Hydroxyalkylation of condensed tannins: Comparison of proanthocyanidin extraction process and epoxide chain length on physicochemical properties

Industrial Crops & Products 140 (2019) 111618

Contents lists available at ScienceDirect

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

Hydroxyalkylation of condensed tannins: Comparison of proanthocyanidin extraction process and epoxide chain length on physicochemical properties

T

James H. Bridson , Marion Sanglard, Ibrar Hussain, Vincent Bouad, Meeta Patel, Kate Parker ⁎

Scion, Private Bag 3020, Rotorua, 3046, New Zealand

ARTICLE INFO

ABSTRACT

Keywords: Flavonoid Condensed tannin Proanthocyanidins Epoxide Polyurethane Polyol

Bio-based chemical feedstocks are increasingly being sought for the polymer industry, including polyols for polyurethane synthesis. Proanthocyanidins, isolated from Pinus radiata bark using different extraction methodologies, were investigated as starting materials for the synthesis of polyols. A mild-base, solvent-free and anhydrous hydroxyalkylation using varying chain length epoxides was employed with the aim of producing biobased polyols with viscosities suitable for polyurethanes. The method of proanthocyanidin extraction influenced the reactivity towards epoxide, with water and ethanol-water extracts providing product yields greater than 80%. Epoxide chain length was increased from propylene to hexylene oxide without detrimental effect on the yield or molar substitution. A key attribute of the method employed was the minimisation of side reactions, with homopolymer contents typically less than 10%. At the highest molar substitution levels, melt and flow behaviour was obtained with sub ambient glass transition temperatures and viscosities suitable for direct application in polyurethane synthesis.

1. Introduction Increasing concern around the sustainability of petroleum derived polymers has led to a growing demand for bio-based renewable feedstocks for the polymer industry (Chen and Patel, 2012). Polyol feedstocks are used in polyurethanes, coatings, adhesives, and sealants and are currently dominated by petroleum derived polyether (69%) and polyester (19%) polyols (Aniceto et al., 2012). Polyether polyols are commonly prepared from ethylene and propylene oxide polymerised onto an initiator molecule, such as ethylene glycol, glycerol, pentaerythritol, or sorbitol, under basic conditions. The initiator molecule and molecular weight determine the functionality of the resulting polyol. Rigid polyurethanes require high functionality polyols such as those resulting from pentaerythritol, sorbitol or sucrose initiators. Pentaerythritol is synthesised from the reaction of acetaldehyde with four equivalents of formaldehyde in alkaline medium producing sodium formate as a by-product (Peters and Quinn, 1955). Based on green chemistry principles of atom economy, less hazardous synthesis, and renewable feedstocks, alternative initiators to pentaerythritol would be advantageous. Sorbitol and sucrose initiators prove advantageous, fulfilling criteria of being renewable and avoiding hazardous synthesis.

However, one disadvantage of these bio-based initiators is direct competition with food production and supply. To avoid competition with food supply, initiators have been sought from agricultural by-products (Aniceto et al., 2012). Of these by-products, polyphenolics are of particular interest due to their high functionality. In addition, the aromatic structure may impart additional benefits of rigidity, desirable thermal properties and flame resistance to the corresponding polymer (Furtwengler and Averous, 2018). Proanthocyanidins (condensed tannins) are the second largest source of natural polyphenolic compounds after lignin. Global production of commercial tannins is approximately 200,000 tons per year (Pizzi, 2008). Well distributed in nature and especially in the wood and bark of trees, they can be easily extracted using polar solvents (Li and Maplesden, 1998). Condensed tannins are oligomers and polymers of the flavan-3-ol monomer with various hydroxylation patterns around both the A- and B-rings (Fig. 1). Proanthocyanidins are well known for their high chemical and biological activities including UV absorption, antimicrobial, and antioxidant properties (De Bruyne et al., 1999; Harborne and Williams, 2000). These properties are exploited in numerous applications, such as wood adhesives (Pizzi, 2006), personal care, nutraceuticals and pharmaceuticals (Reuter et al., 2010;

Abbreviations: HWT, hot water tannin; SUT, sulphite urea tannin; EWT, ethanol water tannin; PO, propylene oxide; BO, butylene oxide; HO, hexylene oxide; TEA, trimethylamine; FTIR, fourier transform infrared; NMR, nuclear magnetic resonance; GPC, gel permeation chromatography; UV, ultraviolet; RI, refractive index; TGA, thermogravimetric analysis; mDSC, modulated differential scanning calorimetry; MS, molar substitution; SD, standard deviation ⁎ Corresponding author at: Scion, 49 Sala Street, Rotorua, 3046, New Zealand. E-mail address: [email protected] (J.H. Bridson). https://doi.org/10.1016/j.indcrop.2019.111618 Received 15 January 2019; Received in revised form 1 July 2019; Accepted 27 July 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

Fig. 1. Reaction scheme for the synthesis of hydroxypropylated tannins using propylene oxide and triethylamine amine at.110–150 °C.

to the extraction media. Ethanol water tannin (EWT) was prepared using a 1:1 (v/v) ratio of ethanol to water at 40 °C for 10 min. The liquor was concentrated by falling film evaporation and subsequently freeze dried to yield a brown powder. All tannin starting materials were re-dried under vacuum at 50 °C before use. Propylene oxide ((PO) > 99%, Sigma-Aldrich), butylene oxide ((BO) 99%, Merck), hexylene oxide ((HO) 97%, Sigma-Aldrich) and triethylamine ((TEA) > 99%, Sigma-Aldrich) were used as received.

