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Antioxidant activity of unmodified kraft and organosolv lignins to be used as sustainable components for polyurethane coatings

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Abstract

The antioxidative capacity of four different kraft lignins (KL) and one additional organosolv lignin (OSL) was studied using the Folin–Ciocalteu (FC) assay. To do so, the corresponding FC assay procedure was adapted and optimized to be appropriate for lignin analysis. Different solvents were tested, and DMSO and saturated sodium carbonate (Na2CO3) for pH adjustment showed the best results. The absorption wavelength of 740 nm was chosen due to highest determination coefficients. Continuous calibration is recommended on a daily basis to guarantee accuracy. The antioxidant capacity and related radical scavenging activity of various lignins were correlated with the biomass nature (soft wood vs grasses) and pulping methods (kraft vs organosolv). The results show higher antioxidant activity for kraft vs organosolv lignins. First lignin-derived polyurethane coatings were prepared using the unmodified kraft lignin. The films, prepared via spin coating, show a high flexibility and transparency.

Introduction

The use of lignocellulosic feedstock (LCF) in biorefineries has been recognized as a promising approach to produce valuable products such as fuels, power, and chemicals (Fig. 1).1,2 Today, there is a rapidly increasing interest in lignin as a substitute for fossil-based phenol derivatives in polyurethanes and phenol-based resins for applications in construction3 and packaging, but also as hydrogels for biomedicine.4 In addition to the utilization of unmodified lignins, various chemical modification methods are currently under investigation including oxidative and reductive depolymerization and fragmentation methods, comprehensively reviewed by Schutyser and Laurichesse.5,6

Fig. 1
figure1

Schematic process of lignocellulosic feedstock exploitation (C: cellulose, HC: hemicellulose, L: lignin) Copyright 2018 MDPI International2

Due to their polyphenolic structure consisting of three monolignols and corresponding units (H, G, and S), lignins possess a distinct antioxidant activity (Fig. 2).

Fig. 2
figure2

Structural monolignol-derived units H, G, and S of lignin

Thus, kraft lignin from wood sources in pulp industry was reported to be as efficient as vitamin E to protect the oxidation of corn oil.7 Most antioxidant effects of lignins are considered as derived from the scavenging action of their phenolic structures on oxygen containing reactive free radicals. As their free radical scavenging ability is facilitated by their OH group, the total phenolic content could be used as a basis for rapid screening of antioxidant activity.8,9 Very recently, we reported an antioxidative capacity study using the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay showing correlations between antioxidant activity and minor structural differences of kraft lignins purified via selective extraction.10 The highest activity was found for lignin fractions with the most narrow molecular weight distribution according to SEC analysis. Antioxidant activity measured via DPPH inhibition of the unmodified kraft lignin fractions was above the values reported in the literature, including commercial BHT, confirming that technical black liquor can be used without further modification.10

Here, we present antioxidant activity studies of kraft lignins isolated at different pH values and one organosolv lignin using the Folin–Ciocalteu assay. Results are discussed regarding the biomass source (wood vs grass) and pulping process (kraft vs organosolv). Based on the studied kraft lignin, first polyurethane coatings were prepared showing high homogeneity and transparency.

Experimental

Chemicals and reagents

Black liquor obtained from kraft pulping (according to the supplier using a mixture of soft and hard wood) was obtained from Zellstoff-und Papierfabrik Rosenthal GmbH (Blankenstein, Germany, MERCER group).

Sulfuric acid was purchased from Fisher Scientific in Loughborough, UK. Ethanol absolute and sodium carbonate were obtained from VWR Chemicals, Germany. The Folin–Ciocalteu reagent, gallic acid, DMSO, sodium hydroxide, and 4,4-diphenylmethanediisocyanate (MDI) were received from Merck in Darmstadt, Germany. Methanol, tetrahydrofuran (THF), triethylamine (TEA), and potassium bromide were obtained from Carl Roth GmbH in Karlsruhe, Germany. PEG425 was obtained from Sigma-Aldrich in Steinheim, Germany. All chemicals were of reagent grade without further purification.

