Cellulose as a Source of Water Dispersible Renewable Film-Forming

Nov 17, 2015 - Cellulose is one of the most abundant renewable polymeric materials on Earth. Given its abundance, there has been a large investment of...
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Cellulose as a Source of Water Dispersible Renewable Film-Forming Materials D. Bradley G. Williams, Jennifer M. Mason, Cameron J. Tristram, and Simon F. R. Hinkley* The Ferrier Research Institute, Victoria University of Wellington, 69 Gracefield Road, Lower Hutt 5010, New Zealand S Supporting Information *

ABSTRACT: Cellulose is found to be a good source of waterborne film-forming materials when modified with the correct type and level of functional groups. The modification of cellulose to incorporate high levels of levulinic functionality and other C2−C6 alkyl esters is reported for the first time. The levulinyl-functionalized cellulose is readily modified to produce oxime or acylhydrazone derivatives, which is particularly useful to fine-tune the physical characteristics of the cellulose ester. This includes the glass transition temperature of the cellulose esters, their ability to produce fine uniform colloidal particles in aqueous media and ultimately provide the principle film-forming component of an architectural coating. The process has been demonstrated at the 500 g level suggesting ready scalability. Lewis acids and Lewis acid-assisted Brønsted acids are employed for the first time in such chemistry, and together with sulfur-based Brønsted acids are all found to be effective catalysts for the esterification of cellulose to produce mixed ester systems. The Lewis acid catalysts demonstrate exceptional activity and produce high molecular weight cellulose derivatives.

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INTRODUCTION

The industry standard for cellulose manipulation is as a “swollen” solid-state,9 in which the cellulose has been pretreated before undergoing chemical reaction. Xanthate intermediates used in the viscose process and N-methylmorpholine-N-oxide used as the solvent in the Lyocell process are equally commercially important.10 Nonaqueous solvents such as LiCl/N,N-dimethylacetamide have been used extensively for chemical modifications of cellulose and present one of the most important breakthroughs which allowed the wider study of cellulose and its chemistry.11 Ionic liquids recently have shown to be very useful for cellulose processing,12 where dissolution of cellulose to achieve 10−20 wt % solutions is readily achievable. Our focus has been the generation of a novel cellulose derivative with the chemical flexibility to be used as the sole film-forming component in a commercial coating application as a waterborne emulsion. Our efforts in this field have led to a novel butyrate-levulinate-acetate functionalized cellulose mixed ester product; its glass transition (Tg) temperatures are such that, when dispersed into an aqueous medium, it is film-forming at ambient temperature.13,14

Cellulose is one of the most abundant renewable polymeric materials on Earth. Given its abundance, there has been a large investment of time and resources to convert cellulose into useful small molecules and polymeric materials. Chemistry related to cellulose and levulinic acid derivatives is receiving unprecedented attention.1,2 On the one hand, these efforts have been rewarded by the development of elegant processes by which small molecules such as levulinic acid (LA),3 hydroxymethylfurfural,4 hexane5 and others can be made. On the other, cellulose esters (CE) are produced at industrial scale and are used in a range of applications including thermoplastics and thermosetting resins, as polyols in curable coatings, controlled release technologies, optical films and membranes for separations technologies.6 Environmental concerns and decades of research, coupled with low cost petrochemical feedstocks, have seen waterborne acrylic and polyurethane formulations7 dominate the commercial market. However, modified CE polymers such as cellulose acetate butyrate find utility in myriad applications from pharmaceuticals to automotive finishes providing metal-flake orientation and desirable curing characteristics.6 More recently, there has been interest in water-based paints which contain binders prepared from renewable materials such as cellulose.8 © XXXX American Chemical Society

Received: September 27, 2015 Revised: October 29, 2015

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DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Levulinoyl Esters and Oxime Derivatives

