Crystallization Behavior Of Polypropylene Research Articles

Concurrent effect of shear flow and CNT presence on crystallization behavior and electrical properties of polypropylene based nanocomposites: A comprehensive structure-property investigation

The loading of nanoparticle in the polymer matrix affects their response to shear flows applied during the fabrication process. With this in mind, in the present study, the concurrent effect of carbon nanotube (CNT) presence and shear application on the rheological and electrical properties as well as on the crystallization behavior of polypropylene (PP) based nanocomposites has been investigated. The results of rheological analyses showed that the solid-like behavior increases in proportion to the CNT content, and a remarkable increase in the solid-like behavior is obtained at a CNT content of 2 wt%. The isothermal crystallization results corroborated that crystallization temperature increases proportionally with the CNT content, however does not change with shearing. The non-isothermal crystallization findings confirmed that crystallization kinetics promotes with increasing CNT content, and this effect becomes more pronounced with shearing. The results of thermal analysis confirmed that the melting temperature decreases slightly, and the crystallinity remains almost unchanged with increasing CNT content up to 2%. However, when the CNT content rose up to 4%, the crystallinity decreased significantly due to confined crystallization. When this nanocomposite was subjected to shearing, the crystallinity increased. The results of electrical conductivity measurement revealed that the conductivity increases with increasing CNT content, while it decreases with shearing and the vulnerability of nanocomposites to shearing decreases with increasing CNT content.

Effects of Pressure and Cooling Rates on Crystallization Behavior and Morphology of Isotactic Polypropylene

Isotactic Polypropylene (iPP) is a widely used polymer due to its excellent mechanical and thermal properties, as well as its chemical resistance. The crystallization behavior of polypropylene is influenced by several factors, such as temperature, cooling rate, and pressure. The effect of pressure is significant for both scientific and technological points of view, since in important industrial processing techniques the polymer solidifies under high pressures. In this paper, the study of the effect of pressure on the crystallization kinetics of iPP was conducted using a dilatometer in the pressure range from 100 to 600 bar and under two cooling rates: 0.1 and 1 °C/s. The morphology of the samples was characterized using DSC, optical microscopy, and X-ray diffraction. The results showed that pressure had a larger effect on specific volume changes at higher temperatures (in the melt state) than at lower temperatures (in the solid state). The polymer crystallization, which determined the transition between the melt and solid state, occurred at higher temperatures with increasing pressure. The cooling rate affected the crystallization process, with higher cooling rates leading to crystallization at lower temperatures. The size of the spherulites decreased with increasing cooling rates. The crystallinity evolution curves showed a linear relationship between the crystallization temperature and pressure. The study used a Kolmogoroff–Avrami–Evans model to describe the evolution into isotropic structures, and the predictions of the model accurately described the effect of pressure and cooling rates on the final spherulite radii.

Controllable Synthesis of a Well-defined Polypropylene grafted Silica Nanoparticles and Its Effect on Crystallization Behavior of Polypropylene

Controllable Synthesis of a Well-defined Polypropylene grafted Silica Nanoparticles and Its Effect on Crystallization Behavior of Polypropylene

Temperature influence on the triboelectric powder charging during dry coating of polypropylene with nanosilica particles

The influence of process temperature on the triboelectric charge during dry coating of cohesive polymer powders is investigated. The purpose of heating is to decrease the surface moisture of the powder, which results in an increase of electrical charge of the powder. The effect of the hydrophilicity of the SiO2 guest particles on the quality of the dry coating of PP host particles was investigated with respect to the integral charge of the dry coated powder. Furthermore, the flowability, crystallization behavior and degree of coverage (DOC) were investigated to determine the influence of the increased charge on these topics. Increased process temperature increases the electrical charge of the powder, which leads to a higher DOC with guest particles and, thus, drastically improved flowability compared to dry coating at ambient temperature. Furthermore, changes in the crystallization behavior of polypropylene are observed, due to more nuclei.

Supercritical CO2 utilization for development of graded cellular structures in semicrystalline polymers

Carbon dioxide (CO2) in a supercritical condition is utilized to develop hierarchical cellular structures in injection molding. Polypropylene is used as sample semicrystalline polymer. First, a single-phase CO2-charged melt was produced under pressure. Then, a gradient cell structure was induced onto the polymer melt by pressure reduction and crystal-induced shrinkage. A visualization mold was employed to monitor real-time nucleation of CO2 cells within the mold cavity. Distinctive cell nucleation behaviour was visualized for the first time in which a myriad of ultra-fine cells nucleated rapidly inside the system in an abnormal manner. This type of cell nucleation was different from that occurring due to a pressure drop during foam processing. This mode of cell nucleation was likely driven by the crystallization of the polypropylene chains in the presence of the supercritical CO2. The crystallization behaviour of polypropylene was then investigated within the mold cavity and in the presence of dissolved CO2 by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The presence of supercritical CO2 affected the size of α and β crystals. Furthermore, γ crystals were detected in the core regions of samples. The formed crystals served as effective nucleating sites which promoted the nucleation of large number of fine cells and produced structurally graded foams. The effects of CO2 content, melt packing, and melt temperature on the cell structure development was studied. It was concluded that these parameters can be regulated to design cellular structures with desired cell size and cell morphologies.

