Article pubs.acs.org/IECR
Flame-Retardant Polyimide Cross-Linked with Polyhedral Oligomeric Octa(aminophenyl)silsesquioxane Haibo Fan and Rongjie Yang* School of Materials Science & Engineering, National Laboratory of Flame Retardant Materials, Engineering Research Center of Fire-Safe Materials and Technology, Ministry of Education, Beijing Institute of Technology, Beijing 100081, People’s Republic of China S Supporting Information *
ABSTRACT: Polyimide (PI) composites cross-linked through octa(aminophenyl)silsesquioxane (OAPS) were thermally cured with a controlled temperature program. The compatibility between OAPS and PI was good. FTIR analysis indicated that the imidization reaction was complete. TGA results indicated that OAPS did not change the decomposition path of PI. The incorporation of OAPS into PI improved a little the tensile strength and Young’s modulus. The fire resistance performances of the PI/OAPS composites were evaluated. The LOI tests showed that incorporation of OAPS into PI improved the flame retardancy significantly. The mechanism of the flame retardancy of the composites was studied. The volatile pyrolysis products of PI/OAPS composites were similar to those of the PI control. However, the decomposition products of OAPS remained in the condensed phase and improved the thermo-stability of the char efficiently. The out-migration effect of silicon was beneficial to protect the compact char.
1. INTRODUCTION Polyhedral oligomeric silsesquioxanes (POSSs) are threedimensional, structurally well-defined cage molecules with the general formula (RSiO1.5)n. The inorganic silica-like core is surrounded by organic groups, and the cage size is about 1.5 nm.1−5 Typical POSS derivatives have the structure of a cubeoctameric framework covalently bonded with eight organic groups, one or more of which is reactive or polymerizable. These POSS derivatives have attracted considerable interest for many years due to their high performance, which originates from the combination of advantages attributable to their inorganic and organic components. POSS cages can be incorporated into polymers via copolymerization,6−9 grafting,10,11 or blending.12−14 They have been used successfully to improve polymer properties such as thermal stability, oxidation resistance, and mechanical properties.8,13−16 The application of POSS to polyimides (PIs) is an obvious development because PIs are among the most successful commercial polymers, and they have been used widely as coatings for microelectronic devices, gas-separation membranes, and high-temperature materials for the aerospace industry due to their excellent thermal stability, high glass transition temperatures, and mechanical properties such as tensile strength and modulus.17−19 Several reports of organic−inorganic hybrid composites involving PIs and POSS have emerged and have mainly concerned POSS with eight functional amino groups, for example, octa(aminophenyl)silsesquioxane (OAPS) and octa(aminopropyl)silsesquioxane (OAPrS). Guo studied the properties of PI/OAPS aerogels,8 while Iyer researched the gas transport properties of PI/OAPS barrier.20 Asuncion also found that PI/OAPS films exhibit excellent barrier properties, competitive with current commercial grade barrier films.21 Tamaki, Iyer, and Huang mainly discussed the thermal stability and mechanical properties of PI/OAPS nanocomposites.22−24 Seckin observed the © 2013 American Chemical Society
