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     

Structure development and interfacial interactions in high‐density polyethylene/hydroxyapatite (HDPE/HA) composites molded with preferred orientation

Composites of high‐density polyethylene (HDPE) filled with sintered and nonsintered hydroxyapatite (HA) powders, designated as HAs and HAns, respectively, were compounded by twin screw extrusion. Compounds with neoalkoxy titanate or zirconate coupling agents were also produced to improve interfacial interaction and filler dispersion in the composites. The composites were molded into tensile test bars using (i) conventional injection molding and (ii) shear‐controlled orientation in injection molding (SCORIM). This latter molding technique was used to deliberately induce a strong anisotropic character to the composites. The mechanical characterization included tensile testing and microhardness measurements. The morphology of the moldings was studied by both polarized light microscopy and scanning electron microscopy, and the structure developed was assessed by wide‐angle X‐ray diffraction. The reinforcing effect of HA particles was found to depend on the molding technique employed. The higher mechanical performance of SCORIM processed composites results from the much higher orientation of the matrix and, to a lesser extent, from the superior degree of filler dispersion compared with conventional moldings. The strong anisotropy of the SCORIM moldings is associated with a clear laminated morphology developed during shear application stage. The titanate and the zirconate coupling agents caused significant variations in the tensile test behavior, but their influence was strongly dependent on the molding technique employed. The application of shear associated with the use of coupling agents promotes the disruption of the HA agglomerates and improves mechanical performance. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2873–2886, 2002     

The enhancement of the mechanical properties of a high-density polyethylene

This paper describes the process optimization in injection molding of high-density polyethylene (HDPE). Both conventional injection molding and shear controlled orientation (SCORIM) were employed in processing. The process optimization was based on design of experiments and complemented with analysis of variance. Mechanical characterization was carried out by tensile testing. Wide-angle X-ray diffraction and differential scanning calorimetry were used for the structural characterization of the moldings. High-density polyethylene exhibits 7.2 GPa Young's modulus and 155 MPa of ultimate tensile strength following the application of SCORIM processing. These results account for a fourfold increase in Young's modulus and a fivefold increase in ultimate tensile strength compared to conventional injection molding. The maintenance of toughness while enhancing stiffness and strength of the SCORIM moldings is attributable to an oriented morphology developed during shear flow, i.e., shish-kebab structure. The frequency of shearing action has the strongest influence on the morphology development. It is also demonstrated that the studied parameters are very much interdependent. It is possible to achieve substantial gains in mechanical properties of HDPE in SCORIM processing without causing a substantial increase in cycle time. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2473–2483, 1999     

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     

Quantification of shish–kebab and β‐crystal on the mechanical properties of polypropylene

The oriented “shish–kebab” structure and β‐crystal can enhance the mechanical properties of polypropylene products. In this regard, equipment and β‐nucleation agents have been developed or modified to form shish–kebab and β‐crystal. However, the effect of shish–kebab/β‐crystal proportion on the mechanical properties of polypropylene remains unclear. The answer is crucial but remains a challenge because of the difficulty in manipulating the shish–kebab proportion. In this work, we used a self‐made multiflow vibrate‐injection molding, which can provide a controllable shear flow, to produce samples with different shear‐layer thicknesses. The shish–kebab proportion was represented by R, which is the thickness ratio of the shear layer to that of the whole sample. Results showed that the tensile strength exponentially increased, whereas the elongation at break exponentially decreased, with R. The impact strength remained constant with R, indicating that the shish–kebab and β‐crystal possessed similar toughening effects. This work proposes a schematic to interpret the strengthening mechanism involved and presents a method of establishing and controlling the mechanical properties of polypropylene samples by using shish–kebab structures and β‐crystals. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45052.     

Injection molding of a starch/EVOH blend aimed as an alternative biomaterial for temporary applications

Biodegradable polymers show great potential to be used as materials for temporary implants and bone replacement applications in orthopedics. However, its use in high load‐bearing applications will depend on the successful development of biodegradable implants with a mechanical performance matching that of human bone. This article describes the optimization of the injection molding process of an alternative biodegradable starch‐based polymer aimed at biomedical applications. A blend of starch with a copolymer of ethylene–vinyl alcohol (SEVA‐C) was studied. Both conventional injection molding and shear controlled orientation (SCORIM) were optimized with the support of design of experiments and analysis of variance techniques. The mechanical characterization was performed by tensile testing. The structure developed within the moldings was assessed by wide‐angle X‐ray diffraction and differential scanning calorimetry. Increases up to 30% in the tangent modulus and 20% in the ultimate tensile strength compared with conventional molding were achieved with the application of SCORIM. The holding pressure and the frequency of the shear applied have the strongest influence on the morphology development and consequently on the mechanical performance. The solidification of SEVA‐C at high cavity pressures enhances stiffness for long durations of the shearing stage in SCORIM. However, the effect of viscous heating of SEVA‐C is important and ought to be considered. A decrease of the material phase miscibility in SEVA‐C occurs as result of the shear fields imposed. The microstructure evaluation suggests that the mechanical properties enhancement in SCORIM molded SEVA‐C is attributable to preferred orientation developed during processing. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1303–1315, 2000     

