AU721466B2 - Process of making a polyurethane roller with high surface resistance - Google Patents

Description

I AUSTRALIA Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: SName of Applicant: Lexmark International, Inc.

Actual Inventor(s): Todd L. Janes Johnny D. Massie II Jean M. Massie Ronald L. Roe Donald W. Stafford *Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: PROCESS OF MAKING A POLYURETHANE ROLLER WITH HIGH SURFACE

RESISTANCE

Our Ref 580559 POF Code: 321975/321975 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): la PROCESS OF MAKING A POLYURETHANE ROLLER WITH HIGH SURFACE

RESISTANCE

The present application is a divisional application from Australian patent application number 17788/97, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD This invention relates to developer rollers used in electrophotography, and, more specifically, to a roller and its process of manufacture having a surface with a high electrical resistivity layer.

BACKGROUND OF THE INVENTION A functional developer roller for use in contact electrophotographic printing having a high resistance surface layer over a semi-conductive core gives excellent print performance independent of the speed of movement of the printing members (termed process speed). This invention is an improvement over a common method which involves making a semi-conductive core and subsequently coating that core with a resistive material in a separate process such as spray or dip coating. For certain applications other developer rollers employ the controlled conductivity of a semi-conductive core having no outer layer, of which U.S. Pat. No. 5,248,560 to Baker et al is illustrative.

By using the novel combination of materials described in this specification a high resistance surface layer over a more conductive core can be produced simply by oxidizing the roll surface. This eliminates the need for coating the conductive roll with a resistive layer in a separate process. This process is an improvement over a more common method which involves making a semiconductive core and then subsequently coating the core with a resistive material in a separate process such as spray or dip coating. Baking is much more cost effective than spray or dip coating and produces a roller with less defects. The baked polydiene-based roll of this invention mimics the electrical performance of the coated roller and gives excellent print performance over a wide range of process speeds.

C:\WINWORD\TONIA\DAVIN\DIVISI-1\DIV17788.DOC 2 DISCLOSURE OF THE INVENTION The roller of this invention is a cast urethane, electrically conductive rubber roller with a surface layer of high electrical resistivity. This roller mimics the electrical properties of a coated roller. The roller is composed of a polydiene, such as polyisoprene and more specifically polybutadiene, either as a polyol or a urethane prepolymer, blended with a second polyurethane prepolymer and a conductive additive such as ferric chloride. The bulk resistivity of the roller is approximately lx10 8 (one times ten to the eighth power) ohm-cmat 720 F. and relative humidity. The surface of the cured roller is oxidized to produce a surface layer of material with high electrical resistivity. Oxidation of the roller is achieved by baking the roller in air at fairly high temperature (approximately 100 degrees for several hours. The reaction of the oxygen with the polybutadiene, catalyzed by the ferric chloride, oxidizes the surface of the roller. The oxidized layer is very resistive. The cost of production is low.

15 BEST MODE FOR CARRYING OUT THE INVENTION ~In electrophotography, the developer roller function is to develop a layer of toner on a photoconductor drum charged in an image pattern. Electrical models of this process have been well reported in the literature. Equations for the development curve, which relates the developed mass of toner per unit area to the development potential, have been derived for several developer roll constructions. The development equations for a semi-conductive roller (lx107 lx109 ohm-cm resistivity) and a semi-conductive roller with a high resistance :coating have been compared. The electrical model developed by H. Tachibana (Conference Record IEEE IAS 1989, p. 2260, "Control of Toner Reproduction Characteristics by Time Constant of Development Roller in Mono-Component Development") can be used to evaluate the print performance of these rolls for different print speeds, roller electrical properties, and other variations.

Results indicate that a two layer, "coated" roll will develop a fixed quantity of toner per volt of development bias that is determined by the dielectric thicknesses of the photoconductor, the toner and the developer roller. This development characteristic is independent of process speed, within limits. In contrast, a solid roll of a single resistivity develops a quantity of toner based on C:\WINWORD\TONIA'DAVIN\DIVISI-1\DIV17788.DOC the dielectric constants of the photoconductor and the toner, and the resistance of the roll in the photoconductor nip. This is dependent on process speed. In addition, a two-layer roll has a longer time constant than a solid roll. Longer time constant materials leave a higher effective development surface potential on the developer roll at the entry to the photoconductor nip. This improves the single pel dot print performance of the roll.

Therefore, the print performance of a two layer roller is superior to that of a solid roll across a wide process speed range and is less sensitive to office environments.

