synergistic effect of mesoporous silica sba-15 on intumescent flame-retardant polypropylene

FIRE AND MATERIALSFire Mater. 2011; 35:83–91Published online 29 April 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.1040

Synergistic effect of mesoporous silica SBA-15 on intumescentflame-retardant polypropylene

Jun Li1, Ping Wei1,∗,†, Linke Li1, Yong Qian1, Chen Wang1 and Nian Hua Huang2

1School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240,

People’s Republic of China2Department of Polymer Science and Engineering, Wuhan University of Science and Engineering, Wuhan 430073,

People’s Republic of China

SUMMARY

Mesoporous silica SBA-15 synthesized from Pluronic P123 and tetraethoxysilane was used as a synergisticagent on the flame retardancy of polypropylene (PP)/intumescent flame-retardant (IFR) system. Limitingoxygen index (LOI), UL-94 rating and thermogravimetric analysis were used to evaluate the synergisticeffect of SBA-15 on PP/IFR system. It showed that PP/IFR system could reach V-0 with loading ofSBA-15 ranging from 0.5 to 3 wt%, while without SBA-15 it had no rating at UL-94 test. The LOI valueincreased from 25.5 to 32.2 when the loading of SBA-15 was 1 wt%. The thermal stability of PP/IFRwas improved in the presence of SBA-15 and the amount of the char residue at 600◦C was increasedfrom 8.96 to 16.42 wt% when loading of SBA-15 varied from 0.5 to 5 wt%. Laser Raman spectroscopy(LRS) and scanning electron microscopy were employed to study the morphology of the char residue ofPP/IFR system with and without SBA-15. Copyright � 2010 John Wiley & Sons, Ltd.

Received 17 December 2009; Revised 9 February 2010; Accepted 9 February 2010

KEY WORDS: polypropylene; mesoporous silica SBA-15; intumescent flame retardant; synergistic effect

1. INTRODUCTION

Polypropylene (PP) has a broad application in transportation, construction, electrical/electronicappliances, and in general household materials. However, it has a low limited oxygen index (LOI)value of 17.4 and is easy to combust and drip when it is ignited, which restrict its application fields.To increase the flame-retardant property of the material, flame retardants (FRs) are often addedto PP. Usually, FRs are classified into halogen-containing FRs and halogen-free FRs. Halogen-containing FRs used in polymers are limited because of their productions of toxic, corrosive, andhalogen gases during combustion although they can provide the material with effective flame-retardant properties. On the contrary, the halogen-free FRs are environmental-friendly and havebeen widely studied in recent years and are mostly used to replace halogen-containing FRs.

In the search for halogen-free flame-retardants, intumescent flame-retardants (IFR) have receivedconsiderable attention recently [1, 2]. IFR systems are designed to swell and form a porous carbona-ceous char layer on the surface of the materials that act as a barrier for hot air and pyrolysisproducts fuelling the fire. The IFR based on the combination of ammonium polyphosphate (APP)with pentaerythritol (PER) has demonstrated good flame-retardant property in PP matrices [3–7],

∗Correspondence to: Ping Wei, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,Shanghai 200240, People’s Republic of China.

†E-mail: [email protected]

Contract/grant sponsor: Shanghai Leading Academic Discipline Project; contract/grant number: B202

Copyright � 2010 John Wiley & Sons, Ltd.

84 J. LI ET AL.

especially when used in further combination with other additives that lead to improved fire retar-dancy. Bourbigot [8–10] has found that clay and zeolite 4A has a good synergistic effect on flameretardancy in PP matrix containing APP/PER mixture. Tang [11, 12] also has made progress on theresearch of synergistic effect of nickel catalyst for PP/IFR system. Besides, Hu [13] has recentlyreported the findings of �-ZrP, which acts as a ‘solid acid’ catalyst as an effective synergistic agentfor PP/IFR system. So the use of synergistic agents can produce more efficient flame retardancy.

In this paper, mesoporous silica SBA-15 [14, 15], a nano-porous material with highly ordered poresizes and good thermal stability, was first used as a synergistic agent to improve flame retardancy ofAPP/PER mixture in PP matrix, which exhibits a distinct synergistic effect with APP and PER. Theflame retardant properties, the formation of charred layers, and their microstructures and morpholog-ical structures in the PP/IFR blends were examined by using the LOI, UL-94 test, thermogravimetricanalysis (TGA), laser Raman spectroscopy (LRS), and scanning electron microscopy (SEM).

