Chapter 11
Selected Aspects of Poly(ethylene terephthalate) Solution Behavior Application to a Selective Dissolution Process for the Separation of Mixed Plastics 1
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Leland M. Vane and Ferdinand Rodriguez School of Chemical Engineering, Olin Hall, Cornell University, Ithaca, NY 14853 The solution crystallization kinetics and crystal dissolution behavior of a commercial grade of poly(ethylene terephthalate) in N-methyl-2pyrrolidinone were characterized using turbidimetric and dilatometric methods. The importance of this information to a selective dissolution process for the separation of poly(ethylene terephthalate) from a 2-liter bottle waste was investigated. A relationship between the photosignal response data from the turbidimeter and the dilatometer extent-of– transformation data was developed which allows for the prediction of the crystallization rate constant using the more easily obtained turbidimeter data. In the United States and, indeed, the world, a ground swell of interest in the recovery and reuse of all post consumer materials has occurred. This public concern has motivated research into processes to separate mixed post-consumer plastics from the overall waste stream as well as from each other. Mixed plastics command a relatively low price compared to virgin resins primarily due to diminished physical properties resulting from polymer-polymer incompatibility, discoloration, and degradation. In addition, the use of mixed plastics in downgrading operations is not truly "recycling" since the material does not replace virgin resins. Using the closure of the recycling loop as a goal, interest has resurfaced in processes to separate mixed post-consumer plastics. Common methods for separating mixed plastics include air classification, hydrocycloning, flotation-sedimentation, depolymerization-purification-repolymerization, and selective dissolution. Of these, only selective dissolution and depolymerization are capable of removing bound impurities from the plastic as well as differentiating plastics based on the chemical properties of the individual polymers. These two methods should yield polymers suitable for reuse in original applications, although both suffer from increased expenses due to the complexity of equipment and higher energy requirements. Selective Dissolution The concept of using solvents to dissolve individual polymers selectively from a 1
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In Emerging Washington, Technologies in D.C Plastics20036 Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
mixture is by no means new, but it is certainly receiving renewed attention. Much of the original work on solvent processes occurred in the 1970's. In one of the original research efforts in solvent processes, Sperber and Rosen (7,2) separated a mixture of polystyrene (PS), polyvinyl chloride) (PVC), high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) into three separate phases using a blend of xylene and cyclohexanone. At about the same time, Seymour and Stahl (3) were studying the use of toluene and methanol at various temperatures in a branched scheme to selectively remove individual polymers from a mixture of PS, PVC, L D P E , poly(methyl methacrylate) (PMMA), and poly(vinyl acetate) (PVAC). Other researchers concentrated on the solvent recovery of individual polymers such as polypropylene (4). In addition, numerous domestic and foreign patents were granted in the 1970's for the solvent separation and purification of thermoplastic polymers (5-9). Interest in plastics recycling diminished in the late 1970's and early 1980's as natural resource concerns lessened and the public settled into carefree throwaway habits. The solid waste problems of the late 1980's and the volatility of the oil export regions has aroused the public and brought new life to plastics separations research (10-19). Research on solvent processes has been a part of this resurgence (10-14). Inclusion of P E T in Mixed Waste. Early research on the solvent separation of mixed plastics did not include PET in the mixture because this versatile and valuable polymer had not penetrated the packaging market. In addition, many of the solvents chosen for use in these processes were not adequately screened for their effects on health and the environment. It was, therefore, necessary to reconsider the concept of selective dissolution with the recovery of PET and the selection of more "friendly" solvents included as research goals. Recently, researchers at Rensselaer Polytechnic Institute have studied the separation of PVC, PS, L D P E , PP, HDPE, and PET using either xylene or tetrahydrofuran (72). The polymers are dissolved in batch mode with the separation based on the solvent temperature which controls the dissolution rate of each resin. Once dissolved, the polymer solution is exposed to elevated temperatures and pressures before the solvent is flash devolatilized. This process suffers from the same limitations which beset all single solvent systems. When only one solvent is used to dissolve a wide range of polymers, the selectivity is significantly decreased due to the unintended partial dissolution of polymer "A" when polymer "B" is the target polymer and also carryover of undissolved "B" when "A" becomes the target polymer. Relying solely on the temperature dependent dissolution rate of polydisperse and semicrystalline polymers significantly reduces the purification capacity of the process. A considerable improvement in selectivity can be achieved by using multiple solvents which are compatible with only a limited number of polymers. In this way, polymer dissolution is not just a function of the solvent temperature, but of the polymer-solvent interaction as well. Combining Separation Technologies. A n even more advantageous separation scheme is the combination of low-cost sink-float separation technology with a multi-solvent selective dissolution process. A sink-float process would serve as the first separation stage for shredded plastic waste, achieving segregation into two or more streams based on the densities of the materials as shown in Figure 1. The streams from the sink-float process are then further purified using solvent-processing trains with the solvents optimized based on the compositions of the sink-float product streams. A general flow sheet of a solution process is shown in Figure 2. The first stage of the solvent process involves the removal of soluble impurities from the stream using the process solvent at a slightly elevated temperature. This temperature should be kept low enough to prevent dissolution of the target polymer. The impurities to be removed in the Solvent Washing stage are materials such as adhesives, coatings, and any soluble non-target polymer. The remaining undissolved materials are then exposed
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
11. VANE AND RODRIGUEZ
2-liter Bottles
Aspects of PET Solution Behavior
Paper & Fines
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Polyolefins (to solvent processing)
Water/Detergent
PET, Aluminum (to solvent processing)
Dirty Water (to recycle)
Figure 1. Schematic diagram of sink-float process for 2-liter bottles.
