Effects of montmorillonite on structures and properties of injection-molded polypropylene

A nanocomposite of polypropylene with montmorillonite (PP-MMT) was prepared via melt blending. The structures and properties of PP-MMT were studied by differential scanning calorimetry, polarized optical microscopy, transmission electron microscopy (TEM), and wide-angle X-ray diffraction (WAXD). The orientation behaviors of both MMT and PP in an injection-molded specimen were revealed by TEM and WAXD. The results indicate that the specimen displays a multilayer structure: in the inner layer, the long axes of MMT stacks are parallel to the transverse direction (TD) of the specimen and the b-axes of PP crystallites are perpendicular to the TD; whereas in the outer layer, the long axes of MMT are perpendicular to the TD and the b-axes of PP crystallites are parallel to the TD. The orientation of PP crystallites in PP-MMT is much lower than that in pure PP, which results in lower molding shrinkage of PP-MMT. Structure–property relationship of PP-MMT was discussed with a conclusion that a moderate improvement in performances of PP-MMT might be derived from the intercalated dispersion and special orientation of MMT stacks. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47442.     

Effects of injection molding conditions on the electrical properties of polycarbonate/carbon nanotube nanocomposites

Polycarbonate/Carbon nanotube (PC/CNT) nanocomposites containing various CNT contents (0–5 wt%) were prepared by injection molding. The effects of CNT contents, injection speed (V) and injection temperature (T) on the electrical resistivity of the PC/CNT nanocomposites were investigated. It was found that the tensile strength of nanocomposites was enhanced slightly with increased CNT contents, and the tensile modulus was 29% greater after the 5 wt% CNT addition, but the brittle tendency became stronger. Aside from tensile properties, the electrical resistivity of the nanocomposites dropped 12 orders of magnitude after the 5 wt% CNT addition. Also, there was a tendency that the electrical resistivity was lower in the case of higher injection temperature and lower injection speed. Scanning electron microscope (SEM) images and the distribution of surface layer electrical resistivity, clearly showed a notable influence by surface layer microstructures on the electrical resistivity, and the injection conditions affected both the value of the maximum electrical resistivity and the position where it occurred. This study offers an alternative green and simple molding process to prepare conductive PC nanocomposites and to achieve the industrialization of PC/CNT nanocomposite products which can be used in electromagnetic shielding and anti‐static fields. POLYM. COMPOS., 37:3245–3255, 2016. © 2015 Society of Plastics Engineers     

Fracture and impact properties of polycarbonates and MBS elastomer-modified polycarbonates

Mechanical properties of polycarbonates (PCs) and elastomer-modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile–brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer-modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (G_c) measured at ?30°C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found at M_n = 6800 or MFR = 135. The G_c of the elastomer-modified PC (MFR = 15, 5% elastomer) is about twice that of thee unmodified one. The presence of elastomer in the PC matrix promotes the plane–strain localized shear yielding to greater extents and thus increases the impact strength and G_c in a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer-toughening mechanism. The plane–strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane–stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer-modified PC (10% elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic-zone volume is proposed.     

The fire-resistance properties of polycarbonates

Summary The present paper describes two types of polycarbonates possessing fire-resistance. The first type is obtained by reacting dialkyl bis(hydroxy-4-phenyl) alkyl phosphonates with phosgene and the second type is a copolymer of bisphenol A and dialkyl bisphenol phosphonates.We show that the incorporation of dialkyl bisphenol phosphonates into the bisphenol A polycarbonate chains can be satisfactorily carried out using interfacial polycondensation.The products with groups-P (OCH₃)₂ possess high oxygen indices (OI).     

