Sheet extrusion of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion of sheets from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used were HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties for in situ composites in which the TLCP fibrils are generated in contractions in the die and the subsequent drawing step. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors affecting the mechanical properties of the composite sheets considered include the effect of in situ composite strand properties and TLCP concentration. In addition, the properties of the extruded sheets are compared to those of microcomposites processed by means of injection molding. It is shown that the sheets produced using microcomposites have a good balance between the machine and transverse direction properties (ratios of these properties ranging from 0.8 to 1.2) and those properties compare well to those obtained by processing microcomposites in injection molding. The tensile modulus of the composite sheets increases with increasing in situ composite strand modulus. The moduli of the 20 wt% Vectra A950 and HX6000 composites are about equal to the modulus of 20 wt% glass reinforced PP (about 2.1 GPa), while the tensile strength of the TLCP reinforced composites is 28% lower than that of the glass reinforced PP. Furthermore, it is shown that the tensile modulus of the 10 wt% TLCP composites approach the predictions of composite theory, while at 20 and 30 wt% TLCP negative deviations from the predictions of composite theory are seen. Finally, it is concluded that the properties of the sheets produced through the extrusion of microcomposites may be further improved by improving the modulus of in situ composite strands and reducing the TLCP fiber diameter.     

Extrusion blow molding of microcomposites based on thermotropic liquid crystalline polymers and polypropylene

This work is concerned with the extrusion blow molding of bottles from pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated in elongational flow fields that occur in polymer processing operations and to determine if TLCP reinforced bottles could be produced by extrusion blow molding of microcomposites. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of TLCP concentration and in situ composite strand properties on the mechanical properties and anisotropy of bottles made from microcomposites. Specifically, strands having three different draw ratios are used to produce bottles at 10 and 20 wt% TLCP. Increasing the in situ composite strand modulus is shown to cause an increase in both the machine and transverse direction moduli of the composite bottles. The mechanical properties of the bottles increase with increasing TLCP composition. Finally, the machine and transverse direction properties are not balanced in the composite bottles produced in this study (degrees of anisotropy ranging from 1.5 to 1.8). The mechanical anisotropy is probably the result of a low blow up ratio (2) in the bottles and the TLCP fibers being oriented primarily in the machine direction due to the shear flow in the die.     

Composites based on drawn strands of thermotropic liquid crystalline polymer reinforced polypropylene

This paper is concerned with the use of two thermotropic liquid crystalline polymers (TLCPs), HX1000 and Vectra B950, to reinforce a thermoplastic matrix of polypropylene (PP). The goal was to pregenerate the optimal TLCP reinforcement in PP and then process the material at a lower temperature than the melting point of the TLCP to form a composite structure. Specifically, strands of the blend were produced using a dual extrusion process, which resulted in the formation of axially continuous TLCP fibrils within the PP matrix. It was found that the mechanical properties of the strands were greatly improved by increased draw ratio and that optimal reinforcement, as predicted by the rule of mixtures, could be achieved. Initial studies indicated that injection molding and sheet extrusion of the pelletized strands caused the TLCP phase to agglomerate and deform, which resulted in a reduction of the mechanical property enhancement. However, the TLCP fibrillar morphology in the pregenerated strands was maintained during compression molding, which resulted in uniaxial composites with properties equal to or greater than properties of the strands. In addition, composites were made using compression molding in which strands were randomly oriented prior to consolidation to show the limits of properties possible in composites produced from the pregenerated strands. It was found that this process could be used to produce composites in which the mechanical properties were isotropic in the plane of the sample and approached the properly limits predicted by composite theory. Additionally, it was found that many of the mechanical properties of the VB/PP materials were greatly enhanced by the addition of a maleated PP throughout the composite forming process.     

Self‐reinforced polypropylene/LCP extruded strands and their moldings

Polypropylenes (PP) of various molecular weights were mixed with a thermotropic liquid crystal polymer (LCP) and strands were prepared by extrusion and stretching. The strands were subsequently pelletized and then injection molded at temperatures below the melting point of LCP. The mechanical properties and the morphology of the strands and injection‐molded specimens were investigated as a function of draw ratio, LCP concentration, and PP molecular weight. The results for strands show that an increase in the draw ratio, LCP concentration and matrix molecular weight in general enhance the modulus and tensile strength. However, the tensile properties of injection‐molded specimens are found to be reduced compared with those of the original strands, in particular at high LCP concentration. The morphology of LCP changes from spherical or ellipsoidal droplets to elongated fibrils in the strands as the draw ratio increases, but this aligned LCP fibrillar morphology was not transferred to the injection‐molded specimens because of the disorientation of fibrils during injection molding. Compatibilization of PP/LCP blends was also studied by using various polymers. Maleic anhydride and acrylic acid modified PPs improved the tensile properties modestly, but maleic anhydride modified EPDM reduced the tensile properties.     

