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Butyrated kraft lignin as compatibilizing agent for natural fiber reinforced thermoset composites
Composites: Part A 35 (2004) 327–338 www.elsevier.com/locate/compositesa
Butyrated kraft lignin as compatibilizing agent for natural fiber reinforced thermoset composites Wim Thielemans, Richard P. Wool* ACRES Group, Colburn Lab, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, USA
Abstract Butyrated kraft lignin was added to an unsaturated thermosetting resin, consisting of a mixture of acrylated epoxidized soybean oil and styrene. Composites were made by the Vacuum Assisted Transfer Molding process with varying amounts of butyrated kraft lignin dissolved in the unsaturated resin system. Butyrated kraft lignin improved the interface between the resin and reinforcing flax fibers. SEM images illustrate a clear improvement in the adhesion of the resin to the fibers by showing the fibers fracturing together with the resin, without fiber pullout. Increases in viscosity upon solubilization of butyrated kraft lignin in the unsaturated resin of up to 150% for 10 wt% butyrated lignin however, resulted in the appearance of dry patches in composites made with dense fiber reinforcement mats. These dry spots decrease the mechanical properties of the overall composite. On the other hand, strength improvement is seen for composites made with short wheat straw fibers, which allow for better resin penetration and fiber wetting. The flexural strength increased by 40% for a 5 wt% butyrated lignin addition. Work currently in progress to reduce the negative effects of the viscosity increase by changing the processing conditions, is also described. q 2003 Published by Elsevier Ltd. Keywords: A. Thermosetting resin; B. Fibre/matrix bond; B. Interface/interphase; A. Fibres
1. Introduction Polymer composites are used in a wide variety of applications, varying from aerospace, automotive, and military applications to uses in sports equipment and civil infrastructure. They are made by embedding strong fibers in a polymer matrix. A variety of reinforcements are in use, such as aramid, glass, carbon, and natural fibers [1]. Several works describe the various possible fiber reinforcements and their mechanical properties [2,3]. Natural fibers are currently getting a lot of attention for replacing synthetic fibers [4]. They have the advantage of being abundantly available, renewable, cheaper, since they are waste products, and exhibit good mechanical properties. With densities comparable to aramid fibers (, 1500 kg m23), they can display specific strength and moduli higher than glass fibers. The composite matrix can be formed of either thermoplastic or thermosetting material. Thermoplastic polymers, such as polyethylene and polypropylene, melt upon heating. Fibers are mixed in with the molten polymer, and the * Corresponding author. Tel.: þ 1-302-831-3312; fax: þ1-302-831-8525. E-mail address: [email protected] (R.P. Wool). 1359-835X/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2003.09.011
composite is then shaped into the desired form at high temperature. Thermosetting polymer composites are formed by injecting monomers into a mold with the desired, final shape. The mold can either contain the fibers in a preform or short fibers can be injected together with the monomers into the mold. The mold is then heated and the monomers polymerize into a cross-linked network entrapping the fibers. The composite part is unable to be reshaped after polymerization. In order to obtain a largely natural-based composite, a thermosetting soybean-oil based resin was used. The soybean oil based resin is obtained by first epoxidizing, then acrylating the free-radically stable double bonds on the triglyceride molecules in these oils. The acrylate double bonds can then be polymerized together with styrene, which is used as a reactive diluent to decrease the resin viscosity. The trygliceride modification reactions and the resulting resin have been well studied and characterized [5,6,7]. The chemical structure of the resin components is given in Fig. 1. Strength improvement in composites depends on stress transfer between the polymer matrix, on which the external force is applied, and the reinforcing fibers. If the interface is weak, failure will occur at the fiber-matrix interface and no
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Fig. 1. Chemical structures of the resin constituents and lignin: (A) styrene, (B) Acrylated epoxidized soybean oil, (C) Butyrated hardwood lignin, based on a simplified structure of Hardwood Beech lignin [16]. Me denotes a CH3 group, But a butyrate group where it could not be completely drawn.
improvement will be seen. While glass fibers are treated with a compatibilizer as it is spun [2], fiber treatment is not as convenient for natural fibers. A recent overview of natural fiber treatments is given by Mohanty et al. [8]. However, any surface treatment would also increase the cost of natural fibers significantly, reducing one of the incentives to use natural fibers in composites with each required extra processing step. Natural wood and plant fibers are constituted of cellulose fibers, consisting of helically wound cellulose microfibrils, bound together by an amorphous lignin matrix. Lignin acts as a sealant to keep water in the fibers, as a protection against
biological attack, and as a stiffener to give the stem its resistance against wind and gravity forces. Hemicellulose is also found in these natural fibers and is generally believed to be a compatibilizer between cellulose and lignin [9]. This make-up of wood suggests the potential of lignin to act as a compatibilizer between hydrophilic natural fiber reinforcement and a hydrophobic matrix polymer. A first approach in which kraft lignin was deposited onto natural fibers has been described elsewhere [10,11]. The strength and modulus of the composite was found to increase when lignin was deposited from an aqueous sodium hydroxide solution onto the natural fibers [10]. The lignin
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layer increased the rigidity and strength of the fiber, resulting in an increase in composite modulus. It also shielded the hydrophilic fibers from the hydrophobic resin, and increased the composite strength, pointing towards a stronger fiber-resin interface. The deposited lignin layer also resulted in a smoother outer fiber surface, compared to the original rough surface of the cellulosic fibers [10]. This could be portrayed as a surface defect treatment. Addition of too much lignin onto the fiber however, resulted in a thicker lignin layer, and a decrease of mechanical properties of the overall composite [10]. A second approach, pursued in this work, is the inclusion of lignin in the resin. The kraft lignin used in this work contains large amounts of hydroxyl groups, both aliphatic and aromatic, as shown in Fig. 2. The aromatic hydroxyl groups have the capability of inhibiting free radical polymerization by forming stable quinonic structures [12,13]. The high polarity of the hydroxyl groups leads to a molecule insoluble in rather non-polar thermosetting resins, while the formation of stable quinonic structures poses problems during free radical polymerization. By reacting these alcohol groups with butyric anhydride, the inhibiting characteristics subside and the molecule becomes soluble in styrene, the reactive diluent in the resin used here, and in a vast amount of free radically polymerizing thermosets [14]. Fig. 1 gives the final lignin structure after butyration. Butyrated lignin (Lignin-BA), dissolved in a monomer mixture, may decrease the negative thermodynamic
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interactions between hydrophilic natural fibers and the hydrophobic polymer matrix, due to its inherent natural compatibility with the fiber elements and its new chemically induced solubility in the polymer matrix. Application of a resin soluble compatibilizer would render natural fiber treatment unnecessary, reducing processing cost. The stronger resin-natural fiber interface will improve the overall composite properties and better use the potential that natural fiber reinforcements promise. In this paper, the effect of different lignin concentrations on the mechanical properties of composites made up of flax and wheat straw fibers, and acrylated epoxidized soybean oil was investigated. The ultimate goal is a lignin-based, resin-soluble additive that gives rise to a very strong resin-natural fiber interface.
2. Experimental 2.1. Materials The hardwood kraft lignin was obtained from WestVaco, and is sold under the trade name PC-1369. Hardwood kraft lignin contains about 1 hydroxyl group per Phenyl Propane Unit (PPU: C3 – C6 unit with molecular weight of 185 g/mol for kraft lignin) with almost 25% aliphatic hydroxyl groups, and 75% aromatic hydroxyl groups. The total hydroxyl group content was taken from
Fig. 2. Proposed chemical structure of hardwood lignin [16].
