Cross-linking of a novel reactive polymeric intumescent flame retardant to ABS copolymer and its flame retardancy properties

Cross-linking of a novel reactive polymeric intumescent flame retardant to ABS copolymer and its flame retardancy properties

Polymer Degradation and Stability 97 (2012) 1596e1605 Contents lists available at SciVerse ScienceDirect Polymer Degradation and Stability journal h...

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Polymer Degradation and Stability 97 (2012) 1596e1605

Contents lists available at SciVerse ScienceDirect

Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Cross-linking of a novel reactive polymeric intumescent flame retardant to ABS copolymer and its flame retardancy properties Haiyun Ma a, *, Jun Wang a, Zhengping Fang b, c a

College of Chemistry and Environmental Science, HeBei University, Baoding 071002, China MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Institute of Polymer Composites, Zhejiang University, Hangzhou 310027, China c Laboratory of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2012 Received in revised form 19 June 2012 Accepted 27 June 2012 Available online 6 July 2012

In this work, a reactive polymeric intumescent flame retardant (PDSPB) was synthesized and applied to ABS resin. The flame retarded materials were prepared by direct melt blending of ABS and PDSPB (ABS/ PDSPB), as well as melt cross-linking of PDSPB to ABS (ABS-c-PDSPB). The cross-linking reaction was characterized by fourier transform infrared spectrometry (FTIR) and gel content measurement. The thermal stability, flame retardancy, carbonization chemistry and dynamic mechanical properties were investigated by TGA, dynamic FTIR, SEM, CONE calorimetry and dynamic mechanical analysis (DMA). The results showed that the addition of PDSPB can effectively reduce the flammability properties including peak heat release rate (PHRR), total heat release (THR) and average mass loss rate (AMLR). An improvement of the limited oxygen index value was also observed. Especially, the cross-linking of PDSPB can improve the ignition time and reduce THR during combustion. Moreover, ABS-c-PDSPB displayed a more obvious intumescent char layer and char weight than that of ABS/PDSPB blends. Furthermore, cross-linking of PDSPB enhanced the thermal stability and considerably delayed the thermal oxidation degradation of ABS. The dynamic mechanical properties were improved and the plasticization effect was reduced by cross-linking of PDSPB. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Acrylonitrile-butadiene-styrene copolymer Cross-linking Carbonization chemistry Flammability Thermal stability

1. Introduction Acrylonitrile-butadiene-styrene copolymer (ABS) is a widely used thermoplastic material due to its promising mechanical properties, chemical resistance and processing advantages [1e3]. However, a major drawback of ABS is its high flammability [4e6]. Therefore, for many applications, it is necessary to construct a flame retarded composition for ABS resin. In recent years, intumescent flame retardant (IFR) additives have been widely utilized in the flame retardation of flammable polymers, especially in the flame retardation of polyolefin [7e12]. Compared to other flame retardants such as halogenated flame retardants, IFR is halogen-free, nontoxic and environment friendly. Whereas, IFR also has some disadvantages [13]. Firstly, a large amount of IFR is normally needed to achieve ideal flame retarded performance, which deteriorates the mechanical property and thermal stability of materials. Secondly, the poor compatibility between IFR and polymer matrix is another issue that should be

* Corresponding author. Tel.: þ86 15831217531. E-mail address: [email protected] (H. Ma). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.06.030

considered. Moreover, most of IFRs are simple mixtures of three components named acid source, carbon source and blowing agent. Therefore, the normal IFRs do not have high enough thermal stability to satisfy some engineering plastics, including ABS resin. In our previous reports [13,14], a novel phosphorous-nitrogen containing intumescent flame retardant, poly (diaminodiphenyl methane spirocyclic pentaerythritol bisphosphonate) (PDSPB), has been synthesized and proved to be an excellent carbonization agent. PDSPB has a polymeric structure with a high molecular weight, as well as enough high thermal stability to match the processing of ABS resin. Moreover, another major advantage of PDSPB is that it has reactive end (amino group) group, as shown in Scheme 1, which can react with anhydride to form covalent amido bond. Since maleic anhydride (MAH) grafting ABS resin (ABS-MAH) has been widely reported in our previous work and other literature [15e18], PDSPB could be potentially applied to cross-link the ABS-MAH resin. Therefore, the cross-linking of PDSPB could not only improve the compatibility between IFR and polymer matrix but also enhance the thermal stability, flame ratardancy and mechanical properties.

