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The composite particles of nano calcium carbonate/polymethyl methacrylate (PMMA) were prepared by soapless emulsion polymerization

surface of nano calcium carbonate to form nano calcium carbonate/PMMA

The materials formed by embedding of inorganic particles

[7,8]. The soapless emulsion polymerization with organicmonomers in the aqueous suspensions of inorganic particles

inorganic particles can be well dispersed into polymer matrix.

In the paper, it was innovatively validated that nano calciumcarbonate without surface modification can also be well coatedby polymers to form composite particles. The effects ofsynthesis parameters on monomer conversion were studied.

06)has been considered as a promising and preponderant approachto prepare nano composite particles, because the organicinto polymeric matrices represent a new class of polymericmaterials that combine the properties of the inorganic particles(in terms of mechanical strength, modulus, and thermalstability) with the processability and the flexibility of organicpolymer matrix. Of course, such materials can be obtained bysimply mixing the organic and inorganic components. However,in order to achieve the best dispersion of the inorganic particlein polymer matrix and interfacial adhesion between the polymerand inorganic particle, the techniques for synthesizing compos-ite particles made of inorganic particles coated by polymershave been developed [1,2], including mini-emulsion [3,4],suspension [5], dispersion [6], and emulsion polymerization

Moreover, due to lack of surfactants, the application fields ofcomposite particles are enlarged compared with the productsfrom emulsion polymerization. The prepared compositeparticles have great potential applications in Painting, Plastics,and Rubber, etc.

The inorganic particles were mainly nano silicon dioxide,nano titanium dioxide, nano aluminum oxide, etc., all of whichhave high surface activities and more active radicals [911].Nano calcium carbonate that has lower surface activity andfewer active radical was used to prepare inorganicorganicnano composite particles by the technology only after its surfacehad been commendably modified by surfactants [1216].composite particles. 2006 Published by Elsevier B.V.

Keywords: Nano calcium carbonate; Composite particles; Soapless emulsion polymerization; PMMA; Mechanism

1. Introduction components on the surface of inorganic particles can besynthesized in accordance with the applying systems. Thecarbonate. PMMA is chemically grafted and physically wrapped on thetechnique in the aqueous suspension of nano calcium carbonate. The effects of synthesis parameters on monomer conversion were studied, and theformation mechanism of composite particles was proposed. The results indicated that nano calcium carbonate that existed in the suspension canboost the monomer conversion. Appropriate stirring rate can enhance the probabilities for polymer to coat on the surface of nano calciumStudy on in situ preparation ofcomposit

Wei Wu a, Taobo He a, Jian-feng Cha Key Lab for Nanomaterials, Ministry of Education; Research Center o

Beijing University of Chemical Teb Advanced Materials Research Institute, Univer

Received 14 November 2Available online

Abstract

Materials Letters 60 (20 Corresponding author. Tel.: +86 10 64446466; fax: +86 10 64434784.E-mail address: chenjf@mail.buct.edu.cn (J. Chen).

0167-577X/$ - see front matter 2006 Published by Elsevier B.V.doi:10.1016/j.matlet.2005.03.077ano calcium carbonate/PMMAparticles

a,, Xueqin Zhang a, Yuxi Chen b

e Ministry of Education for High Gravity Engineering and Technology,ology, Beijing 100029, PR Chinaof New Orleans, New Orleans, LA 70148, USA

; accepted 1 March 2005February 2006

24102415www.elsevier.com/locate/matletThe formation mechanisms of composite particles wereproposed. The applications in epoxy varnish were investigatedpreliminarily.

elements on the surface of nano calcium carbonate andcomposite particles after extraction.

The characteristics of paint film, which were in terms ofadhesive force, impaction, toughness, hardness, were testedaccording to the China National Standards and their units werealso in accordance with the standards.

3. Results and discussions

3.1. The effects of synthesis parameters on the preparation of nanocomposite particles

The basic technology of preparation of composite particles issoapless emulsion polymerization of methyl methacrylate in nano

2411etters 60 (2006) 241024152. Experimental

2.1. Main raw material and reagents

Nano calcium carbonate in the form of filter cake was pro-vided by Nanotech Science and Technology Ltd and producedby high gravity technology with a mean size of 40 nm, cubicshape and crystalline structure of calcite. Chemicals aregenerally reagent grade from commercial sources withoutfurther purification. Analytical-grade methyl methacrylatemade by Beijing Yili Fine Chemicals Ltd was used as monomer.Analytical-grade ammonia persulphate (APS) made by BeijingChemical Plant was used as initiating agent. Analytically gradetoluene made by Beijing Yili Fine Chemicals Ltd was used asextraction solvent of polymer. Sodium salt of poly (acrylic) acidpurchased from market was used as dispersing agent.

