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FIGURE 1 Preparation of DEP-containing benzoxazine compound and its cured resins PBz.
approaches still encounter some problems in storages of the
curing compositions. With the presence of catalytic agents,
benzoxazines might polymerize in the period of storage. To
overcome this problem, thermally latent catalysts for benzox-
azine compounds have received research attention.25–31 The
thermally latent catalysts do not perform the catalytic effect
at ambient or storage temperatures and could promote the
polymerization reaction at high or processing temperatures.
Kimura et al.25–27 reported the uses of complexes of amine
and sulfonic acid compounds as latent curing agents for ben-
zoxazines. The mixture of the catalyst agent and benzoxazine
monomer was stable below 100 ꢀC and proceeded rapid
polymerization at temperatures above 170 ꢀC. Endo et al.28
reported a similar approach using p-toluenesulfonates as the
thermally latent initiators for benzoxazine polymerization.
The initiators undergo thermally induced dissociation reac-
tion to generate alkyl cations and/or p-toluenesulfonic acid,
which are capable to initiate benzoxazine polymerization.28
The mechanism has also been applied for the preparation of
thermally latent catalysts possessing oligomeric silsequiox-
ane groups.29 Another compounds showing thermally latent
effect on benzoxazine polymerization are resorcinol-based
aromatic urethanes. This kind of urethanes could undergo
thermal dissociation to generate resorcinol and phenyl iso-
cyanate. The isocyanate groups are active species for promo-
tion of the benzoxazine polymerization.30
improves the flame retardancy of the resulting polybenzoxa-
zine by an increase in the limited oxygen index (LOI) from
25 to 33. The results demonstrate that DEP-Bz is a multi-
functional agent for preparation of polybenzoxazine resins.
EXPERIMENTAL
Materials
2-Hydroxybenzaldehyde (Aldrich, 99.99% in purity), aniline
(Acros Chemical Co.), paraformaldehyde (Acros Chemical Co.),
and diethylphosphite (DEP, > 95%, Strem Chemical Co.) were
used as received. Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazi-
nyl)isopropane (BPA-FBz) was prepared in our laboratory
according to the reported method.34 Reagent-grade solvents,
1,4-dioxane, n-hexane, chloroform, and tetrahydrofuran (THF)
were all purchased from TEDIA Chem. Co. and purified with
the standard methods prior to use.
Characterization
FTIR spectra were measured with a Perkin–Elmer Spectrum
One FTIR. 1H and 31P NMR spectra were recorded with a
Varian Unityinova-500 (500 MHz) NMR spectrometer. Solid-
31
€
state P NMR spectra were recorded with a Bruker Avance
III 400 NMR spectrometer (400 MHz). Differential scanning
calorimetry (DSC) was performed with a Thermal Analysis
(TA) DSC-Q100 instrument under nitrogen atmosphere. The
gas flow rate was 40 mL min21. The measurements on the
anhydrous and moistured samples were carried out at a
heating rate of 1 and 10 ꢀC min21, respectively. The mois-
tured DEP-Bz sample was obtained by wetting the sample in
small amount of water and the excess water was blotted
with tissue paper. Thermogravimetric analysis (TGA) was
performed with a TA TGA-Q500 thermo gravimetric analyzer
at a heating rate of 10 ꢀC min21 under nitrogen and air
atmosphere. Dynamic mechanical analysis (DMA) was meas-
ured with a TA DMA-Q800 instrument, using the three-point
bending method with an amplitude of 10 lm, a force of 0.1
N, and a frequency of 1 Hz. The samples for DMA analysis
were in the dimension of 3 3 7 3 0.5 mm3. Elemental anal-
ysis (EA) was conducted with a Heraeus CHNAO rapid ele-
mental analyzer using acetanilide as a standard. Limited
oxygen index (LOI) values were measured on a Stanton
From the molecular design viewpoint, incorporation of the
thermally latent groups to benzoxazine structure results in
reactive type catalysts.31 This kind of agent is not only a cat-
alyst but also a co-monomer for the curing compositions of
benzoxazines. As a result, in this work a diethylphosphonate
(DEP)-containing benzoxazine compound (DEP-Bz, Fig. 1)
has been prepared and used as a reactive type thermally
latent catalyst for benzoxazine resins. The thermally induced
dissociation reaction of the diethylphosphonate group of
DEP-Bz generates phosphonic acid group, which is capable
of promotion of the benzoxazine polymerization. Conversely,
DEP-Bz is an efficient reactive-type flame retardant for poly-
benzoxazine and other polymers, as it has a high phosphorus
content of about 9 wt %.32,33 Addition of 20 wt % of DEP-Bz
to the benzoxazine curing composition is able to significantly
3524
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 3523–3530