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.

Pulp Grinders - Mechanical Pulp Production

Types of grinders

Chain grinders

Chain grinders are the most common type of grinder, the chamber between the vertical chains houses the logs ready to be processed. The logs are continuously driven by the chains onto the surface of the revolving pulp stone. The chains apply a force on the log keeping them against the surface of the stone. Due to the surface profile fibers are torn out of the wooden compound. The logs are stacked horizontally due to the orientation of the fibers (minimizes fiber damage - increases fiber length)

Water showers are used to keep the stone clean and dilute the stock suspension. Water temp is usually higher than 80 deg. using a low water temperature can cause the stone surface to crack (due to temperature difference). Using hot water allows the stone to maintain the high temperatures. these high temperatures affect the lignin of the wood softening the material which in turn means the fibers can be separated with less damage

The basin collects the pulp washed off by the water showers. typically this produces a low consistency pulp, a thickening stage is added after to bring the consistency to a more manageable level. 

Pocket grinder

The pocket grinder is a technological advancement of the chain grinder above. processing the logs within a sealed "pocket" allows the sealed area to operate at a higher atmospheric pressure than the pocket grinder.

At higher pressures (approx: 5 bar) the boiling effect of water is affected. the higher temperature of the water will soften the lignin within the logs, longer fibers can be created with less damage done to the fibers. 

Revolving pulp stone is surrounded with a metal housing and 4 pockets, feeding of the pocket grinder was done manually. The debarked logs are pushed against the revolving surface of the pulp stone by hydraulically driven pistons. The pulp is collected underneath the pulp stone.  

 

The entire process of feeding the logs to the groundwood outlet is done under pressure. (up to 5 bar housing pressure, shower water temp up to app 120 deg) pressure affects the boiling temp of water. 


For more info on Mechanical ground wood follow this link
Stone Ground Wood - Mechanical Pulp Production

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.

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).

Consistency Meters - Papermachine Automation

Consistency Meters

Rotor designed consistency meters

The most common consistency meter found, the rotor design CM (Consistency meter) sits just outside of the stock flow stream. A deflector rotor pulls stock into the recess where the measurement device rotates at a constant speed. The stock gets thicker the torque on the motor to maintain that speed increases. The consistency can then be measured against the motor torque. This is known as a strain gauge. These types of measurement devices require a certain flow to function correctly and can measure consistencies of 1% - 10%.

unlike fixed blade consistency meters the rotor design is not affected by the variations in stock flows because the device creates its own flow from the deflection rotor onto the measurment device.

Fixed blade Consistency meters

Fixed blade CM work in the same way as the rotor design except the paddle is placed within the fiber flow stream. The fixed paddles moves with the fiber flow. The consistency increases in the pip, this in turn increases the force against the blade. The force is measured by the meter and calculated to a consistency. This is another instance of a strain gauge. 

The limitation with this type of measurement device is the stationary aspect. As stock flows past the paddle, fiber and rejects can stick/ build up reducing the accuracy of the device.
Variable flow within the pipe will alter the consistency measurment. A higher flow will add a higher force onto the consistency meter resulting in a higher measured consistency.

The major advantage of this type of meaurment is cost - usually customers purchase a fixed blade/ dynamic blade consistency measurment will the intention of replaying it with a better model.

Microwave Consistency meters

Microwave transmitters work on the principle that sending microwaves through water the waves travel at a certain speed. When fiber is introduces the microwaves move faster through the stream. The consistency can therefore be measured depending on the speed of the microwaves being sent and received by the meter. 

Using the calculation;   Velocity = C / sqrt(e)   

Where; C = Speed of light in a vacuum

     E = Dielectric constant of liquid (water)

These devices are more accurate than the rotor/ blade design. The microwave Transmitter also has no moving parts for the pulp to affect/ build up on like the blade transmitter.

The CM is not affected by the flow rate, colour, and brightness, like traditional microwave ovens they are highly affected by metals. these consistency meters are used within very clean pulp systems like the aproach flow because the likleyhood of metals entering the stream are very low. The microwave transmitter needs to be the same size as the pipe being used. Due to the expensive nature of these devices microwave transmitters are typically used on smaller pipe work or substituted for cheaper models. 

For more Info on Instrumentation within paper-making check out my other blog posts!

Level transmitters;
https://www.papermakingbible.co.uk/2018/04/process-level-indicators-papermachine.html

PID controllers;
https://www.papermakingbible.co.uk/2018/02/pid-control-loop-parameters.htm