Effects of moisture on properties of epoxy molding compounds

In this article the effects of moisture on a novel epoxy molding compound, including the mechanical properties, temperature transition, and thermal degradation behavior, are studied. The experimental results reveal that the absorbed water acts predominantly as a crazing agent, continuously decreasing the mechanical strength with the time in water. The glass-transition temperature decreases at the early stage and is finally equilibrated. The thermal degradation behavior of the materials is not greatly influenced by the hydrothermal age. The decomposition of samples in oxygen is composed of two independent steps: the thermal degradation and oxidation at high temperature. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2253–2259, 2001     

Effect of processing conditions on morphology and mechanical properties of injection-molded poly(l-lactic acid)

This work investigates the relationship among the processing, morphology, and the mechanical properties of injection-molded poly(L -lactic acid) (PLLA). Melt processing temperature, mold temperature, injection flow rate, and holding pressure were systematically changed following a design of experiments array. The thermomechanical environment imposed during processing was estimated by computer simulations for the mold-filling phase, which allows the calculation of shear stress, shear rate, and the thickness of frozen skin layer. The morphology was characterized by differential scanning calorimetry and hot recoverable strain measurements. The analysis of variance results of influence of processing factors on the morphology are in good agreement with the analysis of thermomechanical parameters on the morphology. The primary factor for inducing the crystallinity in PLLA product was the stress-induced crystallization, whereas the thermal induced crystallization had a little effect. The morphology–mechanical property relationships were established. The crystallinity developed during processing has little effect on elastic modulus, increases the yield strength, and severely decreases the elongation at break. The level of molecular orientation developed during processing has little effect on elastic modulus, but increases both the yield strength and the elongation at break. POLYM. ENG. SCI., 47:1141–1147, 2007. © 2007 Society of Plastics Engineers     

Effects of PVP incorporation on the properties of injection-molded high-performance ceramics with PEG-based binders

PEG/PMMA binder systems are among the most widely used water-soluble binders for ceramic injection molding. Binder-induced binder segregation in PEG/PMMA systems is mainly caused by the difference between crystallization temperature of PEG (below 50 °C) and glass transition temperature of PMMA (~120 °C). In this study, we have adopted the method of introduction of appropriate amount of PVP to reduce the binder-induced defects. A principle model was thus established for the suppression of PEG crystallization by PVP molecules. It was revealed that the mechanism by which PVP suppresses PEG crystallization is hydrogen-bonded formation between the carbonyl group of PVP and the hydroxyl groups of PEG molecules. This compresses the crystallization space of PEG molecules, and suppresses their crystallization. It was determined that the range of PVP content that can be safely removed via water debinding is 0–30 wt/wt% PEGS. The incorporation of PVP could effectively enhance the homogeneity of the compact, reduce the size of pores formed after thermal debinding in the compact, promote densification, reduce defects after sintering and enhance the mechanical performance of the sintered bodies. Furthermore, the performance of the molded product was optimal when 20 wt/wt% PEGS of PVP was added to the binder; the flexural strength and density of the resulting sintered body were 811 MPa and 98.7%, respectively.     

Mechanical properties of selected injection-molded thermosets

Practical ways of processing thermoset materials by injection molding have been developed. These methods have been used particularly in the automotive and electrical industries. The tensile behavior is discussed for specimens which were injection molded from reinforced and filled phenolic unsaturated polyester and epoxy resin. This behavior aids the layout of flow paths. Under static loads the materials show less creep than most thermoplastics. “Post shrinkage” was observed as a shortening of specimens under prolonged heating. Because of brittleness, the materials failed under dynamic load at low strain. The first microcracks directly initiate the final failure. Before the final failure, the materials craze at a certain strain. By means of different testing methods these strain values were determined. For practical applications, these values should be considered in the designing of thermoset parts.     

Impact of compression molding conditions on the thermal and mechanical properties of polyethylene

Compression molding is a current technique in polymer processing. Despite numerous studies, effect of molding pressure on physical properties has surprisingly not been fully investigated. In this study, the thermal and mechanical behavior of the compression‐molded polyethylene were thus explored to better grasp the relationship between processing parameters and ensuing properties. The effect of the molding temperature, pressure, cooling rate, and temperature profile on the tensile and flexural moduli as well as melting point of polyethylene was studied. We conclude that higher tensile and flexural moduli are obtained by increasing pressure and molding temperature, as well as decreasing the cooling rate. Our results were corroborated by X‐ray diffraction and differential scanning calorimetry measurements. Moreover, the use of a temperature gradient with different temperatures for the upper and bottom plates of the mold leads to asymmetric samples whose tensile and flexural moduli are improved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46176.     

