Xylan and xylan derivatives—Their performance in bio-based films and effect of glycerol addition

Industrial Crops and Products 94 (2016) 682–689

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Xylan and xylan derivatives—Their performance in bio-based films and effect of glycerol addition Sónia Sousa a , Ana Ramos a , Dmitry V. Evtuguin b , José A.F. Gamelas c,∗ a b c

FibEnTech and Department of Chemistry, University of Beira Interior, 6201-001 Covilhã, Portugal CICECO and Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal Department of Chemical Engineering, CIEPQPF, University of Coimbra, Pólo II—R. Silvio Lima, 3030-790 Coimbra, Portugal

a r t i c l e

i n f o

Article history: Received 9 May 2016 Received in revised form 4 August 2016 Accepted 13 September 2016 Keywords: Wood xylan Xylan derivatives Films Mechanical properties Water vapour barrier Oxygen barrier

a b s t r a c t In this work, the successful derivatization of a beechwood xylan (BX) by carboxymethylation and hydroxypropylation was achieved aimed to produce bio-based films. Carboxymethyl xylan (CMX) and hydroxypropyl xylan (HPX) with a substitution degree of 0.3 and 1.1, respectively, as determined by 1 H NMR, were synthesised. The xylan’s characterization by thermogravimetric analysis showed that HPX is thermally more stable than CMX or BX. The ability of the xylan derivatives to form films and the effect of the glycerol addition on the films performance were evaluated. Tensile strength, Young’s modulus and water vapour permeability of self-supporting CMX films were higher and the elongation at break lower than those of the corresponding HPX films. The water vapour barrier properties of CMX and HPX films were improved with 10% glycerol addition. Oxygen barrier property was exceptionably good for a CMX film plasticized with 25% of glycerol (oxygen permeability of 0.5 cm3 ␮m m−2 d−1 kPa−1 ) while higher oxygen permeability values were obtained for HPX films. Favourable characteristics were found that may enable the use of these films in, e.g., coatings for packaging. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The great majority of plastic materials in use are based on fossil raw materials. The limited nature of fossil fuels and the increasing environmental concerns on their utilization have led the researchers to focus on plant biomass as a feedstock for the production of bio-based materials. Over the past decades, plant biomass has thus been recognized as one of the most important and cheap sources of renewable polysaccharides (Fundador et al., 2012). Plant biomass consists mainly of cellulose, hemicelluloses and lignin (Deutschman and Dekker, 2012; Hansen and Plackett, 2008). Hemicelluloses represent about 25–35% of the biomass, being xylans the most common hemicellulose occurring in wood and annual plants (Deutschman and Dekker, 2012; Ebringerová and Heinze, 2000; Mosier et al., 2005; Saha, 2003; Sun and Cheng, 2002). Xylan is formed by a main chain of ␤-(1 → 4)-linked xylopyranose units which can be substituted at the C-2 and/or C-3 positions by 4-O-methylglucuronic acid (MeGlcA) groups, O-acetyl groups

∗ Corresponding author. E-mail addresses: [email protected] (S. Sousa), [email protected] (A. Ramos), [email protected] (D.V. Evtuguin), [email protected], [email protected] (J.A.F. Gamelas). http://dx.doi.org/10.1016/j.indcrop.2016.09.031 0926-6690/© 2016 Elsevier B.V. All rights reserved.

or other sugars. O-acetyl-4-O-methylglucuronoxylan, arabino-4O-methylglucuronoxylan and arabinoxylans are among the most common xylans (Shuaiyang et al., 2013). Hemicelluloses are unwanted wood components in the production of dissolving pulp. The increasing demand for dissolving pulp and the possibility of chemical modification of the xylans has opened new application possibilities for this polysaccharide. Indeed, they have been applied in the production of bioethanol, xylitol and xylo-oligosaccharides, as well as in packaging films, foams, gels, surfactants, paper additives and flocculation aids, antimicrobial agents and coating color components (Alekhina et al., 2014; Bouxin et al., 2010; Chen et al., 2010; Deutschmann and Dekker, 2012; Edlund et al., 2010; Hansen and Plackett, 2008; Kataja-aho et al., 2011; Laine et al., 2013; Mikkonen et al., 2015; PetzoldWelcke et al., 2014; Pohjanlehto et al., 2011; Reis et al., 2005; Sun and Cheng, 2002). The study of polysaccharides for packaging films is a research field that has grown in order to replace non-biodegradable barrier materials such as aluminium foil and synthetic polymers. However, its potential has yet to be fulfilled in practice. Films for packaging should have acceptable barrier properties (low permeability) to oxygen and water vapour besides possessing good strength properties and flexibility. The film formation, barrier and mechanical properties depend on the source and chemical structure of xylan

