Tertiary Alkanolamines as Solvolytic Agents for Poly(ethylene

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Ind. Eng. Chem. Res. 2004, 43, 862-874

Tertiary Alkanolamines as Solvolytic Agents for Poly(ethylene terephthalate). Evaluation of the Products as Epoxy Resin Hardeners Tadeusz Spychaj,* Ryszard Pilawka, Stanisława Spychaj, and Artur Bartkowiak† Polymer Institute, Technical University of Szczecin, ul. Pulaskiego 10, 70 - 322 Szczecin, Poland

Tertiary alkanolamines, i.e., N-methyldiethanolamine, N-ethyldiethanolamine, N-isopropyldiethanolamine, N-(n-butyl)diethanolamine, N-cyclohexyldiethanolamine, N-isopropanoldiethanolamine, triethanolamine, triisopropanolamine, N,N′-bis(2-hydroxyethyl)piperazine, N,N,N′,N′tetra(2-hydroxyethyl)ethylenediamine, and N,N,N′,N′-tetra(2-hydroxyethyl)propylene 1,3-diamine were applied as poly(ethylene terephthalate) (PET) solvolytic agents. Solvolysis (i.e., aminoglycolysis) of PET was performed at 190-220 °C for 90-240 min. PET aminoglycolysates were characterized via hydroxyl number and viscosity measurements, whereas some products were characterized by SEC, HPLC, and FTIR and 1H NMR spectroscopies. Selected products were tested as epoxy resin hardeners for elevated-temperature curing. Rheometric properties, as well as heats of the cross-linking reactions in the systems containing liquid bisphenol A based epoxy resin and PET aminoglycolysates (with a broad ratio of the latter) were investigated and compared. The accessibility of nitrogen atoms expressed by the substituent molar volume is a key factor determining the chemical activity of PET aminoglycolysates toward epoxy resin polymerization. FTIR measurements revealed that the transesterification reaction of bisesters of terephthalic acid and tertiary alkanolamines with bisphenol A diglycidyl ether derivatives bearing secondary OH groups plays an important role in the formation of the epoxy resin network. Introduction The recycling of waste poly(ethylene terephthalate) (PET) can be carried out in many ways.1-3 Among the recycling methods, chemical recycling is of great interest because PET is vulnerable to solvolytic chain cleavage, as is typical for condensation polymers. Important advantages of PET chemical solvolysis are the availability of a wide spectrum of degrading agents and a large variety of potential products, such as monomers for polymer and resin syntheses or additives for polymeric materials. Aside from the conventional solvolytic agents and methods used, such as methanol (methanolysis); glycol (glycolysis); and mineral acids, alkalis, water, or steam (hydrolysis), having a commercial importance, one can observe a growing tendency to use less common chemicals for PET degradation.2-5 This trend is associated with the special transesterification reactions, ammonolysis, aminolysis, and aminoglycolysis, resulting in useful chemicals for polymeric or resinous materials. The respective products can be manufactured on a lowtonnage scale.1 Investigations performed in our laboratory indicate that the products of the deep solvolytic degradation of waste PET with triethanolamine (TEA) are useful as the components for reactive resins processing, especially for epoxy resins6-9 and polyurethanes.10 * To whom correspondence should be addressed. Address: Polymer Institute, Technical University of Szczecin, 70-322 Szczecin, Poland. Tel.: (+48-91) 449 46 84. Fax: (48-91) 449 46 85. E-mail: [email protected]. † Present address: Agricultural University of Szczecin, Faculty of Food Science and Fisheries, Department of Food Packaging and Biopolymers, ul. Papieza Pawla VI/3 71-456 Szczecin, Poland.

The chemical solvolysis of PET with TEA is regarded as a glycolysis reaction catalyzed by a tertiary amine. Therefore, the term aminoglycolysis can be used to describe the process. When the product of this process (i.e., PET/TEA) is used to cross-link epoxy resin it exhibits beneficial technological properties, i.e., low viscosity in compositions with liquid epoxy resins within the temperature range from ambient to ca. 100 °C and a long pot life,7,9 and a relatively wide range of weight ratios in the epoxy resin/hardener system.9 Moreover, it can act as a selfemulsifying cross-linker for liquid epoxy resin/water systems and enables water-thinnable paints to be prepared.8 In addition, the PET/TEA cross-linked materials demonstrate high values of mechanical properties such as flexural, tensile, and impact strengths,7,9 similar to those prepared with a special plastifying rubber or with thermoplastic modifiers.9 Therefore, taking into account the useful features of PET/TEA aminoglycolysate, it seems interesting to investigate the processes of PET aminoglycolysis using other tert-alkanolamines and to evaluate whether such aminoglycolysates can also act as effective epoxy resin hardeners. Experimental Section Materials. Post-consumer PET from bottles in the form of flakes (longitudinal dimension less than 6 mm) was used for the degradation process. Before the reaction, the PET was washed with distilled water and dried at 120 °C for 5 h. The following tertiary alkanolamines were applied: N-methyldiethanolamine (NMDA), N-ethyldiethanolamine (NEDA), N-isopropyldiethanolamine (NIPyDA), N-(n-butyl)diethanolamine (NBDA), N-cyclohexyldieth-

