TYPES OF COATINGS- ACRYLIC ESTER COATINGS

TYPES OF COATINGS- ACRYLIC ESTER COATINGS

Currently, the main types of coatings used for canned goods include epoxy phenolic coatings, epoxy amine coatings, organic solvent coatings, ethylene-based coatings, polyester coatings, acrylic coatings, and epoxy ester coatings. Below is a brief introduction to the characteristics of various resins and the performance and applications of coatings made from them.

Acrylic Ester Coatings:

The acrylic resin currently used for metal surface coating is mainly solvent-based thermosetting. It can be formulated into basecoat magnetic paint and clear varnish for the surface. Due to the relatively poor adhesion and flexibility of acrylic resin compared to polyester in formulated paints, acrylic resin is less commonly used in basecoat magnetic paint. Instead, it finds more applications in the surface varnish of beverage and food cans, as well as various mixed-can products.

Acrylic ester resin demonstrates good ink compatibility. The reactive functional groups in current acrylic resins are primarily provided by introducing monomers such as methyl methacrylate, hydroxyethyl acrylate, glycidyl methacrylate, butoxyethyl acrylamide, and acrylic acid, offering hydroxyl (—OH), carboxyl (—COOH), and amine groups (—NHCH2OBu). These functional groups react with various melamine resins, end-capped isocyanates, or the resin's own active functional groups. Crosslinking occurs during high-temperature baking to achieve the desired properties of the coating.

For three-piece beverage and food cans, acrylic ester varnish needs to meet requirements such as excellent adhesion, pencil hardness, high gloss, abrasion resistance, scratch resistance, impact resistance, and maintaining high gloss after steam sterilization. To meet these requirements, the main acrylic ester resin should have an appropriate molecular weight (Mn: 4000~8000). Resins with too low molecular weight exhibit poor adhesion and impact resistance, while those with too high molecular weight result in high viscosity of the resin solution, leading to low solid content and high viscosity of the coating, which is unfavorable for coating application.

Additionally, the glass transition temperature of the resin should be controlled in the range of 10 to 70°C. A low glass transition temperature indicates the presence of too many long-chain alkyl groups in the monomers, such as butyl acrylate and isooctyl acrylate. These monomers increase molecular rotational space, acting as plasticizers. However, they also reduce the resin's reaction rate, affecting the coating's resistance to soiling and water vapor performance. On the other hand, a too high glass transition temperature, indicative of high intrinsic cohesion, results in poor adhesion and impact resistance. It also leads to high viscosity of the coating, low solid content, and hinders construction, affecting the fullness of the clear varnish.

For acrylic resins applied in can coatings, most amino resins and end-capped aliphatic isocyanates exhibit excellent compatibility. Amino resins contribute good reactivity, anti-soiling properties, hardness, and resistance to chemical corrosion. Aliphatic isocyanates enhance the toughness and abrasion resistance of the varnish, but this system exhibits relatively poor solvent resistance. As thermosetting acrylic resins primarily rely on their terminal hydroxyl or carboxyl groups to react with crosslinking agents, the choice of catalysts for catalysis should vary based on the etherification degree of the amino resin. Different acid catalysts should be selected for curing based on the end-capped isocyanate's end groups and their curing temperature.

Key points for the construction of acrylic ester coatings:

Compatibility and Reactivity: Amino resins and end-capped isocyanates are well-mixed with acrylic ester coatings. Amino resins provide good reactivity, anti-soiling properties, hardness, and chemical corrosion resistance. Aliphatic isocyanates enhance toughness and abrasion resistance, but this system exhibits poorer solvent resistance.

Catalyst Selection: Due to the reliance of thermosetting acrylic resins on terminal hydroxyl or carboxyl groups for reaction with crosslinking agents, different acid catalysts should be chosen for catalysis based on the etherification degree of the amino resin. For end-capped isocyanates, different curing temperatures and metal catalysts with varying activity levels should be chosen based on the end groups.

Construction points for acrylic ester coatings used in can coatings:

Molecular Structure and Crosslinking: Acrylic ester resins have limited intramolecular stretchability, and the crosslinking functional groups are distributed on the side of the molecular chain. The crosslinked network structure formed after heating the coating further restricts intra- and intermolecular movement, resulting in a film with a certain hardness and toughness. Care must be taken to control the curing temperature, especially when using high-amino or moderately etherified melamine resins to avoid film brittleness and poor adhesion.

External Varnish Requirements: External varnishes require good fullness, so viscosity and film thickness should be well-controlled during construction. Too low viscosity can lead to insufficient roller adhesion, resulting in thin film thickness, affecting the gloss and fullness of the product. However, caution is needed to prevent high viscosity, which can cause leveling and adhesion problems.

These considerations are crucial for achieving the desired performance and appearance of acrylic ester coatings in can applications.