A process for preparing a toner resin

Abstract: A toner resin suitable for being used in toner compositions or developer compositions for electrostatic imaging is prepared by polymerization of styrene and butadiene in the presence of a mixture of n-dodecyl mercaptan and t-dodecyl mercaptan. The toner resin has defined weight average and number average molecular weights and a defined polydispersity that provide an improved toner resin that can be attained without blending different copolymers while using conventional apparatus. Additionally, the toner resin can exhibit a suitable melt index and glass transition temperature.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for preparing a toner resin. 2. Description of the Related Art Styrene based copolymers, particularly styrenebutadiene copolymers, have long been known to be useful as toner resins that can be used in toner compositions and developer compositions in electrostatic imaging systems. For instance, U.S. Patent No. 4,469,770 discloses styrenebutadiene plasticizer toner composition blends comprised of from about 40 to about 94.5 weight percent of a blend of styrene-butadiene copolymer resin particles and a plasticizer composition which functions as a surfactant prior to acid coagulation. The copolymer contains from about 85 to about 93 weight percent of styrene and from about 7 to about 15 weight percent of butadiene and has a weight average molecular weight of from about 45,000 to about 155,000 and a number average molecular weight of from about 7,000 to about 25,000. In column 1 of the patent, reference is made to other patents which describe styrene-butadiene copolymers used in toner resins, namely U.S. Patent Nos. 3,326,848, 3,960,737, and 3,766,072. Other references to styrene-butadiene copolymers as toner resins are in U.S. Patent No. 4,148,937 and Japanese Kokai Publication No. 53-25654. The art has generally recognized that the molecular weight of the toner resin is an important consideration. In U.S. Patent No. 4,558,108, a vapor phase-aqueous phase process employing multistage heating is used to prepare styrenebutadiene toner resin particles having a weight average between about 10,000 and 400,000 and a glass transition temperature of between about 50 and 130° C. The toner particles are stated as having a polydispersity up to about 9 which is contrasted with a single stage heating process that has a polydispersity of between about 2 and about 5. Other patents discussing the molecular weight of the toner resin include U.S. Patent Nos. 4,473,628, 4,557,991, 4,565,766, 4,652,511, and 4,702,986 and Japanese Kokai Publication No. 57-5052. U.S. Patent No. 4,564,573 describes an electrostatic image developing toner comprised of a binder resin which contains at least 60 percent by weight of styrene-butadiene copolymer containing a component A having a molecular weight of at least 100,000 and a component B having a molecular weight of at least 500,000. U.S. Patent No. 4,473,628 further describes a toner resin which is prepared from two different types of styrene-butadiene copolymers. In preparing polymers which can be used as toner resins, the art has utilized a variety of materials, such as initiators and surfactants, which are used depending on the process selected. One such material is a chain transfer agent and, in this utility, the art has used mercaptans, particularly dodecyl mercaptan. For example, aforementioned U.S. Patent Nos. 4,473,628 and 4,564,573 disclose the presence of t-dodecyl mercaptan in the polymerization initiation system for a copolymer which is predominantly prepared from styrene and butadiene. In addition to U.S. Patent No. 4,469,770 (EPA- 0,115,219) referred to above which discloses an electrophotographic toner composition containing a styrene butadiene toner resin with particular weight and number average molecular weights, polydispersity, melt flow index, glass transition temperature styrene to butadiene weight ratio, reference may be made to GB-A-2 075 703 and U.S. 3,666,534. The GB specification describes styrene butadiene toner resins with particle weight and number average molecular weights and polydispersity, and the U.S. specification a process of polymerizing styrene and butadiene in the presence of n- and t-dodecyl mercaptan. Despite the extensive art which describes styrene-butadiene toners resins, the search has continued for efficient processes which can prepare toner resins having characteristics which make them particularly suitable in toner and developer compositions. 