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Ind. Eng. Chem. Res. 2005, 44, 4504-4512
Synthesis, Structure, and Properties of Novel Polyols from Cardanol and Developed Polyurethanes† Kattimuttathu I. Suresh* and Vadi S. Kishanprasad Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500007, India
The development of a new class of polyurethane polyols from cardanol, a renewable organic resource obtained as a byproduct of the cashew industry, is reported. For synthesizing polyols the monoglycidyl ether of cardanol was prepared first, followed by ring opening to prepare the diol or reaction with diethanol amine to give a triol. Alternately, another triol was also prepared by reaction of the glycerol monochlorohydrin with cardanol. Polyols having a range of hydroxyl values (140-265 mg of KOH/g) were prepared, and structure characterization was carried out by spectroscopic techniques. Polyurethanes were formulated by reaction of these polyols with diphenyl methanediisocyanate (MDI) at an NCO/OH ratio of 1, and films were characterized for thermal stability and viscoelastic properties by thermogravimetric analysis (TGA) and dynamic rheometry, respectively. The dynamic mechanical studies show a linear increase of the Tg value with an increase in the hydroxyl value of the polyol. In TGA, when temperatures at 50% decomposition in nitrogen atmosphere are compared, polyurethanes based on diol and glycard indicate a higher thermal stability. 1. Introduction Renewable organic resources continue to be in the common interest of both academic and industrial laboratories at all the times.1-5 The topic has attained a renewed interest for reasons of economy and environmental friendliness and contributes well to green chemistry practices. Among the renewable resources, cashew nut shell liquid (CNSL), obtained as a byproduct of the cashew processing industry, is unique in that it contains a phenolic moiety with an unsaturated 15-carbon side chain, as shown in Figure 1.6 This combination offers a wide variety of possibilities for the synthetic chemist. Its extraction, chemistry, and composition have been well-documented.6,7 Considerable attention from polymer scientists throughout the world is devoted to utilize their potential attributes as a substitute for petrochemical derivatives and has found use in phenolic resins for break lining, surface coatings, and other miscellaneous applications.7-9 Of late it has been used in the preparation of many speciality materials, such as liquid crystalline polyesters,10 nanotubes,11 cross-linkable polyphenols,12,13 polyurethanes,14-17 and a range of other speciality polymers and additives.18-25 Polyol is one of the essential raw materials (monomers) in the preparation of any polyurethane product. Depending upon the hydroxyl value and other characteristics of the polyol, it finds application in the development of adhesives, coatings, and flexible or rigid foams. Thus, polyurethane polymers provide a versatile range of properties and applications, and their structure can be tailored to suit specific requirements. However, these materials have low thermal stability, primarily due to the presence of urethane bonds. Among renewable resources, vegetable oils have long served as a source † IICT Communication No. 041113. * To whom correspondence should be addressed. Tel.: +9140-27193149. Fax: +91-40-27193991. E-mail: kisuresh@ iict.res.in.
Figure 1. Structure of cardanol.