Stream from Sink Float Process
to Solvent Recovery
Process Solvent Solvent Washing
(Soluble Impurities)
Target Polymer Dissolution
Solvent Recycle
Solution Purification ΒείϋΓηεηΐΕΐΐοη/Αοΐαϋοη/ΓιΙίΓΕίίοη/αάεοφϋοη
Polymer Recovery
Purified Polymer
Figure 2. Simplified flow diagram for a solution process.
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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1 5 0
EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
to the process solvent at a higher temperature than in the first stage, causing rapid dissolution of the target polymer, but leaving the remaining polymers undissolved. As the target polymer dissolves, impurities such as fillers, catalysts, pigments, and plasticizers which may be bound in the polymer matrix are released. These materials as well as the undissolved remnants of the original feed stream must be removed before the target polymer is recovered from solution. This purification can be achieved utilizing a variety of separation technologies including sedimentation, flotation, filtration, adsorption, and ion exchange. Insoluble impurities such as metals, thermosets, rocks, etc. are easily removed by filtration from the low viscosity polymer solution. This is not true for thefiltrationof these same materials from a polymer melt The selective dissolution process, therefore, has a significant advantage in the area of metals removal. The final stage in the solvent process is the recovery of the target polymer from the solution. This recovery can be achieved in many ways including temperature quenching of the solution resulting in polymer crystallization or precipitation, shock-precipitation by addition of a non-solvent, or flash devolatilization (polymer deposition). Addition of a non-solvent appears to be the most attractive because the polymer is recovered rapidly without the thermal degradation which can occur with flash devolatilization. In addition, judicious selection of the non-solvent and mixing conditions can result in a precipitate which is easilyfilteredand dried. P E T Processing Train. In one example of a combined technology process, postconsumer 2-liter bottle material is divided into polyolefin and poly(ethylene terephthalate) fractions using water in sink-float tanks followed by treatment of these fractions in solvent-processing trains. Research on the interactions of polyolefins with xylene (20-26) indicates that mis solvent would be suitable for use in the polyolefin solvent treatment train. Therefore, the selection and evaluation of solvents for the PET processing train has been the major focus of the present research (27). The selection of a process solvent for the PET train was based on the criteria of: PET-solvent compatibility, HDPE-solvent incompatibility, toxicity, cost, and ability to recover solvent for reuse in the process. The solvent of choice for PET is N-methyl-2pyrrolidinone (NMP). NMP is of low toxicity (28-30), is biodegradable (aerobic wastewater treatment), is easily recovered, and is commonly used in many processes (31). In addition, NMP does not appreciably dissolve HDPE, even at elevated temperatures (13, 27) although it does readily dissolve PET at these same temperatures. Role of the Crystalline Nature of P E T . In order to determine the optimum operating conditions for the PET processing train, the dissolution and crystallization behavior of PET in NMP must be analyzed. To this end, the dissolution rate of 2-liter bottle PET in NMP as a function of solvent temperature was determined from the mass loss of sections of 2-liter bottles exposed to NMP at various temperatures for a range of exposure times (27). As shown in Figure 3, NMP was found to rapidly dissolve PET sections of 2-liter bottles at temperatures greater than 130°C. In fact, at 160°C, the sections dissolved in approximately 5 minutes. Below 130°C, it was found that the sections swelled, but did not dissolve. This dramatic difference in dissolution behavior over a small temperature range is due to the presence of crystallites in the PET which have relatively distinct dissolution temperatures. The crystallites act as cross-links below the crystal dissolution temperature (T^), allowing the polymer to swell, but not to dissolve, despite the fact that the amorphous regions would prefer to be dispersed in the solvent. Once the PET sections are heated above Τ φ the crystallites dissolve and the polymer readily disperses into solution. This dissolution rate information implies that in the first stage of the PET
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF LEEDS on October 5, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch011
11.
VANE AND RODRIGUEZ
Aspects of PET Solution Behavior
151
processing train (Solvent Washing), NMP at temperatures as high as 130°C can be used to remove soluble impurities (adhesives, coatings, and PS, P V C , and polycarbonate (PC) contamination), while leaving the PET sections only slightly swollen. In the second stage (Target Polymer Dissolution), the PET chips can be mixed with recycled NMP at 160°C, bringing about the rapid dissolution of the sections without appreciably dissolving any HDPE which may have left the sink-float process in the the PET stream. The main impurities in the PET processing train will most likely be aluminum and HDPE. As mentioned previously, aluminum removal is easily achieved by filtering the low-viscosity polymer solution. This alleviates the need for low-efficiency melt filtration units or expensive eddy current separators which represent a large fraction of plant costs for non-solvent separation processes (32). Ideally, H D P E should not leave the sink-float unit in the PET stream. However, because the sink-float system is not 100% effective, small amounts of HDPE (and some PP) will be present. The advantage of using NMP in the PET processing train is that HDPE is only slightly soluble at elevated temperatures as indicated by the data in Figure 4. For example, at 160°C, only about 0.1 wt% of the HDPE will dissolve in 30 minutes. At this same temperature, PET sections of 2-liter bottles would be completely dissolved in about 5 minutes. The presence of PVC in recycled PET is also a concern because of the damage caused to injection molding equipment exposed to PVC at PET processing temperatures as well as color formation in the plastic. In a selective dissolution process using NMP, trace amounts of PVC should be removed in the Solvent Washing stage since P V C is quite soluble in NMP at 120°C. The later stages of the PET processing train (Solution Purification and Polymer Recovery) are also impacted by the crystalline nature of PET. If cooled below T
