Enhanced Properties of the Incorporation of a Novel Reactive

Article pubs.acs.org/IECR

Enhanced Properties of the Incorporation of a Novel Reactive Phosphorus- and Sulfur-Containing Flame-Retardant Monomer into Unsaturated Polyester Resin Kang Dai,†,‡,§ Lei Song,† Richard K. K. Yuen,‡ Saihua Jiang,†,‡,§ Haifeng Pan,†,‡,§ and Yuan Hu*,†,§ †

State Key Laboratory of Fire Science, University of Science and Technology of China and USTC-CityU Joint Advanced Research Centre, Suzhou, P.R. China ‡ Department of Civil and Architectural Engineering, City University of Hong Kong and USTC-CityU Joint Advanced Research Centre, Suzhou, P.R. China § Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, Suzhou, P.R. China ABSTRACT: A novel reactive phosphorus- and sulfur-containing flame-retardant monomer [di(allyloxybisphenol sulfone) phenoxy phosphonate, DASPP] was successfully synthesized and well characterized. Various amounts of DASPP were incorporated into the unsaturated polyester by radical bulk polymerization. The thermal properties and flammability of the flameretardant unsaturated polyester resin (FR-UPR) samples were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurements, and cone calorimetry. The results showed that the introduction of DASPP into unsaturated polyester resin (UPR) can substantially improve its fire resistance and hightemperature stability. Interestingly, a linear increase in the glass transition temperature (Tg) with increasing incorporated DASPP content was observed by DSC. Scanning electron microscopy (SEM) and Raman spectroscopy studies revealed that DASPP can effectively improve the microstructure of UPR char residue and increase its graphitization degree, which can enhance UPR’s thermo-oxidative stability and char yield in high-temperature regions. Furthermore, real-time Fourier transform infrared (RTIR) spectroscopy was employed to study the thermo-oxidative degradation reactions of different UPR samples, providing insight into the combustion mechanism. In addition, results from tensile testing demonstrated the improved mechanical properties for the samples containing DASPP.

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INTRODUCTION Given its prominent mechanical and electrical properties, superior chemical and weather resistances, and low cost, unsaturated polyester resin (UPR) has been extensively utilized in industrial settings in recent decades. Although it exhibits many intriguing characteristics, it also has some drawbacks. These problems, such as high flammability and subsequent dripping, combined with the generation of dense toxic smoke during burning, must be overcome to achieve UPR’s full potential. To this end, great effort has been dedicated to solving these difficult issues over the past several years. Initially, halogenated compounds [e.g., decabromodiphenyloxide (DBDPO), tetrabromodiphenyloxide (TBDPO)] or inorganic fillers (e.g., alumina trihydrate, expandable graphite) were added to UPR in an attempt to enhance its flame retardancy,1−3 and some desirable progress has been achieved. However, severe environmental pollution results from the combustion of halogen in compounds. Moreover, an unfavorable interaction between the polymer and particles arises. This issue, coupled with the high loading of additives, gives rise to serious weaknesses in the physical and mechanical properties of the polymer. In recent years, reactive phosphorus-containing flame retardants have become the subject of research on a massive scale.4−6 The principal merits that distinguish them from conventional flame-retardant additives are a low required © 2012 American Chemical Society

loading, environmental friendliness, and minimal compromise of the matrix’s inherent properties. Investigations have shown that the introduction of phosphorus-containing flame-retardant units into UPR molecular chains can significantly improve thermal stability and reduce flammability. Lin et al. synthesized hexa(allylamino)cyclotriphosphazene and introduced it into UPR.7 The resulting UPR demonstrated remarkable enhancement in limiting oxygen index (LOI) and carbonization. Liu et al.8 reported the incorporation of 9,10-dihydro-10[2,3-di(hydroxycarbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP) into UPR by melt polycondensation. This material, with a 1.62% phosphorus content, achieved a grade of V0 according to the UL-94 plastics flammability standard released by Underwriters Laboratories, meaning that burning on a vertical specimen stopped within 10 s. Moreover, sulfurcontaining polymers in general, not least those with sulfone groups (e.g., polyphenylene sulfide and polysulfone), are endowed with excellent thermal stability, outstanding mechanical properties, and extraordinary corrosion resistance,9,10 meeting the rigorous demands for use in hot and hostile environments. The introduction of phosphorus-containing Received: Revised: Accepted: Published: 15918

