(1. Powder Engineering Laboratory of Tsinghua University Department of Materials, Beijing100084; 2. R&D Center for Micro-nano Materials and Resources Utilization, Tsinghua Development Research Institute, Tsinghua University, Langfang 06500 1)
The surface of inorganic mineral filler was coated and modified by chemical method to prepare composite mineral particles with surface nanostructure, which effectively improved the surface morphology of the original particles and increased the specific surface area. Through wet grinding with stirring mill, the bonding mode between coated particles and matrix was discussed. It was preliminarily proved that the bonding mode between coated particles and matrix was chemical adsorption, not physical adsorption, and the bonding between them was firm and the coating was not easy to fall off. When coated mineral particles are filled into PP, the mechanical properties of the composites are greatly improved [1 ~ 15].
Inorganic minerals; Filler; Coating modification; Surface nanocrystallized particles.
Introduction to the first author: Guy Guo Sheng (1958-), male, doctor, associate researcher. E-mail :gaigs@tsinghua.edu.cn.
I. Introduction
Micron-sized ultrafine calcium carbonate and wollastonite are commonly used inorganic mineral fillers in plastics or rubber, and they are in great demand. The plastic industry alone needs calcium carbonate ultrafine powder of100×104t [1] every year. However, calcium carbonate ultrafine powder produced by traditional processing technology has sharp edges and corners, flat crystal cleavage surface and poor compatibility with polymers. Modification with coupling agent or surfactant can not fundamentally solve the inherent morphological defects on the particle surface, but these parts are easy to become weak points in the composite material at the microscopic level, which is one of the reasons for the failure of the composite material [2, 3].
The production cost of nano-calcium carbonate is low and the technology is mature, but the agglomeration is serious and it is difficult to disperse evenly, so it is difficult to reflect the unique properties of nano-particles when filled in polymers [4 ~ 6]. The composite mineral particles prepared by Ca(OH)2-H2O-CO2 system give full play to the respective advantages of micron and nano-particles, and make up for the lack of particle morphology.
Second, methods and steps
640 g of heavy calcium carbonate micropowder (Shandong Hongda Cement Co., Ltd.) with an average particle size of 5.2μm and a concentration of 8%, 760 ml of calcium hydroxide solution and 700 ml of hot water were placed in a reaction kettle, the temperature of the slurry was adjusted to 25 ~ 30℃, and the slurry was fully stirred at a speed of 400 rpm. 30% pure carbon dioxide and 70% air are mixed with slurry circulation flow of 20 ml/s, and the mixture is introduced into the reaction kettle and stirred continuously to fully mix gas-solid-liquid three phases. PB- 10 acidity meter is used to monitor the pH value of pulp on line. When the pH value is 7, the reaction ends and lasts for about 20 min. Stopping aeration, filtering the slurry, and drying the filtered material to obtain a solid material, namely the nano-surface modified composite calcium carbonate filler.
The research shows that composite wollastonite, composite dolomite and composite fly ash with rough surface can also be prepared by reasonably adjusting the operating parameters such as calcium hydroxide concentration, mineral addition, particle size, addition time, CO2 flow rate, stirring intensity and slurry temperature.
The reaction product nano-calcium carbonate will be deposited, nucleated and grown on the surface of inorganic mineral particles according to the heterogeneous nucleation principle, thus realizing nano-surface modification. According to phase transition thermodynamics [7, 8], the more similar the atomic arrangement of nucleation crystals and nuclei is, the smaller the heterogeneous nucleation free energy is compared with the homogeneous nucleation free energy, and the smaller the heterogeneous nucleation free energy is, the more favorable it is for heterogeneous nucleation. After adding inorganic mineral micropowder such as heavy calcium carbonate and wollastonite into the slurry, it can be proved from the thermodynamic point of view that nano-CaCO3 products are easy to nucleate and grow on the surface of these particles, thus achieving the purpose of surface nano-modification.
The test equipment used are: CSM-950 and CJSM-630 1F scanning electron microscope, which are used to observe the particle morphology; NOVA4000 high-speed automatic specific surface area meter is used to measure specific surface area; PHI5300 XPS multifunctional electron spectrometer is used to analyze the elemental composition and chemical state on the surface of solid samples. The bonding strength between coated particles and matrix was tested by a self-made wet mixer. The mechanical properties test spline was prepared by φ30×45 parallel co-rotating twin-screw extruder and 150 ZP injection molding machine.
Three. Results and discussion
(1) surface morphology
In Ca(OH)2-H2O-CO2 system, the author successfully prepared inorganic mineral particles coated with nano-calcium carbonate, and the morphological characteristics of calcium carbonate particles are shown in figure 1.
Figure 1 SEM morphology of heavy calcium carbonate particles before and after surface modification.
