cynoacrylates ,additives ,synthesis ,application ,history ,monomer ,commercial products , plastisizer ,accelerator ,primers ,curing ,properties

1.  Cyanoacrylates were commercially introduced in 1950 by Tennesee Eastman
Company. Cyanoacrylate adhesives are monomeric adhesives. They are generally
quick-setting materials which cure to clear, hard glassy resins, useful as sealants,
coatings, and particularly adhesives for bonding together a variety of substrates.
Polymers of alkyl 2-cyanoacrylates are also known as superglues.
 In addition to their use as adhesives, cyanoacrylates have been reported to have
highly herbicidal properties, as they disrupt photosynthetic electron transportation

2.  In 1895 von Auwers and Thorpe attempted to synthesize diethyl-2,2-
dicyanoglutarate by base-catalyzed condensation of aqueous formaldehyde and
ethyl cyanoacetate. They isolated a mixture of oily oligomers and an amorphous
polymer of higher molecular weight. In fact, ethyl cyanoacrylate monomer was
synthesized as an intermediate, which underwent an immediate polymerization
reaction.
 The condensation of formaldehyde with cyanoacetate is still the most important
method for the commercial production of the monomers. The reaction mechanism
takes place as a base-catalyzed Knoevenagel condensation of cyanoacetate and
formaldehyde to give an intermediate disubstituted methylol derivative.

3.  A.E. Ardis at B.F. Goodrich (in 1947) found that the polymer-oligomer mixture
obtained in the formaldehyde-cyanoacetate condensation reaction could be
thermally depolymerized with acid catalysts. However, the monomer prepared by
utilizing these methods was unstable and the yields were low. Later it was realized
that the water is responsible for polymerization.
 Instead of aqueous formaldehyde, paraformaldehyde was used with an organic
solvent to remove the water by azeotropic distillation. The stability of the
monomer can be enhanced by the redistillation of the crude monomer in the
presence of small quantities of acidic stabilizers, e.g., sulfur dioxide. Several other
methods for cyanoacrylate monomer production have been described, including
the pyrolysis of 3-alkoxy-2-cyanopropionates , trans esterification of ethyl
cyanoacrylate, and displacement of cyanoacrylate monomer from its anthracene
Diels- Alder adduct by treatment with maleic anhydride. This last method is used
for the synthesis of monomers that are not accessible or may be difficult to prepare
by the retro polymerization route, for example difunctional cyanoacrylates ,
thiocyanoacrylates , and per fluorinated monomers.
 To improve the cohesive strength, difunctional monomeric crosslinking agents
may be added to the monomer compositions. These include alkyl bis(2-
cyanoacrylates), triallyl isocyanurates, alkylene diacrylates, alkylene
dimethacrylates, trimethylol propane triacrylate, and alkyl bis(2-cyanoacrylates)

4. Commercial products consist mainly of monofunctional monomers. The monomers are
usually low-viscosity liquids with excellent wetting properties.
The monomers are usually low-viscosity liquids with excellent wetting properties. The
basic structure of cyanoacrylate monomers and polymers is

5.  The synthesis of the monomers and the raw materials are shown
Because of the high electronegativity
of the nitrile group and the carboxylate groups,
they undergo rapid anionic polymerization on
contact with basic catalysts. The anionic
polymerization is facilitated by the possibility of
resonance structures. The polymers formed in
this way exhibit high molecular weights, usually
more than 106 Da.

6. Adhesives based on cyanoacrylate esters are effective bonding agents for a wide variety
of materials, but do not give a permanent bond in joints involving glass. A strong bond
to glass is obtained initially but generally the joint fails after a period of weeks or
months at room temperature conditions. The extremely rapid curing rate on glass
caused by the basic nature of the surface is responsible for high stresses that are
generated in the bond line immediately adjacent to the glass, at a molecular level. These
stresses make the polymer in the bond line uniquely susceptible to chemical or physical
degradation. For glass bonding, dibutyl phthalate is a suitable plasticizer in n-butyl
cyanoacrylate Typical plasticizers include various alkyl esters and diesters and alkyl
and aromatic phosphates and phosphonates, diallyl phthalates, and aryl and diaryl
ethers.
The bond strength decreases with
increasing proportions
of plasticizer. Levels greater than about
40% result in bonds of reduced strength.