water absorption, thermal, and dielectric properties of PI/ OAPrS composites.25 All of their results showed that the thermal and mechanical properties of the composites were improved by the incorporation of POSS. However, the flameretardant properties of those composites were not discussed. Although Koytepe, Seckin, and Mulazim studied the flameretardant properties of PIs using zinc borate, borax, and B2O3,26−28 they only gave some testing results for the flameretardant properties. In our previous studies,29,30 OAPS was prepared in tetrahydrofuran using hydrazine hydrate as reductant from octa(nitrophenyl)silsesquioxane (ONPS) in 1 h on the 5% Pd/C-FeCl3 catalyst (Scheme 1). In the present work, PI composites cross-linked through OAPS were prepared. The effects of OAPS on the thermal stabilities, mechanical properties, and most importantly the flame retardancy of PI/OAPS composites were studied. Furthermore, the mechanism of the flame-retarding action of OAPS on PI was also investigated.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Substrates. Octaphenylsilsesquioxane (OPS, Si8O12(C6H5)8, M = 1033.2, 97%) was purchased from Hybrid Plastics Inc., U.S. 3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-diaminodiphenyl ether (ODA) was bought from Alfa Aesar Ino., China. NaCl, Na2SO4, and solvents such as dimethylacetamide (DMAc), tetrahydrofuran (THF), ethyl acetate, and hexane were of analytical purity and obtained from Beijing Chemical Works, China. 2.2. Synthesis of OAPS. Octa(aminophenyl)silsesquioxane (OAPS) was synthesized from OPS by methods detailed in the Received: Revised: Accepted: Published: 2493
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Scheme 1. Synthesis of OAPS
literature.29,30 OAPS, 1H nuclear magnetic resonance (NMR) spectroscopy (500 MHz, DMSO-d6, δ): 7.4−6.2 (2.0H, Ar H), 5.4−4.5 (1.0H, −NH2). 29Si solid NMR (400 MHz, δ): −68.3, −77.5. 13C NMR (500 MHz, DMSO-d6, δ): 153.2, 147.9, 135.1, 131.4, 128.6, 121.2, 119.3, 116.4, 114.6, 113.3. Fourier transforminfrared (FT-IR) spectroscopy (KBr): ν = 3456 and 3358 (−NH2), 1115 (Si−O) cm−1. Anal. Calcd for OAPS: C, 50.0; H, 4.16; N, 9.71. Found: C, 48.9; H, 4.38; N, 9.31. 2.3. Preparation of PI/OAPS Composites. The PI/OAPS composites were prepared by the procedure depicted in
Scheme 2. In formulating PI/OAPS composites, the molar ratio of −NH2 in OAPS and ODA was equal to the anhydride groups in BPDA. The compositions of the PI/OAPS composite samples are listed in Table 1. A three-neck flask was first purged with nitrogen gas to remove moisture. To the above flask was added a BTDA/DMAc solution, followed by ODA-OAPS/ DMAc solution in five portions. The mixture was stirred under N2 at room temperature for 8 h, and a viscous and transparent polyamic acid solution was obtained. The solution was then cast on a glass substrate and thermally treated at 80 °C for 12 h, 120 °C for 4 h, 200 °C for 2 h, and 250 °C for 2 h. The films were removed from the glass substrates with the aid of deionized water and dried at 80 °C in a vacuum oven; the thickness of the films was about 50−70 μm. 2.4. Characterization. FT-IR spectra were recorded on a Nicolet 6700 IR spectrometer in the attenuated total reflectance mode. The spectra were collected in 32 scans with a spectral resolution of 4 cm−1. The X-ray diffraction analysis was achieved using an XPERTPRP diffractometer system; Cu Kα radiation was used with a copper target over the 2θ range of 5−60°. The mechanical properties were measured by means of a universal testing machine (DXLL-5000, Shanghai D&G machinery equipment Co., Ltd.) according to GB/T 1040.1-2006/ISO 527-1:1993. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204 F1 instrument (Germany) in the temperature range of 30−350 °C with a heating rate was 10 °C/min in a nitrogen flow of 20 mL/min. Thermal gravimetric analysis (TGA) was performed with a Netzsch 209 F1 thermal analyzer. The measurements were carried out under a nitrogen atmosphere at a heating rate of 10 °C/min from 40 to 800 °C. To detect gas species, the TGA was coupled with Fourier transform infrared spectrometry (TGA-FTIR, Nicolet 6700), and the measurements were carried out under a nitrogen atmosphere at a heating rate of 20 °C/min from 40 to 800 °C. The sample weight was 6−7 mg for each measurement.