2.2. The process

2.2.1. The preparation of aqueous suspension of nano calciumcarbonate

The mixtures comprising of filter cakes of nano calciumcarbonate and appropriate amounts of deionized water as well asa small quantity of dispersing agent were stirred withexperimental high-speed disperser at 3000 rpm for 1 h to get8% aqueous suspension. The pH values of suspension wereadjusted to 78 with diluted hydrochloric acid.

2.2.2. The preparations of nano composite particlesThe as-prepared aqueous suspension of nano calcium

carbonate was fed into a three-necked flask equipped with acondenser and then stirred at 600 rpm under the circumstance ofN2 gas. When the temperature of the aqueous suspensionreached 7080 C that was maintained by cycling water fromaqueous thermostat baths, the initiating agent aqueous solutionwas added. The 30 ml monomer was added in dropwise within30 min. After being farther stirred for another 3 h, thesuspension was filtrated and dried at 7080 C in vacuum.Part of the dried product was extracted by toluene for 12 h withSolvet extracting apparatus and used for the characterizations.

2.3. Characterizations

H-800 Transmission Electron Microscopy (TEM) was usedto characterize the particle size and morphologies. PCT-1Adifferential thermal analyzer made by Beijing Optical Instru-ment Factory was employed to conduct Thermo-Gravimetric(TG) analysis of nano calcium carbonate and compositeparticles to determine the content of organic substances incomposite particles and bonding states between nano calciumcarbonate and polymer. Nicolet 60SXB Fourier TransformInfrared (FT-IR) Spectrometer was used to characterize thesurface components of the nano calcium carbonate, thecomposite particles and the composite particles after extraction.

W. Wu et al. / Materials LPHI5300 X-ray Photoelectron Spectrometer (XPS) made byAmerican PERKIN-ELMER Physics Electronics Company wasused to characterize the changes of chemical environment ofcalcium carbonate aqueous suspension. The conversion is defined asthe percentage of the mass of created polymer comparing with the totalmass of monomer; its calculating formula is that

C M1M2M3M

100%

where C denotes the monomer conversion; M1 denotes the dried massof composite particles; M2 denotes the mass of added calciumcarbonate; M3 denotes the mass of added initiating agent; M denotesthe mass of added monomer.

The preparations of composite particles are greatly relevant to themonomer conversion. The monomer conversion in the system isaffected by temperature, stirring rate, the dosage of initiating agent,calcium carbonate concentration.

3.1.1. The effects of temperaturesFig. 1 is the relationships between the systematic temperatures and

monomer conversions. The radical polymeric reaction needs energythat is provided by heating in the system. As the temperature rises, thedecomposing rate of initiating agent is speeded and the collisionprobability among monomers, polymers and particles is increased.Accordingly, the conversion is quickly boosted. When the temperatureis over 80 C, it has no obvious effects on the decomposition ofinitiating agent, so the conversion is basically invariable.

3.1.2. The effects of stirring rateThe mass transfer is one of the important influence factors in the

non-homogeneous phase reaction system, which is reflected in thechanges of stirring rate.Fig. 1. Effects of temperature for monomer conversion.

3.1.3. The effects of the dosages of the initiating agentThe decomposing equation of ammonia persulphate (APS) is that

[14]:

Fig. 4. Time-conversion curves for different CaCO3 concentration.

2412 W. Wu et al. / Materials Letters 60 (2006) 24102415Fig. 2 is the relationships between stirring rate and conversion. Theconversion decreases as the stirring rate rises and becomes stable as thestirring rates go higher than 450 rpm. It was found that incompact flocand amounts of froths were created in the products at lower stirring rate.However, homogeneous suspension with less froth is created at higherstirring rate. The polymerization happens either on the surface of nanocalcium carbonate or in the micelles of polymethyl methacrylate inwater phase. When the stirring rate is lower, the mass transfer ratereduces. Consequently, the Tronumsdorff gel effect is obvious [12,13]and causes high conversion and drastic polymerization reaction inwater mainly other than on the surface of nano calcium carbonate. Atthe higher stirring rate, the Tronumsdorff gel effect is indistinctive,homogeneous suspension is created. Moreover, the polymerizationreaction mainly happens on the surface of nano calcium carbonatealthough the conversion is lower. The stirring rate should be kept inhigher level in order to weaken the effects of mass transfer. The resultsare different from that of Seul et al. [15], in which the conversions riseat the low mixing rates and become stable at the high mixing rates, thedifferences are relevant to the surface properties of calcium carbonatethat result in different reaction mechanisms. In Ref. [15], the surface of

Fig. 2. Relationships between conversion and stirring rate.calcium carbonate used is hydrophobic, on which the monomers areeasy to polymerize, even at a low mixing rate.