7. Also, comonomers for cyanoacrylate polymers and polymeric additives have been
described as plasticizers.
The esters of 2-cyanoacrylic acid are also commonly called quick-set adhesives, since
they generally harden after a few seconds when used or the joined parts exhibit at least
a certain degree of initial strength. However, in the case of some substrates, especially
acidic substrates such as wood or paper, the polymerization reaction may be very
greatly delayed. Acidic materials exhibit a pronounced tendency to draw the adhesive,
which is often highly liquid, out of the joint gap by capillary action before hardening
has taken place in the gap. Therefore, attempts have been made to accelerate the
polymerization for such applications by means of certain additives. The methods used
may roughly be divided into three categories:
1. Addition of accelerators directly to the adhesive formulation. This is possible to
only a very limited extent, however, since substances having a basic or nucleophilic
action, which would normally bring about a pronounced acceleration of the
polymerization of the cyanoacrylate adhesive, are generally used at the expense of
the storage stability of such compositions.

8. 2. The second common method is the addition of the accelerators shortly before
application of the adhesive in virtually a two-component system. However,
this method has the disadvantage that the working life is limited after the
activator has been mixed in. In addition, with the small amounts of activator
that are required, the necessary accuracy of metering and homogeneity of
mixing are difficult to achieve.
3. A third process is the use of activators in the form of a dilute solution. The
solution is sprayed onto the parts before they are bonded onto the places
where the adhesive is still liquid after the substrates have been joined. The
solvents used for such dilute solutions of activators are generally low-boiling
organic solvents.
Silacrown compounds as additives give substantially reduced fixture and cure times on
wood and other deactivating surfaces such as leather, ceramic, plastics, and metals with
chromate-treated or acidic oxide surfaces. Silacrown accelerators have significantly
lower reported acute toxicity than the crown ether compounds. The lower observed
toxicity of silacrowns in comparison to crown ethers may be related to the hydrolytic
instability of the Si–O–C linkage. Silacrowns are prepared by transesterification of
alkoxysilanes with poly(ethylene glycol)s (PEGs), i.e., they are reaction products of
silanes but are not themselves silanes.

9. Cyanoacrylate adhesive compositions that employ calixarene compounds as additives
give substantially reduced fixture and cure times on wood and other deactivating
surfaces such as leather, ceramic, plastics, and metals with chromate-treated or ceramic
oxide surfaces
Solutions of lower fatty amines, aromatic amines, and dimethylamine are used that are
sprayed on the surface before the cyanoacrylate is applied, or at the same time.
Examples are N,N-dimethylbenzylamine, N-methyl morpholine, and N,N-
diethyltoluidine. N,N-Dimethyl-p-toluidine, when subsequently applied to the joined
parts, causes even relatively large amounts of adhesive to harden within seconds. The
poly(cyanoacrylate) so formed is completely free of turbidity. Disadvantages include
the very high volatility of the substance, which does not permit long waiting times
between the application of the accelerator solution to the substrates to be bonded and
the subsequent bonding process. The compound is also toxic
Examples of disulfides are dibenzodiazyl disulfide, 6,6-dithiodinicotinic acid, 2,2-
dipyridyl disulfide, or bis(4-tert-butyl-1-isopropyl-2-imidazolyl) disulfide. The
disulfides have a good accelerating action, but they nevertheless permit a long waiting
time between application of the activator and application of the adhesive. In addition,
they avoid spontaneous, merely superficial hardening.

