Cellulosic Browning

Cellulosic browning can occur on many types of rugs and carpets. Cellulosic browning is a brownish discoloration that appears on fibers after cleaning or during drying. For cellulosic browning to come about, you must first have some cellulose fibers. Cellulose is the main constituent of plant-based fibers like cotton, jute, hemp, sisal, and rayon. So 100% wool rugs or 100% synthetic carpet cannot have cellulosic browning. Sometimes, these non-cellulose fiber rugs can have discoloration after cleaning and drying, but it is not cellulosic browning because there is no cellulose present. We call this situation a “mudslide”, it occurs when a rug is improperly or incompletely cleaned, and the remaining soil wicks up to the surface as the fibers dry. More thorough cleaning is the answer to this situation.

However, there are instances where cellulosic browning can occur even on 100% wool pile rugs, or 100% synthetic pile carpets, this can happen if there are cellulosic fibers in the foundation yarns or backings. We have even seen cellulosic browning come from the rug cushion/pad or wood flooring underneath. This is a way to have cellulosic browning where you may think that it would be impossible.

Another factor to having cellulosic browning is moisture. The thing that causes cellulosic browning is in the cellulose fibers. It needs a vehicle to move to the surface where you can see it. That vehicle is water. Dry cellulosic fibers will not brown out, wet fibers can. So the rate/speed of drying is an important factor. The faster, the drying, the less browning effect, and the slower the drying, the more pronounced browning. The pH of the water is also a crucial factor. The higher, the pH, the more browning and the lower the pH, the less browning.

So what is it that causes cellulosic browning? Specifically it is lignin. Lignin is the secondary constituent of plant-based fibers, following cellulose itself. Lignin is a hydrophobic, complex organic polymer, responsible for the brownish discoloration. Lignin is colorful, light yellow to very dark brown. Its color is pH sensitive, low pH makes it lighter and it gets darker as the pH rises. If you get the pH below 3, the color becomes so light that it seems to disappear, but it is still there. Lignin is also hydrophobic, and is pH sensitive. The lower, the pH, the more hydrophobic, and the higher, the pH, the less hydrophobic. To effectively deal with cellulosic browsing, you need to achieve a pH of 7.8 or higher for lignin to become water soluble. This may be the reason behind why we hear how browning becomes more and more difficult to deal with each time a rug or carpet browns-out. This is because the cellulosic browning is solved by treating with an acid solution. This makes the color disappear, but the lignin is still there! So as the pH changes, the color becomes apparent again. To remove the lignin, you have to wash it away with a pH higher than 7.8. In fact, “mercerized” cotton (and other Page 1 of 2 cellulosic fibers) undergo an aggressive washing with high pH solution under tension, yielding cotton without lignin and improves strength, dye-ability, reduces shrinkage (pre-shrunk), improves luster, and removes cellulosic browning and yellowing. Redox reactions are another way to remove discoloration from lignin . This is why 3% hydrogen peroxide works so well on cellulosic browning. Redox reactions are a wiser choice than acid pH treatments as redox treatments actually destroy the lignin, rather than just hiding it. Reduction is often more effective in removing lignin. So, washing solutions like Problem Reducing Rug Shampoo are much more effective in dealing with browning. Low Odor Dye Stripper can be used on very difficult discoloration.

Keep in mind that many rayon fibers have issues with discoloration during the washing and drying cycles. This is because rayon is almost 100% cellulose. Some rayon fibers, like viscose
rayon, still contain a significant amount of lignin. On the other hand, Lyocell (Tensell) rayon has no lignin. So, depending on which method was used to manufacture the rayon fibers, cellulosic browning can be an issue. (See rayon viscose rugs below). It is crucial to clean rayon fibers with the correct chemistry to address the specific properties of the fiber. This includes cleaning on the alkaline side to keep lignin water soluble. To have the correct polarity (cationic) to keep the fibers from sticking to each other, and to improve grooming results. Grooming and fast drying are essential when cleaning rayon fibers. Proper grooming helps maintain the appearance and texture, restoring that “silky” hand of rayon rugs. Fast drying
can reduce the risk of discoloration.

Much of this is important when cleaning other cellulosic fibers like cotton, jute, linen, sisal, etc. It is often wise to clean on the alkaline side to help wash away lignin. And use redox reactions to remove discoloration. Cotton Rug Shampoo is a wise choice for cotton with color bleeding issues as it contains mordants for cotton (and other cellulosic fibers) mordants can set the dyes to prevent color bleeding. Don’t forget Cotton Rug Rinse for more mordants in the rinse cycle to further enhance results. Problem Reducing Rug Shampoo is great for rugs and carpets with cellulosic browning issues.