2. EXPERIMENTAL

2.1. Materials

Tetraethoxysilane (TEOS) and hydrochloric acid (HCl) were provided by Shanghai ChemicalReagent Co., Ltd., Pluronic P123 was provided by BASF, PP (F401, MFI=2.0g/10 min,density=0.9g/cm3) was provided by the Yangzi Petroleum Chemical Company, China. Ammo-nium polyphosphate (APP) and pentaerythritol (PER) (powder, average size <10�m) were kindlyprovided by Shanghai Gaoqiao Co., Ltd.

2.2. The synthesis of mesoporous silica SBA-15

Four grams of Pluronic P123 was dissolved in 30 g of water and 120 g of 2 M HCl solution withstirring at 35◦C. Then, 8.50 g of TEOS was added into that solution with stirring at 35◦C for 20 h.The mixture was aged at 80◦C overnight without stirring. The solid product was recovered, washed,and dried. After calcination in air for 6 h in 550◦C, the product was denoted as SBA-15 [14].

2.3. Preparation of PP blends

PP, IFR (APP and PER), and SBA-15 were dried in vacuum at 80◦C overnight before use. Theywere melt-mixed at 185◦C in a HAAKE Rheocord 90 (made by HAAKE Mess-Technic GmbH,Germany) as 60 g batches of PP with the desired amounts of IFR and SBA-15 additives. Thecomposites were hot-pressed into sheets of suitable thickness and size for tests.

2.4. Measurements

2.4.1. LOI. The LOI value was measured on sheets 120×6.5×3mm3 according to the standardoxygen index test ASTM D 2863-77.

2.4.2. UL-94 test. The test was measured on sheets 130×12.7×3mm3 according to the UL-94test ASTM D 635-77.

2.4.3. TGA. TGA data were obtained in nitrogen at a heating rate of 10◦C/min using a TAQ5000IR thermogravimetric analyzer. In each case, a 10-mg sample was examined under a nitrogenflow rate of 20 ml/min at temperatures ranging from room temperature to 700◦C.

2.4.4. X-ray diffraction. The X-ray diffraction (XRD) scan was recorded at room temperaturewith a Rigaku D/Max 2000 X-ray diffractometer equipped with a Cu Ka tube and Ni filter(�=0.1542nm).

2.4.5. Transmission electron microscopy (TEM). TEM images were obtained by JEOL 2100 withan acceleration voltage of 200 kV. Specimens for the TEM measurements were obtained by placinga drop of sample suspension prepared by ultrasonic dispersion on a carbon-coated copper grid,and dried at room temperature.

Copyright � 2010 John Wiley & Sons, Ltd. Fire Mater. 2011; 35:83–91DOI: 10.1002/fam

SYNERGISTIC EFFECT OF MESOPOROUS SILICA SBA-15 85

2.4.6. SEM. The SEM micrographs of intumescent chars were observed by a JEOL JSM-7401FSEM. The specimens were previously coated with a conductive gold layer.

2.4.7. LRS. LRS measurements were carried out at room temperature with a ThermoFisher Scien-tific, DXR laser Raman spectrometer with excitation provided in back-scattering geometry by a780 nm argon laser line.

3. RESULTS AND DISCUSSION

3.1. SEM, XRD, TEM, and Nitrogen absorption characterization of SBA-15

SEM images (Figure 1) reveal that SBA-15 consists of many rope-like domains with relativelyuniform sizes of about 1�m, which are aggregated into wheat-like macrostructures. The small-XRD pattern for mesoporous silica SBA-15 prepared with P123 shown in Figure 2 show threewell-resolved peaks that are indexable as (100), (110), (200) reflections associated with p6mmhexagonal symmetry in agreement with literature [14]. And the intense (100) peak reflects ad spacing of 5.7 nm. TEM of SBA-15 is shown in Figure 3. It is found that well-ordered hexagonalarrays of mesopores (1D channels) and further confirmed that SBA-15 has a 2D p6mm hexagonalstructure. And from the high-dark contrast in the TEM image of the sample, the distance betweenmesopores is about 5.1 nm, in agreement with that determined from the XRD data. Three well-distinguished regions of the adsorption isotherm (Figure 4) are evident: (1) monolayer–multilayeradsorption, (2) capillary condensation, and (3) multilayer adsorption on the outer particle surfaces.A clear type-H1 hysteresis loop [16] is observed, and the capillary condensation occurs at a pressure(P/P0=0.6−0.7). According to BJH model, the pore size is about 5.1 nm and pore volume isabout 0.583cm3/g.

