Effect of dimethyl formamide in the synthesis of linear low density polyethylene on branched and molecular structure

In this research work, effect of dimethyl formamide (DMF) as an external electron donor in gas phase ethylene/1‐butene copolymerization process in the presence of Ziegler–Natta catalyst has been investigated. Different experiments were performed using TiCl₄/MgCl₂ as catalyst, triethyl aluminum as cocatalyst and in the presence of various amounts of DMF as an external electron donor. Then, effect of DMF on various parameters such as catalyst activity, molecular weight, and short chain branching content of the samples has been studied. The results showed that the more the ratio of DMF/Ti, the lower the catalyst activity. Moreover, the curves relating molecular weight and short chain branching content of the samples to the ratio of DMF/Ti passed through an extremum point at about DMF/Ti = 0.25. A justification for the occurrence of the extremum point has been proposed in this research work, and some analysis methods are adopted to confirm the suggested justification. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1267–1272, 2012     

Modifying adhesion of linear low‐density polyethylene to polypropylene by blending with a homogeneous ethylene copolymer

Adhesion of a Ziegler–Natta catalyzed ethylene copolymer (ZNPE) to polypropylene (PP) was studied by measuring the delamination toughness G of coextruded microlayers by using the T‐peel test. Low values of G compared to a homogeneous copolymer with approximately the same short chain branch (SCB) content were attributed to an amorphous interfacial layer of low molecular weight, highly branched ZNPE fractions. Blending ZNPE with a homogeneous metallocene catalyzed copolymer (mPE) increased G. In this regard, mPE with higher SCB content was more effective than mPE with slightly lower SCB content. The ZNPE interface was mimicked by microlayering ZNPE and ZNPE blends with polystyrene from which the ZNPE layers were easily separated without damage to the surface. Examination with atomic force microscopy revealed a soft coating about 8 nm thick on the surface of the ZNPE layer. Blending with mPE reduced or eliminated the amorphous interfacial layer. It was proposed that mPE increased miscibility of low molecular weight, highly branched fractions of ZNPE and prevented their segregation at the interface. After blending with mPE eliminated the interfacial layer, G increased to a value comparable to that of a homogeneous copolymer with about the same SCB content as ZNPE bulk chains. The increase in G was attributed to epitaxial crystallization of the ethylene copolymer in the absence of an amorphous interfacial layer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 109–115, 2004     

Influence of temperature,molecular weight,and molecular weight dispersity on the surface tension of polystyrene,polypropylene, and polyethylene. II. Theoretical

Experimental data for the surface tension of polystyrenes of different molecular weights (3400–200,000) and different molecular weight dispersities (1–3) and of different polyolefins are compared with the predictions of the Patterson–Rastogi and Dee–Sauer cell theories, which infer the surface tension from pressure–volume–temperature (PVT) data. PVT data for these polymers were obtained from the literature and experimentally and are fitted to the Flory–Orwoll–Vrij equation of state. Both theories predict that the surface tension will decrease linearly with increasing temperature and increase with molecular weight, thereby corroborating the experimental data. However, both theories underestimate the entropy change in the surface formation per unit area at a constant volume for low molecular weight and polydisperse systems and underestimate the effect of molecular weight dispersity on surface tension. Both theories feature two parameters, m and b, that quantify the enthalpic and entropic contributions to surface tension. The theoretical predictions are fitted to the experimental data for monodisperse polystyrene (with a molecular weight above the molecular weight of entanglement), polypropylene, and linear low‐density polyethylene to quantify the enthalpic contribution to surface tension. b is then evaluated as a function of molecular weight and molecular weight dispersity and is found to decrease with increasing molecular weight and to increase with increasing molecular weight dispersity, showing that end‐group excess at the surface has some effect on surface tension. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2201–2212, 2002     

Effect of filler content and surface treatment on the tensile properties of glass‐bead‐filled polypropylene composites