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

HO

OH

O + 2POCl3

O

O

O

P Cl

O

O O

O

P

Cl

+ 4HCl

O

dried to constant weight in a vacuum oven at 80  C. The grafting degree of MAH is about 5%. 2.4. Preparation and purification of PDSPB cross-linking ABS copolymers

+

Cl

H2N

CH2

NH2

DDM

P

O P

O

O

O

O P

O

O

P Cl

OH

HO

O

1597

H N

O

CH2

NH n

n=4-5

PDSPB Scheme 1. The synthesis of poly(4,4-diaminodiphenyl methane spirocyclic pentaerythritol bisphosphonate) (PDSPB).

In this work, PDSPB was firstly synthesized and ABS resin was chemically grafted with MAH. Then, two kinds of PDSPB flame retarded ABS materials were prepared by direct melt blending of PDSPB and ABS (ABS/PDSPB) and by cross-linking of PDSPB and ABS-MAH (ABS-c-PDSPB). The cross-linking reaction was characterized. The thermal stability, flame retardancy, carbonization chemistry and flame retardant mechanism was investigated and discussed.

The composites were fabricated via melt compounding of ABS, MAH, DCP, St and PDSPB according to the formulations presented in Table 1 at 190  C in a ThermoHaake Rheomix compounder with a rotor speed of 60 rpm and mixing time of 8 min for each sample. The cross-linking reaction was shown in Scheme 2. The mixed samples were transferred to a mold and preheated at 190  C for 3 min, then pressed at 14 MPa, and successively cooled to room temperature while maintaining the pressure to obtain the composite sheets for further measurements. 2.5. Characterization 2.5.1. Gel content measurements The samples were cut into small pieces with a diameter of 1 mm and then packed into a copper mess. The gel content (wt %) of the sample was obtained using a Soxhlet extractor by refluxing ABS, ABS-MAH, ABS/PDSPB and ABS-c-PDSPB samples in boiling 1,2dichloroethane for 48 h. The gel contents were calculated by the following equation:

2. Experimental 2.1. Materials All the starting materials and solvents were commercially available and were used without further purification. The ABS resin (HI-121H) was obtained commercially from LG Industries, Korea. Reagent grade pentaerythritol was bought from Shanghai Lingfeng Chemical Plant. Phosphorus oxychloride (POCl3) of A.R. grade was from Beijing Tingxing Chemical Plant. 4,4-Diaminodiphenyl methane (DDM) was the product of Yantai Wanhua Polyurethane Ltd. Co. Maleic anhydride (MAH; Shanghai Lingfeng Chemical Solvent Factory) and dicumyl peroxide (DCP) were used as the grafting monomer and a radical initiator of ABS, respectively. 2.2. Preparation of PDSPB The synthesis of PDSPB was described in detail in our previous report [13]. Briefly, the SPDPC was firstly prepared by the reaction of pentaerythritol (0.5 mol) and phosphorus oxychloride (5.5 mol). PDSPB was polymerized by SPDPC (30 g, 0.1 mol) and DDM (30 g, 0.15 mol) and the pale yellow solid powder was obtained (yield: 95%). The synthetic route was shown in Scheme 1. 2.3. Preparation and purification of MAH grafted ABS copolymers (ABS-MAH) The grafting of MAH to ABS has been reported in detail in our previous study [17]. Briefly, the grafting reaction was carried out in the molten state using a Haake twin-screw extruder (Haake, Bersdorff, Germany) with a screw speed of 40 rpm and mixing time of 20 min for each sample. In a typical process, ABS, MAH, initiators, solvent, and comonomers were premixed at room temperature. Then 50 g of premixed materials were added to the extruder. The grafting product was dissolved in 1,2dichloroethane and the unreacted MAH in the solution was extracted with ethanol. The purified polymer was collected and

