Optimizing impact properties of pp composites by control of spherulitic morphology

The objective of this work was to investigate methods for increasing the impact resistance of composite sheets made from glassfiber mat–reinforced polypropylene prepeg by varying its thermal history. A 60:40 (wt %) mixture of woven glass fiber–PP was crystallized at various temperatures and times to examine the effect of the thermal history (during cooling from the melt) on the degree of crystallinity and spherulitic morphology and to study the relationship between these factors and mechanical properties. The composite laminates were manufactured within a flat mold using a compression molding press and then crystallized from the melt in the range 106–156°C for 10, 30, 60, and 240 min in an air oven. The degree of crystallinity that developed in the matrix polymer was determined using differential scanning calorimetry (DSC), and the matrix morphology was examined by permanganic etching followed by scanning electron microscopy. The highest peak and failure energies during impact were achieved when maximum crystallinity was produced in the specimen crystallized at 134°C for 4 h. Electron microscopy of etched interior sections of impacted specimens has enabled a more detailed understanding of the impact behavior of these materials. The greatest improvement appears to result from an increase in the propensity of cracks to propagate along spherulitic boundaries by virtue of mechanisms facilitated by the results of the differential contraction of the crystalline and amorphous phases within the polypropylene. Differential contraction of the glass and polypropylene appears to be a less important factor, although voids created by such processes have to be taken into account. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1704–1715, 2001     

Crystallization and fiber/matrix interaction during the molding of PEEK/carbon composites

The objective of this work was to investigate the effects of molding conditions (molding temperature, residence time at melt temperature, and cooling rate) on the crystallization behavior and the fiber/matrix interaction in PEEK/carbon composites made from both prepreg and commingled forms. In order to investigate the crystallization behavior of the PEEK matrix, the molding process was simulated by differential scanning calorimetric analysis, DSC. The results show that the prepreg and commingled systems do not have the same matrix morphology; prepreg tape was found to be at its maximum of crystallinity, whereas the commingled system was found to be only partially crystalline. The results show that processing must be carried out at a temperature sufficiently high to destroy the previous thermal history of the PEEK matrix; this is an essential requirement to produce efficient fiber/matrix adhesion in the commingled fabric system. Optical microscopic observations also suggest that matrix morphology near the fibers is dependent on the melting conditions; a well-defined transcrystalline structure at the interface is observed only when the melt temperature is sufficiently high. However, the high temperature of molding can easily result in degradation of the PEEK matrix such as chain scission and crosslinking reactions. Thermal degradation of the matrix during processing is found to affect the crystallization behavior of the composites, the fiber/matrix adhesion, and the matrix properties. This effect is more important in the case of a commingled system containing sized carbon fibers because the sizing agent decomposes in the molding temperature range of PEEK/carbon composites. This produces a decrease of the matrix crystallinity and an elimination of the nucleating ability of the carbon fibers. A transition between cohesive and adhesive fracture is observed when the cooling rate increases from 30°C/min to 71°C/min for the composite made from the commingled fabric. This critical cooling rate is found to closely correspond to a change in the mechanism of crystallization of the PEEK matrix.     

Effect of crystallization conditions on spherulitic texture and tensile properties of sPS/PPO blends

In the second study on melt‐miscible syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, the effect of processing conditions on morphology, ultimate tensile properties, and the mode of fracture is reported. Bulk samples of the blends were molded and then crystallized from melt as well as from the quenched state at different temperatures. The spherulitic morphology of the melt‐crystallized blends, observed by scanning electron microscopy, revealed formation of complete, well‐developed spherulites whose texture increased in coarseness with increasing crystallization temperatures. In all the cold‐crystallized blends lamellar bundles formed a meshlike structure whose texture did not vary significantly with crystallization temperature. Depending on the crystallization temperature, 50/50 melt‐crystallized blends showed varying tensile properties and different modes of failure. In the samples with the largest amorphous domain size of 0.6 μm, the amorphous ellipsoids were cold drawn into fibrils during tensile loading and very high tensile strengths were recorded. The tensile properties for the other melt‐crystallized and all cold‐crystallized blends did not vary substantially with the changing crystallization temperature. The micrographs of the fractured surfaces of the melt‐crystallized blends suggested that, although intraspherulitic fracture occurred at low crystallization temperatures, interspherulitic fracture took place at high crystallization temperatures. The correlation of the morphology and mechanical properties suggests that melt‐miscible blends have good interfacial adhesion between phases and that, by varying composition and processing conditions, it might be possible to control amorphous domain sizes, which is critical in achieving better mechanical properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1984–1994, 2003     

