Aqueous polyurethane dispersions derived from polycarbonatediols and di(4-isocyanatocyclohexyl)methane.
Aqueous polyurethane dispersions are expanding their applications in coatings, adhesives, paper sizings, etc. because of the trend of the environmental regulations to decrease the level of solvent emissions [1-3]. The incorporation of ionic groups into polyurethanes is a practical method to obtain aqueous polyurethane dispersions, and various polyurethane ionomers have been described in the literature [1-11]. The cost of raw materials and the dispersing technology are two important factors in determining the feasibility of polyurethane dispersions for practical applications. Thus, one system with good commercial potential is the polyurethane dispersions derived from a diisocyanate, a commercial polyol and a carboxylic diol that can provide ionic groups [1, 7-11]. The presence of ionic groups not only provides dispersibility in water, but also increases the intermolecular force and enhances the strength, like other ionomers [12-16]. Typically, an NCO-terminated prepolymer ionomer is first prepared, which is readily dispersible in water. After dispersion, a chain extender such as diamine is added to couple the NCO groups to enhance the molecular weight. Then the solvent is removed to obtain the desired aqueous polyurethane dispersion. The effects of various factors, such as the structure and content of diisocyanates, polyols, carboxylic diols, neutralizing agents, and chain extenders, on the preparation of polyurethane dispersions and their properties have been studied extensively [4-11, 17-20].
Polycarbonatediols are recently commercialized polyols and are claimed to provide good hydrolysis resistance, heat aging resistance, oil resistance, weathering resistance, and fungi resistance . It is our recent effort to study the aqueous polyurethane dispersions derived from polycarbonatediols. Since the mechanical strength is significantly affected by the molecular weight of the polyurethane ionomers, we found a practical method to prepare the aqueous polyurethane dispersions derived from various polycarbonatediols and isophorone diisocyanate (IPDI) in which the dispersing procedure was modified to enhance the molecular weight . It was found that the IPDI based aqueous polyurethane dispersions exhibited rather low temperature resistance. If better heat resistance is required, a stiffer diisocyanate such as bis(4'-isocyanatocyclohexyl)methane (HMDI) may be involved. In this article, the aqueous polyurethane dispersions derived from HMDI, various polycarbonatediols and various carboxylic diols were prepared by our newly developed method. The properties of the aqueous polyurethane dispersions were characterized.
Three polycarbonatediols (L4672, L6002, and L5652) with a molecular weight of 2000 were supplied by Asahi Kasei Corporation. The polycarbonatediols were produced from 1,6-hexanediol and another diol of either 1,4-butanediol or 1,5-pentanediol by transesterification with ethylenecarbonate. The molar ratio of 1,4-butanediol to 1,6-hexanediol in L4672 is 70:30. The molar ratio of 1,5-pentanediol to 1,6-hexanediol in L6002 is 5:95 and that in L5652 is 50:50. L4672 and L5652 are liquid at room temperature, and they are easy to handle. L6002 is a white solid at room temperature. The polycarbonatediols were dried at 80[degrees]C and 5 mmHg for 3 h before use.
Di(4-isocyanatocyclohexyl)methane (HMDI), Desmodur[R] W, and Isophorone diisocyanate (IPDI), Desmodur[R] I, were obtained from Bayer and used as received. Dimethylolpropionic acid (DMPA) was of reagent grade (Aldrich). Dimethylol butyric acid (DMBA) was obtained from Nippon Kasei. A carboxylic polycaprolactonediol, Placcel 205BA, with a molecular weight of 500 was supplied by Daicel. Triethyl amine (TEA) and ethylene diamine(EDA) were of reagent grade (Merck) and treated with molecular sieve before use. N-Methylpyrrolidone (NMP) and dibutyltin dilaurate (T-12) were of reagent grade (Merck) and used as received.