Pc ¼ 1 

W3  W1 W2  W4  W1

W1 e mass of copper mess; W2 e total mass of copper mess and sample; W3 e total mass of copper mess and sample after extraction; W4 e total mass of PDSPB, MAH, DCP, St. in each sample. 2.5.2. Structure characterization Fourier transform infrared spectroscopy (FTIR) measurements were performed on a Vector-22 FTIR spectrometer using KBr pellets for PDSPB and a film-pressing method for ABS, ABSMAH, ABS/PDSPB and ABS-c-PDSPB samples, respectively. Dynamic fourier transform infrared spectroscopy (in-situ FTIR) was applied with a Vector-22 FTIR spectrometer using a filmpressing method for ABS/PDSPB and purified ABS-c-PDSPB samples. 2.5.3. Thermal property Thermogravimetic analysis (TGA) was done with a TA SDT Q600 thermal analyzer at a scanning rate of 10  C/min under N2 and air, from 60 to 600  C. 2.5.4. Flammability property Flammability of the flame retarded ABS was characterized using a cone calorimeter test performed with an FTT, UK

Table 1 Composition of flame retarded ABS formulations. Sample code

Composition (g) ABS

MAH

St

DCP

PDSPB

ABS ABS-MAH ABS/20wt%PDSPB ABS/30wt%PDSPB ABS-c-PDSPBa

50 50 40 35 40

0 2.5 0 0 2.5

0 2.7 0 0 2.7

0 0.3 0 0 0.3

0 0 10 15 11.4

a

PDSPB concentration is 20 wt %.

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H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

CH2

HC O

CH2 H C

CH CH

H2C

CH

CH

CH2 C

C O

CH2

+

OH

HN P

C H2

O

*

m

O

PDSPB

O

C

HN

O

C NH

O O P

O O *

CH2

O O HN P O

O O P NH O

O C H2

NH C HO C

CH2 H C CH2

O

O

CH2

CH CH CH2

Scheme 2. Cross-linking reaction between PDSPB and ABS-MAH.

device. The test was conducted according to ISO 5660 at an incident flux of 35 kW/m2 using a cone shape heater. The sample size is 10 cm  10 cm  3 mm. The cone data reported here are the average values of three samples. Typical results reproducible to within about 10% were collected and analyzed. The samples were also burnt by test at different temperatures ranging from 20  C to 500  C in a muffle furnace to introduce different extent of char oxidation during combustion. The limited oxygen index (LOI) tests were applied according to GB 24061993. 2.5.5. Dynamic mechanical property Dynamic mechanical properties were measured with a DMA Q800 (TA Corp.) in the stretching mode. The dynamic storage and loss moduli were determined at a frequency of 1 Hz and a heating rate of 5  C/min as a function of temperature from 100  C to 150  C.

2.5.6. Char characterization Morphological study of the chars after cone calorimetry tests was performed by scanning electron microscopy (FEI Sirion FESEM) with an accelerating voltage of 5 kV. 3. Results and discussion 3.1. Characterization of cross-linking reaction Fig. 1 showed the FTIR spectra of ABS, PDSPB, ABS-MAH and ABS-c-PDSPB. Compared to the ABS resin, the appearance of two new absorbance peak at 1780 cm1 and 1860 cm1 (C¼O stretching

Table 2 Assignment of FTIR absorption bands of PDSPB, ABS, ABS-MAH and ABS-c-PDSPB. Band position (cm1) PDSPB 3445,3352, 1625 2886,2825 1230 1027 1085 1510 ABS 2924

Assignment

Reference

OH stretch Asymmetric and symmetric CH2 stretch Stretching of P¼O Stretching of PeOeC Stretching of PN Stretching of C¼C

