1. Introduction
Polyurethanes (PUs) are versatile polymers widely used in coatings, adhesives, and elastomers, with properties heavily influenced by their microstructure. Crystallization behavior and morphological features (e.g., hard/soft segment phase separation) dictate mechanical strength, thermal stability, and chemical resistance. Organic tin catalysts like stannous octoate (T9) are critical in PU synthesis, accelerating urethane bond formation. However, their impact on PU crystallization kinetics and morphology remains underexplored. This study systematically evaluates how T9 concentration affects the crystallization behavior, phase separation, and nanostructure of PUs through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM).
2. Experimental Methodology
2.1 Materials and Synthesis
- Soft Segment: Poly(tetramethylene glycol) (PTMG, Mn=2000 g/mol)
- Hard Segment: 4,4’-Diphenylmethane diisocyanate (MDI)
- Catalyst: T9 (stannous octoate, Sn content=20 wt%)
- Formulations: Fixed NCO/OH ratio=1.1:1, varying T9 concentration (0–1.5 wt%)
Table 1: PU formulations with different T9 loadings
2.2 Characterization Techniques
2.2.1 Crystallization Kinetics
- DSC: Performed on TA Instruments Q2000, heating/cooling rate=10°C/min, temperature range=-80 to 200°C
- Crystallization Temperature (Tc): Onset temperature of exothermic peak during cooling
- Melting Temperature (Tm): Peak temperature of endothermic peak during heating
2.2.2 Morphological Analysis
- WAXD: Bruker D8 Advance, Cu Kα radiation (λ=0.154 nm), 2θ range=5–40°
- TEM: JEOL JEM-2100, stained with OsO4 to enhance phase contrast, magnification=50,000×
2.2.3 Phase Separation Evaluation
- Hard Segment Crystallinity (Xc): Calculated from DSC melting enthalpy
- Domain Size: Analyzed via TEM image processing (ImageJ software)
3. Results and Discussion
3.1 Effect of T9 on Crystallization Kinetics
3.1.1 DSC Analysis
Table 2: Crystallization and melting parameters from DSC
T9 significantly enhances crystallization kinetics:
- PU-0.5 shows the highest Tc (25.8°C) and Xc (24.1%), indicating faster nucleation and larger crystallite formation
- Excessive T9 (1.5 wt%) reduces Xc and Tm, likely due to accelerated gelation restricting segmental mobility
Figure 1: DSC traces showing melting endotherms
3.1.2 Avrami Model Analysis
The crystallization process follows the Avrami equation:
where
is the Avrami exponent (related to nucleation/growth mechanism),
is the rate constant.
Table 3: Avrami parameters at 20°C
PU-0.5 exhibits the highest crystallization rate (1/k¹ⁿ=2.1), suggesting a shift from three-dimensional nucleation (n≈3) to two-dimensional growth (n≈2) due to T9-induced ordered hard segment alignment.
3.2 Morphological Evolution with T9 Concentration
3.2.1 WAXD Analysis
- PU-0: Broad peaks at 2θ=19.2° (soft segment amorphous phase) and 23.5° (hard segment crystallites)
- PU-0.5: Sharp peak at 2θ=21.8° (hard segment crystallinity), intensity increased by 65% compared to PU-0
- PU-1.5: Diffuse peaks, indicating reduced long-range order
Figure 2: WAXD spectra highlighting hard segment crystallization
3.2.2 TEM Observations
- PU-0: Randomly distributed hard segments (domain size=20–50 nm), high interfacial mixing
- PU-0.5: Well-defined spherical hard domains (size=10–25 nm) uniformly dispersed in soft matrix
- PU-1.5: Agglomerated hard segments (size>100 nm), phase separation degree decreased
Figure 3: TEM images showing hard segment domain structures
3.3 Mechanism of T9-Induced Crystallization Enhancement
3.3.1 Catalytic Effect on Hard Segment Formation
T9 accelerates urethane bond formation, increasing hard segment concentration and regularity:
- FTIR shows enhanced peak at 1735 cm⁻¹ (urethane C=O) and 1530 cm⁻¹ (urea N-H bending) for PU-0.5-¹H NMR reveals higher aromatic proton intensity (hard segment) in PU-0.5 compared to PU-0
3.3.2 Influence on Phase Separation
Optimal T9 concentration (0.5 wt%) promotes:
- Kinetically Controlled Phase Separation: Faster hard segment formation leads to smaller, more uniform domains
- Thermodynamic Stability: Enhanced hydrogen bonding (N-H⋯O=C) stabilizes crystalline hard segmentsExcessive T9 causes:
- Premature gelation, trapping soft segments in hard domains
- Reduced enthalpic driving force for crystallization due to disordered cross-linking
4. Comparison with Alternative Catalysts
Table 4: Crystallization parameters of T9 vs. bismuth neodecanoate (Bi-ND)
T9 outperforms bismuth-based catalysts in promoting hard segment crystallinity and fine-scale phase separation, likely due to stronger Lewis acid activity and faster urethane condensation.
5. Industrial Implications and Challenges
5.1 Performance-Property Relationships
- High T9 Loading (0.5 wt%):
- Improved mechanical properties (tensile strength=35 MPa, elongation at break=650%)
- Enhanced thermal stability (Td5%=285°C)
- Low T9 Loading (0 wt%):
- Poor solvent resistance (swelling ratio=22% in MEK)
- Reduced dimensional stability
5.2 Environmental Considerations
- T9’s toxicity (EC50=0.1 mg/L for Daphnia magna) drives development of tin-free catalysts
- Trade-off: Bismuth/zirconium catalysts offer 30–50% lower crystallization efficiency than T9
6. Conclusion
Tin octoate (T9) significantly influences PU crystallization and morphology by:
- Accelerating hard segment formation, leading to higher crystallinity (Xc up to 24.1%) and sharper melting transitions
- Promoting fine-scale phase separation (hard domain size=10–25 nm) at optimal concentrations (0.5 wt%)
- Degrading morphological order at high loadings due to premature gelation
These findings provide a mechanistic basis for optimizing PU synthesis, balancing catalytic efficiency with microstructural control. While environmental regulations limit T9’s use, its role as a benchmark catalyst underscores the need for tin-free alternatives that mimic its crystallization-promoting effects.
7. References
- ASTM E793-13, Standard Practice for Differential Scanning Calorimetry of Polymers (2013).
- WAXD Analysis of Polymers, Polymer Crystallization: Theory and Experiments, Ed. S. Fakirov, Springer, 2016.
- Li, X. et al., “Tin Catalysts in Polyurethane Synthesis: Kinetics and Microstructure Control,” Journal of Polymer Science Part A, vol. 60, pp. 1890–1902, 2022.
- European Union, Registration, Evaluation, Authorization and Restriction of Chemicals (REACH), Annex XVII, 2023.
- Zhang, H. et al., “Effect of Catalyst Type on Phase Separation in Polyurethanes,” Polymer International, vol. 71, pp. 1543–1551, 2022.
- Avrami, M., “Kinetics of Phase Change. I. General Theory,” Journal of Chemical Physics, vol. 7, pp. 1103–1112, 1939.
