Thermal properties of charring plasticizers from the biobased alcohols, pentaerythritol and 3,5-dihydroxybenzoic acid

Article abstract of DOI:10.1007/s10973-019-08311-8

To be converted into a useful commercial material, poly(vinyl chloride) (PVC) must generally be heavily plasticized. Owing to its high level of chlorine, PVC is not flammable (limiting oxygen index of 45). However, addition of a plasticizer usually imparts appreciable flammability. The plasticizer is often volatilized from the polymer when it is exposed to heat. In general, plasticizers burn readily and may be the source of the flammability of formulated polymers. This flammability might be controlled by using effective plasticizers that promote char formation at the surface of the burning polymer. Two series of charring compounds, pentaerythritol esters containing aromatic moieties and phosphorus esters of 3,5-dihydroxybenzoates, have been demonstrated to be effective plasticizers for PVC. The thermal degradation of both series of compounds has been examined using thermogravimetry and infrared spectroscopy. For both sets of esters, elimination of an acid is a major degradation process. For the degradation of pentaerythritol esters, a carboxylic acid is a primary degradation product. For the alkyl 3,5-(diphenylphosphato)benzoates, degradation may lead to the formation of either a carbon or a phosphorus acid depending on structure.

Full text of DOI:10.1007/s10973-019-08311-8

Journal of Thermal Analysis and Calorimetry
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Thermal properties of charring plasticizers from the biobased alcohols,
pentaerythritol and 3,5-dihydroxybenzoic acid
1
Aeshah Alrubayyi1 • Eric A. Ostrander1
Bob A. Howell
•
Received: 12 October 2018 / Accepted: 27 April 2019
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2019
Abstract
To be converted into a useful commercial material, poly(vinyl chloride) (PVC) must generally be heavily plasticized.
Owing to its high level of chlorine, PVC is not flammable (limiting oxygen index of 45). However, addition of a plasticizer
usually imparts appreciable flammability. The plasticizer is often volatilized from the polymer when it is exposed to heat.
In general, plasticizers burn readily and may be the source of the flammability of formulated polymers. This flammability
might be controlled by using effective plasticizers that promote char formation at the surface of the burning polymer. Two
series of charring compounds, pentaerythritol esters containing aromatic moieties and phosphorus esters of 3,5-dihy-
droxybenzoates, have been demonstrated to be effective plasticizers for PVC. The thermal degradation of both series of
compounds has been examined using thermogravimetry and infrared spectroscopy. For both sets of esters, elimination of an
acid is a major degradation process. For the degradation of pentaerythritol esters, a carboxylic acid is a primary degradation
product. For the alkyl 3,5-(diphenylphosphato)benzoates, degradation may lead to the formation of either a carbon or a
phosphorus acid depending on structure.
Keywords Biobased plasticizers Á Ester pyrolysis Á Degradative elimination Á Pentaerythritol esters Á Phosphatobenzoic
acids Á Renewables
Introduction
been in poly(vinyl chloride) (PVC) formulation [6]. Huge
amounts of plasticizer are required for PVC formulation.
Approximately 85% of all plasticizer production is used in
the formulation of PVC. Phthalates, particularly DEHP,
have several positive features. They behave as effective
plasticizers and are available at modest cost. However, they
are somewhat volatile and tend to migrate from a polymer
matrix into which they have been incorporated. This gives
rise to two problems. The first is flammability. Unformu-
lated PVC effectively does not burn (LOI of 45). In con-
trast, formulated PVC is often quite flammable because of
volatilization of the plasticizer. More importantly, the
plasticizer may migrate from the polymer and enter the
environment. As a consequence, human populations
worldwide have been exposed to these compounds, par-
ticularly DEHP [7–9]. Because of the widespread distri-
bution of phthalates in the everyday work environment,
human exposure can occur in a variety of ways [10–17].
Exposure to phthalates disrupts endocrine function and
promotes a variety of diseases [18–21]. Because of this, the
use of phthalates is being restricted by regulation and
The development of polymeric materials over the past
several decades has contributed immensely to a high
standard of living around the world [1]. These materials
impact the daily lives for most citizens. Current production
is greater than 300 million tons annually. For processing,
polymers must be plasticized [2–4]. Plasticization
improves the flexibility and workability of polymeric
materials. Most commonly, plasticizers have been esters
because of their ability to strongly interact with a variety of
polymers. Traditionally, phthalate esters have been the
most prominent plasticizers [5]. One of these di-(2-ethyl-
hexyl)phthalate (DEHP) has been widely used, accounting
for at least 60% of plasticizer application. Much of this has
&
Bob A. Howell
bob.a.howell@cmich.edu
1
Department of Chemistry and Biochemistry, Center for
Applications in Polymer Science, Central Michigan
University, Mt. Pleasant, MI 48859-0001, USA
123

