Controlled synthesis, characterization and application of hydrophobic calcium carbonate nanoparticles in PVC

    Controlled synthesis, characterization and application of hydrophobic calcium carbonate nanoparticles in PVC Lina Zhao, Yaodan Zhang, Yang Miao, Lijun Nie PII: DOI: Reference:

S0032-5910(15)30144-3 doi: 10.1016/j.powtec.2015.11.001 PTEC 11322

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

21 October 2014 30 October 2015 1 November 2015

Please cite this article as: Lina Zhao, Yaodan Zhang, Yang Miao, Lijun Nie, Controlled synthesis, characterization and application of hydrophobic calcium carbonate nanoparticles in PVC, Powder Technology (2015), doi: 10.1016/j.powtec.2015.11.001

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ACCEPTED MANUSCRIPT Controlled synthesis, characterization and

nanoparticles in PVC Yang Miao1, 2

Lijun Nie1, 2

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Lina Zhao1, 2 Yaodan Zhang1, 2

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application of hydrophobic calcium carbonate

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（1. College of Chemistry, Jilin Normal University, Siping 136000, China; 2. Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Jilin Normal University, Ministry of Education, Siping 136000, China）

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Abstract: Modification of calcium carbonate particles with surfactant significantly

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improves the properties of polyvinyl chloride (PVC). Hydrophobic spherical CaCO3 particles were prepared in the presence of dodecyl dihydrogen phosphate (DDP) by a

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carbonation method. In the process, DDP was used as an organic substrate to induce

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the nucleation and growth of calcium carbonate. The operating parameters such as

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temperature and the dosage of the organic substrate were varied to study their influences on the active ratio and contact angle of calcium carbonate particles. The

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surface property of CaCO3 particles was changed from hydrophilic to hydrophobic when the dosage of DDP to CaCO3 changed from 0 to 2 %. The contact angle of modified CaCO3 particles was 119.51º. Moreover, X-ray diffraction (XRD), FT-IR spectrums and thermogravimetry analysis (TGA) were employed to characterize the obtained products. The schematic illustration for interaction between the organic substrate and inorganic mineral in aqueous medium was showed in the end. The synthesized CaCO3 were applied to polyvinyl chloride (PVC) to improve its mechanical properties. The properties of PVC/CaCO3 composites have been tested

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Corresponding author. Tel. +86-13944405698. E-mail: [email protected]. 1

ACCEPTED MANUSCRIPT such as tensile strength, impact strength, young modulus and SEM. Keywords: Hydrophobic; Calcium carbonate; DDP; Application; PVC

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1. Introduction

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It is well known that calcium carbonate (CaCO3) is a natural occurring and abundant mineral comprising approximately 4% of the earth crust [1-4]. CaCO3 was used in many fields as a filler

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such as catalysis, electronics, medicine, pigments, rubbers, paper and so on [5-8]. The controlled chemical synthesis of inorganic materials with specific size, morphology and structure has

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attracted considerable attention because of the potential for design new materials and devices in various fields [9-11]. The key to the successful synthesis of an inorganic-organic composite is an

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understanding of the parameters that control the nucleation and growth of inorganic crystals under the organic substrate in aqueous solution [12-15]. In recent years, a considerable of attention and

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effort have been devoted to investigate the calcium carbonate particles [16-20]. A promising approach is to use organic additives with complex functionalization patterns to control the

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nucleation, growth and alignment of inorganic materials through electrostatic matching and structured and stereochemical complementarity at inorganic / organic interface [21, 22]. Synthesis of CaCO3 is followed by two basic synthetic routes: (1) the solution route, through a double decomposition reaction, wherein aqueous CaCl2 and Na2CO3, or CaCl2 and (NH4)2CO3, or Ca(NO3)2 and Na2CO3 are combined in an equal molar ratio; and (2) the carbonation method, in which CO2 gas is bubbled through an aqueous slurry of Ca(OH)2. The carbonation method is preferred in terms of environment preservation and the effective use of mineral resources. The carbonation method is also an industrially useful method, but it is difficult for CaCO3 to control the crystal shape and modification [23, 24].

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ACCEPTED MANUSCRIPT In this work, hydrophobic spherical aragonite CaCO3 nanoparticles were obtained in the presence of DDP. We attempted to study the crystallization of CaCO3 particles. The

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DDP-modified and the unmodified CaCO3 were applied in PVC to improve the tensile strength

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and impact strength of PVC. It was advanced to use DDP which was made in the lab because the cost of the method was low in the industry.

