Roll For Contact Fusing Thermoplastic Particles To Substrates

Abstract

A fuser roll is coated with a silicone elastomer and heat and vacuum cured to remove the cyclic siloxanes to a state of constant weight loss to improve the release of paper and toner from the roll surface.

Description

BACKGROUND OF THE INVENTION

This invention relates to the field of electrophotographic reproduction and more particularly to the field of fusing of thermoplastic toner particles to a porous substrate such as paper in order to render the copies produced by the electrophotographic process, permanent.

Several different techniques have been used in the past to fuse or fix the thermoplastic powder material used to develop electrophotographic images. For example, the thermoplastic powder or toner can be fused by radiant heating, vapor fusing, conductive heating through the paper and fuser roll techniques such as hot rolls or high pressure rolls.

Within the fuser roll technique, there has been two different approaches, that being the use of a rigid, or non-deforming hot roll and a compliant back-up roll and the other technique being the use of a rigid back-up roll and a compliant hot roll. In either case the roll which is compliant is generally a sturdy rigid cylinder which is in turn coated with a layer of an elastomeric material which then is deformed in the pressure nip formed by it and its mating roll in the fuser roll apparatus, thus creating a footprint or width where the two roll surfaces intimately engage. This footprint or nip is necessary due to the fact that two rigid rolls engaging each other would only provide line contact or at best a very very narrow contact when engaging the paper and there would be insufficient residence time in which to transfer enough thermal energy to the toner to adequately soften and fuse the toner particles to the paper. If very high loads are applied to rolls with elastomer coating such as to simulate two high pressure metallic rolls, then the elastomer is subject to premature failure due to the very high stresses applied to the elastomer.

The preferable arrangement within fusing apparatuses implementing the fuser roll technique, is to use a deformable or elastomeric coated fuser hot roll and a relatively rigid back-up roll.

Prior art fuser rolls have been coated with silicone elastomers and in some cases further encased in sleeves of low surface energy materials which do not contain cyclic siloxanes. Other approaches to the release problem include use of low energy fillers such as polytetraflouroethylene. Other fuser rolls use a higher energy level material and apply a release material in liquid form to offset deleterious effects of the higher energy materials, or release properties.

For clarity, the term hot roll, as used herein, may be interchangeably used with that of fuser roll and designates the roll which is heated and is forced into engagement with the surface of the paper carrying the unfused toner image.

The term back-up roll is used in the sense that it engages the reverse side of the sheet from that carrying the unfused toner image and provides the backing force for the paper to cause intimate engagement between the hot roll surface and the toner image.

The deformable or elastomeric coated hot roll is preferable because the radius of the roll surface, when deformed, is a much sharper or shorter radius at the exit point of the nip than the undeformed radius of the roll. This sharp curve or shortened radius is beneficial to release of the copy from the surface of the hot roll since it is very difficult for the toned paper to traverse the short radius thus staying with and wrapping to the hot roll causing a paper feed failure.

Release, and the attendant propensity for wrapping of the copy around the hot roll, is a problem which has plagued hot roll fusing since its inception. This is due to the fact that in order to fuse the toner, the toner must be made soft, tacky, and partially melted to effect fusing and therefore it has a tendency to offset or stick to the surface of anything with which it comes in contact. Because of this tendency of sticking or offset of the toner material to any surface with which it comes in contact while in a soft tacky state, it is necessary to use materials in the hot roll construction which have a very, very low surface energy or have a low tendency to stick to other materials.

It has been discovered that silicone rubbers have a very low surface energy and are generally of the class of materials which are advantageous in this implementation. Unfortunately, silicone rubbers have several qualities which have prevented wide-spread commercialized use as roll fuser coatings. The silicone rubbers have a very low surface energy at room temperature; however, when raised to elevated temperatures for the operation of the roll fuser, the silicone rubbers release properties are greatly deteriorated. Because of the deterioration of the release properties, paper and toner images have a tendency to adhere to the roll and wrap around the roll causing a fuser failure.

This type of failure, it is believed, can be caused by materials in the roll which are adversely effected by the temperatures at which the roll is operated resulting in degradation of release qualities of the roll at elevated temperatures.

It is the primary object of this invention to remove cyclic siloxanes from the elastomeric coating material to extend the useful life of a hot roll within a roll fusing apparatus.

It is an object of this invention to improve hot roll fusing.

