LC Polyester Composite

The macroscopic inorganic fibers worsen the processability of the composites, increasing abrasion on the processing equipment surface. A solution to this problem may be the addition of LC Polymer to ease the processability due to the very low melt viscosity of LC Polymer. Moreover, they also provide reinforcement effect at a microscale. These types of composite, in which thermoplastics are reinforced by both organic and inorganic fibers, are called in-situ hybrid composites. The fillers, whiskers and fibers are used as inorganic reinforcements for this type of composite. 

The LC polyester-amide/carbon fiber composite preparation can be done by several step strategy such as  

Stage 1 The impregnation can be carried out in a crosshead tape die. Molten LC polyester passes out of a twin screw extruder to the tape die channels. The extruder is fitted with three temperature 

Minimum void content in composites is very much required for maximizing mechanical properties. During application of vacuum to the layup prior to compression molding, the air trapped between various plies can be removed to achieve minimum air entrapment. The ply assembly is wrapped with a high-temperature-resistant Kapton film and vacuum applied. The void content in the composite panel can be reduced to 0.8% by volume [66]. 

PP-matrix in capillary flow can be observed at LCP concentrations 20 wt% and temperatures 488 K [67]. 

---

The LC polyester composite and nanocomposites are also reported to make a reinforced system which has also facilitating effect on processing. This is due to the low viscosity of LC Polymer which consists of linear semi-rigid rod-like molecules that can improve the processability of composites and shaped into fibrous structures and get easily oriented in the flow direction [32]. 

LC polyester composition, containing terphenyl additive to the extent of 0.3-15 parts in 100 parts LC polyester, find application as electric and electronic components with complicated shapes as the material is free from blister formation during welding. Without terphenyl the blister formation can not be prevented. It has high flow-ability, low anisotropy and also the gas evolution is suppressed. 

To the LC polyester composition additives can also be used in the form of antioxidants and heat stabilizers such as a hindered phenol, hydroquinones, phosphites etc. Another additives, also added as required, can be mentioned as, ultraviolet absorber (resorcinol, salicylate etc.), color protecting agent (phosphate, hypo-phosphite etc.), lubricant and mold lubricant (stearyl alcohol, stea-ramide etc.) [137]. 

In a typical procedure for reclamation, the LC polyester composite is first granulated into small pieces suitable for extrusion. The granulated composite is, then, mixed with dicumyl peroxide and fed into the extruder. The melt is extruded into a heated mineral oil bath. This solution is stirred vigorously during the entire batch process. The other component (PP) of the composite dissolves in hot oil and the chopped pieces of LC polyester recovered by centrifuging. The remaining oily pieces are then boiled in kerosene to remove the mineral oil. After decanting kerosene/mineral oil solution the LC polyester is washed with hexane to dissolve the kerosene and dried in convection oven. Almost 97% pure LC polyester is reclaimed [150]. 

Recent developments in the substitution of completely aromatic LC polyesters have produced polymers which show improved solubilities and reduced transition temperatures (29). The presence of these side groups provides a method for producing polymers that are compatible with other similarly modified polymers. In this way, blends of rigid and flexible polymers can be prepared. Substituents have included alkyl, alkoxy (30) and phenyl alkyl groups (21), some of which lead to mesophases that have been reported as being "sanidic" or board-like. This approach has been used with both polyesters and polyamides and has lead to lyotropic and thermotropic polymers depending on the particular composition used. Some compositions even show die ability to form both lyotropic and thermotropic mesophases (22). 

A polymeric substituent on LC polyester should be appropriate to exhibit similar influence as in the case of lateral substituents. Moreover, a polymeric substituent can act as a chemically bound solvent [50]. The compatibility of the polyester rods and a random coil matrix polymer should be significantly increased if a substituent exhibiting the same chemical structure as the matrix polymer is used. This may also lead to the reinforcement of the mechanical properties of the matrix polymer which is akin to fiber reinforced composites. 

The chemical resistance of polyester amide glass fibre composite is excellent [126]. A solvent mixture of CF3COOH/CHCI3 was used as a solvent for thermotropic LC polyester, based on 4-chlorocarbonyl phenyl esters of aromatic dicarboxylic acids and phenols or aliphatic diols for viscosity measurement. This indicates thermal stability in various organic solvents. [127]. Unsaturated aromatic LC polyesters, synthesized with the aim to fix the LC state, can be crosslinked by using styrene. The crosslinked matrix can be degraded by refluxing in 3 M aqueous sodium hydroxide solution and methanol in a vol. ratio of 3 2 [128]. 

LC polyester substrate is treated with an aqueous solution comprising an alkali metal hydroxide and a solubilizer as an etchant composition is used for etching the liquid crystal polymer substrate. An aqueous solution of a mixture of 35-55 wt. % of an alkali metal salt, and 10-35 wt. % of a solubilizer dissolved in the solution to provide the etchant composition suitable for etching the LC polyester at a temperature range of 50-120° C. 

