Extruded LCPs

It appears that the application of extruded LCPs as wear resistant... 

Large-diameter melt-extruded LCP rods can be used to replace steel wire and as strength members in optical cable applications. This is because LCP rods have abalance of critical properties for these applications, including low density, flexibility, high tensile properties (which protect optical fibers from breakage), very low coefficient of thermal expansion, good chemical resistance, and almost zero water regain [6]. 

This chapter will briefly describe LCPs, why they are useful, and how processing methods can make effective use of their properties. Next, new processing methods will be discussed that unlock the mechanical, electrical, barrier, and chemical resistance properties of extruded LCP. Later, we will look at applications of tubing, film, and molded containers, along with new work in processing and using LCP-thermoplastic blends. 

This section will describe the process and machinery for making multiaxially oriented film from LCP. As summarized in section 2, the extruded LCP melt can be oriented through axial and transverse shear forces. Now, we will consider the use of stretching, or elongational forces, to improve and control the orientation. 

We saw previously in Figures 11.2 and 11.3 how a uniaxial film is produced by extruding LCP through a fixed slot die. Unlike random coil polymers, if the uniaxial LCP film is heated above Tg and below Tm, then transverse stress is applied, and the LCP film will tear, as in Figure 11.2(a). The reason for this is that the rigid rod molecular segments remain aligned, even near the melt temperature, and, therefore, the LCP film remains weak in the transverse direction... 

Fig. 10 Optical micrograph of the skin region of twin-screw extruded LCP/PP blend processed at a melt temperature of 290°C with (a) low and (b) high draw ratios.

Since the processing conditions and mixing equipment have a crucial effect on the morphology of immiscible polymer blends [45], experiments were carried out in four different types of extruders to find optimal conditions for blend preparation and fibrillation. Nevertheless, the morphologies of PP-LCP blends produced by... 

Figure 1 Optical micrographs in the flow direction of the extruded strands of the PP-LCP blends exhibiting viscosity ratios of (a) t7lcp i7pp = 0,6, and (b) 2.8 [44].

Prior to blending, the LCP was dried at 155°C for 5 h. The melt blending of the materials was carried out with a Berstorff ZE 25 x 33D corotating twin-screw extruder at a melt temperature of 290°C, with a screw speed of 200 rpm, and an output of 6.4 kg/h. The extrudate was immediately quenched in a water bath and repelletized. 

In preliminary tests, melt mixed blends of PP and LCP were processed at six different temperatures (Tcyi 230, 240, 250, 260, 270, and 280°C) with a Brabender Plasti-Corder PLE 651 laboratory single-screw extruder. The measured melt temperatures were about 10°C higher than the cylinder temperatures (Tcyi). The objective was to study the influence of temperature on the size and shape of the dispersed LCP phase. Two different polypropylenes were used to ascertain the effect of the viscosity of the matrix on the final morphology. Different draw ratios were obtained by varying the speed of the take-up machine. 

Figure 2 Optical micrographs of melt mixed PP-LCP blends single-screw extruded at melt temperatures of (a) 250°C, and (b) 260°C.

Figure 3 Twin-screw extruded PP-LCP blend processed at a melt temperature of 290°C with low- (left) and high-draw ratio (right). Upper micrographs are taken from the core and lower ones from the skin region.

Blends of polypropylene (PP) and liquid crystalline polymer (LCP) processed without melting the LCP were compared with conventional melt processed blends. In a first stage, PP was blended with 20 wt% of LCP in a twin-screw extruder with the take-up speed varied to achieve blends with different LCP fiber dimensions. In the second stage, these blends were processed both below and above the Tm of the LCP by extrusion and injection molding. 

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

Liquid crystalline polymers exhibit anisotropy in extruded and molded articles as a result of preferential orientation of LCP domains or individual chains. Reference (21 highlights some of the molecular structural features of LCP s that account for their fundamental anisotropy. These include the large aspect ratio of the individual polymer chains and their tendency to form aligned, highly crystalline domains. 

To date, reports of investigations on the gas transport properties of main chain liquid crystalline polymers appear to have been limited to the work conducted in our laboratory. Chiou and Paul (4.) have briefly described the transport parameters of an extruded film of an LCP having a similar structure to the commercial product Vectra. This copolyester belongs to the family of napthylene thermotropic polymers (NTP s) commercialized by Hoechst-Celanese Corp. whose synthesis and properties have been described previously (iLS.). Transient permeation experiments were conducted with a series of gases. The effective... 

The shear-induced interlayer slip was theoretically predicted — it creates a tree-ring structure in the extrudates [Utracki et al., 1986 Utracki, 1991b Bousmina et al., 1999]. The relation may be used to describe the steady-state viscosity of antagonistically immiscible polymer blends, such as PP/LCP [Ye et al., 1991 Utracki, 1991b]. 

PP was blended with ECP 2 extruders with a static mixer, to stretch LCP into microfibrils Sukhadiaela/., 1991, 1992... 

Kevlar is a lyotropic liquid crystal which can be obtained from a sulfuric acid solution when the concentration reaches a critical value, e.g., 10%. However, this polymer like other LCPs is also anisotropic and its mechanical properties is directional, but less so in fibers than in the extruded plaques. The fiber properties in Table I are compared with other organic fibers and steel ( 15). On an equal weight basis, Kevlar has a strength several times that of steel. Perhaps, Kevlar is the first polymer at least comparable to metals. 

In fiber optic applications, LCPs can be extruded into a variety of shapes and sizes using conventional extrusion equipment found in typical fiber optic wire and cable production plants. 

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