Influence of Lignin Features on Thermal Stability and Mechanical

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Influence of lignin features on thermal stability and mechanical properties of natural rubber compounds Davide Barana, Syed Danish Ali, Anika Salanti, Marco Orlandi, Luca Castellani, Thomas Hanel, and Luca Zoia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00774 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Sustainable Chemistry & Engineering

Influence of lignin features on thermal stability and mechanical properties of natural rubber compounds

Davide Barana1, Syed Danish Ali1, Anika Salanti2, Marco Orlandi2, Luca Castellani3, Thomas Hanel3, Luca Zoia2*

1

Corimav-Pirelli, Department of Material Science, University of Milano-Bicocca, Via R. Cozzi 53, Milan, 20126, Italy

2

Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 1, Milan, 20126, Italy

3

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

AUTHOR INFORMATION * Corresponding author: Luca Zoia. Email: [email protected]. Tel: (0039) 02-6448-2709. 1 ACS Paragon Plus Environment


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Abstract: The aim of this work is to elucidate the relationship between lignin main features and its behavior in natural rubber compounds, in particular focusing on thermal stability and mechanical properties. Five lignins obtained from different sources and through different extraction processes were characterized in terms of purity, sulfur content, molecular weight distribution (GPC), qualitative and quantitative functional group distribution (FTIR and

31

P-NMR). Then the lignins

were incorporated in natural rubber by two different approaches, namely co-precipitation and dry-mixing. Thermal stability and mechanical properties of lignin/natural rubber blends were investigated in both masterbatches and vulcanized compounds. The Oxidation Induction Time (OIT) was used to determine the thermal stabilization of the lignin-NR masterbatchs, while tensile stress-strain properties of the compounds were evaluated after vulcanization. It was found that differences in the chemical and morphological characteristics of lignin influence its antioxidant and reinforcement capability. The addition of lignin to vulcanized compounds demonstrated the possibility to improve mechanical properties hypothetically through a tandem mechanism of protection and reinforcement.

Keywords: Lignin, natural rubber, elastomers, filler, antioxidant, thermal stability.

2 ACS Paragon Plus Environment

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INTRODUCTION The development of polymeric materials from renewable sources ongoing from the second half of the 19th century was halted by the petrochemical boom.1 Nowadays the paradigm has changed: economic and environmental issues raised concerns about the dependence from fossil resources and paved the way for the comeback of renewable feedstocks.2 Elastomers are among the materials that most influenced technological development; they are present as key components in many technologies and are extensively used in several fields for the production of tires, seals, coatings, adhesives, etc. Along with vulcanization, the use of reinforcing fillers is the main strategy adopted to improve the properties of elastomers, as they provide better mechanical qualities and also improve resistance to abrasion and fatigue.3 Furthermore, fillers like carbon black can also protect the rubber from thermal aging and UV degradation.4 The research for sustainable substitutes of fossil-derived materials has gathered a lot of attention upon lignocellulosic biomasses, a promising renewable feedstock constituting the most abundant source of biomass in the world. Lignocellulosic materials are non-edible resources generated in large quantities as side products of agricultural and forestry industries. Lignocellulosics main constituents are cellulose, lignin and hemicellulose but other components such as extractives (terpenes, tannins, fatty acids, resins, etc.) and ashes5 could be recognized in their composition. Lignin is the most abundant renewable source of poly-aromatic moieties as its annual production is second only to cellulose.2 This aromatic polymer is biosynthesized for structural purposes in the plant cell walls through oxidative coupling of mesomeric phenoxy radicals originating from three p-hydroxycinnamic alcohols which differ in the degree of methoxylation of the aromatic ring leading to the formation of an extremely complex three-dimensional network. The three monolignols precursor, i.e., p-coumaryl, coniferyl and sinapyl alcohol, are recognized in the lignin structure as p-hydroxyphenyl (P), guaiacyl (G), and syringyl (S) units, respectively. 3 ACS Paragon Plus Environment


