Ind. Eng. Chem. Res. 2011, 50, 1523–1529
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New Acrylated Oligomers with a Sulfide Group for Radiation-Curable Coatings James H. Aerykssen and Igor V. Khudyakov* Bomar Specialties, Torrington, Connecticut 06790, United States
Dendritic oligomers were prepared using the Michael reaction of thiol with acrylates taken in excess. Liquid oligomers A-D demonstrate exceptional shelf life. When cured, the oligomer produces a coating with relatively high tensile strength and hardness that is akin to high viscosity dendritic urethane acrylates. Oligomers A-C demonstrate a balance of properties in between the developed in our lab earlier dendritic polyester acrylates, BDE-1025 (low viscosity, lower hardness) and the dendritic urethane acrylates D and XDR-1416 (high viscosity, high hardness). Introduction The radical step-growth addition reaction of thiols across double bonds and triple bonds has gained much attention in the recent years.1-6 Among the many interesting properties that these “click” reactions are noted for is the lack of dioxygne inhibition and self-initiation, that is, the ability to photopolymerize without the need of photoinitiators. Polymers produced by thiol-ene polymerization possess valuable properties. Despite active fundamental research in thiol-ene photopolymerization, practical applications of such systems are limited. Unfortunately, due to the affinity of thiols to enes even in the absence of light and at ambient temperatures, these monomers/ oligomers have a relatively short shelf life of only a few weeks. This is consistent with most other thiol-ene systems whose instability has been documented elsewhere.3,4 Apparently, much of the research on thiol-ene systems to date has used freshly prepared mixtures of thiol and ene. However, the coatings industry generally requires a shelf life of six months or more at ambient temperature. To overcome this problem, several methods have been proposed to increase the shelf life stability of the thiol-ene system with the most successful being the use of hindered, secondary thiols.4 We have independently corroborated this result4 and applied it to our thiol-ene oligomers to produce oligomers with a shelf life of much greater than six months. Regrettably, the high cost of the secondary thiols makes these reagents commercially impractical. We decided not to fight the inevitable, namely a dark reaction of primary thiols with acrylates.7 We prepared products of such reactions and studied the products in photopolymerizable performance coatings. Experimental Section Devices and Techniques. Properties of the high MW products were analyzed via GPC, which gave molecular weights as weight average and number average (Mw and Mn) values, and the molecular weight distribution (MWD) was given as8 MWD ) Mw/Mn We used a Polymer Laboratories PL-GPC 50 chromatograph with a RI detector, along with two Polymers Laboratories mixed-D columns, and set the column temperature to 40 °C. We used stabilized THF as an eluent, running at a rate of 1.0 mL/min. We used Polymer Laboratories Cirrus Software, version 3.0, to analyze the data, and we used a 10-point * To whom correspondence should be addressed. E-mail: ikhudyakov@ bomarspecialties.com.
