Thermally assisted transfer of small electrostatographic toner particles

Disclosed is an improved method of making a hard copy in a process where a latent electrostatic image on an image-bearing substrate is developed by applying to the image a dry thermoplastic toner which comprises a binder polymer, and the developed image is transferred to the surface of a receiver by contacting the developed image on the substrate with the surface, then removing the surface from the substrate. The improvement comprises developing the latent electrostatic image with a toner having a particle size less than 8 micrometers, heating the surface before it contacts the developed image to a temperature such that the surface heats the toner particles when it contacts the developed image to a temperature between 10.degree. C. above the T.sub.g of the toner binder and 20.degree. C. below the T.sub.g of the toner binder, where the temperature is sufficient to fuse discrete toner particles that form the image to each other at points of contact between the particles, but insufficient to cause the contacting particles to flow into a single mass, non-electrostatically transferring the developed image to the surface, where the roughness average of the surface is less than the radius of the particles, and heating the developed image after it has been removed from the substrate to a temperature sufficient to fix it.

TECHNICAL FIELD 
This invention relates to a thermally assisted method of transferring and 
fixing electrostatographic toner particles that have a particle size of 
less than 8 micrometers. In particular, it relates to such a process where 
the receiver surface is heated before the transfer occurs, the transfer is 
not electrostatically assisted, and the toner is not fixed during 
transfer. 
BACKGROUND ART 
In a conventional electrostatographic copying process, a laten 
electrostatic image is formed on an insulating substrate, such as a 
photoconductor. If a dry development process is used, charged toner 
particles are applied to the electrostatic image, where they adhere in 
proportion to the magnitude of the electrostatic potential difference 
between the toner particles and the charges on the image. Toner particles 
that form the developed image are transferred to a receiver by pressing 
the surface of the receiver against the developed image. It is 
conventional to use either an electrostatically biased roller or a corona 
to transfer toner particles from the image bearing substrate to the 
receiver. The transferred particles are then fixed to the receiver surface 
by a suitable method such as the application of heat. 
While this conventional process works well with large toner particles, 
difficulties arise as the size of the toner particles is reduced. Smaller 
toner particles are necessary to achieve higher resolution copies but, as 
the size of the toner particles falls below about 8 micrometers, the 
surface forces holding the toner particles to the substrate tend to 
dominate over the electrostatic force that can be applied to the particles 
to assist their transfer to the receiver. Thus, less toner transfers and 
image quality suffers increases in mottle. In addition, as the particle 
size decreases, certain other image defects also begin to increase, such 
as the "halo defect," where tone particles that are adjacent to areas of 
maximum toner density fail to transfer, and "hollow character," where the 
centers of fine lines fail to transfer. "Dot explosion," where toner 
particles comprising half tone dots scatter during transfer, also occurs 
during electrostatic transfer. Some of these defects are believed to be 
due to repulsive coulombic forces between the particles. This, high 
resolution images require very small particles, but high resolution images 
without image defects have not been achievable using electrostatically 
assisted transfer. 
One alternative process of transferring toner particles, without using an 
electrostatic bias, is to melt or fuse the particles to the receiver 
during transfer by heating the toner above its melting point. While this 
process does ameliorate image quality by reducing the defects that are 
aggravated by electrostatically assisted transfer, it, in turn, creates 
new problems that must be overcome. First, that process requires higher 
temperatures than does the conventional process, and these higher 
temperatures subject the substrate (e.g., a photoconductor) to higher 
temperatures. This can alter the electrical and photoconductive 
characteristics of the substrate, and/or cause physical distortions, and 
therefore mandate the use of more thermally stable materials, which may be 
more expensive and/or less suitable for other reasons. The receiver is 
also subjected to higher temperatures over a long period of time which can 
weaken and deteriorate the receiver and blister its surface. Also, because 
of the time required for enough heat to transfer from the receiver to the 
toner to melt it, the process is slow; typical process speeds are of the 
order of only 0.4 meters/minute. Melted toner may also occasionally fuse 
to the substrate, which may permanently damage the substrate. A special 
cleaning process is also needed if the substrate is to be reused, and 
cleaning adds to the cost of the process and subjects the substrate to 
additional thermal cycling. High pressures (about 345 to 760 kPa) are also 
needed in this process. These high pressures, in conjunction with the high 
temperature and long nip duration time, can be especially hard on a 
substrate. 
