Fiber reinforced polypropylene: Influence of iPP molecular weight on morphology,crystallization, and thermal and mechanical properties

The influence of molecular weight and its distribution on the nucleation density, crystallization, thermal and mechanical behavior of isotactic polypropylene based composites has been investigated. The composites were prepared by compression molding. The ability of carbon and Kevlar fibers to nucleate the polypropylene has been studied during isothermal and nonisothermal crystallization, by optical microscopy and differential scanning calorimetry (DSC), as function of crystallization temperature T_c and iPP molecular weight. Two extreme crystallization conditions were tested: quenching and slow crystallization to obtain crystals and amorphous phases of different structure. The ability of fibers to enhance mechanical properties in polypropylene based composites was examined by tensile tests at room temperature. It was found that nucleation density, crystallization parameters, and the results of tensile tests strongly depend on the molecular weight M w of iPP, molecular weight distribution, and thermal history of polypropylene. The numerical values of the nucleation density have been found to strongly depend on the nature of fiber. In fact, Kevlar fiber has shown a better nucleating ability than carbon fiber. The results of tensile tests have been related to the sample morphology. The analysis of fractured specimens also provided useful information about fiber-matrix adhesion.     

Thermal and chemical effects of nucleating agents on α, β, γ-polymorphism of isotactic polypropylene

Isotactic polyprpylene (iPP) occurs in several crystalline forms, denoted as f (monoclinic), g (hexagonal), and n (orthorhombic) phases. Hot-stage microscopy, differential scanning calorimetery, and wide-angle X-ray diffraction were used to investigate the influence of thermal treatment and nucleating agents on the morphology of iPP matrices. The tendency of glass fiber (GF) and Kevlar aramid fiber (KF) to induce transcrystallinity in different iPP matrices was evaluated. The f form was present in all iPP specimens treated by different nucleating agents at different crystallization temperatures (Tc). The g and n forms (impure) were found only in iPP specimens that were treated with g -nucleating agent and n - nucleating agent, respectively. Development of transcrystallization was found to depend on the type of fiber used, nucleating agents, and Tc. It was observed that the crystallinity content, obtained by applying different thermal treatments (slow cooling or quenching), gave rise to different morphologies of iPP matrices.     

Pultrusion of epoxy matrix composites: Pulling force model and thermal stress analysis

Pultrusion processing for epoxy resin with glass, carbon, and DuPont Kevlar fibers has been modeled. The model has three submodels: heat transfer, pressure, and force. The validity of the model was verified by comparison of results obtained on the glass fiber/epoxy system with experimental values in the literature. The application of the model permitted an explanation of the difference in behavior between polyester and epoxy resins. Different process variables were studied. Increasing the pulling velocities increases force values. The influence of the fiber content was analyzed. Different wall temperature profiles were used and the residual thermal stresses were calculated. The use of different fibers (glass, carbon, and Kevlar) was simulated under the same process conditions. The higher pulling force was obtained for composites with carbon fibers.     

Morphologies of iPP induced by its partially carbon-coated homogeneity fibers

Homogeneity fiber/matrix composites of isotactic polypropylene (iPP) were prepared with both partially carbon-coated and non-carbon-coated iPP fibers. Their morphologies produced by melt recrystallization were studied by means of polarized optical microscopy. The results show that through vacuum evaporating a thin carbon film partially on the surface of iPP fiber, the nucleation ability of the molten iPP matrix during the course of recooling has been enhanced tremendously. The early formation and high density of the iPP row nuclei formed along the partially carbon-coated iPP fibers lead to the formation of an apparent iPP transcrystalline zone in the vicinity of its precoated fiber. The high nucleation ability of the carbon-coated iPP fiber towards its homogeneity matrix may originate from the surface fixing effect of the vacuum evaporated carbon layer on the polymer samples.     

