Lignin-based composites with enhanced mechanical properties by acetone fractionation and epoxidation modification

Summary

Epoxy resin is widely used in various fields of the national economy due to its excellent chemical and mechanical properties. Lignin is mainly derived from lignocelluloses as one of the most abundant renewable bioresources. Due to the diversity of lignin sources and the complexity as well as heterogeneity of its structure, the value of lignin has not been fully realized. Herein, we report the utilization of industrial alkali lignin for the preparation of low-carbon and environmentally friendly bio-based epoxy thermosetting materials. Specifically, epoxidized lignin with substituted petroleum-based chemical bisphenol A diglycidyl ether (BADGE) in various proportions was cross-linked to fabricate thermosetting epoxies. The cured thermosetting resin revealed enhanced tensile strength (4.6 MPa) and elongation (315.5%) in comparison with the common BADGE polymers. Overall, this work provides a practicable approach for lignin valorization toward tailored sustainable bioplastics in the context of a circular bioeconomy.

Graphical abstract

Highlights

• •Acetone-soluble lignin revealed excellent homogeneity and low molecular weight
• •Lignin uniformity enhanced the mechanical properties of bio-based epoxies
• •The fabrication of lignin epoxy includes epoxidation modification and cure process
• •Bio-based epoxy resins from lignin display high potential to replace bisphenol A

Chemistry; Chemical engineering; Biotechnology; Biomass

Introduction

Nowadays, plastic provides great convenience and fast services for daily life while consuming large amounts of petrochemical resources.^1,2,3 To ensure the sustainable utilization of the resources, the development of renewable materials has attracted increasing interest. Lignin, the most abundant resource with renewable aromatic structures, is a typical heterogeneous biopolymer with a complex chemical structure and broad molecular weight distribution.^4,5,6 The random polymerization of three monomers during biosynthesis is the main reason for the heterogeneity of lignin.^7,8 Plants on Earth synthesize 150 billion tons of lignin each year. As a result, lignin is deservedly the most abundant renewable bioresources. Among them, the pulp and paper industry produced about 150–180 million tons of industrial lignin annually, but only 2% are commercially used,^9,10 such as energy,¹¹ chemicals,^12,13,14 polymers,¹⁵ and carbon materials.¹⁶ There are mainly two ways for lignin valorization. One strategy was to deconstruct lignin into phenolic compounds with low molecular weights by the strategies of catalytic and pyrolytic approaches.^{17,18,19,20,21} Another alternative route is the crosslinking or blending of lignin with other monomers or polymers to produce thermoset resins,^22,23 adhesives,^24,25 foams,²⁶ and thermoplastics.^27,28

Currently, lignin has been developed to participate in the manufacture of various materials, such as composites,^29,30 hydrogels,^31,32 thermosets, and thermoplastic material.³³ Among them, the preparation of thermoset materials is one of the most promising uses. Epoxy resins are used in various industries with excellent chemical and mechanical properties.^34,35,36 The significance of exploring bio-based polymers is to replace existing petroleum-based polymers, more importantly, to provide significant performance advantages over existing products. According to previous reports, lignin-based thermosetting materials include phenol/formaldehyde resins,^37,38 polyurethanes,³⁹ epoxy,^22,40 and thiol-vinyl resins.⁴¹ Epoxy resins are primarily made from petrochemicals and used in adhesives, coatings, and composite materials due to their versatile features.⁴² Diglycidyl ether bisphenol A (DGEBA) is a commonly used epoxy resin, which is cured under different conditions by adding hardeners to form epoxy resins.⁴³ Bisphenol A, the main raw material of DGEBA epoxy resin,⁴⁴ has been reported to cause endocrine disorders that threaten the health of fetuses and children. Obesity due to metabolic disorders and cancer are also thought to be linked.⁴⁵ Therefore, it is urgent to use renewable aromatic sources to replace BADGE in the preparation of epoxy resins. Industrial lignin, with high aromatic content and low molecular weight dispersion index, is suitable for replacing BADGE in epoxy resin. There are three means to mix lignin with epoxy resins: 1) mixing with petroleum-based epoxy resin,^46,47 2) epoxidation of lignin,^48,49 and 3) epoxidation of lignin after modification.^{22,48,50,51,52,53} In order to prevent bisphenol A from affecting humans and the environment, it is essential not only to ban its use in materials, such as food packaging,^43,54 but also to find alternative, renewable, and sustainable raw materials to replace BADGE in epoxy formulations. Replacing BADGE with lignin still remains challenges due to its high molecular weight, different types of hydroxyl groups, and low solubility in organic solvents and water.²²

