Kraftpaper is the paper grade with the highest strength specification. The technical term for this is "tensile strength", which naturally depends on the basis weight (grammage), which is why it is used to calculate the index. Therefore, the tensile strength index , "TSI" for short is mostly quoted.
Even the wood raw material is graded to achieve these high strengths. Softwoods from coniferous trees (e.g. spruce, pine), and especially slow-growing coniferous trees, are particularly suited. Due to the slow growth, the wood's fibres are stronger so that the Nordic countries have an advantage here, although this is evened out somewhat by the greater period required for reafforestation.
Kraftpapier consists of virtually 100 % pulp fibres, only starch, alum and size are added to achieve surface effects and strength increases. Use of waste-paper in the production of kraftpaper is unthinkable, as waste-paper contains many damaged fibres and would therefore reduce the strength. In addition, fillers (loadings) can also not be used which account for a good proportion of the weight, e.g. in graphic papers.
The pulp is often ground (refined) before the paper was made to improve strength. This causes so-called fibrils (fibrilla) to split off the surface of the fibre and then hook together to form a densely bound, strong fibre network. The strength increases highly and the beginning and then rapidly flattens off asymptotically, until no further strength increase can be achieved even with further increased grinding. Papermakers then talk of overground or "dead-beaten" pulp. The art is in grinding the pulp so that the cost/benefit ratio is optimum (power consumption for grinding/strength increase).