SUMMARY OF THE INVENTION 
In accordance with this invention, toner particles are transferred 
non-electrostatically to a receiver that is heated, but the receiver is 
not heated sufficiently to melt the particles. It has been found that it 
is not necessary to melt the toner particles in order to achieve their 
transfer, but that merely fusing toner particles to each other at their 
points of contact is adequate to accomplish a complete, or nearly 
complete, transfer of the particles. Thus, the toner is not fixed during 
transfer but is instead fixed at a separate location, away from the 
substrate. In this way, the higher temperatures required for fixing the 
toner do not affect the substrate. Since the heat required to merely 
sinter the toner particles at their points of contact is much lower than 
the heat needed to fix the toner, the substrate is not damaged by high 
temperatures during transfer and conventional substrate materials can be 
used. Also, because the transfer in the process of this invention is 
completely non-electrostatic, image defects that are aggravated by an 
electrostatically assisted transfer are not a problem in the process of 
this invention. And, also because the transfer is not electrostatically 
assisted, the electrical conductivity of the toner is much less important, 
so single component developers and more conductive toners can be used, 
while otherwise they could not be used with satisfactory results. 
Moreover, small toner particles (i.e., less than 8 micrometers), which 
cannot be effectively transferred electrostatically, can be transferred 
with high efficiency using this process. 
It has further been found that if the receiver is heated only at the nip, 
the temperature of the receiver surface when it contacts the toner 
particles cannot be controlled. That is, at times insufficient heat 
penetrates through the receiver to fuse the toner particles at their 
points of contact and the toner therefore does not transfer well, while at 
other times so much heat passes through the receiver that the toner melts 
completely and the photoconductor is damaged. It has been found that this 
problem can be overcome by preheating the receiver surface before transfer 
occurs so that the temperature of the receiver surface is always within 
the range required to fuse the toner particles at their points of contact 
without melting them.

In FIG. 1, a receiver sheet 1 is preheated by heater 2 to a temperature 
adequate to fuse toner particles at their points of contact during 
transfer, but inadequate to melt the particles. A photoconductive drum 3 
has been uniformly charged by corona 4, then imagewise exposed to light at 
station 5, which discharged exposed portions of the drum, forming a latent 
electrostatic image on the drum. This image is developed by the 
application of toner particles 6 having a particle size of less than 8 
micrometers, to the image at station 7. The developed image 9 is 
transferred to receiver 1 at nip 10, which is formed between drum 3 and 
backup roller 11. Receiver 1 passes between heated rollers 12 and 13 which 
fix the toner particles to the receiver. 
DETAILED DESCRIPTION OF THE INVENTION 
Toners useful in this invention are dry toners having a particle size of 
less than 8 micrometers, and preferably less than 5 micrometers, as the 
problems that this invention are directed to are not significant when the 
particle size of the toner is much greater than 8 micrometers, while the 
problems are especially intense when the particle size is less than 5 
micrometers. (Particle size herein refers to mean volume weighted diameter 
as measured by conventional diameter measuring devices such as a Coulter 
Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum 
of the mass of each particle times the diameter of a spherical particle of 
equal mass and density, divided by total particle mass.) The toners must 
contain a thermoplastic binder in order to be fusible. The toner binder 
should have a glass transition temperature, T.sub.g, of about 40.degree. 
to about 100.degree. C., and preferably about 45.degree. to about 
65.degree. C., as a lower T.sub.g may result in a clumping of the toner as 
it is handled at room temperature, while a higher T.sub.g renders the 
process of this invention too energy intensive and may heat the substrate 
too much, resulting in damage to the substrate and various transfer 
problems. Preferably, the toner particles have a relatively high caking 
temperature, for example, higher than about 60.degree. C., so that the 
toner powders can be stored for relatively long periods of time at fairly 
high temperatures without individual particles agglomerating and clumping 
together. 
