One-step synthesis of hydrophobic magnesium hydroxide nanoparticles and their ap

Hongyan Shen,Youzhi Liu*

Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering,College of Chemical Engineering and Environment,North University of China,Taiyuan 030051,China

Keywords:Magnesium hydroxide Synthesis Polypropylene Hydrophobic property Rotating packed bed

ABSTRACT Hydrophobic magnesium hydroxide(MH)nanoparticles were prepared by a one-step synthesis method in a high-gravity environment generated by a novel impinging stream-rotating packed bed(IS-RPB)reactor.The reactant solutions were simultaneously and continuously pumped into the IS-RPB reactor,and then Tween 80 was added as a surface modifier.The morphology,structure,and properties of blank and hydrophobic MH were characterized.The effects of MH nanoparticles on the flame retardancy,thermal stability,and mechanical properties of PP/MH composites were also studied.We found that the obtained MH nanoparticles exhibited hexagonal lamella with a mean size of 30 nm,excellent hydrophobic properties(e.g.,high water contact angle of 112°),and improved thermal stability of MH.The limiting oxygen index(LOI)further showed that increased MH loading can significantly improve flame-retardant performance,which reached 29.3%for PP/MHcomposites with 30 wt%hydrophobic samples.The thermal stability and mechanical properties of the PP/MH composites with hydrophobic samples were also much higher than those of PP/MH composites with blank MH.Results showed that the one-step synthesis had high potential application in the large-scale production of hydrophobic MH nanoparticles.

1.Introduction

Polypropylene(PP)materials are widely used to manufacture automobiles,household appliances,and building materials.However,the chemical constitution of PP causes it to have a low limiting oxygen index(LOI)of 17.5%.These materials also tend to produce smoke,poisonous gases,and melt-dropping problems[1],which restrict their application range.Thus,PP materials must be modified with flameretardant additives to improve their flame retardancy and meet firesafety standards.Generally,halogen-based flame retardants are added to provide PP materials with non- flammability,but the environmental and health problems induced by halogen-based flame retardants limit their application scope[2].Thus,considering the high cost and environment-safety problems of halogen-free flame retardants,identifying materials thatcan serve as their alternatives is attracting considerable interest.

Inorganic magnesium hydroxide(MH)is a kind of environmentfriendly halogen-free flame-retardant filler that has been extensively studied because of its advantages,such as low smoke,nontoxicity,and non-generation of carcinogenic gas.MH can also be used at higher processing temperatures than the most widely used aluminum hydroxidefillers.Upon thermal degradation,MH undergoes endothermic dehydration,releasing water to the gas phase with the formation of a thermally stable ceramic-like protective layer(MgO)on the polymer surface,which protects the underlying polymer matrix[3-12].However,compared with traditional halogen flame retardants,MH has a relatively low flame-retardant efficiency,which requires high loading(N60 wt%)to achieve effective flame retardancy[13-16].Such high loading causes the mechanical properties of the polymer matrix to drop down sharply due to its agglomeration and poor compatibility with this matrix.Furthermore,the hydrophilic surfaces of these MH nanoparticles do not favor their dispersion in polymer matrix[17].

The above-mentioned problems may be solved by surface modification.Chen et al.[18]selected titanate and zinc stearate as the interface modifiers between Mg(OH)2particles and the polymer substrate.Song et al.[19]synthesized hydrophobic MH nanoparticles through one-step precipitation aided with ultrasonic irradiation and by using Tween 80 as a stabilizer,dispersant,and surface modifier.Dong et al.[20]prepared nano-MH via wet precipitation by using a mixture of sodium dodecyl sulfate and monoalcohol ether phosphate as surface modifiers.However,these methods often require complicated process conditions,rendering them inconvenient and economically unacceptable for the large-scale production of hydrophobic MH nanoparticles.Therefore,a simple one-step synthesis method of preparing hydrophobic MH nanoparticles in large scale is urgently required.

Our group has previously prepared hydrophobic MH nanoparticles through a one-step synthesis method,and the resultant products exhibit excellent hydrophobic properties compared with blank MH.In the present work,we attempted to synthesize hydrophobic MH nanoparticles through a one-step synthesis method in a novel impinging stream-rotating packed bed(IS-RPB)reactor[21]by using Tween 80 as a modifier.Herein,we reported the preparation and characterization of hydrophobic MH nanoparticles.The effects of hydrophobic MH nanoparticles on the flammability,thermal stability,and mechanical properties of PP/MH composites were investigated.Results indicated the potential applicability of the proposed method to the large-scale production of hydrophobic MH nanoparticles,and that the obtained hydrophobic MH nanoparticles led to a considerable increase in both flame retardancy and mechanical properties of the PP composites.

