聚丙烯硬弹性膜的挤出流延工艺及拉伸成孔性

以某进口均聚聚丙烯(PP)(PP1)为原料,通过挤出流延工艺制备了PP1硬弹性膜并在130℃下对其进行热处理,研究了挤出温度、流延辊转速及温度、热处理温度和时间等工艺参数对PP1硬弹性膜弹性回复率的影响,得到了最佳工艺条件,即挤出温度为195℃、流延辊温度及转速分别为90℃,26 r/min,热处理温度和时间分别为145℃,30 min。将该工艺应用于分子量及其分布与PP1基本相似的国产PP(PP2),发现由此工艺制备的PP2硬弹性膜的弹性回复率可达94.6%,与PP1硬弹性膜相差不大。通过冷热单向拉伸将PP1和PP2硬弹性膜制备成微孔膜,发现两者具有相似且优异的微孔结构。在此基础上,通过扫描电子显微镜研究了冷热拉伸工艺参数对PP2微孔膜拉伸成孔性的影响。结果表明,当冷拉伸倍数为15%,热拉伸倍数为100%,冷热拉伸速率均为50 mm/min时,制备的PP2微孔膜形成了完整规则的多孔结构。     

硬弹性HDPE流延基膜冷拉过程中显微结构的变化

以不同拉伸倍率和拉伸速度在室温下对硬弹性HDPE流延基膜施加冷拉作用,通过原子力显微镜、场发射扫描电镜、X射线衍射以及孔隙率和透气率测定来观察冷拉前后薄膜中片晶结构的变化,并且了解这些显微结构变化对微孔膜结构和性能的影响。研究发现,提高冷拉倍率可以增加拉伸膜中片晶分离程度,有利于形成具有较高孔隙率和透气率的微孔膜,但冷拉倍率过高会使片晶弯曲崩溃,部分微孔闭合;随冷拉速度增加,拉伸膜中片晶分离更加彻底,使得微孔膜中微孔数目增多且分布更加均匀;冷拉过程中拉伸应力下结晶的较长连接链和应力作用下破坏的片晶边缘的微次晶共同构成了拉伸膜中分离片晶间的初始桥结构。     

硬弹性HDPE流延基膜冷拉过程中显微结构的变化

以不同拉伸倍率和拉伸速度在室温下对硬弹性HDPE流延基膜施加冷拉作用,通过原子力显微镜、场发射扫描电镜、X射线衍射以及孔隙率和透气率测定来观察冷拉前后薄膜中片晶结构的变化,并且了解这些显微结构变化对微孔膜结构和性能的影响。研究发现,提高冷拉倍率可以增加拉伸膜中片晶分离程度,有利于形成具有较高孔隙率和透气率的微孔膜,但冷拉倍率过高会使片晶弯曲崩溃,部分微孔闭合;随冷拉速度增加,拉伸膜中片晶分离更加彻底,使得微孔膜中微孔数目增多且分布更加均匀;冷拉过程中拉伸应力下结晶的较长连接链和应力作用下破坏的片晶边缘的微次晶共同构成了拉伸膜中分离片晶间的初始桥结构。     

熔融纺丝拉伸法制备PP/PPOH中空纤维膜的研究

根据片晶分离致孔的机理,利用羟基化改性聚丙烯(PPOH)与聚丙烯(PP)共混,通过熔融纺丝拉伸法制备具有一定亲水性的PP/PPOH中空纤维膜,探究了热处理温度及热拉伸温度对PP/PPOH中空纤维膜的结构与性能的影响。结果表明：当热处理温度为130℃时,PP/PPOH初纺中空纤维膜的结晶度高,力学性能较好,纯水通量为102.4 L/(m²·h);当热处理温度为130℃、热拉伸温度为130℃时,PP/PPOH中空纤维膜的结晶度最高,达39.4%,拉伸强度达93.1 MPa,表面微孔结构完善,纯水通量最高,达118.4 L/(m²·h)。     

吹胀比对聚丙烯微孔膜结构与性能的影响

采用挤出吹塑工艺并依托熔体拉伸成孔机理,制备了聚丙烯微孔膜,研究了不同吹胀比对聚丙烯初始膜及拉伸后微孔膜结构与性能的影响。结果表明,与吹胀比为1的聚丙烯初始吹塑膜及微孔膜相比,吹胀比为1.4的初始膜弹性回复率、取向度、纵向屈服强度以及应变硬化行为均较为接近,但横向强度明显提高;后拉伸制备的微孔膜透气性、孔隙率和热收缩性等性能也较相近,但穿刺性能得到明显改善。因此,吹塑成型过程中横向应力的存在,能够有效提高聚丙烯初始吹塑膜的横向强度,但是,较大的吹胀比(如吹胀比为2.1)降低了初始膜片晶结构规整性,拉伸微孔膜各项性能也出现了一定程度的下降。     

