Propery of Reed Elsevier 13 Cyanoacrylates Cyanoacrylates were commercially introduced in for polymerization. Instead of aqueous formaldehyde, 1950 by Tennesee Eastman Company. Cyanoacrylate paraformaldehyde was used with an organic solvent to adhesives are monomeric adhesives. They are gener- remove the water by azeotropic distillation. The sta- ally quick-setting materials which cure to clear, hard bility of the monomer can be enhanced by the redistil- glassy resins, useful as sealants, coatings, and par- lation of the crude monomer in the presence of small ticularly adhesives for bonding together a variety of quantities of acidic stabilizers, e.g., sulfur dioxide. substrates [1]. Polymers of alkyl 2-cyanoacrylates are Several other methods for cyanoacrylate monomer also known as superglues. production have been described, including the pyro- In addition to their use as adhesives, cyanoacrylates lysis of 3-alkoxy-2-cyanopropionates [8], transester- have been reported to have highly herbicidal prop- ification of ethyl cyanoacrylate [9], and displacement erties, as they disrupt photosynthetic electron trans- of cyanoacrylate monomer from its anthracene Diels- portation [2–4]. Alder adduct by treatment with maleic anhydride. This last method is used for the synthesis of monomers 13.1 Monomers that are not accessible or may be difficult to prepare by the retropolymerization route, for example difunc- 13.1.1 Synthesis tional cyanoacrylates [10], thiocyanoacrylates [11], and perfluorinated monomers. In 1895 von Auwers and Thorpe [5] attempted to synthesize diethyl-2,2-dicyanoglutarate (Figure 13.1) by base-catalyzed condensation of aqueous formalde- 13.1.2 Crosslinkers hyde and ethyl cyanoacetate. They isolated a mixture To improve the cohesive strength, difunctional mono- of oily oligomers and an amorphous polymer of higher meric crosslinking agents may be added to the mono- molecular weight. mer compositions. These include alkyl bis(2-cyano- In fact, ethyl cyanoacrylate monomer was synthe- acrylates), triallyl isocyanurates, alkylene diacrylates, sized as an intermediate, which underwent an imme- alkylene dimethacrylates, trimethylol propane triacry- diate polymerization reaction. The condensation of late, and alkyl bis(2-cyanoacrylates) [15]. formaldehyde with cyanoacetate is still the most imp- ortant method for the commercial production of the 13.1.3 Commercial Products monomers, cf. Figure 13.2. The reaction mechanism takes place as a base-catalyzed Knoevenagel conden- Commercial products consist mainly of monofunc- sation of cyanoacetate and formaldehyde to give an tional monomers. Commonly encountered monomers intermediate disubstituted methylol derivative. are shown in Table 13.1. The monomers are usually A.E. Ardis [6] at B.F. Goodrich (in 1947) found low-viscosity liquids with excellent wetting proper- that the polymer-oligomer mixture obtained in ties. The basic structure of cyanoacrylate monomers the formaldehyde-cyanoacetate condensation reac- and polymers is shown in Figure 13.3. The syntheses tion could be thermally depolymerized with acid cat- of the monomers and the raw materials are shown in alysts. However, the monomer prepared by utilizing Figures 13.4 and 13.5. Because of the high electro- these methods was unstable and the yields were low. negativity of the nitrile group and the carboxylate Later [7] it was realized that the water is responsible CN CN CN CN CN H C H CH2 C CH O + CH C H C CH C H 2 COR 2 2 C OR-H2O CO R R O C C OR O O O O O Figure 13.2 Synthesis of cyanoacrylates: Knoeve- Figure 13.1 2,4-Dicyanoglutaric acid ester. nagel reaction. Fink: Reactive Polymers Fundamentals and Applications. http://dx.doi.org/10.1016/B978-1-4557-3149-7.00013-9 © 2013 Elsevier Inc. All rights reserved. 317 Propery of Reed Elsevier 318 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS Table 13.1 Commercially Available Cyanoacrylates Compound Remarks Methyl cyanoacrylate Strongest bonding to metals, good stability against solvents Ethyl cyanoacrylate General purpose Allyl cyanoacrylate >100 ◦C service temperature n-Butyl cyanoacrylate Flexible, medical applications [12] Isobutyl cyanoacrylate Medical applications [12] 2-Octyl cyanoacrylate Medical applications [13,12] 2-Methoxyethyl cyanoacrylate Weak odor 2-Ethoxyethyl cyanoacrylate Weak odor 2-Methoxy-1-methylethyl cyanoacrylate Weak odor Methacryloyloxyethyl-2-cyanopenta-2,4-dienoate Strong adhesive [14] CN CN Cl Cl CH C CH2 C CC CH CO H 2 n 3 COR COR H Cl O O O H2 O Cl2 Figure 13.3 Basic structure of cyanoacrylate mono- mers and polymers. Cl CH2 CO H O CN Figure 13.5 Synthesis of chloroacetic acid. ++H C H CH2 O C OR N O H CN CN CCH - - 2 Y CH C Y + 2 C O R C O R -H2O O O CN N N N CH2 C H C C C OR Y CCH - 2 Y CH2 C O OC R C O R - O O Figure 13.6 Resonance structures of the growing anions. CN CH C 2 in this way exhibit high molecular weights, usually COR more than 106 Da. O Figure 13.4 Synthesis of cyanoacrylates: Mannich reaction. 