A potentially general approach to aliphatic ester-derived PVC plasticizers with suppressed migration as sustainable alternatives to DEHP

Article abstract of DOI:10.1039/c9gc03077h

The safety of flexible polyvinyl chloride (PVC) materials is now being questioned, partially due to allegations that di-(2-ethylhexyl) phthalate (DEHP), the dominant PVC plasticizer, is an endocrine disruptor and a probable carcinogen. As sustainable alternatives, aliphatic ester-derived plasticizers feature low cytotoxicity, biodegradability, and renewability, but they can only partially replace DEHP in PVC formulations because they have a strong tendency to migrate. Here, using epoxidized fatty acid methyl esters and aliphatic diesters as representatives, we demonstrate that covalently attaching one short ester at each α-position of aliphatic esters via Claisen condensation with judiciously selected dialkyl carbonate significantly suppresses their migration from the PVC matrix. In certain scenarios, the engineered plasticizers even migrate at a rate comparable to, or improved beyond that of DEHP. This approach harnesses the α-proton, a structural feature shared by all aliphatic esters, and dialkyl carbonate, an established "green chemical", thus providing a potentially general and green route towards DEHP alternatives with unprecedented performance that may contribute to sustainability of the old and oft-maligned PVC industry.

Full text of DOI:10.1039/c9gc03077h

Green Chemistry
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A potentially general approach to aliphatic
ester-derived PVC plasticizers with suppressed
migration as sustainable alternatives to DEHP†
Cite this: Green Chem., 2019, 21,
6430
Siyu Pan, a Delong Hou,b Jinming Chang, c Zhou Xu,d Songhang Wang,b
Sunxian Yan,b Qi Zeng,a Zhonghui Wangb and Yi Chen*a,e
The safety of flexible polyvinyl chloride (PVC) materials is now being questioned, partially due to alle-
gations that di-(2-ethylhexyl) phthalate (DEHP), the dominant PVC plasticizer, is an endocrine disruptor
and a probable carcinogen. As sustainable alternatives, aliphatic ester-derived plasticizers feature low
cytotoxicity, biodegradability, and renewability, but they can only partially replace DEHP in PVC formu-
lations because they have a strong tendency to migrate. Here, using epoxidized fatty acid methyl esters
and aliphatic diesters as representatives, we demonstrate that covalently attaching one short ester at each
α-position of aliphatic esters via Claisen condensation with judiciously selected dialkyl carbonate signifi-
cantly suppresses their migration from the PVC matrix. In certain scenarios, the engineered plasticizers
even migrate at a rate comparable to, or improved beyond that of DEHP. This approach harnesses the
α-proton, a structural feature shared by all aliphatic esters, and dialkyl carbonate, an established “green
chemical”, thus providing a potentially general and green route towards DEHP alternatives with unpre-
cedented performance that may contribute to sustainability of the old and oft-maligned PVC industry.
Received 1st September 2019,
Accepted 23rd October 2019
DOI: 10.1039/c9gc03077h
rsc.li/greenchem
decade, multiple regulations have been enacted in both the
European Union and Unite States that restrict the use of DEHP
Introduction
Ever since its commercial debut in 1926, polyvinyl chloride in processing particular PVC products. Accordingly, ortho-
(PVC) has continued to benefit modern civilization in a myriad phthalates accounted for 65% of global plasticizer consump-
of aspects. In recent years, global market demand for PVC tion in 2017, down from approximately 88% in 2005, and are
materials has witnessed continuous growth, from 38.3 million projected to decline sharply to 60% in 2022.5 In this context,
tons in 2013 to 41.3 million tons in 2016.1 PVC comes in two burgeoning academic and industrial interest emerged for
basic forms: rigid (unplasticized) and flexible (plasticized). alternatives with performance equal to DEHP but good toxico-
Despite versatility and cost-efficiency, the safety of flexible PVC logical profiles.
is now being questioned, partially due to allegations that di-(2-
Promising alternatives include those derived from aliphatic
ethylhexyl) phthalate (DEHP), the most widely used PVC plasti- esters, as exemplified by epoxidized fatty acid methyl esters
cizer, is bio-accumulative, and recognized as a human endo- (EFAMEs) and aliphatic diesters (ADEs). EFAMEs are obtained
crine disruptor and a probable carcinogen.2–4 Over the past from unsaturated, single-chain fatty acid methyl esters by
epoxidation. In addition to renewability and biodegradability,
EFAMEs exhibit higher plasticizing efficiency and lower cost
than high-molecular-weight epoxidized triglycerides (e.g., epox-
aCollege of Biomass Science and Engineering, Sichuan University, Chengdu 610065,
idized soybean oil), and are thus now industrially established
P.R. China
as an important coplasticizer.6–8 ADEs are synthesized from
bKey Laboratory of Leather Chemistry and Engineering of Ministry of Education,
linear dibasic acids and alcohols, also known as a bio-
Sichuan University, Chengdu 610065, P.R. China
cChemical Synthesis and Pollution Control Key Laboratory of Sichuan Province,
China West Normal University, Nanchong 637009, P.R. China
dSchool of Life Science and Food Engineering, Yibin University, Yibin 644007,
P.R. China
eDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge,
Massachusetts, 02139, USA. E-mail: chenleon@mit.edu
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc03077h
degradable plasticizer with the degradation products being
generally non-toxic. When combined with DEHP, ADEs are par-
ticularly effective in improving the low temperature flexibility
of the PVC products.5,9 Despite their good toxicological pro-
files, neither EFAMEs nor ADEs can be used in PVC formu-
lations alone, because they have a strong tendency to migrate.
