In order to manufacture environmentally friendly particle boards and enhance local resources, the lignins of Sorghum bicolor and Andropogon gayanus characterized using RMN 13C and MALDI TOF have the same structures. They contents Guaiacyl (G), Syringyl (S), p-hydroxyphenylpropane (H) and functional groups of phenols, flavonoids and secondary alcohols. The total phenol content determinated using Folin-Ciocalteu reagent is respectively 20.97 and 15.42 mg eqgallic acid/g of extract. The power of their adhesives is different. The Internal Bond (IB) of particleboards manufactured with these adhesives are respectively 0.37 MPa and 0.41 MPa. These lignins can be used as antioxydants.
Plants constituents such as lignin, tannin, hemicellulose and others are interesting. Many searchers in various fields such as in the paper industry, pharmaceutical industry, bakeries, wood adhesives etc. [
This idea of upgrading local materials and replacing all synthetic resins in the wood industry with biodegradable resins made from renewable resources such as wood, grasses and bone is of economic importance particularly for African countries in development. It can be of great interest in reducing the effect of climate change in some part of this continent (the sub-Saharan Africa).
Sorghum bicolor is a cereal and forage grass which belongs to the Poaceae family and the Andropogoneae tribe. It is the world's fifth largest cereal, after maize, rice, wheat and barley [
Despite the multiple benefits of Andropogon gayanus and Sorghum bicolor stems, these gramineae are not managed efficiently, they are waste often thrown into the wild, sometimes burned and used as firewood in households in the Northern Cameroon and they simply need to be upgraded. The Andropogon particularly is often a source of environmental hazards because it is often the source of the bush fires. Thus, the extraction of their lignins would constitute a great economic potential for these African countries because resins and biodegradable composite materials will be manufactured locally [
It is in this context that the present paper responds to this approach in investigating the applicability of the lignin of sorghum bicolor and Andropogon gayanus stem. Given the importance of sorghum bicolor and Andropogon gayanus species in Africa in general, and Cameroon in particular, and that of lignin in the wood and pharmaceutical industries, a sound knowledge of the chemical structure of sorghum bicolor and Andropogon gayanus using Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) and Carbon-13 Nuclear Magnetic Resonance (13C NMR) spectroscopy and the mechanical characteristics of particle board made from these lignins resins constitutes our interest center.
140 g of each sample (sorghum bicolor and Andropogon gayanus) stems crushed and 980 ml of NaOH (0.7 - 1 M) were mixed in a reactor of 2 liters, and boiled at 170˚C during 3 hours. After cooling the boiled content during 2 hours, it was sieved to obtained black liquor at pH = 5.5 adjusted using sulfuric acid. Then, the precipitate lignin was filtrated on the vacuum and watch with distillated water till neutral. The lignin obtained was stoved in an oven at 45˚C during 48 hours [
0.0125 g extract of each sample above was diluted in 25 ml of methanol to obtain a final absorbance in the range [0, 0.5]. Portion of 2.5 mL of Folin-Ciocalteu reagent that has been pre-diluted ten times in methanol, then 2 mL of an aqueous sodium carbonate solution (75 g/L) were added to 0.5 mL of the methanolic solution of the extract. Sodium carbonate is added 30 s to 8 min after the Folin-Ciocalteu reagent. The resulting mixture was maintained during 5 min in a water bath at 50˚C and then dipped in a cold water bath. The mixture was centrifuged and the absorbance of the supernatant liquid was determined at 760 nm with a Perkin-Elmer UV Lambda Spectrophotometer. The total phenolic content was calculated as gallic acid equivalent from the calibration curve of a gallic acid standard solution (0 - 25 mg/L) and expressed as milligrams of gallic acid equivalent/g of extract. All tests were repeated three times [
5 mg of each lignin extract were dissolved in 1 mL of acetone and the obtained solution was mixed with another one composed of acetone (10 mg/mL acetone) and 2,5-dihydroxybenzoic acid as matrix. Then, add the sodium chloride (NaCl) to the matrix (10 mg/mL in distilled water) to enhance the ion formation. The resulting solutions were evaporated on the MALDI target before placing into the spectrometer. The KRATOS compact MALDI AXIMA PERFORMANCE MALDI TOF 2 instrument was used to record the spectra. The irradiation source was a pulsed nitrogen laser (wave-length: 337 nm, laser pulse length 3 ns, and target type: ground steel).