calibration curve that was based on polystyrene (Easical PS-2 standards from Polymer Laboratories). The IR spectrometer was a Perkin-Elmer Spectrum One model with a diamond-crystal universal attenuated total reflectance (UATR) accessory. The viscosity (η) was measured with a Brookfield RVT viscometer with a small adapter (spindle SC415 and cup 7R) that was connected to a Neslab circulating water bath at 25 or 50 °C. The tensile properties of cured samples (elongation to break, tensile strength at break, tensile modulus) were measured with a Cheminstruments Tensile Tester-1000 system. The test method was designed to be in compliance with ASTM Standard D 882. The tester was controlled by the Cheminstruments EZ-Lab system program. At least five samples of each cured formulation were studied at ambient temperature, to verify reproducibility of the data. Nuclear magnetic resonance (NMR) spectra were taken with a Varian VXRS 300 MHz NMR spectrometer that was operated at 299.94 and 75.43 MHz for 1H and 13C, respectively, with typical acquisition parameters. NMR spectra were obtained at ambient temperature in CDCl3. 13C NMR spectra were obtained in the presence of paramagnetic chromium(III) acetylacetonate Cr(acac)3, to ensure quantization of the carbon signals. For an additional confirmation of structures of our products we ran 1 H-1H COSY experiments and took DEPT spectra. For the basics of these two techniques (see for example, ref 9) CDCl3 was a solvent in NMR experiments. The relative concentration of fragments in oligomers was obtained by comparing integrals of corresponding signals in 13C NMR spectra. We used ACD/ Laboratories NMR software to predict the NMR spectra and Sadtler database. Synthesized in this work oligomers were diluted by isobornyl acrylate (30.0 wt %) prior to photopolymerization. Irgacure 184, at a concentration of 2.0%, was added as photoinitiator to all samples. Samples were cured with a Fusion 300 W/in. processor with D-bulb in the air. Two passes under the lamp at a conveyor speed of 20 ft/min were performed for all cure experiments. Dynamic mechanical properties were measured with a TA Instruments DMA 2980 controlled stress rheometer. After measurement of the linear viscoelastic region for each material, strain was selected at 1%. The temperature interval was -50 to 100 °C, with a ramp of 2 °C/min. The glass transition temperature (Tg) values of the cured oligomers were of interest. In accordance with a common practice, we identified Tg as a temperature where the relation tan δ reaches a maximum: tan δ ) G′′/G′, where G′′ is a shear loss modulus and G′ is a shear storage modulus. Thermogravimetric analysis (TGA) was performed with TA Instruments TGAQ-500 in the air. The temperature ramp was
10.1021/ie101816b 2011 American Chemical Society Published on Web 12/23/2010
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Figure 1. 1H NMR spectrum of oligomer A and a chemical structure corresponding to observed signals.
Figure 2. 13C NMR spectrum of oligomer A and a chemical structure corresponding to observed signals.
10 °C/min, from room temperature to 700 °C. Chemical resistance to solvent was determined in a manner based on ASTM Method D 5402-93 using methyl ethyl ketone (MEK) as a solvent. A maximum of 200 MEK double-rubs was performed. Chemical resistance to common acids, bases, or staining materials was determined in a manner based on ASTM Method D 1308-87 with a 1.0 mil coating being applied to a stainless steel substrate. The test areas were covered, and the materials were allowed to sit for a maximum 24 h after which the coating was examined for any defect in color, gloss, or integrity. Reagents. We used the following reagents. Ethoxylated pentaerythritol tetraacrylate ePETA or SR-494,
Figure 3. The COSY spectrum of oligomer A.
isobornyl acrylate IBOA or SR-506A, trimethylopropanetriacrylate TMPTA or SR-351HP, tripropyleneglycol diacrylate (TRPGDA) or SR-306HP, tris(2-hydroxyethylisocyanurate triacrylate THIA or SR-368.
1,1′-dimercaptodiethyl sulfide DMDS from Ingeieria y Ciencias Aplicadas (Mexico),
1-thioglycerol was from Aldrich, 2-(methacryloyloxy)ethyl isocyanate MOI from Showa Denko, N- methyldiethanol amine MDEA from Aldrich, dibutyltin dilaurate DBTDL from Atofina, and Irgacure 184 from of Ciba Additives. All compounds were used as received. All SR compounds were from Sartomer. We used also mercaptoacetic acid 1,2-ethanediyl ester or glycol dimercaptoacetate GDMA from Evans Chemetics,
Results and Discussion Synthesis of Oligomers and Their Properties. We prepared dendritic (hyperbranched) (meth)acrylated oligomers in catalyzed addition reactions. All reactions were solventless. Oligomer A was prepared by a reaction of TMPTA with GDMA (ratio equivalents of acrylate to thiol groups was 2.5:1.0). We undermine here and below by ratio of reagents the ratio of
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Table 1. Properties of Oligomers A-D
a
Mw, kg/mol MWDa f η, Pb d
A
B
C
D
8 5.3 18 550c
9 5.4 20 440c
25 10.5 72 300c
2 1.1 6 700d
a Determination error 15%. At 50 °C.
b
Determination error 5%.
c
At 25 °C.
