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The oxypropylation of cork residues: preliminary results
Bioresource Technology 73 (2000) 187±189
Short communication
The oxypropylation of cork residues: preliminary results Margarita Evtiouguina a, Ana Margarida Barros a, Jose J. Cruz-Pinto a, Carlos Pascoal Neto a, Naceur Belgacem b, Claire Pavier c, Alessandro Gandini c,* a Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal Department of Pulp and Paper Engineering, University of Beira Interior, 6200 Covilh~ a, Portugal Inst. Nat. Polytechnique de Grenoble, Ecole Francßaise de Papeterie et des Industries Graphiques, Rue de la Papeterie, BP 65, 38402 St. Martin d ÕH eres, France b
c
Received 21 October 1999; accepted 10 November 1999
Abstract Cork particles were oxypropylated under pressure at a relatively high temperature and yielded a mixture of liquid polyols composed of propylene oxide homopolymer (PPO) and cork macromolecules bearing oligo(propylene oxide) grafts. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Cork; Oxypropylation; Viscous polyols
1. Introduction Several dierent types of polymeric substrates arising from renewable resources have been submitted to oxypropylation treatments in view of modifying their structure, morphology and properties and thereby opening new possible uses as materials or additives. Cellulose and some of its derivatives, lignins, chitosan and, more recently, the polysaccharide mixture constituting sugar beet pulp (Pavier and Gandini, 1999a), are typical examples of such substrates. In the latter publication this topic is brie¯y reviewed. In all instances, the initial solid natural product was converted into a liquid polyol of varying viscosities made up of a mixture of oxypropylated products; macromolecular structures in which at least some of the original OH groups borne by the substrate were grafted with oligo(propylene oxide) chains, and propylene oxide homopolymer. To the best of our knowledge, cork has not been the object of a published study within this context. Cork is a unique natural material constituting the outer bark of Quercus suber L. Its morphology and composition dier substantially from those of most lignocellulosic counterparts, particularly because of the presence of large amounts of suberin (up to 55% by weight), a predomi-
nantly aliphatic biopolyester crosslinked to the other, more common, components of the cell wall matrix, namely polysaccharides and lignin-like structures (Cordeiro et al., 1998a; Lopes et al., 1998). Our laboratories have been involved in a common project aimed at gaining a better knowledge of cork (Gil et al., 1997; Cordeiro et al., 1998a,b) and at making good use of suberin as a macromonomer (Cordeiro et al., 1997, 1999a) or a ®ller (Cordeiro et al., 1999b). Among the reasons for this thorough investigation, lies the fact that the cork industry generates very substantial wastes, in the form of sawdusts, which are usually burnt. It seemed therefore interesting to examine the possibilities of making a better use of these side products. As an important complement to this practical motivation, there was a scienti®c driving force associated with the peculiar nature of cork, in terms of both its chemical composition and macrostructure and its physico-chemical properties. The present study remained entirely within these aims, since it tackled the chemical modi®cation of cork with the purpose of reaching a better understanding of its reactivity in heterogeneous systems, and obtaining novel products with potential applications in the realm of polymeric materials. 2. Methods
*
Corresponding author. Tel.: +33-4-7682-6947; fax: +33-4-76826933. E-mail address: [email protected] (A. Gandini).
