The Bonding Mechanism in Hardboard Manufacture Review Report

A concept of the bonding mechanism in hardborad manufacture is presented with respect to — the types of bonds formed — the means of creating bonding area between fibers — the effects of fiber direction on bonding and strength in the various board planes the moisture resistance of the bonding achieved. To produce sufficient bonding area, especially in absence of a binder, the plasticization of wood polymers above their glass transition temperature is necessary with the coarse type of thermomechanical fiber bundies used for hardboard. Bond formation at the lignin dominated surface of these fibers then has to take place above a critical temperature: — in the wet process this critical temperature is the glass transition of moist lignin, approximately 115°C — in the dry S—2—S process, for hardboard e.g. from dry insulating board, this critical temperature is the glass transitions of the dry wood polymersv which are all near 220°C The term S—2—S Stands for double smooth or two sides smooth hardboard according to a dry process. Press operations in use for hardboard were experimentally developed in relation to this. Hydrogen bonding at lignin and cellulosic surface areas is then considered to be responsible for the main type of bonding in the press-drying Operation. The development of moisture resistance» such äs wet strength and dimensional stability in hardboard, takes place in the täte pari of the hot press-drying Operation and in dry heat treatment. Moisture resistance is understood to be due in pari to the replacements in the bonding region of hydrogen bonds by covalent inter-fiber bonds, thereby partly crosslinking lignocellulosic material. The effect of unsaturated oils or fats in an oil tempering process is understood äs a co-polymerization reaction producing additional crosslinking. Oleophilic material prevents hydrogen bonding between fibers in the press drying Operation. Only van der WaaTs forces operate in such material. Its presence in fiber-fiber contact area reduces the strength of the board, evident especially in the direction perpendicular to the sheet. Some effects of moisture, temperature and Sorption conditions on strength parameters are discussed.