High molecular weight thermoplastic polyether ester elastomer by reactive extrusion.
Because of its superior oil and chemical resistance, as well as its excellent low temperature impact properties, thermoplastic polyether-ester elastomer (TPEE) has been widely used in automobile, electric, and electronic applications [1-2]. Although chloroprene rubber (CR) was widely used in automobile parts, it has been replaced by TPEE due to the former's poor durability. Furthermore, the use of TPEE has been extended in North America and Europe due to its superior lightweight and fatigue-resistant properties, and chemical and ozone resistance. Since TPEE is produced by extrusion blowing in the melt state, the resin should have good melt viscosity and melt tension in order to yield constant thickness distribution during extrusion blowing.
Generally, it is difficult to perform extrusion blending with TPEE, due to its low melt viscosity and melt tension [3-4]. Although the melt viscosity and melt tension of TPEE can be increased by adding a branching agent during melt polymerization, it does not increase the melt viscosity sufficiently for extrusion blowing. To solve this problem, several attempts have been made to increase the melt viscosity and melt tension using a chain extender during reactive extrusion after the melt polymerization. Guo et al.  reported the reduction of the crystallization time by the reactive extrusion of poly(butylene terephthalate) (PBT) in the presence of diepoxy group as a chain extender, and suggested that their approach to chain extending provided a simpler and cheaper method of obtaining high-molecular-weight PBT resins than the conventional solid post polycondensation method. According to Weisskorpf et al. , multifunctional monomers, such as trimethylolpropane (TMP), trimethylolethane (TME), pentaerythritol, and trimesic acid, can be used to synthesize high molecular weight branched poly(ethylene terephthalate) (PET). Tang et al.  reported the effect of the modified 4,4-diphenylmethane diisocyanate (m-MDI) content in the poly(ethylene terephthalate) (PET)/polycarbonate (PC) blending process. As the content of m-MDI increased, the molecular weight of PET increased. However, if the reaction proceeds for a long enough time or excessive m-MDI is added, the product will be cross-linked, due to the excessive reactivity of the isocyanate group and this will affect the ductility of the blend. On the other hand, when the proper amount of m-MDI is used, the molecular weight of PET will decrease with increasing blending time, due to hydrolytic and thermal degradation of isocyanate group. Therefore, it is necessary to complete the reaction within a short period of time to decrease the possibility of degradation. If the residence time is not sufficient to complete the tire thane formation reaction, unreacted isocyanate groups will generate [CO.sub.2] gas during the post extrusion processing. Furthermore, the unreacted isocyanate group may also induce undesirable side reactions during the post processing. Therefore, it is necessary to ensure that the reaction of the isocyanate is completed and the physical properties of the product are maintained during processing. Therefore, the optimum processing parameters for the blending of m-MDI and the branched TPEE precursors need to be found.
A similar situation arises in the case of TPEE. Unfortunately, there are not sufficient reports on the optimum processing conditions for the TPEE and m-MDI blending process. In this article, the synthesis and characterization of hydroxyl terminated branched TPEE elastomers of segmented copolymers based poly(tetramethylene etherglycol) (PTMEG), 1,4-butandiol (1,4-BD) and glycerol as a soft segment, a hard segments, and as a branching agent, respectively, will be discussed . High molecular weight TPEE was further synthesized using m-MDI chain extender system by the reactive extrusion method. This study focuses on finding the optimum conditions required to obtain high molecular weight TPEE elastomers. It was anticipated that the incorporation of m-MDI into the elastomer chain would result in a high molecular weight polymer with enhanced high melt tension and thermal stability and such a process could be applied to commercial polymerization.
Materials and Characterization
Poly(tetramethylene etherglycol) (PTMEG, [M.sub.n] = 2000), 1,4-butandiol(1,4-BD), Irganox 1010 and modified 4,4-diphenylmethane diisocyanate(m-MDI, MM103C) from BASF, dimethylene terephthalate(DMT) from SK-Chemical and poly(butylene terephthalate) (PBT, TRIBIT 1500) in pellet form from Samyang Corporation were used as received. Phenol, 1,1,2,2-tetrachloroethane, [CF.sub.3]COOD (TFA-d) and [CD.sub.2][C1.sub.2], tetrabutoxy titanate(TBT) were purchased from Aldrich and used without further purification.
