Influence of compatibilizers and processing temperature on microcellular injection‐molded polypropylene/(waste tire powder) composites

Nowadays the economic recycling of waste tires has become a global challenge. The use of waste tire powder as a dispersed elastomeric phase in a polypropylene (PP) matrix offers an interesting opportunity for recycling of waste tire rubber. Compatibilized PP/(waste tire powder) composites are microcellularly processed to create a new class of materials with unique properties. Recent studies have demonstrated the feasibility of developing microcellular structures in PP/waste ground rubber tire (WGRT) composites. Microcellular PP/WGRT composites are prepared by an injection‐molding process using a chemical blowing agent. In this study, cell sizes, cell density, void fraction, and mechanical properties of the composite foams were measured, as well as the shear viscosity of the unfoamed composites. The influence of various compatibilizers and processing temperatures on cell morphology and the mechanical properties of injection‐molded PP/WGRT composites were investigated. It was seen that the addition of maleic anhydride‐grafted styrene‐ethylene‐butylene‐styrene (SEBS‐g‐MA) increased the shear viscosity of the composites. The void fraction and cell density of the PP/WGRT composites increased with addition of compatibilizers, whereas the average cell sizes decreased. A processing temperature range of 180–195°C gave finer microcellular structure and regular cell distribution. The SEBS‐g‐MA enhanced the elongation properties and acted as an effective compatibilizer in this particular system. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers     

Effects of nano‐fillers and process conditions on the microstructure and mechanical properties of microcellular injection molded polyamide nanocomposites

This paper presents the effects of process conditions and nano‐clay fillers on the microstructure (namely, size, density, and distribution of microcells within samples) and the resulting mechanical properties of microcellular injection molded polyamide‐6 (PA‐6) nanocomposite and its neat‐resin counterpart. Based on the design of experiments (DOE) matrices, samples were molded at various supercritical fluid (SCF) levels, melt temperatures, shot sizes, melt plastication pressures (MPP), and injection speeds. These samples were then subjected to scanning electron microscope (SEM) analysis, tensile testing, and impact testing. For both materials, the microstructure and the mechanical properties of the molded samples were found to be dependent on the process conditions and presence of nano‐clay, which could serve as microcell nucleating agent. At higher weight reductions, the nanocomposite samples exhibit much smaller microcells and higher cell densities than those obtained in the neat‐resin samples. The SEM micrographs reveal noticeable differences in microcell surface roughness between the nanocomposite and the neat resin. A statistical design analysis was used to identify the optimal process conditions that would result in desirable cell size and density and, thus, better mechanical properties. For example, the highest tensile strengths have been observed at the highest levels of shot size, MPP, injection speed, and SCF level, and at the lowest level of melt temperature.     

The cell forming process of microcellular injection‐molded parts

In this article, we studied the cell forming process of microcellular injection‐molded parts. Using a modified injection molding machine equipped with a Mucell^® SCF delivery system, microcellular‐foamed acrylonitrile–butadiene–styrene parts with different shot sizes were molded. The cell structure on the fractured surfaces along the direction both vertical and parallel to melt flow in the molded parts was examined. The results showed that a regular spherical cells region and a distorted ellipsoidal cells region exist in the molded parts simultaneously. The length of the distorted cells region along the melt flow direction in the molded parts remained basically unchanged for different shot sizes and it is about 195 mm away from the flow front in this study's conditions. The cell formation mechanism was analyzed, two cell forming processes in microcellular injection molding, the “foam during filling” process and the “foam after filling” process, were proposed. It was also found that the melt pressure in the filling stage is the dominant factor affecting the cell forming process, and there is a critical melt pressure value in the filling stage, 20.9 MPa, as the dividing line of the two cell forming processes in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40365.     

Crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 nanocomposites

This article presents the effects of nanoclay and supercritical nitrogen on the crystallization and thermal behavior of microcellular injection‐molded polyamide‐6 (PA6) nanocomposites with 5 and 7.5 wt% nanoclay. Differential scanning calorimetry (DSC), X‐ray diffractometry (XRD), and polarized optical microscopy (POM) were used to characterize the thermal behavior and crystalline structure. The isothermal and nonisothermal crystallization kinetics of neat resin and its corresponding nanocomposite samples were analyzed using the Avrami and Ozawa equations, respectively. The activation energies determined using the Arrhenius equation for isothermal crystallization and the Kissinger equation for nonisothermal crystallization were comparable. The specimen thickness had a significant influence on the nonisothermal crystallization especially at high scanning rates. Nanocomposites with an optimal amount of nanoclay possessed the highest crystallization rate and a higher level of nucleation activity. The nanoclay increased the magnitude of the activation energy but decreased the overall crystallinity. The dissolved SCF did not alter the crystalline structure significantly. In contrast with conventionally injection‐molded solid counterparts, microcellular neat resin parts and microcellular nanocomposite parts were found to have lower crystallinity in the core and higher crystallinity near the skin. POLYM. ENG. SCI., 46:904–918, 2006. © 2006 Society of Plastics Engineers     

The influence of processing parameters on fingering formation in fluid‐assisted injection‐molded disks

One of the problems encountered in fluid‐assisted injection‐molded parts is the gas or water “fingering” phenomenon, in which gas (water) bubbles penetrate nonuniformly into the core of the parts and form finger‐shape branches. Severe fingerings can lead to significant reductions in part stiffness. This study investigated the fingering phenomenon in fluid‐assisted injection‐molded disk parts. Experiments were carried out on a reciprocating injection‐molding machine equipped with gas‐ and water‐injection units. The material used was virgin polypropylene. A disk cavity with two different thicknesses was used for all experiments. The effects of various processing parameters on the fingering were examined. It was found that the melt short shot size and mold temperature were the principal parameters affecting the formation of part fingerings. In addition, the formation mechanism of part fingerings has also been proposed to better understand the formation of part fingerings. It has been shown that the fluid‐assisted filling process is an unstable system by nature. Any small perturbation by material viscosity or by temperature gradient can trigger the unbalance of gas (water) penetrations in the parts and result in fingerings. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Microstructure and crystallography in microcellular injection‐molded polyamide‐6 nanocomposite and neat resin

The effects of adding nanoclay to polyamide‐6 (PA‐6) neat resin, and the effects of processing parameters on cell density and size in microcellular injection‐molded components were investigated. In addition, the crystal sizes, structures, and orientation were analyzed with the use of x‐ray diffraction (XRD) and a polarized optical microscope. The standard ASTM D 638‐02 tensile bars for the analyses were molded according to a fractional four‐factor, three‐level, L9 Taguchi design of experiment (DOE) with varying melt temperature, injection speed, supercritical fluid (SCF) concentration, and shot size. It was found that the presence of montmorillonite (MMT) nanoclay greatly reduced the size of the cells and crystals, but increased their density in comparison with neat resin processed under identical molding conditions. In addition, at the sprue section downstream of the machine nozzle, cell size gradually decreased from the part center toward the skin for both the neat resin and the nanocomposite. It was also found that shot size was the most important processing parameter for both the neat resin and nanocomposite in affecting cell density and size in microcellular injection molding components. Weakly preferred crystal orientations were observed on the surface of microcellular injection‐molded PA‐6/MMT tensile bars. Finally, the addition of nanoclay in PA‐6 neat resin facilitated the formation of γ‐phase crystals in the molded components. Polym. Eng. Sci. 45:52–61, 2005. © 2004 Society of Plastics Engineers.     