August 7, 2012 October 29, 2012 November 20, 2012 November 20, 2012 dx.doi.org/10.1021/ie302106w | Ind. Eng. Chem. Res. 2012, 51, 15918−15926

Industrial & Engineering Chemistry Research

Article

precipitated triethylamine hydrochloride was filtered off. The residue was concentrated by rotary evaporation and then washed with distilled water, extracted with chloroform, and dried over MgSO4, yielding an amber liquid (17.7 g, 82% yield). 1 H NMR (CDCl3), δ (ppm): 6.9−7.9 (ArH), 6.0 (1H, CH CH2), 5.4 (1H, CHCH2, trans), 5.3 (1H, CHCH2, cis), 4.6 (2H, O−CH2). 31P NMR (CDCl3), δ (ppm): −19.0 (singlet peak). The synthetic procedures for ABSE and DASPP are illustrated in Scheme 1.

groups into these polymers to further improve the fire resistance11,12 or glass transition temperature (Tg)13 and the subsequent use of the resulting polymers as flame-retardant additives14 have also been investigated. However, limited attention in research has been paid to the combination of phosphorus and sulfur in a single reactive flame-retardant monomer for polymers with high fire risk. In this study, a novel reactive flame-retardant monomer containing phosphorus and sulfur was synthesized, and then incorporated into resin to prepare a flame-retardant unsaturated polyester resin (FRUPR). The flammability, thermal properties, and tensile performance of the resulting UPR samples were investigated. Additionally, the effects of di(allyloxybisphenol sulfone) phenoxy phosphonate (DASPP) on char formation and thermo-oxidative degradation of UPR samples were examined. This FR-UPR will be of particular interest in those situations requiring high safety standards.

Scheme 1. Synthetic Pathways of ABSE and DASPP

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EXPERIMENTAL SECTION Materials. Phenyl dichlorophosphate (PDCP) was supplied by Deheng Chemical Corp (Shijiazhuang, China) and was distilled before further use. 4,4′-Sulfonyldiphenol and allyl chloride were generously provided by Changsheng Chemical Corp (Jiangyin, China) and used as received. Triethylamine (TEA), tetrahydrofuran (THF), and acetonitrile were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and dried over 4-Å molecular sieves. Benzoyl peroxide (BPO) was purified by recrystallization from methanol. Unsaturated polyester, containing 40 wt % styrene, was used as received from Huamei Composite Corp. (Tongxiang, China). Synthesis and Sample Preparation. Synthesis of Allyl Bisphenol Sulfone Ether (ABSE). In a typical procedure, 4,4′sulfonyldiphenol (25.03 g, 0.1 mol) and TEA (10.63 g, 0.105 mol) were dissolved in 200 mL of THF, and the mixture was then poured into a three-necked flask equipped with a stir bar, a reflux condenser, and a N2 inlet. The reaction mixture was stirred vigorously and heated to 50 °C under a dry N2 atmosphere. After the 4,4′-sulfonyldiphenol had completely dissolved, a mixture of allyl chloride (7.65 g, 0.1 mol) and THF (40 mL) was added dropwise into the reactant mixture over 2 h and maintained at a constant temperature for an additional 4 h. Subsequently, triethylamine hydrochloride was removed by filtering, and the residue was evaporated to remove solvent and unreacted reactants and then washed with distilled water, yielding a white powder. The powder was dried under a vacuum at 50 °C overnight (22.9 g, 79% yield). 1H NMR (DMSO-d6), δ (ppm): 7.7−7.8 (4H, Ar−SO2), 7.1 (2H, Ar− OH), 6.9 (2H. Ar−O−allyl), 6.0 (1H, CHCH2), 5.4 (1H, CHCH2, trans), 5.3 (1H, CHCH2, cis), 4.6 (2H, O− CH2), 3.4 (1H, −OH). Synthesis of Di(allyloxybisphenol sulfone) Phenoxy Phosphonate (DASPP). In one three-necked flask equipped with a stir bar, a reflux condenser. and a N2 inlet, ABSE (17.421g, 0.06 mol), TEA (6.375 g, 0.063 mol), and CuCl (0.017 g) were diluted with acetonitrile (150 mL). The reaction mixture was stirred vigorously and heated to reflux under dry N2. After that, a mixture of PDCP (6.329 g, 0.03 mol) and acetonitrile (40 mL) was added dropwise for 0.5 h and maintained at a constant temperature for an additional 6 h with vigorous stirring. Subsequently, the solvent was evaporated, and another appropriate solvent (THF, 50 mL) was added to the reaction mixture with thorough stirring. The resulting

