Influence of lignin source and esterification on properties of lignin-polyethylene blends

Industrial Crops and Products 86 (2016) 320–328

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Influence of lignin source and esterification on properties of lignin-polyethylene blends Laura Dehne a , Carlos Vila Babarro b,c , Bodo Saake a,∗ , Katrin U. Schwarz a a b c

Department of Wood Science, Chemical Wood Technology, University of Hamburg, Leuschnerstraße 91b, 21031 Hamburg, Germany Thünen Institute of Wood Research, Leuschnerstraße 91b, 21031 Hamburg, Germany Chemical Engineering Department, University of Vigo, Faculty of Sciences, 32004 Ourense, Spain

a r t i c l e

i n f o

Article history: Received 21 December 2015 Received in revised form 31 March 2016 Accepted 1 April 2016 Keywords: Lignin Esterification Lignin-polyethylene blends Mechanical properties Water absorption

a b s t r a c t Lignins from pulp mills and biorefineries will gain increasing commercial importance in the next years. Their utilisation in polyolefin blends is of growing research interest as it offers a value-added usage of lignin as well as substitution of non-renewable resources. However, the low compatibility of lignin and polyolefins restrains a satisfying blend production and leads to poor mechanical properties. This can be overcome by modifying lignin prior to its incorporation into polymers in order to reduce its polarity. In this study, five lignins from different raw material sources and production processes were esterified with acetic, propionic and butyric anhydride and subsequently mixed with polyethylene with a weight ratio of 1:1. The paper provides a comprehensive and systematic evaluation of the influence of esterification as well as chemical composition of lignins on mechanical properties and, for the first time, water absorption of lignin-polyethylene blends. Properties of blends were found to improve upon lignin esterification. Compared to blends with unmodified lignin, a progressive increase of tensile (+45%) and flexural (+30%) strength could be observed with increasing length of the ester carbon chain. Lignin source and chemical composition affect water absorption of blends, but show no significant effect on mechanical properties as similar values were observed for blends produced with the respective lignin esters. © 2016 Published by Elsevier B.V.

1. Introduction Lignin is an amorphous, aromatic, natural organic macromolecule built up from phenyl propane units. It is one of the most abundant biopolymers on earth, next to cellulose, and is obtained as a by-product from pulp and paper industries. Kraft, sulphite and soda are the established processes from which lignin can be obtained (Strassberger et al., 2014). Furthermore, Organosolv lignins are intensively investigated, although the process is not established on a commercial scale (Pan and Saddler, 2013). Currently, most of the available lignin is incinerated for energy production in the chemical recovery system of pulp mills. In the future, biorefineries, e.g. for cellulosic ethanol production, open up novel sources for lignin production. Here, great potential for lignin utilisation can be postulated (Podschun et al., 2015) as technology

∗ Corresponding author. E-mail addresses: [email protected] (L. Dehne), [email protected] (C. Vila Babarro), [email protected] (B. Saake), [email protected] (K.U. Schwarz). http://dx.doi.org/10.1016/j.indcrop.2016.04.005 0926-6690/© 2016 Published by Elsevier B.V.

for lignin recovery in this sector is advancing (Larsen et al., 2012). Lignin structure and its chemical composition are known to differ according to plant material and are furthermore influenced by the conditions of the extraction process and subsequent treatment (Pouteau et al., 2003). Lignins deriving from pulp mills have extensively been characterised. For many biorefinery lignins, on the other hand, the knowledge regarding their structure, chemical composition, and properties is still limited. A detailed characterisation, however, is essential to assess lignin utilisation and to evaluate eligible applications. Besides the purity, molar mass distribution and OH group content decide on lignin usability in polymeric systems (Chiemniecki and Glasser, 1988; Pouteau et al., 2003; Pouteau et al., 2004). A lot of research has been conducted on the use of lignin in polymeric systems and numerous comprehensive reports have been published on recent advances (Chung and Washburn, 2013; Doherty et al., 2011; Duval and Lawoko, 2014; Lora and Glasser, 2002; Ten and Vermerris, 2015). Focus was thereby put on lignin application in resins, adhesives and foams as well as polymer blends. In the field of polymer blends, lignin is a promising alternative to substitute inorganic fillers, since it is not abrasive and

