Comparison of Different Types of Polysilazanes (PHPS / OPSZ / MPSZ) - Part 1
Polysilazane (PSZ), as a high-performance precursor polymer, is receiving increasing attention. It has a main chain structure based on Si-N bonds, possessing excellent high-temperature resistance, corrosion resistance, and mechanical properties, and can be converted into ceramic coatings or materials. Based on the different side chain substituents, polysilazanes are mainly divided into three types: perhydropolysilazane (PHPS), organopolysilazane (OPSZ), and methylpolysilazane (MPSZ).
PHPS (Perhydropolysilazane): The structural formula is [-SiH₂-NH-]ₙ, where all substituents are hydrogen atoms, belonging to inorganic polysilazanes. The molecular chain is highly reactive and sensitive to moisture and oxygen.
OPSZ (Organopolysilazane): At least one organic substituent (such as methyl, vinyl, etc.) is present on the silicon atom in the structure, commonly polymethyl(hydrogen)/polydimethylsilazane. It belongs to the organically modified type; the introduction of organic groups improves stability.
MPSZ (Methylpolysilazane): Usually refers to polysilazanes where the main substituent on the silicon atom is a methyl group (such as polydimethylsilazane or methylhydrogenpolysilazane), which can be considered a specific variant of OPSZ, emphasizing the hydrophobicity and flexibility brought about by methyl substitution.
Comparison of Curing Mechanisms and Conversion Behavior
The curing of polysilazanes usually involves hydrolysis, oxidation, and cross-linking, ultimately converting into ceramic phases such as SiO₂, SiON, or SiCN.
PHPS: Cures under humid air at room temperature or low-temperature heating (<200°C), mainly through the easy hydrolysis/oxidation of Si-H and Si-N bonds, forming Si-O-Si bonds. The conversion product is carbon-free pure SiO₂. The characteristics are thorough and rapid conversion, but the coating is prone to cracking.
OPSZ: Cures at room temperature or with heating (catalysts can be added), mainly through Si-NH-Si hydrolysis, while the organic groups are partially retained or removed. The conversion product is carbon-containing SiOX or SiCN. The characteristics are a relatively slow conversion rate and good flexibility.
MPSZ: The curing conditions are similar to OPSZ, often carried out at room temperature. The methyl group provides hydrophobicity, and a three-dimensional network is formed through cross-linking. The conversion product is SiOx containing methyl residues. Its characteristics include strong easy-cleaning properties and high hardness (up to 9H).
Overall, the coating after PHPS conversion is denser and smoother, but has poor flexibility; OPSZ/MPSZ, due to the retention of organic groups, results in a more flexible and impact-resistant coating.
