Copper-Amine Complex Solution As a Low-Emission Catalyst for Flexible Polyurethane Foam Preparation

e-Polymers 2015; 15(2): 119–126

Duangruthai Sridaeng, Benjatham Sukkaneewat, Nuttawut Chueasakol and Nuanphun Chantarasiri* Copper-amine complex solution as a low-emission catalyst for flexible polyurethane foam preparation

Abstract: A low-emission catalyst for the preparation manufactured in an extremely wide range of grades, of flexible polyurethane (FPUR) foams was developed. from flexible to rigid foams (1, 2). Flexible polyurethane Copper-amine complex solutions in ethylene glycol (FPUR) foams are produced by reacting diisocyanates

(EG), namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG or polyisocyanates with compounds that contain at (en, ethylenediamine; trien, triethylenetetramine), were least two hydrogen atoms that are reactive towards the synthesized and used as catalysts for the preparation isocyanate groups in the presence of blowing agents, of FPUR foams. The synthesis of Cu(OAc)2(en)2-EG and catalysts, surfactants and other additives. High-density

Cu(OAc)2(trien)-EG is convenient because the synthesis of flexible foams are defined as those having a density copper-amine complexes can be done in situ using ethyl- above 100 kg/m3. They can be easily produced in a variety ene glycol as a solvent and no purification step is needed. of shapes by molding or cutting. Flexible polyurethane

It was found that Cu(OAc)2(en)2-EG was a suitable catalyst foams are used for many applications, e.g., acoustic for FPUR foam preparation. In comparison to Dabco EG insulation, furniture upholstery, automotive seating and (or triethylenediamine), which is a commercial catalyst carpet foam backing. for FPUR foam preparation, Cu(OAc)2(en)2-EG had a com- Tertiary amines are the most important cata- parable catalytic activity in gelling reaction and a higher lysts in the manufacture of FPUR foams. The rela- catalytic activity in blowing reaction. The FPUR foam pre- tionship between catalyst structure and activity in pared from Cu(OAc)2(en)2-EG had a lower density than that polyurethane formation was investigated (3). It was prepared from Dabco EG. found that triethylenediamine (TEDA or DABCO) and dibutyltin dilaurate were strong gelling catalysts, while Keywords: copper acetate; copper-amine complex; flex- pentamethyldiethylenetriamine was a strong blowing ible polyurethane foam; low-emission catalyst; synergis- catalyst. N,N-Dimethylcyclohexylamine (DMCHA) and tic effect. tetramethylethylenediamine showed moderate activity between the gelling and the blowing reactions. For the development of new amine catalysts for FPUR foams, pol- DOI 10.1515/epoly-2014-0197 Received October 29, 2014; accepted January 2, 2015; previously yurethane reaction kinetics was followed using Fourier published online February 4, 2015 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 1 Introduction available amines were evaluated for FPUR preparation (5). It was found that DABCO and the mixtures DMCHA- Polyurethane foam has been widely produced in the stannous octoate [Sn(Oct) ] and DABCO-Sn(Oct) -N,N- polymer industry. This is because polyurethane can be 2 2 bis(2-dimethylaminoethyl)methylamine showed the best catalytic results. Simulation of the chemical reactions in *Corresponding author: Nuanphun Chantarasiri, Supramolecular rigid polyurethane foam formation catalyzed by amine Chemistry Research Unit, Faculty of Science, Department of was studied (6). The impact of catalysts, including the Chemistry, Chulalongkorn University, Bangkok, 10330, Thailand, impact of catalyst concentration, was investigated, and e-mail: [email protected] the results provided good agreement with experimen- Duangruthai Sridaeng: Faculty of Science, Department of Chemistry, tal data. Since amine catalysts cause odor problems Rangsit University, Pathumthani, 12000, Thailand Benjatham Sukkaneewat and Nuttawut Chueasakol: Program during the manufacturing process, the development of of Petrochemistry and Polymer Science, Faculty of Science, new catalyst systems for FPUR foams with less volatile Chulalongkorn University, Bangkok, 10330, Thailand organic compound emission is an area of interest. The 120 D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst

