Gas Plasma Treatment of KevlarÔ
and SpectraÔ Fabrics for Advanced Composites
By: Stephen L. Kaplan and
Wally P. Hansen
4th State, Inc., Belmont, CA 94002
The engineering properties, (strength,
stiffness, weight and heat tolerance) of fiber and the fabrics made thereof are the
primary reason for its selection. However, secondary characteristics such as surface
properties are assuming more critical importance. For example, if a polyethylene fabric is
to become the reinforcement in a composite structure, the surface of the fiber needs to be
altered to promote the adhesion of a matrix polymer to the fiber, preventing an otherwise
weak composite structure.
A cold gas plasma process is shown to provide
a dramatic increase of the flexural strength of KevlarÔ and SpectraÔ composites. With
plasma processing, the surface of the material is cleaned and modified by just a few
angstroms in an economical and environmentally safe method. A plasma system capable of
economical treatment of composite reinforcement fabrics up to 60" in width is
available and is being used commercially.
Background
Any process which changes the polymer must
not change the bulk properties or the polymer may lose its primary physical and chemical
characteristics. Prior to gas plasma treatment, various techniques have been used for
fabric treatment such as chemical and/or solvent etch, flame treatment, and corona
discharge. However, these treatment techniques have significant drawbacks.
Wet chemical and solvent treatment, if
effective, often add numerous additional processing steps such as neutralization, washing
and rinsing and drying. These solvents and chemicals are usually hazardous or designated
hazardous, constituting a toxic waste disposal problem and cost.
Corona and flame treatment while a very cost
efficient treatment method is often not effective on many non-woven and fabric substrates.
Because of the potential for rapid high heat generation, treatment is conducted at high
speeds, thus the residence times are insufficient to permit penetration of the active
species that effect change into the fiber bundles or interstices of non-woven webs and
fabrics. Since corona discharge systems depend on ionizing free air, the process may not
produce consistent results from day to day, season to season and location to location.
Further, electrostatic discharge produces ozone as an effluent, which must be properly
processed before venting to the atmosphere, thus adding to the cost of the treatment
process.
Cold Gas Plasma for Re-engineering Polymer
Surfaces
Over the past quarter-century the technique
of re-engineering polymer surface properties through exposure to a gas plasma has been
extended to virtually all polymers. A variety of results can be easily obtained, specific
to the polymer and the gas species employed. Producible effects run the gamut from highly
wettable surfaces exhibiting superior adhesion characteristics and chemical reactivity to
completely unwettable, inert surfaces. More sophisticated plasma processes permit
dissimilar polymers to be "grafted" onto the bulk polymer chain, or the
deposition in-situ of a micro-thin coating via plasma polymerization.
The effect of a plasma on a given
material is determined by the chemistry of the reactions between the surface and the
reactive species present in the plasma. At the low exposure energies typically present in
glow-discharge plasma systems the interactions occur only in the top few molecular layers.
The majority of plasma activation processes are related to preparing the surface for
subsequent operations such as printing or altering the surface wetting characteristics.
Gases, or mixtures of gases, used for cold
plasma treatment of polymers include air, nitrogen, argon, oxygen, nitrous oxide, helium,
tetrafluoromethane, water vapor, carbon dioxide, methane, and ammonia. Each gas produces a
unique plasma composition and results in different polymer surface properties. For
example, the surface energy which is analogous to wettability and chemical reactivity can
be increased very quickly and effectively by plasma-induced oxidation, nitration,
hydrolyzation or amination. Conversely, plasma-induced fluorination depresses surface
energy, producing an inert and non-wettable surface.
Gas Plasma Equipment
The reactor is a vacuum chamber equipped with
vacuum pump, purge plumbing, process gas sources and regulators, a source of
electromagnetic energy and a system controller to orchestrate the process.
The equipment operation cycle is carefully
monitored and controlled by the electronics package, which operates the valves,
pressure/vacuum flow gates and the RF source. In the 4th State system the roll product to
be treated (up to 60" width and 19" package diameter) is loaded in the payoff
chamber and threaded through the chamber to the take-up reel. The plasma treatment
operation is then initiated and entirely controlled by the push of a single button. The
process steps are: 1) pump down to predetermined vacuum pressure (base pressure), 2)
introduce process gas and allow to stabilize at a desired process pressure, 3) initiation
of plasma by providing rf energy, 4) transport product through the system and 5) after
treating the desired length, shutting rf power and process gas delivery, 6) pump down to
base pressure to eliminate residual process gas(es), 7) vent to atmosphere and 8) remove
treated product.
