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Designing bio-based plasticizers: Effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends
Accepted Manuscript Designing bio-based plasticizers: Effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends
Yong Yang, Juncheng Huang, Ruoyu Zhang, Jin Zhu PII: DOI: Reference:
S0264-1275(17)30356-8 doi: 10.1016/j.matdes.2017.04.005 JMADE 2927
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
24 December 2016 31 March 2017 3 April 2017
Please cite this article as: Yong Yang, Juncheng Huang, Ruoyu Zhang, Jin Zhu , Designing bio-based plasticizers: Effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.04.005
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ACCEPTED MANUSCRIPT Designing bio-based plasticizers: effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends Yong Yang a,b, Juncheng Huang a, Ruoyu Zhang a,*, Jin Zhu a,* a
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials
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Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR
University of Chinese Academy of Sciences, No.19, Yuquan Road, Beijing 100049, PR China
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b
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China
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* Corresponding authors. Tel.: +86 574 86685925; Fax: +86 574 86685925.
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E-mail address: [email protected] (R.Y. Zhang); [email protected] (J. Zhu). Abstract: A series of bio-based plasticizers based on isosorbide, a renewable
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monomer derived from glucose, were successfully synthesized. The chemical
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structures of the synthesized isosorbide diesters with different alkyl chain length were confirmed by FTIR and 1H-NMR. The influence of varying alkyl chain length on
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thermal and mechanical properties of isosorbide diesters in poly(vinyl chloride) (PVC)
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was investigated in detail. It was found that the plasticizing efficiency of isosorbide diester decreased as the alkyl chain length increased, reflecting on the gradually increased Tg and the decreased elongation at break of PVC blend. However, the longer alkyl chain length of isosorbide diester improved the thermal stability of PVC blend and simultaneously depressed its volatility from PVC. Besides, the bio-based isosorbide diesters rivaled those petro-based phthalates, and they could be potential candidates to replace phthalates in the future industry. 1
ACCEPTED MANUSCRIPT Keywords: Poly(vinyl chloride); Bio-based plasticizer; Isosorbide diester; Miscibility; Thermal stability; Volatility 1. Introduction
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Poly(vinyl chloride) (PVC) is one of the most useful plastics, and it has been widely used in many areas such as artificial leather, flooring, children toys, packaging
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materials, cables, pipes, window profiles, and medical field [1-3]. Due to the strong
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polar effect among PVC chains, it is rigid and brittle in nature. On one hand, high
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temperature could decrease such interaction, but processing at elevated temperature can cause serious degradation of PVC. On the other hand, the brittleness seriously
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limits the application of PVC. In order to decrease the processing temperature and obtain desirable toughness, a large amount of plasticizer needs to be added into PVC.
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During the last decade, the worldwide production of plasticizers was around 6.4 million tons per year, and it is estimated to exceed more than 13.2 million tons per
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year in 2018 [4]. Among these commercial plasticizers, phthalate esters play an important role and account for more than 80% of the total production owing to their
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excellent performance and relatively low cost [5]. Unfortunately, phthalates are low-molecular-weight compounds, which can easily migrate from the polymer matrix. And they are difficult to degrade in natural environment. Therefore, the leached plasticizers may have potentially toxic effects on humans and are harmful to environment [6-10]. For this reason, US and many European countries have restricted the use of phthalates in flexible PVC products, especially the products used for children’s toys [11,12]. Furthermore, petro-based 2
ACCEPTED MANUSCRIPT products are under big pressure in recent years. Reasons are clear that the mining of petroleum will pollute the environment and it will deplete one day. As a result, the adoption of bio-based products becomes a popular trend. Many efforts have been devoted to developing bio-based plasticizers with low toxicity and low migration
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levels [4].
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Bio-based plasticizers based on epoxidized soybean oil [13-15], epoxidized
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sunflower oil [16,17], epoxidized neem oil [18], epoxidized linseed oil [19], rice fatty acid esters [20,21], low-molecular-weight glycerol esters [22], castor oil based
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plasticizers [23,24], cardanol and its derivatives [5,11,25-27] have been reported. The
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epoxidized vegetable oils represent promising bio-based plasticizers since they are derived from renewable resources. Besides, they are readily available, biodegradable
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and environmental friendly. However, double bonds exist in epoxidized vegetable oils
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and deteriorate the compatibility between PVC and the oils, leading to its easy migration from PVC matrix. As a result, epoxidized vegetable oils could only be used
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as secondary plasticizers [23]. Though cardanol and its derivatives have been
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confirmed to be potential candidates for replacing phthalates due to their versatile chemical structures [5], but they still contain a large number of unsaturated double bonds in their side alkyl chains, further epoxidation is necessary, which would increase the cost. As the plastics industry and environmental awareness continues to grow, there is an urgent and unmet need to develop more bio-based plasticizers with improved properties. Different carboxylic and dicarboxylic acids or diols such as isosorbide are typical
3
ACCEPTED MANUSCRIPT monomers based on renewable resources, which are believed to have large-scale yield in the future [28]. Isosorbide is a nontoxic, biodegradable, and thermally stable heterocyclic diol derived from glucose, and its diesters can be easily synthesized through traditional esterification or lipase catalyzed esterification with fatty acid
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[29-31]. Isosorbide diesters are fully biodegradable and have passed tests such as
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acute toxicity, sensitization, mutagenicity, and estrogenicity [32,33]. Due to their
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similar chemical structures with phthalates, it is reasonable to suspect that they may also have good compatibility with PVC and can act as excellent plasticizers. Yin and
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Hakkarainen [9] have investigated the effect of oligo(isosorbide adipate) (OSA),
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oligo(isosorbide suberate) and isosorbide dihexanoate (SDH) plasticizers on the thermo-mechanical properties of plasticized PVC, and they found that all the
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synthesized isosorbide plasticizers showed the potential as alternative PVC
plasticizers for PVC.
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plasticizers. Their work was a beginning to investigate isosorbide esters as bio-based
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By systematically changing the alkyl chain length of isosorbide diesters, it should
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be possible to tune the properties of the plasticized PVC blends. In this study, four isosorbide diesters with varying alkyl chain length were synthesized, and their plasticizing effects on PVC were investigated by means of glass transition temperatures, mechanical properties, hardness test, thermal behaviors, and volatility properties. The commercial plasticizer, dioctyl terephthalate (DOTP), was used for parallel comparison. 2. Experimental details 4
ACCEPTED MANUSCRIPT 2.1. Materials PVC (S700) was purchased from the SINOPEC Qilu Petrochemical Co.,Ltd. (Zibo, China) with average degree of polymerization of 650-750 and density of 0.57 g/cm3. It was dried at 70 oC under vacuum for 2 h prior to use. Calcium zinc heat stabilizer
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(BZ-105P) was offered by Haide Chemical Co.,Ltd. (Jinan, China). Isosorbide (99%)
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was provided by Rizhao LeDeShi Chemical Co.,Ltd (Beijing, China). n-Butyryl
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chloride (98%), n-hexanoyl chloride (98%), n-octanoyl chloride (99%), n-decanoyl
Chemistry
(Shanghai,
China).
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chloride (98%) and dioctyl terephthalate (DOTP) (97%) were supplied by Aladdin Triethylamine
(AR),
dichloromethane
(CP),
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tetrahydrofuran (AR) and anhydrous MgSO4 (AR) were purchased from Sinopharm Chemical Reagent (Shanghai, China).
