Study On Polymer Reological Behavior And Structure-properties Prepared Via Low Frequency Vibration Injection Molding

Recently the melt vibration technique, as an effective morphology control technique, is widely studied in the world. In this paper, two creatively self-made vibration injection-molding devices are applied to realize shear vibration and pressure vibration, respectively, which are used to study the relationship between vibration injection molding and rheological behavior and structure-morphology of injection moldings. Aim to industrialize the melt vibration technique, effects of low frequency melt vibration on rheology, uniaxially and biaxially mechanical properties and morphologies of conventional injection moldings (CIM) and vibration injection molding (VIM) are researched in detail. The VIM pioneers a novel method to better melt rheological behavior and to obtain self-reinforced and/or self-toughened injection moldings, and the main results are as followed.1.Research and Development of Low Frequency Vibration Injection Molding DeviceAim to realize shear vibration and pressure vibration, respectively, two vibration fields, wittily superimposed to injection molding process, and the developed technique is here called low frequency vibration injection molding. The eachvibration injection molding device can realize conventional injection molding and vibration injection molding, respectively, so the structure-properties prepared by vibration injection molding can be compared to those obtained by conventional injection molding. As for the shear vibration device, the slot-and-crank mechanism was used to realize melt shear vibration, the vibration frequency (0~15Hz) and displacement amplitude (0~3mm) can be conveniently and precisely adjusted. The hydraulic system was first used to realize melt pressure vibration, and the range of vibration frequency is from 0 to 3Hz, and range of pressure vibration amplitude is from 0 to 59.4MPa. The adoptions of pressure vibration amplitude, pressurized oil cylinder are creative designs in the pressure vibration device. The selected scheme of pressure vibration penetrating through barrel, nozzle, runner system and mold cavity is never reported in the literatures.2. Effect of Low Frequency Melt Vibration Field on Rheological Behavior of Polymer MeltThe rheological behavior was changed greatly in the shear vibration field. The average apparent viscosity decreases first with increasing vibration frequency and displacement amplitude, and when the frequency reaches about 5Hz the viscosity begins to pick up and changes in viscosity is very little, which is consistent with the research results studied by J. P. Ibar, the authority of vibration injection molding. But in his researches the effect of melt vibration on the rheological behavior was focused on amorphous polymers. In the paper the studied polymer was expanded to semi-crystalline polymer. The polymer melt temperature and the average pressure are influencing factors for the viscosity, and at lower temperature and/or lower average pressure result in greater decrease in average apparent viscosity. For HDPE a maximal decrease in average apparent viscosity induced by melt shear vibration, 53.2%, is obtained at 5.67Hz, and the corresponding value is 60.3% for PS at 4Hz.In previous studies on effect of melt vibration are mainly focused on melt shear vibration, and the amplitude is also the displacement vibration amplitude. In the paper the pulsing pressure vibration is creatively superposed to the flowing melt, and originates the study on melt properties through pressure vibration.The pressure vibration field can improve polymer melt's Theological properties. At constant pressure vibration amplitude, the apparent viscosity decrease greatly in low ranges of vibration frequency (0~0.7Hz), and then tends to plateau. For HDPE, PP andABS, the maximal decreasing percentages are 48.9%, 82.9% and 66.7%, respectively. At a constant vibration frequency, the apparent viscosity decreases greatly with increasing pressure vibration amplitude. When the vibration amplitude is 29.7MPa, the decrease percentages are 99%, 99% and 94.3% for HDPE, PP andABS, respectively. Compared with the effects of melt shear vibration, the pressure vibration can improve the melt rheological behavior greatly.3. Structures and Properties of Dumbbel I Injection Molding Prepared via Low Frequency Vibration Injection MoldingPrepared by pressure vibration injection molding device, the self-reinforced dumbbell injection moldings, for semi-crystalline materials, are generally obtained. The tensile strength and impact strength are increase with the increasing of pressure vibration amplitude with decreased elongation at break; the tensile strength is enhanced with increasing