Uni‐ and biaxial impact behavior of double‐gated nanoclay‐reinforced polypropylene injection moldings

Polypopylene/nanoclay three‐dimensional parts were produced without intermediate steps by direct injection molding to explore the influence of flow features and nanoclay incorporation in their impact performance. The nanocomposite was obtained by direct compounding of commercial PP with nanoclay masterbatch. The as‐molded morphology was analyzed by X‐ray and TEM analyses in terms of skin‐core structure and nanoclay particle dispersion. The nanoclay particles induced the reduction of β‐form spherulites, a known toughener. The impact behavior was assessed in tensile and biaxial modes. The PP nanocomposite molding toughness was practically unaffected by the processing melt temperature and flow rate. Conversely the nanoclay presence is influent in the impact performance. Under biaxial stress impact, the regions close to weld lines are tougher than the bulk and the fracture develops with main crack paths along the flow direction and the weld line. Cracking along the weld line results from less macromolecular interpenetration and chain entanglement, and unfavorable nanoparticle orientation. It seems that a failure mechanism which involves nanoclay delamination and multiple matrix crazing explains the toughening of PP in the directions where the nanoparticle orientation with respect to loading is adequate. POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

剪切控制取向注射成型概述

对比了剪切控制取向注射成型（ＳＣＯＲＩＭ）与普通注射成型（ＣＩＭ）的基本原理，剪切控制技术（ＳＣＯＴ），可消除制品熔接缝，改善外观质量，还可消除内在应力和空隙等缺陷，提高制品内在质量。     

Crystalline morphology of isotactic polypropylene (iPP) in injection molded poly(ethylene terephthalate) (PET)/iPP microfibrillar blends

The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ microfibrillar blends have been prepared through a “slit die extrusion-hot stretch-quenching” process, in which PET assumes microfibrils with 0.5-15 μm in diameter depending on the hot stretching ratios (HSR, the area of the transverse section of the die to the area of the transverse section of the extrudate). The injection molded specimens of virgin iPP and the PET/iPP blends were prepared by conventional injection molding (CIM) and by shear controlled orientation injection molding (SCORIM), respectively. The effect of shear stress and PET phase with different shape on superstructures and their distribution of injection molded microfibrillar samples were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The shear (or elongational) flow during CIM and SCORIM can induce oriented lamellae (i.e. kebabs induced by shish). The shish-kebab structure appears not only in the skin and intermediated layers of CIM samples, but also in the whole region of SCORIM samples. For the neat iPP samples, a more “stretched” shish-kebab structure with higher orientation degree can be obtained in the interior region (intermediate and core layers) by the SCORIM method; moreover, the SCORIM can result in the growth of β-form crystal both in intermediate layer and in core layer, which only appears in intermediate layer of the neat iPP samples obtained by CIM. For the PET/iPP blends, interestingly, the addition of microfibrils as well as their aspect ratios can affect the orientation degree of kebabs only in the intermediate layers, and the addition of microfibrils with a low aspect ratio can bring out a considerable increase in the orientation degree of kebabs along the flow direction. However, for the SCORIM, the addition of microfibrils seems to be a minor effect on the orientation degree of kebabs, and it tends to hamper the formation of a more “stretched” shish-kebab structure and suppresses the growth of β-form crystal distinctly. Furthermore, It appears from experiment that γ-form crystals can grow successfully in this oriented iPP melt with the synergistic effect of shear and pressure only when the growth of β crystals can be restrained by some factors, such as the PET dispersed phase and thermal conditions (cooling rate).     

Hierarchic structure and mechanical property of glass fiber reinforced isotactic polypropylene composites molded by multiflow vibration injection molding

Maleic anhydride‐grafted polypropylene (Ma‐PP) and β nucleation agents (β‐NA) were used to modify the glass fiber (GF)/isotactic polypropylene (iPP) composite. The interface adhesion, degree of orientation, and crystalline morphologies of the PP/GF composites molded by multiflow vibrate‐injection molding (MFVIM) and conventional injection molding (CIM) were studied by polarized light microscopy (PLM), scanning electronic microscopy (SEM), and X‐ray measurements. Results prove that the interface adhesion was improved by the Ma‐PP; γ crystal was generated by the MFVIM due to the instant high pressure and shear during the multiflow; and a hierarchical structure which has a strengthened skin and a toughened core was formed. As a result, the final PP/GF/β‐NA composite has a 60% increase in tensile strength and 80% improvement in impact strength compare with the CIM pure PP/GF composite. Based on the observations, a modified model is proposed to interpret the strengthening and toughening mechanism. Our work paves the way to obtain high‐performance GF/iPP composites. POLYM. COMPOS., 38:2707–2717, 2017. © 2015 Society of Plastics Engineers     

