Types of hot melt adhesive for footwear: TPU, PES, EVA and PBS

What they have in common

A hot melt is a solvent-free thermoplastic polymer. Kyotex sums up how it works in three steps: melt, press, cool. With no solvent, there's nothing to flash off; and because it's thermoplastic, the adhesive can be softened again with heat.

That last trait cuts both ways: it lets you reactivate a bond, but it also means the finished bond shouldn't be exposed to temperatures close to its activation temperature. That's why the polymer should be chosen with the shoe's end use in mind.

Everything you apply is adhesive, too — no part of it evaporates — so there are no solvent fumes at the station. What you do need to watch is running it at the temperature the manufacturer specifies.

TPU: thermoplastic polyurethane

TPU is a thermoplastic polyurethane. It's generally chosen when the bond needs to stay flexible and move with the material, which is common in footwear and textiles. In footwear manufacturing, TPU is among the most widely used bases for hot melt films.

That flexibility also makes it worth considering where the bond has to survive repeated flexing. Kyotex includes TPU in its K Thermo hot melt powder range.

PES: copolyester

PES is a thermoplastic copolyester — a versatile base widely used to bond and laminate textiles and other materials.

In footwear, it's an option to consider for laminating linings and textiles. Kyotex also offers it as a powder within K Thermo.

EVA: ethylene-vinyl acetate

EVA is one of the most common bases for hot melt adhesives. It generally activates at lower temperatures than other bases, which helps with sensitive materials; the trade-off is that its finished bond usually tolerates less heat. That makes it a common starting point when materials are heat-sensitive.

According to Kyotex, EVA powder is used in large volumes in leather, footwear, filtration, luggage and carpet manufacturing.

PBS: polybutylene succinate

PBS is a polyester that Kyotex offers as the biodegradable option in its powder range. It's an alternative for brands looking to reduce the environmental footprint of their materials, as long as it meets what the part requires. As with any adhesive, confirm with your supplier how it performs in your process and how it should be stored.

The four polymers at a glance

• TPU: flexible bonds that move with the material.
• PES: a versatile base, widely used to bond and laminate textiles.
• EVA: activates at lower temperatures; the finished bond tolerates less heat.
• PBS: the biodegradable option in the Kyotex powder range.

How to choose between them

No polymer is better than the others across the board; one is simply a better fit for each job. Before requesting samples, look at:

• The materials you're bonding and the temperature they can take.
• How flexible the bond needs to be: an insole and a reinforcement have different needs.
• The shoe's end use: whether it will see heat, moisture or washing.
• Your equipment: the press or laminator has to reach the temperature and pressure the adhesive calls for.
• Your sustainability goals: if you're after a biodegradable option, consider PBS.
• Your volume: the format and the equipment should match how many pairs you run per shift.

With those answers, your supplier can recommend a polymer and a format. Then confirm it with a trial on your line and your usual quality tests.

Formats: powder, film, web, net and granules

The polymer is only half the decision; the other half is the form the adhesive comes in. K Thermo is available in five formats:

• Powder: scattered over the material and activated in the press. According to Kyotex, insole bonding is where it performs at its best.
• Film: a continuous layer of adhesive.
• Web: a fine mesh of adhesive fibers.
• Net: an open adhesive mesh.
• Granules: melted in equipment before application.

Film, web and net are common in lamination; powder in solvent-free insole bonding.

Where they're used in footwear

• Insole bonding: powder or film to bond the insole.
• Lamination: joining linings, textiles and synthetics before cutting.
• Reinforcements: K Pads reinforcements come treated with Kyotex hot melt or cold adhesives.
• Leather goods: Kyotex lists bonding PVC to leather among the uses of its powder range.

Kyotex also publishes uses outside footwear, such as lingerie and stretch fabrics, automotive and carpet.

Common mistakes when choosing a hot melt

• Choosing out of habit. The polymer that worked on one style won't always work on another with different materials.
• Thinking only about the press. The bond also has to survive the shoe's life: flexing, heat, moisture or washing.
• Not adjusting the process. Each polymer needs its own mix of temperature, pressure and time; using another adhesive's settings gives weak bonds.
• Testing only straight out of the press. Also evaluate after the part cools and rests.
• Checking only one side. The adhesive has to work with both materials it joins.

How to run the trial

1. Use the same materials and the same style you produce today.
2. Set temperature, pressure and time to the adhesive manufacturer's instructions.
3. Evaluate the bond after cooling and after your normal dwell time.
4. If the part flexes in use, flex it before you sign off on the result.
5. Compare against your current method and log the results for each material combination.

What if the material can't take heat?

Then a hot melt isn't the best choice. Look at K System pressure-sensitive tape or K Next cold bonding, and read how to choose a shoe adhesive by material.

Next steps

To compare hot melt with conventional cement, read hot melt vs. solvent-based shoe adhesive. If you already know what you need, request a quote and a Kyotex specialist will recommend the polymer and format for your process.