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Preparation and evaluation of hydrophobically modified core shell calcium carbonate structure by different capping agents
Powder Technology 235 (2013) 581–589
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Preparation and evaluation of hydrophobically modified core shell calcium carbonate structure by different capping agents Deepika a, b, S.K. Hait a,⁎, J. Christopher a, Y. Chen b, P. Hodgson b, D.K. Tuli a a b
Indian Oil Corporation Limited, R&D Centre, Sector 13, Faridabad, Haryana, 121007, India Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 8 November 2012 Accepted 10 November 2012 Available online 17 November 2012 Keywords: Calcium carbonate Dispersants Surface modification Milling
a b s t r a c t Surface modification of precipitated calcium carbonate particles in a planetary ball mill using different dispersants such as stearic acid, oleic acid, palmitic acid, salicylic acid, oleyl amine, DDSA (dodecenyl succinic anhydride), ODSA (octadecenyl succinic anhydride), TPSA (tetra propenyl succinic anhydride) as modification agent was done in order to find out the potential of dispersant in surface modification and for making dispersion in hydrocarbon oil was investigated. Different dispersants were ball milled keeping all the parameters for processing (milling) like—milling time, ball ratios, sample dosage and milling constant. The physical properties of the hydrophobically modified calcium carbonate were measured; the particle size and morphology of the resulting samples were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface coating thickness was also estimated by geometric calculation using results of TEM and TGA considering the formation of core shell structure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCO3 is one of the cheapest, basic inorganic materials which have got wide applications in oil, paint, plastics and agricultural industries. Some of the applications are stabilizing the pH of the soil, over basing in lubricants, calcium supplement in animal feed stock and filler material in papers and polymers. Over basing is the most important application of CaCO3 in lubricant formulation. The oxidation of sulfurous and nitrogenous impurities of fuel during combustion leads to the formation of inorganic and organic acids. The oxidative degradation of lubricants would also lead to the formation of organo-acids. If allowed to build-up, these acids would cause severe corrosion to engine parts. Over basing or detergents are one of the most important additives in lubricant to mitigate acid build-up, prevent rust formation and promote engine cleanliness, oxidation inhibition and extended trouble-free operation. CaCO3 (and Ca(OH)2) in the form of colloidal nanoparticle stabilized by a surfactant layer is used as over basing additives in lubricant formulation. These additives essentially consist of 15–40% by mass inorganic core (CaCO3 or Ca(OH)2) stabilized by 20–45% oil-soluble surfactants incorporated into lubricating base oil. Various surfactants typically long chain carboxylic acids, glycols, alcohols, phenates, sulfonates, salicylates or phosphonates are used to prepare stable colloidal nanoparticles of Ca compounds. Fig. 1a shows the schematic
⁎ Corresponding author. Tel.: +91 129 2294638. E-mail address: [email protected] (S.K. Hait). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.11.015
of overbasing additive (overbased sulfonate). The structure within bracket represents a neutral calcium sulfonate which stabilizes the calcium carbonate core. The quantity of base incorporated in the particle core is greater than that needed to neutralize the acid surfactant and hence it is termed as overbased. This can be achieved by taking advantage of the excellent dispersant properties of the long chain sulfonate molecule. Excess base in the form of calcium carbonate can be dispersed in micellers to produce so-called overbased sulfonates (Fig. 1b). Thus surface modification by surfactants enables to disperse and/or stabilize CaCO3 in the oil medium to be used for overbasing application. Several studies have been performed on the surface modification of CaCO3 nanoparticles [1–9]. Most of the synthesis routes involve direct carbonation and mineralization of CaO or Ca(OH)2 in the presence of surfactant like fatty acids and solvent medium such as water or alcohol. The process mimics the crystallization of CaCO3 within living system and hence often termed as bio-mimetic synthesis or bio-mineralization of CaCO3. Solution and carbonation routes were employed by many researchers for the synthesis of hydrophobic CaCO3 nanoparticles. Hydrophobic CaCO3 crystals are obtained through carbonation route from Ca(OH)2 slurry under CO2 purging at desired temperature in the presence of different surface modifiers such as fatty acids, octadecyl dihydrogen phosphate, sodium stearate, terpineol etc.[2,3,6,10–22]. There are reports that have mentioned carbonation process assisted by ultrasonication in which by optimizing the synthesis parameters they had obtained desired particle size [23,24]. The carbonation route preparation and surface modification of CaCO3 have extensively been studied and the
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Deepika et al. / Powder Technology 235 (2013) 581–589 Chart 2 Details of the milling process parameters. 1. 2. 3. 4. 5.
Mole ratio of CaCO3 and selected dispersant Medium suitable for capping agent Volume of ZrO2 balls (1 mm, 0.5 mm and 0.1 mm diameter). Rotating speed Milling time
1:1 Three times of sample dosage. 1/3 of vessel volume 1200 rpm 2 hr
method cannot be applied for oil based application as the particles will settle down due to their larger size and incompletely covered coating shell. Synthesis of nano CaCO3 by synthetic procedure involves mainly co precipitation of CaCl2 in the presence of fatty acids as capping agents with sodium- or ammonium carbonates as a precipitant [26–30]. A few works were also found on the surface modification through post synthesis drafting of capping agents on freshly made CaCO3 from co-precipitation method [30]. Such works generally yielded crystallization of vaterite and calcite hitherto there was no control on the phase purity and uniform surface coating. Very recently the microwave assisted formation of non-agglomerated vaterite through bicarbonate decomposition in polyol medium has been reported [31]. Lamellar vaterite was obtained in the presence of surfactants and fatty acid surfactant mixtures. The formation of lamellar morphology as well as vaterite phase was controlled through surfactant concentration and the hydrophobic alkyl chain of cationic surfactant played major role in regulating the CaCO3 growth [28]. Fig. 1. Schematics of over based sulfonate (a) and its micellar form (b).