Cork powder, kindly provided by the Champcork Company of Portugal, was ground to particles not
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 1 5 8 - 3
188
M. Evtiouguina et al. / Bioresource Technology 73 (2000) 187±189
exceeding 0.4 mm in diameter and air dried. Its residual moisture content was 7% w/w. The reactions with pure commercial propylene oxide (PO) were carried out in a 300 cm3 stainless steel vessel equipped with stirring, heating elements and both pressure and temperature sensors. The typical procedure consisted in introducing weighed amounts of cork and high-purity commercial KOH pellets into the reactor, followed by a known volume of PO. The reactor was closed and the solid/ liquid suspension (cork and KOH are insoluble in PO) was heated progressively under stirring to reach temperatures between 100°C and 145°C within about 1 h. The oxypropylation and PO homopolymerisation reactions started progressively as the temperature was raised. Then, a sudden increase in temperature, which attained values ranging between 170°C and 260°C depending on the speci®c conditions, was recorded revealing a strong acceleration of the process. The pressure increased correspondingly up to 8±15 atm. This phenomenon was attributed to the concomitant eects of sheer kinetic activation coupled with a base-catalysed degradation of the cork network which enhanced the accessibility of the OH functions. After a lapse of one to several hours (depending inversely on the chosen reaction temperature and concentrations of KOH and PO), the temperature decreased steadily back to the set value and the pressure dropped more rapidly to near-atmospheric values, indicating the near-complete consumption of PO. After allowing the system to reach room temperature, the reactor was opened and methylene chloride was added to the viscous product. The resulting mixture was neutralised with acetic acid and ®ltered in order to remove any solid residue insoluble in CH2 Cl2 . The latter was dried at 45°C and weighed to establish the percentage of unreacted or poorly oxypropylated cork. The volatile components of the ®ltered solution were removed in a rotary evaporator leaving a viscous polyol containing the potassium acetate formed in the neutralisation step. This product was then submitted to an extraction with boiling n-hexane in order to separate the insoluble oxypropylated cork from the PO homopolymer (PPO) (Pavier and Gandini, 1999b) and the two fractions were ®nally vacuum-dried and weighed. The solid residue was characterised by FTIR only, whereas the two viscous polyols were submitted to a more thorough inspection involving FTIR, 1 H- and 13 CNMR spectroscopy, DSC, GPC (in DMAc, polystyrene standards), vapour pressure osmometry (VPO), bulk viscometry and OH number. 3. Results and discussion Based on previous experience with other oxypropylation systems involving lignocellulosic substrates
(Pavier and Gandini, 1999a), the reaction parameters which were examined in this preliminary study were: the percentage of KOH in the initial ternary mixture, which was varied from 5% to 20% w/w with respect to the amount of cork; the relative proportion of PO with respect to cork, which ranged from 4:1 to 8:1 w/w; the set temperature which went, as mentioned above, from 100°C to 145°C. The following observations describe the general behaviour of this new system: · The extent of the reaction was always very high, as measured by the total weight of the recovered product compared with the total weight of the reagents. Thus, the unreacted PO was typically around 3% and at most 8% with the mildest conditions (lower temperatures and KOH concentrations). · The proportion of CH2 Cl2 -insoluble product was always very modest (between 0.5% and 5%), indicating that the reaction had involved the vast majority of the cork, since the latter was insoluble in that solvent. Moreover, a comparison of the FTIR spectra of the initial cork and of the solid residue clearly showed that even these insoluble fractions had been oxypropylated, albeit poorly, as shown by the presence of a strong CH3 peak at 2980 cmÿ1 , absent in the spectrum of cork. In other words, the vast majority of the cork was thoroughly oxypropylated (transformation into a viscous liquid soluble in CH2 Cl2 ) and the rest also reacted, but to a lesser extent. · The liquid product was made up of oxypropylated cork and PPO. The proportion of the latter tended to be higher than that of the former, ranging from 75% with the higher initial PO proportions to 40% with the lower ones. These results are in tune with those obtained in previous studies carried out with other natural polymeric materials (Pavier and Gandini, 1999a). · The molecular weights of the PPO were typically lower than 1000, whereas those of the oxypropylated cork exceeded the sensitivity of the VPO instrument (>10 000). The viscosities of the two fractions re¯ected this large dierence, with values about 2 orders of magnitude lower for the PPO. This is entirely reasonable considering the dierence in molecular size and architecture between the two products (see below). · The spectroscopic evidence gathered from a large number of products indicated unambiguously the existence of two well-de®ned components in the liquid product; a lower-viscosity hexane-soluble fraction with FTIR, 1 H- and 13 C-NMR spectra virtually identical to those of a commercial sample of PPO with Mn 1000, and a high viscosity hexane-insoluble fraction characterised by spectroscopic features showing the presence of both PO oligomeric chains and of cork constituents, aliphatic chains from suberin and polysaccharide structures.
