Renewable polybenzoxazines based in diphenolic acid

Article abstract of DOI:10.1016/j.polymer.2012.02.026

The reaction of mixtures of renewable diphenolic acid (DPA) and its methylesterbenzoxazine derivative (MDP-Bz) has been studied. The DPA was introduced to lower the high temperature needed to complete the curing of the pure benzoxazine. In this way, sampl

Full text of DOI:10.1016/j.polymer.2012.02.026

Polymer 53 (2012) 1617e1623
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Renewable polybenzoxazines based in diphenolic acid
*
Camilo Zúñiga, Gerard Lligadas, Juan C. Ronda , Marina Galià, Virginia Cádiz
Departament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili, Campus Sescelades, Marcel.lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of mixtures of renewable diphenolic acid (DPA) and its methylesterbenzoxazine derivative
(MDP-Bz) has been studied. The DPA was introduced to lower the high temperature needed to complete
the curing of the pure benzoxazine. In this way, samples with different DPA/MDP-Bz ratio (0, 2, 5, 10 and
25% of DPA) were investigated. Moreover, high performance flame retardant thermosetting resins with
phosphorus were prepared through the mixture of MDP-Bz and a DPA-phosphazene derivative (DPA-
PPZ). The curing behavior of these materials was studied by differential scanning calorimetry (DSC).
Finally, the properties of the materials were evaluated by termogravimetric analysis (TGA), dynamic
mechanical analysis (DMTA), tensile measurements, limiting oxygen index (LOI) and UL-94 Burn Test.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 22 December 2011
Received in revised form
15 February 2012
Accepted 17 February 2012
Available online 23 February 2012
Keywords:
Renewable resources
Diphenolic acid
Polybenzoxazines
1. Introduction
monomers with additional functionality, the synthesis of novel
polymeric precursors and the blending with high performance
Over the last decade, an interesting addition cure phenolic resin,
namely benzoxazine, has attracted significant attention of the
research community because of its unique advantages over most of
the known polymers [1]. Polybenzoxazines have become one of
a rare few new polymers commercialized in the past 30 years.
The chemistry of benzoxazines is responsible for a number of
inherent processing advantages, including low melt viscosity, no
harsh catalyst required, and no volatile release during cure and
minimal cure shrinkage. They also have a good thermal stability
that is demonstrated by their high glass transition temperatures,
degradation temperatures, and char yields. A unique combination
of intramolecular and intermolecular hydrogen bonding also
contributes to the properties of polybenzoxazines.
Although, polybenzoxazines offer a variety of advantages, pure
polybenzoxazine-based polymers suffer a number of disadvantages
too. The disadvantages of the typical polybenzoxazines are the high
temperature needed for complete curing and the brittleness of the
cured materials that can sometimes limit their potential applica-
tions. The crosslinking densities of polybenzoxazine homopoly-
mers are believed to be considerably lower than those of ordinary
thermosetting resins. The tightening of the network structure has
been achieved by introducing additional crosslinkable sites. To
properly address these issues and overcome the associated disad-
vantages, the most successful strategies are the preparation of
polymer, filler or fiber [2].
Until now, benzoxazine monomers are synthesized either in
solution or by a melt-state reaction using a combination of
petroleum-based phenolic derivatives, formaldehyde and primary
amines. The scarcity of nonrenewable resources encouraged the
scientific community to develop and commercialize new biobased
products that can alleviate the wide-spread dependence on fossil
fuels and, enhance security, the environment and the economy. The
literature reports only few examples of renewable resources-based
benzoxazine materials. One of them is the low viscosity cardanol-
based benzoxazine that has been described as possible alternative
to the standard petroleum-based ones [3]. Cardanol is a well known
phenol obtained as a renewable organic resource and harmful by-
product of the cashew industry. Cardanol-based benzoxazines
and their potential applications were studied as a matrix for natural
fiber-reinforced composites [4] and as reactive diluents in the
synthesis process of bisphenol A (BPA) based benzoxazines [5].