Scheme 2. Preparation of PI/OAPS Composites
Table 1. Compositions of PI/OAPS Composites materials
PI control
PI-1
PI-2
PI-3
PI-4
PI-5
BTDA (g) ODA (g) OAPS (g) n(−NH2 in OAPS)/n(−NH2) OAPS (wt %) DMAc (mL)
6.168 3.832 0 0 0 100
6.142 3.721 0.137 2.5% 1.37% 100
6.117 3.6100 0.273 5% 2.74% 100
6.090 3.501 0.409 7.5% 4.09% 100
6.065 3.392 0.543 10% 5.43% 100
6.015 3.177 0.808 15% 8.07% 100
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Vertical burning tests were performed according to UL-94 standard with samples of dimensions 200 × 50 mm2. The limiting oxygen index (LOI) was obtained according to the standard ASTM-D 2863 through measuring the minimum oxygen concentration required to support candle-like combustion of films. An oxygen index instrument (Rheometric Scientific Ltd.) was used on barrels of dimensions 160 × 20 mm2. The rolled film was obtained by winding the film in a spiral of 45°, and the diameter of the testing sample was 2 mm. Microcombustion calorimetry (MCC, FAA-PCFC model) measurements were carried out according to the ASTM D7309-07 standard. The heating rate was 1 °C s−1, and the maximum pyrolysis temperature was 800 °C with a combustion temperature of 900 °C for the evolved gases. The flow was an O2/N2 mixture at a ratio of 20/80 cm3 min−1, and the sample weight was 5 mg. Scanning electron microscopy (SEM; Hitachi TM3000) was used to study the morphology of the samples using an accelerating voltage of 5 kV. Samples for SEM were prepared by dehydrating in vacuum.
Figure 1. Fourier transform-infrared spectra of PI control and PI-4.
anhydride ν(CO) peak at around 1890 cm−1 was not present, indicating that the imidization reaction was complete. 3.4. X-ray Diffraction Analysis. The X-ray diffraction patterns of the OAPS and PI/OAPS composites are displayed in Figure 2. PI control, PI-2, and PI-4 composites showed wide,
3. RESULTS AND DISCUSSION 3.1. Gel Effect of PI/OAPS Cross-Linked Composites. The gel effect appeared in the process of forming polyamic acid solution in the experiments when the n(−NH2 in OAPS)/ n(−NH2 in OAPS and ODA) was 15%. The solution gelled rapidly (in several minites) after adding OAPS. Table 2 lists the Table 2. Gel Points of PI/OAPS Cross-Linked Composites PI control PI-2 PI-4 PI-5
average functionality ( f)
critical reaction degree (Pc)
2 2.04 2.08 2.12
1 0.98 0.96 0.94
gel points of the different PI/OAPS cross-linked composites. The critical reaction degree (Pc) of the composites would decrease with an increased amount of OAPS. In this study, the polyamic acid solution of PI-5 would be gelled because of the low Pc of 0.94 after adding OAPS in some minutes. 3.2. Morphological Properties of PI/OAPS Composites. The compatibility of OAPS molecules in PI is the key to achieving well-dispersed PI/OAPS composites. As can be seen from the photographs of PI/OAPS composites with different OAPS contents (Supporting Information Figure 1), the PI/ OAPS composites exhibited excellent optical transparency. This result is attributed to the existence of covalent bonds between OAPS and BTDA, while obvious OPS particles could be seen in the PI/OPS composites. 3.3. FT-IR Analysis of PI/OAPS Composites. Infrared spectroscopy was performed to characterize the structures of the composites. Figure 1 presents FT-IR curves of the PI control and PI-4, and they are nearly the same due to the small amounts of OAPS added and the overlap of the peaks. Moreover, the FT-IR spectra showed distinct features that clearly indicate imide ring formation and the disappearance of the polyamic acid peak during the thermal cyclization step. The characteristic absorption bands of the imide ring appeared near 1777 cm−1 (asym. CO stretching), 1713 cm−1 (sym. CO stretching), 1373 cm−1 (C−N stretching), 1082 cm−1, and 717 cm−1 (imide ring deformation). The characteristic band of the
Figure 2. X-ray diffraction patterns of OAPS and PI/OAPS composites.