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The peculiar chemical nature makes lignin a potential candidate for the replacement of fossil resources. Moreover, lignin could also impart new desirable properties to the materials as it can act as antimicrobial, antifungal and antioxidant agent, can absorb UV radiation and exhibits flameretardant properties.6-8 Nevertheless, lignin valorization and upgrade to co-product status is often hindered by its complex and heterogeneous chemical and morphological structure which is strongly influenced by numerous factors, such as the botanical source and the extraction process. In fact, the relative abundance of P, G and S units, and thus the degree of crosslinking, is different among softwoods, hardwoods and herbaceous plants, while different extraction process often results in the modification of lignin molecular weight distributions and solubility properties. Since lignin is generally produced as a by-product from processes that focus on the exploitation of other fractions of the lignocellulosic biomass (as in papermaking and lignocellulosic bioethanol production), common delignification techniques such as kraft and soda pulping can alter lignin’s architecture and its physicochemical properties.9,10 Additionally, these refining systems are not optimized to produce pure lignin and the array of aromatic polymers obtained is usually contaminated by a certain degree of impurities, mainly ashes and residual carbohydrates still covalently connected to lignin. This additional source of chemical and physical heterogeneity can be a major issue, precluding complete utilization of lignin.11 Despite the highlighted complications, a great effort is made to exploit

lignin in polymer

composites due to its advantageous properties, such as high abundance, large annual renewability, low average molecular weight, environmental friendliness, CO2 neutrality and reinforcing capability.12 The use of lignin as filler or co-reactant in combination with both synthetic and natural polymers for the fabrication of thermosets, thermoplastics, elastomers, resins and foams is widely reported in literature.12-16 First attempts to use lignin streams produced by sulfate and soda pulp mills as a reinforcing filler for natural and synthetic rubbers date back to 1950’s.17 4 ACS Paragon Plus Environment

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Despite the negative effects on the mechanical properties observed when lignin is used as a replacement of conventional reinforcing fillers18-19, primarily caused by the relatively large particle size and polarity, the polymer is receiving a great deal of attention thanks to its antioxidant properties. It was demonstrated that the use of lignin as filler can increases the resistance of vulcanized Natural Rubber (NR) towards thermo-oxidative degradation18, indeed. Natural rubber is highly unsaturated and very susceptible to oxidation; the presence of sterically hindered phenolic groups in the lignin structure opens the possibility of its application as antioxidant in NR composites. The radical scavenging activity can be evaluated by means of DPPH radical scavenging test. According to this method, it was demonstrated the pivotal importance of phenolic hydroxyl groups in inhibiting the generation of free radicals, however high molecular weight and polydispersity can also influence the radical scavenging activity of lignin.20-23 In the present work, the effect of five lignins, obtained from different botanical sources and extraction methods, inserted as filler in natural rubber composite was assessed. A comprehensive characterization including molecular weight distribution (GPC), qualitative and quantitative functional group distribution (FTIR and

31

P-NMR), and sulfur content was carried out. Thermal

stability and mechanical behavior of lignin-NR blends prepared using different techniques were evaluated on both lignin masterbatches and vulcanized compounds. The oxidative susceptibility of rubber compounds was assessed measuring the Oxygen Induction Time via DSC, while the mechanical properties were evaluated performing static tensile tests.



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EXPERIMENTAL Materials The Soda Grass lignin (SG) lignin used in this work is commercialized by Green Value under the trade name of Protobind 1000®. It is obtained from annual plants using a process based on soda pulping. The Softwood Kraft lignin (SWK) and Hardwood Kraft lignin (HWK) were recovered as a side product of the Kraft process, used for the production of cellulose by the paper industry. The HWK was the only lignin to be subjected to purification due to its high ash content. It was therefore purified by dissolution in alkaline medium (pH 13) at 10 % of consistency and reprecipitated under acidic conditions (pH 1) after addition of sulfuric acid 98%. After removing excess of acid with several centrifugation cycles and freeze-drying, purified HWK lignin powder was obtained. Wheat Straw lignin (WS) was purchased from Chemtex srl and is a by-product generated during the production of bioethanol after steam-explosion pretreatment. Rice Husk lignin (RH) was extracted from rice husk using a simple biorefinery process developed at laboratory scale, based on sodium hydroxide extraction under mild conditions.24 For rubber compounding the following products were used: Stabilized Natural Rubber latex (NR) - 60% solid content [Mn: 4400 - Mw: 23000 - DPI: 5.2] (Latex trade center), soluble sulfur (Zolfoindustria), zinc oxide (Zincol ossidi), stearic acid (Undesa), and N-cyclohexyl-2-benzothiazole sulfenamide (CBS, Zolfoindustria). All other reagents and the solvents (ACS grade) were purchased from SigmaAldrich and used as received without further purification. Lignin characterization Lignin Content. The amount of total lignin (purity degree) was calculated as the sum of the acidinsoluble (Klason lignin) and acid soluble lignin content, measured according to the method reported by Yeh et al.25 The values reported are the average of three analyses ± 1.0 % (P = 0.05, n = 3). 6 ACS Paragon Plus Environment

