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Eur. Polym. J. Vol. 34, No. 8, pp. 1163±1169, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $Ðsee front matter S0014-3057(97)00245-0
POLYURETHANES BASED ON OXYGEN-ORGANOSOLV LIGNIN D. V. EVTUGUIN,1* J. P. ANDREOLETY2 and A. GANDINI2{ University of Aveiro, 3810 Aveiro, Portugal and 2Ecole Franc° aise de Papeterie et des Industries Graphique (INPG), MateÂriaux PolymeÁres, B.P. 65, 38402 Saint Martin d'Heres, France
1
(Received 12 March 1997; accepted in ®nal form 26 June 1997) AbstractÐOxygen-organosolv lignins, isolated from spent liquors after deligni®cation of wood in dierent acidic organic solvent±water media, were used as unmodi®ed macromonomers in conjuction with an oligoethyleneoxide di-isocyanate. This led to crosslinked elastomeric polyurethanes which were thoroughly characterized. It was found that the nature of the organic solvent used for the organosolv pulping had an in¯uence on the reactivity of the isolated lignins towards the di-isocyanate because of the variation in the amount of alcoholic groups caused by reactions with solvent molecules during the deligni®cation. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Lignin is the second most abundant natural renewable polymer after cellulose [1]. Its isolation in papermaking, hydrolysis and other biomass re®neries leads essentially to its use as a fuel, i.e. an economic exploitation based on energy recovery in situ [1, 2]. However, growing research and technological interest has been devoted to alternative ways of utilizing this massive resource. These include simple applications as ®llers, speci®c chemical modi®cations and the use as macromonomers (as such or after appropriate structural changes) in the synthesis of new polymeric materials [1±3]. Lignins are complex macromolecular substances characterized not only by the fact that already within the speci®c vegetal species their crosslinked structure can vary considerably, but also because the numerous techniques available for deligni®cation provide oligomeric or polymeric fragments with very variable chemical and topological features [1±5]. It follows that, when applications are sought and a speci®c lignin is employed to test its viability, one can run into a number of diculties related to complex irregular structures, but also into problems connected with the irreproducibility of results if the source material (and therefore its speci®c structure) is changed. Pulping for paper production is presently mainly based on sulphate and sulphite processes which favor chemical passivation of the resulting technical lignins by the increase in their degree of condensation through reactive groups (which are therefore lost) and consequently of their polydispersity. This leads to poorer reactivity and to the presence of often undesirable physical (e.g. ashes) and chemical impurities (e.g. sulphur-containing moieties). *On leave from St.-Petersburg Forest Academy, Institutski 5, 194018 St.-Petersburg, Russia. {To whom all correspondence should be addressed.
Organosolv technologies, which have been developed in recent years, provide a milder means of deligni®cation and therefore the resulting fragments tend to possess larger proportions of reactive functions and can moreover be easily recovered from the organic medium and puri®ed [6±8]. An additional advantage of these lignins is the fact that they do not contain any sulphur. This state of aairs favors research and development studies on the alternative uses mentioned above and obviously this interest will increase if organosolv technologies reach a wider industrial practice. An example of the interest of organosolv procedures is the recent work on oxygen pulping in organic solvent±water media which allows to obtain as by-products remarkably functionalized lignins [7, 9] with high weight-by-weight contents of carboxy (up to 13%) and hydroxy (10±12%) groups, low DPs and polydispersities and small amounts of carbohydrate impurities (less than 1.5%). Clearly these products represent a much more interesting starting point as macromonomers than more conventional lignins thanks to their enhanced reactivity coupled with lower average molecular size. This dierence has already been shown in two sets of studies concerning lignin-based polyesters [10±12]. In both investigations we adopted the novel strategy of avoiding any chemical treatment between the isolation of the lignins and their use as OH-bearing macromonomers, in order to simplify the approach with respect to previous, more involved practice [1, 3] and to make it more viable in economic terms. This more straightforward approach clearly made organosolv lignins more attractive because of their higher OH contents and better solubility than e.g. kraft counterparts [10±12]. The other obvious way of making good use of lignins as macromonomers is still based on the condensation reactions of its OH groups, but this time with isocyanates, i.e. the synthesis of polyurethanes. A preliminary study on these materials was con-
