Biobased Plasticizers from Carbohydrate-Derived 2,5-Bis

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Biobased Plasticizers from Carbohydratederived 2,5-bis-(Hydroxymethyl)furan Bob A. Howell, and Simone T. Lazar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05442 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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Industrial & Engineering Chemistry Research

Biobased Plasticizers from Carbohydrate-derived 2,5-bis-(Hydroxymethyl)furan Bob A. Howell* and Simone T. Lazar Center for Applications in Polymer Science Department of Chemistry and Biochemistry Central Michigan University Mt. Pleasant, MI 48859-0001 [email protected] Abstract As potential replacements for phthalate plasticizers, a series of esters of biobased 2,5-bis(hydroxymethyl)furan has been synthesized and fully characterized using spectroscopic and thermal methods. These materials are fully compatible with PVC and display good plasticizing effect, comparable to that of common phthalate plasticizers. The presence of these materials does not dimenish the thermal stability of the polymer and they display low migratory potential. These compounds would seem to represent attractive biobased alternatives to traditional phthalate esters for the plasticization of PVC. Key words: PVC plasticization; green plasticizers; polymer additives from renewable biosources; nonmigrating polymer additives; plasticizers from furanics. Introduction There is a great current interest in the development of polymers and polymer additives from renewable biosources.1-18 This is driven by concerns about sustainability, environmental quality, human health and the diminishing reserves of petroleum which serve as a primary source of petrochemical counterparts of these materials. Biomaterials suitable for conversion to useful polymer additives are available from a wide variety of sources, are renewable annually, are generally nontoxic and are independent of fluctuations in petrochemical markets. Furanics derived from saccharidic materials provide a broad base for the development of new polymers and additives.19,20 5-Hydroxymethylfurfural (HMF) is a key intermediate in the production of these products.21-25 It may be generated from fructose, glucose or higher saccharides. These compounds are readily available from a number of crop plants. The oxidation of HMF, either catalytically or enzymatically, to generate 2,5-furandicarboxylic acid (FDCA) has garnered the most attention.28 This compound may serve as a precursor to several useful materials, most notably poly(ethylene furanoate) [PEF].27 This material is now being commercialized as a replacement for poly(ethylene trephthalate) in many applications.28 HMF may also be reduced under a variety of conditions to generate 2,5-bis-(hydroxymethyl)furan (BHMF). BHMF may serve as a base for the generation of a family of plasticizers for polymeric materials, most prominently poly(vinyl chloride) [PVC]. For flexible applications, PVC must be heavily plasticized. Approximately 85% of all plasticizer production is used in the formulation of 1 ACS Paragon Plus Environment