B. A. Howell et al.
societal pressure [22, 23]. Several approaches have been
explored to respond to this problem [24–36]. One of the
most promising is the generation of efficient plasticizers
from nontoxic, renewable biosources [28–36]. The thermal
stability of two sets of new biobased plasticizers, pen-
taerythritol esters containing aromatic units and phospho-
rylated esters of 2,5-dihydroxybenzoic acid, designed to
display low migratory potential and promote char forma-
tion during combustion has been assessed using a variety of
techniques.
(d = 0.00). Phosphorus chemical shifts are in d with respect
to triphenylphosphate as internal reference (d = - 18.00).
Electrospray ionization mass spectrometry (ESI–MS) was
carried out using a Waters Acquity/LCT Premier XE unit
with samples introduced as dilute solutions in acetonitrile/
water. Thermal transitions were determined by differential
scanning calorimetry (DSC) using a TA instruments Q2000
instrument. Samples, contained in standard aluminum pans,
-
1
were analyzed at heating rate of 5 or 10 K min . Ther-
mogravimetry was performed using a TA instruments
Q500 instrument. Typically, a heating rate of 5 or
-
1
1
0 K min was used. Samples (4–10 mg) were contained
Experimental
in a platinum pan. The sample compartment was purged
3
-1
with dry nitrogen or air at 50 cm min during analysis.
Materials
Synthesis
Common solvents and reagents were obtained from the
Aldrich Chemical Company. Tetrahydrofuran (THF) was
distilled from lithium aluminum hydride in a nitrogen
atmosphere prior to use and methylene chloride from cal-
cium hydride. Benzoyl chloride, 2-thiophenecarbonyl
chloride, 2-furoyl chloride, triethylamine, 4-(dimethy-
lamino)pyridine, 3,5-dihydroxybenzoic acid, hexanol,
octanol, decanol and pentaerythritol valerate were from the
Aldrich Chemical Company. Pentaerythritol was obtained
from ACROS Organics. Diphenylchlorophosphate was
provided by ICL-IP America, Inc.
Pentaerythritol tetrabenzoate (PE-B)
In a dry, 100-mL, three-necked, round-bottomed flask fitted
with a 25-mL pressure-equalizing dropping funnel, a
magnetic stirring bar and a Liebig condenser bearing a
nitrogen inlet tube were taken 3.02 g (22.0 mmol) of
pentaerythritol (PE), 15.3 mL (11.14 g, 11.0 mmol) of
triethylamine, 0.019 g (0.16 mmol) of 4-dimethy-
laminopyridine (DMAP) and 40 mL of anhydrous methy-
lene chloride. The mixture was stirred to achieve
dissolution of PE. A solution of 12.38 g (88.0 mmol) of
benzoyl chloride in 15 mL of anhydrous methylene chlo-
ride was added dropwise, over a period of 0.5 h. The
resulting mixture was stirred for 1 h near 273 K (external
ice bath) and then at solvent reflux for 24 h. The mixture
was allowed to cool at room temperature and transferred to
a separatory funnel. The solution was washed successively,
with two 50-mL portions of 10% aqueous hydrochloric
acid solution, 50 mL of saturated aqueous sodium bicar-
bonate solution and 50 mL of saturated aqueous sodium
chloride solution. The solution was dried over anhydrous
sodium sulfate, and the solvent was removed by rotary
evaporation at reduced pressure to afford 6.62 g (52%
yield) of the pentaerythritol tetrabenzoate as a white solid:
Methods and instrumentation
In general, reactions were carried out in a dry (all glass-
ware was dried in an oven overnight at 393 K and allowed
to cool under a stream of dry nitrogen prior to use) three-
necked, round-bottomed flask fitted with Liebig condenser
bearing a gas inlet tube, a magnetic stirring bar (or Trubore
stirrer) and a pressure-equalizing dropping funnel (or syr-
inge port). Chromatography was accomplished using Sila-
Flash P60 (230–400 mesh silica; Silicycle) in a column of
appropriate size and hexane/ethyl acetate as eluant. Silica-
coated Mylar plates (ThermoFisher Scientific) were used
for thin-layer chromatography (TLC). Infrared spectra
were obtained by attenuated total reflectance (ATR) using a
Thermo Scientific Nicolet 380 FTIR spectrophotometer.
-
1
mp 375 K (DSC); Tdec 607 K (TGA); FTIR (cm , ATR)
3056 (m) Csp2–H, 2962 (m) Csp3–H, 1718 (vs) ester car-
-
1
Absorptions were recorded in wavenumbers (cm ), and
absorption intensities were classified in the usual fashion as
very weak (vw), weak (w), medium (m), strong (s) and very
strong (vs) relative to the strongest band in the spectrum.
Nuclear magnetic resonance (NMR) spectra were obtained
bonyl, C=O, 1584 (m) aromatic nucleus and 1261 (s) car-
1
bon to oxygen, C–O; H NMR (d, CDCl ) 4.70 (s, 8H,
3
methylene protons adjacent to the ester oxygen), 7.37, 7.52
1
3
and 8.02 (m, 20H, aromatic nucleus protons); C NMR (d,
CDCl ) 43.0 (quaternary carbon atom), 63.6 (carbon atoms
3
using
a
5–15% solution in deuterochloroform or
of the methylene groups adjacent to the ester oxygen),
128.5, 129.8, 130.0 and 133.3 (aromatic carbon atoms) and
166.0 (carbonyl carbon atoms); ESI MS (C H O ) m/z,
dimethylsulfoxide-d and a Varian Mercury 300 MHz or
6
an INOVA 500 MHz spectrometer. Proton and carbon
chemical shifts are reported in parts per million (d) with
respect to tetramethylsilane (TMS) as an internal reference
3
3 28 8
?
?
?
553 [M ? H] , 575 [M ? Na] and 591.14 [M ? K] .
Other esters were prepared in an analogous manner.
1
23