Because thermotropic wholly aromatic LC polyesters have characteristics such as high strength, low melt viscosity, low shrinkage, ease of processibility, excellent thermal resistance, low water, and gas absorption. They have wide applications in following areas fibers, rods, sheets, composites used in mechanical and chemical industries chip carriers, connectors, switches used in electronics connectors, couplers, buffers used in fiber optics interior components, brackets in aerospace and so on. 

As the properties of LC polyesters can be used for most of the applications from engineering plastics/composite/blend to optical data recording, the research on this material for furthering its applicability is expected to continue for years to come. New LC polyesters are also being reported. 

The next development in liquid crystal polyesters was the preparation by polycondensation based on terephthalic acid (TPA) and hydroquinone (HQ) or p-hydroxybenzoic acid (HBA). The polyesters are insoluble with very high melting temperatures of 600 °C for poly (TPA/HQ) and 610 °C for poly (HBA), which are by far too high to obtain stable liquid crystalline phases for melt processing. In 1972, Economy and coworkers patented several copolyester compositions, and one of these are the copolymerization of poly (4-hydroxybenzoic acid) (PHB) with 4,4 -dihydroxybiphenyl (BP) and terephthalic acid (TPA) due to the need for lower melting, melt-processable polymers. Considerable synthetic efforts have been attempted in order to decrease the melting temperatures of aromatic LC polyesters while retaining LC properties. The copolyester structure was tailored by partial substitution of TPA with isophthalic acid to produce a melt-spinnable material. 

For the separation of the polyesters with respect to functionality, LC-CC was used the critical point of adsorption of the polymer chain corresponding to an eluent composition of acetone/hexane 51 49 (v/v) on silica gel. The critical chromatogram of a polyester sample together with the functionality fraction assignment is given in Fig. 22 and Table 4. The ether peaks are obtained due to the formation of ether structures in the polyester samples. 

The finding that the PEIs of 27b and monosubstituted hydro quinones form broad nematic phases, but show little propensity to crystallize, has prompted various modifications of their structures and properties. In this connection it should be stated that non-crystalline LC-polymers have found little interest in the past decades, but they may be attractive for various applications provided that the Tg can be varied between 90 and 250 °C. For instance, the absence of crystallinity has the advantage that the mechanical properties do not depend on the thermal history, and thus on the processing conditions. The temperature allowing a convenient processing may be reduced below 200 °C, which is of interest for the processing of LC-polymer reinforced blends and composites. Furthermore, non-crystalline nematic FC-polyesters are a useful basis for the synthesis of cholesteric lacquers, films or pigments (Sect. 5). 

Table 2. Application of MI to critical compositions for LC phase formation in random polyesters...

Cycloaliphatic LC PEAs based on commercially poly(l,4-cyclohexanedimelhylene terephthalate) (PCX) have also been synthesized with two cycloaliphatic diamines and a linear counterpart (1,6-hexamethylenediamine) (Figure 8.12) [55]. The compositions of the ester/amide units in the copolymers were varied up to 50% by the adjustment of the amounts of the diol and diamine in the feed. The introduction of amide linkages was found to induce nematic LC properties into the polyester backbone, which in turn increased the polymer chain alignment. Interestingly, the introduction of nematic LC phases into PCT was only possible when a low ratio of amide units (i.e., less than 25 mol%) was incorporated into the polymer backbone. 

The study of LC copolyesters started with the polyester of poly(ethylene tere-phthalate) (PET) and parahydroxy benzoic acid (PHB) by Jackson and Kuhfuss, 1974 (Jackson Jr. and Kuhfuss, 1996). It turned out that the copolyesters are heterogeneous consisting of flexible (ET-rich) and rigid (HB-rich) blocks. Thus, the copolyester itself, with a composition of 0.6 mole fraction of HB, segregates into... 

In practice, two-component solvent mixtures are employed as eluents and sample solvents inLC LC. One constituent of mixture supports elution of interactive polymer from the particular column, while another one induces its retention within column. To adjust polymer interactivity or to cope with the limited solubility of analyzed polymers, multicomponent solvents can be employed. Typical examples are mixtures of hexafluoropropanol with chloroform, which dissolve aromatic polyesters and some polyamides at ambient temperature. The sample solvents, eluents and barriers usually contain the same hquids but their composition is adjusted to fulfil their particular role Sample solvent must dissolve all its constituents and barrier must efficiently decelerate interactive macromolecules. Eluent serves either as a barrier in LC LCS, LC LCA and LC LCP or it promotes unhindered sample elution in LC LCD, LC LCU and LC LCI. 

---