The Key to Improve the Quality and Reduce the Cost of Molded Pulp Products

Principles of pulp molding wet end chemistry are fundamental to the design of equipment, molds and products

One is the key to improving product quality and reducing costs.

Author Liu Zhichen

The pulp molding industry has sprung up, developed rapidly and gradually matured. The lack of relevant theoretical knowledge of new industry practitioners will affect production and product quality. When the production is smooth, it is difficult to accurately find out the cause of the problem when the production is not smooth. Learn some relevant theoretical knowledge, know its but also know its reason. Contribute to the development of new products, new equipment and reasonable design molds, and promote the development of the industry.

In the early stage of the development of the pulp molding industry, most of the raw materials are waste paper, and the products are the inner packaging products that are dried first and then shaped. The form and position tolerances, dimensional tolerances and appearance of the products are not strictly required. In recent years, the development of high-quality packaging, fast tableware and food packaging are the use of raw pulp paper, the product requirements are very high, the study of wet end chemical principle is particularly important.

First, pulp molding wet end chemical definition

The wet-end chemistry of pulp molding is to discuss the laws of mutual reflection and action of various components in pulp, such as fibers, water, fillers, chemical additives, etc., in the mold cavity of the molding machine, such as ice filtration, retention, molding and white water circulation., And affect the operation of the molding machine and product quality of a discipline.

The paper fiber mentioned here refers to the fiber that can only be obtained after cooking and splitting of plant straw. It contains a large amount of hydroxyl and hydrogen-oxygen bonds, with a length of 1~7mm, a diameter of 20 ~ 50um, a wall thickness of 3 ~ 7um, and a hollow micro-material with sharp ends, which is essentially different from the commonly used fiber.

The water in the pulp components mentioned in wet end chemistry acts as a medium, I .e. a water bridge. The various components and ingredients are suspended in water. In the various stages of the process, water acts directly, both as a dissolution aid, hydrating and swelling the fibers, and as a medium environment for the chemical reaction when the wet paper blank is formed.

Fillers and chemical aids refer to retention aids, filter aids, dry strength agents, wet strength agents, defoamers, anti-foaming agents, preservatives and colorants.

Second, the chemical principle and function of pulp molding wet end

Starting from the principle of wet-end chemistry, the formation mechanism and process of fiber retention, filtration and dehydration are studied by means of surface chemistry and colloid chemistry, so as to make the design of pulp molding mold more perfect and rational, improve product quality and reduce costs.

Except for the fibers, the particle diameters of the remaining components of the slurry are within the range of colloidal particles (I. e., the diameter is less than 10um). Due to the large specific surface area of colloidal particles, these components have a strong adsorption capacity, and chemical reactions occur on the surface of these particles. In addition, the reaction of wet-end chemistry is related to the whole slurry hydrophobic colloid system, so the various changes that occur during the wet-end forming process are mainly related to the reaction of colloid chemistry and surface chemistry.

The purpose of the wet end chemistry research is to change the molding process of the article and improve the quality of the article by applying fillers and additives in the slurry. To improve the final use of paper performance and quality as the main role of a class of additives called functional additives; another type of additives to promote and improve the molding process, to prevent interference and reduce raw materials and energy consumption called process additives.

Process additives include retention aids, filter aids, resin control agents, wet strength agents, preservatives and mesh cleaning agents. Functional additives are: dry strength agent, waterproofing agent, oil repellent, dye, brightener, softener, etc. Additives and additives are based on water as a bridge (medium environment), in the form of ions adsorbed on the surface of fibers and fine fibers to improve water filtration, retention or enhance the binding strength between fibers.

In addition to the study of additives, wet end chemistry is mainly to study the chemical characteristics of fibers and fine fibers, mainly for adsorption properties, swelling properties, ion exchange properties, these properties are affected by the specific surface area of fibers and fine fibers, surface charge and surface chemical composition.

Due to the small size of the fines, there is a large surface area per unit mass. According to the Marton research report, the surface area of the fines is 5 to 8 times that of the fibers, and the surface area directly affects the adsorption. Chemical aids work primarily through adsorption processes, so fines play a major role in wet-end chemistry. The adsorption capacity of fine fibers is 2 to 3 times that of fibers, and the fine fibers also play a major role in the filtration of water. The fine fibers account for 40 to 60% of the ingredients. Therefore, the retention of fines is critical to the effective action of the slurry.

