Analysis of the isothermal crystallization of isotactic polypropylene nucleated with sorbitol derivatives

The isothermal crystallization kinetics of isotactic polypropylene (iPP) and iPP nucleated with the sorbitol derivatives 1,3:2,4‐bis(4‐methyldibenzylidene)sorbitol and 1,3:2,4‐bis(3,4‐dimethylbenzylidene)sorbitol was studied, along with the subsequent melting behavior, as a function of the nucleating agent concentration. The influence of the agents on the crystallization rate, crystallization temperature, and crystallization range was examined. The isothermal crystallization temperature increased, along with the crystallization rate, with increasing nucleating agent concentration. The maximum effect of the additives occurred at concentrations of 0.3% or greater. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2261–2274, 2003     

Mechanical,flammability, and crystallization behavior of polypropylene composites reinforced by aramid fibers

In this article, we report the mechanical and thermal properties, together with the crystallization and flammability behaviors, of pure polypropylene (PP) and PP/aramid fiber (AF) composites with AF loadings of 5, 10, 20, 30, and 40 wt %. The mechanical properties of the samples were evaluated by tensile and izod notched impact tests, and the results show that the tensile strength of the composites could reach up to 67.8 MPa and the izod notched impact strength could rise to 40.1 kJ/m². The structure and morphology were observed by scanning electron microscopy and polarized optical microscopy, respectively. This demonstrated that a solid interface adhesion between the matrix and fibers was formed. The thermal and crystalline behaviors of the PP/AF composites were also investigated by thermogravimetric analysis and differential scanning calorimetry analysis, and the results show that the char residue of the PP/AF composites improved greatly with increasing AF loading, and the highest value could reach up to 23.7% in the presence of 40 wt % AF. The supercooling degree, initial crystallization temperature, and crystallization percentage were used to characterize the crystallization behavior of the PP/AF composites, and the results indicate that the AFs had positive effects on the promotion of PP nucleation, which can usually improve the mechanical properties of composites. Moreover, the flammability analysis of the PP/AF composites demonstrated that the presence of AFs could significantly decrease the peak heat release rate and the total heat release and reduce the melt-dripping of the PP/AF composites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Semiquantitative analysis of the thermal degradation of polypropylene

The pyrolysis product distributions of polypropylene (PP) were observed, and the experimental data were semiquantitatively analyzed by computer simulation with the Molic mouse method. Studies by many researchers on PP decomposition and experimental results achieved in this study were examined. The product distributions were classified into three types of hydrocarbon products: 3n, 3n + 1, and 3n + 2 (n = monomer unit). The ratio showed the characteristic tendency, and the thermal degradation of PP appeared to include very complicated scission paths. The Molic mouse method, which was applied to the analysis of the thermal rearrangement and decomposition of poly(phenylene ether), was used to simulate the experimental results. After several steps by which the proper model was constructed, the scission probabilities to generate 2 × 3n, 3n + 1 and 3n + 2, 3n and 3n + 1, 3n and 3n + 2, C9, and C15 hydrocarbons were successfully calculated to be 0.25, 0.67, 0.47, 0.35, 1.0, and 0.33, respectively. This meant that the scission process of PP was very simple except for the C9 and C15 generation paths, for which the cyclic compounds were stable intermediates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1465–1472, 2003     

Effects of rosin‐type cocrystal nucleating agents on the crystallization process and the properties of polypropylene

A new kind of rosin‐type nucleating agent for polypropylene (PP), the cocrystal of dehydroabietic acid, potassium dehydroabietate, and sodium dehydroabietate, was prepared, and the effects of the nucleating agents on the mechanical and crystallization properties of PP were also studied. The results of differential scanning calorimetry and X‐ray diffraction proved that the cocrystal of dehydroabietic acid and compound alkali dehydroabietate was formed rather than a simple blend of dehydroabietic acid and single alkali dehydroabietate. When it was added to PP, the size of the PP spherulite decreased; the mechanical properties, crystallization temperature, and transparency of PP were substantially improved. Thus, the cocrystal of dehydroabietic acid, potassium dehydroabietate, and sodium dehydroabietate acted as a more effective nucleating agent for PP. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2137–2141, 2003     

