The methods of measuring the closed cell content of phenolic foams were studied by densimeter.The system balance times were recorded under different pressures during the measuring processes

and the structural changes were analysed after the measuring processes combined with scanning electron microscopy in order to optimize the measuring pressure.The microstructure of different phenolic foams was characterized by scanning electron microscopy and the relationship between the closed cell content and microstructure of phenolic foams was further discussed.The samples after storing for different time periods underwent constant temperature treatment

and the closed cell content and microstructure of samples before and after treatmeat were compared and analyzed.The results demonstrated that the first prerequisite for obtaining the accurate results was to optimize the measuring condition and pressure for measuring the closed cell content of phenolic foams

the low closed cell content of phenolic foams was resulted from the flaw of foam microstructure (e.g. pinhole and fissure)

and the primary cause for the long measuring time due to the balance difficulty was the inhomogeneous cells in the microstructure and different aftertreatment procedures.