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Ash Content. Accurately weighed and dried samples (around 100 mg) were put in tared, welldesiccated porcelain crucibles and placed in a muffle furnace set at 550 °C for 3 h. The crucibles were then stored in a desiccator until room temperature was reached. The ash content was determined gravimetrically. The values reported in the text and in the tables are the average of three analyses ± 0.1 % (P = 0.05, n = 3). 31

P NMR Analyses. Accurately weighed lignin samples (around 30 mg) were dissolved in a pyridine-

deuterated chloroform stock solution (1.6:1 v/v, 700 μL) containing 1 mg mL-1 of chromium(III) acetylacetonate, [Cr(acac)3] as relaxation agent. 100 μL of an internal standard solution of endo-Nhydroxy-5-norbornene-2,3-dicarboximide (e-HNDI, 121.5 mM, CDCl3/pyridine 4.5:0.5) was then added. 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (100 µl) was used as phosphorus derivatizing agent to quantitate the amount of different hydroxyl groups (aliphatics, phenolics, and acidic).26

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P-NMR spectra were recorded of 800 μL samples on a Bruker Avance 500 MHz

instrument at room temperature. Relaxation delay of 5 s was used between the scans (90° pulse angle). Line broadening of 3 Hz was applied to FIDs before Fourier transform. For each spectrum, typically 100 scans were accumulated. The 31P NMR data reported in this paper are the average of three experiments. The maximum standard deviation was 2 × 10-2 mmol g-1, while the maximum standard error was 1 × 10-2 mmol g-1. Lignin acetylation. Roughly 20 mg of each lignin samples were acetylated in a pyridine-acetic anhydride solution (1:1 v/v, 2 mL) and kept overnight at 40 °C. After stripping with ethanol, toluene, and chloroform (25 mL × 3 each solvent), the sample was dried in vacuum. The acetylated lignin has been solubilized in tetrahydrofuran for GPC analysis. GPC Analyses. Gel Permeation Chromatography analyses were performed on a Waters 600 E liquid chromatography connected to a HP1040 ultraviolet UV detector set at 280 nm. The injection port was a Rheodyne loop valve equipped with a 20 μL loop. The GP-column system was composed by 7 ACS Paragon Plus Environment


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a sequence of an Agilent PL gel 5 μm, 500 Ǻ, and an Agilent PL gel 5 μm, 104 Ǻ. The solvent used was THF (Fluka 99.8%). PL Polymer Standards of Polystyrene from Polymer Laboratories were used for calibration. The evaluation of the number-average molecular weight (Mn) and the weightaverage molecular weight (Mw) of the extracted lignin samples was performed according to the methodology developed by Himmel.27 The peak molecular weight Mp is defined as the molecular weight of the species with maximum absorbance. Moreover, the ratio PDI = Mw/Mn, defined as PolyDispersity Index was also calculated. The Mn, Mw, and Mp values reported are the average of three analyses (Mw: ± 1000 g mol-1; Mn, Mp: ± 100 g mol-1, P = 0.05, n = 3). ATR-FTIR. Attenuated Total Reflectance Infrared Spectroscopy was used for a qualitative characterization of lignin samples. The analyses were performed with a Nicolet iS10 spectrometer (Thermo Scientific) equipped with iTR Smart device (total scan 32, range 4000-800 cm-1, resolution 1 cm-1). Sulfur Content. Sulfur content was assessed using a Leco SC632 Sulfur and Carbon analyzer. The instrument uses an ASTM-approved technique to determine the amount of sulfur contained in different materials. The method refers to sulfur determination according to ASTM D6741-10, ASTM D1619-11 (Method A), ASTM D7679-13 (Method A). Materials preparation. Coprecipitation. To prepare NR-lignin masterbatches the proper amount of lignin, according to the desired final concentration (eg: 7,5 g for masterbatches at 15 PHR), was added to a 0.1 M NaOH aqueous solution (15 mL per gram of lignin) and the pH was adjusted to 13 using 10 wt% NaOH. After being stirred for 1 h, the solution was gently poured in beaker containing 83.4 g of 60 wt% natural rubber latex. The emulsion obtained was kept under stirring for one additional hour and finally 10 wt% sulfuric acid was progressively added to obtain complete coagulation, evaluated by visual inspection. The coagulated rubber was reduced in thin layers using a rubber two roll mixer 8 ACS Paragon Plus Environment