10. Stabilizers have to be added both for the
production and for storage. The stabilizer
systems are added so that no polymerization
occurs during transportation and storage in
sealed drums, even at elevated temperatures
and after long periods. After application
polymerization occurs immediately.
Accordingly, besides radical polymerization
inhibitors, inhibitors against anionic
polymerization are generally added to
cyanoacrylate adhesives. Stabilizers are
summarized in Table 13.6. A typical
stabilizer to prevent radical polymerization is
hydroquinone. Boron trifluoride prevents
anionic polymerization.

11. Thickeners are added to increase the viscosity of 2- cyanoacrylate adhesive compositions. The
2-cyanoacrylate monomer generally has a low viscosity of several centipoise, and therefore
the adhesive penetrates into porous materials such as wood and leather or adherents with a
rough surface. Thus, good adhesion bond strengths are difficult to achieve. Thickeners are
summarized as
It has been proposed to add a strongly acidic cation exchanger as inhibitor. Cation
exchangers are based on crosslinked poly(styrene)-containing sulfonic acid groups. The
disadvantage of this approach is that the ion exchanger added can easily impede the
outflow of the adhesive and that, as a solid, it does not act throughout the entire volume of
the adhesive.

12. It has been proposed to modify the surface of storage containers for cyanoacrylate
adhesives in such a way that they contain acid groups [32]. Although this proposal can
be successfully implemented, it is afflicted by the problem that the inhibition occurs in
the vicinity of the container wall.
 Sulfur Dioxide. Another method of stabilizing cyanoacrylate adhesives is to add
sulfur dioxide as an inhibitor.
 Dioxathiolanes. Cyclic organic sulfates, sulfites, sulfoxides, and sulfinates, for
example 2-oxo-1,3,2- dioxathiolanes, act in raising the ceiling temperature and
hence to improve the thermal stability of the adhesives
It is well known in the adhesive field that there are plastic substrates made from certain
types of plastic materials which are extremely difficult to bond. Such difficult-to-bond
materials include low surface energy plastics such as poly(ethylene) and
poly(propylene) and highly crystalline materials such as polyacetals and poly(butylene
terephthalate). As a consequence of the difficulty in bonding substrates made from
these plastics materials with adhesives, various surface treatments have been employed
where such materials require bonding.

13. 1,5-Diazabicyclo[4.3.0]non-5-ene, 1,8-diazabicyclo- [5.4.0]undec-7-ene, and 1,5,7-
triazabicyclo[4.4.0]dec- 5-ene are shown

14. Compounds with a variety of highly
branched architectures are known, including
cascade, dendrimer, hyperbranched, and
comb-like architectures. The term multi-amine
compounds refers to compounds with such
branched architectures in which branching
occurs via tertiary amine groups. For
example, polyamine dendrimers are prepared
by the condensation of tris(2-
aminoethyl)amine and dodecyl aldehyde
followed by reduction with tetran-
butylammonium cyanoborohydride . The
reaction is shown as

15. Cyanoacrylates can be polymerized both by radical and by anionic mechanisms. The
polymerization of cyanoacrylates has been monitored by Raman spectroscopy. with
free-radical initiators. However, in the presence of catalytic amounts of anionic bases
and in the presence of covalent bases such as amines and phosphines, they polymerize
extremely rapidly. The exceptionally fast rate of anionic polymerization of
cyanoacrylates in the presence of a base, including water, made this class of monomers
unique among all acrylic and vinyl monomers. Consequently, the anionic
polymerization is initiated by traces of moisture which are to be found on almost all
surfaces. Accordingly, cyanoacrylate adhesives set very quickly when introduced
between two surfaces stored under ambient conditions.
Although the predominant mechanism by which cyanoacrylate monomers undergo
polymerization is anionic, free-radical polymerization is also known to occur. Radical
polymerization of cyanoacrylate can be achieved in the presence of a radical-forming
component and a photosensitizer. The radical-generating component can be dibenzoyl
peroxide and the photoinitiator component is 2,4,6-triphenylpyrylium tetrafluoroborate
. The chemical structures of these compounds are shown

