Development of Epoxy Foaming with CO2 as Latent Blowing Agent

Article pubs.acs.org/IECR

Development of Epoxy Foaming with CO2 as Latent Blowing Agent and Principle in Selection of Amine Curing Agent Qiang Ren,†,‡ Haijin Xu,‡ Qiang Yu,‡ and Shiping Zhu*,† †

Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China

‡

ABSTRACT: Some commercially available amines used as curing agents for epoxy resins can capture CO2 to form ammonium carbamates, which represent a novel type of latent blowing and curing agent for epoxy foaming. The principles in the selection of suitable amines are critical for final applications. This work aimed to reveal these principles by screening and comparing the preparation, chemical structure and composition, and curing and foaming performance of several types of ammonium carbamates from typical amine curing agents. It is found that amines with pKa >9 are eligible to react with CO2 to form ammonium carbamates with a high yield. Furthermore, the amines should have rigid cyclic groups in their structures to form a solid ammonium carbamate powder, which can be easily mixed with epoxy resins. The curing rate of the amine should not be much faster than the decomposition of the carbamate to obtain epoxy foams with good pore morphology and mechanical performance.

1. INTRODUCTION

required to keep the supercritical state of CO2. It is not compatible with the processing of thermoset polymers. Epoxy resin represents one of the most important thermoset polymers. It can be made into foams with excellent adhesive, high rigidity, and good chemical resistance properties.16−20 Application of CO2 as a blowing agent in epoxy foaming is quite attractive but has had very limited success.21 Reversible absorption and release of CO2 by amine and its derivatives to adjust the hydrophilic−hydrophobic properties of aminecontaining molecules has provided the opportunity for a number of applications otherwise unimaginable,22 including solvent,23 surfactant,24 latex,25,26 gel,27−29 and polymer assemblies.30,31 Inspired by these works, our group pioneered a novel approach to one-pack epoxy foaming in which a solid ammonium carbamate was prepared from the reaction of liquid N-aminoethylpiperazine and gaseous CO2, and it was used as the latent curing and blowing agent.32 The ammonium carbamate had good stability at room temperature but could be reversibly decomposed back to amine and CO2 upon heating. This approach has several advantages: it avoids the use of volatile organic compound (VOC) blowing agents, which benefits health and the environment. It is in a solid state so that it can be premixed with epoxy resin to form a stable single pack, instead of the three separate packs of epoxy resin, amine curing agent, and VOC blowing agent, and thus, it saves significant costs in storage and transportation. However, it should be pointed out that although there are plenty of types of commercial amines available as curing agents for epoxy resin, not all of the amines are suitable for this application due to their different levels of basicity derived from their chemical structure. What is the principle underlying the

Low density is one of the most attractive characteristics of polymeric materials relative to metals and ceramics. Furthermore, bulk polymer material can be easily processed into polymer foams to further enhance this advantage. Undoubtedly, polymer foams make wonderful contributions in our daily life and in a variety of industrial fields in which weight reduction, impact resistance, and heat or sound insulation matter.1 At the same time, the high surface ratio and controllable mass transferring property of polymer foams endow them new promising applications in gas capture and storage, catalyst carrier, separation,2,3 bioscaffolding4,5 and drug release.6 The development of polymer foams (porous polymers) and composite foams continues to be the most attractive field of materials science.7−12 The blowing agent is essential to produce polymer foams. It can release a gaseous substance under certain circumstances. As one of the most important polymer processing additives, polymer blowing agents can be based on physical or chemical mechanisms. Examples include 1,1-dichloro-1-fluoroethane (HCFC-141b), azodicarbonamide (AC), and hydrogen-containing polysiloxane, which have large annual consumption amounts. These organic blowing agents are made from fossilbased raw materials and are finally released to the atmosphere, which has a negative impact on our environment. Substitution of these blowing agents by environmentally friendly chemicals is of great benefit for the sustainable development of the polymer foaming industry. CO2 represents a good candidate as a blowing agent due to its significant abundance as well as human and environmentally benign characteristics. However, direct use of CO2 as a blowing agent is quite challenging because it is in a gas state under normal conditions and is thus not easy to handle. Supercritical CO2 has been demonstrated to work well with thermoplastic polymers,13,14 but few reports on thermoset polymers can be found.8,15 A possible cause is the high pressure condition © 2015 American Chemical Society

