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Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 1–6

Preparation and characterization of PMMA/flaky aluminumcomposite particle in the presence of MPS

Hui Liu ∗, Hongqi Ye, Yingchao ZhangCollege of Chemistry and Chemical Engineering, Central South University,

Changsha 410083, PR China

Received 2 March 2007; received in revised form 7 June 2007; accepted 27 June 2007Available online 30 June 2007

bstract

In order to enhance corrosion resistance of aluminum pigments, PMMA/flaky aluminum composite particle was prepared in the presence of MPSy means of in situ emulsion polymerization of MMA. It was found that the PC and CE could attain 9.1% and 45.5%, respectively at the conditionsf m(MPS)/m(Al) = 20%, time = 20 h, and temperature = 55 ◦C in the process of coupling. As for the in situ emulsion polymerization, there werebvious increases in both PG and GE when PC was increased from 0 to 1.2% or more, showing that more PMMA had been formed on the surfacef aluminum in the presence of MPS. PG and GE reached the highest value of 20.5% and 54.1%, respectively when C was 0.94 mmol/L (a

CTAB

ittle higher than CMC). The analysis of FT-IR and EDS indicated that PMMA/flaky aluminum composite particle had been successfully prepared,nd evolved hydrogen detection showed that corrosion resistance of aluminum pigments had been remarkably improved in the presence ofPS. 2007 Elsevier B.V. All rights reserved.

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eywords: PMMA; Flaky aluminum powder; Composite particle; MPS; Coupl

. Introduction

During the last two decades, aluminum pigments have beenidely used in automotive coatings, roof coatings, printing inks,

nd plastic materials for their protective and decorative functions1–3]. The aluminum pigments are usually produced by grindingtomized aluminum powder in ball mill together with white spirits solvent and fatty acid as lubricant. The mill operates at apeed that allows the balls to cascade onto the fine, sphericalr irregular atomized aluminum powders and finally to flattenhe powders into an ultrathin flaky particles. After grinding, theubsequent filtration makes the ultrathin flaky aluminum powdern the form of paste for sale with volatile organic compounds4–6].

Growing importance for environmental aspects has lead the

aint and coatings industry to the development of waterborneoating systems with a reduced content of volatile organic com-ounds [7]. It was found that the ordinary aluminum pigments

∗ Corresponding author. Tel.: +86 731 8876605; fax: +86 731 8879616.E-mail address: liuhui@mail.csu.edu.cn (H. Liu).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.06.057

ould not meet the requirements of the waterborne painting andoating systems, because they may be corroded in waterborneystems to emit hydrogen gas for their high specific surface andctivity according to the following reaction [8–10]:

Al + 6H2O → 2Al(OH)3 + 3H2↑The evolution of hydrogen during this corrosion reaction may

ead to a dangerous pressure buildup in container [11]. Further-ore, the color of aluminum pigments changes from silver to

rey [12]. A tremendous research activity has been devotedo inhibit aluminum corrosion reaction by surface modifica-ion of aluminum pigment. Kiehl and Greiwe [7] and Supplitnd Schubert [13] reported that aluminum pigment was coatedith SiO2 via sol–gel method through tetraethyl orthosilicate

TEOS, Si(OC2H5)4) to improve its corrosion resistance. Muller14] investigated the corrosion inhibition by mixing aluminumigment with commercially available styrene-maleic anhydrideopolymers (SMAn) as an inhibitor. Singh et al. [15] prepared

he Al/PMMA composites containing different volume frac-ions of aluminum by attritor milling followed by hot pressingnd investigated their thermal expansion and electrical behav-or. Zhang et al. [16] reported that spherical aluminum powder

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H. Liu et al. / Colloids and Surfaces A:

as encapsulated with polystyrene (PS) by means of in situ dis-ersion polymerization with the aim to preserve its chemicalctivity.

From the detailed discussion of above investigations, it isell known that formation of inhibiting layer on the surfacef aluminum is the key point by inorganic and organic sur-ace modification. In our present work, in order to enhanceorrosion resistance of aluminum pigments, PMMA/flaky alu-inum composite particle was prepared in the presence of silane

oupling agent, 3-methacryloxypropyltrimethoxysilane (MPS).laky aluminum pigments were first coupled with MPS, and

hen encapsulated with polymethylmethacrylate (PMMA) byeans of in situ emulsion polymerization of methyl methacry-

ate (MMA). The aim of the coupling is to bring more PMMAormation on the surface of aluminum powders.