Ververidis et al., 2007). Proanthocyanidins sourced from Pinus radiata exhibit low toxicity (Frevel et al., 2012) and can be sourced from nonfood forestry residues using water based extraction techniques (Feng et al., 2013; Li and Maplesden, 1998). This presents a sustainable alternative initiator for the synthesis of high functionality polyols. Chemical modification of tannins to enhance their properties for industrial application has been widely studied (Arbenz and Averous, 2015). Among these modifications, functionalisation of the hydroxyl groups and more particularly reactions with epoxides (alkylene oxides) have been the focus of numerous studies (Aniceto et al., 2012; Arbenz and Averous, 2014; Arbenz and Avérous, 2015; Bridson, 2007; García et al., 2016, 2013; García et al., 2014). The mechanism of hydroxypropylation typically involves impregnating biomass with aqueous alkali to activate the hydroxyl groups followed by ring opening of the epoxide (Aniceto et al., 2012). In the case of proanthocyanidins, the use of mild-base such as triethylamine is preferable over strong bases to avoid rearrangement reactions of the flavonoid units (Fig. 1) (Bridson et al., 2018; Hashida and Ohara, 2002). This reaction is selective towards the phenolic hydroxyl groups of tannin which are extended and thus become more available for further reactions to produce new polymeric systems (Sandler and Karo, 1980). Hydroxyalkylated tannins have demonstrated potential for use as an alternative polyol source in polyurethane foams or adhesives (García et al., 2015). However, the high viscosity of hydroxyalkylated tannin-based polyols compared to commercial polyol systems is a major limitation to the uptake of this technology (Laurichesse et al., 2015). To advance the sustainability of polyurethane foams using tanninbased polyols, new strategies are required to reduce the polyol viscosity. Current strategies involve heating the tannin-based polyol to reduce the viscosity or blending with a lower viscosity polyol (García et al., 2015; Laurichesse et al., 2015). These approaches are either commercially unviable or compromise the maximum level of tannin that can be substituted into the formulation (Furtwengler and Averous, 2018; Sonnenschein, 2015). The method of tannin extraction and the resulting chemistry of the extract is known to influence the viscosity and molecular weight (Arbenz and Averous, 2015). Furthermore, upon derivatisation alkyl chain length is known to influence the physicochemical properties as demonstrated with esterification of tannin (Bridson et al., 2013; Grigsby et al., 2013). In this study, we hypothesise that the molecular weight and chemistry of the starting proanthocyanidin material and chain length of the epoxide will influence the thermal properties and viscosity of the resulting hydroxyalkylated products.

2.2. Synthesis of hydroxyalkylated tannins Hydroxyalkylated tannins were synthesised using a previously reported method of Bridson (2007), Bridson et al. (2018). Tannin (2 g), triethylamine (48 μL) and epoxide (5–10 mole equivalent based on flavan C15 unit; 1–2 mole equivalents based on total hydroxyl groups) (Bogun, 2007) were charged into a 15 mL sealed pulping reactor with Teflon liner. The reaction vessels were placed in an oil bath at 110 °C or 150 °C for 18 h before cooling to room temperature. Unreacted epoxide was evaporated at 50 °C under vacuum to constant weight. All reactions at 110 °C were performed in triplicate, with reactions at 150 °C performed in duplicate. Hydroxyalkylated tannin products are labelled using the following nomenclature: tannin starting material (HWT, EWT or SUT) – ratio of epoxide (5 or 10) – epoxide (PO or BO) – reaction temperature (110 °C unless stated otherwise). Homopolymer content was determined gravimetrically by cyclohexane extraction as previously reported (Arbenz and Avérous, 2015). 2.3. Chemical characterisation Solution state NMR spectra were recorded using a Bruker Avance DPX400 MHz spectrometer with a 5 mm inverse broad band probe (Bruker). The molar substitution (MS) was determined from both the yield and 1H NMR spectroscopy using Eq. 1 adapted from Bridson et al. (2013).

MS =

I1/ n1 I2/ n2

(1)

Where: I1 is the ether CH3 integral; n1 is the number of protons corresponding to the ether integral; I2 is the polyphenol unit aromatic integral; and n2 is the number of aromatic protons based on the hydroxylation pattern of the monomeric polyphenol unit (pine bark tannin = 3.5) (Bogun, 2007). Samples were analysed in triplicate and the mean reported. The hydroxyl value was determined by 31P NMR spectroscopy using a method adapted from Granata and Argyropoulos (Granata and Argyropoulos, 1995) with endo-N-hydroxy-5-norbornene-2,3-dicarboximide as the internal standard (Zawadzki and Ragauskas, 2001). Approximately 20 mg of sample was accurately weighed into a vial and dissolved in 100 μL of internal standard (25 mg.mL−1 in dimethyl formamide), 400 μL of pyridine : deuterated chloroform (1.6 : 1) stock solution and 100 μL of relaxation solution (6 mg.mL−1 chromium acetylacetonate in stock solution). Once completely dissolved, 100 μL of

2. Materials and methods 2.1. Materials Hot water tannin (HWT) was isolated by hot water (90 °C) extraction of Pinus radiata bark followed by spray drying of the liquor to yield a brown powder. Sulfite urea tannin (SUT) was prepared in the same manner with the addition of sodium sulfite and urea (2% w/w on bark) 2

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

Table 1 Reaction conditions used for the synthesis of hydroxyalkylated tannins, yields and quantitative 1H and Hydroxyalkylated tannins

HWT-5PO EWT-5PO SUT-5PO HWT-10PO EWT-10PO SUT-10PO HWT-10BO HWT-10HO HWT-10PO-150 HWT-10BO-150 1 2

Ratio of tannin to epoxide Weight

Mole1

50:50

1:5

33:66

1:10

28:72 22:78 33:66 28:72

Temp. (°C)

110

150

Yield (%)

96 96 97 86 82 54 90 86 96 93

Homopolymer (%)

0.0 0.2 0.7 2.0 2.4 0.2 3.9 5.6 8.2 24.2

31

P NMR spectroscopy results.

Molar substitution

Hydroxyl quantification by (SD2)

31

P NMR (mmol/g)

Yield based

1

H NMR (SD2)

Aliphatic

Aromatic

4.6 4.6 4.8 7.9 7.2 3.0 8.9 8.4 9.4 9.4

3.95 3.48 3.75 8.22 7.81 3.01 8.86 7.59 13.7 13.7

7.33 6.74 8.56 5.98 6.29 6.79 6.69 5.92 6.02 7.15

0.66 0.47 0.50 0.67 0.32 0.64 0.00 0.01 0.29 0.00

(0.61) (0.82) (0.16) (0.71) (0.73) (0.29) (0.45) (0.59) (1.52) (1.09)

(0.82) (0.50) (0.32) (0.63) (0.12) (0.90) (0.05) (0.10) (0.41) (0.13)

(0.22) (0.14) (0.19) (0.09) (0.03) (0.22) (0.00) (0.00) (0.02) (0.00)

Mole ratio of tannin to epoxide based on flavan C15 unit. Standard deviation.