Modification of the processing window of a thermotropic liquid crystalline polymer by blending with another thermotropic liquid crystalline polymer

This paper is concerned with properties and processing performance of two thermotropic liquid crystalline polymers (TLCPs) produced by DuPont (HX6000 and HX8000) with widely varying melting points and blends of these two TLCPs. This work was carried out in an effort to develop a TLCP suitable for generating poly(ethylene terephthalate) (PET) composites in which the melting point of the TLCP was higher than the processing temperature of PET. Strands of the neat TLCPs and a 50/50 wt % TLCP–TLCP blend were spun and tested for their tensile properties. It was determined that the moduli of the HX8000, HX6000, and HX6000–HX8000 blend strands were 47.1, 70, and 38.5 GPa, respectfully. Monofilaments of PET–HX6000–HX8000 (50/25/25 wt %) were spun with the use of a novel dual extruder process. The strands had moduli as high as 28 GPa, exceeding predictions made using the rule of mixtures and tensile strengths around 275 MPa. The strands were then uniaxially compression molded at 270°C. It was found that after compression molding, the modulus dropped from 28 GPa to roughly 12 GPa due to the loss of molecular orientation in the TLCP phase. However, this represents an improvement over the use of HX8000. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2209–2218, 1999     

Separation of a thermotropic liquid crystalline polymer from polypropylene composites

This work is concerned with determining how to effectively recycle wholly thermoplastic composites comprised of a polypropylene (PP) matrix reinforced with a thermotropic liquid crystalline polymer (TLCP). A novel reclamation process was developed in which the TLCP could be recovered from the PP matrix. Reactive extrusion was used to reduce the molecular weight of the PP (Montell 6523) and to facilitate phase separation. The melt was then extruded into a heated mineral oil bath, which separated the TLCP (DuPont HX8000) from the matrix by dissolving the PP. It was found that greater than 70 wt% of the TLCP could be reclaimed from the PP matrix at a purity of greater than 96 wt%. In order to determine the ability to reuse the reclaimed HX8000, injection molded in situ composites were generated and their mechanical properties were determined. When the neat HX8000 component was partially replaced with reclaimed HX8000, the injection molded TLCP/PP composites showed no discernible difference in mechanical properties.     

Thermoplastic composites reinforced with long fiber thermotropic liquid crystalline polymers for fused deposition modeling

This work is concerned with preliminary studies on developing thermoplastic composites for use in fused deposition modeling (FDM). Polypropylene (PP) strands reinforced with thermotropic liquid crystalline polymer (TLCP) fibrils were generated in a novel dual extruder process. The process allowed the reinforcement of PP with a melting point (T_m) of 165°C with continuous fibrils of a high melting (283°C) TLCP (Vectra A950). The strands were then re-extruded in a capillary rheometer forming monofilaments to simulate piston actuated FDM. The effects of the thermal and deformation histories on the mechanical properties of the re-extruded strands were evaluated. It was found that tensile properties of the strands improved with draw ratio and that the maximum modulus of the composite strands was similar to that predicted by composite theory. Strands were consolidated uniaxially via compression molding at temperatures just above the melting point of the matrix to determine the effect of thermal history. This resulted in a ∼20% reduction in tensile modulus relative to the modulus of the strands. Monofilaments were extruded from a capillary rheometer in which long fiber strands were used as feedstock to study the effects of deformation history on the tensile properties. It was found that the tensile properties of the monofilaments were dependent on capillary diameter, capillary L/D, and apparent shear rate due to fibril alignment.     

Flow‐induced warpage of injection‐molded TLCP fiber‐reinforced polypropylene composites

The most common belief is that warpage in injection‐molded fiber‐reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow‐induced stresses generated during mold filling do not play a significant role. Injection‐molded plaques of polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils were generated in order to investigate the role of residual flow‐induced stresses relative to that of thermal stresses on the warpage. In an effort to relate the material parameters to warpage, the rheological behavior of these fiber‐filled systems was investigated. The shrinkage and the thermal expansion of the TLCP/PP composites, and hence, the thermally induced stresses decreased with an increase in fiber loading while the flow‐induced stresses increased. The increase in the flow‐induced stresses was attributed to increased relaxation times (this is not the only cause, but is a significant factor) with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber‐reinforced thermoplastics, the flow‐induced residual stresses must be accounted for. It is expected that the results reported here can be extended to glass‐reinforced PP composites as well. POLYM. COMPOS., 27:239–248, 2006. © 2006 Society of Plastics Engineers     