Herein, we report a route to fabricate lignin-based thermosetting epoxy resins with excellent mechanical strength by solvent fractionation and lignin modification. Firstly, industrial lignin was purified by acetone separation, and then ethylene oxide structure was introduced into the lignin framework to improve the reactive sites of lignin. Subsequently, the obtained lignin fraction was applied as a substitute for petroleum-based chemical BADGE to fabricate thermosetting resin material. Notably, the acetone-soluble lignin sample revealed better processability as compared to the pristine lignin due to its excellent homogeneity and low molecular weight. Accordingly, the objective of this work was to address the petroleum consumption and environmental issues by replacing the petroleum-based component of epoxy resins with lignin.

Results

Molecular weight

In this work, a way for fabrication of lignin-based epoxy resin through fractionation and modification has been developed to adjust the properties of epoxy resin as depicted in Figure 1. Notably, the lignin sample dissolved in acetone revealed a lighter yellow color whereas the lignin-insoluble fraction in acetone displayed a dark brown color (Figure S1). Clearly, the results in Table 1 showed that F_ASL part has lower average molecular weight (M_w, 1290 g/mol) polydispersity (PDI, 1.7) than those of the F_AIL component (M_w, 1450 g/mol; PDI, 1.8). Moreover, the aggregation of lignin macromolecules occurred during the fractionation processing, which led to the lignin fraction with relatively higher M_w than that of the pristine one.⁵⁵ In addition, the F_AIL curve shifted to the left of the retention time axis with a wider distribution, while the F_ASL curve revealed a narrower distribution similarly to that of F_IL (Figure S2). Consequently, these results disclosed that acetone was an effective solvent for lignin fractionation with a more homogeneous structure and reduced heterogeneity.

Figure 1.

Schematic diagram of epoxidation modification of lignin

Table 1.

Lignin yield, ethylene oxide content, molecular weight, and thermal analysis

Samples	Yield (%)	Ethylene oxide content (mmol−1)	Molecular weight			Thermal analysis
			Mw (g/mol)	Mn (g/mol)	PDI (Mw/Mn)	T5% (oC)	Tmax (oC)	Char900 (%)
FIL	–	–	1280	740	1.7	206.4	342.1	37.1
FASL	48.7	–	1290	780	1.7	230.9	355.7	32.2
FAIL	49.4	–	1450	815	1.8	208.6	359.4	23.8
FILM	86.2	1.1	2690	1375	2.0	275.5	352.9	28.4
FASLM	84.7	2.0	3080	1450	2.1	287.4	380.3	34.7
FAILM	83.0	1.5	3010	1450	2.1	285.0	377.9	33.6

The modification reaction resulted in significantly increased molecular weights and polydispersity of lignin (as shown in Table 1), where M_w nearly doubled and PDI increased by 0.2–0.4, implying side chains of different lengths were grafted onto the lignin. The polydispersity of all fractions was in the range of 1.7–2.1, suitable for the preparation of thermosetting resins.⁴¹ Meanwhile, F_ASL and F_AIL accounted for 48.7% and 49.4% of F_IL. In the modification reaction, the yields of modified lignin were F_ILM: 86.2%, F_ASLM: 84.7%, and F_AILM: 83.0%, and their higher yields provided the possibility to realize the industrialization of this route. As combined with its high yield and increased molecular weight as well as polydispersity, it laid the foundation for the subsequent preparation of thermosetting epoxy resins.

