Biobased Plasticizers from Tartaric Acid, an Abundantly Available

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Applied Chemistry

Biobased Plasticizers from Tartaric Acid, an Abundantly Available, Annually Renewable Material Bob A. Howell, and Wenxiao Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03486 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Industrial & Engineering Chemistry Research

Biobased Plasticizers from Tartaric Acid, an Abundantly Available, Renewable Material

Bob A. Howell* and Wenxiao Sun Science of Advanced Materials Center for Applications in Polymer Science Department of Chemistry and Biochemistry Central Michigan University Mt. Pleasant, MI 48859-0001 [email protected] Abstract Esters/ethers derived from tartaric acid, an abundant renewable biomaterial, have been synthesized, fully characterized and evaluated as plasticizers in PVC. These compounds are fully compatible with PVC, afford good plasticizing effect, display low migration potential, and do not negatively impact the thermal stability of the polymers. These materials may represent attractive biobased alternatives to traditional phthalate plasticizers. Key words: Nontoxic plasticizers; PVC plasticization; tartrate esters; phthalate-free flexible PVC; environmentally-benign plasticizers. Introduction Polymeric materials are immensely important in modern society and are largely responsible for the quality of life enjoyed by most citizens of the world.(1) Current annual production is greater than 300 million tons. For processing and utilization polymers must be blended with a number of additives.(2,3) Plasticizers are required for polymer processing and are the largest volume additives. Plasticization is especially important for poly(vinyl chloride) [PVC]. This material is currently the third most prominent commercial polymer yet it did not become a useful material for more than ninety years after its discovery.(4) This was made possible by the discovery of effective means of plasticization of the material. Huge amounts of plasticizer are required for PVC processing. Approximately, 85% of all plasticizer production is used in the formulation of PVC. Plasticizers improve the flexibility and workability of polymeric materials by reducing barrier to molecular motion which increases the free volume and imparts greater mobility. This may be readily noted by a decrease in the glass transition temperature, Tg, for the material.(5) Several physical properties of the material will be affected by effective plastication.(6) Most commonly, plasticizers have been esters because of their ability to strongly interact with a range of polymers. As a class, phthalate esters have been dominant for several decades.(7) Although, several phthalate esters have been used, one has dominated the market. Di-(2-ethylhexyl) phthalate (DEHP) has accounted for at least 60% of usage. DEHP is derived from two low value materials, o-xylene and butraldehyde. o-Xylene is the least valuable of the mixed xylenes from the reforming of light naphtha and may be readily 1 ACS Paragon Plus Environment

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oxidized to phthalic anhydride. Butraldehyde is a by-product of hydroformulation of propylene. Aldol condensation of butraldehyde followed by hydrogenation affords 2-ethylhexanol. Reaction of phthalic anhydride with this alcohol generates DEHP.(8) Not only is DEHP inexpensive, it is an effective plasticizer for PVC. It has been very widely used. DEHP, and other phthalates, tend to readily migrate from polymer matrices into which they have been incorporated. As a consequence, they have become widely distributed in the environment and human populations worldwide have been exposed to these compounds, particularly DEHP.(9-11) Human exposure can occur in a number of ways including contact with medical devices (intravenous tubing, blood bags, catheters, dialysis equipment), toys, house dust, food packaging or even plants eaten as food.(12-18) Exposure to phthalates disrupts endocrine function and leads to a variety of disease states.(19-22) Because of this the use of phthalates is coming under increasing societal and regulatory pressure.(23,24) Several approaches have been explored to address this problem including the synthesis of polymeric plasticizers and the covalent attachment of plasticizing pendants to the polymer mainchain.(25-28) However, the greatest interest has been in generating efficient plasticizers from nontoxic, renewable biosources.(29-34) Tartaric acid is a white crystalline organic acid found in many fruits, especially in grapes.(35-38) It is a renewable by-product of wine-making and is generated by fermentation and precipitation during wine production. It has been widely used in the food industry to provide tart taste and to extend shelf-life. It has also been used in effervescent antacids and in the synthesis of pharmaceuticals. Nonetheless, it is abundantly available annually. It is tetrafunctional containing two carboxyl and two hydroxyl groups which may be accessed for the generation of useful materials. In this case, it has been converted to esters/ethers which function as effective plasticizers for PVC. EXPERIMENTAL Methods and Instrumentation In general, ester synthesis was carried out using methods previously described.39 Characterization was primarily accomplished using thermal and spectroscopic methods as previously described.39,40 Materials Common solvents and reagents were obtained from ThermoFisher Scientific or the Aldrich Chemical Company. Tetrahydrofuran (THF) was distilled from lithium aluminum hydride in a nitrogen atmosphere prior to use; methylene chloride from calcium hydride. Benzoyl chloride, benzyl bromide, tartaric acid, and diethyl tartrate were used as received from the Aldrich Chemical Company.