For example, even with a high epoxy value of 5.8%, EFAMEs
6430 | Green Chem., 2019, 21, 6430–6440
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still migrate tenfold faster than DEHP, burdening the environ- EFAME plasticizers, and (3) another two industrially established
ment and shortening the lifespan of the end-products.10,11 aliphatic ester-derived plasticizers, diethyl sebacate and
Thus, the performance of these aliphatic ester-derived plastici- dimethyl sebacate, as the feedstocks. They were allowed to react
zers is far from comparable to that of DEHP; an urgent task is with dimethyl carbonate or diethyl carbonate via Claisen con-
to engineer their structure in a manner that efficiently counter- densation, yielding four structurally new plasticizers with one or
acts migration.
two short ester branches. The migration behavior of the engin-
To this end, previous researchers have reported several eered plasticizers was investigated by combining experiments
promising strategies, including increasing the molecular with theoretical prediction from molecular dynamics simu-
weight of EFAMEs by transesterification,12 hyperbranched lation. In particular, the green and sustainable advance of the
polymerization from specific feedstocks,4 and covalently present strategy with respect to our previous report that utilized
linking the plasticizers to PVC by click chemistry.13 Despite an esterification reaction14 was discussed. The rationale under-
their efficiency, these strategies require unique chemistry to be lying the present effort utilizes dialkyl carbonate, an established
developed for each feedstock of interest, and thus are only “green chemical”, as a raw material, and is in principle amen-
useful for a single feedstock composition, ultimately limiting able to any aliphatic ester-derived plasticizer, thus providing
their applicability in industry where a large number of var- access to DEHP alternatives with unprecedented performance
ieties of aliphatic ester-derived plasticizers are available. in a potentially general and green manner that may address the
Therefore, a key challenge in this field is the development of a challenge of sustainability now confronting the PVC industry.
molecular engineering approach with general applicability that
may decelerate migration of any aliphatic ester-derived PVC
plasticizer, regardless of its composition.
Experimental
Previously, we demonstrated that an engineered short ester
Materials
protruding from a linear epoxidized methyl ricinoleate chain
Methyl oleate (MO, 85%, iodine value = 73 g I2 per 100 g),
methyl stearate (MS, >96%), dimethyl sebacate (DMS, 99%),
diethyl sebacate (DES, 98%), dimethyl carbonate (DMC, anhy-
drous, >99%), diethyl carbonate (DEC, anhydrous, >99%),
sodium hydride (NaH, a 60% dispersion in mineral oil,
washed three times with n-hexane before use), N,N-dimethyl-
formamide (DMF, anhydrous, 99.8%), and DEHP (99%) were
purchased from Aladdin reagent Co., Ltd (Shanghai, China).
Phosphoric acid (85%), formic acid (88%), and hydrogen per-
oxide (H2O2, 50%) were provided by Chron Chemicals Co., Ltd
(Chengdu, China). The PVC resin employed herein was an
SG-5 suspension-grade homopolymer powder with a K-value of
68–66, kindly donated by Jinlu Resin Co., Ltd (Chengdu,
was more competent in trapping the plasticizers in the PVC
matrix than the traditional strategy that underscored a high
epoxy value.14 The branched plasticizer, in certain scenarios,
even migrated as slowly as DEHP, pointing a way to address
the leaching problem long haunting EFAME plasticizers.
However, covalently attaching such ester was achieved by ester-
ification using acetic anhydride that entailed a unique mid-
chain hydroxyl in the feedstock. Unfortunately, apart from
methyl ricinoleate, almost all feedstocks for EFAME synthesis,
as well as other sub-classes of aliphatic ester-derived plastici-
zers, bear no hydroxyls. Thus, this approach, similar to any
previously reported strategy mentioned above, could not be
extended to other systems to have a general significance.
China). The thermal stabilizer composition included
a
Claisen condensation is a carbon–carbon bond forming reac-
tion that occurs between two esters in a strong base, yielding a
β-keto ester. A Claisen condensation proceeds on the premise
that at least one of the reactants must have an α-proton, and
thus can be enolized. By combining different ester partners,
this fundamental reaction allows chemists to strategically syn-
thesize a multitude of structurally diverse, complex organic
calcium/zinc system (5096, Goldstab Organics Pvt. Ltd, India)
and a β-diketone compound (Rhodiastab®, Solvay Chemicals,
China) in a weight ratio of 10 : 1.
Synthesis of ester-branched plasticizers via Claisen
condensation
compounds.15,16 Here, we notice that although lacking hydroxyl Four plasticizers branched with one or two short esters were
handles, all aliphatic ester-derived PVC plasticizers contain synthesized via Claisen condensation between MO, MS, DMS
α-protons, and thus theoretically, they may be enolized to or DES, and an excessive amount of DMC or DEC. Here, a
undergo a mixed Claisen condensation with another non-eno- detailed synthesis procedure for ester-branched epoxidized
lizable ester, enabling a specific ester to be covalently attached methyl oleate (EB-EMO) was typically introduced. In brief, into
at the α-position. In this way, we envision that all aliphatic a 500 mL flask were added NaH (12 g, 0.5 mol), DMC (360 g,
ester-derived plasticizers, regardless of their composition, can 4.0 mol), DMF (29 g, 0.4 mol), and MO (60 g, 0.2 mol), which
be branched with one short ester that has been proved efficient was heated up to 55 °C under a nitrogen atmosphere. After
in suppressing migration from the PVC matrix.