低温冷拉对具有排核片晶结构的PVDF微孔膜成孔的影响

采用聚偏氟乙烯（PVDF）材料制备锂电池用微孔膜,使用熔体挤出拉伸法制备了预制膜片,通过牵伸比的调节成功获得了拥有排核片晶结构的预制膜片。结果表明,该预制膜具有沿着流动方向的高取向度。退火后,在拉伸预制膜成孔的过程中,由于PVDF松弛时间较短,即使预制膜具有排核片晶结构,在常温下的成孔效果也较差;根据时温等效原理,降低冷拉时的拉伸温度,可以达到与提高松弛时间同样的效果;对比室温下冷拉和-20 ℃低温下冷拉膜片的扫描电子显微镜照片,低温冷拉后的膜片具有更多的孔结构和更好的孔分布情况,并初步探索了低温拉伸条件对于成孔促进作用的成孔机理。     

挤出吹塑聚丙烯微孔膜的制备及性能

使用挤出吹塑法制备聚丙烯微孔膜,研究了不同熔体牵伸比对聚丙烯微孔膜结构与性能的影响。结果表明:随着熔体牵伸比的增大,聚丙烯从球晶逐步转变为平行排列的片晶结构;聚丙烯初始膜的弹性回复率和取向度逐渐增大,应力应变曲线塑性平台区逐渐变弱,应变硬化行为越来越明显;因此聚丙烯微孔膜透气性能变好,孔隙率增大,热收缩率变小。     

硬弹性材料拉伸成孔机理及PVDF多孔膜的制备

对硬弹性材料结构特征和拉伸成孔机理进行了总结，重点介绍了聚偏氟乙烯（PVDF）硬弹性材料及多孔膜的研究发展现状。结果表明，硬弹性材料是一类经特殊加工的具有排状片晶结构的合成聚合物材料，截然不同于普通弹性材料，具有高弹、高模等特性，并且低温弹性优异，特别是在拉伸时可以形成微孔，该特性已被用于多孔膜的制备。同时指出，目前熔...     

热拉伸温度对熔体拉伸法制备聚丙烯微孔隔膜微观结构的影响

通过小角X射线散射(SAXS)、扫描电子显微镜(SEM)和调制差示扫描量热法(TMDSC)表征了熔体拉伸法制备聚丙烯微孔隔膜在105～145℃的热拉伸过程。发现随拉伸温度升高,拉伸带来的片晶破坏减少,105℃下热拉伸仍然存在片晶被破坏分离; 130和145℃下热拉伸只发生片晶堆分离。微孔隔膜的孔洞尺寸主要由拉伸倍率和热拉伸温度控制;连接分离片晶的架桥直径和长度增幅随拉伸温度的升高而增大。在热拉伸过程,架桥主要由片晶转化形成,与无定型分子链的二次结晶无关。     

热拉伸比对熔融挤出拉伸法制备聚丙烯微孔膜的影响

通过固定冷拉伸比,改变热拉伸比制备了具有不同微孔结构的聚丙烯微孔膜,通过扫描电子显微镜(SEM)观测了微孔的形貌,通过差示扫描量热法(DSC)测试了微孔膜的片晶厚度,研究了热拉伸过程中微孔形成过程以及微孔尺寸和数量,微孔膜孔隙率随热拉伸比的变化情况。结果表明,冷拉伸过程主要是片晶间微裂纹的形成过程,微裂纹均匀的分布在试样内部,但由于不同微裂纹的抗拉伸性能不同,在热拉伸阶段首先被拉开的是较薄弱的微裂纹,形成部分微孔。当微孔的尺寸足够大,微孔中的"桥结构"拉伸至较大尺寸提供足够的张力以维持微孔不再变形。随着热拉伸的继续进行,"桥结构"能将力传递给带状微裂纹之间的堆积片晶,将这些堆积片晶之间的次薄弱缺陷拉开,继续形成微孔。     

Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)-property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was discussed.     

