Polyurethane dispersions derived from polycarbonatediols and m-di(2-isocyanatopropyl)benzene.INTRODUCTION
The emission of organic solvent is a severe problem for solvent-borne polyurethane coatings. It is an important purpose to decrease the level of volatile organic concentration (VOC) for environment protection. Aqueous polyurethane dispersion coating is an example compliant with such trend, and is expanding their applications in various fields (1-3). The incorporation of ionic groups into poly-urethanes is a practical method to obtain aqueous polyurethane dispersions, and various polyurethane ionomers have been described in the literature (1-11). The presence of ionic groups not only provides dispersibility in water but also increases the intermolecular force and enhance 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, 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 (21). It is our recent effort to study the aqueous polyurethane dispersions derived from polycarbonatediols (22-25). As 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 various diisocyanates (22), (24). In this article, the aqueous polyurethane dispersions derived from m-di(2-isocyanatopropyl)benzene (TMXDI), various polycarbonatediols and various carboxylic diols were prepared by the modified 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 are derived from 1,6-hexanediol and another diol of either 1,4-butanediol or 1,5-pentanediol and 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.
TMXDI (m-tetramethylxylylene disocyanate) was obtained from Cytec Industries and used as received. 2,2-Di(hydroxymethyl) propanoic acid (dimethylolpropionic acid; DMPA) was Aldrich reagent grade and 2,2-di(hydroxymelhyl)butanoic acid (dimethylol butyric acid; DMBA) was obtained from Nippon Kasei. A carboxylic polycaprolactonediol, Placcel 205BA with a molecular weight of 500 was supplied by Daicel. The carboxylic polycaprolactonediol is derived from dimethylol propionic acid and caprolactone by ring opening polymerization. Triethyl amine (TEA) and ethylene diamine (EDA) were Merck reagent grade and treated with molecular sieve before use. Butanone (MEK) was Merck reagent grade 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 (22). A typical procedure is as follows.
Into a glass reaction kettle, 36.6 g (0.15 mol) of m-di(2-isocyanatopropyl)benzene (TMXDI), 100 g (0.05 mol) of L6002, 7.4 g (0.05 mol) of DMBA, and 6.1 g (0.05 mol) of TEA were reacted in 30 mL of MEK at 120[degrees]C at a speed of 100 rpm for 2.5 hr to obtain an NCO-terminated prepolymer. The NCO content of this prepolymer was determined by a di-n-butylamine back titration method (2), and the obtained value was 2.34%. The polyurethane prepolymer is cooled down to below 90[degrees]C. 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 the adding of water and the mixture was diluted with MEK 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 (352.9 g), for obtaining a solid content of 30%, was added. A solution of 2.0 g (0.00333 mol) of EDA in 2.0 g of deionized water was added to the prepolymer dispersion and stirred at 30[degrees]C for an hour. Then, the mixture was heated by a rotary film evaporator under 80[degrees]C and 5 mm Hg to remove MEK to obtain a polyurethane dispersion with a solid content of 30%. The polyurethane dispersion is denoted as T6002B, wherein T represents TMXDI, 4672 represents L6002 polycarbonatediol and B means that DMBA is used as the carboxylic diol. For other series, P means that DMPA is used as the carboxylic diol, and PCL means that the carboxylic polycaprolactonediol (Placcel 205BA) is used as the carboxylic diol. The formulations in the preparation of other polyurethane dispersions are summarized in Table 1.
TABLE 1. Formulation of the polyurethane dispersions. (a) Caboxylic diol (g) Water (g) T4672P 6.7 353.3 T6002P 6.7 353.3 T5652P 6.7 353.3 T4672B 7.4 354.9 T6002B 7.4 354.9 T5652B 7.4 354.9 T4672PCL 25.0 396.0 T6002PCL 25.0 396.0 T5652PCL 25.0 396.0 (a) Components of constant values: TMXDI: 36.6 g; Polyol: 100 g; TEA: 6.1 g; EDA: 2.0 g: MEK: 30 mL.
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 particle size of the polyurethane dispersions was measured by a Photal Par-IIIs Photon Correlator (Otsuka Electrics) at 25[degrees]C.
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 Perkin-Elmer 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=0).