10.1021/ie030356u CCC: $27.50 © 2004 American Chemical Society Published on Web 01/17/2004

Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 863 Table 1. Characteristics of the Tertiary Alkanolamines Used for PET Solvolysis

anolamine (NCDA), triethanolamine (TEA), N-isopropanoldiethanolamine (NIPDA), triisopropanolamine (TIPA), N,N′-bis(2-hydroxyethyl)piperazine (BHEP), N,N,N′,N′-tetra(2-hydroxyethyl) ethylenediamine (THEE-

DA), and N,N,N′,N′-tetra(2-hydroxyethyl)propylene 1,3diamine (THEPDA). Their chemical formulas and origins, together with their hydroxyl value characteristics, are given in Table 1.

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Table 2. Epoxy Products Applied in the Experiments property or source epoxy equivalent (g-equiv/100 g) M h n (g/mol) color viscosity (mPa‚s) source

Epidian 6

DGEBA

188

172

∼390 honey-colored ∼15.000 Chemical Works Organika-Sarzyna, Poland

344 colorless solid molecular distillation; see ref 11

Epoxy Resins. The characteristics of the liquid epoxy resin Epidian 6 and diglycidyl ether of bisphenol A are presented in Table 2. The degradation products of PET with tertiary alkanolamines given in Table 1 were obtained at 190220 °C for 90-240 min. A constant PET/alkanolamine molar ratio of 1:2 was kept, and the products were denoted as PET/NMDA, PET/TEA, etc. (i.e., obtained by N-methyldiethanolamine or triethanolamine solvolysis, respectively). These products were then tested as epoxy resin hardeners. Epoxy resin compositions were prepared by mixing the epoxy resin and the respective hardener at ambient or elevated temperature up to 80 °C for several minutes. Various ratios of hardener to epoxy resin were used. An appropriate amount of hardener was related to the ratio of epoxy groups of the resin per tertiary nitrogen atom of the assumed molecular structure of the hardener (8:1-3:1, usually ca. 10-30 g/100 g of the resin). For this purpose, it was assumed that a particular PET/ tert-alkanolamine degradation product is composed exclusively of the bis(alkanolamine)ester of terephthalic acid (see Scheme 1). In other words, the mentioned epoxy group(s)/N ratio should be considered as a first approximation measure of the component relation in the epoxy compositions. Samples for glass temperature measurements were hardened at 80 °C for 2 h (stage I); after 2 h, they were postcured at 140 °C for an additional 2 h. Methods. Chemical Degradation of PET. This reaction was carried out in a glass reactor. The volatile products containing ethylene glycol, water, and some other byproducts were distilled off during the process. Product Characterization. The products of PET aminoglycolysis were preliminarily characterized by the determination of the hydroxyl values and viscosities. The hydroxyl values were determined using a modified method based on Polish Standard PN-C/89052-03, 1993 (dimethyl sulfoxide was used as the solvent and 1-methylimidazol as the catalyst).10 Viscosities of PET/tert-alkanolamine degradation products were measured at the selected temperatures using an ARES (advanced rheometric expansion system) rheometer (Rheometric Scientific). The cross-linking process was controlled by measurements of the viscosity changes and the heat flow variations during the reaction of a particular Epidian 6/PET/tert-alkanolamine degradation product system, using the ARES rheometer (temperature gradient ) 5 °C/min, cone-plate spacing ) 1 mm, φ ) 50 mm,) and a DSC-7 Perkin-Elmer instrument (temperature gradient ) 10 °C/min), respectively. The glass transition temperatures were determined on the basis of DMTA measurements (Mark II, Polymer Laboratories) (temperature gradient ) 3 °C/min, 1 Hz, torsional strain ) 4%).

