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Network structures and thermal properties of polyurethane films prepared from liquefied wood
Bioresource Technology 77 (2001) 33±40
Network structures and thermal properties of polyurethane ®lms prepared from lique®ed wood Y. Kurimoto a,*, M. Takeda b, S. Doi a, Y. Tamura a, H. Ono c a
c
Institute of Wood Technology, Akita Prefectural University, 11-1 Kaieisaka, Noshiro, Akita 016-0876, Japan b Akita Prefecture, 4-1-2 Sanno, Akita 010-0951, Japan Graduate School of Agriculture and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Received 25 January 2000; received in revised form 31 July 2000; accepted 4 September 2000
Abstract Polyurethane (PU) ®lms were prepared by solution-casting after co-polymerization of lique®ed woods (LWs) and polymeric methylene diphenylene diisocyanate (PMDI). The resulting PU ®lms had various [NCO]/[OH] ratios ranging from 0.6 to 1.4 and contained 5.0±16.8% dissolved woody components at the [NCO]/[OH] ratio of 1.0. The crosslink densities of the ®lms with [NCO]/ [OH] ratios of 0.6±1.4 increased remarkably with increasing [NCO]/[OH] ratio. Similarly, there were large increases in glass transition temperatures (Tg ). These characteristics could be attributed to eective incorporation of PMDI into the LW. The crosslink densities and Tg of the PU ®lms prepared at the [NCO]/[OH] ratio of 1.0 increased because the amounts of dissolved woody components in the ®lms increased. It is concluded that the dissolved woody components acted as crosslinking points in PU network formations. The thermal degradation of the PU ®lms at an [NCO]/[OH] ratio of more than 0.8 or with more than 10.6% dissolved wood started above 262°C under an N2 atmosphere. The thermostability was lost at low crosslink density or with large amount of co-polymerized glycerol structures in the PU networks. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Lique®ed wood; Polyurethane ®lm; Crosslink density; Sol fraction; Thermal degradation
1. Introduction Liquefaction of wood materials using polyhydric alcohols (polyols) is one of the processes to utilize wood waste as a raw material for polyurethane (PU) resins. To date, several studies have dealt with the incorporation of lignin or lignin derivatives as the natural polymer into PUs (Saraf and Glasser, 1984; Saraf et al., 1985; Yoshida et al., 1987, 1990). The lignins used for the studies were usually obtained as by-products from pulping. The source, molecular mass distribution and OH group content restrict the types of lignin suitable for the PUs. Wood liquefaction using polyols is a technique to convert whole wood into liquid. The resulting products through solvolysis were directly used as polyol for PU forms without any fractionation (Kurimoto et al., 1992; Yao et al., 1995, 1996). Almost all wood wastes could be used for the liquefaction since the primary components *
Corresponding author. Tel.: +81-185-52-6985; fax: +81-185-526976. E-mail address: [email protected] (Y. Kurimoto).
of various wood species have similar chemical structures (Kurimoto et al., 1999). In an earlier study, we prepared lique®ed wood (LW) containing 9.6±29.8% dissolved woody components by the liquefaction of sugi (Japanese cedar, Cryptomeria japonica D. Don) wood using glycerol±PEG (polyethylene glycol) as co-solvent (Kurimoto et al., 2000). PU ®lms were prepared by solution-casting after co-polymerization of the LWs with polymeric methylene diphenylene diisocyanate (PMDI) at an isocyanate/ hydroxyl group ([NCO]/[OH]) ratio of 1.0, and their mechanical properties were measured as a function of the amount of dissolved woody components. Results showed that increase in the dissolved woody components signi®cantly enhanced the Young's modulus and reduced the ductility of the PU ®lms. Notably, the Young's modulus of the PU ®lm (0.58 GPa) prepared from the 29.8%-LW was 1.9 times larger than that of the wood-free PU ®lm (0.31 GPa), although the amount of PMDI charged in the former was 15.4% less than that of the latter. The reduction of PMDI charge is a great advantage in utilizing LWs in the production of PUs. It is, however,
0960-8524/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 1 3 6 - X
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
postulated that a decrease of PMDI at a constant [NCO]/[OH] ratio is not suitable for thermal stability and/or chemical resistance, since less PMDI charge would result in a decrease in urethane concentration in PU networks, and an increase in the LW charge causes chemical and biological attack. To achieve potential utilization of LWs in the PU industry, a systematic study of the interrelationships among PMDI charge, network structures and thermal properties is required. This paper describes a systematic investigation of the eect of the amount of dissolved woody components and [NCO]/[OH] ratios on the network structures and thermal properties of LW±PU ®lms. The crosslink density and the weight of soluble materials were determined from swelling tests in N ; N -dimethylformamide (DMF) to clarify the network structures in combination with thermal analysis techniques, dierential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).
ratio of glycerol:PEG:sulfuric acid was kept constant at 10:90:3. Thirty grams of oven-dried wood ¯our, 60.0±270.0 g of GP and 1.8±8.1 g of sulfuric acid were placed in a 500 ml separable ¯ask and re¯uxed at 150°C for 75 min with continuous stirring (Kurimoto et al., 2000). The liquefaction products were then diluted to ten times their weight with a dioxane±water mixture (80/20, v/v), and the insoluble wood residues in the solutions were ®ltered using PTFE membrane ®lters (TOYO H050A047A, pore size 0:50 lm). The apparent pHs of the ®ltered solutions were then adjusted to 5.5 with 1 M sodium hydroxide solution. The sodium sulfates precipitated were removed using the membrane ®lters. Dioxane and water in the solutions were evaporated under reduced pressure until the moisture contents were approximately 0.7%. LWs are considered to consist of woody components dissolved through solvolysis and GP co-solvent. The characteristics of each LW are summarized in Table 1.
2. Methods
2.3. Preparation of polyurethane (PU) ®lms
2.1. Materials
Ten grams of LW or GP dissolved in 10 ml of dichloromethane were mixed with PMDI (4.90±10.75 g) in a polypropylene cup according to the formulations listed in Table 2. The solutions were agitated at 1200±1400 rpm for 10 min for polymerization without catalyst and surfactant. The polymerized mixtures were then cast into Petri dishes. The cast ®lms were gradually dried below 7°C for 3 days in a refrigerator and then cured for 11 days at 20°C and 65% relative humidity (RH). Finally, the ®lms were heat-treated at 100°C for 8 h in an oven. All the PU ®lms obtained were about 0.25 mm thick and transparent. No gel particles or bubbles were observed. Although CO2 was generated through a reaction between the water in LWs and the PMDI in the initial reaction stage, all of the CO2 was removed from the solutions by ultrasonic mixing. The [NCO]/ [OH] ratio and the weight percentage of dissolved woody components in PU ®lm (WC±PU) are given as follows:
Sugi wood was ground using a Wiley mill (Yoshida Seisakusyo 1029-C model, Tokyo, Japan). The fractions passing 1000 lm and retained at 106 lm mesh screens were used for the liquefaction process. The PEG used was PEG#400 (average molecular weight: 400) obtained from Wako Pure Chemical Industries (Osaka, Japan). The PMDI used was MR-100 (Japan Polyurethane Industry, Yokohama, Japan) with an NCO group content of 7.43 mmol/g. All other chemicals used were extrapure grades of reagents in accordance with Japanese Industrial Standard (JIS K8576, K8461, K8161, K8500) and used as received. 2.2. Lique®ed woods (LWs) As for the liquefaction solvent, a mixture of glycerol and PEG#400 (GP) co-solvent was used. The sulfuric acid used was a catalyst during liquefaction. The weight Table 1 Characteristics of LWs LW LW-1 LW-2 LW-3 LW-4 LW-5 GPb (control) a b
DWCa (%) 9.6 19.0 23.3 27.5 29.8 0
(mg KOH/g)
Hydroxyl number (mmol/g)
Acid number (mg KOH/g)
Viscosity (Pa s at 25°C)
Moisture content (%)
329.1 292.5 292.4 288.1 278.7 435.2
5.87 5.21 5.21 5.14 4.97 7.76
12.1 17.9 22.1 20.2 21.4 0
0.33 0.93 2.1 9.2 31.6 0.18
0.79 0.61 0.77 0.69 0.73 0.21
Amount of dissolved woody components in LW. The weight ratio of glycerol:PEG was 1:9.