3. Summary of the Invention The present invention provides a process for preparing a toner resin comprising polymerizing styrene and butadiene in a weight ratio of styrene to butadiene of from 85:15 to 97:3 in the presence of a mixture of n-dodecyl mercaptan and t-dodecyl mercaptan in a weight ratio of from 1:9 to 9:1 until a toner resin is formed that has a weight average molecular weight of from 50,000 to 140,000, a number average molecular weight of from 5,000 to 13,000, a poly-dispersity of from 5 to 15 and a melt index of from 5 to 80 g/10 min. Subsidiary features of the present invention are claimed below. In the description, ordinary latitude in numerical limits is expressed by use of the word "about". DESCRIPTION OF THE PREFERRED EMBODIMENTS As stated hereinabove, the present invention relates to preparation of toner resin comprised of a copolymer of styrene and butadiene. Typically, the copolymer contains styrene in an amount from about 85 to about 97%, preferably from about 88 to about 96%, and most preferably from about 90 to about 95% by weight of the copolymer, while the butadiene is present in an amount ranging from about 3 to about 15%, preferably from about 4 to about 12% and most preferably from about 5 to about 10% by weight of the copolymer. Additional comonomers can be used to prepare the toner resin provided that the presence of such comonomers do not substantially adversely effect the advantageous properties of the toner resin. Such comonomers include, for example, acrylic acid esters, methacrylic acid esters, fumaric acid esters, maleic acid esters, vinyl esters and acrylonitrile. In general, the amount of such copolymerizable monomers is less than about 50%, preferably less than about 30%, based on the weight of the copolymer. The toner resin produced by the process of the present invention has a weight average molecular weight determined by gel permeation chromatography which is in the range of from about 50,000 to about 140,000, preferably from about 60,000 to about 120,000 and most preferably from about 70,000 to about 100,000. If the weight average molecular weight is too high, the fixing characteristics of the toner resin are adversely affected while if the weight average molecular weight is too low, the offset characteristics of the toner resin are adversely affected. The toner resin exhibits a number average molecular weight which is also based on gel permeation chromatography that is in the range of from about 5,000 to about 13,000, preferably from about 6,000 to about 12,000 and most preferably from about 7,000 to about 10,000. If the toner resin exhibits a number average molecular weight which is too high, the fixing characteristics of the toner resin are adversely affected, while if the number average molecular weight is too low, the offset characteristics and caking properties of the copolymer resin are adversely affected. In addition to the weight average molecular weight and number average molecular weight, the toner resin of the present invention must exhibit an appropriate polydispersity which is the dimensionless quotient determined by the ratio of the weight average molecular weight to the number average molecular weight. The polydispersity of the toner resin is in the range of from about 5 to about 15, preferably from about 6 to about 13 and most preferably from about 7 to about 10. If the polydispersity is too low, the toner resin typically exhibits a viscosity profile wherein viscosity decreases sharply at higher temperatures which is undesirable for good offsetting properties. Conversely, if the polydispersity is too high, the fixing properties of the toner resin are adversely affected. The toner resin produced by the process of the present invention further exhibits a melt index and preferably glass transition temperature which makes it particularly suitable for use in toner compositions. More specifically, the toner resin has a melt index (MI, also known as the melt flow rate, determined at 150° C and a 2160 gram load) within the range of from about 5 to about 80 g/10 minutes, preferably from about 10 to about 60 g/10 minutes and most preferably from about 15 to about 40 g/10 minutes. The glass transition temperature (Tg) of the toner resin, as determined by a differential scanning calorimeter, is in the range from about 50 to about 70°C, preferably from about 55 to about 65°C and most preferably from about 57 to about 63°C. The toner resin produced by the process of the present invention which exhibits the foregoing characteristics is preferably obtained by conducting polymerization of the copolymerizable monomers in the presence of a mixture of n-dodecylmercaptan and t-dodecylmercaptan (both of which are commercially available). The polymerization can be conducted according to known suspension or emulsion polymerization techniques with emulsion polymerization being preferred. The weight ratio of n-dodecylmercaptan to t-dodecylmercaptan is from about 10:90 to about 90:10, preferably from about 20:80 to about 80:20, and most preferably from about 30:70 to about 70:30. The total amount of the dodecylmercaptans is preferably from about 0.5 to about 6% by weight, preferably from about 1 to about 5% by weight and most preferably from about 1.5 to about 4% by weight of the copolymerizable monomers. It is believed that the diverse chain transfer coefficients and diffusion rates of the n-dodecylmercaptan and t-dodecylmercaptan enable a toner resin to be obtained which has the aforementioned advantageous characteristics. However, it is to be understood that the present invention is not limited by this theory. By using the mixture of n- and t-dodecylmercaptan, a toner resin that has the noted advantageous characteristics in a single reaction operation can be obtained without requiring