Lignin isolation

Black liquor and kraft lignin: The kraft lignin (KL) was extracted through the acidic precipitation from black liquor. First, about 450 mL of black liquor was filtered with a vacuum filter. The filter cake was rejected. The 400 mL filtrate received was heated to 50–60°C. While stirring, 160 mL sulfuric acid (25 vol%) was added. The solution was stirred for another hour without the addition of heat and then vacuum filtrated. The filter cake was washed with distilled water and sulfuric acid (25 vol%) until the requested pH value was reached (pH 2 to pH 5). Finally, the precipitated lignin was dried in a freeze dryer for 48 h. The four kraft lignins to be used were precipitated at pH 2, pH 3, pH 4, and pH 5. The organosolv lignin sample (OSL) was prepared to be used for comparison studies according to an earlier published procedure.11,12 Shortly, approx. 50 g Miscanthus X giganteus passing a 0.5-mm sieve and 400 mL ethanol (80% υ/υ) were mixed and heated at 170°C for 90 min under continuous stirring in a Parr reactor with a Parr 4848 reactor controller. Afterward, the Miscanthus biomass was vacuum filtrated and washed five times with 50 mL ethanol (80% υ/υ). Three volumes of water were added to the filtrate to precipitate the organosolv lignin (OSL), which was collected by centrifugation at 3500 rpm for 5 min and washed three times with dist. water. Finally, the OSL was freeze-dried for 72 h.

Lignin–polyurethane synthesis

Lignin is used to partially replace the polyol component. Lignin–polyurethane (LPU) films were prepared by using different NCO/OH ratios (1.2–2.5) according to Pan and Saddler13 Lignin-based polyurethane coatings were prepared dissolving lignin in THF and PEG425 in an ultrasonic bath followed by the addition of 4,4-diphenylmethanediisocyanate (MDI) and triethylamine (TEA). Thus, 1 g lignin was dissolved in 6 mL THF, MDI was added, and the mixture was transferred on a PE transparency and dried for 1 h at room temperature. Finally, the prefilms were cured at 35°C for 3 h to obtain the final lignin PU films.14

Analytical methods

FTIR and UV–Vis spectroscopy

FTIR spectra of lignin were recorded on a JASCO FTIR 410 spectrometer in a range of 4000–400 cm−1 using a KBr disk containing 1% finely grounded sample. The spectrum was recorded over 64 scans with a resolution of 4 cm−1. A PerkinElmer Lambda 35 UV–Vis spectrophotometer was used for recording the UV–Vis spectra. To obtain a concentration of 50 µL mL−1, 5 mg lignin was dissolved in 100 mL 0.1 mol NaOH solution.

Size exclusion chromatography

The weight average (Mw) and number average (Mn) molecular weights of the lignins as well as their polydispersity (PDI) were determined by size exclusion chromatography (PSS SECurity2 GPC System). Tetrahydrofuran (THF) was used as the mobile phase with a run time of 30 min and an injection volume of 100 µL. Polystyrene standards were used for the calibration at different molecular weights.

OH number analysis

The determination of hydroxyl number was carried out with lignins following ISO 14900:2001(E) developed for polyether polyols with steric hindrance.15 Each lignin sample was boiled under reflux in 25 mL of acetylation reagent solution with a blank sample simultaneously under the same conditions.

After 3 h at reflux, the flasks were left to cool down to room temperature. Twenty-five milliliters of sample and blank, respectively, was filled up with water to 100 mL and was titrated with sodium hydroxide (0.5 mol L−1). The split up of the acetylated samples and blanks allowed a triple determination via titration. Different amounts of sample and blank were needed. The differences were used to determine the total hydroxyl content.

