Composites from Brazilian natural fibers with polypropylene: mechanical and thermal properties

Brazil has a well established ethanol production program based on sugarcane. Sugarcane bagasse and straw are the main by-products that may be used as reinforcement in natural fiber composites. Current work evaluated the influence of fiber insertion within a polypropylene (PP) matrix by tensile, TGA and DSC measurements. Thus, the mechanical properties, weight loss, degradation, melting and crystallization temperatures, heat of melting and crystallization and percentage of crystallinity were attained. Fiber insertion in the matrix improved the tensile modulus and changed the thermal stability of composites (intermediary between neat fibers and PP). The incorporation of natural fibers in PP promoted also apparent T _c and ΔH _c increases. As a conclusion, the fibers added to polypropylene increased the nucleating ability, accelerating the crystallization process, improving the mechanical properties and consequently the fiber/matrix interaction.     

The Preparation and Study on Ultrahigh Molecular Weight Polypropylene Gel‐spun Fibers

A novel polypropylene (PP) fiber was prepared by using gel spinning/crystallization from dilute solutions of ultrahigh molecular weight isotactic polypropylene (i‐UHMWPP), and subsequently drawing at various temperatures. The influence of drawing temperature on the properties of the resulted fibers was investigated. We found that the draw‐ability and mechanical as well as crystallization properties of the fibers obtained were dramatically improved with increasing drawing temperature. When the drawing temperature is below the α‐crystal relaxation temperature of PP, which was measured by wide‐angle X‐ray diffraction (WAXD) analysis as 100–120°C, the fibers are characterized by lower crystallinity and smaller crystals with less perfection, resulting in brittle fracture and subsequently poor mechanical durability. With drawing at temperatures above the α‐crystal relaxation temperature of PP, a novel UHMWPP fiber with Young's modulus of 27 GPa and tensile strength of 1.3 GPa was obtained. Higher crystallinity and larger crystals with better perfection and orientation were observed in this fiber.     

Nucleator-Enhanced Molecular Orientation and Crystallization in Melt-Spun Monofilaments of Syndiotactic Polypropylene

Different from other semi-crystalline polymers, the crystallization of syndiotactic polypropylene (sPP) into helical sequences under flow is largely inhibited due to favorable formation of trans-planar conformations. The results presented in this study, using micro-FTIR, 2D-WAXS, and 2D-SAXS characterization, indicate that molecular orientation and crystallization in melt-spun sPP monofilaments can be dramatically enhanced by a sorbitol-based nucleator. It is suggested that, similar to the oriented crystallization in the nucleated isotactic polypropylene (iPP), the phase-separated nanofibrils, aligned under flow prior to sPP crystallization, act as shish structure (primary nuclei) to mediate the molecular orientation and crystallization of sPP.     

聚丙烯/抗老化剂复合体系的γ-射线辐照降解研究

研究了聚丙烯(PP)及含有受阻酚类抗氧剂和受阻胺类光稳定剂复合体系的PP复合物经过γ-射线辐照后发生的结构变化及抗老化剂所起作用。实验利用红外光谱(FTIR)和示差扫描量热法(DSC)对PP的结构变化进行了系统表征。研究结果表明,当辐照剂量较小(50 kGy)时,纯PP及其复合物体系均未发生明显降解;当辐照剂量较大(≥50 kGy)时,PP及其复合物的羰基指数迅速提高,二者的结晶温度和熔融温度大幅度降低,说明PP发生了严重降解。在相同γ-射线辐照剂量条件下(≥50 kGy),PP复合物的羰基指数高于纯PP,而结晶温度以及熔融温度低于纯PP,表明高辐照剂量下抗老化剂复合物的存在不但没有阻止聚丙烯的降解,反而加快了降解的速率。     

Induction time as a measure for heterogeneous spherulite nucleation: Quantitative evaluation of early-stage growth kinetics

A theory is proposed indicating that the induction time characterizes the nucleation process in polymer crystallization. A microscopic method has been developed to monitor early stages of the spherulite growth. Using the method, based on photometric measurement of the depolarized light intensity during isothermal crystallization from the melt, nonzero nucleation induction time values were found for polypropylene (PP). From comparison of the theory with the experimental results obtained from polypropylene crystallization, it follows that the formation of PP crystal nuclei on admixtures is affected both by the formation of the first crystalline layer and by the size of the critical nucleus.     

