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Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping
Accepted Manuscript Title: Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping Authors: Liyong Zou, Min Zhou, Jiyan Liu, Xueqing Liu, Jia Chen, Quan Hu, Sha Peng PII: DOI: Reference:
S0040-6031(18)30139-4 https://doi.org/10.1016/j.tca.2018.04.016 TCA 77986
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
19-10-2017 13-4-2018 30-4-2018
Please cite this article as: Liyong Zou, Min Zhou, Jiyan Liu, Xueqing Liu, Jia Chen, Quan Hu, Sha Peng, Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping, Thermochimica Acta https://doi.org/10.1016/j.tca.2018.04.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping Liyong Zou, Min Zhou, Jiyan Liu*, Xueqing Liu*, Jia Chen, Quan Hu, Sha peng Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University Flexible Display Materials and Technology Co-innovation Center of Hubei Province 8# Sanjiao Hu Rd, Wuhan, 430056, China
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*Corresponding authors: Prof. Dr. Liu XQ ([email protected]); Prof. Dr. Liu JY ([email protected])
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Highlights
A novel multi-arm flame retardant aluminum salt of phosphinate is
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synthesized.
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Adding multi-arm flame retardant into TPEE can enhance the anti-dripping of
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the TPEE.
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Multi-arm flame retardant combining with MPP shows a synergism effect.
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The anti-dripping and synergism mechanism are proposed.
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novel
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ABSTRACT
multiarm
aluminum
salt
of
pentaerythrityl
ester
of
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tetra(carboxyethylmethylphosphinic acid) (Alcpp) was synthesized and added into thermoplastic polyester elastomer (TPEE). Thermal analysis, evolved gas analysis
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(TGA-FTIR), flammability tests (LOI, UL94), microcombustion calorimeter (MCC) and chemical analyses of residues (SEM-EDX) were used. Alcpp provides TPEE with desired flame retardancy and anti-dripping property and shows a good compatibility with TPEE. TGA-FTIR and SEM-EDX analysis show that Alcpp can depress the heating release and promote the char forming and acts mainly in condensed phase. In addition, Alcpp combining with MPP achieves better fire-resistant performance than
Alcpp alone.When total additives is kept 28 wt%, 25 wt% Alcpp combining 3 wt% MPP exhibits the optimum synergy by increasing char yield and depressing heat release and flammability. The reason for the synergy is due to a strong compact carbonaceous char resulting from reaction between Alcpp and MPP, which effectively prevent the dripping of the TPEE during combustion.
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Keywords: thermoplastic polyester; flame retardancy; metal phosphinate; anti-
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dripping
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1. Introduction
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Thermoplastic elastomers (TPEs), combining the process ability of thermoplastics and
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the physical properties of elastomers, have been widely used in automobile
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component field, electronic & electrical field, home electrical, and medical appliances [1-3]. There are various TPEs including urethane, styrene, olefin, amide, and ester-
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based ones. Among them, thermoplastic polyester elastomer (TPEE) is very important in commercial application due to its excellent flex fatigue and a broad service
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temperature range [4,5]. TPEE is a block copolymer composed of hard segment of
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poly(tetramethylene terephthalate) and soft segment of poly(tetramethylene oxy)terephthalate. Like other polyester-based material, TPEE is inherent flammability and serious dripping during combustion and involves a real hazard to the users for
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these applications. Hence improving its fire resistance to meet the application requirements is very important. Halogen-containing compounds are commercial flame retardants for TPEE [6,7]. With the strict limitation on hazard substance releasing into the environment during the disposal of the electrical and election waste, the industries for these halogen additives based TPEE are under pressure to change to flame
retardants of more environmentally friendly and harmless to health [8-10]. In the exploring halogen-free flame retardants for TPEE, flame retardants of excellent antidripping performance is precious. Metal salts of phosphinate have been developed as an eco-friendly flame retardant in recent years. Aluminum or zinc diethylphosphinate (AlPi or ZnPi) has proved to be
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effective in the fire resistant of polyester [11,12], polyamide [13], and epoxy [14,15]. AlPi alone in TPEE could not get satisfying performance on antidripping. In order to
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improve the antidripping efficiency of AlPi in TPEE, nanoparticles such as
montmorillonite [16], La2O3 [17], MoS2 [18], layered zinc hydroxide [19], melamine
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compounds or organic char forming agent novolac have been tried to combine with
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AlPi [20]. These extra additives show good synergistic effect with AlPi in improving
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the anti-dripping property of TPEE; however, dispersing these tiny nanoparticles into
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matrix homogeneously is very difficult by melt-mixing on industry scale. Additionally, some rare metal-containing additives will bring potential pollution in
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the disposal of the electronic or electric items. The structure of metal salts of phosphinate has a great influence on the fire-resistant
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and other performance of polymers [21-24]. Designing metal salts of phosphinate with an appropriate structure will be a good solution to tune the performance of the
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final materials. In our previous work, we developed a convenient route to synthesize metal salts of carboxylalkyl phosphinate by using commercial available materials 2-
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methy-2,5-dioxo-1,2-oxaphospholane (OP) and isopentyldiol [25]. It endows epoxy with satisfying anti-dropping property and shows good compatibility with matrix. In this
study,
a
multiarm
aluminum
salt
of
pentaerythrityl
ester
of
tetra(carboxyethylmethylphosphinic acid) (Alcpp) was synthesized starting from pentaerythritol and OP, as shown in Figure 1:
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Figure 1. Synthetic route for Alcpp.
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Polyol is often used as a char forming agent in the flame retardant system due to its high carbon content [26-28]. Alcpp contains long and bulk organic pentaerythritol
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ester which is expected to present good compatibility with polyester matrix as well as high carbon forming ability. Considering melamine polyphosphate (MPP) shows a
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good synergy with pentaerythritol and phosphinate salts [29], the performance of Alcpp combining MPP in TPEE was investigated.
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2. Experimental 2.1. Materials Thermoplastic elastomer (TPEE, H6555) with Shore hardness of 55, melting point of 193 oC, density of 1.2 g.cm-3 and melting index of 13 g per 10 min at 230 oC, was provided by Sunshine Plastics Co., Ltd, (Sichuan China). Melaminepolyphosphite
(MPP) was purchased from Nantong Zexi Fine Chemical Ltd. (Jinan, China). 2methy-2, 5-dioxo-1, 2-oxaphospholane (OP) with purity above 98 % was provided by Zhenghao Chemical Ltd. (Wuhan, China). Pentaerythritol (PER), acetone, toluene, ptoluenesulfonic acid and aluminium isopropoxide are chemical reagents and
2.2. Sample Preparation 2.1.1. Synthesis of β-carboxylethylmethylphosphinic acid (CEP)
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purchased from Guoyao Chemical Ltd. (Tianjin,China ) and used as received.
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To a solution of 0.2 mol (26.8 g) OP in 100 mL of acetone, 0.2 mol (3.6 g) water was added dropwise within 30 min at 50 oC. After the addition was completed, the
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mixture was refluxing for 2 h at 60 oC. Then white CEP was precipitated from the solution with the temperature cool down. The solid CEP was filtered off and washed
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thrice, each with 100 mL of acetone. and then dried at 100 oC for 24 h (Yield =93 %).
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2.2.2. Synthesis of pentaerythrityl ester of tetra(carboxyethylmethylphosphinic acid) (CPP)
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A mixture of 0.11 mol CEP (16.7 g), 0.025 mol PER (3.4 g) and p-toluenesulfonic acid 0.01 mol (0.17 g) as catalyst in 100 mL of toluene were added in a round-
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bottom flask. The mixture was stirred at 110 oC for 6 h under refluxing. In order to get the best yield, the byproduct water was removed during the reaction with water
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separator. After the reaction finished, the solution was cooled down to room temperature. White product CPP was collected and washed thrice, each with 100 mL
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of acetone, and then dried at 40 °C under reduced pressure for 24 h. (Yield = 95%) 2.2.3. Synthesis of aluminum salt of pentaerythrityl ester of tetra (carboxyethylmethylphosphinic acid) (Alcpp)
To a solution of 0.15 mol (100.8 g) CPP in 200 mL of isopropanol, a solution of 0.2 mol (41.0 g) aluminum isopropoxide in 50 mL of isopropanol was added. The mixture
was stirred at 85 oC under refluxing for 3 h. The precipitate white Alcpp was filtered and washed thrice, each with 100 mL of alcohol,
and then dried at 100 oC until
constant weight (Yield =97%). 2.2.4. Preparation of samples TPEE pellets were melt-blended with additives using an XK-160 twin-screw internal
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mixture (Jiangsu, China) at 220 oC for 20 min with a screw speed of 120 rpm. The resulting mixture was then hot pressed at 225 oC for 5 min under 10 MPa for the sheet
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of suitable thickness and size for the further measurements. The neat TPEE used as a standard was treated in the same way. The composition of the formulations are shown
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in Table 2. 2.3. Measurements
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The 1H NMR and 31P NMR spectra have been recorded in a solution of DMSO-d6 at
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25 oC with a Mercury VX-300 instrument operating at 400 MHz (Varian, US), using Tetramethylsilane as inner reference and H3PO4 (85 %) as external reference.