Characterization of a maleic anhydride-modified polypropylene as an adhesion promoter for glass fiber composites

Isothermal and nonisothermal crystallizations of isotactic polypropylene (iPP), maleic anhydride (MAH)-grafted PP, and MAH-modified iPP were studied by differential scanning calorimetry (DSC), to evaluate the influence of a small amount of MAH-grafted PP in iPP on its crystallization behavior. Isothermal crystallization was followed in the temperature range from 391 K to 403 K, and the rate constant and Avrami exponents were determined. Nonisothermal crystallization was carried out at different cooling rates (1-20 K/min). It was found that the crystallization kinetics of iPP was significantly altered by modification with the MAH-grafted polymer. A decreased equilibrium melting temperature, as well as decreased surface energy of folding and critical dimensions of a growing nucleus, was determined for the MAH-modified iPP, indicating faster growth of lamellae and a higher rate of crystallization. The improved nucleation ability of the modified polymer was shown to cause a shift in the crystallization peak temperature towards higher values (from 393.7 K to 399.6 K, at a cooling rate of 1 K/min), resulting in crystal structures less disposed to recrystallization. Model composites of iPP and MAH-modified iPP with glass fibers were also analysed. The apparent shear strength of single-fiber model composites with MAH-modified iPP was drastically increased compared with homo-iPP.     

Effects of reinforcing fibers on the crystallization of polypropylene

The effects of the incorporation of different types of fibers on the crystallization kinetics and thermodynamics of isotactic polypropylene (iPP) are investigated. The study is mainly performed by thermal analysis, both in isothermal and constant cooling rate conditions, utilizing differential scanning calorimetry (DSC). The crystallization kinetics of the studied composites is successfully described by the Avrami model. Moreover, the effects of different kind of fibers are studied comparing the values of kinetics parameters such as the Avrami exponent n, the kinetic constant of the crystallization rate, Kn and the half‐time of crystallization, T 1/2. The results of the investigation show that the fibers behave as effective nucleant agents for the crystallization of polypropylene. In fact, a dramatic decrease of the half‐time of crystallization, T1/2, as well as a sensible increase of the overall crystallization rate, Kn, are observed in the presence of all the fibers analyzed. These effects are more marked in the presence of aramid fibers. The Avrami model is successfully applied to describe the crystallization kinetics of the composites. The kinetic curves obtained under non‐isothermal conditions confirm the results obtained under isothermal conditions and demonstrate the nucleating action of the fibers on the PP crystallization. Furthermore, the spherulite growth and the transcrystallinity on the surface of the fibers are investigated by optical polarizing microscopy. It is observed that transcrystallinity takes place in all kind of fibers studied, but also in this case, aramid fibers are the most effective in promoting transcrystallinity.     

Interplay between crystallization behaviors and extensional deformation of isotactic polypropylene and its blend with poly(ethylene-co-octene)

Comparison investigation of the interaction between crystallization behaviors and extensional deformation of both isotactic polypropylene (iPP) and its blend with poly(ethylene-co-octene) (iPP/PEOc) was carried out in this study. The samples of iPP and the iPP/PEOc (80/20, wt.%) blend were prepared by changing the cooling rate during nonisothermal crystallization. Tensile testing showed that with the decrease of cooling rate, the progressive destruction of ductility of the two samples was resulted from the more perfect crystallites formed in the cooling process. The influence of cooling rate on the tensile properties is more prominent for pure iPP than for the iPP/PEOc blend. The crystalline structure was proven to be partially destroyed under the extensional deformation, and such crystalline structure destruction was in close association with the deformation of the specimens. The oriented noncrystalline molecular chains could easily be reorganized into more perfect crystals in postheating runs. The original crystalline structure has been found, to some extent, to determine the extensional deformation and the final crystallization behavior.     

Enhanced interfacial adhesion via interfacial crystallization between sisal fiber and isotactic polypropylene: direct evidence from single‐fiber fragmentation testing