Nature of Shear-Induced Primary Nuclei in iPP Melt

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals.     

Transcrystallization of polypropylene on different modified jute fibers

This article deals with the transcrystallization of polypropylene on untreated, MAH-polypropylene grafted, and alkali treated jute fibers due to isothermal as well as non-isothermal crystallization conditions. In both cases, TCL-thickness as well as TCL growth initiation temperature were dependent on fiber treatment. The chosen T_C in isothermal conditions was of great significance on TCL growth rate. The experimental data were successfully fitted by using an Arrhenius-type relationship.     

CRYSTALLIZATION BEHAVIOR OF PP IN A NEW GENERATION OF COMPOSITE MATERIAL

The microstructure of isotactic polypropylene (iPP) has been investigated during different steps of the processing of commingled PP/unidirectional glass fiber composites. From wide angle x-ray scattering and differential scanning calorimetry analysis, complemented by density measurements, it can be concluded that the crystalline phase in both un-reinforced PP and composite materials is only constituted of the α phase. The morphology of the crystalline phase and the two-dimensional geometric arrangement of fibers within the matrix of commingled composites reinforced by 45, 60, and 75 wt% of fibers have also been investigated using optical microscopy. The composites exhibit heterogeneous morphology whatever the fiber content is. Moreover, large spherulites can be distinguished, but the presence of a transcrystalline phase around the fibers cannot be detected.     

Kenaf/polypropylene biocomposites: effects of electron beam irradiation and alkali treatment on kenaf natural fibers

Thermal and dynamic mechanical properties of kenaf natural fiber reinforced polypropylene (PP) biocomposites were examined to compare the effects of natural fiber treatment by electron beam irradiation (EBI) and alkalization. The alpha cellulose contents, the functional groups on the surfaces and the thermal stability of the untreated and treated kenaf fibers were studied. Kenaf fiber/polypropylene(PP) biocomposites were fabricated by means of a compression molding technique using chopped kenaf fibers treated with electron beam (EB) dosages of 100, 200, 500 kGy or with NaOH concentrations of 2, 5, 10 wt%, respectively. The thermal stability, the dynamic mechanical and the interfacial properties of untreated and treated kenaf/PP biocomposites were also investigated through a thermogravimetric analysis, a dynamic mechanical analysis and a fractographic observation, respectively. The results show that the characteristics of kenaf fibers and biocomposites depended on the different treatment level with the EB dosages or on the NaOH concentrations used. In this study, the modification of kenaf fiber surfaces at 200 kGy EBI and treatment with 5 wt% NaOH was most effective for improving the performance of kenaf/PP biocomposites. This study suggests that EBI can be used for modification of natural fiber as an environmentally friendly process and contribute to an improvement in the performances of kenaf/PP biocomposites.     

STUDIES OF KINETICS OF NONISOTHERMAL CRYSTALLIZATION OF i-POLYPROPYLENE BY WIDE-ANGLE AND SMALL-ANGLE SCATTERING OF X-RAY SYNCHROTRON RADIATION

Crystallization behavior of isotactic polypropylene (i-PP) in nonisothermal conditions was studied by wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) using monochromatized X-rays from synchrotron radiation. It was demonstrated that the crystallization rate during continuous cooling with a constant rate is not only a function of the actual, momentary temperature, but also depends on the cooling rate. Except for temperatures close to the melting temperature, the measured values of the crystallization rate are much smaller than those evaluated from isothermal crystallization. SAXS studies revealed two regions of changes in lamellar morphology occurring during the initial and final periods of the process.     

