Startseite Naturwissenschaften Preparation of rigid polyurethane foams using low-emission catalysts derived from metal acetates and ethanolamine
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Preparation of rigid polyurethane foams using low-emission catalysts derived from metal acetates and ethanolamine

  • Duangruthai Sridaeng , Wannisa Jitaree , Preecha Thiampanya und Nuanphun Chantarasiri EMAIL logo
Veröffentlicht/Copyright: 12. Mai 2016
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e-Polymers
Aus der Zeitschrift e-Polymers Band 16 Heft 4

Abstract

Two metal acetate-ethanolamine complexes, namely Cu(OAc)2(EA) and Zn(OAc)2(EA), were synthesized from metal acetates [M(OAc)2, where M=Cu and Zn] and ethanolamine (EA). These metal acetate-ethanolamine complexes can be used as catalysts in the preparation of rigid polyurethane (RPUR) foams. Both Cu(OAc)2(EA) and Zn(OAc)2(EA) were obtained as viscous liquids, which have very weak odor and could be easily dissolved in the starting materials of RPUR foam formulation. The results were compared with RPUR foam prepared from dimethylcyclohexylamine (DMCHA), which is a commercial catalyst with very strong amine odor. Considering the gel time and rise time, Cu(OAc)2(EA) had higher catalytic activity than Zn(OAc)2(EA) and both metal acetate-ethanolamine complexes had lower catalytic activity than DMCHA. Density and compressive strength of RPUR foam catalyzed by Cu(OAc)2(EA) were comparable to that prepared from DMCHA.

Keywords: copper-ethanolamine complex; low-emission catalyst; metal acetate; rigid polyurethane foam; zinc-ethanolamine complex

1 Introduction

Polyurethanes are the polymers that can be prepared as thermoplastic, elastomeric and thermoset materials which depend on the structure of their starting materials and formulations (1–3). Polyurethane foams can be manufactured in wide range of grades from flexible to rigid foams. Rigid polyurethane (RPUR) foams are prepared from the reaction between polyisocyanates with polyols in the presence of blowing agents, catalysts, surfactants and other additives. There are two main reactions in the foaming process, gelling and blowing reactions. Gelling reaction is the reaction between isocyanates and polyols to give urethane linkages, which increases the molecular weight of the polymer. The catalysts that effectively accelerate gelling reaction are gelling catalysts. Blowing reaction is the reaction between isocyanates and water to give amines and carbon dioxide. Amines react with isocyanates to form urea linkages. Carbon dioxide causes the expansion of the material to give a cellular structure. The catalysts that effectively accelerate blowing reaction are called blowing catalysts.

Two main types of catalysts employed in the manufacture of RPUR foams are amines and organotin compounds (4–6). Examples are N,N-dimethylcyclohexylamine (DMCHA), which shows moderate activity between the gelling and blowing reactions while dibutyltin dilaurate and pentamethyldiethylenetriamine are strong gelling and blowing catalysts, respectively. Other common tertiary amines which effectively catalyze gelling and blowing reactions are 1,4-diazabicyclo-[2,2,2]-octane (DABCO) and 2,2′-bis-(dimethylaminoethylether) (BDMAEE), respectively.

As the amine catalysts cause odor problems during the manufacturing process and organotin is toxic, investigation of new catalyst systems for RPUR foams is an area of interest. The catalysts having weak odor and less volatile organic compound (VOC) emission, namely N-methylmorpholine, 2-morpholinoethanol, N,N-Bis-(3-dimethylaminopropyl)-N-isopropanolamine, N,N-Bis-(3-dimethylaminopropyl)amine and N,N,N′-trimethyl- N′-(2-hydroxyethyl)bis(2-aminoethyl) ether were used as single catalysts and as catalyst mixtures for polyurethane foam preparation (7, 8). Different metal compounds and metal complexes were investigated as catalysts for different polyurethane systems. Metal acetylacetonates (where metal=Zr, Co, Ni, Ti and Zn) and bismuth carboxylate were used as catalysts for waterborne polyurethanes (9–12). The catalyst mixtures of metal acetylacetonates-triethylenediamine (where metal=Mn, Fe, Co, Ni and Cu) (13) and Co(f6acac)2-Co(acac)2 (14) were used in polymerization of isophorone diisocyanate-diethylene glycol and hexamethylene diisocyanate-diethylene glycol systems, respectively. The neodymium chloride Schiff base complex-triethylenediamine system was used for preparation of semi-rigid polyurethane foam (15). Guanidines, amidines and N-heterocyclic carbenes were reported to have comparable catalytic activity to dibutyltin dilaurate and dibutyltin diacetate for polyurethane formation (16).

Our research group synthesized new catalysts with good catalytic activity for the preparation of water blown RPUR foams. These catalysts were copper acetate-amine (17) and copper acetylacetonate-amine complexes (18), namely Cu(OAc)2(en)2, Cu(OAc)2(trien) and Cu(acac)2(trien) (where OAc=acetate, acac=acetylacetonate, en=ethylenediamine and trien=triethylenetetramine). Cu(OAc)2(en)2, Cu(OAc)2(trien) and Cu(acac)2(trien) have weak odor and can be prepared from readily available starting materials. All copper-amine complexes have good solubility in water blown RPUR foam formulation. Although these metal-amine complexes have good catalytic activity, the aliphatic amines, namely ethylenediamine and triethylenetetramine, used in their syntheses have odor.

