Formation of a polyetheramide triblock copolymer by reactive extrusion; process and properties.
Various authors (1-6) have described the synthesis of block copolymers by polymerization of [epsilon]-caprolactam in the presence of isocyanate-terminated poly(propylene oxide) and poly(butadiene) polymeric activators. The immiscibility between hard segments and elastomer segments was a reported experimental limitation. It was found that immiscibility between carboxylic-terminated polyamide-6 and poly(tetramethylene ether) glycol (PTMEG) was a severe problem (7). PTMEG was found to be completely immiscible with carboxylic-terminated polyamide-6 prepolymer within the temperature range from room temperature to 220[degrees]C. Phase separation hindered the chemical reaction and products with poor properties were obtained. The synthesis of polyisobutylene(PIB)-polyamide(PA) [(PIB-PA).sub.n] multiblocks was reported by Zaschke and Kennedy (8). They proceeded by the solution polymerization of HOOC- or OCN- ditelechelic PIBs with various diisocyanates and dicarboxylic acids. In general, they are polyetheramide block copolymers. The properties of the resulting block copolymer can be controlled over a very broad range by selection of elastomers type and total elastomer content, as well as by selection of hard segments. Polyamide-based thermoplastic elastomers based upon flexible segments were first introduced into the market in 1982 and include Pebax from Atochem in France and Estamid from Upjohn/Dow in the United States (9-11).
The use of a starved modular twin-screw extruder as a polymerization reactor can provide industry with a convenient commercial reaction procedure. In the present study, a preparation method of polyamide-6 based polyetheramide triblock copolymer in a modular intermeshing counter-rotating twin-screw extruder is described. Three special mixing elements configured in the counter-rotating twin-screw extruder are used to improve mixing performance and lengthen residence time. Subsequent to polymerization, the ability to directly post-process the product was shown by continuous melt spinning of both polyamide 6 and the polyetheramide triblock copolymer from the twin-screw extruder.
This paper is a part of our research on the synthesis of polyamide-based block copolymers (12, 13). In the course of our fundamental studies aimed at finding a synthetic route to polyamide-based block copolymers in a twin-screw extruder, the present authors decided to explore the possibility of preparing A-B-A type triblock copolymer with poly(tetramethylene ether) soft segments connected to polyamide-6 as the hard segment having melting points of over 200[degrees]C. This combination of soft and hard segments yields not only outstanding thermal and physical properties but also good processability. The crystalline amide blocks contribute high modulus, tensile strength, chemical resistance, and high melting temperature. The synthesis route has several interesting aspects. The process we describe is a one-step, solvent-free process in which no by-product is formed during the reaction procedure. This eliminates the need for a devolatilization process except for removal of residual monomer. This approach to formin g elastomer-toughened polyamides demonstrates an original preparation technique in a continuous manner and on-line fiber forming process. Depending upon elastomer content, the products should be rubber-toughened thermoplastics or thermoplastic elastomers.
On the basis of our examination of background information, we concluded that the target materials could be made by bulk and solvent-free polymerization of isocyanate-terminated PTMEGs according to Scheme 1. However, studies in our laboratories (14, 15) indicate lauryl lactam and other lactams might be used as well. Thus our results can be best discussed in terms of (a) continuous anionic synthesis of PA6 homopolymer, (b) continuous anionic synthesis of polyetheramide block copolymer, and (c) continuous formation of PA6 and PEA melt-spun filaments. The reason that we chose polyamide-6 instead of other polylactams as a hard segment is that caprolactam is more readily available and has a higher melting temperature. Thus, polyamide-6 based TPEs have a high potential for competition. The crystalline melting point of polyamide-6 is higher (215[degrees]C) than that other commercial lactams and should give the material a higher operating temperature range.
