Influence of the adhesive on the mechanical properties in laminated multi-layer paperboard : Evaluation of two common dispersion adhesives, their failure modes, internal strength and influence on bending and tensile properties

Sammanfattning: The objectives of the thesis were to investigate the adhesives influence on the mechanical properties in laminated multi-layer paperboard. The study also strived to find a method to investigate the interface strength of the adhesive layer. This was investigated by constructing handsheets of laminated paperboard by gluing a bulky, CTMP center ply, Chromofoil, of two different grammages, 290 g/m2 and 360 g/m2, together with two outer plies of ordinary copy paper, Multi- Copy 80 g/m2. The sheets were glued with two types of dispersion adhesives, a starch based adhesive and a polyvinyl acetate based adhesive. Twelve types of test specimens were constructed and tested in a series of mechanical tests, which involved a tensile test, a two-point bending test and a Scott Bond test. The method called “digital image correlation” was used trying to investigate the interface strength of the adhesive layer. The study also involved an evaluation of the possibility to predict the bending stiffness of laminated multi-layer paperboard by using calculations according to classic laminate theory. The results show that it is fully possible to use these calculated values as a prediction, with low risk of overestimating the bending stiffness of the paperboard and without involving the adhesive layer in the calculation. The method provided high absolute values in bending and tensile properties, which increased with increasing amount of glue applied. This came at the cost of grammage, which lead to low index values. The overall conclusion is that the effect from the adhesives is somewhat negative, since the mechanical properties would have been improved if the adhesive had been replaced with more fiber. The major source of errors during the experiments was the roughness of the manufacturing method. This lead to an uncertainty about whether the right amount of adhesive had been applied to the test specimens. Due to this the validity of the results could be questioned. Digital image correlation proved to be a rather sensitive method that requires a high accuracy setup and execution for it to be applicable in this type of study. The failure mode of the glue seam correlated well to the amount of glue applied. A lower amount of adhesive applied increases the likeliness of failure of the glue seam.