Volume 37, Issue 4

Gas counter pressure (GCP) technology can impose a reverse pressure to melt and thereby effectively increase the pressure acting on the melt at flow front. Theoretically, it has a potential to solve some defects often occurring in conventional injection molding (CIM) process. This paper designed and manufactured a GCP injection mold. GCP injection molding experiments were conducted. Effects of GCP process on melt flow and density, dimensional accuracy, and mechanical properties of molded samples were investigated. The results showed that GCP process can effectively inhibit the “fountain effect” in melt filling process, decrease the dimensional shrinkage of molded samples, increase dimensional accuracy of samples, and effectively improve impact property of samples. For the samples without weld line, tensile strength and flexural strength of GCP injection molded samples are slightly increased in comparison with those of CIM samples, but for the samples with weld line, GCP process can greatly improve the tensile strength and flexural strength of molded samples. When GCP is 9 MPa and GCP holding time is 10 s, the dimensional accuracy of molded samples without weld line, the tensile strength and flexural strength of the molded samples with weld line all increase up to maximum values. In comparison with CIM samples, the dimensional shrinkage of samples without weld line decreases by 17.2%, the tensile strength and flexural strength of samples with weld line increase by 30.51% and 23.69%, respectively. The impact value of the samples molded by process parameter combination of GCP 9 MPa and GCP holding time 20 s is the highest, and the impact value increases by 18.65%.

Poly(vinyl alcohol) (PVOH) resin is one of the most widely used water-soluble biodegradable polymer. Because of thermal degradation, PVOH exhibits limited melt processing and lacks moldability. The effects of adding glycerol as plasticizer and banana fibers (BF) to PVOH on its moldability and mechanical property were investigated. Melt blending of PVOH with glycerol and/or BF was performed in an internal mixer. The blended materials were then compression molded to produce tensile specimens. Various characterization techniques were employed to study the mechanical properties, compatibility, and crystallization behavior of the PVOH blends. By melt blending with glycerol, PVOH could be processed but decreased the tensile modulus, tensile strength, and crystallization temperature. Furthermore, the addition of BF enhanced the mechanical and thermal properties and crystallization temperature of plasticized PVOH due to compatibility between the two components. Apart from enhancing the mechanical properties and thermal stability, the incorporation of BF can reduce the production cost.

The thermal and mechanical properties of polyamide 46 (PA46) filled with carbon fiber (CF/PA46) composites were studied. CF/PA46 was fabricated by the method of melt blending and injection molding. The results showed that thermal conductivity, tensile strength and impact strength of the composite increased with the increase of weight fraction of CF, however, the elongation at the break decreased as its weight fraction increased. The addition of CF had little effect on the melting temperature of composites, while the crystallization onset (T o ) and crystallization peak (T p ) temperatures of composites shifted to higher points. The scanning electron microscope images showed that when the weight fraction of CF was increased, the CF was more likely to form thermal chains and a network. When the CF weight fraction was 40%, thermal conductivity was 1.49 W/(m·K), approximately 5.54 times as high as that of the pure PA46, and the thermal diffusivity was 0.9755 mm 2 /s, 6.5 times higher than that of the pure matrix. Comparing the experimental data with the three expected thermal conduction models data, the Maxwell-Eucken thermal conduction model was considered more suitable for the PA46/CF composite, in which the weight fraction of the filler was <10% in the thermal conductive system.

Various quantities of carbon fibers (CFs) (from 5% to 20% in weight) were added to matrix by melt blending to produce polylactic acid (PLA)/thermoplastic polyurethane (TPU)/CF composites. Differential scanning calorimetry measurements revealed that the CF content and mixing time had little influence on the crystallization and melting behavior of PLA. Thermogravimetric analysis showed that the introduction of CFs tended to decrease the thermal stability of PLA/TPU/CF composites, and the increase of mixing time tended to increase the thermal stability of PLA/TPU/CF composites when the mixing time is <5 min. Rheological results showed that all the samples exhibited non-Newtonian and shear thinning characteristics. The storage modulus and complex viscosity both increased with the increase of the CF content. It also showed that the increase of mixing time tended to increase the storage modulus and complex viscosity of PLA/TPU/CF composites when the mixing time is <5 min. Scanning electron microscopy images showed that the TPU/PLA blends contain a continuous PLA phase with evenly distributed TPU particles in the size range of 0.25–3 μm, and the blends are immiscible at the micron scale. Mechanical properties showed that the addition of proper CF content could lead to an obvious increase (about 11.43%) in tensile strength.

In this work, an unoxidized aluminum particle/epoxy composite coating was used to cover the surface of pure epoxy resin to reduce the loss of strength after water-heat aging and to improve the wear resistance. The samples were maintained in a water bath at 80°C for 100 days to accelerate the aging process. After water-heat aging, the tensile and bending strengths of the pure epoxy samples were reduced from 30 MPa and 60 MPa, respectively, to 26 MPa and 55 MPa, respectively. However, the tensile and bending strengths of the sample with 6 wt.% aluminum particles were reduced from 39 MPa and 95 MPa to 36 MPa and 89 MPa, respectively. The sample with 6 wt.% aluminum particles exhibited the lowest tensile and bending strength loss rates, 7.2% and 6.1%, respectively, which were 37.4% and 40.8% lower, respectively, than those of the pure epoxy. Additionally, the wear rate and friction coefficient of the samples with 6 wt.% aluminum particles were 2.5×10 -6 cm 3 /min·N and 0.415, respectively, corresponding to a reduction of 35.9% and 30.0%, respectively, compared with the pure epoxy. Thus, it was found that the addition of unoxidized aluminum particles can efficiently increase the water-heat aging resistance and wear resistance of epoxy resin.

