LPCAT treatment

The second gas is introduced in the expansion chamber. Because of an extremely high velocity of gas injecting from a small nozzle (e.g., 3 mm in diameter), the second gas injected into the expansion chamber in vacuum cannot migrate into the cascade arc generator. Thus, the activation of Ar in the cascade arc generator and deactivation of the activated species of Ar in the expansion chamber, which activate the second gas introduced in the expansion chamber, are temporally and spatially separated. The LPCAT treatment and polymerization occur under such a totally (temporally, spatially, and kinetically) decoupled activation/deactivation system. 

The ideal surface modification of powders is the process that breaks down the aggregates and modifies the surface of the primary particles simultaneously. LPCAT treatment could be very close to this ideal situation. The supersonic velocity of reactive species breaks up the existing aggregates to a significant level, if not completely, allowing the chemically reactive species to interact with the primary particle surface. 

Figure 16.20 Schematic illustration of CO2 LPCAT treatment of fine particles.

The treatment by secondary plasma reactor utilizes chemically reactive species created in glow discharge without influences of electron and ion bombardments and luminous gas phase. In-glow LPCAT treatment, on the other hand, utilizes luminous gas phase without the influence of ion and electron bombardment, and chemically reactive species are created on PTFE by energy transfer from the luminous gas phase. Thus, surface treatment by secondary plasma works only with gases that produce relatively long-lived chemically reactive species. Most secondary plasma treatments appear to be surface modifications by air or oxygen. 

Figure 30.9. LPCAT treatment under stronger plasma conditions for a longer time yields the more wettable surface. However, the sessile droplet contact angle of a paint on Parylene C surface (resident time 0) is low and minimal change occurred with LTCAT treatment. Thus, the adhesion problem is not due to the wetting difficulty.

Figures 30.11-30.16 depict the aging aspects of LPCAT-treated TPOs, and Figures 30.17-30.19 depict the durability aspect of LPCAT treatments investigated with three kinds of commercially available TPOs. In figures, LPCAT-air is abbreviated as air plasma, and likewise argon plasma and methane plasma. The results shown in Figures 30.11-30.16 indicate that the wettability or the contact angle of water on a plasma-treated surface is not the major factor that accounts for...

Figures 30.18 and 30.19 show the changes in the longest water boiling time, which still pass the tape test, of primer-coated TPOs with the arc current and plasma exposure time of LPCAT treatment, respectively. From Figure 30.18 it is evident that durable bonding of the primer to TPOs was obtained with argon and methane LPCAT treatments at a lower arc current. Since the arc current represents energy input in the LPCAT process, plasma treatment conducted at a lower arc current may prevent the overtreatment on the TPO and thus give better adhesion results.

As detailed in Figure 30.19, this overtreatment effect was also observed when LPCAT treatment was applied for longer times at the low current of 2 A. 

A single monoatomic gas, e.g., argon, helium, etc., is used as the carrier gas of the cascade arc discharge. When the luminous gas is injected into an expansion chamber in low pressure, e.g., 1 torr or less, the flame extends a significant length (e.g., 1 m), which depends on the flow rate, input power, diameter of the nozzle, and the pressure of the expansion chamber. This mode of cascade arc torch is termed low-pressure cascade arc torch (LPCAT), which is useful in the surface modification by means of LPCAT treatment and LPCAT polymerization. 

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