Aluminum Material Varieties, Properties, And Applications in Can Manufacturing

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Aluminum cans, also known as two-piece cans, consist of a can body and a can lid, both manufactured on different production lines using various aluminum alloys. In practice, the can lid is composed of two components: the lid body and the pull-tab, both produced with different alloys and thicknesses of aluminum strips on highly automated lines.

The can body typically employs Al-Mn series alloys, such as 3004, AA3004, AA3104, and AA3204, with a thickness ranging from 0.25 to 0.29 mm and a condition of H19. For can lids (cover, pull-tab components, etc.), the majority use 5182-H19 strips, while some may use 5052-H19 or H48. Cans with internal pressure, like beer and carbonated beverage cans, and those without internal pressure, like juice cans, utilize 5082 (or 5182) and 5052 alloy strips, respectively, in an H38 condition. The lid body (base lid) typically employs thin strips of approximately 0.27 mm thickness with 5082 or 5182 alloys. The pull-tab material is mostly 5052-H19, although Novelis Aluminum Sheet and Plate Company exclusively offers 5182-H19 and H48 coil strips. Material widths vary, including 45mm, 46mm, and 68mm.Materials for pull-tabs and sealing components are mostly 5052 alloy, occasionally with some instances of 5182 alloy. The combination of the pull-tab and sealing component is commonly referred to as the pull-tab.

Types of Coatings-Epoxy Ester Coatings

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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.

Epoxy Ester Coatings

Epoxy ester resin is a product obtained through the esterification process of plant oil reacting with epoxy resin. This resin imparts flexibility and color to coatings.

Epoxy ester coatings are primarily used for post-printing varnish, suitable for both mixed-can products and products resistant to boiling. Therefore, it is suitable for metal can bodies, deep-drawn cans, twist-off caps, crown caps, and all types of mixed-can products.

Most epoxy ester coatings are colorless, but the addition of dyes can produce golden epoxy ester coatings used for decorating the external surfaces of twist-off caps and crown caps. The key properties and advantages include:

(1) Excellent Gloss:

Exhibits a high level of gloss.
(2) Good Color Fastness:

Maintains good color stability, crucial for external designs.
(3) Combination of Flexibility and Hardness:

Combines flexibility with good hardness.
(4) Excellent Ink Compatibility:

Exceptionally compatible with inks, especially suitable for the “wet-on-wet” production process.
Compared to other synthetic coatings, epoxy ester coatings are one of the most versatile varieties. They can achieve the same level of gloss as oil resin varnish while overcoming the drawbacks of poor color fastness and susceptibility to yellowing.

Curing for epoxy ester coatings is achieved through oxidation and thermal polymerization. For mixed-can production, a peak temperature of 160-180°C is typically used, heated for up to 10 minutes when high ink color fastness is required. The difference in dry film color is minimal within this temperature range. When used for products resistant to boiling, heating with a peak temperature of up to 190°C for a maximum of 10 minutes is needed to maximize water and steam resistance performance.

Types of Coatings-Epoxy Amine Coatings

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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. The characteristics of various resins, as well as the performance and applications of coatings produced from them, are briefly outlined below.

Epoxy Amine Coatings:

Epoxy amine coatings are formulated with epoxy resin and amine resin as the primary components in specific proportions. Widely used as protective coatings for the external surfaces of can lids, can bodies, twist-off caps, and sealing caps, these coatings exhibit the following key properties and advantages:

(1) Colorless Coating:

The coating is transparent, providing a colorless appearance.

(2) Excellent Chemical Resistance and Processability:

Demonstrates good resistance to chemical agents and is easily processed.

(3) High Boil Resistance:

Exhibits strong resistance to boiling conditions.

(4) Good Color Fastness Upon Re-Baking:

Maintains good color stability when subjected to additional baking.

Types of Coatings-Epoxy Amine Coatings

Epoxy amine coatings are highly sterilization-resistant, particularly in strongly alkaline water with a pH range of 9 to 10, making them the preferred choice for protecting the external surfaces of can lids. This property is also utilized in packaging latex paint. The resistance to water absorption and resistance to whitening during the cooking process are significantly better than other modified epoxy products. Their excellent heat resistance prevents mechanical abrasions during heat treatment.

The color fastness and resistance to yellowing of epoxy amine coatings during re-baking are noteworthy, making them widely applied as protective coatings for the external surfaces of three-piece beverage and food cans, including can bodies, can lids, twist-off caps, and crown caps. They are especially useful when the final baking of internal coatings may affect the performance of external coatings.

Epoxy amine coatings maintain their performance well even when exposed to contents that may spill and corrode the external coating of the can. Due to their excellent adhesion, they are also used as base coatings for printing.

Curing for epoxy amine coatings is achieved through thermal polymerization. For mixed-can products, they are typically heated to a peak temperature of 170-180°C for up to 10 minutes. When higher boil resistance and chemical resistance are required, a peak temperature of 200°C is recommended.

Epoxy amine coatings are also frequently used as internal coatings, especially in beverage cans with low corrosive contents such as dairy products and fruit juice beverages.