Controlling Plasticizer Migration

Summer has arrived, and as the days get warmer, the technical phone calls increase with questions concerning the performance of UV inks on vinyl banner and static cling substrates.
It is not uncommon to hear that the inks "have changed" and the same ink that used to be glossy and dry now appears matte and tacky or wet. The problem is not that the ink has changed, it's that the warmer weather conditions are causing changes to the substrates.
Most plastic materials contain plasticizers to make them flexible. While these plasticizers make the material softer and more flexible, they also can cause screen printers many difficulties.
Plasticizers migrate to the surface of the material and change the surface which the ink must adhere to. When the material is exposed to higher temperatures, the migration process occurs more quickly. Since static cling and vinyl banner materials contain high levels of plasticizers, storing them in a hot warehouse, transporting in a hot truck, or curing through a hot unit will quickly cause plasticizers to migrate to the substrate surface, potentially causing ink adhesion and/or blocking problems.
Some vinyl banner manufacturers now offer top-coated vinyl banner materials which prevent any plasticizer from contaminating the surface of the material. This top-coating eliminates the plasticizer migration problems, but it adds cost and may change the UV ink chemistry needed to adhere to the banner.
Printers should weigh the higher material costs against the potential production down time in making their substrate decision.
Another solution to the problems of plasticizer migration may be in determining the best UV ink to use, given your choice of vinyl material, your shop environment, and your specific processing equipment. If the ink film is not flexible enough, it can simply flake off when installed or handled. Intentionally undercuring a harder ink to try to keep it flexible is definitely not the answer as it can cause blocking and/or adhesion problems. A less flexible ink may appear flexible today when under cured, but will post-cure and embrittle with time and cause problems later. Select an ink which can be thoroughly cured and provides the required level of flexibility.
Curing speed is also an issue to consider.
Typically, the more flexible an ink system, the more curing energy is required to achieve a proper cure. When the UV energy is increased through higher power settings or slower belt speeds, more heat is generated which will accelerate plasticizer migration and possible problems with adhesion or blocking. High levels of heat will also cause distortion problems with substrates, especially with static cling. The substrate should be processed as cool as possible and still achieve proper cure, so your decision on which ink system to use will in part depend on the type and condition of your curing equipment.
The effects of plasticizer migration on a printed ink film will depend on the ink system being used. The more flexible/slower ink systems are more susceptible to the plasticizer rising through the soft ink film during the curing process, which can cause a greasy or matte surface of the ink and can contribute to blocking problems. On the other hand, an ink which is faster curing can actually help to block some of this plasticizer migration to the surface of the ink film.
The choice of inks depends on your ability to control the issues which cause plasticizer migration. If you are using vinyl which you and your supplier store and transport under controlled conditions, and you have the equipment to cure a slower ink system, then you can look to the most flexible ink system available. If your material is not so well controlled, you may need to compromise somewhat on ink flexibility.
To allow you to use the best product for your situation, Sericol offers a number of UV and Solvent based inks which are very flexible and adhere well to vinyl materials. To make the best decision, it is important to understand how substrates can change and how to control the variables which cause these changes. As always, Sericol offers the technical assistance to help you identify key issues in making your ink decisions.