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Polymers 2019, 11, 1914; doi:10.3390/polym11121914 www.mdpi.com/journal/polymers
Article
Synthesis of Bisphenol A Based Phosphazene-
Containing Epoxy Resin with Reduced Viscosity
Vyacheslav V. Kireev
1
, Yulya V. Bilichenko
1
, Roman S. Borisov
2,3
, Jianxin Mu
4
,
Dmitry A. Kuznetsov
5
, Anastasiya V. Eroshenko
1
, Sergey N. Filatov
1
and Igor S. Sirotin
1,
*
1
Department of Plastics, Mendeleev University of Chemical Technology of Russia, Miusskaya sq. 9,
125047 Moscow, Russia; kireev@muctr.ru (V.V.K.); julyab2@gmail.com (Y.V.B.);
eroshenko.nast@yandex.ru (A.V.E.); filatovsn@muctr.ru (S.N.F.)
2
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29,
119991 Moscow, Russia; borisov@ips.ac.ru
3
Department of Organic Chemistry, Peoples′ Friendship University of Russia, Miklukho-Maklaya str.6,
117198 Moscow, Russia
4
College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China; emujianxin@163.com
5
Scientific & Research Institute of Natural Gases and Gas Technologies — Gazprom VNIIGAZ, Razvilka,
s.p. Razvilkovskoe, Leninsky dist., Moscow region, 142717 Moscow, Russia; expdk@yandex.ru
* Correspondence: isirotin@muctr.ru; Tel.: +7-(499)-978-9265
Received: 3 October 2019; Accepted: 16 November 2019; Published: 20 November 2019
Abstract: Phosphazene-containing epoxy oligomers (PEO) were synthesized by the interaction of
hexachlorocyclotriphosphazene (HCP), phenol, and bisphenol A in a medium of excess of
epichlorohydrin using potassium carbonate and hydroxide as HCl acceptors with the aim of
obtaining a product with lower viscosity and higher phosphazene content. PEOs are mixtures of
epoxycyclophosphazene (ECP) and a conventional organic epoxy resin based on bisphenol A in an
amount controlled by the ratio of the initial mono- and diphenol. According to
31
P NMR
spectroscopy, pentasubstituted aryloxycyclotrophosphazene compounds predominate in the ECP
composition. The relative content in the ECP radicals of mono- and diphenol was determined by
the MALDI-TOF mass spectrometry method. The organic epoxy fraction, according to gas
chromatograpy-mass spectrometry (GC-MS), contains 50–70 wt % diglycidyl ether of bisphenol A.
PEO resins obtained in the present work have reduced viscosity when compared to other known
phosphazene-containging epoxy resins while phosphazene content is still about 50 wt %. Resins
with an epoxy number within 12–17 wt %, are cured by conventional curing agents to form
compositions with flame-retardant properties, while other characteristics of these compositions are
at the level of conventional epoxy materials.
Keywords: epoxy resin, epoxy oligomer, phosphazene, bisphenol A, phenol
1. Introduction
Out of all the varieties of synthetic polymers, epoxies, developed over 70 years ago, won a special
place in industry and everyday life, not necessarily in terms of overall manufacturing volume, moreso
in their specific role [1]. Classic bisphenol A based epoxy resins have many advantages: low cost, low
curing shrinkage, good chemical resistance, and mechanical properties [1,2]. In terms of valuable
qualities, epoxy polymers are superior to many other classes of synthetic polymers, which makes
them indispensable as a basis for adhesives, paints, coatings, and binders for reinforced plastics [1,3].
The development and application of new epoxy oligomers and binders based on them are expanding
at a rapid pace despite the appearance of new generation binders, such as bismaleimides, polyimides,
and cyanate esters [2]. The latter are characterized by enhanced physical and mechanical properties,
including high heat resistance and low flammability. However, their price is very high, their
Polymers 2019, 11, 1914 2 of 16
processing properties are worse than those of the same epoxides, and a high degree of crosslinking
can adversely affect the mechanical properties [1,2].
However, polymers based on unmodified epoxy resins often have low and unstable
performance characteristics, in particular flammability and relatively low heat resistance [3].
A rather effective way to increase the heat resistance of epoxy matrices is with structural
modification, for example, with compatible oligomers of higher functionality, which are incorporated
into the three-dimensional network structure formed during curing. Examples of such resins are
triglycidyl-p-aminophenol (TGPAP), tetraglycidyl-4,4′-methylenedianiline (TGMDA), and
epoxidized novolacs [3,4].
The solution of the flammability problems of epoxy polymers is a more complex issue and
involves trade-offs. Thus, a traditional and effective way to reduce the flammability of epoxy systems
is to replace bisphenol A based epoxy resins with their brominated analogs. However, brominated
epoxy resins, as well as other halogen-containing compounds, emit toxic gases when they come into
contact with flames, which limits their use, for example, in civil aviation and transport, for
environmental reasons [5,6].
