Recently, Professor Liu Xiaoxuan of Guangdong University of technology, polymer photochemistry team, has made progress in the field of 3D printing self-healing silicone materials. The research results are entitled "self healing, Reprocessing and 3D printing of transparent and hydrologic resistant silicone elastomers "was published in the Chemical Engineering Journal. The first author of the paper is Liu Zhu, a doctoral student of Guangdong University of technology, and the corresponding author is Professor Liu Xiaoxuan.
In the process of processing and using, silicone materials will inevitably encounter microcracks, cracks and other damages. Microcracks are usually difficult to detect, and continuous use will lead to the decline of material performance or even failure, bringing serious security risks. It is of great significance to improve the service life and safety of silicone materials and reduce the replacement cost to realize rapid and efficient curing and self repair and restore the original performance indicators. At present, the commercial silicone materials can not be cured by light, because the polar light curing group is difficult to introduce polysiloxane chain, the preparation and purification is difficult, and the compatibility and storage stability are poor. In addition, the reversible dynamic bond has strong polarity, which is difficult to introduce polysiloxane chain, and the intrinsic self-healing material is realized by constructing reversible dynamic bond. Therefore, at present, it is difficult to realize both light curing and self-healing properties of self-healing silicone elastomers. Generally, thermal curing method is used and the prepared elastomers are opaque, poor compatibility and poor hydrolysis resistance. Therefore, the research of self-healing transparent UV curable silicone material based on excellent hydrolysis resistance is of great significance in the field of 3D printing applications.
Professor Liu Xiaoxuan's team prepared a kind of transparent organic silicon elastomer with rapid UV curing and excellent self-healing properties through the photo induced click reaction between pdms-sh and vinyl terminated silicone oil, and the reversible / irreversible hybrid double network constructed by the thermo reversible dynamic ion crosslinking of carboxylic silicone oil and amino silicone oil. And it is applied to 3D printing to print the silicone elastomer with excellent multiple self-healing properties. The curing mechanism and crosslinking network diagram of the prepared photo thermal double crosslinked silicone elastomer (Fig. 1).
Figure 1 Schematic illustration of photo thermal dual curving and crossed networks of silicone elastomers
The prepared double crosslinked organic elastomer UV can be gel within 3 s under the light radiation. The initial strength of the thiol alkene network is basically completed within 20 s, and the initial strength is formed. Moreover, the ionic crosslinking network does not affect the formation of the sulfhydryl light curing network, which satisfies the prerequisites for SLA 3D printing (Fig. 2).
Figure 2 (a) g 'and G "during in situ logical test; (b) real-time infrared spectra (FTIR) of te – in samples
The excellent dynamic reversibility of the elastomer was verified by the variable temperature infrared (vt-ir), dynamic thermomechanical analysis (DMA) and stress relaxation behavior (Fig. 3). Through the dynamic dissociation and recombination of the ionic bond in the elastomer, the fracture surface of the silicone elastomer can be effectively repaired, and the repair efficiency can reach over 97%, and after multiple repairs, the repair efficiency can still reach over 92% (Fig. 4 and Fig. 5). In addition, the prepared double crosslinked silicone elastomer has excellent recovery performance. After recovery, the tensile strength of the elastomer can be recovered by more than 90%, and after multiple recovery, the tensile strength can still be recovered by more than 85%. After recovery, the elastomer still has excellent self-healing performance, and the repair efficiency can reach more than 90% after multiple repairs (Figure 6).
Fig. 3 vt-ir spectra of te – in10, (a) peak shift as held from 40 to 120 ° C by 10 ° C / min, (b) peak shift during cooled from 120 to 40 ° C by 10 ° C / min. variation of storage module e? Under variable temperatures as a function of time, (C) in samples, (d) te – in10 samples, (g) TE–IN10 samples under different temperatures and (h) elastomers containing various ionic bonds at 60 °C, (c) Linear fitting of relaxation time (τ) versus temperature according to the Arrhenius' equation, (d) Stress relaxation activation energy (Ea) of samples with various ionic bonds.
Figure 4 illustration of self-healing mechanism
Fig. 5 (a) repeated macro repairing of damaged te – in10 samples. Stress strain curves of virgin and self healed silicone elastomers, (b) repeatedly healed te – in10 samples, (c) te – in10, te – in20 and te – in30 samples healed at 100 ° C for 12 h
Figure 6 photos for the reprocessing of te – in10 samples. (B-C) stress strain curves of virgin and reprocessed silicone elastomers, (b) repeated reprocessed te – in10 samples from 80 meshes, (c) self healing properties of repeated reprocessed te – in10 samples
The prepared double crosslinked silicone elastomer has excellent transparency. With the increase of ion network content, the visible light transmittance can still reach more than 90%. After multiple high-temperature heat repair and recovery, the visible light transmittance can still reach more than 85% (Figure 7). The elastomer has excellent anti hydrolysis performance. Even if it is treated with hot water at 80 ℃ for 48h, the mechanical properties, e 'and TG of the elastomer are not significantly reduced. In addition, after being treated with hot water at 80 ℃ for 48h, the elastomer sample still has a repair efficiency of more than 90% (Fig. 7).
