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- Impact of Mechanical Converting Processes on Tissue Paper PropertiesPublication . Vieira, Joana Magalhães Gonçalves Costa; Costa, Ana Paula Nunes de Almeida Alves da; Fiadeiro, Paulo TorrãoTissue paper is truly rooted in the daily life of modern society due to the wide variety of products that make different applications possible. For this type of industry, it is a huge challenge to produce the best products to retain the final consumer. Tissue paper is a paper characterized mainly by its low grammage and tensile strength, and by its high softness, liquid absorption, and elasticity. Depending on the product segment to be produced, it is necessary to consider which of these characteristics are essential, for example, in toilet paper the focus is on softness and absorbency, in kitchen rolls it is on absorbency and wet strength, softness on napkins, absorption and wet strength and softness on facial tissues. These properties must be adapted to meet end consumer requirements, which vary greatly for the different countries in the world. The tissue paper production process uses virgin cellulose as the main raw material and involves two steps: the formation of the tissue paper sheet itself (tissue base paper) and its transformation into different types of finished products. This work essentially addresses this second stage, where the transformation of tissue base paper into finished products takes place and its impact on the properties of the products produced. During the tissue paper base sheet transformation process, also called the conversion process, the properties acquired in the previous step are altered, as the sheet is subjected to successive operations that will permanently deform it. The converting machine is characterized by several operations, the main ones being winding/unwinding, embossing/laminating, perforating, cutting, packaging, and palletizing. The converting process proves to be very complex and has a huge impact on the properties of the finished tissue paper. Embossing is the key operation in the tissue paper transformation process, as it is the one that most affects the final properties of the finished product. This operation consists in marking a pattern on the tissue paper base sheet by applying pressure, with the purpose of producing papers that are more appealing to the final consumer and/or being a mean to recognize a brand. In addition to visually affecting the paper, it also affects the final properties of the finished products, adding a more pronounced third Z dimension with a compression matrix, increasing its liquid absorption capacity and volume, but, on the other hand, reducing its softness and tensile strength. Since embossing is the most impacting operation in the transformation of tissue paper and taking into account the industrial embossing process, a system was developed that would allow the study on a laboratory scale of the impact of this operation on the physical-mechanical properties of tissue paper, depending on the different operating parameters of the converting machine, such as the finishing of the dots and/or lines of the embossing pattern, hardness of the embossing rubber, pressure, temperature and humidity, both on laboratory sheets (isotropic handsheets) and on industrial base tissue paper (anisotropic and creped sheets). As this laboratory set-up makes it possible to control all the operating parameters individually, it was possible to optimize the embossing process at a laboratory level and its validation was carried out using the finite element method. Thus, for each new unique standard that the tissue paper industry intends to implement, they can test it in the laboratory before making the scale-up. Using the laboratory embossing system, we started by studying how pressure affects the main properties of tissue paper. Industrial base tissue paper sheets were used and an optimum pressure of 2.8 bar was achieved for this system. It was possible to distinguish two effects that occur in the tissue paper sheet during the embossing operation with pressure, the densification of the sheet and the permanent deformation of the sheet with the mark of the pattern. The effect of pressure when densifying the paper sheet gives it a gain in mechanical strength, but without differences in terms of liquid absorption. The two embossing patterns (deco and micro) showed different behaviors with the effect of pressure, but both showed losses in both mechanical properties and softness. These losses were less pronounced for the pressure 2.8 bar since the densification is maximum for this pressure. On the other hand, the finite element method failed to show how pressure affects paper strength. Another operating parameter that also impacts the final properties of tissue products is the influence of the hardness of the rubber used in the counter-roller to the embossed steel roll with an engraved pattern. Three different configurations of stacking rubber plates with different hardness on sheets of industrial tissue paper were studied. This study allowed us to conclude that to obtain greater softness, the best solution was where two rubber plates with different hardness were used, and the rubber plate with greater hardness was in contact with the tissue paper sheet. This result corroborates the future industrial trend in which the use of rubber rollers in the embossing operation with an inner layer of low hardness and an outer layer of high hardness is pointed out. The finite element method, in addition to validating the results obtained, proved to be a reliable tool to virtually test other configurations, such as, for example, three or more rubber plates with different