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

本课题研制了两种新型近红外在线传感器。一个用于监测熔融聚乙烯聚合物的密度，另一个用于监测熔融聚合物中溶解的CO2浓度。密度和熔体指数是商用聚烯烃聚合物分级的两个关键性能。为了保证质量，必须在生产工厂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。本课题研制了一种基于近红外光谱的在线密度传感器。采用单螺杆挤出机连接的光纤装置对流动中的熔融聚乙烯进行近红外光谱采集。利用1170 nm处的吸收强度与1213 nm处的吸收强度之比，成功地测量了14种等级聚乙烯的密度。通过在发泡挤出机上安装相同的近红外探头，在那里二氧化碳被引入到聚合物泡沫中，溶解在熔融聚合物中的二氧化碳浓度可以通过近红外光谱在线测量。导出了2019 nm处吸光度光谱与溶解气体浓度的校准曲线，从而从测量的近红外光谱推断出溶解二氧化碳气体浓度。实验结果表明，所研制的传感器可以成功地估计出聚乙烯、聚丙烯和聚丁二酸丁二乙烯三种聚合物中溶解的CO2浓度。这两项发展表明，近红外在线传感技术是聚合物生产过程质量控制中最有前途的方法之一。

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.