Study on the relationship between molecular structure and processing properties of PVC-C resin
As a high performance engineering plastic, PVC-C (CPVC) has many advantages that PVC resin can not be compared with, but also has some shortcomings, mainly reflected in the thermal stability and processing performance. The main factors affecting the thermal stability and processing performance of CPVC resin are the resin itself and the auxiliaries used in the process.
For processing properties, various additives were added to improve the thermal stability, processing fluidity and other properties of CPVC resin. Even adding a second or even a third component (other resins) to the CPVC resin to improve its processing properties has made some progress, but at the same time, it also reduces the other properties of CPVC resin.
From the perspective of CPVC resin processing, the effects of relative molecular mass and distribution of resin, chlorine content and chlorine atom distribution of resin, and sub-micro phase state of particles on processing properties have not been reported. Based on the study of the properties of CPVC resin, the effects of the relative molecular mass and distribution of CPVC resin, the chlorine content of resin and the distribution of chlorine atoms, and the sub-micro phase state of particles on its processing properties were discussed in this paper. It has positive guiding significance for the production of CPVC in China and the selection of CPVC resin during processing.
Study on the Relative molecular mass and distribution of CPVC resin
Table 1 GPC test results of different CPVC resins:
Tree fat
Type number are
Molecular weight average
Molecular weight peak position
Molecular weight viscosity
Molecular weight multiple fraction
divergence
1 # 399008140078800763002.04
2 # 311006570070800617002.11
3 # 341007380074700690002.16
4 # 357007760078200727002.17
5 # 372007960078300753002.14
6 # 563001276001230001160002.27
The larger the relative molecular weight of CPVC resin, the longer the molecular chain, and the more basic units contained in the molecular chain. Therefore, the intermolecular force and internal friction resistance increase, the melting viscosity increases correspondingly, the processing fluidity becomes worse, and the machinability of the material becomes worse, which brings difficulties to the processing and molding. If the relative molecular mass is too small, the interaction force between molecular chains decreases rapidly, resulting in the rapid decline of mechanical properties of materials. Therefore, it is very necessary to study the relative molecular mass and distribution of resin, so as to make it take into account the requirements of both use performance and processing performance.
Table 1 shows the GPC test results of different CPVC resins. It can be seen from Table 1 that the relative molecular weights of 1# ~ 5# resins are close, especially the peak relative molecular weights are around 75,000. The relative molecular weight of 6# resin was the largest, its peak relative molecular weight shifted to the direction of high relative molecular weight, and its molecular weight distribution was the widest, while the other 5 resins had relatively narrow molecular weight distribution and small peak relative molecular weight. The most different molecular weight distribution of the six resins was 1# resin and 6# resin -- 1# resin had the smallest molecular weight distribution, while 6# resin had the largest molecular weight distribution.
Study on chlorine content and chlorine atom distribution of CPVC resin
The content of chlorine in the polymer chain and the distribution of chlorine atoms on the main chain greatly affect the fracture mode and rate of the molecular chain, thus affecting the thermal stability of the resin. The distribution of chlorine atoms also affects the polarity of molecular chains, and the interaction between molecular chains has a great influence on the processing fluidity of resins. With the increase of chlorine content, there are more -CCl2- structures in the CPVC molecular chain, and the corresponding occurrence of -ChCl-CCL2 - structures increases. From the perspective of bond energy, this makes the molecular chain easy to decompose when heated. However, when the chlorine content is the same, if the molecular chain contains more -ChCl - structure, that is, the percentage of -ChCl - content is high, the thermal stability of CPVC resin is relatively excellent. Therefore, it is necessary to study the influence of chlorine content and chlorine atom distribution on the processing property and thermal stability of CPVC resin.
Table 2 Mole content and chlorine content of structural units in different CPVC resins
The molar percentage of each structural unit of resin chlorine content %
-CH2--CHCl--CCl2-
1 # 23.8967.858.2669.48
2 # 23.1467.928.9469.86
3 # 28.8965.165.9667.41
4 # 23.4168.508.0969.56
5 # 21.8968.0710.0470.46
6 # 27.6867.105.2367.55
Table 2 shows the NMR test results of six kinds of resins used in the experiment. It can be seen that the chlorine content of 1#, 2# and 4# resins is similar, and the molar percentage content of -CH2-, -ChCl - and -CCL2 - basic structures is not much different. The chlorine content of 3# resin and 6# resin is relatively low, and the percentage of -CCL2 - structure in 6# resin is slightly lower than that of 3# resin, which is obviously lower than that of the other 4 resins. Therefore, the possibility of forming -ChCl-CCl2 - structure in its molecular chain is low, and its thermal stability should be better than other resins. 5# resin has the highest chlorine linkage, and the percentage of -CCL2 - structure is also significantly higher than that of the other five resins. The large amount of -CCl2- structure and the significant reduction of -Ch2 - content increase the possibility of forming -ChCl-CCl2 - structure in the molecular chain. From the point of view of bond energy, 6# resin should have the best thermal stability.
Study on submicrophase state of CPVC resin particles
CPVC has many problems, such as difficult resin plasticization, large melt viscosity, high processing temperature and narrow temperature range. In the process of processing, CPVC particles, as basic particles, undergo extrusion and friction under the action of equipment when heated, and gradually reach the state of softening and melting. Therefore, it is an important aspect to study the sub-micro phase of particles to improve their machining performance.