Morphological evolution of HDPE parts in the microinjection molding: Comparison with conventional injection molding

To compare the difference of morphological evolution of HDPE micropart and macropart, micropart with 200 μm thickness and macropart with 2000 μm thickness were prepared. The PLM images of micropart and macropart exhibited a similar “skin–core” structure, but the micropart showed a much larger fraction of orientation layer. The SEM observation of core layer of micropart featured an unoriented lamellae structure and shear layer of micropart showed a highly oriented shish‐kebab structure. The 2D‐WAXD patterns of shear layer of macropart indicated twisted oriented shish‐kebab (KM‐I) structures, however that of micropart indicated untwisted oriented shish‐kebab (KM‐II) structures which was firstly found in microinjection molding. The diffraction pattern of the micropart exhibited stronger azimuthal dependence than the shear layer of macropart, indicating the most pronounced orientation of HDPE chains within lamellae. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of high molecular weight component on the hierarchical crystalline structures of injection‐molded bars of polyethylene

A small amount of high molecular weight molecules can have a dramatic influence on the flow‐induced crystallization kinetics and orientation of polymers. To elucidate the effects of the high molecular weight component under a real processing process, we prepared model blends in which high density polyethylene with a high molecular weight and wide molecular weight distribution was blended with a metallocene polyethylene with a low molecular weight and very narrow molecular weight distribution. To enhance the shear strength, gas‐assisted injection molding was utilized in producing the molded bars. The hierarchical structures and orientation behavior of the molded bars were intensively explored by using scanning electron microscopy and two‐dimensional wide‐angle X‐ray diffraction, focusing on effects of the high molecular weight component on the formation of the shish kebab structure. It was found that there exists a critical concentration of high molecular weight component for the formation of a shish kebab structure. The threshold was about 5.5–7.0 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. Moreover, the rheological properties of molten polyethylene melts were studied by dynamic rheological measurements and a critical characteristic relaxation time for shish kebab formation was obtained under the processing conditions adopted in this research. © 2013 Society of Chemical Industry     

Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

All‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m². According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.     

Structural development of HDPE in injection molding

This study investigated some relevant structure/properties relationships in shear‐controlled orientation in injection molding (SCORIM) of high‐density polyethylene (HDPE). SCORIM was used to deliberately induce a strong anisotropic character in the HDPE microstructure. Three grades with different molecular weight characteristics were molded into tensile test bars, which were subsequently characterized in terms of the mechanical behavior by tensile tests and microhardness measurements. The structure developed upon processing was also characterized by polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). SCORIM allows the production of very stiff molded parts, exhibiting a very well‐defined laminated morphology. This morphology is associated with both an M‐shaped microhardness profile and a pronounced mechanical anisotropy. These characteristics are supported by an analogous variation in the crystallinity and a high level of molecular orientation, as indicated, respectively, by calorimetric measurements and X‐ray diffraction results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2079–2087, 2003     

Enhanced orientation of the water‐assisted injection‐molded ipp in the presence of nucleating agent

The nucleated isotactic polypropylene (iPP) was molded by water‐assisted injection molding. The crystalline morphology and orientation distribution were studied. The results show that shear brought by melt filling and pressurized water penetration can separately induce the formation of oriented structures in skin region (i.e., the region near mold cavity wall) and the water channel region. For virgin iPP, slightly oriented lamellae appear exclusively in the above aforementioned regions. However, shish‐kebab structure occurs not only in skin and water channel region of the iPP containing moderate content of nucleator (0.2 wt%) but also in the whole region of the iPP containing a higher content of nucleator (1 wt%). It is well known that nucleator cannot directly induce the development of shish‐kebab in the absence of shear, thus the results indicate: shear flow actually distributes over a much broader range than expected; in shear field, nucleator is significantly helpful for the shear which is not sufficient to solely induce oriented structure to promote the formation of the oriented structure. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Effect of low‐frequency melt vibration on HDPE morphology