Received: Revised: Accepted: Published: 11056

August 21, 2015 October 22, 2015 October 26, 2015 October 26, 2015 DOI: 10.1021/acs.iecr.5b03069 Ind. Eng. Chem. Res. 2015, 54, 11056−11064

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Industrial & Engineering Chemistry Research

could be observed in the solution, indicating that no reaction occurred and that no B-DDM was yielded. 2.3. Preparation of Epoxy Foams and Bulk EP Thermosets. Typically, EP (120 g), PEG-b-PPG-b-PEG (0.6 g), fumed silica (1.2 g), and B-DDCM (43.7 g) were charged in a 500 mL plastic beaker and stirred mechanically at 800 rpm for 5 min, followed by 300 rpm for 5 min to obtain a homogeneous mixture. Then, 23 g of the mixture was charged in an alumina mold with a 1 mm Teflon film lining. The inner cave of the mold was cylindrical in shape and was 60 mm in diameter and 30 mm in depth. Four charged molds were sealed by a cover fixed with screws and then transferred to an oven preheated to 160 °C to perform the foaming and curing for 2 h. After curing, the molds were removed and cooled to room temperature for 2 h. The foams prepared using B-mXDA and B-AEP followed the same procedures with a stoichiometric balance of amine protons and epoxide groups under 140 °C for 1 h and 120 °C for 1 h, respectively. The bulk EP curing by AEP, mXDA, and DDCM were performed with the same temperature profile as their corresponding blocked amines as control samples for comparison. 2.4. Characterization Methods. FTIR spectra were acquired on a Nicolet 6700 FTIR spectrometer using the KBr plate method. TGA measurements was taken on a TGA Q5000 thermogravimetric analyzer from TA Instruments in the temperature range 50−300 °C with a ramp rate of 10 °C/min in a N2 flow rate of 25 mL min−1 with a sample weight of ∼20 mg. DSC testing of blocked-amines, epoxy curing formulation, and resulting foams or bulk thermosets was performed on a DSC 2910 modulated differential scanning colorimeter from TA Instruments in the temperature range 0−250 °C with a ramp rate of 10 °C. All of the tests were performed with a N2 flow rate of 20 mL min−1 with sample weight of ∼12 mg. The foam morphology was imaged by scanning electron microscopy (SEM) (JEOL 6610LV) with an acceleration voltage of 5 kV. The sample was cut by a sharp knife to get a cross section, which was coated by a layer of gold prior to imaging. The average pore diameter was calculated from SEM images with a sample population of 50 pores. The compressive tests of the epoxy foams were performed using a 50 kN MTS Criterion Series 40 Electromechanical Universal Test System according to ATSM D1621-00. The diameter of the cylindrical test specimen was ∼58 mm and the height is ∼29 mm. The crosshead speed was 2.5 mm/min. A total of 5 specimens were tested to obtain the average value and standard deviation of the compressive strength and modulus.

selection of a suitable curing agent? This is an important fundamental question to be answered. It serves as the very objective of this work. This paper reports our findings on the comparison of the preparation, curing, and foaming performance of ammonium carbamates from several typical commercially available amine curing agents. The whole process for the foaming preparation is shown in Scheme 1. Scheme 1. Process for Foam Preparation using DDCM as a Typical Curing Agent