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flexible PVC. Esters of phthalic acid have been commonly used plasticizers. Among these di(2-ethylhexyl) phthalate (DEHP) has been the most prominent. DEHP is an effective plasticizer and is readily available from two inexpensive starting materials, o-xylene and butraldehyde. o-Xylene is the least valuable of the mixed xylenes from the reforming of light naptha and may be oxidized to phthalic anhydride. Butraldehyde is a by-product of the hydroformulation of propylene. Aldol condensation of butraldehyde followed by hydrogenation generates 2-ethylhexanol. Reaction of this alcohol with phthalic anhydride provides DEHP.29 DEHP has been widely used as a plasticizer for PVC. It, and other phthalates, has a tendency to migrate from a polymer matrix into which it has been incorporated. As a consequence, it has become widely distributed in the environment. Almost all human populations have been exposed to DEHP at some level, some quite high.30-32 Human exposure to DEHP and other phthalates may occur in a variety of ways. Contact with medical devices (intravenous tubing, blood bags, catheters, dialysis equipment) or children’s toys (teething rings, pacifiers) may lead to direct introduction of phthalates.33-35 Indirect exposure may result from breathing house dust or eating contaminated food (contamination can occur during processing, handling, transportation, packaging or storage).31,36-38 Alternatively, phthalates may enter the food chain via plants eaten as food.39.40 Human exposure to phthalates has been associated with a number of disease states most arising from endocrine disruption,41-44 Because of this the use of phthalates has come under increasing societal and regulatory pressure.45,46 Efforts to find suitable replacements for phthalates is ongoing. Approaches to limit migratory ability by covalent attachment to a polymer mainchain or by using high molecular weight additives have been explored.47-50 However, most attention has been focused on the development of new plasticizers from renewable biosources.51-60 EXPERIMENTAL Materials Common solvents and reagents were obtained from ThermoFisher Scientific or the Aldrich Chemical Company. Tetrahydrofuran (THF) was distilled from lithium aluminum hydride prior to use; methylene chloride from calcium hydride. 2-Ethylhexanoyl chloride, octanoyl chloride, and benzoyl chloride were obtained from Aldrich Chemical Company and used as received. Poly(vinyl chloride) with Mn = 48,000 was obtained from Fluka AG. Instrumentation Instrumentation used for characterization and analysis has previously been described.61 Melting points were determined by differential scanning calorimetry (DSC) at a heating rate of 10 °C/min using a TA instruments Q2000 DSC instrument. All samples for DSC (5-10 mg) were contained in a crimped aluminum pan. The DSC cell was purged with dry nitrogen at 50 ml/min during analysis. Dynamic storage modulus and tan δ were determined by dynamic mechanical analysis (DMA) using a TA Instruments Q800 DMA instrument. A ramp rate of 5 °C/min and a constant frequency of 1Hz were used. All samples for DMA were thin rectangular films (5 ± 0.5 mm by 15 ± 1 mm with a thickness of 2 ± 0.5 mm). Glass transition temperatures (Tg) were determined from both DMA and DSC measurements. 2 ACS Paragon Plus Environment

