Processing and properties of a fibre reinforced liquid crystalline polymer

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Materials Processing Technology ELSEVIER

Journal of Materials Processing Technology 67 (1997) 126-130

Processing and properties of a fibre reinforced liquid crystalline polymer Xiao Hu *, Shaw Chyi Tan, Indrajit Ray Chaudhary Division of Materials Engineering and Centre for Advanced Materials Research, School o["Applied Science, Nanyang Technological Unirersity, Singapore 639-798, Singapore

Abstract The unique processing and rheological behaviour of filled LCPs are not fully understood, since they are highly sensitive to thermal and flow histories. In this work, extrusion and rheological studies of a thermotropic liquid crystalline polymer reinforced with short glass fibres were carried out using a capillary rheometer. Attempts have been made to correlate the processing conditions with the structure and properties of the extrudates. Microscopic analysis has shown that the glass fibres were mostly oriented in the direction of extrusion and the presence of voids is evident in the extrudates. Good-quality extrudates were generally obtained, although melt fracture occurred in some extrudate as revealed by scanning electron micrographs. The mechanical properties, in particular the fracture stress, strain and Young's modulus of the materials, were found to be related closely to the rheological histories and the processing conditions. © 1997 Published by Elsevier Science S.A. Keywords: Liquid crystalline polymer; Extrusion; Rheology; Glass fibre

1. Introduction Thermotropic liquid crystalline polymers (LCPs) have emerged as high-performance engineering plastics. Their applications are mainly for the moulding of highend electrical/electronic, automotive, and domestic-appliance products [1]. The properties of LCPs are affected significantly by the melt flow during processing, Their rheological behaviour, on the other hand, is sensitive to their mechanical and thermal histories [2]. Whilst the rheology o f thermotropic LCPs requires further investigation, only limited information can be found in literature on the processing of thermotropic LCPs. The melt extrusion of LCP rods and sheets was discussed and some work was done on injection moulded LCPs [3,4]. These studies focused on the morphology and mechanical properties of the materials. The morphology and properties of melt-processed LCP are found to be dependent on the flow and thermal histories. The high strength and stiffness of LCPs are due to the rigid rod-like molecules which develop into a highly ordered structures and result in self-reinforcing characteristics [1,3,5]. Although thermotropic LCPs ex* Corresponding author.

hibit exceptional mechanical properties when oriented, anisotropy is actually a weakness in moulded parts, resulting in poor properties transverse to the melt flow direction and weak weld lines. These factors, coupled with the high cost of the LCP materials, have limited the market growth for moulded products. Commercial LCP resins are often filled with glass or other fillers to negate the anisotropy and to reduce the cost. It is therefore necessary to study the rheologicel and processing behaviour of such filled LCPs. This paper presents the a study on the melt flow behaviour and extrusion process of a thermotropic liquid crystalline polymer reinforced with short glass fibres. Correlation between the flow behaviour, the processing parameters and the mechanical properties of the material will be discussed in detail.

2. Experimental procedure The material used in this study was an aromatic copolyesteramide filled with 30 wt% short glass fibres. Melt transitions was observed at 280°C using differential scanning calorimetry (DSC) analysis. Glass transition was visible from dynamic mechanical thermal

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X. Hu et al./Journal of Materials Processing Technology 67 (1997) 126-130 ~og v~scos~ty (Ha.s)

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Tensile tests were carried out for extrudates obtained under different processing conditions. The equipment was a universal tester (Instron 4206) with a 500 N load cell. A set of Instron pneumatic action grips (Model 2712-001) were used in the experiments. The tests were conducted at 25°C using a cross-head speed of 5 mm rain- I. The gauge length of the samples was fixed at 25