Effect of thermal history on mechanical properties of polyetheretherketone below the glass transition temperature

Tensile properties of polyethertherketone (PEEK) have been studied at 125, 25, and ?100°C for thin films prepared with different thermal histories. Initial morphology was controlled by rate of cooling from the melt. Amorphous films resulted from quenching the melt, while semicrystalline films were obtained by cooling the melt at different rates, or by crystallization of the rubbery amorphous state. The films were characterized using density, X-ray scattering, differential scanning calorimetry, and infrared spectroscopy. Scanning electron microscopy was used to examine fracture surfaces. Degree of crystallinity and rate of cooling from the melt affected the tensile properties at all test temperatures. For films with nearly the same degree of crystallinity, those which were more slowly cooled from the melt fractured at the lowest strain. The amorphous films were most tough, drawing to 233% at ?100°C and to over 500% at 125°C. Films crystallized from the rubbery amorphous state had stress–strain behavior intermediate between that of the amorphous and melt-crystallized films at all test temperatures. Density measurements on the drawn material indicate that void formation occurs simultaneously with the formation of fibrillar crystals. Necking resulted in density increases for amorphous films, and density decreases for the semicrystalline films.     

Crystallization of polymer melts under fast cooling. I: Nucleated polyamide 6

Samples of nucleated polyamide 6 were quenched from the melt so as to monitor cooling histories. Their morphology and crystal structures were discussed in relation to sample thermal history; cooling history relevant to quenched sample morphology was recognized as being confined in the interval 110 to 160°C. In particular, samples were essentially amorphous for cooling rates larger than 200°C/s. The γ-crystalline form was observed for cooling rates in the interval 70 to 200°C/s, whereas α crystalline structure was found dominant for yet lower cooling rates. Crystallinities evaluated from densities of quenched samples were compared with predictions of an isokinetic form of the Avrami model for polymer crystallization kinetics.     

Bleached extruder chemi‐mechanical pulp fiber‐PLA composites: Comparison of mechanical,thermal, and rheological properties with those of wood flour‐PLA bio‐composites

An environmentally friendly bleached extruder chemi‐mechanical pulp fiber or wood flour was melt compounded with poly(lactic acid) (PLA) into a biocomposite and hot compression molded. The mechanical, thermal, and rheological properties were determined. The chemical composition, scanning electron microscopy, and Fourier transform infrared spectroscopy results showed that the hemicellulose in the pulp fiber raw material was almost completely removed after the pulp treatment. The mechanical tests indicated that the pulp fiber increased the tensile and flexural moduli and decreased the tensile, flexural, and impact strengths of the biocomposites. However, pulp fiber strongly reinforced the PLA matrix because the mechanical properties of pulp fiber‐PLA composites (especially the tensile and flexural strengths) were better than those of wood flour‐PLA composites. Differential scanning calorimetry analysis confirmed that both pulp fiber and wood flour accelerated the cold crystallization rate and increased the degree of crystallinity of PLA, and that this effect was greater with 40% pulp fiber. The addition of pulp fiber and wood flour modified the rheological behavior because the composite viscosity increased in the presence of fibers and decreased as the test frequency increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44241.     

Mechanical and thermal properties of environment-friendly “green” composites made from pineapple leaf fibers and poly(hydroxybutyrate-co-valerate) resin

This paper presents the mechanical and thermal properties of unidirectional, degradable, environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) (PHBV) resin. Tensile and flexural properties of the “green” composites with different fiber contents were measured in both longitudinal and transverse directions. Compared to those of virgin resin, the tensile and flexural strengths of “green” composites are significantly higher in the longitudinal direction while they are lower in the transverse direction. However, the mechanical properties are lower than those predicted by simple models. Scanning electron microscope (SEM) photomicrographs of the tensile fracture surfaces demonstrate fibers being pulled out from the matrix, the interfacial failure, fiber fibrillation, and the nonunidirectional nature of the “green” composites. The thermal behavior of the “green” composites, studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), showed that the presence of pineapple fibers does not affect the nonisothermal crystallization kinetics, crystallinity, and thermal decomposition of PHBV resin.     