2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane was added as the phosphitylating agent, the solution stirred for a further 30 s before transferring to a 5 mm NMR tube for analysis. 31P NMR spectra were acquired on a DPX 400 spectrometer (Bruker, Germany) with 256 transients and a 10 s delay. The spectra were processed with exponential line broadening of 2 Hz and the ppm shift calibrated to phosphitylated water (δ 132.2 ppm). The following peaks were integrated: internal standard (δ 152.5 - 151.1 ppm), aliphatic OH (δ 149.8 - 144.6 ppm), phenolic OH (δ 144.6 – 137.4 ppm), carboxylic OH (δ 136.0 - 133.5 ppm). Samples were analysed in duplicate and the mean reported. For all samples, the level of carboxylic acid OH was less than 0.10 and therefore was not reported. Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker Tensor 27 instrument equipped with a single bound diamond attenuated total reflectance (ATR) cell (Bruker). Background and sample spectra were acquired at 4 cm−1 resolution with 32 scans from 400 to 4000 cm−1. Spectra were baseline corrected, normalised to the aromatic peak at 1515 cm−1, and integrated using OPUS 7.2 software (Bruker). Gel permeation chromatography (GPC) was performed using a PLGPC 50 integrated system (Polymer Laboratories) equipped with two PolarGel-L columns connected in series and ultraviolet (UV) and refractive index (RI) detectors. Samples were dissolved and eluted in dimethylsulfoxide with 0.05 mol.L−1 lithium chloride at a flow rate of 1 mL.min−1 at 50 °C with UV detection at 280 nm. Calibration was based on pullulan saccharides (180–194,000 g.mol−1) with samples analysed in triplicate and the mean reported (Mn, Mw and PD). Thermogravimetric analysis (TGA) was performed using a Discovery TGA (TA Instruments) using high temperature platinum pans. The samples (around 40 mg) were heated from room temperature to 600 °C at a constant heating rate of 10 °C. min−1 under a nitrogen flow of 10 mL.min−1. The onset of degradation was determined based on the intercept of tangents to the weight loss profile. All analyses were carried out in not less than duplicate and data analysis was performed using TRIOS v3.3.1.4246 software (TA Instruments) Modulated differential scanning calorimetry (mDSC) experiments were carried out using a Discovery DSC (TA Instruments) with TZero™ standard aluminium pans. The thermograms were recorded while heating from -80 to 200 °C at 2.0 °C.min−1 using a modulation amplitude of 1.272 °C and period of 60 s under a nitrogen flow of 50 mL.min−1. Glass transitions were determined from the second modulated heating cycle reversing heat flow at half height of the step transition. All analyses were carried out in not less than duplicate and data analysis was performed using TRIOS v3.3.1.4246 software (TA Instruments). Rheology measurements were done using a TA Instruments AR 2000

rheometer using 25 mm parallel plates with a geometry gap of 1000 μm. Several successive tests were performed: (1) a stress sweep test was conducted to determine the flow behaviour of the samples, with a torque ranging from 600 to 2000 μN.m (in log mode, 10 points per decade) at 25 °C at a frequency of 1 Hz (after 1 min equilibration time); (2) a frequency sweep test from 0.1 to 10 Hz (in log mode, 10 points per decade) at 25 °C and 1500 μN.m torque (after 1 min equilibration time); (3) a temperature ramp from 25 °C to 100 °C at a constant rate of 2 °C.min−1, a frequency of 1 Hz, and at 1500 μN.m torque (after 1 min equilibration time). 3. Results and discussion With the goal of producing hydroxyalkylated proanthocyanidins of low viscosity, pine bark tannins were modified using previously established chemistry (Bridson, 2007; Bridson et al., 2018). This method was advantageous as it employed a mild-base to minimise possible rearrangement reactions of the proanthocyanidin and was solvent-free, simplifying product work-up. While the tannin starting material was not soluble in the epoxide, initial reactions in the solid state increase the solubility, ultimately leading to a homogenous mixture (Bridson, 2007; Bridson et al., 2018). Using this protocol, different tannin materials (EWT, HWT and SUT) known to have varying molecular weight (Mw 16,100, 10,600 and 8000 g.mol−1 respectively) and chemistry were evaluated (Bogun, 2007) (Supporting information). In addition, the influence of reaction temperature, molar ratio, and epoxide chain length were also investigated (Table 1). Pine bark tannins obtained from different extractions had similar reactivity with PO at a 1:5 mol ratio yielding brown glassy solids in excellent yield (96% and 97%) (Table 1). The molar substitution calculated from 1H NMR spectra for all three tannins at this ratio was equivalent (p > 0.05). At a higher mole ratio of PO (1:10) the molar substitution of HWT and EWT concordantly increased producing tacky products in very good yield (86% and 82%). In contrast, the yield and molar substitution for SUT-10PO was considerably lower (54%) compared with the corresponding products prepared from HWT or EWT. The reduced yield was concordant with the molar substitution determined by 1H NMR spectroscopy. This reduced reactivity may be due to competing reactions with the incorporated sulfonate groups or residual sodium sulfite from the extraction procedure (Dobinson et al., 1969), or the residual urea from the extraction process (Tousignant and Baker, 1957). Given the similar reactivity of the HWT and EWT and the poor substitution achieved with SUT, only HWT was used for subsequent reactions with the longer chain epoxides BO and HO. These were performed at a 1:10 mol ratio to yield glassy to tacky solids in very good 3