13.2 Special Additives groups, they undergo rapid anionic polymerization on 13.2.1 Plasticizers contact with basic catalysts. The anionic polymeriza- Adhesives based on cyanoacrylate esters are effective tion is facilitated by the possibility of resonance struc- bonding agents for a wide variety of materials, but do tures, as shown in Figure 13.6. The polymers formed not give a permanent bond in joints involving glass. Propery of Reed Elsevier 13: CYANOACRYLATES 319 A strong bond to glass is obtained initially but gener- Table 13.4 Comonomers and Polymeric Additives ally the joint fails after a period of weeks or months Compound Reference at room temperature conditions. The extremely rapid curing rate on glass caused by the basic nature of the Methacrylate olefin copolymer [18] Short-chain alternating acrylic copolymers [19] surface is responsible for high stresses that are gener- Polyester from aliphatic/aromatic acids [20] ated in the bond line immediately adjacent to the glass, Elastomer from a core-shell polymer [21] at a molecular level. These stresses make the polymer Acrylate [22] in the bond line uniquely susceptible to chemical or physical degradation [16]. tion of plasticizer needed for good durability is about Cyanoacrylate adhesive bonds also tend to be rela- 30–50%. tively brittle; therefore, the adhesive compositions are Also, comonomers for cyanacrylate polymers and often plasticized [17]. Typical plasticizers include var- polymeric additives have been described as plasticiz- ious alkyl esters and diesters and alkyl and aromatic ers. These are listed in Table 13.4. phosphates and phosphonates, diallyl phthalates, and Components that may form semi-interpenetrating aryl and diaryl ethers. Plasticizers are summarized in polymer networks are based on poly(ethyl-2-cyano- Table 13.2. acrylate) and an oligo(ethylene glycol) diglycidyl For glass bonding, dibutyl phthalate is a suitable ether. These formulations were developed to reduce plasticizer in n-butyl cyanoacrylate [16]. The glass the brittleness of neat poly(cyanoacrylate)s [23]. bonds were tested for durability by subjecting them Some of these materials were found to be transpar- to a sequence of washing cycles in a domestic dish- ent and exhibited great flexibility, which was main- washer. The results shown in Table 13.3 suggest that tained after 24 h of immersion in water and subsequent the bond strength decreases with increasing propor- drying. tions of plasticizer. Levels greater than about 40% result in bonds of reduced strength. The concentra- 13.2.2 Accelerators The esters of 2-cyanoacrylic acid are also com- Table 13.2 Plasticizers [15] monly called quick-set adhesives, since they generally Compound Compound harden after a few seconds when used or the joined Dioctyl phthalate Dimethyl sebacate parts exhibit at least a certain degree of initial strength. Triethyl phosphate Tri(2-ethylhexyl)phosphate However, in the case of some substrates, especially Tr i(p-cresyl)phosphate Glyceryl triacetate acidic substrates such as wood or paper, the polymer- Glyceryl tributyrate Diethyl sebacate ization reaction may be very greatly delayed. Dioctyl adipate Isopropyl myristate Acidic materials exhibit a pronounced tendency to Butyl stearate Lauric acid Dibutyl phthalate Trioctyl trimellitate draw the adhesive, which is often highly liquid, out of Dioctyl glutarate the joint gap by capillary action before hardening has taken place in the gap. Even in cases in which, for reasons of geometry, the Table 13.3 Durability of Bonds to Glass with Various adhesive must be applied in a relatively thick layer Amounts of Plasticizer [16] in the joint gap or in cases where relatively large Dibutyl Phthalate Bond Strength Durabilitya amounts of adhesive are applied and relatively large (%) (N mm−2) drops of adhesive protrude from between the parts to 02.705be joined, rapid hardening throughout may rarely be 10 3.20 3 achieved [24]. 20 3.20 5 Therefore, attempts have been made to accelerate 25 1.94 5 the polymerization for such applications by means of 30 2.46 50 certain additives. The methods used may roughly be 40 1.86 90 50 0.80 90 divided into three categories: 60 0.32 20 70 0.08 10 • Addition of accelerators directly to the adhesive a Number of Dishwasher Cycles. formulation. This is possible to only a very limited Propery of Reed Elsevier 320 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS extent, however, since substances having a basic or nucleophilic action, which would normally bring R1 about a pronounced acceleration of the polymer- (CH2 CH2 O)n Si O ization of the cyanoacrylate adhesive, are generally R2 used at the expense of the storage stability of such compositions. n = 4...10 • The second common method is the addition of the Figure 13.7 Silacrown ethers [25]. accelerators shortly before application of the adhe- sive in virtually a two-component system. How- 13.2.2.2 Calixarenes ever, this method has the disadvantage that the working life is limited after the activator has been Cyanoacrylate adhesive compositions that employ mixed in.