Factors that influence the adhesion - Surface treatment for plastics
Adhesive Abrading
Adhesive abrading is performed by abrading the plastic�s surface in the presence
of liquid adhesive. Two of the abraded, adhesive-coated adherends are then mated,
and the adhesive is allowed to cure. (This increases the bond strengths achieved
on Teflon (PTFE) by approximately 700 percent). When abrasion is carried out
in the presence of the adhesive, free radicals are created which react directly
with the adhesive. This does not normally occur because the free radicals are
scavenged by the oxygen present in air, or decay, before the adhesive is applied.
Common uses fluorocarbons
Chromic Acid Etching
Chromic acid etching increases the bondability of a plastic by introducing reactive
sites, such as hydroxyl, carbonyl, carboxylic acid, and -SO3H groups, to the
plastic�s surface and forming root-like cavities which provide sites for mechanical
interlocking. The effects of this treatment vary from substrate to substrate.
For example, increasing the etch time and temperature increases only the etch
depth when etching polypropylene. On the other hand, both the degree of oxidation
and etch depth increase with time for polyethylene.
Common uses polyolefins, ABS, polystyrene, polyphenyloxide, acetals
Corona Discharge
In a corona discharge process, the plastic is exposed to a corona discharge,
usually in the presence of air and at atmospheric pressure. This roughens the
surface, which provides sites for mechanical interlocking, and introduces reactive
sites on the plastic's surface, consequently increasing the wettability and
reactivity of the surface. The reactive functionalities which are theorized
to be introduced to the surface may include, but are not proven to be, carbonyl,
hydroxyl, hydroperoxide, aldehyde, ether, ester, and carboxylic acid groups,
as well as unsaturated bonds.
Common uses polyolefins
Flame Treatment
Flame treatment increases the bondability of a plastic by oxidizing the surface
through brief exposure to flame. The oxidation proceeds by a free radical mechanism,
accompanied by chain scissions and some crosslinking. The functionalities introduced
by oxidation are hydroxyl, carbonyl, carboxyl, and amide groups with a typical
oxidation depth of approximately 4 to 9 nanometers. The improved bondability
results from increased wettability, due to increased surface energy, and interfacial
diffusivity, caused by chain scissions.
Common uses polyolefins, polyacetals, polyethylene, terephthalate
Iodine Treatment
Iodine treatment increases the bond strengths achieved on a substrate by altering
the surface crystallinity from alpha form (where the N-H groups lie parallel
to the surface) to beta form (where the N-H groups stand perpendicular to the
surface). The surface remains relatively smooth after treatment, so it is believed
that increased chemical reactivity, rather than mechanical interlocking is the
mechanism for improved adhesion.
Common uses nylon
Plasma Treatment
Plasma treatment increases the bondability of a substrate by bombarding the
substrate surface with ions of a gas, such as Ar2, He2, N2, and O2, at low pressure.
Several mechanisms have been proposed to explain the enhanced bondability created
by plasma treating. For example, plasma treatment is hypothesized to crosslink
the substrate�s surface, which strengthens the joint boundary and prevents a
thin layer of substrate from peeling off. In addition, the surface oxidation
caused by plasma treatment is thought to introduce reactive functionalities
which then increase the surface's reactivity and wettability. Another theory
attributes plasma treatment�s effectiveness to an increased interfacial diffusion
which is created by chain scissions in the substrate�s surface. Chain scissions
increase the interfacial diffusion by lowering the surface viscosity and increasing
the molecular mobility of the plastic�s surface.
Common uses polyolefins, polyesters, many more
Primers
Primers typically consist of a reactive chemical species dispersed in a solvent.
To use the primer, the solution is brushed or sprayed onto the substrate surface.