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D. V. Evtuguin et al. Table 1. Some characteristic of spruce oxygen-organosolv lignins Lignin sample and pulping conditions Oxygen±acetone (1508C; 4.5 h) Oxygen±ethanol (1558C; 4.5 h) Oxygen±acetic acid (1508C; 4.0 h)
Formula based on C9
Number of OH groups per C9 (OHalc:OH ph)
Reference
C9O4.24H8.14(OCH3)0.82
1.23(0.85:0.38)
[9]
C9O3.46H9.05(OCH3)0.79
1.18(0.84:0.34)
[17]
C9O3.91H7.84(OCH3)0.74
0.80(0.40:0.40)
[18]
ducted lately in our laboratory [13]. In the present work we used oxygenorganosolv lignins as unmodi®ed macromonomers and made them react with an oligoethyleneoxide di-isocyanate synthesized in the laboratory, to yield (partially) crosslinked elastomeric polyurethanes. The aim of this investigation was to test the reactivity of the new kind of lignin [7] in an original synthetic approach based on the bulk polycondensation between the aliphatic and phenolic hydroxy groups of lignin and the isocyanate moieties of the difunctional oligoethyleneoxide chain-extender. This would moreover allow us to compare the properties of the ensuing materials with those obtained using other lignins [13]. EXPERIMENTAL PROCEDURES
Materials The oxygen-organosolv lignin employed in this work was isolated from the spent liquor arising from the oxidative deligni®cation of aspen chips by oxygen (1458C; 180 min) in an acetone±water medium 60:40 (v/v), according to a previously published procedure [7]. This lignin had the following characteristics: Ð Ash content <0.1%; Ð Carbohydrates contents <1.0%; Ð Aliphatic and phenolic hydroxy group: respectively 0.95 and 0.41 per aromatic group, as obtained from quantitative 13 C-NMR spectra of acetylated samples [14]; Ð Formula based on a classical C9 phenylpropane unit (PPU): C9O3.65H8.30(OCH3)0.98, MPPU=205; Ð Mw=4100, as measured by static low angle laser light scattering [15]; Ð Mn=1400, as obtained from vapor pressure osmometry in methoxyethanol. In the experiments dealing with the comparison of the reactivity of dierent lignins, we used three samples which were spruce oxygen-organosolv lignins isolated from pulping solutions after deligni®cation in (i) acetone±water (60:40, v/v), (ii) ethanol±water (60:40, v/v) and (iii) acetic acid±water (80:20, v/v) (Table 1). These were characterized as reported elsewhere [7, 9, 16, 17]. The oligoethyleneoxide di-isocyanate (ODI) was synthesized from a commercially available polyethyleneoxide diamine (Jeamine, Mn=600) by its reaction with bis(trichloromethyl)carbonate (triphosgene) as already described [13, 18]. All solvents and catalysts used both for the synthesis of ODI and for the extractions of the polyurethanes were high-grade commercial products.
Preparation of polyurethanes Lignins were vacuum dried at 308C before the polycondensations which were carried out under nitrogen in a glass reactor provided with good stirring. The preparation sequence was as follows: lignin, in the form of a ®ne powder, was mixed with ODI and stirred for 30±40 min at 408C in order to achieve a homogeneous viscous solution.
The temperature was then increased to 608C within 20 min and the liquid catalyst (dibutyltin dilaurate) was ®nally added to give a 2% w/w concentration. The stirring was stopped when the viscosity of reaction mixture became too high and the system was kept at 608C for a further period of 8 h. The resulting polyurethanes were extracted with dichloromethane in a soxlhet for 12 h in order to eliminate non-crosslinked products. The yield of the insoluble polymer was calculated with respect to the total product before extraction. In the case of the elaboration of ®lms, the reaction procedure was performed in the same manner but, after the addition of the catalyst, the resulting mixture was poured into a ¯at mould about 1 mm thick. At the end of the reaction, the membranes were demoulded and characterized.