frequency in the low range frequency, but the breaking elongation decreases, thereafter the elongation begins to pick up with increasing vibration frequency, at the same time the tensile strength is also improved. Appeal to the DSC, SEM and WAXD we pioneers study on the morphology of skin, shear layer and core, respectively.As for HDPE injection moldings, the degree of self-reinforcement in tensile strength is greatly dependent on not only the molecular weight but also the molecular weight distribution or the percentage of high molecular weight. The high molecular weight chain first forms the primary nuclei, induced by pressure vibration field, and then other chains begin to grow on the primary nuclei, the SEM photos show that row-nucleated lamella structure or shish-kebab structure exists in the vibration injection moldings. For HDPE 6100M, with lower molecular weight and bimodal molecular weight distribution, the yield strength and Young's modus increase from 39.8MPa, conventional injection molding values, to 60.6MPa, individually, and the corresponding percentages are 52.3%; for HDPE 6098,with the biggest molecularweight, the conventional values in strength is 43.9MPa, and the enhanced value is 62.1MPa, and the corresponding percentage is 41.5%; for HD6070EA, with lowest molecular weight, the biggest yield strength for vibration injection molding is 36.1MPa against 26.5MPa, conventional value, and the corresponding percentage is 36.2%. For HDPE6100M and HDPE 6098 vibration injection moldings, the tensile strength is enhanced, but the breaking elongation decreases, and begins to pick up obtained at high vibration frequency; for HD6070EA the tensile strength is improved with improved plasticity.For PP 1300, a 17.2% increase in yield strength is obtained in vibration injected dumbbells, but we creative find that when the pressure vibration amplitude is beyond 39.6MPa, the impact strength pullups into high stage. Injected at high temperature(230 *C, 250 °C) the self-reinforced and simultaneous self-toughened vibration injected dumbbells are obtain at high vibration frequency, which breaks out the conventional ideas, which is that when the tensile strength increases the material's ductility must decrease, accepted by most polymer specialists, and the increase in ductility is due to the decreased dimensions of spherulites and percentage increase in 3 -form crystalline for polypropylene. SEM photos show that the spherulites deform and orients along the flow direction with decreased dimensions of spherulites. WAXD datas identify the Y -form crystalline induced by vibration injection molding, and the changes of 3 -form crystalline are dependent on the melt temperatures. In the low melt temperature vibration injection molding decreases the percentage of 3 -form crystalline.which is consitent with the results of SCORIM,and when the melt temperature is high the the percentage of 3 -form crystalline increases prepared at high vibration frequency and low pressure vibration amplitude, which is consitent with the facts, which is that the 3 -form crystalline generally occurs in the presence of shearing foreces.accepted by most scientists.With the application of vibration injection molding, a -phase, 3 -phase and Y -phase of isotactic polyprpylene are coexit in the moldings, but in the conventional moldings there are only a -phase, 3 -phase, and the percentages of 3 -phase and v -phase can be adjusted by controlling vibration processing variables.Adjusting the vibration injection molding variables, the percentages of P -form and Y -form crystallines are controlled, thus the controllability of resultant properies are creatively obtained. Based on lots of experience of vibration injection molding we originally plat the a -form, 3 -form and Y -form distribution phase diagram and creatively draw a conclusions of relationship between vibration injection variables and properties, which can realize the designability of properties.The study on the fiber orientation in the vibration injection molding can visually display the effect of vibration injection field on the molecular orientation. The WAXD data realize and justify that the fiber orientation structure is favor in the presence of Y -form crystalline.The Vicat softening point temperature of specimen increase prepared at vibration injection molding, for HDPE 6100M, HDPE6098,HD6070EA, PP 1300 and FRPP, the increase in temperature are 10°C > 11.7°C-. 