Quantification of the Young's modulus for polypropylene: Influence of initial crystallinity and service temperature

In this study, the mechanical properties of isotactic polypropylene (iPP) materials with different crystallinities at room and elevated temperatures were investigated. In order to obtain samples with a certain range of crystallinity, and to ensure a uniform microstructure of these samples, the iPP samples obtained by injection molding required melt compression molding and controlled annealing. In the macromechanical studies, the experimental results showed that the storage modulus and Young's modulus of polypropylene were sensitive to the service temperature. The crystallinity also had a great influence on this relationship. A function was proposed to evaluate the dependence of the Young's modulus of polypropylene on initial crystallinity and service temperature, and tested based on experimental data. The Young's modulus of iPP is reduced by about 90% when the service temperature rises from 25 to 125 °C. Moreover, the reduced value in Young's modulus between polypropylene having the highest and lowest crystallinity was reduced from 214.55 to 56.75 MPa. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48581.     

Non-linear rheological response as a tool for assessing dispersion in polypropylene/polycaprolactone/clay nanocomposites and blends made with sub-critical gas-assisted processing

Polypropylene (PP) was blended with polycaprolactone (PCL) and nanoclay (NC) in a twin-screw extruder (TSE) using a traditional extrusion process and a sub-critical gas-assisted process (SGAP). SGAP is a new and facile processing method that injects compressed gas (CO₂ or N₂) at low pressures (~10 bars) into the barrel of the extruder to induce rapid and repetitive foaming and resolubilization as the melt travels through regions of high pressure and low pressure. Bubble expansion during foaming introduces an equibiaxial elongational flow not otherwise generated in TSE, adding to the total stress the polymer matrix can exert to break up nanoparticle agglomerates and reduce the droplet size of secondary polymers in blends. Impact, morphology, and X-ray diffraction (XRD) properties confirmed a smaller PCL phase droplet size and an increase in dispersion of the NC when SGAP was used. Standard small amplitude oscillatory (SAOS) rheological tests for the storage modulus G′ were not sensitive enough to discern the difference between the traditionally extruded samples and the SGAP samples. However, the zero-strain non-linearity parameter, Q₀, determined by the Fourier-Transform rheology, was able to distinguish the enhanced dispersive and mixing capabilities of SGAP. Practical implications of SGAP and Fourier-Transform (FT) rheology are also discussed in this paper. POLYM. ENG. SCI., 60:55–60, 2020. © 2019 Society of Plastics Engineers     

Processing and characterization of solid and microcellular PHBV/coir fiber composites

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/coir fiber composites were prepared via both conventional and microcellular injection-molding processes. The surface of the hydrophilic coir fiber was modified by alkali- and silane-treatment to improve its adhesion with PHBV. The morphology, thermal, and mechanical properties were investigated. The addition of coir fiber (treated and untreated) reduced cell size and increased cell density. Further decrease in cell size and increase in cell density was observed for treated fibers compared with PHBV/untreated-fiber composites. Mechanical properties such as specific toughness and strain-at-break improved for both solid and microcellular specimens with the addition of coir fibers (both treated and untreated); however, the specific modulus remained essentially the same statistically while the specific strength decreased slightly. The silane-treated coir fiber composites showed the greatest improvement in specific toughness and strain-at-break among the treated-fiber composites. In addition, adding coir fibers (treated and untreated) also increased the degree of crystallinity of the PHBV composites. PHBV with treated coir fibers showed a higher degree of crystallinity compared with untreated coir fibers.     

A review of thermoplastic polymer foams for functional applications

Journal of Materials Science - Recently, functional applications of thermoplastic foams have received extensive attention from the research and materials communities, focusing on their various...     

Flash

Flash, a common injection molding defect, arises when melt flows from the cavity into thin gaps between parting surfaces. Besides rules of thumb for eliminating flash, there are few fundamental papers on flash analysis. Understanding flash as a transport phenomenon provides a systematic basis for solving flash problems. The governing equations for the gap flow are established and solved for an isothermal power law fluid, under constant pressure along the parting line where flash begins. Two shapes are investigated, rectangular and ring slits that respectively correspond to modeling flash from straight and curved parting lines. Our equation for flash length, the distance to which the melt penetrates the gap developed between the parting surfaces, is the main result of this work. Further, adimensionalizing not only unifies the results for straight and curved parting lines, but also provides insight into how rheology, pressure, temperature, and geometry govern flash. Our approach avoids tedious numerical simulation and mold structural analysis. Our theory is validated by our polycarbonate flash experiments. POLYM. ENG. SCI. 46:241–247, 2006. © 2006 Society of Plastics Engineers     

Development of a rheo‐dielectric sensor for online shear stress measurement during the injection molding process

