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Preparation of a UV-attenuating agent using a mechanochemical reaction
Journal of Materials Processing Technology 142 (2003) 131–138
Preparation of a UV-attenuating agent using a mechanochemical reaction Tomohiro Iwasaki∗ , Munetake Satoh, Shunji Ichio Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho Sakai, Osaka 599-8531, Japan Received 26 June 2002; received in revised form 2 December 2002; accepted 26 February 2003
Abstract This paper describes a mechanical preparation method of an ultraviolet (UV)-attenuating agent with a high transparency for visible light using a high-speed elliptical-rotor-type powder mixer (HEM). Ultra-fine zinc oxide particles are used to attenuate UV radiation. To stabilize homogeneous dispersion of zinc oxide particles in a suspension, they were uniformly immobilized on the surface of silica particles by a mechanochemical reaction between zinc oxide and silica, and the composite particles were incorporated into a polymer medium. The optimum processing time in the immobilization was determined by means of a proposed model considering both the activation energy of the mechanochemical reaction and the mechanical energy applied to the particles in the minimum clearance region of the HEM. The relative change in the number of hydroxyl groups on the surface of the silica particles confirmed that a processing time based on the number of zinc oxide particles was suitable for immobilization. The suspension in which the composite particles were dispersed showed more effective UV attenuation performance and had a higher transparency for visible light in comparison with a suspension containing only a mixture in which the zinc oxide and silica particles were simply blended. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Powder processing; Particulate composite; Surface modification; Particle coating; Mechanochemical reaction; Ultraviolet attenuation
1. Introduction Fine particles of oxides of metal (e.g. titanium, zinc, zirconium, iron) are extensively used as agents to attenuate (absorb and/or scatter) ultraviolet (UV) radiation (wavelength 290–400 nm). Various UV-attenuating formulations in which metal oxide particles are incorporated into a liquid or polymer medium, such as sunscreens, paints, anti-UV additives for plastics and skin care cosmetics, are manufactured. Such formulations are required to have multiformity for coloring; high transparency for visible light is desirable in a particle-dispersed suspension before being colored. To increase the transparency and to effectively attenuate the UV radiation with a lower concentration of metal oxide particles, nanometer-sized primary single particles with a lower refractive index must be homogeneously dispersed in a medium [1–3]. However, dispersion of ultra-fine particles is very difficult without any reagent because of a strong interactive force among the particles [2,4]. Dispersion of composite particles, in which fine metal oxide particles are uniformly dispersed and immobilized on the surface of coarser parti-
∗ Corresponding author. Tel.: +81-722-54-9307; fax: +81-722-54-9911. E-mail address: [email protected] (T. Iwasaki).
cles (base material) without agglomeration, into a medium is an effective technique to solve such problems [5,6]. In this study, we attempted to prepare composite particles as a UV-attenuating agent with a high transparency for visible light using a high-speed elliptical-rotor-type powder mixer (HEM) [6–12]. In order to stabilize homogeneous dispersion of single metal oxide particles in a medium, the metal oxide (zinc oxide (ZnO) [1–4,13,14]) guest particles are immobilized mechanically on the surface of host particles (spherical silica (SiO2 ) particles) using a mechanochemical reaction between the guest–host particles. Depending on the number of ZnO particles to be compounded, the optimum processing time in the immobilization process is determined on the basis of the magnitude of both the mechanical energy [10,15] applied to the particles during the process and the activation energy of the mechanochemical reaction. The mechanical energy is estimated by means of a proposed model [9] in which the powder behavior in a high energy zone of the HEM is expressed using two types of phenomena: the powder sliding flow on the vessel wall after plastic deformation and the dynamic powder compression. The UV attenuation performance of a transparent polymer media in which composite particles are dispersed and their transparency for visible light are evaluated on the basis of spectrophotometry.
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00594-6
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2. Theory In the HEM, strong compressive and shear forces are repeatedly applied to the particles in the minimum clearance region, where the rotor tip most approaches the vessel wall, every time a special condition is satisfied, i.e. the particles pass through the minimum clearance region and the rotor’s major axis is superimposed on the vessel’s minor axis [9,11]. In particular, relatively high shear energy is applied to the particles when the vessel’s major axis is horizontal [9] and contributes effectively to the surface modification of particles [16]. In this model, we assume that the shear energy applies on the following two shear planes: (1) the contact surface with the rotor and (2) the sliding plane generated in the powder layer compressed to a maximum bulk density. In the following section, the model for estimating the shear energy is outlined [9], and the optimum processing time in the immobilization process is determined, depending on the number of ZnO particles to be compounded. 2.1. Normal and shear stresses in powder layer Fig. 1 illustrates the powder layer in the minimum clearance region, consisting of ZnO and SiO2 particles. The powder layer is deformed by the motion of the rotor rotating at a constant speed of NR . The vessel’s rotational motion is ignored because the rotational speed of the vessel is quite slow in comparison with that of the rotor. The vessel’s major axis is fixed horizontally in the model calculation. hB represents the thickness of the powder layer before it is compressed by the rotor. dC indicates the minimum clearance. The powder is in contact with the rotor in the region of x1 ≤ x ≤ x2 at an arbitrary inclination angle of the rotor. The powder
Fig. 2. Stresses acting on a powder layer in the plastic equilibrium state.
layer within this region is divided into several infinitesimal sections. The inside wall of the vessel is assumed to be flat within this region. To simplify the model equations, the powder layer is dealt with as a uniform continuous body in the analysis. 2.1.1. Plastic deformation and sliding of the powder layer Fig. 2 shows the normal and shear stresses in an infinitesimal section. The rotor surface is considered to be flat within each infinitesimal section. ψ is the acute angle between the rotor surface and x-axis. dx and hx indicate the width and height of the infinitesimal section, respectively. σ x and τ xy are the horizontal and shear stresses acting on the plane of hx , respectively. σ p is the normal stress that the rotor applies to the powder layer. τ R and τ V are the shear stresses acting on the powder-to-rotor and powder-to-vessel contact surfaces, respectively. From the force balances in the direction of the x-axis, the following equation is obtained: d (hx σx ) = τR + τV − σx tan ψ dx
(1)
It is assumed that the intensity and direction of σ p are constants within the infinitesimal section regardless of y. Fig. 3 illustrates the Mohr’s stress circles expressing the relationship between the normal stress σ and the shear stress τ in
Fig. 1. Powder layer compression by the rotor in the minimum clearance region.