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has a low density (Sánchez and Alvarez, 1999). Early investigations were dedicated to the incorporation of unmodified lignin into thermoplastics (Deanin et al., 1978; Pucciariello et al., 2004; Sánchez and Alvarez, 1999). This attempt resulted in an expected increase in stiffness, yet also displayed a significant reduction of tensile strength and elongation. Pucciariello et al. (2004) ascribed these observations to a poor adhesion between the polar lignin and non-polar polymer (in their case PP), which caused a poor stress transfer and yielding at lower tensile stress compared to neat polymer. Moreover, lignin particles were assumed to cause defects in the polymer matrix, acting as stress concentration points. The use of unmodified lignin is therefore limited (Duval and Lawoko, 2014). In their review on biobased polymers, Laurichesse and Avérous (2014) only recently stated that products based on the simple addition or incorporation of lignin were too brittle. Chiemniecki and Glasser (1988) furthermore related the deterioration of mechanical properties to a poor miscibility of lignin and polymer. Due to differences in the polarity, the solubility of lignin in the polymer matrix is low (Thielemans and Wool, 2005), indicating a low compatibility of lignin and polymer. According to Feldmann (2002), the elucidation of the impact of material interactions on their miscibility in blends is one of the most noticeable advances in this research area over the last decades. Different strategies were developed to enhance lignin compatibility with polymers. One strategy is the addition of coupling agents such as ethylene-vinyl acetate (Alexy et al., 2004; Glasser et al., 1988), maleic anhydride grafted PP and PE (Sailaja and Deepthi, 2010; Toriz et al., 2002) or a mixture of both (Luo et al., 2009). The addition of coupling agents enhanced the compatibility of blends, yielding in higher tensile strengths compared to blends without coupling agent. A further strategy is the chemical modification of lignin, i.e. a derivatisation of its hydroxyl groups, as they are highly reactive and additionally responsible for lignin’s polar character. Hence, their modification can introduce new reactive sites and at the same time reduce lignin polarity. Several approaches can be found in the literature, comprising the grafting of lignin with non-polar polymers (Casanave et al., 1996; Sailaja, 2005), alkylation (Chen et al., 2011; Li and Sarkanen, 2002, 2005; Maldhure et al., 2012), as well as esterification (Maldhure et al., 2011, 2012; Teramoto et al., 2009; Thielemans and Wool, 2005). The two last procedures are the more common ones (Duval and Lawoko, 2014), whereby in a direct comparison, esterified lignin was found to lead to less deterioration of mechanical properties than alkylated one (Maldhure et al., 2011). Regarding the esterification of lignin, Lewis and Brauns (1947) reported on the modification of lignin hydroxyl groups with carboxylic acid anhydrides, varying the chain length from C2 to C18 . A reduction of the melting point and polarity of the lignin was observed with increasing length of the ester carbon chain. A similar approach was followed by Thielemans and Wool (2005), who esterified lignin with carboxylic acid anhydrides and N-methylimidazole as catalyst, showing that the solubility of lignin in non-polar solvent increases with increasing length of the ester carbon chain. Recently, Teramoto et al. (2009) reported a successful increase in the miscibility of Organosolv lignin and poly (-caprolatone) after esterification of lignin with C2 –C5 anhydrides. In the present paper, further investigations on the esterification of lignin with carboxylic acid anhydrides are performed. Therefore, five different technical lignins from conventional and biorefinery processing routes were derivatised and subsequently blended with polyethylene (PE-HD) at a weight ratio of 1:1. Detailed chemical analyses of the unmodified and derivatised lignins as well as mechanical and water absorption tests of lignin-polyethylene (LPE) blends were performed. Based on the results, the influence of lignin source/production process and lignin modification on blend properties was evaluated.

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2. Material and methods 2.1. Materials 2.1.1. Lignins Industrial hardwood (Eucalyptus spp.) and softwood (Pinus spp./Picea spp.) Kraft lignins (HW-KL, SW-KL) were provided by Suzano Pulp and Paper, Brazil, and Stora Enso, Sweden, respectively. Organosolv (Picea spp.) lignin (OSL) from sodium hydroxide assisted methanol-water pulping derived from the former Organocell GmbH, Germany. Commercially available Soda (Triticum spp.) lignin (SGL) was purchased from Green Value SA, Switzerland. Hydrolysis (Triticum spp.) lignin (HL) derived from a bioethanol plant and was obtained after steaming followed by hydrolysis and alkaline purification. 2.1.2. Polymer High density polyethylene (PE-HD) named Hostalen GC 7260 (LyondellBasell, Germany) served as matrix polymer for the ligninpolyethylene blends. Density: 0.96 g/cm3 , MFR: 8 g/10 min (190 ◦ C, 2.16 kg). 2.2. Methods 2.2.1. Esterification of lignin Esterification of lignin samples was performed according to a revised procedure by Thielemans and Wool (2005). Lignin was dissolved in acetic (100%; Roth, Germany), propionic (100%; Roth, Germany), and butyric anhydride (100%, Merck, Germany), respectively, with a 2:1 weight ratio of solvent to lignin, adding 0.01 ml N-methylimidazole (99%; Roth, Germany) as catalyst per gram of lignin. The reaction was performed at 50 ◦ C for 3 h in a stirred glass reactor. Afterwards, the solution was poured into cold deionised water to quench the reaction and precipitate the lignin derivative. The solid was filtered and washed until the pH of the filtrate reached five. Lignin derivatives were dried in an oven at 50 ◦ C. Modification yield was calculated based on the molecular weight, which was calculated from the C9 -formula, assuming a complete esterification of the lignin. Yields between 88 and 99% were achieved, whereby the yield increased with increasing length of the ester carbon chain. Derivatives of HW-KL showed distinctly lower values (65–70%). 2.2.2. Chemical characterisation Lignins were subjected to quantitative 2-step hydrolysis with sulfuric acid (Lorenz et al., 2015). The solid residue after hydrolysis was recovered by filtration and considered as acid-insoluble lignin. Acid-soluble lignin was spectrophotometrically determined at 560 nm as described by Tappi UM250 um-83 (1991). Polysaccharide content of the hydrolysates was determined by borate anion-exchange chromatography with post-column derivatisation and detection at 560 nm as described by Lorenz et al. (2015). Ash content was determined gravimetrically as prescribed by TAPPI T 211 om-93 (1993) after incineration at elevated temperature of 800 ◦ C. Higher temperature was chosen since lignin samples were not completely incinerated at 525 ◦ C. Elemental analysis was performed using CE Instruments CHNS Flash 1112 analyser system, calculating the oxygen as difference from total sum (ash corrected). Standard protocol by Vieböck and Schwappach (1930) was followed to quantify methoxy group content. Size exclusion chromatography (SEC) of lignin samples was performed in dimethyl sulfoxide with 0.1% LiBr as eluent according to a protocol published by Schütt et al. (2013). Hydroxyl group content was determined by 31 P NMR spectroscopy, following the methodology described by Argyropoulos et al. (1993). Changes in the chemical structure of the lignins upon esterification were further analysed by Fourier Transform Infrared Spectroscopy (FTIR) using Bruker Vector 33 in

