Synthesis of N Alkoxy Hindered Amine Containing Silane …myweb.fsu.edu/yli5/index_files/Articles/j8.pdfsynthesized by combining N-alkoxy hindered amine and silane coupling together

Synthesis of N‑Alkoxy Hindered Amine Containing Silane as aMultifunctional Flame Retardant Synergist and Its Application inIntumescent Flame Retardant PolypropyleneKun Cao,†,‡ Shui-liang Wu,‡ Shao-long Qiu,‡ Yan Li,‡ and Zhen Yao*,‡

†State Key Laboratory of Chemical Engineering and ‡Institute of Polymerization and Polymer Engineering, Department of Chemicaland Biological Engineering, Zhejiang University, Hangzhou 310027, China

ABSTRACT: A novel and multifunctional flame retardant synergist, N-alkoxy hindered amine containing silane (Si-NORs), wassynthesized by combining N-alkoxy hindered amine and silane coupling together through sol−gel reaction. The composition ofSi-NORs was characterized by FTIR and XPS. Intumescent flame retardant polypropylene (IFR−PP) composites were preparedwith different contents of Si-NOR and characterized by the limiting oxygen index (LOI), vertical burning tests (UL-94 tests),TGA, the Yellowness Index (YI), mechanical properties, and SEM measurements. The results showed that IFR−PP compositeswith 1 wt % Si-NORs and 25 wt % intumescent flame retardant could reach a V-0 rating in the UL-94 tests. Moreover, thethermal stability, UV stability, mechanical properties, compatibility, and char residue structure were also improved significantly,which proves Si-NOR as a multifunctional flame retardant synergist. The possible synergistic mechanism of Si-NORs was alsodiscussed.

1. INTRODUCTION

Polypropylene (PP) has been widely used in many fields due toits excellent mechanical properties, ease of processing, low cost,etc.1,2 Unfortunately, its application has greatly been limited byits inherent flammability.The addition of flame retardants (FRs) is an effective way to

reduce flammability.3,4 With advantages such as low release ofsmoke and toxic gases and antidripping characteristics, eco-friendly intumescent flame retardants (IFRs) have been welldeveloped as replacements for the halogen-containing flameretardants.5,6 Typically, an IFR consists of three ingredients,namely, an acid resource, usually ammonium polyphosphate(APP), a carbon source, commonly pentaerythritol (PER), anda blowing agent, such as melamine (MEL).7,8 However, it alsohas some disadvantages, such as low compatibility withpolyolefins and heavy loading, which deteriorate the UVstability and mechanical properties of PP greatly.9,10

An efficient approach to resolve these problems is to usesynergists that can enhance the flame-retardant efficiency ofIFR significantly.11−14 Hindered amine has been used as a UVstabilizer for a long period of time.15,16 The recent attempt toimprove the properties of hindered amine reveals that the N-alkoxy hindered amines (NORs) possess excellent flameretardancy resulting from the thermolysis of NORs, whichleads to the formation of efficient and regenerable free radicalscavengers, interrupting and suppressing the free radicalcombustion progress of polyolefins.17−19 Furthermore, NORshave a good synergistic effect in combination with conventionalFRs to improve their efficiency through radical reactions andreduce the loading of conventional FRs.17−19

Some efforts have been made to investigate the synergisticeffect between NORs and other conventional FRs. CibaSpecialty Chemicals (now BASF) disclosed patents describingthe activity of NORs as FR synergists with organic or inorganicbrominated and/or phosphorus containing FRs for polyole-

fins.20,21 Ciba also introduced the first product in the area offlame retardants based on NORs, Flamestab NOR 116.22