Hydrolytic degradation behavior of poly(butylene succinate)s with different crystalline morphologies

The effect of crystalline morphology on the hydrolytic degradation behavior of poly(butylene succinate) (PBS) in an alkaline solution was investigated by using scanning electron microscopy, gel permeation chromatography, and weight loss measurement. Morphological changes were induced on PBS samples by different thermal treatments (i.e., melt quenching or isothermal crystallization) at a constant overall degree of crystallinity. It was found that even with a similar degree of crystallinity, the hydrolytic degradation rate of an isothermally crystallized sample at 60°C was higher than that of a melt‐quenched sample. This was due to the difference in the internal morphology of the spherulites: the internal structure of spherulite in an isothermally crystallized sample consists of coarse and loosely packed fibrils whereas a melt‐quenched sample contains finer and tightly packed fibrils. This result suggested that the internal structure of the spherulite of PBS samples plays an important role in the hydrolytic degradation for this experimental condition. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1025–1033, 2001     

PVDF/PMMA/Basalt fiber composites: Morphology,melting and crystallization,structure, mechanical properties,and heat resistance

This paper is to study the effect of basalt fiber on morphology, melting and crystallization, structure, mechanical properties, melting and crystallization of PVDF/PMMA composites using scanning electron microscopy (SEM), X‐ray, differential scanning calorimeter (DSC), dynamical mechanical analysis (DMA), etc. Basalt fiber may disperse well in PVDF/PMMA matrix and form compact fiber network, and this makes tensile and flexural strength of fiber reinforced PVDF/PMMA composites get to the maximum value of 62 and 102 MPa, respectively. However, the mechanical properties begin to decrease when basalt fiber content exceeds 20 wt %. The α and β phase of PVDF can coexist in composites, and basalt fiber and PMMA can induce β phase of PVDF. The melting temperature of PVDF in composites is kept unchanged, but the degree of crystallinity of composites increases as basalt fiber content increase, and then declines when fiber content exceeds 20%. The DSC results confirm that the nucleation ability of PVDF is enhanced by basalt fiber. Also, the heat resistance of PVDF/PMMA composite is improved from 133 to 146.1°C due to basalt fiber. The DMA shows that basalt fiber increases the storage modulus of PVDF/PMMA composite, and the loss peak of PMMA increases from 116.1 to 130°C. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40494.     

Enhancement of Mechanical Properties of FDM‐PLA Parts via Thermal Annealing

The objective of this study is to investigate the possibility of enhancing mechanical properties of poly(lactic acid) (PLA) samples processed by a rapid manufacturing (RM) technique by increasing PLA crystallinity degree via thermal annealing. The samples are manufactured by fused deposition modeling (FDM) at different temperatures and subsequently evaluated by three‐point bending flexural and tensile tests. The polymer processed at 215 °C is thermally annealed over its glass transition temperature in order to increase the degree of crystallinity to the maximum attainable level as measured by the differential scanning calorimetry and confirmed by X‐ray diffraction. The increase in the degree of crystallinity of FDM‐PLA enhances flexural stress of the samples by 11–17%. The study also demonstrates applicability of radiation sterilization for FDM‐PLA parts. Therefore, thermal annealing might be introduced into a standard RM technology of PLA, particularly for sterilizable customized implants, to efficiently improve their mechanical properties.     

Kinetics of isothermal and non‐isothermal crystallization of poly(butylene terephthalate)/ liquid crystalline polymer blends

The melting behavior and isothermal and non‐isothermal crystallization kinetics of poly(butylene terephthalate) (PBT)/thermotropic liquid crystalline polymer (LCP), Vectra A950 (VA) blends were studied by using differential scanning calorimetry. Isothermal crystallization experiments were performed at crystallization temperatures (T_c), of 190, 195, 200 and 205°C from the melt (300°C) and analyzed based on the Avrami equation. The values of the Avrami exponent indicate that the PBT crystallization process in PBT/VA blends is governed by three‐dimensional morphology growth preceded by heterogeneous nucleation. The overall crystallization rate of PBT in the melt blends is enhanced by the presence of VA. However, the degree of PBT crystallinily remains almost the same. The analysis of the melting behavior of these blends indicates that the stability and the reorganization process of PBT crystals in blends are dependent on the blend compositions and the thermal history. The fold surface interfacial energy of PBT in blends is more modified than in pure PBT. Analysis of the crystallization data shows that crystallization occurs in Regime II across the temperature range 190°C‐205°C. A kinetic treatment based on the combination of Avrami and Ozawa equations, known as Liu's approach, describes the non‐isothermal crystallization. It is observed that at a given cooling rate the VA blending increases the overall crystallization rate of PBT.     

