Mechanism analysis and improvement strategies for solvent-free leather damp-heat aging problems

Solvent-free polyurethane (PU) synthetic leather is gradually replacing traditional solvent-based leather due to its environmental advantages. However, in practical applications, performance degradation caused by hygrothermal aging has become a key bottleneck restricting its widespread adoption. This paper systematically analyzes the aging mechanisms of solvent-free leather under hygrothermal conditions, identifying ester group hydrolysis, interfacial delamination, and filler migration as the primary failure modes. Furthermore, systematic improvement approaches are proposed from three dimensions: molecular structure design, process optimization, and post-treatment modification. The research indicates that through polyether/polyester copolymer modification, compounded use of hydrolysis stabilizers, and crosslinking density regulation, the strength retention rate after hygrothermal aging can be increased to over 85%.

Hygrothermal aging refers to the irreversible performance degradation of materials under long-term service in high-temperature and high-humidity environments. For solvent-free leather, this issue presents particular complexity: on the one hand, solvent-free systems lack the microporous structure formed by solvent evaporation, resulting in significantly different internal stress distribution compared to conventional leather; on the other hand, the sensitivity of the three-dimensional network formed by two-component crosslinking reactions to moisture directly affects the long-term stability of the material. According to relevant research statistics, the tensile strength of some solvent-free leather products can decrease by more than 40% after 500 hours of hygrothermal aging, far exceeding the 20%–30% decline observed in conventional solvent-based products.

Mechanism Analysis of Hygrothermal Aging Issues

The hygrothermal aging of polyurethane is essentially a chemical attack by water molecules on the molecular chains. Under high-temperature and high-humidity conditions, water molecules diffuse into the polymer matrix and interact with the polar groups in the polyurethane molecular chains. The chemical nature of the hydrolysis reaction is the nucleophilic attack of water molecules on ester bonds or urethane bonds, leading to molecular chain scission.

From a thermodynamic perspective, ester groups are far more susceptible to hydrolysis than ether groups. In the presence of water, ester groups undergo hydrolysis to generate carboxylic acids and alcohols, and the carboxylic acids produced further catalyze the hydrolysis reaction, forming an autocatalytic cycle. This mechanism explains why ester-based polyurethanes often exhibit an “accelerated degradation” characteristic during hygrothermal aging. In contrast, the C–O–C bonds in ether groups possess relatively high hydrolytic stability, but their oxidation resistance is poor, making them prone to forming peroxides under prolonged thermal-oxidative environments.

The core characteristic that distinguishes solvent-free polyurethane from solvent-based products lies in its highly crosslinked three-dimensional network structure. This structure imparts excellent initial strength and chemical resistance to solvent-free leather, but it also introduces unique hygrothermal aging behavior.

Within the crosslinked network, hydrolysis reactions can occur at two levels: first, chain scission of the molecular segments between crosslinking points (intra-chain cleavage), and second, cleavage of the crosslinking points themselves. The former leads to the formation of “dangling chains” within the network, while the latter directly reduces the crosslink density. Both modes of cleavage result in a decline in mechanical properties, but their underlying mechanisms differ—intra-chain cleavage primarily affects tensile strength and elongation at break, whereas crosslink point cleavage has a more pronounced impact on elastic recovery and creep resistance.

Solvent-free leather typically adopts a multi-layer composite structure, consisting of a top layer, a foamed layer, and a base fabric layer. Under hot and humid conditions, differences in the thermal expansion coefficient and moisture absorption rate between the layers can generate interfacial stress. When the interfacial stress exceeds the interlayer bonding strength, delamination or blistering will occur.

In addition, to reduce costs or modify the hand feel, inorganic fillers such as calcium carbonate and talc are often added to solvent-free formulations. During hygrothermal aging, water molecules can penetrate along the filler–polymer interface, weakening the interfacial bonding strength. More critically, some fillers dissolve under the action of acidic hydrolysis products, forming void defects that further accelerate the overall deterioration of the material.

Methods of Improvement and Technical Strategies

 Improving the intrinsic hydrolytic stability of polyurethane at the molecular level is the most fundamental way to improve it.

