Lignin based functional additives for natural rubber - ACS Sustainable

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Lignin based functional additives for natural rubber Davide Barana, Marco Orlandi, Luca Zoia, Luca Castellani, Thomas Hanel, Christiaan Bolck, and Richard Gosselink ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02145 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Sustainable Chemistry & Engineering

Lignin based functional additives for natural rubber Davide Barana†, Marco Orlandi‡, Luca Zoia‡*, Luca Castellani§, Thomas Hanel§, Christiaan Bolck∥, Richard Gosselink∥

†

Corimav Pirelli, Material Science Department, University of Milano-Bicocca,

Via R. Cozzi 53, 20126 Milan, Italy ‡

Environmental and Earth Sciences Department, University of Milano-Bicocca,

Piazza della Scienza 1, 20126 Milan, Italy §

Pirelli Tyre SpA, Viale Sarca 222, Milan, 20126, Italy

∥ Wageningen

Food and Biobased Research,

Bornse Weilanden 9, 6708WG Wageningen, The Netherlands

* Corresponding author: Luca Zoia E-mail: [email protected]; phone: +39 02 6448 2709; fax: 02 6448 2835

KEYWORDS: Lignin valorization; Chemical Modification; Fractionation; Natural Rubber; Solubility parameters.

1 ACS Paragon Plus Environment

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ABSTRACT In this work, the possibility to conveniently exploit lignin as a functional additive for natural rubber was pursued following two strategies. The first was based on the fractionation of lignin: extraction with organic solvents is suitable to produce lignin fractions with better defined structural features, molecular weight distributions, and physiochemical properties. The second approach was based on the chemical modification of lignin in the attempt to overcome its poor affinity with the rubber: esterification with anhydrides was selected to modify relatively large samples of lignin at lab scale. The effectiveness of different modifications of lignin as a drop-in replacement for carbon black was evaluated analyzing the tensile mechanical properties of model elastomeric compounds. In addition, the behavior of the modified lignins was rationalized through the Hansen Solubility Parameters predicted with the group-contribution method.

INTRODUCTION On a global scale, a considerable effort is being lavished in the attempt to protect the environment from the side effects of fossil resources massive exploitation.1–3 Main strategies focus on the reduction of GHG (Green House Gases) emissions through the development of renewable energy and alternative carriers.4,5 Similarly, it is necessary to address the issues connected to the mass production of chemicals and materials, as many key products still rely on the refinery of crude oil and share the same concerns of fuels.6 Biobased chemicals and materials might be conveniently produced from fractions of the biomasses that are not suitable to yield food or biofuels, adding further value to the biomasses and enhancing the commercial viability of biorefineries. Commodities produced from renewable resources could be more environmental friendly by design, reducing the footprint caused by incineration (GHG) or accumulation (e.g.


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microplastics).7

Besides

environmental

advantages

as

carbon

dioxide

neutrality

or

biodegradability, renewable materials might also display other interesting characteristics as lightweight, peculiar mechanical properties and competitive cost.8 From the prospective of mass production, lignocellulosic biomasses are particularly alluring due to their large availability and renewability.9 Lignocellulose is a natural composite constituted by three main components, cellulose (25-55%) lignin (10-35%) and hemicellulose (24-50%).10 Carbohydrates are valuable for the production of biofuels, chemicals and nano-materials.11–13 On the contrary, the conversion of the aromatics into valuable products is an open challenge and lignin is still often regarded as a by-product.14 In fact many efforts were made to produce lignin based materials with various degrees of success.15–19 In rubber technology, a major achievement would be the possibility to replace carbon black, a ubiquitous reinforcing agent, with lignin. However, the mechanical properties of natural rubber reinforced with lignin were often found to be limited by the nonoptimal interactions between the biopolymers.20–22 A previous effort focused on the effect of different lignin structures on the properties of lignin-natural rubber composites20: softwood Kraft lignin (SWK) was found to confer the best mechanical properties to vulcanized rubber. In this work the possibility to further improve the effectiveness of softwood Kraft lignin is explored. Two strategies were pursued: i) fractionation via solvent extraction.21,22 and ii) chemical modification. The modifications were performed in the attempt to overcome the poor affinity between lignin and rubber that was supposed to limit adhesion and to hinder lignin’s dispersion. In this regard, it was previously demonstrated that when lignin is mixed with rubbers characterized by an increased polarity it is possible to obtain a finer dispersion and consequently to produce elastomers with improved mechanical properties. The behavior was rationalized by Tran and coworkers23 analyzing the differences between the cohesive energy density or