FTIR spectra were recorded for thin layers of the composition of DGEBA and PET/NMDA aminoglycolysate, kept between KBr plates at 50 °C, directly after mixing of the components and then after 15, 60, and 120 min of the cross-linking reaction. 1H NMR spectra were recorded on a Brooker DPX 400 spectrometer equipped with a 5-mm 1H/BB-inverse probehead, operated at 400.13 and 100.62 MHz with 0.12 and 0.97 Hz per point for 1H and 13C, respectively. CDCl3 was used as the solvent and tetramethylsilane as the internal standard. Size-exclusion chromatography (SEC) and HPLC were applied for comparative analysis of the PET/TEA product series. An SEC system equipped with a Hewlett-Packard PL Gel (500 Å), 5-µm column, operating with methanol at a flow rate of 0.5 cm3/min; a DRI-Shodex RI-71 refractive index detector and a UV Hitachi Lachrom L-7400 detector; and a Rheodyne 7725i injection valve (20 µL) was used. The HPLC system consisted of a Merck LichroCart 250-4z column filled with LiChrospher 100 RP-8 spherical carrier, 5 µm, operating with methanol. Detectors and other parameters were the same as for SEC measurements. Results and Discussion PET Solvolysis with tert-Alkanolamines. As mentioned earlier, the chemical degradation of PET with tert-alkanolamines can be considered as a glycolysis reaction catalyzed by tertiary nitrogen atoms. The catalytic influence of the tertiary nitrogen on both the esterification and deesterification reactions is generally known.12,13 The aminodiester derivatives of terephthalic acid are formed during the solvolysis with an excess of tert-alkanolamines (Scheme 1, part A). However, side reactions, such as intra- and intermolecular dehydratation at high temperature (above 190 °C), cause the formation of morpholine rings from adjacent 2-hydroxyethyl groups and/or ether intermolecular links between hydroxyethyl moieties of the species present in the system14-16 (Scheme 1, part B). These etherification side reactions cause the PET aminoglycolysate hydroxyl value to decrease. In addition to the OH-bearing terephthalic esters of tert-alkanolamines, ethylene glycol (EG, as the solvolysis byproduct) can also take part in the etherification reactions. For the sake of clarity, possible etherification reactions involving EG are not taken into account in Scheme 1. The solvolytic activity of tertiary alkanolamines toward the PET ester bond should depend on their basicity, on the hydroxyl group concentration, and on the types of substituents placed on the carbon atoms adjacent to the nitrogen atoms. The first two factors for a particular tert-alkanolamine can be characterized by pKb (or pKa) and hydroxyl value, respectively. Unfortunately, accurate data on pKb (or pKa) are not available for the majority of the applied tert-alkanolamines.17 The third parameter expresses the access of the OH groups to the polymer backbone ester bonds and the catalytic influence of tertiary nitrogen atom(s) on the C-O bond cleavage (steric factor). There is no a simple measure of the steric factor. It seems that the differences in the molar volume for R substituents in a particular tertalkanolamine molecule (see Scheme 1) used for PET aminoglycolysis can serve as a measure of the steric effect.

Scheme 1. Scheme of PET Aminoglycolysis with Tertiary Alkanolamines and Dehydration of the Products

Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 865

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Table 3. Molar Volumes of the N Substituents in the Tertiary Alkanolamines Used for PET Degradation

a

Two types of 2-hydroxyethyl groups (in various surroundings).

Table 4. Conditions of PET Aminoglycolysis Processes and Description of the Aminoglycolysate Products

no. 1

alkanolamine N-methyldiethanolamine

short description of product PET/NMDA

2 3 4 5

N-ethyldiethanolamine N-isopropyldiethanolamine N-(n-butyl)diethanolamine N-cyclohexyldiethanolamine

PET/NEDA PET/NIPyDA PET/NBDA PET/NCDA

6

triethanolamine

PET/TEA

7

triisopropanolamine

PET/TIPA

8

N-isopropanoldiethanolamine

PET/NIPDA

9

N,N′-bis(2-hydroxyethyl) piperazine N,N,N′,N′-tetra(2-hydroxyethyl) ethylenediamine N,N,N′,N′-tetra(2-hydroxyethyl) propylene 1,3-diamine

PET/BHEP

10 11 a

PET/THEEDA PET/THEPDA

reaction temperature (°C)

reaction time (min)

viscosity at 25 °C (Pa‚s) 11.9a

hydroxyl value (mg of KOH/g) measured

theoretical

480a

304

190 205 205 190 205 190 205 190 205 205 205 220 190 210 205 210 205

180 90 90 120 90 120 90 120 90 120 180 120 240 90 120 140 90

22 093.0 4.5 9.2 2.3 72.1 238.0 556.0 17.6 37.2 36.8 60.9 1211.0 1496.0 37.0 58.0 6875.0

175 299 284 288 238 212 327 296 273 190 121 422 315 362 307 196

205 220 205

120 120 180

26.2 456.4 1.6

426 366 447

282 279 248 224 523

383 491 234 558 533

PET not completely degraded.

