In order to improve water resistance of the plywood made with urea-formaldehyde resin (UF), an adhesive, commonly used in the manufacture of wood-based materials, is modified first with water resistant melamine-formaldehyde (MF) or phenolformaldehyde (PF) adhesives. Despite their obvious advantages, a combined melamine-urea-phenolformaldehyde (MUPF) resin does not completely exhibit the advantageous properties of its individual components, particularly in terms of resistance to the action of variable ambient conditions. As a result, wood based materials manufactured with its use have lower water resistance than when produced using pure PF resin Hence Effort for lowering of energy consumption, for improving of environment and better exploitation of raw materials is the new technologies in future. An another direction leading to an enhancement of water resistance in amino resins with their modification with polymeric isocyanate (pMDI) has been developed to increase the water resistance of the plywood. The pMDI resins are already proved for manufacturing wood panel products like Particle board, Medium Density Fibre board etc. But its use as an adhesive for plywood manufacturing is limited due to several reasons. Analyses showed that a 40 % addition of isocyanates to UF resin makes it possible to manufacture plywood with an even better water resistance than that specifi ed in the respective standard binding for MF resin. It was reported that mixing isocyanate with PF resin or UF resin adhesives enables it to react faster with water than with other resin and makes each adhesive's methylol group and --NCO group react to create urethane bonds with relatively strong bond strengths. The NCO group in pMDI may react with the hydroxyl group present in wood and the curing starts before the assembly of veneers for hot pressing.
p-MDI has a density of 1.23 g/cm3 at 25°C. p-MDI can also tolerate high temperatures and is considered to have a low flammability risk due to its flash point of over 200°C. It will start to decompose at temperatures above 230°C.
A typical composite adhesive was manufactured by mixing emulsifiable polymeric isocyanate with urea formaldehyde resin at a different concentration starting from 1%, 2.5%, 5.0%, 7.5% and 10% on the basis of liquid Urea Formaldehyde resin. Adhesive properties of UF /PMDI hybrid resin like viscosity, gel time , FTIR spectroscopy, free formaldehyde of the resin and the bonding strength property of the plywood manufactured ,formaldehyde emission of the board were analyzed .
The data showed the addition of 1% to 10% polymeric isocyanate (pMDI) resin to the UF resin influenced the gel time. The pH of the hybrid resin decreased slightly, reducing the reactivity of the hybrid resins. The viscosity increased from 82 to 126 cp when 1% pMDI resin at was added into the UF resins and increased to 126cp for the addition of 10%pMDI resin .The addition of 10% pMDI resin yielded a higher viscosity than for those of 1% pMDI, Solid content of UF/PMDI hybrid resin does not have a significant changes was observed from the test data.
As expected, the pMDI resin had an obvious influence on the glue shear strength of plywood with both dry and wet. Glue shear strength values increasing as the concentration increased up to 10%. Both dry and wet glue shear strength values increased by 20% and 25%, respectively, compared with that of the control. These high dry glue shear strength values are attributed to the formation of various bonds between the isocyanate group and the –CH2OH groups of UF resins or those between the isocyanate group and the hydroxyl group of wood. Boiling water resistance properties of the plywood manufactured with UF/ pMDI resin showed better then control samples .This might indicate an improvement in the water resistance of the plywood due to the incorporation of the pMDI resin into the UF resin. Though plywood does not show water resistance properties upto BWR grade plywood when tested as per IS 848(2006), but able to withstand up to 3 hr at 1000C in cyclic test.
The addition of 10% to 7.5% pMDI resin reduced the emission by 30% and 22%, respectively. This was probably due to the lower concentration of –NCO groups in the UF/ pMDI resin with 1% pMDI resin compared with the case of 10% pMDI. The –NCO groups probably could react with the free formaldehyde (HCHO) and formed HNCO.
The results and analysis of studies leads to the conclusion that the plywood with improved glue shear strength, water resistance and considerable limited formaldehyde content may be obtained in the case of Urea formaldehyde modified with emulsifiable polymeric isocyanate (PMDI) at a concentration of 10%, However the plywood does not confirms to the boiling water resistance required as per the BWR Grade when tested as per IS: 848 (2006). It shows better glue shear strength of the plywood in dry state than the value of the plywood made with unmodified UF resin and confirms to MR grade plywood. The gel time of the UF/PMDI hybrid resin shorten when 10% PMDI was introduced into the liquid resin. The results obtained from this study suggest that the modification of UF resins with pMDI resin is a way of improving the water resistance and reduce the formaldehyde emission up to 20 to 30%.
AUTHOR: S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)