The intrinsic viscosity (IV) of the polymer was determined with a capillary Ubbelohde type viscometer at 35[degrees] C, using a polymer solution with a concentration of 0.5 g/ml in a mixed solution of phenol/1,1,2,2-tetrachloroethane (50/50, wt%). Normally, the IV is used as an indication of the molecular weight of the polymers. The melt index ratio (MIR) is the ratio of the throughput under a load of 2.16 kg to that under a load of 21.6 kg and was measured by a melt indexer at 230[degrees]C during a period of 10 min. The NMR spectra were acquired with a Bruker ADVANCE DPX 400 NMR spectrometer at room temperature, using a mixed solution of TFA-d/[CD.sub.2][Cl.sub.2] (1/5, v/v). The pulse width for the proton-nuclear magnetic resonance ([.sup.1] H NMR) measurement was 11.8 [micro]s. The IR spectra were obtained using a Bio-Rad FTS 185 Fourier Transform Infra Red (FTIR) spectrometer at room temperature, with a minimal resolution of 4 [cm.sup.-1] in transmittance mode. Spectra were collected in the 500-4000 [cm.sup.-1] region. The rheological properties were measured with a model Rhcometric ARES rheometer using parallel plates(diameter 25 mm) at 230[degrees]C under nitrogen atmosphere with the frequency of [omega] = 10 rad/s and a strain amplitude of 20%.
Branched TPEE Precursor Preparation
In a 15 1 stainless reactor, 2300 g of DMT, 1488 g of 1,4-BD, 2499 g of PTMEG, 4.56 g of glycerol (0.3 mol%/1,4-BD, usually 0.1-5.0 mol%/1,4-BD component) as a branching agent and 0.025 wt% of TBT as a catalyst were charged under a nitrogen atmosphere. The reactor was initially heated up to 140[degrees]C and kept at this temperature until all of the chemicals were clearly melted. The reaction was further heated to 215[degrees]C and allowed to proceed for 4 hr. The reaction was ended when the reaction conversion rate was over 99%, where the conversion is determined by the amount of methanol extracted by the vacuum trap. In the next step, 0.04 wt% TBT and 0.07 wt% Irganox 1010 as a thermal stabilizer were added to the reactor for the polycondensation reaction. The polycondensation was carried out by gradually increasing the temperature from 215[degrees]C to 250[degrees]C over a period of 2 hr and the reaction was maintained at this temperature for an additional 2 hr . When the reaction started, the pressure was reduced from 760 to 0.3 torr over a period of 2 hr and continued for an additional 2 hr to prepare the branched TPEE precursor.
Reactive Extrusion of TPEE
The reactive extrusion of TPEE was carried out by feeding 0.5-2.0 wt% of m-MDI containing carbodiimide into a pilot-scale twin-screw extruder [10-11], together with 93 wt% of TPEE and 3.0 wt% of PBT as the main components and 4 wt% of thermal-stabilizer, antioxidant, and lubricant [12-13]. During the reactive extrusion, the temperature of the extruder (ZSK-25, W&P) was kept in the range of 170-240[degrees]C with a screw rotational speed of 150 rpm, a feed rate of 12.5 kg/hr and a residence time of 45 s. Reactive extrusions were carried out three times in the same extruder to increase the conversion.
