PARYLENE EXPLANATION FROM WIKIPEDIA
Parylene is the trade name for a variety of chemical vapor deposited poly(p-xylene) polymers used as moisture barriers and electrical insulators. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other manufacturing advantages.
Parylene is a green chemistry, which is self-initiated (no initiator needed) and un- terminated (no termination group needed) with no solvent or catalyst required. The precursor, [2.2]paracyclophane, yields 100% monomer and initiator and does not yield any by-products.
Parylene C and to a lesser extent AF-4, SF, HT (all the same polymer) are used for coating printed circuit boards (PCBs) and medical devices. There are numerous other applications as parylene is an excellent moisture barrier. It is the most bio-accepted coating for stents, defibrillators, pacemakers and other devices permanently implanted into the body.[citation needed]
Parylenes are relatively soft (0.25 GPa) except for Parylene X (1.0 GPa) and they have poor oxidative resistance (~115 oC) and UV stability, except for Parylene AF-4. However, Parylene AF-4 is more expensive due to a three-step synthesis of its precursor with low yield and a poor deposition efficiency.
Parylene N is a polymer manufactured from di-p-xylylene, a dimer synthesized from p- xylylene. Di-p-xylylene, more properly known as [2.2]paracyclophane, is made from p- xylylene in several steps involving bromination, amination and elimination. [1]
Parylene N is an unsubstituted molecule. Heating [2.2]paracyclophane under low pressure (1.0 mTorr- 1.0 Torr) conditions gives rise to a diradical species[2] [3] which polymerizes when deposited on a surface. Until the monomer comes into contact with a surface it is in a gaseous phase and can access the entire exposed surface. It has a variety of uses. In electronics, chemical vapor deposition at low pressure onto circuit boards produces a thin, even conformal polymer coating.
Other derivatives
There are a number of derivatives and isomers of parylene including: Parylene N (hydrocarbon), Parylene C (one chlorine group per repeat unit), Parylene D (two chlorine groups per repeat unit), Parylene AF-4 (generic name, aliphatic flourination 4 atoms), Parylene SF, Parylene HT (AF-4, SCS product), Parylene A, Parylene AM (one methylene amine group per repeat unit), Parylene VT-4 (generic name, fluorine atoms on the aromatic ring), Parylene CF, and Parylene X (a cross-linkable version, not commercially available). Characteristics and advantages
• Hydrophobic, chemically resistant coating with good barrier properties for inorganic and organic media, strong acids, caustic solutions, gases and water vapor • Low leakage current and a low dielectric constant (average in-plane and out-of- plane: 2.67 parylene N and 2.5 parylene AF-4, SF, HT)[4] • A biostable, biocompatible coating; FDA approved for various applications • Dense pin-hole free from 14 Å thickness [5] • Thin highly conformal transparent coating • Coating without temperature load of the substrates as coating takes place at ambient temperature in the vacuum • Highly corrosion resistant • Completely homogeneous surface • Oxidatively stable up to 350°C (Parylene AF-4, SF, HT) • Low intrinsic thin film stress due to its room temperature deposition • Low coefficient of friction (AF-4, HT, SF) • Very low permeability to gases Typical applications
• Dielectric coating (e.g. cores/coils) • Hydrophobic coating (e.g. biomedical hoses) • Barrier layers (e.g. for filter, diaphragms, valves) • Microwave electronics • Sensors in rough environment (e.g. automotive fuel/air sensors) • Electronics for space travel and military • Corrosion protection for metallic surfaces • Reinforcement of micro-structures • Abrasion protection • Protection of plastic, rubber, etc. from harmful environmental conditions • Reduction of friction (e.g. for guiding catheters, also acupuncture needles) • Dissolving deuterated polyethylene for making nuclear targets