Figure 4. The DEPT spectrum of oligomer A.
The oligomer is terminated with R1 ) acrylate group:
Figure 5. IR spectrum of A. Acrylate absorption at 809 cm-1 is marked.
We estimated average acrylate functionality f of A assuming that the molecule has a symmetry axis. Each end of the difunctional thiol repeatedly adds to an acrylate group of trifunctional TMPTA creating an ever branching dendritic structure. The composition of the structure should consist of n moles of triacrylate, n - 1 mols of dithiol, and have an acrylate f ) n + 2. Counting the equivalents that form thioether links, the equivalent ratio of acrylate/thiol is approximately 1.5:1 as the dendritic structure increases in size. Dendritic structures whose branches grew at the same rate should occur at f equal to 2x × 6, where x ) 0, 1, 2, 3, etc. On the basis of Mw of A and Mw of its fragments GDMA and TMPTA, we got f, (Table 1). Scheme 1 presents a generic structure of A. The reaction of thiol with acrylate is a Michael reaction. It occurs as an anti-Markovnikov addition.6,7 In the present work we mixed an excess of acrylates to thiols (in terms of equivalents). That allowed the preservation of acrylate functionality and obtaining a (photo)polymerizable product that we used in this work of difunctional thiols which allowed oligomerization of the reagent. Such reactions were performed at 65 °C under stirring for several hours in air in the presence of 5.10-3 M of MDEA (Scheme 2). Were the reaction to have proceeded by Markovnikov addition a quartet would have arisen due to splitting from the -CH3, and the relative intensities would have been 2:1. Instead, the 1 H sulfide fragment contains two signals of approximate equal intensity appearing as a singlet and triplet. This confirms the mechanism of Scheme 2.
equivalents. NMR spectra, IR spectrum, and a GPC trace of A are presented in Figures 1-6. Based on NMR data (Figures 1-4) we conclude that A has the following structure:
where R is a repeating unit of the following structure:
Figure 6. GPC trace of A.
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Scheme 1. Generic Structure of Oligomer A with f ) 18a
a
It is assumed that A has axial symmetry. The central triacrylate of each f ) 6 fragment is in bold. Internal ester groups are omitted.
Scheme 2. The Mechanism of Thiol-Acrylate Addition Catalyzed by MDEA (Further Abbreviated as B Here)
The reaction (Scheme 2) was deemed complete when the thiol IR peak3 at 2570 cm-1 was no longer detectable (Figure 5) and no change was observed in the GPC chromatogram (Figure 6). NMR data (Figure 1, 2) confirmed completeness of a reaction of acrylate with thiols: the intensity of H-signal adjacent to SHgroup (signal 1) is very low, Figure 1. That testifies to fact of completeness of addition. Obtained values of Mw, MWD, and η for A are presented in the Table 1. The GPC trace of A demonstrates peaks 2-4 besides the main peak 1 (Figure 6). Peak 4 has the same retention time as TMPTA, and it corresponds to a low amount of nonreacted TMPTA. On the basis of Mw corresponding to peaks 2 and 3, we conclude that they belong to adducts with acrylate functionalities f ) 6 and 4, respectively. Products of higher functionalities with the average f ) 18 (peak 1) consists of 70% of all products. Oligomer B and C are similar to oligomer A. B is a product of reaction of TMPTA with DMDS, taken in equivalents of acrylate group to thiol group as 2.5:1.0. We describe B, C, and D in less detail than A. We obtained corresponding character-
Figure 7. 1H NMR spectrum of oligomer B and a chemical structure corresponding to observed signals.
istics of B and C like those obtained for A and estimated acrylate functionality of B and C the same way as for A, (Table 1 and Figures 7-10). Oligomer C (Table 1) is a product of a reaction of ePETA with GDMA in equivalent ratio 3.5:1.0. NMR spectra and a chemical structure of C are presented in Figures 11-14. It is worth remembering that oligomers A-C are products of a reaction of ditiol with triacrylate or tetraacrylate. Oligomer D is not dendritic acrylate but methacrylated dendrimer. First, we got a product of reaction of THIA (cf., the Experimental Section above) with 1-thiolglycerol. That reaction led to a formation of hexol. Hexol was reacted with a stoichiometric amount of MOI with a formation of D (Scheme 3).