equipment necessary for multiphase processing. Furthermore, the need to employ two different polymers in order to obtain a mixture having the desired characteristics can be obviated. Such advantageous results, which cannot be obtained by using either the n-dodecylmercaptan or t-dodecylmercaptan alone, mark a significant advance in the art. The toner resin is conveniently prepared otherwise using conventional polymerization ingredients and techniques known to those of ordinary skill in the art. For example, emulsion polymerization can be conducted by using a standard stainless steel reactor equipped with an agitator which is charged with distilled water, an alkaline agent, such as sodium hydroxide, in order to adjust the pH to from about 9 to about 11, and one or more emulsion stabilizing surfactants such as sodium lauryl sulfate, sodium oleate, a wood resin derivative commercially available under the tradename Dressinate™, and sodium stearate. After the contents of the reactor are heated to obtain a solution, an initiator such as sodium persulfate, potassium persulfate, ammonium persulfate in an amount of from about 0.1 to about 1% by weight of the monomer charge can be added followed by the addition of a mixture of the styrene and the n- and t-dodecylmercaptan. The reactor can then be evacuated and charged with nitrogen in order to remove oxygen from the system. Thereafter, butadiene is introduced into the reactor and, upon completion of the butadiene addition, the temperature is increased to from about 50 to about 80°C for from about 3 to about 8 hours. In the interest of process efficiency, the contents of the reactor can then be transferred to a coagulation vessel and a coagulating solution containing a conventional coagulating agent in an amount of from about 0.1 to about 2% by weight of the initial monomer charge is added. The coagulating agent can be a mineral acid, such as sulfuric acid, or a mineral acid salt, such as calcium chloride, aluminum sulfate or calcium nitrate, with calcium chloride being preferred. After a period of agitation, the coagulated mixture is then filtered and dried in any known manner in order to recover the resin. Depending on the styrene-butadiene ratio, the initiator, the surfactant, pH, reaction temperature and/or the coagulating agent selected, a different ratio of the n- and t-dodecyl mercaptan or total amount of the mercaptans within the defined ranges may be needed in order to obtain a resin with the desired characteristics. However, such determination can be readily determined by those of ordinary skill in the art. Additionally, other known ingredients can be used in the process, including other chain transfer agents, as long as they do not adversely affect the aforementioned characteristics of the toner resin. The toner resin can be used with known ingredients in order to prepare a toner composition that can be used in electrostatic imaging systems. For instance, the toner resin can be combined with known pigments, dyes, charge control agents, etc. under known processing conditions (e.g., mixing and polymerizing) in order to obtain toner compositions that can be used in electrostatic imaging systems. Such materials and techniques of forming toner compositions are, for example, set forth in the aforementioned documents, especially U.S. Patent Nos. 4,473,628, 4,469,770, 4,564,573. Similarly, the toner resins can be combined with such known materials and carrier particles of various types in order to obtain developer compositions that can be used in a manner well known to those skilled in the art. The following inventive Examples and Comparative Examples are presented to illustrate and contrast the present invention. However, the Examples should not be construed as limiting the invention. In Examples 1-8 and Comparative Examples 1-3, the following general procedure was used. A soap solution is prepared by dissolving sodium hydroxide (0.88 g), sodium dodecyl sulfate (0.88 g), Dressinate™ or sodium oleate (21.6 g) in 740 g of deionized water at 45° to 50° C. The soap solution is cooled to 25°C and charged to a 2 liter, stainless steel, stirred pressure reactor. A mixture of styrene (372 g for Examples 3, 4, 5, 6, 7, and 8 and Comparative Example 3; 360 g for Examples 1 and 2 and Comparative Examples 1 and 2,), n-dodecyl mercaptan, and t-dodecyl mercaptan and a solution of 3.6 g of sodium persulfate dissolved in 20 ml of deionized water are charged to the reactor. The reactor head is bolted to the reactor. The heating, cooling, and agitation utilities are connected to the reactor. Oxygen is purged from the reactor by alternately applying vacuum and nitrogen pressure to the reactor. After five cycles vacuum is applied to the reactor. The reactor agitator is turned on and butadiene (28 g for Examples 3, 4, 5, 6, 7 and 8 and Comparative Example 3 and 40 g for Examples 1 and 2 and Comparative Examples 1 and 2) is charged under nitrogen pressure to the reactor from a 300 ml stainless steel cylinder with valves at both ends. The reaction mixture is heated to 55°C over a period of 20 minutes and held at this temperature for 3.5 hours. The conversion of styrene to polymer