What’s the crack?

Regardless of the price paid for them, the reliability and longevity of conveyor belts will ultimately determine their cost. In the cement industry, abrasive wear and heat are the main factors that determine the level of repairs and maintenance and working lifetime. However, what is not well known is that ozone (O₃) and ultraviolet light (UV) also have a huge influence on operational lifetime. Despite the serious damage they cause, more than 90% of all conveyor belts sold in Europe, Asia and Africa are not ozone and UV resistant. This article explains why it is such an important issue.

Ozone exposure: causes and effects

When developing any rubber compound, one of the first considerations should be the effect of ozone. Almost all of the rubber used in conveyor belt manufacturing is synthetic. This is because different mechanical properties and characteristics can be specifically created depending on what the rubber will be used for and the working environments it will be subjected to.

Ozone (O₃) occurs naturally in the upper atmosphere. It is formed continuously by the action of solar ultraviolet radiation on molecular oxygen (O₂). At high altitude, ozone acts as a protective shield by absorbing harmful ultraviolet rays. Wind currents carry O₃ to the atmosphere at the Earth’s surface. However, at low altitude, ozone becomes a pollutant. Ground level or ‘bad’ ozone is not emitted directly into the air but is created by the photolysis of nitrogen dioxide (NO₂) from sources such as automobile exhaust and industrial discharges. This is known as ozonolysis.

Ozonolysis

Ozonolysis is the reaction that occurs between the molecular structure (double bonds) and ozone.

The scientific explanation is that the immediate result is the formation of an ozonide, which then decomposes rapidly so that the double bond molecule is split. The critical step in the breakdown of molecular chains is when polymers are attacked. The strength of polymers depends on the chain molecular weight or degree of polymerisation. The longer the chain length, the greater the mechanical strengths including the highly important tensile strength of the rubber. By splitting the chain, the molecular weight drops rapidly. It is necessary to understand this because there comes a point when very little strength remains and cracks start to form. Further attacks occur inside the freshly exposed cracks, which continue to grow steadily until they complete a ‘circuit’ and the product separates or fails.

Exposure is unavoidable because even tiny traces of ozone in the air will attack the molecular structure of rubber. It increases the acidity of carbon black surfaces with natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber being the most sensitive to degradation. Although the first visible sign is when cracks start to appear in the surface of the rubber, depending on the level of ozone resistance that has been built into the rubber compound, the process of ozonolysis effectively begins when the conveyor belt leaves the production line.

Although the variability of weather, airflow patterns, seasonal changes, the level of emissions and climatic conditions do mean that ozone concentrations can differ from one location to another, the fact is that ground level ozone pollution is ever-present and therefore its effects should never be under-estimated.

Hidden effects

Ozone cracks form in rubber that is under tension. This is sometimes referred to as ‘dry rotting’. It is important to bear in mind that only a very small amount of tension (critical strain) is needed. Even a belt that has not yet been fitted on a conveyor has a certain amount of intrinsic tension. The cracks are always oriented at right angles to the strain axis. The dynamic stress that a conveyor belt undergoes whilst in operation is considerable and ozone attack occurs at the points where the strain is greatest.

The repeated action of the mechanical stress of the conveyor belt and the frictional process from the idlers mean that the rubber molecular chain will break to form what scientists refer to as a ‘free radical’. This triggers the oxidative chain reaction that forms a chemical process that mechanically breaks the molecular chain and activates the oxidation process, magnifying a whole range of more obvious problems such as the inability to resist abrasive wear.

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