Shear controlled orientation in injection moulding of starch based blends intended for medical applications

Abstract

Biodegradable polymeric implants are considered to be good alternatives to metallic implants in several temporary applications. Aliphatic polyesters have been extensively investigated and have been employed in several biomedical applications. More recently, biodegradable starch based polymeric blends have also been considered as alternative materials. In this study, the effect of shear controlled orientation injection moulding (SCORIM) on the mechanical properties and degradation behaviour of starch-polylactide and starch-poly(ethylene-co-vinyl alcohol) blends has been investigated and compared with those produced by conventional injection moulding. The changes in these properties has also been studied when using an hydroxyapatite filler to reinforce the polymeric matrixes. The SCORIM processing enhanced the unidirectional mechanical properties substantially. The incorporation of hydroxyapatite into the polymer matrix had a stiffening effect but also reduced the strength and toughness. Generally, the mechanical properties deteriorated substantially in vitro. Reinforcement by hydroxyapatite was found to be less effective than expected in a wet environment.     

Development of the skin layer in injection moulding: phenomenological model

This work explores the conditions affecting the development of the skin layer in injection moulding. The thermomechanical environment is varied by systematic changes on the operative processing parameters for three moulding geometries: an axi-symmetric tensile bar, a lateral gated disc and a two-point central gated box. The thermomechanical environments are evaluated by computer simulations, by the local bulk temperature and shear stress level. The calculation of dimensionless numbers (Cameron, Brinkman and Nahme) allows the interpretation of the main physical phenomena occurring in the distinct moulding geometries. The relationships between the imposed thermomechanical environment and the thickness of the skin layer are established. For all the mouldings the skin thickness decreases with the increase in the thermal and shear stress levels, although with distinct contributions for the different moulding geometries. The development of the skin layer is interpreted in the light of a phenomenological model involving two time variables: the time allowed for relaxation of the highly oriented melt until the crystallization temperature is reached and the relaxation time of the material. The influence of the operative processing parameters on both time variables are discussed and used to interpret the experimental variations of the thickness of skin layer.     

Shear‐induced crystallization of injection molded vetiver grass‐polypropylene composites

Vetiver grass was used as an alternative filler in polypropylene (PP) composites in this study. Chemical treatment of vetiver grass by alkalization was carried out to obtain alkali‐treated vetiver grass. It was shown that alkali‐treated vetiver grass exhibited higher thermal stability than untreated vetiver grass. Injection molding was used to prepare the composites. The microstructure of injection molded samples showed a distinct skin layer due to shear‐induced crystallization. It was found that normalized thickness of shear‐induced crystallization layer of the composite was lower than that of neat PP. The effect of vetiver particle sizes on shear‐induced crystallization and physical properties of the composites were elucidated. Furthermore, the effect of processing conditions on shear‐induced crystallization, degree of crystallinity, gapwise crystallinity distribution, and mechanical properties of the composite were investigated. It was shown that injection speed and mold temperature affected the normalized thickness of shear‐induced crystallization layer and degree of crystallinity of the composites. However, processing conditions showed insignificant effect on the mechanical properties of vetiver fiber‐PP composites. The degree of crystallinity showed no distribution throughout the thickness direction of the composites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The Tensile Behaviour of an Injection-Moulded Propylene–Ethylene Copolymer: the Effect of the Local Thermomechanical Processing Conditions

In this paper the mechanical behaviour of a propylene–ethylene copolymer was characterised as a function of the local processing conditions using injection-moulded axisymmetrical dumbbell-like specimens of diameter 1·5mm. The moulding programme, designed according to the central composite face technique, included variation of the injection and mould temperatures and the flow rate. The local thermomechanical environment associated with each moulding condition was determined by filling simulations and quantified by the local bulk temperature and the maximum shear stress. Based on these parameters, thermal and thermomechanical indexes were defined to quantify the morphology of the mouldings, which was evaluated experimentally by the skin ratio. The tensile tests were carried out at two different constant velocities: 2 mm min^-1 (0·56×10^-6ms^-1) and 3 ms^-1, in order to assess the tangent modulus, the strain and the energy at rupture. The results concerning processing–structure and processing–properties relationships are presented as 3D plots, based on polynomial interpolations of the experimental data. © 1997 SCI.     