26 h, the paste was neutralized by being poured into excess 1
To demonstrate the feasibility of the proposed strategy, M aqueous solution of hydrochloric acid, followed by washing
proof-of-concept experiments were conducted herein by using with ultrapure water three times. The organic phase was finally
(1) methyl oleate, the main component of the feedstocks for subjected to vacuum distillation at 50 °C to recycle excess
synthesizing EFAMEs, (2) methyl stearate, a fast-migrating, DMC, affording ester-branched methyl oleate (EB-MO) in 95%
undesirable component existing in almost all commercial yield.
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EB-MO was then epoxidized to obtain the plasticizer, Preparation of flexible PVC membranes
employing cheap and environmentally benign hydrogen per-
oxide as an oxidant that gave water as the only byproduct.
First, EB-MO (50 g), formic acid (0.2 g), and phosphoric acid
(0.15 g) were mixed homogeneously at 50 °C, into which hydro-
gen peroxide (40 g) was added dropwise at a constant rate over
a period of 3.0 h by using an automatic dripping kit. The
mixture was continuously stirred at 50 °C for another 4 h, fol-
lowed by washing the organic layer successively with 3% (w/w)
aqueous solution of sodium bicarbonate and ultrapure water.
Once distilled under vacuum, EB-EMO with a yield of 96% was
obtained. For comparison, MO was also epoxidized directly
under the same epoxidation conditions, yielding an ordinary,
linear EFAME plasticizer (EMO).
Ester-branched methyl stearate (EB-MS), dimethyl sebacate
(EB-DMS), and diethyl sebacate (EB-DES) were synthesized in a
similar way except that they required no epoxidation procedure.
The Claisen condensation conditions for all plasticizers
reported herein are tabulated in Table 1, with their synthesis
procedure and chemical structure illustrated in Scheme 1.
To obtain flexible PVC membranes with fully compatible com-
positions that prevented putative artifactual migration of plas-
ticizers, a two-pot dry blending technique suggested by B.I.
Chaudhary et al. was adopted.17 In brief, PVC powder (100 g)
and the thermal stabilizer (2.0 g) were first mixed in a con-
tainer using a spatula, followed by stirring at 90 °C and 40 rpm
for 2 min. The plasticizer (50 g), preheated at 90 °C, was then
poured into the mixture, which was stirred at 90 °C and 40
rpm until complete plasticizer absorption by visual obser-
vation. Thereafter, the blend was heated to 150 °C and melt-
mixed at 40 rpm using HTK-200 cam rotors (Haer Machinery
Co., Ltd, China) for 5 min, followed by being pressed using a
CH-0201 two-roll mill (Chuanghong Instrument Equipment
Co., Ltd, China) with a rotation speed of 12 rpm at 150 °C for
5 min. Finally, a transparent and flexible PVC membrane with
an average thickness of 0.3 mm was obtained, and conditioned
in a desiccator at ambient temperature for five days before any
characterization.
Characterization
Physicochemical parameters including the acid value, iodine
value, epoxy value, and flash point of the as-synthesized plasti-
cizers were measured according to Chinese Standards GB/T
1668-2008,18 GB/T 1676-2008,19 GB/T 1677-2008,20 and GB/T
1671-2008,21 respectively, with the experimental results tabu-
lated in Table 2. Fourier transform infrared (FT-IR) spectra
were recorded on a Nicolet iS10 FT-IR spectrometer (Thermo
Scientific, USA) at a resolution of 2 cm−1 after 32 scans.
Nuclear magnetic resonance (NMR) spectra were collected on
an AV11-600MH NMR spectrometer (Bruker, Swiss) in deute-
rated chloroform (CDCl3-d6, 99.9 atom% D) containing 0.03%
(v/v) tetramethylsilane (TMS). Gas chromatography-mass spec-
trometry (GC-MS) analysis was carried out on an Agilent
7890A/5975C series GC-MS system (Santa Clara, USA) equipped
with a Rxi-5 ms capillary column (30 m, 0.25 mm I.D.,
0.25 μm) containing 5% diphenyl-95% dimethyl polysiloxane
as the stationary phase. Helium (99.999% pure) with a flow
Table 1 Synthesis conditions for ester-branched aliphatic ester-derived
plasticizers via Claisen condensation
[Feedstock]/[NaH]/
Temperature
(°C)
Time
(h)
Yield
(%)
[DMC or DEC]/[DMF]a
EB-EMOb
EB-MSb
1/2.5/20/2
1/2.5/20/2
1/5.0/40/4
1/5.0/40/4
55
55
50
50
26
26
36
36
96
93
95
92
EB-DMSb
EB-DESc
a Molar ratio. b React with DMC. c React with DEC.
