Effects of long chain branches on the crystallization and foaming behaviors of polypropylene-g-poly(ethylene-co-1-butene) graft copolymers with well-defined molecular structures

A series of polypropylene-g-poly(ethylene-co-1-butene) graft copolymers (PP-g-EBR) with well-defined long chain branched (LCB) molecular structures, basing on the same PP–BT precursor (PP–BT2), were used to study effects of EBR LCBs on the crystallization and foaming behaviors of PP-g-EBRs. The kinetics results of isothermal and nonisothermal crystallization verify the opposite effects of LCB structure on the crystallization process of PP backbones in PP-g-EBRs: on one hand, the indolent LCB structure can perform the function of heterogeneous nucleation to facilitate the crystallization; on the other hand, the mobility and reptation ability of PP backbones are restrained by the LCB structure, which hinders the crystallization process. Additionally, the fluctuation-assisted nucleation mechanism caused by microphase separation between the EBR rich phase and the PP rich phase may account, to some extent, for the heterogeneous nucleation effect. The PP–BT2 and PP-g-EBRs were foamed by a batch method under the same conditions, using supercritical CO₂ as blowing agent. The resulting PP-g-EBR foams exhibited closed cell structure and increased cell density compared to the PP–BT2 foam, attributing to the enhanced melt strength. The cell density of PP-g-EBR foam increased first and decreased then with the LCB level increasing. The influence of LCB level on cell size was somewhat complex. Increasing LCB level, which promoted melt strength and strain hardening behavior of PP-g-EBRs, decreased the cell size and narrowed the cell size distribution. However, large cells were observed in PP-g-EBR foams with relatively high LCB level, which could be ascribed to the larger growing space introduced by the higher content of amorphous EBR LCBs. Moreover, the melting behaviors of PP–BT2 and PP-g-EBRs before and after foaming treatment were compared.     

Good extrusion foaming performance of long-chain branched PET induced by its enhanced crystallization property

Poly(ethylene terephthalate) (PET) was modified by regulating different contents of branching agent epoxy-based multifunctional oligomer and chain extender pyromellitic dianhydride in reactive extrusion process. The modified PET with better long-chain branched (LCB) structure boosted its rheological properties, and its enhancement of melt viscoelasticity resulted in excellent foamability in molten-state foaming process using supercritical CO₂ as blowing agent. More importantly, the branched structures acted as crystal sites to accelerate the crystallization kinetic of LCB PET whether under atmospheric pressure or high-pressure CO₂. The shear and elongation flow inside die further quickly induced the crystallization of LCB PET. The rapidly generated fine crystals could both introduce heterogeneous cell nucleation and suppress CO₂ escape, so the cell morphology of LCB PET in continuous extrusion foaming process exhibited a three-fold increase in cell density and smaller uniform cell size with respect to those of other foam-grade PET with long-chain structure.     

Rheological control in foaming polymeric materials: I. Amorphous polymers

The influence of rheological properties, especially melt strength, on foam structures, such as cell size, cell density and cell size distribution, of amorphous polymer was investigated. The rheology of polystyrene (PS) was controlled by molecular modification with free radical reaction, and PS with long chain branching (LCB) level ranging from 0.15 to 1.6 branching point per 10⁴ carbon atom was gotten. The shear and elongational rheology were found to be dependent on the LCB structure, and the strain hardening behavior of modified samples in transient elongational viscosity confirmed the existence of long branched chain. The effects of chain structure and foaming conditions such as temperature and pressure were studied by the analysis on the foam structures obtained by supercritical CO₂. The experimental results revealed that increasing LCB level would decrease cell size, make cell size distribution narrower and slightly increase cell density. The effects of chain topology on the foam structures were also investigated by numerical simulation, where Pom-Pom model was used to describe the effect of backbone length and arm length. The dependence of cell size on the arm length was consistently observed in experiments and simulation. It suggested that the arm length had greater influence on the cell radius than the backbone length. Therefore, the relationship among foam structures, rheological properties and molecular structures can be established from both experiments and simulation, which can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.     

