Sole Bonding: Building Shoes That Last

Current data suggests that, on average, over 15% of footwear fails in testing because of poor sole bonding. In conversations with leading retailers and brands, sole bonding is often identified as the principal reason for returns. It is therefore safe to assume that, in reality, the true rate of failure due to inadequate sole bonding is far higher than the 15% failure rate we see in controlled testing.

What causes the failure?

We first need to understand the type of failure. It is often assumed that failure in sole bonding is caused by the adhesive failing. Since the bond will only ever fail at the weakest point, it is therefore justifiable to ask whether it is a failure in the adhesive or in the material.

When inspecting footwear to determine the cause of the failure, we meticulously analyze the point of failure to determine the root cause of the problem. The most common forms of failure found are:

• 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

How do we solve the problem?

Good factory management, quality assurance and in-line inspections can all help to alleviate the problem. From the development stage, manufacturers should understand the performance of the materials they select and make sure the materials are appropriate for their minimum sole bonding requirements. Failure to do this will create future problems.

It cannot be over emphasized that perfect preparation is absolutely critical. Cutting corners and not following an analytical approach to preparation will cause sole bonding failures.

The following are some typical guidelines on how to prepare uppers and outsoles.

Upper materials

• Leather: must 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

Outsole materials

• 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

In production

During production there is always pressure to produce the maximum number of shoes per day. Brands and retailers want shorter lead times to meet ever increasing demands and reduce overall costs in the supply chain. Sole bonding can be a slow procedure but cutting corners is not the answer. Without correct preparation, issues may not become apparent until the product has left the factory and it becomes too late to make corrections.

Sole bonding operators need to be educated in the footwear technology of sole bonding, not just how the machine turns on and off, and how to present the shoe to the machine. Operators need to be supported in understanding the science behind adhesives and manufacturers must have the correct systems in place to ensure effective sole bonding.

As an example, some factories use a conveyor belt system with the heat activation unit. This will 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.

One way to solve the sole bonding failure problem is to add a UV component to the adhesive solution and then give the operator a UV or Black Light to check the adhesive coverage. Other points that need to be taken into consideration to ensure effective sole bonding are: checking the temperature of the heat activation unit, verifying the correct activation time and open time of the adhesive with the supplier and understanding the correct pressures on the sole attaching machine. A simple in-line test is the carbon paper test which can 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 need to make sure the dwell time on the sole attaching machine is correct and that the operator is not tripping the machine before the set dwell time, as the adhesive needs constant pressure and time to coagulate and start the cross-linking process. Early removal from the sole attaching machine will shorten that time and deactivate the process before it is finished.

Manufacturers also need to consider the machine being used for the application of the outsole. Flat, low-heeled, high-heeled, wedge-platform and walled outsoles will each require different machines to ensure effective application. Using one machine for all types will give inconsistent results.

Good sole bonding is critical in the footwear industry. It is the number one issue in shoe manufacturing. Customers returning broken shoes will damage the reputation of a retailer and a brand, translating into a loss in brand confidence and loss of sales. To avoid this, keep a close eye on production, train your 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.

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For more information contact your local SGS representative, or our global team and visit Softlines and Accessories.

Paul Bridge
Global Business Manager (Footwear and Leather)
SGS United Kingdom Limited
t: +44 (0)203 008 7860