KBr powder.
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FTIR was recorded on a Tensor 27 Bruker spectrometer (Bruker, German) with
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X-ray fluorescent spectroscopy (XRF) measurement was done with a ZSX Primus II (Rigaku, Japan) XRF spectrometer with a 35-kV Rh-anode tube.
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Thermogravimetric analysis (TGA) was carried out with TSDT Q600 spectrometer
(TA, USA) thermogravimetric analyzer. Samples of about 10 mg were heated in
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alumina pans from room temperature up to 700 °C at a heating rate of 20 °C min-1 in N2. TGA was coupled with a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, USA).The complete purge gas flow was transferred to the infrared analysis cell through a transfer tube with an inner diameter of 1 mm.The transfer line and the
gas cell were heated to 200 oC. Spectra were obtained in situ during the thermal degradation of the samples. Limiting oxygen index (LOI) measurements were taken using a HC-2-type instrument (Jiangning, China) in accordance with ASTM D2863-77. UL 94 vertical burning tests were conducted on a CZF-3 instrument (Jiangning,
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China) in accordance with ASTM D3801. The dimensions of the sample were 100×13×3 mm3.
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Heat release rate (HRR) and total heat release (THR) were measured in a MCC-2 microcombustion calorimeter (MCC) (Govmark Organization Inc., USA).Samples of about 5-7 mg were heated in alumina pans from 40 oC to 700 oC at a heating rate of 1 C s-1. The flow rate of N2 and O2 was 80 ml/min and 20 ml min-1, respectively.
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Rheological measurements were carried out on a discovery Hybrid Rheometer-3
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rheometer (DHR-3, TA, USA) equipped with a cone-and-plate at 220 oC at a
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frequency from 0.01 to 100 s-1 in nitrogen atmosphere. A parallel-plate geometry (25
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mm diameter) was used at a gap of 1 mm. Measurements were performed under dynamic oscillatory shear.
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Differential scanning calorimetry (DSC) measurements were carried out on a TA DSC-Q20 (TA,USA) at a heating rate of 10 oC min-1 in N2. The flow rate of N2 was
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50 ml min-1.
Scanning electron microscopy (SEM) measurements were conducted using a
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Philips XL-40 instrument.
3. Results and discussion 3.1. Chemical structure of Al(cpp) Figure 2 shows the FTIR spectra of CEP, PER,CPP and Alcpp. The characteristic signals from the FTIR spectra are assigned and summarized in Table 1.The circles
peaks in the Figure 2 and corresponding underlined values are used for identifying the products. In CEP, the OH absorption of –COOH is about 3000 cm-1, which overlaps with the peak of CH3 (2940 cm-1). The broad peaks at 2700-2500 cm-1 and 1720 cm-1 are attributed to COOH. 1220 cm-1 is for C-O of COOH. Other absorption peaks are 1418 cm-1 (CH2), 1309 cm-1 (P-CH3), 1233 cm-1 (P=O), 990 cm-1 (P-O-H) [20]. The
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characteristic peaks for PER are as following: 3300 cm-1 (OH); 2951, 2882 and 1472
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cm-1(-CH2); 1100 cm-1 (C-O in the CH2OH).
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Figure 2. FTIR spectra for CEP,PER,CPP and Alcpp. After CEP was esterified with pentaerythritol (PER), in the spectrum of CPP, the peak
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at 3300 cm-1, peaks between 2700 and 2500 cm-1 disappear; the peak at 1721 cm-1 for C=O moves to 1731 cm-1 which is the characteristic absorption of C=O of ester. Above results indicate that -COOH has taken part in the reaction and ester group was
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formed. Comparing the spectrum of Alcpp with CPP, the peak at 990 cm-1 for P-OH of CPP was not found in Alcpp but a new peak was found at 1085 cm-1, which is attributed to the P-O-Al.
Table 1. Characteristic attribution of FTIR absorption bands. Assignment
Absorption (cm-1)
Assignment
3300
OH in PER
1233
P=O
2950-2882, 1411,1314
CH2 and CH3
1133, 1052
C-O
3000, 2700-2500 and 1720
COOH and C=O
1085
P-OAl
1309
P-CH3
990
P-OH
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Absorption(cm-1)
The structure of CEP is same as reported in the previous study[25]. 1H NMR and
P NMR spectrum of CPP are presented in Figure 3. In the 1H NMR spectrum of
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CPP, the shifts at 1.3-1.5 ppm are for P-CH3, shifts at 1.80 ppm are for P-CH2 and shifts at about 2.2-2.5 ppm are for C-CH2, shifts at around 3.5-4.0 ppm are assigned
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to -CH2 of pentaerythrityl; the shift for P-OH is observed at 9.76 and 7.64 ppm. The
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shift splitting of POH is due to the inter- or intra-molecular hydrogen bonds. In the 31P
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NMR spectrum of CPP, the shift of P is 47.7 ppm. The composition of Alcpp was
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further analysis with XRF: The Al is 3.75 wt% and the P is 12.39 wt%; the atomic
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ratio of P to Al is 2.88:1 (calculated value is 3:1).
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Figure 3. 1H NMR and 31P NMR spectrum for CPP.
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3.2. Combustion Characteristics: LOI, UL94 and MCC MPP was selected to combine with Alcpp and added into TPEE. The total additive is
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kept at 28 wt%, just change the ratio of Alcpp and MPP. The UL94 test and LOI values of the samples are listed in Table 2. Pure TPEE is easily burning with LOI of 15.8% and flammable dripping. Adding 28 wt% Alcpp increases the LOI of TPEE to 25.6% and no dripping during combustion.The MPP alone at 28 wt% in TPEE has no a flame-retardant action
because it burns completely with several flaming drips. When 3 wt% MPP combining 25 wt% Alcpp were added into TPEE, the LOI of the formulation TPEE/Alcpp/MP3 increases to 27.4% and passes the UL 94 V0 ratio without dripping. Further increment of MPP results in the LOI values of the formulations decreasing and failing in UL 94
Table 2. Formulation and flame retardancy of samples MPP (wt%)
Al (wt%)
P (wt%)
Pure TPEE
100
0
0
-
-
TPEE/Alcpp
72
28
0
1.5
4.9
TPEE/Alcpp/MP3
72
25
3
1.3
5.0
TPEE/Alcpp/MP6
72
22
6
1.1
5.1
TPEE/Alcpp/MP10
72
18
10
0.9
TPEE/ MP28
72
0
28
UL 94
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5.2
Dripping
-
15.8
yes
V1
25.6
no
V0
27.4
no
V1
26.3
no
V2
24.5
yes
-
20.4
yes
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5.7
LOI (%)
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Alcpp (wt%)
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TPEE (wt%)
Sample
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V1 rating and dripping during burning.
Digital photos after the LOI test of the selected samples are shown in Figure 4, It
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can be found that the burned top of TPEE/Alcpp is wrapped by a condensed char. After 3 wt% Alcpp is replaced by MPP, in the case of TPEE/Alcpp/MP3, the char is
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swollen. When the MPP is increased to 6 wt%, for the TPEE/Alcpp/MP6, the char
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become looser and brittler than that of TPEE/Alcpp/MP3; the burned end is deformed, implying the sample has a tendency to flowing during burning. In the TPEE/MP28, after the flame is applied, it burns quickly with serious dripping; only a little loose
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ash remains on the top when the flame is put out.
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Figure 4. Pictures of selected samples after LOI tests
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Above results show that Alcpp plays a important role in the antidripping of the
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TPEE. It can form a condensed char to prevent material from flowing. MPP acts in
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the gas phase and makes the char becoming swollen. In the TPEE/Alcpp/MP formulations, with MPP increasing, there is no enough condensed char layer to
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prevent the materials from progressive burning and flowing, so low LOI value and burning rating are the results with dripping occurring. There exists a balance between
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gas and solid action. In this study, combination of 25 wt% Alcpp with 3 wt% MPP
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achieves the best flame retardancy. The heat release rate (HRR) variation of the formulations with time determined by
MCC is presented in Figure 5. Corresponding data including the peak of the heat
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release rate (PHRR), the temperature of PHRR (TPHRR),the heat release capacity (HRC) and the total heat release (THR) from MCC test are listed in Table 3.
