Volume 26, Issue 4

While the effect of vacuum venting has been reported for injection molding of micro and nanoscale features, the limited research has produced conflicting results. To clarify the positive effect of vacuum venting on replication of microscale features, this work focused on the interactions between vacuum venting and (1) feature size, (2) material type in terms of melt viscosity and wettability, and (3) injection velocity and mold temperature. A metal-polymer hybrid tooling with a range of positive microscale features was employed to mold polystyrene and polymethylmethacrylate parts. Overall, vacuum venting always effective in feature definition (sharpness of edges) enhancement, but provided increases in depth ratio that depended on material (melt viscosity and wettability) and processing conditions (i.e., injection velocity, and mold temperature).

In the current study the dependency of morphology and thermal behavior of medium density polyethylene (MDPE) powder reinforced with 0, 5, 10 and 15 wt.% nano-sized Al 2 O 3 particles on mechanical milling were studied. The effects of milling time on the morphology and the thermal behavior were investigated by Transition Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetery (DSC) and Thermo Gravimetric Analyzer (TGA). The TEM micrographs obtained from the nanocomposites proved that mechanical milling could be an adequate method to disperse nano-sized hard particles throughout the polymer. The SEM micrographs of pure MDPE showed that regular shape powder converted to the flake shapes as milling time increased. Also the results illustrated that an increase in milling time caused to decrease the agglomeration of alumina. The DSC profiles of samples elucidated that ball milling had little effect on crystalline temperature and melting point of all materials including MDPE and its nanocomposites. The nanocomposite exhibited more thermal stability than pure polyethylene proved by TGA tests.

The poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) blend nanofibers at 100/0, 80/20, 50/50, 20/80 and 0/100 weight ratios were obtained via electrospinning process. Intermolecular interactions, miscibility and compatibility of polymer blends were studied by Fourier transform infrared (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and UV-visible spectrophotometer. The results suggest that intermolecular interactions have occurred between pure PAA and PEO in PAA/PEO blend. These interactions made PAA/PEO blend miscible at above-mentioned weight ratios. Additionally, the morphology and the fibers diameter were investigated using scanning electron microscope (SEM) analysis, which indicated beadless fibers with diameter range of about 120 to 300 nm. It was observed that the homogenous nanofibers with the smaller diameter were obtained in PAA/PEO blend with PAA dominant content. Finally, SEM results suggest that the formation of pure PAA nanofibers with concentration of 5.0 wt.% would not occur. Whereas, in the same concentration, pure PEO and PAA/PEO blend nanofibers with no bead defects were obtained. However, in higher pure PAA concentrations electrospun fibers were formed.

The concentration effects of the silane coupling agent bis (triethoxysilylpropyl) disulfide (TESPD) on mechanical properties of silica-filled silicone rubber (SR)/natural rubber (NR) compound are investigated upon torque maximum value, tensile strength, tensile modulus and hardness. The compounds are cured with a conventional vulcanization system. The concentration range of the TESPD used is 1 to 17 phr. The optimum concentration of the TESPD is found to be 13 phr in the SR/NR compound. The degree of dispersion of silica in the rubber matrix is also examined by scanning electron microscopy (SEM).

Microcellular foams have attracted interest in both scientific and industrial communities. Injection molding is perhaps the most versatile industrial method of polymer processing. Microcellular injection molding is a newcomer variant in this class. It is capable to produce microcellular structure. The mechanical behavior of a microcellular polymer is different than conventional foams. Although they exhibit much improved mechanical properties than conventional foams still they are inferior to solid polymer. Incorporation of nanoparticles in matrix may affect the nucleation and bubble growth process. Further, nanoparticles will enhance the performance (stiffness, strength etc.) of matrix. In present study an effort has been made to quantitatively analyze the improvements in mechanical properties of microcellular foam with the addition of nanoclay (MMT). Nanocomposite of polypropylene and nanoclay was prepared in a co-rotating twin screw extruder. Maleic anhydride grafted polypropylene (PP-g-MA) was used as compatibilizer. PP/nanocomposites were foamed in a microcellular injection molding machine, where supercritical nitrogen (SCF-N 2 ) was mixed with plasticized PP/nanocomposite inside the screw – barrel assembly. The molded foamed samples were tested for various mechanical properties. Experimental results show that a considerable improvement occur in the property of microcellular polypropylene by the addition of 2 wt.% nanoclay (MMT).

