Cyanoacrylates polymerisation

Ethyl 2-cyanoacrylate polymerises more rapidly than its butyl counterpart, and the conductivity measurements suggest that this is because the equilibrium concentration of the ethylpyridinium betaine is greater. 

JOH 81] Johnston D.S., Pepper D.C., Polymerisation via macrozwitterions, 3. Ethyl and butyl cyanoacrylates polymerised by benzyldimethyl, triethyl and tribenzylamines , Macromolecular Chemistry and Physics, vol. 182, pp. 421-435, 1981. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation). 

The chemistry of cyanoacrylate adhesives contains no co-reactants but can polymerise at room temperature on any substrate that is exposed to atmospheric moisture or alkaline surfaces. Synthesised cyanoacrylate esters can be methyl, ethyl, n-propyl, n-butyl, allyl, ethoxyethyl and methoxyethyl. The basic structure of the cyanoacrylate monomer is ... 

The industrial manufacturing process for cyanoacrylate monomers is designed to generate pure organic compounds free of metals. The prepared monomer is functionally very reactive and is polymerised by several mechanisms of which the most common is by anionic methods. In most applications the initiation is usually carried out by the nucleophilic contaminant (water or moisture) found on most surfaces. These adhesives differ from other adhesives in that they are monofunctional and can homopolymerise rapidly at room temperature. A number of modifiers have been added to impart a range of desired properties and these include stabilisers, inhibitors, thickeners, plasticisers, tracers, colorants and preservatives. 

With certain exceptions, cyanoacrylate monomer formulations containing additives e.g. rubbers, high-density neutral resins, silicon dioxide, etc., may hinder accurate and precise analysis using dilution methods. In such cases it may be necessary to prepare samples using destructive techniques, particularly where the levels are very low. Solvent selection for dilution of cyanoacrylate adhesive must be compatible for the entire journey of the sample solution from sample vessel to torch. Failure to do this could cause the cyanoacrylate to polymerise locally and block the entire sample transport system in ICP-OES and can cause serious damage requiring expensive replacements. The solvents suggested in the above dilution methods were found to be satisfactory. 

The NMR and MALDI-TOF-MS results have demonstrated that with cyanoacrylate polymers initiated with pyridine and TPP, the initiator is present as the end group. Pyridine is more easily released that TPP on thermal degradation and there is evidence that adventitious initiator can influence the degradation process suggesting the possibility of a melt-phase re-polymerisation. 

Figure 1 Structure of cyanoacrylates monomers and mechanism of polymerisation.

Alvoddinov,.A. (1982) Study of the kinetics of polymerisation of Ethyl-a-cyanoacrylate. Dokl. Akad. Nauk. Uzb. SSJl 7, 41-43. 

Donelly, E.F., Johnston, D.S. and Pepper, D.C. (1977) Ionic and zwitterionic polymerisation of n-alkyl 2-cyanoacrylates. Polymer lett., 15, 399-405. 

Cyanoacrylate adhesives contain an acidic stabiliser, which prevents the adhesive from polymerising. The acid stabiliser is neutralised when the product is in contact with the surface moisture. In general, moisture, which is normally found on all surfaces exposed to the atmosphere, is sufficient to initiate curing within a few seconds. The moisture neutralises the stabiliser and thus initiates the cure. Figure 10.1 represents the cyanoacrylate in liquid form ... 

The fast cure of cyanoacrylates can in many applications offer benefits to the production engineer, as process times will be fast and clamp times are usually short. The thinner the bond line the faster the cure. Note however that the joint must not be disturbed during the cure cycle as the cyanoacrylate will not properly re-configure itself if the molecular chains are broken at the start of the polymerisation. 

In the same way as excess adhesive can cause blooming, a slow cure may give a similar result. The cyanoacrylate at the periphery of the joint will search for available moisture from the surrounding air and may then cure as a white powder on the adjacent surface. A slow cure may be the result of excess adhesive, but is also likely to be caused by acidic deposits on the substrate. These acidic deposits can cancel ont the nentralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely (see also Section 10.4.1). 

Low strengths. Disturbance of partially cured adhesive. Check the production process. Cyanoacrylates should not be moved just as they are polymerising from a liquid to a solid. 

The fact that the polymerisation of anaerobics is initiated from the surface of the substrates being bonded gives them a deficiency in common with cyanoacrylates (see Section 2.7) namely a very limited gap-filling capability. Primers must be used to improve this situation, thus negating the one-component benefit of the adhesives. 

The polymerised cyanoacrylates are rigid thermoplastics with a Tg of 100 °C. As such they have very poor impact and peel strengths (particularly at low temperatures) and have limited resistance to high temperature. 

In contrast to the condensation polymerisation reaction for epoxy resins, another fast-acting fixing agent. Super Glue , uses an addition reaction to stick objects together. The monomer is CH2= C(CN)COOCHj, methyl cyanoacrylate, and the addition takes place across the carbon-carbon double bond. The polymerisation reaction is initiated by the presence of moisture. 

In Figure 1.1 above, the large spheres represent the adhesive monomer and the smaller spheres represent the acidic stabiliser with the dark spheres representing the surface moisture. As the cyanoacrylate comes into contact with the surface moisture, the acidic stabiliser is neutralised and chains of adhesive molecules build up on the surfaces and inter-weave to bind the surfaces together and polymerise the adhesive. 

The cyanoacrylate should not be disturbed during the critical time whilst it is polymerising, as the adhesive may never subseqnently gain its full strength. 

Slow cure can also be overcome by using an activator (or accelerator). The activators increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. UV-curing cyanoacrylates have also been used in applications to accelerate the cure speed and thus eliminate the possibility of blooming. 

Another reason for adhesive failure might be excessively fast cure. Cyanoacrylates will sometimes cure so rapidly on an alkaline surface that they polymerise before they have a chance to properly adhere to the surface. A glazed or glossy appearance to the failed cyanoacrylate is often an indication that the adhesive has cured too quickly. Plated metals sometimes have traces of alkalinity remaining on the surface and washing with an aqueous cleaner can rectify the situation. 

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