Surface analysis of silica gel particles after mechanical dry coating with magnesium stearate

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Surface analysis of silica gel particles after mechanical dry coating with magnesium stearate Laurence Galet, Yamina Ouabbas, Alain Chamayou, Philippe Grosseau, Michel Baron, Gérard Thomas


Introduction
Dry particle coating to change the surface properties of powders is very important in many industries. Typical applications include, but are not limited, to flowability, wettability (hydrophobic/hydrophilic properties), solubility, dispersibility, flavour, shape, electrostatic, optical, electric, magnetic, etc. In dry particle coating processes, materials with relatively large particle size (host particles; 1-500 µm) are mechanically coated with fine particles (guest particles; 0.1-50 µm) in order to create new functionality or to improve their initial characteristics [1]. Since the size of the guest particles are so small, van der Waals interactions are strong enough to keep them firmly attached to the host particles. Thus, either a discrete or continuous coating of guest particles can be achieved depending on a variety of operating conditions including processing time, rotation speed, weight fraction of guest to host particles and particle properties [2]. Figure 1 below is a simple schematic illustrating the process of dry particle coating.
Over the last few years, we have performed experimental investigations of applications of the dry coating technique to study the effect of mechanical dry coating on the surface properties of powders. Our recent work is related to the modification of the flowability and the wettability properties of silica gel particles coated with different weight ratios of magnesium stearate by using a high energy impact coater Hybridizer from Nara Machinery [3] and a high shear mixer Cyclomix from Hosokawa [4]. For example, the flowability of the silica gel powder was significantly decreased after treatment in the Cyclomix mixer with 15% of MgSt 2 . Moreover, it has been found that the coating by hydrophobic MgSt 2 in the Cyclomix can reduce the high affinity between silica gel and water after treatment with 5% and 15% of MgSt. In those papers, it has been demonstrated that a dry particle coating technique can be used to modify the properties of silica gel powder by coating with small quantities of hydrophobic magnesium stearate in both the Hybridizer and the Cyclomix. The more uniform coating has been obtained after treatment in the Hybridizer device. In this paper, we report an investigation of the surface analysis of the coated silica gel particles using atomic force microscopy in tapping mode (TM-AFM) and contact mode (CM-AFM). The TM-AFM method, based on the measurements of the oscillations of a cantilever probe in contact on the sample, has been used for several years to characterize the surface topography of particles. In particular, phase contrast images are related to characterize the attractive and repulsive probe sample interactions [5], the sample surface components [6] and the morphology of composite particles [7]. The CM-AFM method consists in using a colloidal probe fixed on the cantilever to quantify the adhesion force between the particles. As an example, Kani et al report an interesting study on the silicamica adhesion force as a function on the presence of impurities on the silica particles [8].
Meincken et al. report a relationship between contact angle analysis and adhesive forces measurements by the AFM technique in a study of the surface hydrophobicity of polyurethane coatings [9]. This technique has been also used to characterize the adhesion properties of pharmaceutical carriers [10]. In our case, we performed AFM phase-contrast and adhesion forces measurements to characterize the surface of silica particles as a function of the magnesium stearate mass ratio (1 to 30%).  Table 1.

Powders
Table1: Properties of host and guest particles.

Coating process
The hybridization system (HB, type NHS-0; Nara Machinery Co.), a high-speed dry impact blending coater, has been used to make the silica gel-MgSt 2 composite particles. The reactor consists in a very high-speed rotating rotor with six blades, a stator and a powder recirculation circuit. The coating chamber is surrounded with a jacket in which coolant is circulated [1]. The coating process can be summarized as follows: the powder mixture (host and guest particles) is subjected to high impaction and dispersion due to collisions with blades and the walls of the device and continuously re-circulates in the machine through the cycle tube. Particle coating is achieved due to the embedding or filming of the guest particles onto the host particles by high impaction forces and friction heat. Since the rotor of the hybridizer can rotate from 5000 to 16000 rpm, very short processing time is required to achieve coating. The operating conditions used in experiments are 4800 rpm for 5 min. The preparation of the coated particles is described in detail in previous papers [3,4]. Coating experiments have been carried out with 1%, 5%, 15% and 30% of mass fraction of guest MgSt 2 particles.

