ULTRASONICALLY-AIDED EXTRUSION OF BLENDS OF TWO THERMOTROPIC LCPS
© 2010 r. Kaan Gunes and Avraam I. Isayev
Institute of Polymer Engineering, The University of Akron Akron, Ohio 44325-0301, USA e-mail: aisayev@uakron.edu
Abstract—Ultrasonically-aided extrusion of two thermotropic LCPs based on 6-oxy-2-napththoyl and p-oxybenzoyl moieties (LCP1), and p-oxybenzoyl, terephthaloyl and hydroquinone moieties (LCP2) and their blends that are partially miscible was investigated. Ultrasonic treatment induced structural changes in the components, leading to improved mechanical properties of LCP1 moldings, and degradation of LCP2. Also, the ultrasonic treatment was found to further improve the partial miscibility of the blends. Although LCP1 and LCP2 behaved differently under ultrasonic treatment, synergistic effects on fibrillation and mechanical properties of blends were observed in injection moldings. Mechanical properties of melt spun fibers of blends lied between those of the components. Lack of synergism in mechanical properties of fibers was primarily attributed to saturation of LCP2 orientation at low draw down ratios in fiber spinning.
INTRODUCTION
The formation of fibrillated structure in blends of two flexible chain polymers was first described by Vinogradov and co-workers [1]. Later, it was found that liquid crystalline polymers (LCPs) in blends with the flexible or rigid chain polymers may also generate in-situ or self-reinforced composites with superior mechanical properties through the formation of highly oriented fibrillar structures along with reduction of viscosity and pressure during processing [2]. Many commercial LCPs are highly aromatic, and possess high thermal and chemical resistance, and superior mechanical properties in fibers and moldings.
Morphology and mechanical properties of binary blends of LCPs are governed by their chemical structure, interaction, composition, viscosities and processing conditions [2, 3]. Synergism in mechanical properties is also observed in some LCP blends through improved LCP fibrillation [2—5]. Among such systems are blends of 73/27 HBA/HNA copolymer with 60/20/20 HNA/TA/acetoxy-acid aniline [4], and blends of73/27 HBA/HNA copolymer with a terephthalic acid, isophthalic acid, and dioxydiphenyl copolymer [5]. Synergistic improvements in the modulus, tensile strength, and impact strength are observed as a result of enhanced molecular orientation. The current study focuses on the effects of compatibi-lization on the properties of LCP/LCP blends.
Compatibilization of blends of liquid crystalline polyesters has been achieved by addition of copoly-mers, catalysts [3], acids [6] and prolonged annealing [7]. However, these methods of compatibilization are inefficient due to long processing times or the addition
of third components. A fast and efficient alternative to the above-mentioned methods is in situ compatibili-zation, which could be promoted by ultrasonically-aided extrusion [8, 9].
Ultrasonic treatment in the melt state leads to mac-romolecular chain scission and their recombination. The dominating effect of treatment is governed by the chemical structure of polymer, environmental variables, and the ultrasonic amplitude [8]. High power ultrasound was found to promote reactions at short residence times leading to copolymerization at the interfaces in polymer blends [8—11]. Ultrasonic extrusion was also found to induce homopolymerization of PA6 [10] and PET [11]. PET homopolymerization and compatibilization induced with ultrasonic treatment was found to improve mechanical properties of PET/LCP blends through enhanced LCP fibrillation and improved interfacial adhesion [11].
The present paper studies the effects of ultrasonic treatment during extrusion of two LCPs and their blends. It is aimed to induce in situ compatibilization of LCP blends and determine the effect of ultrasonic treatment on their components. The rheological, ther-mo-mechanical, mechanical, and morphological properties of two LCPs and their blends are presented as a function of ultrasonic amplitude.
MATERIALS AND EXPERIMENTAL PROCEDURES
Materials
The LCPs studied were a copolymer of hydroxy-benzoic and hydroxynaphthoic acid (LCP1, Vectra
A950, Ticona) and a copolymer of dioxydiphenyl, terephthalic and isophthalic acid (LCP2, Ultrax KR4002, BASF).
Preparation of LCP1/LCP2 Blends
Both LCP1 and LCP2 were dried at 120°C in a vacuum oven for 24 hours prior to use. Pure LCP1 was prepared in the ultrasonic single screw extruder that was described previously [11]. In the current study, the mixing section after the ultrasonic treatment zone of the extruder was replaced with screw flights to reduce thermo-mechanically induced degradation.
The barrel temperature was set at 260°C in the feeding zone and 300°C in all other zones. It should be noted that extrusion of LCP1 was carried out previously at a temperature of 285°C [11]. In the current study, LCP1 was extruded at 300°C to provide a comparison of processing characteristics with LCP2 and blends. However, all other results reported for LCP1 in this study were from material previously extruded at 285°C. Pressure, temperature and ultrasonic power consumption were recorded by a data acquisition system (Dataq Instruments, DI-715-U, Akron, OH).
