水辅助注塑聚丙烯制品的晶体结构研究

通过偏光显微镜(PLM)观察了水辅助注塑(WAIM)聚丙烯(PP)制品靠近注水喷嘴和远离注水喷嘴两个位置的晶体结构,发现WAIM制品沿壁厚方向都可以分为表层、中芯层和水道层,并且发现两个位置水道层和中芯层的晶体结构比较相似,表层晶体结构出现了较大的差异;靠近注水喷嘴位置表层晶体结构出现明显的取向现象,而在远离注水喷嘴位置没有出现。在晶体结构分析的基础上,初步探讨了水辅助注塑制品晶体结构的形成机理。结果表明:水的穿透对于熔体内部剪切的增加和取向结晶的形成有明显的促进作用。     

水辅助注塑PP/EVOH共混物制品的相形态与阻渗性能

选择3种不同黏度的聚丙烯(PP)与聚乙烯醇(EVOH)共混制备质量比为90/10的共混物,并采用水辅助注塑(WAIM)将这3种共混物成型为中空制品。从WAIM制品靠近浇口(1^#)和末端(2^#)两个位置取出样品,通过扫描电镜观察样品壁厚上3个位置的相形态,并测试所取样品的甲苯渗透率。借助WAIM中高压水作用下模腔内熔体的流场对样品中3个位置的相形态进行了分析。对WAIM的高黏度比共混物制品2^#样品,在外表层和内表层分散相呈粗纤维状,芯层主要呈液滴状,其阻渗性能与相应的WAIM PP样品比有适度提高(约2.4倍);对WAIM的2种低黏度比共混物制品2^#样品,外表层和内表层分散相呈细纤维状,芯层呈粗长纤维状,其阻渗性能与相应的WAIM PP样品比提高幅度较大(其中对黏度比最小的共混物达9.8倍)。1^#位置所取3种WAIM PP/EVOH样品中分散相纤维的平均直径比2^#位置的大,导致1^#位置所取样品的阻渗性能比2^#位置的低。     

水辅助注塑尼龙6制品的结晶行为研究

利用差示扫描量热法(DSC)对水辅助注塑尼龙6(PA6)制品靠近注水喷嘴(P1)位置和远离注水喷嘴(P2)位置的表层、中芯层和水道层的结晶行为进行了研究,并初步探讨了高压水对PA6制品结晶行为的影响。结果表明:对于P1和P2位置,表层与水道层的结晶度均低于中芯层的结晶度;P1位置表层与水道层结晶度均低于P2位置同层的结晶度;P1位置中芯层结晶度高于P2位置中芯层的结晶度;高压水的冷却作用会降低水道层熔体的结晶度并促使水道层形成较小的晶体。     

挤出机头内流场对玻纤取向和分布的影响

本文分析了平直和发散两种挤出机头内的流场,推得发散流道内熔体周向拉伸应变速率的表达式;研究了两种挤出流率下由这两种机头挤出的制品壁内玻纤的取向和分布,并通过流道内的剪切和周向拉伸应变速率,对玻纤取向和分布的形成机理进行解释。结果表明：经平直机头挤出的制品内,玻纤在剪切作用下基本沿流动方向排列。发散机头内熔体受剪切和周向拉伸的共同作用,使制品壁厚方向形成了“表层-次表层-芯层-次表层-表层”的五层结构,并首次发现芯层呈“W”形排列。玻纤的排列不仅受流动过程中的应变影响,更取决于应变速率的大小。     

水辅助注塑PP/EVA共混物的相形态

通过扫描电镜(SEM)观察了水辅助注塑聚丙烯(PP)/乙烯-醋酸乙烯共聚物(EVA)共混物制品近浇口和远浇口处外表层、芯层和内表层的相形态.结果表明,分散相主要以层状分布在基体中,在两个观察位置外表层和内表层的层状分散相厚度比芯层的大,远浇口处外表层和内表层的分散相厚度比近浇口处对应层的大.从温度场和剪切场两个方面分析了分散相的形变过程.     

注射成型纤维取向模拟及实验验证

通过实验分析了短纤维增强复合材料注射成型标准试样中的纤维取向分布,并与Moldflow模拟结果进行了对比.结果表明,注塑试样标定段具有分层取向结构,表层沿流动方向取向强烈,过渡层取向程度下降,但仍为平面取向,芯层则表现为三维取向分布.Moldflow模拟结果与实验结果对比表明,模拟结果高估了芯层的取向程度.     