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2.2. Synthesis of isosorbide esters and preparation of plasticized PVC films Isosorbide (11.8 g, 0.08 mol), dichloromethane (125 ml) and triethylamine (33.8 ml,
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0.24 mol) were fed into a three-neck round bottom flask equipped with a reflux condenser and magnetic stir. After their dissolution under nitrogen atmosphere,
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n-butyryl chloride (16.9 ml, 0.16 mol) was fed drop-wise into the three-neck round bottom which is immersed in ice bath. The reaction lasted for 2 h at 0 oC, and then 45 o
C for another 0.5 h. After removing the precipitate via filtration, the filtrate was
washed against water for three times and dried with anhydrous MgSO4. After removal of the residual solvent, pure SDB product can be finally obtained, and the purity of SDB is 91%. The synthesis route is shown in Scheme 1.
5
ACCEPTED MANUSCRIPT (Insert Scheme 1 here) SDH, SDO and SDD was prepared under the same reaction conditions as described previously for SDB, except that n-butyryl chloride was replaced with n-hexanoyl chloride, n-octanoyl chloride and n-decanoyl chloride by the same molar mass. The
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purity of SDH, SDO and SDD is 96%, 96% and 93%, respectively.
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PVC/plasticizer films were prepared by solution casting. 10 g of PVC, 4 g of
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plasticizer and 0.4 g of calcium zinc heat stabilizer were dissolved in 200 mL THF at room temperature for 24 h, the abbreviation and formulations of all the samples were
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listed in Table 1. The prepared solutions were casted on clean petri dishes and dried at
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ambient pressure and room temperature for 7 days to remove most of THF. Then the solid films were taken out and put into vacuum oven for another 7 days to eliminate
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in desiccators for further test.
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the residual THF. Finally, film samples were well packed in aluminum foils and kept
(Insert Table 1 here)
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2.3. The FTIR measurements
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The chemical structures of isosorbide diesters were confirmed by FTIR (Nicolet FTIR 6700 infrared spectrophotometer, America, KBr powder) over a range of 4000 400 cm−1. 2.4. Nuclear magnetic resonance Isosorbide diesters were analyzed by Bruker Avance Ⅲ 400 Fourier transform nuclear magnetic resonance spectrometer (Bruker, Switzerland) operating at 400 MHz
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ACCEPTED MANUSCRIPT (1H-NMR), and CDCl3 was used as solvent. 2.5. DMA measurement Dynamic mechanical analyses (DMA) of pure PVC and plasticized PVCs were
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studied on a DMA-SDTA861e (Mettler-Toledo, Switzerland), and the dimension of test bars was L× W× H= 50 mm ×5 mm × 0.7 mm. Specimens were subjected to a
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tension with amplitude of 20 μm and at a frequency of 1 Hz. The temperature range
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was set from -20 oC to 100 oC with a heating rate of 3 oC/min. An experiment is
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always repeated by using another sample to make sure the reproducibility.
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2.6. Measurement of mechanical properties
An Instron 5567 (Instron, America) was used for testing the mechanical properties
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of pure PVC and plasticized PVCs by following the Chinese standard
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GB/T1040.1-2006. All the samples with the thickness of 0.7 mm were stretched at a cross head speed of 20 mm/min for tensile test. Tensile strength, tensile modulus, and
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elongation at break were obtained from the stress-strain data. Each test used six
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replicates to obtain a reliable mean and standard deviation. 2.7 Hardness test
Shore A and Shore D hardness of all specimens were measured using Shore Durometer LX-A and LX-D (Handpi, China), respectively. 2.8. Thermogravimetric analysis Thermogravimetric analysis (TGA) was carried out on a METTLER TOLEDO
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ACCEPTED MANUSCRIPT TGA/DSC1 instrument (Mettler-Toledo, Switzerland). The samples were heated from 50 to 600oC at a heating rate of 10 oC/min under N2 atmosphere. 2.9. Volatility test
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Volatility properties were evaluated by placing pure PVC and plasticized PVCs (20mm × 20 mm × 0.7 mm) in a convection oven at 70 oC for 24 h and 72 h,
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respectively. Then they were cooled to room temperature in a desiccator for another 2
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h. The weight changes were recorded before and after the treatment.
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3.1. Synthesis of isosorbide diesters
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3. Results and discussion
The FTIR spectra of isosorbide, SDB, SDH, SDO and SDD are shown in Fig. 1.
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Compared with isosorbide, SDB, SDH, SDO and SDD showed a strong and sharp
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band at around 1740 cm-1, which correspond to C=O. Meanwhile, absorptions at around 1160 cm-1 and 1090 cm-1 of SDB, SDH, SDO and SDD could be assigned to
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the stretching vibration of C-O-C. The above data implied that the ester groups were
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formed and the esterification reactions were successful. Besides, the absorptions of the isosorbide diesters at 2960-2840 cm-1 were much stronger than that of isosorbide, representing the large number of -CH2- and -CH3 in isosorbide diesters. The above data suggested that the isosorbide diesters with varying alkyl chain length were successfully synthesized. (Insert Fig. 1 here) To further verify the chemical structures of the isosorbide diesters, 1H-NMR 8
ACCEPTED MANUSCRIPT spectra of SDB, SDH, SDO and SDD were investigated, which are shown in Fig. 2. In Fig. 2a, proton signals arising in the region of 3.6-5.2 ppm (H1, H2, H3, H4, H5 and H6) were attributed to the protons from isosorbide, in good agreement with the 1
H-NMR spectrum of other isosorbide compounds reported in literatures [9,34]. Peaks
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appeared at 2.3 ppm (H7) and 1.62 ppm (H8) were assigned to -(CH2)n- and the
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proton signals at 0.94 ppm (H9) represented -CH3. Therefore, the 1H-NMR spectrum
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of SDB corresponds well to its chemical structure. Similar analysis could be carried out on samples of SDH, SDO and SDD. With both 1H-NMR and FTIR data, we could
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conclude that SDB, SDH, SDO and SDD were successfully prepared with high purity.
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(Insert Fig. 2 here) 3.2. Miscibility between isosorbide diesters and PVC
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(Insert Fig. 3 here)
FTIR is a reliable and facile method to analyze the miscibility between PVC and
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plasticizer [35,36]. After adding into PVC, the carbonyl group absorption band of plasticizer will shift to a lower position, and this shift is caused by intermolecular
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interactions, attributed to the dipole-dipole interactions of C=O…H-C-Cl bonds, which are important for the formation of miscible blends. Therefore, the shift can be used for evaluating the miscibility of PVC and ester/polyester blends [9]. Here, the positions of carbonyl group absorption band in isosorbide diesters and their PVC blends are recorded by FTIR and shown in Fig. 3. Apparently, the carbonyl group absorption band of plasticized PVCs shifted to a lower frequency compared to pure plasticizer, implied the strong interaction between PVC and plasticizers. A large shift 9
ACCEPTED MANUSCRIPT of the carbonyl group absorption band indicated good miscibility [10]. The position of carbonyl group absorption band before and after the blending are listed in Table 2, and the shift of plasticized PVCs was in the order of SDB (6.3) > SDH (5.0) > SDO (4.5) > SDD (3.5). Therefore, as the alkyl chain length increased, the miscibility
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between the isosorbide diester and PVC decreased. Since the carbonyl group is the
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only reason that the isosorbide diesters are miscible with PVC, and the alkyl chain in
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diesters is thermodynamically incompatible with PVC chain. The controlled change of alkyl chain length of isosorbide diesters induced significant variation of the
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miscibility between isosorbide diesters and PVC, and then it would affect various
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properties of the final blends.
