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Dyes and Pigments 79 (2008) 236–241

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Dyes and Pigments

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Preparation and characterization of poly(trimethylolpropane triacrylate)/flakyaluminum composite particle by in situ polymerization

Hui Liu*, Hongqi Ye, Yingchao Zhang, Xinde TangCollege of Chemistry and Chemical Engineering, Central South University, Lushan Road South, Changsha 410083, PR China

a r t i c l e i n f o

Article history:Received 30 October 2007Received in revised form 21 February 2008Accepted 4 March 2008Available online 19 March 2008

Keywords:Poly(trimethylolpropane triacrylate)Flaky aluminumComposite particleEncapsulationGraftingIn situ polymerization

* Corresponding author. Tel.: þ86 731 8876605; faxE-mail address: liuhui0320@126.com (H. Liu).

0143-7208/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.dyepig.2008.03.001

a b s t r a c t

Poly(trimethylolpropane triacrylate)/flaky aluminum composite particle was prepared by in situ poly-merization in order to improve the corrosion resistance and adhesive property of aluminum pigments.The effect of monomer concentration, initiator concentration and feeding mode on the conversion, per-centage of grafting and grafting efficiency in the preparation was studied. It was found that poly(trimethylolpropane triacrylate) had been successfully grafted on the surface of flaky aluminum throughchemical bond, rather than the simple physical adsorption. Evolved hydrogen detection and pullingexperiment showed that the corrosion resistance and adhesive property of composite particle had beenmarkedly enhanced, implying that flaky aluminum had been compactly encapsulated with a layer ofpoly(trimethylolpropane triacrylate) by in situ polymerization.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

During the last two decades, flaky aluminum powders (alsocalled aluminum pigments) have been widely used in automotivecoatings, roof coatings, printing inks, and plastic materials for theirprotective and decorative functions [1–3]. The aluminum pigmentsare usually produced by grinding atomized aluminum powderin the ball mill together with white spirit as solvent and fatty acidas lubricant. The mill operates at a speed that allows the ballsto cascade onto the fine, spherical or irregular atomized aluminumpowders and finally to flatten the powders into an ultrathin flakyparticles. After grinding, the subsequent filtration makes theultrathin flaky aluminum powder in the form of paste for sale withvolatile organic compounds [4–6]. However, a severe problemis the corrosion reaction of aluminum pigment in aqueous acidicor alkali painting media, which causes the evolution of hydrogen[7,8]:

2Al D 6H2O / 2AlðOHÞ3 D 3H2[

2Al D 6HCl / 2AlCl3 D 3H2[

: þ86 731 8879616.

All rights reserved.

2Al D 2NaOH D 6H2O / 2NaAlðOHÞ4 D 3H2[

Consequently, the corrosion reaction will change the color ofaluminum pigment from silver to grey and may lead to a dangerouspressure buildup in the container [9]. Therefore it is necessaryto inhibit the corrosion. At present, a lot of research has been de-voted to inhibit the corrosion reaction by surface modificationof aluminum pigments. Aluminum pigment was coated with SiO2

via sol–gel method through tetraethyl orthosilicate to improve itscorrosion resistance [10,11]. The corrosion inhibition by mixingaluminum pigment with commercially available styrene–maleicanhydride copolymers as an inhibitor was also investigated [12].Al/poly(methyl methacrylate) composites containing differentvolume fractions of aluminum were prepared and thermalexpansion and electrical behavior of the composites were in-vestigated [13].

In our previous reports, aluminum pigment was encapsulatedwith styrene–maleic acid copolymer by in situ polymerization [14],and poly(methyl methacrylate)/flaky aluminum composite particlewas synthesized and characterized in the presence of silane cou-pling agent, 3-methacryloxypropyl-trimethoxysilane [15]. In thepresent work, poly(trimethylolpropane triacrylate)/flaky alumi-num composite particle (PTMPTA/Al) was prepared by in situ po-lymerization in order to improve the corrosion resistance andadhesive performance of aluminum pigments.

CH3—CH2—C—CH2—O—C—CH=CH2 3

O

Scheme 1. The molecular structure of TMPTA.