结晶特性对微发泡聚丙烯材料发泡行为的影响

以化学发泡注塑成型技术为主线,在二次开模条件下制备微发泡PP材料;通过DSC、XRD技术分析了结晶特性对微发泡聚丙烯材料发泡行为的影响。结果表明:结晶特性对气泡的成核、长大和定型过程具有明显的影响;添加滑石粉的改性PP材料结晶特性较差,发泡质量明显降低,泡孔直径和泡孔密度分别为36.98μm、3.29×107个/cm3;添加云母粉的改性PP材料具有合适的结晶温度和结晶度,发泡质量较理想,泡孔直径和泡孔密度分别为22.09μm、4.76×108个/cm3;能够获得泡孔细小、均匀的微发泡PP材料。     

Fabrication of polyamide 12T micro-cellular foams with ultra-high compressive strength via in situ polytetrafluoroethylene fibrillation using supercritical carbon dioxide foaming

The development of micro-cellular foams with ultra-high compressive strength and high volume expansion ratio (VER) is a challenging task. Herein, polyamide 12T (PA12T) micro-cellular foams with ultra-high compressive strength were fabricated via in situ polytetrafluoroethylene (PTFE) fibrillation using supercritical CO₂ foaming technology and a chain extender. The resulting branched structure showed considerably improved viscoelasticity and foaming performance, thus improving the cell morphology of the PA12T foam and exhibiting high VER. The PTFE fibrillation network induced melt strength enhancement, crystallization nucleation, and cell nucleation. The branched PA12T foam with 1.5 wt% PTFE exhibited the smallest cell diameter (15 μm) and highest cell density (3 × 10⁹ cells/cm³). The compressive strength of the foam (0.50 MPa under 5% strain) was 70% higher than that of pure PA12T. This research offers an effective method for producing high-VER PA12T foams with adjustable micro-cellular structures and excellent mechanical properties.     

线性和长支链聚丙烯/氯化钠共混物的流变性能及发泡形态的研究

制备了线性和长支链聚丙烯/氯化钠(NaCl)共混物,通过流变仪、差示扫描量热仪(DSC)及超临界二氧化碳发泡方法研究了共混物的流变性能、热性能及发泡形态。流变结果表明,紫外辐照挤出改性后的聚丙烯(PP)样品都接入了长支链,NaCl的加入没有改变长支链结构,且NaCl加入得越多,拉伸黏度越小。通过DSC可知,随着长支链的引入,聚丙烯的熔程变宽,结晶温度有所提升,而NaCl的加入使得结晶温度下降,熔点变化不大。通过紫外辐照反应挤出改性引入长支链可有效改善PP的发泡形态。加入致孔剂可制得开孔材料,开孔程度随着致孔剂的含量的增大而增大。     

Influence of polyfunctional monomer on melt strength and rheology of long‐chain branched polypropylene by reactive extrusion

Long chain branching (LCB) were added to linear polypropylene (PP) using reactive extrusion in the presence of selected polyfunctional monomers (PFMs) and a peroxide of dibenzoyl peroxide (BPO). Fourier Transformed Infrared spectra (FTIR) directly confirmed the grafting reaction occurred during the reactive extrusion process. Various rheological plots including viscosity curve, storage modulus, Cole‐Cole plot, and Van‐Gurp plots, confirmed that the LCB structure were introduced into modified PPs skeleton after modification. In comparison with linear PP, the branched samples exhibited higher melt strength, lower melt flow index, and the enhancement of crystallization temperature. The LCB level in modified PPs and their melt strength were affected by the type of PFM used and could be controlled by the PFM properties and structure. PFMs with lower boiling points, such as 1, 4‐butanediol diacrylate (BDDA), could not produce LCB structure in modified PP skeleton. The shorter molecular chain bifunctional monomers, such as 1,6‐hexanediol diacrylate (HDDA), favored the branching reaction if their boiling points were above the highest extrusion temperature. And some polar groups, such as hydroxyl, in the molecule of PFM were harmful to the branching reaction, which might be attributed to the harm of the polarity of groups to the dispersion of PFM in PP matrix. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Solid‐state foaming of isotactic polypropylene and its composites with spherical or fibrous poly(butylenes terephthalate)

