Influence of Clay Chemistry on the Microstructure, Water Sensitivity, and Home-Composting Disintegration of Protein-Rich Wheat Gluten–PBAT Blends

The transition toward a circular plastics economy requires materials that reduce fossil dependency while ensuring realistic end-of-life solutions. Agro-industrial byproducts such as wheat gluten (WG) represent promising feedstocks, although their poor processability and high moisture sensitivity limit practical use. In this work, wheat gluten was plasticized with glycerol and urea to obtain plasticized wheat gluten (fWG), which was then blended with 30 wt % poly(butylene adipate-co-terephthalate) (PBAT), and reinforced with 5 wt % montmorillonite (MMT) clays─either unmodified (CNa), organo-modified (C30B), or a 1:1 mixture─to assess the role of clay chemistry in structural organization and functional performance. PBAT incorporation generated interfacial discontinuities and increased water vapor permeability, whereas clay addition improved phase compatibility and reduced PBAT-dispersed domain size, particularly for C30B. Spectroscopic, diffraction, and scattering analyses revealed that clay incorporation further modified the structural organization of the blends, introducing subtle nanostructural rearrangements without altering the crystalline fraction of the polymers. At the macroscopic level, all clay-containing composites exhibited markedly reduced water vapor permeability, achieving values comparable to neat PBAT despite the lower polyester content, and showed modest improvements in mechanical response. In contrast, solubility was governed primarily by the protein fraction and was not further affected by clays. Under home-composting conditions, PBAT exhibited only minor surface deterioration and remained largely intact, yet the fWG–PBAT blend disintegrated at a rate comparable to the protein matrix alone. Clay-containing composites fragmented more gradually, but all WG-based formulations achieved extensive disintegration within 4 weeks. Altogether, these findings indicate that WG–PBAT–clay composites emerge as a sustainable alternative for the development of films formulated with high agro-industrial byproduct content and realistic end-of-life performance.