by: Don M. Coatesa and
Stephen L. Kaplan
As adaptable as polymeric materials are in their
many applications in our daily lives, there is the need to tailor
the surfaces of polymers to provide yet even more flexibility
in their uses. Plasma treatments offer an unprecedented spectrum
of possible surface modifications to enhance polymers, ranging
from simple topographical changes to creation of surface chemistries
and coatings that are radically different from the bulk polymer.
Furthermore, plasma treatments are environmentally friendly and
economical in their use of materials .
Plasma processing can be classified into at least
four categories that often overlap. These are: (1) surface preparation
by breakdown of surface oils and loose contaminates; (2) etching
of new topographies, (3) surface activation by creation or grafting
of new functional groups or chemically reactive excited metastable
species on the surface; and (4) deposition of monolithic adherent
surface coatings by polymerization of monomeric species on the
surface. Key features of these processes will be briefly discussed,
with a rudimentary introduction to the chemistries involved and
examples. Focus is placed on capacitively-coupled rf plasmas,
since they are most commonly used in polymer treatment.
In many industrial and scientific processes, extremely
clean surfaces are crucial. By "clean" it is usually
meant that the bulk material is also the material that makes up
the actual surface, not foreign, loose or otherwise mechanically
unstable debris. For metal surfaces, plasma treatment can strip
off surface oils and contaminants leaving the surface truly "cleaned"
down to the base metal (see the article by Rie et. at., in this
issue of the MRS Bulletin). This is possible since metals are
typically quite resistant to attack from many plasma chemistries.
In the case of polymers, the use of the word cleaning" takes
on a slightly different context since polymers are readily attacked
by plasma environments. Thus polymer surfaces are not just "scrubbed"
down to the base polymer but are actually altered chemically and
topographically.
Production of high-strength composites requires
surfaces of fiber and plastic insert parts to be stripped of low
molecular weight, poorly attached, surface polymer residues before
matrix resins are applied.1 Plasma "scrubs" surfaces
of unwanted materials largely by a combination of sputtering by
energetic ions and UV photolysis of covalent bonds of the surface
contaminates' molecular structure. For example, the surface of
polyethylene is typically contaminated with low molecular-weight,
wax-like, incompletely polymerized oligomers of ethylene, the
monomer for polyethylene. These poorly adherent fragments must
be removed before printing inks or adhesives can be applied. Since
the fragments are on the surface and they are more easily and
quickly degraded into volatile compounds than the base polymeric
structure. Therefore, the base polymer structure remains essentially
intact and minimally etched if short treatment times are used.
Repetitive cleavage of low molecular weight polyethylene surface
contaminates produces gases that can be pumped from the system,
leaving a stable surface suitable for strong attachment of adhesives:
Noble feed gases such as Argon are used since they
tend to initiate cleavage without grafting to the surface. Treatment
times are typically short so as to reduce further unwanted chemistry.
Excessive treatment results in attack of the base
polymer which ultimately "ashes" the entire polymeric
part resulting in its destruction. A typical cleaning procedure
for polyethylene would be to treat with Argon at a pressure of
0.01 to 0.4 Torr, with a power density of ˜ 0.5 W/cm2 at
13.56 MHz rf on parallel-plate electrodes. Once contaminants are
removed, a more stable polymer surface is exposed to the plasma
environment so as to facilitate further reaction such as etching,
grafting or direct application of the desired surface coating.
More aggressive processing can be achieved by using reactive gases
or by use of electrical biasing to increase the energetics of
the bombarding ions. However, etching into the base polymer begins
to occur, which leads us logically to the next topic.
