Facile modification of aluminum hypophosphate and its flame retardancy for polystyrene

Wensheng Li ,Liangyuan Qi ,Daolin Ye ,Wei Cai,3 ,Weiyi Xing,3,*

1 State Key Laboratory of Fire Science,University of Science and Technology of China,Hefei 230026,China

2 Nano Science and Technology Institute,University of Science and Technology of China,Suzhou 215123,China

3 Suzhou Key Laboratory of Urban Public Safety,Suzhou Institute for Advanced Study,University of Science and Technology of China,Suzhou 215000,China

Keywords:Nanomaterials Safety Thermodynamic properties Aluminum hypophosphite Char formation mechanism

ABSTRACT A phosphorus-containing flame retardant,aluminum hypophosphite (AHPi),has been modified by (3-aminopropyl) triethoxysilane (KH550) to prepare flame-retardant polystyrene (PS).The influence of modified AHPi on the morphology and characterization was investigated,and differences in flame retardant properties of the PS/AHPi and PS/modified AHPi were compared.The PS composite can pass the vertical burning tests (UL-94 standard) with a V-0 rating when the mass content of modified AHPi reaches 20%,compared with the mass content of 25%AHPi.The element mapping of the PS composite shows that modified AHPi has better dispersion in PS than AHPi.Thermogravimetric analysis results indicated that adding modified AHPi can advance the initial decomposition temperature of the composite material.With the addition of modified AHPi,the decrease in peak heat release rate (pHRR) is more evident than AHPi,and the char yield of the resultant PS composites gradually increased.With the addition of 25%modified AHPi,the pHRR and total heat release of PS composites decreased by 81.4%and 37.6%.The modification of AHPi promoted its dispersion in the PS matrix and improved the char formation of PS composites.The results of real-time infrared spectrometry of PS composites,Fourier transform infrared spectra and X-ray photoelectron analysis of the char layer indicated that modified AHPi has flame retardancy in condensed and gas phases.

1.Introduction

Polystyrene (PS) is one of the five major synthetic resins as a typical thermoplastic material.Polystyrene is widely used in various fields due to its excellent chemical resistance,thermal insulation,water resistance,high transparency,and strong rigidity.Unfortunately,its flammability and dripping in burning also bring significant risks to its wide application.To reduce fire risks,flame retardancy PS is urgently needed [1-4].

Due to the difficulty of char formation,brominated flame retardants are still the most widely used commercial flame retardants of PS.Brominated flame retardants,including hexabromocyclododecane (HBCD) [5],decabromodiphenyl oxide (DBDPO) [6],tetrabromo-phthalimide,decabromodi-phenylethane,tetrabromo-bisphenol have been widely used.Brominated flame retardants have high flame-retardant efficiency due to their strong free radical capture ability.Because halogen additives produce a lot of toxic gas and harm the environment during combustion,the research on halogen-free additives has received extensive attention at home and abroad [7-12].Phosphorus-based flame retardants,metals,and montmorillonites have become the research focus worldwide [4,13-18].Metal hydroxides are widely used in polymers because of their excellent non-toxic properties [15,17].It is necessary to integrate different constituents to achieve better flame retardancy.With different modifications,montmorillonite presents different flame-retardant effects,including lamellate structures,charring forming abilities,and catalytic results.For example,Na-montmorillonite shows the best flame resistance in polystyrene resin.According to the previous report,many flame retardants even need a significant additional amount to improve the flame retardancy of PS [19].