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Synthesis Esters/ethers of tartaric acid were prepared using previously described methods.39 Diethyl tartrate wa obtained in 92.4% yield as a clear colorless oil: FTIR (ATR, cm-1) 3488 (m) O-H stretch, 2957(s), 2932 (s), 2860 (m) Csp3-H, 1747 (vs) C=O; 1H-NMR (CDCl3, δ) 0.88 (t, J = 5.0 Hz, 6H) methyl protons, 1.30 (m, 12H) methylene protons, 1.68 (m, 4H) methylene protons beta to carbonyl, 4.24 (td, J = 6.8 Hz, 3.5Hz, 4H) methylene protons alpha to carbonyl, 4.51 (d, J = 10.0 Hz, 2H) methine protons, 3.24 (d, J = 6.4 Hz, 2H) hydroxyl protons; 13C-NMR (CDCl , δ) 13.9 (CH ), 22.4 (CH ), 25.3 (CH ), 28.4 (CH ), 31.3 (CH ), 66.5 (CH ), 3 3 2 2 2 2 2 72.0 (CH), 171.6 (C=O). Diethyl 2,3-dibenzoyltartrate was obtained (94.2% yield) as a colorless oil which crystallized below room temperature, mp 64° C: FTIR (ATR, cm-1) 3064 (w), 3031 (w) Csp2-H, 2982 (w), 2937(w), 2904 (w) Csp3-H, 1764 (vs), 1728 (s) C=O, 1601 (w), aromatic nucleus, 1095 (s), C-O; 1H-NMR (CDCl3, δ) 1.19 (t, J = 7.1 Hz, 6H) methyl protons, 4.23 (qd, J = 7.3, 3.0 Hz, 4H) methylene protons, 6.02 (s, 2H) methine protons, 7.47 (m, 4H) aromatic protons meta to carbonyl, 7.61 (m, 2H) aromatic protons para to carbonyl, 8.13 (m, 4H) aromatic protons ortho to carbonyl; 13C-NMR (CDCl3, δ) 14.0 (CH3), 62.3 (CH2), 71.6 (CH), 128.5 (ArCH), 128.6 (ArCH), 130.0 (ArCH), 133.7 (ArC), 165.1 (C=O), 165.7 (C=O); MS (CI) (C22H22O8) m/z, 415 [M+H]+, 369 [M-C2H5O]+, 341 [M-C3H5O2]+, 293 [C15H17O6]+, 105 [C7H5O]+. Dihexyl 2,3-dibenzoyltartrate was obtained in (74.0% yield) as a white solid (recrystallization from ethyl/acetate, 1/6): mp 63 °C: FTIR (ATR, cm-1) 3072 (w) Csp2-H, 2957 (s), 2931 (s), 2859 (m) Csp3-H, 1767 (s), 1732 (vs), C=O, 1602 (m), aromatic nucleus, 1105 (m), C-O; 1H-NMR (CDCl3, δ) 0.78 (t, J = 7.0 Hz, 6H) methyl protons, 1.12 (m, 12H) methylene protons, 1.53 (qd, J = 13.6, 6.8 Hz, 4H) methylene protons beta to carbonyl, 4.13 (td, J = 6.6, 10.8 Hz, 4H) methylene protons alpha to carbonyl, 6.01 (s, 2H) methine protons, 7.47 (m, 4H) aromatic protons meta to carbonyl, 7.61 (m, 2H) aromatic protons para to carbonyl, 8.12 (m, 4H) aromatic protons ortho to carbonyl; 13C-NMR (CDCl3, δ) 13.9 (CH3), 22.3 (CH2), 25.3 (CH2), 28.4 (CH2), 31.2 (CH2), 66.4 (CH2), 71.6 (CH), 128.5 (ArCH), 128.6 (ArCH), 130.0 (ArCH), 133.7 (ArC), 165.2 (C=O), 165.9 (C=O). MS (CI) (C30H38O8) m/z, 527 [M+H]+, 425 [M-C6H13O]+, 405 [C23H33O6]+, 397 [M-C7H13O2]+, 105 [C7H5O]+. Diethyl 2,3-dibenzyloxytartrate was obtained in 71.9% yield as a nearly colorless oil: FTIR (ATR, cm-1) 3064 (w), 3031 (w) Csp2-H, 2981 (w), 2937(w), 2904 (w) Csp3-H, 1753 (vs), 1730 (s) C=O, 1611 (w), aromatic nucleus, 1101 (s), C-O; 1H-NMR (CDCl3, δ) 1.17 (t, J = 7.2 Hz, 6H) methyl protons, 4.06 (q, J = 10.8 Hz, 2H) and 4.17 (q, J = 10.8 Hz, 2H) methylene protons, 4.38 (s, 4H) benzylic methylene protons, 4.45 (d, J = 12.0 Hz, 1H) and 4.87 (d, J = 12.0 Hz, 1H) methine protons, 7.27- 7.32 (m, 10H) aromatic protons; 13C-NMR (CDCl3, δ) 14.0 (CH3), 61.2 (CH2), 73.1 (CH2), 78.3 (CH), 127.8 (ArCH), 128.2 (ArCH), 128.3 (ArCH), 136.9 (ArC), 169.0 (C=O). MS (CI) (C22H26O6) m/z, 385 [M-H2]+, 295 [M-C7H7]+, 207 [C8H14O6+H]+, 91 [C7H7]+.