To obtain highly adherent coatings on polymeric
surfaces, more than surface cleaning is often required. Plasma
can provide "micro-roughened" surface topographies unattainable
by physical
abrasion methods. One factor that contributes to
the improved adhesion exhibited after plasma treatment is simply
an increase in surface area of the polymer surface so as to provide
a larger contact surface interface to the coating. This can be
achieved either by the opening of micropores, by the ablative
removal of contaminants on the surface that cover or block the
porosity, or through micro-roughening. Micro-roughening occurs
with crystalline polymers or alloys through the process of differential
ablation whereby the crystalline and amorphous regimes or different
components within the alloy ablate at different rates.2 Scanning
electron micrographs shown in Figure 1 illustrate differential
etching due to crystalline differences in the polymer structure
in which Spectra® ultra-high-molecular-weight polyethylene
has been etched in 0.1 Torr oxygen at 0.01 W/cm2 power density.
The gas species being ionized is not the only factor
that determines etching parameters. The polymer composition and
microstructure also plays a key role. Typical reactive gases used
are 02, CF4, SF6, and mixtures of these with each other or with
noble gases. Higher power densities of order 1 W/cm2 and often
electrical biasing are used to increase ion bombardment energies.
Biasing is achieved by using deliberately mismatched electrode
areas, i.e., one electrode of the diode pair is much larger than
the other, giving a "self bias," Alternatively, a steady-state
rf bias may be directly applied to a target electrode that may
be configured either as a solid plate beneath the polymeric material,
or as an highly-transparent screen on top.
Plasma activation is the alteration of surface
characteristics by the substitution or addition of new chemical
groups from active species created in a plasma for groups normally
present in the base polymer. For example, conventional polyethylene
can be made more useful by transforming its surface with simple
plasma treatments as shown in Figure 2. Such groups become 'handles"
that can perform new roles. For example, hydroxyl and carboxylic
acid groups can be grafted to polyethylene to make the once hydrophobic
surface hydrophilic. Amide and amine groups could alternatively
be grafted to make surfaces receptive to dyes for coloration.
Process gases such as 02, N2, He, Ar, NH3, N20,
C02, CF4, and air or some combination of these gases are generally
used in activation treatments. The activation mechanism is believed
to be the creation of free radicals on the polymeric material's
surface molecules and then subsequent coupling of these free radicals
with active species from the plasma environment. Depending on
process gas, a large variety of chemical groups can be incorporated
into the surface (e.g., hydroxyl, carbonyl, carboxylic, amino
or peroxyl groups).
To better understand the complexity of some of
the chemistry involved, consider the case of an oxygen plasma.
The following oxidation reaction scheme is a logical pathway to
produce oxygenated groups grafted on a polymeric surface. First,
hydrogen is abstracted from the polymer backbone, R, by atomic
oxygen present in the plasma leaving the polymer with a free radical
site:
Then, molecular oxygen can couple to the free radical
creating a peroxy radical:
The peroxy radical can then abstract hydrogen from
a neighboring polymer backbone or other source and rearrange into
a carboxylic acid group or an ester:
Not indicated in this reaction scheme are the possible
formation of alcohols, ethers, peroxides and hydroperoxides. The
byproducts, typically C02, H20 and low molecular weight hydrocarbons,
are readily removed by the vacuum pumps.
Additional co-reactants can produce new surface
chemistry or accelerate the reaction kinetics. For example, in
an oxygen plasma, the breaking of the carbon-carbon and carbon-hydrogen
bonds are the rate limiting steps. When tetrafluoromethane is
introduced as a co-reactant, the 02/CF4 plasma yields excited
forms of 0, OF, CO, CF3, C02, and F. Fluorine and fluorine containing
species are more effective in breaking the carbon-carbon and carbon-hydrogen
bonds (than oxygen species), thereby accelerating the reaction
rate. Oxidation by fluorine free-radicals is known to be as effective
as oxidation by the strongest mineral acid etchant solutions,
with one important difference: the plasma byproducts do not require
special handling. As soon as the plasma is shut off, or the excited
species exit the rf field, the species recombine to their original
stable and non-reactive form, usually within a few seconds.
As an example of a commercial application, we consider
paint adhesion to polymers, an important need in automotive manufacturing.