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

fraction (Arbenz and Averous, 2014), leading to overestimation of the level of homopolymer. This was confirmed by analysis of the cyclohexane extract by FTIR spectroscopy with a peak observed at 1515 cm−1 characteristic of tannin aromatic functionality (Supporting information). Quantification of hydroxyl groups by 31P NMR spectroscopy proved an effective method to differentiate aliphatic and aromatic hydroxyl functionality of the products (Table 1). The starting material could not be quantified due to insolubility in the solvent mixture, however the ratio of aliphatic to aromatic hydroxyl groups is expected to be approximately 1:4 (Bogun, 2007). Upon hydroxyalkylation, a decrease in aromatic hydroxyl levels was expected indicating substitution occurring at the phenolic groups. While substitution was also expected to occur at the C-ring aliphatic hydroxyl, this would not be observed as the signal from the substituted alkyl chain primary and secondary hydroxyl groups occurrs in the same spectral region. Indeed, for all products the majority of hydroxyl functionality was present as aliphatic groups with minimal aromatic hydroxyl groups remaining. This indicated near complete reaction of the phenolic groups with the corresponding epoxide. For all propylene oxide products prepared at 110 °C, the level of residual aromatic hydroxyl groups was similar. Increasing the temperature from 110 to 150 °C reduced the level of aromatic hydroxyl groups from 0.67 (HWT-10PO) to 0.29 (HWT-10PO-150). The unreacted aromatic hydroxyl groups may arise from steric hindrance especially around the flavan A-ring where the meta-substitution combined with interflavanoid linkages may inhibit the epoxide reaction at the C-5 or C-7 hydroxyl groups. However, increasing temperature appears to force the reaction, increasing substitution at the aromatic hydroxyls. Analysis by FTIR spectroscopy confirmed the introduction of new functional groups upon hydroxyalkylation of tannin. The band at 2800 to 3000 cm−1 attributed to CH stretching showed a large increase upon hydroxyalkylation due to the presence of methylene and methyl groups of the constituent alkyl chain (Fig. 2). The relative intensity of the CH stretching peak increased with the ratio of epoxide addition and further with increasing reaction temperature concordant with molar substitution values determined by 1H NMR spectroscopy. Methyl asymmetric and symmetric stretching peaks were observed at c.a. 2960 and 2875 cm−1, with methylene asymmetric and symmetric stretching at c.a. 2930 and 2850 cm−1. The intensity of the methylene peaks increased relative to the methyl peak with increasing epoxide chain length (Fig. 3). The CO stretch at c.a. 1100 cm−1 became more prominent upon hydroxylalkylation, attributed to primary and secondary alcohol and alkyl substituted ethers. The two alcohol peaks were differentiated in the second derivative trace, with the signal at c.a. 1100 cm−1 attributed to secondary alcohols and the signal at c.a. 1050 cm−1 attributed to primary alcohols. This reflected the two possible reaction paths under basic condition; SN2 nucleophilic substitution

Fig. 2. Example FTIR spectra of hydroxypropylated HWT at different PO ratios and reaction temperature, where (a) HWT, (b) HWT-5PO, (c) HWT-10PO and (d) HWT-10PO-150.

yields. The molar substitution was not significantly different (p > 0.05) across the three epoxides (PO, BO and HO), indicating that the overall reactivity was not influenced by steric effects under these reaction conditions (Table 1). In an attempt to further improve the yield and increase the molar substitution, the reaction temperature was increased from 110 °C to 150 °C as previously reported for the hydroxypropylation of Gambier tannin (Arbenz and Avérous, 2015). A modest increase in yield was observed, accompanied by a significant increase in the molar substitution (p < 0.05) (Table 1). However, the molar substitution determined by 1H NMR spectroscopy for both products (HWT-10PO-150 and HWT-10BO-150) was greater than theoretically possible assuming a stoichiometric reaction. For reactions carried out using PO (HWT10PO-150) this may be due to intervening side reactions such as hydroxyl dehydration or chain transfer reactions known to occur at temperatures above 130 °C (Di Serio et al., 1996). Such side reactions are less prevalent for longer chain epoxides due to steric hindrance of the larger pendant alkyl group (Malik et al., 2016). The presence of impurities in the starting tannin such as carbohydrates (confirmed by gel permeation chromatography), may also impact the hydroxyalkylation reactions and also affect the subsequent characterisation of the products. Homopolymerisation of the epoxide is an inevitable secondary reaction that yields poly(alkylene oxide) in conjunction with hydroxyalkylated tannin. The relative proportion of products depends on the reaction conditions, and is favoured by high water content, catalyst loading, particle size, epoxide to biomass ratio and temperature (Aniceto et al., 2012). While a mixture of products can beneficially reduce the viscosity and be used directly for the production of polyurethanes, minimising homopolymerisation was desirable to maximize the biomass content (Aniceto et al., 2012). For all samples prepared at 110 °C the homopolymer content was very low (< 6%), which was attributed to the mild-base, solvent-free and anhydrous reaction system employed in this study (Table 1). As expected the level of homopolymer increased with epoxide ratio and reaction temperature for HWT and EWT. Very low homopolymer content was produced using SUT (< 1%) which contributed to the samples hard glassy appearance. These results compare favourably with homopolymer contents reported for similar hydroxypropylation reactions; ranging from 4 to 32% using Gambier tannin (Arbenz and Avérous, 2015), up to 45% using Acacia tannin (Duval and Avérous, 2016) and approximately 60% using an alkaline Pinus contorta bark extract (D’Souza et al., 2015). Avoiding the use of hard-base and aqueous reaction conditions was clearly advantageous for minimising homopolymerisation and maximising reaction efficiency. With increasing epoxide chain length (PO to HO) the level of homopolymer was observed to increase from 2 to 6%. However, the increased hydrophobicity of BO and HO derivatives may result in a portion of hydroxyalkylated tannin being solubilised in the cyclohexane

Fig. 3. Example FTIR spectra of hydroxyalkylated HWT using different epoxides and reaction temperatures, where (a) HWT, (b) HWT-10PO, (c) HWT10BO and (d) HWT-10HO. 4