The tensile properties of polypropylene (PP) filled with two A‐glass beads with the same size, PP/3000 (glass bead surface pretreated with a silane coupling agent) and PP/3000U (no surface pretreatment), have been measured by using an Instron materials testing machine at room temperature, to identify the effects of the filler surface pretreatment and its content on the tensile properties of these composites. The results show that the Young's modulus E_c of the composites increases non‐linearly with increasing volume fraction of glass beads ϕ_f, while the tensile yield strength σ_yc and tensile stress at break σ_bc of the composites decrease with an increase of ϕ_f, in the ϕ_f range 0–30%. Furthermore, the values of E_c and σ_bc of the PP/3000 system are somewhat higher than those of the PP/3000U system under the same test conditions, but this is in contrast to the tensile strain at break ε_bc and tensile fracture energy E_bc, especially at higher ϕ_f values. Good agreement is shown between the measured tensile strength and the predicted value by using an equation proposed in previous work. In addition, ε_bc and E_bc reach maximum values at ϕ_f = 25% for both systems. This indicates that there is a brittle–ductile transition for the composites in tension. © 2000 Society of Chemical Industry     

Influence of organophilic montmorillonite and polypropylene on the rheological behaviors and mechanical properties of ultrahigh molecular weight polyethylene

The effects of organophilic montmorillonite (OM)/poly(ethylene glycol) (PEG) hybrids and polypropylene (PP) on the phase morphology, rheological behaviors, and mechanical properties of ultrahigh molecular weight polyethylene (UHMWPE) were investigated. The presence of the OM/PEG hybrids and PP in UHMWPE was found that it was able to lead to a significant reduction of melt viscosity and enhancement in tensile strength, and elongation at break of UHMWPE. A quantitative analysis indicated a larger affinity of the OM to the PEG than to PP or UHMWPE in the composites, suggesting that OM was intercalated by PEG. This was proposed to be responsible for the reduction of viscosity. Polarizing optical microscopy analysis, on the other hand, indicated that the dispersed OM, which acted as a nucleating agent, lowered the spherulite dimension and increased the spherulite number, resulting in high tensile strength and elongation at break. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Preparation and characterization of high melt strength polypropylene with long chain branched structure by the reactive extrusion process

The reactive extrusion of maleic anhydride grafted polypropylene (PP‐g‐MAH) with ethylenediamine (EDA) as coupling agent is carried out in a corotating twin‐screw extruder to produce long chain branched polypropylene (LCBPP). Part of PP‐g‐MAH is replaced by maleic anhydride grafted high‐density polyethylene (HDPE‐g‐MAH) or linear low‐density polyethylene (LLDPE‐g‐MAH) to obtain hybrid long chain branched (LCB) polyolefins. Compared with the PP‐g‐MAH, PE‐g‐MAH, and their blends, the LCB polyolefins exhibit excellent dynamic shear and transient extensional rheological characteristics such as increased dynamic modulus, higher low‐frequency complex viscosity, broader relaxation spectra, significantly enhanced melt strength and strain‐hardening behaviors. The LCB polyolefins also have higher tensile strength, tensile modulus, impact strength and lower elongation at break than their blends. Furthermore, supercritical carbon dioxide (scCO₂) is constructively introduced in the reactive extrusion process. In the presence of scCO₂, the motor current of the twin extruder is decreased and LCB polyolefins with lower melt flow rate (MFR), higher complex viscosity and increased tensile strength and modulus can be obtained. This indicates that the application of scCO₂ can reduce the viscosity of melt in extruder, enhance the diffusion of reactive species, and then facilitate the long chain branching reaction between anhydride group and primary amine group. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

支化及超支化聚乙烯研究进展

综述了支化及超支化聚乙烯的制备方法及功能化改性研究进展,重点介绍了后过渡金属催化剂催化乙烯"链行走"制备超支化聚乙烯的研究进展,同时介绍了功能性超支化聚乙烯的应用,并展望了其发展前景。     

Interfacial adhesion reaction of polyethylene and starch blends using maleated polyethylene reactive compatibilizer

