Microcellular Foaming Of Thermoplastic Polyimide With Supercritical CO₂

Polyimide (PI) is a kind of high performance engineering plastics which has been widely used in many high-tech fields due to its excellent heat resistant ability and matrix strength. The price of PI raw material has restrained its application in many industries, which imply that getting lighter weight of polymer by cells generation in matrix is significant and urgently. However, the traditional method of polymer foams preparation would utilize chemical blowing agents which might be unfriendly with environment, meanwhile, the foams were always generated with uneven distribution of cells. As a green physical blowing agent, supercritical carbon dioxide (ScCO2) is harmless and non-toxic, able to form stable structure in polymer, particularly suitable for the research of polymer foams.In this study, the foaming feasibility experiments of PI powder synthesized by tetracarboxy diphthalic ether dianhydride (ODPA) and diamino diphenyl ether (ODA) was conducted firstly. The results indicated bad foamability due to extraordinary stiffness of PI matrix, and hard to make cells nucleation and growth under ScCO2 foaming technique, meanwhile, it's invalidity of physical modification because of the poor solubility in many solvents. The PEI from SABIC shows favorable viscoelasticity and appropriate glass-transition temperature for foaming, revealing enormous potential in future market. The expansion ratio of PEI foams can reach to 3.5 through rapid pressure release method, and 0.7 ?m for cell diameter as well as 3.6×1012 cell/cm3 for cell density by temperature rising technique.The addition of co-blowing agent apparently expands the expansion ratio of PEI,8.7 times in water/CO2 system and 7.5 times in ethanol/CO2 system, respectively. The cell diameter increase dramatically to 47? with saturation temperature rising, while the cell density has reverse trend. Moreover, the foaming temperature decreased from 200? to 170? as the co-blowing agent was adding in. Foams which prepared by water/CO2 system show analogical thermal stability with raw material through thermogravimetry measurement, whereas the foams prepared by ethanol/CO2 system perform worse thermal stability than them.