Retention, drainage and formation

The papermaking process uses, together with cellulose fibres as a supporting framework, materials, such as inorganic fillers (calcium carbonate both naturally occurring and synthetic, clay and titanium dioxide amongst others), starches and process and effect chemicals. 

The furnish contains between 0.3% and 1.5% dry papermaking materials so for each ton of paper made between 66 and 150 tons of water need to be removed in a controlled manner. 

Retention is effectively the efficiency of the paper machine. Retention can be divided into mechanical retention and chemically assisted retention. Mechanical retention is caused by both the machine wire used to form the paper web and by the wet paper web itself. Chemically assisted retention is accomplished using retention aids. These water soluble polymers bind the smaller particles like fines and fillers by various mechanisms to the fibre. 

Drainage is the controlled removal of water on the wire and press section. The retention aid removes the free water within sheet interstices and capillaries between fibres and fines by flocculation. 

Depending on the cationic charge of the retention aid, fibre-surface bound water can be removed via charge competition by the retention aid. 

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Retention, drainage and formation were one of the most difficult balances to obtain in papermaking when only one product was used to attain these properties. When a few hundred grams of polyacrylamide were used to flocculate the stock just before the headbox, both the drainage and fines retention increased as more was added. Similarly, paper formation deteriorated with increasing retention aid addition. 

Retention systems have become increasingly more sophisticated as our understanding of wet-end chemistry has improved and paper machines have become more demanding. Often two, three or even four chemicals are necessary to attain the retention, drainage and formation balance on modem machines. This sometimes involves a coagulant to neutralise anionic trash, then silica sol or bentonite, anionic or cationic polyacrylamides (or even both) and cationic starch to flocculate the stock. 

Three-component retention drainage and formation programs... 

At the beginning of the 21st century, RDF programs have been developed that decouple retention, drainage and formation. These programs consist of a cationic high molecular... 

Depending on the paper/board machine, the grade(s) produced and the chemistries involved, choice of retention/drainage aid may be limited by other factors - sheet formation requirements, sheet porosity specifications, sheet linting propensity, sheet bulk density, etc. Due care is required to maintain sheet quality at aU times. 

Generally there is a proportional relationship between retention and drainage and an inverse one between retention and formation. For the paper industry there are serious negative implications for the overdosing of flocculants. 

The headbox distributes the stock across the full width of the wire through a slit called the slice. The slice opening can be adjusted to give a consistent cross machine profile. The "wire" is a continuous belt made of woven polyester or similar synthetic fiber, or of phosphor-bronze for certain paper grades. Many different weave patterns are used, from the basic square weave to complex multilayer weaves designed to optimize water drainage, paper formation, and retention of fines and fillers. 

Cationic starch in a paper mill furnish can have additional benefits beyond ash retention and strength. Properly added cationic starch can improve formation in a sheet. With an even distribution of fibers, the natural attraction of water for ionized anionic groups can be counteracted by the addition of cationic counter ions in the form of cationic starch. The flocculation effect that occurs produces much improved drainage on the paper machine. The result is increased speed on the machine yielding greater production rates and overall efficiency. To a paper mill, increased production means increased profitability. 

Starch is added to the papermaking furnish for a variety of applications paper formation control furnish drainage improvement filler and cellulose fines retention size retention internal paper strength improvement surface strength enhancement and reduction of waste water pollution. 

Paper/board quality All retention and drainage aids products and systems affect the sheet formation. The need to maintain acceptable formation can limit the type of system that can be applied and also the level of retention and drainage performance which can be sustained. As many trials fail due to an over-performing retention aid as fail due to an under-performing retention aid. 

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