LIQUID CRYSTAL POLYMERS (LCP):
History
Liquid crystal polymers, also known as self-reinforced polymers were first introduced in 1965 (@Kevlar from Du Pont de Nemours).
This well-known aramid fiber is a lyotropic (in solution form) macro-molecularly ordered polymer.
This was followed in 1972 by Ekkcel 1-2000 from Carborundum which was Thermotropic (melt-processable) in nature
Development work commenced in 1975 by Celanese led in the mid eighties to Vectra (now a Hoechst product) based on thermotropic polyester.
BASF (Ultrax) and Bayer (KU 1-90 range) have also now commenced production of LCPs. The highly ordered state in solution or in the melt contrasts with the randomly ordered macromolecules of conventional polymers.
Between the liquid and solid states, LCPs may be in a liquid crystalline intermediate state which is described as mesomorphous.
OR
Liquid crystalline polymers are polymers which in the melt state lie between the boundaries of solid substances and liquids. The liquid crystalline structure is called a mesomorphic phase or an anisotropic phase because macroscopically in the melt state the liquid crystalline polymers are fluids.
OR
Liquid crystallinity in polymers may occur either by dissolving a polymer in a solvent (lyotropic liquid-crystal polymers) or by heating a polymer above its glass or melting transition point (thermotropic liquid-crystal polymers). Liquid-crystal polymers are present in melted/liquid or solid form. In solid form the main example of lyotropic LCPs is the commercial aramid known as kevlar. Chemical structure of this aramid consists of linearly substituted aromatic rings linked by amide groups. In a similar way, several series of thermotropic LCPs have been commercially produced by several companies (e.g., Vectran / Ticona).
Today, LCPs can be melt-processed on conventional equipment at high speeds with excellent replication of mold details. In fact, the high ease of forming of LCPs is an important competitive advantage against other plastics, as it offsets high raw material cost.
LCPs are characterized by two superimposed structural principles:
1. Main chain polymers in which the rigid molecular units (mesogens) are arranged in the polymer chain.
2. Side chain polymers in which the mesogens are attached as side groups.
3. Main chain polymers are not suitable for use at high temperatures.
4. The side chain polymers are distinguished by particularly good electrical and optical properties which are exploited in optical storage or liquid crystal displays (LCD).
5. LCPs are based mainly on polyesters but polyestercarbonates, polyesteramides, polyesterimides & polyazomethines can also be used in their production.
LCP – General Properties:
Besides the properties typical of each type of LCP, they all share the following characteristics:
1. melt-processable
2. ordered structures in the melt
3. highly oriented in the solid state (self-reinforcement)
4. anisotropic property profile
5. wide variation in properties possible by selecting the appropriate LCP type.
LCP properties -1
In addition they exhibit the following properties:
1. medium density
2. high tensile strength
3. high notched impact strength
4. high tensile modulus of elasticity, up to wide melting range
5. high continuous service temperature
LCP properties -2
1..good low temperature properties
2. low coefficient of linear expansion
3. orientation easily achieved by injection molding, extrusion and blow molding
4. processable on standard machines
5. low shrinkage
6. low melt viscosity, but high strength and extensibility of melt.
LCP properties -3
1. good electrical properties
2. high resistance to chemicals
3. flammability UL 94 V-0 (0.8 mm) without flame retardant
4. low moisture absorption
5. high resistance to stress cracking.
A recently introduced amorphous LCP exhibits less variation in properties at high temperatures. The processing range is wider and the material can thus be blow molded and thermoformed.
Processable LCPs cannot be obtained from rigid linear macromolecules alone because they melt only above their decomposition temperature. This difficulty has been overcome by incorporating flexible linkages in the main chain.
Many properties, favorable and unfavourable, can be significantly influenced by the choice of processing conditions and molding design, e.g.:
1. At large wall thicknesses, LCPs lose their peak properties in the longitudinal direction,
2. shrinkage can become negative in the direction of flow,
3. anisotropy can be reduced by adding fillers,
4. filler content can be very high (> 40% w/w).
