Pin-holes

The appearance of pin-holes in paper is most often associated with entrained air in the furnish, though the problem also can be associated with formation uniformity, low basis weight, or the use of furnish that happens to be very low in fines content. The consequences of air and foam in the papermaking system are covered in greater detail elsewhere in this guide.

A likely mechanism of pin-hole formation is the tendency of vacuum at a flat-box or couch roll to pull small bubbles of entrained air through the paper. This is a difficult mechanism to prove, but sometimes it can provide a clue as to how to overcome practical difficulties. If the pin-holes come on suddenly, then it is possible that a leak has developed in a pump seal, introducing a greater amount of air into the thin-stock furnish.

Air in the fiber furnish can be minimized by using some of the approaches mentioned elsewhere in this guide. To summarize, the air content of furnish coming from the headbox slice can be reduced by (a) use of an optimum dosage of defoaming chemicals that have been found to work well under the temperature and chemical conditions of the process, (b) attention to the efficient operation of deaerating equipment, including a deculator (if present).

Though it would be asking too much to try to repair a poorly formed base stock by use of the size press, it is possible to partly close up pin-holes with size press starch. To maximize this effect it is recommended that the solution viscosity be relatively high. In addition, a hydroxyethylated starch, which tends to give a tougher, more flexible film than underivatized starch, is likely to be more effective in covering pin-holes. Other premium size-press additives that can help with this kind of problem include polyvinyl alcohol (PVOH) and carboxymethylcellulose (CMC).

References

Avery-Edwards, D. J., Elms, R., and Buckingham, A., "Silicone Antifoams for Nonwoven Applications," Tappi J. 77 (8): 35 (1994).

Lorz, R. H., "Air Content, Retention, and Drainage: Important Parameters in Paper/Board Production," Pulp Paper Can. 88 (10): T361 (1987).

Matula, J. P., and Kukkamaki, E., "New Findings of Entrained Air and Dissolved Gases in PM Wet End: Mill Case Study," TAPPI J 83 (4) no page (2000).

May, O. W., and Buckman, S. J., "Practical Effects of Air in Papermaking," Tappi 58 (2): 90 (1975).

Rauch, R., and Sangl, R., "Latest Findings on Entrained Air and Dissolved Gases in Pulp Suspensions," Proc. TAPPI 2000 Papermakers Conf., 159 (2000).

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.


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