The role of petroleum waxes in the protection of rubber.

The role of petroleum waxes in the protection of rubber

The use of petroleum waxes to protect or assist in the protection of double bonded elastomers from the destructive effects of ozone has long been known, while the mechanism by which these petroleum hydrocarbons fulfill their protective functions has more recently been elucidated, and can be followed by the analytical technique of high temperature gas chromatography.

It is known that in the absence of any effective protective system, vulcanized elastomers absorb ozone, and that is the relaxed state, i.e. without any tension and extension of the vulcanizate, the ozone attacks only the double bonds on the surface of the elastomer where ozonides are formed, the absorption being restricted to the surface layers. Under tension the absorption becomes continuous with formation of surface cracks, and beyond a critical strain, the rate of crack growth continues at a constant rate, proportional to the ozone concentration.

Petroleum waxes function by migrating through the rubber to the surface of the vulcanizate, and there form an inert, thin, non-crystalline, adherent and flexible film, which acts as a barrier to ozone gas and prevents the reaction mentioned above from taking place.

It can be shown that for a given wax of known composition used in this manner, the thickness, character and chemical composition of the protective film, or indeed the very appearance of such a film at the surface, depends to a very large extent on the temperature of that surface.

Detailed knowledge of the composition of petroleum waxes, and this influence of temperature on their activity, allow rubber waxes to be blended from suitable petroleum raw materials to give products offering maximum protection over a wide temperature range and with optimum cost-effectiveness.

Composition of petroleum waxes

The waxes produced by the world's petroleum refineries can be conveniently classified in three groups produced from different lube oil streams in the refinery distillation process. The simplest and analytically best documented are the paraffin or distillate waxes, these consisting mainly of a mixture of "normal" (alkane) paraffin hydrocarbons, having the well known generic formula [C.sub.n][H.sub.2n+2][prime] where n, the "carbon number," ranges from about 18 to about 50, the carbon atoms being joined together in linear fashion. The components being linear and regular, are able to pack together in bundles or crystals, and the paraffin waxes are characteristics in being crystalline products.

While a degree of branching increases with increasing average molecular weight through the range of available paraffin waxes, it rarely exceeds 30 or 35% of the whole, and is usually much less.

The second group, of increasing average molecular weight, with an ever-increasing proportion of slightly branched chain or "iso alkane" hydrocarbons, is the so-called intermediate wax group, where carbon numbers of the components can range from the low 20s to the upper 50s, and where the `normal' or alkane hydrocarbon reduces progressively with increasing average molecular weight from about 70% to about 30% of the whole. Such waxes have very limited packing or crystallizing ability.

Finally there are the microcrystalline waxes, whose average molecular weight is much higher, with n, in the still prevailing generic formula of the vast bulk of the components, ranging from the 30s to well into the 80s. Here the waxes contain only small proportions of linear hydrocarbons, from perhaps 30% down to zero, the rest being predominantly branched hydrocarbon, increasing in complexity and isomeric permutation as the molecular weight increases.

The two earlier groups of paraffin and intermediate can readily be analyzed as regards the distribution of their hydrocarbon components over the whole carbon number range by high temperature gas chromatography and, by incorporating a known amount of a pure alkane hydrocarbon as an internal standard, the alkane content of the wax can be determined quantitatively.

Such analysis can be performed by packed column or capillary column operations using appropriate apparatus, both procedures having particular advantages, but when the proportion of "non normal" or iso alkane hydrocarbon is high, the latter technique, using "cold on column" injection finds favor, due to inherently greater resolving power.

Examples of typical paraffin/intermediate wax traces produced by capillary column technology operating up to 400 [degrees] C are shown in figure 1.

The pattern of carbon distribution for the alkane components of paraffin waxes produced by the world's oil refineries is shown in figure 2.

Microcrystalline waxes do not lend themselves to detailed analysis in this way, since the greater part of the hydrocarbon components is so complex, the isomers being so numerous and high in molecular weight, that even the best capillary columns are unable to resolve them. For many of the same reasons, as will be discussed later, these same waxes find little application in the formulation of high quality and cost effective rubber waxes.

It will be obvious from the foregoing that there are no clear boundaries between these three recognized classes of petroleum wax, and that a continuous spectrum exists, ranging from the lowest melting point simplest paraffin wax of almost 100% alkane content to the somewhat higher melting point and vastly more complex microcrystalline wax containing almost 100% branched chain iso alkane or `non normal' high molecular weight hydrocarbons.

