WO2006117829A1

WO2006117829A1 - Method for processing polymeric yarns and textile materials for modifying their surface resistivity

Info

Publication number: WO2006117829A1
Application number: PCT/IT2006/000311
Authority: WO
Inventors: Claudia Riccardi; Stefano Zanini; Paolo Fracas; Paola Massini
Original assignee: Universita' Degli Studi Di Milano - Bicocca; Saati S.P.A.
Priority date: 2005-05-04
Filing date: 2006-05-04
Publication date: 2006-11-09
Also published as: EP1880050A1; ITMI20050813A1

Abstract

The present invention relates to a method for processing polymeric yarns and textile materials to modify the surface resistivity thereof, said method being characterized in that it comprises a step of performing a plasma treatment directly on said fibers of said polymeric yarns and textile material.

Description

METHOD FOR PROCESSING POLYMERIC YARNS AND TEXTILE MATERIALS FOR MODIFYING THEIR SURFACE RESISTIVITY

BACKGROUND OF THE INVENTION

The present invention relates to a method for processing polymeric yarns and textile materials for modifying their surface resistivity.

As is known, static electricity can be defined as a stationary electric charge due to either an excess or lacking of electrons on the surface of a body.

The accumulation of electric charges on an article is caused by the movement of the electrons inside the article or by a passage said electrons from a body to another body.

Actually, the term "antistatic" indicates the property of a material according to which said material is not electrostatically charged by rubbing, stirring or separating of two surfaces. Thus, an antistatic material will not be electrostatically charged because of the above mentioned events, owing to a continuous dissipation of the electric charge through the encompassing environment . Accordingly, antistatic materials must both prevent any static electricity from being formed and eliminate, in a nearly instantaneous and safe manner, possibly already present electrostatic charges.

At present, enforcing rules related to electrostatic discharging products and processes agree in defining antistatic material classes by their surface resistivity (R₃) . Such a parameter is defined as a ratio of the D. C. voltage and current passing through/over the surface of the body.

In such a case, the surface comprises a squared area.

Actually, the surface resistivity corresponds to the resistance between two opposite sides of the square, and is independent from the square size and metering unit thereof. The surface resistivity is defined in ohms/square (ohm/square) .

A further relevant parameter is the so-called surface "conducibility" which is defined as the reverse of the surface resistivity. Thus, said materials can be classified, as follows : conductive: i.e. those having a Rs < 1 x 10⁵ Ω/square; static-dissipative: i.e. those having 1 x 10⁵ Ω/square < Rs 1 x 10¹² Ω/square; insulating: i.e. those having Rs > 1 x 10¹² Ω/square. In a productive or making environment, an electrostatic charge is generated each time that, during a processing operation, two surfaces in general are contacted to one another and are then separated from one another again.

■ The accumulation of a charge on a textile fabric material causes drawbacks such as electric discharges, difficulties in processing the textile fabric and yarn materials, and an accumulating or depositing of powders .

The sensitivity to the above mentioned drawbacks deriving from an electrostatic charge generated on a textile material contacting a part or a component of a processing machine, depends on the ratio of the material rate and the diffusion of the surface charge on the material itself.

If the material is fed through a long pathlength, then, before a disappearing of the static electricity, the charge is continually accumulated on an increasingly broader surface. . On the contrary, if the surface charge diffusion speed or rate is larger than the material feeding speed or rate, then no charge is accumulated, and, accordingly, no drawback would be generated.

The triboelectric charge value, accordingly, can greatly change depending on: The material type; The surface type; The contact pressure; The separating speed; - The relative humidity.

Depending on the existing or generated charges, the above mentioned drawbacks can be classified as follows :

A mutual repulsion of materials having like-sign charges;

An attraction of materials having opposite-sign charges;

An attraction of a charge-free material, because of an induction effect (powder, dirt particles and so on) .

The above mentioned static electricity eliminating methods are applied in the textile material industry as follows: by preparing mixed fibers, of any desired suitable composition, having a different contact potential, thereby said fibers will be charged by opposite sign charges, to be neutralized one another and prevent static electricity from being generated. A disadvantage of such a method is that it is not always possible to form mixed fibers at will.

In fact, it is necessary to also consider the fiber processing and chemical performance properties, as well as their textile and dyeing properties.

- By suitably selecting the textile material and the material in general subjected to rubbing. A proper selection of the material and apparatus for processing it allows to prevent electrostatic charges from being generated, provided that the charges being formed on said material and apparatus are mutually neutralized on their respective contact surfaces. A main disadvantage of this method is its very low operating flexibility and a technical impossibility of finding and using constructional materials which are compatible to the textile' material antistatic requirement.