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Hildebrand solubility parameter (δ) of lignin and the different rubbers copolymers of butadiene and acrylonitrile (NBR). It was observed that when ∆δ (δlignin - δrubber) fell in the 1.5-3.5 range, the dispersion was excellent, giving rise to superior properties. The gap between the solubility parameters was also successfully used to explain the different solubility of lignin fractions obtained by selective extraction with green organic solvents. Boeriu et al.21 demonstrated that the fractions were characterized by a narrow molecular weight distribution and a defined functional group content resulting in different solubility parameters. It was reported that solubility was maximum when ∆δ < 2 and still acceptable when ∆δ < 4. The solubility parameter of unmodified lignin is usually high, between 24.6 and 31.0, according to different references,24–27 while the solubility parameter of natural rubber is appreciably lower, in the 17.518.2 range,28,29 resulting in a minimum ∆δ of 7.1. As a result, it seemed reasonable to hypothesize that a modified softwood Kraft lignin, characterized by a lower solubility parameter, could be a more effective additive for natural rubber. The modified lignin could be prepared via solubility-driven fractionation or chemical modification, i.e. via esterification with anhydrides. This approach has several advantages: mild reaction conditions, short reaction time, small amount of waste products, and possibility to avoid solvents. Furthermore, esterification was already proved successful to improve the compatibility of lignin with polyethylene (PE) and polylactic acid (PLA).30,31 Several anhydrides were selected to study the effect of different side chains. In addition a methylated lignin was synthetized using the procedure developed by Sen et al.32 For each product, the total (Hildebrand, δ) solubility parameters as well as their dispersive (δD), polar (δP), and hydrogen-bonding (δH) contributions (Hansen Solubility Parameters, HSP) were predicted with the group-contribution method of Stefanis and Panayiotou.33,34 The calculated parameters allowed, through the solubility theory developed by Hansen31, to


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quantitatively describe the changes in the compatibility between lignin and natural rubber due to the modifications and to rationalize the mechanical behavior of the respective elastomeric compounds. The compounds were prepared blending lignin and natural rubber through two distinct techniques: direct mixing and coprecipitation. The mechanical properties were assessed performing tensile tests on vulcanized specimens and evaluated in comparison to suitable references prepared with natural rubber and carbon black reinforced natural rubber.

MATERIALS AND METHODS Materials Industrial softwood (from Pinus spp./Picea spp.) Kraft lignins (SWK) produced using the Lignoboost™ system were provided by Stora Enso, Sweden. The elastomers used to prepare lignin-natural rubber composites and all the references are the following: commercial grade stabilized Natural Rubber Latex (NRL) with 60% solid content HA type (High Ammonia) according to ISO 2004 standard (Latex Trade Center), commercial grade natural rubber type 20 according to Standard Indonesian Rubber (SIR) specifications (Astlett). For the vulcanization: soluble sulphur (Zolfoindustria), zinc oxide (Zincol ossidi), stearic acid (Sogis), N-cyclohexyl-2benzothiazole sulfenamide (CBS, Zolfoindustria). Reference filler: carbon black N375 type according to ASTM (American Society for Testing and Materials) nomenclature (CB, Birla). As antioxidant agents: N-1,3-dimethylbutyl-N’-phenyl-p-phenylenediamine (6PPD) and 2,2,4trimethyl-1,2-dihydroquinoline (TMQ). All other reagents and the solvents (ACS grade) were purchased from Sigma-Aldrich and used as received without further purification.



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Methods Lignin Fractionation Lignin was fractionated by successive extractions with organic solvents. Three fractions were obtained: the fraction soluble in methyl ethyl ketone (F1), the fraction soluble in methanol (F2) and the insoluble residue (F3). The extractions were carried out on 250 g of dried softwood Kraft lignin (SWK) using 1 L of solvent. At each step, the undissolved material was retrieved by vacuum-assisted filtration and re-suspended for a second identical extraction. The solubilized fractions were recovered evaporating the solvent. Lignin chemical modification Lignin esterification. The chemical modification was performed following the method proposed by Thielemans and Wool25 with some marginal adjustments. 30 g of SWK lignin were dispersed in a 250 mL round bottom flask containing 90 mL of 1,4-Dioxane. 40 g of the corresponding anhydride - [Propionic (Pro), Butyric (But), Isobutyric (IBut), Crotonic (Cro) or Methacrylic (Mth)] - were added to the solution and the air was purged with nitrogen. 0.6 mL of 1methylimidazole (1MIM) were added after stabilizing the temperature at 50 °C and the reaction was carried on for 3 hours at the same temperature, applying vigorous magnetic stirring. The reaction was quenched in 500 mL of demineralized water, briefly stirred, and equally divided in 6 plastic bottles (250 mL) for centrifugation (10 min at 5000 rpm). After the first centrifugation, the clear supernatant was disposed, and fresh demineralized water was added to re-suspend the precipitate; then the dispersion was centrifuged again and finally the precipitate was transferred in several wide aluminum pans with the help of a small amount of acetone. The clear and colorless supernatant was drained, and the products were left to dry under the fume hood for several days, until weight was constant.


