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) Saigang is also called polyformaldehyde (r), which belongs to the plastic category.
) (Saigang) is a tough and elastic material that still has good creep resistance, geometric stability and impact resistance even at low temperatures. There are both homopolymer and copolymer materials. Homopolymer materials have good ductility and fatigue strength, but are not easy to process. Copolymer materials have good thermal and chemical stability and are easy to process. Both homopolymer and copolymer materials are crystalline materials and do not absorb moisture easily. The high degree of crystallization results in a very high shrinkage rate, which can be as high as %~%. There are different shrinkage rates for various reinforced materials.
)Product preparation method
)The synthesis of polyformaldehyde generally involves condensation polymerization of an aqueous solution of formaldehyde in the presence of acid. Polyformaldehyde with a polymerization degree of above is obtained, and then heated and decomposed into formaldehyde gas. After purification and dehydration, the monomer is usually purified by partial prepolymerization, and then passed into a dry solvent containing a small amount of initiator for polymerization. Because of the presence of water, the molecular weight is significantly reduced. The initiator can be Lewis acid or alkali. However, most of them use tertium for anionic addition polymerization. The reaction is as follows: the end group of polyoxymethylene is hemiacetal (). When the temperature is higher than ℃, the end group is easy to break, and it generally needs to be stabilized by end group treatment. After stabilization treatment, it can withstand heat up to ℃. Paraformaldehyde can be processed at temperatures of ~℃, such as injection extrusion blow molding, etc.
) Homopolymer formaldehyde has high crystallinity, mechanical strength, rigidity, thermal deformation temperature, etc. than copolymer formaldehyde. Copolymer formaldehyde has a lower melting point, thermal stability, chemical corrosion resistance, flow characteristics, and processability than homopolymer formaldehyde. New The products developed are ultra-high flow (rapid prototyping), impact-resistant and mold deposition-reducing grades, as well as inorganic-filled, reinforced grades.
) Homopolyformaldehyde is prepared from formaldehyde as a monomer. The raw material formaldehyde often contains a large amount of water formaldehyde and other impurities. It needs to be refined to obtain high-purity formaldehyde. It is polymerized into homopolyformaldehyde by passing it into an inert solution containing a cationic catalyst (such as boron trifluoride etherate complex), and then in The terminal hydroxyl groups are esterified in the presence of acetic anhydride to obtain heat-stable polyformaldehyde, and then antioxidants and other additives are added to granulate into homopolyoxymethylene products.
Copolymer formaldehyde is mainly prepared by copolymerization of trimer formaldehyde. The concentration is %~% formaldehyde. Under the catalysis of concentrated sulfuric acid or cation exchange resin, trioxane is obtained and distilled into high purity products. The latter and a small amount of comonomers (such as dioxane) are ring-opening polymerized in the presence of Lewis acid to form copolymers. formaldehyde. Most polymerization methods are bulk polymerization, using twin-screw extruders. Most of the copolymer acetal chain ends are hemiacetal end groups, which are extremely unstable to heat. They need to be end-capping and stabilizing to become a heat-stable polyacetal. Add antioxidants and other additives to granulate copolymer acetal products. .
)Application scope
)Industrial application has a very low friction coefficient and good geometric stability, and is especially suitable for making gears and bearings. Since it also has high temperature resistance, it is also used in plumbing equipment (valve pump housings), lawn equipment, etc.
)Civilian application scope: VCR players, radios, headphones, stereos and other audio equipment, printers, keyboards, R drivers and other machines, washing machines, dryers, hair dryers and other household appliances, seat belt mechanical parts, door exterior handles, reflectors, engine rooms, etc. for automobiles. Parts include precision parts for cameras, watches, etc., as well as building materials, toys such as game consoles, and molding materials for stationery.