Wood species effects on the characteristics of liquefied wood and the properties of polyurethane films prepared from the liquefied wood

Biomass and Bioenergy 21 (2001) 381–390

Wood species eects on the characteristics of lique ed wood and the properties of polyurethane lms prepared from the lique ed wood Y. Kurimotoa; ∗ , A. Koizumia , S. Doia , Y. Tamuraa , H. Onob a Institute

b Graduate

of Wood Technology, Akita Prefectural University, Noshiro, Akita 016-0876, Japan School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan Received 20 June 2000; accepted 1 June 2001

Abstract Polyurethane (PU) lms were prepared by solution-casting through co-polymerization between lique ed wood (LW) and polymeric methylene diphenylene diisocyanate (PMDI) at [NCO]=[OH] ratios of 1.0 and 1.2. The LWs tested were made from six wood species through liquefaction using glycerol–polyethylene glycol (PEG) co-solvent, in the presence of sulfuric ◦ ◦ acid at 150 C. The viscosity of the LWs at 25 C varied from 1.37 to 2:31 Pa s with the wood species, whereas the hydroxyl number, moisture content, and amount of dissolved woody components (DWC) were almost constant. The PU lms prepared from LW with high viscosity were found to be more rigid than the lms prepared from LW with low viscosity. The increase in the viscosity contributed to the increases in the crosslink density of the PU lms. Varying the viscosity is a way to control c 2001 Elsevier Science Ltd. All rights reserved. the mechanical properties of PU lms at a constant [NCO]=[OH] ratio.
Keywords: Wood liquefaction; Species eects; Polyurethane; Crosslink density; Sol fraction; Tensile test

1. Introduction Liquefaction of wood materials using polyhydric alcohol aims to utilize wood wastes from the wood industry for production of polyurethane (PU) resins [1–3]. Such wastes are usually mixtures of dierent softwood and hardwood species. Almost all wood species have to be used as sources for the liquefaction

∗ Corresponding author. Tel.: +81-185-52-6985; fax: +81-18552-6976. E-mail address: [email protected] (Y. Kurimoto).

process. In lique ed wood (LW) regardless of the wood species, same number of -OH groups as well as same molecular weight distributions of polymer fragments having -OH groups are required to produce a physically and chemically homogeneous PU resin. In an earlier study [4], LWs containing 9.6 –29.8% dissolved woody components (DWC) were prepared by the liquefaction of sugi (Japanese cedar, Cryptomeria japonica D.DON) wood using glycerol–PEG (polyethylene glycol) co-solvent. Hydroxyl numbers of the LWs were signi cantly aected by the amount of DWC, that is, the higher the DWC, the

c 2001 Elsevier Science Ltd. All rights reserved. 0961-9534/01/$ - see front matter  PII: S 0 9 6 1 - 9 5 3 4 ( 0 1 ) 0 0 0 4 1 - 1

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Table 1 Chemical constituents of wood species [5] Softwood

Hardwood

Chemical constituent (%)

Karamatsu

Akamatsu

Radiata pine

Udaikanba

Buna

Mizunara

Holocellulose Klason lignin Ethanol–benzene solubles Hot water solubles Ash Total

69.5 22.5 0.9 6.9 0.2 100.0

71.5 23.5 1.1 3.6 0.3 100.0

61.6 24.8 6.5 6.8 0.2 100.0

75.8 20.7 1.0 2.3 0.2 100.0

73.6 22.4 0.9 2.7 0.4 100.0

73.3 20.5 0.8 5.0 0.4 100.0

lower the hydroxyl number. Co-polymerization of each LW with polymeric methylene diphenylene diisocyanate (PMDI) was achieved depending on an isocyanate=hydroxyl group ([NCO]=[OH]) ratio, as is typical in common urethane compounds. The increases in the DWC increased the Young’s modulus and reduced the ductility of the PU lms at 1.0 [NCO]=[OH] ratio. Four softwoods and three hardwoods were separately lique ed using glycerol–PEG co-solvent [5]. The values of DWC and the hydroxyl numbers were almost the same regardless of the wood species tested (23.6 –24.2% and 200 –220 mg KOH=g, respectively). These suggest that PUs prepared from the LWs at a constant [NCO]=[OH] ratio have identical mechanical properties. However, no study has been found focusing on the relationship between the wood species and the mechanical properties. Further, no information related to network structures of the PUs has been reported. In this paper, six kinds of LWs were prepared separately from three softwood and three hardwood species. The eects of wood species on networks and on mechanical properties of PU lms are discussed in relation to the characteristics of LWs.

