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.

Starch Cooking - Technical Papermaking

Starch cooking techniques

Starch Addition – Solids content and Viscosity

Starch is mixed up and cooked to specification according to two specific parameters, the solids content of the final starch slurry and the final slurry’s viscosity.

The viscosity of the starch slurry increases exponentially as the cooked solids content increases. Even at low solids content the starch can be too thick to use efficiently. Therefore reagent is added (ammonium persulphate); this chemical reduces the viscosity of the slurry and allows for higher solids content. The reagent works by breaking down the long starch glucose chains into smaller chains this in turn reduces the viscosity. To stop the reduction, the starch is cooked again at a higher temperature to burn off the chemical.

a Pond starch press is designed to work between 40 – 70 cP viscosity (60 Deg C Spindle 62). Too high viscosity can cause a number of issues at the size press. Thicker starch slurry will not penetrate the sheet as efficiently. The starch as a result coats the paper and forces itself between the size press rolls and the sheet causing wear on the rubber rolls. Increasing the moisture of the sheet entering the size press will allow more starch to penetrate but at a cost of web strength through and especially after the press.

Thermoconversion of starch 

An older crude method of converting Starch molecule is Thermo-conversion. Once uncooked starch granules are mixed into a slurry. A reactant is added to the starch slurry to begin the cooking process. Ammonium Persulphate is the reaction chemical added to the starch. The starch and the ammonium persulphate is heated to 145 degrees to start the reaction. The ammonium persulphate breaks down the glucose chains reducing the overall viscosity of the starch.

The positive of thermochemical conversion is the robustness of the process. The system can withstand process variations more so than enzymatic conversion. For example changes in PH value or variations of starches being used. Enzymatic conversion has a tight tolerance on its process, if the pH value goes too acidic or alkali the enzyme will not function. Enzymatic conversion works best with a consistent process and limited variations.

Enzymatic conversion of starch

Enzymatic conversion of starch involves using an enzyme within the starch cooking process to break down the starch glucose chains allows for a reduction in the starch viscosity while being able to increase the starch solids. Without pre-treatment cooking, as the starch solids increase the viscosity increases in an exponential manner. The viscosity of the starch at the desired solid content would be unusable at a size press.

The cost benefits of using an enzyme would be being able to use a cheaper un-modified wheat starch rather than a modified starch. It also allows for more control on the degree of conversion, the starch chain length and the solid content can almost be tailored made depending on the enzyme dosing.

The Enzyme is dosed with the starch slurry and heated up to around 80 (75 for pm6) Deg C to kick-start the reaction. As the starch goes through the reaction tubes the starch chains break down. The mixture is the flash-heated with steam to 130 Deg C to stop the reaction. If this step did not occur the starch would break down to glucose sugars which have no strength additives.

Using the Enzyme cooking technique allows for higher solid content at a lower viscosity when compared with Thermoconversion of starch resulting in a higher strength gain in the sheet. 

Enzymatic conversion has much lower power consumption when compared to the thermochemical cooking process. The temperature needed for the cook is much lower (80 Deg C). The cookers use a flash cooker method to kill the enzyme. A flash cooker allows hot steam to kill of the enzyme without having to heat the entire mixture up.

Starch retrogression (Papermakers amylose)

Starch retrogression or “amylose crystallization” is a result of the re-association (chains link back together) of linear amylose or straight-chain fragments and amylopectin branched fragments. This is an issue that can occur within starch cooking systems. Papermaker’s amylose can lead to a number of paper making problems including hard drying, picking, dusting, low porosity, and a weak sheet.

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. 

Paper Machine Water Chemistry - Technical Papermaking

Paper Machine Water Chemistry

Process water is the name given to the back water on the paper machine, water that is reused within the papermaking process.

The backwater is a mixture of chemicals and fines that were not used the first time around (one pass retention). The back water can have detrimental effect ts to the process if the parameters of the water fall outside of specific ranges/ conditions. For example if the conductivity of the back water became too low or outside of normal operating conditions it would mean the retention on the wire is reduced.

Low conductivity (-18mV) shows there is a high volume of anionic trash within the system which when mixes with the cationic starch or polymer, hydrocol will bond with the trash rather than the fibres leading to poor formation on the wire, pick outs and deposits forming around the machinery and a reduction in run ability. 

pH of process water

pH can be described as the single most important aspect of wet end chemistry, this is because most/ all aspects of the chemistry relies on in some aspect the pH.