Figure 7. Transmission of (a) silicon elastomers containing variable ionic bonds, (b) te – in10 samples with different healing times, (c) stress strain curves and (d) storage module e 'and Tan δ of te – in10 samples after hydro thermally treated
Silicone elastomer formula can print smooth and clear elastomer entities (such as "GDUT" abbreviation, "gear" and "lucky star") through the form 2 Desktop SLA 3D printer, and the addition of organic pigments will not affect the curing rate. In addition, 3D printed elastomer has excellent self-healing performance. After several times of repair, the self-healing efficiency is still over 99%. The results show that 3D printing layer stacking does not affect the distribution of ion network and the reversible dissociation recombination process.
Figure 8 (a) schematic of SLA based 3D printing. (b) 3D printed variable geometries from te – in 1 0 formula with different segments, "GDUT" with 1.0 wt% RP – 355, "to other gears" in turn with 0.05 wt% BP – 825, 0.05 wt% RP – 355, and 1.0 wt.% BP – 825, “Ascendant” with 0.05 wt% RP–355. (c) Photographs of self-healing 3D printed “Ascendant”. (d) Stress-strain curves of repeatedly self-healed 3D printed samples after 100 °C for 12 h.
Therefore, the self-healing and recyclable 3D printing silicone elastomer with excellent transparency and hydrolysis resistance was successfully prepared by the way of photo thermal double crosslinking of sulfhydryl ene rapid photopolymerization and thermo reversible ion crosslinking. It is of great significance for the long-term printing of complex structure elastomer functional parts, which can save energy and environmental protection, prolong service life and reduce cost. It provides a feasible scheme for rapid curing and self-healing silicone materials based on reversible dynamic ion association induction.
Paper link:
https://doi.org/10.1016/j.cej.2020.124142
The main research direction of the research group of the author is polymer photochemistry, including mercapto olefin photochemistry, 3D printing elastomer, 3D printing self-healing materials and multi-functional photoinitiator. The related achievements are published in progress in polymer science, Journal of Materials Chemistry C, polymer chemistry, Langmuir, Journal of photochemistry & photobiology A: Chemisty, chemisty - chemist and progress in organic costing are the top international mainstream journals in this field.
Yan Yang, Zushan Ye, Xiaoxuan Liu*, Jiahui Su*. A Healable Waterborne Polyurethane Synergistically Cross-Linked by Hydrogen Bonds and Covalent Bonds for Composite Conductor. Journal of Materials Chemistry C, 2020, DOI: 10.1039/D0TC00551G
J. Zhou, X. Allonas, X. Liu*. Zirconium Propoxide: A Coupling Agent for the Synthesis of Multifunctional Photoinitiators. ChemPhotoChem, 2018, 2(1): 18-21.
J. Zhou, X. Allonas, X. Liu*. Synthesis and Characterization of Organozirconiums with Type-II Photoinitiator Ligands as Multifunctional Photoinitiators for Free Radical Photopolymerization. Journal of Photochemistry and Photobiology Part A: Chemistry, 2018, 356: 580-586.
J. Zhou, X. Allonas, X. Liu.* Fluorinated Organozirconiums: Enhancement of Overcoming Oxygen Inhibition in the UV-curing Film. Progress in Organic Coatings, 2018, 120: 228-233.
Yang Y, Zhang T, Liu X*. et al. Preparation and photochromic behavior of spiropyran-containing fluorinated polyacrylate hydrophobic coatings. Langmuir, 2018, 4 (51): 15812-15819
Su J, Huang H, Cui Y, Liu X*, et al. A photo-induced nitroxide trapping method to prepare α, ω-heterotelechelic polymers. Polymer Chemistry, 2016, 7(14): 2511-2520.
Xiang, H.; Yin, J.; Lin, G.; Liu, X*. et al., Photo-crosslinkable, self-healable and reprocessable rubbers. Chemical. Engineering Journal. 2019, 358, 878-890.
H. Xiang, X. Wang, Z. Liu, L. Zhang*, X. Liu*, UV-curable, 3D printable and biocompatible silicone elastomers, Progress in Organic Coatings, 2019, 137: 105372.
https://doi.org/10.1016/j.porgcoat. 2019.105372
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