FIBROUS SUBSTRATE TREATMENT METHOD AND MACHINE FOR IMPLEMENTING THE METHOD

Technical field

This invention relates to a fibrous substrate treatment method and machine, in particular for treating a continuous substrate with atmospheric plasma. The invention addresses the above mentioned method as applied to a continuous substrate made from fibres of any kind: natural, such as, for example, cotton, wool, carded or combed viscose, or other natural fibres in semi-processed form; or substrates made from synthetic fibres or blends of synthetic and natural fibres. Background art

The technology used for treatment with plasma at atmospheric pressure is known, for example from patent application WO2011/095930, and consists in treating a continuous permeable substrate (made from natural or synthetic fibre or fabric or from polymeric membrane) with plasma at atmospheric pressure, obtained by electric discharges in a controlled atmosphere, particularly in air. This technology is proved to be particularly effective because it was found that treating the substrate in this way improves its commercial value, minimizes production cost and improves the performance quality of subsequent treatments such as finishing, dyeing and the like.

In particular, compared to the widely used vacuum plasma treatment installations, installations operating with plasma at atmospheric pressure (or atmospheric plasma) are much simpler and more economical, both to construct and to run.

In effect, in vacuum plasma installations, expensive vacuum chambers must be provided in which to place the material to be treated with the plasma, create the vacuum and feed in the process gas. Furthermore, since the fabric to be treated has to be fed into and out of the different vacuum chambers, the fabric treatment process cannot be a continuous one, which in turn means that the productivity of the installation is greatly reduced.

Moreover, the process gases used are decidedly expensive.

Atmospheric plasma installations advantageously overcome these expensive installation construction and running complications and, besides, allow atmospheric air to be used as process gas, but are not themselves free of disadvantages.

With the plasma in the air at atmospheric pressure, the discharge is not uniform as in plasma in a vacuum but creates unwanted filaments. This characteristic, peculiar to atmospheric plasma in the presence of oxygen, constitutes a problem, as described below, which this invention intends to overcome.

In this specification, the expression plasma at atmospheric pressure also means using air as process gas.

According to the above mentioned patent application, the substrate is fed between two sets of rollers which are suitably offset from each other in such a way that electric discharges can be generated across successive rollers and can be made to pass through the material constituting the substrate.

In particular, as stated, it is known that - at atmospheric pressure in the presence of oxygen - an electric discharge across two rollers which are suitably spaced to define a gap in which the substrate is made to move is not uniform across the gap but, on the contrary, has end parts characterized by a discharge which is "more diffuse" and a central part where the discharge is "filamentary". In other words, in the proximity of the surfaces of the rollers, the discharge is distributed in a more diffuse manner over a spatial region, allowing excellent treatment of the substrate. In the central zone, however, the filamentary configuration of the discharge in the air at atmospheric pressure is excessively concentrated and therefore very aggressive on the material constituting the substrate. That means the substrate is exposed to a high risk of burning and, hence, irreparable damage and, in any case, there is always the risk of the end quality being below expectations on account of the non-uniformity of the substrate treatment.

In addition to this problem, it should be noted that preventing the risk of burns calls for high flows of working gas (air), which means having to adopt extraction . and/or ventilation systems which, besides being cumbersome, also increase the complexity and costs of the installation. In this situation, there is an evident need for an atmospheric plasma treatment method and a machine for implementing the method which overcome the above mentioned disadvantages. The need to optimize the treatment is even more critical in the case of treatment of fibrous substrates made of a material which is very voluminous and hence subject to a higher risk of burning.

Aim of the invention

This invention therefore has for an aim to provide a fibre substrate treatment method and machine which can guarantee an end result of high quality.

It is also an aim of the invention to provide a fibre substrate treatment method which can be implemented by a machine that is constructionally simple and relatively inexpensive.

The aim is fully achieved by the fibre substrate treatment method and machine as characterized in the appended claims.

Brief description of the drawings

The technical features of the invention, with reference to the above aim, are clearly described in the appended claims and its advantages are apparent from the detailed description which follows, with reference to the accompanying drawings which illustrate a preferred non-limiting example embodiment of it, and in which:

- Figure 1 schematically illustrates a fibre substrate treatment machine according to this invention;

- Figure 2 shows an enlarged detail of the machine of Figure 1 ;

- Figure 3 shows an enlargement of a part of the detail of Figure 2.

