Injection-molded Isotactic Polypropylene Research Articles

Injection-Molded Isotactic Polypropylene Colored with Green Transparent and Opaque Pigments.

Polypropylene (PP) belongs among the most important commodity plastics due to its widespread application. The color of the PP products can be achieved by the addition of pigments, which can dramatically affect its material characteristics. To maintain product consistency (dimensional, mechanical, and optical), knowledge of these implications is of great importance. This study investigates the effect of transparent/opaque green masterbatches (MBs) and their concentration on the physico-mechanical and optical properties of PP produced by injection molding. The results showed that selected pigments had different nucleating abilities, affecting the dimensional stability and crystallinity of the product. The rheological properties of pigmented PP melts were affected as well. Mechanical testing showed that the presence of both pigments increased the tensile strength and Young's modulus, while the elongation at break was significantly increased only for the opaque MB. The impact toughness of colored PP with both MBs remained similar to that of neat PP. The optical properties were well controlled by the dosing of MBs, and were further related to the RAL color standards, as demonstrated by CIE color space analysis. Finally, the selection of appropriate pigments for PP should be considered, especially in areas where dimensional and color stability, as well as product safety, are highly important.

Predicting the complex stress-strain curves of polymeric solids by classification-embedded dual neural network

Predicting stress–strain curves is key to facilitate the design of polymer materials and their products with tailored mechanical response. However, due to their structural complexity, polymeric solids generally feature complex stress–strain curves, which renders it challenging to model their stress–strain behaviors. Here, using the categorized knowledge of stress–strain curves, a “Classification-Embedded Dual Neural Network (CDNN)” framework is introduced to accurately predict the mechanical evolution of polymeric solids, by taking the example of injection-molded isotactic polypropylene. Upon built, the dual model is a parallel coupling of a “curve type classifier” and a “curve feature predictor” that predict, respectively, the stress–strain curve categories and their feature points that dictate the extent of similarity between two arbitrary curves in the same category, regardless of the curve complexity. Importantly, with the aid of curve-categorized knowledge, the CDNN strategy offers an update-to-date best balance in model accuracy (20% curve error in maximum) and simplicity (300 neurons in total), which greatly enhances the model’s extrapolability and interpretability and, in turn, mitigates the demanding data requirement (27 samplings from a 4D space, that is, a material design space consisting of 4 design dimensions to tune the structure). Overall, this work establishes a simple, robust methodology in predicting polymeric solids’ stress–strain curves and is potentially generic to a variety of materials.

Experimental Investigation of Effects of the Nucleating Agent on Mechanical and Crystallization Behavior of Injection-Molded Isotactic Polypropylene

In this study, we investigated the effect of talc (magnesium-silicate-monohydrate/3MgO.4SiO2.H2O) nucleating agent (NA) on the thermal and mechanical properties of isotactic-polypropylene (i-PP) at loadings ranging from 0.5 to 5% by weight using the melt compounding method. The results demonstrated that NA/i-PP exhibited significantly improved mechanical performance compared to neat i-PP. In addition, the influence of the concentration of the NA on the melting and crystallization behavior of neat i-PP was also analyzed through DSC curves. The tensile and flexural properties of i-PP enhanced as NA concentration increased. Furthermore, scanning electron microscopy revealed that there is a significant difference in the fracture surface between the NA/i-PP and the i-PP matrix. Consequently, it was determined that the NA (talc) used in this study was a good NA for i-PP, improved the mechanical and thermal properties of the product, affected the crystallization properties and the microstructure, and reduced the mold cycle time. Besides these, the use of various additives between talc and i-PP matrix that can improve interface features are expected to be an alternative approach to the new i-PP design with much higher mechanical and thermal properties.

Annealing induced high‐impact toughness of injection molded isotactic polypropylene filled with liquid‐phase exfoliated graphene

AbstractFavorable dispersibility of graphene (G) nanosheets in isotactic polypropylene (iPP) matrix is a keypoint that affects its toughening effect. In this work, a very low level of liquid‐phase exfoliated G was incorporated into iPP, and the iPP/G composite samples were further annealed at moderate temperature (120–140°C) for 3 h. It was found that liquid‐phase exfoliated G exhibits favorable dispersibility and efficient heterogeneous nucleation ability, significantly reducing the size of iPP spherulites. Characterization of the changes in microstructure of iPP and iPP/G composite upon annealing reveals that the presence of G promotes the perfection of crystalline structure and enhances dramatically the mobility of chain segments in the amorphous region of iPP during the annealing treatment. Moreover, the addition of this kind of well‐dispersed G may be favorable for the stabilization of β‐crystal during the injection molding process, and annealing at 120–140°C is also conducive to the formation of β‐crystal. A possible synergistic toughening mechanism of the incorporation of liquid‐phase exfoliated G and annealing treatment was put forward.