. Experimental

.1. Materials and reagents

The flaky structure of raw aluminum pigmentShanghai Weiye Co. Ltd., China, D50 = 15 �m, diame-er/thickness = 100–200) was shown in Fig. 1. It was driedn vacuum at 100 ◦C for 24 h to remove the volatile organicompounds before use. MMA (Guangzhou Chemical factory,hina) was refined to remove the antipolymerization agent.PS (Wuhan University Silicone New Material Co. Ltd.,ubei, China), cetytrimethylammonium bromide (CTAB,hanghai Chemical Reagent Plant, China) as an emulsifier,nd ammonium persulfate (APS, Tianjin Chemicals Co. Ltd.,hina) as an initiator, were all analytical grade reagents andsed as received.

.2. Flaky aluminum powder coupled with MPS

Into a 250 ml flask, 10 g flaky aluminum powder, 2 g MPS,nd 100 ml toluene were dispersed with ultrasonic vibrations for

Fig. 1. SEM image of flaky aluminum powder.

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cochem. Eng. Aspects 315 (2008) 1–6

0 min at ambient temperature. After 0.05 g hydroquinone as anntipolymerization agent and two drops of 2, 6-dimethylpyridines the catalyst were added, the reactant was refluxed at designedemperature under stirring with a magnetic stirrer at nitrogentmosphere. After the reaction, the mixture was centrifugednd the obtained cake was extracted with acetone for 72 h. Theesidual was dried in vacuum at 100 ◦C and the MPS-coupledluminum powder (called MPS-c-Al) was obtained. The con-ent of coupled MPS was calculated from Si elemental analysesf MPS-c-Al, and the percentage of coupling (PC) and the cou-ling efficiency (CE) were calculated according to the followingquations [17]:

C = Coupled MPS (g)

Aluminum powder used (g)× 100%

E = Coupled MPS (g)

MPS used (g)× 100%

.3. In situ emulsion polymerization of MMA

A certain amount of CTAB, 100 ml distilled water, 6 gMA, and a little acrylic acid were added into a four-necked

ound-bottomed flask equipped with a mechanical stirrer, a ther-ometer, a reflux condenser and a guttate funnel. After complete

mulsification through stirring, 10 g MPS-c-Al was charged intohe flask. When the mixture was heated to 65 ◦C with nitrogenowing, 0.6 g APS aqueous solution was slowly added into flasky guttate funnel. Then the mixture was heated to 85 ◦C andeacted for several hours. After the reaction, the mixture wasrecipitated with methanol and filtered. The cake was dried inacuum for 24 h and PMMA/flaky aluminum composite particlecalled PMMA/Al) was obtained.

In order to determine quantitatively grafted and non-graftedMMA in PMMA/Al, a certain amount of PMMA/Al wasispersed in acetone with ultrasonic vibration and the non-rafted PMMA was dissolved in acetone. After centrifugation10000 rpm) for 1 h, non-grafted PMMA was obtained by pre-ipitating the filtrate with methanol. The cake was dried inacuum, and PMMA-grafted aluminum was obtained. GraftedMMA in PMMA-grafted aluminum can be calculated from

he content of coupled MPS through Si elemental analyses andhe content of aluminum through evolved hydrogen detectionn a reaction of aqua regia described in literature [3]. TotalMMA was the sum of grafted and non-grafted PMMA, and

he conversion of monomers (C), percentage of grafting (PG)nd grafting efficiency (GE) were calculated according to theollowing equations [18–19]:

= Total PMMA (g)

MMA used (g)× 100%

G = Grafted PMMA (g)

Aluminum powder used (g)× 100%

E = Grafted PMMA (g)

Total PMMA (g)× 100%

H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 1–6 3

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Fig. 3. The effect of coupling time on PC and CE (m(MPS)/m(Al) = 20%,coupling temperature = 25 ◦C).

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ig. 2. The effect of MPS amount on PC and CE (coupling time = 24 h, couplingemperature = 25 ◦C).

.4. Characterization

FT-IR spectroscopy patterns were recorded on a NicoletVATAR360 system. Energy dispersive spectroscopic (EDS)nalysis of the samples was carried out on a JSM-6360LV instru-ent. Hydrogen volume was employed to evaluate corrosion

esistance of samples [20], and the evolved hydrogen detectionas carried out as described in the literature [21].