Some pulp molding enterprises have not yet noticed the importance of wet-end chemical research will directly affect the mold design and molding machine operation, the lack of wet-end chemical measurement and control means in the production line, and the use of fiber retention rate meter in mold design and evaluation.

Understand the mechanism of pulp molding, master the change law of wet end chemistry, adjust the addition point of chemical additives, and the addition amount is conducive to mold design and molding machine operation. Without the guidance of pulp molding theory, the designed mold is known, but it is difficult to produce good products. What's more, it will affect the production process and cause a lot of material and energy waste.

Third, keep the relationship with the mold

Retention is a term used for papermaking and refers to the retention of fibers in the slurry by the forming mesh to form a paper product. Its definition includes two contents, one is the ratio of the quantity of materials sent to the wet part of the molding machine to the quantity of paper products after heat setting; the other is the ratio of the quantity of materials contained in the wet paper blank before heat setting to the quantity of materials after heat setting.

The fine component in the slurry is a very fine particulate material, which generally refers to the part that can pass through the 60 mesh mesh is the fine fiber (Jaycoc). For a certain kind of pulp, fine fibers account for about 50% of the furnish, so the research is actually aimed at these components. The retention of fine fibers is ultimately accomplished by mechanical closure. Mechanical entrapment refers to the seizing of fine fibers by the seams where the fibers settle to form a mat on the web form. Initially, only the long fiber portion of the slurry was trapped by the forming mesh mold because the pores of the forming mesh mold were much larger than the fiber particles. More fines will be trapped on a denser pulp mat.

By definition, fines are components that can pass through a 60-mesh mesh. The general mesh mold mesh in 40~65 mesh, in order to prevent a large number of fine fiber loss, the use of wet-end chemical methods to produce colloidal aggregation in the slurry. Colloidal aggregation of the fines is the primary means of keeping the fines. Included are flocs formed of a pure fines component and a fines component flocs containing fibers in which the fines are adsorbed on the surface of the fibers and these fines are trapped with the fibers in the forming pulp mat.

Retention makes the paper product have two sides in the structure, and the fine fiber component in the slurry is distributed much less on one side of the net mold than on the other side. In order to produce paper tableware with smooth surface and easy to print, some tableware enterprises adopt adsorption forming outside the net mold to keep the net marks on the inner surface of the product. The waterproof and oil-proof functions of the products produced by this method are greatly reduced. In order to obtain the same function, the amount of additives must be added, thus increasing the cost. Therefore, when designing the mold, the mold working surface should be selected according to the stress and use of the product.

In order to ensure that the net mold has sufficient passing capacity and improve productivity, the filter hole of the net mold can not be selected too small, and after the mesh is large, a large number of fine fibers will run away in vain. In order to solve this contradiction, it is necessary to select a large enough mesh in the design of the mesh mold, and use the retention agent in the slurry to make the fine fibers colloidal aggregation, thereby improving the retention capacity of the mesh mold. Colloidal polymerization agents seek to promote flocculation between fines, with the hope of reducing flocculation between pure fibers. In order to make the paper product uniform and improve retention, it is desirable to have most of the fines component adhere to the fibers. To achieve this effect, the retention aid is added before the wet-end web mold.

The use of retention aids can increase the mesh of the mesh mold under the same retention rate, thereby increasing productivity. The selection of retention aids is closely related to the weaving method of mesh, the geometric shape of mesh, the material of mesh mold and the diameter of mesh wire.

Retention is an important part of the formation of paper products, paper product strength is from here. The strength of paper products is formed and the binding force of paper fibers. The strength of paper is determined by the combination of one fiber and all fibers, one fiber is the type of fiber, and the combination of all fibers is determined by the production process.

