Effect of sintering on processability vis-a-vis microcellular foaming of ultrahigh molecular weight polyethylene

Processing of ultrahigh molecular weight polyethylene (UHMWPE) involves sintering due to its high melt strength and no flowability above melting temperature. Variations in compression molding pressure during sintering lead to chain rearrangement at the sintered interphase and the boundary, affecting foamability. UHMWPE particles are sintered using compression molding; samples are prepared at two different pressures: UHPE-HP (80 bar) and UHPE-LP (40 bar) at 180°C. The sintering phenomenon of UHMWPE particles is observed through an optical microscope, and their effect on foaming was observed. UHPE-HP foams are systematically studied to obtain the foaming window. Increasing foaming pressure (80–120 bar) made UHPE-HP foams softer (0.350–0.219 g/cm³) with varying average cell size (26.37–46.1 μm) and foam cell density (3.98 × 10⁷–1.06 × 10⁸ cells/cm³), and compression modulus decreased from 9 to 5.4 MPa. DMA results showed a strong dependence of stiffness on crystallinity, and foamed samples exhibit higher stiffness than their unfoamed counterpart. The storage modulus for foamed samples decreases with increase in the gas content. The UHPE-LP foam is relatively softer, with a lower foam density (0.233 g/cm³), a higher expansion ratio, bigger average foam cells (35.13 μm), and lower foam cell density (9.33 × 10⁷ cells/cm³). This is due to constrained crystallinity at the interphase and pre-existing cavities, favoring the foaming.     

Foaming behavior of microcellular foam polypropylene/modified nano calcium carbonate composites

A new process was used to prepare microcellular foams with supercritical carbon dioxide as the physical foaming agent in a batch. The foaming temperature range of the new process was about five times broader than that of the conventional one. Characterization of the cellular structure of the original polypropylene (PP) and PP/nano‐CaCO₃ (nanocomposites) foams was conducted to reveal the effects of the blend composition and processing conditions. The results show that the cellular structure of the PP foams was more sensitive to the foaming temperature and saturation pressure variations than that of the nanocomposite foams. Uniform cells of PP foams are achieved only at a temperature of 154°C. Also, the low pressure of 20 MPa led to very small cells and a low cell density. The competition between the cell growth and cell nucleation played important role in the foam density and was directly related to the foaming temperature. Decreasing the infiltration temperature depressed the initial foaming temperature, and this resulted in significantly larger cells and a lower cell density. A short foaming time led to a skin–core structure; this indicated that a decrease in the cell size was found from skin to core, but the skin–core structure gradually disappeared with increasing foaming time. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of dynamic shear on the microcellular foaming of polypropylene/high‐density polyethylene blends

Dynamic shear in the axial direction of a rotor was vertically superposed on the melt flow direction, and its effects on the shear rate and melt strength were investigated theoretically. Polypropylene/high‐density polyethylene blends were microcellularly foamed with different vibration parameters. The experimental results were compared with those of a theoretical analysis, and the effects of dynamic shear on the foamability and ultimate cell structure were analyzed in detail. The theoretical results showed that the shear rate and melt strength increased with an increase in the vibration amplitude and frequency. The enhanced melt strength could effectively restrict cell growth, prevent cell rupture, and improve foamability. The experimental results showed that the cell orientation decreased and the cell structure was improved when axial dynamic shear induced by rotor vibrations was superposed on the melt flow direction. Furthermore, the cell diameter decreased and the cell density increased with increases in the vibration amplitude and frequency. The experimental results were very consistent with the theoretical analysis. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Investigation on the cell nucleation and cell growth in microcellular foaming by means of temperature quenching

