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SURFACE TREATMENTS FOR PLASTICS

Plastics by character are non wettable (hydrophobic) with low surface energies
resisting adhesion and bonding
Basics of Surface Wetting & Pre treatment 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. 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. Surface pretreatments are used to increase
surface energy and improve the wetting and adhesive properties of polymer
materials. A variety of pre treatment processes are discussed below. Each method
is application specific and possesses unique advantages and potential limitations.

PRINCIPAL SURFACE TREATMENT FOR PLASTICS
Treatment Description Mostly used for
Flame treatment The joint surface is oxidised by brief polypropylene
exposure to an oxygenated flame polyethylene
acetal
Corona discharge The corona discharge creates an ozone polypropylene
generating spark that both roughens the surface polyethylene
& raises the energy level.
Plasma treatment the surface is bombarded at low pressure with most plastics
ions of a gas typically oxygen, nitrogen, argon
or helium
Simple abrasion The surface is mechanically roughened then any most plastics
dust and debris is removed.
Adhesive abrasion The surface is abraded in presence of the fluropolymers
adhesive, immediately before bonding.
Acid etching Etching with chromic acid roughens the surfaces PP, PE, ABS
and increased energy levels Acetal, PPO
Primers There are many proprietary types of primer PP, PE, PBT,
available, generally optimised for specific PET, Acetal,
combinations of adhesive and polymer. fluoropolymers
Surface cleaning Treatment with solvents detergents and other all plastics
cleaning agents to remove surface dust & grease.
Sources: Loctite and Handbook of Plastics Joining
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