2.Experimental

2.1.Materials

Magnesium chloride hexahydrate(MgCl2·6H2O),Tween 80(ethoxylated sorbitan monooleate),absolute ethanol,and liquid paraffin were purchased from Tianjin Guangfu Chemical Reagent Factory(China).Sodium hydroxide(NaOH)was purchased from Tianjin Dalu ChemicalReagentFactory(China).All chemicalreagents used in the experiments were analytically pure and not subjected to further purification.High-purity water prepared with a water-purification system(GWA-UN,China)was used in all procedures.

The PP materials used were injection-molding grade and supplied by Shandong Luke Chemical Industry Co.,Ltd.,China.

2.2.Synthesis of hydrophobic MH nanoparticles

The experimental setup of the preparation of MH nanoparticles is shown in Fig.1.The IS-RPB reactor,which was the key part of a high-gravity system,mainly consisted of a rotating packed bed and an impinging stream liquid distributor.The rotating packed bed was installed inside a stationary shell,with inner and outer diameters of 50 and 125 mm,respectively.The axial height of the rotating packed bed was 210 mm.The reactor was packed with a stainless-steel wire mesh with 0.3 mm diameter and 0.96 porosity.The liquid distributor containing a straight tube with a 1.5 mm hole on one end and an L-type tube with a 1.5 mm hole inside its front end.The two tubes were installed parallel to each other,7 mm apart,in the cavity center of the rotating packed bed.

In a typical preparation of MH nanoparticles,a certain amount of Tween 80 was added to a solution of MgCl2prepared by dissolving MgCl2in 2 L of ethanol solution(68.5%),and the Mg2+concentration was fixed to 0.75 mol·L-1.The as-prepared solution and 1.5 mol·L-1NaOH solution were sequentially preheated to 60°C.Then,the MgCl2and NaOH solutions were simultaneously and continuously pumped from their tanks into the IS-RPB reactor.The intense reaction mixing produced MH as a white precipitate.The reaction temperature,rotation speed,and liquid flow rate were maintained at 60 °C,800 r·min-1,and 40 L·h-1,respectively.After flowing out from the IS-RPB reactor,the MH suspension was separated by centrifugation at 8000 r·min-1for 3 min and washed several times with distilled water and absolute ethanol to remove the soluble products.After drying at 60°C in a vacuum oven for 6 h,the ethanol in the mother liquor was recovered by distillation to reduce operation costs.

2.3.Preparation of PP/MH composites

MH loading in the PP/MH composites was varied from 0 wt%to 30 wt%.Before melt blending,MH and PP materials were dried in an oven at 80°C for 48 h.The PP/MH composites were prepared by melt mixing PP materials and MH nanoparticles on a two-roll mill at 180°C for 15 min.Subsequently,the as-prepared PP/MH composites were hotpressed under10 MPa for10 min at180°C into 3.0 mm-thick sheets and then cut into suitable sizes for various analyses.

2.4.Characterization

2.4.1.MH nanoparticles

The morphologies of blank and hydrophobic MH nanoparticles were investigated using a JSM-6700F field-emission scanning electron microscopy system(FESEM)from JEOL and a Tecnai F20 transmission electron microscopy(TEM)from FEI.The BET specific surface areas of the MH nanoparticles were measured by N2adsorption-desorption analysis with an Autosorb iQ-MP Quantachrome instrument(USA).The phase,purity,and crystallographic structures of blank and hydrophobic MH nanoparticles were analyzed by X-ray diffraction(XRD)on a Bruker D8 Advance diffractometer working with Cu Kαsource(λ=0.15406 nm)at 40 kV operation voltage and 40 mA current,respectively.The infrared spectra of blank and hydrophobic MH nanoparticles were measured with a Spectrum Two Fourier transform infrared(FTIR)spectrometer through the KBr pellet method within the spectral range of 400 cm-1to 4000 cm-1.Thermal analysis was performed on an STA 449F3 simultaneous thermal analyzer from 35°C to 600 °C at a heating rate of 10 °C·min-1in a nitrogen atmosphere.Surface wettability was measured with a JC2000D1 contact-angle analyzer.The powder samples were pressed into thin pellets under 10 MPa,and contact angles were measured by the sessile drop method.The probe fluid used was reverse-osmosis-purified water.

