Metal Can Polyester-IPDI Coating for Corrosion and Sterilization
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Solution Overview
Problem
Existing coatings for metal cans used in food and beverage packaging lack a balanced combination of properties such as adhesion, corrosion resistance, chemical resistance, flexibility, stain resistance, and hydrolytic stability, while also failing to withstand processing conditions during can fabrication and food sterilization, particularly due to the limitations of crosslinking between polyester and phenolic resins.
Innovation Solution
A coating composition comprising a polyester polyol and a modifying polyester, along with isophorone diisocyanate and optionally an amino resin, is applied to the metal cans, with specific monomer ratios and properties to enhance adhesion, corrosion resistance, and flexibility, ensuring stability under processing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polyesters with hydroxyl functionalities are used for coating, then the coating can be applied to metal cans, but the crosslinking with phenolic resin is too poor to provide adequate solvent resistance and corrosion protection
Solution Approach 1:
The patent changes the chemical parameters of the polyester by using carboxyl-terminated polyester instead of hydroxyl-terminated polyester. This parameter change enables adequate crosslinking with phenolic resin, achieving the required crosslinking density for solvent resistance and corrosion protection without complicating the coating formulation.
Solution Approach 2:
The patent creates a composite coating system by combining carboxyl-terminated polyester with phenolic resin. This composite material approach enables effective crosslinking between the two components, providing both the required protective properties and formulation simplicity.
2Reliability
If higher crosslinking is achieved to improve corrosion resistance and retort resistance, then the coating becomes less flexible, which reduces crazing resistance and bending ability during processing
Solution Approach 1:
The patent adjusts the carboxyl content parameter of the polyester to an optimal range (0.5-5.0 mmol/g). This parameter optimization achieves adequate crosslinking for corrosion resistance while maintaining sufficient flexibility, resolving the contradiction between these two properties.
Solution Approach 2:
The patent creates local quality differentiation within the coating by having the carboxyl groups concentrated in the polyester component, which then distributes crosslinking density uniformly throughout the coating matrix. This enables localized crosslinking control that maintains overall flexibility while providing corrosion resistance.
3Temperature
If coatings are designed to withstand high processing temperatures during can fabrication and food sterilization, then the coating may lose flexibility and adhesion, but if too much flexibility is maintained, the coating cannot withstand processing conditions
Solution Approach 1:
The patent creates a composite system where carboxyl-terminated polyester crosslinks with phenolic resin to form a thermally stable network. This composite structure maintains both thermal resistance and adhesion, as the crosslinked network provides heat stability while the phenolic resin ensures strong metal substrate bonding.
Solution Approach 2:
The patent optimizes the carboxyl content parameter and molecular weight of the polyester to achieve a balance between thermal stability and adhesion. The specific parameter ranges enable the coating to withstand processing temperatures while maintaining reliable adhesion to the metal can.
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 balanced set of properties including adhesion, corrosion resistance, flexibility, and hydrolytic stability, enabling the cans to withstand processing and sterilization without compromising performance.
Implementation Method 1
crosslinking between common polyester and phenolic resin is too poor to provide adequate properties
Implementation Method 2
TMCD can also provide good hydrolytic stability of the polyester due to its secondary hydroxyl functionality
Implementation Method 3
Prolonged contact between the metal and the filled product can lead to corrosion of the container. To prevent direct contact between filled product and metal, a coating is typically applied
Data Source
AI summary
A metal can for food, wherein at least a part of the surface is coated with a coating composition comprising: (a) a polyester polyol in an amount of 55-90 wt.%; (b) a modifying polyester in an amount of 1-20 wt.%; and (c) isophorone diisocyanate (IPDI) in an amount of 5-40 wt.%, all wt.% are based on the total weight of (a), (b), (c), and (d).