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.
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
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
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