Abstract
A low-emission catalyst for the preparation of flexible polyurethane (FPUR) foams was developed. Copper-amine complex solutions in ethylene glycol (EG), namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG (en, ethylenediamine; trien, triethylenetetramine), were synthesized and used as catalysts for the preparation of FPUR foams. The synthesis of Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG is convenient because the synthesis of copper-amine complexes can be done in situ using ethylene glycol as a solvent and no purification step is needed. It was found that Cu(OAc)2(en)2-EG was a suitable catalyst for FPUR foam preparation. In comparison to Dabco EG (or triethylenediamine), which is a commercial catalyst for FPUR foam preparation, Cu(OAc)2(en)2-EG had a comparable catalytic activity in gelling reaction and a higher catalytic activity in blowing reaction. The FPUR foam prepared from Cu(OAc)2(en)2-EG had a lower density than that prepared from Dabco EG.
1 Introduction
Polyurethane foam has been widely produced in the polymer industry. This is because polyurethane can be manufactured in an extremely wide range of grades, from flexible to rigid foams (1, 2). Flexible polyurethane (FPUR) foams are produced by reacting diisocyanates or polyisocyanates with compounds that contain at least two hydrogen atoms that are reactive towards the isocyanate groups in the presence of blowing agents, catalysts, surfactants and other additives. High-density flexible foams are defined as those having a density above 100 kg/m3. They can be easily produced in a variety of shapes by molding or cutting. Flexible polyurethane foams are used for many applications, e.g., acoustic insulation, furniture upholstery, automotive seating and carpet foam backing.
Tertiary amines are the most important catalysts in the manufacture of FPUR foams. The relationship between catalyst structure and activity in polyurethane formation was investigated (3). It was found that triethylenediamine (TEDA or DABCO) and dibutyltin dilaurate were strong gelling catalysts, while pentamethyldiethylenetriamine was a strong blowing catalyst. N,N-Dimethylcyclohexylamine (DMCHA) and tetramethylethylenediamine showed moderate activity between the gelling and the blowing reactions. For the development of new amine catalysts for FPUR foams, polyurethane reaction kinetics was followed using Fourier transform infrared spectroscopy (FTIR)-programmed cell to match the foam core temperature profile, which had a similar condition to the actual FPUR foam processing (4). Single catalysts and catalyst mixtures of commercially available amines were evaluated for FPUR preparation (5). It was found that DABCO and the mixtures DMCHA-stannous octoate [Sn(Oct)2] and DABCO-Sn(Oct)2-N,N-bis(2-dimethylaminoethyl)methylamine showed the best catalytic results. Simulation of the chemical reactions in rigid polyurethane foam formation catalyzed by amine was studied (6). The impact of catalysts, including the impact of catalyst concentration, was investigated, and the results provided good agreement with experimental data. Since amine catalysts cause odor problems during the manufacturing process, the development of new catalyst systems for FPUR foams with less volatile organic compound emission is an area of interest. The catalyst-containing reactive groups, namely, amine and hydroxyl groups, which undergo reaction with the isocyanate group to become part of the polymer matrix (7), were investigated.
Besides amines, metal compounds and metal complexes can be used as catalysts in the preparation of polyurethane and waterborne polyurethane (8–13). Our research group synthesized new catalysts with good catalytic activity for the preparation of water-blown rigid polyurethane (RPUR) foams (14). These catalysts were copper-amine complexes, Cu(OAc)2(en)2 and Cu(OAc)2(trien) (where en indicates ethylenediamine; trien, triethylenetetramine). Both Cu(OAc)2(en)2 and Cu(OAc)2(trien) are odorless and can be prepared from inexpensive and readily available starting materials. Cu(OAc)2(en)2 and Cu(OAc)2(trien) were synthesized in acetone, which was removed to obtain a pure copper complex, before using them in the RPUR foam preparation. Copper-amine complexes have good solubility in water-blown RPUR foam formulation since they have good solubility in water and there is sufficient amount of water in RPUR foam formulation to dissolve copper complexes. However, the FPUR foam formulation contains a small amount of water and it would be difficult to obtain a homogeneous mixing of the copper-amine complexes with the other components in FPUR foam preparation.