Caprolactam monomer (obtained from DSM) was dried in vacuum (1 Torr, 60[degrees]C, 48 h) before use. Sodium (Aldrich Chemical Co.) was used as received as an initiator. The chemical 4-4'-diphenylmethane diisocyanate (MDI, 2125M, Dow Chemical Co.) was used as activator without further purification. Isocyanate-terminated PTMEG prepolymer (Baytec-ME04O, [M.sub.n] 4760 and [M.sub.w]/[M.sub.n] = 1.54) was supplied by Bayer. PTMEG prepolymer was a pure reagent and used as received. Parts by weight of NCO per 100 parts of prepolymer was 4.55% ~ 4.94%.
Pebax 6333 is a mutiblock copolymer with alternating polyaimide 12 and PTMEG segments and obtained from Elf-Atochem. The polyamide 6 used in this study was obtained from Honeywell (formerly AlliedSignal, Capron 8200).
Reactive Extrusion Apparatus and Procedure
The polymerization was carried out in LSM-34GG Leistritz modular intermeshing counter-rotating twin-screw extruder. The modular screw configuration used is the same as screw configuration G, which Lim and White (16) found gave good mixing performance. These elements are as described (i) thick flighted highly intermeshing screws (FD), (ii) thin flighted highly intermeshing screws (FF), (iii) screw elements wherein the flight thickness increases in the direction of flow (KFD), and (iv) various non-pumping elements. One of these non-pumping elements is called a "shearing" element, and the second a "slit stowing" element. The "shearing" element consists of two solid cylindrical sections of different diameters placed sequentially on each shaft, contacting the large-diameter section on the other. The "slit stowing" elements instead have a grooved cylindrical section. Barrel diameter as 34 mm and L/D ratio was 32. Typical residence times and variance for this screw configuration with different operating conditions are given in Table 1.
A sodium initiator and MDI activator were used. The same premixing procedure for the caprolactam initiator and, separately, the initiator/activator has been followed as described in our previous paper (17). A volumetric feeder was used to feed the uniformly tumble-mixed reactants into the hopper of a 34-mm Leistritz GG modular intermeshing counter-rotating twin-screw extruder at a feed rate of 1 kg/hr with dry nitrogen gas. The ring-opening polymerization of caprolactam was carried out using different barrel temperature profiles and screw speeds, which are given in Table 2.
The screw configuration is shown in Fig. 1. The barrel temperature profile used was optimized in a manner such that the barrel near the hopper accepted and melted the feed. The temperatures at the intermediate zones must be concerned with the polymerization reaction and those in the later stages to pump the melt.
The emerging extrudates (PA6) were pelletized or collected. The unreacted caprolactam was extracted by water or methanol in a Soxhlet apparatus. The product obtained was dissolved in concentrated formic acid, and precipitated into distilled water. The resulting product was filtered, and the polyamide dried under vacuum. The yields of PA6s ranged from 92% to 93%, which may be compared to earlier studies of polymerization of polyamides by this mechanism in a modular co-rotating twin-screw extruder in our laboratory (14, 18, 19).
For the preparation of polyetheramide block copolymer, a similar premixing procedure was followed, but with various proportions of monomer and PTMEG pre-polymer. Two-tenths (0.2) mole of initiator (sodium) and 20 moles of monomer (caprolactam), and a certain amount of PTMEG prepolymer (MDI-end capped, MDI activator) were premixed separately in various proportions to maintain the content of soft segments at 13 wt% in the final product. The separately pre-pared initiator and activator were then tumble-mixed after a cooling process. These mixtures were then fed into the twin-screw extruder with dry nitrogen at a feed rate of 1 kg/hr. The same screw configuration as for the PA6 synthesis was used. The processing conditions are described in Table 2. The reaction involves ring-opening of caprolactam at the end of the prepolymer, in which PTMEG is end-capped with MDI, forming a triblock copolymer with polyamide end blocks.
The extruded triblock copolymer formed was crushed into granules or powder and boiled with distilled water or methanol, and the conversion of caprolactam was determined from the weight change in the boiling water extraction process. After drying in vacuo, the polymer was extracted with THF or chloroform in a Soxhlet apparatus to isolate the unreacted PTMEG prepolymer as the THF or chloroform-soluble part. The yields of PEA copolymers ranged from 86% to 87%. The solid material obtained was dispersed in formic acid and poured into distilled water or methanol and washed. The product finally was dried overnight in a vacuum oven.
