Xueqin Gao

The Morphology and Tensile Strength of High Density Polyethylene/Nano-Calcium Carbonate Composites Prepared by Dynamic Packing Injection Molding

In this article, dynamic packing injection molding (DPIM) was used to prepare pure high density polyethylene (HDPE) and its composite with nano-CaCO3 samples, whose mechanical properties were improved significantly. Compared with conventional injection molding (CIM), the enhancement of tensile strength of dynamic HDPE/ nano-CaCO3 samples are 122%. According to the SEM, WAXD, and DSC measurement, it was found that a much better dispersion of nano-CaCO3 was achieved by the technology of DPIM. More importantly, DPIM also caused an obvious increase in orientation of HDPE matrix. The crystallinity degree of HDPE in dynamic sample increased by 5.52% compared with a conventional one. The improvement in mechanical properties for dynamic HDPE/ nano-CaCO3 is attributed to the even distribution of nano-CaCO3 particles, the orientation of HDPE crystals, as well as increase of crystallinity degree under the influence of DPIM.

Study on the Improvement of Crystallization in HDPE induced by High-Molecular-Weight Polyethylene through Dynamic Packing Injection Molding

The effect of high–molecular-weight polyethylene (HMWPE) on crystal morphology was investigated for high-density polyethylene (HDPE) through dynamic packing injection molding (DPIM). With the aid of differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), and scanning electron microscopy (SEM) measurements, a typical web-like shish kebab morphology, which markedly increases stiffness and toughness, was found in HMWPE-induced samples through DPIM. The SEM results show that the much better web-like shish kebab structure, in which most of the lamellae connect different columns, compared with conventional shish kebab, was formed in HDPE blends with 4% HMWPE content (B4) through DPIM. The WAXD studies indicate that orientation degrees of crystallographic planes (110) and (200) in the B4 samples were much higher than those of samples molded by static packing injection molding and B0 samples molded by DPIM. A combination of the higher degree of crystal orientation and the formation of web-like shish kebab led to simultaneous great increments of stiffness and toughness, which overcomes the traditional limitation that stiffness and toughness cannot be greatly enhanced simultaneously. All these results show that HWMPE favored for great improvement of crystal structures in HDPE when its content is appropriate through DPIM.

Effect of different morphologies on the creep behavior of high-density polyethylene

With the wide use of polymer materials as pressure parts, people have started paying more attention to the safety and longevity of polymeric materials. Creep is one of the most important factors to evaluate materials. In this study, a self-designed oscillatory shearing injection molding (OSIM) device was utilized to prepare pure HDPE specimens with special morphologies. According to a comparison of the creep behavior of the OSIM specimens with conventional injection molding (CIM) specimens, the distinction between the resistivity to creep due to the special morphologies was observed. Two initial external stress levels (10 MPa and 15 MPa) and three temperatures (ambient temperature 25 °C, 40 °C and 60 °C) were employed in this experiment. Different morphologies resulted in different responses to creep. The deformation and compliance of the CIM specimens were triple or more than those found for the OSIM specimens under the same conditions. The instantaneous deformation of the OSIM specimens was 0.2% compared with 0.6% found for the CIM specimens under 10 MPa at 25 °C. The deformation of the OSIM specimens was 4% after creep for an hour, but the CIM specimens were already necked at less than 50 min under 15 MPa at 40 °C. At 60 °C, too much plastic deformation appears in the creep behavior of the CIM specimens and the creep behavior was nearly not observed under these conditions. In addition, the creep behavior of the OSIM specimens can be observed at 60 °C. According to our tests and analysis, the property of creep resistivity for the OSIM specimens was better than that found for the CIM samples, in both the amorphous phase and crystalline region. In addition, the creep behavior of the OSIM and CIM specimens was satisfactorily described using the generalized Kelvin–Voigt model with one retardation time.