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

at the more or less substituted carbon of the epoxy ring producing either primary or secondary alcohols respectively. The formation of secondary alcohols was favoured with PO as expected (Di Serio et al., 1996), with the proportion of primary alcohols increasing with epoxide chain length. While steric hindrance would favour the formation of secondary alcohols, the longer alkyl chain length may provide increased inductive stabilisation of the transition state intermediate favouring the formation of primary alcohols. Distinction between the different phenolic CO stretching bands was challenging due to vibrational interactions and peak overlap (Ricci et al., 2015). However the peak at 1200 cm−1, typically attributed to phenolic CO stretching, was not present in the hydroxyalkylated products with a new peak evident at 1263 cm−1 attributed to the resulting aromatic ether bonds. This was further evidenced by changes in the region from 900 to 740 cm−1 related to OH wagging of aromatic alcohols and out of plane bending of aromatic CH which are strongly affected by the number and position of substituents (Ricci et al., 2015). The hydroxyl peak centred at approximately 3350 cm−1 can be assigned to the phenolic functional group of the starting material or the aliphatic hydroxyl groups of the hydroxyalkyl derivatives. This band became narrower and shifted to higher frequency upon hydroxyalkylation and with increasing levels of molar substitution, which was indicative of a reduction in hydrogen bonding (Coates, 2000). The hydroxyl peak increased in width and shifted to lower frequency with epoxide chain length (3390 cm−1 for HWT-10PO and 3365 cm−1 for HWT-10BO and HO) indicating a greater level of hydrogen bonding for the BO and HO derivatives compared with the hydroxypropyl derivative. Characterisation of the molecular weight by gel permeation chromatography showed relative changes upon hydroxyalkylation. On hydroxypropylation of HWT the Mn increased from c.a. 1300 g.mol−1 for the starting tannin up to c.a. 2900 g.mol−1 (Supporting information). Similarly, the Mw increased from 10,600 g.mol−1 up to 29,500 g.mol−1. The polydispersity of HWT increased upon hydroxypropylation with a bimodal distribution evident (Fig. 4). The bimodal distribution was unlikely to be auto-condensation given the mild-base reactions conditions used in the synthesis (Arbenz and Averous, 2015) and the presence of a higher molecular weight shoulder in the starting material. While lithium chloride was used during GPC to mitigate flavonoid aggregation, changes upon hydroxyalkylation may influence the tendency towards aggregation and hence cannot be dismissed. Across the HWT series, the molecular weight did not corroborate with the degree of substitution which increased in the order HWT-5PO < HWT10PO < HWT-10PO-150. Similarly for EWT, an increase in molecular weight upon hydroxypropylation was observed, but no trend was observed with the degree of substitution. For SUT only an increase in Mn was observed upon hydroxypropylation. With increasing epoxide chain

Fig. 5. Molecular weight profile (RI detection) of HWT hydroxyalkylated with different chain length epoxides (PO, BO or HO).

length, the Mn increased (c.a. 2900 to 3400) while the Mw decreased (c.a. 29,500 to 9700). This was accompanied by a change in the molecular weight profile where the peak at c.a. 100,000 g.mol-1 decreased in intensity with a corresponding increase in the lower molecular weight peak at c.a. 10,000 g.mol−1 (Fig. 5). This change in distribution pattern may be attributed to varying levels of aggregation with epoxide chain length during GPC. Comparing the molecular weight profiles from the RI and UV detector enabled differentiation between the tannin and the non-tannin components within the product mixture. The aromatic structure of tannins leads to UV absorption at 280 nm (Yanagida et al., 2003), while non-aromatic components such as carbohydrates and homopolymerised epoxide will only be observed by the refractive index detector. In both the UV and RI molecular weight profiles of unmodified tannin the peak at approximately 300 g.mol−1 was attributed to monomeric flavonoid components such as taxifolin with a shoulder at approximately 600 g.mol−1 attributed to dimers (Bogun, 2007). The peak at approximately 180 g.mol−1 was observed in the RI profile only and was attributed to carbohydrate impurities. Upon hydroxyalkylation the monomer flavonoid peak broadened and concordantly shifted to between 400 and 700 g.mol−1 consistent with the observed levels of molar substitution. On comparing the UV and RI molecular weight profiles of all products minimal homopolymer was evident, in agreement with the low levels of homopolymer observed on cyclohexane extraction (Figure 4 & 5 and Supporting information). Thermogravimetric analysis showed typical degradation profiles for unmodified tannin with moisture loss below 100 °C, followed by a main degradation step beginning at approximately 200 °C (Gaugler and Grigsby, 2009). The thermal stability decreased in the order EWT = HWT > SUT, with the onset of degradation at 237, 236, and c.a. 190 °C respectively. The comparatively lower thermal stability of the SUT has been previously attributed to the sulfonate group promoting pyrolytic decomposition (Gaugler and Grigsby, 2009). Hydroxyalkylated tannins exhibited minimal moisture loss below 100 °C compared with the unmodified tannins, indicative of an increase in hydrophobicity. Two degradation steps were observed for all hydroxypropylated tannins, with onsets at approximately 200 °C and 320 °C (Fig. 6 and Table 2). The hydroxypropylated SUT was considerably less stable than the corresponding EWT or HWT derivatives with a lower onset temperature and greater mass loss during the first degradation step. Samples prepared at 150 °C exhibited considerably lower thermal stability (temperature at 20% mass loss) than the corresponding samples prepared at 110 °C. This was attributed to the increased level of homopolymer present in samples prepared at the higher reaction temperature. The epoxide chain length had only a minor influence on thermal stability of products with the second degradation onset and temperature at 20% mass loss generally similar across all three samples

Fig. 4. Molecular weight profile of HWT hydroxypropylated under different reaction conditions (RI detection). 5

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

Fig. 6. Example TGA thermograms of (a) different tannins reacted with 5 mol of PO, (b) different tannins reacted with 10 mol of PO, and (c) HWT reacted with 10 mol of PO, BO and HO at 110 °C and 150 °C.