When the pH of the water increases (becomes more alkaline), the surface charge of the fibres also increases. This will affect the attraction of the retention aids and other cat-ionic substances to the fibres. The most undesired effect would be the substantial increase of bacteria in the system.

Another effect of high pH ius fiber swelling, fiber swelling is useful during refining because the higher surface area and increased flexability of the fiber leads to high de-fibrilation (versus cutting of the fiber). Caustic acid is added to create this effect.

fiber swelling is also used within de-inking plants. swelling of the fibers pre floatation allows the inks and binder to split and break off when the fiber swells this aids in the  bleaching and deinking process leading to high brightness of the finished pulp.

pH can affect quite strongly the dissolving ability of wood components and to changes in the dissolved substances. Increase in pH improves wood components dissolving ability in the water system and thus the amount of anionic particles dissolved and colloidal substances.

Because of the undesirable fractions within water, the water needs to be cleaned at some stage with different kind of methods like for example with disc-filtering and or chemicals. The wet-end of the paper machine contains the highest amount of water and its chemistry has to be controlled by a variety of chemicals like, retention chemicals, fixatives, de-foaming agents and biocides.

A decrease of the pH value leads to deposits on the machine as the precipitation of non-wood materials increases.  A fine balance has to be made; typically a pH of 7 is achieved within the water loop.

Machine Issues caused by change in pH
Increase in pH	Decrease in pH
Level of bacteria in water increases	Deposit precipitation increases – deposits on the machines
Higher amount of anionic trash in the system
Surface charge of fibres increase

Bugs/ Bacteria - Biocides

Paper machines run well when the operating conditions remain constant. Water plays a large part in paper production and any slight disturbances in the water can cause negative effects on the machine.

Bugs, bacteria thrive in water and damp conditions. Bacteria build up within the water system will cause a variety of issues. For example; Slime build up, lowering PH within the water (acidification) and upsetting the chemical balance within the water loops.

Biocides are used to control the bacterial problem. The main goal of the biocides is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.

Papermachine Cleaning Chemicals - Technical Papermaking

Wire conditioner

Wire conditioner is sprayed on to the wires of the fourdrinier machine. The job of the wire conditioner is to coat the wire and prevent stickie’s, glues, pitch and other tacky substances from binding with the wire plugging the holes in the mesh. Blocked parts on the wire will inhibit drainage leading to weight and moisture deviations as well as holes/ weak spots in the sheet. these will cause a lot of issues on the paper machine.

When problems occur on the paper machines wire like marks from stickie’s and hot melts a caustic chemical clean on the headbox can remove the hot melts. Caustic soda is used because it breaks down inorganic materials like chalk, ash and stickie’s.

Felt conditioner

Felt conditioner works in a similar way to the wire conditioner helps prevents stickie’s and pitch from attaching to the felt and blocking the felt. An excessive amount of pitch on the felts not only hinders dewatering but sticks off a small amount of fibers. After time this will build up and cause defects in the sheet, potentially leading to breaks.

Chemical felt cleaning and conditioning

Chemical precipitates found in the press section are inorganic as well as organic. The organic or hydrophobic types include rosin size, wet strength resin, and pitch and hydrocarbon oil. Inorganic precipitates include clay, calcium carbonate.  

Generally speaking, chemicals for cleaning felts are applied using either a continuous or a shutdown cleaning method. Continuous cleaning effectively keeps the felt open during its run rather than relying on the more difficult job of cleaning a plugged felt.

Chemical felt washes are done more infrequently on PM3/6 than on PM4, usually if the machine has been shut for a while a chemical wash is usually done but on crawl speed to allow better penetration of the chemicals in the felt. Using a lower speed allows the chemicals to bed in and reacts with the dirt and contaminates.

 Acid

Acid is used in two ways on the machines, continuously dosing and shock dosing. The acid is used continuously on all of the machines to help keep the felt open, rather than relying on the shock cleaning on a closed felt.

The Shock dosing is done at a high concentration of acid, it is important to ensure the water is running, neat acid will melt the felts.

