The Effect of Fillers Upon the Properties of Electrocinductive Cyanoacrylate Adhesives

The effect of fillers upon the properties of electroconductive cyanoaerylat esflres K.G. Chorbadjiev and D.L. Kotzev (Scientific Research Centre for Specialty Polymers, Bulgaria) The influence of type and size of filler particles and viscosity of the cyanoacrylate component on the volume resistivity, conductivity mechanism and adhesive strength of the resultant bond has been investigated. Various carbon and metal fillers were used to make conductive cyanoacrylate adhesive compositions and the best results were obtained when Ag, Ni or Mo powders (5-10 pm) were incorporated in thickened ethyl 2-cyanoacrylate with a viscosity of 50-100 cP. Key words: electroconductive adhesives; cyanoacrylate adhesives; adhesive-bonded joints; adhesive strength Adhesive compositions with electroconductive properties are finding increased application in the electronics industry for assembly of various electronic components L 2. They are of most use when the components cannot withstand the temperature of soldering. Cyanoacrylate conductive adhesives, when compared to the traditional epoxy and acrylic based conductive adhesives 3 have the following strong points: • short setting time at room temperature and humidity without use of catalysts; • one component adhesives; • strong bonding action towards various materials (metals, plastics, ceramics, etc); • satisfactory electroconductivity of adhesive bond; • easy to work with. The only available commercial product, 'Saikoron B' (Japan), based on ethyl 2-cyanoacrylate and silver powder, has a setting time of 3-4 rain and a volume resistivity of the bond 1-6.10 -4 N.cm 2, 3. The methods for obtaining electroconductive cyanoacrylate adhesives, described in the patent literature 4-s, deal mainly with the compatibility of the highly reactive cyanoacrylate monomers with metal and carbon fillers. As part of the research to synthesize conductive cyanoacrylate adhesives, the objective of the present work was to determine the influence of the type and size of metallic and carbon fillers and the viscosity of the cyanoacrylate component on the volume resistivity, conductivity mechanism and the strength of the resultant bond. Experimental details Ethyl 2-cyanoacrylate (ECA) with a viscosity of 2-4 cP and ECA thickened with poly(methyl methacrylate) with viscosities of 50 cP, 100 cP and 500 cP were used. The material was commercial grade 'Kanokonlit' (Bulgaria) cyanoacrylate adhesive. The materials tested as conductive fillers are shown in Table 1. The adhesive compositions were prepared according to References 7 and 8. The carbon fillers (carbon black and graphite) were mixed directly with ECA at room temperature until homogeneous compositions were obtained. At room temperature the compositions have setting times in the range of 10-20 min and a shelf life of over 4 months. In order to prevent spontaneous polymerization of the adhesive, the metal powder fillers were washed at 30-80°C with diluted solutions of inorganic or organic acids prior to mixing with the cyanoacrylate. The compositions containing metal fillers had setting times of 5-20 min and a shelf life of up to l0 days after mixing. Volume resistivity was determined according to ASTM D 2739-72. Brass tensile adhesion specimens with electrical connections were used. Resistivity was measured 24 h after adhesive application at 23 + I°C and 50 +_ 5% RH with a Kelvin bridge, calibrated to 1% accuracy. Tensile shear strength tests, performed on bonded steel joints having dimensions specified in ASTM D-1002, were conducted 24 h after bonding at room temperature with a Zwick 1474 testing machine with a constant cross-head speed of 50 mm min -l. 0 1 4 3 - 7 4 9 6 / 8 8 / 0 7 0 1 4 3 - 0 4 $03.00 © 1988 Butterworth 8 Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES VOL.8 NO.3 JULY 1988 143 T a b l e 1. P o w d e r fillers u s e d for p r e p a r a t i o n of c o n d u c t i v e c y a n o a c r y l a t e a d h e s i v e s Type Particles mean diameter (pm) Carbon black Graphite 0.04 Up to 1 0 0 Ag Source GDR (P-1250) Bulgaria (technical grade) Laboratory reduction of AgNO 3 with AI Fluka, Switzerland Fluka, Switzerland Fluka, Switzerland Fluka, Switzerland Fluka, Switzerland Fluka, Switzerland Fluka, Switzerland 5 Ni Me Fe Cr W AI Cu 10 10 20-80 20-80 20-8O 5 5 Results and discussion Fig. 1 shows the effect of type and quantity of filler on the conductivity of the adhesive bond. The tested fillers can be divided into four groups with respect to