Secondary or re-pulped fibers, refined to the same degree in terms of freeness, exhibit different physical and optical characteristics than the same fiber in the virgin state. Paper which contains various ·per cent of secondary fibers have better opacity, less sensitivity to moisture and humidity, in the sense of hygro-expansivity, reduction of tendency to curl with addition of moisture to one side of the sheet, and less tensile elongation. The advantages listed above are offset to some extent by a loss in tensile strength, fold endurance and porosity
