Separator for lead acid battery

A separator for a valve-regulated lead acid battery, comprising:

• a sheet initially formed through wet papermaking substantially containing at least glass fibers, the sheet having a length, width, and thickness,
• the orientation of the glass fibers being randomized to allow for strengthening compressive force for the sheet,
• the glass fibers substantially overlapping with each other for suppressing a loss in the thicknesses of the glass fibers,
• the sheet having surface tension being dispersed in many directions for suppressing the lost in the thickness of the separator, and
• the proportion of glass fibers that are in the vertical direction is higher than the proportion of glass fibers that are in the non-vertical direction with respect to the horizontal plane of the sheet resulting in the enhancement of the pressure force of the sheet,
• wherein a paper machine controller is configured to control the jet-wire ratio and the inclination angle of the forming wire, so as to provide the resulting separator for a lead acid battery having an MD/CD strength ratio shown by the following expression (1) of 1.5 or less and a 60 kPa compressive force in liquid immersion shown by the following expression (2) of 65% or more to secure a prolonged cycle lifetime for the separator:
• MD/CD strength ratio=MD strength/CD strength, (Expression 1)
• wherein the sheet formed through wet papermaking cut in a direction in parallel to the flow direction in the papermaking to a length of 15 cm and a width of 2.5 cm is designated as an MD specimen, and the sheet formed through wet papermaking cut in a direction perpendicular to the flow direction in the papermaking to a length of 15 cm and a width of 2.5 cm is designated as a CD specimen, which are measured with a tensile tester under conditions of a chuck distance of 10 cm and a tensile speed of 25 mm/min, and the maximum breaking strengths of the MD specimen and the CD specimen are designated as the MD strength and the CD strength respectively,
• 60 kPa compressive force in liquid immersion(%)=(pressure force after liquid immersion/compressive force in liquid immersion)×100, (Expression 2)
• wherein 10 sheets of the sheet formed through wet papermaking are cut into 10 cm×10 cm and are laminated to prepare a specimen, which is sandwiched between a fixed plate and a movable plate, the movable plate is moved toward the fixed plate at a moving speed of 1 mm/min to compress the sheet under pressure of 60 kPa, which is designated as the compressive force in liquid immersion, and the pressure force of the specimen after immersing in a sulfuric acid solution having a specific gravity of 1.3 and allowing to stand for 1 hour is designated as the pressure force after liquid immersion.