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. 

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).

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

Consistency Measurement - Technical Papermaking

Consistency Measurement


Testing the consistency of a stock is the same as consistency of water solution. Consistency is the term given to the amount of solid matter within a body of liquid. The higher the consistency the more solid matter is present within the same volume of liquid. Consistency is measured in percentage; it’s a percentage/ ratio of solid matter to water in a certain volume.


To find the consistency we need to test the amount of dry content within a set volume of water. For example if we wanted the consistency of 100ml of water, and tested 25 gram of dry solids the consistency would be about 25%.

To capture the solid content within a solution a filter paper is used. Firstly the filter paper is weighed at 0% moisture (previously oven dried) and recorded. The stock sample is weighed (1g = 1ml) and recorded. The weighed stock sample is filtered through a vacuum and rinsed making sure all of the solids from the sample are on the sheet. The filtered solids and the filter paper go into the oven to remove all of the remaining moisture.

Once the sample leaves the oven it is weighed and recorded. This formula is used to calculate the consistency below.

Consistency=  (Mass of Solids)/(Volume of Liquid)

We have to calculate the mass of the solids captured on the filter paper, this is calculated below;

Mass of Solids=Total dry weight-Filter paper weight

Here are a list of results that I took from measuring the consistencies of a pulp preparation plant's stock flows through two fractionators and Long fiber screens.