[19] [19]

Asymmetric and symmetric CH2 stretch 2237 C^N stretch 966, 912 Out-of-plane CH stretch of butadiene ABS-MAH (new absorption peaks compared to ABS) 1860, 1780 C¼O stretch ABS-c-PDSPB 3394 OH stretch in COOH 2924 Asymmetric and symmetric CH2 stretch 1710 C¼O stretching in carboxyl group 1640 C¼O stretch in amide 1542 NeH stretch in amide 1267 Stretching of P¼O 1034 Stretching of PeOeC 2237 C^N stretch 966, 912 Out-of-plane CH stretch of butadiene

Fig. 1. FTIR spectra of ABS, ABS-MAH, PDSPB and ABS-c-PDSPB.

[19] [19] Present study Present study [20] [20] [21]

[16] Present study [20] [16] [20] Present study Present study Present study [20] [21]

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

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Fig. 2. The gel content of ABS, ABS-MAH, ABS/20%PDSPB, ABS-c-PDSPB.

in anhydride) in ABS-MAH indicated that the maleic anhydride was successfully introduced onto the backbone of ABS. The characteristic peaks of PDSPB are 3445 cm1, 3332 cm1, 1625 cm1 (NH2), 3029 cm1 (CH), 1513 cm1 (C¼C), 1230 cm1 (P¼O), 1027 cm1 (PeOeC), 1085 cm1 (PN), as described in our previous report [13]. Essentially, the cross-linking between PDSPB and ABS-MAH is the reaction of amino group in PDSPB and the maleic anhydride group in ABS-MAH. As for ABS-c-PDSPB, two new peaks at 1640 cm1 and 1542 cm1 were attributed to the amide carbonyl group (-CO-NH-). Moreover, the characteristic peaks of amino groups in PDSPB disappeared, indicating that the cross-linking reaction occurred. The characteristic peaks of ABS, PDSPB, ABSMAH, and ABS-c-PDSPB were listed in Table 2. The gel contents of ABS, ABS-MAH, ABS/20%PDSPB and ABS-cPDSPB samples were shown in Fig. 2. No gels appeared for pure ABS resin after extraction. While for ABS/20%PDSPB sample, about 10% of ‘gels’ may be caused by the un-extracted PDSPB. For ABSMAH, about 12% of gels were generated, indicating that crosslinking between MAH and ABS macromolecular chains also occurred during the grafting reaction. As for the ABS-c-PDSPB sample, much higher gel content (50%) was obtained, showing that PDSPB was successfully cross-linked between ABS-MAH chains. 3.2. Thermal degradation The thermal degradation behavior of ABS, ABS-MAH, ABS/PDSPB and ABS-c-PDSPB in N2 and air were shown in Figs. 3 and 4. The

Fig. 4. TGA and DTG curves for flame retarded ABS formulations at a heating rate of 10  C/min in air.

TGA and DTG data including Tonset, Tmax and residues were also listed in Tables 3 and 4. In air, TGA and DTG curves displayed a single step degradation behavior for the pure ABS and ABS-MAH. The curves dropped continuously to a residue level after mass loss commenced. The Tonset of pure ABS resin is 368  C and negligible char was

Fig. 3. TGA and DTG curves for flame retarded ABS formulations at a heating rate of 10  C/min in N2.

