Parylene C film

Improving Adhesion of Parylene C Film on Smooth Surface 

Parylene C, or monochloro-substituted poly(para-xylylene), is a polymer that has excellent bulk mechanical properties as well as excellent barrier properties for 

A very important factor in the utilization of plasma deposition is that both processes are carried out in vacuum. Enhanced adhesion of Parylene C and 

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. 

The reactive species in Parylene C deposition that interacts with the substrate surface is para-xylylene, in which two free radicals exist in the para position of a benzene ring. Para-xylylene is relatively stable and reacts only with other free radicals or with other para-xylylene units. In order to create a good adhesion of Parylene C film to a smooth-surface substrate, it is necessary to create free radicals on the substrate surface. With the aid of plasma interface modification, it is possible to achieve strong adhesion of Parylene coatings to such smooth surfaces. Strong adhesion of Parylene C coating to bare 7075-T6 (an aluminum alloy) panels was achieved with the application of plasma polymers [16]. 

---

The spectral dependence of the light sensitivity (as indicated by yellowing) of free films of Parylene-C was determined. A Heraeus Sun-Test chamber, equipped with a xenon arc lamp filtered to yield a simulated solar spectrum, was used for the irradiation. An additional infrared filter minimized sample heating. The irradiance at the sample location was originally 0.83 W/m2 at 340 nm, but the output decreased approximately 20% after 1500 hours use. Long band-pass optical filters with nominal cut-offs of 305 nm, 345 nm, 385 nm and 400 nm were inserted between the xenon lamp and the Parylene-C film samples to determine the wavelength threshold for yellowing. The sample temperature was maintained at 30+ 2 °C with a water-cooled... 

Color measurements on Parylene-C film were determined with a Minolta Chromameter 221, a colorimeter with output limited to CIE chromaticity or tristimulus values and CIELAB L, a and b color coordinates. Measurements on the films after various exposure times were recorded with the sample mounted over the white calibration plate. 

Figure 13. Color change (AE) of Parylene-C films versus light exposure.

Figure 14. Normalized color change of UV irradiated Parylene-C film.

Y. S. Yeh, W. James, H. Yasuda, Polymerization of para-xylylene derivatives, VI. morphology of parylene N an parylene C films investigated by gas transport characteristics, /. Polym. Sci. Part B 1990, 28, 545. 

Accordingly, the transport of salt requires larger elementary free volume than does the transport of water molecules. Hydrated ions are much larger than water, and hydrated cation and hydrated anion must move together because of coulumbic attractive force between them. Consequently, salt ions cannot permeate an amphoteric hydrophobic/hydrophilic polymer, of which the hydration value is low, i.e., less than few volume percent, by the solution-diffusion principle. Therefore, salt permeation through a hydrophobic polymer film such as low-density polyethylene (LDPE) and parylene C film should not occur. 

The similar breakdown of the surface state was also observed with parylene C film. Parylene C is a semicrystalline polymer and one of the most effective barriers for gases and vapors according to permeability values. It was thought that such an excellent barrier film would provide superior corrosion protection of a metal when it was deposited on the metal surface. Contrary to the expectation, parylene C film didn t provide good corrosion protection due to the surface state breakdown described above. This conclusion was ascertained by studies using electrochemical impedance spectroscopy (EIS), which is described in Chapter 28. 

Parylene C film does not adhere well to any smooth surface due to its unique polymerization mechanisms as described in Chapter 5. A freestanding film can be easily peeled off of the substrate surface, although a film does not peel off by itself in many cases, even if immersed in water. This feature enables us to investigate a system... 

Figure 28.20 Changes of Bode plots as a function of immersion time for freestanding Parylene C film and Parylene C coated Alclad 7075-T6 aluminum sheets in 0.9% NaCl solution 0.1 day indicates the initial run after 2h immersion of the samples.

Thus, without SAIE, Parylene C film, which has excellent barrier and physical properties, cannot be utilized in corrosion protection of a metal. Conversely, SAIE is the key to yield an excellent corrosion protection systems. It is also important to recognize how a nanofilm of hydrophobic amorphous network of plasma coating can prevent the initiation of the salt intrusion process. 

Parylene C film could be utilized in system approach interface engineering (SAIE) if the adhesion of Parylene C film to the substrate and the adhesion of a eoating, whieh is applied on the surface of Parylene C, could be improved. The seheme of utilizing Parylene C film in SAIE is depicted in Figure 30.7. Methods to improve its adhesion have been developed, but improvement is limited due to the lack of specific chemical interactions in the interfaee [13]. Low-temperature plasma deposition has proved to be a very effeetive proeess in improving the adhesion properties of materials while maintaining their desirable bulk properties. 

Figure 30.8 Adhesion improvement of Parylene C to TMS or TMS/CH4 plasma pol5rmer coated 7075-T6 panels with subsequent Ar RF plasma treatment at 100W and SOmtorr (a) and (b) on TMS plasma polymer, treatment time 1.0 min, (c) on TMS/CH4 plasma polymer, treatment time 2.0 min, (d) on TMS/CH4 plasma polymer, treatment time 10.0 min the blue color (dark) in (b) is the color of TMS film, indicating that Parylene C film was peeled off.

Solvent-Borne Primer on LPCAT-Treated Parylene C Films... 

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. 

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. 

The fabrication of carbonaceous electrodes prepared from pyrolyzed parylene-C films has been reported [117]. High aspect ratio carbonaceous micro-electrodes can be prepared by masking pyrolyzed parylene-C coated pipets with an insulating parylene-C film. Carbon electrodes coupled with electrochemical detection have been used extensively for the investigation of biogeiuc amines. 

Wei L, Parhi P, Vogler EA, Ritty TM, Lakhtakia A. Thickness-controlled hydro-phobicity of fibrous parylene-c films. Mater Lett 2010 64(9) 1063-5. 

---