Elucidating the Role of Lignin Type and Functionality in the Development of High-Performance Biobased Phenolic Thermoset Resins

In this work, a series of biobased phenolic resins were developed starting from kraft and soda lignin, suitably functionalized through esterification by means of succinic anhydride. As a result of an extensive optimization study of the functionalization and curing reactions, clear correlations between lignin type and chemical–physical characteristics and the properties of the resulting phenolic resin systems were described. In particular, the esterification reaction through succinic anhydride was found to play a key role in enhancing the chemical reactivity and in facilitating the successful incorporation of lignin into the resin formulations. The obtained high-lignin-content thermoset materials were shown to exhibit tunable chemical (functionality, gel content, and cross-linking density), thermal (glass transition temperature and thermo-oxidative stability), and mechanical (surface hardness, indentation modulus, and creep behavior) characteristics, which could outperform those of fully oil-based reference phenolic resins by judicious control of lignin concentration and chemical characteristics. In particular, succinylated kraft lignin was found to enable more efficient incorporation into the cured systems. This work provides the first demonstration of the incorporation of succinic-anhydride-modified-lignin in the formulation of high-performance phenolic resins, ultimately contributing to the definition of structure–property–performance correlations for rational biobased material design in the context of advanced and sustainable manufacturing.


Characterization of lignins
A Waters 510 HPLC system equipped with a refractive index detector was used for gel permeation chromatography (GPC) analysis, to determine the molecular weight and molecular weight distribution of the lignins before and after fractionation and functionalization.THF was used as eluent.The analyzed lignin samples (volume 200 μL, concentration 1 mg/mL in THF) were injected into a system of three columns connected in series (Ultrastyragel HR, Waters-dimensions 7.8 mm × 300 mm) and the analysis was performed at 30 °C at a flow rate of 0.5 mL/min.The GPC system was calibrated against polystyrene standards in the 102-104 g/mol molecular weight range.To allow complete solubility in the THF eluent, before the analysis the parent lignins KL and SL were acetylated.S1 The estimation of the number-average and weight-average molecular weights (Mn and Mw, respectively) of the obtained lignin fractions was performed excluding the signals related to the solvent (THF) and the solvent stabilizer (butylated hydroxytoluene), visible at long elution times (>29.5 min).Comparing all the spectra before and after the application of the curing cycle, some common changes in the signals recorded can be found.The most evident effect of curing process is the disappearance of the peak located around 1000 cm -1 , typically attributed to the stretching vibration of C-O bond in methylol group.S2,S3 As curing proceeds, in fact, those species are involved in the formation of methylene bridges.This is confirmed by the appearance of a new band around 1475 cm -1 , correlated to the deformation of the formed CH 2 groups.S4,S5 At the same time, the reduction of the signal located at 1370 cm -1 is observed, assigned to aromatic O-H in plane bending, as a consequence of further condensation reactions.S4 Additionally, in all the cured resins, a new peak at 1130 cm -1 appears.This latter is attributed to the formation of a different type of crosslinking linkages (i.e., dimethyl-ether bridges CH 2 -O-CH 2 ), as a result of the polycondensation reaction between hydroxymethyl groups and phenolic rings.S5 In the case of SAn-functionalized lignins, the peak located at 1740 cm -1 , associated with C=O stretching of carbonyl bond and clearly visible in the spectra of uncured LPF SAn-KL and LPF SAn-SL, drastically decreases after crosslinking.This could likely be related to the decarboxylation of carbonyl bond, induced by the temperature/pressure conditions reached during the thermal cycle.S6 Lastly, other characteristic peaks of all cured resins spectra are found at 820 and 760 cm -1 , owing to the out-of-plane bending vibrations of C-H in aromatic rings when ortho-ortho and ortho-para connections are established, respectively, which are typical of PF resins.S7