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3.3. DMA measurements
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(Insert Table 2 here)
Glass transition temperature (Tg) is an effective index for the evaluation of
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plasticizing efficiency, and lower Tg usually means better compatibility between plasticizer and PVC [12,24]. DMA test was employed to investigate the variation of
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Tg in pure PVC and plasticized PVCs. As usual, temperature corresponding to loss tangent peak is taken as Tg, and the results are shown in Fig. 4. Tg values of PVC/40SDB, PVC/40SDH, PVC/40SDO, PVC/40SDD, PVC/40DOTP and pure PVC are 24.3 oC, 26.7 oC, 28.2 oC, 36.1 oC, 32.8 oC and 64.6 oC, respectively. It is clear that each sample has only one tanδ peak, indicating that the isosorbide diesters and DOTP are thermodynamically compatible with PVC. It is also clear that Tg of plasticized PVCs is lower than that of pure PVC. The addition of plasticizers weakened the 10
ACCEPTED MANUSCRIPT intermolecular interactions of PVC chains and increased the free volume of PVC, therefore, they improved the mobility of PVC chains. (Insert Fig. 4 here) With the decrease of the alkyl chain length of isosorbide diester, Tg of the
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corresponding PVC blend decreased accordingly. Therefore, the plasticizing
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efficiency of the isosorbide diesters are in the order of SDB > SDH > SDO > SDD. It
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is worth noting that the Tg of DOTP is intermediate between SDO and SDD. From the viewpoint of industry, the evaluation of the plasticizer is based on the amount we use.
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In this work, we added the same amount of the plasticizers in PVC. When considering
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molecular weight, the molar ratio of isosorbide diesters in blends decreases as the alkyl chain length increases. Although SDB has the smallest molecular weight among
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these plasticizers, but it has the highest molar ratio, so it exhibits the strongest
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dipole-dipole interaction with PVC chains. Consequently, SDB shows the largest depression on Tg in PVC.
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Besides, the peak width of tanδ could be related with the miscibility between PVC
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and plasticizer, and narrow peak indicates good miscibility between PVC and plasticizer [37]. As can be seen from the DMA curves, as the decrease of the alkyl chain length of isosorbide diesters, tanδ peak became sharper and narrower, which implied that PVC and the isosorbide diester with shorter alkyl chain length had better miscibility [32,38]. 3.4. Mechanical properties of pure PVC and plasticized PVCs (Insert Fig. 5 here) 11
ACCEPTED MANUSCRIPT The plasticizing efficiency of the synthesized isosorbide diesters was further evaluated by tensile test. The stress-strain curves of pure PVC and plasticized PVCs are shown in Fig. 5. Tensile strength, tensile modulus and elongation at break of these samples are summarized in Table 3. As can be expected, when compared with pure
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PVC, plasticized PVCs have better elongation at break, but poorer tensile strength and
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tensile modulus. The improved elongation at break is due to the increase of the
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entanglement between PVC chains after the addition of plasticizers. The stiffness of the PVC is soften by these small molecules via separating the long PVC chains.
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Besides, the enlargement in free volume largely depresses the modulus and strength at
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break.
(Insert Table 3 here)
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At the same weight ratio, PVC plasticized with SDB has the highest elongation at
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break and the lowest tensile modulus. With the increase of the alkyl chain length of isosorbide diesters, the elongation at break decreases while tensile modulus and
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tensile strength increase. This trend fits the results of DMA experiments, suggesting
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that the molar ratio of the isosorbide diester is important. Similar to DMA data, mechanical properties of PVC/DOTP blend locates between PVC/SDO and PVC/SDD blends.
3.5. Hardness of pure PVC and plasticized PVCs Shore A and Shore D hardness of pure PVC and plasticized PVCs was measured by Hardness Durometer. The results reported in Fig. 6 show that all the plasticized PVCs exhibited lower hardness than that of pure PVC. Furthermore, the increase of alkyl 12
ACCEPTED MANUSCRIPT chain length of isosorbide diester is responsible of a general increase of the hardness of the material. Again, the trend is similar with mechanical and DMA results. The C=O…H-C-Cl interaction and the molar ratio of small molecules are also important to the hardness. In comparison, the hardness of PVC/DOTP blend was similar to that of
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PVC/SDO.
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3.6. Thermal stability of PVC and Plasticized PVCs
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(Insert Fig. 6 here)
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Thermal stability is very important to PVC, as it easily degrades during the processing. Here, thermal degradation of pure PVC and plasticized PVCs was studied
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by TGA at a heating rate of 10 oC/min. TGA and the corresponding DTG curves are depicted in Fig. 7. It was observed from TGA curves that all of the plasticized PVCs
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were thermally stable in N2 atmosphere below 160 oC and were seen to undergo a two-stage thermal degradation process above this temperature. The first degradation
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stage between 230 oC and 340 oC could be corresponding to the dechlorination of PVC, with the formation and stoichiometric elimination of HCl [23]. The second
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stage above 430 oC may be attributed to the cross linking among C=C bonds, thermal degradation of polyenes involved cyclization and splitting of chains [23,24]. DTG curves of PVC samples showed two degradation peaks at about 280 oC and 460 oC respectively,
corresponding
to
the
two
fast
thermal
degradation
stages
mentioned-above. (Insert Fig. 7 here) Table 4 summarizes the thermal degradation data of the PVC samples, including 13
ACCEPTED MANUSCRIPT temperatures corresponding to the mass loss of 5% (T5), the mass loss of 10% (T10), the mass loss of 50% (T50) and the maximum weight-loss temperature rate (TP1 and TP2). According to T5, it can be seen that pure PVC underwent a much earlier degradation than other plasticized PVC samples, suggesting that the addition of
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plasticizer could improve the thermal stability of PVC. As the increase of the alkyl
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chain length of isosorbide diesters in PVC blends, T5, T10, T50 and TP1 increased
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gradually but TP2 almost kept stable at around 460 oC. It can be concluded that the longer alkyl chain in isosorbide diester could improve the thermal stability of PVC
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blends under 460 oC. In this test, DOTP was better than any other bio-based
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plasticizers in stabilizing PVC, which might be explained by the excellent thermal stability of benzene ring.
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3.7. Volatility test
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(Insert Table 4 here)
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The volatility and exudation properties of plasticizers from polymer strongly depend on the molecular weight, solubility, compatibility, and chemical structure of
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the plasticizers, and are an important parameter on evaluating the migration of plasticizers [27]. Fig. 8 shows the weight losses of plasticized PVC samples by volatility test. Clearly, volatile losses of PVC samples were in the following order: PVC/40SDB > PVC/40SDH > PVC/40SDO ≈ PVC/40DOTP > PVC/40SDD. It was interesting to note that the molecular weights of the plasticizers increased in the order of SDB (286) < SDH (342) < SDO (398) ≈ DOTP (391) < SDD (454). Obviously, in this study, molecular weight was more important than other factors, like interaction 14
ACCEPTED MANUSCRIPT intensity and compatibility etc., on deciding the volatility in PVC. Similar dependence on the molecular weight of plasticizers was also reported in plasticized PVC by Chaudhary [39]. As time goes on, the volatile loss became more and more serious as compared the samples volatilizing for 24 h and 72 h. Since DOTP had similar
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molecular weight with SDO, it also had similar volatile loss with SDO. In the
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following work, the long-term volatility and exudation properties will be investigated
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to evaluate the lifetime of the PVC/isosorbide diester blends.
