Parylene is the generic name for the poly-para-xylylenes. These materials form linear, highly-crystalline polymers but can be usefully produced only as coatings and films. The most commonly used is Parylene C, the mono-chloro substituted compound. Parylene N, the un-substituted compound, has better high-frequency dielectric properties, better penetrating power for coating the bore of very small diameter tubes, and is often preferred in medical applications. Parylene D, the di-chloro-substituted compound, has better high temperature endurance.
The Parylene Coating Process
The Parylene process is unique in coating technology and is best described as a vapor deposition polymerization. It is carried out under vacuum and requires specialized equipment.
1. The process begins with sublimation at about 150°C of the high purity crystalline dimer di-p-xylylene.
2. The vapor is pyrolised at about 650°C to form the gaseous monomer which has an olefinic structure.
3. The coating chamber is at room temperature. The vapor condenses on all surfaces equally and can pass through holes as small as 1μ. It then spontaneously polymerizes to form a product with a high degree of crystallinity. The coating is absolutely conformal and can be laid down in thicknesses from a few angstroms to 50 microns or more depending on the requirements of the end use.
Parylene is applied at room temperature with specialized vacuum deposition equipment that permits control of coating rate and thickness. The deposition process takes place at the molecular level as the chemical, in dimer form, is converted under vacuum and heat to dimeric gas; pyrolized to cleave the dimer; and finally deposited as a clear polymer film.