The cell nucleation and real‐time cell growth with increasing cell growth time in microcellular foaming were investigated by means of temperature quenching in a supercritical CO₂ pressure‐quench process. Samples of uniform size and shape were saturated in a vessel under conditions of 100–180°C and 30 MPa, and then depressurized to the atmosphere in 10 s. After depressurization, these samples were removed from the vessel at prescribed intervals, and immediately immersed in an ice‐water slurry to obtain foamed samples with various cell growth times. It was found that the nucleation density is closely correlated to the gas absorption capacity of the polymer matrix, so that the final cell density should not be adopted as the nucleation density, as done commonly. The change of cell structure and mass density with increasing cell growth time was dominated by gas diffusion behavior, which was strongly influenced by the temperature. The final cell structure was mainly determined by the cell growth step, where gas diffusion played a key role. The final cell density was in direct proportion to the gas remaining in the substrate, which ranged from 6.0 × 10⁹ to 4.7 × 10⁶ cells/cm³. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 163–171, 2004     

Bubble growth in the microcellular foaming of CO2/polypropylene solutions

This article is concerned with bubble growth dynamics in the CO₂/polypropylene microcellular foaming process. The effect of the melt strength on the bubble growth was thoroughly investigated in theory for the first time. The theoretical results indicate that enhanced melt strength effectively restrains the bubble growth and stabilizes the bubble oscillation. Higher melt strength leads to lower bubble growth rate, shorter growth time, and smaller ultimate bubble size. Compared to the melt strength, the viscoelasticity and the gas pressure have less effect on the microcellular foaming process. The bubble growth varies a little as the viscoelasticity is varied. The bubble oscillation and growth rate are enhanced with increasing gas pressure, which leads to the augmentation of the bubble size. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Application of a novel organic nucleating agent: Cucurbit[6]uril to improve polypropylene injection foaming behavior and their physical properties

In this study, polypropylene (PP) foams were prepared with cucurbit[6]uril (Q[6]) used as a novel nucleating agent through a microcellular injection molding foaming process. The effect of Q[6] content (0.5–2.0 wt %) on the foaming behavior, as well as thermal, rheological and mechanical characterizations of the PP/Q[6] (PQ) samples were performed. According to scanning electron microscopy (SEM) images, the microstructure of foams exhibited a smaller cell size, higher cell density, and more homogeneous distribution of cells at a higher Q[6] content. It was found that the peak temperature of crystallization, the crystallinity and the crystallization rate of PP can be obviously improved by adding a low content of Q[6] (0.25–1.0 wt %), whereas further increasing the content of Q[6] (2.0 wt %) would disfavor the effect. With increasing the Q[6] concentration, the PQ composites had higher complex viscosities at low frequencies and higher modulus than that of PP except the content of 2.0 wt %. Furthermore, the introduction of Q[6] into PP can produce a mild increase in tensile strength, flexural strength and impact strength of PQ foams. This can be due to the well dispersion of Q[6], good compatibility between PP and Q[6], as well as the improvement of the cell morphology. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44538.     

Improving polypropylene microcellular foaming through blending and the addition of nano‐calcium carbonate

This article reports an attempt to improve polypropylene (PP) microcellular foaming through the blending of PP with high‐density polyethylene (HDPE) as a minor component and the incorporation of nano‐calcium carbonate (nano‐CaCO₃) into PP and its blends with HDPE. Three HDPEs were selected to form three blends with a viscosity ratio less than, close to, or greater than unity. Two concentrations of nano‐CaCO₃, 5 and 20 wt %, were used. The blends and nanocomposites were prepared with a twin‐screw extruder. The foaming was carried out by a batch process with supercritical carbon dioxide as a blowing agent. The online shear viscosity during compounding and the dynamic rheological properties of some samples used for foaming were measured. The cell structure of the foams was examined with scanning electron microscopy (SEM), and the morphological parameters of some foams were calculated from SEM micrographs. The rheological properties of samples were used to explain the resulting cell structure. The results showed that the blend with a viscosity ratio close to unity produced a microcellular foam with the minimum mean cell diameter (0.7 μm) and maximum cell density (1.17 × 10¹¹ cells/cm³) among the three blends. A foamed PP/nano‐CaCO₃ composite with 5 wt % nano‐CaCO₃ exhibited the largest cell density (8.4 × 10¹¹ cells/cm³). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermoplastic polyurethane (TPU) modifier to develop bimodal cell structure in polypropylene/TPU microcellular foam in presence of supercritical CO2