In this article, the foaming behavior of isotactic polypropylene (iPP) and its composites with spherical or fibrous poly(butylenes terephthalate) (PBT) using supercritical CO₂ as a blowing agent were investigated. Their foaming performances were also compared in relation to the crystal morphology and rheological behavior of PP. Results demonstrate that crystal structures significantly impacted the cell structures of foams. At relatively low temperature, microcells appeared at the centers of PP spherulites where the melting started. Particularly, bi‐modal cell structure formed in the foamed PP with increasing temperature. However, in the foamed PP composites with spherical or fibrous PBT, this structure almost disappeared due to the smaller PP spherulites. In foaming PP/PBT composites, the heterogeneous nucleation of spherical or fibrous PBT was effective at reducing cell size as well as improving cell density and cell uniformity. The fibrous PBT also acted as scaffolds for preserving cell shapes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41801.     

无机粉体形状对微发泡聚丙烯材料发泡行为的影响

选取粒径约为20μm的针状MgSO4晶须、片层状云母粉和粒状SiO2,以5%的用量加入到聚丙烯(PP)中,在二次开模条件下制备微发泡PP复合材料;通过异相成核理论和粉体分布的特性,分析了无机粉体形状对微发泡PP复合材料发泡行为的影响。结果表明,片层状云母粉具有良好的相容性、比表面特性和异相成核作用,发泡效果理想;泡孔直径达到22.10μm左右、泡孔密度为6.92×108个/cm3;聚烯烃类材料发泡的成核剂中,以片层状的云母粉较为理想。     

Rheology and thermal behavior of long branching polypropylene prepared by reactive extrusion

Long‐chain branching polypropylene (LCB‐PP) was achieved by reactive extrusion in the presence of bifunctional monomer [1,6‐hexanediol diarylate (HDDA)] and peroxide of dicumyl peroxide (DCP). Influences of HDDA and DCP concentrations on the branching efficiency were comparatively evaluated. Fourier transformed infrared spectroscopy (FTIR) results indicated that the grafting reaction took place, and HDDA has been grafted on PP skeleton. In comparison with initial PP, some modified samples showed lower melt flow index because of a large number of LCB in their skeleton. Several rheology plots were used to investigate the rheological properties of the initial PP and modified PPs, and the rheological characteristics confirmed the LCB in modified PPs skeleton. DSC results showed that the crystallization temperatures of modified PPs were higher than those of initial PP and degraded PP, suggesting that the modified PPs had long‐chain branched structure. The contrastive investigation in the rheology of modified PPs suggested that proper concentrations of HDDA and DCP were more beneficial to producing LCB during reactive extrusion. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Morphology and Properties of Polypropylene Foaming Sheet with Alternating Multilayered Structure

In this work, the foaming sheet was designed as alternating multilayered foam/film structure of foaming layers and film layers. The foaming layer contained polypropylene (PP)/high density polyethylene (HDPE)/Talc ternary composites. The film layer contained PP only. The rheological data showed that the melt elasticity of PP was obviously improved by the addition of HDPE and talc. The results exhibited that the alternating multilayered structure was well kept and hardly influenced by the foaming layers, and then the mechanical properties were obviously improved. The cell in the alternating multilayered sheet with 16 layers was smaller and more homogenous than that in pure PP foaming sheet.     

Structure and properties of closed‐cell foam prepared from irradiation crosslinked silicone rubber

Silicone rubber foam was prepared through crosslinking with electron beam irradiation and foaming by the decomposing of blowing agent azobisformamide (AC) in hot air. The crosslinking and foaming of silicone rubber was carried out separately, which was different from the conventional method of chemical crosslinking and foaming. After foaming, the silicone rubber foam was irradiated again to stabilize the foam structure and further improve its mechanical properties. The effects of irradiation dose before and after foaming, and the amount of blowing agents on the structure and properties of silicone rubber foam were studied. The experimental results show that with the increase of AC content, the average cell diameter of silicone rubber foam increases a little, the foam density decreases to a minimum value when AC content is 10 phr. With the increase of irradiation dose before foaming from 10 to 17.5 kGy, the cell nucleation density of silicone rubber foam increases, the average cell diameter decreases, and the foam density increases. With the increase of irradiation before foaming, the tensile strength, tensile modulus, and the elongation at break of the silicone rubber foam increase. Through irradiation crosslinking again after foaming, the foam density is decreased and the mechanical properties of silicone foam are further improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The melt grafting preparation and rheological characterization of long chain branching polypropylene

