Plasma coatings, primer adhesion

Postdeposition plasma modifications to the plasma polymer of TMS have been seen to greatly improve bonding to various primers and paints [18-20]. One particular system has been observed to have tremendous adhesion between plasma-coated A1 alloy panels and paint applied to them. This system involves cathodic DC plasma deposition of a roughly 50-nm primary plasma polymer film from TMS onto a properly pretreated alloy substrate, followed by the deposition of an extremely thin fluorocarbon film by DC cathodic deposition of hexafluoroethane (HFE). It was the superadhesion aspect of this particular system that triggered the series of ESR studies [3,21]. 

This film system was seen to outperform others not incorporating the adhesion-promoting HFE film when a primer is applied to the LCVD coated alloy surface. The alloy panels were always treated with O2 plasma to remove any organic contaminants from the alloy surface prior to film deposition. The entire steps involved in the plasma coating process are ... 

Table 28.4 Surface Treatment Effect of TMS by Succeeding Plasmas on Adhesion of Spray Paint Primer [Spraylat EWDY048 (primer D)] to TMS Plasma-Coated [7A](Ace/0) Aluminum Panels ...

Table 30.2 summarizes the tape test results of the Courtauld primer coating on Parylene C surfaces with and without argon LPCAT plasma treatment. The adhesion performance of Parylene C films with respect to the Courtauld primer was improved in varying degrees depending on treatment conditions and treatment time. 

The main aim of SAIE is the complete elimination of heavy metals from the coating systems an approach that primarily relies on tenacious water-insensitive adhesion and good barrier characteristics of a primer has been taken. It should be pointed out that this approach is theoretically incompatible with the approach that utilizes the primers with corrosion inhibitors, e.g., chromated primers. This is because a primer with super barrier characteristics would not allow the migration of inhibitors and would not provide enough water for their electrochemical reaction to form corrosion protection products, as described in Chapter 28. In order to further elucidate the SAIE concept, both chromated and nonchromated spray primers were employed to generate two types of plasma coating-modified systems, and their corrosion protection behaviors were investigated in this study. 

Figure 31.28 Scanned images of Prohesion salt spray-tested A1 panels with chromated plasma coating systems prepared by anode magnetron plasmas Primer G could not be removed from the whole Al panel due to the strong adhesion, and only the portion with pitting corrosion occurred underneath could be stripped off with paint stripper solution.

Since it was observed that fluorine contamination was a possibility and had potentially detrimental effects as described in Chapter 10, the excellent primer adhesion achieved with Tfs/(Ar) and Tcs/(Ar), shown in Table 31.3, has significant importance in the practical application of the plasma technique without any of the potentially deleterious effects of fluorine-based systems. Argon plasma treatments on both flow system TMS (Tfs) and closed system TMS (Tcs) polymers were then investigated as an additional system modification that could provide strong adhesion without the incorporation of fluorine-containing monomers in the quest to produce chromate-free coatings systems. 

Table 31.3 summarizes the adhesion test results of nonchromated primers [Spraylat EWAE118 (D) and Dexter 10-PW-22-2(X)] to plasma coatings deposited without an anode assembly, which were produced in both flow and closed system deposition processes. As noted in Table 31.3, closed system TMS plasma polymers (Tcs) showed superior primer adhesion performance to similar ones obtained from a flow system (Tfs). Similar to chromated primers, summarized in... 

Table 32.4 displays the adhesion test results for different spray primers applied to T/F plasma-treated bare 7075-T6 alloys. The results demonstrate that this special plasma polymer coating gave rise to excellent water-insensitive adhesion of all three spray primers used in this study Turco solution could not delaminate any of the primers over a period of 24h. In addition, up to 6 days aging of T/F plasma coatings in air prior to primer application did not degrade the excellent adhesion performance of the systems. 

The corrosion protection of plasma interface-engineered coating systems relies on the tenacious water-insensitive adhesion and good barrier characteristics of the coatings [3]. DC cathodic polymerization and plasma treatment have been demonstrated as efficient in improving the primer adhesion to metallic substrates. 

Table 32.6 Adhesion Test Results of Chromated Primers (Deft 44-GN-36 and 44-GN-72) to Plasma Coated Alclad 7075-T6 Alloy...