Therefore, we were interested in the synthesis of another type of metal complex by using another ligand that emits less odor than the aliphatic amines. Ethanol-amine (EA) attracted our attentions since it has high boiling point (170°C), emits weak odor and readily available. Ethanolamine contains one amino and one alcohol group. Ethanolamine has many applications and metal-ethanolamine complexes are known, which offers possibility for their applications as catalysts for RPUR foam preparation in our work.

There are reports about the applications of ethanolamine. It can be used as a plasticizer for thermoplastic starch (19) and for aminolytic degradation of post-consumer poly(ethylene terephthalate) (PET) waste in the presence of dibutyltin oxide as a catalyst (20) to give bis(2-hydroxyethylene) terephthalamide (BHETA), which has potential applications in adhesives and coatings. The copper chloride-ethanolamine redox system is used in the addition reactions of a variety of l-octene and polyhaloalkanes to give good yields of addition products (21).

Metal-ethanolamine complexes have been studied by many research groups. Jensen (22) reported the preparation of copper nitrate-ethanolamine and copper perchlorate-ethanolamine complexes, which were characterized by elemental analysis and UV-Vis spectroscopy. Co(II), Ni(II) and Cu(II) complexes of ethanolamine functions supported on 2 mol% DVB-crosslinked polystyrene were prepared by Mathew and Jacob (23). Thermal stability of the polymeric ligand increased on complexation with metal ions and followed the order: Co(II)>Cu(II)>Ni(II)> uncomplexed resin. This resulted from the formation of stable ring structures on complexation. The polymer-anchored Cu(II) complex suggesting a square planar geometry. The proposed structures of the Cu or Ni complexes correspond to the metal-ligand ratio 1:2. When there is only one ligand, the coordination sphere is saturated by anion or solvent molecules. Lin et al. (24) used luminol chemiluminescence in unbuffered solutions with a cobalt(II)-ethanolamine complex immobilized on Dowex-50W resin as catalyst in H2O2 and glucose flow-through sensor systems. Other transition metal ions and alkanolamines were also investigated. The chemiluminescence intensity decreased in the order monoethanolamine>diethanolamine>triethanolamine and Co(II)>Cu(II)>Ni(II)>Fe(III)>Mn(II)>Fe(II). Zabierowski et al. (25) synthesized a series of copper(II) complexes with tridentate salicylidene-2-ethanolamine type ligands and characterized these copper(II) complexes by single crystal X-ray diffraction, elemental analysis, IR and UV-Vis spectroscopy as well as cyclic voltammetry and magnetic susceptibility measurements.

As metal-ethanolamine complexes are known, the purpose of our work is to prepare Cu(OAc)2(EA) and Zn(OAc)2(EA) complexes. Cu(OAc)2(EA) and Zn(OAc)2(EA) can be prepared from metal acetate and ethanolamine. Ethanolamine has a high boiling point and emits less odor than the aliphatic amines. Cu(OAc)2(EA) and Zn(OAc)2(EA) are stable in air, emit very weak odor and have good solubility with other starting materials used in RPUR foam formulation. Therefore, Cu(OAc)2(EA) and Zn(OAc)2(EA) are potential catalysts in the preparation of RPUF foams. The obtained results in RPUR foam preparation catalyzed by Cu(OAc)2(EA) and Zn(OAc)2(EA) were compared with those obtained from DMCHA.

2 Experimental

2.1 Materials

Copper (II) acetate monohydrate [Cu(OAc)2.H2O], zinc acetate dihydrate [Zn(OAc)2.2H2O] and ethanolamine (EA) used for the preparation of metal acetate-ethanolamine complexes were obtained from Sigma-Aldrich (St. Louis, MO, USA) and Merck (Darmstadt, Germany). For RPUR foam preparation, distilled water was used as a blowing agent. Polyol (Daltolac®180, sucrose-based polyether polyol, hydroxyl value=440 mgKOH/g, functionality=4.3), polysiloxane surfactant (Tegostab® B8460) and polymeric 4,4′-methane diphenyl diisocyanate (PMDI, Suprasec® 5005, % NCO=31.0 wt %, average functionality=2.7) were supplied by Huntsman (Samutprakarn, Thailand) Co. Ltd. N,N-dimethylcyclohexylamine (DMCHA), supplied by Huntsman (Thailand) Co. Ltd., was used as a commercial reference catalyst.

2.2 Experimental design

RPUR foams at the fixed isocyanate (NCO) index of 100 were prepared using two methods, namely the cup test and molded methods. The catalysts employed in the preparation of RPUR foams were Cu(OAc)2(EA) and Zn(OAc)2(EA) complexes. Reaction times in the preparation of RPUR foams and RPUR foam properties were investigated. The amount of metal acetate-ethanolamine complexes was varied to investigate the effect on the reaction times. The results were compared to those obtained from the commercial reference catalyst (DMCHA).