2. Materials and methods 2.1. Wood species and chemicals Wood species tested were: karamatsu (Japanese larch, Larix leptolepis GORDON), akamatsu (Japanese red pine, Pinus densi9ora SIEBOLD et ZUCCARINI), radiata pine (Pinus radiata D. DON), udaikanba

(Japanese birch, Betula maximowicziana REGEL), buna (Japanese beech, Fagus crenata BLUME) and mizunara (Japanese oak, Quercus mongolica FISCHER var. grosseserrata REHDER et WILSON). Chemical compositions of the wood species are listed in Table 1 [5]. All wood species were ground to pass through a 1 mm screen using a Wiley mill, Yoshida Seisakusyo Co., Ltd. 1029-C model (Tokyo, Japan). The fractions passing 1000
m and retained at 106
m mesh screens were used for the liquefaction. Glycerol and PEG#400 (average molecular weight, 400) obtained from Wako Pure Chemical Industries Ltd. (Osaka, Japan) were used as liquefaction reagents. PMDI used was MR-100 (Japan Polyurethane Industry Ltd., Yokohama, Japan) with NCO group content of 7:43 mmol=g. All other chemicals used were extra pure grade of reagents in accordance with Japanese Industrial Standard (JIS) and used as received. 2.2. Preparation of LWs LWs were separately prepared according to a procedure shown in Fig. 1. Thirty grams of oven-dried wood Pour, 90:0 g of glycerol–PEG co-solvent and 2:7 g of sulfuric acid were charged into a 500 ml sep◦ arable Pask and rePuxed for 75 min at 150 C with continuous stirring. The weight ratio of glycerol:PEG was kept constant at 1 : 9. After rePux, the liquefaction products were diluted with ten times its weight of a dioxane-water mixture (80=20, v=v). Unliqueed wood residues in the solutions were ltered using PTFE membrane lters (TOYO H050A047A, pore size 0:50
m). The apparent pHs of the ltrates were adjusted to 5.5 with 1 M sodium hydroxide solution [4]. The sodium sulfates precipitated were removed

Y. Kurimoto et al. / Biomass and Bioenergy 21 (2001) 381–390

383

Fig. 1. Procedure used for preparing lique ed wood (LW). Weight ratio of glycerol : PEG#400 was 1 : 9.

using the membrane lters. Dioxane and water in the solutions were then evaporated under reduced pressure until the moisture contents were around 0.7%. LWs are considered to consist of woody components dissolved through solvolysis and the glycerol–PEG co-solvent. The amounts of dissolved woody components in LW (DWC) were de ned by the following equation: DWC = (WW − WU )=(WW − WU + WGP ) × 100; (1) where WU ; WW and WGP are the weights (g) of unlique ed wood residue, wood sample before liquefaction and glycerol–PEG co-solvent, respectively. 2.3. Characteristics measurements of LWs The hydroxyl numbers were determined according to the methods described in our earlier investigation [5]. One gram of each LW was esteri ed for ◦ 20 min at 110 C with 25 ml of a phthalation reagent, a mixture of 150 g phthalic anhydride, 24:2 g im-

idazol, and 1000 g dioxane. After that, the excess reagent was titrated with 1 M sodium hydroxide solution. The hydroxyl number was calculated from the dierence in titration of blank and the sample solutions. The acid numbers were determined as follows [5]: 8 g of LW sample were dissolved in a mixture of 80 ml of dioxane and 20 ml of water. The resulting solution was titrated at room temperature with 1 M sodium hydroxide solution. The acid number was calculated from the dierence in titration of blank and the sample solutions. The moisture contents were determined by Karl Fischer method using a moisture content meter (Kyoto Electronics Ltd. MKS-210 model, Kyoto, Japan). ◦ The viscosities at 25 C were measured using a Brook eld viscometer (Brook eld Engineering Labs. Inc., HAT model, Stoughton, Massachusetts, USA). Average molecular weights of LWs were estimated using LC-6A HPLC system (Shimadzu, Kyoto, Japan)