In-Situ Synchrotron X-ray Study on the Structure Variation of Morphology-Identified Injection-Molded β-Nucleated iPP under Tensile Deformation

The deformation behavior of semi-crystalline polymers is strongly dependent on the morphology formed during processing. In this study, in-situ synchrotron X-ray was firstly used to identify the morphological distributions of injection-molded isotactic polypropylene (iPP) with different concentrations of β-nucleating agent. It was found that under relatively high concentration of β-nucleating agent (i.e., ≥0.03 wt.%), the outer region (skin and shear region) of the iPP was dominated by mainly highly oriented α-phase as well as certain amount γ-phase, while the core region was rich in β-crystals with little if any orientation. The addition of the β-nucleating agent was beneficial for the formation of lamellae with large lamellar stacking distance in the shear layer. Then the synchrotron X-ray was applied to study the structure variation of those morphology-identified samples under tensile deformation. It was found that voids and cavities along the stretching direction existed in the deformed iPP samples and their volume increased with increasing concentration of β-nucleating agent. The increased volume of void and cavity was associated with the β to α phase transition, which mainly occurred at the core region. In addition, upon stretching crystalline fragmentation and rearrangement took place following the formation of thinner lamellae.

Creep and recovery behavior of injection-molded isotactic polypropylene with controllable skin-core structure

The creep behavior is one of the most important properties and should be characterized to evaluate long-term durability and reliability of polymeric materials. An alteration of the crystalline structure and morphology in the semicrystalline polymer leads to a change in the creep behavior as well. In this work, in order to understand the creep behavior of different morphologies, a modified injection molding technology with twice melt filling (M2) was used to prepare isotactic polypropylene (iPP) samples with controllable skin-core structure. The different shear layer thicknesses were obtained by adjusting the time intervals of the twice melt filling. Compared with the conventional injection-molded samples (M1), M2 samples present better efficiencies in reducing the creep and recovery response, and the response was strongly influenced by the shear layer thicknesses. For example, the thickness of skin layer of M2 sample with a time interval of 9 s increased approximately 42% and its creep deformation reduced to 52% at 80 °C. It was also found that the creep and recovery strain reduced with decreased temperature or with applied stress. In addition, several theoretical models were used to describe the creep and recovery behavior of M2 samples for a better understanding of the mechanisms.

Effect of Melt‐Memory on the Crystal Polymorphism in Molded Isotactic Polypropylene

AbstractThe influence of γ‐quinacridone as a β‐crystal nucleating agent in injection molded isotactic polypropylene (iPP) is discussed. Samples are injection molded and characterized via polarized‐light optical microscopy and X‐ray diffraction. Mold‐filling simulation is used to understand the shear and cooling processes during sample preparation. The cooling rate associated with the quench near the mold wall is estimated to be greater than 600 K s−1 using simulation, confirming previous studies that β‐crystal growth is not supported at that cooling rate. The non‐nucleated samples form β‐crystals at a distance of 100–300 µm from the skin and in the core of the sample, which is not expected based on quiescent cooling data. Since the mold‐filling simulation does not predict shear in the core, the formation of the β‐crystals formed in this region is attributed to shear‐induced crystallization effects in the injection unit of the molding machine that are not modeled in flow simulation, as they are typically excluded from any molding simulation analysis. This “melt‐memory” effect has shown to be significant, and it is suggested that the prediction of final properties of injection moldings requires understanding and knowledge of the entire shear history of the material including that of the injection unit.