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Synthesis bis-2,5-(2-Ethylhexanoylmethyl)furan (BHMF-E) Into a dry, 100-ml, three-necked, round-bottomed flask fitted with a 50-ml pressureequalizing dropping funnel, a magnetic stir bar, and a Liebig condenser bearing a gas-inlet tube was placed a solution of 1.00 g (7.80 mmol) of 2,5-bis(hydroxymethyl)furan, and 2.56 ml (1.86 g, 18.4 mmol) of triethylamine in 15 ml of anhydrous tetrahydrofuran (THF). The stirred solution was cooled to near 0°C (external ice bath) and a solution of 3.13 ml (3.00 g, 18.4 mmol) of 2-ethylhexanoyl chloride in 25 ml of anhydrous THF was added dropwise over a period of one hour. The mixture was allowed to warm to room temperature and stirred until reaction was complete as indicated by analysis of an aliquot of the reaction mixture using infrared spectroscopy. When the reaction was complete, the triethylammonium chloride was removed by filtration and the mixture was dissolved in 50 ml of diethyl ether and washed, successively, with three 10-ml portions of 10% aqueous hydrochloric acid solution, 10 ml of 10% aqueous sodium hydroxide solution and 10 ml of saturated aqueous sodium chloride solution. The solution was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation at reduced pressure. The residual material was held at 15 torr and 50°C for 24 hr to provide 2.21 g (74.3% yield) of BHMF-E as an amber liquid: decomposition Tonset 213°C (TGA); FTIR (cm-1, ATR) 2960 (s) 2933 (s) 2874 (m) –Csp3-H, 1734 (vs) –ester carbonyl C=O, 1561 (w) –aromatic furan nucleus, and 1160 (s) –carbon to oxygen C-O; 1H NMR (, CDCl3) 0.80 (m, 12H, methyl protons), 1.17 (m, 8H, methylene protons), 1.46 (m, 8H, methylene protons), 2.28 (m, 2H, methine protons), 5.03 (s, 4H, methylene protons adjacent to the furan nucleus and ester oxygen), 6.34 (s, 2H, protons of the furan nucleus); 13C NMR (, CDCl3) 11.7 (carbon atoms of the methyl groups) 13.9 (carbon atoms of the branched methyl groups) 22.5 (carbon atoms of the methylene groups) 25.3 (carbon atoms of the methylene groups) 29.4 (carbon atoms of the methylene groups) 31.6 (carbon atoms of the methylene groups) 47.1 (carbon atoms of the methine groups), 57.6 (carbon atoms of the methylene groups adjacent to the furan nucleus and ester oxygen), 111.2 (furan carbon atoms), 150.3 (substituted furan carbon atoms), 175.9 (carbonyl carbon atoms); ESI MS (C22H36O5) m/z, 403.1 m/z [M + Na+], 419.1 m/z [M + K+], 783.4 m/z [2M + Na+], and 799.4 [2M + K+]. All other esters were prepared using the method described above. bis-2,5-(Octanoylmethyl)furan (BHMF-O): 85.1% yield, amber semi-solid: mp 29°C (DSC); decomposition Tonset 213°C (TGA); FTIR (cm-1, ATR) 2916 (s) 2849 (m) –Csp3-H, 1732 (vs) –ester carbonyl C=O, 1564 (w) –aromatic furan nucleus, and 1152 (s) –carbon to oxygen CO; 1H NMR (, CDCl3) 0.87 (t, 6H, J = 6.93 ,methyl protons), 1.28 (m, 16H, methylene protons), 1.61 (m, 4H, methylene protons), 2.32 (t, 4H, J = 7.48, methylene protons adjacent to ester carbonyl), 5.03 (s, 4H, methylene protons adjacent to ester oxygen), 6.35 (s, 2H, furan protons); 13C NMR (, CDCl ) 14.0 (carbon atoms of the methyl groups) 22.6 (carbon atoms of the 3 methylene groups) 24.8 (carbon atoms of the methylene groups) 28.8 (carbon atoms of the methylene groups) 29.0 (carbon atoms of the methylene groups) 31.6 (carbon atoms of the methylene groups) 34.1 (carbon atoms of the methylene groups adjacent to carbonyl), 57.9 (carbon atoms in methylene groups adjacent to the furan nucleus and ester oxygen), 111.4 (furan 3 ACS Paragon Plus Environment


