Nitrocellulose-based inks are widely used in flexible packaging printing due to their chemical and physical characteristics that make them ideal for applications on plastic films, paper and metal substrates.
Nitrocellulose is a polymer that is derived from cellulose by treatment with nitric and sulfuric acid, resulting in a material with excellent film-forming and adhesion properties.
Below, we delve into the features, benefits, and limitations of nitrocellulose inks for printing flexible packaging.
Composition of nitrocellulose-based inks
Flexographic and gravure printing inks that contain nitrocellulose include a combination of resins, pigments, solvents and additives. Nitrocellulose serves as the main binder, providing a stable matrix that retains pigments and effectively binds to the substrate. The solvents used are chosen to facilitate application and rapid drying during the printing process.
Typical composition of these inks may include:
- Binding resin: nitrocellulose (10% ÷ 30%).
- Pigments: to give color and opacity.
- Solvents: Ethyl acetate, MEK, isopropyl alcohol, and other fast-evaporating volatile solvents.
- Additives: Dispersants, rheological agents, and surface tension modifiers to improve flowability and adhesion.
Properties of nitrocellulose inks
Nitrocellulose-based inks have several properties that make them particularly suitable for use on flexible packaging:
- Fast drying: thanks to the volatility of solvents, the inks dry quickly even at low temperatures, reducing the risk of smearing during high-speed printing.
- Excellent adhesion: nitrocellulose has excellent adhesion to a wide range of substrates, such as polyethylene, polypropylene, polyesters, and metal films.
- Good mechanical resistance: ink films are flexible but resistant, maintaining integrity even during folding and lamination processes.
- Versatility: the inks can be formulated for flexo and gravure printing, adapting to different printing requirements and speeds.
Applications in flexible packaging
Nitrocellulose-based inks are mainly used for printing on flexible packaging materials for food, pharmaceuticals and consumer products, ideal for:
- Polypropylene packaging: suitable for bags, pouches and sealable packages.
- Laminated films: used in multi-layer structures, where the ink must maintain stability during the lamination process.
- Paper and cartonboard: Good adhesion and fast drying also make them suitable for printing on label and packaging paper.
Advantages and limitations of nitrocellulose-based inks
Advantages:
- Fast drying: minimizes the risk of unwanted transfer and improves print quality.
- High pigment compatibility: nitrocellulose has a high ability to disperse various types of pigments, providing bright and uniform colors.
- Good chemical resistance: to chemicals, greases and oils, important characteristics for food and pharmaceutical packaging.
Limitations:
- Yellowing tendency: under conditions of prolonged exposure to UV light or heat, nitrocellulose can turn yellow over time.
- Safety: nitrocellulose is highly flammable, which requires stringent safety measures during handling and storage.
- Regulatory restrictions: the use of volatile solvents means that these inks must comply with volatile organic compound (VOC) environmental regulations.
Regulations and Safety of Nitrocellulose-Based Inks
Due to their flammability and solvent content, nitrocellulose-based inks are subject to strict regulations. In Europe, REACH requires the controlled use of hazardous solvents and the assessment of the risk of long-term exposure. In the United States, the EPA regulates the emission of VOCs and the safe handling of these inks in industrial processes.
Recent Developments and Low Environmental Impact Inks
With increasing restrictions on VOCs and increasing demand for sustainability, manufacturers are developing nitrocellulose-based alternatives with reduced solvent content or based on water-soluble mixtures. These hybrid inks seek to maintain the drying and adhesion characteristics of nitrocellulose, reducing environmental impact.
How to safely recover waste solvents containing nitrocellulose-based inks
Distillation of waste solvents containing nitrocellulose-based inks can present risks related to the chemical nature of nitrocellulose and the operating conditions of the process. Nitrocellulose, in fact, is known to be a highly flammable and thermally unstable substance, especially when mixed with solvents with a low flash point.
For the safe recycling of nitrocellulose-containing solvents, it is essential to adopt advanced distillation systems and a systematic approach that includes appropriate techniques and safety measures. Here are some best practices for performing a safe and efficient recovery:
Preliminary evaluation of solvent composition
- Perform a chemical analysis of the spent solvent to identify the concentration of nitrocellulose and other potentially unstable components.
- Evaluate for impurities or catalysts that could affect thermal stability.
Filtration and pre-treatment
- Before distilling, remove the solids from the spent solvent. This reduces the risk of clogging and residue build-up in the boiler.
- In some cases, a pretreatment with neutralizing agents can help stabilize nitrocellulose and reduce its reactivity.
Vacuum distillation and inertization
- Vacuum distillation is highly recommended to reduce the boiling point of the solvent, minimizing the exposure of nitrocellulose to high temperatures.
- Maintain a controlled pressure and temperature profile to prevent sudden heat spikes.
- Use inert gases such as nitrogen to maintain an oxygen-free atmosphere within the plant to significantly reduce the risk of fire or explosion during distillation.
Monitoring and safety systems
- Install temperature and pressure sensors to monitor operating conditions in real time.
- Implement alarm systems and automatic safety valves to intervene immediately in case of detection of abnormal conditions.
Safe disposal of residues
- Nitrocellulose-containing residues should be disposed of as hazardous waste and should not be accumulated or allowed to dry in the open air.
- Use anti-static containers and dedicated storage areas to prevent the risk of self-ignition.