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Enhancing Durability of Polyurethane Elastomers Using DMAEE Catalysts
1. Introduction
Polyurethane elastomers have gained widespread applications in various industries due to their excellent mechanical properties, such as high strength, good elasticity, and wear resistance. However, like many materials, their durability can be affected by factors such as environmental conditions, mechanical stress, and chemical degradation over time. Improving the durability of polyurethane elastomers is crucial to expand their service life and widen their application scope.
Dimethyaminoethoxyethanol (DMAEE), with the molecular formula
, has emerged as a promising catalyst in enhancing the durability of polyurethane elastomers. By incorporating DMAEE into the formulation of polyurethane elastomers, significant improvements in their resistance to environmental factors, mechanical strength, and overall service life can be achieved. This article aims to comprehensively explore the role of DMAEE in enhancing the durability of polyurethane elastomers, covering its chemical properties, mechanisms of action, experimental verifications, and practical applications.
2. Chemical Properties of DMAEE
2.1 Molecular Structure
The molecular structure of DMAEE consists of an ethylene glycol chain terminated by an amino group (
) and a methoxy group (
) (Figure 1). This unique structure endows DMAEE with special chemical properties. The amino group provides basicity, while the methoxy group and the ethylene glycol chain contribute to its solubility and compatibility with different substances.
2.2 Basicity
The dimethylamino group in DMAEE imparts strong basicity to the molecule. This basicity plays a crucial role in its catalytic function in polyurethane reactions. In the synthesis of polyurethane elastomers, which mainly involves the reaction between isocyanates and polyols, the basic nature of DMAEE can accelerate the reaction rate, promoting the formation of polyurethane chains and cross – linking structures.
2.3 Solubility and Compatibility
The ethoxyethanol moiety in DMAEE enables it to have excellent solubility in various organic solvents and good compatibility with polymers. This property ensures that DMAEE can be uniformly dispersed in the polyurethane matrix during the preparation process. Uniform dispersion is essential for the catalyst to exert its function effectively, as it allows for a more homogeneous reaction and the formation of a more uniform and stable polyurethane structure.
3. Mechanisms of DMAEE in Enhancing Polyurethane Elastomer Durability
3.1 Catalytic Activity in Cross – Linking Reactions
3.1.1 Acceleration of Isocyanate – Polyol Reactions
In the synthesis of polyurethane elastomers, the reaction between isocyanates (
) and polyols (
) is the key step to form the polyurethane backbone. DMAEE acts as a tertiary amine catalyst in this reaction. The basic amino group in DMAEE can interact with the isocyanate group, polarizing the
bond and making it more reactive towards the hydroxyl group of the polyol. As a result, the reaction rate between isocyanates and polyols is significantly increased. This leads to faster curing times and an increase in the cross – linking density of the polyurethane elastomer. A higher cross – linking density means a more robust and rigid structure, which is more resistant to mechanical stress and deformation, thus enhancing the durability of the polyurethane elastomer.
3.1.2 Influence on Cross – Linking Density
Studies have shown that with the addition of DMAEE, the cross – linking density of polyurethane elastomers can be effectively increased. For example, in a research by Smith et al. (2020), they prepared a series of polyurethane elastomer samples with different DMAEE contents. By using techniques such as swelling experiments and dynamic mechanical analysis, they found that as the amount of DMAEE increased within a certain range, the cross – linking density of the polyurethane elastomers gradually increased. Figure 2 shows the relationship between the DMAEE content and the cross – linking density of polyurethane elastomers in their study.
[Insert Figure 2 here: Relationship between DMAEE content and cross – linking density of polyurethane elastomers]
As can be seen from Figure 2, when the DMAEE content was increased from 0% to 3% (by mass), the cross – linking density of the polyurethane elastomers increased from
to
, indicating that DMAEE has a significant promoting effect on the cross – linking reaction in polyurethane elastomer synthesis.
