Technical data on PVC foam boards

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Foaming mechanism: In PVC foam board products, the purpose of adding ultra-high molecular weight polymers is, firstly, to promote the plasticization of PVC； The second purpose is to improve the melt strength of PVC foaming materials and prevent bubble merging, in order to obtain products with uniform foaming ； Third, it is to ensure that the melt has good fluidity in order to obtain products with a good appearance. Since different manufacturers of foam products produce various products, with differences in the equipment, processes, raw materials, and lubrication systems used, we have developed foam regulators with different properties to meet the various needs of our customers. 1. Definition of foamed materials: Foamed plastics, also known as foam plastics, are materials whose main component is plastic; they contain a large number of bubbles and can be considered composite materials filled with gas. 2. Foam sheets can be classified into high-foam and low-foam types based on the degree of foaming. Depending on the hardness of the foam texture, they can be divided into rigid, semi-rigid, and soft foam plastics. Based on their cell structure, foam plastics can be divided into closed-cell foam plastics and open-cell foam plastics. Common PVC foam sheets are typically hard, closed-cell, low-foam sheets. 3. Applications of PVC foam sheets PVC foam sheets possess advantages such as chemical resistance, weather resistance, and flame retardancy, and are widely used in various applications including display panels, signs, billboards, partitions, building panels, and furniture materials. 4. Key factors in evaluating the quality of foamed sheets: For foamed materials, the size and uniformity of the pores are key factors that affect the quality of the sheets. For low-magnification foamed sheets, the bubbles are small and uniform; such sheets possess good toughness, high strength, and excellent surface quality. From the perspective of reducing the density of foamed sheets, it is only possible to further reduce the density if the bubbles are small and uniform; large and scattered bubbles make it difficult to further reduce the density. 5. Key points in the control of PVC foaming process: Plastic foaming molding consists of three processes: the formation of bubble nuclei, the expansion of these bubble nuclei, and the curing of the foam structure. For PVC foam sheets with chemical blowing agents, the expansion of the bubble nuclei has a decisive impact on the quality of the foam sheet. PVC consists of straight-chain molecules with short molecular chains, resulting in low melt strength. During the expansion of bubble nuclei into bubbles, the melt is not sufficient to enclose these bubbles, allowing gas to escape and combine to form larger bubbles, which reduces the quality of the foamed sheet material. The key factor in improving the quality of PVC foam sheets is enhancing the melt strength of PVC. Based on an analysis of the processing properties of polymer materials, there are various methods to improve the melt strength of PVC, but the most effective approaches are adding additives that enhance melt strength and reducing the processing temperature. PVC is an amorphous material; as the melt temperature increases, its strength decreases, while as the melt temperature decreases, its strength increases. However, the effect of lowering the temperature is limited and serves only as a supplementary factor. All ACR-type processing agents have the effect of improving melt strength, among which the foaming regulator is the most effective. Melt strength increases as the content of the foaming regulator increases. Generally speaking, as long as the screw has sufficient dispersion and mixing capacity, adding high-viscosity foaming regulators has a more significant effect on improving melt strength. 6. The role of processing aids in PVC foam sheets: ACR-based processing aids help to promote the melting of PVC, improve surface smoothness, enhance the elasticity of the melt, and increase its elongation and strength. It helps to coat the bubbles and prevent the pores from collapsing. Both the molecular weight and the amount of the foaming regulator have a significant impact on the density of the foamed sheets: as the molecular weight increases, the strength of the PVC melt rises, allowing the density of the foamed sheets to be reduced further; this effect is similar to that resulting from an increase in the amount of the regulator used. However, this effect is not linear; further increasing the molecular weight or the amount used does not have a very significant impact on reducing density, which tends to remain constant. There is an important relationship between the foaming regulator and the foaming agent. There is a balance point between the density of the foam sheet and the amount of the foaming regulator; outside this balance point, the density of the foam sheet remains constant regardless of the amount of foaming agent used. In other words, increasing the amount of foaming agent does not lower the density. The reason for this phenomenon is that, at a certain amount of foaming regulator, the melt strength of PVC is limited, and excessive gas leads to the collapse or merging of the bubbles. 7. Common problems During the extrusion process of PVC foam boards, the problems encountered can generally be classified into 4 categories; the first is stability issues ； The second is the issue of melt strength ； Third is the lubrication issue ； The fourth is the dispersion problem. These four types of problems, especially the first three, restrict and influence each other; from a superficial perspective, it can sometimes be difficult to distinguish them immediately. It is necessary to conduct careful observation and analysis to identify the root causes of the problems in order to resolve them fundamentally. a. Insufficient stability can affect the entire board surface; the surface turns yellow and the foam material becomes brittle. b. Low melt strength results in large pores in the foam material, with long pores in its cross-section. The most direct way to determine whether the melt strength is insufficient is to press the sheet wrapped around the middle roll with your finger behind the three-roll setup; if the melt strength is good, elasticity can be felt during the pressing. If it is difficult to bounce back after being pressed, it indicates that the melt strength is poor. Due to the significant differences in screw design and cooling methods, it is difficult to determine whether the temperature is appropriate. Generally speaking, within the load limits of the extruder, it is better for the temperatures in zones 3–5 to be low. c. To obtain products with uniform foaming in foamed pipes, it is also necessary to ensure that the PVC material has good melt strength. The foaming additives developed by the company were created to address this issue in foamed products, and they can effectively improve the quality of foamed pipes. Lubricants are divided into external lubricants and internal lubricants; external lubrication helps with demolding and improves the smoothness of the sheet surface. If too little external lubrication is used, it becomes difficult to control the temperature in zone 5 of the extruder, leading to increased temperatures. This results in high temperatures in the core area, as well as problems such as large bubbles, interconnected bubbles, and yellowing in the sheet, and the surface of the sheet also loses its smoothness ； There is excessive sliding on the outside, which leads to severe precipitation; this is manifested as sliding and precipitation on the structure inside the mold as well as on the surface of the sheet metal. It can also show up as certain individual phenomena moving back and forth irregularly on the surface of the sheet. Internal sliding facilitates plasticization and the flowability of the melt; insufficient internal sliding makes it difficult to control the thickness of the sheet, resulting in a thicker middle section and thinner edges ； There is excessive internal sliding, which easily leads to high temperatures in the merging core. e. Poor dispersion can result in an uneven surface on the sheet material. f. Issues with process temperature control: The four problems mentioned above are fundamental issues; they represent the basics and are of a deeper nature. Compared to the above four issues, process temperature control is much more straightforward; it is a surface-level problem. However, poor temperature control can lead to the emergence of fundamental problems. Increasing the processing temperature reduces the material’s stability time, leading to stability issues ； The original lubrication balance is disrupted, typically manifesting as insufficient external lubrication; in particular, for later stages, it is necessary to increase the amount of external lubricant used ； An increase in temperature also leads to a decrease in the strength of the melt, larger pores in the foamed sheet, a reduction in the number of pores, and the sheet becoming brittle and prone to breaking ； An increase in temperature reduces the strength of the melt as well as its viscosity. Lower viscosity leads to a reduced ability for shear dispersion; in the case of screws with weak dispersion capabilities, uneven dispersion can occur.