Evaluating rub, abrasion and scuff resistance
Investigating the SATRA test methods and equipment on which these assessments can be conducted.
There are a number of ways that rubbing or scuffing actions can damage footwear – or footwear can affect other items. The causes of damage to footwear are numerous, including rubbing on chairs and tables, against clothing, against the other shoe or boot, and coming into contact with a range of surfaces encountered in walking. Some problems are the result of one-off incidents (such as striking a kerbstone), whereas other damage is caused over a longer period of repetitive contact – for instance, rubbing against external clothing or against hose inside the shoe.
As highlighted above, the footwear can also cause problems to other surfaces. This is typically the result of colour transfer from footwear to clothing, furniture or floor coverings, or the marking of flooring by soling materials.
While it is not possible to prevent all damage from occurring, it is possible to ascertain if a material provides a good level of resistance to scuffing or colour transfer onto other materials, depending on the application for which the footwear is intended.
Some types of footwear are particularly liable to exposure to scuffing – for example, children’s footwear, safety or industrial boots and footwear for military applications. These will typically require more severe scuffing tests than for general, everyday footwear. SATRA has developed a range of test methods and associated test machines that can be used to simulate the damage caused in a range of situations encountered in use. This article outlines these SATRA tests and the associated test machines.
SATRA STM 633 Martindale abrasion machine
The SATRA TM31:2003 (2014) test method (‘Abrasion resistance – Martindale method’) – shown in figure 1 – allows an assessment to be made of the resistance of a material to the repeated mild abrasions which are encountered during everyday wear, under both dry and wet conditions. It is applicable to textile, coated fabric and coated leather for footwear ‘outers’ (the outer surface of the upper materials), and for all materials used in linings and sockings. The Martindale test is not recommended for conventionally-finished leather for outers, as the wet test in particular is too severe and will remove the finish more readily than will occur in wear. The test involves visual assessments of damage carried out on the test specimens at pre-set intervals. The amount of wear is assessed subjectively, as well as any pilling (in textiles) and/or colour change.
SATRA STM 421 and STM 643B rub fastness testers
Both the SATRA STM 421 (seen in figure 2) and the SATRA STM 643B test machines can be used to conduct tests described in SATRA TM173:1995 – ‘Colour fastness to rubbing – reciprocating method’, which is a to-and-fro rub fastness test (also known as the ‘Veslic’ or ‘IULTCS’ test). The assessment involves a mild form of abrasion similar to the SATRA TM8 circular rubbing test and is also mainly used for outer leather. It reproduces the repeated mild abrasions that are encountered during normal everyday wear and uses wool felt rubbing pads under dry and wet conditions.
In this test, a felt pad moves backwards and forwards over the leather surface. The leather is clamped over a flat horizontal table and stretched linearly, in the direction of rubbing, normally by 10 per cent. This is to keep the specimen taut and prevent it from rucking in front of the rubbing pad. A specified load is applied to the specimen during the travel in the fore-and-aft directions. After testing, in both wet and dry conditions, any marring of the surface of the leather is assessed by comparing the tested and untested areas against a grey scale. Again, the pads may also be assessed for staining, but the SATRA TM167 test using a crockmeter is the best test for this property.
SATRA STM 643B is a two-head machine, one of which can be heated to assess a material’s response to applied heat – for example, from ‘ironing’ or during the lasting process when the footwear is being manufactured.
SATRA circular rub fastness testers
The vulnerability of materials to surface marring can be assessed by using the SATRA TM8:2004 test method (‘Colour fastness to circular rubbing’), using either the SATRA STM 461 or STM 462 circular rub fastness tester (also shown in figure 2). The STM 462 tester is the same as the STM 461 unit, but with the addition of the means to apply cooling to the surface of the test specimen during the test. SATRA TM8 is generally used to assess the outermost surface of conventional finished leathers for uppers, which normally have little resistance to harsh abrasion. It reproduces the repeated mild abrasions that can occur during normal, everyday wear.
When conducting the test, a specimen of leather is rubbed by a revolving wool felt pad under a specified load and in dry conditions. Wet tests are also carried out after soaking the felt pads in distilled water. The wet test is conducted with an applied, but lower, specified force compared to the dry test. On completion of a test, any marring is assessed by comparing the tested and untested areas against a grey scale for colour change. The felt pads may also be assessed for staining using grey scales, but this test is less accurate at predicting this than when testing with the crockmeter in line with SATRA TM167.