Fig.1.Experimental setup of the preparation of MH nanoparticles.1.liquid inlet 1 2.liquid inlet 2 3.shell 4.packing 5.impinging stream 6.liquid outlet

Compatibility with the organic phase wasdetermined by sedimentation test,where 0.1 g of MH nanoparticles was placed in 10 ml of water.The suspension was kept stationary,and nanoparticle sedimentation was observed.

2.4.2.PP/MH composites

The thermal stability properties of PP/MH composites were carried out on a STA 449F3 simultaneous thermal analyzer(Netzsch,Germany)from 50 °C to 900 °C at a heating rate of 10 °C·min-1in a nitrogen atmosphere.

LOI was determined using an LOI instrument(Type HC-2;Jiangning Analysis Instrument Factory,Nanjing,China)on specimens with dimensions of 120×6.5×3.0 mm3based on the standard oxygen index test ASTM D2863-77.LOI was calculated according to the equation below:

where[O2]and[N2]are the O2and N2concentrations,respectively.

Tensile examinationswere performed on a Material Testinstrument(Model WDW-10C,Shanghai,China)at an extensional speed of 50 mm·min-1.Tensile specimens were prepared according to ASTM D638 standard method with an injection-molding machine.

Impacts test were performed using an Izod impact testing machine(Model ZBC-4B,Shenzhen,China)on specimens prepared according to ASTM D256 standard method.

3.Results and Discussion

3.1.MH nanoparticles

3.1.1.XRD analysis

Fig.2.XRD patterns of(a)blank and(b)hydrophobic MH.

The crystalline structure of blank and hydrophobic MH nanoparticles was investigated by XRD,and results are shown in Fig.2.Both XRD patterns can be indexed well as hexagonal-phase MH,in accordance with standard data(JCPDS Card No.7-239).All peaks except that of the(001)lattice plane had no obvious difference in intensity.Compared with blank MH,hydrophobic MH had stronger intensity on the(001)lattice plane,indicating the grafting ofTween 80 onto the(001)lattice plane ofMg(OH)2.The dispersion properties ofhydrophobic MHalso improved because of the increased intensity on the(001)lattice plane.

The crystallite sizes derived from the(001),(101),and(100)lattice planesofthe MHsampleswere calculated according to their XRDpatterns using the Debye-Scherrer formula[22]and are listed in Table 1.Hydrophobic MHcrystals were smallerthan blank MHcrystals and hydrophobic MH crystals synthesized using oleic acid as surface modifier[23].

Table 1 In fluence of reaction temperature on MH particle size

3.1.2.FTIR analysis

Structural characterization was performed by FTIR spectroscopy,and various features can be observed in Fig.3.Spectra b and c in Fig.3 show the characteristic stretching vibration bands for hydroxyl group(--OH)at 3699 cm-1,the bending vibration of the--OH bond at 1430-1650 cm-1,and the stretching vibration of water at 3440 cm-1.Moreover,Spectra c in Fig.3 shows the clearly visible vibration bands of the--CH2--and--CH3groups at 2925 and 2855 cm-1,as well as the C--O--C stretching vibration peak at 1129 cm-1.This finding indicated that MH interacted with Tween 80 during preparation,in agreement with hydrophobicity test results.

Fig.3.FTIR spectra of the(a)modifier Tween 80,(b)blank MH,and(c)hydrophobic MH.

3.1.3.FESEM and EDS analyses

The surface morphology and agglomeration level of MH were studied by field-emission scanning electron microscopy(FESEM),and the micrographs are shown in Fig.4.Blank MH with large particle size and high level of agglomeration was observed(Fig.4a).Conversely,the SEM image of hydrophobic MH(Fig.4b)revealed that these nanoparticles exhibited hexagonal lamella morphology with favorable dispersion.Particle-size distribution ranged from 30 nm to 70 nm,and the statistically average particle size was 30 nm.Some nanoparticles were perpendicular to the picture,clearly showing that the thickness of these MH nanolamellas was 3 nm.This finding demonstrated that the modifier Tween 80 strongly in fluenced the morphology and agglomeration levelofMH.Thus,adding Tween 80 to the synthesis system favored the production of small-sized MH nanoparticles with uniform crystal growth.These observations were consistent with the XRD results.The corresponding EDS results(Fig.5)indicated that hydrophobic MH contained large amounts of C,O,and Mg,which corresponded to Tween 80 and Mg(OH)2.This result confirmed that C existed in the MH nanoparticles,indicating Tween 80 well adhered onto the surface of MH,in agreement with FTIR analysis.

Fig.4.FESEM images of MH:(a)blank MH and(b)hydrophobic MH.

