Researchs On Foaming And Electrical Properties Of PLA/CNTs Composites

In recent decades, microcellular foamed nanocomposites have attracted a great interest benefiting from their high toughness, high stiffness, high thermal stability and low dielectric constant. The mainstream trend is searching for green polymer alternatives to petroleum-based polymers to reduce the dependence on fossil fuels in plastics industry, among which, polylactic acid is favored by researchers. In this work,polylactic acid/carbon nanotubes (PLA/CNTs) composites were prepared by melt blending method. The foamed PLA and PLA/CNTs materials were prepared at different foaming pressure, foaming temperature and saturation time, using supercritical carbon dioxide (ScCO2) as physical foaming agent. The thermal property,cell structure and electrical conductivity of the materials were characterized. The influence of foaming conditions on the thermal property and cell structure of foamed PLA and PLA/CNTs was investigated. The effect of foaming conditions and cell structure on the electrical properties of the foamed PLA/CNTs was discussed, based on the aim of preparing PLA/CNTs foams with better electrical conductivity than the unfoamed ones.(1) Thermal analysis shows that solid PLA/CNTs has a low crystallinity of 17.5%, but it is still higher than that of solid PLA, which is 8.2%. With the increase of foaming pressure, the glass transition temperature, melting point and crystallinity of foamed PLA and PLA/CNTs increase first and then decrease, and both have a double melting peak at 150?, 16 MPa.(2) SEM analysis shows that foaming pressure, foaming temperature and saturation time all have influence on the morphology of foamed PLA and PLA/CNTs.For foamed PLA, with the increase of foaming pressure, the cell size increases first and then decreases, otherwise, the cell density decreases first and then increases; with the increase of foaming temperature, the cell size increases first and then decreases,while the cell density decreases first and then increases. For the foamed PLA/CNTs,with the increase of foaming pressure, the cell size increases first and then decreases,while the cell density decreases first and then increases; with the increase of foaming temperature,the cell size increases but the cell density decreases,being foamed at 150?, 16 MPa, the mean cell diameter of PLA/CNT-3 is the smallest, which is 16.46?m, and the cell density is 1.2×109cell/cm3. The addition of CNTs has a great effect on the cell structure of the foamed PLA/CNTs. Compared with foamed PLA, the cell density of foamed PLA/CNTs increases by about two orders of magnitude to about 109cells/cm3. The mean cell diameter decreases, while the cell density increases, and the cell uniformity increases, but not the more the CNTs content is, the better the cells morphology is.(3) The volume electrical conductivity of the unfoamed and foamed PLA/CNTs all increases obviously with the increase of CNTs content. With the same volume content of CNTs, the volume electrical conductivity of the foamed PLA/CNTs is three orders of magnitude higher than the unfoamed one. With the increase of foaming pressure and foaming temperature, the volume electrical conductivity of foamed PLA/CNTs decreases first and then increases. PLA/CNTs composites containing 3.0 wt% of CNTs were foamed at 150?, 16 MPa, the obtained foaming materials have better cell morphology and better volume electrical conductivity, the cell density of which is 1.2×109 cell/cm3, the volume electrical conductivity of which is 1.6×10-1 S/m.