Tetramethylimidazolidinediylpropylamine (TMBPA) as a Dual-Function Catalyst for Flexible and Rigid Foams

Tetramethylimidazolidinediylpropylamine (TMBPA): A Dual-Function Catalyst for Flexible and Rigid Foams

Introduction

Polyurethane (PU) foams, renowned for their versatility and diverse applications, are produced by the exothermic reaction of polyols and isocyanates in the presence of catalysts, blowing agents, surfactants, and other additives. The catalysts play a crucial role in controlling the two main competing reactions: the gelation reaction (polyol-isocyanate reaction leading to polymer chain extension and crosslinking) and the blowing reaction (reaction of isocyanate with water or other blowing agents to generate carbon dioxide, leading to cell formation). The careful balance of these reactions is essential for achieving the desired foam properties, such as cell size, density, and mechanical strength.

Traditional catalysts, primarily tertiary amines and organometallic compounds, each have their limitations. Tertiary amines, while effective in promoting both gelation and blowing reactions, can contribute to volatile organic compound (VOC) emissions and may exhibit undesirable odor. Organometallic catalysts, such as tin compounds, are potent gelation catalysts but can be toxic and environmentally problematic. This has spurred research and development into alternative catalysts that offer a balance of activity, selectivity, and environmental friendliness.

Tetramethylimidazolidinediylpropylamine (TMBPA) is an emerging catalyst in the polyurethane foam industry, demonstrating potential as a dual-function catalyst capable of promoting both the gelation and blowing reactions. Its unique molecular structure combines the reactivity of a tertiary amine with the potential for reduced VOC emissions due to its relatively high molecular weight and low volatility. This article aims to provide a comprehensive overview of TMBPA, including its properties, mechanism of action, applications in flexible and rigid foams, advantages, and limitations.

1. Product Parameters

Property	Value	Unit
Chemical Name	Tetramethylimidazolidinediylpropylamine	–
CAS Number	6938-22-3	–
Molecular Formula	C₁₀H₂₃N₃	–
Molecular Weight	185.31	g/mol
Appearance	Colorless to light yellow liquid	–
Density	~0.93	g/cm³ at 25°C
Boiling Point	~220	°C
Flash Point	~90	°C
Solubility	Soluble in water and most organic solvents	–
Amine Value	~300	mg KOH/g
Moisture Content	≤ 0.5	%

2. Chemical Structure and Properties

TMBPA belongs to the class of tertiary amine catalysts and possesses a unique imidazolidine ring within its structure. This cyclic structure contributes to its relatively high molecular weight and reduced volatility compared to many other tertiary amine catalysts.

      CH3   CH3
      |     |
  N---CH2-N-CH2
  |       |
  CH2     CH2
  |       |
  CH2     CH2
  |
  CH2-N(CH3)2

Key Features of the TMBPA Molecule:

Tertiary Amine Groups: The presence of three tertiary amine groups provides multiple active sites for catalyzing the urethane and urea reactions.
Imidazolidine Ring: The imidazolidine ring contributes to the molecule’s stability and reduces its volatility. This ring structure may also influence the selectivity of the catalyst towards specific reactions.
Propylamine Side Chain: The propylamine side chain further enhances the molecule’s compatibility with the polyol and isocyanate components of the polyurethane formulation.

3. Mechanism of Action

TMBPA, like other tertiary amine catalysts, functions by accelerating the urethane (gelation) and urea (blowing) reactions. It achieves this by acting as a nucleophile, interacting with the isocyanate group to facilitate its reaction with either the polyol or water.

3.1 Gelation Reaction (Polyol-Isocyanate):

Complex Formation: The nitrogen atom of the tertiary amine in TMBPA attacks the electrophilic carbon of the isocyanate group (-N=C=O), forming a complex. This complex polarizes the isocyanate group, making it more susceptible to nucleophilic attack.
Proton Abstraction: The polyol (R-OH) donates a proton to the negatively charged nitrogen atom in the TMBPA-isocyanate complex.
Urethane Formation: The deprotonated polyol reacts with the isocyanate carbon, forming a urethane linkage (-NH-C(O)-O-).
Catalyst Regeneration: TMBPA is regenerated and available to catalyze further reactions.

3.2 Blowing Reaction (Isocyanate-Water):

Complex Formation: Similar to the gelation reaction, TMBPA forms a complex with the isocyanate group.
Proton Abstraction: Water (H-OH) donates a proton to the negatively charged nitrogen atom in the TMBPA-isocyanate complex.
Carbamic Acid Formation: The deprotonated water reacts with the isocyanate carbon, forming carbamic acid (-NH-C(O)-OH).
Decomposition of Carbamic Acid: Carbamic acid is unstable and decomposes into an amine and carbon dioxide (CO₂), which acts as the blowing agent.
Urea Formation: The amine produced from the carbamic acid decomposition reacts with another isocyanate molecule to form a urea linkage (-NH-C(O)-NH-).
Catalyst Regeneration: TMBPA is regenerated and available to catalyze further reactions.