Continuous melt spinning of PA6 and polyetheramide triblock copolymer was carried out subsequent to polymerization in the twin-screw extruder. A spinneret with a hole diameter of 2 mm was attached as the die section for the extruder. The exit velocity through the spinneret was computed to be 2.95 cm/sec at a prepared reactant feed rate of 1 kg/hr. The emerging extrudates (PA6 and its block copolymer) were melt spun into fibers by a winder at various drawdown ratios ([V.sub.L]/[V.sub.0]) of 100, 200 and 530.
Analysis of Products
The conversion of caprolactam was evaluated by Soxhlet extraction of the extruded polyamide 6 (PA6) with distilled water. This was accomplished by boiling PA6 in a thimble with distilled water or methanol until a constant weight was reached. Then, the conversion of caprolactam was estimated by comparing the weight of the sample before and after extraction. The conversion of polyetheramide triblock copolymer was determined in the same manner. The triblock copolymer formed was boiled with distilled water or methanol, and the conversion of caprolactam was calculated. The content of unreacted PTMEG prepolymer was estimated by further extraction with boiling chloroform or THF.
Number and weight average molecular weights and molecular weight distribution ([M.sub.w]/[M.sub.n]) of PTMEG prepolymer were determined by using a Waters 150-C GPC equipped with RI. Five Ultrastyragel columns (500; 1000; 10,000; 100,000; 1,000,000) and tetrahydrofuran solutions (injection 200 [micro]L and concentration of 11 mg/ml) were used. Their [M.sub.n] and [M.sub.w] were determined by GPC by polystyrene calibration. These molecular weight data are only apparent values. The Styragel columns used were calibrated with polystyrene standard samples; the polydispersity indexes calibrated by this method for nylon-6 and block copolymers have essentially a comparative meaning. Because of strong hydrogen bonds between the amide groups, polyamides and polyetheramide triblock copolymer were insoluble in THF; however, after N-trifluoroacetylation of the amide groups (Scheme 2), both homopolymers and block copolymers became soluble in THF, rendering GPC and light scattering characterizations possible (8). It must be men tioned that it would be difficult to measure the MW of the block copolymer because the refractive index depends on the differing contributions from the polyamide and polyether segments. Thus, the polydispersity indexes obtained for nylon-6 and block copolymers have essentially a comparative meaning.
[H-NMR.sup.1] spectra were obtained at ambient temperatures by a Varian Gemini-200 spectrometer operating at 200 MHz and a Gemini-300 spectrometer operating at 300 MHz using HCOOH/[CDCI.sub.3] (3/2 by volume) mixture solutions in 5-mm tubes. Sample concentrations were 20 mg/ml for proton spectroscopy. Typically, for 200 MHz [H-NMR.sup.1], 128 transients were accumulated for the proton spectra with 31.5[degrees] pulses (18 [micro]s), 3.744 sec acquisition time, and 4 s delay. Sixteen transients were accumulated for 300 MHz [H-NMR.sup.1]. Tetramethylsilane was used as an internal standard.
All the IR spectra were acquired on an ATI Mattson Genesis Series Fourier transform infrared spectrometer at a resolution of 4 [cm.sup.-1] at room temperature in air with 16 scans.
The transition behavior of the polymerized PA6 and polyetheramide triblock copolymer was determined by differential scanning calorimeter (DSC, DuPont Thermal Analyzer 9900) in the temperature range of -100[degrees]C to 260[degrees]C at a scanning rate of 20[degrees]C/min, and the weight of samples was carefully controlled at 10 mg. The flow rate of [N.sub.2] was at 30 ml/min.
Thermogravimetric analysis (TGA, DuPont Thermal Analyzer 9900) was used to study the chemical stability of the products. The scanning rate was 20[degrees]C/min and the initial weights of the samples were approximately 15 to 20 mg. The temperature range investigated was 70[degrees]C to 600[[degrees]C. The flow rate of [N.sub.2] was set at 30 ml/min. A 5% weight loss was considered to be indicative of the decomposition temperature.
