Plasma treatments contact angles

The most obvious result of the plasma treatment of a polymer is the improved wettability of the surface. For most untreated polymers the surface energr is 25 to 50 dynes/cm, with a water contact angle of 95° to 60°. After plasma treatment, with an oxidizing plasma, the contact angle decreases to less than 40°, with some surfaces being so wettable that the contact angle is difficult to measure. 

Figure 16. Dependence of the PE contact angle on the exposure time to 8.3 W plasma discharge. The contact angle was measured at different times elapsed from the plasma treatment. The numbers are the times in hours elapsed from the plasma treatment [71].

The wettability of ABS can be increased by the treatment with an atmospheric plasma torch (64).. This was established by contact angle measurements and other methods. The wettability was increased when the atmospheric plasma treatment was done in a slow manner. The decrease in contact angle with respect to water is explained due to a significant increase in the oxygen content, which is caused by the formation of carboxylic and hydroxyl groups on the polymer surface. 

The modification of the chemical composition of polymer surfaces, and thus their wettability with chemical substances, can be realized in different ways electric discharges more commonly called Corona effect, oxidation by a flame, plasma treatment, UV irradiation and also UV irradiation under ozone atmosphere. Numerous studies have been devoted to the effects of these different treatments. More recently, Strobel et al. [204] compared the effects of these treatments on polypropylene and polyethylene terephthalate using analytical methods such as E.S.C.A., F.T.I.R., and contact angle measurements. They demonstrated that a flame oxidizes polymers only superficially (2-3 nm) whereas treatment realized by plasma effect or Corona effect permits one to work deeply in the polymer (10 nm). The combination of UV irradiation with ozone flux modifies the chemical composition of the polymers to a depth much greater than 10 nm, introducing oxygenated functions into the core of the polymer. 

The contact angles of native PDMS (110°) have been decreased to 5° (02 plasma treatment), 45°(HC1 treatment), and 75° (APTES treatment) [250]. 

FIGURE 9.7 The contact angle of PDMS after various surface treatments (a) 3 h after 02 plasma treatment (28° C), (b) 48 h after 02 plasma treatment (70° C), (c) 144 h after 02 plasma treatment (90° C) and (d) parylene-coated PDMS (92° C)[1000]. Reprinted with permission from the Institute of Physics Publishing. 

The surface wettability by water drops has been clearly linked with the concentration of chemically bound nitrogen in the surface. Conversely, measurements of advancing and receding contact angles for water can provide information regarding surface composition. On the other hand, there appears to be little or no direct correlation between water wettability and adhesive strength of evaporated aluminum films. Substantial improvement in adhesion of Ai to many polymers can be achieved by 02 plasma treatment of the polymer surface before metallization. Other workers have shown this to result in chemical linkages between the metal and polymer surface moieties. 

In DC discharge plasmas, the sudden decrease of contact angle after 15 min treatment has been assigned by S. Okasaki et al. to a structural change of the material surface from crystallised graphite to an amorphous state [93]. It has been shown that the fluorination of PAN-based carbon fibers is more effective in the case of CF4-He plasmas than in 5% F2—He plasmas [94]. The study of the thermal stability of these F-treated fibers has shown however that a 70% loss of fluorine occurred when the samples were heated at 293°C for 10 min. 

Change in contact angles as a function of storage time was also not studied. Studies on polystyrene indicate that the contact angle does change with time after plasma treatment (11). 

Plasma treatment penetrates through a porous structure, i.e., it is not limited to the exposed surface. Contact angle cannot be measured because a water droplet penetrates quickly into the fabric. Contact angle can be measured but a water droplet penetrates slowly into the fabric. 

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