Application of Aldimine Latent Curing Agents in One-Component Moisture-Curing Polyurethane Systems —— Mechanism of CO₂ Bubble Control and Key Formulation Design Points

1 Root Causes of Foaming in Moisture-Curing Polyurethane

1.1 Gas-Generating Reaction Between Moisture and Isocyanate

One-component (one-component, 1K) moisture-curing polyurethane (Polyurethane, PU) adhesives, sealants, and waterproofing coatings are usually based on isocyanate-terminated prepolymers. During sealed storage, moisture ingress into the system should be avoided as much as possible. After application, moisture from the air gradually diffuses into the material and reacts with isocyanate groups (–NCO), thereby driving the curing process.

The typical reaction pathway between isocyanate and water is as follows:

R–NCO + H₂O → R–NH–COOH

R–NH–COOH → R–NH₂ + CO₂↑

R–NH₂ + R'–NCO → R–NH–CO–NH–R'

Here, R–NH–COOH is an unstable carbamic acid intermediate, which subsequently decomposes to form a primary amine and CO₂. The primary amine generated then continues to react with isocyanate, forming a urea linkage structure.

Traditional moisture-curing PU involves two simultaneous outcomes: on the one hand, amines are formed and further converted into urea linkages, promoting material curing; on the other hand, CO₂ is released, creating the risk of bubbles, pinholes, and blistering.

1.2 CO₂ Retention Leading to Bubbles, Pinholes, and Blistering

In thin coatings or open application conditions, part of the CO₂ can escape from the system, and defects may not be obvious. However, under conditions such as thick adhesive layers, thick coatings, high temperature and high humidity, overly rapid surface drying, or a rapid increase in system viscosity, CO₂ may not be able to escape in time and can easily become trapped inside the material. These gas-related defects mainly lead to three types of effects:

Defect Type	Cause of Formation	Effect on the Material
Bubbles	CO₂ is retained inside the material	Reduces material compactness and the effective load-bearing cross-section
Pinholes	Gas escapes toward the surface and leaves micropores behind	Reduces waterproofing continuity and surface integrity
Blistering	Localized gas accumulation lifts the coating or adhesive layer	Weakens bonding stability and service reliability

2 Chemical Nature of Aldimine Latent Curing Agents

2.1 Aldimine Blocking of Primary Amines

Primary amines react very rapidly with isocyanates. If free primary amines are directly added to an isocyanate-terminated one-component PU system, premature chain extension or crosslinking may occur during storage, leading to an increase in viscosity, gelation, or even product failure.

The design concept of aldimine latent curing agents is to allow primary amines to undergo a condensation reaction with aldehydes to form imine structures, thereby temporarily blocking the high reactivity of the primary amines. The basic reaction can be expressed as:

R–CHO + H₂N–R'–NH₂ → R–CH=N–R'–N=CH–R + 2H₂O

The equation above uses the reaction between a diprimary amine and an aldehyde to form a dialdimine structure as an example. After aldimine formation, the originally highly reactive primary amine no longer exists in the form of a free amine. Under anhydrous storage conditions, this can reduce the risk of premature reaction with –NCO. Since the synthesis of aldimines itself generates water, when they are used as raw materials in one-component NCO-containing systems, residual moisture and free amine content should be controlled to avoid affecting storage stability.

2.2 Moisture-Triggered Hydrolysis to Release Primary Amines

Aldimine structures are sensitive to moisture. After application, moisture from the air enters the adhesive layer or coating film, causing the aldimine to undergo hydrolysis and release the primary amine again, while also releasing the corresponding aldehyde blocking agent.

