Synergistic toughening effects of nucleating agent and ethylene–octene copolymer on polypropylene

The synergistic toughening effect of nucleating agent (NA) and ethylene–octene copolymer (POE) on polypropylene was studied in the present work. Two different nucleating agents, such as α-form nucleating agent 1,3 : 2,4-bis (3,4-dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) and β-form nucleating agent aryl amides compounds (TMB-5), were selected to blend with PP or PP/POE blends, respectively. The results show that PP containing 0.5–0.25 wt % DMDBS or 0.5–0.25 wt % TMB-5 has relatively low impact strength. For PP/POE blends, although the impact strength increases gradually with the increasing of POE content, high content of POE is needed to obtain the available PP toughness. However, once nucleating agent and POE are simultaneously added into PP, PP/POE/NA blends show great improvement of toughness even at low POE content. Furthermore, the synergistic toughening effect of POE/TMB-5 is more apparent than that of POE/DMDBS. SEM results show that whether DMDBS or TMB-5 has no apparent effect on the morphologies of POE in the PP/POE/NA blends. Further investigations using DSC and POM indicate that both DMDBS and TMB-5 induce the apparent enhancement of the crystallization temperature of PP and the sharp decrease of spherulites size of PP in the PP/POE/NA blends. The possible synergistic toughening mechanism is discussed in the work. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Cooperative effect of molecular orientation and β-form on toughening injection-molded isotactic polypropylene with β-nucleating agent

The toughness of conventional injection-molded isotactic polypropylene bars has been investigated with respect to the notch location and β-nucleating agent. Superior toughness is achieved in the β-nucleated bars with notches near the gate, but is absent in the near-gate bars without β-nucleating agent or in the β-nucleated ones far from the gate. With detailed structural analysis across the sample thickness, it is indicated that in these tough bars, extensive flow close to the surface suppresses the formation of β-form to a large extent but favors the generation of oriented α-form, whereas toward inner region, an opposite tendency is presented with the decay of flow strength and the activation of β-nucleating agent. Allowing that this peculiar hierarchical structure is absent in other two kinds of bars, it is deduced that the cooperative effect of molecular orientation of α-form in the skin layer and rich β-form in the inner region is responsible for the significant toughness enhancement in the β-nucleated bars near the gate. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The β-nucleating effect of wollastonite-filled isotactic polypropylene composites

Wollastonite (W) with β-nucleating effect (β-W₁₀₀) for iPP crystallization was obtained through reaction between Ca²⁺ in wollastonite and pimelic acid (PA) and the β-iPP composites filled by different content of β-W₁₀₀ were prepared. The effect of PA and wollastonite contents on β-nucleation, crystallization and melting behavior, and crystalline morphology of W and β-W₁₀₀-filled iPP composites was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction, and polarizing optical microscopy. The results indicated that incorporation of W and β-W₁₀₀ increase the crystallization peak temperature of iPP due to its heterogeneous nucleating ability. And iPP/W composites predominantly crystallize in the α-phase iPP and iPP/β-W₁₀₀ composites in the β-phase iPP. The results of DSC multi-scanning in same and different melting temperatures showed that β-W₁₀₀ not only has strong heterogeneous β-nucleating effect but also DSC multi-scanning in same and different melting temperatures has no influence on the heterogeneous β-nucleating effect of β-W₁₀₀. The β-iPP containing high wollastonite content with high β-phase content can be easily prepared.     

Influences of molecular structure on the isothermal crystallization behavior and mechanical properties of β-nucleated isotactic polypropylene

ABSTRACT

To investigate the influence of molecular structure of isotactic polypropylene (iPP) on the crystallization behavior and mechanical properties of its β-nucleated product, in this study, four iPP samples were selected. Results of molecular structure characterizations revealed that the molecular weight of the iPP resins are close, while the average isotacticities and the isotacticity distributions of the samples are quite different from each other. Isothermal crystallization kinetics study suggested that due to their different average isotacticities and isotacticity distributions, the crystallinity difference of the four samples is as high as 5.3%, and the crystallinity of β-crystal difference is as high as 7.2%. Generally, the higher the average isotacticity of the iPP and the wider its distribution are, the stronger the β-crystallization ability of the sample is. Moreover, the mechanical measurement revealed that high average isotacticity and the wide isotacticity distribution of the iPP resulted in the strong the β-crystallization ability and thus better the toughness.     