To the best of our knowledge, there have been few reports about the development of new catalysts for FPUR foams. Therefore, it is of interest to use these copper-amine complexes as catalysts in the preparation of FPUR foams. A new method for the preparation of Cu(OAc)2(en)2 and Cu(OAc)2(trien) was developed in order to improve the solubility of these copper complexes in FPUR foam formulation and to obtain a more convenient procedure in the synthesis of copper-amine complexes. A commonly used catalyst in FPUR foam formulation is Dabco EG, which is a solution of 33 wt.% triethylenediamine in ethylene glycol. Therefore, the objective of this research was to synthesize copper-amine complexes in the form of solutions in ethylene glycol, namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG. During the polymerization, ethylene glycol in Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG underwent polymerization reaction with isocyanate. The pure copper-amine complexes, namely, Cu(OAc)2(en)2 and Cu(OAc)2(trien), remained in the FPUR foam matrix after the polymerization was completed. Based on our previous work, the physical state of pure Cu(OAc)2(en)2 and Cu(OAc)2(trien) was solid and viscous liquid, respectively. Therefore, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG have the potential to be used as low-emission catalysts for FPUR foam preparation.
2 Experimental
2.1 Materials
Ethylene glycol was obtained from Carlo Erba (Italy). Copper(II) acetate monohydrate [Cu(OAc)2·H2O], ethylenediamine (en) and triethylenetetramine (trien) were obtained from Aldrich (USA). Poly(ethylene oxide) triol (Jeffol G-31-35, glycerin initiated ethylene oxide triol) (molecular weight 4800 g/mol, hydroxyl value 35 mg KOH/g, average functionality 3), diphenylmethane diisocyanate prepolymer (MDIP, Suprasec 2449, % NCO 18.9 wt.%), triethylenediamine in ethylene glycol (Dabco EG, commercial reference catalyst), silicone surfactant (Dabco DC193) and chain extender (mono ethylene glycol) were supplied by Huntsman (Thailand) Co., Ltd. (Thailand). Distilled water was used as a chemical blowing agent.
2.2 Analytical method
Fourier transform infrared and attenuated total reflection infrared (ATR-IR) spectra were recorded on a Perkin-Elmer RX I FTIR spectrometer (USA) and a Nicolet 6700 FTIR spectrometer (USA), respectively, over the range 500–4000 cm-1 at a resolution of 4 cm-1. Ultraviolet-visible (UV-Vis) spectra were recorded on a Varian Cary 50 UV-Vis spectrophotometer (USA) over the range 200–800 nm. MALDI-TOF mass spectra were carried out using a Bruker Daltonics mass spectrometer (USA) using 2-cyano-4-hydroxy cinnamic acid as a matrix.
2.3 Synthesis of copper-ethylenediamine complex solution in ethylene glycol [Cu(OAc)2(en)2-EG]
A solution of ethylenediamine (0.42 ml, 6.28 mmol) was dissolved in ethylene glycol (1.8 ml) at room temperature for 15 min. Copper(II) acetate monohydrate (0.624 g, 3.12 mmol) was added, and the reaction mixture was stirred continuously at room temperature for 2 h. A solution of 33 wt.% of Cu(OAc)2(en)2 in ethylene glycol [Cu(OAc)2(en)2-EG] was obtained as an odorless purple solution with low viscosity. UV: λmax (MeOH)=232 nm, molar absorptivity (ε)=5.667. MALDI-TOF m/z of CuC8H22N4O4 [Cu(OAc)2(en)2]+: 301.83; found 303.05 [Cu(OAc)2(en)2+H]+, 242.74; found 242.93 [Cu(OAc)2(en)+H]+.