hardness. The impact of the geometry of the finishing of the lines and dots of the patterns to be embossed, on the final properties of the tissue paper was another operating parameter object of study. This work was carried out using industrial base tissue paper sheets and it was concluded that although the patterns with straight finish geometry individually presented a higher softness value, when the prototype of a finished product with 2 plies (deco + micro) was assembled, the greater softness was obtained for the round finish geometry. It was confirmed that the softness value decreases with increasing bulk, being more pronounced for the micro embossing pattern. No relevant differences were found in the kinetics of liquid droplet scattering over time, from which it can be inferred that the finish geometry of the lines and dots of the embossing patterns does not affect this property in this type of products. The finite element method also in this case, allowed a better understanding of the effect of the pattern finishing geometry on the tissue paper sheet, and the simulation results matches with the experimental results, showing the same trend where the patterns with round geometry marked more tissue paper sheet than patterns with straight finishing. The laboratory embossing system was also used to investigate the effect of this converting operation on industrial base tissue paper sheets and handsheets. To evaluate the influence of the embossing patterns, the fiber composition and the creping process, industrial base tissue paper sheets, handsheets produced from a fibrous suspension obtained from the disintegration of the industrial sheet were used as samples (keeping the same fibrous composition) and handsheets produced from a never-dry bleached eucalyptus industrial kraft pulp. The handsheets were produced with a grammage of 17 g/m2 (grammage similar to the industrial base tissue paper) and not pressed. The results indicated that the embossing process produced more bulky and porous structures, at the expense of losses in mechanical and softness properties, which were more pronounced for the micro pattern than for the deco pattern. The effect of fibrous composition showed that an increase in mechanical properties negatively impacted the softness of handsheets. Handsheets composed of 100% eucalyptus showed greater softness than handsheets whose composition is a mixture of short and long fiber. It was found that the crepe existing in the sheet of industrial base tissue paper, gives it a high elongation capacity that is practically non-existent in the handsheets. Furthermore, due to this operation, industrial paper samples have a higher apparent porosity than handsheets samples. The analysis by the finite element method allowed the validation of the experimental results, proving that the micro pattern has a higher stress field value and, consequently, a lower mechanical strength. In addition to the use of the laboratory embossing system and to deepen the impact of embossing on the properties of finished tissue products, other studies were carried out on industrially and commercialized finished products. The absorption capacity, also being a fundamental property of tissue papers, was one of the studies developed. In this work, the absorption capacities of four different industrial base tissue papers were compared, as well as the respective 2-ply industrial toilet papers that they originated. It was concluded that the embossing operation increased the thickness and, consequently, the bulk of the toilet paper. Furthermore, it was also found that among the various samples of toilet paper there was no perceptible variation in the water absorption time, as the samples presented similar morphology and porosity. However, it was found that where bulk increased the most (about 150%), it resulted in an increase in water absorption capacity (about 60%). Another important study to deepen the embossing operation was the impact of the sequence of stacking a toilet paper with an odd number of plies (in this case 5 plies). The two possible configurations, 1 and 2 (deco:micro pattern of 3:2 and 2:3 plies, respectively) were the object of study. The industrially produced products with both configurations were originated from the same base tissue mother-reels. Overall, the sheet stacking sequence was found to influence the properties of the finished toilet paper. For configurations 1 and 2, after the embossing process, bulk increases of 46% and 40%, respectively, and water absorption capacity increases of 2% and 17%, respectively, were recorded. Regarding the mechanical properties, both configurations had a greater negative impact caused by the deco embossing pattern. For commercial purposes and to meet the preferences of the final consumer, toilet paper with configuration 1 was more suitable for mechanical strength preferences and toilet paper with configuration 2 was the most suitable for preferences of absorbency. Regarding softness, the stacking sequence also affected the results, where configuration 2 proved to be the smoothest and most pleasant to the touch product, with an overall handfeel value of 75.3 HF, and the product produced with configuration 1 presented rougher and less pleasant to the touch, with an overall handfeel value of 68.0 HF. Another operation that takes place in the converting machine and which also impacts both the properties of this type of product and the operability of the machine itself is the perforating operation. A system was then developed that would allow a laboratory-scale study of the impact of this operation on the perforation efficiency of the finished products. This perforation system applies to all tissue paper products, such as