Dihexyl 2,3-dibenzyloxytartrate was obtained in 90.1% yield as a colorless oil (from silica chromatography, ethyl acetate/petroleum ether-1/6): FTIR (ATR, cm-1) 3064 (w), 3032 (w) Csp2-H, 2955 (s), 2930 (m), 2858 (m) Csp3-H, 1755 (vs), 1736 (s) C=O, 1624 (m) aromatic nucleus, 1104 (s), C-O; 1H-NMR (CDCl3, δ) 0.89 (t, J = 7.0 Hz, 6H) methyl protons, 1.27 (m, 12H) methylene protons, 1.55 (m, 4H) methylene protons beta to carbonyl, 4.02 (t, J = 10.6 Hz, 2H) and 4.15 (t, J = 10.6 Hz, 2H) methylene protons alpha to carbonyl, 4.42 (s, 4H) benzylic methylene protons, 4.47 (d, J = 11.9 Hz, 1H) and 4.87 (d, J = 11.9 Hz, 1H) methine proton, 7.27- 7.32 (m, 10H) aromatic protons; 13C-NMR (CDCl3, δ) 13.8 (CH3), 22.3 (CH2), 25.3 (CH2), 28.2 (CH2), 31.2 (CH2),65.3 (CH2), 73.0 (CH2), 78.4 (CH), 127.9 (ArCH), 128.0 (ArCH), 128.3 (ArCH), 136.9 (ArC), 169.1 (C=O). MS (CI) (C30H42O6) m/z, 497 [M-H2]+, 407 [M-C7H7]+, 319 [C16H30O6+H]+, 235 [C10H18O6+H]+, 91 [C7H7]+.

Preparation of Poly(vinyl chloride) [PVC]/Plasticizer Blends PVC/plasticizer films were cast from THF solution. PVC (100 phr; parts per hundred resin) and plasticizer (10, 20, 30, 40, 50, 60 phr) were premixed in tetrahydrofuran (THF, 8 ml) and agitated for six hours to form a homogenous clear, colorless solution. The solution was cast onto a Petri dish and covered with a glass plate. The sets of cast films were placed in an oven at 50 °C to evaporate the solvent. After six hours at atmospheric pressure, the pressure was reduced to 18 torr and the temperature increased to 70 °C. The films were held under these conditions for four days. Results and Discussion Tartaric acid is a biomaterial abundantly available annually as a by-product of the wine industry. It is tetrafunctional and may serve as a base for the development of a family of effective plasticizers. The approach used here was to first convert the carboxyl functionality to alkyl esters and then to generate benzoate ester or benzyl ethers at the hydroxy groups. These compounds were fully characterized using spectroscopic and thermal methods and evaluated as plasticizers in poly(vinyl chloride) [PVC]. The esterification is illustrated below using 1-hexanol (Scheme 1). The infrared spectrum of the ester (Figure 1) contains two characteristic