Fabrication of MgAl LDH@graphene oxide nanohybrids and their effect on the thermal stability and crystallization behavior of polypropylene.

The co-precipitation method is used to fabricate layered double hydroxide (LDH) nanohybrids with surface engineering of graphene oxide (GO) by radially grafting borate-LDH (BLDH) to BLDH@GO nanosheets, aiming at improving the surface characteristics and compatibility of LDH with the polymer matrix. The results prove the successful fabrication of BLDH@GO and LDH@GO nanosheets. The nanosheets are mixed into polypropylene (PP) by melt blending to study the structure and properties of the composites. The PP composites with BLDH@GO and BLDH have both exfoliation structures and aggregation structures, and the two nanosheets show enhanced interfacial interactions with the PP matrix compared with LDH and LDH@GO. The initial decomposition temperatures of the PP composites are lower than those of the neat PP, but the thermal degradation temperatures of the PP composites are higher. Compared with the other samples, BLDH@GO provides a higher nucleation density, reflected in a smaller spherulite size and a higher crystallization temperature confirmed by the differential scanning calorimetry (DSC) results. BLDH@GO shifts the crystallization temperature of PP to higher values (compared to the neat PP) due to the nucleation effect, which is in line with the increase in the nucleation density detected by polarized optical microscopy (POM).

Dual role of PDMS on improving supercritical CO2 foaming of polypropylene: CO2-philic additive and crystallization nucleating agent

A systematic study was done to screen for the polydimethylsiloxane (PDMS) with molecular weight (Mw) from 770 g/mol to 17,000 g/mol to optimize the supercritical CO2 foaming process of semi-crystalline polypropylene (PP). The solubility and diffusion measurements demonstrated that the high solubility and slow desorption rate of CO2 were achieved in PP/PDMS blends via incorporating PDMS. Rheology results showed the viscosity of PP/PDMS blends increased with the increase of Mw of PDMS. The crystallization behaviors of PP and PP/PDMS blends were thoroughly investigated in the presence of CO2. PDMS with Mw from 4000 g/mol to 9000 g/mol enhanced the crystallization of the blend evidently. Since PDMS with medium Mw could work as a CO2-philic additive and crystallization nucleating agent in PP, the resultant foams exhibited a more uniform cellular morphology with smaller cell size (decreased from 23 to 13 μm) and higher cell density compared with that of PP foam.

Rheology and crystallization behavior of polypropylene and high‐density polyethylene in the presence of a low molar mass polyethylene

Rheology and crystallization behavior of polypropylene and high‐density polyethylene in the presence of a low molar mass polyethylene

Effect of sulfur phase transition on polypropylene crystallization

ABSTRACTThe Polypropylene (PP) crystals can be regulated by inorganic additive. In this work, sulfur was blended with PP by dynamical in-situ dispersion to obtain good dispersion and refinement; meanwhile, during the non-isothermal crystallization of PP, the phase state of the dispersed sulfur phase changed from the liquid to the supercooled state, affecting the crystalline morphology and crystallization behavior of PP. The results showed that amorphous sulfur particles were present in the PP spherulites, which was obviously different from the traditional crystal morphology. The crystallization rate of the blend is found to increase with smaller sulfur content and due to the occurrence of the sulfur phase change; the crystallization activation energy are less than that of pure PP.

Foaming Mechanism of Polypropylene in Gas-Assisted Microcellular Injection Molding

In our recent study, it is reported that gas-assisted microcellular injection molding (GAMIM) is a promising method to fabricate plastic foams with good surface quality, high weight reduction, and improved mechanical performance. To clarify the foaming mechanism, the crystallization behaviors of polypropylene in both conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were studied. It is found that GAIM can significantly promote melt crystallization and refine crystals. To explore the improvement mechanism of crystallization, a numerical model was developed to simulate the filling and cooling processes of CIM and GAIM. The simulation results show that the much stronger shear field and the faster cooling rate lead to the refinement of crystals in GAIM. Finally, a crystallization-driven cell nucleation model was proposed to explain the improved foaming behavior of polypropylene in GAMIM. This paper provides a deep understanding of the foaming behavior and hence benefits the foaming process design.

