4. Conclusions
Lowering the ratio of F/U beyond a certain limit decreased not only the reactivity of UF resin but also impaired the physical and mechanical properties of particleboard. The effect of formaldehyde to urea mole ratio (F/U) of UF resin is rather extensive and covers a wide range of resin, board, and testing variables. This range makes it difficult to quantify the dependence of important properties upon mole ratio or to quantify what the lowest mole ratio should be to maintain acceptable levels of a given property.
All the particleboards bonded with UF resins in the trial, including the E1 boards, passed the severe hydrolysis test at 80 °C for 5 days with the exception of the boards bonded with the C UF resin.
Exterior resin, such as type MUPF satisfying the requirements of E1 level, also met all the requirements up to the standard with respect to the mechanical and physical properties, including the boiling test.
When UF resin was used as bonding agent, the properties of particleboard were changed in the following manner within the mole ratio F/U of 1.27–1.01 with an average resin content of face/core of 11%/8%: MOR and IB decreased up to approximately 10% and 30%, TS and WA increased up to about 19% and 30%. Increasing the resin content in the core layer from 7% to 9% increased the values of IB 27.5% on the average.