FT-IR analysis

The Fourier transform infrared spectroscopy (FT-IR) spectra of the fractionated lignin samples were depicted in Figure S3. In particular, the three characteristic peaks at 1613, 1516, and 1426 cm⁻¹ were arising from the lignin aromatic ring skeleton vibrations. Overall, the spectra of all lignin samples showed the vibrations characteristic for the guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units (1612, 1516, and 1426 cm⁻¹), and the intensities of the bands varied slightly between them. Moreover, the FT-IR spectra of the epoxidized lignins were shown in Figure S4. Notably, the bands at 907 and 755 cm⁻¹ were observed to be increased in after the epoxidation modification, indicating the successful grafting of propylene oxide onto the lignin backbone.

NMR analysis

To characterize the detailed lignin structure for the next step in preparing polymer materials, the NMR analysis was exposed as shown in Figure 2. The spectrum contains two regions: aliphatic C–O (δ_C/δ_H 35-90/2.0–5.5) and aromatic (δ_C/δ_H 100–150/6.0–8.0) area. The methoxy signal (δ_C/δ_H 56.3/3.75) is the most obvious signal in this area.⁵⁶ G, S, and H units were clearly observed in the 2D HSQC NMR spectra of all samples along with a significant amount of pCA and FA components. Accordingly, it is indicated that the raw material belongs to Gramineae lignin, which is consistent with the FTIR analysis. Clearly, two typical signals including β-aryl ether (A, β-O-4, stained navy blue) and phenyl-coumaran (B, β-5, stained dark green) were easily identified. Moreover, the semi-quantitative NMR results revealed that the linkage of β-O-4′ in F_ASL was 10.6%, which was significantly lower than that of F_AIL (20.6%).

Figure 2.

The sidechain (δ_C-δ_H 35–90/2.0–5.5 ppm, F_IL, F_ASL, and F_AIL) and aromatic (δ_C-δ_H 100–150/6.0–8.0 ppm, F_IL, F_ASL, and F_AIL) regions of the 2D HSQC NMR spectra

Main structures present in the lignin samples: (A) β-O-4′ alkyl-aryl ethers; (B) plenylcoumaran formed by β-5′ coupling; (G) guaiacyl units; (S) syringyl units; (H) p-hydroxyphenyl units; (pCA) p-coumarates; and (FA) ferulates.

In the aromatic region of the 2D HSQC NMR spectrum (Figure 2D), it was found that F_IL contained SGH units with remarkable amounts of hydroxycinnamic acids (pCA and FA), which were typical characteristics of herbaceous lignins. Notably, F_ASL revealed a significantly lower S/G ratio (1.0, Figure 2E) than that of F_AIL (1.7, Figure 2F), which was in good accordance with the trend of M_w as shown in Table 1. Accordingly, this result demonstrated that the acetone separation of industrial lignin successfully led to the soluble fraction with more homogeneity.

To improve the activity of lignin and prevent the degradation of the main chain structure and the aggregation of lignin macromolecules, the lignin samples were grafted and modified. Epichlorohydrin reacts with three kinds of lignin fractions (F_IL, F_ILM, and F_ASL) to introduce epoxy groups. The modification method was performed according to the optimized parameters in a previous study.⁵⁵ As shown in Figure 3, the 2D HSQC NMR analysis of F_IL, F_ASL, and F_AIL identified peaks are assigned to the introduced functional oxirane moieties (marked with red), indicating the three carbons of the epoxy group after modification. However, there was no significant change in the interunit connections during epoxidation, which confirmed that the lignin skeleton remained intact. The signals of the introduced epoxy group were also used for quantification with ¹H NMR (Figures S8−S10) using p-nitrobenzaldehyde as the internal standard where the C/H correlation peak was at 43.½.8 ppm. As shown in Table 1, the content of ethylene oxide was positively correlated with the increase in molecular weight of modified lignin samples. The F_ASL possessed the highest amount of oxirane with 2.0 mmol per gram of lignin. Therefore, we speculate that the resin material made from F_ASL exhibits good tensile strength and elongation at break in terms of mechanical properties. ¹H NMR and 2D HSQC NMR results confirmed the success of lignin epoxidation modification.

Figure 3.

2D HSQC NMR of the sidechain region (δ_C/δ_H 35–90/2.0–5.5 ppm) before and after lignin epoxidation modification

2D HSQC NMR of the original lignin fractions of F_IL (A), F_ASL (C), F_AIL (E), and the corresponding epoxy-lignin samples of F_ILM (B), F_ASLM (D), and F_AILM (F).

