METHOD FOR PREPARING AN ELECTROPHOTOGRAPHIC TONER

Abstract: A method for the preparation of toner particles having a shell of polymeric material in which a colorant is distributed. The particles are prepared by treating base particles with a shell-polymer and a colorant and contacted to give a uniform distribution of the additives on the base particles whereafter the shell-polymer is fused to the base particles.

Description: METHOD FOR PREPARING AN ELECTROPHOTOGRAPHIC TONER Technical field The present invention relates to a process for pre- 5. paring an electrophotographic toner, especially to the step for incorporating a coloring agent or other additions in the particle. Background of the invention Particles for use as toners in electrophotographic 10. processes must fulfil several requirements in order to func¬ tion in the intended manner. The particles must be sufficient ly hard or they will break down by abrasion during the treat¬ ment in the apparatus and also to prevent caking at ordinary operation conditions. On the other hand, the particles must 15. have a sufficiently low melting point to be fixed to the support by conventional methods such as rolling or applica¬ tion of heat. The particles must have suitable magnetic or electrical properties, and particularly the surface must be capable of accepting and retaining the required charge and, 20. especially in cases when the charge is obtained by rubbing against other materials, it must have stable triboelectrical properties. The surface must, however, not be hygroscopic so that water is retained as this can lead to caking of the par¬ ticles or to a change in the electrical properties. The mate- 25- rial of the particles must be resistant to aging and it must be possible to incorporate an aging resistant colorant. The particles should also have as uniform and spherical form as possible to increase the abrasion resistance, to prevent the particles from being fixed to non-desired spots on the char'- 30. ged surface and to make a higher charge, and a charge which is more uniformly distributed between the particles, possib¬ le. The particles should also have as uniform size distribu¬ tion as possible since variations in the size leads to varia¬ tions in the capability of accepting the charge and thus to a 35. non-uniform deposition on the accepting surface so that the resolution becomes limited. Known toner materials have only fulfilled the above mentioned requirements to a limited extent. The usual method of preparation, incorporation of a colorant, charge-modifying, agents etc. in a plastic mass followed by grinding and s r-t;- ning of the product, certainly makes it possible to choose the material and the additives fairly freely but the obtain¬ ed particles are of very varying shapes and have a very non- 5. -uniform size distribution and this leads to poor abrasion resistance, poor resolution and difficulties in removing, the particles, particularly the smaller ones, from non-desired spots on the surface. Alternative methods for preparing toner particles have been suggested and these methods have given 10. particles of a more rounded form, but these known methods ha¬ ve not given the desired narrow size distribution, they have decreased the freedom of choice as concerns the material and the incorporation of additives has been made more difficult. The preparation of spherical polymer particles having 15. an extremely uniform size distribution by a swelling process to sizes suitable for toner particles is previously known. This method is described in the European patent application 3 90 3 which is incorporated by reference herein. However, the conditions at the swelling are such that conventional 20. methods for coloring and adaption of the properties in other respect to the toner-use cannot be used or carried out quite easily. In the Swedish patent applications 8107421-3 and 8201739-3 are disclosed how to produce very monodisperse and ,2 • spherical particles from monodisperse and spherical base par¬ ticles. It has now been found that many of the advantages disclosed can be obtained also for less monodisperse partic¬ les-and for other than thermoplastic base particles. The invention generally 30. It is an object of the present invention to provide a toner which fulfils the above mentioned requirements better than those hitherto known and which overcomes the deficien¬ cies of those. It is also an object of the invention to pro¬ vide a process for the preparation of such toner. It is a 35- particular object of the invention to provide a toner which satisfies the requirements but has considerably improved pro¬ perties as concerns roundness and monodisperse particle dist¬ ribution. It is another object of this invention to make it possible to use particles obtained according to the process described in the above mentioned patent application as toner material. The above objects are accomplished by the features evi¬ dent from the appended claims. 5. At the production of toner particles very spherical particles having an exceptionally narrow size distribution are obtained by starting from very monodisperse, i.e. small polymer particles of uniform size obtained by emulsion poly¬ merisation and then swelling these to sizes suitable for to- 10. ner application as described more in detail below. By adding a colorant to the particles after swelling and polymerisa¬ tion, the particles can be coloured without any negative in¬ fluence of the addition on the swelling- and polymerisation steps. Further, in this manner a concentration of the additi- 15. ve to the surface of the particles is obtained and this means that a fairly large amount of substance can be added on a small radius increase of the particles so that the uniformi¬ ty and spherical shape of these can be retained, and it also means that a distribution which is favourable with respect to 20. light absorption' is obtained, which further diminishes the required amount of additive so that the influence on the shape and size distribution of the particles can be reduced to a minimum. By including the colorant in a shell or by applying a shell as a cover layer on top of the layer of co- 25- loured substance there is obtained a possibility to influence several properties which are important with respect to the toner-use without giving rise to a negative influence on any swelling- and polymerisation step at the production of the particles. By selection of shell material or by additives to 30. this the surface properties of the particles can be modified for example with respect to hydrophilic properties, charge properties, conductivity and melting capability or tack. The shell also functions as a protection against abrasion of the colorant. Also for the shell, and optional additives 35. in this, a thin layer is sufficient for incorporation- of considerable amounts of modifying agents. Color- and cover layers on the surface of the particles also give great pos¬ sibilities of adapting the same monodisperse original par¬ ticles to different toner applications which is of special 3 advantage since substantial changes in the production pro¬ cess for the original particles entail considerable costs. Other objects adn advantages of the invention will be evident from the detailed description below. 5. Although the description is mainly directed to the ma¬ nufacture of monodisperse particles it is to be understood that the invention also cover production of toner particles with a more polydisperse particle size distribution. Espe¬ cially the invention covers particle size distribution above 10. a coefficient of variation of 30 percent. Also the invention covers other than spherical particles and other than thermo¬ plastic base particles. According to the subclaims the dis¬ closure of the Swedish patent applications 8107^21-3 and 8201739-3 (corresponding British patent application 823^977) 15- can be excluded or disclaimed from the present claim protec¬ tion. Definition of coefficient of variation The monodisperse order of the particles is in this disclosure expressed by the coefficient of variation (C V). 