Sample Preparation. Samples were prepared by mixing DASPP with unsaturated polyester in different ratios, combined with BPO at a concentration of 2 wt % as an initiator. Each mixture was stirred vigorously for 0.5 h at room temperature. A high-intensity Ultrasonic Processor (VGT-2013QT, Ningbo Haishudasheng Instrument) was then employed for 10 min to remove the bubbles generated by stirring. All samples were subsequently cast into polytetrafluoroethylene molds, cured at 70 °C for 2 h, and subjected to postcuring at 120 °C for 1 h. The cured samples are referred to hereafter as UPR0, UPR10, UPR15, and UPR20, according to the concentration of DASPP in the resin. The schematic reaction route between DASPP and unsaturated polyester is presented in Scheme 2. Measurements. 1H NMR and 31P NMR Spectrometry. 1H NMR and 31P NMR spectra were recorded on an AVANCE 400 Bruker spectrometer. The 1H NMR spectrum of ABSE was referenced by solvent shifts (DMSO-d6 = 2.50 ppm), and for DASPP, the 1H NMR and 31P NMR spectra were referenced by solvent shifts (CDCl3 = 7.25 ppm). Fourier Transform Infrared (FTIR) Spectroscopy. FTIR measurements were performed on a Nicolet 6700 FTIR spectrometer. Thermogravimetric Analysis (TGA). TGA was conducted on a Q5000 IR thermogravimetric analyzer (TA Instruments/ Waters) under N2 or air atmosphere, using a heating rate of 20 °C/min from room temperature to 700 °C in a 60 cm3/min stream. All samples were maintained within 3−5 mg. Cone Calorimetry. The test was performed on a cone calorimeter (Fire Testing Technology) in accordance with standard method ISO 5660. Each specimen (100 × 100 × 3 mm3) was wrapped in aluminum foil and exposed horizontally to a 35 kW/m2 external heat flux. Differential Scanning Calorimetry (DSC). DSC measurements of the polymer samples were performed on a MettlerToledo DSC instrument equipped with a liquid nitrogen cooling system and automated sampler. Typical DSC experi15919

dx.doi.org/10.1021/ie302106w | Ind. Eng. Chem. Res. 2012, 51, 15918−15926


Article

Scheme 2. Schematic Reaction Route of Unsaturated Polyester and DASPP

Figure 1. (a) 1H NMR spectrum of ABSE. (b) 1H NMR and (c) 31P NMR spectra of DASPP.

samples were tested for each specimen, and results were averaged in accordance with standard ASTM D638. Scanning Electron Microscopy (SEM). The morphology of the char residues was evaluated with a Hitachi X650 scanning electron microscope. Raman Spectroscopy. Raman spectroscopy measurements were conducted at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co.), and the Gaussian peak type in the Peak Fitting Module of Origin 8.0 software was employed in curve fitting to determine spectral parameters.