3.1.4.TEM analysis

Transmission electron microscopy(TEM)studies were performed to observe the morphology of blank and hydrophobic MH samples,and the images are shown in Fig.6.The blank MH sample was irregularly shaped and had a high level of agglomeration(30-70 nm).Conversely,the hydrophobic MH nanoparticles were more regularly shaped and had uniform particle-size distribution with particle sizes of around 30 nm,similar to those measured by SEM.This finding suggests that adding Tween 80 was favorable to obtaining small-sized MH nanoparticles with regular hexagonal lamella and uniform particle-size distribution.

3.1.5.BET analysis

The surface area of MH was determined by BET method.Fig.7 presents the N2adsorption-desorption isotherms and the corresponding BJH pore-size distribution of blank and hydrophobic MH.The BET surface area of hydrophobic MH was as high as 145.77 m2·g-1,which was higher than that of blank MH(56.028 m2·g-1).The differences in surface area between blank MH and hydrophobic MH can be attributed to the nanoparticle size.Smaller particles can have a larger surface area than larger size particles.This large surface area ofMHcan improve the mechanical property of the materials by enhancing their molecular interactions and chemical reactivity.The most probable pore size of blank MH as evaluated by BJH method was about 17.44 nm,whereas the hydrophobic MH decreased to 9.608 nm.These results indicated that Tween 80 occupied the mesoporous channels of MH[24,25].

3.1.6.TG/DSC analysis

The thermal properties of blank and hydrophobic MH were investigated by TG/DSC analysis,and the TG and DTG curves are shown in Fig.8.Blank MH exhibited a pronounced mass-loss step within 340.2-376.2°C,with a total mass loss percentage of 28.51%,which can be attributed to the thermal decomposition of MH.The theoretical mass loss of Mg(OH)2→MgO transformation was 30.8%,slightly larger than the observed value of 28.51%,and this discrepancy can be ascribed to the incomplete decomposition of the sample within the studied temperature range.

Fig.5.EDS pattern of hydrophobic MH.

Fig.6.TEEM images of MH:(a)blank MH and(b)hydrophobic MH.

Fig.7.Nitrogen adsorption-desorption isotherms and the corresponding BJH pore-size distribution of blank MH and hydrophobic MH.

Fig.8.TG and DTG curves of blank and hydrophobic MH.

By contrast,a two-step successive mass loss was observed for hydrophobic MH from 313.9 °C to 483.1 °C.The first step at 313.9-350.4 °C(26.96%),corresponding to the thermal decomposition of MH.The second step at 423.7-483.1°C(9.02%)corresponded to Tween 80 decomposition.The total mass loss percentage of hydrophobic MH was 35.98%,which was higher than that of blank MH(28.51%).This result was due to the introduction of Tween 80 into the synthesis system that led to a higher amount of Tween 80 absorbed onto the MH nanoparticle surface.This phenomenon can decrease the endothermic decomposition temperature of the MH nanoparticles and enhance their thermal stability.

Fig.9 shows the DSC curves of blank and hydrophobic MH.Given that the thermal decomposition of MH was endothermic,the DSC curve of hydrophobic MH showed an endothermic peak at 342.5 and at 461.4°C,which can be explained by the presence of Tween 80 on the surface of MH nanoparticles.

Fig.9.DSC curves of blank and hydrophobic MH.

3.1.7.Hydrophobicity analysis

The hydrophobic nature of MH nanoparticles was evaluated by contact-angle analysis and sedimentation test.As shown in Fig.10,Tween 80 drastically affected the hydrophobicity index of samples.Blank MH sample had a low water contact angle of＜10°,and the blank MH nanoparticles all sank in water after 10 min.This finding indicated that the surface wetting was very favorable,and water did spread over a large area of the surface.By contrast,the water contact angle of hydrophobic MH was 112°,which was higher than that of hydrophobic MH of 110.4°[23]).Moreover,almost all hydrophobic MH nanoparticles floated on water after 30 d,demonstrating that surface polarity was weaker and surface energy was lower.These observations may be attributed to the large number of Tween 80 molecules present on the surface of the hydrophobic MH nanoparticles with alkyl chains predominantly exposed to the air-water interface.

Fig.10.Behavior of water droplets on the surface of thin MH nanoparticle pellets:(a)blank MH and(b)hydrophobic MH.