      Screening and blending of polyol systems ： Replacing some polyester polyols with polyether-type polyols can significantly improve hydrolysis stability. Research shows that when the polyether chain segment ratio reaches 40%-60%, the strength retention rate after damp-heat aging can increase from 55% to 82%. However, polyether-type polyurethane generally has lower mechanical strength and wear resistance than polyester types, so it needs to be balanced according to specific application scenarios. Copolymer polyols (such as polycarbonate diol and polycaprolactone glycol) combine good hydrolytic stability and mechanical properties, making them an ideal choice for high-end solvent-free leather.

       Selection of isocyanate components : Urethane bonds formed by aliphatic isocyanates (such as HDI, IPDI) have better hydrolytic stability than aromatic isocyanates (such as MDI, TDI) and do not cause yellowing issues. For light-colored or transparent leather products, aliphatic systems have irreplaceable advantages.

       Regulation of crosslinking density ： Moderate crosslinking can suppress the diffusion rate of water molecules, but excessive crosslinking can cause material brittle and increased internal stress. Research shows that when the molecular weight (Mc) between crosslinking points is controlled within the range of 2000-4000 g/mol, good resistance to damp heat aging can be achieved while maintaining flexibility.

 Introduction of hydrolysis stabilizers ： Carbodiamide compounds are the most commonly used hydrolysis stabilizers for polyester-type polyurethane. Its mechanism is that the carbodiimide group can preferentially react with the hydrolyzed carboxylic acid, thereby interrupting the autocatalytic cycle. The typical addition amount is 1%-3% of the resin mass, which can extend the lifespan of damp heat aging by 2-3 times. Epoxy compounds and oxazoliline compounds can also be used as hydrolytic stabilizers, with similar mechanisms of action.

      Synergistic use of antioxidants and UV absorbers ： Damp-heat aging often occurs in conjunction with thermal and oxygen aging and photo-aging processes. The combined system of phenolic antioxidants and phosphite-assisted antioxidants can effectively inhibit oxidative degradation; The synergistic use of benzotriazole UV absorbers and hampered amine light stabilizers (HALS) is suitable for outdoor applications.

       Surface modification of hydrophobic treatment agents : Applying fluorine-containing or silicon-containing hydrophobic agents to the leather surface can reduce surface energy and decrease moisture adsorption. This treatment significantly inhibits early aging, but its effectiveness gradually diminishes with long-term wear of the surface coating.

  Production environment control : Strictly controlling the temperature and humidity conditions in the coating workshop (recommended temperature 25±2°C, relative humidity 40%-50%) is the prerequisite for ensuring product quality. The moisture content of the raw materials must be strictly controlled below 0.05% to avoid prereactions with isocyanate components.

       Post-curing process optimization : The crosslinking reaction of solvent-free polyurethane does not end immediately after coating; heat treatment during the post-curing stage has a decisive impact on the final performance. Research shows that post-curing at 80-90°C for 24-48 hours can complete the crosslinking reaction and significantly improve resistance to damp heat aging.

       Interface design for multilayer composite structures : Introducing a capacity expansion layer or gradient transition layer between the surface layer and the foamed layer can alleviate thermal stress between layers. Pretreating the base fabric with plasma treatment or primer can improve interfacial bonding strength and reduce the risk of delamination in humid and hot environments.

Improvement strategy	Technical principles	Advantages	Limitations	Applicable scenarios
Polyether/polyester copolymer	Enhances the intrinsic stability of molecular chains	Long-lasting results with no risk of migration	Possible impact performance	Long-term use in humid and hot environments
Hydrolysis stabilizer added	Interrupt the autocatalytic hydrolysis cycle	Fast results and relatively low cost	There is a migration loss issue	Medium- to short-term use or cost-sensitive type
Surface hydrophobic treatment	Reduce contact with moisture	The process is simple and the cost is low	Effectiveness attenuates with wear	Low-intensity usage environment
Cross-linking density optimization	Inhibits moisture diffusion	Overall performance balance	Process windows need to be precisely controlled	Products requiring high performance
Post-curing process reinforcement	Improve the cross-link network	Easy to operate with no additional costs	Dependence on formulas	Universally applicable