The market for recycled thermoplastic polymers is undergoing a sharp increase, although recyclate polymers are regarded as materials with inferior properties when compared to the virgin material. This study investigates the behavior of non-contaminated scraps of low density polyethylene (LDPE) from the plastic packaging industry after single processing stage, in terms of the productivity and the thermal and rheological properties. The LDPE used was reprocessed on a single-screw extruder (Miotto) of 90 mm screw diameter and L/D = 25. Three screw speeds (80, 90 and 100 min −1 ) were investigated at three processing temperatures (200, 250 and 300°C). The reprocessed materials were characterized by thermogravimetric analysis TGA, parallel plates rheometry and productivity. The results obtained by TGA showed a typical range of polyolefin degradation (350 to 450°C). The viscoelastic properties did not show significant changes in relation to the rheological behavior. Increasing the temperature and screw speed promoted a productivity gain of approximately 30%. This suggests that under the conditions studied it is possible to reprocess this material with good productivity ensuring its thermal, mechanical and rheological properties.

Nanocomposites based on isotactic polypropylene (PP) and nanosilica (SiO 2 ) were prepared using a co-rotating twin-screw extruder (TSE). The effect of operating variables, such as screw speed and screw configuration on the dispersion of nanosilica in the polymer matrix has been studied, using TEM imaging. High shear stress, sufficient residence time, and high fill ratio in the melting section of the screw were the most important factors in achieving good nanosilica dispersion. Furthermore, the effects of filler loading and amount of a maleated polypropylene (PP-g-MA) compatibilizer on the degree of SiO 2 dispersion were investigated. The foaming performance of the composites was evaluated using a batch foaming simulation system, and an extrusion foaming setup that employed respectively N 2 and CO 2 blowing agents. Well-dispersed surface-modified hydrophobic SiO 2 particles acted as effective nucleating agents for foaming, when used at loadings below 1 phr.

Structure of paint/polymer interface was investigated for the injection molded and compression molded ABS specimens. For the interface of the injection molded specimen, a peculiar “zigzag” structure formation of several tens μm in size was observed. In contrast, the interface structure was not observed for the compression molded specimen and injection molded specimen annealed above the glass transition temperature (T g ). These facts indicate that the interface structure is related to the residual stress and molecular orientation at the surface of the injection molding specimen, because both surface structures would be removed by annealing at temperatures above T g . The interface structure was strongly dependent on the curing agent concentration. The depth of the structure decreased with increasing curing agent concentration because of decreasing in curing time. At painting temperature between 70°C and 130°C, the depth and pitch were constant. However, below 60°C and above 140°C, the depth and pitch were changed. These results strongly suggest that the interface structure is attributable to relaxation of the surface residual stress and molecular orientation by paint soaking.

Pressure and temperature dependency of shear and elongation viscosity and the thermal expansivity and the compressibility determined from PVT data were investigated for propylene homo- and propylene-1-pentene, -1-hexene, 1-heptene and -nonene copolymers and ethylene homopolymer and ethylene-1-butene, 1-pentene and 1-hexene copolymers. The short branching degree dependence of thermal sensitivity and pressure coefficient and the thermal expansivity and the compressibility has been determined. The fractional free volumes were calculated from the viscosity and PVT curves and the thermal expansion coefficient and compressibility factor of fractional free volume were determined. The temperature, pressure and stress dependence of fractional free volume was investigated. The fractional free volume calculated from viscosity data were compared from values comes from PVT measurement. A conversion equation was suggested.

Significant different thermal properties between amorphous and semi-crystalline polymers have a great effect on the selection of proper sintering parameters and the resulting properties of the parts made by selective laser sintering (SLS) process. This paper studied the differences in the part bed temperature (T b ), and relative density, tensile strength and dimensional accuracy of the SLS fabricated parts between semi-crystalline and amorphous polymers, by measuring and comparing the laser sintering properties of polystyrene (PS), a typical amorphous polymer, and nylon-12 (PA12), a typical semi-crystalline polymer. The results show that: the part bed temperatures (T b ) of amorphous polymers and semi-crystalline polymers should be kept close to glass transition temperature (T g ) and initial melting temperature (T im ) respectively, which can be measured by differential scanning calorimetry (DSC), and this rule combined with trial and error experiments can determine T b of a polymer in the SLS process; the amorphous polymer SLS parts have very low relative densities and much lower tensile strengths than the strengths of their fully dense forms, while the semi-crystalline polymer SLS parts have higher relative densities and their tensile strengths are close to the strengths of their fully dense forms; the dimensional accuracy of the SLS parts of amorphous polymers is higher than that of semi-crystalline polymer SLS parts at the same processing parameters. The obtained results will be helpful for the development of new SLS materials and the setting of processing parameters.