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

as well as on the added plasticizers such as polyols (Alekhina et al., 2014; Edlund et al., 2010; Escalante et al., 2012; Goksu et al., 2007; Gröndahl et al., 2004; Mikkonen and Tenkanen, 2012; Mikkonen et al., 2009). Regarding the films formation several parameters have to be considered, namely the xylan solubility, glass transition temperature and its polymerization degree (Alekhina et al., 2014; Gröndahl et al., 2004). The loss of the xylan’s acetyl groups and methylglucuronic acid groups will typically give a significant decrease of their water solubility and thus, the xylan potential to produce films from water solutions will be diminished. For this reason, xylans alkali-extracted from bleached pulps (free of acetyl groups) are not appropriate for the films production. Even the water soluble acetylated xylans, may not provide, as well, films with acceptable properties. The addition of plasticizers such as sorbitol, glycerol or xylitol improves film-forming properties (Goksu et al., 2007; Gröndahl et al., 2004; Hartman et al., 2006; Mikkonen et al., 2009, 2015). The use of chemically modified xylan for films production has been investigated as well. Xylans modification includes esterification (Fundador et al., 2012; Zhong et al., ˇ 2013), carboxymethylation (Alekhina et al., 2014; Simkovic et al., ˇ 2014), hydroxypropylsulfonation and quaternization (Simkovic et al., 2014), acetylation (Stepan et al., 2013), and hydroxypropylation (Jain et al., 2001; Mikkonen et al., 2015). Properties such as hydrophilicity/hydrophobicity, solubility, thermoplasticity and potential for films production can then be changed to targeted values. Other possibility is based on the preparation of composites of xylans with other polymers such as chitosan, carboxymethylcellulose, cellulose, nanocellulose or poly(vinyl alcohol) (Edlund et al., 2010; Gao et al., 2014; Peng et al., 2011; Saxena et al., 2009). In the present work, a commercial beechwood xylan and two modified xylans produced from commercial xylan by carboxymethylation and hydroxypropylation, were thoroughly characterized. Films were prepared from the different xylan derivatives without and with plasticizer (glycerol) and the respective physical properties evaluated and compared. The effect of the glycerol addition on the performance of carboxymethyl xylan and hydroxypropyl xylan based-films is reported here by the first time. 2. Materials and methods 2.1. Materials Beechwood xylan (BX) (Sigma-Aldrich, St. Louis, MO, USA) was used as the xylan source. Deuterium oxide (99.9 at%D), (±)-propylene oxide (ReagentPlus 99%), sodium monochloroacetate, anhydrous calcium chloride, sodium nitrate and sodium azide were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium hydroxide (Pronalab, Lisbon, Portugal), hydrochloric acid, 2-propanol, N,N-dimethylacetamide, lithium chloride, anhydrous glycerol (Merck, Darmstadt, Germany), acetic acid, ethanol (SigmaAldrich, St. Louis, MO, USA), sulphuric acid (Panreac, Barcelona, Spain) and acetone (Scharlau, Barcelona, Spain), were used as reagent-grade chemicals. 2.2. Chemical modification of xylan Carboxymethyl xylan (CMX) was synthesised based on the reaction of BX with sodium monochloroacetate in aqueous sodium hydroxide medium, according to a method reported by Petzold et al. (2006). In detail, 5 g of BX (37.8 mmol of anhydroxylose units (AXU)) was dissolved in 25 mL of 25% aqueous sodium hydroxide solution followed by the addition of 35 mL of 2-propanol. The mixture was stirred for 30 min at 30 ◦ C. Then, 4.39 g (37.8 mmol) of sodium monochloroacetate was added and the temperature raised to 65 ◦ C for 70 min. The reaction mixture was neutralized with diluted acetic

683

acid, and then the CMX was precipitated and washed with ethanol, and finally dried at room temperature. Hydroxypropyl xylan (HPX) was synthesised by the reaction of BX with propylene oxide in aqueous sodium hydroxide, according to the method used by Laine et al. (2013). In detail, 13 g of 50% aqueous sodium hydroxide was added to 3.25 g of BX dispersed in distilled water (16.3%, w/w) and the mixture vigorously stirred in a pressure reactor. 7.6 mL of cooled propylene oxide was then added and the reaction mixture heated at 50 ◦ C for 40 h. The reaction mixture was neutralized with 1 M hydrochloric acid solution, and then the HPX was precipitated by adding acetone. After washing, the HPX was freeze dried. The xylan derivatives were further characterized by 1 H NMR and FTIR spectroscopy, size exclusion chromatography and thermogravimetric analysis. 2.3. Characterization of xylans 2.3.1. Neutral sugar analysis The neutral sugars composition of BX was determined by Saeman hydrolysis (treatment with 72% H2 SO4 at 20 ◦ C for 3 h, followed by 2.5 h hydrolysis with 1 M H2 SO4 at 100 ◦ C) followed by conversion of the released neutral monosaccharides to alditol acetate derivatives and analysis by gas chromatography (Selvedran et al., 1979) using a Varian 3350 gas chromatograph equipped with a FID detector and a DB-225J&W column. The results showed the predominance of xylose (97.1%) and the presence of small amounts of arabinose (1.2%), glucose (0.8%), rhamnose (0.4%), galactose (0.3%) and fucose (0.2%). 2.3.2. 1 H NMR and FTIR spectroscopy 1 H NMR spectra were collected in a Bruker Avance III 400 MHz NMR spectrometer with a Bruker standard pulse program. Spectra of all samples were acquired at room temperature and spectrum of HPX was also acquired at 5 ◦ C (to have a better separation between the water signal and the H-1 signal). Samples, directly in the state of polymer chain, were dissolved in D2 O (10 mg mL−1 ) for the acquisition of the spectra. Sodium 3-(trimethylsilyl)propionate-d4 (ı 0.00) was used as internal standard. FTIR spectra were obtained in a Bruker Tensor 27 spectrometer using potassium bromide pellets. The spectra were recorded in the 500–4000 cm−1 range with a resolution of 4 cm−1 and a number of scans of 128. 2.3.3. Degree of substitution of xylan derivatives The degree of substitution (DS) of carboxymethyl groups of the CMX sample was evaluated based on the intensity (area) ˇ et al., 2014), as follows: of selected 1 H NMR signals (Simkovic DS = IH-1(s) /(IH-1(u) + IH-1(s) + IH-1* ), where IH-1(s) is the area of fitted H-1 signals, at 4.59 ppm and ca. 4.53 ppm, of substituted AXU units at C-2 and C-3 positions, IH-1(u) is the area of fitted H-1 signal at 4.49 ppm, of unsubstituted AXU units, and IH-1* is the area of fitted H-1 signal at 4.63 ppm, of AXU units containing MeGlcA groups at C-2 position. The DS of hydroxypropyl groups of the HPX sample was calculated from the area of the signal due to the three methyl protons in the hydroxypropyl group (at 1.13 ppm) and the area of the H-1 signal (Laine et al., 2013): DS = (Imethylprotons /3)/IH-1 . 2.3.4. Size exclusion chromatography (SEC) analysis Beechwood xylan and HPX solutions for SEC analysis were prepared as following: 5 mg of xylan was dissolved in 50 ␮L of a 8% LiCl solution in N,N-dimethylacetamide (DMAC) at 105 ◦ C for 15 min and further diluted with DMAC to a xylan concentration of about 1% (w/w). The SEC analysis was carried out on a PL-GPC 110 system,

