Hello and welcome to this blog post in which we discuss the high heat polymer Liquid Crystal Polymer (LCP) and its specific properties. In general, defining the part requirements can be seen as a common starting point in polymer material selection. However when selecting high heat plastics, short and long time temperature performance plays a key role too. Also, implementing design features such as thin wall part design is another drive of the usage of high performance polymers. However, what happens if the wall thickness of LCP parts is decreased. Will the tensile strength keeps constant or is there a different behaviour to be observed?
Introduction to Liquid Crystal Polymers
Based on their superiority in high heat solder resistance, high-temperature strength, dimensional stability, overall good chemical resistance, low flammability, and low water absorption, liquid crystalline polymers (LCPs) are widely employed in many types of electric and electronic parts (connectors). Since LCPs have an exceptionally low melt viscosity, they have better thin-wall fluidity and mouldability compared to any other engineering plastics. LCPs are currently utilized for the most highly precise applications, also where Surface Mounting Technology (SMT) is needed. Electric and electronic devices moulded using LCPs have grown in significance in recent years for the IT-related industries as well as many consumer markets. Recent developments of high heat LCPs include the usage as high-heat EV battery module insulation [2].
Not to be overlooked is the fact that every LCP has a unique chemical structure. This implies that although the term "liquid crystalline polymer" refers to the overall set of features, each manufacturer of LCP may have unique chemical structures and this is similar to polyamides. For example, PA6 and PA 4.6 show significantly differing thermal resistance, yet they both absorb more water than polyesters and have poorer dimension stability. While each polyamide has a unique chemical structure that determines its different thermal resistance, the amide-bonding group determines the increased water absorption property.
Looking into the literature [1] of polymer chemistry, we can distinguish between
-Type I LCP (HDT a 1.82 MPa > 260°C),
-Type II LCP (HDT = 210-260°C), and
-Type III (HDT < 210°C).
All three types contain a p-hydroxybenzoic group and are called “thermotropic” LCP ( in contract to “lyotropic” LCP = liquid crystals can be seen in solvent as a solution). The crystals stay solid in the melt phase and can be modelled as matchsticks during the injection moulding filling process. Applying shear to the polymer will result in a very good alignment of the matchsticks.
Summarizing the key pros and cons of LCP:
Pros:
-High thermal resistance (up to 260 °C)
-Barrier properties (due to dense skin layer)
-Excellent soldering resistance for lead-free reflow soldering processes
-Solvent stability (except alkali & steam)
-Superior high flowability
-High flame retardancy (UL 94 V-0 @0.3 mm)
Cons:
-Strong anisotropy in moulded parts
-Lower weld line strength
Thermal properties: CUT vs. HDT of LCP
The short term temperature resistance of engineering polymers can be improved by adding glass-fiber reinforcement, however the long term temperature resistance stays on a similar level. This is different with high heat plastics such as PEEK, PPS, LCP, Polyarylates (PAR), Polysulfones (PSU, PESU, PPSU), and Polyimides (PEI, PAI, PI). They combine a high short- and long term thermal resistance. Figure 1 compares the Continuous Use temperature (CUT) to the Heat Deflection Temperature (HDT; short term temperature resistance) of high performance and engineering polymers. LCP has an excellent short- and long-term temperature stability.
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| Figure 1: Short-term (HDT 1.8 MPa) vs. long-term temperature resistance (CUT) of engineering and high performance polymers such as LCP, PEEK, and PPS. |
What is the glass transition and melt temperature of LCP?
LCPs do not have a “glass transition” temperature in the classical way of definition (Alpha temperature transition enabling the movement of more than 40 C-atoms in backbone [3]). They have a liquid crystal temperature. Figure 2 shows the DMA curves of PESU (amorphous; Tg = 220°C), PEEK (semi-crystalline; Tg=143°C; Tm =334°C), and LCP (Tlc = 300-380°C). LCP does not have a glass transition temperature, nor a melting temperature. It has a liquid crystalline temperature where the crystals remain solid, however the linkages between the solid crystals can move [1]. If you examine in detail the literature, a small transition temperature of LCP was found at 120°C. LCP can keep a high mechanical strength level up to 300 °C, outperforming PEEK and PESU.
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| Figure 2: DMA of high performance polymers - LCP has a liquid crystalline transition area. |
Wall thickness and tensile strength
Moving back to the question from the beginning of this post: what is the relationship between wall thickness and tensile strength of LCP?
The skin layer's thickness of LCP is almost 200 μm and is a result of the strong orientation of the solid crystal elements (“matchsticks”). The ratio of the skin layer to the total thickness increases proportionately as the thickness decreases. The skin layer has strong mechanical properties since it is made up of highly aligned fibrous semi-crystals of stiff rod molecules. Because of this, LCP's strength will progressively rise as its thickness decreases. Figure 3 shows this relationship of a LCP, and comparing it to a PBT and PESU. This is a common and unique feature of LCP that isn't seen in traditional polymers.
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| Figure 3: Tensile strength as a function of wall thickness of LCP, PESU, and PBT. |
Conclusions
Designing parts with high performance polymers such as LCP is not more difficult compared to engineering or commodity polymers. It is different and the dependency of mechanical properties as a function of wall thickness allows applications made out of LCP to be really thin and still fulfill stringent requirements such as temperature, flame retardancy, and strength.
More on high performance polymers can be found here and here.
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig Juster
Literature:
[1] https://www.sumitomo-chem.co.jp/sep/english/products/pdf/lcp_users_manual_v31_e.pdf
[2] https://www.solvay.com/en/press-release/solvay-introduces-new-polymer-high-heat-ev-battery-module-insulation
[3] https://www.findoutaboutplastics.com/2018/12/dynamic-mechanical-analysis-dma-as.html
[4] https://www.azom.com/article.aspx?ArticleID=13872
[5] https://www.ptonline.com/articles/tracing-the-history-of-polymeric-materials-part-27-lcp
