Recycled Board, Rub-Off, and a Food-Contact Declaration
A converter in Noida is moving a dry-biscuit carton from 300 gsm virgin kraft board to a 100% recycled uncoated board of the same caliper. The job is 1/1 black text plus a red spot, sheet-fed offset, 60,000 cartons. On the first 2,000 sheets the red prints unevenly, the black rubs off after 24 hours and the tape test pulls both fibre and ink, and the board shows dark speckles from recycled fibre. The press uses a conventional oxidative sheet-fed ink, a fountain solution made with hard tap water at about 320 ppm CaCO3, and no primer. The customer also asks for a declaration that the ink is suitable for direct food contact. The shop has a corona treater available on another line and a lab that can run tape adhesion, rub and migration screening.
1. Diagnose the likely causes of the poor ink adhesion and uneven red. Distinguish between board, fountain solution, ink and press causes, and say why corona treatment is probably not the answer.
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The board change is the first suspect. Recycled uncoated board has more variable surface sizing, more dust and a weaker surface than virgin kraft, so ink vehicle can be absorbed unevenly, leaving pigment poorly anchored and producing rub-off. The dark speckles are expected from recycled fibre and are not necessarily a printing fault, but they make density reading and visual inspection harder. The tape test pulling fibre as well as ink suggests the test may be too aggressive for an uncoated surface; nonetheless, ink removal means anchorage is inadequate.
The hard tap water at 320 ppm is a second cause. Calcium reacts with fountain solution components, raises conductivity and can emulsify the oxidative ink, giving unstable ink transfer and uneven red. Check pH and conductivity, and switch to RO or deionised water with a fountain concentrate designed for hard water. The red may also be a ink/water balance or duct/roller problem, so compare a control print on the old board before blaming the board alone.
Corona treatment is for low-surface-energy films such as PE or PP, not for paperboard. Using it here would add cost without solving absorption or chemistry, so it is the wrong control.
2. Recommend ink and process changes for the recycled board and the food-contact requirement, with the tests that should support each change.
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For adhesion and rub resistance, move away from a conventional oxidative ink toward an ink matched to absorbent recycled board, or add a suitable primer or hold-out coating if the job allows. Reduce ink film to the minimum that reaches the approved density; excessive black makes rub-off worse. Control fountain solution with RO water, measure pH and conductivity, and keep ink/water balance stable. Check blanket pressure and roller settings, and allow proper oxidation before stacking or converting. If a primer is used, confirm it does not create a food-contact problem.
For food contact, do not treat a normal offset ink as direct-contact safe. The printed surface is not a functional barrier. Specify low-migration, mineral-oil-free inks with a documented composition, and obtain a migration test report from the lab. The tests should include tape adhesion and rub resistance for the print, plus migration screening for the ink system. If the board is uncoated and the print is on the food side, a functional barrier or a different ink system is required. Document the board grade, ink batch, fountain chemistry and test results on the job record so the customer's declaration is traceable.
3. The customer asks for a declaration that the ink is 'suitable for direct food contact'. What can you legitimately claim, and what would you propose instead?
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You cannot legitimately claim direct food-contact suitability from the ink supplier's general brochure or from a print that has only passed a rub test. Direct contact depends on the ink formulation, the substrate, the printed area, the food type, time and temperature, and the possible set-off from the printed outer surface to the food-contact surface. A statement should be based on the ink maker's specific food-contact declaration, the board's compliance, and migration testing of the finished article under the intended conditions.
Propose a compliant route: use low-migration inks and, if needed, a functional barrier such as a PE or PP liner or an approved lacquer on the food-contact side; keep the print on the non-contact side; and have the finished pack tested for migration. Provide the customer with a statement of composition and the test report, not a blanket 'food grade' certificate. If the customer refuses a barrier and demands direct contact, decline the declaration and offer a re-designed pack. This protects the converter legally and avoids a false claim on 60,000 cartons.