EP0001785B1 - Classified electrostatographic toner material, developing mixture and developing process using this mixture - Google Patents

Description

The invention relates to a classified electrostatographic toner material, an electrostatographic developer mixture and a development method using this mixture.

A photoconductor is charged in electrophotography and then exposed imagewise. In the areas in which the photoconductor has been exposed, the charge is discharged or a charge recombination occurs, while the dark areas retain their electrostatic charge.

Due to the different charging in the exposed and unexposed areas of the photoconductor, electric fields arise between them. The resulting latent electrostatic image is then developed on the photoconductor by depositing small colored particles called toner, which have a charge so that the electric fields direct them to the image areas of the photoconductor, on the photoconductor.

A number of methods for developing latent electrostatic images using toner particles are known. One of these methods is called cascade development and is described, for example, in US Pat. No. 2,618,552. Another method is known as a magnetic brush method and is described in U.S. Patent 2,874,063.

A two-component developer material is used in both cascade and magnetic brush development. The developer material consists of a mixture of small toner particles and relatively large carrier particles. The toner particles are held on the surface of the relatively large carrier particles by electrostatic forces which arise from the contact between the toner and carrier particles with the formation of triboelectric charges with opposite polarities on the toner and carrier. When the developer material is contacted with the electrostatic latent image on the photoconductor, the toner particles are attracted to the latent image.

The toner and carrier particles of the developer material are prepared and treated such that the toner particles are charged with the desired polarity and size to ensure that the toner particles are substantially attracted to the desired image areas of the photoconductor. The toner particles are then electrostatically transferred to a desired copy sheet, and thereafter the transferred image of the toner particles is fixed by heat and / or pressure to obtain the fixed copy of the desired image as a final product.

One of the problems in producing the desired image on the copy sheet is to provide the best possible image quality. This is commonly referred to as copy quality. The copy quality includes properties like a clear picture, i.e. a clear record of lines; uniform darkness of the image areas; Background quality, i.e. gray values or the absence of these values in the background areas and other inaccurate properties, all of which are essential for maintaining good copy quality.

Other factors related to the toner that deserve attention in a development process are the use of the toner per copy.

Of course, from an economic standpoint, the less toner used for a given image, the better. Also, in a system in which unused toner is discharged from the air using a filter, it is essential to keep the amount of unused toner as low as possible, thereby extending the life of the filter.

When using a hot fixation process, it is desirable to provide an image that has the best possible heat transfer characteristics to minimize the amount of heat necessary to fix the image. This is not only essential for the sake of energy saving, but also because the fixing time or temperature can be reduced if the toner transfers heat faster. All of these factors play an essential role in designing optimal toner particles.

One of the essential properties of the toner particles that leads to obtaining optimal results in the above-mentioned areas is the size and the size distribution of the toner particles. This fact is known per se, and there are several proposals for different toner particle classification systems.

US Patent 3674736 relates to pigmented polymer particles suitable for use as toners and as developers for electrostatic processes and a method of making such toners. This patent claims materials with an average particle diameter of the order of 1 to 30 μm (NMD) and a geometric mean deviation of the particle size distribution (GSD) of less than 1.5 μm. A specific particle size distribution can be derived from these values by extrapolation and the application of a Gaussian distribution.

German Offenlegungsschrift 2522771 describes toner particles which have essentially the same size distribution as stated in the US patent. In the German patent application a toner with a size distribution according to the number or population is described which contains less than 30% particles with a size below 5 µm, about 25% particles with a size between 8 and 12 µm and less than 5% particles a size over 20! Jm. A fine index ratio of less than 2.50 and a coarse index ratio of less than 1.50 is also specified in the published patent application.

German patent application 2101156 discloses a toner material which has 80%, preferably 90% or more, of particles which are in a size range of approximately 5 to 25 μm. The size of the remaining particles is not characterized. In particular, this published specification contains no information about the ratio of the particles with a diameter below 5 µm to those with a diameter above 16 µm.

The object of the present invention is to provide a classified electrostatographic toner material with which a significantly improved copy quality can be obtained compared to the toner materials according to the prior art.

The invention also relates to an electrostatographic developer mixture and a development process using this mixture.