20. To obtain C V the standard deviation of the sample must first be calculated. n ( Standard deviation (S) = Σ. x - x.)' n - 1 This means that S is the square root of the arithmetic mean of the squares of deviations of the various items from the aritmethic mean of all items. If the diameters of the 25. particles are measured in micrometer, the dimention of S will also be micrometer. S x 100 C V = mean If the diameters of the particles vary according to standardized normal distribution 68 per cent of all the par¬ ticles will have diameters between - 1 C V of the mean. 30. The particles produced according to the present method can have any particle size distribution but preferably the coefficient of variation is below 100, more preferably below 75 percent and best below 50 percent. Preparation of the base particles The invention is not limited to a particular method 5. for the preparation of the base particles but any method can be used. Methods which give monodisperse and spherical par¬ ticles are preferred, for example methods based on swelling of smaller polymer particles, and ecpecially such methods wherein the swelling starts from particles having a narrow 10. size distribution according to the above discussion. The base particles are preferably of thermoplastic material. Several different kinds of monomer can be used for the purpose of producing base particles and among suit¬ able monomers which are polymerisable in water can be men- 15- tioned styrene, vinyl chloride, vinylidene chloride, acrylo- nitrile and acrylates, such as methyl acrylate, ethyl acry- late and methacrylates such as butylmethacrylate. Mixtures of different monomers can also be used. Particularly suitable monomers are styrene, styrene-acryl, styrene-acrylo nitrile- 20. -acryl or vinylidene chloride-acrylo nitrile. The monomer is selected among other things with respect to desired softe¬ ning properties. When the toner particles are to be used in processes wherein they are fixed by application of heat it is desired that at least part of the polymer content of the 25- finished particle has a glass temperature below 100 C, pre¬ ferably below 8θ°C. Most preferably the glass temperature is above 30°C. Suitable hardness properties can also be in¬ fluenced by the degree of polymerisation. A suitable size of toner particles according to the 30. invention is between 2 and 0 urn, preferably between 5 and 25 um and most preferably between 8 and 15 urn. The base par¬ ticles should thus have a size such that they after the shell application described below get a size within these limits. To obtain a toner for use in practice certain additions 5- must be made and the most important of these is the addition of a colorant. However, as mentioned by way of introduction, when using thermoplastic base particles, it is difficult to incorporate additives before the polymerisation without in¬ terfering with these processes and without spoiling the uni- form size distribution. For other types of particles similar problems may be experienced in trying to incorporate additi¬ ves within the particles. If the additives are divided into such which are molecularly soluble in the particle material 5. and such which are not molecularly soluble but form a phase of their own, usually a solid phase, if they are incorpora¬ ted in the particles, both groups will give problems if at¬ tempts are made to incorporate the materials at an early stage in the production process. The soluble substances are 10. as a rule organic dye stuffs, containing reactive functional groups which tend to react with the initiator and make the polymerisation more dif icult, change the formed polymer and be changed themselves in a non-desired manner. Solid additi¬ ves, which are not soluble in the particle, cannot easily be 15- incorporated in the inner of the particle before the polyme¬ risation in a controlled manner as the absorption capacity of the particle is not controlled by conditions of solubili¬ ty or of equilibrium and thus a non-uniform absorption will easily occur leading to a broadening of the size distribut- 20. ion. It is also difficult to select a colloid- or emulsifier system which will both efficiently stabilize the swelled par¬ ticles against coalescence and ' at the same time permit pas¬ sage of the pigment particles through the phase boundary sur¬ face. 25- On account of the above reasons it has been found suit¬ able to incorporate required additives on the ready particle or after the polymerisation reaction. However, after the po¬ lymerisation the colorant can hardly be incorporated homo- genously in the inner of particles without giving at least 30. a concentration at the sur ace. From reasons accounted for above this is per se advantageous. However, if this is the case the requirements on a good adhesion to the particles are great as color material which has come loose from the particles spoils the size distribution of the toner and gi- 35- ves rise to f OMP problems with miscoloration of the finished copies. i'i - problems increase with the amount of color mater: al to be incorporated and it is consequently difficult to provide the particles with a sufficient amount of colorant and at 5. the same time achieve a satisfactory adhesion of this to the polymerised particle as this is solid and little pene¬ trable to additives of different kinds. Still other problems to solve when the additions are made after the polymerisation of the particles are that the final surface must show all the properties required for the toner 10. application, which were mentioned in the introduction, such as suit¬ able electrical properties, reasonable hydrophilic proper¬ ties and suitable softness and other rαechanical properties. The colorant itself is seldom advantageous in these respects. The conductivity is often too high and the charge properties 15. to poor, and at the same time the adhesive properties are un¬ suitable. Finally, it must be possible to carry out the in¬ corporation in such a manner that the spherical and monodis¬ perse character of the particles is not lost since other¬ wise the desired properties will not be obtained. A parti- 20. cular problem in connection with this is that there is a risk of agglomeration of the particles at the incorporation. Preparation of a shell A_ 1 _General_remarks According to the invention an addition of a coloured substance after the polymerisation of the base particles is 25. made possible and in such a manner that the above mentioned requirements on the conditions of the surface can be satis¬ fied. This is achieved by coating the surface of the base particles with a shell of a polymer comprising or covering a coloured substance. By means of the polymer shell a satis- 30. factory adhesion can be secured as the requirements for ad¬ hesion between the base particles and the added colorant are reduced. As the shell in principle can be made to have any thickness, or at least to be considerably thicker than for example a layer of pigment, a sufficient amount of colo- 5- rant can be incorporated in the particles in this manner. The polymer material of the shell can be selected to fulfil the required conditions of the surface. As mentioned in the introduction the shell structure also gives a suitable distri¬ bution of the colorant with respect to light absorption and a particles . The above mentioned requirements should be considered when the polymer material for the shell is selected and this should also be selected with respect to suitable softness of 5- the material and with respect to the fact that the colored substance that is used should be wetted by the polymer. A liquid colorant should be at least partly soluble in the polymer and a pigment should be capable of being dispersed in the polymer. For some of the methods below it is suitable 10. to select a polymer which is soluble in a solvent or stably dispersible in a dispersion agent. Suitable monomer for the polymer material of the shell are the same monomers previous¬ ly mentioned for the base particles. However, different ma¬ terials can be used for the core and the shell respectively. 