For the painting of plastic surfaces, cleanliness alone is not
necessarily sufficient to assure enduring paint adhesion. Rather,
grafting of new surface chemistry is needed. Polymers, such as
polyolefins and polyolefin alloys, e.g., Thermoplastic Olefins
(TPO), are especially difficult to paint due to their "waxy"
surface and require pretreatment to provide paint film adhesion.
The most common pre-treatment for TPO prior to painting has been
either application of oxy/acetylene flames directly onto the surface
(flaming) or the application of chemical adhesion promoters. Flaming,
while effective to a degree, is not practical with more sophisticated
panel designs which have recesses, louvers, or deep accent grooves.
Adhesion promoters, typically low-solids (<5%) solution of
chlorinated olefins in solvent, generally provide a higher level
of effectiveness than flame treatment. Solvent-based adhesion
promoters are not environmentally friendly since they contain
a large proportion of volatile organic compounds. Water-borne
adhesion promoters have not yet proven to be as effective and
are more costly. Plasma treatment outperforms these commercial
pre-treatment processes or combinations of processes3,4. Paint
adhesion has actually been shown to exceed the strength of the
TPO base material, which has never been demonstrated with any
other pretreatment process. Material was treated with an air feed
gas at 0.2-0.4 mTorr, rf energy density from 0.01 to 0.1 W/cm2
and treatment time from 30 to 60 sec. Plasma treatment provided
a 1400 to 1800% improvement in peel strength vs the control, while
the failure mechanism shifted from adhesive between the paint-substrate
interface to cohesive within the TPO substrate.
Polymerization is the creation of very large molecules
by the joining of many small, linkable, molecules called monomers.
Classical monomers, as used in wet chemistry polymerization, have
reactive structures such as double bonds that allow them to bond
to one-another when the appropriate conditions are present. The
double bond in methyl methacrylate provides the linking site for
forming the useful plastic, polymethyl methacrylate (N = a large
number of repeat units, e.g., 100,000) resulting in the reaction
Uv light, free radicals or energetic ions from
the plasma, initiate the polymerization process. The monomer methyl
methacrylate, when used as the feed gas, will begin the polymerization
process by linking repeatedly, increasing its molecular weight
many thousand fold. This plasma polymerization has been studied
by Fourier Transform InfraRed Attenuated Total Reflectance (FTIR/ATR)
spectroscopy in real time.5 The resulting polymer was directly
grown onto a Ge crystal ATR optic element inside the plasma reaction
chamber. The crystal was IR probed through a window in the chamber.
A polymethyl methacrylate film was deposited at 65-W power, 0.2
Torr pressure and 30 sccm flow rate of monomer. Interestingly,
the polymer as deposited continued to change its IR signature
even after the plasma power was turned off. This is not uncommon
in plasma induced reactions due to long-lived free radical species
that continue to react.
In the above example, a known "polymerizible"
monomer was reacted into a polymeric film.
Surprisingly however, plasma conditions can also
create polymer films from materials that ordinarily do not form
polymers by conventional wet chemistry techniques. Plasmas can
fractionate feed gases that lack linkable sites into many new
and reactive compounds that subsequently may polymerize. For instance,
ethane (C2H6) in an rf plasma will deposit as a polyhydrocarbon
that has a density approaching 1.6 g/cc.
The structure of plasma polymers can be varied
by the use of co-reactants or the introduction of 02, N2, or NH3
into the reaction chamber during polymerization. This technique
is commonly employed to incorporate specific atomic species into
the resulting polymeric material that may be missing in the primary
monomer. Ammonia or acrylonitrile are used as the co-reactants
during the deposition of films from a methane plasma to incorporate
nitrogen.6 Similarly, hydroxyl and carboxylic acid functionalities
can be incorporated by plasma co-polymerizing acrylic acid6 or
ally alcohol with the primary monomer to provide oxygen and hydrophilicy.
7,8
Studies have developed correlations between the
power input, the type of monomer feed gas used, and the gas flow
rate to the density and type of active species in the plasma.
