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Condition: Used: Good. Hence, the microstructures of the microcellular parts are further investigated in the following section. It is well known that SEM observation is one of the most popular and reliable ways to check morphological characteristics, which are generally viewed as the determining factors of the final physical and mechanical properties of microcellular foamed parts and include the cell density, size, and distribution.
It can be observed from Figure 5 that the detailed cellular structures are clearly different owing to the addition of PC and compatibilizers. Compared to the neat PP resin shown in Figure 5 a, the number of cells substantially increase with the addition of PC, as shown in Figure 5 b, which means that the addition of PC could improve the foaming cell density of pure PP resins. It can also be seen from Figure 5 that all foams exhibit a completely closed cell cellular structure, which can ensure that the mechanical properties of the samples do not drop too much as a result of the bubbles.
It can also be found from Figure 6 that the PCG sample has the most uniform cell size distribution within the SEM images, which corresponds to the minimum reduction ratio of mechanical properties shown in Figure 4. Hence, it can be concluded that a fine foaming structure can lead to excellent mechanical properties.
In this study, the nucleation density is higher than 3. Therefore, the method of fabricating PP foamed components with the addition of PC and PP- c -GMA might be an effective way to improve the cellular structure of foamed IM parts that actually results in improved mechanical properties and saves materials.
Statistic results of cell size and cell density from the SEM images of Figure 5. It can be concluded from the above discussion that the use of the different compatibilizers made a great difference in the morphological structure and mechanical properties, which can be attributed to the different effects from the compatibilizers and needs to be discussed further. The addition of a compatibilizer is a widely used and efficient method for enhancing the interfacial adhesion between phases of polymer blends, thus resulting in an improvement in the mechanical properties.
Figure 7 shows SEM images of the fracture surfaces of the six representative samples. Figure 7 a,f show the morphological characterization of the pure PP and PC resin, respectively, and both appear to be characteristic of a single homogeneous component. With the addition of PC, the two phases can be easily observed in Figure 7 b, as PP clearly forms a continuous phase while PC forms large domains.
However, the size of the dispersed PC particles varies within a wide range. Not only that, there is an irregularly shaped dark region on the interface, which could be attributed to nonuniformly distributed PC droplets that were debonded and pulled out from the PP matrix during cryogenic deformation. SEM images of the molded sample. Compared with the dispersion in Figure 7 c, it can be shown in Figure 7 d that the PC phase exhibits smaller particle diameters and a more regular and finer dispersion in the PP matrix, which indicates an increase in the compatibility between the PP and PC.
As a result, the interfacial adhesion between the PP and PC phases improved in the presence of PP- c -GMA, subsequently reducing the interfacial tension between the two phases and leading to good mechanical properties. Compared to the dispersion of the PC particles shown in Figure 7 d, the PC particles shown in Figure 7 e have a more uniform dispersion in the PP matrices, and the mean diameter of the dispersed PC particles also clearly decrease, which indicates a further increase in the compatibility between the PP and PC.
The compatibility was further examined by determining the glass transition temperature T g of the PC, which is shown as the loss factor vs. The T g of PC decreases from The loss factor peak temperature further shifted to The improvement of compatibility is generally beneficial for the mechanical properties because it indicates an improved phase adhesion between the blend partners due to a reduction in the interfacial tension and coalescence.
This improvement may be attributed to the number of nucleating sites possibly increasing with the incorporation of the dispersed phase, since the interfacial boundaries between the two immiscible phases can effectively lower the critical energy barrier for bubble nucleation. However, the opposite is actually true. Hence, there are still other possible reasons that remain undetermined, and more investigation of the forming mechanism of PCG fine foam structure needs to be carried out.
It is not easy to characterize the mixture of a rich-gas melt used in foamed IM that generally involves many complex processing conditions, including not only IM processing parameters, such as variable pressure, shear rate, temperature, and temperature gradient but also the interaction between the polymer melt and the gas, such as gas release, gas dissolving in polymer melt, and injection of gas-melt solution into mold cavity. Rheological measurement is generally seen as a conventional and useful method, which has important guiding significance on the processing properties of polymers [ 54 , 55 , 56 ].
The torque vs. The addition of PC, regardless of whether gas was involved, decreases the torque of pure PP resin. For the torque rheological test, the higher the balance torque is, the higher the melt viscosity. It suggested that the torque of PP decreases with the addition of PC. It is also known that the viscosity difference between PP and PC is generally very large. However, the results shown in Figure 9 a indicate that the viscosity of PP and PC are almost the same at the test condition.