Effects of the chemical foaming agents,injection parameters,and melt‐flow index on the microstructure and mechanical properties of microcellular injection‐molded wood‐fiber/polypropylene composites

Wood‐fiber‐reinforced plastic profiles are growing rapidly in nonstructural wood‐replacement applications. Most manufacturers are evaluating new alternative foamed composites, which are lighter and more like wood. Foamed wood composites accept screws and nails better than their nonfoamed counterparts, and they have other advantages as well. For example, internal pressures created by foaming give better surface definition and sharper contours and corners than nonfoamed profiles have. In this study, the microfoaming of polypropylene (PP) containing hardwood fiber was performed with an injection‐molding process. The effects of different chemical foaming agents (endothermic, exothermic, and endothermic/exothermic), injection parameters (the mold temperature, front flow speed, and filling quantity), and different types of PP (different melt‐flow indices) on the density, microvoid content, physicomechanical properties, surface roughness, and microcell classification of microfoamed PP/wood‐fiber composites were studied. A maleic anhydride/polypropylene copolymer (MAH‐PP) compatibilizer was used with the intention of improving the mechanical properties of microfoamed composites. The microcell classification (from light microscopy) and scanning electron micrographs showed that an exothermic chemical foaming agent produced the best performance with respect to the cell size, diameter, and distance. The polymer melt‐flow index and the variation of the injection parameters affected the properties and microstructure of the microfoamed composites. The density of the microfoamed hardwood‐fiber/PP (with a high melt‐flow index) composites was reduced by approximately 30% and decreased to 0.718 g/cm³ with an exothermic chemical foaming agent. Tensile and flexural tests were performed on the foamed composites to determine the dependence of the mechanical properties on the density and microvoid content of the foamed specimens, and these properties were compared with those of nonfoamed composites. MAH‐PP improved the physicomechanical properties up to 80%. With an increase in the mold temperature (80–110°C), the surface roughness was reduced by nearly 70% for the foamed composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1090–1096, 2005     

The mechanical/thermal properties of microcellular injection‐molded poly‐lactic‐acid nanocomposites

This study investigated the influence of montmorillonite (MMT) content on the mechanical/thermal properties of microcellular injection‐molded polylactide (PLA)/clay nanocomposites. Carbon dioxide was the blowing agent. The PLA/MMT nanocomposites were prepared by twin screw extrusion. The results showed that as MMT content is increased, tensile strength, impact strength, and cell density decrease. This is caused by the speed degradation of PLA due to the addition of MMT. MMT decreases the crystallization temperature but increases the decomposition temperature of the nanocomposites. The XRD results showed that the layer spacing of the clay increases as MMT content increases. TEM pictures showed that the MMT is well dispersed within the PLA matrix. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers.     

Micromechanical properties of injection‐molded starch–wood particle composites

The micromechanical properties of injection‐molded starch–wood particle composites were investigated as a function of particle content and humidity conditions. The composite materials were characterized by scanning electron microscopy and X‐ray diffraction methods. The microhardness of the composites was shown to increase notably with the concentration of the wood particles. In addition, creep behavior under the indenter and temperature dependence were evaluated in terms of the independent contribution of the starch matrix and the wood microparticles to the hardness value. The influence of drying time on the density and weight uptake of the injection‐molded composites was highlighted. The results revealed the role of the mechanism of water evaporation, showing that the dependence of water uptake and temperature was greater for the starch–wood composites than for the pure starch sample. Experiments performed during the drying process at 70°C indicated that the wood in the starch composites did not prevent water loss from the samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4893–4899, 2006     

Effects of SCF content,injection speed,and CF content on the morphology and tensile properties of microcellular injection‐molded CF/PP composites