Table 2 Physicochemical parameters of ester-branched plasticizers
synthesized from different aliphatic ester feedstocks via Claisen
condensation
Acid value
Iodine value
Epoxy
Flash
(mg KOH per g) (g I2 per 100 g) value (%) point (°C)
MO
EMO
EB-MO
<0.1
<0.1
<0.1
73
0.5 0.1
62
2
n/a
4.2 0.2
n/a
n/a
180
n/a
210
1
2
2
EB-EMO <0.1
0.5 0.1
3.6 0.1
MS
EB-MS
<0.1
<0.1
n/a
n/a
n/a
n/a
173
184
1
1
DMS
EB-DMS <0.1
<0.1
n/a
n/a
n/a
n/a
158
180
2
1
Scheme 1 Synthesis procedure and chemical structure of (a) EB-EMO;
(b) EB-MS; (c) EB-DMS; and (d) EB-DES.
DES
EB-DES
<0.1
<0.1
n/a
n/a
n/a
n/a
162
186
2
2
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rate of 3.0 mL min−1 was adopted as the carrier gas. The energy-minimized until complete convergence, and subjected
column temperature was initially programmed at 100 °C for to three annealing cycles in which the temperature was esca-
5 min, increased to 250 °C at 10 °C min−1, and finally up to lated stepwise to 500 K and then back to 300 K at intervals of
290 °C at 2 °C min−1. The injection temperature was 290 °C. 10 K. At each temperature interval, an NPT dynamic simu-
The ionization energy was 70 eV with a scan time of 0.5 s. lation was run until full equilibration. Subsequently, the
Identification of the components was performed by matching assembly was equilibrated at 450 K for 10 ns using an NPT
their recorded mass spectra with the NIST08 Mass Spectral ensemble, from which self-diffusion coefficients, Dα, of the
Library. Differential scanning calorimetry (DSC) analysis was plasticizers were calculated according to the Einstein relation:
carried out on a 204 F1 DSC analyzer (NETZSCH, Germany)
Nα
X
that scanned the sample at a heating rate of 10 °C min−1
under a nitrogen atmosphere. Cell proliferation assay was con-
ducted according to ISO 10993-522 to evaluate the in vitro cyto-
toxicity of the engineered plasticizers. Stress–strain curves of
the PVC membranes were obtained using a GT-A1-7000S uni-
versal testing machine (Chengdu GT Test Machine Co. Ltd,
China) according to ASTM D-638.23
1
6Nα
d
dt
2
Dα ¼
lim
jRiðtÞ ꢀ Rið0Þj
ð1Þ
t!1
i¼1
where Nα stands for the number of plasticizers of species α,
Ri(t) − Ri(0) is the vector distance travelled by plasticizer i over
the time interval t, and 〈|Ri(t) − Ri(0)|2〉 represents the aver-
aged mean-square displacement (MSD) of plasticizer i. The
angular brackets indicate averaging over all time origins.
A
HAAKE Phoenix II fogging tester (Thermo Fisher
Scientific, USA) was employed to evaluate plasticizer migration
from PVC membranes by volatilization at 100 0.5 °C for 16 h
according to ISO 6452:200724 and DIN 75201-2011-11.25 To
determine the resistance of the plasticizers to extraction, the
PVC membranes (30 mm × 30 mm × 0.3 mm) were soaked in
Results and discussion
Chemical structure of ester-branched plasticizers
50 mL n-hexane or alcohol and thermostated at 25 1 °C with As a proof-of-concept, MO, the main component of the feed-
a constant stirring speed of 150 rpm. At a fixed time interval, stocks for synthesizing EFAMEs, was first selected as a repre-
0.1 mL solvent was withdrawn and analyzed by the GC tech- sentative to be ester-branched via Claisen condensation. In the
nique. The accumulative amount of plasticizer extracted with first step, MO was deprotonated in the α-position by a strong
increasing time was calculated by using calibration curves, base, yielding a resonance-stabilized enolate. The strong base
expressed as the percentage of the initial weight of the PVC of choice herein was sodium hydride, given that it not only
membrane. The reported data were mean standard deviation promoted a full conversion unachievable by sodium methox-
of triplicate samples for each measurement.
ide, but also precluded transesterification that altered the
backbone structure of the feedstock as sodium ethoxide did.
Designing low-migrating plasticizers from aliphatic esters
Simulation details
All-atom molecular dynamics simulations were conducted should not substantially alter their original structure; other-
using a commercial software package, Materials Studio 8.0, wise their intrinsic performances were compromised. Next, the
provided by Accelry Inc. (San Diego, USA). The interatomic carbonyl carbon in DMC was nucleophilically attacked by the
interactions were described by the COMPASS force field. The enolate, followed by elimination of one methoxy group, yield-
van der Waals term was summed by an atom-based approach ing an alkoxide, where the doubly α-proton generated was
that truncated atom pairs with a distance greater than 11.5 Å. further removed to form a new, highly resonance-stabilized
The truncation was implemented using a quantic switching enolate. Note that non-enolizable DMC was present 20 times
function with a spline width of 1.0 Å. A buffer width of 0.5 Å per MO chain to suppress self-condensation (Table 1). Of
was specified to generate the neighbor list. The coulombic course, DEC or other dialkyl carbonates might work as well
interaction was calculated using the Ewald summation under elegantly designed conditions, but in that case, transes-
method with an Ewald accuracy of 0.01 kcal mol−1 and an terification problem arose again that impaired the backbone
update width of 1.0. Andersen and Berendsen algorithms were structure, and hence, intrinsic performances of MO. Finally,
adopted to maintain a constant temperature and pressure (1 aqueous acid, hydrochloric acid herein, was employed to neu-
bar), respectively. The time step was 1 fs for all dynamics runs. tralize the enolate and any base still present, resulting in an
To calculate the mobility of the plasticizers within the PVC ester-branched methyl oleate that could be further epoxidized
matrix, a series of PVC-plasticizer binary systems were con- by hydrogen peroxide.