Fig. 3. Relationships between normal and shear stresses in a powder layer in the plastic equilibrium state.
T. Iwasaki et al. / Journal of Materials Processing Technology 142 (2003) 131–138
the powder layer in the plastic equilibrium state [17]. The stress circles Ci , CR and CV in this figure indicate the σ–τ relationships in the middle of the infinitesimal section, on the rotor surface and on the inside wall of the vessel, respectively. ϕw is the angle of the wall friction of the powder layer. A simple formula is used as the yield locus (YL): τ = (σ + K) tan ϕ
(2)
where ϕ is the angle of the internal friction of the powder layer. K is the apparent tensile stress and is determined experimentally as a function of the bulk density ρ of the powder layer [18]: ρ K = a1 exp a2 1 − (3) ρt
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where fe = 21 ρ(2πNR r)2
(10)
and r is the distance between the center of the rotor shaft and the rotor surface at a position. β indicates the angle between the directions of σ p and fe . As the boundary condition of Eq. (1), the horizontal stress σ x0 at the position where the rotor contacts the powder layer for the first time is determined as follows: on the assumption that the major principal stress is equal to the average of the normal stress, ρ0 ghB /2, in the powder layer before being compressed, the σ x0 required to cause the plastic deformation is estimated using the following equation: σx0 =
ρ0 ghB 2k
(11)
where ρt is the true density of the powder, and a1 and a2 the constants that depend on the physical properties of the powder. ρ is expressed by Kawakita’s equation [19]:
where g is the gravity acceleration and k [= (1 − sin ϕ)/(1 + sin ϕ)] is the Rankine constant.
b 1 b2 σ y ρ − ρ0 = ρ 1 + b 2 σy
2.1.2. Dynamic compression of the powder layer When the powder layer is compressed by the rotor without sliding, the direction of the major principal stress σ s in the middle of the infinitesimal section coincides with that of the y-axis. The powder layer after compression is assumed to be in a plastic equilibrium state. Fig. 4 shows the σ–τ relationship in the powder layer (Ci ) and that on the rotor surface (CR ). σ s is calculated by substituting the bulk density of the compressed powder layer into Eq. (4). On the assumption that the intensity and direction of σ s are constant regardless of y, the normal stress σ c and the shear stress τ c acting on the rotor surface are determined by using the relationships shown in Fig. 4. By adding fe to σ c and τ c , the actual normal and shear stresses, σ cd and τ cd , in the dynamic compression are expressed as follows:
(4)
where ρ0 indicates the loose bulk density, σ y the normal stress shown in Fig. 3 and both b1 and b2 are constants depending on the physical properties of the powder. If ρ attains a maximum bulk density that is determined experimentally, the sliding plane is considered to be generated in the powder layer and then the powder layer is broken (slides and flows). The following equations are valid from Fig. 3: 2 (σx − σ0 )2 + τxy = (σ1 − σ0 )2
(5)
τxy = (σ1 − σ0 ) sin 2α
(6)
σ 1 indicates the major principal stress. σ 0 is given as follows: σ0 =
σ1 − K sin ϕ 1 + sin ϕ
(7)
Assuming that σ 1 coincides with σ p in the middle of the infinitesimal section where the effect of the rotor and the vessel wall on the stresses is small, α is expressed by the following equation: α=
π +ψ 2
(8)
σ x , τ R and τ V are expressed as functions of σ p by using Eqs. (2)–(8) and Fig. 3. σ p is determined by substituting these functions into Eq. (1). σ p and τ R are the stresses in a static state. In order to express the dynamic compressing and shearing phenomena, these stresses are corrected by a factor relating to the dynamic force, i.e. the stress fe originating in the kinetic energy that the rotor gives to the powder. The corrected normal and shear stresses, σ pd and τ Rd , are expressed by the following equations, respectively: σpd = σp + fe cos β,
τRd = τR + fe sin β
(9)
σcd = σc + fe cos β,
τcd = τc + fe sin β
(12)
Comparison between σ pd and σ cd at each infinitesimal section determines the behavior of the powder layer at the position. For example, in an infinitesimal section where σpd < σcd , the powder layer undergoes plastic deformation and then slides before the intensity of the normal stress attains the intensity of the stress required for the dynamic compression. The normal and shear stresses actually acting on the rotor surface can be determined in the range of x1 ≤ x ≤ x2 according to this procedure.
Fig. 4. σ–τ relationships in a powder layer compressed dynamically.
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where ρtZ is the true density of ZnO particles. Eq. (14) is applicable in the range where the ZnO particles form a mono-particle layer on the surface of SiO2 particles, i.e. when CZ is less than the mass ratio CZ,max corresponding to adhesion in a hexagonal close packing state: 2π [1 + (D/d)]2 CZ,max = √ 3 1 + (D/d)3 (ρtS /ρtZ )
The average number NS of silica particles arranged on the shear planes of a unit area is expressed as a function of the bulk density ρbS of each infinitesimal section in the powder layer: 2/3 ρbS NS = (16) (π/6)D3 ρtS
Fig. 5. ZnO particles arranged on the shear plane.