Calculations of the substituent molar volumes were performed using the method of group contributions to the molar volume according to Fedors.18,19 These data are presented in Table 3. Although the rate of PET degradation was, to some extent, different for a different tert-alkanolamines under the same experimental conditions, nevertheless, it was impossible to quantify the differences. On the other hand, significant differences in the reactivities of aminoglycolysis products toward low-molecular-weight epoxy resins were found. These aspects are described and discussed in the next part of the paper.

During the PET degradation process, some volatiles (ethylene glycol, water, and other components) were distilled off from the reaction system. The final products were obtained as brown-colored semisolid or viscous liquid products. Characteristics of the PET aminoglycolysis processes and the reaction products are given in Table 4. The products of PET aminoglycolysis reactions performed at 190 °C exhibit substantially lower viscosities and higher hydroxyl numbers than those obtained at higher temperature for the same reaction time (Table 4). On the other hand, some of the products of the PET


Figure 1. Size-exclusion chromatograms of the PET/TEA products from the processes performed under various experimental conditions.

aminoglycolysis performed at the lowest temperature (190 °C) still contained incompletely degraded polymer particles. It was also observed that the evolution of volatiles from the reactor at 190 °C was rather low or even no volatiles were obtained. It seems that, under such experimental conditions, the vaporization of ethylene glycol, which is the major byproduct of the solvolysis, is rather low (boiling point ≈ 197 °C) and the dehydratation reactions involving the OH groups are relatively slow (almost no water formation). This could also be confirmed by the higher-than-expected (i.e., theoretical) hydroxyl values that were measured for the products obtained at 190 °C (Table 4). Analysis of the SEC and HPLC R-8 elution curves recorded by the two detectors (DRI and UV) (Figures 1 and 2, respectively) demonstrated that PET/TEA aminoglycolysates are multicomponent mixtures exhibiting some differences. Four SEC peaks in a range of retention time (RT) up to ca. 22 min are located in the region of separation based mainly on the size-exclusion mechanism.20 Above RT ≈ 22 min, the separation in the SEC column is affected to a significant extent by the adsorption of the solutes from the methanol eluent onto the styrene-divinylbenzene copolymer gel. Five peaks representing at least the same number of chemical com-

Figure 2. Liquid chromatography (RP-8) traces of the PET/TEA products from the processes performed under various experimental conditions.

pounds can be found in the separated aminoglycolysates in Figures 1 and 2 (they are especially clear on the UV traces). The presence of species at the lowest retention time in Figure 1 (i.e., the highest-molecular-weight components), ca. 16.5 min, decreases in the order from the product obtained under the mildest conditions (205 °C/1.5 h) up to the product obtained under the more severe parameters (220 °C/2 h). It can be expected that the highest-molecular-weight species present on SEC chromatograms are the result of incompletely degraded polyester chains. The corresponding ethylene terephthalate oligomers ended mainly by TEA moieties (via ester bonds) undergo further degradation in higher temperature/time regimes, and then the respective SEC peaks diminish or disappear. These data show the progress of the chain scission of the PET backbone during aminoglycolysis. The peaks of the most polar substances eluted first on the RP-8 column (Figure 2) appear in the range RT ≈ 4.1-4.2 min. However, these peaks are shifted with respect to the each other. The order of RT is PET/ TEA 205 °C/1.5 h < PET/TEA 205 °C/2 h < PET/TEA 220 °C/2 h. The shifts of the HPLC peaks and their decreasing intensities exhibit the same order (see Figure 2A, UV traces), which allows us to conclude that the

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Figure 3. FTIR spectra of the PET/TEA (205 °C/2 h) product.

Figure 4. NMR spectra of the PET/TEA (205 °C/2 h) product.

longer the process and the higher the temperature, the lower the polarity of the final PET aminoglycolysate. The SEC and HPLC data are in accordance with the results of the hydroxyl value and viscosity measurements. All of these results are consistent with the expectation on inter- and intramolecular dehydration reactions involving the OH groups and causing branching and a decrease of the polarity of the species present in the system (see Scheme 1). Examples of FTIR and 1H NMR spectra for the selected PET aminoglycolysate [PET/TEA (205 °C/2h)] are presented in Figures 3 and 4, respectively. The FTIR spectrum exhibits a broad band in the range 3000-3600 cm-1 (caused by, among others, the hydroxyl groups), 1720 cm-1 (carboxyls), 1500-1650 cm-1 (amide groups), 1170-1300 cm-1 (ether and ester bonds), and 1000-1150 cm-1 (ether links of the type -CH2CH2OCH2CH2O- and hydroxyls). Reasons for the absorbance of the amide band are the presence of monoand diethanolamine in the applied triethanolamine and the formation of the former from TEA at reaction temperatures above 200 °C. As a result, amide derivatives of terephthalic acid can be formed. The 1H NMR spectrum (Figure 4) exhibits three ranges: 8.2-7.6 ppm (aromatic protons and protons of