ments were conducted in aluminum pans under N2 at a heating rate of 20 °C/min from 30 to 250 °C. Limiting Oxygen Index (LOI). LOI measurements were performed on an HC-2 oxygen index meter (Jiangning Analysis Instrument Co.) with sample dimensions 100 × 6.7 × 3 mm3, in accordance with standard ASTM D2863-2010. Real-Time Fourier Transform Infrared (RTIR) Spectroscopy. RTIR spectroscopy was performed on a Nicolet MAGNA-IR 750 spectrophotometer equipped with a temperature-controlled heating device. Ground powdery mixtures of sample and KBr were pressed into a tablet and then positioned in a ventilated oven. The heating rate of the oven was 10 °C/min. Tensile Properties. Tensile testing was conducted at a temperature of 23 ± 2 °C using an Instron universal testing machine with a crosshead speed of 1 mm/min. At least five

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RESULTS AND DISCUSSION Structural Characterization of Monomers. The 1H NMR spectrum of ABSE and the 1H NMR and 31P NMR spectra of DASPP are shown in Figure 1. For ABSE, the 15920



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aromatic signals arising from the protons near the sulfone group lie in the range δ = 7.8−6.9 ppm, whereas the protons on the allyl group give rise to signals at δ = 6.0−4.6 ppm. Moreover, the broader signal of the hydroxyl proton is observed at δ = 3.4 ppm, confirming the expected monomer structure. In the case of DASPP, the aromatic shifts in the range δ = 7.4−7.1 ppm, partly overlapping with the shifts of the protons close to the sulfone group, are attributed to the protons on the phenoxyl near the phosphoryl. Additionally, the complete disappearance of the hydroxyl shift is observed on the 1H NMR spectrum of DASPP, whereas a single signal δ = −19.0 ppm can be seen in the 31P NMR spectrum, manifesting a thorough reaction between ABSE and PDCP. The integrals of the signals fit the respective monomer structures reasonably well. Obvious alterations in the molecular structures between ABSE and DASAPP are observed in the FTIR spectra (Figure 2). Whereas peaks at 1110 and 1160 cm−1 corresponding to the

Table 1. Flame Retardancy and Cone Calorimetry Data for the Different UPRs

a

sample

DASPP content (wt %)

UPR0 UPR10 UPR15 UPR20

0 10 15 20

LOI

pHRR (kW/ m2)

THR (MJ/ m2)

mass residue (%)

EHCaveragea (MJ/kg)

20.5 24.5 25.0 26.0

969 660 604 536

76 66 62 59

2.2 4.7 5.4 6.5

21.4 18.6 17.3 16.1

Average effective heat combustion.

Figure 3. HRR curves of the different UPRs.

UPR20, the total heat release (THR) also declined significantly with increasing content of DASPP in UPR, decreasing from 76 MJ/m2 (UPR0) to 59 MJ/m2 (UPR20). This reduction can be attributed to the incomplete combustion of UPR and is in good accord with the increased mass residue after burning shown in Figure 4, indicating that many more UPR20 molecular chains Figure 2. FTIR spectra of ABSE and DASPP.

symmetric stretching vibration of the sulfone group can be clearly seen for both ABSE and DASPP, the strong, broad peak of hydroxyl at 3430 cm−1 in the ABSE spectrum is fully absent in the case of DASPP. Furthermore, peaks characteristic of (P− O−Ar) at 966 and 1190 cm−1 15 were found after the reaction of ABSE and PDCP, further corroborating the illustrated structure of DASPP. Flame Retardancy. As an important parameter, LOI has been widely employed in determining the flammability of polymers.5,16 In comparison with the LOI value for pristine UPR (UPR0), UPRs containing DASPP (UPR10, -15, and -20) showed greater LOI values (Table 1). The LOI value increased to 24.5 for UPR10 and reached 26.0 for UPR20. Moreover, the dripping during combustion of UPR was eliminated by incorporating a moderate proportion (ca. 15%) of DASPP. Cone calorimetry testing was also employed to evaluate the fire behavior of the UPRs. The related heat release rate (HRR) curves and cone calorimetry data for the UPRs are presented in Figure 3 and Table 1, respectively. Introducing DASPP into UPR had a profound impact, decreasing HRR considerably, as can be observed in Figure 3. Specifically, the peak heat release rate (pHRR) decreased from 969 kW/m2 (UPR0) to 536 kW/ m2 (UPR20). In addition to the ∼44.7% reduction in pHRR for