catalyst-containing reactive groups, namely, amine and Cu(OAc)2(trien)-EG have the potential to be used as low- hydroxyl groups, which undergo reaction with the iso- emission catalysts for FPUR foam preparation. cyanate group to become part of the polymer matrix (7), were investigated. Besides amines, metal compounds and metal com- 2 Experimental plexes can be used as catalysts in the preparation of polyurethane and waterborne polyurethane (8–13). Our 2.1 Materials research group synthesized new catalysts with good catalytic activity for the preparation of water-blown Ethylene glycol was obtained from Carlo Erba (Italy). rigid polyurethane (RPUR) foams (14). These cata- Copper(II) acetate monohydrate [Cu(OAc) ·H O], ethyl- lysts were copper-amine complexes, Cu(OAc) (en) and 2 2 2 2 enediamine (en) and triethylenetetramine (trien) were Cu(OAc) (trien) (where en indicates ethylenediamine; 2 obtained from Aldrich (USA). Poly(ethylene oxide) triol trien, triethylenetetramine). Both Cu(OAc) (en) and 2 2 (Jeffol G-31-35, glycerin initiated ethylene oxide triol) (molec- Cu(OAc) (trien) are odorless and can be prepared from 2 ular weight 4800 g/mol, hydroxyl value 35 mg KOH/g, inexpensive and readily available starting materials. average functionality 3), diphenylmethane diisocyanate Cu(OAc) (en) and Cu(OAc) (trien) were synthesized in 2 2 2 prepolymer (MDIP, Suprasec 2449, % NCO 18.9 wt.%), tri- acetone, which was removed to obtain a pure copper ethylenediamine in ethylene glycol (Dabco EG, commercial complex, before using them in the RPUR foam prepara- reference catalyst), silicone surfactant (Dabco DC193) and tion. Copper-amine complexes have good solubility in chain extender (mono ethylene glycol) were supplied by water-blown RPUR foam formulation since they have Huntsman (Thailand) Co., Ltd. (Thailand). Distilled water good solubility in water and there is sufficient amount was used as a chemical blowing agent. of water in RPUR foam formulation to dissolve copper complexes. However, the FPUR foam formulation con- tains a small amount of water and it would be difficult 2.2 Analytical method to obtain a homogeneous mixing of the copper-amine complexes with the other components in FPUR foam Fourier transform infrared and attenuated total reflection preparation. infrared (ATR-IR) spectra were recorded on a Perkin-Elmer To the best of our knowledge, there have been few RX I FTIR spectrometer (USA) and a Nicolet 6700 FTIR spec- reports about the development of new catalysts for trometer (USA), respectively, over the range 500–4000 cm-1 FPUR foams. Therefore, it is of interest to use these at a resolution of 4 cm-1. Ultraviolet-visible (UV-Vis) spectra copper-amine complexes as catalysts in the prepara- were recorded on a Varian Cary 50 UV-Vis spectrophotometer tion of FPUR foams. A new method for the preparation (USA) over the range 200–800 nm. MALDI-TOF mass spectra of Cu(OAc) (en) and Cu(OAc) (trien) was developed in 2 2 2 were carried out using a Bruker Daltonics mass spectrometer order to improve the solubility of these copper com- (USA) using 2-cyano-4-hydroxy cinnamic acid as a matrix. plexes 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 2.3 Synthesis of copper-ethylenediamine formulation is Dabco EG, which is a solution of 33 wt.% complex solution in ethylene glycol triethylenediamine in ethylene glycol. Therefore, the [Cu(OAc) (en) -EG] objective of this research was to synthesize copper- 2 2 amine complexes in the form of solutions in ethylene A solution of ethylenediamine (0.42 ml, 6.28 mmol) was glycol, namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)- dissolved in ethylene glycol (1.8 ml) at room tempera- EG. During the polymerization, ethylene glycol in ture for 15 min. Copper(II) acetate monohydrate (0.624 g,

Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG underwent 3.12 mmol) was added, and the reaction mixture was stirred polymerization reaction with isocyanate. The pure continuously at room temperature for 2 h. A solution of copper-amine complexes, namely, Cu(OAc)2(en)2 and 33 wt.% of Cu(OAc)2(en)2 in ethylene glycol [Cu(OAc)2(en)2-