Zhang et al.23 investigated the synergistic effect of FlamestabNOR 116 and APP in fiber-forming PP containing nanoclays.They found that the char residue was increased and theantioxidant character of Flamestab NOR 116 was enhanced.The work of Marney et al.24 demonstrated that the addition ofNORs to a PP system, containing tris(3-bromo-2,2-bis-(bromomethyl)propyl) phosphate (TBBPP), improved its UL94 rating from V-2 to V-0 and reduced the onset temperature ofthermal decomposition. The subsequent study of this groupfound that the generation of nitroxyl radicals from NORs caninteract with TBBPP and facilitate the release of bromine,thereby improving the flame retardant performance.25 How-ever, to the best of our knowledge, there are few references tothe investigation of the synergistic effect between NORs andIFRs so far.Considering the great contribution to the UV stability of

polyolefins, research on NORs have also been focused oncombining NORs and other effective ingredients into a newmultifunctional flame retardant. Aubert et al.26,27 synthesized aninnovative and multifunctional flame retardant compound bycombining NORs and diazene into a new molecule (AZO-NOR), which alone can effectively provide flame retardancyand self-extinguishing properties to PP.28

It is well-known that silane coupling agents have a goodcapability of bonding the fillers and polymer matrixes as well asa great contribution to forming a compact and dense charstructure during combustion.29−31 Besides, through the sol−gelreaction of silane couplings, the Si−O−Si network structure

Received: June 27, 2012Revised: November 11, 2012Accepted: November 20, 2012Published: November 20, 2012

Article

pubs.acs.org/IECR

© 2012 American Chemical Society 309 dx.doi.org/10.1021/ie3017048 | Ind. Eng. Chem. Res. 2013, 52, 309−317

pubs.acs.org/IECR

between silane couplings can be easily formed in a less energy-consuming and simple synthetic way.31 In our previous work,the chemical structure, properties, synthesis methods, and flameretardant mechanism of NORs and their latest applications asFR or FR synergists in polyolefins was systematicallyreviewed.19 In this article, a novel and multifunctional flameretardant synergist, N-alkoxy hindered amine containing silane(Si-NORs), is synthesized through combining NORs and silanecoupling agents together based on the sol−gel reaction.Moreover, IFR−PP composites with Si-NORs were charac-terized by the limiting oxygen index (LOI), vertical burningtests (UL-94 tests), thermogravimetric analysis (TGA), theYellowness Index (YI), mechanical properties, and scanningelectron microscopy (SEM) measurements. Based on thestructure of the char residue, a plausible mechanism of Si-NORs synergetic effects is also discussed.

2. EXPERIMENTAL SECTION2.1. Materials. PP (F401, powder) was provided by

SINOPEC Yangzi Petrochemical Co., Ltd. Maleic anhydridegrafted PP (PP-g-MAH, MAH content = 1 wt %) as acompatibilizer was purchased from Ningbo Nengzhiguang NewMaterials Technology Co., Ltd. Antioxidant B215 was providedby Nanjing Hua Lim Co., Ltd. APP with average degree ofpolymerization n > 1000 was supplied by Hangzhou JLS FlameRetardants Chemical Co., Ltd. MEL and PER were purchasedfrom Shanghai LingFeng Chemical Reagent Co., Ltd. Tinuvin

152 (T152), a reactive N-alkoxy hindered amine with ahydroxyl group, was provided by Ciba Specialty Chemicals.Cyanuric chloride (TCT, 99%) was purchased from AcrosOrganics. N,N-Diisopropylethylamine (DIPEA, 99%), used asan acid-binding agent, was purchased from Shanghai DEMOChemical Co., Ltd. KH-553, a silane coupling agent, wasprovided by Hangzhou JessicaChem Co., Ltd. Acetone and 1,4-dioxane were distilled before use, and other reagents were usedas received without further purification.