One of the promising ways of modifying epoxy polymers to increase heat and fire resistance is
the introduction of organometallic compounds, in particular phosphorus compounds, known as
universal flame retardants for a large number of polymeric materials. Refusal of halogen-containing
flame retardants in favor of phosphorus-based ones is a modern global trend [5].
However, the introduction of low-molecular, non-reactive compounds into the material,
including red phosphorus and its inorganic compounds (phosphates, polyphosphates, etc.) worsens
the mechanical properties and transparency of polymers [5]. Organic phosphates, which are better
combined with the base polymer, have proven themselves more functional. Meanwhile, there are
some difficulties associated, for example, with the fact that ethers of phosphoric acids can act as
plasticizers, reducing the heat resistance of the material [7]. The best properties are possessed by
binders containing functional organic phosphorus compounds capable of forming covalent bonds
with an epoxy matrix. [6,8]. For example, glycidyl ethers of phosphorus acids, which not only reduce
the flammability, but also increase the mechanical and adhesive strength of the material, are quite
promising structural phosphorus-containing modifiers [8,9]. However, they are also poorly
compatible with epoxy polymers [9,10]. Finally, most industrial phosphorus-containing flame
retardants have a much lower degradation temperature than epoxy polymers [5,6,11,12], which
makes it impossible to use them in engineering plastics and as part of high-temperature binders.
The combination of the above factors leads to the fact that epoxy resins are not always able to
satisfy the growing needs of high-tech industries, especially in the aerospace industry, automotive
industry, electrical engineering, electronics, etc.
A possible way to solve the problems described above, including the flammability of epoxy
resins, is the use of modifiers based on phosphazenes.
The main chain of organophosphazenes consists of alternating atoms of phosphorus and
nitrogen, and at the phosphorus atom there are organic radicals introduced by the substitution of
halogen in halogenphosphazenes. The nature of organic substituents, usually introduced by the
reaction of nucleophilic substitution of chlorine, can vary widely and determines the properties of
the final polymer or oligomer. The unique properties of various organophosphazenes cause the ever-
growing interest of researchers in phosphazene chemistry [13]. Compared to other
organophosphorus compounds, aryloxyphosphazenes have, as a rule, higher thermal stability and
chemical resistance and are promising non-halogen flame-retardants [6,14,15], characterized by the
synergistic action of phosphorus and nitrogen [14,16]. Thus, phenoxycyclophosphazenes were
commercialized as a flame retardant by Otsuka Chemical and others [14,17].
Researchers have long been trying to combine the exceptional properties of
organophosphazenes as highly effective flame retardants with the function of structural modifiers,
such as polyfunctional epoxy resins. There are two main synthetic approaches that allow to obtain
functional phosphazenes capable of forming covalent bonds with epoxy matrices:
Polymers 2019, 11, 1914 3 of 16
1) The synthesis of organophosphazenes with reactive epoxy groups for addition to the epoxy
component [18–43];
2) The synthesis of organophosphazenes with reactive amine groups for use as a curing agent or
its component [44–52].
Functional epoxyphosphazenes are highly effective flame retardants that not only do not reduce
mechanical properties, since they are well compatible with the epoxy matrix, but can also improve
them, probably due to the formation of a special three-dimensional polymer network, in the nodes of
which phosphazene cycles are located [53].
Currently, the majority of methods for the synthesis of functional epoxyphosphazenes described
in the literature are of primarily scientific interest due to the complexity of scaling and the large
number of intermediate stages [18–22,24,28,30,31,33–36,38–43]. Although there are
epoxyphosphazenes that are fairly easy to synthesize, for example, on the basis of hexachlorocyclo
triphosphazene and glycidol [23,25–27,29,32,37,40]. However, alkoxyphosphazenes, which include
such glycidyloxyphosphazenes, are not thermally stable [13]. Thus, even during their synthesis, an
undesirable phosphazene-phosphazane rearrangement occurs [27]. Thus, only aromatic
organophosphazenes can be used as a component of high-temperature epoxy binders.
In recent years, phosphazene-containing epoxy oligomers (PEO) with reduced flammability on
the base of cyclic chlorophosphazenes have been synthesized and characterized [54]. The most
accessible and promising are PEOs obtained by the reaction of epichlorohydrin with
hydroxyaryloxycyclotriphosphazenes (HAP), the condensation products of
hexachlorocyclotriphosphazene (HCP) and 4,4′-dioxydiphenyl-2,2-propane (Figure 1).
Figure 1. Synthesis of phosphazene-containing epoxy oligomers by the reaction of epichlorohydrin
with hydroxyaryloxycyclotriphosphazenes, the condensation products of
hexachlorocyclotriphosphazene and 4,4′-dioxydiphenyl-2,2-propane [55].
