Simulation And Experimental Study On Preparing Rigid Polyurethane Foams By Compressed Carbon Dioxide

Preparing rigid polyurethane(PU)foams by compressed carbon dioxide(CO2)is a foaming process coupling with curing reaction.To prepare polyurethane foams with favorable morphology,it is of great importance to investigate polyurethane curing reaction in the atmosphere of compressed CO2 and the effect of the physical properties varied with reaction on bubble nucleation and bubble growth.In this paper,the PU isothermal curing kinetics was established,the influence of physical properties on the foaming process was analyzed numerically and the key influence factors on PU cell morphology was investigated experimentally.At first,the isothermal curing reaction between the polyether polyol and the polyaryl polymethylene isocyanate was investigated by high-pressure differential scanning calorimeter method in the range of 40-100? and 0.1-9 MPa CO2 pressures.The results showed that PU curing reaction in high-pressure CO2 atmosphere is accelerated and had higher final curing degrees compared with those in high-pressure helium atmosphere.With the rising of CO2 pressure,both PU curing rate and final curing degree increase.The apparent activation energies obtained by diffusion-amended autocatalytic kinetic Kamal model or calculated by isoconversional method are lower at higher CO2 pressure,which further confirms the promoting effect of compressed CO2 on PU curing reaction.Secondly,the bubble nucleation and growth were simulated based on the classical nucleation theory and cell growth model coupled the curing kinetics and physical properties varied with curing degree.The results showed that the increase of viscosity and relaxation time caused by larger curing degree result in bigger cell density and smaller average cell diameter,and it could be inferred that viscosity plays a greater role in the foaming process than relaxation time.The decrease of diffusion coefficient and surface tension as well as the increase of Henry constant all increase cell density and decrease average cell diameter.Finally,for PU system of polyether polyol and the polyaryl polymethylene isocyanate,the influence of curing degree,foaming temperature,CO2 pressure and release rate of pressure on cell morphology were studied by rapid depressurization foaming method.As curing degree increases,cell density firstly increases and then decreases,the average cell diameter always decreases,and it is owing to the change of viscoelastic properties,Henry constant,diffusion coefficient and surface tension with curing reaction.With the increase of foaming temperature,cell density decreases and average cell diameter increases.With the rising of CO2 pressure,cells density increases and average cell diameter decreases.This is because higher CO2 pressure and temperature not only changes the physical properties of PU system,but also accelerates the curing reaction.Under the foaming condition of 40 ??9 MPa CO2 pressure and 9 MP/s depressure rate,PU foams with more favorable morphology of cell density of 2.83×106 cells/cm3 and average cell diameter 48.86 ?m can be obtained.