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INSIGHT The effects of ultrasonic irradiation on extrusion processing and crystallization behavior of polypropylene (PP) are examined. The results show that appropriate irradiation intensity can prominently decrease die pressure and apparent viscosity, increase output, reduce extrudate swell, and improve apparent quality of PP. DSC and WAX analysis demonstrate that the crystalline structure of PP is changed and the content of the beta crystalline increases through ultrasonic vibration. Special effects can be induced by ultrasound, such as strong stress, shatter and vibration, which alter the growth of PP crystals and may favor a special crystal transformation (alpha -> beta). INTRODUCTION Recently, some new processing techniques have been emerging, such as the direct application of physical (mechanical) fields on polymer melts. The route can be realized by introducing mechanical or ultrasonic vibrations. Compared with low-intensity vibration, ultrasonic waves are widely employed to influence a medium through shatter, cavitation, chemical and mechanical action, with low noise. Lemelson (1) described an apparatus and method for controlling the internal structure of plastics in a mold. By applying ultrasonic energy to the solidifying material, the apparatus can control well the crystalline structure formed during solidification. Pendleton (2) also described an ultrasonic process to modify the crystalline structure of blown polyolefin films. Khamad (3) reported that imposition of ultrasound on HDPE melts containing a small amount of butyl rubber resulted in increase of cystallinity, reduction of structural defects, and enhancement of mechanical properties. Isayev and Levin (4-7) introduced ultrasonic vibrations to an extruder by which they accomplished reclaiming rubber from old tires and found improvements of some mechanical properties of the extrudate, such as strain at break and Young's modulus. However, there is little study on crystalline structure of PP for the case when ultrasound is employed at the die entrance. This article studies PP extrusion with parallel superposition of ultrasonic waves upon the die. The effects of ultrasound vibrations during extrusion on improving processability and crystallization of PP melts are described. EXPERIMENTAL Materials and Experimental Setup PP, 2401, with a melt index of 2.5 g/10 min and a density of 0.905 g/cm^sup 3^ at 25 deg C, was commercially obtained from Yanshan Petrochemical Co., Beijing, China. The Ultrasound-Extrusion experimental setup consists of a single- screw extruder (d = 20 mm, L/D = 25), with a variable cross-section capillary having high-temperature pressure transducer and ultrasonic generator (Fig. 1). The ultrasonic frequency is 20 kHz and power ranges from 0 to 300W. The direction of ultrasonic vibration coincides with that of the melt flow. Samples used for all tests were directly from extruded strands, which are short and very near to the die. Different samples were extruded one by one at the same rotation speed, die temperature (200 deg C) and different ultrasonic intensity. They were then all cooled at the same room temperature. MFI The melt flow index of samples was measured on an American CS- 127 Melt Flow Index measuring device at 230 deg C with a load of 2160 g. Differential Scanning Calorimetry RESULTS AND DISCUSSION Die Temperature Figure 2 shows a prominently exponential ascendance in die temperature by introducing ultrasonic vibration during extrusion of PP. In presence of 250 W ultrasonic intensity, die temperature at 5 rpm jumped 10 deg C higher than without ultrasonic vibration. This is because part of the sound energy is converted into heat energy by the internal friction between the melt molecules. For a given shear rate, the temperature rise is a function of ultrasonic intensity. The higher the intensity, the more violent the molecular motion, and the greater the temperature rise. Die Pressure and Throughput The dependence of die pressure on ultrasonic intensity at different screw speed was examined. Figure 3 shows that higher ultrasonic intensity and lower rotation speed both contribute to a larger reduction of pressure, indicating that the pressure drop depends on the intensity and duration of ultrasonic treatment. An increase of both factors gives the melt the chance to obtain stronger energy from ultrasound. In addition, ultrasonic vibration during extrusion affects the throughput of extrudate; the flow rate increases with an increase in the ultrasonic wave intensity, as shown in Fig. 4. All of the above-mentioned characteristics demonstrate that ultrasonic treatment will significantly improve the extrusion efficiency. Apparent Viscosity The prominent pressure drops after the introduction of ultrasound into melts are surely related to molecular chain structure, which can be reflected in viscosity data. Figure 5 shows the dependence of apparent viscosity on ultrasonic intensity at various rotation speeds. Ultrasonic motions result in a sharp decrease of viscosity of the melt. Since higher screw speed means shorter residence time, the decrease of apparent viscosity of PP is also a function of ultrasonic intensity and time, just like the pressure reduction. Further, Fig. 6 demonstrate that MFR increases after application of ultrasound under the same extrusion conditions. But with low- intensity ultrasound, MFR changes very little-from 4.2 g/10 min (OW) to 4.7 g/ 10 min (100 W). It may be concluded that appropriate ultrasonic irradiation can accelerate