Mechanism of wax migration

The foregoing remarks on wax composition become important and relevant in the context of surface protection, when the behavior of wax within a rubber vulcanizate is examined.

At vulcanization temperatures petroleum waxes dissolve completely in most vulcanizates. As the rubber cools a supersaturated wax solution is formed inside the compound. The concentration gradient between interior and surface of the rubber causes continuous migration of the wax moleculer to that surface, where a uniform and characteristic film is formed, acting as physical barrier to the few pphm of ozone in the atmosphere.

It will be clear at once that the possibility and ease of movement of any insoluble wax hydrocarbon through a rubber vulcanizate will be directly related to its molecular complexity, a low molecular weight alkane component, typical of a simple paraffin, being more mobile than, for example, a C30 alkane predominant in a higher paraffin. This in turn will be more mobile than the short side chain hydrocarbons of the higher intermediates, and very much more so than the bulky branched molecules of the microcrystalline range of the petroleum waxes.

Waxes with a very high alkane level give rise to films, which themselves being wholly alkane will be crystalline and, in consequence, relatively porous to gas permeation. Disruption of molecular regularity by the presence of a small amount of branched hydrocarbon, coinciding at the rubber surface with the arriving alkane, will give an amorphous film of high density and maximum resistance to gas, i.e. ozone, penetration.

The complex, bulky, branched components of microcrystalline waxes, referred to above, migrate to a rubber surface so slowly, if at all, as to offer a negligible degree of protection. Such waxes are never used on their own or even in a major phase in any blended product, although in some commercial products they may be used in minor proportion to assist in giving or being believed to give, ultra long term protection to a rubber surface. Sometimes too they fulfill a useful function as process aids in the compounding operation.

However, reference to commercial rubber waxes in some technical papers, and in the promotional literature of manufacturers and agents as "microcrystalline waxes" is, to say the least, misleading.

It is true that the actual hydrocarbon film at the surface must be of an amorphous or non crystalline character so as to present minimum porosity against ozone penetration, but this is in no way related to the use of a microcrystalline wax, and is achieved simply enough by a few percent of low molecular weight lightly branched hydrocarbons as may be provided by suitable intermediate waxes, these having realistic migration rates, and being well able to disrupt the regular packing of the paraffinic mass.

Thus effective, well designed and cost effective protective waxes are, and always have been, essentially paraffinic in composition with a small but vital branched chain element having the desired migrational characteristics.

Effect of temperature on wax migration or "bloom"

Many factors such as the polymer type, degree of oil extension, amount and type of filler, and degree of cross-linking can and do affect to a small extent the amount of wax migrating to the surface, i.e. the wax "bloom." The actual composition of that wax, as well as its amount, are affected to a much greater degree by the surrounding temperature, and this relationship must be studied in detail by anyone with any involvement in the protection of a rubber surface from ozone attack.

Three factors are involved, to differing degrees at different temperatures, and often conflicting:

* The rate of ozone attack on a double bond increases with temperature up to about 50 - 55 [degrees] C, when ozone itself begins to decompose to harmless oxygen. A threshold temperature exists below which there is insufficient activation energy for the chemical reaction between ozone and the double bond to take place. This temperature is about - 5 [degrees] C.

* Each component of a hydrocarbon wax migrates at its own particular rate, controlled by its own molecular complexity, chain length and degree of branching having their effect as previously described. For any given hydrocarbon, migration rate increases with temperature.

* For a hydrocarbon molecule to contribute to the protection of a rubber surface, it must be present at the surface and not in the interior of the rubber, i.e. it must be insoluble in the rubber at that particular time.

Solubility of any hydrocarbon in rubber, any rubber, increases with temperature, and the simpler the hydrocarbon, i.e. the shorter the chain length and the greater the linearity, the more soluble it will be at any given temperature.

Thus it follows that the composition of the wax film at the rubber surface will vary with the temperature. At or near 0 [degrees] C only the lower carbon number hydrocarbons, e.g. C18 to C24 have rates of diffusion which allow them to appear at the surface at a reasonable rate. At temperatures of 40 [degrees] C and over, these low carbon number fractions are almost completely soluble in the rubber, their place at the surface being taken by higher hydrocarbons, e.g. C30 upwards. With variation in temperature there is a constant variation in the chemical composition of the wax film at the surface as re-sorption into the rubber and re-deposition at the surface takes place.

