Fiberboards 



2855 



2 3 4 5 



NUMBER OF CYCLES (100-65%) 



Figure 23-83. — Dimensional range (DR) and permanent dimensional change (PDC) in 

 thickness swelling of wet-formed hardboard during cyclic exposure. PDC is equiv- 

 alent to springback. (Drawing after Klinga and Back 1964.) 



sequence of relative humidity cycles before and after a 2!/2-hour heat treatment at 

 190°C (375°F). While there is a net contraction of board dimensions as a result of 

 the heat treatment, the actual dimensional changes between 30- and 100-percent 

 relative humidity exposure have actually increased somewhat, whereas the com- 

 ponent corresponding to the 65- to 90-percent relative humidity interval has been 

 reduced. 



Heat stabilization was at first believed to result from a cross-linking reaction 

 in which water is eliminated between hydroxyl groups on two adjacent cellulose 

 chains, with the formation of ether linkages. This has been disproved (Seborg et 

 al. 1953), and Stamm (1964) has proposed that initial thermal degradation of 

 wood results in furfural polymers of breakdown sugars, which are less hygro- 

 scopic than the hemicellulose from which they are formed. Spalt (1977) con- 

 cluded that wax added to fiberboard furnish is redistributed during heat 

 treatment, forming a monomolecular film on all fiber surfaces and increasing 

 water repellency. Other theories ascribe the reduction of both the total swelling 

 and of the permanent expansion component to cross linking between cellulose 

 molecules by acetyl groups (Klinga and Back 1964). 



The effect of tempering on dimensional changes is limited because the mod- 

 em tempering process applies rather small quantities of oil to board surfaces 

 only. Any reduction in water absorption and increase in strength properties is 

 largely due to the heat treatment following application of tempering oil. 