Figure 4. Photographs of the char residues of samples after the cone calorimetry test: (a) UPR0, (b) UPR20.

underwent the carbonization process during combustion.17,18 The char residues of UPR0 and UPR20 after cone calorimetry tests are shown in Figure 4. Compared with the almost burnedout char residue from neat resin, a thick and intact char layer can be observed for UPR20. This more compact char will be better able to hamper produced combustible gases, as well as decrease thermal conductivity in combustion.19 Consequently, a notable improvement in flame retardancy can be obtained. Although all of the UPR samples failed to pass the UL-94 test, because of the high styrene content in the resin (40%), the combination of phosphorus and sulfur in one reactive monomer provides a new approach for improving flame 15921



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retardancy and other properties of polymers and is worthy of further investigation. Thermal Properties. From an industrial application point of view, the Tg value of a material is considered to be a crucial thermal parameter. Both chain rigidity and polarity are predominant factors in the glass transitions of polymers,20 and cross-linking degree also affects the Tg values of thermosetting polymers.21 Thus, taking advantage of these direct correspondences is a feasible approach for the enhancement of Tg in molecular design. For DASPP, in addition to the abundant rigid aromatic rings and high polar sulfone groups, the two double bonds can further increase the cross-linking degree in polymerization. When DASPP was incorporated into UPR, the characteristic molecular structure led to a distinct rise in Tg, as confirmed by DSC in Figure 5. It should be noted that Figure 6. Tg as a function of DASPP content in the different UPRs.

(temperature corresponding to the maximum weight loss) at 384 °C (UPR15) and 367 °C (UPR20) can be explained by the partial degradation of the incorporated DASPP. Meanwhile, a small decrease in the temperature at 10% weight loss (T0.1) can be seen, due to the cleavage of the relatively weak P−O−C linkage12,23 in DASPP, decreasing from 360 to 343 °C as shown in Table 2. Moreover, the UPR char yield was enhanced by the introduction of DASPP. At 700 °C, whereas a limited 1.7% char yield was found for UPR0, the char yield of UPR20 was as high as 7.8%, suggesting that, in this inert atmosphere, the char from the UPRs containing DASPP could be preserved at high temperatures. In air atmosphere, another decomposition stage at 480−680 °C was observed for all of the UPRs, due to further oxidation of the primary carbonaceous char.24 Furthermore, the alterations after incorporation of DASPP can also be distinguished in Figure 8 and Table 2. Remarkably, the carbonaceous char yield increased from 10.7% to 18.7% at 500 °C as the DASPP content increased, demonstrating significant improvement in thermo-oxidative stability. In summary, the reported results reveal that, in addition to enhancing Tg, the introduction of DASPP into UPR can contribute to better high-temperature stability, as well as notable improvement in char yield. The formed char can retard thermal conductivity, constrain mass transfer, and then weaken exothermic reaction. As a consequence, combustion will be substantially suppressed by the interference of the char.25 Char Morphology and Raman Spectroscopy Analysis. Figure 9 shows SEM images of the micromorphologies of the UPR0 and UPR20 char residues. As presented in Figure 9a, a highly porous and pitted surface was observed for the UPR0 char. In contrast, the UPR20 char surface morphology (Figure 9b) was continuous with few pores, and even at a higher magnification, the surface of the char was rather compact and smooth. This optimized char microstructure was further

Figure 5. DSC thermograms of the different UPRs.

the glass transition process becomes less detectable in the DSC curve, because of the increasing hindrance in movement of chain segments. Moreover, the Tg values for all of the UPRs (Table 2) mapped in Figure 6 show a linear increase from 135.8 to 142.9 °C as the DASPP content increased to 20%. The thermal stability and decomposition of the UPRs were evaluated by TGA in both N2 and air atmospheres; the corresponding TG and DTG curves and the detailed data from TGA are presented in Figures 7 and 8 and Table2, respectively. In N2 atmosphere, whereas a one-stage weight loss curve was observed for UPR0, the tendency for weight loss to occur in two distinct stages between 250 and 500 °C became increasingly evident as the proportion of DASPP in the UPRs rose. This implies that the incorporation of DASPP altered the decomposition process. Considering that both dehydration and breakdown of the polyester and polystyrene chains are responsible for the one-stage weight loss of UPR in the temperature range of 250−500 °C,22 the resulting other Tmax

