Immobilization of flame retardant onto silica nanoparticle surface and properties of epoxy resin filled with the flame retardant-immobilized silica

Immobilization of Flame Retardant onto SilicaNanoparticle Surface and Properties of Epoxy Resin Filledwith the Flame Retardant-Immobilized Silica

TAKESHI YAMAUCHI,1,2,3 AKIRA YUUKI,1 GANG WEI,2 KUMI SHIRAI,4 KAZUHIRO FUJIKI,5

NORIO TSUBOKAWA1,2,3

1Graduate School of Science and Technology, Niigata University, 8050, Ikarashi 2-no-cho, Niigata 950-2181, Japan

2Niigata University Venture Business Laboratory, 8050, Ikarashi 2-no-cho, Niigata 950-2181, Japan

3Center for Transdisciplinary Research, Niigata University, 8050, Ikarashi 2-nocho, Niigata 950-2181, Japan

4Department of Material Science and Technology, Faculty of Engineering, Niigata University, 8050, Ikarashi 2-no-cho,Niigata 950-218, Japan

5Department of Environmental Science, Niigata Institute of Technology, 1719, Fujihashi, Kashiwazaki, Niigata 945-1195, Japan

Received 4 June 2009; accepted 22 July 2009DOI: 10.1002/pola.23657Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: To prepare silica nanoparticle having flame retardant activity, the immo-bilization of flame retardant onto hyperbranched poly(amidoamine) (PAMAM)-graftedsilica was investigated. Grafting of PAMAM onto a silica surface was achieved in asolvent-free dry-system using PAMAM dendrimer synthesis methodology. The immo-bilization of bromine flame retardant, poly(2,20,6,60-tetrabromobisphenol-A) diglycidylether (PTBBA), was successfully achieved by the reaction of terminal amino groupsof PAMAM-grafted silica (Silica-PAMAM) with epoxy groups of PTBBA. The immobi-lization of PTBBA was confirmed by FTIR and thermal decomposition GC-MS. Theamount of PTBBA immobilized onto Silica-PAMAM was determined to be 60 wt %.PTBBA-immobilized Silica-PAMAM (Silica-PAMAM-PTBBA) was dispersed uniformlyin a epoxy resin, and the epoxy resin was cured in the presence of hexamethylenedi-amine. Flame retardant activity of the epoxy resin filled with Silica-PAMAM-PTBBAwas estimated by limiting oxygen index (LOI). The LOI of epoxy resin filled withSilica-PAMAM-PTBBA was higher than that filled with untreated silica and freePTBBA. It was confirmed that the flame retardant activity of epoxy resin wasimproved by the addition of the Silica-PAMAM-PTBBA. The elimination of PTBBAfrom the epoxy resin filled with Silica-PAMAM-PTBBA into boiling water was hardlyobserved by immobilization of PTBBA onto silica surface. VVC 2009 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 47: 6145–6152, 2009

Keywords: flame retardant; hyperbranched; nanoparticles; silicas; surface grafting

INTRODUCTION

Most of organic polymers are combustible,because they contain hydrogen and carbon atoms.For their various applications in the building,

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 6145–6152 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: N. Tsubokawa (E-mail: [email protected])

6145

electrical, transportation, mining, and otherindustries, they have to fulfill flame retardantrequirements. Thus the main objectives in devel-opment of flame retarding polymers are toimprove an ignition resistance and a reduce offlame spread when flame retardants were incor-porated into flammable polymers.1

Several types of flame retardant additives areused to improve flame retardant properties ofpolymer materials. These flame retardants can bedivided into halogen, inorganic, phosphorous,nitrogen, and nitrogen-phosphorous flame retard-ant.2 The bromine flame retardants, such as poly(2,20,6,60-tetrabromobisphenol-A) diglycidyl ether(PTBBA) are good flame retardant activities, butthey have problems of blooming phenomena anddeterioration in mechanical properties, becausethey act as a plasticizer of resins.

On the other hand, in our laboratory, it hasbeen reported that the surface grafting of variouspolymers onto silica nanoparticle by using previ-ously introduced initiating groups on the nano-particle surface, such as peroxyester, azo, acyliumperchlorate, and potassium carboxylate groups.3–5

In addition, it has been reported that hyper-branched poly(amidoamine) (PAMAM) can begrown from amino groups on silica nanoparticle,6

chitosan powder,7 and carbon black8 surfacesusing dendrimer synthesis methodology9,10 asschematically shown in Figure 1. Grafting wasachieved by repeating the following two processesin a solution system6–8 or a solvent-free dry-sys-tem:11 (1) Michael addition of amino groups onthe surface to methyl acrylate (MA) and (2) ami-dation of the resulting terminal methyl estergroups with ethylenediamine (EDA) (Scheme 1).