We have recently focused our attention on the synthesis of new
diphenolic acid (DPA) based benzoxazines and its methyl-
esterbenzoxazine derivative (MDP-Bz) pursuing the strategies to
minimize the environmental impact of new materials [6]. DPA has
a structure similar to BPA but incorporates an additional carboxylic
functionality. It must be pointed out that BPA has recently shown
estrogenic properties and endocrine modulating activity [7], so the
replacement of BPA by DPA offers additional advantages due to
these safety concerns. The research interest resulted enhanced as
DPA is commercially available and much cheaper than BPA and it
can introduce functional carboxyl group into the polymer structure.
* Corresponding author. Tel.: þ34 977 55 80 45; fax: þ34 977 558446.
E-mail address: juancarlos.ronda@urv.cat (J.C. Ronda).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.02.026

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C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
Moreover, DPA is a condensation product of phenol and levulinic
acid. Levulinic acid is believed to be a cheap platform chemical and
can be commercially produced from cellulose-rich biomass (espe-
cially from waster biomass) in large scale [8]. With regard to
phenol, which is currently produced exclusively from petrochem-
ical resources, its bioproduction from glucose has been described
[9,10] and so DPA could be considered as a fully renewable
component.
phenyl-3,4-dihydro-2H-1,3-benzoxazine)] pentanoate (MDP-Bz)
were synthesized according to our reported procedures Scheme 1
[6]. Tetrahydrofuran (THF) was distilled over sodium/benzophe-
none. Other solvents were purified by standard procedures.
Chopped Strand Fiberglass Mat (CSM) (E-glass, 73 g/m2, thickness:
0.5 mm) was kindly supplied by Aismalibar S.A.
2.2. Synthesis of DPA-phosphazene salt (DPA-PPZ) Scheme 2
As mentioned, we have recently developed the benzoxazine
monomer derived from the DPA methyl ester (MDP-Bz). This ben-
zoxazine, in addition to its renewable origin, offers a number of
advantages over other conventional benzoxazine monomers due to
the presence of the methyl ester moieties. Upon curing, extensive
transesterification reactions between the ester and phenolic groups
resulting from the benzoxazine ring opening, take place. This
curing process leads to resins with a superior crosslinking degree
which results in materials with much better thermal and
mechanical performance when compared to similar BPA-based
polybenzoxazine materials [6]. The main drawback of this ben-
zoxazine is the high temperature needed for the uncatalyzed curing
process (260 ꢀC).
In this work, we studied the thermal behavior of mixtures of
renewable DPA and MDP-Bz. The presence of a carboxylic group in
DPA monomer is expected to catalyze the benzoxazine ring
opening lowering the temperature needed to complete the curing
of these benzoxazines [11]. Moreover, the DPA phenolic aromatic
rings react with the benzoxazine affording the complete incorpo-
ration of DPA in the polymeric network and avoiding the problems
originated by the use of non reactive catalysts.
Moreover, this paper focuses on developing high performance
flame retardant thermosetting resins through the mixture of MDP-
Bz and DPA-PPZ. As is well known these new phosphorus-
containing polybenzoxazines impacted the properties of the final
material and improved the flame resistance [12].
The curing behavior of these materials was studied by differ-
ential scanning calorimetry (DSC). Finally, the properties of the
materials were evaluated by termogravimetric analysis (TGA),
dynamic mechanical analysis (DMTA), tensile measurements,
limiting oxygen index (LOI) and UL-94 Burn Test.
In a 100 mL round bottom flask equipped with a pressure-
equalized addition funnel and under inert atmosphere 0.53 g
(0.83 mmol) of P4-t-Bu were dissolved in 20 mL anhydrous THF. A
solution of 0.24 g (0.83 mmol) of DPA in 20 mL anhydrous THF were
added dropwise at room temperature with stirring. The reaction
was further carried out for 12 h at room temperature. The excess of
THF was removed under vacuum to obtain DPA-PPZ as a white solid
with 95% yield.
1H NMR (DMSO-d6,
d ppm): 9.73 (2H, OH), 6.89 (4H, d, AreH),
6.60 (4H, d, AreH), 2.62 (54H, d), 2.11 (2H, t), 2.56 (2H, t), 1.40
(3H, s), 1.26 (9H, s).