TGA analysis

The thermal decomposition properties of the three types of unmodified lignin were determined by thermogravimetric analysis (TGA) (Figures S5 and S6) and the results are summarized in Table 1. Clearly, when the temperature was lower than 200°C, the loss of lignin quality was slow. These three kinds of unmodified lignin samples mainly began to degrade rapidly at 200°C, and the degradation rate tended to be flat after the temperature reached 600°C. The reason could be described that the scission of the lignin side chains caused the release of some gaseous products. The internal bonds of lignin macromolecules gradually broke, and the residue reacted to become coke as the temperature rose (Table S1). In contrast, F_ASL displayed higher thermal stability under N₂ (T_5% = 230.9°C, the temperature at 5% reduction in polymer weight) than those of F_IL and F_AIL (206.4°C and 208.6°C). The residual carbon rates of F_IL, F_ASL, and F_AIL were 37.1%, 32.2%, and 23.8%, respectively (Table 1).

The three types of modified lignin (F_ILM, F_ASLM, and F_AILM) firstly degraded slowly at 25°C～300°C and then rapidly at 300°C～500°C. When the temperature reached 500°C, the mass loss became slow again. From the thermogravimetric curves, it was found that the degradation temperature of lignin after the modification increased from 200°C～230°C to about 270°C～290^oC (T_5% in Table 1). Intriguingly, F_ILM, F_ASLM, and F_AILM fractions with higher molecular weight were more thermally stable than F_IL, F_ASL, and F_AIL fractions. This result indicated that the lignin modified by epoxidation showed better thermal stability and was more conducive to the preparation of epoxy resin.

Cured epoxy resins

Lignin and commercial bisphenol A were mixed in different proportions, and then polyetheramine J400 was added as a curing crosslinking agent. Finally, acetonitrile was added as a diluent to obtain a uniform viscous liquid. The fully dissolved solution was cast in a Teflon mold, and the casting process should be carried out slowly to prevent bubbles from affecting the material properties. In order to maintain each NH₂ group contains two oxirane moieties, the amount of diamine needs to be adjusted according to the stoichiometric ratio between epoxy functional groups and amine groups. The manufactured thermosetting epoxy resin revealed dumbbell-shaped patterns as shown in Figure 1. Notably, the lignin content of 5%～20% could be cured into resin material at 60°C, whereas higher lignin content needed to be cured at 140°C. Polyetheramine 400 was chosen as the crosslinking agent because it was relatively non-toxic, colorless, and most importantly, has excellent toughness when fully cured.

SEM analysis of epoxy resin

The fractures of thermosetting epoxy resin with different lignin contents were analyzed by SEM (Figure 4). It was found that epoxy resins with higher lignin contents would automatically burst in an ultra-low temperature environment, while those with lower lignin contents required external force to achieve a brittle fracture. As can be seen from Figure 4, the fracture surface of BADGE was relatively smooth and bright. After the addition of F_IL component, it was observed that there were small granular substances, and the cracks were significantly darkened. This was due to the heterogeneity of lignin and its own dark brown color. However, after adding the modified F_IL component, namely the F_ILM component, the fracture surface was more uniform and the granular materials disappeared. This phenomenon indicated that the modified lignin was more suitable for the preparation of epoxy resin. Moreover, it was observed that the surface of F_ASLM revealed a uniform and compact texture, indicating that the material was dense. However, lignin and bisphenol A were miscible well and no phase separation occurred. It was presumed that the resin material was more elastic according to the deeper cracks of F_ASLM. As compared with F_ASLM-5%, the particle size of F_ASLM-10% was obvious, which affected the mechanical strengths of the material to a certain extent. Altogether, the decreased resin properties could be ascribed to the formation of lignin agglomerates, and F_ASLM-5% was the best raw material for the preparation of epoxy resins.

Figure 4.