2.4 Synthesis of copper-triethylenetetramine complex solution in ethylene glycol [Cu(OAc)2(trien)-EG]
A solution of triethylenetetramine (0.43 ml, 2.89 mmol) was dissolved in ethylene glycol (1.8 ml) at room temperature for 15 min. Copper(II) acetate monohydrate (0.578 g, 2.89 mmol) was added, and the reaction mixture was stirred continuously at room temperature for 2 h. A solution of 33 wt.% of Cu(OAc)2(trien) in ethylene glycol [Cu(OAc)2(trien)-EG] was obtained as an odorless blue solution and low viscosity. UV; λmax (MeOH)=258 nm, molar absorptivity (ε)=4.322. MALDI-TOF m/z of CuC10H24N4O4 [Cu(OAc)2(trien)]+ 327.87; found 268.16 [Cu(OAc)(trien)]+.
2.5 Preparation of free rise flexible polyurethane foam prepared by the cup test method
Table 1 shows the FPUR foam formulation. The free rise foams were prepared in a 700–ml paper cup, which was used for the investigation of the reaction time, free rise density, height of foams and NCO conversion. In the first step, the polyol, the catalysts (Dabco EG or copper-amine complexes), the surfactant, the chain extender and the blowing agent were mixed in a paper cup. In the second step, MDIP was added to the polyol mixture. Then, the mixture was mixed using a mechanical stirrer at 2000 rpm for 10 s to obtain a homogeneous mixture. The mixture was poured into another paper cup. The foam was allowed to rise freely, and during the foaming reaction, cream time (the time when the foam starts to rise or the blowing reaction), gel time (the time when the foam mixture starts to appear as a gel), rise time (the time when the foam stops rising) and tack-free time (the time when the polymerization is finished) were measured. The foams were kept for 2 days before the measurement of free rise density and the investigation of the NCO conversion by infrared spectroscopy. The density of FPUR foam was measured in accordance with ASTM D3574. The size of the specimen was 3.0×3.0×3.0 cm (width×length×thickness), and the average value of the three samples was reported.
FPUR foam formulations (pbwa).
| Formulation (NCO index=100) | pbw |
|---|---|
| Poly(ethylene oxide) triol, glycerin initiated (Jeffol G-31-35, molecular weight=4800 g/mol, OH number=35 mg KOH/g) | 100.0 |
| Metal complex catalyst or reference catalyst (Dabco EG) | 1.5b |
| Silicone surfactant (polysiloxane, Dabco DC193) | 0.5 |
| Chain extender (ethylene glycol) | 5.0 |
| Blowing agent (water) | 0.5 |
| Diphenylmethane diisocyanate prepolymer (MDIP, Suprasec 2449, % NCO=18.9 wt.%) | 69.3 |
aParts by weight or 1 g in 100 g of polyol.
bThe amount of 1.5 pbw consisted of 0.5 pbw of the metal complex and 1.0 pbw of ethylene glycol.
2.6 Molded flexible polyurethane foam
The molded foams were prepared in an aluminum mold slap with a dimension of 20×10×0.6 cm (width×length× thickness), which was used for the investigation of the mechanical properties and morphology. The preparation of the molded foams used the same mixing step as in the preparation of free rise foams. After all the components were mixed by a mechanical stirrer at 2000 rpm for 10 s, the liquid was poured into an aluminum mold. The demolding time for the FPUR foams prepared from Dabco EG, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG was 30, 30 and 60 min, respectively. The FPUR foams were kept for 2 days at room temperature. The foam density was measured. The compression set of the FPUR foams set at 25% thickness was investigated using a universal testing machine (Hounsfield H 10 KM) in accordance with ASTM C165-95. Specimens with a dimension of 0.6×0.6×0.6 cm (width×length×thickness) perpendicular to the foam rise direction were analyzed, and the average values of the three samples were reported. Tensile testing of the FPUR foams was carried out using a universal testing machine (Hounsfield H 10 KM) in accordance with ASTM D412-68, and the average values of the three samples were reported. The morphology of FPUR foams was investigated on a Hitachi/S-4800 scanning electron microscope.
