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Core Principle

Waterborne polyester amino baking paint belongs to the thermal crosslinking system: carboxyl and hydroxyl groups on the polyester molecular chain undergo etherification crosslinking reaction with amino resins (HMMM, etc.) under baking conditions. Hardness is contributed by crosslinking density, resin Tg, and rigid structural units; Flexibility relies on the flexibility of molecular chain segments and the regulation of crosslinking point spacing; The water resistance depends on the content of hydrophilic groups, the density of the cross-linked network, and the polarity of the system. There is a natural balance between the three, which needs to be balanced from multiple dimensions such as molecular design, formula matching, and baking process.

1. Molecular structure design

(1) Main chain monomer: copolymerization of rigid and flexible units

Rigid monomers (such as isophthalic acid IPA, terephthalic acid PTA, hexahydrophthalic anhydride, etc.): introducing benzene and aliphatic ring structures to enhance molecular chain rigidity, improve coating Tg and hardness; But if the proportion of rigid monomers is too high, the movement of the chain segments is limited, the coating becomes brittle, the flexibility decreases, and the internal stress increases, making it easier for water to penetrate and the water resistance deteriorates.

Flexible long-chain diols/diacids (adipic acid, neopentyl glycol, 1,6-hexanediol): The long methylene structure increases the rotational freedom of the chain segment, improving bending and impact flexibility; However, excessive flexible components can reduce Tg and the hardness of the crosslinked coating, resulting in loose molecular chain stacking, easy infiltration of water molecules, and decreased water resistance.

Formula strategy: Adopting soft and hard monomer block/random copolymerization, controlling the ratio of rigid/flexible monomers, and constructing a polyester skeleton that combines rigidity and flexibility.

(2) Regulation of crosslinking density by hydroxyl and carboxyl content

Hydroxyl group: It is the active site that crosslinks with amino resin. Hydroxyl value increase: the number of crosslinking points increases, the crosslinking network becomes denser, the hardness increases, the network voids decrease, and the water resistance improves; But if the hydroxyl group is too high, the cross-linking density is too high, the distance between cross-linking points becomes shorter, the internal stress of the coating increases sharply, the flexibility decreases, and it is easy to crack.

Carboxyl group: used for resin water dispersion (introducing hydrophilicity to achieve water dispersibility). The higher the carboxyl content, the better the water solubility of the resin, but residual polar carboxyl groups will increase the hydrophilicity of the coating and deteriorate its water resistance; The carboxyl group is too low, the resin is difficult to disperse stably, the particle size of lotion becomes larger, and the storage is unstable.

Key: Control the acid value within an appropriate range (water-based baking polyester generally has an acid value of 20-40mg KOH/g), try to block carboxyl groups or neutralize them into salts, evaporate neutralizing agents during the baking stage, and reduce residual polar groups in the coating.

(3) Resin molecular weight

Low molecular weight: good molecular mobility, more complete cross-linking reaction, high hardness; But the molecular chain is short, the network toughness after cross-linking is insufficient, and the water resistance is average. High molecular weight: longer molecular chains, longer segments between crosslinking points, and improved flexibility; But if the molecular weight is too large, the viscosity of the system increases, the compatibility with amino resin decreases, the cross-linking reaction is incomplete, and the hardness and water resistance decrease.

2. Proportion and selection of amino resin

As a crosslinking agent, the amount of amino resin directly determines the final crosslinking density:

1. Low amino ratio: insufficient cross-linking, insufficient hardness, many unreacted polyester hydrophilic groups in the coating, poor water resistance; Flexibility is still acceptable.

2. Excessive amino ratio: Excessive cross-linking leads to a significant increase in hardness, but the coating becomes brittle and flexibility decreases significantly; Excessive unreacted amino resin contains polar functional groups, which are prone to hydrolysis when in contact with water, resulting in decreased water resistance. Selection: Priority should be given to high imine and self etherified HMMM (fully methylated amino resin), which has moderate reaction activity and better hydrolysis stability than partially methylated amino groups. It reduces residual hydrophilic groups after baking and also takes into account water resistance.

3. Synergistic optimization of additives and fillers

1. Film forming aids: High boiling point and low polarity film forming aids are selected to promote the melt flow of polyester resin, make cross-linking reactions more uniform, and reduce coating defects; However, excessive residue of film-forming agents in the film can reduce hardness and water resistance.

2. Color fillers: Inert fillers such as barium sulfate and talc powder can enhance hardness; Flake fillers (mica powder) can form a maze structure, prolong the water penetration path, and improve water resistance; Excessive addition of powder can damage the continuity of the coating and reduce its flexibility.

3. Wetting dispersant: Low hydrophilic and water-resistant dispersants are preferred to avoid small molecule surfactants remaining in the coating film, which can become water penetration channels and damage water resistance.

4. Baking process regulation

The degree of crosslinking reaction is determined by the baking temperature/holding time:

Insufficient baking: incomplete cross-linking, low hardness, large residual hydrophilic groups, poor water resistance; Flexibility tends to be soft.

Excessive baking: Excessive cross-linking, thermal oxidation degradation, brittle and cracked coating, loss of flexibility, and even resin degradation resulting in polar small molecules and decreased water resistance. Reasonable process: Step heating, first low-temperature evaporation of water and film-forming agents, and then high-temperature crosslinking to avoid the rapid escape of water vapor and the formation of pinholes, ensuring the uniform formation of the crosslinked network.

5. Further breaking the balance through modification methods (advanced direction)

1. Silicon modification: Introducing siloxane segments into the polyester main chain results in good Si-O bond flexibility and low surface energy. A small amount of silicon modification can simultaneously improve water resistance and retain flexibility; But a high silicon content can reduce hardness and easily lead to shrinkage.

2. A small amount of aliphatic epoxy modification: consuming some carboxyl groups in the system, reducing the polarity of the coating, improving water resistance, and enhancing the toughness of the cross-linked network.