Plasticizer Migration Inhibits Stain Penetration

Question:
I recently stained a 5-year-old concrete floor. The owner of the building had rubber mats down on the floor near the front entrance for 4 years. After staining, I noticed the stain was significantly lighter in color in exactly the same spot where the rubber mats had been. What caused this, and how can it be fixed?

Answer:
The cause is most likely plasticizer migration. Plasticizers are chemicals added to plastics, vinyl, and rubber to make them softer and more flexible. The softer or more flexible the material, the more plasticizer it contains. Since rubber mats are fairly flexible, they contain a lot of plasticizer. Over time, the plasticizers migrate out of the rubber or plastic and are absorbed into surrounding materials (in this case, your concrete floor). Heat and pressure can speed the process. Since concrete is porous like a sponge, plasticizers migrate into the pores easily, filling them with small amounts of plastic that inhibit the penetration of water or stains. That’s why on this project the stain was lighter only in those areas where the mats had been laying for years.

Unfortunately, removing plasticizers from concrete is very difficult. A good degreaser or stripper can be used if the problem is detected prior to staining. If stain has already been applied, a dye or color wash can be put down to help darken the color in the affected areas. If the area has already been sealed, try applying a tinted sealer. Be sure to soften the edges of the tinted sealer to avoid turning a lighter color problem into a darker color problem.