Basics of Surface Wetting & Pretreatment Methods:
Plastics adhesion problems are wide-spread
throughout industry. A major component of these problems is due to
the fact that many plastics have chemically inert and nonporous
surfaces with low surface tensions. That is, most plastics are
hydrophobic and are not naturally wettable. These properties,
although advantageous to the design engineer, often result in
secondary assembly and decorating concerns – bonding, printing,
coating and painting. Surface pretreatments on today’s high
performance engineering resins solve many adhesion problems while
increasing bond strength performance.
As a general rule, acceptable bonding adhesion is achieved when
the surface energy of a substrate (measured in dynes/cm) is
approximately 10 dynes/cm greater than the surface tension of the
liquid. In this situation, the liquid is said to “wet out” or adhere
to the surface. Surface tension which is a measurement of surface
energy is the property, due to molecular forces, by which all
liquids through contraction of the surface tend to bring the
contained volume onto a shape having the least surface area. The
higher the surface energy of the solid substrate relative to the
surface tension of a liquid, the better its “wettability”, and the
smaller the contact angle. Figure 1.
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The degree or quality of treatment is affected by the
cleanliness of the plastic surface. The surface must be
clean to achieve optimal pretreatment. Surface
contamination such as silicone mold release, dirt, dust,
grease, oils, and fingerprints inhibit treatment.
Material purity is also an important factor. The shelf
life of treated plastics depends on the type of resin,
formulation, and the ambient environment of the storage
area. Shelf life of treated products is limited by the
presence of low molecular weight components such as
antioxidants, plasticizers, slip and antistatic agents,
colorants and pigments, stabilizers, etc. Exposure of
treated surfaces to elevated temperatures increases
molecular chain mobility. The higher the chain mobility
the faster the aging of the treatment. Polymer chain
mobility in treated materials causes the bonding sites
created by the treatment to move away from the surface.
These components may eventually migrate to the polymer
surface, therefore it is recommended to bond, coat,
paint, or decorate the product as soon as possible after
pretreatment.
Surface pretreatments are used to increase surface
energy and improve the wetting and adhesive properties
of polymer materials. A variety of pretreatment
processes are used in industry including RF cold gas
plasma, electrical (corona discharge), and flame plasma.
Each method is application specific and possesses unique
advantages and potential limitations. All of these
processes are characterized by their ability to generate
a “gas plasma” – an extremely reactive gas consisting of
free electrons, positive ions, and other chemical
species. In the science of physics, the mechanisms in
which these plasmas are generated are different but
their effects on surface wettability are similar.
Plasmas can be conceptualized as a fourth state of
matter. If sufficient energy is supplied, solids melt
into liquids, liquids vaporize into gases, and gases
ionize into plasmas.
Free electrons, ions, metastables, radicals and UV
generated in plasma regions can impact a surface with
energies sufficient to break the molecular bonds on the
surface of most substrates. This creates very reactive
free radicals on the polymer surface. These free
radicals can form, cross link, or in the presence of
oxygen react rapidly to form various chemical functional
groups on the substrate surface. Polar functional groups
which can form and enhance bondability include carbonyl
(C=O), carboxyl (HOOC), hydroperoxide (HOO-), and
hydroxyl (HO-) groups. Even small amounts of reactive
functional groups incorporated into polymers can be
highly beneficial to improving surface characteristics
and wettability.
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