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Figure 5. HRR curves for samples.
TPHRR (oC)
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Table 3. MCC data from experiments PHRR (W g-1)
THR (kJ g-1)
HRC (J g-1 oC)
443
148
8.9
114
Pure TPEE
432
795
28.8
609
TPEE/Alcpp
423
496
22.3
482
TPEE/Alcpp/MP3
428
463
19.3
447
TPEE/Alcpp/MP6
422
495
19.9
487
TPEE/Alcpp/MP10
428
538
19.8
509
401
738
25.6
592
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Sample
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Alcpp
TPEE/MP28
Alcpp releases very little heat during heating. The TPHRR is 443 oC with PHRR 148
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W g-1 and THR 8.9 kJ g-1. THR and PHRR value of Alcpp are much lower than that of neat TPEE. Adding 28 wt% Alcpp makes the THR, PHRR and HRC value of TPEE to reduce by 28.5 %, 37.6 % and 20.9%, respectively. When Alcpp combining with MPP in different ratio was added into TPEE, TPEE/Alcpp/MP3 shows the best in depression of heat releasing ; the PHRR, THR and HRC values are reduced by
39.9%, 33.0% and 26.6 % respectively comparing with pure TPEE. However, further increasing MPP to 6 wt% and 10 wt% leads the THR and PHRR and the HRC values to increase again. 3.3. Rheological characterization The melt rheological properties of the TPEE, TPEE/Alcpp, TPEE/Alcpp/MP3 and
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TPEE/MP28 were investigated by the rheological device. The dependence of the complex viscosity (η) and the storage modulus (G′) on the angular frequency are
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presented in Figure 6.
Figure 6. Dependence of complex viscosity (η ) and storage modulus (G′) on angular frequency (ω) for the melting samples at 220 oC
As we know, the rheological performance influences the combustion and processing behavior of composites. For most melt-processed polymeric nanocomposites, adding flame retardants often increase the viscosity and G' of the system. However, in the case of TPEE, the viscosity and G′ is decreased by adding Alcpp or MPP. The reason is that physical cross-linking of the TPEE is destroyed. Compared to the MPP,
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Alcpp based TPEE has lower viscosity η and G. It may be due to multiple-arm
structure of Alcpp, which enlarges the distance of the polymer chains and reducing
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the entanglement network, as reported in literature [30,31].
Figure 7 shows heating subsequent cooling of samples recorded by DSC. The
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melting enthalpy (ΔHm) during heating and crystallization enthalpy (ΔHc) during
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cooling are marked on the corresponding curve. Adding Alcpp or MPP has a little
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influence on the the initial melting temperature of the TPEE but moves crystallization
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of TPEE to a higher temperature. This phenomenon is probably caused by the heterogeneous nucleation effect of Alcpp or MPP. In addition, the ΔHc and ΔHm of
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the TPEE are reduced by introducing Alcpp or MPP , implying that the crystallinity of TPEE is reduced by Alcpp or MPP, which support rheological measurements.
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Usually, the increase of the viscosity and crystallinity would be good for
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preventing the melt dripping and flowing of the polymer, the system with higher viscosity shows better anti-dripping properties. However, the systems containing Alcpp or MPP show lower viscosity and
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crystallinity than pure TPEE, which implies the antidripping properties of Alcpp is not due to the increase of the viscosity.
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Figure 7. DSC curves of heating (a) and subsequent cooling (b) for samples.
3.4. Morphology and EDX analysis of residues Figure 8 illustrates the SEM photos of TPEE/Alcpp,TPEE/Alcpp/MP3 and
TPEE/MP28 before and after LOI test. In the SEM photos of formulation before combustion such as TPEE/Alcpp, Alcpp particles show good dispersion in the matrix
(Figure 8a), almost no naked particles appear on the surface. However in the formulations containing MPP, dispersion of particles is not so homogeneous as in TPEE/Alcpp, especially in TPEE/MP. Big caves and pile-up are observed in the photographs of TPEE/MP28 (Figure 8c) before burning. The caves are the evidence of MPP aggregating. When the force is applied, the materials starts to break along the
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interface of weak adhesion between the particles and matrix. The caves are left after
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the particle aggregates peel off the polymer matrix.
Figure 8. SEM for TPEE/Alcpp (a & a’) , TPEE/Alcpp/MP3(b & b’), TPEE/MP28 (c & c’) before and after burning: a, b, c are before burning; a’,b’, c’ are after burning.
In the SEM photographs of the residues, holes distribute on the
char surface,
indicating volatile gave off during combustion. The holes number and size increases in the order of TPEE/Alcpp/MP3
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Table 4 presents the elements content before and after LOI test of
determined by EDX. The P and Al content from EDX is lower than calculated results
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(as shown in Table 2). EDX is an semi-quantitative analysis for the element content. In addition, it is only can detect the element locating on the surface of the material;
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the Alcpp or MPP embedded within TPEE matrix are difficult to be measured by
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EDX. So it is reasonable that the value of P and Al determined by EDX is lower than
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the calculated results. On the other hand, lower Al and P content outside the sample
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shows Alcpp has a good compatibility with TPEE and most of Alcpp were embedded into matrix.
Before burning(wt%)
C
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91.8
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TPEE/Alcpp
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Table 4. EDX results for formulations before and after burning O
Al
P
P/Al
P (gas)
8.6
0.27
0.38
1.41
-
88.9
2.1
8.2
0.22
0.36
1.64
-
TPEE/MP28
88.5
3.4
7.2
-
0.67
-
-
6.3
0.81
0.58
0.71
0.56
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TPEE/Alcpp/MP3
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After burning(wt%) TPEE/Alcpp
93.3
TPEE/Alcpp/MP3
93.6
2.7
2.8
0.46
0.48
1.04
0.17
TPEE/MP28
89.7
1.4
6.8
-
1.51
-
-
It is supposed that Al stays in the condensed phase during decomposition, so it used as a reference to the element P or N. The ratio of P/Al in the original surface and the
fraction of initial phosphorus released in the gas phase is calculated based on EDX results. In Table 4, The release of P into gas from the TPEE/Alcpp/MP3 is less than that from the TPEE/Alcpp, more P is retained in the residues and takes part in char forming. Hence, char surface from TPEE/Alcpp/MP3 looks firmer than the other two
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formulations. 3.5. TGA-FTIR analysis for the evolved gas
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Figure 9 shows the TGA-DTG curves for TPEE, Alcpp, TPEE/Alcpp,
TPEE/Alcpp/MP3 and TPEE/MP28 in N2 and air atmosphere. Detailed data including temperature at 5% mass loss (T5%), the maximum rate degradation temperature (Tmax),
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the maximum decomposition rate (DTGmax) and char yields at 700 oC are summarized in Table 5.
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In the N2, Alcpp is very thermal stable with T5% 377 oC and Tmax 437 oC. A tiny
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decomposed stage appears at 480 oC after the main stage; 40.3% residues remain at
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700 oC. Pure TPEE starts to lose weight at 362 °C with Tmax 406 °C; only 4.3 % residues remain at 700 °C.Adding 28 wt% Alcpp alone reduces T5% and DTGmax of
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TPEE by 9 oC and 55%, respectively; corresponding char yield at 700 oC is increased by 10.1%. Adding 28 wt% MPP alone results in T5% and DTGmax of TPEE decreasing
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by 7 oC and 9.1%, respectively; char at 700 oC for TPEE/MP28 is increased by 6.6%. Combining 25 wt% Alcpp with 3 wt% MPP makes the DTGmax reducing by
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60.2%, the char yield after about 450 oC is higher than that of formulations containing Alcpp or MPP alone; char yield at 700 oC is increased by 16.7% comparing to pure TPEE, clearly indicating that the combination of Alcpp and MPP results in a greater char formation than the simple superposition of them.
The thermal decomposition of the samples exhibits the similar pattern in the air atmosphere. The T5%, Tmax and char at 700 oC of the samples are lower than that in the
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N2. It indicates that the samples are less thermal stable in the air than in the N2.