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RESULTS AND DISCUSSION Synthesis of Levulinoyl-Containing Cellulose Esters. Commercial scale production of CEs is typically performed under solvent-free conditions in the presence of sulfuric acid catalyst.20 Reactivity of the acylating agent tails off quite sharply with increasing chain length, producing the reactivity series acetic > propionic > butyric ≫ isobutyric.21 The generally accepted rationale for this decrease in reactivity is a combination of increased steric hindrance of the esterifying agent as chain length increases,22 possibly coupled with the reduced ability of the acylating agent to act as solvent due to its lowered polarity. In our work, cellulose, as bleached wood pulp (MW 448 000 g·mol−1 as determined by MALLS23) was subjected to swelling in hot water (12.5 mL of water at 60 °C per g of dry cellulose) for 3 h, and then two exchanges were performed with acetic acid, producing cellulose wet with glacial acetic acid and a final wet mass of around 3 g per g of dry cellulose added. To produce LAC and the other mixed levulinoyl-alkanoyl ester cellulose ester products, the acetic acid was exchanged with levulinic acid (LA, twice with 6.25 mL per g cellulose on a dry basis, 40 °C). LA (>97%) can exist as a supercooled liquid (mp 34 °C) for an extended period at room temperature so heating during such exchanges is not necessary. Esterification was achieved by adding LA, an anhydride and sulfuric acid catalyst, after which the reaction mixture was heated with stirring (using a magnetic follower for small scale reactions or an overhead mechanical stirrer for larger scale reactions, Scheme 1, Table 1). It was found that the reaction mixture proceeded from a stiff slurry to a less viscous slurry and eventually into relatively low viscosity homogeneous mixture as the reaction progressed. This was typically accompanied by yellowing of the reaction mixture leading to various shades of orange or even brown in extreme cases, depending on the reaction conditions. At the end of the reaction, the mixture was cooled and slowly diluted with an equal volume of an aqueous magnesium acetate solution (5 wt % magnesium acetate in acetic acid−water 1:1 v/v). This diluted solution was then slowly poured into a vigorously

Levulinoyl-containing cellulose esters are not unknown: cellulose esterification to generate levulinoyl15,16 or acetyllevulinoyl cellulose species have been described17 and cellulosesurface modifications for wound healing applications have been reported.18 In all cases, the process was mediated by a reaction cosolvent17 or entailed the predissolution of a highly purified cellulose feedstock in typical cellulose-dissolution systems (N,N-dimethylacetamide/LiCl).16 Attempts to transesterify a preformed cellulosic ester with levulinic acid have also been reported, but the resultant material was not well characterized.19 In our hands, transesterification of cellulose esters was found to be a particularly inefficient process. In contrast to these previous disclosures, herein we demonstrate a scaleable process for the production of levulinoyl-containing cellulose esters directly from wood pulp, using methodology closely aligned to accepted industrial practice for cellulose esterification. In addition, this work outlines that while longer alkyl− ester levulinoyl celluloses can be readily synthesized there are significant disadvantages to producing species containing > C4 esters unless they are levulinoyl. In our studies of a trisubstituted butyrate-levulinate-acetate functionalized cellulose mixed ester product, the substrate could be modified at the ketone functionality of the levulinoyl moiety to form oxime or acylhydrazone derivatives that significantly change the physical characteristics of the polymer, most notably by lowering the Tg value thereof. When the oxime contained carboxylic acid functionality, the product was readily dispersible as a colloid in water. Although the polymer demonstrated a relatively high Tg value, the formulation was film-forming at room temperature. Thus, this carboxy-modified polymer could be formulated into a paint. Herein, we detail more extensive studies toward the controlled synthesis of mixed ester cellulose products and their modification. In addition, we discuss the influence of the catalyst on the reaction in terms of rate and molecular weight, detailing the unexpected efficacy of Lewis acids and of Lewis acid-assisted Brønsted acids. B

DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

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The products (Table 1) were characterized by 1H and 13C NMR spectroscopy (Figures 1 and 2), including 2D NMR spectroscopy (HSQC and HMBC), degree of substitution analysis, weight-average molecular weight (MW) determination, dispersity (Đ) and Tg. It is clear from Table 1 that there is quite a dramatic response by the Tg value to the type of substitution and the MW of the product. Clearly, the Tg of the product decreased with increasing chain length of the alkanoyl side chain (Table 1, entries 2−6) but then began to rise again (Table 1, entry 7). This finding is consistent with prior results, in which there is a decrease in Tg as the chain length of the substituent increased in monosubstituted or per-esterified CEs up to C6/C8, after which the Tg increased.24,25 There is some evidence that larger fatty acid moieties display independent chain-melt characteristics segregated from the cellulose backbone.26 Figure 1 shows a BLAC species with low levulinoyl content as compared to a LAC of high DSLev. Apart from the selfevident substituent contributions to the 1H NMR spectrum (δ < 3.0 ppm), it is notable in the spectra that the incorporation of the levulinoyl ester generates a significantly greater range of chemical-shift environments for the cellulosic methines as evidenced by the complexity of the region δ 3.4−5.4 ppm. It was important to target low Tg values, in order to generate a product that would be film forming at room temperature and to minimize the amount of plasticizing or coalescing agents required. The minimum film forming temperature of a waterborne emulsion is dominated by the Tg of the polymeric film-forming component.27 The published work shows that in two otherwise-identical formulations, the Tg of the film-forming polymer determines the level of film-formation (sintering of the particles to form a coherent film) and the quality of the film. Therefore, generating CEs with Tg values approaching near-toambient temperature ( 40 °C, below the detection limit. Film Formation. The ability of the carboxylated BLAC 10a 32 mgKOH·g−1 dispersions to produce films was investigated by spreading the dispersions onto a glass surface, and allowing them to dry at ambient temperature. In all cases, hard, brittle, transparent and highly cracked films were produced. Scanning electron microscopy of the films clearly demonstrated that film formation was accompanied by coalescence of the particles (Figure 4). It is clear from the micrograph that a coherent film