2.3 Characterization of metal acetate-ethanolamine complexes

Ultraviolet-visible (UV-vis) spectra were recorded on a UV-vis spectrophotometer (Varian Cary 50, Palo Alto, CA, USA) over the range 200–500 nm at a medium speed scan. Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrometer (PerkinElmer RX I, Waltham, MA, USA) over the range 500–4000 cm-1 at a resolution of 4 cm-1. Nuclear magnetic resonance (NMR) spectrum were recorded on a Varian Mercury Plus 400 MHz NMR spectrometer. All chemical shifts were reported in part per million (ppm) using the residual proton signal in deuterated DMSO as an internal reference. Matrix-associated laser desorption ionization time of flight (MALDI-TOF) mass spectra were obtained on a Bruker Daltonics mass spectrometer (Billerica, MA, USA) and using 2-cyano-4-hydroxy cinnamic acid (CCA) as a matrix.

2.4 Characterization of RPUR foams

The time of foaming reactions prepared using the cup test method was recorded using a digital stopwatch. The reaction times studied were cream time (the time when the foam height was 5% height as compared to the total height), gel time (the time when the foam mixture changed from viscous liquid to gel), rise time (the time when the foam stopped rising) and tack-free time (the time when the dryness of the foam surface could be observed which indicated that the polymerization reaction was completed). The height of foams prepared by the cup test method at different reaction times was measured to obtain rise profiles. Isocyanate (NCO) conversion was calculated from attenuated total reflection infrared (ATR–IR) spectra recorded on a Nicolet 6700 FTIR spectrometer over the range 500–4000 cm-1 at a resolution of 4 cm-1. For density measurement and the compression test, the foam samples were cut into a cubic shape with dimensions of 3.0 cm×3.0 cm×3.0 cm. Density of foam samples was measured according to ASTM D 1622-09 and the average values of three samples are reported. The cross section of the specimens for density measurements was 9.0 cm2, which was less than that recommended by ASTM (25.8 cm2). Compression testing of the foams in parallel and perpendicular to the foam-rising direction was performed with a Lloyd LRX universal testing machine according to ASTM D 1621-09. The preload cell used was 0.100 N and the rate of crosshead movement was fixed at 2.54 mm/min. Compressive strength was determined by plotting applied force (compressive strength) against deformation of the foam sample (compressive strain) and the data was recorded at 10% deformation. Morphology of foams in parallel and perpendicular to the foam-rising direction was obtained on a Hitachi S-4800 scanning electron microscope at the accelerating voltage of 20 kV. The samples were gold-coated before scanning.

2.5 Synthesis of copper acetate-ethanolamine complex [Cu(OAc)2(EA)]

A solution of ethanolamine (0.138 g, 2.25 mmol) was stirred in acetone (20 ml) at room temperature for 20 min. Cu(OAc)2.H2O (0.450 g, 2.25 mmol) was then added. After the reaction mixture was stirred at room temperature overnight, the reaction mixture was evaporated and dried in vacuo to remove acetone. Cu(OAc)2(EA) was obtained as a dark blue viscous liquid (0.565 g, quantitative yield) and was used without further purification: UV-vis (MeOH): λmax (ε)=235 nm (4630); IR (neat): ν=3213 (m; ν(N-H)), 2924 (w; ν(C-H)), 1555 (s; νas(C=O)), 1397 (s; νs(C=O)), 1330 (w; ν(C-N)), 1041 (s; ν(C-O)) cm-1; MS (MALDI-TOF, m/z): calcd for CuC6H13O5N=[Cu(OAc)2(EA)]+, 242.72; found 261.27 [Cu(OAc)2(EA)+H2O+H]+.

2.6 Synthesis of zinc acetate-ethanolamine complex [Zn(OAc)2(EA)]

Zn(OAc)2(EA) was prepared using a similar procedure as described for Cu(OAc)2(EA). Ethanolamine (0.136 g, 2.23 mmol), Zn(OAc)2.2H2O (0.490 g, 2.23 mmol) and acetone (20 ml) were used in the synthesis. Zn(OAc)2(EA) was obtained as a dark yellow viscous liquid (0.610 g, quantitative yield) and was used without further purification: IR (neat): ν=3240 (m; ν(N-H)), 2934 (w; ν(C-H)), 1560 (s; νas(C=O)), 1397 (s; νs(C=O)), 1333 (w; ν(C-N)), 1015 (s; ν(C-O)) cm-1; 1H-NMR (400 MHz), DMSO-d6; δ (in ppm): 4.15 (s, 2H, NH and OH), 3.42 (t, 2H, J=5.6 Hz, OCH2), 2.64 (t, 2H, J=5.6 Hz, NCH2), 1.76 (s, 6H, CH3); MS (MALDI-TOF, m/z): calcd for ZnC6H13O5N=[Zn(OAc)2(EA)]+, 244.58; found 285.18 [Zn(OAc)2(EA)+H2O+Na]+.

