Monday, October 3, 2016

Thoery: corona discharge method for surface treatment of plastic

Corona discharge: (air plasma)

It is ionized air created by discharging high frequency high voltage energy across a metal or insulated electrode. This electrode is positioned over a grounded roll. The space between the electrode and the roll is typically .060". It is in this air gap that corona is generated.

It is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. The corona plasma is generated by the application of high voltage to an electrode that has a sharp tip. The plasma forms at the tip. A linear array of electrodes is often used to create a curtain of corona plasma. Materials such as plastics, cloth, or paper may be passed through the corona plasma curtain in order to change the surface energy of the material. All materials have an inherent surface energy. Surface treatment systems are available for virtually any surface format including dimensional objects, sheets and roll goods that are handled in a web format. Corona treating forms low-molecular-weight (LMWOM) on film surface; oxidizes film surface; and forms positive and negative sites by adding and deleting electrons.

Basics of high voltage discharge in air and its application to surface treatment.

In the presence of a high voltage discharge in an air gap, free electrons, which are always present in the air, accelerate and ionise the gas. When the electric discharge is very strong, collisions of high velocity electrons with molecules of gas result in no loss in momentum, and electron avalanching occurs. When a plastic part is placed in the discharge path, the electrons generated in the discharge impact the surface with energies 2 to 3 times that necessary to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals. These free radicals in the presence of oxygen can react rapidly to form various chemical functional groups on the substrate surface. Functional groups resulting from this oxidation reaction are the most effective at increasing surface energy and enhancing chemical bonding to the resin matrix. These include carbonyl (-C=O-). carboxyl (HOOC-), hydroperoxide (HOO-) and hydroxyl (HO-) groups.

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