In a paper published in the journal Polymers, researchers explored enhancing polyvinyl alcohol (PVA) for packaging by addressing its moisture vulnerability. They used carbon quantum dots (CQDs) as a solvent to improve the dispersibility of hydrophobic coatings like tetraethyl orthosilicate (TEOS) and hexadecyltrimethoxysilane (HDTMS).
WCAs measured immediately and after 30 min for different TEOS/HDTMS coating suspensions. Image Credit: https://www.mdpi.com/2073-4360/16/17/2513
The CQD suspension produced a smooth hydrophobic layer, resulting in a water contact angle of 110°, confirming improved coating quality. This approach significantly enhanced PVA's hydrophobic properties, making it more suitable for high-humidity conditions.
Background
Past work highlighted the need for biodegradable polymer films, particularly PVA, for packaging due to their transparency and oxygen barrier properties. However, PVA's hydrophilicity limits its use under high-humidity conditions.
Challenges with PVA include its susceptibility to water, which compromises its oxygen barrier properties and overall performance in high-humidity environments; despite various strategies, such as cross-linking, PVA's intrinsic hydrophilicity persists, limiting its effectiveness for packaging applications.
Materials and Methods
Commercial-grade PVA resins, along with chemicals such as boric acid, hydrogen chloride, TEOS, and HDTMS, were used. Green tea was sourced from Bosung Jeda Co., and deionized water was utilized throughout the experiments.
To prepare crosslinked PVA (CPVA) films, PVA was dissolved in water and mixed with a boric acid solution, followed by the addition of hydrochloric acid. The team applied this mixture to a glass substrate, dried it, and then peeled it off for further characterization.
Green tea extract (GTE) and CQD suspensions were prepared and used to enhance the dispersion of TEOS/HDTMS suspensions for film coating. The coating solutions were mixed with ethanol, H2O, GTE, or CQD solution, and then TEOS and HDTMS were added. The resulting suspensions were sprayed onto the PVA films, which were then dried.
Characterization involved high-resolution transmission electron microscopy (HRTEM) for CQD morphology, Fourier transform infrared (FTIR) spectroscopy for chemical structure analysis, and scanning electron microscopy (SEM) for surface imaging. Additional tests included water contact angle (WCA) measurements, UV-vis spectroscopy, and oxygen transmission rates (OTR) to assess the films' properties and performance.
Results and Analysis
The particle analysis of the co-solvent materials revealed that the synthesized CQDs were spherical, well-dispersed, and had an average diameter of 4.1 nm. This size is much smaller compared to GTE particles, which were significantly larger at 10–80 μm. The CQDs demonstrated a narrow particle size distribution and showed lattice fringes of 0.21 nm, confirming their successful synthesis. In contrast, GTE particles were notably larger, which made them less effective in improving the dispersion of the TEOS/HDTMS coating.
The dispersibility of TEOS/HDTMS coating suspensions varied with the solvent used. Suspensions with water (CP–H2O) and GTE (CP–GTE) were opaque and prone to aggregation, with phase separation occurring within three days. Conversely, the CQD suspension (CP–CQD) remained transparent and well-dispersed. The enhanced dispersion with CQDs was attributed to their extremely small and lightweight nature, which provided better stability and reduced aggregation compared to larger particles like those in GTE.
FTIR analysis of the CPVA and TEOS/HDTMS-coated films indicated that the CQD suspension resulted in a more uniform coating. The FTIR spectra showed a decrease in hydroxyl group peaks and an increase in silicon (Si) –O and Si–C bond peaks, suggesting effective coating and reduced presence of PVA and boric acid on the surface. SEM images corroborated these findings, showing a smoother, more uniform coating with CQDs compared to GTE or water, which exhibited aggregation and less effective coverage.
Water contact angle (WCA) measurements confirmed the improved hydrophobicity of the CQD-coated films, with a WCA of 110.1° compared to 16° for uncoated CPVA films. The CP–CQD films showed the least surface degradation and maintained high hydrophobicity, indicating effective water resistance.
Additionally, the oxygen transmission rate (OTR) significantly decreased with TEOS/HDTMS coating, with the CP–CQD film showing the lowest OTR and highest transparency, emphasizing the superior performance of CQDs in maintaining film quality and barrier properties.
Conclusion
To sum up, the study applied a CQD suspension to address aggregation issues in TEOS/HDTMS coatings on PVA surfaces. The coating improved uniformity and hydrophobicity, as evidenced by an increase in the water contact angle from 16° to 110.1°, and enhanced barrier properties and transparency.
SEM analysis showed reduced surface aggregation with CQD suspensions, facilitating superior coating quality. Future research is needed to evaluate the safety and potential toxicity of this technology for packaging applications.
Journal Reference
Oh, Y. et al. (2024). Using a Carbon Quantum Dot Suspension as a New Solvent for Clear Hydrophobic Surface Coating on Hydrophilic PVA Films. Polymers, 16:17, 2513–2513. DOI:10.3390/polym16172513, https://www.mdpi.com/2073-4360/16/17/2513
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