Kaizhi Shen

In‐Situ Microfibrillar PET/iPP Blend via a Slit Die Extrusion, Hot Stretching and Quenching Process: Influences of PET Concentration on Morphology and Crystallization of iPP at a Fixed Hot Stretching Ratio

In‐situ microfibrillar poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) was fabricated via a slit die extrusion, hot stretching, and quenching process. Morphological observation shows that well‐defined microfibrils were generated in‐situ. The microfibrils become larger with the increase of PET concentration. The presence of PET microfibrils shows significant nucleation ability for crystallization of iPP. Higher concentration of PET microfibril corresponds to faster crystallization of the iPP matrix, while the variation of PET concentration has little influence on the onset and maximum crystallization temperatures of iPP matrix during cooling from melt. Optical microscopy observation reveals that transcrystallites form in the microfibrillar blend, in which the PET microfibrils play as the center row nuclei. In the as‐stretched microfibrillar blends, small‐angle X‐ray scattering measurements show that the long period of lamellar crystals of iPP is not affected by the presence of PET micofibers. Wide‐angle x‐ray scattering reveals that the β phase of iPP is obtained in the as‐stretched microfibrillar composites, whose concentration decreases with the increase of PET microfibril concentration. This suggests that PET microfibrils do not promote the occurrence of the β phase.

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.

Simultaneously improving the tensile and impact properties of isotactic polypropylene with the cooperation of co-PP and β-nucleating agent through pressure vibration injection molding

In this article, crystalline morphology and molecular orientation of isotactic polypropylene (iPP), random copolymerized polypropylene (co-PP) and β-nucleating agent (β-NA) composites prepared by pressure vibration injection molding (PVIM) have been investigated via polarized light microscopy, scanning electron microscopy, wide-angle X-ray diffraction and differential scanning calorimetry. Results demonstrated that the interaction between co-PP and iPP molecular chains was beneficial for the mechanical improvement and the introduction of β-NA further improved the toughness of iPP. In addition, after applying the pressure vibration injection molding (PVIM) technology, the shear layer thickness increased remarkably and the tensile strength improved consequently. Thus, the strength and toughness of iPP/co-PP/β-NA composites prepared by PVIM were simultaneously improved compared to those of the pure iPP prepared by conventional injection molding (CIM): the impact toughness was increased by five times and tensile strength was increased by 9 MPa. This work provided a new method to further enhance the properties of iPP/co-PP composites through dynamic processing strategy.

Mechanical Property and Crystal Structure of Nylon6 Samples Prepared by Vibration Injection Molding

An injection apparatus with a pressure vibration field was self-developed to study the effects of vibration frequency and vibration pressure on the mechanical properties and crystal structure of nylon6 during the process of vibration injection molding. Scanning electron microscope (SEM) and wide-angle X-ray diffraction (WAXD) measurements were conducted. Experimental results showed that the amount of spherulites in subsurface of vibration sample is much more than that of static sample, and the spherulites become more uniformly and perfect. In subsurface layer, vibration induced the change of crystal form from γ-form to α-form. Tensile and impact strength of samples obtained via vibration injection molding were improved, while the elongation decreased.

Self-Reinforced High-Density Polyethylene Prepared by Low-Frequency, Vibration-Assisted Injection Molding. 1. Processing Conditions and Physical Properties

Using a low-frequency, vibration-assisted injection molding (VAIM) device, the effects of vibration variables (frequency and amplitude) on mechanical properties and thermal softening temperature of high-density polyethylene (HDPE) injection moldings were investigated. For VAIM-processed samples, the mechanical properties can be improved by changing vibration frequency and vibration pressure amplitude. Injected at a constant vibration pressure amplitude, a low range of frequency (below 0.7 Hz) was favorable for increasing yield strength; in the high range of frequency (0.7 Hz < f < 2.33 Hz) the yield strength remained at a plateau. Injected at a constant frequency (0.7 Hz) the yield strength increased sharply with decreased elongation when applying large vibration pressure amplitude. The maximal yield strength and Youngs modulus were 60.6 MPa and 2.1 GPa for a VAIM sample compared with 39.8 MPa and 1.0 GPa for a conventional injection-molded (CIM) sample, respectively; there was also a 10°C increase in Vicat softening point temperature.

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.