Yankee coatings and thermal spray coatings

There was a time when Yankee coating referred solely to the adhesion and release package sprayed onto the surface of the Yankee during production. Its purpose was to protect the cast iron surface, provide blade lubrication and allow tissue makers to adjust adhesion properties to match the sheet structure development they were targeting.
Since the 90s however, the phrase Yankee coating can just as often be used to describe an applied metal coating via metalized thermal spray.

Connected to innovative technology. Connected to your success

The technology was originally offered as way for tissue makers to avoid grinding their cast iron surfaces to the point of requiring a derate of the operating pressure.
This was a very appealing solution that was widely adopted by the industry and the idea of grinding a cast iron Yankee to the point of derating went the way of the dodo bird.

Today metalized thermal spray coatings might be applied as a preventative measure on cast Yankees or added at the first grind occurrence as opposed to waiting for derate. The coatings are of course original equipment on all stainless steel dryers delivered. As such, they are commonplace.
For the other type of Yankee coating supplier, namely the organic chemical type, suitable chemistry and operating methodologies must be taken into consideration for the new metalized surfaces. The physical and chemical structures of traditional cast and newer thermal spray coatings are very different and this inevitably impacts the coating/Yankee surface bond development. Most adhesive chemistries developed and used for tissue creping evolved in the context of application on a cast iron surface.

The newer thermal sprays present greater challenges to the adhesive due to their ability to be finished to a higher smoothness (less mechanical adhesion potential) and higher chromium/stainless type chemistry for which typical adhesive chemistries have less natural affinity. The porosity of the cast iron Yankee surfaces was often the limiting factor of how smooth such a surface could be made, so the risk of driving smoothness to a point where mechanical adhesion of the organic coating was significantly compromised was unlikely.

Buckman’s purpose is to be your trusted advisor, creating innovative solutions that help you succeed

As such a mantra of “the smoother the better” developed with respect to finishing cast iron surfaces.
Chemical suppliers have had to develop chemistries that can create an effective metal surface/coating bond. If the surface/coating bond is weak, any coating applied will be susceptible to wash-out in the pressure roll nip or mechanically removed at the creping blades, which results in it not being able to build an effective organic coating for the blade to operate in.
When this occurs the best case result is poor quality and runnability; at worst it results in the complete loss of the ability to get the sheet across the Yankee and to the reel.
To adjust for this chemical companies have had to develop new testing and application methods that can allow us to effectively gage adhesive-to-alloy bond potential. By doing this, research into adjusting the structure of common polymers, as well new potential copolymers, can be effectively screened for their improved affinity to newer allows. Testing equipment such as differential mechanical analysis units and peel test units which have been adapted to this purpose provide the data necessary to guide product development.

Over the years suppliers of both the thermal spray coatings and the chemical coatings have learned much about applying both coating types to ensure the best chance of a successful re-metallization or regrind. Due to the difficulty in recreating the creping process in the lab, much of what has been learned over the past twenty years has happened empirically through trial and error. Some newer technology which is providing good guidance on establishing new organic coating package center-lines include on-line vibration monitoring and on-line natural coating analyzers. This sort of online tech provides instant insight into both the thickness and hardness of the organic coating layer. They also provide guidance into whether heavy coating is organic chemistry or mostly cellulose based. This guidance can help redirect add-on levels and ratios of your Yankee coating package early in the process to avoid prolonged run conditions that could damage the newly ground thermal spray on coating. Some of those errors have been costly. I present here some key observations we have made over the years with respect to creating a good organic coating on re-metalized surfaces. These are offered as generalities of which one or more may or may not apply to a particular type of thermal spray coating and as such your mileage may vary when aligning to them. Some of our learnings regarding commissioning of thermal sprays include:

• Finishing a metalized thermal spray coating below 25ra (µin) or 0.625ra (µm) roughness increases the chances of creating a final surface that has too low an affinity for organic coating.
• Systems with higher natural coating levels (e.g. recycled fiber and closed water systems) in the wet end will see less impact than those with low natural coating contents (e.g. virgin fiber and open water systems).
• Newer adhesive chemistries that have been developed for improved adhesion and nip tenacity produce better results.
• The type, addition point and addition rate of phosphate ion can have a very large impact on the development of the bond strength between the surface and organic coating.
• Higher organic coating consumption of 10 to 15% in order to recreate previous base coating levels are common.
• Expect higher adhesive to release ratios on re-metallized vs cast surfaces.
• Data from on-line crepe blade vibration monitoring as well as online natural coating levels in the wet end provides helpful center-lining insights on start up on new surfaces.