Article pubs.acs.org/IECR
Synthesis of N‑Alkoxy Hindered Amine Containing Silane as a Multifunctional Flame Retardant Synergist and Its Application in Intumescent Flame Retardant Polypropylene Kun 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 Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: A novel and multifunctional flame retardant synergist, N-alkoxy hindered amine containing silane (Si-NORs), was synthesized by combining N-alkoxy hindered amine and silane coupling together through sol−gel reaction. The composition of Si-NORs was characterized by FTIR and XPS. Intumescent flame retardant polypropylene (IFR−PP) composites were prepared with 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 composites with 1 wt % Si-NORs and 25 wt % intumescent flame retardant could reach a V-0 rating in the UL-94 tests. Moreover, the thermal 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 also discussed. fins.20,21 Ciba also introduced the first product in the area of flame retardants based on NORs, Flamestab NOR 116.22 Zhang et al.23 investigated the synergistic effect of Flamestab NOR 116 and APP in fiber-forming PP containing nanoclays. They found that the char residue was increased and the antioxidant character of Flamestab NOR 116 was enhanced. The work of Marney et al.24 demonstrated that the addition of NORs to a PP system, containing tris(3-bromo-2,2-bis(bromomethyl)propyl) phosphate (TBBPP), improved its UL 94 rating from V-2 to V-0 and reduced the onset temperature of thermal decomposition. The subsequent study of this group found that the generation of nitroxyl radicals from NORs can interact with TBBPP and facilitate the release of bromine, thereby improving the flame retardant performance.25 However, to the best of our knowledge, there are few references to the investigation of the synergistic effect between NORs and IFRs so far. Considering the great contribution to the UV stability of polyolefins, research on NORs have also been focused on combining NORs and other effective ingredients into a new multifunctional flame retardant. Aubert et al.26,27 synthesized an innovative and multifunctional flame retardant compound by combining NORs and diazene into a new molecule (AZONOR), which alone can effectively provide flame retardancy and self-extinguishing properties to PP.28 It is well-known that silane coupling agents have a good capability of bonding the fillers and polymer matrixes as well as a great contribution to forming a compact and dense char structure during combustion.29−31 Besides, through the sol−gel reaction of silane couplings, the Si−O−Si network structure
1. INTRODUCTION Polypropylene (PP) has been widely used in many fields due to its excellent mechanical properties, ease of processing, low cost, etc.1,2 Unfortunately, its application has greatly been limited by its inherent flammability. The addition of flame retardants (FRs) is an effective way to reduce flammability.3,4 With advantages such as low release of smoke and toxic gases and antidripping characteristics, ecofriendly intumescent flame retardants (IFRs) have been well developed as replacements for the halogen-containing flame retardants.5,6 Typically, an IFR consists of three ingredients, namely, an acid resource, usually ammonium polyphosphate (APP), a carbon source, commonly pentaerythritol (PER), and a blowing agent, such as melamine (MEL).7,8 However, it also has some disadvantages, such as low compatibility with polyolefins and heavy loading, which deteriorate the UV stability and mechanical properties of PP greatly.9,10 An efficient approach to resolve these problems is to use synergists that can enhance the flame-retardant efficiency of IFR significantly.11−14 Hindered amine has been used as a UV stabilizer for a long period of time.15,16 The recent attempt to improve the properties of hindered amine reveals that the Nalkoxy hindered amines (NORs) possess excellent flame retardancy resulting from the thermolysis of NORs, which leads to the formation of efficient and regenerable free radical scavengers, interrupting and suppressing the free radical combustion progress of polyolefins.17−19 Furthermore, NORs have a good synergistic effect in combination with conventional FRs to improve their efficiency through radical reactions and reduce the loading of conventional FRs.17−19 Some efforts have been made to investigate the synergistic effect between NORs and other conventional FRs. Ciba Specialty Chemicals (now BASF) disclosed patents describing the activity of NORs as FR synergists with organic or inorganic brominated and/or phosphorus containing FRs for polyole© 2012 American Chemical Society
Received: Revised: Accepted: Published: 309
June 27, 2012 November 11, 2012 November 20, 2012 November 20, 2012 dx.doi.org/10.1021/ie3017048 | Ind. Eng. Chem. Res. 2013, 52, 309−317
