Abstract:
Creping modifiers (non-adhesive and non-release additives) are commonly used in dry crepe tissue manufacture. However, there are many creping programs that don’t make optimum use of their modifiers. This paper will attempt to explain why some modifiers work well in certain systems, but poorly in others. Several case histories will be presented to demonstrate the advantages- and potential disadvantages- of using creping modifiers in dry crepe tissue manufacture.
A word about creping adhesives
It is important to understand that creping modifiers must work with the type of creping adhesive they are supposed to modify. Creping adhesives are usually cationic coagulants; typical molecular weights of creping adhesives, release agents, and modifiers are listed in Table 1. Component molecular weights can give a basic idea of their relative size, which helps to explain how various components fit into the coating matrix on the Yankee dryer. Coagulants come in three basic shapes:
- Linear
- Branched
- Amorphous
When looking at a particular creping adhesive’s stability on the Yankee, it is helpful to look at the relative bonding area per molecular weight. If a polymer can’t bond with itself, it can’t set up on the dryer surface and won’t be of much use when it comes to holding the sheet against the dryer at the creping blade. As can be seen in Figure 1, linear polymers have plenty of available bonding area, and tend to set up particularly easily on the dryer surface. For example, a popular linear creping adhesive performs quite well at normal sheet moisture, and can even tolerate sheet moistures well over 8%. However, it sets up so well that it is extremely difficult to run at sheet moistures below 3%. On the other hand, branched polymers have less available bonding area per molecular weight, as seen in Figure 2. A commonly used hyperbranched, non-reactive creping adhesive with a low glass transition temperature (Tg) offers a hard, durable coating- until the sheet moisture exceeds 6%, at which point the adhesive fails. Amorphous polymers tend to be represented as forming polymeric balls due to their extremely branched structure, and have the least amount of bonding area per molecular weight (see Figure 3). A popular nonreactive amorphous creping adhesive is surprisingly susceptible to moisture and temperature variation in the cross machine direction because of its inefficient bonding ability. When paired with the proper modifier, however, its moisture tolerance improves dramatically (see Case History #1)
Commonly used creping modifiers
Creping modifiers- those components that often are the third component of a three component creping program- tend to be looked at in a one dimensional way. Polyvinyl alcohol (PVA) is a coating softener; monoammonium phosphate (MAP) and diammonium phosphate (DAP) promote coating development, etc. Modifiers are often used successfully in multi-component creping programs, but their track record is far from perfect. Why is it that too often they do not deliver the intended benefits of their chemistries?
Broadly speaking, there are two basic types of creping modifiers: accelerants and plasticizers. Tackifiers and humectants are also used to modify creping adhesives, but are usually part of the adhesive formulation itself, rather than a separate component in the creping program. Humectants, in particular, have been used to soften particularly “hard” adhesives, such as non-reactive, linear creping adhesives.
Accelerants
Phosphates are the most common type of accelerant. They are used for a number of reasons, but probably are best known as part of the creping package of PAE wet strength, polyvinyl alcohol (PVA), and MAP that was patented in the 1980s. Phosphates tend to react with the Yankee surface, acting as a linkage between the metal of the Yankee and the organic portion of the coating matrix on the Yankee. They also help to prevent Yankee corrosion. Perhaps most importantly, though, phosphates help the Yankee coating set up faster on the dryer surface. The faster the coating sets up, the more robust it is, and the more resistant it is to wet streaks, (relatively) cold Yankee surface temperatures, and chemical interference. A common downside of using a phosphate accelerant is a loss of handfeel. On a 30 point softness scale, significant use of MAP can lead to a loss of 5 or more points- when not paired with an appropriate creping adhesive.
I like to think of the action of phosphates in the coating matrix as analogous to microparticles in a multicomponent retention program: a relatively small agent that helps to link other components by charge interaction. When combined with an appropriate creping adhesive, phosphates can produce surprising results, as shown in Case History #1.
Plasticizers
Plasticizers come in two basic varieties: PVA and a family of chemistries normally referred to as plasticizing release agents. PVA has been in use for decades in combination with MAP and PAE wet strength, where it has acted to bulk up and even out the Yankee coating. Interestingly, typical wet strength agents used as creping adhesives don’t need a lot of plasticizing- they have a reasonably high Tg. The PVA functions more to fill in the low spots that invariably appear in a wet strength-based creping program. Because of their high reactivity, wet strength agents are particularly susceptible to cross machine direction variations in temperature and moisture. Typically, using a wet strength agent as a creping adhesive without PVA will lead to over-cured areas on the dryer that lead to sheet picking and under-cured areas that lead to roll corrugation.