The interfacial reaction of the polyethylene (PE)/starch blend system containing the reactive compatibilizer maleated polyethylene (m‐PE) was directly characterized by Fourier transform infrared (FTIR) spectroscopy. A significant amount of anhydride groups on m‐PE existed as hydrolyzed forms, resulting in a large amount of carboxyl groups. Using a vacuum‐heating‐cell designed in the laboratory, the carboxyl groups were successfully transformed into the dehydrolyzed state (i.e., anhydride group). This result enabled the direct spectroscopic observation of chemical reaction occurring at the interface. For the PE/starch blend system containing m‐PE, the chemical reaction at the interface was verified by the evolution of ester and carboxyl groups in the FTIR spectra. The effect of the reactive compatibilizer on the interfacial morphology was also examined by scanning electron micrography (SEM). Enhanced interfacial adhesion was clearly observed for the blend system containing reactive compatibilizer. Tensile strengths of blend systems containing m‐PE also increased noticeably compared with the corresponding system without compatibilizer. A similar observation was made for the breaking elongation data. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 767–776, 2002     

Study on processing of ultrahigh molecular weight polyethylene/polypropylene blends

The processing of ultrahigh molecular weight polyethylene (UHMWPE) by the addition of polypropylene (PP) and high‐density polyethylene (HDPE) was investigated. The results show that the addition of PP improves the processability of UHMWPE more effectively than does the addition of HDPE. UHMWPE/PP blends can be effectively processed with a twin‐roller and general single‐screw extruder. In the extrusion of UHMWPE/PP blends, PP is enriched at the surface of the blend adjacent to the barrel wall, thus increasing the frictional force on the wall; the conveyance of the solid down to the channel can then be carried out. The melt pool against the active flight flank exerts a considerable pressure on the UHMWPE powder in the passive flight flank, which overcomes the hard compaction of UHMWPE. The PP penetrates into the gaps between the particles, acting as a heat‐transfer agent and adhesive, thus enhancing the heat‐transfer ability in the material. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 977–985, 2004     

抗冲聚丙烯结构与性能研究

对部分国内外抗冲聚丙烯(PP)产品进行了微观形态和结构分析，研究其对材料宏观力学性能的影响。实验结果表明：抗冲PP是一个含有PP均聚物、丙烯与乙烯-丙烯两嵌段共聚物、乙丙橡胶(EPR)、聚乙烯均聚物等的多相体系。EPR的分子序列结构对聚合物抗冲击性能起主要作用。在序列结构中，丙烯、乙烯单体在分子链上的位置交换越频繁，抗冲击性能越得到提高。丙烯序列平均长度的增大对抗冲击性能有一定的削弱作用。     

On the kinetics of isothermal crystallization of branched polyethylene

Isothermal crystallization of branched polyethylene was studied by time-dependent small-angle X-ray scattering experiments and dilatometry. A strict proportionality between scattering intensities and density changes was found for all times. This result indicates complete absence of crystal thickening and perfectioning processes. Isotherms directly reflect the increase in specific inner surface resulting from nucleation and lateral growth of lamellae. The shape of isotherms suggests a time-dependent non-uniform internal structure of spherulites with a decrease in specific inner surface with increasing distance from centre.     

