Preparing a raw yankee surface

By Nick Mike, NGM Tissue Consulting, LLC

SUBJECT: Preparing a new yankee, sanded or newly ground yankee dryer surface

SUMMARY: A "raw", freshly sanded or newly ground yankee dryer requires a prepared surface treatment before loading a conventional steel doctor and before a new sheet of tissue is formed. Whether cast iron, alloy metalized or raw yankee surface, needs to be treated/prepared properly. The best recommendation comes from your yankee dryer supplier. However, there is similarity amongst prepping procedures as indicated below basic steps:

1. Wash or clean the yankee surface

2. Protect the surface with a phosphate

3. To smooth rough edges and produce a runnable surface

4. To enhance the metal surface to accept yankee coating

Newly ground yankees or newly "cast" yankees have surfaces that are microscopically sharp, porous and overly chemically excitable. There is a strong potential for corrosion. A stepwise process is required to prepare the yankee surface for refinement to a process oriented, papermaking surface that prevents mechanical and corrosive wear/buildup. Basically, it's a wash to remove metal dust and debris, chemical protection by hardening the yankee, making the surface more uniform and receptive of yankee coating in the cross direction (CD), prevent corrosion, and burnishing to remove the micro burrs and high buildup of phosphate. Application of yankee coating to develop a strong and tissue receptive yankee surface is the last step. Most chemical and yankee dryer companies all have their own procedures but the basics are covered below. We can summarize these procedures as follows:

1. Clean the yankee surface with soluble mineral oil (release agent) through hand held sprayers or through the yankee spray boom. This will remove dust metal fillings and debris accumulated in the porous surface. You should see gray or black residue dropping from the yankee as this step proceeds. Consider 1 - 2 hours at 200-250 F, depending on the situation and condition of the newly ground, brand new cast, or metalized surface.

2. Once completed with step one, apply a phosphate, preferably Mono Ammonium Phosphate, MAP, at a 5 - 20 % concentration to a warm- medium hot yankee on "Sunday" drive either through hand sprayers or preferably through the yankee spray boom. The yankee will only absorb a set amount of phosphate and residual will slough off and fall to the floor. This step will continue until the yankee starts to discolor maybe mottle in color. Consider about 30 - 180 minutes dependent on yankee temperature, speed and MAP concentration. Phosphate application will harden the yankee surface by almost 25%, will fill gaps and micro-voids in the rough yankee surface, prepare the yankee surface to readily accept yankee coating (think what paint primer is to paint), and eliminate corrosion potential across the yankee by neutralizing the flow of electrons by creating an anodic inhibitor.

3. Once step 3 is completed, "burnish" the yankee by applying a bronze doctor blade loaded at

"barely touching" the yankee. As an example, if the normal run pressure psi of the doctor is 40 psi and the blade pressure needed for the blade to start to load on the yankee is 20 psi, run around 25 psi. This step will take down high spots in the sharp metal edges and phosphate buildup. It will " level" the yankee surface so when a yankee coating is applied it will be uniform in the CD of the yankee surface. Cross directional uniformity (CD) is the single most important criteria for tissue making.

4. Coat the yankee with a solid yankee coating for 15-20 minutes. Intermittently applying a bronze or steel doctor blade again, loaded at less than normal operating pressure. Poly Amino Epichlorohyrin, PAE, the typical yankee coating base, is ideal as it reacts with the surface as a crosslinker (sets on the yankee) with pH and dehydration. This step causes a protective film to form on the yankee that will set the stage for a uniform CD coating application.

5. Bring the sheet over and load the creping blade at normal levels. You should be good to go.

Many variations of this process are available and this is only an explanation of the process. Contact your yankee dryer manufacturer for specific instructions on yankee prepping.

Nick Mike of NGM Tissue Consulting, LLC can be reached by calling 603-616-8931, or by email at nmike03580@yahoo.com.