15. In practice the materials of the core and the shell will be somewhat different in most cases as the preparation process for the base particles demands the presence of certain sub¬ stances which are specific for this process such as the difficultly soluble swelling agent. Strongly hydrophilic 20. polymers should not be used for the shell and neither should emulsifiers be present in any considerable amounts since these might make the surface of the finished particle hydro¬ philic. Alternatively, residues of emulsifiers can be washed away after the coating. Copolymers of styrene and butyl meth- 25. acrylate have been found to be particularly suitable. Addi¬ tives for different purposes can be present in the polymer, as has been mentioned, for example charge regulating addi¬ tives. The material of the shell can advantageously be diffe¬ rent from that of the core as concerns mechanical properties. 30. When the toner particles are to be used in connections whe¬ rein they are crushed by mechanical pressure and fixed to the paper the core can be of an adhering material to get the best adhesion while the shell can be harder to avoid agglomeration of the particles before the fixation. When 35- the fixation instad shall be carried out by heating the core can be harder than the shell and the fusion at the fixa¬ tion will then essentially take place between the outer parts of the particles while the harder core contributes to a matt picture and a generally more durable particle. Although thermoplastics are preferred it is also possible to use curable plastics such as condensation products between formaldehyde and urea, mela- mine, phenol or resorcinol or other curable systems based on amines, acid chlorides or isocyanates/polyols. After the app- 5- lication a curing process must then be carried out. Shell of curable plastics can be of particular value for particles of the kind which, according to the above, are crushed and fixed by pressure. The amount of polymer in the shell should be so great 10. that the desired amount of colorant can be stably covered or included in the shell. However, the amount shluld not be so great that the incorporation thereof detrimentally influences the intended size distribution of the particle mixture. In thos cases where the colorant is distributed in the shell the volume 15. ratio between shell and core is suitably kept between 0.1 and 10, corresponding to an icrease of the radius of the core of . between about 3«2 and 122 per cent, but the volume ratio is preferably kept between 0.2 and 1 (6.3 to 26 per cent increase of the radius) or 0.5 to 5 to keep pigment concentration low to 20. limit conductivity. In those cases where the colorant shall onl be covered by means of the shell the volume ratio can be lowere down to 0.01 but the ratio is preferably kept above 0.05- The colorant can be a molecularly soluble substance e.g. an organic dye but it is preferred to use pigments, particularl 25- inorganic pigments, to get the most satisfactory long term dura bility, and then particularly carbon black or magnetite in thos cases wherein magnetic particles are of importance. For pigments the volume amount based on the volume of the entire particle * should be between 0.5 and 50 per cent, particularly between 1 30. and 25 per cent. When the colorant is incorporated essentially in the shell the amount based on the volume of the shell should be between 2 and 60 per cent and preferably between 5 and 40 per¬ cent. For soluble organic dyes the amount is more easily calcu¬ lated on basis of the weight and in these cases the above given 35. numerical values can be used as per cent by weight values in¬ stead. Used pigments should have a particle size which is clear¬ ly below that of the base particles, e.g. below 3 um and especi¬ ally below 1 ^μm. Very small particles may give rise to an in¬ creased tendency to agglomeration and it is thus preferred that the size is above 0.01 urn and preferably also above 0.1 urn. It is of course possible to arrange several shell struc¬ tures on the base particles. It can for example be suitable to have an inner shell with a colorant and an outer shell without 5. colorant, to protect the colorant against abrasion as much as possible. In these cases it is of course only necessary to adap the surface properties of the outer shell in the manner dis¬ cussed above and the material of the inner shell can be selecte more freely. 10. According to the invention the base particles are provide with a colorant .and a shell, as described above, by adding to them after the polymerisation, the colorant and a powder poly¬ mer, a solution of the polymer in a solvent or a dispersion of t polymer in a dispersion medium and by being contacted with thes 15- components until an even distribution of the components on the surfaces of the base particles is obtained and the solvent is evaporated or the polymer is fused by heat. The expression "polymer solution" includes also partly dissolved or partly dispersed polymers, such as a polymer which has been partly 20. precipitated from a solution. It is, however, preferred that pure solutions are used at the preparation of a shell using solvent according to the invention. The colorant can be added to the base particles before the polymer powder, solution or dispersion is added, e.g. by 25- making a liquid colorant wet or penetrate the base particles, optionally in the presence of a solvent as a means for aiding diffusion. Alternatively, a solid colored substance such as a pigment can be distributed on the base particle surface e.g. by being treated mechanically together with the base particles. 30. A solvent or dispersion agent can optionally be used also in this case to facilitate the adhesion of the pigment particles to the base particles and/or to soften the surface of the base particles to improve the retention of the particles. Methanol has successfully been used as solvent in this case. Any solvent 35- is evaporated thereafter and the polymer solution or dispersion is then charged for formation of the shell. This- method gives the best protection of the added colorant. It is, however, preferred that the colorant is mixed with the polymer powder. solution or dispersion and added to the surface of the base particles together with this. This is a simple method which also as a rule gives the best distribution of the colorant and the best adhesion between base particle, colorant and shell. It also has the least influence on the siz 5- distribution. In a polymer solution the colorant can be either dissolved or slurried. In a polymer dispersion the colorant ca be present outside the polymer particles in the dispersion me¬ dium but for the best adhesion it is preferred that the dye or pigment is in the polymer particles. In a polymer powder the 10. colorant can be present in the polymer particles or be present between the particles as a separate phase. In the later case the colorant must be well mixed with and distributed in the polymer particle mass. A pigment should thus preferably be distributed on the surface of the polymer particles which can 15. achieved by intimately mixing the components and grinding them together. The polymer for the shell can, as has been mentioned, be added in a solvent. Hereby an low viscosity fluid component is obtained, which, when charged to the base particles, can be 20. distributed in a layer surrounding these with maintained spher cality of the ~ particles. By selection of the solvent the vis¬ cosity and thus the amount of polymer deposited on each partic can be controlled within certain limits. The viscosity, in com bination with the surface tension of the solvent, contributes 25- to a levelling of the variations in applied amount on differen particles which is of importance in cases where the size distr bution must not be considerably increased at the shell forma¬ tion. The behaviour of the colorant in the solution can also be influenced to some extent by the selection of the solvent. 