How should we measure Reinforcement?
Some studies have shown that the reinforcement potential of LWC base paper is proportional to the average fiber length and tear strength at 70 Nm/g tensile index and inversely proportional to fiber coarseness. For SC papers the reinforcement potential has been theorized to be described simply by the ratio of fibre length to coarseness. However, there is no general accepted way for measuring the reinforcement potential.
For simplicity, industry has traditionally measured the strength properties of pure chemical pulps and combined them together to evaluate the reinforcement potential. The strength properties have traditionally dealt with tear strength, tensile strength and elastic modulus, and more recently with fracture energy and fracture toughness. From the paper physics perspective the relevant parameter to measure would be one that is in-plane since that is the direction that paper most frequently fails in. Elmendorf or Brecht-Imset tear strength is an out-of-plane test, and in recent years the consensus has grown that such tests are of limited usefulness in predicting paper failure behaviour.
The use of tear strength in some mills producing LWC and SC is declining in popularity. As a measure of reinforcement it may have been more appropriate when the mechanical pulp had high levels of shives and most of the paper machines had open draws. For most papermakers these days are long gone. In many cases the mills that still measure reinforcement on the basis of tear strength will actually be over-using softwood kraft with consequent impacts on formation and cost. These mills are often able to realize further optimization opportunities if they optimize paper properties on the basis of an in-plane test such as tensile or fracture toughness.
A very interesting area of research today concerns the activation of fibre segments within the paper structure. The basis for this approach is that paper should not be considered as an inert composite material, but rather as a network where forces from bonding, drying, shrinkage have acted on the fibres. Fibre elements that are not sharing in load bearing are unable to contribute to the strength of the sheet, regardless of how well bonded they may be to other fibres. This field is an exciting one and will probably reveal new opportunities to optimize furnishes in the near future.