Article pubs.acs.org/IECR
Solid-State Polymerization of Semiaromatic Copolyamides of Nylon4,T and Nylon-4,6: Composition Ratio Effect and Thermal Properties Young Jun Kim,† Kurnia Endah Yohana,†,‡ Hong-shik Lee,† and Jaehoon Kim†,‡,c,* †
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 ‡ Clean Energy and Chemical Engineering, University of Science and Technology, 113 Gwahangno, Yuseong-gu 305333, Daejeon, Korea c Green School, Korea University, 5-1 Anam Dong, Seongbuk-gu, Seoul 136-701, Republic of Korea S Supporting Information *
ABSTRACT: Solid-state polymerization (SSP) of semiaromatic copolyamides composed of nylon-4,T and nylon-4,6 in different molar ratios were investigated using a mixture of nitrogen and steam as the sweep fluid at reaction temperatures in the range of 220−260 °C. Prepolymers having different nylon-4,T contents (28.2, 37.9, and 47.8 mol %) were synthesized by melt polyamidation with stoichiometric ratios of diamine and diacid. The SSP conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts. Under these conditions, a high-molecular-weight semiaromatic copolyamide with an intrinsic viscosity of 2.469 dL/g was obtained upon SSP at 260 °C for 48 h. As the nylon-4,T content increased, the SSP rate decreased owing to the inherently low reactivity of the aromatic diacid and inhibition of chainend mobility. The glass transition, melting, and decomposition temperatures of the polymers synthesized from the higher-nylon4,T-content prepolymer were always higher than those of the polymers synthesized from the lower-nylon-4,T-content prepolymer. terephthalate) (PTT), 17,18 poly(butylene terephthalate) (PBT),19 poly(bisphenol A carbonate) (BPA-PC),20−24 poly(ethylene naphthalate) (PEN),25 and poly(L-lactic acid) (PLA).26−33 Theoretical modeling studies have also been undertaken to provide deeper insight into the behavior of industrially relevant polymers such as BPA-PC,34−36 nylon,37,38 and PET39−41 during SSP. Various SSP approaches have been tested to achieve highmolecular-weight, high-temperature polyamides, and some high-temperature polyamides such as aliphatic diamine− aliphatic diacid polyamides (nylon-6,6, Tm = 262 °C; nylon4,6, Tm = 289 °C) have been commercialized. However, the development of semiaromatic polyamides produced from aliphatic diamine−aromatic diacid (e.g., terephthalic acid and isophthalic acid) with enhanced thermal, chemical, and mechanical properties is still needed to meet market requirements.7,8,19 Semiaromatic polyamides composed of aliphatic diamine−aromatic diacid polymers, which include aliphatic diamines having six carbons or less, might not be commercially attractive materials owing to their extremely high melting temperatures (e.g., nylon-4,T, Tm = 428/475 °C; nylon-6,T, Tm = 370 °C), which makes it difficult for them to be meltprocessed (e.g., using an extruder), and their relatively high water absorption properties. For these reasons, polyamides composed of aromatic/aliphatic dicarboxylic acids and aliphatic diamines have been widely investigated in both the academic
1. INTRODUCTION High-temperature polyamides (or nylons) are important engineering polymers with unique properties including high melting temperature, high heat and chemical resistance, excellent dimensional stability, low water absorption, and high mechanical strength and stiffness. Thus, they are widely used in electronics (e.g., connectors, sockets, switches, surface mount devices, reflectors, and portable electronics housing) and lightweight automobile applications (e.g., powertrain components and electrical systems). Many properties of polymeric materials can be improved by increasing the molecular weights of the polymers, which can widen their applications. However, it is difficult to reach polymers with desired molecular weights for such applications using typical melt polymerization because of their high melting temperatures and the chain degradation that occurs at high reaction temperatures. One of the routes to achieve polyamides with high molecular weights is solid-state polymerization (SSP).1,2 In a typical SSP process, a polymer with low molecular weight (a prepolymer) is first synthesized by melt polymerization at relatively low temperature. If necessary, the prepolymer is then ground or pelletized and then crystallized. Next, the prepolymer is heated at a temperature above its glass transition temperature, but below its melting temperature with a sweep-gas fluid (e.g., inert gases) or with application of vacuum to remove reaction byproducts. Various polyamides have been synthesized by SSP, including nylon-4,2,3 nylon-6,6,4 nylon-4,6,5,6 nylon-4,T,7 nylon-4,I,8 copolyamides,9,10 and terpolyamides.11,12 In addition to polyamides, SSP has been widely used to produce various important engineering plastics including polyesteramide,13 poly(ethylene terephthalate) (PET),14−16 poly(trimethylene © 2012 American Chemical Society
Received: Revised: Accepted: Published: 15801
July 19, 2012 November 14, 2012 November 20, 2012 November 20, 2012 dx.doi.org/10.1021/ie301915h | Ind. Eng. Chem. Res. 2012, 51, 15801−15810
Industrial & Engineering Chemistry Research
Article
Scheme 1. Synthetic Scheme for the Copolyamide of Nylon-4,6/-4,T
calorimetry (DSC) studies. The thermal stability of the synthesized polymers at different aromatic diacid compositions was measured using thermal gravimetric analysis (TGA). The polymer composition was determined using 1H NMR spectroscopy.
