Testing for vulnerability to ozone damage

Highlighting SATRA’s test equipment designed to assess potential problems to footwear caused by this naturally occurring or artificially created gas.

by David Billington

Ozone (O₃) is a colourless and highly reactive form of oxygen that, besides being dangerous to health, can also attack and degrade many types of materials – including some that are commonly used in footwear. It is generally accepted that natural rubber is particularly susceptible to attack by ozone, however other forms of rubber and other polymers are also at risk of ozone degradation which generally appears as cracks in the polymer.

What is ozone?

Ozone is present in the upper levels of the earth’s atmosphere, where it is formed and degraded in a constant cycle by the action of ultraviolet (UV) light on oxygen. It plays an active and important role in absorbing much of the UV radiation from the sun. Without the ozone layer, significantly higher levels of UV light would reach the earth’s surface, making life very difficult (if not impossible) for most forms of animal life. Ozone is also found at ground level, although usually at very low concentrations – around 0.003 parts per million (ppm). Being highly reactive, it usually degrades very quickly.

However, in certain circumstances, ozone can be present at significantly higher levels in areas where there are large numbers of electrical devices. Discharges caused by the action of switches or by the brushes within electric motors can produce high levels of ozone in the locality. It can also be present in areas subject to high pollution caused by internal combustion engines. Of course, internal combustion engines do not directly produce ozone, but the action of sunlight on a mix of hydrocarbons or volatile organic compounds (VOCs), nitrogen oxides present as combustion products (NO_x) and atmospheric oxygen can dramatically increase the concentration of ozone at ground level.

Studies have shown that ozone concentration levels as high as 0.2 ppm can occur for periods of up to one hour in inner city areas. It is worth noting that ozone concentrations of over 0.1 ppm are considered toxic to humans.

Certain types of electric light which produce high levels of UV radiation can also produce significant levels of ozone. Many industrial processes (for instance, certain welding processes) can produce ozone as a by-product in concentrations above 1 ppm.

How ozone reacts with materials

The reaction of ozone with materials is a complex process. One source has stated that over 50 different forms of chemical reaction have been identified as occurring between ozone and various materials. It is a powerful oxidising agent which can cause the rapid oxidation of many metals. Ozone can attack materials, resulting in weakening of their structures. It can attack surfaces and change surface properties such as colour or appearance. Ozone can also attack a range of biological materials. In humans and animals, for example, it attacks the linings of the throat and the lungs, which helps to explain some respiratory health problems caused by pollution in inner city areas.

Typically, certain types of rubber are thought of as being most susceptible to attack by ozone. The ozone molecule is able to attack molecules containing unsaturated bonds (that is, double or triple bonds) between the carbon atoms. Materials are usually most at risk when under strain. The process is complex, but effectively the ozone cuts such bonds in two. The overall effect of this is that rubber – when placed under strain and exposed to ozone – will show cracking and loss of strength. Often, the severity of cracking is such that the component breaks into two pieces.

Because of the susceptibility of rubbers to ozone attack, the obvious footwear component most at risk of attack by ozone is the sole. Levels as low as 0.03 ppm have been found to cause cracking in some types of rubber. Therefore, it is apparent that at times the atmosphere within inner city environments could easily contain a sufficient concentration of ozone to cause damage to rubber soles and other rubber components. As previously mentioned, the susceptibility of materials to damage from ozone can be increased if they are under strain and if the temperature is higher than usual.

Ozone can also attack a wide range of polymers. Even polyethylene, which is relatively stable owing to its saturated bond structure, is not immune from attack. There is also evidence that ozone – in combination with other pollutants such as sulphur dioxide and nitrogen dioxide – can attack a wide range of materials, including polyurethane, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyacrylonitrile, butyl rubber and nylon, and reduce the strength of these materials.

Ozone damage testing

SATRA manufactures, supplies, commissions, services and calibrates ozone test chambers for a wide global market. SATRA automatic ozone test chambers allow for an accelerated test of the effects of ozone. This is done by subjecting specimens to a quantifiable high concentration of ozone (generated by the test chamber) while at the same time controlling the temperature, humidity and airflow. The ozone chamber allows for specimens to be placed under load or subject to cyclic loading for the duration of the test, to simulate operating conditions. The use of an ozone test chamber allows products with design revisions or changes in rubber formulations to be subject to an accelerated ozone test and a revised product quickly brought to market.