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Figure 9.TGA-DTG curves for samples in N2 and Air
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Table 5. TGA-DTG data for formulations in N2 and Air N2
Air
T5% (oC)
Tmax (/oC)
DTGmax (% min-1)
Char at 700oC(%)
T5% (oC)
Tmax (/oC)
DTGmax (% min-1)
Char at 700oC(%)
Alcpp
370
437
28.7
40.3
357
430
23.5
37.7
TPEE
361
406
49.3
4.20
338
396
47.1
0.5
TPEE/Alcpp
352
405
22.3
14.1
336
395
27.9
10.2
TPEE/Alcpp/MP3
346
404
19.6
20.9
335
394
24.4
15.1
TPEE/MP28
354
405
44.8
10.8
335
396
37.9
5.13
A
Sample
TGA analysis supports the results obtained from the LOI (Figure. 4) and the MCC test (Figure 5) as well as SEM-EDX analysis (Figure 8). In the cases of TPEE/Alcpp, TPEE/Alcpp/MP3 and TPEE/MP28, TPEE/Alcpp/MP3 shows a lowest THR and PHRR value in the MCC test; the most compact char are observed in SEM and LOI test, and the highest char yield is obtained in the TGA analysis. TPEE/MP28 shows
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the highest THR and PHRR value in the MCC test; the loosest char residues is observed in the SEM and LOI test and the least char yield is a result in the TGA
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analysis.
Using FTIR monitors the volatile during degradation of the formulations in N2..
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Figure 10 presents the intensity of total volatile released at different time for the
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M
A
N
various formulations.
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Figure 10. Intensity of total volatile with time for samples in N2.
In the Figure 10, the area under the curve presents the total intensity of the peak.
The pure TPEE starts releasing volatile after 16 min, one peak is observed with the peak area 0.3664 and time for the maximum gas releasing rate is 18 min (390 oC). Incorporating 28 wt% Alcpp into TPEE moves the peak to a higher temperature of 400 oC (18.5 min ) with the peak area 0.2458. In TPEE/Alcpp/MP3, time for the
maximum gas releasing rate shifts to 19.3 min with the peak area 0.2430. In TPEE/MP28, the peak locates at 410 oC (19.4 min) with peak area 0.3248. The results from Figure 10 are consistent with the SEM observation: The char of TPEE/Alcpp/MP3 has the least holes number and size because the sample has released the least volatile during the decomposition. TPEE/MPP produces the more
and more holes in the char surface of TPEE/MP28 are results.
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volatile than TPEE/Alcpp or TPEE/Alcpp/MP during the decomposition, so larger
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FTIR spectra of the volatile at the maximum releasing rate and main products at
different time in N2 are presented in Figure 11 and Figure 12 respectively.TPEE
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undergoes ester-linkage breaking in the major degradation step and producing
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compounds containing carboxyl, hydroxyl and unsaturated double bonds; progressing
A
degradation releases CO2, butadiene, hydrocarbon, benzene, acid, ester, and aldehyde [20]. Signals for the volatile coming from TPEE in Figure 11 are as following: 3736
M
cm-1 (water), 3090 cm-1 (benzene), 2973 and 2848 cm-1(hydrocarbon), 2350, 2300 and
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667 cm-1 (carbon dioxide), 1077 cm-1 (ether), 1748 and 1760 cm-1 (carbonyl).The detailed information for the carbonyl compounds is following: 1760 and 2706 cm-1
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(dehyde), 1748 and 1265 cm-1 (ester), 905 cm-1 (butadiene), 734 cm-1 (C-H in aromatic ring). For TPEE/Alcpp, a small peak at 2251 cm-1 is attributed to CO. Peak
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at 1265 cm-1 shifts to 1200 cm-1, as shown in Figure 11, it might be due to overlap of P=O and C-O; but it is difficult to discern the substance coming from Alcpp among
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the volatile.
In Figure 12, the release of hydrocarbon, ether, carbonyl compounds of the
TPEE/Alcpp is reduced greatly except for CO2 comparing with pure TPEE. TPEE/Alcpp begins to release CO2 at about 18 min, the release of CO2 is greater than that from pure TPEE during 18-25 min. Like the TPEE, The decomposition of
the
Alcpp
starts
from
the
breaking
of
ester
linkage,
producing
2-
carboxyethylmethylphosphinic acid (CEP) or aluminum salt. According to the literature[30-32] , elimination of CO2 is the main process in the progressive decomposition. So, high CO2 release rate results from the decomposition of Alcpp. For TPEE/MP, peaks at 2251 cm-1 combining 3502 cm-1 are for HOCN coming
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from MPP, as shown in Figure 11. In Figure 12, a very strong CO2 releasing peak
was observed at 16-20 min. In addition, the release of hydrocarbon (2973 cm-1),
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carbonyl compounds (1748 cm-1) and ether (1074 cm-1) from TPEE/MP are higher than that from TPEE/Alcpp or TPEE/Alcpp/MP3. The decomposition behavior of
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MPP depends on the chemical environment. MPP alone decomposes mainly into
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ammonia and melon formation [35]; MPP in polyamides releases melamine [36],
A
MPP in HIPS only the decomposition to isocyanic acid and CO2 was observed [37].
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ED
M
In TPEE/MPP, isocyanic acid and CO2 are the main products for MPP decomposition.
Figure 11. FTIR spectra of volatile of samples at the maximum release rate in N2.
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Figure 12. Release rate of main products for samples with time in N2. . EDX analysis has shown that partial phosphorous of Alcpp releases into the gas
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phase and some of phosphorous is left in the residues when Alcpp is alone in TPEE. Combining MPP with Alcpp can enhance the char layer and more phosphorus remains
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in the residues. SEM photos indicate the char obtained from TPEE/Alcpp/MP3 is
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denser and has less holes distributing on the surface. TGA-FTIR results further support above investigation. The reason for the synergy between Alcpp and MPP is probably that interaction between MPP and Alcpp forms aluminum polyphosphate in
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a state of condensed char layer to prevent the rest materials from progressive degrading, on the other hand, the non-flammable gas CO2, HNCO monitored by the FTIR is another reason for enhancing the flame retardancy and depressing heat release of the formulation because of their dilution action. A synergy between AlPi and MPP
hasbeen observed in polyamide [20,38] and styrene–ethylene–butylene–styrene based TPEs (TPE-S) [39], poly(methyl methacrylate) (PMMA) as well [40].
4. Conclusions Multiarm aluminum esteralkylphosphinates Alcpp shows a good antidripping property, desired flame retardancy as well as good dispersion in TPEE.
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28 wt% Alcpp alone in TPEE makes TPEE to reach UL94 V1 rating and eliminates dripping. TGA analysis showed that Alcpp catalyzes the decomposition of TPEE at
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the early decomposition stage and promotes char forming at a higher temperature.
Like other phosphorus-containing flame retardants, Alcpp acts both in the gas phase
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through releasing phosphorus-containing compounds and in the condensed phase
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through forming solid char barrier.
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When the total additives is kept at 28 wt%, Alcpp combining with MPP in TPEE
M
shows a obvious synergism effect with respect to the flame retardant, heat release during burning and char formation; the optimum synergy is obtained at a dosage of 25
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wt% Alcpp combining 3 wt% MPP, corresponding formulation TPEE/Alcpp/MP3 passes UL94 V0 ratio; DTGmax is reduced by 60.2% and the char yield at 700 oC is
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increased by 16.7%; PHRR, THR and HRC values are reduced by 39.9%, 33.0% and
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26.6% respectively comparing with the pure TPEE. SEM-EDX as well as FTIR results showed that synergism effect is due to the
reaction between Alcpp and MPP. Products coming from combination of Alcpp and
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MPP stabilize the carbonaceous residues better than that from Alcpp or MPP alone. The condensed char effectively prevent TPEE from the flowing and dripping during combustion.
Acknowledgments This work was financially supported by national key research and development program of China (Grant No. 2016YFB0401500) and national high-tech R&D 863 program of China (Grant No. 2015AA033400) . References
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S0040-6031(18)30139-4 https://doi.org/10.1016/j.tca.2018.04.016 TCA 77986
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
19-10-2017 13-4-2018 30-4-2018
Please cite this article as: Liyong Zou, Min Zhou, Jiyan Liu, Xueqing Liu, Jia Chen, Quan Hu, Sha Peng, Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping, Thermochimica Acta https://doi.org/10.1016/j.tca.2018.04.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Flame-retardant thermoplastic polyester based on multiarm aluminum phosphinate for improving anti-dripping Liyong Zou, Min Zhou, Jiyan Liu*, Xueqing Liu*, Jia Chen, Quan Hu, Sha peng Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University Flexible Display Materials and Technology Co-innovation Center of Hubei Province 8# Sanjiao Hu Rd, Wuhan, 430056, China
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*Corresponding authors: Prof. Dr. Liu XQ ([email protected]); Prof. Dr. Liu JY ([email protected])
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Highlights
A novel multi-arm flame retardant aluminum salt of phosphinate is
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synthesized.
