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
Exterior glue with dark color. Most expensive glue for plywood. Common name is WBP Phenolic glue.
good water solubility)
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 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 Fortified Urea-formaldehyde (MUF)
Melamine-urea formaldehyde (MUF) adhesives are adhesives which combine the properties from urea and melamine polymers, to ensure optimal and cost-efficient performance.
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.
Waterproof Capabilities of Different Plywood Glue
WBP plywood made with melamine glue rather than phenolic resin can maintain lamination in boiling water for between four and eight hours. Higher grade melamine plywood can withstand boiling water for 10 to 20 hours.
WBP plywood using ordinary phenolic resin glue can handle boiling water for one to three days.
It should be noted that the length of time plywood can withstand boiling water without delaminating is not only dependent on the glue – it also depends on the grade of veneers and pressure.
| GLUE TYPE | MR UF | WBP MELAMINE UF | WBP PHENOLIC UF |
|---|---|---|---|
| PRICE | LOW | MEDIUM | HIGH |
| BONDING STRENGTH | MEDIUM | MEDIUM | HIGH |
| WATER BOILING PROOF | 4 HOURS | 10 HOURS | 72 HOURS |
Uses For Different Glue
| Glue Type | Chracteristic | Uses |
|---|---|---|
| Urea-formaldehyde (UF) | Moisture Resistant (MR) | Interior, Furniture, Can not be used exterior |
| Melamine UF | MR & WBP | Interior, Exterior (Can not be in water long time), Formwork |
| Phenolic UF | Great WBP | Structural Plywood, Formwork |
What kind of glue can be used in formwork plywood?
Formwork plywood requires a good strength which can be obtained from structure design and high pressure. Since the formwork is a temporary structure so the film faced plywood doesn’t have to be structural grade. In this case, the glue used for manufacturing formwork plywood can be A-bond, B-bond or C-bond.