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and then washed in excess of water, until pH 6 was reached. The thin layers of rubber-lignin composite were left air drying sheltered from light until constant weight was reached. A reference of coagulated natural rubber (neat natural rubber) was prepared in the same manner in absence of lignin. Dry-mixing: The sample labeled as “Dry-Mix” was prepared adding dry SWK lignin powder to neat natural rubber through mechanical blending, using an internal chamber Brabender mixer (chamber volume 50 mL, 0.9 fill factor). Compounding. Rubber compounds were prepared adding the vulcanizing agents to neat natural rubber and NR-lignin masterbatches with the internal chamber mixer. Rubber was kneaded at 60°C and 70 rpm. After three minutes, stearic acid, zinc oxide, accelerator (CBS), and sulfur were added and mixed with the rubber composites for 5 minutes. After the mixing step the rubber compounds were passed three times through a two roll mill at 40°C for further homogenization. The mixtures were prepared according to the formulations reported in Table 3. The amount of each material is expressed by PHR (Parts per Hundred Rubber), parts by weight per hundred parts of elastomer. Materials characterization. SEM. Dispersion of lignin in the natural rubber was evaluated analyzing gold sputtered cut surfaces of SG lignin masterbatches using a Ultra Plus Zeiss Field Emission Scanning Electron Microscope (WD=3.7 mm, EHT =10.00 kV, ESB-Grid=300 V). Oxygen Induction Time (OIT). The analysis was performed with a Metler Toledo 822 DSC instrument to quantify the thermal stabilization provided by lignin to NR. Air-dried coagulated natural rubber and masterbatches containing lignin at different concentrations were dried in an oven at 35 °C under vacuum for 12 hours. Afterwards, a 3 mg sample was accurately weighted and placed in an aluminum pan. The sample was heated to 170 °C at 15 °C/min under nitrogen 9 ACS Paragon Plus Environment


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atmosphere, it was then kept at 170 °C for 2 minutes to equilibrate at isothermal condition, and finally the oxygen stream was opened and the induction time recorded at the onset of the exothermal peak. Tensile mechanical properties. Rubber compounds prepared according to the procedure reported in the compounding section were left to rest at room temperature for 24 hours. Subsequently, they were reduced into 8 mm thick sheets using a two roll mill and vulcanized in a press at 151 °C, pressure 4.3 bar for 30 minutes using an electrically heated hydraulic press (BM Biraghi). Five dumbbell shaped test specimens were die-cut for each compound sheet and their thickness was accurately measured. Stress-strain curves were recorded as the samples were progressively strained. The tensile stress was recorded at 10, 50, 100 and 300% elongation; tensile strength (ultimate tensile stress) and elongation at break (ultimate elongation) were also recorded. Tensile stress-strain analyses were performed using a Zwick/Roell tensile testing machine according to ISO 37 and UNI 6065. The data reported are the mean of 5 analyses.


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RESULTS AND DISCUSSION Lignin characterization. Five different lignin samples (in term of botanical origin and extraction process) were selected to study the relationship between their main features and their performance in blends with natural rubber. Preliminarily, all the specimens were characterized by gravimetric, spectroscopic and chromatographic techniques. Infrared spectroscopy can be a powerful technique for the analysis of lignin samples as both chemical structure and purity are reflected in the absorption spectra. The identification of the main peaks of the spectrum was based on published data reported in papers dealing with IR characterization of lignins.28,29 The IR spectra of the five lignin samples under investigation are reported in Figure 1.

