2806 Chapter 23 



PRESSING S2S HARDBOARD 



The original patent for S2S hardboard (Mason 1938) reads in the first claim, 

 as follows: 



"The process of making a hardboard product having high dry and wet strength from a light 

 porous sheet of Hgnocellulose fiber containing the natural fiber incrustations including the 

 steps of drying the sheet to a bone-dry condition, and then applying pressure to the bone- 

 dry sheet at a temperature of about 400 to 500°F sufficient to materially consolidate and 

 densify and impart high dry strength and high wet strength to the sheet by activation of the 

 bonding properties of the incrusting substances." 



This claim presents the critical requirements for the successful manufacture of 

 wet formed S2S hardboard, i.e., drying the mat to bone-dry condition and then 

 applying press temperatures in excess of 400°F. The low moisture content (zero 

 percent) is necessary to allow short press cycles without the danger of entrapping 

 steam in the board. The high temperatures are required to soften the lignin and to 

 generate small amounts of water — apparently necessary in the bonding proc- 

 ess — via destructive distillation of the wood fibers. 



Mat handling. — One of the important characteristics of the S2S process is its 

 extremely short press cycle. It is possible to press 30 press loads/hour. In the 

 case of a 4- by 16-foot, 20-opening press, such speed requires 600 mats/hour or 

 10 mats/minute, and effective press line speed of 160 feet/minute. Considering 

 that mats must be spaced, accelerated, and decelerated, maximum mat speeds 

 can easily reach 300 feet/minute. 



The S2S mat coming out of the dryer is a rigid but low density product and 

 does not tolerate rough handling. Any fractures or other injuries of the S2S mat 

 will result in unsatisfactory S2S hardboard. The speed at which mats can be 

 handled without damage probably defines the upper limit of productivity in S2S 

 hardboard plants.^ 



Mats for S2S hardboard are generally 12 feet wide and 16 feet long as they 

 come from the dryer, with allowance for edge trim and kerf from saws cutting 

 mats to hot-press size. Efficiency of the line could be increased somewhat by 

 cutting double-length mats (32 feet) on the forming machine, which would 

 reduce spacing losses in the dryer and trim allowances. It would, however, 

 require longer speed-up sections in the dryer and larger size double trimmers.^ 

 Since it is very difficult to exactly synchronize forming machine and hot press, 

 and an S2S press generally can handle !/4-inch board faster than it can be formed, 

 but not Vs-inch board, intermediate storage is provided, following the double 

 trimmer. Most systems also are designed to permit mat removal between dryer 

 and trimmer to allow for sawblade changes and other down times. 



Pre-drying. — Prior to entering the hot press, the trimmed S2S mats at 1- to 5- 

 percent moisture content pass on edge through a short, picket- type pre-dryer 

 where moisture content is reduced to practically zero, and mat temperature 

 elevated to as close to 300°F as possible. The low mat moisture content and high 

 temperature allow fast application of full hot-press pressure without breathing 

 phases in the press cycle, thereby shortening pressure time. However, if press 

 times are increased to accommodate breathing cycles for steam escape, S2S 

 boards can be made without pre-drying the mats. 