The protective wax film on the sidewall of a tire is therefore a constantly changing dynamic entity, almost disappearing into the rubber as the tire heats to running temperature, and re-appearing as the tire cools on standing.

Investigation of the composition of the wax film appearing under specific conditions at a rubber surface can be carried out by a simple solvent extraction of the wax from that surface, followed by gas chromatography analysis of the concentrated extract. A continuous relationship can then be shown to exist between the prevailing temperature, and the carbon number of the particular hydrocarbon having its maximum migration rate at that temperature.

This is illustrated in table 1, which, over a 72 hour period at a particular controlled temperature, gives the carbon number of the alkane hydrocarbon with maximum migration at that temperature. The wax used in this experiment was a `plateau' type similar to the one depicted in figure 5.

Design of a wax for protection over a particular temperature range

This relationship of film composition with temperature becomes very important in designing a rubber wax to give adequate protection throughout the life of, for example, an automobile tire.

If an average 60/2 paraffin wax, a common enough material, is blended into a rubber compound, and different samples of the cured component stretched and tested in ozone at different temperatures, a graph can be drawn plotting test temperature against sensitivity to ozone attack (figure 4).

At ambient temperature (20-25 [degrees] C), sensitivity to ozone attack is least, and attack is least likely to occur because migration conditions for bulk of the wax components are optimal. At higher temperatures the greater rate of hydrocarbon migration and hence speed of film formation is offset and exceeded, partly by rising ozone activity, but more importantly, by resorption or non-appearance at the surface of the lower carbon number components. Thus at about 45-50 [degrees] C there is an area of maximum sensitivity or likelihood of ozone attack, caused primarily by there being insufficient insoluble hydrocarbon available in a simple paraffin wax to form an adequate film at the surface.

At still higher temperatures ozone itself begins to decompose to oxygen, thus the sensitivity curve shows a dramatic downward trend beyond about 50 [degrees] C.

At a temperature of about 0 [degrees] C, all wax components suffer from reduced mobility, and attack may start at the surface before a protective film can be formed. Even worse, crystalline film formation, afforded in the main by lower carbon number alkanes may result in a porous film through which ozone attack continues indefinitely. In either case the lower reactivity of ozone cannot compensate, and a second area of maximum sensitivity to, or likelihood of, ozone attack appears, and one far too infrequently recognized.

At still lower temperatures the rate of chemical reaction of ozone with the bond reduces to the reactivity threshold, so that even in the complete absence of wax `bloom' there is no recognizable attack.

These areas of maximum sensitivity to, or likelihood of, ozone attack can be reduced or eliminated, or the threshold strain increased, by replacing the wax or waxes used, or by blending them, with other waxes rich in the alkane and isoalkane (non normal) components, known to give their migration maxima at or about the critical temperature.

The ultimate graph will thus assume a more horizontal aspect, the position of which on the ordinate axis can be adjusted to a large extent by altering the total amount of blended wax incorporated into the vulcanizate.

If in its service life a particular rubber component is not expected to meet either of these critical temperatures in an over-threshold strain condition, it may not be necessary to cater for them. A wax designed to give protection in both critical temperature areas may be a "plateau wax" or even a "twin-peak" wax (see figures 5 and 6) and a higher proportion of it will obviously need to be used to give comparable protection to a narrow range wax at its own temperature of optimum effectiveness.

It has been recognized for many years that protection at low temperatures is technically more difficult to provide than at higher temperatures, yet it is only within recent years that low temperature testing for ozone resistance has become accepted by many manufacturers as a vital requirement of tire technology.

Routine testing in the temperature range of 20-30 [degrees] C is still common, yet this is the zone of least sensitivity to, and likelihood of, ozone attack, where an adequate protection level offers no assurance that failure would not occur if the same system were tested at 0 [degrees] C or 45 [degrees] C.

