Ozone cracking crack growth

Antiozonant Properties. Aromatic secondary diamines are the only class of organic chemicals able to reduce efficiently the ozone crack growth of vulcanizates under dynamic conditions and be acceptable at the same time from both the technical and toxicological points of view. The presence of a secondary aromatic amine moiety itself in a molecule is not a sufficient condition to attain antiozonants efficiency. (E.g., secondary monomaines are only antioxidants and flex-crack inhibitors without appreciable antiozonant activity. On the other hand, all N,N -disubstituted PD antiozonants are also efficient antoxidants and most of them also act as flex-crack inhibitors (1). Both these stabilization activities have to be considered in the complex antiozonant mechanism, together with some metal deactivating activity. 

The incidence of cracking due to ozone attack has been investigated for NR-EPDM blends (Tokita, 1977 Avgeropoulos et al., 1976). Andrews (1966) showed that small zones of EPR in an EPR-NR blend provide a barrier that inhibits ozone crack growth. Ambelang et al. (1969) found the importance of small EPM domain size in EPDM-SBR blends. Matthew (1984) has shown that carbon black improves the ozone resistance of NR-EPDM. An improvement was obtained in the blends with a balanced carbon black phase obtained by phase inversion. This is because (1) there is better reinforcement of the EPDM phase and (2) carbon black in the EPDM expands the volume of that phase. In BIMS-BR blends, the ozone failure can be retarded by reducing the size of the BIMS dispersion (Tse et al., 1998). 

The study of ozone crack growth has been an active area of research since the early 1960s. Early work showed that ozone crack growth proceeds at a linear rate for typical elastomers and is proportional to the concentration of ozone. The... 

By contrast, however, some rubber materials exhibit abnormal, strain-dependent ozone crack growth. Here, fewer T4 level ozone cracks appear because of the more rapid depth increase, i. e., the total destruction of a carbon-black filled vulcanized rubber strip specimen protected only by wax under 20% strain occurs 30 days earlier compared to material under 30% strain. When strain is increased from 30 to 200%, the growth of the numerous cracks, which were approx. 1 mm deep at first, increases rapidly. The isochrone charts of different crack depths shown in Figure 5.106 illustrate that the weathering time required to reach T3 and T4 levels in the strain range from 80 to 130% (even to 200%) decreases relatively little however, in the strain range from 30 to 80%, it decreases markedly, while it rises again between 20 and 30% strain. These differences in strain influence are seen in particular at the transition from crack level T3 to T4 [698]. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

If results are obtained as a function of ozone concentration, in theory it should be possible to make extrapolations to ambient conditions by empirically fitting a relation to the concentration against time to cracking/crack growth rate. For natural rubber, there has been evidence that that the relation is broadly linear. 

CSPE have excellent combinations of properties that include total resistance to ozone excellent resistance to abrasion, weather resistance even in light colors, heat, flame, oxidizing chemical, solvents, crack growth, and dielectric properties. Also provide low moisture absorption, resistance to oil similar to neoprene, low temperature flexibility is fair at -40C (-40F), low gas permeability for an elastomer and good adhesion to substrates. Can be made into a wide range of colors. Use includes hoses, roll covers, tank liners, wire and cable covers, footware, and building products (flash, sealing, etc.). 

Two types of stabilizers inhibit crack growth in rubbers microcrystalline waxes and alkylated phenylene diamines. A small quantity of the wax milled into a rubber will gradually diffuse to the surface where it will serve as a barrier impervious to ozone. A combination of wax and alkylated phenylene dicunine antiozonant is generally used for optimum protection. The exact function of antiozonant is still obscure but it is possible that it accelerates scission processes on the polymer surface producing a protective film of viscous products. 

Rubbers having main chain unsaturation can undergo ozone cracking when test pieces are exposed to atmospheric traces of ozone at an elongation above a characteristic threshold strain [63]. For most diene rubbers this strain is typically around 5% in the absence of any protective agent, although under dynamic strain conditions it can be reduced to less than 1%. Antiozonant materials are added to rise the threshold strain above the maximum strain encountered in service life or alternatively, as is usually the case in dynamically strained applications, to reduce the rate of crack growth. 

G < Gq no crack growth occurs by tearing, but only by chemical (ozone) attack. 

The rate at which a crack grows when the critical energy condition is satisfied depends on two factors the rate of incidence of ozone at the crack tip and the rate of segmental motion in the tip region. When either of these processes is sufficiently slow, it becomes rate controlling. The overall rate, R, of crack growth is thus given approximately by... 

Many compounds have been reported in the literature to be chemical antiozonants, and nearly all contain nitrogen. Compound classes include derivatives of 2,2,4-trimethyl-l, 2-dihydroquinoline, N-substituted ureas or thioureas, substituted pyrroles, and nickel or zinc dithiocarbamate salts. The most effective antiozonants, however, are derivatives of p-phenylenediamine (p-PDA). The commercial materials are grouped into three classes N,N -dialkyl-p-PDAs, Nalkyl-N -ary 1-p-PDAs, and NX-diary 1-p-PI) As. The NX-dialkyl-p-PDAs (where the alkyl group may be 1-methylheptyl, l-ethyl-3-methylpentyl, 1,4-dimethylpentyl, or cyclohexyl) are the most effective in terms of their reactivity to ozone. These derivatives increase the critical stress required for the initiation of crack growth, and they also reduce the rate of crack growth significantly. The sec-alkyl group is most active, for reasons that are not yet completely clear. The drawbacks of these derivatives are ... 

Zinc dialkyl dithiocarbamates reduce the rate of crack growth, but their scorch resistances are prohibitively low. A few other classes of compounds have been reported to react rapidly with ozone, such as phosphines and stibines. These materials also exhibit antiozonant activity when swollen into rubber vulcanizates after the cure. However, they cannot be used practically for two reasons ... 

Ozone attack leads to chain scission and the formation and propagation of cracks. As crack growth proceeds, fresh surfaces are continuously exposed for further ozone attack. Degradation continues until failure relieves the inherent stress. It has been calculated that only about 1 % of all the ozone that reacts with the rubber is responsible for the formation of cracks. 

A key result of the early crack growth studies was the "critical stress" effect, i.e., no crack growth occurs unless a specific stress value is exceeded. In practical terms these stress values correspond to threshold tensile strains of 3-5%, depending on stiffness. It has been found that critical stress values are largely unchanged by temperature, plasticization, and ozone concentration. Polychloroprene has a higher critical stress value than other diene rubbers, consistent with its reduced reactivity to ozone. 

The most common form of crystalline alumina is corundum, which has a rhombohedral Bravais lattice with a space group R-3c. These materials have been used in energy storage, alkali metal thermal-to-electric conversion cells, and gas sensors. In recent years, it has been reported that alumina can also reduce heat, humidity, light, ozone and gamma radiation, flame resistance, and the crack growth in NR. ... 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

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