Page 77 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 45

serviceable adhesive bonding and sealing systems is itself a broad field
of technology. Adhesives and sealants can be produced in various
forms: one and two part liquids, solvent based solutions, water based
emulsion, supported or unsupported film, preformed pellets or shaped
extrusions, and numerous other forms. This variety of formulation
possibilities and end-use forms are indicative of the advanced state of
development of adhesives and sealants.
These products are generally developed and prepared for sale by
formulators that range from very small operations to large interna-
tional businesses. An adhesive or sealant formulation will depend on
the base material that is the principal part of the formulation and on
the requirements of the application. Processes and formulations are
often considered proprietary to the adhesive or sealant manufacturer.
However, there are formulas and processing methods that are public
knowledge due to the tremendous amount of published research on
adhesives and sealants. In general, an adhesive or sealant formulation
consists of the following components.
The adhesive base or binder is the primary component of an adhe-
sive that holds the substrates together. The binder is, generally, the
component from which the name of the adhesive is derived. For ex-
ample, an epoxy adhesive may have many components, but the pri-
mary material is epoxy resin.
A hardener is a substance added to an adhesive to promote the cur-
ing reaction by taking part in it. Two-part adhesive systems generally
have one part, which is the base, and a second part, which is the
hardener. Upon mixing, a chemical reaction ensues which causes the
adhesive to solidify. A catalyst is sometimes incorporated into an ad-
hesive formulation to speed-up the reaction between the base and
hardener. Certain adhesive bases only need a source of energy to cure.
This source may come from ultraviolet light, radiation, etc. In these
cases, the adhesive may contain a catalyst but no hardener.
Solvents are sometimes needed to disperse the adhesive to a spread-
able consistency. Solvents used with synthetic resins and elastomers
are generally organic in nature, and often a mixture of solvents is
required to achieve the desired properties. There must be some way
for the solvent to escape the joint after the adhesive is applied and
the assembly is made. Thus, solvents are generally only used in ap-
plications where passages are available for their escape, such as in
the preparation of pressure sensitive coatings or bonding of porous
substrates.
An ingredient added to an adhesive to reduce the concentration of
base material is called a diluent. Diluents are principally used to lower
the viscosity and modify the processing conditions of some adhesives
and sealants. Reactive diluents chemically react with the base mate-
rial during cure, become part of the product, and do not evaporate as