity. Plasticizing a high-damping elastomer (butyl or NBR) can make it more susceptible to ozone attack at room temperature (ref. 4). In oxygen atmospheres, increasing temperature roughly doubles the rate of oxidation for every 10 [degrees] C. Increasing temperature is a common method to accelerate the simulated oxidative aging of elastomers. Ozone attack is sufficiently rapid that no "accelerated aging" is necessary, and it may give misleading results. Nevertheless, as will be shown, it may be desirable to test under a variety of conditions to understand how the vulcanizate will perform during year-round use.
Strain has no effect on oxidative reactions, but it increases the rate of crack growth in the presence of ozone.
The most common ways to provide ozone resistance depend on forming a protective surface barrier against ozone attack. These strategies will be discussed first. Subsequently, vulcanizate modification by various techniques will be discussed.
Waxes form a physical barrier at the surface of the rubber which protects against ozone attack. Waxes work by temperature-dependent solubility. At the dosages used for successful ozone protection, waxes dissolve in the hot rubber mixture but exceed their solubility limit at ambient exposure conditions. Overtime, the wax diffuses to the surface (blooms) to form the protective ozone barrier. There are two types of waxes: straight chain paraffin waxes and branched-chain microcrystalline waxes. The most widely used antiozonant waxes are blends of normal paraffins and microcrystalline waxes for maximum protection over a broad range of exposure temperatures.
The advantages of waxes are that they are relatively low cost, nondiscoloring, do not affect vulcanization and may act as process aids. A major drawback of antiozonant waxes is that wax provides no protection under dynamic conditions in which flexing or stretching breaks the wax film barrier. Ozone will then attack the underlying rubber at the breaks, usually causing the rapid growth of large cracks in a relatively short period of time. Other disadvantages of waxes are that they tend to affect the appearance of the vulcanizate and can reduce building tack or adhesion to metal. Also, as will be discussed, the film's composition is continually changing in response to ambient temperature fluctuations.
Paraffin waxes are straight-chain hydrocarbon molecules containing about 20 to 50 carbon atoms. The carbon distribution of two typical paraffin waxes is shown in figure 1 taken from Jowett (ref. 5). Waxes having a melting point of less than 50 [degrees] C are too soluble to form a barrier and protect against ozone. The 52-54 [degrees] C paraffin wax shown will give good protection at low temperature, but it becomes soluble at higher temperature. The higher melting 66-68 [degrees] C paraffin wax blooms too slowly to protect in winter. Nevertheless, Lewis reports that 1.5 phr of this type of wax protects well at 30 [degrees] C (25 pphm ozone at 20% strain, ref. 6). Paraffin waxes melting above 75 [degrees] C diffuse too slowly to get to the surface and form a barrier of the thickness necessary to protect the underlying rubber.
A comparison in natural rubber of paraffin waxes with different melting points is shown in table 2, from work done by Lewis (ref. 6). The wax with the lowest melting point does not provide good ozone resistance at elevated temperature. As waxes with higher melting points are used, the temperature at which the rubber is protected also increases. But note that the wax with the highest melting point does not protect at the coldest temperature tested, which suggests it will fail in winter. The same behavior expressed as "carbon number waxes" was recently demonstrated in an NR/BR blend by Chikamune, as shown in figure 2 (ref. 7). Chikamune obtained similar results in SBR and all NR compounds. Since waxes protect only against static exposure to ozone, it may seem unnecessary to test at 50 or 60 [degrees] C, but a black compound on a roof can get that hot on a sunny summer day. [TABULAR DATA 2 OMITTED]
Microcrystalline waxes at one time were known as amorphous waxes since the crystals formed can be tiny and in some cases invisible to the unaided eye. Microcrystalline waxes are hydrocarbons similar to paraffin waxes but the carbon chains are branched. Microcrystalline waxes are obtained from the residual oil fraction, after volatile distilling components have been removed, and therefore have higher molecular weight, possessing about 40 to 70 carbon atoms per molecule.
Unlike the paraffins, it is not possible to characterize microcrystalline waxes by a single parameter such as melting point or carbon number. At any given carbon number there are many possible structures, depending on the type and amount of branching present in the micro wax. The branching in microcrystalline waxes increases liquid density, refractive index, melt viscosity and needle penetration (indicating a softer wax). In the solid state, branching reduces crystallinity and thus shows lower density when waxes are compared at equal carbon number. When waxes are compared at the same melting point, solid densities are similar because the reduced crystallinity from branching is counteracted by the increase in density with molecular weight. Table 3 gives a comparison of highly fractionated waxes from ref. 8. The 27 carbon wax shown is a normal paraffin. A list of commercial waxes, ranked by melting point, is given in appendix A from information published in ref. 9. [TABULAR DATA 3 OMITTED]
Some of the benefits of microcrystalline waxes are that they form a more flexible film which is less likely to crack under modest deformation. Lewis reports that microcrystalline waxes are more adherent to synthetic polyisoprene than paraffin waxes (ref. 6). Because of their high molecular weight, microcrystalline waxes bloom too slowly to be used alone. Since diffusion becomes more rapid at elevated temperatures, a blend of paraffin and microcrystalline waxes is commonly preferred to give the best performance over a broad temperature range, in order to provide year-round protection to the rubber vulcanizate.