2. EXPERIMENTAL SECTION 2.1. Materials. Epoxy resin (EP), diglycidyl ether of bisphenol A (DGEBA) with a brand mark of D.E.R. 332, was purchased from Aldrich. DGEBA has an epoxide equivalent weight (EEW) value of 171−175 and viscosity of 4000−6000 mPa s (25 °C). All the amine compounds, including Naminoethylpiperazine (AEP, 99%), m-xylylenediamine (mXDA, 99%), 3-(diethylamino) propylamine (DEAPA, ⩾99%), 4,4′diaminodicyclohexylmethane (DDCM, 95%), and 4,4′-diaminodiphenylmethane (DDM, ⩾97%), anhydrous ethanol, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-b-PPG-b-PEG) with an average Mn of 2800 g/mol, and fumed silica with particle size of 0.007 μm, were from Aldrich and used as received. 2.2. Preparation of Carbamates by Reaction of Amines with CO2. Typically, 60 g of DDCM and 300 mL of anhydrous ethanol were charged into a 1000 mL threenecked flask with a mechanical stirring bar to form a solution. Then, CO2 gas with a flow rate of 60 mL min−1 was introduced to the bottom of the flask by a needle of 1.63 mm diameter fixed in a rubber stopper in one neck to bubble in the solution. A needle of 0.62 mm diameter fixed in a rubber stopper in another neck was used as an outlet of CO2 gas. The transparent solution gradually turned into a milky turbid liquid over 4 h with a stirring rate of 250 rpm. The bubbling was continued for another 3 h. Once stirring was stopped, a white precipitate was obtained. The white precipitate was filtered, washed with 30 mL of anhydrous ethanol three times, dried in fume hood at room temperature for 12 h, and placed under a vacuum for another 12 h to yield 70.5 g (99.6%) of loose, fine white powder. This carbamate product is named CO2-blockedDDCM (abbreviated as B-DDCM). The same procedure was used to perform the reactions of CO2 with AEP, mXDA, DEAPA, and DDM. The yield for B-AEP is 79.9 g (90.2%) and for B-mXDA is 80.6 g (98.8%). For DEAPA, no obvious precipitation was observed, but the solution became viscous. Removing alcohol by rotational evaporation followed by drying gave loose sticky flakes (B-DEAPA). For DDM, no change

3. RESULTS AND DISCUSSION 3.1. Ammonium Carbamates Synthesized by the Reaction of Amines with CO2. Scheme 2 presents the general reaction mechanisms of amines with CO2 with or without water. Primary and secondary amines have strong combinations with CO2, which can be reversed only by heating as shown in eqs 1−4. Tertiary amines have weak combinations with CO2 and can be recovered easily to their original states by repelling CO2 with N2, as shown in eq 5. The weak interactions provide the possibility for the fast switchable applications reported in the literature.26,33,34 Primary and secondary amines with active hydrogen atoms are used as effective curing agents for epoxy resin in a certain range of temperatures. For most of them, the pot life after mixing with epoxy resin only lasts for a couple of hours at the maximum. On the contrary, the ammonium carbamates or carbonates resulting from the 11057

DOI: 10.1021/acs.iecr.5b03069 Ind. Eng. Chem. Res. 2015, 54, 11056−11064

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its structure. DDCM is an alicyclic amine, and DDM is a typical aromatic amine. The experimental results showed that the reactions of CO2 with AEP, mXDA, and DDCM in ethanol are all highly efficienct at producing white salt powder with high yield. The reaction of CO2 with DEAPA resulted in viscous gels, but no obvious precipitation was observed in ethanol. Removal of ethanol by rotational evaporation and successive drying gave loose sticky flakes. No reaction of DDM with CO2 was observed under the ambient conditions. In general, the reaction of amine with CO2 follows the rule of acid−base neutralization. Basicity of amines must be adequate enough because CO2 is a weak acid anhydride. The constant of basicity Kb is an equilibrium constant of the protonation reaction of amine in water, which gives a quantitative description to the basicity of amine. Ka is an equilibrium constant for the dissociation of conjugated acid of the amine. The pKa increases with an increase in the amine basicity. Table 1 gives pKa values of the