NUCLEATING EFFECT OF MONTMORILLONITE NANOPARTICLES IN POLYPROPYLENE

The nucleation effect of two layered montmorillonite silicates of different origin was studied in polypropylene (PP). Composites were prepared as a function of composition in an internal mixer or by homogenization in a twin-screw compounder. Melting and crystallization characteristics were determined by differential scanning calorimetry. The gallery distance of the silicates and the structure of the composites were studied by WAXS. The results prove that montmorillonite may considerably influence the crystallization of PP. The effect depends very much on the origin and treatment of the filler. Both treatment and composite preparation lead to considerable changes in the separation distance of the silicate layers. Even though the composites contain fillers with more than one population of layer distances, the nucleating effect is related to the completely collapsed galleries of 1 nm distance. Besides gallery distance, organophillization also modifies the surface tension of the filler, but this apparently does not influence its nucleating efficiency. The efficient gallery distance is twice as large as the characteristic matching lattice dimension determined by Lotz et al. Although a few observations could not be explained and the tentative explanation given earlier may need further verification, the results prove that nucleation does not occur at the flat surface of the filler but between its galleries, where polymer molecules may adsorb preferentially.     

Effect of Metallic Salts of Malonic Acid on the Formation of β Crystalline Form in Isotactic Polypropylene

The effects of malonic acid and the lithium, sodium, potassium, zinc, magnesium, calcium, strontium, and barium salts of malonic acid on the formation of β crystalline form in isotactic polypropylene at the crystallization temperatures 120 and 130°C have been investigated. It was found that malonic acid and the lithium, sodium, and potassium salts of malonic acid inhibit the formation of β crystalline form in polypropylene. Zinc malonate has a limited positive effect on the formation of β crystalline form, while magnesium, calcium, strontium, and barium salts are β nucleating agents, in descending order. The decreased β nucleation abilities of the alkaline earth metallic salts of malonic acid are attributed to the increasing atomic radii of the combined metals and decreasing crystallization temperatures.     

Morphology,Nonisothermal Crystallization Behavior and Mechanical Properties of Polypropylene Modified by Ionomers

The morphology and nonisothermal crystallization behavior of polypropylene modified by ionomers based on ethylene copolymers (Surlyn 8920 and 9320) were investigated by using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The crystallization rate of polypropylene was accelerated by the ionomers which initiated heterogeneous nucleation of the polypropylene. At low ionomers content (0.25 wt%), Surlyn 8920, neutralized by sodium, was more efficient to enhance the crystallization rate of polypropylene than Surlyn 9320 (neutralized by zinc). The crystallization process of polypropylene modified by the ionomers was analyzed by different kinetics models. The study showed that the Mo approach was applicable for this system, whereas the Avrami, Jeziorny, and Ozawa methods were not. Furthermore, the notched impact strength of polypropylene modified by the ionomers was increased without any reduction of tensile strength and flexural modulus.     

Effect of Metallic Salts of Pimelic Acid and Crystallization Temperatures on the Formation of β Crystalline Form in Isotactic Poly(propylene)

The effect of crystallization temperatures (100–140°C) on the formation of β crystalline form in isotactic polypropylene nucleated by pimelic acid and sodium, magnesium, zinc, calcium, barium and aluminium salts of pimelic acid has been investigated. It is found that β crystalline form can be produced from polypropylene melt, isothermally crystallized at 120 or 130°C. Aluminium pimelate is a α nucleator which suppresses the production of β crystalline form in polypropylene. Pimelic acid and magnesium pimelate are weak β nucleators. Sodium pimelate is a moderate β nucleator. Zinc pimelate, calcium pimelate and barium pimelate are good β nucleators for polypropylene with calcium pimelate being the best.     

Formation of the Excess Free Volume in Triple Junctions during Nickel Crystallization

The formation of an excess free volume in triple junctions during crystallization has been studied by the molecular dynamics model using nickel as an example. It is shown that an excess free volume that forms during nickel crystallization in triple junctions predominantly forms as a result of the fixation of the liquid phase volume when contacting three crystallization fronts that contains, after crystallization, a high fraction of the free volume. In some cases, as the free volume is concentrated in triple junctions, a comparatively small crystalline subgrain (from one to several nanometers in diameter) forms, and the subgrain has the orientation different from those of contacting grains and exists in the extended state.     