684

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

equipped with a PLgel 10 ␮m pre-column and two PLgel MIXEDB 10 ␮m, 300 × 7.5 mm columns, and a refractive index detector. The pre-column, SEC columns and the injection system were maintained at 70 ◦ C during the analysis. The eluent (0.1 M LiCl solution in DMAC) flow was 0.9 mL min−1 and the injected volume of sample was 100 ␮L. The CMX was dissolved in ultrapure water (since it was not soluble in DMAC) to which 0.1 M NaNO3 and 0.02% NaN3 were added, aiming at obtaining a xylan concentration of about 0.5% (w/w). SEC analysis was performed on the PL-GPC 110 system using two PL aquagel-OH MIXED 8 ␮m columns. The temperature of the columns and injection system was maintained at 36 ◦ C. The eluent was ultrapure water with 0.1 M NaNO3 and 0.02% NaN3 at a flow rate of 0.9 mL min−1 . The SEC columns were calibrated with pullulan reference standards in a range of 1–100 kDa. 2.3.5. Thermogravimetric analysis Thermal stability of the xylans was evaluated using a thermal analyser (TGA Q50, TA Instruments). Samples of approximately 5–10 mg were heated in a platinum crucible up to 1000 ◦ C, at a heating rate of 20 ◦ C min−1 under a nitrogen flow of 40 mL min−1 . Thermogravimetric data were used to calculate the activation energies of thermal decompositions, using the equation ln(ln 1/y) = (Ea /R)(1/T), where Ea is the activation energy (kJ mol−1 ), R is the universal gas constant (R = 8.314 J mol−1 K−1 ), T is the temperature in Kelvin, and y is the fraction of undecomposed material remaining at a temperature T, and defined as (WT − W∞ )/(W0 − W∞ ). Here, WT is the sample weight at the temperature T, W0 and W∞ are the sample weights at the beginning and at the end of the thermal decomposition, respectively. The activation energy was obtained from the slope of the linear fit of ln(ln 1/y) versus 1/T (Broido, 1969). 2.4. Preparation of films All films were prepared by the casting method. CMX was dissolved in deionized water (1% w/w) at room temperature, and then 35.2 g of solution was poured onto a glass petri dish (80 mm diameter) and dried at 23 ◦ C and relative humidity (RH) of 50%. HPX was dissolved in deionized water (1% w/w) at 80 ◦ C for 1 h. Films from HPX were prepared by casting 40.6 g of solution on a polystyrene petri dish (85 mm diameter), and allowed to dry at 23 ◦ C and 50% RH. Externally plasticized films of the xylan derivatives were also prepared. For these films, glycerol (10 and 25% w/w of the xylan derivative) was added to each xylan solution and the mixture was heated to 80 ◦ C for 15 min under magnetic stirring. The preparation of plasticized films followed the same procedure of the corresponding unplasticized films. All films were conditioned at 23 ◦ C and 50% RH before analysis. 2.4.1. Structural properties Basis weight was determined by the ratio between the weight and the area of the film. The films were weighed in an analytical balance (±0.0001 g). The thickness of the films was measured with a micrometer (Adamel Lhomargy model MI 20). The average value of the measurements at five different points of the film was considered. 2.4.2. Tensile properties Tensile tests were performed at 23 ◦ C and 50% RH using a tensile testing machine (Thwing-Albert Instrument Co., EJA series) with a 500 N load cell. The initial grip distance was 50 mm and the rate of grip separation was 5 mm min−1 . The sample films were cut in rectangular specimens with a width of 15 mm and length of 70 mm.

Tensile strength, elongation at break in percentage and Young’s modulus were calculated. Values presented are the mean of six measurements for each film. 2.4.3. Water vapour permeability (WVP) The determination of the water vapour transmission rate (WVTR) was based on ASTM E96-1995 method, using the RH gradient of 0/50%. Films were sealed with melted paraffin on aluminium cups containing anhydrous calcium chloride (0% RH) and placed into a climate room at 23 ◦ C and 50% RH. The test sample assemblies were periodically weighed for 32 h, using an analytical balance (±0.0001 g). The WVTR was determined from the ratio between the slope of the linear regression of weight gain versus time and the exposed area of the film. The water vapour permeability (WVP) was obtained by multiplying the WVTR by the thickness of the film and dividing it by the water vapour pressure difference between the two sides of the film. 2.4.4. Oxygen permeability (OP) The oxygen transmission rate (OTR) of the films was measured using the Oxtran ML 2/21 Mocon equipment in accordance with ASTM F1927-14. The area of the samples was 5 cm2 and the measurements were carried out at 23 ◦ C and 50% RH. The oxygen permeability (OP) was calculated by multiplying the OTR by the film thickness and dividing it by the oxygen pressure difference between the two sides of the film. 3. Results and discussion 3.1. Chemical characterization of xylan and xylan derivatives The xylan derivatives (and the original xylan) were firstly characterized by 1 H NMR and infrared spectroscopy to confirm the chemical modification and determine the degree of substitution. The 1 H NMR spectrum of BX (Fig. 1a) showed major signals corresponding to AXU at 4.49 ppm (H-1, anomeric), 3.29 ppm (H-2), 3.56 ppm (H-3), 3.80 ppm (H-4), 3.38 ppm (H-5ax ) and 4.10 ppm (H5eq ) and signals at 5.29 and 3.47 ppm corresponding to H-1 and methoxy protons, respectively, from 4-O-methylglucuronic acid (MeGlcA) groups (Belmokaddem et al., 2011; Evtuguin et al., 2003; Fundador et al., 2012). The ratio of the areas of the signals due to the anomeric protons of MeGlcA and AXU indicates that one MeGlcA group is present per 10 AXU. In the spectrum of CMX (Fig. 1b), close to the signal of the anomeric proton at 4.49 ppm, less intense signals appeared at ca. 4.53 and 4.59 ppm. According to the previous ˇ study of Simkovic et al. (2014), the latter can be attributed to H-1 of functionalized units in O-3/O-2 positions of AXU and is an indication of successful carboxymethylation. The 1 H NMR spectrum of HPX (Fig. 1c) showed a very intense signal at 1.13 ppm clearly attributed to the methyl protons of the substituent hydroxypropyl groups (Laine et al., 2013). For each modification and based on the NMR data, the degree of substitution was calculated (Section 2.3.3). This was found to be of 0.30 for CMX and 1.1 for HPX. The effectiveness of the modification reactions was also verified by FTIR spectroscopy (Fig. 2). The spectrum of BX showed a sharp band at 897 cm−1 (Fig. 2A) due to C1 H bending mode, which is characteristic of ␤-glycosidic linkages between the sugar units (Kacurákova et al., 2000), several bands within the 1500–1000 cm−1 region related to the C H bending and C O stretching vibrations, including the band at 1163 cm−1 due to the asymmetric stretching of C O C glycosidic bonds (Kacurákova et al., 2000) and, a band at 1633 cm−1 mostly due to sorbed water (Belmokaddem et al., 2011; Ren et al., 2008). Note the broadness of the latter band obscuring the band of ionized carboxyl’s from glucuronic acid groups expected at ca. 1610 cm−1 (Buslov et al., 2009). Bands due to the C H stretching were also observed in the region