The melting point of polymers useful as toner binders preferably is about 
65.degree. C. to about 200.degree. C. so that the toner particles can be 
readily fused to a receiver to form a permanent image. Especially 
preferred polymers are those having a melting point of about 65.degree. to 
about 120.degree. C. The polymers useful as toner binders in the practice 
of the present invention can be used alone or in combination and include 
those polymers conventionally employed in electrostatic toners. Among the 
various polymers which can be employed in the toner particles of the 
present invention are polycarbonates, resin-modified maleic alkyd 
polymers, polyamides, phenol-formaldehyde polymers and various derivatives 
thereof, polyester condensates, modified alkyd polymers, aromatic polymers 
containing alternating methylene and aromatic units such as described in 
U.S. Pat. No. 3,809,554 and fusible crosslinked polymers as described in 
U.S. Pat. No. Re. 31,072. 
Typical useful toner polymers include certain polycarbonates such as those 
described in U.S. Pat. No. 3,694,359, which include polycarbonate 
materials containing an alkylidene diarylene moiety in a recurring unit 
and having from 1 to about 10 carbon atoms in the alkyl moiety. Other 
useful polymers having the above-described physical properties include 
polymeric ester of acrylic and methacrylic acid such as poly(alkyl 
acrylate), and poly(alkyl methacrylate) wherein the alkyl moiety can 
contain from 1 to about 10 carbon atoms. Additionally, other polyesters 
having the aforementioned physical properties are also useful. Among such 
other useful polyesters are copolyesters prepared from terephthalic acid 
(including substituted terephthalic acid), a 
bis(hydroxyalkoxy)phenylalkane having from 1 to 4 carbon atoms in the 
alkoxy radical and from 1 to 10 carbon atoms in the alkane moiety (which 
can also be a halogen-substituted alkane), and in the alkylene moiety. 
Other useful polymers are various styrene-containing polymers. Such 
polymers can comprise, e.g., a polymerized blend of from about 40 to about 
100 percent by weight of styrene, from 0 to about 45 percent by weight of 
a lower alkyl acrylate or methacrylate having from 1 to about 4 carbon 
atoms in the alkyl moiety such as methyl, ethyl, isopropyl, butyl, etc. 
and from about 5 to about 50 percent by weight of another vinyl monomer 
other than styrene, for example, a higher alkyl acrylate or methacrylate 
having from about 6 to 20 or more carbon atoms in the alkyl group. Typical 
styrene-containing polymers prepared from a copolymerized blend as 
described hereinabove are copolymers prepared from a monomeric blend of 40 
to 60 percent by weight styrene or styrene homolog, from about 20 to about 
50 percent by weight of a lower alkyl acrylate or methacrylate and from 
about 5 to about 30 percent by weight of a higher alkyl acrylate or 
methacrylate such as ethylhexyl acrylate (e.g., styrene-butyl 
acrylate-ethylhexyl acrylate copolymer). Preferred fusible styrene 
copolymers are those which are covalently crosslinked with a small amount 
of a divinyl compound such as divinylbenzene. A variety of other useful 
styrene-containing toner materials are disclosed in U.S. Pat. No. 
2,917,460; U.S. Pat. Nos. Re 25,316; 2,788,288; 2,638,416; 2,618,552 and 
2,659,670. Preferred toner binders are polymers and copolymers of styrene 
or a derivative of styrene and an acrylate, preferably butylacrylate. 
Useful toner particles can simply comprise the polymeric particles but it 
is often desirable to incorporate addenda in the toner such as waxes, 
colorants, release agents, charge control agents, and other toner addenda 
well known in the art. The toner particle can also incorporate carrier 
material so as to form what is sometimes referred to as a "single 
component developer." The toners can also contain magnetizable material, 
but such toners are not preferred because they are available in only a few 
colors and it is difficult to make such toners in the small particles 
sizes required in this invention. 