Hydroxyaryloxycyclotriphosphazenes (HAP), epoxycyclophosphazene (ECP), diglycidyl ether of
bisphenol A (DGEBA), phosphazene-containing epoxy oligomers (PEO).
The main problem in the synthesis of PEO is the high functionality of HCP, which requires the
use of a more than 10-fold molar excess of diphenol to achieve a more complete replacement of the
chlorine atoms in it and to avoid gelation. As for the industrial use of epoxyphosphazenes, the main
limiting factor complicating the preparation of the formulation and worsening its processing
Polymers 2019, 11, 1914 4 of 16
properties is that most epoxyphosphazenes are solids with a softening temperature of 80–100 ° C and
high melt viscosity. High average functionality can also influence processing properties.
Epoxidation of a mixture of HAP and excess of diphenol with epichlorohydrin produces PEO,
which contains fractions of the usual bisphenol A based epoxy resin and epoxycyclophosphazene
(ECP) oligomers with a variable ratio [54]. In [55,56], single-stage synthesis of PEO was realized by
direct interaction of HCP and an excess of diphenol (bisphenol A [55,56] or resorcinol [57]) in the
epichlorohydrin medium as a reagent and a solvent. The resulting PEO includes ECP consisting
essentially of tetra- and pentaepoxides of the above formula with n = 4 and 5 and a conventional
organic epoxy monomer. These PEOs in comparison with pure epoxyphosphаzenes contain organic
epoxide, which is in fact an active diluent that lowers the viscosity and average functionality to an
acceptable level for further processing. However, the viscosity of such PEO concentrates at ambient
temperature is still more than 200 Pa·s, which is a fairly high value, close to the processing limit.
The content of ECP in a mixture with the organic epoxide may be increased with simultaneous
reducing of the content of residual chlorine, the functionality of HCP and its molecular weight, which
was realized in our previous work [53] by replacing part of the chlorine atoms in chlorophosphazene
with monophenol residues (Figure 2).
Figure 2. Replacement of the part of the chlorine atoms in chlorophosphazene with monophenol
residues in order to increase the content of epoxycyclophosphazenes in a mixture with the organic
epoxide [53].
According to matrix-assisted laser desorption/ionization time-of-flight MALDI-TOF mass
spectrometry, the main components of the oligomer formed are phosphazene-containing di-, tri- and
tetraepoxides, the ratio of which in the reaction mixture can be varied by alternation of the value of
n. The average content of epoxy groups in these PEO is 13%–14% by weight. Compositions based on
diglycidyl ether of bisphenol A (DGEBA) and methyltetrahydrophthalic anhydride, containing 15%
of the obtained oligomers in the epoxy component, compared to pure DGEBA, were characterized
by more than 30% higher glass transition temperature and flexural strength, and at 75% content of
PEO were non-combustible and had limiting oxygen index (LOI) of 30 [53]. However, the need to
obtain and use of phenolates complicates the scalability of the described method.
Thus, the development of new scalable methods for obtaining epoxyphosphazene-containing
epoxy resins based on available starting materials with reduced viscosity and improved technological
properties is of great scientific and practical interest and may contribute to accelerating the
widespread use of phosphazene-containing resins as components of high-tech flame-retardant
polymer composite materials.
In order to exclude phenolate synthesis in the known method [53] (Figure 2) and to obtain pre-
formulated PEOs with lower viscosity and greater phosphorus content in the present work, by
analogy with [55,56], PEO was synthesized by direct interaction of HCP, phenol, bisphenol A, and
epichlorohydrin.
2. Materials and Methods
In the present work PEO was synthesized by direct interaction of HCP, phenol and bisphenol A
(BPA) in epichlorohydrin (ECH) medium as reagent and solvent in the presence of KOH with a role
of both HCl acceptor and epoxy-forming reagent (Figure 3, Method A).
Polymers 2019, 11, 1914 5 of 16
Figure 3. Single-stage synthesis of phosphazene-containing epoxy oligomers by direct interaction of
hexachlorocyclotriphosphazene (HCP), phenol and bisphenol A in epichlorohydrin medium (this
work).
For comparison, stepwise replacement of chlorine atoms in HCP was carried out first on
phenoxy groups, and then on the residues of bisphenol A in the presence of potassium carbonate as
HCl acceptor on the first stage and with KOH on the second (Figure 3, Method B).
2.1. Starting Materials
Hexachlorocyclophosphazene—a white crystalline substance with m.p. of 113 °C; nuclear
magnetic resonance (NMR) 31P-singlet spectrum with δP = 19.9 ppm, was obtained by the method
[58]. Potassium hydroxide in the form of 90.0% pure white pellets (JSC KAUSTIK, Volgograd, Russia)
was used without purification, the content of crystallization water determined by acid-base titration
was about 10%. Epichlorohydrin (Solvay, Tavaux, France) with the content of the main substance of
99.8% was distilled before use as a colorless liquid, b.p. 118 °C. Potassium carbonate (Sigma-Aldrich,
St. Louis, MI, USA) was dried in vacuum at 100 °C before use as a white crystalline substance in the
form of powder, soluble in water. Bisphenol A (PJSC Kazanorgsintez, Kazan, Russia) was purified
via repeated recrystallization from chlorobenzene to yield a product with m.p. of 156.5 °С. Phenol
(Sigma-Aldrich, St. Louis, MI, USA) was distilled before use into white crystals, m.p. 40.0 °C. Solvents
were purified according to known methods, their physical characteristics corresponded to the
literature data [59].