Acid is used to clean the felts and to neutralise the caustic that has been previously dosed. The acid is dosed through the chemical sprays on the felts. Acid is dosed for ten mins with 5 mins flushing time after. The acid removes the broken down pitch, fines etc. in the felts. The acid is delivered in a higher concentration when shock dosed and more dilution water is added when the acid is used continuously to keep the felts cleaned. The caustic id a alkaline which is on the opposite spectrum to the acid.

Caustic Soda

Caustic is used to treat the felts. The chemical is shock dosed through the chemical sprays onto the felts to break up the pitch, ash, fines and dirt collected in the felts. The caustic is applied for 10 mins on each of the felts. The issue with caustic being used on the felts is the chemical can cause the felt to close up, reducing the efficient of water removal. After the chemical dosing the lines are flushed with fresh water to clean the pipe work and sprays.

 Anti-Scale Chemical

Anti-Scale chemical is important chemical because the papermaking process is water based. Scale is made up of minerals (mainly chalk and limestoneare composed mainly of calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), and calcium sulfate (CaSO₄)) within the water precipitating out and building up within water systems.

Industrial water systems using hard water can experience breakdowns as the scale builds up in pipes, boilers, water tanks etc. To maintain the level of scale within the water systems a chemical is doses at specific points to dissolve/ break down the chalky deposits preventing build up and ultimately costly breakdowns.

The effects of scale

Can restrict the flow through pipes as the internal diameter decreases as the scale builds up in the pipe.

Scale impairs the heat exchange between metals into the water. this reduces the cooling/ heating efficiency and can lead to the metal components over heating. This causes issues with the drying cylinders requiring more steam to dry the paper because not al heat can be transferred through the cylinder if a build up of scale occurs.

Below is a list of all the dosing points in the site. There are problematic areas that scale is likely to build up and can cause catastrophic breakdowns for example. If the dosing to the pump seal water failed the pumps can clog up with scale, pumping would be reduced and the mechanical could seize. 

Bentonite addition - Technical Papermaking

Bentonite (anionic smectite clay)

Bentonite is the name given to the anionic smectite clay material used to improve retention and drainage. The composition of the particles can be described as very thin plates caused by the salts in the clay. This gives a large surface area for the particle to bond with other particles and cationic polymer.

The bentonite has two functions, primarily when use either before or after a cationic polymer it serves as a drainage/ retention aid. When bentonite is used with cationic polymer it can be used to control the level of pitch, tacky materials (commonly referred to as Anionic trash).

Bentonite added down-stream to polymer improves dewatering on the wire. Best achieved when high mass cationic polymer is used has been added so that the stock furnish has a momentary net cationic charge. If the stock has a high level of Anionic trash it makes sense to firstly treat the stock with sufficient cationic polymer.

Bentonite works with the polymer to increase the dewatering capabilities of the paper web, the two chemicals work in unison to achieve this. Bentonite works as a drainage aid allowing the water to be removed more easily from the web. When adjusting the chemicals the set points of both the polymer and the hydrocol must be reduced or increased together, having a high polymer dosage and low hydrocol dosage or vice versa for example can decrease the drainage on the machine and cause poor formation.

Diluted Bentonite is added post screen and works by reforming the fiber flocculation’s that have been broken down  by shear forces going through the primary machine screen. Bentonite brings the smaller flocs together for better formation on the fourdrinier.

Wet end polymer addition - Technical Papermaking

Polymer (cationic polyacrylamides)           

Flocculation, retention, and drainage is affected by the quality of the backwater as well as the polymer characteristics, these characteristics affect the electrokinetic energy between the fibers namely the charge density, size of the particles, the weight of the molecules etc.

                                                                  

A balance has to be made between the flocculation of stock and the drainage of the stock. This balance results in a need for fiber flocculation to be limited but the flocculation of fine particles and retention additives should be maximized. As the fibers and fillers flock together to form bonds the drainage on the wire decreases. The molecular weight of the polymer affects the drainage and flocculation. With higher weight flocks are better formed. To improve the drainage the weight of the molecule has to be reduced. It is very important to choose the right polymer to keep the balance.