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

685

Fig. 1. 1 H NMR spectra of beechwood xylan (a), carboxymethyl xylan (b) and hydroxypropyl xylan (c) in D2 O (spectrum of hydroxypropyl xylan at 5 ◦ C and the others at room temperature). Spectrum of carboxymethyl xylan is shown in a more restricted chemical shift range to better evidence the signals from anomeric protons (H-1). “u” and “s” denote unsubstituted and substituted anhydroxylose units, respectively, and “*” denotes anhydroxylose units containing methylglucuronic acid groups.

Table 1 Number average molar mass (Mn ), weight average molar mass (Mw ), polydispersity (Mw /Mn ), and activation energy of thermal decompositions (Ea ) for xylan and xylan derivatives. Sample

Mn (kDa)

Mw (kDa)

Mw /Mn

Ea (kJ mol−1 )a

BX CMX HPX

5.8 13.7 5.7

9.5 39.7 9.7

1.66 2.91 1.70

62.8; 47.6b 17.8; 58.3c 57.7d

a R2 values for the linear regressions of ln(ln 1/y) versus 1/T (see Section 2.3.5) were ≥0.99 for BX and CMX and 0.98 for HPX. b BX showed two different slopes, one in the 191–268 ◦ C range and another in the 268–393 ◦ C range. c CMX with two different slopes, one in 203–232 ◦ C and another in 232–340 ◦ C. d HPX showed only one distinct slope between 200 and 390 ◦ C.

around 2900 cm−1 with an absorption maximum at 2918 cm−1 (Fig. 2B). For CMX as compared to BX, an additional strong band at 1603 cm−1 ascribed to the asymmetric stretching of the COO entities from the carboxymethyl groups (Ren et al., 2008) was observed in the FTIR spectrum (Fig. 2A). A strong band at 1419 cm−1 , already observed in the spectrum of the initial xylan with much lower relative intensity, was also noticeable, which is attributed to the COO− symmetric stretching. This band is also observed in the spectrum of beech xylan because of the presence of some amount of ionized carboxyls from glucuronic acid units (Buslov et al., 2009). The FTIR spectrum of the obtained CMX also resembles that of a commercial sample of carboxymethylcellulose (results not shown). Finally, in the HPX spectrum, an additional band at 2974 cm−1 (Fig. 2B), assigned to the asymmetric C H stretching of the methyl moieties from the hydroxypropyl groups (Varshney and Naithani, 2011) was observed, showing the successful hydroxypropylation. 3.2. Molar mass In order to examine the effect of derivatization on xylan molar mass, their number average (Mn ) and weight average (Mw ) molar masses, and polydispersity (Mw /Mn ) were determined by SEC measurements (Table 1). Note that for CMX it was necessary to use a solvent other than DMAC/LiCl (frequently used as a solvent and

eluent for the determination of molecular weight of polysaccharides) (Evtuguin et al., 2003; Striegel, 1997). On the other hand, the elution of (polar) ionic molecules such as carbohydrates in SEC is complicated by various non-size exclusion effects such as intermolecular electrostatic interactions (ion exchange, ion exclusion and ion inclusion), intramolecular electrostatic interactions, and adsorption, which distort the normal SEC separation mechanism. Ionic interactions can, however, be reduced by increasing the ionic strength through the addition of an electrolyte to the eluent (Glöckner, 1987; Mori and Barth, 1999; Poole, 2003). The results shown in Table 1 suggest that molar mass of CMX increased in comparison with BX. The same result was obtained by Alekhina et al. (2014) that assigned it to the introduction of carboxymethyl groups which may have an impact on the hydrodynamic volume of the polymer molecules and affect chain aggregation. In fact, despite in our study 0.1 M NaNO3 in the eluent was used to prevent the formation of aggregates, the results seem to indicate that this did not happen. The higher value of polydispersity of the CMX sample compared to that of BX indicates less uniform distribution of molar mass. It is also shown in Table 1, that the hydroxypropylation had a minor impact on molar mass, as well as, in polydispersity. Contrarily to found in present study, Mikkonen et al. (2015) observed an increase in molar mass for HPX with several DS, in comparison to the initial xylan. These differences may be due to the different reaction conditions used in both studies such as dry matter content, xylan/propylene oxide molar ratio and reaction time. Overall, the results of Table 1 suggest that only minor xylan degradation occurred during the derivatization process.