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(Insert Fig. 8 here) 4. Conclusions
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In this work, we focused on designing, synthesizing and evaluating four isosorbide diesters with different alkyl chain length as bio-based plasticizers for PVC, namely
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isosorbide dibutyrate (SDB), isosorbide dihexanoate (SDH), isosorbide dioctanoate (SDO) and isosorbide didecanoate (SDD). The effect of varying alkyl chain length of
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isosorbide diesters on thermal and mechanical properties of plasticized PVCs was studied systematically. It was found that the content of carbonyl groups and the
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molecular weight of the isosorbide diester were the two controlling factors on plasticizing PVC. As the alkyl chain length of the isosorbide diester increased, the content of carbonyl groups decreased, and its polarized interaction with PVC decreased, leading to a poorer plasticizing efficiency. However, as the alkyl chain length of the isosorbide diester increased, its molecular weight increased, which improved the thermal stability of PVC blend and depressed its volatility from PVC. Though the cost of the bio-based isosorbide diesters is much higher, their performance 15
ACCEPTED MANUSCRIPT rivaled that of commercial DOTP, and they can be used as potential alternatives to petro-based phthalate plasticizers in PVC, without toxic or environmental problems. Acknowledgements
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The authors are grateful for the financial support from “STS project” of Chinese
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Academy of Sciences (Project No. KFJ-EW-STS-077).
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[17] B. Bouchoul, M.T. Benaniba, V. Massardier, Effect of biobased plasticizers on thermal, mechanical, and permanence properties of poly(vinyl chloride), J. Vinyl.
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Addit. Techn. 20 (2014) 260-267.
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[18] P.K. Gamage, A.S. Farid, Migration of novel epoxidized neem oil as plasticizer from PVC: experimental design approach, J. Appl. Polym. Sci. 121 (2011) 823-838.
[19] O. Fenollar, D. Garcia-Sanoguera, L. Sanchez-Nacher, J. Lopez, R. Balart, Characterization of the curing process of vinyl plastisols with epoxidized linseed oil as a natural-based plasticizer, J. Appl. Polym. Sci. 124 (2011) 2550-2557. [20] A.F. Faria-Machado, M.A.D. Silva, M.G.A. Vieira, M.M. Beppu, Epoxidation of
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ACCEPTED MANUSCRIPT modified natural plasticizer obtained from rice fatty acids and application on polyvinylchloride films, J. Appl. Polym. Sci. 127 (2012) 3543-3549. [21] M.G.A. Vieira, M.A.D. Silva, A.C.G. Macumoto, L.O.D. Santos, M.M. Beppu, Synthesis and application of natural polymeric plasticizer obtained through
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polyesterification of rice fatty acid, Mater. Res. 17 (2014) 386-391.
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[22] O.Y.S. Palacios, P.C.N. Rincon, J.P. Corriou, M.C. Pardo, C. Fonteix,
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Low-molecular-weight glycerol esters as plasticizers for poly(vinyl chloride), J. Vinyl. Addit. Techn. 20 (2014) 65-71.
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[23] P.Y. Jia, M. Zhang, L.H. Hu, G.D. Feng, C.Y. Bo, Y.H. Zhou, Synthesis and
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application of environmental castor oil based polyol ester plasticizers for poly(vinyl chloride), ACS Sustain. Chem. Eng. 3 (2015) 2187-2193.
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[24] P.Y. Jia, M. Zhang, C.G. Liu, L.H. Hu, G.D. Feng, C.Y. Bo, Y.H. Zhou, Effect of
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chlorinated phosphate ester based on castor oil on thermal degradation of poly (vinyl chloride) blends and its flame retardant mechanism as secondary plasticizer,
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[25] E.W. Neuse, J.D. Van Schalkwyk, Cardanol derivatives as PVC plasticizers .Ⅱ. plasticizer evaluation, J. Appl. Polym. Sci. 21 (1977) 3023-3033. [26] E. Calo, A. Gerco, A. Maffezzoli, Effects of diffusion of a naturally-derived plasticizer from soft PVC, Polym. Degrad. Stabil. 96 (2011) 784-789. [27] J. Chen, X.Y. Li, Y.G. Wang, K. Li, J.R. Huang, J.C. Jiang, X.A. Nie, Synthesis and application of a natural plasticizer based on cardanol for poly(vinyl chloride), J. Taiwan. Inst. Chem. E. 65 (2016) 488-497.
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ACCEPTED MANUSCRIPT [28] F. Fenouillot, A. Rousseau, G. Colomines, R. Saint-Loup, J.P. Pascault, Polymers from renewable 1,4:3,6-dianhydrohexitols (isosorbide, isomannide and isoidide): a review, Prog. Polym. Sci. 35 (2010) 578-622. [29] C. Cecutti, Z. Mouloungui, A. Gaset, Synthesis of new diesters of
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1,4:3,6-dianhydro-D-glucitol by esterification with fatty acid chlorides, Bioresour.
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Technol. 66 (1998) 63-67.
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[30] D. Mukesh, D. Sheth, A. Mokashi, J. Wagh, J.M. Tilak, A.A. Banerji, K.R. Thakkar, Lipase catalyzed esterification of isosorbide and sorbitol, Biotechnol.
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Lett. 15 (1993) 1243-1246.
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[31] N.E. Boulifi, J. Aracil, M. Mart´ınez, Lipase-catalyzed synthesis of isosorbide monoricinoleate: Process optimization by response surface methodology,
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Bioresour. Technol. 101 (2010) 8520-8525.
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[32] Y. Yang, Z. Xiang, L.S. Zhang, Z.B. Tang, R.Y. Zhang, J. Zhu, Isosorbide dioctoate as a “green” plasticizer for poly(lactic acid), Mater. Des. 91 (2016)
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262-268.
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[33] E.S. Beach, B.R. Weeks, R. Stern, P.T. Anastas, Plastics additives and green chemistry, Pure. Appl. Chem. 85 (2013) 1611-1624. [34] M. Beldi, R. Medimagh, S. Chatti, Characterization of cyclic and non-cyclic poly-(ether-urethane)s bio-based sugar diols by a combination of MALDI-TOF and NMR, Eur. Polym. J. 43 (2007) 3415-3433. [35] D.L. Tabb, J.L. Koenig, Fourier-transform infrared study of plasticized and unplasticized poly(vinyl chloride), Macromolecules. 8 (1975) 929-934.
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ACCEPTED MANUSCRIPT [36] N. Gonzalez, M.J. Fernandez-Berridi, Application of fourier transform infrared spectroscopy in the study of interactions between PVC and plasticizers: PVC/plasticizer compatibility versus chemical structure of plasticizer, J. Appl. Polym. Sci. 101 (2006) 1731-1737.
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[37] N. Gil, M. Saska, I. Negulescu, Evaluation of the effects of biobased plasticizers
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on the thermal and mechanical properties of poly(vinyl chloride), J. Appl. Polym.
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Sci. 102 (2006) 1366-1373.
[38] Z.J. Ren, L.S. Dong, Y.M. Yang, Dynamic mechanical and thermal properties of
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plasticized poly(lactic acid), J. Appl. Polym. Sci. 101 (2005) 1583-1590.
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[39] B.I. Chaudhary, B.D. Nguyen, A. Zamanskiy, Dialkyl furan-2,5-dicarboxylates, epoxidized fatty acid esters and their mixtures as bio-based plasticizers for
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poly(vinyl chloride), J. Appl. Polym. Sci. 132 (2015) 42382-42387.
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Table 1 The abbreviation and formulations of pure PVC and plasticized PVCs. SDB
SDH
SDO
SDD
DOTP
Stabilizer
PVC
100
/
/
/
/
/
4
PVC/40SDB
100
40
/
/
/
/
4
PVC/40SDH
100
/
40
/
/
/
4
PVC/40SDO
100
/
/
40
/
/
4
PVC/40SDD
100
/
/
/
40
/
4
PVC/40DOTP
100
/
/
/
/
40
4
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Note: “/” means none.