• a) weniger als 2 Gew.% Teilchen grösser 16 µm;
• b) 9 bis 15 Gew.% Teilchen kleiner 5 µm;
• c) Rest Teilchen mit einer Grösse zwischen 5 und 16 µm; und das einen Medianwert der Teilchengrösse nach der Gewichtsverteilung zwischen 8,5 und 9,5 µm aufweist.

The object of the invention is achieved by a toner material which contains particles with the following size distribution:

• a) less than 2% by weight of particles larger than 16 µm;
• b) 9 to 15% by weight of particles smaller than 5 µm;
• c) remainder particles with a size between 5 and 16 microns; and which has a median particle size according to the weight distribution between 8.5 and 9.5 microns.

The toner particles are mixed with carrier particles to form a developer mixture for use in an electrostatic copying process. In the toner used in a magnetic brush development process in the presence of a carrier, a balance in the size distribution of the toner particles is obtained while it hits the photoconductor surface.

With the toner materials according to the invention, in which the ratio of particles with a diameter below 5 µm to those with a diameter above 16 µm deviates from the normal distribution, excellent copying qualities are obtained.

The invention is explained in more detail on the basis of the following description and the exemplary embodiments.

It was found that by using a toner classified according to the invention, significantly better results were obtained than with previously known conventional toners with regard to the copy quality, the filter life, the toner usability and the fixing quality. A common toner used in the copier of the applicant is classified as follows: 0.8 ± 0.4% by weight have a particle size of less than 5 µm, about 35% by weight have a particle size of more than 16 µm and less than 0.5% by weight % have a particle size larger than 32 µm, the median value of the particle size according to the weight distribution being 13.6 + 0.6 µm. A Coulter counter is used in a manner known per se to measure the size distribution.

In order to be able to compare the toners with one another, samples of different toners were produced with the size distribution given in Table I below.

Each toner was made from a mixed resin system used for Applicant's 111 series copiers. Example I relates to known, conventional toners; and Example 11 relates to a toner according to the present invention.

About one part by weight of the toner of each example was mixed with about 99 parts by weight of a known carrier material consisting of steel balls provided with a polytetrafluoroethylene coating. Carrier materials of this type are described in US Pat. No. 3,947,271. Each mixture was tested in Applicant's commercially available 111 series copier and copies were made. Toner was added to each mixture to maintain a substantially constant toner concentration. 10,000 copies were made with the toner / carrier mixture to balance the toner particle size in the mixture. This balance of toner particle size results from the interaction of the toner, the carrier particles and the photoconductor during copying, and in fact the particle size changes until an equilibrium point is reached at a relatively constant toner concentration and the size distribution thereafter remains essentially constant. This balance is desirable because a toner of this type provides a more uniform copy quality than a developer that has only the initial toner size distribution. Furthermore, the copying quality which is obtained with a developer mixture in equilibrium compared with a developer mixture which has not been brought into equilibrium allows more representative conclusions to be drawn about the mode of operation of the copier. The equilibrium values of each sample are given in Table 11 below.

Subsequent to the particle size equilibrium period, additional copies were made to check the copy quality. The following experiments were carried out to check the copy quality and the behavior of the toner.

Background Quality: The quality of the background of the copies was measured using an S-4 luminance meter and colorimeter from Diano Corporation. This device is used to measure the reflectance of a surface. The results indicate the percentage change in reflectance of the paper before and after making a copy. In general, a background measurement resulting from a change in the reflectance of the paper by more than 1.5% is no longer acceptable and the copy quality is poor due to high background formation.

Return speed: The copier is equipped with a filter, which should clean the returned toner. It is a physical cleaning device and the life of the device is inversely proportional to the return speed. In other words, the lower the return speed, the better the toner behavior. A recycled toner is the one that was deposited on the photoconductor but was not transferred to the copy sheet.

Toner yield: Toner yield is the number of copies that can be made at a given optical density per pound (453.6 g) of toner used.

Optical density: The optical density is the measure for the density or the filling of the image lines on a copy after fixing.

Fixing quality on an offset printing plate: Papers for the production of an offset printing plate represent a base on which it is difficult to fix a toner image. The fixing quality test for offset papers of this type consists in making a copy on paper for an offset printing plate and then qualitatively assessing the adhesion of the toner image on the base.