of polyols for making polyurethanes. They have yielded products with better thermal stability and mechanical properties.26-29 The use of cardanol derivatives in polyurethane preparation by a few other methodologies has been reported,14-17 and products with better thermal, mechanical, and chemical characteristics were obtained. Despite such studies on the synthesis, chemical modification, and functionalization of cardanol, detailed studies on the structure-property relationships of cardanol-based monomers are limited. When applied to the synthesis of polyols, cardanol-based polyols will have better hydrolytic stability compared to the triglyceride oil based polyols. The objective of the present work was to develop a series of novel polyurethane polyols from cardanol and study the structure-property relationship. It is shown that using suitable methodologies, starting from cardanol, liquid low-viscosity polyols with a range of hydroxyl value could be prepared and the prepared polyurethanes exhibit enhanced thermal stability. 2. Experimental Section 2.1. Materials. Technical-grade CNSL of Indian Standard specification IS 840 (1964) was obtained from Mercury Engineering, Hyderabad, India. Cardanol was purified by double-vacuum distillation.7 The chemicals epichlorohydrin, diethanol amine, glycerol, anhydrous ZnCl2, NaOH (AR, S. D. Finechem), and diphenyl methane diisocyante (Fluka) were used as received
10.1021/ie0488750 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4505 Scheme 1. Schematic of the Reactions Used for the Synthesis of Polyols
without any further purification. The catalysts BF3etherate and dibutyl tin dilaurate (DBTDL) were from E. Merck. The solvents toluene, ethyl acetate, chloroform, methanol, acetone, and triethylamine (S. D. Finechem) were dried by standard methods.30,31 2.2. Methods. All the products were characterized for structure and physicochemical characteristics. The specific gravity was measured using a specific gravity bottle and the hydroxyl value was determined by the pyridine-acetic anhydride method (ASTM-1957-86). Infrared spectra were recorded as thin films on a Nicolet machine. 1H and 13C NMR measurements were carried out in CDCl3 on a Varian 300 MHz machine using TMS as an internal standard. The mass spectra measurements were carried out on VG micromass 7070H instrument at 70 eV. 2.3. Viscoelastic Properties. Dynamic mechanical analysis was performed, to study the variation of storage modulus (E′), loss modulus (E′′), and tan δ with temperature, on a dynamic mechanical spectrometer (DMTA model Rheometric IV). The test was conducted in rectangular tension mode using specimens of 12 mm length, 8 mm width, and 0.5 mm thickness. A constant strain of 0.002% was applied sinusoidally. The test was conducted at 1 Hz and the heating rate used was 3 °C/ min. 2.4. Thermal Analysis. The thermal behavior of the polyurethane film was studied on a Mettler Toledo Thermogravimetric analyzer at a heating rate of 10 °C/ min in N2 atmosphere. The hardness measurements
were carried out using a Shore D durometer from Blue Star Industries as per ASTM 2240 (1975). 2.5. Synthesis of Polyols. In the following section a description of the synthetic procedures adopted in the present work is given. The general scheme of the reactions used for derivatization to the polyols is summarized in Scheme 1. (a) Synthesis of the Monoglycidyl Ether of Cardanol (Epicard). In the first step cardanol was reacted with epichlorohydrin under alkaline conditions to give the monoglycidyl ether. In a typical experiment, 119.4 g (0.3 mol) of cardanol in a 500-mL round-bottom flask fitted with a mechanical stirrer, thermometer, and dropping funnel was heated to 95 ( 2 °C. Then 0.12 g (∼ 0.1%) of anhydrous ZnCl2 was added. The required quantity of epichlorohydrin (37.3 mL, 0.48 mol) was then added dropwise while the temperature was maintained. After its addition the reaction was continued for 2-3 h. Then a stoichiometric amount of sodium hydroxide (19.3 g/100 mL water) was added dropwise. The reaction temperature was increased to 100 ( 2 °C and heating continued for 2-3 h. The product was separated and washed with excess water to remove the byproduct sodium chloride and other unreacted materials. It was then dried over anhydrous sodium sulfate. The product was obtained in 80-85% yield. (b) Synthesis of Diol. The epoxide ring, when opened up under acidic conditions, will give a glycol. In a typical procedure, epicard (10.21 g, 0. 0.028 mol) was mixed with twice its weight (20 g) of 10% H2SO4 in a 250-mL three-neck round-bottom flask, fitted with a