It is another object of this invention to improve release of copies and images from the roll of a hot roll fuser.

It is an additional object of this invention to cure the elastomeric coating material of the hot roll to facilitate release of copies and images.

It is an important object of this invention to remove cyclic siloxanes to enhance copy release from the surface of the elastomeric material.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome together with the accomplishment of the above objects by the removal of cyclic siloxanes from elastomeric silicone rubber materials used as hot roll fuser coatings. The cyclic siloxanes are removed by a simultaneous, combination treatment of heating the coated roll structure to a temperature above the normal operating temperature of the roll and subjecting the roll and coating to a high vacuum during the heating process. This accelerates the removal of cyclic siloxanes and enhances the breakdown by any metastable crosslinks which would otherwise result in cyclic siloxanes liberation later in service in the fuser apparatus thus adversely effecting release.

A more detailed understanding of the invention will become apparent from the detailed description to follow, and the drawing.

The FIGURE is a general curve illustrating the weight loss with time at a temperature for a silicone elastomer.

DETAILED DESCRIPTION

For the sake of illustration of the preferred embodiment, the description of materials and procedures will be made with respect to a silicone rubber material manufactured and sold by General Electric Corporation designated as RTC-60. Other silicone rubbers behave in the same or similar manner and the procedures taught herein are believed to be applicable to them.

This material is made-up of four constituents pre-combined in one container. The basic or main constituent of the RTV-60 is polydimethyl siloxane with approximately 1,000 Si-O units per a linear chain. ##SPC1##

The above material is of the general form where n equals approximately 1,000. Contained in this material are two filler materials, a first filler material of silicon dioxide (SiO.sub.2) and a second filler material ferric oxide (Fe.sub.2 O.sub.3). In addition to the above three constituents, a fourth constituent is a trifunctional silane of the form ##SPC2##

where R is an organic group.

As a separate material, dibutyltin dilaurate catalyst is provided. The dibutyltin dilaurate catalyst is added to the four constituents listed above, to initiate the vulcanization or crosslinking of the basic composition. As the vulcanization or crosslinking progresses, an elastomeric, network structure is formed.

Due to incomplete crosslinking or vulcanization, there are formed quantities of cyclic siloxanes of the general formula ##SPC3##

where n ranges from 3 to 8, and where the structure is formed into a ring. As an example, the cyclic siloxane wherein n equals 4 is illustrated below: ##SPC4##

The cyclic siloxanes are present as an impurity in the crosslinked polymer and also are formed by degradation of the polymeric crosslinked structure with thermal aging. The chains in the crosslinked structure, by natural phenomena, have some bonds which are weaker than others. When the material is heated, energy is provided by which these bonds may be broken and a segment of the longer chain is then separated from the crosslinked structure and continues to break down until such time as n of the general formula for cyclic siloxanes is in the range of from 3 to 8. At elevated temperatures these materials are then chemically stable and exist in either a liquid or vapor state depending on their individual melting and boiling points and the prevailing conditions. The cyclic siloxanes have varying boiling points depending on the molecular weights. Some boiling points are sufficiently high that it is necessary to reduce the pressure to a vacuum to volatilize these materials rapidly enough to prevent a build-up of these products. The boiling points are set forth in Table I below.

TABLE I ______________________________________ BOILING POINTS OF CYCLIC SILOXANES AT SPECIFIED PRESSURES ______________________________________ n Boiling Point .degree.F Pressure in mm Hg. ______________________________________ 3 271 760 4 340 760 5 400 760 6 456 760 7 297 20 8 334 20 ______________________________________

As one may observe, the cure temperature must be above the operating temperature of the roll, preferably above the highest boiling point of the material to be removed. Thus, under a pressure of about 20 mms of Hg the cure temperature should be above 334.degree.F. The temperature of a roll in operation is approximately 340.degree. to 380.degree.F. A temperature of cure of 500.degree.F. is both adeugate and high enough to provide efficiency of speed when coupled with a vacuum pressure of about 20 mm Hg.

