Table of Contents
1. Aqueous Coating
1.1 What is Aqueous Coating?
Aqueous coating is a water-based liquid applied to the surface of printed materials using either an inline printing press or an offline coating machine. It is characterized by being non-toxic, odorless, having a strong sense of transparency, and emitting no volatile organic compounds (VOCs). This makes it particularly suitable for packaging and printing in industries with high environmental requirements, such as food, pharmaceuticals, and tobacco.
Once dried, aqueous coating forms a soft, natural protective layer that significantly enhances the water resistance, abrasion resistance, and chemical resistance of the printed materials.
1.2 Process Principles
1.2.1 Preparation
Material Preparation: Use water-based varnish with water-soluble resin as the main component, along with other necessary additives like emulsion molecules and abrasion-resistant particles.
Incoming Inspection: Inspect the aqueous varnish to ensure key indicators such as viscosity, solid content, and pH value meet the standards, thus ensuring the stability of batch quality.
1.2.2 Wetting and Coverage
Wetting Process: The aqueous varnish begins to wet and evenly cover the surface of the printed material.
Component Interaction: The free abrasion-resistant particles and molecules in the varnish start to interact with the water-soluble resin.
1.2.3 Diffusion and Penetration
Diffusion: The aqueous varnish spreads evenly on the surface of the printed material, ensuring comprehensive coverage.
Penetration: The components of the varnish begin to penetrate the micro-convexities on the surface of the printed material, laying the foundation for subsequent curing and film formation.
1.2.4 Curing and Film Formation
Drying Process: Use infrared drying and other equipment to remove moisture from the wet film, gradually drying the varnish.
Film Formation: During drying, the film-forming resin and emulsion begin to cure, forming a protective film. Abrasion-resistant particles are also fixed within the film, enhancing its abrasion resistance. This protective film smooths out the micro-convexities on the surface of the printed material, enhancing its visual appeal. Additionally, this layer can reflect light, creating a high-gloss effect that makes the printed material look more vivid and colorful.
1.2.5 Quality Control and Stability
Viscosity Adjustment: In high-temperature drying seasons, due to water volatility, viscosity measurement might be needed to determine the amount of water to add, maintaining varnish stability.
Equipment Control: Use suitable coating machines for coating and drying operations to ensure the varnish is evenly applied and achieves the desired drying effect.
1.3 Product Applications
Food Packaging: Aqueous varnish provides a high-gloss surface effect for food packaging, making it more attractive while meeting hygiene and safety requirements.
Cosmetic Packaging: Coating with aqueous varnish can give cosmetic packaging a more appealing look, enhancing market competitiveness.
Pharmaceutical Packaging: Pharmaceutical packaging has stringent hygiene and safety requirements. The use of aqueous varnish not only improves the gloss but also does not adversely affect the medication.
Label Printing: Labels serve as important identifiers for goods, making their appearance and durability crucial. Aqueous varnish can provide a tough protective layer, increasing the abrasion and scratch resistance of labels, ensuring they remain intact during application and use. It also enhances the gloss, making labels more noticeable and easier to read.
Commercial Printing: In commercial printing, aqueous varnish is widely used for brochures, posters, catalogs, and other printed materials. It enhances the surface gloss and color saturation, making prints more attractive and eye-catching.
Packaging Paper: Aqueous varnish can be used on various packaging papers, including those for food, pharmaceuticals, beverages, and daily necessities, improving surface gloss and abrasion resistance.
1.4 Characteristics of Aqueous Coating
Fast Drying Speed: Utilizes efficient drying technology like infrared drying, significantly improving production efficiency.
High Gloss: Forms an even, transparent film on the surface, significantly enhancing gloss and visual appeal.
Strong Abrasion Resistance: The formed film is highly resistant to abrasion, effectively protecting printed materials from damage.
Good Adhesion: Excellent adhesion to the surface of printed materials, preventing peeling or flaking, thus improving durability and stability.
Wide Application Range: Suitable for various materials, including paper, plastic films, etc., used in food, pharmaceutical, cosmetic packaging, labels, brochures, etc.
Easy to Adjust: The formula is easily adjustable to meet different requirements for viscosity and gloss.
Good Equipment Compatibility: Compatible with various printing equipment, including offset, gravure, and flexographic printers.
Cost-effective: Lower cost and no need for additional solvent recovery and handling equipment, reducing production costs.