Plasma treatment and adhesion properties of a rubber-modified polypropylene

In this study, cold, glow-discharge plasmas were used as a pretreatment method for the lacquering of rubber-modified polypropylene plates. This type of material is also referred to as thermoplastic polyolefins (TPOs). The effects of plasma treatments at radio- and microwave frequencies (RF and MW) and in combined MW-RF modes were studied, as were the effects of plasma power-to-gas flow (P/F) ratios and of discharges in oxygen, nitrogen, air, argon, and hydrogen. Surface characterization was carried out by contact angle measurements with water as the wetting liquid, and by XPS analyses. The adhesion between a two-component polyurethane (PUR) lacquer and plasma-treated TPO plates was evaluated by 180°-peel testing. The wettability of TPO surfaces was not affected by the plasma frequency or the P/F ratio, while the influence of the discharge gas was noticeable. Furthermore, no correlation between wettability and peel force could be found. Instead, lacquer adhesion was shown to be highly dependent on the P/F ratio and on the choice of discharge gas. The peel forces were found to be in the range of 0.1-35 N/15 mm, and the locus of failures was shown (by visual inspection or by XPS analysis) invariably to be in the TPO substrate. Electromagnetic radiation, most likely vacuum-ultraviolet (VUV) emission (<200 nm), was proposed to be a critical factor in plasma treatments. Attributed to VUV radiation was the creation of radicals in the TPO substrate; these lead to severe chain scission reactions and thereby govern the cohesive strength of the near-surface region of the substrate.     

Alternating multilayer structure of polyethylene/polypropylene blends obtained through injection molding

The formation of multilayer structures in the high‐speed thin wall injection‐molded samples of high‐density polyethylene/isotactic polypropylene blends is reported. Based on the morphology development in injection runner and mold, a possible formation mechanism of multilayer structure was proposed in this study. Injection molding could be used as a simple and an effective method for the fabrication of multifunctional multilayer structure. This work is interesting and important for scientific research as well as several potential applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

原位成纤复合法改善聚丙烯的力学性能

采用原位成纤复合法制备了均聚聚丙烯(PP-H)/聚对苯二甲酸乙二酯(PET)原位成纤复合材料,研究了PET原位微纤化对PP-H力学性能的影响.实验采用的设计工艺可实现PET在PP-H基体中原位微纤化;原位生成的PET纤维对PP-H力学性能的影响呈各向异性,表现为拉伸及弯曲强度提高,冲击强度下降,改善效果受PET微观形态...     

Effect of shear history on flow instability at capillary extrusion for long‐chain branched polyethylene

Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low‐density polyethylene (LDPE) having long‐chain branches. It is found that processing history in an internal mixer in a molten state depresses long‐time relaxation mechanism associated with long‐chain branches, which is known as “shear modification.” Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark‐skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long‐chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark‐skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Spherulite nucleation in polypropylene blends with low density polyethylene

Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with low density polyethylene (LDPE) was investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP spherulites in the blend decreases with increasing LDPE concentration to a much greater extent than follows from the decreasing amount of iPP. The shapes of spherulite size distributions indicate that athermal (heterogeneous) primary nucleation is inhibited. The density of primary nucleation in the blends decreases strongly with increasing mixing time. The same effect was observed in the blends with the nucleating agent which was added to iPP or LDPE. These experiments demonstrate that heterogenoeus nuclei migrate across interphase boundaries from the iPP melt to the LDPE melt during the mixing process. It is suggested that the interfacial energy difference between the nuclei and the molten components of the blend is responsible for the migration of nuclei.     

Electrical behavior and structure of polypropylene/ultrahigh molecular weight polyethylene/carbon black immiscible blends

This study investigates the electrical behavior, which is the positive temperature coefficient/negative temperature coefficient (PTC/NTC), and structure of polypropylene (PP)/ultrahigh molecular weight polyethylene (UHMWPE)/carbon black (CB) and PP/γ irradiated UHMWPE (XL‐UHMWPE)/CB blends. As‐received UHMWPE or XL‐UHMWPE particles are chosen as the dispersed phase because of their unusual structural and rheological properties (extremely high viscosity), which practically prevent CB particles penetration. Because of their stronger affinity to PE, CB particles initially form conductive networks in the UHMWPE phase, followed by distribution in the PP matrix, thus interconnecting the CB‐covered UHMWPE particles. This unusual CB distribution results in a reduced electrical percolation threshold and also a double‐PTC effect. The blends are also investigated as filaments for the effect of shear rate and processing temperature on their electrical properties using a capillary rheometer. Because of the different morphologies of the as‐received and XL‐UHMWPE particles in the filaments, the UHMWPE containing blends exhibit unpredictable resistivities with increasing shear rates, while their XL‐UHMWPE containing counterparts depict more stable trends. The different electrical properties of the produced filaments are also related to differences in the rheological behavior of PP/UHMWPE/CB and PP/XL‐UHMWPE/CB blends. Although the flow mechanism of the former blend is attributed to polymer viscous flow, the latter is attributed to particle slippage effects. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 104–115, 2001     