BACKGROUND: A low‐frequency vibration‐assisted injection‐molding (VAIM) device was developed to explore the morphology of high‐density polyethylene (HDPE) injection moldings. Scanning electron microscopy, wide‐angle X‐ray diffraction and differential scanning calorimetry were used to characterize structure‐property relationships of final products prepared under different VAIM processing conditions (vibration frequency and vibration pressure amplitude) with conventional injection molding for comparison. RESULTS: It was found that increasing the vibration frequency at constant vibration pressure amplitude was beneficial for obtaining ‘shish‐kebab’ structures in the core region of VAIM specimens, and increasing the vibration pressure amplitude at constant vibration frequency was a prerequisite for achieving HDPE specimens with large‐scale lamellas, more pronounced orientation and high crystallinity. CONCLUSION: Both preferred orientation lamellas and increased crystallinity allow one to obtain strong injection moldings with the application of the melt vibration technique. Copyright © 2009 Society of Chemical Industry     

An Alternating Skin–Core Structure in Melt Multi‐Injection‐Molded Polyethylene

Shish‐kebab, which is endowed with superior strength and modulus, provides the potential to fabricate self‐reinforced polymer products. However, the injection‐molded product usually exhibits a typical skin–core structure, and the shish‐kebab is only located in an extremely thin shear layer. Therefore, the controlling and tailoring of crystal structures in complex flow field to improve the mechanical properties of the injection‐molded sample are still a great challenge. Herein, for the first time, high‐density polyethylene sample with a novel macroscopic alternating skin–core structure is achieved using a melt multi‐injection molding technique. Results show that, with increasing the amount of melt injection, the layers of skin–core structure increase in the form of arithmetic progression, and therefore the tensile strength of the samples progressively increases due to an increase of shish‐kebab content. This study demonstrates a new approach to achieve multilayer homogeneous materials with excellent tensile strength via macroscopic structural design during the practical molding process.     

Mechanical performance of starch based bioactive composite biomaterials molded with preferred orientation

Composites of blends of starch with ethylene vinyl alcohol copolymer (SEVA‐C) filled with 10, 30 and 50% by weight (wt.) of hydroxyapatite (HA–the major inorganic constituent of human bone) were produced by twin‐screw extrusion (TSE) compounding. These composites were molded into tensile test bars using two molding techniques: (i) conventional injection molding and (ii) shear controlled orientation in injection molding (SCORIM). The bars produced were mechanically characterized by means of tensile testing and dynamical mechanical analysis (DMA). The structure of the moldings was assessed by wide‐angle X‐ray diffraction (WAXD) and the failure surfaces of the moldings analyzed by scanning electron microscopy (SEM). The enhancement of stiffness observed with HA reinforcement results partially from the stiffening effect of the blend associated with the decrease in plasticizer content during the compounding stage. SCORIM was able to further increase the stiffness of SEVA‐C/HA composites, allowing a maximum improvement of 12% for 30% wt. HA as compared to conventional molding. DMA results showed that the reinforcement of SEVA‐C causes the broadening of the relaxation peak of the polymer, suggesting a structural change within the starch fraction that may be related with thermal degradation of the polymer. The addition of HA particles reduces the preferred orientation exhibited by the SEVA‐C matrix, which is believed to limit the maximum mechanical performance that can be attained. Nevertheless, composites based on a biodegradable matrix with modulus above 7 GPa (in the bounds of the lower limit for human cortical bone) could be successfully produced.     

Mechanical properties and transparency of injection‐molded polyacetal with branched and linear structure: Influence of crystalline morphology

Branched and linear polyacetals prepared by cationic bulk polymerization were molded under high‐injection rate and pressure, and the resultant 1‐mm‐thick specimens were investigated regarding the crystalline morphology, mechanical properties, and transparency. The branched polyacetal exhibited shear‐induced transformation of crystalline morphology, namely, the spherulites, the elongated spherulites, and shish‐kebab morphology parallel to the flow direction, with increasing shear viscosity. The degree of orientation of the branched polyacetal, calculated from the intensity distribution on the Debye ring of the (100) diffraction by WAXS, linearly and significantly increased with the increase of the logarithm of the shear viscosity. The difference of the crystalline morphology greatly influenced the mechanical properties and transparency of the branched and linear polyacetals. The branched polyacetal with the shish‐kebab morphology had approximately 20% higher tensile strength and modulus as compared with those with the spherulites morphology, and showed translucent with a higher light transmittance over a wide range of wavelength of incident light. The results indicate that a large number of fibrous crystals in the shish‐kebab morphology result in the self‐reinforcement of specimens parallel to the flow direction and diminishment of the scattering of incident light. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3182–3392, 2006     

Extension of the orientation region of high density polyethylene molded by gas‐assisted injection molding: control of the thermal field