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O

OH OH

HO OH

O

1) SOCl2 THF (DMF)

H3C

H2 C

C H2

H2 C

C H2

H2 C

O

2)

H3C

H2 C

OH O

O OH

O

C H2

C H2

H2 C

H2 C

C H2

C H2

H2 C

H2 C

C H2

OH

CH3

Scheme 1. Esterification of Tartaric Acid. absorptions: strong hydroxyl absorption at 3488 cm-1 and carbonyl absorption at 1747 cm-1. The NMR of the compound (Figure 2) contains a triplet at δ 0.88 (6H), a multiplet at δ 1.30 (12H), a multiplet at δ 1.68 (4H) and a triplet of doublets (td) at δ 4.24 (4H) for the protons 1H

Figure 1. Infrared Spectrum of Dihexyl Tartrate. of the hexyl group, a doublet at δ 4.51 (2H) for the methine protons, and a peak at δ 3.24 for the hydroxyl protons. The 13C NMR (Figure 3) contains six peaks at δ 13.9, 22.4, 25.3, 28.4, 31.3



Figure 2. 1H NMR Spectrum of Dihexyl Tartrate. and 66.5 for the carbon atoms of the hexyl group, a peak at δ 72.0 for the methine carbon atoms, and resonance at δ 171.6 for the carbonyl carbon atoms.

Figure 3. 13C NMR Spectrum of Dihexyl Tartrate. Tartaric acid may be converted into the diethyl ester in a similar manner. Either ester may be converted to the corresponding dibenzoate by treatment with benzoyl chloride


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O Cl O R

OH

O O

O OH

R

R Et2O Py

O

R = H3C

H2 C

C H2

or H3C

H2 C

C H2

O O

O

C C

C C

O

O O

R

O

H2 C

H2 C

Scheme 2. Synthesis of Dialkyl 2,3-Dibenzoyltartrate. (Scheme2). Dihexyl 2,3-dibenzoyltartrate is a white solid, m.p. 63° C, thermally stable to 271° C. The infrared spectrum of this compound (Figure 4) contains aromatic absorption at

Figure 4. Infrared Spectrum of Dihexyl 2,3-Dibenzoyltartrate. 1602 cm-1, strong carbonyl absorption at 1732 cm-1 and 1767 cm-1, and C-O absorption at 1105 cm-1. The 1H NMR spectrum of the compound (Figure 5) contains a triplet at δ 0.78 (6H),



Figure 5. 1H NMR Spectrum of Dihexyl 2,3-Dibenzoyltartrate. a triplet at δ 0.78 (6H), a multiplet at δ 1.12 (12H), a multiplet at δ 1.53 (4H) and a triplet of doublets (td) at δ 4.13 (4H) for the hexyl group protons, a singlet at δ 6.01 (2H) for the methine protons, and resonance at δ 7.47 to 8.12 (10H) for the aromatic protons. The 13C NMR spectrum (Figure 6) contains six peaks at δ 13.9, 22.3, 25.3, 28.4, 31.2 and 66.4 for the carbon atoms of the hexyl group, a peak at δ 71.6 for the methine carbon atoms, a set of peaks at δ 128.5, δ 128.6, δ 130.0 and δ 133.7 for the aromatic carbon atoms, and peaks at δ 165.2 and δ 165.9 for carbonyl carbon atoms.

Figure 6. 13C NMR Spectrum of Dihexyl 2,3-Dibenzoyltartrate. Alternatively, the two hydroxyl groups of the tartrates were converted into benzyl ethers by treating the corresponding alkoxide with benzyl bromide (Scheme 3), which afforded diethyl/dihexyl 2,3-dibenzyloxytartrate.


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O R

O

C

OH CH

CH

OH

C

O

NaH R

R

DMF-dry

O

O

O-

C

C CH H

C

O-

O

O

O

H2 C O R

O

C

H 2C

O CH O

R

Br

CH2

CH

C

O

R

O R = H 3C

H2 C

C H2

or H3C

H2 C

C H2

H2 C

H2 C

Scheme 3. Synthesis of Dialkyl 2,3-Dibenzyloxytartrate. Diethyl 2,3-dibenzyloxytartrate is stable to 229 °C (TGA) and then undergoes a smooth single stage decomposition. The infrared spectrum of this compound (Figure 7) contains Csp2-H stretch at 3064 cm-1 and 3031 cm-1, Csp3-H stretch at 2982 cm-1, 2973 cm-1 and