2.2. Synthesis of Si-NORs. Cyanuric chloride (2.766 g,0.015 mol) and acetone (80 mL) were fed into a four-neck flaskequipped with an ice bath, a stirrer, a thermometer, a refluxcondenser, a microsyringe, and a nitrogen inlet. After themixture was purged with nitrogen atmosphere under vigorousmechanical stirring, a solution of T152 (7.572 g, 0.01 mol) andDIPEA (2.61 mL, 0.015 mol) in acetone (30 mL) was addeddropwise through the microsyringe to the flask within 0.5 h at0−5 °C. The reaction was carried out for 3 h.The mixed solution of both DIPEA (2.61 mL, 0.015 mol)

and KH-553 (1.65 mL, 0.015 mol) in acetone (20 mL) wasadded to the above flask within 0.5 h, and the reactiontemperature was increased to 48 °C simultaneously. Thereaction continued at 48 °C for 2 h. Then, the reaction solutionwas placed in a rotary evaporator (55 °C) to remove acetoneand dried for 12 h at 45 °C in a vacuum oven. The residue waswashed with acetone/water (1:1, volume ratio) to remove the

Scheme 1. Synthetic Route for the Preparation of Si-NORs

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie3017048 | Ind. Eng. Chem. Res. 2013, 52, 309−317310

residual reactants and further dried about 24 h at 55 °C. Theobtained intermediate is light yellow powder (yield 97.56%).The intermediate (5.03 g, 0.005 mol) and 1,4-dioxane (50

mL) were fed into the four-neck flask. The mixed solution ofDIPEA (2.61 mL, 0.015 mol) and KH-553 (1.10 mL, 0.01 mol)in acetone (10 mL) was added to the flask through themicrosyringe within 0.5 h, and the reaction mixture was heatedto reflux simultaneously. After further heating at reflux for 3 h,the reaction mixture was added with saturated ammonia (10mL) in 0.5 h and was kept under reflux for 1 h. Here, saturatedammonia was used as a catalyst to promote the sol−gel reactionof silane coupling agents to form Si−O−Si networks. After thereaction was completed, followed by cooling, filtration, washing,and drying, a novel flame retardant synergist, Si-NORs, wasobtained as yellow powder with 63.38 wt % yield. The route forthe preparation of Si-NORs is presented in Scheme 1. TheFourier transform infrared (FTIR) and X-ray photoelectron(XPS) spectra of Intermediate and Si-NORs are shown inFigures 1 and 2, respectively.

2.3. Preparation of IFR−PP Samples. PP, IFR, and Si-NORs were dried in a vacuum oven at 80 °C overnight beforeuse. The IFR−PP composites were prepared by blending 75 wt% PP powder (with 3 wt % compatibilizer PP-g-MAH and 0.5wt % antioxidant B215), 25 wt % IFR, and different additions of

Si-NORs using a high-speed mixer, and then being extruded bya twin-screw extruder (HAAKE Polylab OS, Thermo ElectronGmbH, Germany) at 190 °C. The resulting samples were hot-pressed into different shapes for further tests. The detailedformulations of IFR−PP composites are listed in Table 1.

2.4. Characterization and Measurements. The FTIRspectra were recorded with a Nicolet 5700 FT-IR spectropho-tometer using a thin KBr disk. The transition mode was used,and the wavenumber range was set from 4000 to 500 cm−1.The X-ray photoelectron spectra (XPS) were recorded with a

VG ESCALAB MARK II spectrometer (Mg Kα, 1253.6 eV;constant analyzer energy (CAE), 50 eV; steps, 0.2 eV, 0.5 eV).Limiting oxygen index (LOI) values were measured using a

XYC-75 oxygen index instrument (Jiaxing Kaibo TestingInstrument Co., Ltd., China) with a sheet dimension of 100× 6.5 × 3 mm3 according to ASTM D2863. UL-94 verticalburning tests were conducted on a vertical burning instrumentDR-I (Chengde Jinjian Testing Instrument Co.,Ltd., China)with a sheet dimension of 130 × 13 × 3 mm3 according toASTM D3801. UV-light irradiation was carried out by using anaccelerated weathering tester, which contains eight UV lamps(Model UVA-313, 40 W, 0.8 W/m2, wavelength 313 nm).Samples were mounted on metallic boards, and the temper-ature was regulated at 60 °C and controlled by a Ptthermocouple.Samples were taken out at regular intervals, and their