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Scheme 1. Synthetic Route for the Preparation of Si-NORs
152 (T152), a reactive N-alkoxy hindered amine with a hydroxyl group, was provided by Ciba Specialty Chemicals. Cyanuric chloride (TCT, 99%) was purchased from Acros Organics. N,N-Diisopropylethylamine (DIPEA, 99%), used as an acid-binding agent, was purchased from Shanghai DEMO Chemical Co., Ltd. KH-553, a silane coupling agent, was provided by Hangzhou JessicaChem Co., Ltd. Acetone and 1,4dioxane were distilled before use, and other reagents were used as 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 flask equipped with an ice bath, a stirrer, a thermometer, a reflux condenser, a microsyringe, and a nitrogen inlet. After the mixture was purged with nitrogen atmosphere under vigorous mechanical stirring, a solution of T152 (7.572 g, 0.01 mol) and DIPEA (2.61 mL, 0.015 mol) in acetone (30 mL) was added dropwise through the microsyringe to the flask within 0.5 h at 0−5 °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) was added to the above flask within 0.5 h, and the reaction temperature was increased to 48 °C simultaneously. The reaction continued at 48 °C for 2 h. Then, the reaction solution was placed in a rotary evaporator (55 °C) to remove acetone and dried for 12 h at 45 °C in a vacuum oven. The residue was washed with acetone/water (1:1, volume ratio) to remove the
between silane couplings can be easily formed in a less energyconsuming and simple synthetic way.31 In our previous work, the chemical structure, properties, synthesis methods, and flame retardant mechanism of NORs and their latest applications as FR or FR synergists in polyolefins was systematically reviewed.19 In this article, a novel and multifunctional flame retardant synergist, N-alkoxy hindered amine containing silane (Si-NORs), is synthesized through combining NORs and silane coupling agents together based on the sol−gel reaction. Moreover, IFR−PP composites with Si-NORs were characterized by the limiting oxygen index (LOI), vertical burning tests (UL-94 tests), thermogravimetric analysis (TGA), the Yellowness Index (YI), mechanical properties, and scanning electron microscopy (SEM) measurements. Based on the structure of the char residue, a plausible mechanism of SiNORs synergetic effects is also discussed.
2. EXPERIMENTAL SECTION 2.1. Materials. PP (F401, powder) was provided by SINOPEC Yangzi Petrochemical Co., Ltd. Maleic anhydride grafted PP (PP-g-MAH, MAH content = 1 wt %) as a compatibilizer was purchased from Ningbo Nengzhiguang New Materials Technology Co., Ltd. Antioxidant B215 was provided by Nanjing Hua Lim Co., Ltd. APP with average degree of polymerization n > 1000 was supplied by Hangzhou JLS Flame Retardants Chemical Co., Ltd. MEL and PER were purchased from Shanghai LingFeng Chemical Reagent Co., Ltd. Tinuvin 310
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residual reactants and further dried about 24 h at 55 °C. The obtained 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 of DIPEA (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 the microsyringe within 0.5 h, and the reaction mixture was heated to reflux simultaneously. After further heating at reflux for 3 h, the reaction mixture was added with saturated ammonia (10 mL) in 0.5 h and was kept under reflux for 1 h. Here, saturated ammonia was used as a catalyst to promote the sol−gel reaction of silane coupling agents to form Si−O−Si networks. After the reaction was completed, followed by cooling, filtration, washing, and drying, a novel flame retardant synergist, Si-NORs, was obtained as yellow powder with 63.38 wt % yield. The route for the preparation of Si-NORs is presented in Scheme 1. The Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectra of Intermediate and Si-NORs are shown in Figures 1 and 2, respectively.
Si-NORs using a high-speed mixer, and then being extruded by a twin-screw extruder (HAAKE Polylab OS, Thermo Electron GmbH, Germany) at 190 °C. The resulting samples were hotpressed into different shapes for further tests. The detailed formulations of IFR−PP composites are listed in Table 1. Table 1. Effect of Si-NORs on Flame Retardancy of IFR−PP Composites flame retardancy
components (%)
sample
PP
IFR
UL-94
second flame time (s)
PP100 PP85/IFR15/ Si-NORs0 PP80/IFR20/ Si-NORs0 PP75/IFR25/ Si-NORs0 PP70/IFR30/ Si-NORs0 PP75/IFR25/ Si-NORs0.5 PP75/IFR25/ Si-NORs1 PP75/IFR25/ Si-NORs3 PP75/IFR25/ Si-NORs5
100 85
0 15
0 0
18 24
failed failed
− −
yes yes
80
20
0
28
failed
−
yes
75
25
0
30
V-2
25
yes
70
30
0
34
V-0
1
no
75
25
0.5
30
V-2
20
yes
75
25
1
32
V-0
5
no
75
25
3
32
V-2
9
yes
75
25
5
32
V-2
6
yes
a
SiNORs
LOI (%)
dripping
IFR was composed of APP, PER, and MEL with the weight ratio fixed at 2:1:1.