Figure 7. Infrared Spectrum of Dihexyl 2,3-Dibenzyloxytartrate. 2858 cm-1, aromatic absorption at 1624 cm-1, strong carbonyl absorption at 1736 cm-1 and 1755 cm-1, and C-O absorption at 1104 cm-1. The 1H NMR spectrum of the compound (Figure 8)



Figure 8. 1H NMR Spectrum of Dihexyl 2,3-Dibenzyloxytartrate. contains a triplet at δ 0.89 (6H), a multiplet at δ 1.27 (12H), a multiplet at δ 1.55 (4H), a set of quartets at δ 4.02 (2H) and δ 4.15 (2H) for the hexyl protons, a singlet at δ 4.42 (4H) for the benzylic methylene protons, a set of doublets at δ 4.47 (1H) and δ 4.87 (1H) for the methine protons, and peaks at δ 7.27 to 7.32 (10H) for the aromatic protons. The 1 3 C NMR spectrum (Figure 9) contains six peaks at δ 13.8, 22.3, 25.3, 28.2, 31.2 and 65.3 for the carbon

Figure 9. 13C NMR Spectrum of Dihexyl 2,3-Dibenzyloxytartrate. atoms of the hexyl group, a peak at δ 73.0 for the benzylic methylene carbon atoms, a peak at δ 78.4 for the methine carbon atoms, a set of resonances at δ 127.9, δ 128.0, δ 128.3 and δ 136.9 for the aromatic carbon atoms, and a peak at δ 169.1 for carbonyl carbon atoms. Characteristic properties of all the tartaric acid derivatives are presented in Table 1. Table 1. Properties of Tartaric Acid Derivatives. OZ R

O

O

O O

R

OZ


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R

Z

Appearance

Decomposition Onset Temperature (°C)a

Carbonyl Absorption (cm-1)

Z Benzylic Protons (δ)

Z Benzylic/Benzoyl Carbon Atoms (δ)

Light 229 1730, 1753 4.40 73.1 yellow oil O White CH3CH2 214 1728, 1764 165.2 C crystal Light CH2 210 1736, 1755 4.41 73.0 CH3(CH2)4CH2 yellow oil O White 272 1732, 1767 165.2 CH3(CH2)4CH2 C crystal a. Extrapolated onset temperature from the derivative plot of thermogravimetric mass loss versus temperature. CH3CH2

CH2

Blends of the tartrate ester/ethers (10-60 phr; parts per hundred parts resin) with PVC were prepared. All films were transparent and flexible suggesting that the distribution of plasticizer in the polymer matrix is uniform. The stiffness of the films decreased with increasing incorporation of any of the potential plasticizers. The blends containing dialkyl 1,2dibenzyloxytartrates seemed to display better flexibility than those containing dialkyl 2,3dibenzoyltartrates. Further, the flexibility of the films is also dependent on the size of the ester alkyl groups as well as the structure of the moieties in the additive. Blends containing diethyl dibenzoyl/dibenzyloxy tartrate at 50 phr do not crease upon folding while those containing the dihexyl compound at 30 phr do not crease upon folding. The efficiency of the plasticizer would seem to be directly related to both the structure of the ester and the nature of the aromatic substituents. Compounds containing large alkyl esters are more efficient than those with smaller alkyl esters. Compounds containing benzyl ether functionality seem to impart better flexibility than do those containing benzoyl ester groups. These observations of gross behavior are supported by the results of more definitive tests as indicated below. Infrared spectra for PVC and the polymer containing increasing levels of tartrate plasticizers are displayed in Figure 10.




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Figure 10. Infrared Spectra of (a) PVC, (b, b-1) PVC Plasticized with Diethyl 2,3Dibenzoyltartrate, (c) Dihexyl 2,3-Dibenzoyltartrate, (d) Diethyl 2,3-Dibenzyloxytartrate, and (e) Dihexyl 2,3-Dibenzyloxytartrate. Absorptions for the additive are clearly present in the spectra and increase in intensity with increasing loading of plasticizer in the polymer. This suggests that the plasticizers have been uniformly blended with the polymer matrix. The effectiveness of plasticization may most readily be reflected in suppression of the glass transition temperature, Tg, for the polymer. The glass transition temperatures of neat PVC and plasticized PVC samples were determined by differential scanning calorimetry (DSC) using a heat-cool-heat cycle to remove any thermal history. Reported Tg values reflect the midpoint of the baseline of a plot of heat flow versus temperature and are listed in Table 5. Neat PVC has a Tg of 83 °C. All plasticizers depressed Tg effectively. The magnitude of the depression increased with increasing level of plasticizer from 10 to 60 phr. The effectiveness of the four tartaric acid derived plasticizers was compared to that of commercial di(2-ethylhexyl)phthalate (DEHP). As expected a larger suppression of Tg is observed with increasing loading of plasticizer in the polymer matrix.