Cu(OAc)2(trien), remained in the FPUR foam matrix EG] was obtained as an odorless purple solution with low after the polymerization was completed. Based viscosity. UV: λmax (MeOH) = 232 nm, molar absorptivity + on our previous work, the physical state of pure (ε) = 5.667. MALDI-TOF m/z of CuC8H22N4O4 [Cu(OAc)2(en)2] : + Cu(OAc)2(en)2 and Cu(OAc)2(trien) was solid and viscous 301.83; found 303.05 [Cu(OAc)2(en)2+H] , 242.74; found + liquid, respectively. Therefore, Cu(OAc)2(en)2-EG and 242.93 [Cu(OAc)2(en)+H] . D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst 121

2.4 Synthesis of copper-triethylenetet- the polymerization is finished) were measured. The foams ramine complex solution in ethylene were kept for 2 days before the measurement of free rise glycol [Cu(OAc) (trien)-EG] density and the investigation of the NCO conversion by 2 infrared spectroscopy. The density of FPUR foam was A solution of triethylenetetramine (0.43 ml, 2.89 mmol) measured in accordance with ASTM D3574. The size of the was dissolved in ethylene glycol (1.8 ml) at room tem- specimen was 3.0 × 3.0 × 3.0 cm (width × length × thickness), perature for 15 min. Copper(II) acetate monohydrate and the average value of the three samples was reported. (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 2.6 Molded flexible polyurethane foam

[Cu(OAc)2(trien)-EG] was obtained as an odorless blue solution and low viscosity. UV; λmax (MeOH) = 258 nm, molar The molded foams were prepared in an aluminum mold absorptivity (ε) = 4.322. MALDI-TOF m/z of CuC10H24N4O4 slap with a dimension of 20 × 10 × 0.6 cm (width × length × + + [Cu(OAc)2(trien)] 327.87; found 268.16 [Cu(OAc)(trien)] . 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 2.5 Preparation of free rise flexible in the preparation of free rise foams. After all the compo- polyurethane foam prepared by the nents were mixed by a mechanical stirrer at 2000 rpm for cup test method 10 s, the liquid was poured into an aluminum mold. The demolding time for the FPUR foams prepared from Dabco

Table 1 shows the FPUR foam formulation. The free rise EG, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG was 30, foams were prepared in a 700–ml paper cup, which 30 and 60 min, respectively. The FPUR foams were kept was used for the investigation of the reaction time, free for 2 days at room temperature. The foam density was rise density, height of foams and NCO conversion. In the measured. The compression set of the FPUR foams set at first step, the polyol, the catalysts (Dabco EG or copper- 25% thickness was investigated using a universal testing amine complexes), the surfactant, the chain extender machine (Hounsfield H 10 KM) in accordance with ASTM and the blowing agent were mixed in a paper cup. In C165-95. Specimens with a dimension of 0.6 × 0.6 × 0.6 cm the second step, MDIP was added to the polyol mixture. (width × length × thickness) perpendicular to the foam rise Then, the mixture was mixed using a mechanical stirrer direction were analyzed, and the average values of the at 2000 rpm for 10 s to obtain a homogeneous mixture. three samples were reported. Tensile testing of the FPUR The mixture was poured into another paper cup. The foam foams was carried out using a universal testing machine was allowed to rise freely, and during the foaming reac- (Hounsfield H 10 KM) in accordance with ASTM D412-68, tion, cream time (the time when the foam starts to rise or and the average values of the three samples were reported. the blowing reaction), gel time (the time when the foam The morphology of FPUR foams was investigated on a mixture starts to appear as a gel), rise time (the time when Hitachi/S-4800 scanning electron microscope. the foam stops rising) and tack-free time (the time when