degree of hydrophobicity was evaluated through combination of techniques such as floating test, contact angle measurement and active ratio test. The synthesis of CaCO3 core – and salt of CaCO3 with capping agent – shell nanoparticles was reported and the monolayer coverage by capping agent was validated using Langmuir adsorption calculation [18]. The coating inhibits the degree of agglomeration compared to uncoated nanoparticles. The coating efficiency had also been calculated through SEM analysis [25]. The hydrophobic particles prepared by this Chart 1 Detailed molecular structures of the dispersants used. Capping agent
Structure
Molecular weight
Oleic acid C17H33COOH
282.46
Stearic acid C17H35COOH
284.48
Palmitic acid C15H31COOH
256.43
Salicylic acid C7H6O3
138.12
Oleyl amine C18H37N
267.50
ODSA C22H38O3 Octadodecenyl succinic anhydride TPSA C16H26O3 Tetrapropenyl succinic anhydride
350.54
DDSA C16H26O3 Dodecenyl succinic anhydride
266.38
266.42
Fig. 2. XRD patterns of CaCO3 particles after milling in different capping agents (a) uncoated CaCO3, (b) OAChex, (c) SAChex, (d) PAChex, (e) SCCmet, (f) OLCmet, (g) ODCmet, (h) TPCmet and (i) DDCxyl.
Deepika et al. / Powder Technology 235 (2013) 581–589 Chart 3 Description of samples.
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Table 1 Crystallite Sizes of calcium carbonate milled with different capping agents.
S. no.
Sample description
Capping agent
Medium
S. no.
Sample
XS (Å)
1. 2. 3. 4. 5. 6. 7. 8.
OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
Oleic acid Stearic acid Palmitic acid Salicylic acid Oleylamine ODSA TPSA DDSA
Hexane Hexane Hexane Methanol Methanol Methanol Methanol Xylene
1. 2. 3. 4. 5. 6. 7. 8. 9.
CaCO3 OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
519.875 299.375 338.75 321.125 281.25 372 344 341.5 333.25
In contrast to the above mentioned soft chemical approach, ball milling technique rarely termed as mechano chemical approach has also been employed to synthesis hydrophobic CaCO3. It is widely used method for the synthesis of nanoparticles and nanocomposites; in which happens the mechanical breakdown of solids into smaller particles (macro to micro or micro to nano or submicro particles) [35]. Generally the surface area of the solids would increase due to the induced defects onto the solid structure during milling. The size reduction through this method is depending upon various milling parameters affecting the process. These factors are material of milling media, ball to solid ratio, filling volume of milling chamber, milling atmosphere, milling speed and time. In mechano chemical approach the newly created surface of the small molecules in-situ modified by the surfactanrt molecules to make them hydrophobic. This small molecule surface modification continues until creation of new surface is ceased due to limitation of the milling parameters and process conditions. The particle size reduction is not as easy as that of controlling these milling factors. The desired size reduction could be achieved by following right and optimized milling parameters. Mechano chemical preparation of nano calcite CaCO3 in NaCl matrix occurred during displacement reaction between CaCl2 and Na2CO3 through ball milling process. The effect of washing and heating over the phase formation was also reported [32]. Further, the effect of milling time on different phase transitions was also studied [33]. Ball milling is a versatile tool for the surface modification of CaCO3 using modifiers like sodium stearate, styrene, sodium oleate, stearic acid under wet condition and researchers examined the effects of different parameters for improved product quality [32–36]. Unlike in solution and carbonation route where there are parameters crucially effecting the crystal growth and morphology of CaCO3; in ball milling process no such significant problems exist except milling parameters mentioned above. However, these parameters can be controlled with
ease to give better tailored materials. The desirable phase and morphology of the solids (for instance calcite and cubic CaCO3) can be selected for milling with suitable surface modifier. Particle size reduction and in situ surface modification through mechano chemical bonding of precipitated calcium carbonate nanoparticles with modifiers and the product application in over basing have not been reported in the scientific literature. In this paper we attempted to produce hydrophobically surface modified core shell type calcium carbonate nanoparticles, where shell is the organic–inorganic hybrid structure by ball milling route and characterized by relevant physicochemical techniques.
2. Experimental work 2.1. Materials and method Stearic acid, oleic acid, palmitic acid (all laboratory grade), salicylic acid (analytical grade),oleyl amine (GR grade), hexane, methanol, xylene solvents (all laboratory grade) were purchased from commercial market. Other materials like DDSA (dodecenyl succinic anhydride), ODSA (octadecenyl succinic anhydride), TPSA (tetrapropenyl succinic anhydride) procured from pentagon U.K. The molecular structures of the dispersants used are detailed in Chart 1. Precipitated calcium carbonate (PCC, THIOX-CARB 300) was purchased from Specialty minerals Lifford, Birmingham. Precipitated CaCO3 and respective dispersant in 1:1 molar ratio were ball milled with suitable solvents in planetary ball mill (Retsch PM-400MA) using ZrO2 balls and vessels. In order to accomplish maximum surface coating, an excess amount of coating agent to be used and hence 1:1 molar ratio was chosen arbitrarily.
Fig. 3. Low-angle XRD patterns for (a) OAChex, (b) SAChex.
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Fig. 4. (i) Showing the thermal decomposition pattern for the (a) uncoated CaCO3 and (b) stearic acid; (ii) showing the decomposition pattern for coated CaCO3 with different capping agents (a) OAChex, (b) SAChex, and (c) PAChex; (iii) showing the decomposition pattern for coated CaCO3 with different capping agents (a) SCCmet and (b) OLCmet; (iv) showing the decomposition pattern for coated CaCO3 with different capping agents (a) ODCmet, (b) TPCmet, and (c) DDCxyl.