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymers
The branched TPEE precursor was synthesized from PTMEG and DMT as a soft segment, DMT and 1,4BD as hard segments and glycerol (0.1-5.0 mol% of butanediol) as a branching agent, as shown Fig, 1. These compounds are stable materials and can be stored for extended periods of time. The introduction of hydroxyl groups in the branched TPEE precursor provides chain extension sites with m-MDI. The reactive extrusion of the polymer with m-MDI is an especially attractive route for the preparation of the high molecular weight TPEE. Hydroxyl and isoeyanate groups can react under mild conditions. Furthermore, the reaction leads directly to the formation of urethane bonds, which are thermally and hydrolytically stable. If m-MDI contains 5% carbodiimide, its stability against hydrolysis in the reactive extrusion process will be enhanced. Figure 2 shows the NMR spectrum of the prepared TPEE. Peak a is corresponding to DMT, peaks b and d to DMT and peaks c and e to PTMEG. From this NMR spectrum, it can be confirmed that the high quality polymer precursor was successfully prepared for the reactive extrusion.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
To confirm the formation of high molecular weight TPEE containing urethane bonds by reactive extrusion, its 1H NMR spectrum was obtained, as shown in Fig. 3. The peak at a chemical shift of 7.2 is related to the protons in the urethane bond formed by m-MDI and the OH terminal groups of the branched TPEE precursor. Thus, it was confirmed that reactive extrusion is a successful process for the preparation of TPEE.
[FIGURE 3 OMITTED]
Chain Extension by Reactive Extrusion
The molecular weight of the polymer was obtained from the IV. In the first reactive extrusion, the IV increases as the m-MDI content increases, reaches a peak at an m-MDI content of 1.5 wt%, and remains constant thereafter, as shown in Fig. 4. This might indicate that all of the hydroxyl groups in the branched TPEE precursor were consumed in the urethane formation reaction. The second extrusion also shows similar behavior to the first extrusion. The decrease in the intrinsic viscosity with increasing number of repeated extrusions may be related to the thermal degradation of TPEE,  as well as to the thermal degradation of the urethane bonds in the TPEE polymer. This means that the repeated extrusion process should be optimized by considering the trade-off between the chain extension reaction and the thermal degradation of the TPEE polymer.
[FIGURE 4 OMITTED]
The MIR is defined as the ratio of the melt index at a load of 21.6 kg to that at a load of 2.16 kg. Normally, the MIR increases with increasing degree of cross-linking. According to Fig. 5, the cross-linking of the polymer precursor with m-MDI is clearly observed in the DSC data. The melting point and enthalpy after the first extrusion decreased with increasing m-MDI content. This is related to the partial cross-linking of the polymer and its changing to an amorphous structure. Although the data is not shown here, the melting point and heat of fusion also decreases as the number of extrusions increases, which could be explained by the increase in the amount of amorphous phase.
[FIGURE 5 OMITTED]
As shown in Fig, 6, the MIR increases as the content of m-MDI increases which is a similar behavior to the intrinsic viscosity. The MIR of TPEE in the first extrusion with 0.5 wt% of m-MDI increases as the m-MDI content increases, reaches a peak at an m-MDI content of 1.5 wt%, and remains constant thereafter. The MIR of TPEE in the first extrusion with 0.5 wt% is higher than that in the second and third extrusions with the same m-MDI content, due to thermal degradation. However, in the case where higher m-MDI content is used, the MIR increases as the number of extrusions increases, because the extent of the chain extension reaction is increased due to the increased residence time of unreacted m-MDI. The reason why the MIR of TPEE after the first extrusion is lower than that after the second extrusion at m-MDI content of 1.0% could be explained by the competition between the content of unreacted m-MDI and the residence time. This means that the first extrusion does not provide sufficient reaction time. On the other hand, the second extrusion provides sufficient reaction time, although a large amount of degradation occurs due to thermal degradation. The optimum content of m-MDI of 15% is consistent with the intrinsic viscosity result. The reactivity of m-MDI in the reactive extrusion was confirmed by FTIR. The amount of nonreacted m-MDI increased as the content of m-MDI increased, Figure 7 shows the FTIR results at the different contents of m-MDI. The peak intensity at a wavelength of 2260-2280 [cm.sup.-1] in the FTIR spectrum, which is the peak of the N=C=O bond of m-MDI. increased with increasing content of m-MDI. This indicates that there is an optimum content of m-MDI for the reactive extrusion and that it is not necessary to input more than 1.5 wt%. At the content below 1.5 wt%, m-MDI will react fully and there will be no residues of the unreacted m-MDI, which is also consistent with the IV and MIR data.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Rheological Behavior From Modulus and Viscosity