3 Results and discussion
3.1 Synthesis of copper-amine complex solutions in ethylene glycol [Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG]
Synthesis of the copper-amine complexes, Cu(OAc)2(en)2 and Cu(OAc)2(trien), was carried out using ethylene glycol as a solvent (Scheme 1). Copper-amine complexes were obtained in the form of solution in ethylene glycol, namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG. The concentration of Cu(OAc)2(en)2 and Cu(OAc)2(trien) in ethylene glycol was chosen to be 33 wt.%, which was similar to that of DABCO EG, which is 33 wt.% solution of triethylenediamine in EG. The important criteria in the synthesis of Cu(OAc)2(en)2 and Cu(OAc)2(trien) are the starting materials and the copper complexes that must be dissoluble in ethylene glycol. It was found that Cu(OAc)2, ethylenediamine, triethylenetetramine and the copper complexes have good solubility in ethylene glycol. Therefore, the copper complex formation could be done in situ using ethylene glycol as a solvent. Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG were obtained as odorless liquids with low viscosity. The solution containing copper-amine complexes could be further used in the preparation of FPUR foam without purification. Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG also have good solubility in FPUR foam formulation. The preparation of Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG was convenient, and large-scale synthesis could be done in a short time.
Synthesis of metal-amine complex solutions in ethylene glycol.
3.2 Characterization of Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG
Due to the high boiling point of ethylene glycol (197.3°C), removal of ethylene glycol from Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG was not possible. Therefore, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG were characterized in the form of solution ethylene glycol. Like in our previous work (14), Cu(OAc)2(en)2 and Cu(OAc)2(trien) were synthesized using acetone as a solvent. After the removal of acetone, the purified Cu(OAc)2(en)2 and Cu(OAc)2(trien) were obtained as a solid and a liquid, respectively. Cu(OAc)2(en)2 was characterized by IR spectroscopy, UV-visible spectroscopy and elemental analysis. Cu(OAc)2(trien) was characterized by IR spectroscopy and UV-visible spectroscopy. The spectroscopic data of Cu(OAc)2(trien) agreed with those reported in the literature (15).
Therefore, the possible characterization methods for Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG are UV-visible spectroscopy and MALDI-TOF mass spectrometry.
To confirm the complex formation in ethylene glycol, the UV-visible spectra of Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG were compared with Cu(OAc)2(en)2 and Cu(OAc)2(trien) synthesized in acetone, and purified copper-amine complexes were obtained. It was found that Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG gave the same λmax as purified Cu(OAc)2(en)2 and Cu(OAc)2(trien) at 230 and 258 nm, respectively. These results indicated that the complexes could be formed in ethylene glycol.
MALDI-TOF mass spectrometry was also used to characterize Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG. The peaks of the mass spectra corresponded to the molecular weight of the copper-amine complexes. A molecular ion of [Cu(OAc)2(en)2+H]+ appeared at m/z 303.05 (Figure 1). Cu(OAc)2(en)2 lost an ethylenediamine (en) unit, and the peak of [Cu(OAc)2(en)+H]+ appeared at 242.93. The molecular ion of [Cu(OAc)2(trien)]+ could not be observed. [Cu(OAc)2(trien)]+ lost an acetate group (OAc), and the peak of [Cu(OAc)(trien)]+ appeared at m/z 268.16 (Figure 2). Therefore, the UV and MS data indicate that the Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG solutions in ethylene glycol contain the Cu(OAc)2(en)2 and Cu(OAc)2(trien).
MALDI-TOF mass spectrum of Cu(OAc)2(en)2-EG.
MALDI-TOF mass spectrum of Cu(OAc)2(trien)-EG.
3.3 Preparation of flexible polyurethane foam
Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG were used as catalysts in the preparation of flexible polyurethane foam. The reaction time of the FPUR foaming reaction, namely, cream time, gel time, rise time and tack-free time, catalyzed by the copper-amine complexes was investigated by the cup test method and compared with that of Dabco EG (Table 2). The catalytic activity of the gelling and blowing reactions in the preparation of RPUR foams was determined by tack-free time and rise time, respectively. Gelling and blowing reactions relate to isocyanate-polyol and isocyanate-water reactions, respectively. The results show that Cu(OAc)2(en)2-EG has a comparable tack-free time to that of Dabco EG. This indicates that Cu(OAc)2(en)2-EG and Dabco EG have a comparable catalytic activity in gelling reaction. Cu(OAc)2(trien)-EG has a much lower catalytic activity than Dabco EG. This might be due to the steric hindrance of the trien unit in Cu(OAc)2(trien).
