Parylene
Parylene is the tradename for a variety of chemical vapor deposited poly(p-xylylene) 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.[1]
Parylenes are relatively soft (0.25 GPa) except for Parylene X (1.0 GPa) and they have poor oxidative resistance (~115 °C) 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.
Contents
Parylene N
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.[2]
Parylene N is an unsubstituted molecule. Heating [2.2]paracyclophane under low pressure (0.01 – 1 Torr) conditions gives rise to a diradical species[3][4] 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 (Kisco product), Parylene HT (AF-4, SCS product), Parylene A (one amine per repeat unit, Kisco product), Parylene AM (one methylene amine group per repeat unit, Kisco product), Parylene VT-4 (generic name, fluorine atoms on the aromatic ring), Parylene CF (VT-4, Kisco product), and Parylene X (a cross-linkable version, not commercially available).
History
Parylene development started in 1947, when Michael Szwarc discovered the polymer as one of the thermal decomposition products of a common solvent p-xylene at a temperature between 700 and 900 °C. Szwarc first postulated the monomer to be para-xylylene which he confirmed by reacting the vapors with iodine and observing the para-xylylene di-iodide as the only product. The reaction yield was only a few percent, and a more efficient route was found later by William F. Gorham at Union Carbide. He deposited parylene films by the thermal decomposition of di-p-xylylene at 550 °C and in vacuum below 1 Torr. This process did not require a solvent and resulted in chemically resistant films free from pinholes. Union Carbide commercialized a parylene coating system in 1965.[5][6]
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)[7]
- A biostable, biocompatible coating; FDA approved for various applications
- Dense pinhole free, with thickness above 1.4 nm[8]
- 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
Parylene films have been used in various applications, including [5]
- Hydrophobic coating (moisture barriers, e.g. for 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, acupuncture needles and Microelectromechanical systems.
- Dissolving deuterated polyethylene for making nuclear targets
- The low dielectric constant of parylene is exploited in dielectric coatings for VLSI interconnects.
References
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- Parylene Specifications and Properties
- Parylene Properties & Characteristics
- Properties of Parylene C & Nde:Parylene
- ↑ James A. Schwarz, Cristian I. Contescu, Karol Putyera (2004). Dekker encyclopédia of nanoscience and nanotechnology, Volume 1. CRC Press. p. 263. ISBN 0824750470.
- ↑ H. E. Winberg and F. S. Fawcett (1973), "Tricyclo[8.2.2.24,7]hexadeca-4,6,10,12,13,15-hexaene", Org. Synth.
- ↑ H. J. Reich, D. J. Cram (1969). "Macro rings. XXXVI. Ring expansion, racemization, and isomer interconversions in the [2.2]paracyclophane system through a diradical intermediate". Journal of the American Chemical SocietyEdition. 91 (13): 3517–3526. doi:10.1021/ja01041a016.
- ↑ P. Kramer, A. K. Sharma, E. E. Hennecke, H. Yasuda (2003). "Polymerization of para-xylylene derivatives (parylene polymerization). I. Deposition kinetics for parylene N and parylene C". Journal of Polymer Science: Polymer Chemistry Edition. 22 (2): 475–491. doi:10.1002/pol.1984.170220218.
- ↑ 5.0 5.1 Jeffrey B. Fortin, Toh-Ming Lu (2003). Chemical vapor deposition polymerization: the growth and properties of parylene thin films. Springer. pp. 4–7. ISBN 1402076886.
- ↑ http://www.svc.org/H/HISTORYA.PDF
- ↑ J.J. Senkevich, S.B. Desu (1999). "Compositional studies of near-room temperature thermal CVD of poly(chloro-p-xylylene)/SiO2 nanocomposites". Chemistry of Materials. 11: 1814. doi:10.1007/s003390051076.
- ↑ J.J. Senkevich and P.-I. Wang (2009). "Molecular Layer Chemistry via Parylenes". Chem. Vapor Dep. 15: 91. doi:10.1002/cvde.200804266.