The ultrasonic amplitude was varied in the range of 0—10 |im at a frequency of 20 kHz. The temperature of the ultrasonic horns was regulated at 80°C by a temperature controller (Sterltronic S8412-4, Stelco, Ontario, Canada) at a water flow rate of 2500 cm3/min. Streamlined reliefs on the inner surface of the barrel guided the polymer melt to flow only through a channel of 2.54 mm thickness between the ultrasonic horns and the screw. Under flood feeding conditions, a screw speed of 15 rpm was used to obtain a mass flow rate of 1 kg/hr. The exiting melt was run into a water bath at room temperature, dried, and then chopped into particles by a grinder (Weima, WSL180/180, Fort Mill, SC).
Impact bars (127 x 12.7 x 3.2 mm3) and dumbbell shaped mini-tensile bars (63.5 x 9.5 x 1.5 mm3, ASTM D 638-03) were injection molded simultaneously in a Van Dorn 55 HP-2.8F (Strongsville, OH). The barrel temperature was set at 285°C in all zones except the feeding zone, which was set at 260°C. A mold temperature of 27°C, a clamping force of 55 tons, an injection speed of15 cm/s, and a holding pressure of 4 MPa were used. In molding of LCP1, holding and cooling times were 20 and 40 s, respectively [11]. In molding of LCP2 and blends, holding and cooling times were 5 and 25 s, respectively.
Fibers were spun following extrusion from a capillary rheometer (Rosand RH7, Malvern Instruments, Westborough, MA) at a barrel temperature of 300°C and a plunger speed of 10 mm/min. A diameter of the barrel was 15 mm; a capillary die having a length of 24 mm and a diameter of 1.5 mm was used. The distance from the exit of the capillary die to the motor shaft, on which a take up bobbin of 287 mm diameter
was mounted, was set at 36 cm. The take up device (Model 12A5BEPM, B&B Motor&Control Corp, NY) consisting of 1 : 20 geared motor (130V DC, 186 W, 1.8 Amp) had electronically controlled variable speed (0—125 rpm), and produced a torque of 8.5 Nm. Fibers were collected at draw down ratios (DDR) of 45, 76.5, and 112.5, as well as without any take up.
Rheological Measurement
A capillary rheometer (Rosand RH7, Malvern Instruments, Westborough, MA) with a capillary having a length to diameter ratio (L/D) of 24 and a diameter of 1 mm was used for viscosity measurements at 300°C. The apparent viscosity as a function of the apparent shear rate was obtained. Viscosity had a standard deviation of 10%.
Dynamic Mechanical Analysis
Dynamic mechanical analysis (Pyris Diamond DMA, Perkin Elmer-Seiko Instruments, Boston, MA) was performed under nitrogen atmosphere at a heating rate of 2°C/min. A 16 mm long section was cut from each end of the injection molded mini-dumbbell samples (63.50 x 9.53 x 1.52 mm3). The grip ends of specimens having a gauge length of10 mm were sanded before clamping in sample holders. DMA experiments were carried out in tension mode under sinusoidal oscillation at a frequency of 1 Hz and an amplitude of 10 ^m. Other DMA parameters were: a minimum force of 200 mN, a tension gain of 15, a force amplitude of 4000 mN, and a position movement wait time of 8 s. The accuracy of DMA temperature measurements was 1°C.
Mechanical Testing
The stress-strain behavior of the injection molded mini-tensile bars were measured using a tensile tester, an Instron 5567 (Norwood, MA) following ASTM D 638-03. A crosshead speed of 5 mm/min, a 10 kN load cell, and an extensometer with a gauge length of 7.62 mm were used. The reported values are averages of a minimum of 5 samples.
Injection molded impact bars were cut into two equal pieces and labeled as a dead-end (DE) and gateend (GE) to conform to the size specifications of ASTM D 256-05. They were tested without notching using a load of 4536 g by means of an Izod impact tester (Testing Machines Inc., Ronkonkoma, NY). The reported values are averages of a minimum of 10 samples.
Morphological Studies
Morphological studies on injection moldings were performed by SEM (Hitachi S-2150, Japan). A rectangular strip was cut from the gate section of injection
W
cLCP^ %
Fig. 1. Average ultrasonic power consumption Wvs. LCP1 concentration cLcPi in untreated and ultrasonically treated LCP1/LCP2 blends.
molded mini tensile bars, and cryogenically fractured along the transverse direction. Specimens were sputter-coated with silver before imaging. Micrographs were taken at the core region of fractured surfaces.
RESULTS AND DISCUSSION
Process Characteristics
The average ultrasonic power consumption as a function of LCP1 concentration at different ultrasonic amplitudes is shown in Fig. 1. The power consumption is seen to increase with the amplitude. The LCP2 and the blends indicated about similar power cons
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