Figure 1. FT-IR spectra of the five lignins under investigation expanded in the 800-1850 cm-1 range.



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Along with the broad and strong band of the hydroxyl groups centered around 3400 cm-1, characteristic peaks related to C-H stretching are easily recognized around 2900 cm-1 (region not reported in Figure 1). The peak around 1700 cm-1 is associated to unconjugated C=O groups of carbonyl and carboxylic groups. Bands associated to aromatic skeletal vibrations are clearly visible at 1595 cm-1 and 1509 cm-1, while peaks connected to C-H deformation in methyl or methylene groups and aromatic ring stretching are found at 1453 and 1426 cm-1, respectively, in all samples. A peak at 1319 cm-1 relative to the C-O stretching of the syringyl units can be spotted in all the samples with the exception of SWK lignin, while a peak at 1265 cm-1, concerning the C-O stretching of guaiacyl units can be found in all the spectra. At 1215 cm-1 there is a strong band associated with the C-O stretching of phenolic functionalities and aromatic ethers. Peaks related to C-H in-plane deformations of guaiacyl and syringyl units are found at 1147 cm-1 and 1114 cm-1, respectively. A strong band reflecting the C-O stretching of primary aliphatic alcohols and ethers is found at 1024 cm-1. Out of plane vibration modes connected to C-H bonds of guaiacyl and syringyl units are displayed at 850 and 832 cm-1, respectively. Structural differences among different lignin specimens are clearly highlighted by the different absorption intensities; every lignin shows a characteristic distribution of intensities, especially in the 1400-1000 cm-1 range, reflecting the characteristic distributions of p-hydroxycumaryl (H), guaiacyl (G) and syringyl (S) moieties. All the lignin specimens examined were characterized by high purity (Table 1), between 94 and 99%. Only the HWK lignin, due to its large ash loading, was subjected to a purification step as described in the experimental section. The quantitative analysis of the distribution of different hydroxyls functionalities into lignin macromolecule was achieved by 31P-NMR analysis (Table1). The distribution of aliphatic hydroxyl groups and carboxylic acids is similar in all the lignins with the exception of the SG lignin that 12 ACS Paragon Plus Environment

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possesses a significant larger number of carboxylic acids. On the contrary, the detected amount of phenolic moieties considerably varies between lignins, both in terms of total phenolic groups and relative abundance of different units (H, G, and S/Condensed), in agreement with the botanical source and the production process. In herbaceous lignins all the three monomeric constituents are well represented, while kraft lignins possess a greater amount of S-type and condensed phenols. It is acknowledged that native softwoods lignin has a limited amount of S units,30 hence its enrichment in condensed phenols must have taken place during extraction process. This would be in agreement with the documented observation that the kraft process could induce substantial modifications in lignin structure promoting condensation.31 As well as the chemical structure, also the molecular weight distribution can significantly impact on lignin properties. Table 1 displays the average molecular indexes calculated after GPC analysis. It is possible to observe substantial and significant variation in the average molecular weight indexes (Mn, Mw, Mp and PDI) for the five lignins under investigation: this reflects the different botanical origins and extraction processes. With the exception of the SG lignin, characterized by an extremely low molecular weight (Mw: 2400, Mn: 1000, Mp: 700), the other samples showed higher polydispersion (except for HWK) and higher molecular weights. In particular, the chromatograms of WS, RH, and SWK lignins (Figure 2) exhibited a shoulder at high molecular weights which greatly affects Mw values. It should be mentioned that, although the purity of all the lignin samples is above 94%, residual oligomeric and monomeric saccharides covalently linked to lignin can significantly alter this large molecule-sensitive index.