The basic reaction can be expressed as:

R–CH=N–R'–N=CH–R + 2H₂O → H₂N–R'–NH₂ + 2R–CHO

After hydrolysis, the corresponding primary amine or polyamine is released. If the dialdimine is prepared from a diprimary amine, a difunctional primary amine is released. This primary amine can rapidly react with isocyanate groups to form urea linkages:

H₂N–R'–NH₂ + 2R–NCO → R–NH–CO–NH–R'–NH–CO–NH–R

This process can be summarized as:

Aldimine undergoes moisture-triggered hydrolysis → difunctional primary amine is released → primary amine reacts with –NCO → urea linkages are formed, leading to chain extension or crosslinking

3 From Gas-Generating Curing to Latent Amine Curing

3.1 Source of CO₂ in the Traditional Moisture-Curing Pathway

Traditional one-component moisture-curing PU relies on the reaction between moisture and –NCO to achieve curing, but this pathway is accompanied by CO₂ release:

–NCO + H₂O → –NH₂ + CO₂↑

–NH₂ + –NCO → –NH–CO–NH–

In this process, water directly attacks the isocyanate group. Although urea linkages can ultimately be formed, CO₂ is an unavoidable by-product. Once the CO₂ generation rate exceeds its escape rate, or the system viscosity rises too quickly, bubble defects will appear. This is also the fundamental reason why many one-component PU sealants and waterproofing coatings are more prone to foaming under high-humidity, thick-coating, or thick-adhesive-layer conditions.

3.2 Aldimines Change the Reaction Pathway of Moisture

After an aldimine latent curing agent is introduced, when the formulation is properly matched, moisture entering the system can trigger aldimine hydrolysis and release primary amines or polyamines. The released primary amines react rapidly with –NCO and can quickly consume isocyanate groups to form urea linkages.

This pathway can be expressed as:

Aldimine + H₂O → Primary amine + Aldehyde

Primary amine + –NCO → Urea linkage

Compared with the traditional pathway, the core difference of the aldimine pathway is that moisture no longer initiates curing only by directly attacking –NCO. Instead, it can hydrolyze the latent amine to release primary amines or polyamines, which then rapidly react with –NCO to complete chain extension or crosslinking. The differences between the two pathways are as follows:

Curing Pathway	First Reaction Involving Moisture	Whether CO₂ Is Generated	Main Result
Traditional moisture curing	Water reacts directly with –NCO	CO₂ is generated	Curing occurs together with gas-generation risk
Aldimine latent amine curing	Water hydrolyzes the aldimine to release primary amine	The main pathway does not generate CO₂	Primary amine is released and rapidly forms urea linkages

It should be noted that aldimines reduce the risk of CO₂-type bubbles, but this does not mean that the system can be completely bubble-free under all formulation and application conditions. If the moisture content of fillers is too high, the application thickness is excessive, the dosage of latent curing agent is insufficient, surface skinning occurs too quickly, or free –NCO is present in excess, isocyanates may still react directly with water and release CO₂. Therefore, aldimines must be matched with raw material drying, NCO content, application thickness, and the catalyst system.

4 Hydrolysis Equivalent for Calculating Active Amine Supply

Taking a hydrolysis equivalent of 154 g/eq as an example, it can generally be understood as follows: every 154 g of this aldimine latent curing agent can provide 1 equivalent of latent amine functionality capable of reacting with isocyanate after complete hydrolysis. The specific definition should be based on the product TDS or COA.

Here, “equivalent” is a formulation calculation concept, not simply molecular weight. Molecular weight describes the mass of a single molecule, whereas equivalent describes how much reactive functionality a given mass of material can provide. For reactive curing agents, formulation engineers are more concerned with how much –NCO the material can consume than with how heavy an individual molecule is.

The equivalent weight of the isocyanate group –NCO is 42 g/eq. Therefore, if the –NCO equivalent that needs to be matched by the aldimine latent curing agent is known, the theoretical dosage can be estimated using the hydrolysis equivalent:

Theoretical dosage of latent curing agent = NCO equivalent to be matched × Hydrolysis equivalent

For example, if 0.05 eq of –NCO in a system needs to be matched by an aldimine latent curing agent, and the hydrolysis equivalent of this latent curing agent is 154 g/eq, then the theoretical dosage is:

0.05 eq × 154 g/eq = 7.7 g

This calculation only represents theoretical equivalent matching. In actual formulations, further adjustments are required based on the target curing speed, open time, storage stability, deep curing, mechanical properties, and system cost.