M. Evtiouguina et al. / Bioresource Technology 73 (2000) 187±189
It can be concluded that PO reacts readily with solid cork particles in the presence of a strong base. This reaction must proceed through at least three pathways: the homopolymerisation of PO induced both by direct initiation (probably from OHÿ generated by KOH plus traces of moisture) and by transfer reactions occurring during the propagation reactions related to this monomer (homopolymerisation and formation of the grafts); oxypropylation from activated OH functions borne by the cork macromolecular moieties, and cork degradation induced by the strongly basic medium. Evidence proving the occurrence of the third reaction came from the fact that the ®nal oxypropylated product was a thermoplastic material (highly viscous liquid) and could not therefore have maintained the cross-linked structure of its natural precursor. The presence of multiple ester groups in the structure of cork, and more particularly within the suberin architecture, leaves little doubt as to the origin of this degradation which relates to their basecatalysed cleavage leading to the breakdown of the cork macromolecular network. The system under study remains, however, hard to control, as shown by the diculty in obtaining a good reproducibility in the results related, for example, to the kinetics of the reaction and to the viscosity of the products. This is attributed to the complexity of the heterogeneous reaction, the diculty in duplicating the temperature pro®le of the process and the possible variation in residual moisture from run to run. Work is in progress to master these and other factors in order to attain a good control of this intricate system. We are also looking into the relative reactivity towards oxypropylation of the various OH groups present in cork, notably from the polysaccharides, suberin and ligninlike polymers. Concurrently, the polyols obtained in this preliminary investigation are being tested as macromonomers in the synthesis of polyurethanes.
189
Acknowledgements We wish to express our gratitude to the French± Portuguese Science Cooperation Programme (ICCTIFrench Embassy) for its support. M. Evtiouguina thanks the CQIM-Inorganic Chemistry and Materials Research Centre (University of Aveiro) for the award of a research grant.
References Cordeiro, N., Belgacem, M.N., Neto, C.P, Gandini, A., 1997. Urethanes and polyurethanes from suberin. 1. Kinetic study. Ind. Crops Prod. 6, 163±167. Cordeiro, N., Belgacem, M.N., Silvestre, A.J.D., Neto, C.P., Gandini, A., 1998a. Cork suberin as a new source of chemicals. 1. Isolation and chemical characterization of its composition. Int. J. Biol. Macromol. 22, 71±88. Cordeiro, N., Belgacem, M.N., Neto, C.P., Gandini, A., 1998b. Cork suberin as a new source of chemicals. 2. Crystallinity, thermal and rheological properties. Bioresource Technol. 63, 153± 158. Cordeiro, N., Belgacem, M.N., Neto, C.P., Gandini, A., 1999a. Urethanes and polyurethanes from suberin. 2. Synthesis and characterization. Ind. Crops Prod. 10, 1±10. Cordeiro, N., Blayo, A., Belgacem, M.N., Neto, C.P., Gandini, A., 1999b. Cork suberin as an additive in oset lithographis printing inks. Ind. Crops Prod., in press. Gil, A.M., Lopes, M., Rocha, J., Neto, C.P., 1997. A 13C solid state NMR spectroscopic study of cork cell wall structure: the eect of suberin removal. Int. J. Biol. Macromol. 20, 293±305. Lopes, M., Neto, C.P., Evtuguin, D., Silvestre, A.J.D., Cordeiro, N., Gandini, A., 1998. Products of the permanganate oxidation of cork, desuberized cork, suberin and lignin from Quercus suber L. Holzforschung, 52, 146±148. Pavier, C., Gandini, A., 1999a. Oxypropylation of sugar beet pulp. 1. Optimisation of the reaction. Ind. Crops Prod., in press. Pavier, C., Gandini, A., 1999b. Oxypropylation of sugar beet pulp. 2. Separation of the grafted pulp from the propylene oxide homopolymer. Carbohydrate Polym., in press.
