Лако-красочные материалы — производство

A. Plasma Processes

There are many definitions of the term plasma, according to the various disciplines with which it is involved. It has often been referred to as the fourth state of matter; the generation of plasma being analogous to the transitions that occur when energy is supplied to a material, causing solids to melt and liquids to become gases. Sufficient additional energy supplied to a gas creates a plasma. In the case of cold gas plasma, typical of that used in this work, the process is excitation of a gas at reduced pressure by RF energy. Typically, a plasma is composed of a large concentration of highly excited atomic, mole­cular, ionic, and radical species. While on an atomic scale, plasma generation cannot be construed as a room-temperature equilibrium process, as the bulk of the material remains near room temperature. The plasma contains free electrons as well as other metastable particles, which upon collision with the surfaces of polymers placed in the plasma environment break covalent chemical bonds, thus creating free radicals on the polymer surface [1]. The free radicals will then undergo additional reactions, depending on the gases present in the plasma or subsequent exposure to gases in the atmosphere. The result is that these gas-radical reactions form a surface that is potentially very different from that of the starting bulk polymer. Since the process is conducted in a reactor under very controlled conditions, the end result is very reproducible.

Plasma processing is not one process but a ‘‘field of opportunities” that can be classified into three overlapping categories: (1) plasma activation, (2) plasma-induced grafting, and (3) plasma polymerization. Plasma activation is the alteration of surface characteristics by the substitution of chemical groups or moieties for groups normally present on the polymer chain being modified. The assumed mechanism is free-radical creation and coupling of these free radicals with active species from the plasma environ­ment. Depending on the process gas selected, a large variety of chemical groups can be incorporated into the surface. These groups may be hydroxyl, carbonyl, carboxylic, amino, or peroxyl groups. Most important, the insertion or substitution of these groups in the polymer chain is under the control of the operation. In this manner, the surface energies and the surface chemical reactivity of plastics can be altered completely without affecting their bulk properties.

Plasma-induced grafting offers another method by which plastic surfaces can be modified. If a noble gas is employed to generate a plasma, a multitude of free radicals are created along the polymer backbone. If after the plasma is extinguished but prior to the introduction of air, an unsaturated monomer such as ally alcohol is introduced into the reaction chamber, it will add to the free radical, yielding a grafted polymer. The range of functional and reactive sites that can be incorporated onto a surface is increased signifi­cantly with this technique. This process differs from activation in that instead of functional modification of the surface polymer chains, material is added on to the polymer backbone.

The third category of plasma processes, plasma deposition, utilizes gases or vapors that fractionate and undergo polymerization under the influence of RF energy. For exam­ple, methane (CH4) under the influence of plasma will deposit as a polyhydrocarbon that has a density approaching 1.6g/cm3. Any material that can be introduced into the process chamber is a potential candidate as a feed material for plasma polymerization. The prop­erties of materials polymerized in this manner are very different from polymers obtained from these materials via conventional polymerization methods. These properties include a high degree of cross-linking and the ability to form pinhole-free films that adhere tena­ciously to various substrates.