a
2.4. Characterization and Measurements. The FTIR spectra were recorded with a Nicolet 5700 FT-IR spectrophotometer 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 Testing Instrument Co., Ltd., China) with a sheet dimension of 100 × 6.5 × 3 mm3 according to ASTM D2863. UL-94 vertical burning tests were conducted on a vertical burning instrument DR-I (Chengde Jinjian Testing Instrument Co.,Ltd., China) with a sheet dimension of 130 × 13 × 3 mm3 according to ASTM D3801. UV-light irradiation was carried out by using an accelerated 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 temperature was regulated at 60 °C and controlled by a Pt thermocouple. Samples were taken out at regular intervals, and their Yellowness Index (YI) and mechanical properties were measured. The YI was measured by Hunterlab ColorQuest XE (Shanghai Shanion Creative Inc., China) with parallel plates of 25 mm diameter and a gap of 1 mm. The tensile and bending properties were measured by a Zwick Z020 universal tensile machine (Zwick, Germany). The impact property was measured by a CEAST impact tester (CEAST, Italy). At least five replicates were conducted for each mechanical property. Thermogravimetric analysis (TGA) was carried out on a Pyris 1 thermoanalyzer instrument under N2 flows. The specimens (about 10 ± 0.2 mg) were heated from room temperature 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.
2.3. Preparation of IFR−PP Samples. PP, IFR, and SiNORs were dried in a vacuum oven at 80 °C overnight before use. The IFR−PP composites were prepared by blending 75 wt % PP powder (with 3 wt % compatibilizer PP-g-MAH and 0.5 wt % antioxidant B215), 25 wt % IFR, and different additions of 311
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Table 2. Assignments of FTIR Absorption Peaks assignment
Tinuvin 152
Intermediate
Si-NORs
references
−OH, N−H, stretching vibration C−H, stretching vibration in-plane deformation triazine ring N−H bending vibration in-plane deformation triazine 4-substituted piperidine ring aryl-N methyl, bending Si−C, wagging N−H, bending vibration N-alkyl Si−O, stretching vibration −OH, stretching vibration CH rocking −Cl Si−C 4-substituted piperidine ring
3454.0 2932.6, 2858.1 1556.1 1529.9 1482.9 1425.7 1365.6 1314.4 − 1243.6 1210.3 − 1054.2 967.1 − − 810.7
3424.1 2933.5, 2858.5 1557.5 1529.5 1480.8 1432.6 1366.6 1317.6 overlapped 1237.6 1208.2 1117.9−1050.7 overlapped 968.1 844.3 806.2 overlapped
3398.8 2934.1, 2861.6 1600.4 1529.8 1482.1 1435.5 1367.6 1320.8 1261.7 1238.2 1202.5 1197.8−1026.2 overlapped 968.7 − 802.8 overlapped
34 31, 34 34 34 34 34 34 34 35 34 34 31, 35 34 34 36 2, 37 34
increase from 30 to 32 when the addition of Si-NORs is more than 0.5 wt %. In the UL-94 test, the flames are selfextinguished more rapidly, and the second flame time becomes much shorter after the addition of Si-NORs. With the addition of 1 wt % Si-NORs and 25 wt % IFR, the LOI value of IFR−PP composites 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 selfextinguished rapidly as the O2 is 30% and 32% while forms notable intumescent char layer during burning process as the O2 was 34%.