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
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Table 2 Formulations and percentages (WC±PU) of dissolved woody components in PU ®lms at [NCO]/[OH] ratios of 0.6±1.4
a
Polyurethane ®lm
LWa
Formulation (%) LW
PMDI
1±1 2±1 3±0.6 3±0.8 3±1 3±1.2 3±1.4 4±1 5±1 0±1 (control)
LW-1 LW-2 LW-3 LW-3 LW-3 LW-3 LW-3 LW-4 LW-5 GP
52.4 55.8 67.1 60.5 55.0 50.5 46.7 55.7 56.2 48.2
47.6 44.2 32.9 39.5 45.0 49.5 53.3 44.3 43.8 51.8
WC±PU (%)
NCO=OH ratio
5.0 10.6 15.6 14.1 12.8 11.8 10.9 15.3 16.8 0
1.0 1.0 0.6 0.8 1.0 1.2 1.4 1.0 1.0 1.0
See Table 1.
NCO=OH ratio MMDI WMDI ; MPoly WPoly WWater 2=18 1000 WC±PU
WPoly weight ratio of DWC 100; WPoly WMDI
The weight percentage of soluble materials (sol fraction) after swelling was calculated from Eq. (4): 1
2
where MMDI is the content of isocyanate group in PMDI (7.43 mmol/g), MPoly the content of hydroxyl group (mmol/g) in LW (see Table 1), DWC is the weight percentage of dissolved woody components in LW (see Table 1) and WMDI ; WPoly and WWater are the weights (g) of PMDI, LW and water in the LW, respectively. 2.4. Measurement of crosslink density Five replicates of oven-dried ®lms having weights of 0.1 g each were placed in 20 ml of DMF and allowed to stand for 1 week at 22°C. After swelling, the samples were removed from the DMF, and the absorbed DMF on the surfaces was blotted using ®lter paper; then the samples were quickly weighed. The ®lm samples were dried again and re-weighed. A 25 ml pycnometer was used to determine each ®lm's density. The crosslink density (mc =V0 , unit: mol=cm3 ) of the ®lm was computed using the following equation (Rials and Glasser, 1984): mc ÿ2 t vt2 ln 1 ÿ t ; V0 V1 2t1=3 ÿ t
Sol fraction
W0 ÿ WD ; W0
4
where W0 is the initial weight of the polymer. 2.5. Dierential scanning calorimetry (DSC) The glass transition temperature (Tg ) of each PU ®lm was measured using a dierential scanning calorimeter (MAC Science DSC3100S model, Tokyo, Japan). A 7± 10 mg sample was ®rst scanned to 100°C in order to eliminate the eect of enthalpy relaxation (Rials and Glasser, 1984; Yoshida et al., 1990), thereafter cooled to ÿ50°C and scanned again to 100°C. All scans were done under dry nitrogen ¯ow of 50 ml/min at a heating or cooling rate of 10°C= min. The Tg in the second scan was de®ned as one-half the total change in heat capacity (Cp ) occurring over the transition region. 2.6. Thermogravimetric analysis (TGA) A sample (8±12 mg) was tested in a thermogravimeter (MAC Science TG-DTA2000S model, Tokyo, Japan) at a programmed rate of 10°C= min from room temperature to 600°C under N2 gas at a ¯ow rate of 200 ml/min.
3
where mc is the eective number of moles of crosslinked chains; V1 the molar volume of the solvent (76.87 ml/mol for DMF); v the polymer±solvent interaction parameter (0.40 according to Yoshida et al., 1987); and t is the volume fraction of polymer in swollen gel (t V0 =V1 ). V0 and V1 are the volume of dry polymer (WD =qp ) and swollen gel (WD =qp W ÿ WD =qS ) at equilibrium. WD and W1 are the weights of de-swollen polymer gel and swollen polymer, respectively. qp and qS are the densities of de-swollen polymer and solvent (0.94 g/ml for DMF), respectively.
3. Results and discussion 3.1. Eect of [NCO]/[OH] ratio and WC±PU on crosslink density and sol fraction of PU ®lms The eect of [NCO]/[OH] ratio on the PU network structures are shown in Fig. 1. The crosslink densities remarkably increased from 0:11 10ÿ3 to 1:36 10ÿ3 mol=cm3 with the increase in the [NCO]/[OH] ratio from 0.6 to 1.4. In contrast, the weights of the sol fraction decreased from 39.2% to 3.1% as the [NCO]/ [OH] ratio increased from 0.6 to 1.4. Evidently, these
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
Fig. 1. Eect of [NCO]/[OH] ratio on crosslink density or sol fraction of PU ®lms prepared from LW-3 with PMDI. Note: The values plotted in the ®gure are average values obtained from measurements of ®ve replicates. Error bars indicated are the ranges of standard deviation for each replicate.
results show that the LW reacts with PMDI, leading to network formations according to the isocyanate chemistry. When [NCO]/[OH] ratio is less than 1.0, urethane linkage between the alcoholic OH and the isocyanate and amines through the reaction of residual water and isocyanate should be formed, where the crosslink density is low. When the [NCO]/[OH] ratio exceeds 1.0, the excess amount of isocyanate reacts with both urethane and amines, resulting in the formation of allophanates and biurets, respectively. The high values of the crosslink density over the range of [NCO]/[OH] ratio higher than 1.0 suggest the formation of allophanates and biurets in the ®lms. In FT-IR analyses on PU ®lms prepared from LW-3 with PMDI, we have clearly con®rmed the presence of absorption bands at 1720, 1702, 1529 and 1216 cmÿ1 in the FT-IR spectra, which are attributable to urethane linkages (Kurimoto et al., 2000). The increases of relative intensities in the hydrogen-bonded C@O band (1702 cmÿ1 ) compared with the non-hydrogen-bonded C@O band (1720 cmÿ1 ) also indicated the formation of urethane linkages. However, clear absorption bands due to urea, allophanates and biurets were not observed in the spectra probably because the bands were weak and/or overlapped with other absorption bands. The eects of WC±PU on the crosslink densities and on the weights of sol fraction in PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0 are shown in Fig. 2. With the exception of the PU ®lm 2±1 (WC±PU: 9.6%), an increase in the crosslink densities was observed when the WC±PU increased from 5.0% to 16.8%. Since no major dierences in the weight of sol fraction and the PMDI charge are found, it is considered that the presence of some amount of dissolved woody components caused the dierence in
Fig. 2. Eect of the WC±PU on crosslink density or sol fraction of PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0.
