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.

Cat-ionic Starch - Technical Papermaking

Paper makers hold starch in high regard as one of the most important papermaking chemicals in producing high quality paper.

Cat-ionic starch is made up of modified maize starch that now has a slight cat-ionic charge (positive charge). Cat-ionic starch is added at the wet end of the machine. The slight cat-ionic demand of the starch allows it to bond with the anionic (negatively charged) fibers, fillers, fines, and other small unusable materials to improve dry strength in the sheet and even improve the retention of the sheet.

Cat-ionic starch is produced when a reactant chemical is treated with slurry of partially swollen starch granules.

Typical starch addition points in a approach flow system occur either before the machine chest in the thick stock system or after the machine chest at the start of the thin stock flow to the head box, Arguably both addition points allow for high strength, depends on the grade and the dosing quantity of starch. 

The problem with adding too much is that it will exceed the adsorption capacity of the surface, based on either the surface area or the limited extent of negative charge of the surfaces of fibers and other solid surfaces in the furnish. Excess starch beyond what adheres to the fibers in likely to cause foam, high biological oxygen demand (BOD) levels in the effluent, and poor retention and drainage. The performance of cationic starch as a strength agent sometimes can be improved by raising the pH; this will tend to make the fibers slightly more anionic and better capable of interacting with the starch.

When adding cat starch in a system with a high level of anionic trash the starch strength qualities can be improved with pretreating the stock with a highly charged cationic material to neutralize the anionic trash. 

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.