2.2. Synthesis of Epoxyphosphazenes
2.2.1. Single-Stage Synthesis of Phosphazene-Containing Epoxy Oligomers (Method A)
A 100 mL three-necked flask, equipped with a reflux condenser, a mechanical stirrer, and a
thermometer, was charged with 1 g (0.0028 mol) of HCP, 2.68 g (0.0144 mol) of bisphenol A, 0.54 g
(0.0058 mol) of phenol, and 65 mL of epichlorohydrin. The reaction mixture was heated to 50–55 °C
and thermostated at this temperature for 30 min until all solids were completely dissolved. After that,
Polymers 2019, 11, 1914 6 of 16
2.32 g (0.0414 mol) of potassium hydroxide was added and the process was conducted for 2 h at a
temperature of 60 °C. At the end of the synthesis, the hot solution was filtered off and excess solvent
was distilled off. The resulting mixture of epoxy oligomers was dried in vacuo at 85 °C. The reaction
product is a slightly colored viscous liquid. The yield was 3.96 g (71%).
2.2.2. Stepwise Synthesis of Phosphazene-Containing Epoxy Oligomers (Method B)
A 100 mL three-necked flask, equipped with a reflux condenser, a mechanical stirrer, and a
thermometer, was charged with 1 g (0.0028 mol) of HCP, 0.54 g (0.0058 mol) of phenol, 0.95 g (0.0069
mol) potassium carbonate, and 65 mL of epichlorohydrin. The reaction mixture was heated to 60 °C
and the process was conducted for 2 h at this temperature. The reaction mixture was then cooled to
50 °C, 3.28 g (0.0144 mol) of bisphenol A was charged and incubated at this temperature for 15
minutes until the bisphenol A was completely dissolved. After that, 2.32 g (0.0414 mol) of potassium
hydroxide was added and the process was conducted for 3 h at a temperature of 60 °C. At the end of
the synthesis, the hot solution was filtered off and excess solvent was distilled off. The resulting
mixture of epoxy oligomers was dried in vacuo at 85 °C. The reaction product was a slightly colored
viscous liquid. The yield was 4.18 g (75%).
2.3. Methods of Analysis
The 31P and 1H NMR spectra were measured in chloroform-d solutions with a Bruker AV-400
spectrometer (Bruker Corporation, Bremen, Germany) operating at 162 and 400 MHz, respectively.
The signals due to the deuterated solvents were used as internal references. The chemical shifts of the
signals were calculated relative to the signals of tetramethylsilane (1H) and phosphoric acid (31P),
which were used as references. The spectra were processed with the help of the MestReNova Lab
software package (Version 12.0.4, MESTRELAB RESEARCH, S.L, Santiago de Compostela, Spain).
MALDI-TOF mass spectrometric analysis was carried out on the Bruker Auto Flex II instrument
(Bruker Corporation, Bremen, Germany).
The gas chromatography-mass-spectrometry (GC-MS) study was carried out on a VARIAN-3800
CP/4000 MS chromatograph-mass spectrometer (manufactured by Varian, Palo Alto, CA, USA).
Separation of the substances was carried out using a VF-5ms chromatographic capillary column with
a length of 30 m, an internal diameter of 0.25 mm, and a fixed-phase layer thickness of 0.25 μm. The
carrier gas was helium. The injector temperature was 280 °C. The volume of the injected sample was
1 μL. The temperature of the column thermostat changed from 50 °C (1 min) to 280 °C (10 min) at a
rate of 10 °C/min. The temperature of the ion source and the interface was 230 and 280 °C. The mass
spectra of the substances were registered in the electron impact regime in the mass range 33–1000 Da.
The processing of the obtained data was carried out with the usage of computer software "Varian MS
Workstation", the identification of the substances was carried out using the NIST 11 mass-spectra
library.
The gel-permeation chromatography (GPC) was carried out on Shimadzu LC-20 Prominence
(Kyoto, Japan) chromatograph equipped with refractometric and UV detector (a wavelength of 264
nm,) and a PSS column (SDV; 300 mm × 8 mm; 1000 A, separation within 100–60,000 Da),
tetrahydrofuran (THF) was used as an eluent (1 mL/min). Molecular mass values were estimated
with the use of a polystyrene calibration curve. Elemental analysis was carried out by
spectrophotometry.
