INTRODUCTION Management of the Yankee surface, or more specifically, the organic coating on the Yankee surface has a great impact on the operation and maintenance of a tissue machine. Tissue quality and machine productivity are both highly influenced by the composition and also adhesive and rheological properties of this surface. For example, a hard coating will make achievement of the bulk target difficult. Likewise, a lack of adhesion (also a by-product of hard coating in some cases) will give a drop in handfeel or softness. Hard coating at the Yankee edges can give rise to edge tears and web breaks, and a hard coating with poor release response makes achievement of a low crepe ratio difficult. Finally, we have the dreaded spectre of chattermarks1, blade-vibration induced CD scoring of the Yankee surface, causing sheet defects, slow running and ultimately a costly regrind or metallisation of the Yankee surface. So clearly, correct management of this thin organic layer is of paramount importance to the majority of tissue makers who use light dry crepe technology. MEASUREMENT OF THE COATING LAYER It is an axiom of control technology that “if you don’t measure it, how can you control it”? So, how to measure such a thin and largely unobservable but also dynamic phenomenon as a Yankee coating? Subjective measurements such as visual observation and even listening to the By Ian Padley, Global Marketing Manager Tissue, BTG Group Advanced Yankee Surface Management the organic coating on the Yankee surface has a great impact on the operation and maintenance of a tissue machine coating are a good starting point. Some classifcations are possible,as shown in Table 1. Many process engineers and operators will have their own classifcation, and with practice this can be quite discriminating, especially in combination with other less subjective methods. An objective measure to add to this is to measure the amount of coating sprayed onto the Yankee. This is extremely useful, but still requires the engineer to make some assumptions. Typically, we start with a flow rate of the various chemicals, which can include a base coating, release, sometimes a modifier and sometimes phosphate additive. Best practice is to define this flow rate as a total solids add-on per unit area, normally expressed in mg/m2 of coating. This requires knowledge of the concentration of the chemicals, their specific gravity and of course sprayed width and Yankee speed. However, this can only measure the chemicals as dispensed; what attaches to the Yankee will be somewhat lower due to misting of the applied spray, boundary air layer effects and so on. Eighty percent (80%) chemical stick- on versus dispensed chemical is a good rule of thumb for a correctly designed spraybar operating at 3-4 bar pressure. Whilst a very useful benchmark, add-on calculation does not take into effect coating rheology or adhesion (this requires some knowledge about the proprietary features of the chemistry) nor does it take ito account so-called natural coating (dealt with later on). Direct measurement of the coating thickness has some promise, but is not widely applied yet. Amongst the techniques proposed is a subtractive measurement using first an eddy current probe to determine the distance onto the metallic surface in tandem with a reflective EMR WORLD PULP&PAPER 25 Table 1. Yankee coating classification by observation Observation Status of Yankee surface Even, translucent grey, quiet, sheet tight Good soft coating coverage, adequate adhesion White but even coating, noisier, sheet tight Some fibre/fines contaminaton of coating, but OK White and dusty, distinct noise, often with sheet flying Highly contaminated coating Streaky white coating, high pitch noise Contaminated coating and poor moisture profile Shiny Yankee, high pitch noise None or little coating, wet sheet or cool Yankee YANKEE SURFACE MANAGEMENT