In the following Table 111, the measurement results regarding the optical density, the background quality, the returned amount, the toner yield and the fixing quality of the copies which were made using the toners of the examples given above are described.

From the table it can be seen that the toner of the example has excellent copy quality and that the toner of example I gives very poor results. It can be seen that the toner according to Example II represents a significant improvement over the toner according to Example I. In fact, background formation is much less, much less toner is recycled, a higher number of copies per pound of toner can be made, and useful offset printing plates can be made with the toner, which is not the case with the toner of Example I.

It can be seen from the results that the toner added to or used in a developer mixture should have a particle size distribution in which particles larger than 16 µm in an amount below 2% by weight, particles smaller than 5 (Lm in an amount between 9 and 15% by weight and the rest should have a particle size between 5 and 16 µm, and in which the median value of the particle size should be between 8.5 and 9.5 µm according to the weight distribution.

It is not entirely clear why there is such a significant improvement with the toner according to the invention, but it is believed that the following factors contribute significantly to this improvement.

The reflectance is a measure of the background quality, and particles on the background can be seen with the naked eye. By reducing the number of particles larger than 16 µm, the number of particles that can be observed with the naked eye is considerably reduced, which produces a better background quality.

It is believed that the amount returned can be reduced as indicated below. Since there are fewer large particles and the majority of the particles are of the same size, the particles will absorb electrostatic charges of approximately the same size to an increased extent. Large particles have a lower charge-to-mass ratio and are less responsive to the force fields during development and transfer; therefore, they are less prone to adhesion and are therefore easier to remove and return. It is also known that large particles have a greater tendency to form dust on the image background due to their low charge-to-mass ratio. Therefore, the lower the proportion of particles larger than 16 µm, the lower the amount returned.

Regarding the higher yield of the toner, it should be stated that a copy is made black by the application of a layer of toner particles which are held in place by electrostatic attraction. The thickness of the layer is irrelevant to the blackness of the copy, as long as the area of the base that is covered is the same size. Therefore, a layer with thinner particles can be used instead of thicker particles, and the weight or volume of the toner used to image the substrate will be lower per layer of particles. By reducing the proportion of particles larger than 16 µm, the weight of the particles per layer will be reduced, resulting in a greater number of copies per pound of toner.

With the toner according to the invention, a significantly improved fixing quality on papers is obtained in the production of an offset printing plate. This result is believed to be due to the better heat transfer characteristic. In theory, it can be assumed that the thinner particle layers of the present invention have a shorter path for heat transfer than the thicker particle layers of previously used toners. This improves the fixing quality of the toner and improves the adhesion of the toner to the base. This property is also essential when using other substrates, because it enables faster fixing than when using thicker layers of toner particles.

It has been found that an exceptionally good copy quality is obtained within the narrow limits of the present invention, which is considerably better than when using previously known toners, and also an excellent utilization of the toner is ensured. However, if the limit values are approximated, in particular if the upper limit of the particle number larger than 16 µm is reached, the improvement in copy quality is no longer so great in comparison with conventional particle size distribution. Nevertheless, a significantly improved toner is still obtained within the large range. An exceptionally good copy quality is obtained within the narrow limits, especially if the number of particles larger than 16 µm is less than 2% by weight.

Claims (3)

1. Classified electrostatographic toner material characterized in that it contains particles having the following size distribution:

a) less than 2% by weight of the particles being greater than 16 microns; .

b) from 9 to 15% by weight of the particles being smaller than 5 microns;

c) the remaining particles being between 5 and 16 microns in size;

and that it has a particle median size by weight between 8.5 and 9.5 microns.

2. Electrostatographic developer mixture containing toner particles and carrier particles of opposite triboelectric charges, characterized in that the toner particles have the values for size distribution and particle median size by weight according to claim 1.

3. Electrostatographic developing process, wherein a developer mixture of toner and carrier particles is applied to a photoconductor with a latent electrostatic image, and wherein the developed image is transferred to a copy sheet, characterized in that the latent electrostatic image is contacted with a developer mixture according to claim 2.