Table 1 FPUR foam formulations (pbwa). 3 Results and discussion

Formulation (NCO index = 100) pbw 3.1 Synthesis of copper-amine complex Poly(ethylene oxide) triol, glycerin initiated (Jeffol G-31-35, 100.0 solutions in ethylene glycol molecular weight = 4800 g/mol, OH number = 35 mg KOH/g) Metal complex catalyst or reference catalyst (Dabco EG) 1.5b [Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG] Silicone surfactant (polysiloxane, Dabco DC193) 0.5 Synthesis of the copper-amine complexes, Cu(OAc) (en) Chain extender (ethylene glycol) 5.0 2 2 Blowing agent (water) 0.5 and Cu(OAc)2(trien), was carried out using ethylene Diphenylmethane diisocyanate prepolymer (MDIP, 69.3 glycol as a solvent (Scheme 1). Copper-amine complexes Suprasec 2449, % NCO = 18.9 wt.%) were obtained in the form of solution in ethylene glycol, aParts by weight or 1 g in 100 g of polyol. namely, Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG. The b The amount of 1.5 pbw consisted of 0.5 pbw of the metal complex concentration of Cu(OAc)2(en)2 and Cu(OAc)2(trien) in and 1.0 pbw of ethylene glycol. ethylene glycol was chosen to be 33 wt.%, which was 122 D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst

and UV-visible spectroscopy. The spectroscopic data of

Cu(OAc)2(trien) agreed with those reported in the litera- ture (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 Scheme 1 Synthesis of metal-amine complex solutions in ethylene complexes could be formed in ethylene glycol. glycol. MALDI-TOF mass spectrometry was also used to char-

acterize Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG. The similar to that of DABCO EG, which is 33 wt.% solution peaks of the mass spectra corresponded to the molecular of triethylenediamine in EG. The important criteria in weight of the copper-amine complexes. A molecular ion + the synthesis of Cu(OAc)2(en)2 and Cu(OAc)2(trien) are of [Cu(OAc)2(en)2+H] appeared at m/z 303.05 (Figure 1). the starting materials and the copper complexes that Cu(OAc)2(en)2 lost an ethylenediamine (en) unit, and + must be dissoluble in ethylene glycol. It was found that the peak of [Cu(OAc)2(en)+H] appeared at 242.93. + Cu(OAc)2, ethylenediamine, triethylenetetramine and the The molecular ion of [Cu(OAc)2(trien)] could not be + copper complexes have good solubility in ethylene glycol. observed. [Cu(OAc)2(trien)] lost an acetate group (OAc), Therefore, the copper complex formation could be done and the peak of [Cu(OAc)(trien)]+ appeared at m/z 268.16 in situ using ethylene glycol as a solvent. Cu(OAc)2(en)2-EG (Figure 2). Therefore, the UV and MS data indicate that the and Cu(OAc)2(trien)-EG were obtained as odorless liquids Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG solutions in eth- with low viscosity. The solution containing copper-amine ylene glycol contain the Cu(OAc)2(en)2 and Cu(OAc)2(trien). 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 3.3 Preparation of flexible polyurethane foam foam formulation. The preparation of Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG was convenient, and large-scale Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG were used as synthesis could be done in a short time. catalysts in the preparation of flexible polyurethane foam. The reaction time of the FPUR foaming reaction, namely,

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 analy- sis. Cu(OAc)2(trien) was characterized by IR spectroscopy Figure 1 MALDI-TOF mass spectrum of Cu(OAc)2(en)2-EG. D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst 123