Effects of ultrasound on the conformation and crystallization behavior of isotactic polypropylene and β-isotactic polypropylene

The effects of ultrasonic irradiation on conformation and crystalline structure of isotactic polypropylene (iPP) and β-isotactic polypropylene (β-iPP) were studied by means of Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results demonstrate that ultrasonic irradiation decreases the helical conformation order and changes the crystalline structures: for iPP, T_m and T_onset decrease, T_m − T_onset becomes larger; for β-iPP, the β_c, (1 − λ)_β and k_β increase, the intensities of α(040) plane and α(130) plane in WAXD profiles decrease evidently. For both iPP and β-iPP, the crystallinity decreases, d-spacing increases, the crystallite size L decreases and ultrasound shows a selective effect on the growth of β-crystal. Furthermore, DSC and WAXD were employed to observe the effect of ultrasound on the “melting memory effect” of β-iPP. The results indicate that ultrasonic irradiation destructs the existing “locally ordered structure” in β-iPP melt, as a result greatly inhibits the βα-recrystallization of β-iPP samples during heating.     

Enhancement of β-nucleated crystallization in polypropylene random copolymer via adding isotactic polypropylene

Polypropylene random copolymer (PPR) is one of important polypropylene types for the application fields needing for excellent toughness. Because of the random copolymer chain configuration, the polymorphic behavior of PPR is difficult to be altered even by adding β-nucleating agent (β-NA). In this study, a promising method was developed by adding isotactic polypropylene (iPP) into PPR/β-NA blend, which has leaded to a surprising enhancement in the β-crystallization capability of PPR. At the optimal component condition, the β-crystal content of PPR can reach the highest level of 92 % and the β-crystallization capability is improved by 56%. As a result of high β-crystal contents, a superior mechanical toughness has been attained. On the other hand, the fractional crystallization experiment suggests that the stereoregular chains of iPP could assist the formation of primary β-nuclei at the very early stage of crystallization. This special crystallization event dominates the final polymorphic composition in PPR. Furthermore, it is demonstrated that impact polypropylene copolymer (IPC) can be used to substitute iPP for the improvement of β-crystal content of PPR. This provides a huge possibility to improve the low temperature properties of PPR to enlarge its applications.     

Changes in optical texture of α-spherulites of isotactic polypropylene upon partial melting and recrystallization

Textural changes in -phase spherulites of isotactic polypropylene (iPP) in a sequence of thermal events were examined by means of polarized light microscopy (PLM). This sequence of thermal events involves isothermal crystallization (atT_c = 117 to 140 °C), followed by heating (at 5 °C/min) to nearly complete melting, and then recrystallization upon cooling (at -40°C/min) to T_c During isothermal crystallization, the a-spherulites were of mixed birefringenceat T_c = 117 to 127 °C or of negative birefringence at T_c = 140 °C; upon heating towards melting, the spherulitec birefringence consistently truned negative. More interestingly, after recrystallization during cooling back to Tc from nearcomplate melting, all spherulites exhibited positive birefringence. The recrystallization could also result in speckles of positive birefringence when T_c was high or upon slower cooling. The changes in optical texture are explained in terms of contributions from tangential (or, cross-hatched) subsidiary lamellae which (as compared to the radial dominant lamellae) are relatively low-melting but thicken and recrystallize more readily in the present temperature range.     

Comparison of premelting and recrystallization behaviour of β-phase isotactic polypropylene by heating and cooling at different rates

Different heat treatments (heating and subsequent cooling with quick or slow rates but up to the same temperature in the melting range) were made on β-polypropylene (β-PP), and premelting and recrystallization behaviour were compared using d.s.c. and WAXS measurements. The results suggest that, under our experimental conditions, lamellar growth during heating and cooling takes place chiefly at the expense of, or under the existence of a melt neighbouring to the lamella, and that the growth in the solid state by molecular refolding plays a minor role. The influence of the recrystallized α-phase, yielded during heating, on the growth of β-form is also discussed.     