In this work, the poly(vinylidene fluoride)/multi-walled carbon nanotubes (PVDF/MWCNTs) hybrid membranes were obtained through vapor induced phase separation (VIPS). In addition, the effects of solution composition on crystal behavior, elemental composite, morphology and hydrophilicity of membranes surface and the viscosity of casting solution were analyzed. The results of field emission scanning electron microscopy and X-ray photoelectron spectroscopy showed that the rough porous surface could be built in all hybrid membranes due to the slower double diffusion mechanism of the VIPS process, which also provided enough time for MWCNTs to move to the surface and be enriched there, to ensure the lowest surface free energy. Furthermore, other analysis displayed that both adding tiny amounts of MWCNTs and a low concentration of PVDF were favorable to the growth and aggregation of PVDF, which were put down to the adverse effect of hydrophobic MWCNTs on the thermodynamic compatibility of polar casting solution, the less nucleation points and the good fluidity with lower concentration of polymer. Then, the diameter of PVDF spherical particles and the membrane surface roughness increased, whereas the hydrophilicity of membrane surface declined effectively.

The usage of biopolymers in developing biodegradable materials for applications that meet demands in society for sustainability and environmental safety has been limited due to the poor mechanical and thermal properties of biopolymers. This study aimed to improve the limited properties of biopolymers, particularly polylactic acid (PLA) films, by investigating the effect of incorporating different concentrations (0–5 wt.%) of halloysite nanoclay and by adding glycerol plasticiser on the mechanical properties (tensile strength, elongation at break, Young’s modulus, and toughness) and thermal properties (glass temperature (Tg), melting temperature (Tm), and crystalline temperature (Tc)) of the produced bio-nanocomposite films. It was found that the addition of halloysite nanoclay and glycerol improved the mechanical and thermal properties of the films. PLA films incorporated with 3 wt.% concentration of halloysite nanoclay resulted in optimum mechanical properties due to the uniform distribution or dispersion of halloysite nanoclay. The addition of halloysite nanoclay and glycerol reduced the Tg, Tm, and Tc of the films, suggesting that they can improve the processability of the biopolymer. The bio-nanocomposite films produced in this work have the potential to replace non-biodegradable films due to the improved properties of the films.

The core-shell oil absorption material (OAM) with fumed silica shell was achieved from Pickering polymerization. The modified fumed silica wall could well stabilize both Pickering emulsion and Pickering polymerization. The particle size of encapsulated OAMs decreased with the increasing concentration of fumed silica and remained unchanged when the concentration was more than 1 wt.%. This fumed silica shell had little effect on the oil absorption rate of OAM. The importance was that the shell reversed the surface property and improved the alkali resistance of OAM. We believe that our core-shell OAMs could reach the self-healing ability of the oil well cement.

The objective of this study was to determine the engineering constants and off-axis tensile stress-strain relation of single-ply quasi-unidirectional (UD) glass fiber (GF)/polypropylene (PP) composites using the new approach. A series of off-axis tensile tests of quasi-UD composites were carried out. In this study, Puck’s interfiber fracture criterion was expanded for the first time to estimate the off-axis tensile stresses of UD composites. With the experimental values, the shear properties were obtained through the curve-fitting methods. Damage mechanisms were demonstrated to evolve with the loading angle. By comparison to experimental data, the Hahn-Tsai equation, together with the transformation equation, was found to be adequate to describe the off-axis tensile stress-strain relation of single-ply quasi-UD GF/PP composites.

In rotational molding, shaping is achieved by elevating the temperature of polymer powder particles above their melting temperature, causing them to adhere to the mold wall. Multi-layer parts can be processed by sequential adding of different polymeric materials to the mold, while the thickness of each layer is defined by the amount of each material. In this work, the influence of adding time of a second component on the shape and development of the interface region between different materials for multi-layer rotational molding is investigated. Therefore, rotational molding experiments were conducted using an uniaxial rotating molding machine and a cylindrical mold. Varying the time of second material addition yielded different specific interface regions. The cross sections of the resulting multi-layer parts were analyzed using transmitted light microscopy and characteristic values were derived to describe the interfaces. Single-layer parts were produced to verify the built-up of the polymer layer and the development of the inner melt surface in rotational molding.

In this paper, experimental and numerical methods were used to determine compressive mechanical properties of multi-walled carbon nanotube (MWCNT) reinforced epoxy. Standard samples with varying weight fractions of MWCNTs were prepared and were tested in compression. Nanocomposite modulus of elasticity, yield strength and compressive strength were determined experimentally. Experimental results show that incorporation of CNTs improves yield and compressive strengths of the epoxy resin to a large extent. Also, numerical simulation of nanocomposites was conducted in ABAQUS finite element (FE) software. In these simulations, the effects of the interface strength between individual nanotubes and between the outer nanotube and matrix were also investigated. Two different mechanisms were used to model these interfaces. In one set of the models, connector constraints were used as the interface. In the second set, an interface consisting of thin shells surrounding the nanotubes was used. The results of this investigation suggest that nanocomposite longitudinal modulus increases with increasing interface strength. Also, numerical results suggest that the connector model predicts values lower than the thin shell interphase model. Finally, experimental and numerical results were compared and a good correlation is observed between the results.