Xylylene Polymers

Abstract

AbstractPinhole‐free coatings of outstanding conformality and thickness uniformity are produced by exposing a substrate to a controlled deposition of pure gaseous monomer through the unique chemistry ofp‐xylylene, which is thermally stable but very reactive toward polymerization with other like molecules, in a process known as vapor deposition polymerization (VDP). A high molecular weight coating of poly(p‐xylylene) (PPX) is produced. Recent VDP developments are stressed in the article.The convenient generation of the extremely reactive monomer is accomplished by thermal cleavage of its stable dimer,cyclo‐di‐p‐xylylene (DPX), commercially in the Gorham process. The PPXs formed as coatings in the Gorham process are referred to generically as parylenes. Parylene N, Parylene C, and Parylene D are polymer coatings produced by the Gorham process using the dimers DPXN, DPXC, and DPXD, respectively. These dimers are the feedstocks for the coating process. The Parylene C monomer, chloro‐p‐xylylene, has become the de facto performance standard owing to its room‐temperature volatility. VDP processes using means other than pyrolytic cleavage of DPX to generate reactive monomer, none practiced commercially, include photopolymerization and plasma polymerization, as well as imidization to produce polyimide films.Manufacture of commercial DPXN is through two routes: direct pyrolysis ofp‐xylene and the Hofmann elimination route. DPXC and DPXD are prepared from DPXN by chlorinating. Thermodynamic considerations of the parylene process by VDP are covered. The importance of the physical processes of condensation and diffusion in VDP are treated, as is the propagation reaction.VDP produces films of uniform thickness having excellent conformality and purity. Electrical properties merit their use in electronic construction. Barrier, spectral, and optical properties are also useful. Crystallinity determines important properties of mechanical strength at elevated temperatures and solvent resistance, which constitute the principal advantages of PPX materials. Uses include circuit boards, hybrid circuits, semiconductors, capacitors, the bathythermograph, medical uses (as in pacemakers), artifact conservation, and in laser fusion targets. They are also used as barrier coatings, in corrosion control, and as dry lubricants. The dimers present little concern in handling and storage to an experienced operator. In ancillary operations such as cleaning or adhesion promotion, suitable precautions given in manufacturers' Material Safety Data Sheets should be observed.