Technical field The present invention relates to a method for the 5. production of toner particles for reprographic applications wherein polymeric base particles are covered with a shell containing a colorant and a polymer.

Background of the invention Particles for use as toners in reprographic methods

10. must fulfil several requirements in order to function in the intended manner. The particles must be sufficiently 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

15. must have a sufficiently low melting point to be fixed to the support by conventional methods such as rolling or heating. The particles must have suitable magnetic or electrical properties, and hereby it is of particular im¬ portance that the surface is capable of accepting and re-

20. taiπing the required charge and especially .in cases where the charge is obtained by rubbing against other materials the surface must have stable triboelectr.i caI properties. However, the surface must not be hygroscopic so that water is retained as this can result in caking of the- particles

25. or in a change of the electrical properties. The material of the particles must be resistant to aging and it must be possible to incorporate a colorant which is resistant to aging. The particles should also have as uniform and spherical form as possible to increase the abrasion resist-

30. ance, to prevent the particles from being fixed to undesired spots on the charged surface and to make it possible to have a higher charge and a charge which is more uniformly distributed between the different particles. The particles should also have as uniform size distribution as possible

35. as variations in size lead to variations in the capability of accepting the charge and thus to a non-uniform deposition on the accepting surface so that the resolution becomes limi ted.

Known toner materials have only fulfilled the above

mentioned requirements to a limited extent. The conventional method of preparation, ie the incorporation of a colorant, charge-modifying agents etc in a plastic mass followed by grinding and screening of the product, certainly makes 5. it possible to choose the material and the additives fairly freely but the obtained particles are of very varying shapes and have a very non-uniform size distribution and this results in a poor abrasion resistance, poor resolution and diffi culties in removing particles, particularly the

10. smaller ones, from undesired spots on the surface. Altei— native methods for the production of toner particles have- been suggested and these have given particles of a more rounded form, but these known methods have not given the desired narrow particle size distribution and they have

15. decreased the freedom of choice with regard to material and they have made the incorporation of additives more difficult.

In the British patent application 2112538 it is described how- advantages tan be gained at the production

20. of toner particles by first preparing the base particles and then add a colorant and optionally other additives in the form of a shell on the base particles. The base particles can here be prepared with desi red properties independent of the process of colouring, the colorant gets

25. a suitable distribution with regard to light absorption and the covering can be made without any considerable effect on the spherical form or size distribution of the base particles.

One problem with the coating with a shell is that

30. the concentration of the colorant in the shell wi ll be high and this can influence among other things the electric¬ al properties of the surface and increase the risk that pigment particles wi ll come loose at abrasion. A certain precision at the shell coating might also be required to

35. avoid agglomeration.

The invention generally The main object of the present invention is to make a shell coating possible according to which the above men¬ tioned disadvantages are avoided. It is a particular object

of the invention to efficiently bind and distribute the colorant in the shell so that the surface properties are not negatively influenced and so that stable particles are obtained. A further ob ect of the invention is to make 5. a simple shell coating technique with a low risk of agg¬ lomeration possible.

The above objects are achieved by the features evident from the appended claims.

As the colorant is applied to a polymeric base part-

10. icle core the mentioned advantages of a shell coating tech¬ nique are maintained. As a monomer, which is later polymer¬ ized, is also supplied to the shell a good anchorage of the colorant in the shell is ensured. In contrast to other binding agents for the shell the monomer can be made to

15. penetrate the base particle somewaht which gives a good binding of the shell to the core. As the core has a li ited swellability or a limited solubility in the monomer the monomer will, however, to the main part be oriented to the shell and secure a good binding and a good incorporation

20. of t e* colorant. These desirable factors are balanced against each other by selection of degree of cross-linking or degree of solubility. The low-viscous monom.er phase is uniformly distributed over the surface of the base part¬ icles already at low temperatures and this contributes

25. to maintaining, or even increasing, the spherical form of the treated particle. A supply of monomer over a water based dispersion phase for the particles gives a good cont¬ rol of the supplied amount of monomer and contributes to maintaining the size distribution of the base particles.