Dynamic mechanical properties were measured using Rheometric Scientific Dynamic Mechanical Analyzer Mark II. Samples were in the form of melt-pressed films. Circular samples were cut, of 0.8-mm thickness and 6.5-mm diameter. Measurements were made at 1 Hz and a heating rate of about 4[degrees]C/min. Samples were initially cooled to ~ -100[degrees]C with liquid nitrogen and heated slowly as measurements were made.
Complex shear viscosities of PA6 and its block copolymer were determined in the parallel disc mode using a Rheometric Mechanical Spectrometer (RMS 800) under nitrogen purging.
The birefringence of the melt spun fibers was measured in a Leitz polarized light microscope (Leitz Labor-luxl2 POL) with a Berek compensator ([lambda] = 546 nm).
Mechanical properties of melt spun fibers of PA6 and block copolymer were also measured with using a Flexsys Tensometer 2000 with 1-KN load cell. A cross-head speed of 5 mm/min was used.
RESULTS AND INTERPRETATION
Reactive Extrusion Process
The experimental conditions for polymerization of polyamide 6 and the results are summarized in Tables 2 and 3, respectively. Table 2 presents feed composition of prepolymer, initiator, activator, and caprolactam monomer and reaction conditions. Screw speeds from 20 rpm to 250 rpm were used at a feed rate of 1 kg/hr. The level of conversion of the polyamides for different polymerization conditions is shown in Table 2. The yield values of PA6s range from 92% to 93%.
The experimental conditions for polymerization of the block copolymer and the results are summarized in Tables 2 and 3, respectively. The block copolymer was crushed into granules or powder and boiled with distilled water or methanol, and conversion of caprolactam was determined. After drying in vacuo, the polymer was extracted with THF or chloroform in a Soxhlet apparatus to isolate the unreacted PTMEG prepolymer as the THF or chloroform-soluble part. Yields 86% to 87% of polymerized polyetheramide triblock copolymer were obtained in the twin-screw extrusion process.
Since caprolactam is miscible with PTMEG at 100[degrees]C or above, a homogeneous system was observed while the prepolymer was premixed with molten caprolactam. Even though the reaction begins and polyamide 6 chains grow at the ends of PTMEG segments, homogeneity and uniform dispersion of reactants can be largely improved by the mixing elements used in the twin-screw extruder. In the first mixing zone, complete melting of caprolactam and polyether prepolymer occurs before the reaction is initiated. The reactants undergo very high shear when they go through the second mixing element, and this high shear improves mixing and uniformity of the reaction medium and increases the possibility for reactants to encounter and react. Thus, the chemical reaction between the two reactants would not appear to be hindered by phase separation arising from immiscibility between the polyether segment and the polyamide segment at the high conversion obtained (Table 2). Caprolactam conversion in copolymerization was 91% ~ 92%. Th e immiscibility between PTMEG and PA6 may increase as the PA6 chains grow along the screw axis toward the die. This reaction hindrance by the existence of immiscible PTMEG elastomer, however, could be reduced by intense mixing of mixing elements in the twin-screw extruder. It is interesting to note that the high yield of copolymerization could be achieved by reactive extrusion process in a very short reaction time (~5 min). This fact is supported by the investigation of Allen and Eaves (20), who also studied the dependence of properties on the nature and amount of the polymeric activators (isocyanate- terminated polycaprolactone), and the conditions of preparation and processing. They draw a conclusion that bulk copolymerization of caprolactam should yield a more desirable product.
GPC traces were shown in Fig. 2 for PA6. The [M.sub.w]/[M.sub.n] of PA6 was 1.53.
DSC thermograms of PA6 compared with that of commercial PA6 (Capron 8200, AlliedSignal) are shown in Fig. 3. The material has a crystalline melting point of 215[degrees]C.
