Paraxylylene

Paraxylylene, also known as para-xylylene, is a hydrocarbon diradical that serves as the monomer unit used to form poly(p-xylylene) coatings, commonly referred to as parylene. The monomer consists of a benzene ring with two methylene groups attached at the para positions, giving a formula around C8H8. In practical use, paraxylylene is generated in situ from a dimer precursor that cleaves thermally to produce the reactive p-xylylene species, which then polymerizes upon contact with a surface to form a thin, conformal film.

In industry, paraxylylene coatings are produced using the Gorham process. In a vacuum deposition chamber, the p-xylylene dimer is sublimed and heated to induce pyrolysis, producing the reactive monomer that deposits on all exposed surfaces and rapidly polymerizes into poly(p-xylylene). This yields uniform, pinhole-free coatings that conform to complex geometries and internal features.

Properties of parylene coatings include chemical inertness, excellent moisture and chemical barriers, and good dielectric properties. The films are typically biocompatible and adhere well to a wide range of substrates, including metals, ceramics, and polymers. The ability to coat intricate shapes at room temperature in vacuum makes parylene particularly suitable for sensitive devices and precision components.

Applications span electronics packaging, medical devices, MEMS, ophthalmic implants, corrosion protection, and protective optical coatings. Parylene is the trade name commonly used for the polymer poly(p-xylylene), noted for its conformal, durable coatings across various industries.