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Y. Kurimoto et al. / Biomass and Bioenergy 21 (2001) 381–390

with Shodex GPC KD-806M and KD-802 columns (Showa Denko, Tokyo, Japan). Dimethyl folmamide (DMF) including 0:01 M lithium bromide (LiBr) was ◦ used as eluent at a Pow rate of 1:0 ml=min at 40 C. The columns were calibrated with authentic polyethylene oxide. 2.4. Preparation of mixed-LW Two kinds of LWs, i.e., one is the LW with highest viscosity and the other is that with lowest viscosity in this experiment, were mixed mechanically using a hand mixer at 1000 rpm. The weight ratio of the two was equal. The hydroxyl number and moisture content were calculated from the hydroxyl numbers and moisture contents of the two LWs, respectively. The ◦ viscosity at 25 C was measured using a Brook eld viscometer. 2.5. Preparation of polyurethane (PU) >lms PU lms were prepared according to the procedures described in our previous investigation [4]. Ten grams of each LW dissolved in 10 ml of dichloromethane were mixed with the PMDI at 1200 –1400 rpm for 10 min at room temperature. No catalyst or surfactant was used. Carbon dioxide generated through a reaction between water and PMDI was removed by ultrasonic mixing. The polymerized mixtures were then casted into Petri dishes for solution-casting to obtain 0:25 mm-thick lms. The PU lms obtained were ◦ gradually dried in a refrigerator below 7 C for 3 days ◦ and were cured for 11 days at 20 C, 65% relative humidity (RH). Finally, the lms were heat-treated at ◦ 100 C for 8 h in an oven. 2.6. Measurements of crosslink density and weight of sol fraction of PU >lms The crosslink densities and the weights of sol fraction, soluble materials, of PU lms after soaking in DMF were measured using the procedures as described in our previous investigation [6]. 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 absorbed DMF on the surfaces were blotted using lter papers then

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 (c =V0 , unit: mol=cm3 ) of the lm was computed using the following equation [7]: c =V0 = − 2(v + v2 + ln(1 − v))=V1 (2v1=3 − v); (2) where c is the eective number of moles of crosslinked chains; V1 is the molar volume of solvent (76:87 ml=mol for DMF); is the polymer–solvent interaction parameter ( = 0:40 according to Yoshida et al. [8]); and v is the volume fraction of polymer in swollen gel (v = V0 =V∞ ): V0 and V∞ are the volumes of dry polymer (WD = p ) and swollen gel (WD = p + (W − WD )= S ) at equilibrium. WD and W∞ are the weights of de-swollen polymer gel and swollen polymer, respectively. p and S are the densities of de-swollen polymer and solvent (0:94 g=ml for DMF), respectively. The weight percentage of sol fraction after swelling was calculated from Eq. (3): Sol fraction = (W0 − WD )=W0 × 100;

(3)

where W0 is the initial weight of polymer. 2.7. Tensile tests of PU >lms ◦

Tensile tests were carried out at 20 C and 65% RH using a universal testing machine (Minebea Co., Ltd. AL-5kN model, Tokyo, Japan). Five replicates were tested for each condition. Tensile strength was calculated from the ultimate tensile force on the basis of initial sample dimensions (Fig. 2). Maximum elongation was taken from the distance between clamps at failure. Young’s modulus in the elastic region was calculated from the following equation since the cross section of the lm was not uniform between the clamps [4]: E = 25:76P=e;

(4)

where: E is the Young’s modulus (Pa). P is the total force applied to the lm (N). e is the total elongation (mm).

Y. Kurimoto et al. / Biomass and Bioenergy 21 (2001) 381–390

Fig. 2. Shape and dimensions of PU lm for tensile test. Note: L = 40; l = 30; B = 8; b0 = 4; t = 0:25; R = 101 (mm).