5 Formation of Urea Linkages and Improvement of Mechanical Properties

5.1 Formation of Urea Linkages Between Primary Amines and –NCO

The primary amine released after aldimine hydrolysis reacts with isocyanate to form a urea linkage structure:

R–NH₂ + R'–NCO → R–NH–CO–NH–R'

Urea linkages have strong polarity and readily form intermolecular hydrogen bonds. For PU adhesives, sealants, and elastic waterproofing coatings, the introduction of urea linkages helps improve cohesive strength, tear resistance, and network stability. However, material performance is not determined by urea linkages alone. If the urea linkage content is too high or the crosslink density is excessive, the material may become harder, and elongation at break may instead decrease.

5.2 Improvement in Strength and Elongation May Come from Three Effects

When the dosage, NCO content, soft/hard segment structure, and crosslink density are properly matched, aldimine latent curing agents may increase tensile strength while maintaining or improving elongation at break. This mainly comes from three aspects:

Source of Effect	Mechanism	Contribution to Performance
Reduction of bubbles	CO₂-type pores are reduced, and internal continuity of the material is improved	Reduces stress concentration and increases effective load-bearing area
More complete chain extension	Difunctional primary amines connect isocyanate-terminated prepolymers	Improves network uniformity and cohesive strength
Enhanced hydrogen bonding	Urea linkages are highly polar, strengthening intermolecular interactions	Improves tensile strength, tear resistance, and elastic recovery

Bubble reduction is the basis for performance improvement. Pores can become crack initiation sites, causing premature failure during tensile deformation. After CO₂-type bubbles are reduced, the material becomes denser internally, and stress distribution becomes more uniform during stretching.

More complete chain extension is the structural reason for performance improvement. Difunctional primary amines react with –NCO and can connect prepolymer chain segments, reducing localized curing nonuniformity and making the network structure more complete.

Enhanced hydrogen bonding from urea linkages is the molecular interaction reason for performance improvement. The strong polarity and hydrogen bonding introduced by urea linkages can improve material cohesion, but they must remain balanced with the soft segment structure, hard segment content, and crosslink density.

6 Applicable Systems and Key Formulation Control Points

6.1 Applicable One-Component Moisture-Curing Systems

Aldimine latent curing agents are mainly suitable for one-component PU systems that contain isocyanate groups and rely on atmospheric moisture to trigger curing, including one-component PU adhesives, one-component PU sealants, and one-component PU waterproofing coatings. Their main functions are as follows:

Application System	Main Function
One-component PU adhesives	Reduces CO₂ bubbles in thick adhesive layers and improves the compactness of the bonded layer
One-component PU sealants	Improves deep curing and reduces blistering and internal voids
One-component PU waterproofing coatings	Reduces the risk of pinholes and bubbles during thick coating or high-humidity application
High-solids moisture-curing PU systems	Improves latent amine chain-extension efficiency and curing uniformity

6.2 Key Factors Affecting Application Performance

Whether an aldimine latent curing agent can effectively reduce bubbles and improve material performance depends on the proper matching of the formulation with the application conditions. In practical applications, particular attention should be paid to the following factors::

Control Factor	Effect
NCO content	Determines the theoretical matching amount of latent curing agent and the risk of residual –NCO
Hydrolysis equivalent	Determines the dosage calculated based on reaction equivalent
Functionality of the latent curing agent	Affects chain-extension ability, crosslink density, and mechanical properties
Moisture content of fillers	Excessive moisture increases the risk of direct gas-generating reaction between –NCO and water
Application thickness	Thick coatings or thick adhesive layers are more likely to trap CO₂
Catalyst system	Affects hydrolysis rate, NCO reaction rate, and open time
Moisture diffusion rate	Affects surface drying, deep curing, and curing uniformity
Compatibility and odor of the released aldehyde	Affects application odor, system compatibility, and user comfort

Among these factors, filler moisture content and application thickness require particular attention. Even after an aldimine latent curing agent is added, if fillers introduce excessive moisture or if the material is applied too thickly in one pass, localized NCO-water reactions may still occur within the system and generate CO₂. Aldimine latent curing agents should be regarded as curing-pathway regulating components rather than simple defoaming additives. They need to be designed together with raw material drying, NCO index, catalysts, application thickness, and ambient humidity.