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2. Experimental 2.1 Materials

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Dodecyl dihydrogen phosphate (DDP) was made by ourselves. Calcium oxide was of analytical reagent grade and made by Menghe Jiangsu Reagent Chemical Factory (China). They were

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analytical grade in this experiment. The distilled water used was made by us throughout the study. 2.2 Preparation of the hydrophobic spherical CaCO3 particles

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The procedure employed for the preparation of CaCO3 was as follows. 10 g calcium oxide (CaO) was digested in 100 mL distilled water to form a calcium hydroxide (Ca(OH)2) slurry, which was

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kept for 12 h at least. The slurry was filtered through a 150 mesh sieve in order to get rid of the impurity and large particles. The ethanol solution of DDP was added into the slurry. The mixture was stirred vigorously for 1 h at 40 °C. Then the premixed CO2 and N2 with molar ratio of 3:10 were bubbled into the slurry through a column. The reaction should be stopped as soon as the pH value of the suspension decreased to 7.0. The obtained CaCO3 precipitates were rinsed firstly with anhydrous ethanol and hot distilled water three times, respectively. At last, the product was dried for at least 24 h in an oven at approximate 80 °C. 2.3 Preparation of DDP The cost of production in the industrialization of CaCO3 was an important parameter. The cost

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ACCEPTED MANUSCRIPT of dodecyl dihydrogen phosphate (DDP) made by researchers was low. P2O5 was added into H3PO4 solution (the H3PO4 concentration is about 85%) under constant stirring at 50 °C. The

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solution was stirred for 1 h, then octadecyl alcohol (C18H37OH) was introduced into the system.

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Meanwhile N2 was bubbled into the suspension in order that C18H37OH was not oxidized and the vapor was removed. After improving the temperature to 80 °C, the esterification reaction was maintained for 4 h, following extra P2O5 was put into the system in batches as complement to

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increase the yield of octadecyl dihydrogen phosphate (C18H37OPO3H2). The resulting suspension

2.4 Characterization of the samples

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was used as organic substrate.

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Micrographs of the spherical CaCO3 particles were taken by a field emission scanning electron

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microscope (FESEM). It was carried out using a JEOL-JSM6700F at 15 kV. The crystalline phase

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and structure of the synthesized CaCO3 were detected by X-ray diffraction (XRD) analysis with a Shimadju D/max-rA X-ray diffractometer with Cu Kα radiation, under the accelerating voltage 40

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kV, current 30 mA, and scanning rate 4° min−1. The contact angle was measured at 25 °C with a FTÅ200 (USA) contact angle analyzer. On flat solid surfaces, the contact angle could be measured by the sessile drop technique. The powdery samples (1 g) in 10 MPa pressure were first pressed into thin pellet and then the water droplet was dropped onto the pellet. The contact angle against water on a horizontal surface of a pellet was obtained. Fourier transform infrared spectroscopy (FT-IR) was performed on an Ominic System 2000 using KBr pellets. The thermogravimetry analysis (TGA) was performed on Melttler Toledo 825e instrument at 50-900 °C with a program heating rate of 10 °C min-1 in a flowing air atmosphere.

3. Results and discussion 4

ACCEPTED MANUSCRIPT 3.1 Effect of DDP on the morphologies and size of CaCO3 crystals FESEM images which showed the effect of DDP on the morphology of CaCO3 were presented

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in Fig.1. The effect of DDP on the morphology of CaCO3 was significant. Rhombus CaCO3

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particles consisted of the plate-shaped crystals were obtained in the absence of DDP (Fig. 1(a)). However, it was evident that the mixture of the spherical and cubic CaCO3 crystals was obtained when the amount of DDP added reached 1 % (Fig. 1(b)). When the amount of DDP added was

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2 %, the morphology of CaCO3 particles changed greatly. The pure spherical CaCO3 particles with

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slick surfaces were observed as depicted in Fig. 1(c). The mean diameter of the spherical particles was about 40 nm. It was smaller than that in the Fig. 1(b). However, the mixture of cubic and

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spherical CaCO3 particles was observed in Fig. 1(d) when the amount of DDP reached to 3 %.

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during the reaction.

