Electrophotographic process using polyamide containing developer

Abstract

A developer mix for use in electrostatic printing to develop latent images, including charged and uncharged areas of an image bearing sheet said developer mix comprising separate granular carrier particles, and a developer powder comprising a coloring agent and a resin having a triboelectric relationship of opposite polarity with respect to said carrier granules, said resin comprising a blend of resins in which the physical properties of the blend are distinct with respect to the physical properties of the resin components which are heat blended together, the principal resin being a polyamide resin which represents the infrangible resin component, and the completed resin being reduced to a melt point within the range of 8.degree., whereby developed images of substantially improved black density may be formed over extended operating periods.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of my copending application Ser. No. 123,065, filed Mar. 10, 1971, now U.S. Pat. No. Ser. 3,764,538, which is a continuation of copending application SEr. No. 692,732, filed Dec. 22, 1967, now abandoned, which in turn is a continuation-in-part of prior copending application Ser. No. 357,743, filed Apr. 6, 1964, now abandoned all assigned to the same assignee as the instant application.

Description

This invention relates to electroscopic powders of the type useful in rendering visible the latent electrostatic images produced by photoelectrostatic or electrostatic copying. More particularly it relates to improved electroscopic powders for use in automated type photoelectrostatic copying apparatus.

In photoelectrostatic copying processes, an electrostatic charge pattern is created on a charge photoconductive layer, such as zinc oxide or selenium, by exposure to a light pattern. Various techniques and devices have been employed to expose the charged surface, such as projection or contact printing methods, fiber optic imaging devices, and various phosphor display devices. Upon exposure of the charged layer to light under darkroom conditions, a latent electrostatic charge image is formed.

Still other image generating devices are employed that directly deposit a charge pattern corresponding to the graphic subject matter by the use of single or multiple styli. The technique of direct imaging may be carried out in the presence of light.

Irrespective of the technique employed for creating a latent electrostatic charge image, it must be rendered visible by development with a suitable resinous, thermoplastic, electroscopic powder and rendered permanent by the application of heat, pressure, solvent vapor or other fixing technique. The developed image may be fixed in place on the surface upon which it is formed, or it may be transferred to a new surface and fixed thereon.

A number of techniques are available and in wide use for carrying out the developing step which brings the electroscopic powder, or toner powder as it is known in the art, into contact with said latent image. These include, for example, powder cascade, powder cloud, and dry magnetic brush development. The advancement represented by the improved electroscopic powders of this invention is applicable to all of the foregoing systems where a charge-sensing powder is brought into contact with an electrostatically charged surface for the purpose of producing a visible image.

Further discussion of the improvements represented by this invention will be explained in terms of the magnetic brush type of apparatus, but the novel electroscopic powders disclosed herein can also be used with equal advantage in other developing apparatus and their use is not limited to magnetic brush techniques.

The magnetic brush method for developing an electrostatic image involves the use of a mixture of magnetically attractable particles and electroscopic powder. This mixture or "developer mix" is formed up into a brush-like mass on the surface of a cylindrical roll under the influence of a magnetic field created by magnetic means disposed within said roll.

The electroscopic powder is held to the magnetically attractable carrier particles by a triboelectric effect which results from frictional contact between the particles. This effect is more fully described in U.S. Pat. No. 2,874,063 dated Feb. 17, 1959. The relative position in the triboelectric series of carrier and electroscopic powder materials will determine the polarity of the charge generated on the electroscopic powder. Hence, particular materials can be selected for either positive or reversal printing. In practice, the electroscopic powders are mixed with larger carrier particles, such as iron, ferrites, magnetites, cobalt, and nickel. The carrier particles align themselves along the lines of magnetic flux provided by the magnetic means so that they stand erect on the surface of the cylinder. In this manner the particles, carrying the electroscopic powder present a uniform and continuous array of developer mix along that portion of the roll which contacts the electrostatic recording member bearing the latent electrostatic image thereon.

