Improving adhesion of an upper sport shoe

Miglioramento dell’adesione su tomaia di calzatura sportiva

One of the peculiarities of the Nadir Plasma Jet is the ease with which it increases the wettability of countless materials. The treatment, called surface activation, favors the increase of the surface energy of a material, be it a metal, a ceramic, a polymer, or even paper, natural or synthetic fabrics, thanks to the characteristic of being a cold plasma.

Plasma activation makes a surface wettable towards a liquid, be it a glue, an over-injected thermoplastic polymer, or an ink. The increase in wettability leads to a better spreading of the adhesive or ink on the treated surface. The Plasma Jet performs the treatment selectively in the areas where an increase in adhesion is necessary, thanks to the easy automation of the device: it can be implemented in a three-axis plotter or in a robotic arm. It is also possible to interface it with other machines thanks to the Ethernet implementation via TCP-IP protocol.

Water-repellent materials such as thermoplastic films for packaging, painted materials, PTFE, or synthetic fabrics can be treated to increase their surface energy until they became hydrophilic, therefore wettable.

The increase in wettability is generally also possible with flaming, but it involves additional steps, such as masking, to prevent the material from being damaged in the most sensitive areas. Furthermore, the Plasma Jet treatment is a cold process, in which there is no combustion, and the process temperatures can easily be kept below 40°C, in order to extend the treatment to heat-sensitive materials such as synthetic sports fabric.

A concrete example of water-repellent material sensitive to temperatures is the synthetic sports fabric of the upper of the shoes. Water repellency is a fundamental characteristic of these technical materials, therefore they have a low surface energy, consequently surfaces that are difficult to glue.

Thanks to the Plasma Jet it was possible to treat only the areas of the upper (edges and wings) that must receive the glue. A drawing followed by a plotter made it possible to make only the areas to be glued hydrophilic in times of the order of few seconds. The increase in surface energy (evident to the naked eye) was characterized through calibrated inks and water contact angle measurememts. Thanks to the Plasma Jet it is therefore possible to modify the surface of the material in a short time, making it wettable and active towards the subsequent process.

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