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Overview
Whether it's standard products like solid wood furniture or customized items like solid wood cabinets and doors, the industry has long struggled to fully resolve various quality and after-sales issues. Due to the characteristics of solid wood products, many appear flawless upon factory inspection, only to develop quality issues after a period of use in consumers' homes. Returns, exchanges, and repairs in these situations not only incur significant additional costs and manpower, but also severely damage brand reputation. Yellowing, cracking, and deformation are among the most common and representative of these issues, all of which are closely related to wood. The following analysis is based entirely on actual cases encountered in the author's work. We hope that this detailed analysis will help factories identify the root causes and find solutions.
All white paints will yellow. This is because the white pigment titanium dioxide turns yellow when exposed to ultraviolet light. This is why white appliances, white plastic-steel windows, and even white cars all gradually yellow. Consequently, white furniture will also inevitably yellow, albeit at a slower rate, making it difficult to detect. This does not constitute a quality issue if explained to consumers at the sales stage.
However, the situation in Figure 1 is completely different. The product has partially yellowed, which is clearly not a result of natural aging. This situation occurs frequently and is unavoidable in almost every factory. This is the localized yellowing we are discussing.
Based on the above situation and experience analysis, local yellowing has the following characteristics:
a. Mainly white paint;
b. Mainly open paint;
c. The wood is mainly red oak;
d. This problem has nothing to do with region; factories in both the south and the north face this problem.
e. Problems usually occur within 3-12 months. If there is no yellowing within 12 months, there will be no further problems.
Blue stain occurs when wood is infected by a discoloring fungus, causing it to turn gray-blue. Blue stain fungi are sensitive to temperature and humidity, and are particularly active in high humidity environments between 23°C and 35°C. This means that in hot, humid summers, if logs are not cut and air-dried promptly after harvesting, they are susceptible to infection. Kiln drying eliminates the blue stain.
It's important to note that the bluing is only a color change; physical properties like strength and hardness remain unchanged. Therefore, it has no impact on the wood's mechanical processing or coating properties. So, why does the paint yellow? The reason is that the bluing area contains a large amount of tannic acid. Tannic acid is highly reactive and is gradually released from the wood. After penetrating the primer, it reacts with the titanium dioxide in the topcoat, resulting in the yellowing of the paint.
Pay attention~~ The characteristics of yellowing are analyzed here:
a. Why white paint?
Other colors of paint will actually change color, but it's very slight and unnoticeable to most people. White paint yellows very obviously, and the yellow and white form a clear contrast.
b. Why is it open paint?
The primer of open paint is thinner and has poorer sealing properties, so open paint is more likely to be penetrated and yellowed. The primer will also yellow, but because it is more difficult for tannins to penetrate the primer and it takes longer, the chance of it happening is lower.
c.Why red oak?
All wood contains tannins, but oak (including white oak and red oak) has a very high tannin content, which is why oak barrels are used to make red wine. Tannins can provide a sour and astringent taste to red wine. The tannin content of other wood species is too low to achieve the same effect as oak.
Red oak is one of the tree species that is prone to blue discoloration. The tannin content in the blue-discolored parts is far higher than that of other tree species, so it is not surprising that it often causes white paint to turn yellow.
d. Is yellowing related to the production process?
Since the yellowing is caused by the blueing of the wood, it has nothing to do with the production process and has nothing to do with the location of the processing plant.
e. When does yellowing occur?
Tannic acid takes time to penetrate, so yellowing doesn't occur immediately after painting; it only occurs after leaving the factory. As for why yellowing rarely occurs after 12 months, it's speculated that tannic acid is highly active, releasing concentrated amounts initially, then gradually decreasing. If the initial high release doesn't cause yellowing, future use is generally safe.
f.Supplement:
The white development effect of red oak has been a very popular style in recent years. For many whole wood custom factories, this product often accounts for more than half of the total order volume. With such a large base, problems are highly likely to occur.
Since the root cause of local yellowing is the blueing of wood, how can manufacturers avoid it?
When purchasing, avoid buying red oak lumber that has bluing. Generally speaking, lumber produced in the summer is more susceptible to bluing than in the winter. Lumber produced in the southern United States, due to its warmer weather, is more susceptible to bluing than lumber produced in the north. When selecting lumber, carefully inspect the sides of the lumber for signs of bluing. If possible, plane the lumber open for inspection.
However, a more common scenario is that the blue-discolored lumber has already been stored. However, there are ways to salvage this situation. The most critical step is color sorting during the blanking process. Avoid using blue-discolored lumber with white paint, and use the selected blue-discolored lumber separately. It's proven that using blue-discolored lumber for dark paint rarely causes quality issues. It's not that tannic acid no longer causes paint discoloration, but rather that the discoloration in dark paint is far less noticeable than in white paint. When it's difficult to detect with the naked eye, it naturally avoids customer complaints.
Many small factories in the custom-made industry still operate in a workshop-like model, cutting materials individually based on orders. Furthermore, due to management issues, cutters often don't pay attention to the color of the finished product, simply cutting to size according to the order sheet. This is the institutional reason why the yellowing problem is difficult to eradicate. To systematically improve quality control, a pre-processing and unified cutting model must be adopted. The sheets are first processed into semi-finished products, and then the semi-finished products are taken when specific orders are received. Only in this way can quality inspection and sorting be carried out at the semi-finished stage.
Another thing to note is that although blue stain itself does not affect the physical properties of wood, if it continues to develop, it will deteriorate into decay. At this time, the material will not only turn black, but the hardness, density, and coating properties of the material will also change. Decayed materials cannot be used. Therefore, it is important to pay attention to the difference between blue stain and decay.