Bioresource natural sisal fiber (SF) was used to prepare single fiber‐reinforced isotactic polypropylene (iPP) composites. Three kinds of interfacial crystalline morphologies, spherulites, medium nuclei density transcrystallinity (MD‐TC) and high nuclei density transcrystallinity (HD‐TC), were obtained in the single fiber‐reinforced composites by implementing quiescent or dynamic shear‐enhanced crystallization and by modulating the compatibility interaction between SF and iPP. The development of interfacial shear strength (IFSS) during the interfacial crystallization process was demonstrated for the first time using a combination of single‐fiber fragmentation testing and optical microscope observation. A close correlation between IFSS and morphological characteristics of interfacial crystallization was well elucidated. The increases in IFSS were very different for spherulitic, MD‐TC and HD‐TC morphologies. The highest IFSS obtained was 28 MPa, after the formation of HD‐TC, which was about 62% of the tensile strength of neat iPP (45 MPa). These results offer powerful and direct evidence that interfacial crystallization could play an important role in the enhancement of interfacial adhesion of real SF/iPP composites. © 2013 Society of Chemical Industry     

Isotactic polypropylene solidification under pressure and high cooling rates. A master curve approach

Solidification in industrial processes very often involves flow fields, high thermal gradients and high pressures: the development of a model able to describe the polymer behavior becomes complex. Recently a new equipment has been developed and improved to study the crystallization of polymers when quenched under pressure. An experimental apparatus based on a modified, special injection moulding machine has been employed. Polymer samples can be cooled at a known cooling rate up to 100°C/s and under a constant pressure up to 40 MPa. Density, Micro Hardness (MH), Wide angle X‐ray diffraction (WAXD), and annealing measurements were then used to characterize the obtained sample morphology. Results on one iPP sample display a lower density and a lower density dependence on cooling rate for increasing pressure. Micro hardness confirms the same trend. A deconvolution technique of WAXD patterns is used to evaluate the final phase content of samples and to assess a crystallization kinetics behavior. A master curve approach to explain iPP behavior under pressure and high cooling rates was successfully applied on density results. On the basis of this simple model it is possible to predict the final polymer density by superposition of the effect of cooling rate and the effect of pressure in a wide range of experimental conditions.     

Crystallization of isotactic polypropylene inside dense networks of carbon nanofillers

In this study, we performed the crystallization of carbon nanotube (CNT)/isotactic polypropylene (iPP) and graphene nanosheet (GNS)/iPP composites with very high nanofiller loadings; these are frequently used in polymer composites for electromagnetic interference shielding and thermal conductivity. Rheology testing indicated that both the high‐loading CNTs and GNSs formed dense networks in the iPP matrix, and transmission electron microscopy showed that their connection types were completely different: the CNTs contacted one another in a dot‐to‐dot manner, whereas the GNSs linked reciprocally in a plane‐to‐plane manner. The carbon nanofiller networks brought about two opposite effects on iPP crystallization: a nucleation effect and a confinement effect. The CNT network showed a stronger nucleation effect; however, the CNT network also revealed a more powerful confinement effect because the CNT network was denser than the GNS network. With increasing content of the carbon nanofillers, the crystallization rates of both the CNT and GNS composites first increased, then decreased, and showed a very high saturation concentration at 50 wt %; this resulted from the competitive relationship between the nucleation effect and confinement effect. The crystallization was facilitated when the carbon nanofiller concentration was below saturation, where the nucleation effect invariably played a dominant role. Although the crystallization was depressed when the carbon nanofiller concentration was above saturation, the nucleation effect was subdued, and the confinement effect was extensive. Compared to the GNS/iPP composites, the CNT/iPP composites showed a more depressed crystallization. The suppression mechanism is discussed with consideration of the local topological structure constructed by those two carbon nanofillers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39505.     

Recrystallization behavior of β‐isotactic polypropylene in homogeneous and heterogeneous matrix–fiber composites

β‐isotactic polypropylene (β‐iPP) cylindritic crystals were produced in homogeneous iPP fiber–matrix composites and heterogeneous polyamide (PA)–iPP fiber–matrix composites under different sample preparation conditions. The melt recrystallization behaviors of the β‐iPP crystals obtained in the homogeneous and heterogeneous composites were studied by optical microscopy. The experimental results show that, by heating the sample to 180 °C, the birefringence contributed by the iPP crystals in both α‐ and β‐forms disappears completely. During the cooling process, the β‐iPP crystals in the homogeneous composite appear again, while the iPP in the heterogeneous composite crystallizes in its α‐form. This demonstrates the different origins of the β‐iPP cylindrites in the homogeneous and heterogeneous composites. While the β‐iPP cylindrites in the heterogeneous composite are associated with the sample preparation procedure, the β‐iPP cylindrites in the homogeneous composite are produced by recrystallization of the molten but incompletely relaxed iPP fibers. In situ observation of the melt recrystallization process shows that the molten iPP oriented fibers crystallize first during the cooling process at relatively higher temperature in the α‐form. Abundant randomly dispersed β‐iPP nuclei formed at the surface of the recrystallized iPP fibers, which generate the β‐iPP cylindrites. Copyright © 2012 Society of Chemical Industry     

