tailieunhanh - Coatings Technology Handbook Episode 2 Part 2

Tham khảo tài liệu 'coatings technology handbook episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Plasma Surface Treatment 39-5 the excited species recombine to their original stable and nonreactive form. In most cases treatment of the exhaust effluent is not required. Gases that contain oxygen are generally more effective at increasing the surface energy. For example plasma oxidation of polypropylene increases the initial surface energy of 29 dynes cm to well over 73 dynes cm in just a few seconds. At 73 dynes cm the polypropylene surface is completely water wettable. Increased surface energy results in a plasma that yields polar groups such as carboxyl hydroxyl hydro-peroxyl and amino. A higher energy hydrophilic surface translates to better wetting and greater chemical reactivity of the modified surface to adhesives paints inks and deposited metallic films providing for improved adhesion and permanency. The enhanced surface reactivity is characterized in the laboratory by studying water wettability. Wettability describes the ability to spread over and penetrate a surface it is measured by the contact angle between the liquid and the surface. The relationship between contact angle and surface energy is inverse the contact angle decreases with increasing surface energy. Wettability can easily be induced on normally nonwettable materials such as polyolefins engineering thermoplastics fluoropolymers thermosets rubbers and fluoroelastomers. Noble gases argon helium etc. generate surface free radicals that react either with other radicals on the surface yielding molecular weight changes or with the air when the part is removed from the chamber thus increasing the surface energy. Process gases such as fluorocarbons will generally provide a lower energy or hydrophobic surface by substitution of abstracted hydrogen with either fluorine or trifluoromethyl radicals to form a fluorocarbon surface. Fluorination is favored in some medical applications where it undesirable to have catheters be wetted by blood. The nonwettable barrier layer also inhibits chemical .

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