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mechanical stirrer, thermometer, and reflux condenser. The reaction mixture was heated under reflux for about 14 h. The product, extracted in ether, was washed with water until neutral to litmus and dried over anhydrous sodium sulfate. The product was isolated in 85% yield. (c) Synthesis of Triol. For preparation of the triol, epicard and diethanol amine were reacted at reflux in the presence of ethanol (cf. Scheme 1). In a typical procedure, epicard (10 g, 0.028 mol) was mixed with an equal weight of ethanol and a molar excess of diethanol amine (3.49 g, 0.033 mol) in a 250-mL round-bottom flask, fitted with a mechanical stirrer, thermometer, and a reflux condenser. After 7 h ethanol was removed from the product on a rotary evaporator. The product was separated and washed with a water-ethanol mixture (1:1) and finally with water to remove excess diethanol amine, if any. It was dried over anhydrous sodium sulfate. The isolated yield was 85%. (d) Synthesis of the Triol Glycard from Glycerol and Cardanol. Apart from the methodology through epicard, described in Scheme 1, the triol was also prepared using a different route. In a standard experiment, 27.6 g (0.3 mol) of glycerol was mixed with BF3etherate catalyst (0.25 mL, ∼1%) in a 250-mL threeneck round-bottom flask fitted with a mechanical stirrer, thermometer, and a dropping funnel under nitrogen atmosphere. The content of the flask was heated to 40 ( 5 °C and then epichlorohydrin (27.76 g, 0.3 mol) was added dropwise, while the temperature was maintained. External cooling was provided whenever required. Afterward the reaction continued at 40 °C for 3 h more. The glycerol monochlorohydrin so prepared was added dropwise to cardanol (92.2 g, 0.3 mol) in 50 mL of ethanol under alkaline conditions. During the addition the reaction temperature was maintained at 78 °C. The reaction was continued at the temperature for 8-10 h. The product was separated and washed with an ethanol-water mixture to remove the byproduct NaCl and any unreacted starting materials. The product was isolated in 80% yield. In all the experiments (cf. procedures a-d above), the components of epicard and the polyols were separated on a 60-120-mesh silica gel column using a solvent system consisting of toluene and varying proportions of ethyl acetate for spectroscopic characterization. 2.6. Polyurethane Synthesis. To study the suitability of using the synthesized polyols and cardanol in the preparation of polyurethanes, the polyol was reacted with diphenyl methanediisocyanate (MDI), and filmforming characteristics of the products were evaluated. After determination of the hydroxyl value, all of the polyurethanes were formulated at an NCO/OH ratio of 1. The required amount of polyol was placed in a 250mL round-bottom flask and purged with N2 for 2 min. Then 2 drops of DBTDL catalyst were added. The required amount of MDI as a solution in toluene or THF was added to the polyol solution over a period of 30 min. The temperature was raised to 70 °C for 4 h. The product was poured into a Teflon Petri dish and dried at 120 °C for 2 h. The films were tested for hardness, thermal stability, and viscoelastic properties. 3. Results and Discussion In this work, the synthesis, characterization, and properties of three different polyols starting from cardanol were undertaken. The mechanical properties of
Table 1. Infrared Transmittance Assignment for Cardanol vibration mode
functional group
wave no. (cm-1)
O-H stretch C-H stretch C-H stretch CdC ring stretch in-plane C-O-H deformation C-O stretch out-of-plane C-H bend
hydroxyl aromatic methyl aromatic aromatic C-O-H C-O-H methylene
3350 3010 2930, 2880 1590, 1450 1350 1150 780, 700
the polyurethanes prepared from them were also evaluated. Owing to the structural complexity of cardanol6 and to ensure raw material consistency, it was thought appropriate to characterize cardanol first. Then structural characterization of the developed polyols was carried out. These results are discussed in the following sections. 3.1. Characterization of Cardanol. The main component of technical grade cashew nut shell liquid is cardanol. Distillation of cardanol was carried out at 210-240 °C under 4-5 mmHg pressure. The distillate obtained was pale yellow in color, which darkened on storage. The distillation was carried out to a yield of 90%. The preparative thin layer chromatogram of the sample was observed on a glass plate coated with silica gel G as reported earlier by Gedam et al.32 The plates were developed in a solvent system consisting of toluene and chloroform. (40:60, v/v), and the spots were observed by spraying concentrated H2SO4 and charring at 80 °C for 10 min. Three spots separated out, the upper spot, with an Rf ) 0.59, corresponds to cardanol, a middle spot appearing at Rf ) 0.38 to 6-methylcardol, and a lower spot with an Rf ) 0.10 to cardol. This is in agreement with the literature values.33 Distilled cardanol was further characterized by IR, 1H and 13C NMR, and mass spectroscopy. In the IR spectrum, peaks at 900 cm-1 are due to vinyl unsaturation in the side chain. The absorption band at 1590 cm-1 indicates the presence of the phenolic group. The complete assignments are given in Table 1. Detailed characterization has been done with NMR and MS, and the spectra with possible assignments are given in Figure 2. 