Because of its many terminal amino groups,the resulting PAMAM-grafted silica (Silica-PAMAM) has potential as a catalyst and enzymesupport. We have pointed out that Silica-PAMAMacts as effective curing agent of epoxy resins.12 Inaddition, norbornadiene moiety was successfullyimmobilized onto Silica-PAMAM to prepare thesilica nanoparticle having solar energy conversionand storage function.13

In the present article, to prepare the silicahaving flame retardant activity, the immobiliza-tion of bromine flame retardant, PTBBA, ontoSilica-PAMAM was investigated (Scheme 2). Inaddition, the epoxy resin filled with PTBBA-im-mobilized Silica-PAMAM (Silica-PAMAM-PTBBA)was prepared, and the flame retardant activity ofepoxy resin filled with Silica-PAMAM-PTBBAwasestimated by LOI. The elimination of PTBBAform the epoxy resin filled with Silica-PAMAM-PTBBA into boiling water was also discussed.

EXPERIMENTAL

Materials and Reagents

Silica nanoparticle used was Aerosil 200 and itwas obtained from Nippon Aerosil, Japan. Theparticle size, specific surface area, and silanolgroup content were 12 nm, 200 m2/g, and1.37 mmol/g, respectively. The content of hydroxyl

Figure 1. Schematical illustration of hyperbranchedPAMAM-grafted silica nanoparticle.

Scheme 1. Grafting of hyperbranched PAMAM ontothe surface of silica nanoparticle by dendrimer syn-thesis methodology.

6146 YAMAUCHI ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

groups was determined by measuring volumetri-cally the amount of ethane evolved by the reactionwith triethylalumimum.14,15 The silica nanopar-ticle was dried in vacuo at 110 �C before use.

Poly(2,20,6,60-tetrabromobisphenol-A) diglycidylether (PTBBA) used was SR-T 1000 (commercialname) obtained from Sakamoto Yakuhin Kogyo,Japan. The molecular weight and bromine contentwere 2.0 � 103 and 51%, respectively. PTBBAwasdried in vacuo at room temperature before use.

Bisphenol-A type epoxy resin (Araldite AER260) was obtained from Asahi-Chiba, Japan. Theepoxy equivalent and viscosity of the epoxy resinwere 180–200 g/eq and 1.2–1.6 � 104 mPa s,respectively. The epoxy resin was dried in vacuoat room temperature before use.

Anisole, tetrahydrofuran (THF), and hexa-methylenediamine (HMDA), which were obtainedfrom Kanto Chemical Co., Inc. Japan, were usedwithout further purification.

Grafting of Hyperbranched PAMAM onto the SilicaSurface in a Solvent-Free Dry-System

The grafting of hyperbranched PAMAM onto thesilica nanoparticle surface was achieved byrepeating the following two steps in a solvent-freedry-system: (1) Michael addition of amino groupsto MA and (2) amidation of the resulting terminal

ester groups with EDA (Scheme 1). Here, wedefined a series of steps that calls reaction cycle(RC), namely, one cycle is RC 1 and two cycle isRC 2. After the reaction, the resulting silica wasdispersed in methanol and then THF. The disper-sion was centrifuged at 1.5 � 104 rpm until silicawas precipitated completely. Silica precipitatedwas dispersed again in those solvents, and thedispersion was centrifuged. The procedures wererepeated until no more ungrafted polymer wasdetected in the supernatant solution. The result-ing silica was stored in vacuo at room tempera-ture. The detailed procedures were described inthe preceding article.11 The resulting silica wasabbreviated as Silica-PAMAM.

Percentage of PAMAM Grafting and Amino GroupContent on Silica-PAMAM

The percentage of PAMAM grafting onto silicasurface was determined by the following equation:

Grafting ð%Þ ¼ ðA=BÞ � 100

where A is the weight of PAMAM-grafted onto thesilica surface and B is the weight of silica. The for-mer value was determined by measuring theweight loss when Silica-PAMAM was heatedat 800 �C using a thermogravimetric analyzer(TGA) under nitrogen.

Scheme 2. Immobilization of flame retardant (PTBBA) onto hyperbranchedPAMAM-grafted silica nanoparticle.