crosslink density. The relationship between the viscosity of LWs and the crosslink densities of LW±PU ®lms (see Fig. 3) also indicates that the presence of dissolved woody components aected the crosslink densities of LW±PU ®lms, because the viscosity of LW was signi®cantly increased by the larger amount of dissolved woody components (Kurimoto et al., 2000). Therefore, the higher crosslink density of the PU ®lm with larger WC±PU suggests that the dissolved woody components in LW acted as crosslinking agents having many OH groups rather than as a chain extender in this polymerization system. The PU ®lms prepared from LWs at the [NCO]/[OH] ratio of 1.0 contain more soluble materials (9.5±13.7%) than the wood-free PU ®lm (4.5%). The soluble materials, which are assumed to consume some PMDI, are excluded from incorporation into PU network
Fig. 3. Relationship between the viscosity of LW and the crosslink density of LW±PU ®lms.
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
37
formations. The soluble materials might be due to small amounts of low molecular mass substances having less than one OH group per molecule (Yamada and Ono, 1999). Further investigations of chemical and physical properties such as the structure of the OH group and the molecular weight distribution are in progress. In general, a higher crosslink density results in a more rigid material (Saraf and Glasser, 1984). In this experiment, the Young's modulus of the PU ®lm 5±1 (0.58 GPa) was 1.9 times higher than that of the PU ®lm 0±1 (0.31 GPa), although there is no large dierence in both the crosslink density values. The rigid molecular structure of the dissolved woody components incorporated into the network formations probably enhanced the stiness of LW±PU ®lms during the tensile tests. 3.2. Thermal properties of PU ®lms 3.2.1. Glass transition temperature Tg The DSC scans of PU ®lms prepared from LW-3 with PMDI at [NCO]/[OH] ratios of 0.8±1.4 are shown in Fig. 4. The glass transition temperature (Tg ) increased with the [NCO]/[OH] ratio. It is expected that the PU ®lms with greater crosslink densities needed more thermal energy to initiate chain movements. There were obvious changes in the appearances of the DSC curves around the glass transition region. When the [NCO]/ [OH] ratio increased, the Tg varied from a sharp transition within a narrow temperature range to an indistinct transition within a broad temperature range. This also resulted in the extension of network formations in LW±PU ®lms. There was a signi®cant correlation r 0:988 between the crosslink densities and the Tg of PU ®lms prepared from various LWs at the [NCO]/[OH] of 1.0 (Fig. 5). It is clear that the polymerization of LWs with
Fig. 4. DSC scans of PU ®lms (see Table 2) prepared from LW-3 with PMDI at [NCO]/[OH] ratios of 0.8±1.4.
Fig. 5. Relationship between the crosslink density and the Tg of PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0. r: signi®cant at 1% level.
PMDI aects not only the crosslink density but also the Tg of PU ®lms. The crosslink density and the Tg of an LW±PU ®lm at a constant [NCO]/[OH] ratio could, therefore, be controlled by varying the amount of dissolved woody components in LW. 3.2.2. Thermal degradation of LW and LW±PU ®lms during heating under an N 2 atmosphere 3.2.2.1. Thermal degradation of LW. The thermal or oxidative stability of LW is estimated from TG curves. The TG curves of LW-3 and GP with the rates of change in weight under an N2 atmosphere are given in Fig. 6. All the peaks observed are due to the decomposition or depolymerization of LW-3 and GP. The peaks ranging from 152°C to 270°C and from 270°C to 422°C observed in the TG curve of GP were caused by the decomposition of glycerol and PEG, respectively. Besides the peaks due to GP, a dierent peak ranging from 252°C to 305°C was observed in the TG curve of LW-3. This could be attributed to the decomposition of dissolved woody components in the LW. The thermal degradation of wood can be represented as the sum of the thermal degradations of the individual wood constituents. It occurs in a step-wise manner, with hemicellulose breaking down ®rst at 200±260°C, cellulose next at 240±350°C and lignin at 280±500°C (Soltes and Elder, 1981). Therefore, it is assumed that the decomposition of the LW fragments that originated in cellulose resulted in the large rate of weight loss with the peak at 276°C. However, the rates of change with other wood fragments that originated in hemicellulose and lignin were not clear since their weight loss regions overlapped with that of GP degradation. The temperature (TS ) at which the measurable polymer degradation started is de®ned as the point where the rate of change in weight goes beyond 0.5%/min. The TS
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
Fig. 6. TG curves and rates of change in weight of LW-3 and GP (see Table 1) with temperature rise under an N2 atmosphere. Note: The inset ®gures are graphs of pure glycerol and PEG#400.
values of LW-3 and GP were 210°C and 175°C, respectively. The lower thermostability of GP is due to its larger proportion of glycerol compared to that of LW-3. There was a 92% weight loss for LW-3 at 550°C, while maximum weight loss was observed at 422°C for the GP. The reason for this is the presence of dissolved woody components in LW-3. The components left charcoal-like materials after thermal treatment just as the individual constituents of wood and wood itself form charcoals through thermal degradation at elevated temperatures (>300°C) (LeVan, 1989). 3.2.2.2. Thermal degradation of PU ®lms. The eects of [NCO]/[OH] ratio on the TG curves of PU ®lms prepared from LW-3 with PMDI at the [NCO]/[OH] ratios of 1.0, 1.2 and 1.4 are illustrated in Fig. 7. An initial weight loss starting at 255°C corresponds to loss of the LW segments in PU ®lms. The second region, indicated `2' in the ®gure, including the peaks at approximately 365°C is caused primarily by the decomposition of isocyanates in urethane polymer. This is based on the report that the isocyanate decomposition of urethane elastomer showed peaks ranging from 290°C to 370°C by the analyses of DSC thermograms (Long and Pisney, 1976). When the [NCO]/[OH] ratio increased from 1.0 to 1.4, broad peaks at approximately 382°C appeared in the region `3' ranging from 375°C to 435°C, followed by decreases in the rate of change at the temperature range from 435°C to 550°C (`4'). The most plausible explanation for these is the decomposition of isocyanates in
Fig. 7. TG curves and rates of change in weight of PU ®lms 3±1, 3±1.2 and 3±1.4 (see Table 2) with the temperature rise under an N2 atmosphere. Labeled regions correspond to 1 ± loss of LW-3 fragments; 2 ± dominate decomposition owing to isocyanates in urethane; 3 ± the sum of the decomposition of the LW-3 fragments and isocyanates in urea; and 4 ± the decomposition of isocyanurate ring structures.