2.2. Determination of processing time When relatively high shear energy is applied to the ZnO–SiO2 powder mixture, the energy is stored inside the particles. This changes their crystallographic, physical and chemical properties. Therefore, immobilization of ZnO particles on the surface of SiO2 particles is assumed to occur by forming hydrophobic groups (–O–) between the hydroxyl groups, i.e. by releasing water molecules. The activation energy of this elementary reaction (OH + OH → O + H2 O) is 4.57 kJ mol−1 [20]. The energy required to cause the reaction can be regarded as the minimum energy for the compounding of ZnO and SiO2 particles. On the assumption that a single ZnO particle is immobilized by the dehydration reaction between a pair of the hydroxyl groups that each ZnO and SiO2 particle has, the energy E required for immobilizing a single ZnO particle is determined to be 7.60 × 10−21 J. Accordingly, the number of ZnO particles immobilized during a single compression by the rotor can be estimated by using the shear energy determined from the model calculation of shear energy and the number of ZnO particles presenting on the shear planes (rotor surface and sliding plane). Assuming that the ZnO particles are uniformly dispersed and adhere to the surface of SiO2 particles before the shear energy is applied, the number of ZnO particles per unit area NZ to which the shear energy applies is approximately expressed by the following equation (see Fig. 5): 2 −2 π −1 D − d −1 κd NZ ≈ √ cos sin NS D+d D+d 2 3
(15)
where ρtS is the true density of SiO2 particles. As illustrated in Fig. 6, the distance δs of shear and the displacement δc of compression are calculated geometrically based on the movement of the rotor surface. By using these parameters and the shear stresses τ R and τ i , determined from the model calculation, the shear energy Es applied on the shear planes per unit area can be estimated: on the contact surface with the rotor τR δ s Es = δc on the sliding plane τi sin ϕ (17) The shear energy EZ applied to a single ZnO particle can be determined by using Eqs. (13), (14), (16) and (17): EZ =
Es NZ
(18)
When EZ exceeds the activation energy E of the mechanochemical reaction, the ZnO particles included within the infinitesimal section are immobilized on the SiO2 particles. By calculating EZ for all the infinitesimal sections, the number nZ of immobilized ZnO particles in a single
(13)
where D and d are the mean diameters of SiO2 and ZnO particles, respectively. κ is the distance between the centers of adhering ZnO particles [21] and is given as a function of the mass ratio CZ of ZnO particles to the composite particles: κ=
√
2π 3CZ [1 + (D/d)3 (ρtS /ρtZ )]
1/2 1+
D d
(14)
Fig. 6. Movement of the rotor surface during shear and compression processes of the powder layer in an infinitesimal section.
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compression by the rotor can be determined. Consequently, the minimum processing time tm required to immobilize all the ZnO particles of total mass m is given by the following equation: m tm = λ (19) (π/6)d 3 ρtZ nZ where λ is the time interval of a single compression [9,11] defined as follows: λ = [2(NR + NV )]−1
(20)
where NR and NV are the rotational speeds of rotor and vessel, respectively.
3. Experimental 3.1. Powders Fine ZnO particles (Sakai Chemical Ind. Co., Ltd., FINEX-50) of nanometer size and spherical SiO2 particles (Asahi Glass Company, SUNSPHERE NP-100) were used as a UV-attenuating ingredient (guest particles) and a base material (host particles), respectively. ZnO particles attenuate the UV radiation more effectively not only in the UVB (290–320 nm) but also in the UVA (320–400 nm) region and have a lower refractive index (about 1.9) than other metal oxides that have been used as the agent. On the other hand, SiO2 particles have several advantageous characteristics (low chemical activity, high transparency for visible light, low sliding friction, etc.) for use in cosmetics and paints. Fig. 7 shows SEM photographs of these powders. The ZnO particles formed agglomerates of about 1–5 m in diameter due to the cohesive force among the particles. Table 1 gives their representative physical properties. The internal and wall friction angles ϕ and ϕw were measured using a friction tester with an inclination plate (Shinto Scientific Co., Ltd., HEIDON-10). The constants b1 and b2 in Eq. (4) were determined experimentally from the static compression test for a SiO2 powder bed using a cylindrical vessel and a piston of 20 mm diameter made of stainless steel as follows: b1 = 0.384 and b2 = 1.75 × 10−4 m2 N−1 . 3.2. Immobilization of ZnO particles on SiO2 particles Fig. 8 illustrates the structure and dimensions of the HEM used in this study. The ratio of the minor axis to the major axis of the elliptical rotor is the same as that of elliptical
Fig. 7. SEM photographs of silica and zinc oxide particles.
barrel vessel. The rotor and the vessel rotate co-axially in opposite directions. The minimum clearance dC between the rotor tip and vessel wall is 0.5 mm. The powder mixture of SiO2 and ZnO that was placed in the vessel occupied 20% of the effective vessel volume (i.e. the difference between the vessel capacity and the rotor volume). The rotational speeds of rotor and vessel were kept at 40 and 0.5 s−1 , respectively. Under these conditions, the powder mixture circulates without stagnation in the vessel and is repeatedly fed to the minimum clearance region [9,11]. The thickness hB of the powder mixture layer before compression was determined as hB = 2.5 mm through the observation of SiO2 powder flow in the vessel. The mass ratio CZ of ZnO particles was varied in the range of 0 to 1.96 × 10−2 (=CZ,max ). Depending on the CZ , the optimum (minimum) processing time tm was determined from the model calculation of Eq. (19) under the same conditions as those in the experiments.
Table 1 Physical properties of silica and zinc oxide particles Powder
Median diameter (m)
True density (kg m−3 )
Bulk density (kg m−3 )
Internal friction angle (◦ )
Wall friction angle (◦ )
Silica Zinc oxide
9.3 0.02
2280 5600
817 200
27.0 –
19.8 –
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4. Results and discussion 4.1. Mechanochemical reaction between ZnO and SiO2 particles
Fig. 8. Schematic diagram of the HEM: 1, vessel; 2, rotor; 3, minimum clearance; 4, shaft; 5, powder.
