Polymeric and elastomeric substrates often can be extremely
challenging to achieve robust bonding adhesion strength to like or dissimilar
materials. Bonding applications are not limited to only adhesives, but include
printing inks, paints and coatings, encapsulant and potting compounds,
metallization and more. This article describes "Low Pressure Cold Gas Plasma"
surface modification as one type of process that can resolve many
three-dimensional bonding problems through plasma chemical surface activation
and functionalization. The science of well known "atmospheric or air" surface
modification processes will be compared.
Particularly notorious tough-to-bond substrates include
silicone and rubber elastomers and engineering thermoplastics including acetals,
fluropolymers, polyolefins, nylons, polycarbonates, polyesters, styrenics, and
more. The underlying reasons why many plastics and elastomers are difficult to
bond are that they are hydrophobic non-polar materials, chemically inert, and
possess poor surface wettability (i.e., low surface energy).
What Is Plasma?
Plasmas can be conceptualized as a fourth state of matter. As
energy is supplied, solids melt into liquids, liquids vaporize into gases, and
gases ionize into "plasmas" – an extremely reactive gas. Free electrons, ions,
metastables, radicals, and UV generated in the plasma can impact a surface with
energies sufficient to break the molecular bonds on the surface of most
polymeric substrates. This creates very reactive free radicals on the polymer
surface which, in turn, 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 that 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 chemical functionality
Cold Gas Plasma Surface Modification
In comparison to atmospheric (air) surface pretreatment
methods, "low pressure cold gas plasma" surface modification is conducted in an
enclosed evacuated chamber. Industrial-grade oxygen gas (O2) commonly is used in
plastics pretreatment applications. Other oxidizing and noble gases include
argon, nitrogen, ammonia NH3, fluorine, and combinations thereof. The selected
gas to be ionized into the reactive gas plasma is released into the chamber
under a partial vacuum and subjected to an electrical field (RF or MW).
It is the response of the highly reactive species generated
with the polymers placed in the plasma field, on inner conductive electrode
aluminum shelves or cage, breaking molecular bonds that results in
chemical/physical surface modifications without affecting the bulk substrate
properties. Cold gas plasma process modifies non-wettable hydrophobic surfaces
to bondable hydrophilic wettable surfaces via chemical and physical mechanisms. See Diagram 1. Micro nano-scale roughened surfaces can be selectively
altered in the plasma environment either by a bombarding effect of ions
accelerated toward the surface, or by a chemical etch process. Roughened
surfaces have higher surface areas (peaks and valleys), equating to a higher
number of binding sites that improve bonding adhesion. See Diagram 2.
Unique to the cold gas plasma is that it successfully can pretreat PTFE.
Further, polymer coatings can be grown on surfaces through a process called
plasma-enhanced chemical vapor deposition (PECVD). Coatings are highly conformal
with thickness (≈2µ) dependant on process time. Examples of coatings include
SiO2-like and Diamond-like. A key application is to coat the insides of plastic
and glass tubes and containers with a quartz-like material that acts as a
barrier to leachables and prevents breakage1.
Diagram 1. Chemical and physical cleaning, surface oxidation, and trace
Diagram 2. Surface roughening by kinetically knocking contaminants from the
surface, increasing surface area topography
Equipment System Set-Up
Low pressure cold gas plasma system equipment consists of a
vacuum chamber made of aluminum with a hinged door for loading and unloading
parts. All systems have similar important component features including vacuum
pump, RF generator, MFC (Mass Flow Controller), and microprocessor controller.
The parts to be cleaned are simply placed on the inner
electrodes which are removable shelves or a cage. The parts/shelves subsequently
are loaded into the reaction chamber in which the process begins by first
pumping down to a vacuum. The process gas then may flow through the system at a
regulated pressure while pumping continues. The RF generator supplies excitation
power. At the end of the plasma process the closed vacuum chamber returns to
ambient pressure, whereby the treated parts are removed and ready for bonding.
Most systems allow for automatic control of process variables including
(typically) pressure, power, gas flow, temperature, and cycle times
(intermediary stage and total elapsed time). See Diagram 3.
Diagram 3. Cold Gas Plasma Schematic
Processing Control Parameters
Operating process parameters are based upon specific
manufacturer model system specifications. Thus, it is important to understand
the application requirements for improved bondability, production throughput,
and downstream manufacturing processes to maintain the treatment shelf-life
(aging) and contamination sources within the work environment. As general
guidelines, total cold gas plasma treatment time will range between 5-10
minutes; gas flow 100-500 sccm; pressure 300-1000 mTorr, RF power 200-1000
Watts; and low temperature heat during processing 75-125°F.
Misnomers of Cold Gas Plasma Pretreatment
It is frequently cited that cold gas plasma treatment is an
"inefficient" method, limited to only off-line batch processing, compared to
atmospheric plasma methods. This is false. Reel-to-reel continuous treatment of
woven polymeric fabric web materials, e.g., threaded polypropylene baby diaper
fabric, is treated continuously within the chamber. Based upon the inside
chamber dimensions, cold gas plasma processes can treat high-volume small and
large parts. While manufacturing process operations can be most efficiently
performed in-line, many product assemblies are conducted in batch or cell
Gas-Phase Surface Oxidation Pretreatments
Surface oxidation modification vis-ŕ-vis surface pretreatments
is used to improve the wettability and bondability of polymers and elastomers
both chemically and physically. Well known process methods are electrical
(corona discharge) and flame plasma. Historically, low pressure cold gas plasma
has been less practiced and discussed for the pretreatment of three-dimensional
parts, due in part to misnomers of the process. In the science of chemistry and
physics, electrical, flame, and cold gas plasma are all "Gas-Phase Surface
Oxidation Processes" characterized by their ability to generate a "gas plasma."