30. The solvent selection is thus decided from the following the polymer shall be soluble in the solvent to the required extent, the colorant shall be soluble or stably dispersible in this, it shall be possible to incorporate optional other addi¬ tives, no residues which are unsuitable for the toner applica- 5- tion shall remain after the evaporation of the solvent Λ and th kind of solvent is also decided from the influence of the solvent o the base particles. It must also be easy to evaporate the sol¬ vent and the solvent should not be absorbed by the base partic les in a too high extent. Solvents which have been tested and found suitable includes hydrocarbons, such as cyclohexane, and chlorinated hydrocarbons such as methylene chloride. Methanol, acetone, methyl acetate and ethyl acetate have also been found suitable. 5. The amount of polymer in the solvent should be kept fair low as this contributes to a good distribution of the composi¬ tion. The amount must however be kept high enough to give the desired thickness of the shell. Suitable amounts of polymer in the solvent are between 1 and 30 per cent by weight and prefer 10. ably between and 20 per cent by weight. When the colorant is to be added with the polymer solution the amounts can be lower somewhat further. It is however essential that the colorant ca be stably included in the solution. For pigments it must be possible to form a stable dispersion and maintain it when the sol- 15. vent is evaporated as otherwise the pigment will not be unifor ly distributed and the adhesion will be at risk. Stabilizers . can be used but their influence on the properties of the final surface must be considered. It is suitable to adjust the hydro phility of the pigment and of the solvent or dispersion agent 20. to each other and to choose for example an oxidized carbon pow der for a polar agent and a non-oxidised, hydrophobic carbon for non-polar agents. As a rule it is suitable to grind, or divide the pigment finely in another manner, in the already prepared solution of the polymer containing optional other 25- additives. The amount of colorant is adjused with respect to the amount of polymer in the solution in such a manner that th above given amounts of colorants are obtained after evaporatio of the solvent. The polymer can also be added in the form of a dispersio 30. of small polymer particles in a dispersion medium. This is pre ferred when it is desirable to avoid solvents for example to lower the risk of agglomeration when a tacky stage must be pas sed at the evaporation of the solvent or to avoid negative In¬ fluence of the solvent on other components in the system. In 35. most cases it is " also more easy to evaporate a dispersion me¬ dium than a solvent and the risk of having the polymer particl contaminated with un-desired components is smaller with a dis¬ persion medium than with a solvent. The dispersion can be pre¬ pared by dividing a colored substance such as carbon powder in a monomer and adding polymerisation aids such as a monomer- soluble initiator. The mixture can then be finely divided in water in the presence of a mild emulsifier such as ' ammonium laurate to as small droplets as possible, e.g. between 0.5 an 5, 1 urn, and the polymerisation of the monomerdrops in the aqueo phase is then triggered. In this manner polymer particles con taining well-included colorant can be obtained. Smaller polym particles in the dispersion can be obtained by conventional emulsion polymerisation in the aqueous phase using water solu le le initiator. Although this method can give particle sizes do to about 0.1 yum it is difficult to incorporate a colorant in the particles and a colorant is then suitably added to the di persion medium in the form of a separate dispersion. At the preparation of a shell the polymer particles and the colorant 15. will then be charged to the base particles at -the same time and due to the finely divided nature of the added components they can be made to fuse in to- a well-sintered shell for exam le by heating. It is easy to change the ratio between polymer and additive with this method. 20. The selection of a dispersion medium is less critical than the selection of a solvent for a polymer solution. Water is normally used. The polymer content in the dispersion is al less critical than the polymer content in a polymer solution but can suitably be kept at between about 5 and 0 per cent b 25. weight. The polymer can also be added in dry powder form to the base particles. The addition of the polymer and a colorant is made under agitation and at a temperature which is sufficient high for allowing adhesion of essentially the entire amount o 30. added shell-polymer to the surface of the base particles and at a temperature which is sufficiently low for preventing any substantial mutual agglomeration between the base particles, both the uncovered and the covered. If the shell-polymer is applied directly in powder form 35. and at a temperature which allows direct adhesion of the powde on the base particles a simple process technique can be used as it is not necessary to use any solvents or dispersion media Further, the added powder accumulates very satisfactorily on the surfaces of the base particles, without leaving free or non-bonded powder in the mixture. If there are any free impuri ties in powder form they are more easily collected than in a system based on solvents or dispersion media. The agglomeratio tendency is " low as the sintering of the small powder particles 5. takes place at lower temperatures and thus at a lower adhesion than required for fusing the larger base particles together. It is not necessary to pass a very tacky stage. Also lightly agglomerated shell-polymer will be decomposed and will adhere satisfactorily to the base particles. 