Unexpected Strength and Toughness Reinforcement of the Injection-Molded Isotactic Polypropylene Parts with Oriented β-Crystals

In this work, three profiles of injection temperatures were adopted to tune the morphology of a β-nucleating agent and further affect the resulting β-crystal morphology of isotactic polypropylene (iPP) during an injection molding process. When the middle injection temperature profile was adopted (the maximum temperature was 220 °C), abundant oriented β-crystals with the c-axis perpendicular to the injection direction were presented throughout the whole injection-molded part, and their existence and distribution were confirmed by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These oriented β-crystals give rise to simultaneously enhanced tensile and impact strength of the β-nucleated iPP part. In a novel outcome, this work transformed the usually mechanically weak and soft component (i.e., the β-crystals) of iPP into one that added to its self-reinforcement and overall strength. This finding provides a new idea that a soft structure can be utilized to reinforce itself.

The influences of a novel shear layer-spherulites layer alternated structure on the mechanical properties of injection-molded isotactic polypropylene

In general, the strength of injection molded objects increases with shear layer thickness or shish-kebab content. However, the effect of the change of the position of shear layer and spherulites layer on the mechanical properties is still unclear, when their thickness are fixed. To answer this question, a novel multilayer structure in which the shear layer mainly containing shish-kebab structure and spherulites layer consisted of spherulites stack alternatively was prepared by a self-designed multi-flow vibration injection molding (MFVIM) device. The results demonstrate that the impact strength can be enhanced significantly by such alternating structure, possibly because this structure is benefit for crack deflection and consequently induces more plastic deformation regions, through altering stress transfer direction and stress distribution. The tensile strength, however, is only affected little by such structure.

Transition from the hierarchical distribution to a homogeneous formation in injection molded isotactic polypropylene (iPP) with dynamic mold temperature control

There is an increasing demand to produce injection molded thermoplastic parts with high performance and more uniform microstructure. In this study, an injection mold with dynamic mold temperature control was developed to create a thermo-mechanical environment in which a high mold temperature and slow cooling rate were retained. Two-dimensional wide angle X-ray diffraction (2D-WAXD) and polarized optical microscopy (POM) studies were carried out to investigate the morphological distribution of isotactic polypropylene (iPP) through the depth. Due to the fast relaxation of polymer chains at a high temperature, the macroscopic orientated structure of iPP in conventional injection molding was eliminated, that is transited from the hierarchical morphology distribution to a more homogeneous formation. A homogeneous appearance without layer boundary was shown and many radial spherulites with loosely packed lamellae distributed uniformly throughout the sample.

Annealing Induced Mechanical Reinforcement of Injection Molded iPP Parts

The mechanical properties and microstructure of injection molded isotactic polypropylene parts with high orientation before and after annealing are analyzed. The mechanical properties of the annealed samples are improved effectively. Through thorough analysis of various structural characterizations, a microstructural model based on the fact that the total length of long period kept constant to analyze the variation of mechanical properties is proposed. It is suggested that the increase of overall crystallinity, the recombination of crystalline phase, and the increase of amorphous phase, respectively, are beneficial for the improvements of the strength, stiffness, and toughness of annealed samples. image

Retrieving the spatially resolved preferred orientation of embedded anisotropic particles by small-angle X-ray scattering tomography

Experimental nondestructive methods for probing the spatially varying arrangement and orientation of ultrastructures in hierarchical materials are in high demand. While conventional computed tomography (CT) is the method of choice for nondestructively imaging the interior of objects in three dimensions, it retrieves only scalar density fields. In addition to the traditional absorption contrast, other contrast mechanisms for image formation based on scattering and refraction are increasingly used in combination with CT methods, improving both the spatial resolution and the ability to distinguish materials of similar density. Being able to obtain vectorial information, like local growth directions and crystallite orientations, in addition to scalar density fields, is a longstanding scientific desire. In this work, it is demonstrated that, under certain conditions, the spatially varying preferred orientation of anisotropic particles embedded in a homogeneous matrix can be retrieved using CT with small-angle X-ray scattering as the contrast mechanism. Specifically, orientation maps of filler talc particles in injection-moulded isotactic polypropylene are obtained nondestructively under the key assumptions that the preferred orientation varies slowly in space and that the orientation of the flake-shaped talc particles is confined to a plane. It is expected that the method will find application inin situstudies of the mechanical deformation of composites and other materials with hierarchical structures over a range of length scales.