Detailed description of preferred embodiments of the invention

In the accompanying drawings, the numeral 1 denotes in its entirety a fibre substrate treatment machine according to the invention.

The machine 1 comprises feed means 2 for a substrate 100 configured to move the substrate 100 along a feed path in feed direction "A".

The feed means comprise at least a first pair of rollers 2a, 2b which are in contact with respective opposite faces of the substrate 100. More specifically, the two rollers 2a, 2b are counter-rotating and rotate at the same speed so as to apply an equal pulling action on both faces of the substrate 100.

Downstream of the first pair of rollers 2a, 2b there is a second pair of rollers 3a, 3b, also counter-rotating and in contact with opposite faces of the substrate 100. Preferably, the rollers 3a, 3b of the second pair also rotate at the same speed so as to apply an equal pulling action on both faces of the substrate 100.

Preferably, the rollers 2a, 2b; 3a, 3b are cylindrical in shape.

Advantageously, the rotation speed of the second pair of rollers 3a, 3b is greater than the rotation speed of the first pair of rollers 2a, 2b so that the substrate 100 is stretched longitudinally (that is to say, along the feed path of the substrate 100) in order to reduce the thickness of the substrate 100. For this reason, the two pairs of rollers 2a, 2b; 3a, 3b define means for stretching the substrate 100 and are located in a station "P" for the pretreatment of the substrate 100.

In the embodiment of Figure 1 , the rollers 2a, 2b; 3a, 3b are the same in diameter. More generally speaking, however, since the rollers 2a, 2b; 3a, 3b might be different in diameter, the stretching action on the substrate 100 might be provided by the higher tangential speed of the second pair of rollers 3a, 3b compared to the tangential speed of the first pair of rollers 2a, 2b.

Downstream of the pretreatment station "P", along the feed path of the substrate 100, there is a treatment station "T" configured to treat the substrate 100 with plasma at atmospheric pressure.

According to the basic operating principle of the treatment with ionized gases and/or plasma at atmospheric pressure, the treatment station "T" comprises at least two electrodes facing respective opposite faces of the substrate 100 and a difference in electrical potential is applied across these electrodes to generate an electric discharge which ionizes a working gas (usually air) surrounding the treatment zone, producing plasma at atmospheric pressure in the proximity of the substrate 100.

Preferably, the electrodes are defined by at least one pair of rollers 5a, 6a, whose respective axes of rotation are parallel to each other and which are mounted transversely, preferably perpendicularly, to the feed path of the substrate 100. The two rollers 5a, 6a are mounted on opposite sides of the substrate 100 so that the outside surfaces of the rollers 5a, 6a are in contact with opposite faces of the substrate 100.

The above mentioned difference in electrical potential is applied across the two rollers 5a, 6a in such a way that the electric discharge generated across the rollers 5a, 6a passes through the material constituting the substrate 100.

Preferably, the cylindrical outside surfaces of the rollers 5a, 6a of the treatment station "T" are covered with a coating of dielectric material, preferably silicone, in order to generate a discharge of the Dielectric Barrier Discharge (DBD) type.

At the treatment station "T", an air flow (as in the above mentioned patent WO2011/095930) crosses the gap between the rollers 5a, 6a and repels the discharge products, thereby reducing the probability of burning the substrate.

In a preferred embodiment, illustrated in Figure 1 , the machine 1 comprises a first array 5 of rollers 5a and a second array 6 of rollers 6a, each array 5, 6 acting on a respective face of the substrate 100 in such a way as to generate the plasma at atmospheric pressure at a plurality of discharge zones located in succession along the feed path of the substrate 100.

The two arrays 5, 6 are longitudinally offset in such a way that the feed path, defined by the rollers 5a, 6a, is made to follow an undulatory course such that the substrate 100 superimposes on a part of the cylindrical outer surface of the rollers 5a, 6a, in particular a section of the cylindrical surface (as shown in Figure 2).

Each roller 5a of the first array 5 is operatively (and electrically) interconnected with a corresponding roller 6a of the second array 6 so as to define pairs of rollers defining respective treatment zones of the substrate 100.

In this configuration, the rollers 5a, 6a of the aforementioned arrays 5, 6 constitute means for guiding the substrate 100 within the treatment station "T".

