Research Progress on Polyformaldehyde Process

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Advances in the research on polyoxymethylene production: Polyoxymethylene was first synthesized by Butletrow in 1851 from the residues resulting from the vacuum distillation of an aqueous formaldehyde solution. However, due to an error in judgment, it was initially thought to be paraformaldehyde, and later considered to be trimethylolpropane. The name polyformaldehyde first appeared in 1888, when Tollens and Mayer named the polymeric residue obtained by evaporating a formaldehyde solution polyformaldehyde. In 1897, after thorough research, Delepine concluded that polyformaldehyde is a mixture of hydroxide polymers formed by the polycondensation of methylene glycol. In the 20th century, extensive research and discussion on polyformaldehyde were conducted. Before the 1950s, the production of polyformaldehyde was mainly at the laboratory research stage. The main aspect of process research is how to concentrate low-concentration formaldehyde to produce polyformaldehyde. From the early 20th century to the late 1920s, the production of polyformaldehyde mainly involved using silver methoxide as a catalyst for oxidative reactions; the resulting gas was then washed with water to produce a formaldehyde solution. This solution was cooled and crystallized, followed by precipitation and separation, and finally filtered to yield polyformaldehyde with a concentration of around 60%. The product had a viscous paste-like consistency, contained low levels of aldehydes, had a low yield, and also resulted in significant emissions of volatile aldehydes. In the 1930s, researchers such as Otto Fuchs and Erich Naujoks developed a method for producing polyformaldehyde by adding co-solvents (ethyl acetate or dichloroethane) to low-concentration formalin solutions and then carrying out azeotropic dehydration to concentrate the formalin. This was the first approach to using azeotropic distillation for the production of PF; however, it had drawbacks such as low dehydration efficiency of the co-solvents, partial miscibility with the mother liquor, and low rates of recovery and reuse. From the 1950s to the 1970s, the industrial production of polyformaldehyde began to develop rapidly. Most of the literature reports that polyformaldehyde is produced by using formaldehyde subjected to various concentration processes to create concentrated aldehydes; after cooling and solidification in a rotating drum, it is then scraped off to obtain the product. The concentration of this product is not high, so further drying is required. In 1950, Bertrang W. and others first proposed a production method for polyformaldehyde by vacuum-concentrating formalin solutions. In 1951, Alexander F., Walter E. and others prepared polyformaldehyde by subjecting formalin to two stages of evaporation concentration followed by vacuum drying in a rotary dryer, in order to reduce the loss of aldehydes during the drying process; this was the earliest method used for producing polyformaldehyde. In 1950, Joseph F. and others proposed a method for producing polyformaldehyde from formaldehyde gas, involving passing an aldehyde-containing gas through a stirred-tank dryer in which concentrated formaldehyde was being dried; the formaldehyde in the gas phase was absorbed and polymerized on the surface of the solidified concentrated formaldehyde, thereby enabling further concentration to produce high-concentration polyformaldehyde. However, this method has not been put into industrial use due to pipeline blockages caused by the easy condensation and solidification of the aldehyde-containing gas. Allen G. et al. used ether in countercurrent absorption to capture the reaction product gas containing formaldehyde and inert gases generated during formaldehyde production; after absorption, the resulting suspension was separated by precipitation, and then the ether was evaporated to obtain solid polyformaldehyde. This approach provides a method for producing polyformaldehyde by absorbing, solidifying, and separating formaldehyde using a cold inert organic liquid. Studies by Hughes W., Bertrand W., and others have shown that drying a mixture of α-polyoxymethylene and pasty polyoxymethylene not only enables the production of modified polyoxymethylene with high concentration, good water solubility, and excellent reactivity, but also results in minimal loss of formaldehyde during the drying process, thereby **increasing the yield of the product**. Among them, α-polyoxymethylene can be obtained by adding concentrated aldehyde to sulfuric acid or caustic alkali under controlled conditions. Alexander G. and Yankers were the first to study and propose the heating depolymerization process in 1956; by applying heat before the polymerization of formaldehyde solution, the oligomers present in the solution were depolymerized into methylene glycol or formaldehyde molecules, thereby avoiding the problems of excessive polymerization degree and poor water solubility of paraformaldehyde products caused by the presence of oligomers. Harold F. and others reported that a paddle dryer was used, along with the adjustment of a certain vacuum level, to produce polyformaldehyde; the aldehyde content in the resulting product could reach 95%. A traditional vacuum rake dryer process was also developed for the production of PF, but this process requires sophisticated equipment, makes it difficult to obtain the product, and results in a lumpy product that is hard to crush. A patent filed by Sumitomo Chemical in 1961 described a process for producing flaky polyformaldehyde: after formaldehyde was concentrated, it was cooled and solidified using a drum; thereafter, the solid formaldehyde was scraped onto a metal conveyor belt where it underwent polymerization and drying. The patent also explored the conditions for the initial cooling and solidification of the formaldehyde solution, providing a basis for subsequent polymerization of formaldehyde solutions and controlled-temperature drying to produce polyformaldehyde. In a study by Friedrich L. and others, industrial formaldehyde was vacuum-concentrated and then sprayed into a cold, inert organic liquid with a low boiling point; the concentrated formaldehyde droplets cooled and solidified into granules, which were separated by sedimentation. The remaining organic liquid was then removed by evaporation, and polyformaldehyde was obtained through vacuum drying. This represented one of the earlier methods to use spray granulation for the production of polyformaldehyde with good flow properties. In 1966, Hans J. Mainz and others proposed the use of spray drying to produce flowable granular polyformaldehyde, thus introducing a competitive method for its production – the spray drying process. Entering the 1970s, Italian joint-stock companies further developed and refined the production process for polyformaldehyde via azeotropic distillation, enabling the production of polyformaldehyde with good water solubility, a porous structure, and a high aldehyde content (90%–99%). Moller et al. used spray granulation, or the method of cooling and solidifying concentrated formaldehyde before extruding it into granules, followed by two-stage fluidized bed drying with controlled temperatures, to produce polyformaldehyde that is flowable, non-sticky, and uniform in quality, with a purity of over 95%. This is one of the several processes that are well-suited for commercial use in industrial production today. In 1985, Celanese introduced a process for the production of polyformaldehyde using spray fluidized bed composite drying. Spray drying enables the production of high-quality PF products with a purity of over 90%, and by using a fluidized bed, the purity can be increased to 99% with uniform product flow. A Japanese patent from 1992 described a process for producing polyformaldehyde using microwave drying. After cooling and solidifying 78–83% concentrated formaldehyde and subjecting it to pre-drying, microwave and far-infrared radiation were used for penetrating drying. This method ensures uniform heating of the material, resulting in homogeneous products, and it also allows for **shorter drying times; it is therefore a promising new processing technique. In 2000, Zhao Yuming and others used the catalytic air oxidation of methylal to produce polyformaldehyde: the gas containing formaldehyde and water obtained after oxidizing methylal was subjected to hydropolymerization, followed by separation of gas, solid, and liquid phases; the products obtained were concentrated formaldehyde as well as solid polyformaldehyde. However, the equipment required for cooling the product gas takes up a lot of space, product separation is difficult, and the formaldehyde content in the resulting paraformaldehyde is less than 90%. In summary, research on the production of polyformaldehyde is moving in the direction of low consumption, low cost, and cleanliness with no pollution.