Yellowness Index (YI) and mechanical properties weremeasured. The YI was measured by Hunterlab ColorQuestXE (Shanghai Shanion Creative Inc., China) with parallel platesof 25 mm diameter and a gap of 1 mm. The tensile and bendingproperties were measured by a Zwick Z020 universal tensilemachine (Zwick, Germany). The impact property wasmeasured by a CEAST impact tester (CEAST, Italy). At leastfive replicates were conducted for each mechanical property.Thermogravimetric analysis (TGA) was carried out on a

Pyris 1 thermoanalyzer instrument under N2 flows. Thespecimens (about 10 ± 0.2 mg) were heated from roomtemperature to 600 °C at a linear rate of 10 °C/min.

Figure 1. Fourier transform infrared spectra of TI52, Intermediate,and Si-NORs.

Figure 2. XPS spectra of Intermediate and Si-NORs.

Table 1. Effect of Si-NORs on Flame Retardancy of IFR−PPComposites

components (%) flame retardancy

sample PP IFRaSi-

NORsLOI(%) UL-94

secondflame time

(s) dripping

PP100 100 0 0 18 failed − yes

PP85/IFR15/Si-NORs0

85 15 0 24 failed − yes

PP80/IFR20/Si-NORs0

80 20 0 28 failed − yes

PP75/IFR25/Si-NORs0

75 25 0 30 V-2 25 yes

PP70/IFR30/Si-NORs0

70 30 0 34 V-0 1 no

PP75/IFR25/Si-NORs0.5

75 25 0.5 30 V-2 20 yes

PP75/IFR25/Si-NORs1

75 25 1 32 V-0 5 no

PP75/IFR25/Si-NORs3

75 25 3 32 V-2 9 yes

PP75/IFR25/Si-NORs5

75 25 5 32 V-2 6 yes

aIFR was composed of APP, PER, and MEL with the weight ratio fixedat 2:1:1.



Scanning electron microscopy (SEM) was performed on thecross sections of LOI samples and their char residues after LOItests using a SIRION scanning electron microscope (FEI,Netherlands) with 25.0 kV beam voltage.

3. RESULTS AND DISCUSSION

3.1. Structure Characterization. The FTIR spectra ofTI52, Intermediate, and Si-NORs are shown in Figure 1. Thethree curves are similar except in the range of 800−1300 cm−1.The detailed assignments of IR absorption peaks are presentedin Table 2. The peak at 1054.2 cm−1 in the curve of T152corresponds to the stretching vibrations of −OH. Theemerging peak at 844.3 cm−1 in the curve of Intermediate isattributed to the triazine−Cl, and its disappearance from thecurve of Si-NORs indicates the occurrence of chloridedisplacement reaction and that the Cl atoms attached on thetriazine ring have been totally replaced. Meanwhile, the peaks at1050.7−1117.9 cm−1 in the curve of Intermediate correspondto the stretching vibrations of Si−O, which become muchstronger in the curve of Si-NORs. Moreover, the strongabsorption bands attributed to Si−C at 802.8 and 1261.7 cm−1

in the curve of Si-NORs prove the occurrence of the sol−gelreaction of KH-553.The XPS spectra of Intermediate and Si-NORs are shown in

Figure 2. For Intermediate, the six peaks at 102.5, 152.5, 199.5,284.0, 398.5, and 531.0 eV are attributed to Si 2p, Si 2s, Cl 2p,C 1s, N 1s, and O 1s, respectively.32,33 After the sol−gelreaction, the Si 2p and Si 2s peaks of Si-NORs are greatlyincreased and the Cl 2p peak almost disappears compared withthat of Intermediate, which further confirms Si-NORs issynthesized through the chloride displacement reaction ofcyanuric chloride and sol−gel reaction of silane couplings.3.2. Flame Retardancy and Thermal Stability. The