Preferably, the rollers 5a, 6a of each pair of rollers of the arrays 5, 6 are spaced from each other to define a gap of minimum thickness "s3" between 0.3 mm and 3.0 mm, preferably between 0.4 mm and 0.8 mm. In a preferred embodiment, the rollers 5a, 6a of the arrays 5, 6 to do not subject the substrate 100 to further longitudinal stretching but, at most, the rollers 5a, 6a contribute to keeping the substrate 100 in a state of tension identical to that given to the substrate 100 by the stretching means. In other words, the rollers 5a, 6a of the arrays 5, 6 may be driven by a suitable drive motor (not illustrated) which causes each roller 5a, 6a to rotate at the same tangential velocity as all the other rollers 5a, 6a of the arrays 5, 6. That way, within the treatment station "T" the substrate 100 does not undergo any further longitudinal elongation and the substrate 100 is not further reduced in thickness.

In the embodiment of Figure 1 , each array 5, 6 comprises six rollers 5a, 6a, thus defining six pairs of rollers 5a, 6a defining the same number of treatment zones. There may, however, be any number of rollers for each of the arrays 5a, 6a without departing from the scope of the inventive concept. There may be two of more arrays 5, 6 along the feed path "A". Described below is a fibre substrate treatment method which can be implemented by the machine 1 described above.

In use, the substrate 100 is made to move along the feed path and, advantageously, before being subjected to the treatment with ionized gases and/or plasma at atmospheric pressure at the treatment station "T", the substrate 00 is reduced in thickness until the substrate 100 reaches a predetermined thickness.

Preferably, the substrate 100 is reduced in thickness until it has a final thickness "s2" of between 0.05 mm and 1 mm, preferably between 0.1 mm and 0.5 mm. Advantageously, for reasons which will be explained below, the final substrate thickness "s2", obtained in the preparation station "P", is less than the set thickness "s3" of the gap between the rollers 5, 6 in the treatment station "T".

In the preferred embodiment, the reduction in the thickness of the substrate 100 is accomplished by stretching the substrate 100 longitudinally along the feed path, in particular in the pretreatment station In the pretreatment station "P" the substrate 100 is preferably subjected to longitudinal stretching characterized by a final stretch factor of between 1.5 and 10. This stretch factor is calculated as the ratio between the length of a piece of substrate 100 leaving the pretreatment station "P" and the length of the same piece of substrate 100 before it entered the pretreatment station "P". Alternatively, the stretch factor can be calculated as the ratio between the tangential speed of the second pair of rollers 3a, 3b and the tangential speed of the first pair of rollers 2a, 2b.

It should, however, be noted that the thickness reduction of the substrate 100 upstream of the treatment station "T", which constitutes an inventive aspect of the invention, might also be performed in different ways from that described. For example, the thickness might be reduced by making the substrate 100 pass between two counter-rotating rollers defining between them a distance whose minimum thickness is equal to the final value to be given to the substrate 100, or even less, if allowance is made for the possible elastic return of the material constituting the substrate 100. After its thickness has been reduced, the substrate 100 is subjected to the treatment with plasma at atmospheric pressure, which is done by feeding the substrate between two opposite arrays 5, 6 of rollers 5a, 6a, the arrays being longitudinally offset in such a way that the feed path, defined by the rollers 5a, 6a, is made to follow an undulatory course.

Each roller 5a of a first array 5 is operatively coupled to a corresponding roller 6a of the other array 6 to define a respective treatment zone in which the electric discharge generated across the two rollers 5a, 6a passes. In this treatment zone, the working gas used (which is ionized by the discharge) is preferably air. The working gas might, however, also be a mixture of a main gas and any other special gas suitable for the atmospheric plasma treatment of the substrate 100, such as, for example, reactive gases (nitrogen, carbon dioxide or different precursor gases) or inert gases (helium, argon or others) or mixtures of these.

Preferably, as already stated above, the rollers 5a, 6a of the arrays 5, 6 do not subject the substrate 100 to further longitudinal stretching but, at most, the rollers 5a, 6a contribute to keeping the substrate 100 in a state of tension identical to that given to the substrate 100 by the stretching means, with a substantially constant thickness.

Figure 3 shows the structure of the electric discharge across the two coupled rollers 5a, 6a forming part of the arrays 5, 6, at three different points on the selfsame rollers 5a, 6a. It may be noticed that the discharge at atmospheric pressure has end parts "R1" where its structure is more diffuse, and a central part "R2" where the structure is more filamentary. Advantageously, thanks to the reduced thickness values of the gap between the rollers 5a, 6a, the length of the (inclined) part of substrate 100 which crosses the gap and hence travels the central part "R2" with the more filamentary structure, is reduced.