2
31P NMR (DMSO-d6,
d
ppm, H3PO4): ꢁ23.5 ppm (q, 1P, JPP
:
48.7 Hz), 12.6 ppm (d, 3P, 2JPP: 48.7 Hz).
2.3. Crosslinking reaction
MDP-Bz was degassed in a glass oven for 30 min at 140 ꢀC. After
this, the appropriate amount of melted (190 ꢀC) DPA was mixed by
hand for 5 min with the MDP-Bz. The used amounts of DPA were 0,
2, 5, 10 and 25 wt.% of the total weight mixture. Polymerization of
the mixtures was carried out into a manual hydraulic press 15 ton
sample pressing (SPECAC) equipped with water cooled heated
platens. Samples were compressed in a rectangular teflon mold
(80 mm ꢂ 60 mm ꢂ 0.5 mm) under a pressure of 2.3 MPa to obtain
films. Samples of 30 mm ꢂ 7 mm ꢂ 0.5 mm were taken for
measurements. As every mixture has different thermal polymeri-
zation behavior, the curing temperatures were adjusted according
to DSC thermogram of the corresponding formulations.
Fiberglass reinforced polybenzoxazines were also prepared. An
appropriate amount of degassed MDP-Bz and DPA were melted at
190 ꢀC and mixed by hand for 5 min. Subsequently, this mixture at
140 ꢀC was manually spread with a brush of each of the sides of
a fiberglass sheet, which was previously cut to desired dimensions.
The resultant materials were placed into a teflon mold (cavity
dimension: 50 mm ꢂ 50 mm ꢂ 0.5 mm) and compressed at 2.3 MPa
using a manual hydraulic press (SPECAC). The used amounts of DPA
and curing temperatures were as above mentioned for non-
reinforced samples. The fiberglass content was c.a 15 wt.% in all
mixtures.
2. Experimental part
2.1. Materials
The following chemicals were obtained from the sources indi-
cated and used as received: ammonium sulphate (Probus), para-
formaldehyde (Probus), 4,40-bis(4-hydroxyphenyl) pentanoic acid
(Aldrich), trimethyl orthoformate (Fluka), p-toluenesulfonic acid
monohydrate (Panreac), 1-terc-butyl-4,4,4-tris(dimethylamino)-
Flame retardant polybenzoxazines were prepared by dissolving
appropriate amounts of DPA, MDP-Bz and DPA-PPZ in acetone/
methanol and subsequently evaporating the solvents at 45 ꢀC for
60 h and finally under vacuum at 140 ꢀC. The weight ratios of MDP-
Bz, DPA and DPA-PPZ were 82.5/10/7.5 (1.7% P) and 80/10/10 (2.3%
2,2-[tris(dimethylamino)phosphoranylideneamino]-2
nadi(phosphazene) (P4-t-Bu) solution 1 M in hexane (Aldrich).
1,3,5-Triphenylhexahydro-1,3,5-triazine, methyl
4,40-bis(4-
hydroxy-phenyl) pentanoate (MDP) and methyl 4,40-bis-[6-(3-
l l
5,4 5-cate-
Scheme 1. Synthesis of MDP and MDP-Bz.

C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
1619
Scheme 2. Synthesis of DPA-phosphazene derivative (DPA-PPZ).
P). Resin bars were obtained by placing the compounds into a steel
mold (cavity dimension: 70 mm ꢂ 6 mm ꢂ 3 mm) and compressing
(SPECAC equipment) at 9.2 MPa. The curing temperatures were
adjusted according to each DSC thermogram.
bar was 10 mm. The sample was subjected to two 10 s ignitions
with a calibrated methane-fueled flame in a controlled-size unit
that was free of passing air currents. After the first ignition, the
flame was removed and the time the polymer took to self-
extinguish was recorded. The second ignition was then performed
on the same sample and the self-extinguishing time/dripping
characteristics were recorded. If the sample self-extinguished in
less than 10 s with no dripping, we considered it to be a V-
0 material, which is an industry standard for flame retardancy.
2.4. Extraction and pyrolysis experiments
About 200 mg of each MDP-Bz/DPA resin were extracted with
boiling THF during 12 h. The soluble fractions were negligible in all
cases except for MDP-Bz-25/wt.% DPA in which 13 mg (6.3%) were
obtained starting from 210 mg of sample. This fraction was
analyzed by 1H and 13C NMR.