- By fitting or mating the textile article to the friction generating material, through antistatic agents.

Such a fitting or mating can be achieved by a chemical surface treatment. Disadvantages: the need of using chemical products, performing drying operations and a comparatively high power and water consume. -" By ionizing air. The radioactive substances adapted to eliminate static electricity can be considered as pertaining to two groups: those emitting α rays and those emitting β rays. These two groups have specific characteristics and are moreover differentiated from one another in their related apparatus. Actually, the α ray emitting apparatus generally use radium as an active material; on the other hand, the β ray emitting apparatus use thallium- 204 or^* strontium-90 and yttrium-90. The apparatus are constructionally simple and have a satisfactory efficiency; however, they present dangers, in particular do not allow an immediate detection of possible malfunctions or damages.

Of the above disclosed methods, that using chemical antistatic products is the most-commonly used since it can be easily carried out. The chemical treatments, actually, can reduce the electrostatic charges both by providing the fiber surface with a sufficient conductivity, and preventing charges from passing from a contact surface to the other. . However, a high efficiency does not represent the sole main factor affecting a practical use of an antistatic agent or material. In fact, the chemical products must also meet other requirements, such as, for example: - must not alter the fiber properties; must not affect the dyeing material lightfastness and chromatic tone; must not modify the hand, draping and sliding characteristics of the fabric; - must not interact with other materials contacted thereby.

The setting or fixation of the product on the above mentioned fabric materials is achieved by a reaction of the antistatic substance and fiber, or by a polymerization of the water soluble compounds in a water insoluble antistatic film adhering to the fiber. . Some antistatic products comprise surface active agents, including a water hydrophobic hydrocarbon portion, having an affinity for the fiber, and and end hydrophilic group, projecting from the fiber.

In the surface active product groups, the individual compounds have very remarkable differences with respect to their use as antistatic materials.

Molecules having double-bonds or linkages have been found as particularly efficient; moreover, it has been found that an increase of the polarized or polarizable groups and salt bonds contribute to preventing electric charge phenomena from occurring.

With respect to a permanent type of product, it is necessary that a polymerization reaction occurs for fixing.it on the fiber materials. This type of product too bears water hydrophobic hydrocarbon chains and free side chain polarizable groups.

The permanent effect of such a product, moreover, is based on a forming of a film on its surfaces; moreover, its lightfastness greatly depends on the adhering characteristics of said film, and on the resistance against mechanical actions and solvents.

For cloth article fabric materials, which are affected by several modes of wear during their daily use, it is necessary that the mentioned antistatic products be able of resisting against dry and wet washing operations, rubbing and so on, without impairing the properties of the fabric material such as its hand, draping, dyeing fastness and anti-dirt characteristics .

However, the above mentioned chemical treatments have a lot of drawbacks due to the use of chemical products which frequently have a polluting nature or require complex disposal of processes, together with the requirements of using water for applying the chemical products, and, moreover, of performing drying operations with a comparatively high electric power consume.

Moreover, a treatment of the above mentioned type causes the product organoleptic and volume properties to change, and, accordingly, such a treatment is not advantageous as applied to textile or yarn materials for making cloth articles.

SUMb-ARY OF THE INVENTION

Accordingly, the aim of the present invention is to overcome the above mentioned problems, by providing a method for processing polymeric yarns and textile materials to modify their surface resistivity, which does not use conventional chemical products and water, and which, moreover, has a very low environmental impact, without modifying the volume and organoleptic properties of the materials being processed.

Within the scope of the above mentioned aim, a main object of the invention is to provide such a method allowing to obtain a quick charge dissipation, to be easily fitted to a broad range of fabric material products to be processed, thereby providing said products with novel functional properties. A further object is to provide such a method which άs not related to any modifications of the material surface static charge, i.e. of the material contact potential, but to a reduction of the surface resistivity or, equivalently, an increase of the surface conducibility, thereby, each time electric charges are created on the material surface because of rubbing or contacting with other charged bodies, said surface charges do not accumulate, but are dissipated owing to said increased surface electric conducibility.

A further object of the present invention is to provide such a method for processing polymeric yarns and textile materials which, owing to its specifically designed characteristics, is very reliable and safe in operation.

Yet another object of the present invention it to provide such a method which can be easily carried out and which, moreover, is very competitive from a mere economic standpoint. According to one aspect of the present invention, the above mentioned aim and objects, as well as yet^' other objects, which will become more apparent hereinafter, are achieved by a method for processing polymeric yarns and textile materials to modify their surface resistivity, characterized in that said method comprises a step of directly processing by a plasma the textile material fibers.

Further characteristics and advantages of the method according to the present invention will become more apparent hereinafter from the following detailed disclosure.

