Crystalline Polyester Can Coating for Retort-Resistant Aqueous Dispersion
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Solution Overview
Problem
Existing coatings for metal cans, such as epoxy-based and vinyl chloride-based coatings, pose health and environmental risks due to the use of harmful substances like bisphenol A, formaldehyde, and organic solvents, and lack sufficient processability and retort resistance.
Innovation Solution
A crystalline polyester resin aqueous dispersion with specific compositional ratios of polycarboxylic acid and polyalcohol components, along with a melting point of 120 to 160°C, is used to form a coating film that is solvent-soluble and retort-resistant, eliminating the need for curing agents and reducing organic solvent use.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional epoxy-based coatings or vinyl chloride-based coatings are used to achieve corrosion resistance and flavor properties, then the coating provides adequate protection, but harmful substances such as bisphenol A, formaldehyde, and organic solvents are introduced that pose health and environmental risks
Solution Approach 1:
The invention changes the chemical composition parameters of the coating by using crystalline polyester resin with specific molecular weight (10,000-50,000) and specific copolymerization ratios (orthophthalic acid 10-50 mol%, 1,4-butanediol 70-90 mol%), eliminating harmful substances while maintaining coating performance through precise parameter control
Solution Approach 2:
The invention uses a composite aqueous dispersion system combining crystalline polyester resin with water-soluble organic solvents and basic compounds, creating a new material system that replaces harmful epoxy and vinyl chloride-based coatings while providing equivalent or superior performance
2Reliability
If the melting point of crystalline polyester resin is increased to improve retort resistance, then the coating can withstand high-temperature sterilization, but solvent solubility deteriorates and the resin cannot be made into an aqueous dispersion
Solution Approach 1:
The invention optimizes multiple parameters simultaneously: melting point (120-160°C), molecular weight (10,000-50,000), and copolymerization ratios (orthophthalic acid 10-50 mol%, 1,4-butanediol 70-90 mol%), achieving a balance where the resin has sufficient heat resistance for retort sterilization while maintaining solubility in water-soluble organic solvents for aqueous dispersion formation
Solution Approach 2:
The invention uses excess water-soluble organic solvent and basic compound in the aqueous dispersion formulation to ensure complete dissolution and dispersion of the crystalline polyester resin, compensating for the reduced solubility that results from optimizing the melting point for retort resistance
3Reliability
If high-acid-value acrylic modified polyester resin is used to achieve adequate coating properties, then the coating can be formed, but sufficient processability is not achieved during can forming
Solution Approach 1:
The invention changes from high-acid-value resin to controlled low-acid-value crystalline polyester resin (acid value 10-50 mg KOH/g), and modifies the molecular structure by using specific copolymerization ratios with 1,4-butanediol predominating, achieving both adequate film formation and excellent processability during can necking and threading operations
4Object-affected harmful factors
If aqueous coating is used to eliminate harmful organic solvents, then environmental pollution is reduced, but the coating lacks sufficient performance for can application
Solution Approach 1:
The invention uses water-soluble organic solvents and basic compounds as intermediaries to enable the crystalline polyester resin to form a stable aqueous dispersion, allowing the resin to be applied in aqueous form (reducing harmful solvent pollution) while the intermediary substances facilitate adequate coating performance through proper dissolution and film formation
Solution Approach 2:
The invention optimizes the pH value (7-14) and solid content (5-50%) parameters of the aqueous dispersion, along with the resin composition parameters, to achieve a formulation that eliminates harmful organic solvents while providing sufficient adhesion, processability, and retort resistance for can coating applications
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 film exhibits excellent processability, retort resistance, and solvent solubility, while avoiding harmful substances, making it suitable for beverage and food cans.
Implementation Method 1
a crystalline polyester resin aqueous dispersion containing a crystalline polyester resin (A)... the crystalline polyester resin has excellent solvent solubility
Implementation Method 2
the coating film is required not only to maintain adhesiveness to a metal material but also to cause no whitening (retort resistance)... the coating film exhibits excellent processability, retort resistance
Data Source
AI summary
An object of the present invention is to provide a crystalline polyester resin aqueous dispersion containing a crystalline polyester resin that has excellent solvent solubility and therefore needs only a small amount of an organic solvent used to form an aqueous dispersion, and being capable of forming a coating film that has excellent properties such as processability and retort resistance; a coating composition containing the crystalline polyester resin aqueous dispersion; a coating film; and a metal can. Another object of the present invention is to obtain a coating that contains no curing agent and therefore excludes a harmful substance such as bisphenol A and formaldehyde. A crystalline polyester resin aqueous dispersion containing a crystalline polyester resin (A) described below. Crystalline polyester resin (A): a crystalline polyester resin containing a polycarboxylic acid component and a polyalcohol component as copolymerization components, wherein the crystalline polyester resin contains 4 to 55 mol% of orthophthalic acid as the polycarboxylic acid component, and 50 mol% or more of 1,4-butanediol and less than 40 mol% of ethylene glycol as the polyalcohol component, and wherein the crystalline polyester resin has an acid value of 200 eq/ton or more and a melting point of 120 to 160°C.