Scheme 2. General Reaction Mechanisms of Amines with CO2

Table 1. pKa Values of the Amines Used in This Work reactions of primary and secondary amines with CO2 are stable at room temperature, which makes it possible to design a onepack foaming formulation as demonstrated in our previous study.32 Epoxy-amine systems have been well-developed, and there are many commercially available amine curing agents for the formulation of epoxy-based material fabrications with versatile physical properties. As the approach illustrated in Scheme 1 demonstrates, the fundamental question remaining to be answered is which types of amine curing agents are good candidates for the one-pack foaming application? We chose and evaluated five types of representative commercially available amines as examples, as shown in Scheme 3, in the hope of

pKa

DEAPA

AEP

mXDA

DDCM

DDM

10.6

9.6

9.2

10.9

4.8

five amines used in this work.35,36 On the basis of our knowledge, the amines having a pKa value in the range of 5−9 are mainly tertiary aliphatic species, which are not suitable as a curing agent for epoxies. It appears that pKa > 9 is required for amines to properly react with CO2 and to generate ammonium carbamate salt powders. To further verify the chemical structures of the obtained carbamate salts, FTIR spectra of the blocked amines were recorded. Figure 1 shows an example of B-mXDA. The strong

Scheme 3. Five Kinds of Amine Compounds Used in This Work

Figure 1. FTIR spectra of mXDA and B-mXDA.

answering this question. The underlying principle in the amine selection is critical for the design of newly developed latent curing-foaming agents for epoxy resin. The most important traits of amine curing agents differ in two aspects. The first is the active hydrogen reactivity derived from electron-withdrawing or -donating behavior of the group attached to the amine. Aliphatic amines are more reactive than aromatic amines and thus can trigger curing at lower temperatures. The second is the rigidity of the molecular structure, which greatly influences the heat deformation temperature of the final thermosets. We took both aspects into account in optimizing the amine selection. DEAPA is a representative of aliphatic amine. mXDA also belongs to the aliphatic class but with a rigid phenyl group. AEP has both aliphatic and piperazine amines in

two NH2 stretching peaks at 3360 and 3290 cm−1 of the original mXDA (Figure 1b) represent the asymmetric and symmetric vibrations, respectively. After the reaction with CO2, the strong and wide peaks around 3300 cm−1 turned into a single sharp peak. The peaks at 1323−1416 and 1464−1607 cm−1 were derived from the symmetric CO2−2 bands and the symmetric NH3+ deformation, respectively.37 All of the changes revealed that the amine groups were blocked, and mXDA was turned into ammonium carbamate. Furthermore, the FTIR measurement of B-mXDA after 90 days of storage (Figure 1c) gave the same result as the newly prepared sample, verifying good stability of the blocked amine under ambient conditions. 11058


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Figure 2. DSC curves of (a) B-DEAPA, (b) B-AEP, (c) B-mXDA, and (d) B-DDCM.

two amine moieties capture one CO2 molecule on average, each mXDA molecule would theoretically capture one CO2 molecule and generate ammonium carbamate free of water, as shown by eq 1 in Scheme 2. CO2 content slightly higher than the theoretical value could be derived from the participation of water in the reaction to generate ammonium carbonate as shown in eq 2 in Scheme 2. However, the contribution from water was very limited in the anhydrous alcohol used in our work. The molecular structure of B-mXDA could be equivalently expressed as mXDA·1.1CO2. The amine hydrogen equivalent weight (AHEW) is a gram of the curing agent containing one equivalent of the latent N−H groups. The average molecular weight of B-mXDA was 185.1 (136.7 + 1.1 × 44). The AHEW of B-mXDA was 46.3, calculated as 185.1 divided by the number of latent N−H groups of 4. On the basis of the same principle, data of the other blocked amines were also calculated. The onset of decomposition temperature of B-AEP was 108 °C (Figure 3a), its latent CO2 content was 33%, and AHEW was 63.6.32 The onset of decomposition temperature of B-DDCM was 123 °C (Figure 3c), and the latent CO2 content was 15.2%. The B-DDCM molecular structure could be equivalently expressed as DDCM· 0.86 CO2, and AHEW was 62.05 g/mol. 3.2. Curing Kinetics of Epoxy Resin by Blocked Amines Revealed by DSC. The elucidation of chemical composition and decomposition behavior of the blocked amines paved the way for the design of the foaming formulation. The designed curing formulation was based on the stoichiometric balance of amine protons and epoxide groups. The kinetics of blocked-amine decomposition and that

