Liquid crystal polymers as a 3D printing feedstock
Khai Trung Le talks to Dr Kunal Masania on the development of a liquid crystal polymer feedstock for 3D printing.
A new means of 3D printing using liquid crystal polymers (LCP) compatible with desktop fused deposition modelling (FDM) 3D printers and able to create strong and complex objects could represent the next FDM step change.
While 3D printing has found a space in industry for several decades already, it has yet to achieve a ubiquity found through commercial success. Some ascribe this to the calibre of available 3D printers, the low strength of polymers and adhesive strength of print lines, or the technical hardship of creating complex geometries with strong mechanical properties. Adding glass or carbon fibre to reinforce feedstock is commonplace, but this can make the material brittle. Fibre-reinforced polymers are also difficult to recycle.
A team from ETH Zürich, Switzerland, looked to LCPs, described by Dr Kunal Masania, Senior Scientist at the ETH Zürich Department of Materials, as very short polymer chains that are stiff and prefer to reside in an aligned manner as minimum entropy, as a potential new feedstock. The team took inspiration from the structure of natural materials, specifically the molecular alignment of silk proteins along the fibre directions in spider silk, and the way living tissue such as wood arranges fibres along stress lines throughout loaded structures, as in a wood knot.
The team used Vectra A950 as the LCP, comprising p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid, chosen for its strong mechanic properties after fibre spinning. Masania told Materials World, ‘It was possible to reproduce this high alignment during the extrusion from an FDM nozzle by using LCP as an FDM feedstock material. The anisotropic fibre properties were utilised by tailoring the local orientation of the print path according to the specific loading conditions imposed by the environment.’
While the team states this process is compatible with commercial 3D printers, the LCP feedstock must be processed at temperatures of around 300˚C. However, by introducing additional flexible monomer units to the LCP, the melting temperature can be brought down to around 200˚C and maintain the mechanical properties of spun fibres. Masania added, ‘LCPs with different melting temperatures all exhibit high molecular alignment and mechanical properties after fibre spinning. Therefore, we are very confident that our findings will be reproducible with a broad range of LCPs.’
Their work is detailed in the paper, Three-dimensional printing of hierarchical liquid-crystal-polymer structures, published in Nature. The team claims the printed LCP structures are stronger than state-of-the-art 3D-printed polymers, and without labour- and energy-intensive steps involved in composite manufacturing technologies. Print speeds are similar to those of acrylonitrile butadiene styrene and polylactic acid, two of the most common FDM feedstock materials.
Always the bridesmaid
LCPs were first developed in the 1960s. They are nematic – molecules are oriented in loose parallel lines – in solution, and have lower degradation than melt temperatures. Their use in FDM usually involves dissolving the polymer in a solvent before it is spun, stretched and dried, resulting in the aligned fibre. However, by producing a co-polymer, the nematic structure is disturbed enough to allow the mixture to melt and flow before degrading.
Masania said, ‘These thermotropic LCPs were always a bridesmaid to the lyotropic types which are used to make high-performance polymer fibres. The main reason for this is because they are extremely anisotropic so, when injection moulding, the mechanical properties follow the orientation of the flow within the object. While this is also challenging to work with industrially, it’s a very advantageous phenomena for directed assembly using 3D printing.’
Next, the team will look to generating optimum designs using the LCP feedstock.
The paper, Three-dimensional printing of hierarchical liquid-crystal-polymer structures, can be read at http://go.nature.com/2y7mJlC