Preparation of the Polyurethane Dispersions
The polyurethane dispersions were prepared by a method in which the dispersing procedure was modified as described previously . A typical standard procedure for obtaining HM4672B (HM represents the HMDI diisocyanate, 4672 represents the L4672 polycarbonatediol, and B represents the DMBA carboxylic diol) is as follows. Into a 1 L glass reaction kettle (equipped with a mechanical stirrer containing a torque meter, a thermometer, a condenser for reflux and a nitrogen gas inlet), 39.3 g (0.15 mol) of HMDI, 100 g (0.05 mol) of L4672, 7.4 g (0.05 mol) of DMBA, 6.1 g (0.05 mol) of TEA and 30 mL of NMP were added. Under nitrogen, the mixture was stirred at a speed of 100 rpm and heated to 80[degrees]C for 2.5 h to obtain an NCO-terminated prepolymer solution. Its NCO content was determined to be 2.29% using a di-n-butylamine back titration method . Then, the prepolymer solution was mixed with a small amount (0.5 g) of deionized water for dispersion step by step. The torque of the stirrer increased on adding of water and the mixture was diluted with NMP if necessary. After 2.0 g of water was added, the torque of the stirrer became steady and no significant increase in torque was observed, then enough water (356.8 g), for obtaining a solid content of 30%, was added. A solution of 1.5 g (0.0025 mol) of EDA in 2.0 g of deionized water was added to the prepolymer dispersion and stirred at 500 rpm at 30[degrees]C for 1 h. Then, the mixture was heated by a rotary film evaporator under 80[degrees]C and 5 mmHg to remove NMP so as to obtain a polyurethane dispersion with a solid content of 30%. The formulations in the preparation of the polyurethane dispersions are summarized in Table 1.
The particle size of the polyurethane dispersions was measured by a Photal Par-IIIs Photon Correlator (Otsuka Electrics) at 25[degrees]C. The molecular weight and molecular weight distribution of the polyurethane dispersions were determined by the gel pemeation chromatography(GPC) performed on a Tosoh HLC-8220 GPC equipped with a TSK gel Super HM-H*4 column and a RI detector. The eluent was dimethyl formamide (DMF), the flow rate was 0.5 mL/min, the operation temperature was set to be 40[degrees]C, and the molecular weight was calibrated with polystyrene standards.
The polyurethane dispersions were cast into films and dried. The tensile stress-strain data of dumb-bell shaped film specimens were determined by an Instron 4469 Universal Testing Machine at an extension rate of 100 mm/min at 23[degrees]C, and the gauge length used was 25 mm.
The IR spectra of the cast films were measured by a PerkinElmer 1600 series FTIR. The characteristic peaks are at 3375 [cm.sup.-1] (N-H), 2950 [cm.sup.-1] (C-H) and 1745 [cm.sup.-1] (C=O).
The DSC heating curves of the cast film samples from -100 to 250[degrees]C were determined by a DuPont DSC 910 at a heating rate of 20[degrees]C/min under nitrogen.
The dynamic mechanical properties of strip specimens (20 x 5 x 1 [mm.sup.3]) were measured by Dynamic Mechanical Analysis (DMA7, PerkinElmer), with a tensile mode at a forced vibration frequency of 1 Hz and a heating rate of 5[degrees]C/min.
RESULTS AND DISCUSSION
The formulations in the preparation of the polyurethane dispersions are summarized in Table 1. Scheme 1 shows the reaction procedure. In the preparation of the polyurethane dispersions, the molar composition of the reactants was held constant (molar ratio of HMDI:polycarbonatediol:carboxylic diol:TEA:EDA = 3.0:1.0:1.0:1.0:0.5). HMDI, polycarbonatediol, carboxylic diol, and TEA were mixed and reacted in NMP to form a prepolymer solution. To lower the over hydrolysis of the NCO groups, the prepolymer solution was mixed with a small amount (about 0.5 g) of water for dispersion step by step in a modified procedure. By this procedure, the amine groups formed by the hydrolysis of the NCO groups had enough time to couple the remaining NCO end groups, as indicated by the increasing viscosity of the dispersing medium. Afterwards, enough water was added. The obtained prepolymer dispersion exhibited a rather high content of NCO groups and this let the chain extension to be more effective as described previously . After the prepolymer dispersion was chain extended with EDA, the solvent was removed so as to obtain the desired polyurethane dispersions.
The modified dispersing procedure is an important factor to enhance the molecular weight of the polyurethane dispersions, as indicated by the GPC data. The [M.sub.n] (number average molecular weight) values of the polyurethane dispersions prepared by a conventional dispersing procedure were always found to be less than 10,000. The GPC data of the polyurethane dispersions are shown in Table 2. The [M.sub.n] values of these polyurethane dispersions, which were prepared by a modified dispersing procedure, were around 20,000, indicating that they have significantly higher molecular weights.
The particle size data of the polyurethane dispersions are summarized in Table 2. Since the dispersing procedure is rather complex, the effect of the structure of the polycarbonatediols on the particle size of the polyurethane dispersions shows no obvious trend. However, the chemical structure of the carboxylic diol seems to affect the average particle size of the polyurethane dispersions significantly. The polyurethane dispersions derived from the carboxylic polycaprolactonediol is finer than that derived from DMPA or DMBA, as shown in Table 2. This might be due to the more flexible nature of the carboxylic polycaprolactonediol unit, which might enhance the penetration of water molecule into polyurethane ionomers, and finer dispersions were obtained.