and industrial sectors. Examples include nylon-6,6/-6,n (where n is the number of carbons ranging from 2−10),42 nylon-4,T/4,6,9 nylon-4,T/-6,T,10 nylon-10,T/-6,T and nylon-12,T/6,T,43 nylon-4,6/-4,T/-4,I,11 and nylon-4,6/-4,T/-6,T and nylon-4,6/-4,T/-6,6.12 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) intrinsic reaction kinetics of the chain extension reaction through collision of functional end groups, (3) internal diffusion of the reaction byproducts through the polymer matrix to the polymer surface, and (4) external diffusion of the reaction byproducts from the polymer surface into the sweep-gas phase. In step 2, end-group migration through interchange reactions of amine− amide or alcohol−ester in the amorphous region can facilitate the chain extension reaction.44,45 As we and other groups previously reported for the SSPs of nylon-6,6,4 PET,14−16 PTT,17,18 and BPA-PC,20−24 the SSP rate is dependent on one or more SSP process parameters (temperature, pressure, and sweep-gas flow rate) and prepolymer properties (particle size, crystallinity, molecular weight of the prepolymer, catalysts, and composition). Thus, it is very important to decouple the factors influencing the SSP rate to gain better insight into the molecular weight evolution during SSP and to predict attainable molecular weights. However, only a few fundamental studies of SSP processes for semiaromatic polyamides have been conducted. Gaymans et al.9 reported thermal and mechanical properties of copolyamides of nylon-4,T/-4,6 at different molar ratios synthesized under one set of SSP conditions (4 h, 260 °C, stream of N2 and steam with ratio of 2:1, with no information provided on the sweep-gas flow rate and prepolymer properties). In this research, we investigated the effect of prepolymer properties (particle size and aromatic diacid composition) and SSP parameters (temperature, time, sweep-gas flow rate, and steam content of the sweep fluid) on SSP nylon-4,T/-4,6. Scheme 1 shows nylon-4,T/-4,6 synthetic chemistry. To examine the effect of aromatic diacid composition, the SSP conditions were carefully controlled to eliminate any influence of either internal or external byproduct diffusion. The polymer properties, including glass transition temperature (Tg), melting temperature (Tm), and heat of fusion (ΔHf), were measured during the first and second scans of differential scanning
2. EXPERIMENTAL SECTION 2.1. Materials. 1,4-Butanediamine (BDA, purity > 98%) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Terephthalic acid (TPA, purity > 99%) and adipic acid (AA, purity > 99%) were purchased from Junsei Chemical Co., Ltd. (Tokyo, Japan). 1,6-Hexamethylene diamine (HMDA purity > 99%), ethanol (purity > 99.9%), methanol (purity > 99.9%), and concentrated sulfuric acid (purity > 98%) were purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Korea). Trifluoroacetic acid-d (CF3COOD, purity > 99%), as an NMR solvent, was purchased from Sigma-Aldrich (St. Louis, MO). Deionized ultrafiltered (DIUF) water was prepared using a Millipore Milli-Q Ultrapure water purification system with a 0.22-μm filter (Billerica, MA). Nitrogen (purity > 99.99%) was purchased from Shinyang Sanso Co. (Seoul, Korea) and passed through an oxygen trap (model 20601, Restek Corp., Bellefonte, PA) before introduction into the reactor. 