Scanning electron microscopy (SEM) was performed on the cross sections of LOI samples and their char residues after LOI tests 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 of TI52, Intermediate, and Si-NORs are shown in Figure 1. The three curves are similar except in the range of 800−1300 cm−1. The detailed assignments of IR absorption peaks are presented in Table 2. The peak at 1054.2 cm−1 in the curve of T152 corresponds to the stretching vibrations of −OH. The emerging peak at 844.3 cm−1 in the curve of Intermediate is attributed to the triazine−Cl, and its disappearance from the curve of Si-NORs indicates the occurrence of chloride displacement reaction and that the Cl atoms attached on the triazine ring have been totally replaced. Meanwhile, the peaks at 1050.7−1117.9 cm−1 in the curve of Intermediate correspond to the stretching vibrations of Si−O, which become much stronger in the curve of Si-NORs. Moreover, the strong absorption 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−gel reaction 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−gel reaction, the Si 2p and Si 2s peaks of Si-NORs are greatly increased and the Cl 2p peak almost disappears compared with that of Intermediate, which further confirms Si-NORs is synthesized through the chloride displacement reaction of cyanuric 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−PP composites was evaluated by LOI and the UL-94 test. The results together with the formulations of IFR−PP composites are shown in Table 1. Pure PP is highly combustible as well as easy dripping and is not classified in the UL-94 rating. High loading of IFR is necessary to provide adequate flame retardancy. It takes 30 wt % IFR to obtain a LOI value of 34 and pass the V-0 rating. The results of the LOI test show that the LOI values of IFR−PP composites with 25 wt % IFR
Figure 3. Digital photos of PP75/IFR25/Si-NORs1 after LOI tests. O2 was (a) 30, (b) 32, and (c) 34%.
However, when the loading of Si-NORs increases, the LOI values are just kept at 32 and samples exhibit burning dripping behavior during the UL-94 test, only passing the V-2 rating. This is a result of the inverse concentration effect of flame retardants based on N-alkoxy hindered amines as reported in other 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 atmosphere are presented in Figure 4. The relevant data are shown in Table 3. It can be seen that Si-NORs has a lower weight loss rate and more residual char compared to T152. The initial decomposition temperature (Tinitial) of Si-NORs is 255.19 °C, which is lower than that of T152 due to the dehydration condensation reaction of Si−OH. The degradation process of Si-NORs can be divided into two stages. The first stage is 312
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of decomposition for PP75/IFR25/Si-NORs0 is almost completed at about 450 °C, whereas that of PP75/IFR25/SiNORs1 is up to 500 °C. These results indicate that the incorporation of 1 wt % Si-NORs into IFR−PP composites can improve the thermal stability of the IFR−PP composites at higher temperature. However, the char residue left at 600 °C is at the same level for both samples. 3.3. UV Stability. To estimate the effect of Si-NORs on the UV stability of IFR−PP composites, the Yellowness Index (YI) and mechanical properties of IFR−PP samples resulting from exposure 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
Figure 5. Effect of exposure time on Yellowness Index of PP75/ IFR25/Si-NORs0 and PP75/IFR25/Si-NORs1. Figure 4. TGA and DTG curves of T152, Si-NORs, PP75/IFR25/SiNORs0, and PP75/IFR25/Si-NORs1.
addition of 1 wt % Si-NORs can effectively protect IFR−PP from aging and yellowing. Before UV-light irradiation, the YI of PP75/IFR25/Si-NORs1 is 26.10, higher than the 18.26 of PP75/IFR25/Si-NORs0, which shows that the addition of SiNORs can cause the yellowing of IFR−PP by itself.28 After 5 days of UV-light irradiation, the YI of PP75/IFR25/Si-NORs0 is increased by 189.0%, up to 57.74, whereas the YI of PP75/ IFR25/Si-NORs1 is 37.52, increased by only 43.75%. Moreover, the YI of PP75/IFR25/Si-NORs0 is increased further while the YI of PP75/IFR25/Si-NORs1 remains stable with the extended exposure time. The effects of exposure time on tensile, bending, and impact strengths of PP75/IFR25/Si-NORs0 and PP75/IFR25/SiNORs1 are shown in Figure 6. It is found that 30 days of UV-light irradiation can decrease the tensile, bending, and impact strengths of PP75/IFR25/Si-NORs0, by 25, 27, and 34%, respectively. The corresponding decline of PP75/IFR25/ Si-NORs1’s properties is much lower, by only 5, 16, and 10%,
caused by the further dehydration condensation as well as initial pyrolysis of Si-NORs (the fracture of nonaromatic alkyl) in the temperature range 280−320 °C. The second stage can be attributed to the pyrolysis of Si-NORs and the formation of a ceramic-like network structure containing Si−O−Si around 360 °C. The stability of that structure makes the second maximum rate decomposition temperature (Tpeak) of Si-NORs 30 °C higher than that of T152. The char residue of Si-NORs is 23.06 wt % at 600 °C, while there is only 1.637 wt % left for T152. These results indicate that Si-NORs shows higher thermal stability 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 and 474.65 °C, respectively, increased by 32.53 and 86 °C compared to those of PP75/IFR25/Si-NORs0. The process
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 Si-NORs PP75/IFR25/Si-NORs0 PP75/IFR25/Si-NORs1
277.9 255.2 278.0 310.6
8.421/303.0 4.212/299.8
8.831/333.5 4.763/360.7 20.83/388.7 22.38/474.7
3.998/423.6 −
1.637 23.06 9.135 8.883
a
Tinitial, temperature where 5 wt % weight loss occurred. bRpeak, maximum weight loss rate of samples; Tpeak, temperature where maximum weight loss rate occurred. cChar residue was obtained at 600 °C. 313
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Figure 7. Effect of Si-NORs loading on tensile strength, bending strength, and impact strength of IFR−PP composites.
