Cellulose is the oldest and most abundant natural polymer on the earth, and it is an inexhaustible and the most precious natural renewable resource for human beings. Cellulose chemistry and industry started more than 160 years ago and was the main research object during the birth and development of polymer chemistry. The research results of cellulose and its derivatives have made significant contributions to the establishment, development and enrichment of polymer physics and chemistry. There was a time when interest in cellulose research declined due to the rise of petrochemical synthetic products and materials due to the water sensitivity, insolubility and infusibility of cellulose, but after the 1970s, due to the oil crisis and the There is a shortage of synthetic chemical raw materials, prices are rising, and with the increasing emphasis on environmental pollution worldwide, renewable cellulose resources with low price, biodegradability, non-toxicity, good biocompatibility, and The research, development and application of derivatives ushered in the second spring.

First, the source and type of cellulose

As early as 1838, the French botanist Anselme Payen used wood to be treated alternately with nitric acid and sodium hydroxide solution to separate a white substance with a uniform structure. obtained substance. Its polymer form was not determined by Sdaudinger until 1932.

As the most abundant natural organic renewable resource on earth, cellulose comes from green terrestrial and submarine plants and animals. Plant cellulose is divided into cotton, wood, hemp and various straw plant cellulose and other types according to the source. It is the main component of plant fiber cell wall; in addition, some are derived from animal bacteria, seabed organisms and various animals. cellulose. Cellulose mainly exists in the world of green plants. Through photosynthesis, plants can synthesize hundreds of millions of tons of cellulose every year, including about 100'106t of wood pulp cellulose and 12'106t of cotton cellulose. In other words, according to the differences in resources in various countries, the main sources of raw cellulose used are these two categories.

In my country, cellulose ether manufacturers mainly use cotton cellulose, which is often referred to as refined cotton. The source is mainly cotton linters with a length of less than 10mm remaining on the cottonseed husk after removing the long lint, and obtained after refining. Cotton linters on cottonseed are rich in cellulose, about 65% to 80%, and the rest are fat, wax, pectin and ash. The purpose of refining is to remove these components and impurities through chemical treatment to obtain refined cotton with a cellulose content of 99.5%. The process is completed in the refined cotton factory. Refining is firstly to open and dedust the cotton linter raw material, then immerse it in a dilute caustic soda solution and heat and cook it under pressure to remove fat, wax, residual seed husk, pectin and ash, etc., and at the same time destroy the fiber. The outer layer of primary cell wall makes the cells expand, and can also reduce the crystallinity of cellulose, increase the gap between cellulose fibers and their specific surface area, and help improve the chemical reaction ability of cotton pulp. The cooked pulp is then washed, de-sanded, beaten, bleached, dehydrated and dried to obtain a refined cotton product with qualified cellulose content. The cellulose content mainly refers to the a-cellulose content, which is defined as the macromolecular cellulose that is insoluble in 17.5% NaOH aqueous solution at 20oC.

Wood contains 35%~45% cellulose, the rest is hemicellulose (25%~35%), lignin (20%~30%), fat, wax, residual seed husk, pectin and ash, etc. ,quite complicated.

Due to differences in climate and region, the types of wood fibers owned by various countries are also different. The main natural wood fibers in the world come from various softwoods and hardwoods. In the mixed forests in the southern United States, mainly hardwoods, such as birch (Betula sp. ), aspen (Populus tremula and P.tremuloides), oak (Quercus sp.), rubber tree (Nyssa sp.), maple (Acer sp.) and beech (Fagus sp.) and many other species. The cork belt of the northern hemisphere includes Russia, northern Europe, Canada, and parts of North America. The main coniferous forests of Asia and Europe include Eastern Siberian larch (Larix sibirica), European red pine (Pinus silvestris), and Norway spruce (Picea abies). North America mainly includes white spruce (Picea glauca) and black spruce (P.mariana), balsam fir (Abies balsamea), Jack (P.contorta), black pine (Pinus banksiana) and so on. In the United States, there are hemlock (Tsuga sp.), Douglas fir (Pseudotsuga sp.), as well as western and southern pine trees. The southern pine trees mainly include Pinus taeda, P.elliottii and P. palustris and other varieties.