10. The method of adding the polymer powder, solution or dis persion and of fusing the polymer to the base particles by hea ing or by evaporating the solvent or dispersion medium respec¬ tively are very important for the final properties of the par¬ ticles. If the particles are allowed to agglomerate during the 15. stages of the production the final spherical form of the par¬ ticles will be impaired and the monodisperse distribution will be ruined. Even if agglomeration does not occur, the size dist ribution can be impaired if varying amounts of polymer are ad¬ sorbed on the different particles and It is thus desirable to 20. control this adsorption to a certain extent. Generally, suitable processes for the preparation of a shell should thus include a step in which the polymer is added to the surface of the base particles in a as well-defined and uniformly thick layer as possible and a further step in which 25- heat is added or the solvent or dispersion medium is removed under such conditions that agglomeration of the particles can be avoided to the greatest possible extent. The ways of carryi out these steps will necessarily vary somewhat depending on whether the particles are kept apart from each other by means 30. of a gas phase or a liquid phase. B_._Gas_phase separation techniques Using a gas phase for separation of the particles usuall gives a good control of the growth of the shell thickness and makes it possible to have a simple design of the production 35. process equipment apparatus. When dispersing the particles in a gas known distributing methods can be used. It is particular suitable to fluidise the particles with a gas stream. Although an ideal separation without any contact between the particles Is desirable in principle, this condition is not necessary in practice. The minimum requirement is, however, that the part les are kept in continuous movement with respect to each oth and this can most simply be attained by a vigoruou ' s agitatio of a particle bed. The purpose of the separation is to preve 5. the particles from agglomerating during fusion by heating or the removal of the solvent or dispersion medium and to preve them from having such an influence on each other that the sp cal form is impaired or the narrow particle size distributio is broadened. 10. A preferred manner of carrying out the process comprise several steps. Microscopically, the base particles shall firs be contacted with the powder or wetted by the added solution or dispersion then the added components shall be uniformly distributed between the base particles in a smoothening step 15. and finally the additives shall be adhered to the base partic les in a fusing step. A preferred manner for the contact step comprises spraying the solution or dispersion in the form of fine drops on a bed of the particles or slowly adding the pol mer powder on a bed of the particles. 20. A smoothening operation is for solutions or dispersions then must simply carried out by selecting consistency or surface tension of the sol tion or dispersion so that an essentially uniform amount of the solut will be left on the surface of each particle when these are separated from each other after careful wetting and mixing. 25. Repeated contacts or collisions of the particles are however desired in order to get a good distribution. The evaporation operation can partly be carried out at same time as the other operations. The risk of agglomeration is at its greatest during the last part of the evaporation an 30. at least this last part must thus be carried out after or dur the separation described above. During this phase it can thus suitable to increase the distance between the particles and/o decrease the relative movement between these and the gas by adding more gas or increasing the gas velocity. Heat can be 35. applied in order to facilitate the evaporation. When a bed is vigorously agitated according to what has been said above the agitation work is often sufficient for heating. Inert gas can also be blown through to facilitate the evaporation and this is particularly suitable when it is primarily desired to in- -g * 0REΛ OMPI crease the separation of the particles. Vacuum can also be app¬ lied and this is preferred when it is desired to avoid particle losses in the leaving inert gas or when it is desired to in¬ crease the friction in a bed under agitation, e.g. to heat the 5. bed. For the final fusion and smoothening of the surface of th shell particles can be heated rapidly for melting of the sur¬ face and the particles shall then be well dispersed and separa ted in the gas phase. 10. It is not necessary to carry out the above described operations in the form of steps which are separate from each other or in separate apparatuses but it is possible to carry out the steps at essentially the same time and in the same apparatus space. It is for example possible to carry out wetti 15. smoothening and evaporation in an agitated bed with separation of the particles and also with collisions of these. It has bee found particularly suitable to charge the base particles to a fluidised bed and spray the solution or the dispersion into this in the form of fairly small droplets, for example of about 20. 50 ; um, and the wetting and smoothening will then take place essentially at the same time while the particles are in a flui dised condition and separated from each other. The evaporation of the solvent or dispersion medium is carried out at the same time by application of heat, vigorous gas passage or applica- 25. tioπ of vacuum. A shell formation on the microscopic level wil in this manner take place in several steps by repeated wetting smoothening and evaporation. When the shell-polymer is added in dry powder form the contact step follows by making the addition directly to a bed 30. of base particles and smoothening and distribution over the su face of these is obtained by means of the repeated contacts resulting from the agitation of the bed. To secure adhesion of the powder to the base particles and to avoid free powder in t finished product the powder should be charged continuously and 35» not faster than it is adsorbed on surfaces of the base particl The most important control parameter for the adhesion and the shell formation is hereby the temperature. The temperature should be selected high enough to obtain a sufficient adhesion between the powder particles and the bas 4=- 4 ct P μ- ct Ω μ- O Φ 3 o Ω P 3 3 O s: P 3 co α O μ> to P ct pi 4 < μ- P cr 3 P t t P μ- oq O Φ 3 o p 3 to Φ ct 3 * O cr α Φ co μ. Ω 3 P Ω 3 4 Φ cr P φ o Φ P α O φ Pi Pi P 4 P μ. 3 3 oq μ. Ct ct er μ « μ- co to o TJ J P Φ ticularly suitable to fluidise the particles by means of a 15au stream. The above described operations for the shell preparation can, if desired, be repeated one or several times. As it is 5. more easy to obtain a shell tickness which is uniformly distri buted between the different particles if the applied amount is small, It is suitable to use a repeated treatment when a thick shell is to be formed. Also in those cases when it is desired to have an outer shell with other properties than an Inner she 10. according to what has been said above, the operation manner ca be repeated. When the formation of a shell takes place under conditio which allow contact between the particles a certain agglomera¬ tion cannot be entirely avoided. The smaller the particles are 15. the bigger are the problems. However, if any bridges have been formed these can if desired, be broken by careful ginding e.g. in a pin mill or by temporarily increasing the agitation of the bed. One way of avoidning agglomeration and increasing the 20. free-flowing properties which can be used during the entire production process and which is particularly suitable for small particle sizes below about 10 yam comprises mixing the base particles with considerably larger, preferably spherical, particles of e.g. a polymer material as auxiliary material 25. during the process and these particles can later be separated. To increase the free-flowing properties of the powder mass these auxiliary particles should be considerably larger than the base particles, suitably between 10 and 100 times as larg or for example from about 0.5 to 2 mm. Better free-flowing 30. properties are obtained with a larger amount of auxiliary particles but at the same time the production capacity decrea ses and it is thus suitable to have a content of auxiliary particles between about 20 and 90 per cent by weight in the powder mass. As the auxiliary particles are present at the co 35- ing operation these will also be coated by a layer of the add polymer. The layer on the auxiliary particles will however no be thicker than that on the base particles and since the size difference is pronounced the surface of the auxiliary particl does not represent more than a fraction of the total surface of- the powder mixture and the polymer loss will thus be negli gible. Also "Che radius increase of the auxiliary particles, calculated as per cent, will be minor and the auxiliary par¬ ticles can consequently be re-used several hundred times 5. without any unacceptable size increase. After the coating pro cess the auxiliary particles can be removed and this can be done with simple method, for example by screening, since ther is a considerable difference in size between these and the to ner particles. 