PROCESSING AND USE OF 2-COMPONENT INKS

High-performance physical characteristics include lightfastness, weather resistance, adhesion, abrasion resistance and superior chemical resistance. 2c inks are indispensable for substrates, such as glass, metals, durable plastics and polyolefins. For these materials, single-component (1c) inks often fail to meet the required performance criteria. Despite 2c ink systems’ clear benefits, there are inherent challenges. These include formulation principles, process control and post-processing requirements. 

1c systems are used – supplied or diluted – to meet the viscosity requirements of printing presses. Once dried, 1c inks are ready for post processing. However, they often have physical and chemical resistance property deficiencies. Whilst 1c solutions are simpler to use, they sometimes fail to achieve high performance requirements.

Unlike 1c inks – which rely solely on solvent evaporation for film formation – 2c inks chemically react to cross-link polymers. 

Physical drying – Solvent evaporation leads to film formation. Typically, solvent reducer is added to control ink viscosity and rheological behaviour.

Chemical cross-linking – A reactive cross-linking agent (hardener) is added to the system to cross-link functional groups in the ink binder. This forms a chemically resistant polymer network. To achieve optimum results it is critical to only add the recommended dose of hardener and follow the recommended process conditions.

This cross-linking reaction is time- and temperature-dependent. Whilst a print may be touch dry, the reaction can continue for 
up to 72 hours (or longer) – at ambient conditions (20°C) – to achieve full cure. Full cure is required before the ink film achieves its final resistance properties.

“Inks need to be thoroughly stirred to homogenise pigments, binders and additives”

Ink compatibility – Ink systems need to be selected based on substrate surface energy, porosity and thermal stability. For low-energy surfaces – such as polyolefins – pre-treatment (for example, flame or corona) may be necessary.

Hardener type – This should be selected based on cure temperature (room temperature versus elevated), chemical resistance requirements (acid/base, solvents) and mechanical properties (flexibility versus hardness).

Mix ratio precision – Deviations from specified ratios (typically 4:1 to 10:1 ink:hardener by weight) can result in incomplete cross-linking, poor adhesion or brittleness. 

It is important to carefully manage the use of 2c ink systems as they can have a more limited time of use.

Unmixed inks and hardeners degrade over time due to moisture sensitivity and reactive group instability. They need to be stored in sealed containers under controlled conditions for optimum shelf life.

Pot life begins upon mixing, with a typical range of 2–8 hours. It is influenced by temperature and humidity. An increase in gelation or viscosity indicates the end of usable life.

When preparing a 2c ink, it is necessary to adhere to the following steps to ensure best on-press performance.

Pre-mixing – Inks need to be thoroughly stirred to homogenise pigments, binders and additives.

Weighing – Precision, calibrated scales must be used to measure ink and hardener. Batch sizes that are too large to consume before exceeding pot life, should be avoided.

Catalyst addition – Hardener should be added slowly, while stirring, to prevent localised reaction zones.

Post-mix resting – 5–10 minutes needs to be allowed for degassing and partial pre-reaction.

Viscosity adjustment – Compatible solvents need to be used after hardener incorporation, to maintain stoichiometric balance.

It is important to condition the substrate prior to printing. Substrates below ambient temperature may cause condensation. This can lead to premature reaction with moisture-sensitive hardeners, such as isocyanates. In addition, oils, dust and residues need to be removed using appropriate solvents or plasma (corona) treatment. For glass and metal substrates, pre-heating to 20–25°C improves ink wetting and reaction kinetics.

2c inks are suitable for screen, pad and inkjet systems with solvent-resistant components. For multi-pass printing, interlayer adhesion depends on timing. Overprinting should occur within 12–24 hours to avoid surface passivation.

Additionally, table temperature and humidity should be maintained. High humidity (>60%) can interfere with curing, especially for moisture-reactive systems.

Screens, stencils and tools need to be cleaned immediately before the pot life expires. Hardened ink residues are insoluble and may require mechanical removal.

Using manufacturer-recommended cleaning agents will avoid damage to mesh or stencil materials.

“2c inks are suitable for screen, pad and inkjet systems with solvent-resistant components”

Although an ink may appear dry, it has not necessarily reached full cure where the cross-linking reaction is complete. Effective cross-linking is only achieved once the printed film is fully dried with controlled temperature and humidity. This happens in two phases.

Drying phase – Initial solvent evaporation occurs before most of the cross-linking. Forced air or infrared drying may be used.

“Full cure is required before the ink film achieves its final resistance properties”

Curing phase – Cure can be possible in 24–72 hours at 20°C depending on the system. All hardeners will have a minimum activation temperature below which the reaction may not complete. Therefore,
it is important to follow recommended temperature settings for each ink. Oven curing at 50–80°C can reduce cure time and improve cross-link density. A 10°C increase halves the reaction time. Humidity sensitivity is also an important factor to consider. Isocyanate-based hardeners react with water, forming urea and CO₂ which can cause bubbling or an incomplete cure.

Moving prints to cold storage immediately after printing should be avoided. The curing temperature needs to be maintained until full reaction is complete. Stacking and packaging should be delayed until ink is fully dry to prevent blocking or imprinting.

Timing of tests – Conduct adhesion, chemical resistance and abrasion tests should only be carried out after full cure. Premature testing may yield false negatives.

Accelerated quality control – Oven-cured samples can be used for rapid assessment. However, validation with ambient-cured samples for production equivalence is advised.

Standard tests – Cross-hatch adhesion (ISO 2409), solvent rub resistance (MEK double rub), abrasion resistance (Taber test) and weathering (QUV exposure).

Manufacturer specifications should be followed for mix ratios, drying conditions and pot life. Precision tools should be used for weighing and mixing. Control environmental factors to consider are temperature, humidity and substrate condition. It is important to implement robust quality-control protocols post-curing. Technical data sheets can be consulted for substrate-specific recommendations.

A special thank you to Johann Bauer, now retired, who put together the original content for this article.