Fiberboards 



2859 



1^ 



10 



20 



30 



40 



50 



TREATING TIME (HOURS) 



Figure 23-87. — Bending strength of Vs-inch hardboard as affected by temperature and 

 treating time. (Drawing after Voss 1952.) 



INDUSTRIAL PRACTICES 



A large percentage of all hardboard is heat treated, but tempering is limited to 

 products where its benefits are essential. In some plants, all boards go through 

 the heat treating line. Only a few small plants have no heat treating facilities. 



Abitibi Corp. and U.S. Gypsum Co. temper about 80 percent of their S2S- 

 wet-formed hardboard which is prefinished for use as interior wall paneling. 

 Tempering provides improved paint hold-out, high abrasion, scratch and scar 

 resistance, and general wear quality. Tempering also increases surface water 

 resistance, which is very important for hardboard used to enclose showers, for 

 instance. Most S2S boards are tempered on both surfaces. 



A very much smaller percentage of S 1 S board is tempered. Siding is generally 

 not tempered. Unfinished board used as drawer bottoms, furniture backs, and 

 similar applications, is generally not tempered. 



Tempering. — Oil tempering was patented by the Masonite Corp.; its objec- 

 tive was to substantially improve board properties by forcing considerable oil 

 into the pressed hardboard. This was done by soaking the hot boards in heated 

 oils for periods up to ^/z-hour. Most common is linseed oil, but soybean oil, tung 

 oil, and tall oil are also used. Synthetic resins are sometimes blended with the 

 oils. Quantities of oil absorbed by soaking are about 6 percent by weight, which 

 for a 4- by 8-foot, !/8-inch board with specific gravity of 1 .0 would be equivalent 

 1.25 pounds of oil. 



