Test equipment for leather assessment

Describing a number of machines and devices used to test the suitability of leather.

by Peter Allen

The article 'Determining leather shrinkage temperature' describes the SATRA STD 114 test equipment and the associated SATRA TM17:1997 – ‘Shrinkage temperature of leather’ test method. In this article, the theme of leather testing is continued with an overview of a number of test methods and the associated SATRA test machines which allow a range of different leather characteristics to be assessed.

The properties covered in this article are thickness, tear strength, the ability for the leather to accommodate two-dimensional stretching, resistance to flex cracking, water resistance, permeability and moisture absorption, and rub fastness.

Knowing the thickness of leather is important for footwear manufacture – for example, to determine the allowance in a sole mould or other pattern or when setting a skiving allowance. Linking tear strength with thickness is a good way to assess the intrinsic quality of the leather – the higher the strength for a given thickness of a particular type of leather, the better the leather. SATRA’s STM 483 thickness gauge can be used to measure thickness to SATRA TM1:2004 (2013) – ‘Thickness of leather and insole materials’. With the STM 483 gauge, the thickness is determined using a deadweight and a contact foot of specified size, whilch applies a pressure of 49kPa. Other SATRA gauges are available for measuring textile thickness, although these apply a much lower pressure through the contact foot.

Tear strength

Tear strength is another important parameter to be determined for leather. A number of different tests are available, including the ‘trouser leg’ and ‘wing rip’ tests. However, an assessment developed specifically for leather is the ‘Baumann tear’ test. This is covered by the SATRA TM162:1992 test method – ‘Tear strength – Baumann method’. When conducting this test, a defined shape is punched out of each leather specimen. Specialist jaws (for instance, STM 566ST), fitted to a tensile testing machine (such as SATRA STM 566) allows a load to be applied pulling across the cut-out in the specimen. Typically, two tears are produced – one from each end of the cut-out. The maximum load achieved is recorded, and an average is taken from three specimens in both of the ‘along’ and ‘across’ directions.

For many types of footwear, it is important to be able to stretch leather uppers over the last to create the required shape. The ‘lastometer’ test (sometimes referred to as a ‘ball burst’ test) provides a means of assessing a leather upper’s suitability for lasting by measuring how much it can be stretched in two directions simultaneously (‘distended’) without the leather sustaining physical damage – together with the associated force applied. The SATRA TM24:1992 – ‘Lastometer ball burst test’ sets out the details of the assessment, which can be conducted using the SATRA STM 463 digital lastometer (figure 1).

Flex cracking

One of the important durability tests for footwear leather is the assessment of the material’s resistance to flex cracking. When in use, footwear uppers are repeatedly flexed by the walking action – often many thousands of times a day. The SATRA STM 601 vamp flex test machine (figure 2) was developed specifically to reproduce the inward and outward flexing patterns found in the vamp when shoes are flexed. SATRA’s TM25:1992 test method – ‘Vamp flex test – resistance to creasing and cracking’ may be conducted using the STM 601 machine. It is SATRA’s preferred test for assessing the flexing resistance of footwear upper materials, as the flexed pattern is representative of the flexing patterns which occur in use. The test can be conducted either dry or wet. Testing wet leather also allows an assessment of the risk of salt spue to be made (the release of salts from the leather which result in white ‘tide’ marks).

Another flex test which can be used for leathers is the Bally flexometer test. However, this test is not considered as representative as the vamp test, as it only produces an outward fold. Although it is widely specified for footwear, the Bally flexometer test is more suitable for non-footwear applications, such as the assessment of furniture and automotive leather seat coverings. The SATRA Bally flexometer test is TM55:1999 – ‘Flexing resistance of upper materials – Bally flexometer’, which can be conducted using the SATRA STM 701 machine.

Both the vamp flex test machine and the Bally flexometer are also available as variants, which allow testing to be conducted at low temperatures. SATRA STM 101F is the low temperature vamp flexometer, and the SATRA STM 477F is the low-temperature Bally flexometer.

Resistance to water penetration

Water penetration resistance of leather is an important characteristic for many types of footwear. The two most widely used test methods are referred to by the equipment used, namely the Bally penetrometer and the Maeser. The Maeser (SATRA test method TM34:1993 (2009) – ‘Resistance to water penetration – Maeser test’), which may be conducted using the SATRA STM 606D test machine, is SATRA’s preferred method, mainly because it reproduces the flexing seen in wear better than the Bally penetrometer test. Experience shows that the Maeser test is a better test for the more highly water-resistant leathers. SATRA TM171:1995 – ‘Resistance to water penetration – penetrometer test’, may be conducted using the STM 703 Bally penetrometer test machine and can be used as a general leather water resistance assessment.