effect of Si-NORs on the flame retardancy of IFR−PPcomposites was evaluated by LOI and the UL-94 test. Theresults together with the formulations of IFR−PP compositesare shown in Table 1. Pure PP is highly combustible as well aseasy dripping and is not classified in the UL-94 rating. Highloading of IFR is necessary to provide adequate flameretardancy. It takes 30 wt % IFR to obtain a LOI value of 34and pass the V-0 rating. The results of the LOI test show thatthe LOI values of IFR−PP composites with 25 wt % IFR

increase from 30 to 32 when the addition of Si-NORs is morethan 0.5 wt %. In the UL-94 test, the flames are self-extinguished more rapidly, and the second flame time becomesmuch shorter after the addition of Si-NORs. With the additionof 1 wt % Si-NORs and 25 wt % IFR, the LOI value of IFR−PPcomposites is 32 and a UL-94 V-0 rating can be reached.The digital photos of PP75/IFR25/Si-NORs1 after LOI tests

are shown in Figure 3. As we can see, the samples are self-extinguished rapidly as the O2 is 30% and 32% while formsnotable intumescent char layer during burning process as theO2 was 34%.

However, when the loading of Si-NORs increases, the LOIvalues are just kept at 32 and samples exhibit burning drippingbehavior during the UL-94 test, only passing the V-2 rating.This is a result of the inverse concentration effect of flameretardants based on N-alkoxy hindered amines as reported inother literature,28 and we also discuss it in section 3.6.Therefore, the optimum addition of Si-NORs is 1 wt %.The TGA and DTG curves of T152, Si-NORs, PP75/IFR25/

Si-NORs0, and PP75/IFR25/Si-NORs1 in nitrogen atmos-phere are presented in Figure 4. The relevant data are shown inTable 3. It can be seen that Si-NORs has a lower weight lossrate and more residual char compared to T152. The initialdecomposition temperature (Tinitial) of Si-NORs is 255.19 °C,which is lower than that of T152 due to the dehydrationcondensation reaction of Si−OH. The degradation process ofSi-NORs can be divided into two stages. The first stage is

Table 2. Assignments of FTIR Absorption Peaks

assignment Tinuvin 152 Intermediate Si-NORs references

−OH, N−H, stretching vibration 3454.0 3424.1 3398.8 34C−H, stretching vibration 2932.6, 2858.1 2933.5, 2858.5 2934.1, 2861.6 31, 34in-plane deformation triazine ring 1556.1 1557.5 1600.4 34N−H bending vibration 1529.9 1529.5 1529.8 34in-plane deformation triazine 1482.9 1480.8 1482.1 344-substituted piperidine ring 1425.7 1432.6 1435.5 34aryl-N 1365.6 1366.6 1367.6 34methyl, bending 1314.4 1317.6 1320.8 34Si−C, wagging − overlapped 1261.7 35N−H, bending vibration 1243.6 1237.6 1238.2 34N-alkyl 1210.3 1208.2 1202.5 34Si−O, stretching vibration − 1117.9−1050.7 1197.8−1026.2 31, 35−OH, stretching vibration 1054.2 overlapped overlapped 34CH rocking 967.1 968.1 968.7 34−Cl − 844.3 − 36Si−C − 806.2 802.8 2, 374-substituted piperidine ring 810.7 overlapped overlapped 34

Figure 3. Digital photos of PP75/IFR25/Si-NORs1 after LOI tests. O2was (a) 30, (b) 32, and (c) 34%.



caused by the further dehydration condensation as well as initialpyrolysis of Si-NORs (the fracture of nonaromatic alkyl) in thetemperature range 280−320 °C. The second stage can beattributed to the pyrolysis of Si-NORs and the formation of aceramic-like network structure containing Si−O−Si around 360°C. The stability of that structure makes the second maximumrate decomposition temperature (Tpeak) of Si-NORs 30 °Chigher than that of T152. The char residue of Si-NORs is 23.06wt % at 600 °C, while there is only 1.637 wt % left for T152.These results indicate that Si-NORs shows higher thermalstability after sol−gel reaction of silane couplings.As for IFR−PP composites, the Tinitial and Tpeak of IFR−PP

