Лако-красочные материалы — производство

While most, if not all, plasma equipment consists of similar components, the design of the reactor chamber, the distribution of power, the excitation frequency, and the gas dynamics can all be critical parameters influencing the efficiency and properties of plasma reactions. An extensive amount of work has been published that shows a direct correlation between excitation frequency and plasma reactivity. Manufacturers of plasma equipment employ­ing radio-frequency (RF) excitation use either low frequencies (i. e., less than 400 kHz) or the higher frequencies at 13.56 or 27.12 MHz as specified by the Federal Communications Commission. For applications involving the treatment of plastics, 13.56 MHz is the pre­ferred frequency. Also important is whether the material being treated is in a primary or a secondary plasma. Older equipment using large cylindrical barrels typically comprises secondary plasma systems (Fig. 1). The plasma is created either between closely spaced, paired electrodes that may function as shelves or in the annulus between the vessel’s outer wall and a ring electrode, when employed. Treatment of materials placed within the working volume depends on the diffusion of active species created in the primary

Chamber Wall

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plasma (i. e., within the RF field). Diffusion of these active species is very dependent on pressure; the higher the pressure, the shorter the mean free path. The mean free path is the distance that active species can probably travel before undergoing collisions that deacti­vate radicals or neutralize ions. Therefore, when using a secondary plasma, the concentra­tion of active species varies either across the diameter of a barrel system or between electrode pairs, as the case may be. Thus, by the physical laws of nature, the treatment within the working volume of a secondary plasma system cannot be uniform. By contrast, when working within the RF field, or primary plasma, the gas is constantly being excited. Thus polymeric articles being treated are immersed in a constant concentration of active species. Further, since diffusion is not a mechanistic limitation, significantly higher oper­ating pressures may be used. This allows higher process gas flow rates, assuring that off­gassing species from the polymer are sufficiently diluted, providing the full benefits of the desired process gas. In addition, the primary plasma is rich in ultraviolet (UV) radiation, which is often an important initiation step in polymer reactions. Since UV radiation is line of sight, uniform treatment of multiple parts can only be obtained when working within the primary plasma. Otherwise, any part in the shadow of another will receive different radiation, and therefore the effectiveness of the treatment is expected to vary.

The types of reactors used for the deposition of plasma polymers have been varied. Glass and/or quartz reactors or aluminum chambers with metal parallel-plate electrodes seem to predominate in the literature, although several investigators have used inductively or capacitively coupled systems with external electrodes. High rates of deposition are found in the glow area, with the rate of deposition decreasing as we move farther away from the glow discharge region. Consequently, primary plasma systems that use a 13.56-MHz RF source are favored. The RF excitation used by various equipment man­ufacturers can be as low as 2 to 4 kHz or can be the more typical 13.56 MHz (high frequency). Microwave plasma systems have also been used for the deposition of plasma polymers. Previous studies have shown that the densities of films deposited by low-frequency systems are significantly lower than those of films deposited by either the high-frequency or microwave plasma systems. The choice of equipment used for plasma polymerization and deposition is thus dictated by the rate of deposition desired, the film properties that can be obtained by the various systems, and practical considerations such as the size of the parts to be treated and processing rates that are feasible in any given system.