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McCarthy and Islam [15] and equaled the total butyrate amount after total butyration, determined by 1H-NMR spectroscopy. Aromatic hydroxyl group content was also taken from the same reference [15] and confirmed by 1HNMR spectroscopy of the unmodified lignin. A proposed structure is given in Fig. 2 [16]. Butyric anhydride (BA, 98% pure) and methacrylic anhydride (MA, 94% pure) were both obtained from Sigma-Aldrich. The catalyst for the lignin-anhydride reaction was 1-Methyl Imidazole (1MI), Sigma Aldrich, 99% pure. Cyclohexane (99% pure) and ethyl ether (anhydrous) for the purification of reacted lignin were obtained from Fisher Scientific. The polymerization catalyst used in this study was Trigonox 239A from Akzo Nobel (45% Cumyl Hydroperoxide solution in carboxylic ester [17]), with a commonly used activator Cobalt Naphtenate [18] (6% solution in mineral spirits, from Mahogany Company) to allow for room temperature curing (Fig. 3). Cobalt naphtenate is a Cobalt Salt of Naphtenic Acid [19]. The initiator used for high temperature cure was tert-Butyl PeroxyBenzoate, from Sigma Aldrich (98% pure). The resin used was an Acrylated Expoxidized Soybean Oil (AESO), from UCB Chemical, sold under the trade name Ebecryl 860. This was diluted with 99% pure styrene from Sigma-Aldrich. Flax fiber mats were obtained from Cargill Limited, USA. The continuous flax fiber mats are 1/400 thick, with a weight of 680 g/m2 and contain 85% flax and 15% glass fiber binder. Short wheat straw fibers were obtained by milling seedless long fibers at 1800 rpm. They were subsequently passed through a 1/200 screen to separate out the desired, broken-down short fibers from the original long ones. The average length of the short straw fibers was 1.2 cm. All fibers were stored at ambient conditions with relative humidity of 73% and used without prior drying or surface treatment. Deuterated dimethyl sulfoxide (DMSO-d6) and deuterated water (D2O) for 1H-NMR studies were obtained from Cambridge Isotope Labs and were 99.9% pure. Spectroscopic grade acetone for dissolution of lignin-BA before FTIR analysis was purchased from Sigma-Aldrich.
2.2. Lignin modification (Butyration) Kraft lignin, and butyric anhydride were added in a 1:2 weight ratio to an Erlenmeyer flask. Per 40 g of lignin, 1 g of the catalyst 1-MI was added. The reaction mixture was heated up to 50 8C and reacted for several hours while stirring vigorously. Initially, the kraft lignin does not dissolve in butyric anhydride. However, upon conversion of the lignin hydroxyl groups to butyrates, lignin becomes soluble in the reaction mixture. This is an easy way to monitor the progress of the reaction. The reaction was found to be essentially completed after about 20 min but the reaction mixture was left for several hours to ensure full conversion of hydroxyl groups [14]. The reaction mechanism, based on the 1-MI catalyzed reaction of anhydrides with hydroxyl groups is given in Fig. 4 [20]. Ethyl ether was then added to the reaction mixture (1:1 volume ratio) to reduce the viscosity and allow for a clear phase separation when water is added. The ethyl ether/lignin/BA/1MI was then washed with deionized water to extract the 1MI out of the reaction mixture. Three water washes with 1:1 volume ratio were sufficient. Adequacy of the water washes was determined by the disappearance of the 1MI peaks in the 1 H-NMR spectra of the dried butyrated lignin. The 1MI signals were determined by taking a 1H-NMR spectrum of 1MI in DMSO-d6 (Fig. 5). Lignin was then sedimented out of the ethyl ether/BA/lignin mixture with cyclohexane. The lignin recovery depends on the amount of cyclohexane used. Recoveries of up to 80% could easily be obtained. The sedimented lignin was then filtered out and dried under vacuum for 24 h. Residual amounts of 1MI in the reaction mixture after the water washes, effect in a slimy lignin-BA/1MI sediment, rather than a granular sediment obtained in the absence of 1MI, due to the insolubility of 1MI in cyclohexane. Recovery of butyrated lignin from this slimy sediment is troublesome. One has thus the possibility to probe the efficiency of the water washes on a small sample before adding cyclohexane to the whole reaction mixture. 2.3. Characterization of butyrated lignin The butyration reaction was followed by Fourier Transform Infrared Spectroscopy (FTIR). Dried lignin-BA was
Fig. 3. Chemical structures of cumyl hydroperoxide [17] (A) and cobalt– naphtenate [18], based on its literature definition [19] (B).
Fig. 4. Reaction mechanism of butyration of the lignin hydroxyl groups, based on the similar 1MI catalyzed reaction mechanism of acetic anhydride with hydroxyl groups. Butyric anhydride reacts first with 1MI to form an Nbutyryl-N0 -methylimidazolium ion, which then further reacts with the lignin hydroxyl groups [20].
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Fig. 5. 1H-NMR spectrum of 1MI in DMSO-d6. Signals can be seen at d ¼ 3.6, 6.9, 7.1 and 7.6 ppm. The small signal at 2.5 ppm is due to the solvent DMSO-d6.
dissolved in acetone. A thin film of the acetone/lignin-BA mixture was applied to a NaCl disk, after which the disk was left to dry under vacuum for several minutes to evaporate off all acetone. The FTIR spectra were then recorded with a Genesis Series FTIR spectrometer from ATI Mattson (Madison, WI). The spectrometer uses a Deuterated Triglycine Sulfate (DTGS) detector and Potassium Bromide (KBr) beam splitter. The resolution was 4 cm21, and 128 scans were taken per spectrum. Interpretation of the spectrum was done with the software program WinFirst. Complete butyration was confirmed by 1H-NMR on a 250 MHz Bruker AC250 NMR spectrometer with 0.01 g lignin-BA dissolved in 0.1 g DMSO-d6. The magnetic field strength was 250.13 MHz, with a resolution of 0.427 Hz/pt and a spectral window of ^ 2500 Hz. Twenty-four spectra with a relaxation delay of 10 s and pulse width of 908 were taken at 25 8C. Confirmation of complete reaction of alcohol groups was checked by integration of the butyrate peaks (CH3: , 0.9 ppm, CH2: , 1.5 and 2.2 ppm) [23 – 26], and the broad aromatic alcohol peak (8.5 – 11 ppm range) [27]. Due to the low amount of scans, distinguishing between the signals of aliphatic or aromatic butyrate signals is not possible. For our purpose here, only confirmation of conversion of all hydroxyl groups into butyrate groups was necessary. The low number of scans gave sufficient accuracy while keeping the analysis time low. However, for detailed analysis, differentiation between aromatic and aliphatic esters can be done by increasing the amount of scans to values of 1000 or more [14,25]. Twenty percent by volume of D2O was then added to the DMSO-d6/Lignin-BA mixture to replace any possible hydrogens remaining on the aromatic alcohol groups. Subtractions of the aromatic hydroxyl group signal
with D2O from the signal obtained without D2O, has to be zero if all aromatic alcohol groups are reacted. Hydrogendeuterium substitution is only seen for functional groups with active hydrogen such as acid and aromatic hydroxyl groups, but not for aliphatic hydroxyl groups [28]. 2.4. Monomer mixture Butyrated lignin (lignin-BA) in different quantities was dissolved in styrene. Pure styrene has a much lower viscosity than the AESO/styrene resin, which helps to speed up the solubilization of lignin-BA. Lignin-BA/styrene was then mixed with AESO to obtain 1.0, 2.5, 5.0 and 10 wt% ligninBA in the AESO/styrene/lignin-BA resin. The AESO/styrene weight ratio was 7/3. Three wt% of the total resin weight of Trigonox 239A and 0.8 wt% Cobalt-Naphtenate were mixed into the monomer-lignin mixture for room temperature cure. For high temperature cure, 1.5 wt% tert-Butyl PeroxyBenzoate was added to the AESO/styrene/lignin-BA mixture. 2.5. Viscosity measurements of monomer mixtures The viscosity of the monomer mixtures was measured with an Anton Paar Automated Microviscometer (AMVn), based on the rolling ball principle. The capillary used has an internal diameter of 4 mm, the ball inside a diameter of 3 mm. This setup is capable of measuring viscosities in the range of 80– 800 mPa.s. The viscosity was measured at three different angles of the capillary, resulting in three different shear rates. The angles were 50, 60 and 708 from the horizontal. Experiments at each angle were repeated a minimum of four times.