SEM micrographs of thermosetting epoxy resin (BADGE, F_AIL-5%, F_IL-5%, F_ILM-5%, F_ASLM-10%, and F_ASLM-5%)

Thermal properties of epoxy resin

The thermal stabilities of thermosetting epoxy resins with different lignin contents were evaluated by TGA analysis. In the tensile test, the material of F_ASLM lignin-based resin showed better stretching effect. Under nitrogen atmosphere, all the epoxy resins were degraded in a continuous single step process. It was obverted that the thermosetting epoxy resins with 20% lignin content were thermally stable at 289°C (the temperature at which the weight of the resin was reduced by 5%), but the degradation temperatures were stable up to 315°C when the lignin content was reduced to 5%. We speculate that the lower the lignin content, the better the thermal stability of the thermosetting epoxy resin. The fastest degradation rates for resins with 5%, 10%, 15%, and 20% lignin contents were 1.9%/^oC, 1.7%/^oC, 1.6%/^oC, and 1.5%/^oC, respectively (Figure 5B). Accordingly, it can be concluded that the decrease in the maximum degradation rate was related to the addition of lignin. As compared with the initial weight, the weight loss rates of F_ASLM-5%, F_ASLM-10%, F_ASLM-15%, and _FASLM-20% were 96.0%, 92.4%, 86.9%, and 83.3%, respectively. On this account, the stability of resins was improved whereby the addition of lignin.

Figure 5.

TGA of resins with different lignin contents

The TGA (A) and DTG (B) spectra of BADGE, F_ASLM-5%, F_ASLM-10%, F_ASLM-15%, and F_ASLM-20%.

Mechanical properties

The mechanical properties of six thermosetting resin materials were tested by tensile test. Figures S11−S16 showed the relationships between tensile strength and extension at the break of different samples. To form anticipated epoxy resin after curing, the lignin content was limited. When the modified lignin was used to prepare the resin, its content could reach up to 40%, which significantly differed from the pre-modified lignin (the maximum content is only 25%). The tensile test results of the three samples demonstrated that the epoxy resin based on F_ASLM had better performance than the others (Figure 6C). As compared with pure BADGE, the elongation at break and tensile strength of F_ASLM-5% revealed improvement of about 95.8% and 10.3%, respectively (Table 2). After increasing the proportion of lignin in F_ASLM to 10%, the elongation at break and tensile strength increased by 58.5% and 29.4%, respectively. Among the other F_ILM and F_AILM samples, F_ILM-5% and F_AILM-5% displayed enhanced performance in terms of elongation at break and tensile strength in comparison with pure BADGE. Nonetheless, when the lignin content was further increased to 10%, a significant decrease in their performance was observed.

Figure 6.

Preparation principle and tensile properties of epoxy resin

(A) Schematic diagram of the fabrication of thermosetting epoxy resin by mixing unmodified lignin and epoxidized lignin with bisphenol A using polyetheramine 400 as crosslinking agent.

(B) Comparison of tensile strength and extension at break of epoxy resins with those of state-of-the-art bio-based composite.

(C) Typical tensile stress-strain curves of BADGE, F_ILM-5%, F_ILM-10%, F_ASLM-5%, F_ASLM-10%, F_AILM-5%, and F_AILM-10%.

(D) Tensile tested epoxy material. (i) The resin is fixed on the universal tension machine and starts to be stretched. (ii) The resin is stretched to lengthen. (iii) Dumbbell shape of unstretched resin after curing. (iv) Resin quickly returns to its original length after being pulled off.

Table 2.

Comparison of tensile properties of neat BADGE and BADGE/lignin composites

Sample	Tensile strength (N/m)	Tensile stress (MPa)	Tensile strain (%)	Young’s modulus (MPa)	Rate of change (%)a
					σ	ε
BADGE	3528.70	4.19 ± 0.12	161.16 ± 5.63	3.47	–	–
FILM-5%	5715.55	5.75 ± 0.09	259.08 ± 5.81	3.26	+37.23	+60.82
FILM-10%	3495.04	3.22 ± 0.13	186.81 ± 1.24	2.14	−23.15	+15.96
FASLM-5%	4633.36	4.62 ± 0.15	315.52 ± 6.40	3.46	+10.26	+95.83
FASLM-10%	5444.70	5.42 ± 0.34	255.32 ± 3.16	3.29	+29.36	+58.49
FAILM-5%	6258..84	6.31 ± 0.28	156.05 ± 5.63	5.52	+50.60	−3.14
FAILM-10%	2616.71	2.65 ± 0.21	150.36 ± 6.12	2.62	−36.75	−6.67

^aCompared with entry 1.