Fig. 3. Relationships between conversion and dosage of APS.S2O28 2SO

4 1

This reaction coincides with the regulation of first order reaction.The equation of reaction rate is as follow:

dS2O28

dt k S2O28

2

where t is reaction time with the unit of hour; [S2O82] is the

concentration of ammonia persulphate with the unit of mol/L; k isreaction rate constant.

During the preparation of composite particles, ammonia persulphatedecomposes by heating to create free radicals that initiateFig. 5. TEM of nano CaCO3 particles.

2413W. Wu et al. / Materials Letters 60 (2006) 24102415polymerization of methyl methacrylate. The high concentration ofpersulphate radicals can speed the polymerization reaction to raiseconversion. It can be known from Fig. 3 that when the dosages ofammonia persulphate are lower than 0.4% of monomer mass, theconversion rise as the dosage of APS increase; when the dosages arehigher than 0.4% of monomer mass, the conversion is hardlychanged. The existence of an optimal value is owed to the monomer

Fig. 6. TEM of nano CaCO3/PMMA composite particles.dosage and concentration of calcium carbonate being fixed in thesystem and the reaction time being constant for different dosage ofAPS.

3.1.4. The effects of calcium carbonate concentrationsFig. 4 is the relationships between monomer conversion and

reaction time in the suspensions of different concentrations of nanocalcium carbonate. The rate of polymerization reaction increases

Fig. 7. Thermogravimetric analysis curves for pure CaCO3 and CaCO3/PMMAcomposite particles.apparently in the presence of nano calcium carbonate. It was reportedthat the stable emulsions could be formed when there were particles onthe interfaces between water and oil [17]. Based on the experimentalphenomenon and results, it is deduced that nano calcium carbonate canbe located on the interfaces of monometer-water to stabilize the formedmicelles that provide more polymerization sites than that without nanocalcium carbonate.

3.2. Characterizations of composite particles

Figs. 5 and 6 are TEM micrographs of nano calcium carbonatebefore and after modification. The particle size becoming large aftermodification indicates that there are polymers coated on the surface ofnano calcium carbonate.

Fig. 7 is thermogravimetric analysis curves of composite particlesand nano calcium carbonate. The lost weight above 270 C forcomposite particles is about 43% of the total weight, which is due to thedecomposition of polymer in the composite particles.

3.3. The bonding mechanisms between nano calcium carbonate andPMMA

Fig. 8. Relationships between extraction time and mass of composite particles.In order to study the bonding status between nano calciumcarbonate and polymer in composite particles, the relationships

Fig. 9. FT-IR curves of samples. 1: Pure CaCO3; 2: composite particles aftercalcination; 3: composite particles after extraction; 4: composite particles.

excellent anti-chemicals and excellent electrical isolation. However,the solidifying velocity of epoxy varnish has strong effects on itsapplications. Generally, the quicker its solidifying velocity varnishespecially at lower temperature, the wider its application fields andthe better its application properties. In the paper, the compositeparticles of CaCO3/PMMA were used to improve performances ofepoxy varnish. Table 2 shows the performances of epoxy varnishafter adding composite particles of 3.7% of epoxy varnish mass. Thesolidifying velocity at 20 C is quickened after composite particleswere added.

Table 1The results of XPS analysis for pure nano-CaCO3 and composite particles afterextraction

Sample Element Bondingenergy (eV)

Chemicalshift (eV)

Atom relativecontent

Pure CaCO3 C1S 289.6 19.94O1S 531.3 61.68Ca2P 3/2 347.0 18.39Ca2P 1/2 350.4

Composite particlesafter extraction

C1S 289.0 0.6 33.96O1S 531.5 0.2 56.94

2414 W. Wu et al. / Materials Letters 60 (2006) 24102415between the extraction time and the mass of composite particles wereinvestigated and the results are shown in Fig. 8. The mass of compositeparticles is stable after extraction for 24 h. The lost mass is 38.7% of thetotal mass, which is less than the lost weight obtained by thermo-gravimetric analysis, i.e., 43%. 7.54% of the total mass bonds hardlywith nano calcium carbonate.