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ATR (attenuated total reflection) mode. Spectra were collected from 3750 cm−1 to 700 cm−1 with 60 cumulative scans and a resolution of 2 cm−1 . 2.2.3. Manufacturing and testing of lignin-polyethylene blends Melt mixing of PE-HD with unmodified and modified lignins was accomplished using a Haake Rheomix OS 3000 torque-rheometer with Banbury rotors (Thermo Fisher Scientific, Germany). Lignin content was kept constant at 50 wt.%. Mixing was performed for 15 min at 160 ◦ C and 50 rpm, adding the lignin powder after stabilisation of the PE-HD torque curve. Test specimens were injection moulded using a Haake minijet II (Thermo Fisher Scientific, Germany). Melt temperature was adjusted between 160 and 168 ◦ C depending on the applied acid anhydride. Casting moulds had a constant temperature of 90 ◦ C; injection pressure and holding pressure were kept at 500 and 300 bar, respectively. For the tensile tests, dumbbell-shaped specimens (type 1BA) were prepared according to DIN EN ISO 527-2:2012-06. Rod-shaped specimens were made according to DIN EN ISO 178:2013-09 for flexural, Charpy impact and water storage tests. Investigations of tensile and flexural properties were conducted using a Zwick Roell Z050. Tensile tests were performed at 1 mm/min test speed and 20 mm gauge length. For the flexural test, 2 mm/min test speed and 64 mm supporting width were set. Test specimens were conditioned and tested at 20 ◦ C/␸ = 65%. Charpy impact strength was investigated according to DIN EN ISO 179:2010-11 with a Zwick Roell HIT 5.5P equipped with a 1 J hammer (ISO 179-1/1eU). Determination of water absorption was evaluated according to EN ISO 317:1993 and DIN EN ISO 62:200805. Rod-shaped specimens were stored in water (20 ◦ C) for 28 days. Dimensions and weight of conditioned (20 ◦ C/␸ = 65%) specimens were recorded before and right after water storage in a 5-fold determination with a digital calliper and balance, respectively. Graphic illustrations of test results depict mean values and respective 95% confidence levels. 3. Results and discussion Unmodified and derivatised lignins were subjected to chemical analyses to evaluate alterations due to the esterification with acid anhydrides and to correlate the observed chemical characteristics to the resulting mechanical properties and water absorption of lignin-polyethylene blends. 3.1. Chemical characterisation of unmodified lignins Purity of lignins was evaluated based on lignin (acid soluble and acid insoluble), carbohydrate and ash content as depicted in Table 1. Lignin content was determined to be higher than 88% for SW-KL, OSL, and SGL, while HW-KL and HL showed distinctly lower values. For HW-KL this can be explained by the high amount of ash. In case of HL, this is related to the big portion of remaining carbohydrates (13%). The significantly high amount of glucose found in HL indicates an incomplete enzymatic hydrolysis of cellulose in

Table 1 Lignin, carbohydrate and ash content of unmodified lignins. Lignin

HW-KL SW-KL OSL SGL HL

Carbohydrates

Ash

acid insol. %

acid sol. %

 %

Xyl %

Glu %

%

72.8 90.2 83.5 85.0 70.8

10.0 5.1 4.7 7.8 3.3

1.3 2.2 1.4 2.9 13.0

0.7 0.8 0.4 1.6 1.4

0.2 0.2 0.1 0.5 10.8

11.6 0.6 6.5 2.7 3.8

the production process. Carbohydrate content of the other studied lignins was low, ranging from 1.3% (HW-KL) to 2.9% (SGL), corresponding to observations recently made by Schorr et al. (2014). Sugar compositions of the studied lignins differed according to their raw material. In all samples but HL, xylose was the predominant sugar (up to 1.6% in SGL); glucose, mannose, arabinose, and galactose were present at small amounts. Ash content of the studied lignins varied distinctively from 0.6% (SW-KL) to 11.6% (HW-KL) as a result of the different extraction and purification procedures. Purity of lignins was furthermore evaluated based on their elemental composition, whereby the sulphur and nitrogen content were of main interest (Table 2). The nitrogen content can be seen as an indicator for remaining proteins in the lignin. Highest nitrogen content was found in HL and SGL, which may arise from the naturally high protein, and therefore nitrogen, content of wheat straw. Elemental analyses of Soda grass lignin (Protobind 2400) published by Sahoo et al. (2011) and Gonugunta (2012) both give a nitrogen content of 0.62%, while Schorr et al. (2014) detected 0.93%. Differences among these values and our findings can be related to different batches and purification procedures. In case of HL, the high nitrogen content may as well derive from residual protein structures from the biotechnical processing steps. Kraft lignins (HW-KL and SW-KL) showed highest sulphur content of around 3% which was expected due to the use of sodium sulphide (Na2 S) in the pulping process. Deviations from lower values (1–2%) reported in the literature (Laurichesse and Avérous, 2014) may result from differences in the raw materials and purification protocols. A noticeably high amount of sulphur was as well detected in SGL (1.2%), which is surprising since the wheat straw was processed in sulphur-free medium. In the literature, sulphur content of Soda grass lignin (Protobind 2400) is given in a range from 0.5% (Sahoo et al., 2011) to 2.0% (Schorr et al., 2014). It could be assumed that the sulphur derives from H2 SO4 used in the precipitation protocols of the manufacturer or from proteins in the wheat straw. To further analyse the source of sulphur, SGL was purified and analysed as described by Podschun et al. (2015). The ash free lignin still contained 0.8% sulphur, which is most likely bound to the lignin and can therefore not be removed. Since wheat straw contains only up to 0.21% sulphur (Byers and Bolton, 1979), residual sulphur must have been enriched in the lignin fraction. As was expected, OSL was found to be sulphur-free since neither the pulping process nor subsequent precipitation and washing steps involved any sulphur containing chemicals.