Notably, it was observed that the elongation or tensile strain increased significantly but the tensile strength was not (Figure 6C). The reasons could be explained that the incorporation of appropriate rigid lignin segment into the epoxy networks led to the alteration of crosslink structure,⁵⁷ thus improving the interfacial compatibility between lignin and BADGE. However, a higher addition of lignin could result in agglomeration as displayed in the SEM images in Figure 4, and then decrease the compatibility and weaken the interactions of lignin and BADGE.⁵⁸

In the present work, the most prominent effect of adding F_ASLM lignin is to make the epoxy resin flexible, while F_AILM increased the rigidity of the material. The mechanical properties of thermosetting resin materials prepared by addition of the modified lignin were also improved. All polymer materials exhibited high elongation at break and excellent ductility due to the inclusion of flexible chain amine curing agents. The addition of large amounts of lignin produced internal agglomerates, and lignin agglomerates led to strong stress concentration effects and phase separation between lignin and BADGE, which were the main reasons for the degradation of mechanical properties.

The intermolecular connection mechanism of BADGE and lignin complexed polymers is shown in Figure 6A. As compared with unmodified lignin, the epoxidized lignin was subtly involved in further enhancing the mechanical properties of lignin-based composites. Moreover, the strain at break and tensile strength of the BADGE/lignin thermosetting epoxy resin prepared in this study was compared with the data reported in literature.^{55,59,60,61,62,63,64} Gratifyingly, the prepared thermoset epoxy material of this work revealed excellent elongation at break even though without outstanding properties in terms of tensile strength (Figure 6B). Furthermore, it was found that the thermosetting epoxy resin quickly returned to its original length when pulled off, and the shape before and after stretching was depicted in Figure 6D. In short, there are two main reasons for the addition of lignin in this work to obtain epoxy resins with more desirable tensile properties than that of pure BADGE. Firstly, the acetone fractionation yielded a more homogeneous fraction, and then the modification treatment introduced epoxy groups to improve the active site of the lignin. The reduction of lignin aggregation during this process helped to maintain the excellent properties of the material.

Discussion

In conclusion, this work prepared a lignin-based thermosetting epoxy resin with better mechanical strength than commercial BADGE. First, the molecular weight, polydispersity, and structural heterogeneity of lignin were significantly reduced by acetone fractionation. Subsequently, a large amount of propylene oxide structure was introduced into the lignin framework to improve the reactivity. Finally, lignin was used as a substitute for petroleum-based chemical BADGE to prepare epoxy thermosetting resin materials. This polymer material was fabricated by mixing lignin and BADGE in different proportions and crosslinking as well as curing with a flexible polyether diamine (M_w 400 g/mol). It was found that the elongation at break and tensile strength of lignin-based epoxy resin were improved by 95.8% and 50.6% as compared with commercial bisphenol A with the addition of 5% lignin content. This work bridges lignin and traditional thermosetting resins, affording a sustainable strategy that prepares excellent epoxy resins benefit from the advantages of the addition of lignin, and will vastly broaden the application of lignin-based epoxy resin.