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Lignin methylation. The procedure was developed on the basis of the alternative method for lignin methylation optimized by Sen et al.32, 30 g of SWK lignin were pre-dispersed into a 500 mL round bottom flask containing 200 mL of dimethyl sulfoxide (DMSO) and 9.6 g of NaOH (2 eq. to the phenolic hydroxyl groups). 60 g of dimethyl carbonate (DMC, 2 eq.) were added and after few minutes of magnetic stirring, the solution was transferred in a 600 mL Hastelloy Parr reactor. The system, provided with mechanical stirring, was heated at 150 °C and the reaction was protracted for 15 h. After the completion of the reaction, the sealed reactor was cooled to room temperature. The product was precipitated acidifying the solution with 2 N HCl and washed with a large excess of demineralized water to restore a neutral pH and to remove salts and other impurities. Finally, the solids were transferred in several aluminum pans and air-dried. Prediction of the solubility parameters for lignin and modified lignins The Hansen Solubility Parameters (HSP), characterized by the dispersive (δD), polar (δP), and hydrogen-bonding (δH) contributions of lignin and modified lignins were predicted with the group-contribution method developed by Stefanis and Panayiotou limited at first-order groups. The original model was proposed in 2008.34 Whereas the values reported in this work were calculated using the improved version published in 2012.33 To calculate the HSP for a complex polymer like lignin an approximation was made, assuming that lignin was the product of the polymerization of G-type phenylpropane units, as already proposed by Boeriu et al.21 However, in the simplified structure the concentrations of the functional groups is not well represented, for this reason, to obtain more accurate predictions of the HSP, the concentrations of chemical functionalities per repeating unit were corrected using the actual concentrations measured via 31

P-NMR spectroscopy. The HSP of chemically modified lignins were calculated under the

assumption that all hydroxyl groups were modified.



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Characterization of lignin, lignin fractions and chemically modified lignins Water content. Water content was determined measuring the weight loss on three 250 mg lignin samples at 105°C for 2 hours. The results are the average of three measurements. Ash content. The ash content was also determined gravimetrically in the same fashion, heating dry lignin powder at 550°C for three hours under air. FT-IR spectroscopy. The spectra of chemically modified lignins were obtained with a Nicolet iS10 spectrometer (Thermo Scientific) set in ATR configuration and equipped with an iTR Smart device (total scan 32, range 4000-800 cm-1, resolution 1 cm-1). Samples (ca. 200 mg) were dried in an oven for few hours at 40 °C, until constant weight, before the analysis. 31

P-NMR spectroscopy. Pristine SWK lignin and its fractions were quantitatively analyzed by

31

PNMR35 using the same experimental procedure described in the supporting information of

Costant et al.36 Alkaline Size Exclusion Chromatography. The molecular weight distributions of pristine SWK lignin and its fractions were determined via Size Exclusion Chromatography (SEC) at 25°C, using a 0.5 M NaOH aqueous solution as mobile phase. The molar mass distribution was measured with a TSK gel Toyopearl guardcolumn PWxl, column size: 6.0 mm I.D. x 4 cm, particle size: 12 µm and two TSKgel GMPWxl, columns connected in series, size: 7.8 mm I.D. x 30 cm, particle size: 13 µm, calibrated with sodium-polystyrene sulfonates (PSS standards), UV detection at 280 nm. Preparation of the elastomeric composites The elastomeric composites were prepared using two distinct techniques: coprecipitation and direct-mixing. The coprecipitation technique (Cop) was used to improve lignin dispersion and consists of two steps. In the first step, a lignin/natural rubber masterbatch is prepared by