Figure 2 MALDI-TOF mass spectrum of Cu(OAc)2(trien)-EG. Figure 3 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) (trien)-EG/Dabco EG. cream time, gel time, rise time and tack-free time, cata- 2 lyzed by the copper-amine complexes was investigated by the cup test method and compared with that of Dabco EG Cu(OAc) (en) -EG had a long reaction time in the initial (Table 2). The catalytic activity of the gelling and blowing 2 2 stage and a fast rise curve in the latter stage. In compari- reactions in the preparation of RPUR foams was deter- son to Cu(OAc) (en) -EG, Dabco EG had a shorter reaction mined by tack-free time and rise time, respectively. Gelling 2 2 time in the initial stage and a slower rise curve in the latter and blowing reactions relate to isocyanate-polyol and stage. isocyanate-water reactions, respectively. The results show The isocyanate conversion (NCO conversion) of that Cu(OAc) (en) -EG has a comparable tack-free time to 2 2 FPUR foams catalyzed by Dabco EG, Cu(OAc) (en) -EG, that of Dabco EG. This indicates that Cu(OAc) (en) -EG and 2 2 2 2 Cu(OAc) (trien)-EG, Cu(OAc) (en) -EG/Dabco EG and Dabco EG have a comparable catalytic activity in gelling 2 2 2 Cu(OAc) (trien)-EG/Dabco EG was investigated by ATR-IR reaction. Cu(OAc) (trien)-EG has a much lower catalytic 2 2 spectroscopy. NCO conversion was determined from the activity than Dabco EG. This might be due to the steric absorption band of the isocyanate group at 2277 cm-1 as hindrance of the trien unit in Cu(OAc) (trien). 2 shown in the following equation (16): The density of foam varied inversely with foam t i height. FPUR foams prepared from Cu(OAc)2(en)2-EG had NCOconversion(%)=×[1-(NCO/NCO] 100 a much lower density than those prepared from Dabco EG. t This suggested that Cu(OAc)2(en)2-EG had a higher cata- where NCO is the area of the isocyanate peak at time t, lytic activity in blowing reaction than Dabco EG. Figure 3 which is the time after the foam was kept at room tempera- shows the rise profiles of the FPUR foams prepared from ture for 48 h to complete the polymerization reaction. NCOi different catalysts. The data agree with the reaction times is the area of the isocyanate peak at the initial time. The shown in Table 2. The polymerization reactions using isocyanate peak area was normalized by the aromatic ring

Table 2 Reaction times, density and foam height of FPUR foams catalyzed by Dabco EG/metal-amine complexes obtained from the cup testa.

Catalysts Cream time Gel time Rise time Tack-free Free rise Foam (min:s) (min:s) (min:s) time (min:s) density (kg/m3) 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. 124 D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst 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 Scheme 2 Dissociation of Cu(OAc) (en) to give Cu(OAc) (en) and higher temperature causes the metal complexes to dis- 2 2 2 proposed polymerization mechanism catalyzed by Cu(OAc) (en). sociate to give metal complex cations and anions. These 2 reactive cations and anions are able to undergo polymerization reaction with epoxy oligomer to give metal- could be observed. Therefore, it is proposed that containing epoxy polymers. The temperature of the metal Cu(OAc)2(en) can also catalyze the foaming reaction by complex dissociation corresponds to the temperature at a similar mechanism to that of Cu(OAc)2(en)2 (Scheme 2). the beginning of the curing reaction of the epoxy oligomer The dissociated en unit could then undergo a reaction with with metal complexes. Metal-amine complexes having a the isocyanate group in MDIP to give the urea group. This higher dissociation temperature will give a slower rate of should not have an impact on the foaming reaction since curing. the amount of en group is small and thus is negligible. The

Therefore, the reaction mechanism catalyzed by catalytic mechanism of Cu(OAc)2(trien)-EG is similar to

Cu(OAc)2(en)2-EG is proposed to proceed from two mecha- that of Cu(OAc)2(en)2-EG. Cu(OAc)2(trien)-EG showed less nisms, namely, the mechanisms at low and high reaction catalytic activity than Cu(OAc)2(en)2-EG due to the steric temperature. The foaming reaction was done at room tem- effect of the trien group, which was larger than that of the perature. At the start of the foaming reaction, the metal en group. 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 3.5 Synergistic effect of Dabco EG with atom in Cu(OAc) (en) acts as a Lewis acid and coordinates 2 2 Cu(OAc)2(en)2 and Cu(OAc)2(trien) with the oxygen atom of the NCO group, which causes the NCO carbon to be more electrophilic. The nitrogen atom Tertiary amines are known to act in a synergistic manner in Cu(OAc)2(en)2 interacts with the proton of the hydroxyl with tin catalysts (2). Therefore, the synergistic effect of group and causes the hydroxyl oxygen to be more nucleo- Dabco EG with Cu(OAc)2(en)2 and Cu(OAc)2(trien) was philic, which then reacts with the isocyanate group to investigated. Mixtures of Cu(OAc)2(en)2-EG/Dabco EG and give a urethane linkage. As the foaming reaction pro- Cu(OAc)2(trien)-EG/Dabco EG were used as catalysts in ceeds, the reaction temperature becomes higher due to the FPUR foam formulation. The weight ratio of copper- the exothermic polymerization reaction. The heat causes amine complex/Dabco EG was 1:1, and the total amount

