BIAS TRANSFER ROLL MATERIAL ELECTRICAL LIFE EXTENSION
Original Publication Date: 1993-Jun-30
Included in the Prior Art Database: 2004-Apr-06
Xerox Disclosure Journal
In general, photoreceptor based electrophotographic printing and copying machines suffer from transfer deletions associated with nonflat papers or those whose mechanical properties prohibit good contact with a photoreceptor in a corotron transfer zone. Examples of these types of substrates include cockled papers, perforated papers, and tab stock whereby the deletions result from an inability of the toner to transfer at low electrostatic field levels. On the other hand, when a combination of higher field requirements and larger air gaps between the paper and photoreceptor are applied, a Paschen breakdown produces a subsequent loss of image transfer. The problem of transfer deletion can be remedied by utilizing bias transfer roll technology so that the roll flattens the paper before the fields are high enough to enable Paschen breakdown. Existing bias transfer rolls possess a resistivity value which is chosen to accommodate a desired field relaxation time and power supply requirements. The magnitude of the resistivity is controlled by the addition of an organic salt (THAB) to the base urethane material. The THAB dissociates when a field is applied across the roll sandwich structure to cause resulting ions to move through the bulk rubber. This migration mechanism manifests itself as an increase in resistivity. Since roll nip corona current is desirable for paper tracking, it is a regulated parameter during the transfer process. Higher applied voltages are necessary to maintain an operating level when the resistivity increases. As the THAB deletion process increases in proportion to an increasing field, the operational limits of the power supply are stressed. Consequently, hardware design problems are imposed because of the associated arcing from biased elements to grounded surfaces.