Method of improving properties of mechanical paper pulp without chemical reaction therewith

In the production of mechanical paper pulp from natural vegetable fibrous raw material such as wood by frictionally acting upon a natural vegetable fibrous raw material containing plasticizable constituents to at least partially separate the fibers thereof from each other while heating the plasticizable constituents thereof to a temperature sufficiently high to plasticize the same rendering the plasticizable constituents soft and sticky, the method of improving the quality and properties of pulp produced therefrom which comprises adding at least one paper filler material which is not chemically reactive with the pulp when added and at such time that it comes into contact with the plasticizable constituents of the pulp while the same are in softened and sticky condition so that the filler material adheres to but does not chemically react with the fibers of the pulp to result in improvement of the properties of the pulp.