Crystalline Polyester Can Coating Without Curing Agents
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Solution Overview
Problem
Existing coatings for metal cans, such as epoxy-based and vinyl chloride-based coatings, contain harmful substances like bisphenol A and formaldehyde, and lack sufficient processability and retort resistance, while aqueous coatings require organic solvents and have poor solvent solubility.
Innovation Solution
A crystalline polyester resin aqueous dispersion is developed using specific polycarboxylic acid and polyalcohol components, with terephthalic acid and 1,6-hexanediol as major components, achieving high crystallinity and solvent solubility, and omitting curing agents to ensure excellent processability and retort resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional epoxy-based coatings or vinyl chloride-based coatings are used, then corrosion resistance and flavor properties are improved, but harmful substances (bisphenol A, formaldehyde) are introduced and processability deteriorates
Solution Approach 1:
The patent removes harmful substances (bisphenol A, formaldehyde, isocyanate) from the coating composition by using an aqueous dispersion of crystalline polyester resin without curing agents, thereby eliminating carcinogenic and flavor-affecting components while maintaining protective functions
Solution Approach 2:
The patent changes the chemical composition parameters by using specific ratios of terephthalic acid (75-95 mol%) and 1,6-hexanediol (55-85 mol%) to achieve a melting point of 120-160°C and acid value of 180-300 eq/ton, which enables excellent processability and retort resistance without harmful additives
2Object-affected harmful factors
If aqueous coating is used to eliminate harmful substances, then health safety is improved, but solvent solubility deteriorates and organic solvents are still required
Solution Approach 1:
The patent changes the physical parameters of the polyester resin by controlling the melting point (120-160°C) and acid value (180-300 eq/ton) through specific monomer ratios, enabling the resin to form stable aqueous dispersions without organic solvents and achieve excellent solvent solubility
Solution Approach 2:
The crystalline polyester resin structure itself provides the necessary solubility and dispersion properties in water through its specific chemical composition, eliminating the need for external organic solvents or dispersing agents
3Quantity of substance
If high-acid-value acrylic modified polyester resin is used to improve solvent solubility, then aqueous dispersion is achieved, but processability deteriorates and retort resistance decreases due to unreacted crosslinking agent
Solution Approach 1:
The patent removes the crosslinking agent (β-hydroxyalkylamide) from the coating composition, using only the crystalline polyester resin component, thereby eliminating the source of unreacted residues that cause poor retort resistance while maintaining adequate processability through controlled acid value
Solution Approach 2:
The patent optimizes the acid value parameter to 180-300 eq/ton, which provides sufficient reactivity for processability while maintaining solvent solubility and eliminating the need for additional crosslinking agents that would compromise retort resistance
4Quantity of substance
If crystalline polyester resin with low melting point is used to improve solvent solubility, then aqueous dispersion is achieved, but retort resistance deteriorates
Solution Approach 1:
The patent establishes an optimal melting point range of 120-160°C through specific monomer composition (75-95 mol% terephthalic acid and 55-85 mol% 1,6-hexanediol), which balances solvent solubility for aqueous dispersion with sufficient thermal stability for retort resistance
Data Source
AI summary
An object of the present invention is to provide: a crystalline polyester resin aqueous dispersion that contains substantially no curing agent and that is therefore free from harmful substances such as bisphenol A and formaldehyde, wherein the crystalline polyester resin therein exhibits excellent solvent solubility and is therefore able to be formed into an aqueous dispersion with a relatively small usage of an organic solvent, and wherein the crystalline polyester resin aqueous dispersion is capable of forming a coating film that exhibits excellent properties such as processability and retort resistance; a coating composition containing the crystalline polyester resin aqueous dispersion; a coating film obtained from the coating composition; and a metal can containing the coating film. The crystalline polyester resin aqueous dispersion containing a crystalline polyester resin (A) described below. Crystalline polyester resin (A): the crystalline polyester resin containing a polycarboxylic acid component and a polyalcohol component as copolymerization components, wherein the crystalline polyester resin contains 75 mol% or more of terephthalic acid as the polycarboxylic acid component, and 55 mol% or more of 1,6-hexanediol as the polyalcohol component, wherein the crystalline polyester resin has an acid value of 180 eq/ton or more, and wherein the crystalline polyester resin has a melting point of 120 to 160°C.