solvent

20 wt %

ethyl levulinate butyl levulinate 1-acetoxy-2-butoxyethane 4-hydroxy-4methylpentan-2-one diethylene glycol propylene glycol monobutyl ether

13.3 wt % 8.9 wt %

5.6 wt %

− − − −

− − + +

− − ++ ++

++ + N/A ++

− ++

+ N/A

+ N/A

++ N/A

a Visual quality of the film after drying; − means the absence of cracks; + means fine cracks and some heavier cracking; ++ means heavy cracking; N/A means not investigated.

were not stable for more than 2 days, and so this method to improve film quality would not provide a usable solution. We next investigated the use of sucrose acetate isobutyrate (SAIB) as a plasticizer. SAIB is a partly renewable plasticizer with a range of uses,51 for example resins, coatings and inks.52 Unlike coalescing solvents, SAIB is expected to remain in the polymer film long after drying and curing and so exerts its plasticizing effect over an extended period of time. SAIB was added to the BLAC 10a 32 mgKOH·g−1 dispersion in different amounts from 10 to 80 wt %, according to Table 7. Table 7. Effect of SAIB on Films of BLAC 10a 32 mgKOH·g−1

Figure 4. Scanning electron micrograph of carboxylated BLAC film. The structure of the PTFE filter is also evident (top left of the micrograph).

SAIB (wt %)

Tg (°C)

film qualitya

film qualityb

0 10 20 40 60 80

92 70 59 41 N/A 18

++ ++ ++ ++ − −

++ ++ ++ ++ + −

Visual quality of the film after drying. bVisual quality of the film after 4 weeks; − means the absence of cracks; + means fine cracks and some heavier cracking; ++ means heavy cracking; N/A means not investigated. a

had been formed in which the particles retain much of their original rod-like morphology. Film formation was unexpected since the minimum film forming temperature and the Tg of a substance are closely related.48 Since the Tg of BLAC 10a 32 mgKOH·g−1 had been found to be 92 °C, this material was considered to be an unlikely candidate for spontaneous film formation at room temperature. It is possible that hydroplasticization49 contributed to film formation in the present instance and that this effect was lost upon drying, leading to cracking. Several approaches were investigated to improve the quality of the film produced from BLAC 10a 32 mgKOH·g−1. In the first, the use of coalescing solvents was investigated.50 The coalescing solvent evaporates slower than water, remains in the film during curing and evaporates slowly from the film. It therefore removes the buildup of stresses in the film during the drying and curing phase, leading to improved film formation and film qualities. The high boiling solvents that formed part of our investigation are listed in Table 6. Ethyl levulinate and butyl levulinate proved useful and provided crack-free films at incorporation levels as low as 8.9 wt %. Usefully, butyl levulinate, which with a boiling point of 252 °C is not classified as a volatile organic compound, provided excellent films when incorporated as low as 8.9 wt %. However, dispersions of BLAC 10a 32 mgKOH·g−1 to which levulinate solvents had been added

SAIB added to the BLAC 10a dispersion at above 20 wt % had no effect on the average particle size compared to the parent BLAC 10a dispersion. Below this value of added SAIB, the dispersions were unstable and tended to gel. As expected, increasing amounts of SAIB led to depressed Tg values and improved film quality. Scanning electron microscopy of films formed which included 20 wt % SAIB and 40 wt % SAIB (Figure 5) indicated that the film formation was accompanied by the creation of spherical particles with cylindrical interconnections, and that the size of the spherical particles increased with increasing levels of SAIB. This is quite in contrast to the film formed from the parent BLAC 10a 32 mgKOH·g−1 in which the particles retained their original elongated shape to some extent. Formulation of a Paint. Finally, BLAC 10a 32 mgKOH· g−1 was formulated into a low sheen paint. To this end, a standard mill base (a mixture of pigments, surfactants, dispersants, fungicides, etc.) was prepared which would produce a white paint.53 Into this mill base was blended a dispersion which contained BLAC 10a 32 mgKOH·g−1 (26.1 wt %), SAIB (20.9 wt %), triethylamine (0.9 wt %) and water (52.1 wt %). This water-based paint contained 12 g·L−1 of volatile organic compounds, falling well within the limits of 45− H

DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

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polymer could be formulated into paints. The chemistry employed was readily amenable to scale and can generate a binder which contains >75% renewable carbon content.

