In recent years, due to the serious environmental pollution caused by plastic films, the development and use of biodegradable films has attracted more and more attention. Cellulose, with its rich source and excellent biodegradability, has become an important raw material for synthesizing biodegradable films. According to statistics, nature produces about 1,000 to 150 billion tons of cellulose per year through photosynthesis.
From a chemical structural point of view, cellulose is a linear polymer that is linked to each other by Î²-1,4-glucosidic bonds from D-glucopyranosyl (ie, anhydroglucose), and each glucose group in a cellulose macromolecule. The ring contains three alcoholic hydroxyl groups, so that the cellulose molecules have a strong hydrogen bond between and within the molecule, resulting in cellulose is insoluble in common solvents and difficult to be directly used. Therefore, in the past cellulose cellulose preparation process, cellulose was always chemically modified and then used, the two most commonly used methods are viscose method and copper ammonia method, and the trade names of the products are celluloids. (Cellophane) and Cuprohane. The viscose process for producing cellulose film has a long history and mature technology, but it has long process routes, complex production, and high raw material and energy consumption. At the same time, it has disadvantages such as high prices, poor tear resistance, and limited sturdiness. Therefore, its application Has been greatly limited. The new method for producing thin films using NMMO (N-methylmorpholine-N-oxide) as a direct solvent for cellulose is not only simple and has no pollution during the entire production process. The produced films are highly competitive in terms of transparency, air permeability, water permeability, strength, chemical stability, etc., and can be used as packaging films for foods, medicines, etc., reverse osmosis separation membranes, and electrode separators for batteries. , is a promising green polymer material.
Dissolution Mechanism of Cellulose in NMMO
The dissolution mechanism of cellulose in NMMO is a direct dissolution mechanism, which is performed by breaking hydrogen bonds between cellulose molecules without the production of cellulose derivatives. The two pairs of lone pair electrons of the oxygen atom on the strong polar functional group Nâ†’O in the NMMO molecule can form 1 to 2 hydrogen bonds (secondary bonds) with the hydrogen nuclei of the two hydroxyl groups, for example, can be combined with NMMOÂ·H2O. The water molecule and ethanol can also form a strong hydrogen bond, Cell-OH...O<-N, with the hydroxyl group (Cell-OH) in the cellulose macromolecule to form a cellulose-NMMO complex. First, in the amorphous region of cellulose, the original hydrogen bonds between cellulose macromolecules are destroyed, and then due to the presence of excess NMMO solvent, the complexation gradually penetrates into the crystalline region, and then the aggregate structure of cellulose is destroyed. Finally, the cellulose is dissolved.
NMMO film manufacturing process and its characteristics
The traditional process for producing cellulose films is a viscose process. In the production process, the release of harmful gases such as byproducts H2S and CS2 complicates the film production process and pollutes the environment. Although the last-formed film has a water-washing process before drying, a part of the sulfur-containing compound still remains in the film. At the same time, these harmful gases can also cause other problems in production. For example, air bubbles in the film cause uneven strength, and the accumulation of gas causes unpredictable changes in the size of the gel-like membrane tube, thereby deteriorating product performance.
The NMMO process of cellulosic films is a new process for the production of cellulose films without chemical reactions. In this process the pulp is mixed directly with a high concentration aqueous solution of NMMO and an antioxidant is added to prevent oxidative degradation of the cellulose during dissolution. Dehydration under reduced pressure at 85 Â°C ~ 120 Â°C, until the water content reached about 13.3%, and then dissolved under stirring at a certain temperature to obtain a higher concentration of the solution, after defoaming through the filter, through the annular die or slit The die is extruded, solidified in an NMMO low-consistency water bath, and a cellulosic film is formed by processes such as drawing, washing, plasticizing, and drying.
Compared with the viscose method, the NMMO process simplifies the technological process, reduces the use of chemical raw materials and energy consumption; the production process is completely a physical process, there is no chemical reaction; the biochemical toxicity of the solvent used NMMO is benign, does not cause variation, And 99.15% to 99.17% can be recycled without polluting the environment. Therefore, the law is a "green production process."
Development Status of NMMO Process Green Cellulose Film at Home and Abroad
The cellulose/NMMO stock was originally used for the production of green fiber Lyocell. Due to the non-pollution of its production process and the excellent performance of the fiber itself, great success has been achieved in the industry. From the study of several companies abroad that have industrialized the production of Lyocell fiber, the NMMO solution of cellulose can not only be spun, but also can be used to prepare biodegradable green packaging films by extrusion. At present, there are not many companies that have started to produce and research NMMO cellulose films abroad. There are mainly the following:
Akzona company in 1979 has proposed NMMO cellulose film can be used for packaging tape and packaging materials. TITK Laboratories, Germany also conducted a narrow strip of cellulose film for the research of agricultural packaging tapes, and proposed that the film has advantages that are unmatched by flax and polypropylene packaging tapes. Lenzing and Courtaulds proposed the feasibility of using the NMMO process to produce cellulose films. Among them, Courtaulds also discussed the feasibility of using NMMO process films for packaging and battery-insulated barrier films, but since the two companies focus on fibers, research on thin films has been slow. At present, only the Viskase company in Chicago, the United States, is really launching the industrialization of the film. The company is the world's largest supplier of cellulose casings, and is the world's leading producer of specialty plastic films for food packaging, and began specialized research on this film prior to Lenzing and Courtaulds. Viskase originally used viscose process to produce cellulose casings for food packaging. While other companies are still studying Lyocell fiber, they foreseeably started to study the application of NMMO process films on casings, and they are now leading the field. . Akozo Nobel in the Netherlands is Europe's largest manufacturer of copper-ammonia dialysis membranes. It is currently exploring the use of NMMO to produce cellulosic membranes. It is expected to be used in artificial kidneys, the largest application of traditional cellulose membranes. At present, UCB films company in the United Kingdom and Terumo company in Japan also studied the cellulose separation membrane of NMMO method, but the film for packaging application has not been studied yet.