In the present paper, epoxy group content (in wt %) is the weight of oxirane groups (–CHCH2O)
divided by the total molecular weight of epoxy resin. Thus, the theoretical content of epoxy groups
was calculated by the formula:
𝐸 =
43 × 𝑛
𝑀100% (1)
where 43 and M are the molecular weights of the oxirane group and the whole epoxy resin molecule,
respectively, and n is the number of oxirane groups in the epoxy resin molecule. The experimental
epoxy group content was determined by the method of reverse acid-base titration [60–62]. In these
Polymers 2019, 11, 1914 7 of 16
methods, glycidyl groups are converted to chlorohydrin groups by dissolving the sample of epoxy
resin in hydrochloric acid acetone solution followed by titration of the excess of hydrochloric acid
with NaOH solution. The experimental epoxy group content is calculated by the formula:
𝐸 =𝑉−𝑉
×43×𝑁
𝑔× 1000 100% (2)
where V is the amount of NaOH solution for titration of a sample containing epoxy resin in mL; V0 is
the amount of NaOH solution for titration of blank sample without epoxy resin in mL; 43 is the
molecular weight of the oxirane group; N is the concentration of NaOH solution, mol·L−1; and g is the
mass of epoxy resin sample in g.
The viscosity of epoxy oligomers was evaluated on a Reotest-2 rotational viscometer with a
working cone-plane unit. The viscosity of the PEOs obtained in present and earlier papers was
compared with conventional low molecular weight bisphenol A based epoxy resin D.E.R.TM 332
epoxy resin (Dow Chemical, Midland, TX, USA) with the epoxy group content of 24.6%–25.1% and
epoxy equivalent of 171–175 which is referenced as DGEBA and with ERISYS® Resorcinol diglycidyl
ether RDGE (CVC Thermoset Specialties, Moorestown, NJ, USA) with the epoxy group content of
34.4%–36.4% and epoxy equivalent of 118–125.
3. Results and Discussion
The composition and structure of the phosphazene component of the resulting mixture of
epoxides were evaluated by combination of 31P NMR spectroscopy and MALDI-TOF spectrometry
(Figures 4 and 5) and the organic component by gas chromatography–mass spectrometry.
According to 31P NMR spectroscopy, irrespective of the initial mole ratio of
HCP:phenol:bisphenol A compounds with pentasubstituted triphosphazene rings predominate in
the phosphazene fraction (AB2 system with triplet δp = 20–26 ppm and doublet δp = 8–10 ppm)
together with insignificant amounts of tetraaryloxy-substituted compounds (AB2 system with
doublet at δp = 20–22 ppm and triplet at δp = 5–8 ppm).
Laser mass spectrometry data confirm the presence in the phosphazene fraction of mainly
pentaaryloxy-substituted HCP, containing residues of phenol and diphenol in various proportions
in the substituents near phosphorus atom (Figure 4, Table 1).
The content of compounds with different ratios of mono- and diphenolic aryloxy radicals as
substituents near phosphorus atom can be controlled by varying the initial ratio of
HCP:phenol:bisphenol A, as follows from Figure 4.
Figure 4. MALDI-TOF mass spectra of epoxycyclophosphazenes synthesized according to scheme A
at molar ratios of HCP:phenol:bisphenol A = 1:2:5 (a), 1:2:6 (b), 1:4 :3 (c), and 1:4:4 (d).
750 1000 1250 1500 1750 m/z
1587
1207
636
827
1017 1397
(a)
(b)
(c)
(d)
Polymers 2019, 11, 1914 8 of 16
The quantitative composition of PEOs was determined from the relative intensity of the peaks
on the MALDI-TOF spectra. On the example of the initial molar ratio of HCP:phenol:bisphenol A
equal to 1:3:5 the content of individual compounds in the phosphazene fractions formed by methods
A and B (Figure 3) is compared in Table 1.
More homogeneous is the composition of PEOs obtained according to method A (Figure 3): It
contains four basic compounds with one, two, three, and four epoxy groups (Figure 5а and Table 1).
It is noteworthy that there are no compounds with four and five epoxy groups in the B synthesis
product, but they appear in an amount up to 10 wt % of aryloxyphosphazenes with unsubstituted
OH groups of bisphenol A radicals (peaks with m/z = 960 and 1150 in Figure 5b).
Figure 5. MALDI-TOF mass spectra of epoxycyclophosphazenes synthesized according to schemes A
(a) and B (b) at molar ratios of HCP: phenol: bisphenol A = 1:3:5.
