Sustainable Bio-based Polyurethane Adhesives Utilizing PPG2000 and Renewable Polyols: Synthesis, Characterization, and Mechanical Properties

Developing high-performance, sustainable adhesives for automotive, aerospace, and industrial applications remains a major challenge due to the inherent trade-off between mechanical strength and thermal stability in bio-based materials. While previous studies have explored bio-based polyurethane (PU) adhesives, achieving superior adhesion and durability remains challenging when compared to petroleum-based counterparts. This study presents a novel bio-based polyurethane adhesive system utilizing polypropylene glycol (PPG2000), isophorone diisocyanate (IPDI), and renewable polyols (isosorbide-derived polyols, diglycerol, and glycerol). The adhesives were synthesized via a controlled one-shot polymerization process with 4-tert-butylphenol as an end-capping agent, enabling precise modulation of crosslink density and molecular architecture. Fourier-transform infrared (FT-IR) spectroscopy confirmed complete urethane bond formation, and isocyanate group (NCO%) titration validated stoichiometric conversion. Gel permeation chromatography (GPC) revealed distinct molecular weight distributions, which influence adhesive performance by affecting crosslink density, elasticity, and mechanical strength depending on polyol structure. Thermal analysis showed that isosorbide-derived polyol formulations exhibited up to a 25°C higher degradation onset temperature and a 10°C increase in glass transition temperature (Tg) compared to petroleum-based adhesives. Meanwhile, formulations containing diglycerol and glycerol demonstrated up to 39% higher shear strength (32.5 MPa) and 77% improved impact resistance (36.8 MPa) relative to the reference system, attributed to optimized segmental mobility and crosslinking effects. This work establishes a strategic framework for designing bio-based polyurethane adhesives, while acknowledging limitations such as potential variability in raw material sources and suggesting future research into long-term environmental performance, that not only surpasses conventional petroleum-based systems in thermal and mechanical performance but also aligns with the principles of green chemistry and sustainable material innovation. These findings offer a pathway for next-generation structural adhesives in automotive, aerospace, and industrial applications.