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coagulating a pre-dispersion obtained adding an alkaline lignin solution to natural rubber latex. In the second step, curing agents and additives are added using an internal chamber mixer. In direct-mixing (Mix), dry lignin powders were directly blended in the mixer with the solid natural rubber and other additives. Preparation of lignin/natural rubber masterbatches. At first, lignin was dissolved in a 0.1 M NaOH solution using 15 mL of alkaline solution per g of lignin. The solution was then gently poured in a beaker already containing the natural rubber latex under stirring. The lignin solution and the natural rubber latex were stirred for one hour at room temperature and subsequently coagulated adding 10 % w/w sulphuric acid. The amounts of lignin and natural rubber latex were calculated to obtain masterbatches with a final lignin concentration of 15 PHR (Part per Hundred Rubber) or 13% w/w. The solids produced during the coagulation phase were recovered, washed thoroughly, and reduced in thin layers using an open two-roll mixer. The sheets were additionally rinsed with fresh water several times to remove the excess of acid used during the coagulation. The rubber samples were left air drying sheltered from light until constant weight was reached. Preparation of rubber model compounds. Rubber composites were prepared mixing additives to neat natural rubber or lignin/natural rubber masterbatches according to the schemes of Table 1.



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A

NR

CB

Cop

Mix

Natural Rubber (Sir20 or coagulated NRL)

100

100

0

100

Carbon Black (N375)

0

15

0

0

Lignin (SWK or F1, F2, F3)/Natural Rubber Masterbatch

0

0

115

0

Lignin (SWK or F1, Me, Pro, But, IBut, Cro or Mth)

0

0

0

15

Soluble sulphur

2

2

2

2

Zinc Oxide

5

5

5

5

Stearic acid

2

2

2

2

Accelerator (CBS)

2

2

2

2

Antioxidant 1 (6PPD)

1

1

1

1

Antioxidant 2 (TMQ)

1

1

1

1

Total PHR

113

128

128

128

B

time (min)

RPM

Temperature (°C)

Action

1st step

0

50

60

Loading rubber or masterbatch

3

50

60

Loading filler (none / lignin / CB)

5

50

60

Loading Stearic Acid and Zn Oxide

7

50

60

Loading Antioxidants

12

50

60

Dump

0

40

40

Loading compound from step 1

3

40

40

Loading Sulphur and Accelerator

8

40

40

Dump

2nd step

Table 1. Formulations used to prepare lignin-natural rubber compounds and reference compounds (A) and mixing procedure for the preparation of lignin-natural rubber compounds and reference compounds (B).

The mixing was carried out in a Haake mixer having an internal chamber of 250 mL. The fill factor, defined as the volume of the ingredients divided by the volume of the chamber, was 0.8. The rubber or the lignin/rubber masterbatches were introduced into the mixer at time = 0, the other ingredients were loaded subsequently according to the two-step procedure. At the end of the second step, the green compounds were passed three times through an open two roll mixing


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mill to ensure homogeneity in the samples (roll distance 5 mm, temperature 60 °C, speed 10 rpm). Evaluation of the mechanical properties of rubber model compounds. After mixing, rubber compounds were left to rest at room temperature for 24 h. Then, the compounds were reduced to roughly 8 mm thickness with a two-roll open mill and were vulcanized in a hydraulic press (4.3 bar at 150°C for 30 minutes). For each sample, five dumbbell shaped test specimens were die-cut from vulcanized sheets. Stress-strain analyses were performed using a Zwick/Roell tensile testing machine. The parameters used are ISO-37 and UNI-6065 compliant. The data reported in the result and discussion section are the mean of 5 analyses; plotted curves refer to the median specimen.

RESULTS AND DISCUSSION Lignin fractionation Lignin fractionation and characterizations Sixty-six % of SWK lignin was found to be soluble in methyl ethyl ketone (MEK, F1), 13% was dissolved by methanol (MeOH, F2) and finally, 21% of the initial lignin precipitated in both solvents (F3). The variation of the chemical group distributions was quantitatively assessed via 31

P-NMR. The results of the analysis are reported in Table 2 and Figure 1A.



31

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SWK

F1

F2

F3

1.97

1,42 ↓

2,18 ↑

2,76 ↑

Total

4,78

4,94 ↑

4,01 ↓

3,34 ↓

Cond.