Reinforcing effect and isothermal crystallization kinetics of poly(3‐hydroxybutyrate) nanocomposites blended with organically modified montmorillonite

Organically modified montmorillonite (OMMT) has been incorporated up to 7 wt% in poly(3‐hydroxybutyrate) (PHB) by melt compounding in a twin screw extruder. PHB nanocomposites reinforced with C93A showed significant increase in tensile and flexural modulus and impact strength comparatively. Wide angle X‐ray diffraction showed an increase in overall d‐spacing indicating intercalated structure. The intercalation morphology was further supported by transmission electron microscope images indicating formation of intercalated structure in case of PHB/OMMT and a mixture of Intercalated/exfoliated structure in case of PHB/TMI‐MMT nanocomposites. Thermogravimetric analyses indicate that the thermal stability of PHB/TMI‐MMT nanocomposites is higher among all other nanocomposites under investigation and virgin PHB. Differential scanning calorimetry (DSC) analysis of PHB nanocomposites shows marginal increase in glass transition temperature and decrease in crystallization temperature compared to virgin PHB. The isothermal crystallization kinetics of PHB/C93A nanocomposites was investigated by DSC in the temperature range of 100–120°C and the development of relative crystallinity with the crystallization time was analyzed by Avrami equation. POLYM. COMPOS., 35:999–1012, 2014. © 2013 Society of Plastics Engineers     

Preparation and Assessment of Phase Morphology,Rheological Properties,and Thermal Behavior of Low-Density Polyethylene/Polyhexene-1 Blends

In this work, polyhexene-1 (PH-1) is synthesized by polymerization of hexane-1 with Ziegler–Natta catalyst and melt blended with low-density polyethylene (LDPE). The phase morphology, rheology, crystallization, and thermal behavior of (LDPE)/PH-1 blends are investigated. A good compatibility is observed in the blends up to 10?wt% PH-1 and the most of the droplets in the fractured surface are covered with and buried in the LDPE matrix and at higher percentage the droplet particle size significantly increased. The effect of microstructure of the blends on the flow behavior is studied by small amplitude oscillation rheology. By decreasing the compatibility and increasing the particle size, the Cole–Cole plots are deviated from the semi-circular shape at higher percentages than 10?wt% of PH-1. The change in the crystallization and melting behavior of LDPE in the blends are studied by differential scanning calorimetry and X-ray diffraction (XRD). It is found that by increasing the PH-1 the melting temperature of LDPE decreased from 112.5 to 110.8°C and crystallization temperature increased from 95.2 to 97.7°C which is evident of the nucleation effect. The intensity of (110) peak in XRD test declined as a remake of amorphous part of LDPE and the degree of crystallinity of LDPE decreased from 28 to 22% at 20?wt% PH-1.     

Effect of annealing on the structure and properties of polyvinylidene fluoride hollow fiber by melt‐spinning

Polyvinylidene fluoride hollow fibers were prepared by melt‐spinning technique under three spinning temperatures. The effects of annealing treatment on the structure and properties of hollow fiber were studied by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), tensile test, and scanning electron microscopy (SEM) measurements. DSC and WAXD results indicated that the annealing not only produced secondary crystallization but also perfected primary crystallization, and spinning and annealing temperature influenced the crystallinity of hollow fiber: the crystallinity decreased with the increase of spinning temperature; 140°C annealing increased the crystallinity, and hardly influenced the orientation of hollow fiber; above 150°C annealing increased the crystallinity as well, and furthermore had a comparative effect on the orientation. The tensile tests showed that the annealed samples, which did not present the obvious yield point, exhibited characteristics of hard elasticity, and all the hollow fiber had no neck phenomenon. Compared with the annealed sample, the precursor presented a clear yield point. In addition, the annealed samples had a higher break strength and initial modulus by contrast with the precursor, and the 140°C annealed sample showed the smallest break elongation. SEM demonstrated the micro‐fiber structure appeared in surface of drawn sample. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 935–941, 2007     

Crystallization of polymer melts under fast cooling. II. High-purity iPP

Samples of a high-purity isotactic polypropylene (iPP) were quenched from the melt so as to monitor cooling history. A continuous variation of morphology and crystal structure was obtained with cooling rate. This is discussed in relation to sample thermal history evidencing that cooling history relevant to quenched samples is in the neighborhood of 90°C. In particular the samples are essentially mesomorphic when at this temperature cooling rates larger than 80°C/s were adopted, while below a few tens of °C/s only α-monocline form is obtained. Densities of quenched samples were compared with predictions of an isokinetic extrapolation of Avrami model of polymer crystallization kinetics.     

