New rub fastness test machines
Describing the new SATRA STM 643B and STM 643R test equipment.
by Peter Allen
The ability of footwear leather to resist damage from rubbing is important to the shoemaker, as this can help to avoid product returns as a result of unhappy customers. SATRA provides test machinery to help manufacturers assess the leather used in their products, with the design of two models having recently been updated.
As a part of SATRA’s ongoing development of test equipment, these two new models – STM 643B and STM 643R – have been produced following a review and update of the machine design and methods of construction. In the process, they are brought in line with the constructional style applied to a number of other SATRA machines which have been recently updated.
STM 643B and STM 643R are designed to carry out a rub fastness test on the surface of leather to determine the amount of ‘marring’ of the leather surface or the finish, and to assess the amount of colour transfer from the sample to the rubbing pad. The test can be conducted under dry or wet conditions by using a dry rubbing pad or pre-wetting the rubbing pad in distilled water or a simulated sweat solution, and it can also be carried out using wet leather samples. These machines can also be used to assess the abrasion resistance of insole boards.
STM 643B and STM 643R can be used for a number of tests that require a ‘to-and-fro’ movement across a material surface. The rubbing element, the number of cycles and the load can be adjusted to suit the test requirements. Both of these new machines can be used to conduct tests to EN ISO 11640:2012 (‘Leather – tests for colour fastness – colour fastness to cycles of to-and-fro rubbing’) and VESLIC – C4500 (testing the rub fastness of leather colour), as well as SATRA TM177 (resistance to scuffing by mild to-and-fro abrasion) and EN ISO 20344:2011 (‘PPE – test methods for footwear, clause 7.3: determination of abrasion resistance of insoles’). The latter two test methods are used to assess the wear on insole boards, simulating the scuffing action of damp hose on footwear insoles.
The new machines
STM 643B (which replaces the STM 705) is a two-station model with one station which can be heated. This station contains a heated head which can be used to determine the reaction of the leather surface to the application of heat and pressure, which can be experienced during ‘ironing’ (a process which can be used during footwear manufacture to apply heat to a leather to remove wrinkles). It is also a condition associated with the normal lasting process. Thermoplastic-based finishes are particularly susceptible to damage through ironing. The ironing characteristics of the leather can be influenced by the addition of agents in the finish which help them achieve a leather surface favourable to ironing. A metal foot is provided with the machine to facilitate the ironing process and the temperature of the heated head can be set via the machine controls. This test is covered by VESLIC – C4580.
The STM 643R test machine (which replaces the STM 421) has the same functions as the STM 643B, except it is a single-station test machine and does not have a heated head. Therefore, it is unsuitable for conducting tests to VESLIC – C4580.
The ability to pre-tension the test sample is a feature of both models. This is particularly useful when testing stretchy leathers, and is a requirement of the tests referenced within this article.
Testing a sample
To conduct a test, the sample is placed on the test surface and clamped using the clamps provided. Pre-tensioning can be effected by adjustment of the knob or screw, in conjunction with the percentage scale engraved on the testing table. A counter is fitted to the machine, which allows a predetermined number of cycles to be set and which stops the test once the set number is reached.
When conducting a rub fastness test, a felt pad (such as SATRA STM 421P) which meets the specified characteristics defined within the test standards, is inserted into the machined holder on the end of the rubbing head. This is then lowered to the surface of the test sample. In the case of testing to EN ISO 20344 clause 7.3, an abradant fabric is secured over the felt pad and a load is applied as specified within the test method. The test commences with the activation of the reciprocating motion. On completion of a specified number of cycles, visual inspection is carried out of the material sample to identify signs of marring, and the rubbing pad for evidence of colour transfer, as specified within the individual test method. Grey and colour staining scales are provided with each machine.
The abrasion effect of footwear rubber soling rubbing onto the leather upper can also be tested using these test machines. A small profiled rubber pad is used in place of the felt pad, this test being covered by VESLIC – C4505.
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This article was originally published on page 50 of the July/August 2013 issue of SATRA Bulletin.