What do we mean by Reinforcement?
The main use of softwood kraft pulps in papermaking is to strengthen weaker papermaking furnishes without significantly impairing other end-use properties. This function is referred to as reinforcement. Reinforcement fibres increase the strength of the wet web, the dry sheet, and the sheet when rewetted. Reinforcement fibres also improve the drainage of the furnish. Because of the generally higher cost of reinforcement fibres when compared to other furnish ingredients; it is normally good practice to use the minimum amount possible while ensuring that the desired strength properties are achieved.
While the paper industry tends to be rather casual in the way in which it uses the term strength, the generally used mechanical strength properties of paper are its elastic modulus and its fracture resistance. Additionally we commonly include its tensile strength and extensibility when referring to its strength. The manner in which fibres interact and the physical and chemical ways in which they tangle or bond together within a sheet to provide these strength properties have been the subject of extensive research and is quite well understood (see Further Reading for more reference). The particularly crucial role of fibre morphology in defining the reinforcement potential of a fibre is described in our Morphology section.
So what makes one pulp better than another from the point of view of reinforcement on the paper machine, the coater or the pressroom?