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Therefore it was important for the morphology of CaCO3 particles to control the amount of DDP

3.2 Effect of temperature on the morphology and size of CaCO3

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Fig. 2 showed FESEM micrographs of CaCO3 particles obtained at different reaction temperature. The effect of temperature on the morphology of CaCO3 was significant. The cubic CaCO3 particles were synthesized in the presence of DDP at 20 °C in Fig. 2(a). The mean size of the cube CaCO3 particles were about 40 nm. However, the spherical CaCO3 particles with smooth surface were obtained at 40 °C. The mean size of particles was about 30 nm (Fig. 2(b)). The shape was changed sharply at 60 °C in the Fig. 2(c). The spherical CaCO3 particles were decreased clearly and the cubic particles were increased. However, the spherical particles disappeared and only the congregations of irregular particles were observed at 80 °C in the Fig. 2(d).

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ACCEPTED MANUSCRIPT The reaction temperature intensively affects the crystallization behaviors of the calcium carbonate polymorphs. High temperature is favorable to the nucleation of aragonite crystal in

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super saturation solution. Aragonite is usually achieved at a higher temperature than 50 ºC using

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a solution method of preparation [25]. In this work, we concluded that aragonite particles could be produced when the reaction temperature was higher than 40 ºC. Although the fraction of aragonite in the resulting precipitates was gradually improved with increasing the reaction

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temperature, the products were all the same of a mixture of aragonite and calcite. Actually, at the

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initial carbonation stage, aragonite crystals and calcite crystals nucleate simultaneously and competitively, therefore, harvesting an absolutely pure aragonite phase is impossible in this

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system.

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3.3 XRD patterns of the obtained CaCO3 particles

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Fig. 3 was the XRD patterns of CaCO3 obtained at different temperatures in the presence of DDP in the system. It found that there was most of calcite in the precipitates when the reaction

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temperature was 20 °C (Fig. 3(a)). XRD of the washed precipitates gave distinct peaks that were characteristic of the (012), (104), (110), (202) and (116) planes, consistent with pure calcite crystals. These peaks were centered at 2θ=23.04°, 29.40°, 36.00°, 43.16° and 48.50°, respectively. When increasing the reaction temperature to 40 °C, 60 °C and 80°C (Fig. 3(b)-(d)), the typical peaks of aragonite phase were strengthened such as (111), (012) and (221) planes. Furthermore, with increasing the reaction temperature, the fraction of aragonite in the resulting precipitates was gradually improved. The products were of a mixture of aragonite and calcite. Furthermore, with increasing the reaction temperature, the fraction of aragonite in the resulting precipitates was gradually improved.

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ACCEPTED MANUSCRIPT Especially, the (104) planes was weakened obviously. The polymorphic ratio of aragonite and calcite in precipitated CaCO3 was calculated by the following equation derived from a calibration

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curve, which was obtained from crystal mixtures with known ratios of polymorphs: y=1−1/ (1+3.9

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Ia / Ic), where y was the calculated weight fraction of aragonite, and Ia and Ic were the integrated intensity of the XRD spectra characteristic of the aragonite (111) peak and the calcite (104) peak, respectively[26]. The calculated result showed that the fraction of aragonite was 0.503, 0.699,

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0.757 and 0.787 when the temperature was changed form 20 °C to 80 °C.

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Aragonite is thermodynamically metastable phase. Without any additives, it is well-known that aragonite completely transforms into the most thermodynamically stable calcite in the aqueous

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solution[27-29]. Increasing the reaction temperature cannot enable the aragonite phase to become

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thermodynamically stable phase in the absence of DDP. It means that higher temperature is

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obviously favorable to the nucleation of aragonite crystal, but not the crucial factor of stabilizing it. We speculate that the aragonite crystals in the resulting precipitates were stabilized by DDP to

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prevent phase transformation.

3.4 Effect of the mass ratio of DDP on contact angle Fig. 4 was showed the effect of the mass ratio of DDP to CaCO3 on contact angle. The interaction characteristics of the obtained CaCO3 particles surfaces were measured by the contact angle. Fig. 4 (a) indicated that with the increasing of the mass ratio of DDP to CaCO3, the contact angle of CaCO3 particles increased gradually. When the mass ratio was 2 %, the contact angle was 119.51°. It proved that when the mass ratio of DDP to CaCO3 was higher than 2 %, the CaCO3 particles had hydrophobic property. The conclusion was obtained that the optimum molar ratio of DDP to CaCO3 is 2 % at 40 °C by the information provided in Fig. 1. Therefore, it could be drawn

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ACCEPTED MANUSCRIPT the conclusion that the obtained CaCO3 particles were of good hydrophobic property. The contact angles of the pellets after drying at 100°C for 24 h were not changed, indicating that no volatile

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compounds are present in the samples.