. Results and discussion

.1. Coupling of MPS on the surface of flaky aluminumowder

The selection of MPS is based on the fact that there are OCH3nd C C bonding in the molecular structure of MPS. The OCH3an condense with hydroxyl on the surface of aluminum, and the

C bonding can copolymerize with MMA, which is helpfulo bridge aluminum and MMA and enhance the formation ofMMA/Al composite particle.

According to Fig. 2, it is found that PC increases with thencreasing m(MPS)/m(Al). When m(MPS)/m(Al) rises to 20%,he increasing extent of PC is becoming faint and CE attains

he maximum of 26.0%, which indicates the achievement ofalanced coupling. The condensation reaction may take placeetween OCH3 in MPS and hydroxyl on the surface of alu-inum, which can be described in Scheme 1.

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Scheme 1. Schematic representation of MPS cou

ig. 4. The effect of coupling temperature on PC and CE (m(MPS)/m(Al) = 20%,oupling time = 20 h).

The curves in Fig. 3 show that both PC and CE increase withhe increasing coupling time. Meanwhile, PC keeps a plateauevel of 5.2% and CE reaches the culmination of 26.0% in thease of the coupling time over 20 h. It is concluded that MPSas been saturated onto the surface of aluminum when time isver 20 h, which is agreement with the literature [17].

Because coupling is a chemical reaction, temperature shouldlay an important role in coupling process. As expected in Fig. 4,oth PC and CE increase linearly with the increasing tempera-

pling on the surface of aluminum powder.

4 H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 1–6

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Fig. 5. The effect of PC on C, PG and GE at CCTAB = 1.13 mmol/L.

ure. It is implied that coupling is an endothermic reaction. At5 ◦C, PC and CE can attain 9.1% and 45.5%, respectively. Theeason for the coupling temperature did not exceed 55 ◦C is dueo the possible homogenous polymerization of MPS at higheremperature according to literature [17].

.2. Effect of PC and emulsifier concentration on in situolymerization of MMA

As indicated above, MPS can be coupled on the surface ofluminum and the amount of coupled MPS is reflected by theercentage of coupling. Following is the investigation on theffect of PC and emulsifier concentration on in situ emulsionolymerization of MMA.

As shown in Fig. 5, although conversion of MMA (C) keepsbout 62.0%, there are obvious increases in both PG and GEhen PC is increased from 0 to 1.2% or more, showing thatore PMMA has been formed on the surface of aluminum in

he presence of MPS. The mechanism may be that MMA wouldopolymerize with the C C bonding of MPS-c-Al and thenelf-polymerize to form PMMA on the surface of aluminum.he formation of copolymers on the surface of aluminum isery helpful to preparation of PMMA/Al composite particle. AtC = 5.2%, PG and GE can reach 18.2% and 48.8%, respectively.t can also be seen from Fig. 5 that the relationship between PGnd PC may accord with the following experiential equation:

G = PG∞kPC

1 + kPC

here PG∞ is the saturated percentage of grafting and k is aonstant (calculated as 4.89 by simulation). As a result, linearelationship between PG and PC occurs when PC is very lownd PG keeps a stable value of PG∞ when PC is high enough.

Because PMMA/Al composite particle is prepared by meansf in situ emulsion polymerization, it is necessary to control themount of emulsifier strictly. Fig. 6 shows the effect of emulsifier

oncentration on C, PG and GE. Unexpectedly, the conversionf MMA (C) is independent of CCTAB and keeps a stable valuef 62.0%, which is different with general emulsion polymeriza-ion. It can be attributed to two reasons: firstly, due to possible

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ig. 6. The effect of CTAB concentration on C, PG and GE at PC = 5.2%.

dsorption of aluminum powder, effective CTAB concentrationnd the number of latex would decrease, which prevents thencrease of C; secondly, the existence of aluminum prolongs theath of primary radicals to latex particle, and reduces the initi-ting efficiency, which is known as “cage effect” [22]. It is alsoound from Fig. 6 that PG and GE reaches the highest valuef 20.5% and 54.1%, respectively when CCTAB is 0.94 mmol/L.ince, it is impossible for polymerization occurs entirely ontourface of aluminum, the existing mode of aluminum powder inmulsion system may be divided into three types: (a) insertedn the polymer; (b) attached on the surface of latex particles; (c)reely suspended in the latex system. With the development ofolymerization and growth of latex particles, it is imperative toncrease the emulsifier amount in order to avoid agglomerationnd gelatin. On one hand, when CCTAB is lower than criticalicelle concentration (CMC), increasing CCTAB may accelerate

he formation of more latex particles, which is importance of initu emulsion polymerization. On the other hand, too high CCTABan result in nucleation of micelles and increasing proportion ofomogenous nucleation, which may lead to the decrease of PGnd GE. Since CMC of CTAB is 0.85 mmol/L [23], it is summa-ized that satisfying PG and GE can be obtained when CCTAB islittle higher than CMC.

