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     

Mechanical properties of injection‐molded short‐fiber thermoplastic composites. Part 1: The elastic moduli and strengths of glass‐filled poly(butylene terephthalate)

The effects of processing and part geometry on the local mechanical properties of injection‐molded, 30 wt% short‐fiber‐reinforced filled poly(butylene terephthalate) (PBT) are characterized by mechanical tests on specimens cut from rectangular plaques of different thicknesses injection molded at several different processing conditions. Stiffness data from tensile tests at 12.7‐mm intervals on 12.7‐mm‐wide strips cut from injection‐molded plaques—both along the flow and cross‐flow directions—and flexural tests on these strips show consistency of plaque‐to‐plaque local properties. Also, in addition to the well‐known anisotropic properties caused by flow‐induced fiber orientation, injection‐molded short fiber composites exhibit in‐plane and through‐thickness nonhomogeneity—as indicated by in‐plane property variations, by differences between tensile and flexural properties, and by the flexural strength being significantly higher than the tensile strength. The sensitivity of these mechanical properties to process conditions and plaque geometry have also been determined: the flow‐direction tensile modulus increases with fill time, the differences between flow and cross‐flow properties decrease with increasing thickness, and both the flow and cross‐flow flexural moduli decrease with increasing plaque thickness. While the flexural modulus is comparable to the tensile modulus, the flexural strength is significantly higher than the tensile strength. POLYM. COMPOS., 26:428–447, 2005. © 2005 Society of Plastics Engineers     

Tensile properties of injection molded long fiber thermoplastic composites

This paper deals with the mechanical performances of a new class of injection molded long fiber composites based on PP and PBT matrices. Effects of material parameters such as fiber concentration, breakage, orientation, and matrix composition are analyzed. The critical fiber length, l, of the PP long fiber composite, evaluated from the pull-out length of the tensile fracture surface, was found to be much higher than those previously reported. Tensile strength calculated from the measured ll and fiber length distribution in the molded samples was found to be in agreement with the measured values. From this work it is concluded that higher mechanical performances of the long fiber reinforced thermoplastics will be attained by the injection molding process to further reduce fiber breakage.     

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     

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     

Improvement of the short‐ and long‐term mechanical properties of injection‐molded poly(etheretherketone) and hydroxyapatite nanocomposites

Nanocomposites of polyetheretherketone (PEEK) and hydroxyapatite (HA) nanoparticles treated with a silane coupling agent were successfully prepared by twin screw extrusion and injection molding. Some of the samples were annealed after the injection molding. The silane treatment promoted an improvement of the short‐ and long‐term mechanical properties of the nanocomposites. A higher stress and a six times higher deformation at break and a higher impact strength were observed in the silane‐treated nanocomposites when compared to the nontreated ones. The number of cycles to fail of the treated nanocomposites was almost 200% higher than the number of cycles to fail of the nontreated samples. The treatment also decreased the glass transition temperature and amount of crystallinity of the samples. This improvement in mechanical properties obtained from the silane treatment was attributed to the strengthening of the PEEK/HA interfacial bond, to the plasticization of the PEEK matrix by silane oligomers produced during the processing and to a better dispersion of the HA nanoparticles within the PEEK matrix. Samples annealing, however, diminished all these properties due to the increase in crystallinity. Studies of the short‐ and long‐term mechanical properties of these nanocomposites under physiological conditions and of the proliferation of stem cells are under way. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44476.     

Effect of chitin addition on injection‐molded thermoplastic corn starch

A new type of biodegradable starch‐based composites was prepared by injection‐molding using glycerol and water as plasticizers. Chitin flakes, obtained from shells of Penaeusschmitti, were used as reinforcing phase. The effect of chitin content on the structural and tensile properties of the composite samples was examined after conditioning at 28°C and 80% relative humidity for 30 days. In general, chitin incorporation into the starch matrix resulted in materials with higher modulus and decreased elongation at break. Wide‐angle X‐ray diffraction and differential scanning calorimetry evidenced a significant decrease in crystallinity in the composite samples in relation to the unfilled starch material. Contact angle measurements revealed that the addition of chitin contributes to the improvement of water resistance of the composite samples when compared to injection molded starch alone. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2706–2713, 2004     

