Modification of cyanoacrylate adhesives with β-vinyl-α-cyanoacrylates

ISSN 1995-4212, Polymer Science, Series D. Glues and Sealing Materials, 2008, Vol. 1, No. 1, pp. 36–40. © Pleiades Publishing, Ltd., 2008. Original Russian Text © D.A. Aronovich, A.M. Vetrova, 2007, published in Klei. Germetiki. Tekhnologii, 2007, No. 4, pp. 10–14. PROPERTIES OF MATERIALS Modification of Cyanoacrylate Adhesives with b-Vinyl-a-Cyanoacrylates D. A. Aronovich and A. M. Vetrova Federal State Unitary Enterprise Kargin Institute of Polymers, Dzerzhinsk, Nizhegorodskaya obl., 606000 Russia e-mail: niip@kis.ru Received December 20, 2006 Abstract—Adhesive properties of β-vinyl-α-cyanoacrylates and their anionic copolymerization with αcyanoacrylates are studied. The ability of bifunctional β-vinyl-α-cyanoacrylates to enhance the thermal stability of cyanoacrylate adhesives is demonstrated. DOI: 10.1134/S1995421208010127 Cyanoacrylate adhesives are widely used for bonding in various branches of technology due to their high rate of curing at room temperature, adhesion to versatile materials, and possibilities of automating the bonding processes. Extensive application of cyanoacrylate adhesives is also promoted by a single-component composition, the absence of solvent, low toxicity, and sufficient stability in organic solvents. At the same time, adhesives based on monomeric cyanoacrylates are characterized by a number of drawbacks, the main of which are low heat and moisture stability, elasticity, and impact resistance. As is seen from this table, compositions with β-vinyl-α-cyanoacrylates are characterized by higher initial adhesive strength and enhanced strength in the 100–150°C range compared to unmodified cyanoacrylates. Strength properties of compositions modified with β-vinyl-α-cyanoacrylates are also higher than those of compositions modified with other compounds containing double bonds (TEGMA, maleic anhydride). It was also shown that the addition of β-vinyl-αcyanoacrylates to adhesive composition based on propyl cyanoacrylate by increasing several times over the impact strength within the temperature range of 25– 175°C [11]. One of the most efficient procedures for the modification of cyanoacrylate adhesives aimed at the enhancement of their mechanical properties is the addition of various monomeric compounds to adhesive compositions [1–3]. For such monomers, it was proposed to use also different β-substituted cyanoacrylates, the most promising of which are β-vinyl-α-cyanoacrylates of the general formula CH2=CHCH=C(CN)COOR (esters of 2-cyanobutadiene carbonic acid) [4–7]. These compounds, formed through the reaction of acrolein with corresponding cyanoacetates, were first described by Gerber [8, 9] and are mainly low-melting crystalline substances that can readily be polymerized by moisture or heating. As was shown, β-vinyl-α-cyanoacrylates are also capable of bonding different materials, although with lower strength than α-cyanoacrylates; additional double bonds in ester radical [10] make it possible to enhance the strength of adhesive joints at 120°C (Table 1). In view of the fact that polymerization during bonding with cyanoacrylates proceeds by the anionic mechanism due to adsorbed moisture, it was of interest to establish whether β-vinyl-α-cyanoacrylates are copolymerized with α-cyanoacrylates under the conditions of Table 1. Strength properties of Steel St.3–Steel St.3 adhesive joints bonded by β-vinyl-α-cyanoacrylates Tensile