A common problem associated with the use of PVA is coating slough off. Part of the reason for this problem is PVA is often used with harder, more moisture tolerant creping adhesives in order to soften them and improve sheet handfeel. Unfortunately, the PVA molecule is often larger than the polymer it is supposed to be modifying. It is not surprising that PVA is unable to prevent the self bonding of lower molecular weight, linear polymers- instead bulking the coating matrix to the point that pieces of coating travel with the sheet to the roll (see Case History #2).
Plasticizing release agents function to soften and bulk up the coating matrix by associating with the creping adhesive. Significantly, plasticizing releases are much smaller than creping adhesives, and help to control over-bonding of the adhesive in the coating matrix. As opposed to oil-based release agents, plasticizers have a minimal concentration gradient in the Z-direction of the coating matrix. The lack of a concentration gradient helps to keep the cured portion of the coating softer than would be possible with an oil-based release. It is common to see an improvement in hard edge buildup, blade life, and Yankee coating uniformity when replacing an oil-based release with a plasticizing release. Plasticizing release agents are often used as part of a two component creping program, but are sometimes used in conjunction with a standard, oil-based release in particularly problematic systems. An example of using a plasticizing release as part of a three component program is on a tissue machine using particularly “dirty” furnish- one that contains a lot of ash from unwashed recycle fiber. The bulking property of plasticizing release agents tends to trap ash in the coating; sometimes it is necessary to use some release oil to strip excess coating from the dryer surface. The softening action of plasticizing release agents can have a profound effect on machine runnability, as shown in Case History #3.
Case history 1
Location: China
Machine type: suction former
Grade: Facial tissue
Furnish: 100% virgin fiber
Speed: 650 m/minute
Yankee: 5.5 bar, with an unheated hood
Incumbent creping program
Creping adhesive: medium Tg, slightly hard, branched, reactive; fed at 0.54 mg/m2
MAP at 0.45 mg/m2
Oil-based release at 3.5 mg/m2
This tissue machine was feeding a low rate of creping adhesive, and it showed: the Yankee coating was light and uneven. The press felt was streaky from wet strength use, which led to cold areas on the Yankee. Consequently, there was significant roll corrugation, especially on the drive side of the roll. Blade wear was poor, with around 5 mm at the edges. A creping adhesive was trialed to improve roll building and blade wear. The trial adhesive was a soft, non-reactive, amorphous polymer with a high Tg. It works particularly well at low sheet moisture, and is useful for developing good sheet handfeel. Unfortunately, the adhesive’s structure allows only limited bonding to build the coating matrix.
Because of the adhesive’s sensitivity to temperature and moisture streaks, even 1.5 mg/m2 (nearly three times as much adhesive as the pre-trial program) could not improve roll building. Clearly, the new adhesive was in need of help in setting up on the Yankee. Increasing the MAP (working as an accelerant) to 0.75 mg/m2 allowed the feed rate of the adhesive to be reduced to 0.75 mg/m2. At this point, the roll became flat, while handfeel was maintained and blade wear was improved. The Yankee coating was now even and more robust than pre-trial. In this case, the MAP didn’t simply add phosphates to the Yankee surface- extra phosphates do not build up the coating matrix enough to show a heavier and more even coating, especially while reducing the adhesive feed rate by 50%. The MAP helped the adhesive form a more robust coating by contributing to polymer bonding, which increased the bonding efficiency of the amorphous adhesive. It should be noted that feeding MAP at 0.75 mg/m2 did cause a reduction in crepe bar count from 80 to 75 bars/ cm, but in this case there was no significant loss of handfeel.