30. It is easy to avoid agglomeration as the monomer addition can be carried out without being hindered by the presence of surface active stabilisers for the particles in the aqueous phase.

Further objects and advantages of the invention will

35. be e ident from the detailed description below.

Detailed description of the invention In principle, the coating technique according to the invention can be applied to very varying types of base particles. It is, for example, possible to coat particles

of inorganic origin such as particles of glass, metal or magnetite. As a rule, base particles of organic material are preferred and the organic material can comprise thermo- setting resins and waxes, for example to form a capsule 5. with soft contents which can be fized to a paper by pressing to crushing. Thermoplastics are particularly preferred, among other things as they can be produced i h different properties for different purposes and as the shell, accord¬ ing to what has been stated earlier, can be made to adhere

10. well to these materials. Among suitable monomers for the thermoplastic material can be mentioned styrene, vinyl chloride, vinylidene chloride, acrylo nitrite and acrylates, such as meth lacrylate and ethylac rylate . Mixtures of diffei— ent monomers can also be used. Particularly suitable mono-

15. mers are styrene, styrene-acryl, styrene-acrylo nitrile- acryl or -vinylidene chloride-acrylo nitrile.

As has been mentioned, the choice of material for the base particle is made with regard to the fact that it must not ^' be too sw.ellable or soluble in the later added

20. monomer. One way of achieving this is to cross-link the base particle by incorporating a monomer with more than one bond forming function such as divinyl benzene, poly- functional acrylates, diallyl phthalate etc. A high amount of such polyfunctional addition monomers in the total mono¬

25 mer amount, eg more than 30% active content, gives highly cross-linked particles which do not permit any monomer penetration worth mentioning. A low content, eg below 1%, gives particles which will absorb too much of the later added monomer. Between 3 and 20% can be a suitable interval,

30, but it can vary depending on the types of monomer which are admixed. An alternative way of limiting the monomer absorption is to choose components for the base particles which are difficultly soluble. Polyacryloni tri le is for example difficultly soluble in later added styrene monomer.

35. A suitable size of toner particles according to the invention is between 1 and 50 μm , particularly between 2 and 25 The base particles should thus have such a size that they after the coa ing have a size within these ranges.

Although the invention is not limited to any part¬ icular size distribution of the base particles it is pre¬ ferred that they have a narrow size distribut on. The co¬ efficient of var ation for the particles (standard deviation 5. x 100/mean diameter) s suitably below 75, preferably below 50 and most preferably below 30.

The coating techn que according to the invention can be used ndependent of the shape of the base particles but it is preferred that the base particles are spherical.

10. Particles below 1 micron which have the same size and are spherical can be produced by conventional emulsion polymerization if small amounts of tensides are used and persulfate is used as initiator. It is more difficult to produce larger particles of the same size but a couple

15. of methods have been described, eg Ugelstad European patent 3905 or Barret: Dispersion Polymerization in Organic Media p. 137, Lehigh University: Graduate Research Progress Re¬ ports No 20, July 1983. However, for both mentioned methods the base processes do no-? allow coloring of the particles,

20. either with organic or inorganic colorant ^' s. For reprographic purposes it s, however, usually necessary that the part¬ icles are coloured .

Particles prepared according to the method described in the European patent specification 3905 are particularly

25. suitable as base particles according to the present in¬ vention as spherical particles with a narrow molecular weight distribution, eg with coefficients of variation below 30, also below 10 or below 5 per cent, can be obtained according to this known method. The European patent 3905

30. is hereby incorporated as reference.