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Figure 8. C NMR spectrum of oligomer B and a chemical structure corresponding to observed signals.
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Figure 11. 1H NMR spectrum of oligomer C and a chemical structure corresponding to observed signals.10
Figure 12. 13C NMR spectrum of oligomer C and a chemical structure corresponding to observed signals.10
of 2230 cm-1 peak of-NCO.11 As expected, D is an individual compound with the MWD being close to 1.0 (Table 1).
Figure 9. The COSY spectrum of oligomer B.
Figure 10. The DEPT spectrum of oligomer B.
A common catalyst of urethane synthesis DBTDL11 was added to the mixture of hexol and MOI in the concentration of 5 × 10-3 M. The reaction with MOI was performed at 60-80 °C. Completion of the reaction was verified by IR: disappearance
Figure 13. The COSY spectrum of oligomer C.
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Figure 14. The DEPT spectrum of oligomer C.
Scheme 3. Chemical Structure of D
Figure 16. 13C NMR spectrum of oligomer D and a chemical structure corresponding to observed signals.
NMR spectra (Figures 15-18) confirm the structure of D presented in Scheme 3. Properties of the Photopolymers. It is described in the Experimental Section how oligomers A-D were cured with UVlight. Table 2 presents physical (mechanical) properties of the cured oligomers. All four cured oligomers demonstrated more chemical resistance than that of 200 MEK double rubs. The cured new oligomers manifest high chemical oxidative resistance, high toughness, and high chemical resistance estimated by MEK double rubs. In our preliminary communication we reported resilience of A to high temperatures demonstrated in TGA experiments. Air-saturated solutions of A-D are stable for six or more months at 60 °C. The sulfide group -S- apparently serves as a secondary antioxidant decomposing accumulating
Figure 17. The COSY spectrum of oligomer D.
Figure 18. The DEPT spectrum of oligomer D.
hydroperoxides by a nonradical path with a formation of sulfoxide -S()O)-.4,13 Conclusions Figure 15. 1H NMR spectrum of oligomer D and a chemical structure corresponding to observed signals.
In this work, we have prepared dendritic oligomers using the Michael reaction of thiol with acrylates taken in excess. Unlike
Ind. Eng. Chem. Res., Vol. 50, No. 3, 2011 Table 2. Physical Properties of Cured Oligomers A-D and of Two Commercially Available Dendritic Acrylates at Room Temperaturea
A tensile strength at 27 break, MPae elongation at 5 break, %e durometer 80D hardnesse f Tg, °C 46 η, cPf, h 3100
B
C
D
BDE-1029b,c XDR-1406b,d dendritic dendritic polyester urethane acrylate acrylate
29
18
63
38
62
3
12
3
4
4
83D
68D
86D
58D
87D
83 95
91 13750
73 18 67; 159g 2300 2000 73000
a All six oligomers were cured being diluted by 30 wt % of IBOA (cf. Experimental Section). b We synthesized and described oligomer earlier, (cf. ref 12). c f ) 14. d f ) 10. e Determination error is 15%. f Determination error is 5%. g Oligomer demonstrated two Tg. h Viscosities of diluted solutions, prior cure at 25 °C.
