Long-Term Environmental Durability of Recycled Textile-Low-Density Polyethylene Composites Reinforced with Plant Fibers: Mass Uptake, Flexural Stability, and Microbial Colonization

Recycled textile-polyethylene composites combine heterogeneous polymer and cellulosic phases whose long-term response to moisture and microbial colonization determines their suitability for building products. To the best of the authors’ knowledge, this is the first study to couple one-year, real-environment (non-accelerated) exposure of product-scale composites made from mixed post-consumer textile waste with mechanical testing and amplicon-based profiling of the viable surface microbiota, thereby linking material durability and biological colonization in a single dataset. In this study, recycled textile-low-density polyethylene (LDPE) composites produced by intrusion or extrusion and reinforced with 3 wt.% coconut or sisal fibers, together with an unreinforced reference and selected brushed-surface variants, were evaluated after 12 months in soil or water. Mass change and three-point flexural properties were quantified using five specimens per condition, and the culturable surface communities were characterized by 16S rRNA (V3–V4) and ITS1 amplicon sequencing. Mass gain remained low (0.116–0.580%), although water exposure and surface brushing significantly increased uptake. Across all material-exposure variants, flexural strain at break, flexural stress at break, flexural modulus, and maximum flexural stress decreased by only 0.3–1.1%, and every uncertainty-normalized shift remained below 0.5 propagated standard errors. Bacterial communities were dominated by Pseudomonas, Serratia, or Bacillus, whereas fungal profiles separated into Naganishia/Filobasidium-rich and Mucor-dominated assemblages. A literature-based trait analysis revealed high cumulative abundances of genera with documented cellulose- and ester-hydrolyzing capacity and, among bacteria, polyethylene/alkane-oxidizing capacity. The results indicate persistent biological colonization of the composite surfaces, whereas bulk flexural performance remained essentially unchanged after one year of environmental exposure.