A typical TGA curve for PA6 is shown in Fig. 4 and compared with that of commercialized PA6 supplied by AlliedSignal. They are designated by caprolactam (uptriangle), isocyanate-terminated PTMEG prepolymer (downtriangle), PA6(B) (open circle), PEA polyetheramide triblock copolymer (open square), and commercial PA6 (solid line) (Capron 8200). It can be seen that the decomposition temperature for commercial PA6 is 420[degrees]C and 414[degrees]C for the PA6 polymerized in the twin-screw extruder. The caprolactam monomer has a volatilization temperature of 136[degrees]C.
Melt viscosities of PA6 are compared with those of commercial PA6s in Fig. 5. PA6 polymerized in the twin-screw extruder exhibited a higher viscosity than one of the commercial PA6s studied, but a lower viscosity than the other.
The yields of polyetheramide triblock copolymer were as high as 87%. The influence of feed rate and screw rotation speed on conversion was not thoroughly investigated. It was, however, found that yield could be strongly influenced by feed rate. Increasing the feed rate and screw speed both lower the yield. However, in Table 2, the feed rate is constant, and the conversion remains basically unchanged (86.6% - 86.0%) when the screw speed changed from 20 to 250. This situation might be explained by the fact that, even in case of phase separation, the polyamide forms the continuous phase in which the monomer is dissolved and feeds the growing chains without being dramatically affected by the processing conditions. The results are summarized in Table 1.
Figure 2 shows representative GPC traces obtained. The molecular weight of the block copolymer has lower elution counts than the OCN-[PTMEG.sub.4000]-NCO prepolymer because of the PA6 chains at its ends. The PEA block copolymer exhibits [M.sub.w]/[M.sub.n] of 1.74.
Two small peaks of PTMEG prepolymer in the 27-29 elution count range suggest a contribution by lower molecular weight components and excess MDI. Assuming that all of the caprolactam and unreacted PTMEG prepolymer were removed after the series of extraction procedures. the weight percent of the PTMEG elastomer segment existing in the block copolymers can be readily calculated. Since Baytec ME040 has very broad molecular weight distribution (MWD), very broad GPC peaks of PEA were obtained. The narrow GPC peak of PEA is attributed to the nature of difunctional initiator used for polymerization of caprolactam (2, 4). The polydispersities of PA6 and PEA were 1.53 and 1.74, respectively. PEA showed a broader MWD than PA6 because of the introduction of broad MWD PTMEG prepolymer. Since Baytec-ME040 has excess MDI, PA6 homopolymer and its block copolymer were formed at the same time. It is very difficult to remove or separate PA6 homopolymer from its block copolymer. Thus, the GPC peak for PEA presumably has contribu tions from the block copolymer and the PA6 homopolymer. It would be more difficult to measure the MW of the block copolymer because refractive index depends on molecular structure, not just concentration. The values obtained in this study have contributions from polyamide hompolymer, diblock copolymer and triblock copolymer. Thus, the polydispersity indexes obtained for nylon-6 and block copolymers have essentially a comparative meaning.
The infrared spectrum of polyetheramide triblock copolymer (PEA) in Fig. 6 shows the absence of isocyanate absorption observed at 2260 [cm.sup.-1]. Figure 7 provides the [H.sup.1] spectra of PA6 homopolymer, PTMEG prepolymer, and their block copolymer. Proton shifts in PA6, PEA and prepolymer are distinguishable, which are summarized in Fig. 7 and Scheme 1.
Dynamic mechanical moduli of the block copolymer as functions of temperature are shown in Fig. 8. The storage and loss moduli are almost equivalent for polyamide 6 and the block copolymer under the conditions investigated. We also include Atochem Pebax 6333, which is made up of multiblocks of PTMEG and polyamide 12. Both the storage and loss moduli of Pebax are lower than those of PEA.
DSC thermograms of our PEA block copolymer are compared with that of commercial PA6, which is shown in Fig. 3. The crystalline melting point ([T.sub.m]) of PA6 segments is 215[degrees]C, Based upon the DSC thermograms, polyethearmide triblock copolymer shows another crystalline melting point of 28[degrees]C, which probably corresponds to the polyether elastomer segment.