10. Another way of avoidning agglomeration is to add a powd of a substance which is not sticky at the conditions used. In fact, a colorant in the form of a pigment can act as a powder stabilizer in the present method. An enhancement of this effe can, however, be obtained by using a larger amount of powder, 15. a powder of smaller particle size than the pigment or a powde less wettable by the polymer than the pigment. Generally the powder is inorganic, like colloidal silica or colloidal alumi¬ nium oxide. It Is preferred that the added .powder can later be removed which is possible if the substance selected can be de- 20. composed and evaporated, e.g. ammonium or calcium carbonates, or is soluble, like aluminium or magnesium hydrates. Use of powder stabilizers is of greatest value at the fusion step and the addition can preferably be made in this step. The powder stabilizer is further of greatest value for dry powder polymer 25. or dispersed polymer. 5i_^i2Hi^_S^ a --C-.-ϊ-cE§ϊ-^i22--^-i2l-:2i9y§§ Use of a liquid phase for keeping the particles apart at the production of the shell generally gives low risks of agglomeration of the particles, a simple process with respect 30. to production technique and not so great problems with respect to solvents. A preferred manner of carrying our the process comprises mixing the base particles in dispersed form with an emulsion or dispersion of the shell polymer, optionally con¬ taining a colorant as has been said above, precipitating the 35. particles of shell polymer and colorant on the surface of the base particles, sintering the so formed shell and separating the shell-covered base particles from the dispersion. In this process a polymer solution of the above type can be added in the form of an emulsion to the dispersion of the base particles. After the emulsion drops have been precipitat on the base particles a certain sintering of the shell takes place as a direct consequence of the liquid, character of the precipitated particles. ' The solvent can then be removed in an 5. known manner, for example by being allowed to slowly diffuse through the phase of dispersion medium and this process is facilitated by using a water soluble solvent. The shell poly¬ mer can also be added in the form of a dispersion of polymer particles, according to the above, and then either microsus- 10. pensions of the polymer with included colorant or a dispersio of emulsion polymerised particles, which does not contain a colorant but with this additive in the dispersion medium, can be used. In this case solvent Is not required and consequentl neither removal of solvent but there are higher requirements 15. on the sintering step for the shell. The dispersion medium is suitably based on water in all the cases. Precipitation or flocking of the shell-polymer particle on the surface of the base particles is suitably carried out by changing the colloidal conditions in the dispersion. If fo 20. example, a fatty acid type emulsifier is used for the disper¬ sion of base particles and shell polymer particles flocculati can be accomplished by addition of salts , especially salts of alkaline earth metals, or by lowering the pH by means of addi acid. The change of the conditions should be carried out slow 25. ly to obtain a good control of the flocculation process. Alte natively, it is possible to add the shell-polymer dispersion slowly to the base particle dispersion, in which the conditio are already adapted so that the shell-polymer will flocculate when introduced therein. The process shall be interrupted whe 30. a sufficient amount of shell-polymer has been precipitated on the base particles but before there is a mutual agglomeration of the base particles. To facilitate the flocculation of shell-polymer on the base particles, and to improve the adhesion to these, the sur 35. face of the base particles can be pretreated with substances which improve the adhesion to the shell-polymer particles. Th adhesion agent should have a low water solubility so that it will not diffuse also to the surface of the shell-forming pol mer particles. Suitable agents are organic liquids such as di octyl adipate, or insoluble soaps such as claciu soaps of fatty acids. A steric stabilizer, for example polyvinyl alcohol, ca be added in order to stabilize the flocculated shell or to 5. interrupt the flocculation. This will among others facilitat the sintering step when this is made in water suspension. After stabilization and sintering the shell covered ba particles can be removed from the dispersion. The particles can thereafter optionally be washed to remove emulsifier resi 10. dues. If desired a further sintering of the shell can be acco plished after the drying by heating in a gas stream to get a maximum strength and sphericity. Although the shell covering methods have been descri¬ bed above in relation to monodisperse particles of thermo- 15. plastic materials, which are to prefer, it is possible to use the methods for covering also other particles. The base particles can thus for example be thermoplastic particles produced in other ways, of another organic material, such as thermosetting resins, or of inorganic material such as 20. glass or metal. The described particles can be used as toner particles in a manner known per se in electrographic, magnetigraphic and especially electrophotographic methods. They can be mixe with conventional developing- and carrier particles, for 25. example of steel or glass, or be used as one-component toners They can be used for wet methods but are particulary suitable for dry methods. Example 1 77 ml water, 11.7 ml of chlorodoαecan, 1.8 g of benzoyl- peroxide, 9-3 ml of dichloroethane and 0.2 g of sodium lauryi- sulfate were homogenised in a two-step Manton Gaulin hornogeniser, 5. model 15 M, with 200 kg/cm 2 in the first step and 80 kg/cm2 in the last step for about 1 to 1.5 minutes and this resulted in an emulsion with particles having a size in the order of 0.1 to 0.2 yum. A seed latex of monodisperse polystyrene par¬ ticles having a diameter of 0.65 yum (determined by electron 10. microscopy) was added to the emulsion. The amount of seed latex added was 83-8 ml, containing 77 ml of water and 6.8 ml of sty¬ rene particles. Additionally 6 ml of water and 8.5 ml of acetone were added under agitation at 35 to 40 C. After agitation for 12 hours at 40 C the acetone was removed by evaporation under 15. vacuum. When the acetone and the dichloroethane had been evapo¬ rated 1.0 g of sodium laurylsul ate and 840 ml of water were added so that the amount of water was 1000 ml. Further 275 ml of distilled styrene were added under agitation at 30 C. After agitation for 2 hours at 30°C the temperature was increased to 20. 