Using good reference materials

The reference materials used in vital footwear tests can greatly affect the results.

Ensuring reliable, accurate and repeatable testing relies on many factors, including the quality of any reference (or quality control) materials associated with the test method. However, the importance of using appropriate, tightly specified and controlled reference materials is often overlooked or misunderstood.

What are reference materials?

For the purposes of this article, a reference material is taken as being an example of a ‘consumable’ that is used as part of the test procedure in many footwear and material standards.

These materials are by necessity carefully specified in the relevant test methods, and such specifications must be complied with if the test results are to be reliable and correlate well with other laboratories. Of course, no documents are ever totally free from errors, and there have been instances of problems in material specifications in published test methods. However, these anomalies are rare and should become apparent when the materials are checked – as discussed later in this article.

Examples of common reference materials are:

abradant cloth (or paper), supporting felt and backing foam for the Martindale abrasion
control rubber and abrasive paper for solings abrasion
calibration rubber, abrasive paper and flooring surfaces for footwear slip resistance
control cloth and blades for glove cut testing
colour transfer pads and fabrics for rubbing, water and perspiration fastness
‘grey scales’ used to assess colour changes
analytical reagents used in chemical testing
lamps for light fastness and ozone test chambers.

While this list is far from exhaustive, it clearly demonstrates the wide-ranging role these materials play in the performance of so many common test procedures. Whatever their role, if these materials do not conform to the specifications required by the test methods, the test will either not proceed correctly or the result will be incorrectly interpreted. Test results will therefore be unreliable, highly variable or both.

Fitting a test tile into a SATRA STM 603 slip resistance tester

Specification checks

Whenever new supplies of materials are delivered to a laboratory, it is essential that they are checked before use in order to ensure that they meet with the requirements given in the test procedures. Indeed, if the laboratory is working to a formal quality management system (for example, as a SATRA accredited laboratory or ISO 17025), it is a requirement that all reference materials are fully ‘controlled’. This means that they have to be registered (along with all relevant details such as the supplier, delivery date, batch number, grade or quality and shelf life if relevant), marked with their name, delivery date and batch number and then checked in some way to ensure that they conform to the specification. Evidence of this conformity must also be recorded, to create an audit trail back which proves at a later date that the correct materials were used during testing done on an earlier occasion.

Reputable suppliers will ensure that all their reference materials are supplied with a ‘Certificate of Conformity’. When the supplier is known to be reliable, these certificates can be used as evidence of suitability. The best way to establish reliability is to conduct in-house checks on the supplier’s materials. The frequency and scale can be reduced if all checks yield good results. Conversely, they should be increased if problems are then found.

Where certificates are not available from suppliers, the laboratory’s quality management system will require specification checks on the delivered materials. However, if the packaging displays information confirming that the materials conform to a relevant standard or have particular properties then the packaging itself (or a copy of it) may be used in lieu of a formal certificate when backed up by periodic testing as described above.

In all cases, the level of checks conducted on materials should be based on a risk assessment. This should take into account the criticality of the material to the test method, the manufacturing controls in place, previous results and information available from the supplier.

Test methods vary

It must be remembered that the requirements for reference materials are specified in the individual test method. It must not be assumed, however, that if a laboratory is carrying out similar test procedures to two or more different methods (perhaps required by different customers), then the requirements for the reference materials are the same for all. It is very often the case that similar methods will have different specifications for the reference materials, which makes it essential that the materials are checked against all the relevant specifications.

A good example of this may be found in the requirements for the Martindale abrasion supporting felt. There are three commonly quoted test methods for Martindale, all of which have slightly different requirements, as shown in box 1.

Box 1: Differing requirements for Martindale supporting felt
Test method	Specified thickness (mm)	Specified mass per unit area (g/m²)
SATRA TM31	2 to 3.5	575 to 800
EN ISO 12947-1	2.5 ±0.5	750 ±50
EN ISO 20344	3 ±0.5	750 ±50

Although it is possible to acquire felt which meets all three specifications, it is essential that each batch is checked against the requirements of the method or methods which are being performed in the laboratory.

Counterfeit materials

In recent years, SATRA has become increasingly aware of the proliferation of poor quality or counterfeit reference materials being sold in many countries. Some reference materials are inherently expensive to produce to tight specifications, and where these materials are offered at lower prices, they find a ready market among budget-conscious laboratories. If checks are not made, sub-standard materials could contribute to incorrect results.

The use of cheaper, non-compliant reference materials is a false economy. This is because the resulting unreliable results could easily lead to good quality products being rejected from production, or poor-quality materials being passed for finished product use (which could subsequently not pass another test or even fail while being worn). Both of these outcomes could be very expensive for the manufacturer in terms of both financial losses and growth of a poor reputation.

Storage and care of reference materials

Being such a valuable and necessary resource, laboratory reference materials must be stored correctly. For some, this will require keeping them in a cool, dry and dark environment to prevent damage from excessive heat, moisture or light. However, the best conditions for each will depend on the type of material. Most materials may be stored in the controlled environment of the laboratory.

Laboratory staff must also be aware of any limitations on the shelf life of these materials. While fabrics and abrasive papers will remain in good condition for many years if stored correctly, rubber materials are prone to ageing and will have specified time limits on their usage. This ageing may be slowed by storing at lower temperatures, but expert advice must be obtained about any effects this may cause. Also, lamps used in test equipment will always have recommended maxima on their hours of use, as their light output will diminish with use. Reference standard chemicals may require additional safety measures for their storage, as these may be toxic, flammable or highly reactive. Whatever the material, reputable reference material suppliers will be able to advise on storage conditions and SATRA is always willing to help in this respect.

SATRA quality reference materials

SATRA can help with all aspects of this important area of laboratory testing. Advice can be given on the requirements, use, storage and expiry of all necessary reference materials.

It is important to always use high-quality reference materials to ensure accurate test results

Most importantly, however, SATRA can supply many different reference materials for a wide range of tests applicable to leathers, textiles, rubbers, footwear, leathergoods, gloves and many other consumer products, as well as personal protective equipment (PPE) items. SATRA reference materials are ‘second to none’ – manufactured to quality standards, as well as checked and certified (where applicable) before despatch, with relevant certificates supplied with each order.

Purchasers of SATRA reference materials can be confident that the materials they buy have benefitted from SATRA’s extensive research programme and our many years’ experience of test method and machine development, coupled with the experience of testing in our own laboratories.

How can we help?

Please contact test.equipment@satra.com for more information or to order reference materials.

Publishing Data

This article was originally published on page 10 of the November 2021 issue of SATRA Bulletin.