Reaction times, density and foam height of FPUR foams catalyzed by Dabco EG/metal-amine complexes obtained from the cup testa.
| Catalysts | Cream time (min:s) | Gel time (min:s) | Rise time (min:s) | Tack-free time (min:s) | Free rise density (kg/m3) | Foam height (cm) |
|---|---|---|---|---|---|---|
| Dabco® EG | 0:18±0.01 | 0:34±0.01 | 1:04±0.02 | 1.33±0.01 | 266±12 | 11.0 |
| Cu(OAc)2(en)2 | 0:17±0.01 | 0:30±0.01 | 0:45±0.03 | 1:13±0.03 | 179±8 | 13.0 |
| Cu(OAc)2(trien) | 1:02±0.03 | 2:02±0.03 | 3:27±0.08 | 11:30±0.09 | 227±3 | 11.6 |
| Dabco® EG/Cu(OAc)2(en)2 | 0:10±0.01 | 0:19±0.01 | 0:33±0.03 | 0:41±0.02 | 199±7 | 12.0 |
| Dabco® EG/Cu(OAc)2(trien) | 0:15±0.01 | 0:24±0.01 | 0:39±0.04 | 0:45±0.02 | 215±12 | 11.5 |
aThe data reported are average values with standard deviation of <5% from the average values.
The density of foam varied inversely with foam height. FPUR foams prepared from Cu(OAc)2(en)2-EG had a much lower density than those prepared from Dabco EG. This suggested that Cu(OAc)2(en)2-EG had a higher catalytic activity in blowing reaction than Dabco EG. Figure 3 shows the rise profiles of the FPUR foams prepared from different catalysts. The data agree with the reaction times shown in Table 2. The polymerization reactions using Cu(OAc)2(en)2-EG had a long reaction time in the initial stage and a fast rise curve in the latter stage. In comparison to Cu(OAc)2(en)2-EG, Dabco EG had a shorter reaction time in the initial stage and a slower rise curve in the latter stage.
Rise profiles of FPUR foams catalyzed by (A) Dabco EG, (B) Cu(OAc)2(en)2, (C) Cu(OAc)2(trien), (D) Cu(OAc)2(en)2-EG/Dabco EG and (E) Cu(OAc)2(trien)-EG/Dabco EG.
The isocyanate conversion (NCO conversion) of FPUR foams catalyzed by Dabco EG, Cu(OAc)2(en)2-EG, Cu(OAc)2(trien)-EG, Cu(OAc)2(en)2-EG/Dabco EG and Cu(OAc)2(trien)-EG/Dabco EG was investigated by ATR-IR spectroscopy. NCO conversion was determined from the absorption band of the isocyanate group at 2277 cm-1 as shown in the following equation (16):
where NCOt is the area of the isocyanate peak at time t, which is the time after the foam was kept at room temperature for 48 h to complete the polymerization reaction. NCOi is the area of the isocyanate peak at the initial time. The isocyanate peak area was normalized by the aromatic ring peak area at 1595 cm-1. It was found that all copper-amine complexes and Dabco®EG/copper-amine complexes gave quantitative NCO conversion.
