Cationic Starch - Technical paper making

Cat-ionic Starch - Technical paper making

Starch is used to increase the strength properties of paper and improve the sizing by means of coating the sheet with starch slurry or adding starch at the wet end of the machine.

Starch much like cellulose is a chain made up of glucose molecules, bridged together by oxygen atoms called “glycosides”. This makes starch very soluble in water but also easily digestible by bacteria.

There are two types of starch used in paper making, Cat-ionic wet end starch, and oxidized starch.

Wetend Starch Cookers (Cationic maize starch)

Wetend starch is modified using a derivatized quaternary ammonium compound to add a cat-ionic charge to the starch. This allows the starch to naturally bond with the anionic fibers, Fillers and fines (all anionic).

In a way cat-ionic starch can help improve drainage as well as improving dry strength properties in the sheet.

Cat-ionic starch can be added to different locations in the machine approach flow or Thick stock loop. Dosing in different locations can influence the advantages you get from cat-ionic starch.

Adding starch to the Pulp storage chest of the machine acts like a Fixative. The cat-ionic starch binds to the anionic trash in the pulp, cleaning the water loop, improving Wetend retention and in some cases acts like a dry strength agent.

For the strength impact cat-ionic starch should be dosed close to the thick stock pump. The short dwell time prevent the starch from attracting only the anionic trash. The starch can form bridges between the fibres fillers and fines to create a stronger better formation sheet as well as  improving formation. 

Uncooked cat-ionic starch can be added between the layers of a multi ply machine. The starch is sandwiched between the ply’s. Its not until the sheet is passed through the drying section that the starch cooks and acts like an inter layer glue. This can be important if the paper has a specific ply bond strength parameter that needs to be adhered to.

Size Press's - Papermaking process overview

Papermaking process overview

Size press section

The role of the size press in a paper machine is to apply a thin layer of starch and or other chemicals to the paper web.

A typical starch press comprises two rolls with a starch application system to coat the rolls.

Starch addition can improve several paper properties for example;

• reduce dusting of the sheet or sealing lose material to the sheet
• increase in paper stiffness
• increase in burst strength property
• increase in short span compression (SCT)
• reduction in air permeability. 

Starch or size being added at the press has 100% retention. This is higher than applying the chemicals at the wet end. The retention rate would be roughly the same as the fiber retention (80%). More starch can be added at the size press compared to applying it using cationic starch.

Although not all the starch is absorbed in the pond/ addition point, the excess starch/ chemicals are recirculated back to the starch storage tank by the machine To be reused. 

There are typically two different size press designs, Pond size press (also known as a Size press) and Sym sizers. The basic principle is the same the only difference is the application to the sheet.

Sym sizers

These are cleaner than the pond method and are more suited for certain types of paper and certain strength properties that want to be achieved. Sym sizers consist of two rolls with the paper being fed between them. Both sets of rolls need to run at a matching speed to prevent skidding and possibly breaking the paper.

Applicators on either side of the rolls apply starch to a thin metal rod with a tapered edge. The roll then applied a specified amount of start to the larger roll which presses the starch onto the paper as it passes through. This creates a film of starch on the paper. The other advantage of this than the pond method would be the sheet doesn’t get as saturated with starch and takes less drying time after the size press (20% moisture leaving)

This method is used on lightweight grades of paper and paper that need a better burst strength and tensile strength (paper liners)

Pond Sizers

pond style size press is an older designed surface size coaters. Similar to the Sym sizers but instead of an applicator adding a thin layer to the role the starch is filled in the spaces between the roll and where the paper enters the nip.
The paper will become more saturated but greater strength can be achieved. Pond Size press allows starch to further penetrate the sheet compared to a Sym sizer This also means the paper is wetter when leaving the Size press (30% moisture)

Size Press Advantages to paper machine

Dusting reduction

Dust/ lint is produced from the paper when small particles become loose from the sheet and “picked out” from the hot cylinders. Sometimes dust is transferred onto the paper surface from rolls or fabrics, so it makes sense to inspect the surfaces in the wet-press, dryer, and calendar sections of the paper machine. Tacky materials in the wet-press area or on the early dryer cans sometimes pull fibers and other solid materials from the sheet, and these materials can appear later as dust.

Starch is a tacky substance and when applied to the paper web can be used to “stick down” loose materials in the sheet. On the other side of this depending on the dryers set up the starch can aid in pick outs from making the web tacky.