7 Related Raw Materials, Additives, and Representative Products for Aldimine Latent Curing Systems in One-Component Moisture-Curing Polyurethane

Table 1 Soft-Segment and Hydroxyl-Functional Raw Materials for Polyurethane Prepolymers

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Polyether flexible soft-segment polyol	25322-69-4	P103212	Polypropylene glycol (PPG)	Average molecular weight 4000	Used to prepare isocyanate-terminated polyurethane prepolymers. Provides flexible polyether segments and helps adjust the flexibility, low-temperature elasticity, and elongation at break of adhesives, sealants, and waterproofing coatings.
High-molecular-weight polyester soft-segment diol	36890-68-3	P478032	Polycaprolactone diol	Average Mn 10000	Used to prepare polyester-type polyurethane prepolymers. Introduces polycaprolactone segments to improve cohesive strength, abrasion resistance, and mechanical support of the coating film.
Polytetrahydrofuran ether elastic soft-segment diol	25190-06-1	P432410	Polytetrahydrofuran (PTHF)	Average Mn ~2900	Used to build elastic polyether soft segments, improving rebound resilience, hydrolysis resistance, and dynamic deformation capability of polyurethane systems. Suitable for research on elastic sealants and waterproofing coating films.
Bio-based branched hydroxyl-functional raw material	8001-79-4	C110663	Castor oil	Chemically pure (CP)	Contains naturally occurring hydroxyl structures and can participate in polyurethane prepolymer synthesis. Introduces branched segments and is used in waterproofing coatings, sealants, and bio-based polyurethane systems.

Table 2 Aldehyde Raw Materials Related to the Synthesis of Aldimine Latent Curing Agents

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Branched aliphatic aldehyde blocking raw material	78-84-2	I434189	Isobutyraldehyde	Distilled grade, ≥99.5%	Condenses with primary amines to form isobutylidene-type aldimine structures. Used for the synthesis of low-viscosity latent amine curing agents and for research on hydrolysis-triggered amine release reactions.
Linear aliphatic aldehyde blocking raw material	123-72-8	B100302	n-Butyraldehyde	Distilled grade, ≥99.5%	Used to synthesize aliphatic aldimine latent curing agents and to adjust the hydrophobicity, hydrolysis response, and compatibility of imine structures with polyurethane prepolymers.
Aromatic aldehyde blocking raw material	100-52-7	B110464	Benzaldehyde	Distilled grade, ≥99.5%	Condenses with diamines to prepare aromatic aldimine latent amines. After hydrolysis, primary amines are released to participate in isocyanate chain extension and form urea linkages.
Furan-based aldehyde blocking raw material	67-47-0	H301743	5-Hydroxymethylfurfural	Moligand™, refined grade, ≥99.5%	Contains a furan ring and a hydroxymethyl structure. Can be used as a research-type aldehyde source candidate for the design of bio-based aldimine latent curing agents. Because it contains hydroxyl groups, its side reactions, compatibility, and storage stability should be evaluated before it is introduced into NCO-containing systems.
Furfural aldehyde blocking raw material	98-01-1	F100546	Furfural	AR, ≥99%	Condenses with aliphatic or cycloaliphatic diamines to prepare furan-based aldimines. Used in research on latent amine hydrolysis, urea linkage formation, and bio-based curing agents.
Long-chain branched aliphatic aldehyde blocking raw material	123-05-7	E156034	2-Ethylhexanal	≥95% (GC)	Used to prepare branched alkyl aldimine latent curing agents, improving the compatibility of latent amines in hydrophobic polyurethane prepolymers and the stability of application systems.

