It is well known that in extrusion coating the adhesion of PE to paperboard and other porous substrates decreases with decreasing thickness. Several hypotheses are proposed for the origin of this decrease, including a reduction in time for oxidation, faster cooling in the air gap and more rapid quenching in the nip. A model of the penetration of the molten polymer into the substrate shows that the greatest effect is cooling in the nip; thinner coatings have less time to flow into the interstices of the substrate once contact with the chill roll is made. The model results agree well with experimental adhesion data from the literature and lead to practical suggestions for improving adhesion performance.