A Bruker MSL spectrometer (Bruker Biospin, Wissembourg, France) was used to analyze the Carbon-13 Nuclear Magnetic Resonance (13C NMR). The spectra were acquired with 5 s recycle delays, a 90˚ pulse of 4.2 µs and a contact time of 1 ms. The number of transients used was 3000 [
A water solution containing 35% lignin extract was mixed with 5% paraformaldehyde powder and pH was adjusted to 10 adding sodium hydroxide solution (NaOH). All components weight are on solids lignin. The components of each formulation was mixed in a beaker at 25◦C temperature, process was stopped when viscosity reached 750 mPas using Brookfield viscometer RV. Spindle Nr, 27 at 25◦C. Two formulations of adhesives with those lignins extract where developed. These adhesives were used to manufacture particle boards.
Two particleboards of 350 × 300 × 14 mm3 dimension were prepared using particles of Sorghum bicolor and Andropogon gayanus (moisture content = 2%) at 28 kg/cm2 maximum pressure and 190◦C - 200◦C press temperature during 7.5 min. The total adhesive resin solids load on dry wood was 10% w/w of the total mix of different lignin. The particleboards were tested for dry internal bond (IB) strength in accordance with the standard EN312.
The total phenolic content of Sorghum bicolor lignin and Andropogon gayanus in the solvent measured by the Folin-Ciocalteu method are respectively 20.97 and 15.42 mg eqgallic acid/g of extract. The possibility to uses this lignin as adhesive is showed by the existence of phenol in this extract [
The examination of the MALDI TOF spectra of Sorghum bicolor and Andropogon gayanus lignin in
these monomers are: 94; 139 Da (for H); 151; 182 and 197 Da (for G) and 210 (for S) Da (
Further information on the chemical structure of lignin extracts of Andropogon gayanus (LA) and lignin sorghum bicolor (LS) was obtained by 13C NMR. The peaks of the spectra in
structure | Calculated M.W | Calculated M.W +Na | Experimental peak | Peak relative proportion (%) | |
---|---|---|---|---|---|
G 1H | monomer | 181 | 204 | 204 | 15 |
G-2H | monomer | 195 | 218 | 218 | 20 |
S-5H | monomer | 205 | 228 | 228 | 5 |
S + 4H | monomer | 214 | 237 | 237 | 18 |
S + 6H | monomer | 224 | 247 | 247 | 5 |
H-G-OH | dimer | 228 | 251 | 251 | 15 |
H-G-7H | dimer | 238 | 261 | 261 | 8 |
H-G | dimer | 245 | 268 | 268 | 28 |
H-G + 1H | dimer | 246 | 269 | 269 | 5 |
H-G + 5H | dimer | 250 | 273 | 273 | 37 |
S-S-CH3 | dimer | 251 | 274 | 274 | 15 |
G-H-2OH | dimer | 257 | 280 | 280 | 65 |
S-S-7H | dimer | 259 | 282 | 282 | 97 |
G-H + 6H | dimer | 260 | 283 | 283 | 10 |
S-S + 2H | dimer | 268 | 291 | 291 | 5 |
S-S + 3H | dimer | 279 | 302 | 302 | 21 |
G-G + 4H | dimer | 280 | 303 | 303 | 15 |
G-H | dimer | 290 | 313 | 313 | 10 |
G-H + 7H | dimer | 297 | 320 | 320 | 5 |
H-S G-G | dimer | 304 | 327 | 327 | 10 |
H-S G-G + 2H | dimer | 306 | 329 | 329 | 5 |
H-S G-G-CO2 | dimer | 316 | 339 | 339 | 9 |
S-H-CH2 | dimer | 323 | 346 | 346 | 5 |
G-G + 2OH | dimer | 330 | 353 | 353 | 5 |
H-G + 3H | dimer | 339 | 362 | 362 | 100 |
H-G-2H | dimer | 340 | 363 | 363 | 3 |
G-H-H2O S-H | dimer | 342 | 365 | 365 | 3 |
H-S G-H-CH3 | dimer | 345 | 368 | 368 | 12 |
S-H | dimer | 349 | 372 | 372 | 5 |
H-G-1H | dimer | 356 | 379 | 379 | 4 |
G-G S-H + 3H | dimer | 363 | 386 | 386 | 5 |
G-G-2H | dimer | 371 | 394 | 394 | 4 |
G-G-2H | dimer | 380 | 403 | 403 | 12 |
G-G-H2O | dimer | 384 | 407 | 407 | 68 |
G-G + 2H | dimer | 387 | 410 | 410 | 82 |
