Supercritical CO₂ Foaming Behavior And Property Regulation Of Typical General Polymer Multiphase System

Polymer foams are defined as a two-phase system composed of a continuous solid polymeric phase and either a continuous or discontinuous gaseous phase.The solid polymer could be a thermoplastic or a thermoset.In the foaming process,a gaseous phase is introduced into the softened or molten polymer,foaming is carried out,and the polymer is solidified so as to stabilize the porous structure.Polymer foam has the advantages of lightweight,high specific strength,excellent heat insulation,sound insulation,electrical insulation,impact resistance,and so on.Due to these excellent properties,polymer foams are widely used and have a very promising future in important technological sectors such as the automotive and aeronautical industries,renewable energies,construction,transportation and packaging,and biotechnology.At present,the preparation of polymer foam by the supercritical carbon dioxide(sc-CO2)foaming process has attracted widespread attention.The sc-CO2 holds the unique combination of the gas-like diffusion coefficient and liquid-like solubility,which makes it an ideal physical blowing agent.It can dissolve in most polymers,and has a good plasticizing effect on polymer melt,reducing the glass transition temperature(Tg)of the polymer matrix.Nowadays,with the rapid development of society and industry,there is a growing need for the higher comprehensive performance of polymer foams in various fields.The single polymer foam has gradually been unable to meet the needs of industrial development.Foaming of polymer blends is an effective way to expand the function of foam.This method can not only improve the comprehensive performance of polymer foam but also obtain polymer blend foam with customized properties.However,the preparation of polymer blend foams using sc-CO2 foaming presents many challenges,both in terms of the compatibility of polymer blends and the complexity of the foaming behavior.Thus,it provides great opportunities and challenges for the preparation of high-performance foams.In this paper,from the perspective of foaming behavior and performance regulation of polymer blends,a series of studies were carried out on the sc-CO2 foaming behavior and mechanical properties of miscible and immiscible polymer blends with different general polymers(including polyethylene(PE),polypropylene(PP),polystyrene(PS),and polyvinyl chloride(PVC))as the matrix.Furthermore,a new strategy for constructing thermoplastic composite foams with excellent ablation resistance and flame retardancy was designed from the perspective of blending.The main contents of this thesis are concluded as follows:1.Rigid PVC foam was prepared through sc-CO2 foaming by blending polymethyl methacrylate(PMMA)and epoxy resin(EP)with PVC matrix,respectively.It was found that PMMA had good compatibility with the PVC matrix.PVC/PMMA blends could obtain foam materials with complete cell structure through the sc-CO2 process.With the increase of PMMA content,the optimal foaming temperature of the blends showed an upward trend.The expansion ratio of PVC/PMMA foaming materials was up to 5,but due to the high foaming temperature,the materials appeared to decompose and discolored to a certain extent.While maintaining good compatibility with the PVC matrix,EP could play a good role in plasticizing the PVC matrix.When the EP content was above 50.0 wt%,the Tg of PVC/EP blends was reduced from 85℃ of pure PVC to below 30℃.The processability of the PVC matrix was greatly improved.Under the same sc-CO2 foaming condition,the expansion ratio and cell size of PVC/EP blends increased with the increase of EP content.The cured PVC/EP(50/50)foam with NH3 had a gel content of 82.8 wt%,its density was 0.17g cm-3.The compressive strength of the foam was close to 3.0 MPa(under 30%deformation).The above results indicated that the rigid PVC foam with light weight and good cell morphology could be prepared by plasticizing the PVC matrix with EP,foaming,and finally curing.2.Compared to miscible polymer blends,immiscible polymer blends showed significantly different foaming behavior.The effect of one-dimensional carbon nanotubes(CNTs)on the phase structure and foaming performance of immiscible LLDPE/PS blends during batch foaming of sc-CO2 was studied.It was found that the phase structure of LLDPE/PS(50/50)blends with CNTs could be transformed from cocontinuous to sea-island morphology,when a sufficient quantity of CNTs nanofillers was added.As a result,the foaming ability of