This method comprises the preparation of an aqueous dispersion of monodisperse seed particles having a mean diameter below about 3 μm , which, in addition to polymer, contains a fairly low-molecular substance which is diffi-

35. cultly soluble in water but can be absorbed by the polymer, eg as described below, addition of a monomer which is less difficultly soluble in water than the difficultly soluble substance but which is capable of being absorbed by the seed particles containing the difficultly soluble substance

under conditions which permit molecular migration of the monomer to the particles and absorption therein, and poly¬ merization of the monomer particles. The thus obtained particles are suitable as base particles for the toner 5. of the present invention.

When the base particles have been prepared, for ex¬ ample according to any of the mentioned methods, it might be suitable, depending on the system of surface active agents that have been used, to wash away rests of emulsi-

10. fiers from the surfaces of the core particles of uniform size before the subsequent treatment is carried out. How¬ ever, if for example a fatty acid soap or metal hydroxides have been used as colloidal stabilisers it is not necessary to wash the particles before the following treatment. A

15. lowering of the pH might here be sufficient.

The colorant can be a molecularly soluble substance, for example an organic colorant, but it is preferred to use pigments, and particularly inorganic pigme-nts, to get the best long term durability, and then particularly carbon

20. black or magnetite in those 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 and 25 per cent. When the colorant is incorporated essentially in

25. 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. The used pigments should have a particle size which is clearly below that of the base particles, for example below 1 μm and preferably below 0.3 μ . For soluble

30. organic dyes the amount is more easily calculated on basis of the weight and in these cases the above given numerical values can instead be used as per cent by weight values.

The coating of the base particles with a colorant can be made either in a dry state or in a liquid phase.

35. in the first case dried base particles are mixed with pig¬ ment. To obtain an even distribution of the pigment the powder should be subjected to shearing forces during the mixing. If the coating is carried out in a liquid - prefer¬ ably water - a dispersion of the pigment and optionally

uniform size and the addition hereby makes for the surface of the core- particles. By selecting suitable shearing forces it is then possible to get a uniform d stribution on the 5. core particles.

In connection with this different measures can be taken to facilitate the absorption of the colorant on the surface of the base particle. The surface can be softened by the addition of a solvent or by increasing the temp-

10. erature. In aqueous phase flockulation can be facilitated by flockulating the stabiliser systems. The cores can also be made to have a structured composition with an outer part which is less cross-linked than the main part of the core particle. In this manner it is possible to obtain

15. core particles ith very Little swelling but where it never¬ theless is easy to make a pigment layer become fixed to the surface.

Good results have been obtained by addition of an adhesi e agent together with the colorant. The most varying

20. substances can be used as adhesive liquid, for example alcohols, organic esters, aromatic or aliphatic hydrocar¬ bons, different waxes ith suitable melting points etc.

When the monomer is added later by migration in a dispersion agent phase it is particularly suitable to use

25. substances which are sparingly soluble as adhesive agents as these can faci litate the monomer migration to the colour¬ ed particles, ^" even after that a separate monomer phase has been formed on these, and contribute to a good distri¬ bution of the colorant in the monomer. The solubility of

30. the substance in water should be greater than that of the base particle but it should be below 1 Q~2 , preferably below 10 ^" ^ and most preferably below 10"^ g/l. The molecular weight should be below 5000 and preferably also below 500. The substance should preferably be organic for ^' the best

35. compatibility with other components and, for solubility reasons, it should be non-ionic. Hydrocarbons and substi¬ tuted hydrocarbons can be used. Examples of substances are chlorododecane, dioctyl adi pate, stearylmethacrylate or an initiator which is sparingly soluble such as dioc- tanoylperoxide. It can be particularly suitable to use-

very sparingly soluble monomers as the adhesive l quid. Besides the mentioned effect of swelling also the advantage that the substances are polymerised is hereby obtained and there will thus be no rests of adhesive liquid in the 5. particles, which among other things can lead to exudation on the surfaces of the finished particles. It is also poss¬ ible to use an oligomer as the substance which is difficult¬ ly soluble. The difficultly soluble substance should be a liquid under the conditions at addition and monomer mig-

10. ration.

