Page 451 - Handbook of Adhesives and Sealants

Adhesives Families 389

They have good resistance to moisture and solvents, and salt spray
resistance is excellent. Tough anaerobic adhesives have been prepared
from resins having hard but flexible properties. 25

10.3.11 Cyanoacrylates
Cyanoacrylate adhesives became widely known after their introduc-
tion in the early 1970s in the consumer market as ‘‘superglue’’. Cy-
anoacrylate adhesives are generally methyl or ethyl cyanoacrylate-
base, single component liquids that have fast cure, excellent tensile
shear strength, and good shelf life. Although very similar in curing
characteristics to the anaerobic adhesives, cyanoacrylate resins are
more rigid and less resistant to moisture.
Cyanoacrylate adhesives are available only as low-viscosity liquids
that cure in seconds at room temperature without the need of a primer.
Cyanoacrylates normally polymerize by means of an anionic mecha-
nism that requires moisture in the air to initiate polymerization. The
cure mechanism does not work well on acidic substrates that can in-
hibit or terminate the polymerization or on wet substrates which re-
sult in formation of many low molecular weight chains. Factors that
influence curing include the amount of moisture on the surface to be
bonded, relative humidity, pH, and bond-line thickness. The curing
mechanism is water initiated. Lack of complete moisture penetration
in thick bond-lines can prevent curing of the center section of the bond.
Cyanoacrylate adhesives bond well to many substrates as shown in
Table 10.16. When bonding metals and other rigid surfaces, methyl
cyanoacrylate bonds are stronger and more impact resistant than
ethyl cyanoacrylate bonds. However, ethyl cyanoacrylate is preferred
on rubber or plastic surfaces.
Cyanoacrylates normally have poor heat and moisture resistance;
their peel and impact strengths are also low. Although cyanoacrylate
adhesives cure, they have many of the characteristics of a thermo-
plastic resin. They find use as structural adhesives where there is
minimal environmental stress and great value is placed on fast setting
times. Cyanoacrylate adhesives also have relatively high cost.
A new generation of cyanoacrylates claims to have improved tough-
ness and high shear strength (3,500 psi). 27 New cyanoacrylate resin
monomers have been introduced to provide faster cures, higher
strengths with some plastics, and greater thermal and impact resis-
tance. Products have been developed which have 5–10 times higher
peel strength than conventional compositions. Rubber toughening has
been accomplished using various tougheners. 28,29 The use of phthalic
anhydride has been reported to improve both the moisture resistance
and heat resistance of the cyanoacrylates. 30 Cyanoacrylates are now
available in gel form in addition to the conventional low viscosity liq-