The milling time for different ball sizes (1, 0.5 and 0.1 mm) was fixed for 2 hours. The details of the milling process parameters are given in tabular form in Chart 2. After milling, the samples were rinsed and washed several times with suitable solvent so as to remove excess dispersant followed by centrifugation and drying. The samples are described in Chart 3 with six alphabetical letters in which first three refer to capping agent and CaCO3 (C) followed by the last three are milling medium hexane (hex), methanol (met) and xylene (xyl).
analyzing machine (TA Instruments, USA) under a flow of air. Nearly 5–10 mg of the sample were taken in the Platinum pan and heated in air at the heating rate of 10 deg/min up to 900 °C. 2.2.3. Transmission electron microscopy (TEM) analysis TEM analysis was performed to view the morphology and particle size of the milled product. TEM measurement was carried out in a Jeol (JEM 2100) instrument at 200 kV having a LaB6 filament with a point resolution of 0.194 nm and lattice resolution of 0.14 nm. TEM samples were prepared by dispersing the milled sample in heptane and followed
2.2. Characterization 2.2.1. XRD studies XRD studies were carried out in an 18 kW X-ray diffractometer (Rigaku, Japan) having copper rotating anode. XRD patterns were recorded at 50 kV and 250 mA, at a scan rate of 2 deg/min with a step size of 0.01 deg. The XRD patterns were processed and peak search was conducted by search match to find out different phases present in the sample. 2.2.2. Differential thermal studies The thermal stability and phase transitions of the samples as function of temperature were measured using TGA model 2960 thermal
Table 2 The weight loss during the temperature range of 250 °C–450 °C corresponding to decomposition of calcium salt of capping agent for different capping agents. S. no.
Calcium salts of capping agent
Weight % loss of shell (approximate)
1. 2. 3. 4. 5. 6. 7. 8.
Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium
27 52 49 3 6 50 15 14
salt salt salt salt salt salt salt salt
of oleic acid of stearic acid of palmitic acid of salicylic acid of oleylamine of ODSA of TPSA of DDSA
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Fig. 5. The transmission electron microscopy images of CaCO3 particles: (a) uncoated CaCO3, (b) OAChex, (c) SAChex, (d) PAChex, (e) SCCmet, (f) OLCmet, (g) ODCmet, (h) TPCmet, and (i) DDCxyl.
by ultrasonication for 5 min. A drop of the solution was poured on a copper grid placed on a filter paper and kept for drying. 3. Results and discussion The experiments were carried out to investigate the effect of different dispersants on the shape, size and surface characteristics of the CaCO3 after milling treatment. 3.1. X-ray diffraction (XRD) results
Fig. 6. Representation of cubic core–shell CaCO3 nanoparticle.
The XRD patterns in Fig. 2 show that the products obtained after milling with different capping agents were typically calcite in nature despite of their different morphologies and sizes; Fig. 3a and b shows the low angle diffraction of representative samples (SACHex and OACHex). The distinct patterns observed are matching well with calcium stearate and calcium oleate reflections (JCPDS File). In this process CaCO3 powders were first de-agglomerated and then further reduced to smaller particles by grinding actions and at
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Fig. 7. Showing the histogram plots for (a) OAChex, (b) SAChex, (c) PAChex, (d) SCCmet, (e) OLCmet, (f) ODCmet, (g) TPCmet and (h) DDCxyl.
Deepika et al. / Powder Technology 235 (2013) 581–589
the same time the newly created surfaces of smaller particles were covered by the agents under ball milling impact [36]. Table 1 comprises the crystallite sizes of the fresh uncoated and milling coated CaCO3 with different capping agents; a decrease in crystallite size was observed after milling for all the samples. The impact of ball milling on the particle size reduction is a well known fact as well as the coating by capping agents reduces the surface energy of calcite and could control the particle-particle interaction which was confirmed by Anshan and Taylor using IGC (inverse gas chromatography) [37]. 3.2. Thermo gravimetric analysis (TGA) results TGA results from uncoated and coated CaCO3 are given in Fig. 4. Fig. 4i (a) shows TGA curve of the uncoated CaCO3 and stearic acid. The uncoated CaCO3 undergoes two decomposition steps: a weight loss of 8.514% below 600 °C and a large weight loss of 42.03% above 600 °C indicating the complete decomposition of CaCO3 to CaO and CO2. TGA curve of stearic acid (Fig. 4i (b)) shows about 95% weight loss below 250 °C indicating the complete thermal decomposition of organics under air atmosphere. Fig. 4 (ii–iv) shows TGA curves for the coated CaCO3 samples; overall there are three weight loss steps. The first step with minimal weight loss was primarily due to the desorption of adsorbed solvents and excess of dispersants occurred below 250 °C. Second step was in the temperature range from 250 °C to 650 °C where the weight loss may be attributed to the thermal decomposition of coating agent. The final third weight loss observed after 650 °C was due to the thermal decomposition of CaCO3 into CaO and CO2. The weight loss of third step was found to be about 15–40% for all of the samples. The percentage loss depends up on the second step weight loss, more the second step less the third step weight loss or less the second step more the third step weight loss. The second step weight loss was found to be an important characteristic of the coated CaCO3. Table 2 indicates clearly that the weight loss was more where efficacious reaction takes place between the calcium carbonate and capping agent. Typical acid–base reaction is expected with fatty acid and succinic anhydride based coating agents. However, in the case of salicylic acid and oleylamine minimal weight loss was observed indicating an ineffective reaction with calcite under milling condition. The reaction of salicylic acid might be hindered due to the presence of intramolecular hydrogen bonding whereas no reaction is expected with oleyl amine an organic base. Thus the reaction of calcite with dispersants except salicylic acid and oleyl amine are given (Eqs. (1) and (2)). 2−
2CaCO3 þ 2RCOOH→2CaOCOR þ 2CO3
ð1Þ
ð2Þ
