Injection-molded Tensile Bars Research Articles

Effect of High Molecular Mass Tail in the Mol Mass Distribution of Polypropylene on Injection Molding

Abstract The effect of high molecular mass (MM) tail in the molecular mass distribution (MMD) of polypropylene (PP) on injection molding has been studied using injection molded tensile bars from a commercial PP resin and two samples of the above resin deprived of the high MM tail in different degrees by controlled degradation during extruder reaction with an added processing aid named Promax. The central portions of the tensile bars were studied by polarizing microscopy of microtomed sections parallel and perpendicular to the melt flow direction, and by density and birefringence profiles across the thickness of the bar. Mechanical properties and acoustic emission behavior in stretching of these injection molded tensile bars are also discussed. The experimental results show that by stripping off the high MM tail in the MMD the injection molded tensile bar has smaller degree of crystallization, less frozen orientation, improved uniformity across the thickness and the thickness of the skin part is greatly reduced. Better quality of the fabricated articles is to be expected by using Promax with PP for injection molding.

Mechanical properties and thermal characteristics of poly(lactic acid) and paraffin wax blends prepared by conventional melt compounding and sub-critical gas-assisted processing (SGAP)

In this study, poly(lactic acid) (PLA)/paraffin wax (PW) blends containing different amounts of PW were investigated. The blends were prepared by a twin-screw extruder using two different methods: conventional melt compounding extrusion and sub-critical gas-assisted processing (SGAP). Then, the blends and neat PLA were injection molded into ASTM 638 Type V tensile bars for evaluation. To observe the effects of the different melt compounding processes and the effects of paraffin wax added to the blends, the rheological properties, crystallization behavior, and thermal stability of the extruded blends, as well as the phase morphology and tensile properties of the injection molded tensile bars, were characterized. The results showed that the addition of paraffin wax yielded tremendous improvements in elongation compared to neat PLA. In addition, samples made by the sub-critical gas-assisted processing (SGAP) extrusion method exhibited more homogeneous phase morphologies and paraffin wax dispersion, better tensile properties and thermal stability, and more consistent material properties as compared to their conventionally compounded counterparts.

Influence of processing conditions on the weld line in doubly injection‐molded glassy polycarbonate and polystyrene: Microindentation hardness study

AbstractThe microhardness (H) technique has been used to characterize the quality of the weld line in injection‐molded tensile bars from a two‐component machine in which both melt streams from the same material can be independently controlled. More specific, the influence of melt temperature and indentation location (closer or further from the sample edge parallel to the injection direction and across the weld line) has been followed on polycarbonate (PC) and polystyrene (PS) glassy samples. For both polymers at lower melt temperatures, a strong H decrease (between 15 and 50%) followed by a sharp increase in a narrow distance (around 0.10 mm), is observed. When the melt temperature increases up to 300°C (for PC) and 270°C (for PS), a much smaller H decrease is observed in the central part of the samples. However, closer to the tensile bar edges (2 mm) the weld line remains undetectable by microhardness measurements. The present results reveal that the processing temperature affects the broadening of the weld line through the conditions for effective mutual interdiffusion of chains from the two fronts coming from opposite sides. © 2005 Wiley Periodicals, Inc. Adv Polym Techn 24:14–20, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20028

Biodegradation of starch/polylactic acid/poly(hydroxyester-ether) composite bars in soil

Injection molded tensile bars composed of native corn starch (0–70%), poly( d, l-lactic acid) (95% L) (PLA, 13–100%) and poly(hydroxyester-ether) (PHEE, 0–27%) were buried in soil for 1 year in order to study the effects of starch and PHEE on rates of biodegradation. Rates of weight loss increased in the order pure PLA (∼0%/year)<starch/PLA (0–15%/year)<starch/PHEE/ PLA (4–50%/year) and increased with increasing starch and PHEE contents. Weight losses were due to starch only with the degradation proceeding from outside to inside along a narrow zone. Tensile strength did not change with time for pure PLA and, after an initial decline, did not change much for the other compositions. Some formulations containing PHEE and lower (40%) starch levels had higher tensile strengths after initial exposure to soil than those without PHEE.