Compound variables can affect the amount of wax needed to maintain a protective barrier. Higher levels of fillers may require a corresponding increase in wax level to maintain a sufficient bloom layer. The addition of plasticizer may assist wax bloom at low temperatures by increasing the mobility of polymer chains. At elevated temperatures, the use of plasticizer can increase wax solubility, requiring additional wax dosage.
Ozone reacts more rapidly with para-phenylenediamines than with double bonds. Layer and Lattimer (ref. 10) have shown that the ozonized paraphenylenediamine film becomes part of the effective ozone barrier for these materials. This oxygen-containing polar film is less likely to be reabsorbed at higher temperature, making PPDs less sensitive to temperature changes than waxes. The PPDs provide ozone protection under dynamic flexing conditions. The disadvantages of PPDs are that they all stain and discolor, that they may be subject to losses (by volatilization, oxidation or water leaching), that they can contribute to scorch and, like waxes, may detract from adhesion to metal. Some typical paraphenylenediamines and their properties are shown in table 4. The cohesive energy density of PPDs, as measured by density and melting point, increases from dialkyl < alkyl/aryl<diaryl. The basicity of the PPDs decreases in the same order. For the alkyl group to show good antiozonant activity, the carbon attached to the nitrogen must have one and only one hydrogen atom, i.e. a secondary alkyl amine functionality. [TABULAR DATA 4 OMITTED]
Di-secondary-alkyl p-phenylenediamines are liquids which increase the threshold strain that a part can withstand without ozone cracking (figure 4). Dialkyl PPDs migrate rapidly and show high ozone reactivity. The high basicity of the dialkyl PPDS contributes to scorch in many compounds and may make them susceptible to leaching by acid rain. They are often used in blends with the alkyl/aryl PPDS. A dicyclohexyl PPD is a solid suggested for low migration staining. Also available are several dialkyl PPDs based on lower-alkyl ketones. These low molecular weight PPDs are used as lubricant additives but are too volatile for rubber application. The volatility of typical PPDs which are used in rubber is shown in figure 3, taken from Dean and Kuczkowski (ref. 11). For thermal gravimetric analysis (TGA), the heating rate was 10 [degrees] C/min. in [N.sub.2]. Volatility is somewhat in line with molecular weight (MW) although the liquid dialkyl DEMPD is more volatile than its MW would suggest.
Alkyl/aryl p-phenylenediamines are commercially the most important PPDs due to the prevalence of HPPD in tires. Several liquid blends containing HPPD and dialkyl PPDs are offered for high-volume uses such as polymer stabilization and oil-injection mixing. As a class, alkyl/aryl PPDs are intermediate in basicity, scorch characteristics and susceptibility to oxidation. The solid alkyl/aryl PPDs do not show an increase in critical strain unless used in combination with wax to form a continuous protective barrier as shown in figure 4 according to Fogg (ref. 12). The time to oxidize half of a PPD antiozonant has been determined in [O.sub.2] at 100 [degrees] C by Lorenz and Parks (ref. 13). The low molecular weight IPPD migrates rapidly, which provides good protection under high ozone test concentrations. IPPD stains profusely and is readily lost from an NR vulcanizate by extraction at low pH, as demonstrated by Collonge and coworkers (ref. 14). The cycloalkyl CHPD melts above 100 [degrees] C and is therefore not suitable for compounds which will be mill mixed. CHPD is suggested for applications requiring low migration staining.
Diaryl p-phenylenediamines are slow diffusing but are the least volatile PPDs, providing more permanent protection. They are often used in combination with other antiozonants to provide long-term protection. Diaryl PPDs are the preferred antiozonant for polychloroprene (CR), which has a slow rate of crack growth in the presence of ozone (ref. 4). Diaryl PPDs are the least scorchy class of p-phenylenediamines and the most resistant to loss by oxidation. DPPD is commonly offered in blends to reduce the melting point and provide easier dispersion. DTPD is generally more soluble than DPPD and can be used at higher levels. However, diaryl PPDs have limited solubility in natural rubber (NR) and will bloom as crystals, allowing ozone cracking around the particles (ref. 11).
Other chemical antiozonants
1,2-dihydro-2,2,4-trimethylquinoline (TMQ) 6-ethoxy TMQ was one of the first antiozonants used, according to Miller, et al (ref. 15). Like the dialkyl PPDs, monomeric TMQs are volatile liquids which stain and discolor but can be used to make liquid blends with HPPD.