strength, MPa Type of radical R in monomer 5.4 8.8 C2H5OC2H4– 4.7 4.2 C6H5CH2– 6.2 7.4 CH2=CH– 5.5 15.8 CH2=CH–CH2– 5.2 14.9 CH2=CH–CH2OC2H4– 4.8 13.5 CH2=C(CH3)COOC2H4– 36 after 3-h storage in water at 120°C C2H5– The addition of β-vinyl-α-cyanoacrylates to compositions based on ethyl-, butyl-, and ethoxyethyl-αcyanoacrylates demonstrated that these compounds promote the enhancement of heat- and moisture stability of adhesive joints without a decrease in setting time and working life (Table 2). initial 6.6 17.8 MODIFICATION OF CYANOACRYLATE ADHESIVES 37 Table 2. Strength properties of cyanoacrylate adhesive compositions containing β-vinyl-α-cyanoacrylates* Adhesive composition Tensile strength, MPa, after type of radical type of radical in cyanoacrylate in β-vinyl-α-cyanoacrylate heating at temperature, °C**, for 1 h 20 100 150 170 200 3-h storage in water at 100°C C2H5– 26.5 21.5/16.0 12.0/5.1 3.5/2.0 0/0 4.5 n-C4H9– 23.5 22.0/13.0 12.5/4.5 3.6/1.0 0/0 9.6 C2H5OC2H4– 21.2 18.8/7.0 1.2/0 0/0 5.4 6.0/2.5 C2H5– C2H5– 28.0 32.0/17.0 27.0/8.5 17.5/4.6 1.8/0.5 17.5 C2H5– CH2=CH–CH2– 29.0 34.0/22.0 24.0/10.5 19.2/6.0 4.4/2.1 19.6 C2H5– CH2=C(CH3)CO2C2H4– 30.5 35.8/18.4 26.2/12.0 17.5/7.5 5.4/2.6 21.2 n-C4H9– C2H5– 31.4 33.0/17.0 24.0/11.0 13.7/6.5 2.0/0.8 23.2 C2H5OC2H4– CH2=CH–CH2– 29.0 30.2/13.0 11.0/5.5 7.0/2.6 0/0 19.8 C2H5– TEGMA*** 21.0 20.5/15.0 19.8/10.5 11.5/3.7 0/0 17.0 C2H5– Maleic anhydride*** 2.8 31.6/15.4 24.0/6.4 17.6/3.6 2.1/1.2 14.0 *Content of β-vinyl-α-cyanoacrylate in composition is 20 wt %. **Numerator is the strength at 20°C, denominator is the strength at heating temperature. ***For comparison. anionic initiation. For this purpose, we studied the copolymerization of ethyl cyanoacrylate (ECA) with ethyl ether of β-vinyl-α-cyanoacrylic acid (EVC) in the presence of water (weak nucleophile) as an initiator. The IR spectral analysis of copolymers and EVC polymer showed that the absorption band at 980 cm–1 assigned to extraplanar deformation vibrations of =CH groups located in trans-position relative to double bond is sensitive to the variations in the amount of β-vinylα-cyanoacrylate, and the measurement of the optical density of this band makes it possible to determine the composition of copolymers: Content of EVC in the initial mixture, wt % 8.5 20.3 20.3 47 Content of EVC according to IR spectra, wt % 7.9 19.4 19.1* 45.3 31 (mixture of homopolymers) 32 31 (mixture of homopolymers) 14* * After triple reprecipitation. It turned out that, upon the reprecipitation of copolymers, in contrast to mechanical mixture, the intensity of such band remains practically unchanged and the composition of copolymer is close to that of initial mixture. The analysis of differential thermogravimetric curves of the degradation of copolymers and the mixture of homopolymers also proves the formation of copolymer. There is one maximum (245°C) at the curve of copolymer degradation, whereas, for the mixture (70 : 30) of homopolymers, two maxima are observed, one at 210°C for polyECA and the other at 265°C for polyEVC. POLYMER SCIENCE Series D Vol. 1 No. 1 2008 The addition of synthesized bifunctional β-vinyl-αcyanoacrylates to α-cyanoacrylates [12] increases thermal stability to an even greater extent [13–15] (Table 3). Bifunctional β-vinyl-α-cyanoacrylates are crosslinking agents that lead to a substantial increase in the initial strength at 20°C, heat- and water resistance of adhesive joints. The glass transition temperature of the