Focus on creping modifiers

By Chris Rozett, technical marketing manager, contaminant control and tissue, Amazon Papyrus Chemicals

Creping modifiers: practical application

Some modifiers work well in certain systems, but poorly in others. Amazon Papyrus Chemicals’ Chris Rozett discusses case histories that demonstrate the advantages – and potential disadvantages – of using creping modifiers in dry crepe tissue manufacture

While creping modifiers – nonadhesive and non-release additives – are commonly used in dry crepe tissue manufacture, there are many creping programs that don’t make optimum use of their modifiers. 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 (Table 1).

Fig 1: Self bonding in low molecular weight, linear polymers

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 and 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.

Fig 2: Self bonding in medium molecular weight, branched polymers

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 (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 (Figure 3).

Fig 3: Self bonding in medium molecular weight, amorphous polymers

A popular non-reactive 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, 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 multicomponent 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 plasticisers. 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, 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 five 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 multi-component 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.

PLASTICISERS

Plasticisers come in two basic varieties: PVA and a family of chemistries normally referred to as plasticising 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 plasticising as 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).

Plasticising release agents function to soften and bulk up the coating matrix by associating with the creping adhesive. Significantly, plasticising 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, plasticisers 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 build-up, blade life, and Yankee coating uniformity when replacing an oil-based release with a plasticising release. Plasticising 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 plasticising 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 fibre. The bulking property of plasticising 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 plasticising release agents can have a profound effect on machine runnability, as shown in Case history 3.

CONCLUSION

Just as all chemistries have their place, so no one chemistry is useful under all conditions. Creping modifiers have their particular rolls to play in helping to build up, soften, or stabilise 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; plasticising release can help to soften coatings while making them more bulky – with both high and low molecular weight adhesives.