SATRA STM 423 chisel scuff tester
SATRA TM140:1996 – ‘Scuff resistance – chisel method’ is a very challenging test for a material, and it reproduces severe snagging. The test is too severe for many materials, and is generally used only for those claimed to have a high level of abrasion resistance such as polyurethane (PU)-coated leather and heavy-duty coated fabrics. The assessment can be conducted using the SATRA STM 423 chisel scuff tester (figure 3). The test specimen is mounted on a turntable and clamped down. A weighted metal chisel head is lowered carefully onto the surface of the material as soon as the specimen begins to rotate, and is lifted off before the rotating turntable is stopped. The test is terminated when a small amount of the coating is completely removed, exposing the underlying substrate. The number of revolutions produced is used to classify the material being tested with regard to its abrasion/scuff resistance – whether ‘moderate’, ‘good’ or ‘excellent’.
SATRA STM 425 drum abrasion-snag tester
The SATRA TM95:2020 test method (‘Abrasion and snagging resistance – drum method’) is a fairly severe abrasion test, which falls in terms of severity between the Martindale abrasion test and the chisel scuff test. The SATRA TM95 test reproduces rapid glancing blows or direct impacts against various hard objects – such as may be encountered by children’s footwear in everyday wear and in footwear worn during many sporting activities. The test is too severe for many materials, and is generally used only for those materials claiming to have some degree of abrasion resistance, such as PU-coated leather or heavy-duty coated fabrics. The test can be conducted using the SATRA STM 425 drum abrasion-snag tester (figure 4).
Specimens to be tested are wrapped around and bonded to short cylindrical bars. Four of these assemblies are placed in a rotating drum with internally fixed bars placed around the inside circumference. Granite chippings as specified in the test method are placed in the drum, which is then sealed. The drum is fitted to the test machine and rotated at a defined speed for a specific time. The internal fixed bars have the effect of lifting the granite chippings which then fall back. The chippings impact with and rub across the surface of the specimens under test. On completion of the test, the specimens are removed from the steel bars and cleaned to remove the granite dust. They are then assessed as set out in the test method. The assessments are used to classify the materials with regard to their abrasion/snagging resistance, and also to assess how easily the damage may be repaired by simulating treatment with a shoe care product, such as a shoe polish.
SATRA STM 638 three-station rotary abrader
SATRA TM163:1996 – ‘Abrasion resistance – Taber method’ (figure 5) was adapted for use with footwear upper and lining materials from a general abrasion test developed in the USA for assessing floorcoverings. However, it is not used by SATRA as a routine test for footwear materials, although Taber abrasion tests do appear in some specifications used in the footwear industry. To conduct a SATRA TM163 test, the specimen is secured to a turntable which rotates horizontally. Two specified abrasive wheels mounted on swinging arms are lowered onto the specimen with a defined applied load. The positioning of the abrasive wheels produces a rolling and shearing action across the material under test. Various grades of abrasive wheels are available for different applications.
SATRA STD 165 impact scuff tester
The SATRA STD 165 device operates by allowing a falling pendulum to strike a surface. There are two SATRA test methods associated with this machine. One is SATRA TM38:1999 – ‘Impact scuff test for upper leather’, which is a less commonly used test method for scuff assessment compared to other tests referred to in this article. In the SATRA TM38 test method, the head of a swinging pendulum of defined geometry is allowed to impact the footwear upper material that is mounted in the SATRA STD 165 device. Scuffing of the leather is assessed by comparing scuffed and unscuffed areas against a grey scale along with recording a visual assessment of any other type of surface damage.
The other test method (SATRA TM223:2020 – ‘Floor marking by solings or top pieces’) allows the assessment of the risk of a shoe bottom material marking floorings during wear. In this test, prepared specimens of the soling material to be assessed are mounted to a swinging pendulum. Standard reference surface specimens (black or white) can be mounted to act as the contact surface. The results of these tests permit a determination to be made of the risk of floor marking from the soling material under investigation.
SATRA STD 422 crockmeter
In the course of use, coloured materials such as leathers or fabrics are often subject to rubbing against clothing or other materials. It is important to assess the risk of colour transfer from these materials onto the contacting surface from rubbing. SATRA TM167:2017 – ‘Colour fastness to rubbing – crockmeter test’ allows an assessment to be made of colour transfer. The STD 422 crockmeter is a hand-operated device which produces a backwards and forwards rubbing action between a reference material and the specimen under a specified load. The test is conducted under wet and dry conditions. On completion of the test, the reference material is examined for signs of colour transfer and graded against grey scales for staining.
Selecting the appropriate test – and the correct test machine – for a particular type of material and its application is important to obtain meaningful assessments of products with respect to their colourfastness or resistance to marring or scuffing.
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This article only gives an outline of the individual tests and the associated test machines. Please email email@example.com to find out more about a particular test or firstname.lastname@example.org for more information on SATRA test equipment.
This article was originally published on page 12 of the October 2020 issue of SATRA Bulletin.