---|---|---|---|---|---|
G-S-H2O | dimer | 389 | 412 | 412 | 15 |
G-G-2H | dimer | 397 | 420 | 420 | 16 |
G-G-4H | dimer | 399 | 422 | 422 | 100 |
G-S-5H | dimer | 402 | 425 | 425 | 22 |
G-S | dimer | 407 | 430 | 430 | 12 |
G-S + 8H | dimer | 415 | 438 | 438 | 15 |
S-S-OH | dimer | 419 | 442 | 442 | 42 |
G-S + H2O | dimer | 425 | 448 | 448 | 13 |
S-S-11H | dimer | 426 | 449 | 449 | 32 |
S-S-5H | dimer | 432 | 455 | 455 | 12 |
S-S-3H | dimer | 434 | 457 | 457 | 18 |
S-S + H2O | dimer | 448 | 471 | 471 | 62 |
S-H-G-4H | trimer | 459 | 482 | 482 | 10 |
S-H-G-2H | trimer | 461 | 484 | 484 | 28 |
H-H-G-3H | trimer | 472 | 495 | 495 | 65 |
S-H-G | trimer | 491 | 514 | 514 | 5 |
S-H-G | trimer | 500 | 523 | 523 | 100 |
S-H-G + 7H | trimer | 507 | 530 | 530 | 5 |
S-S-H + 2H | trimer | 515 | 538 | 538 | 20 |
S-S-H-2H | trimer | 521 | 544 | 544 | 7 |
S-S-H-1H | trimer | 528 | 551 | 551 | 82 |
G-G-G-3H | trimer | 537 | 560 | 560 | 8 |
S-S-S + CH3O | trimer | 676 | 699 | 699 | 5 |
group OCH3 in the syringyl and guaiacyl units. The very high intensity of the signal at 75.04 ppm corresponds to the aliphatic group -CH2OH of the side chain of the lignin unit. The signal associated with the α and β-methylene group on the n-propyl side chains appears at 31.25 and 30.42 ppm [
Particleboards manufactured with the two formulations of lignin resins of Andropogon (LPA4055) and Sorghum (LPS4055) had good IB strength. Result
structure | Calculated M.W | Calculated M.W +Na | Experimental peak | Peak relative proportion (%) | |
---|---|---|---|---|---|
G -5H | Monomer | 177 | 200 | 200 | 85 |
G -4H | Monomer | 193 | 216 | 216 | 8 |
H-G + 12H | dimer | 257 | 280 | 280 | 12 |
G-H + 1H | dimer | 277 | 300 | 300 | 5 |
S-H-10H | dimer | 339 | 362 | 362 | 18 |
S-H +4H | dimer | 353 | 376 | 376 | 5 |
S-S-OH | dimer | 419 | 442 | 442 | 5 |
S-S-H-1H | trimer | 528 | 551 | 551 | 10 |
S-S-S-2H | trimer | 628 | 651 | 651 | 3 |
H-G-H-G + 11H | tetramer | 689 | 712 | 712 | 3 |
G-G-G-S + 1H | tetramer | 751 | 774 | 774 | 65 |
S-S-S-S-S-S | hexamer | 998 | 1021 | 1021 | 45 |
S-H-S-H-S-H-1H | hexamer | 1060 | 1083 | 1083 | 28 |
S-H-S-H-S-H-4H | hexamer | 1073 | 1096 | 1096 | 100 |
S-H-S-H-S-H-S-H-4H | octamer | 1336 | 1359 | 1359 | 82 |
S-S-S-S-S-S-S-S-2H | heptamer | 1468 | 1491 | 1491 | 70 |
S-H-S-H-S-H-S-H-S-H | decamer | 1600 | 1623 | 1623 | 60 |
G-S-G-S-G-S-G-S -CH3-OH | octamer | 1732 | 1755 | 1755 | 42 |
G-G-G-G-G-G-G-G-G-G + H2O | decamer | 1864 | 1887 | 1887 | 35 |
H-S-H-S-H-S-H-S-H-S-H-S-H-S-1H | pentadecamer | 2127 | 2150 | 2150 | 22 |
H-G-H-G-H-G-H-G-H-G-H-G-H-G + H2O | tetradecamer | 2391 | 2414 | 2414 | 18 |
G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G-G | octadecamer | 2655 | 2678 | 2678 | 10 |
S-S-S-S-S-S-S-S-S-S-S-S-S-S + CH3 | tetradecamer | 2918 | 2941 | 2941 | 10 |
S-H-S-H-S-H-S-H-S-H-S-H-S-H-S-H-S-H + 6H | octadecamer | 3183 | 3206 | 3206 | 8 |
G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-OH | hexadecamer | 3447 | 3470 | 3470 | 3 |
G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S + Na+ | octadecamer | 3712 | 3735 | 3735 | 5 |
satisfied the dry IB strength requirement of the standard NF EN 312-2 (1996). Dry IB strength of particle boards manufactured with particles of Andropogon gayanus and the LPA4055 formulation was 0.37 MPa (
MALDI?TOF MS and 13˚C NMR analysis were used for the characterization of lignin extracted from the stem of Sorghum bicolor and Andropogon gayanus.
