1. Contact Adhesives
(Material)
There is no previous
definition.
Definition:
Contact
adhesives are made from polymer components which are already in a highmolecular
state, but which are not yet chemically crosslinked. By means of the addition
of solvents, a low-viscosity state is obtained which allows the adhesive to wet
the surface of the solid material. Contact adhesives must be applied to both adherents,
and solidification takes place by a drying process before the materials are joined.
The adherents are pressed together as soon as the adhesive layers are dry to
the touch.
During
the application of pressure, and even afterwards, a diffusion process takes place
between the adhesive molecules of both coats, firmly joining the adherents together.
The strength of the bond can increase, often within hours, after the application
of pressure. If at least one adherent is porous, the adherents can be joined before
the drying process has taken place at all or only partly, because the remaining
solvent is capable of evaporating through the porous adherent. In this case, it
is also possible to reposition the adherents after having joined thembecause,
in the semi-dry condition, the adhesive still largely presents the properties
of a liquid.
In
the dried or cured condition, the adhesive coat is generally in the thermoplastic
state – that is, the molecules are not three-dimensionally crosslinked. As a
consequence, the adhesive coats applied do not present the same resistance to
heat and creep as chemically cured adhesives. It is possible, however, to add
slowly acting hardeners to these polymer systems; this induces a
three-dimensionally widemeshed crosslinking of the systems over the course of
several days. While the addition of hardeners does not alter the processing
properties of the contact adhesives, the durability and strength of the cured
system is considerably improved. One disadvantage of contact adhesives is an
occasional very high solvent content (up to 80% by weight), so that appropriate
protective measures must be taken during processing.
As
a consequence, solvent-containing contact adhesives are now being replaced, when
possible, by so-called .dispersion adhesives. These consist of a dispersion of the
adhesives in water, during the evaporation of which the dispersed polymer particles
combine by diffusion. Unfortunately, dispersion adhesives have the disadvantage
that the drying process of water takes considerably longer than that of
solvents, and an acceleration can only be obtained by the application of heat,
which requires additional energy input. Furthermore, during the drying process,
dispersion adhesives develop inadequate tack, a property which is desirable in
many cases.Adrawback of this approach is that the strength and durability of
dried adhesive coats are generally poorer than those of solvent-containing
contact adhesives; this occurs because the polymer layer created is not
absolutely homogeneous and has a tendency to redisperse under the influence of
moisture.
(Brockman,
W. et al, Adhesive Bonding, p. 33)
2. One-Part Adhesives (Material)
Previous Definition:
One-part adhesives are preferred over two part adhesives for surface mounting because it is a nuisance to have to mix two-part adhesives in the right proportions for the right amount of time. One-part adhesives, eliminating one process variable in manufacturing, are easier to apply, and one does not have to worry about the short working life(pot life) of teh mixture. The single- part adhesives have a shorter shelf life, however. The terms “shelf life” and “pot life” can be confusing. “Shelf life” refers to the usable life of the adhesive as it sits in the container, whereas “pot life,” as indicated above, refers to the usable life of the adhesive after the two main components( catalyst and resin) have been mixed and catalysis has begun.
One-part adhesives are preferred over two part adhesives for surface mounting because it is a nuisance to have to mix two-part adhesives in the right proportions for the right amount of time. One-part adhesives, eliminating one process variable in manufacturing, are easier to apply, and one does not have to worry about the short working life(pot life) of teh mixture. The single- part adhesives have a shorter shelf life, however. The terms “shelf life” and “pot life” can be confusing. “Shelf life” refers to the usable life of the adhesive as it sits in the container, whereas “pot life,” as indicated above, refers to the usable life of the adhesive after the two main components( catalyst and resin) have been mixed and catalysis has begun.
(Ray P. Prasad, Surface Mount Technology: Principles
and Practice, p. 340)
New Definition (Better):
A.
Hot-Setting, One-Part Adhesives
Although,
one-part systems are considerably easier to process than either two-part or multi-part
adhesives, on most occasions it will be necessary to apply heat. One-part adhesives
consist of low-molecular and plastified substances. In general, the base resins
are phenolic resins, epoxy resins and, for the creation of high-temperature adhesives,
polybenzimidazoles and polyimides.
With
regard to phenolic resins and polyimides, the crosslinking process takes place in
the form of polycondensation, and consequently additional pressure must be exerted
on the bond-line during the setting process. Heat must also be applied in order
to remove water from the bond-line, while the pressure must be higher than the water-vapor
pressure at the setting temperature (i.e. generally >500 kPa). In contrast,
contact pressure is dispensable with hot-setting epoxy resins, because the
setting reaction takes place as polyaddition. Only in the case of extended-surface
bonds need contact pressure be applied to the adherents because, in the
noncured state, these adhesives are not capable of transferring any mechanical
forces, and any deformation of the adherents may result in destruction of the
bond.
B.
Cold-Setting, One-Part Adhesives
Besides
setting via heat, several other physical effects like absence of oxygen in the glue
line or presence of moisture at the adherent surfaces can be utilized in order
to initiate a curing reaction. The most common one-part adhesives are based on
an initiation by irradiation (e.g. UV light), by an absence of oxygen
(anaerobic), or by exposure to moisture. Owing to the precise
control of the curing process, systems setting under the influence of UV light
or light (most of which are based on acrylates or epoxy resins) have been
increasingly used to date. In lowmolecular base resins, photoinitiators are
dissolved or chemically incorporated which initiate the crosslinking process
under irradiation with UV light or visible light. Two such groups of adhesive
may be distinguished: those that set exclusively under continuous
irradiation (radical reaction mechanism); and those in which crosslinking
is initiated by short irradiation of the open bond-line and continues to take
place after joining, without further irradiation (ionic reaction mechanism).
Cold-setting,
one-part adhesives which crosslink under the effect of moisture (e.g. one-part
polyurethane and silicon resin systems) are often used as sealing compounds. In
the noncured state these adhesives are low-molecular and noncrosslinked. However,
at the bond-line they are set by the effect of moisture diffusing from the
outside, which results in a three-dimensionally, chemically crosslinked system
with high plasticity. Silicon resins, for example, have a high plasticity and
are characterized by the fact that their strength and deformation properties
are virtually unaffected over a temperature range from –55 to +250 C.
(Brockman,
W., et al, Adhesive Bonding, p. 36)
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