Typical TGA curves of the block copolymer are shown in Fig. 4 and compared with that of commercial PA6 supplied by AlliedSignal. It can be seen that the decomposition temperature is 370[degrees]C for the polyetheramide triblock copolymer. It is observed that polyetheramide triblock copolymer decomposes at a lower temperature than our reactive extrusion produced PA6 and commercial PA6.
The melt viscosities of its block copolymer, polymerized in a twin-screw extruder, are compared with commercial PA6s in Fig. 5. The polyamides exhibit Newtonian behavior at low frequencies and a decreasing viscosity at higher frequencies. It is interesting to note that the block copolymer shows non-Newtonian behavior in which viscosity decreases as frequency increases with no tendency to low shear Newtonian asymptote.
The complex shear viscosities [[eta].sup.*][omega] of PA6 and the block copolymer were compared with commercial PA6. Our PA6 has an equivalent or higher molecular weight than that of the commercial PA6. Polyetheramide triblock copolymer shows a much higher viscosity than PA6. A very striking feature of polyetheramide triblock copolymers is their melt viscosities. These block copolymers show non-Newtonian behavior under all conditions, that is, their viscosities increase as the shear rate is decreased. The lack of a zero shear viscosity may be attributed to the persistence of a two-phase structure in the PEA melt. In such a structure, flow can take place only when the PA6 segments at the ends of the elastomer chains are pulled out of the domains. This is also seen to a very striking degree in S-B-S and S-EB-S block copolymers, which have very high (and very non-Newtonian) viscosities because of their extreme segmental incompatibility. In this context, the polyetheramide triblock copolymer of higher content of elastomer gives higher viscosity because of the increased segmental incompatibility, as demonstrated in Fig. 5.
Melt-spun filaments were produced from the 2-mm-diameter spinneret by drawing the melt stream down onto a take-up roll. The filament diameters were in the range of 180 to 400 [micro]m. Filament diameter decreased with increasing draw clown ratio [V.sub.L]/[V.sub.0].
The birefringence of melt-spun fibers was measured as a function of drawdown ratio. As shown in Fig. 9, the bireflingence is an increasing function of draw-down ratio. The maximum [DELTA]n is 0.02 for PA6 and almost zero for PEA. As shown In Fig. 9, the birefringence is an increasing function of drawdown ratio. In the course of the fiber spinning process, it was found that with greater elastomer content, spinnability becomes more difficult. Banker et al. (21) have corrected birefringence with spinline stress using their correlation.
Typical fiber stress-strain curves are shown in Fig. 10. The ultimate tensile strength and yield strength of PA6 at break are higher than those of a polyetheramide triblock copolymer. The initial moduli of these polyamide elastomers should depend on the amide content. The PEA filament shows higher elongation to break than the PA6 filament owing to the existence of the elastomer segment.
The mechanical properties of melt-spun filaments were determined. Uniaxial stress-strain curves of the filaments are presented in Fig. 10. In general, with an increase in take-up speed, elongation to break decreases while modulus and tensile strength increase. The Young's modulus and tensile strength of the PA6 filament are higher than those of polyetheramide block copolymer filament. However, the elongation to break of the PA6 filament is less than that of the block copolymer filament.
Polyetheramide Triblock Copolymer
The solubility parameters of PEA in various solvents are listed in Table 4. It showed similar solubility behavior except for C[H.sub.3]COOH. Acetic acid is a good solvent for PA6 but a poor solvent for PTMEG. The PEAS were completely soluble in C[H.sub.3]COOH at elevated temperatures (e.g., at the boiling point of C[H.sub.3]COOH, 102 ~ 4[degrees]C).
