Design And Control Of The Foaming Behavior Of Crystalline Polymers With Low Melt Strength

Polymer foams, which could be considered as a polymer/bubble composites have been applied on sound and thermal insulation; packaging; transportation; wind energy and tissue engineering etc. Microcellular foams, characterized by cell size smaller than 10μm and cell density larger than 109 cells/cm3, are drawing increased attention. It has been shown that by keeping the foam cell size uniformly less than 10μm, one can reduce the material usage and improve mechanical properties simultaneously. In this study, supercritical carbon dioxide (scCO2), a potential replacement of the traditional foaming agents (chlorofluorocarbons), is applied to produce microcellular foams from typical crystalline polymers with linear molecular structure and low melt strength. By studying and utilizing the non-isothermal crystallization behavior of polymers under scCO2, three types of polymer foams with special bubble structures are successfully fabricated:microcellular PET foams with sandwich structure; PLA foams with open cell structure and microcellular iPP foams with bi-modal structure.Fabrication of PET microcellular foams with sandwich structure This work is aimed at manipulating sandwich-structure of poly(ethylene terephthalate) (PET) microcellular foams using coupling of CO2 diffusion and CO2-induced crystallization. The intrinsic kinetics of CO2-induced crystallization of amorphous PET at 25℃and different CO2 pressures were detected using in-situ high-pressure Fourier transform infrared spectrum and correlated by Avrami equation. Sorption of CO2 in PET was measured using magnetic suspension balance and the diffusivity determined by Fick’s second law. A model coupling CO2 diffusion in and CO2-induced crystallization of PET was proposed to calculate the CO2 concentration as well as crystallinity distributions in PET sheet at different saturation times. It was revealed that a sandwich crystallization structure could be built in PET sheet, based on which a solid-state foaming process was utilized to manipulate the sandwich-structure of PET microcellular foams with two microcellular or even ultra-microcellular foamed crystalline layers outside and a microcellular foamed amorphous layer inside.Foaming of iPP and PLA utilizing their non-isothermal crystallization behavior under scCO2 The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N2 and compressed CO2 (5～50 bar) at cooling rates of 0.2～5.0℃/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO2 had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO2 as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40℃and the upper and lower temperature limits were 155 and 105℃, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO2, respectively. Due to the acute depression of CO2 solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated. The non-isothermal crystallization behaviors of poly(lactic acid) under ambient N2 and compressed CO2 (5～50 bar) at cooling rates of 0.2～2.0℃/min were carefully studied using the high-pressure differential scanning calorimeter. The presence of compressed CO2 postponed the crystallization peak to lower temperature region while effectively reduced the half-crystallization time and enhanced the crystallinity of the PLA specimen. Based on these findings, the new foaming strategy was also utilized to fabricate PLA foams using the ordinary un-modified PLA. The upper and lower temperature limits of this foaming strategy were 105 and 60℃, which were determined by the melt strength and crystallization behavior of the un-modified PLA specimen, respectively. In the temperature range of 90～110℃, PLA foams with inter-connected structures, porosity of 67.9～91.4% and expansion ratio of 15～30 times were controllably produced. The obtained PLA foams have widely distributed average bubble size of 80～270μm and CO2 enhanced crystallinity of 32～38%. When the foaming temperature was lower than 90℃, PLA foams with closed cells could be controllablly fabricated.Extrusion foaming of low melt-strength iPP and its nano composites Aiming at enhance the viscosity of low melt-strength iPP, nanoparticles (Layered nanoclay, tubular nanoclay, carbon nanofiber multiwall carbon nanotube) with diffenent matrix and geometry are utilized to produce iPP nanocomposites. The rheology property of different iPP nanoparticle systems are compared and iPP/LNC as well as PP/CNF systems are choosen to investigate the influence of the addition of nanoparticles on the extrusion foaming behavior of low melt-strength iPP. Working as a heterogeneous nucleation agent, the addition of LNC effectively reduced the bubble size and increased the bubble density of obtained iPP foams. High loading of LNC also increase the viscosity of iPP matrix, so as to provent the bubble rapture during bubble growth and reduce amount of lost CO2 and finally effectively increased the expansion ratio of iPP foams. Although the addition CNF increased the viscosity of iPP at one magnitude, the presence of CNF also increase the crystallization temperature and crystallization rate during non-isothermal cooling process, which leads to the hardening of iPP matrix and decrease the final expansion ratio.