The desired electrical properties during normal operation of a two-layer roller are a core resistivity less than 1x10 9 ohm-cm, preferably less than 3x10 8 ohm-cm, at 72 0 F. and 50% relative humidity a coating resistivity of 5x10 9 2X1012 ohm-cm, preferably lx1011 ohm-cm, at 72 0 F. and 50% RH. The time constant should be about 5-2,000 milliseconds, preferably about 100 15 milliseconds, at 720 F. and 50% RH.

A common technique to produce a semi-conductive roll with a resistive layer is to prepare a core using any standard rubber molding technique, such as casting liquid urethanes or rubber transfer molding. The core is then ground to the correct dimensions and either spray or dip coated with a resistive material to the desired thickness. The coating is usually applied in several layers to build up to the desired thickness of 100 microns. Problems with this process include its higher cost due to the multiple coating steps and the defects introduced into the surface layer during the coating process.

Using the unique combination of materials described in this specification, a resistive surface layer can be produced on a cast urethane roll simply by baking in air at elevated temperature. The oxidation of the polybutadiene, in the presence of ferric chloride, produces a high resistive layer at the surface. The thickness and resistivity of this layer can be controlled by varying the polybutadiene level, the ferric chloride level, the baking time, the baking temperature and the oxygen level.

This invention describes the use of a blend of a urethane prepolymer with polybutadiene, either in diol or urethane prepolymer form and ferric chloride as a C:\WINWORD\TONIA\DAVINMIVISI-I DIV17788.OOC conductivity modifier. The blend of materials is cured in roll form and then bakes at elevated temperatures 80 0 for various times to oxidize the surface of the roll. This oxidation produces a layer of high resistivity material on the surface of the roll.

Polycaprolactone urethane prepolymer, such as Vibrathane 6060 (trademark product of Uniroyal Chemical), is the preferred base urethane because of its stable electrical resistivity with temperature and humidity changes.

Vibrathane 6060 is a polycaprolactone ester toluene-diisocyanate prepolymer.

Ferric chloride is added to the urethane to reduce the electrical resistivity of the roll core to <1x10 9 ohm-cm. The combination of polycaprolactone urethane and ferric chloride produces a roller with a single resistivity from the roll surface to the center or core. In order to produce a roller with a high resistivity surface layer, a polydiene must be included in the formation.

S:In an alternative formulation, polybutadiene prepolymers are prepared by 15 the reaction of a polybutadiene diol (PBD) with toluene diisocyanate (TDI). This PBD-TDI prepolymer can be blended with the caprolactone prepolymer in various proportions. A suitable polybutadiene prepolymer is an experimental product of Uniroyal Chemical. The blend of prepolymers is cured with polyol curatives, such as Voranol 234-630, (trademark product of Dow Chemical Co., Inc.), a trifunctional polyether polyol. Typical polycaprolactone/polybutadiene blend ratios range from 95/5 parts by weight per hundred parts of total rubber which includes the polycaprolactone and the polybutadiene to 60/40 parts by weight.

Polybutadiene can be added in either prepolymer or diol form. The polycaprolactone urethane can be cured by using a combination of polybutadiene diol (such as polybd (trademark) R45HT, a product of Elf Atochem) with a trifunctional curative such as the Voranol 234-630. Voranol 234-630 is a polyether polyol with a functionality of 3. In this case, the polybutadiene diol acts as a polymer chain extender for the urethane. Typical weights ratios of the Voranol to the polybutadiene diol range from 1/0 up to 1/7 by weight, preferably 1/3 by weight. The polybd R45FfT polybutadiene has a molecular weight Mn, of 2800 and a microstructure of 20% cis-1, polybutadiene, 60% tans-1, 4polybutadiene and 20% 1,2-polybutadiene.

C:AWINWOROITONIA\DAVIN\DIVISI-1\IV7788DOC The polybutadiene prepolymer is a very highly resistive material. The addition of high levels of conductive additives in powder form such as copper (II) chloride of ferric chloride does not lower the electrical resistivity of this material.

In contrast, addition of 0.1 parts per hundred rubber by weight ferric chloride powder to one hundred parts by weight polycaprolactone urethane reduces the electrical resistivity from the 5x1010 ohm-cm range to approximately 1.5x108 ohmcm. Ferric chloride is not soluble in the polybutadiene prepolymer.

