Box makers buy kraft paper on numbers: burst factor, ring crush (RCT), Cobb and GSM. Meet the numbers and the order repeats. Miss them and the next truck goes to someone else. With most kraft made from recycled fibre, chemistry is what closes the gap.
The strength numbers that matter
- Burst factor – resistance to bursting under pressure.
- RCT / CMT – how well the paper resists crushing in a box stack.
- Tensile – runnability on the corrugator.
- Cobb – water absorption; lower is better for humid storage.
Chemicals that help
Cationic starch
Cationic starch is the first and cheapest strength lever. It bonds fibres and improves retention of fines that also add strength.
Dry strength resin
Where starch alone stops working, dry strength resin adds more fibre bonds – often letting mills cut GSM while keeping burst and RCT.
Surface starch
Oxidised starch at the size press stiffens the surface, improving RCT and printability.
Sizing
Rosin sizing with alum lowers Cobb so boxes stay strong in the monsoon.
Retention, drainage and cleanliness
A retention aid, defoamer and antislime agent keep the system clean, so strength chemicals are not wasted on dirt and air.
Machine clothing matters too
Good pressing improves bonding and density. The right endless press felt and a healthy dryer screen help as much as any chemical.
A practical approach
- Measure current strength on reels and know your target.
- Fix retention and cleanliness first – cheapest gains.
- Optimise starch dose and cooking.
- Add DSR only for the remaining gap.
- Try lowering GSM in small steps once strength is stable.
Use cases
- Kraft paper mills making test liner and fluting.
- Corrugated box makers who want consistent paper.
Need more burst at lower GSM? Share your grade, furnish and strength results – we will suggest a strength programme. Talk to us →