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PVC
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Table 2 Shifting of the FTIR carbonyl group absorption band in the plasticized PVCs. SDB
SDH
SDO
SDD
40
1735.6
1736.7
1736.9
1736.9
100 (Neat)
1741.9
1741.7
1741.4
1740.4
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Plasticizer (wt %)
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Table 3 Tensile strength, tensile modulus and elongation at break of pure PVC and plasticized PVCs. Tensile modulus
Elongation at break
(MPa)
(MPa)
(%)
PVC
21.4±0.9
1172±18
98.1±0.9
PVC/40SDB
14.9±0.6
13.1±0.4
PVC/40SDH
16.9±0.5
17.7±1.2
PVC/40SDO
18.2±0.7
PVC/40SDD
17.2±0.1
PVC/40DOTP
18.4±0.2
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Tensile strength
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283.7±12.7 256.6±7.4
18.4±0.1
246.9±4.7
39.3±0.5
226.0±4.5
38.1±2.0
238.3±2.2
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Table 4 TGA and DTG data of pure PVC and plasticized PVCs. T5 (oC)
T10 (oC)
T50 (oC)
TP1 (oC)
TP2 (oC)
Residual (%)
PVC
156.6
207.5
298.5
274.1
462.6
12.79
PVC/40SDB
200.3
233.0
277.6
276.2
466.8
12.29
PVC/40SDH
220.9
248.0
285.8
282.2
463.1
11.30
PVC/40SDO
232.7
253.4
297.4
284.3
PVC/40SDD
240.0
261.6
308.7
PVC/40DOTP
243.7
261.4
306.6
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Samples
296.6
460.2
10.46
460.8
9.87
460.5
9.84
ACCEPTED MANUSCRIPT Figure captions Scheme 1 Synthesis of isosorbide diesters with different acyl chloride (R = C3H7, C5H11, C7H15, C9H19). Fig. 1. The FTIR spectra of isosorbide and isosorbide diesters.
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Fig. 2. 1H-NMR spectra of SDB (a), SDH (b), SDO (c) and SDD (d).
Fig. 4. DMA curves of pure PVC and plasticized PVCs.
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corresponding plasticized PVC with 40 wt % plasticizer.
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Fig. 3. FTIR spectra showing the carbonyl region of (a) SDB, (b) SDH, (c) SDO, (d) SDD and
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Fig. 5. The stress-strain curves of pure PVC and plasticized PVCs.
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Fig. 6. The Shore A and Shore D hardness of pure PVC and plasticized PVCs. Fig. 7. The TGA and DTG curves of pure PVC and plasticized PVCs.
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Fig. 8. Weight losses of pure PVC and plasticized PVCs after volatilization at 70 oC under 24h and
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Figure 2
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights ·Isosorbide diesters with varying lengths of the alkyl chain were evaluated as bio-based alternatives to petro-based phthalate plasticizers for poly(vinyl chloride). ·The plasticizing efficiency of bio-based isosorbide diesters followed the order of
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dibutyrate > dihexanoate > dioctanoate > didecanoate.
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·The thermostability of bio-based isosorbide diesters followed the order of dibutyrate
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< dihexanoate < dioctanoate < didecanoate.
·The bio-based isosorbide diester with longer alkyl chain length is more difficult to
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volatilize.
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Yong Yang, Juncheng Huang, Ruoyu Zhang, Jin Zhu PII: DOI: Reference:
S0264-1275(17)30356-8 doi: 10.1016/j.matdes.2017.04.005 JMADE 2927
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
24 December 2016 31 March 2017 3 April 2017
Please cite this article as: Yong Yang, Juncheng Huang, Ruoyu Zhang, Jin Zhu , Designing bio-based plasticizers: Effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.04.005
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ACCEPTED MANUSCRIPT Designing bio-based plasticizers: effect of alkyl chain length on plasticization properties of isosorbide diesters in PVC blends Yong Yang a,b, Juncheng Huang a, Ruoyu Zhang a,*, Jin Zhu a,* a
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials
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Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR
University of Chinese Academy of Sciences, No.19, Yuquan Road, Beijing 100049, PR China
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b
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China
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* Corresponding authors. Tel.: +86 574 86685925; Fax: +86 574 86685925.
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E-mail address: [email protected] (R.Y. Zhang); [email protected] (J. Zhu). Abstract: A series of bio-based plasticizers based on isosorbide, a renewable
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monomer derived from glucose, were successfully synthesized. The chemical
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structures of the synthesized isosorbide diesters with different alkyl chain length were confirmed by FTIR and 1H-NMR. The influence of varying alkyl chain length on
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thermal and mechanical properties of isosorbide diesters in poly(vinyl chloride) (PVC)
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was investigated in detail. It was found that the plasticizing efficiency of isosorbide diester decreased as the alkyl chain length increased, reflecting on the gradually increased Tg and the decreased elongation at break of PVC blend. However, the longer alkyl chain length of isosorbide diester improved the thermal stability of PVC blend and simultaneously depressed its volatility from PVC. Besides, the bio-based isosorbide diesters rivaled those petro-based phthalates, and they could be potential candidates to replace phthalates in the future industry. 1
ACCEPTED MANUSCRIPT Keywords: Poly(vinyl chloride); Bio-based plasticizer; Isosorbide diester; Miscibility; Thermal stability; Volatility 1. Introduction
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Poly(vinyl chloride) (PVC) is one of the most useful plastics, and it has been widely used in many areas such as artificial leather, flooring, children toys, packaging
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materials, cables, pipes, window profiles, and medical field [1-3]. Due to the strong
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polar effect among PVC chains, it is rigid and brittle in nature. On one hand, high
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temperature could decrease such interaction, but processing at elevated temperature can cause serious degradation of PVC. On the other hand, the brittleness seriously
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limits the application of PVC. In order to decrease the processing temperature and obtain desirable toughness, a large amount of plasticizer needs to be added into PVC.
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During the last decade, the worldwide production of plasticizers was around 6.4 million tons per year, and it is estimated to exceed more than 13.2 million tons per
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year in 2018 [4]. Among these commercial plasticizers, phthalate esters play an important role and account for more than 80% of the total production owing to their
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excellent performance and relatively low cost [5]. Unfortunately, phthalates are low-molecular-weight compounds, which can easily migrate from the polymer matrix. And they are difficult to degrade in natural environment. Therefore, the leached plasticizers may have potentially toxic effects on humans and are harmful to environment [6-10]. For this reason, US and many European countries have restricted the use of phthalates in flexible PVC products, especially the products used for children’s toys [11,12]. Furthermore, petro-based 2
ACCEPTED MANUSCRIPT products are under big pressure in recent years. Reasons are clear that the mining of petroleum will pollute the environment and it will deplete one day. As a result, the adoption of bio-based products becomes a popular trend. Many efforts have been devoted to developing bio-based plasticizers with low toxicity and low migration
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levels [4].