3. Results and discussion
In our previous work, we reported a facile synthesis of meth-
yldiphenolic acid-based benzoxazine (MDP-Bz) by the reaction of
diphenolic acid methyl ester (MDP), 1,3,5-triphenylhexahydro-
1,3,5-triazine and formaldehyde [6]. DPA is a commercially avail-
able condensation product of phenol and levulinic acid, containing
the functional carboxyl group (Scheme 1).
240 mg of 80 wt.% MDP-Bz/10 wt.% DPA/10 wt.% DPA-PPZ were
pyrolized in a quartz tube at 350 ꢀC under vacuum for 30 min. The
condensates, 57.6 mg (24%) were collected in a frozen trap and
analyzed by 1H and 31P NMR.
2.5. Instrumentation
It is well known that 1,3-benzoxazines exhibit exotherm ring
opening reaction around 200e250 ꢀC, which can be monitored by
DSC. MDP-Bz showed an exothermic peak with an onset at 220 ꢀC,
a maximum temperature at 261 ꢀC and 129 kJ as exothermic energy
(Fig. 1a). As it has been mentioned, these high temperatures
required for curing is one of the main drawbacks of poly-
benzoxazines preparation and can be addressed by using acidic
catalyst, such as carboxylic acids, to catalyze the thermal ring
opening of the benzoxazine group.
1H (400 MHz), 13C (100.5 MHz) and 31P (161.9 MHz) NMR spectra
were obtained using a Varian Gemini 400 spectrometer with
Fourier transform and DMSO-d6 as solvent.
Calorimetric studies were carried out on a Mettler DSC821e
thermal analyzer using N2 as a purge gas (100 ml/min) at scanning
rate of 20 ꢀC/min. Thermal stability studies were carried out on
a Mettler TGA/SDTA851e/LF/1100 with N2 or air as a purge gas at
scan rates of 10 ꢀC/min.
In this work we used DPA to low the MDP-Bz curing tempera-
ture. In this way, we studied the thermal behavior of mixtures of
MDP-Bz with different amounts of DPA. Fig. 1a shows the DSC
Mechanical properties were measured by triplicate using
a dynamic mechanical thermal analysis (DMTA) apparatus (TA
DMA 2928). Reinforced and nonreinforced specimens
(10 mm ꢂ 7 mm ꢂ 0.45 mm) were tested in a three point bending
configuration. In the case of phosphorus-containing samples, the
dimensions of the bars were 10 mm ꢂ 6 mm ꢂ 2.5 mm. The thermal
transitions were studied in the 30e350 ꢀC range at a heating rate of
3
ꢀC/min and at a fixed frequency of 1 Hz. Tensile testing was
performed using an Instron machine 3366 with a 100 N load cell at
crosshead speed of 5 mm/min and at 25 ꢀC. The dimensions of the
test specimens were 50 mm ꢂ 4 mm ꢂ 0.35 mm. The tensile
properties were determined from the average of at least five
samples.
LOI measurements were performed on a Stanton Redcroft FTA
flammability unit provided with an Oxygen Analyzer. Samples
(70 mm ꢂ 6 mm ꢂ 3 mm) were prepared by compression molding
at 9.2 MPa and tested by triplicate.
2.6. General procedure for modified UL-94 burn test
Three sample bars (0.5 mm ꢂ 10 mm ꢂ 40 mm) were used for
this test. The Bunsen burner flame height was 25 mm and the
height from the top of the Bunsen burner to the bottom of the test
Fig. 1. DSC plots of MDP-Bz with different a) DPA and b) DPA-PPZ contents.

1620
C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
Table 1
DSC data and curing conditions of mixtures of MDP-Bz with DPA, MDP and DPA-PPZ.