Table 2. Tg and TGA Data in Air and N2 Atmospheres for the Different UPRs T0.1 (°C)

char (500 °C,wt %)

Tmax (°C)

char (700 °C, wt %)

sample

Tg (°C)

air

N2

air

N2

air

air

N2


135.8 139.2 140.9 142.9

356 346 345 345

360 350 344 343

400/566 398/559 390/563 365/385/566

399 403 384/402 367/402

10.7 12.4 13.9 18.7

1.6 1.9 2.0 3.9

1.7 4.0 5.3 7.8

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Figure 7. (a) TG and (b) DTG curves of the different UPRs in N2 atmosphere.

Figure 8. (a) TG and (b) DTG curves of the different UPRs in air.

underwent a distinct ∼40% increase, as compared to that of neat UPR, indicating a much improved char microstructure. Thus, the resulting char with better thermal stability can serve as an excellent insulator, protecting the substrate from degradation. RTIR Analysis on the Thermo-Oxidative Degradation of UPR0/20. To gain further insight into the different thermooxidative degradation behaviors of the UPRs after incorporation of DASPP, RTIR spectroscopy was employed to analyze the solid-state pyrolysis products of UPR0 and UPR20. The characteristic absorption bands of pristine UPR (UPR0) shown in Figure11a are as follows: 3543 cm−1, stretching of −OH; 2960 cm−1, asymmetrical stretching of −CH2;29 1735 and 1167 cm−1, CO stretching and C−O−C symmetrical stretching of the ester groups, 30 respectively; 1450 cm −1 , aromatic absorption band; 767 and 701 cm−1, C−H deformation vibrations of the monosubstituted aromatic ring. Although all of these peaks were visible at ambient temperature, alterations were observed with increasing temperature. The relative intensity of the peak at 3543 cm−1 decreased markedly at 250 °C and then vanished at 350 °C, corresponding to the dehydration in the first stage of degradation,22 as illustrated by TGA. Meanwhile, little variation below 350 °C was detected for the other peaks. With the further elevation of temperature,

Figure 9. SEM images of char residues for (a) UPR0 and (b) UPR20.

validated by Raman spectroscopy. Generally, the spectrum of carbonaceous material reveals two strongly overlapping diffusion bands, at ca. 1580 cm−1 (G band) and 1360 cm−1 (D band). The former derives from the E2g-symmetry stretching vibrational mode of the ideal graphitic lattice, and the latter corresponds to the A1g-symmetry vibrational mode in the disordered graphite structure.17,26 Accordingly, the ratio of the intensities of the G and D bands (IG/ID) can be taken as a parameter quantifying the degree of graphitization or disorder in carbonaceous material, with a high value of IG/ID signifying a good structure with few defects.27,28 As shown in Figure 10, the calculated value of IG/ID (UPR20) obtained by peak fitting 15923



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Figure 10. Raman spectra of char residues for UPR0 and UPR20.

Figure 11. RTIR spectra of (a) UPR0 and (b) UPR20 at different temperatures.

demonstrated by both the SEM and Raman spectroscopy results. Tensile Properties. Although many flame retardants can improve the fire resistance of UPR, the resulting severe reductions in mechanical properties limit their use for demanding applications. In the case of DASPP, after incorporation into UPR, an evidently positive effect on the mechanical response was observed; see Figure 12 and Table 3. The typical engineering stress−strain curves depicted in Figure 12 show brittle behavior for all of the samples. As the DASPP content increased, the Young’s modulus of the samples increased correspondingly. For UPR15, the Young’s modulus increased by 8.7%, with a 27% improvement in tensile strength. The tensile strength, however, declined from this maximum with higher content of DASPP (UPR20). More attractively, a further increase of 19% in stiffness occurred as the Young’s modulus reached 4173 MPa, in comparison with the relatively less rigid neat UPR. From the evident molecular structural alterations of UPR, the enhancement in the observed mechanical properties of the samples can be elaborated as follows: With the introduction of DASPP, primarily the increased cross-linking degree and numerous aromatic rings and sulfone groups can account for the additional stiffness, contributing to the increased Young’s modulus. Meanwhile, the aryl−ether linkage in DASPP also facilitates rotation about these O−C links20 and improves the toughness of the UPR

