Effect of melt temperature and skin‐core morphology on the mechanical performance of nylon 6

The crystalline texture and mechanical (tensile and flexural) properties of injection molded nylon 6 were evaluated to understand the influence of the melt temperature (T_mlt), one of the key‐processing variables. The mechanical properties are found to be sensitive to T_mlt only below ～ 250°C. Rapid quenching of the surface produces a skin with lower crystallinity than the core, which cools more slowly; because of this difference in the rate of cooling, the crystalline component in the skin is rich in γ and that in the core is rich in α. The thickness of this skin decreases from about 1.25 mm to 0.75 mm as T_mlt increases from 225°C to 310°C. Higher tensile strength at yield, lower elongation at break and higher flexural strength were observed in specimens molded at lower T_mlt. These characteristics are associated with thicker and less ordered skin, and a lower crystallinity core. The role of the T_mlt, on microstructure and mechanical properties of injection molded nylon 6, the development of skin‐core morphology, and the role of the residual stresses in the core are discussed.     

ELIMINATING FLOW INDUCED BIREFRINGENCE AND MINIMIZING THERMALLY INDUCED RESIDUAL STRESSES IN INJECTION MOLDED PARTS*

Eliminating flow-induced birefringence and stresses and reducing thermally induced stresses in the injection molded parts have been studied using rapid thermal response (RTR) molding technique. In the RTR molding, mold surface temperature can be rapidly raised above T _g in the filling stage, while the normal injection molding cycle time is still maintained. Therefore, the melt can fill the cavity at temperatures above T _g, which enables the flow-induced stresses to relax completely in a short time after filling and before vitrification. Residual stresses and birefringence in a RTR molded strip specimen are compared with the conventional molded parts by applying layer removal method and retardation measurement. For the material (Monsanto® Lustrex Polystyrene) and process conditions chosen, the birefringence level decreased as the RTR temperature approached and exceeded the glass transition temperature until it almost disappeared at a RTR temperature of 180°C. Reduction of magnitude and shift of peak location were observed in the gapwise stress profile for RTR molded specimen.

     

The phase behavior and mechanical properties of polyarylate and polycarbonate blends

The phase behavior and mechanical properties of a series of polyarylate/polycarbonate blends were studied. The polymers are known to transesterify, the extent of which depends upon the thermal and shear history and affects phase behavior and properties. Single screw extrusion, twin screw extrusion, and solution casting were employed for blend preparation. Two transition temperatures, corresponding to a polycarbonate-rich phase and to a polyarylate-rich phase, were seen in blends that were solution cast or compounded in a single screw extruder at 285°C. But after injection molding a single T_g was observed, When annealed at 180°C for several hours the molded blend was found to phase separate. Blends that were compounded in a twin screw extruder exhibited a single T_g and could not be phase separated. The flexural and tensile properties of blends that were prepared in a twin screw extruder show a small positive synergism. But the impact properties were substantially below the rule of mixtures values, probably the result of advanced exchange reaction and thermal degradation.     

Effect of crystallization temperature on crystal modifications and crystallization kinetics of poly(L‐lactide)

The crystalline structure of poly(L ‐lactide) (PLLA) have been found to quite depend on the crystallization temperatures (T_cs), especially in the range of 100?120°C, which is usually used as the crystallization temperature for the industrial process of PLLA. The analysis of wide‐angle X‐ray diffraction and Fourier transformed infrared spectroscopy revealed that 110°C is a critical temperature for PLLA crystallization. At T_c < 110°C and T_c ≥ 110°C, the α′ and α crystals were mainly produced, respectively. Besides, the structural feature of the α′‐form was illustrated, and it was found that the α′‐form has the larger unit cell dimension than that of the α‐form. Moreover, the crystallization kinetics of the α′ and α crystals are different, resulting in the discontinuousness of the curves of spherulite radius growth rate (G) versus T_c and the half time in the melt‐crystallization (t_1/2) versus T_c investigated by Polarized optical microscope and Differential scanning calorimetry, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microcellular injection molding of in situ modified poly(ethylene terephthalate) with supercritical nitrogen

