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The Barrier Effect: High-Performance Zinc-Free Primers

How a 2007 allnex study showed that a well-engineered waterborne 2K epoxy binder can match, and in some cases beat, zinc-phosphate primers in standardized corrosion tests, without the zinc.

Table of Contents

Close-up of turquoise paint peeling and flaking from a rusted steel beam, exposing the corroded substrate beneath
Paint peeling from a corroded steel beam: the failure mode that a well-engineered barrier film is designed to prevent.

Editor's note: a legacy of sustainable coatings innovation

Long before "zinc-free", "label-free" and "low-VOC" became marketing buzzwords, allnex researchers were already asking uncomfortable questions: can you get high-end corrosion protection without relying on zinc-based pigments at all?

Back in 2007, allnex (then part of a predecessor organization) published a technical article in FARBE UND LACK titled "Zinkfrei und wässrig" (Zinc-free and Waterborne). It presented 2K epoxy primers that matched the performance of established zinc-containing systems in standardized lab tests.

Nearly two decades later, the industry is still wrestling with the same issues: aquatic toxicity labels, zinc restrictions, and the push toward waterborne coating technologies. This summary revisits that early work and shows how allnex was already pioneering solutions to problems the market is only now fully confronting.

Full experimental data

Download the original article (German PDF)

Access the full experimental data, salt-spray panel images, and detailed formulations from the 2007 FARBE UND LACK publication.

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The market challenge: beyond zinc phosphate

For decades, zinc phosphate (ZPO) has been the workhorse anti-corrosion pigment. It replaced older, highly toxic lead and chromate pigments and is still used widely as a benchmark in many coating systems.

However, under modern CLP/REACH frameworks, zinc phosphate is classified as very toxic to aquatic life with long-lasting effects. That classification typically triggers environmental hazard pictograms (the "dead fish" label) and makes it harder to meet eco-label and internal "restricted substances" requirements.

Formulators today face a familiar dual challenge:

  • Eliminate zinc-based pigments to avoid environmental hazard labels and align with sustainability targets.
  • Maintain demanding performance levels, such as 1,000-hour neutral salt-spray resistance, required by industrial specifications.

The allnex answer (2007): the "barrier effect"

In the original study, researchers set out to challenge a core assumption of corrosion protection: do we really need active zinc pigments if the binder film is engineered well enough?

The work focused on internally flexibilized waterborne epoxy dispersions from the BECKOPOX™ range. Unlike standard resins relying on external plasticizers (which migrate over time), these binders were designed to deliver:

  • High cross-link density — creating a tight, low-permeability film.
  • Permanent flexibility — resisting cracking even after aging.

This combination creates what the authors called a "barrier effect": a dense, elastic film that acts as a physical shield, blocking water and ions from reaching the metal substrate. The hypothesis was that if the barrier is strong enough, the active chemical inhibition of zinc pigments can be reduced, or in some cases eliminated altogether.

Key findings: when the binder does the heavy lifting

The study compared multiple waterborne 2K epoxy primer formulations, applied on steel and tested under rigorous standards including DIN EN ISO 9227 (neutral salt spray) and DIN EN ISO 6270-2 (condensation water).

Zinc-free systems achieved benchmark performance

Waterborne 2K epoxy primers based on BECKOPOX binders delivered corrosion protection at the level of established zinc phosphate systems. In some cases, zinc-free formulations even showed comparable or better blistering ratings. This proved that with the right resin design, zinc-free systems can reach the performance space traditionally occupied by heavy metals.

A full panel of pigments tested

The work didn't stop at a single pigment swap. The authors evaluated a broad spectrum of anticorrosive pigments, alongside a pigment-free reference formulation containing only titanium dioxide and inert fillers.

Pigment Chemistry Zinc-based?
ZPO Organo-modified basic zinc orthophosphate Yes
CHP Calcium hydrogen phosphate No
SAPP Strontium aluminium polyphosphate hydrate No
ZPA Modified zinc aluminium ortho-phosphate hydrate Yes
BW 191 Calcium barium phosphosilicate No
SZP 391 Strontium zinc phosphosilicate Yes
K-W 140W Zinc aluminium polyphosphate Yes
Act 106 Molybdenum zinc phosphate Yes
AC 5 Non-toxic anticorrosive pigment (proprietary) No
Reference Pigment-free — TiO₂ only Titanium dioxide (no active corrosion pigment) No
Figure 1. The nine anticorrosion pigments tested in the 2007 study, plus the pigment-free TiO₂ reference formulation used as a control. All pigments were compared at equal loading in the same waterborne 2K epoxy base formulation. Data transcribed from Tab. 2 of the original FARBE UND LACK article.