1,71

1,77

1,50

1,41

Syringyl

0,45

0,47

0,39

0,36

Guaiacyl

2,15

2,39

1,91

1,41

p-Hydroxyl

0,33

0,31

0,21

0,15

Carboxylic -COOH

0,52

0,55 ↑

0,30 ↓

0,15 ↓

Mn

700

600

800

1300

Mw

3300

1900

2800

9000

Mp

2000

1500

2750

5900

PDI

4.6

3.2

3.5

7.0

P-NMR Aliphatic -OH Aromatic -OH

SEC

Cond. = Condensed phenolic moieties

Table 2. Quantitative distribution of chemical functionalities (mmol/g) obtained via

31

P-NMR,

average molecular weights (g/mol) and polydispersity indexes determined via SEC, for extracted samples: F1, F2, F3 and pristine SWK lignin.

The MEK soluble fraction, F1 was found to be enriched in phenolic hydroxyls and carboxylic groups. On the contrary, the concentration of aliphatic hydroxyls was inferior to that of the starting lignin. In F2 the trend was symmetrical to that of F1 and was characterized by a greater concentration of aliphatic hydroxyls in conjunction with a lower concentration of aromatic hydroxyls and carboxyl groups. The same trend became more pronounced in F3 samples. To obtain a clearer understanding of the fractionation products, the characterization of the chemical groups was flanked with the analysis of the molecular weight distributions. In Figure 1B it is possible to observe the overlay of the SEC chromatograms relative to the three fractions: F1, F2, F3 and the starting lignin, SWK. The normalized signal intensity was scaled according to the


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reported yields, to obtain a representation of the quantitative contribution of each fraction to the composition of the initial lignin. The numerical values calculated from the SEC chromatograms are reported in table 2. In the molecular distributions, it was possible to clearly identify a trend that is well reflected by the average molecular weights: F3 > F2 > F1. The molecular weight clearly influenced the solubility and the extraction yield of the lignin molecules in the organic solvents used for the fractionation. Moreover, F1 and F2 were characterized by a smaller polydispersity index, indicating that the solubilized fractions were more homogenous. High molecular weights possibly precluded solubility, in fact both soluble fractions, F1 and F2, displayed an upper molecular weight threshold of approximately 25 kDa, corresponding to a retention time of 17 minutes.



Figure 1.

31

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P-NMR spectra (A) and SEC chromatograms analyses (B) of pristine SWK lignin

and F1, F2 and F3 fractions. (IS: internal standard; Cond: condensed phenols; G-OH: guaiacyl units; H-OH: p-hydroxycoumaryl units).


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Mechanical properties with fractionation products In the first tests on compounds, coprecipitation was used to overcome the detrimental effect of poor dispersion. The stress/strain curves relative to the samples filled with the original lignin (SWK), fractions (F1, F2 and F3) and the two references: neat natural rubber (NR) and carbon black (CB) are displayed in Figure 2A. The plots obtained with the fractions were rather alike to that produced by pristine SWK lignin, however it was possible to notice a subtle but clear trend (F1>SWK>F2>F3). In comparison to SWK, F1 conferred to samples slightly enhanced mechanical properties, whereas the use of F2 and F3 led to a minimal deterioration of the performance. The trend was in good agreement with the progressive higher solubility of the fractions in media having lower polarity (F1>F2>F3) and might be solely explained by the small increases of compatibility with the rubber, while the behavior of SWK lignin seemed to be consistent with the sum of the contributions from its fractions. Another explanation of the mild trend could be found in the differences of antioxidant capability. In fact, F1 was characterized by lower molecular weight and higher concentration of phenolic moieties and it was previously demonstrated that the capability of lignin to protect natural rubber from oxidative stress is proportional to the initial concentration of active functional groups, i.e. hindered phenolic moieties, and the ability of such functional groups to migrate through rubber which is inversely proportional to the average molecular weight of the lignin chains.20



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Figure 2. Stress/strain curves of composites prepared with: A) fractionated lignins through coprecipitation and suitable references, B) F1 lignin via direct-mixing and suitable references, C) chemically modified lignins and suitable references via direct mixing. 16 ACS Paragon Plus Environment