Table 2 Elemental composition, OH and MeO group content of unmodified lignins, data are corrected by ash content. N.d. not determined.

HW-KL SW-KL OSL SGL HL

Elemental composition

MeO

OH content

N %

S %

 phen. OH mmol/g

 aliphat. OH mmol/g

phen./aliphat. OH

%

0.3 0.2 0.5 0.8 1.9

2.8 3.1 0.0 1.2 0.2

18.8 12.7 11.2 13.8 n.d.

3.16 5.23 2.47 2.87 0.12

1.54 2.74 2.43 1.51 3.14

2.05 1.91 1.02 1.90 0.31

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Fig. 2. Normalised FTIR spectra of unmodified and derivatised HW-KL lignins.

Fig. 1. Molar mass distribution curve, weight average molar mass (MW (g/mol)) and polydispersity (PDI) of unmodified lignins.

The results of the methoxy group determination are shown in Table 2. The values detected for HW-KL (18.8%), SW-KL (12.7%) and OSL (11.2%) are in good agreement to a compilation of data published by Fengel and Wegener (1984), giving methoxy group contents of 18–22% for hardwood and 12–16% for softwood lignin. The methoxy group content of SGL amounted to13.8%. Phenolic and aliphatic hydroxyl group contents are summarised in Table 2. The amount of phenolic OH groups was highest in Kraft lignins and particularly in SW-KL. This can be explained by the harsh conditions during Kraft pulping, leading to a cleavage of ether bonds and thus resulting in an increased number of hydroxyl groups. Lower OH group contents observed for OSL and SGL can be related to milder pulping conditions that lead to less fragmentation of the lignins. Significantly lowest amount of OH groups was determined for HL, which can as well be ascribed to the mild processing conditions that do not allow for cleavage of ether bonds, leaving the lignin macromolecules intact. From the phenolic and aliphatic OH groups, the total number of OH groups can be calculated, giving similar data for HW-KL (4.70 mmol/g), OSL (4.90 mmol/g), and SGL (4.38 mmol/g). As expected, SW-KL and HL showed distinctly higher (7.97 mmol/g) and lower (3.26 mmol/g) values, respectively. Phenolic/aliphatic OH ratios, listed in Table 2, prove the higher amount of phenolic OH in HW-KL, SW-KL, and SGL with OH group ratios close to two, while OSL contains phenolic and aliphatic OH groups in equal amounts. HL has a noticeably low OH group ratio due to high amount of aliphatic groups. Size exclusion chromatography was performed to determine the molecular weight of lignins. In Fig. 1 the weight average molar mass (MW ), polydispersity index (PDI = MW /Mn ) and molar mass distribution curves of the studied unmodified lignins are shown. The lignins varied significantly in their weight average molar masses, which can be related to the biomass and to the different production processes of the lignins. Hydrolysis wheat straw lignin (HL) showed the by far highest MW . This fits well to the low amount of phenolic OH groups detected by 31 P NMR spectroscopy (Table 2), which already indicated little cleavage of ether bonds and fractionation of the lignin during the production process. The molar mass distribution curve displayed a distinct portion of lignin with high molar mass. Second highest weight average molar mass was determined for SW-KL lignin. This is surprising, considering the high amount of phenolic OH groups for this sample and might be related to a naturally more condensed structure. The distribution curve displays a comparably low amount of small lignin fragments and a significant shift to higher molar mass when compared to HW-KL, SGL,

and OSL. HW-KL was determined to have lowest MW , which can be ascribed to the lower condensation of hardwood lignin as well as harsh conditions during Kraft pulping. In the molar mass distribution curve of HW-KL a distinct fraction of small fragments can be seen. The weight average molar masses observed for SGL and OSL are in between those determined for HW-KL and SW-KL. For all lignins, polydispersity index (PDI) was high, ranging from 3.3 for HW-KL to 4.7 for SW-KL. These data are in good agreement with data recently published by Schorr et al. (2014). 3.2. Chemical characterisation of derivatised lignins Complete esterification of the five lignins with acid anhydrides was confirmed by 31 P NMR (not shown) and FTIR spectroscopy. FTIR spectra, exemplarily shown for HW-KL, prove a complete disappearance of hydroxyl group signals (3050–3700 cm−1 ) in the spectra of lignin derivatives (Fig. 2). New peaks corresponding to the carbonyl group in the ester were detected at 1757 cm−1 (phenolic) and 1740 cm−1 (aliphatic) (Glasser and Jain, 1993; Olsson et al., 2011). Moreover, an increasingly precise shaping of signals at 2876 and 2937 cm−1 (CH2 -valence vibrations) and 2966 cm−1 (CH3 -valence vibrations) can be noted with increasing length of the ester carbon chains. In the spectra of the butyrated lignin, a further new peak is visible at 748 cm−1 , which can also be assigned to the aliphatic chain of the substituent. The signal of the methoxy group at 2843 cm−1 was not affected by the modification of lignins. The signal at 2300 cm−1 derives from CO2 in the surrounding air and can be neglected. Modification with acid anhydrides led to a purification of the lignins, as was evaluated based on lignin (Table 3), carbohydrate (Fig. 3) and ash content (Fig. 4). The total lignin content, determined from the amount of acid soluble and acid insoluble lignin, decreased by 9% (HW-KL) to 23% (SGL) compared to the unmodified lignins. For each of the lignins, the reduction of lignin content was observed to be less pronounced with increasing length of the ester carbon chain. This is due to the fact that the esters are cleaved during acid hydrolysis and are not included into the gravimetric lignin-determination. Comparing the carbohydrate content of unmodified and derivatised lignins, a reduction of the sugar content can be observed. Similar to the lignin content, a constant decrease of the amount of sugar was observed with increasing length of the ester carbon chain, which can again at least partly be related to the weight differences of the ester substituents. Xylose remained the major sugar in all lignins, except for HL that mainly contains glucose. It can be assumed that the carbohydrates detected in HL are mainly cellulose, which is derivatised as well. Ash was almost entirely removed from the derivatives, irrespective of the initial ash content of the unmodified lignins, which furthermore explains the lower yield of the HW-KL derivatives after modification. However,