3.3. Evaluation of ZnO-immobilized SiO2 particles 3.3.1. Relative change in the number of hydroxyl groups A Fourier transform infrared spectrometer (JASCO Corp., FT/IR-410) was used to investigate the relative change in the number of hydroxyl groups on the surface of SiO2 particles due to the mechanochemical reaction [22,23]. The infrared absorption spectra of samples were measured in the wavenumber region of 600–4000 cm−1 according to the diffuse reflection method. Before measurement of the absorption spectra, the samples were diluted with a potassium bromide powder; the concentration of the SiO2 particles was kept at 2.0% by weight in the mixture. The extent of compounding of ZnO–SiO2 particles throughout the immobilization process was assessed by morphological observation with a SEM and ultrasonic separation of ZnO and SiO2 particles in a water bath (Nihonseiki Kaisha Ltd., US-150T). 3.3.2. UV attenuation effect and transparency of visible light For evaluation of the optical characteristics, ZnO–SiO2 particles were mixed with a petrolatum medium (Tatsumi Pharmaceutical Ind. Co., Ltd.) using a laboratory muller (Toyo Seiki Seisaku-sho, Ltd.) at room temperature. The mass of ZnO–SiO2 particles in the mixed paste was 20% by weight. The paste was placed between a pair of quartz glass plates with a clearance of 28 m, as illustrated in Fig. 9. The transmittance in the wavelength range of 250–800 nm was measured with a spectrophotometer (JASCO Corp., Ubest V-550DS).
Fig. 9. Schematic illustration of the test cell: 1, quartz glass plate; 2, polypropylene film (spacer); 3, sample.
Fig. 10 shows the IR-absorption spectra of the ZnO–SiO2 particles of CZ = 0.011 (corresponding to immobilization in an ideal random packing state) together with that of ZnO–SiO2 particles hand-mixed with a spoon for 10 min. In the wavenumber range of 1000–1150 cm−1 among the ranges of absorption indicating the presence of silanol groups (i.e. 820–880, 1000–1150 and 3650–3690 cm−1 ), change in absorption with processing time was clearly observed. The absorption of the ZnO–SiO2 particles processed for 10 min was smaller than that of the hand-mixed particles processed for the same length of time. The absorbance decreased as the processing progressed. These results demonstrate that the mechanical energy of the HEM created a chemical bond between the ZnO and SiO2 particles and that the number of silanol groups decreased with an increase in the number of immobilized ZnO particles. Therefore, absorbance A1100 at 1100 cm−1 was noted as a representative value of absorbance within the range of 1000–1150 cm−1 . Fig. 11 shows the change in A1100 with the processing time t in the case of CZ = 0.011. A1100 decreased with t and remained constant over the optimum processing time tm = 86.0 min determined from the model calculation. It was confirmed that the immobilization of all the ZnO particles was completed at t = tm . Fig. 12 shows a SEM photograph of the surface of a ZnO–SiO2 particle processed for tm = 86.0 min. The ZnO particles were dispersed uniformly on the surface of SiO2 particles and hardly extricated from the SiO2 particles after 5 min soaking in an ultrasonic water bath, proving that the ZnO particles were immobilized on the surface of SiO2 particles by the mechanochemical reaction between them.
Fig. 10. IR-absorption spectra of ZnO–SiO2 particles.
T. Iwasaki et al. / Journal of Materials Processing Technology 142 (2003) 131–138
Fig. 11. Change in absorbance A1100 at 1100 cm−1 with processing time.
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Fig. 14. Transmission characteristics of suspensions containing ZnO–SiO2 particles.
agreed fairly well with that obtained at tm . These results prove that the proposed model is useful for the determination of the processing time. 4.2. Transmittance of a suspension containing ZnO and SiO2 particles
Fig. 12. Surface morphology of a ZnO–SiO2 particle processed for 86.0 min.
Absorbance A1100 of ZnO–SiO2 particles of various mass ratios CZ is shown in Fig. 13 together with A1100 in the case of the constant processing time (120 min) regardless of CZ . In the case of the larger CZ , more silanol groups were used for immobilization, and the number of silanol groups remaining after the processing decreased with an increase in CZ . A1100 of the excessively processed ZnO–SiO2 particles
Fig. 14 shows the change in the transmittance ratio T/Tb with the wavelength of incident light, where T is the transmittance of the paste mixed with the ZnO–SiO2 particles of CZ = 0.011 obtained under various processing times and Tb is that of petrolatum (i.e. the blank). In this figure, the result for the just blended paste with SiO2 particles of 19.8% by weight and ZnO particles of 0.2% by weight is shown together. The UV radiation was difficult for the SiO2 particles to attenuate in the range of 300–400 nm, whereas the transparency in the visible light region (400–800 nm) was relatively high. The UV attenuation of the ZnO–SiO2 particles prepared by blending was small due to inhomogeneous dispersion of the ZnO particles in the medium. On the contrary, in the ZnO-immobilized SiO2 particles prepared with the HEM, the UV attenuation was maintained, and the transparency for visible light was improved with the processing time, i.e. the progress of the dispersion of the ZnO particles. The transmittance of the ZnO–SiO2 particles processed for tm = 87.0 min almost agreed with those excessively processed (120 min). This result also demonstrates that determination of the optimum processing time using the proposed model is valid.
5. Conclusions
Fig. 13. Change in absorbance A1100 at 1100 cm−1 with mass ratio CZ of ZnO particles.
To prepare a UV-attenuating formulation with a high transparency for visible light, a compounding process in which ultra-fine ZnO guest particles are immobilized on the surface of SiO2 host particles using a mechanochemical reaction was proposed. The optimum operating conditions of the HEM were determined from the model calculation
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based on the minimum energy required for the immobilization. Under these conditions, uniform dispersion and immobilization of the ZnO particles were achieved, and a suspension including the composite particles had excellent UV-attenuating performance and transparency for visible light.