Analysis of polyurethanes The FTIR photoacoustic spectra (FTIR-PAS) of polyurethanes were recorded in a helium atmosphere with a BRUCKER IFS 55 spectrophotometer equipped with an MTEC Model 200 photoacoustic detector. Spectra were collected (250 scans) using a mirror velocity 0.1 cm/s after calibration with a MTEC carbon black reference at 8 scan and 8 cmÿ1 resolution with a medium Beer±Norton apodization function. The thermogravimetric analyses were carried out with a SHIMADZU TGA-50 Analyzer with a heating rate of 108C/min in a ¯owing nitrogen environment. Dierential scanning calorimetry (DSC) was performed in an inert atmosphere on 10±20 mg samples using a ``SETARAM DSC91'' working at a heating rate of 108C/ min in the temperature range of ÿ1508C to 1008C.
RESULTS AND DISCUSSION
General features of polyurethane preparation The advantage of calling upon an oligoether diisocyanate (ODI) stemmed from the fact that this compound played both the role of crosslinking reagent with respect to the OH groups of lignin and that of solvent, since it was know from previous experience [11±13] that these types of structure can dissolve up to 50% of lignin, depending on the origin of the latter. This double property greatly facilitated the operating mode for the synthesis of the polyurethanes and at the same time gave the possibility of introducing directly soft interlignin bridges through the straightforward bulk reaction involving only two reagents. The use of ODI provided the ¯exible segments necessary to reduce the Tg of the ®nal materials to levels well below room temperature, i.e. elastomeric products. In fact, all lignins show glass transition temperatures well above room temperature because of their aromatic and polar moieties and in a previous study in the laboratory, the lowering of Tg
Polyurethanes based on oxygen-organosolv lignin
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Fig. 1. FTIR-PAS spectrum of aspen lignin.
had required the use of a more complex system involving kraft lignin, a conventional commercial di-isocyanate and a softening comacromonomer consisting of oligoethyleneoxide glycol [19]. Preliminary experiments using quasi-stoichiometric [NCO]/[OH] conditions showed that the polycondensation of lignin with ODI did not occur in the absence of catalysts within a wide temperature interval, namely from 40 to 1608C. This lack of reactivity can be explained by (i) steric diculties related to the encounter of OH and NCO functions, both attached to macromolecular reagents; (ii) diffusion problems imposed by the high viscosity of the medium. Moreover, the system involved aliphatic isocyanate moieties, which are well known for their kinetic sluggishness in condensation reactions with OH groups. The addition of dibutyltin dilaurate to these reaction mixtures facilitated the condensation and led to the formation of crosslinked polyurethanes when working above 558C for relatively long periods. The positive eect of dibutyltin dilaurate on these systems can be related to two independent causes, namely: Ð its well-known action as Lewis-acid catalyst for the activation of isocyanate moieties and Ð its very eective role in breaking donor/acceptor interactions, resulting in the activation of the lignin OH groups through their increased accessibility (particularly in the case of phenolic OHs). Figures 1 and 2 show a comparison of the photoacoustic infrared spectra of a sample of our standard aspen oxygen±acetone lignin and of the
corresponding crosslinked polyurethane obtained under stoichiometric conditions, after extraction with dichloromethane. The major dierences which clearly proved that the polycondensation had taken place (and indeed given 76% of crosslinked material, as shown in Table 2) are: (i) the decrease in the relative intensity of the OH band at 3470 cmÿ1 and its shift to lower frequencies, namely around 3380 cmÿ1, indicating the consumption of the former groups to give the corresponding NH functions in the urethane moieties; (ii) the strong increase in the relative intensity of the bands at 2800± 2950 cmÿ1 showing the incorporation of the ODI and the corresponding inclusion of numerous aliphatic moieties; (iii), the relative increase in the carbonyl peaks which was however a more modest feature, because the lignin itself was obviously strongly oxidized [9]; nevertheless, the predominance of a peak around 1720 cmÿ1 in the polyurethane suggests that the aliphatic OH groups had reacted preferentially with the NCO functions of ODI; (iv) the presence of stronger bands around 1550 cmÿ1 constitutes an additional evidence for the formation of O±CO±NH groups; (v) the appearance of a strong wide band at 1150 cmÿ1 typical of the vibration of C±O single bonds arising from the polyether segments from the ODI-type bridges among lignin macromolecules. In conclusion, the bulk catalyzed reaction of our unmodi®ed organosolv lignins with aliphatic di-isocyanates takes place without any apparent complications, albeit quite slowly.