carbon atoms), 150.3 (substituted furan carbon atoms), 173.4 (carbonyl carbon atoms); ESI MS (C22H36O5) m/z, 403.2 m/z [M + Na+], 419.2 m/z [M + K+], 783.5 m/z [2M + Na+], and 799.4 [2M + K+]. bis-2,5-(2-Phenylacetoxymethyl)furan (BHMF-P): 85.6% yield beige crystalline solid (from ethyl acetate): mp 66°C (DSC); decomposition Tonset 202°C (TGA); FTIR (cm-1, ATR) 3133 (vw) 3064 (m) 3033 (m) –Csp2-H, 2962 (w) 2920 (w) –Csp3-H, 1731 (s) –ester carbonyl C=O, 1604 (w) –aromatic phenyl nucleus, 1563 (w) –aromatic furan nucleus, and 1129 (s) – carbon to oxygen C-O; 1H NMR (, CDCl3) 3.66 (s, 4H, benzylic protons adjacent to carbonyl), 5.07 (s, 4H, methylene protons adjacent to the furan nucleus and ester oxygen), 6.34, (s, 2H, furan protons), 7.26 (m, 2H, phenyl protons), 7.28 (m, 4H, phenyl protons), 7.30 (m, 4H, phenyl protons); 13C NMR (, CDCl3) 41.1 (benzylic carbon atoms), 58.4 (carbon atoms of the methylene group adjacent to the furan nucleus and ester oxygen), 111.6 (furan carbon atoms), 127.2, 128.5, 129.3, 133.6 (phenyl carbon atoms), 150.0 (substituted furan carbon atoms), 171.2 (carbonyl carbon atoms); ESI MS (C22H20O5) m/z, 387.1 m/z [M + Na+] and 403.1 m/z [M + K+] bis-2,5-(Benzoylmethyl)furan (BHMF-B): 86.6% yield beige crystalline solid (from ethyl acetate): mp 77°C (DSC); decomposition Tonset 232°C (TGA); FTIR (cm-1, ATR) 3124 (w) 3071 (w) –Csp2-H, 2968 (w) –Csp3-H, 1721 (vs) –ester carbonyl C=O, 1600 (m) –aromatic phenyl nucleus, 1559 (m) –aromatic furan nucleus, and 1250 (s) –carbon to oxygen C-O; 1H NMR (, CDCl3) 5.31 (s, 4H, methylene protons adjacent to the furan nucleus and oxygen atom), 6.48 (s, 2H, furan protons), 7.45 (m, 4H, phenyl protons), 7.56 (m, 2H, phenyl protons), 8.07 (m, 4H, phenyl protons); 13C NMR (, CDCl3) 58.5 (carbon atom in methylene group adjacent to ester oxygen), 111.7 (furan carbon atoms) 128.4, 129.7, 129.8, 133.1 (phenyl carbon atoms) 150.3 (substituted furan carbon atoms), 166.2 (carbonyl carbon atom); ESI MS (C20H16O5) m/z, 359.1 m/z [M + Na+], 375.1 m/z [M + K+], 695.1 m/z [2M + Na+], and 711.2 [2M + K+]. bis-2,5-(Stearoylmethyl)furan (BHMF-S): 69.3% yield, beige crystalline solid (from ethyl acetate): mp 78°C (DSC); decomposition Tonset 285°C (TGA); FTIR (cm-1, ATR) 2915 (vs) 2848 (vs) –Csp3-H, 1733 (vs) –ester carbonyl C=O, 1565 (w) –aromatic furan nucleus, 1151 (s) –carbon to oxygen C-O; 1H NMR (, CDCl3) 0.88 (t, 6H, J = 6.39 methyl protons), 1.25 (s, 56H, methylene protons), 1.60 (m, 4H, methylene protons), 2.33 (t, 4H, methylene protons adjacent to ester oxygen), 5.03 (s, 4H, methylene protons adjacent to the furan nucleus and ester oxygen) and 6.46 (s, 2H, furan protons); 13C NMR (, CDCl3) 14.1 (carbon atoms of methyl groups), 22.7-34.1 (carbon atoms of methylene groups), 57.9 (carbon atoms of methylene groups adjacent to the furan nucleus and ester oxygen) 111.4 (furan carbon atoms), 150.3 (substituted furan carbon atoms), 173.4 (carbonyl carbon atoms). bis-2,5-(Dodecanoylmethyl)furan (BHMF-D): 71.7% yield, beige fluffy solid: mp 58°C (DSC); decomposition Tonset 250°C (TGA); FTIR (cm-1, ATR) 2917 (vs) 2848 (vs) –Csp3-H, 1734 (vs) –ester carbonyl C=O, 1565 (w) –aromatic furan nucleus, 1151 (s) –carbon to oxygen C-O; 1H NMR (, CDCl ) 0.88 (t, 6H, methyl protons), 1.25 (s, 32H, methylene protons), 1.62 (m, 4H, 3 methylene protons), 2.33 (t, 4H, methylene protons adjacent to ester carbonyl), 5.03 (s, 4H, methylene protons adjacent to the furan nucleus and ester oxygen), 6.35 (s, 2H, furan protons); 13C NMR (, CDCl ) 14.1 (carbon atoms of the methyl group), 22.7-34.1 (carbon atoms of the 3 4 ACS Paragon Plus Environment

