Mg(OH)2Complex Nanostructures with Superhydrophobicity and Flame Retardant Effects
Huaqiang Cao,*He Zheng,Jiefu Yin,Yuexiang Lu,Shuisheng Wu,Xiaoming Wu,and Baojun Li
Department of Chemistry,Tsinghua Uni V ersity,Beijing 100084,China Recei V ed:May 23,2010;Re V id Manuscript Recei V ed:September 6,2010
Complex Mg(OH)2nanostructures are synthesized via a biomolecule-assisted hydrothermal route.The as-synthesized Mg(OH)2nanostructures are disperd into an acrylonitrile-butadiene-styrene (ABS)copolymer by mechanical kneading,which shows an excellent flame-retardant behavior.Also we demonstrated for the first time that the complex Mg(OH)2nanostructures can find application in lf-cleaning for its superhydro-phobicity with water contact angle over 150°and sliding angle of 1°.
1.Introduction
Magnesium hydroxide [Mg(OH)2,MH]is an important inorganic material,which has many industrial applications,including synthesizing halogen-free flame retardants for polymers,1,2preparing foodstuff starch esters,3treating waste-water,4and desulfurizing waste gas.5Recently,it has become of grea
t interest to synthesize MH nanostructures with various sizes,shapes,and dimensions.Many methods have been developed to synthesize nanostructures with different morphologies,for example,one-dimensional (1D)nanorods synthesized via a solvothermal method,61D needle-like nanocrystals synthesized via rever precipitation,7two-dimensional (2D)platelike nanocrystals synthesized via a hydrothermal route,8spherelike nanostructures synthesized via a solution-bad chemical process,9and three-dimensional (3D)flowerlike nanostructures synthesized via a hydrothermal reaction,10which prent different physicochemical properties.Acrylonitrile-butadiene-styrene (ABS),being a noncharring polymer upon combustion,is an important commercial engi-neering thermoplastic material due to its excellent mechanical properties,chemical resistance properties,ea of processing,and recycling ability.11However,it still has some shortcomings,such as inflammability.It is difficult to obtain a halogen-free retardant system for ABS.12Thus,it is a great challenge for scientists to improve the properties of ABS.13It is necessary to solve this problem by modifying polymers by adding flame-retardant compounds to decrea their flammability.2,11,14,15However,many environmental laws have been enacted to prohibit the application of halogen-containing flame retardants in polymer materials around the world.MH crystal is an important inorganic halogen-free and smoke-suppressing flame retardant,due to its better thermal stability,smoke suppression property,and flame retardancy,compared with other inorganic fla
me retardants,even in comparison with aluminum trihydrox-ide.16It undergoes decomposition at 340-490°C,which is one of the most important merits.Furthermore,MH is an environ-mentally friendly additive which has been extensively ud in the halogen-free flame-retarded (HFFR)polymeric materials.It was demonstrated that fiber-and lamellalike MH,being a flame retardant,prented excellent functions in polymers.16However,many rearch studies indicated that the MH crystals
should be improved,such as decreasing the usage amount and increasing the flame-retardant efficiency.17,18It was reported that the addition level of MH reached up to 60wt %in order to achieve acceptable combustion resistance,19,20which in turn led to the decrea of the mechanical properties of polymers.It is possible to solve the problems by using micro/nanostructured MH as a flame retardant in ABS.
It is known that the focus of nanotechnology rearch has,in recent years,been steadily moving away from the preparation of high-quality nanomaterials and the understanding of their physicochemical properties to practical applications.21Herein,we report a synthesis of a flowerlike MH complex nanostructure ud as an additive in an ABS matrix.In the prent paper,novel MH nanoflowers with flame retardant effect were designed and successfully fabricated by a simple amino-acid-assisted hydrothermal approach.We also demonstrated for the first time that the MH nan
oflowers may find applications in lf-cleaning for its superhydrophobicity.2.Experimental Section
逆行Synthesis.Growth of flowerlike MH nanostructures was performed through a lf-asmbly manner using a hydrothermal synthesis route.MgCl 2·6H 2O (analytical pure,AR)and glycine [CH 2(NH 2)COOH,AR]were ud without further purification.In a typical synthesis,4mmol of MgCl 2·6H 2O was added into 20mL of deionized water forming solution A,while 8mmol of glycine was dissolved into19.14mL of deionized water,with 0.86mL of 10M NaOH solution added,forming solution B.Solution B was added into the stirred solution A within 30min at room temperature.The resulting mixture was transferred to and aled in a Teflon-lined autoclave,heated to 240°C,and maintained at this temperature for 48h.After the autoclave was cooled down to room temperature naturally,the products were collected and washed with deionized water one time,and then absolute alcohol three times,followed by drying at 60°C for 7h.
