Dokument: Light-Curing Polymer Systems As Novel Pressure-Sensitive Adhesives With Innovative Crosslinking Mechanisms
| Titel: | Light-Curing Polymer Systems As Novel Pressure-Sensitive Adhesives With Innovative Crosslinking Mechanisms | |||||||
| URL für Lesezeichen: | https://docserv.uni-duesseldorf.de/servlets/DocumentServlet?id=67933 | |||||||
| URN (NBN): | urn:nbn:de:hbz:061-20241218-090550-5 | |||||||
| Kollektion: | Dissertationen | |||||||
| Sprache: | Englisch | |||||||
| Dokumententyp: | Wissenschaftliche Abschlussarbeiten » Dissertation | |||||||
| Medientyp: | Text | |||||||
| Autor: | Moritz, Arnold [Autor] | |||||||
| Dateien: |
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| Beitragende: | Prof. Dr. Hartmann, Laura [Gutachter] PD Dr. Schaper, Klaus [Gutachter] | |||||||
| Dewey Dezimal-Klassifikation: | 500 Naturwissenschaften und Mathematik » 540 Chemie | |||||||
| Beschreibung: | UV curing of inks, coatings and adhesives developed to one of the fundamental curing
mechanisms during the past decades as it is known as a rapid and comparable sustainable curing technology, compared to competitive hardening processes. The curing via UV irradiation produced by mercury bulb technologies became known as the robust curing method, combined with reduced oxygen inhibition and better surface curing as the byproduct ozone which is generated and quenches the oxygen molecules reliable. However, since the world strives for a more sustainable and economically friendly industry including not only the origin of energy but also the technologies used by it, the future of mercury bulb curing is more than uncertain. This is not only connected to the fact that with LEDs a worthy replacement was developed, but also by the fact that especially the European Union puts a lot of pressure on the mercury bulb technology and already bans them in several industry sectors. Even though there is still an exemption for the adhesive market so the mercury bulbs can still be used there, it is expected to come to an end in the next years and adhesive customers are more than willing to change to LED curing technology. Since the switch to LED technology is connected to a reduction in the broadness of the wavelength where photons are emitted, a simple switch from mercury bulb curing to LED cannot be simply achieved by changing the UV source only. As the overall curing behavior strongly depends on the photoinitiator technology, it is necessary to have suitable photoinitiators available which show a high reactivity at the wavelength the LED emits. When having a look onto the printing industry, in which it is very common to cure prints with UV irradiation, the switch to LED technology did already take place in the past years. Since the formulations consist of unsaturated monomeric and oligomeric systems here, they can be cured well with commercially available Norrish Type I photoinitiators like BAPO, TPO, etc. Of course, this was not done by simply switching the photoinitiator, but also the concern of oxygen inhibition at the surface needed to be considered and was achieved by using synergistic monomer formulations reducing oxygen inhibition. In the market of pressure sensitive adhesive (PSA), where UV hotmelt based PSA have a not to be neglected share, the switch from mercury bulb curing to LED curing cannot be achieved by using the same photoinitiators which are used by the printing industry. However, as the UV hotmelt PSA market share is expected to rise tremendously in the future, mainly because there is a big pressure on solvent based PSA because of sustainability reasons, the switch from mercury bulb curing to LED is essential to enable a UV hotmelt technology which is not only viable in the future but also is truly dedicated to the idea of being more sustainable. Scheme 1: Overview of chapters dealt with in this PhD thesis Within this thesis, the state of the art for LED curable hotmelts has been investigated including possibilities for rapid LED curing with commercially available photoinitiators. Since there is a big pressure on the change to LED technology for the UV hotmelt market and it is expected to happen during the next few years already, it is essential to keep the process as close to the possibility of commercialization in the next years, including the fact that for UV hotmelt PSA there is a strong limitation for the final product price. Two different possible curing mechanisms, cationic and free radical, have been used to investigate if they are suitable for LED curing of hotmelt PSA. Both curing technologies are already known to the industry and demonstrated over the past years, that they are working properly with regard to high curing speed and curing of thick coatings for the cationic curing, as well as low migration and great product variety in case of free radical curing. Different commercially available cationic photoinitiators have been investigated if they can be used as alternative photoinitiator while keeping rapid LED curing and high temperature stability. Due to the hotmelt application process at ≈120°C it was immediately found out that, it is not possible to use Iodonium based cationic photoiniators since the temperature stability of this photoinitiator class is not sufficient to be used in hotmelt based PSAs. Unfortunately, this leads to the fact, that two types of recent for LED developed cationic photoinitiators cannot be used. Following that, the only solution is to use Sulfonium based cationic photoinitiators, however, it was shown that they alone are not sufficient to enable rapid LED curing while keeping the temperature stability high. Here the implementation of a sensitizer enables a more rapid curing while sticking to the same amount of cationic photoinitiator. Again, focusing on commercial availability, different potential photosensitizers have been investigated in combination with the best cationic photoinitiator and it was possible to speed up the curing tremendously. With that combination and a slight change in the polymer composition it was possible to achieve an UV LED curable hotmelt which not only cures as fast as the mercury cured reference but also shows very similar PSA performance. During the investigation it could be observed that a combination of cationic curing and silane technology leads to even faster curing, enabling lower viscosities which result in reduced application temperature while enabling new performance characteristics on plastic substrates. In case of free radical curing PSA, it was quickly shown that simply formulating a polyacrylate with a Norrish Type I or Type II photoinitiator which demonstrates reactivity at the LED photon emission spectrum did not enable LED curable hotmelt PSAs with sufficient adhesive performance. It is expected that the photoinitiator molecules favor recombination reactions with the activated polymer chains since they are more mobile in the matrix than the polymer chain. This thesis showed the boundaries of commercially available photoinitiator technology. Using only the already commercially available photoinitiator technology it is not possible to achieve LED curable hotmelt PSA showing low migration and sufficient PSA performance. Following that, a new copolymerizable photoinitiator has been developed which enables low migration and rapid curing. The new LED photoinitiator is copolymerized with other acrylic monomers, eliminating any competing recombination reaction of loose photoinitiator molecules. When switching to the new LED photoinitiator, it is possible to cure thicker coatings than before while staying at a very high level of performance and a proven low level of migration. This technology can be used in a variety of different products and opens the door for a sustainable UV LED hotmelt technology in the future as the application range can be seen as very broad, ranging from tapes and labels to medical and food contact for those adhesives. | |||||||
| Lizenz: |  Dieses Werk ist lizenziert unter einer Creative Commons Namensnennung 4.0 International Lizenz | |||||||
| Fachbereich / Einrichtung: | Mathematisch- Naturwissenschaftliche Fakultät | |||||||
| Dokument erstellt am: | 18.12.2024 | |||||||
| Dateien geändert am: | 18.12.2024 | |||||||
| Promotionsantrag am: | 22.05.2024 | |||||||
| Datum der Promotion: | 06.11.2024 |
