Wood Adhesives Based on Natural Resources: A Critical Review: Part III. Tannin‐ and Lignin‐Based Adhesives

Influence of Lignin Content and Pressing Time on Plywood Properties Bonded with Cold-Setting Adhesive Based on Poly (Vinyl Alcohol), Lignin, and Hexamine

“…It is obtained as a by-product of the paper pulp industry with an estimated annual global production level of 50–75 million tons, of which less than 2% is utilized in value-added applications, while the rest is mostly burned for energy or directly managed as waste [ 27 ]. The development of adhesives for manufacturing low-emission wood-based panels is a feasible option for the efficient utilization of technical lignins (lignosulfonates, hydrolysis, organosolv, ionic, or kraft lignin) [ 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. It should be noted that most lignin-based wood adhesives are tested mainly in laboratory conditions and still have not been introduced into industrial practice.…”

“…It should be noted that most lignin-based wood adhesives are tested mainly in laboratory conditions and still have not been introduced into industrial practice. The simplest way to use lignin in the form of wood-based panel adhesives is based on the partial substitution of phenol in phenolic resins [ 30 , 32 , 35 ]. Although the phenolic structure of lignin provides high hydrophobicity and low dispersion properties for the development of bio-based adhesives, its aromatic structure decreases its reactivity, so the development of lignin-based adhesives requires previous stages of functionalization of the lignin structure through phenolic and aliphatic groups as well as aromatic rings [ 30 , 32 , 35 ].…”

“…Another alternative to formaldehyde in the production of wood adhesives is hexamethylenetetramine or hexamine. The dominant use of hexamine is in the production of phenolic resin moulding compounds, where it is added as a hardening component [ 29 , 35 ]. Glyoxalated wheat straw lignin mixed with mimosa tannin and hexamine as a hardener was used as a wood adhesive for interior particleboard panels with a reduced formaldehyde emission with proportions of 40–60% and 50–50% for lignin and tannin, respectively [ 36 ].…”

“…Glyoxalated wheat straw lignin mixed with mimosa tannin and hexamine as a hardener was used as a wood adhesive for interior particleboard panels with a reduced formaldehyde emission with proportions of 40–60% and 50–50% for lignin and tannin, respectively [ 36 ]. Another study showed that softwood kraft lignin was glyoxylated and formulated with tannin-hexamine [ 35 ]. The lignin-based adhesive was prepared with a 60% tannin–40% lignin proportion.…”

“…The highest rate of chemical cure was achieved using paraformaldehyde as hardener followed by glyoxal and hexamine, which required higher curing temperatures to achieve a complete chemical cure. Furthermore, the rate of increase of adhesion strength decreased in the order hexamine > glyoxal > formaldehyde, where the rigidity after curing increased in the same order [ 35 ].…”