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Table 3 Lignin content, elemental composition (corrected by ash content), molar mass and polydispersity of derivatised lignins. Lignin

aHW-KL pHW-KL bHW-KL aSW-KL pSW-KL bSW-KL aOSL pOSL bOSL aSGL pSGL bSGL aHL pHL bHL

Elemental composition

Molar mass

Polydispersity

acid insol. %

acid sol. %

N %

S %

MW g/mol

PDI

71.8 73.1 76.0 76.1 81.6 83.1 73.5 76.0 82.7 76.0 75.7 75.1 65.5 62.8 66.2

7.8 8.7 0.0 2.7 1.9 0.0 7.0 5.0 0.0 4.1 0.0 0.0 2.6 3.2 2.5

0.4 0.4 0.5 0.4 0.4 0.3 0.7 0.9 0.9 0.4 0.5 0.4 1.8 1.8 1.7

1.4 1.4 1.4 2.3 2.0 2.0 0.6 0.5 0.5 0.0 0.0 0.0 0.2 0.1 0.1

5400 5400 5400 9000 8900 9500 4700 4700 5200 5300 5000 5400 15,400 8900 5400

4.3 4.2 4.1 6.8 6.4 6.5 4.2 4.2 4.3 4.6 4.3 4.4 9.8 6.2 4.2

Fig. 3. Carbohydrate content of unmodified and derivatised lignins.

contents upon esterification, the remaining contents of both are lower than expected. This may be related to a purification effect by the acidic conditions hydrolysing the lignin-carbohydrate-complex bonds and removing ash. Elemental composition of the derivatives is shown in Table 3. In all samples, sulphur content was reduced up to 50% upon modification. The nitrogen content was slightly increased, which might be due to traces of the catalyst. No distinctive differences could be observed for different lengths of the ester carbon chains. Weight average molar masses (MW ) and PDI of the derivatives are listed in Table 3, showing mostly an increase compared to the respective unmodified lignin (Table 1). Thereby, MW tended to be higher for higher esters, which can be explained by the increasing weight gain with increasing length of the ester carbon chains. For HL, however, a significant reduction of the weight average molar mass was observed, which can be related to a cleavage of the lignin macromolecules and an acid hydrolysis of the lignincarbohydrate-complexes during esterification. Molar mass of the starting HL lignin was very high due to the low severity of the steaming process. Accordingly, the lignin still contained acid labile bonds, which were cleaved during esterification. Nevertheless, for all other lignins the observed increase of MW was much lower than could theoretically be expected. The reason for this can be found in the molar mass distribution curves that are exemplarily shown for unmodified and derivatised HW-KL in Fig. 5. Apart from the expected increase of the high molar mass fraction due to the formation of esters, a simultaneously increasing amount of smaller lignin fragments can be detected, which results from degradation reactions during modification. In the SEC both effects are superimposed, altogether leading to an increased polydispersity compared to the raw lignin.

3.3. Lignin-polyethylene blends

Fig. 4. Ash content of unmodified and derivatised lignins.

derivatives of HL were determined to still contain approximately 2% of ash. Regarding the theoretical weight changes that occur due to esterification of lignin related to the altered sugar and ash

Applicability of lignins to manufacture lignin-polyethylene blends (L-PE blends) was evaluated based on mechanical properties and water absorption of blends. The amount of lignin in L-PE blends was set at 50% to clearly see differences among the lignins in the blends. Melt mixing of unmodified and derivatised lignins with polyethylene (PE-HD) resulted in 20 formulations that were encoded according to the lignin type (HW-KL, SW-KL, OSL, SGL, HL) and the applied modification (acetylation a, propionation p, butyration b). Blends containing unmodified lignin will be solely encoded by lignin type.

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nevertheless, comparison of properties between different reports in the literature is limited due to differences in the raw materials, manufacturing and especially in the applied test methodology. Comparing the studied blends with unmodified lignin among each other indicates that the raw material source of the lignin does not have any remarkable effect on the mechanical properties (Fig. 6). With exception of blends with HL, no significant differences in the test results could be observed. Thus, it can be assumed that even though the unmodified lignins show distinct differences in their chemical composition and structure, the lignin source is not the decisive factor to influence the mechanical properties.

Fig. 5. Molar mass distribution curve of unmodified and derivatised hardwood Kraft lignin (HW-KL).