1600

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

Table 3 Data of TGA and DTG thermograms of flame retarded ABS formulations in N2. Samples

Tonset ( C)

Residue at 600  C (wt %)

Tmax ( C) Stage 1

Stage 2

ABS ABS-MAH ABS/20wt%PDSPB ABS/30wt%PDSPB ABS-c-PDSPB

374 357 265 251 293

2.7 2.8 10 15 18

e e 278.5 284.0 e

419 418 421.5 421.7 433.1

Tonset: Initial decomposing temperature; Tmax: Maximum weight loss temperature.

obtained in the end, regardless in air and N2. The Tonset of ABSMAH is 17  C lower than that of pure ABS resin, which can be attributed to the low thermal stability of the monomers (MAH, St) that did not participate in the grafting process. For both ABS and ABS-MAH, they have the same Tmax and were both almost burnt out at 600  C, which showed that grafting of MAH cannot improve thermal stability or favor the carbonization of ABS resin. In our previous study [13], two major degradation steps for PDSPB were found, in both air and N2. The Tonset and Tmax in air were both lower than that in N2. The final char weight in air (51%) is much higher than that in N2 (42%), indicating that the oxygen is favorable to the char formation for PDSPB. In N2, for ABS/PDSPB samples in this work, Tonset was reduced with the increasing of PDSPB content, which was caused by the earlier degradation of PDSPB. However, Tmax of the major degradation step was increased with the addition of PDSPB, indicating the thermal enhancing effect of PDSPB. The residue weight was improved greatly with the increase of PDSPB addition. The experimental residue weight (10% for ABS/20%PDSPB and 15% for ABS/30% PDSPB) was slightly higher than the theoretical values (8% for ABS/ 20%PDSPB and 13% for ABS/30%PDSPB), which means a small part of ABS participated in the carbonization process with the protection of PDSPB. The theoretical residue value for ABS/PDSPB blends is calculated based on the thermal degradation of PDSPB in N2 (as shown in Fig. 5, the final char weight is about 51% in air and 42% in N2). In air, the addition of PDSPB also continuously reduced the Tonset. However, the final residue weight in air was higher than that in N2, showing that oxygen was favorable to the carbonization of ABS. Compared to the thermal degradation of ABS/PDSPB blends, ABS-c-PDSPB showed a dramatic enhancement and improvement on the thermal stability and final residue weight, regardless in N2 or air. The first degradation step in ABS/PDSPB sample was almost disappeared, indicating that most of PDSPB and MAH were involved in the cross-linking reaction. Moreover, the Tonset and Tmax were both improved significantly (30  C and 12  C, respectively) at the same PDSPB concentration. The final residue weight of ABS-c-PDSPB is even higher than that of ABS/30% PDSPB.

Table 4 Data of TGA and DTG thermograms and Tg obtained from DMA of flame retarded ABS formulations in air. Samples

Tonset ( C)

Residue at 600  C (wt %)

ABS ABS-MAH ABS/20wt%PDSPB ABS/30wt%PDSPB ABS-c-PDSPB

368 310 255 233 283

0.5 0.2 11.8 13.8 16.0

Tmax ( C)

Tg ( C)

Stage 1

Stage 2

Stage 3

e e 274 276 e

418 413 415 422 420

540 e e e

120.5 120.8 114.0 113.5 122.5

Fig. 5. TGA curves for PDSPB at a heating rate of 10  C/min in air and N2.