is about 99% under these reaction conditions. To increase the conversion of styrene to polymer to > 99.9% the reaction temperature is increased to 80°C for one hour after the 3.5 hour hold at 55°C. The reaction mixture is cooled to 35° to 40°C, and vented to atmospheric pressure. The reactor head is then removed. The contents of the pressure reactor are added over 20 to 30 minutes to a stirred coagulation solution that contains 4000 g of water and 6 g of coagulant. The coagulation solution is held at 50° to 60° C during the addition. The coagulation mixture is stirred for 20 minutes and then filtered. The toner resin is washed with 2000 g of water and then dried at 45° to 55° for 16 hours. The molecular weight is determined by gel permeation chromatography at 40°C on four 7.5 by 300 mm columns with pore sizes of 0.05, 0.1, 1.0 and 10.0 micron (500, 1000, 10000, and 100000 angstroms). The carrier solvent is tetrahydrofuran at a flow rate of 1 ml/minute. Detection is by ultraviolet at 220 nm. The column set is calibrated with polystyrene calibration standards. The melt index of the toner resin is measured in accordance with ASTM Method D 1238 (Procedure B) at a temperature of 150°C and a load of 2160 g. The glass transition temperature of the toner resin is determined by differential scanning calorimetry wherein 10 to 15 mg of toner resin is heated from 25° to 200°C at a rate of 10°C/minute. The sample is cooled to 10°C at a rate of 200°C/minute and held at 10°C for 5 minutes. The sample is heated to 25°C and held at this temperature for 3 minutes. The sample is then heated from 25° to 200°C at a rate of 10°C/minute. The glass transition temperature is calculated from the second scan. The styrene-butadiene ratio, surfactant, coagulating agent and the amount of n- and t-dodecyl mercaptan used in each Example are provided in Table 1 and Table 2 along with the test results. Commercially available t-alkane thiols are referred to for example in Kirk Othmer (1983 ed) 22 946-964, where in particular t -dodecyl mercaptan is noted (page 959) as a mixture of C 12 mercaptans and obtainable from propylene tetramer. The particular material used in the examples herein was obtained from Atochem North America Inc. and was of the following specification:- t -dodecyl mercaptans 98.5% minimum mercaptan sulphur 15.6% minimum distillation range - IBP 227°C minimum - 95% 248°C maximum specific gravity (15.5°C) 0.858 - 0.860 flash point - COC 95°C (205°F) - TCC 865°C (188°F) vapour pressure 37.5°C (100°F) below 700N/m 2 (0.1 p.s.i.) freezing point - 45°C

Claims: A process for preparing a toner resin comprising polymerizing styrene and butadiene in a weight ratio of styrene to butadiene of from 85:15 to 97:3 in the presence of a mixture of n-dodecyl mercaptan and t-dodecyl mercaptan in a weight ratio of from 1:9 to 9:1 until a toner resin is formed that has a weight average molecular weight of from 50,000 to 140,000, a number average molecular weight of from 5,000 to 13,000, a polydispersity of from 5 to 15 and a melt index of from 5 to 80 g/10 min, determined at 150°C and a 2160 gram load. The process of claim 1 wherein the toner resin has a weight average molecular weight of from 60,000 to 120,000, a number average molecular weight of from 6,000 to 12,000 and a polydispersity of from 6 to 13. The process of claim 2 wherein the toner resin has a weight average molecular weight of from 70,000 to 100,000, a number average molecular weight of from 7,000 to 10,000 and a polydispersity of from 7 to 10. The process of claim 1, 2 or 3 wherein the toner resin has a melt index in the range of from 10 to 60 g/10 min. The process of claim 4 wherein the toner resin has a melt index in the range of from 15 to 40 g/10 min. The process of any preceding claim wherein the toner resin has a glass transition temperature of 50°C to 70°C. The process of claim 6 wherein the toner resin has a glass transition temperature of 55° to 65°C. The process of claim 7 wherein the toner resin has a glass transition temperature of 57° to 63°C. The process of any preceding claim wherein the total weight of n-dodecylmercaptan and t-dodecyl mercaptan is from 0.5 to 6% by weight of the total weight of the styrene and butadiene. The process of any preceding claim wherein upon completion of the reaction, a coagulating agent is added. The process of claim 10 wherein the coagulating agent is a mineral acid or a salt thereof. The process of claim 11 wherein the salt is aluminum sulfate or calcium nitrate. The process of claim 11 wherein the salt is calcium chloride. The process of any preceding claim wherein the reaction is conducted in the presence of a surfactant. The process of any preceding claim wherein the weight ratio of styrene to butadiene is from 88:12 to 96:4. The process of claim 15 wherein the weight ratio of styrene to butadiene is from 90:10 to 95:5. The process of any preceding claim wherein the styrene and butadiene are emulsion polymerized.

Inventors: HIVELY RICHARD L, KOBAYASHI NOBUKI, HIVELY, RICHARD L., KOBAYASHI, NOBUKI
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1992
Main classification: G03G9
IPC codes: G03G9, C08F236, C08F212, C08F2