Effect of Modified Diatomite on Crystallinity and Mechanical Properties of Polypropylene

The different coupling agent is used for diatomite modification. The diatomite with different content are melt and mixed with polypropylene to prepare diatomite/polypropylene composites. The modification effects of different coupling agent on diatomite and the influence of agent and diatomite contents on crystallization behavior of polypropylene and mechanical properties of composites are investigated. The results show that different coupling agents have the modification effects on diatomite influence crystallization of polypropylene but the aluminum agent is better than other agents. With the increase of the content of aluminum acid ester coupling agent, the size of bubble pore is uniform, and then the compressive strength reaches to a maximum when aluminum acid ester coupling agent content is 1%. With the increase of the content of diatomite, the porosity of bubble pore of diatomite/polypropylene composite material is gradually increases. The crystallinity of polypropylene and yield strength reach to a maximum (48.15% and 21.5%) when diatomite content is 30%. In addition, the diatomite/polypropylene composites have better compressive strength than the others.

Non-isothermal crystallization properties of polypropylene composites filled with multi-walled carbon nanotubes

The influence of the filler size and content on the crystallization behavior of polypropylene (PP) composites filled with three multi-walled carbon nanotubes (MWCNTs) with different sizes was investigated by means of a differential scanning calorimetry (DSC). It was found that the values of the onset crystallization temperature, crystallization peak temperature and end crystallization temperature increased with increasing MWCNT weight fraction and with decreasing filler average diameter; the value of the crystalline degree of the composites increased slightly with increasing filler weight fraction. The values of the crystalline degree of the composites filled with MWCNTs with wide length-diameter ratio and small specific surface area were the lowest when the MWCNT weight fraction was lower than 4%, which can be attributed to weak interaction between the matrix and inclusions due to small specific surface area. Moreover, the total crystallization rate of the composites filled with a small amount of the MWCNTs could be significantly increased.

Multiwalled carbon nanotubes-based polypropylene composites: Influence of interfacial interaction on the crystallization behavior of polypropylene

Composites of polypropylene (PP) and multi-walled carbon nanotubes (MWCNTs) were prepared via melt-mixing utilizing Li-salt of 6-amino heaxanoic acid (Li-AHA) modified MWCNTs in the presence of a compatibilizer (polypropylene-g-maleic anhydride; PP-g-MA). Improved interaction between the anhydride group of PP-g-MA and the amine functionality of Li-AHA was confirmed via FTIR and Raman spectroscopic analysis. A higher glass transition temperature (Tg) of the PP phase has been observed in these composites as compared to pristine MWCNTs based composites. The crystallization temperature (Tc) of the PP phase was increased as a function of pristine MWCNTs concentration in PP/MWCNTs composites indicating hetero-nucleating action of MWCNTs. However, Tc value was decreased in the presence of Li-AHA modified MWCNTs indicating the adsorbed Li-AHA on the MWCNTs surface. Moreover, Tc value was higher in the presence of Li-AHA modified MWCNTs with PP-g-MA as compared to that of without PP-g-MA, suggesting the desorbed Li-AHA from the MWCNTs surface due to melt-interfacial reaction. Further, MWCNTs were extracted by hot vacuum filtration technique from PP/MWCNTs composites containing Li-AHA and PP-g-MA. The isothermal crystallization kinetics showed a variation in crystallization behavior of the PP phase in the corresponding composites as compared to the “extracted MWCNTs.” POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Effect of functionalization on non-isothermal crystallization behavior of polypropylene

ABSTRACTThe non-isothermal crystallization kinetics of three functionalized polypropylenes (PPs; polypropylenes-g-acrylic acid [PP-g-AA], polypropylenes-g-glycidyl methacrylate [PP-g-GMA], polypropylenes-g-maleic anhydride [PP-g-MAH]) at different cooling rates were investigated by differential scanning calorimetry, using the Jeziorny method, Ozawa method, and Mo method. The result showed that Mo method can adequately describe the non-isothermal crystallization kinetics of pure PP and functionalized PPs, and at a given relative crystallinity, the crystallization rate obtained using Mo method followed an order of PP-g-AA > PP-g-GMA > PP > PP-g-MAH. The crystallization activation energy for these samples was calculated using Kissinger's method, which indicated that the introduction of monomers had a confinement effect on the motion of PP chains.