are increased remarkably with the addition of 1 wt % Si-NORs.The Tinitial and Tpeak of PP75/IFR25/Si-NORs1 are 310.57 and474.65 °C, respectively, increased by 32.53 and 86 °Ccompared to those of PP75/IFR25/Si-NORs0. The process

of decomposition for PP75/IFR25/Si-NORs0 is almostcompleted at about 450 °C, whereas that of PP75/IFR25/Si-NORs1 is up to 500 °C. These results indicate that theincorporation of 1 wt % Si-NORs into IFR−PP composites canimprove the thermal stability of the IFR−PP composites athigher temperature. However, the char residue left at 600 °C isat the same level for both samples.

3.3. UV Stability. To estimate the effect of Si-NORs on theUV stability of IFR−PP composites, the Yellowness Index (YI)and mechanical properties of IFR−PP samples resulting fromexposure to UV-light irradiation were measured.The effect of exposure time on the Yellowness Index of IFR−

PP samples is shown in Figure 5. It can be seen that the

addition of 1 wt % Si-NORs can effectively protect IFR−PPfrom aging and yellowing. Before UV-light irradiation, the YI ofPP75/IFR25/Si-NORs1 is 26.10, higher than the 18.26 ofPP75/IFR25/Si-NORs0, which shows that the addition of Si-NORs can cause the yellowing of IFR−PP by itself.28 After 5days of UV-light irradiation, the YI of PP75/IFR25/Si-NORs0is increased by 189.0%, up to 57.74, whereas the YI of PP75/IFR25/Si-NORs1 is 37.52, increased by only 43.75%. More-over, the YI of PP75/IFR25/Si-NORs0 is increased furtherwhile the YI of PP75/IFR25/Si-NORs1 remains stable with theextended exposure time.The effects of exposure time on tensile, bending, and impact

strengths of PP75/IFR25/Si-NORs0 and PP75/IFR25/Si-NORs1 are shown in Figure 6. It is found that 30 days ofUV-light irradiation can decrease the tensile, bending, andimpact strengths of PP75/IFR25/Si-NORs0, by 25, 27, and34%, respectively. The corresponding decline of PP75/IFR25/Si-NORs1’s properties is much lower, by only 5, 16, and 10%,

Figure 4. TGA and DTG curves of T152, Si-NORs, PP75/IFR25/Si-NORs0, and PP75/IFR25/Si-NORs1.

Table 3. Thermal Degradation Data under Pure Nitrogen by TGA

Rpeak/Tpeakb (%·min−1/ °C)

sample Tinitiala (°C) stage 1 stage 2 stage 3 char residuec (wt %)

T152 277.9 8.421/303.0 8.831/333.5 3.998/423.6 1.637Si-NORs 255.2 4.212/299.8 4.763/360.7 − 23.06PP75/IFR25/Si-NORs0 278.0 20.83/388.7 9.135PP75/IFR25/Si-NORs1 310.6 22.38/474.7 8.883

aTinitial, temperature where 5 wt % weight loss occurred. bRpeak, maximum weight loss rate of samples; Tpeak, temperature where maximum weight lossrate occurred. cChar residue was obtained at 600 °C.

Figure 5. Effect of exposure time on Yellowness Index of PP75/IFR25/Si-NORs0 and PP75/IFR25/Si-NORs1.



respectively. The decrease in PP75/IFR25/Si-NORs1’s me-chanical properties becomes stable after 10 days of UV-lightirradiation, while the PP75/IFR25/Si-NORs0 keeps deteriorat-ing.All results indicate that Si-NORs, as a flame retardant

synergist, can also provide IFR−PP composites with a good UVstability.3.4. Mechanical Properties. The effects of Si-NORs

loading on tensile, bending, and impact strengths of IFR−PPcomposites are presented in Figure 7. The tensile strength ofpure PP is 32.2 MPa, and is reduced to 24.4 MPa after added25 wt % IFR. It is mainly due to the poor compatibility between

