Lignin-based phenolic adhesives, related compositions, and related methods

A method for forming an adhesive composition, the method comprising:

• providing an aqueous reaction mixture comprising
• water,
• a lignin comprising aromatic hydroxyl groups and ortho-reactive carbon atoms relative to the hydroxyl groups,
• a base catalyst to solubilize the lignin in the aqueous reaction mixture, and
• an aldehyde having at least 2 carbon atoms and having at least 2 aldehyde functional groups (—CHO), wherein:
• (i) the aldehyde is added to the aqueous reaction mixture in a controlled manner and in the presence of the base catalyst solubilizing the lignin, thereby beginning a condensation reaction between the lignin and the aldehyde, (ii) adding the aldehyde in the controlled manner to comprises adding the aldehyde batchwise in 2 or more batch additions or continuously over time, while maintaining the aldehyde concentration in the aqueous reaction mixture below a level that would induce gelation, and (iii) a molar ratio of lignin aromatic hydroxyl groups to aldehyde functional groups in the aqueous reaction mixture is in a range of 1:1.1 to 1:4;
• adding further base catalyst in a controlled manner to the aqueous reaction mixture, thereby catalyzing the condensation reaction between the lignin and the aldehyde while maintaining a viscosity of the aqueous reaction mixture below a gelation point of the aqueous reaction mixture, wherein:
• (i) a molar ratio of total aromatic hydroxyl groups originally present in the lignin to total aldehyde functional groups added to the aqueous reaction mixture is less than 1 during the condensation reaction, and
• (ii) adding the further base catalyst in the controlled manner to the aqueous reaction mixture comprises adding the further base catalyst batchwise in 2 or more batch additions or continuously over time to the reaction mixture, while maintaining the base catalyst concentration in the reaction mixture below a level that would induce gelation;
• continuing the condensation reaction between the lignin and the aldehyde to form a resin reaction product while maintaining the viscosity of aqueous reaction mixture below the gelation point of the aqueous reaction mixture until a completion point is reached, wherein, at the completion point, at least some unreacted ortho-reactive carbon atoms remain, at least some substituted methylol functional groups have been formed by the condensation reaction, and the resin reaction product is not crosslinked and has a gelation time measured at 25° C. in a range of 2 minutes to 6 minutes.