In addition to several large companies, some foreign researchers have also conducted preliminary research on such cellulose films in the laboratory. Fink et al. used the blown film process to prepare NMMO process cellulose film, and pointed out that the NMMO process produces cellulose films that have better water permeability, wet tear strength, and tensile strength than films produced using the viscose process. strength. Askew et al. found that the alkali and chemical stability of the films prepared with the NMMO process also improved, but unfortunately none of these researchers' studies continued.
Domestic research in this area has only just begun. Judging from the literature review, only the research group of Donghua University and Sichuan University are conducting preliminary research in this field, and at present, some preliminary researches on separation membranes have been conducted. Previous studies have shown that by adjusting the relevant conditions of the process, it is possible to obtain a cellulosic film with controlled pore size and distribution, which can be applied to various filter materials.
In summary, the current domestic and international NMMO process cellulose film is only in the product development and research stage, especially for packaging films, foreign countries only see a small number of patent reports, the field has not yet seen in-depth and detailed systematic theoretical research reports, and In addition to our research group, there is almost nothing left in China.
Development Prospects of NMMO Process Green Cellulose Film
I. Application in food processing
For the casings made by the viscose process, due to the overflow of H2S and other gases during the production process, micropores of various sizes will appear on the surface, and some gas will remain in the membrane walls, resulting in uneven film thickness and strength. The distribution creates fragile spots that cause the casing to break easily during filling processing. The NMMO film manufacturing process solves this problem because no chemical reaction occurs and no gas is released. The NMMO process casing has a more uniform film thickness, higher tear strength and tensile strength, so that it will not break even when subjected to large external force during the deflation and filling process. Allegedly, the meat sausage made by Viskase using this film was still intact at 75Â°C for 75 min. In addition, the film has better moisture vapor permeability, so that the sausage can be easily penetrated into the sausage by steam and seasoning during the cooking process. Compared with the viscose film, cellulose NMMO solution can penetrate into the paper network better, so that the paper-plastic reinforced casing has better binding performance. All this shows that the NMMO process film is very suitable for casing.
Second, in other packaging applications
Gas permeability in food packaging plays an important role in prolonging the shelf life of foods or preservation of freshness. Both Fink and Gregory have pointed out that using alcohol as a coagulant can produce a porous cellulose film, which has better air permeability. It can prevent CO2 produced by the metabolism of vegetables, fruits, sausages and other respiratory foods from staying in the package, causing food fermentation and spoilage. Therefore, in addition to its application in the field of casings, this new type of film can also be expected to be used as a packaging material for candies, cakes, medicines, and garbage. In addition, porous NMMO membranes with good air permeability are also expected to be used in fruit production. The use of the film as a result set can protect the fruit from pests and bird pests, and can also prevent direct contact between the pesticide and the fruit, ensure fruit taste, and have no pollution to the fruit or the environment. The use of biodegradable cellulose film can greatly reduce the "white pollution" caused by the synthetic polymer film, and bring great opportunities for the development of green packaging industry.
Third, the application of separation membrane
Since the NMMO method cellulose membranes of different micropore sizes and distributions can be prepared by changing the production process conditions, the membrane can be expected to be used as a seawater desalination membrane and an artificial kidney dialysis membrane, and replace the traditional copper ammonia cellulose membrane. Akzo Nobel in the Netherlands is the largest manufacturer of copper ammonia dialysis membranes in Europe. It is currently exploring the use of NMMO to produce cellulose membranes and is expected to be applied to artificial kidneys, the largest application of traditional cellulose membranes.
Fourth, the application of the battery
It is known that the use of a regenerated cellulose film as an electrode separator for batteries has a long history. In application, it is not only required that the cellulose film can play a role of isolation between the positive and negative electrodes to prevent short circuit, but also that the film has desired electrolyte permeability so that the chemical reaction in the battery can take place. However, under the harsh conditions such as strong alkali, the traditional cellulose insulation film is easily corroded, resulting in weakened strength. Moreover, the crystalline zinc oxide in the electrolyte is also easy to puncture the film with low strength, so that the battery life is greatly shortened.
A battery separator made of a cellulosic film produced by the NMMO process not only has a long-term charge retention property, but also has a longer discharge time after being charged for a long time. In addition, since the film has good alkali resistance, chemical stability and mechanical strength, it is very suitable for use as an electrode separator for ordinary, rechargeable button batteries and alkaline batteries.
V. Application in Agriculture
The NMMO cellulose film is also expected to find applications in agriculture, mainly in the following areas:
It can be used as a film covering film (abbreviated as a plastic film) to prevent frost and heavy rain, as well as to regulate light and save irrigation water. It can be used for seedling raising, early rice sowing and harvesting, land re-use index, and grain increase; The greenhouse greenhouse film has the effect of increasing temperature and keeping warm, which is beneficial to the growth of high-temperature sunshine vegetables; it is used as a storage film for green feed to reduce the influence of air and humidity on feed, and to make the proper amount of sugar under the condition of oxygen deficiency. It can produce lactic acid-rich fermentation and keep the fresh color of the feed color, so that the cattle can eat fresh feed in the winter, so as to increase the output of milk and speed up the growth of burdock. It is used as a film for farmhouses, dormitory rooms, and feed sheds. , Lightweight and cheap, no environmental pollution problems, especially suitable for temporary shelter construction in the field; used as agricultural water storage film; used as soil sterilization film to reduce damage to crops caused by pests.
In summary, NMMO
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