resistivity of the bond as a function of their quantity in the composition: Group Group Group Group 1 -2 -3 -4 -- carbon black and graphite Ag, Me, Ni Cu, A1 W, Cr, Fe. Fig. 2 shows the dependence of adhesive strength on filler quantity. In this case the grouping of the metal powders is also maintained, and for clarity the dependence for only one typical representative of each group is shown. From the results shown in Figs 1 and 2 it is evident that high conductivity cannot be achieved with carbon fillers. The adhesive strength for graphite-filled composition is lower than the carbon black composition. For Group 2 fillers (Ag, Me, Ni) conductivity of the bond increases with the increase of filler content, while the adhesive properties are maintained within reasonable values. Tensile shear ~7 X & • strength of the bond drops sharply when the filler content exceeds 70%. Group 3 (AI and Cu) based adhesives cannot achieve a high degree of filling and conductivity and tensile shear strength is low. Even at low filler content cohesive failure mode occurs. Group 4 fillers (Fe, Cr, W) are powders with larger particle size and high density. The conductivity of their adhesive bonds reaches acceptable values when the filler content is above 75%. However, at this filler concentration the adhesive strength of the bond drops sharply. The adhesive compositions of this group of fillers shows a tendency towards aggregation of the metal particles. The value 'critical degree of filling' was used as a measure of the viscosity of the adhesive compositions. It represents the highest viscosity limit of the composition at which adhesive application is still acceptable. Practical determination was carded out in accordance with the procedure described in Reference 9. Some adhesive (0.5 _+ 0.01 g) is placed between two glass discs and a load of 100 g is placed in the centre for 60 s. The diameter of the spread adhesive is then measured. The composition viscosity at which the adhesive spreads in a diameter of 7 mm is considered as the 'critical degree of filling'. Fig. 3 shows the dependence of the quantity of filler on the viscosity of the cyanoacrylate component at the critical degree of filling. Increasing the viscosity of the cyanoacrylate 2 20; el -'> 0 \ __ ° _A == o Ni iMo • Fe -( I0 ~7 Cu • Cr AW I 3(3 • I 50 Filler (weight%) I 70 \:%:. ~ ! "~'~_ v 0 I 90 Fig. 1 Effectof type and quantity of filler on the conductivityof cyanoacrylateadhesivebond. (ethyl 2-cyanoacrylatewith viscosityof 50 cP used) 144 • AI x Groph,~ \; INT.J.ADHESION AND ADHESIVES JULY 1 9 8 8 0 +,. I 20 I 40 1 60 Filler (weight %) Fig. 2 / Dependence of adhesive strength o n filler content in cyanoacrylate conductive adhesive. (ethyl 2-cyanoacrylatewith viscosityof 50 cP used) i 80 ,, , • 90.A.....,, vCu x Grophite + Corbon block • Cr Weight % 60 40 30 80 • Ag 5 z 7¢ ii 60 ,V cl .:> ~ o 512 ~ 3C ~ + / ÷ Corbon block o AJ • Ag x Graphite • Cr I ~ o ~ t Weight % • Ag • + ~ ~ ~ - e . . o x ~ m e~al I 5 O O Fig. 3 Dependence of filler content at critical degree of filling on the viscosity of cyanoscrylate component 15 ÷ o, + I i I00 200 300 400 Viscosity of cyonoocrylote component (cP) 0 o, X i == -2 ¢D ~ 60 Cr 80 Corbon 40 block AI 50 Grophite 60 Q. =S -! 0 • I I00 I 200 I 300 I 400 Viscosity of cyanoocrylote component (cP) Fig. 5 Dependence of adhesive bond's volume resistivity on the viscosity of cyanoacrylate component transfer through a thin polymer layer, the so-called tunnel effect ,u. Determining the value of n in the •equation: I = c~ ..--O f 0 O f I 5O K~O 5O0 Viscosity of cyonoocrylote component (cP) Fig. 4 Dependence of tensile shear strength of adhesive bond on the viscosity of cyanoscrylate component lowers the maximum quantity of filler that can be incorporated into the adhesive composition. More pronounced is the dependence of the filler content, at critical degree of filling, on the size of the filler particles. Increasing the filler particle size increases the amount of filler that can be accommodated in the composition. Figs 4 and 5 show the dependence of tensile shear strength and volume resistivity of the adhesive bond on the viscosity of the cyanoacrylate component. In most cases the tensile shear strength has a maximum when the cyanoacrylate component has a viscosity in the range of 50-100 cP. On the other hand, the volume resistivity of the adhesive bond is not affected to a great extent by the viscosity of the cyanoacrylate base component. Only in the case of carbon fillers is conductivity reduced by the increase of the cyanoacrylate