Isothermal crystallization behavior of isotactic polypropylene blended with small loading of polyhedral oligomeric silsesquioxane

The isothermal crystallization kinetics and morphology development of isotactic polypropylene (iPP) blended with small loading of nanostructure of polyhedral oligomeric silsesquioxane (POSS) were studied with differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). The crystallization behaviors of iPP/POSS composites presented an unusual crystallization behavior during isothermal and nonisothermal crystallization conditions. The exothermic morphologies of isothermal and nonisothermal crystallization of iPP/POSS composites changed remarkably with increasing POSS. Moreover, the developments of spherulitic morphology for iPP/POSS composites showed that the major dispersed POSS molecules became nanocrystals first and then aggregated together forming thread- or network-like morphologies, respectively, depending on POSS content, which was observed. It implies that these major POSS nanocrystals' morphologies appeared as an effective nucleating agent and promoted the nucleation rate of iPP, whereas the minor dispersed POSS molecules that had slight miscibility between iPP retarded the nucleation and growth rates of iPP in the remaining bulk region. Therefore, the isothermal crystallization showed a single exothermic peak at pure iPP and POSS-1.0, whereas at POSS-2.0 and POSS-3.0, displayed the multi-exothermic peaks during isothermal crystallization. These faces indicated that POSS molecules were both influence on the transport of iPP chain in the melted state and on the free-energy of formation the critical nuclei of iPP assisted by the POSS structures were observed. Therefore, we postulated that the crystallization mechanisms of multi-exothermic peaks in isothermal crystallization may proceed to combine the “nucleating agent inducing nucleation of iPP event assisted by the POSS domains” that the nucleation of iPP does occur preferentially on the surfaces of the POSS “threads” or “networks” structures, and “nucleation and growth of iPP in the remaining bulk melted iPP region retarded by dispersed POSS molecules”. Therefore, effects of POSS content on the isothermal and nonisothermal crystallization behaviors of iPP/POSS composites due to the POSS molecules partially miscible with iPP, at very small loading of POSS molecules, promoted or retarded the rates of nucleation and growth of iPP depending on the POSS content and crystallization temperature were discussed.     

α/β复合成核剂对等规聚丙烯结晶形态的影响及非等温结晶动力学

采用偏光显微镜(POM)及示差扫描量热(DSC)法考察了3种α/β复合成核剂NA40/NABW、NA40/HHPA-Ba、NA40/PA-03对成核等规聚丙烯(iPP)的结晶形态及非等温结晶动力学的影响。对成核iPP结晶形态的研究结果表明：α/β复合成核剂的加入能够减小iPP的球晶尺寸。影响α/β复合成核剂成核iPP结晶形态的主要因素是ΔT_Cp（ΔT_Cp为成核iPP结晶峰值温度与iPP结晶峰值温度的差值）,即复合体系中ΔT_Cp较大的成核剂在iPP结晶过程中起主导作用,最终的结晶形态与单独添加这一成核剂时iPP的结晶形态相类似;当两种成核剂的ΔT_Cp接近相同时,两者竞争成核,成核iPP的结晶形态表现为两种成核剂共同作用的结果。因此,通过改变α/β复合成核剂的复合比例即改变两种成核剂的添加浓度,进而改变其ΔT_Cp,可以得到结晶形态完全不同的iPP。采用Caze法对非等温动力学进行了研究,结果表明：添加α/β复合成核剂能够提高iPP的结晶温度,缩短半结晶时间。复合成核剂成核iPP的结晶行为也同样受成核剂ΔT_Cp的影响,复合成核iPP的Avrami指数接近于复合体系中ΔT_Cp较大的成核剂单独添加时iPP的Avrami指数。     

