Colour fastness and rub testing

SATRA test machines for the assessment of colour fastness and resistance to marring.

by Peter Allen

In use, footwear and other leathergoods are often subject to rubbing from clothing or other materials. For example, garments may rub on shoe uppers; wallets and purses might rub against the linings of pockets; shoulder straps of bags may rub on a coat and belts might rub where they make contact with the buckle.

To provide durable products which maintain quality of appearance over their intended life, it is important to ensure that the surface of the materials used demonstrate colour fastness and resistance to surface finish marring (rub-induced surface damage). Materials which are not colour fast can also result in unsightly colour transfer from the leathergoods or footwear onto wearer’s hose or other clothing or surfaces with which they come into contact, such as furniture upholstery or carpets.

SATRA has developed a number of test methods and manufactures a range of test machines to help determine colour fastness and marring for leather or synthetic materials used in footwear and leathergoods products. This article gives an overview of these SATRA test machines.

Assessing colour transfer

SATRA’s STD 422 Crockmeter determines the degree of colour transfer from one material to another. The SATRA TM167 test method is conducted using this test equipment. STD 422 is a hand-operated machine which applies a known load between the material to be investigated and a white or black reference material (depending on the colour of the material being investigated). During operation of the machine, a translation (forwards and backwards) rubbing action is introduced between the material and the reference material. This test is a mechanisation of the traditional method used for checking colour fastness which involves rubbing the material with a finger covered in white cloth. On completion of the test, the reference material is inspected for evidence of colour transfer from the material under test. The degree of any colour transfer to the cloth is graded using comparative ‘Grey Scales’. The Grey Scale rating was developed to allow a repeatable numerical value to be assigned to the subjective assessment of the staining of fabrics by dyed materials. It can also be used to assess the actual change in colour of the same dyed materials when subjected to various treatments – for instance, rub fastness. The Grey Scale covers the numerical values from one to five, including half steps. Grade 5 indicates that no change has taken place, and Grade 1 shows the greatest change. SATRA supplies two versions of Grey Scale: STD 461GC (for colour assessment) and STD 461GS (for staining).

The SATRA STM 461 and STM 462 test machines provide another method of assessing colour fastness, as well as marring (surface marking) resistance of material surfaces. The test methods SATRA TM8 for colour fastness and SATRA TM14 for scuffing resistance are conducted using these machines. Carried out in wet or dry conditions, these tests simulate the condition where a more significant level of repeated rubbing can occur between the material in question and another surface, and were developed originally for footwear applications.

Both SATRA STM 461 and STM 462 machines consist of a motorised rotating head holding a felt pad which rubs on the material under test. A controlled load is applied between the pad and the material under test and a pre-determined number of rotations are set. On completion of the test, the change in appearance of the finish of the material on test and any colour transfer to the felt pad are assessed by reference to a standard scale. Additionally, the STM 462 test machine provides an automatic raise and lower facility between the felt pad and the specimen, as well as an adjustable airflow across the specimen to prevent overheating of the sample during the test. This facility is valuable, as heating can occur due to the friction energy generated between the two surfaces.

In addition to the STD 422 Crockmeter, SATRA also manufactures two other versions of a reciprocating motion rub fastness tester – SATRA STM 421 and STM 705. These machines were originally developed in Switzerland and became popular with tanners for assessing colour fastness and scuffing resistance. The STM 421 machine, to which the SATRA TM173 (colour resistance) and SATRA TM177 (scuffing resistance) test methods are applicable, applies a set load between the sample material under evaluation and a felt pad. When activated, the machine applies a reciprocating rubbing action between the pad and sample material for a predetermined number of cycles. The tests can be conducted under dry or wet conditions. On completion of the test, the felt pad is then visually checked for colour transfer and can also be checked against Grey Scales.

The SATRA STM 705 test machine is a two-station version of the STM 421, but with the additional feature of one station being heated for ironing tests. This test is used to determine the reaction of the leather surface to the application of heat and pressure – conditions encountered during ironing, as well as being associated with the normal lasting process. Thermoplastic-based finishes can also be susceptible to damage when ironed. A metal foot is provided with the machine for use in the ironing process.

Another use of these reciprocating motion rub fastness testers is to test for the rubbing of uppers by soling material, which can typically occur when the wearer crosses his or her legs, placing one shoe on top of the other. To conduct this test, the felt pad is replaced with a profiled rubber pad. An additional feature of these test machines is the ability to pre-tension the sample material as specified in the tests. This is particularly useful when testing leathers characterised by high elasticity.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 36 of the October 2011 issue of SATRA Bulletin.

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