Influence of viscosity ratio on processing and morphology of thermotropic liquid crystal polymer‐reinforced poly(ethylene 2,6‐naphthalate) blends

Thermotropic liquid crystal polymer (TLCP) microfibril‐reinforced poly(ethylene 2,6‐naphthalate) (PEN) composites with various intrinsic viscosities were prepared by a melt compounding method. Polymer composites consisting of bulk cheap polyester with a small amount of expensive TLCP are of interest from a commercial perspective. The TLCP acts as a nucleating agent in the TLCP/PEN composites, enhancing the crystallization of the PEN matrix through heterogeneous nucleation. The structural viscosity index of the TLCP/PEN composites was lower than that of PEN and TLCP, which was attributed to the formation of TLCP fibrillar structures with elongated fibrils in the PEN matrix. The TLCP/PEN composites with higher intrinsic viscosity than the polymer matrix contained these elongated fibrils, and had a TLCP component with a smaller average diameter, and a narrower diameter distribution than TLCP/PEN composites with lower intrinsic viscosity. The higher intrinsic viscosity of the polymer matrix, the higher shear rate and the lower viscosity ratio of TLCP to PEN can all favour TLCP fibrillation in the polymer composites. Copyright © 2006 Society of Chemical Industry     

PA66／TLCP原位复合材料的热性能、形貌及力学性能研究

通过挤出和注射成型制备了聚酰胺66／热致液晶聚酰胺（PA66／FLCP）原位复合材料，研究了其热性能、形貌及力学性能。DSC分析表明，PA66和TLCP相容性较好，随着TLCP含量的增加，PA66的结晶度、结晶速率下降；SEM分析表明，TLCP在PA66基体中分散均匀，两相相容性较好，当加入10％（质量分数，下同）的TLCP时，TLCP形成长径比比较大的纤维；拉伸试验结果表明，当加入TLCP后，PA66的力学性能有明显的改善。当加入10％的TLCP时，共混物的力学性能增幅最大，拉伸强度增加79．6％，拉伸模量增加120．4％，断裂伸长率明显下降。     

Morphology and mechanical properties of thermoplastic composites containing a thermotropic liquid crystalline polymer

Blends of two thermotropic liquid crystalline polymers (TLCPs), with brittle and ductile matrix materials were both injection molded and spun into fibers, in order to investigate the mechanism of in-situ mechanical reinforcement. In the injection molded samples, the TLCP was only moderately elongated into fibrils, and the mechanical properties were below predictions of the rule of mixtures. Fibers spun out of the blends contained numerous fine fibrils with nearly infinite aspect ratio, and as expected, the modulus increased linearly with the TLCP volume fraction, obeying the Tsai-Halpin equation for transversely isotropic composites. Wide angle X-ray diffraction measurements, as well as determination of the fiber-moduli, revealed that during spinning not only a macroscopic elongation of the fibrils was achieved, but also a considerable molecular orientation within the TLCP domains.     

PPS/TLCP共混体系结构与流变研究

采用热致液晶聚合物（TLCP）与聚苯硫醚（PPS）熔融共混的方式制备了PPS／TLCP复合材料,研究了PPS／TLCP共混体系的形貌、流变性能以及加工参数对微纤形成的影响。结果表明：TLCP可明显改善体系的加工特性,并能原位生成微纤化复合材料,TLCP对体系黏度有较大影响,在低剪切速率区黏度下降幅度较大,在高剪切速率区,黏度降低幅度小。PPS／TLCP复合材料存在皮芯结构,工艺参数对TLCP微纤的形成起着重要作用,通过提高注塑速度,对TLCP微纤的形成特别有利。     

Effects of processing conditions on the mechanical performance of maleic anhydride compatibilized in-situ composites of polypropylene with liquid crystalline polymer

Maleic anhydride compatibilized blends of isotactic polypropylene (PP) and thermotropic liquid crystaline polymer (LCP) were prepared either by the direct injection molding (one-step process), or by twin-screw extrusion blending, after which specimens were injection molded (two-step process). The morphology and mechanical properties of these injection molded in situ LCP composites were studied by means of scanning electron microscopy (SEM), Izod impact testing, static tensile, and dynamic mechanical measurements. SEM observations showed that fine and elongated LCP fibrils are formed in the maleic anhydride compatibilized in situ composites fabricated by means of the one-step process. The tensile strength and modulus of these composites were considerably close to those predicted from the rule of mixtures. Furthermore, the impact behavior of LCP fibril reinforced composites was similar to that of the glass fiber reinforced polymer composites. On the other hand, the maleic anhydride compatibilized blends prepared from the two-step process showed lower mechanical performance, which was attributed to the poorer processing behavior leading to the degradation of PP. The effects of the processing steps, temperatures, and compatibilizer addition on the mechanical properties of the PP/LCP blends are discussed.     

