Hydrophobic magnesium hydroxide nanoparticles via oleic acid and poly(methyl methacrylate)

Hydrophobic magnesium hydroxide nanoparticles via oleic acid and poly(methyl methacrylate)-grafting surface modi ficationHong Yan a ,b ,Xue-hu Zhang a ,b ,Li-qiao Wei a ,b ,Xu-guang Liu a ,c ,Bing-she Xu a ,b ,⁎a Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology),Ministry of Education,Taiyuan,Shanxi 030024,PR Chinab College of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan,Shanxi 030024,PR China cCollege of Chemistry and Chemical Engineering,Taiyuan University of Technology,Taiyuan,Shanxi 030024,PR Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 19August 2008Received in revised form 22January 2009Accepted 26January 2009Available online 10February 2009Keywords:PMMA graftingMagnesium hydroxide Nanocomposite HydrophobicHydrophobic magnesium hydroxide nanoparticles were obtained by means of grafting poly(methyl methacrylate)(PMMA)onto the surface of nanoparticles after oleic acid (OA)modi fication.The introduction of functional double bonds was firstly conducted on the surface of nanoparticles by OA modi fication,followed by dispersion polymerization on the particles surface in ethanol solution using methyl methacrylate (MMA)as monomer,azoisobutyronitrile (AIBN)as initiator and polyvinylpyrrolidone (PVP)as stabilizer to graft PMMA on the surface of OA-modi fied magnesium hydroxide.The obtained composite particles were characterized by XRD,FTIR,TGA,FESEM –EDS,and the compatibility with organic solution was determined by sedimentation test.The results show that the organic macromolecule PMMA could be successfully grafted on the surface of OA-modi fied magnesium hydroxide nanoparticles,with the dispersibility and the compatibility of nanoparticles greatly improved in organic phase.©2009Elsevier B.V.All rights reserved.1.IntroductionMagnesium hydroxide is a good green flame retardant for its high decomposition temperature,smoke suppressibility,nontoxicity and neutralization with acid gas from polymer combustion [1–6].However,the fatal disadvantage of Mg(OH)2is its high loading,which would sharply decrease the mechanical properties of a flame-retardant polymeric material [7–9].Furthermore,hydrophilic surfaces of Mg (OH)2nanoparticles are not favorable for their dispersion in polymers.The traditional method of treating filler surface by low-molecular weight coupling agents or surfactants has been found to be reasonably effective [10–13].However,low-molecular weight compounds have a tendency to migrate out from the interface to the surface of the com-posites.As a consequence,there is degradation in the mechanical and physical properties of the composites.Also,an undesirable surface appearance caused by surface scorching or bulging will result [14].Recently,polymer modi fication on inorganic particles has become an effective method to improve their dispersibility and compatibility in organic solvents and polymer matrices [15–24].Whereas,few reports focus on polymer modi fication on Mg(OH)2nanoparticles.Herein,we report a facile method to synthesize hydrophobic Mg(OH)2nanoparticles with good dispersibility and compatibility in organic phase via PMMA grafting by dispersion polymerization after oleic acid modi fication.2.Experimental 2.1.MaterialsPoly (N -vinyl-2-pyrrolidone)(PVP;MW 50,000)was commercially obtained from Shanghai Chemical Reagent Co.Ltd,China.Monomeric methyl methacrylate (MMA),oleic acid (OA),ethanol and N,N-dimethyl formamide (DMF)were purchased from Tianjin Kermel Chemical Reagents Development Center,China.All the above chemi-cals were analytically pure and used as received without further puri fication except MMA,which was distilled at a reduced pressure.Azoisobutyronitrile (AIBN),chemical pure from Beijing Chemical Reagents Company,was dissolved in DMF and then recrystallized in ethanol twice for puri fication.2.2.CharacterizationField-emission scanning electron microscope (FESEM;JSM-6700F,JEOL,Tokyo,Japan),equipped with an energy dispersive spectrometer (EDS),was used to observe the morphologies of Mg(OH)2samples.The phase and crystallographic structures of the samples were examined by X-ray diffraction (XRD)on a Y-2000(Dandong,China)Powder Technology 193(2009)125–129⁎Corresponding author.Key Laboratory of Interface Science and