CH2 groups bonded to O or N heteroatoms and OH protons), 4.2-4.8 ppm, and 3.3-2.4 ppm. The latter are attributable to protons of CH2 groups bonded to O or N heteroatoms and OH protons.14 The NMR spectrum shows that the PET/TEA product is a mixture of numerous chemical compounds. PET Aminoglycolysates as Epoxy Resin Hardeners. Tertiary amines are seldom applied as independent cross-linking agents for epoxy resins. More often, they are used as accelerators when carboxylic acid anhydrides are employed as hardeners. A mechanism for epoxy resin cross-linking in this case is the anionic polymerization accelerated by the hydroxyl groups present in the tertiary amine molecules and/or in the epoxy oligomers.21-25 Nonstoichiometric amounts of tertiary amines are applied for epoxy resin cross-linking; recommended amounts are 10-15 parts per 100 parts of the resin (phr)24 or a molar ratio of 6:1-8:1 epoxy groups of the resin per nitrogen atom of the hardener.21 In our earlier works,6,7,9 we found that the product of PET aminoglycolysis with triethanolamine can be used as an effective epoxy resin hardener for various types of liquid epoxies derived from bisphenol A, bisphenol F, and novolac resins. Hardened epoxy materials based on Epidian 6 or Araldit F resins exhibit good mechanical

Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 869 Table 5. Comparative Thermal and Heat Flow Data for Epidian 6 Compositions with the Selected PET Aminoglycolysates at Various Ratios from DSC Measurements reaction temperature (°C) no.

composition

epoxy groups per N atom

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

E6/PET/NMDA 8 E6/PET/NMDA 7 E6/PET/NMDA 6 E6/PET/NMDA 5 E6/PET/NMDA 4 E6/PET/NEDA 8 E6/PET/NEDA 5 E6/PET/NEDA 4 E6/PET/TEA 8 E6/PET/TEA 7 E6/PET/TEA 6 E6/PET/TEA 5 E6/PET/TEA 4 E6/PET/BHEP 6 (12b) E6/PET/BHEP 5 (10b) E6/PET/BHEP 4 (8b) E6/PET/BHEP 3 (6b) E6/PET/BHEP 2 (4b) E6/PET/THEPDA 6 (12b) E6/PET/THEPDA 5 (10b) E6/PET/THEPDA 4 (8b) E6/PET/THEPDA 2 (4b)

8:1 7:1 6:1 5:1 4:1 8:1 5:1 4:1 8:1 7:1 6:1 5:1 4:1 6:1 (12:1) 5:1 (10:1) 4:1 (8:1) 3:1 (6:1) 2:1 (4:1) 6:1 (12:1) 5:1 (10:1) 4:1 (8:1) 2:1 (4:1)

hardener content (phr)

start

end

at maximum rate

reaction enthalpy (J/g)

12.1 13.8 16.1 19.3 24.2 13.0 20.8 26.0 14.4 16.5 19.2 23.0 28.8 10.5 12.6 15.7 20.9 31.5 13.9 16.7 20.8 41.7

88.6 83.7 101.9a 89.3 89.7 116.7 96.6 116.2 116.9 104.8 103.7 105.1 105.7 84.6 96.8 89.6 87.8 78.4 125.4 107.0 100.5 113.3

159.7 162.0 162.0 163.6 160.7 188.7 205.9 200.9 153.3 164.4 154.3 154.9 161.0 161.1 163.4 163.6 168.9 170.0 168.6 172.4 160.0 160.0

125.2 129.6 128.1 132.3 129.3 155.9 151.1 155.4 136.7 135.9 134.5 134.3 137.5 126.6 132.5 129.8 136.5 112.6 147.2 145.2 132.9 145.3

-204.8 -233.7 -245.1 -263.0 -278.8 -34.6 -175.7 -260.6 -85.2 -123.7 -125.7 -145.7 -183.8 -81.4 -82.6 -97.8 -149.8 -138.1 -60.4 -87.1 -85.7 -59.0

a Value seems anomalous. b Values in parentheses are numbers of epoxy groups per active N atom of the PET/BHEP or PET/THEPDA aminoglycolysate.

Figure 5. Typical rheometric curves for the system Epidian 6/PET/tert-alkanolamine aminoglycolysates from 205 °C. A description of the compositions is provided in Table 5.

