Reasons and preventive measures for peeling (bubbling) or weak bonding of plywood. Laboratory testing report
Introduction: The occurrence of delamination (blistering) or weak bonding (insufficient bonding strength) in each layer of plywood is a common defect in the production process of plywood and an important factor affecting the quality of plywood. Therefore, management personnel at all levels and relevant operators must understand and master the causes and preventive measures of delamination, so as to apply it to production practice, identify and solve problems, and ensure product quality.
The occurrence of delamination (blistering) or weak bonding (insufficient bonding strength) in each layer of plywood is a common defect in the production process of plywood and an important factor affecting the quality of plywood. Therefore, management personnel at all levels and relevant operators must understand and master the causes and preventive measures of delamination, so as to apply it to production practice, identify and solve problems, and ensure product quality.
There are many factors that affect the opening (bubbling) or weak bonding of plywood. Whether it is raw materials, glue types, glue liquid indicators, operating methods, process parameters, production time control, and other factors, they are all influencing factors. The following describes the main influencing factors and preventive measures.
- Wood moisture content: The amount of wood moisture content directly affects the strength of plywood. If the moisture content of wood exceeds 20% or more, it has a significant impact on the strength of plywood, until it causes blistering or weak bonding. Therefore, when producing plywood, the moisture content should be controlled below 14%. The same applies to veneers with added surfaces, and the moisture content should not be too high. Especially in the production of phenolic adhesive (with low solid content), the moisture content of birch based veneer should be controlled below 8%.
- Hot pressing temperature and time: The main reason for adhesive opening and weak adhesion is that the pressure plate temperature is too low or the hot pressing time is short. Generally, the hot pressing temperature of urea formaldehyde adhesive is not less than 105 ℃, and the time is 1-1.2 minutes/mm The hot pressing temperature of triamine adhesive shall not be lower than 115 ℃, and the time shall be 1.2-1.5 minutes/mm The hot pressing temperature of phenolic adhesive shall not be lower than 120 ℃, and the time shall be 1.2-1.5 minutes/mm. The high hot pressing temperature is an important cause of blistering. Therefore, in the production process of plywood, when meeting the bonding temperature, try to limit the temperature of the pressing plate to be too high.
- Glue application amount: The amount of glue applied also affects the bonding strength and is an important factor causing product quality defects, generally controlled at 320-340g/㎡ of core board glue application. The veneer overlay is controlled between 290 to 320g/㎡. If there is a certain temperature (not room temperature) for the core board with added surface, the adhesive amount should be increased appropriately to avoid dry adhesive. For phenolic adhesives with low solid content, one is to increase the flour content (about 35%), and the other is to reduce the adhesive amount while meeting the bonding strength.
- Aging time: The aging time of the coated slab not only affects the pre pressing effect, but also may cause drying of the adhesive due to the temperature of the adhesive solution during the aging process, combined with factors such as wind blowing, which can affect the aging time too long.