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Table 2. DSC Glass Transition Temperatures for Plasticized PVC (20, 40, 60 phr): Diethyl 2,3Dibenzoyltartrate (DBzDET), Diethyl 2,3-Dibenzyloxytartrate (DBnDET), Dihexyl 2,3Dibenzoyltartrate (DBzDHT), Dihexyl 2,3-Dibenzyloxytartrate (DBnDHT) and Di(2ethylhexyl)phthalate (DEHP). Plasticizers (phr) 10 20 30 40 50 60

DBzDET 67 57 48 44 41 37

Tg (°C) DBnDET 66 54 41 36 27 26

DBzDHT 65 49 41 32 23 19

DBnDHT 63 45 31 24 15 4

DEHP 55 39 18 -1 -8 -20

Tartrate esters containing longer alkyl chains are more effective in suppressing Tg than are those with shorter alkyl chains (at 40 phr loading, dihexyl 2,3-dibenzoyltartrate induced a suppression of 51 °C while diethyl 2,3-dibenzoyltartrate induced a suppression of 39 °C; for dihexyl 2,3-dibenzyloxytartrate at the same loading the suppression was 59 °C and for diethyl 2,3-dibenzyloxytartrate, 47 °C). Further, the benzyloxy derivatives are more effective than the corresponding benzoyl esters in suppressing the Tg for PVC (at 40 phr loading, the suppression for dihexyl 2,3-dibenzoyltartrate is 51 °C while that for dihexyl 2,3-dibenzyloxytartrate is 59 °C). At 40 phr loading DEHP suppress the Tg for PVC by 81 °C. While all the tartrate derivatives are effective in suppressing the observed glass transition of PVC the magnitude of the suppression is somewhat dependent on the exact structure of the additives. In all cases, these compounds function as effective plasticizers for PVC, in many cases approaching the efficiency of DEHP. Glass transition temperatures (Tg) of plasticized and unplasticized PVC were determined using dynamic mechanical analysis (DMA). A temperature ramp rate of 5 °C/min and a constant frequency of 1Hz (Figure 11) were used. Values for Tg were taken as the maximum of the tan δ peak and are listed in Table 3. Unplasticized PVC film has a Tg (DMA) of 100 °C.


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Figure 11. DMA Tan δ of Plasticized PVC (20, 40, 60 phr): a) Diethyl 2,3-Dibenzoyltartrate (DBzDET), b) Dihexyl 2,3-Dibenzoyltartrate (DBzDHT), c) Diethyl 2,3-Dibenzyloxytartrate (DBnDET), d) Dihexyl 2,3-Dibenzyloxytartrate (DBnDHT) and e) Di(2-ethylhexyl)phthalate (DEHP).

Table 3. Glass Transition Temperatures of Plasticized PVC Films Determined Using DMA. Plasticizers (phr) 10 20 30 40 50 60

DBzDET 95 85 83 79 73 64

Tg (°C) DBnDET 96 89 75 70 61 56

DBzDHT 97 78 76 70 67 59


DBnDHT 95 81 72 61 51 46

DEHP 84 76 69 52 34 28


The results parallel those obtained using DSC. Among the four tartaric acid-derived plasticizers, the benzyl ethers are more effective than the benzoyl esters in lowering Tg. Larger alkyl tartrate esters are more effective than small alkyl esters, ethyl versus hexyl. Additional information is available from DMA analysis. Compatibility of the plasticizer with the polymer matrix may be reflected by a single tan δ peak.(41,42) Moreover, the width of the tan δ peak can further reflect the homogeneity of the polymer system.(43) The tan δ peak (Figure 11) for PVC plasticized with any of the tartrate esters is relatively narrow and sharp. This may be compared with the relatively broad tan δ peak for PVC plasticized with DEHP. This suggests that these esters are more compatible with the PVC matrix than is DEHP. The storage modulus (E’) of plasticized PVC was evaluated using DMA. A temperature ramp rate of 5 °C/min, a temperature sweep from -50 °C to 150 °C, and a constant frequency of 1Hz was used (Figure 12). Values for E’ obtained at 25 °C are listed in Table 4. A plot of E’ as