Folin–Ciocalteu (FC) assay

The FC assay was used to determine the total phenolic content based on a redox reaction. In our studies, the literature procedures reported by Garcia, dos Santos, and Kiewning, respectively, were optimized and adapted to obtain a suitable test method (Method 3) for kraft and organosolv lignin, see Table 1.16,17,18 In detail, the methods differ in the amount of individual chemicals, the base used for pH adjustment, incubation temperatures, and absorption wavelengths. In all cases, gallic acid is used as reference.19

Table 1 Parameters used for the antioxidant activity measurements according to the FC assay

The measuring procedure was the following: Reagents and samples according to Table 1 were kept either in the drying oven at 40°C or at room temperature before measuring the absorbance with a UV–Vis DR6000 Hach spectrometer against a blank (the same reagents but solvent instead of sample). The results were expressed as gallic acid equivalents (GAE), calculated from a calibration curve with gallic acid in DMSO with six different concentrations. Afterward, the total phenol content (TPC) of the lignins was calculated according to equation (1):

$${\text{TPC}} (\% ) = \frac{{{\text{GAE}} \left( {{\text{mg}}\; {\text{L}}^{ - 1} } \right)}}{{c_{\text{Lignin}} \left( {{\text{mg}} \;{\text{L}}^{ - 1} } \right)}} * 100\%$$
(1)

Atomic force microscopy (AFM)

Imaging measurements were taken on the PU films by means of a scanning thermal microscopes (ThermoMicroscopes system) driven by SPM software (ThermoMicroscopes ProScan version 2.1) in tapping mode with a scan rate of 0.3 Hz and 0.5 Hz using commercially available silicon contact MPP-Rotated probes purchased from Bruker (Bruker AFM probes) Tip (radius: 8 mm, front angle: 15°, back angle 25°) conditions by room temperature in the 20–22°C range.

Transmission electron microscopy

Transmission electron microscopy (TEM) experiments were performed using a Zeiss EM 10 electron microscope (Oberkochen, Germany) operating at 60 kV with a CCD camera (Tröndle, Moorenweis, Germany) with a bright field microscope (JEOL, Tokyo, Japan). The field emission gun was operated at a nominal acceleration voltage of 200 kV.

Results and discussion

Characterization of lignins

FTIR spectra (Fig. 3) show significant differences in lignin structure for kraft (KL) vs organosolv-derived (OSL) lignins. In detail, kraft lignins display a stronger C=O stretch (1704 cm−1), while in OSL the O–CH3 stretching (2849 cm−1) is more pronounced, which supports the observation of more signals belonging to S units (1329, 1124, 833 cm−1) in the OSL (isolated from Miscanthus). KL consists mainly of G units (1271, 857, 815 cm−1), as expected for softwood.2,5,10 The assignment of the most important IR absorption bands is summarized in Table 2.

Fig. 3
figure3

Comparison of the FTIR spectra of the investigated lignins with red lines indicating the major differences

Table 2 Summary of the most important FTIR signals. Comparison of kraft lignin (KL) isolated at pH 2 and organosolv lignin (OSL)

The UV–Vis spectra of the different lignins are very similar to each other showing two absorption maxima (Fig. 4). One significant band appears at 220–230 nm representing aromatic groups in general. The second maximum occurs at 280–300 nm indicating nonconjugated phenolic fragments such as sinapyl-, coniferyl-, and p-coumaryl alcohols. Moreover, there is a shoulder between 240 and 250 nm assigned to conjugated phenolic groups. In accordance with the literature results, the UV absorption bands in accordance with FTIR and SEC results indicated a softwood lignin containing mainly G units. In the case of hardwood lignins with a higher ratio of S units, the UV–Vis absorption bands were shifted to lower wavelengths. In principle, the results confirm the literature data for kraft lignin absorption.20,21,22 Slight differences are obtained regarding the absorption curve due to different isolation procedures working in aqueous media.

Fig. 4
figure4

UV–Vis spectra of lignins precipitated at different pH values

Weight average (Mw) and number average (Mn) molecular weights and corresponding polydispersity (Mw/Mn) determined via size exclusion chromatography (SEC) as well as the OH content according to ISO 14900 are summarized in Table 3.

Table 3 Characterization of the kraft lignins: molecular weight Mw, Mn and polydispersity (PDI), and OH content according to ISO 14900

The hydroxyl numbers for the four lignins varied between 2.67 mmol g−1 (150 mg KOH g−1) for KL pH 2 and 5.35 mmol g−1 (300 mg KOH g−1). In addition, Fig. 5 shows the relationship between hydroxyl number and pH value of the isolated lignins: The higher the pH value, the higher the hydroxyl number. We observed the optimal PU crosslinking for lignin precipitated at pH 4.

