Page 727 - Carrahers_Polymer_Chemistry,_Eighth_Edition

690 Carraher’s Polymer Chemistry

21. The molds are relatively inexpensive, and large articles can be easily produced by the thermo-
forming process.
22. The length is limited by the ability to store and transport the extrudate. Actually, pipe can be
made in continuous lengths by extruding on the job site. Flexible pipe such as LDPE can be
coiled allowing for very long continuous pipes to be made and used.
23. Formica, safety glass, plywood, Macintosh raincoat.
24. Most paints coat and most coatings contain a pigment that allows it to “look nice.”
Relatively nontoxic, easy clean up.
25. Relatively nontoxic with easy clean up.
26. Oil Paints—Oil paints consist of a suspension of pigments in a drying oil, such as linseed
oil. The fi lm is formed by a reaction involving atmospheric oxygen, which polymerizes and
cross-links the drying oil. Oil Varnishes—Varnish coatings consist of a polymer, either nat-
ural or synthetic, dissolved in a drying oil together with appropriate additives as catalysts to
promote cross-linking with oxygen. Enamels—Classical enamel is an oil varnish with a pig-
ment added. Lacquers—Lacquers consist of polymer solutions to which pigments have been
added. Latex Paints—Latex paints are polymer latexes to which pigments have been added.
27. Adhesion occurs generally through one or more of the following mechanisms. Mechanical

adhesion with interlocking occurs when the adhesive mixture flows about and into two rough
substrate faces. This can be likened to a hook and eye, where the stiff plastic hooks get caught in
the fuzz-like maze of more fl exible fibers. Chemical adhesion is the bonding of primary chem-

ical groups. Specific or secondary adhesion occurs when hydrogen bonding or polar (dipolar)

bonding occurs. Viscosity adhesion occurs when movement is restricted simply due to the vis-
cous nature of the adhesive material.
Solvent-Based Adhesives—Solvent-based adhesion occurs through action of the adhesive on
the substrate. Solidification occurs on evaporation of the solvent. Latex Adhesives—Latex

adhesives are based on polymer latexes and require that the polymers be near their T so that
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they can flow and provide good surface contact when the water evaporates. Pressure-Sensitive

Adhesives—Pressure-sensitive adhesions are actually viscous polymer melts at room temper-
ature. The polymers must be applied at temperatures above their T to permit rapid fl ow. The
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adhesive is caused to flow by application of pressure. When the pressure is removed, the viscos-

ity of the polymer is sufficient to hold and adhere to the surface. Hot-Melt Adhesives—Hot-melt

adhesives are thermoplastics that form good adhesives simply by melting, followed by subse-
quent cooling after the plastic has filled surface voids. Reactive Adhesives—Reactive adhesives

are either low molecular weight polymers or monomers that solidify by polymerization and/or
cross-linking reactions after application. Thermosets: A number of thermosets have been used
as adhesives. Phenolic resins are still used to bind together thin sheets of wood to make ply-
wood. Elastomers—Solutions of natural rubber have been used for laminating textiles for over
a century. Pressure-Sensitive Tape—Pressure-sensitive tape, consists of a coating of a solution
of a blend of natural rubber and an ester of glycerol and abietic acid (rosin) on cellophane, was
developed over half a century ago. More recently, natural rubber latex and synthetic rubber have
been used in place of the natural rubber solution. The requirement for pressure-sensitive adhe-
sives is that the elastomers have a T below room temperature. Contact Adhesives—Contact
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adhesives are usually applied to both surfaces, which are then pressed together. Liquid copo-
lymers 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 copolymers 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 hydroperoxides (initiators) that
polymerize in the absence of oxygen. Cyanoacrylates—Cyanoactylates polymerize spontane-
ously in the presence of moist air, producing excellent adhesives.

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