Ferric chloride is added to the polybutadiene polycaprolactone urethane blend to reduce the blend bulk resistivity to <1x10 9 ohm-cm. Typical concentrations of ferric chloride range from 0.05-0.30 parts by weight per hundred rubber, preferably 0.1-0.25 parts by weight per hundred by weight rubber. Other conductive additives in powder form, such as ferrous chloride, calcium chloride and cobalt hexafluoroacetylacetonate are alternatives to the ferric chloride.

.:The urethane formulation is then cast into a mold around a central, metal 15 shaft and then cured at approximately 100 degrees C. for 16 hours using a "combination of curing in a mold, demolding and postcuring in an oven to produce a rubber roller. The roller is then ground to the correct dimensions. This roller :does not have a resistive layer on the surface. The resistive layer is produced by baking the ground roll in air at an elevated temperature for some length of time.

This baking procedure oxidizes the polybutadiene. The polybutadiene is highly unsaturated (60% trans 1,4; 20% cis 1,4 structure) which makes it very susceptible to oxidation. The presence of ferric chloride is necessary to catalyze this oxidation process. A highly resistive layer is not formed in the presence of copper chloride since copper chloride doe not sufficiently catalyze the oxidation reaction to produce a high resistance surface layer. Alternatives which do catalyze this oxidation process are ferrous chloride, calcium chloride and cobalt hexafluoroacetylacetonate.

The oxidation of polybutadiene in the presence of ferric chloride produces a highly resistive surface layer. The thickness and electrical resistivity of this surface layer can be controlled by varying the concentration of ferric chloride, concentration of polybutadiene, the baking temperature, the level of oxygen and the baking time.

C:XWINWORD\TONIA\DAVIN\DIVISI1'fIV7788.DOC 6 The rollers are tested and characterized by a variety of electrical techniques. A roll is typically cleaned with isopropyl alcohol and painted with conductive carbon paint in a 10 mm strip down the roll. A circuit is made by making electrical contact with the painted surface and the roller shaft. The DC resistivity of the roll at 100V, the AC resistivity of the roll at 1 Khz, and the time constant are measured. The time constant is measured by applying a 100 volt bias to the roll, removing the voltage and measuring the time for voltage on the roll to decay to 1/e of its original value. This time constant is related to the thickness and resistivity of the surface layer on the roll. The roller is modelled as two parallel RC circuits in series. One RC circuit represents the core and the second represents the coating. Based on this model, the following equations apply: tau=R*C=rhoc* Kc*epsilono 15 rhoc=tau/(Kc*epsilon) S T=R*A/rhoo.

where tau=time constant rho=coating resistivity C=capacitance Kc=dielectric constant of coating epsilono=8.85x10-1 2 Coulombs 2 /Newtons x Meters 2 (permittivity of free space) T=thickness of resistive layer R=roll DC resistance A=surface area of roll Therefore, the coating thickness and resistivity can be calculated from the time constant and DC resistance measurements. The dielectric constant of the coating is assumed to be 10, a typical value for polyurethane rubber.

Increasing the polybutadiene level increases the resistivity of the coating.

Increasing the time and temperature of baking increases both the coating thickness and the coating electrical resistivity. By the correct combination of polybutadiene level and baking conditions, a roller with a resistive surface layer of C:\WINWORD\TONIA\DAVIN\DIVISI-1\DIV17788.DOC between 5x109 and 2x1012 ohm-cm and a surface layer thickness of approximately 50-200 microns measured at 72°F. and 50% relative humidity can be produced.

It is not uncommon for an office environment to have a high humidity and for a printer, particularly a developer roller inside a printer, to be exposed to a high operating temperature (>40 0 Polyurethane can degrade when exposed to a high level of moisture such as 80% relative humidity for a prolonged period of time and high temperature can accelerate the degradation of urethane rubber.

Also, an acid source will accelerate the degradation. The addition of ferric chloride, which is a highly acidic material, to polyurethane will accelerate the degradation of the urethane. Degradation is defined as the loss in durometer hardness over time and is characterized herein as a loss in durometer hardness when exposed to a high temperature and high humidity environment such as 60 0 C. and 80% relative humidity for a specified period of time. The use of a 15 hydrolytic stabilizer is required to maintain the roll physical and electrical properties over a long period of time and at various environmental conditions.

The addition of TIPA (trademark of Dow Chemical Co.) (chemically, triisopropanolamine 99) acts to hydrolytically stabilize the described urethanebased developer roll.