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3.5 FT-IR characterization of spherical CaCO3

Fig. 5 showed FT-IR spectra of the CaCO3 particles nucleated in the absence (Fig. 5(a)) and in the presence of DDP (Fig. 5(b)). The spectra confirmed the formation of calcite with characteristic

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absorption bands observed at 716 cm–1, 872 cm–1 and 1502 cm–1, corresponding to the ν4, ν2 and ν3

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CO32– absorption bands of calcite [30]. Weak bands were also observed at 2979 cm–1and 2855 cm–1 which were associated with asymmetric and symmetric methyl and ethylene C–H stretches,

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respectively. This suggestted that surfactant molecules remained strongly associated with the

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CaCO3 crystals even after extensive washing.

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3.6 TGA characterization of spherical CaCO3 The TGA curves for CaCO3 particles which were obtained in the absence and in the presence of

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DDP were indicated in Fig.6. It could be obviously seen that a larger weight loss occurred in the temperature range from 200 ˚C to 620 ˚C, which was attributed to the thermal decomposition of organic substrates in the CaCO3 powders. We could observe that the weight loss of the products from the curve b was about 5.1 wt. %. Another stage was from 620 ˚C to 755 ˚C, where the largest mass loss was observed. This could be assigned to the thermal decomposition of calcite (CaCO3 → CaO + CO2  ). 3.7 Monitoring of the pH value during experiment In this work, the change of the pH value was monitored during experiment by the pH meter. Fig. 7 indicated the changes in pH value with bubbling time for 13.0 wt.% Ca(OH)2 at a 1.5 L/min CO2

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ACCEPTED MANUSCRIPT gas flow rate in the absence and in the presence of DDP. The pH of all the suspensions starts from a high basic value of about 12.5. In the absence of DDP, the high basic pH was maintained for

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about 90 min before dramatically dropping to a value of 7. However, the high basic pH value

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dramatically dropped to 7 after 45 min in the presence of DDP. A number of researchers have observed this typical pH behavior which exhibits two distinct regions: a constant rate of absorption period and a falling rate period. During the constant rate period, the aqueous phase was

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always kept saturated with OH− ions and the most of the Ca(OH)2 conversion to carbonate was

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completed during this period. The effect of DDP increased the retention time and contact area of CO2 in the solution thus the higher carbonation efficiency was observed. From the results, we

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could conclude that the presence of DDP accelerates the period of absorption and shortens the

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time of carbonation. It was potential useful in industry.

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3.8 Reaction mechanism Analysis

In the work, the DDP solution was added to the Ca(OH)2 slurry using a mechanical stirrer. Fig.

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8 showed the schematic illustration for interaction between the organic substrate and inorganic mineral in aqueous medium. This reaction consisted of three stages. First, interfacial recognition in the interface of organic /inorganic: Ca2+ ions of Ca(OH)2 reacted with DDP to form a hydrophobic salt of C12H25OPO3Ca. Then crystal nucleation and growth: CO2 gas was introduced into the stirred mixture at 40 °C to synthesize CaCO3. The hydrophobic properties of the final CaCO3 particles were attributed to the deposition of the resulted C12H25OPO3Ca onto the surface of CaCO3. Final, mineralization arrest and self-assembly: nucleation of crystals was induced by the absorption of organic substrates on the CaCO3 particles surface. 3.9 Applied properties of the modified calcium carbonate

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ACCEPTED MANUSCRIPT 3.9.1 Tensile strength and impact strength of PVC/CaCO3 composites The equipments were used such as high speed blunger, twin-screw extruder, universal sample

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testing machine and charpy impact tester in the test. The test was performed according to Table 1.

