Article pubs.acs.org/IECR
Solid-State Polymerization of Poly(trimethylene terephthalate): Reaction Kinetics and Prepolymer Molecular Weight Effects Young Jun Kim,†,‡ Jaehoon Kim,*,† and Seong-Geun Oh‡ †
Clean Energy Research Center, National Agenda Research Division, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea ‡ Department of Chemical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea S Supporting Information *
ABSTRACT: The reaction kinetics and the effects of prepolymer molecular weight on the solid-state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) were investigated using nitrogen as the sweep fluid. The synthetic conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts (1,3-propanediol and water), so that the reaction kinetics were controlled by the forward chain extension reaction. Higher forward reaction rate constants were consistently obtained for the SSP of the high-molecular-weight prepolymer, compared to that of the lowmolecular-weight prepolymer. The activation energy of the polymerization reaction was determined to be 15−26 kcal/mol, depending on the prepolymer molecular weight. The slower reaction rate for polymerization of the low-molecular-weight prepolymer may be attributed to the inhibition of the chain-end mobility due to the higher crystallinity and larger lamellar thickness of the obtained polymers. In addition, the high concentration of carboxylic end groups in the low-molecular-weight prepolymer may also decrease the reaction rate by preferential transesterification between 3-hydroxyl propyl end groups over the esterification reaction between 3-hydroxyl propyl and carbonyl end groups. High-molecular-weight PTT with an intrinsic viscosity of 2.05 dL/g (that corresponds to a number average molecular weight of 57600 g/mol) can be obtained via the SSP of the high-molecular-weight prepolymer at a relatively low temperature of 190 °C under the conditions in which byproduct diffusion resistance is eliminated.
1. INTRODUCTION Poly(trimethylene terephthalate) (PTT) is an important engineering polymer with unique properties such as excellent chemical stability, strain resistance, facility of dyeing, splendid elastic recovery, and softness when fabricated into fibers; thus, it is widely used in various textile and carpet applications.1 PTT was commercialized in the late 1990s by Shell Chemical Co. and DuPont under the brand names of Corterra and Sorona, respectively. Commercial-grade PTT is conventionally synthesized by a two-step melt-polymerization process. The first step involves esterification of terephthalic acid (TPA) and 1,3propanediol (PDO) 2 or transesterification of dimethyl terephthalate (DMT) and PDO3 in the presence of titaniumbased catalysts to produce bis-2-hydroxypropyl terephthalate and/or low-molecular-weight polyesters. In the second step, melt polycondensation of the products synthesized from the first step is carried out in vacuo at high temperatures to remove the reaction byproducts (water and PDO). Melt polycondensation suffers from the limitation of extremely high melt viscosities,4 which gives rise to numerous problems including difficulty in removing the byproducts, which often limits the attainable molecular weight of the polyester, and the requirement of a specially designed reactor (e.g., disk-ring reactor) to provide a large liquid surface area and the application of high vacuum for rapid byproduct removal. Attempts to decrease the melt viscosity by increasing the reaction temperature (typically in the range 260−280 °C) often result in chain degradation and consequent deterioration of the molecular weight of the polymer. © 2012 American Chemical Society
Solid-state polymerization (SSP) can be a very promising alternative to melt polycondensation in the synthesis of highmolecular-weight PTT at relatively low temperatures (typically less than 230 °C). In a typical SSP process, a low-molecularweight polymer (prepolymer) is first synthesized by melt esterification or melt transesterification at low temperatures. The prepolymer is then ground or pelletized and is crystallized to prevent particle agglomeration during SSP, with subsequent heating to a temperature above the glass transition temperature (Tg) and below the melting temperature5 of the prepolymer. The reaction byproducts, water and PDO, are removed by the flow of a sweep fluid (e.g., inert gases) or by applying vacuum. It has been demonstrated that SSP at temperatures of 200−225 °C produced a significant increase in the molecular weight of PTT to ∼20000 g/mol (number average molecular weight, Mn), which is suitable for fiber applications. The temperatures that can be used in SSP are much lower than those of melt polycondensation,4 and thus, the high reaction temperatures and the specialized reactor systems associated with melt polycondensation can be avoided by using SSP to produce PTT. In addition to PTT, SSP has been widely used to produce various important engineering plastics including poly(ethylene terephthalate),6 poly(butylene terephthalate),7 poly(bisphenol A carbonate),8 nylon 6,9 high-temperature nylons,10 and Received: Revised: Accepted: Published: 2904