The rheological properties were observed by the time sweep method of ARES at 230[degrees]C, which is the post-processing temperature of the TPEE. The variations of the modulus and viscosity were obtained with respect to the m-MDI content and time. Figure 8 shows the viscosity measurement in the first reactive extrusion. The viscosity increased when the unreacted m-MDI contents were increased, due to the reaction of the unreacted m-MDI during the ARES measurement. However, the viscosity did not vary with the analysis time at m-MDI content lower than 1.5 wt%. as shown in the figure. This implies that all of the m-MDI reacted with the TPEE and that the viscosity did not change during the measurement. Figure 9 shows the viscosity measurement in the second reactive extrusion as a function of time. The viscosity increased with increasing of m-MDI contents. This phenomenon was similar to the first extrusion. However, the initial increase of viscosity is smaller than that of the first reactive extrusion because of the decrease of the unreacted m-MDI contents in the second reactive extrusion. Although the storage modulus data is not shown herein, its behavior is very similar to that of the viscosity. This is an important result for the production of high molecular weight TPEE. If the modulus and viscosity change in the melt state during the process, it is difficult to achieve good processability for blowing applications which need a product of constant thickness.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
We synthesized a reactive hydroxyl containing branched TPEE precursor using a multifuctional monomer, and obtained high molecular weight TPEE by reactive extrusion. The molecular weight and degree of cross-linking of the prepared TPEE increased as the m-MDI content increased. The molecular weight of the polymer could be obtained from the intrinsic viscosity. The viscosity of TPEE increases as the m-MDI content increases. At 1.5 wt% m-MDI with TPEE. the viscosity increase from 2.26 to 3.20 dl/g for the first reactive extrusion. The melt index ratio is directly related to the degree of cross-linking. The MIR of TPEE in the first extrusion with 0.5 wt% of m-MDI is 41, which increases as the m-MDI content increases. It reaches up to 60 at the m-MDI content of 1.5 wt% and remains constant thereafter. According to the variation of the intrinsic viscosity and MIR with the m-MDI content, it is desirable to use less than 1.5 wt% of m-MDI and the minimum number of extrusions to obtain the optimum blowing conditions, due to the undesirable thermal degradation. In spite of the short reaction time during the reactive extrusion, thermal degradation was not the main obstacle to the processing and the extent of chain extension was reasonably high. The results of FTIR and NMR were consistent with the results of the IV and MIR. The number of reactive extrusions and reaction time were optimized by the rheological properties measurement of viscosity and storage modulus. The high molecular weight TPEE was successfully obtained under the optimized reactive extrusion conditions.
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Sunghwan Cho, (1), (2) Yunjoo Jang, (1) Dongmin Kim, (3) Taeyoung Lee, (4), (5) Dongho Lee, (5) Youngkwan Lee (2)
(1) Advanced Polymeric Materials R&D Center, Samyang Corporation, 63-2 Hwaam, Yuseong, Daejeon 305-717, Korea
(2) Department of Chemical Engineering, Sungkyunkwan University, Suwon, Kyunggi 440-746, Korea
(3) Department of Materials Science and Engineering, Hong lk University, Yeongi, Chungnam 339-701, Korea
(4) R&D Support/Technology Center, Korea Delphi Automotive Systems Corporation., 580-1 Buk-ri, Nongong-eup, Dalseong-gun, Daegu 711-712, Korea
(5) Department of Polymer Science, Kyungpook National University, Daegu 702-701, Korea
Correspondence to: Youngkwan Lee; e-mail: firstname.lastname@example.org
Contract gram sponsor: Korea Research Foundation; contract grant number: KRF-2006-005-J04603.
Published online in Wiley InterScience (www.interscience.wiley.com).
[C]2009 Society of Plastics Engineers
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|Author:||Cho, Sunghwan; Jang, Yunjoo; Kim, Dongmin; Lee, Taeyoung; Lee, Dongho; Lee, Youngkwan|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Jul 1, 2009|
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