Melt Processing of Metastable Acrylic Copolymer Carbon Precursors

Abstract

This thesis is concerned with the development of engineering technologies that facilitate melt spinning of carbon fiber precursors in both an environmentally sound and cost effective manner. More specifically, methods were developed to avoid a degradative process in acrylonitrile copolymers (typically used in textiles and as carbon fiber precursors) that occurs as melt spinning temperatures are approached. The following set of analyses was developed to define the rheological properties required for a melt processable acrylic copolymer suitable for use as a carbon fiber precursor, and accordingly facilitated development of a processing window: measurement of steady shear viscosity as a function of both temperature and time, measurement of the magnitude of the complex viscosity (|η*|) as a function of temperature using a temperature sweep, and measurement of the angular frequency dependence of |η*|. Through a systematic screening process, the following properties were identified to afford melt spinnable acrylic precursors suitable for conversion to carbon fibers: emulsion polymerization, 85-88 mole % acrylonitrile, 11-14 mole % methyl acrylate, 1 mole % acryloyl benzophenone, intrinsic viscosity < 0.6 dL/g, steady shear viscosity ≤ 1000-2000 Pa*s at a shear rate (γ) of 0.1 s⁻¹, viscosity increases ≤ 45% over a period of 1800 seconds at 200-220°C and γ=0.1 s⁻¹. Use of the rheological analyses assisted in development of a melt spinnable carbon fiber precursor, which resulted in carbon fibers possessing a tensile strength and modulus of approximately 1.0 and 120 GPa, respectively. A second approach was evaluated using carbon dioxide (CO₂) to plasticize AN copolymers to an extent that facilitates processing at reduced temperatures, below where thermal degradation is significant. A batch saturation method to absorb CO₂ in AN copolymers was developed. Differential scanning calorimetry and thermogravimetric analyses were used to measure the glass transition temperature (Tg) reduction and amount of absorbed CO₂ (respectively). A pressurized rheometer and measurement procedure was designed to obtain viscosity measurements of saturated AN copolymers. Up to 6.7 wt. % CO₂ was found to absorb into a 65 mole % AN copolymer with the saturation method used, resulting in a 31°C glass transition temperature (Tg) reduction, 60% viscosity reduction, and 30°C potential processing temperature reduction. It was found that CO₂ can absorb into copolymers containing up to 90 mole % AN (with the absorption methods used) with the following results (for a 90/10 mole % AN/MA copolymer): 3.0 wt. % uptake, 27°C Tg reduction, 56% viscosity reduction, and potential processing temperature reduction of 9°C. Via estimates of the required pressure, sealing fluid flow rate, and length of a pressure chamber to prevent foaming of the saturated polymer melt during extrusion, melt spinning of saturated AN copolymers appears feasible.