The thermal stability of the coated CaCO3 was found to be less than 200 °C under air atmosphere. The physical stability of these samples is verified in the oil medium to form stable dispersion which is essential for the lubricant application. All the samples except SCCmet and OLCmet (5–10%) were mixed with group II base oil to give stable partially clear nano dispersion. These dispersions on left undisturbed in the glass cylinders were found to be stable up to a month time. 3.3. Transmission electron microscopy (TEM) results The morphology and particle size of milled samples were characterized using TEM. Fig. 5a shows that original CaCO3 particles are
587
agglomerated and cubic in shape with an average particle size of approximately 200 nm. With the addition of capping agents the CaCO3 powders were de-agglomerated and their shape was little distorted due to breaking down of primary crystals (Fig. 5b, c, d, e, f, g, h, and i). Sample obtained after milling was in a size range of 85 nm to 120 nm. The dispersants have the reactivity of different extent for binding with CaCO3 and results in different efficacy to stabilize the reduced size. From the images it can be inferred that different dispersants gave different impressions to CaCO3 particle shape and sizes. Most of the images show that particles were de-agglomerated and coated by dispersants. It was seen that the maximum rupturing and de-agglomeration of cubes were observed for ODSA capping agent (Fig. 5g). 3.4. Calculation for shell thickness and surface density From the results of TGA and TEM analyses the shell thicknesses was estimated along with the surface density for every dispersant. Two quantities were obtained from TGA: i. The weight of CaO after the CaCO3 decomposition, ii. The amount of capping agent used during the reaction to form shell. The cubic core–shell CaCO3 structure can be represented as shown in Fig. 6, where a and b respectively, are length of the sides of core and shell. The particle size histogram derived from TEM images (Fig. 7) gives the size of cubic particles of maximum distribution. From the length a and b were obtained the volume of the cubic structure for n number of cubic nanoparticles is: 3
ð1Þ
3
ð2Þ
V core ¼ a n V total ¼ b n
The shell structure is made up of CaCO3 and capping agent and if the shell volume is Vshell then it can be obtained from Eqs. (1) and (2) as V core ¼ V total −V shell
ð3Þ
The total volume and shell volumes are calculated from TGA weight loss by using respectively the bulk density of CaCO3 (0.675 g cm−3) and the dry bulk density of the prepared samples. Thus the coating thickness is obtained from length of the sides a and b as follows: t ¼ b−a=2
ð4Þ
If we consider the reaction stoichiometry as 2 moles of capping agent involved per mole of CaCO3 (except in the case of ODSA, DDSA, TPSA and oleyl amine where one mole utilized) then weight
Table 3 Shell thickness and surface density calculated using above calculations with the help of TEM and TGA results. S. no.
Sample
TEM particle size (nm)
Thickness of coating (nm)
Surface density (molecules/nm2) (1013)
1. 2. 3. 4. 5. 6. 7. 8.
OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
102 103 101 102 118 97 95 87
1.85 3.35 3 0 0.35 16.3 1 1.69
2.65 5.04 2.41 0 0.9 4.25 5.7 2.9
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Fig. 8. Showing the TEM images (a) OAChex, (b) OAChex after 10 s of beam exposure and (c) OAChex after 30 s of beam exposure.
(W) of the capping agent in the shell can be obtained from the TGA weight loss. Thus the surface density of the capping agent (the number of capping molecules coated over one cube) is given as follows: W N A =M 6a
2
to carry out this research work; they also express their gratitude for participation and supporting in this work. Deepika is very grateful to Institute for Frontier Materials for supporting her research work under Deakin University Doctoral Program.
ð5Þ
where NA is Avogadro number and M is formula weight of capping agent. Histogram derived from six representative TEM images was recorded at different positions of a grid. From Table 3 we concluded that maximum coating was obtained for ODSA coated CaCO3 i.e. 16.3 nm followed by stearic acid coated CaCO3 gives a value of 3.35 nm while the surface density calculated for both these samples shows different effect due to stoichiometric mole ratios are different for both the capping agents. However, salicylic acid coated sample displays no surface coating as per the calculation done and likewise oleylamine shows a very minute surface coverage having surface density of 0.9 × 10 13 molecules/nm 2 with coating thickness of 0.35 nm only. Similar findings were observed in TGA results also. Below are the TEM images (Fig. 8) for coated oleic acid samples (Fig. 8a) taken at 40,000× resolution showing cuboidal crystals. It was observed that surface of these crystals starts crumpling (Fig. 8b) when the sample was exposed to beams for a time of 10 s. Further, when the exposure time was increased to 30 s it was discovered that the entire surface film has been crumpled (Fig. 8c). Similar observation has also been reported earlier [38]. This observation further confirms the presence of organic coating on the calcium carbonate surface generated by mechano chemical reaction which has been decomposed by the high electron beam energy inside TEM. 4. Conclusion This paper reports the synthesis of hydrophobic core shell type CaCO3 particles via ball milling approach using different capping agents as a surface modifier. Results from this work prove that coating was successfully done that helps in reducing the agglomeration of NPCC to different extent. It has also been demonstrated that organic substrate (capping) alters the crystal morphology and surface properties of calcium carbonate by making it hydrophobic. It was found that different capping agent shows different tendencies to coat the CaCO3 surface and also have different influence on the particle size, morphology and surface hydrophobic property. These coated calcite samples are potential candidate for the over base application in lubricant technology. Acknowledgments The authors wish to acknowledge the financial support of Indian Oil R&D Centre in the form of Indian Oil Golden Jubilee Fellowship
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Preparation and evaluation of hydrophobically modified core shell calcium carbonate structure by different capping agents Deepika a, b, S.K. Hait a,⁎, J. Christopher a, Y. Chen b, P. Hodgson b, D.K. Tuli a a b
