Page 458 - Handbook of Adhesives and Sealants

Adhesives Families 395

truly non-structural systems and will creep extensively under load
even at low temperatures. They are generally used in low strength or
temporary applications.
In addition to pressure-sensitive adhesives, elastomers go into mas-
tic compounds that find wide use in the construction industry. Neo-
prene and reclaimed-rubber mastics are used to bond gypsum board
and plywood flooring to wood-framing members. Often the adhesive
bond is much stronger than the substrate. These mastic systems cure
by evaporation of solvent through the porous substrates.
Elastomer-adhesive formulation is more complex than that of other
adhesives because of the need for antioxidants, tackifiers, and other
additives to the base resin. Each formulation is generally designed to
fulfill a specific set of properties. As a result, commercial elastomeric
adhesives have widely differing properties.

10.4.1.1 Natural rubber. Natural rubber is one of the earliest materi-
als used in formulating adhesives. The long tack retention properties
of natural rubber have made it ideal for pressure sensitive adhesive
formulations. Chemical catalysts or accelerators may be used at am-
bient temperature, or heat curing can be used to vulcanize the natural
rubber to improve the strength and temperature resistance. The rel-
ative strength of vulcanized natural rubber is low relative to other
adhesives. Natural rubber adhesive formulations are heavily modified
with synthetic resins and additives to obtain tailored properties.
Natural rubber adhesives are commonly used in bonding nonmetal-
lic materials such as leather, fabrics, paper, and other rubber products.
Natural rubber has good resistance to water, but poor resistance to
oils, organic solvents, and chemical oxidizing agents. The natural
rubber molecule crystallizes at about 30 F, and water latex emul-
sions must be protected from freezing.
Often chlorinated natural rubber is used in adhesive formulations.
The outstanding property of chlorinated rubber is its resistance to
water, some oils, salt spray, and acids and alkalis. Resistance to or-
ganic solvents in general is poor, and the practical temperature limit
is 280 F. In adhesive formulations, chlorinated rubber has been used
to bond natural and synthetic rubbers to metals. Such adhesives have
been used to prime metal surfaces prior to bonding with neoprene or
nitrile based adhesives.
Other methods of processing natural rubber for use in adhesive for-
mulations are: by cyclizing (treating with acids or metal salts) to form
hard, tough adhesives for metal surfaces; and by reacting with hydro-
gen chloride (rubber hydrochloride adhesives) to form thin, transpar-
ent adhesives used in the packaging industry.