Characterization.The pha analysis was performed with an X-ray diffractometer (XRD)(Bruker D8advance)operating at 40kV ×40mA.Morphology was studied with scanning electron microscopy (SEM,KYKY 2000),field emission scanning electron microscopy (FE-SEM)(JSM-7401F),and transmission electron microscopy (TEM)(JEM-2010F).The high-resolution TEM (HRTEM)images and lected area electron diffraction (SAED)analysis were taken with a JEOL
*To whom correspondence should be addresd.E-mail:hqcao@mail.
J.Phys.Chem.C 2010,114,17362–17368
感动为话题的作文1736210.1021/jp107216z 2010American Chemical Society
Published on Web 09/20/2010
JEM-2010electron microscope,operating at 200kV.Fourier transform infrared (FT-IR)analysis of the sample coated in KBr taken from a compresd pellet was performed by using NICOLET 560Fourier transform infrared spectrophotometer.Wetting Behavior Test.Water contact angle (CA)and sliding angle measurements were carried out on water droplet (drop volume 9µL)and water droplet on an optical contact angle meter (Data physics Inc.,OCA 20)at ambient temperature.The as-synthesized MH (5mg)was disperd in 5mL of ethanol with ultrasonic treating for 10min,followed by drying at 80°C for 30min.The glass was modified by the MH ethanol solution,followed by treating with 1H ,1H ,2H ,2H -perfluorode-cyltriethoxysilane methanol solution ([V(1H ,1H ,2H ,2H -perfluo-rodecyltriethoxysilane):V(methanol))2:98]),air drying for 1h,and then drying at 120°C for 1h.The CA measurement was carried out after the preparation of samples of four days’exposure to an atmosphere at room temperature (ca.32°C)as well as relative humidity of 21%.
Fire Test.ABS-bad nanocomposites (filled with different content of MH nanoflowers)were prepared in a two roll mixing mill (SK-160B,Shanghai Rubber Machinery Works)at 190°C for 30min with a speed of 18rpm,then procesd in a Platen Vulcanizing Press (QLB-350×350×2,Shanghai First Rubber Machinery Works)at 150°C under 20MPa for 10min.The filler contents of were 1and 5wt %in terms of the as-synthesized MH nanoflowers.The Cone calorimeter tests were carried out on 100mm ×100mm ×3mm compresd-molded samples placed horizontally under a heat flux of 35kW m -2according to ISO 5660.
Mechanical Properties Test.ABS and ABS-bad nano-composites (filled with different content of MH nanoflowers)were measured on a 125mm ×7mm ×3mm (length ×width ×height)compresd-molded sample by an electronic universal testing machine (Z004,Zwick/Roll,Zwick GmbH &Co.)with an acting load velocity of 20mm min -1at room temperature.Thermogravimetric Analysis (TGA).TGA experiments were performed using a TGA Q5000V3.5Build 252thermal analyzer in air under air flow rate of 10mL min -1.The samples were heated in platinum pans from room temperature up to about 900°C at a heating rate of 10°C min -1.3.Results and Discussion
To investigate the reaction evolution of MH nanostructures in our system,a ries of contrast experiments were performed.The first study was the effect of the reaction time.Figure 1shows SEM i
mages of the final MH products synthesized in identical concentrations (Mg 2+concentration )4mmol/40mL,glycine concentration )8mmol/40mL)and the identical reaction temperature of 240°C,but for different reaction times,that is,30min,8h,24h,and 48h,with the being denoted as MH-1,MH-2,MH-3,and MH-4,correspondingly.Accord-
ing to SEM obrvation,the morphology of the product appeared as a platelike structure when the reaction time was 30min (Figure 1a).When the reaction time was prolonged to 8,24,or 48h,flowerlike structures were obtained (Figure 1b -d).