(flexural, tensile, and impact strengths) and thermomechanical properties, for a rather broad range of hardener/epoxy resin weight ratios (∼14-28 phr).9,14 In this work, an evaluation of the cross-linking activity for the series of PET/tert-alkanolamine degradation products toward epoxy resin Epidian 6 was performed. The cross-linking reactions in the systems involving epoxy resin/PET/tert-alkanolamine degradation products were followed by rheometry, differential scanning calorimetry, and 1H NMR and FTIR spectroscopies. Typical rheometric curves, i.e., viscosity changes during heating of the representative epoxy compositions (E6/PET/NMDA, E6/PET/BHEP, and E6/PET/NCDA), are presented in Figure 5. A constant ratio equal to 4:1 epoxy groups per nitrogen atom of the respective PET degradation products was used in all of these systems. Two of the compositions exhibit a high viscosity increase at temperature ranges of 115-120 °C (E6/PET/NMDA) and 125-130 °C (E6/PET/BHEP), which indicates the oc-

currence of gelation. The third system based on Epidian 6 and PET/NCDA does not react at temperatures up to 200 °C under the experimental conditions investigated. In similar tests, some other PET degradation products were found to act as epoxy resin hardeners, namely, PET/NEDA, PET/THEEDA, PET/THEPDA, and PET/ TEA (the last product was tested as an epoxy resin hardener in our recent works6-9,14). However, other PET/tert-alkanolamine degradation products with bulky substituents in the direct neighborhood of nitrogen (in addition to PET/NCDA, also PET/NIPyDA, PET/NBDA, PET/NIPDA, and PET/TIPA) did not act as the hardeners under the experimental conditions of our rheometric and microcalorimetric measurements. To compare the cross-linking efficiency and behavior of the epoxy compositions with various ratios of the reactive aminoglycolysates, rheometric and differential scanning calorimetric measurements were made. Five aminoglycolysates were selected for comparative investigations, namely, PET/NMDA, PET/NEDA, PET/ TEA PET/THEPDA, and PET/BHEP. The ratios of the hardeners to Epidian 6 resin were kept in a broad range of ca. 13-32 g per 100 g of the resin (Table 5). The two sets of rheometric curves for PET/NMDA and PET/BHEP are presented in Figure 6. It can be seen from Figure 6 that the rheometric characteristics of the E6/PET/NMDA compositions are rather similar. The dependences for the E6/PET/BHEP systems are, to some extent, different. However, small shifts in the composition minimum viscosities and the corresponding temperatures preceding the gelation point to higher values are observed in both cases as the hardener content increases. Figure 7 shows an example of a DSC thermogram (for E6/PET/NMDA 6 composition) with the characteristic temperatures of the start, end, and maximum of the cross-linking reaction, as well as the heat of the reaction. A similar investigation for E6/PET/TEA has already been performed, and the results were published elsewhere.9 These results, together with those for E6/PET/

870


Figure 8. Diagram of hydroxyl value versus nitrogen substituent molar volume for PET aminoglycolysates (b, active; O, inactive as an epoxy resin hardener).

Figure 6. Set of rheometric curves for the systems (A) Epidian 6/PET/NMDA and (B) Epidian 6/PET/BHEP (hardeners used in various amounts). A description of the compositions is provided in Table 5.

Figure 7. DSC thermogram for the E6/PET/NMDA 6 composition. A description of the compositions is provided in Table 5.

NMDA, E6/PET/NEDA, E6/PET/BHEP, and E6/PET/ THEPDA, are collected in Table 5. A comparison of the thermal and heat data from Table 5 allows the following conclusions to be formulated: (i) The most reactive material is the PET/NMDA product (cross-linking reaction enthapy in the range of ca. 205280 J/g). (ii) The most significant differences in the reaction enthalpies for the epoxy compositions differing in epoxy resin/hardener ratios (from 8:1 to 4:1 epoxy groups per nitrogen) were found for the E6/PET/NEDA system (ca. 35 f 260 J/g). (iii) The reactivities of the

