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.

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.

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.