60 C and the polymerisation started. After 25 hours polymerisa¬ tion a monodisperse latex having a particle diameter of 2 yam was obtained. This latex was used as a starting material for a second swelling and polymerisation step. 28.5 ml of the latex, contain- 25. ing 25 ml of water and 3-5 ml of polystyrene particles, were mixed with an emulsion of 35 ml of water, 4 ml of Perkadox SE-8 (dioctanoyl peroxide), 3 ml of chlorododecane and 0.2 g of sodium laurylsulfate, which emulsion had been homogenized using the same equipment and under the same conditions as stated above 30. for the earlier swelling step. Further, 10 ml of water and 7 ml of acetone were added under agitation at 25-30 C. After 14 hours the acetone was removed under vacuum. Thereafter 1.2 g of sodium laurylsulfate, 1.0 g Berol 267 and 930 ml of water were added to a total amount of water of 1000 ml. 175 ml of distilled styrene 35- were added under agitation at 25-30°C and after 3 hours the tem¬ perature was increased to 70 C to get a complete polymerisation. The final latex was monodicp-ercc and had a particle diameter of about 7 um. Example 2 This example illustrates the preparation of a black micro- suspension. A paste-like carbon dispersion was prepared from butyl ethacrylate, the pigment wetting agent Paraloid D -5^ from 5. Rohm & Haas, Philadelphia, USA (an acrylate polymerisate) and Spezialschwarz 4 from Degussa, Frankfurt, F.R. Germany. For the preparation of this dispersion the following were charged: Butyl methacrylate 251 g Paraloid DM-54 157 g 10. Spezialschwarz 4 157 g 565 g The mixture was ground on a ground mill equipment until a smooth structure was obtained. The ground dispersion was then diluted with the following: 15. Styrene 1098 g Butyl methacrylate 220 g Oleic acid 47 g Porofor N (Bayer, F.R. Germany) 23 g azobisisobutyronitrile 20. 1388 g An aqueous solution was prepared from the following: Water 5830 g Ammonia ( 2 % ) 170 g 6000 g 25- Under slow dosage and under agitation with a high-speed Ultra-Turrax-mixer the carbon-monomer dispersion was charged to the aqueous phase. A fine black emulsion having a droplet • size of about 3 / Um was formed. This emulsion was then further divided by being pumped twice through a 2-step ho ogeniser, 30. Gaulin model 15M-8TA, with a pressure drop of 475 kp/cm . Hereby a fine emulsion with a mean droplet size below 1 μm was obtained. This emulsion was charged to a 14 litre autoclave equipped with an agitator. After evacuation polymerisation was carried out at 75°C for 8 hours. A black dispersion was obtained. ? 4 Example 3 Illustrates coating of monodisperse base particles with a black microsuspension. 1.5 kg of monodisperse particles having a diameter of 10 urn 5. which had been prepared as described in Example 1, were charged to a 8 liter fluid-bed mixer, made by Papenmeyer. The particles consisted of a copolymerisate of 95 per cent styrene and 5 per cent divinylbenzene. Vacuum was applied to the mixer and the jacket temperature was adjusted to 30 C. The microsuspension 10. from Example 2 was added in portions of 10 ml. About 0 ml/min. were added and the addition of dispersion was stopped after 3-5 litres. The mixer was operated until the mixture became dry. The vacuum pumping was then interrupted and the temperature increased to about 50 C. After cooling a black free-flowing 15. polymerisate was obtained. By microscopic inspection it could be established that the monodisperse base particles had been covered with a uniform layer of particles from the black micro¬ suspension. Only a few of the base particles had agglomerated and these agglomerates could later be separated using an air 20. separator. The thus obtained carbon pigmented particles are suitable for use as toners. To control the triboelectric charge it can, in certain cases, be suitable to add charge regulating substances at the preparation of the black microsuspension. 25. Example 4 Illustrates the preparation of a polymer solution contain¬ ing pigment. The following were charged to a ball-mill of 8 litres: Cyclohexane 800 g 30. Methylene chloride 1600 g Methylmethacrylate- butylmethacrylate- copolymerisate 70/30 240 g Carbon black, Degussa FW1 24 g 35- Steel spheres, diameter 5 mm 5000 g The container was rotated for 24 hours and a black polymer- solution was then obtained. The solution was filtered through a 60 /am strainer screen. GMΓI Examp le 5 Illustrates coating of monodisperse base particles with the polymer solution of Example 4. 2000 g of monodisperse particles with a diameter of 10 yam 5- were charged to a fluid-bed mixer. Vacuum was applied. Under fluidlsed conditions the solution of Example 4 was sprayed onto the mixture. The nozzle was of the 1-phase type and had a capa¬ city of 20 ml/min. at 0.5 MPa. The jacket temperature was 40 C. After that 2.5 litres of solution according to Example 4 had 10. been sprayed on a black powder was obtained. A few monodisperse particles had agglomerated. These could be separated using an air separator. An advantage with this method, compared with that of Example 3, is that the coated particles do not contain residues of emulsifier on the surface. 15. Example 6 Illustrates coating of monodisperse base particles with a black microsuspension containing free carbon black. Example 3 was repeated but differing in that 2 per cent additional carbon black Spezialschwarz 4, based on the dry con- 20. tent of the suspension, had been added to the microsuspension. This mixture was later used for coating of base particles. The particles became somewhat blacker than when no additional carbon black was used. The tendency to agglomerate formation was smalle Example 7 25. Illustrates the preparation of an unpigmented latex and the use of this for coating. The following were charged to an autoclave: Methylmethacrylate 700 g Butylmethacrylate 300 g 30. Alkylarylethersulfate ' 20 g Ammoniumpersulfate 8 g Laurylmercaptan 10 g Water 3000 g After evacuation polymerisation was carried out at 6θ°C for 35- 12 hours. A white latex with very small particles (about 0.1 to 0.2 μm ) was obtained. Monodisperse base particles were charged to a fluid-bed mixer. The above prepared latex and a dispersion of magnetite (magnetite in water + 1 % alkylarylethersulfate) were charged to the mixture as two separate flows. The magnetite dispersion had a dry content of 50 per cent. The latex was added twice as rapidly as the magnetite dispersion (35 and 17 ml per minute, respectively). The temperature at the coating was 30 C. The 5. charging was stopped when the added weight of the dry contents of the two dispersion was the same as the weight of the base particles. The batch was then dried under continued mixing. The temperature was increased to 50°C under agitation and the batch was finally cooled. 10. Black monodisperse particles with magnetic properties were obtained. Example 8 Unpigmented latex according to Example 7 was charged to a fluid-bed mixer with base particles and a dispersion of Spezial- 15- schwarz 4 (10 % carbon black, 0.5 % alkylarylethersulfate) was charged parallelly. The charge flows were adjusted so that a shell containing 10 per cent carbon black was formed around the monodisperse base particles. When the dry content of the charged volume corresponded to 60 per cent of the weight of the base 20. particles the additions were stopped. The mixture was dried and then heated under fluidisation to 50°C. Black particles which were free from agglomerates were obtained. Example 9 Example 3 was repeated but after the black microsuspension 25- had been charged to the mixer, the batch was dried and unpigment¬ ed latex according to Example 7 was then added in an amount of 1000 g. After drying heat was applied to 50°C. Black monodis¬ perse particles were obtained. As the outer layer of the partic¬ les is unpigmented such particles can be advantageous in that 30. the dust which is formed in the copying machines from the abra¬ sion of the particles is less coloured and thus the tendency to greying of the background of copies is reduced. Example 10 At the preparation of polymer solution according to 35- Example 4 the carbon black was replaced by an organic colorant, 20 g of Ceresschwarz. This solution was then used for coating of monodisperse base particles according to Example 5- Bluish- -black particles were obtained. l U4 ro ro H Ul o Ul • • O Ul o Ul les. After an addition corresponding to 30 per cent dry content in the microsuspension, on the weight of the base particles, the addition of microsuspension was stopped. The mixture was stirred for 4 hours at a pH of 10.2. In a microscope it could 5- be seen that the microsuspension had been flocculated around the monodisperse base particles. 