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pour point depressant (PPD) is a kind of petroleum additive,the application of which is to be added in minute quantities to diesel to prohibit freezing under low temperature.In present,all kinds of PPDs in market are esters such as Polyvinyl Acetate Resin,polyacrylic ester and crotonates et al.These esters maybe have the same groups or structure and are hard to identify and analyze by existing testing method.As a new detection technology,Terahertz time-domain spectroscopy (THz-TDS) can effectively obtain amplitude,phase and structural information of the organism.Many attentions have been paid to THz-TDS studies of Petroleum products.