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2.6. Composite fabrication Polymer composites were obtained by a VARTM (Vacuum Assisted Resin Transfer Molding) process [21,22]: Three sheets of Cargill Durafiber flax (approx. 1 £ 1 ft) were layed up on a table, and covered with a plexiglass sheet to give a top flat surface of the finished part. The table was pretreated with the release agent Frekote (Loctite, Rocky Hill, CT) to allow easy removal of the composite part after polymerization. This lay-up was covered by a plastic sheet and closed off airtight. A vacuum was then applied on one side while the other side was connected to a container with the resin mixture. After the resin was completely infused through the fiber mats by the vacuum, and no air was present, both ends were closed off airtight and the resin was left overnight at room temperature to cure. The composite was not post-cured. The composite was then cut into parts with the desired dimensions to be tested. The fiber content of the samples was approximately 30 wt%. A second batch of composites was made with short wheat straw fibers. These short fibers were put in a silicone mold after which the resin was poured in. No pressure drop to enhance resin infusion was applied. The resin constituted of the earlier described mixtures of AESO, styrene and lignin-BA and the high temperature initiator tert-Butyl PeroxyBenzoate, with lignin-BA concentrations of 0, 1 and 5 wt%. The composite mixture was cured for 2 h at 110 8C, followed by a post-cure at 160 8C for an extra 2 h. The temperature was always ramped at a rate of 5 8C/min in between the different set point temperatures. After curing, the composites were cut and polished to the desired dimensions for testing. The fiber content was around 8 wt%. There was still a slight increase in air bubbles as lignin-BA amount was increased but not as severe as with the flax composites. The existence of air bubbles can be attributed to the absence of a pressure driving force during resin infusion. 2.7. Characterization of composites The flexural modulus and strength were measured according to ASTM D790 [29] in 3 point bend mode. Five samples of sizes 63.7 £ 2.51 £ 3.2 mm3 were tested for each lignin content. The crosshead speed on the Instron (Model 4201) was 1.3 mm/min with a span of 50.8 cm. Tensile modulus and strength were measured according to ASTM D3093/D3039M [30]. Five samples, without tabs, with measurements 250 £ 2.5 £ 4 mm2 were tested for each lignin content. The crosshead speed was 1.3 mm/min. Only data from samples that did not break at the grips were used. The straw composites were not tested in tensile mode due to inconsistencies of fiber orientation and dispersion between samples of different lignin content in the tensile sample mold. This could be the result of different resin viscosities. The critical fiber length dependence on the lignin-BA content of the resin could not be determined. The fragmentation test is considered to be the most reliable
technique to obtain the critical fiber length and interfacial shear strength [31,32]. However, to determine this critical fiber length and the interfacial shear strength, it is required to measure the broken fragment lengths after testing [31– 35]. Addition of lignin-BA colors the resin dark brown, making it impossible to accurately measure the broken fragment lengths, and thus, determine the critical fiber length. Scanning Electron Microscopy (SEM) micrographs were taken of the fracture surface of the flexural test samples. The fracture surface was gold coated by sputtering for 75 s using a current of 25 mV. The SEM was a JEOL JXA-840 Scanning Microscope. The probe voltage was 6 £ 10210 Amps, with an acceleration voltage of 3 kV. The electron source consisted of a Tungsten wire.
3. Results and discussion 3.1. Butyration FTIR can give a quick and qualitative indication of the extent of lignin butyration. Fig. 6 gives an example of the spectrum recorded for lignin-BA, with the important bands assigned. Upon butyration, the alcohol bands (3300 – 3700, 1097, and 1035 cm21) reduce in size while the CyO bands (1740 and 1600 cm21) and the methyl and methylene C –H bands (2966, 2940, 2880, and 1464 cm21) increase. The C – O band at 1097 and 1035 cm21 are assigned to secondary and primary alcohols respectively. They do not disappear because they fall together with the C – O stretch from aliphatic ethers and the aromatic guaiacyl type C –H stretch respectively, both of which are unaffected by the butyration reaction. Residual acetone may also explain some residual peak at 3300 –3700 cm21 [36]. Band assignments were made based on previously published literature values [37,38]. The 1H-NMR spectrum of fully butyrated lignin with signal assignments is given in Fig. 7. It was found that the butyrate groups matched the amount of original hydroxyl groups (both aromatic and aliphatic). No increase in acid peak (11 –14 ppm) was seen, so no residual butyric acid was left in the dried lignin-BA sample. The amount of aromatic hydroxyl groups was found to be zero for all samples after butyration. We can therefore conclude that all the alcohol groups were converted into butyrate groups. 3.2. Resin viscosities The viscosities of the monomer mixtures as a function of lignin-BA content are depicted in Fig. 8. The three points for each lignin-BA load represents measurements at the three angles of the capillary. The recorded viscosities do not vary significantly for the different angles, and thus shear rates, except at a 10 wt% lignin load. The close values for 60 and 708 angles, however,
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Fig. 6. FTIR spectra of fully butyrated hardwood lignin with important band assigments. Lignin-BA was deposited on an NaCl from an acetone solution.
suggest that the 508 angle measurement is inaccurate. The resin viscosity is found to increase significantly upon lignin-BA addition: an increase from 165 mPa.s without any lignin-BA to 400 mPa.s with addition of 10 wt% lignin-BA. The viscosity thus increases 10% with a 1 wt% lignin-BA mixture, and 150% for 10 wt% ligninBA. This large viscosity rise will affect negatively the ability of the resin to flow through the fiber mats during resin infusion into the mold. It may result in an increase in the amount of air bubbles in the polymerized composite part.
3.3. Mechanical properties The results from tensile tests of the butyrated hardwood lignin in the AESO/styrene/lignin-BA/flax composite are shown in Fig. 9. As the lignin-BA amount in the resin increases, both the modulus and strength decrease. Lignin-BA seems to have a negative effect on the tensile properties. The same effect is seen for the flax composites in flexural mode. The negative lignin-BA effect can occur for several reasons: (a) Lignin-BA could have lost its affinity for cellulosic fibers upon butyration,
Fig. 7. 1H-NMR spectrum of butyrated hardwood lignin with peak assignments. Ar represents aromatic ring, Al aliphatic chain. The signal for aliphatic alcohols coincides with the much bigger methoxyl (– OCH3) hydrogen signal. Therefore, aliphatic alcohols cannot be determined accurately.
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Fig. 8. Effect of lignin-BA content on the resin viscosity as measured by an Anton Paar AMVn viscometer. The three different points at each lignin load represent measurements at different angles of the capillary to obtain different flow rates.
(b) it can decrease the crosslink density of the matrix material since it is a plasticizer and thus lower the matrix integrity or (c) it can decrease the wetting of the fiber by increasing the resin viscosity. It has been shown that lignin-BA addition to the plain resin increased the flexural modulus and strength of the AESO/styrene system with a maximum of 75 and 60%, respectively upon addition of 5 wt% lignin-BA [39]. The increase of resin viscosity upon lignin-BA addition was measured and found to increase significantly. A rise in air bubbles entrapped in the polymerized composite was seen as the lignin-BA content in the resin was raised. Significant dry spots in the fiber mats were also seen as the lignin content increased to 10 wt%. The viscosity increase is therefore believed to be the reason for the declining mechanical properties of the composite part with increasing ligninBA addition. The straw composites allowed for better fiber wetting due to the lower density of the original fiber lay-up, even without any applied pressure drop. The fiber content of the composites is just 8 wt%. This results in significant increase in flexural strength when 5 wt% lignin-BA is added (Fig. 10). Flexural strength increased with 40% to 29 MPa with only a 5 wt% lignin-BA addition. The flexural strength for the resin with 5 wt% lignin-BA approaches the values for the flexural strength of wheat straw of about 30 MPa [40]. The resin thus comes close to the flexural strength of wheat straw, indicating a good resinfiber interface.
3.4. Microscopy SEM was used to examine the resin-fiber interface. Micrographs were taken of the fracture surface of flexural samples. Comparison of Figs. 11(A) and 12(A) show very little improvement upon lignin-BA addition. It even looks like the amount of fiber pullout increased as 10 wt% ligninBA was added, because of the small amount of resin one can see at the surface. It is however, important to look closer at the fracture surface. Fig. 11(B) shows a SEM micrograph of the fracture surface of an AESO/styrene/flax composite without any lignin-BA added. It can clearly be seen that fiber pullout occurred. Clear separation between the fiber
Fig. 9. Tensile modulus and strength of an AESO/styrene/Lignin-BA/flax composite with 30 wt% flax as function of butyrated hardwood lignin.