50 °C.min−1 offered no improvement in the resolution of glass transition or melting features. Therefore, modulated DSC was used in an attempt to improve the resolution and identify transitions. A clearly defined glass transition was observed in the reversing heat flow for all hydroxyalkylated products ranging from -42 °C to 14 °C (Fig. 7). Two broad transitions (T1 and T2) were observed in the non-reversing heat flow at approximately 100 and 150 °C. No trends were observed for the T1 or T2 transitions across the data set. The glass transition temperature decreased with increasing molar substitution, attributed to internal plasticisation by the grafted

(HWT-10PO, HWT-10BO and HWT-10HO). Samples with higher levels of substitution and therefore a lesser proportion of proanthocyanidin, typically exhibited lower residual mass at 600 °C. Similar observations have been made for acetylated hot water tannin from Pinus radiata (Gaugler and Grigsby, 2009). Differential scanning calorimetry was used to assess changes in glass transition temperature across the samples. Initially, DSC was performed using a heat-cool-heat cycle ramped at 10 °C.min−1. This gave featureless thermograms with no clearly identifiable glass or melting transitions (Supporting information). Increasing the heating rate to Table 2 Thermal properties as assessed by mDSC and TGA. Hydroxyalkylated tannins

HWT EWT SUT HWT-5PO EWT-5PO SUT-5PO HWT-10PO EWT-10PO SUT-10PO HWT-10BO HWT-10HO HWT-10PO-150 HWT-10BO-150 1 2

Tg (°C) (SD1)

ND2 ND ND 14.4 (7.0) 0.7 (1.9) −15.1 (3.6) −24.8 (5.8) −12.9 (2.1) 8.09 (8.9) 13.2 (6.1) 8.7 (0.1) −41.7 (3.7) −29.1 (0.5)

Degradation onset point (°C) (SD)

Temperature (°C) at 20% mass loss (SD)

First

Second

50.1 (3.5) 52.9 (1.7) 55.6 (2.0) 193.1 (4.2) 214.5 (8.7) 171.1 (7.6) ND 202.3 (3.3) 169.7 (3.9) 186.0 (4.7) 142.5 (3.2) 208.9 (4.1) 112.2 (4.2)

236.3 236.7 181.9 334.0 352.5 288.1 348.6 348.8 310.5 341.3 349.9 369.8 331.2

Standard deviation. Not determined (feature not evident in thermogram). 6

(7.1) (2.3) (6.2) (5.9) (0.7) (32.8) (1.2) (2.4) (2.0) (2.3) (2.7) (2.6) (3.5)

278.9 283.8 251.5 284.8 305.0 211.2 262.9 326.3 222.1 290.9 275.4 219.4 184.4

(2.8) (2.9) (0.4) (9.4) (6.6) (2.9) (14.0) (2.1) (6.8) (1.4) (52.7) (5.2) (14.1)

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

samples. The rheological behaviour of the hydroxyalkylated tannins was examined to assess their suitability for substitution into polyurethane formulations. When preparing polyurethanes it is important to quickly obtain a homogeneous mixture of polyol and isocyanate to provide optimal reaction conditions (D’Souza et al., 2015). This is dictated by the viscosity of the reagents, therefore it is essential to know and control the viscosity of the polyol component. Commercial polyols, which are mostly polyether polyols (69%) and polyester polyols (19%) often based on sorbitol, glycerol and sucrose, generally exhibit viscosities below 300 Pa.s (typically ranging from 10−2 to 100 Pa.s) (Aniceto et al., 2012). Rheology showed melt and flow behaviour for most of the tannin hydroxypropyl ethers, with the exception of hydroxypropylated SUT (SUT-5PO). The temperature ramp from 25 to 100 °C showed a similar trend for all samples, with a decrease in viscosity on increasing temperature (Fig. 8). For hydroxypropylation, an increasing charge of PO resulted in products with a markedly lower viscosity. For HWT, the viscosity at 30 °C decreased from 1.1 × 106 Pa.s to 8.2 × 104 Pa.s for sample HWT-5PO and HWT-10PO respectively (Table 3). This was similarly observed for EWT. The decrease in viscosity was attributed to increasing molar substitution and homopolymer content (Table 1), which is also known to have a significant impact on viscosity (Aniceto et al., 2012; Arbenz and Avérous, 2015). The more fluid-like behavior at higher substitution levels was confirmed by an increased frequency dependency (Supporting information). The epoxide chain length impacted the viscosity of the products, with the lowest viscosity generally observed for PO derivatives. For the samples prepared with 10 mol equivalents of epoxide at 110 °C, the viscosity was approximately 10 fold lower for the PO sample (8.2 × 104 Pa.s at 30 °C) compared with the BO and HO derivatives (1.0 × 106 Pa.s and 8.8 × 105 Pa.s at 30 °C respectively). Molecular weight is the main determining factor of viscosity at T > Tg for amorphous materials. This together with the increase in hydrogen bonding, shown by FTIR spectroscopy, likely accounts for the differing viscosities with alkyl chain length. This finding contradicts the molecular weight characterisation by GPC, and indicates that the molecular weight profiles obtained by GPC were likely impacted by aggregation. Increasing the reaction temperature from 110 to 150 °C resulted in products with lower viscosity. The decrease in viscosity was attributed to higher levels of molar substitution and the increased level of homopolymer. The influence of molar substitution and homopolymer content on viscosity are concordant with previous studies on the hydroxyalkylation of

Fig. 7. Example mDSC thermogram of hydroxyalkylated HWT showing enhanced resolution of thermal transitions.

hydroxyalkyl groups (Table 2). The internal plasticisation was attributed to a disruption in the intramolecular hydrogen bonding between the tannin chains and also to an increase in free volume (Lora and Glasser, 2002). This was in accordance with observations by FTIR spectroscopy. A negative linear relationship between molar substitution and glass transition temperature was exhibited (HWT-PO R2 = 0.96; EWT-PO R2 = 0.89; SUT-PO R2 = 0.98), indicating that the level of substitution is the primary determinant of glass transition temperature as previously reported for monomeric flavonoids (Bridson et al., 2018). On comparing the different types of tannin, the gradient and y-intercept of the trend line was similar for HWT and EWT. In contrast, the gradient for the SUT trend line was greater, implying a more effective internal plasticisation of the SUT compared with the HWT or EWT for a given level of MS. The more effective internal plasticisation may be attributed to the lower molecular weight of the SUT compared with the HWT and EWT. If a higher level of MS was able to be achieved using SUT, it could be expected that the glass transition would be considerably lower than that achieved using HWT or EWT. On comparing different chain length products, BO and HO were less effective at reducing the glass transition temperature compared with PO. During the TGA and mDSC analysis, several of the samples showed intumescent properties; swelling and forming a char after analysis, particularly the hydroxypropylated EWT

Fig. 8. Example complex viscosity profiles over temperature ramp from 25 to 100 °C for all modified tannins (excluding SUT-5PO). 7

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al.