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methylene groups), 57.9 (methylene carbon atoms adjacent to the furan nucleus), 111.7 (furan carbon atoms), 150.3 (substituted furan carbon atoms), 166.2 (carbonyl carbon atoms). Plasticizer/PVC Blends Blends of the plasticizer compounds with PVC were prepared using a wet blending method. Appropriate levels of the additive (parts per hundred resin; phr) and PVC (750 mg) were dissolved in 8.00 ml of THF. The solutions were placed in petri dishes and held at 50°C for two hours for evaporation of most of the solvent. The sample was then held at 50°C and 15 torr for five days to afford circular, transparent films from which samples for analysis could be cut. Results and Discussion Biobased 2,5-bis-(hydroxymethyl)furan has been converted to a series of esters with good plasticizing efficiency in PVC. This is illustrated in Scheme 1. The identity and physical form for the esters are presented in Table 1.

Scheme 1. Conversion of 2,5-bis-(Hydroxymethyl)furan (BHMF) to Carboxylate Esters. Table 1. Esters of 2,5-bis-(Hydroxymethyl)furan

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Ester

Physical Form, Designation Melting Point (°C)

bis-2,5-(2-Ethylhexanoylmethyl)furan (1)

BHMF-E

Liquid

bis-2,5-(Octanoylmethyl)furan (3)

BHMF-O

Solid, 29

bis-2,5-(2-Phenylacetoxymethyl)furan (6)

BHMF-P

Solid, 66

bis-2,5-(Benzoylmethyl)furan (4)

BHMF-B

Solid, 77

bis-2,5-(Stearoylmethyl)furan (2)

BHMF-S

Solid, 78

bis-2,5-(Dodecanoylmethyl)furan (5)

BHMF-D

Solid, 58

The esters were fully characterized using spectroscopic and thermal methods. This is illustrated for bis-2,5-(2-ethylhexanoylmethyl)furan (BHMF-E). The infrared spectrum of BHMF-E (Figure 1) contains bands at 2960 cm-1, 2933 cm-1, and 2874 cm-1 – Csp3-H stretching vibrations; 1734 cm-1 –very strong ester carbonyl C=O stretching vibration; 1561 cm-1 – aromatic furan nucleus; and 1160 cm-1 –strong carbon to oxygen C-O stretching vibration.

Figure 1. Infrared Spectrum of bis-2,5-(2-Ethylhexanoylmethyl)furan. The 1H NMR spectrum of BHMF-E is shown in Figure 2. It contains a multiplet at δ 0.80 (12H) for the methyl protons, broad peaks at δ 1.17 (8H) and δ 1.46 (8H) for the methylene protons of the ethylhexyl chain, a broad peak at δ 2.28 (2H) for the methine protons adjacent to the ester carbonyl, and a singlet at δ 5.03 (4H) for the methylene protons adjacent to the furan nucleus and ester oxygen. A singlet for protons of the furan nucleus is present at δ 6.34 (2H). The resonances at δ 2.43 and δ 3.31 are respectively from DMSO and water in the solvent. The 13C NMR spectrum is shown in Figure 3. It contains resonances at δ 11.7 and δ 13.9 for the carbon atoms of the methyl groups, δ 22.5, δ 25.3, δ 29.4 and δ 31.6 for the carbon atoms of the 6 ACS Paragon Plus Environment

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methylene groups, δ 47.1 for the methine carbon atoms, δ 57.6 for the methylene carbon atoms adjacent to the furan nucleus, and δ 111.2 and δ 150.3 for the carbon atoms in the furan ring. Resonance for the carbonyl ester carbon atoms is present at δ 175.9.

Figure 2. Proton NMR Spectrum for bis-2,5-(2-Ethylhexanoylmethyl)furan.

Figure 3. Carbon-13 NMR Spectrum for bis-2,5-(2-Ethylhexanoylmethyl)furan.



BHMF-E is a liquid at room temperature and the onset decomposition temperature for the compound is 213°C. The electrospray ionization (ESI) mass spectrum BHMF-E contains molecular ion peak at 403.1 m/z [M + Na+].

Figure 4. Thermal Decomposition of BHMF Esters. The thermal degradation of all the esters is depicted in Figure 4. As may be noted, all have degradation onset temperatures above 200 °C. Onset tempertures for degradation and melting points for the low-melting solids are collected in Table 2. Blends of four of the esters, Table 2. Melting Points and Degradation Onset Temperatures for BHMF Esters. Plasticizer

Melting Point DSC (°C)

Degradation Onset Temperature TGA (°C)a BHMF-E Liquid at room temp. 213 BHMF-O 29 223 BHMF-P 66 242 BHMF-B 77 231 BHMF-S 78 284 BHMF-D 58 268 a. Extrapolated onset temperature from the derivative plot of mass loss versus temperature. BHMF-E, BHMF-O, BHMF-P and BHMF-B, with PVC (20, 25 and 30 phr) were prepared. Flexible transparent films were obtained. BHMF-D and BHMF-S are not fully compatible with PVC and were excluded from further consideration. The behavior of these two compounds 8 ACS Paragon Plus Environment

