Over Self-replenishing/repairing coatings based on siloxane chemistry
Avondlezing door drs. Yujing Zhang georganiseerd door de Chemische Kring Zwolle.
Most of the hydrophobic coatings currently available are vulnerable in maintaining their hydrophobicity upon surface damage or wear, due to the irreversible loss of the low surface energy chemical groups. Therefore, the concept of self-restoring hydrophobic functionalities on polymeric surfaces, by replenishing spontaneously the functional groups on the damaged surfaces, is of high interest for the coatings industry. The proof-of-principle of self-replenishing hydrophobic surfaces was previously reported for a “model” system based on a Poly(urethane)-based cross-linked soft (low-Tg) network with a small amount of fluorinated dangling chains. Driven by the surface energy difference between the functional groups and the bulk, the dangling chains are able to self-reorient towards the air-polymer interfaces created upon damage, so that surface hydrophobicity is spontaneously repaired.
The simplicity and versatility of the self-replenishing (SER) concept makes it attractive for coating applications. However, 1) the time- and energy-consuming preparation methods reported, namely thermal curing by isocyanate chemistry; 2) low hardness and poor mechanical properties of the model systems and 3) the usage of Rf8 fluorinated block, which cause environmental concerns nowadays, are still hindering the practical applications of the self-replenishing concept. In order to overcome these challenges for further industrial applications, improvements on the SER concept by utilizing photo-chemistry, interpenetrating polymer network (IPN) technology and organosilicon low surface energy building blocks will be covered in this presentation.
First, photo-curing, which is known for its advantages of speed and eco-friendliness, was introduced to the self-replenishing concept by incorporating methacrylate-terminated fluorinated dangling chains into polyethylene glycol diacrylate (PEGDA)-based networks via UV or visible range LED-initiated radical polymerization. It is demonstrated that the self-replenishing mechanism can be implemented successfully in this new diacrylate-based network architecture, created rapidly and efficiently by photo-initiated polymerization. In addition, the influence on the PEGDA network structures caused by the difference in the photoinitiation procedure (UV and visible range LED) is discussed.
Further efforts were made to improve the mechanical properties of SER coatings while maintain the SER ability. Based on the learning that the SER ability cannot be maintained when Tg is above room temperature in the photo-cured single networks, a soft/hard dual network approach is selected to form diacrylate/diepoxide IPN-based coatings cured by visible range LED. These systems aim to combine the flexibility of the diacrylate, which is necessary to offer enough flexibility for dangling chains to reorient towards the surfaces created after damage, and the stronger mechanical properties of the diepoxide to improve the coatings hardness.
Last but not the least, the Rf8 low surface energy group in the self-replenishing model PU system was substituted with a more ecofriendly poly(dimethyl siloxane) (PDMS) building block. For this purpose, the new dangling chains consisting of a PDMS chain end and a PCL spacer were synthesized. The surface segregation behavior of the PDMS-based dangling chains and their surface reorganization ability in the crosslinked network are demonstrated by surface tension measurements and dynamic recovery contact angle experiments. The current investigations have proved the self-replenishing ability in these new coating by the self-recovery of the PDMS content and hydrophobicity after the intentional damages.
Yujing Zhang was born on the 11th of September 1987 in Luoyang, China. In 2010, he graduated with a Bachelor degree in Engineering, granted with Chu Kochen honor degree, at the Zhejiang University, China. Awarded with full scholarship by the TU Eindhoven Talent Scholarship Program, he continued his master study in the track of Polymer and Composites at TU Eindhoven, the Netherlands. In 2012, he obtained his MSc. degree with high appreciation (met lof) with the master thesis entitled “structure-property relationship of stereo-complex poly(lactic acid)s” under the supervision of prof. Piet Lemstra and dr. Denka Hristova-Bogaerds. From September 2012, he started a PhD project at the Laboratory of Materials and Interface Chemistry (SMG), TU Eindhoven, under the supervision of prof. Gijsbertus de With and dr. Catarina Esteves, on the topic of self-replenishing hydrophobic coatings. This project was financed by Dutch Polymer Institute and cooperated with the research group of prof. Céline Croutxé-Barghorn and prof. Xavier Allonas at Université Haute-Alsace, France. Also, he obtained the Coating Technologist certificate by the National Dutch Research School. In 2015, Yujing won the Young Scientist Award 2016 of the Nederlandse Verreniging van Verftechnici (NVVT). In his spare time, Yujing is a senior fellow of the Melton Foundation, a USA-based NGO promoting intercultural communication and global citizenship. Since September 2016, Yujing is employed by the Allnex Corporate Innovation Group, as a Senior Materials Specialist.
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