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- Vessel passivation strategies to reduce vessel pickingPublication . Coelho, João Pedro Rodrigues; Vaz, Álvaro Frederico CamposIn recent decades, the paper industry has faced several challenges, both in terms of decarbonisation of the industrial sector and in terms of innovation and concomitant evolution of the digital era. One of the problems associated with the paper industry, and in particular the study case, Eucalyptus globulus (hardwood), is the presence of vessel elements in the wood structure. This problem stands out not in production (paper industry) but in the use (consumer) of the final product. In the case of printing from paper produced with this pulp, the poor adhesion of these elements to the surface of the paper can cause them to come loose and contaminate the printing system, a phenomenon called vessel picking and ink refusal. It is a problem commonly found in printing industries that need to make large quantities of prints and associated with the paper industry for the production of paper. During the printing process, this phenomenon occurs due to the stickiness of the inks involved in the printing process and can cause vessel elements torn from the paper surface to contaminate the printing machine circuit. This contamination can result in a drop in production due to the stopping of printing machines for cleaning, which can result in increased costs. Although it is not a directly felt problem in the paper industry, there is a continuous need to optimize the characteristics of a product, reducing the rate of complaints due to this problem. There is also a need to follow the evolution of the digital era and technologies, including printing technologies that increasingly print faster and with more colours, posing other types of challenges to the paper structure while at the same time accompanying the decarbonisation of the sector. This thesis aimed to understand the phenomenon allowing the creation of papers with a low level of vessel picking and following the studies carried out over the last decades on the subject, some approaches to this problem were attempted. The physical separation of vessel elements was attempted, using the Jacquelin method and also the Britt-Jar method. Some adaptations were made to separation methods (Britt-Jar) and others were created for the same purpose (adapted Strainer). The objective was to create a pure vessel fraction (>90%) to be able to use different analytical means and better understand the differences in behaviour between fibres and vessels, and to create a pulp fraction sufficiently rich in vessels to enable the vessel picking study. Morphological studies were carried out on the characteristics of the pulp and after achieving satisfactory separation levels (>90%) it was possible to carry out more concrete studies on the surface and global chemistry of the vessel elements and fibres. It was also possible to advance two treatment hypotheses, mechanical and enzymatic. With these studies it was possible to discover similarities and differences that could justify the different interaction between the elements (fibres and vessels) and thus try to better understand the vessel picking effect. It was also possible to perceive the effect of the treatments on the different pulp elements. The results reveal differences between vessels and fibres and point to a greater hydrophobicity of the vessels in relation to fibres (μ-FTIR spectrum with differences in the peak relative to the CH bonds and the COC ether group; it is possible that the presence of a higher concentration of hydrocarbons may be related to the greater hydrophobicity shown by the vessels, in agreement with surface free energy values, where the vessels present a higher dispersive/polar energy ratio, and with the higher values of water contact angle for the vessels (more hydrophobic)), a higher content of lignin and extractives in the vessels (low O/C content in XPS and EDX observed in the vessels can be explained by a higher content of lignin and extractives), a higher content of hemicelluloses in the vessels (having a higher content of total acids and hexenuronic acids) and a greater porosity for the vessels in relation to fibres (vessels with a higher content of mesopores and lower content of micropores). The use of enzymatic treatments revealed to have a positive effect on some of the characteristics presented by fibres and vessels (reduction in hemicellulose content and superficial reduction in the content of extractives and hemicelluloses) and also on the selectivity of enzyme action (greater effect on vessels, probably due to difference in porosity). In this work, paper with different quantities of vessels was also simulated and treatments were carried out on them. The effect of beating was studied, the effect of two enzymes (xylanase and an enzymatic cocktail containing cellulase and laccase) and the two treatments simultaneously were studied. Beating proved to be the most effective method in reducing vessel picking (>90%), although both enzymes, with greater relevance for the enzymatic cocktail, showed some effectiveness (vessel picking decrease: xylanase ≈ 38%; enzymatic cocktail ≈ 53%). Enzymatic treatments mainly revealed changes in the behaviour of the vessels and consequently in their integration into the paper structure. A preferential attack on the vessels was detected, probably due to the greater porosity of the vessels; there was a consequent increase in porosity with the enzymatic treatment, where the effect was more pronounced in the vessels; reduced hydrophobicity, proven by water contact angle measurements as well as by the increase in the polar component of the sample’s surface energy. However, the combination of the two methods proved to be the most effective methodology (>91%), and there may be a balance between beating and the use of enzymes, resulting in a reduction in operating costs. Within the tests carried out, enzymatic tests were also carried out directly on a fraction of vessels, which revealed an effectiveness of 94% in reducing vessel picking. In short, the problem of vessel picking can be controlled through beating, which is not a 100% effective solution. Enzymatic treatments improve the beating effect, proving to be efficient in terms of reducing vessel picking, however, it is necessary not to neglect side effects in the tensile properties. From the point of view of the paper production industry, when subjected to a raise in vessel problems, enzymatic treatments should be considered, before beating, presenting a good solution to mitigate the vessel picking problem.