Peak (ppm) LS | Intensity % | Peak (ppm) LA | Intensity % | Peak (ppm) | Assignments |
---|---|---|---|---|---|
177.22 | 2 | 176.60 | 2 | 178.0 - 167.5 | Unconjugated-CO2 H |
152.75 | 10 | 153.37 | 10 | 154 - 152 | C3/C5 S etherified |
148.23 | 15 | 148.03 | 15 | 148 - 147 | C4 G etherified, C3C5 S non etherified |
133.63 | 5 | 134.04 | 5 | 134 | C1 S or C1 in G β-O-4 |
128.08 | 3 | 128 | C2/C6 H | ||
115.54 | 3 | 114.92 | 5 | 115 | C5 G |
102.59 | 38 | 102.38 | 38 | 105 - 104 | C2/C6 S |
82.23 | 10 | 82.65 | 5 | 89 - 78 | Cβ in β-O-4, Cα in β5 and ββCα and Cβ in β-O-4/α-O-4 units |
75.04 | 95 | 75.04 | 100 | Aliphatic-CH2OH | |
73.60 | 80 | 73 - 71 | Cα in β-O-4 | ||
63.94 | 30 | 63.73 | 30 | 64 | Cγ in β5, Cγ with Cα = O, Cγ-O PC ester |
55.92 | 18 | 55.71 | 25 | 56 | OCH3 in G and S |
31.25 | 2 | 30.42 | 2 | 31 - 29 | α, β méthylène groups |
H =Hydroxyphenyl; S = Syringyl; G = Guaiacyl; LS = Sorghum lignin; LA = Andropogon lignin
The analysis carried out had shown that these lignins consist of Guaiacyl (G), Syringyl (S), p-hydroxyphenylpropane (H) units and of phenolic, flavonoid and secondary alcohols functional groups. It is mainly composed of Guaiacyl (G) and Syringyl (S) units β-O-4 linked. The presence of phenols and flavonoids in this lignin extract shows that it could be used as wood adhesives and as antioxidant. They are good for wood adhesive because IB for their particleboards is respectively 0.37 and 0.41 MPa. A study aimed at increasing the mechanical characteristics of these bio-based panels by combining for example tannin and lignin in the elaboration of bioadhesives since tannin and lignin form the essential of polyphenols present in the plant species.
The authors acknowledge Dr. FRANCISCO JOSE SANTIAGO MEDINA for his valuable support during laboratory experimentation carried out at the LERMAB, at the University of Lorraine (France), Pr NTEDE Head of the Laboratory of Chemistry of Higher National School of Engineering, University of Yaoundé 1, Dr. CHEMENI the Head of the Assistant Laboratory of Macromolecular Chemistry at the University of Yaoundé 1, Dr. Jean-Bosco Saha Tchinda, Mr. FokoStephan, Mr Agueda Fabrice and Mr. Njom Abel for their assistance.
The authors declare no conflicts of interest regarding the publication of this paper.
Karga, L.T., Raidandi, D., Konai, N., Pizzi, A. and Meva’a, L. (2019) Characteristics and Mechanicals Potentials of Wood Adhesives Manufactured with Grasses’ Lignins. Journal of Materials Science and Chemical Engineering, 7, 35-47. https://doi.org/10.4236/msce.2019.71004