Papermaking craft

Papermaking craft

Papermaking technology is an outstanding invention in ancient China. Figure 1(a) shows the process flow chart of the Han Dynasty in China. Since the development of papermaking technology, it has the characteristics of simplicity, maturity, low cost, and large-scale output. The modern papermaking process is derived from papermaking technology, which is a potential industrialized production of diaphragm technology. However, the non-woven fabric prepared by it has the disadvantage of low mechanical strength and needs to be further improved. The technology is mainly divided into the main steps of pulping, modulation and papermaking [Figure 1(b)].

Papermaking craft
Figure 1 – Chinese Han Dynasty papermaking process flow chart (a) and modern papermaking process flow chart (b)

Pulping mainly includes mechanical pulping method, chemical pulping method and semi-chemical pulping method. Mechanical pulping is to make wood or plant fibers and other raw materials into fine fibers through mechanical grinding, and its production efficiency is as high as 90% to 95%. At the same time, no chemicals are added in the pulping process, so the pulp contains a large amount of lignin, and the paper made is more difficult to bleach and is easy to turn yellow and brittle under the action of light and heat. The chemical pulping method is to mix chemicals to cook the wood so that the fiber of the raw material is easily separated. The purpose of cooking is to cause chemical reactions between chemicals and raw materials to eliminate lignin and leave cellulose as pulp material. Different chemicals can be added in the cooking process to make the properties of the pulp produced vary. Pulp can be divided into sulfite pulp, alkaline pulp and kraft pulp. The semi-chemical pulping method is a method between mechanical pulping and chemical pulping. The bonding force between the non-fibrous components and the fibers in the raw material is first dissolved, and then the fibers are separated by mechanical methods. The chemicals used in the semi-chemical pulping method are Na2SO3, Ca(HSO)2, plus Na2CO3, NaHCO3 acts as a buffer, and the fiber obtained from the raw material can reach 65%-85%. The obtained paper is harder, has a wide range of uses, is more flexible, and has a lower cost than other methods.

Because the pulp itself contains a large amount of cellulose, if the pulp is used directly to make paper, the physical properties of the paper will be very weak, and the surface will appear loose and porous. Therefore, the preparation process is an important process of papermaking, which is directly related to the strength, color tone, printability of the finished paper, and the length of the shelf life of the paper. The common preparation process can be roughly divided into beating and filler sizing. Beating is the use of mechanical methods to treat the fibers in the pulp to make them broomed and appropriately cut, which can increase the hydrogen bonding between the fibers. More importantly, the fiber absorbs water and swells during beating, and has high elasticity and plasticity, which meets the requirements of paper machine production and can make the produced paper reach the expected quality index. The filler procedure is to add proper amount of minerals and pigments to the pulp, which can increase the smoothness of the paper, increase the opacity and whiteness, increase the thickness and density, and improve the ink absorption capacity and printing adaptability of the paper. Commonly used types are clay, asbestos, talc, gypsum, calcium carbonate, zinc sulfide and magnesium carbonate, etc., to fill the gaps between fibers. The gluing process is mainly to make the paper have the ability to resist moisture penetration, and can strengthen the paper’s hardness, toughness, tension resistance, and moisture resistance, improve the smoothness of the paper surface and reduce the fluffing phenomenon, and also improve the printing adaptability of the paper. Commonly used types are rosin, starch, wax emulsion and so on.

The papermaking process is to add a large amount of water to the prepared pulp to make the fibers hydrated. The fibers are distributed on the metal mesh with the water flow to form longitudinal and transverse fiber directionality, and then enter the paper machine to make the fibers adhere to the papermaking felt tightly to reduce most of the moisture. After filtering, pressing, drying, calendering, coiling, rewinding and slitting, the paper is made into paper.

Studies have shown that the cellulose/polyarylsulfone composite membrane (Figure 2) successfully prepared by wet papermaking technology has high tensile strength, electrolyte infiltration rate and high temperature resistance. Chun et al. used isopropanol-water (IPA-water) to disperse cellulose pulp under high-pressure refining conditions to obtain a nano-cellulose membrane, which has better electrolyte wettability and liquid retention. In addition, colloidal nano-SiO2 inorganic particles can also be used to improve the interface stability of the diaphragm, or a pore former can be used to control the porosity and pore structure of the diaphragm.

Papermaking craft
Figure 2 – Descending diagram of the preparation process of cellulose-PSA composite diaphragm

Qingdao Energy Storage Industry Technology Research Institute uses cellulose acetate to obtain cellulose/PVDF-HFP nanocomposite membranes through electrospinning and dip coating methods with smaller pore sizes and pore size distribution. It is beneficial to reduce the self-discharge of the battery and improve the cycle life of the battery. In addition, the addition of PVDF-HFP is beneficial to improve the hygroscopicity of the cellulose membrane. The research institute also prepared flame-retardant cellulose membrane (FCCN, Figure 3) for the first time using flame retardants, sodium alginate, SiO2, etc. It has excellent flame retardant properties and thermal stability, which is of great significance for improving the safety performance of batteries.

Papermaking craft
Figure 3 – Schematic diagram of the preparation process of FCCN diaphragm

In addition, the research institute also used polydopamine to modify the cellulose separator, which can improve the battery’s charge and discharge performance, rate performance, and cycle performance. The o-diphenol group and amino group in dopamine (3,4-dihydroxy-phenethylamine) have strong adhesion ability, which can improve the mechanical strength of the matrix material and improve the wettability and liquid absorption rate of the diaphragm to the electrolyte .

Algae is also rich in cellulose. For example, the crystallinity of Cladosporium cellulose (CC) is as high as 95%, which makes it extremely low hygroscopicity under ambient humidity and avoids the influence of moisture on the battery. In addition, its another feature is that it can maintain more pore structure after drying without agglomeration. As a separator of lithium ion batteries, CC has good thermal stability and is suitable for high voltage systems (Figure 4 ).

Papermaking craft
Figure 4 – Schematic diagram of the preparation process of Cladosporium cellulose diaphragm

In order to further increase the energy density of the battery, it is necessary to use high-voltage electrode materials and electrolyte, which means that it is necessary to develop high-voltage resistant diaphragm materials. Common materials such as PVDF, PMMA, PET, etc. cannot meet the needs of high-voltage batteries. Zhao et al. used cellulose matrix and polypropylene carbonate (PPC) polymer to develop a gel electrolyte resistant to 5V high voltage, breaking through the high voltage limitation of liquid electrolyte. When metal lithium is used as a negative electrode, it is easily corroded by organic electrolyte, and is continuously pulverized during the charge-discharge cycle, producing lithium dendrites, piercing the diaphragm and contacting the positive and negative electrodes, causing internal short circuits. Goodenough’s team used facial mask paper as a cellulose-based porous membrane, which can effectively inhibit the formation of lithium dendrites in lithium-sulfur batteries. Low-cost, porous structure of cellulose membrane, rich in nanopores and nanorods, and high electrolyte wettability. Its function is similar to a solid electrolyte membrane, and its close contact with the negative electrode can effectively inhibit the formation of lithium dendrites, which greatly improves the safety performance and battery capacity of the battery. In addition, due to the solvation of lithium ions, the migration number of lithium ions is reduced, which affects the rate performance of the battery. The use of lithium tartrate borate (PLTB) single ion conductor to coat the cellulose membrane can increase the lithium ion migration number from 0.31 to 0.48, effectively reducing the concentration polarization and side reactions caused by the accumulation of anions at the electrode interface, and it is beneficial to improve rate performance and cycle life.