Effect of Melt Vibration on Mechanical Properties of Injection Molding and Rheology

A pulse pressure was superimposed on the melt flow in injection molding, called vibration injection molding (VIM); the mechanical properties of the resulting samples were compared with the values of conventional injection molding (CIM). A die (L/D = 17.5) was attached to this device to study rheology. Properties of an amorphous polymer (ABS) and a semicrystalline polymer (PP), prepared in the vibration field, were compared to each other. Applying VIM, the mechanical properties can be improved whether the material is amorphous or not. Increasing with vibration frequency, the tensile strengths of PP were improved. The processing parameters to obtain self‐reinforcing and self‐toughening moldings were found at high vibration frequency Fr (Fr > 1.2 Hz). For ABS, the improvement of tensile strength is very small. For both PP and ABS, the yield strength, Youngs modulus, and impact strength are all improved by increased vibration pressure amplitude. The elongation at break of PP moldings, however, decreases sharply; but the corresponding value decreases little for ABS. So long as the pulse pressure is superimposed on the melt, the average apparent viscosity decreases sharply for both crystalline and amorphous polymers, and the decreases obtained at increased vibration pressure amplitude are bigger than are those obtained at increased vibration frequency. The changes in viscosities for the amorphous material, ABS, are smaller than the are values for the semicrystalline polymer, PP. The amounts of changes in the mechanical properties and rheology depend greatly on the melt temperature. Contact grant sponsor: National Natural Science Funds of China; contract grant number: 54473053; Contact grant sponsor: Special Fund for Major State Basic Research Projects of China; contract grant number: G1999064809.

Mechanical Properties and Morphology of Polypropylene/Nano-Montmorillonite Composites Prepared by Dynamic Packing Injection Molding

In this article, dynamic packing injection molding (DPIM) technology was used to prepare injection samples of pure polypropylene and its composites (PP/OMMT). The DPIM technology led to remarkable mechanical enhancement from 43.23 MPa and 26.16 J/g of conventional injection-molded PP(CIM-PP) to 60.46 MPa and 29.03 J/g of DPIM-PP for tensile strength and impact strength, respectively. More importantly, the samples containing OMMT (termed PP/OMMT) exhibited excellent dispersion under the same flow condition. This special structure gave rise to further reinforcement from 40.91 MPa and 39.23 J/g of CIM-PP/OMMT to 54.87 MPa and 168.19 J/g of DPIM-PP/OMMT for tensile strength and impact strength, respectively. The morphology of DPIM samples was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). It was found that a much better dispersion of nano-OMMT was achieved by DPIM. Moreover, γ-crystal was generated in shear layer of DPIM samples. The crystallinity of PP matrix in DPIM sample increases by 7% compared to CIM sample. The excellent mechanical properties of DPIM PP/OMMT nanocomposites sample could attribute to the homogeneous distribution of OMMT particles, the intercalation or exfoliation of montmorillonite lamellas as well as the morphology change of PP matrix under the influence of dynamic shear stress.

Morphologies and Mechanical Properties of High-Density Polyethylene Induced by the Addition of Small Amounts of Both Low- and High-Molecular-Weight Polyolefin under Shear Stress Applied by Dynamic Packing Injection Molding

This paper focuses on the mechanical properties and crystal morphology of a self-reinforced high-density polyethylene 5000S (HDPE 5000S) by simultaneously blending with 9 wt% high-molecular-weight polyethylene (HMWPE) and 9 wt% low-molecular-weight polyethylene (LMWPE) (A9) under the shear stress field which was engendered by a self-made dynamic packing injection molding (DPIM) machine. The results of mechanical properties, differential scanning calorimetry, and scanning electron microscopy characterization were as follows: (1) The tensile strength of the dynamic samples increased to 112.1 MPa, 4.85 times as much as that of static packing injection molding (SPIM) samples (23.1 MPa), as a result of realizing polyethylenes self-enhancement; (2) Shish-kebab structure was found in the dynamic samples; (3) The crystallinity of the DPIM A9 sample reached 68.6%, on increase by 18.7% compared with that of the SPIM sample. The formation of the shish-kebab structure and enhancement of mechanical properties are explained.

