Premature breakdown of rubber exposed to ozone

Describing problems that can occur with rubber footwear components.

by Austin Simmons

There are a number of environmental factors that can cause the onset of ‘ageing’ in a material. These factors include extremes of temperature (especially rapid changes in temperature), sunlight or moisture, as well as the actions of wearing the products, such as flexing. However, in some cases a material can come under attack from an ageing influence without the user even being aware of it. One particular ageing factor of this type is ozone attack.

Ozone (O₃) is a form of oxygen which comprises three oxygen atoms rather than the two atoms that make up molecular oxygen (O₂). Normally encountered as a gas, ozone is a solid below -250°C, a deep-blue liquid between -250°C and -112°C and a gas (with a distinctive odour) above -112°C. It is a naturally occurring constituent of the atmosphere, but in much smaller proportion than O₂ (in the order of 0.003 parts per million).

Ozone is produced in the upper atmosphere by ultraviolet radiation from the sun, and at the lower level by the electric discharge associated with lightning. Ozone is an unstable gas, readily decomposing into molecular oxygen. The ability of ozone to readily yield up one of its oxygen atoms makes it a powerful oxidising agent. Ozone can also be man-made – either purposely or as a bi-product of another activity (such as by electric discharges from arc welding or sparks from electric motors). These man-made sources can contribute to significant increases in ozone concentrations in the working and living environment.

Ozone – friend or foe?

In fact, it is both an ally and an enemy. In the upper atmosphere, ozone is beneficial as it blocks damaging ultraviolet light from reaching the Earth. This explains why there is such concern regarding chlorofluorocarbons (CFCs), which are believed to be responsible for damage to the ozone layer.

Another advantage of ozone is in its use as a bactericide and algaecide. For example, ozone generated at low concentration levels is used in swimming pool disinfection plants, or in hotel rooms to remove odours. The negative aspect of ozone relates to both its toxicity to man and its detrimental effect on a wide range of materials, including rubbers, printing inks and textiles. Ozone’s toxicity – primarily affecting the eyes and lungs – can be identified commencing at levels of 0.1 parts per million (ppm), with a variety of symptoms identifiable as ozone concentrations rise. As an example, welders exposed to ozone at 9ppm have suffered pulmonary oedema (fluid accumulation in the lungs).

Rubber technologists have long known of ozone’s potential to cause damage, and rubbers under strain are particularly vulnerable. Many rubbers have weak points – unsaturated bonds – in their chemical structure which ozone will attack, causing breakage of the polymer chain and cracks to develop perpendicular to any applied strain. Rubber has been used as a soling material for many decades, and ozone attack can lead to premature failure occurring during wear.

The tendency today to produce more intricate designs (especially those that mould around the upper), can lead to additional stress points, and the manufacturing process inevitably causes rubber materials incorporated into some shoe designs to be put under stress. If attacked by ozone, this can lead to cracking, ranging from superficial surface crazing, to deeper cracking causing significant weakening of a material, to complete fracture of a material or component. The photograph at the top of this article shows a rubber outsole after 48 hours’ exposure in the SATRA test chamber, reproducing cracking observed on footwear affected over time by ozone in the atmosphere.

Although ozone attack is not typically a rapid process, it is not something an end user would expect and can lead to consumer dissatisfaction, complaints and returns.

Testing for ozone resistance is best carried out using relatively high concentrations of ozone in a controlled atmosphere, as a higher level of ozone will accelerate the process of attack. Typically, ozone concentrations used for testing are in the range 50 parts per hundred million (pphm) to 5ppm, although there is occasionally scope for significantly higher concentrations for specific applications. In a typical untreated rubber component under a small amount of stress, significant cracking can often be seen following only 24 hours of ozone exposure at 50pphm.

Tests can be conducted against ASTM, BS or ISO test methods or specific manufacturers’ specifications using tightly specified ozone generating chambers. The ozone resistance of different rubber formulations or rubber products with design differences can be compared, with design revisions or changes in rubber formulations to be subject to an accelerated ozone test and a revised product quickly brought to market. In addition, ozone chamber testing (figure 1) can be used by a supplier to validate batches of material or components against a customer’s specification.

Publishing Data

This article was originally published on page 52 of the November 2018 issue of SATRA Bulletin.

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