Flammability and mechanical properties of wood flour‐filled polypropylene composites

Polypropylene (PP) composites filled with wood flour (WF) were prepared with a twin‐screw extruder and an injection‐molding machine. Three types of ecologically friendly flame retardants (FRs) based on ammonium polyphosphate were used to improve the FR properties of the composites. The flame retardancy of the PP/WF composites was characterized with thermogravimetric analysis (TGA), vertical burn testing (UL94‐V), and limiting oxygen index (LOI) measurements. The TGA data showed that all three types of FRs could enhance the thermal stability of the PP/WF/FR systems at high temperatures and effectively increase the char residue formation. The FRs could effectively reduce the flammability of the PP/WF/FR composites by achieving V‐0 UL94‐V classification. The increased LOI also showed that the flammability of the PP/WF/FR composites was reduced with the addition of FRs. The mechanical property study revealed that, with the incorporation of FRs, the tensile strength and flexural strength were decreased, but the tensile and flexural moduli were increased in all cases. The presence of maleic anhydride grafted polypropylene (MAPP) resulted in an improvement of the filler–matrix bonding between the WF/intumescent FR and PP, and this consequently enhanced the overall mechanical properties of the composites. Morphological studies carried out with scanning electron microscopy revealed clear evidence that the adhesion at the interfacial region was enhanced with the addition of MAPP to the PP/WF/FR composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

Thermal stability and crystallization kinetics of isotactic polypropylene/organomontmorillonite nanocomposites

The thermal stability and crystallization kinetics of isotactic polypropylene (iPP) and iPP/organomontmorillonite (organo‐MMT) nanocomposites were investigated with differential scanning calorimetry and thermogravimetry. The incorporation of organo‐MMT up to a concentration of 4 wt % did not affect the melting temperature of iPP but did increase the peak thermal degradation temperature by 60°C. The isothermal crystallization kinetics showed that the addition of organo‐MMT increased the crystallization rate of iPP but reduced the isothermal Avrami exponent. The crystallization temperature of the nanocomposites measured with nonisothermal crystallization was higher than that of plain iPP, and this indicated an enhanced crystallization rate. The nonisothermal Avrami exponent, like the isothermal exponent, decreased with the addition of organo‐MMT, and this suggested changes in the crystallite growth geometry. Subsequently, the tensile yield strength and the tensile modulus both increased, but the elongation at break and the notched Izod impact strength did not change significantly. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3404–3415, 2003     

Influence of a novel β‐nucleating agent on the structure,morphology, and nonisothermal crystallization behavior of isotactic polypropylene

The crystalline structure, morphology, and nonisothermal crystallization behavior of isotactic polypropylene (iPP) with and without a novel rare earth‐containing β‐nucleating agent (WBG) were investigated with wide‐angle X‐ray diffraction, polar optical microscopy, and differential scanning calorimetry. WBG could induce the formation of the β form, and a higher proportion of the β form could be obtained by the combined effect of the optimum WBG concentration and a lower cooling rate. The content of the β form could reach more than 0.90 in a 0.08 wt % WBG nucleated sample at cooling rates lower than 5°C/min. Polar optical microscopy showed that WBG led to substantial changes in both the morphological development and crystallization process of iPP. At all the studied cooling rates, the temperature at which the maximum rate of crystallization occurred was increased by 8–11°C in the presence of the nucleating agent. An analysis of the nonisothermal crystallization kinetics also revealed that the introduction of WBG significantly shortened both the apparent incubation period for crystallization and the overall crystallization time. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Morphology and properties of isotactic polypropylene modified with hydrocarbon resin MBG273. I. Binary blends