To study the rheological properties of long chain branching (LCB) polypropylene (PP), long chain branches (LCB) were grafted onto the linear PP by melt grafting reaction in the presence of a novel chain extender, poly(hexamethylendiamine-guanidine hydrochloride) (PHGH). The branching reactions between the functionalized PP and PHGH were confirmed by transient torque curves and FTIR. By differential scanning calorimetry (DSC) and polarized microscope measurements, the presence of long chain branching structures was further confirmed. Also, the viscoelastic properties of the LCB PP and linear PP under shear flow were investigated for distinguishing LCB PP from linear PP. It was found that the elastic response of LCB PP at low frequencies was significantly enhanced in comparison with that of the linear PP, implying a presence of a long relaxation time mode that was not revealed in linear PP. Moreover, the branching levels of LCB PP were quantified using a detailed method, which was in correspondence with the molar amount of PHGH grafted on PP.     

本征特性对聚苯乙烯及聚丙烯发泡行为的影响

以化学发泡注塑成型技术为主线,在二次开模条件下制备微发泡聚苯乙烯（PS）及微发泡聚丙烯（PP）;通过流变性、加工性分析了树脂本征特性对PS及PP发泡行为的影响。结果表明：本征特性对气泡的长大和定型过程、气体扩散具有明显的影响;熔体强度越高的材料,阻碍泡孔长大的趋势越明显,所得到的泡孔越细小而均匀;PS具有合适的熔体强度和熔体流动速率（MFR）,发泡质量较理想,泡孔直径和泡孔密度分别为41.4μm、8.7×106个/cm3;PP（K9026）熔体强度较低,而熔体流动速率过大,发泡质量明显降低,泡孔直径和泡孔密度分别为65.94μm、5.82×105个/cm3。     

Effect of long chain branching on nonisothermal crystallization behavior of polyethylenes synthesized with constrained geometry catalyst

Nonisothermal crystallization behavior of linear and long chain branched (LCB) polyethylene (PE) samples having similar molecular weight but different long‐chain branching densities (LCBD) up to 0.44 C per 1000 carbons was investigated using differential scanning calorimetry (DSC) at various scanning rates. The LCB PE samples were prepared in our high‐temperature, high‐pressure continuous stirred‐tank reactor (CSTR) system using the constrained geometry catalyst. The existence of LCB was found to affect the PE crystallization behavior considerably. The enthalpy of crystallization and the ultimate degree of crystallinity decreased with the increase of LCBD. At the relatively low cooling rates, the small amount of LCB promoted nucleation but restrained chain movement and reduced the crystal growth rate. There was ～ 17% of crystallinity generated from a secondary crystallization. The energy barrier became significant with the LCB structure, resulting in chain diffusion limitations and lower LCB PEs overall crystallization rates than their linear counterpart. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

聚四氟乙烯原位纤维化对TPEE/PTFE复合材料发泡行为的影响

通过熔融共混法研究了不同加工工艺对聚四氟乙烯（PTFE）纤维化的影响,制备了热塑性聚酯弹性体（TPEE）/PTFE纳米纤维复合材料。在此基础上,研究了PTFE含量对TPEE的结晶行为、流变行为以及发泡行为的影响。结果表明,PTFE纤维可以促进TPEE结晶,改善TPEE的流变性能。纳米纤维的高比表面积为泡孔成核提供了大量成核位点,有效提高了泡孔密度,降低了泡孔尺寸;当PTFE含量为2份时,制备了平均泡孔直径为3.2 μm,平均泡孔密度为3.11×10¹⁰ 个/cm³的复合材料泡沫。     