Effects of mold surface conditions on flow length in injection molding process

In this study, to clarify the influence of a mold on thin‐wall molding, the effect of different mold surface conditions on the flow length and mobility (i.e., ease with, which melted plastics can be filled into the mold) in an injection molding process was investigated. Three different coatings were used for the mold surface. Several degrees of roughness were also selected for the mold surface. The results were evaluated by comparing flow length with interfacial tensions, which were derived from Young's formula. Although the interfacial tension exhibited different values, the influence on flow length was generally found to be small. On the other hand, in the mold that gives surface roughness, though the change of interfacial tension was small compared with coatings, the flow length increased linearly with the surface roughness when the roughness exceeded a certain level roughness. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Effects of molding conditions on the electromagnetic interference performance of conductive ABS parts

Polymers filled with conducting fibers to prevent electromagnetic interference (EMI) performance have recently received great attention due to the requirements of 3C (computer, communication, and consumer electronics) products. In the present article, the effect of fiber content and processing parameters, including melt temperature, mold temperature, and injection velocity, on the electromagnetic interference shielding effectiveness (SE) in injection molded ABS polymer composites filled with conductive stainless steel fiber (SSF) was investigated. The influence of fiber orientation and distribution resulting from fiber content and molding conditions on EMI performance was also examined. It was found from measured results that fiber content plays a significant role in influencing part EMI SE performance. SE value can reach the highest values of approximately 40 dB and 60 dB at 1000 MHz frequency for fiber content 7 wt % and 14 wt %, respectively, under the best choice of molding conditions. Higher melt and mold temperature would increase shielding effectiveness due to a more uniform and random fiber orientation. However, higher injection velocity leading to highly‐orientated and less uniform distribution of fiber reduces shielding effectiveness. Among all molding parameters, melt temperature affects SE performance most significantly. Its influence slightly decreases as fiber content increases. Injection speed plays a secondary importance in affecting SE values, and its influence increases as fiber content increases. Upon examination of fiber distribution via optical microscope and subsequent image analysis, it was found that the fiber becomes more densely and random distributed toward the last melt‐filled region, whereas fiber exhibits less concentration around the middle way of the flow path. This can be attributed to the combined effects of fountain flow, frozen layer thickness, and gapwise melt front velocity. The results indicate that molding conditions, instead of fiber content alone, are very important on the SE performance for injection molded SSF filled ABS composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1072–1080, 2005     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Effect of molecular weight on the thermal properties of polycarbonates

The relationships between the molecular weight and several thermal properties of 2,2-bis(4-hydroxy-phenyl)propane polycarbonate are reported. It is shown that the amount of thermal degradation, measured thermogravimetrically, increases with decrease in molecular weight and is affected by the nature of the end groups present. The heat of fusion at the crystalline melting point has been measured by differential scanning calorimetry and correlated with density and X-ray diffraction estimates of total crystallinity. The heats of melting obtained were 30.4 cal/g and 32.9 cal/g respectively. The effect of molecular weight on specific heat, annealing peaks and glass transition temperature are also reported.     

Effects of molding temperature and pressure on properties of soy protein polymers

Because of the worldwide environmental pollution problem with petroleum polymers, soy protein polymers have been considered as alternatives for biodegradable plastics. The objective of this research was to study the curing behavior of soy protein isolates (SPIs) for that application. The molding variables of temperature, pressure, and time and curing quality factors of tensile strength, strain, and water resistance were evaluated. The maximum stress of 42.9 MPa and maximum strain of 4.61% of the specimen were obtained when SPI was molded at 150°C and 20 MPa for 5 min. The water absorption of the specimen decreased as molding temperature and time increased. Glycerol greatly improved the flexibility of the specimen but decreased its strength. For SPI with 25% glycerol added, the maximum stress and strain of about 12 MPa and 140%, respectively, were achieved when the specimen was molded at 140°C for 5 min. Molding temperature, pressure, and time are major parameters influencing the curing quality of soy protein polymers. At fixed pressure, the molding temperature and time had significant interactive effects on curing quality. At high temperature (e.g., at 150°C) it took about 3 min to reach optimum curing quality; however, at low temperature (120°C) it took about 10 min to reach optimum curing quality. The maximum strength and strain of the cured protein polymer occurred at the molding temperature close to its phase transition temperature or about 40°C below its exothermic temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2595–2602, 1999     