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EXPERIMENTAL SECTION

General Experimental Information. Commercially sourced solvents and reagents were used without further purification. The Tmixer used a Norgren pneumatic actuator (RT-57232/m/80) set up to depressed two syringes (3, 6 or 12 mL) at a constant rate and the pressure was controlled by a regulator (1−6 bar). The T-mixer had two 1/16” i.d. inlets, a mixing cavity and outlet. The syringes were connected to the mixer using stainless steel tubing and Leuer lock connectors. NMR Spectroscopy. NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer. Each sample (15 mg) was dissolved in CDCl3 (unless otherwise specified, 0.7 mL) in a 5 mm NMR tube. The spectra were collected at 30 °C with chemical shifts referenced to solvent peaks at 1H δ 7.26 ppm and 13C δ 77.08 ppm. 2D NMR experiments were performed to assign the peaks. These included HSQC, HMBC and COSY spectroscopy. Differential Scanning Calorimetry. Determination of Tg values was performed using differential scanning calorimetry on a Mettler Toledo DSC1 STARe System with a GC200 gas controller using an autosampler and operating under a constant flow of nitrogen (30 mL/ min). Samples of cellulose esters were weighed (0.5−15 mg) into aluminum crucibles (40 μL, P/N ME-26763), the lid punctured and the crucible crimp-sealed. A 7-stage temperature method was used and an empty crucible was used as the blank: − 30 to 180 °C (all ramprates were set at 10 °C/min); +180 to −30 °C; hold 10 min; −30 to +180 °C; +180 to −30 °C; hold 10 min; −30 to +180 °C. The glass transition temperature was assigned electronically and also by assessment of the second-order inflection points of the DSC heatflow vs temperature plot with the centroid of the inflection points taken as the Tg. Infrared Spectroscopy. Infrared spectra (IR) were recorded on a PerkinElmer Spectrum One instrument in a frequency range from 4000 to 450 cm−1 using ATR (diamond) technology. Mass Spectrometry. Positive-ion high resolution electrospray mass spectroscopy was performed on a Waters Q-TOF PremierTM Tandem Mass Spectrometer. Residual Solvent Analysis. Gas chromatography analysis for residual acetone using sample injection volumes of 2 μL (volatile organic compound method ASTM D6886−03) was conducted on an Agilent Technologies GC 6890N equipped with a flame ionization detector using hydrogen as the carrier gas at a constant flow rate of 1 mL·min−1 and with a split ratio of 100:1. It was fitted with a J and W Scientific HP-5 column (length 30 m, i.d. 0.32 mm, film thickness 0.25 μm) and was operated at an inlet temperature of 260 °C and detector temperature of 270 °C. The oven temperature profile was as follows: 40 °C (2 min) then ramp to 200 °C (10 °C.min−1). Standard solutions of acetone and 1-propanol (internal standard) were prepared in THF (HPLC grade) with concentrations ranging from 0.15 to 1.5 mg·mL−1 with 7 evenly spaced concentration increments. Samples were prepared by weighing approximately 0.5 g (±0.05 g) of the carboxyfunctionalized cellulose ester (20−30% w/v) dispersed into water. A stock solution of an internal standard (0.5 mL of 1 mg·mL−1 1propanol in THF) was added to the sample and the mixture diluted with THF up to 2 mL. The sample was sealed, mixed thoroughly for 5 min and then centrifuged at 14000 × g for 3 min. An aliquot was removed and tested immediately. Particle Size Analysis. Particle size analysis of the cellulose esters in the aqueous dispersion was performed using a Shimadzu SALD2001 laser diffraction particle size analyzer (minimum measurable particle size of 30 nm), employing deionized water as the dispersion medium. Shimadzu software (SALD-2001-WEA2: V1.00) was used to calculate particle size distribution by volume, using a refractive index of 1.45−0.10i. Samples were run in duplicate and an average value was recorded. The particle size distribution is reported as the average particle size.

Figure 5. Scanning electron micrograph of films of BLAC plasticized with SAIB (A, 20 wt % SAIB; B, 40 wt % SAIB). The structure of the PTFE filter is also evident in part B.