NMR spectral analysis was used in determining the composition of the block copolymers. Figure 7 provides the [H.sup.1] spectra of PA6 homopolymer, PTMEG prepolymer, and their block copolymer, along with the assignment of each signal. The protons of methylene are distinguishable from each other in polyamide 6 homopolymer. The resonance of [H.sup.a] is deshielded by strong electron-withdrawing carbonyl groups to a down field position ([delta]2.2), as compared to the protons [H.sup.b] ([delta]1.6 ~ 1.1). The resonance of [H.sup.c] is seen centered at [delta]3.15. The chemical shifts of [H.sup.b] and [H.sup.d] in PTMEG prepolymer exhibit absorption centered at [delta]1.61 and [delta]3.41, respectively. In the spectrum of polyetheramide triblock copolymer, the absorption which appeared at [delta]1.6 ~ 1.1 was the result from the overlap of [H.sup.b] existing in polyamide 6 and PTMEG prepolymer. The proton on a methylene adjacent to oxygen is shifted and seen at [delta]3.62. In spite of overlapped peaks, the mole f raction of PTMEG segments in block copolymers can be determined by comparing the relative areas of peak [H.sup.a] and peak [H.sup.d]. Since the molecular weight of PTMEG prepolymer is known from GPC analysis the molecular weight of polyamide 6-segment block in its block copolymer can be calculated. The results are in Table 3.
The thermal stability PEA is shown in Fig. 4 and Table 3. Thermal stabilities by TGA in nitrogen ranged from 302[degrees]C for the isocyanate-terminated PTMEG prepolymer, from 411[degrees]C lee to 415[degrees]C for PEAs, and from 420[degrees]C for commercial PA6 (AlliedSignal, Capron 8200) at 5 wt% loss starting point.
DSC curves for polyetheramide triblock copolymer and PTMEG prepolymer are shown in Fig. 3. The crystalline melting point ([T.sub.m]) of PA6 is generally 215[degrees]C. Two main endotherms corresponding to the melting of poly(tetramethylene ether) and PA6 segments were observed. The melting temperature of PA6 segments increased as the caprolactam content increased as a result of the increase of PA6 sequence length in the block copolymer. This explains the fact that polyetheramide triblock copolymer shows a lower [T.sub.m] than that of PA6 homopolymer. The melting temperatures of PTMEG prepolymer used for PEA were 28[degrees]C.
A novel continuous preparation method for polyamide-based block copolyetheramides via one-step reaction was developed. The synthetic route proposed in the present study provides several advantages over those methods developed in industry. Since ring opening of caprolactam is employed in the process, the overall reaction complexity has been reduced. Since it is a solvent-free process, solvent recovery is not required. Unlike the conventionally available preparation methods developed in the industry, no by-product is formed throughout the reaction. Devolatilization may not be necessary except for further removal of residual monomer. Monomer conversions over 90% were readily achieved by this process. The polymerization process was carried out involving caprolactam and polyether segments in a twin-screw extruder. Isocyanate terminated polymers were found to be effective activators for the continuous anionic polymerization of polyetheramide triblock copolymer in a twin-screw extruder. The conversions obtained in t his study are close to the equilibrium conversion of caprolactam, which is typically 94%. The conversion can be enhanced with devolatilization setup. Residual monomer can be easily removed by applying vacuum right after the third mixing element in a twin-screw extruder. The preparation method proposed in the present study seems to be affected by neither immiscibility nor phase separation. Homogeneity and uniform dispersion of reactants was largely improved by the mixing elements used in a twin-screw extruder. Thus, the lack of hindrance of chemical reaction in twin-screw extruder was seen by the high conversion obtained. The mechnical properties of the triblock copolymer indicate that it is a good rubber-reinforced polyamide or a polyamide thermoplastic elastomer depending upon the polyether content.