structed using the amorphous cell module in Materials Studio
Successful attachment of an extra methyl ester in the epoxi-
8.0. In brief, one atactic PVC chain with 100 repeat units that dized product could be validated by a combination of the fol-
were aligned in a head-to-tail orientation with random torsion lowing experimental results. First, from the FT-IR spectra in
was first constructed using a literature procedure.26 It was then Fig. 1, we calculated the absorption intensity ratios of carbonyl
enclosed, together with ten plasticizers, in a cubic simulation (1739 cm−1) and C–O (1196 cm−1) stretching to that of methyl-
cell at 300 K via the “self-avoiding” random-walk method. ene (2854 cm−1). Methylene remained stoichiometrically con-
Finite-size effects were prevented by periodic boundary con- sistent throughout consecutive reactions of Claisen conden-
ditions in all three dimensions. After construction, the cell was sation and epoxidation, thus serving as an internal standard
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ponding to methyl protons (1) whose position and integral
remained constant throughout the functionalization pro-
cedure, the integral under the methoxy resonance signal (7)
doubled in EB-MO and EB-EMO, suggesting that those extra
methyl esters were in stoichiometric quantities. One could
surmise that the ester branch was covalently attached at the
α-position, given the integral under the α-proton signal (6) was
halved after Claisen condensation. Meanwhile, the attached
ester thinned the electron clouds on adjacent protons, and
thus the resonance position of the α- (6) and β-protons (5)
shifted downfield from 2.2 and 1.5 to 3.3 and 1.8 ppm, respect-
ively. The intermediate EB-MO could be further epoxidized as
ordinary, unsaturated fatty acid methyl esters, as evidenced by
the appearance of a new resonance signal (4) at 2.9 ppm in
EB-EMO assignable to oxirane protons, at the expense of the
double bond-associated multiplets (4) ranging from 5.2 to
5.3 ppm in EB-MO.
FT-IR and NMR analysis provided a rough structural snap-
shot of the engineered plasticizer, but the GC-MS technique
unequivocally showed generation of the α-ester, as well as the
conversion ratio at each step of the functionalization pro-
cedure. According to the GC-MS data presented in Fig. 3(a)
and Table 3, the feedstock was primarily composed of methyl
oleate (1) with a small amount of methyl stearate (2), in good
agreement with specification of the feedstock supplier. These
components disappeared completely after Claisen conden-
sation, whereas two new components (3 and 4) with a reten-
tion time between 23.1 and 23.8 min were detected (Fig. 3(b)).
They were identified respectively as methyl oleate and methyl
Fig. 1 FT-IR spectra of (a) MO; (b) EB-MO; and (c) EB-EMO. The ration-
ale behind the selection of methylene as the internal standard for quan-
titative comparison was based on the fact that they remained stoichio-
metrically consistent throughout Claisen condensation and subsequent
epoxidation.
for quantitative analysis. Compared with MO (Fig. 1(a)),
EB-MO (Fig. 1(b)) showed a noticeable increase in these two
ratios, from 1.15 to 1.45 for carbonyl, and 0.36 to 0.70 for C–O,
indicating that extra esters were generated after condensation
with DMC. Upon epoxidation, the double bond-derived
absorption (3003 cm−1) disappeared completely (Fig. 1(c)),
accompanied by a notable decrease of the iodine value from
62 to 0.5 g I2 per 100 g (Table 2), yielding a methyl ester-
branched EFAME plasticizer.
stearate, both bearing
a methyl ester at the α-position
1
Analyzing the H NMR spectrum (Fig. 2) of the epoxidized
plasticizer at each step of the functionalization procedure con-
firmed its engineered structure. Relative to the signal corres-
Fig. 2 1H NMR spectra of (a) MO; (b) EB-MO; and (c) EB-EMO. The
signal corresponding to methyl protons (1) was qualified as an internal
standard for quantitative comparison in that it was undisturbed with
respect to the position and integral throughout the two-step
functionalization procedure.
Fig. 3 Gas chromatograms of (a) MO; (b) EB-MO; and (c) EB-EMO. By
matching the recorded mass spectra against the NIST08 Mass Spectral
Library, the chemical composition of each component was inferred and
is tabulated in Table 3.
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Table 3 Composition of each component in Fig. 3 as identified by comparing their recorded mass spectra against the NIST08 Mass Spectral Library
Area (%)
Retention time
(min)
Molecular
formula
Matching
degree (%)
Peak
Composition
Feedstock
Intermediate
Product
1
2
3
4
5
20.2
20.4
23.3
23.6
Methyl oleate
Methyl stearate
Ester-branched methyl oleate
Ester-branched methyl stearate
Ester-branched epoxidized methyl oleate
C19H36O2
C19H38O2
C21H38O4
C21H40O4
C21H38O5
83.8
16.2
95
94
84.1
15.9
n/aa
85
14.3
85.7
26.0–26.3
n/aa
a Indeed, components 3 and 5 could not be explicitly identified by the NIST08 Mass Spectral Library. However, their chemical structures could be
inferred from their retention time and peak area in Fig. 3, characteristic fragment ions in the recorded mass spectra (ESI, Fig. S10,† m/z 132 and
145, corresponding to dimethyl malonate and dimethyl methylmalonate anionics, respectively), and composition of the feedstock.
