M.M. Sotzing, J. Toribio, G. Sotzing, A. Chortos
Purdue University,
United States
Keywords: conductive composite, circularity, bio-based feedstock, biopolymer, copper, additive manufacturing, sustainable electronics
Summary:
Low-cost alternatives to conventional printed circuit conductors such as gold and silver are limited by the susceptibility to corrosion of alternatives with comparable conductivity, namely copper. Copper inks have seen success when sintered or cured in reducing environments, but further effort is required to achieve sustainable and ambiently processable conductors. A new biopolymer platform, poly(cannabinoids), that utilizes cannabinoids as feedstock shows promise as novel hydrophobic, inherently antioxidant, high glass transition temperatures polymers, capable of matching properties of petroleum-based plastics with the additional functionality of hydrolytic degradation compatibility. Poly(cannabinoid)s, together with hemp-derived cannabinoids have shown promise in preventing oxidation of metals which are now applied to conductive copper inks. p(CBD)-Adipate, a polyester synthesized through step-growth polymerization of cannabidiol with adipoyl chloride, maintains the inherent antioxidant capacity of CBD monomers. Poly(cannabinoid)s used in this technology also benefit from hydrophobicity, uncommon in conventional biobased polymers, to provide active (antioxidant) and passive (barrier properties) oxidation protection. Challenges in individual particle stability before ink formulation were overcome through a new metal salt reduction synthesis method whereby a copper salt with ascorbic acid reducing agent, ammonium hydroxide pH modifier and cannabidiol surfactant/capping ligand was used to generate copper particles with a yield of over 70% (metals basis) and surface composition of >99% elemental copper by XRD. The alkyl chain of CBD is shown to coordinate through weak electrostatic interaction, which suggests improved interparticle surface contact that is not limited by covalent bonds (BTA, PVP, etc.) or thick capping layers (fatty acids). Additionally, the terpene limonene moiety and phenolic hydroxyl groups remain free to scavenge oxidants. CBD-CuP, when compared to other conventional ligands, shows improved oxidation resistance up to 220 C in TGA. CBD-capped copper particles (CBD-CuP) are hydrophobic, in contrast to the hydrophilic surface of copper, and disperse well into poly(cannabinoid)s. Ink formulations are blade-coated and dried at 80 C to form thin-layer conductive traces that have shown up to 2.5% conductivity of bulk and up to 6% conductivity of bulk at 200 C drying. Conductive composites were subsequently dissolved in solvents to reclaim copper that could be redispersed in the polymer phase and exhibit conductivity. The polymer phase was also demonstrated to degrade hydrolytically within a day of submersion in basic environment substantiated by mass spectrometry. Cannabinoids and cannabinoid-derived biopolymers illustrate a comprehensive capability to inhibit corrosion through inherent antioxidant properties of these materials, while maintaining inter-particle conductivity of copper particles both in and outside of ink formulations. Inks formulated can be used to make high conductivity traces at low temperatures compatible with many flexible substrates. Reclamation and degradation of feedstock materials opens pathways to circularity of a new bio-derived materials platform.