Method of producing multi-ply paper and board products exhibiting increased stiffness

A process for making a multi-ply paper or board product having two or more superposed plies formed of cellulosic fibers which comprises:

• depositing a first furnish of fibers on a moving foraminous support to provide a first layer of slurried fibers on the support;
• progressively dewatering the first layer to provide a first forming ply of fibers on the support;
• heating fibers to be used in the formation of at least one ply of the product at a temperature of from above about 170.degree. C. to about 250.degree. C. in the presence of water for at least about 0.1 minutes;
• incorporating the heat treated fibers in a second furnish of fibers for providing at least a portion of at least one ply of the product;
• depositing the second furnish of fibers atop the first forming ply to provide a second layer of slurried fibers containing the heat treated fibers on the support atop the first forming ply;
• and
• progressively dewatering the second layer through the first forming ply to provide a second forming ply containing heat treated fibers atop of the first forming ply and, thereafter, further dewatering the plies to provide a multi-ply product having superposed dewatered fibrous plies interbonded along their interface wherein the product exhibits increased stiffness for a given basis weight as compared to a multi-ply product wherein the plies do not contain the heat treated fibers.