Preparation process and reaction process of Parylene film
Parylene is a polymer material obtained by vacuum vapor deposition polymerization. Its unique technology and performance advantages have been continuously used in many fields. According to the different molecular structures, parylene can be divided into N-type, C-type, D-type, F-type, and so on.
Preparation technology of parylene film
Different from the general construction method of preparing coatings from liquid coatings, the parylene film is prepared by chemical vapor deposition (CVD), the whole process is carried out under vacuum conditions, and the dimer of para-xylene is decomposed at a high temperature. The active monomer is deposited on the surface of the object and polymerized to form a fully coated film.
It can be used for any shape of surface coating, including sharp edges and corners, the interior of crevices, and extremely fine pinholes, and the film thickness deposited on any part of the product is very uniform and does not produce dead spots. When the thickness of the deposited film exceeds 0.2 μm, pinholes in the film can be eliminated. This feature is difficult to achieve with other coating methods.
The process of preparing the parylene film is divided into three steps:
(1) Under the vacuum condition of 133Pa (1torr), the cyclic dimer of p-xylene is heated and sublimated into gaseous cyclic dimer in the form of solid powder at about 150 ℃;
(2) Under the vacuum condition of about 650℃ and the air pressure of 67Pa, the dimer of the gaseous p-xylene is cracked into the active intermediate p-xylene;
(3) The gaseous active intermediate enters the vacuum deposition chamber with a low temperature and a pressure of 13.3Pa, adsorbs on the substrate, and polymerizes to form a linear polymer material (ie, a parylene film). The conversion of dimer to parylene was nearly 100%. The above three vacuum states can be achieved industrially by a mechanical vacuum pump, which is connected to the vacuum deposition chamber through a -70 ℃ cold trap. The function of the cold trap is to capture free small molecules to protect the vacuum pump.
The sublimation, cracking and deposition polymerization temperature of different types of cyclic dimers will be different, but the film-forming mechanism is the same. Para-xylene benzene is a highly reactive intermediate, stable in a gaseous state at low pressure and high temperature, and rapidly polymerized into para-xylene polymer once coagulated at room temperature.
Therefore, after the p-xylene benzene produced by pyrolysis enters the deposition area at room temperature, it first diffuses on the surface of the workpiece. After being adsorbed to each surface, vapor deposition occurs and at the same time, it rapidly polymerizes into a high molecular weight polymer film, and the polymerization reaction proceeds according to a free radical mechanism.
To a certain extent, the reaction is terminated due to the termination of the diradical or the transfer to a chain transfer agent (such as oxygen), or it can also be terminated by the active radicals being embedded in the polymer chain.
Since the liquid phase is avoided directly from a gaseous monomer to a high-molecular-weight solid polymer, and the polymerization is carried out at room temperature, this process removes the defects commonly found in liquid coatings of uneven thickness.
In addition, this process does not involve solvents in common liquid coatings and does not generate volatile gases, making it an environmentally friendly film-forming process.
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