Currently, the fabrication of microcell and bimodal cell structures (BCS) in polymer foams by using supercritical fluids has become a hot as well as a challenging research area worldwide. In this work, an environmentally friendly, effective, facile, and CO₂-based foaming technique was presented to fabricate microcellular polypropylene (PP) foams with BCS via blending with thermoplastic polyurethane (TPU). The toughness, thermal properties, rheological properties, and foamability of PP were systematically investigated with gradual incorporation of TPU. Representative sea-island structure was observed in the scanning electron microscopy (SEM) images for the fracture surface of various PP/TPU samples. Rheological measurement results demonstrated that the viscoelasticity of various PP/TPU samples was improved remarkably compared with that of pure PP and pure TPU. The impact strength of various PP/TPU samples possessed the highest value as 12.4 kJ/m² with the TPU content of 15 wt%. After the addition of TPU, an ameliorative cellular morphology was observed in the SEM micrographs of various PP/TPU samples and their volume expansion ratio was enhanced significantly thanks to their improved melt elasticity. Moreover, it is worth noting that BCS appeared in various PP/TPU foams when the TPU content exceeded 5 wt%.     

Effect of nano‐silica filler on the rheological and morphological properties of polypropylene/liquid‐crystalline polymer blends

A fumed hydrophilic nano‐silica‐filled polypropylene (PP) composite was blended with a liquid‐crystalline polymer (LCP; Rodrun LC5000). The preblended polymer blend was extruded through a capillary die; this was followed by a series of rheological and morphological characterizations. The viscosity of the PP matrix increased with the addition of the hydrophilic nano‐silica. At shear rates between 50 and 200 s^?1, the composite displays marked shear‐thinning characteristics. However, the incorporation of LC5000 in the PP composite eliminated the shear‐thinning characteristic, which suggests that LC5000 destroyed the agglomerated nano‐silica network in the PP matrix. Although the viscosity ratio of LCP/PP was reduced after the addition of nano‐silica fillers, the LCP phases existed as droplets and ellipsoids. The nano‐silicas were concentrated in the LC5000 phase, which hindered the formation of LCP fibers when processed at high shear deformation. We carried out surface modification of the hydrophilic nano‐silica to investigate the effect of modified nano‐silica (M‐silica) on the morphology of the PP/LC5000 blend system. Ethanol was successfully grafted onto the nano‐silica surface with a controlled grafting ratio. The viscosity was reduced for PP filled with ethanol‐M‐silica when compared to the system filled with untreated hydrophilic nano‐silica. The LC5000 in the (PP/M‐silica)/LC5000 blend existed mainly in the form of fibrils. At high shear rates (e.g., 3000 s^?1), the LC5000 fibril network was formed at the skin region of the extrudates. The exclusion of nano‐silica in the LC5000 phase and the increased viscosity of the matrix were responsible for the morphological changes of the LCP phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1484–1492, 2003     

改性剂对聚丙烯流变性能和发泡行为的影响

在改性聚丙烯PPFP1920中加入丙烯酸类树脂改性剂(记作G200)进行共混改性,制备了PPFP1920/G200复合材料,研究了G200含量对聚丙烯流变性能、发泡行为的影响.结果表明:随着G200含量的增加,复合材料的熔体流动速率大幅降低,复数黏度和低频储能模量提高,低频损耗因子降低,熔体弹性变好,复合材料可发泡性提...     