urea polymer and isocyanurate ring structures, because it is well established that the isocyanate-based polymers provide their thermostability in the following order: isocyanurate > urea > urethane > biuret > allophanate (Fabris, 1976). In this investigation, the PU ®lms were cured under 65% RH at 20°C for 11 days, and were then heated at elevated temperature 100°C. Therefore, the excess amount of PMDI at more than the [NCO]/[OH] ratio of 1.0 would probably lead to the formation of more urea through the NCO±H2 O reaction and isocyanurate ring structures through the polymerization by which the NCO groups react with one another during the heat-treatment procedure. The decomposition of PU ®lms 3±0.6 and 3±0.8 started below 257°C, where the temperature is 5°C lower than that of the PU ®lm 3±1 (Table 3). This re¯ects that the PU ®lms 3±0.6 and 3±0.8 contain less crosslink density and more soluble materials than the PU ®lm 3±1 (see Fig. 1). At a [NCO]/[OH] ratio of more than 1.0, there were little dierences in the temperatures at which the thermal degradation started. The eects of WC±PU on the TG curves of the PU ®lm 0±1, 1±1 and 5±1 are shown in Fig. 8. The WC±PU in the ®lm was 0%, 5.0% and 16.8%, respectively (see Table 2). Each ®lm had four successive changes in weight losses owing to polymer decomposition, i.e. 1 ± loss of the LWs or the GP fragments, 2 ± the decomposition owing to isocyanates in urethane, 3 ± the sum of
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
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Table 3 Thermostability and percentage residues, after heating to 550°C, of PU ®lms prepared from various LWs with PMDI Polyurethane ®lm
WC±PUa (%)
TS b (°C)
Residuec (%)
1±1 2±1 3±0.6 3±0.8 3±1 3±1.2 3±1.4 4±1 5±1 0±1
5.0 10.6 15.6 14.1 12.8 11.8 10.9 15.3 16.8 0
253 256 239 257 262 265 263 263 262 253
22.3 24.8 ) ) 25.9 ) ) 26.6 27.3 17.9
a
See Table 2. The temperature at which the polymer decomposition started (>0.5%/min). c Values are based on dried PU ®lm weight after heating to 550°C. b
Fig. 9. TG curves and rates of change in weight of PU ®lms prepared from two kinds of GPs with PMDI at the [NCO]/[OH] ratio of 1.0. Glycerol:PEG 5:95 ( ± ); 15:85 (- - -).
Fig. 8. TG curve and rate of change in weight of PU ®lms (a) 0±1, (b) 1±1 and (c) 5±1 (see Table 2) with the temperature rise under an N2 atmosphere. The values of WC±PU were 0%, 5.0% and 16.8%, respectively. The labeled regions (1±4) are the same as in Fig. 7.
the decomposition of isocyanates in urea and the LWs or the GP fragments, and 4 ± the decomposition due to isocyanurate. The peaks, which correspond to the decompositions of urea and isocyanurate ring structures, decreased when the WC±PU increased from 0% to 16.8%. It is expected that the concentration of urea and isocyanurate formation in PU ®lm are aected by the
quantity of glycerol. Glycerol has two types of ±OH group. The polymerization of those groups with PMDI does not proceed at an even rate, i. e. primary > secondary (Grigat and Dieterich, 1993). If some secondary ±OH group could not react with PMDI in the initial mixing stage because of reduced reactivity or steric hindrance, the excess PMDI would undergo the polymerization with moisture and isocyanate even at the [NCO]/[OH] ratio of 1.0. As shown in Fig. 9, the increase in glycerol proportion to PEG obviously aects the peaks corresponding to the decomposition of urea and isocyanurate ring structures. Thus, the PU ®lms prepared from GP or LW having large amounts of glycerol formed many urea and isocyanurate ring structures.
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
The thermal degradation of the PU ®lms 0±1, 1±1 and 2±1 (0±10.6% WC±PU) started at 253±256°C, which are 6±10°C less than those of the PU ®lms 3±1, 4±1 and 5±1 (12.8±16.8% WC±PU) (Table 3). The reduced thermostability of the ®lms corresponded to the decomposition of the glycerol fragment in the ®lms. The decrease in contribution of the glycerol to PU network formations resulted in high thermostability. The PU ®lm with larger WC±PU yielded more charcoal-like residues after heating to 550°C. The order of increasing yield of the charcoal-like residues is reasonable because the higher proportion of aromatic compounds like lignin tends to produce more char residues. 4. Conclusions The crosslink density and the glass transition temperature (Tg ) of LW±PU ®lm were controlled by varying the [NCO]/[OH] ratio as in common urethane compounds. These can be attributed to the incorporation of PMDI into LW according to [NCO]/[OH] stoichiometry. The crosslink density and the Tg of LW±PU ®lm can be controlled by varying the amount of dissolved woody components at a constant [NCO]/[OH] ratio. Linear relation was found between crosslink density and Tg . The sol fraction, which is about 10±14%, is excluded from incorporation into the network formations of LW± PU ®lms at the [NCO]/[OH] ratio of 1.0. There were four successive changes in weight losses owing to polymer decomposition caused by heating under an N2 atmosphere. The greatest in¯uence on the weight loss was the decomposition of isocyanates in urethane with the peak at approximately 365°C. The thermal degradation of LW±PU ®lms at more than 0.8 [NCO]/[OH] ratio or with larger than 10.6% WC±PU at the [NCO]/[OH] ratio of 1.0 started above 262°C. This temperature is 9°C higher than that for the wood-free PU ®lm. Thermostability was lost by the reduced crosslink density (more sol fraction) or large amount of co-polymerized glycerol structures in PU networks.
References Fabris, H.J., 1976. Thermal and oxidative stability of urethanes. Advances in Urethane Science and Technology 4, 89±111. Grigat, E., Dieterich, D., 1993. In: Oertel, G. (Ed.), Polyurethane Handbook, second ed. Carl Hanser, New York, pp. 11±17. Kurimoto, Y., Doi, S., Tamura, Y., 1999. Species eects on woodliquefaction in polyhydric alcohols. Holzforschung 53, 617±622. Kurimoto, Y., Shirakawa, K., Yoshioka, M., Shiraishi, N., 1992. Liquefaction of untreated wood with polyhydric alcohols and its application to polyurethane foams. In: New Zealand Forest Research Institute, Bulletin No. 176, pp. 163±172. Kurimoto, Y., Takeda, M., Koizumi, A., Yamauchi, S., Doi, S., Tamura, Y., 2000. Mechanical properties of polyurethane ®lms prepared from lique®ed wood with polymeric MDI. Bioresource Technology 74, 151±157. LeVan, S.L., 1989. In: Schniewind, A.P. (Ed.), Concise Encyclopedia of Wood and Wood-based Materials. Pergamon, New York, pp. 271±273. Long, T.S., Pisney, J.J., 1976. Characterization of polyurethane using infrared and thermal analysis. Plastics Engineering 22, 398±403. Rials, T.G., Glasser, W.G., 1984. Engineering plastics from lignin. IV. Eects of crosslink density on polyurethane ®lm properties ± variation in NCO:OH ratio. Holzforschung 38, 191±199. Saraf, V.P., Glasser, W.G., 1984. Engineering plastics from lignin. III. Structure property relationships in solution cast polyurethane ®lms. Journal of Applied Polymer Science 29, 1831±1841. Saraf, V.P., Glasser, W.G., Wilkes, G.L., McGrarh, J.E., 1985. Engineering plastics from lignin. VI. Structure±property relationships of peg-containing polyurethane networks. Journal of Applied Polymer Science 30, 3809±3823. Soltes, E., Elder, T., 1981. Pyrolysis. In: Organic Chemicals from Biomass. CRC Press, Boca Raton, FL, pp. 82±87. Yamada, T., Ono, H., 1999. Rapid liquefaction of lignocellulosic waste by using ethylene carbonate. Bioresource Technology 70, 61±67. Yao, Y., Yoshioka, M., Shiraishi, N., 1995. Rigid polyurethane foams from combined liquefaction mixture of wood and starch. Mokuzai Gakkaishi 41 (7), 659±668. Yao, Y., Yoshioka, M., Shiraishi, N., 1996. Water-absorbing polyurethane foams from lique®ed starch. Journal of Applied Polymer Science 60, 1939±1949. Yoshida, H., Morck, R., Kringstad, K.P., Hatakeyama, H., 1987. Kraft lignin in polyurethanes. I. Mechanical properties of polyurethanes from kraft lignin±polyether triol-polymeric MDI system. Journal of Applied Polymer Science 34, 1187±1198. Yoshida, H., Morck, R., Kringstad, K.P., Hatakeyama, H., 1990. Kraft lignin in polyurethanes. II. Eects of the molecular weight of kraft lignin on the properties of polyurethanes from a kraft lignin± polyether triol-polymeric MDI system. Journal of Applied Polymer Science 40, 1819±1832.