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Adding multi-arm flame retardant into TPEE can enhance the anti-dripping of
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the TPEE.
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Multi-arm flame retardant combining with MPP shows a synergism effect.
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The anti-dripping and synergism mechanism are proposed.
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novel
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ABSTRACT
multiarm
aluminum
salt
of
pentaerythrityl
ester
of
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tetra(carboxyethylmethylphosphinic acid) (Alcpp) was synthesized and added into thermoplastic polyester elastomer (TPEE). Thermal analysis, evolved gas analysis
A
(TGA-FTIR), flammability tests (LOI, UL94), microcombustion calorimeter (MCC) and chemical analyses of residues (SEM-EDX) were used. Alcpp provides TPEE with desired flame retardancy and anti-dripping property and shows a good compatibility with TPEE. TGA-FTIR and SEM-EDX analysis show that Alcpp can depress the heating release and promote the char forming and acts mainly in condensed phase. In addition, Alcpp combining with MPP achieves better fire-resistant performance than
Alcpp alone.When total additives is kept 28 wt%, 25 wt% Alcpp combining 3 wt% MPP exhibits the optimum synergy by increasing char yield and depressing heat release and flammability. The reason for the synergy is due to a strong compact carbonaceous char resulting from reaction between Alcpp and MPP, which effectively prevent the dripping of the TPEE during combustion.
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Keywords: thermoplastic polyester; flame retardancy; metal phosphinate; anti-
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dripping
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1. Introduction
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Thermoplastic elastomers (TPEs), combining the process ability of thermoplastics and
A
the physical properties of elastomers, have been widely used in automobile
M
component field, electronic & electrical field, home electrical, and medical appliances [1-3]. There are various TPEs including urethane, styrene, olefin, amide, and ester-
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based ones. Among them, thermoplastic polyester elastomer (TPEE) is very important in commercial application due to its excellent flex fatigue and a broad service
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temperature range [4,5]. TPEE is a block copolymer composed of hard segment of
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poly(tetramethylene terephthalate) and soft segment of poly(tetramethylene oxy)terephthalate. Like other polyester-based material, TPEE is inherent flammability and serious dripping during combustion and involves a real hazard to the users for
A
these applications. Hence improving its fire resistance to meet the application requirements is very important. Halogen-containing compounds are commercial flame retardants for TPEE [6,7]. With the strict limitation on hazard substance releasing into the environment during the disposal of the electrical and election waste, the industries for these halogen additives based TPEE are under pressure to change to flame
retardants of more environmentally friendly and harmless to health [8-10]. In the exploring halogen-free flame retardants for TPEE, flame retardants of excellent antidripping performance is precious. Metal salts of phosphinate have been developed as an eco-friendly flame retardant in recent years. Aluminum or zinc diethylphosphinate (AlPi or ZnPi) has proved to be
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effective in the fire resistant of polyester [11,12], polyamide [13], and epoxy [14,15]. AlPi alone in TPEE could not get satisfying performance on antidripping. In order to
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improve the antidripping efficiency of AlPi in TPEE, nanoparticles such as
montmorillonite [16], La2O3 [17], MoS2 [18], layered zinc hydroxide [19], melamine
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compounds or organic char forming agent novolac have been tried to combine with
N
AlPi [20]. These extra additives show good synergistic effect with AlPi in improving
A
the anti-dripping property of TPEE; however, dispersing these tiny nanoparticles into
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matrix homogeneously is very difficult by melt-mixing on industry scale. Additionally, some rare metal-containing additives will bring potential pollution in
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the disposal of the electronic or electric items. The structure of metal salts of phosphinate has a great influence on the fire-resistant
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and other performance of polymers [21-24]. Designing metal salts of phosphinate with an appropriate structure will be a good solution to tune the performance of the
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final materials. In our previous work, we developed a convenient route to synthesize metal salts of carboxylalkyl phosphinate by using commercial available materials 2-
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methy-2,5-dioxo-1,2-oxaphospholane (OP) and isopentyldiol [25]. It endows epoxy with satisfying anti-dropping property and shows good compatibility with matrix. In this
study,
a
multiarm
aluminum
salt
of
pentaerythrityl
ester
of
tetra(carboxyethylmethylphosphinic acid) (Alcpp) was synthesized starting from pentaerythritol and OP, as shown in Figure 1:
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Figure 1. Synthetic route for Alcpp.
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Polyol is often used as a char forming agent in the flame retardant system due to its high carbon content [26-28]. Alcpp contains long and bulk organic pentaerythritol
PT
ester which is expected to present good compatibility with polyester matrix as well as high carbon forming ability. Considering melamine polyphosphate (MPP) shows a
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good synergy with pentaerythritol and phosphinate salts [29], the performance of Alcpp combining MPP in TPEE was investigated.
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2. Experimental 2.1. Materials Thermoplastic elastomer (TPEE, H6555) with Shore hardness of 55, melting point of 193 oC, density of 1.2 g.cm-3 and melting index of 13 g per 10 min at 230 oC, was provided by Sunshine Plastics Co., Ltd, (Sichuan China). Melaminepolyphosphite
(MPP) was purchased from Nantong Zexi Fine Chemical Ltd. (Jinan, China). 2methy-2, 5-dioxo-1, 2-oxaphospholane (OP) with purity above 98 % was provided by Zhenghao Chemical Ltd. (Wuhan, China). Pentaerythritol (PER), acetone, toluene, ptoluenesulfonic acid and aluminium isopropoxide are chemical reagents and
2.2. Sample Preparation 2.1.1. Synthesis of β-carboxylethylmethylphosphinic acid (CEP)
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purchased from Guoyao Chemical Ltd. (Tianjin,China ) and used as received.
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To a solution of 0.2 mol (26.8 g) OP in 100 mL of acetone, 0.2 mol (3.6 g) water was added dropwise within 30 min at 50 oC. After the addition was completed, the
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mixture was refluxing for 2 h at 60 oC. Then white CEP was precipitated from the solution with the temperature cool down. The solid CEP was filtered off and washed
A
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thrice, each with 100 mL of acetone. and then dried at 100 oC for 24 h (Yield =93 %).
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2.2.2. Synthesis of pentaerythrityl ester of tetra(carboxyethylmethylphosphinic acid) (CPP)
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A mixture of 0.11 mol CEP (16.7 g), 0.025 mol PER (3.4 g) and p-toluenesulfonic acid 0.01 mol (0.17 g) as catalyst in 100 mL of toluene were added in a round-
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bottom flask. The mixture was stirred at 110 oC for 6 h under refluxing. In order to get the best yield, the byproduct water was removed during the reaction with water
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separator. After the reaction finished, the solution was cooled down to room temperature. White product CPP was collected and washed thrice, each with 100 mL
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of acetone, and then dried at 40 °C under reduced pressure for 24 h. (Yield = 95%) 2.2.3. Synthesis of aluminum salt of pentaerythrityl ester of tetra (carboxyethylmethylphosphinic acid) (Alcpp)
To a solution of 0.15 mol (100.8 g) CPP in 200 mL of isopropanol, a solution of 0.2 mol (41.0 g) aluminum isopropoxide in 50 mL of isopropanol was added. The mixture
was stirred at 85 oC under refluxing for 3 h. The precipitate white Alcpp was filtered and washed thrice, each with 100 mL of alcohol,
and then dried at 100 oC until
constant weight (Yield =97%). 2.2.4. Preparation of samples TPEE pellets were melt-blended with additives using an XK-160 twin-screw internal
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mixture (Jiangsu, China) at 220 oC for 20 min with a screw speed of 120 rpm. The resulting mixture was then hot pressed at 225 oC for 5 min under 10 MPa for the sheet
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of suitable thickness and size for the further measurements. The neat TPEE used as a standard was treated in the same way. The composition of the formulations are shown
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in Table 2. 2.3. Measurements
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The 1H NMR and 31P NMR spectra have been recorded in a solution of DMSO-d6 at
M
A
25 oC with a Mercury VX-300 instrument operating at 400 MHz (Varian, US), using Tetramethylsilane as inner reference and H3PO4 (85 %) as external reference.
KBr powder.
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FTIR was recorded on a Tensor 27 Bruker spectrometer (Bruker, German) with
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X-ray fluorescent spectroscopy (XRF) measurement was done with a ZSX Primus II (Rigaku, Japan) XRF spectrometer with a 35-kV Rh-anode tube.