Phenolic rigid organic filler/isotactic polypropylene composites. II. Tensile properties

A novel phenolic rigid organic filler (KT) was used to modify isotactic polypropylene (iPP). The influence of KT particles on the tensile properties of PP/KT microcomposites was studied by uniaxial tensile test and the morphological structures of the stretched specimens were observed by scanning electron microscopy (SEM) and polarized optical microscopy (POM). We found that the Young’s modulus of PP/KT specimens increased with filler content, while the yield and break of the specimens are related to the filler particles size. The yield stress, the breaking stress and the ultimate elongation of PP/KT specimens were close to those of unfilled iPP specimens when the maximal filler particles size is less than a critical value, which is 7 μm at a crosshead speed of 10 mm/min and 3 μm at 200 mm/min, close to that of glass bead but far more than those of other rigid inorganic filler particles. The interfacial interaction was further estimated from yield stress, indicating that KT particles have a moderate interfacial interaction with iPP matrix. Thus, the incorporation of small KT particles can reinforce iPP matrix and simultaneously cause few detrimental effects on the other excellent tensile properties of iPP matrix, due to their organic nature, higher specific area, solid true-spherical shape and the homogenous dispersion of the ROF particles in microcomposites.     

Influence of polymerization conditions on the structure and properties of polyethylene/polypropylene in‐reactor alloy synthesized in the gas phase with a spherical Ziegler–Natta catalyst

A spherical TiCl₄/MgCl₂‐based catalyst was used in the synthesis of in‐reactor polyethylene/polypropylene alloys by polyethylene homopolymerization and subsequent homopolymerization of propylene in the gas phase. Different conditions in the ethylene homopolymerization stage, such as monomer pressure and polymerization temperature, were investigated, and their influences on the structure and properties of in‐reactor alloys were studied. Raising the polymerization temperature is the most effective way of speeding up polymerization and regulating the ethylene content of polyethylene (PE)/polypropylene (PP) alloys, but it will cause a greater increase in the PE‐b‐PP block copolymer fraction (named fraction D) than in the fraction of PP‐block‐PE in which the PP segments have low or medium isotacticity (named fraction A). Although changing ethylene monomer pressure could influence the ethylene content of PE/PP alloys slightly, it is an effective way of regulating the structural distribution. Reducing the monomer pressure will evidently increase fractions A and D. The mechanical properties of the alloys, including impact strength and flexural modulus, can be regulated in a broad range with changes in polymerization conditions. These properties are highly dependent on the amount, distribution, and chain structure of fractions A and D. The impact strength is affected by both fraction A and fraction D in a complicated way, whereas the flexural modulus is mainly determined by the amount of fraction A. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2136–2143, 2006     

仿生材料多巴胺对橡胶/钢丝帘线黏合性能的影响

选用子午线轮胎胎体胶料配方作为基本配方,以钢丝帘线作为骨架材料,采用自行研发的橡胶与钢丝帘线动态黏合性能的测试方法,考察了多巴胺及传统黏合体系对橡胶/钢丝帘线动态黏合性能的影响,并探讨了多巴胺替代间苯二酚作为无毒环境友好型黏合剂的可行性。结果发现,钴盐黏合体系赋予了橡胶与钢丝帘线较好的动态黏合,多巴胺体系赋予了较好的静态黏合,间甲白体系居中,3种黏合体系并用可赋予最好的动、静态黏合性能。橡胶/钢丝帘线的动、静态黏合力均随着多巴胺用量的增加而增大,但在拉伸疲劳后出现下降。