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Bio-based plasticizers based on epoxidized soybean oil [13-15], epoxidized
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sunflower oil [16,17], epoxidized neem oil [18], epoxidized linseed oil [19], rice fatty acid esters [20,21], low-molecular-weight glycerol esters [22], castor oil based
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plasticizers [23,24], cardanol and its derivatives [5,11,25-27] have been reported. The
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epoxidized vegetable oils represent promising bio-based plasticizers since they are derived from renewable resources. Besides, they are readily available, biodegradable
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and environmental friendly. However, double bonds exist in epoxidized vegetable oils
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and deteriorate the compatibility between PVC and the oils, leading to its easy migration from PVC matrix. As a result, epoxidized vegetable oils could only be used
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as secondary plasticizers [23]. Though cardanol and its derivatives have been
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confirmed to be potential candidates for replacing phthalates due to their versatile chemical structures [5], but they still contain a large number of unsaturated double bonds in their side alkyl chains, further epoxidation is necessary, which would increase the cost. As the plastics industry and environmental awareness continues to grow, there is an urgent and unmet need to develop more bio-based plasticizers with improved properties. Different carboxylic and dicarboxylic acids or diols such as isosorbide are typical
3
ACCEPTED MANUSCRIPT monomers based on renewable resources, which are believed to have large-scale yield in the future [28]. Isosorbide is a nontoxic, biodegradable, and thermally stable heterocyclic diol derived from glucose, and its diesters can be easily synthesized through traditional esterification or lipase catalyzed esterification with fatty acid
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[29-31]. Isosorbide diesters are fully biodegradable and have passed tests such as
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acute toxicity, sensitization, mutagenicity, and estrogenicity [32,33]. Due to their
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similar chemical structures with phthalates, it is reasonable to suspect that they may also have good compatibility with PVC and can act as excellent plasticizers. Yin and
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Hakkarainen [9] have investigated the effect of oligo(isosorbide adipate) (OSA),
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oligo(isosorbide suberate) and isosorbide dihexanoate (SDH) plasticizers on the thermo-mechanical properties of plasticized PVC, and they found that all the
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synthesized isosorbide plasticizers showed the potential as alternative PVC
plasticizers for PVC.
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plasticizers. Their work was a beginning to investigate isosorbide esters as bio-based
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By systematically changing the alkyl chain length of isosorbide diesters, it should
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be possible to tune the properties of the plasticized PVC blends. In this study, four isosorbide diesters with varying alkyl chain length were synthesized, and their plasticizing effects on PVC were investigated by means of glass transition temperatures, mechanical properties, hardness test, thermal behaviors, and volatility properties. The commercial plasticizer, dioctyl terephthalate (DOTP), was used for parallel comparison. 2. Experimental details 4
ACCEPTED MANUSCRIPT 2.1. Materials PVC (S700) was purchased from the SINOPEC Qilu Petrochemical Co.,Ltd. (Zibo, China) with average degree of polymerization of 650-750 and density of 0.57 g/cm3. It was dried at 70 oC under vacuum for 2 h prior to use. Calcium zinc heat stabilizer
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(BZ-105P) was offered by Haide Chemical Co.,Ltd. (Jinan, China). Isosorbide (99%)
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was provided by Rizhao LeDeShi Chemical Co.,Ltd (Beijing, China). n-Butyryl
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chloride (98%), n-hexanoyl chloride (98%), n-octanoyl chloride (99%), n-decanoyl
Chemistry
(Shanghai,
China).
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chloride (98%) and dioctyl terephthalate (DOTP) (97%) were supplied by Aladdin Triethylamine
(AR),
dichloromethane
(CP),
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tetrahydrofuran (AR) and anhydrous MgSO4 (AR) were purchased from Sinopharm Chemical Reagent (Shanghai, China).
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2.2. Synthesis of isosorbide esters and preparation of plasticized PVC films Isosorbide (11.8 g, 0.08 mol), dichloromethane (125 ml) and triethylamine (33.8 ml,
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0.24 mol) were fed into a three-neck round bottom flask equipped with a reflux condenser and magnetic stir. After their dissolution under nitrogen atmosphere,
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n-butyryl chloride (16.9 ml, 0.16 mol) was fed drop-wise into the three-neck round bottom which is immersed in ice bath. The reaction lasted for 2 h at 0 oC, and then 45 o
C for another 0.5 h. After removing the precipitate via filtration, the filtrate was
washed against water for three times and dried with anhydrous MgSO4. After removal of the residual solvent, pure SDB product can be finally obtained, and the purity of SDB is 91%. The synthesis route is shown in Scheme 1.
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purity of SDH, SDO and SDD is 96%, 96% and 93%, respectively.
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PVC/plasticizer films were prepared by solution casting. 10 g of PVC, 4 g of
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plasticizer and 0.4 g of calcium zinc heat stabilizer were dissolved in 200 mL THF at room temperature for 24 h, the abbreviation and formulations of all the samples were
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listed in Table 1. The prepared solutions were casted on clean petri dishes and dried at
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ambient pressure and room temperature for 7 days to remove most of THF. Then the solid films were taken out and put into vacuum oven for another 7 days to eliminate
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in desiccators for further test.
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the residual THF. Finally, film samples were well packed in aluminum foils and kept
(Insert Table 1 here)
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2.3. The FTIR measurements
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The chemical structures of isosorbide diesters were confirmed by FTIR (Nicolet FTIR 6700 infrared spectrophotometer, America, KBr powder) over a range of 4000 400 cm−1. 2.4. Nuclear magnetic resonance Isosorbide diesters were analyzed by Bruker Avance Ⅲ 400 Fourier transform nuclear magnetic resonance spectrometer (Bruker, Switzerland) operating at 400 MHz
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ACCEPTED MANUSCRIPT (1H-NMR), and CDCl3 was used as solvent. 2.5. DMA measurement Dynamic mechanical analyses (DMA) of pure PVC and plasticized PVCs were
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studied on a DMA-SDTA861e (Mettler-Toledo, Switzerland), and the dimension of test bars was L× W× H= 50 mm ×5 mm × 0.7 mm. Specimens were subjected to a
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tension with amplitude of 20 μm and at a frequency of 1 Hz. The temperature range
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always repeated by using another sample to make sure the reproducibility.
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2.6. Measurement of mechanical properties
An Instron 5567 (Instron, America) was used for testing the mechanical properties
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of pure PVC and plasticized PVCs by following the Chinese standard
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GB/T1040.1-2006. All the samples with the thickness of 0.7 mm were stretched at a cross head speed of 20 mm/min for tensile test. Tensile strength, tensile modulus, and
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elongation at break were obtained from the stress-strain data. Each test used six
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replicates to obtain a reliable mean and standard deviation. 2.7 Hardness test
Shore A and Shore D hardness of all specimens were measured using Shore Durometer LX-A and LX-D (Handpi, China), respectively. 2.8. Thermogravimetric analysis Thermogravimetric analysis (TGA) was carried out on a METTLER TOLEDO
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ACCEPTED MANUSCRIPT TGA/DSC1 instrument (Mettler-Toledo, Switzerland). The samples were heated from 50 to 600oC at a heating rate of 10 oC/min under N2 atmosphere. 2.9. Volatility test
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Volatility properties were evaluated by placing pure PVC and plasticized PVCs (20mm × 20 mm × 0.7 mm) in a convection oven at 70 oC for 24 h and 72 h,
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respectively. Then they were cooled to room temperature in a desiccator for another 2
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h. The weight changes were recorded before and after the treatment.
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3.1. Synthesis of isosorbide diesters
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3. Results and discussion
The FTIR spectra of isosorbide, SDB, SDH, SDO and SDD are shown in Fig. 1.
