Phthalate plasticizers covalently bound to PVC: Plasticization with suppressed migration

Article abstract of DOI:10.1021/ma902740t

The internal plasticization of PVC by displacement of chlorine with phthalate-based thiol additives, that is, the covalent attachment of the plasticizer to the PVC chain, is described for the first time. Using this methodology, a good plasticization efficiency is achieved although flexibility is reduced compared with that of commercial PVC-phthalate systems. However, the migration is completely suppressed. This approach may open new ways to the preparation of flexible PVC with permanent plasticizer effect and zero migration.

Full text of DOI:10.1021/ma902740t

Macromolecules 2010, 43, 2377–2381 2377
DOI: 10.1021/ma902740t
Phthalate Plasticizers Covalently Bound to PVC: Plasticization with
Suppressed Migration
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Rodrigo Navarro, Monica Perez Perrino, Myriam Gomez Tardajos, and Helmut Reinecke*
Institute of Polymer Science and Technology (ICTP-CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain
Received December 11, 2009; Revised Manuscript Received January 11, 2010
ABSTRACT: The internal plasticization of PVC by displacement of chlorine with phthalate-based thiol
additives, that is, the covalent attachment of the plasticizer to the PVC chain, is described for the first time.
Using this methodology, a good plasticization efficiency is achieved although flexibility is reduced compared
with that of commercial PVC-phthalate systems. However, the migration is completely suppressed. This
approach may open new ways to the preparation of flexible PVC with permanent plasticizer effect and zero
migration.
Introduction
Some strategies imply the surface modification of flexible PVC
articles with peroxides, azides,10-12 sulfides,13 or acrylates.
Poly(vinyl chloride) (PVC) places in the second position in the
market share of polymeric materials,1 which is principally due to
its high versatility and excellent balance between low production
costs and general properties. Flexible PVC formulations are
extensively used for the production of many different articles in
the medical field, such as blood or urine bags, transfusion tubing,
etc., and are further used for packaging purposes, toys, bathroom
curtains, or kitchen floors. In order to obtain the desired
flexibility and durability, PVC is sometimes mixed with large
amounts of plasticizers. Most of them are based on esters of
phthalic acid or adipic acid with linear or branched aliphatic
alcohols of long chain length. Di(2-ethylhexyl) phthalate (DOP)
is by far the most commonly used plasticizer for PVC.2 Other
important phthalate-based additives are di(isodecyl) phthalate
(DIDP) or di(2-ethylhexyl) adipate (DEHA).
Others describe physical treatments of the surface with γ-radiation
or plasma exposure.14-17 In all these cases the intention is to cross-
link the article surface leaving the additive behind “bars”.
Another approach is to replace the classical plasticizers by
materials that are (a) biocompatible and/or (b) have an oligo-
meric character.18 The most extensively studied polymeric mate-
rials for this approach are based on functionalized poly(ethylene
oxide) (PEO) or poly(ε-caprolactone) or their combinations.
According to the nature of these systems, the devices should be
nontoxic. However, the migration of the plasticizers is still not
completely avoided. Furthermore, crystallization or phase sepa-
rations are possible, and the plasticizing efficacy is limited.
The most challenging strategy to solve any migration problems
is the covalent linkage of the additive to the polymer chains. PVC
can be chemically modified by nucleophilic substitution of its
chlorine atoms. Many studies in this respect have been carried out
in the past showing that aromatic para-substituted thiol com-
pounds are the most appropriate nucleophiles to achieve high
degrees of modification (up to 80 mol %) without any type of
undesired side reactions.19-21 Another interesting property of
thiol groups in a modifier is their ability to efficiently deactivate
radicals. In fact, Starnes et al. have proposed the use of aliphatic
and aromatic thiol compounds as heat/color stabilizers, replacing
in this way the use of heavy metal salts in these formulations.22 In
this context it should be mentioned that the same authors have
also shown that radical scavenging is not the only stabilizing
function of the thiols but that the major function of these
stabilizers actually is to destroy thermally labile structural
defects.23
In 1985, Michel et al. made use of the high reactivity of thiols
toward nucleophilic substitution reactions and described the use
of thiol compounds as internal plasticizers for PVC.24 However,
the compounds chosen by these authors were unfortunate as they
synthesized esters of mercaptoacetic and thiosalicylic acid. On the
one hand, although aliphatic thiols may lower the glass tempe-
rature of PVC more efficiently than the stiffer aromatic analo-
gues,25,26 it is nowadays known that aliphatic thiols do not show
the appropriate balance between nucleophilicity and basicity ne-
cessary to achieve high degrees of modification and avoid degrada-
tion by elimination side reactions. On the other hand, the strong
steric hindrance of the thiol moiety in ortho position to the ester
The amount of additives in PVC formulation changes accord-
ing to the required properties. As a function of the final applica-
tion, the quantities vary between 15 and 60 wt %, being the most
common quantity for most of the devices around 35-40% of
plasticizer.