U.S. Pat. No. 3,003,462 discloses a typical magnetic brush development apparatus wherein the developer mix is deposited in a trough, thereafter is picked up on the periphery of an applicating cylinder having the magnetic means therein, and is formed into a brush in the environment of said magnetic field. As the rotating applicator cylinder carries the developer mix outside the magnetic field, the magnetic brush collapses and developer mix falls back into the reservoir. This cycle of brush formation and collapse is repeated as long as the developer roll rotates.

Electroscopic powders available heretofore have left much to be desired when used in automated electrostatic copying machines, particularly where the magnetic brush-type apparatus is employed. One of the major problems is that of deterioration of the electroscopic powder component of the developer mix. One evidence of such deterioration appears in the photoelectrostatic copies which begin to show adherence of the electroscopic powder indiscriminately in both image and non-image areas.

Another evidence of mix deterioration is a fall-off or loss in copy density, that is, the developed image appears gray rather than having an intense black color.

Also, carrier particles may begin to deposit on the copy sheet as a result of mix deterioration giving the photoelectrostatic copy a "gritty" feel.

Still further problems caused by mix deterioration relate to improper mixing and impairment of the mechanical mixing means of the developer apparatus.

The automated photoelectrostatic office copying equipment under discussion is designed especially for high production, high quality copying. Equipment of this type is required to produce up to 6000 copies in a typical work day. The deteriorated condition referred to above can take place rapidly. Deterioration of known developer mixes has heretofore necessitated complete and frequent replacement with fresh material.

Deterioration is caused by physical changes in the electroscopic powder. These physical changes primarily concern the particle size of the powder. The first such change relates to particle size fracture or comminution, and the second relates to agglomeration or clumping of small particles into larger ones.

The forces which operate in the magnetic brush developer exert a grinding or milling action on the developer mix. Electroscopic powder particles may be split or fractured so that the new fragments do not have the same electroscopic properties as the particles from which they were formed. Each of these fragments is present as a spurious particle which serves only to impair the performance of the developer system.

Further attrition of the particulate matter generates excessively small particles referred to as dust or fines that are incapable of discriminating between the charged and uncharged areas. These fines tend to become airborne and create an undesirable condition from a housekeeping standpoint.

Agglomeration or clumping is caused by an increase in mix temperature. This temperature rise may be due to the absorption by the powder of frictional energy developed through impact between the particles as they are mixed and churned within the developer unit. Another source of heat is the high temperature fusing unit within the apparatus. The thermoplastic toner particles begin to clump or agglomerate as they reach their softening or tackifying temperature. In a severe condition the agglomerates may occlude some of the iron carrier particles. These clumps, containing both iron and softened electroscopic powder, completely disrupt the developing step.

These clumps often become deposited in clearances between moving and stationary mechanical parts thereby increasing the power required to drive the magnetic brush roller. The additional power is dissipated as heat so that the process of agglomeration becomes progressively worse.

Generation of the triboelectric charge on the electroscopic powder depends upon proper contact between toner and carrier particles. Clumping and agglomeration prevent the proper circulation and blending of toner with the carrier in the developer apparatus. A free-flowing condition is particularly necessary during replenishment when fresh toner is added to a depleted mix. Poor circulation gives rise to a non-uniform powder mixture which produces copies that are unevenly developed. Poor blending of electroscopic powder and iron particles reduces the level of triboelectric charge generated on the powder.

The demands placed on electroscopic powders suitable for high speed, continuous automatic electrostatic copying, are exacting and have heretofore not been met. From the foregoing discussion, it is seen that the thermoplastic, resinous toner particle must have certain distinct properties if it is to achieve a practical mix life in a magnetic brush developing apparatus.

The resin blend should be sharp-melting so that it is converted from discrete, solid pieces to a flowable material over a temperature range not greater than about 5.degree. to 8.degree.F. Such a resin blend will flow smoothly onto the paper to form a permanent image and solidify rapidly when removed from the fuser, thus producing an image which will not smear. The thermoplastic resin should remain in a solid state at temperatures substantially higher than room temperature so that it does not soften, become tacky, and form cakes, clumps, or agglomerates. Such a divergence of requirements demands that the thermoplastic resin be at once tough and resilient enough to withstand the grinding action in the developer apparatus, yet sufficiently brittle and frangible to permit its reduction to powder on conventional grinding equipment.

