Adding some filler to the plastic can improve certain properties of the plastic and is therefore more suitable for certain specialized applications. In order to reduce the density and hardness of the plastic, or to enhance its thermal insulation or sound insulation, the most desirable filler is the void. Plastics containing voids or cells, classified as foam. As the degree of foaming, that is, the difference in the volume fraction of the foam caused by the voids, the properties of the foam may differ considerably from the base plastic. A blowing agent is a chemical that can be added to a plastic to release a gas at a suitable time during the process to form cells in the plastic.

The formation of plastic foam can generally be divided into four stages.

In the first stage, the blowing agent must be completely uniformly dispersed in the polymer, and the polymer is usually in a liquid or molten state. The blowing agent can now form a true solution in the polymer or simply disperse uniformly in the polymer to 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 agents are generally very fine inert particles which provide a site for the formation of a new gas phase.

In the third stage, the initially formed cells are constantly increasing because more gas diffuses and penetrates the cells through the polymer. If this period of time is long enough, the individual cells will come into contact with each other. If the walls separating the individual cells are broken, then by this coalescence, larger cells are formed. If the foam is formed mainly by cell interconnection, it is called an open cell foam. If it is a foam formed by cells that are not connected to each other, it is called a closed cell foam. If cell agglomeration is allowed to proceed without limitation, the foam collapses because the gas is completely separated from the polymer.

In the fourth stage, when the viscosity of the polymer increases and the cells can no longer grow, the foam stabilizes. The viscosity of the polymer can be increased by cooling, crosslinking or other methods.

The last three stages of the foaming process can be as short as a fraction of a second, and no longer than a few seconds. The formation of the foam requires the polymer to be in a liquid state. For this purpose, the polymer can be dissolved or plasticized by heating. Foam production is almost the same as any ordinary plastics production process, usually through extrusion, rotational molding and injection molding, as well as plastisol processing and thermoforming processes. For the same reason, essentially any type of plastic can be made into a foam. Polyvinyl chloride (both hard and soft), polystyrene, polypropylene, ABS and polyethylene have all been made into foam on an industrial scale. The same is true for heat-resistant engineering plastics and thermoset polymers.

The AC foaming agent is obtained by oxidation of diurea and is a pale yellow or orange crystalline powder. Its molecular weight is 116, the decomposition heat is 359. 9J / g ° C, the liberated gas is mainly nitrogen (65%), carbon monoxide (32%) and a small amount of carbon dioxide (3%). The solid residue is mainly diurea, cyanuric acid and uridine. It has a slight ammonia smell, is not flammable, and has self-extinguishing properties when decomposed. Stable at room temperature. This product can be considered as non-toxic.

AC foaming agents have good properties. Therefore, it is widely used, especially in hard PVC foam profiles.

First, the impact of various components of hard PVC foam profiles on AC foaming

1.1 PVC resin

For general profiles, PVC resins with a K value in the range of 58-65 are used. Foamed materials are often selected from PVC resins with a lower K value, which facilitates rapid gelation during processing, ensuring that the melt has a homogeneous structure at the foaming temperature.

1.2 Stabilizer

In addition to the heat stabilizing effect, the heat stabilizer also acts to reduce the decomposition temperature of the hair agent, acts as an activator, promotes the decomposition of AC, and makes the foaming temperature suitable for the processing temperature.

Experiments have shown that the amount of tri-salt increases significantly reduces the decomposition temperature of AC and increases the amount of gas generated, as shown in Table 1. Hard and hard lead also have the effect of lowering the decomposition temperature of AC, but it affects the amount of AC gas generated. Therefore, comprehensive consideration should be given to the reasonable selection of stabilizer dosage.

1.3 AC foaming agent

As the amount of AC foaming 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 PVC product has a low density and a smooth surface. However, as the amount of AC continues to increase, the amount of gas generated is too large, so that the strength of the PVC melt cannot enclose the cells, causing the cells to rupture, and the appearance of the product is collapsed, and the product density is increased. In contrast, the amount of AC foaming agent must be selected according to different products.