SG SWK WS

Absorbance (AU)

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RH HWK

100.000

10.000 1.000 Molecular Weight (g/mol)

100

Figure 2. Molecular weight distributions of the 5 lignins (GPC)

It is widely acknowledged that kraft lignin contains about 1.5 to 3% sulfur, but its nature has not been fully established yet. Recently, it was reported that sulfur may be present in lignin as organically bound sulfur, sulfate ions, elemental sulfur, and adsorbed polysulfide forms. It was also mentioned that the influence of the sulfuric acid used for lignin precipitation on the sulfur content is negligible (up to 0.2 %) and that consequently the sulfur content in lignin samples is mainly derived from the cooking process.32 For instance, thiol groups are introduced in the structure of lignin during the Kraft process.10 In the present work, the sulfur content was measured to evaluate the influence of sulfur concentration on the properties of lignin-NR blends. As expected, the sulfur content was found to be higher in kraft lignins, with total concentrations in line with the values already reported.33


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SG

SWK

HWK

WS

RH

94.3

96.7

99.2

94.1

95.0

Aliphatic -OH

1.69

2.23

1.46

1.84

1.95

S-OH + Cond.

2.02

2.13

3.07

0.82

0.27

G-OH

1.12

2.36

1.01

0.74

0.36

H-OH

0.48

0.34

nd

0.24

0.32

Total phenolic -OH

3.62

4.83

4.08

1.80

0.95

Carboxylic acids groups

1.07

0.59

0.63

0.51

0.52

Mn

1000

4700

4400

4900

5500

Mw

2400

27500

7800

41000

19500

Mp

700

1450

2550

1300

2200

PDI

2.4

5.9

1.8

11.1

3.5

0.7

2.4

2.3

1.5

0.1

2.5

1.7

0.6

0.2

1.0

Purity degree (%) 31

P NMR Assign.

(mmol/g)

GPC MW Indexes (g/mol)

Sulfur Content (%) Ash Content (%)

Table 1. Quantification of hydroxyl groups as detected by

31

P NMR analysis, average molecular

weight indexes according to GPC analyses, % purity based on lignin content and sulfur content for the five lignins under investigation.

Lignin-NR composites. Lignin Incorporation. Since lignin aromatic backbone contains polar functional groups, a relatively poor interaction with hydrophobic rubber can take place and lead to the formation of large 15 ACS Paragon Plus Environment


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particles and aggregates. Insufficient filler dispersion can compromise its antioxidant and reinforcing capability and undermine the overall properties of the compound. It is already reported that high purity of lignin was enhancing dispersion in rubber, nonetheless it was not possible to achieve a satisfactory dispersion with unmodified lignin due to incompatibility with hydrophobic rubber.19 Chemical modifications of lignin such as acetylation of hydroxyl groups are reported to improve the compatibility with the polymeric matrix and to promote a better dispersion.16 However, integrity of phenolic hydroxyl groups must be safeguarded as their modification results in the inhibition of antioxidant properties.23 In order to prepare lignin-rubber blends two strategies can be used: dry-milling and coprecipitation. As previously demonstrated the incorporation of purified lignin into dry rubber does not lead to reinforcement.19 On the other hand, lignin-natural rubber compounds obtained after co-precipitation method demonstrated the possibility to achieve improved mechanical properties.34,35 Coprecipitation takes advantage of the akin pH-responsiveness of natural rubber latex and lignin. Natural rubber latex is a stable dispersion of polymer micro-particles in a alkaline solution. Ammonia is added to the latex to prevent premature coagulation; in fact, by neutralizing the activity of micro-organisms it hinders acidification and ensures long term preservation. Phenolic hydroxyl and carboxylic groups confer to lignin the ability to dissolve in alkaline solutions, at lower pH values, on the contrary, the (re)protonation of the dissociated functionalities induces the precipitation of the biopolymer. This implies that an alkaline solution containing lignin can be added to natural rubber latex. Subsequently lignin and rubber can be co-precipitated adding an acidic solution. As a result, lignin is incorporated in the coagulating rubber. The outcome is a composite material where smaller particles of lignin are more homogenously dispersed in the rubber matrix. The quality of the dispersions was qualitatively assessed analyzing SEM images of cut surfaces (Figure 3). Comparing the dispersions obtained (dry-mixing, left (a), coprecipitation, 16 ACS Paragon Plus Environment

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middle (b) and coprecipitation magnified, right(c)) it is clear that the coprecipitation is much more effective in providing a homogeneous distribution of lignin. During dry-mixing, the internal chamber mixer was not able to break down relatively large particles and, in the SEM image, it is possible to observe heterogeneous lignin aggregates with irregular shape and sizes in the 1-10 μm range. In coprecipitation, the ability of lignin to dissolve in alkaline solutions is exploited in order to achieve dispersion at the molecular level. During the coagulation of natural rubber latex, the solubility of lignin drops and the growing particles start to flocculate, remaining entangled in the coagulating matrix. From the SEM images (c), it is indeed possible to observe more regular lignin aggregates with a sub-micrometer dimension.