(Table 3). This observation indicated that the condensation with equal reactivity, their reaction with dialkyl carbonate
condition designed herein was rational such that a full conver- yields two ester branches attaching at one feedstock molecule.
sion to the anticipated intermediate was achieved. Since satu- For the same reason, it is likely that DMS and DES can react
rated fatty acid methyl ester was not epoxidizable, the gas chro- not only through intermolecular Claisen condensation, but
matogram peak (4) corresponding to ester-branched methyl also intramolecularly to give cyclic β-keto esters. Thus, the
stearate in the final product remained unchanged with respect dosage of dialkyl carbonates in these two cases was enhanced
to retention time and area (Fig. 3(c) and Table 3). Instead, mul- to 40 times per feedstock chain so as to provide enough reac-
tiple new chromatogram peaks (5) associated with ester- tants and suppress self-condensation (Table 1). Likewise, the
branched epoxidized methyl oleate were discerned in the chemical structures of the as-synthesized plasticizers were sys-
1
retention time range of 25.8–26.5 min, accompanied by a com- tematically characterized by FT-IR, H NMR, and GC-MS tech-
plete consumption of the unsaturated intermediate (3).
niques. Similar results (ESI, Fig. S1–S12 and Tables S1–S3†)
EFAME plasticizers of industrial grade are indeed a mixture were obtained as in EB-EMO, demonstrating that the feed-
that also contains varying concentrations of saturated fatty stocks were successfully engineered with one short ester at
acid methyl esters (e.g., methyl stearate).27,28 These saturated each α-position.
components cannot be epoxidized, and hence, barely interact
with PVC chains. As such, no matter how leaching-resistant
Suppressed migration of ester-branched plasticizers
the epoxidized components in an EFAME plasticizer become, The design rule that had been established in our previous
the migration problem persists as long as the saturated com- study based on ester-branched epoxidized methyl ricinoleate
ponents remain. The results from foregoing experiments provided access to an EFAME plasticizer whose migration was
revealed that a small amount of methyl stearate in the methyl impeded to a level comparable to DEHP.14 To demonstrate
oleate feedstock was also engineered with an extra methyl that this rule was also applicable to the engineered plasticizers
ester over the Claisen condensation procedure. We believed herein, their migration behavior from the PVC matrix was
that this branch, although unlikely to qualify methyl stearate determined by a standardized fogging test and an extraction
as a primary plasticizer, might at least assist interaction with procedure, respectively. In a PVC-decorated car parked under
the PVC matrix, contributing to suppressed migration of the direct sunlight whose interior temperature can quickly climb
EFAME plasticizers as a whole. Thus, the fundamental pro- up to 78 °C,29 or in a plasticizing oven where the temperature
perties of such ester-branched methyl stearate deserve our par- is typically between 130 and 170 °C,30 plasticizer migration by
ticular attention. In addition to EFAMEs, ADEs are another volatilization expedites, fogging the car window, leading to a
industrially established sub-class of aliphatic ester-derived substantial waste of plasticizers, and finally, burdening the
plasticizers. They are efficient in endowing the PVC products environment. Thus, a fogging tester was first employed to
with low temperature flexibility, but their application as evaluate how the short esters generated from Claisen conden-
primary plasticizers is problematic since they are confronted sation influenced plasticizer volatilization from PVC mem-
with the same leaching problem as EFAMEs. In this context, branes at high temperature. In all cases, the engineered plasti-
following the preparation of EB-EMO, we decided to synthesize cizers displayed remarkably enhanced resistance to fogging
another three branched plasticizers via Claisen condensation relative to the parent ones (Fig. 4(a)). In particular, 20.79
using MS, DMS or DES as the feedstock, and an excessive 1.56 mg of EMO evaporated from the PVC membrane upon
amount of DMC or DEC as the non-enolizable ester, to assess heating at 100 °C for 16 h, approximately 16 times as high as
general applicability of the strategy proposed herein. Again, DEHP that exhibited a fogging value of 1.32
0.05 mg.
the design concept was to ester-branch the feedstocks without Compared with EMO, EB-EMO turned resistant to volatiliz-
altering their original backbone structures. Accordingly, the ation, with its fogging value decreasing more than tenfold to
non-enolizable ester of choice for DES was DEC instead of 2.01 0.08 mg, a level quite close to that of DEHP. A similar
DMC. In addition, since DMS and DES possess two α-positions trend occurred for the other two ADE plasticizers; the presence
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Fig. 4 (a) Fogging values of PVC membranes containing ester-branched and the parent plasticizers. (b) and (c) illustrated accumulative percentage
leaching of different plasticizers from the PVC membranes as a function of time in n-hexane and alcohol, respectively. The solid lines in (b) and (c)
represented the fitting results of the experimental data using the simplified version of Crank’s migration model (eqn (2)) by linear regression. For
comparison, experimental data of DEHP-plasticized PVC membranes were also included in (a), (b), and (c). Error bars show standard deviations cal-
culated from measurements in triplicate.
