A roll for fusing heat softenable toner material

Abstract: A roll for hot fusing having a coating comprising a cross-linked elastomeric random copolymer of dimethylsiloxane and diphenylsiloxane and three additives, (1) from 40 to 55% by weight of small zinc oxide particles, (2) from 5 to 10% by weight of small graphite particles and (3) from 1 to 5% by weight of ceric dioxide particles.

Description: The present invention is concerned with rolls to be used in heat softenable fusing toner material. In particular it is concerned with the formulation for the coating on such rolls to make them both thermally stable and abrasion resistant. For an example of the prior art see EP-A-0 312 468. Hot roll fusing is used in electrophotographic printing and copying to fix the toner on the paper. At higher printing speeds it is desirable to have the hot roll softer than the back-up roll. This necessitates the use of a thermally conductive layer on the surface of the heated roll, which is simultaneously elastically deformable and thermally conductive. Many composite elastomer materials which provide these properties have been described in the literature. However, a major problem with all such materials is that they are liable to abrasive wear due to contact with the paper. Thus they have limited life and must be replaced at regular intervals, adding significantly to the maintenance cost of the printer or copier. The present invention describes an elastomer formulation which exhibits significant improvements over previous material combinations. In designing a hot roll fuser elastomer it is desirable to ensure functional properties of the material, namely compliance, thermal conductivity and toner release characteristics, while at the same time maximising the lifetime abrasion resistance and thermal stability. It is also desirable to ensure that the material is processable into the final device at reasonable cost. According to the present invention there is provided a roll for fusing heat softenable toner material to a substrate, said roll comprising a core and an outer covering which comprises a cross-linked elastomeric random copolymer of dimethylsiloxane and diphenylsiloxane, characterised in that the covering incorporates from 40 to 55% by weight of particles of zinc oxide in the range of 100 to 500 nm diameter, from 5 to 10% by weight of graphite particles with major diameters no greater than 10 microns, and from 1 to 5% by weight of ceric dioxide particles from 0.2 to 1.0 microns in size. Random copolymers of dimethyisiloxane and diphenyl-siloxane have been described in the literature. For the purposes of the present invention, it is preferred that the weight concentration of phenyl be in the range of 20-50%, with the most preferred concentration being 25%. The molecular weight of this resin preferably should be in the range of 10,000 to 30,000 g/mol, and most preferable 20,000 g/mol. The polymer resin is terminated with a vinyl group at each end. Particulate fillers are used to provide both thermal conductivity and reinforcement of the neat elastomer resin. There is a critical volume fraction of filler above which the abrasion resistance is severely reduced. On the other hand, thermal conductivity and heat capacity are both enhanced by high filler concentration. Therefore it is necessary to find a filler, or combination of fillers, which provide sufficient thermal conductivity and capacity at concentrations low enough that abrasion resistance is not adversely affected. The siloxane elastomers of the present invention containing more than 55% by weight of zinc oxide exhibit rapid wear. However, even at 55% loading the thermal properties are marginal for use in hot roll fuser systems. Therefore, it is necessary to add a second filler to provide additional thermal conductivity. Graphite is such a material, not only because of high intrinsic thermal conductivity, but also because its platelet morphology results in higher conductivity of the composite than spherical particles at similar weight concentrations. The concentration of this second filler is also dictated to some extent; by the effect on wear resistance. Particles of high aspect ratio have lower critical concentrations for rapid increase in wear than spherical particles--around 10% for graphite. This then provides an upper limit on the concentration of the second filler. Graphite also provides additional thermal stabilisation of siloxane elastomers. Therefore it is the preferred material in this application. A key aspect of the present invention is the use of ceric dioxide to assist the degassing of the formulation which, because it contains phenylsiloxane and a high fraction of zinc oxide, is unusually difficult to degas and process without inclusion of bubbles and voids. It is important that any additive be compatible with the rest of the formulation, and not lead to deterioration of the mechanical and thermal properties. Ceric dioxide fulfills all of these requirements. Since CeO₂ is not so reinforcing as zinc oxide, it is important that the amount present be kept to a minimum, so that the abrasion resistance is not compromised. We found it necessary to use at least 1% but amounts greater than 5% degrade the properties. 2% is optimal. In addition, ceric dioxide is also acceptable because it is non-toxic and non-volatile. In fact, it seems to be uniquely suitable for this purpose. In order to form the elastomer layer around the core of the fuser hot roll, a liquid injection moulding technique is commonly used and is particularly convenient. This requires that the resin be cured at high temperature with a cross- linking agent, typically by chemical reaction between vinyl groups attached to the siloxane resin and silane groups in the lower molecular weight curing agent. Preferably, the vinyl siloxane moities are at terminal positions in the polymer chains of the base resin. Tile vinyl-hydride addition reaction is catalysed by platinum, which may be hindered in its action by complexation with an acetylinic alcohol (e.g. propargyl alcohol) which has a boiling point well above room temperature, but below the preferred temperature for the cross-linking reaction. Air is readily dissolved and trapped in polydimethyldiphenylsiloxane resin, and is therefore difficult or even impossible to remove after the necessary mixing of fillers and curing agent with the base resin. The removal of air can be achieved by the use of a particulate anti-foaming agent, namely ceric dioxide at at least 1% by weight. We find that the addition of 2% ceric dioxide reduces by a factor of five the time necessary to thoroughly degas an uncured batch of mixture. However, ceric dioxide is deleterious to the mechanical properties of the resulting elastomer, and concentrations higher than 5% are undesirable. Mixing of the fillers and additives into the resin is done using standard techniques, except that special attention must be paid to degassing the mixture. A cross-linking agent is added immediately prior to casting, which also is achieved by standard liquid injection moulding techniques onto roll cores primed with a suitable adhesion promoter. A comparison of samples prepared in this way with other formulations sold for use in hot roll fusers shows significant improvement in mechanical and thermal properties, as shown in the following Table. The combination of softness, high thermal conductivity and low wear rate are ideal for use in hot-roll fuser application. Previous materials have sacrificed one of these properties at the expense of others; for example, higher thermal conductivity can be achieved by increasing the filler concentration, but this raises the Durometer value and increases the wear rate. It is clear that the overall properties of the formulation described herein are superior. Results in an operating machine confirm this.