From the XPS analysis results, it was found that TMS plasma coatings prepared in a closed system have a carbon-rich surface, which is similar to TMS polymers followed by methane plasma deposition, i.e., T/C as described in a flow system. The adhesion study also showed that the T/C plasma treatment on aluminum alloy provided excellent adhesion to spray paint primers. The primer adhesion performance of closed system TMS plasma polymers was also examined. Since the porous surface... 

Structure of IVD A1 coating may conceal the plasma treatment effect on primer adhesion, Alclad 7075-T6 A1 alloy panels were selected as the substrates for adhesion investigation based on the fact that they can provide smooth surfaces while still providing an A1 coating similar to that of IVD A1 coatings. 

In order to produce chromate-free plasma coating systems, the adhesion of closed system TMS coatings to nonchromated primers (Spraylat EWAE118 and Dexter lO-PW-22-2) was also investigated, and the adhesion test results are summarized in Table 32.9. Closed system TMS plasma polymers showed superior primer adhesion performance to those obtained from a flow system. Similar to chromated primers, summarized in Table 32.8, excellent primer adhesion was always achieved with closed system TMS plasma polymers treated with subsequent Ar plasma applications. 

It should be emphasized that the primers in the plasma coating systems applied to the IVD Al-coated A1 alloys could not be removed by the commercial Turco paint stripper solution. This tenacious and water-insensitive adhesion at the primer/IVD interface achieved by TMS cathodic polymerization in a closed reactor system must be responsible for the excellent corrosion protection performance of these plasma coating systems. In other words, excellent corrosion protection of IVD Al-coated A1 alloys can be accomplished with chromate-free primer coatings with the aid of tenacious and water-insensitive interface adhesion. 

DC cathodic polymerization of TMS mixed with argon improved the primer adhesion performance of the closed system TMS plasma polymers. Moreover, the addition of a certain amount of argon into the TMS plasma system further increased the plasma coating quality, reflected in the increase in refractive indices. Based on the higher compatibility with the IVD process, the excellent adhesion performance, and the benefit of one process combining TMS plasma polymerization and the postdeposition plasma treatment, DC cathodic polymerization of TMS mixed with argon in a closed system is being considered as a more realistic and favorable approach in practical applications. 

However, the poor adhesion of the surface does not allow a further painting stage. The surface of Parylene coatings can be modified by a special plasma coating. In this way, an interlayer is placed to provide good adhesion to the substrate as well as to a subsequent primer. After this procedure, the hydrophobic Parylene polymer again becomes paintable with both solventborne and waterborne spray primers." ... 

Pretreatment primers. In this method of use the silane may be applied from a solvent solution, by vapour phase deposition or by plasma deposition although solvent application is the more usual. The solution usually contains water and silane at a concentration of 1-2 wt%. The applied film may be water washed before subsequent coating/bonding and/or heat cured. The solvent(s) used may be important in both the stability of the solution and the performance, particularly in the wet adhesion. It has been shown that the presence of water either in the solution or as a final rinse is important, particularly in the case of AAMS and presumably other silanes [1]. Other factors which are important include the concentration of silane the pH of the solution the thickness of the silane film deposited. 

Parylene N to smooth surface materials has been reported with the application of plasma depositions [13,14]. It was reported that excellent adhesion of Parylene C coating to a cold-rolled steel surface was achieved using plasma polymer coatings, in turn giving rise to corrosion protection of the metal [15]. Another major deficiency of Parylene C is its poor painting properties when paint is applied on a Parylene C film, due to its extremely hydrophobic surface. Because of this, surface modification of Parylene films is necessary to enhance their adhesion performance with spray primers. 

Plasma treatment of Parylene C films has proved to be very effective in improving the painting properties of Parylene C polymers with respect to a solvent-borne primer as described above. The similar effect of LPCAT plasma treatment on the adhesion of Parylene C polymer to water-borne primer (44-GN-36, Deft Corp.) was also observed. Table 30.3 summarizes the tape test results for the Deft primer coatings on Parylene C surfaces treated by LPCAT under different plasma conditions. 

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