Wax blooms

Rubber is protected against ozone attack by addition of physical and/or chemical antiozonants. Hydrocarbon waxes are the most common type of physical antiozonants, and p-phenylenediamine derivatives are the prevalent chemical antiozonants. Waxes bloom to the rubber surface and form a protective barrier. 

Bloom - A discoloration or change in appearance of the surface of a rubber product caused by the migration of a liquid or solid to the surface. (Examples sulfur bloom, wax bloom.) Not to be confused with dust on a surface from external sources. 

The composition of cuticular lipids varies at all levels of organization in insects, from among species to within individuals. The amount of cuticular lipid can also vary substantially. For example, wax blooms of desert tenebrionid beetles are associated with reduced water-loss (Hadley, 1994). High densities of wax may also serve to reduce heat load by reflecting solar radiation (Hadley, 1994) or to deter predators (Eigenbrode and Espelie, 1995) thus, it cannot be assumed that water conservation is the primary function of wax... 

A critical level of wax bloom is required to form a protective film for static ozone protection. 

Excess levels of wax bloom can have a detrimental effect on fatigue resistance, because the thick layer of wax can crack under strain and the crack can propagate into the product. 

Waxes act as internal lubricants in rubber and increase the scorch safety somewhat. Unfortunately, waxes have a number of shortcomings. First, they are ineffective under dynamic stress conditions. This is likely due to a lack of adhesion between the wax film and the rubber and to the inextensibility of the wax bloom. 

Figure 1. Shows thickness of wax bloom as a function of temperature. Rubber is butadiene/a-methyl-styrene copolymer. Curve 1, paraffin wax Curve 2, microcrystalline wax Curve 3, blended wax.

Several techniques were proposed, and some are used commercially, to reduce the effect of oxygen. One is inert gas atmosphere over the material cured, while it is being irradiated. Another is addition of paraffin waxes to the formulation. The waxes bloom to the surface and form a protective shield against oxygen. A third us making use of oxygen scavengers, like amines, phosphines, and thiols. 

Rubber is protected against ozone attack by addition of physical and/or chemical antio-zonants. Hydrocarbon waxes are the most conunon type of physical antiozonants, and p-phenylenediamines derivatives are the prevalent chemical antiozonants. Waxes bloom to the rubber surface and form a protective barrier. Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. A connnon loading for antiozonants in rubber formulations is 1.5 to 3 phr. Combinations of waxes and chemical antiozonants are generally used. 

Rubber products may be protected against ozone attack by the use of a highly saturated rubber molecule, the use of a wax inhibitor which will "bloom" to the surface, and the use of paper or plastic wrappings to protect the surface. Despite these efforts, rubber products still crack more on the West Coast than on the East Coast of the United States. 

This situation is identical to the previous one and occurs for example when paraffin wax is mixed into rubber above the melting point of the wax. On cooling, the wax starts to crystallise, some of it forming a bloom on the rubber surface. Such a bloom assists in protecting a diene rubber from ozone attack. 

When diene rubbers are exposed to ozone under stressed conditions cracks develop which are perpendicular to the direction of stress. Whilst ozone must react with unstressed rubber no cracking occurs in such circumstances nor when such rubber is subsequently stressed after removal of the ozone environment. For many years such rubbers were protected by waxes which bloomed on to the surface of the rubber to form an impermeable film. This was satisfactory for static applications but where the rubber was operating under dynamic conditions the wax layer became broken and hence less effective. 

The effect of ozone is complicated in so far as its effect is largely at or near the surface and is of greatest consequence in lightly stressed rubbers. Cracks are formed with an axis perpendicular to the applied stress and the number of cracks increases with the extent of stress. The greatest effect occurs when there are only a few cracks which grow in size without the interference of neighbouring cracks and this may lead to catastrophic failure. Under static conditions of service the use of hydrocarbon waxes which bloom to the surface because of their crystalline nature give some protection but where dynamic conditions are encountered the saturated hydrocarbon waxes are usually used in conjunction with an antiozonant. To date the most effective of these are secondary alkyl-aryl-p-phenylenediamines such as /V-isopropyl-jV-phenyl-p-phenylenediamine (IPPD). 

Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. For serving conditions which involve continuous flexing, /j-phenylenediamines (A, A -alkyl-aryl derivatives) can be added. These chemical antiozonants scavenge the ozone before it reacts with the rubber. A barrier of ozonized products is created which protects both the rubber and antiozonant from further attack. However, p-phenylenediamines are staining compounds. Whenever colour is an important concern, blends of elastomers can be used elastomers loading should be higher than 30 phr to provide sufficient effectiveness. 

Although the true waxes, such as beeswax, are esters of fatty acids with alcohols, the term is often applied to certain solid substances which melt at fairly low temperatures, e.g., paraffin wax. Paraffin waxes (crystalline and microcrystalline) are obtained from the refining of petroleum, and are used in rubber compounding as protective agents. They have the property of blooming to the surface of the product, forming a thin film which replenishes itself if removed in service. 

Gloss enhancers are additives that when added to PET or PBT produce a glossy surface finish that is similar to that of acrylonitrile-butadiene-styrene (ABS) ter-polymer. Such additives include AC wax, such as AC316 (Honeywell).1 These waxes have limited compatibility with PET and bloom to the surface to give a... 

Ozone (qv) reacts with double bonds so rapidly that it has no chance to diffuse into the mbber and therefore all action is at the surface. Thus surface-protective agents are most useflil against ozone attack. For example, waxes that bloom to the surface of mbber to form an inert film are used effectively for static protection (34). 

Yeasts and Bacteria. One of the purposes of adding S02 is to inactivate bacteria and wild yeast so that the fermentation may be conducted with a chosen desirable strain of yeasts. Fortunately the wild yeast and the bacteria on grape berries (frequently confused in the older literature with the wax-like bloom which is naturally present on some berries) are susceptible to inactivation by relatively low doses of S02. A clear field is thus available to the large inoculum of S02-tolerant pure culture yeast added by the enologist. 

This property can be improved by the addition of antioxidants. Higher zinc oxide loadings increase resistance to heat ageing. Paraffin wax offers surface protection by blooming to the surface during curing. Low sulphur or sulphurless cure improves heat ageing considerably. 

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. 

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