If a colorless image is desired, it is not necessary to add colorant to the 
toner particles. However, more usually a visibly colored image is desired 
and suitable colorants selected from a wide variety of dyes and pigments 
such as disclosed for example, in U.S. Pat. No. Re. 31,072 are used. A 
particularly useful colorant for toners to be used in black-and-white 
electrophotographic copying machines is carbon black. Colorants in the 
amount of about 1 to about 30 percent, by weight, based on the weight of 
the toner can be used. Often about 8 to 16 percent, by weight, of colorant 
is employed. 
Charge control agents suitable for use in toners are disclosed for example 
in U.S. Pat. Nos. 3,893,935; 4,079,014; 4,323,634 and British Patent Nos. 
1,501,065 and 1,420,839. Charge control agents are generally employed in 
small quantities such as about 0.1 to about 3, weight percent, often 0.2 
to 1.5 weight percent, based on the weight of the toner. 
Toners used in this invention can be mixed with a carrier vehicle. The 
carrier vehicles, which can be used to form suitable developer 
compositions, can be selected from a variety of materials. Such materials 
include carrier core particles and core particles overcoated with a thin 
layer of film-forming resin. Examples of suitable resins are described in 
U.S. Pat. Nos. 3,547,822; 3,632,512; 3,795,618; 3,898,170; 4,545,060; 
4,478,925; 4,076,857; and 3,970,571. 
The carrier core particles can comprise conductive, non-conductive, 
magnetic, or non-magnetic materials. See, for example, U.S. Pat. Nos. 
3,850,663 and 3,970,571. Especially useful in magnetic brush development 
schemes are iron particles such as porous iron particles having oxidized 
surfaces, steel particles, and other "hard" or "soft" ferromagnetic 
materials such as gamma ferric oxides or ferrites, such as ferrites of 
barium, strontium, lead, magnesium, or aluminum. See for example, U.S. 
Pat. Nos. 4,042,518; 4,478,925; and 4,546,060. 
The very small toner particles that are required in this invention can be 
prepared by a variety of processes well-known to those skilled in the art 
including spray-drying, grinding, and suspension polymerization. 
The image-bearing substrate can be in the form of a drum, a belt, a sheet, 
or other shape, and can be made of any of the conventional materials used 
for such purposes. While dielectric recording materials can be used, 
photoconductive materials are preferred, and organic photoconductive 
materials are preferred over inorganic photoconductive materials, because 
they produce an image of superior quality. While the image-bearing 
substrate can be a single use material, reusable substrates are preferred 
as they are less expensive. Of course, reusable substrates must be 
thermally stable at the temperature of transfer. The surface properties of 
the substrate and the receiver should be adjusted so that at the operating 
temperature of the transfer the toner adhesion to the substrate is less 
than the toner adhesion to the receiver. This can be accomplished by using 
substrates having low surface energy, such as polytetrafluoroethylene 
coated polyesters, or by incorporating low surface adhesion (LSA) 
materials, such as zinc stearate, into the substrate or coating the 
substrate with an LSA material. 
In order to insure that the toner adhesion to the receiver is greater than 
the toner adhesion to the substrate at the temperature of transfer, the 
properties of the receiver surface can also be selected so as to increase 
the adhesion of the toner particles to that surface. This can most 
advantageously be accomplished by coating the receiver with a 
thermoplastic that will not stick to the photoconductor, or by coating the 
receiver with a thermoplastic polymer over which is coated a release agent 
which preferably has a lower surface energy than said substrate, as is 
described in copending application Ser. No. 230,381, titled "Improved 
Method Of Non-Electrostatically Transferring Toner," filed Aug. 9, 1988, 
herein incorporated by reference. If a receiver is coated with a 
thermoplastic polymer, it is important that the T.sub.g of the 
thermoplastic polymer be less than 10.degree. C. above the T.sub.g of the 
toner binder and that the receiver be heated to a temperature above the 
T.sub.g of the thermoplastic polymer, so that the thermoplastic coating 
softens and the toner particles become embedded therein. 