85 g·L−1 required to comply with the Environmental Choice New Zealand program labeling requirements in paints.54 All of the volatile organic compound content originated in the mill base, so with an improved mill base the paint formulation would meet even stricter regulations. A gray paint was similarly constituted using the BLAC 10a 32 mgKOH·g−1 dispersion. These two paints were applied to an opacity card using a draw down bar (Figure 6). After aging for 2 months, the films

Figure 6. Opacity cards with white and gray paints formed using BLAC binder.

remained intact and were subjected to qualitative film adhesion, flexibility, toughness and water susceptibility tests. The films passed the adhesion, flexibility and toughness tests and demonstrated some water susceptibility, indicating that paints based on this system would be restricted to areas where moisture contact is limited, such as ceilings. We are presently working toward improving the film’s moisture susceptibility profile to provide a more widely applicable solution.

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CONCLUSION Cellulose in the form of Kraft bleached pulp is readily converted into its levulinoyl−acetyl and butyroyl−levulinoyl− acetyl cellulose ester products in reactions catalyzed by sulfurbased acids. An important discovery was that some metal triflates are effective catalysts for this reaction and particularly that Lewis acid-assisted Brønsted acids are remarkably efficient catalysts. This finding is unusual since the individual parent constituents (lanthanide triflates and phosphoric acid) are poor catalysts for this transformation. The ketone functionality of the levulinoyl moieties provided a readily accessible handle for manipulation of the performance characteristics of the cellulose ester products. This allowed the key parameters Tg, solubility and flexibility (internal plasticization) of the material to be finetuned, such that a dispersion of the BLAC 10a 32 mgKOH·g−1 I

DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

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7), the residual acetic acid was exchanged with levulinic acid (LA, 2 × 100 mL, 40 °C, 90 min). Synthesis of LAC. Levulinic acid (17.2 g, 148 mmol), acetic anhydride (11.3 g, 111 mmol, 3 equiv per OH), and sulfuric acid (9.06 mmol) were added to a round-bottom flask, leading to a mild exotherm. To this mixture was added the pretreated cellulose (6.41 g, containing 2.0 g of cellulose (12.4 mmol) and 4.41 g (37.4 mmol) of residual levulinic acid from the swelling process; equivalent to 12.3 mmol of anhydroglucose with 36.9 mmol of OH groups). The slurry was mixed with an overhead mechanical stirrer at 120 °C for 2 h during which it developed into an orange solution. After cooling the mixture was diluted with an aqueous mixture of Mg(OAc)2 (25 mL, 5 wt % in 1:1 water−acetic acid v/v). The resulting solution was poured slowly into water (600 mL) with stirring. The precipitate was isolated by filtration, resuspended in water (250 mL), stirred for 3 h and filtered. If the precipitate was found to contain entrained reagents, it could be purified by dissolution in acetone (50 mL) and reprecipitation in water (500 mL) to generate an off-white solid, which was isolated by filtration. Residual solvents were removed at 45 °C under vacuum (15 mmHg) to give LAC (3.92 g, 10.2 mmol, 85%). Synthesis of BLAC Using Sulfur-Based Catalysts (Larger Scale). To acetic acid preswelled cellulose (53.2 g dry weight, equivalent to 328 mmol of anhydroglucose and 984 mmol of OH) was added butyric anhydride (470 g, 2.97 mol, 3.02 eq./OH), levulinic acid (448 g, 3.86 mol, 3.92 equiv/OH) and sulfuric acid (0.47 mL). The reaction mixture was stirred using an overhead mechanical stirrer for 1 h at 120 °C and worked up in a similar fashion to the LAC, to produce BLAC as an off-white solid (97.5 g, 79%). Synthesis of BLAC Using Metal Triflate Catalysts. The reaction mixture was prepared in a similar fashion to the reaction above, except that the reaction was conducted using 250 mg of cellulose (1.5 mmol of anhydroglucose) and the catalyst in this instance was a metal triflate (Al(OTf)3 or Yb(OTf)3 or Gd(OTf)3, 3.3 mol %/OH (or according to Table 6), equating to 10 mol % per anhydroglucose unit). The reaction was permitted to proceed for the allotted time and at the desired temperature (Table 6), and was worked up as described above producing BLAC as a bright white solid. Synthesis of BLAC Using Lewis Acid-Assisted Brønsted Acid Catalysts. The reaction mixture was prepared in a similar fashion to the reaction above, except that the reaction was conducted in the presence of a mixed acid system present at 3.3 mol %/OH (or according to Table 6), equating to 10 mol % per anhydroglucose unit, and constituted by adding each acid (the metal triflate and the phosphoric acid) to the reaction vessel. The reaction was permitted to proceed for the allotted time and at the desired temperature (Table 6) and worked up as described above, to produce BLAC as a bright white solid. Carboxylation of BLAC. The synthesis of BLAC 10a 32 mgKOH· g−1 was prepared as previously described.13 To prepare BLAC 10a 63 mgKOH·g−1, BLAC (20.7 g) and O-(carboxymethyl)hydroxylamine hemihydrochloride (2.56 g, 23.4 mmol) were individually dissolved in DMSO (200 and 20 mL, respectively). The two solutions were combined and the mixture stirred for 2 h at ambient temperature. The carboxylated BLAC was precipitated by addition of the DMSO solution to stirring deionized water (1.3 L), the mixture of which was stirred for 2 h. The precipitate was recovered on a PTFE membrane (200 μm pore size), resuspended in deionized water (1 L) with gentle stirring for 2 h and collected by filtration. The precipitate was dried at 60 °C under vacuum for 18 h, yielding 19.7 g of product 10a with an acid number of 63 mgKOH·g−1. To prepare BLAC 10a 100 mgKOH·g−1, BLAC (4 g) and O(carboxymethyl)hydroxylamine hemihydrochloride (984 mg, 8.97 mmol) were individually dissolved in DMSO (60 and 5 mL, respectively). The two solutions were combined and the mixture stirred for 2 h at ambient temperature. The carboxylated BLAC was precipitated by addition of the DMSO solution to stirring deionized water (500 mL), the mixture of which was stirred for 2 h. The precipitate was filtered through a PTFE membrane (200 μm pore size), resuspended in deionized water (300 L) for 2 h with gentle agitation and collected by filtration. The precipitate was dried at 60 °C