Deformation behavior of oriented β-crystals in injection-molded isotactic polypropylene by in situ X-ray scattering

Peculiar oriented β-crystals with its c-axis perpendicular to the injection molding direction were formed in the injection molding processing. Simultaneous with the uniaxial elongation, in situ synchrotron X-ray scattering was employed to investigate the influence of oriented β-crystals on the deformation behavior of the skin and core layers of the injection-molded parts. The presence of oriented β-crystals accelerated the appearance of the yield point, and the yield point became the transition point from lamellar slip to fragmentation. Oriented β-crystals with more regular orientation in the skin layer were favorable for the earlier occurrence and delayed enlargement of microvoids. The formation of microvoids under loading turned out to be related with the separation or slip of oriented β-crystals, which is earlier than the destruction of β-crystals (β-α polymorphous transformation).

Enhanced mechanics in injection molded isotactic polypropylene/polypropylene random copolymer blends via introducing network-like crystal structure

The crystallization behavior, rheological behavior, mechanical properties and microstructures of injection molded isotactic polypropylene (iPP), polypropylene random copolymer (co-PP) and iPP/co-PP blends were investigated. Differential scanning calorimetry (DSC) and dynamic rheological analysis illustrated that iPP and co-PP were compatible in the blends and co-PP uniformly dispersed in the iPP phase. Polarizing optical microscope (POM) was adopted to observe the crystal size and morphology evolution. The results of mechanical properties and scanning electron microscopy (SEM) indicated that the crystal size of iPP in iPP/co-PP blends (10 wt% co-PP + 90 wt% iPP and 30 wt% co-PP + 70 wt% iPP) radically decreased after the incorporation of co-PP. During crystallization, the molecular chain segments of co-PP could penetrate iPP spherulites and form a network-like crystalline structure. The network-like crystal structure could effectively transmit stress and consume more energy to overcome intermolecular forces to resist stretching. In this way, the strength would improve to a certain degree. The impact fracture mechanism of iPP/co-PP blends is quasi ductile fracture by multiple crazes. Our work discovered that the blends containing 10 wt% and 30 wt% of co-PP exhibited prominent toughness and reinforcement.

Tailoring microstructure and mechanical properties of injection molded isotactic–polypropylene via high temperature preshear

In this work, isotactic–polypropylene (iPP) specimens were prepared by a modified injection molding machine, in which high temperature preshear (HTPS) can be imposed on the molten polymer during the plasticizing stage. The effect of HTPS on the microstructure and mechanical property of iPP was investigated. It was found that spherulite size in core region of iPP part decreased steadily with the increasing HTPS duration, indicating that HTPS could substantially enhance iPP nucleation. Moreover, β‐iPP formation correlated strongly with HTPS duration. That is, in the absence of HTPS, β‐iPP existed only in intermediate region; with moderate HTPS duration, β‐iPP could be unexpectedly formed in core region; however, long HTPS duration inhibited β‐iPP formation in both intermediate region and core region. Based on the relationship between β‐iPP formation and HTPS duration, metastable nuclei, instead of α‐row nuclei, were proposed to be responsible for the development of β‐iPP. Notched Izod impact test showed that moderate HTPS duration enhance the impact strength of injection molded iPP by decreasing the thickness of shear region and elevating β‐iPP crystallinity in core region. Dynamic mechanical test indicated that with the increase of HTPS duration, the storage modulus of injection‐molded iPP improves drastically. POLYM. ENG. SCI., 55:2714–2721, 2015. © 2015 Society of Plastics Engineers

Structure and mechanical anisotropy of injection-molded polypropylene with a plywood structure

The injection-molded isotactic polypropylene (PP) containing a specific nucleating agent exhibits unique molecular orientation. Although the skin layer has ‘woven-cloth’ structure, in which α- and β-form crystals orient perpendicular to each other, it shows positive birefringence on average because of the stronger orientation of α-form crystals. In the core, both crystals show perpendicular orientation to the flow direction. As the average orientation direction in the skin layer is perpendicular to that in the core, the anisotropy in mechanical properties and thermal expansion is greatly reduced.

