Covid-19 drives recycled tissue makers toward virgin fiber streams

The Covid-19 crisis has resulted in significant structural changes in recycled fiber stream collection and availability due to the move toward work from home. In particular, sorted office papers (SOP)/mixed office waste (MOW) generation has been significantly reduced with some estimates suggesting that collection of this stream has declined by 30-50%. Supply shortages and rising MOW prices are the result. In the current market conditions, many recycled fiber (RF) tissue makers are either having to supplement their fiber supply with virgin content or, in some cases, having to switch over completely. Moving your base sheet fiber mix from RF to virgin presents both opportunities and challenges to the tissue maker.

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Fiber differences

The fiber that most tissue makers are turning to is southern cone Eucalyptus (EUC). EUC is known for possessing advantages in product handfeel characteristics over recycled fiber, but it is significantly more expensive than the deinked pulp streams that integrated tissue mills produce from MOW. It is worthwhile to ensure that if more expensive virgin fiber must be used, it is done in a way that maximizes benefits and minimizes operational upsets so that production efficiency and quality can be maintained.

This article will concentrate on the two key areas where operational and quality impacts result from the fiber mix changes and best practices to deal with them.

Refining considerations

The first thing to be understood is the difference in basic fiber characteristics. The cell wall thickness and overall fiber length is vastly different for EUC vs the northern hemisphere hardwoods that are used to manufacture office papers. This difference is much greater for the softwoods. Replacing MOW with eucalyptus results in a drastically different base fiber. This difference means the tissue maker has to adjust the way he generates strength. The initial temptation is to achieve the strength needed via higher refining intensities, but the following negatives cascade from this approach: increased fiber cutting, higherfines generation, increased dusting and sheet densification, reduced caliper/bulk and decreased softness. Fines also reduce drainage and increase the drying load on the tissue machine. Where drying capacity is limited, this can result in reduced production capacity.

The negative impact can be reduced but not eliminated with lower intensity refining, however this approach requires more available refining capacity, or in some cases, completely different equipment. The best way to circumvent this problem is to employ an enzyme-based technology to modify the fiber. This technology allows for maximum fibrillation of the fiber with minimal input of refiner energy. The result is the ability to maximize tensile generation without the fiber cutting.
The tissue maker gets the strength required while preserving bulk to basis weight, absorbency and handfeel, all of which suffer when the base sheet is densified.

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Coating impact

The second area of the process that is significantly impacted when substituting recycled fiber for virgin is the difference in what is referred to as “natural coating.” Natural coating is the soluble and colloidal material coming from the wet end that remain on the Yankee dryer once all of the water and other volatiles have evaporated. Organic (applied) coating is the synthetic adhesive sprayed onto the dryer that acts like cement in concrete, gluing the aggregate (rocks and sand in concrete and fillers/fines in a coating) together.

The natural coating is an important part of the Yankee coating matrix that affects machine runnability, creping efficiency and sheet quality. Generally, natural coating will account for 50% or more of the total coating matrix on the dryer surface so it is expected that changes in natural coating level will be seen and felt at the dry end. Different fiber sources and mill closure rates result in differing natural coating levels.
RF furnish contributes a high level of a hard natural coating. In order to manage coating hardness with RF, it is not uncommon to see release-to-adhesive ratios from 3:1 to as high as 5:1.

Replacing RF with virgin EUC will reduce natural coating levels in the water circuit of the tissue machine, and this will have an impact on the Yankee coating build rate, hardness and performance. When moving to EUC, previous add-on rates and ratios cannot be supported. The EUC will contribute much less natural coating, and the nature of that natural coating is much softer. As a result, the tissue maker will need to tighten the release-to-adhesive ratio from where he normally runs with 100% RF. If this action is not taken, the substitution will result in a thinning of the coating. If the coating thins to the point that there is not room for the creping blade to get under the sheet, it will result in picks and tears as depicted in Figure 1.

Potential creping process issues and remedial actions when moving from RF to virgin include:

• Picking. Coating is too hard or there is a lack of coating thickness to allow the blade to get under the sheet. Adjust ratio or increase overall add-on.
• High Blade Wear. Generally, need to increase adhesive/reduce release to build coating to protect the blade.
• Sheet Plugging. Likely that adhesion is high, but coating is too thin. The sheet is sticking directly to the dryer. Adjust to build a thicker coating for the blade to swim in.
• Uneven Profile. Decrease release as coating is likely stripping in high moisture or cooler areas.
• Flashing/blistering. Decrease release/increase adhesive to set coating on dryer to pick up sheet.

For a more complete explanation of the issues related to substituting RF fiber with eucalyptus, and more detailed approaches on how to mitigate them, a webinar on the topic can be viewed at Buckman website – webinars page.