We also investigated the effect of the reaction temperature on the morphology of the products.For this ries of experi-ments,we carried out the experiment in identical concentrations (Mg 2+concentration )4mmol/40mL,glycine concentration )8mmol/40mL)and the identical reaction time (8h),but for different reaction temperatures,that is,210and 240°C,the being denoted as MH-5(Figure 1e)and MH-2(Figure 1b),respectively,and similar flowerlike superstructures were obtained.The effect of concentration of Mg 2+and glycine on the morphology of the products was also investigated.After changing the concentration of Mg 2+from 4mmol/40mL (denoted as MH-2,Figure 1b)to 20mmol/40mL (denoted as MH-6,Figure 1f)with other reaction conditions remaining constant (the identical reaction time of 8h,the identical reaction temperature of 240°C,and identical r
atio of reactions (Mg 2+/glycine)of 1:2,respectively),similar flower-like superstructures were obtained.
The effect of the ratio of reaction agents (Mg 2+/glycine)changing from 1:1(Mg 2+concentration )8mmol/40mL,denoted as MH-7,Figure 1g)to 1:2(Mg 2+concentration )8mmol/40mL,denoted as MH-8,Figure 1h),1:4(Mg 2+concentration )4mmol/40mL,denoted as MH-9,Figure 1i),and 1:8(Mg 2+concentration )4mmol/40mL,denoted as MH-10,Figure 1j),with the identical reaction temperature of 240°C and the identical reaction time of 12h,was that platelike structures (Figure 1g,h)and flowerlike superstructures (Figure 1i,j)were obtained,respectively.
Figure 2shows the XRD patterns for the as-synthesized samples.In the 2θrange 10-70°,various diffraction peaks were obrved,which were assigned to (001),(011),(012),(110),(111),and (103)planes of the hexagonal pha of MH (JCPDS 83-0114).No impurity peaks were found in Figure 2,indicating that the MH crystals obtained via our method consist of a pure pha.
The Fourier-transform infrared (FT-IR)spectrum recorded from the MH nanoflowers shows absorption bands at
3700,
Figure 1.SEM images of products (a)MH-1,(b)MH-2,(c)MH-3,(d)MH-4,(e)MH-5,(f)MH-6,(g)MH-7,(h)MH-8,(i)MH-9,and (j)MH-10
.
Figure 2.XRD patterns of (a)MH-1,(b)MH-2,(c)MH-4,(d)MH-5,(e)MH-7,(f)MH-9,and (g)MH-10.
Mg(OH)2Complex Nanostructures J.Phys.Chem.C,Vol.114,No.41,201017363
3460,1640,1410,582,and 428cm -1(Figure 3).The sharp and strong peak appearing at 3700cm -1can be attributed to the O -H stretching vibrations in the Mg(OH)2crystal structures.22-26It is known that infrared technique relying on shifts and the width of the -OH stretching bands can be ud to detect hydrogen bonds.27Usually,hydrogen bonding results in decread frequency and a broadening of the absorption band.28A broad band between 3100and 3600cm -1,centering at 3460cm -1,indicates that it belongs to the O -H stretching vibrations of adsorbed water molecules and the surface hy-droxyls disturbed by the hydrogen bonds.29The absorption band at 1640and 1410cm -1can be attributed to bending vibrations of Mg -OH and -OH bonds in the crystal structure,respec-tively.24The bands at 582and 428cm -1are assigned to deformation vibrations of Mg -O -Mg.23,26From the FT-IR data,we can further identify the as-synthesized sample to be MH.The morphology and size of a typical MH nanostructure were further characterized by low magnification SEM and high magnification FE-SEM,as shown in Figure 4.Interesting,low-magnification SEM images (Figure 4a,b)show that the products consist of flowerlike complex nanostructures ca.11.4µm in diameter of a single superstructure (Figure 4b)asmbled by nanoplates,as a cond order structure,60nm in thickness (Figure 4d).The structure and morphology
of MH were further studied by TEM (Figure 4e)and electron diffraction (ED)(Figure 4f).According to the TEM obrvation,the MH nanoflowers are compod of platelike structures.The ED indicates that the MH nanoflowers belong to crystalline hex-agonal MH.The pha identification of the sample has been corroborated by using XRD technique.
