Foamed PVC pipe
DATE：2022/3/4 8:46:03 / READ： / SOURCE：This station
Foamed PVC pipe
PVC-U core layer foamed pipe is a new type of plastic pipe. It is formed by multi-layer co extrusion process with an appropriate amount of additives added to PVC resin. At present, this process has been widely used in the world. Two extruders are used to co extrude to one batching die at the same time. The extruded PVC material is divided into two molten material streams in the batching die, which form the inner and outer skin layers of the pipe; Another extruder inputs foamed PVC into the same batching die, which is in the middle of the inner and outer layers, that is, the core layer. Because PVC-U core layer foamed pipe has more advantages than rigid PVC solid wall pipe, the application field is expanding.Our country has produced PVC-U core foaming pipes in 1990s. In recent years, more than 10 core foam pipe production lines have been produced in France, Austria, Korea and Taiwan, China, and more than 20 production lines have been produced in Dalian, Qingdao, Weifang, Wuhan and Nantong. It has effectively promoted the production and application of core foamed pipes in China.
1. Material selection and formula design
(1) Foaming principle of PVC pipe during the processing of PVC-U, an appropriate amount of foaming agent is added to the material. At a certain temperature, the gas released by the decomposition of foaming agent forms a gap in the melt, which increases the volume of PVC polymer and reduces the density of products accordingly. In the process of extrusion foaming, the expansion of foaming agent occurs in an environment close to the melting temperature of polymer (extruder barrel), and the expansion must occur after the melting of polymer. Therefore, the two processes of expansion and melting at the same temperature must be separated from each other, so as not to interfere with each other and affect the quality of products.
Through the test of different foaming agents and their addition amount, the best formula must ensure the following points: ① the foaming ratio and cell size in the melt are uniform, and there are no continuous holes, so as to ensure the quality of the product
Good mechanical properties: ② no strength decrease caused by micro change; ③ The surface shall be free of bubbles and the surface shall be as dense as possible.
When the PVC melt containing foaming agent is extruded through the screw groove and die at high temperature, the pressure exerted by the screw on the melt shall be evenly released in the die. In order to prevent the melt from foaming early, the pressure in the die should be higher than that of the foaming agent gas, and the foaming stage should be pushed back as much as possible. The melt containing foaming agent must expand when extruding the die head. If the melt expands in the die head, it will produce shear force and destroy the formed bubble holes, resulting in rough surface of the product.
PVC-U co extrusion foaming system is composed of PVC resin, modifier, stabilizer, lubricant, foaming agent, nucleating agent, etc. Compared with ordinary PVC formula, the influence of its components on AC foaming effect and the relationship between melt strength and cell structure should be considered in the formula design of this system.
The following focuses on several influencing factors. The quality of the general formula is as follows:
PVC resin 100 HST 0.5
Stabilizer 5 ~ 10 TiO2 ~ 4
Modifier 5 ~ 10 CaCO; ten
Paraffin 0.5 foaming agent 0.1 ~ 0.4
(2) The influence of stable system on AC foaming system C is used as foaming agent in production The higher the decomposition temperature of AC, the greater the gas generation. In CO extruded PVC foamed pipe, AC decomposition temperature is required to be between PVC plasticization and decomposition temperature. In the PVC foaming system, the stabilizer is not only used to prevent the decomposition and deterioration of the resin in the process of processing and use, but also can activate the AC foaming agent due to the presence of metal ions, which can improve the foaming property of AC. The experimental results show that the addition of three salts can significantly reduce the decomposition temperature of AC, but the decomposition temperature increases with the increase of the amount. Similarly, the gas generation increases slightly with the increase of the amount of three salts, but if the amount is too much, the gas generation decreases.
In addition, the experimental results also show that the increase of PS dosage can also reduce the decomposition temperature of AC, but the foaming amount decreases, as shown in table 1-52.
Table 1-52 effect of PBST dosage on foaming No. ns dosage (mass fraction) AC decomposition temperature / ℃ gas generation / ml * g '
Bast is a stabilizer that can inhibit AC. if the amount of bast is small, the decomposition temperature of AC will be low and the amount of outgassing will be large; When the content is high, due to the inhibition effect, the amount of AC outgassing decreases and the decomposition temperature is also high, as shown in table 1-53.
Table 1-53 effect of bast dosage on foaming
S / N BST dosage (mass fraction) AC decomposition temperature / ℃ 185 220 gas generation / ml.g - '
It can be seen from the experimental results in table 1-53 that different lead salts have different effects on AC foaming. Therefore, a single lead salt stabilizer cannot be used in formula design, and synergistic effect must be adopted. Therefore, in the actual production, the stable system should be adjusted to find out the best formula.
(3) The influence of processing modifier the addition of ACR modifier improves the melt strength of PVC resin, so that the cell wall can withstand the cell gas pressure in the foaming process without cracking and causing large holes. With the increase of ACR dosage, the melt viscosity increases, which limits the formation of holes and is conducive to the formation of fine bubbles. Therefore, with the increase of ACR dosage, the foaming effect of PVC is better. The foaming effect is closely related to the apparent viscosity. With the decrease of the apparent viscosity, the product density decreases. Generally speaking. When the melt viscosity is high, the bubble expansion speed decreases, which limits the formation of holes and is conducive to the formation of small bubbles. The density of foam tube also increases with the increase of ACR dosage, because the large bubbles are reduced. It can be seen that ACR can improve foaming molding, as shown in table 1-54.
Table 1-54 effect of ACR dosage on foaming
ACR dosage (mass fraction) 2 3
Density / GAM-1 0.72 0.88 0.97
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