samples cured at ambient temperature increases from 132 to 173°C when adding 5 wt % of ethylene glycol diester and β-vinyl-α-cyanoacrylic acid to ethyl cyanoacrylate. TGA of cured polymers demonstrates 38 ARONOVICH, VETROVA Table 3. Strength properties of Steel St.3–Steel St.3 adhesive joints bonded by compositions of ethyl-α-cyanoacrylate and bis(β-vinyl-α-cyanoacrylates) Tensile strength, MPa, after Type of radical R in bis(β-vinyl-αcyanoacrylates)* heating at temperature, °C, for 1 h 3-h storage in water at 100°C 20 100 150 170 200 –CH2CH2– 38.0 45.0 34.2 24.6 8.0 23.2 –CH2CH2CH2– 36.8 42.5 34.0 21.0 6.5 21.8 –CH2CH(CH3)– 34.5 40.0 32.0 20.5 3.6 25.8 CH2C(CH3)2CH2– 30.5 35.0 31.0 20.0 3.5 –(CH2CH2O)2– 33.0 38.0 33.0 19.2 7.0 24.4 –(CH2CH2O)3– 27.5 29.0 28.0 17.0 3.2 19.7 –(CH2CH2O)12– 16.6 20.0 8.5 6.4 1.0 9.5 –CH2CH2–** 32.0 27.3 16.4/5.0*** *Formula CH2=CH–CH=C(CN)COOROCOC(CN)=CH–CH=CH2 . **Upon the addition to allyl-α-cyanoacrylate. ***After heating at 250°C for 24 h. that β-vinyl-α-cyanoacrylates enhance the resistance to thermal-oxidative degradation (figure). Upon the modification of allyl-α-cyanoacrylate by β-vinyl-α-cyanoacrylic acid diesters, the thermal stability of adhesive joints increases to a greater extent and the addition of peroxide compounds to such compositions enables us to increase the stability of adhesive joints to 250°C. Mass loss, % 100 1 2 3 20 10 0 120 140 160 180 200 220 240 260 280 Temperature, °C Thermogravimetric curves of cured polymers: (1) poly(ethyl-α-cyanoacrylate, (2) copolymer (95 : 5) of ethyl-α-cyanoacrylate and ethyl-β-vinyl-αcyanoacrylate, and (3) copolymer (95 : 5) of ethyl-αcyanoacrylate and ethylene glycol diester of β-vinylα-cyanoacrylic acid. Copolymers of β-vinyl-α-cyanoacrylates and α-cyanoacrylates can also be used as thickeners of cyanoacrylate adhesive compositions. In this connection, we studied the copolymerization of ethyl cyanoacrylate and butyl cyanoacrylate (BCA) in the presence of anionic initiators (water and triethylamine, TEA) in acetone and tetrahydrofuran (TGF) within the 10–40°C temperature range with ethyl (EVC), benzyl (VVC), allyl (AVC), and (methacryloyloxy)ethyl (MVC) esters of β-vinyl-α-cyanoacrylic acid. To study the influence of copolymerization conditions on the properties of formed polymers, we applied the method of the mathematical design of experiments with the use of hyper-Grecian-Latin 4 × 4 square combined with half-complete factorial 25 experiment [16]. Properties of copolymers were characterized by the number-average molecular mass Mn, glass transition temperature Tg, viscoelastic transition temperature Tv , the temperature of thermal degradation onset Td, and the activation energy of thermal degradation E. Experimental data were processed using the analysis of variance and the analysis of means (Tables 4 and 5). The analysis of variance allows us to conclude that the molecular mass and thermomechanical properties of copolymers are mostly affected by the nature of the initiator of polymerization, whereas the thermal stability of copolymers depends primarily on the temperature of copolymerization and comonomer ratio. According to the analysis of means (Table 5), copolymers with higher molecular masses (Y1), glass transition temperatures (Y2), and viscous flow temperatures POLYMER SCIENCE Series D Vol. 1 No. 1 2008 MODIFICATION OF CYANOACRYLATE ADHESIVES 39 Table 4. Summarized data of the analysis of variance Squared ratio Variability factor