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.

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. 

Hand Sheet Making - Paper Testing

Hand sheet making

Hand sheets are created for a variety of uses; to determine the composition of stock/ pulp in a tank, to look at the cleanliness of papermaking stock, to test the strength properties of the pulp etc. There are a variety of ways to calculate the hand sheet weight and volume of stock needed to form a specific hand sheet.

Consistency method

Hand sheets are made to a grammage of 120gsm for testing to achieve this weight the stock volume needed is calculated. The hand sheet weight should be about 2.4g. (this depends on the diameter of the final hand sheet) Firstly the consistency of the stock sample is calculated. Once this is known a simple calculation can be made to work out the stock volume.

  

There will be a tolerance error using this calculation as the consistency of the stock sample can be inaccurate due to pouring the sample/ collecting the sample. A more accurate method can be used.

A good consistency measurement example can be found here; Technical Papermaking Consistency

Test sheet Method

Before making a hand sheet a test sheet is created to determine the amount of stock need to make a specific weight hand sheet. This method is ideal if the consistency is unknown.

First the stock needs to be diluted to around 0.3% consistency in a bucket. Using the same calculation above a volume of stock is calculated for the test sheet. The test sheet is formed and dried. Once the sheet has been dried the test sheet is weighed and recorded.

The weight is used in another calculation to determine how much extra or less stock is needed to achieve the desired weight. This step removes the errors made when creating the 0.3% solution.

 

This calculation will give you the new volume needed to make the hand sheet.

Forming Hand sheets for physical test of pulp

Using the test method as described by Tappi T205 hand sheets can be made from pulp ready for physical testing.

The stock samples are taken. The first step is to identify the amount of stock needed to make a standard 80GSM hand sheet. This can be achieved in two ways, either by conducting a consistency test on the pulp sample, which takes time and not always accurate.

The other method requires making a test sheet before the hand sheets.

Creating a hand sheet

With the hand sheet cylinder down (open) the hand valve is opened to allow water to pass through the wire, gently rubbing the surface allows any remaining fibres to be removed. The wire is now clean.

The cylinder is locked back in place. The valve is opened slightly to allow some water to fill up, next add 500ml of 0.3% stock solution. The water is filled up to 350mm from the wire to the inscribed line.

Insert the perforated stirrer 5 times for about 6 seconds trying not to spin the stirrer and remove the perforated disk from the liquid. Carefully remove the stirrer on the last up movement and wait for 5 seconds allowing entrapped air to leave and the fibres to settle. Fully open the drain and let the water drain under the vacuum from the water leg. The hand sheet will now form on the wire.

Blotting paper is placed on the formed hand sheet as well as a couch plate on top; the couch roll is applied with no extra force apart from the rolls weight. The couch roll is rolled five times across the plate. With the movement likened to opening a book, the disk and blotting paper is removed and placed onto the drying rings or hot plate.

Cleanliness

Creating hand sheet can be a good indication on the cleanliness of the stock; they can be used especially when troubleshooting quality problems on the machine. For example if a screen was passing/ the slots or holes were damaged you can see an increased level of contaminates in the hand sheet of the accepted stock line. It is a good idea to have a base line hand sheet of the process to be used as a comparison when trouble shooting appearance quality issues.