Calcium dicarboxylates nucleation of β‐polypropylene

Seven dicarboxylates of calcium were synthesized. The effect of dicarboxylate on the formation of β‐form polypropylene was investigated by X‐ray diffraction. Calcium pimelate, calcium suberate, calcium phthalate, and calcium terephthalate have been found to be an effective β‐nucleator. The K_x values of the isotatic propylene samples with 0.5 wt % of the nucleators above are 0.95, 0.96, 0.93, and 0.62, respectively. Calcium succinate, calcium adipate, and calcium sebacate behave invalidly on the nucleating of the β‐phase. We conducted an investigation on the affect of particle shape, crystal form, and crystallinity upon the level of the β‐form. The X‐ray diffraction of the effective nucleators reveals a common character that their first reflection locate at the d‐spacing between 10 to 13 Å, indicating structural similarity of the nucleators with β‐polypropylene. The nucleation mechanism is explained by the cooperative effect of the nonpolar and polar part of nucleating agents in the crystallization of polypropylene. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 633–638, 2002     

Preparation of polyethylene‐octene elastomer/clay nanocomposite and microcellular foam processed in supercritical carbon dioxide

Polyethylene‐octene elastomer (POE)/organoclay nanocomposite was prepared by melt mixing of the POE with an organoclay (Cloisite 20A) in an internal mixer, using poly[ethylene‐co‐(methyl acrylate)‐co‐(glycidyl methacrylate)] copolymer (E‐MG‐GMA) as a compatibilizer. X‐ray diffraction and transmission electron microscopy analysis revealed that an intercalated nanocomposite was formed and the silicate layers of the clay were uniformly dispersed at a nanometre scale in the POE matrix. The nanocomposite exhibited greatly enhanced tensile and dynamic mechanical properties compared with the POE/clay composite without the compatibilizer. The POE/E‐MA‐GMA/clay nanocomposite was used to produce foams by a batch process in an autoclave, with supercritical carbon dioxide as a foaming agent. The nanocomposite produced a microcellular foam with average cell size as small as 3.4 µm and cell density as high as 2 × 10¹¹ cells cm^?3. Copyright © 2005 Society of Chemical Industry     

Effect of bark flour content on the hygroscopic characteristics of wood–polypropylene composites

The effects of the bark content on the water absorption and thickness swelling of wood–plastic composites prepared from polypropylene, wood flour, and bark flour were studied. Samples were made with a laboratory twin‐screw extruder. The results showed that among composites free of maleic anhydride polypropylene, those composites containing a higher bark flour content exhibited lower water absorption and lower thickness swelling. Maleic anhydride polypropylene reduced water absorption and thickness swelling in composites containing wood flour and a lower content of bark flour but had no influence on the hygroscopic properties of composites made with higher bark contents. Adding maleic anhydride polypropylene had no effect on the water diffusion coefficients and swelling rate parameters of composites made with a higher bark flour content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of the high‐density polyethylene melt index on the microcellular foaming of high‐density polyethylene/polypropylene blends

The effect of high‐density polyethylene (HDPE)/polypropylene (PP) blending on the crystallinity as a function of the HDPE melt index was studied. The melting temperature and total amount of crystallinity in the HDPE/PP blends were lower than those of the pure polymers, regardless of the blend composition and melt index. The effects of the melt index, blending, and foaming conditions (foaming temperature and foaming time) on the void fractions of HDPEs of various melt indices and HDPE/PP blends were also investigated. The void fraction was strongly dependent on the foaming time, foaming temperature, and blend composition as well as the melt index of HDPE. The void fraction of the foamed 30:70 HDPE/PP blend was always higher than that of the foamed 50:50 HDPE/PP blend, regardless of the melt index. The microcellular structure could be greatly improved with a suitable ratio of HDPE to PP and with foaming above the melting temperature for long enough; however, using high‐melt‐index HDPE in the HDPE/PP blends had a deleterious effect on both the void fraction and cell morphology of the blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 364–371, 2004     

聚丙烯/粉煤灰微发泡复合材料的制备及发泡性能研究

采用化学微发泡法制备了聚丙烯/粉煤灰(PPFP1919/FA)微发泡复合材料,并对不同FA含量的PPFP1919/FA复合材料的流变性能、发泡质量及力学性能进行研究.结果表明,FA分散在PPFP1919基体中提高了复合材料的熔体强度,使熔体弹性变好,可发泡性提高;FA在发泡过程中还起到了异相成核作用,提供了成核点,从而...     