To meet the needs of environmental protection,phosphorusbased flame retardants have been widely studied,showing excellent performance in gaseous and condensed phase flame retardants.Currently,methyl-9,10-dihydro-9-oxa-10-phosphaphe nanthrene-10-oxide (methyl-DOPO) has been used to improve the flame retardancy of PS [20].Alicyclic compounds containing phosphorus can be decomposed before PS decomposition,thus presenting free radical capture.Phosphorus-based flame retardants are also often used as acid sources in intumescent flame retardants (IFRS).Ammonium polyphosphate (APP),phytic acid,and aluminum hypophosphite (AHPi) are common acid sources in intumescent flame retardants (IFRs) [21-24].Flame-retardant PS contained 22.5% APP and 7.5% char-forming agent passes vertical burning tests rating[23].As a commercial flame retardant,AHPi has been widely used in various polymer materials due to its high flame retardancy efficiency,smoke suppression,low toxicity,and environmental friendliness.The phytic acid 6 (PA6) passed the UL-94 V-0 rating with an AHPi of up to 20% [25].There is a report that AHPi needs to be added to 25%,thus making the PS pass the UL94 V-0 rating [26].And 20% AHPi can produce polylactic acid(PLA),giving the UL-94 V-0 rating[27].Combining AHPi and other flame retardants can also improve flame retardant efficiency.The combination of 10.3%APP and 1.5%AHPi enhances the flame retardancy of unsaturated polyester resin (UPR)/APP/AHPi composites[22].Combining AHPi and zinc borate,the composition system can make polyethylene (PE) pass the UL-94 with a V-1 rating[28].As an excellent flame retardant,aluminum hypophosphate(AHPi) can work in gaseous and condensed phases.However,aluminum hypophosphate is incompatible with PS and cannot be well-dispersed in the matrix.A large number of additions will lead to the degradation of the mechanical properties of the polymeric composites.Therefore,modification of AHPi can effectively avoid this problem,and the compatibility between AHPi and matrix can improve its flame retardant efficiency.

In recent years,the surface treatment of additive flame retardants has attracted extensive attention [29,30].The surface modification of additive flame retardants can promote the dispersity of flame retardants in polymers and improve the flame retardancy of polymeric composites [31].Jiangetal.used (3-aminopropyl) triethoxysilane (KH550) to polymerize PSinsituon the surface of boron nitride (BN),thus improving the dispersion of BN in PS[32].Liuetal.used KH550 and silicone to modify the surface of APP to improve the water resistance and flame retardant of polypropylene (PP) [33].However,the influence of AHPi modified by KH550 as an additive flame retardant on the flame retardancy and morphology of polystyrene has not been investigated.

In this study,modified aluminum hypophosphite with high compatibility was well synthesized.To improve the flame retardant efficiency of AHPi,KH550 was used to modify the surface of AHPi to enhance the compatibility of the interface between AHPi and PS.The modified AHPi and the unmodified AHPi were compared with different additives to observe the effect of modified AHPi on the flame retardancy of PS.The flammability,combustion behavior,thermal degradation process,and flame retardancy mechanism of PS/modified AHPi composites were investigated in detail.

2.Experimental

2.1.Materials

PS was purchased from Yangzi Petrochemical Co.,Ltd.(Nanjing,China) and was dried in the oven at 80 °C for 8 h before blending.AHPi was purchased from Qingzhou Yichao Chemical Co.,Ltd.,China.KH550 was purchased from Aladdin Reagent,China.

2.2.Preparation of modified AHPi (MAHPi)

AHPi was modified by the pretreatment packing method.A modified solution composed of 20% KH550 (8 g),72% ethanol(28.8 g),and 8% deionized water (3.2 g) was stirred at room temperature for 30 min.AHPi was mixed with a modified solution in a solid blender.The modified solution was sprayed directly onto the AHPi and stirred for 30 min.Then,the modified AHPi(MAHPi)was dried at 120 °C (2 h).The KH550-treated AHPi is the MAHPi.

2.3.Sample preparation

PS/MAHPi were prepared by melt blending at 180 °C with a twin-roller mill (roller speed of 60 r.min-1,8 min mixing).Meanwhile,the ratios are 20% and 25%.The composite was hotpressed into sheets at 185 °C and cut to the required dimensions for tests.

2.4.Characterization

Field emission scanning electron microscopy (SEM,Philip’s XL 30E,Netherlands) was used to study the surface morphology of untreated and LBL-treated foam under high vacuum conditions at 8 kV acceleration and energy dispersive X-ray spectroscopy (EDX,Oxford,UK).

Thermogravimetric analyzer(TGA,Q5000,TA instrument,USA)conducts thermogravimetric analysis under nitrogen atmosphere(sample gas flow is 20 ml.min-1).Place the sample of about(10 ± 0.2) mg in an open alumina disk and heat it from 20 to 800 °C at a heating rate of 10 °C.min-1.These measurements obtainedT5%(the temperature at 5%mass loss rate),Tmax(the temperature at maximum mass loss rate),and 800 °C residues.