The effect of chemical-vapor-deposition conditions on the properties of carbon-carbon composites

Properties are given for as-deposited and heat-treated carbon-felt, carbon-matrix composites infiltrated at deposition temperatures of 1100 and 1400°C, and pressures of 20 and 630 Torr. A thermal stress figure of merit was calculated for each material, with the heat-treated composite infiltrated at 1400°C and 630 Torr yielding the highest value. As with most graphitizing carbon materials, heat-treatment resulted in a decrease of the flexural strengths and moduli. The strength-to-modulus ratios, however, increased, being highest for deposition conditions of 1400°C and 630 Torr. Heat-treatment also resulted in an increase in thermal conductivity and a decrease in thermal expansion. These changes were related to the degree of crystallinity and to the formation of cracks within the matrix.     

Effect of Vibration Extrusion on Mechanical Properties and Structure of HDPE/OMMT Nanocomposites

A vibration technique was applied in extrusion molding of HDPE 6100M/OMMT nanocomposites. The results from the study suggest that samples obtained by vibration extrusion were strengthened effectively. The maximum increase percentage of tensile strength at 180°C and 200°C reached 25.14% and 21.43% respectively. It was found from microscopic structures measured by DSC, WAXD and SEM that the crystalline grains of polyethylene matrix became fine, that the orientation degree of crystalline increased and that crystallinity became perfect under the vibration field. Moreover, vibration can make nano-OMMT disperse more homogeneously in the HDPE matrix.     

Effect of poly(styrene-co-maleic anhydride) on physical properties and crystalline behavior of nylon-6/PEBA blends

This study investigated the effect of blending poly(styrene-co-maleic anhydride) (SMA) on the mechanical and thermal properties of nylon-6/polyether block amide (PEBA) blend. In these blends, nylon-6 was toughened with PEBA using SMA as the compatibilizer. All the blends were prepared via direct melt compounding using a co-rotating twin screw extruder. The amount of PEBA added affected the crystallization characteristics and the relative ratio of γ and α crystalline phases of Nylon 6. The crystallization rate of Nylon 6 was also affected by the cooling rate and the amount of PEBA added. The results of mechanical tests showed that the tensile properties, flexural properties, and impact strengths of the nylon-6/PEBA were all increased when blended with 1 wt% of SMA, at both 23 and ?20 °C. However, for neat nylon-6, the impact strength was not affected despite that both tensile and flexural properties were increased by the blending of SMA. The results indicated that SMA can increase the compatibility between nylon-6 and PEBA, thus expanding the usage of nylon-6/PEBA blend in low-temperature applications.     

Morphology and mechanical properties of injection molded poly(ethylene terephthalate)

This work reports on the relationships between processing, the morphology and the mechanical properties of an injection molded poly(ethylene terephthalate), PET. Specimens were injection molded with different mold temperatures of 30°C, 50°C, 80°C, 100°C, 120°C, 150°C, while maintaining constant the other operative processing parameters. The thermomechanical environment imposed during processing was estimated by computer simulations of the mold‐filling phase, which allow the calculation of two thermomechanical indices indicative of morphological development (degree of crystallinity and level of molecular orientation). The morphology of the moldings was characterized by differential scanning calorimetry (DSC) and by hot recoverable strain tests. The mechanical behavior was assessed in tensile testing at 5 mm/min and 23°C. A strong thermal and mechanical coupling is evidenced in the injection molding process, significantly influencing morphology development. An increase in the mold temperature induces a decrease of the level of molecular orientation (decrement in the hot recoverable strain) and an increment in the initial crystallinity of the moldings (decrement in the enthalpy of cold crystallization), also reflected in the variations of the computed thermomechanical indices. The initial modulus is mainly dependent upon the level of molecular orientation. The yield stress is influenced by both the degree of crystallinity and the level of molecular orientation of the moldings, but more significantly by the former. The strain at break was not satisfactorily linked directly to the initial morphological state because of the expected morphology changes occurring during deformation. Polym. Eng. Sci. 44:2174–2184, 2004. © 2004 Society of Plastics Engineers.     

Effects of glass‐fiber content and coolant temperature on temperature and crystallinity profiles of PP melt during cooling

This investigation studied the temperature gradients and degree of crystallinity of polypropylene melt across a circular duct during the cooling process, where the coolant used was chilled water. The effects of glass‐fiber content, varying from 0 to 44 wt %, and coolant temperature, varying from 5 to 20°C, were our main interest. The results suggested that the rate of cooling of the polymer of each position across the duct was not significantly affected by the temperature of the coolant and glass‐fiber contents, although the rate of cooling was influenced by the size of the duct. The crystallization temperature and degree of crystallinity of the polymer increased with increasing glass fiber contents and the coolant temperature. These phenomena were associated with the heat transfer between the coolant and the polymer, crystallization temperature, exothermic crystallization process, and thermal properties of the polymer. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2087–2097, 2001