The relative rates of the gelation and blowing reactions are influenced by several factors, including the catalyst concentration, temperature, and the specific components of the polyurethane formulation.

4. Applications in Flexible Polyurethane Foams

Flexible polyurethane foams are widely used in applications such as mattresses, furniture cushioning, automotive seating, and carpet underlay. TMBPA can be employed as a catalyst, either alone or in combination with other catalysts, to achieve the desired foam properties.

4.1 Dosage and Performance:

The optimal dosage of TMBPA in flexible foam formulations typically ranges from 0.1 to 1.0 parts per hundred parts of polyol (php). The specific dosage depends on the desired foam density, cell structure, and overall reactivity of the system.

Property	Typical Range	Notes
TMBPA Dosage (php)	0.1 – 1.0	Lower dosage for slower reaction; higher dosage for faster reaction.
Foam Density (kg/m³)	15 – 50	Controlled by water content and other blowing agents. TMBPA influences cell opening and uniformity, impacting density.
Cell Size (μm)	100 – 500	Affected by surfactant type and concentration, as well as the balance between gelation and blowing reactions. TMBPA influences cell size.
Airflow (CFM)	1 – 5	Indicates cell openness. TMBPA can contribute to more open cells.
Tensile Strength (kPa)	50 – 200	Depends on polymer structure and crosslinking density. TMBPA indirectly affects tensile strength by influencing the polymer network.
Elongation (%)	100 – 300	Depends on polymer structure and crosslinking density. TMBPA indirectly affects elongation by influencing the polymer network.

4.2 Advantages in Flexible Foams:

Good Balance of Gelation and Blowing: TMBPA promotes both the gelation and blowing reactions, leading to a well-balanced foam structure with desirable cell size and density.
Improved Cell Opening: TMBPA can contribute to more open-celled structures, which are beneficial for breathability and comfort in applications like mattresses and furniture.
Reduced VOC Emissions: Compared to some other tertiary amine catalysts, TMBPA has a relatively high molecular weight and low volatility, leading to potentially lower VOC emissions.
Good Processability: TMBPA is compatible with most common polyol and isocyanate systems, making it easy to incorporate into existing foam formulations.

4.3 Examples of Flexible Foam Formulations with TMBPA:

Table 1: Example Flexible Foam Formulation (Conventional Polyether Polyol System)

Component	Parts by Weight
Polyether Polyol (3000 MW)	100
Water	3.5
TMBPA	0.3
Surfactant (Silicone)	1.0
TDI 80/20	45

Table 2: Example Flexible Foam Formulation (Polymer Polyol System)

Component	Parts by Weight
Polymer Polyol	80
Conventional Polyether Polyol (3000 MW)	20
Water	3.0
TMBPA	0.4
Surfactant (Silicone)	1.2
TDI 80/20	40

Note: These are just example formulations, and the specific amounts of each component may need to be adjusted depending on the desired foam properties and the specific raw materials used.

5. Applications in Rigid Polyurethane Foams

Rigid polyurethane foams are characterized by their closed-cell structure and high thermal insulation properties, making them suitable for applications such as building insulation, refrigerator insulation, and structural panels. TMBPA can also be used as a catalyst in rigid foam formulations, although its role may be more nuanced compared to flexible foams.

5.1 Dosage and Performance:

The typical dosage of TMBPA in rigid foam formulations ranges from 0.2 to 1.5 php. Higher dosages may be required in formulations using high levels of blowing agents or low reactivity polyols.

Property	Typical Range	Notes
TMBPA Dosage (php)	0.2 – 1.5	Higher dosage often needed for faster rise times and improved cell structure in rigid foams.
Foam Density (kg/m³)	25 – 60	Primarily controlled by the type and amount of blowing agent. TMBPA influences the cell structure and can impact density.
Cell Size (μm)	50 – 300	Influenced by blowing agent type and surfactant. TMBPA contributes to finer cell structure.
Closed Cell Content (%)	90 – 98	Key property for thermal insulation. TMBPA contributes to a high closed-cell content.
Compressive Strength (kPa)	100 – 400	Depends on density and cell structure. TMBPA indirectly affects compressive strength by influencing the polymer network.
Thermal Conductivity (W/mK)	0.020 – 0.030	Primary measure of insulation performance. Good cell structure, facilitated by TMBPA, is crucial for low thermal conductivity.

