A paper machine runs on fibre and water, but what decides whether the sheet comes out strong, clean and printable is the small list of chemicals added along the way. Most mills use eight to twelve of them. Each has one job, and when one is wrong the whole machine feels it.
This guide walks through the main ones in the order they usually meet the stock, with notes on what to watch. It is written for production heads and purchase teams, not chemists.
1. Starches – the backbone of strength
Starch is the most used additive after fillers. Cationic starch goes in at the wet end, where its positive charge lets it stick to fibres and fines. It improves dry strength and helps hold filler in the sheet. Oxidised starch and maize starch are mainly used at the size press for surface strength and to reduce linting.
Benefit: stronger sheet without adding more fibre.
2. Sizing agents – controlling water pickup
Writing paper must take ink without feathering; liner board must not soak up water. Fortified rosin with alum is the traditional choice for acidic systems, while surface sizing with surface size chemicals works on the face of the sheet.
Benefit: sharper print, better water resistance, fewer complaints from converters.
3. Retention and drainage aids
These polymers hold fines and filler in the sheet and help water leave faster on the wire. A good retention and drainage aid can raise machine speed, lower steam use and cut solids in the white water.
Benefit: less waste to the ETP and more output from the same furnish.
4. Strength resins
Dry strength resin helps mills using recycled fibre recover burst and tensile strength. Wet strength resins such as melamine formaldehyde are used where paper must hold up when wet – tissue, napkins and some packaging.
5. Defoamers and biocides
Foam traps air in the sheet and causes pinholes. Slime builds up in tanks and breaks off as spots and breaks. A suitable paper machine defoamer and an antislime agent keep the system clean and running.
6. Fillers and coating pigments
China clay, calcite powder and GCC improve opacity and smoothness and replace part of the costly fibre. Coating grades such as Kaolite-90 give board its white, printable top.
7. Dyes and whitening agents
Shade must stay the same reel after reel. Direct and basic dyes colour the stock, while an optical whitening agent lifts brightness on white grades.
How to tell if your chemical programme is working
- White water is clear and solids are stable.
- Breaks and felt filling are going down, not up.
- Ash in the sheet stays close to target.
- Steam per tonne is steady or falling.
- Customers are not returning reels for shade or print issues.
If two or more of these drift, look at the chemistry before you look at the machine.
A note on buying
Most problems we see come from changing suppliers or grades without a trial. Always run new chemicals on one machine first, keep samples of every lot, and agree packing (usually 50 Kg bags, or carboys for liquids) before the first order.
Need help choosing the right grades? Tell us your paper grade, furnish and machine speed – our team will suggest a programme and send a quotation. Talk to us →