The amount of adhesi e liquid should be large enough for all the pigment to be picked up by the core particles but not so large that the core particles will agglomerate. The required amount can vary, for example depending on

15. amount of pigment and the degree of retention of the ad¬ hesive liquid by the base particle. Suitably the amounts can be between 1 and 50 per cent and particularly between 5 and 25 per cent of the weight of the base particle.

To achieve an efficient mixing it has been found

20. suitable to add coarser particles, for example glass beads, to the mixture of core particles and adhesi e agent, part¬ icularly at dry blending methods. To improve the free-flow¬ ing properties of the powder mass these auxiliary particles should be considerably larger than the base particles,

25. suitably betwee.n 10 and 100 times larger or for example about 0.5 to 5 mm. Better free-flowing properties are ob¬ tained with larger amounts of auxiliary particles but at the same time the production capacity decreases and it is thus suitable to keep the content of auxiliary particles

30. between about 20 and 90 per cent by weight in the powder mass .

If correct conditions for the mixing have been select¬ ed the pigment will be quantitatively picked up on the surface of the core particles and no free pigment particles

35. are visible at microscopy. For use in reprography it is, however, necessary that the pigment layer on the surface is protected by being mixed into a polymer layer or covered by a polymer layer. According to the invention a covering layer is produced by adding a monomer and initiator to

the pigmented particles in such a manner that the monomer is included in the pigment layer or forms a circle surround¬ ing this. The monomer is then polymerized and hereby a pigmented particle wherein the pigment layer is protected 5. is obtained.

Suitable monomers for the shell are the same ones that have been listed as suitable for the base particles. It is, however, possible to choose different materials for the core and the shell. The monomer should among other

10. things be selected with regard to desired softening pro¬ perties. The shell shall normally not be cross-linked. When the toner particles are to be used in processes where adhesion is obtained by heating it is desirable that at least a part of the polymer content of the finished particle

15. has a glass temperature (.Tg) below 100°C, and preferably below 80°C. The glass temperature should suitably be above 30°C. Suitable hardness properties can also be influenced by the degree of polymerisation. When a substance which is difficultly soluble, as described above, is used to

20. facilitate the monomer addition the monomer should be more soluble in water than the substance which is difficultly soluble, preferably at least 10 times and most preferably at least 100 times as soluble in water.

The shell monomer can be added in any manner which

25. gives a good protection of the colorant. It is preferred to disperse the coated base particles in an aqueous liquid. Hereby dispersing agents, tensides, steric stabilisers, powder stabilisers, alcohols, ammonia etc can be used. The dispersion is then mixed with the monomer, which subsequent-

30. Ly can be made to migrate to the coloured base particles by means of conventional swelling technique used in polymer¬ isation processes. If the base particles permit swelling the added monomer will first be absorbed in the cores and these will grow in size. The pigment layer will then still

35. be on the surface of the particles. When more monomer, in addition to the amount which has been absorbed, is gra¬ dually added this additional monomer will form a separate external phase. If the monomer drops are smaller than the diameter of the monomer coated base particles the swelling

can continue also without the addition of special auxiliary swelling agents. At the polymerisation the pigment layer is encapsulated. To obtain a good wetting of the entire particle surface so that a spheric monomer coating is ob- 5. tained the surface of the particle is suitably prepared so that it has a surface tension which is suitable with regard to the monomer.