In order to analyze the differences between different resin particles, the submicrophase states of CPVC resin particles used in the experiment were observed by SEM. As shown by SEM photos of 6 kinds of resins, the surface morphology of 1#, 4# and 5# resins is similar, and the cortex is complete and dense. The surface of 2# and 3# resins was attached with a large number of tiny particles, and the cortex was relatively complete. The cortex of 6# resin was almost completely destroyed, and a large number of subparticles were exposed on the surface, and the subparticles were evenly stacked. From the SEM photos of CPVC resin section, the internal structure of 3# and 4# resins was similar, and the stacking density between sub-particles was different. The distribution of sub-particles in 1#, 2# and 5# resins is relatively good, but there is also the problem of different packing density. 6# resin inside the sub-particle accumulation uniform.
Study on processing properties of different resins
After chlorination of PVC, due to the introduction of a large number of chlorine atoms in the molecular chain, the polarity of the molecular chain increases, the whole molecular chain is relatively stiff, and the flexibility becomes poor. With the increase of chlorine content, the rigidity of molecular chain increases, the melt viscosity increases, and the processing difficulty increases accordingly. Therefore, it is very important to study the processing properties of different CPVC resins.
In order to study the relationship between the molecular structure of CPVC resin and the sub-microphase state of particles and the processing properties of the resin, the Brabender torque rheometer was used to study the processing properties of six CPVC resins, and the structural characterization of the resin was analyzed. FIG. 2 shows the rheological curves of various CPVC resins. From the rheological curves, the processability and thermal stability of resins under the same conditions can be judged, so that a CPVC resin with excellent processing fluidity and thermal stability can be found.
At the early stage of processing, CPVC resin starts to plasticize under the action of heat and shear, and the torque increases rapidly with the plasticizing process. After about 3 minutes of continuous heating and shear action, the particles are plasticized evenly and enter the stable processing stage. Before the resin is completely plasticized and fused, the flow curves of 1# ~ 5# resins have similar shapes, but the torque is different. At the early stage of processing, only the flow curves of 6# resins have no obvious plasticized melting peak, and no obvious maximum value, and the torque directly reaches equilibrium, indicating that the plasticization has been completely completed. After resin plasticization, all the six resins entered a relatively stable stage of processing, with relatively balanced torque and little fluctuation. The equilibrium torque of 3# resin is the smallest, and the curves of 2# and 4# resin are almost completely coincident after entering the stable stage of processing. The torque of 1# and 5# resins was the largest, while the torque of 6# resins decreased slightly with the progress of processing. From the point of view of stabilization time, 6# resin is the best, 3#, 5# second, 1#, 2#, 4# three kinds of resin stabilization time is the shortest.
The similar shape of the flow curve of 1# ~ 5# resin is mainly determined by the sub-micro phase state of the resin particles. The cortex structure of these five kinds of resin is similar to the structure and stacking state of the sub-particles. The flow curve of 6# resin almost does not have the maximum peak value, and the torque directly reaches equilibrium. This is mainly because 6# resin particles have submicro phase suitable for processing -- the cortex is completely destroyed, and the surface and internal subparticles are stacked evenly -- this particle structure is conducive to rapid and uniform resin plasticization, stable processing production. Therefore, 6# resin has better processing performance.
The equilibrium torque of resin is mainly affected by the chlorine content of resin, the distribution of chlorine atoms in the molecular chain, the relative molecular mass and its distribution. The equilibrium torques of 2# and 4# resins are almost the same, mainly because the relative molecular mass and distribution of the two resins are not very different, and the chlorine content of the resins and the molar percentage of the three basic structural units are similar. The relative molecular mass size and distribution of 3# resin and 4# resin are similar, and the material temperature is almost the same during the experiment, while the equilibrium torque is different. This is because the chlorine content of 3# is small, especially the structure content of the more polar -ChCl -, and the polarity of the whole molecular chain is relatively small. The interaction between chain forging and chain forging and between molecules is smaller than that of 4#. Therefore, the torque of 3# resin is smaller than that of 4# resin under the same conditions. The chlorine content and chlorine atom distribution of 1# and 2# resins are similar, and the difference of equilibrium torque is mainly caused by the relative molecular mass. The relative molecular mass of 1# resin is larger, and the distribution is relatively narrow, so the equilibrium torque is larger. The balance torque of 1#, 5# and 6# resin is relatively large, among which 6# resin is mainly caused by the high molecular weight, 1# and 5# resin is caused by the high chlorine content.
The thermal stability of resin is mainly determined by the chlorine content and the distribution of chlorine atoms in the molecular chain. As can be seen from Figure 2 and Table 2, the stability time of resin basically decreases with the increase of chlorine content. Under the same chlorine content, the distribution of chlorine atoms of the resin has a great influence on the stability of the resin. When the resin molecular chain contains more -ChCl - structure and less -CCl2- structure, the thermal stability of the CPVC resin is relatively good. 6 # resin chlorine content is the lowest, distribution of chlorine atoms are largely in line with the above requirements, so its stability in the experiment the longest time, and distribution of chlorine and chlorine atoms and 6 # resin similar thermal stability of 3 # resin time may be relatively short because of the surface of resin particles caused by fine particles (SEM photos can be clear).