3. Results and discussion 3.1. Mechanical properties of PU >lms prepared from di@erent LWs Two types of PU lms, which comprised 1.0 or 1.2 [NCO]=[OH] ratio, were prepared through the co-polymerization of the LWs and PMDI. The formulation, Young’s modulus (YM), tensile strength (TS), and maximum elongation (ME) of each PU lm are shown in Table 2. There was no large difference in the amount of LW charged at the same [NCO]=[OH] ratio, because the hydroxyl number and the moisture content, which are the parameters that control the theoretical amount of PMDI, were almost constant (Table 3). This means that the masses of dissolved woody components included in the lms are equal.

385

The YM and TS of the PU lms at 1.2 [NCO]=[OH] ratio are larger than those of the lms at 1.0 [NCO]=[OH] ratio, whereas the ME of the former are smaller than those of the latter. These could be attributed to increases in crosslink densities of PU lms (Table 4), because a higher crosslink density results in a more rigid material [9]. In our earlier investigations [4,6], it has been reported that the LW prepared from sugi wood reacted with PMDI, leading to network formations according to the [NCO]=[OH] stoichiometry. When the [NCO]=[OH] ratio is 1.0, urethane linkages between alcoholic -OH and isocyanate and urea linkages through reaction of residual water and isocyanate should be formed. When the [NCO]=[OH] ratio exceeds 1.0, excess amount of isocyanate reacts with urethanes and ureas, resulting in formations of allophanates and biurets, respectively. In the case of LWs, similar network formations were possibly formed and resulted in the increases of crosslink densities depending on the introduction of PMDI into each LW. Analysis of variance showed signi cant dierences in the YM, TS and ME among the PU lms tested (Table 5), indicating that the mechanical properties of PU lms varied depending on the wood species. The dierences were attributed to the crosslink densities of PU lms. The increase in the crosslink density of PU lms at 1.0 [NCO]=[OH] ratio increased the YM, and decreased the ME as shown in Fig. 3. The relationships between the viscosity and the crosslink densities or the weights of sol fraction of PU lms are shown in Fig. 4. At the viscosities from 1.37 to 1:95 Pa s and [NCO]=[OH] ratios of 1.0 and 1.2, there are no dierences in the crosslink densities and the weights of sol fraction. However, the crosslink densities increased and the weights of sol fraction decreased with increases in the viscosities from 1.95 to 2:31 Pa s. Since the sol fraction is excluded from incorporation into the network formations of PU lm, the increases in the sol fraction decreased the crosslink densities. The sol fraction might be components originated from small amount of low molecular weight substances having less than one -OH group per molecule such as levulinic acid esters [10]. The low molecular weight substances are probably the reason that the LW with low viscosity produced the PU lm with large amount of sol fraction at a same [NCO]=[OH] ratio.

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Y. Kurimoto et al. / Biomass and Bioenergy 21 (2001) 381–390

Table 2 Formulations and mechanical properties of PU lms prepared from dierent LWs at [NCO]=[OH] ratios of 1.0 and 1.2 Young’s modulus (GPa)

LW

Tensile strength (MPa)

Maximum elongation (mm)

Weighta (%)

Mean

SDb

Mean

SDb

Mean

SDb

[NCO]=[OH] = 1:0 1 Karamatsu 2 Akamatsu 3 Radiata pine 4 Udaikanba 5 Buna 6 Mizunara

55.1 55.8 56.5 56.1 56.6 56.4

0.39 0.28 0.22 0.22 0.26 0.36

0.07 0.05 0.04 0.06 0.08 0.07

36.3 36.7 34.6 34.5 32.7 35.9

4.2 4.6 3.6 2.5 2.3 2.5

28.1 34.7 33.5 34.5 31.6 27.8

3.3 2.6 2.4 1.5 1.1 1.7

[NCO]=[OH] = 1:2 7 Karamatsu 8 Akamatsu 9 Radiata pine 10 Udaikanba 11 Buna 12 Mizunara

50.5 51.3 52.0 51.6 52.1 51.9

2.05 1.70 1.74 1.84 1.67 2.04

0.38 0.39 0.16 0.29 0.28 0.09

59.5 53.2 55.2 50.4 54.8 57.8

3.8 2.6 2.0 2.1 6.5 4.1

4.0 5.0 3.9 5.3 5.3 3.2

1.5 1.6 1.0 1.5 1.0 0.6

Polyurethane

a Weight

Type

percentage in PU lm. deviation.

