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Summary
Whether it's standard products like solid wood furniture or custom-made products like solid wood cabinets and doors, a long-standing and difficult-to-resolve issue in the industry is the various quality and after-sales problems. Due to the characteristics of solid wood products, many products that pass factory inspection develop quality issues after a period of use in the consumer's home. Returns, exchanges, and repairs not only incur significant additional manpower and material costs but also severely damage brand reputation. Among these problems, yellowing, cracking, and warping are particularly prevalent and representative. These issues are all closely related to the wood itself. The following analysis is based entirely on actual cases encountered in my work, and I hope that through detailed analysis, I can help factories clarify the root causes and find solutions.
All white paint will yellow eventually because the white pigment, titanium dioxide, turns yellow when exposed to ultraviolet light. Therefore, white appliances, white PVC windows, and white cars will all gradually yellow. Consequently, white furniture will also yellow, although the process will be slow and difficult to detect under normal circumstances. If this is explained to consumers beforehand during the sales process, it will not constitute a quality issue.
However, the situation in Figure 1 is completely different; the product shows localized yellowing, which is clearly not due to natural aging. This phenomenon occurs very frequently, and almost every factory cannot completely avoid it. This is the localized yellowing we will be discussing.
Based on the above information and experience, localized yellowing has the following characteristics:
a. Primarily white paint;
b. Primarily open-pore finish;
c. The wood is mainly red oak;
d. This problem exists in both southern and northern factories, regardless of location;
e. Problems usually occur within 3-12 months. If there is no yellowing within 12 months, there will generally be no further problems.
Blue stain occurs when wood is infected with a discoloration fungus, turning it a grayish-blue color. This fungus is highly sensitive to temperature and humidity, becoming very active in high-humidity environments (23℃-35℃). This means that in hot, humid summers, logs are easily infected by blue stain if they are not promptly cut and dried after felling. Wood will not develop blue stain after being dried in a kiln.
It's important to note that blue stain only changes color; its physical properties, such as strength and hardness, remain unchanged. Therefore, it has no impact on the wood's machinability or coating performance. So why does it cause paint to yellow? The reason lies in the fact that the blue stain area contains a large amount of tannic acid. Tannic acid is highly reactive and gradually releases from the wood. After penetrating the primer, it undergoes a complexation reaction with the titanium dioxide in the topcoat, resulting in the paint turning yellow.
Pay attention! Here's an analysis of the characteristics of yellowing:
a. Why is it white paint?
Other colors of paint can also change color, but very slightly, to the point that most people don't notice. White paint, however, yellows very noticeably, creating a stark contrast between the yellow and white.
b. Why is it an open-pore finish?
Open-pore paints have a thinner primer and less sealing properties, making them more susceptible to penetration and yellowing. Sealing primers can also yellow, but because tannins have more difficulty penetrating the primer and require more time, the likelihood of yellowing is lower.
c. Why red oak?
All woods contain tannins, but oak (including white and red oak) has a particularly high tannin content. This is why oak barrels are essential for making red wine; tannins provide the wine with a slightly acidic and astringent flavor. Other tree species have too low a tannin content to achieve the same effect as oak.
Red oak is one of the tree species that is very prone to blue stain. The tannin content in the blue staining parts is much higher than that in other tree species, so it is not surprising that white paint often turns yellow.
d. Is yellowing related to the production process?
Since yellowing is caused by the blue staining of wood and is unrelated to the production process, it is also unrelated to the location of the processing plant.
e. When does the yellowing process take?
Tannic acid takes time to penetrate, so it doesn't yellow immediately after coating; it only yellows after the product leaves the factory. As for why yellowing rarely occurs after 12 months, it's speculated that tannic acid is quite reactive and is released in a concentrated manner in the early stages, with the release gradually decreasing thereafter. If yellowing doesn't occur after the initial high release, it's generally safe to apply it thereafter.
f. Supplement:
The white finish on red oak has become a very popular style in recent years. For many solid wood custom furniture factories, this product often accounts for more than half of their total orders. With such a large volume, problems are highly likely to occur.
Since the root cause of localized yellowing is blue staining of wood, how can manufacturers avoid it?
During the procurement stage, try to avoid buying red oak lumber that has developed blue stain. Generally speaking, lumber produced in the summer is more prone to blue staining than that produced in the winter. Lumber from the hotter southern United States is also more prone to blue staining than that from the north. When purchasing lumber, carefully observe the sides for signs of blue staining; if possible, plan the surface open for inspection.
However, a more common situation is that the blue stain substrate has already been received into the warehouse, but there are still ways to salvage the situation. The most crucial step is to sort the materials by color during the cutting process. Blue stain materials should be avoided when using white paint, and the selected blue stain materials should be used separately. It has been proven that using blue stain materials for dark-colored coatings rarely causes quality problems. This isn't because tannins no longer cause paint discoloration, but rather because the discoloration of dark paints is far less noticeable than that of white paint. When it's difficult to discern with the naked eye, it naturally avoids customer complaints.
Many small factories in the custom furniture industry still operate on a workshop model, cutting materials individually to order. Furthermore, due to management issues, workers often don't pay attention to the color of the finished product; they simply cut the materials to the specified dimensions. This is the systemic reason why yellowing is so difficult to eradicate. To systematically improve quality control, a pre-processed, standardized material cutting model must be adopted. The boards should be processed into semi-finished products, and materials should only be used when a specific order is placed. Only in this way can quality inspection and sorting be carried out at the semi-finished product stage.
It's also important to note that while blue stain itself doesn't affect the physical properties of wood, if it continues to develop, it will deteriorate into decay. At this stage, the material not only turns black, but its hardness, density, and finishing properties will also change. Decayed material is unusable. Therefore, it's crucial to distinguish between blue stain and decay. (From *Asian Panel & Furniture*)