【Long Article】The most detailed introduction to plastic packing materials

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This post was last edited by jxkelley on 2017-4-13 at 16:26. Plastic fillers: Introduction to plastic fillers: Plastic fillers are a type of chemical industry filler; their mass transfer efficiency is among the highest among all types of fillers. Moreover, they have a high porosity, and these advantages mean that they are essential in many fields where their absence would be unacceptable. Plastic fillers have a high specific surface area, which also ensures good gas-liquid contact with them. Of course, the advantages of plastic fillers go beyond these; it has high mass transfer units and pressure drops, and these characteristics also determine its excellent gas-liquid contact properties. Therefore, plastic materials are an excellent filler, and their various properties make them suitable for a wide range of industrial fields, where they serve as an indispensable component. The materials of plastic fillers mainly include polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC), among which polypropylene is commonly used in China. Plastic fillers have good corrosion resistance and can withstand the corrosion of common inorganic acids, bases, and organic solvents. It has good heat resistance and can be used for a long time at temperatures below 100°C. Plastic fillers are lightweight and inexpensive, possess good toughness, are impact-resistant and not easy to break, allowing them to be used in thin-walled structures. It features high throughput and low pressure, and is widely used in devices for absorption, desorption, extraction, dust removal, etc. The disadvantage of plastic fillers is their poor surface wettability, but this can be improved through appropriate surface treatment. Plastic fillers are widely used in industries such as environmental protection, chemicals, petroleum, and gas, thanks to their unique properties. It is mainly used in tower packing, and the operating temperature must be between 60 degrees and 280 degrees; otherwise, it cannot function effectively. Types of plastic fillers: Plastic fillers are generally divided into loose plastic fillers and structured plastic fillers. Loose plastic fillers mainly include: multi-faceted hollow spheres, wreaths, Herringbone rings, stepped rings, conjugate rings, Bower rings, rectangular saddle rings, irregular saddle rings, conjugate rings, flat rings, Lassie rings, snowflake-shaped rings, hexagonal rings, rotating rings, pentagonal rings, conical rings, mesh rings, cross-shaped spherical rings, floating balls, surface-covering balls, and diamond-shaped covering balls. Plastic structured packing has advantages such as light weight, large capacity, low pressure drop, high specific surface area, and easy replacement. The wire mesh corrugated filler is composed of a mesh woven from a mixture of polypropylene and polyacrylonitrile wires. The inclined corrugated packing is made of polypropylene or PVDF sheets, and holes may be made in the sheets to improve mass transfer efficiency. Processing technology of plastic fillers: The molding of plastic fillers generally involves several steps, including the preparation and preparation of raw materials, molding, and post-processing of the finished products; molding is the process of shaping plastics in various forms into the desired shape or prototype. There are many molding methods, including extrusion molding, injection molding, compression molding, calendering, casting, compression sintering, transfer molding, foaming molding, and others. 1) Preparation of the molded material: Since plastics with simple components have limited properties and are difficult to meet the required standards, additives and polymers are combined to form a uniform composite. Thus, it is possible to meet various requirements for the products. For ease of use and processing, the materials used for molding are mainly granules and powders. They are all made from resin and additives. The main additives include: plasticizers, antioxidants, fillers, lubricants, colorants, curing agents, etc. Polymers or resins are the main components of powdered plastics, and their properties have a significant impact on both the processing characteristics and the performance of the final products, primarily through effects on molecular weight, molecular weight distribution, particle structure, and particle size. 　　Plasticizers are generally a class of organic compounds that are stable to both heat and chemical reagents. The plasticization process can be regarded as a process of mutual dissolution between polymers and small molecules, which enhances the flexibility and cold resistance of plastics. During the molding and processing process or over long-term use, polymers can undergo degradation or cross-linking as a result of various external factors, which leads to a deterioration in their properties and renders them unusable. 　　Substances added to prevent or suppress such destructive effects are collectively referred to as antioxidants. It mainly includes stabilizers, antioxidants, light stabilizers, etc.; it functions to inhibit degradation, oxidation, photo-degradation, and to remove impurities through catalysis. Substances added to improve the molding properties of plastics, enhance certain technical specifications of the products, endow them with new characteristics, or reduce costs and the consumption of polymer monomers are known as fillers. Additives that are incorporated into plastics in order to improve the flow properties of the plastic melt, reduce or prevent adhesion to the equipment, and enhance the surface finish of the products are known as lubricants. Substances that come into contact with the plastic during molding and are similar to lubricants, but are used solely to prevent adhesion to plastic-metal equipment and to facilitate demolding, are commonly referred to as demolding agents, or also as lubricants. A substance added to give products various bright colors and enhance their appearance is called a colorant. Some colorants also help improve resistance to weathering, thereby extending the service life of the products. During the molding of thermosetting plastics, it is sometimes necessary to add a substance that enables the resin to undergo cross-linking or accelerates this process; such a substance is called a curing agent. 　　In the production of plastic products, while only a few polymers can be used alone, most must be mixed with other materials and compounded before they can be used in molding processes. By \"ingredients,\" it is meant mixing various components together to create as homogeneous a system as possible. To this end, mixed operations must be employed, and mixing, kneading, and plasticizing are all common mixing processes used in plastic formulation, which are carried out through diffusion, convection, and shear. The preparation process generally consists of four steps: preparation of raw materials, initial mixing, plasticization of the initially mixed material, and crushing and granulation of the plasticized material. 　　2) Plastic molding: In most cases, molding is a method of transforming plastic into products of various shapes by heating it to a viscous flow state, allowing it to flow, be shaped, and then cooled to harden. 　　