The formation mechanism of paper strength believes that a large number of hydroxyl and hydroxyl bonds are free on the surface of fibers in water. Under the action of water bridge, the hydroxyl groups are arranged properly. The surface tension of water pulls the fibers together, and finally forms hydrogen bonding between fibers. This hydrogen bonding is manifested as the binding force between fibers. Hydrogen bonding can only be achieved when the distance between adjacent hydroxyl groups is less than 27.6 minus 8 centimeters.

Fourth, the influence of wet-end chemistry on the running stability of the molding machine

Wet-end chemistry has both positive and negative effects on the operation of the molding machine. On the one hand, wet-end chemistry can be used to enhance water filtration, reduce air entry and eliminate foam, keep the molding machine clean, and keep the solid content in white water low. On the other hand, if these factors are out of control, the same wet chemical phenomenon will cause the molding machine to run abnormally, produce spots and bubbles in the product, and reduce the water filtration, so that the molding machine is not clean, thereby reducing production efficiency, which is mainly manifested in the following aspects:

1, the paper material of the water

Water drainage is an important performance in the operation of the molding machine. The degree of water filtration will be affected by the flocculation between fiber and fiber and fine fiber. If the flocs formed are large and porous, the paper material will become viscous and hinder the passage of water, thus reducing the water filtration.

2, precipitation and scaling

Precipitation and scaling often occur when the wet part chemistry is out of control. The common ones are excessive use of chemical additives, charge imbalance, chemical incompatibility, and unstable chemical balance. Therefore, these phenomena can lead to precipitation and scaling. At present, there are many ways to clean up precipitation and scaling, but the best way is to find the cause of runaway and correct it.

3, the formation of foam

The fibers contain substances that can stabilize the air into the pulp (some chemical additives also play the same role), which will reduce the water filtration of the paper stock, creating stickiness and foam. If it happens, the best way is to find the root cause and eliminate it. If it is impossible to do so, mechanical and chemical methods can generally be used to eliminate it. At this time, the effect of wet end chemistry will be small.

Fifth, wet-end chemical measurement, control and development trends

In recent years, the biggest interest of the pulp molding industry in the field of wet cloth chemistry is to try to add and use wet end chemicals and establish a set of technologies and methods that can be fully controlled. At present, the solution to this problem is to establish a complex physical and chemical model to try to explain the adsorption, retention and other process runnability in multi-component chemical systems. This aspect mainly includes the test measurement technology and online measurement of wet-end chemistry, the operation data of the molding machine and the process control technology of wet-end chemistry.

Trends in wet-end chemistry A major trend in wet-end chemistry research over the past decade has been the completion of the transition from acidic to alkaline papermaking. Paper products are formed at a higher PH value. The advantage is that the corrosion of the equipment is reduced, the fiber is well swollen, the strength is greatly improved, the filler content is improved, the wet paper blank is easy to dry, the forming speed is accelerated, the output is increased, and the energy consumption is reduced. Therefore, the application and promotion of neutral or alkaline papermaking technology has had a huge impact on wet-end chemistry. In this case, it is necessary to make a comprehensive summary and research on the application of chemical additives in papermaking. The current development trend of papermaking wet-end chemistry is mainly manifested in the following aspects.

The first major trend is CaCO3, filler and filler modification.

The second trend is that wet-end chemistry is actually used in waste paper pulp. As the pollutants brought into the waste paper increase in the slurry, these pollutants consume chemical additives at the same time, and the problem of net mold precipitation also arises.

The third trend, that is, chemical additives used in high-precision products and food packaging, the use of fillers, retention agents, reinforcing agents, etc., must choose the most advanced paper industry technology, pulp molding is a rising star, do not forget the paper industry technology is far ahead.

The fourth trend is the development of new wet-end chemical additives such as cationic dry strength agent, wet strength agent, cationic dispersion rosin size and other paper chemicals.

The fifth trend is the future of wet-end chemical process measurement and control, with the development of new online monitoring technology, practitioners in the actual use of new control and the wet-end chemical process control and molding machine other control programs separately, so as to achieve the wet-end chemical action of pulp molding control.

The development of new technologies in the pulp molding industry requires the establishment of a corresponding wet-end chemical control system. At present, the supply of papermaking chemical additives on the market is complex and diverse. Therefore, how to make good use of these chemical additives in wet end chemistry to promote the development of pulp molding industry is very important.