amorphous diffraction peaks. As compared to PI/OAPS, OAPS showed a diffraction peak at 2θ = 7.75°, indicating a d-spacing of 11.4 Å (by Bragg’s equation). This diffraction peak is related to the local order among OAPS molecules of the Si−O caged structure.31,32 The cross-linking reactions between BTDA and OAPS eliminate the local order of the individual OAPS molecules. It could be speculated that the reaction between OAPS and BTDA made OAPS well-distributed in the PI matrix. 3.5. Thermal Properties of PI/OAPS Composites. The glass transition temperatures (Tg) of the PI/OAPS composites were measured by DSC (Supporting Information Figure 2), and the results are summarized in Table 3. A glass transition Table 3. TGA and DSC Data of PI/OAPS Composites samples
Tg (°C)
Tonset (°C)
Tmax (°C)
residues at 800 °C (%)
PI control PI-1 PI-2 PI-3 PI-4
281
547 542 538 539 540
590 587 592 590 592
61.5 62.4 63.0 63.2 63.9
286 290
was identified for PI/OAPS composites with 2.74 and 5.43 wt % OAPS compositions; moreover, the PI-2 and PI-4 2495
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composites exhibited a higher Tg than did the PI control. These features could be attributed to the reaction between the OAPS and BTDA. Tg could provide a direct insight into the mobility of polymer chains. These results indicated that the large Si−O cage of OAPS limits the mobility of the PI chains in the crosslinking network. Thermal stabilities of the PI/OAPS composites were evaluated by TGA as seen in Figure 3. The relevant thermal
clusters could enhance the tensile strength, while high crosslinking density in the PI/OAPS composites decreased the tensile strength. Moreover, the cross-linked structures in the PI/OAPS composites could lead to a decrease in the elongation at break, and the higher is OAPS, the lower is the elongation. However, the cross-linking structures improved the Young’s moduli of the PI/OAPS composites considerably. 3.7. Fire Resistance of PI/OAPS Composites. To evaluate the fire resistance of the PI/OAPS composites, vertical burning classifications (UL-94) and LOI were carried out. The PI control and PI/OAPS composites attained a UL-94 rating of VTM-0. The first igniting flames of all of the PI/OAPS composites were quenched in 3 s, and the second igniting flames were quenched in 1 s. The effects of OAPS on the LOI values of BTDA/ODA system are presented in Figure 4. The
Figure 3. Thermogravimetric analysis curves of PI/OAPS composites in the nitrogen atmosphere.
decomposition data, including the Tonset, defined as the temperature at 5% weight-loss, the Tmax defined as the temperature at maximum weight-loss rate, and the char residues at 800 °C, are given in Table 3. According to Figure 3, the OAPS loading did not change the PI degradation path, and the thermal properties of PI/OAPS composites were similar to those of PI. As compared to the PI control, the PI/OAPS composites exhibited a little lower Tonset values, which resulted from the initial degradation of the OAPS at a relatively low temperature mainly. However, the residues of the PI/OAPS composites increased with the OAPS load. It has been reported that the presence of some POSS as a synergist could increase char yields. The chemical bonding with silicon in the condensed phase could lead to the formation of a glassy char.33,34 These results indicate that the interaction between OAPS and PI results in a high char yield. 3.6. Mechanical Testing of PI/OAPS Composites. The effects of OAPS content on the mechanical properties of the PI/OAPS cross-linked composites were studied. The tensile strength, the elongation at break, and the Young’s modulus of the PI/OAPS composites are shown in Table 4. When the amine group concentration of OAPS was 2.5% (PI-1), the tensile strength at break increased from 107.1 to 111.6 MPa. However, it declined with the increasing OAPS ratios; for example, the tensile strength of PI-4 is only 101.3 MPa. Thus, a small amount of the cross-linked rigid structures of POSS
Figure 4. Limited oxygen index of the PI/OAPS composites.
LOI values increased with the increases in OAPS loading. When 2.74 wt % OAPS is incorporated, the LOI value of the PI-2 increased from 46.5% to 55.5%, and the LOI of PI-4 with 5.43 wt % OAPS increased by 10.5% (LOI = 57.0%). The high LOI values indicate that the flame retardancy of PI was effectively improved by OAPS. 3.8. Flame Retardancy Mechanism of PI/OAPS Composites. 3.8.1. Analysis of Volatile Pyrolysis Products of PI/OAPS Composites. The combined TG-FTIR method was used to analyze how OAPS affected the release of the volatile products from the thermal decomposition of PI/OAPS composites. Figure 5 shows the FT-IR spectra of the PI control and PI-4 at certain temperatures. The major pyrolysis gases detected from the decomposition processes of the PI control and PI-4 were phenol derivatives/water (3670 cm−1), methane (3016 cm−1), CO2 (2360 cm−1), CO (2180 cm−1, 2090 cm−1), and aromatic components (1690 cm−1, 1510 cm−1). As shown in Figure 5, all of the evolving gas species for PI-4 were similar to those for the PI control. No other characteristic absorptions of the gas products of OAPS were found in the FT-IR spectra of PI-4 in Figure 5. This implies that most of the decomposition products of OAPS remained in the
Table 4. Mechanical Properties of the PI/OAPS Composites
tensile strength (MPa) elongation at break (%) Young’s modulus (GPa)
PI control
PI-1
PI-2
PI-3
PI-4
107.1 ± 4.8 5.87 ± 0.20 2.61 ± 0.10
111.6 ± 3.7 5.49 ± 0.31 2.90 ± 0.07
107.9 ± 4.0 5.72 ± 0.24 2.70 ± 0.07
108.1 ± 4.6 5.39 ± 0.31 2.85 ± 0.06
101.3 ± 5.8 4.52 ± 0.40 2.79 ± 0.04
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Figure 5. Fourier transform-infrared spectra of pyrolysis products of PI control and PI-4 at different temperatures.