Effect of the impact conditions on the mechanical properties of injection‐molded parts

This work focused on the study of the impact event on molded parts in the framework of automotive components. The influence of the impact conditions and processing parameters on the mechanical behavior of talc‐filled polypropylene specimens was analyzed. The specimens were lateral‐gate discs produced by injection molding, and the mechanical characterization was performed through instrumented falling weight impact tests concomitantly assisted with high‐speed videography. Results analyzed using the analysis of variance (ANOVA) method have shown that from the considered parameters, only the dart diameter and test temperature have significant influence on the falling weight impact properties. Higher dart diameter leads to higher peak force and peak energy results. Conversely, higher levels of test temperatures lead to lower values of peak force and peak energy. By means of high‐speed videography, a more brittle fracture was observed for experiments with higher levels of test velocity and dart diameter and lower levels of test temperature. The injection‐molding process conditions assessed in this study have an influence on the impact response of moldings, mainly on the deformation capabilities of the moldings. POLYM. ENG. SCI., 52:1845–1853, 2012. © 2012 Society of Plastics Engineers     

The relationship between mechanical properties and morphology of vibration‐injection‐molded polyethylene

This paper introduces a novel melt vibration‐injection molding. The effect of mid‐frequency melt vibration on mechanical properties was introduced, and SEM, WAXD and DSC investigations had been employed to provide evidence for explaining the relationship between mechanical properties and morphology of vibration‐injection‐molded specimens. The results show that the effect of vibration frequency is very different from that of vibration pressure amplitude. At a given vibration pressure amplitude, the increase of vibration frequency is beneficial for obtaining preferential orientation, more perfect lamellae and enhanced mechanical properties. For a given vibration frequency, increase of vibration pressure amplitude is a pre‐requisite for the achievement of a large‐scale lamella, more pronounced orientation, increase of cyrstallinity and high strength of high‐density polyethylene, but part of the toughness is lost. Copyright © 2004 Society of Chemical Industry     

Thermal properties of glass fiber reinforced polyamide 6 composites throughout the direct long‐fiber reinforced thermoplastic process

The direct long‐fiber reinforced thermoplastic (D‐LFT) process offers a streamlined material processing technique and decreases the degradation of the material. To ensure product consistency and process optimization, it is imperative to understand how the process sequence affects degradation and thermal properties of the material during the D‐LFT process. This study investigates variation in molecular weight and thermal properties of the glass fiber reinforced polyamide 6 (PA6) composites throughout the D‐LFT process. Viscosity number (VN) measurements, thermogravimetric analyses (TGA), and differential scanning calorimetry (DSC) analyses were performed on samples taken from different locations along the D‐LFT process. It was found that VN, which is a measure of molecular weight of the PA6 base resin, decreased throughout the processes. In contrast, TGA results suggested that apparent activation energy for decomposition increased during consecutive process stages. Non‐isothermal DSC results showed that there were no significant changes to the degree of crystallization; however, isothermal DSC results indicated that later stages of the process showed a decrease in crystallization half‐time, and the largest changes were observed in areas after the two extrusion portions of the process. POLYM. ENG. SCI., 58:46–54, 2018. © 2017 Society of Plastics Engineers     

The mechanical properties of insert‐molded bimaterial composites

Bimaterial composite samples were constructed by injecting various polymers into a mold containing a fraction of a pre‐molded specimen. The resulting series composites were tested in tension. Breaking stresses were independent of fractional length. Conversely, both elongation to break and apparent stiffness varied with Although samples broke at or near the interface, adhesion was reasonably good, as indicated by transfer of material across interfaces.     

Ultraviolet–ozone surface cleaning of injection‐molded,thermoplastic microcantilevers

Ultraviolet–ozone (UVO) is used for the cleaning of labware from organic contamination and includes sterilization, surface roughening, and activation. A range of polymers has been treated without major effects on surface topography and surface chemistry. Even for those, which are subject to physical and chemical aging, a trade‐off between damages and surface cleaning was often found. This communication presents a comprehensive overview on how to UVO‐treat selected polymers known from applications in biology and medicine, in particular microcantilevers used for biosensing. The study provides well‐defined thresholds below which degradation and surface damages are avoided. The impact of UVO treatments on the surface and bulk properties of the injection‐molded polymers was examined by means of differential scanning calorimetry and Fourier transform infrared spectroscopy measurements. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41922.     