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conforms to the previously established trend that plasticizer compatibility with PVC generally decreases with an increasing alkyl portion of aromatic esters.62 Suppression of the glass transition temperature of PVC commonly reflects the effectiveness of a plasticizer.63,64 Hence, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to determine the glass transition temperatures of unplasticized and plasticized PVC samples. DSC is the most common and traditional technique, but DMA is the more sensitive technique for determining Tg.65 A heat-cool-heat method was used to obtain the glass transition temperature of the films from DSC. The Tg values from DSC were reported using the midpoint temperature between the extrapolated onset and extrapolated endpoint. Multi-strain tests using DMA were also used to determine Tg values for PVC and the four plasticized films. The Tg values for DMA were reported as the temperature at the maximum for the tan delta peak. The glass transition temperatures for blends of the four miscible esters (20, 25, 30 phr) with PVC are collected in Table 3. The comparable temperatures for PVC plasticized with the same level of DEHP are included for comparison. As may be noted, the presence of any of the BHMF ester is sufficient to induce a strong reduction Table 3. Glass Transition Temperatures for Plasticized and Unplasticized PVC Determined Using Differential Scanning Calorimetry. Plasticizer BHMF-E BHMF-O BHMF-P BHMF-B DEHP PVC

20 phr 41 39 40 40 31

DSC –Tg (°C) 25 phr 28 23 36 36 24 81

30 phr 26 20 27 27 14

in Tg. In fact, these reductions are nearly as great as the corresponding change for DEHP present at the same level. Glass transition temperatures for the PVC/BHMF ester blends determined using DMA are presented in Table 4. The trend observed here is the same as from DSC. All the BHMF esters are effective plasticizers for PVC. In fact, two of them, BHMF-P and BHMF-B, are nearly as effective as DEHP.



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Table 4. Glass Transition Temperatures for Plasticized and Unplasticized PVC Determined Using Dynamic Mechanical Analysis. Plasticizer BHMF-E BHMF-O BHMF-P BHMF-B DEHP PVC

20 phr 81 75 69 69 69

DMA – Tg (°C) 25 phr 78 69 67 66 64 100

30 phr 72 66 62 62 59

Plasticizer volatility was determined by monitoring mass loss versus time at 125° C for blends containing 20 phr plasticizer. Results are presented in Figure 5. It is clear that the

Figure 5. Loss of BHMF Plasticizer from PVC at 125°C. BHMF ester plasticizers are not readily volatilized from the PVC matrix. Two, BHMF-P (2% weight loss at 125o C after 15 hr) and BHMF-B (3% weight loss at 125o C after 15 hr) display particularly low volatility Thus, they not only serve as effective plasticizers but may be expected to display good durability. The loss of any of these plasticizers during polymer processing would be no greater than that observed for DEHP. Since plasticizer volatility is a major cause of flammability for formulated PVC samlpes, the use of these plasticizers could mitigate flammability of finished items.


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Figure 6. Thermal Degradation of Plasticized PVC Films. The thermal stability of PVC/BHMF ester blends is presented in Figure 6. As may be noted, the presence of any of the plasticizers does not significantly alter the degradation for PVC. This may be better reflected in the temperatures for the onset of degradation collected in Table 5. Table 5. Onset Decomposition Temperatures for Plasticized PVC. TGA – Onset Decomposition Temperature (°C)a 20 phr 25 phr 30 phr BHMF-E 233 233 241 BHMF-O 233 236 238 BHMF-P 231 234 235 BHMF-B 235 236 237 DEHP 259 260 263 PVC 254 a. Extrapolated onset from the derivative plot of mass loss versus temperature. Plasticizer



In summary, these biobased 2,5-bis-(hydroxymethyl)furan esters function as effective plasticizers for PVC, are relatively nonvolatile, and do not degrade the thermal stability of the polymer. Conclusions A series of four esters of biobased 2,5- bis-(hydroxymethyl) furan that are fully compatible with the PVC matrix has been prepared and fully characterized using spectroscopic and thermal methods. These materials display good plasticizing effect when present at 20, 25 and 30 phr. These esters do not readily migrate from the PVC matrix and their presence does not alter the thermal stability of the polymer. These compounds may represent attractive, biosourced alternatives for phthalate esters as plasticizers for PVC. Acknowledgement A sample of 2,5- bis-(hydroxymethyl)furan was graciously provided by PennAKem, Memphis, TN.


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Biobased Plasticizers from Carbohydrate-Derived 2,5-Bis

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