The effects of molding conditions on the surface composition of sheet molding compound

An improved internal reflection infrared spectroscopy (ATR) technique (1) has been found to be effective for measuring the relative concentrations of polyester and polystyrene (resin) and calcium carbonate filler on sheet molding compound (SMC) surfaces. The technique has been used to determine the effect of molding conditions on the surface compositions of three commercial SMC materials. The surface compositions of two of the materials, of the same formulation but obtained from different sources, were the same and were unaffected by molding conditions. The surface of the third material (or a different formulation) was found to have substantially less resin than the first two materials. The surface composition of the third material varied with molding conditions, the greatest uniformity being obtained with high molding temperatures and pressures. This study has shown that the ATR technique is suitable for determining the relative surface compositions of SMC formulations. This method will be used to correlate the SMC surface composition to SMC properties, such as surface appearance, paintability, and adhesive bond durability.     

Phase morphology of injection-molded blends of nylon-6 and polyethylene and comparison with compression molding

An experimental study is presented of the development of phase morphology in injection and compression molding of 80/20 and 60/40 nylon-6/polyethylene blends and its evolution during annealing. The phase morphology of the compression-molded part is isotropic, while that of both screw and ram injection-molded parts is both heterogeneous and anisotropic through the cross section. In the injection-molded parts, the morphology exhibits its greatest level of isotropy in the core and becomes increasingly anisotropic approaching the mold wall (through the thinnest cross section); i.e., the 2 direction. There is an intermediate position of maximum anisotropy. Sections made in various directions indicate that the dispersed phase forms platelets oriented in the plane of the machine and width directions. The position of maximum anisotropy seems to represent a frozen layer thickness that increases with decreasing mold temperature and with decreasing injection rate into the mold. Annealing studies show coalescence in both compression- and injection-molded bars. The greatest coalescence occurs in the injection-molded parts near the positions of greatest anisotropy of morphology. This suggests that coalescence is associated with collisions between dispersed phase globules as their sphericity increases.     

提高注塑件性能的熔体振荡技术

目前有一种新开发的注射熔体振荡技术,无需添加填料和助剂,就可提高注塑部件的机械特性。这项技术还可转移、隐藏或减少部件表面的注射熔接痕,并纠正制件取向应力等缺陷。     

Influence of crystallinity on rheological properties of unfilled and particle-filled polycarbonates

Polycarbonates are modified, nowadays, with various kinds of fillers from the micro to the nano range in order to improve properties like the electrical conductivity, for example. Rheological measurements are often performed to support the characterisation of the compounds. In this communication it is demonstrated that the crystallisation of polycarbonates in the molten state has to be taken into account for a comprehensive assessment of rheological experiments. This is particularly evident in the case of filled materials as particles can act as nucleating agents. The crystallisation behaviour of neat and filled polycarbonates is investigated by differential scanning calorimetry and its influence on rheological properties presented by dynamic-mechanical and creep experiments. From the results, the role of crystallisation processes becomes evident for a reliable interpretation of rheological investigations on polycarbonates and their modelling. Comparative measurements on the non-crystallisable polymethylmethacrylate underline the importance of a thorough discussion of the crystallinity in the case of polycarbonates.     

Effect of molding conditions on crystallization kinetics and mechanical properties of poly(lactic acid)

Although Poly(lactic acid) (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly, to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a post‐molding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate. POLYM. ENG. SCI., 57:306–311, 2017. © 2016 Society of Plastics Engineers