5°C % 8.6°C and 4.5°C, respectively.For conventional moldings, a spherulitic morphology predominates, and the core region is very big; the pressure vibration injection molding can enlarge the domain of shear layer with thinner core region, and even the core region disappears.The vibration injection molding is beneficial to obtain decreased crystalline particle, perfect crystal and increasing crystallinity, for HDPE 6100M, HDPE6098, HD6070EA and PP 1300, the increased percentages are 9.6%,7.9%,10.2% and 12.1%, respectively.4. Structures and Properties of Square Specimen Prepared via Low Frequency Vibration Injection MoldingWith application of pressure vibration injection molding, the mechanical properties in both longitudinal direction and transverse direction are reinforced, we pioneer a novel method to prepare biaxial self-reinforcement at uniaxial stress field, and such method is never reported in the literatures. In the longitudinal direction, for HDPE, PP and FRPP, the increase percentages in tensile strength are 25.8%, 17.4% and 11.8%, respectively; in the transverse direction the corresponding increase percentages are 14.3%, 9.6% and 14.9%.SEM photos identify the occurrence of interlocking shish-kebab structure which isfavor to realize biaxial self-reinforcement; the spherulites are still dominant in the isotactic polypropylene vibration square specimen, the effect of pressure vibration field results in deformation and orientation of spherulites with decreased dimensions. Such resultant morphology and the proposed morphology development model never appear in the literatures.The states of glass fibers are random distributed in the shear layer, and almost perpendicular to the flow direction in the core region, for FRPP conventional moldings; for the vibration injection molded FRPP, the combined effects of flow field and vibration force field orientates the fibers at an angle with the flow direction, and the glass fibers in the core regions prepared at low vibration frequency and high pressure vibration amplitude, are nearly at an angle of 45° with the flow direction, which results in obtaining biaxial self reinforcement in tensile strength.For PP, vibration force field is favor in occurrence of Y-form and decrease percentage of P -form, and the contents of P -form and Y -form can also be controlled by adjusting the vibration injection molding variables.When the spuare specimens injected via vibration injection molding, for the Vicatsoftening point temperature, the enhanced temperatures are 4.2 *C, 3.5 °C and 2.5 °C, for HDPE6100M, PP 1300 and FRPP, respectively, which means that the vibration injection molding results in stereo self-reinforcement, which pioneers a novel method to obtain isotropic properties.The melt peak shifts to high temperature for HDPE 6100M vibration injection molding, and the crystal are more perfect with enhanced crystallinity, e.g. in the shear layer about a 10% increase in crystallinity is obtain; for PP 1300 the enthalpy and crystallinity of vibration injected dumbbells increase, and a 11% increase in crystallinity is obtained in the core region.5. Structures and Properties of LLDPE/PP Copolymer Injection Molding Prepared via Low Frequency Vibration Injection MoldingAddition of LLDPE to PP results in improved toughness of PP, at the same time the yield strength decreases. Based on the experience to obtain self-reinforced dumbbell via vibration injection molding, the PP/LLDP copolymer was appealed tovibration injection molding to strengthen the copolymer. With the application of vibration injection molding, the Y -form crystalline of PP is obtained. As shown in SEM photos the crystal band, also like shish kebab structure, is widely distributed in the skin-core structure of vibration injection molding, and also oriented in the flow direction. The occurrence of Y -form crystalline and the shish kebab structure and increase in crystallinity are beneficial of enhanced tensile strength, and the crystal particle dimensions are obviously decreased at low vibration frequency and high pressure vibration amplitude, and the resultant property is improved toughness.When the ratio of LLDPE/PP is 35/65, there are a 5.7% increase in tensile strength, and simultaneous a 47.4% increase in impact strength. So the addition of LLDPE improves impact strength, prepared via vibration injection molding the toughness of PP/LLDPE copolymer is enhanced again, with a little increase in the tensile strength. The Vicat softening point temperature increases 5.2 "C for LLDPE/PP (35/65) copolymer is also due to the application of vibration injection molding.From above self-reinforcement of PP/LLDPE copolymer, it is foreseeable that the melt vibration technique can widely strengthen the copolymer and the technique provides a novel new idea to obtain high performance of copolymer.In a word, with the application of vibration injection molding, the rheological property of polymer is improved with self-reinforced injection moldings, and when injected at high temperature the isotactic polypropylene is self-reinforced and self-toughened simultaneously.