Foam products may turn yellow during use for various reasons. Here are several possible causes:

Oxidation: When the material in sponge products comes into contact with oxygen, an oxidation reaction may occur, causing the sponge to turn yellow. This is typically due to the polymer material in the sponge reacting with oxygen in the air, resulting in a change in color.

Dirt and Contaminants: Sponge products are commonly used for cleaning and wiping surfaces, thus coming into contact with various dirt and contaminants. These particles may penetrate the sponge's fibers, making it difficult to thoroughly clean them. Over time, the accumulation of dirt can cause the sponge to yellow.

Exposure to Sunlight and UV Radiation: When sponge products are exposed to prolonged sunlight or UV rays, certain components in the material may undergo photochemical reactions. These reactions can lead to changes in the molecular structure, causing the sponge to turn yellow.

Some foam products may be sensitive to certain chemicals, leading to yellowing upon contact. These chemicals could include cleaners, bleaches, dyes, or other substances.

Improper Storage and Maintenance: Failure to store and care for foam products properly, such as leaving them exposed to damp conditions for extended periods or storing them with colored materials, can result in the foam becoming yellowed.

It's important to note that the yellowing of sponge products doesn't necessarily mean their performance or functionality is compromised, but it may affect appearance. To maintain the original color of sponge products, regular cleaning and maintenance are recommended, as well as avoiding contact with substances that could lead to yellowing.