1.4 modifier

There are many types of hard PVC modifiers, among which CPE and ACR modifiers are mainly used. The addition of ACR can improve the melt strength of the PVC resin, so that the cell wall in the foaming process can withstand the gas pressure in the bubble without causing large holes. The amount of ACR is increased, and the melt strength is increased to make the cells small and large. Therefore, ACR not only affects the cell structure, but also affects the density of the product. See Table 3.

The main role of CPE is to increase the impact strength of hard PVC materials. Different types of CPE and dosage have great influence on the performance of PVC system. Generally, CPE-135A is preferred, 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 function, which helps to form suitable cells. However, excessive addition of CaCO3 causes the cells to be irregular, and the material density is correspondingly increased, which is also unfavorable for the impact strength, and generally is 10%-15%.

Second, the impact of hard PVC foam profile molding process on foaming

The main factors affecting the molding process of PVC foaming quality are as follows.

2.1 Extrusion temperature

Extrusion temperature is an important factor affecting the quality of foaming. It has been proven that high quality foams are only available in the proper temperature range. The higher the melt temperature, the lower the melt strength of the extruded material itself, and the foaming pressure in the bubble may exceed the limit of the surface tension of the foam, thereby causing the cells to rupture, resulting in a rough foamed surface.

The temperature of the melt has the greatest effect on the foaming, and during the foaming process, the gas pressure acts on the melt structure. If the temperature of the material is too low, only a incomplete foam structure is formed due to the high melt viscosity, resulting in a high profile density. If the temperature of the material is too high, the cell is torn due to the low melt viscosity, and most of the foaming gas can quickly escape and cause the cell to collapse.

2.2 The influence of die temperature on foaming

The gas generated by the thermal decomposition of the blowing agent is dissolved in the melt under a high pressure in the barrel to form a supersaturated solution. The highly supersaturated state of the gas is an unsteady state, and it is easy to form a bubble nucleus in the melt (CaCO 3 and TiO 2 act to form a nucleating agent which is easy to foam by bubble formation). If the nucleus formed in the extrudate is insufficient, the formation of bubbles is closely related to the viscosity, elasticity, and solubility and diffusion rate in the melt of the PVC. Practice has shown that as the die temperature rises, the product density decreases, but the mold 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 has shown that the cell size and foaming density become very small as the extrusion pressure increases, and the number of cells increases as the extrusion pressure increases. Therefore, the extrusion pressure can effectively control the foaming density.

2.4 Influence of material residence time in the 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 prolonged, and the number of pores gradually increases, but starts to decrease after reaching the maximum value. In fact, the degree of decomposition of the blowing agent is highly dependent on the ratio of gas to core in the melt as it exits the die. At shorter residence times, the decomposition temperature is also smaller and the density is higher. If the residence time increases, the ratio of gas to core increases and the foaming density decreases. 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 to core becomes very large, resulting in an over-foamed product with insufficient nucleation.

2.5 Screw speed affects foaming

The screw speed has a large influence on the product density. First, the screw speed determines the amount of shear that the PVC melt is subjected to, which affects the strength of the PVC melt. Secondly, the rotation speed causes shear heat generation to increase the temperature of the material, which has an effect on the decomposition of the foaming agent and the stability of PVC degradation. Therefore, excessively high rotational speed is likely to cause problems such as uneven cell structure and rough surface of the foamed material. On the contrary, the excessively low rotational speed not only reduces the productivity, but also is disadvantageous for the melt plasticization and the uniform foaming of the material. Therefore, it is very important to choose a reasonable speed. Under the same process conditions of the same formula, as the rotation speed increases, the density of the product decreases. Increasing the screw speed can increase the head pressure, forcing the material to foam after leaving the die, and a good product can be obtained, but excessively high speed can result in The cell structure is poor.

Third, the conclusion

(1) In the formulation of hard PVC foam profile, the type of PVC resin, the amount of stabilizer and the choice, and the choice of modifier CaCO3 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 rotation speed in the extrusion molding process all affect the AC foaming system, so the extrusion process conditions must be strictly controlled.

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