Table 3 Amine Raw Materials Related to the Synthesis of Aldimine Latent Curing Agents

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Linear aliphatic diamine raw material	124-09-4	H103908	1,6-Hexanediamine (HMDA)	Chemically pure (CP), ≥98%	Condenses with aldehydes to prepare difunctional aldimines. After hydrolysis, linear diamines are released and react with isocyanates to form urea-based chain-extension structures.
Small-molecule polyamine crosslinking raw material	112-24-3	T103762	Triethylenetetramine (TETA)	Chemically pure (CP), ≥68%	Contains multiple amine reaction sites and can be used for the structural design of multifunctional latent amines. Suitable for experimental research aimed at increasing the crosslink density of polyurethane-urea networks.
Flexible polyether diamine raw material	9046-10-0	P108072	Polyetheramine D-400	Average Mn ~400	Forms flexible aldimine latent curing agents with aldehydes. After hydrolysis, polyether diamines are released to improve the flexibility and elongation at break of the cured material.
Small-molecule polyamine chain-extension and crosslinking raw material	111-40-0	D100059	Diethylenetriamine	≥99%	Can be used for latent amine structure design. After hydrolysis and release, it participates in isocyanate chain extension and crosslinking reactions. Suitable for research on cohesive strength and curing speed.
Cycloaliphatic diamine latent-curing raw material	2855-13-2	A104545	Isophoronediamine, mixture of cis and trans isomers (IPDA)	≥99%	Condenses with aldehydes to prepare cycloaliphatic aldimine latent curing agents. After hydrolysis, diamines are released and react with isocyanates to form urea linkage structures with relatively good weatherability.
Araliphatic diamine latent-curing raw material	1477-55-0	X107227	m-Xylylenediamine (MXDA)	≥99%	Can be used to synthesize araliphatic aldimine latent amines. After hydrolysis, it reacts rapidly with isocyanates and is suitable for research on polyurethane-urea structures with high cohesive strength.
Rigid cycloaliphatic diamine raw material	1761-71-3	M158600	4,4'-Methylenebis(cyclohexylamine), mixture of isomers	≥97%	Condenses with aldehydes to prepare cycloaliphatic latent amines. After hydrolysis, relatively rigid chain segments are formed. Used in research on low-yellowing polyurethane-urea networks and weather-resistant systems.

Table 4 Main Isocyanate Raw Materials

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Aromatic polyisocyanate prepolymer raw material	9016-87-9	P304914	Polymeric methylene diphenyl diisocyanate	NCO content ~30%; viscosity ~200 mPa·s (25°C)	Provides relatively high NCO content. Used in high-solids polyurethane prepolymers and reactive systems, and reacts with amines released by aldimine hydrolysis to form polyurethane-urea networks.
Aliphatic diisocyanate raw material	822-06-0	H106723	Hexamethylene diisocyanate (HDI)	Moligand™, ≥99%	Used in weather-resistant, low-yellowing polyurethane systems. Forms urea linkages with primary amines released from latent amines after hydrolysis, and is suitable for research on transparent or light-colored coatings.
Cycloaliphatic diisocyanate raw material	4098-71-9	I109582	Isophorone diisocyanate, mixture of isomers (IPDI)	≥99%	Used in weather-resistant isocyanate-terminated prepolymers. Suitable for use with cycloaliphatic aldimine latent curing agents to study curing speed, yellowing, and mechanical properties.
Aromatic diisocyanate raw material	26471-62-5	T135411	Toluene diisocyanate, 2,4- and 2,6-isomers (TDI)	≥98% (GC)	Used in polyurethane prepolymer synthesis. Residual NCO can react with amines released by aldimine hydrolysis. Suitable for experiments on aromatic moisture-curing polyurethane systems.
Araliphatic diisocyanate raw material	3634-83-1	M158033	m-Xylylene diisocyanate (MXDI)	≥98% (GC)	Used in polyurethane systems that require a balance between reactivity and yellowing resistance. Reacts with amines released from latent amines to form urea-containing chain segments.
Aromatic diisocyanate raw material	101-68-8	M106783	4,4'-Methylenebis(phenyl isocyanate) (MDI)	≥98%	Used in the synthesis of high-strength polyurethane prepolymers. Undergoes chain-extension reactions with diamines released by aldimine hydrolysis to form hard-segment structures containing urea linkages.
Cycloaliphatic diisocyanate raw material	5124-30-1	D155475	Dicyclohexylmethane 4,4'-diisocyanate, mixture of isomers (HMDI)	≥90% (GC)	Used in weather-resistant and elastic polyurethane systems. Works with cycloaliphatic latent amines to build low-yellowing polyurethane-urea curing networks.

