Effect of Lcp Addition on the Properties of Hybrid Composites

A hybrid composite consisting of rubber toughened nylon 6,6, glass fiber and LCP was investigated by varying LCP content. The hybrid system exhibited better processability than the glass fiber reinforced composite. A decrease in the total torque was observed with the LCP content indicating the reduction in the energy consumed during the processing of the hybrid composites. Thermal stability of the glass fiber reinforced composites improved with LCP addition. SEM observation of the tensile fracture urface revealed LCP fibrillation in the toughened matrix. s Introduction Reinforcing toughened polymer matrices with short glass fibers is an attractive route to minimize the reduction in strength and stiffness arisng from the compliant elastomeric phase [1,2]. Elastomers toughen the polymer matrix by promoting matrix plasticity through rubber cavitation, which first relieve the triaxial tension at the crack tip [3]. The advantage of fiber reinforcement is two-fold: (1) enhance the mechanical properties such as strength, stiffness and heat deflection temperature and (2) promote fiber-induced toughening in the matrix [4]. Presence of inorganic fillers raises the melt viscosity of the polymer, resulting in higher energy consumption and lower processability. Fiber reinforcement often catalyzes the tear and wear of the processing equipment. The shear forces during the compounding process lead to fiber breakage affecting the reinforcement effect of glass fibers. Blending of a reinforced matrix with LCP offers the potential to reduce the viscosity thereby improving the processability. The mechanical performance of such a hybrid composite is also comparable to that of glass fiber reinforced polymers [5]. He and coworkers [6,7] reported synergistic properties for LCP/glass fiber/thermoplastic hybrid composites. However, little is understood on materials containing an engineered combination of short glass fibers, LCP and elastomeric phase. Through a series of studies we seek to investigate the desirable properties of such a complex hybrid system. This paper addresses the effect of addition of LCP on the processability, thermal stability and mechanical properties of glass fiber reinforced toughened thermoplastics. * Corresponding author Experimental work Rubber-toughened nylon 6,6 (Zytel ST801 from Du Pont) was dry blended with 20 wt% of short E-glass fibers (length=12mm; diameter=17μm) and independently with 5, 10, 15 and 20 wt% of LCP (Vectra A950 from Hoechst-Celanese). The LCP comprised 27-mol% of 2hydroxy-6-naphthoic acid (HNA) and 73-mol% of phydroxy benzoic acid (HBA). Melt blending was carried out using a high shear rate, inter-meshing, co-rotating twin-screw extruder (Leistritz Micro 18; with a screw diameter of 18 mm and L/D ratio = 30). The tempersture profile in the extruder was 260-280-285-285-292C. Screw speed was kept at 200 rpm. The extruded pellets were injection molded into 3.5 mm thick dog-bone specimens (ASTM D638 type I) using a Battenfeld BA 300 CD injection-molding machine. The temperatures at zone 1 and zone 2 were kept at 285°C and 292°C respectively. The nozzle temperature was kept at 275°C and the mold temperature at 30°C. An injection pressure of 70 bar and holding time of 50 s were used. The screw speed was kept at 140 rpm. Rubber toughened nylon 6,6 containing 20 and 30 wt% glass fibers were processed and compared to the properties pertaining to the hybrid composites. All the materials were dried at 80°C for at least 72 h in a vacuum oven before processing. Torque measurements were carried out in a Haake Rheocord-90 intermeshing, counter-rotating, twinscrew extruder. 100 g each of 20 wt% reinforced rubbertoughened nylon 6,6 and it’s independent blends with 5, 10, 15, 20 wt% LCP were extruded at a temperature of 290C while maintaining a screw speed of 100 rpm. Accumulated torque for a time of 2 min and the instantaneous torque were measured. Thermal stability of the blends was assessed by thermogravimetric analysis (TGA) using a TA TGA 2950 equipped with TA thermal analysis software. TGA measurements were done at a scan rate of 20°C/min purged with a stream of nitrogen. Dynamic mechanical properties were assessed by dynamic mechanical thermal analysis (DMTA) using a TA DMA 2980 equipped with TA thermal analysis software. The measurements were done on the injection molded specimens at a scan rate of 3C/min with a frequency of 1Hz. A double cantilever clamp was used in a bending mode Scanning Electron Microscopy (SEM) observations of the tensile fracture surfaces were performed on a JEOL 5410 LV model SEM. The fracture surface was coated with gold in an SPI sputter coater. The tensile fracture surface was also studied using SEM. Morphological observation of the extrudates was conducted after cryogenic fracture in liquid nitrogen. Tensile tests were conducted according to ASTM D638M using an Instron Model 5565 computercontrolled testing machine and the tensile strength, tensile modulus and tensile strain were simultaneously recorded. The crosshead speed was 5 mm/min at room temperature. Five specimens of each blend composition were tested. Results and Discussion