Specific Process Knowledge/Polymer Processing/Polymer Processing/Coating/Parylene Coater: Difference between revisions
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A parylene coater is the integrated CVD system — vaporiser, pyrolysis furnace, deposition chamber, cold trap, and vacuum system — used to run this process. | A parylene coater is the integrated CVD system — vaporiser, pyrolysis furnace, deposition chamber, cold trap, and vacuum system — used to run this process. | ||
===Chemistry in Brief=== | |||
The precursor for each parylene type is a cyclic dimer of the corresponding p-xylylene unit (di-para-xylylene, “DPX,” for Parylene N; the chlorinated dimer for Parylene C; and so on). Heating this solid dimer under vacuum sublimes it directly to a vapour without melting. Pyrolysis at high temperature homolytically cleaves the dimer's two methylene–methylene bonds, producing a pair of reactive p-xylylene monomers (diradicals). These monomers do not react with each other in the gas phase because the chamber is kept cold and dilute; instead, they diffuse to any surface at or near room temperature, adsorb, and rapidly propagate into a linear, high-molecular-weight polymer directly on that surface. There is no solvent, no initiator, and no exotherm large enough to damage typical substrates — polymerisation is driven by the radical reactivity of the monomer itself, not by added chemistry. | |||
Because polymerisation happens directly on the substrate surface rather than in solution, the resulting film has very high molecular weight, high crystallinity (grade-dependent), and essentially no seams, pinholes, or coating-induced stress from solvent evaporation or shrinkage, which is the root reason parylene outperforms liquid coatings as a barrier film. | |||
=== Working Principle === | === Working Principle === | ||