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Fig. 10. Flexural modulus and strength of AESO/styrene/lignin-BA/straw composites with 8 wt% straw as function of Lignin-BA addition.
and the AESO/styrene can be observed in the close-up picture. One can also notice stress fractures starting from the interface between the matrix and fiber, going into the matrix material to relieve stress buildup. The interface between the AESO/styrene matrix and the flax fibers is weak and thus allows fiber pullout to occur to relieve the stress buildup in the material. The result is only a limited strength improvement. Adding lignin-BA to the AESO/styrene/flax system has beneficial effects as seen in the micrograph of Fig. 12(B). The diameter of the fibers shown is of the order of 1 –10 mm. The fibers show no pullout and fracture occurs at the surface, together with the matrix material. A decrease in fiber pullout was seen for the whole surface as the ligninBA content increased. This points towards a strong fibermatrix interface with good stress transfer from the polymer to the natural fibers. It can thus be seen that lignin-BA is a compatibilizing agent and did not lose all of its affinity for cellulosic fiber upon butyration. The strength of the composite material should reach the strength of the natural fiber and show a significant increase when compared to the samples without lignin-BA. A large improvement in flexural strength is seen for the straw composites. The exact compatibilization process is not yet known. It could be that there is some form of self-assembly of the lignin molecules around the natural fibers. Lignin-BA would then shield the polarity of the natural fibers from the rest of the resin. Grafting of polymer radicals onto lignin-BA is possible [41,42] and could improve the lignin-BA-resin interface even more. Self-assembly and the diffusion of lignin-BA towards the fiber surface however, will be significantly hindered by the high viscosity of the polymerizing resin and the large molecular weight of lignin-BA. The decrease of the mechanical properties for the flax composites is thus due to the increased resin viscosity giving
Fig. 11. SEM micrographs of the fracture surface of an AESO/styrene/flax composite: (A) large overall picture, (B) is zoomed in on the fiber-matrix interface, with the fiber diameter about 20 mm.
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table and on top of the sample, the 1 atm pressure drop can be achieved over a much shorter distance, across the thickness of the fiber mats. The price, however, is the loss of two smooth surfaces on top and bottom of the sample. The use of less dense fiber reinforcement has already shown to work with the straw composites. These composites had large gaps between the fibers and thus, allowed for easier fiber wetting by a viscous resin. The result was an anticipated increase in flexural strength.
4. Conclusions
Fig. 12. SEM micrographs of the fracture surface of an AESO/styrene/ lignin-BA/flax composite with lignin-BA constituting 10 wt% of the resin weight: (A) large overall picture, (B) fractured at point where good fiber wetting had occurred (400 £ magnification).
poor wetting of the fibers. As more lignin is added to the resin, the viscosity was found to increase more than twofold. Increases in viscosity posed problems with fiber wetting and resulted in dry patches of fiber mats in the composites. These dry patches form weak links with little structural integrity against deformation where fracture will occur preferentially. Extra proof is given by the decrease of both the modulus and the strength. The modulus depends largely on the moduli of the composite constituents and not the resin-fiber interface. Since lignin-BA has been found to increase the modulus of the pure resin [39], a decrease in composite modulus can only be due to the encapsulation of a higher amount of low-modulus regions: small air bubbles and dry patches. Solutions under investigation are the use of Resin Transfer Molding (RTM) which allows for higher pressure drops than the 1 atm that can be attained by VARTM, a change in the VARTM technique used, and the used of less dense fiber reinforcement. The VARTM technique utilized solid material on top and bottom of the sample, resulting in a pressure drop across the fiber mats. By using a low resistance breather cloth on the supporting
Kraft lignin can be fully butyrated and dissolves in the AESO/styrene (7/3 weight ratio) resin. The viscosity of the pure AESO/styrene resin was 164 mPa.s, steadily increasing with increasing lignin-BA contents, up to 400 mPa.s for 10 wt% lignin-BA, a 2.5 fold increase. This increasing viscosity becomes problematic during resin injection into a mold filled with dense natural fiber mats. Composites made with AESO/styrene/lignin-BA and flax fibers showed an increase in entrapped air bubbles and the appearance of large dry patches with increasing ligninBA weight fraction due to the viscosity increase. The poor fiber wetting resulted in a decrease in tensile and flexural properties with increasing lignin-BA load. Short fiber composites consisting of the AESO/styrene/lignin-BA resin and short wheat straw fibers, allowed for better fiber wetting. The short fibers form a less dense structure than the flax fiber mats and the viscosity effect on fiber wetting is less pronounced. The effect was a 40% increase in flexural strength with 5 wt% lignin-BA in the resin compared to the neat resin (no lignin-BA). This points to a beneficial effect of lignin-BA addition on the resin-fiber interactions. The increase in flexural strength indicates an improvement in stress transfer between the resin and natural fiber, which has to be the result of a stronger resin-fiber interface. SEM micrographs of the flax-AESO/styrene/lignin-BA composites also showed an improvement in the resinnatural fiber interface upon lignin-BA addition, even though the composite mechanical properties declined due to the formation of dry patches. The combination of these results suggests that addition of lignin-BA to the AESO/styrene resin results in a significant improvement in the natural fiber-AESO/styrene interface, by compatibilizing the resin and the natural fiber.
5. Work in progress Work is currently in progress to find solutions to the viscosity increase with lignin-BA addition. Solution being investigated include the use of less dense fiber mats even though this will lower the fiber content, the increase of styrene content in the pure resin to 35 wt%, which results in a viscosity drop of 45%, and changes in the VARTM setup
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to get better resin dispersion. The use of lower molecular weight lignin is also being considered. A lower molecular weight lignin should have a lower viscosity effect than the current lignin used.
Acknowledgements The authors would like to thank the Department of Energy and the Environmental Protection Agency for funding and WestVaco for supplying lignin samples. The authors would also like to acknowledge Dr Mahmoud Dweib for his help with the VARTM technique and the Wagner Research Group at the University of Delaware, Department of Chemical Engineering for use of their viscometer.
References [1] McCrum NG. Principles of polymer engineering. Oxford: Oxford University; 1988. [2] Jones FR. Glass fibres-surface treatment. In: Jones FR, editor. Handbook of polymer-fibre composites. Essex, UK: Longman Scientific and Technical; 1994. p. 42–8. [3] Lewin M, Pearce EM, editors. Handbook of fiber chemistry, 2nd ed. New York: Marcel Dekker; 1998. [4] Mohanty AK, Misra M, Hinrichsen G. Biofibres Biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276(3-4):1 –24. [5] Khot SN, Lascala JJ, Can E, Morye SS, Williams GI, Palmese GR, Kusefoglu SH, Wool RP. Development and application of triglyceride-based polymers and composites. J Appl Polym Sci 2001;82(3): 703–23. [6] LaScala JJ. The effects of triglyceride structure on the properties of plant oil-based resins. Newark, DE: University of Delaware. PhD Thesis; 2002. [7] Wool RP, Khot SN, LaScala JJ, Bunker SP, Lu J, Thielemans W, Can E, Morye SS, Williams GI. Affordable composites and plastics from renewable resources: part I: synthesis of monomers and polymers. In: Lankey RL, Anastas PT, editors. Advancing sustainability through green chemistry and engineering. ACS Symposium Series 823, Washington, DC: American Chemical Society; 2002. p. 177 –204. [8] Mohanty AK, Misra M, Drzal LT. Surface modifications of natural fibers and performance of the resulting biocomposites: an overview. Comp Intf 2001;8(5):313–43. [9] Hansen CM, Bjo¨rkman A. The ultrastructure of wood from a solubility parameter point of view. Holzforschung 1998;52(4): 335– 44. [10] Thielemans W, Can E, Morye SS, Wool RP. Novel applications of lignin in composite materials. J Appl Polym Sci 2002;83(2):323–31. [11] Wool RP, Khot SN, LaScala JJ, Bunker SP, Lu J, Thielemans W, Can E, Morye SS, Williams GI. Affordable composites and plastics from renewable resources: part II: manufacture of composites. In: Lankey RL, Anastas PT, editors. Advancing sustainability through green chemistry and engineering. ACS Symposium Series 823, Washington, DC: American Chemical Society; 2002. p. 205 –24. [12] Lu FJ, Chu LH, Gau RJ. Free radical-scavenging properties of lignin. Nutr Cancer 1998;30(1):31–8. [13] Barclay LRC, Xi F, Norris JQ. Antioxidant properties of phenolic lignin model compounds. J Wood Chem Technol 1997;17(1-2): 73–90.
