Heat-resistant safety footwear

The first of two articles describing SATRA machines used to assess heat-resistant safety footwear materials.

by Peter Allen

Safety footwear often needs to protect the wearer against heat from a number of different transmission routes and in different scenarios. These include radiant heat, heat transmitted through direct contact (through the sole or the upper construction) and heat sources relating to a specific activity (such as from molten metal splashes).

A number of international standards specify tests required to verify footwear’s resistance to heat for a particular application. Examples of these are: ‘Wildfire firefighting personal protective equipment – requirements and test methods’ (ISO 16073:2011), ‘Footwear for firefighters’ (EN 15090:2006) and ‘Personal protective equipment – footwear protecting against thermal risks and molten metal splashes as found in foundries and welding – requirements and test methods’ (EN ISO 20349:2010).

SATRA supplies four pieces of test equipment applicable to a number of requirements within these standards: STM 531 Contact Heat Test machine, STM 518 Radiant Heat Test machine, STM 471 Heated Sandbath for testing insulation of sole complex, and STM 524 Molten Metal Splash Test machine. The first two are described in this article, with the latter two to be covered in a subsequent SATRA Bulletin article.

The STM 531 machine

The SATRA STM 531 Contact Heat Test machine allows tests to be conducted to the requirements of EN 702:1995 (referenced, for example, in EN ISO 20349:2010). An assessment can be made of the transmission of direct contact heat for materials used in footwear conforming to these standards.

The STM 531 machine comprises a welded frame on which the contact heat mechanism is mounted. A block, which is electrically heated to a controlled temperature (for instance, 500°C), is lowered onto the surface of the test specimen at a defined speed. The heated block is of a size and weight to apply a force of 49N onto the specimen, as required by the test method. A timing system allows the load to be applied for a pre-set period, after which it is automatically raised. The test sample is mounted on a calorimeter incorporating a heat sensor, which is connected to a display unit to show the calorimeter’s temperature and, therefore, a measure of the transmitted heat through the sample.

As the test begins, a thermal guard (between the heat source and the test specimen) is automatically removed. The test start time is determined from when the bottom edge of the heating cylinder is within 10mm of the test specimen. A threshold time is determined, and is defined as ‘the time between the start time and the moment of time at which the calorimeter has increased by 10°C above its starting temperature’. Standards which reference EN 702 will specify a minimum threshold time, which must be achieved for materials required to conform to that particular standard. Other characteristics can also be assessed – for example, a requirement in EN ISO 20349:2010 calls for no melting of the test specimen’s inner surface. A signal output from the machine can plot a temperature/time coefficient to aid the determining of the threshold time.

The Radiant Heat Test

The SATRA STM 518 Radiant Heat Test machine enables the assessment of footwear materials for resistance to transmission of radiant heat. Using this equipment, tests can be conducted to the requirements of EN ISO 6942:2002 (‘Protective clothing – protection against heat and fire’) which, in turn, is referenced in other test standards, including the previously-mentioned ISO 16073:2011 and EN 15090:2006.

The SATRA STM 518 Radiant Heat Test machine comprises a source of radiant heat, a specimen holder and a calorimeter (which also incorporates a heat sensor). A water-cooled mounting frame and movable screen, positioned between the radiant source and the specimen, allows the temperature of the specimen and calorimeter to be maintained at room temperature before the test commences. The specimen holder, associated calorimeter and guard are mounted on rails, allowing them to be moved closer or further away from the radiant source. This is used to adjust the level of heat (the ‘incident heat flux density’) applied to the specimen.

A specimen is positioned between the heat source and the calorimeter, clamped on one side of the calorimeter, and tensioned across the calorimeter’s curved surface at a controlled force by means of weights and pulleys. Timing for the test commences when the movable screen is raised and the temperature time characteristic obtained during a 30-second exposure period. The temperature recorded by the heat sensor 12 seconds and 24 seconds after the start of the test is used in the calculation of the transmitted heat flux density, transmission factor and the Radiant Heat Transfer Index (RHTI). Referencing test methods will typically specify the particular requirements. For instance, EN 15090:2006 specifies a requirement for RHTI for a specified heat flux density, in addition to assessments to be carried out on the footwear material samples subjected to the test.

These two specialist pieces of SATRA test equipment are invaluable in demonstrating compliance with the relevant standards relating to safety footwear. They also allow the comparison of alternative materials or combinations of materials with respect to resistance to heat transmission, as part of a product development programme.

Publishing Data

This article was originally published on page 34 of the January 2012 issue of SATRA Bulletin.

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