kitchen paper or toilet paper, that need to be portioned according to the needs of the end consumer. The perforations facilitate this portioning, promoting the separation of sheets or services, by the perforation line without tearing them. However, the perforated paper must be strong enough to hold together under a certain tension, but on the other hand it should be weak enough that the sheet or service can be detached from the roll easily, without tearing, with little effort along the straight or patterned horizontal perforated line. This balance is given by the perforation efficiency. The higher the perforation efficiency, the easier the service separation will be. In this context, the developed laboratory perforating system allows testing new types of cuts distances on a laboratory scale, allowing the results to be transposed to an industrial scale, and allows to evaluate problems associated with the perforation of products, as well as testing new cutting patterns. As customer satisfaction can depend on the perforation performance, the laboratory perforation system was used to perforate different commercial toilet papers (in brands and number of plies) to evaluate their perforation efficiency. With this study, it was verified a stabilization of the perforation efficiency from a cut distance of 6 mm and a 15% increase in the cut distance for the laboratory blade to correspond to the industrial perforation efficiency. The finite element method was also used to simulate the progression curve of perforation efficiency as a function of cut distance. This analysis confirmed the behavior of the evolution of the perforation efficiency with the increase of the cut distance and its stabilization from the cut distance of 6 mm. Another study with interest to understand the impact of perforation was its evaluation in commercial toilet products. In this work, the mechanical behavior of 15 commercial toilet papers from different European producers, with equally different compositions and number of plies, was studied. A qualitative analysis of the quality of the cuts, together with a quantitative analysis of the dimensions of the same cuts, was performed through an optical system. An analysis using the finite element method was carried out where it was possible to examine the behavior of the stress concentration in the cut hole and the influence of the cut distance. The results showed that a cut distance equal to or less than 2.0 mm should not be used in these types of papers, and the perforation efficiency increased with the increasing of the cut distance, regardless of the number of plies that make up the toilet paper. The stress concentration factor was also determined and a limit value of 0.11 was reached. Toilet papers to tear at the perforation line, as desired, need to have a stress concentration factor above this limit value. Resuming, it is in the converting machine where value is added to tissue paper products, which is why these machines are constantly evolving. At the beginning of the tissue paper transformation, the paper was rewound by hand on a mandrel and when the first semi-automatic machine appeared, only a few LOGs were wound per minute. Currently, product design plays a very important role, as in addition to its apparent sophistication, it is also the key to optimizing its properties. Therefore, more and more products are embossed and printed, and design patterns are constantly changing and optimizing. Due to market demands, the adaptability of this type of machine and its rapid updating is imperative for the producer, as meeting the requirements of the final consumer is his main motivation. The fact that the automation of the converting machines goes all the way to the end of the line (palletization), allows the producer to better control the quality and price of the product he presents to the consumer, who, in the last analysis, has a greater probability of success for the producer who have the right products that reflect the needs and wants of customers. The advantage will be found in producers who use the latest technology, because with the slowdown in the economy, products will have to be redesigned to find a lower price, which means making an additional effort to produce the products at the lowest cost possible as well as lowering the cost of transportation through distribution channels. This work thus helps the producer to optimize the operating parameters along the converting machine, pointing out some modifications, such as replacing the single hardness rubber roller with one of variable hardness, or finding the optimal pressure of the machine in which the resistance mechanics is maximized, and that when implemented will improve the quality of the product produced, adding value. The digital twining of the various converting processes, presented here by the finite element method, proved to be a reliable modeling tool to test changes that are intended to be introduced in the process, virtually and with reduced costs. This procedure is a trend in the near future, because it allows optimization in a digital environment, without having to make several attempts and errors to determine the optimal parameters of the processes. Due to the high competition and secrecy between different tissue paper producers and suppliers, there is little research and publications related to the production and its impact on the final properties of these types of tissue paper products. This thesis shows some advances that have been made in this area of research, since most of the studies referenced here are very recent, which indicates a slight opening of the industry to create partnerships to deepen these mechanical impacts on the properties of finished products.