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[14] Thielemans W, Wool RP. Modifications of Lignin for Use in Plant-Oil and Petroleum-based Thermosetting Resins. Presented at AIChE Annual Meeting, Reno, NV; 2001. [15] McCarthy JL, Islam A. Lignin chemistry, technology and utilization: a brief history. In: Glasser WG, Northey RA, Schultz TP, editors. Lignin: historical, biological, and materials perspectives. ACS Symposium Series 742, Washington, DC: American Chemical Society; 1999. p. 2 –99. [16] Nimz HH. Chemistry of potential chromophoric groups in beech lignin. Tappi J 1973;56(5):124–6. [17] Trigonox 239A, Product Data Sheet. Chicago, IL: Akzo Nobel; 2002. [18] Ziaee S, Palmese GR. Effects of temperature on cure kinetics and mechanical properties of vinyl-ester resins. J Polym Sci, Part B: Polym Phys 1999;37:725–44. [19] Alger MSM. Polymer science dictionary. 2nd, New York: Chapman and Hall; 1997. [20] Connors KA. N-Methylimid dazole as a catalyst for analytical acetylations of hydroxyl compounds. Anal Chem 1978;50(11): 1542– 5. [21] Loos AC. Low-cost fabrication of advanced polymeric composites by resin infusion processes. Adv Compos Mater 2001;10(2-3):99–106. [22] Beckwidth SW, Hyland CR. Resin transfer molding: a decade of technology advances. SAMPE J 1998;34(6):7–19. [23] SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ Butyric Anhydride 1 H-NMR spectrum. National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan 11/25/02 [24] SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ Butyric Acid 1HNMR spectrum. National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan 11/25/02 [25] Li S, Lundquist K. Analysis of lignins as propionate derivatives by NMR spectroscopy. J Wood Chem Technol 1997;17(4):391–7. [26] Glasser WG, Jain RK. Lignin derivatives I Alkanoates. Holzforschung 1993;47:225 –33. [27] Tiainen E, Drakenberg T, Tamminen T, Kataja K, Hase A. Determination of phenolic hydroxyl groups in lignin by combined use of 1H-NMR and UV spectroscopy. Holzforschung 1999;53: 529 –83. [28] Fales HM, Robertson AV. Exchange of active hydrogen for deuterium in spectroscopic studies. Tetrahedron Lett 1962;3:111 –5. [29] ASTM D790-95a. Standard test methods for flexural properties of unreinforced and reinforced and electrically insulating materials; 1996 [30] ASTM D3093/D3039M. Standard test method for tensile properties of polymer matrix composite materials; 1995 [31] Herrerafranco PJ, Drzal LT. Comparison of methods for the measurement of fiber/matrix adhesion in composites. Composites 1992;23:2–27. [32] Herrerafranco PJ, Rao V, Drzal LT, Chian MYM. Bond strength measurement in composites—analysis of experimental techniques. Compos Engng 1992;2(1):32–45. [33] Zhao FM, Okabe T, Takeda N. The estimation of statistical fiber strength by fragmentation tests of single-fiber composites. Compos Sci Technol 2000;60:1965–74. [34] Tripathi1 D, Lopattananon N, Jones FR. A technological solution to the testing and data reduction of single fibre fragmentation tests. Compos, Part A: Appl S 1998;29A:1099–109. [35] Rashkovan IA, Korabelnikov YG. Critical length and distribution of fibrous fillers and composites. Mech Compos Mater 1997;33(1): 70–3. [36] SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ Acetone FTIR spectrum. National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan 11/25/02 [37] Pandey KK. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J Appl Polym Sci 1999;71: 1969– 75. [38] Hergert HL. Lignins: Occurrence, formation, structure and reactions. In: Sarkanen KV, Ludwig CH, editors. New York: Wiley-Interscience; 1971. p. 267 –97.
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[39] Thielemans W, Wool RP. Modified kraft lignin as property-enhancer for unsaturated thermosetting resins. Presented at AIChE Annual Meeting 2002, Indianapolis, IN; 2002 [40] O’Dogherty MJ, Huber JA, Dyson J, Marshall CJ. A study of the physical and mechanical properties of wheat straw. J Agric Engng Res 1995;62:133 –42.
[41] Meister JJ. Polymer modification: Principles, techniques and applications. In: Meister JJ, editor. New York: Marcel Dekker; 2000. p. 112–29. [42] Phillips RB, Stannett VT, Brown W. Graft copolymerization of styrene and lignin.2 Kraft softwood lignin. J Appl Polym Sci 1972; 16(1):1– 14.
Butyrated kraft lignin as compatibilizing agent for natural fiber reinforced thermoset composites Wim Thielemans, Richard P. Wool* ACRES Group, Colburn Lab, Department of Chemical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, USA
Abstract Butyrated kraft lignin was added to an unsaturated thermosetting resin, consisting of a mixture of acrylated epoxidized soybean oil and styrene. Composites were made by the Vacuum Assisted Transfer Molding process with varying amounts of butyrated kraft lignin dissolved in the unsaturated resin system. Butyrated kraft lignin improved the interface between the resin and reinforcing flax fibers. SEM images illustrate a clear improvement in the adhesion of the resin to the fibers by showing the fibers fracturing together with the resin, without fiber pullout. Increases in viscosity upon solubilization of butyrated kraft lignin in the unsaturated resin of up to 150% for 10 wt% butyrated lignin however, resulted in the appearance of dry patches in composites made with dense fiber reinforcement mats. These dry spots decrease the mechanical properties of the overall composite. On the other hand, strength improvement is seen for composites made with short wheat straw fibers, which allow for better resin penetration and fiber wetting. The flexural strength increased by 40% for a 5 wt% butyrated lignin addition. Work currently in progress to reduce the negative effects of the viscosity increase by changing the processing conditions, is also described. q 2003 Published by Elsevier Ltd. Keywords: A. Thermosetting resin; B. Fibre/matrix bond; B. Interface/interphase; A. Fibres
1. Introduction Polymer composites are used in a wide variety of applications, varying from aerospace, automotive, and military applications to uses in sports equipment and civil infrastructure. They are made by embedding strong fibers in a polymer matrix. A variety of reinforcements are in use, such as aramid, glass, carbon, and natural fibers [1]. Several works describe the various possible fiber reinforcements and their mechanical properties [2,3]. Natural fibers are currently getting a lot of attention for replacing synthetic fibers [4]. They have the advantage of being abundantly available, renewable, cheaper, since they are waste products, and exhibit good mechanical properties. With densities comparable to aramid fibers (, 1500 kg m23), they can display specific strength and moduli higher than glass fibers. The composite matrix can be formed of either thermoplastic or thermosetting material. Thermoplastic polymers, such as polyethylene and polypropylene, melt upon heating. Fibers are mixed in with the molten polymer, and the * Corresponding author. Tel.: þ 1-302-831-3312; fax: þ1-302-831-8525. E-mail address: [email protected] (R.P. Wool). 1359-835X/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2003.09.011
composite is then shaped into the desired form at high temperature. Thermosetting polymer composites are formed by injecting monomers into a mold with the desired, final shape. The mold can either contain the fibers in a preform or short fibers can be injected together with the monomers into the mold. The mold is then heated and the monomers polymerize into a cross-linked network entrapping the fibers. The composite part is unable to be reshaped after polymerization. In order to obtain a largely natural-based composite, a thermosetting soybean-oil based resin was used. The soybean oil based resin is obtained by first epoxidizing, then acrylating the free-radically stable double bonds on the triglyceride molecules in these oils. The acrylate double bonds can then be polymerized together with styrene, which is used as a reactive diluent to decrease the resin viscosity. The trygliceride modification reactions and the resulting resin have been well studied and characterized [5,6,7]. The chemical structure of the resin components is given in Fig. 1. Strength improvement in composites depends on stress transfer between the polymer matrix, on which the external force is applied, and the reinforcing fibers. If the interface is weak, failure will occur at the fiber-matrix interface and no
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Fig. 1. Chemical structures of the resin constituents and lignin: (A) styrene, (B) Acrylated epoxidized soybean oil, (C) Butyrated hardwood lignin, based on a simplified structure of Hardwood Beech lignin [16]. Me denotes a CH3 group, But a butyrate group where it could not be completely drawn.
