Dibutyltin Dilaurate: Influence on the Crystallization Behavior of Polyurethane Polymers
Abstract
Polyurethanes (PUs) are a class of versatile polymers widely used in industrial and commercial applications due to their excellent mechanical properties, thermal stability, and chemical resistance. The performance of polyurethane materials is significantly influenced by their microstructure, particularly crystallinity. Among various factors affecting PU crystallization, catalysts play a crucial role. Dibutyltin dilaurate (DBTDL), a commonly used organotin catalyst, accelerates the urethane-forming reaction between hydroxyl and isocyanate groups. However, its influence extends beyond kinetics—it also impacts the polymer’s morphology and crystallization behavior.
This article provides a comprehensive review of how dibutyltin dilaurate affects the crystallization behavior of polyurethane polymers. It explores the chemical structure, physical parameters, catalytic mechanisms, and the resulting morphological changes that occur during polymer synthesis. Furthermore, this work presents data from both international and Chinese studies, incorporating tables for comparison and clarity. Finally, several strategies for optimizing DBTDL usage in PU formulations are discussed.
1. Introduction
Polyurethanes are formed via step-growth polymerization reactions involving diols (polyols) and diisocyanates. Depending on the monomers and processing conditions, PUs can be thermoplastic or thermosetting and exhibit varying degrees of crystallinity. Crystallinity plays a pivotal role in determining mechanical strength, melting temperature, and solvent resistance—properties critical for applications such as foams, coatings, adhesives, and elastomers.
Catalysts are essential in controlling the rate and selectivity of urethane bond formation. Among the most effective catalysts is dibutyltin dilaurate (DBTDL), which promotes the reaction between isocyanate (-NCO) and hydroxyl (-OH) groups. Despite its widespread use, the side effects of DBTDL on polymer morphology and crystallization have not always been thoroughly considered. This paper aims to fill that gap by presenting an in-depth analysis of DBTDL’s influence on PU crystallization behavior, supported by experimental evidence and literature reviews.
2. Chemical Structure and Product Parameters of Dibutyltin Dilaurate
2.1 Chemical Properties
| Property | Value |
|---|---|
| Name | Dibutyltin dilaurate |
| CAS Number | 77-58-7 |
| Molecular Formula | C₂₈H₅₄O₄Sn |
| Molecular Weight | ~563.4 g/mol |
| Appearance | Colorless to pale yellow liquid |
| Density | 1.09 g/cm³ at 25°C |
| Solubility | Insoluble in water; soluble in organic solvents |
| Viscosity | ~20–30 mPa·s at 25°C |
DBTDL consists of a central tin atom bonded to two butyl groups and two laurate (C₁₁H₂₃COO⁻) chains. As a weak Lewis acid, it facilitates the nucleophilic attack of hydroxyl groups on isocyanate groups, thereby accelerating the formation of urethane linkages:
This reaction is fundamental to polyurethane synthesis and influences both the kinetics and the final morphology of the polymer.
3. Role of Catalysts in Polyurethane Synthesis
3.1 Types of Catalysts Used in Polyurethane Reactions
| Catalyst Type | Function | Common Examples |
|---|---|---|
| Amine-based | Promote gelation (gelling reaction) | DABCO, TEDA |
| Organotin compounds | Promote urethane formation | DBTDL, T-12, Tin(II) octoate |
| Bismuth carboxylates | Low toxicity alternatives | Neostann U-100 |
Among these, organotin compounds like DBTDL are preferred when high reactivity and control over phase separation are desired. However, their presence can alter hydrogen bonding patterns and segmental mobility, which in turn affects crystallization.
4. Crystallization Mechanisms in Polyurethanes
Polyurethanes typically consist of alternating soft and hard segments. The soft segments (usually polyether or polyester chains) contribute flexibility, while the hard segments (urethane moieties) form semi-crystalline domains through hydrogen bonding and microphase separation.
Crystallization occurs predominantly in the hard segments, especially when aromatic diisocyanates like MDI (methylene diphenyl diisocyanate) are used. The degree of crystallinity depends on several factors:
- Chain regularity and symmetry
- Intermolecular hydrogen bonding
- Degree of phase separation
- Cooling rate and annealing conditions
- Presence of catalyst residues
DBTDL primarily affects the latter two points by influencing the kinetics of crosslinking and the mobility of chains during solidification.
5. Impact of DBTDL on Crystallization Behavior
5.1 Effect on Reaction Kinetics and Thermal History
The addition of DBTDL accelerates the reaction rate, which can lead to faster gelation and reduced time for molecular rearrangement. This can result in kinetically trapped structures with lower crystallinity.
Table 1: Effect of DBTDL Concentration on Gel Time and Crystallinity
Data adapted from Tang et al., 2021 [1]
| DBTDL (% wt) | Gel Time (min) | Crystallinity (%) | Melting Temp. (°C) |
|---|---|---|---|
| 0 | 25 | 38 | 138 |
| 0.1 | 18 | 34 | 136 |
| 0.3 | 12 | 27 | 132 |
| 0.5 | 9 | 21 | 129 |
As shown, increasing DBTDL concentration shortens gel time but simultaneously reduces the extent of crystallization, likely due to restricted chain mobility and premature vitrification.
5.2 Influence on Hydrogen Bonding and Microphase Separation
Xu et al. (2020) [2] investigated the effect of DBTDL on hydrogen bonding using FTIR spectroscopy and found that higher catalyst content led to weaker hydrogen bonds between urethane groups. This reduction was attributed to residual catalyst molecules interfering with hydrogen bond alignment.
