Every summer, deinked pulp mills in the northern hemisphere watch brightness fall without any obvious change in raw material or process settings. The cause often sits in the waste paper itself, in the way printing ink ages before the bales ever reach the pulper. Papermakers know this pattern as the summer ink effect.
What Drives the Summer Ink Effect
Printing ink is a mix of pigment and binder, with fillers and additives added for flow, drying and rub resistance. Heat changes that mixture. Once ambient temperature climbs, the binder migrates and cures differently on the sheet, so the ink film that arrives at the recycling mill is no longer the film that left the press.
Mineral Oil and Vegetable Oil Binders Age in Different Ways
Black ink for newsprint usually relies on mineral oil as its binder, while colored inks more often use vegetable oil. The two behave very differently over time. In black ink, the mineral oil sits at the paper surface at first. Over four to six months of aging, almost all of it migrates into the sheet. Mineral oil takes up oxygen poorly, so the small amount of binder left on the surface crosslinks very little. It forms no stiff network, only weak and brittle structures that crumble into a mass of very small ink particles.
Vegetable oil ink follows another path. Its binder also starts at the surface, but it absorbs oxygen readily. After the same aging period, part of the oil has moved into the paper while much of it stays on the surface, still bound to pigment. The unsaturated double bonds in vegetable oils allow crosslinked, rigid structures to form, and those structures cling to the paper fibers.
Why Old Newsprint Behaves Differently in the Pulper
Those two aging paths decide what happens during repulping. When old newspapers printed with mineral oil ink and with vegetable oil ink are disintegrated in the same pulper, the mineral oil ink fragments more easily. The result is a higher ERIC value, the effective residual ink concentration measured after disintegration. After thorough washing, the stock also carries a higher concentration of ink particles that deposit back onto the fibers and cannot be removed again. That irreversible redeposition is what pulls brightness down.
Mill Records Show the Same Seasonal Pattern
The pattern repeats across regions. Deinked pulp brightness in Canada reaches its yearly low from June to August. Mills in the northern United States record the lowest values in July and August, about 1.4 percent below the normal level. Our own waste paper deinking line follows the same curve, with the weakest brightness in July and August. Low point qualification rates were recorded at 46.2 percent in 1998, 71.2 percent in 1999, 2.6 percent in 2000 and 11.3 percent in 2001. The figures move from year to year, yet the seasonal shape does not.
Practical Reading for Deinking Operations
The summer ink effect shows up in every country and region where waste paper is deinked, so it belongs in normal planning rather than in the list of surprises. Mills that expect the July and August dip can adjust furnish blends, track ERIC values more closely and tighten washing control before brightness drifts. Knowing how mineral oil and vegetable oil binders age gives the process team a sound starting point for the changes that matter most.