Process for making high-strength, high-yield sulfite-modified thermomechanical pulp and a linerboard composition produced therefrom

A process for producing a linerboard composition having a percent mullen of at least 80%, including a replacement quantity of sulfite-modified thermomechanical pulp, which comprises:

• (a) initially imparting mechanical attritional forces to undefibered lignocellulose which has been subjected to an elevated temperature and corresponding pressure for a residence time period sufficient to soften the lignin portion of the lignocellulose causing substantial fiber separation to occur in the middle lamella layer so that the fibers remain essentially intact with a lignin coating on their outer surfaces, the power imparted to said undefibered lignocellulose by said initial mechanical attritional forces being sufficient to provide said substantial fiber separation without causing significant disassembly of the cellulose layers which form the fibers themselves;
• (b) adding a sulfite chemical to the lignocellulose prior to, during, or subsequent to said initial mechanical attrition step so that a substantial quantity of carbon-to-sulfur covalent bonds are created when the sulfite interacts with the lignin, the degree of sulfonation being controlled in a manner adequate to produce sulfite-modified thermomechanical pulp having a percent bound sulfur level of at least about 0.15% by weight, based on the total weight of lignocellulose employed in producing said sulfite-modified pulp, while at the same time preventing dissipation of the lignin content of the sulfite-modified thermomechanical pulp to the point where the yield is detrimentally affected;
• (c) subjecting said sulfite-treated lignocellulose to a second mechanical attrition step, conducted at substantially atmospheric temperature and pressure conditions, to produce a sulfite-modified thermomechanical pulp having a yield of at least about 85% by weight, the average fiber length of the undefiberized lignocellulose being substantially maintained on formation of the sulfite-modified thermomechanical pulp to facilitate the maintenance of a critical bursting strength level;
• (d) forming an aqueous linerboard furnish, including at least about 25% by weight of said sulfite-modified thermomechanical pulp, capable of being formed into linerboard having a percent mullen of at least about 80%, without altering the ability of the furnish to properly drain;
• (e) depositing said furnish on a foraminous surface to produce a wet linerboard web;
• and
• (f) drying said linerboard web.