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.     

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.     

Rheological and mechanical behavior of long‐polymer‐fiber reinforced thermoplastic pellets

Polymer–polymer materials consist of a thermoplastic matrix and a thermoplastic reinforcement. Recent research activities concentrate on the manufacturing of semi‐finished polymer–polymer materials in other shapes than the commercially available tapes and sheets. In particular, a pellet‐like form provides the possibility of processing the polymer–polymer material by injection and compression molding. Nevertheless, the thermoplastic reinforcement is vulnerable to excessive heat and the processing usually needs special attention. The current study investigates the processing of long‐polymer‐fiber reinforced thermoplastic pellets, namely polypropylene‐polyethylene terephthalate and a single‐polymer polyethylene terephthalate, by extrusion for subsequent compression molding applications. The flow characteristics of the material as well as the preservation of the polymer reinforcement can be handled by accurate temperature control. The tensile and impact properties decrease with increasing process temperature though. Moreover, the results prove that the use of a common long‐fiber reinforced thermoplastic process chain is applicable to the newly developed polymer–polymer material. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39716.     

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     

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     

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     

Study on thermal expansion in injection‐molded isotactic polypropylene and thermoplastic elastomer blends

The linear thermal expansion coefficients (CLTEs) along flow direction (FD) for the injection‐molded blends composed of isotactic polypropylene (iPP) and various ethylenic thermoplastic elastomers (TPEs) were investigated using a thermo‐mechanical analyzer. The iPP/TPE blends with higher comonomer contents in the TPE showed extremely low CLTE. TEM observation revealed that the array of the TPE whose MFR was adjusted to be higher than the iPP matrix was in lamella‐like sheet stacked normal to normal direction (ND) with being elongated along both FD and transverse‐to‐flow direction. At higher magnification of TEM, the iPP lamellae in the blend with higher comonomer contents in the TPE deeply penetrated into the TPE phase as a consequence of the faster iPP crystallization before the completion of the phase‐separation. Hence, the location of the iPP amorphous chains would change depending on the comonomer contents in the TPE; in the case of the iPP/TPE blend with higher comonomer contents, large amount of the iPP amorphous chains would be trapped inside the TPE phase because of incomplete phase‐separation arrested by faster crystallization. Therefore, the extremely low CLTE for the iPP/TPE blend with higher comonomer contents was accounted for by the simultaneous suppression of the thermal expansions from both the TPE phase and the iPP amorphous chains trapped inside the TPE by rigid iPP crystalline lamellae connecting in parallel with the TPE phase. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Shear‐induced fibrillation and resultant mechanical properties of injection‐molded polyamide 1010/isotactic polypropylene blends

It is feasible to control the phase morphology and orientation for immiscible polymer blends to manipulate their properties. In this paper, the blend of polyamide 1010 (PA1010) and isotactic polypropylene (iPP) (mainly at a fixed ratio of PA1010/iPP = 80/20) was used as an example to demonstrate the effect of shear on the morphology and resultant mechanical properties. After being melt blended, the injection‐molded bars were prepared via a dynamic packing equipment to impose a prolonged shearing on the melts during the solidification stage. By controlling the shear time, the structure evolution and morphological development of the blends can be well controlled. Mechanical measurement of the molded bar showed a dramatically improved tensile property and impact strength with increasing shear time. Morphological examination revealed that the iPP droplets are elongated and become thin fibrils along the shear direction with increasing shear time. The shear‐induced fibrillation, instead of orientation, is believed to be responsible for the largely improved properties of the blend, particularly for the impact strength. The toughening mechanism is discussed based on the combined effect of hindrance of crack propagation and the transferring and bearing of the load due to the existence of the fibrils. This was further proved by changing the blending ratio and using low molecular weight iPP. Finally, we propose a concept for designing blending materials with good comprehensive properties. Copyright © 2011 Society of Chemical Industry