Studies of microcellular nanocomposites with supercritical fluid assisted injection moulding process

Abstract

In this study, the microstructure, thermal behaviour and mechanical properties of microcellular nanocomposites were studied. Microcell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, was based on the material system and moulding conditions. Nanoclay in the microcellular, supercrtitical fluid assisted injection moulding process promoted the γ form and suppressed the α form crystalline structure of polyamide 6 (PA6). In the crystallisation kinetics studies, the Avrami equation and the modified Ozawa equation with the Mo method were used to model and analyse isothermal and non-isothermal crystallisation processes respectively. The existence of nanoclay increased the magnitude of the activation energy for both isothermal and non-isothermal crystallisation processes. This suggests the fast crystallisation process and the small crystalline size for microcellular nanocomposite processing. Interestingly, the dissolved gas lowered the crystallinity of the cores of moulded microcellular parts, but the addition of nanoclay reduced the crystallinity of both the cores and the skins of parts. The collective effect of the dissolved gas and nanoclay acted to shorten the moulding cycle time greatly with a reduction in the overall crystallinity of microcellular nanocomposite parts.     

Synergistic enhancement of crystallization and mechanical performance of polypropylene random copolymer by strong shear and β‐nucleating agent

Investigation of microstructure and properties is critical for the development and application of polymer materials. Polypropylene random copolymer (PPR) and β‐nucleated PPR are widely used in water pipe production. The effect of melt shear flow on the crystalline structure and mechanical properties of PPR containing β‐nucleating agent needs in‐depth understanding. In this paper, we demonstrated the preparation of PPR and PPR containing 0.1 wt% calcium pimelate (Ca‐Pim) samples by conventional injection molding (CIM) and oscillation shear injection molding (OSIM). The multilayer structures and morphologies of the samples were characterized by SEM, two‐dimensional X‐ray scattering and DSC. The mechanical properties and the microstructures of samples prepared by these two injection molding methods were compared. Compared with samples prepared by CIM, the stronger shear provided by OSIM induced the formation of a thicker layer of a shish‐kebab structure and a higher content of γ crystals, and dramatically suppressed the β‐nucleating effect of Ca‐Pim. The OSIM samples have more shish‐kebab structures and higher crystallinities than CIM samples and therefore the former exhibit better rigidity than the latter. The β crystals in the core layer and the thicker layer of shish‐kebab structure endow OSIM‐PPR/0.1 wt% Ca‐Pim with excellent impact toughness. © 2017 Society of Chemical Industry     

Compounding and injection molding of polybutene‐1/polypropylene blends

The effect of shear‐controlled orientation injection molding (SCORIM) was investigated for polybutene‐1/polypropylene blends. This article reports on the methods and processing conditions used for blending and injection molding. The properties of SCORIM moldings are compared with those of conventional moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated with mechanical tests, differential scanning calorimetry studies, and polarized light microscopy. The application of SCORIM improved Young's modulus and the ultimate tensile strength. The gain in stiffness was greater for higher polybutene‐1 content blends. A drastic decrease in the strain at break and toughness was observed in SCORIM moldings. The enhanced molecular orientation of SCORIM moldings resulted in a featureless appearance of the morphology. Interfacial features due to segregation were visible in the micrographs of SCORIM moldings. Both conventional and SCORIM moldings exhibited form I′ in polybutene‐1. This article explains the relationship between the mechanical properties and micromorphologies. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 806–813, 2003     

Integrated compounding and injection moulding of short fibre reinforced composites

Abstract

Composites of high density polyethylene (HDPE) and carbon fibre (C fibre) were compounded and moulded into tensile test bars in compounding injection moulding (CIM) equipment that combines a twin-screw extruder and an injection moulding unit. Two HDPE grades exhibiting different rheological behaviours were used as matrices. The mechanical properties of the moulded parts were assessed by both tensile and impact tests. The respective morphologies were characterised by scanning electron microscopy (SEM) and the semicrystalline structures of the matrices investigated by X-ray diffraction. The final fibre length distribution and fibre orientation profiles along the part thickness were also quantified. The composites with lower viscosity exhibit higher stiffness, higher strength and superior impact performance. Both composites exhibit a three layer laminated morphology, featuring two shell zones and a core region. Interfacial interaction is favoured by a lower melt viscosity that enhances the wetting of the fibre surfaces and promotes mechanical interlocking. The composites display a bimodal fibre length distribution that accounts for significant fibre length degradation upon processing. The dimensions of the transversely orientated core differ for the two composites, which is attributed to the dissimilar pseudoplastic behaviour of the two HDPE grades and the different thermal levels of the compounds during injection moulding. Further improvements in mechanical performance are expected through the optimisation of the processing conditions, tailoring of the rheological behaviour of the compound and the use of more adequate mould designs.     