Crystallization and morphology in blends of isotactic polypropylene and linear polyethylene

Thermooptical, wide-angle x-ray diffraction and morphological investigations of blends of isotactic polypropylene with linear polyethylene revealed an increased crystallization rate of the polyethylene blend component, compared to crystallization of polyethylene alone. Crystallization behavior of the polyethylene component was markedly dependent on the blend thermal history and on the circumstances of the polyethylene phase—whether it was disperse or continuous. The higher crystallization rate of the polyethylene component was related to the presence of various types of heterogeneous crystallization nuclei in the blend and to the stabilizing action of the solid polypropylene matrix on minute polyethylene crystallites which survived above melting temperature.     

Fibrillar crystals of isotactic polystyrene. II. Aggregation in stirred suspensions

Macroscopic particles of various morphological forms are created by the crystallization of isotactic polystyrene from rapidly stirred solutions. Crystallization from trimethylbenzene over a 150° C temperature range produced spherical particles, globules, fibers, and rings depending upon the growth temperature. Crystallization from cyclohexanol (θ = 83.5°C) produced fibers in the temperature range of 85° to 140° C. Globular structures, similar to those formed by stirrer crystallization in trimethylbenzene developed during the Ziegler-Natta polymerization of styrene in trimethylbenzene between 25° and 75°C. Highly birefringent conical appendages appeared on many of the globules and fibers. The fibers often exhibited a skin-core effect with an average longitudinal and transverse orientation of the chain molecules in the skin and core, respectively. The skin frequently displayed periodic banded extinctions. All of these macrostructures consisted of a highly porous assembly of constituent microfibrillar units. It is believed that the morphogenesis of these structures involves the gradual aggregation of microfibrils under the influence of the flow patterns of the system.     

New Possibilities in the Description of Overall Crystallization of Polymers

Applications of the probabilistic model of spherulitic structure formation are demonstrated. The model permits prediction of the conversion of melt into spherulites in complicated crystallization conditions, including crystallization in a finite volume, in the presence of reinforcing fibers, and in a temperature gradient. The progression of interspherulitic boundary formation can also be described, thus giving insight into development of the spherulitic pattern. The predictions based on the model are verified by a computer simulation of the spherulitic structure formation. They are also in agreement with experimental data.

The modeling of polymer crystallization during cooling of thick-walled products is also reported. The obtained information allows for the geometry of a product and its size to be related with a time and temperature range of polymer crystallization.     

Pd,Ge薄膜体系中的分形晶化行为

利用透射电子显微镜（ＴＥＭ）对有化合物生成（Ｐｄ₂Ｇｅ和ＰｄＧｅ等）的Ｐｄ，Ｇｅ薄膜体系中的分形晶化行为进行了系统研究实验结果和分析表明：在各种退火温度条件下，Ｐｄ－Ｇｅ共蒸膜都难以发生分形晶化，Ｐｄ／ａ－Ｇｅ双层膜较ａ－Ｇｅ／Ｐｄ双层膜更容易导致分形结构的产生Ｚ化合物Ｐｄ₂Ｇｅ和ＰｄＧｅ的形成对薄膜中的分形晶化有明显的抑制作用，体系中能否出现分形结构，取决于非晶Ｇｅ的成核生长和Ｐｄ，Ｇｅ化学反应两种过程的相互竟争． 关键词：     

Crystallization Kinetics of Linear and Long‐Chain Branched Polypropylene

Isothermal crystallization kinetics of linear polypropylene (PP) and long‐chain branched (LCB) PPs were investigated on the basis of the Avrami theory. The Avrami exponents of LCB PPs are smaller than that of linear PP; moreover, they decrease with an increasing LCB level. The crystallization half‐time of LCB PP depends more strongly on the crystallization temperature than does that of linear PP. The Lauritzen‐Hoffman theory was used to study the effect of LCB on the crystal growth rate of PP. The fold surface free energy of LCB PP is lower than that of linear PP; moreover, it decreases with an increasing LCB level. However, when the LCB level is over a certain value, the fold surface free energy increases again. Furthermore, the crystal structures of linear PP and LCB PPs were studied by wide‐angle X‐ray diffraction (WAXD); it was observed that linear PP can crystallize in the α and β forms, while LCB PPs have only the α crystalline form. Moreover, the relative intensities of different α peaks were also influenced by the LCB level.