Mechanical behaviour of resins
Figure S8 Geometry used for wood joint adhesion test.
The adhesive strength of wood-to-wood joint was tested in accordance with normative BS ISO 6237:2003.1 g of resin blend was homogeneously spread on each wood layer.The wood sheets were overlayed paying close attention to the direction of the fibers, in such a way that the middle panel fibers were oriented orthogonally to the outer ones.After the curing step, panels were notched according to the normative, as schematized in Figure S8.The specimens as obtained were then loaded in tension on a Zwick-Roell dynamometer.Loading was applied at a uniformly increasing rate of 2.5 kN/min, hence a crosshead speed of 0.5 mm/min.The failure of a plywood can generally occur for either wood or adhesive failure.In the event of exposed adhesive after failure, a percentage assessment of the visible adhesive can be made.At least five testes for each sample were performed. η IT : amount of elastic work done by micro-indenting the sample, computed as the ratio between the elastic deformation work and the total deformation work performed during indentation, expressed in %.
 C IT,1 : Indentation creep, expressed in %.It describes the material behaviour at a constant maximum test load, calculated as indicated by Equation S1:  C IT,2 : Recovery creep, expressed in %.It describes the material behaviour at a constant minimum test load after load reduction, calculated as expressed by Equation S2: SEM analyses were performed on all cured systems with the aim of studying their microstructure and morphology and to assess potential effects related to the introduction of different types and contents of lignin in the resins.The reference PF resin (Figure S11A) exhibits a regular and ladder-likely surface, with no evidence of surface defects or porosity, thus indicating efficient crosslinking and phase homogeneity.A sufficiently homogeneous surface is observed also in case of LPF KL 5 and LPF SL 5 (Figure S11B and S11C, respectively).A few lignin particles are clearly distinguishable in these samples, which are responsible for making the section rougher, likely contributing to initiate brittle fracture.Also in these cases, the surface does not present any kind of porosity, validating the suitability of the crosslinking process adopted.At higher pristine lignin substitution rate (i.e., 20 wt%, Figure S11D), the appearance of the resin cross-section is quite different.More kraft lignin particles become clearly visible within the resin matrix, as a result of their high concentration in the thermoset.
Considering the images related to modified lignin-based resins, similar remarks can be done.Indeed, at lower lignin substitution rate (i.e., LPF SAn-KL 5 and LPF SAn-SL 5 in Figure S11E and S11F, respectively), the fracture surface appears more regular and homogeneous, and no micro-or macro-porosity can be noted.
Additionally, lignin particles are barely distinguishable, as a result of their excellent distribution and high incorporation level within the matrix.Conversely, approaching 10 wt% of lignin content in the resin, agglomerates of lignin particles are visible (Figure S11G and S11H) as well as some micro-porosities, as a possible consequence of lignin dehydration and/or presence of trapped volatile reaction byproducts (i.e., water vapour and ammonia).The agglomeration effect appears to be more pronounced when lignin is present at 20 or 30 wt% in the matrix (Figure S11I and S11L).Moreover, in the case of LPF SAn-KL 20, detachment of the lignin particles from the matrix is also observed, while in LPF SAn-KL 30 some microfractures originating from lignin particles embedded in the rigid matrix appear on the surface (highlighted with a dashed white circle in the micrographs).

Figure S1
Figure S1 SEM images of (A) pristine KL (B) milled KL and (C) pristine SL.

Figure S4
Figure S4 Ozawa linear regression for the evaluation of the activation energy associated with the crosslinking reaction of (A) pristine kraft lignin-based resins, (B) succinylated kraft lignin-based resins, (C) pristine soda lignin-based resins and (D) succinylated soda lignin-based resins.

Figure S5
Figure S5 Kissinger linear regression for the evaluation of the activation energy associated with the crosslinking reaction of (A) pristine kraft lignin-based resins, (B) succinylated kraft lignin-based resins, (C) pristine soda lignin-based resins and (D) succinylated soda lignin-based resins.

Figure S6 FTIR
Figure S6 FTIR of uncured resinoid blends and cured resins in the region 1800-600 cm -1 of (A) reference resin PF, (B) LPF based on pristine kraft lignin, (C) LPF based on succinylated kraft lignin, (D) LPF based on pristine soda lignin and (E) LPF based on succinylated soda lignin.

Figure S7
Figure S7 DSC of cured resins of (A) reference resin PF, (B) LPF based on pristine kraft lignin, (C) LPF based on succinylated kraft lignin, (D) LPF based on pristine soda lignin and (E) LPF based on succinylated soda lignin.

Figure S9
Figure S9Example of typical microindentation cycle (penetration depth as function of applied load).Relevant penetration depths used for the calculations are marked.

Figure S10
Figure S10 Representative example of microindentation cycle where penetration depth (h) is represented as a function of applied load (F) of (A) pristine kraft lignin-based resins, (B) succinylated kraft lignin-based resins, (C) pristine soda lignin-based resins and (D) succinylated soda lignin-based resins.

Table S1
GPC analysis of all lignin samples.

Table S2
Characteristic mass loss temperature related to 5% (T 5 ), 10% (T 10 ) and 50% (T 50 ) mass loss, maximum degradation rate temperature (DTG max ) and final residue at 750 °C (R 750 ) of reference resin PF, LPF based on pristine kraft lignin, LPF based on succinylated kraft lignin, LPF based on pristine soda lignin and LPF based on succinylated soda lignin.