BPA-Free Metal Can Coating for Flexible Corrosion Resistance
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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, a modifying polyester, isophorone diisocyanate, a resole phenolic resin, and optionally an amino resin, with specific monomer ratios and properties to achieve a balanced set of coating properties, including a glass transition temperature of 50 to 110°C, acid number of 0 to 10 mgKOH/g, hydroxyl number of 8 to 40 mgKOH/g, and molecular weights ranging from 5,000 to 100,000 g/mol.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polyesters with hydroxyl functionalities are used for crosslinking with phenolic resins, then the coating can be applied, but the crosslinking density is too poor to provide adequate solvent resistance and corrosion resistance
Solution Approach 1:
The patent modifies the polyester structure by incorporating TMCD units with secondary hydroxyl groups, which have different reactivity characteristics compared to conventional primary hydroxyl groups. This parameter change in the chemical structure enables adequate crosslinking density with phenolic resins while maintaining the desired coating properties including solvent resistance and corrosion resistance.
Solution Approach 2:
The invention creates a composite coating system combining TMCD-based polyester with phenolic resins. This composite material approach leverages the complementary properties of both components: the TMCD polyester provides flexibility and adhesion, while the phenolic resin contributes to crosslinking density, corrosion resistance, and heat resistance when properly crosslinked.
2Reliability
If higher crosslinking is achieved to improve corrosion resistance and retort resistance, then these properties are enhanced, but the coating becomes less flexible and more prone to crazing during processing
Solution Approach 1:
The patent optimizes the crosslinking parameters by controlling the polyester molecular weight, hydroxyl functionality, and composition to achieve adequate crosslinking density without excessive crosslinking. The TMCD-based polyester with secondary hydroxyl groups provides a balanced crosslinking rate that achieves corrosion resistance while maintaining flexibility and preventing crazing during can fabrication processing.
3Reliability
If epoxy and phenolic resin coatings are used to provide good balance of protective properties, then adhesion and corrosion resistance are achieved, but the coating contains BPA which is being phased out by industry sectors
Solution Approach 1:
The patent removes BPA and epoxy resins from the coating formulation, replacing them with alternative materials. The TMCD-based polyester provides the necessary adhesion and protective properties without containing BPA, while the phenolic resin component is used in a manner that avoids BPA formation, thus eliminating the harmful substance while maintaining coating performance.
4Temperature
If the coating must withstand processing conditions during can fabrication and food sterilization, then heat resistance is achieved, but the coating may lose flexibility and adhesion
Solution Approach 1:
The patent optimizes the polyester parameters including molecular weight (5,000-20,000 g/mol), hydroxyl number (8-40 mg KOH/g), and composition to achieve adequate crosslinking that provides heat resistance during sterilization while maintaining adhesion and flexibility. The TMCD structure with secondary hydroxyl groups enables this balanced performance under thermal processing conditions.
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 enhanced adhesion, corrosion resistance, chemical resistance, flexibility, and hydrolytic stability, ensuring the metal cans can withstand processing conditions and food sterilization without compromising on flexibility or resistance to crazing.
Implementation Method 1
crosslinking between common polyester and phenolic resin is too poor to provide adequate properties
Implementation Method 2
Coatings based on a combination of epoxy and phenolic resins are known to be able to provide a good balance of the required properties
Implementation Method 3
the coating must be able to withstand processing conditions during can fabrication and food sterilization
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 35-85 wt.%; (b) a modifying polyester in an amount of 2-45 wt.%; (c) isophorone diisocyanate (IPDI) in an amount of 5-30 wt.%; (d) a resole phenolic resin in an amount of 4-35 wt.%; and (e) an amino resin in an amount of 0-40 wt.%, all wt.% are based on the total weight of (a), (b), (c), (d), and (c).