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Figure 12. Storage Modulus (E’) for Plasticized PVC Film (20, 40, 60 phr Loading of Plasticizer): a) Diethyl 2,3-Dibenzoyltartrate (DBzDET), b) Dihexyl 2,3-Dibenzoyltartrate (DBzDHT), c) Diethyl 2,3-Dibenzyloxytartrate (DBnDET), d) Dihexyl 2,3-Dibenzyloxytartrate (DBnDHT) and e) Di(2-ethylhexyl)phthalate (DEHP). a function of temperature reflects the change from rigid to flexible for the sample.(44) Neat PVC film has a storage modulus (E’) of 3.1*103 MPa at 25 °C. As may be seen from Figure 12, the storage modulus (E’) decreases with an increasing loading of plasticizers, indicating that the stiffness of the films decreases and the material becomes more flexible. The benzyl ethers are more effective than the corresponding benzoyl esters in lowering E’. Larger alkyl tartrate esters for either benzoyl esters or benzyl ethers are more effective than are smaller esters in lowering the storage modulus.

Table 4. Storage Modulus (E’) of Plasticized PVC Films Determined using DMA at 25 °C. Plasticizers (phr) 10 20 30 40 50 60

DBzDET 3.1 3.2 2.5 3.2 2.9 2.8

Storage modulus (E’, *10-3 MPa) DBzDHT DBnDET DBnDHT 2.6 3.4 3.9 2.8 2.7 3.0 2.4 2.2 2.9 2.3 2.9 2.4 2.2 2.3 2.5 2.2 2.6 1.0


DEHP 2.8 2.7 1.6 0.6 0.3 0.1


The volatilization of plasticizer from compounded PVC films (50 phr) at 220 °C was determined using a mass loss procedure.(45) Results are shown in Figure 13. Three of the tartrate plasticizers are volatilized from PVC film at rates lower than that for DEHP volatilization. Volatilization of DBnDET occurs at marginally greater rate than that observed for DEHP. Clearly, the volatility of all the tartrate esters is in an acceptable range for use as plasticizers. 4.4

5

3.5

4

2.8

3

2.1

2 1

0.3

P EH D

D

Bn D

H

T

ET D

Bn D

T H Bz D D

Bz D

ET

0

D

Mass Loss (Percent of Initial Sample Mass)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 13. Volatilization of Plasticizers from PVC Film. The thermal stability of PVC films plasticized with tartrate esters is depicted in Figure 14.


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Figure 14. Thermal Degradation of PVC Films Plasticized with a) Diethyl 2,3Dibenzoyltartrate (DBzDET), b) Dihexyl 2,3-Dibenzoyltartrate (DBzDHT), c) Diethyl 2,3Dibenzyloxytartrate (DBnDET) and d) Dihexyl 2,3-Dibenzyloxytartrate (DBnDHT). It is clear that thermal stability is not much impacted by the presence of any of the plasticizers. This observation is also reflected in the decomposition onset temperatures collected in Table 5. Table 5. Decomposition Onset Temperatures for Plasticized PVC Films. Plasticizers Decomposition onset temperature (°C)a (phr) DBzDET DBzDHT DBnDET DBnDHT DEHP 10 263 268 263 266 264 20 263 267 271 272 270 30 265 265 263 269 273 40 258 254 261 253 276 50 256 265 266 258 277 60 265 264 262 264 278 a. Extrapolated decomposition onset temperature from the derivative plot of mass loss versus temperature. 19 ACS Paragon Plus Environment


All of the tartrate esters are fully compatible with PVC, behave as effective plasticizers in PVC, are relatively nonvolatile and do not negatively impact the thermal stability of the polymer. Conclusions A set of four esters/ethers derived from tartaric acid, an abundantly available and renewable biosource, has been synthesized and fully characterized using spectroscopic and thermal methods. These materials are fully compatible with PVC and provide good plasticization at acceptable loadings. They display low migratory potential and do not negatively impact the thermal stability of the polymer. These materials may represent attractive biobased alternatives to phthalates as plasticizers for PVC.


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Table of Contents Graphic


Biobased Plasticizers from Tartaric Acid, an Abundantly Available

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