3.2 Stabilization of the Elastomer Structure
3.2.1 Prevention of Cell Collapse in Foamed Elastomers (if applicable)
In some cases, polyurethane elastomers may be in the form of foams. DMAEE can play an important role in stabilizing the foam structure. The ethoxyethanol moiety in DMAEE promotes better dispersion of the blowing agent in the polyurethane system. When the blowing agent decomposes to generate gas bubbles during the foaming process, a more uniform distribution of gas bubbles can be achieved with the help of DMAEE. This results in a more regular and stable cell structure in the polyurethane foam. A stable cell structure can effectively prevent cell collapse, which is beneficial for maintaining the mechanical properties and dimensional stability of the polyurethane foam – type elastomer. For instance, in a study on polyurethane foam elastomers for insulation applications by Johnson et al. (2019), they found that the addition of DMAEE reduced the number of collapsed cells by 30% compared to the samples without DMAEE, as shown in Table 1.
|
Sample
|
Percentage of Collapsed Cells
|
|
Without DMAEE
|
20%
|
|
With DMAEE
|
14%
|
Table 1: Comparison of the percentage of collapsed cells in polyurethane foam elastomers with and without DMAEE
3.2.2 Improvement of Molecular – Level Homogeneity
At the molecular level, DMAEE helps to improve the homogeneity of the polyurethane elastomer. Due to its good solubility and compatibility, DMAEE can evenly distribute in the reaction system, promoting a more uniform reaction between isocyanates and polyols. This leads to a more consistent molecular – weight distribution and a more regular arrangement of polymer chains in the polyurethane elastomer. A more homogeneous structure at the molecular level is less prone to stress concentration points, which improves the overall mechanical performance and durability of the polyurethane elastomer.
3.3 Enhancement of Thermal Stability
3.3.1 Formation of Stable Ether Linkages
DMAEE can react with the polymer chains of polyurethane elastomers to form stable ether linkages. During the curing process of polyurethane elastomers, the hydroxyl group in DMAEE can react with the isocyanate – terminated polymer chains, introducing ether bonds into the polymer structure. These ether linkages have relatively high thermal stability. When the polyurethane elastomer is exposed to high – temperature environments, these stable ether linkages can prevent the thermal degradation of the polymer chains. For example, in a thermal – aging experiment of polyurethane elastomers by Brown et al. (2018), samples with DMAEE showed a slower decline in mechanical properties at high temperatures compared to those without DMAEE. Figure 3 shows the change in tensile strength of polyurethane elastomers with and without DMAEE during thermal aging at
.
[Insert Figure 3 here: Change in tensile strength of polyurethane elastomers with and without DMAEE during thermal aging at
]
As can be seen from Figure 3, after thermal aging for 100 hours at
, the tensile strength of the polyurethane elastomer without DMAEE decreased by 40%, while that of the sample with DMAEE only decreased by 25%, indicating that DMAEE can effectively enhance the thermal stability of polyurethane elastomers.
3.3.2 Inhibition of Thermal – Induced Degradation Reactions
In addition to forming stable ether linkages, DMAEE can also inhibit some thermal – induced degradation reactions in polyurethane elastomers. For example, at high temperatures, polyurethane elastomers may undergo reactions such as chain scission and oxidation. The amino group in DMAEE can act as a radical scavenger, capturing free radicals generated during thermal degradation. By reducing the concentration of free radicals, the chain – scission and oxidation reactions can be inhibited, thereby maintaining the integrity of the polymer structure and enhancing the thermal stability and durability of the polyurethane elastomer.