We investigated a system formed of isotactic polypropylene (iPP) and hydrogenated hydrocarbon resin MBG273 (up to 30 wt % resin) to study the influence of the composition on the morphology, structure, and properties of its blends and derived films. All the blends, after the mixing of the components in the melt and cooling at room temperature, were formed by a crystalline phase of iPP and by one homogeneous phase formed by amorphous iPP and the MBG273 resin. The presence of MBG273 did not influence the crystalline structure of iPP, which remained, for every blend, α‐monoclinic, but it reduced the crystallization temperature and nucleation density of iPP. Differential scanning calorimetry and dynamic mechanical thermal analysis showed an increase in the glass‐transition temperature with the resin content, confirming the formation of one amorphous phase. Tensile property analysis indicated an increase in Young's modulus and a decrease in the elongation at break of films as a function of the resin content in the blends. The water vapor permeability and tensile mechanical properties were related to an increase in the glass transition with the addition of MBG273. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3454–3465, 2004     

Morphology,crystallization and melting behaviour of films of isotactic polypropylene blended with ethylene-propylene copolymers and polyisobutylene

The crystallization, the morphology and the thermal behaviour of thin films of isotactic polypropylene (iPP) blended with elastomers such as random ethylene-propylene copolymers (EPM) with different ethylene content and polyisobutylene (PiB) were investigated by means of optical microscopy, differential scanning calorimetry and wide angle X-ray diffractometry. During crystallization EPM copolymers are ejected on the surface of the film forming droplet-like domains. A different morphology is observed in iPP/PiB blends. For these mixtures the elastomers separate from the iPP phase forming spherical domains that are incorporated in the iPP intraspherulitic regions. Both EPM and PiB elastomers act as nucleant agents for iPP spherulites. This nucleation efficiency is strongly dependent on the chemical structure and molecular mass of the elastomers. The addition of EPM causes an elevation of the observed and equilibrium melting temperature of iPP. This unusual effect may be accounted for by assuming that the elastomers are able to extract selectively the more defective molecules of iPP. The depression of the growth rate of spherulites and the observed and equilibrium melting temperature of iPP, noted in iPP/PiB blends, suggests that these two polymers have a certain degree of compatibility in the melt.     

Effects of calcium carbonate and its purity on crystallization and melting behavior,mechanical properties,and processability of syndiotactic polypropylene

Calcium carbonate‐filled syndiotactic poly(propylene) (CaCO₃‐filled s‐PP) was prepared in a self‐wiping, co‐rotating twin‐screw extruder. The effects of CaCO₃ of varying particle size (1.9, 2.8 and 10.5 μm), content (0–40 wt %), and type of surface modification (uncoated, stearic acid‐coated, and paraffin‐coated) on the crystallization and melting behavior, mechanical properties, and processability of CaCO₃‐filled s‐PP were investigated. Non‐isothermal crystallization studies indicate that CaCO₃ acts as a good nucleating agent for s‐PP. The nucleating efficiency of CaCO₃ for s‐PP was found to depend strongly on its purity, type of surface treatment, and average particle size. Tensile strength was found to decrease, while Young's modulus increased, with increasing CaCO₃ content. Both types of surface treatment on CaCO₃ particles reduced tensile strength and Young's modulus, but improved impact resistance. Scanning electron microscopy (SEM) observations of the fracture surfaces for selected CaCO₃‐filled s‐PP samples revealed an improvement in CaCO₃ dispersion as a result of surface treatment. Finally, steady‐state shear viscosity of CaCO₃‐filled s‐PP was found to increase with increasing CaCO₃ content and decreasing particle size. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 201–212, 2004     

Polymorphism in a metallocenic isotactic polypropylene as revealed by means of FTIR spectroscopy: Influence of the processing conditions

The polymorphic structure in specimens of a metallocenic isotactic polypropylene, processed under different conditions, has been studied by means of wide‐angle X‐ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The proportions of the different polymorphs have been evaluated, and the influence of the processing parameters (nucleating agents, cooling rate, and nature of the surface of the molds) has been analyzed. The combination of WAXS, DSC, and FTIR results confirms the adequacy of this last technique to obtain quantitative information about the competition between the crystalline phases of polypropylene. It has also been proved that the nature of the mold can enhance considerably the activity of beta‐nucleating agents. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of vibration on crystal morphology and structure of isotactic polypropylene in nonisothermal crystallization