A critical point in fabricating thermoset resin-based foams is the matching of bubbling and curing rates. The curing process must occur soon after bubbling with a very small time gap. In this work, the decomposition of blocked-amine provided CO2 as the bubbling source and amine as the curing agent. Figure 2 shows DSC curves of all the blocked-amines. Two strong endothermic peaks were detected below 200 °C, indicating the occurrence of decomposition. It was found that B-DEAPA was not adequately stable under fume hood airflow conditions. Upon being stored for 2 weeks, the samples experienced decomposition, and thus, no further characterization was carried out. The possible reason is that the weak combination of tertial amine with CO2 in DEAPA triggered the gradual decomposition of B-DEAPA. Figure 3 shows TGA characterization for B-AEP, B-mXDA, and B-DDCM. Taking B-mXDA as an example, the onset of decomposition was 113 °C (1% weight loss temperature), as shown in Figure 3b. The TGA curve could be divided into three stages as indicated by three peaks (155, 179, 236 °C) of the derivative thermogravimetric (DTG) curve. In comparison, there was only one stage with mXDA at 252 °C. The first and second stages of the blocked amine were clearly derived from the decomposition of BmXDA, that is, the release of CO2. The total weight loss of the first and second stages was 33%, whereas mXDA exhibited only 9% weight loss in the same temperature range due to volatilization. Deducting the volatile component of mXDA gave an estimate of 26% latent CO2 content in B-mXDA. If B-mXDA had the molecular structure of mXDA·xCO2, x could be estimated as 1.1 based on 26% CO2 release. Considering that 11059


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Figure 4. DSC curves of the nonisothermal curing of (a) AEP/EP, BAEP/EP; (b) mXDA/EP, B-mXDA/EP; and (c) DDCM/EP, BDDCM/EP.

Figure 3. TG and DTG curves of (a) B-AEP, (b) B-mXDA, and (c) BDDCM.

exothermal amounts of the epoxy/blocked-amine formulations were much lower than those of the free amines. This was because the decomposition of the blocked amines was strongly endothermic, as revealed in Figure 2, and the epoxy group contents in the blocked-amine formulations were lower. The curing degree α of the epoxy resin at temperature T can be calculated by eq 1

of subsequent epoxy resin curing are vital for the foaming process. DSC is helpful to understand the kinetic processes.38 Figure 4 shows DSC curves of the non-isothermal curing processes. AEP, mXDA, and DDCM cured the epoxy resin with the starting temperatures at approximately 25, 30, and 40 °C, and the peak temperatures at 98, 106, and 113 °C, respectively. Their CO2-blocked counterparts, B-AEP, B-mXDA, and BDDCM, could also cure the epoxy resin but at the much higher starting temperatures of 95, 126, and 113 °C and peak temperatures of 131, 143, and 140 °C, respectively. The blocked-amine/epoxy formulations were very stable at room temperature, which allowed the formulation of the single-pack foaming system. The single packs were safe and convenient for storage. The epoxy resin mixed with B-AEP was stored for 180 days, and those with B-mXDA and B-DDCM were stored for 60 days under ambient conditions. No change in appearance of the packs was observed. There was also not much change in their curing behavior, as shown in Figure 4. The curing

T

( ddHT ) dT α= T ∫T ( ddHT ) dT ∫T

i

e

i

(1)

where dH/dT is the heat flow rate in the DSC curve, and Ti and Te are the start and end temperatures of curing. The decomposition degree of blocked amine can also be calculated by the same equation based on the endothermic amount. Figure 5 shows the decomposition degree of each blocked amine, together with the curing degree of the blocked amine as well as the curing degree of the corresponding free amine. The 11060


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Table 2. Half-Life Times of the Decomposition for Blocked Amines and the Curing of Epoxy Resin by Blocked Amines in Comparison to Free Amines reaction AEP/EP curing B-AEP decomp B-AEP/EP curing

decomposition profiles were calculated from the first two endothermic peaks in Figure 2, and curing profiles were from the exothermic peak in Figure 4. The half-life times of the blocked-amine decomposition and the epoxy curing (t1/2) can be calculated by eq 2 Tα= 1/2 − Ti β