Fig. 2. The effect of initiator concentration on conversion, percentage of grafting andgrafting efficiency.

H. Liu et al. / Dyes and Pigments 79 (2008) 236–241 237

2. Experimental

2.1. Materials and reagents

Flaky aluminum (Al, from Shanghai Weiye Co. Ltd., China) wasdried in vacuum at 100 �C for 24 h to remove the volatile organiccompounds, and trimethylolpropane triacrylate (TMPTA, fromNanjing Chemical factory, China) was refined to remove theantipolymerization agent before use. Azobisisobutyronitrile (AIBN,from Tianjin Chemicals Co. Ltd., China) as the initiator, 3-meth-acryloxypropyl-trimethoxysilane (from Wuhan University SiliconeNew Material Co. Ltd, China) as the coupling agent, poly-vinylpyrrolidone (from Tianjin Chemicals Co. Ltd., China) as thedispersant, and toluene (from Changsha chemical factory, China)as the solvent, were all analytical grade reagents and used asreceived.

2.2. Preparation of PTMPTA/Al composite particle

A certain amount of flaky aluminum, toluene, TMPTA, poly-vinylpyrrolidone, 3-methacryloxypropyl-trimethoxysilane, anda little acrylic acid were added into a four-necked flask equippedwith a mechanical stirrer, a thermometer, a reflux condenser anda guttate funnel. Toluene solution of AIBN was slowly added intothe flask by guttata funnel at 65 �C under nitrogen flowing. Thenthe mixture was heated to 85 �C and reacted for several hours.After the reaction, the mixture was precipitated with methanoland filtered. The cake was dried in vacuum for 24 h to obtainPTMPTA/Al composite particle.

In order to determine quantitatively grafted and non-graftedPTMPTA in PTMPTA/Al, a certain amount of PTMPTA/Al wasdispersed in tetrahydrofuran with ultrasonic vibration and thenon-grafted PTMPTA was dissolved in tetrahydrofuran. After thecentrifugation at 10,000 rpm for 1 h, non-grafted PTMPTA wasobtained by precipitating the filtrate with methanol. The cake wasdried in vacuum, and PTMPTA-grafted aluminum was obtained.

Fig. 1. The effect of monomer concentration on conversion, percentage of grafting andgrafting efficiency.

The evolved hydrogen detection in a reaction of aqua regia withaluminum as described in literature [1] was used to measure thecontent of aluminum in PTMPTA-grafted aluminum. TotalPTMPTA was the sum of grafted and non-grafted PTMPTA.Therefore, the conversion, percentage of grafting and grafting effi-ciency were calculated according to the following equations [16–18]:

Conversion ¼ total PTMPTAðgÞTMPTA usedðgÞ � 100%

grafted PTMPTAðgÞ

Percentage of grafting ¼

flaky aluminum usedðgÞ � 100%

grafted PTMPTAðgÞ

Grafting efficiency ¼

total PTMPTAðgÞ � 100%

2.3. Characterization and testing

Fourier transform infrared (FTIR) spectroscopy patterns wererecorded on a Nicolet AVATAR360 system. The surface character-ization of PTMPTA/Al was accomplished by using an ESCALAB MK IIMulti-functional X-ray photoelectron spectrometer (XPS) with passenergy of 29.35 eV and an Mg Ka line excitation source. The bindingenergy of C1s (284.6 eV) was used as a reference. Hydrogen volumewas used to evaluate the corrosion resistance of samples [15], andthe evolved hydrogen detection was carried out as described in theliterature [19]. The adhesion of PTMPTA/Al was tested throughpulling experiment [20]: the flaky aluminum or PTMPTA/Alwas firstly dispersed in thinner solution, and then the mixture wassprayed onto a black plastic board, finally the plastic boardwas pulled by adhesive tape after air-drying.

Table 1The effect of feeding mode on conversion, percentage of grafting and grafting efficiency

Feedingmode

Conversion(%)

Percentage ofgrafting

Grafting efficiency(%)

All reactantsadded together (A)

52.3 5.6 26.8

Only AIBN dropped (B) 56.5 10.9 48.3Only TMPTA dropped (C) 62.6 9.7 38.7AIBN and TMPTA dropped

together (D)66.8 12.1 45.3

(a)

(b)

(c)

Fig. 3. FTIR spectra of flaky aluminum (a), pure PTMPTA (b) and PTMPTA/Al compositeparticle (c).