LLDPE/PS(50/50)blends was improved effectively.This was because the phase interface of the island structure was closed,which prevented the escape of CO2 from the phase interface of the blends.Meanwhile,the critical value of CNTs for significantly improving the foaming ability of blends decreased slightly with the increase in foaming temperature.In contrast,for the LLDPE/PS blends(such as 80/20)with sea-island phase structure,the addition of CNTs did not significantly change the morphology and foaming capacity of the blends.Furthermore,the foam with a semi-open cellular structure(meaning that there were small pores within the large cell wall)was obtained in the foamed LLDPE/PS blends after the tetrahydrofuran(THF)etching PS phase.The size of the small pore on the cell wall could be adjusted by the content of the PS dispersed phase in the LLDPE/PS blends and LLDPE/PS/CNTs nanocomposites.It was found that foaming temperature,PS phase content,and the presence of CNTs had an important influence on the mass loss rate and relative mass loss in the solvent etching process of foaming samples.3.The toughening of cycloolefin copolymer(COC)materials by POE and EPDM elastic particles and the foaming behavior of toughened COC blend materials were investigated.It was found that the phase structures of COC/POE(EPDM)blends were all island phase structures when the mass ratio was 90/10.The POE(or EPDM),as the dispersed phase,showed good dispersion in the COC matrix with an average phase area size of～1 μm.There is a strong interfacial adhesion between the dispersed phase particles and the COC matrix.Compared with the pure COC material,the tensile elongation at the break of COC/POE(90/10)and COC/EPDM(90/10)blends was improved while the rigidity was maintained.The impact strength of blends was significantly improved,and the toughness greatly improved.Moreover,the dielectric constant and dielectric loss of both blends were still low.The sc-CO2 foaming behavior of the pure COC material and its blends was investigated.It was found that the blends had higher foaming performance compared with the single COC material.In particular,the optimal expansion ratio of COC/EPDM(90/10)foams increased from 11.7 to 15.4 and had lower cell size and higher cell density.In addition,the COC/EPDM(90/10)blend maintained stable and good foaming performance in the temperature range of 170℃ to 200℃.This was mainly because the dispersed phase EPDM provided enough heterogeneous nucleation sites in the foaming process to improve the density and porosity of the pores.On the other hand,the EPDM and COC matrix had strong interfacial adhesion,showing a certain degree of stretching orientation along the pore wall direction,which reduced the degree of pore tearing at high temperatures to a certain extent.4.A new strategy for rapidly forming a three-dimensional network porous charlayer during combustion of PP composite foam was demonstrated,based on the synergistic effect of the intumescent flame retardant(IFR)with two-dimensional filler expanded graphite(EG)and one-dimensional filler carbon nanotubes(CNTs).The designed PP/10EG/5CNTs/25IFR composite foam showed good flame retardancy,excellent ablation resistance,and excellent thermal insulation in both small-scale and large-scale flames.The mechanism for improving flame retardancy and ablation resistance was contributed by the formation of a complete porous char layer on the surface of the PP/EG/CNTs/IFR composite foam during the combustion.The porous char layer completely inhibited the collapse and droplet phenomenon and provided an ability for the composite foam to maintain the whole shape during combustion.It was found that among three components,the IFR quickly expanded and formed char residue during combustion and improved the flame retardancy of PP composite foams in the case of small flame(LOI and UL-94).The EG aggregates violently expanded during combustion and offset the shrink of cellular structure,which blocked the transfer of heat from outside to inside and protected the inner material from the fire.At the same time,the CNTs bound expanded EG aggregates tightly through the interaction between CNTs and EGs.Very interestingly,the IFR was found to form char residue on the surface of CNTs/expanded EG aggregates to not only improved the compactness of the porous char layer but also promoted the thermal oxidation resistance of this char layer.It is believed that this strategy can be applied to improve the flame retardancy and excellent ablation resistance of other thermoplastic polymer composite foams.