Because of thermodynamic reasons, plasticizers tend to
migrate to the surface of an article. On one hand, this leads to
a progressive loss of its initial properties and, on the other hand,
implies serious health hazards when PVC articles for biomedical
applications or children toys are dealt with because additives
migrated to the article’s surface can contaminate physiological
fluids like blood, serum, or plasma. In many studies it is described
that DOP is liable of producing toxic and adverse effects,
especially in animal or human tissues such as the pituitary gland,
liver, or testicles.3-7 For this reason DOP is one of several
phthalate plasticizers that has been recently (February 10,
2009) banned by the U.S. Consumer Product Savety Commision
(CPSC) for the manufacture of child care articles and toys
containing phthalates.
Furthermore, it has been described that the metabolic products
of DOP are able to act as potential carcinogenic agents.8,9 Because
of this high risk that implies the exposure of DOP to the human
organism, several approaches have been developed to reduce
plasticizer migration from flexible PVC to the environments.
*Corresponding author: Tel 34-91-5622900; Fax 34-91-5644853.
r 2010 American Chemical Society
Published on Web 01/21/2010
pubs.acs.org/Macromolecules

2378 Macromolecules, Vol. 43, No. 5, 2010
Navarro et al.
Figure 1. Chemical structures of the plasticizers described in the present contribution.
Scheme 1. Synthetic Route for the Preparation of Mercapto-Functionalized Plasticizers
group in thiosalicylic esters prevented an efficient attachment of the
aromatic mercapto derivative onto the polymeric backbone.
Results and Discussion
Synthesis of Functionalized Plasticizers. In order to avoid
migration of plasticizers, we have developed a synthetic
route to functionalized plasticizers with physicochemical
properties similar to those of commercial DOP, but with
an additional functional group able to establish a covalent
bond to the polymeric backbone. For these reasons, we
have synthesized the di(2-ethylhexyl) 4-mercaptophthalate
(DOP-SH) and the di(2-ethylhexyl) 5-mercaptoisophthalate
(
isoDOP-SH). The latter one has been synthesized before by
Starnes;22 DOP-SH is a new compound. The structures of
both functionalized additives are schematically shown in
Figure 1.
In Scheme 1, the synthetic route to the desired compounds
which is different than that described in the literature22 is
depicted. As starting materials, we have used 4-sulfophthalic
acid and 5-sulfophthalic acid sodium salt. In the first step,
the activation of the functional acid groups has been carried
out using PCl5 as halogenating agent. Then, using 2 equiv of
the desired alcohol (2-ethylhexanol), the selective esterifica-
tion of acyl groups (COCl) was carried out using triethyl-
amine as an acid scavenger. The formation of the sulfonate
ester can be completely avoided working at low temperatures
and adding first the alcohol and then the amine catalyst.
Finally, the selective reduction of the chlorosulfonyl group
to a thiol moiety was performed by SnCl2 as the reducing
agent. Using this Lewis acid, the ester groups remain
unaffected during the reduction. The final overall yield of
Figure 2. 1H NMR spectrum of DOP-SH.
progressively with the reaction time. These protons arise
from the modified units of the copolymers. The signal of the
CH-Cl protons at 4.5 ppm decreases with conversion while
two new signals appear in the same region at 4.8 and 4.2 ppm,
which are formed due to CH-S and COOCH2 protons,
respectively. In the case of higher modification degrees,
further peaks are observed. These peaks are due to the effect
of the chemical composition distribution, which becomes
broader when the number of modifier groups in the polymer
backbone increases.