Some thermoplastic compositions, which are tough and infrangible, have excellent electroscopic properties, but cannot be manufactured on conventional milling equipment. Other resins which may have excellent electroscopic properties and which may be readily ground to a desired particle size in conventional milling equipment are too brittle or frangible for the magnetic brush apparatus. These are reduced to an inordinate amount of dust and fines by the mixing action of the developer unit.

It is a primary object of this invention to provide an electroscopic powder particularly suitable for use in automated and continuous photoelectrostatic copying machines.

It is an object of this invention to provide an electroscopic developing powder having greatly improved resistance to deterioration in magnetic brush developer apparatus.

It is a further object of this invention to provide an electroscopic developing powder that will fuse in a narrow temperature range below the char point of paper and will resist clumping or agglomeration.

It is a further object of this invention to provide electroscopic developing powder which is resistant to attrition or grinding when used in a magnetic brush developer apparatus.

It is a still further object of this invention to provide an electroscopic developing powder comprised of a blend of thermoplastic resins having suitable frangibility properties and temperature response characteristics that will produce consistently high quality electrostatic copies having a high contrast between image and non-image areas.

These and other objects are apparent from and are achieved in accordance with the following disclosure.

The electroscopic powders which constitute this invention comprise a blend of a tough, infrangible synthetic resin with a highly frangible thermoplastic synthetic resin which melts between about 70.degree.C. (158.degree.F.) and 165.degree.C. (329.degree.F.), preferably in the range of 213.degree.-235.degree.F., said blend having the critical property of going from discrete particles to flowable material in a range from 50.degree.-8.degree.F. The blend of synthetic thermoplastic resin materials preferably should have a correspondingly high softening point, that is, the thermoplastic particles should remain discrete at temperatures up to 130.degree.F. and not adhere to one another or form agglomerates. The preferred average particle size of the electroscopic power ranges from 4 to 10 microns with the over-all range of particle sizes ranging from 1 micron to 74 microns.

The invention is directed to a novel thermoplastic resinous electroscopic powder comprising a tough, infrangible resin component, such as a thermoplastic polyamide resin, which is chemically blended with a highly frangible, brittle substance such as a rosin-modified maleic anhydride-polyhydric alcohol resin, an unsaturated co-ester resin such as a diphenol resin esterified with a fatty acid, or a pure non-heat reactive phenolic resin.

The powder blends may optionally include additives such as polyol resins, toluenesulfonamides, or butylated-hydroxy-toluene which enter into the blends as fluxing agents, tending to decrease the melt viscosity of the thermoplastic blend.

The preferred polyamide resins are produced by the reaction of high molecular weight polyene fatty acids and their esters with an amine. By reacting ammonia, a primary or secondary amine, a hydroxyamine or an alkanolamine, with a high molecular weight carboxylic acid or an ester thereof, either saturated or unsaturated, said acid or ester being obtainable by polymerizing at elevated temperature said polyene fatty acid or esters thereof, and in the case of the esters, converting the polymers to the corresponding acid if desired, there are produced the preferred polyamides. Examples of polyene fatty acids in esterified form are 9,11- and/or 9,12-octadecadienoic acid (obtainable from soybean oil and dehydrated castor oil), linoleic acid, alpha and beta-eleostearic acid (obtainable from tung oil). The preferred esters are those derived from methanol, ethanol, and propanol. Primary or secondary amines may be used such as, for example, methylamine,ethylamine, propylamine, ethylenediamine, tetraamethylenediamine, pentamethylenediamine, piperazine, and diethylenetriamine. The class of thermoplastic, polyamide resins is disclosed in U.S. Pat. No. 2,379,413 and sold by the General Mills Company under the trademarks "Versamide" and "Omamid". Other suitable polyamide resins are also available from the Krumbhaar Resin Division of Lawter Chemicals, Inc., under the trademark "Polymid".

