ISO 340 Testing
The ISO 340 test standard makes the distinction between fire resistance with covers, which is grade EN 12882 Class 2A and fire resistance ‘with or without covers, which is grade EN 12882 Class 2B. The relevance of “with or without covers” is that as the belt cover wears during its operational life it becomes thinner so the amount of fire resistant rubber protecting the flammable carcass reduces.
The actual tests involve exposing 6 individual samples of belt to a naked flame causing them to burn. The source of the flame is then removed and the combustion time (duration of flame) of the test piece is recorded. A current of air is then applied to the test piece for a specified time after the removal of the flame. The flame should not re-ignite.
The time it takes for each belt sample to self-extinguish after the flame has been removed is precisely measured. This is the crucial aspect of the test because the duration of continued burning (visible flame) should be less than 15 seconds for each sample. The absolute maximum cumulative duration for each group of six sample pieces is 45 seconds. In other words, an average duration of less than 7.5 seconds per test sample. As has already been mentioned, this factor is of paramount importance because it effectively determines how far the fire can be carried by a moving belt. The effects of fire being literally ‘conveyed’ to adjoining buildings can be seen in the photograph.
Even if a manufacturer states that their fire resistant/fire retardant belt has passed the ISO 340 test, the buyer should still exercise caution. A typical conveyor belt can easily travel more than 20 meters within the 7.5 seconds average allowed to pass the test so this would still allow the belt to carry flames over a potentially dangerous distance. Dunlop Conveyor Belting in the Netherlands are widely recognised as the inventors of fire resistance rubber conveyor belts and it is interesting to note that their self-imposed time limit standard for a sample to self-extinguish is less than one second. It is always a wise move to ask to see copies of the test results before placing an order.
Given the highly flammable nature of biomass, in terms of actual fire resistance, it is recommended that EN 12882 Class 2B be regarded as the minimum standard. For conveyors that are in enclosed areas a higher level of fire resistance is needed. Here, DIN 22109 part 4, which is Class 4A of EN 12882, is the recommended standard.
Resistance to Oils and Resins
Biomass, especially the wood and wood waste content, can contain vegetable oils and resins that can have a very detrimental effect on the performance and life expectancy of a conveyor belt. Over time the oils and resins penetrate the rubber causing it to swell and distort, resulting in serious running problems.
Oil (including fat and grease) resistance can be divided into two sources – mineral and vegetable & animal. Rather surprisingly, ISO or DIN international standards for oil & grease resistance do not yet exist. As a consequence, the American ASTM ‘D’ 1460 test method is generally regarded as being the most demanding test of its kind in the world.
The level of oil and resin present depends very much on the type (origin) of the wood itself. For most wood from Scandinavia, good resistance to oil is necessary as these trees are mostly pine trees, which have high turpentine content. In South-European countries and in Latin America, Eucalyptus trees are commonly used. The wood from these trees contains little or no turpentine so oil resistance is not so important. If the origin of the wood used for the biomass can be from variable (or unknown) sources then we would recommend the use of conveyor belts that have a combined resistance to both fire and oil.
Optimising Safety and Economy
The ingredients used to create a fire-resistant (self-extinguishing) and oil resistant rubber compound almost invariably have an adverse effect on the wear-resistance of the rubber. In plain speak, fire resistant and oil resistant rubber usually wears significantly faster than belting designed purely to be resistant to abrasion.
However, it is possible to have the best of both worlds. One such example is Dunlop who have developed fire (and oil) resistant rubber compounds that also have extremely good resistance to abrasion. This means that the belt retains its resistance to fire for much longer while at the same time considerably extending its operational lifetime. Sadly, laboratory tests consistently reveal that this is very much an exception to the rule within the conveyor belt industry.
Fire resistant conveyor belts are a very significant investment so for reasons of both safety and value for money, buyers should always request technical datasheets before placing an order because they include data on the level of abrasion (wear) resistance. It is important to remember that for abrasion, lower figures represent better resistance to wear.
Ozone & UV Resistance
The fourth essential characteristic of belting used to carry biomass (indeed for all types of rubber conveyor belt) is ozone and UV resistance. Ozone (O3) occurs naturally in the upper atmosphere, where 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. 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 automobile exhaust and industrial discharges. The effects are known as ozonolysis.
Ground level ozone pollution is an ever-present fact of life that should never be under-estimated, especially in ports because coastal areas invariably have a much higher level of ozone pollution. Even tiny traces of ozone in the air will attack the molecular structure in rubber. It also increases the acidity of carbon black surfaces. Natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber are the most sensitive to this kind of degradation, which can have seriously detrimental effects on the belt including surface cracking and a marked decrease in the tensile strength of the rubber.
Even more significant are the environmental and health and safety consequences, especially when carrying biomass because the dust particles penetrate the surface cracks and are then discharged (shaken out) on the return (underside) run of the belt thereby increasing pollution and the risk of explosion.
At first glance, fine cracks in the surface rubber may not seem to be a major problem but over a period of time the rubber becomes increasingly brittle. Transversal cracks deepen under the repeated stress of passing over the pulleys and drums and, if the conveyor has a relatively short transition distance, longitudinal cracks can also begin to appear. There are also hidden long-term effects. One of those hidden effects is that moisture (as well as oils and resins from the wood waste) seep into the cracks and penetrate through the belt covers down to the carcass of the belt. The belt starts to distort and all sorts of difficult, expensive problems ensue.
To make matters worse, ‘bad’ ozone has a partner in crime that also has a seriously detrimental effect on rubber. Ultraviolet radiation causes chemical reactions to take place within rubber and the rapid decline in the ozone layer in the upper atmosphere over the past several decades is allowing an increasing level of UV radiation to reach the earth’s surface. Ultraviolet light from sunlight and fluorescent lighting accelerates deterioration because it produces photochemical reactions that promote the oxidation of the surface of the rubber resulting in a loss in mechanical strength.
ISO 1431 international Standards
Preventing the problems caused by ozone and UV is surprisingly easy using special anti-oxidant additives within the rubber compound during mixing that act as highly efficient anti-ozonants. The test used to measure resistance to the effects of ozone is ISO 1431, which involves placing samples under tension inside an ozone cabinet (50 pphm, strain 20%) for 96 hours. The pass criteria is that there should be no signs of cracking when the test is completed.
Unfortunately for the users of conveyor belts, most belt manufacturers completely ignore the issue of ozone and UV resistance because the protective additives cost money making their belts less price competitive. Would-be purchasers therefore need to always insist that the belt supplier provides written verification that their belts successfully undergo testing according to EN/ISO 1431 and at which conditions the test is performed.
Perfect Storms demand a perfect Solution
Conveyor belts carrying biomass really do have to face the perfect storm. Firstly; they operate in highly explosive, combustible environments so they need to be completely anti-static and able to self-extinguish as quickly as possible if ignited. Secondly, they convey materials that contain potentially damaging materials in terms of oils and resins. Thirdly, they are under constant attack by the elements including ozone pollution and ultra violet. In short, they have to be safe, reliable and provide an operational life that is as long as possible in order to be economic.
Such belts are, of course, available but you need to be absolutely sure of their providence. You also need to be as sure as you can be that what the manufacture has promised will actually be delivered. When it comes to carrying biomass, belts that are not of the highest standard are a very dangerous and expensive liability indeed.
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