What is plasticizer migration and how can it affect your finished product? Plasticizer migration has been mentioned in previous blogs, however a better understanding of what plasticizers are, why they are used, how they can migrate, and what that means to you will help you understand what is happening if the problem arises with the finishes you use.
Plasticizers are resins that provide flexibility to finishes. These resins can be classified as primary or secondary plasticizers. We will focus on primary plasticizers as they are the type that leads to plasticizer migration from a coating. Primary plasticizers are classified as resins since they do not evaporate. They are left in the finish after the solvents evaporate. Primary plasticizers do have some characteristics of solvents since they are liquid and will dissolve some other resins. While they are typically classified as a resin, they are in-between resins and solvents.
Their characteristics between resins and solvents are what make them very useful in some coatings. Many lacquers make good use of their special qualities. Many lacquers would be too hard and brittle for use on wood since wood is a “breathing” substrate. Wood absorbs and releases humidity over time, making the wood expand and contract. A coating must be flexible enough to withstand this expansion and contraction repeatedly. Without primary plasticizers, many lacquer-based coatings would crack quickly. The special nature of wood has allowed primary plasticizers to be used to improve the flexibility of lacquers, so the overall durability is improved.
Primary plasticizers are commonly used in lacquers and hybrids of lacquers including pre and post-catalyzed lacquers. Typically, lacquers are the most susceptible to plasticizer migration, and in a descending order pre-catalyzed lacquer then post-catalyzed lacquer. They are seldom used or non-existent in other types of coatings. Many catalyzed varnishes do not need primary plasticizers and are inherently more resistant to plasticizer migration from an outside source.
Primary plasticizers can migrate from a coating because they are liquids and just entwined in the other resins in the coating. Over time, the plasticizers can move out of the finish. This typically takes many years to happen and over the course of decades the finishes that use primary plasticizers can become brittle and crack or print, and even sometimes “melt” into other items the finish is in contact with for a prolonged period.
Plasticizer migration can also occur from an outside source into the coating. Many plastic materials use plasticizers to improve flexibility. When these products are in contact with the finish the plasticizer can migrate from the plastic to the coating causing a printing or in a worse case they “melt” together. Different plastic or rubbery materials may contain different amounts of primary plasticizers and have very different results when in contact with the finish. Also, there are many different primary plasticizers. Each of them will have a different degree of potential plasticizer migration.
It is important to check for plasticizer migration with your finish. A common test to determine how resistant the finish is to plasticizer migration from an outside source is ASTM D2199 - 03(2013). This test method uses a piece of vinyl fabric under pressure to determine the finishes’ ability to resist the migration of the plasticizer from the vinyl into the coating.
You should distinguish plasticizer migration from residual solvent in the coating causing a print or “melt”. Residual solvent can cause the same printing problems that primary plasticizers can cause. Therefore, it is important to make sure that each coat of finish applied is thoroughly dried before putting another coat of finish on or putting other objects in contact with the finish.
The resistance to plasticizer migration can usually be improved by moving to a catalyzed coating, especially a catalyzed varnish that does not use plasticizers. Also, making sure that anything that comes in contact with the finish doesn’t have plasticizers, or very little of them if possible.