2002 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, 2002 through December 31, 2003
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 would ordinarily 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 rapid saturation of the acrylic copolymers with CO2; and (3) optimize the copolymer structure to accelerate the saturation process.
It has been found that CO2 can be adsorbed into acrylonitrile copolymers and serves to plasticize them leading to a reduction in the 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 dimethyl-formamide (DMF) or dimethylacetamide (DMAC) to prevent them from crosslinking and cyclizing. When introduced at elevated temperatures and pressures, the gas can diffuse into the acrylic copolymers, resulting in a reduction of Tg. Copolymers of acrylonitrile (AN) and methyl acrylate (MA) have been adsorbed with CO2 and tested via thermal analyses (discussed in detail below). AN copolymers containing between 65 and 88 mole percent AN have all been shown to adsorb CO2.
As measured by differential scanning calorimetry (DSC), it has been found that up to a roughly 25°C reduction in Tg can be obtained by adsorption of approximately 5 weight percent CO2, confirmed using thermogravimetric analyses (TGA). This was confirmed in copolymers containing up to 88 mole percent AN. The reduction in Tg obtained from CO2 adsorption should result in a viscosity reduction and the possibility of processing these polymers at lower temperatures where degradation is insignificant.
To confirm that the lowering of the Tg truly leads to a viscosity reduction at a given temperature, viscosity measurements were conducted in a capillary rheometer, which was modified to facilitate pressurizing of the acrylonitrile copolymers containing adsorbed CO2. Traditional measurement methods are not suitable, because a static pressure must be applied during the rheological testing to prevent the gas from foaming the polymer upon exiting the capillary. This is accomplished by using a pressurized chamber modified to collect the extrudate during testing. Two copolymers have been tested, one containing 65 mole percent AN, and the other containing 88 mole percent AN. Both materials exhibit a significant viscosity reduction of roughly 35 percent across the range tested. The Williams Landel Ferry (WLF) equation has been used to back calculate estimates of approximately a 20°C reduction in processing temperature for the 88 mole percent AN copolymer. High levels of adsorbed CO2 could lead to further reductions in the temperature at which the AN copolymers can be processed and thereby avoid degradation problems.
Further studies are being conducted to obtain information pertaining to the ability to absorb higher levels of CO2 into the AN copolymers and reduce the Tg even further. The initial method of adsorbing CO2 into the sample is inadequate for generating higher levels of CO2. A method for measuring the viscosity reduction of plasticized acrylonitrile copolymers in a continuous process has been designed. A high-pressure liquid pump is used to inject CO2 into a single-screw extruder with a two-stage screw. A slit die attached to the extruder facilitates viscosity measurements. The results will be confirmed by means of viscosity measurements in the modified capillary rheometer. These data are needed to complete the design of the pressure chamber required to prevent foaming of spun fibers upon exiting the spinneret.
Future activities include: (1) developing a procedure for increasing the level of CO2 in the AN copolymers; (2) confirming that increased levels of CO2 lead to further reduction in the Tg and processing temperature; (3) designing a static step-down chamber, which allows us to extrude the AN copolymers without significant foaming; and (4) using the above information to design the step-down chamber, which allows continuous spinning of AN copolymer fibers.