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Fig. 1. Flow curves of the glass fibre filled liquid crystalline polymer. analysis. The flow behaviour of the glass filled LCP was analyzed using an instrumented Shimatzu CFT-500C capillary rheometer operating in a constant temperature mode. The capillary extrusion was carried out at different temperatures and shear rates. The material was dried in a convection oven for 12 h at 85°C prior to extrusion and rheological measurement. The dynamic mechanical properties, storage modulus E' and energy dissipation factor tan ~, of the extrudates were measured as a function of temperature. The equipment used was a Perkin Elmer 7 series thermal analyser operating in a three-point-bending mode. Tests were conducted at a frequency of 1.0 Hz. A Nikon HFX-DX optical microscope was used to examine the structures of the extrudates. Polished oblique- and cross-sections of the extrudates were analyzed and the distribution of the glass fibres and the existence of voids were observed. Surface morphology and fracture surfaces of the extrudates after tensile test were studied using a Cambridge SEM-360 scanning electron microscope.

3.1. Rheological behaviour

Melt flow tests of the fibre filled LCP were conducted at four different temperatures (285, 290, 300 and 310°C). Measurements at below 280 or above 320°C were unsuccessful, since no flow curve can be constructed due respectively to incomplete melting of the LCP in the former and limitations of the instrument in the latter case. The melt-flow properties of the melt are presented in Fig. I. where the apparent viscosity was plotted against the apparent shear rate to a logarithmic scale. The shear thinning or pseudo-plastic behaviour of the melt is clearly visible. The extent of shear thinning is usually described by the slope of the flow curve. Over a range of shear rates, the relationship between the apparent viscosity and the apparent shear rate can be expressed using an empirical power law [6]. It is also noted from Fig. 1 that the material exhibits dilatant behaviour at high shear rates for the melt flow tested at 300 and 310°C. The viscosity was found increasing sharply with shear rates at the range above 104 s-~, this phenomenon being attributed to the separation of the polymer and the glass fibres [7]. The details of the rheological properties ~f the material are discussed elsewhere [8]. In this presentation, the discussion will be

Fig. 2. SEM surface image of the LCP extrudates ( x 100).

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;ei ~L ~Journal of Mate~ials ProCessing Technology 67 (1997) 126,130

Fig. 3, SEM micrograph of a melt fractured extrudate ( x 100).

focused on the relationships between the processing parameters and the mechanical strength of the extrudates. The effect of the flow history on the mechanical properties will also be examined closely.

3.2. Dynamic mecha~:dcalproperties The storage modulus, E', and dissipation factor, tan ti, of the glass filled LCP as functions of temperature were obtained from the samples extruded at 310"C, but are typical for all samples extruded at other temperatures, i.e., 285, 290 and 300°C. From the E'-temperature curve, a deflection in modulus was noted at around 65 to 100*C, which can be attributed to glass transition. There was little change in modulus until the temperature rose to 220"C, above which the modulus dropped

ever, the melting process cannot be followed tully in this case since the tests are valid only at temperatures of up to 270"C. Stress (MPa)

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3.3. Microstructures Optical micrographs taken from the polished surfaces of the glass filled LCP extrudates show that the fibres are largely oriented in the direction parallel to the direction of flow. Fig. 2 shows a typical SEM micrograph of the surface morphology of the extrudates. Melt fracture was observed for a sample extruded at high shear rate, as illustrated in Fig. 3, which reveals the poor cohesion between the glass fibres and the LCP matrix.

3.4. Mechanical properties and fractography Fig. 4 shows a typical stress-strain curve of the glass fibre filled LCP. The material deformed elastically and no yield point was observed. This type of stress-strain behaviour is observed commonly in brittle polymers. However, the SEM fractograph given in Fig. 5(a) shows a rough fracture surface with significant fibre pull-out. A micrograph at higher magnification, Fig. 5(b), reveals the failure at the glass fibre/LCP resin interface. Poor interfacial bonding and the presence of voids are probably responsible for the brittle fracture behaviour of the material. The effects of shear rate and extrusion temperature on the mechanical properties of the material were studied in detail. The tensile strength was plotted against logarithmic shear rate as shown Fig. 6, for extrudates processed at 285, 290, 300 and 310°C. The dependence of the tensile strength on the processing conditions, i.e., temperature and shear rate, is demonstrated dearly. For samples processed at 285 and 290°C, the tensile strength increases somewhat monotonously with shear rate over the entire ranges investigated. The increase in tensile strength with shear rate may be due to the enhanced orientation of both the LCP molecules and

