Taking a closer look

Outlining some principles of effective microscopy and applications for the footwear and leather industries.

by David Smith

SATRA uses microscopes equipped with high-quality digital cameras to assist customers with sample and specimen analysis. Optical microscopy offers an efficient and economical solution to many quality-control issues and complements other inspection techniques used at SATRA. It is practically instant, requires little or no sample preparation and items can be viewed by the customer as well as the operator at the point of inspection.

Software associated with the microscope allows the taking of measurements from a recorded digital image, thus enabling particle counts, shape perimeter measurements, and the measuring of lengths and areas. It also allows for the superimposition of a scale or a grid over a digital microscope image.

A relatively low magnification and a large specimen viewing area make a good combination for examining features on whole footwear or other products that are not practical to cut and prepare for viewing on a stage-based microscope.

SATRA’s most recent acquisition in microscopy equipment is an Omni-Core system. This has a number of practical features, including interchangeable lenses providing optical magnification up to around 600x, as well as digital magnification possible in excess of this figure.

A live, lag-free video display on an associated monitor permits viewing by multiple users, thus allowing technical discussions to take place while considering microscope images.

One of the main limitations of conventional optical microscopy is the lack of depth-of-field in microscope images, so that often only a thin ‘slice’ of the sample is clearly in focus at any one time. This problem can be overcome with the Z-axis stitching software on SATRA’s microscope, which combines multiple images taken at different depths of focus to create an image that is sharply in focus throughout, from front to back.

Powerful image analysis software allows for the onscreen superimposition of measurements taken from samples, which can be a most useful and striking addition to any technical report.

The light-emitting diode (LED) ring light used for most microscopic examinations can be removed and replaced with an ultraviolet (UV) light. This latter form of illumination has specific use in water resistance analysis, where a small amount of UV tracer dye is introduced into either the water tank of a water resistance test machine, or into a pipette of treated water that is droppered onto the upper of a sample of footwear.

The subsequent passage of this mixture through the outer layers of the footwear and into the interior can then be monitored by observing the footwear under the UV light, where the presence of the tracer dye becomes very obvious (see figure 1).

The imaging head itself also offers some versatility in mounting. The camera head can be removed from the sturdy mount and affixed in any orientation to other brackets, allowing bespoke analysis of samples that could not otherwise be accommodated under a more conventional microscope.

A microscopic examination

SATRA can carry out microscopic inspections of fabrics and threads for quality control purposes, or to examine in detail the effects of simulated weathering or abrasion on materials or products. The inspection of individual footwear components, along with accompanying detailed and annotated photographs, is another valuable service that SATRA can provide. As part of our work on mould prevention, SATRA has used optical microscopy to examine leathers and fabrics contaminated with mould spores.

Other examples of inspection work carried out on our microscopes include the viewing of defects in plastic mouldings to examine the quality of surface finishes – for instance, whether paint or adhesive is evenly applied – and the measuring of hairline cracks or crazing in plastic coatings. The evaluation of crack propagation in weathered or tested plastic items can also benefit from microscopic examination.

By examining cross sections of some coated leathers or materials such as metals or plastics which may be painted, it is possible to take accurate readings of the thickness of the layers of paint or coating. These can be presented pictorially in a SATRA test report.

Stitching, decorative trim or other embellishments are also good subjects for microscopy. This is because the distance between stitches or decorative broguing needs to be consistent between shoes, and can easily be measured by the system.

The human eye has a remarkable range of vision, and remains one of the most effective tools for quality control and specimen inspection. However, viewing an item under even low microscopic magnification can make a significant difference to the level of detail visible and, as a result, to the corresponding value of the assessment.

The inclusion of one or two well-chosen and concisely annotated microscope images within a technical report can greatly assist the reader to understand the technical content, and can also convey technical findings much more effectively than a block of text.

Leather identification

Of particular interest in recent years is the use of microscopy to distinguish between genuine leather and synthetic alternatives. The globally-accepted definition of leather found in European Standard EN 15987:2015 – ‘Leather – Terminology – Key definitions for the leather trade’, states: ‘A general description is a hide or skin with its original fibrous structure more or less intact, tanned to be imputrescible. The hair or wool may, or may not, have been removed. Leather is also made from a hide or skin which has been split into layers or segmented, either before or after tanning’.

One of the key factors that points to a material not being leather is a very consistent appearance or ‘uniformity’ through the cross section. To illustrate this, the cross section of bovine leather (figure 2) shows the non-uniform nature of the collagen fibre structure through the thickness of the leather. In comparison, examples of typical materials designed to look like leather – a fabric coated with polyvinyl chloride (PVC) is shown in figure 3 and a polyurethane (PU)-coated fabric in figure 4. It is obvious from the cross section that the structure is much more consistent in appearance than that of the natural variation noticeable in leather, and the separate layers of coating and textile substrate (base material) are easily distinguished.

Production of fibre board material – also called ‘bonded leather fibre’ – involves tanned hides or skins being disintegrated mechanically and/or chemically into fibrous particles, after which small pieces or powders are then (with or without the combination of chemical binding agent) made into sheets. This contains a minimum of 50 per cent by weight of dry leather fibre. If there is any other component apart from leather fibre, binding material and leather auxiliaries, this must be declared as part of the description.

While any of the imitation leather materials discussed above are perfectly acceptable materials to use in many applications instead of leather, it is important that the user understands that the performance in use will be quite different to that of leather. In many instances, these materials do not demonstrate the same level of durability, tear resistance, flex resistance, breathability or ‘mouldability’ – all of which are benefits of using leather, especially in footwear production. One advantage that these materials do have, however, is the fact that they are produced on a roll or in sheets. Therefore, they can be nested and cut easily with low levels of waste.

As long as there is no intention of misleading the consumer or incorrectly describing a product as ‘leather’ when it is an imitation, there is a place on the market for both leather and synthetic alternatives. However, it is important that technologists fully understand what they are specifying and what performance levels are realistic for that material. Microscopy can assist with accurate product labelling, and SATRA can assist with this if further clarification is required.

Publishing Data

This article was originally published on page 12 of the February 2021 issue of SATRA Bulletin.

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