Chemical groups of the samples were tested by Fourier transform infrared spectroscopy (FTIR) spectra on a Nicolet 6700 spectrometer (Nicolet Instrument Corp.,USA) with a wavenumber range of 4000-400 cm-1.

The real-time Fourier transform infrared (RTIR) spectra were recorded using a Nicolet MAGNA-IR 750 spectrophotometer equipped with a heating device and a temperature controller.The samples were evenly mixed with KBr powders,pressed into tablets,and placed in a ventilated oven with a heating rate of 10 °C.min-1.

The sample was burned on a cone calorimeter (Fire Testing Technology,UK) according to ISO 5660 standard procedure with a dimension of 100 mm × 100 mm × 25 mm.Each sample layer is exposed to an external heat flow of 35 kW.m-2.The following parameters were recorded:time to ignite(TTI,s),peak heat release rate (HRR,kW.m-2),and total heat release (THR,MJ.m-2).Before the flammability and cone calorimetry tests,the samples were conditioned in a climate chamber (23 °C) for 24 h.

The element and chemical state of materials were studied by Xray photoelectron spectroscopy(XPS,ESCALAB MK-II,VG Scientific ltd.,UK).

The sample was conducted on CFZ-2 horizontal and vertical combustion tester (Jiangning Instrument Analysis Company,China) to confirm the vertical combustion test level (UL-94),and the sample size was 130 mm × 13 mm × 3 mm.

3.Results and Discussion

3.1.Characterization of AHPi/MAHPi

The FTIR spectra of AHPi and MAHPi are shown in Fig.1.According to the FTIR spectrum,the peaks at 2382 and 2409 cm-1corresponded to the stretching vibration of the-PH2band of AHPi[34].The peaks at 1194 cm-1were attributed to the stretching vibration of the P=O band ofof AHPi.The peaks at 1078 cm-1were attributed to the stretching vibration of P-O ofof AHPi.The characteristic peaks of MAHPi locate at 3481 cm-1,which was attributed to the stretching vibration of the-NH2band.The peaks at 1633 cm-1were attributed to the in-plane bending vibration of-NH2of MAHPi.The other peaks were the same as the characteristic peaks of AHPi.

Fig.1.FTIR spectra of AHPi and MAHPi.

The surface element compositions of MAHPi are listed in Table 1.The proportion of silicon in the MAHPi can be seen in Table 1.It occupies a minimal component in the composite material.As shown in Fig.2,after the surface modification of KH550,MAHPi was smaller than AHPi,and aggregation was improved.It can be proved that MAHPi was dispersed in PS with finer particles with the help of KH550,and its uniformity was also enhanced.

Table 1 Surface elemental compositions of MAHPi

Fig.2.SEM images of (a) AHPi and (b) MAHPi.

3.2.Morphology

The influence of AHPi and MAHPi on the morphology of PS composites was studied by SEM,as shown in Fig.3.The brittle fracture of PS/20AHPi and PS/20MAHPi was obtained by SEM photograph. The dispersion of AHPi in the PS matrix is not good,as is shown in Fig.3(a) and (b).At low magnification,there are noticeable bulges and bulks on the cross-section.However,there was no apparent aggregation of MAHPi.There is no noticeable bump on the cross-section.The EDX spectra were used to check the elemental distributions of the samples.Fig.3(c)and(d)show the elemental mapping of Al distributions Fig.3(a) and (b).The elemental mapping of Al shows that the distribution of AHPi in PS is not uniform,while the dispersion of MAHPi obtained by modification in PS is well-dispersed.

Fig.3.SEM images of (a) PS/20% AHPi and (b) PS/20% MAHPi.EDS element maps of Al of (c) PS/20%AHPi and (d) PS/20% MAHPi.

