Pond Size Press - Paper making Overview

Pond Sizer - Paper making Overview

pond style size press is an older designed surface size coaters. Prior to Sym Sizers Starch was applied directly between two rolls creating a "pond" of starch. 

Paper passes through the liquid phase and is pressed between the size press rolls. prior to the Size press the Paper is dried to a Moisture content between 8 to 12 %. this is the ideal moisture range to accept liquid back into the paper. 

The moment the paper comes into contact with the Starch it starts to be absorbed into the paper. the advantage to pond size presses is that more starch can be applied to the paper sheet than sym sizers. this is due to the increased contact time with the starch. the pond level, or amount of starch within the two rolls typically can be adjusted to control the amount of starch absorbed into the sheet. 

With greater starch pick up from the pond the sheet will also contain more water after application. Leaving the Pond the sheet can contain almost as much as 40% Moisture.

The higher moisture content leaving the size press does have its disadvantages. the high moisture content will weaken the sheet. this section of the paper machine can be responsible;le for majority of the paper breaks incurred. 

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. 

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.