H.-B. Zhao et al. / Polymer 55 (2014) 2394e2403
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relationship between cross-linking and flame retardation as well as
2.3. Characterization
anti-dripping. The flame retardance affected by cross-linking has
been investigated in detail. Simultaneously, to highlight the flame-
retardant effect of cross-linking, more functional cross-linkable
monomers (PEPE) were incorporated into the copolyesters via
melt polycondensation, and the flame-retardant properties of those
more highly cross-linkable polymers were investigated [13].
NMR spectra (1H, 400 MHz) were obtained at room temperature
using a Bruker AVANCE AVII 400 NMR instrument, with CF3COOD
as the solvent, and tetramethylsilane as the internal reference.
Fourier transform infrared spectroscopy (FTIR) was performed us-
ing a Nicolet 6700 spectrometer. The intrinsic viscosities of copo-
lyesters were determined with an Ubbelodhe viscometer with a
concentration of 0.5 g/dL at 25 ꢂC in 1:1 (v/v) phenol-1, 1, 2, 2-
tetrachloroethane solution.
2. Experimental
Cross-linking behavior was examined using a NETZSCH simul-
taneous TGA-DSC (449C) with ꢃ0.1 ꢂC temperature error at a
heating rate of 10 ꢂC minꢁ1 in N2.
2.1. Chemicals and substrates
4-Phenylethynylphthalic anhydride (PEPA) was purchased from
Changzhou Sunlight Pharmaceutical Co., Ltd. (Changzhou, China).
Antimony trioxide (Sb2O3, AR), ethylene glycol (EG), dimethyl tere-
phthalate (DMT), methanol, phenol, tetrachloroethane, hexafluoro
isopropanol, zinc acetate (CP) were all manufactured by Chengdu
Chemical Industries Co. (Chengdu, China) and used as received.
Dynamic oscillatory rheological measurements for neat PET and
copolyesters were preformed with a parallel-plate fixture (25 mm
diameter and 1 mm thickness) using an Advanced Dynamic
Rheometric Expansion System (ARES, Bohlin Gemini 200) in an
oscillatory shear mode. Temperature scanning tests at a fixed fre-
quency of 0.1 Hz were in the range from 230 ꢂC, 170 ꢂC, 170 ꢂC, and
260 ꢂC to 330 ꢂC for P(ET-co-P)20, P(ET-co-P)40, P(ET-co-P)80, and
neat PET, respectively. The temperature error is about ꢃ1 ꢂC. Time
scanning tests at a fixed frequency of 0.1 Hz were performed at a
fixed 300 ꢂC for 550 s.
2.2. Sample preparation
Poly(ethylene
terephthalate-co-4-phenylethynylphthalate)
abbreviated as P(ET-co-P)s were synthesized using established
procedures [12]. The preparation of P(ET-co-P)40 is presented here as
a representative example, where the number 40 denotes the molar
parts of PEPE per hundred of DMT (say, PEPE: DMT ¼ 40: 100 in mol).
44.4 g (0.229 mol) DMT, 35.5 g (0.573 mol) EG and 0.0799 g
(4.35 ꢀ 10ꢁ4 mol) zinc acetate were added to a 250-mL bottle
equipped with a DeaneStark trap with a condense. The reaction
systemwas firstlyheated to 180 ꢂC for 3 h. As the reactionproceeded,
methanol was released. After a stoichiometric amount of methanol
was removed, the mixture was poured into an excess of warmwater,
and the precipitate bis(2-hydroxyethyl) terephthalate (BHET) was
obtained. The precipitate was filtrated, washed by water, and dried
overnight. Then, stoichiometric BHET (0.229 mol) was added to a
250 mL polymerization bottle. And ethylene glycol solution of PEPE/
PEPA (0.0916 mol) and antimony trioxide (3.43 ꢀ 10ꢁ4 mol, 0.100 g)
was also added. The mixture was heated to 240 ꢂC for 2 h under a
steady stream of nitrogen. Finally, the pressure in the vessel was
reduced to lower than 60 Pa and the temperature was raised to
270 ꢂC over 0.5 h and maintained for 2e4 h. Other examples were
obtained in a similar way. IR (KBr): 2882e2997 (w), 1722 (s), 2210
Thermal decomposition behavior of the copolyesters was con-
ducted using a NETZSCH TGA (209 F1) with ꢃ0.1 ꢂC temperature
error at a heating rate of 10 ꢂC minꢁ1
.
