Reaction Kinetics and Process Model of the Polyacrylonitrile Fibers Stabilization Process Based on Dielectric Measurements.

Julia Hofele,Guido Link,John Jelonnek

doi:10.3390/ma15031222

Abstract

Microwave-based dielectric heating is a suitable method for energy- and time-efficient processes. Considering the energy required in the production of carbon fibers, it is evident that microwave-based dielectric heating during the different phases of the production needs to be considered too. Nevertheless, the dielectric properties of the processed material needs to be known for the design of an appropriate microwave applicator. When looking at the first stage in the production, the stabilization stage of the PAN fiber, the important data about the dielectric properties is very limited in literature. For this reason, first in-situ temperature-dependent measurements of the dielectric properties during the stabilization stage are presented. The impact of raising temperatures and chemical reactions on the dielectric properties of the heated PAN fiber is discussed. Secondly, the steps taken to set up the reaction kinetics from the dielectric loss point of view are given. This enables determination of the reaction degree as a function of the measured dielectric loss for the first time. The established correlation opens the potential for the application to processes such as an in-situ quality determination. The strong temperature impact on the process is shown, and reaction kinetics are analyzed accordingly. In a final third step, a heat transfer model is presented. It utilizes the evaluated reaction kinetics data and microwave heating, creating a first modelling approach for monitoring and controlling the desired fiber temperature, leading towards an online process.

Highlights

Carbon fibers are used in multiple lightweight applications due to their good physical properties [1]
Their tensile strength to density ratio is superior to other materials such as aluminum or steel
A spun PAN fiber gets stabilized in normal air and using a conventional oven at temperatures ranging from 200 ◦C up to 300 ◦C as a first step in the production process

Summary

Introduction

Carbon fibers are used in multiple lightweight applications due to their good physical properties [1]. Later in the process, when dielectric loss of the fibers is increasing with temperature, to some extent a direct microwave heating effect might take place. For designing a suitable microwave applicator, it is key to understand the dielectric properties of the material depending on chemical composition, temperature and chemical reaction. For this reason, dielectric measurements are presented first, followed by an investigation of the reaction kinetics in the stabilization stage from a dielectric point of view. To the knowledge of the authors, so far microwave heating has not been included in the modelling of the stabilization process, and no connection between reaction kinetics and dielectric properties was made. It and may help in design and development of appropriate microwave applicators and optimize process parameters that allow avoiding process instabilities due to thermal runaway effects

Dielectric Measurements with the Perturbation Method

Findings

Thermal Process Model