Furthermore, by introducing gas of the system, the ability of the polymer chains to move freely is also changed, which is reflected in a change of shear viscosity. Compared to the torque values shown in Figure 9 a, the torque of the corresponding rich-gas samples shown in Figure 9 b are clearly lower, which means a lower shear viscosity due to the gas dissolved in the polymer melt. Torque rheological curves torque vs. Although a decrease in the torque with the presence of the gas is expected and can be attributed to the plasticization effect of the gas, there is still a strange phenomenon concerning the different variance of the torque between the PCG and PCE samples that needs to be clarified.
Microcellular Injection Molding : Jingyi Xu :
To make it clearer, the difference of the torque between the rich-gas and the no-gas samples was expressed as the reduction ratio, which can be calculated by the following expression:. Also, the low interfacial bonding at the two-phase interface of the incompatible blends may provide a relatively easy passage for gas escape [ 60 ]. Therefore, the gas increases the chance of separation between the two phases, which leads to a higher reduction ratio.
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It is also observed from Figure 9 c that the reduction ratio of PCG is smaller than that of PCM and then PPC, indicating the reduction ratio varies with the incorporation of the different compatibilizers. However, a strong compatible system may mean that the influence of the gas will be reflected by the blended matrix if the compatibility exceeds a limit, as occurred in the PCE samples.
As a result, the reduction ratio of the PCE melt viscosity instead increases due to the excellent compatibilization as discussed before. It can be concluded that a moderate compatibility is beneficial for the increase of rich-gas melt viscosity. Moreover, it can also be deduced that another reason for the improved foamability of PP with the addition of PC and compatibilizer is the increase of the rich-gas melt viscosity, which results in more cell growth resistance and prevents gas escape. As discussed before, the existence of the incompatible second phase improves the foamability of pure PP resin and leads to a finer and denser cell structure.
This improvement can be attributed to the number of nucleating sites possibly increasing with the incorporation of the dispersed phase, since the interfacial boundaries between the two immiscible phases can effectively lower the critical energy barrier for bubble nucleation. For the strong compatible system, PCE; however, the rich-gas melt viscosity decreases drastically resulting in its cellular structure instead deteriorating slightly. Therefore, according to the above analysis, the influences of PC and compatibilizer on the foaming behavior of PP can be attributed to a comprehensive effect of the interface between the PP and the PC phases and the variation of the rich-gas melt viscosity.
It was found that PC can improve the cellular foam structure of a pure PP resin, and it can be attributed to a possible mechanism, i. The addition of different compatibilizers makes the influence of PC on the PP foaming behavior more complex.
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One can explain that it is due to a comprehensive effect of the compatibility on the rheological behavior. Accordingly, an effective way of improving the foaming behavior of PP foamed injection molded components was suggested and proposed. Conceptualization, Y. National Center for Biotechnology Information , U. Journal List Polymers Basel v. Polymers Basel. Published online Feb Find articles by Bei Su.
Find articles by Cao Yan. Author information Article notes Copyright and License information Disclaimer. Received Dec 25; Accepted Feb 1. Abstract To improve the foaming behavior of a common linear polypropylene PP resin, polycarbonate PC was blended with PP, and three different grafted polymers were used as the compatibilizers.
Introduction Polypropylene PP foams are often viewed as one of the most popular lightweight thermoplastic materials. Experimental 2. Open in a separate window.
Figure 1. Table 1 The main processing parameters of the mixing and the extrusion. Figure 2. Table 2 The selective processing parameters of injection molding. Results and Discussion 3. Figure 3. Figure 4. Foaming Behavior It is well known that SEM observation is one of the most popular and reliable ways to check morphological characteristics, which are generally viewed as the determining factors of the final physical and mechanical properties of microcellular foamed parts and include the cell density, size, and distribution.
Figure 5. Figure 6. Compatibility The addition of a compatibilizer is a widely used and efficient method for enhancing the interfacial adhesion between phases of polymer blends, thus resulting in an improvement in the mechanical properties. Figure 7.
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Figure 8. Relative Rheological Behavior It is not easy to characterize the mixture of a rich-gas melt used in foamed IM that generally involves many complex processing conditions, including not only IM processing parameters, such as variable pressure, shear rate, temperature, and temperature gradient but also the interaction between the polymer melt and the gas, such as gas release, gas dissolving in polymer melt, and injection of gas-melt solution into mold cavity.
Figure 9. Author Contributions Conceptualization, Y. Conflicts of Interest The authors declare no conflict of interest. References 1. Park C. Saiz-Arroyo C. Mohebbi A. Part B. Foaming of Polypropylene with Supercritical Carbon Dioxide. Zhou S. Zheng W. Ding S. Antunes M.
Multifunctional Polymer Foams with Carbon Nanoparticles. Zhou Y. Rubber Compos. Zhai W. Sharudin R. Zhang P. Rachtanapun P. Sun X. Gunkel F. Tan X. Rudin A. Polycarbonate Blends with Polystyrene and Polypropylene. Laoutid F.