The lead photograph in this article shows a sample of rubber after 48 hours’ exposure to ozone in the SATRA test chamber, which reproduced cracking caused over time by ozone in the atmosphere. For example, 48 hours’ testing can provide an equivalent of approximately ten years’ exposure to ozone under normal atmospheric conditions, depending on the materials being analysed.

Tests can be conducted against ASTM, BS or ISO test methods or specific manufacturers’ specifications. The ozone resistance of different rubber formulations or rubber products with design differences can be compared, and product design and material specifications changed accordingly. In addition, ozone chamber testing can be used by a supplier to validate batches of material or components against a customer’s specification. SATRA has supplied ozone test chambers to many leading rubber manufacturers associated with the automotive industry worldwide, as well as to the aerospace sector, ink manufacturing industry and to research and academic institutions.

It is probable that a number of failures across a wide range of rubber-based products have ozone as a hidden contributing factor. SATRA not only designs and makes ozone test chambers, but also offers a consultancy and ozone testing facility using SATRA ozone test chambers to investigate the effects of ozone in product failures.

SATRA HTE 703/903 ozone test chambers

SATRA ozone testing technology has evolved as a result of more than 30 years of experience. State-of-the-art technology and construction techniques have been combined with expertise to produce a reliable, accurate, fully automated, labour and cost saving unit which is truly environmentally safe.

An integral closed loop air-ozone system ensures that the ozone is reformed into oxygen (O₂) after use within the test chamber, eliminating the need for additional external venting or filtering of spent gases. Ozone is generated as required for the test by means of a UV lamp, as its highly unstable molecule means that it cannot be stored.

The heavy-duty aluminium exposure chamber has an anti-corrosive interior and is complete with racking system to accept sample-mounting apparatus. A range of static and dynamic specimen holders to international standards is available. Access is via a double glazed, airtight and safety interlocked door. Entry to the test area is prevented until the interior environment reverts to safe ambient levels. Exterior lighting is excluded from the test area via tinted glass, while the interior can be lit by the integral test inspection lamp.

Controls and digital displays are logically laid out on a control panel and reflect the simplicity of operation required to set up a test routine. The only requirement is to set the ozone concentration, temperature, relative humidity (HTE 903 model only), test start/finish/purge time sequence.

SATRA ozone chambers technical information
Model	HTE 703	HTE 903
Temperature ranges	0°C to 70°C 10°C to 70°C Ambient (5°C) to 70°C	23°C to 70°C
Temperature control accuracy	±1°C at 40°C and ±3°C at 70°C
Ozone concentration	Variable from 1 to 500 parts per hundred million (pphm) with a 1 to 2,000 pphm option
Ozone concentration      measurement accuracy	±2 pphm, and control accuracy is 4 per cent of set point (±2 pphm at 50 pphm)
Control accuracy	±5 per cent full scale deflection (FSD) or better, being typically      ±2 pphm at 50 pphm, measured under stable absorption conditions
Ozone generation	Generation is via ultraviolet (UV) ozone lamps with a maximum ozone generation output of approximately 50,000 pphm ÷ litres/min – for example, 200 pphm at 250 litres/min. A high ozone option produces 2,000 pphm (in conjunction with UV type analyser)
Ozonised airflow	40 to 440 litres/min infinitely variable, allowing a maximum of three air changes per minute and an effective velocity of 3.3 to 33.3 mm/sec
Temperature/ozone recovery period	Following specimen insertion <10 per cent of set point after 15 minutes and 4 per cent after 30 minutes (cold start times will vary)
Relative humidity (RH)	N/A	50 to 80 per cent RH over the exposure chamber temperature range of 23°C to 40°C
Water supply	N/A	Requires a maximum temperature of 15°C, a minimum pressure of 2 bars and a maximum flow of 80 litres/hour

The ozone concentration is automatically controlled and monitored. The set concentration and monitored concentration are digitally displayed, with the monitored concentration simultaneously recorded. A digital display proportional controller allows an automatic servo system to control ozone across the whole range.

Publishing Data

This article was originally published on page 42 of the March 2021 issue of SATRA Bulletin.

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