3.3. Thermal stability The thermal stability of BX and xylan derivatives was investigated using thermogravimetric analysis (Fig. 3). It was observed an initial weight loss due to the presence of small amount of moisture in the sample. All samples started to decompose at around 200 ◦ C, but the decomposition rates differed between xylan derivatives and BX, as indicated by the Tmax (the decomposition temperature corresponding to the maximum weight loss rate). The degradation of

686

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

A

HPX

Trasmittance, a.u

CMX

1603 1419

BX

897

1633

1419 1163 1045

1900

1500

1100

700

-1

Wavenumber (cm )

Trasmittance, a.u

B

2974

HPX

CMX

2931

BX

2921

2918

3800

3400

3000

2600

2200

Fig. 3. Thermogravimetric analysis and the corresponding derivative curve for unmodified xylan (BX), carboxymethyl xylan (CMX) and hydroxypropyl xylan (HPX).

the second part to decomposition of the xylan backbone, as stated by Ünlü (2013) for carboxymethylcellulose samples. HPX exhibited only one kind of decomposition with Ea of 57.7 kJ mol−1 . In Fig. 3 it was also observed a weight loss in the temperature range of 700 to 900 ◦ C, decreasing in the following order: CMX > HPX > BX. This weight loss can be assigned to the decomposition of salts initially present in the samples. At the end of the analysis (1000 ◦ C), it was noted that BX had a higher residual weight compared to xylan derivatives, especially in comparison with CMX. This result was probably due to differences in the salts content of the initial samples.

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of beechwood xylan (BX), carboxymethyl xylan (CMX) and hydroxypropyl xylan (HPX) in the region of the characteristic polysaccharide bands (A) and at higher wavenumbers showing the C H stretching vibrations (B).

BX occurred in two stages, with Tmax at 248 and 297 ◦ C. In the first stage it is assumed that the cleavage of the glycosidic bonds and the decomposition of side-chain structural elements such as the 4-O-methyglucuronic acid groups occur, and in the second stage the fragmentation of other xylan depolymerized units takes place (Shafizadeh et al., 1972). The xylan derivatives had Tmax at 205 and 287 ◦ C (CMX) and at 318 ◦ C (HPX). The results indicated that the HPX is thermally more stable than the unmodified xylan. The activation energy for the thermal decompositions in the temperature range for which the main thermal degradation occurs (from about 200 to 390 ◦ C) was also calculated (as explained in Section 2.3.5). The results are presented in Table 1. As seen in Fig. 3, BX had two distinct decomposition ranges in 191–268 ◦ C and 268–393 ◦ C indicating two kinds of decomposition reaction having Ea values of 62.8 and 47.6 kJ mol−1 , respectively. Also CMX sample showed two kinds of decomposition separated at 232 ◦ C; the first with an Ea value of 17.8 kJ mol−1 and the latter with Ea of 58.3 kJ mol−1 . The first part of decomposition may be assigned to decarboxylation of the CMX and

3.4. Properties of films of xylan derivatives The inability of pure hemicelluloses to form continuous films from aqueous suspensions has been reported in the literature for birchwood xylan (Alekhina et al., 2014; Goksu et al., 2007), oat spelt arabinoxylan (Mikkonen et al., 2009), aspen wood glucurunoxylan (Gröndahl et al., 2004) or wood hydrolysate (Edlund et al., 2010), and was also confirmed in our preliminary experiments. This result is likely due to low solubility of unmodified xylan in water, an insufficient chain length of the polymer or high glass transition temperature (Alekhina et al., 2014; Gröndahl et al., 2004). The film-forming performance can be improved by addition of external plasticizers or by derivatization of xylans. Self-supporting films obtained from xylan derivatives, as CMX and HPX with different DS are reported in literature (Alekhina et al., 2014; Mikkonen ˇ et al., 2015; Simkovic et al., 2014). In the present work, the selfsupporting film-forming ability of CMX and HPX was also tested. Concerning the CMX, although forming a continuous film this is brittle. Even so, the film properties could be tested. The effect of external plasticizer on film-forming performance of xylan derivatives was investigated by the addition of glycerol at 10 and 25% (w/w). At 25% glycerol content, the two xylan derivatives formed continuous films, but the HPX film adheres so strongly to the petri dish that could not be detached. Table 2 shows the results of the

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

687

Table 2 Structural, tensile and barrier properties of xylan derivatives films, with standard deviations. Filma

Basis weight (g m−2 )

Thickness (␮m)

Tensile strength (MPa)

Elongation (%)

Young’s modulus (MPa)

WVPb (g mm m−2 d−1 kPa−1 )

OPc (cm3 ␮m m−2 d−1 kPa−1 )

CMX CMX/10G CMX/25G HPX HPX/10G

69.2 ± 2.8 68.2 ± 2.8 62.4 ± 3.5 64.1 ± 0.4 63.2 ± 0.2

50.1 ± 2.7 50.7 ± 1.0 45.6 ± 0.6 63.4 ± 2.5 54.9 ± 3.0

41.5 ± 7.0 28.0 ± 1.9 16.8 ± 1.5 12.7 ± 0.9 7.3 ± 1.2

1.9 ± 0.5 1.6 ± 0.1 2.6 ± 0.4 3.0 ± 0.5 3.0 ± 0.8

3173.4 ± 516.6 2223.2 ± 179.5 917.2 ± 113.1 628.5 ± 62.2 380.2 ± 42.1

3.31d 1.41 ± 0.32 1.41 ± 0.15 2.23 ± 0.39 1.75 ± 0.23

e

a b c d e

e

0.5d 23.6 ± 5.5 37.1 ± 1.0

The code after slash indicates the amount of added glycerol. WVP refers to the water vapour permeability. OP refers to the oxygen permeability. Only one successful measurement was made. No result was obtained due to leaking of the film.