November 15, 2011 January 26, 2012 January 27, 2012 January 27, 2012 dx.doi.org/10.1021/ie202635z | Ind. Eng. Chem. Res. 2012, 51, 2904−2912
Industrial & Engineering Chemistry Research
Article
Table 1. Experimental Synthesis Conditions and Properties of Prepolymers esterification prepolymer PTTP1 PTTP2 PTTP3 a
stage 1 25−230 °C, 1.5 h, 0.10−0.26 MPa, TBT: 20 ppm 25−250 °C, 1.5 h, 0.10−0.26 MPa, TBT: 20 ppm 25−250 °C, 1.5 h, 0.10−0.26 MPa, TBT: 20 ppm
stage 2
polycondensation
230 °C, 8.5 h, 0.26 MPa 250 °C, 4 h, 0.26 MPa 250 °C, 4 h, 0.26 MPa
230 °C, 1 h, N2, 580 mL/min, TBT: 50 ppm 250 °C, 1 h, N2, 580 mL/min, TBT: 50 ppm 250 °C, 8 h, N2, 580 mL/min, TBT: 50 ppm
IV (dL/g)
Tm (°C)
crystallinity (%)
COOH concn (μmol/g)
OH concna (μmol/g)
0.121
207.96
53.4
41
872
0.205
222.72
53.6
12
521
0.416
222.18
56.2
14
209
OH end group concentrations were estimated by subtracting the COOH concentrations from C0 values.
poly(ethylene naphthalate).11 Theoretical modeling studies have also been undertaken in order to provide deeper insight into the behavior of industrially relevant polymers such as BPAPC,12 nylon,13 and PET14 during SSP. The overall rate of SSP depends on the following four chemical or physical steps: (1) functional end-group diffusion in the amorphous region of the polymer matrix (2) the intrinsic reaction kinetics of the chain extension reaction via collision of functional end groups (3) internal diffusion of the reaction byproducts through the polymer matrix to the polymer surface (4) external diffusion of the reaction byproducts from the polymer surface into the sweep gas phase Depending on the prepolymer properties (e.g., molecular weight, particle size, crystallinity, catalyst concentration, and end-group ratio) and on the reaction conditions (e.g., sweep fluid flow rate, temperature, and pressure), the SSP reaction rate and the obtainable molecular weight are controlled by one or more of these steps. For example, we showed that both the prepolymer property and the reaction conditions had a significant effect on the SSP rate of BPA-PC.8b−d In addition, the intrinsic SSP rates that were obtained by eliminating any influence of either internal byproduct diffusion or external byproduct diffusion were significantly dependent on the prepolymer crystallinity and on the prepolymer molecular weight.8c,d Notwithstanding the numerous potential advantages of using SSP to produce PTT compared with the conventional melt polycondensation, only a few studies on the former have been reported to date.4,15 Moreover, a detailed study of the intrinsic reaction kinetics for SSP of PTT has not been undertaken so far. In the previous studies, internal diffusion and external diffusion of byproducts were not decoupled from the chain extension reactions. Duh4 studied the SSP of PTT using prepolymers with intrinsic viscosities (IVs) of 0.445−0.660 dL/ g and with particle sizes of 1.89−1.92 g per 100 pellets (g/100). The IVs of the final polymer after 22 h SSP were 1.35−1.65 dL/g, which corresponded to an Mn of 35000−45000 g/mol (estimated based on the Mark−Houwink equation, [η] = 3.13 × 10−4 Mn0.80). The apparent reaction rate constants were in the range 2.05−2.39 × 10−3 (μmol/g)−1·h−1 at reaction temperatures of 220−225 °C, depending on the prepolymer molecular weight. Kim et al.15 used commercialized PTT chips with IVs of 0.93−1.02 dL/g (Mn of 21900−24600 g/mol) and with particle sizes of 2.5−3.2 g/100 as prepolymers for the SSP of PTT. The use of prepolymers with initial high molecular weights in the SSP process produced high-molecular-weight PTT at relatively short reaction times; for example, after 9 h SSP, the IVs of PTT increased from the initial 0.93−1.02 dL/g up to 1.48−1.63 g/
dL (Mn of 39200−44200 g/mol).15 The apparent reaction rate constants reported (11.47−14.27 × 10−3 (μmol/g)−1·h−1 at 220 °C) were much higher than those reported by Duh. In this study, we investigated the intrinsic reaction kinetics of the SSP of PTT with nitrogen as the sweep gas with careful control of the SSP experimental conditions to eliminate any influence of either internal byproduct diffusion or external byproduct diffusion on the reaction kinetics. The forward rate constants were determined at temperatures of 170−190 °C using the approach presented by Duh,4 and the activation energy was derived. Lastly, the effects of the prepolymer molecular weight on the SSP rate were discussed on the basis of the polymer properties including melting temperature,5 crystallinity, and end-group species.