Relationship of polymorphic crystalline phase texture to strain recovery and stiffness of a propylene‐based elastomer

A stretching process to enhance the stiffness of an elastomeric propylene‐ethylene copolymer through orientation was examined. The tensile extension was performed at various temperatures within the unusually broad melting range of the copolymer. Stretching transformed the unmelted lamellar crystals into shish‐kebab fibers that acted as a scaffold for an elastomeric matrix of entangled, amorphous chains. Density measurements indicated that the process did not significantly affect the amount of crystallinity, which was about 23%. If the specimen was recrystallized by cooling after it recovered from the stretched state, the amount of orientation decreased with increasing stretching temperature. However, if recrystallization occurred in the stretched state, it led to the formation of a second crystalline network that prevented contraction of the oriented crystalline structure during strain recovery. It was suggested that the second network was anchored by α′‐PP daughter lamellae that crystallized epitaxially on the α‐PP mother crystals of the extended fibrils. Although the manner in which the films were stretched and recrystallized strongly affected the modulus, good elasticity of the stretched films revealed the persistence of an elastomeric network. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Properties of uniaxially stretched polypropylene films

Polypropylene (PP) films have been prepared through two different cast extrusion processes: one using a machine direction orientation (MDO) unit and the other stretching the films at the die under high cooling conditions (lab unit). Films for two PP resins different in molecular structure have been prepared using both processing techniques. The effect of the resin structure and the processing conditions on the film properties has been examined. It was found that the MDO unit generated a highly oriented fibrillar crystalline structure with a distribution of elongated thick fibrils while extrusion under high cooling conditions generated an oriented row nucleated lamellar structure. The films showed distinctive tensile responses in stretching, with a strong solid‐elastic response for the oriented MDO films and a steady strain hardening after yielding for the sample obtained from lab unit cast extrusion. It was found that the strength in the transverse direction (TD) was particularly very low for the oriented MDO films made of the bimodal PP. The oxygen permeability was reduced with increasing draw ratio (DR) for the MDO films. The haze property for the MDO samples reduced to a plateau for DR up to 5 while clarity improved continuously with DR.     

Impact of different die draw ratio on crystalline and oriented properties of polypropylene cast films and annealed films

Four kinds of polypropylene (PP) cast films with different die draw ratios (DDR) were prepared. The impact of different DDR on the crystalline and oriented properties of PP cast films and annealed films was explored herein. Wide angle X-ray diffraction (WAXD) and fourier transform infrared (FTIR) methods were adopted to examine the orientation degree of crystalline and amorphous phases. Long period distance (L_p) of the crystalline structure was tested by small angle X-ray scattering (SAXS). Crystallization was determined by differential scanning calorimeter (DSC). The oriented and crystalline behaviors of the samples were carried out by the elastic recovery (ER) testing. Then, samples after being annealed were examined by the same methods. The influence of annealing process on the films’ structures and properties was explored. Besides, the final stretched microporous membranes manufactured via stretching the annealed films along machine direction were examined by scanning electronic microscope (SEM). No matter for cast films or for annealed films, it is found that the films’ orientation degree of crystalline and amorphous phases, as well as L_p and crystallinity are larger at higher DDR and relatively lower at lower DDR. When the DDR is overly high (DDR?=?170), both the oriented and crystalline properties will decline. Elastic recovery testing indicates that a film with better orientation of the crystalline and the amorphous phases as well as with higher crystallinity can be obtained at an appropriate DDR. SEM images show that stretched membranes with better microporous structure can be obtained when the precursor film is prepared at a proper DDR.     

退火处理对成核聚丙烯性能的影响

采用退火处理对磷酸酯钠盐成核的聚丙烯（PP）体系进行了性能调控,研究了退火时间和退火温度对成核PP力学性能、耐热性能和结晶性能的影响。结果表明,随着退火温度的提高和退火时间的延长,PP的力学性能、耐热性能和结晶能力均得到了明显提升,特别是在120 ℃下,退火0.5 h,PP的拉伸强度从38.0 MPa提高到39.4 MPa,弯曲模量从1227 MPa提高到1882 MPa,热变形温度从112.2 ℃提高到122.7 ℃,结晶度从36 %提高到44 %。     

Studies on the biaxial stretching of polypropylene film. XI. Type II orientation during [2-2FI]-type stretching and its change by subsequent thermal contraction