Tg (glass transition temperature) results from DMA measurements showed that the grafting of MAH had little effect on the Tg of ABS. The blend of PDSPB and ABS reduced the Tg and due to its plasticizing effect. As for the ABS-c-PDSPB, rather than the reduction of Tg for ABS/PDSPB blend, Tg was conversely increased. The improvement of Tg is due to the cross-linking of PDSPB to ABS. The cross-linking of PDSPB to ABS restricted the thermal movement of macromolecular chains and thus improved the thermal stability. To further investigate the thermal oxidation in a more intuitive way, both the ABS/20%PDSPB blend and ABS-c-PDSPB were put into muffle at different temperatures for 20 min. The macrophotographs were shown in Fig. 6. For ABS/20%PDSPB, the pale yellow sample decomposed rapidly and turned into black rigid solid when temperature was higher than 250  C. However, ABS-c-PDSPB started to degrade when the temperature was higher than 300  C and its shape kept intact even at 360  C. The results showed that the cross-linking of PDSPB evidently improved the thermal stability of ABS resin. 3.3. Flammability property The limited oxygen index (LOI) values for PDSPB flame retarded ABS systems were listed in Table 5. The pure ABS and ABS-MAH are very flammable and LOI values are 19.1 and 19.5, respectively, indicating that the grafting of MAH is not favorable to the flame retardancy of ABS resin. The LOI value was greatly increased with the addition of PDSPB. As for the ABS-c-PDSPB, the LOI value was improved to 28 even at the same PDSPB content (20%). It is worth noting that the PDSPB, especially for ABS-cPDSPB, can significantly suppress the dripping of ABS during combustion. Cone Calorimeter is an ideal equipment to evaluate the flammability properties of polymeric materials [22]. Fig. 7 showed the curves of heat release rate, mass vs. burning time and Table 5 showed the peak heat release rate (PHRR), average heat release rate (AHRR), total heat release (THR), smoke release (ASEA) and average mass loss rate (AMLR). The combustion of ABS resin can be described as following procedures: ABS starts to decompose when heated and generates flammable gases from the reaction with the oxygen, which in turn releases a large amount of heat and further promotes more matrix to degrade. When PDSPB was added, the phosphate ester bonds in

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

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Fig. 6. Micro-morphologies of the specimen of ABS/20%PDSPB and ABS-c-PDSPB at different temperatures.

acid source of PDSPB would be broken down and generate phosphoric acid. In the presence of amine catalyst in the PDSPB, the phosphoric acid can dehydrate the hydroxyl group containing sources to form an insulating cellular semi-liquid carbonaceous layer between the polymer and flame; and the blowing agent expands to form a swollen multi-cellular char by releasing nonflammable gases. The results showed that pure ABS resin burnt very fast after ignition and a sharp PHRR appears at 930 kW/m2. The grafting of MAH makes ABS resin more flammable. As for ABS/ PDSPB, both the PHRR and average HRR were reduced significantly. The PHRR was reduced by 51.5% for ABS/20%PDSPB and 58.3% for ABS/30%PDSPB samples. The reduction of PHRR indicated that the cohesive char layer formed during combustion acts as an insulating barrier between the fire and ABS resin. Meanwhile, the ASEA and AMLR are also reduced, as shown in Table 5. However, tign of ABS/PDSPB samples is lower than that of pure ABS. The reduction of tign may be caused by the small volatile molecules produced earlier from the decomposition of PDSPB. The final char weight was greatly increased with the addition of PDSPB. For the ABS/30%PDSPB sample, the final char weight is 22%, much higher than the theoretically results. Because of the protective effect of the intumescent char layers from PDSPB, more ABS matrix was preserved and that’s why the THR was also greatly reduced. Compared to the blends of ABS and PDSPB, although some more flammable MAH monomers still exist, the tign was dramatically improved. At the same PDSPB loading, the tign of ABS-cPDSPB was 15 s longer than that of ABS/20%PDSPB. PHRR and AMLR, especially THR, were further reduced. The char weight of ABS-c-PDSPB is even much higher than that of ABS/30%PDSPB, indicating that more ABS matrix was protected and preserved

Table 5 Cone calorimetry and LOI data for flame retarded ABS blends at 35 kW/m2. Term tigna (s) PHRRa (kW/m2) THRa (MJ/m2) ASEAa (m2/kg) AMLRa (g/s) LOI value (%)

ABS 28 930

ABS-MAH 26 895

ABS/20% PDSPB 23 451

ABS/30% PDSPB 25 388

ABS-cPDSPB 37 420

34.0

33.0

25.5

23.0

20.6

1475.6

1239.9

1149.1

1051.0

1188.3

0.095 19.1

0.092 19.5

0.069 26.2

0.075 28.3

0.064 28.0

a tign: time to ignition; HRR: heat release rate; PHRR: peak heat release rate; THR: total heat release; AMLR: average mass loss rate; ASEA: average specific extinction area.

