news

1、 Reaction principle (key: only reacts with carboxyl groups)

The resin (acrylic lotion, PUD) is introduced with acrylic acid/DMPA monomer, with carboxyl – COOH; Carboxyl groups are neutralized by amines to form carboxylate ions, which are highly hydrophilic and are the main culprits of poor water resistance and easy swelling and hydrolysis of water-based paint films.

Reaction equation:

\(\ boldsymbol {R-N=C=N+R ‘- COOH \ rightarrow R-NH-CO-NR’ – R ‘\ quad (N-acylurea)} \)

1. Polycarbodiimide is a multifunctional oligomer with multiple – N=C=N – chains, which can simultaneously connect multiple resin macromolecules to form a three-dimensional network cross-linking; The reaction produces no small molecule by-products and does not release gas

2. Latent reaction characteristics: When the pH of the feed solution is relatively high (8.5~9.5), the carboxyl group is carboxylate, the reaction is slow, and the applicable period is long (24~72 hours); Film formation and drying, evaporation of water and volatile amines, decrease in system pH, recovery of carboxyl groups, and accelerated cross-linking (cross-linking can be done at room temperature, greatly accelerated by heating to 60-80 ℃)

❗ Prerequisite: The resin molecule contains carboxyl groups, and without carboxyl groups, the carbodiimide has little cross-linking effect

2、 Two pathways to enhance water and chemical resistance

1) Eliminating hydrophilic carboxyl groups (core contribution)

Uncrosslinked paint film: a large amount of – COOH/carboxylate ions, which easily absorb water and swell when exposed to water; After water infiltration, it will also catalyze the hydrolysis of resin lactone bonds, causing the paint film to gradually turn white, bubble, and lose strength. PCDI directly consumes carboxyl groups, converts hydrophilic groups into hydrophobic and stable acylurea structures, no longer adsorbs water molecules, significantly reduces water absorption, and inhibits long-term hydrolysis.

Carbodiimide itself is also known as an anti hydrolysis additive, and this is the mechanism behind it

2) Build a dense three-dimensional cross-linked network with physical barriers

After crosslinking, the polymer chains are covalently locked and the segments cannot slide freely:

Chemicals such as water, ethanol, cleaning agents, and MEK are difficult to penetrate into the interior of the paint film;

When immersed in solvents, polymers are not easily swollen or dissolved;

Enhance MEK’s resistance to wiping, alcohol, acid and alkali, and stains;

The higher the crosslinking density, the better the chemical resistance; But excessive PCDI paint film becomes brittle and elongation decreases.

3、 Key points of formula operation (coating/ink high-frequency pit)

1. Addition amount (based on resin solids)

Conventional: 2-6%; High water resistance requirement of 4-6%; Exceeding 8% is prone to increased brittleness and cost, and excess carbodiimide will slowly hydrolyze upon contact with water, which in turn affects water resistance.

2. Compatibility taboos

The presence of free carboxyl group additives (some carboxyl group dispersants, wetting agents, thickeners) in the system will consume PCDI and reduce crosslinking efficiency;

Avoid strong acidic raw materials, which will consume carbodiimide in advance in the tank and shorten the service life;

Applicable pH window: The recommended pH for the mixed solution is 8.0~9.2; Low pH results in rapid hydrolysis of carbodiimide and significantly shortened shelf life.

3. Side reaction risk

The carbodiimide group can slowly react with water to form urea, so there is a shelf life after pre mixing and it cannot be stored for a long time; Prepare and use up within 24 hours as much as possible.

\The hydrolysis product of (\ boldsymbol {- N=C=N -+H2O \ rightarrow \; disubstituted urea} \) does not damage the paint film itself, but consumes active groups and reduces crosslinking density.

4. Compare with Aziridine

PCDI: low toxicity, long shelf life, generates acylurea, and has good hydrolysis stability; Medium cross-linking density;

Aziridine: extremely active, short shelf life, and higher toxicity; Higher cross-linking density and higher chemical resistance limit.

4、 Performance change trend

✅  Successful cross-linking: water absorption rate ↓, MEK wiping ↑, alcohol resistance ↑, salt spray resistance ↑, paint film swelling rate ↓, hydrolysis stability ↑, improved adhesion ⚠️  Insufficient addition: There are many residual carboxyl groups, and the improvement in water resistance is limited;   ⚠️  Excessive addition: excessive cross-linking → brittleness ↑, decreased impact, decreased flexibility;   ⚠️  Expired shelf life: hydrolysis consumption of active groups, insufficient cross-linking, and failure to meet expected water and chemical resistance.