Extrusion molding, also known as extrusion or extrusion casting, is a molding method in which heated and melted plastic is forced to pass through a die under pressure, thanks to the squeezing action of a screw or plunger, thereby forming a continuous profile with a constant cross-section. It can produce tubes, rods, wires, sheets, films, wires and cables, as well as coated products. This method is characterized by high production efficiency and strong adaptability, and can be used for almost all thermoplastics as well as certain thermosets. 　　The most commonly used extrusion equipment today are single-screw extruders and twin-screw extruders; the latter are particularly suitable for the molding of hard polyvinyl chloride powder or plastics in other multi-component systems. The common one is a single-screw extruder. It mainly includes five parts: the transmission system, the feeding device, the barrel, the screw, and the die head and die. 　　There are many types of plastics suitable for extrusion molding, and the shapes and sizes of the resulting products vary greatly, but the extrusion molding process remains essentially the same. The process includes drying and shaping of the materials, setting and cooling of the finished products, drawing and winding (or cutting) of the products, and sometimes also post-treatment of the products. The moisture in the raw materials or that absorbed from the surroundings can affect the proper progress of the extrusion process and the quality of the final product. In mild cases, this can cause defects such as bubbles and a dull surface on the product, as well as a reduction in its physical and mechanical properties; in severe cases, it can prevent the extrusion process from taking place at all. Therefore, the raw materials should be dried before use, with the moisture content typically kept below 0.5%. It varies depending on the molecular weight of the polymer, the shape and size of the product, as well as the type of extruder. Moreover, during the extrusion process, the rotation speed of the screw, along with the pressure and temperature inside the barrel, influence one another; adjustments must be made according to specific circumstances. The distribution of temperature and pressure within the barrel, die head, and nozzle during extrusion generally follows the patterns shown in the diagram. The process conditions during the extrusion process have a significant impact on the quality of the final product. In particular, the degree of plasticization directly affects the physical and mechanical properties as well as the appearance of the product. The factors that determine the degree of plasticization include increasing the rotational speed of the screw, which enhances the shearing action on the material and facilitates its mixing and plasticization. Thermoplastic plastics suitable for extruding pipes include PVC, PP, PE, ABS, PA, PC, PTFE, etc. 　　Injection molding, abbreviated as injection molding process, is a manufacturing method in which material is plasticized in the heated barrel of an injection machine, and then injected into the cavity of a closed mold via a screw or plunger, where it cools to form the final product. It is widely used in the molding of thermoplastic plastics, as well as in the molding of certain thermosetting plastics (such as phenolic plastics and amino plastics). The advantage of injection molding is its ability to produce plastic parts with complex shapes and precise dimensions in a single step, including those with metal or non-metallic inlays, and it is even possible to fill them with gas to create hollow structures. It offers high production efficiency and a high degree of automation. The principle of injection molding is to place the pellets in the barrel of an injection machine, where they are heated and brought to a viscous flow state under shear forces. Then, pressure is applied using a plunger or screw to force the melt to rapidly pass through the nozzle and fill the mold cavity, where it cools and solidifies. Its production process includes the following steps, which are repeated over and over again. Clean and prepare the mold -> Close the mold -> Inject material -> Cool -> Open the mold -> Eject the product. 　　Compression molding is one of the oldest and most important methods in plastic molding technologies, and it is primarily used for molding thermosetting plastics. Based on the properties of the material and the characteristics of the molding process, it can be further divided into compression molding and laminating. Mold pressing, also known as compression molding, is a process in which plastic in powder, granular, fragmented, or fibrous form is placed into the heated cavity of a female mold. After the male mold is closed, heat is applied to melt the plastic, and under pressure the material fills the mold cavity, forming a product with the shape of that cavity. The product is then either heated (to facilitate further cross-linking reactions and solidification) or cooled (for thermoplastics, cooling is used to harden them), after which it is removed from the mold to be obtained as the final product. Compared to injection molding, compression molding involves simpler control of the production process, as well as simpler equipment and molds, making it easier to manufacture large-sized products. Thermosetting plastic molded products have advantages such as good heat resistance, a wide operating temperature range, and minimal deformation. However, their disadvantages include a long production cycle, low efficiency, and difficulty in achieving automation; as a result, the workload for workers is high, they are unable to produce items with complex shapes, and thick-walled products cannot be molded using these materials. 　　Calendering: This is the main method used for producing films and sheets. It involves passing the already plasticized thermoplastic, which is at a temperature close to its viscous flow temperature, through a series of horizontally rotating rollers. This process subjects the material to compression and stretching, resulting in thin sheet-like products with a certain thickness, width, and surface finish. Plastics used for calendering are mostly thermoplastic amorphous plastics, among which polyvinyl chloride is the most commonly used. It is suitable for producing soft PVC films with thicknesses in the range of 0.05–0.5 millimeters, as well as hard PVC sheets with thicknesses in the range of 0.25–0.7 millimeters. When the product thickness is greater than or lower than this range, the calendering method is generally not used; instead, extrusion blow molding or other methods are employed. Roll forming offers high production capacity (it allows for continuous production and is easy to automate), good product quality (the films produced are of higher quality than those made by blow molding or T-extrusion), and it can also be used to create composite materials (such as artificial leather and coated paper), as well as to apply patterns onto surfaces. However, the required processing equipment is large, high precision is demanded, there are many auxiliary devices, and at the same time, the width of the product is limited by the maximum working length of the rolling mill rolls.