Page 114 - Handbook of Adhesives and Sealants

82 Chapter Two

or characteristics whose value will determine their general usefulness
as adhesives or sealants in specific applications.

2.6.1 Polymeric materials
It is natural to ask why so many polymeric materials with greatly
different chemical composition and structure make good adhesives or
sealants. The reason is that: all polymeric materials have fundamental
properties that are important in adhesion; polymers can be made to
flow and then set via several reaction mechanisms to form a cohesively
strong solid, and additives can be used to modify these properties.
The performance of a polymeric adhesive or sealant is dependent
upon the physical properties of the polymeric base from which it is
made and the various formulating agents used to modify the base
polymer. Relationships between the chemical structure of a polymer,
its physical properties, and its adhesive performance have interested
scientists for many years. This has resulted in many excellent tech-
nical studies on property-structure relationships and has helped to
improve understanding of the factors controlling adhesive and sealant
performance.

2.6.2 Properties important for adhesives
and sealants
The fundamental parametric properties that are important in suc-
cessful adhesives and sealants are the glass transition temperature,
the solubility parameter, the surface free energy, and the viscosity.
Added to these are the non-parametric qualities of microstructure or
organization. Some of these properties are mainly involved in the ap-
plication of adhesive to the substrate, and they play little direct part
once intimate contact on a molecular scale has been achieved.

2.6.2.1 Properties important for adhesion. Surface energetics are con-
trolled largely by the general chemical composition of the polymer
molecules. Factors such as the polarity and surface functionality of
the molecules affect the surface tension and often account for the great
improvement in adhesion. These properties are usually determined by
the base polymer in the adhesive or sealant formulation.
Addition of carboxyl functionality to polymers is well known as an
adhesion promotion mechanism for polar substrates. Thus, -COOH
groups improve the adhesion of polyesters, acrylics, and olefin poly-
mers to these substrates. Other electronegative atoms, such as chlo-
rine and nitrogen, have been noticed to improve the chances for ad-
hesion via hydrogen bonding across the interface.