The present invention relates generally to an electrostatographic printing apparatus and more particularly to a fusing system for fixing toner to support substrate. In particular the present invention relates to a release member for a toner fixing station in such apparatus.
In the process of electrophotography, a light image of an original to be produced is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. The residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support or receiver, such as a sheet of plain paper with subsequent affixing of the image thereto.
In order to fix or fuse the toner onto a support member permanently by heat, it is necessary to elevate the temperature of the toner to a point at which constituents of the toner coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner cools, solidification of the toner occurs causing the toner to be bonded firmly to the receiver.
One approach to thermal fusing of toner images onto the supporting substrate has been to pass the receiver with the unfused toner images thereon between a pair of opposed roller members at least one of which is heated. During operation of a fusing system of this type, the receiver to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roller thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roller systems wherein the fusing roller is coated with an adhesive mixture, such as a silicone. The silicone rubbers which have been used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e., room temperature vulcanization silicone rubber referred hereinafter referred to as RTV silicone rubber, liquid silicone rubber to as LSR rubber, and high temperature vulcanization type silicone rubber referred to as HTV rubber. All these silicone rubbers or elastomers arc well known in the art and are commercially available.
In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the receiver will be retained by the heated fuser roller and not penetrate into the receiver surface. This tackified mixture will stick to the surface of the fusing roller and come in contact with the subsequent receiver sheet bearing a toner image to be fused. A tacky image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tacky image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused there between, toner may be transferred from the fuser roller to the pressure roll. These conditions are referred to in the copying art as "offset". Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the adhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It is well known in the art to provide toner release agent s such as silicone oil, in particular, poly(dimethylsiloxane), which is applied on the fuser roller to a thickness of the order of about 1 micron to act as a polymeric release agent. These mixtures possess a relatively low surface energy and have been found to be mixtures that arc suitable for use in the heated fuser roller environment. In practice, a thin layer of poly(dimethylsiloxane) (silicone oil) release agent is applied to the surface of the heated roller to form an interface between the roller surface and the toner image carried on the support mixture. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roller surface. While silicone elastomer fuser members have been commercially successful in some commercial applications they suffer from certain difficulties in that they tend to swell by being in contact with a silicone oil release agent which migrates or is absorbed into the silicone nibber. While a small degree of swelling may be acceptable if it is uniform, failure of such rolls has been observed by excessive swelling over a period of operation. Under such circumstances, the fuser member may no longer function in providing a uniform fusing and may lead to poor receiver handling. An additional failure mode is referred to as debonding wherein the swelling of the silicone rubber has become so significant that it actually delaminates from the core as described in U.S. Pat. No. 4,777,087 by Heeks et al.
To reduce damage to the fuser member by the release agent, it is known to dispose an intermediate layer beneath the outer silicone layer. Such barrier layer should be impervious to release agent swell. Applicable materials are described in U.S. Pat. No. 5,200,284, which is polyester based, U.S. Pat. No. 5,766,759 which is a polyurethane, U.S. Pat. Nos. 5,582,917, and 5,534,347 which are fluorosilicone interpenetrating networks. There are advantages as disadvantages for each of the above. The nonfluorosilicone lack temperature stability. The others loose adhesion with release agent swell.
There is a need for processable, cost-effective fuser member intermediate layer which are resistant to release agent swell and whose adhesion to the outer layer is not significantly compromised by release agent swell.