Figure 3. SEM microscopy images of SG lignin incorporated in NR by Dry-Mixing a) and coprecipitation b) - c).

The average particle size is one of the main parameters affecting mechanical properties of rubber composites. It is well known that the filler particle size determines the effective contact area between the filler and polymeric matrix: in term of mechanical properties, fillers with particle size larger than 10 µm do not have reinforcement capabilities or have negative effects. Fillers with



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particle size between 1 to 10 µm are used as diluents. Finally, semi-reinforcing fillers range from 100 to 1000 nm.36 The low dispersion of lignin in the dry-mixing approach is clearly due to the strong intermolecular interactions (hydrogen bonds and π-π stacking) that hold together lignin particles, preventing adequate dispersion. Moreover, the particle size affects the filler available surface area and it could produce a detrimental effect in the antioxidant properties, reducing the concentration of the active chemical functionalities such as phenolic hydroxyl groups. Hence, coprecipitation technique allows obtaining a better dispersion of the lignin while retaining its desirable characteristics; accordingly, it was selected for the preparation of lignin-NR masterbatches to be used for the evaluation of thermal stabilities and mechanical properties. Antioxidant properties in lignin-NR masterbatches. The main target of the present work was to study the influence of lignin structure and composition on the thermal stability of natural rubber. The stabilization effect provided by the presence of antioxidants on natural rubber can be assessed by differential scanning calorimetry (DSC), measuring the oxidation induction time (OIT) in isothermal conditions.37 Few milligrams of the sample are heated under nitrogen until a constant temperature is reached (170 °C). The atmosphere is then brought back to standard oxidative conditions and the onset time of the first exothermic peak is measured. This time span is defined as the OIT (Oxidative Induction Time). The protection time was assumed to be the difference between each sample OIT and the corresponding OIT of the blank reference constituted by neat natural rubber. According to the results reported in Table 2, a substantial variance in the antioxidant properties of the different lignins is observed.

The protection time spans in a

considerable range, from the lower value of the RH lignin-NR masterbatch (2.7 minutes) to the higher value of the SG-NR one (54.6 minutes).


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Onset time

Protection time

(min)

(min)

SG

55,6

54,6

SWK

44,2

42,4

HWK

23,6

21,7

WS

18,5

16,6

RH

4,5

2,7

NR (neat)

1,9

0

Table 2. Oxidative induction times (OIT) and protection times (in minutes) measured for lignin-NR masterbatches at lignin loadings of 15 PHR and neat NR reference (natural rubber only).

The data seems to confirm the pivotal role of the phenolic moiety, however, a consistent linear correlation between protection time and phenolic hydroxyl groups concentration was not found. Higher phenolic contents in lignin (total phenolic content: SWK>HWK>SG>WS>RH) did not necessarily produce a more effective protection against NR degradation, even if their abundance seems to be clearly related to the OIT’s extent. It was already proven that also solubility and mobility in the rubber phase affect the effectiveness of synthetic antioxidants.38 The detrimental effect of high molecular weights on antioxidant properties of lignin in polypropylene composites was reported and correlated to the lower solubility of heavier fractions.39 Lignin solubility was reported to play a major role and higher phenolic concentrations were also found to induce a negative effect on antioxidant properties due to the reduced compatibility with the polymeric matrix. Using the coprecipitation technique we obtained a good dispersion of lignin and indeed we assumed that the mobility of lignin in the rubber phase could play a primary role. The ability to migrate through the rubber is a desirable characteristic of antioxidants: they can move from the bulk of the sample towards the surface, 19 ACS Paragon Plus Environment