of one short ester at each α-position resulted in a sharp drop where Mt is defined as the amount of plasticizer leached out at
of their fogging values from 121.61 2.18 mg (DMS) and 93.53 time t, CP0 stands for the initial plasticizer concentration in
2.66 mg (DES) to 27.02
1.89 mg (EB-DMS) and 9.16
the polymer, while D denotes the diffusion coefficient. Using
1.33 mg (EB-DES), respectively. These results indicated that the least-squares procedure, diffusion coefficients of different
even though the short esters were attached at the α-position, plasticizers were estimated from the slope of the linear
rather than the midchain of the feedstock as in our previous regression lines fitted to the experimental data in Fig. 4(b)
report,14 they were still implicated in additional polar inter- and (c). In terms of high coefficients of determination
action and physical interdigitation with PVC at the molecular approaching unity (Table 4), eqn (2) provided fairly good fit to
level, leading to restrained plasticizer migration.
the accumulative leaching data. Moreover, the diffusion coeffi-
Not only at elevated temperature but also in solvents can cients of all ester-branched plasticizers were significantly lower
plasticizers leach out of the PVC membrane. To evaluate than those of the parent ones; the extent of decline was a
whether the engineered plasticizers also became resistant to complex function of the plasticizer type, the number of
extraction, the PVC membranes were soaked in nonpolar α-esters covalently attached, and the solvent where the extrac-
n-hexane or alcohol with high polarity, where the accumulative tion experiment was carried out. Particularly in the case of
percentage leaching of different plasticizers was measured by
the GC technique as a function of time. According to the pro-
files in Fig. 4(b) and (c), all ester-branched plasticizers deceler-
ated in migration relative to the parent ones, regardless of the
Table 4 Measured diffusion coefficients (D) and simulated self-
diffusion coefficients (Dα) of different plasticizers from the PVC matrix,
solvents. In some cases, the engineered plasticizers even
obtained by linear-fitting the experimental data in Fig. 4(b) and (c) using
migrated at a rate compared to, or much slower than DEHP. eqn (2), and from the slope of the MSD vs. time plots shown in Fig. 5,
respectively
For example, the leaching rates of EB-DMS and EB-DES
remained quite low in n-hexane, with only 1.9% of EB-DMS
and less than 0.3% of EB-DES leached out at the end of the
Coefficient of
determination
D × 10−11 (cm2 s−1
)
Dα × 10−8
experiment, as against 18.4% of DMS, 6.9% of DES, and more
than 20.5% in the case of DEHP. Over the initial phase of the
migration or when only a small percentage of plasticizers was
extracted throughout the experiment, the migrant in the
solvent remained far lower than that in the polymer. Under
both circumstances, the migration should follow the simpli-
fied version of Crank’s migration model:31
n-Hexane
Alcohol
43.2
n-Hexane
Alcohol
(cm2 s−1
15.9
)
DEHP
4346.6
0.9825
0.9921
EMO
EB-EMO
3988.3
1347.8
896.9
213.4
0.9463
0.9990
0.9970
0.9884
45.4
9.4
DMS
EB-DMS
1493.1
5.8
1874.3
30.1
0.9993
0.9647
0.9996
0.9689
32.7
5.3
rffiffiffi
pffiffiffi
Mt
CP0
t
π
DES
EB-DES
43.5
0.08
1018.4
107.4
0.9816
0.9850
0.9923
0.9605
16.3
11.1
¼ 2 ꢁ D ꢁ
ð2Þ
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EB-DMS and EB-DES, the presence of two ester branches in bioaccumulation; in particular, they have negligible irritating
each plasticizer reduced their diffusion coefficients in and mutagenic effects either by contact or inhalation.34 It is
n-hexane to 5.8 × 10−11 and 0.08 × 10−11 cm2 s−1, respectively, equally important that Claisen condensation with DMC or
not only several hundred times lower than that of DMS and DEC yields relatively safe and recyclable methanol or ethanol
DES, but also incomparably lower than that of DEHP. In as the only by-products, rather than hazardous acetic acid,
alcohol, similar results were visible, demonstrating that the recovery of which from the wastewater remains
a
α-esters generated from Claisen condensation had appreciable challenge.35,36 Since DMC and DEC display reasonably low
effects on migration behavior of both EFAME and ADEs. boiling temperature (90 °C for DMC; 126 °C for DEC), and are
Typically, DMS, in its pristine form, was otherwise susceptible immiscible with water, excess DMC and DEC can be recycled
to extraction by alcohol, displaying a high diffusion coefficient from the reactants by simple vacuum distillation, which is also
of 1874.3 × 10−11 cm2 s−1, but Claisen condensation with DMC in line with the concept of green chemistry.
yielded a significantly decreased diffusion coefficient (30.1 ×
10−11 cm2 s−1), which was quite close to that of DEHP (43.2 × full-atomic molecular dynamics simulation that reproduced a
10−11 cm2 s−1) under the same experimental conditions.
series of PVC amorphous cells containing a fixed number of
The measured phenomenology was further verified by a
Here, we believed that EB-EMO became migration-resistant different plasticizers. Accurate simulation of the PVC material
partially because the small amount of methyl stearate in the was validated by comparing specific properties estimated from
feedstock was also engineered with an α-ester branch. To calculation with experimental values (ESI, Table S4†).
confirm this assertion, we used methyl stearate of reagent Following annealing, the binary cells were equilibrated for 10
grade to synthesize another ester-branch fatty acid methyl ns using an NPT (T = 450 K; P = 1 bar) ensemble, from which
ester, EB-MS. Unfortunately, even with an extra ester displaying the MSD vs. time plots of the plasticizers were obtained (Fig. 5)
strong and permanent polarity, the product was not qualified for self-diffusion coefficient calculation. From the results tabu-
as a primary plasticizer such that EB-MS alone failed to plasti- lated in Table 4, we found that the ester branches, from a com-
cize the PVC powder by the two-pot dry blending technique
employed herein. Then, we turned to investigate the compat-
ibility of the engineered methyl stearate with PVC by measur-
ing the temperature at which 2 g of PVC powder dissolved
completely in 100 mL EB-MS. As expected, the PVC powder
was insoluble in MS even when heated above 200 °C, but the
polymer dissolved completely in EB-MS at 160 °C, suggesting
notably enhanced compatibility.