Thermal properties of charring plasticizers from the biobased…
Fig. 1 Structures of
pentaerythritol esters containing
aromatic units and
phosphorylated esters of 3,5-
Ar
O
O
O
Ar
Ar
O
dihydroxybenzoic acid
P
O
O
O
O
O
O
O
O
OR
Ar
O
O
O
P
O
O
Ar =
R = – CH
S
3
–
–
–
CH (CH ) CH
2 2 4
3
3
3
CH (CH ) CH
2
2 6
CH (CH ) CH
2
2 8
O
Fig. 2 Characteristic infrared
C–H (sp2)
3056
Ester
C = O
1718
1713
1719
C–O
1261
1263
1289
Aromatic nucleus
-1
absorptions (cm ) for
pentaerythritol esters containing
aromatic units
PE-B
PE-T
PE-F
1584
1525
1571
3053
3056
100
100
8
6
0
0
8
0
0
6
4
2
0
4
0
0
4
2
0
4
000
3000
2000
1000
000
3000
2000
1000
–
1
cm
–1
cm
1
00
8
6
0
0
4
2
0
0
4
000
3000
2000
1000
–1
cm
Pentaerythritol tetra(thiophene-2-carboxylate) (PE-T)
(m) Csp3–H, 1713 (vs) ester carbonyl, C=O, 1525 (m) aro-
matic thiophene nucleus and 1263 (s) carbon to oxygen, C–
1
Pentaerythritol tetra(2-thiophenecarboxylate) was obtained
O; H NMR (d, CDCl ) 4.59 (s, 8H, methylene protons
3
as an amber solid (61% yield): mp 395 K (DSC); T
adjacent to the ester oxygen), 7.06, 7.55 and 7.79 (m, 12H,
13
dec
-
39 K (TGA); FTIR (cm , ATR) 3053 (m) Csp2–H, 2963
1
6
thiophene protons); C NMR (d, CDCl ) 43.0 (quaternary
3
123