Figure 6. Effects of exposure time on tensile strength, bending strength, and impact strength of PP75/IFR25/Si-NORs0 and PP75/ IFR25/Si-NORs1.
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 of IFR−PP composites increases to 26.2 MPa. Further increase in Si-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−PP composites, the bending strength increases from 44.3 to 50.0 MPa 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 addition within 1 wt %. However, since there is a serious deterioration in
respectively. The decrease in PP75/IFR25/Si-NORs1’s mechanical properties becomes stable after 10 days of UV-light irradiation, while the PP75/IFR25/Si-NORs0 keeps deteriorating. All results indicate that Si-NORs, as a flame retardant synergist, can also provide IFR−PP composites with a good UV stability. 3.4. Mechanical Properties. The effects of Si-NORs loading on tensile, bending, and impact strengths of IFR−PP composites are presented in Figure 7. The tensile strength of pure PP is 32.2 MPa, and is reduced to 24.4 MPa after added 25 wt % IFR. It is mainly due to the poor compatibility between 314
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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.
compatibility. In this case, the mechanical properties of IFR− PP composites must be decreased due to poor interfacial bonding between IFR particles and PP matrix. As is shown in Figure 8B1,B2 (samples with 1 wt % Si-NORs), most of the IFR particles are uniformly embedded in the PP matrix and the interface 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-NORs has many alkoxy silane groups and unreacted silanol groups capable of reacting with HO-rich surfaces of IFR particles. At the other end, it has a large number of alkyl groups which have a good compatibility with PP matrix. The compatibilization of Si-NORs can effectively improve the interfacial compatibility between IFR and PP matrix and enhance the mechanical properties of the IFR−PP composites, which is in agreement with 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 essential factor for improving the flame retardancy of IFR−PP composites. Figure 9 shows SEM image of the intersection of char residues after the LOI test. It can be found that both chars have cellular structure. Compared to the sample without SiNORs (Figure 9a), the char residue of sample with 1 wt % SiNORs (Figure 9b) has a higher cell number and thinner cellular
impact strength of IFR−PP composites after the addition of 25 wt % IFR, the impact strength of IFR−PP composites is still much lower than that of pure PP even added with 5 wt % SiNORs. Seen from the above results, Si-NORs can also improve the mechanical properties of IFR−PP composites to different extents and the optimum addition of Si-NORs is 1 wt %. The explanation is that Si-NORs improves the compatibility between the IFR and PP matrix, which is proved by the morphological structures of the IFR−PP composites shown in section 3.5. 3.5. Morphological Structure of Composites and Char Residues. From the above analysis, a conclusion can be drawn that the flame retardancy thermal, UV stability, and mechanical properties of IFR−PP composites are improved with the introduction of Si-NORs. The mechanical properties are correlative to the microstructures of composites. Figure 8 presents the morphologies of PP75/IFR25/Si-NORs0 (Figure 8A1,A2) and PP75/IFR25/Si-NORs1 (Figure 8B1,B2) observed by SEM measurement. From Figure 8A1,A2 (the micrographs of samples without Si-NORs), the interface between IFR and PP matrix can be clearly observed (marked by black arrows). Moreover, a lot of IFR particles aggregate severely on the surface of PP matrix (marked by white arrows), suggesting poor 315
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Figure 10. Possible flame retardant mechanism of IFR−PP composite with Si-NORs.
