All three are external crosslinkers for waterborne resins that contain carboxyl or hydroxyl groups, but the active centers differ. Aziridine works by three-membered-ring strain, carbodiimide by the cumulative double bond N=C=N, and isocyanate by –NCO. The comparison below follows structure → reaction → performance → selection.

1. Aziridine Crosslinker

Structural features
Typical grade: TMPTA type, a trimethylolpropane backbone plus 3 aziridine rings
Core functional group: a three-membered nitrogen heterocycle (aziridine)
Very high ring strain → high activity in electrophilic ring opening
Multifunctional (3 to multi-arm), miscible with water, alcohols and ketones, and stable at pH 7.5–10

Reaction mechanism
Main partner: carboxyl –COOH (acrylic emulsions and carboxyl-functional PU)
Acid-catalyzed ring opening → crosslinking through amide / tertiary-amine bonds
It reacts at room temperature and can keep crosslinking in a dry film
Below pH 7 the reaction speeds up and the mix gels easily; pH 8–9 is safer

Performance profile
✅ Fast cure at room temperature, at a low dosage (1–3% on resin solids)
✅ Water resistance, wet- and dry-scrub resistance, and adhesion improve quickly
❌ Short pot life (8–24 h; it loses activity easily after mixing)

Typical uses:
Waterborne printing pastes, leather finishes, wood lacquers, inks and protective-film adhesives

2. Polycarbodiimide (PCDI / Carbodiimide)

Structural features
A cumulative double bond –N=C=N– in the backbone (similar to the product left after an isocyanate loses CO₂)
Made by polycondensation of a diisocyanate; a multifunctional polymer
Far less reactive toward water than –NCO
Supplied as a waterborne dispersion or emulsion, with low toxicity and low VOC

Reaction mechanism
Selectivity: carboxyl groups react first
–COOH attacks the central carbon of N=C=N → an O-acylisourea → rearrangement to a stable N-acylurea
It does not need strong-acid catalysis. Crosslinking starts after free carboxyl groups are released as the film forms
Side reaction: slow hydrolysis to a urea of low activity

Performance profile
✅ Long pot life (24–72 h, and it can even be stored as one component)
✅ Low toxicity, no formaldehyde and no NCO release
✅ The acylurea bond resists hydrolysis → it also acts as a hydrolysis stabilizer
✅ More flexible than aziridine
⚠️ Crosslinking is slower than with aziridine. It needs time at room temperature, or a low-temperature bake, to speed up
⚠️ It reacts only with carboxyl groups and does nothing in a purely hydroxyl-functional resin
Higher dosage
Crosslink density: medium to high (depends on equivalent weight)

Typical uses:
Waterborne PU/acrylic coatings, adhesives, hydrolysis-resistant coatings, food-contact packaging and export leather

3. Waterborne Isocyanate Crosslinker (wb-NCO / Blocked)

Structural features Two ways to use it:
Two-component hydrophilic modified NCO
(HDI trimer + a polyether hydrophilic chain) → reacts with –OH
Blocked NCO
(blocked with an oxime, caprolactam or an alcohol) → one-component bake
Active center: –N=C=O
Aliphatic types (HDI) resist yellowing; aromatic types cost less but yellow
Blocked type: “dormant at room temperature, unblocked by heat”

Reaction mechanism
Free NCO: with –OH → a urethane; with –NH₂ → a urea; with water → a urea + CO₂
Blocked type: heat removes the blocking agent → free NCO → crosslinking
Typical unblocking temperature: 100–150°C (oximes: 120–140°C)

Performance profile
The highest crosslink density and the densest film
✅ The strongest gains in chemical resistance, solvent resistance, abrasion resistance and hardness
✅ Aliphatic systems weather well
❌ The shortest pot life (4–8 h for the two-component system, and shorter still in heat and humidity)
❌ Sensitive to water; water beaten in during mixing causes foam or deactivation
❌ Needs a bake (blocked type) or mixing just before use (free type)
❌ Free NCO, odor from the blocking agent, and REACH restriction risk

Typical uses:
Waterborne 2K PU wood finishes, automotive refinish, industrial baking coatings, metal protection and synthetic-leather finishes

4. Core Comparison of the Three

5. How to Choose

• Carboxyl-functional PU or acrylic emulsion + air dry at room temperature + fast shipment
  → Aziridine (the short pot life has to be accepted)
• Acrylic emulsion + a long pot life
  → Carbodiimide first (the dosage is higher, and a bake is needed)
• Hydroxyl-functional waterborne PU + a highly abrasion-resistant industrial coating + an oven line
  → Blocked isocyanate
• 2K waterborne wood coatings + maximum performance
  → Aziridine crosslinker

6. Easy Mistakes

• Aziridine: do not let the pH fall below 7.5, or the mix thickens and gels as soon as the two parts are combined
• Carbodiimide: carboxyl groups in the ammonium-salt form do not react. Crosslinking starts only after the amine evaporates during film formation. Do not test “water resistance straight after mixing”
• Isocyanate: when water is added during dispersion, do not stir so hard that air is beaten in. Do not leave it for long with a highly alkaline carboxyl ammonium salt, or water will consume the NCO

Source: WeChat account Yuxuan5275