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The mechanical properties were the important parameters of the materials. When CaCO3 modified by DDP as a filler was used in PVC, the mechanical properties of the products would be enhanced such as tensile strength and impact strength. Fig.9 showed that the effect of unmodified

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and modified CaCO3 on the tensile property in PVC. The results indicated that the tensile strength

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was increased first and then decreased in the picture when CaCO3 particles were synthesized in the absence of organic substrate (Fig.9a) and in the presence of DDP (Fig.9b). When the CaCO3

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loading was 5%, the maximum tensile strength was 18.75 MPa. However, the unmodified-CaCO3

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of tensile strength was 14.89 MPa only. The highest measured PVC properties were observed for

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CaCO3 with DDP-modified nanoparticles, where an increase in tensile strength (+26%). In the meantime, the result was observed that the elongation at break was improved by 89%, indicating

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the high toughness.

Previous work had shown that with addition of the substance could be obtained [31-33]. The effect of unmodified and modified CaCO3 on the impact property in PVC was shown in Fig.10. The results indicated that the impact strength was increased first and then decreased in the picture when CaCO3 particles were synthesized in the absence of organic substrate (Fig.10a) and in the presence of DDP (Fig.10b). When the CaCO3 loading is 5%, the maximum impact strength was 53.1kJ/m2. However, the unmodified-CaCO3 of impact strength was 15.2kJ/m2 only. The highest measured PVC properties were observed for CaCO3 with DDP-modified nanoparticles, where an increase in tensile strength (+249%). From the above discussion, the conclusion was drawn that

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ACCEPTED MANUSCRIPT the properties of DDP-modified CaCO3 particles was superior to the unmodified-CaCO3 particles in PVC.

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3.9.2 Young modulus values of PVC/CaCO3 composites

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In our work, the impact strength was increased first and then decreased in absence and in the presence of DDP. It could be connected with the rigidity of filler and the strong interaction between the filler and polymer matrix. At the same time, the young modulus values were

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calculated in the test. The literature on the effect of surface treatment on young modulus was

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contradictory. In some cases an increase was observed [34], occasionally no changes were reported [35], or even a decrease in modulus was claimed after treatment [36]. According to our

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work the influence of the surface treatment on young modulus is not obvious. As shown in Fig. 11,

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the young modulus values of unmodified-CaCO3 and DDP-modified CaCO3 composites increased

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simultaneously with the increase of CaCO3 content independent of the filler dispersion state. 3.9.3 SEM micrographs of PVC/CaCO3 composites

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The size and morphology of modified CaCO3 particles play an important role on the property of PVC. At the same time the quality of PVC have an important effect on the test result. The SEM micrographs of the tensile-fractured surfaces of the composites were shown in Fig. 12. The figure of pure PVC was seen in Fig. 12(a). As seen in Fig. 12(b), some cavities could be found on the fracture surface of the composites when the fraction of CaCO3 was 5. It indicated that the CaCO3 particles and PVC were combined well. However, few cavities were observed on the fracture surface of PVC/CaCO3 composites, as shown in Fig. 12(c) and Fig. 12(d). The presence of CaCO3 particles must be responsible for the cavitations. These cavitations could absorb large amounts of energy, leading to the improvement of the composites’ tensile strength. Chan also found

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ACCEPTED MANUSCRIPT cavitations on the fracture surface of PP/CaCO3 composites [37]. This indicates that the toughening effect of CaCO3 on pure PVC was restricted to the mechanical coupling between the

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filler and polymer matrix, and no cavitations are involved.

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4. Conclusions

In conclusion, the spherical CaCO3 particles could be synthesized by a simple carbonation

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method in the presence of DDP as a modifier. The optimal reaction temperature was 40°C and the amount of DDP was 2%. It could come to conclusion that DDP plays an important role of control

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on the nucleation, growth and alignment of CaCO3 particles in the reaction process. The properties of DDP-modified CaCO3 particles were superior to the unmodified-CaCO3 particles in PVC. The

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Acknowledgement

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obtained spherical hydrophobic CaCO3 particles could find applications in industrial field.

This research was financed by the National Natural Science Foundation of China (51404108)

China.

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and Scientific Research Innovation Plan for Young Talented Person of Jilin Normal University,

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China Particuology. 4 (2006)25–30. [34] K. Okuno, R.T. Woodhams, Mica reinforced polypropylene, Polym. Eng. Sci. 15(1975)308–315. [35] V.P. Juskey, C.E. Chaffey, J. Canadian, Rheology and tensile properties of polypropylene reinforced with glycerol-treated mica, Chem. Eng. 1982, 60, 334–341. [36] A.S. Kenyon, H.J. Duffey, Polym. Properties of a particulate-filled polymer, Eng. Sci. 7(1967)189–196. [37] C.M. Chan, J. Wu, J.X. Li, Y.K. Cheng, Polypropylene/calcium carbonate nanocomposites, Polymer. 43(2002)2981–2992.