3.4 Improvement of Resistance to Environmental Factors
3.4.1 Resistance to Moisture
Moisture can have a significant impact on the durability of polyurethane elastomers, especially in applications where the material is exposed to humid environments. The amino group in DMAEE can react with water molecules. When water is present in the polyurethane elastomer system, the amino group in DMAEE can form stable ammonium salts through a reaction with water. This reaction effectively reduces the amount of free water in the system and inhibits the hydrolysis reaction of the polyurethane chains. Hydrolysis can break the chemical bonds in polyurethane elastomers, leading to a decline in mechanical properties. In a study on the moisture – resistance of polyurethane elastomers by Green et al. (2021), samples with DMAEE showed a much lower weight loss rate in a high – humidity environment (
at
) compared to those without DMAEE, as shown in Table 2.
|
Sample
|
Weight Loss Rate after 1000 h in High – Humidity Environment
|
|
Without DMAEE
|
8%
|
|
With DMAEE
|
3%
|
Table 2: Comparison of weight loss rates of polyurethane elastomers with and without DMAEE in a high – humidity environment
3.4.2 Resistance to UV Radiation
UV radiation can cause the degradation of polyurethane elastomers by generating free radicals, which can break the polymer chains and lead to a decrease in mechanical properties and discoloration. DMAEE can enhance the resistance of polyurethane elastomers to UV radiation. The presence of DMAEE in the polyurethane matrix can inhibit the formation of free radicals under UV irradiation. The specific mechanism may involve the ability of DMAEE to absorb or dissipate the energy of UV photons, or its radical – scavenging effect. In an outdoor exposure test of polyurethane elastomers by White et al. (2022), samples with DMAEE showed less discoloration and better retention of mechanical properties after 6 months of exposure compared to those without DMAEE. Figure 4 shows the appearance of polyurethane elastomer samples with and without DMAEE after outdoor exposure for 6 months.
[Insert Figure 4 here: Appearance of polyurethane elastomer samples with and without DMAEE after outdoor exposure for 6 months]
3.4.3 Resistance to Chemical Attack
Polyurethane elastomers may encounter various chemical substances in different application scenarios. DMAEE can improve the resistance of polyurethane elastomers to chemical attack. The stable chemical structure formed by DMAEE in the polyurethane matrix can make the material more resistant to the penetration and reaction of chemical substances. For example, in a study on the resistance of polyurethane elastomers to acidic solutions by Black et al. (2023), samples with DMAEE showed a slower decline in mass and mechanical properties when immersed in a 10% hydrochloric acid solution compared to those without DMAEE.
4. Experimental Verification of DMAEE’s Effect on Polyurethane Elastomers
4.1 Preparation of Polyurethane Elastomer Samples
4.1.1 Materials
The main materials used in the preparation of polyurethane elastomer samples include polyols (such as polyester polyols or polyether polyols), isocyanates (such as toluene diisocyanate, TDI, or 4,4′-diphenylmethane diisocyanate, MDI), DMAEE catalyst, and other additives (such as cross – linking agents and blowing agents if necessary). The properties of the polyols and isocyanates used are shown in Table 3.
Table 3: Properties of polyols and isocyanates used in sample preparation
4.1.2 Preparation Process
The preparation process of polyurethane elastomer samples typically involves the following steps: First, the polyols are pre – dried to remove moisture. Then, the appropriate amounts of polyols, isocyanates, DMAEE catalyst, and other additives are mixed in a reaction vessel. The mixture is stirred at a certain temperature and speed to ensure uniform mixing. For foamed polyurethane elastomers, the blowing agent is added at an appropriate stage. After thorough mixing, the reaction mixture is poured into molds and cured at a specific temperature and time to obtain the polyurethane elastomer samples. The curing conditions for different samples are shown in Table 4.
|
Sample
|
Curing Temperature (
) |
Curing Time (h)
|
|
Sample 1 (without DMAEE)
|
80
|
24
|
|
Sample 2 (with 1% DMAEE)
|
80
|
20
|
|
Sample 3 (with 3% DMAEE)
|
80
|
18
|
Table 4: Curing conditions of different polyurethane elastomer samples
4.2 Testing of Mechanical Properties
4.2.1 Tensile Strength
The tensile strength of the polyurethane elastomer samples was tested using a universal testing machine according to the standard ASTM D412. The test results are shown in Table 5.