b Standard

Table 3 Characteristics of LWs LW-type

DWCa (%)

Hydroxyl number (mg KOH=g)

Moisture content (%)

Acid number (mg KOH=g)

Karamatsu Akamatsu Radiata pine Udaikanba Buna Mizunara

24.1 24.5 24.4 24.3 24.3 24.3

294.2 284.9 273.8 282.0 275.4 277.8

0.74 0.72 0.75 0.70 0.70 0.71

23.2 26.5 20.1 21.4 25.3 24.3

2.11 1.84 1.95 1.44 1.37 2.31

Mean CVb (%)

24.3 0.4

281.4 2.7

0.72 2.9

23.5 10.3

1.84 20.2

a (30−

Viscosity ◦ (Pa s at 25 C)

unlique ed wood residue) ÷ (90− unlique ed wood residue) ×100. of variation.

b CoeRcient

3.2. Viscosities of LWs Varying the [NCO]=[OH] ratio is a general way to control the mechanical properties of PU lm. However, it is hard to decide actual amount of PMDI-charge according to the desired mechanical properties of PU lm at a constant [NCO]=[OH] ratio, when dierent LWs were used as the starting materials. An easy way to produce a PU lm with constant mechanical properties from dierent LWs is required.

The relationships between lignin and holocellulose contents of 90 Japanese wood species and those used in this study are shown in the appendix. Molecular weight distribution of LW-Karamatsu, which is typical curve among the LWs tested, is shown in Fig. 5. The curve was divided into two ranges by its molecular weight. The peak at higher molecular weight, Peak H principally resulted from partly degraded woody components through solvolysis with Glycerol and=or PEG#400. The other, Peak L seemed to be the

Y. Kurimoto et al. / Biomass and Bioenergy 21 (2001) 381–390

387

Table 4 Crosslink densities and weights of sol fraction of PU lms prepared from dierent LWs Polyurethanea

Crosslink density (×10−3 mol=cm3 )

Sol fraction (%)

Mean

SDb

Mean

SDb

1 2 3 4 5 6

1.06 1.01 0.99 0.91 1.04 1.19

0.04 0.12 0.16 0.06 0.14 0.09

7.2 7.9 7.5 7.4 7.5 6.4

1.0 0.1 0.7 0.4 1.0 0.8

7 8 9 10 11 12

1.50 1.44 1.45 1.42 1.58 1.76

0.04 0.11 0.08 0.15 0.08 0.07

4.2 4.9 4.4 4.7 4.8 3.9

0.3 0.4 0.7 0.7 0.2 0.7

a See

Fig. 3. Relationship between the crosslink density (CD) and the Young’s modulus (YM) or the maximum elongation (ME) of PU lms at a [NCO]=[OH] ratio of 1.0.

Table 2. deviation.

b Standard

Table 5 Variance ratio values obtained from analyses of variance of the mechanical properties of PU lms prepared from dierent LWs at [NCO]=[OH] ratios of 1.0 and 1.2 [NCO]=[OH] ratio Young’s modulus Tensile strength Maximum elongation 1.0 1.2 a Signi

b Signi

6.19a 1.71

0.92 3.52b

9.89a 2.34

cant at 1% level. cant at 5% level.

mixtures of un-reacted glycerol-PEG#400 co-solvent and the substances having less than one -OH group per molecule. The number average molecular weights (Mn ) and polydispersities (Mw =Mn ) of LWs are summarized in Table 6. The total Mn ranged from 583 to 667, although the Peak H of LWs showed so high Mn ranging from 1:46E + 4 to 1:86E + 4. This was because that the area percentages of Peak L were superior to those of Peak H in the LW’s molecular weight distributions. Viscosities of LWs may be inPuenced by both degree of polymerization and branched molecular structures. No clear relationship between the viscosity and the Mn of Peak H or Peak L was observed (compared to

Fig. 4. Eects of the viscosity on the crosslink density and the weight of sol fraction of PU lms at [NCO]=[OH] ratios of 1.0 and 1.2. Note: The values plotted in the gures are mean values obtained from measurements of ve replications, and error bars indicated are the ranges of standard deviation for each replication.

