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 Kevlar® and Spectra® 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.

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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