3.4 Proposed polymerization mechanism catalyzed by Cu(OAc)2(en)2-EG
Metal-amine complexes have been previously used as curing agents in the preparation of metal-containing epoxy polymers (15, 17–19). These metal-amine complexes give a high rate of curing at a comparatively low temperature. The obtained metal-containing epoxy polymers have improved mechanical properties and thermal oxidative stability. The polymerization mechanism is proposed in two conditions, namely, the reactions below and above the metal complex dissociation temperature. When epoxy oligomer is cured at low temperature, metal complexes do not dissociate and act as initiators for the ionic polymerization of epoxy oligomer. Curing of epoxy oligomer at higher temperature causes the metal complexes to dissociate to give metal complex cations and anions. These reactive cations and anions are able to undergo polymerization reaction with epoxy oligomer to give metal-containing epoxy polymers. The temperature of the metal complex dissociation corresponds to the temperature at the beginning of the curing reaction of the epoxy oligomer with metal complexes. Metal-amine complexes having a higher dissociation temperature will give a slower rate of curing.
Therefore, the reaction mechanism catalyzed by Cu(OAc)2(en)2-EG is proposed to proceed from two mechanisms, namely, the mechanisms at low and high reaction temperature. The foaming reaction was done at room temperature. At the start of the foaming reaction, the metal complex did not dissociate and Cu(OAc)2(en)2-EG catalyzed the reaction according to the proposed mechanism reported in our previous work as follows (14): Copper atom in Cu(OAc)2(en)2 acts as a Lewis acid and coordinates with the oxygen atom of the NCO group, which causes the NCO carbon to be more electrophilic. The nitrogen atom in Cu(OAc)2(en)2 interacts with the proton of the hydroxyl group and causes the hydroxyl oxygen to be more nucleophilic, which then reacts with the isocyanate group to give a urethane linkage. As the foaming reaction proceeds, the reaction temperature becomes higher due to the exothermic polymerization reaction. The heat causes Cu(OAc)2(en)2 to dissociate into Cu(OAc)2(en) and ethylenediamine (Scheme 2). The structure of Cu(OAc)2(en) is proposed based on the MALDI-TOF mass spectrum of Cu(OAc)2(en)2-EG where the peak of [Cu(OAc)2(en)+H]+ could be observed. Therefore, it is proposed that Cu(OAc)2(en) can also catalyze the foaming reaction by a similar mechanism to that of Cu(OAc)2(en)2 (Scheme 2). The dissociated en unit could then undergo a reaction with the isocyanate group in MDIP to give the urea group. This should not have an impact on the foaming reaction since the amount of en group is small and thus is negligible. The catalytic mechanism of Cu(OAc)2(trien)-EG is similar to that of Cu(OAc)2(en)2-EG. Cu(OAc)2(trien)-EG showed less catalytic activity than Cu(OAc)2(en)2-EG due to the steric effect of the trien group, which was larger than that of the en group.
Dissociation of Cu(OAc)2(en)2 to give Cu(OAc)2(en) and proposed polymerization mechanism catalyzed by Cu(OAc)2(en).
3.5 Synergistic effect of Dabco EG with Cu(OAc)2(en)2 and Cu(OAc)2(trien)
Tertiary amines are known to act in a synergistic manner with tin catalysts (2). Therefore, the synergistic effect of Dabco EG with Cu(OAc)2(en)2 and Cu(OAc)2(trien) was investigated. Mixtures of Cu(OAc)2(en)2-EG/Dabco EG and Cu(OAc)2(trien)-EG/Dabco EG were used as catalysts in the FPUR foam formulation. The weight ratio of copper-amine complex/Dabco EG was 1:1, and the total amount of copper-amine complex/Dabco EG in the FPUR foam formulation was 1.5 parts by weight. A synergistic effect of copper-amine complex and Dabco EG was observed. The catalytic activity of copper-amine complex/Dabco EG was higher than those of copper-amine complex and Dabco EG. The reaction times of copper-amine complex/Dabco® EG (Table 2) agree with the data of the rise profile (Figure 3). The catalytic mechanism is proposed to show that the copper atom in copper-amine complex acts as a Lewis acid and coordinates with the oxygen atom of the NCO group; therefore, the NCO carbon is more electrophilic. In contrast, the nitrogen atom of Dabco EG is a tertiary amine, which is active towards removing the proton of the hydroxyl group and causes the hydroxyl oxygen to become more nucleophilic. The hydroxyl oxygen then reacts with the NCO group to produce a urethane linkage.
