27. May 2013   4:45 pm
Andy Stecher

Andy Stecher
Elgin, IL

The second installment in this series addresses the opposite of the first, creating a hydrophobic surface.  First we must ask what they are and why such a surface would be desired.  There are a number of answers but as a colleague of mine wisely says ‘Plasma is not a panacea’.

Hydrophobic surfaces are by-definition surfaces with lower energy states than the 72 mN/m (dyne) energy level at which water is attracted.  In essence, this is a surface water does not like to be on.  Droplets may form through condensation or be placed on them directly, but they will not spread.  They prefer their own level of energy and therefor contract to have the smallest contact with the surface they can muster.  This is what you see when water beads up on the freshly-waxed hood of your car.  The water may be held in place by gravity, but like a kid in the Principal’s office, they don’t want to be there.

So the first thing that comes to mind is that these coatings are designed to keep things dry.  That is the ‘How’, and here are some of the reasons why:  By repelling water on the edges of a case, you are keeping it away from damaging what is inside.  This same property can be used to divert small flows to the areas where you want them in micro-fluidic devices, such as medical test apparatus.  They will also resist water-based liquids such as paints or adhesives and minimize their ability to be permanently bonded to surfaces.  This makes a material easier to clean.

The coatings applied using our PlasmaPlus deposition system are based on the same SiOx chemistry used for hydrophilic coatings, with modifications to the process to make the surface energy as low as possible.  In most cases this is not below the as-molded surface energy of less expensive polymers such at polypropylene or HDPE, but it is much lower than the energy levels of most metals.  For this reason, the hydrophobic properties of these coatings are best used to inhibit corrosion.  They can resist the accumulation of physi-adsorbed water which can be a driver for corrosion.  Due to it’s other bonding characteristics, this same layer can act to promote adhesion in non-water-based systems.  The end result is a water-repellent bondline that is also chemically bonded by adhesive used.  This can be used to seal metal surfaces with much greater reliability than just cleaning and adhesive, thus extending part life dramatically!

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