Лако-красочные материалы — производство

In comparison to the processes described above, plasma polymerization offers an entirely new avenue for adhesion improvement when bonding different materials. For example, films deposited from a methane plasma have been shown to improve dramatically the adhesion properties of many materials when tested in both the dry and wet state [41]. The process of plasma film deposition is often called plasma polymerization, although the process that takes place is not polymerization in the classical sense. Gases in plasma may undergo polymerization, usually through a free-radical initiation process. When a gas is ionized by RF energy, the resulting plasma contains free electrons as well as other metastable particles. When the process gas mixture used consists wholly or in part of hydrocarbon gases, the hydrocarbon molecule is fractured into free-radical fragments. These free-radical fragments become the sites at which the polymerization process is initiated. As the molecular weight of the plasma polymerized product increases, it is deposited onto the substrate placed within the plasma chamber. Since the fragmentation of the feed gas in the plasma generates free-radical species for initiating the polymerization process, gases such as methane (CH4), which have zero functionality, can be used to form plasma polymers. In addition to methane, plasma polymers have been formed from other hydrocarbon gases, such as ethylene or propylene, fluorocarbon monomers such as tetra — fluoroethylene, and organosilicon compounds such as hexamethyldisiloxane (HMDSO) or

vinyltrimethylsilane (VTMS). Due to the complex nature of the fragmentation process, the resulting polymer structure is unlike any that can be deduced from conventional polymer­ization mechanisms [43].

The physics of plasma polymerization processes has been described in depth else­where in sufficient detail for the interested reader [41,44]. The conditions used during glow discharge polymerization determine not only the structure of the resulting film but also the rate at which these films are deposited onto the target substrate materials [41,45,46]. The degree to which the monomer is fragmented is dependent on the amount of energy sup­plied per unit weight of monomer that is allowed to flow through the reactor. When sufficient energy is supplied to break all the bonds of the monomer molecule, the recom­bination or polymerization process becomes atomic in nature. In addition, the structure of the plasma polymers can be varied by changing reaction conditions, including the use of comonomers or the introduction of oxygen, nitrogen, or ammonia into the reaction chamber during the polymerization process. These studies have developed a correlation between the power input, type of monomer used, and monomer flow rate to the density and the type of active species in the plasma. These factors, in turn, determine the rate of deposition and the film structure [46,47]. Table 2 shows typical deposition rates for some common plasma-polymerized films.

While plasmas of ammonia, mixtures of hydrogen and nitrogen, and oxides of nitrogen have been used to incorporate nitrogen atoms into the surface layers of the polymer [28,29], the level of nitrogen incorporation has been less than 10 at.% [29]. In contrast, films deposited from allyl amine have been shown to contain up to 25 at.% nitrogen as measured by spectroscopic methods [48]. Despite this high nitrogen content, however, the authors report a lower than expected concentration of amino groups. Other studies have shown concentrations of up to 2 molecules/nm2 of reactive amine groups on the surface of films deposited from allyl amine onto FEP substrates. These surface con­centrations were determined by derivatization of the amine groups with fluorescein iso­thiocyanate and subsequent detection of the fluorescein chromophore by optical spectroscopic methods [49]. Since electron spectroscopy for chemical analysis does not always allow precise determination of functional sites, the earlier data may reflect limita­tions of the analytical methods used.

In a similar vein, hydroxyl and carboxylic acid functionalities can be incorporated by plasma-polymerizing acrylic acid [50] or ally alcohol [48]. Another technique commonly employed to incorporate specific atomic species is the use of co-reactants along with the primary monomer. In one such example, ammonia or acrylonitrile was used as the

co-reactant during the deposition of films from a methane plasma [51]. Two additional techniques that are available to the surface engineer interested in modifying plasma- deposited films are plasma surface modification of the deposited film in a second process step and wet chemical reaction methods. As an example, carbonyls formed during the plasma deposition of films from N-vinylpyrrolidone were reacted with lithium aluminum hydride and sodium borohydride to convert these carbonyls to hydroxyl groups [52]. It should be noted that the use of plasma-deposited films for adhesion enhancement is not limited to polymeric substrates. Such films have also been deposited onto inorganic mate­rials such as mica [50] and metal substrates such as aluminum and steel in an effort to improve adhesion of these materials to polymers [41].