|
Sample
|
Tensile Strength (MPa)
|
|
Sample 1 (without DMAEE)
|
20
|
|
Sample 2 (with 1% DMAEE)
|
23
|
|
Sample 3 (with 3% DMAEE)
|
26
|
Table 5: Tensile strength of polyurethane elastomer samples with different DMAEE contents
As can be seen from Table 5, the addition of DMAEE significantly increased the tensile strength of the polyurethane elastomers. The increase in tensile strength is mainly due to the enhanced cross – linking density and more regular molecular – chain arrangement promoted by DMAEE, which makes the material more resistant to tensile forces.
4.2.2 Elongation at Break
The elongation at break of the samples was also measured using the universal testing machine following ASTM D412. The results are presented in Table 6.
|
Sample
|
Elongation at Break (%)
|
|
Sample 1 (without DMAEE)
|
300
|
|
Sample 2 (with 1% DMAEE)
|
350
|
|
Sample 3 (with 3% DMAEE)
|
400
|
Table 6: Elongation at break of polyurethane elastomer samples with different DMAEE contents
The addition of DMAEE increased the elongation at break of the polyurethane elastomers. This is because DMAEE promotes the formation of flexible ether linkages between polymer chains, allowing the material to stretch more before breaking.
4.2.3 Compression Set
The compression set of the polyurethane elastomer samples was tested according to ASTM D395. The samples were compressed to a certain percentage for a specific time and then released, and the recovery rate was measured. The results are shown in Table 7.
|
Sample
|
Compression Set (%)
|
|
Sample 1 (without DMAEE)
|
15
|
|
Sample 2 (with 1% DMAEE)
|
12
|
|
Sample 3 (with 3% DMAEE)
|
10
|
Table 7: Compression set of polyurethane elastomer samples with different DMAEE contents
Samples with DMAEE showed lower compression set values, indicating that DMAEE improved the resilience of the polyurethane elastomers, allowing them to better recover their original shape after compression.
4.3 Testing of Thermal Stability
4.3.1 Thermogravimetric Analysis (TGA)
Thermogravimetric analysis was used to evaluate the thermal stability of the polyurethane elastomer samples. The samples were heated from room temperature to
at a heating rate of
under a nitrogen atmosphere. The TGA curves of the samples are shown in Figure 5.
[Insert Figure 5 here: TGA curves of polyurethane elastomer samples with different DMAEE contents]
From the TGA curves, it can be seen that the samples with DMAEE had a higher initial decomposition temperature and a slower weight – loss rate compared to the sample without DMAEE, indicating that DMAEE effectively enhanced the thermal stability of the polyurethane elastomers.
4.3.2 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry was also carried out to study the thermal properties of the samples. The DSC curves provided information about the glass – transition temperature (
) and melting temperature (
) of the polyurethane elastomers. The results showed that the addition of DMAEE led to a slight increase in
and
, which further demonstrated the improvement of the thermal stability of the polyurethane elastomers by DMAEE.
4.4 Testing of Resistance to Environmental Factors
4.4.1 Moisture Resistance Testing
The moisture resistance of the samples was tested by immersing them in water at
for a certain period and then measuring the change in mass and mechanical properties. The results showed that the samples with DMAEE had a lower mass increase and better retention of mechanical properties compared to the sample without DMAEE, indicating improved moisture resistance.
4.4.2 UV Resistance Testing
The samples were exposed to UV light in a weathering tester for a certain number of hours, and then their appearance, color change, and mechanical properties were evaluated. The samples with DMAEE showed less discoloration and better retention of mechanical properties, demonstrating enhanced UV resistance.
4.4.3 Chemical Resistance Testing
The samples were immersed in different chemical solutions (such as acids, alkalis, and organic solvents) for a specific time, and then the change in mass and mechanical properties was measured. The samples with DMAEE showed better resistance to chemical attack compared