The possible growth mechanism of the complex flowerlike MH nanostructures is prented in Figure 5.It is compod of three stages:nucleation,growth,and asmbly,termed as the N-G-A mechanism.30The first stage is the nucleation ,the initial reaction between Mg 2+and glycine [CH 2(NH 2)COOH]ions [CH 2(NH 3+)COO -],which is too fast to generate the nuclei.The CH 2(NH 3+)COO -plays an important role in controlling nucleation and growth of MH nuclei.Glycine ions may act as a bidentate ligand to form relatively stable Mg 2+complex 1{Mg(OH)2[CH 2-(NH 2)-COOH]2}.31It can be ex-plained that the sp 3d 2hybrid orbital of Mg 2+is empty and ready to accept an electron pair.Mg 2+ion is a well-defined electron pair accepting a hard Lewis acid.16It is believed to generate complex 1(in the solution as a precursor prior to the hydro-thermal process),abiding by R.G.Pearson’s hard -soft acid ba theory (HSAB theory).32After a hydrothermal treatment process,MH nuclei are generated.The cond stage is the MH nanoplate formation.The nanoplates can be regarded as 2D nanoparticles.The formation of nanoplates is determined by not only the surface area but also the surface energy.31It is known that MH
is a lamellar structure with a plane compod of oxygen and magnesium ions and each Mg 2+ion is O-6-fold coordinated.23Layered (2D)MH (Figure 5b)is typified by its anisotropic character.The atoms within the layers (ab plane)are covalently bonded,while the layers are stacked together by hydrogen bonding along the c axis via OH-,which leads to the forming of nanoplates through the oriented attachment mechanism.33The third stage is the lf-asmbly process of nanoplates via hydrogen bonding (aggregation),leading to the formation of hierarchical architecture:flowerlike superstructures.All lf-asmbling systems are driven by some principle of energy minimization.34Hydrogen bonds are weaker energy (20kJ mol -1)compared with covalent bonds (about 500kJ mol -1);however,they are favorable for lf-asmbling superstructures without chemical reactions.Furthermore,compared with the thermal energy (2.4kJ mol -1),the hydrogen bonds are strong enough to hold the superstructures together.18The hydrogen bonds between the plates of MH hold the flowerlike superstruc-tures together,which has been demonstrated by the FT-IR data (Figure 3)and SEM and TEM obrvations (Figure 1and 4).Surface wettability of the as-synthesized MH nanostructures was studied by measurement of the water contact angle (CA)using a water droplet of 9µL.The CA value of the glass surface was 91.2°(1.8°after its initial coating with a metha-nol solution of 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane [(C 2H 5O)3Si(CH 2)2C 8F 17][V(1H ,1H ,2H ,2H -perfluorodecyltri-ethoxysilane):V(methanol))2:98](Figure 6a showing the CA of 9
2.9°).The CA value changed to 146.5°(4.9°after the glass surface was coated with as-synthesized MH nanostructures ethanol solution four times,then treated with a methanol solution of 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane [V(1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane):V(methanol))2:98](Figure 6b showing the CA of 146.8°).However,after the glass being treated with as-synthesized MH nanostructures ethanol solution eighttimes,thentreatedwithamethanolsolutionof1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane [V(1H ,1H ,2H ,2H -perfluorodecyl-triethoxysilane):V(methanol))2:98],the CA value changed to 151.1(1.5°(Figure 6c showing the CA of 150.6°),as well as corresponding sliding angle of 1°(Figure 6d -h and Sup-porting Information,Movie S1).The results imply the real possibility of introducing large-scale industrial fabrication of superhydrophobic surfaces with novel lf-cleaning and anticor-rosion properties.35
It has been demonstrated that the wetting phenomenon is determined by both the chemical composition and the textured topography of the surface.36Usually,the solid surfaces with CA over 150°are defined as superhydrophobicity.37Lotus leaf prents the lotus ,superhydrophobicity,allowing water droplets to freely roll off the leaf into nearly perfect spheres,while allowing oil to lectively spread.The lotus effect can be attributed to the surface struct
ure of lotus leaf,which is compod of unique micro-and nanosized cond-order struc-tures.This leads to a lotus leaf natural low-energy surface.This nature phenomenon of a plant has inspired materials scientists to biomimetically synthesize complex nanostructures for gen-erating superhydrophobicity which can be applied in an environ-ment protection field.31,38-41This work indicates that the surface wettability of a solid can be changed via combing the generation of physical roughness (such as complex MH nanostructures with micro-and nanosized cond-order structures)with chemical surface treatment.