Table 3. Effect of Substance on the Cured Coating of A substance
color
gloss
integrity
Drano bleach yellow mustard 25% trisodium phosphate (aq) 10% iodine 31.45% HCl (aq) 10W-30 motor oil
no effect no effect slight stain no effect no effect moderate discoloration no effect
no effect no effect no effect no effect no effect no effect no effect
no effect no effect no effect no effect no effect no effect no effect
our previous thiol-ene systems,5 liquid oligomers A-D demonstrate exceptional shelf life. When cured, the oligomer produces a coating with relatively high tensile strength and hardness that is akin to high viscosity dendritic urethane acrylates (Tables 2 and 3). Oligomers A-C demonstrate a balance of properties in between the dendritic polyester acrylates developed earlier in our lab, BDE-1025 (low viscosity, lower hardness), and the dendritic urethane acrylates D and XDR1416 (high viscosity, high hardness). It is safe to assume that oligomers A-D, BDE-1025, and XRD-1416 will meet the diverse demands of the coatings formulator. The clustering of thioether links within the dendritic core in the studied oligomers imparts unsurpassed chemical and solvent resistance required for high performance coatings (Tables 3). The cured new oligomers manifest high chemical oxidative resistance, high toughness, and high chemical resistance. Oligomers A-D are high f acrylates which form tough polyacryl-
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ates with sulfide groups. These oligomers are finding application in protective UV-curable coatings. Dendrimer D, like other similar-structured dendrimers,14 is expected to find a more sophisticated application in nanotechnology, microelectronics, and drug delivery. Literature Cited (1) Hoyle, C. E.; Bowman, C. N. Thiol-Ene Click Chemistry. Angew. Chem., Int. Ed. 2010, 49 (10), 1540. (2) Kade, M. J.; Burke, D. J.; Hawker, C. J. The Power of Thiol-Ene Chemistry. J. Polym. Sci., Part A 2010, 48 (3), 743. (3) Hoyle, C. E.; Lee, T. Y.; Roper, T. Thiol-Enes: Chemistry of the Past with Promise for the Future. J. Polym. Sci., Part A 2004, 42 (7), 5301. (4) Li, Q.; Zhou, H.; Hoyle, C. E. The Effect of Thiol and Ene Structures on Thiol-Ene Networks: Photopolymerization, Physical, Mechanical and Optical Properties. Polymer 2009, 50 (4), 2237. (5) Khudyakov, I. V.; Leon, J. A.; Zopf, R. D. Self-Initiated Allyl Ether and Vinyl Ether Urethane Monomers and Oligomers. American Coatings Conference, Charlotte, NC, 2008. (6) (a) Aerykssen, J. H.; Leon, J. A.; Zopf R. D.; Khudyakov, I. V. New Self-Initiating Oligomers Based on Thiol-Ene Chemistry. RadTech Europe, Nice, France, 2009. (b) Aerykssen, J. H.; Nebioglu, A.; Zopf R. D.; Khudyakov, I. V. Patent pending. (7) Chan, J. W.; Wei, H.; Zhou, H.; Hoyle, C. E. The Effects of Primary Amine Catalyzed Thio-Acrylate Michael Reaction on the Kinetics, Mechanical and Physical Properties of Thio-Acrylate Networks. Eur. Polym. J. 2009, 45 (9), 2717. (8) MALDI-TOF experiments would lead to a precise MWD. (9) Sathyanarayana, D. N. Introduction to Magnetic Resonance Spectroscopy. ESR, NMR, NQR; International Publishing House: New Delhi, India, 2009; Chapters 15, 19. (10) The origin of the CH signal labeled with a question mark is not obvious. We attentively ascribe it to impurities. (11) Swiderski, K. W.; Khudyakov, I. V. Synthesis and Properties of Urethane Acrylate Oligomers: Direct versus Reverse Addition. Ind. Eng. Chem. Res. 2004, 43 (11), 6281. (12) www.bomarspecialties.com. (13) Denisov, E. T.; Khudyakov, I. V. Mechanisms of Action and Reactivities of the Free Radicals of Inhibitors. Chem. ReV. 1987, 87 (10), 1387. (14) Killops, K. L.; Campos, L. M.; Hawker, C. J. Robust, Efficient, and Orthogonal Synthesis of Dendrimers via Thiol-Ene “Click” Chemistry. J. Am. Chem. Soc. 2008, 130 (6), 5062.
ReceiVed for reView August 30, 2010 ReVised manuscript receiVed November 29, 2010 Accepted December 2, 2010 IE101816B
