Continuous synthesis and application of novel, archaeoinspired tackifiers from birch bark waste

A new family of sustainable tackifying agents with adjustable glass transition temperatures was prepared via torrefaction of outer birch bark waste. The application of the obtained betulin esters as a tackifier was tested in combination with a semicrystalline ethylene vinyl acetate (EVA) copolymer in a standard hot-melt adhesive formulation. Three torrefied birch bark products with different glass transition temperatures (23, 45, and 66 °C) showed excellent compatibility with the EVA copolymer in a 1/1 (w/w) mixing ratio. A high T_g tackifier resin (66 °C) was prepared with a rotary tube furnace in a continuous production process at 240 °C. High product yields of about 25% (w/w) could be obtained after solvent extraction of the torrefied bark with acetone. The birch bark tackifying agents change from a stiff, brittle material below T_g to a low viscosity fluid at higher temperatures accompanied by a change of storage and loss modulus over several decades. The T_g of the adhesives could be exactly tailored by the choice of the birch bark tackifier. The resulting adhesives showed excellent lap shear strength on zinc-plated steel and on wood, shifting to values up to 10 MPa with increased T_g of the archaeoinspired adhesives.