Table 1. The content of the phosphazene fractions of products synthesized by schemes A and B
according to the data of MALDI-TOF-spectrometry. The molar ratio of HCP: phenol: bisphenol A =
1:3:5.
m/z Compound Formula
1
Relative Content of the
Compound (% weight) in
the Product, Obtained via
Method A Method B
636 P
3
N
3
Cl(OPh)
5
12.3 3.6
654 P
3
N
3
Cl
4
(OPh)(OArOGly) - 3.0
768 P
3
N
3
Cl
2
(OPh)
3
(OArOGly) 1.6 7.2
826 P
3
N
3
Cl(OPh)
4
(OArOGly) 14.0 8.6
958 P
3
N
3
Cl(OPh)
3
(OArOGly)(OArOGly′) - 5.0
960 P
3
N
3
Cl(OPh)
3
(OArOH)(OArOGly) - 8.5
1016 P
3
N
3
Cl(OPh)
3
(OArOGly)
2
22.2 9.2
1150 P
3
N
3
Cl(OPh)
2
(OArOH)(OArOGly)
2
- 3.0
1206 P
3
N
3
Cl(OPh)
2
(OArOGly)
3
25.0 19.3
1397 P
3
N
3
Cl(OPh)(OArOGly)
4
16.7 -
1587 P
3
N
3
Cl(OArOGly)
5
1.0 -
Polymers 2019, 11, 1914 9 of 16
Since the organic part of the synthesized epoxy-oligomers could not be evaluated well by
MALDI-TOF, a gas chromatography-mass spectrometry method was used for its analysis (Figures 6
and 7). As follows from Figure 6, the organic epoxide includes several compounds with different
retention times in a chromatographic column.
There is insufficient information in the literature about the analysis of epoxy monomers by gas
chromatography-mass spectrometry. However, given the fact that in order to achieve a satisfactory
separation and to ensure the release of substances from the column, it was necessary to increase the
temperature of the latter up to 280 °C, it can be assumed that fragmentation of the product
components could occur not only as a result of electron impact during ionization, but also as a result
of thermal exposure in a column.
The molecular weights of starting compounds and their derivatives formed as a result thermal
exposure and electron impact estimated from the mass spectra allowed us to propose their chemical
formulas which are listed in Table 2.
As follows from the data of gas chromatography-mass spectrometry, the main compounds in
the organic part of the epoxide synthesized according to both schemes are mono- and diglycidyl
ethers of bisphenol A with a predominant content of the latter (~70% and ~20% for methods A and B,
respectively).
Phenylglycidyl ether (PGE) is present in a small amount only in the product synthesized
according to method B (5%–10%). This indicates the preferential interaction of phenol not with
epichlorohydrin, but with HCP at the initial stages of the process.
The results of the analysis of the electron impact mass spectra of the fractions (Figure 6) isolated
from the chromatographic column allow us to draw the following conclusions (Table 2). The reaction
products eluted from the column as a result of electron impact upon registration of the mass spectra
may undergo some transformations, and the main among them is the transformation of the
isopropylidene group into ethylidene:
As a result, the majority of compounds fixed on mass spectra (peaks 3–8 in Figure 6) have a
molecular weight of 16 units less than expected, which in our opinion is due to the elimination of
CH4.
A similar decomposition of the isopropylidene group with the formation of a double bond was
observed earlier under conditions of the synthesis bisphenol A based
hydroxyaryloxycyclotriphosphazenes at 170 °C [60]:
The main component of the organic fraction of the resulting epoxides is diglycidyl ether of
bisphenol A (DGEBA), the highest yield (~70%) of which is achieved in the synthesis of method A
with the simultaneous feeding of starting reagents.
The compounds' formulas given in the sixth column of Table 2 show the elimination of methane
from the corresponding compounds of the formulas given in the third column of this table.
H3CCH3CH2
+CH
4
Polymers 2019, 11, 1914 10 of 16
Figure 6. Chromatograms of organic epoxides obtained by schemes A (a) and B (b).
Figure 7. Mass spectra of fractions of organic epoxides synthesized according to methods A (Figure
3) and B (Figure 4). Spectrum numbers (1–9) correspond to the numbers of fractions on
chromatograms, Figure 6.
Elution time, min
8 101214161820222426286
(1)
(3)
(4)
(5)
(7)
(8)
(9)
(a)
Elution time, min
8 101214161820222426286
(1)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(2)
(b)
(1) (2) (3) (4) (5)
(6) (7) (8) (9)
50 200 350
0
50
100
118 212
305
319
50 150 250 350 450
0
50
100
118 349
397
411
306
50 150 250 350
0
50
100
119 212 270
361
375
50 150 250 350
0
50
100
119 213 270
325
340
50 190
0
50
100
66
93
149
50 490
0
50
100
119
212
50 170 290
0
50
100
119
213
268
283
100
50 200
0
50
93
185
100
50 220 390
0
50
119
250
265
m/z m/z m/z m/z
m/z m/z m/z m/z
m/z
Polymers 2019, 11, 1914 11 of 16
Table 2. Results of chromatography-mass spectrometric analysis of organic fractions of reaction
products obtained according to schemes A and B. Molar ratio of HCP:phenol:bisphenol A = 1:3:5.
