The hierarchical structure of water‐assisted injection molded high density polyethylene: Small angle X‐ray scattering study

Abstract

High density polyethylene (HDPE) was molded by a new polymer processing method, that is, water‐assisted injection molding (WAIM), and its hierarchical structure was studied by two‐dimensional small angle X‐ray scattering (SAXS). For comparison, the hierarchical structure of HDPE molded by conventional injection molding (CIM) was also characterized. The result shows that the WAIM part exhibits a distinct skin‐core‐water channel structure which is different from the skin‐core structure for the CIM part. In the skin layer of both WAIM and CIM parts, the shish‐kebab structure was formed due to the shear stress brought by melt filling, but the lamellar orientation parameter of CIM part is smaller than that of WAIM part. The spherulites with random lamellar orientation are dominant at the core of both parts owing to the low cooling rate and feeble shear stress therein. Interestingly, the shish structure and the lamellae with low level of orientation can be found at the water channel layer of WAIM part. They are attributed to the shear stress brought by water penetration. Moreover, the lamellar orientation parameter in water channel layer is smaller than that of skin layer. In addition, the long period of WAIM part first increases and then decreases with the elevating distance from the skin surface, while that of CIM part tends to increase monotonously. In a word, one can conclude that the rapid cooling rate and shear brought by the injected water have significant influence on the structural evolution for the WAIM part. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

An Alternating Skin–Core Structure in Melt Multi‐Injection‐Molded Polyethylene

“…Clearly, a large number of well‐developed banded spherulites were formed in the core region of M1 sample regardless of the distance from the outer surface (Figure ). The size of the banded spherulites increased as the distance increased from the outer surface due to thermal gradient . For the depths of 800 and 1500 µm of the M3 sample in Figure a, c are corresponding to the region 3/9 and 5/7 in Figure b, respectively.…”

Suppression of the hierarchical structure of water-assisted injection moulded iPP in the presence of a β-nucleating agent and lamellar branching of a β-crystal

“…[1][2][3][4][5][6][7][8][9][10] It has been well proved that there exists a "skin-core-water-channel" structure in the waterassisted injection moulded (WAIM) parts due to the melt lling and water penetration. [6][7][8][9][10] However, from a practical point of view, the heterogeneous structure is not benecial for the mechanical properties due to the residual stress produced by different levels of crystal orientation across the thickness direction. 4 Therefore, the homogeneous structure is expected.…”

“…4,5 In WAIM, the rapid cooling rate and shear brought by water penetration are the two principal factors in the oriented structures formation, and they have a dramatic effect on the morphological development through the thickness of WAIM part. [6][7][8][9][10] Thus, in this study, the objective is to study the effect of b-NA content on the hierarchical crystalline structure of WAIM parts.…”

Overview of the Experimental Trends in Water‐Assisted Injection Molding

“…More importantly, the injected water has a significant effect of rapid cooling rate on the interior melt. Obviously, it is unambiguous that the thermo‐mechanical field in WAIM process is more complicated than that in GAIM and CIM processes …”

“…The hierarchical structure of high‐density polyethylene (HDPE) molded by WAIM was studied by Liu et al using 2D small‐angle X‐ray scattering (2D‐SAXS). The results showed that the WAIM part was characterized with a distinct skin–core–water channel structure through the thickness, including a shish‐kebab structured skin, a typically spherulitic core, and a shish structure with low oriented lamellar water channel region (see Figure ).…”