IMMOBILIZATION OF FLAME RETARDANT ONTO SILICA 6147


The amino group content on silica nanoparticlesurface was determined by titration. Into a flask,0.10 g of silica having amino groups and 50 mL of0.01 mol/L HCl aqueous solution was changed,and the mixture was stirred with a magnetic stir-rer for 3 h. After the reaction, the mixture wascentrifuged and 2.0 mL of the supernatantsolution was titrated with the aqueous solution ofsodium hydroxide using phenolphthalein as anindicator.

Immobilization of PTBBA onto Silica-PAMAM

The immobilization of PTBBA onto Silica-PAMAM was achieved by the reaction of terminalamino groups on the surface of Silica-PAMAMwith epoxy groups of PTBBA as shown in Scheme2. A typical procedure was as follows. Into a flaskwere placed 0.01 g of Silica-PAMAM, 0.10 g ofPTBBA, and 30 mL of anisole, and the mixturewas stirred at 100 �C for 24 h under nitrogen. Af-ter the reaction, the resulting silica was centri-fuged at 1.5 � 104 rpm and the supernatant solu-tion containing unreacted PTBBA was removed.The precipitated silica was dispersed in THF andcentrifuged again, and the procedure wasrepeated until no more PTBBA could be detectedin the supernatant solution. The resulting silicawas stored in vacuo at room temperature. ThePTBBA-immobilized Silica-PAMAM was abbrevi-ated as Silica-PAMAM-PTBBA.

Determination of the Amount of ImmobilizedPTBBA on Silica-PAMAM

The percentage of immobilized PTBBA onto silicasurface was determined by the following equation:

Immobilized PTBBA ð%Þ ¼ ðC�DÞ=B � 100

where B is weight of silica as mentioned above, Cis the weight of Silica-PAMAM-PTBBA, and D isthe weight of PAMAM-grafted onto silica. Theweight of C was determined by measuring theweight loss when Silica-PAMAM-PTBBA washeated at a 800 �C (heating rate of 10 �C/min)using a TGA (Shimadzu Model TGA-50) innitrogen.

Characterization

The immobilization of PTBBA onto Silica-PAMAM was identified by FTIR spectra and ther-mal decomposition gas chromatograms and massspectra (GC-MS). The FTIR spectra were recorded

on a FTIR Spectrophotometer (Shimadzu Model8200-A) using KBr pellet. Thermal decompositionGC-MS were recorded on a gas chromatographmass spectrometer, Shimadzu Manufacturing,GPMS-QP2010, equipped with a double shot pyro-lyzer, Frontier Laboratories, PY-2020. Heliumwas used as a carrier gas. The column was pro-grammed from 70 to 320 �C at a heating rate of20 �C/min and then held at 320 �C for 5 min.

Preparation of Epoxy Resin Filled withSilica-PAMAM-PTBBA

Silica-PAMAM-PTBBA was uniformly dispersedin a mixture of epoxy resin and HMDA by use of arotation and revolution super-mixer (Thinkymodel AR-100). The mixture was poured into aTeflon mold, which was coated with a fluorine-releasing agent. The curing reaction was carriedout at 100 �C for 1 h and then at 120 �C for 2 h.Size of sample was 80 � 6.5 � 3.0 mm.

Estimation of Limited Oxygen Index (LOI) of theEpoxy Resin Filled with Silica-PAMAM-PTBBA

The flame retardant activity of the epoxy resinfilled with Silica-PAMAM-PTBBA was estimatedby limited oxygen index according to the methodsof JIS K-7201. In the LOI test, a vertical strip ofmaterial is burned within a glass chimney con-taining a vertically moving column of mixture ofoxygen and nitrogen. LOI is defined as the mini-mum percent of oxygen, in a mixture of oxygenand nitrogen, which will just support combustionof a material initially at room temperature.

Estimation of PTBBA Elimination from Epoxy ResinFilled with Silica-PAMAM-PTBBA

Epoxy resin filled with Silica-PAMAM-PTBBAand that with free PTBBA were refluxed in waterfor 24 h and the PTBBA eliminated into waterwas detected qualitatively by a Beilstein test.

RESULTS AND DISCUSSION

Grafting of Hyperbranched PAMAM onto theSilica Surface

Hyperbranched PAMAM was grafted onto thesilica nanoparticle surface by repetition of the twoprocesses6–8,11 in a solvent-free dry-system: (1)Michael addition of MA to amino groups on the



surface and (2) amidation of the resulting termi-nal ester groups with EDA.