7.2 5.3 2.9

reaction mXDA/EP curing B-mXDA decomp B-mXDA/EP curing

t1/2 (min) 7.5 5.6 2.6

reaction DDCM/EP curing B-DDCM decomp B-DDCM/EP curing

t1/2 (min) 7.6 5.2 4.2

the piperazine ring and the relatively lower steric effect of the ethylamine group. DDCM was the most inactive with a start curing temperature of ∼40 °C and t1/2 of 7.6 min. This relatively low reactivity was derived from a high steric effect of the large cyclohexyl group. In comparison, among the blocked amines as curing agents, the B-mXDA/EP formulation showed the fast curing rate with t1/2 of 2.6 min, and B-DDCM/EP showed the slowest rate with t1/2 of 4.2 min. These kinetic parameters played an important role in the development of the foaming processes, and they will be discussed in combination with the pore morphology and foam properties. 3.3. Morphology and Properties of Epoxy Foams with Different Blocked Amines. Epoxy foaming was accomplished using the blocked amines as foaming and curing agents. Fumed silica was employed as the nucleating agent and PEG-b-PPG-bPEG as the foam stabilizer. B-DEAPA was found to not be stable at ambient temperature, and it could not be homogeneously mixed with epoxy resin to fabricate foams. With B-AEP, B-mXDA, and B-DDCM, foam samples with target densities of ∼0.3 g/cm3 were obtained. Table 3 summarizes all of the foaming formulations and conditions, as well as basic properties of the foams. It was unexpected that the foams from B-AEP and B-DDCM had strong mechanical performances that were comparable with those from the literature data,17,39,40 whereas those from B-mXDA were quite weak. Figure 6 shows the compressive stress−strain curves. For samples having a density of ∼0.3 g/cm3, B-DDCM cured foam exhibited a typical rigid mechanical behavior with high modulus and yield strength, whereas B-mXDA cured foam exhibited rather low modulus and no remarkable yield point. Figure 7 shows SEM images. Clear closed-cell morphologies could be seen in both B-AEP and B-DDCM cured foams. Some deformation to oval-shape pores was also evident in B-AEP cured foams, whereas perfect round pores were found in BDDCM cured foams. In B-mXDA cured foams, an irregular morphology with a small portion closed cells with most pores interconnected was observed. The irregular morphology was responsible for the poor mechanical performance. The interconnected pore morphology was not as good for mechanical loading as the closed-cell morphology. The question is why the foam prepared from B-mXDA was of poor irregular morphology. An examination of the kinetic data in Table 2 reveals that the decomposition of B-mXDA was the slowest and the curing of the B-mXDA/EP process was the fastest among the three blocked amines. As a result, excessive curing occurred before bubbling was completed. Partially cured resins were poor in flow and deformation, generating inhomogeneous interface forces and making it difficult to obtain homogeneous bubbling. Moreover, the latent CO2 content of blocked amine is adequate to sufficiently expand

Figure 5. Decomposition kinetics of blocked amines and the curing kinetics of epoxy resin by blocked amines in comparison to free amines: (a) B-AEP, B-AEP/EP, AEP/EP; (b) B-mXDA, B-mXDA/EP, mXDA/EP; and (c) B-DDCM, B-DDCM/EP, DDCM/EP.

t1/2 =

t1/2 (min)

(2)

where Tα=1/2 is the temperature for α reaching 1/2, and β is the temperature ramping rate. Table 2 summarizes all of the calculated t1/2 values. Regarding decomposition of the blocked amines, B-DDCM had the fastest rate with a t1/2 of 5.2 min, and B-mXDA had the slowest rate with a t1/2 of 5.6 min. Regarding epoxy resin curing with the free amines, AEP was the most active with a start curing temperature of ∼25 °C and t1/2 of 7.2 min. This could be attributed to a catalytic curing role of the tertiary amine on 11061


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Industrial & Engineering Chemistry Research Table 3. Foaming Formulation, Conditions, and Basic Properties of the Prepared Foams formulation and conditionsa 1%/120 1%/140 1%/160 1%/120 1%/120

