CAE DS – Plastics for Injection Moulding Liquid Crystalline Polymers (LCP) Tampere University of Technology‐ Sanna Nykänen Liquid crystalline polymers are classified as crystalline, special plastics and are quite expensive materials. Liquid crystal state means interstate between a crystal, solid material and an isotropic (amorphous) liquid. These materials are called liquid crystalline polymers because they have liquid flow‐ properties. When liquid crystalline polymers are in a melt –state, part of their degree of organization main‐
tains. Liquid crystalline polymers are called self‐reinforcing materials because their orientation‐effect. Figure 1:
Morphology of plastic solidi‐
fication a) liquid crystalline polymer, b) normal plastic Mainly polyesters are used for manufacturing liquid crystalline polymers. Also polyester carbonates and polyester amides and –imides are suitable materials for manufacturing LCPs. Usually starting materials are aromatic substances. Only small amount of energy is needed when liquid crystals are formed. Therefore even small impureness in material can lead to a remarkable change in a structure of liquid crystal. Figure 2:
Possible starting materials’ structures of LCPs. Liquid crystalline polymers can be divided in to three different groups. These groups are called lyotropic and thermotropic LCPs and blends of polymers and small molecule liquid crystals. General Manufacturing Classification Lyotropic LCPs If the forces inside the molecule chains are so big that bringing enough heat energy will break the chemical structure of the composition, material have to be dissolved for bringing it to a liquid crystal state. This kind of material is called lyotropic liquid crystalline polymer. Liquid Crystalline Polymers ‐ 1 CAE DS – Plastics for Injection Moulding Thermotropic LCPs Thermotropic liquid crystalline polymers can be processed with heat. These plastics are usually manufactured from aromatic copolyesters. Polymerchains contain stiff and rod‐shaped parts (mesogens). These rod‐shaped parts act like fibres during processing. This means that plastic’s structure will become defibrated. Blends Thermoplastic polymers can be blended with liquid crystals that have small mole‐
cules. Properties Thermotropic liquid crystalline polymers are one of the promising new technical plastics because most of their properties are better than properties of thermoplastics used nowadays. At the same time self‐reinforcing polymers’ physical properties do not differ much from filled or reinforced technical plastics’ properties. Main properties The main properties for LCPs’ usage: − Price/ performance – ratio − Relatively low processing temperatures − Good chemical resistance − Fluidity of melt − Small shrinkage − Fire endurance − Dimension stability in high temperatures − Ability to reach excellent properties on certain direction ( direction of flow) − Low coefficient of heat expansion LCPs have as good mechanical properties as fibre reinforced composites, excellent heat stability and chemical resistance. LCPs’ properties are anisotropic and the properties are better in longitudinal direction than in lateral direction. Anisotropy can be reduced by using additives. The main disadvantages of LCPs are anisotropy of properties, weak strength of weld lines and low ultimate tensile strain (from 1.2 to 6.9 %). Mechanical properties Liquid crystalline polymers have good stiffness and strength (especially on flow Thermal direction). Stiffness can be improved by using reinforcements, but on the same time properties ultimate tensile strength and impact strength will become smaller. Temperature range of LCPs’ usage is large. Very good mechanical properties main‐
tain even in ‐160 °C. For example impact strength remains constant until ‐80 °C and HDT‐ temperatures (1.8 MPa) are in a range of 120‐355 °C. Temperature stability of liquid crystalline polymers depends on molecular struc‐
ture, crystal defects, impurities and processing conditions. Amorphous liquid crystalline polymers are durable until + 185 °C and partly crystalline liquid crystal‐
line polymers endure even + 275 °C. Liquid Crystalline Polymers ‐ 2 CAE DS – Plastics for Injection Moulding LCPs’ coefficient of heat expansion is extremely small. It is only 0.1‐0.5‐fold com‐
pared to normal thermoplastic. Melting temperatures may vary from 275 to 435 °C. The amount of heat needed to melt LCPs is only 1/10 of normal thermoplastics. This also means that cooling takes less energy and so their processing is cheaper than normal plastics. Melt also solidifies quicker and it enables short manufacturing times. Liquid crystalline polymers have remarkably good chemical resistance. They do not dissolve in any common solvent even on high temperatures. They have also a good resistance of strong acids and bases. LCPs have a small water absorption and high stress‐cracking resistance. Their weather properties are also good. Chemical properties LCPs’ electrical properties are remarkable good and they maintain on large tem‐
perature range. Electrical properties LCPs have good flow properties that are depended on injection pressure and thick‐
ness of the walls. Their viscosity is 0.25‐0.5 ‐fold compared to normal thermoplastics. This makes possible of injection mould long, uniform parts and the rate of injection can be very high. Very small details can be injection moulded because melt flows even in the smallest mould’s cavities. Rheological properties Some other properties of LCPs: Other properties −
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Very small mould shrinkage No mould shrinkage in flow direction Shrinkage can be negative in flow direction if orientation is remarkable Using large amounts of additives is possible Small coefficient of friction Good abrasion resistance Small permeability Injection moulded parts do not need any after‐treatment Processing LCPs are processed mainly by injection moulding or extruding. LCPs need relatively high temperatures for them to melt. However they have quite low enthalpy so they can be processed rather easy. Temperatures in the mould can change from + 50 ° C to 180 ° C depending on LCP ‐grade. Also the mass tempera‐
ture is depended on LCP‐grade. When processing LCPs it is very characteristics for them to have a low viscosity and their structure will orientate very strongly. Because of their orientation‐ effect, parts manufactured from LCPs have to be designed as thin walled as possible. If parts have heavy walls, the inner parts of the product will not orientate and the strength properties of inner parts will be weak. Also strength of weld lines is low because of this orientation effect. These factors set great requirements for product design. Solidifying of liquid crystalline polymer happens quickly because molecular struc‐
ture has organized already in melt‐state. This enables very short (10 seconds) injection moulding times. With large wall thicknesses, properties on flow direction weaken. Liquid Crystalline Polymers ‐ 3 CAE DS – Plastics for Injection Moulding LCPs have very low shrinkages because they maintain their crystal structure even in solid state. Crystals do not reorganize in molten state like happens with partially crystalline polymers. Liquid crystalline polymers’ small shrinkages enable using small draft angles on their moulds. If the mould has been polished properly, parts without clearances can be pushed out of the mould. Very sensitively flowing mate‐
rial however copies the shapes of mould surface very carefully, so it is recommended to use draft angle of 0.5 °. Again even not‐ reinforced LCPs are stiff and back drafts need moving cores. The nozzle of the mould should be small, if the moulded product is thin walled. LCPs are normally injected very fast to the mould so the gas removal of the mould has to be good. Optimal dimensions for gas‐escape channels are depth of 0.015‐
0.025 mm and width of 2‐3 mm. Applications − Liquid crystal displays − Optical filters − Optical temperature measuring − Fibres − Films − Coatings − Shaped pieces Trade names − Vectra (Hoechst; Ticona, DE) − Laxtar (LATI S.p.A., IT) − Zenite (Du Pont, USA) − Xydar (Solvay Advanced Polymers, USA) References: Järvelä et al., Ruiskuvalu, Plastdata, Tampere, 2000 Plastic‐materials and their technical applications, Course material, Tampere Uni‐
versity of technology; Plastics and elastomers laboratory. Liquid Crystalline Polymers ‐ 4