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Compared with isosorbide, SDB, SDH, SDO and SDD showed a strong and sharp
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band at around 1740 cm-1, which correspond to C=O. Meanwhile, absorptions at around 1160 cm-1 and 1090 cm-1 of SDB, SDH, SDO and SDD could be assigned to
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the stretching vibration of C-O-C. The above data implied that the ester groups were
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formed and the esterification reactions were successful. Besides, the absorptions of the isosorbide diesters at 2960-2840 cm-1 were much stronger than that of isosorbide, representing the large number of -CH2- and -CH3 in isosorbide diesters. The above data suggested that the isosorbide diesters with varying alkyl chain length were successfully synthesized. (Insert Fig. 1 here) To further verify the chemical structures of the isosorbide diesters, 1H-NMR 8
ACCEPTED MANUSCRIPT spectra of SDB, SDH, SDO and SDD were investigated, which are shown in Fig. 2. In Fig. 2a, proton signals arising in the region of 3.6-5.2 ppm (H1, H2, H3, H4, H5 and H6) were attributed to the protons from isosorbide, in good agreement with the 1
H-NMR spectrum of other isosorbide compounds reported in literatures [9,34]. Peaks
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appeared at 2.3 ppm (H7) and 1.62 ppm (H8) were assigned to -(CH2)n- and the
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proton signals at 0.94 ppm (H9) represented -CH3. Therefore, the 1H-NMR spectrum
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of SDB corresponds well to its chemical structure. Similar analysis could be carried out on samples of SDH, SDO and SDD. With both 1H-NMR and FTIR data, we could
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conclude that SDB, SDH, SDO and SDD were successfully prepared with high purity.
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(Insert Fig. 2 here) 3.2. Miscibility between isosorbide diesters and PVC
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(Insert Fig. 3 here)
FTIR is a reliable and facile method to analyze the miscibility between PVC and
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plasticizer [35,36]. After adding into PVC, the carbonyl group absorption band of plasticizer will shift to a lower position, and this shift is caused by intermolecular
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interactions, attributed to the dipole-dipole interactions of C=O…H-C-Cl bonds, which are important for the formation of miscible blends. Therefore, the shift can be used for evaluating the miscibility of PVC and ester/polyester blends [9]. Here, the positions of carbonyl group absorption band in isosorbide diesters and their PVC blends are recorded by FTIR and shown in Fig. 3. Apparently, the carbonyl group absorption band of plasticized PVCs shifted to a lower frequency compared to pure plasticizer, implied the strong interaction between PVC and plasticizers. A large shift 9
ACCEPTED MANUSCRIPT of the carbonyl group absorption band indicated good miscibility [10]. The position of carbonyl group absorption band before and after the blending are listed in Table 2, and the shift of plasticized PVCs was in the order of SDB (6.3) > SDH (5.0) > SDO (4.5) > SDD (3.5). Therefore, as the alkyl chain length increased, the miscibility
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between the isosorbide diester and PVC decreased. Since the carbonyl group is the
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only reason that the isosorbide diesters are miscible with PVC, and the alkyl chain in
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diesters is thermodynamically incompatible with PVC chain. The controlled change of alkyl chain length of isosorbide diesters induced significant variation of the
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miscibility between isosorbide diesters and PVC, and then it would affect various
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properties of the final blends.
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3.3. DMA measurements
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(Insert Table 2 here)
Glass transition temperature (Tg) is an effective index for the evaluation of
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plasticizing efficiency, and lower Tg usually means better compatibility between plasticizer and PVC [12,24]. DMA test was employed to investigate the variation of
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Tg in pure PVC and plasticized PVCs. As usual, temperature corresponding to loss tangent peak is taken as Tg, and the results are shown in Fig. 4. Tg values of PVC/40SDB, PVC/40SDH, PVC/40SDO, PVC/40SDD, PVC/40DOTP and pure PVC are 24.3 oC, 26.7 oC, 28.2 oC, 36.1 oC, 32.8 oC and 64.6 oC, respectively. It is clear that each sample has only one tanδ peak, indicating that the isosorbide diesters and DOTP are thermodynamically compatible with PVC. It is also clear that Tg of plasticized PVCs is lower than that of pure PVC. The addition of plasticizers weakened the 10
ACCEPTED MANUSCRIPT intermolecular interactions of PVC chains and increased the free volume of PVC, therefore, they improved the mobility of PVC chains. (Insert Fig. 4 here) With the decrease of the alkyl chain length of isosorbide diester, Tg of the
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corresponding PVC blend decreased accordingly. Therefore, the plasticizing
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efficiency of the isosorbide diesters are in the order of SDB > SDH > SDO > SDD. It
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is worth noting that the Tg of DOTP is intermediate between SDO and SDD. From the viewpoint of industry, the evaluation of the plasticizer is based on the amount we use.
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In this work, we added the same amount of the plasticizers in PVC. When considering
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molecular weight, the molar ratio of isosorbide diesters in blends decreases as the alkyl chain length increases. Although SDB has the smallest molecular weight among
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these plasticizers, but it has the highest molar ratio, so it exhibits the strongest
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dipole-dipole interaction with PVC chains. Consequently, SDB shows the largest depression on Tg in PVC.
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Besides, the peak width of tanδ could be related with the miscibility between PVC
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and plasticizer, and narrow peak indicates good miscibility between PVC and plasticizer [37]. As can be seen from the DMA curves, as the decrease of the alkyl chain length of isosorbide diesters, tanδ peak became sharper and narrower, which implied that PVC and the isosorbide diester with shorter alkyl chain length had better miscibility [32,38]. 3.4. Mechanical properties of pure PVC and plasticized PVCs (Insert Fig. 5 here) 11
ACCEPTED MANUSCRIPT The plasticizing efficiency of the synthesized isosorbide diesters was further evaluated by tensile test. The stress-strain curves of pure PVC and plasticized PVCs are shown in Fig. 5. Tensile strength, tensile modulus and elongation at break of these samples are summarized in Table 3. As can be expected, when compared with pure
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PVC, plasticized PVCs have better elongation at break, but poorer tensile strength and
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tensile modulus. The improved elongation at break is due to the increase of the
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entanglement between PVC chains after the addition of plasticizers. The stiffness of the PVC is soften by these small molecules via separating the long PVC chains.
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Besides, the enlargement in free volume largely depresses the modulus and strength at
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break.
(Insert Table 3 here)
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At the same weight ratio, PVC plasticized with SDB has the highest elongation at
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break and the lowest tensile modulus. With the increase of the alkyl chain length of isosorbide diesters, the elongation at break decreases while tensile modulus and
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tensile strength increase. This trend fits the results of DMA experiments, suggesting
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that the molar ratio of the isosorbide diester is important. Similar to DMA data, mechanical properties of PVC/DOTP blend locates between PVC/SDO and PVC/SDD blends.
3.5. Hardness of pure PVC and plasticized PVCs Shore A and Shore D hardness of pure PVC and plasticized PVCs was measured by Hardness Durometer. The results reported in Fig. 6 show that all the plasticized PVCs exhibited lower hardness than that of pure PVC. Furthermore, the increase of alkyl 12
ACCEPTED MANUSCRIPT chain length of isosorbide diester is responsible of a general increase of the hardness of the material. Again, the trend is similar with mechanical and DMA results. The C=O…H-C-Cl interaction and the molar ratio of small molecules are also important to the hardness. In comparison, the hardness of PVC/DOTP blend was similar to that of
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PVC/SDO.