The carrier solvent is then allowed to flash off, leaving the active species
behind. Depending on the type of primer, the surface may be ready to bond immediately,
as in the case of polyolefin primers for cyanoacrylates, or may require time
to react with atmospheric moisture before the application of the adhesive. Primers
that must react with atmospheric moisture include silane nd isocyanate-based
primers which are typically used for silicone and polyurethane-based adhesives,
respectively. Surface primers generally improve substrate bondablility by acting
as a chemical bridge between the substrate and the adhesive. Typically, the
reactive species in a primer will be multifunctional, with one set of reactive
groups that will preferentially react with the substrate surface, and additional
groups that will have a high affinity for the adhesive.
Common uses acetals, fluoropolymers, polybutylene, terephthalate, polyolefins,
polyurethane, silicone
Sodium Treatment
Sodium treatment is carried out by immersing the substrate in an aggressive
etching solution containing either a sodium-naphthalene complex dissolved in
tetrahydrofuran or a sodium-ammonia complex dissolved in ammonia. The etching
process results in the dissolution of the amorphous regions of the substrate�s
surface, consequently increasing the substrate�s surface roughness and potential
for mechanical interlocking. Moreover, sodium treatment introduces unsaturated
bonds, carbonyl groups, and carboxyl groups to the substrate�s surface, which
increases the substrate's reactivity and wettability. Due to carbonaceous residue
which results from the defluorination of the surface, sodium treatment darkens
the surface to an approximate depth of 1 micrometer. The on-part life of the
treatment is very long (years), however, heating and UV exposure rapidly degrade
the treated surface. Major disadvantages of using sodium treatments are that
extended exposure to the solution will result in a substantial degradation of
the substrate�s surface, the etchants are highly hazardous, and that the solution
degrades very rapidly in the presence of oxygen.
Common uses fluorocarbons
Surface Grafting
Surface grafting is accomplished by grafting a chemical species to the substrate�s
surface which increases the substrate�s bondability. For example, polyethylene
can be exposed to gamma radiation in the presence of vinyl acetate monomer,
which then becomes chemically grafted to the polyethylene surface.
Common uses vinylic compounds on polyolefins
Surface Roughening
Surface roughening is a simple, low cost method of increasing the bondability
of many plastics. Surface roughening increases the bondability by dramatically
increasing the number of mechanical interlocking sites.
Common uses effective for many plastics
Thermal Treatment
Thermal treatment increases the bondability of plastics by exposing the plastic
to a blast of hot air approximately 500�C), which oxidizes the surface. This
mainly introduces carbonyl, carboxyl, and amide groups to the surface, but some
hydroperoxide groups are also formed. Very similar to flame treatments, this
process also utilizes a free radical mechanism accompanied by chain scission
and some crosslinking. The improved bondability results from increased wettability,
due to the introduction of polar groups, and interfacial diffusivity, caused
by chain scissions.
Common uses polyolefins
Transcrystalline Growth
Transcrystalline growth improves bondability of a plastic by molding adherends
against a high energy metallic substrate that induces trancrystalline growth
in the plastic's surface regions. The metallic substrate induces the formation
of crystallites at the plastic�s surface and results in rod-like or columnar
spherulites that form inward from the interface. This is thought to strengthen
the surface by driving low molecular weight material into the interior. In addition,
some metallic substrates may oxidize the plastic�s surface, resulting in a substantial
increase in the reactivity and wettability of the plastic's surface. The effectiveness
of this treatment is dependent on such molding conditions as the cooling rate
and mold surface.
Common uses polyolefins, polyamides, polyurethanes
UV Exposure
UV exposure increases the bondability of plastics by irradiating them with high
intensity UV light. However, the effectiveness of UV exposure is very dependent
on the wavelength of light being used. For example, light with a wavelength
of 184 nm will crosslink the surface of polyethylene, while light at 253.7 nm
will not. UV irradiation causes chain scissions, crosslinking, and oxidation
of the polymer�s surface, even in inert gases. Many different mechanisms describing
why UV exposure increases the bondability of plastics have been proposed, including:
increasing the wettability; strengthening the plastic's boundary layer through
crosslinking; and inducing hydrogen bonding. The predominant view is that the
bondability is improved by the formation of polymeric scission products, which
promote interfacial flow, interdiffusion, and polar interactions.
Common uses polyolefins