Table 2Surface elemental composition of flaky aluminum and PTMPTA/Al compositeparticle

Samples Al (%) C (%) O (%)

Flaky aluminum 30.1 40.5 29.4PTMPTA/Al 4.2 62.4 33.4

H. Liu et al. / Dyes and Pigments 79 (2008) 236–241238

3. Results and discussion

3.1. Preparation of PTMPTA/Al composite particle

Firstly, the selection of TMPTA is based on the fact that there arethree vinyl bonds in the molecular structure of TMPTA (Scheme 1),so the in situ polymerization of TMPTA may lead to the formation ofcrosslinked netty PTMPTA [21,22], which is helpful to improve thecorrosion resistance and adhesive performance of flaky aluminum.Fig. 1 shows the effect of TMPTA concentration on conversion,percentage of grafting and grafting efficiency in the preparation ofcomposite particle. As can be seen, conversion decreases with in-creasing m(TMPTA)/m(Al), which can be explained with the re-duction of relative initiator (AIBN) concentration. It is easy to findthat when m(TMPTA)/m(Al)¼ 40%, percentage of grafting remains ata plateau level of 11.3%, and grafting efficiency reaches the highestvalue of 49.4%. The increase of m(TMPTA)/m(Al) may result in theaugment of TMPTA concentration around flaky aluminum, which

(a)

(b)

Fig. 4. XPS full-survey spectra of flaky aluminum (a) and PTMPTA/Al composite par-ticle (b).

enhances the probability of integrating between TMPTA and flakyaluminum. Nevertheless, too high TMPTA concentration may giverise to the agglomeration of flaky aluminum, which is harmful tothe in situ polymerization.

Secondly, the effect of initiator concentration on conversion,percentage of grafting and grafting efficiency is demonstrated inFig. 2. It can be seen that conversion increases with the increasingm(AIBN)/m(TMPTA), which can be attributed to the increase ofradical concentration. Moreover, both percentage of grafting andgrafting efficiency achieve their respective culmination at m(AIBN)/m(TMPTA)¼ 5%. Because the amounts of radical and active chainincrease with the increasing initiator concentration, the graftingbetween PTMPTA and flaky aluminum bears high probability. At thesame time, too much initiator may lead to the formation of low-molecular-weight PTMPTA, which is unfavorable to the preparationof PTMPTA/Al composite particle with high percentage of grafting.

Lastly, feeding mode is of importance to the synthesis of PTMPTA/Al composite particle, and its effect on conversion, percentage ofgrafting and grafting efficiency is listed in Table 1. As shown in Table 1,mode A (all reactants added together) has the worst effect, whilemode D (AIBN and TMPTA dropped together) has the best effect. Theexperimental result can be interpreted as follows: when AIBN and

Fig. 5. High-resolution C1s XPS spectrum and curve-fitting of flaky aluminum (a) andPTMPTA/Al composite particle (b).

Table 3C1s XPS analysis of flaky aluminum and PTMPTA/Al composite particle

Samples Standardposition (eV)

Actualposition (eV)

Chemicalshift (eV)

Content(%)

Attribution

Flaky aluminum 285.0 284.6 �0.4 71.1 C–C286.9 286.4 �0.5 28.9 C–O

PTMPTA/Al 285.0 284.7 �0.3 52.2 C–C286.9 286.0 �0.9 38.5 C–O288.6 288.1 �0.5 9.4 C]O

Table 4O1s XPS analysis of flaky aluminum and PTMPTA/Al composite particle

Samples Standardposition (eV)

Actualposition (eV)

Chemicalshift (eV)

Content(%)

Attribution

Flaky aluminum 531.5 531.2 �0.3 81.3 Al–O533.1 533.3 þ0.2 18.7 C–O

PTMPTA/Al 531.5 531.9 þ0.4 54.5 Al–O532.0 533.3 þ1.3 12.8 C]O533.1 533.8 þ0.7 32.7 C–O

H. Liu et al. / Dyes and Pigments 79 (2008) 236–241 239

TMPTA are dropped together, the polymerizing system is actuallycomplemented with initiator and monomer, which is beneficialto the in situ polymerization of TMPTA on the surface of flakyaluminum.