In addition to natural forests, there are also cultivated softwood and hardwood species such as the Radiata pine (Pinus radiata) that grows in Australia and New Zealand. Many eucalyptus species grown in subtropical regions also play an important role in the pulp and paper industry. Various other non-wood fiber raw materials, mainly grasses, such as cereal straw (rice, wheat, etc.) straw, bagasse and bamboo, etc., form an important source of fiber, but have not been fully utilized.

There are several methods for making cellulose pulp from wood, but the purpose is to dissolve hemicellulose and a large amount of residual lignin, and then bleach to remove the residue, and finally obtain high-purity pulp with high a-cellulose content. There are mainly bisulfite process, sodium sulfite process and pre-hydrolysis Kraft process. The removal of lignin by various sulfite pulping processes is actually based on sulfur dioxide, which changes the type of cation, pH value of the solution and cooking temperature. The acid calcium bisulfite pulping process is used worldwide, but its use is limited due to the production of insoluble calcium sulfate during chemical regeneration. Later, with the introduction of so-called soluble cations, such as magnesium, sodium and ammonium ions, the pH of the solution increased from 1~2 in the traditional calcium bisulfite process to 5 in the magnesium bisulfite process, and even reached sodium bisulfite/sulfuric acid Alkaline conditions for the sodium hydrogen process.

The acid bisulfite process and improved two- or three-step sodium sulfite processes, such as the Rauma process, have played an important role in the dissolving pulping industry for a long time, and the acid bisulfite process is still in use. The main feature of the multi-step process is the alternation of bisulfite/sulfite stages and alkaline stages. The process can start or end with an alkaline stage, the latter requiring selective alkaline extraction to reduce residual hemicellulose content.

The Kraft pulping process is commonly used worldwide and is the main process for grading paperboard pulp. In order to obtain dissolving wood pulp grades, pre-hydrolysis is carried out before Kraft cooking. Prehydrolysis is the steam treatment of wood chips or cooking with water at 140~170°C, or the treatment with dilute acid at 110~120°C. Steam or water treatment can destroy the acetyl and formic acid groups in the wood, form acetic acid and formic acid, and make the pH value of the wood reach 3.5 to promote the decomposition of wood components, and the mass can be reduced by 5%~20% with different hydrolysis time and temperature . Nearly half of the cork hemicellulose, mainly glucomannan, dissolves after hydrolysis, but the lignin hardly changes. Relatively speaking, a large amount of hardwood lignin was dissolved. If the hydrolysis time is prolonged, the cellulose will change, which will lead to a decrease in the yield of a-cellulose and more lignin to condense. It also makes lignin removal more difficult later in the process, requiring stronger bases and higher temperatures. During the prehydrolysis stage, wood loses 20% to 22%, and beech (fagus silvatica) can obtain higher a-cellulose content (95% to 96%). Increasing the prehydrolysis and Kraft cooking temperature of wood pulp can reduce the treatment time, and at the same time, the viscosity can be significantly reduced at the same a-cellulose content. All things being equal, the alpha-cellulose content (slightly below 96%) obtained from pine and birch is the same, while eucalyptus is slightly above 97%, with about the same viscosity as hardwood pulp, but significantly higher than pine pulp .

The raw material is from softwood to hardwood, and the process is from acidic sulfurous acid to alkaline prehydrolysis Kraft method. The modern dissolving wood pulp production process has been greatly developed. The use of hardwood can produce wood pulp with high a-cellulose content, and it is easy to achieve complete chlorine-free bleaching (ie, the TCF process, which means that no chlorine-containing substances are added in each process stage) process bleaching. However, in the final analysis, regenerated cellulose with excellent performance requires high cellulose activity, high a-cellulose content, narrow polymerization degree distribution and easy control of solution viscosity, etc.

Table 1 compiles the weight average molecular weight Mw and the degree of polymerization DP of several celluloses and their derivatives from different sources.

Table 1. Mw and DP ranges of some celluloses and derivatives

Depending on the source, the molecular weight and distribution of cellulose will directly affect the mechanical properties, solubility, aging and chemical reaction properties of the material such as strength, modulus and deflection. The commonly used methods for determining the molecular weight of cellulose include viscosity method, osmotic pressure method, ultracentrifugation sedimentation method and light scattering method.