molecular motion and make long entangled macromolecular chains unravel with a low degree of degradation. Die Swell The swelling of the extrudate in capillary extrusion is a manifestation of the elastic properties and the fading memory of the melt, and is a result of unrelaxed deformations generated during the extrusion process. Figure 7 shows that die swell ratios D/D^sub 0^ decrease with increasing ultrasonic irradiation power. Since swelling is a relaxation process, energy from ultrasonic irradiation can affect the relaxation of polymer melts by shortening the relaxation time of chain segments, thus decreasing the elasticity of the melt at the die entrance. It is also observed that ultrasonic waves contribute to a smoother appearance of extrudates. Crystalline Structure Ultrasound can greatly improve the processing of PP, and this effect must be a result of structural changes of the PP chain. At the same time, PP is well known for its different crystalline forms: alpha, beta and gamma forms. So the ultrasonic irradiation probably induces the change of the crystal structure. To verify this hypothesis, samples were analyzed by DSC and WAXD. Table 1 summarizes the values obtained from DSC thermograms. T^sub c^ is the crystallization temperature, T^sub onset^ is the temperature at the cross point of the megathermic tangent line and the base line at angle of alpha. Si, the tangent of alpha, is a measure of the rate of nucleation. (T^sub onset^-T^sub c^) is a measure of overall rate of crystallization of the system, which decreases with the increase of the crystallization rate (11). It has been found that with the use of ultrasonic vibration, (T^sub onset^-- T^sub c^), Si and crystallinity are lower than their respective values for a sample without ultrasonic treatment. This indicates that ultrasound prevents nucleation as well as accelerating the growth of PP crystal, and the decreased rate of nucleation may lead to lower crystallinity. Furthermore, it is also noted from Fig. 8 that in the case of ultrasound, the peak occurring at lower temperature, which corresponds to fusion of beta-form (temperature of the peak is about 145 deg C), becomes stronger and the enthalpy is higher. The other hightemperature peak due to fusion of the alpha-form is correspondingly weaker. These investigations may imply that processing PP with ultrasound causes a preference for the growth of the beta-form relative to that of the alpha-form. Table 1. It can be seen from the X-ray diffraction patterns that PP samples with and without ultrasonic treatment are different (Fig. 9 and Table 2). By introducing ultrasonic effects, peaks at theta = 16 deg, 21.3 deg increase and become markedly wider (Mg. 9). In addition, Table 2 shows the values of the increase of K. According to Turner-Jones (9), these indicate an increase of the beta-forms of polypropylene. Moreover, the data listed in Table 2 show that nearly all the d spacing of crystal planes of PP decrease and values of S increase with ultrasonic treatment. The maximum contribution (K = 29.5%) and maximum order (S = 47.9%) of the beta phase are in the samples affected by ultrasonic power of 180 W. All the above imply that ultrasonic vibrations can shorten the distances between molecular chains and decrease the crystal defects and disordered structure, so that the lattices of PP have an increased order. The enhancement of the beta-form combined with the decrease of crystallinity implies that the polymorphic transitions alpha -> beta of PP may exist in the system. CONCLUSION The application of ultrasound to extrusion can improve the processing of PP by raising the die temperature and decreasing pressure and viscosity, thus enhancing the throughput and appearance of extrudate\s. The results show that this ultrasonic process can modify the structure of crystallizable thermoplastic materials like PP during extrusion. Ultrasonic irradiation results in a reduction of the crystallinity of extruded PP. Ultrasound also shows a selective effect on the growth of different crystal forms of PP, which causes an increase of the relative amount and order of the beta-form as well as transitions of the alpha-form to the beta-form. ACKNOWLEDGMENT This paper is based on results from the subject supported by the Special Funds for Major State Basic Research Projects (G1999064809). Table 2. REFERENCE 1. J. Lemelson, U.S. Patent 4,288,398 (1981). 2. J. W. Pendleton, U.S. Patent 3,298,065 (1965). 3. S. I. Khamad, E. N. Popova, and Z. I. Salina, Deposited Doc. (RUSS), VINITI, 1829 (1984). 4. A. 1. Isayev, C. M. along, and X. Zeng, Adv. Polym TechnoL, 31, 10 (1990). 5. A. I. Isayev and J. Chen, Rubber Chemistry and Technology, 68, 267 (1995). 6. A. I. Isayev, S. P. Yushanov, and J. Chen, J. Appl. Polym. ScL, 59, 803 (1996). 7. V. Yu. Levin and S. H. Kim, Rubber Chemistry and Techno/ogy, 70, 120 (1997). 8. M. A. Gomez, H. Tanaka, and A. E. Tonelli, Polymer, 28, 2227 (1987). 9. A. Tuner-Jones, J. M. Aizlewood, and D. R. Beckett, MakromoL Chem., 75, 136 (1964). 10. G. Zhou, Z. He, J. Yu, Z. Han, and G. Shi, MakromoL Chem., 187, 633 (1986). 11. A. K. Gupta and V. B. Gupta, J. AppL Polym. ScL, 27, 4669 (1982). YURONG CAO and HUILIN LI* The State Key Laboratory of Polymer Materials Engineering Polymer Research Institute of Sichuan University Chengdu 610065, People's Republic of China Correspondence to: HuiLin Li
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Source: Polymer Engineering and Science - Copyright Society of Plastics Engineers Jul 2002
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