熔体温度对HDPE微制品结构及性能的影响

采用微注塑成型技术,在不同熔体温度下制备高密度聚乙烯(HDPE)微制品。通过拉伸测试考察了熔体温度对力学性能的影响,并利用差示扫描量热法(DSC)、广角X射线衍射(WAXD)、小角X射线散射(SAXS)和扫描电子显微镜(SEM)对制品的微结构变化进行系统研究。结果表明:随着熔体温度的升高,shish结构和shish-kebab晶体的数量增加,制品的结晶度增加,分子链和片晶取向程度增大;取向度和结晶度的增加是HDPE制品拉伸强度和模量随熔体温度升高而增加的主要原因。     

振动力场对注射制品取向结构影响研究

对影响聚合物制品取向结构的传统加工因素做了分析总结,从微观结构形态分析论证了振动力场对制品取向结构的影响,指出动态成型加工与传统稳态加工相比能够有效提高制品的取向;对振动力场作用下的聚合物注塑制品进行力学性能测试和偏光红外测试。结果表明,与传统稳态加工相比,制品的拉伸强度有了明显的提高;采用红外光谱仪对动态注射成型制品的取向函数进行测定后,发现振动力场能够显著提高制品芯层的取向,尽管对制品皮层的取向提高不是很明显,但缩小了皮/芯层之间的取向差异,从总体上提高了制品取向;研究比较了在振动力场影响下制品取向函数与拉伸强度的变化规律,指出振动力场通过提高制品的取向结构来提高制品的力学性能。     

微型注塑POM/MoS_2复合材料微观形貌和结晶行为研究

制备了聚甲醛/二硫化钼(POM/Mo S_2)复合材料并对其进行微型注塑,考察了POM/Mo S_2微型制品的微观形貌,同时研究了注射条件对其取向和结晶行为的影响。扫描电子显微镜(SEM)和偏光显微镜(PLM)分析结果表明:POM/Mo S_2微型制品具有典型的皮芯结构,其中Mo S_2片状粒子在剪切层沿流动方向取向排列,而在芯层则呈无规排列。二维广角X射线衍射分析(2D-WAXD)和差示扫描量热分析(DSC)的结果表明:微型注塑的模具温度和注射速率对微型制品的取向和结晶行为都有较大的影响。随着模具温度的提升,相应微型注塑制品的取向程度呈下降趋势,但POM的熔点和结晶度提高;随着注射速率的提高,相应微型制品的取向程度及POM的结晶度均呈上升趋势,但POM的熔点变化不明显。     

车用聚丙烯复合材料结晶行为研究

以聚丙烯为基体,滑石粉、乙烯/辛烯共聚物为添加成分,制备了聚丙烯复合材料。采用广角X射线衍射表征了复合材料注塑样板皮层和芯层,以及注塑样板不同区域表层的结晶行为。结果表明:聚丙烯复合材料注塑样板皮层具有显著的b轴结晶取向趋势;复合材料注塑样板芯层的b轴结晶取向趋势较弱;注塑样板正中位置表层b轴结晶取向趋势最明显。     

The hierarchical structure of water-assisted injection molded high density polyethylene: Small angle X-ray scattering study

High density polyethylene (HDPE) was molded by a new polymer processing method, that is, water-assisted injection molding (WAIM), and its hierarchical structure was studied by two-dimensional small angle X-ray scattering (SAXS). For comparison, the hierarchical structure of HDPE molded by conventional injection molding (CIM) was also characterized. The result shows that the WAIM part exhibits a distinct skin-core-water channel structure which is different from the skin-core structure for the CIM part. In the skin layer of both WAIM and CIM parts, the shish-kebab structure was formed due to the shear stress brought by melt filling, but the lamellar orientation parameter of CIM part is smaller than that of WAIM part. The spherulites with random lamellar orientation are dominant at the core of both parts owing to the low cooling rate and feeble shear stress therein. Interestingly, the shish structure and the lamellae with low level of orientation can be found at the water channel layer of WAIM part. They are attributed to the shear stress brought by water penetration. Moreover, the lamellar orientation parameter in water channel layer is smaller than that of skin layer. In addition, the long period of WAIM part first increases and then decreases with the elevating distance from the skin surface, while that of CIM part tends to increase monotonously. In a word, one can conclude that the rapid cooling rate and shear brought by the injected water have significant influence on the structural evolution for the WAIM part. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Shear induced shish-kebab structure in PP and its blends with LLDPE

In order to better understand the effect of shear stress on the crystal morphology and orientation of polyolefins, dynamic packing injection molding was used to prepare oriented pure polypropylene (PP) and its blends with linear low density polyethylene (LLDPE). The obtained samples were characterized via 2d-SAXS, 2d-WAXD and AFM. Macroscopically, shear induced morphology with surface skin, central core and oriented layer between the skin and the core was observed in the cross-section areas of the samples. For pure PP, a highly oriented structure was seen in the sheared layer but much less oriented structure exists in the core. The orientation in the skin lies in between. The shish-kebab structure, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), was found in the sheared layer. Shish structure exists mainly in the skin layer and oriented spherulits dominates in the core. For PP/LLDPE (50/50) blends, a change of phase morphology from less-phase-separated structure (homogeneous) in the skin, to co-continuous structure in the sheared layer and sea-island structure in the core was observed. PP formed a shish-kebab structure in all the three layers. And on the other hand, a very unique crystal morphology and lamellar orientation of LLDPE were obtained, with the lamellar stack oriented either perpendicularly or 45-50° away from the shear flow direction.     