Moreover, apart from the reduced zone of the substrate 100 which crosses the central part "R2" with more filamentary structure, the air flow used for the process is concentrated in the gap between the rollers 5a, 6a, so as to further reduce the probability of the discharge producing burns on the fibre of the substrate 100.

In short, with the specific reduced values of the gap "s3" and with the thickness "s2" of the substrate 100 less than the gap s3 between the rollers 5, 6, there is less probability of the filamentary part causing burns on the fibres of the substrate 100. In other words, with the reduced thicknesses in play and with the reduced thickness "s2" of the substrate 100 smaller than the gap "s3" it is possible to keep the substrate 100 adjacent to the surface of the rollers 5, 6 of the treatment station "T", where the electric discharge has a more diffuse structure, for most of the time during its passage between the rollers 5, 6, whereas in the more central zone of the path, the filamentary discharge passes through the thinned substrate without causing burns. This advantage is particularly appreciable precisely with an offset roller array configuration as in this invention, which causes the feed path to follow an undulatory course such that the substrate impinges on a part of the cylindrical outer surface of the rollers.

Preferably, the method according to the invention uses air as the working fluid to be ionized and can therefore be identified as a method of treatment with atmospheric plasma.

More in detail, atmospheric plasma is a particular state of gas (air) at atmospheric pressure and is created when the electrical discharge is applied to it. The gas (air) in the plasma state is characterized, above all, by the presence of numerous radicals (of oxygen, nitrogen, OH groups and others) which attack the surface of the substrate and produce surface modifications both chemical (activation of the substrate) and physical (transforming the morphology at nanometric level) type.

Example embodiment:

Web of cotton carded fibre in line, treated by a machine that uses atmospheric plasma

A web (100) of carded cotton fibre is thinned in the preparation station (T) where a stretch factor of 4 (stretch ratio of 1 :4) is applied. The treatment station (T) comprises two consecutive zones, each with two opposite arrays (5, 6) of eight electrode-rollers (5a, 6a) each, each zone producing eight discharges across the electrode-rollers. The total length of the area affected by the atmospheric plasma in machine direction is approximately 17 cm and the specific plasma power applied to the fibre is approx. 12 J/cm ² . The gap between the electrode-rollers is set to a thickness value (s3) of 0.75 mm. The working gas used is air with RH 45% humidity and is set at a total flow rate (through two treatment zones) of 100 m ³ /h.

The web treated in this atmospheric plasma machine does not show signs of burning or other defects on the cotton fibres.

Next, the treated fibre sliver is returned to the thickness it had prior to treatment in order to assess the changes in its properties obtained thanks to the treatment.

Prior to treatment, the breaking strength of the web is 0.7 ± 0.1 N. After treatment, the breaking strength of the web is 2.5 ± 0.4 N.

An increase in affinity to water is also noticed. Prior to treatment, the water immersion time of the web is infinite. After treatment, the water immersion time of the web is 12 seconds.

The present invention achieves the preset aims and overcomes the disadvantages of the prior art.

Reducing the thickness of the substrate to extremely small values such as those stated herein makes it possible to treat the substrate mainly with the part of the electrical discharge with the more diffuse structure, thus significantly reducing the burnings typical of interaction with the filamentary part of the discharge and thus without damaging the material. Moreover, with the reduced thickness of the material, the reduced interaction of the substrate with the filamentary part of the electrical discharge makes the treatment more efficient because it reduces energy dispersion and non-uniformity of the treatment and therefore allows use of lower power compared to prior art systems, which in turn means that the part of the discharge with the more diffuse structure also has a less aggressive action on the substrate. Setting a gap, between the rollers of the treatment station smaller but always greater than the thickness of the substrate to be treated, reduces discharge energy dispersion outside the substrate and thus allows energy saving. Treating the fibre substrate in the manner described herein is more energy efficient and that in turn produces a smaller variation in the tactile properties of the treated fibre (what is commonly referred as the "handle" of the finished fabric)"hand" of the finished fabric. There is thus an evident improvement in the quality of the final product. Further, only a minimum amount of ventilation is needed to remove the ozone produced by the electrical discharge in the air and powerful extraction systems are no longer necessary.

Moreover, of the possible ways of reducing the thickness of the substrate, longitudinal stretching is particularly advantageous because it is already widely used in several processes for treating fibres in the textile industry.