Cyanoacrylates durability

Aerobic adhesives were developed as an advance over anaerobic and cyanoacrylate adhesives. Anaerobic (threadblocking) adhesives are used to augment mechanical fasteners such as screws, bolts, or press-fits. Cyanoacrylates tend to be used for nondurable bonding on rigid surfaces and for durable rubber bonding. 

Figure 7 Durability of cyanoacrylate adhesives. Asterisk denotes substrate failure.

Assemblies joined with cyanoacrylate adhesives exhibit good long-term durability, particularly when the materials are somewhat flexible, such as rubbers and most plastics (see Fig. 7). Bonded lap shear specimens have been aged outdoors for 7 years with good retention of strength (see Table 8). 

The main reason for the use of plasticizers in formulations is to increase flexibility. More extensive stu has been conducted with dioctyl phthalate, DOP, where appearance, curing time, film formation, flexibility and durability have been evaluated for n-butyl cyanoacrylates containing from 15 to 50 wt% plasticizer. When DOP was at a 35 wt% or above, no film was formed. With the plasticizer at 15 wt%, flexibility and durabihty suffered. A 20-25 wt% range has produced the best overall performance. 

Adhesives recommended include modified epoxies, modified phenoUcs, epoxy-phenolics, neoprene-phenolics, second-generation acrylics, cyanoacrylates, silicone rubbers, and vinyl plastisols. Sell has ranked a number of adhesives in the order of decreasing durability with aluminum adherends as follows ... 

Environmental performance The durability (see Durability - fundamentals) of cyanoacrylate adhesive bonds is reasonably good on rubbers and some polymer substrates. However, on glass and metals, both thermal and moisture durability are low. 

Moisture resistance This may be increased on metal and glass substrates by including cross-linking agents, which may yield a more hydrolytically stable polymer or by using hydrophobic monomers such as fluorinated cyanoacrylates. Silane adhesion promoters also improve moisture durability. There is also evidence to suggest that inclusion of some of the anhydrides described above has a beneficial effect. 

Printing processes may include a post-processing infiltration step in order to increase the strength of the printed article using two-component casting resins or adhesives or one-component cyanoacrylate adhesives to achieve greater durability in a three-dimensional article (17). 

Reactive liquid adhesives can be chemically cross-linked to form durable, three-dimensional molecular structures. These bind chemically and/or mechanically with the fabric surface to generate high joint strength. They are generally one-part elevated-temperarnre-setting or two-part room-temperature-setting polymer systems. However, one-part systems that cure at room temperature when in contact with the substrates are also available (e.g., cyanoacrylates). 

In summary, it can be stated that a large number of different surface treatments exist to improve the adhesional properties of PP. It should be stated here that some manufacturers of cyanoacrylate adhesives recommend special chemical primers which produce adhesion on PP without other treatment. In the experience of the author, this type of primer improves the adhesion but produces bonded joints that are insufficiently durable, especially under humid enAuronmental conditions. 

Cyanoacrylates were shown to form a strong durable bond between bones in vitro. Tensile adhesive strength betw een smooth bovine cortical bone specimens bonded together with the isobutyl monomer and tested after one day storage in water was approximately 6.5 mPa (Brauer et al, 1979). The monomer was used, without evidence of histotoxicity, to repair osteochondral fractures (Harper and Ralston, 1983) and recently to improve meniscal repairs (Koukabis et al, 1995). Butylcyanoacrylate was also used in facial bone surgery for frontal bone reconstitution (Avery and Ord, 1982). 

There are several reasons why cyanoacrylates are attractive as adhesives. They are easy to apply, one-part, 100% reactive, storage-stable adhesives. They cure rapidly at room temperature when spread in thin films between substrate surfaces, and they form strong bonds between a variety of substrates. However, cyanoacrylates do have several serious shortcomings including poor heat resistance, poor moisture resistance, poor peel and impact resistance, and limited ability to fill gaps and to bond porous substrates. The poor durability and impact resistance have been particular limitations in metal-to-metal bonding. 

Durability is one of the most important aspects of the performance of a structural adhesive. The durability of an adhesive joint is the sum total of its responses to environmental effects such as heat, moisture, other chemicals, radiation, and mechanical stresses. Cyanoacrylate-based adhesives have a reputation for poor durability, especially when bonding metals... 

