As the proportion of recycled pulp keeps rising, the foam problem rises along with it. The ink, stickies, plastic particles, and residual chemicals carried in by waste paper already contain large amounts of surface-active substances, and with the mechanical agitation during deinking, bleaching, and washing, stock foaming has become something paper mills deal with almost every day. The same defoamer that works well on a virgin pulp line may not perform nearly as well when switched to a recycled pulp line, and that is determined by the complexity of secondary fibers.
Recycled Fiber Foam Is More Stubborn
The stability of foam in recycled pulp comes to a large extent from the invisible impurities in the recovered raw materials. Hot-melt adhesives, pressure-sensitive adhesives, and printing inks are partly broken down during pulping, turning into fine hydrophobic particles that both stabilize foam and readily entangle with the defoamer to consume its activity. As e-commerce packaging drives up the output of containerboard and corrugated base paper, the use of recycled pulp keeps rising, and mills' demands on defoamers have shifted from simply being able to break foam to being able to control it stably and over the long term.
Different Sections Need Different Grades
Foam in the pulping section comes from cooking, washing, and bleaching, where the system is hot and strongly alkaline, so the defoamer must be heat- and alkali-resistant and break foam powerfully; foam in the papermaking section is more related to wet-end chemicals such as retention and drainage aids and sizing agents, where compatibility matters more, to keep oily substances from leaving spots on the sheet or interfering with sizing. Using one product across the whole line often fails to satisfy either end. On recycled pulp lines, particular attention must be paid to secondary foaming at the wet end, because the surface-active substances carried in by recovered fibers make foam recur in the papermaking section.
The Environmental Red Line Keeps Rising
Paper defoamers are shifting from high-residue, hard-to-degrade types toward low-VOC, readily biodegradable types. As wastewater discharge standards and green factory assessments tighten, the impact of defoamers on stock retention, sheet appearance, and effluent COD is being taken into account. Mills increasingly favor products that are both efficient and environmentally friendly, coordinating foam control with cleaner production.
There is one more easily overlooked stage on recycled pulp lines, namely deinking. Flotation deinking itself relies on foam to carry ink particles out, so the foam here must be carefully controlled: too little and the ink is not removed, too much and fiber yield suffers. The choice of defoamer for the deinking stage has to work with the parameters of the flotation process, and it is a completely different matter from the pulping and papermaking sections.
Foam control in recycled pulp papermaking is essentially a long battle against complex impurities. Only by combining defoamer selection with the characteristics of secondary fibers, the differences between sections, and environmental requirements, and by verifying actual performance through pilot trials, can you stabilize machine speed and improve sheet quality while bringing overall costs down. For mills with a high proportion of recycled pulp and volatile raw materials, it is also advisable to set up a foam inspection log that links the foaming situation of each section to defoamer consumption; over time this builds up a set of foam-control experience of your own.