Tests show the following: 12 days exposure at 60*C./80%rh Roller based on polycaprolactone- TDI polybutadiene diol with -0.20 phr ferric chloride 40% loss of hardness* -0.20 phr ferric chloride +0.12 4% loss of hardness phr TIPA phr parts per hundred by weight *Durometer hardness is measured according to ASTM D2240-86 using type A hardness (Shore A) C:\WINWORD\TONIA\DAVIN\DIVISI-1\DIV17788.DOC The addition of a low concentration of TIPA shows no influence on the electrical properties of the roll.

The resistive surface layer produced by the oxidation process is permanent. Oxidized rolls have been analyzed for several months without a significant change in electrical properties.

Print test results of the described oxidized polybutadiene-based rolls indicate that they have excellent print performance across a wide speed range.

Their performance mimics that of a conductive roll coated in a separate process with a resistive material.

Specific working applications of this invention include: EXAMPLE 1 V Vibrathane 6060( 100 phr 15 (phr is parts per hundred by weight) Voranol 234-630(2) 5.5 phr poly bd R-45HT' 3 16.5 phr Ferric chloride 0.20 phr 20 Triisopropanolamine 99 0.12 phr DC 200 silicone oil, 50 cs 4 3.0 phr Reaction stoichiometry of Baked 10 hours at 100 0

C.

25 (1Vibrathane 6060 having an NCO content of 3.44% 2 Voranol 234-630 having a hydroxyl number of 630.6 (3)poly bd R-45HT having a hydroxyl value of 0.86 (4)DC 200 is a trademark of Dow Coming Corporation C:\WINWORD\TONIA\DAVIN\DIVISI-1\DIV177.DOC EXAMPLE 2 Vibrathane 6060 Uniroyal Chemical PBD-TDI prepolymer Voranol 234-630 Ferric chloride DC 200 silicone oil, 50 cs Reaction stoichiometry of Baked 20 hours at 100 0

C.

90 phr (phr is parts per hundred by weight) 10 phr 6.9 phr 0.1 phr 3.0 phr EXAMPLE 3 r Vibrathane 6060 Voranol 234-630 poly bd R-45HT Ferric Chloride Reaction stoichiometry of Baked 6 hrs at 100 0

C.

100 phr (phr is parts per hundred by weight) 6.5 phr 6.5 phr 0.1 phr Variations will be apparent and can be anticipated. Patent coverage is sought as provided by law, with particular reference to the accompanying claims.

a a.

a C:\WINWORD\TONIADAVIN\DIVISI~ \DIV17788.DOC

Claims (12)

1. The process of making a developer roller comprising casting a mixture of polydiene diol or a prepolymer of polydiene diol with toluene diisocyanate, a urethane prepolymer, a polyol with functionality greater than 2, and a powder of a conductive metal salt which catalyzes the oxidation of said polydiene during baking to form a roller having an outer surface, curing said cast mixture and then baking said cured mixture to oxidize said outer surface of said roller.

2. The process as in claim 1 in which said polydiene diol or a prepolymer of polydiene diol is polybutadiene diol.

3. The process as in claim 2 in which said urethane prepolymer is 15 polycaprolactone ester toluene-diisocyanate prepolymer. S

4. The process as in claim 3 in which said conductive metal salt is ferric chloride used in an amount of about 0.1 to 0.25 parts by weight to about 100 parts by weight of said polycaprolactone ester toluene-diisocyanate prepolymer.

5. The process as in claim 4 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture. S

6. The process as in claim 3 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture.

7. The process as in claim 2 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture. C:\WINWORD\TONIA\DAVIN\DIVISI-1\DIV17788.DOC 11

8. The process as in claim 1 in which said urethane prepolymer is polycaprolactone ester toluene-diisocyanate prepolymer.

9. The process as in claim 8 in which said conductive metal salt is ferric chloride used in an amount of about 0.1 to 0.25 parts by weight to about 100 parts by weight of said polycaprolactone ester toluene-diisocyanate prepolymer.

The process as in claim 9 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture.

11. The process as in claim 8 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture.

12. The process as in claim 1 in which said mixture contains a minor amount of a triisopropanolamine sufficient to stabilize the urethane of said cured mixture. DATED: 22 April 1999 PHILLIPS ORMONDE FITZPATRICK Attorneys for: LEXMARK INTERNATIONAL INC. •o C:\WINWORD\TONIA\DAVIMDIVISI\DIV17788.DOC