To take the inverse concentration effect of Si-NORs into consideration, it is possible that increasing addition of Si-NORs (more than 1 wt %) promotes scissions of PP as a sequence of higher free radical concentration after heating and decomposition of the NORs component. When the addition of SiNORs is more than 1 wt %, above effect even becomes the dominant role which reduces the molecular weight and hence the melt viscosity of the condensed phase, so promoting the change from V-0 to V-2.
wall, which can slow down heat and mass transfer between gas and condensed phases and enhance the flame retardancy and thermal stability of IFR−PP composites more effectively. This result is in accordance with the LOI, UL-94 tests, and TGA analysis 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 synergistic mechanism of Si-NORs in IFR−PP composites is attributed to both a gas phase mechanism and a condensed phase mechanism. It is well-known that the pyrolysis of PP is a free radical chain reaction through β-scission of PP chains, while the produced free radicals in return speed up the degradation of PP.38 In the gas phase, the efficient and regenerable free radical scavengers, nitroxyl radicals, generated from the thermal decomposition of Si-NORs, can be involved in the free radical chemical reactions during the combustion process and reduce the free radical concentration by converting them into relatively stable alcohols and ketones.17−19 This is the main reason why the flames are often self-extinguished rapidly and the second flame time of samples with Si-NORs became much shorter during the UL-94 test. In the condensed phase, Si-NORs improves the quality and structure of char residues considerably. During combustion of IFR−PP composites, nonflammable gases such as NH3 and H2O will be generated from the blowing agent of IFR and the melting char residues will be foamed. As shown in Figure 10, a number of nitroxyl radicals are generated in one Si-NOR molecule during the thermal decomposition of Si-NORs (namely Si[̵NO•]n). On the basis of the efficient radicaltrapping ability of nitroxyl radicals, it is reasonable to conclude that a cross-linking network will be formed in situ. The network can improve the melt viscosity of condensed phases, stabilize cell growth, prevent cell coalescence, and result in the char layer with better insulation properties.
4. CONCLUSIONS A novel and multifunctional flame retardant synergist, N-alkoxy hindered amine containing silane (Si-NORs), was synthesized based on the sol−gel reaction. It is proved that Si-NORs can replace part of IFR and endow IFR−PP composite with better flame retardancy. With 1 wt % loading of Si-NORs, the LOI value of IFR−PP composites (with 25 wt % IFR) is increased from 30 to 32 and the UL-94 rating is V-0. The synergistic effect of Si-NORs is attributed to the capture of active free radicals in the gas phase as well as the formation of a crosslinking network in the condensed phase by free radical scavengers generated from the thermal decomposition of SiNORs, which improves the morphology of the cellular charred layer and enhances the flame retardancy of IFR−PP composites. Si-NORs also acts as the UV stabilizer and compatibilizer to improve the UV stability and mechanical properties of IFR−PP.