(a) raw material heavy calcium carbonate particles; (b) and (c) composite heavy calcium carbonate particles
As can be seen from Figures 1(b) and (c), the coated particles are uniform in size, with a particle size of about 80nm, and the coating rate is high. Compared with before coating, the sharp edges and corners of the particles are passivated and the surface roughness is improved. The flat cleavage plane formed in the grinding process no longer exists, but is replaced by nano-particle coating. By BET measurement, the specific surface area of coated calcium carbonate increased from 0.66 m2 g-1to 2.06 m2 g-1,which increased by more than 2 times. The specific surface area of composite wollastonite particles also increased from1.74m2 g-1to 7.36m2 g-1.
(II) Bonding strength between coating and substrate
1. The surface energy δ E when the daughter particle actually falls off.
In order to further test the bonding strength between the coating and the substrate, the composite heavy calcium carbonate was wet ground in a stirring mill, and the peeling of the coating under the action of ball milling medium was studied.
The experiment adopts a self-made wet stirring mill, which consists of a φ 1 10 mm static grinding cylinder and a multi-blade agitator. Φ1mm zirconia ball is used as grinding medium, and 100 g material and proper amount of water are added. The motor drives the agitator to rotate at a speed of 355 rpm through the speed change device. The grinding medium and material move in multi-dimensional circular rotation, resulting in violent displacement up and down, left and right, and the material is subjected to friction, impact, shear and other effects [2]. The morphological changes of composite calcium carbonate powder after grinding for 30 minutes, 45 minutes and 60 minutes are shown in Figure 2-(a), (b) and (c).
Fig. 2 SEM morphology of composite calcium carbonate powder after grinding
(a) 30 minutes; 45 minutes; 60 minutes
As can be seen from Figure 2, after grinding for 30 min, the surface is still covered with nanoparticles, and there is almost no change. A small amount of coated particles fell off at 45 minutes; After grinding for 60 minutes, the coating all fell off and obvious dents were visible. In a stirred mill, the kinetic energy EiB per unit volume of the grinding ball can be expressed by the following formula [9]:
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Where (1): d is the diameter of the stirring mill, 0.11m; DR is the diameter of agitator, 0.09 m; ; Zeta is a constant, 0.0082; U is the circumferential speed, 0.836 m/s; ρB is the density of the grinding ball, 63 10 kg/m3. The energy absorbed by particles in the effective area can be derived from the kinetic energy EVB per unit volume of the grinding ball:
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In formula (2), VB is the volume of the grinding ball, 7.924×10-5m3; -5 m3； VB is the effective area volume,1.161×10-4m3; ρM is the relative density of particles, 2710 kg/m3; εM is the porosity of abrasive particles in the state of natural accumulation, which can be ignored. Assuming that the particles are uniformly distributed in the effective area and the particle size is uniform, the average absorbed energy of a single particle can be calculated from EM:
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In formula (3), m is the mass of particles within the effective area, 0.1kg; ; Da is the average particle size of the treated material, 5.36×10-6m; -6m； N 1 is the number of particles in the effective area, 2.625×1010; Then em = 8.46× 10- 13j.
As can be seen from Figure 2, after grinding for 45 min, the coated particles begin to fall off, and the absorption energy E of a single particle at this time is
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In formula (4), t is the ball milling time, 2700s.
According to the grinding theory of particles, 5% ~ 25% of the energy absorbed during particle crushing is converted into a new surface energy δ E [10 ~ 14]. If calculated by 5%, the new surface energy δ e =1.14×10-10j when the composite particles begin to fall off. That is to say, only when the surface energy reaches Δ e, the particles coated on the surface begin to fall off.
2. Predict the surface energy Δ e' when the coated particles fall off.
It is assumed that the parent particles of heavy calcium carbonate are cubic, the surface coating is a single layer coating, and the sub-particles in the coating are all spherical particles with the same diameter. The change value of particle surface area before and after shedding can be expressed as δ s (m2):
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In formulas (5), (6) and (7), S 1 is the surface area of particles before the coating falls off, m2; S2 is the total surface area of the daughter particles and the mother particles after the coating falls off, m2; Dc is the particle size of the parent particle, 5.2×10-6m; D is the daughter particle diameter, 8×10-8m; N2 is the number of daughter particles.
When the daughter particle is completely separated from the surface of the parent particle, the increase of surface energy δ e ′ should be
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In formula (8), γc is the surface energy of calcium carbonate, 0.08 j/m2 [1 1], and δ e' = 3.894×10-1j can be obtained. That is to say, when the surface energy of particles increases to Δ e', the daughter particles can fall off the surface of the parent particles.