Chemical protective agents

No article on the protection of rubber from ozone attack could avoid at least a brief mention of this vitally important group of products which is used, in combination with wax, to give enhanced long term protection and resistance to dynamic stress. While suitably formulated wax films will withstand appreciable flexing, they have no resistance to dynamic elongation. For most dynamic applications involving high stress, such as in tires, driving belts and large diameter hose, it is necessary to use chemical antiozonants along with suitable waxes.

It has long been known that such selected combinations offer greater protection under static as well as dynamic stress than larger amounts of either type of protective when used alone. This synergistic effect has been widely studied, and it is just as important and relevant to select and blend a chemical antiozonant package for particular applications and compounds as it is to choose a particular wax system.

The largest and most important groups of the aging protection agents are the substituted para phenylene diamines. These would be even more widely used if they were not staining to a significant extent. Nevertheless they are essential components of automobile tires, and are used in combinations of varying degrees of effectiveness by every manufacturer of black tires.

Compared to waxes, the substituted ppds are very expensive. It is often a valuable cost effective exercise to balance the amounts of selected ppds with a suitable wax system to produce the optimum protective package. Generally a ratio of 1 ppd to 2 or 3 of wax gives the best balance of protection and cost.

Testing of the protective package for ozone resistance

The standard methods of testing rubber for ozone resistance, e.g. ASTM D1149, DIN 53509, BS 903, GOST 6949, etc., were written many years ago before the mechanism of wax protection was properly understood, and they have never been revised to keep pace with current knowledge. Consequently, adherence to the stipulated techniques of sample preparation, conditioning and temperature of testing can produce very misleading and inappropriate results.

All these methods call for test temperatures of between 23 and 30 [degrees] C, i.e. in the range where ozone attack is least likely to occur. Provision is made by some methods for testing at higher temperature, e.g. 40 [degrees] C, but prior to the actual exposure to ozone the sample is required to be conditioned at room temperature for long periods, 24 hours in the case of D1149 and for 72 hours by DIN 51509. Such times are far too long to give any measure of the rate of build-up of a protective film. Since the conditioning is at room temperature whereas the samples are introduced to ozone at 40 [degrees] C, a redistribution of hydrocarbons at the surface takes place, the speed of such movement varying widely with the particular laboratory conditions. Thus what is supposed to be a standard test commences under non-standard conditions.

It would be far more relevant and informative if testing in an ozone atmosphere were to be carried out at 0 [degrees] C and 45 [degrees] C, i.e. in the critical temperature zones previously discussed. However, it must be stressed that stretching of the rubber and conditioning of the freshly stretched rubber surface must take place at the test temperature and not at room temperature. This is particularly important if the test temperature is sub-ambient, e.g. 0 [degrees] C, 5 [degrees] C, etc. If conditioning was to be carried out at ambient, the wax film would be one composed of hydrocarbons with their migration maxima at ambient, many of which would never have reached the surface at 0 [degrees] C, thus a misleading and optimistic picture of protective quality would be obtained.

In practice an automobile tire sidewall which is scuffed, scraped or even cleaned a 0 [degrees] C will have to rely for immediate protection on the protective elements (both antiozonant and wax) capable of blooming quickly to the surface to repair the damaged film at the existing temperature, i.e. 0 [degrees] C.

Thus sample preparation and conditioning should be at 0 [degrees] C, and in a multi-sample operation, for varying lengths of time from zero up to 1 hour or more. Results of such ozone exposure from samples prepared in this way give practical, meaningful and reproducible information on rate of film damage repair and the overall performance of the rubber in practice.

Specification of rubber waxes and their quality control

It will be obvious from what has been said, that the single factor controlling the performance of a rubber wax is its chemical composition. This is defined by the twin parameters of the carbon number distribution and the ratio of straight chain to branched chain hydrocarbon present.

These, therefore, are the two factors which should be specified and subject to tight quality control by any supplier. If these two factors are excluded from a purchase specification and a batch inspection report, any other data covering physical properties such as melting point, density, color, flash point, etc., are so much useless and meaningless "padding," and need have no relevance at all to the chemical composition, and hence to the performance of the rubber wax.

Thus, with detailed knowledge of the composition of the raw materials available throughout the world, and the equipment and ability to monitor the consistency of such materials, a reputable blender will supply, to a reliable and relevant specification, rubber waxes capable of giving consistent performance and effective protection against atmospheric ozone attack for the lifetime of the rubber component. [Figures 1 to 6 Omitted] [Tabular Data Omitted]