The mechanism of how the plasmas are generated is a distinguishing factor. Each
method can be application-specific and each possesses advantages and
Atmospheric Surface Modification Systems
In contrast to cold gas plasma, which is conducted in a low
pressure vacuum chamber, flame treatment and electrical (corona discharge)
pretreatment processes are conducted at atmospheric conditions. Electrical
treatments are further distinguished as "blown air" plasma and "blown ion."
Atmospheric processes can be used in both batch and continuous line production.
In terms of nomenclature it is important to understand the science of each
Flame Plasma Treatment uses the highly reactive species
present in the combustion of air and hydrocarbon gas (to create the plasma).
Flame treatment is exothermic; however, heat does not create the chemical
functionality and improved surface wetting. Flaming will clean dirt, debris, and
some hydrocarbons from the substrate. Flaming will not remove silicones, mold
releases, and slip agents. Flame treatment can impart higher wetting, oxidation,
and shelf-life than electrical pretreatments due to its relative shallower depth
of treatment from the surface, 5-10nm. Ozone is not produced2.
Classical Electrical Corona Discharge is obtained using a
generator and electrode(s) connected to a high voltage source, a counter
electrode at potential zero, and a dielectric used as a barrier. That is, high
frequency-high voltage discharge (step up transformer) creating a potential
difference between two points requiring earth ground 35+kV and 20-25kHz. This
pretreatment process has virtually no cleaning capabilities. Ozone (NOx) is
Electrical "Air Plasma" is Corona discharge spot treatment
(also termed blown air plasma/forced air corona/blown arc). This treatment head
consists of two hook electrodes in close proximity to each other connected to a
high voltage transformer generating an electric arc of approximately 7-12 kV,
lower frequency 50-60 cycles/sec (relative to electrical corona discharge). Then
using forced air, a continuous electric arc produces a corona discharge,
"plasma." No positive ground needed. This pretreatment process has virtually no
cleaning capabilities. Ozone is produced.
Electrical Blown ion plasma also is termed Focused Corona
plasma/Potential-free plasma. This treatment utilizes a single narrow nozzle
electrode, powered by an electrical generator and step-up transformer and high
pressurized air in which an intense focused plasma is generated within the
treatment head and streams outward. Since the process is potential-free, it can
be used to pretreat conductive products. This pretreatment process can clean
dirt, debris, and some hydrocarbons from the substrate but not most silicones
and slip agents. New research indicates that fine etching of the surface can
create new topographies for increased mechanical bonding. Ozone is not a
UV radiation/Ozone pretreatment and chemical primers also are
performed at atmospheric conditions. These pretreatment methods will be
discussed in a future article.
Factors Influencing Adhesion and Aging
The degree or quality of pretreatment for strong adhesion
strength is affected by the cleanliness of the plastic/elastomeric surfaces.
Contamination sources on product surfaces that inhibit treatment include dirt,
dust, grease, and oil. Low molecular weight materials such as silicones, mold
release, and anti-slip agents are particularly deleterious for bonding. Further,
certain soluble or nonsoluble compound agents used in pigment and dye colorants
can adversely affect adhesion. Exposure of treated surfaces to elevated
temperatures increases molecular chain mobility. The higher the chain mobility
is, the faster the aging of the pretreatment.
Plasma-treated surfaces age at different rates and to varying
extents relative to factors with the surrounding environment. Aging
characteristics and their storage shelf-life are essential to manufacturing
process operations. Activated surfaces may have a shelf-life of hours, days,
months, or longer. The cold gas plasma process demonstrates significantly
extended activation lifetimes3 and more uniform treatment compared to
atmospheric methods4. It is recommended to bond, coat, paint, or
decorate products as soon as possible following pretreatment.
The global need to achieve robust bonding adhesion on plastic
and elastomeric products demands surface modification "activation"
pretreatments. Chemical and physical functionality and surface roughening, via
gas phase surface oxidation processes, can resolve most adhesion problems. Each
method can be application-specific and may possess unique advantages and/or
limitations. Plasma-oxidized surfaces can deleteriously affect downstream
assembly processes, such as poor heat sealing/welding, when overtreatment
occurs. While cold gas plasma is the most expensive method, in some applications
it has the capability to yield the most robust results. Atmospheric treatments
are lower cost. However in some cases, day-to-day humidity variation can affect
the treatment uniformity. Careful evaluation of all process factors and an
understanding of the alternative surface modification technologies are
1. Demetrius Chrysostomou, PVATepla, March 2009
2. Scott R. Sabreen, Joe Digiacomo, Plastics Decorating
3. Edward M. Liston, GaSonics International, Journal of
4. Dori & Kushner, University of Illinois and 3M
Scott R. Sabreen is founder and president of The Sabreen
Group, Inc. (TSG). TSG is a global engineering company specializing in secondary
plastics manufacturing processes – surface pretreatments, bonding, decorating
and finishing, laser marking, and product security. For more information, call
(888) SABREEN or visit www.sabreen.com or