improvement will be seen. While glass fibers are treated with a compatibilizer as it is spun [2], fiber treatment is not as convenient for natural fibers. A recent overview of natural fiber treatments is given by Mohanty et al. [8]. However, any surface treatment would also increase the cost of natural fibers significantly, reducing one of the incentives to use natural fibers in composites with each required extra processing step. Natural wood and plant fibers are constituted of cellulose fibers, consisting of helically wound cellulose microfibrils, bound together by an amorphous lignin matrix. Lignin acts as a sealant to keep water in the fibers, as a protection against
biological attack, and as a stiffener to give the stem its resistance against wind and gravity forces. Hemicellulose is also found in these natural fibers and is generally believed to be a compatibilizer between cellulose and lignin [9]. This make-up of wood suggests the potential of lignin to act as a compatibilizer between hydrophilic natural fiber reinforcement and a hydrophobic matrix polymer. A first approach in which kraft lignin was deposited onto natural fibers has been described elsewhere [10,11]. The strength and modulus of the composite was found to increase when lignin was deposited from an aqueous sodium hydroxide solution onto the natural fibers [10]. The lignin
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layer increased the rigidity and strength of the fiber, resulting in an increase in composite modulus. It also shielded the hydrophilic fibers from the hydrophobic resin, and increased the composite strength, pointing towards a stronger fiber-resin interface. The deposited lignin layer also resulted in a smoother outer fiber surface, compared to the original rough surface of the cellulosic fibers [10]. This could be portrayed as a surface defect treatment. Addition of too much lignin onto the fiber however, resulted in a thicker lignin layer, and a decrease of mechanical properties of the overall composite [10]. A second approach, pursued in this work, is the inclusion of lignin in the resin. The kraft lignin used in this work contains large amounts of hydroxyl groups, both aliphatic and aromatic, as shown in Fig. 2. The aromatic hydroxyl groups have the capability of inhibiting free radical polymerization by forming stable quinonic structures [12,13]. The high polarity of the hydroxyl groups leads to a molecule insoluble in rather non-polar thermosetting resins, while the formation of stable quinonic structures poses problems during free radical polymerization. By reacting these alcohol groups with butyric anhydride, the inhibiting characteristics subside and the molecule becomes soluble in styrene, the reactive diluent in the resin used here, and in a vast amount of free radically polymerizing thermosets [14]. Fig. 1 gives the final lignin structure after butyration. Butyrated lignin (Lignin-BA), dissolved in a monomer mixture, may decrease the negative thermodynamic
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interactions between hydrophilic natural fibers and the hydrophobic polymer matrix, due to its inherent natural compatibility with the fiber elements and its new chemically induced solubility in the polymer matrix. Application of a resin soluble compatibilizer would render natural fiber treatment unnecessary, reducing processing cost. The stronger resin-natural fiber interface will improve the overall composite properties and better use the potential that natural fiber reinforcements promise. In this paper, the effect of different lignin concentrations on the mechanical properties of composites made up of flax and wheat straw fibers, and acrylated epoxidized soybean oil was investigated. The ultimate goal is a lignin-based, resin-soluble additive that gives rise to a very strong resin-natural fiber interface.
2. Experimental 2.1. Materials The hardwood kraft lignin was obtained from WestVaco, and is sold under the trade name PC-1369. Hardwood kraft lignin contains about 1 hydroxyl group per Phenyl Propane Unit (PPU: C3 – C6 unit with molecular weight of 185 g/mol for kraft lignin) with almost 25% aliphatic hydroxyl groups, and 75% aromatic hydroxyl groups. The total hydroxyl group content was taken from
Fig. 2. Proposed chemical structure of hardwood lignin [16].
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McCarthy and Islam [15] and equaled the total butyrate amount after total butyration, determined by 1H-NMR spectroscopy. Aromatic hydroxyl group content was also taken from the same reference [15] and confirmed by 1HNMR spectroscopy of the unmodified lignin. A proposed structure is given in Fig. 2 [16]. Butyric anhydride (BA, 98% pure) and methacrylic anhydride (MA, 94% pure) were both obtained from Sigma-Aldrich. The catalyst for the lignin-anhydride reaction was 1-Methyl Imidazole (1MI), Sigma Aldrich, 99% pure. Cyclohexane (99% pure) and ethyl ether (anhydrous) for the purification of reacted lignin were obtained from Fisher Scientific. The polymerization catalyst used in this study was Trigonox 239A from Akzo Nobel (45% Cumyl Hydroperoxide solution in carboxylic ester [17]), with a commonly used activator Cobalt Naphtenate [18] (6% solution in mineral spirits, from Mahogany Company) to allow for room temperature curing (Fig. 3). Cobalt naphtenate is a Cobalt Salt of Naphtenic Acid [19]. The initiator used for high temperature cure was tert-Butyl PeroxyBenzoate, from Sigma Aldrich (98% pure). The resin used was an Acrylated Expoxidized Soybean Oil (AESO), from UCB Chemical, sold under the trade name Ebecryl 860. This was diluted with 99% pure styrene from Sigma-Aldrich. Flax fiber mats were obtained from Cargill Limited, USA. The continuous flax fiber mats are 1/400 thick, with a weight of 680 g/m2 and contain 85% flax and 15% glass fiber binder. Short wheat straw fibers were obtained by milling seedless long fibers at 1800 rpm. They were subsequently passed through a 1/200 screen to separate out the desired, broken-down short fibers from the original long ones. The average length of the short straw fibers was 1.2 cm. All fibers were stored at ambient conditions with relative humidity of 73% and used without prior drying or surface treatment. Deuterated dimethyl sulfoxide (DMSO-d6) and deuterated water (D2O) for 1H-NMR studies were obtained from Cambridge Isotope Labs and were 99.9% pure. Spectroscopic grade acetone for dissolution of lignin-BA before FTIR analysis was purchased from Sigma-Aldrich.
2.2. Lignin modification (Butyration) Kraft lignin, and butyric anhydride were added in a 1:2 weight ratio to an Erlenmeyer flask. Per 40 g of lignin, 1 g of the catalyst 1-MI was added. The reaction mixture was heated up to 50 8C and reacted for several hours while stirring vigorously. Initially, the kraft lignin does not dissolve in butyric anhydride. However, upon conversion of the lignin hydroxyl groups to butyrates, lignin becomes soluble in the reaction mixture. This is an easy way to monitor the progress of the reaction. The reaction was found to be essentially completed after about 20 min but the reaction mixture was left for several hours to ensure full conversion of hydroxyl groups [14]. The reaction mechanism, based on the 1-MI catalyzed reaction of anhydrides with hydroxyl groups is given in Fig. 4 [20]. Ethyl ether was then added to the reaction mixture (1:1 volume ratio) to reduce the viscosity and allow for a clear phase separation when water is added. The ethyl ether/lignin/BA/1MI was then washed with deionized water to extract the 1MI out of the reaction mixture. Three water washes with 1:1 volume ratio were sufficient. Adequacy of the water washes was determined by the disappearance of the 1MI peaks in the 1 H-NMR spectra of the dried butyrated lignin. The 1MI signals were determined by taking a 1H-NMR spectrum of 1MI in DMSO-d6 (Fig. 5). Lignin was then sedimented out of the ethyl ether/BA/lignin mixture with cyclohexane. The lignin recovery depends on the amount of cyclohexane used. Recoveries of up to 80% could easily be obtained. The sedimented lignin was then filtered out and dried under vacuum for 24 h. Residual amounts of 1MI in the reaction mixture after the water washes, effect in a slimy lignin-BA/1MI sediment, rather than a granular sediment obtained in the absence of 1MI, due to the insolubility of 1MI in cyclohexane. Recovery of butyrated lignin from this slimy sediment is troublesome. One has thus the possibility to probe the efficiency of the water washes on a small sample before adding cyclohexane to the whole reaction mixture. 2.3. Characterization of butyrated lignin The butyration reaction was followed by Fourier Transform Infrared Spectroscopy (FTIR). Dried lignin-BA was
Fig. 3. Chemical structures of cumyl hydroperoxide [17] (A) and cobalt– naphtenate [18], based on its literature definition [19] (B).
Fig. 4. Reaction mechanism of butyration of the lignin hydroxyl groups, based on the similar 1MI catalyzed reaction mechanism of acetic anhydride with hydroxyl groups. Butyric anhydride reacts first with 1MI to form an Nbutyryl-N0 -methylimidazolium ion, which then further reacts with the lignin hydroxyl groups [20].