Carbon fiber/polypropylene composite foams were prepared by microcellular injection molding using nitrogen as a foaming agent. The effects of nitrogen content, injection speed, and CF content on the morphology and tensile properties of the composite foams were investigated. A three‐layer structure was formed in the microcellular foams: the skin layer was solid, the intermediate layer contained stretched cells parallel to the flow direction, and the core layer consisted of spherical cells. The average cell diameter of the machine direction decreased from 41 to 34 μm as the nitrogen content increased from 0.5 to 1 wt%, increased from 34 to 43 μm as the injection speed increased from 50 to 150 mm/s, and decreased from 34 to 25 μm as the CF content increased from 10 to 30 wt%. Thus, the microcellular structure was improved by increasing the nitrogen and CF content and by decreasing the injection speed. Furthermore, when the CF content increased from 10 to 30 wt%, the Young's modulus of the solids and foams increased by 78% and 113%, respectively. Thus, the Young's modulus of the foams improved by 35% due to the improvement in the cellular structure. POLYM. ENG. SCI., 59:1371–1380 2019. © 2019 Society of Plastics Engineers     

Heterogeneity and anisotropy of injection‐molded discs of polypropylene and polypropylene composites

The anisotropy and heterogeneity of injection‐molded discs of polypropylene, talc‐filled polypropylene composites, and silane‐treated talc‐filled polypropylene composites are studied by means of dynamic mechanical analysis and thermomechanical analysis. The aims of this work are to discover the relationships between the structure of the composites, their anisotropic properties, and the heterogeneity of the molded discs. The experimental results show that although the discs are almost homogeneous, they present a high degree of anisotropy. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1275–1283, 2000     

Flow‐induced warpage of injection‐molded TLCP fiber‐reinforced polypropylene composites

The most common belief is that warpage in injection‐molded fiber‐reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow‐induced stresses generated during mold filling do not play a significant role. Injection‐molded plaques of polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils were generated in order to investigate the role of residual flow‐induced stresses relative to that of thermal stresses on the warpage. In an effort to relate the material parameters to warpage, the rheological behavior of these fiber‐filled systems was investigated. The shrinkage and the thermal expansion of the TLCP/PP composites, and hence, the thermally induced stresses decreased with an increase in fiber loading while the flow‐induced stresses increased. The increase in the flow‐induced stresses was attributed to increased relaxation times (this is not the only cause, but is a significant factor) with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber‐reinforced thermoplastics, the flow‐induced residual stresses must be accounted for. It is expected that the results reported here can be extended to glass‐reinforced PP composites as well. POLYM. COMPOS., 27:239–248, 2006. © 2006 Society of Plastics Engineers     

Mechanical properties of injection‐molded isotactic polypropylene/roselle fiber composites

Natural fiber‐thermoplastic composite materials, based on their cost‐effectiveness and environmental friendliness, have attracted much interest both scientifically and technologically in recent years. Other advantages of natural fibers are good specific strength, less abrasion, and less irritation upon inhalation (in comparison with some common inorganic fillers). In the present contribution, roselle (Hibiscus sabdariffa L.) fibers were chosen and used as reinforcing fillers for isotactic polypropylene (iPP) for the first time, due mainly to the cost‐effectiveness and natural abundance on Thai soil. Processibility and mechanical properties of the resulting composites were investigated against the type and the mean size of the fibers. The results showed that the highest mechanical properties were observed when roselle bast fibers were incorporated. When whole‐stalk (WS) fibers (i.e., the weight ratio of bast and core fibers was 40 : 60 w/w) were used, moderate mechanical properties of the resulting composites were realized. The optimal contents of the WS fibers and the maleic anhydride‐grafted iPP compatibilizer that resulted in an improvement in some of the mechanical properties of the resulting composites were 40 and 7 wt %, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3291–3300, 2006     

Hygrothermal aging and tensile behavior of injection‐molded rice husk‐filled polypropylene composites

The effect of the filler volume fraction on the tensile behavior of injection‐molded rice husk‐filled polypropylene (RH–PP) composites was studied. Hygrothermal aging behavior was also investigated by immersing the specimens in distilled water at 30 and 90°C. The kinetics of moisture absorption was studied from the amount of water uptake by specimens at regular interval times. It was found that the diffusion coefficient and the maximum moisture content are dependent on the filler volume fraction and the immersion temperatures. Incorporation of RH into the PP matrix has led to a significant improvement in the tensile modulus and a moderate improvement in the tensile strength. Elongation at break and energy at break, on the other hand, decreased drastically with the incorporation of the RH filler. The extent of deterioration incurred by hygrothermal aging was dependent on the immersion temperature. Both the tensile strength and tensile modulus deteriorated as a result of the combined effect of thermal aging and moisture attack. Furthermore, the tensile properties were not recovered upon redrying of the specimens. Scanning electron microscopy was used to investigate the mode of failure of the RH–PP composites. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 742–753, 2001     