In order to study the deformation mechanism of type II stretching, the change in orientation during the restretching and subsequent thermal contraction was investigated by x-ray diffraction method. When a uniaxially oriented film is restretched, the lamellae which are stacked in the stretching direction by the stretching rotate as a whole toward the restretching axis. They rotate backward nearly reversibly during the thermal contraction, unless the restretching exceeds a balancing state, where the orientation in the film plane are equal in all directions. However, when the restretching degree is so high and the film orientation exceeds the balancing state, the lamellar rotation is accompanied by a complex phenomenon. It is considered from the wide-angle and small-angle x-ray diffraction patterns that the lamellar surface becomes indented because of slippage between microfibrils composing the lamellae, and the microfibrils themselves bend at the boundary between the amorphous and crystalline regions within which the tilting of c-axis also occurs. Upon contracting of the film; these changes recover, but even in the last stage of contraction the orientation approaches the symmetrical biaxial orientation but not the uniaxial orientation from which the biaxial orientation is started. These orientation and disorientation behaviors are not affected basically by a slight change in the restretching temperature and the degree of stretching.     

Microporous membranes of polyoxymethylene from a melt‐extrusion process: (II) Effects of thermal annealing and stretching on porosity

A two‐part study utilizing polyoxymethylene (POM) was undertaken to investigate a three‐stage process (melt‐extrusion/annealing/uniaxial‐stretching) utilized to produce microporous films. In this report, the thermal annealing (second stage) and subsequent uniaxial‐stretching (third stage) results of selected POM films from two commercial resins, labeled D & F, are discussed. Specifically, the annealing and uniaxial stretching effects on film morphology, orientation, and other pertinent film properties are addressed. Additionally, sequential analysis was performed regarding the influence each stage had on the resulting microporosity. It was found that the melt‐extruded precursor morphology and orientation, as a consequence of the first stage extrusion parameters and resin characteristics, are crucial to controlling the membrane permeability. The annealing parameters were also deemed critical, where a temperature of 145°C applied for 20 min under no tension was the optimum annealing condition for producing a highly microporous film upon stretching. For the conditions studied, the stretching parameters that were found to be optimum for producing the desired characteristics in the final film were a cold temperature of 50°C and hot stretch temperature of 100°C. The optimum extension levels were concluded to be 90% for both the cold and hot stretch steps, and thus a total overall extension level of 180%. However, these results were only with respect to resin F films. Because the resin D melt‐extruded precursors possessed twisted lamellar morphologies and relatively low crystal orientation, their samples could not be produced into microporous films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1762–1780, 2002; DOI 10.1002/app.10587     

Effect of external field on the lamellar crystalline structure and properties of poly(4-methyl-1-pentene) casting film

In this work, lamellae structures of poly(4-methyl-1-pentene) casting films with different draw ratios and properties of corresponding stretched membranes were discussed. The crystalline structure was quantitatively analyzed in terms of crystallinity, long period, crystalline orientation, and lamellae lateral dimension, which were largely influenced by the draw ratio. Apparently, as draw ratio increased, the elapsed time of samples declined rapidly and porosity showed an upward tendency. In this article, these results can be used to predict the relationship between properties of casting films (such as orientation, lamellar structure, and elastic recovery) and properties of stretched membrane (average pore size, permeability, and porosity). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47293.     

Novel polypropylene microporous membranes via spherulitic deformation – Processing perspectives

The processing boundaries for initiating intra-/inter-spherulitic deformation to create microporous polypropylene membranes by lamellar separation are delineated. The processing parameters are: annealing temperature, extension ratio, stretching rate, and stretching temperature. A fixed set of extrusion conditions was chosen for producing precursor films having similar spherulitic morphologies. The morphological changes indicating the occurrence of intra-/inter-spherulitic transition on a spherulitic scale can also be detected on a lamellar scale by WAXS examination via analysis of the α-form orientation index. Membrane porosity measurements showed a consistent correlation with the observed values of the α-form orientation index. Increasing the extension ratio did not change the microstructure in the non-annealed sample; however, the lamellae can be further oriented in the annealed samples. Inter-spherulitic deformation became obvious at slow stretching rates; intra-spherulitic deformation was favored at fast stretching rates. The DSC thermal analysis of the precursor films showed two significant endothermic discontinuities (T₁ at 0 °C and T₂ at 40 °C) in both non-annealed and annealed precursor films; T₁ is believed to be the conventional T_g of polypropylene whereas T₂ appears to originate from the rigid-amorphous fraction trapped within the lamellar “wells” where the amorphous phase is surrounded by the R-lamellae and the T-lamellae. The lamellae could break down or slip from the lamellar knots as stretching temperatures are high enough to minimize the influence of the rigid-amorphous fraction, and the annealed lamellae can still be oriented without a catastrophic cold-drawn deformation.