IFR and PP matrix. With the increasing addition of Si-NORs,the tensile strength of IFR−PP composites shows a slight rise.When loaded with 1 wt % Si-NORs, the tensile strength ofIFR−PP composites increases to 26.2 MPa. Further increase inSi-NORs loading has an insignificant effect on composites’tensile strength.Si-NORs can improve the bending strength of IFR−PP

composites significantly. With 1 wt % Si-NORs in IFR−PPcomposites, the bending strength increases from 44.3 to 50.0MPa and even reaches 52.7 MPa with 5 wt % Si-NORs in IFR−PP composites, which is almost as good as pure PP.Similarly, the improvement of impact strength of IFR−PP

composites by Si-NORs is also concentrated on the additionwithin 1 wt %. However, since there is a serious deterioration in

Figure 6. Effects of exposure time on tensile strength, bendingstrength, and impact strength of PP75/IFR25/Si-NORs0 and PP75/IFR25/Si-NORs1.

Figure 7. Effect of Si-NORs loading on tensile strength, bendingstrength, and impact strength of IFR−PP composites.



impact strength of IFR−PP composites after the addition of 25wt % IFR, the impact strength of IFR−PP composites is stillmuch lower than that of pure PP even added with 5 wt % Si-NORs.Seen from the above results, Si-NORs can also improve the

mechanical properties of IFR−PP composites to differentextents and the optimum addition of Si-NORs is 1 wt %. Theexplanation is that Si-NORs improves the compatibilitybetween the IFR and PP matrix, which is proved by themorphological structures of the IFR−PP composites shown insection 3.5.3.5. Morphological Structure of Composites and Char

Residues. From the above analysis, a conclusion can be drawnthat the flame retardancy thermal, UV stability, and mechanicalproperties of IFR−PP composites are improved with theintroduction of Si-NORs. The mechanical properties arecorrelative to the microstructures of composites. Figure 8presents the morphologies of PP75/IFR25/Si-NORs0 (Figure8A1,A2) and PP75/IFR25/Si-NORs1 (Figure 8B1,B2) observedby SEM measurement. From Figure 8A1,A2 (the micrographs ofsamples without Si-NORs), the interface between IFR and PPmatrix can be clearly observed (marked by black arrows).Moreover, a lot of IFR particles aggregate severely on thesurface of PP matrix (marked by white arrows), suggesting poor

compatibility. In this case, the mechanical properties of IFR−PP composites must be decreased due to poor interfacialbonding between IFR particles and PP matrix. As is shown inFigure 8B1,B2 (samples with 1 wt % Si-NORs), most of the IFRparticles are uniformly embedded in the PP matrix and theinterface between IFR and PP matrix becomes blurred.These results indicate that Si-NORs also acts as a

compatibilizer in IFR−PP composites. At one end, Si-NORshas many alkoxy silane groups and unreacted silanol groupscapable of reacting with HO-rich surfaces of IFR particles. Atthe other end, it has a large number of alkyl groups which havea good compatibility with PP matrix. The compatibilization ofSi-NORs can effectively improve the interfacial compatibilitybetween IFR and PP matrix and enhance the mechanicalproperties of the IFR−PP composites, which is in agreementwith the results from the mechanical properties tests.It is known that the formation of a dense and compact

intumescent charred layer during combustion is the essentialfactor for improving the flame retardancy of IFR−PPcomposites. Figure 9 shows SEM image of the intersection ofchar residues after the LOI test. It can be found that both charshave cellular structure. Compared to the sample without Si-NORs (Figure 9a), the char residue of sample with 1 wt % Si-NORs (Figure 9b) has a higher cell number and thinner cellular

Figure 8. SEM micrographs for (A1, A2) PP75/IFR25/Si-NORs0 and (B1, B2) PP75/IFR25/Si-NORs1.

