Instand Adhesives

Three Bond Technical News Issued January 1, 1996 46 Instant Adhesives (Cyanoacrylate Adhesives) Introduction _______________________________________________ • It has passed 38 years since instant adhesives were first introduced to the global market in 1957. Today, instant adhesives are used in a variety of applications for industrial, medical, and general domestic purpose. The main reason that usage fields of instant adhesives have expanded can be found on its ease of use, which is one-part adhesive and bonds any materials strongly and instantaneously at room temperature. As it can be used easily, demands of instant adhesives are high, however, especially in industrial fields it is not adopted in many cases due to the bonding performance and durability. Therefore, it is demanded to develop instant adhesives with higher performances. Three Bond has developed and marketed ThreeBond 1731 / 1733, and ThreeBond 1737 / 1738 as the products improving peeling strength and shock resistance that are disadvantages of instant adhesives. This issue will introduce the background and history of the development of these products and will describe their characteristics. Contents Introduction 1. Polymer properties and strengthening ............................................................ 2 2. Instant adhesive properties and strengthening ............................................... 3 3. Polymerization-inducing phase-separated structure of cyanoacrylate/rubber alloy .......................................................................... 5 4. Second-generation instant adhesives-ThreeBond 1731 and 1733................. 6 5. Third-generation instant adhesives-ThreeBond 1737 and 1738 ..................... 7 6. The ThreeBond 1700 Series ........................................................................... 8 1 1. Polymer properties and strengthening _______________________ • Wu has reported that the properties of thermoplastic polymers can be classified into the following types based on two molecular parameters: entanglement density υe (mmole/cc) of the polymer chain and the characteristic ratio C∞1). (1) Brittle polymers (hard and brittle polymers) υe ≤ ~ 0.15, C∞ ≥ ~ 7.5 (2) Pseudo-ductile polymers (soft polymers) υe ≥ ~0.15, C∞ ≤ ~7.5 (3) Intermediate polymers υe = ~0.15, C∞ = ~7.5 Figure 1 shows how various polymers are grouped according to this classification scheme. Also, regarding polymers alloyed by adding rubber components, Wu shows optimal dispersion morphology of the rubber components for strengthening polymers, as follow. (1) Optimal diameter of rubber particle exists for alloys consisting of brittle polymers/rubber component (Figure 2). (2) Optimal surface-to-surface distance between rubber particles exists for alloys consisting of pseudo-ductile polymer/rubber component (Figure 3). (3) Special rubber phase morphology is required for alloys consisting of intermediate polymer/rubber component. Entanglement density Pseudo-ductile polymers Intermediate polymers Brittle polymers Characteristic ratio Figure 1: Classification of various thermoplastic polymers 2 Optimal particle diameter do (µm) Rubber Polymer components Entanglement density υe (mmole/cc) Critical distance between particle surfaces τc (µm) Figure 2: Brittle polymers and optimal particle size Polymer Rubber components Characteristic ratio C∞ Figure 3: Pseudo-ductile polymers and critical distance between particle surfaces. 2. Instant adhesive properties and strengthening ________________ • Cyanoacrylate (cyanoacrylic acid ester), the main constituent of instant adhesives, reacts with trace amounts of moisture on the surface of the materials to be bonded and cures by polymerization to form cyanoacrylate polymers (Figure 4). Because these polymers are hard and brittle, they feature high shearing strength in the shearing direction, but are weak in strength for peeling and shock. Figure 5 shows the results of calculation according to Wu's method2) of two molecular parameters representing polymer properties for ethyl cyanoacrylate, the most popular adhesive on the market today. Curing Cyanoacrylate Cyanoacrylate polymer Figure 4: Curing of cyanoacrylate Ethyl cyanoacrylate polymer Figure 5: Ethyl cyanoacrylate polymer and values of molecular parameters 3 Entanglement density υe (mmole/cc) When strengthening poly (ethylcyanoacrylates) (PECA), which is a brittle polymer according to Wu's classification, by adding rubber components, it is concluded that there an optimal rubber particle diameter exists. Figure 6 shows the brittleness of PECA compared to other polymers, and Figure 7 shows the results of calculation for the optimal diameter of rubber particle. According to Wu's theory, for alloy consisting of PECA/rubber, the optimal particle diameter for strengthening PECA is around 1 µm. Brittleness Characteristic ratio C∞ Figure 6: Brittleness of PECA Polymer Optimal particle diameter do (µm) Rubber component Entanglement density υe (mmole/cc) Figure 7: Entanglement density and the optimal particle diameter for PECA 4 3. Polymerization-inducing phase-separated structure of cyanoacrylate/rubber alloy3,4) ______________________________ • Figure 8: Dispersion of rubber particles within cured adhesive Initial stage of curing Cyanoacrylate monomer Figure 8 is a transparence type electron microscope image of the state of rubber component dispersion within the cured cyanoacrylate adhesive to which a rubber component has been added. The rubber is concentrated in the central portion of the cured adhesive, and it was found that the rubber particles were dispersed in spheres of diameters ranging from the submicron level to around 1 micrometer. This concentrated morphology is understandable in that the curing of cyanoacrylate is initiated by contact with moisture on the surface of the materials to be bonded. As stated above, curing progresses from the bonding surface toward the interior parts of the adhesive. Therefore, the rubber particles are concentrated at the central portions of the adhesive, ultimately leading to phase separation (Figure 9). Progress of curing and concentration of rubber components Curing complete Cyanoacrylate polymer Rubber components Figure 9: Polymerization of cyanoacrylate and phase separation We developed instant adhesives with higher peeling strength and excellent shock resistance, applying the examination for strengthening polymers through alloying with rubber, and the results of analysis of the phase separation structure based on the unique polymerization mechanism of cyanoacrylate, 5 4. Second-generation instant adhesives −ThreeBond 1731 and 17335, 6, 7) __________________________ • ThreeBond 1731 and 1733 are second-generation high-performance instant adhesives with non-conjugated rubber components dissolved in the main constituent, cyanoacrylate; these components undergo phase separation as curing progresses (Figure 10). Table 1 shows the properties and general characteristics of these products. In addition to the properties shown in Table 1, alloying with rubber improves cold resistance, heat cycle resistance based on reduction of thermal stress, and heat resistance. Furthermore, with the improvements of bonding ability for the bonding resistant materials such as ethylene-propylene-diene terpolymers (EPDM), it became possible to apply these adhesives on semi-structure applications that could not use conventional instant adhesives. First-generation instant adhesives: ThreeBond 1781, 1782, and 1783 Curing Compatible system Compatible system Second-generation instant adhesives: ThreeBond 1731 and 1733 Curing Incompatible system Compatible system Figure 10: Phase structures of first-generation and second-generation instant adhesives Table 1: Properties and general characteristics of ThreeBond 1731 and 1733 Items Units TB1731 TB1733 Color and appearance - Lemon yellow Lemon yellow Viscosity mPa•s {cP} 20 {20} 150 {150} Seconds 10 20 19 {190} 16 {160} 10 {100} 11 {110} 3.0 {8.0} 3.0 {8.0} 2.0 {5.0} 2.0 {5.0} 34 {35} 34 {35} Set time Fe/Fe Fe/Fe Shearing adhesive strength 2 Mps {kgf/cm } Al/Al Fe/Fe Peeling adhesive strength kN/m {kgf/25mm} Al/Al Impact adhesive strength Fe/Fe 2 2 kJ/m {kgf•cm/cm } (TB: abbreviation for ThreeBond) 6 5. Third-generation instant adhesives −ThreeBond 1737 and 17388) _____________________________ • is not consistent on such surfaces. In other words, the changes on curing speed result changes of dispersion state, and appeared as changes of performance. Furthermore, ThreeBond 1737 and 1738 are semi-gel-type and gel-type formulations, respectively, with high thixotropic properties. Thus, these products