Chain orientation in melt-extruded samples of vectra A, vectra B, and blends in relation to the mechanical properties

The profile of molecular orientation within injection-molded tensile bars of the liquid crystalline copolyesters Vectra A, Vectra B, and Vectra C, as well as blends of these polymers, was investigated by means of wide-angle X-ray scattering (WAXS) using synchrotron radiation (HASYLAB, Hamburg). The local variation of chain orientation was resolved into steps of 100 μm. An even higher resolution was obtained by using the microfocus camera (focal spot 2 μm) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. In Vectra A and in the blend of Vectra A and Vectra B, a smooth variation of the orientation was found being almost zero at the surface and showing its maximum at a distance of 0.6 mm from the surface. The orientation in Vectra B was rather fluctuating. The average chain orientation in the blend samples processed under the same conditions was higher than in samples of the pure liquid crystalline copolyesters. The mechanical properties of the different layers within the injection moldings were determined by cutting the samples into slices and measuring the stress-strain curves. For specimens of comparable orientation, it turned out that the blend samples had the largest values of Young's modulus and tensile strength. The synergism of orientation and mechanical strength was also found for different blend compositions, as well as in blends of Vectra B and Vectra C. Annealing the injection moldings above the melting point resulted in a rapid relaxation of the orientation, whereas the chain alignment persisted at lower temperatures. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 531–545, 1998

Chain orientation in melt‐extruded samples of vectra A, vectra B, and blends in relation to the mechanical properties

The profile of molecular orientation within injection-molded tensile bars of the liquid crystalline copolyesters Vectra A, Vectra B, and Vectra C, as well as blends of these polymers, was investigated by means of wide-angle X-ray scattering (WAXS) using synchrotron radiation (HASYLAB, Hamburg). The local variation of chain orientation was resolved into steps of 100 μm. An even higher resolution was obtained by using the microfocus camera (focal spot 2 μm) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. In Vectra A and in the blend of Vectra A and Vectra B, a smooth variation of the orientation was found being almost zero at the surface and showing its maximum at a distance of 0.6 mm from the surface. The orientation in Vectra B was rather fluctuating. The average chain orientation in the blend samples processed under the same conditions was higher than in samples of the pure liquid crystalline copolyesters. The mechanical properties of the different layers within the injection moldings were determined by cutting the samples into slices and measuring the stress-strain curves. For specimens of comparable orientation, it turned out that the blend samples had the largest values of Young's modulus and tensile strength. The synergism of orientation and mechanical strength was also found for different blend compositions, as well as in blends of Vectra B and Vectra C. Annealing the injection moldings above the melting point resulted in a rapid relaxation of the orientation, whereas the chain alignment persisted at lower temperatures. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 531–545, 1998

Elastic Moduli of Injection-Molded Short-Glass-Fiber-Reinforced Poly(ethylene terephthalate)

The nine independent elastic constants of injection-molded tensile bars of poly(ethylene terephthalate) reinforced with 30%, 45% and 55% by weight of glass fibers have been determined by ultrasonic techniques. The tensile and shear moduli increase with increasing fiber volume fraction, with the tensile modulus along the longitudinal direction showing the greatest change. For a composite containing 55 weight% of glass fibers, the longitudinal tensile and shear modulus exceed those of the matrix by factors of 5 and 3, respectively. By regarding the injection-molded bar as a multidirectional laminate comprising a large number of unidirectional plies, the elastic moduli have been calculated and found to agree closely with the experimental data.

Effective thermal conductivity of misoriented short-fiber reinforced thermoplastics

Abstract This paper deals with the effective thermal conductivity of composites containing misoriented fibers. Statistical distribution functions such as Weilbull can be used to represent this kind of distribution. The orientation distribution of fibers in FRTP (fiber-reinforced thermoplastics) can be characterized by a single parameter exponential function F(θ) = 1 - e−λθ A large λ indicates a highly oriented material whereas a small λ represents a quasi-isotropic material. A method based on the idea of Mori-Tanala's mean field theory in conjunction with the Eshelby's equivalent inclusion method for steady-state heat conduction in composite including the effects of fiber length and orientation to predict thermal conductivity of FRTP is presented. The effective thermal conductivity of planar orientation distribution and transversely isotropic distribution of fibers are studied. For planar orientation case, we have examined the thermal conductivity constants of injection-molded tensile bars of poly (pheylene sulfide) reinforced with 30 and 40% by weight of carbon or glass fibers which have been measured by Choy et al. (1992). For transversely isotropic orientation case, the thermal conductivity constants of a transversely isotropic composite are explicitly derived. These thermal conductivity constants are given in terms of the length-diameter aspect ratio, the volume fraction, and the orientation-distribution parameter of the short fibers. Numerical results are presented to demonstrate the effects of the fiber thermal property, aspect ratio, volume fraction and orientation distribution function on composite thermal conductivity. The fiber orientation distribution and aspect ratio have a more significant effect on composite longitudinal thermal conductivity than does the fiber volume fraction within the range examined.

