Thermotropic LCPs processing

Four companies (trade name in parentheses), Amoco (Xydar), Hoechst-Celanese (Vectra), Du Pont, and Granmont (Granlar), make thermotropic LCPs for various types of extmsion and mol ding processes. Six companies have discontinued TCP materials that were either commercial or under development. These companies include ICl, BASF, Eastman, Bayer, General Electric, and Monsanto. 

Other properties of thermoplastics as well. Owing to the relatively low-melt viscosity of thermotropic LCPs, often a small amount of LCP decreases the blend viscosity significantly and renders the matrix thermoplastics easier to process [9-211. 

Since thermotropic LCPs have recently been used in applications requiring high modulus and strength, much effort has been expended to improve the processability... 

Working with a solution is needed for polymers which above their melting point would degrade (example aromatic polyamide fibres such as Kevlar and Twaron). For fibres the removal of the solvent is not too problematic. In e.g. injection moulding applications solvents caimot be used here thermotropic LCP s have to be used. Since these would degrade during processing, they are diluted by copolymerisation (example poly-hydroxy-benzoic acid - co - PETP)... 

Processing. Relatively little has been published on the processing of thermotropic LCPs. The morphology of melt-processed articles is dependent on the deformation and thermal histories. Extensional flows produce fibrillar structures with high orientation in the machine direction. Flows with complicated stress distributions and temperature gradients, such as encountered in injection molding, yield complicated morphologies. 

The orientation and morphology of thermotropic LCP parts are strongly dependent on the process conditions near the melt temperature because of the inherent molecular anisotropy and very long relaxation times. Random coil thermoplastic polymers are much less dependent on processing history near the melt temperature, and orientation requires a secondary step such... 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

Thermotropic LCPs can be processed and molded to sfiuctural parts of different shapes by means of conventional processing techniques for thermoplastics, such as extrusion and injection molding. Due to the rigidity of the molecular chains of thermotropic LCPs, injection-molded parts of pure thermotropic LCPs usually show strong anisotropy, evidenced by higher strength and modulus along the flow direction and weak lateral adhesion transverse the flow direction. To overcome this deficiency, for injection-molded parts, thermotropic LCPs are blended with other thermoplastics to reduce the anisotropy of the performances of the final products and to widen their application areas. 

Compared to most isotropic polymers, thermotropic LCPs possess outstanding mechanical properties due to their rigid rod-like backbones. It has been reported that LCPs have tensile strength in a range of 150 to 400 MPa and tensile modulus from 13 to 25 GPa, while LCP fibers have tensile strength ranging from 500 to 650 MPa and tensile modulus from 50 to 100 GPa. When LCPs are blended with thermoplastics in a process where an elongational flow field exists, the LCP phase will deform into molecularly oriented fibers that reinforce the thermoplastic matrix. So, until now, many isotropic polymers have been blended with LCPs. Table 7.5 summarizes various in situ composites reported, with their machine direction mechanical properties. 

It should be noted that there is not simply an addition of thermotropic LCPs into fiber-reinforced plastics to get in situ hybrid composites. Bafna et al. used glass fibers to decrease the anisotropy of LCP fibril-reinforced polyetherimide [136]. He et al. improved the processability and mechanical performances of glass fiber-reinforced polypropylene by the addition of LCPs [159]. However, these two works did not actively and purposely generate a reinforced composite with the reinforcements having their diameters at two orders of magnitude. The key point for in situ hybrid composites is the formation of LCP fibrils in the material system. As a combination of in situ composite and hybrid fiber reinforcing, the fabrication of in situ hybrid composites utilizes fabrication... 

A disadvantage of Kevlar is that the polymer Is a "lyotropic" liquid crystal, that Is, one that becomes liquid crystalline when dissolved In an appropriate solvent. The need to remove solvent during the fabrication process limits lyotropic LCP s to formation of thin fibers or films. In order to Form thick articles or parts with more complicated shapes requires a "thermotropic" LCP, that Is, one that becomes liquid crystalline upon heating.[4]... 

For a review of the development of thermotropic LCP s and some of their processing and characterization, see 6. Calundann and M. Jaffe, Welch Foundation Lecture, Houston, 1982. 

The second section is devoted to self-reinforced composites prepared through the melt processing of thermoplastics and thermotropic LCPs. The individual chapters address various aspects of rheology, processing compatibilization, and performance characteristics of in situ composites. Attention is directed to preparation of fibers and molded products, with the special objective of optimal performance properties through processing. 

---