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3.6. Thermal stability of PVC and Plasticized PVCs
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(Insert Fig. 6 here)
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Thermal stability is very important to PVC, as it easily degrades during the processing. Here, thermal degradation of pure PVC and plasticized PVCs was studied
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by TGA at a heating rate of 10 oC/min. TGA and the corresponding DTG curves are depicted in Fig. 7. It was observed from TGA curves that all of the plasticized PVCs
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were thermally stable in N2 atmosphere below 160 oC and were seen to undergo a two-stage thermal degradation process above this temperature. The first degradation
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stage between 230 oC and 340 oC could be corresponding to the dechlorination of PVC, with the formation and stoichiometric elimination of HCl [23]. The second
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stage above 430 oC may be attributed to the cross linking among C=C bonds, thermal degradation of polyenes involved cyclization and splitting of chains [23,24]. DTG curves of PVC samples showed two degradation peaks at about 280 oC and 460 oC respectively,
corresponding
to
the
two
fast
thermal
degradation
stages
mentioned-above. (Insert Fig. 7 here) Table 4 summarizes the thermal degradation data of the PVC samples, including 13
ACCEPTED MANUSCRIPT temperatures corresponding to the mass loss of 5% (T5), the mass loss of 10% (T10), the mass loss of 50% (T50) and the maximum weight-loss temperature rate (TP1 and TP2). According to T5, it can be seen that pure PVC underwent a much earlier degradation than other plasticized PVC samples, suggesting that the addition of
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plasticizer could improve the thermal stability of PVC. As the increase of the alkyl
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chain length of isosorbide diesters in PVC blends, T5, T10, T50 and TP1 increased
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gradually but TP2 almost kept stable at around 460 oC. It can be concluded that the longer alkyl chain in isosorbide diester could improve the thermal stability of PVC
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blends under 460 oC. In this test, DOTP was better than any other bio-based
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plasticizers in stabilizing PVC, which might be explained by the excellent thermal stability of benzene ring.
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3.7. Volatility test
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(Insert Table 4 here)
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The volatility and exudation properties of plasticizers from polymer strongly depend on the molecular weight, solubility, compatibility, and chemical structure of
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the plasticizers, and are an important parameter on evaluating the migration of plasticizers [27]. Fig. 8 shows the weight losses of plasticized PVC samples by volatility test. Clearly, volatile losses of PVC samples were in the following order: PVC/40SDB > PVC/40SDH > PVC/40SDO ≈ PVC/40DOTP > PVC/40SDD. It was interesting to note that the molecular weights of the plasticizers increased in the order of SDB (286) < SDH (342) < SDO (398) ≈ DOTP (391) < SDD (454). Obviously, in this study, molecular weight was more important than other factors, like interaction 14
ACCEPTED MANUSCRIPT intensity and compatibility etc., on deciding the volatility in PVC. Similar dependence on the molecular weight of plasticizers was also reported in plasticized PVC by Chaudhary [39]. As time goes on, the volatile loss became more and more serious as compared the samples volatilizing for 24 h and 72 h. Since DOTP had similar
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molecular weight with SDO, it also had similar volatile loss with SDO. In the
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following work, the long-term volatility and exudation properties will be investigated
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to evaluate the lifetime of the PVC/isosorbide diester blends.
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(Insert Fig. 8 here) 4. Conclusions
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In this work, we focused on designing, synthesizing and evaluating four isosorbide diesters with different alkyl chain length as bio-based plasticizers for PVC, namely
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isosorbide dibutyrate (SDB), isosorbide dihexanoate (SDH), isosorbide dioctanoate (SDO) and isosorbide didecanoate (SDD). The effect of varying alkyl chain length of
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isosorbide diesters on thermal and mechanical properties of plasticized PVCs was studied systematically. It was found that the content of carbonyl groups and the
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molecular weight of the isosorbide diester were the two controlling factors on plasticizing PVC. As the alkyl chain length of the isosorbide diester increased, the content of carbonyl groups decreased, and its polarized interaction with PVC decreased, leading to a poorer plasticizing efficiency. However, as the alkyl chain length of the isosorbide diester increased, its molecular weight increased, which improved the thermal stability of PVC blend and depressed its volatility from PVC. Though the cost of the bio-based isosorbide diesters is much higher, their performance 15
ACCEPTED MANUSCRIPT rivaled that of commercial DOTP, and they can be used as potential alternatives to petro-based phthalate plasticizers in PVC, without toxic or environmental problems. Acknowledgements
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The authors are grateful for the financial support from “STS project” of Chinese
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Academy of Sciences (Project No. KFJ-EW-STS-077).
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Table 1 The abbreviation and formulations of pure PVC and plasticized PVCs. SDB
SDH
SDO
SDD
DOTP
Stabilizer
PVC
100
/
/
/
/
/
4
PVC/40SDB
100
40
/
/
/
/
4
PVC/40SDH
100
/
40
/
/
/
4
PVC/40SDO
100
/
/
40
/
/
4
PVC/40SDD
100
/
/
/
40
/
4
PVC/40DOTP
100
/
/
/
/
40
4
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Note: “/” means none.
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PVC
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Table 2 Shifting of the FTIR carbonyl group absorption band in the plasticized PVCs. SDB
SDH
SDO
SDD
40
1735.6
1736.7
1736.9
1736.9
100 (Neat)
1741.9
1741.7
1741.4
1740.4
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Plasticizer (wt %)
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Table 3 Tensile strength, tensile modulus and elongation at break of pure PVC and plasticized PVCs. Tensile modulus
Elongation at break
(MPa)
(MPa)
(%)
PVC
21.4±0.9
1172±18
98.1±0.9
PVC/40SDB
14.9±0.6
13.1±0.4
PVC/40SDH
16.9±0.5
17.7±1.2
PVC/40SDO
18.2±0.7
PVC/40SDD
17.2±0.1
PVC/40DOTP
18.4±0.2
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Tensile strength
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283.7±12.7 256.6±7.4
18.4±0.1
246.9±4.7
39.3±0.5
226.0±4.5
38.1±2.0
238.3±2.2
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Table 4 TGA and DTG data of pure PVC and plasticized PVCs. T5 (oC)
T10 (oC)
T50 (oC)
TP1 (oC)
TP2 (oC)
Residual (%)
PVC
156.6
207.5
298.5
274.1
462.6
12.79
PVC/40SDB
200.3
233.0
277.6
276.2
466.8
12.29
PVC/40SDH
220.9
248.0
285.8
282.2
463.1
11.30
PVC/40SDO
232.7
253.4
297.4
284.3
PVC/40SDD
240.0
261.6
308.7
PVC/40DOTP
243.7
261.4
306.6
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Samples
296.6
460.2
10.46
460.8
9.87
460.5
9.84
ACCEPTED MANUSCRIPT Figure captions Scheme 1 Synthesis of isosorbide diesters with different acyl chloride (R = C3H7, C5H11, C7H15, C9H19). Fig. 1. The FTIR spectra of isosorbide and isosorbide diesters.
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Fig. 2. 1H-NMR spectra of SDB (a), SDH (b), SDO (c) and SDD (d).
Fig. 4. DMA curves of pure PVC and plasticized PVCs.
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corresponding plasticized PVC with 40 wt % plasticizer.
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Fig. 3. FTIR spectra showing the carbonyl region of (a) SDB, (b) SDH, (c) SDO, (d) SDD and
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Fig. 5. The stress-strain curves of pure PVC and plasticized PVCs.
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Fig. 6. The Shore A and Shore D hardness of pure PVC and plasticized PVCs. Fig. 7. The TGA and DTG curves of pure PVC and plasticized PVCs.
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Fig. 8. Weight losses of pure PVC and plasticized PVCs after volatilization at 70 oC under 24h and
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights ·Isosorbide diesters with varying lengths of the alkyl chain were evaluated as bio-based alternatives to petro-based phthalate plasticizers for poly(vinyl chloride). ·The plasticizing efficiency of bio-based isosorbide diesters followed the order of
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dibutyrate > dihexanoate > dioctanoate > didecanoate.
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·The thermostability of bio-based isosorbide diesters followed the order of dibutyrate
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< dihexanoate < dioctanoate < didecanoate.
·The bio-based isosorbide diester with longer alkyl chain length is more difficult to
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volatilize.
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