Fig. 5
figure5

Dependency of the pH value (precipitated lignins) on the hydroxyl number (mmol g−1) of kraft lignin

Folin–Ciocalteu assay

According to published studies so far, the antioxidant capacity of lignins has mainly been investigated using the DPPH assay and the Folin–Ciocalteu method: The DPPH assay uses the redox reaction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with an antioxidant, resulting in reduced color intensity proportional to the antioxidant concentration.23 The Folin–Ciocalteu assay (or total phenolics assay) is used to measure the TPC of natural products. In detail, the Folin–Ciocalteu reagent (a mixture of phosphomolybdate and phosphotungstate) reacts with an antioxidant, changing the color intensity proportionally to the antioxidant concentration. Gallic acid is used as a reference compound, and results are expressed as GAE or TPC.24

Thus, Dizhbite et al. developed structure–property relationships regarding antioxidant activity, proposing that the π-conjugation systems of lignin operate as catalysts/activators in the interaction with DPPH radicals, and heterogeneity and polydispersity critically decrease the antioxidant efficiency.25 Using electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic polyconjugated clusters in lignin samples, paramagnetic polyconjugated clusters were confirmed to result in a linear increase in antioxidant capacity, whereas aromatic OH and OCH3 contents were less influential. Santos et al. studied isolation and purification effects including solvent influence, comparing water and organic solvents vs alkaline solution. They found that lignins with a low percentage of phenols showed the highest elimination of DPPH radicals.26

Here, the antioxidant activity of kraft and organosolv lignins is studied using the FC assay, following a modified procedure in order to adapt the assay (originally developed for small phenolic derivatives) to rather complex macromolecules such as kraft and organosolv lignins.17,18,19

According to solubility tests (Table 4), DMSO was chosen as suitable solvent for KL, OSL, and gallic acid. Furthermore, the absorption maxima reported in the literature were verified to find the most appropriate absorption wavelength.

Table 4 Solubility of lignins in suggested solvents

The results are shown in Table 5. For method 1, the curves are similar, but differ significantly in their slope (Fig. 6). For gallic acid, the absorption maximum is 742 nm, for KL it is between 737 and 746 nm, and the OSL has a maximum in the range of 751–768 nm. The resulting absorptions are in line with data reported for lignin studies using the FC assay.27

Table 5 Absorption maxima of lignins studied via different methods
Fig. 6
figure6

Wavelength maxima of gallic acid, KL, and OSL according to Method 1

The recorded UV–Vis spectra from Method 2 are shown in Fig. 7. Significant changes were observed compared to Method 1: The absorption maxima of both lignins were shifted to lower wavelengths (450–460 nm for KL, 460–464 nm for OSL). For both lignins, the absorption increases at higher wavelengths (> 700 nm), resulting in a greenish color. Due to the different curves of gallic acid and the lignin, this method is not suitable for measuring the absorption, as there is no consistent maximum.

Fig. 7
figure7

Wavelength maxima of gallic acid, KL, and OSL according to Method 2

Thus, Method 3 was developed based on these preliminary results. The quantities of chemicals were kept identical to Method 2, as well as the order in which they were added. Instead of 0.1 molar NaOH, a saturated solution of Na2CO3 was added. The solutions obtained after incubation at room temperature are shown in Fig. 8, in comparison with solutions with NaOH addition: It can be seen that the blank is again colorless, while both KL and gallic acid have a dark blue color when adding Na2CO3. With NaOH, the blank is yellow, while gallic acid and KL have a green-yellowish color. This yellowish color indicates that no reaction of the FC reagent took place, as a blank solution quickly fades from yellow to colorless unless the reagent is partially reduced. However, if a reaction with a reducing agent takes place, the originally yellow color turns blue.28

Fig. 8
figure8

Comparison of samples (from left to right): blank, gallic acid, and KL in NaOH (1) and Na2CO3 (2)

The recorded spectra from Method 2 and Method 3 are shown in Fig. 9. There is a certain similarity of the curves for Method 1 and Method 3: The absorption maxima are just slightly shifted (bathochromic shift). Both methods use Na2CO3 as base, but in different amounts. Since the absorption maxima are generally very broad, it can be concluded that the results are comparable despite the lower batch volume and the different proportions of the chemicals used.

