Glue is an essential part of plywood manufacturing. You can also say it is the most important element of plywood. Different glue is used for plywood in different applications.
As a plywood manufacturer in China, we use three types of glue in practice.
- Type A Bond – Phenol Formaldehyde
- Type B Bond – Melamine Urea Formaldehyde
- Type C Bond – Urea Formaldehyde
- Type D Bond – Polyvinyl acetate – PVAc Glue
- Type E Bond – MDI
Type A Bond – Phenol Formaldehyde (PF)
Phenolic resins are the strongest bond and are also waterproof. The polymer is thermosetting and has to be cured at elevated 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 structural plywood or film faced plywood. It is weather and boil proof (WBP) because phenolic resins have no melting point but only a decomposing point in the temperature zone of 220 °C.
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
- Glue layer is too crispy
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.
MUF glue is a condensation product of melamine, urea and formaldehyde.
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.
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 easy to produce and bond well. It can only withstand occasional exposure to dampness. It should not be used for structural applications or wet areas. UF glue is chosen for its fast curing and wide operating window. Urea adhesives are cured at lower pH and elevated temperatures.
Its price is the lowest among the three UF glues. UF is also the most widely used glue in the plywood industry.
Type D Bond – Polyvinyl acetate
PVA or PVAc is short for Polyvinyl acetate. It is best known as wood glue, white glue, or carpenter’s glue.
It is mainly used for interior applications. Polyvinyl acetate is used for the manufacturing of low formaldehyde plywood.
Type E Bond – MDI
Wood panel industry usually use MDI for Laminated Veneer Lumber, OSB, Finger Joint Board, MDF. MDI glue has great WBP capabilities. Its quantity used in chipboard can be much less than the UF glue.
Melamine vs Phenolic Resin
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.
Uses For Different Glue
|Urea Formaldehyde||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.