E6/PET/BHEP and E6/PET/TEA compositions are similar if the same epoxy group/active nitrogen ratios are considered. For steric reasons (see Table 3), some aminoglycolysates, i.e., PET/BHEP and PET/THEPDA, exhibit only half of the active nitrogen content in epoxy cross-linking reaction (active nitrogens, Table 5). (iv) The cross-linking reaction starts with a measurable rate (under DSC measurement conditions) at a temperature of about ca. 84-116 °C. According to literature data, the reaction rate of the homopolymerization of epoxy compounds increases along with increasing steric accessibility of the nitrogen in the tertiary amine, basicity, and hydroxyl group acidity.21,26 Two of these parameters seem to be particularly important in their influence on PET aminoglycolysates catalytic activity toward epoxy resin polymerization, i.e., in the order of influence, the steric accessibility of the nitrogen atoms [expressed here by the molar volume of the substituents (SMV)] and the hydroxyl value. The remaining parameter, i.e., basicity, plays only a minor role in the discussed context. In the case of PET aminoglycolysates, the OH groups are inherently incorporated into their molecules. Figure 8 presents a diagram of hydroxyl value (HV) versus SMV for the alkanolesters of terephthalate acid (see data in Scheme 1, Tables 3 and 4), constructed on the basis of the calculated data. The entire field of the diagram is divided into two parts: below an SMV of ca. 50 cm3/mol are located the points for real epoxy resin hardeners, and above ca. 58 cm3/mol are placed the points for PET aminoglycolysates inactive in the epoxy cross-linking reaction. It also seems that the value of HV does not play as important a role as SMV does. The diagram in Figure 8 shows that the bulky substituents with SMVs above ca. 50 cm3/mol in the neighborhood of nitrogen atoms of PET aminoglycolysates make steric barriers, thus negatively influencing their catalytic activity in the anionic polymerization of the epoxy resin. The glass temperatures of the epoxy materials hardened with PET/NMDA, PET/BHEP, and PET/TEA determined by DMTA measurements are within the range of ca. 100-122 °C (Figure 9). The results from Figure 9 indicate that the Tg values of the epoxy materials are dependent on the type of hardener used. Almost-constant values were found for the E6/PET/ BHEP systems (102-103 °C), whereas the Tg values increases with the extent of PET/TEA hardener in the E6/PET/TEA system (103-122 °C) and, in contrast,


Figure 9. Glass transition temperatures of the epoxy materials hardened with various ratios of the selected PET aminoglycolysates.

decreased with increasing amount of PET/NMDA hardener in the respective epoxy materials (99-115 °C). To elucidate the influence of the hardener type on the thermomechanical properties of the epoxy materials, independent experiments should be performed. Epoxy Resin Cross-Linking Reaction as Followed via FTIR Spectroscopy. A mechanism for the anionic polymerization reaction of epoxy compounds catalyzed by the tertiary amines has been investigated by numerous authors.24-28 The hydroxyl groups necessary to start the reaction and to achieve its acceleration can be introduced into the system in the following way: (i) alcohol or phenol additive; (ii) presence in higher-molecular-weight epoxy resins or formation as a result of epoxy ring opening, thus forming secondary hydroxyls; or (iii) tertiary alkanolamine. It is generally accepted that the mechanism of epoxy compound polymerization catalyzed by tertiary amines proceeds through zwitterions R3N+CH2-CH(O-)-R1, where quaternary ammonium alcoholates are the active sites.26

On the other hand, Funashi29 also studied the ringopening reactions of oxirane with aryl carboxylates catalyzed by tertiary amines and found that these reactions proceed through zwitterions of quaternary ammonium alcoholates. As a result of epoxy resin homopolymerization catalyzed by tertiary amines, an accumulation of hydroxyl, ether, carbonyl, and unsaturation band intensities is observed, along with a decrease of the epoxide band.26,30 FTIR spectra recorded during the cross-linking reaction of the diglycidyl ether of bisphenol A (DGEBA) with PET/NMDA at 50 °C (Figure 10) revealed increasing intensities of the above-mentioned bands, i.e., ether bonds (ca. 1120 cm-1), carbonyls (ca. 1720 cm-1), double bonds (ca. 1580 cm-1), and hydroxyl groups (3360-3335 cm-1), as a function of the reaction time (Figure 11). However, the most intense increase in IR bands was found for the regions 1404-1410 and 1559-1550 cm-1 (Figure 12). The first of these bands are assigned to the secondary hydroxyl groups and the stretching vibrations of the aromatic ring. The second bands are assigned to the ester groups with the N atom placed in the γ position [-N+-CH2-CH(OCO)-R]. The former bands with a relatively high intensity (∼400% of that for the starting mixture of the two components, i.e., DGEBA and PET/NMDA) are characteristic of higher-molecular-weight epoxy resins, e.g., Epikote 100131 or the ring-opening derivatives of DGEBA. In this range, high-temperature-degraded polyesters, such as poly(ethylene terephthalate) and poly(diethylene glycol terephthalate),32 also absorb. The latter, which corresponds to the N-ester band is present in PET/NMDA aminoglycolysate (1559 cm-1). However, its intensity substantially increases while its position is shifted to ca. 1550 cm-1 for the reaction products with DGEBA (increase of ∼580% after 120 min at 50 °C) in comparison with the band of the initial E6/ PET/NMDA composition. The reasons for the above findings are associated with the transesterification reactions proceeding between bis(N-esters of terephthalic acid) (i.e., PET/NMDA component) and hydroxyl

Figure 10. FTIR spectra of DGEBA, PET/NMDA, and their compositions DGEBA/PET/NMDA 4 after various times of reaction at 50 °C: 30 and 120 min.