Short communication
The oxypropylation of cork residues: preliminary results Margarita Evtiouguina a, Ana Margarida Barros a, Jose J. Cruz-Pinto a, Carlos Pascoal Neto a, Naceur Belgacem b, Claire Pavier c, Alessandro Gandini c,* a Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal Department of Pulp and Paper Engineering, University of Beira Interior, 6200 Covilh~ a, Portugal Inst. Nat. Polytechnique de Grenoble, Ecole Francßaise de Papeterie et des Industries Graphiques, Rue de la Papeterie, BP 65, 38402 St. Martin d ÕH eres, France b
c
Received 21 October 1999; accepted 10 November 1999
Abstract Cork particles were oxypropylated under pressure at a relatively high temperature and yielded a mixture of liquid polyols composed of propylene oxide homopolymer (PPO) and cork macromolecules bearing oligo(propylene oxide) grafts. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Cork; Oxypropylation; Viscous polyols
1. Introduction Several dierent types of polymeric substrates arising from renewable resources have been submitted to oxypropylation treatments in view of modifying their structure, morphology and properties and thereby opening new possible uses as materials or additives. Cellulose and some of its derivatives, lignins, chitosan and, more recently, the polysaccharide mixture constituting sugar beet pulp (Pavier and Gandini, 1999a), are typical examples of such substrates. In the latter publication this topic is brie¯y reviewed. In all instances, the initial solid natural product was converted into a liquid polyol of varying viscosities made up of a mixture of oxypropylated products; macromolecular structures in which at least some of the original OH groups borne by the substrate were grafted with oligo(propylene oxide) chains, and propylene oxide homopolymer. To the best of our knowledge, cork has not been the object of a published study within this context. Cork is a unique natural material constituting the outer bark of Quercus suber L. Its morphology and composition dier substantially from those of most lignocellulosic counterparts, particularly because of the presence of large amounts of suberin (up to 55% by weight), a predomi-
nantly aliphatic biopolyester crosslinked to the other, more common, components of the cell wall matrix, namely polysaccharides and lignin-like structures (Cordeiro et al., 1998a; Lopes et al., 1998). Our laboratories have been involved in a common project aimed at gaining a better knowledge of cork (Gil et al., 1997; Cordeiro et al., 1998a,b) and at making good use of suberin as a macromonomer (Cordeiro et al., 1997, 1999a) or a ®ller (Cordeiro et al., 1999b). Among the reasons for this thorough investigation, lies the fact that the cork industry generates very substantial wastes, in the form of sawdusts, which are usually burnt. It seemed therefore interesting to examine the possibilities of making a better use of these side products. As an important complement to this practical motivation, there was a scienti®c driving force associated with the peculiar nature of cork, in terms of both its chemical composition and macrostructure and its physico-chemical properties. The present study remained entirely within these aims, since it tackled the chemical modi®cation of cork with the purpose of reaching a better understanding of its reactivity in heterogeneous systems, and obtaining novel products with potential applications in the realm of polymeric materials. 2. Methods
*
Corresponding author. Tel.: +33-4-7682-6947; fax: +33-4-76826933. E-mail address: [email protected] (A. Gandini).