3.2. LOI and UL-94 rating of PP/IFR/SBA-15 blends

Table I and Figure 5 present the LOI values and UL-94 testing results of the flame retarded PPComposites. Keeping the IFR as the constant (25 wt%), different ratios of SBA-15 were added tothe PP/IFR/SBA-15 system. It can be seen that for PP/IFR, the LOI value is 25.5 and there isno rating in UL-94 test. The LOI values of the composites increase with the loading of SBA-15,and the highest LOI value is 33.2, obtained at 1 wt% SBA-15, increased 30% by comparison withPP/IFR. Besides, the PP/IFR/SBA-15 passed the UL-94 test when the loading of SBA-15 variedfrom 0.5 to 3 wt%. However, PP7 failed to pass the UL-94 test with 5 wt% SBA-15.

The above results illustrate that IFR used alone in PP does not have good flame retardancy whileincorporating SBA-15 into PP/IFR has good flame retardancy. The most proper loading of SBA-15

Figure 1. SEM images of hexagonal mesoporous silica SBA-15 with different magnification.


86 J. LI ET AL.

Figure 2. XRD pattern of SBA-15.

Figure 3. TEM images of SBA-15.

Table I. Formulations of the PP/IFR containing SBA-15 (PP:IFR=3:1, IFR: APP/PER=3/1, by weight).

Sample no. Composites LOI UL-94

PP1 PP/IFR 25.5 No ratingPP2 PP/IFR/0.1 wt% SBA-15 28.6 No ratingPP3 PP/IFR/0.5 wt% SBA-15 31.5 V-0PP4 PP/IFR/1 wt% SBA-15 33.2 V-0PP5 PP/IFR/2 wt% SBA-15 31.3 V-0PP6 PP/IFR/3 wt% SBA-15 31.2 V-0PP7 PP/IFR/5 wt% SBA-15 28.8 No rating

in PP/IFR/SBA-15 system is 1 wt%. It is explained by the fact that SBA-15 plays a role of catalystin the PP/IFR system for good char layer formation. While the loading of SBA-15 reaches 5 wt%,the esterification [2, 17] speed of the system does not match well with the foaming speed, resulting inunstable and broken cell structure char layer, which will be further confirmed in the paper.



Figure 4. Nitrogen adsorption isotherm and pore volume plots for SBA-15.

Figure 5. Effect of content of SBA-15 on LOI value of PP/IFR/SBA-15 system(PP/IFR/SBA−15=75/25/x; IFR: APP/PER=3/1, by weight).

3.3. Thermogravimetric behavior of PP and PP/IFR/SBA-15 blends

The TGA results of PP/IFR/SBA-15 system on nitrogen atmosphere at a heating rate of 10◦C/minare shown in Figure 6. The onset weight loss temperature(Tonset), 10% weight loss temperature(T−10wt%), maximum decomposition temperature (Tmax), and char residue at 600◦C are listed inTable II. The curve for pure PP shows one-step degradation of total weight loss, in which thethermal degradation takes place in the range of 400–500◦C, and has no char residue at 500◦C.In the presence of SBA-15, the thermal stability of PP/IFR system is improved significantly(Table II), and it can been seen that the decomposition temperature of PP/IFR system is shifted tohigher temperature with an increase in the addition of SBA-15. T−10wt% and Tmax has increasedfrom 385 to 396◦C and 455 to 488◦C respectively, based on the addition of 1 wt% SBA-15.The amount of char residue of PP/IFR system at 600◦C increases from 8.96 to 16.42 wt%. It ispossible that the presence of SBA-15 can promote to form a char layer of better quality in thePP/IFR system, which can endure the higher temperature and protect PP from decomposing. Sothe flame retardant property is improved. Also, the onset temperature of PP/IFR/5 wt% SBA-15


88 J. LI ET AL.

Figure 6. Effect of content of SBA-15 on the thermal stability of PP/IFR/SBA-15 system measured byTG under a nitrogen atmosphere at a heating rate of 10◦C/min.

Table II. TGA data of PP/IFR/SBA-15.

Sample no. Toneset (◦C) T−10wt% (◦C) Tmax (◦C) Char residue at 600◦C (wt%)

PP0 429 426 420 0PP1 213 385 455 8.96PP4 231 396 488 10.33PP7 170 395 486 16.42

was about 170◦C lower than PP/IFR, indicating that SBA-15 might act as a catalyst to increasethe formation of a mixture of ester [17, 18], which would bring less compact char layer and worseflame retardancy, which agrees with the results from LOI and UL-94 test.

3.4. Morphology of the char residue

The morphologies of the char residue after the flame extinguished during the LOI test was furtherstudied with a digital camera, LRS, and SEM shown in Figures 7–9.

