a) Water absorption: refers to the ratio of the mass of water absorbed by the stomata of all openings to the mass of the dry sample;b) Air permeability: refers to the degree to which the product allows gas to pass under a pressure difference. Under a pressure difference of 10 Pa, the amount of air passing through the product layer with a thickness of 1 m and an area of 1m2 in 1hr is called the "air permeability coefficient". The high air permeability of the product will greatly accelerate the ability of corrosive gas to pass and accelerate the corrosion rate of the product. Therefore, the smaller the air permeability the better.