Wide distribution of shish-kebab structure and tensile property of micro-injection-molded isotactic polypropylene microparts: a comparative study with injection-molded macroparts

In this paper, injection-molded isotactic polypropylene (iPP) microparts and macroparts were respectively fabricated by the molds with different thickness under the same processing conditions. Comparative study on microstructure and mechanical property was carried out by means of scanning electron microscopy (SEM), two-dimensional wide-angle X-ray diffraction (2D-WAXD), two-dimensional small-angle X-ray scattering (2D-SAXS), differential scanning calorimetry (DSC) and tensile test. SEM images reveal that the two parts show distinctly different hierarchical structure. An obvious ‘skin–core’ structure is present for the macroparts, while a wide distribution of shish-kebab structure develops in both shear and core layer for microparts, exhibiting a specific ‘core-free’ morphology. 2D-WAXD, 2D-SAXS and DSC results show that microparts have higher orientation degree and crystallinity as compared to macroparts, which are responsible for the remarkably high tensile strength and modulus. Our work provides a good example for better understanding processing structure–property relationship of iPP through tuning their internal microstructure.

Effect of β-Nucleating Agent on the Crystallization, Mechanical Properties, and Heat Resistance of Injection-Molded Isotactic Polypropylene

Injection-molded β-isotactic polypropylene (β-iPP) was prepared with a commercial β-nucleating agent (NT-A). The effect of NT-A on the crystallization, mechanical properties, and heat resistance of β-iPP was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized light microscopy (PLM), and mechanical and heat deflection tests. DSC and WAXD analysis showed that the content of β-crystals in the nucleated iPP was higher than that of pure iPP, and the content of β-crystals of the core was higher than that of the skin. PLM observations showed that injection-molded iPP had an obvious skin-core structure. NT-A induced abundant β-crystals and resulted in small spherulites which improved the Izod notched impact strength. When the content of NT-A was 0.075wt%, the Izod notched impact strength reached a maximum, 2.6 times more than that of pure iPP. The heat distortion temperature was also improved by NT-A.

Effect of Structural Inhomogeneity on Mechanical Behavior of Injection Molded Polypropylene Investigated with Microbeam X-ray Scattering

Relationship between the structure of injection-molded isotactic polypropylene and its tensile mechanical properties, necking and fracture behaviors in particular, was investigated in terms of micrometer-scale structural inhomogeneity of nanometer- and subnanometer-scale structures. To clarify the micrometer-scale inhomogeneity, we employed scanning microbeam wide-angle X-ray diffraction and small-angle X-ray scattering technique. Four isotactic polypropylene samples were studied, produced using different injection-condition and thermal treatments. The results of scanning microbeam X-ray scattering measurements showed the presence of two types of micrometer-scale structural inhomogeneity in addition to the orientation of molecules: the distribution of polymorphs and of crystalline ordering. The results of scanning microbeam X-ray scattering of deformed sample showed the disappearance of the β-form isotactic polypropylene crystals at the outer regions accompanied by the plastic deformation. It is indicated that the inhomogeneous distribution of crystalline ordering and the existence of different polymorphs are highly related to the tensile mechanical behavior.

Suppressing the Skin–Core Structure of Injection-Molded Isotactic Polypropylene via Combination of an in situ Microfibrillar Network and an Interfacial Compatibilizer

Injection-molded semicrystalline polymer parts generally exhibited a so-called skin-core structure basically as a result of the large gradients of temperature, shear rate, stress, and pressure fields created by the boundary conditions of injection molding. Suppression of the skin-core structure is a long-term practical challenge. In the current work, the skin-core structure of the conventional injection-molded isotactic polypropylene (iPP) was largely relieved by the cooperative effects of an in situ microfibrillar network and interfacial compatibilizer. The in situ poly(ethylene terephthalate) microfibrils of 1-8 μm in diameter and large aspect ratios of above 40 tended to entangle with each other to generate a microfibrillar network in the iPP melt. During injection molding, the iPP molecules experienced confined flow in the microchannels or pores formed by the microfibrillar network, which could redistribute and homogenize the flow field of polymer melt. Addition of the compatibilizer, glycidyl methacrylate-grafted iPP, restrained the molecular orientation but facilitated preservation of oriented molecules due to the chemical bonds at the interface between PET microfibrils and iPP. The cooperative effects of in situ microfibrillar network and interfacial compatibilizer led to almost the same molecular orientation across the whole thickness of the injection-molded parts. Additionally, the content of β crystals in different layers of injection-molded iPP parts depended on the combined effects of the molecular orientation, the amount of oriented crystals, and the crystallization time between 105 and 140 °C. The presence of the interfacial compatibilizer facilitated formation of the β crystals because of preservation of the oriented molecules.