Parylene is the common name of a polymer whose backbone consists of para-benzenediyl rings −C6H4− connected by 1,2-ethanediyl bridges −CH2−CH2−. It can be obtained by polymerization of para-xylylene H2C=C6H4=CH2. The name is also used for several polymers with the same backbone, where some hydrogen atoms are replaced by other functional groups. Some of these variants are designated in commerce by letter-number codes such as "parylene C" and "parylene AF-4". Some of these names are registered trademarks in some countries. Coatings of parylene are often applied to electronic circuits and other equipment as electrical insulation, moisture barriers, or protection against corrosion and chemical attack (conformal coating). They are also used to reduce friction and in medicine to prevent adverse reactions to implanted devices. These coatings are typically applied by chemical vapor deposition in an atmosphere of the monomer para-xylylene. Parylene is considered a "green" polymer because its polymerization needs no initiator or other chemicals to terminate the chain; and the coatings can be applied at or near room temperature, without any solvent.
History Parylene was discovered in 1947 by Michael Szwarc as one of the thermal decomposition products of para-xylene H3C−C6H4−CH3 above 1000 °C. Szwarc identified para-xylylene as the precursor by observing that reaction with iodine yielded para-xylylene di-iodide as the only product. The reaction yield was only a few percent. A more efficient route was found in 1965 by William F. Gorham at Union Carbide. He deposited parylene films by the thermal decomposition of [2.2]paracyclophane at temperatures exceeding 550 °C and in vacuum below 1 Torr. This process, referred to as vacuum deposition polymerization (VDP), did not require a solvent and resulted in chemically resistant films free from pinholes. Union Carbide commercialized a parylene coating system in 1965. Union Carbide went on to undertake research into the synthesis of numerous parylene precursors, including parylene AF-4, throughout the 1960s into the early 1970s. Union Carbide purchased NovaTran (a parylene coater) in 1984 and combined it with other electronic chemical coating businesses to form the Specialty Coating Systems division. The division was sold to Cookson Electronics in 1994. There are parylene coating service companies located around the world, but there is limited commercial availability of parylene. The [2.2]paracyclophane precursors can be purchased for parylene N, C, D, AF-4 and VT-4. Parylene services are provided for N, C, AF-4, VT-4 and E (copolymer of N and E).
Varieties
Parylene N Parylene N is the un-substituted polymer obtained by polymerization of the para-xylene intermediate.
Chlorinated parylenes
Derivatives of parylene can be obtained by replacing hydrogen atoms on the phenyl ring or the aliphatic bridge by other functional groups. The most common of these variants is parylene C, which has one hydrogen atom in the aryl ring replaced by chlorine. Another common variant is parylene D, with two such substitutions on the ring. Parylene C is the most used variety, due to its low cost of its precursor and to the balance of its properties as dielectric and moisture barrier properties and ease of deposition. A major disadvantage for many applications is its insolubility in any solvent at room temperature, which prevents removal of the coating when the part has to be re-worked. Parylene C is also the most commonly used because of its relatively low cost. It can be deposited at room temperature while still possessing a high degree of conformality and uniformity and a moderate deposition rate in a batch process. Also, the chlorine on the phenyl ring of the parylene C repeat unit is problematic for RoHS compliance, especially for the printed circuit board manufacture. Moreover, some of the dimer precursor is decomposed by breaking of the aryl-chlorine bond during pyrolysis, generating carbonaceous material that contaminates the coating, and hydrogen chloride HCl that may harm vacuum pumps and other equipment. The chlorine atom leaves the phenyl ring in the pyrolysis tube at all temperatures; however, optimizing the pyrolysis temperature will minimize this problem. The free-radical (phenyl radical) generated in this process is not resonance-stabilized and mitigates the deposition of a parylene-like material on the downside of the pyrolysis tube. This material becomes carbonized and generates particles in situ to contaminate clean rooms and create defects on printed-circuit boards that are often called "stringers and nodules". Parylene N and E do not have this problem and therefore are preferred for manufacturing and clean room use.
Fluorinated parylenes Another common halogenated variant is parylene AF-4, with the four hydrogen atoms on the aliphatic chain replaced by fluorine atoms. This variant is also marketed under the trade names of parylene SF (Kisco) and HT parylene (SCS). The −CF2− unit that comprises the ethylene chain is the same as the repeating unit of PTFE (Teflon), consistent with its superior oxidative and UV stability. Parylene AF-4 has been used to protect outdoor LED displays and lighting from water, salt and pollutants successfully. Another fluorinated variant is parylene VT-4 (also called parylene F), with fluorine substituted for the four hydrogens on the aryl ring. This variant is marketed by Kisco with the trademark Parylene CF. Because of the aliphatic −CH2− units, it has poor oxidative and UV stability, but still better than N, C, or D.
Alkyl-substituted parylenes The hydrogen atoms can be replaced also by alkyl groups. Substitution may occur on either the phenyl ring or the ethylene bridge, or both. Specifically, replacement of one hydrogen on the phenyl ring by a methyl group or an ethyl group yields parylene M and E respectively.
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