Y1 Y2 Y3 Y4 Y5 Mn Tg Tv Td E X1 1st comonomer group 28.97/0.99 0.4 0.089 28.98 116/0.90 X2 2nd comonomer group 10.06 1.63 0.52 5.54 23 X3 Monomer ratio 14.46 0.13 0.26 33.8/0.99 2 X4 Solvent 1 2.33/0.75 0.28 17.45 28 X5 Monomer : solvent ratio 1 0.003 0.40 X6 Type of initiator 54.43/0.99 2.89/0.75 6.27/0.75 X7 Amount of initiator 3.63 0.53 0.47 X8 Process temperature 11.49 0.62 0.13 1 4 21.23 69/0.90 1 4.7 71.12/0.99 62/0.90 Note: Denominator indicates the probability factor. Table 5. Summarized data of the analysis of means Mean values for property levels Factors Factor level 129600 95 147 194 27.88 91900 93 147 194 24.00 97800 114 159 188 25.25 143800 90 146.5 186 28.50 105700 95 143 190 23.00 95800 96 151 194 27.88 123800 100 146 186 25.60 111300 105 151 193 26.50 108300 92 146 187 23.88 107700 98 146 190 24.75 111900 98 152 190 25.62 Water 137000 105 161 193 27.25 TEA Amount of initiator, % 22.50 1 : 10 X7 186 1:5 Type of initiator 150 TGF X6 101 Acetone Monomer : solvent ratio 90000 95/5 X5 E, kcal/mol 80/20 Solvent Td , °C AVC X4 Tv , °C MVC Monomer ratio (1/2) Tg , °C BVC X3 Y5 EVC 2nd comonomer group Y4 BCA X2 Y3 ECA 1st comonomer group Y2 Mn X1 Y1 82600 91 137 186 23.12 Water TEA 1.0 0.1 120000 105 158 191 24.50 1.5 0.5 90200 98 148 189 25.00 2.5 0.8 64600 90 144.5 189 26.75 1.0 3.5 Series D Vol. 1 144 191 24.50 92.5 153 181 21.75 140800 94 149 188 22.75 89400 99 148 194 25.75 40 POLYMER SCIENCE 99 119500 20 Polymerization temperature, °C 120000 10 30 X8 89700 107 145 197 30.50 No. 1 2008 40 ARONOVICH, VETROVA Table 6. Strength properties of the joints bonded by modified adhesive compositions Tensile strength/Shear strength, MPa, at 20°C Bonded material Steel 12Cr18N10T Brass Duralumin D16AT Kovar Fluoroplastic F-4 (Teflon) after initial 3 thermal treatment cycles from –60 to +100°C 30 days at 40°C and 98% humidity 200 h at 100°C 36/22 33/11 31/16 42/17 8/4 40/25 29/6.5 31/14 48/15 5/4 29/18 14/8 23/12 34/11 4/3 38/19 31/16 32/15 43/18 8/6 (Y3) are formed when water (X6) is used as initiator, while copolymers with the highest thermal stability (Y4, Y5) are synthesized upon the copolymerization of butyl-α-cyanoacrylate (X1) at 80/20 ratio (X3) in acetone solution (X4) in the presence of water (X6) at 40°C (X8). It is also seen from Table 5 that the alkyl radical of β-vinyl-α-cyanoacrylate (X2) affects the activation energy of copolymer thermal degradation (Y5) and that its largest value is achieved for copolymers with MVC (X2, Y5). The incorporation of an aromatic ring into ester radical (X2) of β-vinyl-α-cyanoacrylate (BVC) leads to an increase in temperatures of glass transition (Y2) and viscous flow (Y3), but virtually does not affect its thermal stability (Y4, Y5). Copolymers synthesized are well dissolved in ECA and allow us to prepare high-viscosity adhesive compositions (viscosity is up to 5000 cP) [17]. In this case, we did not observe a decrease in strength parameters; however, the stability of adhesives prepared by thickening with copolymer in the presence of triethylamine decreases during storage. Properties of thickened adhesive compositions are presented in Table 6. Thus, the properties of cyanoacrylate adhesives can be substantially improved by their modification with β-vinyl-α-cyanoacrylates. REFERENCES 1. N. N. Trofimov, D. A. Aronovich, V. S. Etlis, and N. M. Pinchuk, Plast. Massy, No. 9, 55 (1976). 2. L. M. Pritykin, D. A. Kardashov, and V. L. 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