Virgin Softwood Pulp Ratio Verification in Technical Packaging Papers
Verifying virgin softwood content in packaging paper relies on fiber morphology analysis, ISO chemical pulp identification, and trace chemical markers
Tensile Index
Sheet Toughness
A standardized measure of the tensile strength of paper is normalized by the basis weight of the sample to allow comparison across different grades. This calculated value, known as tensile index, expresses the tensile force per unit width divided by the grammage of the sheet. It reflects the inherent strength of the fiber network rather than the thickness of the paper.
Paper buyers use this property to evaluate the structural quality of packaging materials.
Mathematical Determination
Calculating this value divides the measured tensile strength in kilonewtons per meter by the basis weight in grams per square meter. The result is expressed in newton meters per gram, which represents the theoretical length of paper that could suspend itself without breaking. Because this method eliminates the effect of basis weight, tensile index is an excellent tool for comparing the efficiency of different fiber blends.
For example, a mill can determine whether a lighter sheet with a high index can replace a heavier sheet with a lower index. This helps mills reduce material usage while maintaining the required strength of the finished product. A higher value indicates superior fiber bonding and refining, which are necessary for lightweight papers.
Sourcing Specification
Sourcing agreements for high speed packaging lines often specify a minimum index to ensure runnability and prevent sheet breaks. When purchasing sack kraft paper or folding boxboard, specifying the tensile index helps the buyer secure material that will not fail under tension. The value is highly sensitive to the degree of refining and the moisture content of the paper.
Technical specifications must define both the required index and the testing conditions to ensure fair comparisons between suppliers.