Composition
Tonset (ꢀC)
Tmax (ꢀC)
DHo (kJ/mol)
Curing conditions
MDP-Bz
MDP-Bz + 2% DPA
MDP-Bz + 2% MDP
MDP-Bz + 5% DPA
220
180
173
165
158
150
163
135
150
140
140
260
239
246
228
240
209
234
179
220
217
216
122
147
145
142
143
134
139
108
115
142
117
190 ꢀC (2 h), 215 ꢀC (2 h), 235 ꢀC (8 h), 260 ꢀC (2 h)
170 ꢀC (2 h), 190 ꢀC (2 h), 225 ꢀC (8 h), 240 ꢀC (2 h)
160 ꢀC (2 h), 180 ꢀC (2 h), 210 ꢀC (8 h), 230 ꢀC (2 h)
150 ꢀC (2 h), 165 ꢀC (2 h), 195 ꢀC (8 h), 210 ꢀC (2 h)
135 ꢀC (2 h), 150 ꢀC (2 h), 170 ꢀC (8 h), 180 ꢀC (2 h)
MDP-Bz + 5% MDP
MDP-Bz + 10% DPA
MDP-Bz + 10% MDP
MDP-Bz + 25% DPA
MDP-Bz + 25% MDP
MDP-Bz + 10%DPA + 7.5% DPA-PPZ
MDP-Bz + 10%DPA + 10% DPA-PPZ
140 ꢀC (2 h), 160 ꢀC (2 h), 200 ꢀC (8 h), 215 ꢀC (2 h)
140 ꢀC (2 h), 150 ꢀC (2 h), 195 ꢀC (8 h), 215 ꢀC (2 h)
thermograms of the mixtures of MDP-Bz and 2, 5, 10, and 25 wt.% of
DPA. A shifting of the polymerization exotherm to lower temper-
atures can be seen as DPA amount increases. A similar behavior was
observed in the onset temperatures of the exothermic peaks.
Table 1 summarizes all these values. As can be seen, when 2 wt.%
DPA is added to MDP-Bz the enthalpy value undergoes an increase,
suggesting a more complex polymerization mechanism in which
carboxylic and phenolic groups are involved. For higher DPA
amounts enthalpy values decrease progressively due to the lower
content of the benzoxazine monomer in the mixture. As it has been
reported previously, phenolic hydroxyl compounds produce an
outstanding decrease in the benzoxazines polymerization
temperature, acting hydroxyl groups as acid cationic initiators in
the ring opening polymerization [13,14]. As DPA contains both
phenolic hydroxyl and carboxyl groups a test was carried out with
MDP, that only contains hydroxyl groups, to prove the higher
catalytic effect of carboxylic acid group in DPA. So, mixtures of 2, 5,
10, and 25 wt.% of MDP-Bz/MDP were prepared and their thermal
behavior was analyzed by DSC. Data were summarized in Table 1. A
notable difference in the maximum temperatures of curing was
observed. The MDP-Bz/25 wt.% DPA compared to MDP-Bz/25 wt.%
MDP shows a decrease of 40 ꢀC, due to the presence of catalytic
carboxylic acid.
The curing conditions, collected in Table 1, were selected
according the DSC traces. Temperature cycles starting just below
the onset temperature of the exotherm (Tonset) and up to the
maximum temperature of the exotherm (Tmax) were used. In all
cases DSC measurements were carried out showing the
completeness of the curing process. To evaluate the crosslinked
fraction, samples were extracted with THF showing no noticeable
soluble materials with the exception of the MDP-Bz/25 wt.% DPA
which contained a 6.3% of soluble fraction.
The dynamo-mechanical, mechanical and thermogravimetric
properties of the polybenzoxazines were investigated. According to
DSC data, the curing of the benzoxazine resins was carried out in
a
mold under compression by heating samples at different
temperatures and times collected in Table 1. Due to the brittleness
of some samples fiberglass reinforced polybenzoxazines were also
Fig. 2. Storage modulus and Tan
d plots of a) polybenzoxazines with different DPA and DPA-PPZ contents and b) reinforced polybenzoxazines with different DPA content.

C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
1621
prepared to improve their mechanical properties. The dynamic
mechanical behavior of the cured benzoxazine resins was obtained
as a function of the temperature beginning in the glassy state of
each composition to the rubbery plateau of each material. The plots
are shown in Fig. 2.
with fiberglass respectively, and Table 3 summarizes the ther-
mogravimetric data.