Indian Oil Corporation Limited, R&D Centre, Sector 13, Faridabad, Haryana, 121007, India Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 8 November 2012 Accepted 10 November 2012 Available online 17 November 2012 Keywords: Calcium carbonate Dispersants Surface modification Milling
a b s t r a c t Surface modification of precipitated calcium carbonate particles in a planetary ball mill using different dispersants such as stearic acid, oleic acid, palmitic acid, salicylic acid, oleyl amine, DDSA (dodecenyl succinic anhydride), ODSA (octadecenyl succinic anhydride), TPSA (tetra propenyl succinic anhydride) as modification agent was done in order to find out the potential of dispersant in surface modification and for making dispersion in hydrocarbon oil was investigated. Different dispersants were ball milled keeping all the parameters for processing (milling) like—milling time, ball ratios, sample dosage and milling constant. The physical properties of the hydrophobically modified calcium carbonate were measured; the particle size and morphology of the resulting samples were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface coating thickness was also estimated by geometric calculation using results of TEM and TGA considering the formation of core shell structure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCO3 is one of the cheapest, basic inorganic materials which have got wide applications in oil, paint, plastics and agricultural industries. Some of the applications are stabilizing the pH of the soil, over basing in lubricants, calcium supplement in animal feed stock and filler material in papers and polymers. Over basing is the most important application of CaCO3 in lubricant formulation. The oxidation of sulfurous and nitrogenous impurities of fuel during combustion leads to the formation of inorganic and organic acids. The oxidative degradation of lubricants would also lead to the formation of organo-acids. If allowed to build-up, these acids would cause severe corrosion to engine parts. Over basing or detergents are one of the most important additives in lubricant to mitigate acid build-up, prevent rust formation and promote engine cleanliness, oxidation inhibition and extended trouble-free operation. CaCO3 (and Ca(OH)2) in the form of colloidal nanoparticle stabilized by a surfactant layer is used as over basing additives in lubricant formulation. These additives essentially consist of 15–40% by mass inorganic core (CaCO3 or Ca(OH)2) stabilized by 20–45% oil-soluble surfactants incorporated into lubricating base oil. Various surfactants typically long chain carboxylic acids, glycols, alcohols, phenates, sulfonates, salicylates or phosphonates are used to prepare stable colloidal nanoparticles of Ca compounds. Fig. 1a shows the schematic
⁎ Corresponding author. Tel.: +91 129 2294638. E-mail address: [email protected] (S.K. Hait). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.11.015
of overbasing additive (overbased sulfonate). The structure within bracket represents a neutral calcium sulfonate which stabilizes the calcium carbonate core. The quantity of base incorporated in the particle core is greater than that needed to neutralize the acid surfactant and hence it is termed as overbased. This can be achieved by taking advantage of the excellent dispersant properties of the long chain sulfonate molecule. Excess base in the form of calcium carbonate can be dispersed in micellers to produce so-called overbased sulfonates (Fig. 1b). Thus surface modification by surfactants enables to disperse and/or stabilize CaCO3 in the oil medium to be used for overbasing application. Several studies have been performed on the surface modification of CaCO3 nanoparticles [1–9]. Most of the synthesis routes involve direct carbonation and mineralization of CaO or Ca(OH)2 in the presence of surfactant like fatty acids and solvent medium such as water or alcohol. The process mimics the crystallization of CaCO3 within living system and hence often termed as bio-mimetic synthesis or bio-mineralization of CaCO3. Solution and carbonation routes were employed by many researchers for the synthesis of hydrophobic CaCO3 nanoparticles. Hydrophobic CaCO3 crystals are obtained through carbonation route from Ca(OH)2 slurry under CO2 purging at desired temperature in the presence of different surface modifiers such as fatty acids, octadecyl dihydrogen phosphate, sodium stearate, terpineol etc.[2,3,6,10–22]. There are reports that have mentioned carbonation process assisted by ultrasonication in which by optimizing the synthesis parameters they had obtained desired particle size [23,24]. The carbonation route preparation and surface modification of CaCO3 have extensively been studied and the
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Deepika et al. / Powder Technology 235 (2013) 581–589 Chart 2 Details of the milling process parameters. 1. 2. 3. 4. 5.
Mole ratio of CaCO3 and selected dispersant Medium suitable for capping agent Volume of ZrO2 balls (1 mm, 0.5 mm and 0.1 mm diameter). Rotating speed Milling time
1:1 Three times of sample dosage. 1/3 of vessel volume 1200 rpm 2 hr
method cannot be applied for oil based application as the particles will settle down due to their larger size and incompletely covered coating shell. Synthesis of nano CaCO3 by synthetic procedure involves mainly co precipitation of CaCl2 in the presence of fatty acids as capping agents with sodium- or ammonium carbonates as a precipitant [26–30]. A few works were also found on the surface modification through post synthesis drafting of capping agents on freshly made CaCO3 from co-precipitation method [30]. Such works generally yielded crystallization of vaterite and calcite hitherto there was no control on the phase purity and uniform surface coating. Very recently the microwave assisted formation of non-agglomerated vaterite through bicarbonate decomposition in polyol medium has been reported [31]. Lamellar vaterite was obtained in the presence of surfactants and fatty acid surfactant mixtures. The formation of lamellar morphology as well as vaterite phase was controlled through surfactant concentration and the hydrophobic alkyl chain of cationic surfactant played major role in regulating the CaCO3 growth [28]. Fig. 1. Schematics of over based sulfonate (a) and its micellar form (b).
