Retention aids are used to retain paper making stock components in the sheet during the forming process, while at the same time promoting the free drainage of water.
Long paper making fibers are naturally retained on the forming wire. It is the short fibers, or fines, plus the fillers and other wet end chemicals, which have a tendency to be washed through the wire as the water drains from the stock. When applied correctly, a retention aid system will aid water release from the stock, improving machine speeds and reducing drying costs. In turn, this result in more stable running on the paper machine and a reduction in the solids lost to effluent.
Function: To maintain adequate efficiency, drainage, and cleanliness of a paper machine; to achieve adequate Z-directional uniformity; to retain additives, including fillers.
Strategies for Use: Retention aids can have a profound effect on the efficiency of the process and on the quality of the product. For instance, an overdose of retention aid is likely to produce a highly flocced sheet that is unacceptable to the customer. So the most general rule is to add only the amount of retention aid that is needed to optimize run ability and avoid excessive problems of two-sidedness of the sheet structure. Relatively high retention is desirable, however, when using ASA size or when using rosin emulsion size under neutral or alkaline pH, since those additives will tend to gradually decompose if they are not retained in the first pass. Additional strategies are mentioned under the headings for several of the additives that are often used in retention aid programs.
Retention Aid – Product Characteristics
|Appearance||Water Soluble Powder Polymer|
|Ionic character (Cationic)||Medium|
|Use concentration suggested (g/l)||1.0|
|Concentration of stock solution (g/l)||10 – 15|
Retention Aid – Benefits
· Increased machine speed
· Improved retention of fines and fillers
· Reduced steam usage
· Improved paper formation
· Reduction in foaming
· Cleaner white water system
Cautions: Consult the MSDS for each additive. Be aware that many different patents exist that can affect one’s right to use different combinations of additives