Both in nitrogen and air the decomposition temperature
decrease with DPA content. In nitrogen, a maximum weight loss
rate, at approximately 420 ꢀC appears in all the samples except
when 25 wt.% DPA is added. This behavior indicates that there is
a single decomposition mechanism which is similar for all resins.
However, for the resin from MDP-Bz/25 wt.% DPA, two maximum
weight loss rates appear at 344 ꢀC and 417 ꢀC. The first one could be
related to DPA polyester decomposition. The analysis of the soluble
extract by 1H and 13C NMR indicates as main component the
hyperbranched polyester produced by self condensation of DPA
molecules that seems to occur when a large excess of DPA is used
[15]. No significant differences of behavior are observed with the
reinforced samples. In air two stages of weight loss appear in all the
samples except when 25 wt.% DPA is added that another stage, at
lower temperature, is also observed as explained above. Tempera-
tures of 10% weight loss decrease with the DPA content being
especially significant the decrease for the MDP-Bz/25 wt.% DPA.
Char yields at 800 ꢀC are quite similar for all the resins, although as
expected the reinforced counterparts showed higher values of char
yield.
The preparation of reinforced benzoxazines with fiberglass was
necessary for the mechanical characterization of these resins. Fig. 4
shows the stress-strain curves for the different reinforced poly-
benzoxazines and Table 2 summarizes their mechanical properties.
Young’s modulus and tensile strength at yield elongation decrease
as the DPA increases. As it is known, molecular flexibility, inter-
molecular packing, molecular architecture and molecular weight
between crosslinks affect the tensile strength and elongation at
break [16]. As above noted an increase in DPA percentage in MDP-
Bz mixtures decreases crosslinking density, thus generating a fall of
the tensile strength and Young’s modulus.
There are a number of important material parameters that can
be derived from the dynamic mechanical data. The storage
modulus, E0, of a solid sample at room temperature provides
a measure of material stiffness under shear deformation, and is
plotted for the mixtures as a function of temperature. For all the
thermosets polymerized from MDP-Bz and DPA (Fig. 2a), the
storage modulus maintains approximately the same value for
a wide temperature range up to 220 ꢀC with the exception of the
sample with 25 wt.% DPA. A same behavior is observed in those
reinforced with fiberglass (Fig. 2b). Moreover, as expected, it is also
observed that when samples are reinforced the storage modulus is
greater. The glass transition temperatures (Tg’s) of the crosslinked
materials can be detected as the maximum of the loss modulus (E00),
which corresponds to the initial drop from the glassy state into the
transition, or as the
a relaxation peak of the loss factor, tan d, which
corresponds to the transition midpoint of the log of the E0 curve.
Table 2 shows the Tg values of both measurements.
As expected, Tg as tan
is also collected the Tg value, as the half-height of the heat capacity
increase (½ Cp), determined by DSC, which is in good accordance
with DMTA values. The analysis of the height of the tan peak
values indicates in general a lower crosslinking density for the
mixtures compared to MDP-Bz. The height of tan peak increases
d
peak is higher than E00 peak. In this table
D
d
d
with the content of DPA in the mixtures, indicating that the
crosslinking density decreases due to the decrease of the MDP-Bz in
the mixture. Polybenzoxazines reinforced with fiberglass show
a different behavior being the height of tan
d peaks lower than
nonreinforced ones. Moreover, the higher the DPA content the
lower the Tg’s. This could be explained again by the lower
percentage of MDP-Bz and consequently a decrease in the cross-
linking density which takes place through transesterification
reactions between the ester groups and the phenolic OHs, either
resulted from ring opening of benzoxazine or from DPA [6]. The
One of the main concerns about the use of polymeric materials
is their flammability. For this reason fire retardants such as phos-
phorus or silicon are usually introduced into polybenzoxazines to
enhance their flame retardancy [12]. Thus, we studied the possi-
bility of inferring flame retardancy to MDP-Bz by mixing with
a phosphazene diphenolic acid derivative (DPA-PPZ). The synthesis
of this compound was carried out by mixing DPA and P4-t-Bu in
THF. Its 31P NMR spectrum was consistent with the protonated form
tan
d peak width at half-height is quite narrow and similar for all
samples suggesting a similar homogeneity.