3.3.Thermal stability

TGA is considered to be one of the most effective tools for studying the thermal stability and thermal decomposition behavior of polymer composites.Fig.4.presents the TGA and DTG thermograms of PS and PS-filled AHPi/MAHPi under air atmosphere.Several characteristic degradation temperatures at 5% mass loss(T5%)and the maximum mass loss rate are listed in Table 2 for comparing the thermal stability of PS composites.As shown in Fig.4,the thermal decomposition process of PS in the air shows onestep mass loss behavior in the temperature range of 330 to 445 °C.TheT5%of AHPi-filled PS shows no difference with neat PS,but the decomposition process was divided into two steps.It is due to the decomposition temperature of AHPi being lower than that of PS.The peaks at 300-350 °C shown in Fig.4(b) represents the thermal decomposition of AHPi.In the presence of oxygen,AHPi decomposes to release PH3,which can be oxidized to H3PO4.H3PO4can promote PS to form a stable char layer,playing a role in blocking heat and oxygen[24].Compared with AHPi filled PS,the modified AHPi shows lowerT5%and lowerTmax,1.This is due to its better dispersion of modified AHPi within the PS matrix.As can be observed in Fig.4(b),the mass loss rate of AHPi and MAHPi-filled PS was decreased by 31%and 35%,respectively,compared with pure PS.It can be seen from Table 2 that the residual yield of PS composites with 25% AHPi is as high as 31.31% at 800 °C,and the residual yield of PS composites with 25% MAHPi is 31.38%at 800°C.The modified AHPi can improve its dispersibility in the PS matrix and decompose faster when heated.Before PS decomposition,MAHPi releases P.for free radical capture and promotes the formation of a stable char layer.

Table 2 Data obtained from TGA measurements of PS and PS filled with AHPi/MAHPi under air atmosphere

Fig.4.(a) TGA and (b) DTG thermograms of PS and PS filled by AHPi/MAHPi under air atmosphere.

3.4.Flame retardancy

The vertical burning tests (UL-94 standard) and limit oxygen index (LOI) were used to investigate the flame retardancy of the specimens,as is shown in Table 3.The pristine PS is flammable,accompanied by severe dripping behavior.The UL-94 rating of PS composites with 20% AHPi reached the V-2 rating.However,a V-1 rating can be reached when the addition amount of MAHPi in PS composites is up to 18%.As shown in Fig.5,MAHPi effectively inhibited the dripping of PS.UL-94 rating of PS composite is increased to V-0 rating with 20% MAHPi from a V-2 rating of PS composite with 20%AHPi.The LOI of PS composites increased from 20% to 24% when adding MAHPi reached 25%.The results indicate that the modified AHPi significantly improved the flame retardancy of PS composites compared with AHPi.

Table 3 Flame retardancy performance of PS composites

Fig.5.Digital photos of the composites after the vertical burning test: (a) PS/20%AHPi,(b) PS/25% AHPi,(c) PS/18% MAHPi,(d) PS/20% MAHPi,(e) PS/25% MAHPi.

As the most effective fire safety test method,the cone calorimeter test with a heat flux of 35 kW.m-2was used to test the flame retardancy behavior of PS,AHPi-filled PS,and MAHPi-filled PS.The data of time to ignition (TTI),peak heat release rate (pHRR),total heat release (THR),and total smoke rate (TSR),which were obtained from the cone calorimeter,were summarized in Table 4.Fig.6(a) showed the heat release rate (HRR) versus the burning time curves of the samples.The heat release rate of pure PS rapidly reached a peak value of 860 kW.m-2after ignition.With the addition of AHPi,pHRR value of AHPi-filled PS shows a significant decrease.Compared with the pure sample,the pHRR value of PS composites with 25% AHPi was decreased by 79.2%.In addition,the pHRR of PS composites with 25% AHPi was reduced to 179 kW.m-2.However,modified AHPi has a more obvious effect on reducing the peak heat release of PS.Specifically,pHRR values of PS composites with 25% MAHPi decreased to 160 kW.m-2,corresponding to an approximately 81.4%reduction compared to pure PS.With the addition of AHPi/MAHPi,the shape of the pHRR curves of PS becomes flat,as shown in Fig.6(a).The addition of MAHPi could effectively promote the char formation of PS during combustion,while the effect was better than that of AHPi.The char formation can isolate combustible material from oxygen and heat,thus presenting a condensed phase flame retardancy effect.Therefore,the heat release rate of PS composites was reduced.