Polymeric TMQs also have antiozonant activity, but their slow diffusion limits their use as sole antiozonants. Polymeric TMQs are used as antioxidants in combination with PPDs to prevent oxidation of the PPDs, as well as to protect the polymer. Polymeric TMQs are used to protect against ozone in wire and cable applications because they are compatible with peroxide cure systems. Unlike waxes and PPDs, rubber parts containing polymeric TMQs do not require preconditioning before ozone exposure.
Nickel di-n-butyldithiocarbamate confers ozone protection and is non-volatile. Its green color precludes use in light-colored goods. It is not used in natural rubber because it acts as a pro-oxidant in this polymer. It can be especially helpful in nitrile rubber (NBR) where the PPDs are ineffective. Other ways to protect NBR will be mentioned later.
N-alkyl thioureas have antiozonant activity but are seldom used for this purpose. Thioureas are very active accelerators causing scorch problems. The bloom formed for antiozonant effectiveness readily washes away. The liquid tributylthiourea gives a tacky surface.
Applying an ozone-resistant rubber coating can offer relatively durable protection. Such a coating is insensitive to temperature changes and is not subject to leaching, volatilization or other losses, but once the coating is scuffed or punctured, the underlying rubber is rapidly degraded. Additional equipment and expense are required to apply the coating to the rubber part. Several types of coatings are used.
Extruded veneers are used by the tire industry to protect the sidewall from ozone and weather. Other laminated articles such as boots and protective garments can also use this approach to ozone resistance.
Elastomeric solutions can provide protection against ozone. Commercial preparations are available. Alternatively, home-made versions can be formulated from fully-saturated rubbers. Suppliers of chlorosulfonated polyethylene, polyacrylates, polyurethanes and other elastomers can offer guidance on the preparation of solution coatings. Solvent recovery equipment may be needed to comply with volatile organic compounds (VOC) regulations.
Latex-based coatings can avoid the VOC problems. Parker and Roberts recently announced the availability of a hydrogenated nitrile latex with ozone resistance (ref. 16). Latex emulsions of selected solution polymers are also offered commercially (ref. 17), including EPDM, chlorosulfonated polyethylene and polyisobutylene.
Polymer blends using >30% ozone-resistant rubbers have proven successful in providing permanent, non-staining ozone protection. As with all polymer modification techniques, this requires a sacrifice in physical or other performance properties. At one time, polychloroprene was used for ozone resistance in white sidewall blends. Today, blends with EPDM are used. One important requirement is to obtain good covulcanization of the different polymer phases. According to Lewis (ref. 6), EPM copolymer provides ozone resistance only if the blend is peroxide cured. For typical sulfur cures, fast curing grades of EPDM are preferred.
Gardner and coworkers have discussed a fully saturated polyisobutylene with a benzylic bromide cure site which gives good covulcanization with diene polymers (ref. 18). Since nitrile rubber (NBR) is difficult to protect with PPDs, blends with polyvinylchloride (PVC) are offered or can be mixed to make ozone-resistant articles from NBR. Like EPDM blends, these depend on careful control of mixing conditions to get good dispersion of the polymers for optimum ozone resistance (ref. 19). Also, curing must occur above the flux temperature of the PVC to avoid any built-in stresses which would lead to ozone cracks.
Hydrogenation of NBR is practiced by several suppliers. Improved resistance to ozone is one of the many attributes of H-NBR. Similarly, butadiene-based S-B-S block copolymers are hydrogenated to ozone-resistant S-EB-S thermoplastics.
Elastomer selection is a broader description of the preceding section on hydrogenated polymers. Suppliers of hydrogenated polymers are vigorously contending with suppliers of other fully saturated rubbers for the same markets. The compounder should carefully consider all the properties required for a given application. This may involve trade-offs in properties or economics. The end result should be the most cost-effective rubber compound which will satisfy the application.
Several approaches can be used to prevent ozone attack on the surface of rubber. Wax incorporation forms a stiff protective barrier for static conditions. Substituted para-phenylenediamines provide staining ozone protection in dynamic use. Quinolines and dithiocarbamates find use in specialized applications. As a separate operation, flexible rubber coatings can be applied to the part. Using blends with ozone-resistant polymers or bulk modification will give non-staining ozone protection with changes in physical properties.
Appendix A - commercial waxes (ref. 9)
Density melt. Refr. Needle Trade name Mg/[m.sup.3] pt., C index penetr.
