Thermochromic properties of composite materials made from polylactic acid reinforced with recovered liquid crystals from waste LCD/LED laptops screens

Abstract

The use of flat panel displays has permeated daily life since the breakthrough in the usage of liquid crystals (LCs) in display technology. However, the presence of chemical compositions such us cyano-groups, benzene, fluorine, cyclohexyl, etc., in LCs has potential adverse effects on humans and their environment as well. The environmental issue can be resolved by recovering the LCs for use in different applications. In this study, solvent extraction recovery process was directly used to simultaneously recover and purify LCs from waste liquid crystal display (LCD)/light emitting diode (LED) screens of laptops. The physicochemical (Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC)) and morphological properties (scanning electron microscopy (SEM)) of the purified and unpurified recovered liquid crystals (RLCs) were compared. The RLCs were incorporated in polylactic acid (PLA) using 0, 5, 10, 15, 20, 25 and 30 wt% of RLC. Thermochromic properties of the formulated PLA-RLC composites were studied from room temperature (25 °C) to 100 °C using a colorimeter. The changes in colour with change in temperature were measured and recorded. A recovery rate of 92.46% LCs was obtained from the laptop screens, making the purification process very effective as ascertained by the physicochemical characterization techniques. The difference in crystallinity of the purified and unpurified RLCs was 2.55% with the solid to liquid crystalline transition temperatures of the purified and unpurified RLCs being 45 °C and 65 °C, respectively. The thermochromic studies of the formulated PLA-RLC composites showed an appreciable colour change with temperature improvements in all the colour variables (i.e., the lightness, a* and b*). The colour changes were however not visible to the eyes due to the small energy difference (ΔE*) values. The implication of the results is that the RLC from waste screens of LCDs/LEDs exhibits enhanced thermochromic properties when different purification stages are used.