Plasma treatment apparatus and method

Abstract

A plasma treated gas permeable material is produced by applying an alternating voltage between spaced electrodes, at least one of which is covered with a dielectric barrier and at least one of which comprises a plurality of discrete electrode segments, to generate plasma microdischarges between the spaced electrodes. A gas permeable material is passed between or adjacent to the spaced electrodes. A gas is moved between the electrode segments into and through the space between the electrodes and through the gas permeable material. The gas flows over plasma generation surfaces of the respective electrode segments and is moved at a rate whereby the gas flow between the spaced electrodes is turbulent and so randomises the plasma microdischarges and disperses plasma products that would otherwise give rise to burning instabilities in the gas permeable material, whereby the randomized plasma microdischarges provide a generally uniform plasma treatment of the gas permeable material. Also disclosed is an apparatus for laying out the process.

Description

FIELD OF THE INVENTION [0001] This invention relates to a method and apparatus for the plasma treatment of gas permeable materials, for instance, fibrous materials. The invention is particularly useful when applied to wool. BACKGROUND OF THE INVENTION [0002] Plasma treatment of materials is widely used to alter the surface characteristics of the material. The treatment is generally useful where it is uniformly distributed over the surface of the material. When applied to wool fibres, plasma treatments are used to oxidize a lipid layer on the surface of the fibres. Oxidation of the lipid layer makes the wool fibres more receptive to subsequent surface treatments, for instance anti-shrink and pilling-prevention treatments. [0003] Removal of the lipid layer also increases friction between the fibres. This benefits yam production processes as less twisting is required to form the yam. The lower twisting level enables the yarn to be produced at a greater rate, enabling downstream process...

AI Technical Summary

Benefits of technology

[0012] Further, the localisation of microdischarges increases the probability of generation of a high temperature instability within the plasma that burns the material to be treated. This seems to occur due to the local release by the plasma of particular chemicals from the material surface, possibly where a high concentration of contaminant occurs. The generation of these chemicals changes the nature of the plasma and causes localised excessive absorption of plasma energy and a rapid rise in temperature. Localisation of the microdischarges is believed to result in two effects. Firstly, ions remaining stationary relative to the electrodes cause localisation of the plasma microdischarges between the electrodes and result in uneven and less effective surface treatment. Secondly, trapping of ions and plasma-material by-products in a gas permeable material moving relative to the electrodes causes localisation of the plasma relative to the material and can result in burning of the material.

[0044] The uniform contact between the liquid medium and the dielectric sheath ensures that the current is distributed evenly across the surface of the dielectric sheath. This facilitates randomized electrical breakdown of the gas between the electrodes, and in consequence a more uniform plasma. It also avoids undesirable concentration of the electric field at sharp points within the electrode structure that increase the probability of dielectric breakdown. This is especially important when thin dielectric media are used, which is preferred in order to maximise the efficiency of coupling of electrical energy to the plasma. The liquid conductor also helps to uniformly distribute heat in the electrodes and so minimise thermal stresses.

Problems solved by technology

The ions from each microdischarge do not normally disperse easily and, having lower electrical resistance compared to the surrounding gas, cause the microdischarges to occur repeatedly at the same locations.

This provides very poor uniformity of treatment and fails to use all of the available gas for generation of useful chemical species by the plasma.

Further, the localisation of microdischarges increases the probability of generation of a high temperature instability within the plasma that burns the material to be treated.

The generation of these chemicals changes the nature of the plasma and causes localised excessive absorption of plasma energy and a rapid rise in temperature.

Firstly, ions remaining stationary relative to the electrodes cause localisation of the plasma microdischarges between the electrodes and result in uneven and less effective surface treatment.

Secondly, trapping of ions and plasma-material by-products in a gas permeable material moving relative to the electrodes causes localisation of the plasma relative to the material and can result in burning of the material.

It has further been recognised that the localisation of the plasma microdischarges prevents use of the whole gas volume available in generating the plasma and thus limits the concentration of the active species that beneficially modify the surface chemistry of the material.

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