1 per cent of polyvinyl alcohol, with a degree of hydro¬ lysis of 88, based on the base particles, was added. The pH was lowered to 3.5 by slow addition of hydrochlorid acid and 10. the temperature was increased to 50°C. These operations made the flocculated layer around the base particles more dense. After dewatering and washing the particles were dried. Example lq This example describes the preparation of a latex for 15. preparation of a powder for shell formation. A water soluble initiator was used. The following were charged to a vessel: 70 parts of styrene 30 parts of butylmethacrylate 20. 1.5 parts of hydrogen peroxide 0.002 parts of iron, in the form of iron chloride 1 part of citric acid 4 parts of lauric acid - 0.6 parts of mercapto ethanol 25. 240 parts of water The pH was adjusted to 8.5 using ammonia. The emulsion was finely divided by means of a high-speed agitator and then charge to a polymerisation reactor. After evacuation the temperature was adjusted to 65 C. After polymerisation for four hours 0.75 30. parts of hydrogen peroxide were charged additionally. The poly¬ merisation was stopped after 12 hours. A white latex having par¬ ticles below 1 jam was obtained. OMPI Example 15 100 parts of the latex from example 14 was mixed with g of carbon, Spezialschwarz 4 from Degussa, Frankfurt, F.R. German When a paste-like state had been reached the mixture was trans- 5. ferred to a three roll mill and milled twice. The paste spread in a thin layer and allowed to dry the air. The pro¬ duct was then ground. Using microscopic inspection the powder was found to consist of several black-coloured small particles of less than 1 yum, but also of larger particles of up to 20-30 /ur 10. Both the smaller and larger particles are supposed to be formed from small latex primary particles in admixture with carbon. The ground powder was for two hours added to a mixer con¬ taining the monodisperse base particles mixed with polystyrene particles of 1 mm. The mixture consisted of the following: 15. Base particles 500 g 1 mm particles 500 g Powder was added thereto (ground as above) 500 g The temperature at the addition was 55°C After completed addi- 20. tion of powder for the shell formation the temperature was in¬ creased to 6θ°C under continued agitation. In this manner a black free-flowing powder was obtained. By microscopic investi¬ gation the powder was found to consist of base particles covered with a black shell. The large particles were finally screened off 25- to give the powder. Example lβ 100 parts of latex from example 2 was mixed according to example 15 and then used for shell ' formation. Example 17 30. Latex according to example ^ was dried at a low tempera¬ ture without addition of carbon or magnetite powder. Coated base particles according to example 15 were charged to a mixer. The temperature was adjusted to 55°C. Powder prepa¬ red from latex according to example 14 was added for 1 hour. The mixture consisted of the following: Coated base particles 500 g 5. 1 mm particles 500 g Powder, from the latex according to example 14 was added thereto 100 g ^^uB-^A OMPI The temperature at the addition was 55 J C. After completed addi¬ tion of the powder the temperature was increased to 58 C. A black powder coated with an unpigmented shell was obtained. Example 18 5. A latex was prepared from the following: Methylmethacrylate 70 parts Butylmethacrylate 30 parts Mercapto ethanol 0.57 parts Laurie acid 4 parts - 10. Methylethylketone hydroperoxide 0.3 parts Copper, in the form of copper chloride 0.0002 parts Water 240 parts Ammonia (to pH 8.5) 0.22 parts 15. Polymerisation was carried out at 6θ°C. A white latex v/as ob¬ tained. This latex was used for the preparation of a black-pig- mented powder according to the method described in example 15 This powder was then used for coating base particles with a shell. 20. Example 19 100 parts of latex from example 14 were mixed with 30 g of finely dis¬ persed magnetite. The viscosity of the mixture was increased gradual ly . When a paste-like condition had been reached the mixture was spread in a thin layer and dried . 25. The product was then ground . Microscopy showed that the ground powder consisted of several black-coloured small partic¬ les of less than 1 μm but also of larger particles of up to 20-40 um . Both the smaller and the larger part icles are supposed to be formed from small latex primary particles in admixture 30 . with magnetite . The ground powder was added for two hours to a mixer con¬ taining monodisperse base particles mixed v/ith polystyrene par¬ ticles of 1 mm. The mixture consisted of the following : Base particles 500 g 35. 1 mm particles 500 g Powder according to the above was added thereto 1000 g JB. i The temperature at the addition was 55 C. After completed tion of powder for the shell formation the temperature was creased to 65 C under continued agitation. In this manner a black free-flowing powder was obtained. Microscopy showed t 5. the powder consisted of base particles covered with a black shell. The large particles were finally screened off to yie the powder. CMPI.

Claims: Claims 1. A process for the preparation of toner particles including that base particles are treated with polymer and colorant until a uniform distribution of polymer and 5. colorant on the base particles has been obtained and that the polymer is fused to the base particles. 2. A process according to claim 1, characterized in, that the toner particles have a coefficient of variation above 30 percent. 10. 3- A process according to claim 1 -to the parts it is not disclosed in the Swedish patent applications 8107421-3 and 8201739-3. 4. A process according to claim 1, characterized in, that the base particles are kept separated in a continuous 15. gas phase during treatment and fusion. 5. A process according to claim 4, characterized in, that the polymer is added in the form of a solution or a dispersion and that fusion is obtained by removal of the solvent or the dispersion medium. 20. 6. A process according to claim 4, characterized in, that the polymer is added in powder form to the base par¬ ticles and that a temperature is provided which is suffi¬ ciently high for allowing adhesion of essentially the en¬ tire amount of the shell-polymer to the surface of the ba- 25- se particles and at a temperature which is sufficiently low for preventing any substantial agglomeration mutually bet¬ ween the base particles. 7. A process according to claim 6, characterized in, that the temperature is increased after the addition of-the 30. shell-polymer for sintering the polymer. 8. A process according to claim 3j characterized in, that the base particles are kept separated in a continuous liquid phase during treatment and fusion. 9- A process according to claim 3 » characterized in, 35. that a substantial amount of auxiliary particles, having a considerably larger size than the base particles, are mixed with the base particles. 10. A process according to claim 1, characterized in, that the toner particles have a mean size between 2 and 50 yam.

Inventors: AXELSSON ANDERS, BYLANDER LOLO, PORRVIK STEN, TANNER OLOF, AXELSSON, ANDERS, BYLANDER, LOLO, PORRVIK, STEN, TANNER, OLOF
Applicants: KEMA NORD AB, KEMANORD AB
Application year: 1983
Main classification: G03G9
IPC codes: G03G9