Figure 6. Microcombustion calorimetry tests for the PI/OAPS composites.
condensed phase. This phenomenon might be attributed to the interactions between OAPS and PI networks. MCC testing was carried out on PI/OAPS composites to evaluate their heat release capacity. Figure 6 shows heat release rate (HRR) versus temperature curves for PI-2 and PI-4 as compared to the PI control. It can be seen that the MCC results were in accordance with the TGA results in Figure 3. The initial exothermic temperatures of PI-2 and PI-4 were lower than that of the PI control, which was caused by the lower Tonset of PI-2 and PI-4. Moreover, the addition of OAPS to PI slightly reduced the HRR due to the residues of PI/OAPS composites being increased a little as compared to PI control. Accordingly, the volatile pyrolysis products of PI/OAPS composites had hardly any change as compared to those of the PI control because of the small amount of OAPS added. 3.8.2. Analysis of Condensed Products of PI/OAPS Composites. Combustion processes of the PI control and PI-4 at the oxygen concentrations of 47% and 57.5% (LOI + 0.5%) are illustrated in Figure 7. For the PI control, when the testing oxygen concentration was 47%, the sample ignited quickly, and many sparks were released from the flame throughout the combustion process. However, for PI-4 tested at an oxygen concentration of 57.5%, when the sample ignited, the flame propagation was stable and no sparks were observed. Moreover, after the LOI testing, some interesting information was established by the visual observation of the end of the test residues. Only a little obvious char for the PI control could be
Figure 7. Combustion processes of PI control and PI-4 at the oxygen concentrations of 47% and 57.5%, respectively.
observed, whereas a white and firm char layer for PI-4 was created during the combustion. The char layer generated is thought to act as a barrier for both heat flow and mass transport. The PI control sample burned rapidly when it was ignited at an oxygen concentration of 47% (LOI of PI control +0.5%). However, the flame propagation of the PI-4 sample was apparently slow; then the flame gradually became small, and finally extinguished within 5 s. As compared to the shiney black char surface for the PI control, a white char layer of PI-4 was generated at the surface of the char residues (Supporting Information Figure 3), preventing the heat flow and mass transport. The surface morphology of the char could be very important for solid-phase flame retardancy. Figure 8 presents the SEM images of the residues from the matrix of the PI control and PI-4 after combustion at oxygen concentrations of 46.5% and 57%, respectively. Many holes could be seen in the char of the PI control, and the holes adjacent to the flame were much bigger 2497
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Figure 8. Scanning electron micrographs of the char residues of PI control and PI-4 after combustion at the oxygen concentrations of 46.5% and 57%, respectively.
