Where to Add Retention Aid: Issues of Time and Shear, 2001 Papermakers Conference Proceedings

Papermakers continually wrestle with the question of whether to add high-mass acrylamide copolymer retention aids before or after pressure screens in the approach flow to a paper machine forming section. Early addition of a retention aid provides more opportunity for breakage of chemically induced fiber flocs, possibly leading to more uniform formation. Early addition also provides more chance for chemicals to become degraded or lost in the porosity of fiber cell walls. Later addition tends to maximize chemical efficiency in terms of first-pass retention. A new method of fiber floc evaluation was applied in the case of headbox-consistency fiber slurries to help understand what happens to chemically induced fiber flocs when they are exposed to increased time and shear. The extent of flocculation was determined by the force required to move a pair of 6-mm probes through a slurry. Supplementary tests of the streaming potential of fibers were used to help explain the separate effects of time and hydrodynamic shear on the state of flocculation.