The NMR analysis reveals some important features of this
modification reaction: DOP-SH and isoDOP-SH are suitable
agents to form flexible PVC with grafted plasticizers; under
the experimental conditions used these modification reac-
tions are free of undesired side reactions like cross-linking or
elimination of HCl what can be deduced from the absence of
corresponding olefinic protons at 5.8 and 6.2 ppm and the
white color of the products.
1
this synthetic route was 84%. The H NMR spectrum of
DOP-SH is shown in Figure 2.
Modification of PVC with Plasticizers. The modification
reaction of PVC with the novel functionalized additives was
carried out in cyclohexanone solution at 60 °C. In Figure 3
1H NMR spectra of PVC modified with DOP-SH to different
degrees of modification are shown. With increasing reaction
time, new aromatic (7.0-8.0 ppm) and aliphatic (1.8-
0.5 ppm) proton peaks arise increasing their intensities
The modification degree MG (%) can be easily calculated
from the 1H NMR data using formula 1, where I(aromatic)
and I(5-3 ppm) are the integrals of the respective proton

Article
Macromolecules, Vol. 43, No. 5, 2010 2379
Figure 3. 1H NMR spectra of PVC modified with DOP-SH at different reaction times.
signals. This formula is applicable to both kinds of plastici-
zers.
MG ð%Þ ¼ 100 ꢀ ðIðaromaticÞ=3Þ=
ðIð5 -3 ppmÞ -4=3IðaromaticÞÞ
ð1Þ
Under the reaction conditions used, the molar percentage of
plasticizer that can be linked to the PVC chains was 31 and
23 mol % for isoDOP-SH and DOP-SH isomer, respectively.
It has to be pointed out that these molar conversions
correspond to much higher weight percentages. A 30%
molar percentage for example corresponds to 75 wt %. That
means that it is possible to incorporate similar amounts of
thiol plasticizer via a covalent bond into the polymer back-
bone than is usually employed in commercial plastisol
formulations.
The composition of the obtained copolymers was also
determined by elemental analysis using the ratio of sulfur
and chloride contents and by FT-IR spectroscopy. The
results obtained from the three techniques agree very
well and thus confirm the proposed structure of the new
materials.
Glass Transition Temperature. We have studied the plas-
ticizer efficiency of the novel additives. Therefore, we have
measured the evolution of the glass temperatures (Tgs) of the
novel copolymers as a function of the degree of modification.
The measured Tgs for commercial PVC/DOP mixtures and
PVC-S-DOP copolymers with different degrees of modifica-
tions are plotted in Figure 4 with respect to the molar
percentage of DOP.
For both functionalized additives, the glass temperature is
progressively reduced with the grafted amount of plastici-
zers. Although the plasticizing efficiency in these systems is
worse than that of conventional PVC-DOP mixtures, which
is a consequence of the higher structural stiffness of the
modified monomeric units, it can be seen that the copolymers
with high amounts of plasticizer present Tgs around 0 °C.
This means that these materials are completely flexible at
room temperature.
Figure 4. Variation of glass temperature with the content of conven-
tional DOP, DOP-SH, and isoDOP-SH.
glass temperature was lower than room temperature. How-
ever, the PVC/DOP-SH series has lower Tgs than that of
PVC/isoDOP-SH. This is probably due to the relative posi-
tion of the ester groups in the aromatic ring. However, it
should also be possible to explain this fact with the increase
of generated free volume of DOP-SH with respect to
isoDOP-SH.
Migration of Plasticizers. In order to study the migration
tendency of the plasticizer, the migration behavior of the new
materials was tested by extraction experiments using heptane
at room temperature. In order to quantify the amount of
migrated plasticizer, strips of the plasticized films were
placed in a flask containing heptane and a small amount of
1,6-hexamethylene diisocyanate as internal reference. From
time to time aliquots of the solution were extracted and
the amount of plasticizer determined by IR spectroscopy.
