Geneva, Switzerland – The number one reason for shoe failure is a weak bond between the outsole and the upper – 15% of footwear fails in testing are caused by poor sole bonding. Leading retailers and brands report that sole bonding is often identified as the principal reason for shoe returns. We can assume, therefore, the true rate of failure is probably far higher than the 15% failure rate we experience in controlled testing.
The causes of failure are numerous. The common assumption is that sole bonding failure is a result of adhesive failure but, since the bond will only ever fail at the weakest point, it is justifiable to ask whether it is a failure of the adhesive or of the material.
SGS has meticulously analyzed the point of failure in many shoes and found a number of different causes:
- Adhesive peeling from upper – identified by no adhesive remaining on the upper material and a solid coating of adhesive on the outsole
- Upper surface tear – upper material is the weak point and has torn away from the adhesive bond, leaving all adhesive and a small amount of upper material still attached to the outsole
- Adhesive non-coalescence – adhesive bond has completely failed, leaving adhesive on both the outsole and the upper material
- Adhesive peeling from outsole – adhesive has not bonded to the outsole, leaving the adhesive securely attached to the upper material, with little or no material remaining on the outsole material
- Sole surface tear – outsole material has torn away and the adhesive layer remains attached to the upper material
- Adhesive breakdown – adhesive has failed to cross link and coagulate, leading to breakdown of the bond
To solve this problem, businesses must employ good factory management, quality assurance and in-line inspections. During the development stage, manufacturers should look at the performance parameters of the materials they select and make sure the materials are appropriate for their minimum sole bonding requirements. Failure to do this will create problems in the future.
The key to manufacturing good quality shoes is perfect preparation. Cutting corners and avoiding an analytical approach to preparation will cause sole bonding failures. Here are some basic guidelines on how to prepare uppers and outsoles.
- Leather: needs to be roughened to the corium layer and must not simply have its surface coating and grain layer removed
- Coated fabrics: the finishing layer can be removed either by roughening or by using a solvent wipe. Care must be taken to not damage the base substrate
- Textiles: materials with a smooth surface or a top finish will need a light scouring to remove the surface material and potential contaminants
- Leather: normally split to provide a uniform material for bonding. The surface should be roughened and any excess fibers removed
- Thermoplastic rubber (TR or TPR): treat with a halogenation primer
- Resin rubber and micro cellular rubber: scour, roughen or split the rubber and treat with a halogenation primer
- Molded rubber units: halogenation is required (may also roughen or scour prior to halogenation)
- PU: where possible roughen the surface. Alternatively apply a solvent wipe
- Microcellular EVA: lightly roughen the surface and apply an appropriate EVA primer
- PVC: remove surface contaminants with a solvent wipe. The cloth should be replaced, at least, after every ten uses to avoid contaminants being transferred from one outsole to the next
Once the materials have been prepared, care should also be taken during the production phase. This includes being aware of problems resulting from too much pressure being placed on the production line. Brands and retailers always want shorter lead times to meet ever increasing demands and reduce overall costs in the supply chain but it is a false economy to cut corners. Sole bonding can be a slow procedure but, without correct preparation, untold damage can be done to a brand if a faulty product is released onto the market.
Sole bonding operators must also be educated in the technology behind sole bonding, not just how the machine is turned on and off. Operators need to know how to present the shoe to the machine correctly and understand the science behind adhesives.
Manufacturers need to also have the correct systems in place to ensure effective sole bonding. For example, in a factory using a conveyor belt system with a heat activation unit, the system may produce three or four shoes at a time. Because the factory is hot, the operator will use a fan to cool themselves down. The ambient temperature of the factory and the effect of the fan will immediately cool the surface temperature of the adhesive. This is guaranteed to cause a sole bond issue on the shoes that have cooled the most – for example, the third or fourth shoe in each batch. The problem is not the fan, it is the bottle-neck of shoes coming out of the heat activation unit at a speed beyond the ability of a single operator to move all shoes to the next step in the same amount of time.
The problem of poor sole bonding can be answered by adding a UV component to the adhesive solution and then giving the operator a UV or Black Light to check the adhesive coverage.
Manufacturers should also consider:
- Checking the temperature of the heat activation unit
- Verifying the correct activation time and open time of the adhesive with the supplier
- Understanding the correct pressures on the sole attaching machine
A simple in-line test, the carbon paper test, can also be used to see whether sufficient pressure is being applied in the correct areas. The carbon paper creates a clear image of the pressure areas.
Manufacturers must also ensure the dwell time on the sole attaching machine is correct and that the operator is not tripping the machine before the set dwell time, because the adhesive needs constant pressure and time to coagulate and start the cross-linking process. Removing the shoe too early will deactivate the process before it is finished.
At the same time, manufacturers need to consider the machine being used for the application of the outsole. Different types of outsole, flat, low-heeled, high-heeled, wedge-platform and walled, will all require a different machine to ensure effective, consistent application.
With poor sole bonding being the number one issue in poor shoe manufacturing, it is important companies put into practice good processes. Broken shoes will damage the reputation of a retailer and a brand, and this translates into a loss in brand confidence and a loss of sales. Manufacturers are advised to keep a close eye on production, train their operators to understand the science behind adhesion and then learn to identify the root cause of sole bonding failures. This will help solve the problem and help build consumer confidence.
SGS Softline & Accessories Services
With a global network of over 40 laboratories, manned by a team of professionals from multi-disciplinary backgrounds, SGS provide a comprehensive range of physical, chemical and functional testing services for components, materials and finished products. They help manufacturers ensure quality, performance and compliance with international, industrial and regulatory standards worldwide. To learn more about SGS’s Softlines & Accessories Services.