Page 116 - Handbook of Adhesives and Sealants

84 Chapter Two

assume new alignments under mechanical or thermal-expansion
stress. This movement spreads the applied energy over a greater num-
ber of atoms and, thus, gives the bond a better chance to resist stress.
Brittleness is, therefore, reduced and flexibility is increased. Molecu-
lar flexibility can be controlled by the following conditions.

Effect on flexibility

Molecular weight Negative
Crosslink density Negative
Crystallinity Negative
Negative
T g
Fillers Negative
Plasticizers Positive
Flexibilizers Positive

Increasing molecular weight improves cohesive strength but often
weakens adhesion. Crystallinity can significantly improve the cohe-
sive strength of a polymer up to its melting point, but too much crys-
tallinity can also embrittle the adhesive. Heterogeneous nucleation
(crystallinity occurring at the melt surface of a polymer) has been
shown to improve the surface cohesive strength of FEP Teflon, Nylon-
6, and polyethylene when cast against high energy surfaces.
Crosslinked adhesives have cohesive properties that are found to
depend on the molecular weight between crosslinks, M . Shear
c
strengths of epoxy aluminum joints decreased as M increased; how-
c
ever, flexibility and toughness are increased. Figure 2.19 shows the
relationship between crosslink density and the physical state of epoxy
resins.
The excellent cohesive strength of polyamides compared to other
common polymers of equivalent molecular weight is due to the pres-
ence of interchain hydrogen bonding. Excellent adhesion of epoxies to
aluminum, of surface treated rubber and other polymers to glass, and
of polymers to cellulosics are also attributed to interfacial hydrogen
bonding. Hydrogen bonding can be considered a special case of cross-
linking.
Like all polymers, adhesive and sealant materials undergo constant
thermally induced vibration. The amplitude of these vibrations is de-
termined primarily by temperature, chain flexibility, and crosslinking,
and to a lesser extent by fillers and physical stresses. A certain
amount of chain flexibility is desirable since it imparts resiliency and
toughness to the adhesive film. Too much flexibility, however, may lead
to ‘‘creep’’ (i.e., plastic flow under load) or poor temperature resistance.