structural, tensile and barrier properties of the films. This table shows that basis weight ranged from 62.4 to 69.2 g m−2 mainly dependent on the amount of film that becomes adhered to the petri dish walls. In turn, thickness ranged from 45.6 to 63.4 ␮m and the values were used to calculate the tensile properties, WVP and OP. The tensile strength and Young’s modulus of CMX film were higher than those of HPX film but opposite result was obtained for elongation at break. The tensile strength observed for CMX film can be attributed to a larger number of hydrogen bonds between CMX chains than between HPX chains, resulting in higher apparent density of CMX films compared to HPX films, which also contribute to the low flexibility of the film. On the other hand, as stated by Alekhina et al. (2014), Mw significantly affects the mechanical strength properties. The higher Mw of the CMX compared to HPX (Table 1), may, thus, also explain the differences in tensile properties. The tensile strength and Young’s modulus values of our CMX films (produced from CMX with DS = 0.30) are higher than those obtained by Alekhina et al. (2014) for CMX (DS = 0.36 and 0.58) from bleached birch kraft pulp. In addition, the present values of elongation at break are similar or lower than those found by Alekhina et al. (2014) (3–8%). Probably, these differences can be explained by the lower DS of the CMX produced in the present work. The high number of carboxymethyl groups lowers tensile strength of the films, since carboxymethyl groups act as internal plasticizers and as well increase the water content that in turn also acts as plasticizer, and thereby the intermolecular forces between the polymer chains ˇ decrease (Alekhina et al., 2014). However, according to Simkovic et al. (2014), films of CMX from xylan extracted from beech sawdust holocellulose with a high DS (1.03), exhibited high tensile properties (93 MPa, 2.7% and 6090 MPa for tensile strength, elongation at break and Young’s modulus, respectively). In present work, the tensile testing results of HPX film were lower than those obtained for HPX films (with DS = 1.1) by Mikkonen et al. (2015), probably due to the lower Mw of our HPX compared to that used by those authors. Concerning the effect of external plasticizer it was observed that the addition of glycerol at 10 and/or 25% reduces the tensile strength and Young’s modulus of the films but can increase elongation at break. A similar trend was found by other authors using glycerol, sorbitol or xylitol as plasticizers in films produced from arabinoglucuronoxylan, xylan, glucuronoxylan, arabinoxylan and HPX (Escalante et al., 2012; Goksu et al., 2007; Gröndahl et al., 2004; Mikkonen et al., 2009, 2015). The external plasticizers are molecules of low molecular size which occupy intermolecular spaces between xylan chains, reducing the formation of hydrogen bonding between the chains, thus allowing that the polymer chains have more freedom of motion (Hartman et al., 2006; Vieira et al., 2011). As consequence, the strength and the stiffness of the films decreases and the flexibility typically increases. For HPX/10G film, all tensile properties were worse (lower tensile strength, elongation at break and Young’s modulus) than those obtained by Mikkonen et al. (2015) for HPX films plasticized with the same

amount of sorbitol, regardless of the DS (0.3–1.1), probably due to the aforementioned differences in Mw of the HPXs used in the two studies. Sensitivity to moisture and its relation to the amount of the glycerol can be inferred from WVP values shown in Table 2. CMX film showed poorer water vapour barrier than HPX film. The WVP values of CMX and HPX films were lower than those of previously studied CMX and HPX films (19–38 and 6.6 g mm m−2 d−1 kPa−1 , respectively) (Alekhina et al., 2014; Mikkonen et al., 2015). The water vapour barrier properties of CMX and HPX films were improved with the addition of 10% glycerol. This trend was also found by Mikkonen et al. (2015) for HPX films plasticized with sorbitol or by Talja et al. (2007) for starch-based films plasticized with different polyols. However, the present results differ from those reported by other authors, regardless of the used plasticizer (Goksu et al., 2007; Heikkinen et al., 2014; Mikkonen et al., 2009). The results obtained in the present work may be explained as follows: the presence of micro voids or other irregularities in the structure of unplasticized films increases water diffusion. As the addition of glycerol improves film formability, a decrease in WVP is observed; the formation of hydrogen bonds between xylan derivative chains and the hydroxyl groups of glycerol decreases the sorption sites for water compared to unplasticized films (Talja et al., 2007). In the case of CMX film, a greater amount of added glycerol (25%) did not affect the WVP. All plasticized films had similar water vapour barrier properties, probably indicating that the structural modifications of the xylan network induced by glycerol are similar, whatever the new functional groups introduced into xylan backbone. In comparison with previously studied hemicelluloses films, the films produced in the present work showed similar or even superior water vapour barrier properties (1.1–38 g mm m−2 d−1 kPa−1 ) (Alekhina et al., 2014; Hansen and Placket, 2008; Mikkonen et al., 2009, 2015; Péroval et al., 2002). In addition, WVP values were lower than those obtained for cellophane (6.0 g mm m−2 d−1 kPa−1 ) but 9–21 times higher than those of commercial plastic films such as lowdensity polyethylene (LDPE) (0.16 g mm m−2 d−1 kPa−1 ) (Péroval et al., 2002). Polysaccharides are generally good oxygen barriers at low and moderate RH because hydrogen bonds contribute to the dense packaging of the polymers and thus to a low permeability (Alekhina et al., 2014; Gröndahl et al., 2004). The OP of the films is shown in Table 2. The CMX/25G film exhibited the lowest value of OP (0.5 cm3 ␮m m−2 d−1 kPa−1 ) although this result was based on only one successful measurement. The good barrier property against oxygen of this film may be due to its more dense structure relatively to the HPX films being reflected in the higher tensile strength and lower elongation at break (Table 2). The impossibility of measuring OTR in CMX and CMX/10G films due to the leaking in the measurement was owing to the rigid and brittle structure. The OP of CMX/25G film was better than the OP of CMX films without external plasticizer addition produced by Alekhina et al. (2014).

