Structural Property
Void space within the fiber network of a paper sheet determines how liquids and gases penetrate the finished material. Papermakers manipulate furnish porosity to control how the sheet interacts with printing inks and barrier coatings. This structural characteristic depends on the blend of softwood and hardwood fibers, as well as the concentration of mineral fillers.
A high fraction of long fibers increases the volume of interconnected voids, while short fibers and fillers pack together to restrict air flow.
Refining Effect
Mechanical refining of the pulp reduces the average fiber length and creates fine fibrillated surfaces, which decreases the overall furnish porosity of the wet web. This action increases the bonded area between fibers, making the paper denser and less permeable. Refined fibers hold water more tightly, which prolongs the pressing and drying stages on the paper machine.
Conversely, unrefined pulp maintains a higher fraction of open channels, allowing air to escape easily during sheet formation. Controlling the energy applied during refining is therefore the primary method for adjusting sheet density.
Drainage Behavior
High permeability speeds up the drainage of water on the forming wire, allowing the paper machine to run at higher speeds. When furnish porosity is too low, the wet sheet retains water, which limits production throughput and increases steam consumption in the drying section.