Influence of TiO2 nanoparticles on nonisothermal crystallization of PP in a wide range of cooling rates analyzed by fast scanning DSC

ABSTRACTThe influence of titanium dioxide (TiO2) nanoparticles on the crystallization behavior of polypropylene was investigated by conventional differential scanning calorimetry (DSC) and fast scanning DSC measurements. The data obtained from both methods were estimated for the first time using the Lauritzen‐Hoffmann equation to analyze the behavior over a wide cooling range under nonisothermal conditions. This provides more reliable values of nucleation parameters (Kg) and surface free energy (σe). The variation of the effective energy (ΔE) was determined with the Kissinger method. Regardless of the cooling rate, both Kg and σe indicate the role of titania as a nucleating agent enhances the crystallization rate. However, the ΔE denotes that TiO2 acts as an obstacle to the mobility of chain segments at cooling rates below 150 °C/s, while, in contrast, the presence of titania enhances the chain mobility at cooling rates above 150 °C/s. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43944.

Thermal properties and crystallization kinetics of a new polypropylene–polyamide 6 composite

The crystallization behavior of polypropylene (PP)-polyamide 6 (PA6) composites prepared by melt blending was analyzed by differential scanning calorimetry. Nonisothermal experiments were used to evaluate the effect of lithium chloride (LiCl) on the crystallization process of PP from molten state. The addition of LiCl dramatically modified the nonisothermal crystallization behavior of the PP matrix, the crystallization temperature first increased and then decreased with the increase of LiCl, and crystallization rate and effective activation energy decreased compared with PP-PA composite. The influence of LiCl on the thermal stability of PP-PA6 composites was investigated by thermogravimetry. The results showed the addition of LiCl increased the temperatures at the maximum degradation rate and improved the thermal stability of PP-PA6 composites. In addition, the hydrophilicity of PP was largely improved due to the introduction of hygroscopic PA6 and LiCl. The water contact angle of PP-PA 2.25 decreased from 103° to 65° compared with pure PP.

A new crystallization process in polypropylene highly filled with calcium carbonate

The influence of high amounts of calcium carbonate filler on the crystallization behavior of polypropylene (PP) is investigated by differential scanning calorimetry (DSC) and fast scanning DSC measurements. The non-isothermal crystallization process at industrially relevant cooling rates of about 100 K/s is significantly influenced by the calcium carbonate filler. Isothermal crystallization measurements indicate a new crystallization process in the temperature range between 45 and 80 °C caused by the filler content. To find an explanation for the origin of this process, we have analyzed the interaction between polymer and filler, the crystalline structure, and the crystallization kinetics. From the experimental results, we conclude that the newly observed crystallization process is governed by an additional nucleation process for the growth of α-phase crystals.

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.

Comparison on properties of lignocellulosic flour/polymer composites by using wood, cellulose, and lignin flours as fillers

In this study, cellulose flour (CF) and lignin flour (LF) were separated from wood flour (WF) by nitric acid–ethanol method and ball-milling coupled with dioxane extraction method, respectively. The separated CF, LF, and the original WF were blend with two types of polymers, namely, polypropylene (PP) and poly (lactic acid) (PLA) to make composites. The physical, mechanical, and thermal properties of the thus-filled composites were tested. The results showed that: (1) both CF and LF had significant influence on physical, mechanical, and thermal properties of PP or PLA composites; (2) CF benefited to most mechanical properties of the composites, while LF reduced water uptake and improved thermal stability. WF was more hydrophilic and was easier to thermal degrade than CF and LF. However, the flexural modulus was high in WF groups; (3) for different polymer matrix, the interface bonding between WF components and polymer was different. The interaction between LF and PP was stronger, while it was more compatible for CF and PLA, which showed totally contrary influence on the crystallization behavior of PP and PLA.

Preparation and characterization of polypropylene/silica composite particle with interpenetrating network via hot emulsion sol–gel approach

A novel interpenetrating structural ultrafine polypropylene-silica nanocomposite particles were synthesized by a modified sol–gel approach in the presence of the melt polypropylene emulsion. A series of samples with different polypropylene content was prepared to investigate the unique characteristics of this original nanocomposite. The thermal gravimetric analysis and differential scanning calorimetry results showed that the nanocomposites had the interpenetrating structure and good thermal stability, and the crystallization behavior of polypropylene was confined by the silica matrix. The interpenetrating structure of nanocomposites was also suggested by the nitrogen adsorption–desorption measurement results. The scanning electronic microscope and transmission electron microscopy images indicated that the nanocomposites had irregular particle morphology. The nanoparticle tracking analysis results show that the mean size of the nanocomposites was around 160nm. According to the results obtained from different measurements, a reasonable formation mechanism was proposed.