The polyurethane dispersions were cast into films and their tensile properties were determined. Typical tensile stress vs. strain curves of the cast films of the polyurethane dispersions are shown in Fig. 1. The elongation at break of the films is high, indicating that the films are very ductile. The results of some tensile properties are summarized in Table 3. It seems that the difference in the chemical structure of these polycarbonatediols does not affect the tensile properties greatly. However, the chemical structure of the carboxylic diols affects the tensile properties in an interesting way. The chemical structure of DMPA is more or less similar to that of DMBA, and it is reasonable that a polyurethane dispersion derived DMPA exhibits tensile properties similar to those of the corresponding polyurethane dispersion derived DMBA. However, the films of the polyurethane dispersions derived from the carboxylic polycaprolactonediol are significantly softer, exhibiting lower tensile strength and tensile moduli when compared with that of the polyurethane dispersion derived from DMPA and DMBA, as shown in Fig. 1 and Table 3. The thing is the more flexible nature of the carboxylic polycaprolactonediol. Thus, it is easy to design softer products with high elongation through the use of the carboxylic polycaprolactonediol.
[FIGURE 1 OMITTED]
Typical second run DSC heating curves of the polycarbonatediols and the cast films of the polyurethane dispersions are shown in Fig. 2. The DSC heating curve of L6002 exhibits a step inflection and a melting endotherm. The mid-point of the step inflection is taken as the glass transition temperature ([T.sub.g]) and the peak temperature of the melting endotherm is taken as the melting point ([T.sub.m]). The heat of fusion ([DELTA][H.sub.m]) of L6002 is rather high, as shown in Table 4. This indicates that L6002 can crystallize significantly after cooling from the molten state at a slow cooling rate. The DSC heating curves of L4672 and L5652 show only a [T.sub.g], but no [T.sub.m]. These two polycarbonatediols are liquid at room temperature. All the second run DSC heating curves of the cast films of the polyurethane dispersions exhibit only a step inflection corresponding to the glass transition temperature of the soft segments ([T.sub.g]S). The results are summarized in Table 4. It can be seen that the [T.sub.g]S values of the polyurethane dispersions are about 20[degrees]C higher than those of the corresponding polycarbonatediols, as shown in Table 4. This may be due to the presence of the ionic groups which hinder the motion of polycarbonate soft segments. The presence of the ionic groups might also hinder the crystallization of the polycarbonate soft segments in H6002P and H6002B, and thus no [T.sub.m]S was found.
[FIGURE 2 OMITTED]
Typical DMA (dynamic mechanical analysis) curves of the cast films of the polyurethane dispersions are shown in Fig. 3. The cast films of these HMDI based polyurethane dispersions derived from DMPA and DMBA show similar real modulus (E') vs. temperature curves (see H4672B curve in Fig. 3). There is a great drop in E' around -30[degrees]C, corresponding to the [T.sub.g]S. Moreover, there is another drop in E' around 100[degrees]C. The DMA measurements could not be done successively if the films became too loose because of the drop of E' upon the increasing of temperature. The end temperature of DMA testing (summarized in Table 4) may be a limit service temperature for the polyurethane dispersions, which means that the higher temperature of the HMDI based polyurethane dispersion films derived from DMPA or DMBA is likely to be in the range of 110-120[degrees]C. Thus, they exhibit higher heat resistance than the IPDI based polyurethane dispersion films derived from DMPA or DMBA with final temperatures of DMA testing of about 80[degrees]C .
The E' vs. temperature curves of the cast films of the HMDI based polyurethane dispersions derived from the carboxylic polycaprolactonediol are similar (see H4672PCL curve in Fig. 3). These E' vs. temperature curves show a plateau region below -40[degrees]C, and the curves become inclined up to the end point. The great drop in E' around -30[degrees]C corresponds to the [T.sub.g]S. Another great drop in E' is above 50[degrees]C. The final temperatures in DMA testing are around 70[degrees]C (listed in Table 4). The main difference of E' vs. temperature curve of the polyurethane dispersion films derived from the carboxylic polycaprolactonediol and from those derived from DMPA or DMBA (see Fig. 3) could be due to the more flexible nature of the carboxylic polycaprolactonediol. Cast films of the polyurethane dispersion films derived from the carboxylic polycaprolactonediol are softer, and exhibit very much lower moduli above 50[degrees]C. Thus they may lower the limit service temperature when compared with those derived from DMPA or DMBA.
All the tan [delta] vs. temperature curves of the HMDI exhibit a major damping peak around -30[degrees]C, corresponding to the glass transition temperature of the soft segments ([T.sub.g]S). The results are summarized in Table 4. It can be seen that the observed [T.sub.g]S data are consistent with the DSC data.