Engineering in Advanced Materials of Taiyuan University of Technology,Ministry of Education,Taiyuan,Shanxi 030024,PR China.Tel.:+863516014852;fax:+863516010311.E-mail address:xubs@ (B.Xu).0032-5910/$–see front matter ©2009Elsevier B.V.All rights reserved.doi:10.1016/j.powtec.2009.01.024Contents lists available at ScienceDirectPowder Technologyj o ur n a l h o m e p a ge :ww w.e l s e v i e r.c o m/l o c a t e /p o wt e cpowder diffractometer working with CuK αradiation and Ni filter (λ=1.54178Å)at a scan rate of 0.05°/s.FTIR spectrum was recorded on a Bio-Rad (Hercules,CA)FTS-165spectrometer.Thermogravimetric analysis (TGA)was carried out on a TG-209thermoanalyzer (Netzsch,Germany),Ar atmosphere,heating rate 10°C/min.Sedimentation was conducted as follows:0.2g of nanoparticles was dispersed in 20ml of liquid paraf fin/water under vigorous stirring for 2h.Then the suspension was kept stationary and sedimentation was observed.2.3.Sample preparationAt first,100ml of 0.5mol/l magnesium sulphate solution was injected into 200ml of 0.5mol/l sodium hydroxide solution by a peristaltic pump with a constant flow rate of 1ml/min under vigorous electromagnetic stirring.Then Mg(OH)2precipitate was filtered,washed with deionized water,and freeze-dried in vacuum at −55°C for 24h.One gram of dried Mg(OH)2nanoparticles was modi fied by 100ml of OA ethanol solution (5w/v%OA)at 70°C under vigorous stirring for 40min.OA-modi fied powder was filtered,washed with ethanol for several times and then dried at 60°C in vacuum.The OA-modi fied powder and 2g of MMA monomer were added into 100ml of PVP ethanol solution (0.5w/v%)under ultrasonic stirring for 10min.Then the suspension was injected into a three-neck flask equipped with a thermometer,a nitrogen inlet,and a re fluxing condenser.The mixture was purged with nitrogen for 5min and then heated to 60°C for 30min,Fig.1.Scheme of the polymer grafting on the solid surface of Mg(OH)2.Fig.2.FTIR spectra of (a)as-precipitated and (b)OA-modi fied magnesiumhydroxide.Fig.3.FTIR spectra of (a)PMMA-grafted Mg(OH)2and (b)PMMA.Fig.4.XRD patterns of (a)as-precipitated and (b)PMMA-grafted Mg(OH)2.126H.Yan et al./Powder Technology 193(2009)125–129followed by the addition of AIBN (0.2g).Then the temperature was elevated to 80°C for 6h.The resultant milky dispersions were vacuum-filtered,washed with deionized water and ethanol and finally freeze-dried in vacuum at −55°C for 24h,yielding white composite powders.3.Results and discussion 3.1.OA modi ficationMagnesium hydroxide is an inorganic material,while PMMA is an organic matter.In order to easily polymerize MMA monomer on the surfaces of magnesium hydroxide nanoparticles,OA was used to modify the surfaces of nanoparticles to make them lipophilic.OA is a surface treatment agent with a carboxyl group and an unsaturated double-bond,which facilitates the polymerization of MMA monomers on magnesium hydroxide.Firstly,the carboxyl group reacted with hydroxyls of magnesium hydroxide to introduce unsaturated double-bond onto the surface of magnesium hydroxide,and then the un-saturated double-bond opened to reacted with MMA monomers during polymerization initiated by AIBN,achieving the grafting of PMMA on the magnesium hydroxide.The scheme of this process is shown in Fig.1.Fig.2shows the FTIR spectra of magnesium hydroxide before and after OA modi fication.Apart from the characteristic stretching vibration bands for hydroxyl group (3698cm −1)and Mg –O (441cm −1)of Mg(OH)2,the band at 1737cm −1for OA-modi fied Mg(OH)2is assigned to the characteristic stretching vibration of C f O in carboxylic acid,and the two bands at 2920and 2850cm −1are assigned to the asymmetric and symmetric stretching vibration of aliphatic CH 2group,respectively.The tender band of surface adsorbed hydroxylgroupsFig.5.TGA curves of (a)as-precipitated Mg(OH)2and (b)PMMA-grafted Mg(OH)2.Fig.6.FESEM images of Mg(OH)2(a),(b)as-precipitated and (c),(d)PMMA-grafted in different magni fications and EDS spectra (inset).127H.Yan et al./Powder Technology 193(2009)125–129reveals the presence of OA molecules on the surface of magnesium hydroxide by substituting hydroxyl groups [25].3.2.PMMA graftingBy comparison of the FTIR spectra of PMMA-grafted Mg(OH)2(a)and PMMA (b),as shown in Fig.3,the bands at 1735and 1439cm −1are assigned to C f O