degree of hydrophobicity was evaluated through combination of techniques such as floating test, contact angle measurement and active ratio test. The synthesis of CaCO3 core – and salt of CaCO3 with capping agent – shell nanoparticles was reported and the monolayer coverage by capping agent was validated using Langmuir adsorption calculation [18]. The coating inhibits the degree of agglomeration compared to uncoated nanoparticles. The coating efficiency had also been calculated through SEM analysis [25]. The hydrophobic particles prepared by this Chart 1 Detailed molecular structures of the dispersants used. Capping agent
Structure
Molecular weight
Oleic acid C17H33COOH
282.46
Stearic acid C17H35COOH
284.48
Palmitic acid C15H31COOH
256.43
Salicylic acid C7H6O3
138.12
Oleyl amine C18H37N
267.50
ODSA C22H38O3 Octadodecenyl succinic anhydride TPSA C16H26O3 Tetrapropenyl succinic anhydride
350.54
DDSA C16H26O3 Dodecenyl succinic anhydride
266.38
266.42
Fig. 2. XRD patterns of CaCO3 particles after milling in different capping agents (a) uncoated CaCO3, (b) OAChex, (c) SAChex, (d) PAChex, (e) SCCmet, (f) OLCmet, (g) ODCmet, (h) TPCmet and (i) DDCxyl.
Deepika et al. / Powder Technology 235 (2013) 581–589 Chart 3 Description of samples.
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Table 1 Crystallite Sizes of calcium carbonate milled with different capping agents.
S. no.
Sample description
Capping agent
Medium
S. no.
Sample
XS (Å)
1. 2. 3. 4. 5. 6. 7. 8.
OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
Oleic acid Stearic acid Palmitic acid Salicylic acid Oleylamine ODSA TPSA DDSA
Hexane Hexane Hexane Methanol Methanol Methanol Methanol Xylene
1. 2. 3. 4. 5. 6. 7. 8. 9.
CaCO3 OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
519.875 299.375 338.75 321.125 281.25 372 344 341.5 333.25
In contrast to the above mentioned soft chemical approach, ball milling technique rarely termed as mechano chemical approach has also been employed to synthesis hydrophobic CaCO3. It is widely used method for the synthesis of nanoparticles and nanocomposites; in which happens the mechanical breakdown of solids into smaller particles (macro to micro or micro to nano or submicro particles) [35]. Generally the surface area of the solids would increase due to the induced defects onto the solid structure during milling. The size reduction through this method is depending upon various milling parameters affecting the process. These factors are material of milling media, ball to solid ratio, filling volume of milling chamber, milling atmosphere, milling speed and time. In mechano chemical approach the newly created surface of the small molecules in-situ modified by the surfactanrt molecules to make them hydrophobic. This small molecule surface modification continues until creation of new surface is ceased due to limitation of the milling parameters and process conditions. The particle size reduction is not as easy as that of controlling these milling factors. The desired size reduction could be achieved by following right and optimized milling parameters. Mechano chemical preparation of nano calcite CaCO3 in NaCl matrix occurred during displacement reaction between CaCl2 and Na2CO3 through ball milling process. The effect of washing and heating over the phase formation was also reported [32]. Further, the effect of milling time on different phase transitions was also studied [33]. Ball milling is a versatile tool for the surface modification of CaCO3 using modifiers like sodium stearate, styrene, sodium oleate, stearic acid under wet condition and researchers examined the effects of different parameters for improved product quality [32–36]. Unlike in solution and carbonation route where there are parameters crucially effecting the crystal growth and morphology of CaCO3; in ball milling process no such significant problems exist except milling parameters mentioned above. However, these parameters can be controlled with
ease to give better tailored materials. The desirable phase and morphology of the solids (for instance calcite and cubic CaCO3) can be selected for milling with suitable surface modifier. Particle size reduction and in situ surface modification through mechano chemical bonding of precipitated calcium carbonate nanoparticles with modifiers and the product application in over basing have not been reported in the scientific literature. In this paper we attempted to produce hydrophobically surface modified core shell type calcium carbonate nanoparticles, where shell is the organic–inorganic hybrid structure by ball milling route and characterized by relevant physicochemical techniques.
2. Experimental work 2.1. Materials and method Stearic acid, oleic acid, palmitic acid (all laboratory grade), salicylic acid (analytical grade),oleyl amine (GR grade), hexane, methanol, xylene solvents (all laboratory grade) were purchased from commercial market. Other materials like DDSA (dodecenyl succinic anhydride), ODSA (octadecenyl succinic anhydride), TPSA (tetrapropenyl succinic anhydride) procured from pentagon U.K. The molecular structures of the dispersants used are detailed in Chart 1. Precipitated calcium carbonate (PCC, THIOX-CARB 300) was purchased from Specialty minerals Lifford, Birmingham. Precipitated CaCO3 and respective dispersant in 1:1 molar ratio were ball milled with suitable solvents in planetary ball mill (Retsch PM-400MA) using ZrO2 balls and vessels. In order to accomplish maximum surface coating, an excess amount of coating agent to be used and hence 1:1 molar ratio was chosen arbitrarily.
Fig. 3. Low-angle XRD patterns for (a) OAChex, (b) SAChex.
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Fig. 4. (i) Showing the thermal decomposition pattern for the (a) uncoated CaCO3 and (b) stearic acid; (ii) showing the decomposition pattern for coated CaCO3 with different capping agents (a) OAChex, (b) SAChex, and (c) PAChex; (iii) showing the decomposition pattern for coated CaCO3 with different capping agents (a) SCCmet and (b) OLCmet; (iv) showing the decomposition pattern for coated CaCO3 with different capping agents (a) ODCmet, (b) TPCmet, and (c) DDCxyl.