2. UV Coating
2.1 What is UV Coating?
UV coating, short for ultraviolet coating, involves applying a special coating to the surface or specific areas of printed materials using UV-specific coatings. These coatings are cured and hardened rapidly under ultraviolet light, typically within 0.1-0.3 seconds, forming a protective layer on the surface of the printed material.
2.2 Process Principles
UV coating involves the use of ultraviolet light to cure coatings on material surfaces. UV light activates photoinitiators in the coating, triggering a cross-linking reaction that cures the coating into a film.
2.2.1 Preparation
Choosing Coatings and Light Sources: The choice of coating affects product quality and performance. Common coatings include acrylic resin and polyurethane, known for fast curing, abrasion resistance, and high gloss. The light source should provide adequate UV intensity and uniformity while ensuring safety and stability. Common sources are mercury lamps and LED lights.
Preparing the Work Environment: Ensure safety and product quality, including cleaning the work area and checking equipment status.
2.2.2 Surface Treatment
Removing Contaminants: Ensure the coating adheres well and looks good.
Pre-treatment Processes: Include de-powdering, flattening, and corona treatment, ensuring the UV coating achieves a smooth, glossy, and firm adhesion.
2.2.3 Applying UV Coating
Using Special Equipment: Evenly apply UV coating on the material surface using special spray equipment. Controlling the application amount is crucial, typically adjusted via equipment parameters and coating concentration.
2.2.4 UV Curing
Curing Under UV Light: Place the coated material under UV curing equipment. UV light triggers the cross-linking reaction in the photoinitiators, curing the coating into a film within seconds.
2.3 Product Applications
Printing Industry: Enhances the gloss and abrasion resistance of printed materials, including packaging, labels, cards, high-end business cards, and posters. UV coating creates unique artistic effects like spot gloss or matte, enhancing the appeal and marketability of printed materials.
Packaging Industry: Widely used for various packaging materials like paper, plastic, and metal. UV coating provides high gloss and abrasion resistance, making packaging more attractive and durable. It also enhances water, oil, and stain resistance, protecting the contents from external damage.
2.4 Characteristics of UV Coating
Environmental Friendliness: UV coatings are almost solvent-free, significantly reducing air pollution and improving the working environment. They are safe and meet high environmental standards.
High Efficiency: UV curing requires no thermal energy, is low-energy, and completes the curing process instantaneously, improving production efficiency. The fast drying speed allows direct stacking or proceeding to the next process.
Superior Appearance: UV-coated surfaces are smooth and glossy, with high hardness and abrasion resistance, bright and vibrant, and resistant to chemicals.
Cost-effectiveness: High active ingredient content, minimal evaporation, and low usage costs. For example, coating consumption on typical coated paper is about 4g/m², costing roughly 60% of lamination costs.
Wide Applicability: Suitable for a variety of substrates, including paper, aluminum foil, plastic, and more.
Resource Saving: Compact UV curing equipment saves space and energy, requiring no additional drying tunnels.
Convenient Operation: No issues like curling, bubbling, or delamination; can be stacked easily for subsequent processing.
3. Comparison Between Aqueous and UV Coating
3.1 Environmental Friendliness
Aqueous coating is non-toxic, odorless, and reduces VOC emissions. UV coatings have a slight odor.
3.2 Drying Speed
Aqueous coatings dry quickly, enhancing production efficiency. Modern aqueous coatings can achieve drying speeds of about 200m/min. UV coatings cure even faster.
3.3 Surface Performance
Aqueous coatings provide good gloss and fluidity, with excellent water resistance, adhesion, and abrasion resistance once dry. UV coatings are glossier and more abrasion-resistant.
3.4 Adaptability
Both coatings can be applied to various substrates like paper, board, aluminum foil, and gold/silver cardboard.
3.5 Cost
Aqueous coatings are cheaper and easier to use, requiring only water for dilution and cleaning. UV coatings are more expensive.
3.6 Issues
Aqueous coatings may cause size deformation in low-grammage paper. Proper handling of viscosity, printing speed, application amount, and drying time is necessary. UV coating quality control is more complex. Domestic UV coatings may have slight yellowing, affecting color fidelity.
Therefore, aqueous coating emphasizes environmental friendliness, quick drying, strong adaptability, and low cost. However, attention should be paid to the dimensional deformation of low-weight paper. UV coating stands out for its fast curing speed, high gloss, excellent physical and chemical resistance, and environmental benefits, although it involves more complex quality control and relatively higher costs.
