Strength deformation and relaxation of joints bonded with modified cyanoacrylate adhesives

Strength, deformation and relaxation of joints bonded with modified cyanoacrylate adhesives C. Petrov, B. Se raf i mo v and D.L. Kotzev (Scientific Research Centre for Speciality Polymers, Bulgaria) The introduction of high molecular weight poly(methyI methacrylate) or poly(butadiene-co-acrylonitrile) into ethyl 2-cyanoacrylate produced viscous adhesives with a homogeneous or heterogeneous structure after cure. Steel joints bonded with these adhesives are shown to have improved tensile shear strength, deformability and stress relaxation of bonds compared with pure cyanoacrylate adhesive. Poly(methyl methacrylate)-modified adhesive is recommended for static load-bearing joints while poly(butadiene-co-acrylonitrile)-modified adhesive is more suited to cyclic or vibrating loads. Key words: adhesives; modified cyanoacrylates; adhesive-bonded joints; adhesive strength; mechanical tests size ~ 1/lm) is finely dispersed in the cured acrylic One method which has been used to improve the matrix 8. Thus, the applied load is borne by the glassy properties (such as viscosity and brittleness after cure) portion of the structure while the fracture energy is of cyanoacrylate adhesives has been the incorporation absorbed and dissipated in the rubbery phase which of polymeric material into the liquid cyanoacrylate distorts during the dissipation of that energy9. monomer. As early as 1957, Coover et a l I used The objective of the work described below was to poly(methyl methacrylate) (PMMA) as thickener for study and compare the strength, strain and relaxation cyanoacrylate adhesives. Introducing high molecular properties of adhesive bonds based on ethyl weight PMMA into the cyanoacrylate achieves the 2-cyanoacrylate (ECA)adhesive modified with PMMAor desired viscosity of the adhesive composition, even at poly(butadiene-co-acrylonitrile) (PBAN). low modifier content, without detrimental effect on the setting time or strength properties 2. Studies on the properties of resultant bonds led the present authors to Experimental details the supposition that the morphological structure of the 100% GC pure ECA, obtained by distilling the cured adhesive resembled an interpenetrating network commercial grade product (Kanokonlit E, Bulgaria) system 3. Toughened cyanoacrylates are obtained when was stabilized with 250 ppm hydroquinone and elastomeric polymers, such as methacrylate-butadiene200 ppm p-toluenesulphonic acid. The PMMA-modified styrene terpolymers 4, butadiene-acrylonitrile adhesives were obtained by dissolving specified copolymer~, or methyl acrylate-ethylene copolymer6, amounts of commercial grade PMMA(bulk polymer, are incorporated into the cyanoacrylate composition. molecular weight 1.5 x 106) directly into the ECA by As well as achieving the desired viscosity modification, mixing at 50°C. The PBAN (commercial brand, the elastomers impart significant improvement on Perbunan 3807NS from BASF, FRG) was first impact resistance, peel strength, strength at higher dissolved in distilled and dried CH2C12 to form a 10 temperatures, and resistivity to multiple cyclic deformations of the adhesive bond 6' 7. Although no weight % solution and then mixed with the ECA. The CH2C12 was removed under vacuum (5 mmHg) at 40°C specific data are reported it can be assumed that the leaving the PBAN-modified adhesive composition. elastomeric phase is finely dispersed in the cured The amount of modifier used and the viscosities of cyanoacrylate matrix, resembling the structure of the compositions obtained are given in Table 1. toughened acrylic adhesives which has been well Adhesive-bonded steel (0.2% C content) joints were described. In these, the rubber phase (particle 0143-7496/88/100207-04 $03.00 © 1988 Butterworth 8 Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES VOL.8 NO.4 OCTOBER 1988 207 Table 1. Viscosity of adhesive c o m p o s i t i o n s Modifier Type Viscosity at 20°C (cP) Amount (weight %) -- PMMA PBAN 2.5 1 2 3 4 23 52 150 495 1 3 5 8 10 I I i 70 mm IJ 25 -96 180 450 620 I i i 0.05 mm - - 1 mm I 1.5 mm 147. i PMMA poly(methyl methac~late) PBAN poly(butadiene co-acrylonitrile) I i i used in all the mechanical tests. The surfaces to be bonded were roughened with Igel 400 sandpaper and degreased with trichloroethylene (chemical treatment or activation were not employed). After application of the adhesive the joint was clamped and left for 24 h at j 70 mm i i I F • r 12mm 4 F Fig. 2 Test specimen for tensile strain determination (Reference 12) J 1 I0 mm" ~ , 5 ml I I .L 12mm ~ F Fig. 1 208 Test specimen for shear strain determination (References 10, 1 1 INT.J.ADHESION AND ADHESIVES OCTOBER 1988 20-22 C and 55-65% RH. The thickness of the glue-line (0.05 mm) was controlled with the use of calibrated copper wire. Tensile shear strength was determined in accordance with ASTM D-1002 on single overlap specimens. A Zwick 1474 Universal testing machine was used. The test specimen for shear strain had dimensions as shown in.Fi~. 