Network structures and thermal properties of polyurethane ®lms prepared from lique®ed wood Y. Kurimoto a,*, M. Takeda b, S. Doi a, Y. Tamura a, H. Ono c a
c
Institute of Wood Technology, Akita Prefectural University, 11-1 Kaieisaka, Noshiro, Akita 016-0876, Japan b Akita Prefecture, 4-1-2 Sanno, Akita 010-0951, Japan Graduate School of Agriculture and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Received 25 January 2000; received in revised form 31 July 2000; accepted 4 September 2000
Abstract Polyurethane (PU) ®lms were prepared by solution-casting after co-polymerization of lique®ed woods (LWs) and polymeric methylene diphenylene diisocyanate (PMDI). The resulting PU ®lms had various [NCO]/[OH] ratios ranging from 0.6 to 1.4 and contained 5.0±16.8% dissolved woody components at the [NCO]/[OH] ratio of 1.0. The crosslink densities of the ®lms with [NCO]/ [OH] ratios of 0.6±1.4 increased remarkably with increasing [NCO]/[OH] ratio. Similarly, there were large increases in glass transition temperatures (Tg ). These characteristics could be attributed to eective incorporation of PMDI into the LW. The crosslink densities and Tg of the PU ®lms prepared at the [NCO]/[OH] ratio of 1.0 increased because the amounts of dissolved woody components in the ®lms increased. It is concluded that the dissolved woody components acted as crosslinking points in PU network formations. The thermal degradation of the PU ®lms at an [NCO]/[OH] ratio of more than 0.8 or with more than 10.6% dissolved wood started above 262°C under an N2 atmosphere. The thermostability was lost at low crosslink density or with large amount of co-polymerized glycerol structures in the PU networks. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Lique®ed wood; Polyurethane ®lm; Crosslink density; Sol fraction; Thermal degradation
1. Introduction Liquefaction of wood materials using polyhydric alcohols (polyols) is one of the processes to utilize wood waste as a raw material for polyurethane (PU) resins. To date, several studies have dealt with the incorporation of lignin or lignin derivatives as the natural polymer into PUs (Saraf and Glasser, 1984; Saraf et al., 1985; Yoshida et al., 1987, 1990). The lignins used for the studies were usually obtained as by-products from pulping. The source, molecular mass distribution and OH group content restrict the types of lignin suitable for the PUs. Wood liquefaction using polyols is a technique to convert whole wood into liquid. The resulting products through solvolysis were directly used as polyol for PU forms without any fractionation (Kurimoto et al., 1992; Yao et al., 1995, 1996). Almost all wood wastes could be used for the liquefaction since the primary components *
Corresponding author. Tel.: +81-185-52-6985; fax: +81-185-526976. E-mail address: [email protected] (Y. Kurimoto).
of various wood species have similar chemical structures (Kurimoto et al., 1999). In an earlier study, we prepared lique®ed wood (LW) containing 9.6±29.8% dissolved woody components by the liquefaction of sugi (Japanese cedar, Cryptomeria japonica D. Don) wood using glycerol±PEG (polyethylene glycol) as co-solvent (Kurimoto et al., 2000). PU ®lms were prepared by solution-casting after co-polymerization of the LWs with polymeric methylene diphenylene diisocyanate (PMDI) at an isocyanate/ hydroxyl group ([NCO]/[OH]) ratio of 1.0, and their mechanical properties were measured as a function of the amount of dissolved woody components. Results showed that increase in the dissolved woody components signi®cantly enhanced the Young's modulus and reduced the ductility of the PU ®lms. Notably, the Young's modulus of the PU ®lm (0.58 GPa) prepared from the 29.8%-LW was 1.9 times larger than that of the wood-free PU ®lm (0.31 GPa), although the amount of PMDI charged in the former was 15.4% less than that of the latter. The reduction of PMDI charge is a great advantage in utilizing LWs in the production of PUs. It is, however,
0960-8524/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 1 3 6 - X
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
postulated that a decrease of PMDI at a constant [NCO]/[OH] ratio is not suitable for thermal stability and/or chemical resistance, since less PMDI charge would result in a decrease in urethane concentration in PU networks, and an increase in the LW charge causes chemical and biological attack. To achieve potential utilization of LWs in the PU industry, a systematic study of the interrelationships among PMDI charge, network structures and thermal properties is required. This paper describes a systematic investigation of the eect of the amount of dissolved woody components and [NCO]/[OH] ratios on the network structures and thermal properties of LW±PU ®lms. The crosslink density and the weight of soluble materials were determined from swelling tests in N ; N -dimethylformamide (DMF) to clarify the network structures in combination with thermal analysis techniques, dierential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).
ratio of glycerol:PEG:sulfuric acid was kept constant at 10:90:3. Thirty grams of oven-dried wood ¯our, 60.0±270.0 g of GP and 1.8±8.1 g of sulfuric acid were placed in a 500 ml separable ¯ask and re¯uxed at 150°C for 75 min with continuous stirring (Kurimoto et al., 2000). The liquefaction products were then diluted to ten times their weight with a dioxane±water mixture (80/20, v/v), and the insoluble wood residues in the solutions were ®ltered using PTFE membrane ®lters (TOYO H050A047A, pore size 0:50 lm). The apparent pHs of the ®ltered solutions were then adjusted to 5.5 with 1 M sodium hydroxide solution. The sodium sulfates precipitated were removed using the membrane ®lters. Dioxane and water in the solutions were evaporated under reduced pressure until the moisture contents were approximately 0.7%. LWs are considered to consist of woody components dissolved through solvolysis and GP co-solvent. The characteristics of each LW are summarized in Table 1.