The microcellular injection molding (commercially known as MuCell) of in situ polymerization‐modified PET (m‐PET) was performed using supercritical nitrogen as the physical blowing agent. Based on the design of experiment matrices, the influence of operating conditions on the mechanical properties of molded samples was studied systematically for two kinds of m‐PETs, namely, n‐m‐PET and m‐m‐PET synthesized using pentaerythritol and pyromellitic dianhydride (PMDA) as modifying monomers, respectively. Optimal conditions for injection molding were obtained by analyzing the signal‐to‐noise (S/N) ratio of the tensile strength of the molded samples. The specific mechanical properties, especially the impact strength, of the microcellular samples under those optimal conditions increased significantly. Scanning electron microscope analyses showed a uniform cell structure in the molded specimens with an average cell size of around 35 µm. The m‐m‐PET modified with PMDA generated a slightly finer cell structure and a higher cell density than the n‐m‐PET. POLYM. ENG. SCI., 54:2739–2745, 2014. © 2013 Society of Plastics Engineers     

Regulation of trans‐1,4‐polyisoprene crystallinity and mechanical properties of styrene‐butadiene rubber/trans‐1,4‐polyisoprene vulcanizate

The effects of processing temperature and bis‐[γ‐(triethoxysilyl)‐propyl]‐tetrasulfide (Si69) on crystallization, morphology, and mechanical properties of styrene‐butadiene rubber (SBR)/trans‐1,4‐polyisoprene (TPI) vulcanizate are investigated. The crystallinity and crystalline melting temperature (T_m) of TPI in the vulcanizates with TPI/silica/(Si69) pre‐mixed at 150 °C are much lower than that pre‐mixed at 80 °C. At the same pre‐mixing temperature, the presence of 1 phr Si69 leads to a decreased crystallinity and T_m. The TPI domains with phase size of about 1 μm and silica are well dispersed in the vulcanizate, and TPI crystals get smaller in size and less in amount by pre‐mixing TPI, silica and Si69 at 150 °C. The vulcanizates with TPI/silica/(Si69) pre‐mixed at 150 °C have decreased tensile strength and modulus at a given extension than that pre‐mixed at 80 °C. At the same pre‐mixing temperature, the tensile strength and modulus of the vulcanizate increase with the addition of 1 phr Si69. The crystallinity of TPI component in SBR/TPI vulcanizate is effectively controlled by changing processing temperature and adding Si69, which is important for theoretical research and practical application of TPI. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44395.     

Effect of high‐temperature annealing on the elastoplastic response of isotactic polypropylene in loading–unloading tests

A series of uniaxial tensile loading–unloading tests is performed on isotactic polypropylene at room temperature. Prior to mechanical testing, injection‐molded specimens are annealed for 24 h at temperatures T = 145, 150, 155, 158, 160, 163, and 165°C, which cover the entire region of high‐temperature annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer at small strains. The stress–strain relations are determined by six adjustable parameters that are found by matching observations in cyclic tests. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. It is shown that all material constants are affected by the annealing temperature, which is explained by changes in the crystalline morphology driven by thermal treatment. Some of the adjustable parameters experience finite jumps in the vicinity of the critical temperature T_c = 159°C. These jumps are attributed to the α₂ → α₂′ phase transformation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 186–196, 2003     

The temperature and strain-rate dependence of mechanical properties in polyoxymethylene

The computer aided design approach used in current applications of semicrystalline polyoxymethylene (POM) requires high strain-rate mechanical data. The primary aim of this work has been to measure the room temperature modulus and tensile strength of injection molded samples of POM of different molecular weights at cross-head speeds of between 10^?5 ms^?1. We observe no major transition in bulk mechanical behavior in this range of test speeds, the Young's modulus E, in particular, showing little strain rate dependence. This is rationalized on the basis of tensile tests over a range of temperatures, these indicating room temperature to correspond to the plateau in the E(T) curves (T_g for these materials is taken to be ?70°C, and the DSC melting onset occurs at ～ 170°C). The tensile strength increases as ～log(d?/dt) and the behavior is found to be highly nonlinear, strains to fail of the order of 1 being observed even at the highest strain rates, depending on the molecular weight. It is believed that the yield stress of th crystalline regions determines the tensile strength above T_g, the higher degree of crystallinity associated with lower molecular weights resulting in a slightly higher tensile strength. Nevertheless, failure is qualitatively brittle, with no necking and relatively little permanent deformation. This behavior is discussed in terms of morphological investigations of the fractured samples by optical and scanning electron microscopy (SEM). In attempting to relate ultimate failure to the molecular/crystalline structure of the samples, measurements of the critical stress intensity for crack initiation in mode I opening, K_IC, as a function of crystallization temperature T_c have been carried out using compact tension specimens machined from injection molded and compression molded plaques. K_IC increases with molecular weight and decreases with T_c at low test speeds (in spite of an increase in crystallinity with T_c). This is accounted for in terms of a crack shielding model for crack initiation and of molecular rearrangements occurring during crystallization which lead to a decrease in the effective entanglement density with T_c. The implications of this model are then compared with K_IC results over a range of cross-head speeds and temperatures.     

