Time Will Tell

The cement industry uses more heat resistant conveyor belting than any other industry. Unfortunately, it also has the highest rates of belt replacement, as nothing destroys conveyor belts faster than heat. High temperatures accelerate the thermal ageing process of the outer covers. How long this process takes depends on how well the rubber can withstand the heat without incurring damage while also protecting the inner carcass. Consequently, there are huge disparities between the operational lifetime and whole life cost provided by belts supplied by different manufacturers and suppliers.

The ageing process

Although rubber is obviously not a lifeform, the one thing that it has in common with organic life is that it ages. And as it ages, it deteriorates. As far as rubber conveyor belts are concerned, there are two forms of ageing – thermal and oxidative. High temperature materials and working environments rapidly accelerate the ageing process of rubber, causing it to harden and crack. Known as thermal ageing, high temperatures also have an extremely destructive effect on the inner carcass of the belt because heat gradually destroys the adhesion between the rubber covers on the top and bottom of the carcass, as well as between the inner plies contained within it. The result is ‘de-lamination’, which occurs when the layers of the belt literally become detached from one another.

As rubber becomes harder and less elastic, its tensile strength and elongation (stretch) can decrease by as much as 80%. This effectively destroys the operational strength and flexibility of the belt, and seriously weakens the splice joints. At the same time, surface cover wear is accelerated because the rubber’s resistance to abrasive wear decreases by as much as 40% or more. As the covers become thinner, their ability to protect the inner carcass from temperature build-up diminishes, creating a vicious cycle that further accelerates both the de-lamination and ageing processes.

Apart from thermal ageing, the other major cause of rubber deterioration and degradation is oxidative ageing, which is primarily caused by exposure to ultraviolet light (sunlight and fluorescent light) and other reactive gases, especially ground level ozone. These factors cause rubber to become brittle and crack. The dynamic stress caused by the belt flexing around pulleys and drums under tension greatly accelerates the formation of the cracks, which then allow more heat to penetrate down into the carcass.

Heat resistance – definitions and classes

Not all heat resistant conveyor belts of a stated specification provide a similar performance or operational lifetime. In fact, alarming differences are commonplace. To understand the reasons, it is important to first understand the different specifications applicable to heat resistant conveyor belts and the associated terminology, which can be very confusing.

The temperature limits that a belt can withstand are viewed in two ways – the maximum continuous temperature of the conveyed material and the maximum temporary peak temperature. Heat resistant grades are historically identified as grade ‘T’. The continuous and peak temperature limits are commonly combined into classes T1, T2, and T3 respectively, although some prefer to use T100, T150, and T200 instead. The most important thing to remember is that there are only two main classifications of heat resistance recognised in the market. These are T150, which relates to a maximum continuous temperature of 150°C, and T200, which is for more extreme heat conditions up to 200°C.

T150 should consistently withstand continuous material temperatures up to a maximum of 150°C over long periods, but it should only be expected to cope with peak temperatures in excess of that limit for a very short period of time (literally minutes). For T150 classification belts, the necessary physical properties can be achieved using a styrene butadiene rubber (SBR) and a process of chemical alteration of the various chemical polymers together with the use of other components and accelerants. The same principle applies to the T200 classification for peak temperatures higher than 200°C. Any period longer than just a few minutes will almost certainly cause irreparable damage to the belt.

For the higher temperature T200 classification, it is necessary to replace the SBR with more complex and expensive polymers such as EPM (ethylene propylene rubber) and EPDM (ethylene propylene diene monomer rubber), which have a superior resistance to heat. A different, more complex vulcanising mechanism involving hydrogen peroxide is also required.

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