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Post finishing shrinkage and cracks in Hardwood plywood panels

 

By:

Pierre Walsh Lebel

Director engineering, R&D

Perfecta Plywood

 

Which of us has not ever faced hardwood plywood claims due to surface shrinking and veneer cracks? This problem is well known in the industry but it is more commonly found on hard maple and birch; species that are known to be unstable after finishing. These occurrences can be explained, in these species, within two categories: the first one is pertinent to the wood specie itself while the second is related to the finishing process.

 

Hard maple and yellow birch are two species characterized by high FSP[1] (27% and higher). This characteristic enables them to absorb significant quantities of water in their « cell wall ». Keep in mind that we are not talking about « free water », which is easier to evaporate within the wood grain itself. Wood cell walls are a location where evaporation becomes problematic. As tension is created in the wood structure, the water absorbed by these wood species will necessarily generate shrinkage and cracks when evaporation is forced.

 

This sensitivity of maple and yellow birch makes them quite vulnerable when there is water absorption during the pressing cycle. It becomes a much greater issue when finishes are subsequently applied such as water based stains (aniline solutions) and latex based flats. All these products make the wood structure expand. Inadequate drying between applications, especially in the case of water based products, makes maple and yellow birch finishing a riskier prospect.

 

First of all, we have to understand what happens during the plywood laminating process while working with species at risk, especially maple and yellow birch. For example, let’s take two maple veneer sheets laminated on particleboard. The face and back sheets will see their moisture content pass from 10% to 30% and increase even more during hot press lamination. Once laminated, a veneer sheet will have a natural tendency to stabilize its moisture content with the use of ambient air. Humidity transfer will be non existant from the veneer to the core due to the glue line itself which has created a waterproof barrier blocking water migration to the core. So, this means all the water and liquids contained within the adhesive will be transferred to the veneer.

 

Current market trends for environmentally friendly water based stains and latex flat finishes, are now an integral parts our finishing processes, particularly for the furniture industry. In a just in time marketplace, these panels are being brought to end use very rapidly. Once the lamination process is concluded, sufficient drying time must be given to these panels before these types of finishes are applied. If this does not occur and the panels are not allowed to reach their hydroscopic equilibrium, this excess water will lodge itself in the veneer and create the appropriate natural conditions for cracks and shrinkage. As maple and yellow birch are recognized to have FSP high values, as discussed above, compared to other wood species, their capacity to capture water in their wood cell walls will substantially increase the risk that this water will create cracks and defect on the veneer surface.

 

When you add a fast finishing, varnishing application and a catalyst process such as cooking chambers, this will create ideal conditions where water will be trapped between the veneer glue line and the finishing surface. As maple and yellow birch wood tissues have accumulated relatively more water in their cell walls, catalyzed finishing will increase steam pressure in the veneer. Being saturated by the veneer lamination adhesive first and water based finishing products second, there would be a lot of water accumulated within the cell wall. This is precisely where the problem lies. In wood technology, cell wall water by opposition to cell cavities’ water (free water) is known to create tension and introduce deformations and cracks while evaporating. Again, the wood has this natural capability to stabilize its water content with its environment but artificial conditions blocking this hydroscopic equilibrium creates tensions in the wood structure. Is there a solution? Should we avoid water-based products and focus more on toluene, xylene and urethane aromatic products?

 

It has to be understood that water based products that we are referring too, are here to stay. They can be found in automobile paints, wood stain/finishes and several other green products. There is however evidence that bad conditioning during the steps of “finishing”, along with catalyzed drying, makes the product more vulnerable to cracking and shrinkage. Why are we taking about this as a risk proposal as opposed to a certainty when finishing maple and yellow birch? Simply because there no two identical pieces of wood and cracking and shrinkage also depends on wood density, porosity and grain arrangement.

 

 

 

 

 

 



[1] FSP: Fibre saturation point. This corresponds to the moisture content achieved where water is being evaporated from the cell cavities (free water) while the cell walls remain fully saturated.

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