Biomacromolecules 2005, 6, 1895-1905
1895
Lignin Esters for Use in Unsaturated Thermosets: Lignin Modification and Solubility Modeling Wim Thielemans† and Richard P. Wool* Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, Delaware 19716 Received January 17, 2005; Revised Manuscript Received May 8, 2005
Kraft lignins from hardwood and softwood were esterified with several anhydrides to alter their solubility behavior in nonpolar solvents, such as styrene-containing thermoset resins. The esterification reaction was facile, it reduced the amount of waste products, and can be readily scaled up. Increasing the carbon chain length on the ester group improved the solubility of kraft lignin in nonpolar solvents, with butyrated lignin being completely soluble in styrene. Esterification with unsaturated groups such as methacrylic anhydride, improved the solubility to a lesser extent than the saturated analogues. The solubility behavior of the modified lignin was described using the Flory-Huggins solubility theory, combined with the predictive method of Hoy. The main goal to obtain a styrene soluble kraft lignin that could be used in unsaturated polyesters and vinyl esters was achieved with fully butyrated kraft lignin and a butyrated/methacrylated kraft lignin. The solubility of the latter is governed by the butyrate/methacrylate ratio. The reaction rate constants for the butyration and methacrylation reactions were also determined and the aromatic hydroxyl groups were found to be consistently three times more reactive than the aliphatic ones. Introduction Lignin is the second most abundant, naturally occurring macromolecule amidst cellulose and natural oils. It is found as a cell wall component in all vascular plants and in the woody stems of arborescent angiosperms (hardwoods) and gymnosperms (softwoods).1 The lignin content in woody stems varies between 15 and 40% where it acts as a water sealant in the stems and plays an important part in controlling water transport through the cell wall. It also protects plants against biological attack by hampering enzyme penetration. Lignin is also a permanent glue, bonding cells together in the woody stems and thus giving the stems their well-known rigidity and impact resistance. Lignin used in this work was obtained using the kraft pulping process. The use of kraft lignin as a copolymer or polymer additive has received a considerable amount of attention.2-4 The most straightforward application is the use of lignin as a filler material in thermoplastic5-9 and thermosetting10,11 polymers and rubbers12 with limited positive to negative effects on mechanical properties. Co-reaction of lignin with phenol-formaldehyde resins,13,14 epoxy-resins,15-17 polyurethane precursors,18-27 and polyester precursors28,29 has proven to be more successful. Mechanical property improvement or no property deterioration was obtained up to a certain lignin load. Chemical modification of lignin is another area of significant scientific work. It is largely based on the * Corresponding author E-mail:
[email protected]. † Current address: Ecole Franc ¸ aise de la Papeterie et des Industries Graphiques (EFPG-INPG), 461 Rue de la Papeterie, BP 65, 38402 SaintMartin d’He`res, France.
knowledge of lignin modifications used to dissolve lignin in organic solvents and applied in the determination of lignin functional groups. Dissolution is needed for various characterization techniques, and different chemical modifications are therefore already well-established.30,31 Chemical modification of lignin can thus be used to improve polymerlignin compatibility and to introduce reactive sites. The available hydroxyl groups on the lignin molecule are reactive, plentiful, and local centers of high-polarity capable of hydrogen bonding.32 The modification of these reactive nuclei results in an effective alteration of the lignin solubility behavior.30,33 Lignin has been reacted with carboxylic acids, carboxylic acid halides, or anhydrides33 among others. Various acetylation procedures have been described in the literature.34-38 Propionation and esterification with larger chain derivatives have also been successful but have so far received relatively little attention.35,39,40 The general modification method using pyridine is both solvent and timeconsuming.34,38 The purpose of this work is 2-fold: to describe a simple and fast esterification pathway for lignin modification and to obtain an esterified kraft lignin soluble in styrene. Since styrene is the most commonly used reactive diluent in unsaturated thermosetting polymers, in which it is the most nonpolar component, styrene solubility will allow these lignins to be used as additive to this polymer class. Lignin is expected to improve the properties of bio- and petroleumbased composites by (a) acting as a toughening agent, (b) improving the connectivity in the network, (c) adding additional stiffening groups to the matrix, (d) acting as a sizing agent between natural fibers and the resin matrix, (e) behaving as a comonomer for the resin, (f) inducing plasticity
10.1021/bm0500345 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/07/2005
1896
Biomacromolecules, Vol. 6, No. 4, 2005
