Paper Machine Runnability – Retention Aid

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Retention Aid is the key to cost savings in a paper mill. Retention aids are used at the wet end of a paper mill to improve the retention of fine particles, fillers, additives & sizing agents during the formation of paper. Particularly retention aid is needed to be added in the alkaline paper-making process, but not in the acidic paper-making process because in this process alum helps in retention. Modern paper-making technology is an alkaline paper-making process. Moreover, modern mills operate high-speed paper machine with increased usage of recycled fibers. In this case, the task of retaining is more challenging and paper machine runnability will be troublesome. As a result, the papermaking process becomes less economical. Hence the demand for retention aid is increased to save costs.

The chemical most used as retention aids are polyacrylamide and polyethyleneimine.

Troubles of low retention:

Papermaking is in essence a filtration process. The paper machine wire cloth can be considered as a continuous filter; where a proportion of the solids in the stock is retained and some fine particles are drained through the wire cloth mesh with water and formed white water. The greater part of the white water is returned to the stock, with the result that the concentration of fines and fillers gradually increases. It should be retained otherwise following troubles arise:

  • Fine fiber particles and filler drainage
  • Higher additives and sizing cost
  • Increased deposits
  • Decreased runnability
  • More downtime for cleaning
  • Filling of wet-press felts
  • Bad paper sheet formation
  • Higher sewer losses

 

The function of retention aids

The best retention of fine particles and colloids in the papermaking process normally occurs when the zeta potential is near to zero and retention aid takes care of that. The main functions of retention aids are the following:-

  • Accelerate drainage, hence increasing machine speed
  • Improved retention of fines and fillers
  • Reduced wet-end additives and sizing usage
  • Reduced steam usage due to better surface contact
  • Reduce deposits
  • Reduce sheet breaks
  • Improves paper machine runnability
  • Improved paper sheet formation
  • Increased brightness
  • Cleaner white-water system
  • Reduce downtime for cleaning
    Overall, Increase the productivity

 

Disadvantages of retention aid

If retention aid is used in higher doses, as a result highly flocked sheet is formed. Also, observed are increased paper web break, two-sidedness, and decreased paper machine runnability. Hence to optimize runnability the dosing needs to be optimized.