Production of polystyrene microcellular foam plastics and a comparison of late‐ and quick‐heating

A batchwise process for the production of microcellular plastics was studied in the polystyrene–nitrogen system. The effects of saturation temperature, saturation pressure, and late‐ and quick‐heating on the microcellular structure were investigated by considering the solubility of the gas in the polymer. It was found that the mean cell diameter was reduced and the cell number density increased with increase in the gas solubility. Variation in the saturation temperature showed that the cell number density had a minimum and the mean cell diameter had a maximum at about 350 K, which was related to the minimum solubility of nitrogen in polystyrene. The long heating time at 393 K of a solution saturated under 25 MPa increased the cell diameter, reduced the cell number density, and gave a maximum volume expansion ratio at about 300 s. Further heating caused the cell size and volume expansion ratio to be decreased, which might be caused by diffusion of the gas out of the polymer sample. The effect of the saturation temperature under high saturation pressure on the cell number density was qualitatively well predicted by the nucleation theory. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2383–2395, 2000     

A study on the onset surface melt fracture of polypropylene materials with foaming additives

The melt fracture behaviors of linear and branched polypropylene resins with foaming additives were investigated. The effects of branching, processing temperature, additives, and blowing agent on the surface melt fracture of polypropylene materials were thoroughly studied. A CCD camera was installed at the die exit to precisely observe the onset of surface melt fracture of extruded foams. The critical wall shear stress was determined for various linear and branched polypropylene resins using a capillary die. It was found that the branching required to foam polypropylene resins also promotes melt fracture: the critical shear stress was decreased by 0.0175 MPa with an increase of 0.1 n/1000c in long‐chain branching. It was also observed that the dissolved blowing agent (butane) significantly suppressed the melt fracture of both linear and branched polypropylene resins. On the other hand, a noticeable increase in the critical shear stress of branched polypropylene materials was observed with the nucleating agent (talc) and the aging modifier (glycerol mono stearate), whereas almost negligible effect of the additives on the critical shear stress was observed for linear polypropylene materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene/alumina nanocomposites

Nonisothermal melt crystallization kinetics of syndiotactic polypropylene (sPP)/alumina nanocomposites were investigated via differential scanning calorimetry. The addition of alumina nanoparticles significantly increases the number of nuclei and promotes the crystallization rate of sPP. Nonisothermal melt crystallization kinetics was analyzed by fitting the experimental data to a Nakamura model using Matlab. The average values of Avrami exponent n are 1.7 for both sPP and sPP/Al₂O₃ nanocomposites during slow cooling, which implies a two‐dimensional growth is the predominant mechanism of crystallization following a heterogeneous nucleation. The two nanocomposites give n values equal to 2.3 during faster cooling, indicating that the main growth type taking place for sPP/alumina nanocomposites is also the two‐dimensional growth. The subsequent melting behavior shows that the presence of alumina nanoparticles changed both the cold crystallization and the recrystallization of sPP. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

聚丙烯/超临界氮气微孔注塑充模过程工艺参数研究

以聚丙烯(PP)为原料,超临界氮气(N2)为发泡剂,通过自行设计的一种扁平螺旋线形流道模具探究了注塑制品充模长度与减重比、泡孔结构之间的关系,并设计正交试验研究了注气压差、注气时间、注射压力和注射速率对制品充模长度的影响.结果表明,在一定范围内,熔体背压越高,制品充模长度越长,且随着充模长度增加,制品的减重比呈先缓慢增...     

PP/HDPE/EPDM复合体系微孔发泡实验

聚丙烯(PP)是结晶性聚合物,熔体强度低,发泡性能差.为了提高PP的微孔发泡性能,首先将PP和聚乙烯(PE)共混,然后在PP/PE共混体系中加入少量EPDM,研究EPDM的质量含量对PP/PE共混体系熔体强度和最终泡孔结构的影响.分析机理,寻找能够提高PP熔体强度和改善发泡性能的材料.