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Table 1 Comparison of Mean Residence Time and Variance for Different Screw Speeds. Screw speed (min.sup.-1) t (sec) [[sigma].sup.2.sub.1] Feed rate 20 582 93240 1.0 kg/hr 150 480 66600 250 474 51120 Table 2 Experimental Conditions and Results. Feed Composition (mole) PTMEG PTMEG Product prepolymer content Designation (mole) sodium MDI caprolactam (in wt%) PA6 (A) (a) 0 0.2 0.2 20 0 PA6 (B) (a) 0 0.2 0.2 20 0 PA6 (C) (a) 0 0.2 0.2 20 0 PA6 (D) (a) 0 0.2 0.2 20 0 PA6 (E) (a) 0 0.2 0.2 20 0 PA6 (F) (a) 0 0.2 0.2 20 0 PA6 (G) (a) 0 0.2 0.2 20 0 PEA (1) (b) 0.2 0.2 20 13 PEA (2) (b) 0.2 0.2 20 13 PEA (3) (b) 0.2 0.2 20 13 Reaction Feed Screw Product temp rate speed Convsion Designation ([degrees]C) (kg/hr) (rpm) (%) PA6 (A) (a) A (d) 1 20 92.5 PA6 (B) (a) B (e) 1 20 92.7 PA6 (C) (a) B (e) 1 150 92.1 PA6 (D) (a) B (e) 1 250 92.3 PA6 (E) (a) C (f) 1 20 91.8 PA6 (F) (a) C (f) 1 150 92.3 PA6 (G) (a) C (f) 1 250 92.0 PEA (1) (b) B (e) 1 20 86.6 PEA (2) (b) B (e) 1 150 86.0 PEA (3) (b) B (e) 1 250 86.1 (a)Polyamide 6. (b)Polyetheramide triblock copolymer. (d)(Hopper) 170 230 230 230 230 230 230 230 230[degrees]C (die). (e)(Hopper) 170 250 250 250 250 250 250 250 250[degrees]C (die). (f)(Hopper) 170 270 270 270 270 270 270 270 270[degrees]C (die). Table 3 PA6 and Polyetheramide Triblock Copolymer. Water or THF or Methanol- Chloroform- PTMEG Soluble Caprolactam soluble prepolymer Product fraction conversion fraction conversion Designation (g) (%) (g) (%) PA6(B) 92.7 Baytec ME040 Prepolymer PEA(1) (b) 0.43 91 0.13 79 THF or Chloroform-insoluble fraction Copoly- % PA6 merization Product weight PA6 yield yield Designation (g) (calc) (c) (%) (d) (%) (e) PA6(B) Baytec ME040 Prepolymer PEA(1) (b) 4.3 88 91 89 THF or Chloroform-insoluble fraction PTMEG PA6 Decom- Soft Hard position Phase phase in Product [T.sub.m] [T.sub.m] [N.sub.2] Designation ([degrees]C) ([degrees]C) ([degrees]C) PA6(B) 213 414 Baytec ME040 Prepolymer 22 302 PEA(1) (b) 27 210 413 THF or Chloroform-insoluble fraction Product GPC Designation [M.sub.N] [M.sub.W]/[M.sub.N] PA6(B) 19,000 1.53 Baytec ME040 Prepolymer 4,760 1.54 PEA(1) (b) 22,900 1.74 PEA (1)(b)Initial weight before extraction (4.86 g) (c)[THF-soluble fraction of weight]/[THF-insoluble fraction of weight] (d)Weight of polyamide 6 in copolymer/caprolactam weight in feed (e)[THF-insoluble fraction of weight]/[caprolactam weight in feed + PTMEG weight in feed] Table 4 Solubility of PAs and PEAs. Solvents Product designation Methanol [CHCI.sub.3] THF (c) PA6s - - - PEA(1)s - - - PEA(2)s - - - Solvents Product designation [CH.sub.3]COOH (a) [CH.sub.3]COOH (b) HCOOH PA6s - + + PEA(1)s - + + PEA(2)s - + + Solvents Product designation m-Cresol PA6s + PEA(1)s + PEA(2)s + (a)Solubility: + soluble at room temperature. - insoluble at room temperature. (b)Solubility: + soluble at boiling point of [CH.sub.3]COOH; - insoluble at boiling point of [CH.sub.3]COOH. (c)THF = Tetrahydrofuran.
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BYUNG H. LEE (1) and JAMES L. WHITE
(1.) Present address: Newell Rubbermaid, 1427 William Blount Drive, Maryville, TN 37801-8249.
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|Author:||Lee, Byung H.; White, James L.|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2002|
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