viscosity. The conductivity mechanism of polymeric compositions containing conductive fillers is based on two possible effects; electric charge transfer through direct contact of filler particles and/or electric charge I 500 (l) where I = current intensity, Y = applied potential, c and n are constants, can clarify the conductivity mechanism. When n = 1 the so called ohmic conductivity occurs and the volume resistivity is not dependent on the applied potential. When n > 1 the conductivity is determined by the 'tunnel effect' charge transfer through the polymer layer and the volume resistivity becomes dependent on the applied potential. Conductivity increases with the increase of the potential. Table 2 shows the values of n as a function of type and quantity of filler for various cyanoacrylate adhesive compositions. The value of n was determined by linear approximation. It can be seen that, even at critical degree of filling, n > 1 for carbon fillers. The transfer of electric current is determined by the polymer matrix. Similar results for n are obtained for Al and Cu based compositions. This can be explained by the exceptionally good wettability of these metal particles by the cyanoacrylate monomer. The subsequent polymerization of the monomer eliminates the possibility for direct contact of the metal particles. For the third group fillers (Fe, Cr, W) in the critical degree of filling range, n becomes almost 1. This can be explained by the amount of filler accepted into the composition, its relatively large particle size and their tendency to aggregate. The best results and ohmic conductivity are displayed when Ag, Ni and Mo powders are used as fillers. Also, it can be concluded from the data in Table 2, that the increase of viscosity of the cyanoacrylate component raises the value of n. This is a result of the uniform distribution of the filler particles in the adhesive and, hence, in the formed polymer bond. Conclusions The conductivity and strength characteristics of the obtained cyanoacrylate adhesives depend primarily on INTJ.ADHESION AND ADHESIVES JULY 1988 145 Table 2. Conductivity mechanism of cyanoacrylate adhesive bonds as a function of type and quantity of filler and viscosity of cyanoacwlate component Value of n where / = c-V n Type of filler Filler content (weight %) cyanoacrylate with 2 cP viscosity cyanoacrylate with 50 cP viscosity cyanoacry(ate with 100 cP viscosity cyanoacrylate with 500 cP viscosity Carbon black 20 30 40 50 2.15 2.09 1.83 1.42 2.32 2.18 2.01 1.64 2.40 2.30 2.00 1.83 2.55 2.40 2.20 2.06 Graphite 30 40 50 60 70 2.04 1.84 1.52 1.34 1.20 2.22 2.10 1.72 1.53 1.40 2.32 2.15 2.00 1.42 1.56 2.44 2.30 2.10 1.85 1.42 Ag 20 30 40 50 60 70 1.47 1.15 1.02 1.00 1.00 1.00 1.58 1.20 1.05 1.06 1.02 1.02 1.64 1.34 1.12 1.07 1.08 1.06 1.65 1.44 1.15 1.10 1.12 1.10 Cu 40 50 60 70 1.92 1.98 1.96 1.94 ! .94 2.12 2.32 2.00 2.18 2.23 2.13 2.12 2.07 2.01 2.06 2.00 Cr 40 50 60 70 80 85 2.54 2.00 1.89 1.42 1.32 1.10 2.60 2.20 2.05 1.43 1.21 1.05 2.64 2.10 1.84 1.58 1.15 1.04 2.58 1.88 1.70 1.25 1.05 1.03 the type, quantity and size of the conductive filler. The viscosity of the cyanoacrylate component has little influence on the conductivity, but a more pronounced effect on the adhesive strength of the bond. The best results are obtained when Ag, Ni or Mo powders (5-10pm) are incorporated into thickened ethyl 2cyanoacrylate with a viscosity of 50-100 cP. References I Gul, V.E. and Shenfild, L.Z. 'Adhesives and coatings for the electronics" in "Electroconductive Polymer Compositions"(Himia, Moscow, USSR, 1984) pp 217-222 2 Suzuki, Y. "Characteristics and use of electroconductive adhesive Saikoron B" Kogyo Zaityo 32 No 2 (1984) lap 106-109 3 Suzuki, Y. 'Electroconductive instant setting adhesives' Kogyo Zairyo 33 No 2 (1985) pp 4 6 - 5 0 4 Inoue, K. Japanese Patent 73 036 (1976) 146 INT.J.ADHESION AND ADHESIVES JULY 1988 5 Inoue, K. Japanese Patent 74 626 (1977) 6 Taoka Gosei Chemical Co. Japanese Patent 118 776 (1985) 7 Chorllxidjiev, IL, Kotzev, D. end Kabeivanov, V. Bulgarian Patent 41054 (1987) 8 Chorbedjiev, K., Kotzev, D. end Kabeivanov, V. Bulgarian Patent 41055 (1987) 9 Gul, V.E. end Shenfild, L.Z. 'Electroconductive adhesives' in "Electroconductive Polymer Compositions"(Himia, Mescow, USSR, 1984) p 70 10 Van Beek, L.K. end Van Pul, B.I. 'Non-ohmic behaviour of carbon black loaded rubbers' Carbon 2 No 2 (1964) pp 121-126 Authors The authors are with The Scientific Research Centre for Specialty Polymers, Kliment Ohridski St 4A, 1156 Sofia, Bulgaria. Enquiries should be addressed to Dr D.L. Kotzev.