Table 3). Unfortunately, the branched molecular structures of LWs were not elucidated in this study. The total polydispersities (Mw =Mn ) of LWs from softwoods were larger than those from hardwoods.

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Table 6 Number average molecular weights (Mn ) and polydispersities (Mw =Mn ) of LWs Peak Hb

Total

Peak Lc

LW-typea

Mn

Mw =Mn

Area%

Mn

Mw =Mn

Area %

Mn

Mw =Mn

Area %

Karamatsu Akamatsu Radiata pine Udaikanba Buna Mizunara

605 667 628 609 583 610

35.0 33.4 39.3 30.4 20.2 24.2

100.0 100.0 100.0 100.0 100.0 100.0

1.66E+04 1.72E+04 1.76E+04 1.86E+04 1.46E+04 1.49E+04

3.3 3.3 3.5 2.8 2.3 2.6

38.1 38.9 39.0 35.2 33.5 38.0

380 413 389 399 393 390

1.7 1.6 1.7 1.7 1.7 1.7

61.9 61.1 61.0 64.8 66.5 62.0

a See

Table 3. range of molecular weight fractions in LW. c Lower range of molecular weight fractions in LW. b Higher

Fig. 6. Relationship between the area percentages of Peak H and viscosities of LWs. See Table 6. Fig. 5. Molecular weight distribution curve of LW-Karamatsu. See Table 3.

One possible reason of the dierence is considered to be the reactivity of lignin. Of the major components of lignocellulosics, lignin is the most chemically susceptible and condensation reactions occur during liquefaction [5,10,11]. The condensation occurs more frequently in softwoods than in hardwoods since uncondensed C-5 position of guaiacyl propane unit of softwood lignin contributed to higher reactivity compared to that of syringyl propane unit of hardwood. Therefore, it is assumed that condensed structures from softwoods increased their polydispersities compared to the structures from hardwoods.

The relationship between the area percentage of Peak H and viscosities of LWs is shown in Fig. 6. The increases in the area percentage increased the viscosities. The viscosity of glycerol–PEG#400 co-solvent ◦ at 25 C, which seemed primary components of the Peak L, was 0:18 Pa s. Therefore, the LW with low area percentage of Peak H, that is, high area percentage of Peak L, showed low viscosity. As shown in Fig. 4, the PU lm prepared from LW with high viscosity gave rigid mechanical properties. The rigid mechanical properties of PU lms with higher area percentage of Peak H suggests that the fragments of Peak H (i.e., partly degraded woody components) acted as crosslinking agents rather than as a chain extender in this polymerization system. It is considered

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389

Table 7 Mechanical properties of PU lm prepared from mixed-LW at a [NCO]=[OH] ratio of 1.0 Mixed-LWa

PU lms 4,6

Mechanical property

Mean

SD

Averageb

c

Young’s modulus (GPa) Tensile strength (MPa) Maximum elongation (mm)

0.31 37.7 32.5

0.04 3.4 2.3

0.29 35.2 31.2

0.14 1.4 6.7

a Equal weights of LW-Udaikanba and LW-Mizunara were mixed. b Average values calculated from the PU lms 4 and 6 (see Table 2). c Dierences between the PU lms 4 and 6 (see Table 2).

that a ratio of area percentage between Peak H and Peak L would aect the mechanical properties of PU lm. Equal weights of the LW-Udaikanba (1:44 Pa s) and the LW-Mizunara (2:31 Pa s) were mixed, and PU lm was then prepared from the resulting LW at the [NCO]=[OH] ratio of 1.0. The viscosity of ◦ the mixed-LW at 25 C was 1:90 Pa s. The YM, TS and ME of the PU lm prepared were 0:31 GPa, 37:7 MPa and 32:5 mm, respectively (Table 7). These values are close to the averages of the PU lm 4 (LW-Udaikanba) and 6 (LW-Mizunara), indicating that using the mixed-LW contributed to the reduction of the variations in the YM, TS and ME. It can be concluded that varying the viscosity is a way to easily control the mechanical properties of PU lm without changing the [NCO]=[OH] ratio. 4. Conclusions LWs were separately prepared from six wood species using glycerol–PEG co-solvent in the presence of sulfuric acid. Every LW could form the networks through the co-polymerization with PMDI depending on the [NCO]=[OH] ratio. The mechanical properties (Young’s modulus, tensile strength and maximum elongation) of the PU lms varied with the wood species tested. The use of the mixed-LW could reduce the variations of each mechanical property. Controlling the viscosity seems to be an eective way to produce a PU lm with stable mechanical properties, even if a wide variety of wood species is used as sources for the liquefaction.