Effects of injection moulding induced morphology on the fracture behaviour of virgin and recycled polypropylene

The effect of injection moulding induced morphology on the fracture behaviour of virgin and recycled polypropylene (PP) has been studied. Skin-core morphology has been analysed by microhardness measurements, since the microhardness and the degree of crystallinity are directly related. Virgin PP has shown higher microhardness values and bigger plastically deformed zone at the crack tip than recycled one. These two differences are due to the higher crystallinity of the recycled PP.Otherwise, in both materials, skin layer has shown lower microhardness values and smaller plastic zone extension than the core region. The former phenomenon is suggested to be governed by the different degrees of crystallinity between both regions, whereas the latter is related to the stress-state (triaxial stress-state) rather than to morphological parameters.     

Gas‐assisted injection molded polypropylene: The skin‐core structure

In this article, gas penetration‐induced skin‐core structure of isotactic polypropylene(iPP), which is molded by gas‐assisted injection molding at different gas pressures, was investigated. For comparison, the counterpart was also molded by conventional injection molding (CIM) using the same processing parameters but without gas penetration. They were characterized via PLM, DSC, and SEM. And the crystal morphology at different gas pressures was principally concerned. For the GAIM parts, highly oriented structure is formed in the skin zone, and much less oriented structure in the inner zone (near the gas channel surface). Furthermore, it is suggested that the naked shish structure can be developed in the skin zone of GAIM part, which is molded at higher gas pressures, and shish‐kebab structure is mainly formed in the skin zone of that, which is molded at lower gas pressure. However, for the CIM part, from the skin to the core zone, the dominant morphological feature is spherulite. In a word, the presence of gas penetration notably enhances the oriented structure formation and gives rise to the skin‐core structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Interfacial instabilities in multimaterial co-injection mouldings: Part 2 - Interfacial mixing in transparent mouldings

Abstract

Previous research in co-injection moulding has produced viscosity ratio guidelines for skin and core components, which must be followed if good core distribution is to be achieved. However, by examining two phase systems of PMMA-PC, which fall within the recommended viscosity range, this work shows that viscosity matching of materials is not a sufficient requirement for interfacial stability. The transparency of the materials allows areas of interfacial mixing to show up as streaks in the mouldings, so that factors affecting stability can be determined. One system is found to be more stable than the others. Explanations for such effects are given by consideration of interfacial mixing, viscosity, tooling geometry, injection speeds, interfacial stresses and shear in multilayered mouldings.     

Prediction of mechanical properties of polyethylene mouldings based on laminate theory and thermomechanical indices

Abstract

The properties of moulded plastic products are dependent on the processing technology used in their manufacture and in particular on the structural morphology resulting from the thermomechanical environment imposed on the melt. This paper presents a unified approach to describe the behaviour of the products based on knowledge of the thermomechanical conditions imposed during processing. A linear medium density polyethylene has been processed using rotational moulding, compression moulding, and injection moulding in order to achieve different thermomechanical conditions (i.e. shear rates and cooling rates). The processing conditions used were typical of those in common use in the respective industries. The moulding parts were mechanically tested to determine the tensile, flexural, and impact properties. These measurements were performed both on samples corresponding to the entire thickness of the moulding and on slices taken from across the section of the mouldings. On the basis of these measurements, two models were developed. One is based on laminate theory, in which, from a knowledge of the mechanical properties of the individual layers through the wall thickness, it is possible to predict the tensile and flexural properties of the full thickness moulding. The other is an empirical model that predicts the tensile modulus of a plastic part as a function of two thermomechanical indices. It is shown that the type of dependence of the mechanical performance on the thermomechanical conditions imposed during processing is similar for the three moulding techniques used. A good agreement is achieved between the experimental data and those predicted by the thermomechanical model. It is also shown that via the combined use of the thermomechanical indices concept and the laminate analysis, good predictions of the mechanical behaviour of plastic mouldings with complex microstructures can be achieved. It is proposed that this approach could provide a very valuable addition to existing melt flow simulation packages. This would enable not only processing conditions to be optimised but the properties of the end product could be predicted.     