Crystalline morphology of isotactic polypropylene (iPP) in injection molded poly(ethylene terephthalate) (PET)/iPP microfibrillar blends

The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ microfibrillar blends have been prepared through a “slit die extrusion-hot stretch-quenching” process, in which PET assumes microfibrils with 0.5-15 μm in diameter depending on the hot stretching ratios (HSR, the area of the transverse section of the die to the area of the transverse section of the extrudate). The injection molded specimens of virgin iPP and the PET/iPP blends were prepared by conventional injection molding (CIM) and by shear controlled orientation injection molding (SCORIM), respectively. The effect of shear stress and PET phase with different shape on superstructures and their distribution of injection molded microfibrillar samples were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The shear (or elongational) flow during CIM and SCORIM can induce oriented lamellae (i.e. kebabs induced by shish). The shish-kebab structure appears not only in the skin and intermediated layers of CIM samples, but also in the whole region of SCORIM samples. For the neat iPP samples, a more “stretched” shish-kebab structure with higher orientation degree can be obtained in the interior region (intermediate and core layers) by the SCORIM method; moreover, the SCORIM can result in the growth of β-form crystal both in intermediate layer and in core layer, which only appears in intermediate layer of the neat iPP samples obtained by CIM. For the PET/iPP blends, interestingly, the addition of microfibrils as well as their aspect ratios can affect the orientation degree of kebabs only in the intermediate layers, and the addition of microfibrils with a low aspect ratio can bring out a considerable increase in the orientation degree of kebabs along the flow direction. However, for the SCORIM, the addition of microfibrils seems to be a minor effect on the orientation degree of kebabs, and it tends to hamper the formation of a more “stretched” shish-kebab structure and suppresses the growth of β-form crystal distinctly. Furthermore, It appears from experiment that γ-form crystals can grow successfully in this oriented iPP melt with the synergistic effect of shear and pressure only when the growth of β crystals can be restrained by some factors, such as the PET dispersed phase and thermal conditions (cooling rate).     

Tailoring the crystalline structure of polypropylene parts molded through water‐assisted injection molding: Effects of melt temperature and polymeric nucleating agent

The melt temperature and a special polymeric nucleating agent [acrylonitrile–styrene copolymer (SAN)] were investigated to find an effective way for tailoring the crystalline structures of the water‐assisted injection‐molded polypropylene (WAIM PP) parts. The results showed that lowering the melt temperature led to the formation of a small amount of β‐form crystals in both outer and core layers of the WAIM PP parts. Nevertheless, the melt temperature had little effect on tailoring the crystalline structures of the WAIM PP parts. The addition of a low content (6 wt%) of the SAN was interestingly found to gradually influence the crystalline structures as lowering the melt temperature. WAIM PP/SAN blend parts with high contents of β‐form in both outer and core layers (30.7 and 18.4%, respectively), and high contents of transcrystals in the inner layer were molded at relatively low melt temperature (180°C), whereas the SAN had little influence on the crystalline structures at higher melt temperature (230°C). The formation of the transcrystals was ascribed to the in situ fibrillation of the SAN, which was resulted from high shear and cooling rates caused by high‐pressure water penetration during WAIM. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

The hierarchy structure and orientation of high density polyethylene obtained via dynamic packing injection molding

The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI).     

Formation mechanism of high-pressure water penetration induced fiber orientation in overflow water-assisted injection molded short glass fiber-reinforced polypropylene

Recently, there has been growing interest in water-assisted injection molding (WAIM) not only for its advantages over gas-assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water-assisted injection molding (OWAIM) parts of short glass fiber-reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short-shot water-assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.     

Process‐induced phase and crystal morphologies in water‐assisted injection molded polypropylene/polymeric β‐nucleating agent blend parts

By adding a polymeric β‐nucleating agent (acrylonitrile–styrene copolymer, SAN), in situ microfibril reinforced isotactic polypropylene (iPP)/SAN blend parts with high contents of β‐form crystals and transcrystals were molded via water‐assisted injection molding (WAIM). Thanks to the unique stress and temperature fields occurring during the WAIM, SAN microfibers formed across the whole residual wall of iPP/SAN blend parts with relatively large thickness. Numerical simulations on high‐pressure water penetration and cooling stages of the WAIM were carried out to reveal the stress and temperature fields. Comprehensive analysis of both experimental and simulated results showed that not only the shear flow field but also elongational flow field occurring during the WAIM was responsible for the formation of SAN microfibers and unique crystal morphology distribution in the WAIM iPP/SAN blend part. Moreover, during the WAIM, the high cooling rate also played an important role in the formation of both phase and crystal morphologies. The preferential formation of transcrystals in the inner layer of WAIM iPP/SAN blend part could be ascribed to the strong elongation, rather than the strong shear. It was believed that the quantification of stress and temperature fields of the WAIM via numerical simulation could provide a guidence for molding high‐performance products. POLYM. ENG. SCI., 55:1698–1705, 2015. © 2014 Society of Plastics Engineers