Cork powder, kindly provided by the Champcork Company of Portugal, was ground to particles not
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 1 5 8 - 3
188
M. Evtiouguina et al. / Bioresource Technology 73 (2000) 187±189
exceeding 0.4 mm in diameter and air dried. Its residual moisture content was 7% w/w. The reactions with pure commercial propylene oxide (PO) were carried out in a 300 cm3 stainless steel vessel equipped with stirring, heating elements and both pressure and temperature sensors. The typical procedure consisted in introducing weighed amounts of cork and high-purity commercial KOH pellets into the reactor, followed by a known volume of PO. The reactor was closed and the solid/ liquid suspension (cork and KOH are insoluble in PO) was heated progressively under stirring to reach temperatures between 100°C and 145°C within about 1 h. The oxypropylation and PO homopolymerisation reactions started progressively as the temperature was raised. Then, a sudden increase in temperature, which attained values ranging between 170°C and 260°C depending on the speci®c conditions, was recorded revealing a strong acceleration of the process. The pressure increased correspondingly up to 8±15 atm. This phenomenon was attributed to the concomitant eects of sheer kinetic activation coupled with a base-catalysed degradation of the cork network which enhanced the accessibility of the OH functions. After a lapse of one to several hours (depending inversely on the chosen reaction temperature and concentrations of KOH and PO), the temperature decreased steadily back to the set value and the pressure dropped more rapidly to near-atmospheric values, indicating the near-complete consumption of PO. After allowing the system to reach room temperature, the reactor was opened and methylene chloride was added to the viscous product. The resulting mixture was neutralised with acetic acid and ®ltered in order to remove any solid residue insoluble in CH2 Cl2 . The latter was dried at 45°C and weighed to establish the percentage of unreacted or poorly oxypropylated cork. The volatile components of the ®ltered solution were removed in a rotary evaporator leaving a viscous polyol containing the potassium acetate formed in the neutralisation step. This product was then submitted to an extraction with boiling n-hexane in order to separate the insoluble oxypropylated cork from the PO homopolymer (PPO) (Pavier and Gandini, 1999b) and the two fractions were ®nally vacuum-dried and weighed. The solid residue was characterised by FTIR only, whereas the two viscous polyols were submitted to a more thorough inspection involving FTIR, 1 H- and 13 CNMR spectroscopy, DSC, GPC (in DMAc, polystyrene standards), vapour pressure osmometry (VPO), bulk viscometry and OH number. 3. Results and discussion Based on previous experience with other oxypropylation systems involving lignocellulosic substrates
(Pavier and Gandini, 1999a), the reaction parameters which were examined in this preliminary study were: the percentage of KOH in the initial ternary mixture, which was varied from 5% to 20% w/w with respect to the amount of cork; the relative proportion of PO with respect to cork, which ranged from 4:1 to 8:1 w/w; the set temperature which went, as mentioned above, from 100°C to 145°C. The following observations describe the general behaviour of this new system: · The extent of the reaction was always very high, as measured by the total weight of the recovered product compared with the total weight of the reagents. Thus, the unreacted PO was typically around 3% and at most 8% with the mildest conditions (lower temperatures and KOH concentrations). · The proportion of CH2 Cl2 -insoluble product was always very modest (between 0.5% and 5%), indicating that the reaction had involved the vast majority of the cork, since the latter was insoluble in that solvent. Moreover, a comparison of the FTIR spectra of the initial cork and of the solid residue clearly showed that even these insoluble fractions had been oxypropylated, albeit poorly, as shown by the presence of a strong CH3 peak at 2980 cmÿ1 , absent in the spectrum of cork. In other words, the vast majority of the cork was thoroughly oxypropylated (transformation into a viscous liquid soluble in CH2 Cl2 ) and the rest also reacted, but to a lesser extent. · The liquid product was made up of oxypropylated cork and PPO. The proportion of the latter tended to be higher than that of the former, ranging from 75% with the higher initial PO proportions to 40% with the lower ones. These results are in tune with those obtained in previous studies carried out with other natural polymeric materials (Pavier and Gandini, 1999a). · The molecular weights of the PPO were typically lower than 1000, whereas those of the oxypropylated cork exceeded the sensitivity of the VPO instrument (>10 000). The viscosities of the two fractions re¯ected this large dierence, with values about 2 orders of magnitude lower for the PPO. This is entirely reasonable considering the dierence in molecular size and architecture between the two products (see below). · The spectroscopic evidence gathered from a large number of products indicated unambiguously the existence of two well-de®ned components in the liquid product; a lower-viscosity hexane-soluble fraction with FTIR, 1 H- and 13 C-NMR spectra virtually identical to those of a commercial sample of PPO with Mn 1000, and a high viscosity hexane-insoluble fraction characterised by spectroscopic features showing the presence of both PO oligomeric chains and of cork constituents, aliphatic chains from suberin and polysaccharide structures.