1H NMR in Figure 2a shows aromatic proton peaks at δ 6.5-7.2 ppm range. The peaks in the range δ 4.95.4 ppm point to vinyl unsaturation. The peaks in the upfield region, i.e., between δ values 3.0 and 0.8 ppm, suggest the presence of alkyl chain unsaturation. In the 13C NMR (Figure 2b) the peaks due to olefinic unsaturation appear at δ values in the range 114-132 ppm. There are also aromatic carbon peaks in the range 112128 ppm. The peak at 156 ppm is for the aromatic carbon attached to the oxygen atom. The aliphatic carbons of the side chain appear in the range 14-36 ppm. Further, the mass spectra (Figure 2c) give the molecular ion peak, base peak, and other fragment ions. 3.2. Synthesis of Polyols. In recent years, various methodologies useful for the design of monomers based on cardanol have been reported.34 In the present work we chose modification through the phenolic hydroxyl group (cf. Scheme 1) for preparation of the polyols. In one of the approaches the monoglycidyl ether (designated as epicard) was prepared first. The 2,3-epoxy propyl function is a versatile one, and its synthetic applications are well-documented.35,36 The developed polyols were designated as diol and triol. In the second approach, glycerol was reacted with epichlorohydrin to
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4507
Figure 2.
1H
and
13C
NMR and EI-MS of cardanol.
give the monochlorohydrin. This, when reacted with cardanol under alkaline conditions, gave the polyol (glycard). Though both triol and Glycard were trifunctional, they were prepared by two different methodologies (cf. Scheme 1) and it is expected that the functional groups, due to the varying number of primary and secondary -OH groups show different reactivity when used in the preparation of polyurethanes. All reaction products were purified by column chromatography on 60-120-mesh silica gel columns, using TLC to check the progress of the column separation and analyzed by spectroscopic techniques. The spectral characteristics of the monoglycidyl ether and the developed polyols are summarized in Table 2. The important features, as evident from Table 2, along with the IR characteristics of the monoglycidyl ether (epicard) are discussed below. The IR spectrum of epicard has characteristic absorption bands at 1060 and 1260 cm-1, indicating the formation of the phenolic ether linkage. In the 1H NMR spectra (cf. Table 2), the peaks characteristic to the epoxide group appear at δ (ppm) values 4.10, 3.4, and 2.9-3.1. The 13C NMR spectra confirm the formation of the epoxy derivative
as it shows characteristic peaks at δ 44.3 and 49.9 ppm for the glycidyl ether group and a peak at δ 68.6 ppm due the carbon attached to the phenolic oxygen.37,38 The mass spectrum (cf. Table 2) showed a molecular ion peak at m/z 358, corresponding to the monoglycidyl ether of cardanol. Epicard was further reacted with appropriate reagents to produce polyols with a range of hydroxyl values (cf. Scheme 1). For example, to prepare the diol, the monoglycidyl ether of cardanol was hydrolyzed by refluxing with 10% aqueous H2SO4 for 14 h. The epoxide ring, when opened up under acidic conditions, will give a glycol. The completion of the reaction is checked by the products’ solubility in ethanol. The diol is soluble in ethanol, but not epicard. The 1H NMR spectra (cf. Table 2) of the purified product showed the absence of characteristic peaks due to epoxide ring protons; instead, peaks characteristic for the -CH2 and -CH groups attached to the -OH group appears. The 13C NMR showed characteristic peaks due to the polyol groups at δ 63.5 ppm due to the primary hydroxyl group and a peak at δ 70.8 ppm for the secondary hydroxyl
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Table 2. Spectral Data for the Monoglycidyl Ether and the Polyolsa
a In the 1H and 13C NMR only the assignments characteristic for the functional groups are tabulated. Other peaks that are present in the cardanol spectra (Figure 2) are common in the derivative spectra also.
