Glue is an essential part of plywood manufacturing. You can also say it is the most important element of plywood. Together with the veneer material, glue determines the strength, quality, and durability of the plywood panels.
Different glue is used for different quality plywood. UF glue is for interior plywood production and it is the most consumed glue in the Chinese plywood industry. Phenolic formaldehyde glue is relatively rare as there is lower demand for phenolic bonded plywood.
There are many types of new glue emerging in the Chinese plywood industry. Such as MDI, Soybean Glue, etc. As a plywood manufacturer in China, we mainly use PF, MUF and UF glue in the production of our wood panels.
Type A Bond – Phenol Urea-formaldehyde (PUF)
Type B Bond – Melamine Urea-formaldehyde(MUF)
Type C Bond – Urea-formaldehyde(UF)
Type A Bond – Phenol Urea-formaldehyde (PUF)
Phenol urea-formaldehyde (PUF) thermosetting resin was synthesized by reacting methylol phenol with urea under acidic conditions. PUF resin made by this process performed better bonding strength than commercial UF resin and mechanical blending of PF and UF resins in both dry and wet conditions. Its bonding strength was found to be intermediate to UF and PF resins.
PUF thermosetting resin is thermosetting and has to be cured at very high temperatures. The result is a permanent bond that can be exposed to heat, cold or wet conditions for a long time without deteriorating.
This glue bond is used for the manufacture of exterior plywood, structural plywood or premium film faced plywood. It features a dark glue line like below.
Phenolic Glue Line
Phenolic glue is weather and boiling proof (WBP) because phenolic resins have no melting point but only a decomposing point in the temperature zone of 220 °C. Phenolic resin are either water soluble or alcohol soluble. The phenolic resin used for making plywood glue are the better one, water-soluble.
Fillers should be added into the PF glue to increase performance.
Important Knowledge
Not every plywood factory can produce plywood with phenolic glue. The phenolic glue is very picky for the press machine and production techniques: 1) High-pressure hot press machine is required.
2) Workers should be aware of PF glue’s short curing time.
Advantages of Phenolic Glue
Great Bonding Strength
Water Resistant
High Temprature Resistant
Chemically Stable
Fungal and termite Resistant
Disadvantages of Phenolic Glue
Color is too dark will may not look suitable in some uses.
The glue layer is too crispy
Require high temperature and high pressure for production line
Not easy to control production due to short time of glue curing
Type B Bond – Melamine Urea-formaldehyde(MUF)
Melamine UF glue is a condensation product of melamine, urea and formaldehyde. It is a UF glue modified with melamine and PVA. The melamine and PVA are added when plywood factory produces E0 plywood.
The quality of MUF depends on the mol ratio of urea and melamine. Melamine weight is usually 5%-20% of the urea weight. If melamine is too many, the cost is too high. If melamine is too less, the glue will be less adhesive.
When PVA weight is 1% of urea weight, and melamine weight is 3% of urea weight, the melamine UF can perform at the highest level. Bonding strength can reach 0.70MPa and formaldehyde emission can reach E0 grade.
Type B glue bond is waterproof but will deteriorate after several years of being exposed continuously to weather. Its curing process may be performed both at room temperature and higher temperature depending on the application and performance requirements. Generally, melamine adhesives give very good water resistance and light-colored glue line. Plywood with a B-Bond is best used in situations of limited weather exposure, e.g. exterior door skins, marine plywood, and concrete formwork. It is waterproof but not weatherproof. Melamine glue plywood can be used in damp or wet places but can not last long under the rain.
Type C Bond – Urea-formaldehyde (UF)
Urea-formaldehyde (UF) adhesives are thermosetting, cost-efficient polymers, mainly used for interior applications. It is the condensation reaction product of urea and formaldehyde.
UF glue is chosen for its fast curing and wide operating window. Urea adhesives are cured at lower pH and elevated temperatures. As the most used glue in the plywood industry, 80% of plywood are manufactured with UF glue. UF glue can be modified with melamine to create the MUF glue.
UF glue is easy to produce and bond well. But it has its shortback. UF glue can only withstand occasional exposure to dampness. It should not be used for structural applications or wet areas.
Curing Agent is important for the quality of UF glue. Curing agent can redue the PH of UF glue and accelerate the curing process of the adhesive in production. Acid curing agent are added such as Ammonium chloride.
Urea-formaldehyde can be blended with soy protein concentrate. Soy protein concentrate can alter the UF glue for better performance. Added at a reasonable ratio, the plywood can be improved in shear strength and formaldehyde emission.