Figure 9. SEM micrographs of the residues of inner surface of (a) PP75/IFR25/Si-NORs0 and (b) PP75/IFR25/Si-NORs1 after LOI test.



wall, which can slow down heat and mass transfer between gasand condensed phases and enhance the flame retardancy andthermal stability of IFR−PP composites more effectively. Thisresult is in accordance with the LOI, UL-94 tests, and TGAanalysis of IFR−PP composites in section 3.2.3.6. Possible Synergistic Mechanism. On the basis of the

above analysis, it is reasonable to speculate that the synergisticmechanism of Si-NORs in IFR−PP composites is attributed toboth a gas phase mechanism and a condensed phasemechanism.It is well-known that the pyrolysis of PP is a free radical chain

reaction through β-scission of PP chains, while the producedfree radicals in return speed up the degradation of PP.38 In thegas phase, the efficient and regenerable free radical scavengers,nitroxyl radicals, generated from the thermal decomposition ofSi-NORs, can be involved in the free radical chemical reactionsduring the combustion process and reduce the free radicalconcentration by converting them into relatively stable alcoholsand ketones.17−19 This is the main reason why the flames areoften self-extinguished rapidly and the second flame time ofsamples with Si-NORs became much shorter during the UL-94test.In the condensed phase, Si-NORs improves the quality and

structure of char residues considerably. During combustion ofIFR−PP composites, nonflammable gases such as NH3 andH2O will be generated from the blowing agent of IFR and themelting char residues will be foamed. As shown in Figure 10, anumber of nitroxyl radicals are generated in one Si-NORmolecule during the thermal decomposition of Si-NORs(namely Si[NO•]n). On the basis of the efficient radical-trapping ability of nitroxyl radicals, it is reasonable to concludethat a cross-linking network will be formed in situ. The networkcan improve the melt viscosity of condensed phases, stabilizecell growth, prevent cell coalescence, and result in the char layerwith better insulation properties.

To take the inverse concentration effect of Si-NORs intoconsideration, it is possible that increasing addition of Si-NORs(more than 1 wt %) promotes scissions of PP as a sequence ofhigher free radical concentration after heating and decom-position of the NORs component. When the addition of Si-NORs is more than 1 wt %, above effect even becomes thedominant role which reduces the molecular weight and hencethe melt viscosity of the condensed phase, so promoting thechange from V-0 to V-2.

4. CONCLUSIONS

A novel and multifunctional flame retardant synergist, N-alkoxyhindered amine containing silane (Si-NORs), was synthesizedbased on the sol−gel reaction. It is proved that Si-NORs canreplace part of IFR and endow IFR−PP composite with betterflame retardancy. With 1 wt % loading of Si-NORs, the LOIvalue of IFR−PP composites (with 25 wt % IFR) is increasedfrom 30 to 32 and the UL-94 rating is V-0. The synergisticeffect of Si-NORs is attributed to the capture of active freeradicals in the gas phase as well as the formation of a cross-linking network in the condensed phase by free radicalscavengers generated from the thermal decomposition of Si-NORs, which improves the morphology of the cellular charredlayer and enhances the flame retardancy of IFR−PPcomposites. Si-NORs also acts as the UV stabilizer andcompatibilizer to improve the UV stability and mechanicalproperties of IFR−PP.

■ AUTHOR INFORMATION

Corresponding Author*Tel./fax: ++86-571-87951832. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

Figure 10. Possible flame retardant mechanism of IFR−PP composite with Si-NORs.



mailto:[email protected]

■ ACKNOWLEDGMENTS

This work was supported by the National Natural ScienceFoundation of China through Project 51173166, ZhejiangProvincial Natural Science Foundation through ProjectY4110134, the Program for Changjiang Scholars and InnovativeResearch Team in University, and the Fundamental ResearchFunds for the Central Universities.

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Synthesis of N Alkoxy Hindered Amine Containing Silane …myweb.fsu.edu/yli5/index_files/Articles/j8.pdfsynthesized by combining N-alkoxy hindered amine and silane coupling together

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