Table 1 shows the percentage of PAMAM graft-ing onto the silica surface and the amino groupcontent of the silica surface after the graftingreaction in a solvent-free dry-system. Both ofthese values increased with the number ofrepeated reaction cycles. However, whenuntreated silica was used, no grafting of PAMAMonto the surface and no increase in the number ofsurface amino groups was observed, even afterrepeated reaction cycles (RC6).

However, the amount of grafting and aminogroups at every reaction cycles was considerablysmaller than that of theoretical value. This maybe due to the fact that the Michael addition of MAto amino groups on the surface and amidation ofthe resulting terminal ester groups with EDA arecarried out in heterogeneous system and that thegrafted chains on the surface interfere with thepropagation of PAMAM from the surface becauseof steric hindrance.

Figure 2 shows FTIR spectra of PAMAM-grafted silica obtained from repeated reactioncycles (RC3 and RC6). The absorptions at 1650and 1550 cm�1, which are characteristic of amideand amino groups, respectively, increased withthe number of reaction cycles. These results indi-cated that PAMAM grows from amino groups onthe silica surface in a solvent-free dry-system.

Effect of Amino Group Content on theImmobilization of PTBBA onto Silica-PAMAM

Figure 3 shows the effect of amino group contentof Silica-PAMAM on the immobilization ofPTBBA. It was found that the amount of PTBBAimmobilized onto Silica-PAMAM was increasedwith increasing reaction time. It is interesting tonote that the amount of PTBBA immobilized onto

Silica–PAMAM obtained from RC6 (amino groupcontent ¼ 5.9 mmol/g) was less than that fromRC3 (amino group content ¼ 2.6 mmol/g) despitehigher content of amino groups. This may be dueto the fact that terminal amino groups of Silica-PAMAM were readily blocked by immobilizedPTBBA. Therefore, it became apparent thatSilica-PAMAM obtained from RC3 was the mostefficient to immobilize PTBBA.

Effect of Temperature on the Immobilization ofPTBBA onto Silica-PAMAM

Figure 4 shows the effect of temperature on theimmobilization of PTBBA onto Silica-PAMAM

Table 1. Percentage of Grafting and Amino Group Content of Silica-PAMAM

SilicaReactionCycle

Experimental Value Theoretical Value

Grafting(%)

AminoGroup

(mmol/g)Grafting

(%)

AminoGroup

(mmol/g)

Untreated RC6 0 0 0 0Silica-NH2 RC0 2.3 0.4 2.3 0.4Silica-NH2 RC3 50.7 2.6 56.6 2.8Silica-NH2 RC6 94.7 5.9 508.9 22.7

Figure 2. FTIR spectra of (A) Silica-NH2, (B) Silica-PAMAM (RC3; grafting ¼ 50.7%), and (C) Silica-PAMAM (RC6; grafting ¼ 94.7%).



(RC3) surface. The amount of PTBBA immobilizedonto the Silica-PAMAM increased with increasingreaction temperature and reached to 60 wt %at 150 �C after 48 h.

Confirmation of Immobilization of PTBBAonto Silica-PAMAM

Figure 5 shows FTIR spectra of Silica-PAMAMand Silica-PAMAM-PTBBA. In FTIR spectra

of Silica-PAMAM-PTBBA, new absorptions 1450cm�1 and 730 cm�1, which are characteristic ofaromatic rings of PTBBA, were observed. On thecontrary, the absorption of Silica-PAMAM at1550 cm�1, which is characteristic of aminogroups, decreased by immobilization of PTBBAonto Silica-PAMAM. These results suggestedthat the terminal amino groups on the Silica-PAMAM were reacted with epoxy groups ofPTBBA.

Gas chromatograms (GC) of thermal decom-posed gas of (A) Silica-PAMAM, (B) Silica-PAMAM-PTBBA, and (C) PTBBA are shown inFigure 6. In comparison with these chromato-grams, the thermal decomposition gas of Silica-PAMAM-PTBBA was similar to that of PTBBA.In addition, the mass spectrum of the decom-posed gas of Silica-PAMAM-PTBBA at retentiontime 5.3 min was in agreement with that ofPTBBA, as shown in Figure 7. Figure 7 alsoshows that Silica-PAMAM-PTBBA was decom-posed to produce fragments of PTBBA moieties(m/z ¼ 252). These results clearly show thatPTBBA was successfully immobilized onto Silica-PAMAM.

Figure 4. Effect of temperature on the immobilizationof PTBBA onto Silica-PAMAM (RC3). Silica-PAMAM(RC3), 0.01 g; PTBBA, 0.10 g; anisole, 30 mL.

