2003 Progress Report: Benign Processing of Polymers Plasticized with Absorbed Carbon DioxideEPA Grant Number: R829555
Title: Benign Processing of Polymers Plasticized with Absorbed Carbon Dioxide
Investigators: Baird, Donald G.
Institution: Virginia Polytechnic Institute and State University
EPA Project Officer: Richards, April
Project Period: January 1, 2002 through December 31, 2004 (Extended to July 31, 2005)
Project Period Covered by this Report: January 1, 2003 through December 31, 2004
Project Amount: $325,000
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Sustainability , Nanotechnology , Pollution Prevention/Sustainable Development
The objectives of this research project are to: (1) develop a melt spinning process for acrylic polymers, which ordinarily would have to be solution spun using toxic organic solvents, using supercritical carbon dioxide (CO2) as a plasticizer (i.e., replace the toxic organic solvents with benign CO2); (2) develop a technique that will allow for rapid saturation of the acrylic copolymers with CO2; (3) optimize the copolymer structure to accelerate the saturation process; and (4) design a chamber that prevents foaming of the extruded filaments.
It has been found that CO2 can be adsorbed into and serves to plasticize acrylonitrile (AN) copolymers, leading to a reduction in glass transition temperature (Tg). Plasticization leads to the reduction of viscosity of the acrylic copolymers at a given temperature. Presently, acrylic copolymers used in the manufacture of textile fibers and as precursors for carbon fibers must be solution spun in toxic solvents such as dimethylformamide or dimethylacetamide to prevent them from crosslinking and cyclizing. When introduced at elevated temperatures and pressures, the gas can absorb and diffuse into the investigated copolymers, resulting in a reduction of the Tg. Copolymers of AN and methyl acrylate, which traditionally are solution processed to produce acrylic fibers or carbon fiber precursors, have been adsorbed with CO2 and tested via thermal analyses. AN copolymers containing between 65 and 88 mole percent AN all have been shown to adsorb CO2. Although the diffusivity of CO2 in polyacrylonitrile copolymers is very low, the solubility is relatively high.
A Tg reduction of 31°C (measured using differential scanning calorimetry) and a CO2 uptake of 6.7 weight percent (measured using thermogravimetric analysis) were observed in the 65 mole percent AN copolymer by using a batch method to saturate the copolymer in a 17.237 MPa (2,500 psi) CO2 environment at 120°C for 6 hours. Pressurized capillary rheometry shows a viscosity reduction of 25 percent to 50 percent over the range of shear rates tested, increasing with weight percent of CO2. Weight percent uptake of CO2 was found to depend on the saturation conditions; however, pressure was limited to 17.237 MPa (2,500 psi) for batch testing because of equipment limitations. Even with this limitation, it has been found that a 6.7 weight percent uptake of CO2 yields a processing temperature reduction of roughly 30°C.
Our desired goal is to melt process high AN content copolymers. A measurement of viscosity and process temperature reduction in a continuous process is required to further verify the feasibility of this goal. A high-pressure liquid pump was used to inject CO2 into a single-screw extruder with a two-stage screw. Viscosity reduction of the 65 mole percent copolymer was measured using slit die rheometry, with 5-10 weight percent CO2 uptake measured using a Coriolis mass flow meter. A viscosity reduction of up to 50 percent was obtained over the range of shear rates tested. Further studies are to be conducted to investigate the viscosity reduction and processing temperature reduction in specially designed higher AN content copolymers.
To prevent the polymer from excessively foaming upon exit of the extrusion die, a pressurized step-down chamber has been designed and built to cool the polymer while under pressure to a temperature below its Tg. Once cooled, the polymer may enter atmospheric conditions as a solid fiber where it later can be processed into textile fiber or carbon fiber. The specifications for a cooling water pump as well as a method for incorporating it have been developed to allow this process to run continuously.
We will: (1) investigate the viscosity reduction and processing temperature reduction in higher AN content copolymers; (2) use the above information to tune the process to allow continuous spinning of high AN content copolymer fibers; and (3) investigate the effect of CO2 as a processing aid for promoting higher draw of fibers and hence improved mechanical properties.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 21 publications||6 publications in selected types||All 6 journal articles|
||Bortner MJ, Baird DG. Absorption of CO2 and subsequent viscosity reduction of an acrylonitrile copolymer. Polymer 2004;45(10):3399-3412.||
||Bortner MJ, Bhanu VA, McGrath JE, Baird DG. Shear rheological properties of acrylic copolymers and terpolymers suitable for potentially melt processable carbon fiber precursors. Journal of Applied Polymer Science 2004;93(6):2856-2865.||