2.7 Preparation of RPUR foams using cup test method

The isocyanate (NCO) index of 100 was used to prepare all RPUR foams. Table 1 shows the foam formulation in parts by weight (pbw) unit. Parts by weight are the weight of components (in grams) per 100 g of polyol. For an example of DMCHA catalyst, the weights of polyol, DMCHA, surfactant, blowing agent (water) and PMDI used are 100, 1.0, 2.5, 3.0 and 151 g, respectively.

Table 1:

RPUR foam formulation at NCO index of 100 (in parts by weight unit, pbw).

ChemicalsAmount (pbw) used for different catalysts
DMCHAMetal-ethanolamine complexes
Polyether polyol (Daltolac®180)100.0100.0
Catalysts (DMCHA or metal-ethanolamine complexes)1.00.5, 1.0, 2.0
Surfactant (Tegotab® B8460)2.52.5
Blowing agent (water)a3.04.0
PMDI (Suprasec® 5005)151.0167.0

aThe most suitable amounts of blowing agent (water) for RPUR foams catalyzed by DMCHA (reference catalyst) and metal-ethanolamine complexes are 3 and 4 pbw, respectively.

The NCO index is a measure of excess isocyanate used relative to the theoretical equivalent amount required to react with polyol and water (blowing agent). The isocyanate index can be calculated from the following equation.

NCO index=(Actual amount of isocyanate used/Theoretical amount of isocyanate)×100

The cup test method used in this work was modified from ASTM D7487-13. RPUR foam was prepared in a 700 ml paper cup using a two-step method. In the first step, catalysts (metal acetate-ethanolamine complexes or DMCHA), surfactant, and blowing agent (water) were added to polyol and the components were mixed to obtain the mixture called the “polyol blend”. In the second step, PMDI was added to the “polyol blend” and the components were then mixed by a mechanical stirrer at the speed of 2000 rpm for 20 s. The foam was allowed to rise freely. Cream time, gel time, tack-free time and rise time were measured during the foaming reaction. After the foaming reaction was completed, the foam was kept at room temperature for 48 h before measurements of density and NCO conversion.

RPUR foams obtained from DMCHA and metal-ethanolamine complex catalysts have similar IR absorptions as follows: IR (ATR-IR): ν=3320 (m; ν(N-H)), 2907 (m; ν(C-H)), 2874 (m; ν(C-H)), 2272 (w; ν(free NCO)), 1711 (s; ν(C=O)), 1595 (m; ν(Ar-H)), 1075 (s; ν(C-O urethane)) cm-1.

2.8 Preparation of RPUR foams in the mold

RPUR foam samples were prepared in 10.0 cm×10.0 cm×10.0 cm plastic mold by mixing all starting materials in a paper cup and poured into a mold. The foam was allowed to rise freely and after the foaming reaction was completed, it was kept at room temperature for 48 h before investigations of properties, namely density, compressive strength and morphology. It was found that RPUR foams prepared in a mold have the same density as those prepared using the cup test method.

3 Results and discussion

3.1 Synthesis and characterization of metal acetate-ethanolamine complexes

The reaction of ethanolamine with copper and zinc acetate gave Cu(OAc)2(EA) and Zn(OAc)2(EA), respectively, Scheme 1. The molar ratio of metal acetates to ethanolamine was fixed at 1:1. Cu(OAc)2(EA) and Zn(OAc)2(EA) were obtained as viscous liquids that could not be further purified, for example, by recrystallization or chromatography. Therefore, possible characterization methods for metal-ethanolamine complexes are UV-visible spectroscopy, IR spectroscopy and MALDI-TOF mass spectrometry.

Scheme 1: Synthesis of metal-ethanolamine complexes [M(OAc)2(EA)].
Scheme 1:

Synthesis of metal-ethanolamine complexes [M(OAc)2(EA)].