The total branching of the polymer affects the flocculation. Branching is the term used to describe the polymers ability to bond with multiple chemicals, fibers, fines etc. with industrial water (Backwater system/ dirty water) it was found that polymer with a high degree of branching and a higher weight had more resistance against shear forces and held overall better bonds. This made a positive effect on the retention and drainage of the stock. The dosing point of the polymer is before the primary screen where it will experience high shear forces. Using a polymer with both of these qualities will benefit the formation/ retention.

  Polymer used is a dry chemical mixed with dilution water and stored in a chemical tank before being dosed. The issue with the liquid polymer is that it can affect the chemical balance in the thick stock system (flocculation/ retention is reduced) a higher dosing rate is needed to achieve the same parameters as the dry polymer.

The Diluted polymer is added pre-Primary screen and used to create flocks of fibres by reducing the negative charge between the fibres. The polymer is made up of Nano particles and chains which bond together fillers and fines to create flocks. Polymer bonds between the individual fibres forming hydrogen bonds between them. The screen breaks these down into smaller flocks/ chains. This aids in the formation and will improve strength. 

Fourdrinier Retention Mechanisms - Technical Papermaking

Mechanisms of retention/ drainage

Retention aids traditionally based on Alum, Alum neutralizes the charge of the paper making furnishes and was seen a "fix all" regarding wet end chemistry problems. 

Modern Retention aids based on PEI single polymers used "bridging" as the dominant mechanism for retention. the first type of polymers were of high molecular weight which brougfht fibers and filler together and formed "bridges". Nowa days new micro particle systems follow a complex flocculation system to improve wire retention.

What is of interest are the flocculation properties of polymers (retention aids) because many components of the stock furnish (sludge, fillers, fines etc) are too small to be mechanically retained on the wire and need to be bound to the larger fibers through flocculation. The ideal scenario would be to restrict fiber to fiber flocculation and encourage the smaller particles and additives to flock to the fibers. This would give the best retention and dewatering of the sheet.

Bellow is a table that describes the elements of papermaking that will affect the retention on a fourdrinier machine, they can be catagorised into, Pulp conditions, Wire conditions and the additives added to the stock/ furnish

Stock Factors	Conditions of Wire	Additives
pH	Sheet grammage	Types and amounts of fillers
Consistency	Sheet formation	Shape and density of mineral particles
Temperature	Fabric characteristics	Types and amounts of other additives
Fiber characteristics	Type of dewatering elements	Order of addition
Degree of system closure	Machine speed	Ionic balance
	Shake (if used) – old technique used on high quality paper machines	Level of anionic trash

Improving retention on the wire has many benifits, primarly cost. Papermachines producing News print have a retention of about 50%, increasing this retention to 55% for example will reduce the amount of primary stock needed to an extra 45%. for example at a retention of 50% producing 25t/hr throughput the mass of stock through the headbox needs to be 50t/hr, by increasing the retention to 55% the mass through the head box is reduced to 45T/hr

Colloid Chemistry - Technical Papermaking

Colloid chemistry

The word "colloids" (used in science) describes materials that have at least one dimension that is smaller than 1 micro-meters. The word colloid does not give any indication on the chemical makeup of the particle.

Almost everything the papermakers deal with can be considered to be colloidal. Although fibers are larger than the classical definition, the fiber surface is highly porous, and micro fibrils of colloidal dimensions extend out into solution from the surface of a refined cellulose fiber.

Other colloidal particles common in papermaking furnish include fiber fines, filler particles, sizing emulsion particles, and retention aid molecules (for example; molecules so big that they no longer behave like regular molecules – high molecular weight polymers).

The average end-to-end distance of a retention aid polymer (500 nm) is much larger than the size of a typical colloidal particle (2 to 5 nm ).

When papermakers refer to colloids, they usually are most interested in the colloidal organic materials, including fatty acids, lignin by-products, and oxidized hemicellulose. These are often called "DSC" for "dissolved and colloidal materials," or "anionic trash."

Deposits form on papermaking equipment due to the “thermodynamic instability” of many materials suspended in water (oils, pitch, hot melts etc.). We can combat this by getting those materials to deposit/ bind onto fibers, thereby keeping their concentration low in the liquid, ergo less deposits on the machine.

Retention of colloidal materials is best achieved by a combination of coagulation (treatment to neutralize charges (conductivity), causing the particles to come out of the suspension) and flocculation (treatment with polyelectrolytes (high molecular polymers) so large that they can bridge between the surfaces).