Cu(OAc)2(en)2 to dissociate into Cu(OAc)2(en) and ethyl- of copper-amine complex/Dabco EG in the FPUR foam enediamine (Scheme 2). The structure of Cu(OAc)2(en) formulation was 1.5 parts by weight. A synergistic effect is proposed based on the MALDI-TOF mass spectrum of of copper-amine complex and Dabco EG was observed. + Cu(OAc)2(en)2-EG where the peak of [Cu(OAc)2(en)+H] The catalytic activity of copper-amine complex/Dabco D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst 125

EG was higher than those of copper-amine complex and catalyzed by Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG Dabco EG. The reaction times of copper-amine complex/ had a lower tensile strength and elongation at break than ® Dabco EG (Table 2) agree with the data of the rise profile those catalyzed by Dabco EG. The use of Cu(OAc)2(en)2- ® ® (Figure 3). The catalytic mechanism is proposed to show EG/Dabco EG and Cu(OAc)2(trien)-EG/Dabco EG as that the copper atom in copper-amine complex acts as a catalysts improved both the tensile strength and the Lewis acid and coordinates with the oxygen atom of the elongation at break. This is because the mold density of

NCO group; therefore, the NCO carbon is more electro- FPUR foams obtained from Cu(OAc)2(en)2-EG/Dabco EG philic. In contrast, the nitrogen atom of Dabco EG is a ter- and Cu(OAc)2(trien)-EG/Dabco EG was higher than those tiary amine, which is active towards removing the proton obtained from Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG, of the hydroxyl group and causes the hydroxyl oxygen respectively. The foam catalyzed by Cu(OAc)2(en)2-EG/ to become more nucleophilic. The hydroxyl oxygen then Dabco EG had a comparable tensile strength and elonga- reacts with the NCO group to produce a urethane linkage. tion at break to that obtained from Dabco EG. The morphology of molded FPUR foams catalyzed ® by Cu(OAc)2(en)2 and Cu(OAc)2(en)2-EG/Dabco EG was 3.6 Characterization of flexible investigated by using a scanning electron microscope

polyurethane foams (Figure 4). The FPUR foam catalyzed by Cu(OAc)2(en)2- EG/Dabco EG showed better morphology than that pre-

The mechanical properties of FPUR foams were investi- pared from Cu(OAc)2(en)2-EG. This might be because gated using molded foams (Table 3). FPUR foams cata- Cu(OAc)2(en)2-EG is a better catalyst for the blowing reac- lyzed by all catalysts had a similar compression set. tion than Dabco EG, which means that the CO2 generation

For tensile strength and elongation at break, the foams rate of Cu(OAc)2(en)2-EG is faster than that of Dabco EG.

Table 3 Mechanical properties of molded FPUR foams catalyzed by various catalysts.

Catalysts Molded densitya Compression set Tensile Elongation (kg/m3) at 25% (MPa) strength (MPa) 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.

Figure 4 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. 126 D. Sridaeng et al.: Copper-amine complex solution as a low-emission catalyst