PP/IFR/1 wt% SBA-15 (Figure 7(b)) forms a very big and compact char residue at the endof the sample bar, whereas there is little char residue left on PP/IFR (Figure 7(a)). The changesin the morphologies of char residue are useful to explain the differences of the flame retardancyamong PP/IFR/SBA-15 system, and well support the results discussed in this paper.

Raman scattering is especially sensitive to the structural disorder of graphite. In the previouswork [19, 20] LRS was used to characterize the different types of carbonaceous structure formedin the intumescent char.

Figure 8 shows the diffusion spectra of the char residues obtained from Raman spectroscopy.The spectra of the char residue with or without SBA-15 exhibits two broad bands around 1580and 1350cm−1, similar to the graphite- like species [9]. The peak observed at 1580cm−1 (G peak)could be due to the C–C vibration in the aromatic layers. The peak at 1330cm−1 (D peak) canbe attributed to the vibration of carbon atoms with dangling bonds in the plane termination ofdisordered graphite or glassy carbon [21, 22].

Tuinstra and Koenig (T-K) [22] found that the relative intensity ratio R of the D peak to theG peak is inversely proportional to an in-plane microcrystalline size and an in-plane phononcorrelation length obtained from Raman spectroscopy. Therefore, the T-K relation has frequentlybeen used to evaluate in-plane microcrystalline size and to characterize the carbonaceous species.The spectra from PP/IFR/1 wt% SBA-15 shows that the intensity ratio of the D peak to theG peak is greater than that of PP/IFR. Therefore, the size of carbonaceous microstructures from



Figure 7. Digital photos of the chars formed after extinguishing in LOI test:(a) PP/IFR and (b) PP/IFR/1 wt% SBA-15.

Figure 8. Laser Raman spectra of the intumescent char residue obtained fromPP/IFR and PP/IFR/1 wt% SBA-15 blends.

PP/IFR/SBA-15 system could be smaller than that from PP/IFR system. These results are in goodagreement with those reported from Bourbigot’s work [9] in which the higher protective shieldefficiency was related to the smaller size of carbonaceous microstructures. It can be proposed thatthe SBA-15 inhibit the increase of the carbonaceous micro-domain in size during burning, whichleads to the formation of more compact char layers.

As seen in Figure 9, the inner surface of char residue of PP/IFR/1 wt% SBA-15 seems morecompact and provides a much better barrier to the transfer of the heat, combustible gases, and freeradicals [23] during combustion and is also less susceptible to cracking. For PP/IFR/5 wt% SBA-15, the char is still tighter and more stable than PP/IFR, but not as good as that of PP/IFR/1 wt%SBA-15. Figure 8(d)–(f) show that the outer morphologies of residual chars show irregular swollenstructures [24].The swollen charred layers obtained from the PP IFR/SBA-15 blends (d and f)


90 J. LI ET AL.

Figure 9. SEM micrographs of the inner surface (a)–(c) and outer surface (d)–(f) of intumescent charobtained from PP/IFR/SBA-15 blends: (a) PP1; (b) PP4; (c) PP7; (d) PP1; (e) PP4; and (f) PP7.

were constituted of closed cells, but the cells of char layer obtained from the PP/IFR blends(d) were broken. Also, comparing e and f, with more loading of SBA-15, part of the cells wasbroken may be due to the negative catalyst effect of the reaction in PP/IFR system. These resultscan be explained by the fact that the presence of SBA-15 improves the stability of the char layer.The closed structural cells also promote the thermal protection properties of char layer, which isin good agreement with the above-mentioned LRS results.

4. CONCLUSION

Mesoporous silica SBA-15 was synthesized and used as an efficient synergistic agent in PP/IFRsystem. The LOI value of the system was increased and reached the highest value of 32.2 with1 wt% SBA-15. Besides, the blends PP/IFR could pass UL-94 V-0 rating with 0.5–3 wt% SBA-15loading. The TGA results showed that the thermal stability of PP/IFR blends has been significantlyimproved while the maximum decomposition temperature increased from 455 to 488◦C based onthe addition of 1 wt% SBA-15. Meanwhile, SBA-15 has played a role of promoting the char layerformation in PP/IFR blends with char residue at 600◦C increased from 8.96 to 10.33 wt% with1 wt% SBA-15 addition, and to 16.42 wt% with 5 wt% SBA-15 addition respectively. The LRS and



SEM results provided positive evidence that the PP/IFR blends with SBA-15 could form smallersize of carbonaceous microstructures in char layer than the PP/IFR, which act as a barrier andeffectively protect the PP matrix from combustion.

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synergistic effect of mesoporous silica sba-15 on intumescent flame-retardant polypropylene

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