Morphology development for single‐site ethylene copolymers in rotational molding

The morphology development of ethylene copolymers was modeled with the modified phase‐field theory. The metastability of polymer crystallization was also considered in the modeling. Modeling and experimental work were simultaneously undertaken to compare the crystallization kinetics of single‐site‐catalyzed and Ziegler‐catalyzed resins and their influence on morphology development in the rotational‐molding process. With a more uniform short‐chain branch distribution, the single‐site copolymers developed well‐defined spherulitic structures. The Ziegler–Natta catalyst resins were characterized by a higher nuclei density and a faster crystallization rate and produced finer structures in the molded parts. The modeling approach proposed in our work allowed an evaluation of the processing and the material effects on the development of morphological features during melt solidification. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Analysis of the influence of the injection molding process on the crystallization kinetics of a HDPE

Thermal analyses of microparts in high density polyethylene (HDPE) have shown that the specific processing conditions used in microinjection molding have irreversible consequences on the polymer morphology. This result has been demonstrated with the analysis of the non‐isothermal crystallization behavior of a HDPE with different thermal histories. The evolution of the absolute crystallinity has been analyzed with a relevant model able to separate the primary and secondary mechanisms all over the crystallization duration. This model has emphasized that the evolution of the primary crystallinity with time is different for the microparts compared to the conventional objects. These differences were attributed to variations of the crystallization mechanisms, especially within the nucleation phase, where a persistent melt memory effect of the former chains orientation/extension was assumed. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44239.     

Metallocene polypropylene crystallization kinetic during cooling in rotational molding thermal condition

This article is part of an ambitious project. The aim is to simulate mechanical properties of rotomolded part from microstructure consideration. Main objective here is to consider metallocene polypropylene crystallization kinetic (PP) during cooling stage in rotational molding. Crystallization kinetic of metallocene PP is so rapid that microscopy cannot help to observe nucleation and growth. Crystallization rate can be estimated by a global kinetic. Given that cooling in rotational molding is dynamic with a constant rate, Ozawa law appears more appropriate. Ozawa parameters have been estimated by differential scanning calorimetry. In rotational molding thermal condition, Avrami index identifies a complex nucleation intermediate between spontaneous and sporadic. Ozawa rate constant is 68 times higher than this obtained for Ziegler–Natta PP. By coupling transformation rate from Ozawa model and a thermal model developed earlier, the difference between theory and experimental is less than 1%. To optimize rotational molding, study has been completed by sensitivity to adjustable parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Surface morphology alignment of block copolymers induced by injection molding

Block copolymers are of increasing interest because of their nanometer-scale morphologies, which can be utilized in a range of applications, including nanolithography. Orientation of the domains can be controlled by part design and processing conditions in injection molding. In this work the surface morphology and alignment of block copolymers by mechanical flow fields from injection molding was investigated using a styrene-ethylene/butylene-styrene triblock copolymer (SEBS) and compared with the morphology induced by spin coating. Compared with the isotropic morphology found by spin coating and annealing, the surface domains were oriented in the flow direction. Increasing mold temperature and injection velocity enhanced the degree of orientation, whereas melt temperature had little effect. Smaller characteristic lengths were produced with higher mold temperatures and injection velocities.     

Motion control for uniaxial rotational molding

Motion control parameters of rotational molding can affect process efficiency and product quality. Different motion control schemes will lead to varied powder flow regimes exhibiting different levels of mixing and temperature uniformity. The change in nature of powder flow during a molding cycle suggests that varying the rotational speed could improve the powder mixing and temperature uniformity, therefore potentially reducing processing time and energy consumption. Experiments completed investigating powder flow under uniaxial rotation show that savings of up to 2.5% of the heating cycle time can be achieved. This validates the hypothesis that altering the rotational speed to maintain the ideal powder flow throughout the heating cycle can be utilized to reduce the time taken for all the polymer powder to adhere to the mold wall. The effect of rotational speed on wall thickness uniformity and impact strength were investigated and discussed. Results show a strong influence of rotational speed (and powder flow) on the wall thickness uniformity of the moldings with wall thickness uniformity deviations of up to 50% found (within the 2–35 RPM speed range tested).     

