A new type of extrusion coating layer
Herrala, Eveliina (2022)
2022
Materiaalitekniikan DI-ohjelma - Master's Programme in Materials Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2022-11-08
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202210117577
https://urn.fi/URN:NBN:fi:tuni-202210117577
Tiivistelmä
The aim of this thesis is to study foaming in extrusion coating. The purpose was to find out how bubble formation can be improved in chemical foaming when the goal is to reduce the density of the coating by 30%. In addition, the thesis searched for answers to how foaming parameters affect density, bubble size and number of bubbles.
This thesis consists of a theoretical and an experimental part. The theoretical part begins with a discussion of extrusion coating on a general level and then low-density polyethylene coating on paperboard. Then, the theory of foaming; foam structure and characterization, the foaming process and different foaming methods are discussed. After that, the use of different additives such as nucleating agents and nanoparticles is described briefly since they can be used as an aid in foaming. The rest of the theoretical part focuses on extrusion foaming. The rheology related, the influencing variables and the characteristics of low-density polyethylene foaming are discussed.
In the experimental part, five film foaming tests were carried out with a single-screw extruder and two coating tests were carried out on the experimental extrusion pilot line. Four different polymers and one foaming agent were used as materials in the single-screw extruder tests. From the results, the density of the film and the effect of different variables on foaming were analyzed. Materials in the coating tests on the pilot line were three different polymers and the same blowing agent as in the film foaming tests. The results were analyzed by measuring the coating amount, adhesion and pinhole amount and taking microscopic images. Based on these measurements, the density of the coating and the parameters affecting foaming were determined.
Film foaming tests performed with a single-screw extruder revealed that the amount of blowing agent greatly affects the density achieved. It was possible to reduce the density in the film foaming experiments and based on the results, a few factors that improve foaming were observed. These include e.g., increasing the blowing agent concentration, optimizing the process temperature according to the foaming agent and the polymer used, increasing the pressure in the extruder, and decreasing the line speed.
In foam co-extrusion coating tests, these same factors had a positive effect on foaming. These factors, however, did not seem to have a strong influence on coating density. In fact, the density of both the reference samples and the foamed samples yielded extremely low values and thus a 30% lower density of the cover was not achieved in this work. Presumable reasons for this include bubble deformation caused by intense stretching and compression, and the choice of the density measurement method. The compression of the bubbles can cause an error for example in the measurement of the film thickness between the reference and foam samples and thereby affect the measured density values.
A method for measuring density that produces more accurate results should be developed for further studies. In addition, the adhesion of the coating to the substrate was weak in the co-extrusion coating of the foam, and therefore adhesion should be improved in further studies.
This thesis consists of a theoretical and an experimental part. The theoretical part begins with a discussion of extrusion coating on a general level and then low-density polyethylene coating on paperboard. Then, the theory of foaming; foam structure and characterization, the foaming process and different foaming methods are discussed. After that, the use of different additives such as nucleating agents and nanoparticles is described briefly since they can be used as an aid in foaming. The rest of the theoretical part focuses on extrusion foaming. The rheology related, the influencing variables and the characteristics of low-density polyethylene foaming are discussed.
In the experimental part, five film foaming tests were carried out with a single-screw extruder and two coating tests were carried out on the experimental extrusion pilot line. Four different polymers and one foaming agent were used as materials in the single-screw extruder tests. From the results, the density of the film and the effect of different variables on foaming were analyzed. Materials in the coating tests on the pilot line were three different polymers and the same blowing agent as in the film foaming tests. The results were analyzed by measuring the coating amount, adhesion and pinhole amount and taking microscopic images. Based on these measurements, the density of the coating and the parameters affecting foaming were determined.
Film foaming tests performed with a single-screw extruder revealed that the amount of blowing agent greatly affects the density achieved. It was possible to reduce the density in the film foaming experiments and based on the results, a few factors that improve foaming were observed. These include e.g., increasing the blowing agent concentration, optimizing the process temperature according to the foaming agent and the polymer used, increasing the pressure in the extruder, and decreasing the line speed.
In foam co-extrusion coating tests, these same factors had a positive effect on foaming. These factors, however, did not seem to have a strong influence on coating density. In fact, the density of both the reference samples and the foamed samples yielded extremely low values and thus a 30% lower density of the cover was not achieved in this work. Presumable reasons for this include bubble deformation caused by intense stretching and compression, and the choice of the density measurement method. The compression of the bubbles can cause an error for example in the measurement of the film thickness between the reference and foam samples and thereby affect the measured density values.
A method for measuring density that produces more accurate results should be developed for further studies. In addition, the adhesion of the coating to the substrate was weak in the co-extrusion coating of the foam, and therefore adhesion should be improved in further studies.