The second thermoplastic constituent in the electroscopic powder is extremely frangible but it is sharp melting. A suitable frangible constituent may be a rosin-modified phenolic resin, such as those prepared by modifying a phenol formaldehyde resin with the reaction product of maleic anhydride and rosin or a polyhydric alcohol such as glycerol or pentaerythrytol. Such rosin-modified phenolic resins are sold under the trademark "Amberol" by Rohm & Haas Company. Diphenolic resin materials esterified with a soya fatty acid and certain thermoplastic phenolformaldehyde resins exhibit satisfactory frangible properties. The esterified diphenolics are available from the Johnson Wax Company Chemical Division of Racine, Wis., and the thermoplastic phenol-formaldehyde resins are available from the Krumbhaar Resin Division of Lawter Chemicals, Inc., and from Nelio Chemicals, Inc., Jacksonville, Fla., as their VBR-800 series resins.

The blend of infrangible and frangible resins with coloring materials forming the electroscopic powder should be highly infrangible and should have a fracturing value of at least 400 gram-centimeters when measured on a wafer of resin 3.75 cm, in diameter and 0.5 cm. in thickness at 100.degree.F. by the falling ball method. In this method, as adapted from American Institute of Mining and Metallurgical Engineers, Vol. 87, p. 35, 1930, the resin wafer is subjected to impact by a falling ball and the energy (measured in gram-centimeters) required to just fracture the wafer is measured. The infrangible resin component of the resin blend should preferably have a fracturing value of at least 1000 gram-centimeters in the foregoing test while the frangible resin component may have low fracturing values in the range of 100-200 gram-centimeters.

The resin blend forming the electroscopic powder should not soften or become tacky at temperatures below 130.degree.F. The softening properties of resins can be measured with a penetrometer by the procedure of A.S.T.M. Standard No. D5-61. By this procedure, it has been found that resin blends which permit a maximum penetration not greater than 1.0 millimeter at 130.degree.F. with a standard needle (No. 4103) in a standard "Lab-Line" penetrometer (No. 4100) at a force of 100 grams for 5 seconds resist softening and do not clump or agglomerate during use as electroscopic powder.

The action of the fluxing agent, as an optional component, is believed to lower the melting point of the blend without broadening the melting point range. Successful fluxing agents are provided by the group of polyhydric alcohols sold by the Shell Chemical Company under the trademark "Polyol X-450" of the general formula ##EQU1## having molecular weights in the range from 1000 to 2000. Purified wood resins such as those sold by Hercules Powder Company under the name "M-Wood Rosins," and toluenesulfonamides available under the trademark "Santicizer 8" and "Santicizer 9" from Monsanto Chemical Company are also suitable.

The various thermoplastic resins are compounded by reducing the resinous materials to the molten state and then blending in the required pigments, dyes, and coloring agents and the fluxing materials where they are to be included, using conventional mixing equipment.

______________________________________ FORMULATION NO. 1 (Major percentage of tough, infrangible resin) Polyamide Resin 50-90% Frangible resin component (phenolic maleic anhydride- polyhydric alcohol resin 5-40% Polyols 0-10% Nubian resin black 1-5% Carbon black pigment (Neo-spectra, Mark III) 1-5% FORMULATION NO. 2 (Minor percentage of tough, infrangible resin) Polyamide Resin 9-50% Frangible resin component (Pure phenolics Krumbhaar K-254 50-90% Polyols 0-10% Nubian resin black 1-6% Carbon black pigment (Neo-spectra, Mark III) 1-2% ______________________________________

The following examples are given to illustrate preferred embodiments and process for producing electroscopic powders embodying this invention. It will be understood that this invention is not limited to these examples.

In these examples, all percentages are given on a weight basis.

EXAMPLE 1

______________________________________ Electroscopic powder formulation is: Synthetic, polyamide, thermo- plastic resin (Versamide 930) 33% Polyol (Shell X-450) 9.9% Maleic anhydride-polyhydric alcohol rosin-modified resin (Amberol 800, Rohm & Haas Company) 49.5% Nubian resin black dye 6.6% Carbon black pigment (Neo-spectra, Mark II) 1.0% ______________________________________

The quantity of polyamide resin called for is heated in a suitable vessel equipped with a conventional impeller type mixer until the resin is just molten so that it can be stirred. To the molten polyamide resin is added the Polyol while the mass is being agitated. Agitation continues during the addition of the maleic anhydride-polyhydric alcohol resin-modified resin. After the maleic anhydride-polyhydric alcohol resin is completely melted, the quantity of black dye is added to the batch followed by the addition of the carbon black.