Absolute values of DOP were obtained using a calibration
curve obtained from mixtures of known compositions of
diisocyanate and DOP. In Figure 5 the result of these extrac-
tion measurements are compared for a conventional PVC/DOP
Another interesting aspect is the difference in the plastici-
zer efficiency between both isomers. In both cases, the lowest

2380 Macromolecules, Vol. 43, No. 5, 2010
Navarro et al.
product was purified by distillation under reduced pressure (1a:
bp0.2 Torr: 127 °C; yield: 95%; 1b: bp0.3Torr: 132 °C, yield: 95%).
b. Synthesis of Di(2-ethylhexyl) 4-Chlorosulfonylphthalate
(2a) or Di(2-ethylhexyl) 5-Chlorosulfonylisophthalate (2b). The
chlorosulfonyl derivative ester (2) was prepared according to
a standard esterification method from 1 (10.0 g, 33.2 mmol,
1 equiv), using chloroform (500 mL) with 2 equiv amounts of
2-ethylhexanol (10.4 mL, 66.4 mmol) and triethylamine (9.2 mL,
66.4 mmol) as an acid scavenger. The reaction mixture was
heated to 60 °C for 2 h. Then, the organic phase was washed with
HCl (1 M), saturated NH4Cl aqueous solution, and Milli-Q
water. The organic phase was dried over anhydrous MgSO4 and
filtered, and the halogenated solvent was evaporated. The yields
were 94% (2a) and 97% (2b).
2a: 1H NMR (CDCl3, 300 MHz) δ: 8.85 (d, 1H, J=1.3 Hz,
Ar-H), 8.40 (d, 1H, J=6.1 Hz, Ar-H), 8.22 (dd, 1H, J=1.3 and
6.1 Hz, Ar-H), 4.31 (d, J = 5.7 Hz, 4H, 2 ꢀ COOCH2),
1.86-1.71 (m, 2H, 2 ꢀ (CH2)2CH-CH2), 1.52-1.33 (m, 16H,
CH2), 0.99-0.89 (m, 12H, 4 ꢀ CH3).
Figure 5. Extraction of plasticized PVC sheets with heptane at room
temperature: conventional PVC-DOP (black) and PVC-DOP-SH (red).
2b: 1H NMR (CDCl3, 300 MHz) δ: 8.96 (s, 1H, Ar-H), 8.82
(s, 2H, Ar-H), 4.31 (d, J = 5.7 Hz, 4H, 2 ꢀ COOCH2),
1.86-1.71 (m, 2H, 2 ꢀ (CH2)2CH-CH2), 1.52-1.33 (m, 16H,
CH2), 0.99-0.89 (m, 12H, 4 ꢀ CH3).
mixture and PVC modified with DOP-SNa. As shown by the
black symbol, PVC/DOP loses nearly the complete amount
of additive after less than 3 h. In contrast, the loss of
plasticizer in PVC modified chemically with thiol-functionalized
DOP is zero. This demonstrates that the plasticizer in these
systems is covalently linked to the polymer chain.
c. Synthesis of Di(2-ethylhexyl) 4-Mercaptophthalate (DOP-
SH, 3a) or Di(2-ethylhexyl) 5-Mercaptoisophthalate (isoDOP-
SH, 3b). The diester compound 2 (5.0 g, 10.2 mmol) was
dissolved in glacial acetic acid (5.0 mL), and then 2 equiv
(4.6 g, 20.4 mmol) of the reductive solution of SnCl2/HCl
(100 mL HCl concentrated) was added under stirring. The
reaction temperature was 60 °C for 12 h. The mercapto deriva-
tive 3 was obtained by extracting the water phase several times
with dichloromethane (250 mL). The combined portions were
washed once with NaHCO3 (1 M) and twice with Milli-Q water.
The organic phases were dried over anhydrous MgSO4, filtered,
and concentrated under vacuum. The resulting residue was
purified via column chromatography on silica gel and CH2Cl2
as the eluent to afford the pure functionalized plasticizers
(yields: 3a: 91%; 3b: 85%).
Conclusions
In the present contribution we have developed alternative and
promising additives, derivatives of phthalic and isophthalic acid
that guarantee plasticization of PVC avoiding any migration of
the additive. The structure of the modified polymers and the
evolution of the modification degrees with reaction time were
studied by NMR, ATR-FTIR spectroscopies, and elemental
analysis. Under the selected experimental conditions, the substi-
tution in solution takes place without secondary reactions
(elimination or cross-linking).
