Flexible Can Coating Composition for Crack-Resistant Preforming
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Solution Overview
Problem
Current methods for coating aluminum or steel sheets before forming metal cans often result in coating failures such as cracking and peeling, especially after the forming process, which increases production costs and reduces the durability of the finish.
Innovation Solution
A coating composition comprising 60-90% active hydrogen-containing polyester, 0.1-10% polyanhydride, and 5-30% curing agent is applied to a metal substrate, then cured to form a flexible and ductile finish that can withstand the forming process without fracturing or peeling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polyvinyl chloride coatings are used as a formable finish on aluminum sheets, then the coating can be applied before forming, but the finish cracks and peels after forming the can and bead
Solution Approach 1:
The patent changes the chemical composition parameters of the coating system by using a two-component polyester-polyisocyanate system instead of polyvinyl chloride. This parameter change enables the coating to maintain flexibility and adhesion through the forming process while achieving durable finish without cracking or peeling.
Solution Approach 2:
The invention uses a composite coating system consisting of polyester resin and polyisocyanate crosslinking agent. This composite material provides both the formability needed during can manufacturing and the durability required for the final product, resolving the contradiction between ease of manufacture and reliability.
2Reliability
If a coating is applied after can forming, then the coating can be applied to the finished can body, but production costs increase due to the separate coating step
Solution Approach 1:
The patent applies the coating to the aluminum sheet before the can forming process (pre-coating). This preliminary action eliminates the need for a separate post-forming coating step, reducing production costs while maintaining coating quality through the entire manufacturing process.
Solution Approach 2:
The coating formulation is specifically designed with polyester and polyisocyanate components that enable pre-coating application. The chemical parameters of this system allow the coating to be applied to flat sheets and remain flexible during subsequent forming operations, making pre-coating feasible and cost-effective.
3Strength
If the coating is made rigid to provide durable finish, then the coating provides hard protection, but it fractures and peels during the forming process
Solution Approach 1:
The patent adjusts the chemical parameters of the coating system by using polyester resin with controlled molecular weight and flexibility, combined with polyisocyanate crosslinking. This parameter optimization creates a coating that balances hardness for durability with sufficient flexibility to withstand forming operations without fracturing or peeling.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The coating composition provides a durable, flexible finish that adheres well to the metal substrate, reducing the likelihood of cracking and peeling during can formation and subsequent use, thereby lowering production costs and enhancing the cans' durability.
Implementation Method 1
a curing agent reactive with the active hydrogens associated with (a) and (b)
Data Source
AI summary
A coating composition includes: (a) 60 to 90 percent by weight of an active hydrogen-containing polyester having a number average molecular weight of at least 23,000, an acid value below 5 and a hydroxyl value below 10, (b) 0.1 to 10 percent by weight of a polyanhydride, and (c) 5 to 30 percent by weight of a curing agent reactive with the active hydrogens associated with (a) and (b), the percentages being based on weight of resin solids of the coating composition, where the reaction product of the coating composition forms a single coating without including a thermoplastic polymer. The coating composition is useful in coating food and beverage containers, particularly 2-piece cans formed by drawing and redrawing (DRD).