group. The mass spectra (cf. Table 2) gave the molecular ion peak at m/z 378. Triol. The facile reaction between the epoxy and amine group was exploited for the preparation of the triol. This reaction has been found to be very useful in synthetic chemistry, and the mechanism and rates of this reaction have been studied extensively.39,40 In the present work, epicard was reacted with diethanol amine and the product structure analyzed. In the IR spectrum, the hydroxyl absorption at 3350 cm-1 has increased in intensity, so the absorption band (at 1050 cm-1) due to the stretching vibration of ether linkage has decreased. The 1H NMR spectra show the characteristic triol groups (viz. -CH2 attached to -N and -CH2 attached to -OH function) in the range δ 3.6-4.3 ppm. In the 13C NMR (cf. Table 2 for structure) the peaks at δ values 68.5, 68.6, and 69.8 ppm due to two primary and secondary alcohol groups, respectively, also support this observation. The carbon atoms attached to the nitrogen appear at δ values 57.3, 59.3, and 58.5 ppm. The FABmass spectrum shows the molecular ion peak at m/z 465. Glycard. As a different methodology to prepare the triol, glycerol was reacted with cardanol through the chlorohydrin route. Spectral analysis of the purified product by IR spectroscopy showed an increase in -OH absorption at 3350 cm-1 and ether absorption at 1050 cm-1 in comparison to the spectrum of epicard and cardanol, respectively. The NMR (cf. Table 2) data shows the characteristic peaks due to the presence of
-CH2 OH and -CHOH groups at δ 3.5-4.1 ppm in the 1H NMR and δ 63.5, 70.9, and 72.6 ppm in the 13C NMR, respectively. The peaks at δ 69.4 and 69.3 ppm appear as doublets due to two carbons forming the ether linkage. The mass spectrum shows the molecular ion peak at m/z 452. The hydroxyl value of the polyol was determined by the pyridine-acetic anhydride method.41 In summary, the physicochemical characteristics of the synthesized polyols are given in Table 3. 3.3. Synthesis and Properties of Polyurethane. In the next step, polyurethanes (PUs) were synthesized from these polyols, and products are labeled as PUD, PUT, and PUG based on diol, triol, and glycard, respectively. Table 4 provides the details of PU synthesis from the cardanol-based polyols. These polyurethanes appear to be transparent, amorphous singlephase systems, which are further confirmed in DMTA studies. Though the reaction of epoxy and isocyanate groups is known,42 epicard did not give any integral film due to its monofunctionality. 3.3.a. Dynamic Mechanical Properties. The dynamic mechanical analysis of the films in tensile mode helps in evaluating the viscoelastic properties of the materials. The technique is very useful in evaluating the material properties.43 Typical DMTA traces showing variation of the dynamic mechanical properties are shown in Figure 3. The samples appear to be homogeneous, as indicated by a narrow damping (tan δ) peak.
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4509 Table 3. Physicochemical Characteristics of the Polyols product epicard diol triol glycard a
OH value (mg KOH/g)
sp gr (g/cm3)
ref indexa
140 265 251
0.9652 0.9807 0.9875 1.008
1.519 1.524 1.521 1.518
functionalityb
mol wtc
appearance
2 3 3
358 378 465 452
pale yellow light brown golden yellow brownish
Measured with an Abbe´ refractometer. b Theoretical. c From m/z value.
Table 4. Reactions of Polyols with MDIa polyol
nature of film
Shore D hardness
cardanol epicard diol triol glycard
no integral film no integral film semiflexible film semirigid film rigid film
65 70 80
a In all the cases, NCO/OH ) 1 and DBTDL (0.5% of polyol weight) was employed as catalyst.
