DEVELOPMENT OF ON-LINE SENSOR FOR PROPERTIES OF MOLTEN POLYMER USING NEAR INFRARED SPECTROSCOPY

本项目开发了两种新型的近红外(NIR)在线传感器。一个用于监测熔融的聚乙烯聚合物的密度，另一个用于监测溶解在熔融聚合物中的二氧化碳浓度。密度和熔体指数是对商品聚烯烃聚合物分级的两个关键属性。为了保证质量，必须在生产工厂中尽可能精确地控制这些特性。然而，缺乏适合这些特性的在线传感器使得反馈质量控制变得困难。在本项目中，开发了一种基于近红外(NIR)光谱的在线密度传感器。用连接在单螺杆挤出机上的光纤装置采集了熔融聚氯乙烯流动过程中的近红外光谱。通过在1170 nm和1213 nm处的吸收强度之比，成功地测量了14个等级的聚乙烯的密度，通过在发泡挤出机上安装相同的近红外探头，在发泡挤出机中引入二氧化碳来制造聚合物泡沫，可以从近红外光谱中在线测量熔融聚合物中的二氧化碳浓度。推导出2019 nm吸收光谱与溶解气体浓度之间的校正曲线，从而从测量的近红外光谱中推断溶解CO2气体浓度。实验结果表明，该传感器最多可以检测出聚乙烯、聚丙烯和聚丁二酸丁二醇酯三种聚合物中的溶解二氧化碳浓度，说明近红外在线传感技术是聚合物生产过程中较有前景的质量控制方法之一。

Two new on-line sensors using near infrared (NIR) spectroscopy are developed in this project. One is for monitoring density of molten polyethylene polymer and the other is for monitoring CO2 concentration dissolved in the molten polymers.Density and melt index are two key properties in grading commercial polyolefin polymers. For quality assurance, these properties must be controlled as accurately as possible in the production plant. However, the lack of suitable in-line sensors for these properties makes feedback quality control difficult. In this project, an in-line density sensor using near infrared (NIR) spectroscopy was developed. The NIR spectra of molten pokyethylene in flow are collected by a fiber-optic device attached to a single screw extruder. By the ratio of the absorption intensity at 1170 nm to that at 1213 nm, the densities of 14 grades of polyethylene were successfully measured.By installing the same NIR probe at the foaming extruder, where carbon dioxide is introduced to make polymeric foams, concentration of carbon dioxide dissolved into the molten polymer could be measured on-line from the NIR spectra. The calibration curve relating the absorbance spectrum at 2019 nm to the dissolved gas concentration is derived so as to infer dissolved CO2 gas concentration form measured NIR spectra. Experimental reults show the developed sensor can successfully estimate dissolved CO2 concentration in at most three kinds of polymer, polyethylene, polypropylene and polybuthylene succinate.Those two developments illustrate that the NIR on-line sensing technique is among the more promising method for quality control of polymer production processes.