°C/B-AEP °C/B-mXDA °C/B-DDCM °C/B-AEPc °C/B-AEPc c

ρfb (g cm−3) 0.27 0.30 0.33 0.11 0.46

± ± ± ± ±

0.02 0.01 0.02 0.02 0.03

Pb (%)

davgb (μm)

± ± ± ± ±

145 ± 86 N.D.d 300 ± 221 N.M.e N.M.e

0.77 0.74 0.72 0.91 0.61

0.02 0.01 0.02 0.01 0.02

σ (MPa) 3.1 1.8 3.7 0.5 9.1

± ± ± ± ±

0.2 0.3 0.5 0.1 0.8

E (MPa) 110 56 115 1.85 217

± ± ± ± ±

10 6 12 0.2 19

The foaming formulation is expressed as silica filling level/foaming temperature/curing agent. bρf, P, and davg denote foam density, porosity, and average cell diameter, respectively. The density of bulk EP thermoset is 1.17. P = (1 − ρf/1.17) × 100%. cFoams with different porosities obtained by adjusting loading of the same formulation in closed molds. dN.D.: no detection. eN.M.: no measurement. a

The selection of curing agent is an important parameter in adjusting glass transition temperature (Tg) of the foams, that is, the heat resistant property of the final thermosets. As shown in Figure 8, the foams prepared from the blocked amines exhibited Tg values of 20−30 °C lower than their bulk thermosets. This suggested that molecular chain segments in the porous polymers were freer in movement than those in the bulk. It could also be seen that the Tg of the epoxy foams was adjustable with the same tendency of the bulk polymers.

4. CONCLUSIONS Some liquid amines can react with gaseous CO2 through bubbling to form solid ammonium carbamates. The carbamates are CO2-blocked amines, which can decompose and release CO2 at high temperatures, and at the same time, free the amines. The carbamates can be premixed with epoxy resin diglycidyl ether of bisphenol A to form the foaming formulation and are stable under ambient conditions with little change found in long-term storage. This represents a novel single-pack technology for epoxy foaming. In the foaming process at high temperature, the released gaseous CO2 acts as the blowing agent and the freed amines act as the curing agent. This avoids the use of volatile organic compound (VOC) blowing agents. However, the coordination of blowing and curing actions is critical in this technology. The decomposition rate of the blocked amine and the curing rate of the freed amine with

Figure 6. Typical compressive strength−strain curves of the foams prepared by different blocked amines.

the foaming materials to a high porosity, that is, to achieve a low density. It is possible to control the porosity of foams by adjusting the loading of the foaming formulation in closed molds (Table 3, entries 1, 4, and 5). The mechanical performance can be adjusted as well (Figure 6).

Figure 7. Foam morphology of the epoxy foams prepared from different blocked amines: (a, b) B-AEP/EP, (c, d) B-mXDA/EP, and (e, f) BDDCM/EP. 11062



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Corresponding Author

*Tel.: +1-905-525-9140, ext 24962. E-mail: zhuship@ mcmaster.ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We sincerely thank the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant Program and the Canada Research Chair program of the Federal Government of Canada for supporting this research work. Q.R. acknowledges the sponsorship of Jiangsu Overseas Research, Training Program for University Prominent Young, Middle-Aged Teachers and President, and Qing Lan Project of Jiangsu Provincial Department of Education, China, for supporting his visit to McMaster University.

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Figure 8. DSC curves of the obtained epoxy foams and bulk thermosets.

epoxy must be matched with a small gap between their onset temperatures. The principle in selection of the proper amine type for synthesis of carbamate as the latent blowing and curing agent is revealed in this work. Five types of commercially available amine curing agents are studied. The results show that a pKa value higher than 9 is required for the amine to have an adequate level of basicity to react with CO2 under normal conditions. The amine must also have a rigid molecular structure to obtain carbamate salt in the form of solid powder rather than viscous gel. The decomposition and curing kinetics of the CO2-blocked amines are investigated by DSC, which provides good understanding of the relationship between amine type and the resulting foam morphology. It is found that curing too fast results in a poor morphology and mechanical performance. It is also found that the influence of the blocked amines on glass transition temperature of the final thermoset foams follows the same trend as their bulk polymers. 11063


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