Fig. 6. Tensile strength, elongation and Young’s modulus of lignin-polyethlyene blends with unmodified and derivatised lignins. PE-HD—polyethylene (reference), a—acetylation, p—propionation, b—butyration; *—sample could not be injection moulded properly. Hatched areas: bHW-KL with addition of 1% lubricant. Error bar: 95% confidence level.

3.3.1. Tensile tests 3.3.1.1. Lignin-polyethylene blends with unmodified lignins. Results of the tensile tests are displayed in Fig. 6; neat polyethylene (PE-HD) serves as reference. Lignin-polyethylene blends with 50% unmodified lignin showed tensile strengths between 9 MPa (SGL) and 14 MPa (HL), i.e. a 40–60% reduction compared to neat PE-HD (␴M = 23 MPa). The elongation at maximum strength decreased to 22% of PE-HD (␧M = 11%) in case of HW-KL and to approximately 10% for all other lignins. Corresponding to this, Young’s moduli were observed to increase by 36% (SGL) to 45% (HW-KL). Brittle fractures were observed for all blends with unmodified lignin. The described mechanical properties of L-PE blends can be related to the amorphous structure and inherent mechanical properties of the lignin, bearing a high Young’s modulus and low tenacity as cohesion forces among lignin macromolecules are limited (Toriz et al., 2002). Moreover, a poor compatibility of lignin and matrix polymer can be seen as further explanation (Pouteau et al., 2004). Properties of blends with unmodified lignin have extensively been reviewed in the literature (Pucciariello et al., 2004; Sánchez and Alvarez, 1999);

3.3.1.2. Lignin-polyethylene blends with derivatised lignins. The performed esterification of lignin prior to blending it with PE-HD is beneficial for the tensile strengths (␴M ) of the resulting blends, when compared to blends with unmodified lignin (Fig. 6). Up to 45% higher tensile strengths can be achieved using derivatised lignin. The increase in tensile strength is thereby influenced by the type of lignin modification, i.e. acetylation, propionation, or butyration. With increasing length of the ester carbon chain an increase of the tensile strength was observed. This can possibly be explained by an enhanced miscibility and a more even distribution of lignin particles in the matrix as the lignins become less polar upon esterification of hydroxyl groups. It is moreover conceivable that a better entanglement of the lignin is achieved with increasing length of the ester carbon chains. Altogether, this contributes to an enhanced stress-transfer and therefore higher yielding of the tensile strength. For HW-KL and SGL highest tensile strengths were observed for propionated L-PE blends (␴M = 17 N/mm2 ), while blends with SW-KL and OSL benefit most from acetylation (␴M = 17 N/mm2 ). In contrast to this, esterification of HL is not convenient as blends displayed 16–24% lower tensile strengths than the blend with unmodified HL. This can be attributed to the residual carbohydrates, leading to a mixture of lignin and polysaccharide derivatives. The results suggest that in a derivatised form the mixture of lignin and cellulose imparts proper blending and processing of HL and PE-HD. For the other lignins, Fig. 6 furthermore displays an unexpected reduction of the strength properties of blends with butyrated lignins. A reason for this can be found in the injection moulding process as butyrated L-PE blends tend to stick to the metal surface of the casting mould. Removing the specimen leads to irregularities of its surface and might also induce slight defects in the microstructure of the blend, leading to the deterioration of strength properties. Nevertheless, it has to be pointed out that the tensile strengths of these butyrated L-PE blends were still higher than the ones determined for the respective unmodified L-PE blends. To overcome the affinity of the blends to the casting moulds, 1% of a paraffin waxlike lubricant was added to the butyrated L-PE blends during melt mixing. This enabled an unscathed removal of the specimens. The amended blends yielded tensile strengths of up to 21 MPa, which is a 40% increase compared to butyrated blends without lubricant. As illustrated by the striped area in Fig. 6, the tensile strength is nearly as high as the one determined for neat PE-HD. For reasons of clarity, only the result for the L-PE blend with butyrated HW-KL is shown. In a comparative study with acetylated and propionated L-PE blends no such improvement was observed upon the addition of the lubricant. Thus, it can be stated that best results are achieved for blends with butyrated lignin when a lubricant is added to the blends. Modification of lignin led to a further slight decline of the elongation at maximum strength, which is particularly visible for blends with derivatised HW-KL and SW-KL. This coincides with observations made by Maldhure et al. (2012), who reported lower elongation values for modified blends than for the reference. In the present study, elongation values fluctuated around 10% of the value determined for neat PE-HD. Yet, with increasing length of the

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Fig. 7. Flexural strengths, deflection and fluexural modulus of lignin-polyethylene blends with unmodified and derivatised lignins. PE-HD—polyethylene (reference), a—acetylation, p—propionation, b—butyration; *—sample could not be injection moulded properly. Hatched areas: bHW-KL with addition of 1% lubricant. Error bar: 95% confidence level.