3.6 Characterization of flexible polyurethane foams
The mechanical properties of FPUR foams were investigated using molded foams (Table 3). FPUR foams catalyzed by all catalysts had a similar compression set. For tensile strength and elongation at break, the foams catalyzed by Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG had a lower tensile strength and elongation at break than those catalyzed by Dabco EG. The use of Cu(OAc)2(en)2-EG/Dabco® EG and Cu(OAc)2(trien)-EG/Dabco® EG as catalysts improved both the tensile strength and the elongation at break. This is because the mold density of FPUR foams obtained from Cu(OAc)2(en)2-EG/Dabco EG and Cu(OAc)2(trien)-EG/Dabco EG was higher than those obtained from Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG, respectively. The foam catalyzed by Cu(OAc)2(en)2-EG/Dabco EG had a comparable tensile strength and elongation at break to that obtained from Dabco EG.
Mechanical properties of molded FPUR foams catalyzed by various catalysts.
| Catalysts | Molded densitya (kg/m3) | Compression set at 25% (MPa) | Tensile strength (MPa) | Elongation at break (%) |
|---|---|---|---|---|
| Dabco EG | 484 | 26.45±0.48 | 2.158±0.104 | 252±2 |
| Cu(OAc)2(en)2 | 454 | 26.53±0.52 | 0.982±0.021 | 137±5 |
| Cu(OAc)2(trien) | 465 | 27.45±0.96 | 1.602±0.074 | 167±8 |
| Dabco EG/Cu(OAc)2(en)2 | 497 | 26.82±1.23 | 2.539±0.066 | 255±4 |
| Dabco EG/Cu(OAc)2(trien) | 520 | 26.71±0.58 | 2.067±0.053 | 197±4 |
aThe data reported are average values with standard deviation of <5% from the average values.
The morphology of molded FPUR foams catalyzed by Cu(OAc)2(en)2 and Cu(OAc)2(en)2-EG/Dabco® EG was investigated by using a scanning electron microscope (Figure 4). The FPUR foam catalyzed by Cu(OAc)2(en)2-EG/Dabco EG showed better morphology than that prepared from Cu(OAc)2(en)2-EG. This might be because Cu(OAc)2(en)2-EG is a better catalyst for the blowing reaction than Dabco EG, which means that the CO2 generation rate of Cu(OAc)2(en)2-EG is faster than that of Dabco EG. When Cu(OAc)2(en)2-EG/Dabco EG was used as a catalyst, the rise time was faster than that of Cu(OAc)2(en)2-EG and Dabco EG (Table 2); therefore, a higher amount of CO2 can get into the bubble cells before the viscosity reaches the critical value. As a result, the cells in Figure 4B are larger than those in Figure 4A. For external appearance, the colors of FPUR foams prepared from Dabco EG and Cu(OAc)2(en)2-EG are white and pale blue, respectively. The blue color is due to the color of Cu(OAc)2(en)2.
Scanning electron microscopy images of FPUR foams (prepared in mold slap) catalyzed by (A) Cu(OAc)2(en)2-EG and (B) Dabco EG/Cu(OAc)2(en)2-EG.
4 Conclusions
Copper-amine complex solutions in ethylene glycol, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG, were synthesized using a simple procedure and could be used as catalysts for FPUR foam preparation. The copper-amine complex solutions were characterized by UV-visible spectroscopy and MALDI-TOF mass spectrometry. The catalytic activity of gelling and blowing reactions in the preparation of RPUR foams was determined by tack-free time and rise time, respectively. Cu(OAc)2(en)2-EG was a good catalyst for FPUR foam preparation, while Cu(OAc)2(trien)-EG was a poor catalyst. A synergistic effect was observed when a mixture of Cu(OAc)2(en)2-EG/Dabco EG and Cu(OAc)2(trien)-EG/Dabco EG at a weight ratio of 1:1 was used as a catalyst.
Acknowledgments
The authors would like to thank Huntsman (Thailand) Ltd. for supplying the chemicals used in this research and IRPC Public Company Limited for financial support (AL.0963/2554).
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