Belt member, transfer unit incorporating same, image forming apparatus incorporating same, and method of evaluating same

A multi-layer endless belt member for use in an image forming apparatus and including a volume resistivity and a surface resistivity, the multi-layer endless belt member comprising:

• a base layer having a resistance;
• a surface layer having a resistance higher than the resistance of the base layer, the surface layer for carrying a toner image thereon, wherein:
• the volume resistivity of the multi-layer endless belt ranges from approximately 8.0 to approximately 11.0 in a common logarithm value (log [Ω·cm]),
• the multi-layer endless belt member has a resistivity on a first surface which is an outer surface of the surface layer of the multi-layer endless belt member and a resistivity on a second surface which is as an inner surface of the base layer of the multi-layer endless belt member, and
• an amount of resistivity change of the first surface of the multi-layer endless belt member is greater than an amount of resistivity change of the second surface of the multi-layer endless belt member, where the amount of resistivity change of the first surface indicates a difference between a surface resistivity value measured after a given voltage is applied for 1 second and a highest surface resistivity value measured after the given voltage is applied between 1 second and up to and including 100 seconds on the first surface of the multi-layer endless belt member;
• and the amount of resistivity change of the second surface indicates a difference between a surface resistivity value measured after the given voltage is applied for 1 second and a highest surface resistivity value measured after the given voltage is applied between 1 second and up to and including 100 seconds on the second surface of the multi-layer endless belt member.