11°' II. The rate of loading was 0.025 mm m m - ' . The relaxation modulus was determined with the help of the test specimen for tensile strain determination (shown in Fig. 2) 12. The multilayer structure was chosen because it provided a means for increasing the absolute value of deformation within measurable limits, since cyanoacrylates polymerize only in very thin films. It consists of five cylindrical discs, 12 mm in diameter and 1.5 mm thick, assembled between two cylindrical rods of the same diameter. The adhesive is applied between the discs and rods, thus providing six adhesive lines. All adhesive layers had thicknesses of 0.05 ram. To obtain the necessary co-axiality, a specially-cut Teflon jacket was used for the assembly of the specimen after adhesive application. The specimen was loaded at a rate of 20 mm min -] to a specified deformation and held loaded for 600 s, after which, stress and deformation values were recorded. 25 2100 p 20 1800 60 e~ 15 - 50 D o 5 -I ro ~- 1500 40 ~ m C 10 /% 30 } 1 c 1200 ~q d -~ 20 (/I 10 900 I I I 2 I 3 I q Content of PMMA(%) Fig. 3 Dependence of shear stress and shear strain on content of PMMA in adhesive 600 3 I 2s I I I I I 20 40 60 80 100 le failure (%) Fig. 5 Dependence of relaxation modulus on relative strain: 1, pure ECA; 2, ECA containing PBAN (3 weight %); 3, ECA containing PMMA (4 weight %) 20 60 22 eo Q. D 121 2 D -- 40 o C L to .C - 30 oJ ru3 18 o -20 16 I I I I I I I 5 I I I i lO C o n t e n t o f PBAN (%) Fig. 4 Dependence of shear stress and shear strain on content of PBAN in adhesive I 50 I 100 Rate of stress increase (mmmin - 1 ) Fig. 6 Dependence of stress at failure on rate of applied load: 1, ECA containing PBAN (3 weight %); 2, ECA containing PMMA (3 weight %); 3, pure ECA INT.J.ADHESION AND ADHESIVES OCTOBER 1988 209 Strain was measured using an Instron G-51-11-M extensiometer and the strain/time dependence was recorded with the help of an additional Tacussel EPL-2 recorder. Results end discussion The dependence of tensile shear strength and relative elongation at break of bonded joints on the content of PMMA in the ECA adhesive and PBAN in the ECA adhesive is shown in Figs 3 and 4 respectively. Increasing the content of PMMA causes slight and almost uniform increase of the tensile shear strength and deformability of the bond. PBAN-modified adhesive show increased strength when the elastomer content is in the range 0.5-4.0 weight %, with a well pronounced maximum at 1.0 weight %. Further increase in modifier content affects the strength value detrimentally. The deformability of the adhesive bond increases with the increase of PBAN content. On comparing Fig. 3 with Fig. 4 over the 0-4 weight % modifier content range, it can be seen that the adhesive containing PBAN has better strength and a more pronounced susceptibility of the joint to deformation. The change of relaxation modulus with the increase of the deformation relative to deformation at failure of adhesive bond (e/efailure) is shown in Fig. 5. The curves for pure ECA bonds and PMMA-modified ECA bonds have similar shape. The relaxation modulus steadily decreases in value up to 70-80% of failure strain; above this deformation its value drops sharply. In the case of PBAN-modified adhesive the decrease of modulus value up to e/efailure = 50% is less pronounced, but falls steeply at higher values ratios. This is associated with the change of mechanism of the deformation process caused by the beginning deformation of the elastomeric phase. Fig. 6 shows the dependence of the tensile shear strength of bonded joints on the rate of applied stress. Decreasing the stress rate decreases the value of the tensile shear strength for all adhesive systems. The PBAN-modified adhesive bonds, however, are the least affected. 210 INT.J.ADHESION AND ADHESIVES OCTOBER 1988 Conclusions The data obtained in this study confirm practical results reported previously2. 5 for cyanoacrylate adhesives containing PMMA or PBAN. The modified adhesives display improved strength properties, deformation susceptibility, and stress relaxation. Adhesives modified with PMMA would be appropriate for static load-bearing joints whereas joints bonded with PBAN-modified cyanoacrylate adhesives could better withstand cyclic or vibrating loads, References 1 Coover, Jr, H.W. et e l U S Patent 2 794 788 (1957) 2 Kotzev, D.L. and Dichava, L.B, 'Cyanoacrylate adhesives with increased viscosity' in First National Conference on Chemistry (Ministry of National Education, Sofia, Bulgaria, 1985) p 4 1 5 3 Petrov, C., Sarafirnov, S. and Kotzav, D. 'Adhesive bond properties of ethyl 2-cyanoacrylate modified with poly(methyl methacwlate)' J Adhesion (submitted) 4 Gleave, V. US Patent 4 102 945 (1978) 5 Kabaivenov, V. at al Bulgarian Patent 29487 (1979) 6 O'Connor, J.T. US Patent 4 440 910 (1984) 7 Kotzev, D.L. and Kabaivanov, V.S, 'Improvement and diversification of cyanoacrylate adhesives' in Adhesion-12 edited by K,W. Alien (Elsevier Applied Science Publishers, London, UK, 1988) pp 82-105 8 Chernoek, R.S. and Martin, F.R. Adhesion and Adhesives, Durham University, September 1980 (The Plastics and Rubber Institute, London, UK, 1980) Reprints paper 16 9 Lees, W.A. The British Polymer J 11 (June 1979) p 69 10 Jenadrau, J.P. 'Intrinsic mechanical characterization of structural adhesives' Int J Adhesion and Adhesives 6 No 4 (October 1986) PD 229-231 11 Althof, W. and Brockmann, W. Adhesives Age 20 No 11 (1977) p27 12 Freidin, A.E. in Strength and Durability of Adhesive Joints (Hirnia, Moscow, USSR, 1981 ) pp 113-114 Authors The authors are with the Scientific Research Centre for Speciality Polymers, Kliment Ohridski Street 4A, 1156 Sofia, Bulgaria. Enquiries should be addressed to Dr D.L. Kotzev.