Structural Flakeboards and Composites 3003 



TREATMENTS TO REDUCE THICKNESS SWELL 



Thickness swell of flakeboards can be reduced by pre-treatment of flakes 

 (impregnating them with resin), by injecting steam into the mat during pressing, 

 and by post-pressing exposure of panels to superheated steam, heated air, or hot 

 oil. 



Steam injection during pressing. — Shen (1973) found that injection of high- 

 pressure steam into the mat during hot pressing can significantly reduce press 

 time and thickness swell. For example, 1 -inch-thick maple (Acer saccharum 

 Marsh.) phenolic particleboard at 45-pounds/cu ft density was fully cured in 1 

 minute at a steam pressure of 300 psi. Thickness swell of this board after a 24- 

 hour water soak was only about 10 percent, whereas conventionally hot-pressed 

 board had thickness swell of about 33 percent. 



Impregnation of flakes. — Brown et al. (1966) and Hujanen (1973) found 

 that impregnating flakes with phenol-formaldehyde resin prior to board manu- 

 facture reduced both reversible and irreversible thickness swell. Between 50 and 

 90 percent RH, and in a 50-percent RH-VPS cycle, thickness swell was approxi- 

 mately halved compared to control boards without impregnation. 



Steam post-treatment.— Hann (1965) and Heebink and Hefty (1968, 1969) 

 found that flakeboards steamed at 360°F for 10 minutes in a stack with ventilated 

 cauls between boards, and without restraint, had significantly reduced thickness 

 swell. After water soaking for several days to equilibrium, thickness swell of 

 steamed softwood flakeboards was about 10 percent, while that of unsteamed 

 flakeboards was about 20 percent. MOR was reduced by the treatment, 

 however. 



Hujanen (1973) found that hardwood flakeboards {Populus balsamifera L.) 

 similarly steamed also had reduced post-treatment thickness swell, but density 

 and MOR of steamed boards were significantly reduced; moreoever, steaming 

 produced irregular surfaces, excessive edge swelling, and board thickness in- 

 creased 16 percent during the steam treatment. 



Heat treatment. — Suchsland and Enlow (1968) found that phenolic-bonded 

 flakeboard ovenheated in air for 2 hours at 425°F had only about 60 percent of 

 the thickness swell of untreated controls when both were exposed to 90-percent 

 RH. This heat treatment slightly reduced MOE, but increased internal bond 

 strength slightly. The face layers of their flakeboard were of Pinus banksiana 

 Lamb, with 12-1/2-percent resin content; the core was Populus tremuloides 

 Michx. with 7 percent resin content. 



Oil tempering. — Hall and Gertjejansen (1974) immersed Populus tremu- 

 loides Michx. waferboard in a blend of hydrocarbon drying oil and boiled 

 linseed oil heated to 170°F and left it until they had retentions of 5 and 10 percent 

 (ovendry- weight basis) of oil. The treatment effectively reduced irreversible 

 thickness swelling and loss of MOR, MOE, and IB in samples subjected to 

 accelerated aging. Tempering oil increased MOR and MOE of this waferboard, 

 but had no effect on IB. Most of the benefits were obtained at 5-percent oil 

 retention. Thickness swelling of the waferboard after OD-VPS exposure was 

 reduced. 