Discussion and Results
Typical composite results for plasma treated
and untreated (as received) fabric are presented in Tables I & II. These fibers are as
dissimilar as one could ever anticipate in synthetic polymers. Spectra is ostensibly only
carbon and hydrogen, an analog of wax but a polymer of extremely high molecular weight and
orientation (30:1 draw ratio). Kevlar is a polyaramid with a variety of chemical elements
and groups and is primarily aromatic in structure. By the judicious selection of process
gas the fiber surface of either fiber is reengineered to make it compatible with and, if
desired, reactive to the resin matrix of choice. The improvements in flexural strength and
modulus are the result of an increase in interlaminar shear strength which in the case of
the Kevlar was measured only for the plasma treated fabric composites.
As is readily seen a plasma treatment provides significant
improvements over untreated material, 200 to 300% and more is not uncommon. Since there is
a myriad of fabric styles in use, as well as different grades of both Spectra and Kevlar,
the above data is presented as representative of typical improvement obtained across a
broad matrix of fabric styles and fiber grades. Because the construction of the fabrics
are different one should not compare the properties of these different composites, but
that similar improvements are realized with all constructions.
Conclusion
The outstanding specific strength and modulus
characteristics of advanced fibers can now be more effectively realized in reinforced
composites with plasma surface treatment. The plasma treatment process can be readily
tailored by the judicious selection of the process gas and process parameters to permit
the "reengineering" of the top molecules of the fiber to a specific surface
energy, chemical compatibility or reactivity to specific resin matrices. In addition, for
fibers such as Kevlar where moisture absorption is known to have deteriorating effects,
the plasma process is inherently an effective drying process providing further benefits.
4th State’s plasma system shown has the
capability of treating 60" wide products and roll diameters to 19.5". It is
available to conduct development trials or toll treatment. Consider your product
possibilities by reengineering the reinforcement fiber.
Table I
Kevlar 49 Composites
Style 120
normalized to 60% fiber volume
Lot
|
Fabric Treatment
|
Flexural Strength
MPa |
Flexural Modulus
GPa |
Interlaminar
Shear Strength
MPa |
| 1 |
none |
131.6 |
17.5 |
|
| 2 |
none |
88.5 |
19.8 |
|
| 3 |
none |
130.8 |
36.1 |
|
| 4 |
none |
167.3 |
25.4 |
|
| Mean |
|
129.6 |
24.7 |
|
| Std. Dev. |
|
32.2 |
8.3 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Lot
|
Fabric Treatment
|
Flexural Strength
MPa |
Flexural Modulus
GPa |
Interlaminar
Shear Strength
MPa |
| 5 |
plasma treated |
389.6 |
32.4 |
47.6 |
| 6 |
plasma treated |
393.7 |
34.5 |
28.3 |
| 7 |
plasma treated |
356.5 |
33.1 |
31.7 |
| 8 |
plasma treated |
366.1 |
33.8 |
31.7 |
| Mean |
|
389.6 |
32.4 |
34.8 |
| Std. Dev. |
|
18.1 |
0.9 |
2.7 |
Table II
Properties of Spectra 900 / Epoxy Composites1
Fabric: 8 Harness Satin
| Property |
Plasma |
Untreated |
|
| Fiber Volume (%) |
70 |
67 |
| Flexural Strength (MPa) |
153 |
47 |
| Flexural Modulus (GPa) |
21 |
3 |
| Interlaminar Shear Strength (MPa) |
13 |
4 |
Plasma Fabric Treater
60" width capacity
Acknowledgements
The authors wish to thank Mr. Sean Johnson of
YLA, Inc. for generating and providing the data on the Kevlar composites.
References
1. Kaplan, S.L., Rose, P.W., Nguyen, H.X. and Chang, H.W.,
Gas Plasma Treatment of Spectra Fiber, SAMPE Quarterly, Vol. 19, No. 4, July 1988
Kaplan, S.L., Rose, P.W., Nguyen, H.X.
and Chang, H.W., Gas Plasma Treatment of Spectra Fiber, SAMPE Quarterly, Vol. 19, No. 4,
July 1988
