Parylene is a conformal coating exhibiting extraordinary properties such as high mechanical strength and biocompatibility. It is a transparent (colorless) film in the UV-V is range of the solar spectrum (Parylene N and C absorb below ≈280 nm). The high transmittance of the polymers in the visible region (90%) make them eligible for use in optical applications. For further information on the optical properties of Parylene you can visit “Parylene’s Optical Properties and Performance”.
Whether Parylene can be colored or not is an interesting question and requires a different approach to answer. Each Parylene derivative (C, N, D, F, AF4) Diamond MT Inc. offers has a certain chemical composition and comes with a certain list of properties. Among them the basic parylene monomer is Parylene N (poly-para-xylylene). The derivatization of new varieties can be achieved by the addition of functional groups to Paryelene N main-chain phenyl ring and its aliphatic carbon bonds. Parylene N’s modification by a functional group such as Chlorine and Fluorine leads to Parylene C (poly(2-chloro-para-xylylene)) and Parylene F, respectively. Derivatization results in a set of new material properties: %crystallinity, melting temperature, resistivity, mechanical and electrical properties.
It is stated that the addition of a chromophore to the paracyclophane base molecule can give color to the Parylene . [2.2]paracyclophane is a special type of molecule and exhibits interesting optical properties such as photochromism under different chemical modification conditions . Octafluoro[2.2]paracyclophane (parylene AF4) on the other hand is a transparent parylene. A detailed explanation on the chemical structure and how derivatization works in parylene can be found in the literature . The important aspect that must be understood is that once the chemical structure changes all the mechanical, physical, optical and dielectric properties also changes. This leads us to different ways of using color on the surfaces if that is desirable.
One way of adding color is by depositing micro particles onto the surface that will be parylene coated and encapsulate them in a parylene film in a CVD reaction chamber . Also, fluorescent dyes are possible to be coated under the Parylene if required. However, there are two bottlenecks to consider 1- parylene adhesion might be affected leading to delamination, and 2- the color is an important indicator of Parylene degradation in time (becomes yellowish) due to exposure to UV light and oxygen after a long time , . In order to observe this kind of degradation and protect the products from further exposure to ambient conditions it is advised that a coloration process is not applied.
Fluoresence of Parylene-C: Parylene-C is an aromatic polymer which exhibits autofluorescence, the fluorescence intensity of thin-films (<1 μm) of Parylene-C are relatively weak. A group of researchers have developed an annealing method to improve the fluorescence strength to become visible. Annealing the Parylene-C in a quartz tube furnace under a nitrogen flow of 4.5 L min−1 avoiding possible oxidization at 270 °C they demonstrated that the peak intensity of parylene-C annealed at 270 °C was 42.4 times higher than that of the as-deposited sample. Also they showed that the use of different commonly used wavelengths including 375 nm (47-fold enhancement), 480 nm (38-fold) and 540 nm (17-fold), was possible (Figure 1).
Figure 1 Visibility of the parylene-C fluorescent marker coated on a glass pipette .
In many cases, transparency of Parylene derivatives is viewed as a plus such as encapsulations for optical applications. In conclusion, practically, color can not be added to Parylene.
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 “Parylene,” Wikipedia. Jun. 23, 2020, Accessed: Jun. 25, 2020. [Online]. Available: https://en.wikipedia.org/w/index.php?title=Parylene&oldid=964092525.
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