TYPES OF COATINGS-POLYESTER COATINGS

TYPES OF COATINGS-POLYESTER COATINGS

Polyester resin is a product of the combination of polyfunctional alcohols and polyfunctional acids (anhydrides). Unlike alkyd resins, polyester resins do not contain fatty acids and are sometimes referred to as "non-oil alkyd resins." They offer better color stability and resistance to yellowing, surpassing the alkyd resin series coatings. Polyester coatings are used for various applications, including beverage and food cans, twist-off caps, crown caps, and general-purpose varnishes and topcoats.

Due to the composition of polyester, resin manufacturers can design resins with different hardness and flexibility properties. Polyester coatings exhibit excellent adhesion and are particularly advantageous for adhering to tinplate. They can be used as primers, topcoats, or colored varnishes. Additionally, the presence of more polar functional groups enhances wetting properties, effectively improving the coating's covering power and gloss. The higher molecular chain stretchability compared to acrylic and epoxy resins results in better impact resistance and processability. While polyester coatings may have slightly lower hardness as clear varnishes compared to acrylic esters or epoxy coatings, they demonstrate better toughness and abrasion resistance after curing.

As clear varnishes, polyester coatings can compensate for the inherent poor water vapor resistance by choosing suitable crosslinking agents and adjusting their amounts. This makes them well-suited for applications on high-temperature sterilized cans, maintaining a high gloss after water vapor sterilization. The key properties and advantages of polyester coatings include:

(1) High flexibility;

(2) Excellent adhesion to metal substrates, so no primer is required;

(3) Good color fastness of coating film;

(4) High solid content.

The selection of the main resin is crucial for designing qualified polyester coatings. Firstly, the molecular weight of the resin is a critical parameter. The molecular weight for polyester used in exterior coatings is generally controlled within the range of Mn: 3000 to 10000. Too low molecular weight can result in poor adhesion of the coating, and an increase in polar groups may affect the gloss after sterilization of food cans. Conversely, excessively high molecular weight may lead to characteristics similar to thermoplastic coatings, inadequate crosslinking, poor water vapor resistance after sterilization, loss of gloss in the coating, issues with plate sticking during coating application, and elevated viscosity with low solid content, which hinders coating application.

Secondly, the glass transition temperature of polyester is an important parameter. If the glass transition temperature is too high, the resin's viscosity becomes too large, making it difficult to formulate coatings. Additionally, the rapid increase in the resin's intrinsic cohesive energy can reduce adhesion and impact resistance of the film.

Combining polyester resin with various etherified amino resins can achieve a good balance of hardness and toughness, enhancing resistance to water vapor, anti-soiling, and solvent resistance. Benzene amino resins can provide the coating with initial gloss and certain chemical resistance. Combining polyester with isocyanates can improve the flexibility and processability of the film. The low activation energy of isocyanates can lower the film formation temperature. The choice and combination of these crosslinking agents can be tailored according to the specific requirements of the coating to achieve polyester coatings that meet the demands of metal can applications.

Construction points for polyester coatings: Polyester coatings have relatively lenient construction conditions due to the broad reaction conditions of polyester itself. The curing temperature of the coating can be adjusted based on the specific application scenario.

Recently developed coatings, crosslinked with polyester and phenolic resins, can be used for the internal coating of lightly corrosive food cans. If a non-PVC system or no epoxy resin is required, polyester-phenolic coatings serve as a good alternative. However, their chemical resistance may be slightly inferior to epoxy-phenolic coatings, limiting their use in applications involving highly corrosive foods.

Polyester-Organic Solvent Combination as Internal Coating for Acidic and Sulfur-Containing Food Packaging:

The combination of polyester and organic solvent is also used as an internal coating for the packaging of many acidic and sulfur-containing food products.

Polyester, as the main component of powder coatings, is also applied in the touch-up coating of weld seams on the inner walls of three-piece cans. After the three-piece can body is formed by resistance welding, the welded areas still have exposed metal substrate, and powder touch-up coatings provide protection at these weld seams. The advantages of powder coatings are as follows:

（1）100% Solid Technology: No volatile organic compounds (VOC) are present, unlike liquid coatings.

（2）Environmental Compliance: Powder coatings comply with environmental regulations.

（3）Excellent Coverage: Due to the thicker film, it offers superior coverage, forming a dense coating layer (higher than that of liquid touch-up coatings).

Powder touch-up coatings are typically white but can also come in gray and transparent varieties.

Powder coatings are applied using electrostatic spraying, where the powder is fluidized by airflow, and powder particles become charged (either by corona charging or tribocharging). Due to opposite charge nature, the charged particles are transferred to the substrate and adhere to the metal. The coated cans then enter a furnace, typically requiring baking at 232–260°C for 8–10 seconds to melt the fine powder particles, forming a uniform and continuous film. The heating process involves no chemical reactions, only melting, flowing, and subsequently forming a thermoplastic coating upon cooling. As thermoplastic polyester exhibits excellent chemical resistance and machinability, it can be used in almost all three-piece food cans.

To achieve protection for the weld seams on the inner walls, the quality of powder touch-up coatings must meet the following standards for physical, chemical, mechanical processing performance, and regulatory compliance.

(1) Good fluidity and appearance;

(2) Achieve good protection with the minimum coating thickness;

(3) Can be charged;

(4) Liquidity;

(5) Corrosion resistance/low porosity;

(6) Mechanical strength/flexibility;

(7) Resistance to cooking;

(8) Chemical resistance (food packaging test, solvent resistance);

(9) Low migration degree;

(10) Comply with FDA 175.300 and EU regulations.