Self-neutralizing tannin-citric acid wood adhesives

Tannin-based adhesives have historically relied on formaldehyde or hexamine hardeners, but recent research has focused on sustainable alternatives, such as tannin-citric acid adhesives, which avoid harmful chemicals and show prospect to provide a viable eco-friendly solution for wood bonding applications. However, these adhesives typically suffer from low pH, leading to long-term degradation of wood substrates. This study aims to regulate the pH of tannin-citric acid adhesives by introducing morpholine complexes with weak acids (succinic and malic acids), thereby improving the adhesive's mechanical properties and durability. The structure and performances of the adhesive were analyzed by some analytical methods, including Thermogravimetric (TG) analysis, Electrospray Ionization-Mass Spectrometry (ESI-MS) analysis, Fourier-Transform Infrared Spectroscopy (FTIR) analysis, and X-ray Photoelectron Spectroscopy (XPS) analysis. The mechanical and water resistance properties of the adhesives were tested using industry-standard methods, with additional rapid aging assessments. The results showed that adding morpholine/succinic acid (malic acid) complexes effectively increased the pH of the cured adhesive from 3.89 to 5.22 (5.03). The optimum addition level (10 %, based on the mass of citric acid) resulted in significantly enhanced bonding strength and water resistance, while higher amounts of the complexes led to diminishing returns. Overall, the integration of self-neutralizing complexes in tannin-citric acid adhesives not only mitigates the low-pH issue but also enhances the durability and mechanical properties, offering a viable eco-friendly solution for wood bonding applications.