Surface pre-treatments – flexing and abrasion – can also be applied prior to water resistance testing. It is important to note that to obtain highly water-resistant footwear, other factors need to be considered, in addition to using water-resistant leathers. These factors include seam sealing and the avoidance of wicking paths from the outside to the lining. For this reason, complete shoe water resistance tests should also be conducted, such as conducting a test to SATRA TM230:2014 – ‘Dynamic footwear water penetration test’ – a test for which the SATRA STM 505 dynamic footwear water resistance tester was developed.

Assessing moisture problems

Another key property of leather is its ability to absorb and transpire moisture. This can contribute to the comfort of footwear with respect to the management of internal moisture, which is present primarily because of sweating. SATRA’s TM47:2002 – ‘Water vapour permeability and absorption test’ allows an assessment of both these properties. The test may be conducted using SATRA STM 175 test machine (figure 3). When conducting the SATRA TM47 test on a specimen of the leather (or the leather together with a lining and hose), the specimen or specimen stack is clamped over the opening of a pot containing a known amount of water. The inside of the leather, (or leather lining and hose combination), is placed towards the inside of the pot. The pot is placed in a water bath within the test machine which maintains the base of the pot at a temperature of 32ºC. The specimen pot represents a sweating foot and the test specimens are exposed to moisture vapour in the same way as they would be in actual use.

During the test (which is conducted in a conditioned atmosphere), the test machine provides a controlled airflow which passes over the surface of the specimen. By weighing the sample material(s) and the pot before and after the test, both the moisture absorption and moisture transpiration through the composite sample can be determined. The weight increase of the leather, linings and hose gives a measure of the moisture absorbed by each of these materials. The overall loss in weight of the pot – complete with specimen – on completion of the test gives the amount of moisture transpired through the sample to the atmosphere.

Having selected leathers and linings based on the SATRA TM47 test, it is also valuable to assess the performance of the whole shoe for moisture management. This is because aspects of the footwear design or manufacture can affect the moisture absorption and transpiration of the footwear overall. A whole shoe test can be conducted using SATRA’s STM 567 Endofoot test machine to SATRA test method TM376:2009 – ‘Advanced moisture management test’.

Colour transfer

In the course of use, leather is often subject to rubbing against clothing or other materials. It is important to assess the risk of colour transfer from the leather onto the contacting surface, as well as the vulnerability to surface marring from rubbing. A well-recognised method to check the colourfastness is using a Crockmeter. This can be carried out to SATRA test method TM167:2001 – ‘Colour fastness to rubbing – crockmeter test’, using the SATRA STD 422 test device. The hand-operated test device induces a backwards and forwards rubbing action between a reference material and the specimen, under a specified load. The test can be conducted wet or dry.

On completion of the test, the reference material is examined for signs of colour transfer. The degree of colour transfer is graded against ‘grey scales’. SATRA supplies grey scales for both colour assessment and staining. The crockmeter – while providing an effective method of assessing colour fastness – is not used to assess surface marring. This can be conducted with either a Veslic or a circular rub fastness machine as described below. SATRA STM 421 and STM 643B are veslic-type rub fastness machines. These apply a backward-and-forward rubbing motion between a reference material and the specimen. The STM 421 is a single-head machine, and the STM 643B is a two-headed machine –one head of which can be heated to assess the surface resistance to heat. SATRA TM173:1995 – ‘Colour fastness to rubbing – reciprocating method’ is applicable to these testing machines. Two other SATRA machines can be used for both colour fastness and marring assessment which use a circular rubbing motion. These are the STM 461 and STM 462 Circular Rub Fastness Test Machines. These machines conduct the same test as each other. However, the STM 462 has additional features to keep the sample cool if it is in danger from overheating due to the rubbing friction. SATRA test method TM8:2004 – ‘Colour fastness to circular rubbing’ is the test method applicable to leather.

These mechanised testers are particularly useful for assessing the marring resistance of leather surfaces – tests which can be conducted wet or dry. They also allow an assessment of colour fastness, although the preferred colour fastness test for this characteristic would be conducted using the crockmeter (SATRA STD 422).

This article has not given an exhaustive list of leather test methods and machines. However, it has focused on a number of the main physical and fastness test assessments which can be conducted on leather.

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Publishing Data

This article was originally published on page 8 of the April 2015 issue of SATRA Bulletin.

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