Chromatography Data (Figure 6) Mass-Spectrometric Data (Figure 7)
Peak No
Elution Time (min)
Probable Formula1 of
the Compound at the
Output of the
Column
Calculated
molecular Weight
Content2 of
the
Compound
(wt%)
The Most Likely
Formula
Calculated
molecular Weight
Observed m/z
Values of the
Products
Obtained by
Method
A B
1 7.8 PhOH 93 2.0/7.6 PhOH 93 93 93
7.9 PhOCH2CH=CH2 134 0.5/ - PhOCH2CH=CH2 134 133 none
2 9.3 PhOGly 150 0.9/4.8 PhOGly 150 none 149
3 14.6 HOArOH 228 1.0/6.3 HOAr′OH 212 212 212
4 15.8 HOArOGly 284 0.8/1.9 HOAr′OCH2CH=CH2 252 250 250
5 16.7 HOArOGly 284 2.0/8.2 HOAr′OGly 268 268 + 283 268 + 283
6 18.7 HOAr′OGly′ 306.5 - /4.1 HOAr′OGly′ 306.5 none 305
7 19.8 GlyOArOGly 340 66.8/45.9 GlyOArOGly 325 325 + 340 325 + 340
8 22.4 Gly′OAr′OGly 361 21.2/2.,1 Gly′OAr′OGly 361 361 361
9 26.8 Gly′OAr′OGly′ 397 4.6/ - Gly′OAr′OGly′ 397 397 none
1 Ar’ = ;
2 in the numerator for method A and in the denominator for method B.
It is noteworthy that in the reaction mixture there is practically no monoglycidyl ether of
bisphenol A (MGEBA) with a molecular weight of 284, which corresponds to peaks with an
insignificant intensity in the mass spectra of fraction 5. The main compound in fraction 5 for both
synthetic schemes is the compound with a molecular weight of 268—monoglycidyl ether of 4,4′-
dihydroxydiphenol-1,1-ethenyl.
As for compounds with one chlorohydrin group and the central ethenyl group (peak 8 on the
chromatogram), the lack of a tendency to their dehydrochlorination is apparent due to both the
heterogeneity of the process (KOH is insoluble in the reaction medium) and its insignificant duration
(2 h).
From Figures 6 and 7, it follows that the organic epoxide formed according to Method A (Figure
3) together with epoxycyclophosphazene is more pure—it contains about 90% mono- and diglycidyl
ethers with minimal amounts of initial phenol and bisphenol A.
The properties of different PEO obtained in this work and other PEOs [55–57] that can be
synthesized using the one-pot method are summarized at Table 3. GPC was used (Figure 8) to
determine the relative content of phosphazene and organic fractions (Figure 8a, Table 3). For
comparison, the fractional composition of epoxyphosphazenes based on resorcinol obtained in a
previous study [57] was also determined (Figure 8b, Table 3).
Polymers 2019, 11, 1914 12 of 16
Figure 8. Gel-permeation chromatograms of PEO obtained in this by scheme A (a) with molar ratio
of HCP:phenol:bisphenol A = 1:2:6 (1), 1:4:4 (2), 1:3:5 (3); and of resorcinol-based PEO (b) obtained in
previous work [57] by interaction of HCP with resorcinol with molar ratio of HCP:resorcinol = 1:12
(1), 1:16 (2) and 1:24 (normalized to the height of the main peak).
The phosphazene component contents found from GPC data and calculated from the
phosphorus content value are close to each other and are about 50%–60% for phenol-bisphenol A
based product and are up to 45% for resorcinol-based product. The values of the weight average
molecular weight according to GPC exceed the values obtained by the MALDI-TOF method, which
can indicate both the limited applicability of the standard polystyrene calibration for these systems
and the fact that high molecular weight particles may not be fixed by the MALDI-TOF method due
to their limited volatility. However, the tendency of an increase in the average molecular weight with
an increase in the content of the phosphazene component is confirmed by both GPC and MALDI-
TOF, regardless of the nature of the starting reagents. It should be noted that the molecular weight
values of the low molecular weight organic fraction determined by GPC and GC-MS are close and
correspond to the theoretical assumption that its main component is diphenol diglydyl ether. From
elemental analysis and GPC data it follows that with a comparable content of undesirable chlorine,
the PEO obtained in this work is characterized by the both highest content of phosphorus (up to 5.4%)
and phosphazene component (up to 61%).
The usage of phenol as additional reagent lead to reduced average fuctionality of the product of
1.9–2.2, which is closer to commercial DGEBA or RDGE resins when compared to 2.2–2.5 for the
mixtures based on only bisphenol A or resorcinol. When compared to resorcinol-based phosphazene-
containing epoxy oligomers, the values of viscosity of PEO obtained in this work are slightly higher.
