Printing Inks on Paper Cups: Migration Limits and the 0.01 mg/kg Line
Paper cup printing is on the outside, so it feels safe. The problem is the stack: 50 to 100 cups nested together, printed surface against the inside of the cup below, for weeks in a warehouse and months on a shelf. Ink components can transfer by contact and then migrate into the beverage. The regulatory line for a non-evaluated substance is 0.01 mg per kilogram of food, and it is closer than most print buyers assume.
Where the Ink Actually Goes
A cup stack applies 20 to 60 grams of load per contact point and the surfaces rub during transport. Offset and flexo inks transfer 0.5 to 3 percent of their mass to a contacting surface under that load, and the inside coating of the cup below is the receiving surface. From there, components diffuse through the 18 to 25 gsm PE layer, slowly, but over 6 months at 25 degrees Celsius they arrive.
Set-off is the technical term, and it is measured by pressing a printed sample against an unprinted one at 2 kilograms for 24 hours, then analysing the unprinted surface.
The Migration Arithmetic
Assume an ink film of 2 grams per square metre, a set-off of 1 percent, and a contact area of 30 square centimetres per cup. That is 0.6 milligrams of ink on the food-contact surface. In a 250 ml cup, a substance at 5 percent of the ink mass gives 0.03 mg per cup, or 0.12 mg per kilogram of beverage, which is 12 times the 0.01 mg per kilogram line.
The arithmetic shows why "food-grade ink" is not sufficient. The number depends on film weight, set-off, contact area and the fraction that migrates, and each of those can be controlled.
Mineral Oil Saturation
Recycled paperboard and conventional offset inks both carry mineral oil. Measured MOSH and MOAH levels in printed food packaging commonly run 0.5 to 2.0 mg per kilogram of food after storage, against a German draft limit of 0.5 mg per kilogram for MOSH C20 to C35. A cup with a recycled board wall and a mineral-oil-based ink is a predictable failure.
Switch to virgin board for the food-contact layer and low-migration inks with mineral oil below 0.1 percent. Both cost more: 3 to 6 percent on board, 5 to 10 percent on ink, and both are cheaper than a recall.
Photoinitiators and UV Ink
UV-cured inks give excellent rub resistance, but residual photoinitiators migrate readily. The EU limit for a specific photoinitiator in food is 0.01 mg per kilogram, and a poorly cured UV ink can leave 20 to 100 milligrams per square metre on the surface, enough to fail by two orders of magnitude.
If UV ink is used, specify low-migration photoinitiators, verify cure with a thumb twist test plus an FTIR conversion check above 90 percent, and run a migration test on the finished cup, not on a printed sheet.
Testing the Finished Article
Use Tenax as the food simulant for dry contact, 2 hours at 70 degrees Celsius, then analyse by GC-MS for volatile migrants and LC-MS for non-volatiles. For a beverage cup, also run 2 hours at 70 degrees Celsius with 50 percent ethanol to simulate hot coffee with a little alcohol, and 3 percent acetic acid for acidic drinks.
Report each substance against 0.01 mg per kilogram, and report the total migration against 60 mg per kilogram. A supplier who reports only "passes FDA" has not answered the question.
Ink Types Ranked for Cups
Water-based inks with a low-migration formulation are the default for hot beverage cups: 1 to 3 percent set-off, no photoinitiators, and a rub resistance of 30 to 50 cycles. Solvent-based inks give better gloss but leave residual solvent, typically 2 to 10 milligrams per square metre, and that is a sensory problem as well as a regulatory one.
UV inks are best reserved for the outer surface of a double-wall cup, where the food-contact path is blocked by an air gap and a second wall.
What to Put in the Specification
Specify: low-migration ink, mineral oil below 0.1 percent, no benzophenone or ITX, set-off below 0.5 percent at 2 kilograms for 24 hours, and a migration test report on the finished cup for the intended beverage.
Add a stack storage condition: 40 degrees Celsius for 28 days, which is the accelerated equivalent of 12 months at 23 degrees Celsius. If the cup passes that, the shelf life claim is defensible; if not, the ink or the board has to change.