Wider zones with close‐knit orientation crystals in high density polyethylene (HDPE) parts prepared via the gas‐assisted injection molding (GAIM) process were obtained under high cooling gas pressure. In this study, compressed nitrogen, as a cooling medium, was introduced to retain a high cooling rate of the polymer melt. The high gas pressure leads to fast cooling of the polymer melt, which contributes to the stability of more oriented and stretched chains during the cooling stage. Then many more oriented structures are formed. SEM shows that many more oriented structures and interlocking shish‐kebab structures are achieved in parts under highest cooling gas pressure (P3). The P3 parts possess a higher degree of orientation than the corresponding regions of parts under lowest cooling gas pressure (P1). Moreover, tensile testing indicates that, compared with P1 parts, although P3 parts have lower crystallinity, the mechanical properties are improved because of the wider orientation zone and many more interlocking shish‐kebab structures. Combining the HDPE molecular parameters with the characteristics of the GAIM flow field and temperature field, the stability of oriented or stretched chains and the formation of orientation structures in various zones of the parts were analyzed. © 2014 Society of Chemical Industry     

Shish Kebab Morphology induced in Polyhydroxybutyrate under Shear Flow

Summary: In‐situ rheo small‐angle X‐ray scattering (SAXS), rheo‐light scattering, and rheo‐optical methods were employed to investigate the resultant morphology of polyhydroxybutyrate (PHB) under varying shear flow conditions. Immediately after shear flow application, a highly orientated structure emerged and row nucleation was identified at high shears. Only the initial stages of shish growth (we term the partial shish) were confirmed at excessively high shear conditions. However, only the kebabs were identified at medium shears, below this neither the shish nor kebab were observed. We believe this partial shish is a result of insufficient stability resulting from using such a low‐molecular‐weight species. We conclude that from our observations the shish kebab mechanism appears to display similarities to the Janeschitz‐Kriegl model of precursor formation.

Left: In‐situ rheo‐SAXS two‐dimensional pattern; kebab morphology observed at 100 s^?1 for 1 s shear after 160 s. Right: In‐situ rheo‐optical micrograph; PHB row‐nucleated morphology observed at 100 s^?1 for 1 s shear after 1 min.     

Study on the Improvement of Crystallization in HDPE Induced by High–Molecular-Weight Polyethylene Through Dynamic Packing Injection Molding

The effect of high–molecular-weight polyethylene (HMWPE) on crystal morphology was investigated for high-density polyethylene (HDPE) through dynamic packing injection molding (DPIM). With the aid of differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), and scanning electron microscopy (SEM) measurements, a typical web-like shish kebab morphology, which markedly increases stiffness and toughness, was found in HMWPE-induced samples through DPIM. The SEM results show that the much better web-like shish kebab structure, in which most of the lamellae connect different columns, compared with conventional shish kebab, was formed in HDPE blends with 4% HMWPE content (B4) through DPIM. The WAXD studies indicate that orientation degrees of crystallographic planes (110) and (200) in the B4 samples were much higher than those of samples molded by static packing injection molding and B0 samples molded by DPIM. A combination of the higher degree of crystal orientation and the formation of web-like shish kebab led to simultaneous great increments of stiffness and toughness, which overcomes the traditional limitation that stiffness and toughness cannot be greatly enhanced simultaneously. All these results show that HWMPE favored for great improvement of crystal structures in HDPE when its content is appropriate through DPIM.     

Crystallization behavior and molecular orientation of high density polyethylene parts prepared by gas‐assisted injection molding

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas‐assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two‐dimensional wide‐angle X‐ray scattering as well as dynamic rheological measurements. The non‐isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish‐kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two‐dimensional wide‐angle X‐ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish‐kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry     

Fiber Formation in Medium and Ultra‐High‐Molecular‐Weight Polyhydroxybutyrate Blends under Shear Flow

Summary: Polyhydroxybutyrate (PHB) is an ideal bioplastic, however, this polymer undergoes a severe embrittlement process because of its spherulitic structure, rendering the material brittle. Using a series of in‐situ rheo techniques, we have previously observed only the rapid initial stage of shish formation, we term a partial shish, which existed at high shears in medium‐molecular‐weight PHB, = 360 000. The shish kebab morphology is anticipated to remove or severely lessen this embrittlement process whilst providing new properties and applications. For medium and ultra high‐molecular‐weight (MMWT, = 360 000/UHMWT, = 5 × 10⁶) PHB 99/1 and 99.5/0.5 blends only a partial shish is identified. However, the initial shish formation stage and subsequent stages were observed at 98/2 and 97/3 blend ratios resulting in a complete shish, we term the full shish, and fiber formation was evident. We believe this fiber morphology achieved by high molecular weights is crucial to sustaining the shish kebab structure for an excessive period.

Left: In‐situ rheo‐light scattering micrograph; 97/3 MMWT/UHMWT PHB at 100 s^?1 for 1 s shear shish held at 75 s. Right: In situ rheo‐optical micrograph; PHB fiber morphology observed at 50 s^?1 for 2 s shear 98/2 MMWT/UHMWT PHB after 1 min.