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Fig. 5. 1H-NMR spectrum of 1MI in DMSO-d6. Signals can be seen at d ¼ 3.6, 6.9, 7.1 and 7.6 ppm. The small signal at 2.5 ppm is due to the solvent DMSO-d6.
dissolved in acetone. A thin film of the acetone/lignin-BA mixture was applied to a NaCl disk, after which the disk was left to dry under vacuum for several minutes to evaporate off all acetone. The FTIR spectra were then recorded with a Genesis Series FTIR spectrometer from ATI Mattson (Madison, WI). The spectrometer uses a Deuterated Triglycine Sulfate (DTGS) detector and Potassium Bromide (KBr) beam splitter. The resolution was 4 cm21, and 128 scans were taken per spectrum. Interpretation of the spectrum was done with the software program WinFirst. Complete butyration was confirmed by 1H-NMR on a 250 MHz Bruker AC250 NMR spectrometer with 0.01 g lignin-BA dissolved in 0.1 g DMSO-d6. The magnetic field strength was 250.13 MHz, with a resolution of 0.427 Hz/pt and a spectral window of ^ 2500 Hz. Twenty-four spectra with a relaxation delay of 10 s and pulse width of 908 were taken at 25 8C. Confirmation of complete reaction of alcohol groups was checked by integration of the butyrate peaks (CH3: , 0.9 ppm, CH2: , 1.5 and 2.2 ppm) [23 – 26], and the broad aromatic alcohol peak (8.5 – 11 ppm range) [27]. Due to the low amount of scans, distinguishing between the signals of aliphatic or aromatic butyrate signals is not possible. For our purpose here, only confirmation of conversion of all hydroxyl groups into butyrate groups was necessary. The low number of scans gave sufficient accuracy while keeping the analysis time low. However, for detailed analysis, differentiation between aromatic and aliphatic esters can be done by increasing the amount of scans to values of 1000 or more [14,25]. Twenty percent by volume of D2O was then added to the DMSO-d6/Lignin-BA mixture to replace any possible hydrogens remaining on the aromatic alcohol groups. Subtractions of the aromatic hydroxyl group signal
with D2O from the signal obtained without D2O, has to be zero if all aromatic alcohol groups are reacted. Hydrogendeuterium substitution is only seen for functional groups with active hydrogen such as acid and aromatic hydroxyl groups, but not for aliphatic hydroxyl groups [28]. 2.4. Monomer mixture Butyrated lignin (lignin-BA) in different quantities was dissolved in styrene. Pure styrene has a much lower viscosity than the AESO/styrene resin, which helps to speed up the solubilization of lignin-BA. Lignin-BA/styrene was then mixed with AESO to obtain 1.0, 2.5, 5.0 and 10 wt% ligninBA in the AESO/styrene/lignin-BA resin. The AESO/styrene weight ratio was 7/3. Three wt% of the total resin weight of Trigonox 239A and 0.8 wt% Cobalt-Naphtenate were mixed into the monomer-lignin mixture for room temperature cure. For high temperature cure, 1.5 wt% tert-Butyl PeroxyBenzoate was added to the AESO/styrene/lignin-BA mixture. 2.5. Viscosity measurements of monomer mixtures The viscosity of the monomer mixtures was measured with an Anton Paar Automated Microviscometer (AMVn), based on the rolling ball principle. The capillary used has an internal diameter of 4 mm, the ball inside a diameter of 3 mm. This setup is capable of measuring viscosities in the range of 80– 800 mPa.s. The viscosity was measured at three different angles of the capillary, resulting in three different shear rates. The angles were 50, 60 and 708 from the horizontal. Experiments at each angle were repeated a minimum of four times.
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2.6. Composite fabrication Polymer composites were obtained by a VARTM (Vacuum Assisted Resin Transfer Molding) process [21,22]: Three sheets of Cargill Durafiber flax (approx. 1 £ 1 ft) were layed up on a table, and covered with a plexiglass sheet to give a top flat surface of the finished part. The table was pretreated with the release agent Frekote (Loctite, Rocky Hill, CT) to allow easy removal of the composite part after polymerization. This lay-up was covered by a plastic sheet and closed off airtight. A vacuum was then applied on one side while the other side was connected to a container with the resin mixture. After the resin was completely infused through the fiber mats by the vacuum, and no air was present, both ends were closed off airtight and the resin was left overnight at room temperature to cure. The composite was not post-cured. The composite was then cut into parts with the desired dimensions to be tested. The fiber content of the samples was approximately 30 wt%. A second batch of composites was made with short wheat straw fibers. These short fibers were put in a silicone mold after which the resin was poured in. No pressure drop to enhance resin infusion was applied. The resin constituted of the earlier described mixtures of AESO, styrene and lignin-BA and the high temperature initiator tert-Butyl PeroxyBenzoate, with lignin-BA concentrations of 0, 1 and 5 wt%. The composite mixture was cured for 2 h at 110 8C, followed by a post-cure at 160 8C for an extra 2 h. The temperature was always ramped at a rate of 5 8C/min in between the different set point temperatures. After curing, the composites were cut and polished to the desired dimensions for testing. The fiber content was around 8 wt%. There was still a slight increase in air bubbles as lignin-BA amount was increased but not as severe as with the flax composites. The existence of air bubbles can be attributed to the absence of a pressure driving force during resin infusion. 2.7. Characterization of composites The flexural modulus and strength were measured according to ASTM D790 [29] in 3 point bend mode. Five samples of sizes 63.7 £ 2.51 £ 3.2 mm3 were tested for each lignin content. The crosshead speed on the Instron (Model 4201) was 1.3 mm/min with a span of 50.8 cm. Tensile modulus and strength were measured according to ASTM D3093/D3039M [30]. Five samples, without tabs, with measurements 250 £ 2.5 £ 4 mm2 were tested for each lignin content. The crosshead speed was 1.3 mm/min. Only data from samples that did not break at the grips were used. The straw composites were not tested in tensile mode due to inconsistencies of fiber orientation and dispersion between samples of different lignin content in the tensile sample mold. This could be the result of different resin viscosities. The critical fiber length dependence on the lignin-BA content of the resin could not be determined. The fragmentation test is considered to be the most reliable
technique to obtain the critical fiber length and interfacial shear strength [31,32]. However, to determine this critical fiber length and the interfacial shear strength, it is required to measure the broken fragment lengths after testing [31– 35]. Addition of lignin-BA colors the resin dark brown, making it impossible to accurately measure the broken fragment lengths, and thus, determine the critical fiber length. Scanning Electron Microscopy (SEM) micrographs were taken of the fracture surface of the flexural test samples. The fracture surface was gold coated by sputtering for 75 s using a current of 25 mV. The SEM was a JEOL JXA-840 Scanning Microscope. The probe voltage was 6 £ 10210 Amps, with an acceleration voltage of 3 kV. The electron source consisted of a Tungsten wire.
3. Results and discussion 3.1. Butyration FTIR can give a quick and qualitative indication of the extent of lignin butyration. Fig. 6 gives an example of the spectrum recorded for lignin-BA, with the important bands assigned. Upon butyration, the alcohol bands (3300 – 3700, 1097, and 1035 cm21) reduce in size while the CyO bands (1740 and 1600 cm21) and the methyl and methylene C –H bands (2966, 2940, 2880, and 1464 cm21) increase. The C – O band at 1097 and 1035 cm21 are assigned to secondary and primary alcohols respectively. They do not disappear because they fall together with the C – O stretch from aliphatic ethers and the aromatic guaiacyl type C –H stretch respectively, both of which are unaffected by the butyration reaction. Residual acetone may also explain some residual peak at 3300 –3700 cm21 [36]. Band assignments were made based on previously published literature values [37,38]. The 1H-NMR spectrum of fully butyrated lignin with signal assignments is given in Fig. 7. It was found that the butyrate groups matched the amount of original hydroxyl groups (both aromatic and aliphatic). No increase in acid peak (11 –14 ppm) was seen, so no residual butyric acid was left in the dried lignin-BA sample. The amount of aromatic hydroxyl groups was found to be zero for all samples after butyration. We can therefore conclude that all the alcohol groups were converted into butyrate groups. 3.2. Resin viscosities The viscosities of the monomer mixtures as a function of lignin-BA content are depicted in Fig. 8. The three points for each lignin-BA load represents measurements at the three angles of the capillary. The recorded viscosities do not vary significantly for the different angles, and thus shear rates, except at a 10 wt% lignin load. The close values for 60 and 708 angles, however,
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Fig. 6. FTIR spectra of fully butyrated hardwood lignin with important band assigments. Lignin-BA was deposited on an NaCl from an acetone solution.
suggest that the 508 angle measurement is inaccurate. The resin viscosity is found to increase significantly upon lignin-BA addition: an increase from 165 mPa.s without any lignin-BA to 400 mPa.s with addition of 10 wt% lignin-BA. The viscosity thus increases 10% with a 1 wt% lignin-BA mixture, and 150% for 10 wt% ligninBA. This large viscosity rise will affect negatively the ability of the resin to flow through the fiber mats during resin infusion into the mold. It may result in an increase in the amount of air bubbles in the polymerized composite part.
3.3. Mechanical properties The results from tensile tests of the butyrated hardwood lignin in the AESO/styrene/lignin-BA/flax composite are shown in Fig. 9. As the lignin-BA amount in the resin increases, both the modulus and strength decrease. Lignin-BA seems to have a negative effect on the tensile properties. The same effect is seen for the flax composites in flexural mode. The negative lignin-BA effect can occur for several reasons: (a) Lignin-BA could have lost its affinity for cellulosic fibers upon butyration,
Fig. 7. 1H-NMR spectrum of butyrated hardwood lignin with peak assignments. Ar represents aromatic ring, Al aliphatic chain. The signal for aliphatic alcohols coincides with the much bigger methoxyl (– OCH3) hydrogen signal. Therefore, aliphatic alcohols cannot be determined accurately.