Limitations of the study

The main content of our work is the fabrication of thermosetting epoxy resin from industrial lignin. However, the amount of lignin substitute for bisphenol A diglycidyl ether is limited. We should further improve the interface compatibility of lignin and increase the amount of lignin to reduce the cost. Such tasks are currently ongoing in our laboratory.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Bisphenol A diglycidyl ether	Macklin Biochemical	CAS: 1675-54-3
Epichlorohydrin	Macklin Biochemical	CAS: 106-89-8
Polyetheramine D400	Macklin Biochemical	CAS: 9046-10-0
Potassium bromide	Macklin Biochemical	CAS: 7758-02-3
Deuterated dimethyl sulfoxide	Macklin Biochemical	CAS: 2206-27-1
Tetrahydrofuran	Macklin Biochemical	CAS: 109-99-9
p-nitrobenzaldehyde	Macklin Biochemical	CAS: 555-16-8
Sodium hydroxide	Macklin Biochemical	CAS: 1310-73-2
Acetonitrile	Macklin Biochemical	CAS: 75-05-8
Acetone	Macklin Biochemical	CAS: 67-64-1
Software and algorithms
Orignin	OriginLab	Orignin 9.0
Chemdraw	ChemBioOffice	Chemdraw 18.0
Adobe Illustrator	Adobe	Adobe Illustrator CC 2019
TopSpin	Bruker	TopSpin 4.0
TGA Analyzer	TA Instruments	Q500
DSC Analyzer	TA Instruments	Q250
Gel Permeation Chromatography	Waters	Agilent 1260
Fourier Transform Infrared Spectroscopy	Perkinelmer	GX
Scanning Electron Microscope	Jeol	JSM-7800F
Tensile Testing	Instron	Instron-5965
Nuclear Magnetic Resonance	Bruker	Ascend 400 MHz

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Ling-Ping Xiao (lpxiao@dlpu.edu.cn).

Materials availability

All materials generated in this study are available in the article and supplemental information or from the lead contact without restriction upon reasonable request.

Method details

Fractionation of lignin

The pristine industrial lignin (denoted as F_IL) was isolated with acetone to obtain two different fractions. 5.0 g lignin powder was firstly dissolved in 50 mL of acetone at 25°C for 24 h. Subsequently, the acetone-soluble and acetone-insoluble fractions were easily obtained by centrifugation, which were defined as F_ASL and F_AIL, respectively. Notably, to eliminate the solvent residue, the F_AIL lignin was dried in a vacuum drying oven at 40°C for 24 h followed by grinding with a pestle and mortar. Acetone was removed by evaporation under reduced pressure to collect the acetone dissolved fraction (F_ASL). After dropping the viscous liquid into the acidic aqueous solution with pH 2, the insoluble residue was obtained by centrifugation and washed four times with acidified water (pH 2) and then freeze-dried for further processing.

Epoxidation modification of lignin fractions

Lignin was modified by epoxidation according to the following method: 6.0 g lignin samples (F_IL, F_ASL, and F_AIL) were firstly sufficiently dissolved in 900 mL of acetone-water (1:1, v/v). Then 3.7 g of sodium hydroxide and 56.3 g of epichlorohydrin were added into the solution with a mechanical stirring of 800 rpm at 55°C for 5 h. Subsequently, 800 mL of deionized water was added to quench the reaction and the pH of this solution was adjusted to pH 3.5 with 6 M HCl. The resulting precipitate of modified lignin samples were obtained by centrifugation, and lyophilization. The final three kinds of lignin fractions were named as F_ILM, F_ASLM, and F_AILM, respectively.

Preparation of thermosetting epoxy resin

The thermosetting epoxy resin was fabricated with lignin and BADGE in different proportions. According to the different lignin contents, the corresponding quality of BADGE was calculated. The JD400 was added as the same quality as that of BADGE. Moreover, acetonitrile accounted for a quarter of the total mass of the system. The mixture was stirred at room temperature to obtain a homogeneous viscous liquid. After that, this mixture was introduced into the GB/528-2009 standard PTFE mold and placed in an oven at 60°C for 2 h to remove the solvent. Finally, the resin material was obtained by continuous curing at 60°C or 140°C for 4 h. Notably, the thermosetting epoxies with different contents of modified lignins were named as F_ILM-5%, F_ILM-10%, F_ASLM-5%, F_ASLM-10%, F_AILM-5%, and F_AILM-10%.

Characterizations

Molecular weight determination by GPC

The molecular weight distributions of the samples were determined using a Waters system equipped with an autosampler, a Waters 2487 Dual absorbance UV detector, and a Waters 1515 Iso-strength pump, and tetrahydrofuran (THF) as the mobile phase (1 mL min^-1) at 30°C. 2 mg lignin samples were dissolved into 1 mL THF. After filtering the sample with a 0.45 μm PTFE filter, it was injected into the GPC system and detected with a UV detector set at 280 nm. Seven standard polystyrene samples which purchased from Agilent were used for establishing the calibration curve.