UV-visible spectrum of Cu(OAc)2(EA) shows the maximum absorption at 235 nm (Figure 1), which shifts from the maximum absorption of Cu(OAc)2 at 247 nm. UV-visible spectra of Zn(OAc)2(EA) could not be observed as the electron configuration of zinc atom is 4s23d10. IR spectrum of Cu(OAc)2(EA) (Figure 2) exhibits characteristic asymmetric and symmetric C=O stretching of acetate group at 1555 and 1397 cm-1, respectively. These C=O peaks are different from asymmetric and symmetric C=O stretching of Cu(OAc)2, which appear at 1594 and 1420 cm-1, respectively. IR spectrum of Zn(OAc)2(EA) is similar to that of Cu(OAc)2(EA). Asymmetric and symmetric C=O stretching of acetate group in Zn(OAc)2(EA) appear at 1560 and 1397 cm-1, respectively. These C=O stretching bands are different from those of Zn(OAc)2, which appear at 1531 cm-1 (asymmetric C=O stretching) and 1440 cm-1 (symmetric C=O stretching). 1H NMR spectrum of Zn(OAc)2(EA) (Figure 3) shows triplet signals of -OCH2- and -NCH2- groups at δ 3.42 and 2.64, respectively. These signals are different from those of -OCH2- and -NCH2- in ethanolamine, which appear at δ 3.32 (t) and 2.51 (t), respectively. Both -NH and -OH signals appear as a broad singlet peak at δ=4.15. A singlet peak at δ 1.76 is assigned as two -CH3 groups. 1H NMR spectrum Cu(OAc)2(EA) could not be obtained since Cu2+ is paramagnetic. From MALDI-TOF mass spectra of Cu(OAc)2(EA) and Zn(OAc)2(EA) (Figures 4 and 5, respectively), molecular ions of [Cu(OAc)2(EA)+H2O+H]+ and [Zn(OAc)2(EA)+H2O+Na]+ appear at m/z 261.27 and 285.18, respectively. This indicates that the molecular formula of metal-ethanolamine complexes are Cu(OAc)2(EA) and Zn(OAc)2(EA). Both Cu(OAc)2(EA) and Zn(OAc)2(EA) have four coordinations with two acetate ions and one molecule of ethanolamine, which binds to the metal atom as a bidentate ligand. The m/z of Cu(OAc)2, Zn(OAc)2, Cu(OAc)2(EA)2 and Zn(OAc)2(EA)2 could not be observed. This indicates that Cu(OAc)2 and Zn(OAc)2 undergo a complete reaction with ethanolamine.

Figure 1: UV-Vis spectra of Cu(OAc)2 and Cu(OAc)2(EA).
Figure 1:

UV-Vis spectra of Cu(OAc)2 and Cu(OAc)2(EA).

Figure 2: IR spectra of metal acetates and metal acetate-ethanolamine complexes.
Figure 2:

IR spectra of metal acetates and metal acetate-ethanolamine complexes.

Figure 3: 1H NMR spectrum of Zn(OAc)2(EA).
Figure 3:

1H NMR spectrum of Zn(OAc)2(EA).

Figure 4: Mass spectrum of Cu(OAc)2(EA).
Figure 4:

Mass spectrum of Cu(OAc)2(EA).

Figure 5: Mass spectrum of Zn(OAc)2(EA).
Figure 5:

Mass spectrum of Zn(OAc)2(EA).

The structure of Cu(OAc)2(EA) and Zn(OAc)2(EA) was proposed based on the work reported in the literature as follows: Farraj et al. (26) and Shin et al. (27) prepared various copper formate-alkanolamine complexes by mixing the copper formate with various alkanolamine at a molar ratio of 1:2, respectively. The alkanolamines used as the complexing ligands were diisopropanolamine, ethanolamine, diethanolamine, 1-amino-2-propanol and 2-amino-2-methyl-1-propanol. The color of these copper formate-alkanolamine complexes was deep-blue to purple. Nieuwpoort and Groeneveld studied complexes of nickel(II) and copper(II) halides, nitrates and thiocyanate with 1-amino-2-propanol (28). It was found that 1-amino-2-propanol is randomly connected to the metal either by a bidentate bond or as a monodentate ligand through the nitrogen group. Muhonen and Hämäläinen reported the crystal and molecular structure of bis(2-amino-2-methyl-1-propanol)copper(II) dibenzoate [Cu(C4H11NO)2](C7H5O2)2 (29). The structure consists of [Cu(C4H11NO)2]2+ cations and C7H5O2- anions. Cu(II) has a square planar coordination with 2-amino-2-methyl-1-propanol and benzoate connect by means of hydrogen bonds.

From the literature described above, ethanolamine can bind with metals as either monodentate or bidentate ligand. For metal acetate-ethanolamine complexes, the integration ratio in 1H NMR and molecular ions in MALDI-TOF mass spectra indicate that their structures are Cu(OAc)2(EA) and Zn(OAc)2(EA). This suggests that both Cu2+ and Zn2+ bind with two acetate ions and one molecule of ethanolamine. Therefore, ethanolamine should connect to the metal atom as a bidentate ligand.

3.2 Preparation of RPUR foams catalyzed by metal acetate-ethanolamine complexes

The reaction time of RPUR foam formation catalyzed by metal acetate-ethanolamine complexes is shown in Table 2. These RPUR foams were prepared using the cup test method. The data from the cup test method was then used to prepare RPUR foams in a mold. It was found that RPUR foams prepared from both methods had the same density. Considering the reaction times, metal acetate-ethanolamine complexes had longer gel time and rise time than DMCHA. Therefore, DMCHA had better catalytic activity in both gelling and blowing reactions than metal acetate-ethanolamine complexes. RPUR foams catalyzed by metal acetate-ethanolamine complexes had comparable tack free time to that catalyzed by DMCHA. RPUR foams catalyzed by Cu(OAc)2(EA) gave similar foam height and foam density to that catalyzed by DMCHA. Therefore, these metal-ethanolamine complexes are appropriate for RPUR foam preparation in a large mold, which requires longer time for the starting materials to fill up the mold.

