Determination Of Vitrification Time Of Epoxy Resin Curing Process By In Situ Near Infrared Spectroscopy

Glass transition refers to a segmental relaxation of amorphous portion of polymer from a high elastic state to a glassy state or vice versa.The vitrification of epoxy resin curing process will seriously affect the quality of product.Accurate judging the vitrification phenomenon and its corresponding vitrification time is essential to guide curing process and avoid vitrification to achieve products with excellent performance.At present,vitrification time of curing process is usually determined by experience or laboratory methods such as differential scanning calorimetry before actual production.These results deviate from the actual vitrification time in production process,and the laboratory methods are cumbersome and time consuming.In-situ measurement of glass transition during curing process will aid in the effective control of the curing process.Near-infrared spectroscopy(NIR)origins from overtone and combination absorption of vibrations of hydrogen-containing groups.It can reflect composition and structure of substances at molecular level,and has the characteristics of fast and non-destructive,which is very suitable for in-situ online measurement.Therefore,in-situ measurement of vitrification time in the curing process of epoxy resin was carried out by NIR spectroscopy.The interaction between NIR light and polymer mainly include absorption and scattering.When glass transition happens,changes in molecular conformation and intermolecular forces may cause changes in molecular vibration absorption,while changes in physical properties such as volume and refractive index may cause scattering changes.These spectral changes should be used to determine vitrification time.However,there are also absorption changes caused by chemical reactions and scattering changes from liquid to solid state during the curing process.It is the main research content of this paper to study the appropriate spectral processing methods to separate the spectral information that can reflect the glass transition from these complex spectral changes,and then use this information to achieve accurate determination of vitrification time.First,a polyvinyl alcohol without chemical reaction during glass transition was selected to study a suitable spectral treatment method.Discrete-wavelet transform was used to decompose the temperature-increasing spectral signal of the polyvinyl alcohol film into signals of different frequencies related to noise,chemical group absorption and light scattering,respectively.It was found that the absorbance chemical groups showed a significant turning point with temperature.This turning point is considered to be caused by glass transition,where the temperature is the glass transition temperature.The result is consistent with that of DSC test.However,the light scattering signal is disordered with temperature and cannot be used to judge the glass transition.Maybe because the surface scattering of the film covers the scattering change caused by glass transition.In addition,the change sequence of hydrogen bonds and CH groups during the heating process and their relationship with glass transition were analyzed by two-dimensional correlation.Then,the in-situ NIR spectra of the curing process of epoxy resin with different curing temperature and with different content of hyperbranched polyphenylene ether resin was processed by discrete wavelet transform.It was found that the change of absorbance of no reactive CH group of benzene ring appeared a turning point,and where the time was considered as vitrification time.This result is consistent with that of DSC.The scattering signal can not accurately determine the vitrification time of the system.According to the above research,it can be concluded that the absorption signal of the chemical no-reactive group separated from the scattering information by the discrete wavelet transform can be used to accurately determine the vitrification time of the epoxy curing process.In addition,such absorption signal can also be used to determine the glass transition temperature of polymer.