气体辅助注塑成型带加强筋平板的翘曲试验研究

注塑制品翘曲变形是一种严重缺陷,本文基于带加强筋平板制品实验研究了气体辅助注塑成型工艺参数对制品翘曲的影响规律及其原因,结果表明:熔体预注射量、气体压力及气体保压时间和气体压力清零时间对制品翘曲影响较大。熔体温度、延迟时间和保压压力影响较小。     

长玻纤增强PET复合材料的力学性能研究

采用自制的浸润装置，以PET浸渍长波纤，经切粒后得到长度为6mm的长纤维增强PET预浸料切片，经一定温度热处理，可得到长纤增强PET复合材料。研究了注塑样条中玻纤含量对其力学性能及玻纤长度分布的影响，并采用SEM观察了长玻纤增强PET注塑样条的断面形貌。结果表明，复合材料力学性能随玻璃纤维含量的提高均有不同程度的提高，当玻纤的质量分数在40％～50％时，力学性能基本达到最佳，且由本方法制备的长玻纤增强PET复合材料的力学性能已达到并超过了国外同类产品的水平。     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

Morphological development during injection molding of self‐reinforcing composites. II: Numerical simulations and analytical predictions

In this study, combined numerical simulation of injection molding and analytical calculations have been used to determine the velocity and elongational strain in the advancing melt front (AMF) region, during the molding of PET/LCP blends, at various injection molding conditions. A model is proposed that establishes the relationship between the aspect ratio of LCP fibers and elongational strain, based on the assumption of an affine deformation of the LCP domains. This model enables us to predict the processing dependent morphology of injection molded PET/LCP blends. The effect of processing parameters on the morphology development during injection molding were investigated. The studies show that injection speed and mold temperature have significant effects on the morphological development of the blends, compared with the effect of the melt temperature. A good correlation between calculated and scanning electron microscopy results was obtained.     

Morphological development during injection molding of self‐reinforcing composites. I: Experimental results

The correlation between structure development during injection molding and the modulus of injection molded PET/LCP blends were studied. Process parameters such as injection speed and mold and melt temperatures were varied to determine the effect of these parameters on the tensile modulus and structure development of the blends. The skin/core structure in the cross section of injection molded samples was observed with both optical and scanning electron microscopy techniques. Injection molding experiments show that the thickness of the skin layer increases with decreasing injection speed and decreasing melt and mold temperatures. The trends in morphological developments in the injection molded specimens correlate with the measured tensile moduli.     

Processing-property relationships of polycarbonate/graphene composites

Polycarbonate composites reinforced with graphite and functionalized graphene sheets (FGS) were produced using melt compounding. Composite samples with different degrees of graphite orientation were processed via injection, compression molding and long-term annealing. Electron microscopy and X-ray scattering revealed that FGS was nearly exfoliated. However, graphite remained multi-layer even after melt processing. Flow induced orientation of graphite was observed from both injection and compression molded samples. Graphite particles in samples after long-term annealing exhibited more random orientation. Composites with the exfoliated FGS required a smaller amount of reinforcement for rigidity and connectivity percolation, as determined by melt rheology and electrical conductivity measurements. FGS also showed better performance in suppressing gas permeability of polycarbonate. However, improvements by FGS dispersion in tensile modulus and dimensional stability were not as significant. This may be due to defects in the sheet structure formed during oxidation and pyrolysis used to exfoliate.     

The occurrence of surface roughness in gas assist injection molded nylon composites

Gas assist injection molding has increasingly become an important industrial process because of its tremendous flexibility in the design and manufacture of plastic parts. However, there are some unsolved problems that limit the overall success of this technique. The purpose of this report was to study the surface roughness phenomenon occurring in gas assist injection molded thermoplastic composities. The materials used were 15 % and 35% glass‐fiber filled nylon‐6 composites. Experiments were carried out on an 80‐ton injection molding machine equipped with a high‐pressure nitrogen‐gas injection unit. Two “float‐shape” axisymmetric cavities were used. After molding, the surface quality of molded parts was measured by a roughness meter. Various processing variables were studied in terms of their influence on formation of surface roughness: melt temperature, mold temperature, melt filling speed, short‐shot size, gas pressure, and gas injection delay time. Scanning electronic microscopy was also employed to characterize the composites. It was found that the surface roughness results mainly from the exposure of glass fiber in the matrix. The jetting and irregular flows of the polymer melt during the filling process might be factors causing the fiber exposure.