It has been discovered that the cyclic siloxanes present in the polymer and formed as degradation products are apparently detrimental to release properties of the silicone rubber coating. These cyclic siloxanes are very similar in general structure to materials which have been known as release agents, for example, Dow Corning 200 silicone liquid which is a polydimethylsiloxane. The primary difference is that the cyclic siloxanes have a detrimental effect on the surface characteristics of the silicone rubber. The cyclic siloxanes have a tendency to make the rubber tacky or sticky whereas it generally has a relatively low surface energy level and thus would not be tacky, sticky, or adherent. The prior art teaches the use of polydimethylsiloxanes as release agents, but these are taught with materials other than the silicone rubbers. Examples are the use of polydimethylsiloxanes either on steel rolls or on a Teflon (Trademark of DuPont Corporation) surface. It becomes apparent from the difference in performance, that although these cyclic siloxanes may in some environments have a beneficial effect, in the present environment of a dry silicone elastomer hot roll, a deliterious effect is observed.

As temperature increases in the roll and coating, the weak bonds of the structure will break and the cyclic siloxanes will form adding to any cyclics in the original polymer. As they form, their state changes to a liquid or vapor depending upon the number of Si-O units in the particular siloxane and also the temperature at which the material is maintained.

It has been discovered that the removal of the cyclic siloxanes which are present after the vulcanization phase of preparation along with those formed at elevated service temperatures at which hot rolls must of necessity operate due to the breaking of metastable crosslinks, is beneficial to the release properties of the roll and thus extends the life of the roll to the point where a silicone rubber coated roll is economically and functionally feasible. The procedures for rapid removal and curing of the roll are best understood by following the fabrication procedure from the initial phases.

A roll core is fabricated to the desired dimensions and mounted in a lathe. The RTV-60 silicone material is catalyzed using the dibutyltin dilaurate catalyst up to approximately 0.50 percent by weight. This mixture is then doctored onto the roll as the roll is revolved by the lathe, yielding a coating 30 to 60 mils in thickness. The roll is then continued to be rotated and cured at room temperature for approximately 24 hours. After the 24 hour curing at room temperature, the roll temperature is elevated to approximately 150.degree. F. for a four hour period to insure substantially complete vulcanization. After heat treating at 150.degree.F. for four hours, the coating of silicone rubber is ground to about a 25-50 mil thickness.

After grinding to the desired thickness of coating, the roll is placed in a vacuum oven and the temperature of the roll is elevated to approximately 500.degree. F. for a period of 24 hours. The vacuum oven is operated at approximately 29 inches of Hg vacuum.

Insofar as the invention is fully understood, it's believed that the high temperatures of approximately 500.degree. F., being substantially above the normal operating temperature of the hot roll, drives off as vapor the cyclic siloxanes which are the reaction residue of the crosslinking action as well as any cyclic siloxanes which were present in the original polymer. Additionally, the temperature at which the roll is maintained, breaks any weak bonds in the crosslinked structure of the silicone elastomeric material, thus forming additional cyclic siloxanes. The bonds which are broken are those which are least stable and which would break most easily upon the normal heating of the hot roll during operation. The high vacuum which is imposed upon the roll during the heating, assists the heat in volatilizing the cyclic siloxanes and removing them from the rubber roll. It is necessary to rapidly remove the cyclic siloxanes by the heating and vacuum procedure to effectively remove those cyclic siloxanes with the higher molecular weights as they are formed. Otherwise the cyclics are formed and are slow to dissipate thus building at least an increased transient level of cyclic siloxanes which is detrimental to continued reliable performance.

In subsequent work performed with rolls fabricated according to the above example, it has been found that the simultaneous heat and vacuum treatment of the coated roll structure, produces a satisfactory roll when the cyclic siloxane level is reduced to a point where the cyclic siloxanes are removed at substantially the same rate as they are formed.

This condition is best described with respect to the weight loss of the roll, as it is treated or operated. A characteristic of the silicone rubber elastomeric material formed by the vulcanization of RTV-60 and other silicone elastomer materials is that as long as the structure is subject to heat, some polymeric degradation will occur and cyclic siloxanes form. Thus it is impossible, under present technology, to remove all cyclic siloxanes and prevent further formation of these materials when the elastomeric structure is subjected to elevated service temperatures. FIG. 1 is a general graph of weight loss as a function of time at a preselected temperature and pressure for a sample of RTV-60. The region of the curve, in FIG. 1, indicated as 10 illustrates what can be characterized as a transient condition during the period of time when the residual products of vulcanization or crosslinking specifically the cyclic siloxanes are being removed and during which time the temperature of the curing process is generating additional cyclic siloxanes due to the breakage of the weakest or metastable polymeric bonds of the crosslinked elastomeric material. Region 12 of the curve is the region at which stabilization occurs and the rate of removal is reduced due to the fact that excessive residual vulcanization products are no longer available for removal and that the only materials remaining for removal are those being generated on a continuing basis due to the elevated service temperature.