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In general terms, fractionation coupled with coprecipitation was not found to be a suitable technique to dramatically improve the mechanical properties of lignin reinforced natural rubber. It is worth noticing that the composites filled with F1 showed particularly interesting mechanical properties at low strains (0 - 100%), where the capability to reinforce rubber was comparable to that of carbon black. These results interestingly indicate that for niche applications a fractionated lignin could be a suitable replacement for carbon black. Coprecipitation is an effective technique to obtain fine dispersion of lignin in natural rubber; however, considering operations at an industrial scale, it might not be practically or economically viable and its applicability is limited to rubber grades available as latex suspensions. Therefore, the possibility to add lignin directly to solid rubber in the mixing stages is particularly alluring. The results of the tensile tests performed on rubber compounds prepared via dry-mixing using SWK, F1 and the two references: neat natural rubber (NR) and carbon black (CB) are displayed in Figure 2B. The addition of unmodified lignin (SWK) directly in the mixer resulted in poor dispersion and detrimental properties. In fact, SWK lignin essentially behaved as inert filler. On the contrary, F1 clearly improved the mechanical properties of the rubber compounds. The enhanced compatibility with rubber, which influences dispersion, contact area and quality of the interactions at the interfaces between lignin and rubber, was supposed to be the main reason for to the improved performance achieved with F1. Nevertheless, the sole distribution of functional groups seemed to be unable to explain the improved performances. In fact, according to the solubility parameters (Table 3), F1 lignin was predicted to be more “compatible” with rubber with respect to SWK lignin, but only to a limited extent. It is hence realistic that the molecular weight played a predominant role in the determination of the mechanical properties. In fact, F1 was essentially constituted by oligomeric



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species, as highlighted by the Mw. Perhaps, the smaller average size of molecules allowed the fractionated lignin to rearrange differently in the solvent, reducing the concentration of polar groups at the interface and exposing the hydrophobic skeleton. This behavior was already reported by Qian et al.37 and could explain the “excess” of affinity between extracted lignin and substances of lower polarity. Once more it was confirmed that to achieve specific properties and confer an elevated added value, technical lignins must be further refined. Fractionation was found to be a viable option to modify lignin and to enhance specific characteristics. For instance, the concentration of phenolic hydroxyls, connected to an improved antioxidant capability, and the compatibility with rubber, responsible for better dispersion and stronger polymer-filler interactions, that ultimately promoted reinforcement.

Lignin chemical modifications The chemical conversion of the polar hydroxyl groups allows a more pervasive structural modification of lignin than fractionation. It is hence possible to aim at an enhanced optimization of the compatibility with rubber to ultimately produce elastomeric compounds with superior performances. However, it is important to select reactions that do not ultimately hinder the advantages connected to the utilization of a low-cost, renewable raw material. In this work, esterification with anhydrides was selected to easily modify relatively large samples of lignin at lab scale. Potentially, the reaction could also be extended at industrial scale, since expensive or dangerous reactants, solvents, equipment, and harsh conditions can be avoided.


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Yield and characterization The isolated yields for the modified lignins are the following: Pro = 96% , But = 89% , IBut = 91% , Cro = 80% , Mth = 98%, Me = 84%. The reactants were in excess with respect to the stoichiometric amount required for the full functionalization of lignin’s hydroxyls and theoretical yields were calculated assuming complete modification. The yields obtained with crotonation and methylation were somehow lower than the yield of the other reactions. This might be related with the fact that butyric and isobutyric anhydrides are reported to give essentially complete modification,30 while, in the same conditions, esterification with crotonic anhydride was reported to leave 25% of the aliphatic hydroxyls and 22% of phenolic hydroxyls unmodified. In addition, crotonated lignin was also found to be more difficult to separate from the supernatant after centrifugation, maybe due to the higher density (>1) of crotonic anhydride. The lower yield of methylation with dimethyl carbonate was probably also connected with incomplete functionalization. In fact, using the same conditions, Sen et al.32 found that 91% of the phenolic hydroxyls and 60% of the aliphatic hydroxyls were successfully modified. The solid products were characterized via ATR/FT-IR (Figure 3).



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Figure 3. FT-IR spectra of pristine (SWK) and modified (Me, Pro, But, IBut, Mth, Cro) lignins.

Between 3200 and 3600 cm-1 the peaks associated to the stretching of the different hydroxyl groups disappears in the samples modified with propionic, butyric and isobutyric anhydrides, indicating that the modification was complete. A residual amount of hydroxyl functionalities is clearly present in methylated and crotonated lignins, in agreement with the previous observations. Between 2800 and 3000 cm-1, it was possible to observe that, in esterified samples, the intensity of the bands associated with CH stretching of methyl and methylene groups increases with chain length and chain saturation. In the methylated product, the same bands are detectable confirming the effectiveness of the methoxylation, as highlighted by the higher intensity of the peak at 2830 cm-1 ascribed to the stretching of the methoxy group. Between 1700 and 1800 cm-1 esterified lignins display strong peaks attributable to the stretching of conjugated and unconjugated carbonyl groups. A minor increase in the absorbance in this region was also detected for the methylated lignin. This might be caused by the presence of degradation by20 ACS Paragon Plus Environment