The crystallization of polypropylene in multiwall carbon nanotube‐based composites

The crystallization process of isotactic polypropylene (iPP) was studied under both dynamic and isothermal conditions for a series of multiwall carbon nanotube (MWCNT) composites with nanotube concentrations between 0.1 and 1.0% by weight. The nucleation activity of the nanofillers was confirmed for both dynamic and isothermal crystallization, and was shown to be composition dependent. The effect of the nanofiller on the crystallization of iPP was discussed using the temperature coefficients obtained to determine the interfacial free energy and free energy of nucleation. The basal interfacial free energy decreased with respect to that of neat iPP by up to 15% for as little as 0.1% MWCNT, subsequently decreasing linearly with increasing nanotube concentration. This behavior is in line with the crystallization behavior of iPP with conventional nucleating agents. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Crystallization behaviors in the isotactic polypropylene/graphene composites

Crystallization behaviors and kinetics of iPP in an in-situ prepared isotactic polypropylene/graphene (iPP/G) composites were studied in this paper. In samples used in this study, the graphene fillers were well dispersed, and the interfacial adhesion exhibited enhanced features between graphene and iPP components. The thermal stability of the composites was improved by about 100 °C compared to the pristine iPP. It was found that the crystallization morphology, crystallization rate and kinetics of the iPP/G composites were significantly influenced by the presence of graphene. The nucleation and epitaxial growth of iPP on the graphene surface were observed and studied in detail. It was observed that the nucleation of iPP favored to occur at the wrinkles and edges due to the good match of the lattice parameters and the weak spatial hindrance compared to the smooth surface. Numerous nuclei epitaxially formed and the size of the crystals was very small. The schematic diagram was also proposed for the nucleation and growth process of iPP on the graphene surface in the iPP/G composites. Meanwhile, the overall crystallization kinetics and crystals growth were analyzed through Avrami equation. The obtained Avrami index n decreased with the graphene loadings and was close to 2 for the iPP/G composites, which implied that the growth of iPP in the composites was in two-dimension. And this was caused by the structure of graphene and the spatial confinement effect of graphene platelets in the iPP/G composites.     

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

The crystallization morphologies and mechanical behaviors of iPP/sPP blends and the corresponding fibers were investigated in the present work. For all the investigated iPP/sPP blends, the starting crystallization temperature of sPP during cooling process was significantly increased with increasing iPP content. The iPP/sPP blends are strongly immiscible at the conventional melt processing temperatures, in consistence with the literature results. As isothermally crystallized at 130 °C, sPP still keeps melt state, while iPP component is able to crystallize and the spherulites become imperfect accompanied by decreasing of the crystallite size as sPP content increases. The addition of sPP decreases the crystallinity of iPP/sPP blends and fibers. The storage modulus, E′, of the iPP/sPP blends is higher than that of sPP homopolymer in the temperature range from −90 to 100 °C. The iPP/sPP fibers can be prepared favorably by melt-spinning. As sPP content exceeds 70%, the elastic recovery of the iPP/sPP fibers is approximately equal to that of sPP homopolymer fiber. The drawability of the as-spun fiber of iPP/sPP (50/50) is better than that of sPP fiber, which improves the fiber processing performance and enhances the mechanical properties of the final product. The drawn fiber of sPP presents good elastic behavior within the range of 50% deformation, whereas the elastic property of the iPP/sPP (50/50) fiber slightly decreases, but still much better than that of iPP fiber.     