From the calculation, it can be known that the δ e of the daughter particle is greater than δe', so it is inferred that the combination between the daughter particle and the parent particle should be chemical adsorption rather than physical adsorption, that is, the daughter particle and the parent particle are integrated. The same experiment was carried out on composite wollastonite powder, and the results were consistent
(3) XPS analysis
In order to further analyze the surface characteristics of coated particles, the raw wollastonite and composite wollastonite were analyzed by X-ray photoelectron spectroscopy (XPS). Experimental conditions: 600 g wollastonite powder, with an average particle size of 4.89μm, was provided by Beijing Guoli Ultrafine Powder Company. The concentration of calcium hydroxide solution was 6%, 850 mL, the slurry temperature was 30℃, the rotation speed was 400 r/min, and the slurry circulation rate was 20 ml/s. The surface of wollastonite particles before and after coating contained four elements, namely Ca, Si, C and O. The changes of their element content and binding energy were listed in Table/kloc respectively.
By analyzing the table 1, it can be found that the content of Ca element increases obviously after the surface of wollastonite particles is nano-modified. The ratio of calcium to silicon also increased obviously, from about 1∶ 1 of raw materials to 2∶ 1 after coating.
Table 1 surface element content of wollastonite particles (wB/%)
Note: the sample taken after 10 min is 1 #, and the sample at the end of the reaction is 2 #.
Table 2 Binding Energy (eV) of Surface Elements of Wollastonite Particles
Analysis of Table 2 shows that the peaks of C, Ca, Si and O all have certain chemical shifts. The peak position of element C on the surface of raw wollastonite is 284.8, which should be polluted carbon, and its surface itself has no carbon bond. In the process of surface nanocrystallization of wollastonite particles, the binding energy of calcium decreases. In the initial stage, Ca element is mainly in the chemical environment of > > SiO3. Due to the large electronegativity of Si element, the electron concentration around Ca atom is low, which weakens the shielding effect on its internal electrons, and the internal electron binding energy of Ca atom is large. With the progress of the reaction, nano-calcium carbonate is continuously deposited on the surface of wollastonite particles, that is, the chemical environment around Ca atoms on the surface gradually changes from > > SiO3 to > > CO3. However, the electronegativity of element C is smaller than that of element Si, so the electron density around Ca atom will increase, and the shielding effect on its internal electrons will be enhanced, thus reducing the internal electron binding energy of Ca atom, indicating that its XPS peak will be reduced. After the reaction, the surface of wollastonite is gradually covered with nano-calcium carbonate, and the binding energy of Ca element is consistent with that of pure calcium carbonate. Combined with XRD phase analysis [15], it can be inferred that the coated particles on the surface of wollastonite particles should be nano-calcium carbonate.
(4) Filling
Polypropylene was filled with uncoated and coated heavy calcium carbonate, and modified with stearic acid before filling. According to GB 1040-92, injection molding was carried out by twin-screw extruder and injection molding machine, frozen in liquid nitrogen atmosphere, rapidly impacted, and the fracture surface was sprayed with gold, and the fracture morphology was observed by SEM, as shown in Figure 3.
Figure 3 shows that uncoated calcium carbonate is directly filled in PP, and the interface between its particles and PP matrix is loose, and obvious gullies and cracks can be seen, as shown in Figure 3-(a). However, the interface between coated calcium carbonate particles and PP matrix is closely combined and has good compatibility, as shown in Figure 3-(b). This is because the rough surface and passivated edges and corners of composite particles increase the chance of contact with PP matrix and improve the interface bonding performance.
Fig. 3 SEM morphology of fracture surface of polypropylene-based composites
(a) fil uncoated heavy calcium carbonate particles; (b) fil coated heavy calcium carbonate particle
Four. conclusion
1) In Ca(OH)2-H2O-CO2 system, the surface morphology of inorganic mineral particles can be effectively improved by using heterogeneous nucleation principle, and the surface is rough, and the specific surface area is increased by more than 2 times.
2) The coated particles are firmly combined with the coated particles through chemical adsorption and are not easy to fall off.
3) The coating powder is used as filler, which improves the interfacial bonding performance of PP composites.
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Coating and Characterization of Inorganic Mineral Fillers with Nanoparticles
Gai Guosheng 1, Yang Yufen 2, 1, Hao Xiangyang 1, Fan Shimin 1, Cai Zhenfang 1
(1. R&D Group of Powder Technology, Department of Materials Science and Engineering, Tsinghua University, Beijing100084; 2. R &D Center for Micro/Nano Materials and Resource Utilization, Tsinghua University Research Institute, Jinyuan Road, Langfang Economic Development Zone, Hebei Province, China, 06500 1)
The composite mineral particles with nano-structure on the surface were successfully prepared by chemical method, which effectively improved the surface morphology of the original material and increased the specific surface area. The coalescence between coating particles and substrate was studied by wet grinding with stirring mill. The preliminary conclusion shows that the coated particles are not easy to peel off from the substrate due to chemical adsorption. When coated mineral particles were filled into polypropylene, the mechanical properties of the composites were greatly improved.
Key words: inorganic minerals, fillers, coatings, surface nanostructured particles.