ester carbon chain the elongation of modified L-PE blends tends to increase, which may as well indicate an improved compatibility of lignin and polyethylene upon modification. This can possibly be ascribed to the fact that with increasing length of the ester carbon chain the aliphatic content of the modified lignins increases, resulting in a better compatibility with polyethylene, which is of aliphatic nature. As illustrated by the hatched area in Fig. 6, a slight increase of the elongation value could be observed for the blend with butyrated HW-KL after lubricant was added. Lignin-polyethylene blends with modified lignins are characterised by higher stiffness than neat PE-HD (Et = 1094 MPa). However, comparing the derivatives among each other, Young’s modulus decreased with increasing length of the ester carbon chain, which agrees to the observed increase of elongation values. The reduction of stiffness proposes an enhanced compatibility of lignin and PE-HD, which in turn might reduce the stiffening effect of the lignin. This coincides with observations made by Maldhure et al. (2012). Upon the addition of the lubricant, a slight increase of Young’s modulus was observed (hatched circle, Fig. 6). Comparing the blends with modified lignins among each other, again no distinct impact of the lignin raw material source/processing on the mechanical properties can be seen, particularly since blends with hardwood Kraft (HW-KL) and wheat straw (SGL) lignin show similar results. The deterioration of tensile properties observed for SW-KL and OSL can rather be related to the derivatisation and affinity to the metal surface than to the raw material source/processing. However, L-PE blends with HL indicate that the purity of the lignin has to be considered when modification of lignin is desired. Altogether, esterification of lignin can be seen as the decisive factor to amend the properties of L-PE blends. 3.3.2. Flexural tests 3.3.2.1. Lignin-polyethylene blends with unmodified lignins. The results of the flexural tests shown in Fig. 7 confirm the observations made during the tensile tests. Compared to pure PE-HD as reference, lignin-polyethylene blends altogether showed lower flexural strengths and deflections as well as higher stiffness. Blends with 50% unmodified lignins displayed flexural strengths between 17 MPa (SW-KL) and 23 MPa (HL), which is a reduction of 10–30% compared to neat PE-HD (26 MPa). For all blends, the reduction of flexural strength is less pronounced than of tensile strength, which might be related to the fact that compression forces can be absorbed

Fig. 8. Charpy impact strength of lignin-polyethylene blends with unmodified and derivatised HW-KL. PE-HD—polyethylene (reference), a—acetylation, p—propionation, b—butyration. Error bar: 95% confidence level.

by the lignin. Deflection values decreased to approximately 50% of neat PE-HD, while flexural moduli increased by 53% (HW-KL) to 70% (OSL). Both can be ascribed to the inherent stiffness of lignin as discussed previously. With exception of blends with HL, no distinct differences in the properties could be observed. 3.3.2.2. Lignin-polyethylene blends with derivatised lignins. Upon lignin modification, strength properties of L-PE blends increased. Again, best results were determined for L-PE blends with propionated HW-KL (25 MPa) and SGL (24 MPa), as well as acetylated SW-KL (23 MPa) and OSL (25 MPa) when no lubricant was used. As previously indicated by the tensile tests, modification of HL lignin led to a deterioration of strength properties. Deflection of blends was as well decreased upon lignin modification, showing approximately 30% of the deflection determined for neat PE-HD. A slight increase could, though, be observed with increasing length of the ester carbon chain. Flexural modulus is higher than of neat PE-HD, related to the inherent higher stiffness of lignin. Similar to Young’s modulus, values decrease with increasing length of the ester carbon chain making assume an improved compatibility between lignin and polyethylene. Flexural properties of butyrated L-PE blends with addition of lubricant (bHW-KL) are indicated by the striped/hatched areas in Fig. 7. They show even higher flexural strength than neat PE-HD. This furthermore demonstrates the positive effect of lignin butyration on blend properties. Flexural modulus remained constant upon the addition of lubricant and is therefore not separately illustrated in Fig. 7. Moreover, it can be seen that esterification of lignin rather than its raw material source is the deciding factor to improve blend properties. 3.3.3. Charpy impact tests Charpy impact tests were performed to evaluate the capability of L-PE blends to resist the initiation and propagation of cracks under shock loading. In Fig. 8, Charpy impact strengths of unnotched specimens are depicted; the reference PE-HD did not break under the applied load. The test was exemplarily performed with material containing unmodified and derivatised HW-KL, as the impact strength can qualitatively be estimated based on the elongation values (cf. Fig. 6). Blends with unmodified HW-KL had an impact strength of 4 kJ/m2 . Modification of lignin resulted in a reduction of impact strength similarly to the reduction of elongation values. A slight increase of the impact strength can, however,

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Fig. 9. Volume swelling and weight gain of lignin-polyethylene blends with unmodified lignins and derivatised HW-KL after 28 days water storage. a—acetylation, p—propionation, b—butyration. Error bar: 95% confidence level.

be determined with butyrated blends showing highest values (2.4 kJ/m2 ) of blends with derivatised lignins. 3.3.4. Water storage tests Fig. 9 displays the volume swelling (VS) and weight gain (WG) of L-PE blends with unmodified lignins as well as derivatised HWKL after 28 days of deposition in deionised water. Pure PE-HD as a reference showed no significant volume swelling and weight gain after water storage, which was expected due to its hydrophobic character, and was reported by processors (Kern GmbH, 2015). In contrast to that, all L-PE blends display a distinct VS and WG. This can be ascribed to the chemical features of the lignin, which render it more hydrophilic than PE-HD and promote water absorption; moreover, lignin has an amorphous structure, which could be assumed to loosen the network structure of the L-PE blend in comparison to pure PE-HD. Investigations displayed significant differences among L-PE blends with unmodified lignins, which can be related to the different chemical compositions of the lignins, especially their hydroxyl group content and the amount of remaining sugar and ash. SW-KL showed least water absorption with VS of 0.3% and WG of 1.2%. This lignin sample has high hydroxyl group content, yet shows highest purity among all tested lignins (Tables 1 and 2). By far highest VS and WG were determined for OSL with an increase of approximately 21% for both parameters. Blends with OSL were the only ones to show bulges on the surface which were filled with water after the storage. It can therefore be assumed that in this case water absorption is primarily promoted by a loose blend structure rather than by the hydrophilicity of the lignin. Against expectations, blends with HL showed a low volume swelling, even though sugar content is higher than for all other lignins. This can be related to the fact that the carbohydrates are mainly cellulose, which in contrast to hemicelluloses binds some water but does not swell distinctly. This also explains the distinctly higher WG than VS that was observed for HL. Regarding the chemical composition of the unmodified lignins, the ash content can be assumed to be the main influencing factor, as VS and WG were observed to be higher when ash content was high. Comparing blends with unmodified and derivatised HW-KL, a distinct reduction of volume swelling and weight gain can be observed for the derivatised ones. This can be related to the esterification of hydroxyl groups as well as significantly reduced sugar and ash contents (Figs. 3 and 4). Weight gain was equal for all three derivatives, whereas volume swelling was reduced with increasing length of the ester carbon chain, indicating a progressively more hydrophobic character of the lignin. Based on these observations, similar results can be expected for the other blends with derivatised lignins.