in the deformation zone at crack tips to improve toughness, (g) acting as a free radical trap to reduce radical scission effects during fracture in highly cross linked polymers, (h) improving flame resistance, (i) modifying biodegradability, (j) providing enhanced photoresistance and thermal stability, (k) expanding fatigue lifetime, and (l) contributing to the green engineering of materials.41-43 A following publication will describe the effect of the modified lignin on the polymerized unsaturated thermosetting resins.44 The reaction method described here is based on the earlier reported method using a 50:50 anhydride:acid mixture and a reaction time of 48 hours.38 The more effective catalyst 1-methyl imidazole is used in a pure anhydride mixture resulting in reaction times on the order of minutes and hours. Experimental Techniques Lignin functionalities and extent of reaction were determined by FTIR and 1H NMR spectroscopy. 13C NMR spectroscopy and Gel Permeation Chromatography (GPC) were performed on fully modified KL samples as extra confirmation of complete esterification. FTIR spectroscopy was performed on a Genesis Series FTIR spectrometer from ATI Mattson with a deuterated triglycine sulfate (DTGS) detector and a potassium bromide (KBr) beam splitter. A total of 128 cumulative scans were taken with a resolution of 2 cm-1 in transmission mode. A small amount of KL dispersed or dissolved in acetone was deposited on a NaCl disk (International Crystal Labs, NJ) and placed under vacuum to ensure all solvent evaporation. 1 H NMR spectra were accumulated on a Bruker AC250 spectrometer operating at 250 MHz. The amount of scans varied between 24 and 1000, with pulse delays varying between 10 and 12 s depending on the studied lignin. The pulse width was 90°, with a spectral width of 2500 Hz and a resolution of 0.427 Hz/pt. Approximately 100 mg of KL was dissolved in 1 mL of DMSO-d6 (99.9% deuterated, 0.05% tetramethylsilane, Cambridge Isotope Labs, Inc.) and left overnight over molecular sieves to reduce water contamination and to obtain complete dissolution. Chemical shift assignments are taken from the literature.45-48 Aromatic protons (8-6.2 ppm) were used as a reference signal. To maleated lignin samples was added dichloromethane as internal reference due to interference of maleate and fumarate protons with the aromatic protons. Total esterification was determined from the ester protons. Individual aromatic and aliphatic esters could not be determined due to insufficient signal separation, even when increasing the amount of scans. Li and Lundquist found that, even for high amount of scans, well separated lignin propionate signals only allow for approximate determination of the amounts of aliphatic and aromatic propionates due to upfield shifts caused by biphenyl structures.39 Aliphatic ester substitution was thus calculated from the total conversion and aromatic hydroxyl conversion. Important interference occurs between aromatic alcohol (ArOH) and aldehyde (CHO) proton signals in the 8-11 ppm range. After the initial spectrum of KL in DMSO-d6, addition of 15-20 vol % deuterated water (D2O) ensured the exchange of a deuterium nucleus from D2O with the ArOH
Thielemans and Wool Table 1. Published Kraft Lignin Data32,55,56 pine KL
hardwood KL
analytical composition C9H7.9O2.1S0.1(OCH3)0.82 C9H7.2O1.8S0.1(OCH3)1.15 PPU molecular weight 178 g/mol 183 g/mol Mn 1600 g/mol 1050 g/mol32 1260 g/mol56 Mw 2700 g/mol55 2400 g/mol56 3500 g/mol32 2900 g/mol32 polydispersity 2.2 1.956 - 2.832
and -COOH KL protons.49 Aldehydic protons do not substitute for deuterium. Subtraction of the signal with D2O from the plain DMSO-d6 signal cancels out the aldehydic proton contribution. This method has been proven to be effective in lignin ArOH and -COOH determination.45 13C NMR experiments were performed in CDCl and 3 DMSO-d6, depending on the lignin solubility. Approximately 300 mg of lignin was dissolved in 1 mL of these solvents containing 0.05% TMS. The spectral width was 8000 Hz. All lignin spectra were obtained on a Bruker AC 250 MHz spectrometer operating at 62.9 MHz with 100 000 scans with a delay of 1 s. Derivatized KL is dissolved in THF with 0.1 M LiCl for GPC (100 mg of KL per 3 mL of THF/LiCl). LiCl was used to guarantee the absence of associative structures.50 Association of lignin molecules, even modified, has been found to significantly increase the measured Mw, and polydispersity,51 and can result in solvent-dependent molecular weights.52 No change in Mw was measured for modified lignins dissolved in THF/LiCl for different time periods as has been shown to occur for acetylated KL in pure THF.51 The GPC system consisted of a setup by Waters Corporation: a separation module (Waters 2695), HPLC pump (Waters 515), a refractive index detector (Waters 2414) and 3 Styragel columns in series. The three columns (Waters HR1, HR4, and HR6) with 5 µm Styragel packing allow for the determination of molecular weights ranging from 102 to 107 g/mol. The column was operated at a flow rate of 0.06 mL/min and was calibrated with polystyrene standards (Pressure Chemical, Pittsburgh, PA) over the range of studied lignin molecular weights. Materials Two KLs were used: a softwood, pine KL (Indulin AT), and an experimental hardwood KL. Both were obtained from Westvaco (Charleston, SC). Published data on these kraft lignins are combined in Table 1. For pine KL, some significant scatter does exist on the weight average molecular weight and polydispersity. Polydispersities between 2 and 12 have been reported. This scatter is believed to be due to molecular aggregation and differences in GPC detectors53 more than large changes in KL production. The higher values tend to be found with lignin esters in THF while unmodified KL in high pH aqueous NaOH, with lower aggregation54 results in lower polydispersities. Values reported here represent the lowest reported range. Data on hardwood lignin varied more as it is generally obtained from a blend of varying wood species. However, a similar lignin was tested by Kubo and Kadla56 and is relatively close to older values from Marton.32 Functional
Biomacromolecules, Vol. 6, No. 4, 2005 1897
Lignin Modification and Solubility Modeling Table 2. Functional Groups Present in KL Used in This Worka
aromatic hydroxyl (ArOH) aliphatic hydroxyl (AlOH) carboxylic acid (-COOH) methoxyl (-OCH3) aromatic hydrogen (ArH) aldehyde (-CHO)
softwood KL
hardwood KL
0.64-0.643 (0.64) 0.439 (0.44) 0.11 (0.11) 0.76-0.786 2.5 (2.5)