An experimental study on crystal structure and morphology of isotactic polypropylene (iPP) subjected to vibration was carried out on a laboratory apparatus. Crystallite size, crystal structure, and crystallinity of iPP under vibration or nonvibration were investigated through differential scanning calorimeter (DSC), wide angle X‐ray diffraction (WAXD), and polarized optical microscopy (POM). The results reveal that at high cooling rate, the crystallinity of samples under vibration decreases, and at low cooling rate it remains constant because of chain relaxation. On the other hand, the sizes of the iPP spherulites under vibration decrease as compared with those without vibration. Taking the relaxation of the iPP chain into consideration, we believe that the influence of vibration conditions on the main α‐form of the iPP crystal is rather complex. An obvious increase of β‐form content in the crystal phase results from the imposition of vibration. The results indicate that the content of β‐iPP is dependent on vibration amplitude and time. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2187–2195, 2004     

Effects of rosin‐type nucleating agent and low density polyethylene on the crystallization process of polypropylene

In this article, the influence of rosin‐type nucleating agent (Nu–Na) and low density polyethylene (LDPE) on the crystallization process of polypropylene (PP) from the melt state was studied by differential scanning calorimeter and polarization microscope. It was found that LDPE obstructed the crystallization of PP, decreased the crystallization rate of PP. The rosin‐type nucleating agent Nu–Na substantially improved the rate of crystallization, and decreased the size of spherulites also. The cooperative effect of LDPE and Nu–Na made the crystallization rate of PP increase greatly, the spherulites of PP became much smaller and dispersed more uniformly, and the transparency of PP was further improved evidently. The crystallization temperature (T_c) and melting temperature (T_m) of PP and LDPE in PP/LDPE/Nu–Na (97:3:0.5) were not affected by the number of mixed passes—the nuclei migration from PP to PE had not happened in the mixed passes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2804–2809, 2003     

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

Molten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nonisothermal crystallization and multiple melting behaviors of β‐nucleated impact‐resistant polypropylene copolymer

As a substitute of isotactic polypropylene in applications requiring excellent fracture resistance, impact‐resistant polypropylene copolymer (IPC) has attracted much attention in recent years. In this study, a highly effective β‐form nucleating agent (β‐NA; an aryl amide compound) was introduced into IPC, and our attention was focused on the nonisothermal crystallization and subsequent melting behaviors of the nucleated samples. The nonisothermal crystallization behaviors were investigated on the basis of the different cooling rates and different concentrations of β‐NA with differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), and polarized optical microscopy. The results show that both the cooling rate and concentration of β‐NA greatly determined the nonisothermal crystallization process and subsequent multiple melting behaviors. Further results show that the multiple melting behaviors were related to the transition in β crystallites and those between the β and α crystallites. The morphologies of the dispersed particles and the supermolecular structure of the matrix were characterized with scanning electron microscopy. Finally, the effect of the β‐NA concentration on the fracture resistance of IPC was evaluated by measurement of the notched Izod impact strength. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Nonisothermal crystallization,melting behavior,and morphology of polypropylene/metallocene‐catalyzed polyethylene blends

The nonisothermal crystallization, melting behavior, and morphology of blends of polypropylene (PP) and a metallocene‐catalyzed polyethylene (mPE) elastomer were studied with differential scanning calorimetry, scanning electron microscopy, polarized optical microscopy, and X‐ray diffraction. The results showed that PP and mPE were partially miscible and could form some cocrystallization, although the extent was very small. A modified Avrami analysis and the Mo method were used to analyze the nonisothermal crystallization kinetics of the blends. The values of the Avrami exponent indicated that the crystallization nucleation of the blends was homogeneous, the growth of the spherulites was three‐dimensional, and the crystallization mechanism of PP was not affected by mPE. The crystallization activation energy was estimated with the Kissinger method. Interesting results were obtained with the modified Avrami analysis and Mo and Kissinger methods, and the conclusions were in good agreement. The addition of less mPE increased the overall crystallization rate of PP. The relationship between the composition and morphology of the blends was examined. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1203–1210, 2004     