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AUTHOR INFORMATION
Corresponding Author
*Tel./fax: ++86-571-87951832. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 316
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(19) Cao, K.; Wu, S. L.; Li, Y.; Zhu, F. J.; Yao, Z. Flame Retardant Based on N-Alkoxyoxy Hindered Amines and Their Application in Polyolefins. Prog. Chem. 2011, 23 (6), 1189. (20) Horsey, D. W.; Andrews, S. M.; Davis, L. H.; Dyas, D. D., Jr.; Gray, R. L.; Gupta, A.; Hein, B. V.; Puglisi, J. S.; Ravichandran, R.; Shields, P.; Srinivasan, R. Hindered amine flame retardant compositions. WO 9900450, 1999-01-07, 1999. (21) Roth, M. Flame retardant compositions comprising sterically hindered amines. WO 2009080554, 2009-07-02, 2009. (22) Basbas, A. I.; Alvisi, D.; Cordova, R.; Difazio, M. P.; Fischer, W.; Kotrola, J. A.; Nocentini, T.; Robbins, J.; Schöning, K. U. Process for the Preparation of Sterically Hindered Nitroxyl Ethers. WO 2008003605, 2007-06-25, 2008. (23) Zhang, S.; Horrocks, A. R.; Hull, R.; Kandola, B. K. Flammability, degradation and structural characterization of fibreforming polypropylene containing nanoclay-flame retardant combinations. Polym. Degrad. Stab. 2006, 91 (4), 719. (24) Marney, D. C. O.; Russell, L. J.; Soegeng, T. M.; Dowling, V. P. Mechanistic analysis of the fire performance of a fire retardant system. J. Fire Sci. 2007, 25 (6), 471. (25) Marney, D. C. O.; Russell, L. J.; Stark, T. M. The influence of an N-alkoxy HALS on the decomposition of a brominated fire retardant in polypropylene. Polym. Degrad. Stab. 2008, 93 (3), 714. (26) Aubert, M.; Roth, M.; Pfaendner, R.; Wilén, C. E. Azoalkanes: A novel class of additives for cross-linking and controlled degradation of polyolefins. Macromol. Mater. Eng. 2007, 292 (6), 707. (27) Nicolas, R. C.; Wilén, C. E.; Roth, M.; Pfaendner, R.; King, R. E., III. Azoalkanes: A novel class of flame retardants. Macromol. Rapid Commun. 2006, 27 (12), 976. (28) Aubert, M.; Wilén, C.-E.; Pfaendner, R.; Kniesel, S.; Hoppe, H.; Roth, M. Bis(1-propyloxy-2,2,6,6-tetramethylpiperidin-4-yl)-diazene An innovative multifunctional radical generator providing flame retardancy to polypropylene even after extended artificial weathering. Polym. Degrad. Stab. 2011, 96 (3), 328. (29) Shokoohi, S.; Arefazar, A.; Khosrokhavar, R. Silane Coupling Agents in Polymer-based Reinforced Composites: A Review. J. Reinf. Plast. Compos. 2008, 27 (5), 473. (30) Wang, B.; Tai, Q.; Nie, S.; Zhou, K.; Tang, Q.; Hu, Y.; Song, L. Electron Beam Irradiation Cross Linking of Halogen-Free FlameRetardant Ethylene Vinyl Acetate (EVA) Copolymer by Silica Gel Microencapsulated Ammonium Polyphosphate and Char-Forming Agent. Ind. Eng. Chem. Res. 2011, 50 (9), 5596. (31) Qian, Y.; Wei, P.; Jiang, P.; Zhao, X.; Yu, H. Synthesis of a novel hybrid synergistic flame retardant and its application in PP/IFR. Polym. Degrad. Stab. 2011, 96 (6), 1134. (32) Chen, X.; Jiao, C.; Zhang, J. Microencapsulation of ammonium polyphosphate with hydroxyl silicone oil and its flame retardance in thermoplastic polyurethane. J. Therm. Anal. Calorim. 2011, 104 (3), 1037. (33) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (34) Roberts, A. J. Bonding of additives to functional polyolefins by reactive blending. Doctorate Dissertation, The University of Auckland, New Zealand, 2009. (35) Kurth, D. G.; Bein, T. Optical Effects in Reflection-Absorption IR Spectroscopy of Thin Films of Silane Coupling Agents on Metallic Surfaces. Langmuir 1995, 11 (2), 578. (36) Nie, S.; Hu, Y.; Song, L.; He, Q.; Yang, D.; Chen, H. Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym. Adv. Technol. 2008, 19 (8), 1077. (37) Bum, Y.; Ho, L.; Park, B.; Kie, J.; Young, S.; Lee, M. Synthesis and characterization of polyamideimide-branched siloxane and its gasseparation. J. Appl. Polym. Sci. 1999, 74 (4), 965. (38) Song, P. A.; Zhu, Y.; Tong, L.; Fang, Z. C60 reduces the flammability of polypropylene nanocomposites by in situ forming a gelled-ball network. Nanotechnology 2008, 19 (22), 225707.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China through Project 51173166, Zhejiang Provincial Natural Science Foundation through Project Y4110134, the Program for Changjiang Scholars and Innovative Research Team in University, and the Fundamental Research Funds for the Central Universities.
