Plywood is first manufactured without the face layer, followed by surface calibration or sanding. This process helps remove oil from the surface before applying the face veneer to the mat ply.
In plywood manufacturing, core veneer timbers play a crucial role in providing structural integrity and stability to the plywood panels. Different types of wood species like poplar and eucalyptus, birch, okume, meranti are used as core veneers, each offering unique characteristics and performance attributes. Pine is another commonly used wood species for core veneers. It offers good strength-to-weight ratio, stability, cost effective and ease of machining, making it suitable for structural applications and hence used as a cost effective to traditional core materials like poplar and eucalypatus.
Pine, a widely available and economically viable softwood, is a popular choice in the plywood and furniture industries. However, not all pine species are created equal when it comes to industrial use. One of the lesser-discussed but significant challenges in plywood manufacturing is the oily nature of certain pine species.
This article explores the specific issues caused by the oily characteristics of pine veneer in plywood production.
1. Adhesion Issues with Resin or Glue
The most critical challenge is the reduced effectiveness of adhesives. Plywood is composed of multiple layers of veneer bonded together using synthetic resins, such as urea-formaldehyde or phenol-formaldehyde. Oily surfaces hinder proper bonding by acting as a barrier between the veneer and the adhesive.
- Reduced bonding strength: The oily extractives interfere with the glue’s ability to penetrate the veneer surface, weakening the structural integrity of the final product.
- Glue line failures: Over time, poor adhesion can lead to delamination—where the veneer layers begin to separate.
2. Uneven Surface Finishing
The presence of natural oils can also affect the aesthetics and finish of plywood. Finishes such as Preservative solution, varnishes, stains, or paints may not adhere evenly, leading to blotchy or uneven coloring.
- Surface contamination: Oils may migrate to the surface during drying, making it harder for finishes to bond.
- Inconsistent absorbency: Different parts of the veneer may absorb preservative solution unevenly, resulting in a non-uniform appearance like oil-patches on the surface of the face veneer of the plywood.
3. Complications During Pressing
During hot pressing, high temperatures are used to cure the adhesive and bond the veneers. The oily nature of pine can introduce additional variables.
- Oil Exudation: Under heat, oils can be drawn out of the veneer, contaminating the press plates and reducing efficiency.
- Blistering or Resin Bleed: Excess oil can create steam pockets or mix with the adhesive, causing visible defects like blisters or resin bleed on the surface of the plywood.
Possible Solutions and Mitigation Strategies
Despite these challenges, pine remains a viable raw material when processed correctly. Some methods to mitigate the issues include:
- Veneer Treatment: Applying a chemical treatment to the veneer prior to drying either through vapor pressure treatment (VPT) or immersion in a chemical solution to effectively remove surface oil.
- MAT Technology: Plywood is first manufactured without the face layer, followed by surface calibration or sanding. This process helps remove oil from the surface before applying the face veneer to the mat ply.
- Surface Treatment: To remove oil stain from the plywood surface can use vinegar water solution or backing soda paste on the stain surface and then gently scrub with a soft brush or cloth. or can use fine grift sand paper.
CONCLUSION
While pine veneer offers many advantages in terms of availability, workability, and cost, its oily nature presents unique challenges in the manufacture of high-quality plywood. Adhesion problems, surface finish inconsistencies, and pressing complications are some of the critical concerns faced by manufacturers. However, with proper material preparation and processing strategies, these challenges can be effectively managed, allowing for the continued use of pine as a sustainable and versatile option in the plywood industry.
[Published in Ply Reporter's May 2025 Print Issue]