The DSC heating curves of the cast film samples from -100[degrees]C to 250[degrees]C were determined by a Du Pont DSC 910 at a heating rate of 20[degrees]C/min under nitrogen.
RESULTS AND DISCUSSION
The procedure of the preparation of the polyurethane dispersions is shown in Scheme 1. The molar ratio of TMXDI:polycarbonatediol:carboxylic diol: TEA was held as 3.0:1.0:1.0:1.0. The reaction of TMXDI, polycarbonatediol, carboxylic diol, and TEA in MEK gave polyurethane prepolymers with isocyanate end groups. Theoretically, one-third of isocyanate groups would remain unreacted. The prepolymer solution was then dispersed in enough water, and water soluble EDA was added to couple the NCO end group for chain extension. The molar amount of isocyanate group used was set to be stoichiometrically greater than that of hydrozy group and amine groups (OH + [NH.sub.2]) due to the hydrolysis of the NCO groups by water during dispersing. Fortunately, the hydrolytic reaction formed amine groups which reacted with isocyanate group to enhance the molecular weight.
When the [[NH.sub.2]/NCO] ratio during chain extension was 0.5:1.0 as used previously (22), (23), the Mn values of the obtained polyurethane dispersions were always less than 12,000. It has been found that the [[NH.sub.2]/NCO] ratio during chain extension was one of the important factors governing the molecular weight of the polyurethanes. Thus, the [[NH.sub.2]/NCO] ratio during chain extension was varied to obtain polyurethane dispersions for comparison. Figure 1 shows the effect of the [[NH.sub.2]/NCO] ratio during chain extension on the Mn of a typical polyurethane dispersion (T6002B). It can be seen that the molecular weight tends to reach a maximum around an [[NH.sub.2]/NCO] ratio during chain extension of 0.67:1.0. This value is greater than the previous composition for the Isophorone diisocyanate (TPDI) and Di(4-isocyanatocyclohexyl)methane (HMDI) system (22), (24). This should be due to the lower reactivity of TMXDI with respect to water as compared with IPDI and HMDI (26). Thus, this [[NH.sub.2]/NCO] ratio was used for the preparation of other polyurethane dispersions. After chain extension, MEK was removed to form polyurethane dispersions with a solid content of 30%.
[FIGURE 1 OMITTED]
The formulations in the preparation of the polyurethane dispersions arc summarized in Table 1. The molecular weight data determined by GPC are listed in Table 2. The [M.sub.n] values are all around or greater than 20,000. This indicates that the use of an [[NH.sub.2]/NCO] ratio during chain extension of 0.67:1.0 was also effective in enhancing the molecular weight for this TMXDI system. The molecular weight distribution index ([[M.sub.w]/[M.sub.n]]) is close to 2, which seems a result of the step polymerization conducted in a homogeneous system (27). This might be also due to the low reactivity of TMXDI.
TABLE 2. GPC data and particle size of polyurethane dispersions. Sample [M.sub.n] [M.sub.w] [[M.sub.w]/ Particle size (nm) [M.sub.n]] T4672P 21,300 44,000 2.1 135 T6002P 21,000 39,500 1.9 112 T5652P 23,000 46,600 2.0 126 T4672B 29,700 51,100 1.7 147 T6002B 21,600 44,000 2.0 93 T5652B 22,100 41,700 1.9 118 T4672PCL 20.100 37,400 1.9 41 T6002PCL 42.000 89,500 2.1 73 T5652PCL 21,100 38,400 1.9 60
The average particle size data of the polyurethane dispersions are summarized in Table 2. The particle size of the dispersion may be affected by the ionic content of the resin, the chemical structure of the resin, and the dispersing procedure. It seems that the effect of the chemical structure of the polycarbonatediol on the particle size of the polyurethane dispersions shows no obvious trend, as shown in Table 2. As the ionic content of the resin was similar for the polyurethane dispersions (in Table 1, molar content of carboxylic diol), so the effect of this factor can be excluded. However, the chemical structure the carboxylic diol seems to affect the average particle size of the polyurethane dispersions significantly. The polyurethane dispersions derived from the carboxylic polycaprolactonediol (Placcel 205BA) exhibit significantly smaller particle size 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, similar to the cases described previously (23), (24).
