To produce homopolymer formaldehyde with formaldehyde as the monomer, high-purity formaldehyde monomers must be used. The production process mainly consists of three steps: purification of formaldehyde, polymerization and end sealing.
Purification of formaldehyde
Although formaldehyde is highly prone to polymerization, in the presence of impurities and water, its chain can easily terminate, resulting in only low-molecular-weight polymers. The purification of formaldehyde can be achieved by the thermal decomposition of α -polyoxymethylene or by the thermal decomposition of hemiacetal.
The production of pure formaldehyde from α -polyoxymethylene involves converting polyoxymethylene into α -polyoxymethylene under acidic or alkaline conditions first to remove methanol. After washing, vacuum dry to constant weight. The purified α -polyoxymethylene has a moisture content of less than 0.2% and can be thermally decomposed at around 168℃ to obtain high-purity formaldehyde.
The preparation of pure formaldehyde from hemiacetal involves reacting a formaldehyde aqueous solution with cyclohexanol, followed by vacuum distillation to azeotrope out water and part of the cyclohexanol, resulting in hemiacetal with extremely low water content. Then, the hemiacetal is heated to 130-160 ℃ to decompose. Cyclohexanol is condensed and recovered. The formaldehyde gas passes through a cooler with a temperature of 0℃ to remove the trace impurities it contains, and pure formaldehyde is obtained.
2. Aggregation
Polymerization reactions with formaldehyde as the monomer are generally carried out in inert solvents such as refined cyclohexane. Tri-n-butylamine is used as the initiator. Pure formaldehyde gas is diluted with ultra-pure and dry nitrogen, passed through a purifier at -15℃ to remove residual impurities, and then introduced into a polymerization reactor where solvents and initiators have been added. The amount of initiator used is approximately 1/4000 of the solvent volume. The reaction temperature can be within the range of -20 to 80℃, but room temperature or slightly higher is preferred. The pressure is atmospheric pressure. During the reaction process, vigorous stirring is required, and the polymer precipitates out. After filtration, washing and drying, white powder-like homopolymer formaldehyde can be obtained.
3. End sealing
The end groups of homopolymer formaldehyde obtained by the polymerization of formaldehyde are hydroxyl groups. When the polymer is heated, it is prone to depolymerization starting from the end hydroxyl groups, significantly reducing the molecular weight. Therefore, groups such as acyl, ether or isocyanate groups are used to replace the hydroxyl groups to improve the thermal stability of the resin. In the process, this is called end-capping.
In industry, acetic anhydride is commonly used to undergo esterification reactions with the hydroxyl groups of polymers to obtain polyoxymethylene containing acetate ester end groups. The yield of acidification reaction is inversely proportional to the concentration of acetic acid in acetic anhydride. Therefore, the content of acetic acid in acetic anhydride should be strictly controlled and generally should not exceed 0.05%. Acidification catalysts such as sodium acetate and tertiary amines can react with acetic acid, which is conducive to the progress of esterification reactions.
Esterification takes place in a drum reactor. The temperature of the feed polymer is 25 to 75℃, and the vapor temperature of acetic anhydride is 140 to 160℃. Inert gas or vapor of inert medium can be mixed into acetic anhydride. The ratio of acetic anhydride to polymer is 0.1 to 1:1. The dosage of sodium acetate is 0.01% to 0.1% of the mass of acetic anhydride. The esterification reaction can be completed at around 160℃ for 1 hour. The esterification product was repeatedly washed with acetone and water and fully dried to obtain stable homopolymer formaldehyde powder.
After esterification and capping of homopolymer polyoxymethylene powder, it is mixed with antioxidants, stabilizers and other additives, and then extruded and granulated by an extruder to obtain polyoxymethylene particles for molding.