The addition of some fillers to the plastic can improve some of the properties of the plastic, which makes it more suitable for certain special purposes. In order to reduce the density and hardness of the plastic, or to enhance its thermal insulation or sound insulation, the most ideal Fillers are voids. Plastics containing voids or cells, classified as foamed plastics. With the degree of foaming, ie, the difference in the volume fraction of the foam caused by the voids, the performance of the foam may differ considerably from that of the base plastic. Foaming agent is a chemical that can be added to plastics. At the appropriate time during the process, it will release gas and form bubbles in the plastic.
The formation of plastic foam can generally be divided into four stages.
In the first stage, the foaming agent must be completely and uniformly dispersed in the polymer, and the polymer is usually in a liquid or molten state. The foaming agent can then form a true solution in the polymer, or just be uniformly dispersed in the polymer. In, form a two-phase system.
In the second stage, after a large number of individual bubbles are formed, the system is transformed into a system in which the gas is dispersed in the liquid. At this time, a nucleating agent is often added to promote the formation of a large number of small bubbles. The nucleating agent is generally very fine. Inert particles, which provide sites for the formation of a new gas phase.
In the third stage, the initially formed cells continue to increase in size, because more gas diffuses through the polymer and enters the cells. If this time is long enough, the individual cells will contact each other. If the wall separating the individual cells ruptures, then by this type of coalescence, larger cells will be formed. If the cell is formed mainly by interconnecting the cells, it is called an open cell foam. If it is a bubble formed by cells that are not connected to each other, it is called a closed-cell foam. If the coalescence of cells is allowed to proceed indefinitely, the foam collapses because the gas is completely separated from the polymer. opened.
In the fourth stage, when the viscosity of the polymer increases and the cells cannot grow any longer, the foam will stabilize. Using cooling, cross-linking or other methods can increase the viscosity of the polymer.
The last three stages of the foaming process, from the time point of view, can be as short as a fraction of a second and not more than a few seconds at most. The formation of foam requires the polymer to be liquid. For this purpose, it can be heated. Dissolving or plasticizing polymers. The production process of foams is almost the same as any ordinary plastics production process, usually through extrusion, rotomolding and injection molding, as well as plastisol processing and thermoforming processes. For the same reason, basically any All kinds of plastics can be made into foam plastics. Polyvinyl chloride (both hard and soft), Polystyrene, Polypropylene, ABS and Polyethylene, have all been made into foams on an industrial scale. Heat-resistant engineering plastics and The same is true for thermosetting polymers.
The AC foaming agent is obtained by oxidation of diurea as a pale yellow or orange crystalline powder. Its molecular weight is 116, and the heat of decomposition is 359. 9 J/g °C. The gas released is mainly nitrogen (65%), carbon monoxide ( 32%) and a small amount of carbon dioxide (3%). Decomposition of solid residues is mainly biurea, cyanuric acid, urine azole. Slight ammonia smell during decomposition, non-flammable, self-extinguishing. Storage stability at room temperature. It is non-toxic.
AC foaming agent has good performance. Therefore, it is widely used, especially in hard PVC foamed profiles.
I. Effect of various components of hard PVC foamed profiles on AC foaming
1.1 PVC resin
General profiles use PVC resin with a K value in the range of 58-65. Foam materials often use PVC resin with a lower K value, which facilitates rapid gelation during processing and ensures that the melt has a homogeneous structure at the foaming temperature .
1.2 Stabilizers
In addition to the heat stabilizer in the formula, the heat stabilizer also plays a role of reducing the decomposition temperature of the hair agent, acting as an activator, promoting the decomposition of AC, and making the foaming temperature suitable for the processing temperature.
Experiments show that the use of three salt additions will significantly reduce the decomposition temperature of AC, and increase the amount of gas generated, see Table 1. Hard helium, hard lead also has the effect of reducing the AC decomposition temperature, but it will affect the AC gas volume. Therefore, comprehensive consideration should be given to the reasonable choice of stabilizer dosage
1.3 AC blowing agent
As the amount of AC blowing agent increases, the amount of gas generated increases and the density of the product decreases. When the amount of AC is at a certain value, the density of the PVC product is low and the surface is smooth. However, as the AC usage continues to increase, the amount of gas generated is too high. As for the strength of PVC melt, the cells cannot be encapsulated and the cells are broken. The appearance of the product collapses and the density of the product increases. Instead, the optimum amount of AC blowing agent must be selected according to different products.