688

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689

The obtained OP value for HPX film was similar to that found in the study of Mikkonen et al. (2015). The addition of 10% glycerol increased the oxygen permeability (37.1 cm3 ␮m m−2 d−1 kPa−1 ), contrarily to that found with the addition of 10% sorbitol in the Mikkonen et al. (2015) study (10 cm3 ␮m m−2 d−1 kPa−1 ). This result may indicate that the solid or liquid state of the plasticizer at room temperature influences the structure and properties of the films. Heikkinen et al. (2014) found a slight difference in the OP values of glycerol, sorbitol and glycerol-sorbitol blend plasticized films from oat spelt arabinoxylan. The OP of the films in the present study is significantly better than the OP of often used synthetic polymer films such as low or high-density polyethylene (1870 and 427 cm3 ␮m m−2 d−1 kPa−1 , respectively) (Salame, 1986). 4. Conclusions A commercial beechwood xylan was carboxymethylated and hydroxypropylated to DS levels of 0.3 and 1.1, respectively. The hydroxypropylation had a minor impact on molar mass, while carboxymethylation increased significantly the xylan molar mass. It was found that the hydroxypropylated xylan is thermally more stable than the carboxymethylated or the unmodified xylan. Selfsupporting continuous films were formed with CMX and HPX. The CMX film had higher tensile strength and Young’s modulus but lower elongation at break than the HPX film, presumably due to the high number of hydrogen bonds between CMX chains. The glycerol addition to CMX and HPX improves film-forming performance by enhancing water vapour barrier property, although reducing tensile strength and Young’s modulus. Additionally, oxygen permeability of CMX film produced with 25% glycerol addition was very low showing an exceptionally good oxygen barrier property. Overall, the films prepared from CMX and HPX with glycerol addition showed acceptable barrier properties, making them suitable for application as barrier coatings for paperboard. Acknowledgement This work was carried out in the framework of the FCOMP-010202-FEDER-011500 (NMC) project. References Alekhina, M., Mikkonen, K.S., Alén, R., Tenkanen, M., Sixta, H., 2014. Carboxymethylation of alkali extracted xylan for preparation of bio-based packaging films. Carbohydr. Polym. 100, 89–96. Belmokaddem, F.-Z., Pinel, C., Huber, P., Petit-Conil, M., Perez, D.S., 2011. Green synthesis of xylan hemicellulose esters. Carbohydr. Res. 346, 2896–2904. Broido, A., 1969. A simple, sensitive graphical method of treating thermogravimetric analysis data. J. Polym. Sci. Part A-2 7, 1761–1773. Bouxin, F., Marinkovic, S., Le Bras, J., Estrine, B., 2010. Direct conversion of xylan into alkyl pentosides. Carbohydr. Res. 345, 2469–2473. Buslov, D.K., Kaputski, F.N., Sushko, N.I., Torgashev, V.I., Solov’eva, L.V., Tsarenkov, V.M., Zubets, O.V., Larchenko, L.V., 2009. Infrared spectroscopic analysis of the structure of xylans. J. Appl. Spectrosc. 76, 801–805. Chen, X., Jiang, Z.-H., Chen, S., Qin, W., 2010. Microbial and bioconversion production of D-xylitol and its detection and application. Int. J. Biol. Sci. 6, 834–844. Deutschmann, R., Dekker, R.F.H., 2012. From plant biomass to bio-based chemicals: latest developments in xylan research. Biotechnol. Adv. 30, 1627–1640. Ebringerová, A., Heinze, T., 2000. Xylan and xylan derivatives—biopolymers with valuable properties, 1. Naturally occurring xylans structures, isolation procedures and properties. Macromol. Rapid Commun. 21, 542–556. Edlund, U., Ryberg, Y.Z., Albertsson, A.-C., 2010. Barrier films from renewable forestry waste. Biomacromolecules 11, 2532–2538. Escalante, A., Gonc¸alves, A., Bodin, A., Stepan, A., Sandström, C., Toriz, G., Gatenholm, P., 2012. Flexible oxygen barrier films from spruce xylan. Carbohydr. Polym. 87, 2381–2387. Evtuguin, D.V., Tomás, J.L., Silva, A.M.S., Neto, C.P., 2003. Characterization of an acetylated heteroxylan from Eucalyptus globulus Labill. Carbohydr. Res. 338, 597–604. Fundador, N.G.V., Enomoto-Rogers, Y., Takemura, A., Iwata, T., 2012. Syntheses and characterization of xylan esters. Polymer 53, 3885–3893.