[8] [9] [10] [11] [12] [13] [14]
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Preparation of a UV-attenuating agent using a mechanochemical reaction Tomohiro Iwasaki∗ , Munetake Satoh, Shunji Ichio Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho Sakai, Osaka 599-8531, Japan Received 26 June 2002; received in revised form 2 December 2002; accepted 26 February 2003
Abstract This paper describes a mechanical preparation method of an ultraviolet (UV)-attenuating agent with a high transparency for visible light using a high-speed elliptical-rotor-type powder mixer (HEM). Ultra-fine zinc oxide particles are used to attenuate UV radiation. To stabilize homogeneous dispersion of zinc oxide particles in a suspension, they were uniformly immobilized on the surface of silica particles by a mechanochemical reaction between zinc oxide and silica, and the composite particles were incorporated into a polymer medium. The optimum processing time in the immobilization was determined by means of a proposed model considering both the activation energy of the mechanochemical reaction and the mechanical energy applied to the particles in the minimum clearance region of the HEM. The relative change in the number of hydroxyl groups on the surface of the silica particles confirmed that a processing time based on the number of zinc oxide particles was suitable for immobilization. The suspension in which the composite particles were dispersed showed more effective UV attenuation performance and had a higher transparency for visible light in comparison with a suspension containing only a mixture in which the zinc oxide and silica particles were simply blended. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Powder processing; Particulate composite; Surface modification; Particle coating; Mechanochemical reaction; Ultraviolet attenuation
1. Introduction Fine particles of oxides of metal (e.g. titanium, zinc, zirconium, iron) are extensively used as agents to attenuate (absorb and/or scatter) ultraviolet (UV) radiation (wavelength 290–400 nm). Various UV-attenuating formulations in which metal oxide particles are incorporated into a liquid or polymer medium, such as sunscreens, paints, anti-UV additives for plastics and skin care cosmetics, are manufactured. Such formulations are required to have multiformity for coloring; high transparency for visible light is desirable in a particle-dispersed suspension before being colored. To increase the transparency and to effectively attenuate the UV radiation with a lower concentration of metal oxide particles, nanometer-sized primary single particles with a lower refractive index must be homogeneously dispersed in a medium [1–3]. However, dispersion of ultra-fine particles is very difficult without any reagent because of a strong interactive force among the particles [2,4]. Dispersion of composite particles, in which fine metal oxide particles are uniformly dispersed and immobilized on the surface of coarser parti-
∗ Corresponding author. Tel.: +81-722-54-9307; fax: +81-722-54-9911. E-mail address: [email protected] (T. Iwasaki).
cles (base material) without agglomeration, into a medium is an effective technique to solve such problems [5,6]. In this study, we attempted to prepare composite particles as a UV-attenuating agent with a high transparency for visible light using a high-speed elliptical-rotor-type powder mixer (HEM) [6–12]. In order to stabilize homogeneous dispersion of single metal oxide particles in a medium, the metal oxide (zinc oxide (ZnO) [1–4,13,14]) guest particles are immobilized mechanically on the surface of host particles (spherical silica (SiO2 ) particles) using a mechanochemical reaction between the guest–host particles. Depending on the number of ZnO particles to be compounded, the optimum processing time in the immobilization process is determined on the basis of the magnitude of both the mechanical energy [10,15] applied to the particles during the process and the activation energy of the mechanochemical reaction. The mechanical energy is estimated by means of a proposed model [9] in which the powder behavior in a high energy zone of the HEM is expressed using two types of phenomena: the powder sliding flow on the vessel wall after plastic deformation and the dynamic powder compression. The UV attenuation performance of a transparent polymer media in which composite particles are dispersed and their transparency for visible light are evaluated on the basis of spectrophotometry.
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2. Theory In the HEM, strong compressive and shear forces are repeatedly applied to the particles in the minimum clearance region, where the rotor tip most approaches the vessel wall, every time a special condition is satisfied, i.e. the particles pass through the minimum clearance region and the rotor’s major axis is superimposed on the vessel’s minor axis [9,11]. In particular, relatively high shear energy is applied to the particles when the vessel’s major axis is horizontal [9] and contributes effectively to the surface modification of particles [16]. In this model, we assume that the shear energy applies on the following two shear planes: (1) the contact surface with the rotor and (2) the sliding plane generated in the powder layer compressed to a maximum bulk density. In the following section, the model for estimating the shear energy is outlined [9], and the optimum processing time in the immobilization process is determined, depending on the number of ZnO particles to be compounded. 2.1. Normal and shear stresses in powder layer Fig. 1 illustrates the powder layer in the minimum clearance region, consisting of ZnO and SiO2 particles. The powder layer is deformed by the motion of the rotor rotating at a constant speed of NR . The vessel’s rotational motion is ignored because the rotational speed of the vessel is quite slow in comparison with that of the rotor. The vessel’s major axis is fixed horizontally in the model calculation. hB represents the thickness of the powder layer before it is compressed by the rotor. dC indicates the minimum clearance. The powder is in contact with the rotor in the region of x1 ≤ x ≤ x2 at an arbitrary inclination angle of the rotor. The powder
Fig. 2. Stresses acting on a powder layer in the plastic equilibrium state.
layer within this region is divided into several infinitesimal sections. The inside wall of the vessel is assumed to be flat within this region. To simplify the model equations, the powder layer is dealt with as a uniform continuous body in the analysis. 2.1.1. Plastic deformation and sliding of the powder layer Fig. 2 shows the normal and shear stresses in an infinitesimal section. The rotor surface is considered to be flat within each infinitesimal section. ψ is the acute angle between the rotor surface and x-axis. dx and hx indicate the width and height of the infinitesimal section, respectively. σ x and τ xy are the horizontal and shear stresses acting on the plane of hx , respectively. σ p is the normal stress that the rotor applies to the powder layer. τ R and τ V are the shear stresses acting on the powder-to-rotor and powder-to-vessel contact surfaces, respectively. From the force balances in the direction of the x-axis, the following equation is obtained: d (hx σx ) = τR + τV − σx tan ψ dx
(1)
It is assumed that the intensity and direction of σ p are constants within the infinitesimal section regardless of y. Fig. 3 illustrates the Mohr’s stress circles expressing the relationship between the normal stress σ and the shear stress τ in
Fig. 1. Powder layer compression by the rotor in the minimum clearance region.