Table 3 Complex viscosity of the hydroxyalkylated tannin mixtures at different temperatures. Entry

HWT-5PO EWT-5PO SUT-5PO HWT-10PO EWT-10PO SUT-10PO HWT-10BO HWT-10HO HWT-10PO-150 HWT-10BO-150 1

Complex viscosity (103 Pa.s) at temperature

Temperature when G” > G’ (°C)

30 °C

50 °C

70 °C

80 °C

100 °C

1,122.2 1,434.0 ND1 82.0 468.4 1,333.0 1,001.2 881.7 36.4 3.3

754.7 1,058.6 ND 6.7 87.0 674.7 613.1 230.0 4.1 0.20

247.6 316.6 ND 1.2 12.9 165.5 71.2 18.7 0.68 0.025

130.2 136.2 ND 0.590 4.8 79.2 20.4 4.7 0.33 0.012

23.5 23.4 ND 0.150 0.685 14.2 2.4 0.430 0.098 0.003

> 77 ND < 58 93 6 51 < <

100 25

25 25

Not determined (samples unable to be tested due to insufficient melt/flow behaviour).

tannins (Arbenz and Avérous, 2015; D’Souza et al., 2015). Considering application of the hydroxyalkylated tannins as a polyol component in polyurethane formulations, only the products with high substitution levels would be possible candidates due to their lower viscosity. This is required for efficient mixing of the polyol and isocyanate components to obtain a homogeneous polyurethane formulation. To use these materials, for example in the manufacturing of polyurethane foams, the polyol component would need to be gently heated (i.e. at 50 °C) or blended with a polyol of lower viscosity to obtain an acceptable viscosity. Of the samples with high substitution levels, HWT-10BO-150 exhibited the lowest viscosity (200 Pa.s at 50 °C). This material could be used ‘as is’ negating the requirement to blend with a polyol of lower viscosity, as would be required for HWT10PO or HWT-10PO-150. However, the higher level of homopolymer and additional expense of butylene oxide present disadvantages to the application of HWT-10BO-150. The derivative alkyl chain length would also impact the mechanical properties of the final polyurethane product.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the Biopolymer Network Limited who funded the work under the New Zealand Ministry of Business, Innovation and Employment contract BPLY1302. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.111618. References Aniceto, J.P.S., Portugal, I., Silva, C.M., 2012. Biomass-based polyols through oxypropylation reaction. ChemSusChem 5, 1358–1368. Arbenz, A., Averous, L., 2014. Synthesis and characterization of fully biobased aromatic polyols - oxybutylation of condensed tannins towards new macromolecular architectures. RSC Adv. 4, 61564–61572. Arbenz, A., Averous, L., 2015. Chemical modification of tannins to elaborate aromatic biobased macromolecular architectures. Green Chem. 17, 2626–2646. Arbenz, A., Avérous, L., 2015. Oxyalkylation of gambier tannin—synthesis and characterization of ensuing biobased polyols. Ind. Crops Prod. 67, 295–304. Bogun, B.R., 2007. Molecular Weight Characterisation of Pinus radiata Bark Condensed Tannins, Department of Chemistry. University of Waikato, Hamilton, New Zealand. Bridson, J.H., 2007. Derivatisation of Polyphenols. Department of Chemistry. University of Waikato, Hamilton. Bridson, J.H., Grigsby, W.J., Main, L., 2013. Synthesis and characterization of flavonoid laurate esters by transesterification. J. Appl. Polym. Sci. 129, 181–186. Bridson, J.H., Grigsby, W.J., Main, L., 2018. One-pot solvent-free synthesis and characterisation of hydroxypropylated polyflavonoid compounds. Ind. Crops Prod. 111, 529–535. Chen, G.Q., Patel, M.K., 2012. Plastics derived from biological sources: present and future: a technical and environmental review. Chem. Rev. 112, 2082–2099. Coates, J., 2000. Interpretation of infrared spectra, a practical approach. In: Meyers, R.A. (Ed.), Encyclopedia of Analytical Chemistry. John Wiley & Sons Ltd., Chichester, pp. 10815–10837. D’Souza, J., George, B., Camargo, R., Yan, N., 2015. Synthesis and characterization of biopolyols through the oxypropylation of bark and alkaline extracts of bark. Ind. Crops Prod. 76, 1–11. De Bruyne, T., Pieters, L., Deelstra, H., Vlietinck, A., 1999. Condensed vegetable tannins: biodiversity in structure and biological activities. Biochem. Syst. Ecol. 27, 445–459. Di Serio, M., Vairo, G., Iengo, P., Felippone, F., Santacesaria, E., 1996. Kinetics of ethoxylation and propoxylation of 1- and 2-Octanol catalyzed by KOH. Ind. Eng. Chem. Res. 35, 3848–3853. Dobinson, B., Hofmann, W., Stark, B.P., 1969. The Determination of Epoxide Groups. Pergamon Press, Oxford. Duval, A., Avérous, L., 2016. Oxyalkylation of condensed tannin with propylene carbonate as an alternative to propylene oxide. ACS Sustain. Chem. Eng. 4, 3103–3112. Feng, S., Cheng, S., Yuan, Z., Leitch, M., Xu, C., 2013. Valorization of bark for chemicals and materials: a review. Renew. Sust. Energ. Rev. 26, 560–578. Frevel, M.A., Pipingas, A., Grigsby, W.J., Frampton, C.M., Gilchrist, N.L., 2012. Production, composition and toxicology studies of Enzogenol(R) Pinus radiata bark

4. Conclusions A range of different strategies were evaluated to reduce the viscosity of hydroxyalkylated tannins to support incorporation into polyurethane and other industrial processes. The method of tannin extraction influenced the reactivity towards PO, with HWT and EWT exhibiting similar reactivity and properties on hydroxyalkylation. In contrast, SUT was less reactive, likely due to the sulfonate groups or impurities, and subsequently exhibited inferior physicochemical properties. However, the lower molecular weight of SUT resulted in a comparatively more effective internal plasticization for a given level of molar substitution. The epoxide chain length did not affect the overall reactivity, however differences in isomer specificity were observed. In contrast, the epoxide chain length had a significant effect on the physicochemical properties increasing the glass transition temperature and viscosity with increasing chain length. Homopolymerisation was minimized using a mild-base, solvent-free and anhydrous synthesis route, maximizing the efficiency of tannin derivatisation. Raising the reaction temperature increased the level of homopolymer formation, however homopolymerisation was still considerably lower compared with other literature methods. These findings highlight the influence of the tannin starting material and reaction conditions on the hydroxyalkylation route to produce polyols for industrial applications. Future research should focus on the synthesis of rigid polyurethane foams using these polyols. In addition, techno-economic and life cycle analysis would be beneficial to inform the economic viability and sustainability credentials of these materials.