Case history 2:
Location: Southeast Asia
Machine type: Crescent former
Grade: Toilet tissue
Furnish: 70% virgin fiber + 30% recycle fiber
Speed: 1150 m/minute
Yankee: 6.9 bar; hood temperature ~375oC
Incumbent creping program
Creping adhesive: high Tg, branched, reactive; fed at 1.5 mg/m2
Oil-based release at 6 mg/m2
The creping program produced a relatively soft sheet with a crepe bar count of around 60 bars/cm. However, the program was sensitive to moisture variations (once again, felt contamination led to wet streaks and cold areas on the Yankee). A moisture tolerant creping adhesive was trialed to improve roll building. The adhesive selected was a popular choice for machines with moisture profile problems: a low molecular weight, linear, low Tg, non-reactive polymer. Unfortunately, this adhesive has a reputation for hard edge buildup on the Yankee. It is also very difficult to run when sheet moistures fall below 3%. While the average sheet moisture was above 3.5%, there was a 15 cm area in the center of the roll where the sheet moisture was approximately 2.7%.
At 1.0 mg/m2, the hard creping adhesive quickly solved the problem of poor roll building. However, there were immediate problems with sheet picking in the low moisture area. In order to plasticize the hard coating, a modified PVA was fed at levels from 0.2 to 1.0 mg/m2. The oil-based release was fed at around 12 mg/m2. The PVA bulked up the Yankee coating, producing soft tissue in the non-defect area. However, even though excess coating was sloughing off onto the sheet, there was no effect on sheet picking in the dry area. The large plasticizer molecule was not able to keep the small, linear creping adhesive from bonding too strongly in the coating matrix. Interestingly, an identical problem on a different tissue machine was controlled by pairing a plasticizing release with this same creping adhesive. The much smaller plasticizing release molecule was able to control excessive polymer bonding.
Case history 3:
Location: Indian subcontinent
Machine type: crescent former
Grade: Facial tissue
Furnish: 100% virgin fiber
Speed: 1300 m/minute
Yankee: 4.7 bar; hood temperature ~370oC
Incumbent creping program
Creping adhesive: moderately soft, branched, reactive; fed at 1.4 mg/m2
MAP at 0.2 mg/m2
Oil-based release at 2.9 mg/m2
This tissue machine is unusually susceptible to vibration problems. Every time the mill tried to increase the machine speed above 1300 m/minute, the Yankee would vibrate above 5 mm/sec; the control limit for vibration was 3 mm/sec. The Yankee vibration was caused by hard coating on the dryer making the creping and cleaning blades bounce against the Yankee surface. After six months of unsuccessfully trying to increase machine speed, a second coating supplier was invited to work with the mill. Their trial, consisting of a soft, reactive coating, accelerant modifier, and oil-based release, only lasted a few hours before it failed due to excessive Yankee vibration.
The combination of a soft, high Tg, branched, reactive polymer with a plasticizing release produced far different results. The creping adhesive was fed at 0.76 mg/m2, while the plasticizing release was fed at 1.3 mg/m2. The machine speed was increased to 1500 m/minute, while the Yankee vibration was maintained at or below 2 mm/sec. The Yankee coating became more even, and roll building was flat, where before there were wrinkles in the roll that caused problems during rewinding.
The first two creping programs made use of an accelerant creping modifier in order to help even out the Yankee coating. Unfortunately, in the process of building up the Yankee coating in wet/cold areas on the dryer the accelerant also hardened the coating, which contributed to vibration. The plasticizing release built up the coating while softening it, which allowed the mill to control Yankee vibration without limiting machine speed.
Conclusion
Just as all chemistries have their place- no one chemistry is useful under all conditions- creping modifiers have their particular rolls to play in helping to build up, soften, or stabilize Yankee coatings. What is important to keep in mind is how the particular modifier works with the creping adhesive to meet the needs of the papermaker. Accelerants tend to harden the Yankee coating, but can dramatically boost the performance of a soft adhesive; PVA can be useful in softening and evening out a coating that uses a higher molecular weight creping adhesive; plasticizing release can help to soften coatings while making them more bulky- with both high and low molecular weight adhesives.
Appendix
Table 1: Approximate molecular weights of common creping components
Component Approximate molecular weight
Linear, low molecular weight adhesive 20,000 – 50,000
Branched, medium molecular weight adhesive 200,000 – 500,000
Amorphous, medium molecular weight adhesive 500,000 – 2,000,000
Oil-based release 200 – 500
Plasticizing release 500 – 1,000
PVA 100,000+
MAP 115
DAP 132
Figure 1: Self bonding in low molecular weight, linear polymers
Figure 2: Self bonding in medium molecular weight, branched polymers
Figure 3: Self bonding in medium molecular weight, amorphous polymers
Nguồn: Tissue world