Figure 3. Effect of amino group content on theimmobilization of PTBBA onto Silica-PAMAM (RC3and RC6). Silica-PAMAM, 0.01 g; PTBBA, 0.10 g;anisole, 30.0 mL, 100 �C.

Figure 5. FTIR spectra of (A) Silica-PAMAM (RC3)and (B) Silica-PAMAM (RC3)-PTBBA. [Color figurecan be viewed in the online issue which is availableat www.interscience.wiley.com.]



Flame Retardant Properties of Epoxy Filled withSilica-PAMAM-PTBBA

To estimate the effect of Silica-PAMAM-PTBBAon the flame retardant activity of epoxy resin,LOI values of epoxy resin filled with Silica-PAMAM-PTBBA was investigated. The samplecode and the corresponding composition of the ep-oxy resins filled with Silica-PAMAM-PTBBAwereshown in Table 2. Table 2 also shows LOI valuesof (A) unfilled epoxy resin, (B) epoxy resin filledwith free PTBBA, (C) epoxy resin filled withuntreated silica and free PTBBA, and (D) epoxyresin filled with Silica-PAMAM-PTBBA. The LOIvalue of epoxy resin filled with Silica-PAMAM-PTBBA shows the highest value, despite that ofepoxy resin filled with free PTBBA and that of ep-oxy resin filled with untreated silica and freePTBBA contains same amount of free PTBBA.This may be due to the fact that compatibility of

Silica-PAMAM-PTBBA with epoxy resin wasimproved because of immobilization of PTBBAhaving aromatic moieties.

Based on the aforementioned results, it wasconcluded that by the addition of Silica-PAMAM-PTBBA into epoxy resin, the flame retardantactivity of epoxy resin was considerably improved.The effects of the Silica-PAMAM-PTBBA as addi-tive on the mechanical properties and grass tran-sition temperature of the epoxy resin are nowunder investigation.

Estimation of PTBBA Elimination from Epoxy ResinFilled with Silica-PAMAM-PTBBA

Epoxy resin filled with Silica-PAMAM-PTBBAwas refluxed in water for 24 h and eliminatedPTBBA into boiling water was detected by a Beil-stein test. As a result, the test was negative,

Figure 6. Gas chromatograms of thermal decom-posed gas of (A) Silica-PAMAM, (B) Silica-PAMAM-PTBBA, and (C) PTBBA. Figure 7. Mass spectra of thermal decomposed gas

of (A) Silica-PAMAM-PTBBA and (B) PTBBA.

Table 2. Compositions of the Epoxy Resins Filled with Silica-PAMAM-PTBBAand Flame Retardant Activity (LOI)

CuringAgent (phr) Composition (phr)

SampleCode HMDA

Silica-PAMAM-PTBBA Silica-NH2 PTBBA Sb2O3 LOI

A 20 – – – – 19.9B 20 – – 5 4 22.4C 20 – 10 5 4 21.7D 20 20 – – 4 23.4



indicating no elimination of PTBBA into boilingwater. On the contrary, the elimination of PTBBAinto boiling water from epoxy resin filled with freePTBBA was confirmed. The results clearly showthe anchor effect of silica nanoparticle for theimmobilization of PTBBA.

Therefore, it is expected that the bloomingphenomena and deterioration in mechanical prop-erties can be controlled by use of the flame re-tardant-immobilized silica. The effects of theflame retardant-immobilized silica on the bloom-ing phenomena are now under investigation.

CONCLUSIONS

1. The immobilization of bromine flame re-tardant, PTBBA, onto Silica-PAMAM wassuccessfully achieved by the reaction of ter-minal amino groups on the Silica-PAMAMsurface with epoxy groups of PTBBA.

2. The amount of PTBBA immobilized ontoSilica-PAMAM was increased with increas-ing reaction time. The amount of PTBBAimmobilized onto Silica-PAMAM was deter-mined to be 60 wt %.

3. The flame-retardant property of the epoxyresin was considerably improved by theaddition of Silica-PAMAM-PTBBA.

4. The elimination of PTBBA from epoxyresin filled with Silica-PAMAM-PTBBAinto boiling water was hardly observed.The results clearly show the anchor effect

of silica nanoparticle for the immobilizationof PTBBA.

REFERENCES AND NOTES

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Immobilization of flame retardant onto silica nanoparticle surface and properties of epoxy resin filled with the flame retardant-immobilized silica