The thermomechanical environment and the microstructure of an injection moulded polypropylene copolymer

The microstructure of an injection moulding propylene copolymer is varied through systematic changes on the processing conditions (melt and mould temperatures and injection flow rate). The skin-core structure was characterised by several experimental techniques. The skin ratio was assessed by polarised light microscopy. The morphological features of the skin layer (level of crystalline phase orientation, degree of crystallinity, β-phase content and double texture) were evaluated by wide-angle X-ray diffraction. The core features (degree of crystallinity and lamella thickness) were assessed by differential scanning calorimetry. The thermomechanical environment imposed during processing was characterised by mould filling simulations. The thermal and shear stress levels were evaluated by a cooling index and the wall shear stress. The results show the relationship between these and the microstructural features. The microstructure development is then interpreted considering the constrictions imposed during processing, being assessed by thermomechanical indices. Furthermore, the direct connections between these indices and the degree of crystallinity of the core and the level of orientation of the skin are verified.     

Hierarchy structure in injection molded polypropylene/ethylene–octane copolymer blends

In this article, the phase morphology and mechanical properties of polypropylene (PP)/ethylene–octane copolymer (POE) blends with fixed ratio (60/40) obtained via different processing conditions, including barrel temperature, injection speed, and mold temperature, have been investigated. SEM was carried out for detailed characterization of phase morphology from the skin to the core, layer by layer. It was interesting that for all the processing conditions no dispersed POE elastomer was observed in the skin layer but elongated POE particles with large size were observed in the subskin layer. From the transition zone to the core layer, an increased phase separation was observed, which could lead to a formation of cocontinuous morphology, depending on the processing condition used. Higher barrel temperature, lower mold temperature, and higher injection speed could result in a smaller size of POE phase. The tensile strength and impact strength were found not sensitive to barrel temperature and mold temperature but to the low injection speed, both tensile strength and impact strength had a higher value for specimen obtained via low injection speed. The formation of the skin‐core morphology and the effect of processing conditions on the phase morphology were discussed based on crystallization kinetics of PP matrix, rheology, and shear induced phase mixing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Structure and physical property relationships in processed polybutene‐1

The effect of SCORIM was investigated on three grades of polybutene‐1 and one grade of ethylene–butene‐1 copolymer. The methods and processing conditions used for injection molding and the properties of the moldings are reported. Phase transformations and their relationship with mechanical properties are discussed in detail. Both, conventional and shear‐controlled orientation injection molding (SCORIM) were employed to produce moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated by performing mechanical tests, fractographic analysis, differential scanning calorimetry studies, wide‐angle X‐ray diffraction, polarized light microscopy, and atomic force microscopy. The application of SCORIM improves the mechanical performance. Molecular orientation results in the formation of shish‐kebab morphology. One grade of polybutene‐1 exhibited a greater than fivefold increase in Young's modulus. The application of high cavity pressures favored the formation of the stable Form I' in polybutene‐1. The formation of Form I' led to a decrease in crystallinity and mechanical properties. However, this loss was by far smaller than the gain obtained via the formation of shish‐kebab morphology. The relationship between mechanical properties and micromorphologies of the investigated materials is explained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 814–824, 2003     

Studies on blends of high‐density polyethylene and polypropylene produced by oscillating shear stress field

Tensile strength and morphology of blends of high‐density polyethylene (HDPE) and polypropylene (PP) obtained by oscillating packing injection molding were investigated via Universal Testing Machine, DSC, and SAXS. Tensile strength is greatly enhanced from 24.5 MPa to more than 90 MPa for pure HDPE and for blends with PP content less than 10 wt %. There exists a sharp decrease of tensile strength when PP content is more than 10 wt %. The shear‐induced morphologies with core in the center, oriented zone surrounding the core and skin layer are observed in the cross‐section areas of the samples. Interestingly, a sharp decrease of oriented zone is seen when PP content is more than 10 wt %, associated with the sharp decrease of tensile strength. DSC result shows double melting peaks with a high‐temperature melting peak that is not present in the endotherm obtained from the central core and obtained from the samples by static packing injection molding, which indicates the existence of shish‐kebab structure in the oriented zone. However, there is no difference of crystallinity between the samples by oscillating and by static packing injection molding. SAXS was used to analyze the complicated morphologies induced by shear stress, and results show that the crystal thickness could be greatly increased under shear stress. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 58–63, 2002