offer improved workability and are better suited for automated adhesive application. That thixotropic properties are high mean that there exists large difference between adhesive viscosities in stressed conditions (such as when squeezing the adhesive out of the container) and unstressed conditions (such as after coating onto the material to be bonded). Thus, these adhesives are good in fluidity, but also proof against dripping. ThreeBond 1737 and 1738 are third-generation high-performance instant adhesives in which the conjugated rubber components are dispersed within the main constituent, cyanoacrylate, with a pre-designed phase separation structure regardless curing of the adhesive (Figure 11). The properties and general characteristics of these adhesives are shown in Table 2. A special feature of these third-generation instant adhesives is the pre-designed dispersion state of the rubber components. Thus, consistent performance is achieved at all times. One example of where this is advantageous is on materials to be bonded with acidic surfaces, such as wood, on which adhesives cure more slowly; as a result, the dispersion of the rubber components in second-generation adhesives Third-generation instant adhesives: ThreeBond 1737 and 1738 Curing Incompatible system Incompatible system Figure 11: Phase structure of third-generation instant adhesives Table 2: Properties and general characteristics of ThreeBond 1737 and 1738 Items Color and appearance Form Specific gravity Viscosity Thixotropic index NBR/NBR Setting time Fe/Fe Fe/Fe Shearing adhesive strength Al/Al Peeling adhesive strength Al/Al Impact adhesive strength Fe/Fe Dielectric breakdown voltage Volume resistivity Surface resistivity Dielectric constant Dielectric loss tangent Units TB1737 TB1738 Pa•s {cP} - Light blue Semi-gel form 1.04 2 {2000} 5.2 20 30 16 {160} 15 {150} 1.3 {3.3} 13 {13} 26 {26} 11 3.53 × 10 13 {3.53 × 10 } 14 2.51 × 10 3.287 0.033 Light blue Gel form 1.04 5 {5000} 5.6 30 40 16 {160} 15 {150} 2.0 {5.0} 13 {13} 48 {48} 11 1.59 × 10 13 {1.59 × 10 } 14 4.40 × 10 3.375 0.036 Seconds 2 MPa {kgf/cm } kN/m {kgf/25mm} 2 2 kJ/m {kgf•cm/cm } MV/m {kV/mm} Ω •m { Ω •cm} Ω - (TB: abbreviation for ThreeBond) 7 6. The ThreeBond 1700 Series ______________________________ • Three Bond instant adhesives and related products are introduced. These include a variety of products with a wide range of properties and functions suited to the workability and usage. Related products include peelers for removing cured adhesives, curing accelerators, and primers to enable adhesion of bonding resitant materials such as polyethylene, polypropylene, and polyacetal. We also provide automatic coaters, the S-II Coater, which ejects adhesives via pressurized air, and the SMF-02B Coater, which ejects adhesives using a tubing pump mechanism. Table 3: Three Bond instant adhesives and related products ThreeBond 1700 Series Color Viscosity mPa•s [cP] Colorless 3 {3} 1743 Colorless 100 {100} 20 g 1745 Lemon yellow 500 {500} 20, 500 g 1741 Multi - purpose grade Ethyl type Content 2, 2, 50 g 1747 3 {3} 1702 Colorless 35 {35} 2, 20, 50 g Blue 35 {35} 50 g 1741 D Blue 3 {3} 1743 D Colored type 2000 {2000} Colorless 1702 B Methyl type Lemon yellow 1701 20 g Blue 100 {100} 20, 50 g 20 g 20, 500 g Slow curing type Colorless 100 {100} 1721 Colorless 10 {10} 1731 Functionality grade 1713 Low-odor low-blooming type Lemon yellow 20 {20} 1733 Lemon yellow 150 {150} 1737 Light blue 2000 {2000} 50 g 1738 Light blue 5000 {5000} 20 g 1739 Colorless Gel form 1781 Colorless 3 {3} 20, 50, 500 g 20, 50, 500 g High peeling strength and impact resistant type Gel type Impact-resistant and heat-resistant type 1782 Colorless 80 {80} 1783 Lemon yellow 1000 {1000} 20 g 20, 50 g 20, 50 g 20, 50, 500 g 20, 500 g 50, 500 g 1785 B Curing accelerators 3 {3} Colorless 150 {150} 20, 50 g Lemon yellow 1100 {1100} 20, 50 g 1795 Colorless 1.0 {1.0} 50 g 1796 Peeler Colorless 1786 1787 Fast curing woodwork type 20 g Lemon yellow 0.9 {0.9} 45 g 1796 B Lemon yellow 0.9 {0.9} 45 g 1796 E Adhesive primers for hard-to-bond surfaces Lemon yellow 0.9 {0.9} 500 ml 1797 Lemon yellow 0.85 {0.85} 100 ml Hiroyuki Mikuni Development Division Structural Materials and Public Works Development Department 1456 Hazama-cho, Hachioji-shi, Tokyo 193-8533, Japan Tel: 81-426-61-1333 8