AVAILABILITY
1. Liquid crystal polymers are available as a self-reinforcing, glass, glass/mineral, or mineral carbon fibre filled resin
2. LCP is supplied as pellets in transparent and opaque grey, dark and black colorations.
3. Injection molding grades and extrusion grades for rod, tube, film and for extrusion coating.
4. The physical properties of self-reinforced polymers do not differ significantly from those of filled or reinforced engineering plastics.
5. The ability to align the orientation tendency in molded or semi-finished material with the direction of the greatest stress to obtain high performance moldings is unmatched
6. The cost/performance ratio is also favorable – low shrinkage, low processing temp.
LCP Processing
LCP Injection Moldings
1. The original point of using LCPs was to exploit the increase in strength which can be achieved by orientation of flow.
2. The usual flat shaped design of injection moldings leads, however, to a multi-axial stress state which oriented LCP cannot cope with isotropically.
Injection Moldings
1. The unavoidable shear and stretching flow lead to different localised orientations. This behavior can be compensated only to a limited extent by suitable choice of the position and shape of the gate.
2. Instead of stiffness and strength, the main motives nowadays for using LCPs as engineering materials are flowability, flame retardance, dimensional stability at high temperatures, low thermal expansion, resistance to chemicals and rigidity with low wall thickness.
LCP resins have
1. Low outgassing even at extreme temperatures, a factor that's critical for sensitive electronics and optical applications.
2. When it comes to precision, LCP resins have excellent molding properties, provide the design freedom for extremely small and intricate parts with cross sections as thin as 0.25 mm.
LCP – Machining & Fabrication
1. LCPs can be machined and joined with conventional tools and machinery.
2. They can be bonded with the usual adhesives for polyesters
3. They can be welded with ultrasonics.
4. They can be metallized by spraying or electroplating.
LCP – Machining & Fabrication
1. LCPs can be machined and joined with conventional tools and machinery.
2. They can be bonded with the usual adhesives for polyesters
3. They can be welded with ultrasonics.
4. They can be metallized by spraying or electroplating.
LCP - Recycling:
1. Up to 25% w/w scrap can be added to virgin material.
2. Material which has absorbed moisture must be predried.
3. Plasticizing cylinders can be purged with PE-HD.
LCP – Typical Applications
Electrical & Electronic:
1. Substrates for chips and printed circuits,
2. Coil cores, sockets, encapsulations, Sensors,
3. switches and other electronic components requiring outstanding performance at elevated temperatures,
4. Buffer films, couplings and connectors for fiber optics,
5. Precision high temperature applications such as optical lens components, sensors, LED housings, and motor components
Miniaturized moldings (particularly for use in aggressive environments),
Under-bonnet components in contact with fuel,
Numerous components in aircraft cabins, fuel systems, radomes, electrical components.
1. Drive belt pulleys, packings, bearings, seals,
2. Accessories for oil drilling, pumps,
3. Packings for distillation columns and scrubbing towers,
4. Measuring instruments,
5. Surface mount devices
6. Dual ovenable cookware
7. Heat sinks containing graphite fibers
LCP -Tradenames
1. Ekce1, Ekonol (Carborundum Co., Metallic Div., US)
2. Granlar (Montedison Inst. Guido, IT)
3. Novoaccurate (Mitsubishi Chem. Ind., lP)
4. Rodron (U nitika, lP)
5. Ultrax (BASF, DE)
6. Vectra (Hoechst, DE)
7. Victrex (ICI, GB)
8. Xydar (Amoco Performance Products
1. Anisotropic physical properties,
2. Low strength of joints,
3. Low elongation at break (1.2 to 6.9%).
PROPERTIES
1. Possess superior elevated temperature performance.
2. High mechanical strength upto 300 Deg.C
3. Very good flame retardant
4. Better electrical properties
5. Good chemical resistance
APPLICATIONS
Appliances: Cook wears used in microwave ovens.
Microwave oven parts like, floor, ceiling parts, stirrer assembly and turn table assemble parts.
Machinery/Equipment: Circuit boards, Semi-conductor handling devices, parts for electric motors.
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