2. Methods
2.3. Preparation of polyurethane (PU) ®lms
2.1. Materials
Ten grams of LW or GP dissolved in 10 ml of dichloromethane were mixed with PMDI (4.90±10.75 g) in a polypropylene cup according to the formulations listed in Table 2. The solutions were agitated at 1200±1400 rpm for 10 min for polymerization without catalyst and surfactant. The polymerized mixtures were then cast into Petri dishes. The cast ®lms were gradually dried below 7°C for 3 days in a refrigerator and then cured for 11 days at 20°C and 65% relative humidity (RH). Finally, the ®lms were heat-treated at 100°C for 8 h in an oven. All the PU ®lms obtained were about 0.25 mm thick and transparent. No gel particles or bubbles were observed. Although CO2 was generated through a reaction between the water in LWs and the PMDI in the initial reaction stage, all of the CO2 was removed from the solutions by ultrasonic mixing. The [NCO]/ [OH] ratio and the weight percentage of dissolved woody components in PU ®lm (WC±PU) are given as follows:
Sugi wood was ground using a Wiley mill (Yoshida Seisakusyo 1029-C model, Tokyo, Japan). The fractions passing 1000 lm and retained at 106 lm mesh screens were used for the liquefaction process. The PEG used was PEG#400 (average molecular weight: 400) obtained from Wako Pure Chemical Industries (Osaka, Japan). The PMDI used was MR-100 (Japan Polyurethane Industry, Yokohama, Japan) with an NCO group content of 7.43 mmol/g. All other chemicals used were extrapure grades of reagents in accordance with Japanese Industrial Standard (JIS K8576, K8461, K8161, K8500) and used as received. 2.2. Lique®ed woods (LWs) As for the liquefaction solvent, a mixture of glycerol and PEG#400 (GP) co-solvent was used. The sulfuric acid used was a catalyst during liquefaction. The weight Table 1 Characteristics of LWs LW LW-1 LW-2 LW-3 LW-4 LW-5 GPb (control) a b
DWCa (%) 9.6 19.0 23.3 27.5 29.8 0
(mg KOH/g)
Hydroxyl number (mmol/g)
Acid number (mg KOH/g)
Viscosity (Pa s at 25°C)
Moisture content (%)
329.1 292.5 292.4 288.1 278.7 435.2
5.87 5.21 5.21 5.14 4.97 7.76
12.1 17.9 22.1 20.2 21.4 0
0.33 0.93 2.1 9.2 31.6 0.18
0.79 0.61 0.77 0.69 0.73 0.21
Amount of dissolved woody components in LW. The weight ratio of glycerol:PEG was 1:9.
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
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Table 2 Formulations and percentages (WC±PU) of dissolved woody components in PU ®lms at [NCO]/[OH] ratios of 0.6±1.4
a
Polyurethane ®lm
LWa
Formulation (%) LW
PMDI
1±1 2±1 3±0.6 3±0.8 3±1 3±1.2 3±1.4 4±1 5±1 0±1 (control)
LW-1 LW-2 LW-3 LW-3 LW-3 LW-3 LW-3 LW-4 LW-5 GP
52.4 55.8 67.1 60.5 55.0 50.5 46.7 55.7 56.2 48.2
47.6 44.2 32.9 39.5 45.0 49.5 53.3 44.3 43.8 51.8
WC±PU (%)
NCO=OH ratio
5.0 10.6 15.6 14.1 12.8 11.8 10.9 15.3 16.8 0
1.0 1.0 0.6 0.8 1.0 1.2 1.4 1.0 1.0 1.0
See Table 1.
NCO=OH ratio MMDI WMDI ; MPoly WPoly WWater 2=18 1000 WC±PU
WPoly weight ratio of DWC 100; WPoly WMDI
The weight percentage of soluble materials (sol fraction) after swelling was calculated from Eq. (4): 1
2
where MMDI is the content of isocyanate group in PMDI (7.43 mmol/g), MPoly the content of hydroxyl group (mmol/g) in LW (see Table 1), DWC is the weight percentage of dissolved woody components in LW (see Table 1) and WMDI ; WPoly and WWater are the weights (g) of PMDI, LW and water in the LW, respectively. 2.4. Measurement of crosslink density Five replicates of oven-dried ®lms having weights of 0.1 g each were placed in 20 ml of DMF and allowed to stand for 1 week at 22°C. After swelling, the samples were removed from the DMF, and the absorbed DMF on the surfaces was blotted using ®lter paper; then the samples were quickly weighed. The ®lm samples were dried again and re-weighed. A 25 ml pycnometer was used to determine each ®lm's density. The crosslink density (mc =V0 , unit: mol=cm3 ) of the ®lm was computed using the following equation (Rials and Glasser, 1984): mc ÿ2 t vt2 ln 1 ÿ t ; V0 V1 2t1=3 ÿ t
Sol fraction
W0 ÿ WD ; W0
4
where W0 is the initial weight of the polymer. 2.5. Dierential scanning calorimetry (DSC) The glass transition temperature (Tg ) of each PU ®lm was measured using a dierential scanning calorimeter (MAC Science DSC3100S model, Tokyo, Japan). A 7± 10 mg sample was ®rst scanned to 100°C in order to eliminate the eect of enthalpy relaxation (Rials and Glasser, 1984; Yoshida et al., 1990), thereafter cooled to ÿ50°C and scanned again to 100°C. All scans were done under dry nitrogen ¯ow of 50 ml/min at a heating or cooling rate of 10°C= min. The Tg in the second scan was de®ned as one-half the total change in heat capacity (Cp ) occurring over the transition region. 2.6. Thermogravimetric analysis (TGA) A sample (8±12 mg) was tested in a thermogravimeter (MAC Science TG-DTA2000S model, Tokyo, Japan) at a programmed rate of 10°C= min from room temperature to 600°C under N2 gas at a ¯ow rate of 200 ml/min.
3
where mc is the eective number of moles of crosslinked chains; V1 the molar volume of the solvent (76.87 ml/mol for DMF); v the polymer±solvent interaction parameter (0.40 according to Yoshida et al., 1987); and t is the volume fraction of polymer in swollen gel (t V0 =V1 ). V0 and V1 are the volume of dry polymer (WD =qp ) and swollen gel (WD =qp W ÿ WD =qS ) at equilibrium. WD and W1 are the weights of de-swollen polymer gel and swollen polymer, respectively. qp and qS are the densities of de-swollen polymer and solvent (0.94 g/ml for DMF), respectively.
