Parylene Chemistry, Grades and Properties

Parylene is a widely used material that is mostly applied on critical electronic circuit to provide a strong protection in harsh environment. Especially for the medical device, aerospace and space industry, parylene is the material of choice.

Parylene is a polymer with unique properties and deposition process. The vapor phase polymerization combined with a particular molecular structure allows the production of highly impermeable thin films ideal for high performance barrier coatings.

As job coating service, COAT-X produces thin films of Parylene N, C, F-VT4, F-AF4 under the name CX PROTECT and multilayer composite barrier coatings as CX ULTRA and CX ULTIMA.

Deposition machines for parylene thin films are available in various sizes from 30L to 600L (CX-30, CX-300 and CX-600).

For best barrier properties, we have developped the CXC-20, a cluster system combining a CVD reactor for Parylene films and an ALD (Atomic Layer Deposition) reactor for defect-free ultra-thin ceramic films.

Parylene Chemistry

Parylene is the generic name for a family of linear thermoplastic polymers derived from poly-para-xylylene. Its base structure consists of aromatic benzene rings connected by methylene groups (–CH₂–), forming a linear, semi-crystalline thermoplastic polymer chain. The fundamental repeating unit for Parylene N is: –[ CH₂–C₆H₄–CH₂ ]n–

This structure imparts high molecular symmetry, enabling tight chain packing, a high degree of crystallinity, and exceptional barrier properties.

The degree of polymerization (n) typically reaches values in the tens of thousands, yielding high-molecular-weight films (Mw ~500,000–1,000,000 g/mol) with excellent mechanical integrity. Crystallinity provides rigidity, chemical resistance, and moisture barrier properties.

Unlike conventional liquid coatings, parylene never exists in liquid form: the polymer forms directly on the substrate surface through the polymerization of gaseous monomers at room temperature, a unique process that gives it its perfectly conformal coating properties.

Parylene Types

Five grades are commercially available. Each results from a modification of the base chemical structure through substitution of hydrogen atoms with halogens (chlorine, fluorine) or other functional groups.

• Parylene N is the founding member of the family, composed only of carbon and hydrogen. Its highly crystalline linear structure gives it the lowest refractive index and the best crevice penetration of all grades.
• Parylene C is the most widely used grade industrially. The addition of one chlorine atom at position 2 of the aromatic ring significantly improves the moisture and corrosive gas barrier compared to grade N, while maintaining excellent dielectric properties.
• Parylene D carries two chlorine atoms (positions 2 and 5). This double substitution provides superior thermal resistance over grade C, at the cost of a slightly higher dielectric constant. Used for applications exposed to sustained elevated temperatures.
• Parylene VT4 all four aromatic ring hydrogens are replaced by fluorine, with unmodified methylene bridges (–CH₂–). Ring fluorination markedly lowers the surface energy of the polymer, reduces the UV absorption cross-section of the aromatic chromophore, and passivates the ring against electrophilic chemical attack.
• Parylene AF4 is fluorinated: the hydrogen atoms of the –CH₂– group are replaced by fluorine atoms. This modification delivers exceptional thermal stability (up to 450 °C peak) and UV resistance, as well as an extremely low coefficient of friction. It is the most advanced grade for aerospace, space, and demanding biomedical applications.
• Multilayer composite coatings combine parylene with thin ceramic layers deposited by ALD (Atomic Layer Deposition), like aluminium oxide (Al₂O₃), silicon dioxide (SiO₂), or titanium dioxide (TiO₂). These hybrid architectures reduce the water vapour transmission rate (WVTR) by several orders of magnitude compared to parylene alone.

Parylene Deposition Process

Parylene is deposited by a three-step CVD (Chemical Vapour Deposition) process carried out under vacuum. This unique process, solvent-free, catalyst-free, at room temperature allows it to uniformly coat all surfaces, including complex 3D geometries, cavities, and sub-surfaces.

The parylene coating process is a three-stage vacuum CVD process. Each stage has distinct thermodynamic and kinetic characteristics.

Stage 1 — Dimer Sublimation (Vaporization)

~150 °C · under vacuum

The solid dimer (di-para-xylylene powder) is heated until it sublimates, transitioning directly from solid to gas phase without passing through a liquid state.

Stage 2 — Pyrolytic Cleavage (Cracking)

~680 °C · quartz tube furnace

The dimer gas passes through a high-temperature furnace that breaks the central C–C bond of the dimer, producing two highly reactive para-xylylene monomer radicals.

Stage 3 — Deposition & polymerization

Room temperature – deposition chamber

The monomer vapour enters the room-temperature chamber. As it condenses on all surfaces, it spontaneously polymerizes, growing molecule by molecule into a continuous film.

This vapor-phase surface polymerization requires no solvent, no initiator, no catalyst, and occurs at room temperature on the substrate. The resulting film is exceptionally pure (>99.9wt%), defect-free, and grows simultaneously on all exposed surfaces regardless of geometry, including re-entrant features, capillary gaps, and the undersides of components— a conformality that no liquid-applied coating can replicate.