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1、? ? This article deals with the typical problems encountered in keeping Tissue felts clean and the most common mechanical and chemical approaches to maintaining clean felts.The need for a tissue-specific approach What makes felt conditioning on a tissue machine special? There are three main factors:
2、 Light weight, relatively delicate felts are used in Tissue production. Sheet grammages are much lower than in non-tissue grades, making the sheet much more susceptible to disruption. Yankee creping programs can be affected by cleaning and conditioning programs. The delicate nature of tissue felts i
3、s probably explained best by looking at the operation of their high pressure showers. Most mills limit the pressure of continuous high pressure showers to 12 bar (most of these operate at or below 10 bar). There are some mills that operate their high pressure showers at up to 25 bar, but the high op
4、erating pressures are explained by the worn condition of their shower nozzles. High pressures and unworn nozzles lead to damaged tissue felts. Picture 1 shows the cross section of a felt that is in reasonably good condition. The felt retains much of its original caliper and there is still much of th
5、e batt fibre content. Picture 2 shows a cross section of a felt that has sustained mechanical damage from overly aggressive showering. Notice the lack of batt fibres on the top side of the felt. Such fibre loss can impact the felts water handling ability. It can also impact sheet formation due to po
6、or pickup (Fourdrinier or twin wire formers) or a poor formation surface (crescent former). Most tissue mills that operate their high pressure showers at over 12 bar limit their use to 1-2 hours per shift.Typical contaminants in tissue felts Press felts are excellent contaminant collection devices.
7、It is usually a fairly simple matter of determining what is contaminating a particular felt: whatever goes into the papermaking system ends up in the felt. All inputs into the papermaking system can contribute to felt contamination: Stock system oPitcho Stickies (from recycled fibre or contaminated
8、converting broke) oFiller (especially talc, which is used by some pulp mills for pitch control)? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Tissue?Felt Conditioning Considerations in tissueoFibres and fibre fines Water system oBacteria oSand/grit oOxidizing biocides Additives oWet strength oDry stre
9、ngth oStarch oCrepe aids and release agents oAntifoam Miscellaneous oLubricant leaks oInsects, etc. oDebris For most tissue mills, the most troublesome felt contaminants are stickies, wet strength, and oxidizers.Stickies Picture 3 shows a typical felt sticky; Picture 4 shows the inevitable result of
10、 that sticky: a felt-related sheet hole. Stickies (and other organic contaminants) have three possible “life cycles” in a paper mill: Small stickies deposit on a collecting surface, such as a Uhle box cover. In this case, there is minimal agglomeration in the stock system and deposits build graduall
11、y. Small stickies agglomerate in the stock and/or water systems, then form a discrete deposit. This is the most common source of fabric contamination, and is documented in Picture 3. Large stickies enter the stock system via the incoming stock and form discrete deposits. This case is most often foun
12、d in recycled fibre mills that lack proper fine screens or in mills that allow adhesives to enter their converting broke stream. It is worth noting that some “water soluble” converting tapes can become insoluble over time and lead to serious problems on the paper machine. While stickies can- and sho
13、uld- be controlled in the stock system, they can also be addressed on the Tissue machine.Wet strength As wet strength agents are functional chemicals, there is little choice as to their use. It is analogous to sizing agents in fine paper mills: it is not a question of whether or not the additive will fill the felt; it is a matter of minimizing its impact on quality and productivity. Wet strength additives are cationic, so unretained wet strength polymer is subst