3.2.PP/MH composites

3.2.1.SEM of PP/MH composites

Fig.11 shows the SEM images of the surfaces of PP/MH composites with blank MH and hydrophobic MH.Compared with blank MH nanoparticles(Fig.11a),hydrophobic MH nanoparticle size(Fig.11b)progressively decreased,and MH nanoparticle distribution in the PP matrix became narrower.Moreover,MH nanoparticle attachment to the PP matrix improved.These findings can be ascribed to the enhanced interfacial adhesion between filler and matrix through the surface modification of MH nanoparticles,thereby increasing the composites'ductility.

3.2.2.Flame retardance of PP/MH composites

The LOI test is widely used to evaluate the flammability of materials.The LOIs of material were＜20%,indicating material combustibility.At LOIs between 22%and 25%,combustion did not easily occur.With increased LOI to N26%,the material was considered as noncombustible.

Table 2 displays the LOIs of the PP/MH composites with different contents of hydrophobic MH nanoparticles.Results revealed that the LOI of PP materials were 17.5%;moreover,the LOIs of PP/MH composites increased with increased content of hydrophobic MH and reached 29.3%when PP/MH composites contained 30 wt%hydrophobic MH.These results indicated thatincreased hydrophobic MHcontentcan significantly improve the flammability of PP materials.This increase may have been due to the better dispersion of hydrophobic MH in PP materials and the formulation of compact chars.

3.2.3.Thermal stability of PP/MH composites

Fig.12 presents the TG and DTG curves of PP materials and PP/MH composites under N2atmosphere at a heating rate of 10 °C·min-1.The thermal decomposition of PP materials occurred in a one-step process within 435.6-473.9°C,and the PP materials underwent complete thermo-oxidative degradation at 473.9°C.Hardly any residue remained at the end of degradation.By contrast,the thermal decomposition behavior of PP/MH composites occurred in a two-step process at 319.9-482.8°C.The first step involved MH dehydration at 319.9-422.9 °C,whereas the second degradation step at 422.9-482.8 °C was due to the degradation of PP materials.Fig.12 shows that the thermal-degradation curves of PP/MH composites at 422.9-482.8°C shifted toward higher temperatures.This result showed that the PP/MH composites showed better thermal stability than PP and had about 17%residue left after degradation.

3.2.4.Mechanical properties of PP/MH composites

The incorporation of MH caused a deterioration in the mechanical properties of the polymer matrix.To assess the effect of hydrophobic MH on the mechanical properties of PP/MH composites,tensile strength and impact strength were measured,as shown in Fig.13.The tensile strength and elongation at break of the PP composites with hydrophobic MHwere much higherthan those ofthe composites with blank MHatthe same MH loading.The tensile strength of PP/MH composites with hydrophobic MH increased by 3%compared with that of PP/MH composites with blank MH when PP/MH composites contained 30 wt%MH.The impact strength of PP/MH composites with hydrophobic MH increased by 10.7%compared with that of PP/MH composites with blank MH when PP/MH composites contained 30 wt%MH.These results indicated that Tween 80 enhanced the compatibility between filler and PP matrix.

Fig.11.SEM of PP/MH composites with(a)blank MH and(b)hydrophobic MH.

Table 2 Limiting oxygen index of PP/MH with hydrophobic MH

Fig.12.TG and DTG curves of PP MATERIALS and PP/MH composites.

Fig.13.Tensile strength and elongation atbreak of PP/MH composites with blank MHand hydrophobic MH.

4.Conclusions

Hydrophobic MH nanoparticles with lamella morphology were synthesized using Tween 80 as a surface modifier by online modification method in a novel IS-RPB reactor.SEM images showed that the obtained nanoparticles had hexagonal lamella morphology with favorable dispersion and an average diameter of 30 nm.XRD revealed that the hydrophobic MH product presented a brucite structure,and that the intensity on(001)lattice plan was higher than that of blank MH.FTIRanalysis showed that Tween 80 interacted with MH through chemical bonding.Compared with blank MH,the productobtained through the proposed method had a higher water contact angle(112°).The improved hydrophobicity and surface dispersibility ofMHnanoparticles were also verified by sedimentation tests.TG/DSC analysis indicated that the total percentage of weight loss of hydrophobic MH(35.98%)was higher than that of blank MH(33.18%),which indicated improved flame-retardant effectiveness.When PP/MH composites contained 30 wt%hydrophobic MH nanoparticles,PP/MH composites had better thermal stability than PP materials,and the LOIs reached 29.3%,which was considered as noncombustible.Moreover,the mechanical properties of PP composites with hydrophobic MH were much higher than composites with blank MH.Thus,this one-step synthesis method in a novel reactor(IS-RPB)was a promising process for the large-scale production of hydrophobic MH nanoparticles.