Study on the Morphology and Properties of PP/HDPE Blend Prepared by Vibration Injection Molding

A self-designed pressure vibration injection molding device was used to study the effect of vibration frequency and vibration pressure on tensile strength and impact strength of PP/HDPE (70/30) samples prepared by vibration injection molding. Furthermore, DSC and scanning electron microscopy (SEM) observations were conducted. The tensile strength and impact strength increase with increasing vibration frequency and vibration pressure. SEM micrographs show that the orientation degree of the vibration sample obtained at 190°C obviously increases compared with the static sample, and the clusters of lamellae are stretched along the flow direction, whereas their lateral sizes decrease correspondingly. There is no evident orientation in the core layer of the vibration sample obtained at 230°C. DSC testing results show that the degree of crystallinity of vibration samples are higher than those of static samples.

Effect of different morphologies on slow crack growth of high-density polyethylene

With the wide application of polymer pipes in the world, people pay more attention to the longevity of the pipes. Slow crack growth is one of the most important factors that influence the longevity of materials, especially PE pipes. Thus, the study of slow crack growth has become deep and concrete. In this paper, slow crack growth in specimens with different morphologies was studied. A self-designed oscillatory shear injection molding (OSIM) device was utilized to prepare pure HDPE 5000s specimens with different morphologies. The process of slow crack growth in OSIM specimens was compared with that in specimens prepared by conventional injection molding (CIM); so it was possible to study the influence of the different morphologies in specimens on the process of slow crack growth under the same external conditions. Different morphologies have great influences on the ability of materials to resist slow crack growth. The time taken for the slow crack growth to fracture in the OSIM specimens is considerably longer than that in the conventional ones. Under the same conditions of 80 °C and 9.0 MPa, the time of complete fracture is 650 min in OSIM specimens, in contrast to 60 min in conventional specimens. Moreover, in the OSIM specimens, the initial stress of the brittle–ductile transition is increased. At 80 °C, the brittle–ductile transition takes place at 4.7 MPa for conventional specimens, whereas for OSIM specimens, it occurs at 5.6 MPa. Furthermore, the mechanisms of their slow crack growth processes are different. The SCG process in conventional specimens is that stress concentration initiates craze, which grows rapidly and becomes a crack. However, the process in OSIM specimens is a cyclic process, in which stress concentration initiates craze, the crack grows, a new craze appears, and the new crack grows until the specimens fracture.

Suppression of the Skin-Core Structure of Injection-Molded Polypropylene Part: Role of Balance Effect Caused by the Incorporation of Glass Fiber

The skin-core hierarchy structure of isotactic polypropylene (iPP) injection-molded parts was successfully suppressed by the introduction of glass fibers (GFs) as a result of the “balance effect.” The pure iPP presents a large fraction of spherulitic core layer, while the thickness of the core layer of the iPP/GF composites was greatly thinner. For pure iPP, the morphology can be divided into three regions along the thickness direction: skin layer, shear layer, and core layer. However, the morphology of the sample with 7 wt% GF was so homogenized that it could not be roughly divided into the three regions. Furthermore, the area of the shear layer becomes larger with increasing GF content. It was full of shish-kebab-like cylindrite structures. These results indicated that GF can homogenize the gradient of shear stress perpendicular to the flow direction. It was confirmed that the GF could be used to stabilize the shear-induced nuclei, especially in the core region, and resulted in the enhanced crystallinity of the β-form. Based on our investigation, a schematic model was proposed to interpret the “balance effect” of GF on suppression of the skin-core structure.

Vibration-dependent morphology and crystal structure of isotactic polypropylene

A small homemade device was used to study the influence of mechanical vibration on the crystal structure and morphology of isotactic polypropylene (iPP) under different melting temperatures, vibration times, vibration frequencies, and cooling rates. The crystallite size, crystal structure, and crystallinity of iPP under or without vibration treatment were investigated by means of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and polarized microscopy observation (PLM). The crystallization of iPP varied with the length of vibration time, vibration frequency, cooling rate, and melt temperature. Compared with the data of conventional samples measured by DSC, vibration could increase the crystallinity of iPP, make melting peak of α-crystal move toward higher temperature and make that of β-crystal shift to lower temperature. Meanwhile, WAXD measurements showed that the vibration could reduce the content of β-crystal evidently, particularly at the lower vibration frequency, lower cooling rate, and higher melting temperature. Furthermore, PLM measurements showed that the vibration made the spherulite size smaller.