16. The particular advantage of cyanoacrylate adhesives in terms of adhesives technology
lies precisely in the high reactivity coupled with the high bond strengths of the final
materials, especially to polar substrates. Due to high molar mass, good wetting
properties, and polarity, poly(cyanoacrylate)s exhibit excellent adhesive properties. In
addition, they have been found useful as polymeric binding agents in controlled drug
delivery systems. They are also useful for dry etching processes.
Of the alkyl cyanoacrylate family of monomers, the methyl and ethyl esters are used
extensively in industrial and consumer-type adhesives.
 Rapid Prototyping:
A three-dimensional printing process uses a powder material as a substrate and a liquid
binder as the ink. The three-dimensional printed specimens as such exhibit a porous
structure and low mechanical strength. Therefore, three-dimensional prints are usually
finished by an infiltration agent prior to their final use. For this, an epoxy resin, a
cyanoacrylate, and a poly(urethane) are commonly used . The type of infiltration agent
greatly influences the final properties of the prints.
 High-resolution Lithography:
In the microelectronics industry involving the construction of micro machines or
magneto resistive heads, there is a continued need to reduce the size of structural
features . The ability to obtain a finer resolution is limited by the wavelength of the light
used to create the lithographic pattern. The trend has been to move from so-called I-line
radiation at 350–248 nm radiation with the goal to further reduce the wavelength.

17.  Cosmetic and Medical Applications:
Cyanoacrylates are also used as adhesives in the ordinary household. A case of an
accidental aspiration of a cyanoacrylate adhesive has been reported . The use of
cyanoacrylate compounds in medical applications has been reviewed . Poly(alkyl
cyanoacrylate)s are biodegradable and biocompatible polymers. Ethyl cyanoacrylate,
butyl cyanoacrylate, and octyl cyanoacrylate have been approved by the US Food and
Drug Administration (FDA) . Such polymers have also been proposed as raw materials
for the synthesis of nanoparticles to be used for drug delivery.
 Manicure Composition:
Cyanoacrylate compositions are used as manicure compositions in treating chapped
nails. When nails are manicured, it is generally observed that the moisture content in
the nails becomes out of balance or lipids are eluted out from the nails. As a result, nail
chapping proceeds under the manicure coating. Therefore, the nail chapping can be
prevented by adding to manicure compositions a substance capable of keeping nails in
good health or improving the nail health.
 Leak Control Embolization of Blood Vessel Ruptures:
A traumatic rupture of the abdominal aorta commonly results in a rapid
exsanguination and death before the patient reaches the hospital. In certain cases an
open surgical repair can be performed. However, there are other methods, such as a
repair using stent grafts. An embolization can be done using N-butyl- 2-cyanoacrylate
with or without associated metallic coils

18. 2-Octyl cyanoacrylate was used as an adjunct to control air leaks after lung resection by
application directly onto the visceral pleura. The indications of this treatment included
a lack of standard sealants in patients with emphysematous lung that would not hold
suture and vast air leaks after decortications or extensive nonanatomic resections in
patients that would not tolerate the loss of tidal volume. All the air leaks sealed
immediately.
Cyanoacrylates can be used for skin surface biopsy because the polymers are formed
with moisture and adhere to the skin . The stratum corneum is responsible for
maintaining a skin barrier function. In a medical study, samples of the stratum
corneum were collected for in vitro investigations . One drop of a cyanoacrylate resin
was placed on a glass slide. Afterwards the glass slide was attached to the test site with
slight pressure and after a short time of 1 min removed.
The isobutyl, n-butyl, and n-octyl cyanoacrylate esters are used clinically as blocking
agents, sealants, and tissue adhesives due to their much lower toxicity as compared
with their more reactive methyl and ethyl counterparts. Cyanoacrylate ester
compositions can be sterilized using visible light irradiation at room temperature.