stretching vibration and CH 3stretching vibration from PMMA.The bands at 1150,1196,1241and 1270cm −1represent C \O \C stretching vibration in PMMA [26].This result indicates that PMMA has been grafted onto the surface of magnesium hydroxide.The interaction between Mg(OH)2and PMMA can also be concluded from the XRD patterns,shown in Fig.4.All diffraction peaks of each sample can be indexed as the hexagonal structure of magnesium hydroxide with the lattice constants comparable to the values of JCPDS 7-239.All of the peaks,except the lattice plane of (001),have no obvious change in intensity,suggesting that PMMA grafted on the lattice plane of (001)of magnesium hydroxide.Effective chemical bonding between Mg(OH)2and PMMA is also evident from the comparison of mass loss data obtained by TGA between Mg(OH)2and PMMA-grafted Mg(OH)2,as shown in Fig.5.For as-precipitated Mg(OH)2,the decomposition occurred in one single step from 340°C to 560°C with the maximum weight loss of 31.8wt.%,which is due to the decomposition of Mg(OH)2to MgO.For PMMA-grafted Mg(OH)2,the decomposition occurred in three steps from 160°C to 565°C.The weight loss from 160°C to 240°C was 2.1wt.%,due to desorption of residual surfactant.The weight loss from 240°C to 340°C was 7.1wt.%,corresponding to the decomposition of PMMA.The weight loss from 340°C to 565°C was 29.3wt.%,corresponding to the de-composition of Mg(OH)2to MgO.3.3.FESEM and EDS analyses of surface modi fied Mg(OH)2Fig.6shows the FESEM images of as-precipitated and PMMA-grafted magnesium hydroxide in different magni fications.The mean size of magnesium hydroxide was about 100nm,as calculated statistically from the FESEM image in the visual field.The Effect of PMMA on the dis-persibility of Mg(OH)2nanoparticles were also observed.It is obviously seen that the nanoparticles easily aggregated to bigger ones for as-precipitated Mg(OH)2(Fig.6a),while PMMA-grafted ones showed good dispersion (Fig.6c),indicating the steric hindrance caused by the stretching of carbon –hydrogen long chains on the surface of magnesium hydroxide effectively avoided agglomeration.From the EDS spectra,it can be seen that carbon element appeared and the content ratio of O to Mg increased after surface modi fication.3.4.Dispersion stability of PMMA-grafted Mg(OH)2nanoparticles in liquid paraf finIn order to check the compatibility of PMMA-grafted Mg(OH)2nanoparticles with organic phase and their hydrophobic nature,the sedimentation of the nanoparticles in liquid paraf fin and water was observed.Fig.7a and b show the sedimentation of as-precipitated and PMMA-grafted nanoparticles in water,respectively.The as-precipitated nanoparticles all sank in water after 2h,while PMMA-grafted ones floated over water for 30days.Fig.7c shows the sedimentation of PMMA-grafted nanoparticles in liquid paraf fin.The nanoparticles dispersed homogeneously in liquid paraf fin and no noticeable sedimen-tation was found after two weeks,indicating that PMMA grafting and OA modi fication on magnesium hydroxide surface can obtain hydrophobic magnesium hydroxide and improve its compatibility with organic phase.4.ConclusionsMagnesium hydroxide nanoparticles were prepared by wet preci-pitation with the mean size of 100nm.PMMA was grafted on the magnesium hydroxide nanoparticles after oleic acid modi fication.MMA monomers polymerized on the nanoparticle surfaces via the double bond of the oleic acid.PMMA-grafted magnesium hydroxide is hydrophobic and disperses homogeneously in liquid paraf fin without noticeable sedi-mentation for 2weeks.The compatibility of surface modi fied magnesium hydroxide nanoparticles will be greatly improved when added in the polymer matrix as fillers.It is predictable that good dispersibility and compatibility in polymer matrix will be bene ficial to the flame retardant performance of magnesium hydroxide nanoparticles.AcknowledgementsThe work was financially supported by the National Basic Research Program of China (No.2004CB217808),National Natural Science Foundation of China (No.90306014and 50673072),and Natural Science Foundation of Shanxi Province (No.20051016).References[1]S.Zhang,A.R.Horrocks,A review of flame retardant polypropylene fibres,Prog.Polym.Sci.28(2003)1517–1538.[2]J.P.Hsu, A.Nacu,Preparation 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