Today, fine chemical raw material manufacturers introduce the process and research results of preparing cellulose membrane with methyl morpholine oxide
In the process of preparing cellulose membrane by NMMO, there are two ways to dissolve cellulose in NMMO: direct dissolution method and indirect dissolution method. The direct dissolution method is to reduce the water content of NMMO solvent to below 13% by vacuum distillation, and then add cellulose into NMMO solvent for dissolution. Indirect dissolution method is to add cellulose into NMMO solvent without thickening for swelling, and then dehydrate by distillation under reduced pressure.
Cellulose mixed with NMMO solution is dehydrated and dissolved under reduced pressure at 85 ~ 120℃ to obtain cellulose solution with higher concentration; After filtering and defoaming, the cellulose solution with high concentration is extruded through an annular die or a slit die, solidified and formed in a low-temperature water bath or a mixed bath of NMMO and H 2 O, and finally, NMMO cellulose membrane is prepared by stretching, washing with water, plasticizing, drying and other processes
First, the choice of raw materials
Cellulose pulp used for film formation can be wood pulp or cotton pulp. Like other cellulose products, cellulose pulp requires low impurity content and narrow molecular mass distribution. Lyocell fiber was produced with NMMO as solvent and unbleached pulp containing a large amount of hemicellulose and lignin as raw material. This process reduced the processing process of fiber, increased the yield of cellulose, reduced the production cost, and kept the excellent characteristics of traditional Lyocell fiber.
Bamboo-based cellulose film was prepared by NMMO with bamboo pulp as raw material. It was found that the bamboo-based cellulose film prepared by NMMO had similar physical properties to original bamboo fiber.
Lyocell fiber was produced from bamboo pulp containing lignin, hemicellulose and calcium, magnesium and iron plasma, and NMMO was used as solvent. The technological conditions of dissolving bamboo cellulose with NMMO were studied, and how to select bamboo pulp raw materials for preparing bamboo Lyocell fiber was expounded.
Bagasse pulp was prepared by ethanol method, and cellulose of bagasse pulp was dissolved into a film with NMMO as solvent, and the structure of the prepared cellulose film was explored.
Second, the dissolution of cellulose in NMMO
When cellulose is added to NMMO, swelling occurs first, and the fast moving NMMO molecules diffuse into cellulose and reach the surface of amorphous region and crystallization zone. At this time, the hydrogen bond of amorphous region first changed, and the hydrogen bond between cellulose molecules was opened and replaced by the hydrogen bond between cellulose and NMMO molecules. With the breaking of hydrogen bond in amorphous region, NMMO molecules continuously enter the amorphous region and crystallization zone of cellulose, and form swelling compounds, which destroy the supramolecular junction of cellulose. The research progress of preparing cellulose membrane by using methyl morpholine oxide, the hydrogen bond in the crystallization zone is also continuously opened, and finally NMMO molecules enter indefinitely, resulting in the dissolution of cellulose.
Microwave (power 105 ~ 490 W, frequency 2450 MHz) was used to heat NMMO dissolved cellulose. Microwave heating in the process of cellulose dissolution can greatly shorten the cellulose dissolution time and reduce the energy consumption
Different concentrations of cellulose /NMMO/ water system were used to prepare membranes, and the crystallization zone and polymerization degree of cellulose membranes were measured. It was found that the microwave heating power was 210 W, which was most beneficial to the dissolution of cellulose in NMMO.
Cellulose films were prepared by blowing film method, and cellulose films with different mechanical properties were obtained by changing the blowing ratio and drawing speed. With the increase of the strength of cellulose membrane, the cellulose membrane can be blown very thin, thus reducing the cost per unit area of cellulose membrane.
Cellulose membranes were prepared by NMMO method with water, methanol, ethanol and their two-component solutions as coagulation baths. The results show that the cellulose films prepared by different coagulation baths have obvious differences in dry morphology. The wet porosity of cellulose membrane is basically not affected by the composition of coagulation bath, while the dense region in dry state presents porous structure after water swelling. The pore diameter increases with the increase of crystallinity of cellulose, and the crystallinity of cellulose determines the pore diameter of wet film to a certain extent, so it has great influence on the average pore diameter of wet film. By changing the coagulation bath composed of methanol and ethanol, the dry or wet structure of cellulose membrane can be controlled separately or synergistically.
Asymmetric cellulose membrane was prepared by L-S phase inversion method with NMMO as solvent. The results show that with the increase of scraping speed, the average surface roughness of the prepared cellulose membrane becomes smaller, the membrane surface becomes flat, the pore diameter becomes smaller, the crystallinity increases, the orientation degree in vertical and horizontal directions increases to a certain extent, the water flux and transparency decrease, while the rejection rate and tensile strength increase.
3. Forming of NMMO cellulose membrane
Cellulose membrane with good gas permeability was prepared by using cotton fiber as raw material. It is found that when the cellulose concentration is 5%, the cellulose membrane has good apparent quality and high transparency.
NMMO dissolves cellulose, and inorganic fillers are added into cellulose solution to prepare cellulose membrane. The effects of different filler types and amounts on the mechanical properties of cellulose membrane are studied. The results showed that the mechanical properties of inorganic filled cellulose membrane were better when SiO 2, porous zeolite, CaCO _ 3 and TiO _ 2 were added at 1. 5%, 1. 0%, 0. 5% and 0. 1% respectively, and the mechanical properties of inorganic filled cellulose membrane treated by silane coupling agent with 1% addition were better. Organic clay (dodecyl triphenyl-phosphorus-mica, C12PPh-mica) was added into cellulose /NMMO solution to improve the mechanical properties and gas barrier properties of cellulose membrane. It is found that adding a small amount (1% ~ 7%) of organic clay can achieve good results.