Fourier transform infrared spectroscopy (FT-IR)

FT-IR spectra of lignin were recorded using a Perkin-Elmer spectrophotometer. The spectra of powdered lignin supported by KBr pellets were recorded in the range of 400–4000 cm⁻¹ with 32 scans averaged at 4.0 cm⁻¹ resolution at room temperature.

Thermal analysis by TGA

The thermal stability of lignin samples and mass loss of epoxy polymers were determined by a Q500 thermogravimetric analyzer (TA Instruments). Approximately 10.0 mg sample in a small crucible were performed under a nitrogen atmosphere with temperature ranging from 25 to 800°C at a heating rate of 10°C/min. The statistic heat-resistant index temperature (Ts) was calculated according to the following equation:

Ts = 0.49[T5% + 0.6 (T30% - T5%)]

where T5% and T30% was the temperature of weight loss of 5% and 30%, respectively.

Glass transition temperature (Tg) determination by DSC

Differential scanning calorimetry (DSC) was performed using a TA Q250 (TA Corporation, US). 5 mg dried lignin powders were sealed into the aluminum pan. The heating procedure was a three-cycle process: ramp 10°C/min to 120°C and isothermal hold for 2 min as the first cycle, ramp 10°C/min to 300°C as the second cycle, and then ramp up to 220°C as the third cycle. The glass transition temperature (T_g) of each lignin fraction was analyzed based on heat flow as a function of temperature in the third cycle (Figure S7).

Chemical structure determination by ¹H NMR

30 mg of epoxidized lignin and 5.0 mg of internal standard (p-nitrobenzaldehyde) were dissolved in 0.5 ml of DMSO-d₆ and proton nuclear magnetic resonance (¹H NMR) spectra were recorded at 25°C on a Bruker Ascend-400 MHz NMR spectrometer.

Linkage and subunit analyses by 2D HSQC NMR

The linkages and subunits in lignin were analyzed using the 2D HSQC NMR spectrum, which was obtained from lignin/DMSO-d₆ solution using a Bruker Ascend-400 Hz spectrometer (Bruker, Germany) at 25°C. The test used the q-hsqcetgp pulse program (n_s = 32, d_s = 16, increments = 256, d₁ = 1.0 s) and used 5 mm reverse gradient ¹H/¹³C cryoprobe. The obtained spectrum was processed using TopSpin 4.0 software.

The lignin samples (50 mg) were added to 0.5 mL DMSO-d₆ and sonicated until homogenous. The semi-quantitative analysis method was applied to calculate the contents of lignin linkages (β-O-4, β-5 and so on) and the ratio of S and G units. The detailed procedures are shown as follows:

I(C₉) units = 0.5I(S_2,6) + I(G₂) + 0.5I(H_2,6)

where IS_2,6 is the integration of S_2,6, including S and S'. IG₂ is the integral value of G₂. IH₂ is the integral value of H_2,6. IC₉ represents the integral value of the aromatic ring. According to the internal standard (IC₉), the amount of I_X% could be obtained by the following formula:

I_X% = I_X/IC₉ × 100%

where I_X is the integral value of the α-position of A (β-O-4), B (β-5) and), the integration should be in the same contour level. In the aromatic region, S/G ratio was calculated according to the following formula:

S/G = 0.5I(S_2,6)/I(G₂)

Scanning electron microscope (SEM)

A scanning electron microscope (SEM, JEOL JSM7800F) was used to analyze the surface and cross-sectional characteristics of epoxy resin doped with different lignin contents. Before the SEM observation, the samples were sputtered with a thin layer of gold coating.

Mechanical property analysis by tensile tests

Tensile tests were conducted on an Instron-5965 machine with a load cell of 5000 N, an initial gauge length of 24 mm, and an across head speed of 10 mm/min. The system automatically records the stress-strain curve. The dumbbell-shaped mold was prepared according to the GB/T 528-2009 standard, and each sample was measured twice.

Published: February 13, 2023

Data and code availability

• •All data reported in this paper will be shared by the lead contact upon request.
• •This study did not generate original code.
• •Any additional information required to reanalyze the data reported in this work is available from the lead contact upon reasonable request.