19 Thioacetal Reactive Oligomers (TARO)

Faculty Mentor: Chen Wang (Materials Science & Engineering, University of Utah)

Introduction

Carbon fiber reinforced polymers (CFRP) are incredibly versatile materials with properties such as light weight and high strength, making them applicable in many different fields, such as automotive, aerospace, or construction. However, these properties often come at the cost of recyclability, and growing demand for CFRP products poses future environmental risks. (1) As a result, covalent adaptable networks (CANs), which are often recyclable by design, are being incorporated more frequently into composite materials. Due to their dynamic covalent bonds, CANs have similar durability to traditional thermoset plastics (2) but still remain reprocessable under specific stimuli. While a variety of CANs have been developed, balancing durability, recyclability, and ease of production remains a challenge. (3)

This project aimed to create thioacetal oligomers using biomass derived chemicals (vanillin and furfural). These oligomers would then be used to create a low viscosity, high glass transition temperature (T_g ) (>80 °C), quick curing resin based on thioacetal epoxy chemistry to be used in CFRPs.

Methods

Dithioacetal Formation: Aldehyde (1 mol equivalent) was dissolved in a minimum amount of methanol. Tris[2-(3- mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC, 2 mol equivalents) was added and mixed thoroughly. Trifluoroacetic acid (TFA, 1 wt%) was added and the reaction stirred at room temperature under vacuum overnight to yield clear, viscous liquid.

Dithioacetal/Epoxy Polymerization: Dithioacetal (1 mol equivalent) and bisphenol A diglycidyl ether (BADGE, 2 mol equivalents) were combined using a speed mixer. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU,1 wt%) was added and combined on the speed mixer again. The resulting liquid was cast into silicone molds and cured at 70 °C for 1-4 hours, yielding yellow, slightly bendable solids.

Depolymerization: Dithioacetal/epoxy polymers were broken into smaller pieces, submerged in THF (400wt%) water (100 wt%) and 6M HCl (10 wt%) and stirred at 90 °C for four hours. (4)

Results

The dithioacetal reaction completion was verified by 1H NMR spectroscopy (Figure 2). Each aldehyde showed a high conversion percentage (Table 1) and did not require further purification.

After the resulting oligomers were polymerized with BADGE, an IR spectrum was taken to verify reaction completion (Figure 3).

While the polymers successfully cured in a relatively short amount of time, their thermal properties did not align with the desired values. As shown in Table 2, their Tgs were between ten to thirty degrees below the necessary temperatures, making them unsuitable for practical applications.

Conclusion

While simple and relatively inexpensive to create, these thioacetal based polymers lacked the mechanical properties needed to be successfully applied to composite materials. Optimizing the resin formulation to increase its Tg will allow for further exploration of its applications in relevant fields, and possible biodegradability.

Acknowledgement

This work was supported by SPUR from the Office of Undergraduate Research at the University of Utah, awarded to Julia Rzepecka.

Bibliography

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Kamarulzaman, S., Png, Z. M., Lim, E. Q., Lim, I. Z. S., Li, Z., & Goh, S. S. (2023).Covalent Adaptable Networks from Renewable Resources: Crosslinked Polymers for a Sustainable Future. Chem, 9(10), 2771–2816. DOI:10.1016/j.chempr.2023.04.024.

Shi, G. L.; Li, T. C., Zhang, D. H., & Zhang, J.H.  (2024). Recyclable High-Performance Carbon Fiber Reinforced Epoxy Composites Based on Dithioacetal Covalent Adaptive Network. Chinese Journal of Polymer Science, 42(10), 1514–1524. DOI:10.1007/s10118-024-3191-8.

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