Foaming behaviour and cell structure of poly(lactic acid) after various modifications

Improving the melt strength of poly(lactic acid) (PLA) is of continuing strategic research interest since a low melt strength results in poor processability of foaming and blowing film. PLA was modified in various ways including crosslinking, chain extension, grafting, blending, plasticizing and nucleation to improve its melt strength. This work focuses on the effect of melt strength after various modifications on foaming behaviour and cell structures of PLA foam. Cell density was increased and cell structure became more uniform with increasing melt strength, in particular when the melt strain was also increased after modification. With increasing melt strength the phenomenon of open and ruptured cells was decreased. The foaming window was widened with increasing melt strength, in particular after crosslinking, chain extension and blending with elastomer. Molecular weight, thermal properties and viscosity of PLA after various modifications were also studied and used to explain the foaming behaviour and foam structures. Copyright © 2012 Society of Chemical Industry     

聚丙烯挤出发泡中的关键技术——发泡体系的性能和发泡机理研究

从聚丙烯挤出发泡体系的性能包括聚丙烯熔体的黏弹性、发泡剂的溶解度和扩散系数、聚丙烯的结晶行为和成核剂的性能以及聚丙烯挤出发泡的气泡成核机理和气泡增长机理系统介绍了聚丙烯挤出发泡中的一些关键技术。研究表明：具有显著应变硬化行为和高熔体强度的长链支化聚丙烯是获得优质PP发泡材料的前提；发泡剂的溶解度和扩散系数、聚丙烯的结晶行为和成核剂的种类和性能对发泡材料的泡孔密度、泡孔尺寸和泡孔尺寸分布有显著影响；气泡成核和气泡增长机理对于聚丙烯挤出发泡的配方设计、工艺确定和设备选型具有极其重要的意义。     

微发泡PP/竹塑复合材料发泡行为及性能研究

采用注塑成型方法,在二次开模条件下制备微发泡聚丙烯(PP)/竹粉复合材料,分析了不同含量的竹粉对微发泡PP复合材料力学性能和发泡行为的影响。结果表明:随着竹粉用量的增加,泡孔平均直径逐渐减小,泡孔密度逐渐增加。竹粉用量为30%时,泡孔直径最小,为23.4μm,泡孔密度达到10.4×106个/cm3,具有理想的发泡效果。竹粉用量在20%时,发泡材料的冲击强度和纯PP基本相同,在性能不降低情况下,节约了原材料成本。     

Determination of modified polyamide 6's foaming windows by bubble growth simulations based on rheological measurements

Reactive extrusion was used to modify virgin polyamides 6 (v-PA6) and to prepare chain extended PA6 (CE-PA6) and long-chain branched PA6 (LCB-PA6) for the melt foaming process. This was done using a twin-screw extruder and the following modifiers: a chain extender ADR-4368 and a branching agent maleic anhydride grafted polypropylene. A reaction mechanism was proposed to explain the chain extension and long-chain branching reactions and was verified by the Fourier transform infrared spectroscopy data. The analysis of the gel permeation chromatography data showed that LCB-PA6 presented a strong increase in the molecular weight and in the dispersity index. Moreover, the rheological properties of the v-PA6 and modified PA6 resins were characterized by a dynamic shear test. The LCB-PA6 compared with CE-PA6 showed much higher shear viscosity and longer characteristic relaxation times, indicating the presence of an LCB structure. A uniaxial elongation test showed that the LCB-PA6 had the highest melt viscosity and melt strength as well as most obvious strain-hardening behavior. A high-pressure differential scanning calorimeter under compressed CO₂ was used to investigate the PA6's crystallization properties so as to analyze its minimum temperature of foaming windows. The melt foamability of the CE-PA6 and the LCB-PA6 was verified by batch melt foaming experiments with CO₂ as the blowing agent and maximum temperature of foaming windows was also quantitatively determined by numerical simulation of bubble growth based on the rheological measurements. The results showed that the LCB-PA6 foams had a smaller cell diameter, a larger cell density, a greater expansion ratio, and wider foaming temperature window than the CE-PA6. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48138.