3. Results and discussion 3.1. Eect of [NCO]/[OH] ratio and WC±PU on crosslink density and sol fraction of PU ®lms The eect of [NCO]/[OH] ratio on the PU network structures are shown in Fig. 1. The crosslink densities remarkably increased from 0:11 10ÿ3 to 1:36 10ÿ3 mol=cm3 with the increase in the [NCO]/[OH] ratio from 0.6 to 1.4. In contrast, the weights of the sol fraction decreased from 39.2% to 3.1% as the [NCO]/ [OH] ratio increased from 0.6 to 1.4. Evidently, these
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Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
Fig. 1. Eect of [NCO]/[OH] ratio on crosslink density or sol fraction of PU ®lms prepared from LW-3 with PMDI. Note: The values plotted in the ®gure are average values obtained from measurements of ®ve replicates. Error bars indicated are the ranges of standard deviation for each replicate.
results show that the LW reacts with PMDI, leading to network formations according to the isocyanate chemistry. When [NCO]/[OH] ratio is less than 1.0, urethane linkage between the alcoholic OH and the isocyanate and amines through the reaction of residual water and isocyanate should be formed, where the crosslink density is low. When the [NCO]/[OH] ratio exceeds 1.0, the excess amount of isocyanate reacts with both urethane and amines, resulting in the formation of allophanates and biurets, respectively. The high values of the crosslink density over the range of [NCO]/[OH] ratio higher than 1.0 suggest the formation of allophanates and biurets in the ®lms. In FT-IR analyses on PU ®lms prepared from LW-3 with PMDI, we have clearly con®rmed the presence of absorption bands at 1720, 1702, 1529 and 1216 cmÿ1 in the FT-IR spectra, which are attributable to urethane linkages (Kurimoto et al., 2000). The increases of relative intensities in the hydrogen-bonded C@O band (1702 cmÿ1 ) compared with the non-hydrogen-bonded C@O band (1720 cmÿ1 ) also indicated the formation of urethane linkages. However, clear absorption bands due to urea, allophanates and biurets were not observed in the spectra probably because the bands were weak and/or overlapped with other absorption bands. The eects of WC±PU on the crosslink densities and on the weights of sol fraction in PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0 are shown in Fig. 2. With the exception of the PU ®lm 2±1 (WC±PU: 9.6%), an increase in the crosslink densities was observed when the WC±PU increased from 5.0% to 16.8%. Since no major dierences in the weight of sol fraction and the PMDI charge are found, it is considered that the presence of some amount of dissolved woody components caused the dierence in
Fig. 2. Eect of the WC±PU on crosslink density or sol fraction of PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0.
crosslink density. The relationship between the viscosity of LWs and the crosslink densities of LW±PU ®lms (see Fig. 3) also indicates that the presence of dissolved woody components aected the crosslink densities of LW±PU ®lms, because the viscosity of LW was signi®cantly increased by the larger amount of dissolved woody components (Kurimoto et al., 2000). Therefore, the higher crosslink density of the PU ®lm with larger WC±PU suggests that the dissolved woody components in LW acted as crosslinking agents having many OH groups rather than as a chain extender in this polymerization system. The PU ®lms prepared from LWs at the [NCO]/[OH] ratio of 1.0 contain more soluble materials (9.5±13.7%) than the wood-free PU ®lm (4.5%). The soluble materials, which are assumed to consume some PMDI, are excluded from incorporation into PU network
Fig. 3. Relationship between the viscosity of LW and the crosslink density of LW±PU ®lms.
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
37
formations. The soluble materials might be due to small amounts of low molecular mass substances having less than one OH group per molecule (Yamada and Ono, 1999). Further investigations of chemical and physical properties such as the structure of the OH group and the molecular weight distribution are in progress. In general, a higher crosslink density results in a more rigid material (Saraf and Glasser, 1984). In this experiment, the Young's modulus of the PU ®lm 5±1 (0.58 GPa) was 1.9 times higher than that of the PU ®lm 0±1 (0.31 GPa), although there is no large dierence in both the crosslink density values. The rigid molecular structure of the dissolved woody components incorporated into the network formations probably enhanced the stiness of LW±PU ®lms during the tensile tests. 3.2. Thermal properties of PU ®lms 3.2.1. Glass transition temperature Tg The DSC scans of PU ®lms prepared from LW-3 with PMDI at [NCO]/[OH] ratios of 0.8±1.4 are shown in Fig. 4. The glass transition temperature (Tg ) increased with the [NCO]/[OH] ratio. It is expected that the PU ®lms with greater crosslink densities needed more thermal energy to initiate chain movements. There were obvious changes in the appearances of the DSC curves around the glass transition region. When the [NCO]/ [OH] ratio increased, the Tg varied from a sharp transition within a narrow temperature range to an indistinct transition within a broad temperature range. This also resulted in the extension of network formations in LW±PU ®lms. There was a signi®cant correlation r 0:988 between the crosslink densities and the Tg of PU ®lms prepared from various LWs at the [NCO]/[OH] of 1.0 (Fig. 5). It is clear that the polymerization of LWs with
Fig. 4. DSC scans of PU ®lms (see Table 2) prepared from LW-3 with PMDI at [NCO]/[OH] ratios of 0.8±1.4.
Fig. 5. Relationship between the crosslink density and the Tg of PU ®lms prepared from various LWs with PMDI at the [NCO]/[OH] ratio of 1.0. r: signi®cant at 1% level.
PMDI aects not only the crosslink density but also the Tg of PU ®lms. The crosslink density and the Tg of an LW±PU ®lm at a constant [NCO]/[OH] ratio could, therefore, be controlled by varying the amount of dissolved woody components in LW. 3.2.2. Thermal degradation of LW and LW±PU ®lms during heating under an N 2 atmosphere 3.2.2.1. Thermal degradation of LW. The thermal or oxidative stability of LW is estimated from TG curves. The TG curves of LW-3 and GP with the rates of change in weight under an N2 atmosphere are given in Fig. 6. All the peaks observed are due to the decomposition or depolymerization of LW-3 and GP. The peaks ranging from 152°C to 270°C and from 270°C to 422°C observed in the TG curve of GP were caused by the decomposition of glycerol and PEG, respectively. Besides the peaks due to GP, a dierent peak ranging from 252°C to 305°C was observed in the TG curve of LW-3. This could be attributed to the decomposition of dissolved woody components in the LW. The thermal degradation of wood can be represented as the sum of the thermal degradations of the individual wood constituents. It occurs in a step-wise manner, with hemicellulose breaking down ®rst at 200±260°C, cellulose next at 240±350°C and lignin at 280±500°C (Soltes and Elder, 1981). Therefore, it is assumed that the decomposition of the LW fragments that originated in cellulose resulted in the large rate of weight loss with the peak at 276°C. However, the rates of change with other wood fragments that originated in hemicellulose and lignin were not clear since their weight loss regions overlapped with that of GP degradation. The temperature (TS ) at which the measurable polymer degradation started is de®ned as the point where the rate of change in weight goes beyond 0.5%/min. The TS
38
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
Fig. 6. TG curves and rates of change in weight of LW-3 and GP (see Table 1) with temperature rise under an N2 atmosphere. Note: The inset ®gures are graphs of pure glycerol and PEG#400.
