1 Why do we choose PF plywood as flooring core material?
The main adhesives for flooring plywood are primarily three types of aldehyde adhesives, namely: melamine-formaldehyde resin(MUF), phenol-formaldehyde resin(PF), and urea-formaldehyde resin (UF). The performance characteristics of these three adhesives are as follows:
UF resin has abundant raw materials, low cost, good bonding strength, and the widest range of applications. However, the durability and water resistance of the wooden panels made from urea-formaldehyde resin are relatively poor, and there are also disadvantages such as the presence of VOCs in the adhesive, which can harm human health.
MUF resin is wear-resistant and heat-resistant, but it is relatively expensive, and the cured adhesive layer is hard and brittle, making it prone to cracking.
PF resin can form a very high bonding strength between wood materials, and it also has good acid resistance, water resistance, and aging resistance. Most importantly, PF resin has a low formaldehyde emission, meeting the requirements of the F **** standard in the JAS standard. Therefore, PF resin is considered the preferred adhesive for flooring plywood.
2. What makes delamination in PF plywood?
Although PF resin provides high bonding strength, its long curing time and high temperature demand precise plywood production. Delamination or reduced bonding occurs due to premature adhesive curing, insufficient temperature, short curing time, or excessive veneer moisture. Factories carefully control adhesive application and curing parameters. Still, uneven high moisture in veneers often causes weak bonding or adhesive failure.
Veneers for plywood come from rotary-cut logs. Machines slice logs into large sheets while green, with moisture over 30%. Workers air-dry and grade the sheets before using them in plywood production.
Before bonding, workers use roller dryers to lower veneer moisture below 10%. Knots and dense regions dry slowly, leaving localized high moisture even after drying.
PF resin is a thermosetting adhesive with a curing temperature above 130 ℃. After the veneers are glued and laminated alternately, they are fed into a multi-layer hot press. Heat is conducted to the billets through upper and lower hot plates, causing the adhesive to cure. Excessive moisture content in localized areas of the veneer during the hot pressing process can generate steam, affecting the penetration of the adhesive into the wood cells and cell walls of the veneer, thereby preventing effective bonding between veneers and leading to delamination.
3.How do we reduce the risk of PF plywood delamination?
3.1 Plywood Production
To address the delamination issue caused by localized high moisture content in wood veneers, plywood factories need to strictly control the moisture content of each veneer. This can be achieved by controlling the feeding rate of the roller drying machine to ensure that each veneer is thoroughly dried.
3.2 Raw Material Inspection
Strict inspection procedures should be conducted for PF plywood purchased by the factory. The laboratory performs a delamination test on the bonding performance using the hot water delamination method. The sample size is determined based on the total incoming quantity.
According to JAS standards, for incoming quantities of over 3000 sheets of plywood, 5 sheets are selected for testing. The plywood is cut into experimental samples of 75mm*75mm, soaked in hot water at 70±3°C for 2 hours, and then dried in a hot air oven at 60±3°C for 3 hours. The bonding layers on all four sides are observed for any delamination. If the total length of delamination exceeds one-third of the total length, it is considered as not meeting the standard.
The determination of incoming inspection qualification is also based on JAS standards. A batch is accepted when over 90% of the tested samples are qualified. If fewer than 70% meet the standard, the batch fails. Results falling within the 70–90% range require further sampling and retesting.
For plywood raw materials from the same supplier that have delamination issues, the number of inspections should be increased.
3.3 Production Quality Control
Since hot water delamination testing is a destructive test and cannot be performed on every plywood sheet, the quality of raw materials can only be inspected through sampling. During production, the risk of delamination can be roughly assessed by tapping and listening to the sound of the plywood.
3.4 Floor Rebalancing
Before installation, the flooring needs to be ventilated and placed in the environment for several days to achieve moisture content rebalancing, releasing the internal stress of drying and swelling. This reduces the risk of delamination during installation.
4. What if delamination occurs?
Although the risk of delamination of plywood has been greatly reduced through quality control measures, it still CANNOT be guaranteed 100%.
Facing the possibility of non-compliance, proposing targeted solutions, and striving to improve the passing rate is our commitment to customers.
