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. 

Approach Flow systems - Papermaking Process Overview

Approach flow systems - Basic overview of the Process

Approach flow systems describe the part of the papermaking process between the final stock preparation storage tanks and the headbox of the paper machine.

All paper machines will have an approach flow system, the only difference will be the configuration of that system.

The primary objective of the approach flow system is to prepare the stock for entering the headbox, of the paper machine the tasks are as follows;

• Dilute the stock down to headbox consistency (0.5 - 1.5%)
• Provide protection for the headbox
• Reduce flakes and contaminants from the stock (flakes, dirt, air) entering the Headbox
• Maintain a uniform mixture of pulp within the approach flow

Bellow is a simple diagram depicting the Approach flow leading to a Fourdrinier Machine. 

Notice the approach flow of a paper machine can be divided into two areas, Thick stock, and Thin stock system.

Thickstock

Thick stock describes the process where the stock consistency is greater than 1% and Thin stock denotes the process that uses 1% and below. The partition between the two system is the Fan pump where thick stock (3-5%) is diluted to >1%.

The thick stock Process typically starts at the storage chest from the stock preparation plant. depending on the process this pulp can be mixed with other variations of pulp to create more desirable paper properties. One method is to "blend" the fiber streams together using a pipe. the fiber streams all converge on a pipe with a dilution water addition point at one end. this pipe feeds into a blending chest where the pulp is again sufficiently mixed using agitators. in an ideal scenario, the pulp needs to be mixed uni formally to prevent paper properties variations on the machine, for example, Strength, Shade etc.

The thick stock flows into the blend chest via the blending tube with the other fiber streams. Typically this is where machine Broke is added to the primary fiber. The stock then passes through to the machine chest. Here there is a consistency dilution valve that controls the consistency entering the machine chest. The operator typically runs the consistency at around 3-4.5%. 

A constant overflow is created in the machine chest to the blend chest, this allows the pulp in the machine chest to be constantly mixed and prevent a "crust" from forming within the tank which can cause flake and lump issues on the machine.

The consistency can be increased when on heavier grades to allow the fan pump speed to slow down. At higher weights, the fan pump can ramp close to its max output flow speed. This speed is controlled by the dry stock control. The theory is that as the consistency increases entering the fan pump less stock is needed to match the desired weight because there is more fiber present in the same volume of water.

The drawback with altering the dry stock control say to 5% at the machine chest is that stock can become flocky entering the headbox and the formation of the sheet can worsen compared with  4.5% consistency. The quality issues are not major but can reduce the quality reading by a few points which can mean a reel failing. By opening the slice the formation can improve on the wire but this leads to its own set of issues on heavier weight grades. 

Thin stock System

Diagram of a Board machine 3 ply thin stock system

The thin stock system starts at the machine chest. The thick stock from the machine chest flows to the fan inlet of the fan pump (m-70). This is where the thick stock meets the dilution water from the primary silo. The two mix in the fan pump and pump forward to the primary screen. The stock at this moment is about 1% fiber content. The accepted fiber from the primary screen pump forward to the PE (Pulse elimination) tank and then onto the headbox. Rejects from the primary screen go through a 2 stage screen system to remove the final amount of rejects.

Monitoring reject flows helps determine if a blockage has occurred on the screen on the reject pipework. The differential pressure between the inlet of the screen and the outlet should be different but not too high, If the DP is too high it will indicate an issue with the screens.

Air in stock and in the thin stock system can cause forming issues with the paper web once it hits the wire. Uneven fiber speed across the wire, thin patches and holes will form. Air is problematic in stock due to the rapid expansion from leaving the headbox. 

Air is removed in a number of ways from the thin stock supply. Each of the pressure screens has an air bleed line from the top center of screen casing. The air is released as it builds up and flows to the open top rejects tank.

De-foamer chemical is added at any point where white water is collected from the wire drainage. drainage from the wire is where the majority of air gets trapped. 

Stone Ground wood Pulp - Mechanical Pulp Production

Mechanical Wood Pulping

All mechanical wood pulping makes no attempt to remove an impurity known as lignin from the fibers. A product made from mechanical wood pulp will not be durable and will degrade rapidly especially in the sunlight.