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Fig. 2. FTIR-PAS spectrum of an aspen ODI-lignin-based crosslinked polyurethane.
In¯uence of NCO/OH ratio on polyurethane properties A series of experiments was conducted in which the [NCO]/[OH] initial ratio was varied around the stoichiometric conditions while all other parameters were maintained unchanged, except of course for the inevitable corresponding change in lignin proportion. The results of these runs are given in Table 2. Although the extent of crosslinking did not change in a drastic manner, a modest trend is visible, indicating that in order to optimize this parameter, which is directly related to the overall reaction eciency, an excess of isocyanate is necessary, namely about 50%. In¯uence of the origin of the lignin on its reactivity The extensive studies on oxygen-organosolv deligni®cation using on the one hand dierent lignocellulosic materials (hard- and soft-wood, annual plants, agricultural wastes, . . . ) and on the other hand a wide range of conditions and speci®c organic solvents [7, 9, 16, 17] have provided a compreTable 2. Eect of NCO/OH ratio on polyurethane yield NCO/OH ratio 0.8 1.0 1.2 1.5 2.0
% of lignin in polyurethane 36.7 31.7 27.6 23.6 18.0
Yield of polyurethane after extraction (%) Tg (8C) 76.2 76.1 78.3 82.9 77.8
ÿ28 n.d. ÿ36 ÿ41 ÿ45
hensive knowledge of the relationships between a given system and the structural features of the ensuing lignins, viz. molecular weight, polydispersity and the quantitative appraisal of the various functional groups. This is a very important point in the present context, since the optimization of speci®c syntheses and properties related to the elaboration of polymers based on lignin macromonomers, will obviously be strongly related to the above features. We decided to start looking into this aspect by preparing a series of polyurethanes using ODI and the lignins shown in Table 1 and comparing the phenomenology of these syntheses as well as the properties of the corresponding products with those of the standard aspen lignin used in all the previous experiments. Table 3 gives the results of these comparative experiments, all carried out with the optimum [NCO]/ [OH] initial ratio of 1.5. As can be seen from these data, the yields of the CH2Cl2-insoluble polyurethanes is quite sensitive to the origin of the lignin used and the best results were obtained with oxygen±acetone and oxygen±ethanol lignins, whereas the oxygen±acetic acid sample gave the lowest yield. The comparison of hardwood (aspen) and softwood (spruce) oxygen±acetone lignins showed that the former was more reactive. All these features can be explained by considering the changes in hydroxy group contents in lignin during the oxygen deligni®cation in organic media of dierent composition [9, 16, 17]. Thus, when this process takes place in an acetone±water medium, the lignin does not interact with the organic solvent,
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Table 3. Eect of lignin origin on polyurethane properties (NCO/OH = 1.5, key to designation of samples in Table 1) Lignin sample Aspen oxygen±acetone Spruce oxygen±acetone Spruce oxygen±ethanol Spruce oxygen±acetic acid
Yield after extraction (%) 82.9 75.6 75.5 68.4
whereas in the ethanolwater mixture, a certain proportion of secondary hydroxy groups (0.1±0.3 OH/ C9), namely those predominantly at the a-position with respect to the aromatic ring, were consumed through etheri®cation promoted by the alcohol [16]. In the case of oxygenacetic acid deligni®cation, the interaction of lignin with the acid was also signi®cant as indicated by the fact that nearly 50% of primary (ca. 0.3 OH/C9) and 30% of secondary OH groups (ca. 0.05 OH/C9) were acetylated during the pulping process [17]. It is well known that the reactivity of OH groups towards isocyanates depends strongly on steric factors and therefore primary moieties give higher rate constants than secondary ones, which in turn are more reactive than tertiary functions [20]. Taking this into account, together with the fact that primary lignin OH groups placed at the g-position with respect to the aromatic ring are more accessible than both secondary and phenolic counterparts, the important positive role of the g-OHs in reactions with isocyanate moieties could be anticipated. Indeed, the proof came from oxygen±acetic acid lignin, which contains low amounts of primary OH groups and thus gave poor results in its polycondensation with ODI (Tables 1 and 3). The negligible
