Understanding Porous vs Non-Porous Printing Applications

"Porous" and "non-porous" is one of the first categorisations a printer applies when reviewing an unfamiliar substrate. It sets the direction for ink chemistry, drying, and long-term adhesion. Getting this categorisation wrong is a common root cause of coding and printing problems on pharmaceutical and food manufacturing lines.

At Axion Group we supply specialist inks for a range of substrates. This article explains the practical significance of porosity, the common porous and non-porous surfaces encountered in pharmaceutical manufacturing, and how the categorisation guides ink selection.

What porosity means for print

A porous surface has micro-scale openings that allow ink solvent — and, to a lesser extent, ink resin — to be absorbed into the substrate. Absorption contributes to drying and to mechanical anchoring of the printed image. A non-porous surface offers no absorption; the ink must dry entirely through evaporation and adhere through surface interactions such as wetting, surface energy match, and, where used, corona or plasma pre-treatment.

The distinction is not always binary. A coated paper carton can behave as porous on the uncoated inside and non-porous on the outer coated face. A varnished label can shift from porous to non-porous depending on the coverage of the varnish. Treat the printed surface, not the base material, as the reference.

Common porous substrates in pharma

Substrates typically treated as porous in pharmaceutical secondary packaging include:

• Uncoated paperboard cartons
• Uncoated label stock (cellulose-based)
• Corrugated shipping outers
• Tyvek and similar non-woven sterile-barrier materials
• Some medical-grade release papers

For these substrates, water-based or alcohol-based coding inks are often serviceable, provided the solvent balance is compatible with the substrate's absorbency and the line speed.

Common non-porous substrates

Non-porous substrates commonly seen include:

• Aluminium blister foils (lacquered or unlacquered)
• PVC / PVDC blister film
• Glass vials
• HDPE and PET bottle surfaces
• Coated / varnished cartons
• Polyester-based labels

Non-porous substrates typically require solvent-based inks with tuned resin systems and adequate flash drying. Adhesion is more sensitive to surface contamination — release oils, mould-release compounds, or handling residues can all cause code failure.

Choosing the right ink chemistry

The chemistry of a coding ink is chosen against the substrate and the code's expected environment. A code that will pass through a subsequent process (autoclaving, hot-water rinse, high-humidity storage) has to be evaluated against those conditions. A code that is only expected to survive dry warehouse storage has a wider window of acceptable chemistries.

We generally recommend the following logic when short-listing:

• Porous substrate, dry environment — water-based coding inks or mild-solvent inks are typically first candidates.
• Non-porous substrate, dry environment — solvent-based inks with MEK or acetone base are typically first candidates.
• Non-porous substrate, wet or heat-exposed environment — evaluate solvent-based inks with additional resistance additives; expect a longer trial.

The final selection is confirmed by application testing, not by product datasheet alone.

Testing methodology

A minimum test protocol we suggest with customers is:

1. Print a representative code with the candidate ink under normal line conditions.
2. Allow the code to cure for the documented interval.
3. Perform an abrasion test (rub with a defined material, count strokes to failure).
4. Perform a solvent-rub test appropriate to the intended environment.
5. Age the sample for at least 24 hours before repeating the abrasion and solvent tests.
6. Where relevant, expose to the actual downstream process (autoclave, warm-water bath, high-humidity chamber) and repeat.

Any change of substrate — even from the same supplier — is a valid reason to re-run this sequence. Product suitability remains subject to application testing.

If you are trying to work out whether a substrate should be treated as porous or non-porous, or whether the current ink on your line is the right chemistry for the surface it is being printed on, Axion Group can help you structure the evaluation. A short conversation about substrate, print process, and downstream conditions is usually enough to define a productive first trial.