After the batch has been thoroughly mixed, it is removed from the mixing vessel, cooled, crushed and pulverized to an average particle size ranging from 4-10 microns. Understandably there will be particle sizes ranging from under one micron to 50 microns and larger. As a final step, the electroscopic powder is passed through a 200 mesh screen so that the largest particle size possible in such an electroscopic powder would be under 75 microns.

The softening point of the electroscopic powder was measured by placing a quantity of the electroscopic powders in a constant temperature oven for 12 hours. A series of oven tests, at different temperature levels, revealed that powder produced in Example 1 remained in particulate form, and did not clump or agglomerate until tested at a 155.degree.F. level. The melt point of the electroscopic powder was in the range of 215.degree.-220.degree.F. measured in accordance with A.S.T.M. Method No. E28-58T.

The electroscopic powder was combined with iron particles in a ratio of one part powder to 15 iron, making a developer mix suitable for developing electrostatic images. The developer mix was charged into the developer unit of a photoelectrostatic copier. More than 50,000 copies were developed with this developer mix requiring the periodic addition of fresh electroscopic powder to replace the amount taken out by the making of copies. Otherwise the developer unit did not require servicing such as complete replacement of the charge, or cleaning of the unit to remove clumps or agglomerates.

The image copies were of uniform density indicating complete and thorough mixing between the carrier and powder. A high image density was maintained throughout the run while non-image areas remained clean and free of spurious toner deposition. A copy is considered to have proper image density if reflectance density measurements, taken by a standard Photovoltmeter, are above 1.0 units.

Similarly, reflectance readings can give a measure of the contrast between the image and non-image areas. The non-image area on a processed copy should not measure more than 0.05 Photovolt units. The copies were clean in the non-image areas giving Photovolt reading less than 0.05 units.

The copies were not "gritty" indicating that the carrier particles were being retained in the system and not occluding on the powder.

EXAMPLE 2

______________________________________ Ingredients: Polyamide resin (Versamide 930) 74% Maleic anhydride-polyhydric alcohol rosin-modified resin (Amberol 800) 19% Nubian resin black dye 5.6% Carbon black pigment (Neo-spectra, Mark II) 1.4% ______________________________________

The ingredients were processed in accordance with the steps set forth for Exammle 1 above. The melting point range and softening point of the above electroscopic powder were 213.degree.-220.degree.F. and greater than 140.degree.F., respectively. It will be noted that this example incorporates a major percentage of the tough, infrangible polyamide material. Photovolt readings were all above 1.0 units. Such an electroscopic formulation finds particular utility in environments where the copying equipment is used for extended periods of time and where there is a high ambient temperature.

EXAMPLE 3

An electroscopic powder was prepared in accordance with the procedure of Example 1 wherein the electroscopic powder was comprised largely of polyamide material.

______________________________________ Ingredients: Polyamide resin (Versamide 930) 80% Maleic anhydride-polyhydric alcohol rosin-modified resin (Amberol 800) 7% Nigrosine dye 6% Polyols 8% Carbon black pigment (Neo-spectra, Mark II) 1% ______________________________________

The melting point range and the softening point of the granular mass were 217.degree.-222.degree.F. and greater than 140.degree.F., respectively. The formulation of Example 3 is suitable for use under high temperature conditions. The inclusion of a fluxing agent tends to lower the melt point slightly and give a more uniform image.

EXAMPLE 4

______________________________________ Ingredients: Polyamide resin (Versamide 930) 33% Polyol (Shell X-450) fluxing agent 9.9% Phenolic Resin (No. K-254 Krumbhaar Chem. Div. of Lawter Chemicals, Inc.) 49.5% Nubian resin black (National Aniline) 6.6% Carbon black pigment (Neo-spectra, Mark II) 1% ______________________________________

The above formula has substituted for the maleic anhydride rosin-modified resin a pheno-formaldehyde resin which is a highly frangible, sharp melting thermoplastic material. The performance of this formulation in respect of print quality and resistance to developer mix deterioration was fully equivalent to that observed in Example 1. The melting point range and softening point of the above example were 215.degree.-220.degree.F. and greater than 140.degree.F., respectively.

