ADHESIVES, GLUES AND SEALANTS
What is a cyanoacrylate adhesive?
Cyanoacrylate adhesives are the most famous and widespread of all the large family of adhesives and glues available today, after a curious story during its discovery, its success is based on the fast drying rate with ease of adhesion on a wide range materials and outstanding mechanical strength achieved with only small drops of adhesive, all these features make the cyanoacrylate the most commonly glue used in secondary joins. Who has not ever used the superglue at home?.
But not everything are benefits in cyanoacrylates, these adhesives have very low resistance to chemical action with water and and physical actions such as sunlight, which is why they are not suitable for joints that are exposed to the weather; this explains why when we bond with superglue (cyanoacrylate trade name) the sole of a shoe, it soon comes off.
Currently on the market there are highly resistant cyanoacrylates with a small curing time ( rigid cyanoacrylates) and a new range of cyanoacrylates that are modified to make them more flexible but with higher curing times (flexible cyanoacrylate), the rigid cyanoacrylate adhesives are the most widespread in the industrial and consumer market.
Note - All information about cyanoacrylate adhesives described are referenced to the basic principles. For each adhesive the valid information is contained in the technical data sheet do it by the manufacturers, due that the formulation of the adhesive changes its properties.
Components of cyanoacrylate adhesives and polyreaction.
Resin – Cyanoacrylate
Hardener - Water, specifically the OH-group
The polyreaction is carried out by anionic polymerization, resulting in a lattice of polymers with thermoplastic properties.
Considerations of cyanoacrylate adhesives
As discussed above, the curing of cyanoacrylate adhesives is the water or the moisture found on the surface of the work piece or on the environment. This is why we have to take special care in the short time that we have to apply these adhesives, due that cyanoacrylate glues react when they contact with humity, passing from liquid to solid state, preventing them from the wet of the substrate and therefore creating poor adhesion, all of this limits the use of these adhesives to the realization of joints with small areas.
Due to the anionic polymerization that occurs during the curing process, the cyanoacrylates also react in base areas, areas that are rich in OH-groups. By contrast, the application of cyanoacrylate in acid surfaces has no effect, causing the adhesive remains in the liquid state without reacting.
The maximum thickness that can be used with these stickers is 0.05 mm for thicknesses over this type of adhesive is greatly diminished its resistance mechanism is a fragile and easily breakable material. Because of that, the use of cyanoacrylates is not recommended for joints that require gap filling or highly porous materials joined.
In order to use cyanoacrylate chemistry and its advantages in requiring unions to fill thicknesses, it has been developed cyanoacrylate adhesives cured by ultraviolet radiation, creating rigid adherence bonds, transparent and high thickness.
Finally the cyanoacrylates exhibit the phenomenon known as "shock-curing" or spontaneous cure, which consists of a presence in excess of hardener (groups OH-) either by:
Presence of excessive humidity (over 80%).
Application of adhesive over very basic materials
Combination of both cases described above.
This excess of hardener reacts with the resin to produce the following effects:
Generation of polymer chains too small, this produces a considerable decrease in the mechanical properties of the adhesive.
Reducing dramatically the time of the skin formation, making it difficult to wet and therefore the adhesion to the substrate.
Typical properties of cyanoacrylate adhesives
They are fragile and resistant adhesives
Resistance to normal stress of 25 Mpa
10% maximum elongation
Low temperature resistance, do not expose to temperature conditions above 80 º C.
Poor resistance to moisture/ humity.
Poor resistance to ultraviolet light.
High adhesive power adhesion in a wide range of substrates, especially plastics
Adhesives are very liquid, very low viscosity, but there are versions of pasty cyanoacrylates .
They have a highly pungent odor.
Applications and examples of cyanoacrylate adhesives
As with all adhesives and glues, properties and characteristics of cyanoacrylates restrict their use in different applications where they can be employed.
The disadvantages of these adhesives are its low elasticity and its very low resistance to chemical and physical attacks, this is the reason why these adhesives are not used in the performance of joints that require elasticity or that are exposed to the exterior.
But, his little cure time and the ease of adhesion a wide range of different materials, make cyanoacrylate adhesives the perfect candidate for making multiple connections.
Cyanoacrylates are widely used in the general consumer market, to repair broken pieces such as ceramics, plastics or metals in a simple and reliable way.
Cyanoacrylates are also used in medical applications because of their short curing time and high adhesion to the skin, the cyanoacrylates are ideal for suturing wounds, this application was first used by surgeons and physicians who cared for the soldiers who fought during the Vietnam War.
On the other hand the cyanoacrylates are used in the industrial market in those bonds where the substrates are plastics that do not require the exposure to chemicals or physical agents such as water or sunlight, for example the interior fitting of equipment and vehicles indicators, displays, fasteners, assembly parts that are hidden, etc ...
Particularly suitable for plastic joints and rubber or rubber caps for non-polar plastics is necessary the use specific primers.
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