Fig. 9 shows typical FITR spectrums of nano calcium carbonate,composite particles, composite particles after calcination and compos-ite particles after extraction. Characteristic peaks for PMMA in thespectrums of the composite particles and the composite particles afterextraction suggest that the polymer chains are covalently bonded to thesurface of nano calcium carbonate. For example, the peaks at bands of30002900 cm1, 1388 cm1 and 1452 cm1 are assigned to thestretching vibrations and bending vibrations of CH3 of PMMA,respectively. The peak at 1735 cm1 is assigned to the characteristicvibration adsorption of carbonyl.

Table 1 shows the XPS characterization results of nano calciumcarbonate and composite particles after extraction. The ratio ofrelative atom numbers of Ca, O, C on the surface of nano calciumcarbonate is 1:3.35:1.08, which is close to that of CaCO3. However,the ratio of relative atom numbers of Ca, O, C on the surface ofcomposite particles after extraction is 1:6.26:3.73, showing thatthere exists PMMA on the surface of composite particles afterextraction and great changes have taken place on the surface ofcalcium carbonate after modification. The relatively great change ofchemical shift of C1s suggests that the PMMA are grafted on thesurface of calcium carbonate after reacting with C atoms ofcarbonate radicals.

3.4. The preliminary investigations of applications of nano compositeparticles in epoxy varnish

Ca2P 3/2 346.9 0.1 9.10Ca2P 1/2 350.4 0.1The epoxy varnish with high quality has found extensiveapplications because of its excellent adhesion to polar substances, carbonate, where polymer has been grafted on and further

Table 2Comparisons of the properties of epoxy resin varnishes (20 C)

Adhesive force(grade)

Impact resistance(J)

Pure epoxy varnish 1 4.9Adding 3.7% Nanocalcium carbonate

12 4.9

Adding 3.7% compositeparticles

1 4.9

Test methods (Chinanational standards)

GB/T1720-1993 GB/T1732-1993Toughness(mm)

Hardness(after 1.5 days)

Hardness(after 7 days)

1 0.34 0.531 0.33 0.45

1 0.42 0.555

At the beginning, there are few radicals in the system. Theprobabilities of termination reactions among increasing radicalschains is low. The collision probabilities of increasing radicalschains with nano calcium carbonate particles are high. AlthoughCaCO3 is a electrovalent bonded compound, the CO3

2 iscovalence radicals which contain CfO double bonds. Theincreasing polymer chains react with the CfO double bonds tocreate more stable radicals and then graft on the surface of nanocalcium carbonate. The remained monomers and radicals'chains are adsorbed easily on the surface of nano calcium4. Discussions of formation mechanism of nano compositeparticles

Based on the previous investigation, a formation mechanismof composite particles is proposed:

S2O28 SO

4 3

4GB/T1731-1993 GB/T1720-1993

Fig. 10. The formation process of composite particles.

2415W. Wu et al. / Materials Letters 60 (2006) 24102415polymerize and wrap on the surface of nano calcium carbonate.Finally, composite particles are formed. The whole process isshown in Fig. 10.

5. Conclusions

In this paper, the effects of synthesis parameters on monomerconversions were studied. The formation mechanism ofcomposite particles was proposed. The conclusions obtainedin the paper are as follows:

(1) Appropriate amounts of nano calcium carbonate canpromote soapless emulsion polymerization of methylmethacrylate. Increasing stirring rate can enhance theprobabilities for polymer to coat on the surface of nanocalcium carbonate. When the temperature is lower than 80C and the dosages of initiating agent is lower than 0.4%of monomer mass, increasing temperature and dosage ofinitiating agent can raise monomer conversion.

(2) There are chemical bonds to create between carbon atomsof nano calcium carbonate and polymethyl methacrylate.Polymers are chemically grafted and physically wrappedon the surface of nano calcium carbonate to form nanocalcium carbonate/PMMA composite particles.

Acknowledgments

Prof. J.F. Chen would like to thank the support by theNational Natural Science Foundation of China (no. 20236020,20325621), the National R&D Program of China (no.2001BA310A01), the Talent Training Program of the BeijingCity (no. 9558103500), and the Fok Ying Tung Foundation (no.81063).

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Study on in situ preparation of nano calcium carbonate/PMMA composite particlesIntroductionExperimentalMain raw material and reagentsThe processThe preparation of aqueous suspension of nano calcium carbonateThe preparations of nano composite particles

Characterizations

Results and discussionsThe effects of synthesis parameters on the preparation of nano composite particlesThe effects of temperaturesThe effects of stirring rateThe effects of the dosages of the initiating agentThe effects of calcium carbonate concentrations

Characterizations of composite particlesThe bonding mechanisms between nano calcium carbonate and PMMAThe preliminary investigations of applications of nano composite particles in epoxy varnish

Discussions of formation mechanism of nano composite particlesConclusionsAcknowledgmentsReferences