3.2. Characterization of PTMPTA/Al composite particle

Fig. 3 exhibits the FTIR spectra of flaky aluminum, pure PTMPTA,and PTMPTA/Al composite particle. Hydroxyl absorption peak at3500 cm�1 in the curve of flaky aluminum attests to the existenceof OH on the surface of flaky aluminum. The characteristic ab-sorption peak corresponding to C]O stretching vibration shiftsfrom 1739 cm�1 in the FTIR curve of pure PTMPTA to 1721 cm�1 inthe curve of PTMPTA/Al, and the absorption band of –CH2– shiftsfrom 2967 cm�1 in the FTIR spectrum of pure PTMPTA to2949 cm�1 in the spectrum of PTMPTA/Al, which is possibly as-cribed to the formation of chemical bond between PTMPTA andflaky aluminum. Furthermore, the characteristic absorption peak at1400 cm�1 corresponding to AlO2

� is also observed in the curve of

Fig. 6. High-resolution O1s XPS spectrum and curve-fitting of flaky aluminum (a) andPTMPTA/Al composite particle (b).

PTMPTA/Al. All these results indicate that PTMPTA/Al compositeparticle has been successfully prepared by in situ polymerization.

XPS is generally employed to analyze the relative elementalcontent and chemical bond on the surface of samples. The XPS full-survey spectra of flaky aluminum and PTMPTA/Al are illustrated inFig. 4, and the surface elemental composition is listed in Table 2. Ascan be seen, compared with flaky aluminum, the content of Al, Cand O for PTMPTA/Al changed from 30.1%, 40.5% and 29.4% to 4.2%,62.4% and 33.4% respectively, manifesting that PTMPTA has beengrafted on the surface of flaky aluminum.

In order to clarify the status of surface chemical bond, the high-resolution XPS spectra of C1s and O1s have been scanned, and theyare fitted through the software ‘‘XPSPEAKFIT’’ in the light of Lor-entzian–Gaussian principle [23,24].

The high-resolution C1s XPS spectra and curve-fitting of flakyaluminum and PTMPTA/Al are shown in Fig. 5a and b respectively,and the binding energies and the attributions of C1s peaks are listedin Table 3. Clearly, compared with flaky aluminum, the content ofC–C on the surface of PTMPTA/Al decreases while the content of C–O increases. Moreover, C]O (288.1 eV) appears in the curve-fittingspectrum of PTMPTA/Al, which can be attributed to the encapsu-lation of PTMPTA onto flaky aluminum. It is also found from Table 3that the negative chemical shifts take place in the curve-fittingspectrum. Furthermore, PTMPTA/Al has more negative shift thanflaky aluminum, which can be explained from the fact that theexistence of PTMPTA reduces the outer electron density, weakensthe shielding effect, and enhances the inner electron bindingenergy.

Fig. 6a and b presents the high-resolution O1s XPS spectra andcurve-fitting of flaky aluminum and PTMPTA/Al respectively, andTable 4 illustrates the binding energies and the attributions of O1speaks. Similar to C1s, the decrease of Al–O, the increase of C–O andthe emergence of C]O (533.8 eV) are essentially caused by thegrafted PTMPTA onto the surface of flaky aluminum. Opposite toC1s, positive chemical shifts of binding energies are found in the

Fig. 7. Evolved hydrogen trends of flaky aluminum and PTMPTA/Al composite particle.

Fig. 8. The image of non-pulled board (a) and pulled board (b) of flaky aluminum. Fig. 9. The image of non-pulled board (a) and pulled board (b) of PTMPTA/Al com-posite particle.

H. Liu et al. / Dyes and Pigments 79 (2008) 236–241240

O1s peaks, which is possibly relevant to the uncrystallized poly-meric layer and special core–shell structure of PTMPTA/Al.

In a word, FTIR and XPS indicate that PTMPTA/Al compositeparticle has been successfully prepared by in situ polymerization,and PTMPTA has been grafted on the surface of flaky aluminumthrough chemical bond, rather than the simple physical adsorption.