Effective elastic moduli of misoriented short-fiber composites

This paper deals with the effective moduli of composites containing misoriented fibers. The orientation distribution of fibers in a composite can be characterized by a single parameter exponential function, F(θ) = 1—e−λπ. A large λ indicates a highly oriented material, whereas a small λ represents a quasi-isotropic material. Based on the Eshelby-Mori-Tanaka theory, the effective elastic moduli of planar orientation distribution and transversely isotropic distribution of fibers are studied. For planar orientation case, we have examined the nine independent stiffness constants of injection-molded tensile bars of poly (pheylene sulfide) reinforced with 30 and 40% by weight of carbon or glass fibers which have been measured by Choy et al. [Polymer Composites 13, 69–80 (1992)]. For the transversely isotropic orientation case, five elastic constants of a composite are derived. These moduli are given in terms of the length-diameter aspect ratio, the volume fraction, and the orientation distribution parameter λ of the short fibers. And they are given in very simple, explicit forms. Numerical results are presented to demonstrate the effects of the fiber elastic property, aspect ratio, volume fraction and orientation distribution function on composite elastic moduli. The fiber orientation distribution and aspect ratio have a more significant effect on composite longitudinal Young’s modulus than does the fiber volume fraction within the range examined.

Characterization of molecular orientation in polymeric products by Fourier transform infrared spectroscopy using diffuse reflectance optics

Orientation in injection-molded polymeric products was investigated by infrared (IR) dichroism Due to differences in reflectivity of parallel and perpendicular polarized light on polymers in the diffuse-reflectance optics, IR dichroism can be measured without using a polarizer, simply by turning the oriented sample through 90 o . Orientation functions were calculated from measured IR dichroic ratios for (strained) injection-molded tensile bars of high-density polyethylene and liquid-crystalline polymer

Effect of High Molecular Mass Tail in the Mol Mass Distribution of Polypropylene on Injection Molding

Abstract The effect of high molecular mass (MM) tail in the molecular mass distribution (MMD) of polypropylene (PP) on injection molding has been studied using injection molded tensile bars from a commercial PP resin and two samples of the above resin deprived of the high MM tail in different degrees by controlled degradation during extruder reaction with an added processing aid named Promax. The central portions of the tensile bars were studied by polarizing microscopy of microtomed sections parallel and perpendicular to the melt flow direction, and by density and birefringence profiles across the thickness of the bar. Mechanical properties and acoustic emission behavior in stretching of these injection molded tensile bars are also discussed. The experimental results show that by stripping off the high MM tail in the MMD the injection molded tensile bar has smaller degree of crystallization, less frozen orientation, improved uniformity across the thickness and the thickness of the skin part is greatly reduced. Better quality of the fabricated articles is to be expected by using Promax with PP for injection molding.

Effect of oven aging on polypropylene

AbstractInjection‐molded tensile bars of unstabilized polypropylene were oven aged at 90°C. After an induction period, tensile elongation decreased catastrophically. During the induction period no evidence of oxidation was observed by infrared reflectance; however, a definite surface etching was noted by electron microscopy. Differential scanning calorimetry measurements indicated that, for samples aged beyond the induction time, oxidative degradation caused a decrease in the melting temperature of polypropylene. Toluene immersion was used to remove selectively material from the surface of tensile bars aged for various times. Using this technique, changes in melting behavior were observed for partially aged polypropylene even prior to the catastrophic decrease in tensile elongation. Similarly, oxidation products and a reduction in molecular weight were detected for the dissolved surface layer during the apparent induction time. Results indicate that molecular weight measurement is the most sensitive indicator of oxidative degradation during the induction period.