Determining leather shrinkage temperature

The use of SATRA’s STD 114 test device.

by Peter Allen

The process of tanning a hide or skin confers a number of important properties on the leather produced – the primary one being the resistance to decay. Another property is that the leather can be much more resistant to shrinkage when subjected to moist heat, compared to raw or untanned hide or skin. There are a variety of different processes which can be used to tan leather such as chrome tanning, vegetable tanning and aldehyde tanning. Different types of tanning (both primary tanning and re-tanning) produce different physical properties, including levels of resistance to moist heat in the resulting leather.

The way to determine that the tanning process has been carried correctly is to measure the ‘hydrothermal stability’ – its resistance to wet heat – more commonly referred to as the ‘shrinkage temperature’. A characteristic of hides, skins and leathers is that if they are gradually heated in water, they reach a temperature at which they are subject to sudden, irreversible shrinkage. Raw hides or skins shrink very easily at temperatures of about 65ºC, whereas chrome tanning, for example, increases the point at which shrinkage occurs to temperatures up to a maximum of around 120ºC. This increased resistance to wet heat is an important requirement for leather when making a wide range of types of footwear in which the leather is subjected to moisture and high temperatures as part of the manufacturing process.

In order to determine the shrinkage temperature for a leather – that is, the temperature when the shrinkage transition occurs – the SATRA TM17:1997 – ‘Shrinkage temperature of leather’ test method can be conducted using SATRA’s STD 114 test apparatus. This will help to determine the suitability of the leather for shoe manufacture with respect to hydrothermal stability.

The principle of SATRA TM17 is that a strip of leather is immersed in water and slowly heated, until a sudden shrinkage occurs. The temperature at which this shrinkage occurs is characteristic of the tannage of the leather. The higher the shrinkage temperature, the better the moist heat resistance of the leather. Therefore, measurement is valuable for judging the suitability of the leather for different footwear manufacturing processes in which moist heat may be applied to the leather.

As the shrinkage temperature of tanned leathers (for instance, chrome-tanned leathers) will often be above 100ºC, it is necessary to conduct the test under pressure. This raises the temperature at which the water will boil, hence allowing tests to be conducted above 100ºC. SATRA STD 114 allows tests to be conducted up to 115ºC by incorporating a test chamber and connecting pipework, and a water reservoir which can be pressurised.

The test apparatus consists of a closed convection circuit comprising the transparent sample chamber (sight glass) connected by pipes to a small water reservoir which can be heated with a Bunsen burner or a similar heat source such as a propane burner. Prior to the test, the sight glass, reservoir and connecting tubes are filled with water. A safety valve is fitted to the apparatus which limits the pressure to that corresponding to a temperature of 120ºC. A transparent plastic guard is fitted around the sight glass as a safety precaution.

Preparing the specimen

When conducting a test to SATRA TM17 and using SATRA STD 114, strips of leather 80mm x 10mm are cut from the leather to be assessed – one specimen in the ‘along’ direction, with its longer side parallel to the direction of the backbone, and one specimen at 90º in the ‘across’ direction. If the backbone direction is not known, the tighter direction of the leather should be used as the along direction. Holes are punched in the ends of the leather to allow the specimen to be held vertically in the test chamber. Before conducting the test, the samples are wetted by immersion in deionised or demineralised water for at least five minutes.

The apparatus is filled with water and the wetted ‘along’ direction specimen is suspended in the sight chamber. The apparatus is closed, creating a pressure-tight system. After an initial dwell period, the water is heated (at approximately 4ºC/min) by applying the external heat source to the boiler compartment. The apparatus includes a thermometer which allows the temperature of the water surrounding the test specimen to be measured. One end of the leather is suspending from a hook in the water within the sight chamber and a small weight is attached to the lower end. The position of the lower end is indicated by an adjustable marker outside the tube to help judge when shrinkage occurs. The test continues with the specimen under observation until the length of the test specimen rapidly decreases by at least several millimetres.

The temperature at which this occurs, to the nearest degree Celsius, is recorded as the ‘shrinkage temperature’ for the along direction. The water in the apparatus is allowed to cool down before opening and the specimen replaced with the across direction specimen. The test is repeated and the shrinkage temperature recorded for the across direction. The shrinkage temperature for the leather under assessment is the average of the two shrinkage temperatures obtained.

Not all tanned leathers are suitable for the high temperatures associated with important footwear manufacturing processes. For example, alum-tanned and vegetable-tanned leathers will typically reach shrinkage temperatures no higher than 85ºC. Alum can be combined with vegetable tanning to produce semi-alum leather which can often achieve shrinkage temperatures in excess of 100ºC. There are a number of other tanning combinations which are available, such as re-tanning with chromium salts after a primary vegetable tanning. Due to this variation in leather tanning processes available and the range of shrinkage temperatures which can be imparted to the leather, it is important to specify and then verify the leathers selected for footwear production have the required heat stability for the manufacturing processes to which they will be subjected.

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Publishing Data

This article was originally published on page 48 of the March 2015 issue of SATRA Bulletin.

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