Table 4 Data obtained from cone calorimeter measurements of PS and PS-filled AHPi/MAHPi under 35 kW.m-2

Fig.6.(a) HRR,(b) THR,and (c) TSP versus burning time plots of PS and PS-filled AHPi/MAHPi.

Fig.6(b)shows the total heat release(THR)versus burning time curves of the samples.Pure PS showed a rapid heat release,and total heat release achieved 109 MJ.m-2after complete combustion.With the increase of MAHPi,the THR of the composites showed a downward trend.The THR of PS composites with 25% MAHPi was 68 MJ.m-2,which was decreased by 37.6% compared with the pure PS.The decrease of THR also indicated that MAHPi played a role in the condensed phase flame retardancy effect and promoted the formation of a char layer.

As shown in Table 4,the AHPi/MAHPi-filled PS exhibited lower TTI than the pure PS.This phenomenon could be explained as the accumulation of combustibles of the AHPi/MAHPi filled PS before ignition is less than that of the pure PS.Compared with AHPi,the modified AHPi could effectively reduce the total flue gas emission(Fig.6(c)).The TSR decreased by 11.1%from 2591 m2.m-2(PS filled with 25% AHPi) to 2303 m2.m-2(PS filled with 25% MAHPi).The decrease in flue gas emission is caused by the formation of Si-O-Si structure during combustion.This structure confined the char residue in the condensed phase.

3.5.Thermal decomposition analysis

The changes in the chemical structure of PS and PS/MAHPi during thermal oxidative degradation in a condensed phase were measured by FTIR,as presented in Fig.7.For pure PS,the peak at 3436 cm-1was attributed to the stretching vibration of O-H bonds.It nearly disappears at the temperature of 100 °C,which can be explained by the release of water.The peaks at 2919 and 2850 cm-1were attributed to the stretching vibration of -CH2-.The peak at 3025 cm-1corresponded to the stretching vibration of H-C=C.Both of those bonds disappeared at 400 °C.The peaks at 1448,1490,and 1600 cm-1were attributed to the stretching vibration of C=C (aromatic hydrocarbons).The peaks at 754 and 698 cm-1were attributed to the out-of-plane bending vibration of C-H (aromatic hydrocarbons).It was considered that the complete degradation of the main chain of PS occurred at 400 °C.

The RTIR spectra of the PS/MAHPi at different degradation temperatures are shown in Fig.7.The peaks at 2408,1191,1078,and 824 cm-1are the characteristic absorption peaks of AHPi.The peak at 2408 and 824 cm-1decrease quickly in relative intensities at 330°C.Combined with the TGA test,it can be seen that the thermal decomposition of AHPi occurs before the degradation of PS.The reduction of the peak at 2408 and 824 cm-1indicates the degradation of the P-H structure,which was formed by the decomposition of aluminum hypophosphate [34].The reduction of the peak at 1191 and 1078 cm-1indicates the degradation of the P-O structure [35,36],which is considered evidence for the decomposition of aluminum hypophosphate.The earlier decomposition of AHPi is deemed evidence that it promotes the carbonization of PS.

3.6.Flame retardancy mechanism

Fig.8 shows the digital photos of char residue samples after the Cone tests.As shown in Fig.8,no char residue is left for PS (Fig.8(a)).With the addition of AHPi or MAHPi,there appeared to be a prominent char layer after combustion.The char layer is continuous,expanding,and dense,which means that AHPi and MAHPi can promote PS to form a stable char layer [37].This effect in the condensed phase separates the combustible from oxygen and heat,thus having an excellent flame retardancy performance [38-39].

Fig.8.Digital photos of the char residue after cone test: (a) PS,(b) PS/20% AHPi,(c) PS/25% AHPi,(d) PS/18% MAHPi,(e) PS/20 % MAHPi,(f) PS/25 % MAHPi.

