| Poly(ethylene terephthalate)(PET) is an important polymer for significant practical use in the field of engineering plastic. Besides excellent properties, light-weight is a new demand for engineering plastic. Thus, PET foams with both low density and high mechanical performance has been drawing great interests. Traditional PET is a sort of linear polymer with relative low molecular weight and narrow molecular weight distribution, and is characterized by its insufficient melt strength which is not compatible with foaming process. During bubble growth stage, bubble walls tend to fracture and bubbles tend to collapse. PET matrix, therefore, should be modified to improve its melt strength. Besides, PET has high affinity with moisture, and quite easy to degrade when exposed to heat and oxygen, making molecular weight and melt strength decrease sharply and thus, difficult to be foamed. In this study, modified PET with excellent foaming properties has been successfully prepared. Based on the determination of solubility of CO2in the modified PET melt and surface tension of the modified PET melt in CO2, cell nucleation in CO2foaming process has been simulated. A novel integrated process of supercritical CO2assisted melt polycondensation modification and foaming of PET is proposed, and the feasibility and advantage of this integrated process have been confirmed. The detailed works are as following:In-situ preparation of foamable high melt strength modified PET. Acid M and alcohol N are selected as modifying monomers for the preparation of modified PET with long chain branching structure. The results show that the melt strength of modified PETs has been improved effectively. When the amount of alcohol N is0.3wt%and acid M is0.8wt%, the intrinsic viscosity (Ⅳ) of the modified PET can be increased to0.860dL/g and0.865dL/g, respectively as well as their melt flow index (MFI) can be decreased to15.1g/10min and15.0g/10min. Differential scanning calorimeter (DSC) tests reveal that the crystallization temperature T., enthalpy of crystallization△Hc, the melting temperature Tm and enthalpy of melting△Hm of modified PETs are all somewhat decreased due to the presence of the branched structures. Rheological test demonstrates that the complex viscosity, the storage modulus and loss modulus all increase compared with unmodified PET. A rapid depressurization batch molten PET foaming test using CO2as blowing agent is carried out and it is found that both modified PETs can be foamed in the temperature range of265-280℃. PET foams with average cell diameter of35～57μm, cell density of106-107cells/cm3can be successfully prepared, whereas traditional PET cannot be foamed under the same condition. In addition, the average cell diameter of alcohol N modified PET foam is smaller than that of acid M modified PET.Solubility of CO2in PET melt. CO2solubility in PET is studied using magnetic suspension balance (MSB) combined with high-temperature and high-pressure view cell. The former is used to obtain apparent solubility and the latter to determine the swelling volume through direct observation in the presence of CO2, which is necessary to correct the gas buoyancy acting on PET melts in the MSB measurement. The effects of temperature and CO2pressure on the swelling ratio and CO2solubility in traditional and modified PET melt are investigated, respectively. The results show that swelling ratio of PET and CO2solubility both decrease with increasing temperature and increase with CO2pressure, however, the swelling ratio increases slowly and approaches a plateau region under higher pressure. Compared with traditional linear PET, modified PET displays both lower swelling ratio and solubility. Solubility value of CO2in PET melt has an order of magnitude of10-2(g CO2/g PET melt) at4～6MPa CO2pressure in the temperature range of250～280℃. and confirms with Henry’s law within the pressure range of1～6MPa. S-L EOS is adopted to fit the swelling ratio in the CO2pressure range of1～14MPa, and CO2solubility at higher pressure is also predicted.Surface tension of PET melt in CO2. Using the Axisymmetric Drop Shape Analysis-Profile method, surface tension of PET melt in CO2at temperatures of250to290℃and CO2pressure of0to14MPa is measured using high-temperature and high-pressure view cell. It is found that surface tensions of modified PET melt decrease with increase of either temperature or CO2pressure, and with CO2pressure increasing, the dependence of surface tension on temperature weakens. Modified PET shows a higher surface tension than traditional linear PET due to the more conformation restriction exhibited by its branched structure. Based on experiment, an empirical equation is proposed to predict the surface tension of modified PET in CO2.Simulation and analysis of cell nucleation in modified PET foaming process. Classical nucleation model is adopted with the introduction of the correction factors both for the free energy barrier (F) and the Zeldovich factor (f0). With only F factor introduced, it is found that F value increases with saturation pressure once the foaming temperature is fixed, and when saturation pressure is fixed. F value increases linearly with temperature. With both F and f0introduced, it is observed that under the three foaming temperatures, fo values all have an order of magnitude of10-21and F values have an order of magnitude of10-5～10-4. When fo is fixed at10-21. it is found that F value also increases with saturation pressure and linearly increases with temperature.Integrated process of supercritical CO2assisted melt polycondensation modification and foaming of PET. The influences of CO2flow rate, CO2pressure and treating time on melt strength of the modified PETs and cell morphologies of foamed PETs are investigated respectively. It is proved that compared with N2sweeping, CO2sweeping can effectively enhance the melt polycondensation of preliminary modified PET. The viscosity of the modified PET increases with CO2flow rate, CO2pressure and CO2treating time. It is found that foam density will decrease with increasing CO2flow rate for the same treating time while it will increase with increasing treating time due to thermal degradation. The influence of CO2pressure on the foam density is non-monotonic. For our experimental system, PET foams with an average diameter of32～62μm and cell density of1×107～4×107cells/cm3can be obtained under the saturation pressure8-14MPa, CO2flow rate3-5L/min, treating time30～50min and temperature280℃. Meanwhile,"one step" foaming method and separated process of supercritical CO2assisted melt polycondensation modification and foaming are carried out for comparison, and the results demonstrate that better cell morphologies can be obtained from the integrated process within the same or shorter time. |
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