Branched Polyester Powder Coatings for Corrosion Resistance
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Solution Overview
Problem
Thermosetting powder coatings comprising polyesters and crosslinkers with oxirane groups face issues with corrosion resistance due to the presence of epoxy cure catalysts, particularly when high amounts of cheaper polyhydric alcohols like ethylene glycol are used, leading to surface defects and decreased corrosion resistance on metal substrates.
Innovation Solution
A carboxylic acid functional branched polyester is developed, prepared from monomers such as terephthalic acid, isophthalic acid, and neopentyl glycol, with a specific molar ratio and functionality, which is used in thermosetting powder coating compositions with a crosslinker and optional epoxy cure catalyst, resulting in enhanced corrosion resistance and improved appearance on metal substrates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high amounts of cheaper polyhydric alcohols such as ethylene glycol are used in polyester resins, then cost is reduced, but surface defects such as cratering, fish-eyes and pinholes occur
Solution Approach 1:
The invention changes the chemical composition parameters of the polyester by incorporating specific ratios of ethylene glycol (10-50 mol%), diethylene glycol (10-50 mol%), and neopentyl glycol (20-60 mol%), along with controlling the TPA:IPA molar ratio (1:9 to 9:1). These parameter changes allow the use of cheaper polyhydric alcohols while preventing surface defects through optimized molecular structure and reactivity balance.
2Speed
If epoxy cure catalysts are used to promote curing between polyester and crosslinker with oxirane groups, then curing speed is improved, but corrosion resistance of the coating decreases
Solution Approach 1:
The invention changes the polyester structure parameters (incorporating specific polyhydric alcohols and aromatic acids in controlled ratios) to achieve optimal balance between curing speed and corrosion resistance. The modified polyester structure allows for reduced catalyst amounts while maintaining adequate cure rates, thereby preserving corrosion resistance.
Solution Approach 2:
The invention creates a composite coating system combining the modified polyester with crosslinker and catalyst, where the synergistic interaction between components achieves both fast curing and good corrosion resistance. The specific composition ratios of the composite materials optimize overall performance.
3Productivity
If high amounts of epoxy cure catalyst are used in fast or low temperature curable thermosetting powder coating compositions, then curing speed is improved, but corrosion resistance on metal substrates decreases
Solution Approach 1:
The invention modifies the polyester parameters (composition, functionality, molecular weight) to enable fast or low temperature curing with reduced catalyst amounts. The optimized polyester structure provides inherent reactivity that reduces dependence on high catalyst concentrations, thus maintaining corrosion resistance while achieving high productivity.
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 solution provides low-cost, corrosion-resistant powder coatings with good appearance and impact resistance, suitable for untreated metal substrates, reducing the need for expensive surface pretreatment and lowering energy consumption, while maintaining smoothness and corrosion resistance on cold roll steel.
Implementation Method 1
the speed of cure of the polyester with the crosslinker in this type of thermosetting powder coating compositions can be controlled by varying the amount and type of the epoxy cure catalyst
Data Source
AI summary
The invention relates to a carboxylic acid functional branched polyester for thermosetting powder coating composition, wherein said polyester is prepared from at least the following monomers: a difunctional aromatic acid, the difunctional aromatic acid being a combination of terephthalic acid (TPA), isophthalic acid (IPA), an at least trifunctional monomer and a difunctional alcohol, the difunctional alcohol being a combination of at least ethylene glycol (EG) and/or diethylene glycol (DEG) and neopentyl glycol (NPG), the molar ratio of TPA to IPA in the carboxylic acid functional branched polyester is at most 12.4, and the functionality of the carboxylic acid functional branched polyester is from 2.15 to 4.0, and the carboxylic acid functional branched polyester has an acid value of from 14 to 80 mg KOH/g resin, and the carboxylic acid functional branched polyester has a glass transition temperature of at least 40C. The powder coating compositions of the present invention derived upon curing of the thermosetting powder coating compositions which comprise the polyester according to the invention together with a crosslinker with oxirane groups and an epoxy cure catalyst present good corrosion resistance to steel substrates. Furthermore, said low cost and corrosion resistant powder coatings may present good reverse impact resistance (RIR) and /or good smoothnes.