Toughened Cyanoacrylates

In recent years, the range of adhesive materials used in automotive manufacture has expanded to include polyurethanes, plastisols, phenolics, hot melts, anaerobics, cyanoacrylates, toughened acrylics and epoxies (see Structural and Hot melt adhesives). Selection criteria are based principally upon the nature of the adherends, the mechanical properties required under service conditions and application and curing characteristics. 

Manufacturers Comments Ethyl cyanoacrylate. General purpose, Manufacturers Comments Modified ethyl cyanoacrylate. Toughened ... 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

The addition of Vamac B-124 to ethyl cyanoacrylate has a more pronounced effect on peel strength, both at ambient temperature and after thermal exposure. After 24 h at ambient temperature, the peel strength of the rubber-toughened formulation is almost 40% greater than the control formulation A without rubber. After heating the test specimens for 2 h at 121°C, the peel strength of formulation A, is almost non-existent, while that of C has increased significantly, as seen in Fig. 7. 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

Adhesives formulated from tile 2-cyanoacrylic esters typically contain stabilizers and thickeners, and may also contain tougheners, colorants, and other special property-enhancing additives. 

Many cyanoacrylates give rather rigid bonds which may in consequence be brittle and not able to resist peeling forces. However, in recent years more flexible toughened grades have been developed for applications in which resistance to peeling is required. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

The use of adhesives/sealants in both the industrial and consumer spheres has increased dramatically in the past 20 years. In the industrial segment, both reactive and nonreactive systems are used in a wide variety of applications. The increasing use of reactive systems has, however, tended to overshadow that of the nonreactive systems. Most prominent amongst the reactive systems favored in industrial applications are anaerobic sealants (methacrylate ester based), instant adhesives (alkyl cyanoacrylate ester based), acrylic (toughened) adhesives, epoxy resin adhesives, polyurethane/isocyanate-based adhesives, silicone adhesives/sealants, and phenolic resin adhesives. 

Plasticizers These are required to reduce the inherent brittleness of poly(alkyl-2-cyanoacrylates). This can be achieved by using non-copolymerizing plasticizers such as esters or higher alkyl cyanoacrylates, which copolymerize with the basic adhesive monomer. Toughness properties can be improved by the inclusion of rubber toughening materials such as ABS (acrylonitrile-butadiene-styrene) or MBS (methacrylate-butadiene-styrene) copolymers. Whichever approach is adopted, toughness is only achieved at the expense of reduced cure speed. 

Very light plastics fans may often be bonded satisfactorily with either cyanoacrylate adhesives or toughened, cold-curing acrylics. Dismantling is not normally practicable when these adhesives are used. 

Suitable plastics pulleys may be bonded satisfactorily with cyanoacrylate adhesives or, where large components are involved, with toughened acrylics -many of which are unsuitable for use on small parts. The stronger anaerobic adhesives and single-part epoxies should be used for all-metal assemblies. 

Quite contrary to popular belief - a belief founded on the use of traditional adhesives - reliable joints can be obtained from unprepared surfaces. Of course, there is no denying that the better the preparation the better the overall performance. But, providing contamination is not gross, perfectly adequate levels of performance can normally be obtained from Anaerobic Cyanoacrylate Plastisol Toughened acrylic and Toughened, heat-cured, epoxide-based adhesives. 

While both cyanoacrylate and toughened acrylic adhesives contain no solvents - all the liquid present is converted to solid - their common base material is an excellent solvent in its own right and so both function well on unprepared surfaces. Toughened acrylics are noteworthy in this respect, though the tolerance of the cyanoacrylates is partially limited because contamination may inhibit hardening. Nonetheless, the cyanoacrylates generally cope well with the unprepared surfaces of the small plastics, rubber and metal parts that they are usually used on. 

Both rigid and flexible PVC may be bonded with toughened acrylics, cyanoacrylates and the solvent-based adhesives. However, although not as convenient as the cyanoacrylates, the toughened acrylics give the best overall performance particularly in severe environments. 

Despite these strictures, a few examples of toughened cyanoacrylates can be found in the patent literature. The earliest mention of a copolymeric toughener is found in a Japanese patent issued to Kato et The toughener... 

Toughened cyanoacrylate adhesives containing random copolymeric elastomers have been patented. The only elastomer whose performance has been quantified is an ethylene-methyl acrylate copolymer sold by Du Pont under the name Vamac B-124. Several other elastomers are said to be useful tougheners, but no details have been given beyond solubility data. 

Brittleness. Cyanoacrylate adhesives are known for their lack of toughness, especially in metal-to-metal bonds. This problem is less severe in plastic or rubber joints. The inclusion of adhesion promoters or tougheners in the adhesive can mitigate this weakness. 

In the last ten years, a number of improvements in cyanoacrylate adhesive technology have been published. Some of these modifications have been translated into new products. For instance, a series of adhesives is being sold with improved performance in the following areas contaminated surface bonding, hard-to-bond plastics, operating temperatures, moisture durability, impact strength, and chlorosis. A toughened cyanoacrylate based on a methyl acrylate-ethylene copolymer has been marked recently. An allyl cyanoacrylate-based adhesive with improved heat durability has also been introduced. A survey of recently patented modifications and improvements for cyanoacrylate adhesives is outlined in Table XIX. 

Tbe lower esters of alkyl 2-cyanoacrylates (eg, methyl, ethyl esters) provide adhesives with excellent lap-shear strengths to a variety of different substrates (3). They are, however, brittle materials and generally require the addition of a rubber-toughening agent to improve peel strength and impact resistance (4). The low esters have pungent odors whereas the higher esters, in particular the alkoxyalkyl esters, are virtually odorless (3). 

---