Disc extruders have been employed for blending, mixing, dispersing, coloring, homogenizing, degassing, and granulating of polymers. One of the difficulties in the design of disc extruders is the evaluation of dynamic characteristics such as required power, momentum transfer rate, and applied torque, which indeed indicates the capability of the machine and its components. In this paper the required conditions and model for developing of elastic deformation of polymers in the disc extruders are studied. By rheological modeling of disc extruders and related elements, dynamic behavior of this processing system which highly depends on elastic properties of polymeric materials is obtained. These data are necessary for a proper design the system, and more accurate calculation its moving parts.

Microcellular injection molding is an emerging process for producing foamed plastic parts with complex geometries. It has many advantages including material, energy, and cost savings as well as greater design freedom and dimensional stability. In spite of these advantages, this technique has been limited to non-aesthetic applications by its propensity to create parts with swirling patterns on the surface, a common characteristic of all foamed parts produced. This paper describes a novel method for achieving microcellular injection molded parts free of surface defects. By controlling the amount of supercritical fluid (SCF) and adding the proper type of polymer additives into the polymer, the onset and rate of cell nucleation were properly controlled so that swirl-free microcellular injection molded parts could be successfully molded. This paper presents the theoretical background and experimental results that demonstrate the effect of polymer additives on the surface quality of microcellular injection molded parts.

Foams from high performance polymers find more and more interest. The processes to generate them can be difficult, however. It is shown how physical foaming with CO 2 can be used as a first step to assess the potentials of such materials. For investigations of such kind an autoclave on a laboratory scale which allows pressure variations up to 300 bars and temperatures up to 300°C was set up. The samples are saturated with supercritical carbon dioxide (s.c. CO 2 ) which acts as a foaming agent. Depending on the process and material parameters different foam characteristics and cell morphologies were obtained and characterised. The potential of this method is demonstrated for two different classes of advanced polymer materials, thermoplastic fluoropolymers (FEP), and a silicone resin. In the case of the fluoropolymer, previously prepared films were foamed and the effects of various process parameters on the foam characteristics were investigated. Besides the general potential of foams from fluoropolymers, they are candidates for polymeric piezoelectric materials with a relatively high temperature stability. Silicone polymers possess some properties superior to common organic polymers. First results on the foaming behaviour of a silicone resin are presented.

Poly(ethylene terephthalate)/organo-modified-montmorillonite (o-MMT) nanocomposites were prepared via melt compounding. A polyester ionomer was used as a compatibilizer to increase the interaction between the nanoclay and PET. The nominal o-MMT content was 2 wt.% and the ionomer/organoclay (mass ratio) was 3:1. The samples were characterized by WAXD, SEM, TEM, TGA, rheometry, DSC, O 2 permeation and tensile testing. It was found that the addition of the ionomer improved the dispersion of the nanoclay particles in the PET matrix, leading to an exfoliated structure for the samples prepared by twin screw extrusion and by an internal mixer (Brabender). This was confirmed by larger complex viscosity and storage modulus at low frequency for molten samples. However, a subsequent processing using single screw extrusion to produce films resulted in thermal degradation of the organo-modifier of the clay and collapse of the gallery spacing. DSC results revealed that the cold crystallization temperature of nanocomposites-based films decreased and the melt crystallization temperature increased with the introduction of the organoclay, due to the strong heterogeneous nucleation effect of the clay particles. The tensile modulus of extruded films increased, while the yield strength remained constant with the incorporation of the organoclay. The oxygen permeability of PET-ionomer nanocomposites decreased as compared with samples containing no ionomer.

This study dealt with operating windows and scale-up rules of a compounding twin screw extruder for polypropylene (PP)/multi-walled carbon nanotubes (MWCNT) composites with required conductivity. In this study we showed that the experimentally obtained boundary line separating either conductive or non-conductive region is essentially linear, and thus the operating windows, in which the PP/MWCNT composite is consistently conductive with statistically 95% reliability level, can be drawn as a triangle. The use of this triangle rule of the operating windows to achieve required volume resistivity was proposed. With similarity of the length and diameter ratio (L/D) and screw configuration between small (laboratory-scale) and large (production-scale)) extruders, our investigation revealed that the most effective scale-up factors for the volume resistivity are (D large /D small ) 3 for the throughput, and (D small /D large ) for the screw speed as a new scale-up rule generally valid for any machine size.

As is well-known, characteristic complementary colors are obtained, if a λ-plate is introduced in white polarized light. This fact is used for an unexpected characterization of nucleation occurring during short term shearing of supercooled melts of iPP. Blue specks show up some time after cessation of flow on a red background. The blue color indicates that these tiny nuclei are oriented in the direction of previous flow. With prolonged waiting these specks extend over a large volume. At their centers spherulites start growing.