5.2 Advantages in Rigid Foams:

Improved Cell Structure: TMBPA can contribute to a finer and more uniform cell structure in rigid foams, leading to enhanced thermal insulation properties and compressive strength.
Faster Cure Rate: In some formulations, TMBPA can accelerate the curing process, reducing demolding times and increasing productivity.
Compatibility with Different Blowing Agents: TMBPA can be used with a variety of blowing agents, including water, hydrocarbons, and hydrofluorocarbons (HFCs), allowing for flexibility in formulation design.
Good Flowability: TMBPA can improve the flowability of the foam formulation, ensuring complete filling of complex molds and reducing the risk of voids or imperfections.

5.3 Examples of Rigid Foam Formulations with TMBPA:

Table 3: Example Rigid Foam Formulation (Polyester Polyol System with Water Blowing)

Component	Parts by Weight
Polyester Polyol	100
Water	1.5
TMBPA	0.5
Surfactant (Silicone)	1.5
Flame Retardant	10
MDI (Polymeric)	120

Table 4: Example Rigid Foam Formulation (Polyether Polyol System with Hydrocarbon Blowing Agent)

Component	Parts by Weight
Polyether Polyol	100
n-Pentane	8.0
TMBPA	0.7
Surfactant (Silicone)	1.8
Flame Retardant	12
MDI (Polymeric)	130

Note: These are illustrative examples and require adjustments based on specific application requirements and raw material characteristics.

6. Advantages and Limitations of TMBPA

6.1 Advantages:

Dual-Function Catalysis: Promotes both gelation and blowing reactions, simplifying formulation design.
Reduced VOC Emissions: Lower volatility compared to some other tertiary amine catalysts.
Good Compatibility: Compatible with a wide range of polyols, isocyanates, and blowing agents.
Improved Cell Structure: Contributes to finer and more uniform cell structure.
Faster Cure Rate: Can accelerate the curing process in some formulations.
Versatile Application: Suitable for both flexible and rigid polyurethane foams.

6.2 Limitations:

Potential for Discoloration: Under certain conditions, TMBPA can contribute to discoloration of the foam, particularly in the presence of light or heat.
Odor: While lower than some amines, TMBPA can still have a characteristic amine odor.
Cost: TMBPA may be more expensive than some traditional tertiary amine catalysts.
Hydrolytic Stability: In some humid environments, TMBPA can be prone to hydrolysis, which can reduce its catalytic activity.
Yellowing: Some reports indicate a potential for yellowing in the foam, particularly under UV exposure.

7. Safety and Handling

TMBPA is a moderately alkaline compound and should be handled with care. Avoid contact with skin and eyes. Wear appropriate personal protective equipment (PPE), such as gloves, safety glasses, and a lab coat. In case of contact, flush immediately with plenty of water. Consult the Material Safety Data Sheet (MSDS) for detailed safety information.

8. Conclusion

Tetramethylimidazolidinediylpropylamine (TMBPA) represents a promising dual-function catalyst for the polyurethane foam industry. Its unique molecular structure offers a balance of activity, selectivity, and environmental friendliness, making it a viable alternative to traditional tertiary amine and organometallic catalysts. While TMBPA exhibits advantages in terms of reduced VOC emissions, improved cell structure, and versatility in both flexible and rigid foam applications, its limitations, such as potential for discoloration and odor, need to be carefully considered during formulation design. Further research and development are ongoing to optimize the performance of TMBPA and address its limitations, paving the way for its wider adoption in the polyurethane foam industry. The future of TMBPA lies in its ability to contribute to more sustainable and high-performance polyurethane foam products. 🧪

9. References

[1] Rand, L., & Frisch, K. C. (1962). Polyurethane. Interscience Publishers.
[2] Oertel, G. (Ed.). (1993). Polyurethane Handbook. Hanser Gardner Publications.
[3] Woods, G. (1990). The ICI Polyurethanes Book. John Wiley & Sons.
[4] Szycher, M. (1999). Szycher’s Handbook of Polyurethanes. CRC Press.
[5] Hepenstrick, J. T., & Markovs, R. A. (1970). U.S. Patent No. 3,547,851. U.S. Patent and Trademark Office. (Example of imidazolidine catalysts in PU)
[6] Technical Data Sheet: Huntsman JEFFCAT® ZF-10. (Example of commercial imidazolidine catalyst).
[7] Elwell, D. & Bots, G. (2009). Polyurethane flexible foam: A guide to processing. Smithers Rapra Publishing.
[8] Ashida, K. (2006). Polyurethane and Related Foams. CRC Press.
[9] Prociak, A., Ryszkowska, J., & Uramiak, M. (2017). Synthesis, properties and applications of polyurethane foams. Woodhead Publishing.
[10] Brydson, J. A. (1999). Plastics Materials. Butterworth-Heinemann.