TOF‐SIMS study of injection‐molded polystyrene

The purpose of this study was to provide experimental evidence of the separation of the polymer components at different scales during conventional processing. This was achieved by characterizing the surface and the bulk (cross section) of moldings manufactured with a high‐flow grade and a low‐flow grade of commercial polystyrene by the time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) analytical technique. Owing to the geometric constraints of the mold used, a weld was also obtained. Different surface spectra were observed for the two molded polystyrenes. The surface of the high‐flow grade moldings showed the spectral features of low‐molar polyolefin (paraffin) contaminants, whereas the bulk was dominated by polystyrene. Spectra from both the surface and the bulk of the low‐flow grade moldings were characteristic of polystyrene. Mold‐filling effects on the surface composition were observed in the flow front region of molded short‐shots of the low‐flow grade. The spectral changes indicated the abundance, in the surface, of the high end of the molar‐mass distribution of the material during the mold filling process. Two‐dimensional maps of the secondary ions from the low‐flow grade also showed an occasional alkali contamination, preferentially along the notch of the weld.     

Effects of interfacial morphology on the welding strength of injection‐molded polyamide

The effect of interfacial morphology controlled by injection‐molding conditions on the welding strength of injection‐molded polyamide was investigated in this article. The experimental results showed that the first injection‐molding conditions had distinct influence on the welding strength at the low secondary injection‐molding temperature T₂ (≤265°C), but the influence vanishes at T₂ ≥ 285°C. On the other hand, no matter what the first injection‐molding conditions are, the welding strength increases with increasing T₂, and when T₂ ≥ 285°C, the highest welding strength reaches 45 MPa, due to the formation of trans‐crystals at the interface. Morphology studies showed that trans‐crystals grow along the perpendicular direction of the interface, and their nuclei are formed at the surface of the first injection‐molded specimens. For the specimens with high‐welding strength, the welding strength relies on the skin layer of the first injection‐molded specimens in which the fracture induced by shear stress happens. POLYM. ENG. SCI., 47:2164–2171, 2007. © 2007 Society of Plastics Engineers     

Microhardness and water sorption in injection‐molded starch

The microhardness of injection‐molded potato starch was investigated in relation to the water sorption mechanism. The creep behavior under the indenter and the temperature dependence of the microhardness are reported. The influence of the drying time on microhardness, weight loss and density changes for materials with different injection‐molding temperatures is highlighted. Results reveal the role of the various mechanisms of water evaporation involved. The occurring structural mechanisms are discussed in terms of the gradual transformation of single helices of amylose and amylopectin into a network structure of double helices and the partial destruction of this structure. Experiments on starch samples, heated at 200°C, suggested the occurrence of an extreme densification of the network hindering the water adsorption in a humid atmosphere. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1246–1252, 2002     

Melting and crystallization behaviors of injection‐molded polypropylene

The melting and crystallization behaviors of the skin layer in an injection‐molded isotactic polypropylene (PP) have been studied, mainly in comparison with those of the core layer and subsidiarily in comparison with those of a compression‐molded PP and a nucleator (talc)–added PP. The skin layer contains about 5% crystals, which have a high melting point of up to 184°C. They thermally vanish by melting once. The subsequent melting history will scarcely affect the melting behaviors. On the other hand, crystallization behaviors are strongly affected by the melting history. The skin layer crystallizes in a wide temperature range at high temperature. This tendency weakens with increasing melting temperature, approaching a constant and that of the core layer above 230°C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230°C. In explaining these experimental results, it is assumed that the residual structure substance is a melt orientation of molecular chains that works as crystallization nuclei and that the vanishing of the residual structure is nothing but a relaxation of the melt orientation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1751–1762, 2000     