Fig. 3. Relationships between normal and shear stresses in a powder layer in the plastic equilibrium state.
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the powder layer in the plastic equilibrium state [17]. The stress circles Ci , CR and CV in this figure indicate the σ–τ relationships in the middle of the infinitesimal section, on the rotor surface and on the inside wall of the vessel, respectively. ϕw is the angle of the wall friction of the powder layer. A simple formula is used as the yield locus (YL): τ = (σ + K) tan ϕ
(2)
where ϕ is the angle of the internal friction of the powder layer. K is the apparent tensile stress and is determined experimentally as a function of the bulk density ρ of the powder layer [18]: ρ K = a1 exp a2 1 − (3) ρt
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where fe = 21 ρ(2πNR r)2
(10)
and r is the distance between the center of the rotor shaft and the rotor surface at a position. β indicates the angle between the directions of σ p and fe . As the boundary condition of Eq. (1), the horizontal stress σ x0 at the position where the rotor contacts the powder layer for the first time is determined as follows: on the assumption that the major principal stress is equal to the average of the normal stress, ρ0 ghB /2, in the powder layer before being compressed, the σ x0 required to cause the plastic deformation is estimated using the following equation: σx0 =
ρ0 ghB 2k
(11)
where ρt is the true density of the powder, and a1 and a2 the constants that depend on the physical properties of the powder. ρ is expressed by Kawakita’s equation [19]:
where g is the gravity acceleration and k [= (1 − sin ϕ)/(1 + sin ϕ)] is the Rankine constant.
b 1 b2 σ y ρ − ρ0 = ρ 1 + b 2 σy
2.1.2. Dynamic compression of the powder layer When the powder layer is compressed by the rotor without sliding, the direction of the major principal stress σ s in the middle of the infinitesimal section coincides with that of the y-axis. The powder layer after compression is assumed to be in a plastic equilibrium state. Fig. 4 shows the σ–τ relationship in the powder layer (Ci ) and that on the rotor surface (CR ). σ s is calculated by substituting the bulk density of the compressed powder layer into Eq. (4). On the assumption that the intensity and direction of σ s are constant regardless of y, the normal stress σ c and the shear stress τ c acting on the rotor surface are determined by using the relationships shown in Fig. 4. By adding fe to σ c and τ c , the actual normal and shear stresses, σ cd and τ cd , in the dynamic compression are expressed as follows:
(4)
where ρ0 indicates the loose bulk density, σ y the normal stress shown in Fig. 3 and both b1 and b2 are constants depending on the physical properties of the powder. If ρ attains a maximum bulk density that is determined experimentally, the sliding plane is considered to be generated in the powder layer and then the powder layer is broken (slides and flows). The following equations are valid from Fig. 3: 2 (σx − σ0 )2 + τxy = (σ1 − σ0 )2
(5)
τxy = (σ1 − σ0 ) sin 2α
(6)
σ 1 indicates the major principal stress. σ 0 is given as follows: σ0 =
σ1 − K sin ϕ 1 + sin ϕ
(7)
Assuming that σ 1 coincides with σ p in the middle of the infinitesimal section where the effect of the rotor and the vessel wall on the stresses is small, α is expressed by the following equation: α=
π +ψ 2
(8)
σ x , τ R and τ V are expressed as functions of σ p by using Eqs. (2)–(8) and Fig. 3. σ p is determined by substituting these functions into Eq. (1). σ p and τ R are the stresses in a static state. In order to express the dynamic compressing and shearing phenomena, these stresses are corrected by a factor relating to the dynamic force, i.e. the stress fe originating in the kinetic energy that the rotor gives to the powder. The corrected normal and shear stresses, σ pd and τ Rd , are expressed by the following equations, respectively: σpd = σp + fe cos β,
τRd = τR + fe sin β
(9)
σcd = σc + fe cos β,
τcd = τc + fe sin β
(12)
Comparison between σ pd and σ cd at each infinitesimal section determines the behavior of the powder layer at the position. For example, in an infinitesimal section where σpd < σcd , the powder layer undergoes plastic deformation and then slides before the intensity of the normal stress attains the intensity of the stress required for the dynamic compression. The normal and shear stresses actually acting on the rotor surface can be determined in the range of x1 ≤ x ≤ x2 according to this procedure.
Fig. 4. σ–τ relationships in a powder layer compressed dynamically.
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where ρtZ is the true density of ZnO particles. Eq. (14) is applicable in the range where the ZnO particles form a mono-particle layer on the surface of SiO2 particles, i.e. when CZ is less than the mass ratio CZ,max corresponding to adhesion in a hexagonal close packing state: 2π [1 + (D/d)]2 CZ,max = √ 3 1 + (D/d)3 (ρtS /ρtZ )
The average number NS of silica particles arranged on the shear planes of a unit area is expressed as a function of the bulk density ρbS of each infinitesimal section in the powder layer: 2/3 ρbS NS = (16) (π/6)D3 ρtS
Fig. 5. ZnO particles arranged on the shear plane.
