A sixty or seventy percent solids slurry arrives in the lab and the stirring rod stands up in the drum. The solids figure on the test sheet looks good, and the cost works out. Most of what you bought is active material, and the solvent load is light.

On the line the trouble wears a different face. The wet film is thin, and fine lines and thick patches show up across the web. The gauge at the pump outlet swings. Stop for ten minutes, restart, and the first stretch of flow is a mess that has to be cut off. The web leaves the oven looking dry, but press a sheet of paper against it and there is a mark. Blocking and odour on the roll follow from there.

Those symptoms look unrelated. All of them come back to the solids figure.

So what actually makes high solids hard?

High Solids Is Not High Viscosity

At the same solids, viscosity can move by a factor of several.

Solids says how much of the slurry is solid. Viscosity says how hard it is to make it flow. The two travel separately. Hold the solids at sixty percent by weight and change the resin grade, and viscosity can jump from two thousand to twenty thousand centipoise. On a fixed viscosity, solids can run anywhere from thirty to seventy percent.

The pair that gets confused is solids by weight and solids by volume. Weight solids is what the test sheet reports, and the drying load follows it. How the particles pack depends on volume solids, the space the solid actually takes up. Take a drum at sixty percent solids by weight, with a solid density of 2.0 and a liquid density of 1.0. The volume solids works out near forty-three percent. The two figures are far apart.

A supplier usually quotes weight solids, so ask which one the formulation file means. Estimate particle spacing from weight solids alone and the slurry looks looser than it is.

High solids carries its own advantage. For the same dry coat weight the wet film is thinner, so less solvent has to leave. The oven can be shorter and the line can run faster. The trouble sits somewhere else. A viscosity range in a coating machine manufacturer's proposal is usually measured on their own reference formulation. The next drum may not match it.

When the Particles Get Too Close

Past a certain solids, the particles start touching each other.

Mono-sized spheres dumped together fill space up to about sixty-four percent by volume, and a blend of sizes goes higher. Take volume solids from forty-five to fifty-five percent and the average surface spacing falls from roughly one particle diameter to a fraction of one. At that distance the particles cannot move independently. Each one has to push its neighbours aside.

That is where shear thickening starts. At low shear rate the particles still have time to rearrange, and the slurry flows. Above a critical rate they cannot get out of the way, viscosity jumps, and pressure climbs through the pipe and the head. Fine lines and thick patches on the web come from there.

Yield stress appears in the same window. Below a certain stress the slurry will not move at all. Let the line sit for ten minutes and the material locks up in the pipe. On restart, the first stretch of flow is unstable until it works through.

Two things on site tell you what the drum will do. Let it stand for ten minutes and pour it. If it takes effort to start moving, yield stress is high. Read viscosity at two or three spindle speeds. A reading that falls quickly with speed means the slurry thins under shear. A reading that climbs means you risk thickening in the head. Where a shear rate range appears in the data sheet, that column answers the question outright. Whether a machine builder supplies it is another matter.

A coating line manufacturer marks a circulation flow on the piping drawing, sized for one viscosity. Change the shear behaviour and that figure has to be recalculated. Short runs, wide bores and a circulation pump that keeps running all keep the material moving, so it does not lock up. A viscosity a supplier quotes is a low-shear reading. In a high-solids slurry that figure sits furthest from what the head actually sees.

A Thin Wet Film Raises the Bar

Thin wet film leaves machine error a bigger share of the total.

At fifty percent solids by volume, a 50 micron dry film needs a 100 micron wet film. Take it to sixty percent and the same dry film needs 83 microns. The film is thinner, so the same roll error becomes a larger fraction of it. Twenty microns of roll runout is twenty percent of a 100 micron wet film, and twelve percent of a 167 micron film.

That eats the margin in the roll train. The tolerance grade a roll grinder quotes and the figure measured after assembly are often a step apart. The measured figure is the one that matters. The runout data in the assembly file usually reaches you from a coating machine manufacturer with the machine. Read it and you can see which film thickness the roll train was built for.

Tension has to hold steady as well. With a thin substrate and a thin wet film, tension variation writes itself straight into thickness. Above two metres of web width, the tension difference between edge and centre becomes visible, and the edges go first.

Runout measured by a coating line supplier during installation, set against the factory figure, tells you whether transport moved anything. That check takes a few hours and saves weeks of guessing later.

Less Solvent, Slower Release

High solids means less solvent.

By total volume that is good news. By path length it is not. Solvent has to travel out through the gaps between particles, and the route is tortuous. The higher the solids, the more twisted it gets. Effective diffusion scales roughly with porosity and inversely with the tortuosity of the path. Go from forty to sixty percent solids and porosity drops while the path lengthens, so diffusion capacity may fall to a third.

The result is less solvent overall and the last few percent leaving more slowly. The web comes out of the oven looking dry, but a sheet of paper pressed against it leaves a mark. Blocking and odour on the roll follow from there.

Shrinkage happens in the same window. A wet film at fifty-five percent volume solids keeps a little over half its volume once dry. That is close to twenty percent linear shrink. That stress lands on the contact points between particles. Run the film thick or the temperature fast and cracks join up from those points.

Ramp the zone temperatures gently and hold the first zone back. Open the air volume fully at the start and the surface skins over, leaving the solvent below nowhere to go. The drying reference table a machine builder supplies is measured at one solids level, and changing solids means rebuilding it. An equipment supplier's drying curve works the same way, so ask for the test conditions with it. Where an OEM coordinates the project, keep the formulation side's test reports in one place. Then the two sides work from one set of numbers.

Before the Proposal, Measure These

Start by separating the two solids figures.

Get weight solids from the formulation, convert volume solids yourself, and write both down. The test sheet a manufacturer issues usually carries weight solids only, so the volume figure is yours to calculate.

Read viscosity at two or three spindle speeds and watch which way it moves. A slurry that thickens as speed rises carries the most risk at the head. An instrument that reads yield stress is better still. Without one, letting the drum stand for ten minutes and pouring it gives you a rough answer.

Work out the wet film thickness that matches your dry film target, using volume solids rather than weight. That figure sets the gap, the roll accuracy and the drying section.

When the proposal arrives from a coating machine manufacturer, check first whether those figures are in it. A coating line manufacturer that sizes the circulation pump and agitator in the proposal has the piping section covered. A coating line supplier that measures start-up pressure on site gives you a number for that section. Whether the machine builder sends a full temperature profile or only the first zone matters more than it looks. Where an equipment supplier states test conditions alongside the figures, they are easier to use. With an OEM running the project, one contact holds those documents, and handover gets a lot quieter.

Whether a high-solids slurry runs does not come down to the solids figure. Whether the two solids figures are kept apart, how close the particles sit and how thin the film has to be. Whether the solvent can still move is the fourth. Match all four and a seventy percent slurry runs continuously. Miss one and dropping the solids only moves the problem somewhere else.

That set of requirements comes down to how the formulation, the roll accuracy and the drying curve are matched. Problems like this come up often in custom work. Innater works precision, thickness and uniformity out against your slurry and your target thickness, machine by machine.

Write both the weight and the volume solids into the technical agreement, and give the two side by side. Send only one and the wet film thickness on the other side of the table will not be the one you asked for.

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Published by Innater Intelligent Equipment Co., Ltd, a maker of non-standard coating machines and automated lines. Email 13316631159@163.com | Website https://www.getinnater.com/ | Tel +8613316631159