Claims: A roll for fusing heat softenable toner material to a substrate, said roll comprising a core and an outer covering which comprises a cross-linked elastomeric random copolymer of dimethylsiloxane and diphenylsiloxane, characterised in that the covering incorporates from 40 to 55% by weight of particles of zinc oxide in the range of 100 to 500 nm diameter, from 5 to 10% by weight of graphite particles with major diameters no greater than 10 microns, and from 1 to 5% by weight of ceric dioxide particles from 0.2 to 3.0 microns in size. A roll as claimed in Claim 1 wherein the copolymer of dimethylsiloxane and diphenylsiloxane has a weight concentration of phenyl in the range of from 20 to 50%. A roll as claimed in Claim 2 wherein the weight concentration of phenyl is 25%. A roll as claimed in Claim 1, 2 or 3 wherein the copolymer has a molecular weight in the range of 10,000 to 30,000 g/mol. A roll as claimed in Claim 4 wherein the copolymer has a molecular weight of 20,000 g/mol. A roll as claimed in any of the preceding Claims wherein the covering incorporates 50% by weight of zinc oxide. A roll as claimed in any of the preceding Claims wherein the covering incorporates 8% by weight of graphite. A roll as claimed in any of the preceding Claims wherein the covering incorporates 2% by weight of ceric oxide.

Inventors: AYALA-ESQUILIN JUAN, DICKSTEIN WILLIAM HARVE, HEDRICK JAMES LUPTON JR, SCOTT JOHN CAMPBELL, YANG ARNOLD CHANG-MOU, AYALA-ESQUILIN, JUAN, DICKSTEIN, WILLIAM HARVE, HEDRICK, JAMES LUPTON, JR., SCOTT, JOHN CAMPBELL, YANG, ARNOLD CHANG-MOU
Applicants: LEXMARK INT INC, LEXMARK INTERNATIONAL, INC.
Application year: 1991
Main classification: G03G15
IPC codes: C08K3, G03G15