Hydrophobic coating, the answer to exercising in the rain

September 19, 2017 Sandra Wassink

It happens to all of us…. You want to go out for a jog, a walk or a bike ride and it rains! Luckily waterproof clothing or raincoats do exist. What interests me is how this type of clothing is manufactured, how does a fabric become waterproof or water-repellent and yet breathable? In other words, what is responsible for this anti-wetting behaviour? An answer to this question can be hydrophobic coating.

In today’s blog I would like to share a successful application with you regarding hydrophobic coating using a Controlled Evaporation and Mixing system (CEM).

Waterproof jacket

Hydrophobic coating

Waterproof yet breathable clothing prevents rainwater drops from penetrating, and at the same time allows perspiration vapour to pass – a very desirable feature when going out exercising in the rain. How can you make fabrics and textiles hydrophobic or add other functionalities to them without affecting the bulk properties of their fibers?

Controlled Evaporator Mixer CEM

Empa, the Swiss Federal Laboratories for Materials Science and Technology

Empa, a research institute of the ETH Domain and devoted to materials science and technology development, investigates and applies plasma polymerisation to deposit thin, nanoscale layers on top of fabrics and fibers, in order to functionalise their surface - and more specific: to make them water-repellent.

A Controlled Evaporation and Mixing system, also called CEM system, played an important role in this process, especially in the controlled supply of polymer precursors. A CEM system is an innovative Liquid Delivery System (LDS) that can be applied for atmospheric or vacuum processes. The vapor generation system consists of a (thermal or Coriolis) liquid flow controller, an MFC for carrier gas and a temperature controlled mixing and evaporation device.

Plasma polymerisation process

In this laboratory scale setup of a low-pressure (0.1 mbar) plasma polymerisation process, the liquid polymer precursor hexamethyldisiloxane (HMDSO) is evaporated and successively activated by the plasma with the aim to be polymerised and deposited onto the fiber surface as a hydrophobic coating. In order to obtain a stable and repeatable polymer precursor vapour flow, the liquid HMDSO flow as well as a carrier gas flow have to be controlled accurately.

A CEM system is used to evaporate the HMDSO. In this setup, liquid HMDSO is drawn from a container at room temperature and measured by a Coriolis flow meter. Then the liquid HMDSO is mixed with argon carrier gas from a thermal mass flow controller and vaporised inside a heat exchanger for controlled heating. The vapour flow is introduced into the plasma reaction chamber operated at 0.1 mbar absolute pressure. All controlled by a PLC system and visualised by LabView software.

HMDSO allows the deposition of polysiloxane coatings at low temperatures, which makes it feasible to coat textile fibres that cannot withstand high temperatures. Empa’s attempts to conduct the plasma polymerisation at low pressure aim at increasing the production yield by promoting heterogeneous deposition on the fiber surface, and by reducing the amount of chemicals.

After a successful experimental setup and test it’s Empa’s aim to upscale the process from laboratory scale to industrial scale.

Vapour flow process

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