EXAMPLE 5

The formula of Example 1 was modified by substituting the same percentage of a purified wood rosin identified as M-wood rosin manufactured by the Hercules Powder Company for the fluxing agent identified as Shell X-450. Over-all, the performance of the fluxing agent contributed some improved flowout onto the paper of the molten resin. The melting point range and softening point were comparable to the formula in Example 1.

EXAMPLE 6

The electroscopic powder prepared in this example conformed to the formula set forth in Example 1 above with the exception that a mixture of ortho- and para-N-ethyl-toluenesulfonamides (Santicizer 8, Monsanto Chemical Company) was substituted as a fluxing agent for the polyol (Shell X-450), The copy quality obtained was fully equivalent to the copy quality obtained from the formulation of Example 1.

EXAMPLE 7

This example differs from Example 1 chiefly in the use of lesser percentages of polyamide resin blended with the highly frangible thermoplastic material. It has been found that the addition of polyamide in amounts less than 9% by weight in the electroscopic powder formula has little or no effect on improving its resistance to the grinding and milling action present in the developer apparatus.

______________________________________ Polyamide resin (Versamide 930) 9% Maleic anhydride-polyhydric alcohol rosin-modified resin (Amberol 800) 83.2% Nubian resin black 6% Carbon black (Neo-spectra, Mark II) 1% ______________________________________

The above composition had a melting point range and softening point of 220.degree.-228.degree.F. and above 155.degree.F., respectively. Electroscopic powders formulated with 5-6% polyamide deteriorated after 3,000-5,000 electrostatic prints. Improvements begin to show when the level of 9%, and above, of the polyamide thermoplastic synthetic resin is included in the formula.

All of the foregoing examples when used in a magnetic brush developer of the type described in U.S. Pat. No. 3,003,462 gave consistently dense uniform images. The first copy from a batch of developer mix, and the later copies made after 100 hours of continual use, produced prints having a print density greater than 1.0 Photovolt readings. Reflectance in the non-image area on the developed copy of photoelectrostatic paper was less than 0.05 Photovolt units.

With the developer mix of this invention, the formation of clouds of developer powder or "throwout", in the vicinity of the developer mix, is greatly minimized, if not completely prevented. Hence, the areas where such machines are located are kept substantially clean.

A further advantage of the electroscopic powder of this invention is that it does not agglomerate or cake during storage, and therefore remains uniform, ready for use. It is not uncommon for materials in shipment to be exposed to a wide variety of climatic conditions, including extremely high temperatures, which often cause the powder to actually "cake" into a solid mass in the shpping container. The material of this invention has been found to retain its free-flowing granular consistency, making it ready for use immediately by the operator.

The discussion of the electroscopic powder has been limited to the technique of positive printing wherein the triboelectric relationship of the electroscopic powder to the iron carriers particles is such that the particles acquire the necessary positive charge so that they will adhere to the negatively charged electrostatic image on the photoconductive member. The advantages provided by the electroscopic powders of this invention may be applied with equal success to the technique of reversal printing as described in co-pending application Ser. No. 221,888 and assigned to the same assignee.

The present invention has been described in great detail, having presented the best mode of formulating the electroscopic powders. Other useful materials and formulations will occur to one skilled in the art over the particular embodiments described herein which are exemplary and not intended to limit the invention, but are intended to cover the invention broadly within the spirit and scope of the appended claims.

Claims

What is claimed is:

1. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of electrostatically charging said member in the dark, exposing the charged member to a light pattern to produce thereon an electrostatic charge image, developing said charge image by applying thereon an electroscopic powder, said electroscopic powder comprising a blend of infrangible, sharp-melting, thermoplastic polyamide resins and a sharp-melting highly frangible, thermoplastic resin, said blend being accomplished by melting the resin components together to form a miscible mixture, said infrangible resin component having a fracturing value of at least 1000 gram centimeters when measured by the falling ball method at 100.degree. F. on a wafer of resin 3.75 centimeters in diameter and 0.5 centimeters in thickness, a melting range not greater than 8.degree. F. being present in an amount ranging from 9 to 90% by weight of said granular powder, said highly frangible resin having a fracturing value not greater than 200 gram centimeters when measured by said falling ball method and a melting range not greater than 8.degree. F. and thereafter fixing said developed charge image to form a permanent image on said recording member.

2. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon an electrostatic charge image;

developing said charge image by applying thereon an electroscopic powder comprising a resin blend of:

i. at least 9% by weight of said powder of an infrangible thermoplastic polyamide resin formed by reacting a high molecular weight polyene fatty acid or ester thereof with an amine, said polyamide resin having a fracturing value of at least 1,000 gram centimeters, and

ii. a highly frangible thermoplastic resin having a fracturing value not greater than 200 gram centimeters,

said resin blend being formed by mixing the resin components in their molten state and said blend further having a sharp melting point within the range of about 150.degree. F. to about 329.degree. F., said fracturing values being measured by the falling ball method on a wafer of resin maintained at about 100.degree. F. and 3.75 cm. in diameter and 0.5 cm. in thickness.

3. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon an electrostatic charge image;

developing said charge image by applying thereon a developer mix comprising:

a. separate granular triboelectrically chargeable carrier particles, and

b. a developer powder comprising:

1. a coloring agent selected from the group consisting of dyes and pigments,

2. a resin blend having a triboelectric relationship of opposite polarity with respect to said carrier particles and having a sharp melting point within the range of about 70.degree. C. to about 165.degree. C., said resin blend being formed by mixing the resin components in their molten state and being composed of:

i. an infrangible thermoplastic polyamide resin formed by reacting a high molecular weight polyene fatty acid or ester thereof with an amine, said polyamide resin having a fracturing value of at least 1,000 gram centimeters, and

ii. a highly frangible thermoplastic resin having a fracturing value not greater than 200 gram centimeters, said fracturing values being measured by the falling ball method on a wafer of resin maintained at 100.degree. F. and 3.75 cm. in diameter and 0.5 cm. in thickness, whereby developed images of substantially improved density can be formed over extended operating periods.

4. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon an electrostatic charge image;

developing said charge image by applying thereon a developer mix, said developer mix comprising:

a. separate granular triboelectrically chargeable carrier particles,

b. a developer powder comprising:

1. a coloring agent selected from the group consisting of dyes and pigments, and

2. a resin blend having a triboelectric relationship of opposite polarity with respect to said carrier particles and having a sharp melting point within the range of about 70.degree. C. to about 165.degree. C., said resin blend being formed by mixing the resin components in their molten state and being comprised of:

i. a major portion of an infrangible thermoplastic polyamide resin formed by reacting a high molecular weight polyene fatty acid or esters thereof with an amine, and

ii. a minor portion of a highly frangible thermoplastic resin having a fracturing value not greater than 200 gram cm., the frangibility of the resins being measured by the falling ball method on a wafer of resin maintained at 100.degree. F. and 3.75 cm. in diameter and 0.5 cm. in thickness, whereby developed images of substantially improved density can be formed over extended operating periods.

5. The method as claimed in claim 4 wherein the frangible resin is a maleic anhydride rosin-modified rosin.

6. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon an electrostatic charge image;

developing said charge image by applying thereon an electroscopic powder suitable for developing said electrostatic charge images, said powder consisting essentially of:

i. frangible and infrangible thermoplastic resins combined when in their molten state to form a miscible mixture, said mixture having physical properties which are distinct from either of the components comprising said mixture, a softening point not less than 130.degree. F., and a melting point below the char point of paper to which it is applied and changing from discrete particles to a flowable state within 8.degree. of said melting point, said frangible resin constituting in the range of from 10 percent to 91 percent by weight of the electroscopic powder and having a fracturing value not greater than 200 gram cms.,

ii. said infrangible resin being a polyamide resin formed by reacting a high molecular weight polyene fatty acid or esters thereof with an amine and constituting in the range of from 9 percent to 90 percent by weight of the electroscopic powder and having a fracturing value of at least 1000 gram centimeters, the fracturing value of the resin being measured at 100.degree. F. by the falling ball method on a wafer of a resin 3.75 cm. in diameter and 0.5 cm. in thickness.

