Bonding quality is one of the most important attributes of plywood. It can determine whether the plywood is good for its intended use.
All plywood must be suitable for dry conditions. Plywood with better bonding quality can be used for humid conditions. Plywood with the best WBP bond can perform in very humid or wet conditions.
Bonding Quality Classification
By European Standard (EN 314-2): Class 1, Class 2 and Class 3
|EN 314-2||Class 1||For dry conditions: normal interior use.|
|EN 314-2||Class 2||For protected exterior conditions: exposure to weather for short time. Also can be used for Class 1.|
|EN 314-2||Class 3||Exterior conditions: exposure to weather over a long time. Also can be used for Class 1 and Class 2.|
By Japanese standard (JAS): Type Special, Type 1 and Type 2
|Japanese JAS||Type Special||Plywood for outdoors where wet conditions continuously happen.|
|Japanese JAS||Type 1||Plywood for places where wet condition intermittently occurs.|
|Japanese JAS||Type 2||Plywood for places where wet condition occasionally occurs.|
Bonding Quality Tests
- BONDING SHEAR TEST: Shear strength and wood failure rate are determined after the plywood sample is exposed to a certain wet treatment. shear load is applied to the wet test sample using a universal testing machine until the test piece breaks.
- Delamination TEST: Measuring veneer delamination after plywood samples are exposed to wet environment.
- KNIFE TEST: A concise method to estimate the bonding quality of plywood. A knife is pushed onto the face ply with its cutting edge parallel to the grain such that the veneer is prised upwards and removed. Bonding quality is assessed visually based on the amount of wood fiber (wood failure) remaining.
What affects the bonding quality?
Almost every process in plywood production affects the bonding quality. Hereby we write about the main factors that affect bonding quality.
If your plywood delaminate or has other bonding problem, you can find your answers below.
Fillers are added in the glue to reduce production cost.
Solid content of the glue shall be 40% – 50%. Thick and rough veneers require low solid content glue with large amount. If the veneer has a high moisture content, the glue solid content should be high as well.
Glue PH should be 4.5-5.0.
Veneer shall be softened for plywood production.
Moisture content and flatness are important factors for veneer quality.
Now manufacturers use different wood, but it is better to use the same veneer species at one plywood production line.
Too many or too less glue applied on the veneer is not good for bonding.
Glued veneers shall go through aging before they are pressed. Overtime aging could reduce the glue resin on veneer surface. Aging time should be shorter in Summer and longer in Winter.
Pressure reducing in hot press can not be too fast otherwise there will be bubbling created inside the plywood.