The milling time for different ball sizes (1, 0.5 and 0.1 mm) was fixed for 2 hours. The details of the milling process parameters are given in tabular form in Chart 2. After milling, the samples were rinsed and washed several times with suitable solvent so as to remove excess dispersant followed by centrifugation and drying. The samples are described in Chart 3 with six alphabetical letters in which first three refer to capping agent and CaCO3 (C) followed by the last three are milling medium hexane (hex), methanol (met) and xylene (xyl).
analyzing machine (TA Instruments, USA) under a flow of air. Nearly 5–10 mg of the sample were taken in the Platinum pan and heated in air at the heating rate of 10 deg/min up to 900 °C. 2.2.3. Transmission electron microscopy (TEM) analysis TEM analysis was performed to view the morphology and particle size of the milled product. TEM measurement was carried out in a Jeol (JEM 2100) instrument at 200 kV having a LaB6 filament with a point resolution of 0.194 nm and lattice resolution of 0.14 nm. TEM samples were prepared by dispersing the milled sample in heptane and followed
2.2. Characterization 2.2.1. XRD studies XRD studies were carried out in an 18 kW X-ray diffractometer (Rigaku, Japan) having copper rotating anode. XRD patterns were recorded at 50 kV and 250 mA, at a scan rate of 2 deg/min with a step size of 0.01 deg. The XRD patterns were processed and peak search was conducted by search match to find out different phases present in the sample. 2.2.2. Differential thermal studies The thermal stability and phase transitions of the samples as function of temperature were measured using TGA model 2960 thermal
Table 2 The weight loss during the temperature range of 250 °C–450 °C corresponding to decomposition of calcium salt of capping agent for different capping agents. S. no.
Calcium salts of capping agent
Weight % loss of shell (approximate)
1. 2. 3. 4. 5. 6. 7. 8.
Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium
27 52 49 3 6 50 15 14
salt salt salt salt salt salt salt salt
of oleic acid of stearic acid of palmitic acid of salicylic acid of oleylamine of ODSA of TPSA of DDSA
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Fig. 5. The transmission electron microscopy images of CaCO3 particles: (a) uncoated CaCO3, (b) OAChex, (c) SAChex, (d) PAChex, (e) SCCmet, (f) OLCmet, (g) ODCmet, (h) TPCmet, and (i) DDCxyl.
by ultrasonication for 5 min. A drop of the solution was poured on a copper grid placed on a filter paper and kept for drying. 3. Results and discussion The experiments were carried out to investigate the effect of different dispersants on the shape, size and surface characteristics of the CaCO3 after milling treatment. 3.1. X-ray diffraction (XRD) results
Fig. 6. Representation of cubic core–shell CaCO3 nanoparticle.
The XRD patterns in Fig. 2 show that the products obtained after milling with different capping agents were typically calcite in nature despite of their different morphologies and sizes; Fig. 3a and b shows the low angle diffraction of representative samples (SACHex and OACHex). The distinct patterns observed are matching well with calcium stearate and calcium oleate reflections (JCPDS File). In this process CaCO3 powders were first de-agglomerated and then further reduced to smaller particles by grinding actions and at
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Fig. 7. Showing the histogram plots for (a) OAChex, (b) SAChex, (c) PAChex, (d) SCCmet, (e) OLCmet, (f) ODCmet, (g) TPCmet and (h) DDCxyl.
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the same time the newly created surfaces of smaller particles were covered by the agents under ball milling impact [36]. Table 1 comprises the crystallite sizes of the fresh uncoated and milling coated CaCO3 with different capping agents; a decrease in crystallite size was observed after milling for all the samples. The impact of ball milling on the particle size reduction is a well known fact as well as the coating by capping agents reduces the surface energy of calcite and could control the particle-particle interaction which was confirmed by Anshan and Taylor using IGC (inverse gas chromatography) [37]. 3.2. Thermo gravimetric analysis (TGA) results TGA results from uncoated and coated CaCO3 are given in Fig. 4. Fig. 4i (a) shows TGA curve of the uncoated CaCO3 and stearic acid. The uncoated CaCO3 undergoes two decomposition steps: a weight loss of 8.514% below 600 °C and a large weight loss of 42.03% above 600 °C indicating the complete decomposition of CaCO3 to CaO and CO2. TGA curve of stearic acid (Fig. 4i (b)) shows about 95% weight loss below 250 °C indicating the complete thermal decomposition of organics under air atmosphere. Fig. 4 (ii–iv) shows TGA curves for the coated CaCO3 samples; overall there are three weight loss steps. The first step with minimal weight loss was primarily due to the desorption of adsorbed solvents and excess of dispersants occurred below 250 °C. Second step was in the temperature range from 250 °C to 650 °C where the weight loss may be attributed to the thermal decomposition of coating agent. The final third weight loss observed after 650 °C was due to the thermal decomposition of CaCO3 into CaO and CO2. The weight loss of third step was found to be about 15–40% for all of the samples. The percentage loss depends up on the second step weight loss, more the second step less the third step weight loss or less the second step more the third step weight loss. The second step weight loss was found to be an important characteristic of the coated CaCO3. Table 2 indicates clearly that the weight loss was more where efficacious reaction takes place between the calcium carbonate and capping agent. Typical acid–base reaction is expected with fatty acid and succinic anhydride based coating agents. However, in the case of salicylic acid and oleylamine minimal weight loss was observed indicating an ineffective reaction with calcite under milling condition. The reaction of salicylic acid might be hindered due to the presence of intramolecular hydrogen bonding whereas no reaction is expected with oleyl amine an organic base. Thus the reaction of calcite with dispersants except salicylic acid and oleyl amine are given (Eqs. (1) and (2)). 2−
2CaCO3 þ 2RCOOH→2CaOCOR þ 2CO3
ð1Þ
ð2Þ
