Hydrophobic silica for electrophotographic toner composition

Abstract

The invention relates to a hydrophobic silica powder which is unprecedentedly insensitive to environmental humidity with which electrophotographic toner can possess stable electro static charge that leads to stable quality of printed image. The hydrophobic silica powder has the following physicochemical properties; average primary particle size (D) is 30-2000 nm, B*D<430 nm, while B stands for weight % of adsorbed water vapor on silica (100 weight %) when the partial pressure of water to the equilibrium vapor pressure of water at 25 C. is 80% and D stands for average primary particle size (nm) of the silica powder, B/C<2.7, while C stands for weight % of adsorbed water vapor on silica (100 weight %) when the partial pressure of water to the equilibrium vapor pressure of water at 25 C. is 20%, and carbon content>0.30 wt.-%.

Claims

1. A hydrophobic silica powder, which has the following physicochemical properties; average primary particle size (D) is 30-2000 nm, 65<B*D<430, whilst B stands for weight % of adsorbed water vapor on silica (100 weight %) when the partial pressure of water to the equilibrium vapor pressure of water at 25 C. is 80% and D stands for average primary particle size (nm) of the silica powder, B/C<2.7, whilst C stands for weight % of adsorbed water vapor on silica (100 weight %) when the partial pressure of water to the equilibrium vapor pressure of water at 25 C. is 20%, and carbon content (weight %)>0.30.

2. The hydrophobic silica powder according to claim 1, characterized in that its hydrophobicity is higher than 50%.

3. The hydrophobic silica powder according to claim 1, characterized in that it is a colloidal silica powder.

4. The hydrophobic silica powder according to claim 1, characterized in that the aspect ratio is 1.0-1.5.

5. The hydrophobic silica powder according to claim 1, characterized in that the hydrophobicity is higher than 55%.

6. A process for the preparation of hydrophobic silica according to claim 1, characterized in that it comprises the following steps: a. preparation of the silica dispersion, b. drying the dispersion of step a to obtain hydrophilic silica powder, c. aging treatment of the silica powder of step b at a temperature between 100 and 170 C. and the temperature of step c is higher than the temperature of step b, d. hydrophobizing the silica powder of step c.

7. The process for the preparation of hydrophobic silica according to claim 6, characterized in that the silica dispersion in step a is prepared by reacting an alkoxy silane or alkaline water glass and acid.

8. The process for the preparation of hydrophobic silica according to claim 6, characterized in that the drying in step b is carried out in a freeze dryer.

9. The process for the preparation of hydrophobic silica according to claim 6, characterized in that the aging treatment of step c is carried out in a drying oven.

10. The process for the preparation of hydrophobic silica according to claim 6, characterized in that the hydrophobizing of step d is carried out by spraying a hydrophobing agent on the silica of step c.

11. The process for the preparation of hydrophobic silica according to claim 10, characterized in that the hydrophobing agent is an alkylsilane, a polydimethyl siloxane or a silazane.

12. A toner composition for electrophotography containing at least one hydrophobic silica powder according to claim 1.

Description

EXAMPLES

(1) Hereinafter, the present invention is more specifically described by referring to Examples and Comparative Examples.

(2) Commercially available aqueous dispersions IDISIL EM13530P, EM7530P or EM5530P (Evonik Industries AG) were de-ionized by a cation exchanger (Lewatit S 108 H, purchased from Lanxess AG) so that a pH-value of 2.5-3.0 was obtained. Afterwards the dispersion was frozen with liquid nitrogen and freeze-dried under room temperature at a pressure of 0.2 mbar with a Christ Alpha 2-4 LDplus Freeze Dryer for longer than 10 hours in the first drying step. For the second drying step the pressure was decreased to 0.05 mbar for at least 2 hours to give hydrophilic silica powder.

(3) The powder underwent the aging treatment for 1 hour at a temperature that is mentioned in Table 1 in a drying oven.

(4) In a reaction container 100 parts by weight of the aged silica from step c was added. The powder was fluidized by mixing. The described amount by weight of hydrophobing reagent was sprayed on it under nitrogen atmosphere as shown in Table 1. This reaction mixture was fluidized for the period at the temperature as listed under nitrogen atmosphere. The resulted mixture was cooled to yield a hydrophobic silica powder.