Any conductive or nonconductive material can be used as the receiver, 
including various metals such as aluminum and copper and metal coated 
plastic films, as well as organic polymeric films and various types of 
paper. If a transparent polymeric receiver, such as polyethylene 
terephthalate, is used, good transparencies can be made using the process 
of this invention. Paper is the preferred receiver material because it is 
inexpensive and the high quality image produced by the process of this 
invention is most desirably viewed on paper. In order to achieve an 
acceptably high transfer efficiency and good image quality the receiver 
must have a roughness average that is less than the radius (i.e., one-half 
the herein defined diameter) of the toner particles, where the roughness 
average is an indication of surface roughness, the value of which is the 
average height of the peaks in micrometers above the mean line between 
peaks and valleys. A suitable device to measure this value directly is a 
profilometer, such as the Surtronic 3 surface roughness instrument 
supplied by Rank Taylor Hobson, P. O. Box 36, Guthlaxton Street, Leicester 
LE205P England. Also see U.S. Pat. No. 4,737,433, herein incorporated by 
reference, which describes advantages to using a receiver surface that is 
smooth compared to toner particle size. 
In the process of this invention, the receiver is preheated to a 
temperature such that the temperature of the receiver during transfer will 
be adequate to fuse the toner particles at their points of contact but 
will not be high enough to melt the toner particles, or to cause 
contacting particles to coalesce or flow together into a single mass. That 
is, the particles must appear as in FIG. 2. The temperature range 
necessary to achieve that result depends upon the time that a receiver 
resides in the nip and the heat capacity of the receiver. In most cases 
the result shown in FIG. 2 can be achieved if the temperature of the 
receiver immediately after the receiver contacts the substrate is below 
the T.sub.g of the toner binder but above a temperature that is 20 degrees 
below that T.sub.g. However, receiver temperatures up to 10.degree. C. 
above the T.sub.g of the toner binder are tolerable when nip time is small 
or the heat capacity of the receiver is low. Although either side of the 
receiver can be heated, it is preferable to heat only the front surface of 
the receiver, that is, the surface of the receiver that will contact the 
toner particles, as this is more energy efficient, it is easier to control 
the temperature of that surface when the heat does not have to pass 
through the receiver, and it usually avoids damage to the receiver. Such 
heating can be accomplished by any suitable means, such as radiant heat in 
an oven or contacting the receiver with a heated roller or a hot shoe. The 
preheating of the receiver must be accomplished before the heated portion 
of the receiver contacts the substrate because, if the receiver is heated 
only in the nip, its temperature may fluctuate over a wide range and its 
temperature cannot easily be kept within the narrow critical range 
required for the successful practice of this invention. Thus, if the 
backup roller, which presses the receiver against the substrate, is used 
to heat the receiver, the receiver must be wrapped around the backup 
roller sufficiently so that the receiver is heated to the proper 
temperature before it enters the nip. The backup roller is preferably not 
the sole source of heat used to effect the transfer, however, because the 
backup roller heats the back of the receiver, which means the heat must 
pass through the receiver to reach the toner. As a result, depending upon 
the receiver used, the process speed, and the ambient temperature, at 
times too much heat will pass through the receiver and it will melt the 
toner, while at other times insufficient heat will pass through the 
receiver and the toner will not transfer well. Thus, while the backup 
roller can be heated if desired, it is preferable to use an unheated 
backup roller. 
It has been found that pressure aids in the transfer of the toner to the 
receiver, and an average nip pressure of about 135 to about 1000 kPa is 
preferred. Lower pressures may result in less toner being transferred and 
higher pressures may damage the substrate and can cause slippage between 
the substrate and the receiver, thereby degrading the image. In any case, 
the toner must not be fixed during transfer but must be fixed instead at a 
separate location that is not in contact with the substrate. In this way, 
the substrate is not exposed to high temperatures and the toner is not 
fused to the substrate. Also, the use of the lower temperatures during 
transfer means that the transfer process can be much faster, 6 
meters/minute or more being feasible. Either halftone or continuous tone 
images can be transferred with equal facility using the process of this 
invention. Because the electrostatic image on the substrate it not 
significantly disturbed during transfer it is possible to make multiple 
copies from a single imagewise exposure. 
