Evaporation flow controllers for hydrophobic coating

Since the Corona pandemic led to restrictions on our movements and activities, walking has become a good alternative for a lot of people to get some fresh air and exercise, myself included.
In the beginning I enjoyed my walks to the fullest, as the Spring just began. Now the weather is changing which makes it less appealing to go for a walk.

Luckily water-repellent clothing and raincoats 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 behavior? An answer to this question is hydrophobic coating.

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

Hydrophobic coating

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

Evaporation for plasma polymerization

The Swiss Federal Laboratories for Materials Science and Technology, Empa, is a research institute of the ETH Domain and devoted to materials science and technology development. They investigate and apply plasma polymerisation to deposit thin, nanoscale layers on top of fabrics and fibers, in order to functionalize their surface - and more specific: to make them water-repellent.

A Bronkhorst evaporation system plays an important role in this process, especially in the controlled supply of polymer precursors. The evaporation system contains a Liquid Delivery System (LDS) that can be applied for atmospheric or vacuum processes. The vapour generation system consists of a liquid mass flow controller, a gas mass flow controller - for the carrier gas - and a temperature-controlled mixing and evaporation device.

Plasma polymerization process

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

An evaporation 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 mass flow meter. Then the liquid HMDSO is mixed with argon carrier gas from a thermal mass flow controller and vaporized inside a heat exchanger for controlled heating. The vapor 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.

After a successful experimental setup and test the process has been upscaled from laboratory scale to industrial scale.