The influence of lignin raw material, production process, and esterification was evaluated for blends of polyethylene and five conventional and biorefinery lignins. Chemical characterisation of unmodified lignins showed distinct differences related to plant origin and processing conditions. Modification rendered lignin derivatives more pure and less polar, as sugar and ash content were decreased and hydroxyl groups were completely esterified. Melt mixing of blends showed that derivatives exhibit better miscibility with polyethylene than unmodified lignins. Mechanical properties were determined by tensile, flexural and Charpy impact tests. The results show no distinct influence of the raw material source on mechanical blend properties. Esterification of lignin, though, turned out to be beneficial for blend properties, which are progressively enhanced with increasing length of the ester carbon chains. Highest strengths were achieved with butyrated lignins, when adding a lubricant to the blend formulation. Water storage tests showed that the incorporation of lignin promotes water absorption of blends. Weight gain and volume swelling differed significantly according to the raw material source. For derivatised blends, water absorption was reduced to a minimum. Acknowledgements This research was funded by the Federal Ministry of Food and Agriculture (BMEL) and supported by WoodWisdom-EraNet and the Fachagentur Nachwachsende Rohstoffe e.V. (FNR project ProLignin, no.: 22020811). The authors would like to thank Stora Enso and Suzano for provision of lignins as well as Heimo Kanerva and Tiina Liitia¨ from VTT (Espoo, Finland) for purification of the hydrolysis lignin. The authors gratefully acknowledge Jacob Podschun for determination of residual sulphur content in Soda grass lignin. References Alexy, P., Kosikova, B., Crkonova, G., Gregorova, A., Martis, P., 2004. Modification of lingin-polyethylene blends with high lignin content using ethylene-vinylacete copolymer as modifier. J. Appl. Polym. Sci. 94, 1855–1860. Argyropoulos, D.S., Bolker, H.I., Heitner, C., Archipov, Y., 1993. 31 P NMR spectroscopy in wood chemistry part V. Qualitative analysis of lignin functional groups. J. Wood Chem. Technol. 13 (2), 187–212. Byers, M., Bolton, J., 1979. Effects of nitrogen and sulfur fertilizers on the yield, N and S content, and amino acid composition of the grain of spring wheat. J. Sci. Food Agric. 30, 251–263. Casanave, S., Aït-Kadi, A., Riedl, B., 1996. Mechanical behaviour of highly filled lignin/polyethylene composites made by catalytic grafting. Can. J. Chem. Eng. 74 (2), 308–315. Chen, F., Dai, H., Dong, X., Yang, J., Zhong, M., 2011. Physical properties of lignin-based polypropylene blends. Polym. Compos. 32 (7), 1019–1025. Chiemniecki, S.L., Glasser, W.G., 1988. Multiphase materials with lignin: 1: blend of hydroxypropyl lignin with poly(methylmethacrylate). Polymer 29, 1021–1029. Chung, H., Washburn, N.R., 2013. Chemistry of lignin-based materials. Green Mater. 1 (3), 137–160. DIN EN ISO 178:2013-09. Kunststoffe—Bestimmung der Biegeeigenschaften (ISO 178:2010 + Amd.1:2013), Deutsche Fassung EN ISO 178:2010 + A1:2013. DIN EN ISO 179:2010-11. Kunststoffe—Bestimmung der Charpy-Schlageigenschaften—Teil 1: Nicht instrumentierte Schlagzähigkeitsprüfung (ISO 179-1:2010), Deutsche Fassung EN ISO 179-1:2010. DIN EN ISO 527-2:2012-06. Kunststoffe—Bestimmung der Zugeigenschaften—Teil 2: Prüfbedingungen für Form- und Extrusionsmassen (ISO 527-2:2012), Deutsche Fassung EN ISO 527-2:2012. DIN EN ISO 62:2008-05. Kunststoffe—Bestimmung der Wasseraufnahme (ISO 62:2008), Deutsche Fassung EN ISO 62:2008. Deanin, R.D., Driscoll, S.B., Cook, R.J., Dubreuil, M.P., Hellmuth, W.N., Shaker, A., 1978. Lignin as a Filler in Commodity Thermoplastics. Technical Papers, Society of Plastics Engeneers, ID gnd/3411-3, Greenwich, Conn. Soc. 0096-8773, Bd. 24. Doherty, W.O.S., Mousavioun, P., Fellows, C.M., 2011. Value-adding to cellulosic ethanol: lignin polymers. Ind. Crops Prod. 33 (2), 259–276. Duval, A., Lawoko, M., 2014. A review on lignin-based polymeric, micro- and nano-structured materials. React. Funct. Polym. 85, 78–96. EN ISO 317:1993. Spanplatten und Faserplatten, Bestimmung der Dickenquellung nach Wasserlagerung, Deutsche Fassung EN 317:1993.

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