(5) TABLE-US-00001 TABLE 1 Aging Surface modification Hydro- Silica temp. time Surface Amount temp. time phobicity Example dispersion ( C.) (h) modifier (parts) ( C.) (h) (%) pH C 1 EM13530P HMDS 5 200 1 22 8.3 C 2 EM13530P 90 1 HMDS 5 200 1 35 7.6 E 1 EM13530P 110 1 HMDS 5 200 1 66 7.5 E 2 EM13530P 130 1 HMDS 5 200 1 81 6.3 E 3 EM13530P 160 1 HMDS 5 200 1 79 6.6 C 3 EM13530P 190 1 HMDS 5 200 1 51 6.4 C 4 EM13530P 220 1 HMDS 5 200 1 42 6.4 C 5 EM13530P 90 1 OCTMO 5 200 1 45 4.9 E 4 EM13530P 130 1 OCTMO 5 200 1 80 4.7 E 5 EM13530P 160 1 OCTMO 5 200 1 83 4.9 C 6 EM7530P 90 1 HMDS 8 200 1 24 8.5 E 6 EM7530P 110 1 HMDS 8 200 1 64 7.0 E 7 EM7530P 130 1 HMDS 8 200 1 75 7.1 E 8 EM7530P 160 1 HMDS 8 200 1 77 6.9 C 7 EM7530P 190 1 HMDS 8 200 1 51 7.2 C 8 EM5530P 90 1 HMDS 10 200 1 36 6.9 E 9 EM5530P 110 1 HMDS 10 200 1 72 6.4 E 10 EM5530P 130 1 HMDS 10 200 1 70 6.5

(6) Two-component toner powder was used which consisted of negatively charged styrene-acrylic resin with average particle size at 8 m manufactured by a grinding method. The toner powder and each hydrophobic silica powder in the table 2 were mixed together to have the ratio which was calculated by the following equation:
Silica (parts by weight)=Average primary particle size of the silica powder (nm)/40

(7) The above mixture was added to a Henschel-type mixer (Super Mixer Piccolo SMP-2 by Kawata MFG Co., Ltd.), then stirred for 1 minute at 600 rpm followed by 3 minutes at 3,000 rpm to yield a toner composition.

(8) Tribo-electrostatic charge of the toner composition was measured under HH and LL conditions. The result is shown in Table 2. The absolute value of the charge was always bigger in LL conditions for any specimen. The smaller the ratio of their charge amount means the more stable toner composition under various environment.

(9) TABLE-US-00002 TABLE 2 Carbon content C B D B*D LL HH LL/ Ex. (wt %) (wt %) (wt %) B/C (nm) (nm) (C/g) (C/g) HH C 1 0.84 1.9 5.2 2.7 132 686 14 4.9 2.9 C 2 0.81 1.4 3.9 2.8 132 515 20 6.4 3.1 E 1 0.81 1.3 2.6 2.0 132 343 22 15 1.5 E 2 0.78 1.3 2.4 1.8 132 317 22 15 1.5 E 3 0.8 1.2 2.3 1.9 132 304 23 16 1.4 C 3 0.73 0.73 2.4 3.3 132 317 29 10 2.9 C 4* 0.65 0.71 2.4 3.4 132 317 C 5 1.6 1.2 3.3 2.8 132 436 22 6.5 3.4 E 4 1.4 0.81 1.3 1.6 132 172 27 19 1.4 E 5 1.4 0.74 1.3 1.8 132 172 28 18 1.6 C 6 1.5 2.6 7.9 3.0 77 608 30 3.5 8.6 E 6 1.3 2.2 4.5 2.0 77 347 38 24 1.6 E 7 1.3 2.1 3.9 1.9 77 300 37 21 1.8 E 8 1.4 1.7 3.9 2.3 77 300 37 22 1.7 C 7 1.2 1.2 3.9 3.3 77 300 55 20 2.8 C 8 1.5 3.1 10.7 3.5 55 589 45 8 5.6 E 9 1.6 2.7 5.8 2.1 55 319 53 29 1.8 E 10 1.4 2.5 5.7 2.3 55 314 52 30 1.7 *C 4 was so aggregated that it was not dispersible on the surface of toner particle.

(10) In Table 2 the inventive hydrophobic silica powders show lower LL/HH ratios, which result in an improved insensitiveness to environmental humidity.