The cone calorimeter is one of the most effective methods for evaluating the flammability of materials.12,42The evolution of heat relea rate (HRR),in particular its maximum
peak
Figure 3.FT-IR spectrum of MH-4nanostructure.
17364J.Phys.Chem.C,Vol.114,No.41,2010Cao et al.
(PHRR),is regarded to be an important parameter to evaluate the fire safety,and so is the time to ignition (TTI,t ign ),the period required for the entire surface of the sample to burn with a sustained luminous flame,which reflects the degree of difficulty in igniting a material.43HRR,TTI,and other parameters were recorded simultaneously.The cone calorimeter experimental results for ABS (sample-ABS)and corresponding ,ABS filled with 1wt %MH (sample-AM-1)and 5wt %MH (sample-AM-2),as prepared by simple melt blending are shown in Figure 7and Table 1.
The behavior of the pristine ,unfilled ABS,is typical of noncharring thermoplastics.The peak HRR reaches ca.663
(17kW m -2,and the combustion is complete after ∼400s.The thermal behavior of sample-AM-1is similar to that of ABS.However,the thermal behavior of another sample,sample-AM-2,is quite different.Although the MH content is only incread from 1wt %to 5wt %in the sample,the maximum
values of HRR are changed from 652(6for sample-AM-1to 430(19kW m -2for sample-AM-2and the corresponding combustion time is changed from ∼400to ∼580s,which indicates that the combustion time of sample-AM-2is more extended.
针织衫怎么洗
It is interesting that the time to ignition (TTI,t ign )is also affected by the content of filled MH.The t ign of ABS (sample-ABS)is only 30(2s,and the t ign of sample-AM-1is 29
(
Figure 4.(a)and (b)SEM images,(c)and (d)FE-SEM images,(e)TEM image,and (f)corresponding electron diffraction (ED)pattern of MH
nanostructures.
Figure 5.(a)Formation of MH flowerlike superstructures,including three stages:nucleation,oriented growth,and lf-asmbly,termed as N-G-A mechanism.(b)Model fragment of a structural layer of M
H.
Mg(OH)2Complex Nanostructures J.Phys.Chem.C,Vol.114,No.41,201017365
1s,while sample-AM-2reaches 67(5s,being almost double that of t ign(ABS)and t ign(S-MH-1).Another important parameter is the time taken to reach the PHRR (t PHRR ).The t PHRR of sample-ABS is ∼169s,and that of sample-AM-1is ∼190s,while that of sample-AM-2is ∼245s,reaching a 44.9%increa compared with sample-ABS.The increa of t PHRR is very important becau it provides crucial time for saving lives and property.The above results validate 5wt %MH loading (sample-AM-2)endowing excellent fire retardancy on ABS.
For ABS/MH nanocomposites,when the MH contents is 1wt %(sample-AM-1),PHRR displays a 1.7%reduction and the t ign and average mass loss rate (AMLR)exhibit little change compared to pure ABS copolymer.However,when 5wt %MH loading was added into ABS copolymer (sample-AM-2),compared with ABS resin,the peak heat relea rate (PHRR)and average specific extinction area (AMLR)prent ∼35%and ∼31%reduction,respectively,while the total heat relea (THR)prents ∼10%reduction.The results indicate that sample-AM-2nanocomposite does not burn out.Both AMLR and THR
阿胶产地
data of sample-AM-2are reduced significantly,suggesting the network structures of MH formed at 5wt %MH loading.The improved fire resistance displayed by the nanocomposite sample-AM-2could be explained by the chemical structures of the nanofillers and the combustion ,by the stabilizing π-πelectronic interactions between the unsaturated structures of the carbonaceous amorphous char.