2.2. Preparation of Nylon Salts. Steps in nylon-4,6 salt preparation were as follows: BDA (88.1 g, 1 mol) was dissolved in 705.3 g of methanol with vigorous stirring at 25 °C. Next, 146.1 g of AA (1 mol) was slowly added to the solution of BDA in methanol for 2 h to synthesize the nylon-4,6 salt. The solution was cooled to 25 °C to precipitate the synthesized nylon-4,6 salt, which was filtered off using 7−9-μm-pore-size filter paper, washed with cold methanol, and dried at 50 °C in a vacuum oven for 48 h. The mass yield of nylon-4,6 salt was 96.2%. Nylon-4,T salt preparation was performed as follows: BDA (88.1 g, 1 mol) was dissolved in 1 L of DIUF water with vigorous stirring at 25 °C. Next, 166.1 g of TPA (1 mol) was slowly added to the aqueous BDA solution for 2 h to synthesize the nylon-4,T salt. After the solution had been cooled to 25 °C, the nylon-4,T was precipitated by addition of 2 L of cold ethanol to the solution. The precipitated nylon-4,T salt was filtered off, washed with cold ethanol, and dried at 50 °C in a vacuum oven for 48 h. The mass yield of Nylon-4,T salt was 15802
dx.doi.org/10.1021/ie301915h | Ind. Eng. Chem. Res. 2012, 51, 15801−15810
Industrial & Engineering Chemistry Research
Article
Table 1. Properties of the Prepolymers
a
prepolymer
nylon-4,T/-4,6 ratioa (mol/mol)
IV (dL/g)
Tm,first (°C)
Tm,second (°C)
ΔHf,first (J/g)
ΔHf,second (J/g)
Td,5 (°C)
Td,10 (°C)
PAP1 PAP2 PAP3
28.2:71.8 37.9:62.1 47.8:52.2
0.085 0.068 0.070
273 253 246
307 305 306
93.6 161.7 147.9
64.6 14.0 4.8
236 232 237
340 277 287
Determined by NMR spectroscopy.
Figure 1. Schematic diagram of the SSP apparatus: (1) nitrogen source, (2) water source, (3) gas mass flow controller, (4) liquid mass flow controller, (5) mixer, (6) forced-convection oven, (7) preheater coil, (8) reactor, (9) controller, and (10) water trap.
96.4%. During the nylon-4,6 and nylon-4,T salt preparations, unreacted BDA, AA, and TPA can cause stoichiometric imbalance of the acid and amine end groups. 2.3. Prepolymer Synthesis. Prepolymers composed of nylon-4,T and nylon-4,6 at different composition ratios were synthesized by polyamidation of the nylon salts using DIUF water as the solvent. The prepolymerization was carried out in a custom-built, 1200 cm3 high-pressure stirred reactor. The reactor was equipped with a heat furnace, magnetic stirrer, purge line, and vent line. In a typical prepolymerization, a mixture of nylon-4,T salt and nylon-4,6 salt at different molar ratios (samples PAP1, PAP2, and PAP3) and 30 g of DIUF water were added to the reactor. The total amount of nylon salts used in prepolymerization was maintained at 300 g. The reactor was then purged with nitrogen for 30 min at a flow rate of 300 mL/min and pressurized at 0.5 MPa of nitrogen. The temperature of the reactor was increased from 25 to 210 °C with stirring at 900 rpm for 1.5 h. Over the course of temperature ramping, the reactor pressure increased to 2 MPa. The polyamidation was allowed to proceed at 210 °C for 6 h at 2 MPa. After prepolymerization, the reactor was rapidly depressurized to atmospheric pressure to remove water. The properties of the prepolymers are listed in Table 1. Hereafter, the solid-state-synthesized samples obtained using PAP1, PAP2, and PAP3 as the prepolymers are designated as PA1, PA2, and PA3, respectively. 2.4. Solid-State Polymerization. Prior to SSP, the prepolymers were ground into powders and separated into three different particle sizes (75, 250−500, and 1000−1180 μm; sweep-gas flow rate = 3000 mL/min; and N2/steam ratio = 2:1 (v/v).
When the particle size decreased from 1000−1180 to