2. EXPERIMENTAL SECTION 2.1. Materials. 1,3-Propanediol (PDO, purity of >99.6%) was purchased from Sigma-Aldrich (St. Louis, MO). Terephthalic acid (TPA, purity of >99%) and o-cresol were purchased from Junsei Chemicals (Tokyo, Japan). Titanium(IV) nbutoxide (TBT, purity of >98%) was purchased from Strem Chemicals (Mewburyport, MA). 1,1,2,2-Tetrachloroethane (TCE, purity of >98%), 1,2-dichlorobenzene (DCB, purity of >99%), phenol (purity of >99%), benzyl alcohol (BA, purity of >99.5%), and 0.025 N methanolic potassium hydroxide (KOH) were purchased from Daejung Chemicals & Metals (Siheung, Korea). Nitrogen (purity of >99.9999%) was purchased from Shinyang Sanso (Seoul, Korea) and was passed through a gas dehumidifier (model 20686, Restek, Inc., Bellefonte, PA) and an oxygen trap (model 20601, Restek, Inc., Bellefonte, PA) before introduction into the reactor. 2.2. Prepolymer Synthesis. PTT prepolymer was synthesized by the esterification and subsequent polycondensation of PDO and TPA using TBT (2 wt % solution in PDO) as a catalyst. The prepolymerization was carried out in a custombuilt, 1200 cm3 high-pressure reactor. The reactor was equipped with a heat furnace, magnetic stirrer, catalyst injection port, and sweep gas fluid line. The experimental conditions used for the prepolymer synthesis and the prepolymer properties are listed in Table 1. In a typical experiment, 206 g of PDO (2.7 mol) and 302 g of TPA (1.8 mol) were introduced into the reactor. The reactor was purged with N2 at a flow rate of 580 mL/min for 20 min, after which the temperature was increased from 25 to 80 °C with stirring at 900 rpm for 15 min. At this step, 20 ppm of TBT (based on PTT) was introduced into the reactor through the injection port and the reactor temperature was further increased to 230 °C (Table 1, sample PTTP1) or 250 °C (Table 1, samples PTTP2 and PTTP3). The total amount of catalyst used for the polymer synthesis was 70 ppm based on PTT. In the course of ramping temperature, the reactor pressure increased to 0.26 2905
dx.doi.org/10.1021/ie202635z | Ind. Eng. Chem. Res. 2012, 51, 2904−2912
Industrial & Engineering Chemistry Research
Article
prepolymers were measured using a potentiometric titrator (model 848 Titronic plus, Metrohm AG, Herisau, Switzerland). The polymer solutions were prepared by dissolving 0.3 g of each polymer in 12 mL of a mixture of o-cresol and benzyl alcohol (5:1 parts by weight). The COOH end-group concentrations were determined by potentiometric titration with 0.025 N methanolic potassium hydroxide solution.
MPa due to evaporation of the byproducts. The esterification reaction was allowed to proceed at 230 °C for 8.5 h (PTTP1) or 250 °C for 4 h (PTTP2 and PTTP3), and the reactor was depressurized to atmospheric pressure to remove water that formed during the esterification. The lower esterification temperature to synthesize PTTP1 may lower catalytic activity, leading to higher COOH and OH concentrations in the endgroup when compared to those of PTTP2 and PTTP3 (see Table 1). Polycondensation was then continued by allowing N2 to flow at 580 mL/min for 1 h (PTTP1 and PTTP2) or for 8 h (PTTP3) over the reaction mixture to remove PDO that formed as the byproduct of transesterification. By adjusting the reaction time and the reaction temperature, prepolymers with three different IVs were obtained (0.121 dL/g, PTTP1; 0.205 dL/g, PTTP2; 0.416 dL/g, PTTP3). 2.3. Solid-State Polymerization (SSP). Prior to SSP, the prepolymers were ground into powder and separated into two different particle sizes (