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products promoted by the harsher reaction conditions. For crotonated and methacrylated lignins a new peak appeared at ~1650 cm-1 identifying the presence of the C=C double bond. In the crotonated sample, also a strong peak at 960 cm-1 highlighted the presence of the alkene. Finally, increased absorption was detected in the 1000-1200 cm-1 region, coherently with the formation of the ester bonds. Modeling of lignins-rubber compatibility The solubility parameters were used to predict the compatibility between rubber, lignin, and modified lignins. The solubility parameters and the solubility ranges of a substance can be conveniently determined with simple experiments and are already available in literature for a great number of solvents and materials.27 Due to this pragmatic approach, solubility parameters can be effectively employed to understand interactions in complex systems. The solubility parameters were calculated with the group contribution method of Stefanis and Panayiotou.34 The values obtained for natural rubber (polyisoprene), original lignin (SWK), lignin fractions (F1, F2 and F3) and chemically modified lignins (Pro, But, IBut, Cro, Mth and Me) are summarized in Table 3.



Page 22 of 33

δd

δp

δh

δ

∆δ

Ro

Ra

RED

Natural Rubber

15,7

4,2

5,2

17,1

-

7,3*

SWK Lignin

19,4

8,1

14

25,3

8,2

12,1

1,66

F1

19,5

8,0

13,5

25,0

7,9

11,8

1,62

F2

19,4

7,9

13,2

24,8

7,7

11,5

1,57

F3

19,3

7,8

13

24,5

7,4

11,2

1,53

Pro

18,4

8,9

6,0

21,3

4,2

7,2

0,98

But

18,4

8,5

5,6

20,8

3,7

6,9

0,95

IBut

18,1

8,0

5,3

20,5

3,4

6,0

0,82

Cro

18,7

8,6

6,4

21,5

4,4

7,4

1,01

Mth

18,2

8,1

5,6

20,7

3,6

6,2

0,85

Me

18,6

7,1

6,3

20,9

3,8

6,6

0,90

Table 3. Predicted solubility parameters (δd , δp , δh , δ), difference with NR (∆δ), solubility range (Ro), Hansen distance from polyisoprene (Ra) and relative energy difference (RED) calculated for starting lignin (SWK), lignin fractions (F1, F2, F3) and chemically modified lignins (Pro, But, IBut, Cro, Mth, Me).*29

The calculated total, or Hildebrand solubility parameters (δ) of rubber and unmodified lignin were found to be in good agreement with the values reported in literature (polyisoprene 17.518.2, lignin 24.6-31.0)24,25,27–29, whereas the δ values obtained for modified lignins were consistent with the values calculated by Thielemans and Wool using the Hoy model (Pro 22.1, But 21.9, and Mth 21.8).25 As anticipated in the introduction, lignins are mostly soluble when the difference between their solubility parameter and the solubility parameter of the solvent is lower than four.21 Hence ∆δ was already indicative of the different compatibility of modified lignins


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and revealed that chemical modifications had greatly enhanced the affinity with natural rubber. The effects of the modifications were better depicted when three different contributions dispersive (δd), polar (δp), and a hydrogen bonding (δh) - to the solubility parameter δ were individually considered. As expected, the greatest effect due to the modification of the hydroxyl groups was a strong reduction in lignin’s capability to form hydrogen bonds. Therefore, beside the enhanced compatibility with rubber, the lesser cohesiveness of lignin might also have hindered self-aggregation, easing the dispersion into the matrix. The affinity between lignin and rubber was quantified according to the HSP theory, calculating the distances in the Hansen space (Ra) and comparing them with the experimental solubility radius of polyisoprene (Ro).29 The relative energy difference (RED) is as a measure of compatibility. For a substance, a good solvent has a RED < 1

and progressively poorer solvents have increasingly higher RED

numbers.26 The RED between unmodified lignin and natural rubber was found to largely exceed 1, well reflecting the sub-optimal compatibility between the two biopolymers. Fractionated lignins had small variations in the solubility parameters according to the different concentration of chemical functionalities, however the differences were small and their specific behavior in the elastomeric compounds was mainly attributed to the heterogeneity observed in the molecular weight distributions. On the contrary, chemically modified lignins displayed appreciable changes in the solubility parameters and lower RED values. Accordingly, all the modifications were predicted to confer at least partial miscibility and lignin-rubber compatibility was predicted to increase following the trend: Cro < Pro < But < Me < Mth < IBut. The enhanced compatibility of chemically modified lignins with polyisoprene was also depicted through the analysis of the fractional parameters. Fractional, or Teas parameters express the percent contribution of each Hansen parameter (δd ; δp ; δh) to the whole Hildebrand value (δ)38 (Figure 4).