Thermogravimetric analysis-infrared spectrometry (TG-IR) was
performed using the NETZSCH TGA (209 F1) thermogravimetric
analyzer that was linked to the Nicolet 6700 FTIR spectrophotom-
eter. The heating rate was 10 ꢂC minꢁ1 from 40 ꢂC to 700 ꢂC (ni-
trogen atmosphere, flow rate of 50 ml/min).
Heat release rate (HRR), total heat release (THR), and the
burning residues were measured using an FTT cone calorimeter
according to ISO 5660-1 at a heat flux 50 kW/m2. The samples were
molded to size of 100 ꢀ 100 ꢀ 3 mm3. The molding temperature
was 260 ꢂC, 210 ꢂC, 200 ꢂC, 200 ꢂC, 200 ꢂC for PET, P(ET-co-P)20
,
P(ET-co-P)40, P(ET-co-P)60, P(ET-co-P)80 respectively. The experi-
ment error of cone is about ꢃ10%.
The limiting oxygen index (LOI) values (ꢃ0.5) were performed
using an HC-2C oxygen index measurement (Jiangning, China) with
sheet dimensions of 130 ꢀ 6.5 ꢀ 3.2 mm3 according to ASTM D
2863-97. The samples were compression molded at 10 MPa and
then cut to a size of 130 ꢀ 6.5 ꢀ 3.2 mm3.
(m).1H NMR (CF3COOD,
d): 7.7 (AreH in terephthalic and isophthalic
The UL-94 vertical test was performed using a vertical burning
test instrument (CZF-2) according to ASTM D 3801. The samples
were compression molded at 10 MPa and then cut to a size of
125 ꢀ 12.7 ꢀ 3.2 mm3.
structural units), 6.7w7.4 (AreH in the 4-phenylethynyl structural
unit) and 4.1w4.4 (eCH2O). Basic characteristics of the copolyesters
are listed in Table 1.
Cross-linked specimens for testing were obtained from the copo-
lyester heated in a muffle furnace at 320 ꢂC (ꢃ3 ꢂC) for 60 min in air.
Samples were prepared at different temperatures for XPS tests
in a tube furnace in nitrogen. The samples were heated to each
specific temperature at a heating rate of 10 ꢂC minꢁ1, and the
sample was held isothermal for 10 min at each temperature: 330 ꢂC,
380 ꢂC, 430 ꢂC and 550 ꢂC (ꢃ3 ꢂC), respectively.
X-ray photoelectron spectroscopy (XPS) was carried out with
XSAM 800 spectrometer (Kratos Co., UK), using Al K
a excitation
radiation (1486.6 eV), operated at 12 kV and 15 mA. Bing energies
were referenced to the carbonaceous carbon at 285.0 eV.
The microstructures of the residual char collected after the cone
calorimeter tests wereobservedusingscanningelectron microscopy
(JEOL JSM 5900LV) with an acceleration voltage of 10 kV. A thin layer
of gold was sprayed at the surface prior to SEM observation.
Raman spectroscopy measurement was carried out at room
temperature with LabRAM HR800 laser Raman spectrometer (SPEX
Co., USA) using a 532 nm helium-neon laser line.
Table 1
Basic characteristics, LOI and UL-94 for neat PET and P(ET-co-P) copolyesters.
Samples
PEPE content (mol%)
[h
] (dL gꢁ1
)
LOI
UL-94
Calculated
Testa
3. Results and discussion
PET
e
e
0.68
1.17
1.24
1.49
1.87
22.0
24.5
28.0
29.0
30.0
NR
P(ET-co-P)20
P(ET-co-P)40
P(ET-co-P)60
P(ET-co-P)80
16.7
28.6
37.5
44.4
16.3
28.0
36.7
44.7
V-2
V-2
NR
3.1. Flammability and anti-dripping behaviors
V-0
The flammability of copolyesters has been widely evaluated by
cone calorimeter testing [14,15]. Fig. 1 contains cone calorimetric
a
Testing results were calculated from NMR.