The thermal stability of the coated CaCO3 was found to be less than 200 °C under air atmosphere. The physical stability of these samples is verified in the oil medium to form stable dispersion which is essential for the lubricant application. All the samples except SCCmet and OLCmet (5–10%) were mixed with group II base oil to give stable partially clear nano dispersion. These dispersions on left undisturbed in the glass cylinders were found to be stable up to a month time. 3.3. Transmission electron microscopy (TEM) results The morphology and particle size of milled samples were characterized using TEM. Fig. 5a shows that original CaCO3 particles are
587
agglomerated and cubic in shape with an average particle size of approximately 200 nm. With the addition of capping agents the CaCO3 powders were de-agglomerated and their shape was little distorted due to breaking down of primary crystals (Fig. 5b, c, d, e, f, g, h, and i). Sample obtained after milling was in a size range of 85 nm to 120 nm. The dispersants have the reactivity of different extent for binding with CaCO3 and results in different efficacy to stabilize the reduced size. From the images it can be inferred that different dispersants gave different impressions to CaCO3 particle shape and sizes. Most of the images show that particles were de-agglomerated and coated by dispersants. It was seen that the maximum rupturing and de-agglomeration of cubes were observed for ODSA capping agent (Fig. 5g). 3.4. Calculation for shell thickness and surface density From the results of TGA and TEM analyses the shell thicknesses was estimated along with the surface density for every dispersant. Two quantities were obtained from TGA: i. The weight of CaO after the CaCO3 decomposition, ii. The amount of capping agent used during the reaction to form shell. The cubic core–shell CaCO3 structure can be represented as shown in Fig. 6, where a and b respectively, are length of the sides of core and shell. The particle size histogram derived from TEM images (Fig. 7) gives the size of cubic particles of maximum distribution. From the length a and b were obtained the volume of the cubic structure for n number of cubic nanoparticles is: 3
ð1Þ
3
ð2Þ
V core ¼ a n V total ¼ b n
The shell structure is made up of CaCO3 and capping agent and if the shell volume is Vshell then it can be obtained from Eqs. (1) and (2) as V core ¼ V total −V shell
ð3Þ
The total volume and shell volumes are calculated from TGA weight loss by using respectively the bulk density of CaCO3 (0.675 g cm−3) and the dry bulk density of the prepared samples. Thus the coating thickness is obtained from length of the sides a and b as follows: t ¼ b−a=2
ð4Þ
If we consider the reaction stoichiometry as 2 moles of capping agent involved per mole of CaCO3 (except in the case of ODSA, DDSA, TPSA and oleyl amine where one mole utilized) then weight
Table 3 Shell thickness and surface density calculated using above calculations with the help of TEM and TGA results. S. no.
Sample
TEM particle size (nm)
Thickness of coating (nm)
Surface density (molecules/nm2) (1013)
1. 2. 3. 4. 5. 6. 7. 8.
OAChex SAChex PAChex SCCmet OLCmet ODCmet TPCmet DDCxyl
102 103 101 102 118 97 95 87
1.85 3.35 3 0 0.35 16.3 1 1.69
2.65 5.04 2.41 0 0.9 4.25 5.7 2.9
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Fig. 8. Showing the TEM images (a) OAChex, (b) OAChex after 10 s of beam exposure and (c) OAChex after 30 s of beam exposure.
(W) of the capping agent in the shell can be obtained from the TGA weight loss. Thus the surface density of the capping agent (the number of capping molecules coated over one cube) is given as follows: W N A =M 6a
2
to carry out this research work; they also express their gratitude for participation and supporting in this work. Deepika is very grateful to Institute for Frontier Materials for supporting her research work under Deakin University Doctoral Program.
ð5Þ
where NA is Avogadro number and M is formula weight of capping agent. Histogram derived from six representative TEM images was recorded at different positions of a grid. From Table 3 we concluded that maximum coating was obtained for ODSA coated CaCO3 i.e. 16.3 nm followed by stearic acid coated CaCO3 gives a value of 3.35 nm while the surface density calculated for both these samples shows different effect due to stoichiometric mole ratios are different for both the capping agents. However, salicylic acid coated sample displays no surface coating as per the calculation done and likewise oleylamine shows a very minute surface coverage having surface density of 0.9 × 10 13 molecules/nm 2 with coating thickness of 0.35 nm only. Similar findings were observed in TGA results also. Below are the TEM images (Fig. 8) for coated oleic acid samples (Fig. 8a) taken at 40,000× resolution showing cuboidal crystals. It was observed that surface of these crystals starts crumpling (Fig. 8b) when the sample was exposed to beams for a time of 10 s. Further, when the exposure time was increased to 30 s it was discovered that the entire surface film has been crumpled (Fig. 8c). Similar observation has also been reported earlier [38]. This observation further confirms the presence of organic coating on the calcium carbonate surface generated by mechano chemical reaction which has been decomposed by the high electron beam energy inside TEM. 4. Conclusion This paper reports the synthesis of hydrophobic core shell type CaCO3 particles via ball milling approach using different capping agents as a surface modifier. Results from this work prove that coating was successfully done that helps in reducing the agglomeration of NPCC to different extent. It has also been demonstrated that organic substrate (capping) alters the crystal morphology and surface properties of calcium carbonate by making it hydrophobic. It was found that different capping agent shows different tendencies to coat the CaCO3 surface and also have different influence on the particle size, morphology and surface hydrophobic property. These coated calcite samples are potential candidate for the over base application in lubricant technology. Acknowledgments The authors wish to acknowledge the financial support of Indian Oil R&D Centre in the form of Indian Oil Golden Jubilee Fellowship
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