The process of this invention is also applicable to the formation of color 
copies. If a color copy is to be made, successive latent electrostatic 
images are formed on the substrate, each representing a different color, 
and each image is developed with a toner of a different color and is 
transferred to a receiver. Typically, but not necessarily, the images will 
correspond to each of the three primary colors, and black as a fourth 
color if desired. After each image has been transferred to the receiver, 
it can be fixed on the receiver, although it is preferable to fix all of 
the transferred images together in a single step. For example, light 
reflected from a color photograph to be copied can be passed through a 
filter before impinging on a charged photoconductor so that the latent 
electrostatic image on the photoconductor corresponds to the presence of 
yellow in the photograph. That latent image can be developed with a yellow 
toner and the developed image can be transferred to a receiver. Light 
reflected from the photograph can then be passed through another filter to 
form a latent electrostatic image on the photoconductor which corresponds 
to the presence of magenta in the photograph, and that latent image can 
then be developed to the same receiver. The process can be repeated for 
cyan (and black, if desired) and then all of the toners on the receiver 
can be fixed in a single step. 
The following examples further illustrate this invention. 
EXAMPLES 1 TO 7 
Latent electrostatic images were formed by standard electrophotographic 
techniques on an inverted multilayer photoconductive element as described 
in Example 5 of U.S. Pat. No. 4,701,396, herein incorporated by reference, 
which had a zinc stearate rubbed surface. The images were developed with 
dry electrographic toners in combination with a lanthanum doped ferrite 
carrier. The toners used were: 
(A) A toner having a particle size of 3.5 micrometers prepared by a 
suspension polymerization process. The toner contained 8 weight percent 
carbon black sold by Cabot Corp. as "Sterling R," a polystyrene binder 
having a T.sub.g of 62.degree. C., sold as "Piccotoner 1221" by Hercules, 
and 0.2 weight percent of a quaternary ammonium charge agent sold by Onyx 
Chemical Co. as "Ammonyx 4002." 
(B) A toner having a particle size of 7.5 micrometers. The toner contained 
6 weight percent carbon black sold by Cabot Corp as "Regal 300," 1.5 
weight percent phosphonium charge agent, and a polyester binder having a 
T.sub.g of approximately 60.degree. C., made from 90 weight percent 
terephthalic acid, 10 weight percent dimethyl glutarate, and a 
stoichiometric amount of 1,2-propanediol. 
Each of the toner imates was transferred according to the process of this 
invention, as is illustrated in FIG. 1, to one of three receivers. Except 
for Example 1, which is a control, the receivers were preheated to about 
90.degree. C. so that the receiver temperature during transfer was 
approximately 60.degree. C., which heated the toner to that temperature. 
The following receivers were used: 
(A) Polyethylene coated paper having a surface roughness average of 0.45 
micrometers, sold as "Photofinishing Stock 486V" by Eastman Kodak. 
(B) A clay coated graphic arts printing paper having a surface roughness 
average of 1.65 micrometers. 
(C) An uncoated copy paper having a surface roughness average of 3.5 
micrometers. 
The following table gives the experiments performed and the results: 
______________________________________ 
Dmax 
Trans- Resid- % Trans- 
Example 
Toner Receiver ferred ual ferred 
______________________________________ 
1 A A 0.33 0.39 46 
2 A C 0.12 0.40 23 
3 A A 0.86 0.03 97 
4 A B 0.51 0.15 77 
5 B A 1.53 0.00 100 
6 B B 1.56 0.00 100 
7 B C 1.06 0.05 95 
______________________________________ 
In the above table, Example 1 is outside the scope of this invention 
because the receiver was not preheated and Example 2 is outside the scope 
of this invention because the roughness average of the receiver was 
greater than the radius of the toner particles. The table shows that 
Example 1 had a transfer efficiency of only 46%, and that Example 2 had a 
transfer efficiency of only 23%, while Examples 3 to 7, which illustrate 
this invention, had transfer efficiencies between 77 and 100%. FIG. 2 is a 
scanning electron micrograph of toner particles from Example 6 after 
transfer. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.