Flow marks in injection molding of polypropylene and ethylene–propylene elastomer blends: Analysis of morphology and rheology

This paper reports an investigation of asynchronous flow marks on the surface of injection molded parts and short shots made from two different blends of polypropylene and ethylene–propylene random copolymer elastomers. Flow marks were observed on the surface with both blends; the spatial frequency of flow marks on the surface was greater in the blend B1, which also exhibited a greater contrast between the surface regions. The same blend was distinctly faster in the linear viscoelastic tests of shear creep recovery and shear viscosity growth. The degree of contrast between the flow‐mark regions and the out‐of‐flow‐mark regions was examined with a detailed analysis of SEM micrographs of the surface regions as well as the near wall regions from short shots. This revealed that the dispersed phase was highly stretched to cylindrical strands in the glossy surface regions of both blends and retracted in the dull regions to different extents in the two cases. A comparison of the particle size distributions and aspect ratio distributions in different regions established that rapid retraction of the suspended elastomer phase was the dominant cause of changes in particle shape between surface regions. Nonlinear shear creep and creep recovery curves of the two elastomer components showed that at a time of 1 s, the fractional strain recovery of the elastomer in B1 was much higher than that of the elastomer in B2. Hence, the nonlinear elastic recovery of the elastomer phase at short times is an important factor in flow mark formation with blends of polypropylene and olefinic elastomers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 423–434, 2005     

Structure and morphology of linear polyethylene extrudates induced by elongational flow

This study focuses on the structure, morphology, and properties of linear polyethylene (PE) profiles manufactured by continuous extrusion. High level of chain orientation was achieved using specific flow and processing conditions. An extrusion die with semihyperbolic convergency was used to generate high percentage of elongational flow and chain extension. Simultaneously, high extrusion pressure and relatively low melt temperature led to flow‐induced crystallization of PE extended chains. The structure of PE tapes consists of crystal aggregates with different level of orientation and crystallinity. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Dimensional stability of single‐site ethylene copolymers in rotational molding

The dimensional stability of ethylene copolymers in rotational molding was studied by comparing the warpage observed for a series of conventional and single‐site catalyzed ethylene copolymers. Bench‐scale molding trials were carried out under controlled molding conditions. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single‐site resins showed less warpage compared to the Ziegler‐catalyzed copolymers with otherwise comparable densities. The Ziegler‐catalyzed copolymers were characterized by a faster crystallization rate, and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single‐site and Ziegler‐catalyzed copolymers are discussed based on their intermolecular comonomer distributions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

The morphology evolution and crystallization behavior of microinjection molded polyoxymethylene/molybdenum disulfide nanocomposite

Microinjection molding was carried out on polyoxymethylene (POM)/molybdenum disulfide (MoS₂) nanocomposite prepared by solid state shear milling (S³M) technology. The morphology evolution and crystallization behavior were then investigated under different conditions of mold temperature and injection speed. The comparison between the microinjection molded micropart with conventional injection molded macropart was also conducted. Results showed that the higher mold temperature could improve the filling property and replication quality. The MoS₂ particles played a heterogeneous nucleation role and remarkably affected the crystallization process of POM. Meanwhile, the crystallization orientation and skin‐core structure induced by the shear stress presented the similar evolution tendency under different microinjection conditions. The multi‐melting behaviors of microparts occurring under different microinjection conditions were attributed to the formation of shish‐kebab structures. The increase of crystallinity and the reduction of mechanical property occurring in macropart are related to its crystallization morphology formed, which is different from that of micropart. The results of this work would lay a foundation for preparation of POM/MoS₂ transmission microparts with good comprehensive performance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44625.     