3.3. Property evaluation of PTMPTA/Al composite particle

Evolved hydrogen volume is usually used to evaluate the cor-rosion resistance. Lesser the hydrogen volume, better the corrosionresistance [25]. Fig. 7 shows the evolved hydrogen trend of somesamples in 0.1 mol L�1 HCl aqueous solution. As can be seen, theevolved hydrogen volume of 0.5 g flaky aluminum after 48 h is65 ml, while PTMPTA/Al releases no hydrogen at all whenm(TMPTA)/m(Al) attains 20%. Clearly, the polymeric layer on thesurface of composite particle prevents aluminum from reactingwith the corrosive media. A conclusion can be drawn that, com-pared with flaky aluminum, the corrosion resistance of PTMPTA/Alhas been markedly improved, implying that flaky aluminum hasbeen compactly encapsulated with a layer of PTMPTA by in situpolymerization.

The pulling experiment is industrially employed to assess theadhesive property of samples, and the images of some non-pulledand pulled samples are demonstrated in Fig. 8a and b, and Fig. 9aand b. It can be observed that the non-pulled board of PTMPTA/Al(Fig. 9a) is slightly denser than that of flaky aluminum (Fig. 8a).Furthermore, after pulling, there is evident desquamation on theboard of flaky aluminum (Fig. 8b), while the board of PTMPTA/Al(Fig. 9b) almost keeps unchanged. There are three vinyl bonds inthe molecular structure of TMPTA, so it is likely that the resultingPTMPTA is in the possession of crosslinked network structure,which is helpful to the improvement of adhesion of PTMPTA/Al. Thepulling results suggest that PTMPTA/Al composite particle hasmuch better adhesive property than flaky aluminum. It may beaffirmed that a compact polymeric layer has been formed on thesurface of composite particle by in situ polymerization, which is ingood agreement with the analysis of corrosion resistance.

4. Conclusion

In order to improve the corrosion resistance and adhesiveproperty of aluminum pigments, PTMPTA/Al composite particle

H. Liu et al. / Dyes and Pigments 79 (2008) 236–241 241

had been successfully prepared by in situ polymerization. It wasfound that both percentage of grafting and grafting efficiency couldattain satisfactory results at the following conditions: m(TMPTA)/m(Al)¼ 40%, m(AIBN)/m(TMPTA)¼ 5%, and TMPTA and initiatorwere dropped together. Compared to flaky aluminum, the contentof Al, C and O for PTMPTA/Al changed from 30.1%, 40.5% and 29.4%to 4.2%, 62.4% and 33.4% respectively, indicating that PTMPTA layerhad been grafted onto the surface of flaky aluminum. Moreover, theemergence of C]O in the curve-fitting XPS spectrum of PTMPTA/Alcomposite particle strongly supported the fact that the drivingforce between PTMPTA and flaky aluminum was chemical bond,rather than the simple physical adsorption. Evolved hydrogendetection and pulling experiment indicated that the corrosionresistance and adhesive property of PTMPTA/Al composite particlehad been markedly enhanced, implying that flaky aluminum hadbeen compactly encapsulated with a layer of PTMPTA by in situpolymerization.

Acknowledgements

The authors thank Dr. Zhao QN from Wuhan University ofTechnology for his help in XPS experiment and Ms. Luo LL fromCentral South University for her help in paper revision. Thanks arealso expressed to their former students, Chen ZL, Chen JN, Bao R,Wang H, Li T, Zhou X and Lin TQ, for their skillful experimentalwork.

References

[1] Liu H, Ye HQ, Zhang YC. Preparation of PMMA grafted aluminum powder bysurface-initiated in situ polymerization. Applied Surface Science 2007;253(17):7219–24.

[2] Zhu JL, Wu SN. Pigments technology. 2nd ed. Beijing: Chemical Industry Press;2002. p. 375–8.

[3] Hirth U. Aluminum pigments for powder coating applications. Focus onPowder Coatings 2005;2005(6):2–3.

[4] Hong SH, Lee DW, Kim BK. Manufacturing of aluminum flake powder from foilscrap by dry ball milling process. Journal of Materials Processing Technology2000;100(1–3):105–9.