Table 2:

Reaction times and physical properties of RPUR foams prepared at NCO index of 100 and catalyzed by metal-ethanolamine complexes (catalyst amount=1 pbw).a

CatalystsCream time (s)Gel time (s)Tack free time (s)Rise time (s)Density(kg/m3)Foam height (%)b
DMCHA (reference catalyst)223218714139.7085
Cu(OAc)2(EA)418216818042.4385
Zn(OAc)2(EA)4611821023334.4197
Cu(OAc)2(EA):Zn(OAc)2(EA)369517018033.52100

aThe data reported are average values with standard deviation <5% from the average values.

bCu(OAc)2(EA):Zn(OAc)2(EA) gave the highest foam height and was designated as 100%.

When considering both tack free time and rise time, both Cu(OAc)2(EA) and Zn(OAc)2(EA) are good catalysts as they catalyze both gelling and blowing reactions. RPUR foams catalyzed by Cu(OAc)2(EA) and Zn(OAc)2(EA) have stable foam structure that do not collapse. In the reference system, when copper acetate, zinc acetate and ethanolamine are used as catalysts, the reactivity in polymerization is poor and RPUR foams have collapsed cell structure.

Cu(OAc)2(EA) gives shorter tack free time than Zn(OAc)2(EA), which indicates that Cu(OAc)2(EA) has better catalytic activity in gelling reaction than Zn(OAc)2(EA). Although Zn(OAc)2(EA) gives longer rise time than Cu(OAc)2(EA), the RPUR foam catalyzed by Zn(OAc)2(EA) has higher foam heights than that catalyzed by Cu(OAc)2(EA). From the density of RPUR foams, Cu(OAc)2(EA) and Zn(OAc)2(EA) are suitable catalysts for preparing RPUR foams having high density and low density, respectively.

As Cu(OAc)2(EA) has good catalytic activity and Zn(OAc)2(EA) gives RPUR foam with low density, Cu(OAc)2(EA) and Zn(OAc)2(EA) were mixed at the mole ratio of 1:1 to give the mixed metal acetate-ethanolamine complexes, namely Cu(OAc)2(EA):Zn(OAc)2(EA). This experiment was done to investigate the effect of Cu(OAc)2(EA):Zn(OAc)2(EA) on catalytic activity and foam density. It was found that Cu(OAc)2(EA):Zn(OAc)2(EA) gave comparable reaction times to Cu(OAc)2(EA) and RPUR foam catalyzed by Cu(OAc)2(EA):Zn(OAc)2(EA) had similar density to that catalyzed by Zn(OAc)2(EA). However, RPUR foam obtained from Cu(OAc)2(EA):Zn(OAc)2(EA) had large holes at the bottom of the mold which resulted in poor morphology. This might be because Cu(OAc)2(EA) and Zn(OAc)2(EA) had different solubility in RPUR foam formulation, therefore, the distribution of catalyst in the foam formulation was not homogeneous. The bottom of the mold might have larger amount of catalyst and underwent faster gelling and blowing reaction. Therefore, Cu(OAc)2(EA):Zn(OAc)2(EA) is not a suitable catalyst.

The rise profile of RPUR foams prepared from metal acetate-ethanolamine complexes shows a similar trend to that of DMCHA (Figure 6). DMCHA is a tertiary amine-based catalyst and has strong catalytic activity towards both blowing and gelling reactions. The results agree with the rise time shown in Table 2. The rise time of RPUR foams follows the order DMCHA<Cu(OAc)2(EA)<Zn(OAc)2(EA). DMCHA shows shorter initial time than the metal-ethanolamine complexes and exhibits a fast rise curve in the latter stage.

Figure 6: Rise profile of RPUR foam catalyzed by metal-ethanolamine complexes.
Figure 6:

Rise profile of RPUR foam catalyzed by metal-ethanolamine complexes.

The polymerization reaction is an exothermic reaction. The maximum core temperature of RPUR foams prepared from metal acetate-ethanolamine complexes is in the range 121–133°C, which is similar to that of DMCHA.

NCO conversion of all RPUR foams was investigated by ATR-IR spectroscopy. NCO conversion was determined from the absorption band of isocyanate group at 2277 cm-1 as shown in the following equation (30).

NCO conversion (%) =[1-(NCOt/NCOi)]×100

Where NCOt is the area of 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 isocyanate peak at the initial time. The isocyanate peak area is normalized by the aromatic ring peak area at 1595 cm-1. It was found that all RPUR foams had quantitative isocyanate conversion.

3.3 Effect of catalyst contents on reaction time

The effect of catalyst contents on reaction times of RPUR foam were investigated. Table 3 show the reaction times of RPUR foams catalyzed by Cu(OAc)2(EA), Zn(OAc)2(EA) and Cu(OAc)2(EA):Zn(OAc)2(EA). For all metal acetate-ethanolamine complexes, increasing the amount of catalyst resulted in the decrease of reaction times and foam density. When the catalyst amount is increased from 0.5 to 2.0 pbw, gel time and tack free time decrease. The effect of catalyst amount on reaction times follows the order Cu(OAc)2(EA):Zn(OAc)2(EA)>Zn(OAc)2(EA)>Cu(OAc)2(EA). Although a high catalyst content of Cu(OAc)2(EA) and Zn(OAc)2(EA) at 2.0 pbw gives faster reaction times, there are large holes at the bottom of the mold which resulted in poor property of foams. From this reason, the optimum catalyst content for the foam formulation is 1.0 pbw.