Optical monitoring of polyesters injection molding

An optical fiber sensor similar to the one developed by Thomas and Bur 1 was constructed for the monitoring of the crystallization of three polyesters during the injection molding process. The polyesters studied were: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene terephthalate (PET). With this optical system it was possible to obtain, in real time, some essential parameters of the polyester crystallization kinetics at different processing conditions. Thus, a study of the influence of injection molding variables on the nonisothermal crystallization kinetics of these polyesters was done. The processing variables were: mold wall and injection temperatures, T_w and T_i, respectively; flow rate, Q; and holding pressure, P_h. The experiments were done following a first order central composite design statistical analysis. The morphology of the samples was analyzed by polarized light optical microscopy, PLOM. The signal of the laser beam during the filling and the crystallization stages of the injection molding of these materials was found to be reproducible. The measurements showed that this system was sensitive to variations of the crystallization of different types of polymers under different processing conditions. The system was not able, however, to monitor the crystallization process when the crystallinity degree developed by the sample was very low, as in the PET resin. It was also observed that T_w and T_i were the most influential variables on the crystallization kinetics of PBT and PTT. Due to its slower crystallization kinetics, PTT was found to be more sensitive to changes in these parameters than the PBT. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 563–579, 2006     

Modeling parison formation in extrusion blow molding by neural networks

A series of experiments were carried out on the parison formation stage in extrusion blow molding of high‐density polyethylene (HDPE) under different die temperature, extrusion flow rate, and parison length. The drop time of parison when it reached a given length and its swells, including the diameter, thickness, and area swells, were determined by analyzing its video images. Two back‐propagation (BP) artificial neural network models, one for predicting the length evolution of parison with its drop time, the other predicting the swells along the parison, were constructed based on the experimental data. Some modifications to the original BP algorithm were carried out to speed it up. The comparison of the predicted parison swells using the trained BP network models with the experimentally determined ones showed quite a good agreement between the two. The sum of squared error for the predictions is within 0.001. The prediction of the parison diameter and thickness distributions can be made online at any parison length or any parison drop time within a given range using the trained models. The predicted parison swells were analyzed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2230–2239, 2005     

Crystallization kinetics and morphology of poly(ethylene suberate)

The crystallization kinetics and morphology of poly(ethylene suberate) (PESub) were studied in detail with differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction. The Avrami equation could describe the overall isothermal melt crystallization kinetics of PESub at different crystallization temperatures; moreover, the overall crystallization rate of PESub decreased with increasing crystallization temperature. The equilibrium melting point of PESub was determined to be 70.8°C. Ring‐banded spherulites and a crystallization regime II to III transition were found for PESub. The Tobin equation could describe the nonisothermal melt crystallization kinetics of PESub at different cooling rates, while the Ozawa equation failed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43086.     

Hydrogenated petroleum resin effect on the crystallization of isotactic polypropylene

The influence of the hydrogenated petroleum resin P125 on the crystallization behavior, crystallization kinetics, and optical properties of polypropylene (PP) were investigated. The results of differential scanning calorimetry, successive self‐nucleation, and annealing fractionation demonstrated that P125 reduced the interaction between the PP molecules, decreased the crystallization, prevented PP from forming thick lamellae, and encouraged the formation of thin lamellae. The isothermal crystallization kinetics, self‐nucleation isothermal crystallization kinetics, and polarized optical microscopy observations showed that P125 slightly decreased the nucleation rate, significantly decreased the crystal growth rate, generally reduced the overall crystallization rate, and effectively deceased the crystallite sizes of PP. The optical properties studies showed that P125 effectively decreased the haze and increased the surface glossiness and yellowness index of PP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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     

Investigation of the crystallization behavior of isotactic polypropylene polymerized with different Ziegler‐Natta catalysts