.3. FT-IR analysis

Fig. 7 shows the FT-IR spectra of flaky aluminum pow-er, MPS-c-Al and PMMA/Al. Hydroxyl absorption peak at500 cm−1 in the curve of flaky aluminum powder shows thexistence of OH on surface of aluminum. In the IR spec-rum of MPS-c-Al, characteristic absorption of CH3 O Si at845 cm−1and weakening of OH at 3500 cm−1 indicate thatPS has been successfully coupled with aluminum by means of

hemical bonding, which is consistent with Scheme 1. The vibra-ion bands corresponding to PMMA (C O stretching vibration

t 1731 cm−1, characteristic absorption of C O C at 989, 1064,149, 1243 cm−1, the aliphatic C H stretch at 2843, 2951, 1243,271, 1387 and 1484 cm−1) are all found in the FT-IR curve ofMMA/Al. All these results definitively attest to the covalent

H. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 1–6 5

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Fig. 9. EDS spectrum of PMMA/Al composite particle.

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ig. 7. FT-IR spectra of flaky aluminum powder (a), MPS-c-Al (b), andMMA/Al composite particle (c).

onding of PMMA to aluminum through copolymerization ofMA with the methacrylate group of MPS initially coupled on

he surface of aluminum.

.4. EDS analysis

In order to confirm the formation of composite particle, EDSesting is employed to analyze relative elemental percentage ofamples’ surface. As can be seen from Fig. 8, 6.34% carbon inaky aluminum powder is due to the existence of some organicdditives on the surface of aluminum during the manufactur-ng process. Compared to flaky aluminum powder, the contentf carbon and oxygen become higher and the content of alu-inum become lower in the EDS spectrum of composite particle

Fig. 9), indicating that a lot of PMMA has been grafted ontourface of aluminum and PMMA/Al composite particle has beenuccessfully prepared.

Fig. 8. EDS spectrum of flaky aluminum powder.

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ig. 10. Relationship between hydrogen volume of some samples and evolvedime.

.5. Assess of corrosion resistance

Evolved hydrogen volume is used to assess the corrosionesistance. The less hydrogen volume is, the better corrosionesistance is. Because PMMA is an effective corrosion inhibitorf aluminum pigments [14], it can be seen from Fig. 10 that thevolved hydrogen volume of composite particle without MPSoupling decreased to 20 from 65 ml for flaky aluminum powderithin 48 h. The evolved hydrogen volume of composite particle

n the presence of MPS is only 4 ml. The remarkable improve-ent in corrosion resistance also confirmed that more PMMA

as been formed on the surface of aluminum in the presence ofPS.

. Conclusion

In order to enhance corrosion resistance of aluminum pig-ents, PMMA/flaky aluminum composite particle was prepared

n the presence of MPS. Flaky aluminum pigments were firstoupled with MPS, and then encapsulated with PMMA by means

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f in situ emulsion polymerization of MMA. It was found thathe PC and CE could attain 9.1% and 45.5%, respectively at theonditions of m(MPS)/m(Al) = 20%, time = 20 h, and temper-ture = 55 ◦C when flaky aluminum powder was coupled withPS. As for the in situ emulsion polymerization, there were

bvious increases in both PG and GE when PC was increasedrom 0 to 1.2% or more, showing that more PMMA had beenormed on the surface of aluminum in the presence of MPS. Theechanism may be that MMA would copolymerize with theC bonding of MPS-c-Al and then self-polymerize to form

MMA on the surface of aluminum. PG and GE reached theighest value of 20.5% and 54.1%, respectively when CCTABas 0.94 mmol/L (a little higher than CMC). The analysis ofT-IR and EDS indicated that PMMA/Al composite particlead been successfully prepared, and evolved hydrogen detec-ion showed that corrosion resistance of aluminum pigmentsad been remarkably improved in the presence of MPS.

cknowledgements

The author thanks W. Tian from Central South University foris great help in EDS experiments. Thanks are also expressedo Y. Li and Z.L. Chen for their skillful experimental works.

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