[FIGURE 3 OMITTED]
The modified dispersing procedure can be successful in enhancing the molecular weight in the preparation of the aqueous polyurethane dispersions derived from polycarbonatediols, HMDI, and carboxylic diols. The particle size of the polyurethane dispersions derived from the carboxylic polycaprolactonediol is finer than those derived from DMPA and DMBA, which is possibly due to the more flexible nature of Placcel 205BA. The films of the polyurethane dispersions derived from the carboxylic polycaprolactonediol were significantly softer, and had lower tensile strength and moduli, and higher elongation than those derived from DMPA and DMBA. The DMA data indicate that the HMDI based polyurethanes dispersion films derived from DMBA and DMPA can exhibit higher temperature resistance than the IPDI based ones.
The authors thank Asahi Kasei Corporation for the raw material supply of this work.
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Da-Kong Lee, Hong-Bing Tsai
Department of Chemical and Materials Engineering, National Ilan University, Taiwan, Republic of China
Department of Chemical and Materials Engineering, Ta Hwa Institute of Technology, Hsin-Chu, Taiwan, Republic of China
Correspondence to: Hong-Bing Tsai: e-mail: firstname.lastname@example.org
TABLE 1. Formulation of the polyurethane dispersions. HMDI (g) Polyol (g) Caboxylic diol (g) TEA (g) T-12 (g) H4672P 39.3 100 6.7 6.1 0.15 H6002P 39.3 100 6.7 6.1 0.15 H5652P 39.3 100 6.7 6.1 0.15 H4672B 39.3 100 7.4 6.1 0.15 H6002B 39.3 100 7.4 6.1 0.15 H5652B 39.3 100 7.4 6.1 0.15 H4672PCL 39.3 100 25.0 6.1 0.15 H6002PCL 39.3 100 25.0 6.1 0.15 H5652PCL 39.3 100 25.0 6.1 0.15 EDA (g) NMP (mL) Water (g) H4672P 1.5 30 358.8 H6002P 1.5 30 358.8 H5652P 1.5 30 358.8 H4672B 1.5 30 360.4 H6002B 1.5 30 360.4 H5652B 1.5 30 360.4 H4672PCL 1.5 30 401.5 H6002PCL 1.5 30 401.5 H5652PCL 1.5 30 401.5 P: DMPA; B: DMBA; PCL: Placcel 205BA. TABLE 2. GPC data and particle size of the polyurethane dispersions. [M.sub.n] [M.sub.w] [M.sub.w]/[M.sub.n] Particle size (nm) H4672P 19,200 76,000 4.0 93.3 H6002P 21,900 70,400 3.2 132.1 H5652P 16,300 65,800 4.0 123.2 H4672B 19,200 68,200 3.6 131.2 H6002B 21,700 75,100 3.5 74.8 H5652B 21,400 65,600 3.1 75.6 H4672PCL 21,600 89,600 4.1 58.0 H6002PCL 17,400 67,200 3.9 39.0 H5652PCL 16,900 68,500 4.1 41.3 TABLE 3. Tensile properties of the cast films. 100% 300% Tensile Elongation Modulus Modulus strength at break (MPa) (MPa) (MPa) (%) H4672P 6.6 12.0 59.9 1,190 H6002P 5.9 11.8 57.2 1,080 H5652P 6.6 11.0 48.0 1,100 H4672B 7.4 13.5 70.7 1,180 H6002B 6.5 14.0 55.0 1,020 H5652B 6.0 10.3 52.5 1,290 H4672PCL 2.8 4.6 41.0 1,440 H6002PCL 2.5 4.4 35.4 1,510 H5652PCL 2.6 4.0 31.6 1,920 TABLE 4. Thermal transitions of polycarbonatediols and casted films of the polyurethane dispersions determined by DSC and DMA. DSC, DSC, [T.sub.g]S DSC, [T.sub.m]S [DELTA][H.sub.m]S Transition method ([degrees]C) ([degrees]C) (J/g) L4672 -46 - - L6002 -54 42 43.5 L5652 -51 - - H4672P -29 - - H6002P -33 - - H5652P -28 - - H4672B -27 - - H6002B -33 - - H5652B -29 - - H4672PCL -30 - - H6002PCL -35 - - H5652PCL -34 - - DMA, [T.sub.g]S DMA testing end Transition method ([degrees]C) temperature ([degrees]C) L4672 - - L6002 - - L5652 - - H4672P -23 112 H6002P -29 124 H5652P -24 111 H4672B -23 108 H6002B -28 107 H5652B -24 109 H4672PCL -27 68 H6002PCL -34 64 H5652PCL -26 72