Evaluation of molecular orientation in injection molded parts with microscale features by Raman spectroscopy

Molecular orientation of polycarbonate (PC) in injection‐molded parts with microscale features was characterized by means of polarized Raman spectroscopy, and the relationship between microstructure and replication was discussed. The microscale feature size of continuous v‐groove was 20 μm in depth and 50 μm in width. PC injection‐molded parts were molded with various molding conditions. The molecular orientation distribution along flow direction on the cross‐section of molding parts were evaluated by the intensity ratio of the bands at 635 to 703 cm^?1 (I₆₃₅/I₇₀₃) in the Raman spectra. Molecular orientation along the flow direction inside the v‐groove was higher than that of the core and the opposite surface region. In particular, the highest molecular orientation was at the surface of the v‐groove. Among the injection molding conditions, the mold temperature showed significant effect on the molecular orientation and replication. Higher mold temperature caused high replication and low molecular orientation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Structural changes of injection molded starch during heat treatment in water atmosphere: Simultaneous wide and small‐angle X‐ray scattering study

Native starches with wide varying amylose content were processed by injection molding. The injection‐molded materials were conditioned in water for 20 days and sealed in glass capillaries. Simultaneous wide‐ and small‐angle X‐ray scattering (WAXS and SAXS, respectively) were recorded during thermal heating using a synchrotron source. Crystallinity, SAXS invariant, Q, and long period, L, were measured as a function of heating temperature. The injection‐molding process provokes a destruction of the crystal forms A (cereal starch) and B (tubercle starch) but favors a development of the crystal form V_h. After wet conditioning, WAXS of the injection‐molded samples shows again the appearance of the crystal forms A or B, and crystallinity reaches values similar or larger than those of native starch. A constant heating rate (5°C/min) was particularly used for a comparison of potato and corn starch with a similar amylose content. While the crystallinity associated to forms A and B slowly decreases below 55°C and then rapidly decreases until its disappearance at 85–90°C, the invariant shows a maximum around 40°C and rapidly decreases thereafter. The total nanostructure disappearance occurs at temperatures about 10°C higher for the case of potato starch. In addition, a recovery of the WAXS and SAXS maxima during the subsequent cooling process before reaching room temperature was observed only for potato starch. Analysis of WAXS and SAXS for the rest of the starch materials reveals clear differences in the structural parameters of the samples that cannot be easily explained solely on the basis of the amylose content. Thus, for Cerestar and Roquette, it is noteworthy that there was a continuous decrease of L until its total disappearance as well as the persistence of crystallinity (form B), presumably stabilized by the presence of the V_h structure (12–15%). Real‐time crystallization experiments on two amorphous injection molded samples, waxy maize (free amylose starch) and potato starch, are also discussed. It is shown that the absence of amylose delays the recrystallization of amylopectine during the experiment. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 301–309, 2004     

Poly(p‐phenylene sulfide) nanofibers prepared by CO2 laser supersonic drawing

Poly(p‐phenylene sulfide) (PPS) nanofibers are prepared by irradiating a PPS fiber with a carbon dioxide (CO₂) laser while drawing it at supersonic speeds. A supersonic jet is generated by blowing air into a vacuum chamber through the fiber injection orifice. Nanofibers obtained at a laser power of 30 W and chamber pressure of 10 kPa exhibit an average diameter of 600 nm and a draw ratio of 110,000. Scanning electron microscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction analyses are employed to investigate the relationships among the chamber pressure, fiber morphology, and crystallization behavior. The nanofibers exhibit two melting temperatures (T_m): approximately 280°C and 320°C. The endothermic peak at T_m = 280°C is ascribable to lamellar crystals and that at T_m = 320°C to the highly complete crystals, since the polymer molecular chain is highly oriented. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40922.     