A variety of non-cyanoacrylate-based modifiers have also been proposed over the last two decades. Some of the earliest of these were the dialkenyl phthalates. Addition of 1-25% concentrations of the phthalate ester to the cyanoacrylate was claimed to improve wet heat resistance. A similar type of heat durability promoter consists of acrylate or methacrylate esters of phosphonates, polyols, or cyanuric acid. Here too, the improvement in heat resistance depends upon curing at elevated temperature, presumably to form a network of the additive, which may also coreact radically with the cyanoacrylate. The use of such additives results in improved heat resistance at 1-20% concentrations. 

The discussion of heat durability in this section should make clear that this is a complex phenomenon. Embrittlement, retropolymerization, thermoplasticity, and the loss of adhesion are all factors affecting the adhesive s performance on metal surfaces. Based on the published state of the art, optimum cyanoacrylate heat durability could be achieved using a combination of a heat-resistant adhesion promoter, a crosslinking agent, and a plasticizer. The heat durability promoters discussed in this section are summarized in Table XI. 

The resistance to hydrolysis of a polycyanoacrylate can also be improved by copolymerization with another monomer. In fact, the cyanopentadienoates cited above should also be considered comonomers. An electron-rich vinyl monomer like styrene and a cyanoacrylate ester will spontaneously copolymerize a short time after mixing. The product is a one-to-one alternating copolymer. This reaction has been used to make a two-part adhesive with better hydrolytic stability than the standard cyanoacrylate. " The carbon dioxide evolved from a suspension of the polymer in boiling water was measured to follow the degradation. After eight hours, polymethyl cyanoacrylate had lost 28% carbon dioxide by hydrolysis and decarboxylation of the polycyanoacrylate ester groups, while the copolymer had lost none. However, this improvement in moisture durability is achieved at the expense of the convenience of a one-part adhesive. 

Another class of cyanoacrylate comonomers which improve moisture durability are the fluorinated ethers of dimethylvinylethynylcarbinol (29). When these monomers are added to cyanoacrylates in 5 to 30% concentration, from 70 to 90% of the bond strength on aluminum is retained after ten days immersion in water. The control composition retained about 20% of the original bond strength under these conditions. 

Discriminating between the various effects of heat and moisture on the strength of cyanoacrylate metal-to-metal bonds is not easy. The environment can affect the adhesive, the metal surface, or the interface between them. The reduction in strength may be due to heat alone, or to water, or to both. The most durable adhesive possible today would probably contain a room temperature active crosslinking agent and an adhesion promoter resistant to both heat and moisture. Table XII lists the water durability modifiers discussed in this section. 

Aluminum can be cleaned in the same fashion as steel. Alternatively, it may be prepared for bonding by chemical etching. For instance, cyanoacrylates cure rapidly to give strong, durable bonds on aluminum etched by Forest Products Laboratory (FPL) chromic acid etching solution. 

Vitreous materials and ceramics respond best to cleaning with water-based cleaners followed by water rinsing and drying. The alkalinity of these substances makes them poor candidates for cyanoacrylate bonding because the durability is so poor. Also, thin adhesives tend to wick into the porous surface of the ceramic before curing, further weakening the bond. 

Cyanoacrylates bond well to wood despite the porous and acidic nature of the material. A viscous adhesive cured with a surface activator will prevent the adhesive from wicking away from the bondline and overcome the natural acidity of the wood. The nature of the initiator influences the durability of the cured bond. Strongly alkaline initiators promote hydrolysis of the adhesive, thus shortening the useful lifetime of the bonded joint. The non-basic initiators described by Robins or the accelerators described in Section II.E.2. should be employed in wood bonding. The wood surface should be freed from grease and loose dirt before bonding. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

The data in Tables XVI and XVII show that cyanoacrylates lack durability on metal surfaces and that the durability on chloroprene is moderate. On the other hand, the cyanoacrylate is more durable than the epoxy and as durable as the polyurethane on polystyrene under moist aging conditions. 

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. 

In contrast to the attention given to the bonding of aluminum and wood, very few studies have been reported dealing with the durability of other structurally bonded systems." A limited amount of sustained load durability data is available for steel bonded with epoxies and aluminum bonded with urethanesin recent years, reports have appeared dealing with the environmental resistance of modified acrylics and cyanoacrylates. Brockmann has also reported on the durability of steel joints exposed to hostile environments. " ... 

Polyamides (nylon) can be bonded well with cyanoacrylates, epoxies, and acrylics. Careful durability testing of joints is recommended because of the tendency of the plastic to absorb atmospheric moisture. 

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