Dynamic crystallization and melting behavior of β-nucleated isotactic polypropylene polymerized with different Ziegler-Natta catalysts

Large amount of work has be published on the dynamic crystallization and melting behavior of β-nucleated polypropylene (β-PP). However, the relationship between molecular structure and dynamic crystallization behavior of β-PP is still not clear. In this study, the dynamic crystallization and melting behavior of two β-nucleated isotactic polypropylene (β-iPP) with nearly same average isotacticity but different stereo-defect distribution, were studied by differential scanning calorimetry (DSC), wide angel X-ray diffraction (WAXD) and temperature modulated DSC (TMDSC). The results indicated that stereo-defect distribution of iPP can significantly influence the dependence of the β-crystal content and thermal stability on the cooling rate. NPP-A with less uniform stereo-defect distribution favors the crystallization at higher temperature region and the formation of β-crystal with high thermal stability in all cooling rates concerned, moreover, the β-crystal content is influenced by cooling rate; for NPP-B with more uniform distribution of stereo-defect, the crystallization temperature and the regular insertion of molecular chains can be reduced in a larger extent. NPP-B is more suitable for the formation of high proportion of β-crystal in both low and high cooling rates, meanwhile, the thermal stability of crystal is sensitive to the cooling rate. This work provides a new insight into the design of β-iPP in dynamic crystallization.     

Influences of molecular weight on the non-isothermal crystallization and melting behavior of β-nucleated isotactic polypropylene with different melt structures

Polymer Bulletin - Previous studies reported that by tuning the fusion temperature T f (i.e., controlling the melt structure), the β-nucleation efficiency of β-nucleated isotactic...     

Effect of α-, β-, γ-, and δ-Tocotrienol on the Oxidative Stability of Lard

Despite the structural similarity of tocopherols, the antioxidative activities of tocotrienol homologues have not been studied often. In this study, the antioxidant activities of α-, β-, γ-, and δ-tocotrienols at various concentrations from 100 to 1,000 ppm in lard were evaluated. Headspace oxygen content of the lard without tocotrienol decreased from 21.1 to 10.7 % and the peroxide value increased from 0.4 to 33.4 mequiv/kg after 7 days of storage at 55 °C in the dark. α-Tocotrienol at 100 ppm and β-tocotrienol at 100 and 200 ppm effectively improved the oxidative stability of lard; however, the antioxidative activities of α- and β-tocotrienol reduced as the concentration increased to 1,000 ppm. The γ- and δ-tocotrienols improved the oxidation stability of lard and the effectiveness was essentially same at all concentrations (p > 0.05). The antioxidative activities of tocotrienols in the autoxidation of lard increased in the order of α-, β-, γ-, and δ-tocotrienols. The activities of tocotrienols in lard were different depending on the type of homologues and concentrations.     

Reinforcing and toughening isotactic polypropylene through shear-induced crystallization and β-nucleating agent induced crystallization

In this article, iPP with β-nucleating agent was molded by sequential co-injection molding (SCIM), in which skin and core melt were injected into the mold cavity one after the other. The microstructure and mechanical properties of samples were investigated by polarized optical microscope (POM), wide angle X-ray diffraction (WAXD) and mechanical property test. Results show that plastic parts molded by SCIM have double shear layers due to twice shear induced by filling flow of skin and core melt. In the shear layers especially the layer at the overlap of skin and core material, shear promoted the formation of highly oriented structures (shish-kebabs) but inhibited the produce of β-form. In core layer of skin material and core layer of core material, β crystals are predominant. The combination of oriented structures (shish-kebabs) and β crystals endow iPP with high strength and toughness. This work demonstrates a new approach to achieve high-performance polymer materials based on general plastics by manipulation strategy for morphology and structure.     