Table 5 Catalysts, Water Scavengers, and Interface-Modification Additives

Category	CAS No.	Aladdin Item No.	Name	Specification or Purity	Product Features and Applications
Tertiary amine polyurethane catalyst	280-57-9	T105635	1,4-Diazabicyclo[2.2.2]octane (DABCO/TEDA)	Moligand™, ≥98%	Used to regulate the reaction rate of isocyanate-related reactions. Suitable for studying the effect of catalysts on surface drying, deep curing, and urea linkage formation after latent amine release.
Organobismuth polyurethane catalyst	34364-26-6	B283250	Bismuth(III) neodecanoate	≥99.9% metals basis, 60% in neodecanoic acid (15–20% Bi)	Used in low-tin or tin-free polyurethane curing systems. Participates in regulating isocyanate reactions, moisture-curing speed, and coating-film formation.
Morpholine ether moisture-curing catalyst	6425-39-4	B102060	Bis(2-morpholinoethyl) ether (DMDEE)	≥97%	Used in one-component moisture-curing polyurethane sealants and adhesives. Regulates surface drying, deep curing, and open time, and is suitable for formulation optimization of latent amine curing systems.
Organotin polyurethane catalyst	77-58-7	D100274	Dibutyltin dilaurate (DBTDL)	≥95%	Promotes the reactions of isocyanates with hydroxyl groups, water, and amines. Can be used to compare the influence of catalyst type on curing speed and mechanical properties after aldimine hydrolysis.
Reactive water scavenger	4083-64-1	T106377	p-Toluenesulfonyl isocyanate	≥96%	Consumes trace moisture in the system, reducing direct NCO gas-generating reactions caused by moisture introduced from raw materials and fillers. Used to improve storage stability and foaming control in one-component polyurethane systems.
Vinyl methoxysilane water scavenger	2768-02-7	V162969	Vinyltrimethoxysilane	≥98% (GC)	Consumes trace moisture through alkoxysilane hydrolysis. Used to control moisture introduced by fillers, improve system storage stability, and enhance wetting of inorganic components.
Vinyl ethoxysilane water scavenger	78-08-0	T103647	Vinyltriethoxysilane (TEVS)	≥97%	Used for moisture scavenging and interface modification. Suitable for screening polyurethane systems that are sensitive to small molecules released by methoxysilanes.
Aminosilane coupling agent	919-30-2	A107147	3-Aminopropyltriethoxysilane (APTS)	≥99%	Improves interfacial bonding between inorganic fillers, mineral substrates, and polyurethane systems. Because it contains an amino group, it can react with NCO and is suitable for research on adhesion and interfacial reactions.
Epoxy silane coupling agent	2530-83-8	G107576	3-Glycidyloxypropyltrimethoxysilane	≥97%	Used to improve inorganic filler dispersion and substrate adhesion. Suitable for interface-reinforcement experiments in polyurethane adhesives, sealants, and waterproofing coatings.
Diaminosilane coupling agent	1760-24-3	T101385	N-[3-(Trimethoxysilyl)propyl]ethylenediamine	≥95%	Provides silane hydrolysis-condensation sites and diamine reaction sites. Used to strengthen the bonding between fillers, substrates, and polyurethane networks. In NCO-containing systems, it is suitable for research on interfacial reaction activity.
Isocyanate-functional silane coupling agent	24801-88-5	T106834	Isocyanatopropyltriethoxysilane	≥95%	Contains both an isocyanate group and an alkoxysilane structure. Can participate in polyurethane reactions and inorganic interface coupling, and is used for research on substrate adhesion and reactive interface modification.

Note: The above are representative Aladdin products related to scientific research and formulation studies. Actual product selection should be based on the Aladdin official website, the COA of the corresponding batch, and the SDS. More information on product specifications, grades, and COA can be searched on the Aladdin official website by product name, CAS number, or item number.

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