however, the intensities of all of the absorption peaks decreased markedly. These alterations satisfactorily describe the principal decomposition of polyester and polystyrene chains in the temperature range of 350−480 °C,22,24 fitting well with the TGA results in air. For the UPR20 spectra in Figure11b, as compared to those of UPR0, in addition to the emergence of characteristic peaks at 1100 and 966 cm−1, the intensities of the peaks at 1592, 1500, and 838 cm−1 increased. This can be ascribed to the abundant para-disubstituted aromatic rings of the introduced DASPP. When the temperature rose to 250 °C, the intensity of the 1100 cm−1 peak, corresponding to the symmetric stretching vibration of the sulfone group, decreased, and it was barely observable at higher temperatures, indicating cleavage of the Ar−SO2 group. Then, the resulting fragments will evolve in the form of volatiles, such as sulfone-containing aromatic compounds or sulfur dioxide.11,28,31 Although the absorption peak of the P O group cannot be clearly detected at ambient temperature, because of overlap with the C−O stretching band, the peak at 966 cm−1 for P−O−Ar was found to disappear as temperature increased. Concurrently with the degradation of the P−O−Ar structure, the appearance of a peak at 900 cm−1 was observed, which can be assigned to the asymmetric stretching vibration of P−O−P.32,33 This indicates the formation of more thermally stable polyphosphate species. The presence of polyphosphates can promote the formation of highly structured char, as 15924



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work described in this paper was substantially supported by a grant from the Research Grant Council of the Hong Kong special Administrative Region (China Project No. CityU 122612), the National Basic Research Program of China (2012CB719701), and the joint fund of NSFC and CAAC (61079015). We would like to thank L. Zhang and G. Carton for helpful discussions.