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where they are effectively depleted. It is well established that the rate of diffusion is inversely proportional to the molecular weight of the diffused material.40 In fact, the higher OIT was achieved by the lignin-NR masterbatch containing the smaller SG lignin and a good correlation between phenolics concentration and antioxidant activity could be envisaged when also molecular weight (expressed by the Mp value: SGSG). In particular, the compounds containing SWK, HWK and SG lignin displayed a greater value of Young’s modulus - especially at high strain - and superior elongation at break, whereas compounds filled with WS and RH lignin have roughly comparable modulus at low deformations but lower values of tensile strength and elongation at break. The hypothesis was confirmed confronting the ultimate properties of SG lignin filled NR compounds (15 PHR) processed at different temperatures (60 and 90°C) as reported in Figure 6. At high temperature (90°C) tensile strength and elongation at break of the resulting sample (expressed in percent 24 ACS Paragon Plus Environment

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relatively to the neat NR reference) are higher respect to that of the sample processed at lower temperature (60°C). This indicates that when natural rubber is exposed to mechanical and thermooxidative stresses the protective effect of lignin exerts a decisive role in the in the definition of the mechanical properties.

223% 200

150

139% 116% 98%

100

50

0 Tensile Strength

Elongation at break 60 °C

90 °C

Figure 6. Tensile strength and elongation at break of SG lignin filled NR compounds (15 PHR) for different processing temperatures (60 and 90°C). Values are expressed as a % relatively to the values of neat NR references.

The microscopic mechanisms accountable for the changes observed in the macroscopic properties of lignin filled natural rubber compounds are not simple to rationalize. Nonetheless it seems reasonable that beside the protection against degradative processes provided by phenolic moieties, other characteristics of lignin can exert a decisive role in the definition of the overall mechanical properties. In fact, in tensile tests the more interesting results were obtained by HWK and SWK, while the best result in term of protection time (OIT) was obtained for SG. This behavior could be associated with the peculiar chemical structure of the SWK and HWK characterized by a higher amount of condensed structures. That may improve the stiffness of the filler providing 25 ACS Paragon Plus Environment


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good resistance against deformation. Another possibility is that the organic sulfur could react during vulcanization taking into account the high modulus measured for the WS lignin filled compounds at 300% elongation. This hypothesis would be supported by the sulfur % content of the WS lignin, which is second only to kraft lignins. As a matter of fact, the presence of thiol groups in the structure of kraft lignins is well recognized as well as the capability of mercaptans to react during the vulcanization creating new carbon-sulfur covalent bonds with the unsaturated chains of the polymer matrix. It is supposed that presence of above said groups could promote stronger rubber-filler interactions, which are related to the reinforcing behavior of particulate fillers.41 However, a comprehensive study of the lignin reinforcement ability, when used as filler in rubber compounds, lies outside the aim of this work.

CONCLUSIONS It was confirmed that the chemical and morphological structure of lignin, when used as a filler in composites, exerts a strong influence on the properties of natural rubber compounds. The antioxidant capability in natural rubber blends remarkably varies among different lignin specimens. The thermal stabilization mechanism was rationalized relating its effectiveness to the concentration of active antioxidant species and their ability to migrate through the polymeric matrix. It was assessed that the antioxidant effect of lignin results in improved mechanical properties of natural rubber compounds also before aging, especially at high strains. It was also noted that various structural features of lignin, unrelated to antioxidant capability and molecular weight, could also affect the tensile strength of the lignin filled compounds. In the light of the collected evidences, it is possible to assert that abundant and low-priced technical lignins are suitable for an effective utilization in natural rubber compounds to increase both thermal stability and mechanical properties. Moreover, it is clear that these results could be useful if additional 26 ACS Paragon Plus Environment

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purification step, fractionation processes and chemical modifications will be undertaken with the aim to produce tailored-lignin to conveniently improve the handling and performance of the compounds.

Funding Sources The PhD Scholarships of Davide Barana and Syed Danish Ali are funded by Corimav-Pirelli.



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For Table of Contents Use Only

TOC graphic

Synopsis Thermal stability and mechanical properties of lignin-filled natural rubber compounds are affected by lignin botanical source and extraction process.

Manuscript Title Influence of lignin features on thermal stability and mechanical properties of natural rubber compounds

Name of all the authors Davide Barana, Syed Danish Ali, Anika Salanti, Marco Orlandi, Luca Castellani, Thomas Hanel, Luca Zoia


Influence of Lignin Features on Thermal Stability and Mechanical

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