As has been mentioned, it is generally desirable that the colorant is encapsulated or covered by the added

10. monomer and that it does not settle on the surface of the particle, neither before nor in connection with the poly¬ merisation. The colorant should thus be soluble or dis- persible in the monomer or have such a surface tension that it makes for the phase boundary surface between the

15. base particle and the shell rather than for the surface of the shell. Organic colorants can thus advantageously be soluble or dispersible in monomer or polymer. To obtain the proper surface properties a surface coating can be required for pigments. The pigments can for example be

20. coated with a hydrophobing substance to increase the com¬ patibility of the particles with for example the polymer material in the shell or in the base particle. For this purpose a polymer coating can be used. For magnetite a silane coating is especially suitable as this makes the

25. surface Less sensitive to moisture, decreases the conduct¬ ivity and gives good compatibility with other materials. Particularly after being cured the silane coating is not tacky and thus the risk of agglomeration is not increased. The amount of added silane can be between 0.1 and 10 per

30. cent by weight and particularly between 0.5 and 5 per cent by weight.

A method of controlling the distribution of pigment in the shell which is generally usable is to pre-treat the pigments by direct polymerising of a layer of cross-

35. linked polymer on their surfaces. Such a layer on the pig¬ ment particles secures a good wetting of the entire pigment coating on the base particles with monomer so that an ess¬ entially uniform and spheric monomer layer is obtained. A further improvement in the dispersibility of the pigments

and reduction of the tendency to agglomeration is obtained if the monomers for the cross-Ii nkabLe polymer on the pig¬ ment particles contain Long side-branches, for example aliphatic chains with 10 to 20 carbon atoms, or long a I- 5. cohols such as Lauryl- or stearylalcohol esterified with the monomer as these branches will be oriented out from the_ pigment surface at the polymerisation. The -coating can be carried out by mixing the pigment, the monomers and the initiator. Well-known coupling agents, such as

10. silanes, can suitably be used to improve the adhesion be¬ tween the pigment surface and the polymer. A suitable poly¬ mer content on the pigment is between 0.1 and 10 per cent.

The amount of monomer added to the shell shall be sufficient for firmly covering or containing the used amount

15. of colorant. The volume amount of monomer should thus not be less than 0.1 times the volume of the base particle. If the base particle is highly cross-linked and thus diffi¬ cult to melt at the fixation of the toner particle when it is used it can be necessary to add a larger amount of

20. monomer to the shell, for example above 0.5 and preferably above 1 times the volume of the base particle. If the in¬ crease in radius of the shell is very large this can give rise to problems with agglomeration, influence the particle size distribution and give difficulties ith the distri-

25. bution of colorant. The shell should thus constitute less than 10 and preferably less than 5 times the volume of the base particle.

For the polymerisa ion it is preferred to use a mono- mei—soluble initiator and this can thus be added together

30. with the monomer if it is sufficiently soluble in water to be able to migrate through the aqueous phase to the covered base particles. If the initiator is very hydrophobic it can advantageously be added directly to the base part¬ icle, suitably in connection with the addition of colorant,

35. eg together ith this or as adhesive agent.

As a rule it will suffice to add the colorant once, followed by formation of shell, but if it is desired it is of course possible to repeat these steps. Hereby it might be necessary to choose a polymer for the inner shells

which is difficultly soluble or which is cross-linked. Example 1

This example illustrates the preparation of monodis¬ perse base particles. 5- 77 ml water, 11.7 ml of c lorododecan, 1.8 g of ben- zoylperoxi de, 9.3 ml of di ch loroethane and 0.2 g of sodium lauryIsulfate were homogenised in a two-step Manton Gaulin ho ogeniser, model 15 M, with 200 kg/cm^ in the first step and 80 kg/cm^ in the last step for about 1 to 1.5 minutes

10. and this resulted in an emulsion with particles having a size in the order of 0.1 to 0.2 μm . A seed latex of mono - disperse polystyrene particles having a diameter of 0.65 urn (determined by electron microscopy) was added to the emulsion. The amount of seed latex added was 83.8 ml, con-

15. taining 77 ml of water and 6.8 ml of styrene 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 vacuum. When the acetone and the di ch loroethane had been evaporated

20. 1.0/ g of sodium lauryI sulfate 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 60°C and the polymerisation started. After

25. 25 hours polyme.ri sati on a monodisperse latex having a part¬ icle diameter of 2 rim was obtained.