Mechanical properties study of micro‐ and nano‐hydroxyapatite reinforced ultrahigh molecular weight polyethylene composites

This is a comparative study between ultrahigh molecular weight polyethylene (UHMWPE) reinforced with micro‐ and nano‐hydroxyapatite (HA) under different filler content. The micro‐ and nano‐HA/UHMWPE composites were prepared by hot‐pressing method, and then compression strength, ball indentation hardness, creep resistance, friction, and wear properties were investigated. To explore mechanisms of these properties, differential scanning calorimetry, infrared spectrum, wettability, and scanning electron microscopy with energy dispersive spectrometry analysis were carried out on the samples. The results demonstrated that UHMWPE reinforced with micro‐ and nano‐HA would improve the ball indentation hardness, compression strength, creep resistance, wettability, and wear behavior. The mechanical properties for both micro‐ and nano‐HA/UHMWPE composites were comparable with pure UHMWPE. The mechanical properties of nano‐HA/UHMWPE composites are better compared with micro‐HA/UHMWPE composites and pure UHMWPE. The optimum filler quantity of micro‐ and nano‐HA/UHMWPE composites is found to be at 15 wt % and 10 wt %, separately. The micro‐ and nano‐HA/UHMWPE composites exhibit a low friction coefficient and good wear resistance at this content. The worn surface of HA/UHMWPE composites shows the wear mechanisms changed from furrow and scratch to surface rupture and delamination when the weight percent of micro‐ and nano‐HA exceed 15 wt % and 10 wt %. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42869.     

Thermal properties of extruded/injection‐molded poly(lactic acid) and biobased composites

To determine the degree of compatibility between poly(lactic acid) and different biomaterials (fibers), poly(lactic acid) was compounded with sugar beet pulp and apple fibers. The fibers were added in 85 : 15 and 70 : 30 poly(lactic acid)/fiber ratios. The composites were blended by extrusion followed by injection molding. Differential scanning calorimetry and thermogravimetric analysis were used to analyze the extruded and extruded/injection‐molded composites. After melting in sealed differential scanning calorimetry pans, the composites were cooled through immersion in liquid nitrogen and aged (stored) at room temperature for 0, 7, 15, and 30 days. After storage, the samples were heated from 25 to 180°C at 10°C/min. The neat poly(lactic acid) showed a glass‐transition transition at 59°C with a change in heat capacity (ΔC_p) value of 0.464. The glass transition was followed by crystallization and melting transitions. The enthalpic relaxation of the poly(lactic acid) and composites steadily increased as a function of the storage time. Although the presence of fibers had little effect on the enthalpic relaxation, injection molding reduced the enthalpic relaxation. The crystallinity percentage of the unprocessed neat poly(lactic acid) dropped by 95% after extrusion and by 80% for the extruded/injection‐molded composites. The degradation was performed in air and nitrogen environments. The degradation activation energy of neat poly(lactic acid) exhibited a significant drop in the nitrogen environment, although it increased in air. This meant that the poly(lactic acid) was more resistant to degradation in the presence of oxygen. Overall, injection molding appeared to reduce the activation energy for all the composites. Sugar beet pulp significantly reduced the activation energy in a nitrogen environment. In an air environment, both sugar beet pulp and apple fibers increased the activation energy. The enzymatic degradation of the composites showed a higher degradation rate for the extruded samples versus the extruded/injection‐molded composites, whereas the apple composites exhibited higher weight loss. The thermogravimetric analysis data showed that the degradation of unprocessed and extruded neat poly(lactic acid) followed a one‐step mechanism, whereas extruded/injection‐molded composites showed two‐step degradation. A higher fiber content resulted in up to three‐step degradation mechanisms. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008