values of LW-3 and GP were 210°C and 175°C, respectively. The lower thermostability of GP is due to its larger proportion of glycerol compared to that of LW-3. There was a 92% weight loss for LW-3 at 550°C, while maximum weight loss was observed at 422°C for the GP. The reason for this is the presence of dissolved woody components in LW-3. The components left charcoal-like materials after thermal treatment just as the individual constituents of wood and wood itself form charcoals through thermal degradation at elevated temperatures (>300°C) (LeVan, 1989). 3.2.2.2. Thermal degradation of PU ®lms. The eects of [NCO]/[OH] ratio on the TG curves of PU ®lms prepared from LW-3 with PMDI at the [NCO]/[OH] ratios of 1.0, 1.2 and 1.4 are illustrated in Fig. 7. An initial weight loss starting at 255°C corresponds to loss of the LW segments in PU ®lms. The second region, indicated `2' in the ®gure, including the peaks at approximately 365°C is caused primarily by the decomposition of isocyanates in urethane polymer. This is based on the report that the isocyanate decomposition of urethane elastomer showed peaks ranging from 290°C to 370°C by the analyses of DSC thermograms (Long and Pisney, 1976). When the [NCO]/[OH] ratio increased from 1.0 to 1.4, broad peaks at approximately 382°C appeared in the region `3' ranging from 375°C to 435°C, followed by decreases in the rate of change at the temperature range from 435°C to 550°C (`4'). The most plausible explanation for these is the decomposition of isocyanates in
Fig. 7. TG curves and rates of change in weight of PU ®lms 3±1, 3±1.2 and 3±1.4 (see Table 2) with the temperature rise under an N2 atmosphere. Labeled regions correspond to 1 ± loss of LW-3 fragments; 2 ± dominate decomposition owing to isocyanates in urethane; 3 ± the sum of the decomposition of the LW-3 fragments and isocyanates in urea; and 4 ± the decomposition of isocyanurate ring structures.
urea polymer and isocyanurate ring structures, because it is well established that the isocyanate-based polymers provide their thermostability in the following order: isocyanurate > urea > urethane > biuret > allophanate (Fabris, 1976). In this investigation, the PU ®lms were cured under 65% RH at 20°C for 11 days, and were then heated at elevated temperature 100°C. Therefore, the excess amount of PMDI at more than the [NCO]/[OH] ratio of 1.0 would probably lead to the formation of more urea through the NCO±H2 O reaction and isocyanurate ring structures through the polymerization by which the NCO groups react with one another during the heat-treatment procedure. The decomposition of PU ®lms 3±0.6 and 3±0.8 started below 257°C, where the temperature is 5°C lower than that of the PU ®lm 3±1 (Table 3). This re¯ects that the PU ®lms 3±0.6 and 3±0.8 contain less crosslink density and more soluble materials than the PU ®lm 3±1 (see Fig. 1). At a [NCO]/[OH] ratio of more than 1.0, there were little dierences in the temperatures at which the thermal degradation started. The eects of WC±PU on the TG curves of the PU ®lm 0±1, 1±1 and 5±1 are shown in Fig. 8. The WC±PU in the ®lm was 0%, 5.0% and 16.8%, respectively (see Table 2). Each ®lm had four successive changes in weight losses owing to polymer decomposition, i.e. 1 ± loss of the LWs or the GP fragments, 2 ± the decomposition owing to isocyanates in urethane, 3 ± the sum of
Y. Kurimoto et al. / Bioresource Technology 77 (2001) 33±40
39
Table 3 Thermostability and percentage residues, after heating to 550°C, of PU ®lms prepared from various LWs with PMDI Polyurethane ®lm
WC±PUa (%)
TS b (°C)
Residuec (%)
1±1 2±1 3±0.6 3±0.8 3±1 3±1.2 3±1.4 4±1 5±1 0±1
5.0 10.6 15.6 14.1 12.8 11.8 10.9 15.3 16.8 0
253 256 239 257 262 265 263 263 262 253
22.3 24.8 ) ) 25.9 ) ) 26.6 27.3 17.9
a
See Table 2. The temperature at which the polymer decomposition started (>0.5%/min). c Values are based on dried PU ®lm weight after heating to 550°C. b
Fig. 9. TG curves and rates of change in weight of PU ®lms prepared from two kinds of GPs with PMDI at the [NCO]/[OH] ratio of 1.0. Glycerol:PEG 5:95 ( ± ); 15:85 (- - -).
Fig. 8. TG curve and rate of change in weight of PU ®lms (a) 0±1, (b) 1±1 and (c) 5±1 (see Table 2) with the temperature rise under an N2 atmosphere. The values of WC±PU were 0%, 5.0% and 16.8%, respectively. The labeled regions (1±4) are the same as in Fig. 7.
the decomposition of isocyanates in urea and the LWs or the GP fragments, and 4 ± the decomposition due to isocyanurate. The peaks, which correspond to the decompositions of urea and isocyanurate ring structures, decreased when the WC±PU increased from 0% to 16.8%. It is expected that the concentration of urea and isocyanurate formation in PU ®lm are aected by the
quantity of glycerol. Glycerol has two types of ±OH group. The polymerization of those groups with PMDI does not proceed at an even rate, i. e. primary > secondary (Grigat and Dieterich, 1993). If some secondary ±OH group could not react with PMDI in the initial mixing stage because of reduced reactivity or steric hindrance, the excess PMDI would undergo the polymerization with moisture and isocyanate even at the [NCO]/[OH] ratio of 1.0. As shown in Fig. 9, the increase in glycerol proportion to PEG obviously aects the peaks corresponding to the decomposition of urea and isocyanurate ring structures. Thus, the PU ®lms prepared from GP or LW having large amounts of glycerol formed many urea and isocyanurate ring structures.
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The thermal degradation of the PU ®lms 0±1, 1±1 and 2±1 (0±10.6% WC±PU) started at 253±256°C, which are 6±10°C less than those of the PU ®lms 3±1, 4±1 and 5±1 (12.8±16.8% WC±PU) (Table 3). The reduced thermostability of the ®lms corresponded to the decomposition of the glycerol fragment in the ®lms. The decrease in contribution of the glycerol to PU network formations resulted in high thermostability. The PU ®lm with larger WC±PU yielded more charcoal-like residues after heating to 550°C. The order of increasing yield of the charcoal-like residues is reasonable because the higher proportion of aromatic compounds like lignin tends to produce more char residues. 4. Conclusions The crosslink density and the glass transition temperature (Tg ) of LW±PU ®lm were controlled by varying the [NCO]/[OH] ratio as in common urethane compounds. These can be attributed to the incorporation of PMDI into LW according to [NCO]/[OH] stoichiometry. The crosslink density and the Tg of LW±PU ®lm can be controlled by varying the amount of dissolved woody components at a constant [NCO]/[OH] ratio. Linear relation was found between crosslink density and Tg . The sol fraction, which is about 10±14%, is excluded from incorporation into the network formations of LW± PU ®lms at the [NCO]/[OH] ratio of 1.0. There were four successive changes in weight losses owing to polymer decomposition caused by heating under an N2 atmosphere. The greatest in¯uence on the weight loss was the decomposition of isocyanates in urethane with the peak at approximately 365°C. The thermal degradation of LW±PU ®lms at more than 0.8 [NCO]/[OH] ratio or with larger than 10.6% WC±PU at the [NCO]/[OH] ratio of 1.0 started above 262°C. This temperature is 9°C higher than that for the wood-free PU ®lm. Thermostability was lost by the reduced crosslink density (more sol fraction) or large amount of co-polymerized glycerol structures in PU networks.
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