TGA data under nitrogen and air atmospheres of these materials
were determined and analyzed. Fig. 3a and b show the weight loss
with the temperature for the different compositions without and
2
of P4-t-Bu with chemical shifts at ꢁ23.5 ppm (q, 1P, JPP: 48,7 Hz)
and 12.6 ppm (d, 3P, 2JPP: 48.7 Hz) [17].
Two mixtures of DPA-PPZ/DPA/MDP-Bz with weight ratios of
82.5/10/7.5 and 80/10/10 were prepared to give resins with 1.7%
and 2.3% of phosphorus content respectively. The DSC plots of MDP-
Bz with DPA-PPZ are shown in (Fig. 1b). A similar behavior in both
the onset temperatures and the enthalpies of the exothermic peaks
was observed in both exotherms and that corresponding to MDP-
Bz/10 wt.% DPA (Fig. 1a). The presence of DPA-PPZ does not seem to
have any significant influence.
Table 2
Thermal and mechanical properties of polybenzoxazines.
Composition
Tg (ꢀC)
Mechanical properties
½
D
Cp E"max Tan
Young
Tensile
strengthb
(MPa)
dmax modulus
(MPa)
To examine the effect of phosphorus content on thermal
stability and the decomposition behavior, TGA data, under nitrogen
and air atmospheres, of phosphazene-based polymers were
determined and analyzed. MDP-Bz/10 wt.% DPA was also consid-
ered for comparison. Fig. 5 shows the weight loss with the
temperature and the similar decomposition behavior either in
nitrogen or in air, for all resins.
Table 3 summarizes the thermogravimetric data. Decomposition
temperatures (T10%) for the phosphorus-containing resins are lower
than for the phosphorus free-resin due to the decomposition of
PeN bonds which have lower thermal stability than CeC bonds.
Because of its lower atomic bond energy (279.7 kJ/mol) [18], the
PeN bond will be destroyed before the CeC bond (347 kJ/mol) in
the main macromolecular chain. In nitrogen (Fig. 5a) the three
MDP-Bz
301
298
272
274
249
255
224
223
159
163
282
296
274
279
260
256
232
234
166
152
163
303
e
e
MDP-Bza
301 1484.6 ꢃ 58 41.6 ꢃ 2.4
288
293 1146.6 ꢃ 28 25.2 ꢃ 3.2
269
MDP-Bz þ 2% DPA
MDP-Bz þ 2% DPAa
MDP-Bz þ 5% DPA
MDP-Bz þ 5% DPAa
MDP-Bz þ 10% DPA
MDP-Bz þ 10% DPAa
MDP-Bz þ 25% DPA
MDP-Bz þ 25% DPAa
e
e
e
e
271 1074.8 ꢃ 88 23.8 ꢃ 4.7
244
247
190
171
191
e
e
967.3 ꢃ 98 17.2 ꢃ 3.5
e
e
996.0 ꢃ 85 21.5 ꢃ 4.3
MDP-Bz þ 10% DPA þ 7.5% 196
e
e
DPA-PPZ
MDP-Bz þ 10% DPA þ 10% 208
DPA-PPZ
176
210
e
e
a
Fiberglass reinforced benzoxazine mixtures (15 wt.%).
Tensile strength at yield elongation.
b

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C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
Fig. 3. TGA plots of a) polybenzoxazines with different DPA content and b) reinforced polybenzoxazines with different DPA content, in nitrogen and air.
samples exhibit a single major break in their decomposition while
in air two step break in the decomposition curves suggest a more
complex decomposition pathway. The decomposition process
observed at temperatures above 600 ꢀC came from the oxidation of
the formed char. In nitrogen, it is also noticed that the char yields of
the phosphorus-containing resins are slightly higher than that of
the free-phosphorus one. This result indicates that the incorpora-
tion of phosphazene into the resins can increase the char yield
because of the formation of phosphorus-rich char during the
decomposition of the phosphazene moiety. However, in air the char
yield is near 0% for all resins indicating that the flame retardant
mechanism of the phosphazene-based polymers must be also a gas
phase mechanism [19]. The phosphazene moieties can release non-
flammable gases such as CO2, NH3 and N2 during burning to dilute
the hot atmosphere and cool the pyrolysis zone at the combustion
surface. These non-flammable gases can cut off the supply of
oxygen. To unravel whether if phosphorus compounds are released
to gas phase during the early stage in the decomposition process,
we performed a pyrolysis experiment at 350 ꢀC. The 31P NMR
analysis of the collected volatile products showed the total absence
of phosphorus species. 1H NMR analysis shows that the degradation
products are mainly aromatic compounds with methyl amino
moieties.