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(1) Galip, H.; Hasipoglu, H.; Gunduz, G. Flame-retardant polyester. J. Appl. Polym. Sci. 1999, 74, 2906−2910. (2) Kicko-Walczak, E. New generation of fire retardant polyester resins. Macromol. Symp. 2003, 199, 343−350. (3) Shih, Y. F.; Wang, Y. T.; Jeng, R. J.; Wei, K. M. Expandable graphite systems for phosphorus-containing unsaturated polyesters. I. Enhanced thermal properties and flame retardancy. Polym. Degrad. Stab. 2004, 86, 339−348. (4) Tai, Q. L.; Chen, L. J.; Song, L.; Nie, S. B.; Hu, Y.; Yuen, R. K. K. Preparation and thermal properties of a novel flame retardant copolymer. Polym. Degrad. Stab. 2010, 95, 830−836. (5) Song, P. A.; Shen, Y.; Du, B. X.; Peng, M.; Shen, L.; Fang, Z. P. Effects of Reactive Compatibilization on the Morphological, Thermal, Mechanical, and Rheological Properties of Intumescent FlameRetardant Polypropylene. ACS Appl. Mater. Interfaces 2009, 1, 452− 459. (6) Qian, X.; Song, L.; Hu, Y.; Yuen, R. K. K.; Chen, L.; Guo, Y.; Hong, N.; Jiang, S. Combustion and Thermal Degradation Mechanism of a Novel Intumescent Flame Retardant for Epoxy Acrylate Containing Phosphorus and Nitrogen. Ind. Eng. Chem. Res. 2011, 50, 1881−1892. (7) Kuan, J. F.; Lin, K. F. Synthesis of hexa-allylamino-cyclotriphosphazene as a reactive fire retardant for unsaturated polyesters. J. Appl. Polym. Sci. 2004, 91, 697−702. (8) Zhang, C.; Huang, J. Y.; Liu, S. M.; Zhao, J. Q. The synthesis and properties of a reactive flame-retardant unsaturated polyester resin from a phosphorus-containing diacid. Polym. Adv. Technol. 2011, 22, 1768−1777. (9) Jutemar, E. P.; Jannasch, P. Influence of the Polymer Backbone Structure on the Properties of Aromatic Ionomers with Pendant Sulfobenzoyl Side Chains for Use As Proton-Exchange Membranes. ACS Appl. Mater. Interfaces 2010, 2, 3718−3725. (10) Abate, L.; Blanco, I.; Cicala, G.; Recca, A.; Restuccia, C. L. Thermal and rheological behaviours of some random aromatic aminoended polyethersulfone/polyetherethersulfone copolymers. Polym. Degrad. Stab. 2006, 91, 3230−3236. (11) Braun, U.; Knoll, U.; Schartel, B.; Hoffmann, T.; Pospiech, D.; Artner, J.; Ciesielski, M.; Doring, M.; Perez-Gratero, R.; Sandler, J. K. W.; Altstadt, V. Novel phosphorus-containing poly(ether sulfone)s and their blends with an epoxy resin: Thermal decomposition and fire retardancy. Macromol. Chem. Phys. 2006, 207, 1501−1514. (12) Chen, H.; Zhang, K.; Xu, J. Synthesis and characterizations of novel phosphorous−nitrogen containing poly(ether sulfone)s. Polym. Degrad. Stab. 2011, 96, 197−203. (13) Lin, C. H.; Chang, S. L.; Wei, T. P. High-Tg Transparent Poly(ether sulfone)s Based on Phosphinated Bisphenols. Macromol. Chem. Phys. 2011, 212, 455−464. (14) Wang, Y. Z.; Yi, B.; Wu, B.; Yang, B.; Liu, Y. Thermal behaviors of flame-retardant polycarbonates containing diphenyl sulfonate and poly(sulfonyl phenylene phosphonate). J. Appl. Polym. Sci. 2003, 89, 882−889.

Figure 12. Stress−strain curves for the different UPRs.

Table 3. Tensile Properties of the Different UPRs sample

tensile strength (MPa)

Young’s modulus (MPa)

elongation (%)


19.07 21.93 24.25 18.26

3509 3612 3815 4173

0.56 0.87 1.13 0.53

REFERENCES

samples,34 leading to increased elongation within a certain DASPP content range, as shown in Figure 12. Consequently, the tensile strength exhibits an increasing tendency. When DASPP rises to 20%, the overdeveloped cross-linking and chain rigidity can offset the flexibility of the relatively limited number of ether groups, and then significantly increase the brittleness of the sample.35 As a result, a decrease in both elongation and tensile strength can be observed, although there is still a marked improvement in Young’s modulus, because of the further reinforced stiffness induced by the deeper cross-linking degree and the increasing numbers of aromatic rings and sulfone groups.

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CONCLUSIONS In this work, a novel reactive phosphorus- and sulfur-containing flame retardant monomer (DASPP) was synthesized, and various amounts of DASSP were incorporated into unsaturated polyester by radical bulk polymerization. The resulting UPR samples demonstrated significant enhancements in thermal properties, flame retardancy, and mechanical properties. The TGA results revealed that the introduced DASPP can contribute improved thermal and thermo-oxidative stability at high temperatures, as well as higher char yield, to UPR. Meanwhile, much lower values of pHRR and THR were observed for samples with DASPP from cone calorimetry, corresponding to an enhancement of the LOI value. Results from both SEM and Raman spectroscopy corroborated the improved microstructure and graphitization degree of UPR char residues, further suggesting enhanced thermo-oxidative stability in high-temperature regions by DASPP. After RTIR analysis, the thermo-oxidative degradation mechanisms of different UPR samples were presented, shedding light on the results obtained by cone calorimetry, TGA, and so on. Furthermore, the characteristic molecular structure of DASPP benefited UPR in terms of Tg and mechanical properties. 15925



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Enhanced Properties of the Incorporation of a Novel Reactive

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