2.2. Determination of processing time When relatively high shear energy is applied to the ZnO–SiO2 powder mixture, the energy is stored inside the particles. This changes their crystallographic, physical and chemical properties. Therefore, immobilization of ZnO particles on the surface of SiO2 particles is assumed to occur by forming hydrophobic groups (–O–) between the hydroxyl groups, i.e. by releasing water molecules. The activation energy of this elementary reaction (OH + OH → O + H2 O) is 4.57 kJ mol−1 [20]. The energy required to cause the reaction can be regarded as the minimum energy for the compounding of ZnO and SiO2 particles. On the assumption that a single ZnO particle is immobilized by the dehydration reaction between a pair of the hydroxyl groups that each ZnO and SiO2 particle has, the energy E required for immobilizing a single ZnO particle is determined to be 7.60 × 10−21 J. Accordingly, the number of ZnO particles immobilized during a single compression by the rotor can be estimated by using the shear energy determined from the model calculation of shear energy and the number of ZnO particles presenting on the shear planes (rotor surface and sliding plane). Assuming that the ZnO particles are uniformly dispersed and adhere to the surface of SiO2 particles before the shear energy is applied, the number of ZnO particles per unit area NZ to which the shear energy applies is approximately expressed by the following equation (see Fig. 5): 2 −2 π −1 D − d −1 κd NZ ≈ √ cos sin NS D+d D+d 2 3
(15)
where ρtS is the true density of SiO2 particles. As illustrated in Fig. 6, the distance δs of shear and the displacement δc of compression are calculated geometrically based on the movement of the rotor surface. By using these parameters and the shear stresses τ R and τ i , determined from the model calculation, the shear energy Es applied on the shear planes per unit area can be estimated: on the contact surface with the rotor τR δ s Es = δc on the sliding plane τi sin ϕ (17) The shear energy EZ applied to a single ZnO particle can be determined by using Eqs. (13), (14), (16) and (17): EZ =
Es NZ
(18)
When EZ exceeds the activation energy E of the mechanochemical reaction, the ZnO particles included within the infinitesimal section are immobilized on the SiO2 particles. By calculating EZ for all the infinitesimal sections, the number nZ of immobilized ZnO particles in a single
(13)
where D and d are the mean diameters of SiO2 and ZnO particles, respectively. κ is the distance between the centers of adhering ZnO particles [21] and is given as a function of the mass ratio CZ of ZnO particles to the composite particles: κ=
√
2π 3CZ [1 + (D/d)3 (ρtS /ρtZ )]
1/2 1+
D d
(14)
Fig. 6. Movement of the rotor surface during shear and compression processes of the powder layer in an infinitesimal section.
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compression by the rotor can be determined. Consequently, the minimum processing time tm required to immobilize all the ZnO particles of total mass m is given by the following equation: m tm = λ (19) (π/6)d 3 ρtZ nZ where λ is the time interval of a single compression [9,11] defined as follows: λ = [2(NR + NV )]−1
(20)
where NR and NV are the rotational speeds of rotor and vessel, respectively.
3. Experimental 3.1. Powders Fine ZnO particles (Sakai Chemical Ind. Co., Ltd., FINEX-50) of nanometer size and spherical SiO2 particles (Asahi Glass Company, SUNSPHERE NP-100) were used as a UV-attenuating ingredient (guest particles) and a base material (host particles), respectively. ZnO particles attenuate the UV radiation more effectively not only in the UVB (290–320 nm) but also in the UVA (320–400 nm) region and have a lower refractive index (about 1.9) than other metal oxides that have been used as the agent. On the other hand, SiO2 particles have several advantageous characteristics (low chemical activity, high transparency for visible light, low sliding friction, etc.) for use in cosmetics and paints. Fig. 7 shows SEM photographs of these powders. The ZnO particles formed agglomerates of about 1–5 m in diameter due to the cohesive force among the particles. Table 1 gives their representative physical properties. The internal and wall friction angles ϕ and ϕw were measured using a friction tester with an inclination plate (Shinto Scientific Co., Ltd., HEIDON-10). The constants b1 and b2 in Eq. (4) were determined experimentally from the static compression test for a SiO2 powder bed using a cylindrical vessel and a piston of 20 mm diameter made of stainless steel as follows: b1 = 0.384 and b2 = 1.75 × 10−4 m2 N−1 . 3.2. Immobilization of ZnO particles on SiO2 particles Fig. 8 illustrates the structure and dimensions of the HEM used in this study. The ratio of the minor axis to the major axis of the elliptical rotor is the same as that of elliptical
Fig. 7. SEM photographs of silica and zinc oxide particles.
barrel vessel. The rotor and the vessel rotate co-axially in opposite directions. The minimum clearance dC between the rotor tip and vessel wall is 0.5 mm. The powder mixture of SiO2 and ZnO that was placed in the vessel occupied 20% of the effective vessel volume (i.e. the difference between the vessel capacity and the rotor volume). The rotational speeds of rotor and vessel were kept at 40 and 0.5 s−1 , respectively. Under these conditions, the powder mixture circulates without stagnation in the vessel and is repeatedly fed to the minimum clearance region [9,11]. The thickness hB of the powder mixture layer before compression was determined as hB = 2.5 mm through the observation of SiO2 powder flow in the vessel. The mass ratio CZ of ZnO particles was varied in the range of 0 to 1.96 × 10−2 (=CZ,max ). Depending on the CZ , the optimum (minimum) processing time tm was determined from the model calculation of Eq. (19) under the same conditions as those in the experiments.
Table 1 Physical properties of silica and zinc oxide particles Powder
Median diameter (m)
True density (kg m−3 )
Bulk density (kg m−3 )
Internal friction angle (◦ )
Wall friction angle (◦ )
Silica Zinc oxide
9.3 0.02
2280 5600
817 200
27.0 –
19.8 –
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4. Results and discussion 4.1. Mechanochemical reaction between ZnO and SiO2 particles
Fig. 8. Schematic diagram of the HEM: 1, vessel; 2, rotor; 3, minimum clearance; 4, shaft; 5, powder.
