Abstract：Fourier infrared spectrum was used to differentiate raw lac, bleached las and lac mixed with resin. The results indicate that the infrared spectra of raw lac and lac mixed with resin are similar, so the authors can differentiate them by the following items: (1) the position and the shape of absorbing peak between 3 500 and 2 500 cm-1, (2) the difference of the vibration peak at 1 700 cm-1 for CO，(3) characteristic peak between 1 200 and 1 400 cm-1, (4) the number of peaks and the change in the position of peak. But with bleached and graft copolymerization lacs the properties have been changed by chemical processing． Graft copolymerization of lac is made from methyl-acrylic acid-methy fat by using potassium permanganate as stimulant. Their main differences are： the change of vibration peak of 1 700 cm-1 for CO, a sharp small peak at 1 558 cm-1, a peak with middle strength at 681 cm-1, and spliting to two small peaks at 1 151 cm-1. The experiment shows that the method of infrared spectrum is more scientific and effective than traditional method.
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