3.8.3. Out-Migration Effect of the Silicon. The outmigration effect of the silicon was discovered after adding POSS to the polymers in previous studies.35,36 In this study, the effect could be demonstrated much easier because the LOI testing sample was tubular. Figure 9 shows photographs of the inner and outer carbon residues of the PI-4 after combustion at an oxygen concentration of 47%. It can be observed that the inner of the residues was shiney black while the outer had some white areas. The higher oxygen concentration led to a much stronger combustion of the outer surface of the testing PI pipe after ignition. In this case, more silicon may migrate to the outer surface. More compact and fiber-like char residues could be formed as a result of the out-migration effect of the silicon to the outer surface, which is shown in the SEM image in Figure 10b. Furthermore, the compact char in the outer plays an important protecting role for the inner surface. It can be observed in Figure 10a that the inner surface was smooth and neat, and it would not result in sparks like the PI control. The FT-IR spectra of the char of the carbon residues of PI control and PI-4 after combustion at an oxygen concentration of 47% in Figure 11 further confirmed the out-migration effect of the silicon. In Figure 11, the broad peak at 1538 cm−1 indicates the formation of polyaromatic carbons. The carbon residues of the PI control and the inner carbon residue of PI-4 did not show any different structures with just a small quantity of polyaromatic compounds remaining according to the FT-IR.
Figure 9. Photographs of the inner (left) and outer (right) carbon residues of the PI-4 after combustion at the oxygen concentration of 47%.
than others. However, for PI-4, the holes were much smaller than those for the PI control, and a honeycomb-like and compact char formation can be observed. Accordingly, the different phenomena during the combustion process of the PI control and PI/OAPS composites shown in Figure 7 could be explained. For the PI control, many holes formed under the heat action after ignition. When the holes became bigger, they connected with each other. So the char residues could be divorced from the sample, and this is the reason for the sparks seen during the combustion process of the PI control. However, for the PI/OAPS composites, the OAPS decomposition products could restrain the spread of the holes and form the honeycomb-like and compact char, which exhibited excellent heat insulation. In conclusion, the increase in the flame retardancy of the PI/OAPS composites is attributed to the improvement of the thermostability of the char after incorporation of OAPS.
Figure 10. Scanning electron micrographs of the inner (a) and outer (b) carbon residues of the PI-4 after combustion at an oxygen concentration of 47%. 2498
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residues after combustion at the oxygen concentration of 47%. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-6891-2927. Fax: +86-10-6891-4862. E-mail: yrj@ bit.edu.cn. Notes
The authors declare no competing financial interest.
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Figure 11. Fourier transform-infrared spectra of the carbon residues of PI control and PI-4 after combustion at an oxygen concentration of 47%.
Furthermore, the increasing absorption peaks of the Si−O (1047, 787, 449 cm−1) imply that more elemental silicon emerged in the outer carbon residues of PI-4.
4. CONCLUSIONS Flame-retardant PI/OAPS composites were obtained by a thermal curing reaction with a controlled temperature program. The critical reaction degree of the system would be ahead as the increasing of the OAPS. The gel effect appeared during the formation of polyamic acid solution when the n(−NH2 in OAPS)/n(−NH2 in OAPS and ODA) was 15%. The compatibility between the PI networks and OAPS was good, due to the existence of covalent bonds between OAPS and BTDA. FT-IR analysis indicated the imide ring formation and the disappearance of the polyamic acid peak. XRD results provided proof of cross-linking reactions between OAPS and BTDA. An increase in the Tg of the PI/OAPS composites was confirmed by DSC; this improvement might be attributable to the large Si−O cage of OAPS. Thermal stabilities of the PI/ OAPS composites were analyzed by TGA. OAPS did not have an evident influence on the decomposition path of PI, and the residues of the PI/OAPS composites saw little increase with the OAPS load. For the mechanical properties, the incorporation of OAPS into PI improved the elongation tensile strength and the Young’s modulus due to the cross-linking rigid structures of POSS clusters, but as a consequence, the elongation at break was reduced. According to the LOI tests, OAPS loading improved the flame retardancy significantly. The LOI was increased from 46.5% to 57% by the addition of 5.43 wt % OAPS. According to the TG-FTIR and the MCC results, the volatile pyrolysis products of PI/OAPS composites were hardly changed as compared to the PI control. However, it should be pointed out that for the PI/OAPS composites, the cross-linking reactions in the condensed phase, the formation of compact char residues, and the out-migration effect of the silicon contributed to the good flame retardancy. The out-migration effect of the silicon was essential to protect the compact char. For the PI control, the char residues could be released in sparks during the combustion process.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Photographs of PI composites. Differential scanning calorimetry curves of PI/OAPS composites. Photographs of the char 2499
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dx.doi.org/10.1021/ie303281x | Ind. Eng. Chem. Res. 2013, 52, 2493−2500