Table 3:

Reaction times and densities of RPUR foams prepared at NCO index of 100 and catalyzed by different amounts of metal-ethanolamine complexes.a

Reaction time and densityAmount of catalyst in RPUR foam formulation (pbw)
Cu(OAc)2(EA)Zn(OAc)2(EA)Cu(OAc)2(EA):Zn(OAc)2(EA)
0.51.02.00.51.02.00.51.02.0
Cream time (s)504132704636553630
Gel time (s)1008261180118811459556
Tack free time (s)18016813524521015027517099
Rise time (s)185180140270233160280180120
Density (kg/m3)48.3242.4335.4140.5334.4130.5341.2733.5229.87

aThe data reported are average values with standard deviation <5% from the average values.

3.4 3.4 Proposed polymerization mechanism catalyzed by Cu(OAc)2(EA)

In our previous work [12], copper acetylacetonate-amine complex [Cu(acac)2(trien)] was used as a catalyst in the preparation of rigid polyurethane foam. The catalytic mechanism of Cu(OAc)2(EA) (Scheme 2) is proposed to be similar to that of Cu(acac)2(trien). Cu(OAc)2(EA) catalyzes the gelling reaction between PMDI and polyol to give urethane linkages, which is represented by R-NCO and R′-OH, respectively, by the mechanism described as follows: copper atom in Cu(OAc)2(EA) acts as a Lewis acid and makes the NCO carbon to be more electrophilic by coordination to the oxygen atom of the R-NCO. The nitrogen atom in Cu(OAc)2(EA) makes the hydroxyl oxygen to be more nucleophilic by interaction with the hydrogen of R′-OH. The reaction between isocyanate and hydroxyl groups gives the urethane group. The catalytic mechanism of the blowing reaction between PMDI and water to give CO2 is proposed to be similar to that of PMDI with polyol. For Zn(OAc)2(EA), it is proposed that the catalytic mechanism is similar to that of Cu(OAc)2(EA).

Scheme 2: Proposed mechanism of urethane formation catalyzed by copper-ethanolamine complex [Cu(OAc)2(EA)].
Scheme 2:

Proposed mechanism of urethane formation catalyzed by copper-ethanolamine complex [Cu(OAc)2(EA)].

3.5 Compressive properties of RPUR foams

Compression stress-strain curves of RPUR foams catalyzed by metal acetate-ethanolamine complexes in parallel and perpendicular to the foam rising direction are shown in Figures 7 and 8. The compressive strength in parallel to the foam rising direction are higher than those in perpendicular to the foam rising direction which confirms that the cell structures of the all RPUR foams are anisotropic structures. RPUR foams catalyzed by Cu(OAc)2(EA) and DMCHA have comparable compressive strength as they have similar foam density. Although the density of RPUR foam catalyzed by Zn(OAc)2(EA) is lower than that of Cu(OAc)2(EA), the compressive stress of RPUR foam catalyzed by Zn(OAc)2(EA) is still suitable for application. For rigid polyurethane foam applications, the compressive stress (at 10% deformation) value of 100 kPa is sufficient (31). RPUR foams obtained from Cu(OAc)2(EA):Zn(OAc)2(EA) and Zn(OAc)2(EA) have similar density, however, Cu(OAc)2(EA):Zn(OAc)2(EA) gives RPUR foam with lower compressive strength than that obtained from Zn(OAc)2(EA).

Figure 7: Compressive stress-strain curves of RPUR foams in parallel to the foam rising direction.
Figure 7:

Compressive stress-strain curves of RPUR foams in parallel to the foam rising direction.

Figure 8: Compressive stress-strain curves of RPUR foams in perpendicular to the foam rising direction.
Figure 8:

Compressive stress-strain curves of RPUR foams in perpendicular to the foam rising direction.

3.6 RPUR foam morphology

SEM micrographs of RPUR foams catalyzed by DMCHA and metal acetate-ethanolamine complexes are shown in Figure 9. It was observed that cell morphology showed spherical cells and elongated cells in the (A) perpendicular and (B) parallel direction, respectively. The cell structure of all RPUR foams are anisotropic structure and the foam cells are elongated in the direction of the rise. The cell size in parallel to the foam rising direction (side view) is larger than that in perpendicular to the foam rising direction (top view). Both Cu(OAc)2(EA) and Zn(OAc)2(EA) give RPUR foams with good morphology while RPUR foam obtained from Cu(OAc)2(EA):Zn(OAc)2(EA) has poor morphology.