4. Chaffanjon P, Grisgby RA, Jr., Rister EL, Jr., Zimmerman RL. Use When Cu(OAc)2(en)2-EG/Dabco EG was used as a catalyst, of real-time FTIR to characterize kinetics of amine catalysts and the rise time was faster than that of Cu(OAc)2(en)2-EG and to develop new grades for various polyurethane applications, Dabco EG (Table 2); therefore, a higher amount of CO 2 including low emission catalysts. J Cell Plast. 2003;39(3): can get into the bubble cells before the viscosity reaches 187–210. the critical value. As a result, the cells in Figure 4B are 5. Strachota A, Strachotová B, Špírková M. Comparison of larger than those in Figure 4A. For external appearance, environmentally friendly, selective polyurethane catalysts. the colors of FPUR foams prepared from Dabco EG and Mater Manuf Process. 2008;23(6):566–70. Cu(OAc) (en) -EG are white and pale blue, respectively. 6. Zhao Y, Zhong F, Tekeei A, Suppes GJ. Modeling impact of cata- 2 2 lyst loading on polyurethane foam polymerization. Appl Catal The blue color is due to the color of Cu(OAc) (en) . 2 2 A-Gen. 2014;469:229–38. 7. Rothe J, Cordelair H, Wehman C. New catalysts for low VOC in flexible slabstock foam. J Cell Plast. 2001;37(3):207–20. 8. Cakic SM, Stamenkovic JV, Djordjevic DM, Ristic IS. Synthesis 4 Conclusions and degradation profile of cast films of PPG-DMPA-IPDI aqueous polyurethane dispersions based on selective catalysts. Polym Copper-amine complex solutions in ethylene glycol, Degrad Stabil. 2009;94(11):2015–22. 9. Cakic SM, Nikolic GS, Stamenkovic JV. The thermal degradation Cu(OAc)2(en)2-EG and Cu(OAc)2(trien)-EG, were synthe- of waterborne polyurethanes with catalysts of different selectiv- sized using a simple procedure and could be used as ity. Polym-Plast Technol. 2007;46(3):299–304. catalysts for FPUR foam preparation. The copper-amine 10. Sardon H, Irusta L, Fernández-Berridi MJ. Synthesis of iso- complex solutions were characterized by UV-visible spec- phorone diisocyanate (IPDI) based waterborne polyurethanes: troscopy and MALDI-TOF mass spectrometry. The catalytic comparison between zirconium and tin catalysts in the polym- activity of gelling and blowing reactions in the prepara- erization process. Prog Org Coat. 2009;66(3):291–5. tion of RPUR foams was determined by tack-free time and 11. Inoue S, Nagai Y. Efficient cobalt complex on the reaction between isophorone diisocyanate and diethylene glycol. Polym rise time, respectively. Cu(OAc) (en) -EG was a good cata- 2 2 J. 2005;37(5):380–3. lyst for FPUR foam preparation, while Cu(OAc)2(trien)-EG 12. Inoue S, Nagai Y, Okamoto H. Amine-manganese complex was a poor catalyst. A synergistic effect was observed as an efficient catalyst for polyurethane syntheses. Polym J. 2002;34(4):298–301. when a mixture of Cu(OAc)2(en)2-EG/Dabco EG and 13. Semsarzadeh, MA, Navarchian, AH. Kinetic study of the bulk Cu(OAc)2(trien)-EG/Dabco EG at a weight ratio of 1:1 was reaction between TDI and PPG in presence of DBTDL and FEAA used as a catalyst. catalysts using quantitative FTIR spectroscopy. J Polym Eng. 2003;23(4):225–40. Acknowledgments: The authors would like to thank 14. Pengjam W, Saengfak B, Ekgasit S, Chantarasiri N. Copper- Huntsman (Thailand) Ltd. for supplying the chemicals amine complexes as new catalysts for rigid polyurethane foam used in this research and IRPC Public Company Limited preparations. J Appl Polym Sci. 2012;123(6):3520–6. for financial support (AL.0963/2554). 15. Kurnoskin AV. Metalliferous epoxy chelate polymers: 1. Synthe- sis and properties. Polymer 1993;34(5):1060–7. 16. Modesti M, Lorenzetti A. An experimental method for evaluating isocyanate conversion and trimer formation in polyisocyanate- polyurethane foams. Eur Polym J. 2001;37(5):949–54. References 17. Kurnoskin AV. Reaction mechanisms of the metal chelates with epoxy oligomers and the structures of the epoxy-chelate metal- 1. Woods G. The ICI polyurethanes book. The Netherlands: Wiley; containing matrixes. J Appl Polym Sci. 1992;46(9):1509–30. 1990. 18. Kurnoskin AV. Epoxy polymer modification with metals. Polym 2. Randal D, Lee S. Huntsman polyurethanes – The polyurethanes Compos. 1993;14(6):481–90. book. UK: Wiley; 2002. 19. Ghaemy M. Study of the reaction mechanism of the cop- 3. Maris RV, Tamano Y, Yoshimura H, Gay KM. Polyurethane catalysis per chelate with DGEBA using DSC. J Therm Anal Calorim. by tertiary amines. J Cell Plast. 2005;41(4):305–22. 2003;72(2):743–52.