However, the viscosity of PEO obtained in this work at a ratio of HCP:phenol:bisphenol A = 1:4:4 and
industrial bisphenol A based epoxy resin (such as diglycidyl ether of bisphenol A, DGEBA) are
comparable (Table 3). At the same time, the viscosity of PEOs obtained at any ratio of
HCP:phenol:bisphenol A is significantly lower than that of bisphenol A based phosphazene-
containing epoxy resins. Thus, it can be expected that the processing properties of the PEO
synthesized in this work will be as close as possible to ordinary bisphenol A based epoxy resins. As
was shown in [53], such PEO are cured by conventional curing agents to form compositions with
reduced flammability, increased glass transition temperature, flexural strength, and modulus while
other characteristics of these compositions are at the level of commercially available epoxy materials.
(a) (b)
1
2
3
log(M. W.)
1
2
3
log(M. W.)
1.0 1.5 2. 0 2.5 3.0 3.5 4.0 4.5 5.0 1.5 2.0 2.5 3.0 3.5 4. 0 4.5
Polymers 2019, 11, 1914 13 of 16
Table 3. The properties of phosphazene-containing epoxy oligomers1.
Raw Reagents Ratio
Mixture Average
Functionality6
Content (wt%) Average Molecular Weight
Viscosity (Pa∙s)
at the
Temperature of
(°С)
Epoxy Group
P Cl
Phosphazene
Fraction2
Entire Mixture3
Mw /Mn
Organic
Fraction
Phosphazene
Fraction 4
20 40 70
DGEBA
- 2.0 24.6–25.1 - - - 346/- - - 5.83 0.86 0.06
RDGE
- 2.0 34.4–36.4 - - - 243/- - - 1.10 0.11 0.03
HCP:BPA PEOs obtained by interaction of HCP with BPA and ECH [55,56,61]
1:8 2.5 17.1 3.1 2.7 49/49 1487/717 3405 1473 - 220 3
1:12 2.3 20.0 1.8 1.5 36/30 1212/681 1486 - 130 2
1:16 2.2 21.4 1.5 1.3 30/ 25 931/627 1492 440 78 2
HCP:Resorcinol PEOs obtained by interaction of HCP with resorcinol and ECH [57]
1:12 2.4 21.0 4.0 4.4 45/43 2350/380 2203 1054 8.33 6.15 0.36
1:16 2.3 28.6 3.0 2.4 30/32 1260/260 999 2.43 1.94 0.15
1 : 24 2.2 29.6 2.0 1.9 23/21 1130/260 957 1.71 0.45 0.05
HCP:PhOH :BPA PEOs obtained by interaction of HCP with BPA, phenol and ECH7 (this work)
1:2:6 2.2 16.1 4.6 2.2 61/54 4246/288 3405 1211 64.6 13.7 0.8
1:3:5 2.0 15.5 5.0 2.3 61/51 3459/413 1058 58.6 10.6 0.8
1:4:4 1.9 14.7 5.4 2.7 57/47 3248/317 930 9.4 6.0 0.8
1 This table in bold italics shows the literature data; 2 Found by gel-permeation chromatography (GPC)
/ Phosphorus content; 3 By GPC (See supplementary data for GPC curves); 4 By MALDI-TOF; 5 By GC-
MS; 6Calculated from phosphazene fraction content found from GPC, MALDI-TOF data, assuming
that organic component’s functionality is 2; 7By method A.
4. Conclusions
The phosphazene-containing epoxy oligomers obtained in the present work have an epoxy
group content within 15–16 wt %, phosphorus content within 4.6%–5.4% and epoxyphosphazene
component content of 50%–60%. These phosphazene-containing epoxy oligomers may be cured by
conventional curing agents to form materials with reduced flammability, while other characteristics
of these compositions are at the level of commercially available epoxy materials [53]. The viscosity of
obtained epoxyphosphazene-containing resins is comparable to conventional bisphenol A based
epoxies, and is much lower in comparison to similar epoxyphosphazene resins based on bisphenol
A. Thus, the obtained epoxyphosphazene resins may be used as a component of a binder for
composite materials, adhesives, and paints.
Author Contributions: Conceptualization, V.V.K and I.S.S.; experiment design, synthesis and data analysis,
Y.V.B., I.S.S. and A.V.E.; GC-MS analysis, D.A.K; MALDI-TOF analysis, R.S.B.; resources and materials, M.J. and
Y.V.B.; writing—original draft preparation, I.S.S.; writing—review and editing, V.V.K. and S.N.F.
Funding: The research is supported by the Ministry of Science and Higher Education and of The Russian
Federation within the framework of state contract NO. 14.574.21.0171 (unique identifier RFMEFI57417X0171).
Acknowledgments: We thank K.A. Brigadnov for technical assistance and participation in the discussions; A.S.
Tupikov and I.A. Sarychev for GPC analysis of PEO.
Conflicts of Interest: The authors declare no conflict of interest.
Polymers 2019, 11, 1914 14 of 16
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