How hydrophobic coating works
Since the Covid-19 pandemic introduced restrictions on our movements and activities, for many people, myself included, taking a walk has become a good alternative for getting fresh air and exercise.
At first, I thoroughly enjoyed my walks, as spring had just begun. Now, however, the weather is changing, making the idea of going for a walk less appealing.
Thankfully, water-repellent and waterproof clothing exists.
What fascinates me is how this type of clothing is made. How does fabric become waterproof or water-repellent while remaining breathable? In other words, what process is responsible for this feature? Hydrophobic coating is one answer to this question.
In today’s post, I’d like to share a successful application of hydrophobic coating using the controlled evaporation and mixing (CEM) system.
Hydrophobic coating
Waterproof yet breathable clothing prevents rain from penetrating the fabric while allowing water vapor to escape, a highly desirable feature in sportswear. But how can fabrics and textiles be made hydrophobic or given other functionalities without affecting the fibers’ properties?
Plasma polymerization through evaporation
The Swiss Federal Laboratories for Materials Science and Technology, Empa, is a research institute under the ETH domain, dedicated to materials science and technology development. They study and apply plasma polymerization to deposit nanometer-thin layers on fabrics and fibers to functionalize their surfaces, specifically to make them water-repellent.
A Bronkhorst evaporation system plays a key role in this process, especially in the controlled delivery of polymer precursors. The evaporation system includes a liquid delivery system (LDS) that can be used in atmospheric and vacuum processes. The vapor generation system consists of a liquid mass flow controller, a gas flow controller for the carrier gas, and a temperature-controlled mixing and evaporation device.
The plasma polymerization process
In this experimental setup for a low-pressure plasma polymerization process (0.1 mbar), hexamethyldisiloxane (HMDSO), a liquid polymer precursor, is evaporated and then activated by plasma to be polymerized and deposited onto the fiber surface as a hydrophobic coating.
To achieve a stable and repeatable vapor flow of the polymer precursor, it is necessary to precisely regulate the flow rates of liquid HMDSO and the carrier gas.
To evaporate HMDSO, an evaporation system is used. In this setup, liquid HMDSO is drawn from a container at room temperature and dosed using a Coriolis mass flow meter. The liquid HMDSO is then mixed with argon carrier gas through a thermal mass flow controller and finally vaporized within the CEM system. The vapor flow is introduced into the plasma reaction chamber, operating at an absolute pressure of 0.1 mbar. The entire process is controlled by a PLC system and visualized through LabView software.
HMDSO enables polysiloxane polymer coatings to deposit at low temperatures, allowing the coating of textile fibers that cannot withstand high temperatures.
Following the success of laboratory experiments and testing, the process was scaled up for industrial applications.
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