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Thermogravimetric analysis (TGA) was carried out with TSDT Q600 spectrometer
(TA, USA) thermogravimetric analyzer. Samples of about 10 mg were heated in
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alumina pans from room temperature up to 700 °C at a heating rate of 20 °C min-1 in N2. TGA was coupled with a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, USA).The complete purge gas flow was transferred to the infrared analysis cell through a transfer tube with an inner diameter of 1 mm.The transfer line and the
gas cell were heated to 200 oC. Spectra were obtained in situ during the thermal degradation of the samples. Limiting oxygen index (LOI) measurements were taken using a HC-2-type instrument (Jiangning, China) in accordance with ASTM D2863-77. UL 94 vertical burning tests were conducted on a CZF-3 instrument (Jiangning,
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China) in accordance with ASTM D3801. The dimensions of the sample were 100×13×3 mm3.
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Heat release rate (HRR) and total heat release (THR) were measured in a MCC-2 microcombustion calorimeter (MCC) (Govmark Organization Inc., USA).Samples of about 5-7 mg were heated in alumina pans from 40 oC to 700 oC at a heating rate of 1 C s-1. The flow rate of N2 and O2 was 80 ml/min and 20 ml min-1, respectively.
N
U
o
Rheological measurements were carried out on a discovery Hybrid Rheometer-3
A
rheometer (DHR-3, TA, USA) equipped with a cone-and-plate at 220 oC at a
M
frequency from 0.01 to 100 s-1 in nitrogen atmosphere. A parallel-plate geometry (25
ED
mm diameter) was used at a gap of 1 mm. Measurements were performed under dynamic oscillatory shear.
PT
Differential scanning calorimetry (DSC) measurements were carried out on a TA DSC-Q20 (TA,USA) at a heating rate of 10 oC min-1 in N2. The flow rate of N2 was
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50 ml min-1.
Scanning electron microscopy (SEM) measurements were conducted using a
A
Philips XL-40 instrument.
3. Results and discussion 3.1. Chemical structure of Al(cpp) Figure 2 shows the FTIR spectra of CEP, PER,CPP and Alcpp. The characteristic signals from the FTIR spectra are assigned and summarized in Table 1.The circles
peaks in the Figure 2 and corresponding underlined values are used for identifying the products. In CEP, the OH absorption of –COOH is about 3000 cm-1, which overlaps with the peak of CH3 (2940 cm-1). The broad peaks at 2700-2500 cm-1 and 1720 cm-1 are attributed to COOH. 1220 cm-1 is for C-O of COOH. Other absorption peaks are 1418 cm-1 (CH2), 1309 cm-1 (P-CH3), 1233 cm-1 (P=O), 990 cm-1 (P-O-H) [20]. The
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characteristic peaks for PER are as following: 3300 cm-1 (OH); 2951, 2882 and 1472
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M
A
N
U
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cm-1(-CH2); 1100 cm-1 (C-O in the CH2OH).
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Figure 2. FTIR spectra for CEP,PER,CPP and Alcpp. After CEP was esterified with pentaerythritol (PER), in the spectrum of CPP, the peak
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at 3300 cm-1, peaks between 2700 and 2500 cm-1 disappear; the peak at 1721 cm-1 for C=O moves to 1731 cm-1 which is the characteristic absorption of C=O of ester. Above results indicate that -COOH has taken part in the reaction and ester group was
A
formed. Comparing the spectrum of Alcpp with CPP, the peak at 990 cm-1 for P-OH of CPP was not found in Alcpp but a new peak was found at 1085 cm-1, which is attributed to the P-O-Al.
Table 1. Characteristic attribution of FTIR absorption bands. Assignment
Absorption (cm-1)
Assignment
3300
OH in PER
1233
P=O
2950-2882, 1411,1314
CH2 and CH3
1133, 1052
C-O
3000, 2700-2500 and 1720
COOH and C=O
1085
P-OAl
1309
P-CH3
990
P-OH
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Absorption(cm-1)
The structure of CEP is same as reported in the previous study[25]. 1H NMR and
P NMR spectrum of CPP are presented in Figure 3. In the 1H NMR spectrum of
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31
CPP, the shifts at 1.3-1.5 ppm are for P-CH3, shifts at 1.80 ppm are for P-CH2 and shifts at about 2.2-2.5 ppm are for C-CH2, shifts at around 3.5-4.0 ppm are assigned
U
to -CH2 of pentaerythrityl; the shift for P-OH is observed at 9.76 and 7.64 ppm. The
N
shift splitting of POH is due to the inter- or intra-molecular hydrogen bonds. In the 31P
A
NMR spectrum of CPP, the shift of P is 47.7 ppm. The composition of Alcpp was
M
further analysis with XRF: The Al is 3.75 wt% and the P is 12.39 wt%; the atomic
A
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ED
ratio of P to Al is 2.88:1 (calculated value is 3:1).
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Figure 3. 1H NMR and 31P NMR spectrum for CPP.
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3.2. Combustion Characteristics: LOI, UL94 and MCC MPP was selected to combine with Alcpp and added into TPEE. The total additive is
A
kept at 28 wt%, just change the ratio of Alcpp and MPP. The UL94 test and LOI values of the samples are listed in Table 2. Pure TPEE is easily burning with LOI of 15.8% and flammable dripping. Adding 28 wt% Alcpp increases the LOI of TPEE to 25.6% and no dripping during combustion.The MPP alone at 28 wt% in TPEE has no a flame-retardant action
because it burns completely with several flaming drips. When 3 wt% MPP combining 25 wt% Alcpp were added into TPEE, the LOI of the formulation TPEE/Alcpp/MP3 increases to 27.4% and passes the UL 94 V0 ratio without dripping. Further increment of MPP results in the LOI values of the formulations decreasing and failing in UL 94
Table 2. Formulation and flame retardancy of samples MPP (wt%)
Al (wt%)
P (wt%)
Pure TPEE
100
0
0
-
-
TPEE/Alcpp
72
28
0
1.5
4.9
TPEE/Alcpp/MP3
72
25
3
1.3
5.0
TPEE/Alcpp/MP6
72
22
6
1.1
5.1
TPEE/Alcpp/MP10
72
18
10
0.9
TPEE/ MP28
72
0
28
UL 94
N
5.2
Dripping
-
15.8
yes
V1
25.6
no
V0
27.4
no
V1
26.3
no
V2
24.5
yes
-
20.4
yes
M
A
5.7
LOI (%)
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Alcpp (wt%)
U
TPEE (wt%)
Sample
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V1 rating and dripping during burning.
Digital photos after the LOI test of the selected samples are shown in Figure 4, It
ED
can be found that the burned top of TPEE/Alcpp is wrapped by a condensed char. After 3 wt% Alcpp is replaced by MPP, in the case of TPEE/Alcpp/MP3, the char is
PT
swollen. When the MPP is increased to 6 wt%, for the TPEE/Alcpp/MP6, the char
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become looser and brittler than that of TPEE/Alcpp/MP3; the burned end is deformed, implying the sample has a tendency to flowing during burning. In the TPEE/MP28, after the flame is applied, it burns quickly with serious dripping; only a little loose
A
ash remains on the top when the flame is put out.
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Figure 4. Pictures of selected samples after LOI tests
N
Above results show that Alcpp plays a important role in the antidripping of the
A
TPEE. It can form a condensed char to prevent material from flowing. MPP acts in
M
the gas phase and makes the char becoming swollen. In the TPEE/Alcpp/MP formulations, with MPP increasing, there is no enough condensed char layer to
ED
prevent the materials from progressive burning and flowing, so low LOI value and burning rating are the results with dripping occurring. There exists a balance between
PT
gas and solid action. In this study, combination of 25 wt% Alcpp with 3 wt% MPP
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achieves the best flame retardancy. The heat release rate (HRR) variation of the formulations with time determined by
MCC is presented in Figure 5. Corresponding data including the peak of the heat
A
release rate (PHRR), the temperature of PHRR (TPHRR),the heat release capacity (HRC) and the total heat release (THR) from MCC test are listed in Table 3.
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N
Figure 5. HRR curves for samples.
