What is the purpose of adding an extender to urea-formaldehyde resin?

Extenders are added primarily to reduce the cost of the adhesive mix and to control its viscosity without unacceptably degrading the bond quality.

What are the most commonly used extenders in plywood manufacturing?

Starchy materials like wheat flour, tapioca flour, and maize starch are the most commonly used extenders.

Does adding extenders affect the water resistance of plywood?

Yes, increasing the proportion of extenders generally decreases the water resistance of the urea-formaldehyde adhesive bond.

IS 1508

: 1972

Extenders for Use in Synthetic Resin Adhesives (Urea-formaldehyde) for Plywood- Code of Practice

CurrentSpecializedCode of PracticeMaterials Science · Wood and Other Lignocellulosic Products

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This code of practice provides guidelines on the selection, requirements, and use of extenders (such as wheat flour and tapioca starch) in urea-formaldehyde synthetic resin adhesives for the manufacture of plywood. It covers the preparation of the adhesive mix and outlines precautions to maintain optimal bond strength.

Extenders for Use in Synthetic Resin Adhesives (Urea-formaldehyde) for Plywood- Code of Practice

Quick Reference Values

maximum moisture content of extender12%

maximum ash content in wheat flour1.5%

typical mesh size requirement100 percent passing 150-micron IS Sieve

Practical Notes

Always ensure extenders are free from lumps, dirt, and foreign matter, as these can drastically reduce the bonding strength of the plywood.

Higher extension ratios (amount of extender added) reduce the cost but also significantly decrease the water resistance and durability of the cured adhesive bond.

Extenders should be mixed uniformly to prevent localized weak spots in the adhesive line.