The aqueous polyurethane dispersions were cast into films and their tensile properties were determined. Typical stress (load/original cross-sectional area) versus strain curves are shown in Fig. 2. The tensile properties of the aqueous polyurethane dispersions are summarized in Table 3. The elongation at break of the films is high, indicating that the films arc very ductile. It seems that the difference in the chemical structure of the polycarbonate-diols 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. On the other hand, the films of the polyurethane dispersions derived from Placcel 205BA behave significantly softer as shown in Fig. 2, and their tensile strength and tensile moduli are also significantly lower when compared with the polyurethane dispersion derived from DMPA and DMBA and shown in Table 3. The thing is the more flexible nature of Placcel 205BA. Thus, it is easy to design softer products with high elongation through the use of Placcel 205BA.
[FIGURE 2 OMITTED]
TABLE 3. Tensile properties of casted films of polyurethane dispersions. Sample 100% modulus (a) 300% modulus (b) Tensile Elongation (MPa) (MPa) strength at break (%) (MPa) T4672P 4.9 11.3 65 680 T6002P 4.0 13.2 67 640 T5652P 3.8 8.4 63 740 T4672B 6.7 16.4 74 670 T6002B 6.3 18.9 69 600 T5652B 4.2 9.5 63 710 T4672PCL 3.1 5.4 43 780 T6002PCL 2.3 3.8 45 830 T5652PCL 2.9 4.9 38 910 (a) Engineering stress at engineering strain of 100%. (b) Engineering stress at engineering strain of 300%.
Typical second run DSC heating curves of the polycar-bonatediols and the casted films of the polyurethane dispersions are shown in Fig. 3. The DSC heating curve of L6002 exhibits a step inflection and a melting endotherm. The midpoint 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]. 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 20-30[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 and better miscibility between the polycarbante soft segment and polyurethane segment (28). The presence of the ionic groups might also hinder the crystallization of the polycarbonate soft segments in T6002P, T6002B and T6002PCL. and thus no [T.sub.m]S was found.
[FIGURE 3 OMITTED]
TABLE 4. Thermal transitions of polycarbonatediols and cast films of the polyurethane dispersions determined by DSC. Sample [T.sub.g]S ([degrees]C) [T.sub.m]S ([degrees]C) [DELTA] [H.sub.m]S (J/g) L4672 -46 -- -- L6002 -54 42 43.5 L5652 -51 -- -- T4672P -25 -- -- T6002P -35 -- -- T5652P -28 -- -- T4672B -27 -- -- T6002B -23 -- -- T5652B -28 -- -- T4672PCL -33 -- -- T6002PCL -36 -- -- T5652PCL -37 -- --
TMXDI was successfully used to prepare aqueous polyurethane dispersions derived from various polycarbonatediols and various carboxylic diols by a method in which the dispersing procedure was modified to enhance the molecular weight. The [[NH.sub.2]/NCO] ratio during chain extension affected the molecular weight of the polyurethanes, significantly. The molecular weight tends to reach a maximum around an [[NH.sub.2]/NCO] ratio during chain extension of 0.67: 1.0 and this optimum ratio was used in preparations. The effect chemical structure of the polycarbonatediols on the properties shows no obvious trend. However, the polyurethane dispersions derived from the carboxylic polycaprolactonediol exhibit smaller particle size and softer tensile properties when compared with those derived from DMPA and DMBA.
The authors thank Asahi Kasei Corporation for the raw material supply of this work.
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Da-Kong Lee, (1) Zong-Da Yang, (1) Hong-Bing Tsai, (1) Ruey-Shi Tsai (2)
(1) Department of Chemical and Materials Engineering, National llan University, I-Lan 260, Taiwan, Republic of China
(2) Department of Chemical and Materials Engineering, Ta Hwa Institute of Technology, Chung-Lin, Hsin-Chu, Taiwan, Republic of China
Correspondence to: Hong-Bing Tsai; e-mail: email@example.com
Contract grant sponsor: The National Science Council of the Republic of China; contract grant number: NSC-97-2216-E-197-005-CC3.
Published online in Wiley InterScience (www.interscience.wiley.com).