What are the key points for controlling solvent residue in gravure printing machines?

Currently, solvent-based inks remain the mainstream in gravure printing. Despite over two decades of promotion of water-based inks in China, there has been no breakthrough in their application for plastic flexible packaging composite films. Even today, most water-based inks still use alcohol-water as diluents, failing to fully and effectively address VOC emissions. Moreover, under the future "dual carbon" requirements, water-based inks have not been able to outperform solvent-based inks, while UV inks face unresolved issues with free monomers. Therefore, solvent-based inks are likely to dominate the flexible packaging industry in the foreseeable future, and the path to solvent residue control remains challenging for the industry.

Using solvent-based inks for printing inevitably involves two issues:

Solvent emissions, which relate to environmental protection and national dual-carbon policies;

Solvent residue, which concerns food hygiene and safety.

Both issues are unavoidable hurdles in gravure plastic printing and are interconnected.

How are solvent emissions and solvent residue linked?

Managing solvent emissions requires VOC treatment, whether through solvent recovery or RTO combustion. To achieve efficient treatment, the solvent concentration in printing exhaust must be increased to above 2–3 g/m³; otherwise, it imposes significant cost pressures.

The mainstream VOC treatment method today involves reducing airflow and increasing concentration on the printing machine. This approach reduces air volume while increasing solvent content in the exhaust, allowing the drying hot air to be recycled continuously. While this improves emission reduction and thermal efficiency, it also raises the risk of solvent residue.

Let’s first examine the relevant national standards for the industry:

The Emission Standard of Volatile Organic Compounds for Printing Industry (GB 37822-2019) regulates VOC emissions for printing enterprises: "Non-methane total hydrocarbons, benzene, toluene, and xylene in exhaust gases from printing enterprises must not exceed 40 mg/m³, 0.5 mg/m³, and 8 mg/m³, respectively. Additionally, if the initial emission rate of non-methane total hydrocarbons exceeds 2 kg/h, the VOC treatment facility must achieve at least 80% efficiency."

For solvent residue, the national standard GB/T 10004-2008 stipulates that total solvent residue must not exceed 5.0 mg/m², with benzene-based solvents undetectable (<0.01 mg/m²).

These two standards set clear requirements for solvent residue in flexible packaging. While managing VOCs, the VOC concentration in drying air increases exponentially, making solvent residue control even more demanding for manufacturers.

Solvent residue originates from printing and compounding processes. To ensure total residue stays within limits, the practical solvent residue during printing should ideally be controlled below 2 mg/m². Key factors affecting solvent residue include pre-printing processes, printing inks, solvent ratios, drying conditions, and the solvent release properties of films. Let’s analyze each factor:

I. Pre-Printing Process

Pre-printing process design is critical for solvent residue control, as proactive measures play a decisive role in final outcomes.

Pre-printing design: Minimize large-area overprinting. Opt for spot colors instead of multi-color overprinting to avoid excessive ink layering, which can lead to incomplete drying and residue exceeding limits.

Avoid special inks: Limit large-area use of metallic, pearlescent, transparent, or matte inks. If unavoidable, plan the entire process (plate → ink → printing → compounding) in advance to prevent residue issues.

Plate engraving parameters: Scientifically set parameters like screen count and engraving depth to avoid excessive ink layering, which hinders drying and increases residue risks.

High-color-concentration inks: Use high-solid, low-viscosity inks to reduce overall ink and solvent usage, lowering residue risks.

Optimal process path: Based on equipment and drying capacity, set the best process considering ink, solvent, speed, drying, and film materials.

Pre-production assessment: Evaluate production capabilities (personnel, workspace, equipment, materials) to prevent residue issues during printing.

II. Printing Inks

Gravure inks are volatile-drying inks composed of pigments, resins (binders), solvents, fillers, and additives. Typical compositions include 10–30% resin, 5–20% pigment, 50–75% solvent, with the rest as fillers and additives. In practice, solvent dilution further increases the solvent proportion, all of which must fully evaporate to meet residue standards.

1. Pigments

Finer pigment particles mean larger surface areas, slowing solvent evaporation. Gravure inks use organic/inorganic pigments and dyes, with particle sizes: inorganic > organic > dyes. Ink production reduces solvent evaporation speed due to added pigments and fillers. In the same ink system, solvent release varies by color—magenta and black inks have poorer release due to small, highly adsorbent particles. Pigment selection and grinding affect solvent release, requiring comprehensive evaluation.

2. Resins

Resins act as dispersing media, ensuring uniform pigment distribution and forming protective films on substrates. They determine ink properties like viscosity, drying, gloss, flow, water resistance, transfer, film formation, and durability. Resin choice is paramount—changing resins alters ink systems, while changing pigments only affects color. Gravure inks use volatile-drying resins, which must meet five criteria:

High molecular weight, transparent/light color (no impact on ink hue);

Good printability, soluble in common solvents;

Uniform pigment dispersion and encapsulation;

Suitable viscosity for rapid solvent release;

Cost-effectiveness.

Common resins include polyamide, nitrocellulose, PVB, CPP, and polyurethane. Some resins have poor solvent release but good printability and affordability, making them mainstream—yet they increase residue risks. Solvent evaporation slows when resins dissolve, especially with polar solvents like alcohols, which bond strongly with polar resins (e.g., polyamide), increasing residue. Thus, inks with better solvent-release resins are preferred.

3. Solvents

Solvents are low-viscosity liquids that adjust viscosity and drying speed. They dissolve resins, disperse pigments, and improve flow. Solvents are classified as true solvents (dissolving resins) or co-solvents (non-dissolving). Some combinations become effective solvents when mixed, hence mixed solvents are common.

Gravure solvents fall into esters, benzenes, ketones, alcohols, and ethers, categorized by polarity (polar/non-polar), activity (active/inert), or boiling point (low/medium/high). Some use non-flammable hydrocarbons as "eco-solvents," but their high density and cost make them impractical for residue control.

Plastic gravure inks typically use mixed solvents, where evaporation rates depend on interactions, not just individual boiling points. Solvent selection must balance printability and minimal residue. Slow-drying solvents in mixtures can cause severe residue, so formulating solvent blends is crucial. Solvent purity also affects evaporation—water content has the greatest impact. Ink viscosity changes influence solvent release, further affecting residue.