Effect of α-, β-, γ-, and δ-Tocotrienol on the Singlet Oxygen Oxidation of Lard

The impacts of four different types of tocotrienol homologues on the singlet oxygen oxidation of lard were evaluated by measuring the headspace oxygen content and the peroxide value. Singlet oxygen oxidation of lard was induced by chlorophyll photosensitization. Samples of 0.100, 0.250, and 0.400 M lard in methylene chloride containing chlorophyll and α‐, β‐, γ‐, or δ‐tocotrienol were prepared and stored under light at 3,000 lux for 4 h. All tocotrienol homologues at 1.20 mM significantly prevented the singlet oxygen oxidation of lard. Chlorophyll under light produced singlet oxygen at 1.09 μmol oxygen/mL headspace/h. A steady state kinetic study showed that tocotrienols reduced the singlet oxygen oxidation of lard by quenching the singlet oxygen. Singlet oxygen reacted with lard at 6.50 × 10⁴M^?1 s^?1. α‐, β‐, γ‐, and δ‐tocotrienol quenched singlet oxygen with the rate of 2.16, 1.99, 2.05, and 0.800 × 10⁷M^?1 s^?1, respectively. Among them, α‐tocotrienol significantly prevented singlet oxygen oxidation of lard.     

Removal of Cadmium and Chromium Ions Using Modified α, β, and γ-Cyclodextrin Polymers

The adsorption of cadmium and chromium from aqueous solutions with epichlorohydrin cross-linked α, β, and γ-cyclodextrin polymers (CDPs) was investigated under a wide range of experimental conditions including pH, metal concentration, and CDP amounts. Recent studies have shown that α, β, and γ-CDPs follow the Freundlich, Langmuir, and Brunauer–Emmett–Teller models and are capable of reducing heavy metal ion concentrations in water to desirable levels with enhanced sorption capabilities. The removal of cadmium and chromium achieved equilibrium in 4 h. The pH of effective cadmium removal was 6.5 to 7.5, and phosphate buffer favored the removal of cadmium in the presence of potassium ion. Chromium removal was optimal at pH 3.6 to 6.5. Current technologies can remove 99% cadmium ions from solution, yet this process is not very efficient for chromium ions. Furthermore, a small amount of CDPs can remove large quantities of heavy metal ions and can then be regenerated with acid for reuse. The present results are promising for using inexpensive CDPs as a low-cost material that is effective in remediating waters contaminated with heavy metal species. The sorption kinetics of CDPs along with the conditions to adsorb cadmium and chromium are reported here for the first time.     

Formation of β-cylindrites under supercooled extrusion of isotactic polypropylene at low shear stress

Formation of β-cylindrites of isotactic polypropylene under various wall shear stress (σ_w), supercooled temperature of melt (T_e) and crystallization temperature (T_c) has been investigated by polarized light microscopy (PLM), wide angle X-ray diffraction (WAXD), and differential scanning calorimeter (DSC). To have better control over the thermomechanical history, instead of a reciprocating screw, the samples were prepared by extruding supercooled melt through capillary die. β-cylindrites can be observed by PLM in the extruded specimen even at a lower σ_w (0.020 MPa), and the number of β-cylindrites nuclei increases rapidly with the lowering of T_e. The nucleation density of β-cylindrites increases with the raising of wall shear stress under a given T_e of 160 °C. Furthermore, at lower supercooled temperature of melt (145 °C), the radius of β-cylindrites decreases with the increasing of σ_w, and the number of β-cylindrites nuclei almost remain invariant. At relatively higher σ_w (0.090 MPa), a saturation of β-cylindrites nuclei is observed with decreasing T_c. A modified model based on above results has been proposed to explain the effect of the original structure of quiescent supercooled melt on the formation of β-cylindrites under low shear stress.     