It is believed that the redox reaction,taking place during the burning of MH flame-retarded ABS,may play a part in the smoke suppression of MH.34,44,45MgO generated via the decomposition of Mg(OH)2owns the catalytic activity which can catalyze the redox reaction C +O f CO 2as well as CO +O f CO 2,via promoting C and CO decompod from polymer ABS to CO 2.This behavior leads to the decrea of the concentration of toxic fumes.The excellent flame retardant effects of the as-synthesized MH can be attributed to the complex micro/nanosuperstructures.The complex MH superstructure is compod of nanoplates as a cond structure with large surface area,which favors the catalytic reaction.By the way,the MgO,being an excellent refractory material,can cover the surface of burning ABS and form barrier,which functions as thermal insulation,isolating oxide and preventing molten drops.43
The TGA and differential TG (DTG)curves for sample-ABS and sample-AM-2are shown in Figure 8.T
学习园地图片he detailed TGA and DTG data for pure ABS and its nanocomposite are summarized in Table 2.The ABS undergoes two-step thermal decomposition (Figure 8a),where end-chain and random scission occurs.There is a remarkable mass loss in the 300-500°C temperature range,attributed to the main chain pyrolysis.46,47The first step is its main decomposition process,beginning at ∼378°C with a maximum rate of weight loss at ∼427°C,leading to about ∼89%weight loss and the formation of ∼11wt %charred residue.It is attributed to the main chain pyrolysis.46The cond decomposition step is obrved between 500and 650°C,starting at 529°C,accompanied by the maximum rate of weight loss at 546°C.It can be attributed to the degradation of the carbonaceous residue formed during the first step.ABS polymer gives no char residue upon combustion.However,sample-AM-2undergoes its first decomposition step appearing in the temperature range 250-450°C (Figure 8b).The first step is its main decomposition process,beginning at ∼369°C with a maximum rate of weight loss at ∼421°C,which can be attributed to the decomposition of MH.This is partly caud by the degradation of Mg(OH)2at lower temper-ature.It is known that the thermal decomposition
temperature
Figure 6.Surface wetting behavior of the glass and the membranes modified by the as-synthesized MH nanostructures.Water contact angle (CA)measurements of (a)glass coated with 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane,(b)glass treated with the Mg(OH)2nanostructures methanol solution four time-cycle,and then 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane molecules.(c)Glass treated with the MH nanostructures methanol solution eight time-cycle,and then 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane molecules.(d -h)The sliding angle measurement of the membrane treat
ed with the MH nanostructures methanol solution eight time-cycle,and then 1H ,1H ,2H ,2H -perfluorodecyltriethoxysilane
molecules.
Figure 7.Heat relea rate curves for ABS resin and its nanocom-posites with different Mg(OH)2content.
TABLE 1:Cone Calorimetry Data for ABS and ABS/MH Nanocomposites at 35kW/m 2
term
为善最乐
好句子二年级a
ABS (sample-ABS)1wt %Mg(OH)2(sample-AM-1)5wt %Mg(OH)2(sample-AM-2)曾经心痛
t ign [s]
30(229(167(5t PHRR [s]169
190245
PHRR [kW/m 2
]
663(17652(6430(19THR [MJ/m 2]107.2(5.4113.0(3.196.8(1.5ASEA [m 2/kg]1322(211342(351190(5AMLR [g/s]0.078(0.006
0.083(0.002
0.054(0.004
a
t ign )time to ignition;PHRR )peak heat relea rate;t PHRR )time to reach the PHRR;THR )total heat relea;ASEA )average specific extinction area;AMLR )average mass loss rate.
17366J.Phys.Chem.C,Vol.114,No.41,2010Cao et al.