7. The method of making an electrostatic copy as claimed in claim 6 wherein said electroscopic powder has a melting point in the range of from 250.degree. to 235.degree. F.

8. The method of making an electrostatic copy as claimed in claim 7 wherein said electroscopic powder is constituted of from 10 percent to 50 percent by weight of said polyamide resin and 50 percent to 90 percent by weight of a maleic anhydride rosin-modified resin.

9. The method of making an electrostatic copy as claimed in claim 6 wherein said electroscopic powder is comprised of from 30 percent to 40 percent by weight of a polyamide resin, 40 percent to 50 percent by weight of a maleic anhydride rosin-modified resin and from 1 percent to 10 percent by weight of a polyhydric alcohol fluxing agent.

10. The method of making an electrostatic copy as claimed in claim 6 wherein said electroscopic powder is comprised of from 50 percent to 90 percent by weight of said thermoplastic infrangible polyamide resin and from 5 percent to 40 percent by weight of a frangible thermoplastic resin component selected from the group consisting of maleic anhydride rosin-modified resin, esterified diphenolic resin, and phenolformaldehyde resins.

11. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon on electrostatic charge image;

developing said charge image by applying thereon a developer mix comprising a granular electroscopic powder of colored thermoplastic particles mixed with larger carrier particles, said thermoplastic particles and said carrier particles having charges of opposite polarity, said thermoplastic particles having as one component at leat 9% by weight of a polyamide resin formed by reacting a high molecular weight polyene fatty acid or esters thereof with an amine heat-blended with a second frangible resin component to form a composite electroscopic powder having a fracturing value of at least 400 gram cms., said polyamide resin having a fracturing value of at least 1000 gram cms., the blend having a melting point in the range of about 158.degree. F. to about 329.degree. F., and

fixing the developed electrostatic image by exposing the recording member to heat.

12. The method of making an electrostatic copy as claimed in claim 11 wherein said polyamide resin is present in an amount of 50 percent to 90 percent by weight of said electroscopic powder.

13. The method of making an electrostatic copy as claimed in claim 11 wherein the second frangible resin component is a maleic-anhydride-polyhydric alcohol rosin-modified resin and is present in an amount of from 9 percent to 50 percent by weight of said electroscopic powder.

14. The method of making an electrostatic copy as claimed in claim 11 wherein the polyamide resin is a reaction product of polymerized linoleic acid and ethylenediamine.

15. The method of making an electrostatic copy as claimed in claim 11 wherein the electroscopic powder has an average particle size in the range of from four microns to ten microns.

16. The method of making an electrostatic copy on an electrostatic recording member comprising the steps of:

electrostatically charging said member in the dark;

exposing the charged member to a pattern of light and shadow to produce thereon an electrostatic charge image;

developing said charge image by applying thereon a developer mix comprising:

A. separate granular triboelectrically chargeable carrier particles, and

B. a developer powder comprising:

i. a coloring agent selected from the group consisting of dyes and pigments,

ii. a resin blend having a triboelectric relationship of opposite polarity with respect to said carrier particles, a melting point in the range of about 158.degree. F. to about 329.degree. F. wherein the blend changes from discrete particles to a flowable state within 8.degree. F. of said melt point, said resin blend being formed by mixing the resin components in the molten state and being composed of

a. at least 9% by weight of said powder of an infrangible thermoplastic polyamid resin formed by reacting a high molecular weight polyene fatty acid or ester thereof with an amine, said polyamide resin having a fracturing value of at least 1000 gram centimeters,

b. a highly frangible thermoplastic resin having a fracturing value not greater than 200 gram centimeters,

said blend having a fracturing value of at least 400 gram centimeters, a particle size in the range of 1 to 74 microns, and a softening point of about 130.degree. F., said fracturing values being measured by the falling ball method on a wafer of resin maintained at 100.degree. F. and 3.75 centimeters in diameter and 0.5 centimeters in thickness,

whereby developed images of substantially improved density can be formed over extended operating periods.