TPHRR (oC)
A
Table 3. MCC data from experiments PHRR (W g-1)
THR (kJ g-1)
HRC (J g-1 oC)
443
148
8.9
114
Pure TPEE
432
795
28.8
609
TPEE/Alcpp
423
496
22.3
482
TPEE/Alcpp/MP3
428
463
19.3
447
TPEE/Alcpp/MP6
422
495
19.9
487
TPEE/Alcpp/MP10
428
538
19.8
509
401
738
25.6
592
M
Sample
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PT
ED
Alcpp
TPEE/MP28
Alcpp releases very little heat during heating. The TPHRR is 443 oC with PHRR 148
A
W g-1 and THR 8.9 kJ g-1. THR and PHRR value of Alcpp are much lower than that of neat TPEE. Adding 28 wt% Alcpp makes the THR, PHRR and HRC value of TPEE to reduce by 28.5 %, 37.6 % and 20.9%, respectively. When Alcpp combining with MPP in different ratio was added into TPEE, TPEE/Alcpp/MP3 shows the best in depression of heat releasing ; the PHRR, THR and HRC values are reduced by
39.9%, 33.0% and 26.6 % respectively comparing with pure TPEE. However, further increasing MPP to 6 wt% and 10 wt% leads the THR and PHRR and the HRC values to increase again. 3.3. Rheological characterization The melt rheological properties of the TPEE, TPEE/Alcpp, TPEE/Alcpp/MP3 and
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TPEE/MP28 were investigated by the rheological device. The dependence of the complex viscosity (η) and the storage modulus (G′) on the angular frequency are
A
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PT
ED
M
A
N
U
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presented in Figure 6.
Figure 6. Dependence of complex viscosity (η ) and storage modulus (G′) on angular frequency (ω) for the melting samples at 220 oC
As we know, the rheological performance influences the combustion and processing behavior of composites. For most melt-processed polymeric nanocomposites, adding flame retardants often increase the viscosity and G' of the system. However, in the case of TPEE, the viscosity and G′ is decreased by adding Alcpp or MPP. The reason is that physical cross-linking of the TPEE is destroyed. Compared to the MPP,
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Alcpp based TPEE has lower viscosity η and G. It may be due to multiple-arm
structure of Alcpp, which enlarges the distance of the polymer chains and reducing
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the entanglement network, as reported in literature [30,31].
Figure 7 shows heating subsequent cooling of samples recorded by DSC. The
U
melting enthalpy (ΔHm) during heating and crystallization enthalpy (ΔHc) during
N
cooling are marked on the corresponding curve. Adding Alcpp or MPP has a little
A
influence on the the initial melting temperature of the TPEE but moves crystallization
M
of TPEE to a higher temperature. This phenomenon is probably caused by the heterogeneous nucleation effect of Alcpp or MPP. In addition, the ΔHc and ΔHm of
ED
the TPEE are reduced by introducing Alcpp or MPP , implying that the crystallinity of TPEE is reduced by Alcpp or MPP, which support rheological measurements.
PT
Usually, the increase of the viscosity and crystallinity would be good for
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preventing the melt dripping and flowing of the polymer, the system with higher viscosity shows better anti-dripping properties. However, the systems containing Alcpp or MPP show lower viscosity and
A
crystallinity than pure TPEE, which implies the antidripping properties of Alcpp is not due to the increase of the viscosity.
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Figure 7. DSC curves of heating (a) and subsequent cooling (b) for samples.
3.4. Morphology and EDX analysis of residues Figure 8 illustrates the SEM photos of TPEE/Alcpp,TPEE/Alcpp/MP3 and
TPEE/MP28 before and after LOI test. In the SEM photos of formulation before combustion such as TPEE/Alcpp, Alcpp particles show good dispersion in the matrix
(Figure 8a), almost no naked particles appear on the surface. However in the formulations containing MPP, dispersion of particles is not so homogeneous as in TPEE/Alcpp, especially in TPEE/MP. Big caves and pile-up are observed in the photographs of TPEE/MP28 (Figure 8c) before burning. The caves are the evidence of MPP aggregating. When the force is applied, the materials starts to break along the
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interface of weak adhesion between the particles and matrix. The caves are left after
A
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PT
ED
M
A
N
U
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the particle aggregates peel off the polymer matrix.
Figure 8. SEM for TPEE/Alcpp (a & a’) , TPEE/Alcpp/MP3(b & b’), TPEE/MP28 (c & c’) before and after burning: a, b, c are before burning; a’,b’, c’ are after burning.
In the SEM photographs of the residues, holes distribute on the
char surface,
indicating volatile gave off during combustion. The holes number and size increases in the order of TPEE/Alcpp/MP3
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Table 4 presents the elements content before and after LOI test of
determined by EDX. The P and Al content from EDX is lower than calculated results
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(as shown in Table 2). EDX is an semi-quantitative analysis for the element content. In addition, it is only can detect the element locating on the surface of the material;
U
the Alcpp or MPP embedded within TPEE matrix are difficult to be measured by
N
EDX. So it is reasonable that the value of P and Al determined by EDX is lower than
A
the calculated results. On the other hand, lower Al and P content outside the sample
M
shows Alcpp has a good compatibility with TPEE and most of Alcpp were embedded into matrix.
Before burning(wt%)
C
N
91.8
PT
TPEE/Alcpp
ED
Table 4. EDX results for formulations before and after burning O
Al
P
P/Al
P (gas)
8.6
0.27
0.38
1.41
-
88.9
2.1
8.2
0.22
0.36
1.64
-
TPEE/MP28
88.5
3.4
7.2
-
0.67
-
-
6.3
0.81
0.58
0.71
0.56
CC E
TPEE/Alcpp/MP3
A
After burning(wt%) TPEE/Alcpp
93.3
TPEE/Alcpp/MP3
93.6
2.7
2.8
0.46
0.48
1.04
0.17
TPEE/MP28
89.7
1.4
6.8
-
1.51
-
-
It is supposed that Al stays in the condensed phase during decomposition, so it used as a reference to the element P or N. The ratio of P/Al in the original surface and the
fraction of initial phosphorus released in the gas phase is calculated based on EDX results. In Table 4, The release of P into gas from the TPEE/Alcpp/MP3 is less than that from the TPEE/Alcpp, more P is retained in the residues and takes part in char forming. Hence, char surface from TPEE/Alcpp/MP3 looks firmer than the other two
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formulations. 3.5. TGA-FTIR analysis for the evolved gas
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Figure 9 shows the TGA-DTG curves for TPEE, Alcpp, TPEE/Alcpp,
TPEE/Alcpp/MP3 and TPEE/MP28 in N2 and air atmosphere. Detailed data including temperature at 5% mass loss (T5%), the maximum rate degradation temperature (Tmax),
N
U
the maximum decomposition rate (DTGmax) and char yields at 700 oC are summarized in Table 5.
A
In the N2, Alcpp is very thermal stable with T5% 377 oC and Tmax 437 oC. A tiny
M
decomposed stage appears at 480 oC after the main stage; 40.3% residues remain at
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700 oC. Pure TPEE starts to lose weight at 362 °C with Tmax 406 °C; only 4.3 % residues remain at 700 °C.Adding 28 wt% Alcpp alone reduces T5% and DTGmax of
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TPEE by 9 oC and 55%, respectively; corresponding char yield at 700 oC is increased by 10.1%. Adding 28 wt% MPP alone results in T5% and DTGmax of TPEE decreasing
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by 7 oC and 9.1%, respectively; char at 700 oC for TPEE/MP28 is increased by 6.6%. Combining 25 wt% Alcpp with 3 wt% MPP makes the DTGmax reducing by
A
60.2%, the char yield after about 450 oC is higher than that of formulations containing Alcpp or MPP alone; char yield at 700 oC is increased by 16.7% comparing to pure TPEE, clearly indicating that the combination of Alcpp and MPP results in a greater char formation than the simple superposition of them.
The thermal decomposition of the samples exhibits the similar pattern in the air atmosphere. The T5%, Tmax and char at 700 oC of the samples are lower than that in the
ED
M
A
N
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N2. It indicates that the samples are less thermal stable in the air than in the N2.
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Figure 9.TGA-DTG curves for samples in N2 and Air
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Table 5. TGA-DTG data for formulations in N2 and Air N2
Air
T5% (oC)
Tmax (/oC)
DTGmax (% min-1)
Char at 700oC(%)
T5% (oC)
Tmax (/oC)
DTGmax (% min-1)
Char at 700oC(%)
Alcpp
370
437
28.7
40.3
357
430
23.5
37.7
TPEE
361
406
49.3
4.20
338
396
47.1
0.5
TPEE/Alcpp
352
405
22.3
14.1
336
395
27.9
10.2
TPEE/Alcpp/MP3
346
404
19.6
20.9
335
394
24.4
15.1
TPEE/MP28
354
405
44.8
10.8
335
396
37.9
5.13
A
Sample
TGA analysis supports the results obtained from the LOI (Figure. 4) and the MCC test (Figure 5) as well as SEM-EDX analysis (Figure 8). In the cases of TPEE/Alcpp, TPEE/Alcpp/MP3 and TPEE/MP28, TPEE/Alcpp/MP3 shows a lowest THR and PHRR value in the MCC test; the most compact char are observed in SEM and LOI test, and the highest char yield is obtained in the TGA analysis. TPEE/MP28 shows
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the highest THR and PHRR value in the MCC test; the loosest char residues is observed in the SEM and LOI test and the least char yield is a result in the TGA
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analysis.