This latex was used as a starting material for a second swelling and polymerisation step. 28.5 ml of the latex, containing 25 ml of water and 3.5 ml of polystyrene

30 particles, were mixed with an emulsion of 35 ml of water, 4 ml of Perkadox SE-8 (dioctanoyl peroxide), 3 ml of chloro- dodecane and 0.2 g of sodium lauryIsulfate, which emulsion had been homogenised using the same equipment and under the same conditions as stated above for the earlier swelling

35 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 lauryl- sulfate, 1.0 g Berol 267 and 930 ml of water were added to a total amount of water of 1000 ml. 150 ml of distilled

styrene and 25 ml of divinyl benzene with 50% active content were added under agitation at 25-30°C and after 3 hours the temperature was increased to 70°C to get a complete polymerisation. The final latex was monodisperse and had 5. a particle diameter of about 7 μm . Example 2

The particle from example 1 were washed and dried. They were then ixed with 800 g 1.5 mm large glass beads, 25 g of carbon and also 20 ml of a mixture of styrene and

10. stearylmethacrylate ⁽ 70:30) and further 10 ml of methanol. After mi ing for 30 minutes all of the carbon had been retained on the surface of the core particles. The glass beads were sieved off and the pigmented particles were dispersed in 800 ml of water containing 5 g of low molecular

15. polyvinyl alcohol having a degree of hydrolysis of 80%.

140 g of styrene, 60 g of butyImethacrylate and 8 g of tertiary butylperoxioctoate (Luperox 26) were mixed and then emulsified in 800 ml of water containing 5 g of polyvinyl alcohol. The emulsion was then mixed with the

20. dispersion of pigmented particles. After 5 hours all monomer had swelled over to the pigmented particles which under microscope looked like dark-coloured cores surrounded by a circle of liquid. The temperature was raised to 80°C. After 10 hours black particles with a diameter of 9 μm

25. were obtained. .After washing the particles were slurried in water and a solution of a colorant of chromium-complex type in methanol was added. The particles were hereby cover¬ ed with a thin layer of colorant and obtained a suitable charge for copying in a copying machine using a selene-drum

30. and Hδganas-ca rri er . Copies with very little background and improved resolution, compared with the normal toner, were obtained. Example 3

As example 2 but 25 g of carbon were replaced by

35. 200 g of magnetite (Bayferox 8600) which had been hydrofobed by treatment with silane. As a final step the toner part¬ icles were treated with a solution of nigrosin in methanol and they hereby obtained a suitable charge for copying on a mono-component copying machine with a photodrum covered

with an organic photo-sensitive layer. Copies with good resolution were obtained. Example 4

This example relates to the production of monodisperse 5. core particles having a structured composition. According to example 1 but with the difference that 125 ml of styrene and 50 ml of divinyl benzene with 50% active content were used. After polymerisation for 10 hours at 70°C the tem¬ perature was lowered and 100 ml of a 25% emulsion of styrene

10. containing 3% of divinyl benzene were added. Sodium lauryl- sulfate was used as emulsifier for the emulsion. After 1 hour all of the styrene had been adsorbed on the pre-form- ed core particles. The temperature was raised to 85°C for 5 hours for polymerisation of the styrene. These core part-

15. icles were used for the preparation of pigmented particles according to the method of example 2. The core particles swelled less than those in example 2 but did, however, satisfactory pick up the pigment. Example 5

20. Example 2 was repeated but the suspension from Example 1 was not dried after the washing but mixed wet with glass beads, carbon, styrene and stea rylmethacrylate (without addition of methanol). The additives were picked up on the base particles under agitation and heating to 60°C.

25. Polyvinyl alcohol was added to the dispersion and this was further treated in the same manner as in Example 2 and resulted in toner particles with essentially the same properti es .