These factors in turn determine the rate of deposition and the
film structure.9,1O Depending on the monomer used, deposition
rates typically range between 5 and 100 nm/min, at 100 W rf power
levels and monomer flow rates of a few sccm.11 Benzene is observed
to have a relatively high deposition rate13 even though it lacks
a conventional polymerization linking site, and, thus, would not
form a polymer under usual wet chemical means. The properties
of materials polymerized in this manner can be very different
from polymers obtained from these same materials via conventional,
wet
chemical polymerization methods (if indeed, such
polymers can even be made by wet chemical means). The physics
and chemistry of plasma polymerization processes have been described
in sufficient detail elsewhere for the interested reader.10-13
The modification that occurs to a polymeric material
by exposure to a plasma is largely determined by: (1) the process
gas(es), (2) the exposure time to the plasma, (3) the energy and
power densities, and, to a lessor degree, (4) the original composition
of the surface. Types of modifications span from relatively simple
surface morphological roughening or smoothing changes, to complex
grafting of radically different functional groups or molecular
moieties, to totally enveloping coatings that completely alter
the surface properties of the bulk material. Free radical chemistry
appears to be the dominant mechanistic pathway for achieving most
surface modifications. In spite of the high complexity of the
ensuing chemistry in a typical plasma, it is possible to tailor
the process to perform specific targeted changes to polymeric
surfaces.
1. S. Kolluri, HIMONT Plasma Science Technical
Note (1992, unpublished).
2. T. Yasuda, T. Okuno, M. Miyama, H. Yasuda, Polymeric
Mat Sci. Eng, 62 (1990) p. 457.
3. S. Kaplan, P. Rose, P. Sorlien, 0. Styrmo, A
AUTOPLAS '92, (Schotland Group,
Dusseldorf, 1992) p. 255.
4. S. Kaplan and M. Hozbor, Society of Plastic
Engineers 1995 RETEC Ypsilanti, MI, Mar.
1995 (ECM Inc., Plymouth, Michigan, 1994) p. 23.
5. Y. Pan, E.Barrios, and D. Denton, (private communication
1996).
6. R. Engelman and H. Yasuda, Polymeric Mat. Sci.
Eng. 62 (1990) p. 19.
7. W. Gombotz and A. Hoffman, Polymeric Mat. Sci.
Eng. 56 (1987) p. 720.
8. H. Griesser and R. Chatelier, Polymeric Mat.
Sci. Eng. 62 (1990) p. 274.
9. D. Schram, G. Kroesen, and J. Beulens, Polymeric
Mat. Sci. Eng. 62 (1990) p. 25.
10. G. Smolinsky and M. Vasile, Symposium on Plasma
Chemistry of Polymers, edited by M.
Shen (Marcel Decker, Inc., New York, 1976) p. 105.
11. H. Yasuda, Plasma Polymerization (Academic
Press, Orlando, 1985).
12. R. d'Agostino, F. Fracassi, F., and F. Illuzi,
Polymeric Mat. Sci. Eng. 62 (1990) p. 15 7.
13. N. Morosoff, B. Crist, M. Bumgarner, T. Hsu,
T., and H. Yasuda, Symposium on Plasma Chemistry of Polymers,
edited by M. Shen (Marcel Decker, Inc., New York, 1976) p. 83.
Don M. Coates is a Senior
Research Associate with DuPont Central Research and Development,
Materials Science and Engineering Division. He received his PhD
in Physical Organic Chemistry from Florida State University.
Stephen L. Kaplan received
his BS degree in Plastics Engineering from Lowell Technological
Institute. In 1985 he founded Plasma Science, Inc., a company
specializing in the use and the manufacture of gas plasma systems
for the surface treatment of plastics. In 1996, he established
4th State. Inc.
* Chapter IV of Plasma Processing of Advanced Materials,
edited by George A. Collins and Donald J. Rej, MRS Bulletin, August
1996
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