Sunolite 130 0.90 54 Antilux 500 & L 0.91 55 Antilux 550 0.91 58 Sunproof Jr. 0.91 59 Vanwax OZ 0.90 59 1.426 Antilux 600 0.91 60 Antilux 110 0.92 62 Okerin 1868 & 2709 0.90 62 1.427 Sunproof Improved 0.91 62 Okerin 1891 0.90 63 1.428 Antilux 660 0.92 64 Okerin 1933/1 0.90 64 1.427 Okerin 2134 64 1.426 Sunproof Extra 0.92 64 Antilux 654 0.92 65 CS-2013 0.92 65 Sunolite 100 & 666 0.92 65 Sunolite 150 0.90 65 Antilux 111 0.92 66 CS-2027 0.92 66 Sunolite 127 0.92 66 CS-CSL 0.92 68 Sunolite 240 0.92 68 Sunproof Regular 0.92 68 Ross Sunproofing 0.93 72 Sunproof Super 0.92 73 Akrowax 5050 74 20 Vanwax H 0.90 76 1.433
Akrowax 130 0.90 55 Okerin 1885 59 1.424 Recco 140 60 Akrowax 145 0.90 62 Sun Wax 12 65 Vanwax H Special 0.93 66 1.429 CS-2043 0.92 67 12 Okerin 236 67 1.427 14 Sun Anti-chek 55 68 Sun Wax 6514 69 Recco 159B 72 Recco Sun Check 74
Nochek 4607 72 11 Vanfre M 0.88 73 1.4385 Antilux 750 0.90 75 Akrowax Micro 23 0.92 80 Be Square 175 82 19 Star Wax 100 86 16 Amorphius Micro 97
(1.) M. Braden and A.N. Gent, J. Applied Polymer Science, Vol. 3(7), pp. 100-106, (1960). (2.) William H. Davis, Paper #8, presented at ACS Rubber Division Meeting in Las Vegas, NV, May 29-June 1, 1990. (3.) A.N. Gent and H. Hirakawa, J. Polymer Science, Vol. A2, p. 157, (1967), reprinted in Rubber Chemistry and Technology, Vol. 41, pp. 1294-1299 (1968). (4.) M. Braden and A.N. Gent, J Applied Polymer Science, Vol. 3 (7), pp. 90-99, (1960). (5.) F. Jowett, The protection of rubber by petroleum waxes, Elastomerics, Vol. III (9), pp. 48-52, (Sept. 1979), based on paper #4 presented at ACS Rubber Division meeting in Atlanta, GA, May 27-30, 1979. (6.) P.M. Lewis, NR Technology, Rubber Developments Supplement, Part 1, pp. 1-35 (1972). (7.) Kaoru Chikamune, "Protecting rubber," Chemtech, pp. 620-623 Oct. 1990), adapted from paper #56 presented at the ACS Rubber Division meeting in Mexico City, Mexico, May 9-12, 1989. (8.) Anon., Sun Petroleum Products Co., Petroleum waxes: Composition, properties and types (1977). (9.) Rubber World Magazine's 1992 Blue Book, materials, compounding ingredients, machinery and services for the rubber industry, Don R. Smith, Ed., Lippincott and Peto Publ. (1992). (10.) Robert W. Layer and Robert P. Lattimer, Rubber Chemistry and Technology, Vol. 63, pp. 426-450 (1990) (11.) Paul R. Dean and Joseph A. Kuczkowski, "The function and selection of antidegradants," paper presented at the ACS Rubber Division meeting in Chicago, IL, Oct. 5-8, 1982. (12.) S.G. Fogg, Trans. Instn. Rubb. Ind., Vol. 6, pp. 234-257, 1962). (13.) O. Lorenz and C.R. Parks, Rubber Chemistry and Technology, Vol. 34, pp. 816-833 (1961) paper presented at the ACS Rubber Division meeting in Louisville, KY, April 19-21, 1961. (14.) E. Spirk, P. Stefanik, J. Orlik and J. Collonge, Caoutch. Plast., Vol. 613, p. 127 (1981). (15.) Donald E. Miller, Robert W. Dessent and Joseph A. Kuczkowski, "Long-term antiozonant protection of sidewalls," Rubber World, pp. 31-48 (Oct. 1985). (16.) Dane K. Parker and Robert F. Roberts, Rubber Chemistry and Technology, Vol. 65, p. 225-258 (1992) paper #58 presented at the ACS Rubber Division meeting Toronto, Ontario, Canada May 21-24, 1991. (17.) Victor Humphries, Ch. 4, "Latexes produced by solvent solution emulsification," Vanderbilt Latex Handbook, Robert F. Mausser, Ed., (1987). (18.) I.J. Gardner, H.C. Wang, R.R. Eckman and J.M.T Frechet, paper #6 presented at the ACS Rubber Division meeting Nashville, TN, Nov. 3-6, 1992. (19.) R.M. Gallagher, Analysis of ozone attack on PVC/NBR elastomers, paper 40 presented at the ACS Rubber Division meeting Nashville, TN, Nov. 3-6, 1992.