Acid Number Determination. The acid number of the polymers was determined following published methods,42 making use of an Eutech Instruments pH510 m with glass electrode (ECFG7451901B). Molecular Weight Determination Using Size Exclusion Chromatography. An Agilent Technologies 1260 HPLC equipped with RI and UV detectors was fitted with TSKgel SuperHM-L and TSKgel Super HM-H columns inline and was operated at 60 °C with a TSKgel SuperH-H guard column. An isocratic solvent system (N,Ndimethylacetamide, HPLC grade) was used at a flow rate of 0.25 mL.min−1, using a sample size of 5 μL with an analyte concentration of 10 mg/mL. Samples were prepared by dissolution of the cellulose ester in N,N-dimethylacetamide (10 mg/mL) followed by centrifugation at 14 000 × g for 5 min. Measurements were taken at 210, 254, 450, 475, and 550 nm for the UV detector, and the refractive index detector was operated at 40 °C. Cellulose ester MW analysis was executed by comparison to monodisperse polystyrene standards (TSK-gel standards A500 through F128). Size exclusion chromatography analysis of the wood pulp (BKT, Kinleith Pinus, bleached) was performed as follows: Cellulose (100 mg) was successively swollen in water (3 × 10 mL each for 1 h) and then the liquid phase consecutively exchanged with acetone (3 × 10 mL each for 1 h) and N,N-dimethylacetamide (3 × 10 mL each for 1 h). A solution of the swollen cellulose was prepared by dissolution of the cellulose in a water-free 8% solution of LiCl in N,Ndimethylacetamide to generate a 10 mg·mL−1 solution. The sample was shaken for 1 h and stored at 4 °C for 18 h. SEC analysis was conducted using a water-free 8% solution of LiCl in N,Ndimethylacetamide as the mobile phase and employing TSKgel SuperHM-L and TSKgel Super HM-H columns inline and a TSKgel SuperH-H guard column with MALLS detection. Degree of Substitution. HPLC analysis was used to determine the degree of substitution (DS) using an Agilient Technologies 1260 HPLC loaded with a Rezex ROA organic acid column operating using an isocratic aqueous (5 mmol L−1 H2SO4) method; flow rate 0.5 mL· min−1 and column temperature of 60 °C. The UV detector was set to monitor samples at 210, 254, 450, 475, and 550 nm and the refractive index detector cell was operated at 40 °C. Standard aqueous solutions (5 mmol L−1 H2SO4) were prepared of levulinic acid, butyric acid, acetic acid and propanoic acid (internal standard) with concentrations of between 0.05 and 20 mg·mL −1 in eight evenly spaced concentrations. The cellulose ester sample was dried and weighed (20 ± 0.1 mg) into a sealable 7 mL test tube and charged with the internal standard stock solution (500 μL of 10 mg·mL−1 aqueous propanoic acid solution in 5 mmol L−1 H2SO4) after which NaOH solution (2 mL, 2 N) was added. The tube was sealed and heated to 105 °C and vortex-mixed every 45 min over a period of 3 h. The mixture was allowed to cool to ambient temperature before treating it with sulfuric acid (∼3 mL, 1 mol·L−1). The tube was resealed and thoroughly mixed and left to stand for 5 min. Samples were centrifuged at 4000 × g at 10 °C for 5 min before a 2 mL aliquot of the supernatant was taken and centrifuged at 14 000 × g for 10 min. The supernatant was taken for HPLC analysis using a sample injection size of 10 μL. The carboxylic acid content was calculated using the relative response factor generated from the standard solutions, normalized against the internal standard. Raw Materials. Cellulose fiber was kindly provided by Carter Holt Harvey, Kinleith (NZ), Kinleith Kraft grade Pinus Bleached. Levulinic acid (>97%) was supplied by Sigma-Aldrich. All other reagents and solvents were of analytical grade and used without further purification. Synthesis of Cellulose Esters. LAC and BLAC were prepared according to our earlier paper for all instances using sulfur-based acids and the metal triflates.24 The methods are given here for ease of reference. Swelling of Cellulose. To swell the cellulose, bleached, mediumcoarse, Pinus radiata wood pulp (16 g) was soaked in warm water (200 mL, 60 °C) for 3 h and then filtered. The residual water was exchanged with acetic acid (2 × 200 mL, 40 °C, 90 min). Excess acetic acid was removed by vacuum filtration and the cellulose was recovered. This material was used in the reaction process to synthesize LAC and BLAC. For the reaction to synthesize the diesters (Table 1, entries 3− J

DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules under vacuum for 18 h affording 4.2 g of product: acid number 100 mgKOH·g−1. Preparation of Model Compound 9. Isobutyryl levulinate and O-(carboxymethy)hydroxylamine (1 equiv) were dissolved in a mixture of acetonitrile:acetic acid:DCM:isopropyl alcohol (1:0.1:1:0.2). The solution was heated (50 °C, 1 h) and the product (E:Z ratio 3:1) recovered by distillation of the solvents. 1H NMR (500 MHz, CDCl3, δ): 0.93 (d, 6H, J = 6.7 Hz, H-3′), 1.90 (s, 3H, H-5Z), 1.92 (s, 3H, H-5E), 1.92 (m, 1H, H-2′), 2.54 (m, 2H, H-2E), 2.67 (m, 2H, H-2Z), 2.61 (m, 2H, H-3Z), 2.54 (m, 4H, H-2E and H-3E), 3.86 (d, 2H, J = 6.7 Hz, H-1′E), 3.88 (d, 2H, J = 6.7 Hz, H-1′Z), 4.59 (s, 2H, H-6E), 4.61 (s, 2H, H-6Z). 13C NMR (125 MHz, CDCl3, δ): 14.9 (C-5E), 19.0 (C-3), 19.7 (C-5Z), 25.1 (C-2Z), 27.8 (C-2′), 30.1 (C3Z), 30.3 (C-3E), 30.9 (C-3E), 69.7 (C-6), 70.7 (C-1′), 158.3 (C-4E), 159.1 (C-4Z), 172.8 (C-1), 175.1 (C-7E), 174.9 (C-7Z); TOF− HRMS found, 286.1159; C11H19NO5Na+ calcd, 286.1161. General Method for Preparation of Oxime and Acyl Hydrazone Derivatives of LAC. The experimental procedure has been previously detailed and is provided here for convenience. LAC (0.2 g) was dissolved in CH3Cl or EtOAc (2 mL) and the alkoxyamine or acyl hydrazide (0.43 or 0.23 mmol; 2.2 or 1.2 mmol·g−1 LAC, respectively) and acetic acid (0.002 mL) were added. The reaction mixture was stirred at ambient temperature until completion of the reaction (2−10 h, based on tlc analysis). The solvents were removed under vacuum to give the oxime or acyl hydrazone. A mixture of 10b and LAC prepared by dissolution and evaporation of solvent (external plasticizing) did not display a decrease in Tg. Dispersion of BLAC 10a (Acid Number 32 mgKOH·g−1); TMixing and Effect of Neutralization Using TEA. Dispersions were prepared as follows; BLAC 10a 32 mgKOH·g−1 was dissolved in acetone (3.8 mL) and the mixture was agitated until a homogeneous solution had formed. To the organic polymer solution were added triethylamine (the amount varied with the level of neutralization) and deionized water (200 μL) and the mixture was stirred. The organic polymer solution was transferred into a 6 mL syringe and an equal amount of deionized water (4.5 mL) was charged to a second 6 mL syringe. Using a pneumatic syringe pump set at 6 bar pressure, the antisolvent and polymer solution streams were brought together and combined at a T-mixer interface. The dispersion was collected in a separate flask and concentrated using a rotary evaporator at 25 °C with a starting pressure of 30 kPa ramping down to 1 kPa over 1 h and maintaining 1 kPa for 1 h. Dispersion of BLAC 10a (Acid Number 63 mgKOH·g−1). BLAC (acid number 63 mgKOH·g−1, 0.5 g) was dissolved in acetone (3.3 mL) and the mixture stirred until a homogeneous solution was obtained. Deionized water (200 μL) and dimethylethanolamine (25 μL) were added and the solution thoroughly mixed. The polymer solution was added dropwise to vigorously stirring deionized water (3.75 mL). The dispersion was concentrated using a rotary evaporator at 25 °C with a starting pressure of 30 kPa ramping down to 1 kPa over 1 h and maintaining 1 kPa for 1 h. Dispersion of BLAC 10a (Acid Number 100 mgKOH·g−1). To BLAC (acid number 100 mgKOH·g−1, 0.5 g) was added acetone (3.3 mL), and the mixture was agitated until a homogeneous solution was obtained. Deionized water (200 μL) and dimethylethanolamine (25 μL) were added, and the solution was thoroughly mixed. Deionized water (3.6 mL) was added dropwise over a 2 min period to the BLAC−acetone solution under mild agitation. The dispersion was concentrated using a rotary evaporator at 25 °C with a starting pressure of 30 kPa ramping down to 1 kPa over 1 h and maintaining 1 kPa for 1 h. Post Addition of Coalescing Solvents. A BLAC dispersion (500 mg of 18.1% solids dispersion of BLAC acid number 32 mgKOH·g−1, average particle size 120 nm) was weighed into a 2 mL vial. The coalescing solvent was added to this dispersion according to Table 8 and the vials were sealed and the contents mixed for 3 min. The dispersion was allowed to rest for 20 min and was visually assessed for signs of destabilization. Films were cast from the dispersions. SAIB Plasticizing of BLAC Dispersions. BLAC 10a 32 mgKOH· g−1 and SAIB (Table 8) were dissolved in acetone (8 mL·g−1) and the