1.4 Modifiers
There are many types of hard PVC modifiers, among which are CPE and ACR modifiers. The addition of ACR improves the melt strength of PVC resin, so that the cell walls in the foaming process can withstand the pressure of the gas in the foam and do not rupture. Holes. The amount of ACR increases, the melt strength increases, and the number of cells is small. Therefore, ACR not only affects the cell structure, but also affects the product density. See Table 3
The main role of CPE is to improve the impact strength of hard PVC materials. The use of different types of CPE and the amount of the impact on the performance of the PVC system is greater. Generally, CPE-135A is better, and the amount added is 5%-15%.
1.5 CaCO3 dosage
In the low-foaming system, when the amount of CaCO3 is small, it has a nucleating agent and helps to form suitable cells. However, excessive CaCO3 addition makes the cells irregular, the density of the material increases accordingly, and the impact strength is also unfavorable. Use 10%-15% of the amount is good.
Second, the hard PVC foam profile molding process impact on the foam
Affect the quality of PVC foam molding process factors are the following.
2.1 Extrusion temperature
Extrusion temperature is an important factor affecting foam quality. Practice has proved that high-quality foam can only be obtained within a suitable temperature range. The higher the melt temperature, the lower the melt strength of the extruded material itself, then The foaming pressure in the foam may exceed the limit of the surface tension of the foam, so that the foam breaks, resulting in a rough foamed surface.
The temperature of the melt has the greatest influence on the foaming. During the foaming process, the gas pressure acts on the melt structure. If the temperature of the material is too low, the density of the profile is high because the higher melt viscosity only results in an incomplete foam structure. If the temperature of the material is too high, most of the foaming gas can escape quickly and cause the cells to collapse because the melt viscosity is low and the cells are torn.
2.2 Die Temperature Effect on Foaming
The gas generated by the thermal decomposition of the blowing agent dissolves in the melt at a high pressure in the barrel to form a supersaturated solution. The highly supersaturated state of the gas is a non-steady state, and bubble nuclei are easily formed in the melt ( CaCO3 and TiO2 act as nucleating agents for the formation of bubble-forming bubbles.) If the generated nucleus is insufficient in the extrudate, the formation of bubbles on the PVC melt will be viscous, elastic, and solubility in the melt of the gas. It is closely related to the diffusion rate. Practice has shown that as the die temperature increases, the product density decreases, but the die temperature is too high (>200°C), and the surface of the product is yellow. Therefore, the die temperature is preferably 180-185°C. .
2.3 Extrusion pressure
Practice shows that the cell size and foaming density quickly decrease as the extrusion pressure increases, and the number of cells increases with the extrusion pressure. Therefore, the extrusion pressure can effectively control the foaming density.
2.4 Influence of Material Residence Time in Extruder
The residence time of the material in the extruder is different, and the foaming quality also changes greatly. The residence time of the material in the extruder is increased, the number of pores gradually increases, but decreases after reaching the maximum value. In fact, the foaming agent The degree of decomposition is closely related to the ratio of gas to core in the melt when leaving the die. With a shorter residence time, the decomposition temperature is also smaller and the density is greater. If the residence time increases, the ratio of gas and core is It will increase, and the foaming density will decrease. If the residence time is too long, it will cause premature decomposition, which will affect the nucleation result, reduce the amount of foam, and the ratio of gas and nucleus becomes very large. Insufficient nuclear foam products.
2.5 Effect of screw speed on foaming
The screw speed has a great influence on the product density. First, the screw speed determines the size of the shear of the PVC melt, which affects the strength of the PVC melt. Second, the rotational speed causes the shear heat to increase the temperature of the material. Decomposition of the foaming agent and the stability of the PVC degradation have an impact. Therefore, excessively high rotation speed may cause uneven foam structure and surface roughness, etc. On the contrary, too low speed not only reduces productivity, but also does not facilitate material melting and molding. It is very important to choose a reasonable rotational speed. It is very important to choose a reasonable rotational speed. Under the same technological conditions of the same formulation, as the rotational speed increases, the product density decreases. Increasing the rotational speed of the screw can increase the head pressure, forcing the material to foam after leaving the die. Good product can be obtained, but high rotational speed will lead to poor cell structure.
Third, the conclusion
(1) In the formulation of hard PVC foamed profiles, the type of PVC resin, the amount and choice of stabilizer, and the selection of modifiers all have a significant impact on the AC foaming system. The best formula must be selected through testing.
(2) Extrusion pressure, extrusion temperature, die temperature, residence time, and screw speed in the extrusion process all affect the AC foaming system. Therefore, the extrusion process conditions must be strictly controlled.