Comparative effects of α- and γ-linolenic acids on rat liver fatty acid oxidation

It has been reported that both n−3 and n−6 octadecatrienoic acids can increase hepatic fatty acid oxidation activity. It remains unclear, however, whether different enzymes in fatty acid oxidation show a similar response to n−3 and n−6 octadecatrienoic acids. The activity of hepatic fatty acid oxidation enzymes in rats fed an oil mixture rich in α-linolenic acid (18:3n−3) and borage oil rich in γ-linolenic acid (18:3n−6) was therefore compared to that in rats fed an oil mixture rich in linoleic acid (18:2n−6) and a saturated fat (palm oil) in this study. Linseed oil served as the source of 18:3n−3 for the oil mixture rich in this octadecatrienoic acid and contained 30.6% 18:3n−3 but not 18:3n−6. Borage oil contained 25.7% 18:3n−6 and 4.5% 18:3n−3. Groups of seven rats each were fed diets containing 15% various fats for 15 d. The oxidation rate of palmitoyl-CoA in the peroxisomes was higher in rats fed a fat mixture rich in 18:3n−3 (3.03 nmol/min/mg protein) and borage oil (2.89 nmol/min/mg protein) than in rats fed palm oil (2.08 nmol/min/mg protein) and a fat mixture rich in 18:2n−6 (2.15 nmol/min/mg protein). The mitochondrial palmitoyl-CoA oxidation rate was highest in rats fed a fat mixture rich in 18:3n−3 (1.93 nmol/min/mg protein), but no significant differences in this parameter were seen among the other groups (1.25–1.46 nmol/min/mg protein). Compared to palm oil and fat mixtures rich in 18:2n−6, a fat mixture rich in 18:3n−3 and borage oil significantly increased the hepatic activity of carnitine palmitoyl-transferase and acyl-CoA oxidase. Compared to palm oil and a fat mixture rich in 18:2n−6, a fat mixture rich in 18:3n−3, but not fats rich in 18:3n−6, significantly decreased 3-hydroxyacyl-CoA dehydrogenase activity. Compared to palm oil and a fat mixture rich in 18:2n−6, borage oil profoundly decreased mitochondrial acyl-CoA dehydrogenase activity, but a fat mixture rich in 18:3n−3 increased it. 2,4-Dienoyl-CoA reductase activity was significantly lower in rats fed palm oil than in other groups. Compared to other fats, borage oil significantly increased Δ³, Δ²-enoyl-CoA isomerase activity. Activity was also significantly higher in rats fed 18:2n−6 oil than in those fed palm oil. It was confirmed that both dietary 18:3n−6 and 18:3n−3 increased fatty acid oxidation activity in the liver. These two dietary octadecatrienoic acids differ considerably, however, in how they affect individual fatty acid oxidation enzymes.     

Heating rate effects on the crystallization behavior of isotactic polypropylene from mesophase – A de-polarized light transmission study

It was ever reported in a communication of this journal that the large crystal grains having “bamboo leaf-like (BL)” morphology were produced by a rapid heating of isotactic polypropylene (iPP) from the mesophase. In order to optimize the condition to generate the BL crystals, heating rate effects on the crystallization behavior from the mesophase of iPP have been studied by utilizing a de-polarized light transmission (DPLT) method. The DPLT sensitively detected not only the cold crystallization from the mesophase around 100–120 °C but also the crystal grain growth in a narrow temperature region just below the melting temperature. With increasing the heating rate, both the temperature regions of the cold crystallization and the crystal grain growth shifted toward the higher temperatures. When the heating rate is slow (<20 °C/min), the crystal grain growth was not conspicuous. With increasing the heating rate, the rate of the crystal grain growth increased and showed a maximum when the heating rate is approximately 60–80 °C/min. However, excessively fast heating (>100 °C/min) also suppressed the crystal grain growth.     

Effect of annealing on the microvoid formation and evolution during biaxial stretching of β nucleated isotactic polypropylene

ABSTRACT

Cast films, based on β-iPP, were prepared via melt extrusion, and, then annealed at different temperature. The effect of annealing on the properties of the cast films, as well as on microvoid formation and evolution was investigated. After annealing, the initial lamellar structure was improved, and more secondary crystals were formed which decreased the numbers of tie chains, which was beneficial to cavitation. And the stability of β-crystal and β lamellar thickness distribution was improved. Finally, the membrane obtained by unannealed film had inferior pore-size distribution, while the annealed sample had uniform pore-size distribution.