The peak temperature (75 °C) could be taken as a measure of the glass transition temperature, and all the samples have Tg values well above room temperature. The tan δ value of the polyurethanes is taken to be a direct measure of the Tg value of the material, and it shows a linear relationship to the hydroxyl value of the polyol. The temperature at which the tan δ peak occurs increases linearly with the hydroxyl value of the polyol. A comparison of the tan δ behavior of the samples in Figure 4 shows that glass transitions of cardanol-based polyurethanes increase more or less linearly with increasing OH number. This behavior is analogous to that of vegetable oil based polyurethanes as observed by Petrovic et al.29 The lowest value of the Tg was observed with PUD (75 °C, cf. Figure 4) and the highest value for PUT (98 °C). The variation of the storage modulus is shown in Figure 5. Initially, when the scan was performed up to 200 °C, the samples were intact after the test. However, it can be seen from Figure 5 that in all samples the moduli exhibit a tendency to increase after 150 °C. This suggested that the thermal stability of the PU is higher than 200 °C. So in one case (glycard) the
final temperature was 350 °C and the test sample was found to degrade at the end of the test. In the variation of tan δ, also a second peak was observed at 314 °C (cf. Figure 4) in that case. This second peak (in both tan δ and E′) could be due to the cross-linking of the side chain unsaturation and could be assigned to the beginning of the cross-linking reaction, and later as the temperature increases, decomposition takes place.44,45 This also shows that the damping property (smaller value of tan δ at 314 °C) decrease with increasing temperature. Reduced damping means the chains are more tightly held together by cross-linking and less capable of dissipating the absorbed energy. The hardness values of the films (cf. Table 4) show a mixed trend, and the value was least for PUD and highest for PUG. 3.3.b. Thermal Stability. The thermogravimetric analysis of polyurethanes based on petroleum-based polyols suggest poor thermal stability.29,46,47 The onset of urethane bond dissociation is somewhere between 150 and 220 °C, depending upon the type of substituent or the isocyanate and polyol side. In the present study, TGA investigation was carried out for all polyurethanes from ambient temperature to 600 °C. The TGA curves are shown in Figure 6. The parameters studied are the temperature corresponding to initial decomposition (Ti), 50% weight loss (T1/2), and char yield at 600 °C.48 Comparison of these values in Table 5 shows that there is an overall increase in the thermal stability, when the decomposition behavior of the different polyurethanes are compared. Petrovic et al.28 in the case of vegetable oil based urethanes observed a similar behavior. It was shown
Figure 3. Typical DMTA trace of polyurethane prepared from diol and MDI showing the variation of dynamic mechanical properties E′, E′′, and tan δ with temperature.
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Figure 4. Comparison of the tan δ traces of the polyurethanes prepared from the diol, triol, and glycard.
Figure 5. DMTA traces showing the variation of storage modulus (E′) for the synthesized polyurethanes with temperature.
Figure 6. Thermogravimetric traces of the cardanol-based polyurethanes in N2 atmosphere at a heating rate of 10 °C/min.
that poly(oxypropylene) (PPO) based polyurethanes degrade in a single step, whereas vegetable oil based polyurethane show a two-step decomposition. Derivative TGA of the same samples in our work actually revealed two and three main degradation processes. The first
stage of the decomposition was associated with the first 20% of weight loss and the second stage with the remaining weight loss. The shape of the weight loss curves of all the PUs was almost identical except for PUT, which decomposed at a slower rate.
Ind. Eng. Chem. Res., Vol. 44, No. 13, 2005 4511 Table 5. Thermal Decomposition Characteristics of Cardanol-Based Polyurethanes sample code PUD PUT PUG
onset of dec (°C) 210 150 210
T10 (°C)
Eaa (kJ/mol)
T50 (°C)
% char yield at 600 °C
310 250 305
48 42 46
440 380 422
30 10 22
a The activation energy was computed from Coats-Redfern plots.