Stone ground wood Pulp

This is the most commonly used method mainly due to the simplicity of the machine. one large grindstone which breaks up the descending log into individual fibers and fiber bundles.

Mechanical pulp is produced from fiber defibrillations from a stone grindstone. the temperature around of the pulp is around 80 to 125 deg, locally at the stone where the fibers are being broken down the temperatures can reach up to 170 deg.

this high temperature is an advantage because the lignin in the logs binding the fibers together starts to become more malleable allowing for less damage to the fibers, more intact fibers are produced. 

Thermo-ground wood pulp

The thermo-ground wood mechanical pulp is an advance on the stone groundwood process where the logs are treated with hot water. the logs sit above the stone grinder in a hot water bath. the hot water is used to make the lignin softer. when the lignin becomes soft the break down of the logs into fibers becomes easier leading to less energy being needed and higher quality pulp.
Temperature around 80 to 95 can be generated locally at the grindstone. The water temperature is usually around 80 deg C. 

Pressurized groundwood

This is another advance on the original stone groundwood pulp. this is mechanical pulp production where the grinding takes place under compresses air pressure usually about 1 bar above atmospheric pressure. the water pressure is there for greater than 95 deg C (due to high-pressure atmosphere raises the boiling point of water). this allows for higher grinding temperatures without steam flashing. the high temperature promotes the softening of lignin. this improves fiber separation and reduces the specific energy consumption of the pulp.
Grindstone temp is about 124 – 130 deg.

Basic principles – Stone groundwood process

The fibers are torn from the logs and washed from the stone by means of spray showers. The supply of fibers and fiber fragments go through a screening process to remove the large particles these are known as shives. The stock is then thickened.

At grinding the logs are heated up from the frictions of the grind stone caused by pressing the logs against the stone the wooden structure is softened, the lignin becomes more malleable when heated. The bonding between the fibers will be less. The shearing forces between the stone surface and the wood mean the fibers are torn out.

This process uses very little or no chemicals but is very energy intensive (1200 – 2100kw/ dry ton)

Under the pulp stone, a basin collects the groundwood pulp

The pulp is screened after this process to remove shives (over sized particles) and then thickened

Varieties of Stone groundwood pulp

Fine groundwood pulp

·         Low moisture content in the logs

·         Gentle pressing of the log in the grinding zone (low speed of the chain drives) leads to a lower load consumption

·         Grit size – small dia grit size

·         Less sharpening of the grindstone with small dia grit on the abrasive layer

·         Less sharpening of the grindstone

·         Higher consistency in the basin

·         High specific energy consumption
As a result, the shopper riegler value is high, low dewatering ability

Coarse groundwood pulp

·         Fresh wood high moisture content

·         Intense pressing of the log in the grinding zone – increased the load on grindstone

·         Grit size :usage of grind stone with large diameter grit

·         Sharp stone surface

·         Low consistency in the basin

·         Low specific energy consumption

·         Result in good dewatering ability – high shopper riegler

Consistency Measurement - Technical Papermaking

Consistency Measurement


Testing the consistency of a stock is the same as consistency of water solution. Consistency is the term given to the amount of solid matter within a body of liquid. The higher the consistency the more solid matter is present within the same volume of liquid. Consistency is measured in percentage; it’s a percentage/ ratio of solid matter to water in a certain volume.


To find the consistency we need to test the amount of dry content within a set volume of water. For example if we wanted the consistency of 100ml of water, and tested 25 gram of dry solids the consistency would be about 25%.

To capture the solid content within a solution a filter paper is used. Firstly the filter paper is weighed at 0% moisture (previously oven dried) and recorded. The stock sample is weighed (1g = 1ml) and recorded. The weighed stock sample is filtered through a vacuum and rinsed making sure all of the solids from the sample are on the sheet. The filtered solids and the filter paper go into the oven to remove all of the remaining moisture.

Once the sample leaves the oven it is weighed and recorded. This formula is used to calculate the consistency below.

Consistency=  (Mass of Solids)/(Volume of Liquid)

We have to calculate the mass of the solids captured on the filter paper, this is calculated below;

Mass of Solids=Total dry weight-Filter paper weight

Here are a list of results that I took from measuring the consistencies of a pulp preparation plant's stock flows through two fractionators and Long fiber screens.