8

Industrial Crops & Products 140 (2019) 111618

J.H. Bridson, et al. extract. Food Chem. Toxicol. 50, 4316–4324. Furtwengler, P., Avérous, L., 2018. Renewable polyols for advanced polyurethane foams from diverse biomass resources. Polym. Chem. 9, 4258–4287. García, D.E., Fuentealba, C.A., Salazar, J.P., Pérez, M.A., Escobar, D., Pizzi, A., 2016. Mild hydroxypropylation of polyflavonoids obtained under pilot-plant scale. Ind. Crops Prod. 87, 350–362. García, D.E., Glasser, W.G., Pizzi, A., Osorio-Madrazo, A., Laborie, M.P., 2013. Hydroxypropyl tannin derivatives from Pinus pinaster (Ait.) bark. Ind. Crops Prod. 49, 730–739. García, D.E., Glasser, W.G., Pizzi, A., Paczkowski, S., Laborie, M.-P., 2015. Hydroxypropyl tannin from Pinus pinaster bark as polyol source in urethane chemistry. Eur. Polym. J. 67, 152–165. García, D.E., Glasser, W.G., Pizzi, T.A., Osorio-Madrazo, A., Laborie, M.P.G., 2014. Synthesis and physicochemical properties of hydroxypropyl tannins from maritime pine bark (Pinus pinaster Ait.). Holzforschung 68, 411–418. Gaugler, M., Grigsby, W.J., 2009. Thermal degradation of condensed tannins from Radiata pine bark. J. Wood Chem. Technol. 29, 305–321. Granata, A., Argyropoulos, D.S., 1995. 2-Chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, a Reagent for the Accurate Determination of the Uncondensed and Condensed Phenolic Moieties in Lignins. J. Agric. Food Chem. 43, 1538–1544. Grigsby, W.J., Bridson, J.H., Lomas, C., Elliot, J.A., 2013. Esterification of condensed tannins and their impact on the properties of poly(lactic acid). PolymersBasel 5, 344–360. Harborne, J.B., Williams, C.A., 2000. Advances in flavonoid research since 1992. Phytochemistry 55, 481–504. Hashida, K., Ohara, S., 2002. Formation of a novel catechinic acid stereoisomer from base-catalyzed reactions of (+)-catechin. J. Wood Chem. Technol. 22, 11–23. Laurichesse, S., Arbenz, A., Barraud, V., Perrin, R., Averous, L., 2015. Synthesis of Innovative Materials Based on Tannins and Lignins: Biobased Polyurethane Foams for Insulation Applications, Biofoams 2015, Sorrento, Italy. Li, J., Maplesden, F., 1998. Commercial production of tannins from radiata pine bark for wood adhesives. Transactions of the Institution of Professional Engineers New Zealand. Electrical, Mechanical, and Chemical Engineering Section 25, 46–51.

Lora, J.H., Glasser, W.G., 2002. Recent industrial applications of lignin: a sustainable alternative to nonrenewable materials. J. Polym. Environ. 10, 39–48. Malik, M.I., Irfan, M., Khan, A., Rahim, S., Abdul-Karim, R., Hashim, J., 2016. Alkylene oxide poylmerizations: identification of side reactions and by-products. J. Polym. Res. 23, 258–265. Peters, M.S., Quinn, J.A., 1955. Pentaerythritol production yields. Ind. Eng. Chem. 47, 1710–1713. Pizzi, A., 2006. Recent developments in eco-efficient bio-based adhesives for wood bonding: opportunities and issues. J. Adhes. Sci. Technol. 20, 829–846. Pizzi, A., 2008. Tannins: Major sources, properties and applications. In: Belgacem, M.N., Gandini, A. (Eds.), Monomers, Polymers and Composites from Renewable Resources. Elsevier Limited, Oxford, UK. Reuter, J., Merfort, I., Schempp, C.M., 2010. Botanicals in dermatology: an evidencebased review. Am. J. Clin. Dermatol. 11, 247–267. Ricci, A., Olejar, K.J., Parpinello, G.P., Kilmartin, P.A., Versari, A., 2015. Application of Fourier transform infrared (FTIR) spectroscopy in the characterization of tannins. Appl. Spectrosc. Rev. 50, 407–442. Sandler, S.R., Karo, W., 1980. Polymer Syntheses. Academic Press, New York. Tousignant, W.F., Baker, A.W., 1957. Reaction of ethylene oxide with urea. J. Org. Chem. 22, 166–167. Sonnenschein, M.F., 2015. Polyurethane building blocks. In: Sonnenschein, M.F. (Ed.), Polyurethanes: Science, Technology, Markets, and Trends. John Wiley & Sons, Inc. Ververidis, F., Trantas, E., Douglas, C., Vollmer, G., Kretzschmar, G., Panopoulos, N., 2007. Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: chemical diversity, impacts on plant biology and human health. Biotechnol. J. 2, 1214–1234. Yanagida, A., Shoji, T., Shibusawa, Y., 2003. Separation of proanthocyanidins by degree of polymerization by means of size-exclusion chromatography and related techniques. J. Biochem. Bioph. Methods 56, 311–322. Zawadzki, M., Ragauskas, A., 2001. N-hydroxy compounds as new internal standards for the 31P-NMR determination of lignin hydroxy functional groups. Holzforschung 55, 283–285.

9