When the onset temperature of decomposition is considered, it can be seen that polyurethanes based on the triol start decomposition at 150 °C, suggesting that they have lower thermal stability compared to conventional polyurethanes. But for the diol- and glycard-based polyurethanes, the decomposition temperature is slightly higher (cf. Table 5). This trend is visible throughout, as T10 and T50 temperatures indicate. This suggests a higher thermal stability of the polyurethanes, probably due to the capability of the polyol to undergo thermal cross-linking at higher temperatures,44,45 agreeing with the observation in DMTA studies. But in the case of the triol, the labile C-N bond may be causing decomposition to take place at lower temperature (cf. 150οC). It is worth mentioning that the final decomposition temperature of this polyurethane is higher than that of PPObased polyols (in PPO-based polyurethanes decomposition is complete at around 250 °C).29 Between PUD and PUG not much difference in thermal stability is observed when the temperatures at 50% decomposition are compared; this suggest that the reason for increased thermal stability is the same in both cases (viz. thermal cross-linking).44,45 The derivative TGA curves (not given here) show that the PUs based on the diol and glycard degrade in a single step, whereas PUT shows a twostep decomposition (two decomposition peaks at 320 and 370 °C). The difference in thermal stability could be the reason for the increased char yields at 600 °C for PUD and PUG. The TGA studies thus show that the cardanolbased polyurethanes essentially exhibit a low rate of decomposition. All the samples were stable above 300 °C. One of the limitations of oil-based polyurethanes is their inferior thermal stability, normally about 250 °C. A kinetic analysis of the thermal decomposition was done using the equation of Coats and Redfern,49 assuming first-order reaction, which was established since the plots resulted in straight lines. The Coats-Redfern equation is given below
ln
[
]
-ln(1 - R) T
2
)
[
]
ln AR 2RT E 1φE E RT
(1)
for the first-order reaction, where R is the fraction decomposed at temperature T, φ is the heating rate, and A is the Arrhenius frequency factor. The activation energy was obtained from the slope of a plot of ln[-ln(1 - R)/T 2] vs 1/T. Typical Coats-Redfern plots for the polyurethane samples are shown in Figure 7. The activation energy values (cf. Table 5) suggest that PUT decomposes early, and although PUD and PUG decomposition starts at a higher temperature, the decomposition rates are high, as indicated by the higher activation energy values. 4. Conclusions The development of three different polyols and their polyurethanes starting from cardanol is reported. The
Figure 7. Coats-Redfern plots for the cardanol-based polyurethanes.
structure elucidation of the polyols was done using spectroscopic techniques. The hydroxyl value of the polyols was found to be in the range 140-265 mg of KOH/g, and all polyols were liquid at room temperature. The polyols were used in the preparation of the polyurethanes by reacting with MDI, and the properties such as hardness, thermal stability, and dynamic mechanical properties were evaluated. The following are the main conclusions drawn from this study. Cardanol-based polyols yield tough, rigid polyurethanes. PU based on diol was semirigid, whereas the PUs based on triol and glycard were rigid, and a similar trend was observed in the Shore D hardness values. Dynamic mechanical (DMTA) studies indicate an increase of the modulus (E′) in the rubbery region, attributed to cross-linking of the side chain unsaturation that takes place at higher temperatures (>150 °C). The determination of Tg values by DMTA shows that the Tg value increases with increasing OH value. The films appeared homogeneous, as indicated by a uniform damping peak. When temperatures for 10% decomposition are compared, polyurethanes based on diol (PUD) and glycard (PUG) were stable above 300 °C, but the triol-based one (PUT) was less stable by 50 °C. But all polyurethanes have enhanced thermal stability in comparison to the PPG-based polyurethanes. Acknowledgment Financial support from ICI Polyurethanes, Everberg, Belgium, is gratefully acknowledged. Literature Cited (1) Sperling, L. H.; Manson, J. A.; Qureshi, S.; Fernandez, A. M. Tough plastics and Reinforced elastomers from Renewable Resource industrial oils: a short review. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 163. (2) Chen, Y.; Zhang, L.; Du, L. Structure and Properties of composites compression molded from polyurethane prepolymer and various soy products. Ind. Eng. Chem. Res. 2003, 42, 6786. (3) Anastas, P. T.; Kirchhoff, M. M. Origin, Current Status and Future Challenges of Green Chemistry. Acc. Chem. Res. 2002, 35, 686. (4) Hoefer, R.; Daute, P.; Gruetzmacher, R.; Westfechtel, A. Oleochemical polyolssA new raw material source for polyurethane coatings and floorings. J. Coat. Technol. 1997, 69, 65.
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Received for review November 22, 2004 Revised manuscript received March 10, 2005 Accepted April 14, 2005 IE0488750