3.3. Evaluation of ZnO-immobilized SiO2 particles 3.3.1. Relative change in the number of hydroxyl groups A Fourier transform infrared spectrometer (JASCO Corp., FT/IR-410) was used to investigate the relative change in the number of hydroxyl groups on the surface of SiO2 particles due to the mechanochemical reaction [22,23]. The infrared absorption spectra of samples were measured in the wavenumber region of 600–4000 cm−1 according to the diffuse reflection method. Before measurement of the absorption spectra, the samples were diluted with a potassium bromide powder; the concentration of the SiO2 particles was kept at 2.0% by weight in the mixture. The extent of compounding of ZnO–SiO2 particles throughout the immobilization process was assessed by morphological observation with a SEM and ultrasonic separation of ZnO and SiO2 particles in a water bath (Nihonseiki Kaisha Ltd., US-150T). 3.3.2. UV attenuation effect and transparency of visible light For evaluation of the optical characteristics, ZnO–SiO2 particles were mixed with a petrolatum medium (Tatsumi Pharmaceutical Ind. Co., Ltd.) using a laboratory muller (Toyo Seiki Seisaku-sho, Ltd.) at room temperature. The mass of ZnO–SiO2 particles in the mixed paste was 20% by weight. The paste was placed between a pair of quartz glass plates with a clearance of 28 m, as illustrated in Fig. 9. The transmittance in the wavelength range of 250–800 nm was measured with a spectrophotometer (JASCO Corp., Ubest V-550DS).
Fig. 9. Schematic illustration of the test cell: 1, quartz glass plate; 2, polypropylene film (spacer); 3, sample.
Fig. 10 shows the IR-absorption spectra of the ZnO–SiO2 particles of CZ = 0.011 (corresponding to immobilization in an ideal random packing state) together with that of ZnO–SiO2 particles hand-mixed with a spoon for 10 min. In the wavenumber range of 1000–1150 cm−1 among the ranges of absorption indicating the presence of silanol groups (i.e. 820–880, 1000–1150 and 3650–3690 cm−1 ), change in absorption with processing time was clearly observed. The absorption of the ZnO–SiO2 particles processed for 10 min was smaller than that of the hand-mixed particles processed for the same length of time. The absorbance decreased as the processing progressed. These results demonstrate that the mechanical energy of the HEM created a chemical bond between the ZnO and SiO2 particles and that the number of silanol groups decreased with an increase in the number of immobilized ZnO particles. Therefore, absorbance A1100 at 1100 cm−1 was noted as a representative value of absorbance within the range of 1000–1150 cm−1 . Fig. 11 shows the change in A1100 with the processing time t in the case of CZ = 0.011. A1100 decreased with t and remained constant over the optimum processing time tm = 86.0 min determined from the model calculation. It was confirmed that the immobilization of all the ZnO particles was completed at t = tm . Fig. 12 shows a SEM photograph of the surface of a ZnO–SiO2 particle processed for tm = 86.0 min. The ZnO particles were dispersed uniformly on the surface of SiO2 particles and hardly extricated from the SiO2 particles after 5 min soaking in an ultrasonic water bath, proving that the ZnO particles were immobilized on the surface of SiO2 particles by the mechanochemical reaction between them.
Fig. 10. IR-absorption spectra of ZnO–SiO2 particles.
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Fig. 11. Change in absorbance A1100 at 1100 cm−1 with processing time.
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Fig. 14. Transmission characteristics of suspensions containing ZnO–SiO2 particles.
agreed fairly well with that obtained at tm . These results prove that the proposed model is useful for the determination of the processing time. 4.2. Transmittance of a suspension containing ZnO and SiO2 particles
Fig. 12. Surface morphology of a ZnO–SiO2 particle processed for 86.0 min.
Absorbance A1100 of ZnO–SiO2 particles of various mass ratios CZ is shown in Fig. 13 together with A1100 in the case of the constant processing time (120 min) regardless of CZ . In the case of the larger CZ , more silanol groups were used for immobilization, and the number of silanol groups remaining after the processing decreased with an increase in CZ . A1100 of the excessively processed ZnO–SiO2 particles
Fig. 14 shows the change in the transmittance ratio T/Tb with the wavelength of incident light, where T is the transmittance of the paste mixed with the ZnO–SiO2 particles of CZ = 0.011 obtained under various processing times and Tb is that of petrolatum (i.e. the blank). In this figure, the result for the just blended paste with SiO2 particles of 19.8% by weight and ZnO particles of 0.2% by weight is shown together. The UV radiation was difficult for the SiO2 particles to attenuate in the range of 300–400 nm, whereas the transparency in the visible light region (400–800 nm) was relatively high. The UV attenuation of the ZnO–SiO2 particles prepared by blending was small due to inhomogeneous dispersion of the ZnO particles in the medium. On the contrary, in the ZnO-immobilized SiO2 particles prepared with the HEM, the UV attenuation was maintained, and the transparency for visible light was improved with the processing time, i.e. the progress of the dispersion of the ZnO particles. The transmittance of the ZnO–SiO2 particles processed for tm = 87.0 min almost agreed with those excessively processed (120 min). This result also demonstrates that determination of the optimum processing time using the proposed model is valid.
5. Conclusions
Fig. 13. Change in absorbance A1100 at 1100 cm−1 with mass ratio CZ of ZnO particles.
To prepare a UV-attenuating formulation with a high transparency for visible light, a compounding process in which ultra-fine ZnO guest particles are immobilized on the surface of SiO2 host particles using a mechanochemical reaction was proposed. The optimum operating conditions of the HEM were determined from the model calculation
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based on the minimum energy required for the immobilization. Under these conditions, uniform dispersion and immobilization of the ZnO particles were achieved, and a suspension including the composite particles had excellent UV-attenuating performance and transparency for visible light.
[8] [9] [10] [11] [12] [13] [14]
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