Polyester Can Coating Composition for Crack-Resistant DRD Forming
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Solution Overview
Problem
Current methods for coating aluminum or steel sheets before forming cans result in high failure rates due to cracking and peeling of the finish during the forming process, leading to increased production costs and reduced durability of the coating.
Innovation Solution
A novel coating composition comprising 60-90% active hydrogen-containing polyester, 0.1-10% polyanhydride, 5-30% curing agent, and epoxidized polybutadiene polymer is applied to the metal substrate, which is then cured to form a flexible and ductile finish that withstands 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 can forming, but the finish cracks and peels during the forming process
Solution Approach 1:
The patent changes the chemical composition parameters of the coating system by using reactive polyester resins with specific hydroxyl values (5-20 mg KOH/g) and acid values (2-10 mg KOH/g), combined with polycarboxylic acid curing agents. This parameter optimization enables the coating to maintain flexibility and adhesion during severe forming operations while achieving proper cure and durability.
Solution Approach 2:
The invention creates a composite coating system combining polyester resins, polycarboxylic acid curing agents, and various additives in specific ratios. This composite formulation integrates multiple functional components: the polyester provides flexibility and adhesion, the polycarboxylic acid enables crosslinking for durability, and the combination achieves both formability and coating integrity that single-component systems cannot provide.
2Productivity
If a coating is applied before can forming, then production costs are reduced by eliminating the post-forming coating step, but the coating fractures and peels during can formation
Solution Approach 1:
The patent applies the coating composition to the aluminum sheet before can forming operations, allowing the coating to be cured and adhered to the substrate in its flat state. The specially formulated polyester-resin system with controlled molecular weight and functionality maintains flexibility during subsequent drawing, redrawing, and beading operations, preventing the coating defects that would otherwise require post-forming rework or replacement.
Solution Approach 2:
By optimizing the hydroxyl value (5-20 mg KOH/g) and acid value (2-10 mg KOH/g) of the polyester resin, the coating achieves the right balance of flexibility for forming and crosslinking density for durability. This parameter control ensures the coating can withstand the mechanical stresses of can formation without fracturing or peeling, eliminating production losses from defective cans.
3Reliability
If the coating is made more flexible to prevent cracking during forming, then coating integrity is improved, but the coating may lack sufficient durability and adhesion
Solution Approach 1:
The patent precisely controls the hydroxyl value (5-20 mg KOH/g) and acid value (2-10 mg KOH/g) of the polyester resin to achieve optimal flexibility. The lower functionality polyester provides the necessary flexibility and elongation to prevent cracking during severe forming operations, while the controlled acid and hydroxyl values ensure adequate crosslinking density for adhesion and durability.
Solution Approach 2:
The coating system combines low-functionality polyester resin (for flexibility) with polycarboxylic acid curing agents (for crosslinking and durability). This composite approach balances the conflicting requirements: the polyester matrix provides flexibility and crack resistance, while the crosslinked network from polycarboxylic acid consolidation provides adhesion strength and chemical resistance, achieving both formability and long-term durability.
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 significantly reduces the failure rate of the finish during can formation, providing a durable and flexible finish that maintains integrity throughout the manufacturing process and subsequent use, thereby reducing production costs and enhancing the can's performance.
Implementation Method 1
a coating composition which forms flexible and ductile finishes on metal
Implementation Method 2
comprising: (a) 60 to 90 percent by weight of an active hydrogen-containing polyester... (b) 0.1 to 10 percent by weight of a polyanhydride... (c) 5 to 30 percent by weight of a curing agent reactive with the active hydrogens
Implementation Method 3
an epoxidized polybutadiene polymer
Data Source
AI summary
A coating composition comprising a high molecular weight polyester, a trimellitic anhydride-polyol adduct and a curing agent is disclosed. The compositions are useful in coating food and beverage containers, particularly 2-piece cans formed by drawing and redrawing (DRD).