Ozone surface resistance

Erisman, J. W A. Van Pul, and P. Wyers, Parameterization of Surface Resistance for the Quantification of Atmospheric Deposition of Acidifying Pollutants and Ozone, Atmos. Environ., 28, 2595-2607 (1994). 

The interaction between the montmorillonite and non-polar molecules such as isooctane or toluene is limited, while the interaction between the montmorillonite and polar molecules such as water is very strong. The roles of montmorillonite to prevent access of non-polar solvents, as shown above, and to keep DPD on the surface or in the interlayer region, are considered to prolong the effective life of DPD, thus providing rubbers with good ozone-cracking resistance. 

Silicones 25% to 50% Excellent UV/ozone/heat resistance no shrinkage 20-30 year life excellent recovery nonstaining Surface preparation critical slight curing odor dirt pickup concrete/aluminum adhesion difficult... 

Ozone attack of rubbers can be prevented in three ways (1) coating the surface (2) adding a chemical antioxidant (3) relieving internal stresses by adding ozone-resistant polymers. 

Based on these considerations, Croft prepared six formulations containing various combinations of NBR and NBR/PVC with CR and SBR and measured their oil, heat and ozone resistance, physical properties, and adhesion characteristics. Whereas the physicals are satisfactory for aU compounds, formulations based on NBR, NBR/PVC with CR performed better on heat and oil aging than the compounds containing SBR as shown in Tables 11.6 and 11.7. However, the adhesion is better with the latter compounds. It has been suggested that cuprous sulfide formed on the wire surface interacts with the double bond in SBR to provide the improvement in adhesion. 

Table 11.14 gives critical stress, critical strain, and critical stored energy of NR-EPDM blends for initiation of ozone cracking. All the properties show an increase on addition of TOR, especially the critical stored energy. Stored energy is a strong indicator of ozone resistance and shows an increase by about 80% on addition of 20 parts of TOR to the compound. This was confirmed by SEM pictures of surface ozone cracks and the results of dynamic mechanical moduli and tan d measurement. 

These waxes have branched structures of higher molecular weight (40-70 carbon atoms) than paraffin waxes and form a quite different crystalline structure on the surface of the rubber when emerging from solution from within the vulcanised rubber. Microcrystalline waxes form smaller crystals, which pack tighter together to form a more coherent, much more flexible film on the rubber surface which is more resistant to ozone penetration. 

Bennett etal. have presented a model for gaseous pollution sorption by plants. The model includes all the known factors that might have a significant effect on pollution sorption by plant leaves, including gas concentration (ambient air and internal leaf), gas fluxes (external and internal), resistance to flow (leaf boundary layer, stomatal, and internal), nature of leaf surfaces (stomatal presence, cutin, and surface properties), importance of gas solubility and thus solute concentration within the leaf, and ability of the plant to metabolize pollutants (decontaminate itself). They mentioned the reactivity of ozone as another factor to consider. They believe that surface sorption may be important, at least over short periods. They presented a possible mathematical representation of these factors, which they suggested is equivalent to the mathematical statement of Ohm s law. This material is well int ated in the review by Bennett and Hill. ... 

Additional curing is often achieved with sulfur, peroxide, or maleimide formulations. Chloro-sulfonated polyethylene has improved resistance to oil, ozone, and heat compared to many other elastomers. Applications include harrier membranes and liners, surface coatings on fabrics, automobile air-conditioner hose, electrical cable insulation, and spark-plug boots [Andrews and Dawson, 1986],... 

Antiozonants (qv) also protect mbber surfaces by the formation of a protection layer on the surface of mbber by reaction of the antiozonant with ozone, the ozonides (4) formed in reaction with the mbber. Certain polymers also provide good ozone protection. The use of 20—30 parts of EPDM, alow diene mbber, in natural mbber or SBR compounds significantly increases ozone resistance. 

The ozone concentration in the atmosphere is only a few pphm. In certain chemical plants as in electrolytic mercury cell houses in the chloralkali industry, the ozone concentration is higher. Although the atmospheric ozone level is low, it reacts with rubber double bonds rapidly and causes cracking of rubber products. Especially when rubber is under stress (stretching and bending as in the case of flexible cell covers), the crack development is faster. Neoprene products resist thousands of parts per hundred million of ozone for hours without surface cracking. This nature of neoprene is quite suitable for cell house application in chlor-alkali industries. Natural rubber will crack within minutes when subjected to ozone concentration of only 50 pphm. 

Regime 5 - instantaneous reactions at an reaction plane developing inside the film For very high reaction rates and/or (very) low mass transfer rates, ozone reacts immediately at the surface of the bubbles. The reaction is no longer dependent on ozone transfer through the liquid film kL or the reaction constant kD, but rather on the specific interfacial surface area a and the gas phase concentration. Here the resistance in the gas phase may be important. For lower c(M) the reaction plane is within the liquid film and both film transfer coefficients as well as a can play a role. The enhancement factor can increase to a high value E > > 3. 

The areas where a test piece is attached to clamps and cut edges are preferential sites for cracking. It is generally good practice to coat clamped areas with an ozone resistant paint (which does not affect the rubber in any way) but cut edges are best left. For most purposes a Hypalon-based paint is satisfactory. Clamps, even when made of material such as aluminium, should be soaked in ozone prior to use. Any pattern or flaws on the test piece surface will also tend to act as stress raisers and show preferential cracking. 

Because antiozonants and waxes, which to be effective must form a surface bloom, are used to enhance ozone resistance it is usual to condition test pieces in the strained state before exposure. The usual conditioning period is between 48 and 96h and the test pieces should be kept in the dark and in an ozone-free atmosphere. For this treatment to be effective, the test piece surface must not of course be touched in the course of subsequent handling. Where specifications wish to specifically exclude compounds which rely on an adequate wax film for protection, the conditioning period is dispensed with. Hill and Jowett47 in a criticism of ozone test methods strongly make the point that the conditioning process should be relevant to service conditions if a discriminating evaluation of waxes is to be made. 

There are three principal categories of oxidizing agents that occur as air pollutants. These are ozone, nitrogen oxides and nitric add. and organic peroxide. Many materials that are relatively resistant to attack by the free oxygen of the air are far less resistant to attack by such oxidants and peroxides. These dissolve in the surface film and thus convert metals to their oxides which react readily even with such relatively weak acids as carbonic acid and sulfurous acid. For instance, copper tarnishes rapidly forming the oxide, which dissolves readily in dilute acids. 

A mode of wear is microcutting, where a sharp edge will cut through the surface. The general mechanism is shown in Figure 7.16. Polyurethane under tension will wear at a greater rate than one that is under slight compression. This is similar to other polyurethane properties such as cut and ozone resistance. 

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