Fig. 9
figure9

Comparison of the UV–Vis absorption maxima of gallic acid and kraft lignin in DMSO in Method 2 (yellow and red) vs Method 3 (gray and blue) (Color figure online)

The curves of the solutions according to Method 2 with NaOH have a different course with no maximum in the expected range. Based on these observations, Na2CO3 appears to be the better choice resulting in absorption maxima for both gallic acid and lignins.

The differences in curves and colors are due to different bases and corresponding pH. The solutions with NaOH have a pH of approx. 12.8, while solutions with Na2CO3 have a pH of approx. 10.5. The optimum pH of 10 for the reaction is only achieved by the addition of Na2CO3.29,30 Since the reaction is faster in the alkaline than in the acidic medium, it can be assumed that a reaction time of 30 min is insufficient for Method 2.19,31

To determine the TPC of lignins, six calibration levels in the range of 10–100 mg L−1 gallic acid in DMSO and one sample solution each of the lignins with a concentration of 100 mg L−1 were measured, both after incubation at room temperature and at 40°C at 740 nm, as this wavelength displays highest values for R2 for the calibration curve. All measurements were taken in triplicate. Standard deviations of the TPC values for the different lignins are summarized in Table 6 and illustrated in Fig. 10. It is noticeable that the TPCs measured after incubation at 40°C show a lower standard deviation for the majority of the lignins, compared to the measurements after incubation at room temperature. One explanation might be that the samples in the drying cabinet are exposed to more constant temperatures, while the room temperature, however, varies depending on weather and daytime, respectively. Nevertheless, it can be stated that the same tendencies can be observed both at room temperature and at 40°C: KL pH 2 has the highest TPC, followed by KL pH 5, KL pH 4, and KL pH 3. Due to the standard deviations, however, statistically significant differences cannot be established between the KLs. The OSL has by far the lowest TPC and thus the weakest antioxidant capacity.

Table 6 Mean and standard deviation of the TPC of the investigated lignins for measurements at 40°C and at room temperature
Fig. 10
figure10

Comparison of the TPC of the investigated lignins for measurement at 40°C and at room temperature

Furthermore, there is no correlation between the TPC and pH value for KL precipitation. Similar results are reported by dos Santos et al. and Faustino et al., who also could not specify any relationship between pH and TPC.17,32 Most obviously, the pH value does not affect the total phenol content or the antioxidant capacity of the lignin.

Comparing the TPCs with the literature values of dos Santos et al., whose original method is modified in this work, slight differences could be detected.17 In detail, the authors precipitated KL from black liquor by means of H2SO4 at pH 2, pH 4, and pH 6. TPC is (29.61 ± 1.61)% for KL pH 2 and (19.95 ± 0.74)% for KL pH 4, compared to (33.5 ± 1.9)% for KL pH 2 and (30.4 ± 2.6)% for KL pH 4. In both cases, the TPC of KL pH 2 is higher than the TPC of KL pH 4, but the difference in TPCs determined here is significantly lower. These differences might be caused by the raw materials used, as our biomass is a mixture of hard and soft wood due to the supplier, but also small differences in the black liquor treatment and further lignin processing steps influence the lignin purity and thus the TPC.

According to García et al., lignins precipitated under acidic conditions should have a higher percentage of total phenols, as they contain fewer impurities and a higher proportion of phenolic components resulting from cleavage of β–O–4 bonds during the kraft process.16 However, the delignification via the kraft process also causes the depolymerization of polysaccharides, which can lead to interferences by also reducing the FC reagent. Therefore, results should only be considered as relative values in order to compare the various lignins to each other.

Lignin–polyurethane coatings

Finally, first lignin-derived polyurethane coatings were successfully produced in an efficient one-step synthesis using unmodified KL as environmentally benign component in contents up to 80 wt% (Fig. 11).

