Corrugated board bonding defect visualization and characterization

Adhesive bonding of fluted medium to linerboard is a fundamental process in manufacturing of wood-fiber based corrugated combined board for packaging. The quality of bonds requires frequent testing since production speed and container box stacking capacity are affected. Several different testing techniques are investigated in this paper that complement the standard tests commonly used: bond tensile strength “pin adhesion” (Technical Association of the Pulp and Paper Industry (TAPPI) method T 821), visual qualitative inspection of adhesive distribution by iodine stained separated components (TAPPI method T 610), and examination of manually peeled separated boards. A specially prepared corrugated board sample set was made on a pilot corrugating machine producing a range of bonding defects associated with common inadvertent improper corrugating operating conditions. A thermoelastic stress analysis using infrared image processing applied to the produced samples demonstrates a patterned localization of strains that is associated with adhesive distribution and bond strength. Analysis of load-displacement data of the pin adhesion test indicates significant measurable changes in the elastic properties of the corrugated board structure corresponding to bond quality. The experimental results supported by Finite Element Analysis indicate that increased bond strength arises from an increased localized modulus attributable to a combination of adhesive penetration into the substrate and formation of covalent bonds. Infra-red imaging of back-lighted board samples provides a complementary convenient means to assess the glue distribution.