[5] Hong SH, Kim BK. Fabrication of aluminum flake powder from foil scrap bya wet ball milling process. Materials Letters 2001;51(2):139–43.

[6] Wendon GW. Aluminum and bronze flaky powders. London: ElectrochemicalPublications Limited; 1983. p. 36–9.

[7] Karlsson P, Palmqvist AEC, Holmberg K. Surface modification for aluminumpigment inhibition. Advances in Colloid and Interface Science 2006;128–130:121–34.

[8] Muller B. Citric acid as corrosion inhibitor for aluminum pigment. CorrosionScience 2004;46(1):159–67.

[9] Muller B. Corrosion inhibition of aluminum and zinc pigments by saccharides.Corrosion Science 2004;44(7):1583–91.

[10] Kiehl A, Greiwe K. Encapsulated aluminum pigments. Progress in OrganicCoatings 1999;37(3–4):179–83.

[11] Supplit R, Schubert U. Corrosion protection of aluminum pigments by sol–gelcoatings. Corrosion Science 2007;49(8):3325–32.

[12] Muller B, Shahid M, Kinet G. Nitro- and aminophenols as corrosion inhibitorsfor aluminium and zinc pigments. Corrosion Science 1999;41(7):1323–31.

[13] Singh V, Tiwari AN, Kulkarni AR. Electrical behaviour of attritor processed Al/PMMA composites. Materials Science and Engineering, B 1996;41(3):310–3.

[14] Liu H, Ye HQ, Tang XD. Aluminum pigment encapsulated by in situ co-polymerization of styrene and maleic acid. Applied Surface Science 2007;254(2):616–20.

[15] Liu H, Ye HQ, Zhang YC. Preparation and characterization of PMMA/flakyaluminum composite particle in the presence of MPS. Colloids and Surfaces A:Physicochemical and Engineering Aspects 2008;315(1–3):1–6.

[16] Liu P. Facile preparation of monodispersed core/shell zinc oxide@polystyrene(ZnO@PS) nanoparticles via soapless seeded microemulsion polymerization.Colloids and Surfaces A: Physicochemical and Engineering Aspects 2006;291(1–3):155–61.

[17] Luna-Xavier JL, Guyot A, Bourgeat-Lami E. Synthesis and characterization ofsilica/poly(methyl methacrylate) nanocomposite latex particles throughemulsion polymerization using a cationic azo initiator. Journal of Colloid andInterface Science 2002;250(1):82–92.

[18] Liu P, Guo JS. Polyacrylamide grafted attapulgite (PAM–ATP) via surface-ini-tiated atom transfer radical polymerization (SI-ATRP) for removal of Hg(II) ionand dyes. Colloids and Surfaces A: Physicochemical and Engineering Aspects2006;282–283:498–503.

[19] Liu H, Ye HQ, Chen ZL, Li Y. Effect of encapsulation of aluminum pigments byin situ polymerization of acrylic acid on corrosion resistance. MaterialsProtection 2006;39(10):8–11.

[20] Satas D, Tracton AA. Coatings technology handbook. 2nd ed. New York: MarcelDekker Press; 2001.

[21] Kim SI, Kim HS, Na SH, Moon SI, Kim YJ, Jo NJ. Electrochemical characteristicsof TMPTA- and TMPETA-based gel polymer electrolyte. Electrochimica Acta2004;50(2–3):317–21.

[22] Drumheller PD, Hubbell JA. Analysis of phase mixing in aged polymer net-works of poly(ethylene glycol) and poly(trimethylolpropane triacrylate).Polymer 1995;36(4):883–5.

[23] Rajagopalan R, Iroh JO. Characterization of polyaniline–polypyrrole compositecoatings on low carbon steel: a XPS and infrared spectroscopy study. AppliedSurface Science 2003;218(1–4):58–69.

[24] Perruchot C, Khan MA, Kamitsi A, Armes SP, Watts JF, Werne TV, et al. XPScharacterisation of core–shell silica–polymer composite particles synthesisedby atom transfer radical polymerisation in aqueous media. European PolymerJournal 2004;40(9):2129–41.