Fiberboards 



2857 



90% R.H 



CYCLES 



Figure 23-85. — Linear changes in response to cyclic exposure of Vs-inch hardboard 

 before and after heat treatment at 375°F for Vh hours. (Drawings after Klinga and 

 Back 1964.) 



Improvement of strength properties. — Heat treatment improves mechani- 

 cal properties by continuation of the bonding process started in the hot press. 

 Figure 23-86 shows the hypothetical development of fiber bonding in the hot 

 press and the simultaneous deterioration of the fiber strength. At the end of the 

 press cycle the board has sufficient strength to maintain the target caliper after 

 pressure release. The process of bond formation may continue during subse- 

 quent heat treatment even while fiber strength deteriorates from thermal degra- 

 dation. The strength properties of the board will thus rise until the fiber becomes 

 the weak link in the system. This explains the characteristic relationship between 

 treating temperature, heating time, and board strength (figs. 23-87 and 23-88). 

 Figure 23-89 shows the concurrent weight loss. 



Oil tempering contributes little to most strength properties with the exception 

 of bending stiffness and bending strength, properties very sensitive to surface 

 quality improvement, particularly where both surfaces are coated, which is the 

 practice in S2S manufacture. It substantially improves surface hardness, how- 

 ever, which is of great importance in premium quality factory finished wall 

 panels. 



The importance of tempering and heat treatment is reflected in the Commer- 

 cial Standard for Basic Hardboard (U.S. Department of Commerce 1973a). Of 

 five classes of hardboard, two are designated as "tempered" hardboard (table 23- 

 9). 