Gao, C.-d., Ren, J.-l., Wang, S.-y., Sun, R.-c., Zhao, L.-h., 2014. Preparation of polyvinyl alcohol/xylan blending films with 1,2,3,4-butane tetracarboxylic acid as a new plasticizer. J. Nanomater. 2014, 1–8, Article ID 764031. Glöckner, G.J., 1987. Polymer characterization by liquid chromatography. Journal of Chromatography Library, vol. 34. Elsevier B.V., pp. 3–558. Goksu, E.I., Karamanlioglu, M., Bakir, U., Yilmaz, L., Yilmazer, U., 2007. Production and characterization of films from cotton stalk xylan. J. Agric. Food Chem. 55, 10685–10691. Gröndahl, M., Eriksson, L., Gatenholm, P., 2004. Material properties of plasticized hardwood xylans for potential application as oxygen barrier films. Biomacromolecules 5, 1528–1535. Hansen, N.M.L., Plackett, D., 2008. Sustainable films and coatings from hemicelluloses: a review. Biomacromolecules 9, 1494–1505. Hartman, J., Albertsson, A.-C., Lindblad, M.S., Sjöberg, J., 2006. Oxygen barrier materials from renewable sources: material properties of softwood hemicellulose-based films. J. Appl. Polym. Sci. 100, 2985–2991. Heikkinen, S.L., Mikkonen, K.S., Koivisto, P., Heikkilä, M.I., Pirkkalainen, K., Liljeström, V., Serimaa, R., Tenkanen, M., 2014. Long-term physical stability of plasticized hemicellulose films. Bioresources 9, 906–921. Jain, R.K., Sjöstedt, M., Glasser, W.G., 2001. Thermoplastic xylan derivatives with propylene oxide. Cellulose 7, 319–336. Kacurákova, M., Capek, P., Sasinkova, V., Wellner, N., Ebringerová, A., 2000. FTIR study of plant cell wall compounds: pectic polysaccharides and hemicelluloses. Carbohydr. Polym. 43, 195–203. Kataja-aho, J., Haavisto, S., Asikainen, J., Hyvärinen, S., Vuoti, S., 2011. The influence of cationized birch xylan on wet and dry strength of fine paper. Bioresources 7, 1713–1728. Laine, C., Harlin, A., Hartman, J., Hyvärinen, S., Kammiovirta, K., Krogerus, B., Pajari, H., Rautkoski, H., Setälä, H., Sievänen, J., Uotila, J., Vähä-Nissi, M., 2013. Hydroxyalkylated xylans—their synthesis and application in coatings for packaging and paper. Ind. Crops Prod. 44, 692–704. Mikkonen, K.S., Tenkanen, M., 2012. Sustainable food-packaging materials based on future biorefinery products: xylans and mannans. Trends Food Sci. Technol. 28, 90–102. Mikkonen, K.S., Heikkinen, S., Soovre, A., Peura, M., Serimaa, R., Talja, R.A., Helén, H., Hyvönen, L., Tenkanen, M., 2009. Films from oat spelt arabinoxylan plasticized with glycerol and sorbitol. J. Appl. Polym. Sci. 114, 457–466. Mikkonen, K.S., Laine, C., Kontro, I., Talja, R.A., Serimaa, R., Tenkanen, M., 2015. Combination of internal and external plasticization of hydroxypropylated birch xylan tailors the properties of sustainable barrier films. Eur. Polym. J. 66, 307–3018. Mori, S., Barth, H.G., 1999. Size Exclusion Chromatography. Springer laboratory, Springer. Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y., Holtzapple, M., Ladisch, M., 2005. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 96, 673–686. Péroval, C., Debeaufort, F., Despré, D., Voilley, A., 2002. Edible arabinoxylan-based films. 1. Effects of lipid type on water vapour permeability, film structure, and other physical characteristics. J. Agric. Food Chem. 50, 3977–3983. Peng, X.-w., Ren, J.-l., Zhong, L.-x., Sun, R.-c., 2011. Nanocomposite films based on xylan-rich hemicelluloses and cellulose nanofibers with enhanced mechanical properties. Biomacromolecules 12, 3321–3329. Petzold, K., Schwikal, K., Heinze, T., 2006. Carboxymethyl xylan—synthesis and detailed structure characterization. Carbohydr. Polym. 64, 292–298. Petzold-Welcke, K., Schwikal, K., Daus, S., Heinze, T., 2014. Xylan derivatives and their application potential—mini-review of own results. Carbohydr. Polym. 100, 80–88. Pohjanlehto, H., Setälä, H., Kammiovirta, K., Harlin, A., 2011. The use of N,N -diallylaldardiamides as cross-linkers in xylan derivatives-based hydrogels. Carbohydr. Res. 346, 2736–2745. Poole, C.F., 2003. The Essence of Chromatography. Elsevier B.V., pp. 267–429 (Chap. 4). Reis, A., Pinto, P., Evtuguin, D.V., Neto, C.P., Domingues, P., Ferrer-Correia, A.J., Domingues, R.M., 2005. Electrospray tandem mass spectrometry of underivatised acetylated xylo-oligosaccharides. Rapid Commun. Mass Spectrom. 19, 3589–3599. Ren, J.-L., Sun, R.-C., Peng, F., 2008. Carboxymethylation of hemicelluloses isolated from sugarcane bagasse. Polym. Degrad. Stab. 93, 786–793. ˇ Simkovic, I., Kelnar, I., Uhliariková, I., Mendichi, R., Mandalika, A., Elder, T., 2014. Carboxymethylated-, hydroxypropylsulfonated- and quaternized xylan derivative films. Carbohydr. Polym. 110, 464–471. Saha, B., 2003. Hemicellulose bioconversion. J. Ind. Microbiol. Biotechnol. 30, 279–291. Salame, M., 1986. Barrier polymers. In: Bakker, M. (Ed.), The Wiley Encyclopedia of Packaging Technology. John Wiley & Sons, pp. 48–54. Saxena, A., Elder, T.J., Pan, S., Ragauskas, A.J., 2009. Novel nanocellulosic xylan composite films. Compos. Part B: Eng. 40, 727–730. Selvedran, R.R., March, J.F., Ring, S.G., 1979. Determination of aldoses and uronic acids content of vegetable fiber. Anal. Biochem. 96, 282–292. Shafizadeh, F., McGinnis, G.D., Philpot, C.W., 1972. Thermal degradation of xylan and related model compounds. Carbohydr. Res. 25, 23–33. Shuaiyang, W., Huiling, L., Junli, R., Chuanfu, L., Feng, P., Runcang, S., 2013. Preparation of xylan citrate—a potential adsorbent for industrial wastewater. Carbohydr. Polym. 92, 1960–1965.

S. Sousa et al. / Industrial Crops and Products 94 (2016) 682–689 Stepan, A.M., King, A.W.T., Kakko, T., Toriz, G., Kilpeläinen, I., Gatenholm, P., 2013. Fast and highly efficient acetylation of xylans in ionic liquid systems. Cellulose 20, 2813–2824. Striegel, A.M., 1997. Theory and applications of DMAC/LiCl in the analysis of polysaccharides. Carbohydr. Polym. 34, 267–274. Sun, Y., Cheng, J., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83, 1–11. Talja, R.A., Helén, H., Roos, Y.H., Jouppila, K., 2007. Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohydr. Polym. 67, 288–295. Ünlü, C.H., 2013. Carboxymethylcellulose from recycled newspaper in aqueous medium. Carbohydr. Polym. 97, 159–164.

689

Varshney, V.K., Naithani, S., 2011. Chemical functionalization of cellulose derived from nonconventional sources. In: Kalia, S., Kaith, B.S., Kaur, I. (Eds.), Cellulose Fibers: Bio- and Nano-polymer Composites—Green Chemistry and Technology. Springer, pp. 43–60. Vieira, M.G.A., da Silva, M.A., dos Santos, L.O., Beppu, M.M., 2011. Natural-based plasticizers and biopolymer films: a review. Eur. Polym. Sci. 47, 254–263. Zhong, L.-X., Peng, X.-W., Yang, D., Cao, X.-F., Sun, R.-C., 2013. Long-chain anhydride modification: a new strategy for preparing xylan films. J. Agric. Food Chem. 61, 655–661.