B. A. Howell et al.
Fig. 3 Proton NMR spectra for
pentaerythritol esters containing
aromatic units
Ester
PE-B
PE-T
PE-F
CH3
4.70
4.59
4.57
Aromatic protons
7.39 - 8.01
7.06 - 7.79
6.48 - 7.56
8
7
6
5
4
3
2
1
δ
8
7
6
5
4
3
2
1
δ
7
6
5
4
3
2
1
δ
9
8
7
6
5
4
3
2
1
ppm
9
8
7
6
5
4
3
2
1
ppm
Fig. 5 Proton
(
NMR
diphenylphosphato)benzoate
spectrum
for
hexyl
3,5-
Pentaerythritol tetra(furan-2-carboxylate) (PE-F)
160
140
120
100
80
60
40
20 ppm
Pentaerythritol tetra(furan-2-carboxylate) was obtained as a
brown solid (77% yield): mp 412 K (DSC); T 582 K
Fig. 4 NMR spectra for hexyl 3,5-dihydroxylbenzoate
dec
-
1
TGA); FTIR (cm , ATR) 3056 (m) Csp2–H, 2988
(
(
carbon atom), 63.1 (carbon atoms of the methylene groups
adjacent to the ester oxygen), 127.9, 132.8, 133.0 and 133.9
w) Csp3–H, 1719 (vs) ester carbonyl, C=O, 1571 (m) aro-
matic furan nucleus and 1289 (s) carbon to oxygen, C–O;
1
(
thiophene carbon atoms) and 161.5 (carbonyl carbon
?
atoms); ESI MS (C H O S ) m/z, 577 [M ? H] , 599
H NMR (d, CDCl ) 4.59 (s, 8H, methylene protons
3
2
5 20 8 4
?
M ? Na] and 615 [M ? K] .
?
adjacent to the ester oxygen), 6.48, 7.17 and 7.56 (m, 12H,
1
[
3
furan protons; C NMR (d, CDCl ), 42.7 (quaternary
3
carbon atom), 62.8 (carbon atoms of the methylene groups
adjacent to the ester oxygen), 111.9, 118.8, 143.9 and 146.7
1
23

Thermal properties of charring plasticizers from the biobased…
1
1
20
00
2.5
2.0
1.5
1.0
0.5
8
6
4
2
0
0
0
0
0
0.0
3
09.27 °C
4
000
3000
2000
1000
– 0.5
0
200
400
600
cm–1
Temperature/K
Fig. 8 Changes which accompany thermal degradation of pentaery-
thritol esters
Fig. 6 Thermal decomposition of pentaerythritol tetra(furan-2-
carboxylate)
Methyl 3,5-di(diphenylphosphato)benzoate
Table 1 Thermal properties of pentaerythritol esters containing aro-
matic units
Methyl 3,5-di(diphenylphosphato)benzoate was obtained
as a white solid (90.8% yield): mp 341 K (DSC); IR
a
Ester
Melting point/K
DSC)
Decomposition onset temperature/K
(TGA)
-
1
(cm ) 3083 (w) Csp2–H, 2948 (w) Csp3–H 1724 (s) ester
(
C=O, 1588 (s) aromatic nucleus, 1300 (s) P=0, 1183 (s) and
PE-B
PE-T
PE-F
a
375
395
412
607
639
582
1
9
7
7
03 (s) P–O–C; H NMR (d, DMSO-d ) 3.85 (s, 3H)
.24–747 (m, 20H, protons of diphenylphosphato groups),
6
.54 (m, 1H) and 7.69 (m, 2H) (aromatic protons of tri-
13
Extrapolated onset temperature from the derivative plot of mass loss
versus temperature
hydroxybenzoate nucleus); C NMR (d, DMSO-d ) 53.3
6
(methoxy carbon atom), 117.4, 118.4, 133.4 and 150.5
(aromatic carbon atoms of trihydroxybenzoate nucleus),
116–152 (additional peaks for carbon atoms of the
(
furan carbon atoms) and 157.9 (carbonyl carbon atoms);
?
diphenylphosphato groups) and 164.5 (ester carbonyl car-
31
?
ESI MS (C H O ) m/z, 513 [M ? H] , 535 [M ? Na]
2
5
20 12
bon atom); P NMR (d, DMSO-d ) –17.6.
6
?
and 551 [M ? K] .
Fig. 7 FTIR spectra of
pentaerythritol esters
a pentaerythritol tetrabenzoate,
b pentaerythritol
tetra(thiophene-2-carboxylate),
c pentaerythritol tetra(furan-2-
carboxylate) and d Pevalen
undergoing thermal degradation
(a)
(b)
0h
0
h
3
6
1
h
h
2h
3h
6
1
2
h
2h
4h
24h
(
0, 3, 6, 12 and 24 h) at 453 K
4000
3000
2000
1000
4000
3000
2000
1000
–1
–1
cm
cm
(c)
(d)
0
3
6
1
2
h
h
h
2h
4h
0
3
6
1
2
h
h
h
2h
4h
4000
3000
2000
1000
4000
3000
2000
1000
–1
–1
cm
cm
123