Table 8. BLAC-SAIB Preparations for Investigating Plasticization SAIB (mg/wt %)

BLAC (mg)

TEA (μL)a

distillation time (h); pressure (kPa); T (°C)

54/4.5 108/9.0 216/18.0 216/36.0 300/50.0 360/60.0 480/80.0 600/100.0

1200 1200 1200 600 600 600 600 600

55 55 55 27.5 27.5 27.5 27.5 27.5

2; 10; 30 2; 10; 30 2; 10; 30 1; 5; 40 1; 5; 40 1; 5; 40 1.25; 10; 40 2; 10; 30

a

TEA = triethylamine.

mixture was stirred to form a homogeneous solution. Triethylamine and deionized water (200 μL) were added and the blend thoroughly mixed. The organic polymer solution was aspirated into a 12 mL syringe and an equal amount of deionized water was charged into a second 12 mL syringe. Using a pneumatic syringe pump set at 5 bar the antisolvent and polymer solution streams were brought together and converged at a T-mixer interface. The dispersion was concentrated using a rotary evaporator at a predetermined temperature, from a starting pressure 30 kPa ramping down to the final pressure over 30 min and maintaining final pressure for the desired time (Table 8). Low-Sheen White Paint. This BLAC−SAIB dispersion was blended with a generic gloss mill-base55 (combined in the ratio of 1.4:1 w/w dispersion/mill-base) containing the standard ingredients such as titanium dioxide, water, anionic dispersants, defoamer and antimicrobial agents. A film was produced on an opacity card using a standard draw-down bar.56 The film so generated was a flexible, continuous, opaque, and uniform film.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02131. Proton and carbon NMR spectra for key cellulose esters; synthesis of plasticizers and characterization data for cellulose (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.F.R.H.) Telephone: +644 463-0052. E-mail: simon. [email protected]. Author Contributions

All authors have contributed to the manuscript and given approval of the final version. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Business, Science and Innovation for funding (Grant C08X1001), Dr. Ian Sims for HPLC analysis of native cellulose and Dr. Graham Caygill for reaction calorimetry and scale-up assistance as well as Dr. Mark Glenny of Resene Paints Ltd for fruitful discussions, assistance with the formulations, and qualitative testing of paint swatches.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.5b02131 Macromolecules XXXX, XXX, XXX−XXX