Quantitative Relationship Between Mechanical Degradation and Biodegradation in Washi-Reinforced PHBH Laminated Composites

• Washi–PHBH exhibits rapid loss of mechanical function at early degradation stages • Continuous cellulose networks promote interface-dominated failure mechanisms • Mechanical reliability is governed by degradation-stage-dependent mechanisms • Results highlight design trade-offs between reinforcement and functional lifetime In this study, an eco-friendly composite comprising marine-biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) reinforced with traditional Japanese washi paper was fabricated via hot compression. Tensile and biodegradation properties were evaluated, including the loss of mechanical properties during biodegradation. Specimens fabricated from two layers of washi paper and one layer of PHBH exhibited higher mechanical properties, with an ultimate tensile strength of 56.44 MPa, which was 163.86% higher than that of neat PHBH. A biodegradation test was conducted under controlled composting conditions and in seawater at room temperature. After 6 weeks in compost, the composite reached 60% biodegradation and lost its tensile properties, becoming extremely fragile after only 3 weeks. In seawater, the composites showed a consistent reduction in tensile properties, although at a slower rate than in compost, likely due to lower microbial activity. A quantitative relationship between tensile strength retention and the biodegradation rate was established in compost using direct mineralization measurements. This relationship was subsequently used to assess equivalent biodegradation behavior in aquatic environments from tensile strength retention. The estimated results showed higher degradation in seawater than in tap water and revealed that a composite fabricated from two layers of washi paper and one layer of PHBH degrades faster than the more layered laminate. Sensitivity analysis confirmed that the estimated trends are robust against reasonable variations in the coupling parameter. These findings demonstrate that washi-reinforced PHBH composites combine enhanced mechanical performance with predictable, environment-dependent biodegradation behavior, and that tensile strength retention can serve as a practical indicator for screening degradation severity in aquatic environments where direct measurements are difficult.