The emptying behaviour of highly viscous liquids. Part II: Development of test methods and evaluation of untreated and coated films
Liquid food products stick to solid surfaces. This problem occurs in all supply chain steps from production to recycling. Although the basic problem has been known for a long time, the parameters influencing the interfacial adhesion have not been clarified yet. Here, two methods (product depletion test and tack test) to evaluate the adhesion of highly viscous liquids to solid surfaces have been developed. The product depletion test allows the assessment of the residue amounts and covered surface areas during the gravity-driven flow on an inclined plane. The tack test is conducted by filling the gap between two parallelly mounted, horizontally oriented plates with a liquid product and measuring the normal force while separating the plates by pulling off the upper plate under defined conditions. The influence of the experimental parameters on the maximum normal force (F-max) could be shown. While the correlation between the pull off velocity of the upper plate and F-max follows a power law, the dependency between the gap size and F-max follows an exponential function. Although no significant correlation between F-max or the work of adhesion (W-a) and the solid's properties could be identified, a correlation between F-max and W-a was observed. As shown by product depletion with ketchup, although the surface coverage is only slightly influenced by the packaging material, the opposite is the case for the covered surface area which, therefore, determines the residue amounts. For some coated samples a large residue reduction was observed.