B. A. Howell et al.
Scheme 1 Mode of degradation
of pentaerythritol esters
containing aromatic units
O
O
O
O
Ar
Ar
O
Ar
Ar
O
O
O
O
O
Ar
O
O
Ar
+
O
Ar
O
Ar
O
O
Multiple
transformations
Hydrogen
abstraction
Ar =
S
O
O
Char
HO
Ar
Fig. 9 Changes which
accompany thermal (573 K)
degradation of methyl 3,5-
(
diphenylphosphato)benzoate
4000
3500
3000
2500
2000
1500
1000
500
–1
Wavenumber/cm
Results and discussion
also contains a peak at d 167.8 for the ester carbonyl car-
bon atom. The proton NMR spectrum for the correspond-
ing diphosphate ester, hexyl 3,5-(diphenylphosphato)
benzoate, is shown in Fig. 5.
The thermal degradation characteristics of two sets of
biobased plasticizers have been established. These mate-
rials were designed to be relatively nonmigrating and to
promote char formation during combustion. Structures are
shown in Fig. 1. Both the sets of esters were fully char-
acterized using spectroscopic and thermal methods. The
infrared spectra for aromatic esters of pentaerythritol are
shown in Fig. 2. The most characteristic absorption is for
the ester carbonyl: pentaerythritol benzoate (PE-B),
The most notable change in the spectrum compound for
the diphenol is the presence of peaks for the aromatic
protons of the diphenylphosphato group at d 7.2. The
spectra for all the other benzoate esters are quite similar,
differing only in the number of resonances for methylene
groups.
The aromatic esters of pentaerythritol are all low-melt-
ing solids with relatively high decomposition temperatures.
The thermal decomposition of pentaerythritol tetra(furan-
2-carboxylate) is illustrated in Fig. 6. Melting points and
decomposition onset temperatures from dynamic TGA are
collected in Table 1.
-
1
1
1
1
718 cm ; pentaerythritol tetra(thiophene-2-carboxylate),
-
713 cm ; and pentaerythritol tetra(furan-2-carboxylate),
1
-
1
718 cm . Proton NMR spectra for these compounds are
displayed in Fig. 3. The most characteristic peak in these
spectra is for the methylene protons at approximately d 4.6.
Proton and carbon-13 NMR spectra for hexyl 3,5-dihy-
droxybenzoate are shown in Fig. 4. The most characteristic
peak in the proton spectrum is a triplet at d 4.28 for the
methylene protons adjacent to oxygen. The corresponding
These esters undergo slow thermal degradation at 453 K
(Fig. 7). The degradation of Pevalen (pentaerythritol
valerate), a fully aliphatic ester, is included for comparison.
The infrared spectra for all the compounds undergoing
degradation suggest that the mode of degradation is the
1
3
peak in the C spectrum appears at d 66. The spectrum
1
23

Thermal properties of charring plasticizers from the biobased…
R = –CH (CH ) CH
2
2 4
3
3
3
–
–
CH (CH ) CH
2 2 6
R = CH
3
CH (CH ) CH
2
2 8
H C=CH(CH ) CH
+
+
2
2 4
3
3
3
H C=CH(CH ) CH
2
2 6
H C=CH(CH ) CH
2
2 8
Hydrogen atom
abstraction
Char formation
Scheme 2 Mode of thermal degradation of alkyl 3,5-(diphenylphosphato)benzoates
same for all the pentaerythritol esters. The most prominent
feature of the degradation is carboxylic acid formation.
This is illustrated in Fig. 8.
using infrared spectroscopy to detect changes which cor-
respond to decomposition. For both sets of esters, elimi-
nation of an acid is a major degradation process. For the
degradation of pentaerythritol esters, a carboxylic acid is a
primary degradation product. For the alkyl 3,5-
(diphenylphosphato)benzoates, degradation may lead to the
formation of either a carbon or a phosphorus acid
depending on structure.
The formation of carboxylic and large amounts of char
is reminiscent of previously reported behavior of pen-
taerythritol derivatives under thermal stress [37, 38].
Because of the absence of b-protons in the structure, the
ester pyrolysis is complex but the main features are clear
and illustrated in Scheme 1.
Acknowledgements A sample of diphenylchlorophosphate was gen-
erously supplied by ICL-IP America, Inc.
The thermal degradation of the alkyl (methyl, hexyl,
octyl and decyl) esters of 3,5-(diphenylphosphato)benzoic
acid was similarly studied. The longer chain alkyl esters
undergo standard pyrolytic elimination of benzoic acids to
generate terminal alkenes and are unremarkable. However,
degradation of the methyl ester is more complex. This ester
has no hydrogen atom b to the carboxyl group and,
therefore, cannot undergo standard ester pyrolysis.
Instead, it degrades at higher temperature to generate a
phosphorus acid and char. This transformation is depicted
in Fig. 9. The degradation processes for the alkyl 3,5-
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