3a: 1H NMR (CDCl3, 300 MHz) δ: 7.62 (d, 1H, J=6.3 Hz,
Ar-H), 7.45 (d, 1H, J=1.2 Hz, Ar-H), 7.34 (dd, 1H, J=6.3 and
1.2 Hz, Ar-H), 4.23-4.12 (m, 4H, 2 ꢀ COOCH2), 3.62 (s, 1H,
-SH), 1.70-1.60 (m, 2H, 2 ꢀ (CH2)2CH-CH2), 1.44-1.23 (m,
16H, CH2), 0.91-0.85 (m, 12H, 4 ꢀ CH3).
The efficiency of the novel plasticizers has been demonstrated.
The glass transition temperature of modified PVC is largely
reduced and is around 0 °C for the highest modified samples.
Experimental Part
3b: 1H NMR (CDCl3, 300 MHz) δ: 8.35 (s, 1H, Ar-H), 8.02
(s, 2H, Ar-H), 4.19 (m, 4H, 2 ꢀ COOCH2), 3.59 (s, 1H, -SH),
1.71-1.58 (m, 2H, 2 ꢀ (CH2)2CH-CH2), 1.44-1.12 (m, 16H,
CH2), 0.90-0.78 (m, 12H, 4 ꢀ CH3).
Materials. Commercial bulk polymerized PVC with a weight-
average molecular weight of Mw=112 000 g/mol was obtained
from ATOCHEM, Spain. The tacticity measured by 13C NMR
spectroscopy was syndio=30.6%, hetero=49.8%, and iso=
19.6%.
4-Sulfophthalic acid solution 50 wt % in water was purchased
from Aldrich. The corresponding sodium salt was obtained by
reaction with stoichiometric amounts of sodium hydroxide in
methanol. After 3 h at room temperature, the white salt was
filtered, washed with methanol, and dried.
5-Sulfoisophthalic acid sodium salt was purchased from
Aldrich and used as received (purity 95%). Cyclohexanone
was bidistilled prior to use. The rest of the materials were
used as received: THF and methanol from Scharlau, sodium
hydride (60% dispersion in mineral oil), phosphorus penta-
chloride (PCl5), tin(II) chloride hydrate (SnCl2), acetic acid
(CH3CO2H) from Aldrich, and potassium carbonate from
Panreac.
Preparation of Plasticizers. a. 4-Chlorosulfonylphthaloyl
Chloride (1a) and 5-Chlorosulfonylphthaloyl Chloride (1b). To a
2 L three-necked round-bottom flask 200 g of the corresponding
carboxylic acid or its sodium salt and 440 g of PCl5 were added
under stirring. The reaction temperature was 110 °C for 3 h.
After this period of time, 500 mL of toluene was added, and the
reaction mixture was refluxed overnight. Then, the white solid
(NaCl) was eliminated by filtration, and the halogenated
Modification of PVC with Thiol Compounds. PVC (0.5 g,
8 mmol) and 8 mmol of mercapto compound were dissolved in
50 mL of cyclohexanone. Potassium carbonate (1.6 g) was
added, and the reaction started under a N2 atmosphere at
60 °C (route I). PVC modification according to route II was
performed using the corresponding thiolate salt prepared in a
previous step. The procedure of the reaction between sodium
hydride and thiol compound is described elsewhere.20
The reactions were stopped by precipitating the mixture in
cold methanol/water (2:1). The modified flexible PVC was
purified using THF/hexane as a solvent-precipitant system
and yielded a slightly yellow material able to form opaque films
similar to those of conventional PVC/DOP blends.
Characterization. NMR spectra of the compounds or modi-
fied polymers were recorded at 25 °C on a 300 MHz Varian
spectrometer operating at 300 MHz using deuterated chloro-
form or deuterated dimethyl sulfoxide as the solvent.
The IR measurements were performed on thin solvent cast
films of the modified polymers using a Nicolet 520 FTIR
spectrometer.
Calorimetric measurements of the modified PVC samples
were carried out using a Perkin-Elmer differential scanning
calorimeter DSC-7. Samples (10 mg) were heated up to 150 °C

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Macromolecules, Vol. 43, No. 5, 2010 2381
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cooling rate of 200 °C/min. The Tg values reported were taken
from the second runs and correspond to the midpoint of the
DSC curves measured from the extension of the pre- and
posttransition baseline.
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