The LOI values, which can be taken as an indicator to evaluate
the polymer’s flame retardancy of the resins were measured and
shown in Table 3. As can be seen the presence of phosphorus
slightly increases the LOI values even when the phosphorus content
Table 3
Thermogravimetric and LOI values of polybenzoxazines.
Composition
TGA (O2)
TGA (N2)
LOI
T10% Tmax (ꢀC)c
Y800 T10% Tmax
Y800
(ꢀC)b
(%)d (ꢀC)b (ꢀC)c
(%)d
MDP-Bz
386 418, 693
380 427, 603
379 409, 620
375 427, 618
379 410, 641
369 428, 616
366 416, 600
362 427, 660
329 332, 425, 651
0
375 417
12 369 409
374 421
11 366 416
371 426
11 359 421
32 27.6
MDP-Bza
40
29
39
30
36
e
e
e
e
e
MDP-Bz þ 2% DPA
MDP-Bz þ 2% DPAa
MDP-Bz þ 5% DPA
MDP-Bz þ 5% DPAa
MDP-Bz þ 10% DPA
MDP-Bz þ 10% DPAa
MDP-Bz þ 25% DPA
MDP-Bz þ 25% DPAa
MDP-Bz þ 10%
0
0
0
8
0
352 439
355 426
319 344, 417 26
30 25.8
36
e
e
e
331 330, 419, 603 11 323 346, 415 35
356 369, 625
348 368, 643
1
1
348 377
342 377
36 27.5
DPA þ 7.5% DPA-PPZ
MDP-Bz þ 10%
36 28.4
DPA þ 10% DPA-PPZ
a
Fiberglass reinforced benzoxazine mixtures.
Temperature of 10% weight loss.
Temperatures of maximum weight loss rate.
Char yield at 800 ꢀC.
b
c
d
Fig. 4. Stressestrain plots of reinforced polybenzoxazines with different DPA content.

C. Zúñiga et al. / Polymer 53 (2012) 1617e1623
1623
and its methylesterbenzoxazine derivative (MDP-Bz) with low
polymerization temperatures. A shifting of the polymerization
exotherm to lower temperatures can be seen as DPA amounts
increase being this low temperature attributable to the presence of
carboxylic acid. Fiberglass reinforced samples had to be prepared to
achieve good tensile properties. High performance flame retardant
thermosetting resins through the mixture of MDP-Bz and DPA-PPZ
were obtained. These phosphorus-containing materials increased
their LOI values and showed a V-0 grade according to the UL-94
test, even with low phosphorus content, corresponding to their
excellent flame retardant properties.
Acknowledgments
The authors express their thanks to CICYT (Comisión Inter-
ministerial de Ciencia y Tecnología) (MAT2008-01412) for financial
support for this work and to Dr. Miguel Angel López-Manchado for
his help in the mechanical properties measurements.
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is low (1.7% and 2.3%) according to the expected behavior for fire-
retarded phosphorus-containing materials.
We also tested the flame retardancy of the synthesized material
with the UL-94 flame test, in which the sample was suspended
above cotton [20]. According to the UL-94 test, the samples with
a phosphorus content produced a V-0 material, as can is to be ex-
pected. These materials, therefore, have outstanding fire-
retardancy properties.
4. Conclusions
[19] Liu R, Wang X. Polym Deg Stab 2009;94:617e24.
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materials for parts in devices and appliances. 4th ed. Research Triangle Park,
NY: Underwriters Laboratories, Inc; 1991.
We have synthesized renewable thermosetting poly-
benzoxazines from mixtures of renewable diphenolic acid (DPA)