Fig. 11
figure11

Schematic urethane linkage formation for the reaction of 4,4-diphenylmethanediisocyanate (MDI) and lignin (with R = monolignol functionalities)

Transparent homogeneous films of high flexibility resulted from lignins isolated at pH 4, possessing a temperature resistance up to 160°C. Swelling tests revealed a resistance against water and THF. Swelling in DMSO depends on index, pH of precipitation, and catalyst utilization for PU preparation.14 According to AFM studies, surface roughness is between 10 and 28 nm (Fig. 12).

Fig. 12
figure12

Left: AFM figure of the coating surface. Right: flexible LPU coating; due to scanning electron microscopy, the thickness of spin-coated films was determined to range between 150 and 160 µm14

Lignin-derived polyurethane coatings have been prepared using different material surfaces: metal, wood, and glass (Fig. 13). All samples have been prepared using lignin isolated at pH 4 (see Table 3). In addition, the LPU coatings were placed onto a structured surface (Fig. 14). Transmission electron microscopy confirmed the homogeneity of the lignin-derived films. In ongoing studies, the antioxidant and antimicrobial capacity of lignin-derived polyurethanes is investigated.

Fig. 13
figure13

Lignin-derived polyurethane coatings on different surfaces: metal, wood, and glass (from left to right). Samples prepared with lignin isolated at pH 4 (see Table 3; NCO/OH 1.7 using Desmodur® and 20 wt% Lupranol®)

Fig. 14
figure14

TEM figure of a LPU coating prepared on a structured surface (bar 2500 nm)

Conclusions

The structure of four kraft lignins (KL), isolated at different pH values, and one organosolv lignin (OSL) was analyzed and compared using FTIR und UV–Vis spectroscopy. Molar masses and polydispersities differ depending on biomass (wood vs grass) and pulping process (kraft vs organosolv). The total phenolic content was determined using an optimized Folin–Ciocalteu assay. The results reveal higher TPC values for kraft lignin compared to organosolv lignin supporting recently published results for lignins purified via selective solvent extraction. Additional studies using alternative test methods (i.e., DPPH) are ongoing for confirmation and verification of the obtained results. Using the kraft lignin, first polyurethane coatings of high transparency and flexibility were prepared. In contrast to published studies, all KLs were isolated at room temperature from aqueous solution. The lignin content in the LPU coatings could be increased up to 80 wt%. Ongoing studies focus the antioxidant behavior and mechanical properties of the LPU coatings.

Abbreviations

AFM:

Atomic force microscopy

BHT:

Butyl hydroxy toluene

DPPH:

2,2-Diphenyl-1-picrylhydrazyl

FC:

Folin–Ciocalteu

GAE:

Gallic acid equivalent

KL:

Kraft lignin

LCF:

Lignocellulose feedstock

MDI:

4,4-Diphenylmethanediisocyanate

OSL:

Organosolv lignin

PEG:

Polyethylene glycol

PU:

Polyurethane

SEC:

Size exclusion chromatography

TEA:

Triethylamine

TEM:

Transmission electron microscopy

TPC:

Total phenol content

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Acknowledgments

This research was supported by the Federal Ministry of Education and Research (BMBF) program “IngenieurNachwuchs” project LignoBau (03FH013IX4). The authors gratefully acknowledge Zellstoff- und Papierfabrik Rosenthal GmbH (Blankenstein, Germany, MERCER group) for providing the black liquor. We gratefully thank Manuela Schebera, Bonn-Rhein-Sieg University of Applied Sciences, performing the AFM measurements, and Christian Rüttiger, Technical University of Darmstadt for TEM studies. Erasmus-Mundus Avempace-II and Bonn-Rhein-Sieg University/Graduate Institute for scholar ship (Abla Alzagameem).

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Klein, S.E., Rumpf, J., Alzagameem, A. et al. Antioxidant activity of unmodified kraft and organosolv lignins to be used as sustainable components for polyurethane coatings. J Coat Technol Res 16, 1543–1552 (2019) doi:10.1007/s11998-019-00201-w

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Keywords

  • Antioxidant activity
  • Biomass
  • Folin–Ciocalteu assay
  • Kraft lignin
  • Lignocellulose feedstock
  • Organosolv lignin
  • Total phenol content