Region 14 of the curve illustrates the steady state condition which is accomplished as a result of the removal of the vulcanization products and the originally produced cyclic siloxanes due to an initial temperature increase. The generation of cyclic siloxanes through degradation of the basic elastomeric layer has been stabilized to a constant rate and the region of the curve in FIG. 1 designated 14, can best be described mathematically as d(WT loss)/dt = K. The derivitive of weight loss with respect to time yields a constant after the transient cyclic siloxanes have been removed.

The constant rate of weight loss may be secured under service conditioned by either attaining the constant rate under care, or curing to a high enough point on the curve in either region 10 or 12 that under a reduced service temperature and atmospheric pressure the rate of weight loss in service is substantially constant.

It is in that state or condition represented by region 14 where acceptable and reliable release properties of the roll fuser can be obtained. During cure, the rate of weight loss in regions 10 and 12 may be increased by higher elevations of temperature and/or an increase in the vacuum applied. A decrease in vacuum and/or temperature will decrease the rate of weight loss in these regions 10 and 12 conversely.

The curve is a general one and with appropriate scale would be applicable to a roll under treatment or in service.

For satisfactory removal of the cyclic siloxanes to insure reliable initial use and extended life, it is necessary to apply both a temperature equal to or greater than the normal operating temperature of the fuser roll and at the same time apply the vacuum.

The simultaneous treatment is necessary because the application of vacuum alone will only withdraw those cyclic siloxanes which are in a liquid or vapor state under normal room temperatures which are those of the lower molecular weight while leaving uneffected the higher molecular weight cyclic siloxanes.

Similarly the application of heat alone, especially at extremely high temperatures, tends to degrade the rubber into the easily formed cyclic siloxanes while relying purely on volatilization at atmospheric pressure to drive off these materials. The rate of vaporization of the cyclic siloxanes is relatively low when compared with other highly volatile materials and thus a large quantity of cyclic siloxanes can be formed within the rubber without their adequate removal when no vacuum is employed together with the elevated temperature curing.

The foregoing explanation and example has been relative to the General Electric silicone rubber material marketed under the designation RTV-60. It should be recognized that other similar materials are available from other vendors and that they are of the same general type of materials, ie., that of a long chain silicone elastomer. The major variation between one vendor's silicone rubber and another vendor's silicone rubber is generally in the use of either different amounts or types of filler materials in lieu of the Fe.sub.2 O.sub.3 and the SiO.sub.2 fillers in the present illustrations. These materials generally have little or no effect on the polymerization and are actually only physical fillers. They do effect thermal properties and other physical properties and effect the rate of formation of cyclic siloxanes. The exact conditions of curing for each particular material and the optimum curing conditions for any particular material will depend upon, (1) the amount of time available for curing, (2) the operating temperature of the fusing roll in its normal operating environment, (3) the particular type of silicone elastomer used. The important point is that the rate of weight loss for the silicone material must stabilize at approximately a constant before an equilibrium state is reached and it is in that state that the enhanced properties referred to above are most reliably found.

It may be possible in some cases to secure the properties of good release in that portion of the curve designated as region 12 or referred to as the knee of the curve, in as much as the cyclic siloxane level in the material has been reduced to a relatively low level. It is not possible to determine percentage levels of cyclic siloxanes with normal laboratory equipment and testing techniques and therefore the rate of weight loss is used as a definitive measure of the structure of the materials involved which have material effects on performance in the hot roll fusing art.

Claims

I claim:

1. A roll for fusing thermoplastic granular material to a substrate, said roll having a cylindrical core, and a coating on said core of a silicone elastomeric material, the improvement comprising:

said coating having the volatile components thereof, reduced to a level at which the coating has, under in service conditions, a substantially constant rate of weight loss.

2. A roll for fusing thermoplastic granular material to a substrate, said roll having a cylindrical core, and a coating on said core of a silicone elastomeric material, the improvement comprising:

said coating having the volatile components thereof, reduced to below the level at which said coating has a substantially constant rate of weight loss for a selected temperature and under a selected nip pressure.

3. The roll of claim 2 wherein said volatile components are cyclic siloxanes having the basic unit structure of ##SPC5##

where n equals to 3 to 8.