Page 642 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Technology 605

Contact Adhesives—Contact adhesives are usually applied to both surfaces, which are then pressed
together. Liquid copolymers of butadiene and acrylonitrile with carboxyl end groups are used as
contact adhesives in the automotive industry.
Thermoplastics—A number of thermoplastics have been used as adhesives. Polyamides and copo-
lymers of ethylene and vinyl acetate (EVA) are used as melt adhesives. Copolymers of methyl
methacrylate and other monomers are used as adhesives in the textile industry. Poly(vinyl acetate)
is often used in school glues.
Anaerobic Adhesives—Anaerobic adhesives consist of mixtures of dimethacrylates and hydroper-
oxides (initiators) that polymerize in the absence of oxygen. They are used for anchoring bolts.
Cyanoacrylates—One of the most interesting and strongly bonded adhesives are cyanoacrylates
(Super Glue; Krazy Glue). These monomers, such as butyl-alpha-cyanoacrylate (18.3), polymerize
spontaneously in the presence of moist air, producing excellent adhesives. These adhesives, which
have both the cyano and ester polar groups, are used for household adhesive problems as well as in

surgery, mechanical assemblies, and as a “fast fix” for athletic cuts such as in boxing.
N

H C
2
O (18.3)
O

H C
3
Many seemingly simple adhesive applications are actually complex. The labels on commercial

dry cell batteries can contain over a dozen layers each present for a specific purpose. While price is
a major consideration, ease of application is another. Thus, while many naturally derived adhesives
are less expensive, synthetic materials may be chosen because of ready application and consistency
of the end product.
One common use of adhesives is as the “working” ingredient of tapes. Numerous tapes are avail-
able, many with interesting stories. The ingredients vary with the intended use. Table 18.9 contains
the main ingredients of some important tapes. But, knowing the ingredients is only the start. There
is much science involved. For instance, important factors involved with pressure-sensitive adhesion
are a balance between allowing molecular interaction between the adhesive and adherent (often
referred to as “wetting”) and the dynamic modulus of the adhesive mixture. This also involves a
balance between “pull-off-rate” and “wetting rate.” Mechanical adhesion with interlocking and
diffusion factors is less important than for permanent adhesion. Pressure-sensitive adhesives, such
as present in “pull-off” tabs such as Post-It Notes, contain components similar to those present in
more permanent Scotch tape, except that particles of emulsified glass polymer are added to reduce

the contact area between the adhesive and the substrate. Some polymers, such as PE, might appear
to be decent adhesive materials, but even in its melt, it is not exceptionally tacky. This is believed a
result of the high degree of chain entanglement. Since the dynamic modulus increases with increas-
ing chain entanglement, PE is not “tacky” (does not easily contact and wet a substrate), and is not a
useful pressure-sensitive adhesive.
Super glue was initially discovered in 1942 as part of the war effort in a search to make clear
plastic gun sights. Super glue was discovered but it stuck to everything so was discarded. In 1951,

Harry Coover and Fred Joyner, Eastman Kodak researchers, rediscovered it. It was first sold as a
commercial product in 1958.

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