Using FTIR monitors the volatile during degradation of the formulations in N2..
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Figure 10 presents the intensity of total volatile released at different time for the
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ED
M
A
N
various formulations.
A
Figure 10. Intensity of total volatile with time for samples in N2.
In the Figure 10, the area under the curve presents the total intensity of the peak.
The pure TPEE starts releasing volatile after 16 min, one peak is observed with the peak area 0.3664 and time for the maximum gas releasing rate is 18 min (390 oC). Incorporating 28 wt% Alcpp into TPEE moves the peak to a higher temperature of 400 oC (18.5 min ) with the peak area 0.2458. In TPEE/Alcpp/MP3, time for the
maximum gas releasing rate shifts to 19.3 min with the peak area 0.2430. In TPEE/MP28, the peak locates at 410 oC (19.4 min) with peak area 0.3248. The results from Figure 10 are consistent with the SEM observation: The char of TPEE/Alcpp/MP3 has the least holes number and size because the sample has released the least volatile during the decomposition. TPEE/MPP produces the more
and more holes in the char surface of TPEE/MP28 are results.
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volatile than TPEE/Alcpp or TPEE/Alcpp/MP during the decomposition, so larger
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FTIR spectra of the volatile at the maximum releasing rate and main products at
different time in N2 are presented in Figure 11 and Figure 12 respectively.TPEE
U
undergoes ester-linkage breaking in the major degradation step and producing
N
compounds containing carboxyl, hydroxyl and unsaturated double bonds; progressing
A
degradation releases CO2, butadiene, hydrocarbon, benzene, acid, ester, and aldehyde [20]. Signals for the volatile coming from TPEE in Figure 11 are as following: 3736
M
cm-1 (water), 3090 cm-1 (benzene), 2973 and 2848 cm-1(hydrocarbon), 2350, 2300 and
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667 cm-1 (carbon dioxide), 1077 cm-1 (ether), 1748 and 1760 cm-1 (carbonyl).The detailed information for the carbonyl compounds is following: 1760 and 2706 cm-1
PT
(dehyde), 1748 and 1265 cm-1 (ester), 905 cm-1 (butadiene), 734 cm-1 (C-H in aromatic ring). For TPEE/Alcpp, a small peak at 2251 cm-1 is attributed to CO. Peak
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at 1265 cm-1 shifts to 1200 cm-1, as shown in Figure 11, it might be due to overlap of P=O and C-O; but it is difficult to discern the substance coming from Alcpp among
A
the volatile.
In Figure 12, the release of hydrocarbon, ether, carbonyl compounds of the
TPEE/Alcpp is reduced greatly except for CO2 comparing with pure TPEE. TPEE/Alcpp begins to release CO2 at about 18 min, the release of CO2 is greater than that from pure TPEE during 18-25 min. Like the TPEE, The decomposition of
the
Alcpp
starts
from
the
breaking
of
ester
linkage,
producing
2-
carboxyethylmethylphosphinic acid (CEP) or aluminum salt. According to the literature[30-32] , elimination of CO2 is the main process in the progressive decomposition. So, high CO2 release rate results from the decomposition of Alcpp. For TPEE/MP, peaks at 2251 cm-1 combining 3502 cm-1 are for HOCN coming
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from MPP, as shown in Figure 11. In Figure 12, a very strong CO2 releasing peak
was observed at 16-20 min. In addition, the release of hydrocarbon (2973 cm-1),
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carbonyl compounds (1748 cm-1) and ether (1074 cm-1) from TPEE/MP are higher than that from TPEE/Alcpp or TPEE/Alcpp/MP3. The decomposition behavior of
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MPP depends on the chemical environment. MPP alone decomposes mainly into
N
ammonia and melon formation [35]; MPP in polyamides releases melamine [36],
A
MPP in HIPS only the decomposition to isocyanic acid and CO2 was observed [37].
A
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PT
ED
M
In TPEE/MPP, isocyanic acid and CO2 are the main products for MPP decomposition.
Figure 11. FTIR spectra of volatile of samples at the maximum release rate in N2.
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Figure 12. Release rate of main products for samples with time in N2. . EDX analysis has shown that partial phosphorous of Alcpp releases into the gas
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phase and some of phosphorous is left in the residues when Alcpp is alone in TPEE. Combining MPP with Alcpp can enhance the char layer and more phosphorus remains
PT
in the residues. SEM photos indicate the char obtained from TPEE/Alcpp/MP3 is
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denser and has less holes distributing on the surface. TGA-FTIR results further support above investigation. The reason for the synergy between Alcpp and MPP is probably that interaction between MPP and Alcpp forms aluminum polyphosphate in
A
a state of condensed char layer to prevent the rest materials from progressive degrading, on the other hand, the non-flammable gas CO2, HNCO monitored by the FTIR is another reason for enhancing the flame retardancy and depressing heat release of the formulation because of their dilution action. A synergy between AlPi and MPP
hasbeen observed in polyamide [20,38] and styrene–ethylene–butylene–styrene based TPEs (TPE-S) [39], poly(methyl methacrylate) (PMMA) as well [40].
4. Conclusions Multiarm aluminum esteralkylphosphinates Alcpp shows a good antidripping property, desired flame retardancy as well as good dispersion in TPEE.
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28 wt% Alcpp alone in TPEE makes TPEE to reach UL94 V1 rating and eliminates dripping. TGA analysis showed that Alcpp catalyzes the decomposition of TPEE at
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the early decomposition stage and promotes char forming at a higher temperature.
Like other phosphorus-containing flame retardants, Alcpp acts both in the gas phase
U
through releasing phosphorus-containing compounds and in the condensed phase
N
through forming solid char barrier.
A
When the total additives is kept at 28 wt%, Alcpp combining with MPP in TPEE
M
shows a obvious synergism effect with respect to the flame retardant, heat release during burning and char formation; the optimum synergy is obtained at a dosage of 25
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wt% Alcpp combining 3 wt% MPP, corresponding formulation TPEE/Alcpp/MP3 passes UL94 V0 ratio; DTGmax is reduced by 60.2% and the char yield at 700 oC is
PT
increased by 16.7%; PHRR, THR and HRC values are reduced by 39.9%, 33.0% and
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26.6% respectively comparing with the pure TPEE. SEM-EDX as well as FTIR results showed that synergism effect is due to the
reaction between Alcpp and MPP. Products coming from combination of Alcpp and
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MPP stabilize the carbonaceous residues better than that from Alcpp or MPP alone. The condensed char effectively prevent TPEE from the flowing and dripping during combustion.
Acknowledgments This work was financially supported by national key research and development program of China (Grant No. 2016YFB0401500) and national high-tech R&D 863 program of China (Grant No. 2015AA033400) . References
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forecast 2012–2018, TMR. 2014.http://www.transparencymarket research .com
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[2] K.E. Kear, Developments in thermoplastic elastomers: Recent advances in
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engineering mechanics and their impact on civil engineering practice, Gordana
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BARI.ASCE. (2015) p539-544.
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ED
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[6] S.V. Levchik, E.D. Weil, Flame retardancy of thermoplastic polyesters—a review of the recent literature, Polym. Int. 54 (2005) 11-35.
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[8] E.D. Weil, S.V. Levchik, Flame retardants for thermoplastic polyesters in commercial use or development, Flame Retardants 25 (2009) 105–119. [9] K. Langfeld, A. Wilke, A. Sut, S. Greiser, B. Ulmer, V. Andrievici, P. Limbach, M. Bastian, B. Schartel, Halogen-free fire retardant styrene-ethylene-butylenestyrene-based
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using
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diethylphosphinate-based combinations, J. Fire Sci. 33 (2015) 157-177.
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environmental occurrence, toxicity and analysis, Chemosphere. 88 (2012) 11191153.
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[11] S. Brehme, T. Koeppl, B. Schartel, V. Altstädt, Competition in aluminium
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phosphinate-based halogen-free flame retardancy of poly(butylene terephthalate)
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and its glass-fibre composites,e-Polymers , 14 (2014) :193-208
M
[12] U. Braun, H. Bahr, H. Sturm, B. Schartel, Flame retardancy mechanisms of metal phosphinates and metal phosphinates in combination with melamine cyanurate in
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glass-fiber reinforced poly(1,4-butylene terephthalate): the influence of metal cation, Polym. Adv. Technol. 19 (2008) 680-692.
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[13] U. Braun, B. Schartel, M.A. Fichera, C. Jäger, Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc
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