Synergistic effect of phosphorus-containing nanosponges on intumescent flame-retardant polypropylene

A novel nanosponge (NS) was synthesized via the crosslinking of β-cyclodextrin with epoxy resin. Subsequently, a phosphorus-containing nanosponge (P–NS) was prepared by the absorbance of resorcinol bis(diphenyl phosphate) into the NS, and it was used as a synergistic agent of intumescent flame retardance in a polypropylene (PP)/melamine pyrophosphate/pentaerythritol composite. The synergistic effect between P–NS and the intumescent flame retardant (IFR) was investigated by thermogravimetry, limiting oxygen index (LOI) testing, vertical burning (UL-94) testing, cone calorimeter testing, and scanning electron microscopy (SEM). The results show that P–NS significantly improved the flame retardancy of the PP/IFR composite. When 3.0 wt % P–NS replaced the same amount of IFR in the composite, the LOI value increased from 29.0 to 32.5%, the UL-94 rating was enhanced from V-1 to V-0, and the peak heat release rate decreased substantially from 343 to 235 kW/m². Simultaneously, the total heat release and mass loss rate decreased dramatically. Furthermore, the SEM results show that the quality of char formation of the PP/IFR/P–NS was superior to that of the PP/IFR composite. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fire-retardant synergy of tris(1-methoxy-2,2,6,6-tetramethyl-4-piperidinyl)phosphite and aluminum hypophosphite/melamine hydrobromide in polypropylene

Fire-retardant synergy of tris(1-methoxy-2,2,6,6-tetramethyl-4-piperidinyl) phosphite (NORPM) and aluminum hypophosphite (ALHP)/melamine hydrobromide (MHB) in polypropylene (PP) was researched by limiting oxygen index (LOI) measurement, vertical burning test, and cone calorimetric test (CCT). The results reveal that NORPM, together with ALHP/MHB, has an exceptional fire-retardant synergy in PP. For the FR-PP containing 1.0 wt% of ALHP and 1.5 wt% of MHB, combining NORPM in 0.20 wt% concentration can make its LOI value increases from 19.0% to 26.1%, the fire-retardant efficiency (EFF) increases from 0.48 to 3.07, synergistic efficiency (SE) is as high as 2.18, the vertical combustion rating can be improved from none to UL 94 V-2, fire performance index (FPI) increases from 58.7 to 84.9, and various heat release parameter values are reduced markedly. NORPM induces the synergy with ALHP/MHB primarily by the mechanisms as follows: active free radicals formed through the decomposition of NORPM, such as CH₃, CH₃O, nitro radicals and amino radicals, together with PH₃ and HBr, trap active radicals (HO·) for combustion. Consequently, the reaction chain of combustion is terminated, which makes gaseous phase fire retardancy more efficient.     

Improvement of the photostability of isotactic polypropylene by the incorporation of ethylene

The effect of the incorporation of ethylene on the photostability of isotactic poly(propylene) (iPP) was studied with the aim of improving the photostability. iPP was prepared with a random ethylene sequence (ethylene–propylene random copolymer, rPP), and the photooxidative degradation behavior was compared with that of homogeneous iPP. Both samples were thermally post‐treated under vacuum to ensure the same crystallinity. The degradation behavior was studied by infrared spectroscopy (IR), gel permeation chromatography (GPC), and temperature rising elution fractionation (TREF) measurements. The rates of hydroperoxide and carbonyl formation in the irradiated iPP increased with irradiation time for > 192 h, whereas those in the irradiated rPP are almost constant after 96 h. The change in molecular weight with the irradiation time showed similar behavior, suggesting that the degradation reaction in the irradiated rPP was suppressed after 96 h. The degradation behavior of rPP was thought to be due to the dissociation of the methyl group, which leads to the termination of degradation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1863–1867, 2002