This study presents the development of a novel rheo‐dielectric sensing technique, based on dielectrostriction measurement, for online injection molding process monitoring. Dielectrostriction, defined as a variation of dielectric properties of material under deformation, detects evolution of molecular orientation during injection molding and enables online shear stress measurement. The dielectrostriction effect resembles the well‐known birefringence phenomenon. As birefringence in polymers is described by the stress‐optical relationship (Fuller, Optical Rheometry of Complex Fluids, Oxford University Press, New York (1995); Janeschitz‐Kriegl, Polymer Melt Rheology and Flow Birefringence, Springer, Berlin (1983); Saiz and Rainde, Dipole Moments and Birefringence of Polymers, Prentice‐Hall, New Jersey (1992)), a stress‐dielectric relationship exists for and is applicable to dielectrostriction. In addition,dielectrostriction measurements can be performed on both transparent and opaque materials with a much simpler data acquisition technique. To demonstrate the feasibility of the dieletrostriction measurement, a planar capacitor sensor rosette has been developed and attached to the surface of an injection mold to obtain the dielectrostriction signal under various processing conditions. The calculated shear stresses of polymer melts based on dielectrostriction signals were validated by agreement with simulation predictions. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

The degradation rate of polyanhydride (poly(sebacic acid), diacetoxy terminated, PSADT)

The in vitro degradation rate of polyanhydride (poly(sebacic acid), diacetoxy terminated), also known as PSADT, was investigated. PSADT tablets with a circular cross-section were formed using a compression molding device, and then immersed into phosphate buffer saline (PBS) for in vitro degradation experiments. The mechanisms of degradation and the degradation rate were characterized by the change in molecular weight and reduction in specimen mass. In addition, the effects of processing temperature and the geometry of the formed PSADT tablets on the rate of degradation were studied. The surface morphology at different degradation times was observed by scanning electron microscopy (SEM). The experimental results showed that PSADT exhibited surface erosion due to the fact that near zero-order degradation kinetics was observed during its degradation process. Moreover, it is found that the geometry of tablets played an important role on the rate of degradation, while the processing temperature had no significant effect on the PSADT degradation rate.     

Adaptive online quality control for injection‐molding by monitoring and controlling mold separation

This paper presents a unique method that makes use of a small signal (of the magnitude of microns) measured by a precision linear displacement transducer mounted on the outside of mold plates to monitor the momentary separation of the core and cavity plates. The maximum value of the separation is found to be highly correlated with part weight, one of the important quality indices. The whole profile of the mold separation (MS) is monitored and used in order to control the process adaptively and to keep the part quality consistent. Adaptive algorithms are developed to control the switchover point from filling to packing in terms of hydraulic pressure from shot to shot. Within a shot, the hydraulic pressure during the holding stage is manipulated to control the MS profile. The experimental results with different resins and mold geometries show that the variation of part weight is reduced significantly as compared to the conventional hydraulic pressure control. POLYM. ENG. SCI. 46:569–580, 2006. © 2006 Society of Plastics Engineers     

Preparation of fast‐degrading poly(lactic acid)/soy protein concentrate biocomposite foams via supercritical CO2 foaming

To increase the degradation rate of poly(lactic acid) (PLA), soy protein concentrate (SPC) was introduced via melt compounding using a self‐developed, co‐rotating, non‐twin‐screw extruder. Poly(2‐ethyl‐2‐oxazoline) (PEOX) and diphenyl methane diisocyanate (MDI) were added to plasticize the melt and improve the compatibility between PLA and SPC. The PLA/SPC blends were subsequently foamed using supercritical carbon dioxide (CO₂) as a blowing agent to produce porous composites. The involvement of SPC promoted cold crystallization of PLA but reduced the thermal stability of the blends. PLA showed a strong interfacial bonding with modified SPC, and the SPC formed continuous three‐dimensional networks when its proportion reached 30 wt%. In the foaming process, SPC domains acted as heterogeneous nucleation sites, which resulted in enhanced cell densities and reduced cell diameters. The PLA/SPC (70:30) sample showed the finest cell structure due to the presence of the SPC network. For the same blends, increasing the foaming pressure from 16 to 20 MPa enhanced the cell density by about 5 times. The water absorption rate and the biodegradation rate of the PLA/SPC foams were much higher than that of neat PLA due to the hydrophilicity of SPC and the porous structure of the foams. POLYM. ENG. SCI., 59:1753–1762, 2019. © 2019 Society of Plastics Engineers     

Residual wall thickness of water‐powered projectile‐assisted injection molding pipes

Water‐powered projectile‐assisted injection molding (W‐PAIM) is an innovative molding process for the production of hollow shaped polymer parts. The W‐PAIM utilizes high pressure water as a power to drive a solid projectile to displace the molten polymer core to form the hollow space. The residual wall thickness (RWT) and its distribution are the important quality criteria. The experimental and numerical investigations were conducted. Experimental specimens showed that the RWT of a W‐PAIM pipe was much thinner than that of a water‐assisted injection molding pipe. The cross‐section size of the projectile defined the basic penetration section size. The software FLUENT was used to obtain the instantaneous distributions of the flow field, which revealed the forming mechanism of the RWT. The experiments indicated that the processing parameters, such as melt temperature, melt injection pressure, mold temperature, and water injection delay time had obvious effects on the RWT, while the water pressure had little effect on it. The RWT of curved pipes was thin at the inner concave side while thick at the outer convex side. The RWTs at the bend portion are influenced by the deflection angle and bending radius, which is due to the pressure difference between the two sides. POLYM. ENG. SCI., 59:295–303, 2019. © 2018 Society of Plastics Engineers