Figure 9: SEM micrographs of RPUR foams catalyzed by metal acetate-ethanolamine complexes in perpendicular (top view) and parallel (side view) to the foam rising direction (45×): (A) DMCHA, perpendicular; (B) DMCHA, parallel; (C) Cu(OAc)2(EA), perpendicular; (D) Cu(OAc)2(EA), parallel; (E) Zn(OAc)2(EA), perpendicular; (F) Zn(OAc)2(EA), parallel; (G) Cu(OAc)2(EA):Zn(OAc)2(EA), perpendicular; (H) Cu(OAc)2(EA):Zn(OAc)2(EA), parallel.
Figure 9:

SEM micrographs of RPUR foams catalyzed by metal acetate-ethanolamine complexes in perpendicular (top view) and parallel (side view) to the foam rising direction (45×): (A) DMCHA, perpendicular; (B) DMCHA, parallel; (C) Cu(OAc)2(EA), perpendicular; (D) Cu(OAc)2(EA), parallel; (E) Zn(OAc)2(EA), perpendicular; (F) Zn(OAc)2(EA), parallel; (G) Cu(OAc)2(EA):Zn(OAc)2(EA), perpendicular; (H) Cu(OAc)2(EA):Zn(OAc)2(EA), parallel.

4 Conclusions

Copper and zinc acetate-ethanolamine complexes, namely Cu(OAc)2(EA) and Zn(OAc)2(EA), were synthesized from readily available starting materials using a simple procedure. The reaction times in RPUR foam preparation catalyzed by Cu(OAc)2(EA) and Zn(OAc)2(EA) were investigated, namely cream time, gel time, tack-free time and rise time. Both Cu(OAc)2(EA) and Zn(OAc)2(EA) gave complete polymerization reactions, which was indicated by quantitative isocyanate conversion. In comparison to DMCHA, both Cu(OAc)2(EA) and Zn(OAc)2(EA) had longer gel time and slower rise profile and therefore preparation of RPUR foams in the mold could be easily carried out.

Acknowledgments:

The authors would like to thank Huntsman (Thailand) Ltd. for supplying the chemicals used in this research.

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Received: 2016-1-25
Accepted: 2016-4-4
Published Online: 2016-5-12
Published in Print: 2016-7-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Artikel in diesem Heft

  1. Frontmatter
  2. In this Issue
  3. Full length articles
  4. Preparation of rigid polyurethane foams using low-emission catalysts derived from metal acetates and ethanolamine
  5. Biodegradation of crosslinked polyurethane acrylates/guar gum composites under natural soil burial conditions
  6. Influence of phthalocyanine pigments on the properties of flame-retardant elastomeric composites based on styrene-butadiene or acrylonitrile-butadiene rubbers
  7. Synthesis and properties of low coefficient of thermal expansion copolyimides derived from biphenyltetracarboxylic dianhydride with p-phenylenediamine and 4,4′-oxydialinine
  8. Thermal behavior of modified poly(L-lactic acid): effect of aromatic multiamide derivative based on 1H-benzotriazole
  9. Functionalized magnetic Fe3O4 nanoparticles for removal of heavy metal ions from aqueous solutions
  10. Effect of oil palm ash on the mechanical and thermal properties of unsaturated polyester composites
  11. Effect of carbon sources on physicochemical properties of bacterial cellulose produced from Gluconacetobacter xylinus MTCC 7795
  12. Investigation into the effect of the angle of dual slots on an air flow field in melt blowing via numerical simulation
  13. Simulation study on the assembly of rod-coil diblock copolymers within coil-selective nanoslits
https://doi.org/10.1515/epoly-2016-0021
Schlagwörter für diesen Artikel
copper-ethanolamine complex; low-emission catalyst; metal acetate; rigid polyurethane foam; zinc-ethanolamine complex
Creative Commons
BY-NC-ND 3.0

Artikel in diesem Heft

  1. Frontmatter
  2. In this Issue
  3. Full length articles
  4. Preparation of rigid polyurethane foams using low-emission catalysts derived from metal acetates and ethanolamine
  5. Biodegradation of crosslinked polyurethane acrylates/guar gum composites under natural soil burial conditions
  6. Influence of phthalocyanine pigments on the properties of flame-retardant elastomeric composites based on styrene-butadiene or acrylonitrile-butadiene rubbers
  7. Synthesis and properties of low coefficient of thermal expansion copolyimides derived from biphenyltetracarboxylic dianhydride with p-phenylenediamine and 4,4′-oxydialinine
  8. Thermal behavior of modified poly(L-lactic acid): effect of aromatic multiamide derivative based on 1H-benzotriazole
  9. Functionalized magnetic Fe3O4 nanoparticles for removal of heavy metal ions from aqueous solutions
  10. Effect of oil palm ash on the mechanical and thermal properties of unsaturated polyester composites
  11. Effect of carbon sources on physicochemical properties of bacterial cellulose produced from Gluconacetobacter xylinus MTCC 7795
  12. Investigation into the effect of the angle of dual slots on an air flow field in melt blowing via numerical simulation
  13. Simulation study on the assembly of rod-coil diblock copolymers within coil-selective nanoslits
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