March 2009

In general terms, surface modification refers to any change to a surface that affects adhesion and wettability. Before you paint a wall, you clean it and prime it. These modifications, cleaning and priming, increase the ability for the paint to stick and result in a more attractive and longer lasting surface.

Polymeric materials in particular are frequently treated to overcome their innate hydrophobic low surface energy properties. The surface energy of a material directly relates to the material's ability to be wetted. Contact angle is used to measure surface energy. When a liquid drop beads up and produces a large contact angle, surface energy is relatively low and adhesion is poor. To improve adhesion and wetting, surface modifications make molecular and physical changes to the polymer surface to functionalize the material and improve wetting and adhesion. A successfully modified surface will exhibit a lower contact angle and higher surface energy.

Surface modifications to polymer materials is thus to improve surface energy and promote adhesiveness prior to the application of films, coatings, inks, and adhesives. Industries that are most concerned with polymer surface modifications include: automotive, medical devices, printing, and electrical cable manufacturers. There are many different types of surface modification treatments including: plasma, corona, laser, vacuum, flame, UV, photochemical, photografting, and chemical. The end result is a modified polymer surface that can be bonded with other materials, coated, or printed upon.  

Plasma surface treatment is one of the most common methods used to increase polymer surface wettability. One method employs an electrical discharge in a vacuum environment. The process may take anywhere from 2 seconds to 2 minutes in a vacuum environment between 2 and 15 mbar. An alternate method of plasma treatment employs a high pressure nozzle that operates in an atmospheric environment as shown below. Different tips are available for specific applications. 

Corona treatment is another popular method used to increase adhesion of polymer surfaces. This method employs electrode elements that discharge high voltage in air at high frequencies between 16-24 kHz. The free electrons in the air ionize the gas which results in electron avalanching. The subsequent stream of electrons hit the surface with enough force to break the molecular bonds on the polymer surface resulting in reactive free radicals. The high voltage discharge affects only the surface while preserving the mechanical and physical properties of the bulk material. Since ozone is emitted during the high-voltage, high-frequency discharge, ozone filter systems are used to neutralize the ozone.

Because shelf life varies for plasma and corona treated parts, application of subsequent coatings, inks, and adhesives is often carried out shortly after the surface treatment process. Once the treated surface has interfaced with the mating material however, the bond is permanent.

Contact angle is an important metric for polymer researchers and material scientist providing a simple measure of wettability and adhesion. Contact angle is also used to measure surface energy which predicts the efficacy of adhesive bonding, cleanliness, absorption, and "printability". A good bond on a polymer solid requires the surface energy to be improved to exceed the surface tension by 4-10 mN/m. Since most polymers have a native surface energy of 20-40 mN/m; and since most adhesives, inks, and coatings require a solid with a surface energy of 45-55 mN/m in order for application to be successful, surface modification treatments must improve the surface energy by 5-35 mN/m. Low surface energy solids such as PTFE and PDMS present particular challenges due to the increased surface energy improvement necessary to achieve bonding. In our June 2006 newsletter, we discussed two surface treatment methods that are being used to improve adhesion of PTFE: laser irradiation and laser surface treatment.

Polymer surface modification is used to treat medical vessels, syringes, and needles to improve liquid flow, treat the external surfaces of cables and insulation to allow for printing, treatment of lids, bottles, covers, and automotive parts prior to printing and application of gasket, flocking bristles, and functional and decorative films and coatings.
 

If you're involved in polymer surface modification in any way, you may be interested in the Seventh International Symposium on Polymer Surface Modification: Relevance to Adhesion. This event will be held July 12-15, 2009, at the University of Maine. The symposium is hosted by MST Conferences, the same organization that brings us the Contact Angle, Wettability, and Adhesion Symposium for which we have been a premier sponsor for some years. To learn more or to register for the Polymer Surface Modification symposium, please visit: http://www.mstconf.com/surfmod7.htm