Crystallization morphologies and non-isothermal crystallization kinetics of isotactic polypropylene modified by α/β compounded nucleating agents

采用偏光显微镜(POM)及示差扫描量热(DSC)法考察了3种α/β复合成核剂NA40/NABW、NA40/HHPA-Ba、NA40/PA-03对成核等规聚丙烯(iPP)的结晶形态及非等温结晶动力学的影响。对成核iPP结晶形态的研究结果表明：α/β复合成核剂的加入能够减小iPP的球晶尺寸。影响α/β复合成核剂成核iPP结晶形态的主要因素是ΔT_Cp（ΔT_Cp为成核iPP结晶峰值温度与iPP结晶峰值温度的差值）,即复合体系中ΔT_Cp较大的成核剂在iPP结晶过程中起主导作用,最终的结晶形态与单独添加这一成核剂时iPP的结晶形态相类似;当两种成核剂的ΔT_Cp接近相同时,两者竞争成核,成核iPP的结晶形态表现为两种成核剂共同作用的结果。因此,通过改变α/β复合成核剂的复合比例即改变两种成核剂的添加浓度,进而改变其ΔT_Cp,可以得到结晶形态完全不同的iPP。采用Caze法对非等温动力学进行了研究,结果表明：添加α/β复合成核剂能够提高iPP的结晶温度,缩短半结晶时间。复合成核剂成核iPP的结晶行为也同样受成核剂ΔT_Cp的影响,复合成核iPP的Avrami指数接近于复合体系中ΔT_Cp较大的成核剂单独添加时iPP的Avrami指数。     

Morphology,Thermal Stability,and Electrical Conductivity of Polymer Nanocomposites of Isotactic Polypropylene/Multi-Walled Carbon Nanotubes

Molecular weight of isotactic polypropylene (iPP) and concentration of multi-walled carbon nanotubes (MWCNT) effects on the morphology, thermal stability, and electrical conductivity for iPP/MWCNT nanocomposites were evaluated. Nanocomposites were prepared by solution mixing followed by non-isothermal crystallization from the melt. The samples were characterized by different physical-chemical techniques. Electrical conductivity was obtained from electrical resistance measured using a source meter. It was determined that the morphology of the nanocomposites shows a change from spherulitic to fibrillar to undefined depending on the molecular weight of iPP and concentration of MWCNT. Morphology was correlated with thermal stability and electrical conductivity.     

Dynamic mechanical behavior of LLDPE composites reinforced with kevlar fibers/short glass fibers

Kevlar fibers (DuPont) and glass fibers have been used to reinforce linear lowdensity polyethylene (LLDPE) by using an elastic melt extruder and the compression molding technique. The impact behavior of hybrid composites of different compositions is compared and has been explained on the basis of volume fraction of fibers. The addition of glass fibers decreases the Izod impact strength of LLDPE. The Izod impact strength of the composite increses when glass fibers are replaced by Kevlar fibers. Dynamic mechanical α‐relaxation is studied and the effect of variation of fiber composition on the relaxation is reported in the temperature range from −50°C to 150°C at 1 Hz frequency. The α‐relaxation shifts towards the higher temperature side on addition of fibers in LLDPE. The addition of fibers increases the storage modulus, E′, of LLDPE. The hybridization of Kevlar and glass fibers helps in desiging composites with a desirable combination of impact strength and modulus. At the low temperature region, E′ increases significantly with glass fibers as compared to that noted with the addition of Kevlar fibers. The α‐transition temperature of composites increases significantly with Kevlar fibers as compared to that observed with addition of glass fibers.     

New understanding for the formation of conductive network in the nanocomposites during the crystallization of matrix

Crystallization behavior of conductive composite has a great effect on the formation of conductive network. But very few studies have exposited, specially on the micro level, the evolution of conductive network during the crystallization of matrix. In this study, the conductive network was found to be destroyed by crystallization behavior of isotactic polypropylene (iPP) matrix, and the carbon black (CB) particles were rejected to the amorphous region or the inter-lamellar of spherulite. By comparison, the low-structure carbon black (LCB) filled system was more sensitive to the crystallization of matrix than the high-structure carbon black (HCB) filled system because of the morphology and interaction force of the CB primary aggregate. A secondary increase in volume resistivity during terminal crystallization was observed in iPP/LCB composite when it isothermally crystallized at a certain temperature. In that case, an analysis of crystallization kinetics of composites through a modified Lauritzen-Hoffman model indicated that the transition from regime I→II in the isothermal crystallization process of iPP matrix showed significant influence on the network formation of LCB particles.