The chemical bond formation of char residue obtained after CCT for PS composites with 25% AHPi,and PS composites with 25%MAHPi were characterized by XPS analysis in Fig.9.The XPS spectra of C 1s of char residues are shown in Fig.9(a) and (d).Three bands are observed with binding energy at 285,286,and 288 eV for C 1s of PS composites with 25% MAHPi,which are assigned to the C-C,P-O-C,and C=O groups.The peaks of P 2p spectra of PS composites with 25% MAHPi with binding energy at 135 and 136 eV are assigned to P=O and P-O groups.The XPS spectra of O 1s of PS composites with 25%MAHPi presents a significant difference from PS composites with 25%AHPi.The peaks around 534 eV can be attributed to the P-O-P and P-O-C groups.The peaks around 533 eV can be attributed to P=O and C=O groups [40].The peaks around 532 eV can be attributed to Si-O-Si and Si-O-P groups.The element compositions are shown in Table 5.The char residue of PS composites with 25% AHPi contained 34.9%C,39.6%O,19.0%P,and 6.5%Al.Compared with the PS composites with 25%AHPi,the C content in the char residue of PS composites with 25%MAHPi was higher.Combined with the results of XPS analysis [41],element compositions,and elemental mapping of PS composites with 20% AHPi and PS/MAHPi,the modified MAHPi shows better dispersion in the PS matrix and can form a broader range of physical barriers in the flame to prevent the combustibles from contacting the flame [42,43].The role of AHPi in condensed flame retardancy was confirmed.

Table 5 Element composition (%) of the char residues for PS/25% AHPi and PS/25% MAHPi

Fig.9.XPS spectra of char residues for PS/25% AHPi: (a) C 1s,(b) O 1s,(c) P 2p and PS/25% MAHPi: (d) C 1s,(e) O 1s,(f) P 2p after CCT.

Fig.10 shows FTIR spectra of the solid decomposition products of PS composites with 25% AHPi and PS composites with 25%MAHPi.According to the FTIR spectrum,these peaks at 2922 and 2852 cm-1were attributed to the symmetry and asymmetry are telescopic vibrations vibration of -CH2-.The broad absorption at 1200 cm-1was attributed to the stretching vibration of the P-O-P bond ofThe peak at 1023 cm-1corresponded to the stretching vibration of the P-O-P bond of[26].Combined with Fig.2,it is shown thatwas oxidized toin the combustion process [44].This is also evidence that MAHPi captures O radicals in the gas phase [45,46].The peak at 481 cm-1was attributed to the Al-O bond of AHPi.However,due to the presence of Si,the peak shifts to a lower wave number.The peak at 457 cm-1was attributed to the Al-O-Si bond of MAHPi.

Fig.10.FTIR spectra of char residue of PS/25% AHPi and PS/25% MAHPi.

4.Conclusions

In this work,melt blending modified AHPi and incorporated it into PS.LOI and UL-94 tests were used to evaluate the flame retardancy properties of composites.For PS composites with 20%MAHPi,the LOI value reaches 23%,and the UL-94 test can pass the V-0 rate.In addition,the modified MAHPi can also inhibit the dripping phenomenon of PS during combustion.Elemental mapping of PS composites with 20% AHPi and PS composites with 20% MAHPi prove that the modification of AHPi can promote its distribution in PS.The results of TGA and DTG confirmed that adding MAHPi can advance the initial decomposition temperature of the composite material.Compared with pHRR,THR,and TSR of PS/AHPi,PS/modified AHPi decreased significantly.According to the XPS results of char residues for PS composites with 25% AHPi and 25%MAHPi,the modified MAHPi can effectively promote char formation because the modified MAHPi forms a more stable Si-O-Si and Si-O-P structure during combustion.Through the RTIR test of composites,XPS test,and FTIR test of char residues,the results show that the effect of MAHPi on the flame retardancy of PS mainly comes from promoting the formation of stable char layer in the condensed phase and capturing free radicals in the gas phase.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research was financially supported by the Youth Innovation Promotion Association CAS (2019448),Fundamental Research Funds for the Central Universities (WK2480000007),the Excellent Young Scientist Training Program of USTC (KY2320000018),USTC Tang Scholar,Youth Innovation cross-team fund project of Qinghai Salt Lake Research Institute (LJCTD-2022-3).This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication,and we thank Hai-Tao Liu for his help on micro/nanofabrication.We thank Dr.Jin Yi at the Experimental Center of Engineering and Material Sciences at USTC for their assistance with thermophysical property analysis.