Fig. 7. Relationship between the lignin and the holocellulose content of dierent wood species. open circles, ninety wood species in Japan [12]; >lled squares, species tested in this experiment.

The structural characterization of the hydroxyl groups, i.e., primary or secondary, and the condensed fractions of each LW is essential to understand the role of viscosity on overall behaviors of networks and mechanical properties of PU lms. Acknowledgements The authors would like to thank Dr. Tatsukiko Yamada, Forestry and Forest Products Research Institute, for his experimental support on GPC analysis of LWs. Appendix The relationships between the lignin and the holocellulose contents are shown in Fig. 7. Open circles plotted in the gure were the values obtained from ninety wood species in Japan [12]. The variation of the lignin content is extremely wide within a range of 14.5 –33.7% (average, 22.8%). While, the lignin contents of the wood species tested were 20.5 –24.8% ( lled squares), indicating that these are most common wood species. The narrow range of wood constituents probably protected a clear understanding of the relationship between the molecular weight distribution and the viscosity of LW. Further investigation using wood species with higher or lower lignin content is required.

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References [1] Shiraishi N. Plasticization of wood and its application. In: Shiraishi N, Kajita H, Norimoto M, editors. Recent research on wood and wood-based materials. UK: Elsevier Applied Science, 1993. p. 155 – 67. [2] Kurimoto Y, Shirakawa K, Yoshioka M, Shiraishi N. Liquefaction of untreated wood with polyhydric alcohols and its application to polyurethane foams. FRI (New Zealand) Bull 1992;176:163–72. [3] Yao Y, Yoshioka M, Shiraishi N. Rigid polyurethane forms from combined liquefaction mixture of wood and starch. Mokuzai Gakkaishi 1995;41(7):659–68. [4] Kurimoto Y, Takeda M, Koizumi A, Yamauchi S, Doi S, Tamura Y. Mechanical properties of polyurethane lms prepared from lique ed wood with polymeric MDI. Bioresource Technology 2000;74(2):151–7. [5] Kurimoto Y, Doi S, Tamura Y. Species eects on wood-liquefaction in polyhydric alcohols. Holzforschung 1999;53(6):617–22. [6] Kurimoto Y, Takeda M, Doi S, Tamura Y, Ono H. Network structures and thermal properties of polyurethane

[7]

[8]

[9] [10] [11] [12]

lms prepared from lique ed wood. Bioresource Technology 2001;77(1):33–40. Rials TG, Glasser WG. Engineering plastics from lignin IV. Eects of crosslink density on polyurethane lm properties-variation in NCO : OH ratio. Holzforschung 1984;38:191–9. Yoshida H, Morck R, Kringstad KP, Hatakeyama H. Kraft lignin in polyurethanes I. Mechanical properties of polyurethanes from kraft lignin–polyether triol-polymeric MDI system. Journal of Applied Polymer Science 1987;34:1187–98. Saraf VP, Glasser WG. Engineering plastics from lignin III. Structure property relationship in solution cast polyurethane lms. Journal of Applied Polymer Science 1984;29:1831–41. Yamada T, Ono H. Rapid liquefaction of lignocellulosic waste by using ethylene carbonate. Bioresource Technology 1999;70:61–7. Yao Y, Yoshioka M, Shiraishi N. Combined liquefaction of wood and starch in a polyethylene glycol=glycerin blended solvent. Mokuzai Gakkaishi 1993;39(8):930–8. Sugihara H. In: Handbook of wood technology, Tokyo: Kogyo Shuppan, 1982. p. 775 –8 (in Japanese).