The result was telling. In condensation-water testing (DIN EN ISO 6270-2), the barrier properties of the binder dominated, meaning all systems performed well — most reached or approached the 1,000-hour mark regardless of which pigment was used. In salt-spray testing (DIN EN ISO 9227), several zinc-free systems matched or exceeded the zinc-phosphate benchmarks. The highest salt-spray result came from the reference formulation containing no corrosion protection pigment at all.

The "inert filler" surprise: BaSO₄ vs. ZPO

In a particularly striking case, the researchers compared two otherwise identical primers:

  • One pigmented with zinc phosphate (active).
  • One utilizing barium sulfate (BaSO₄) and titanium dioxide (inert).

In this specific formulation, the BaSO₄-based, zinc-free primer achieved equal or better neutral salt-spray resistance than the zinc phosphate version. This clearly demonstrated that with a high-performance barrier resin, an inert filler can be sufficient to meet standard corrosion tests.

Waterborne vs. solventborne

The study also compared the optimized waterborne system against a conventional solventborne epoxy primer. At comparable film builds, the waterborne system delivered equivalent overall corrosion protection and better edge coverage. Furthermore, it retained flexibility (cupping values > 3 mm after aging per DIN EN ISO 1520), whereas the solventborne system became brittle.

Salt spray resistance by corrosion protection pigment Bar chart of salt spray test results by corrosion protection pigment. The pigment-free reference containing only titanium dioxide is the tallest bar and the clear winner. ZPO and AC 5 tie for second. CHP, BW 191, SZP 391, and Act 106 group in the middle. SAPP and K-W 140W sit lower, and ZPA is the shortest. The result supports the barrier effect thesis: with a well-engineered binder, removing the corrosion pigment and relying on inert fillers can outperform every active pigment tested. Salt spray resistance by corrosion protection pigment Pigment-free reference (TiO₂ only) Corrosion protection pigment Ref TiO₂ ZPO CHP SAPP ZPA BW 191 SZP 391 K-W 140W Act 106 AC 5 0 200 400 600 800 1000 Corrosion protection pigment Salt spray resistance (hours)
Figure 2. Salt spray test results (DIN EN ISO 9227) for the waterborne 2K epoxy base formulation with different corrosion protection pigments at equal loading. Substrate: Gardobond OC. Film thickness: ~50 μm. Failure criterion: ≥10 mm undermining or ≥m1g1 blistering. The pigment-free TiO₂ reference beat every zinc-containing and zinc-free pigment tested — the key evidence for the barrier effect thesis. Redrawn from the original 2007 FARBE UND LACK article (Abb. 3).

Why this matters in 2026

By shifting the focus from "which zinc pigment do we use?" to "how do we engineer the optimal barrier film?", allnex provides a roadmap for coatings formulators to:

  • Simplify compliance — remove environmental hazard labels by eliminating zinc.
  • Strategize formulation — use zinc-free pigments more strategically on top of a strong binder backbone.
  • Adopt waterborne — embrace waterborne 2K epoxy systems as serious, long-term solutions rather than compromises.

Key benefits for zinc-free formulators

Pass eco-labels

Zinc-free formulations avoid the CLP/REACH "dead fish" aquatic-toxicity pictogram.

Engineer the film first

Start with a flexibilized barrier binder; pigment choice then becomes secondary.

Waterborne as equal

2K waterborne epoxies match solventborne corrosion performance at comparable film builds.

Permanent flexibility

Internally flexibilized dispersions resist cracking even after extended oven aging.

Pioneering solutions

When this study was published, zinc-free, waterborne 2K epoxy primers were a niche concept. Today, they are a strategic necessity. This work underscores how allnex has long been a pioneer in sustainable resin technology, providing the technical foundation that many formulators are only now starting to build upon.

If you would like to find out how allnex resins can improve your coating formulations, please feel free to contact our experts below.