Page 24 of 33

Figure 4. Teas ternary plot graphically representing the percent contribution of each Hansen parameter (δd ; δp ; δh) to the total Hildebrand value (δ).

Chemically modified lignins were appreciably closer to polyisoprene than unmodified or fractionated lignins, hence they were forecasted to have greater chance to fall in the solubility window of natural rubber. According to this approach, Methylated lignin was predicted to potentially display a greater affinity with natural rubber, whereas the esterified lignins were characterized by very similar contributions with only minor discrepancies.


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Mechanical properties with chemical modification products The mechanical properties of the rubber compounds filled with chemically modified lignins are displayed in Figure 3C. The chemical modifications clearly affected the capability of lignin to reinforce rubber and it was possible to observe a neat differentiation in the macroscopic mechanical properties of the composites filled with different modifications of the biofiller. In general, all modified lignin granted reinforcement, increasing the stiffness of the compounds. It was supposed that the effect was due to the lower polarity and hindered hydrogen networking capability of lignin that resulted in better dispersion and stronger interactions at the filler-matrix interface. It is worth to highlight that the ultimate elongation (UE) of the specimens was not sensibly improved by the presence of modified lignins in contrast with what was observed for unmodified and fractionated lignins. This could be a hint indicating that the behavior was related to the antioxidant capability of lignin, hampered by the modifications. Lignins esterified with linear chains (Pro, But, Cro) seemed to provide higher elongations at break than the ramified counterparts (IBut, Mth), while methylated lignin produced the samples with the lower UE. The early failure of the composites filled with methylated lignin was unexpected and it was hypothesized that it was related to the presence of a poorly dispersed lignin that acted as defects in the materials. Perhaps larger agglomerations arose from the presence of a rather insoluble fraction, generated under the harsher conditions required for methylation that are consistent with the initiation of a partial condensation of small lignin fragments into larger, less soluble macromolecular structures. The reinforcing capability of esterified lignins seemed to increase with chain length, ramification, and the presence of double bonds. Longer chains were expected to enhance hydrophobicity and improve the compatibility with rubber. The presence of unsaturated moieties seemed to furtherly enhance the interactions between lignin and rubber,



Page 26 of 33

possibly through the formation of covalent bonds during vulcanization. When the compatibility of the various fillers predicted with the solubility parameters was compared with the reinforcing effect, quantified as the increase in stiffness at increasing elongations, it was possible to envisage a neat relationship (Figure 5).

Figure 5. Relative energy difference (RED) and stresses at different strains for unmodified and chemically modified lignins (*unsaturated products).

Lower energy differences (RED, large grey bar)) corresponded to higher moduli (TS 100%, TS 300% and TS 700%) for lignin esters. However, it possible to observe two outlier in the relationship, marked by an asterisk: Mth and Cro modified lignin. This is due to the presence of double bonds that significantly contributed to increase the reinforcement demonstrating that the chemisorption of rubber on the surface of lignin’s particles is desirable if not mandatory to produce reinforcing lignin, as already reported for lignin allylation39 in rubber compouds.40


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CONCLUSIONS Two distinct approaches to improve the performances of natural rubber composites reinforced with softwood Kraft lignin were explored. Extraction with organic solvents allowed preparing three distinct products: better-defined features characterized the soluble fractions. Moreover, chemical modifications were proved suitable to improve lignin’s reinforcing potential in elastomeric composites. The prediction of the solubility parameters of lignin and its modifications was found to be a useful tool to forecast and rationalize the behavior in complex systems. The connections between some specific structural features of the modified lignins and their reinforcement capability were identified.

AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The Ph.D. Scholarships of Davide Barana was funded by Corimav-Pirelli.

ACKNOWLEDGMENT The authors would like to thank the contributions of Guus Frissen, Martien van den Oever, Sten van Lanen, Jacinta van der Putten and Herman de Beukelaer (Wageningen Food & Biobased Research) to this paper.



Page 28 of 33

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TOC/Abstact Graphic

Synopsis Two strategies, namely solvent fractionation and chemical esterification, has been investigated for the valorization of softwood Kraft lignin as biofiller in tire industry applications.


Lignin based functional additives for natural rubber - ACS Sustainable

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