All About Liquid-Crystal Polymer
Learn about the material's properties, uses, advantages, limitations, and toxicity.
Liquid crystal polymers (LCPs) are a class of advanced polymers that retain a highly ordered molecular structure both in their solid and liquid phases. This results in materials and parts with exceptional mechanical strength, temperature resistance, and flame resistance. This article will delve into more detail on the production and application of LCPs.
Thermotropic liquid crystal polymers.
Image Credit: MalcolmMackley.com
What is a Liquid-Crystal Polymer?
A liquid crystal polymer is a material that retains molecular order in both liquid and solid states. Specifically, the transition from order to disorder during the melting of an LCP occurs well above the temperature at which it loses its fully crystallized structure.
This effectively means that the material has two distinct melting points or, more accurately, two-phase changes. The first phase change takes it from solid to liquid crystal and the next from liquid crystal to a full liquid. The phase between liquid and crystal is referred to as the mesophase and the specific molecules that can form a mesophase are termed mesogens. LCPs can be divided into two main categories, namely lyotropic systems and thermotropic systems.
- Lyotropic systems - A liquid crystal that appears with the addition of a solvent.
- Thermotropic system - A liquid crystal that appears when heated.
A liquid-crystal polymer is a polymer in which the various mesogen molecules link to form long polymeric chains.
How is Liquid-Crystal Polymer Made?
Most commercial LCPs incorporate p-hydroxybenzoic acid as one of the monomers that build molecular chains through various condensation methods. Monomer additives such as bisphenol are then introduced into the p-HBA to lower processing temperatures and allow for easier manufacturing. In addition to these monomers, fillers can be added to the material to further enhance its properties. These fillers can include graphite, fiberglass, or carbon.
What Are the Properties of Liquid-Crystal Polymer?
LCPs are used in a wide array of applications. Listed below are some of the more common Vectra LCPs as well as their relevant mechanical properties.
| 30% glass Filled | 40% glass Filled | |
|---|---|---|
30% glass Filled Specific Gravity | 40% glass Filled 1.61 | 1.70 |
30% glass Filled Tensile Strength (MPa) | 40% glass Filled 164 | 140 |
30% glass Filled Elongation (%) | 40% glass Filled 3.0 | 1.8 |
30% glass Filled Flexural Strength (MPa) | 40% glass Filled 163 | 198 |
30% glass Filled Flexural Modulus (MPa) | 40% glass Filled 9237 | 8674 |
30% glass Filled IZOD Notched Impact strength (J/M2) | 40% glass Filled 106 | 85 |
30% glass Filled Rockwell Hardness (R) | 40% glass Filled R77 | R79 |
30% glass Filled Heat Deflection Temp (C) | 40% glass Filled 280 | 290 |
30% glass Filled Molding Shrinkage TD (%) | 40% glass Filled 0.01 to 0.94 | 0.05 to 0.88 |
Table 1. Liquid Crystal Polymer Properties
What Are the Uses of Liquid-Crystal Polymers?
Liquid crystal polymers have a wide range of potential uses thanks to their exceptional physical properties. Some examples are listed below.
- Electrical connectors: LCP plastic can be used to manufacture conductive electrical connectors. They function to eliminate static build-ups and discharges that would otherwise create noise interference in electrical signals.
- Vascular catheter reinforcement braiding: Vascular catheters with LCP plastic braiding can be important for patients undergoing MRI scans. Catheters with metallic braiding would interact with the magnetic field generated by the machine.
- Surgical instruments: Surgical instruments are sterilized after use via radiation. LCPs are ideal for this because they can withstand the radiation without breaking down.
- Cookware coatings - Cookware with non-stick surfaces need to withstand the high-temperature environment of the stove as well as the corrosive effects of dishwashers and acidic foods. LCPs can withstand temperatures up to 280℃ and the coating is not harmed by dishwasher machines.
What Are the Advantages of Using Liquid-Crystal Polymers?
LCP plastics are very customizable. Changing the molecular structure and the molecular constituents can drastically change the performance of the material. Additionally, additives can further tweak the performance of LCPs.
- Self-reinforcing: LCP resin molecular chains retain their order when injection molded or extruded. Once cooled, these chains add directional strength to the final parts.
- Temperature resistant: LCPs have extremely high operating temperatures with some being able to function up to 340℃ for short periods.
- Flame resistant: LCPs are inherently fire resistant.
- Chemical resistance: LCPs are resistant to a wide range of chemicals.
- Dimensional stability: Linear expansion in the direction of flow is extremely low when compared to other plastics.
- Resistance to sterilization radiation: LCPs are not severely damaged when exposed to sterilizing radiation and can therefore be used in medical applications.
What Are the Disadvantages of Using Liquid-Crystal Polymers?
Some typical disadvantages must be understood before considering LCPs for a specific application. Some of these are listed below.
- High cost: LCPs are advanced materials with exceptional properties. As such, they come at a higher cost than more conventional polymers.
- Anisotropic properties: Properties may vary depending on the direction of the applied load as polymer chains tend to align longitudinally in the direction of extrusion during processing.
How Does Liquid-Crystal Polymer Affect Human Health?
LCPs are typically safe once processed. The manufacturing of LCP raw material makes use of dangerous chemicals but once that is complete, LCPs become inert, much like other polymers. This means that the only potential adverse effect on the skin and eyes comes from mechanical injury. However, inhalation of liquid-crystal polymers will irritate the respiratory tract.
Is Liquid-Crystal Polymer Toxic?
Like other polymers, the combustion products of liquid crystal polymers are very hazardous and toxic. Though most LCPs are flame-resistant, care must be taken to ensure they are not burnt or otherwise ignited.
What is the Difference Between Liquid-Crystal Polymer and Nylon?
Nylon is a semi-crystalline polymer. This should not be confused with a liquid-crystal polymer. Semi-crystalline refers to an equilibrium state within a polymer. Semi-crystalline polymers have sharp melting points where the polymer moves from an ordered crystalline structure to an unordered liquid. This contrasts with LCP polymers which remain ordered even after changing from their solid crystalline phase to a liquid one. For more information, see our guides on Nylon and What Are Nylon 6 and Nylon 6/12.
What Is the Difference Between Liquid-Crystal Polymer and Thermoplastic?
Liquid crystal polymers represent a specific form of thermoplastic whose properties over a well-defined temperature range are somewhere between those of highly ordered solid crystalline materials and amorphous disordered liquids. Three of the most common forms of liquid crystal polymers include semi-aromatic copolyesters, copolyamides, and polyester-co-amides.
By contrast, the term thermoplastic refers to a class of polymer materials that are characterized by their ability to retain their chemical properties after repeated cycles of heating and cooling. Unlike thermosets, whose properties are permanently changed once the material has been heated and cooled, thermoplastics can be repeatedly put through cycles of heating and cooling. This capability enables thermoplastics to be recycled and the material to be repurposed. They will soften when heated and then harden once cooled. Thermosets, once cooled after initially being heated, undergo a permanent chemical change and will not remelt if exposed to heat, and therefore, are not recyclable.
Liquid crystal polymer is one specific type of the broad class of polymers that are known as thermoplastics.
For more information, see our guides on thermoplastic and thermoplastic polyurethane.
Summary
This article summarized the properties, uses, advantages, limitations, and toxicity of liquid crystal polymer.
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