872


Scheme 2. Scheme of the Idealized Crosslinked Network of DGEBA/PET tert-Aminoglycoly satea

a

Four epoxy groups per nitrogen atom.

derivatives of DGEBA (Scheme 2). A tendency toward a similar transesterification has been claimed for other low-molecular-weight systems, e.g., phenylglycidyl ether/ triethylamine26 or resinous/polymeric ones, e.g., bisphenol A epoxy resin/benzyldimethylamine/polycarbonate,33 tetraglycidyl-4,4′-diaminodiphenylmethane/polycarbonate (here, N-epoxy resin acts itself as autocatalyst),34 and PET/polycarbonate.35 The FTIR band at ca. 1550 cm-1 can also be seen in the cited work,34 although it is neither identified nor discussed.

On the basis of literature data and our FTIR measurements, a scheme for epoxy resin network formation in the presence of the active PET aminoglycolysates is proposed (Scheme 2). It can be seen from Scheme 2 that the aminoester derivative of terephthalate acid (TA) and tertiary alkanolamine is bonded within the polymer network of the cross-linked epoxy resin, even if the transesterification of the primary TA ester (of tertiary alkanolamine) proceeds with the hydroxyl derivatives of DGEBA. This is a special feature of the PET aminoglycolysates used as epoxy hardeners distinguishing


Figure 11. FTIR band intensities versus reaction time for the DGEBA/PET/NMDA 4 system at 50 °C: ether links (A1120/A1120(0)), double bonds (A1580/A1580(0)) carbonyl groups (A1720/A1720(0)), and hydroxyl groups (A3360f3335/A3360(0)).

elevated temperatures (up to ca. 80 °C) and rather low viscosities before gelation. These products can be used for epoxy resin hardening in broad ratios of the components. The major factor determining PET aminoglycolysate catalytic activity toward epoxy resin is the accessibility of N atoms. The FTIR spectra revealed that the transesterification of the bisaminoesters of terephthalic acid by the hydroxyl derivatives at DGEBA proceeds during the polymerization reactions of epoxy resins catalyzed by PET aminoglycolysates. PET aminoglycolysate used as a catalyst is incorporated into the epoxy network, causing its modification, e.g., upgrading its impact strength. Acknowledgment This work was supported by The State Committee for Scientific Research (Warsaw, Poland), Grant 3 T09B 089 19. Literature Cited

Figure 12. FTIR band intensities versus reaction time for the DGEBA/PET/NMDA 4 system at 50 °C: ester groups (A1559f1550/ A1559(0)) and secondary hydroxyl groups (A1414f1404/A1414(0)).

them from conventional low-molecular-weight tertiary amines. As a result of these findings, such aminoesters of TA-modified epoxy matrixes thus favorably influence some useful properties, especially the impact strength.9 Such an upgrading of the epoxy materials is usually obtained by using other special hardeners (polyamineamides of dimerized fatty acids) or physically incorporated elastomers (e.g., liquid rubbers, thermoplastic polyesters etc.). Conclusions Waste poly(ethylene terephthalate) can be readily degraded to low-molecular-weight or oligomeric products by tertiary alkanolamines at temperatures above 190 °C. These products comprise mixtures of aminoesters of terephthalic acid. The products of PET aminoglycolysis are viscous liquids with different viscosities and hydroxyl values. Higher viscosities and lower hydroxyl values were found as the temperature and reaction time were increased. These features are results of the side reactions, including dehydration, that occur in the PET/tert-alkanolamine system at high temperatures above 190 °C. Some products of PET chemical degradation with tertiary alkanolamines can be used as epoxy resin hardeners for curing at elevated temperature. The most active PET aminoglycolysates are terephthalic derivatives of N-methyl- and N-ethyldiethanolamines, triethanolamine, and bis(2-hydroxyethyl)piperazine. Compositions of liquid epoxy resin with PET aminoglycolysates exhibit long pot lives at ambient and

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Received for review April 24, 2003 Revised manuscript received October 20, 2003 Accepted November 5, 2003 IE030356U

Tertiary Alkanolamines as Solvolytic Agents for Poly(ethylene

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