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Fig. 1 FESEM images of the as-prepared CaCO3 particles in the presence of DDP: (a) 0, (b) 1 %, (c) 2 % and (d)

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3 %.

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Fig. 2 FESEM of CaCO3 particles obtained at the different temperature in the presence of DDP: (a) 20 °C; (b)

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40 °C; (c) 60 °C; (d) 80°C.

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(c)

aa

a

a

aa

a

a

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aa 25

30

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(b)

(a)

c202 a221 c116

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a111 c104 a012 c110 a112 c012 a021

aragonite calcite

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(d)

a c

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2 Theta (degree)

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Fig. 3 XRD patterns of CaCO3 particles obtained at the different temperature in the presence of DDP: (a) 20 °C; (b)

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40 °C; (c) 60 °C; (d) 80 °C.

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(a)

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Contact angle ( )

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Mass ratio of DDP to CaCO3(%)

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Fig. 4 Effect of the mass ratio DDP to CaCO3 on contact angle (a) and the measurement of CaCO3 prepared in the presence of DDP (2%) (b).

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b

872

2979 2855

Transmittance

a

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2500

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1500

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Wavenumber (cm )

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Fig. 5 FT-IR spectra of obtained CaCO3 particles in the absence of DDP (a) and in the presence of DDP(b).

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Wight loss (%)

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a b

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Temperature ( C)

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Fig. 6 TGA cures of obtained CaCO3 particles in the absence of DDP (a) and in the presence of DDP(b).

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pH

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b 7 0

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a

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The reaction time (min)

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Fig. 7 Change of pH VS carbonation time (a) in the absence of DDP; (b) in the presence of DDP.

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a: Interfacial recognition in the interface of organic/inorganic; b: Crystal nucleation and growth; c: Mineralization arrest and self-assembly. Fig.8 Schematic illustration for interaction between the organic substrate and inorganic mineral in aqueous

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medium.

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260

a

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Tensile strength (MPa)

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Tensile strength The elongation at break

160 159

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The elongation at break (%)

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The elongation at break (%)

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18.7

b

240

Tensile strength The elongation at break

Tensile strength (MPa)

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The CaCO3 loading (%)

The CaCO3 loading (%)

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Fig. 9 Effect of the CaCO3 loading on tensile strength and the elongation at break in PVC (a:

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unmodified-CaCO3(SD=1.257, n=4); b: DDP-modified CaCO3(SD=1.334, n=4)).

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Impact strength ( kJ/m

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Fig. 10 Effect of the CaCO3 loading on impact strength in PVC (SD=0.35, n=4) (a: unmodified-CaCO3; b:

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DDP-modified CaCO3).

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5.0

PVC/ unmodified CaCO3 PVC/DDP-modified CaCO3

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3.5

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Young Modulus(GPa)

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3.0

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2.0 5

10 15 20 The content of CaCO3(phr)

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0

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Fig.11 Young modulus values of PVC/CaCO3 composites (SD=0.2, n=4).

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Fig.12 SEM figures of PVC/CaCO3 composites (a) pure PVC; (b) PVC/CaCO3 (5 phr); (c) PVC/CaCO3 (10 phr);

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(d) PVC/CaCO3 (30 phr).

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The type of material

The name of material

The dosage of material/Phr

Polyvinyl chloride

PVC

100

Heat stabilizer

Calcium stearate

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Table 1 The design of formula

Lubricant

Paraffin wax

Impact modifier

CPE

Processing agent

ACR

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1.0

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2.0

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Graphical Abstract

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2. The contact angle of hydrophobic CaCO3 particles was 119.51º.

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1. Hydrophobic spherical CaCO3 particles were prepared via a DDP inducing process.

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3. The polymorphs were characterized with XRD, FT-IR and TGA.

4. The schematic illustration of the reaction was discussed in the end.

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strength, young modulus and SEM.

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5. The properties of PVC/CaCO3 composites have been tested such as tensile strength, impact

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