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REFERENCES
(1) Song, P.; Fang, Z.; Tong, L.; Xu, Z. Synthesis of a novel oligomeric intumescent flame retardant and its application in polypropylene. Polym. Eng. Sci. 2009, 49 (7), 1326. (2) Li, Q.; Jiang, P.; Su, Z.; Wei, P.; Wang, G.; Tang, X. Synergistic effect of phosphorus, nitrogen, and silicon on flame-retardant properties and char yield in polypropylene. J. Appl. Polym. Sci. 2005, 96 (3), 854. (3) Wilkie, C. A.; Morgan, A. B. Fire Retardancy of Polymeric Materials, 2nd ed.; CRC Press: Boca Raton, FL, 2009. (4) Nie, S. B.; Song, L.; Guo, Y. Q.; Wu, K.; Xing, W. Y.; Lu, H. D.; Hu, Y. Intumescent Flame Retardation of Starch Containing Polypropylene Semibiocomposites: Flame Retardancy and Thermal Degradation. Ind. Eng. Chem. Res. 2009, 48 (24), 10751. (5) Wang, X.; Hu, Y.; Song, L.; Xuan, S. Y.; Xing, W. Y.; Bai, Z. M.; Lu, H. D. Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Poly(lactic acid)/Starch Biocomposites. Ind. Eng. Chem. Res. 2010, 50 (2), 713. (6) Wu, K.; Hu, Y.; Song, L.; Lu, H. D.; Wang, Z. Z. Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Starch-Based Biodegradable Composites. Ind. Eng. Chem. Res. 2009, 48 (6), 3150. (7) Cao, K.; Wu, S. L.; Wang, K. L.; Yao, Z. Kinetic Study on Surface Modification of Ammonium Polyphosphate with Melamine. Ind. Eng. Chem. Res. 2011, 50 (14), 8402. (8) Wang, Z.; Wu, K.; Hu, Y. Study on flame retardance of comicroencapsulated ammonium polyphosphate and dipentaerythritol in polypropylene. Polym. Eng. Sci. 2008, 48 (12), 2426. (9) Bourbigot, S.; Duquesne, S. Fire retardant polymers: recent developments and opportunities. J. Mater. Chem. 2007, 17 (22), 2283. (10) Bourbigot, S.; Le Bras, M.; Duquesne, S.; Rochery, M. Recent Advances for Intumescent Polymers. Macromol. Mater. Eng. 2004, 289 (6), 499. (11) Chen, Y. J.; Zhan, J.; Zhang, P.; Nie, S. B.; Lu, H. D.; Song, L.; Hu, Y. Preparation of Intumescent Flame Retardant Poly(butylene succinate) Using Fumed Silica as Synergistic Agent. Ind. Eng. Chem. Res. 2010, 49 (17), 8200. (12) Liu, Y.; Zhao, J.; Deng, C. L.; Chen, L.; Wang, D. Y.; Wang, Y. Z. Flame-Retardant Effect of Sepiolite on an Intumescent FlameRetardant Polypropylene System. Ind. Eng. Chem. Res. 2011, 50 (4), 2047. (13) Shan, X.; Zhang, P.; Song, L.; Hu, Y.; Lo, S. Compound of Nickel Phosphate with Ni(OH)(PO4)2Layers and Synergistic Application with Intumescent Flame Retardants in Thermoplastic Polyurethane Elastomer. Ind. Eng. Chem. Res. 2011, 50 (12), 7201. (14) Wang, X.; Song, L.; Yang, H.; Lu, H.; Hu, Y. Synergistic Effect of Graphene on Antidripping and Fire Resistance of Intumescent Flame Retardant Poly(butylene succinate) Composites. Ind. Eng. Chem. Res. 2011, 50 (9), 5376. (15) Desai, S. M.; Pandey, J. K.; Singh, R. P. A novel photoadditive for polyolefin photostabilization: Hindered amine light stabilizer. Macromol. Symp. 2001, 169 (1), 121. (16) Gijsman, P. New synergists for hindered amine light stabilizers. Polymer 2002, 43 (5), 1573. (17) Pfaendner, R. Nitroxyl radicals and nitroxylethers beyond stabilization: radical generators for efficient polymer modification. C. R. Chim. 2006, 9 (11−12), 1338. (18) Pfaendner, R. How will additives shape the future of plastics? Polym. Degrad. Stab. 2006, 91 (9), 2249. 317
dx.doi.org/10.1021/ie3017048 | Ind. Eng. Chem. Res. 2013, 52, 309−317