According to the aforementioned results, to ester-branch
aliphatic ester-derived plasticizers for suppressing migration,
Claisen condensation presents the following advance over our
previous effort14 that utilizes acetic anhydride-involved esterifi-
cation. First, esterification requires specific feedstock that
bears unique midchain hydroxyls, whereas Claisen conden-
sation centers around the α-proton, a structural feature shared
by all aliphatic esters, thus being applicable to a variety of
feedstocks. Second, in the case of EFAMEs that are typically a
mixture, esterification only engineers the hydroxy-bearing
component, while Claisen condensation affects all com-
ponents, saturated and unsaturated, contributing to sup-
pressed migration of the plasticizer as a whole. Third, com-
pared with acetic anhydride that has been listed as a U.S. DEA
List II precursor,32 short-chain dialkyl carbonates, particularly
DMC and DEC herein, are well-established environmentally
benign reagents. Currently, the large-scale production of
dialkyl carbonates is carried out through a clean (phosgene-
free) process, which includes direct insertion of CO2 into epox-
ides, allowing possibility and opportunity to alleviate global
climate changes and to reduce the dependence of the chemical Fig. 5 MSD of different plasticizers within a well-equilibrated PVC
industry on petroleum-based carbon resources.33 Furthermore,
unlike acetic anhydride that is highly corrosive and irritant,
amorphous cell over a 10 ns NPT (P = 1 bar; T = 450 K) dynamics run. To
exclude the initial anomalous diffusion regime as well as quality decline
of statistics over a long time, only the parts with simulation time in the
range of 1–5 ns, where the slope of the double logarithmic plots of MSD
DMC and DEC themselves are important organic compounds
and chemical intermediates with the label of “green chemi-
vs. time approached unity, were chosen for self-diffusion coefficient
cals” by virtue of their biodegradability, low toxicity, and low calculation.
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putational perspective, were particularly effective in counter-
acting migration of EMO and DMS. In their pristine form,
EMO and DMS migrated much faster than DEHP over the NPT
dynamics run, manifested by their self-diffusion coefficients
being 2–3 times higher than that of DEHP. Once engineered
with one ester branch at each α-position, their self-diffusion
coefficients decreased sharply to 9.4 × 10−8 and 5.3 × 10−8 cm2
s−1, respectively, indicating that the engineered plasticizers
migrated within the PVC model now at a rate much slower
than DEHP (15.9 × 10−8 cm2 s−1).
Other performances of ester-branched plasticizers
In general, one unique advantage discriminating aliphatic
ester-derived plasticizers from other alternatives is that they
display much higher plasticizing efficiency than DEHP.37 Since
the α-esters attached on EFAMEs and ADEs in this study were
only three or four atoms long, we envisioned that those short
esters might not substantially influence the intrinsic perform-
ances of the parent plasticizers. To evaluate this assertion, we
measured the glass transition temperature of the PVC mem-
branes using the DSC technique (Fig. 6). We observed no sig-
nificant difference in glass transition temperatures of the PVC
membranes containing the same amount of EB-EMO and
EMO. Even in the case of EB-DMS and EB-DES that were engin-
eered with two α-esters, the glass transition temperatures of
the PVC membranes were only 4–5 °C higher than those con-
taining DMS and DES, indicating hardly effected plasticizing
efficiency. This conclusion was also confirmed by molecular
simulation, from which we found an almost identical frac-
Fig. 7 (a) Atomistic simulation of binary PVC-plasticizer cells showing
the free volume enclosed by Connolly surfaces that were generated with
a Connolly radius of 1.5 Å. The atoms and bonds constituting the
polymer and plasticizers were rendered as lines. The Connolly surfaces
facing the free volume side were in blue while the opposite side in gray.
(b) Fractional free volume of the binary PVC-plasticizer cells, calculated
as the ratio of free volume to the total volume of the simulation cell.
Error bars show standard deviations calculated from the last 100 ps of
the dynamics simulation.
tional free volume of the PVC-plasticizer binary cells before
and after the plasticizers were ester-branched (Fig. 7). In
addition, the stress–strain curves of the PVC membranes were
also experimentally obtained. It was found that the mechanical
properties of the membranes were not compromised by the
ester-branched plasticizer relative to the corresponding control
(ESI, Fig. S13†).
Finally, we evaluated the in vitro cytotoxicity of the engin-
eered plasticizers using MTT assay. As shown in Fig. 8, incu-
bation with EB-EMO, EB-DMS or EB-DES did not cause a sig-
Fig. 6 DSC curves of PVC membranes containing ester-branched and
the parent plasticizers. The glass transition temperature was determined
as the midpoint of the endothermal displacement.
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Central Universities, China (20826041C4159). We also acknowl-
edge the Analytical & Testing Center of Sichuan University for
providing Materials Studio 8.0 for MD simulations.
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