Internal Sizing Agents: AKD vs ASA vs Rosin
TL;DR Internal sizing is dosed into the stock at the wet end to make the whole sheet resist water. Three chemistries dominate, and pH decides most of the choice: rosin is the low-cost acid-system size (pH below 7, set with alum); AKD is the neutral-to-alkaline size (pH 7–9) that gives durable, high-brightness sizing but cures slowly; ASA is the neutral/alkaline size that cures fast, suiting high-speed machines, but must be emulsified on site and used immediately. Pick by your pH system first, then by machine speed and cost.
What internal sizing is
Internal sizing is added to the stock before the sheet forms, so the size is distributed through the body of the paper and controls how liquid penetrates the whole sheet. That is different from surface sizing, which is applied to the dried web at the size press and works on the surface. For the wider picture of all sizing chemistries, see the sizing agents overview.
The practical question this page answers is not "what is sizing" — it is "which internal size do I run". That comes down to three agents.
The one thing that decides it: your pH system
Sizing chemistry follows the machine's pH. Rosin is an acid-type size — it works below pH 7 and is set with alum (aluminium sulphate). AKD and ASA are neutral-to-alkaline sizes (roughly pH 7–9), which is where modern machines run because they can use calcium carbonate filler. If you run alkaline with CaCO₃, rosin is off the table; if you run acid, AKD/ASA lose efficiency. Establish the pH system, and the shortlist writes itself.
How each one works
- Rosin — a natural resin; reacts with fibre via alum under acid pH to form a hydrophobic layer. Cheapest, but less effective as pH rises and it can pull brightness down. Still the workhorse for lower-grade, acid-system grades.
- AKD (alkyl ketene dimer) — bonds to cellulose by esterification for durable, permanent sizing with good strength, brightness and print properties. Its trade-off is a slow cure — sizing can keep developing after the reel — and it needs good retention.
- ASA (alkenyl succinic anhydride) — reacts by anhydride ring-opening, so it cures fast, which suits high-speed machines and gives immediate sizing without hurting brightness. The catch: ASA is unstable in water, so it must be emulsified on site and used at once, or it hydrolyses and deposits.
AKD vs ASA vs rosin, side by side
| Rosin | AKD | ASA | |
|---|---|---|---|
| pH system | Acid (<7) | Neutral–alkaline (7–9) | Neutral–alkaline (7–9) |
| Needs alum | Yes | No (retention aid) | Yes, as promoter/retention |
| Cure speed | Immediate (with alum) | Slow (post-reel) | Fast |
| Stability | Stable | Stable emulsion | Emulsify on site, use at once |
| Brightness | Can reduce | Good | Good |
| Machine speed fit | Low–medium | Medium | High-speed |
| Relative cost | Lowest | Medium | Medium |
| Best for | Acid grades, cost-driven | Durable alkaline sizing, fine paper | High-speed alkaline, packaging/liner |
How to choose
- Acid machine, cost-driven, lower grades → rosin + alum.
- Alkaline with CaCO₃ filler, fine paper, brightness matters → AKD.
- High-speed machine needing immediate sizing → ASA (with on-site emulsification).
- Surface, not bulk, water resistance → that is surface sizing, not an internal size.
Where sizing sits in the wet end
Sizing does not act alone — it competes for charge and retention with your other wet-end additives. AKD in particular needs good retention to hold onto the fibre, and the addition point relative to dry strength and retention chemistry changes the result. Get the sizing chemistry right first, then tune the order.
Watch
Frequently asked questions
What is an internal sizing agent?
An internal sizing agent is added to the stock at the wet end, before the sheet forms, so it is distributed through the body of the paper and controls how liquid penetrates the whole sheet. The three main chemistries are rosin, AKD (alkyl ketene dimer) and ASA (alkenyl succinic anhydride). It differs from surface sizing, which is applied to the dried web at the size press.
What is the difference between AKD and ASA?
Both are neutral-to-alkaline sizes (pH 7–9), but AKD bonds to cellulose by esterification and cures slowly, giving durable, high-brightness sizing that keeps developing after the reel. ASA reacts by anhydride ring-opening and cures fast, which suits high-speed machines, but it is unstable in water and must be emulsified on site and used immediately or it hydrolyses.
When should I use rosin sizing instead of AKD or ASA?
Use rosin when the machine runs an acid system (below pH 7) and it is set with alum. Rosin is the lowest-cost option and is common on lower-grade acid grades. If the machine runs neutral or alkaline with calcium carbonate filler, rosin loses efficiency and AKD or ASA are the right choice.
Does sizing agent choice depend on pH?
Yes — pH is the first thing to fix. Rosin is an acid-type size that works below pH 7 with alum; AKD and ASA are neutral-to-alkaline sizes that work at roughly pH 7–9, which is where machines using calcium carbonate filler operate. Establish the pH system and the shortlist of usable sizing agents follows directly.
Does VCYCLETECH supply internal sizing agents?
Yes. VCYCLETECH manufactures paper sizing chemistries — AKD, ASA and rosin sizing agents — along with wet-end and surface sizing products in China, factory-direct, with a batch Certificate of Analysis on every lot, ISO 9001/14001/45001 certification, free laboratory samples for trials and OEM/ODM service. Email sales@vcycletech.com.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment and process chemicals — paper chemicals, surfactants, biocides, coagulants and flocculants, phosphonates and dispersants — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
- Sizing — Wikipedia
- Alkyl ketene dimer (AKD) — Wikipedia
- Rosin — Wikipedia
- Effects of hydrophobic sizing on paper strength (AKD vs rosin) — BioResources
Related: Surface sizing agents · Sizing agents overview · Wet-end additive order · Dry strength agents · Paper chemicals