X. Hu et al. / Journal o1 Materials Process#lg Technology 67 (1997) 126-130

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Fig. 5. Fractographs of a glass fibre filled extrudate showing: (a) a generally rough fracture surface with glass fibre pull-out ( x 100): and (b) failure at the glass fibre/LCP resin interface.

underwent a transition from pseudo-plastic to dilatant flow. Detailed investigation reveals that the shear rates at which the maximum strengths were observed are around 104 s-~. This shear rate is, in fact, coincident with the point at which the transition of the rheological behaviour occurs. Hence, it seems that the mechanical properties of the glass fibre filled LCP are related closely to its rheological flow history. Dilatant flow due to resin fibre separation is responsible for the drop in the mechanical strength of the extrudates. Similar shear-rate and extrusion-temperature dependence was observed for the tensile fracture strain. There is a steady increase in Young's modulus with extrusion shear rate, but the effect of the extrusion temperature on the modulus is less significant.

the glass fibres along the direction of extrusion at higher shear rates [3]. The results for the samples processed at 300 and 310°C are more complex. In these cases, the tensile strength increases initially with shear rate, similarly to that for the samples extruded at 285 and 290°C, in the low shear-rate range. In the higher shear-rate range, however, the strength decreases with increasing shear rate after passing through a maximum. The effects of shear rate on the fibre and molecular orientation may again be responsible for the initial strength increase. However, the decrease in strength at higher shear rates is probably due to the inhomogeneity caused by the separation of the fibres and the LCP resin [7]. As discussed earlier, the rheological behaviour of the glass fibre filled LCP

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Fig. 6. Ultimate tensile strength of the glass fibre filled LCP extruded at different shear rates and temperatures: (A) 285: IB) 290: (C) 300; and (D) 310°C.

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X. Hu et al,/Journal of Materials Processing Technology 67 (1997) 126-130

4. Concl~ions The glass fibre reinforced liquid crystalline polymer was characterised using both thermal and microscopic analysis. The capillary extrusion and rheological measurement of the material were carried out for various conditions, i.e,, various tem~ratures and extrusion shear rates. Both pseudo-plast[c~, and dilatant flow characteristics were o[aserved for different conditions. The effects of the processing parameters on the mechanical properties of the extrudates were examined in detail. Close correlation has been found between the rheological history and the mechanical strength. Dilatant flow behaviour was found to be responsible for the deterioration of the mechanical properties due to the inhomogeneity caused by resin fibre separation.

Acknowledgements The authors would like to thank Aircraft Marine Product Inc. for the supply of materials

and are grateful to Dr. M.H. Liang for helpful discussion.

References [1] C.K. Ober and R.A. Weiss, in R.A. Weiss and C.K. Ober (eds.), Liquid Crystalline Polymers, ACS, Washington DC, 1990. [2] S. Onogi and T. Asada, in G. Astarita, G. Marucci and L. Nicolais (eds.), Rheology, Vol. 1, Plenum Press, New York, 1980, p. 127. [3] L.C. Saywer and M. Jaffe, J. Mater. Sci., 21 (1986) 1897. [4] H. Thapar and M. Bevis, J. Mater. Sci. Lett., 2 (1983) 733. [5] B. Bassett and A.F. Yee, Polym. Compos., ll (1) (1990) 10. [6] P.C. Powell, Engineering with Polymers, Chapman and Hall, London and New York, 1983, p. 221. [7] R.M. German, Powder Injection Moulding, MPIF, Princeton and New Jersey, 1990, p. 147. [8] X. Hu, M.H. Liang, S.C. Tan and I.R. Chaudhary, in H.S.O. Chan et al. (eds.), Proc. 3rd NUS Symp. Materials Science and Engineering, Singapore, Sept. 1994, p. 63.