Study of PET/PP/TiO2 microfibrillar‐structured composites,Part 2: Morphology and mechanical properties

Uncompatibilized and compatibilized (polypropylene grafted maleic anhydride as compatibilizer) polyethylene terephthalate (PET)/polypropylene (PP)/TiO₂ microfibrillar composites (MFC) were prepared by injection molding of the pelletized PET/PP/TiO₂ drawn strands. The morphology of PET fibrils and the distribution of TiO₂ particles in the composites were examined. After injection molding the preferential location of TiO₂ particles is still preserved. Because of the reinforcement effect of PET fibrils, the tensile properties and impact strength of the PET/PP MFC are improved compared with the pure PP. Incorporation of TiO₂ particles results in decrease of both tensile strength and impact strength of the composites. However, the compatibilized PET/PP/TiO₂ MFC demonstrate better mechanical properties compared with the uncompatibilized ones. DMA analysis shows that the glass transition temperature (T_g) of PET in the uncompatibilized PET/PP/TiO₂ MFC and the T_g of PP in the compatibilized PET/PP/TiO₂ MFC are elevated by about 2°C. The elevation of T_g is attributed to the preferential location of TiO₂ particles in the composites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Non‐isothermal crystallization and multiple melting behavior of syndiotactic polystyrene—pre‐melting temperature effects

This work investigated how pre‐melting temperature (T_max) and cooling rate (C) affected the non‐isothermal melt crystallization, melting behavior and crystal structure of syndiotactic polystyrene (sPS) by using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD) techniques. Experimental results indicated that raising T_max or C decreased the crystallization peak temperature (T_p) and crystallization initiating temperature (T_i). The crystallization kinetics was analyzed through the Ozawa equation. Although the Ozawa exponent n and cooling function K(T) were determined for T_max = 340°C and T_max = 315°C specimens, for T_max = 290°C specimens, the Ozawa equation was not applicable. Activation energies for the non‐isothermal crystallization processes of different T_max specimens were estimated to be approximately 418 kJ/mol. As T_max was raised the nucleation rate of sPS became slower. The multiple melting peaks were associated with different polymorphs as well as recrystallized crystals that formed during heating scans. The percentage content of α polymorph formed in the crystals under various crystallization conditions was estimated through WAXD experiments.     

Processing-property relationships in compression molding of sheet molding compounds

An experimental investigation was conducted into establishing relationships between the processing variables and the mechanical properties of compression-molded parts of sheet molding compounds (SMC). Emphasis was placed on investigating the effects on the tensile properties, impact strength, and dynamic mechanical properties of composite specimens, of low-profile additives, and of treating glass fibers (for reinforcement) with sizing chemicals. The processing variables investigated were cure time, mold temperature, and mold pressure. It was found that: (1) An optimum cure time and mold temperature exist for achieving molded SMC composites of the greatest tensile and impact strengths; (2) Of the four different types of low-profile thermoplastic additives employed, the poly(vinyl acetate) modified with acrylic acid gives rise to molded SMC composites having the greatest tensile and impact strengths; (3) An optimum cure time and mold temperature exist for achieving the highest glass-transition (T_g) of the low-profile additive; (4) The values of cure time and mold temperature that have yielded the greatest tensile and impact strengths also yield molded specimens having the highest T_g of the low-profile additive.     

Properties of molded soy protein isolate plastics

Thermal property of soy protein isolates (SPI) was studied with differential scanning calorimetry and thermogravimetric analysis. The weight loss of pure SPI is about 300°C. The glass transition temperature (T_g) is above 200°C. The best molding temperature of glycerin plasticized SPI plastics were then given. It is between 125 and 140°C. Subsequently the special property of molded SPI plastics was investigated. Results show that the atmosphere humidity affects the mechanical property and thermal property of SPI plastics. With the increasing humidity, the tensile strength decreases. While the elongation at breakage and peak area of the differential scanning calorimetry curve increases. At high temperature even at 140°C the molding temperature SPI plastics still have tensile strength though it decreases with the increasing test temperature while elongation at breakage increases. Dynamic mechanic thermal analysis test show that the storage modulus decreases with the rising temperature. The mechanical loss peak appears at lower temperature with the increasing amount of glycerin content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

The effect of extrusion processing conditions on EVOH/clay nanocomposites at low organo‐clay contents