M. Evtiouguina et al. / Bioresource Technology 73 (2000) 187±189
It can be concluded that PO reacts readily with solid cork particles in the presence of a strong base. This reaction must proceed through at least three pathways: the homopolymerisation of PO induced both by direct initiation (probably from OHÿ generated by KOH plus traces of moisture) and by transfer reactions occurring during the propagation reactions related to this monomer (homopolymerisation and formation of the grafts); oxypropylation from activated OH functions borne by the cork macromolecular moieties, and cork degradation induced by the strongly basic medium. Evidence proving the occurrence of the third reaction came from the fact that the ®nal oxypropylated product was a thermoplastic material (highly viscous liquid) and could not therefore have maintained the cross-linked structure of its natural precursor. The presence of multiple ester groups in the structure of cork, and more particularly within the suberin architecture, leaves little doubt as to the origin of this degradation which relates to their basecatalysed cleavage leading to the breakdown of the cork macromolecular network. The system under study remains, however, hard to control, as shown by the diculty in obtaining a good reproducibility in the results related, for example, to the kinetics of the reaction and to the viscosity of the products. This is attributed to the complexity of the heterogeneous reaction, the diculty in duplicating the temperature pro®le of the process and the possible variation in residual moisture from run to run. Work is in progress to master these and other factors in order to attain a good control of this intricate system. We are also looking into the relative reactivity towards oxypropylation of the various OH groups present in cork, notably from the polysaccharides, suberin and ligninlike polymers. Concurrently, the polyols obtained in this preliminary investigation are being tested as macromonomers in the synthesis of polyurethanes.
189
Acknowledgements We wish to express our gratitude to the French± Portuguese Science Cooperation Programme (ICCTIFrench Embassy) for its support. M. Evtiouguina thanks the CQIM-Inorganic Chemistry and Materials Research Centre (University of Aveiro) for the award of a research grant.
References Cordeiro, N., Belgacem, M.N., Neto, C.P, Gandini, A., 1997. Urethanes and polyurethanes from suberin. 1. Kinetic study. Ind. Crops Prod. 6, 163±167. Cordeiro, N., Belgacem, M.N., Silvestre, A.J.D., Neto, C.P., Gandini, A., 1998a. Cork suberin as a new source of chemicals. 1. Isolation and chemical characterization of its composition. Int. J. Biol. Macromol. 22, 71±88. Cordeiro, N., Belgacem, M.N., Neto, C.P., Gandini, A., 1998b. Cork suberin as a new source of chemicals. 2. Crystallinity, thermal and rheological properties. Bioresource Technol. 63, 153± 158. Cordeiro, N., Belgacem, M.N., Neto, C.P., Gandini, A., 1999a. Urethanes and polyurethanes from suberin. 2. Synthesis and characterization. Ind. Crops Prod. 10, 1±10. Cordeiro, N., Blayo, A., Belgacem, M.N., Neto, C.P., Gandini, A., 1999b. Cork suberin as an additive in oset lithographis printing inks. Ind. Crops Prod., in press. Gil, A.M., Lopes, M., Rocha, J., Neto, C.P., 1997. A 13C solid state NMR spectroscopic study of cork cell wall structure: the eect of suberin removal. Int. J. Biol. Macromol. 20, 293±305. Lopes, M., Neto, C.P., Evtuguin, D., Silvestre, A.J.D., Cordeiro, N., Gandini, A., 1998. Products of the permanganate oxidation of cork, desuberized cork, suberin and lignin from Quercus suber L. Holzforschung, 52, 146±148. Pavier, C., Gandini, A., 1999a. Oxypropylation of sugar beet pulp. 1. Optimisation of the reaction. Ind. Crops Prod., in press. Pavier, C., Gandini, A., 1999b. Oxypropylation of sugar beet pulp. 2. Separation of the grafted pulp from the propylene oxide homopolymer. Carbohydrate Polym., in press.