Although footwear design does not appear to have changed significantly over the last century, other than to meet the demands of fashion, the techniques used in the manufacture of shoes have undergone revolutionary changes in that time. The construction of modern footwear involves a widespread use of adhesives, the major exceptions being the sewing of uppers and in some cases, the attachment of heels with nails, see below.
This has resulted in a reduction in the production times for each item, as well as a reduction in the skill levels required for many of the operations. Of these adhesive joints, probably the most demanding joint is that between the sole and the upper material, and it is this joint that is considered in the forensic study. For regularly used items of footwear, the life expectancy is on average one year from purchase. During this time, the adhesive joint is subjected to a range of chemical and environmental conditions, depending on the nature of the shoe. The joint is often subjected to moisture, both from the environment, and perspiration from the wearer. Despite this, the joint generally performs well in service, with relatively few failures.
Five pairs of shoes, obtained from the Clarks customer services department, were selected for detailed investigation. The shoes had typically seen up to between six months and a year of use before being returned. The shoes were subsequently stored in a protected, internal environment. For the study, shoes were selected where the disbonding of the sole did not appear to be due simply to adherend failure, and those where the wear on the soles and in-soles indicated that the shoes had been extensively worn prior to their return
Leather to PVC
One part polyurethane
Mechanical roughing of leather
Solvent wipe / halogenation of sole
As with all adhesive joints, correct surface preparation is of paramount importance to achieve the level of performance required from the adhesive bond. In this case, the area of the leather uppers to be bonded was prepared by a process of mechanical roughing. This removes the surface finishes, as well as removing the weakly attached surface grain layer of the leather and reveals the stronger underlying fibres. The preparation of the soles depended on the material being used. The most common processes used were solvent wiping of PVC soles to remove surface plasticiser and a halogenation process which saturated the butadiene in rubbers, thus making the material more compatible with the adhesive.
The adhesive generally used was a one part polyurethane, containing approximately 20% solids in a solvent base, often methyl ethyl ketone (MEK), which had the desired properties of reasonable gap filling properties, combined with good spot strength (green strength). A layer of adhesive was applied to the perimeter of both components (approximately 10 mm wide) and allowed to dry. Prior to assembly the adhesive was heat activated, on at least one surface, by passing it under infra red heaters to raise the adhesive temperature to 80ºC. The two components were then brought together and pressed for approximately 15 seconds to form the joint.
Activation at 80 ºC, clamped for approximately 15 seconds
The bonding excluding later repairs would have been icarried out at the factory.
Laboratory tests indicated that typically the sole/ upper joint peel strength was around 5 N/mm.
A pair of boys school shoes was selected for analytical testing, on the grounds that these were the most likely to have seen the most demanding environmental exposure. Inspection following peel testing showed what appeared to be interfacial failure. Results of SEM and XPS analyses, however, showed that some of the surface PVC of the sole had been removed with the adhesive, indicating a good level of bonding. The various experimental techniques were unable to identify environmental degradation as a significant cause of joint strength degradation. This is backed up by laboratory experimentation, where it was shown that, in the medium term, polyurethane adhesive joint strength is relatively un-affected by exposure to moisture at natural ambient temperatures. The samples taken, however, are not fully representative as they all come from shoes returned within one year. Many shoes last longer than this and it is not known what the eventual cause of failure of these shoes would be.
Additional peel test data was as follows:
Regularly used items of footwear, have an average life expectancy of one year from purchase. During this time, the joint between the sole and the shoe upper is subjected to a range of chemical and environmental conditions, depending on the nature of the shoe.
The use of adhesives in this demanding application has significantly reduced the time and cost of assembling shoes. Despite the joint being regularly subjected to moisture, both from the environment, and perspiration from the wearer, the results of the investigation have shown that the majority of premature failures that occur are due to errors in the original bonding process, for example insufficient or over enthusiastic surface preparation, rather than environmentally provoked failure of the adhesive.
The majority of premature failures that occur were due to errors in the original bonding process rather than environmentally provoked failure of the adhesive.
Common causes of failure were insufficient or over enthusiastic surface preparation,
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints. Part 1 - General Introduction and Conclusions Part 4 – Footwear, NPL February 1996