Table 2: Hydrogen Bonding Index vs DBTDL Content
Xu et al., 2020 [2]
| DBTDL (% wt) | H-Bond Index (cm⁻¹ shift) | Phase Separation Index |
|---|---|---|
| 0 | 1632 | 0.75 |
| 0.2 | 1628 | 0.68 |
| 0.5 | 1622 | 0.61 |
A lower phase separation index indicates poorer domain segregation between soft and hard segments, which directly affects the development of crystalline regions.
5.3 Morphology Analysis via SEM and AFM
Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that DBTDL-modified PUs tend to form smaller and less defined crystalline domains.
Table 3: Morphological Features Observed via SEM/AFM
| Sample | Domain Size (nm) | Regularity | Crystallinity (%) |
|---|---|---|---|
| No DBTDL | 120–150 | High | 38 |
| 0.3% DBTDL | 70–90 | Medium | 27 |
| 0.5% DBTDL | 50–70 | Low | 21 |
These findings suggest that DBTDL inhibits the growth of large, well-ordered crystals by introducing structural disorder and limiting segmental diffusion.
6. Comparative Studies: International vs Domestic Research
While much of the foundational research on PU crystallization comes from Europe and North America, recent studies from China have corroborated and expanded upon these findings.
6.1 Key International Studies
| Study Author | Institution | Year | Key Finding |
|---|---|---|---|
| M. Szycher | Szycher’s Handbook of Polyurethanes | 2012 | DBTDL enhances reaction speed but compromises network order |
| J. Zhang et al. | University of Manchester | 2018 | DBTDL alters glass transition and crystallization onset temperatures |
| L. Wang et al. | ETH Zurich | 2019 | Residual tin causes long-term degradation of crystalline domains |
6.2 Notable Domestic Contributions
| Study Author | Institution | Year | Key Finding |
|---|---|---|---|
| Y. Liu et al. | East China University of Science and Technology | 2020 | DBTDL dosage correlates inversely with spherulite size |
| Z. Chen et al. | South China University of Technology | 2021 | Non-isothermal crystallization kinetics affected by DBTDL |
| X. Yang et al. | Tsinghua University | 2022 | Tin residue disrupts lamellar stacking in PU hard segments |
Domestic researchers have increasingly focused on quantifying the trade-off between catalytic efficiency and structural integrity, often proposing alternative catalyst systems that mitigate the negative effects of DBTDL.
7. Strategies to Mitigate Negative Effects of DBTDL
Given the dual role of DBTDL—as both a powerful catalyst and a potential disruptor of crystallization—several strategies have been proposed to optimize its use:
7.1 Use of Hybrid Catalyst Systems
Combining DBTDL with amine-based or bismuth catalysts allows for fine-tuning of reactivity and morphology. For example:
- DBTDL + Dabco: Enhances both gelation and urethane formation.
- DBTDL + Bi(III) Carboxylate: Reduces tin content while maintaining catalytic activity.
7.2 Post-Treatment and Annealing
Controlled post-curing or annealing at elevated temperatures can partially restore crystallinity by promoting chain relaxation and hydrogen bond reorganization.
7.3 Selection of Hard Segment Chemistry
Using symmetric diisocyanates (e.g., HDI) or introducing long-chain extenders can improve crystallizability even in the presence of DBTDL.
8. Conclusion
Dibutyltin dilaurate remains one of the most effective catalysts for polyurethane synthesis. Its ability to accelerate urethane bond formation has made it indispensable in many industrial applications. However, its impact on PU crystallization behavior cannot be ignored. Through numerous studies, both domestic and international, it has been established that DBTDL tends to reduce crystallinity by altering micromorphology, weakening hydrogen bonding, and limiting phase separation.
To fully harness the benefits of DBTDL while minimizing its drawbacks, a balanced approach involving hybrid catalyst systems, controlled curing protocols, and careful monomer selection is recommended. Future research should focus on developing low-toxicity alternatives that preserve both catalytic efficiency and structural integrity.
References
- Tang, Y., Li, J., & Zhou, W. (2021). “Effect of dibutyltin dilaurate on the crystallization behavior of polyester-based polyurethanes.” Journal of Applied Polymer Science, 138(12), 50345.
- Xu, L., Sun, H., & Zhao, Q. (2020). “Influence of organotin catalysts on hydrogen bonding and microphase separation in segmented polyurethanes.” Polymer Testing, 88, 106531.
- Zhang, J., Smith, A., & Williams, R. (2018). “Catalyst effects on the thermal and mechanical properties of polyurethane networks.” European Polymer Journal, 105, 123–132.
- Wang, L., Schmid, M., & Müller, C. (2019). “Residual metal catalysts and their long-term effects on polyurethane aging.” Macromolecular Materials and Engineering, 304(7), 1900145.
- Liu, Y., Chen, G., & Wu, X. (2020). “Spherulite morphology evolution in DBTDL-modified polyurethanes.” Chinese Journal of Polymer Science, 38(4), 398–407.
- Chen, Z., Huang, T., & Lin, F. (2021). “Non-isothermal crystallization kinetics of polyurethanes with different catalyst contents.” Acta Polymerica Sinica, 12, 1455–1464.
- Yang, X., Li, K., & Zhao, Y. (2022). “Tin-induced disruption of lamellar structures in polyurethane hard segments.” Tsinghua Science and Technology, 27(3), 456–465.
- Szycher, M. (2012). Szycher’s Handbook of Polyurethanes. CRC Press.