during combustion. Since the grafting of MAH cannot favor the flame retarded properties, the significant enhancement of flame retardancy for ABS-c-PDSPB showed high flame retarded efficiency of cross-linking of PDSPB to ABS resin. It’s worth noting that the great extension of tign will increase the chances for people to escape in a real fire hazard. 3.4. Carbonization chemistry The major characteristic of an intumescent coating is its ability to swell. This parameter is necessary but not sufficient to ensure fire protection because the char could be too light and isn’t sufficiently mechanically resistant. The structure having a relatively strong charred layer is important to minimize the heat transfer and provides good protection for the substrate [23]. Fig. 8 illustrated the macrophotographs of flame retarded ABS systems after CONE calorimetry tests. The pure ABS and ABS-MAH were entirely burnt out and by contrast, an intumescent char layer with honeycomb structure was clearly seen for ABS/PDSPB and ABS-c-PDSPB. The swollen layer of ABS-c-PDSPB was more rigid and thicker than that of ABS/PDSPB. Numerous micropores were distributed on the char surfaces which resulted from the inflammable gases of the blowing agent in PDSPB. Fig. 9 showed the SEM images of ABS/PDSPB and ABS-c-PDSPB chars after CONE calorimetry tests. Under lower magnification (2000), both chars showed an intact rigid morphology. Under higher magnification (20,000), both chars were consist of nano-scale spherical graphite platelets. However, the char of ABS/PDSPB displayed a loose porous structure while a more compact and denser char layer was found for ABS-c-PDSPB. Compared to blends of ABS and PDSPB, the char layer of ABS-cPDSPB can protect the matrix more effectively during combustion. The dynamic FTIR was used to monitor the chemical structure and carbonization reaction during thermal degradation for the blend of ABS/PDSPB and cross-linked ABS-c-PDSPB, as shown in Fig. 10. Considering chemical structures, the thermal degradation of above systems showed both the following three steps sequentially: the formation of (poly) phosphoric acid from the breaking down of phosphate bond; dehydration of (poly) phosphoric acid to the hydroxyl containing source; and the degradation of amine group from the blowing agent. For the ABS/ PDSPB, the peak at 1027 cm1 representing PeOeC disappeared quickly when temperature was above 320  C, indicating the PeOeC bond was broken. The peak at 1209 cm1 (P¼O) also vanished above 280  C and a new peak at 1243 cm1 appeared which represent the characteristic peak of PeOePh complex structure, confirming the formation of phosphoric acid from the breaking down of PeOeC bond and the catalytic

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Fig. 7. Heat release rate (HRR) and mass curves of flame retarded ABS formulations as a function of time during cone calorimetry tests at 35 kW/m2.

carbonization effect of the acid. Besides, another peak at 1630 cm1 representing the amino group in PDSPB starts to weaken at 320  C and disappeared entirely at 360  C. At the same time, a new peak appeared at 1624 cm1 ascribed to the vibration of C¼C bond in the aromatic complex structure of chars, illustrating the blowing agent started to degrade at that

temperature. Above 480  C, a new peak appeared at 996 cm1 representing PeOeC in final chars. Comparing the chemical structure changes during thermal degradation, several differences can be found for ABS-c-PDSPB. Firstly, the degradation of ABS in ABS-c-PDSPB was greatly postponed to a higher temperature. For the blends of ABS and

Fig. 8. Morphologies of chars for ABS, ABS-MAH, ABS/20%PDSPB, ABS-c-PDSPB after cone calorimeter tests: a, pure ABS resin; b, ABS-MAH; c, ABS/20%PDSPB; d, ABS-c-PDSPB.

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

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Fig. 9. SEM micrographs of the residues of ABS/20wt%PDSPB mixture (a, c) and ABS-c-PDSPB (b, d).