Lignin Tg (8C) 130 121 116 122
Polyurethane Tg (8C) ÿ41 ÿ42 ÿ44 ÿ38
dierence in the results related to spruce lignins obtained using either aqueous ethanol or acetone can be rationalized by the very similar aliphatic OH content of these two materials, which con®rms moreover that the phenolic OH groups are less reactive in these systems, as already suggested above. Finally, when the same deligni®cation procedure was adopted, but the wood species changed, namely spruce and aspen, the higher reactivity of the lignin derived from the latter is attributed to its higher aliphatic OH content, as given in the experimental part and Table 1. Glass transition temperatures Table 2 also gives the Tg values of the various crosslinked polyurethanes obtained in the series of runs based on changing the [NCO]/[OH] ratio. Figure 3 shows a typical DSC thermogram related to one of these materials. As expected, all these polymers behaved as elastomers because the ®lms prepared exhibited dynamic mechanical features typical of polymers on the rubbery plateau and now their low values of Tg con®rmed this property. Moreover, it was interesting to examine the trend in the Tgs which indicated a progressive lowering as
Fig. 3. Typical DSC thermogram of an ODI-lignin-based crosslinked polyurethane.
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Fig. 4. TGA thermogram of a sample of aspen lignin.
Fig. 5. TGA thermogram of a crosslinked polyurethane prepared with the aspen lignin of Fig. 4.
Polyurethanes based on oxygen-organosolv lignin
the yield of crosslinked product increased (see Table 2). This is in excellent agreement with the fact that the more frequent the ODI bridges, the higher the contribution of soft segments in the material and, therefore, the lower its glass transition temperature. The Tg values related to the crosslinked polyurethanes obtained with dierent lignins are given in Table 3 together with those of the lignins themselves. The comparison among materials derived from lignins arising from the same wood species, namely spruce, con®rms the remarks made above, viz. the decrease in Tg with increasing extent of crosslinking (this comparison is valid because the dierent starting lignins had essentially the same glass transition temperature, particularly with acetone and acetic acid). The comparison between the two crosslinked polyurethanes prepared with lignins from dierent wood species, namely aspen and spruce, is biased by the fact that the former lignin had a higher Tg. Thus, the fact that the material at a higher yield gave the same Tg as that with a lower one, can be rationalized by the higher stiening eect of aspen lignin. Thermal stability Figures 4 and 5 give the TGA thermograms of aspen lignin and of a crosslinked polyurethane derived from it. As usual, lignin alone exhibited a minor weight loss caused by solvent departure followed by the onset of decomposition around 2508C. The corresponding ODI-based polyurethane was stable up to about 3008C, i.e. crosslinking the lignins macromonomers leads to a considerable improvement in the thermal stability. All other materials prepared in this work behaved likewise. CONCLUSION
Organosolv lignins, used as obtained, lend themselves readily to the preparation of elastomeric polyurethanes. This study provided moreover a means of establishing clearcut correlations on the one hand between the speci®c pulping conditions and the reactivity of the ensuing lignins, and on the other hand between the structure and extent of crosslinking of these materials and their glass transition temperature. The dynamic mechanical properties of these polyurethanes constitute the next step in this general investigation on lignin-based polymers.
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