Ethylene vinyl alcohol (EVOH) copolymer is studied as a host for low concentrations, up to 1 wt%, of organically treated clay. The clay develops a high interaction level with EVOH and thus high torque levels accompany the structuring process leading to the formation of nanocomposites. Extrusion residence time, successive extrusion passes, screw rotational speed, and processing temperature were all found to affect the morphology and the thermal and mechanical properties of the resulting composites. The extrusion compounded composites were subsequently injection molded. A subtle balance of processing parameters is required to achieve improved properties. Long extrusion residence times were found important for good clay dispersion in some cases, whereas in other cases an exfoliated structure was obtained already after the first extrusion pass. Two organically treated clay types processed at the same conditions were examined, and found to result in different morphology and mechanical behavior. Compression molding of extrusion compounded materials, under several extrusion conditions, was studied to illustrate the effect of shear level on the resulting morphology. The delamination level was higher after compression molding compared to that after injection molding. EVOH thermal properties and thermal stability of the related composites were also examined using differential scanning calorimetry and thermal gravimetric analysis. Higher extrusion processing temperature (220 compared to 200°C) was found to change the crystallization process of EVOH in the presence of clay, leading to significant decrease in T_m and T_c compared to that of the neat EVOH. POLYM. COMPOS., 26:343–351, 2005. © 2005 Society of Plastics Engineers     

Effect of molecular weight on brittle‐to‐ductile transition temperature of polyetherimide

The fracture and yield strength of polyetherimide was evaluated over a temperature range of 23 to 140°C for materials with number‐average (M_n) and weight‐average molecular weight (M_w) ranging from 15.6 to 22.8 and 36.6 to 52.3 kg/mol, respectively. The brittle‐to‐ductile transition temperature, where an equal probability exists that an impact will result in a brittle or ductile failure, was determined by evaluating the temperature at which fracture and yield strength are equal. The transition temperature decreased from 155 to 60°C with increasing molecular weight and provided a measure of relative ductility between material samples. As a case study, the practical impact strength of an injection‐molded food service tray was determined at 20°C and correlated with fracture strength as a function of molecular weight. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1666–1671, 2004     

Polylactide‐recycled wood fiber composites

Polylactide (PLA)‐recycled wood fiber (RWF) composites with a small amount of silane were compounded using a kinetic‐mixer and molded using an injection molding machine. The molded PLA‐RWF composites were characterized using gel permeation chromatography, scanning electron microscope, X‐ray diffraction, differential scanning calorimeter, tensile testing machine, and a dynamic mechanical analyzer. As observed in the stress–strain plots, the amount of necking before fracture decreased with an increasing RWF content. Similarly, the strain‐at‐break also decreased with the RWF content. The tensile strength remained the same irrespective of the RWF content. Both the tensile modulus and the storage modulus of the PLA‐RWF composites increased with the RWF content. The degree of crystallinity of the PLA increased with the addition of RWF. No reduction in the number–average molecular weight (M_n) was observed for pure PLA and PLA‐10%RWF‐0.5%Silane composites after injection molding; however, substantial reduction in M_n was found in PLA‐20%RWF‐0.5%Silane composites. Finally, a theoretical model based on Halpin–Tsai empirical relations is presented to compare the theoretical results with that of the experimental results. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical properties and morphology of polypropylene‐calcium carbonate nanocomposites prepared by dynamic packing injection molding

In this article, dynamic packing injection molding (DPIM) technology was used to prepare injection samples of Polypropylene‐Calcium Carbonate (PP/CaCO₃) nanocomposites. Through DPIM, the mechanical properties of PP/nano‐CaCO₃ samples were improved significantly. Compared with conventional injection molding (CIM), the enhancement of the tensile strength and impact strength of the samples molded by DPIM was 39 and 144%, respectively. In addition, the tensile strength and impact strength of the PP/nano‐CaCO₃ composites molded by DPIM increase by 21 and 514%, respectively compared with those of pure PP through CIM. According to the SEM, WAXD, DSC measurement, it could be found that a much better dispersion of nano‐CaCO₃ in samples was achieved by DPIM. Moreover, γcrystal is found in the shear layer of the DPIM samples. The crystallinity of PP matrix in DPIM sample increases by 22.76% compared with that of conventional sample. The improvement of mechanical properties of PP/nano‐CaCO₃ composites prepared by DPIM attributes to the even distribution of nano‐CaCO₃ particles and the morphology change of PP matrix under the influence of dynamic shear stress. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Molding of syndiotactic polystyrene under its melting temperature

Syndiotactic polystyrene (SPS), a thermoplastic polymer that exhibits a high T_m in some crystalline forms, can be conveniently processed by a cold‐compaction technique. Processing temperatures in the range of 150–210°C, well below the T_m, gives rise to physicomechanical properties comparable and even better than those obtained by thermal compression or injection molding. The optimum treatment temperature seems to fall around 175°C. X‐ray diffraction analysis, thermal analysis, and density measurements suggest that such behavior is connected to phase transitions of SPS and favored by the presence of styrene included in the crystalline fraction. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 377–383, 2001