PPI vs Porosity in Foam Ceramic: Why Two 20 PPI Products Can Perform Differently

PPI and porosity are often listed together on foam ceramic datasheets.

Because both describe the internal structure, they are sometimes treated as two ways of expressing the same property.

They are not.

PPI describes how fine the pore network is.

Porosity describes how much of the total foam volume is empty space.

Two foam ceramics can have the same PPI while behaving very differently if their porosity and strut geometry differ.

What Is Porosity?

Porosity is the fraction of total material volume occupied by void space.

If a foam ceramic has 85% porosity, approximately 85% of its geometric volume consists of open or closed void space, while the remainder is solid ceramic.

For an open-cell process ceramic, most useful porosity should participate in the connected flow network.

Why High Porosity Matters

Higher open porosity generally supports:

• greater flow passage;
• lower bulk density;
• lower hydraulic resistance.

But extremely high porosity can also mean:

• thinner struts;
• reduced mechanical strength.

The material must therefore balance openness with structural integrity.

Why PPI Is Different

PPI says something about pore frequency.

Imagine two foams, both 20 PPI.

Foam A has:

• thin ceramic struts;
• large interconnected windows;
• very high open porosity.

Foam B has:

• thicker struts;
• smaller windows;
• lower porosity.

They have similar pore counts but very different amounts of solid ceramic.

Their pressure drop and mechanical strength can therefore differ substantially.

Strut Thickness Is the Missing Variable

The ceramic struts form the skeleton of the foam.

Thicker struts generally increase:

• ceramic mass;
• density;
• mechanical robustness.

At the same time, they reduce:

• open volume;
• pore-window size.

Therefore PPI should ideally be interpreted together with:

• porosity;
• density;
• strut geometry.

Pore Size Is Not Fully Defined by PPI Either

An irregular foam structure does not contain identical circular pores.

There are:

• cells;
• connecting windows;
• strut junctions.

A nominal 20 PPI rating therefore does not imply that every flow passage has one exact diameter.

The actual pore distribution matters.

Why Pressure Drop Can Differ at the Same PPI

Pressure drop depends on how easily fluid can pass through the connected pore network.

If one foam has:

• thicker struts;
• narrower windows;
• lower porosity,

it may produce greater resistance than another foam with the same PPI.

This explains why catalogue PPI alone cannot predict operating pressure drop.

Porosity and Mechanical Strength

More solid ceramic usually provides more load-bearing structure.

Therefore reducing porosity can increase mechanical robustness.

However, strength also depends on:

• ceramic composition;
• firing;
• defects;
• strut continuity.

A lower-porosity foam is not automatically stronger if it contains poorly formed or cracked struts.

Porosity and Catalyst Coating

A catalyst coating deposits material on the internal skeleton.

This changes the effective structure.

After coating:

• struts become thicker;
• pore windows become smaller;
• total open volume decreases.

The more coating applied, the more the hydraulic characteristics can shift.

This is why catalyst-carrier design should evaluate the final coated state, not only the bare ceramic carrier.

Porosity and Filtration

High porosity is attractive because it provides open volume for captured solids.

But filtration efficiency depends on more than total void space.

Particle capture also depends on:

• pore size;
• tortuosity;
• foam thickness;
• particle properties.

A very porous coarse foam may have excellent capacity but low capture efficiency for fine particles.

Bulk Density as a Practical Cross-Check

Foam ceramic density can help indicate how much solid ceramic exists per unit geometric volume.

If two products have identical external dimensions and PPI but one is much heavier, it may have:

• thicker struts;
• lower porosity;
• denser ceramic material.

Density therefore provides additional information beyond PPI.

Why Manufacturers Need to Confirm Production Data

Foam ceramic manufacturing involves a complex three-dimensional structure.

Small production changes can affect:

• coating thickness during forming;
• firing shrinkage;
• final porosity.

For critical projects, actual production data should be confirmed rather than relying only on a generic PPI label.

Which Parameter Should Engineers Prioritize?

It depends on the question.

For:

pore fineness → PPI is useful.

For:

open volume → porosity matters.

For:

weight → density matters.

For:

hydraulics → all of them matter together.

Common Purchasing Mistake

A buyer may specify only:

“30 PPI foam ceramic.”

That leaves several important variables undefined.

A better specification may include:

• PPI;
• porosity range;
• material;
• dimensions;
• minimum strength;
• service temperature.

For catalyst carriers, coating requirements should also be included.

Engineering Takeaway

PPI and porosity describe different aspects of foam geometry.

Neither should substitute for the other.