Detailed characterization of the crystallization behavior is important for obtaining better structure property correlations of the isotactic polypropylene (iPP), however, attributed to the complexity in ZN‐iPP polymerization, the relationship between crystallization behavior and the stereo‐defect distribution of iPP is still under debate. In this study, the crystallization kinetics of the primary nucleation, crystal growth and overall crystallization of two iPP samples (PP‐A and PP‐B) with nearly same average isotacticity but different stereo‐defect distribution (the stereo‐defect distribution of PP‐B is more uniform than PP‐A) were investigated. The results of isothermal crystallization kinetics showed that the overall crystallization rate of PP‐A was much higher than that of PP‐B; but the analysis of self‐nucleation isothermal crystallization kinetics and the polarized optical microscopy (POM) observation indicated that the high overall crystallization rate of PP‐A was attributed to the high primary nucleation rate of the resin. The stereo‐defect distribution plays an important role in determining both the nucleation kinetics and crystal grow kinetics, and thus influence the overall crystallization kinetics. A more uniform distribution of stereo‐defects restrains the crystallization rate of iPP, moreover, it has more influence on nucleation kinetics, comparing with the crystal growth. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crystallization behavior and morphology of double crystalline poly(trimethylene terephthalate)/poly(ethylene oxide terephthalate) copolymers

Double crystalline poly(trimethylene terephthalate)/poly(ethylene oxide terephthalate) copolymers (PTT/PEOT), with PTT content ranging from 16.5 to 65.5 wt%, were synthesized by melt copolycondensation. The morphological transformation of samples from microphase separation to macrophase separation was investigated by gel permeation chromatography and transmission electron microscopy. Differential scanning calorimetry and in situ wide‐angle X‐ray diffraction suggested that all copolycondensation samples displayed double crystalline behavior. The melt‐crystallization peak temperatures (T_{m, c} values) of PTT chains monotonously increased with increasing PTT content and were higher than that of homo‐PTT when the content of PTT was above 30.6 wt%. Interestingly, T_{m, c} values of PEOT chains were also increased with increasing PTT content. Polarized optical microscopy revealed that all copolycondensation samples studied could form ring‐banded spherulites and band spacing increased with increasing T_c values. In addition, band spacing decreased with increasing PTT content at a given T_c. Strangely, although PEOT was the main component in all copolycondensation samples, spherulitic morphology formed by the advance crystallization of PTT did not change after PEOT crystallization. Only a subtle change of quadrant tones was detected. © 2012 Society of Chemical Industry     

Kinetic modeling of polypropylene thermal oxidation during its processing by rotational molding

The main drawback of rotational molding is a long stay (several dozens of minutes) of polymer in melt state at high temperature in atmospheric air. To prevent any significant polymer thermal degradation, it is necessary to define, preliminary, a processing window in a temperature‐molar mass map. The objective of this article is to elaborate and check the validity of a general thermal degradation model devoted to determine, in a near future, some important boundaries of this processing window. This model is composed of two distinct levels: (i) The first level is derived from the thermal transfer mechanisms occurring during a processing operation, polymer phase changes (i.e., melting and crystallization) being simulated by the enthalpy method; and (ii) The second level is derived from the oxidation mechanistic scheme of free additive polymer in melt state established in a previous study, but completed, here, by adding the main stabilization reactions of a common synergistic blend of antioxidants, widely used for rotational molding polymer grades. By juxtaposing such “thermal” and “chemical” levels, it is possible to predict the polymer thermal degradation during a whole processing operation. The validity of both levels is successfully checked in real rotational molding conditions for polypropylene (PP). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci 000: 000–000, 2010     

Fabrication of highly isotactic polypropylene fibers to substitute asbestos in reinforced cement composites and analysis of the fiber formation mechanism

Highly isotactic polypropylene (PP) is currently studied as a cement‐reinforcement fiber that could potentially be substituted for asbestos because of its resistance to prolonged high‐temperature curing. The higher the isotacticity of the PP fiber is, the higher the tensile modulus and breaking strength of the cured fiber are. The PP fiber that exhibits a isotacticity of 99.6% (XI) and draw ratio of 6.0 retains a tensile modulus of 4.23 GPa, even after high‐temperature curing at 175°C for 5 h. PP fiber is cut into 6‐mm lengths and dispersed throughout a cement mixture to prepare a reinforced cement composite. The mixture is cured in an autoclave at 175°C for 5 h. The Charpy impact strength and flexural strength of the obtained cement composite tends to increase with increasing PP isotacticity. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 981‐988, 2013