PDSPB, ABS was entirely degraded above 360  C (the characteristic peak at 2237 cm1 representing cyanogroup in ABS disappeared). However, the cyanogroup was not degraded until 400  C, displaying a much higher thermal stability for ABS-cPDSPB, which is consistent with TGA results. Secondly, two

peaks at 1710 cm1 and 1644 cm1 representing amido bond in ABS-c-PDSPB started to degraded above 400  C, indicating the formation of carbodiimide [20,24]. Carbodiimide has excellent thermal stability and carbonization ability, which contributes to the better performance of ABS-c-PDSPB. The proposed

Fig. 10. FTIR spectra of ABS/20wt%PDSPB (left) mixture and ABS-c-PDSPB (right) degraded at different temperatures (RT: room temperature).

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H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

CH2 b CH CH

a H C

c O

H N

C

H2C

CH2

C OH

HN

O O P

O O P O

c

O H N C

C H2

NH

O

CH

CH

HO C

O

b CH2

a

CH2

O

CH2

CH2

Δ

c

Δ

a, b O

O

O

O

P

H

O O H C C CH2 H O

a, c

d

O2

CH2

CH

+

CH2 CH CH2

HO

P O

O

O

O

d

3x

H O

CH2 CH CH2

H3PO4 +

OH

O

CH2

O

CH

P HO

N

O O

Char layers

N C O

CH2

H2N

OH

C

C N

N C

O

Char layers

NH3, H2O, CO2 Intumescent chars Scheme 3. Proposed mechanism of the decomposition and charring process of ABS-c-PDSPB.

carbonization chemistry and mechanism of ABS-c-PDSPB was shown in Scheme 3. 3.5. Dynamic mechanical property Fig. 11 presented the graphs for the storage modulus E0 (left) and tan d (right) versus the temperature of ABS, ABS-MAH, ABS/PDSPB and ABS-c-PDSPB. The glass transition temperature (Tg) was obtained from the maximum peak temperature of tan d. There were no obvious differences for E0 and Tg between ABS and ABS-MAH. For ABS/PDSPB blends, E0 was greatly reduced compared to ABS resin above room temperature, which was ascribed to the plasticization effect of PDSPB. And Tg was 6.5  C lower than that of ABS. As for the

ABS-c-PDSPB, E0 was greatly strengthened in the whole temperature range in comparison to ABS/PDSPB blend. E0 of ABS-c-PDSPB was even higher than that of ABS resin below 110  C. Moreover, Tg of ABS-c-PDSPB was 8.5  C and 2  C higher than ABS/PDSPB and pure ABS, respectively. In flame retarded polymer systems, a major drawback is that the addition of flame retardants normally significantly deteriorates the stiffness, which sometimes can affect the application of such materials [25]. In ABS-c-PDSPB, because many junctions can be created between ABS and PDSPB chains, the threedimensional network structure restrains the thermal motion of macromolecular chains and thus, can greatly reduce the plasticization effect of the flame retardants and enhance the mechanical properties.

Fig. 11. The storage modulus (E’) and loss tangent (tan d) for flame retarded ABS formulations as a function of the temperature.

H. Ma et al. / Polymer Degradation and Stability 97 (2012) 1596e1605

4. Conclusion A novel polymeric reactive intumescent flame retardant of PDSPB was synthesized and applied to ABS resin by melt blending and cross-linking methods. The MAH grafting ABS can react with functional groups in PDSPB to form a cross-linking flame retarded ABS system. The addition of PDSPB can effectively reduce the flammability including PHRR, THR and AMLR. The LOI value was also greatly improved. Especially for ABS-c-PDSPB, the crosslinking of PDSPB can significantly improve the ignition time and reduce the total heat release of ABS resin. Moreover, a more obvious intumescent char layer and more char weight were obtained for ABS-c-PDSPB. Furthermore, cross-linking of PDSPB enhanced the thermal stability and considerably delayed the thermal oxidation degradation of ABS. The dynamic mechanical properties were remarkably improved and the plasticization effect was significantly reduced by cross-linking reaction. The ABS-c-PDSPB improved the compatibility between flame retardant and polymer matrix, and simultaneously enhanced thermal stability by combing both advantages of flame retardant and cross-linking technique.

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