Application Of Raman Spectrum On Polymer Properties Detection

Polyethylene(PE), polypropylene(PP) and polyvinyl chloride(PVC) were the most significant downstream products of petrochemical industry. However, there were still several imperfect places in their industrial manufacturing and recycling process, like the ineffective product quality detection and system status monitoring. In order to remedy the defects of existing technologies, a novel detection method based on Raman spectrum was proposed. Raman spectrum could offer several advantages for solid and liquid analysis, including the fast acquisition, high accuracy, and non-destruction. In this work, density and melt index(MI) of PE, ethylene contents in PP and the concentration of PVC solution were measured by Raman spectrum. And the innovative results could be summarized as following.(1) The detection methods for PE density were studied. The prediction models for density were established by using characteristic peak analyzing method, principle component analysis(PCA) and partial least squares(PLS) method respectively. Meanwhile, PCA and PLS performed better than characteristic peak analyzing method, the correlation coefficient(r) between predictive densities and actual values were 0.925 and 0.919, the average relative error(ARE) were 0.53% and 0.45%, and the root mean square error(RMSE) were 5.67 and 5.06.(2) Density and MI of high density polyethylene(HDPE) were detected simultaneously based on Raman spectrum. By analyzing the spectra between 2700 and 2970 cm^-1, it was found that the amount of short branched chain had a negative correlation with HDPE density. The prediction model for density was obtained by PLS regression. The values for r, ARE and RMSE between predictive densities and actual ones were 0.944, 0.10% and 1.02, respectively. Knowing the positive correlation between the vinyl content and the MI value of HDPE, the spectra between 1288 and 1650 cm^-1 was used to build the MI prediction model. The values for r, ARE and RMSE between predictive MI and actual ones were 0.957, 10.23% and 1.06. Comparing with the performance of Near Infrared(NIR) detection, Raman spectrum gave a relative better result, which indicated that it's feasible to detect density and MI of HDPE by Raman Spectrum.(3) Ethylene content in PP was studied by Raman spectrum, combining with PLS method. Raman spectras for PE and PP were compared in this work. And several models for ethylene content prediction were set up, based on different range of Raman spectra respectively. According to PLS analysis, loadings of factor 1 could reflect the relationship between the composition of PP molecular chain and ethylene content, that was, ethylene content had a positive correlation with the amount of CH2, but a negative correlation with CH3, CH, and C-C. Although Raman spectrum gave a less precise prediction result than NIR, the values for r, ARE and RMSE between predictive ethylene contents and actual ones were 0.985, 4.48% and 0.56.(4) Quantitative determination of the concentration of PVC solution was studied. According to the comparison between the spectra for PVC and cyclohexanone, the characteristic peaks for PVC and cyclohexanone were found, including the C-Cl bond for PVC (620 and 695 cm^-1) and the alicyclic ketone for cyclohexanone (1709 cm^-1). By using internal standard method, a mathematics model for PVC content prediction was built. And the values for r, ARE and RMSE were 0.994, 11.03% and 3.15.The experiment results indicated that, it's feasible to use Raman spectrum to realize the fast and accurate detection of the properties of polymers and their solutions, which had a significant importance on on-line monitoring and controlling in the manufacturing and recycling of polymers.