Lignin thermal softening

All of the chemical evidence that can be marshalled indicates that wood fiberboard manufacture exploits the thermoplastic properties of lignin. Defibering is effected by the thermal softening of lignin in the middle lamella at saturated steam pressures above 130C. Interfelted fiber mats are consolidated with or without densification pressure by the thermoplastic fusion of lignin-rich fiber surfaces at high board conversion temperatures. 

Goring DAI (1963) Thermal softening of lignin, hemicellulose and cellulose. Pulp Pap Mag Can 64 T-517-527... 

Goring, D.A.I., 1963. Thermal Softening of Lignin, Hemicellulose and Cellulose, Pulp Paper Research Institute of Canada, Montreal, Quebec, Canada, pp. T517-T527. 

More recently, Ohkoshi et al. [103] studied the mechanism of thermoplasticization of wood by allylation. They considered that decrystallization of cellulose within wood during allylation permits the wood to soften thermally and the allylated lignin within wood increases the softening through acting as a plasticizer. 

The preparation of fibre for MDF uses an impact device (the refiner) to separate the fibres but this follows a thermal pretreatment that softens the lignin. Consequently separation oeeurs in the lignin-rich middle lamella between the fibres so preserving as mueh of the eellulosic fibre structure as possible. The preparation of MDF fibre is eonsidered later. 

A unique explosion process for defibering wood was developed by W. H. Mason. Wood chips, about inch long, prepared in conventional chippers and screened, are subjected to high pressure, in a cylinder, commonly called a gun, about 2x6 feet in size, and ejected through a quick-opening valve. The elevated temperature softens the chips, and, upon ejection, they explode into a fluffy mass of fibers and fiber bundles. The process involves thermal plasticization of the lignin, partial hydrolysis, and disintegration by the sudden expansion of the steam within the chip. 

---