Particles characterization
The uncoated and coated silica gel particles were examined by environmental scanning electron microscopy (ESEM) to study the surface morphology and the covering by the MgSt 2 particles (XL 30 Philips). The ESEM images reveal that greater MgSt 2 coverage is observed on the surface of silica gel particles as the percentage of MgSt 2 is increased ( Figure 2). Additional investigations were done to analyse the surface atomic composition of the coated particles by X-ray spectroscopy confirming the presence of magnesium atoms when analysis is performed on a black spot [11]. At 15%, the surface coverage of the MgSt 2 is intense but seems discrete. Thermal analysis was used to measure the MgSt 2 mass fraction after treatment (TG-DSC 11 Setaram). Samples are analyzed under nitrogen gas, from 20°C to 600°C, with a ramp of 5°C/min. Figures 3 and 4 show the heat flow, the mass lost and the differential mass lost as a function of the temperature diagram for pure silica gel and MgSt 2 , respectively. We observe that the silica gel particles have a main first mass loss of about 4-5% near 100°C, due to dehydration, and a second near of 2% 400-600°C, probably due to a thermal decomposition. The MgSt 2 has a first small mass loss near 100°C due to dehydration and a main mass loss of 85-90% between 250°C and 450 °C due to a thermal degradation. This mass loss can be used to quantify the MgSt 2 in the powder mixtures.   of surface sample has been scanned: from 250x250 nm 2 to 5x5 mm 2 , for silica gel and silica gel-MgSt 2 coated particles (1%, 5% and 15% MgSt 2 mass ratio).
In contact mode AFM, the tip scans the sample in close contact with the surface. The force between the tip and the surface sample is measured as the distance, by maintaining a constant deflection. To examine the mechanism of interaction between silica gel and MgSt 2 particles, the CM-AFM measurements were performed with a NP silicon nitride tip with a spring constant of 0.32 N/m on which a MgSt 2 particle was glued under optical microscope.
The adhesion forces were measured between the MgSt 2 particles attached on the end of the cantilever and the different samples of uncoated and coated particles with 1% to 30% MgSt 2 mass ratio. For each sample 450 force curves were obtained. The presence of the magnesium stearate particle on the tip was checked after each series of measurements.
Influence of the coating on the surface property of the silica gel particles was also studied.
The sessile drop method was used to study the wettability of the different samples. We report the contact angle value of a water drop (10 µl) deposited on different particulate systems: pure silica gel, pure MgSt 2 and coated particles. The method performed is described in details in previous papers [3,4].

Results and Discussion
The real concentration of the MgSt 2 after coating is calculated from the TG-DSC measurements and the calibration straight line shows in Figure 5. The results are reported in the Figure 6. This loss of MgSt 2 , due to a deposit of MgSt 2 on the coating chamber surface, varies as a function of the introduced MgSt 2 mass ratio. That is from 30% for the sample containing initially 30% of MgSt 2 in the mixture, to 40% for the sample containing initially 5% MgSt 2 in the mixture.  The contact mode analyses bring some pertinent information about the silica gel covering by the magnesium stearate. Figure 8 gives an adhesion force-displacement curve for the pure products. The force measured between a particle of MgSt 2 fixed onto the AFM tip is about 10 nN for the pure silica gel particle. That is near 120 nN for the MgSt 2 -MgSt 2 , revealing a high interaction.  The mean adhesion force calculated from the 450 measurements for each system is reported in the Table 2. The results show an increasing of the mean value of the force as a function of the increasing of the MgSt 2 concentration. The mean adhesion force seems to evolve progressively to the mean value of the pure MgSt 2 (Figure 10). This no linear evolution could be due to a random distribution of the MgSt 2 particles on the silica gel surface.  The affinity of water is evaluated by the behaviour of a water drop deposited on the particulate systems. The initial contact angle values are reported in Table 3. The Figure 11 shows the evolution of the contact angle as a function of the MgSt 2 ratio. We observe a increasing of the contact angle of the water drop as the presence of the MgSt 2 on the silica gel surface. This result is of course in agreement with the hydrophobicity of this compound.
However, even with a low content of MgSt 2 , the contact angle measured is significatively modified: 70° and 110° for 0.1% and 1.1% MgSt 2 respectively. The surface property of the silica gel particles is modified by dry coating even with a very low ratio of MgSt 2 . This result reveal that at the water drop scale, a very few presence of MgSt 2 is enough to modify the hydrophilic property of the silica gel.