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Fig. 8. Effect of lignin-BA content on the resin viscosity as measured by an Anton Paar AMVn viscometer. The three different points at each lignin load represent measurements at different angles of the capillary to obtain different flow rates.
(b) it can decrease the crosslink density of the matrix material since it is a plasticizer and thus lower the matrix integrity or (c) it can decrease the wetting of the fiber by increasing the resin viscosity. It has been shown that lignin-BA addition to the plain resin increased the flexural modulus and strength of the AESO/styrene system with a maximum of 75 and 60%, respectively upon addition of 5 wt% lignin-BA [39]. The increase of resin viscosity upon lignin-BA addition was measured and found to increase significantly. A rise in air bubbles entrapped in the polymerized composite was seen as the lignin-BA content in the resin was raised. Significant dry spots in the fiber mats were also seen as the lignin content increased to 10 wt%. The viscosity increase is therefore believed to be the reason for the declining mechanical properties of the composite part with increasing ligninBA addition. The straw composites allowed for better fiber wetting due to the lower density of the original fiber lay-up, even without any applied pressure drop. The fiber content of the composites is just 8 wt%. This results in significant increase in flexural strength when 5 wt% lignin-BA is added (Fig. 10). Flexural strength increased with 40% to 29 MPa with only a 5 wt% lignin-BA addition. The flexural strength for the resin with 5 wt% lignin-BA approaches the values for the flexural strength of wheat straw of about 30 MPa [40]. The resin thus comes close to the flexural strength of wheat straw, indicating a good resinfiber interface.
3.4. Microscopy SEM was used to examine the resin-fiber interface. Micrographs were taken of the fracture surface of flexural samples. Comparison of Figs. 11(A) and 12(A) show very little improvement upon lignin-BA addition. It even looks like the amount of fiber pullout increased as 10 wt% ligninBA was added, because of the small amount of resin one can see at the surface. It is however, important to look closer at the fracture surface. Fig. 11(B) shows a SEM micrograph of the fracture surface of an AESO/styrene/flax composite without any lignin-BA added. It can clearly be seen that fiber pullout occurred. Clear separation between the fiber
Fig. 9. Tensile modulus and strength of an AESO/styrene/Lignin-BA/flax composite with 30 wt% flax as function of butyrated hardwood lignin.
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Fig. 10. Flexural modulus and strength of AESO/styrene/lignin-BA/straw composites with 8 wt% straw as function of Lignin-BA addition.
and the AESO/styrene can be observed in the close-up picture. One can also notice stress fractures starting from the interface between the matrix and fiber, going into the matrix material to relieve stress buildup. The interface between the AESO/styrene matrix and the flax fibers is weak and thus allows fiber pullout to occur to relieve the stress buildup in the material. The result is only a limited strength improvement. Adding lignin-BA to the AESO/styrene/flax system has beneficial effects as seen in the micrograph of Fig. 12(B). The diameter of the fibers shown is of the order of 1 –10 mm. The fibers show no pullout and fracture occurs at the surface, together with the matrix material. A decrease in fiber pullout was seen for the whole surface as the ligninBA content increased. This points towards a strong fibermatrix interface with good stress transfer from the polymer to the natural fibers. It can thus be seen that lignin-BA is a compatibilizing agent and did not lose all of its affinity for cellulosic fiber upon butyration. The strength of the composite material should reach the strength of the natural fiber and show a significant increase when compared to the samples without lignin-BA. A large improvement in flexural strength is seen for the straw composites. The exact compatibilization process is not yet known. It could be that there is some form of self-assembly of the lignin molecules around the natural fibers. Lignin-BA would then shield the polarity of the natural fibers from the rest of the resin. Grafting of polymer radicals onto lignin-BA is possible [41,42] and could improve the lignin-BA-resin interface even more. Self-assembly and the diffusion of lignin-BA towards the fiber surface however, will be significantly hindered by the high viscosity of the polymerizing resin and the large molecular weight of lignin-BA. The decrease of the mechanical properties for the flax composites is thus due to the increased resin viscosity giving
Fig. 11. SEM micrographs of the fracture surface of an AESO/styrene/flax composite: (A) large overall picture, (B) is zoomed in on the fiber-matrix interface, with the fiber diameter about 20 mm.
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table and on top of the sample, the 1 atm pressure drop can be achieved over a much shorter distance, across the thickness of the fiber mats. The price, however, is the loss of two smooth surfaces on top and bottom of the sample. The use of less dense fiber reinforcement has already shown to work with the straw composites. These composites had large gaps between the fibers and thus, allowed for easier fiber wetting by a viscous resin. The result was an anticipated increase in flexural strength.
4. Conclusions
Fig. 12. SEM micrographs of the fracture surface of an AESO/styrene/ lignin-BA/flax composite with lignin-BA constituting 10 wt% of the resin weight: (A) large overall picture, (B) fractured at point where good fiber wetting had occurred (400 £ magnification).
poor wetting of the fibers. As more lignin is added to the resin, the viscosity was found to increase more than twofold. Increases in viscosity posed problems with fiber wetting and resulted in dry patches of fiber mats in the composites. These dry patches form weak links with little structural integrity against deformation where fracture will occur preferentially. Extra proof is given by the decrease of both the modulus and the strength. The modulus depends largely on the moduli of the composite constituents and not the resin-fiber interface. Since lignin-BA has been found to increase the modulus of the pure resin [39], a decrease in composite modulus can only be due to the encapsulation of a higher amount of low-modulus regions: small air bubbles and dry patches. Solutions under investigation are the use of Resin Transfer Molding (RTM) which allows for higher pressure drops than the 1 atm that can be attained by VARTM, a change in the VARTM technique used, and the used of less dense fiber reinforcement. The VARTM technique utilized solid material on top and bottom of the sample, resulting in a pressure drop across the fiber mats. By using a low resistance breather cloth on the supporting
Kraft lignin can be fully butyrated and dissolves in the AESO/styrene (7/3 weight ratio) resin. The viscosity of the pure AESO/styrene resin was 164 mPa.s, steadily increasing with increasing lignin-BA contents, up to 400 mPa.s for 10 wt% lignin-BA, a 2.5 fold increase. This increasing viscosity becomes problematic during resin injection into a mold filled with dense natural fiber mats. Composites made with AESO/styrene/lignin-BA and flax fibers showed an increase in entrapped air bubbles and the appearance of large dry patches with increasing ligninBA weight fraction due to the viscosity increase. The poor fiber wetting resulted in a decrease in tensile and flexural properties with increasing lignin-BA load. Short fiber composites consisting of the AESO/styrene/lignin-BA resin and short wheat straw fibers, allowed for better fiber wetting. The short fibers form a less dense structure than the flax fiber mats and the viscosity effect on fiber wetting is less pronounced. The effect was a 40% increase in flexural strength with 5 wt% lignin-BA in the resin compared to the neat resin (no lignin-BA). This points to a beneficial effect of lignin-BA addition on the resin-fiber interactions. The increase in flexural strength indicates an improvement in stress transfer between the resin and natural fiber, which has to be the result of a stronger resin-fiber interface. SEM micrographs of the flax-AESO/styrene/lignin-BA composites also showed an improvement in the resinnatural fiber interface upon lignin-BA addition, even though the composite mechanical properties declined due to the formation of dry patches. The combination of these results suggests that addition of lignin-BA to the AESO/styrene resin results in a significant improvement in the natural fiber-AESO/styrene interface, by compatibilizing the resin and the natural fiber.
5. Work in progress Work is currently in progress to find solutions to the viscosity increase with lignin-BA addition. Solution being investigated include the use of less dense fiber mats even though this will lower the fiber content, the increase of styrene content in the pure resin to 35 wt%, which results in a viscosity drop of 45%, and changes in the VARTM setup
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to get better resin dispersion. The use of lower molecular weight lignin is also being considered. A lower molecular weight lignin should have a lower viscosity effect than the current lignin used.
Acknowledgements The authors would like to thank the Department of Energy and the Environmental Protection Agency for funding and WestVaco for supplying lignin samples. The authors would also like to acknowledge Dr Mahmoud Dweib for his help with the VARTM technique and the Wagner Research Group at the University of Delaware, Department of Chemical Engineering for use of their viscometer.
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