From a lot of experimental tests, it has been surprisingly found that the conducibility properties of the above mentioned materials can be advantageously achieved if said materials, processed according to conventional processing operations, are subjected to a further plasma processing step to be carried out at any desired time.

More specifically, said plasma is a "cold" plasma, i.e. a mixture of charged particles (electrons and ions) and neutral species (atoms, molecules, radicals), the temperature of which is of the same order as the environment temperature.

Moreover, said plasma can be generated under vacuum, or in suitable plasma generating chambers containing gases at pressures less than the atmospheric pressure, and preferably from 0.1 to 20 mbars, or equal to the atmospheric pressure, while using gas control chambers .

In particular, the plasma can be generated by different electromagnetic plasma generating sources, i.e different frequency and different geometrical shape sources .

The physical-chemical processes occurring on the surface of the materials to be processed mainly depend on the plasma parameters, i.e. the plasma gas, gas pressure, unit surface power, plasma material exposition time, and residue pressure of the evacuation step fo'r evacuating the material to be processed vacuum chamber.

The typical operating parameters are as follows : - gas: CO₂, SF₆, hydrocarbons, acrylic acid, ammonia, fluorocarbons, even in mixtures thereof, noble gases, nitrogen, oxygen, hydrogen; - operating gas pressure: 10² mbars at an atmospheric pressure

- residue pressures P > 10^~6 mbars

- Processing powers Pw < 0.5 W/cm² for vacuum treatments and powers of Pw < 10 W/cm² for atmospheric pressure treatments

- a treatment duration or period less than 15 minutes. Description of the method

The plasma processing method can be carried out either in vacuum or at an atmospheric pressure.

The plasma can be generated by any plasma generating source (low frequency, radio frequency, high frequency) . The plasma process does not depend on the geometric shape or frequency of the plasma source, and, moreover, does not depend on the vacuum chamber geometric shape and configuration, but on the above specified operating or working parameters.

By way of an example, two plasma processing methods will be hereinafter disclosed, i.e. a vacuum processing method and an atmospheric pressure processing method. Treatment under vacuum

Locating with respect to the plasma source. A polymeric textile material sample is supplied to a cylindric reactor (having for example a diameter of 20 cm and a height of 40 cm) and is arranged in front of the operating antenna, at a distance therefrom from 2 to 10 cm. Evacuation and degassing step A pumping step is at first performed, in which the overall system is evacuated and brought to a low pressure, in each case however larger than 10^"6 mbars. In this step, the polymeric sample is degassed, thereby- removing therefrom air and moisture contained therein. Filling-in step

Then, a filling-in step is carried out, in which the reactor is filled-in by the process gas (CO₂, SF₆, CH₄) . The target or desired processing pressure (from 10^~2 mbars and 1 mbar) is achieved under flow conditions inside the reactor. Plasma generation step The plasma is supplied through a RF generator or source (for example: Huettinger PFG-300, 13.56 MHz) coupled to the antenna through a semi-automatic matching device. The RF generator or source can be used both in a continuous mode of operation and in a pulse mode of operation. In this latter case, it is possible to change the times t_on (plasma switching on time) and toff (plasma switching-off time) . The applied powers are varied from 0.1 W/cm² to 1 W/cm². Processing step The polymeric fabric sample is exposed to the plasma for a time variable from 1 second to 15 minutes. Filling-in step

At the end of such a processing, a chamber filling^'-in step is carried out, after having switched- off the radiofrequency, and the filling-in step can be carried out by using different types of gases, air, from air to inert or noble gases. At atmospheric pressure

The sample is entrained between two electrodes coated by a ceramics material, spaced from one another at 1 mm to 5 mm. The electrodes are supplied with a low frequency f = 30 kHz - 100 kHz, and a power from 200W to 500W. The samples are cyclically processed, each processing cycle comprising a treatment having a duration of fractions of second.

The typical speeds or processing rates, under continuous-regimen roll-to-roll conditions, vary from 1 m/minute to 100 m/minute. The gases used in noble gas, air or nitrogen mixtures are CO₂, SF₆, CH₄, fluorocarbons, acrylic acid.

" The above disclosed plasma processing method can be applied after any desired yarn or fabric enhancing or nobilitating step. RESULTS

Definition and description of the conducibility measurement This parameter is defined as the ratio of the D. C. voltage and current passing on a surface of a body. In this case, the surface comprises a square area. Actually, the surface resistivity corresponds to the resistance between the two opposite sides of a square and is independent from the square size and size unit thereof. As stated, the surface resistivity is measured in ohms/square. In particular, the surface resistivity is calculated by using a measurement of the electric resistance of the sample on which are arranged two concentric ring elements operating as electrodes and therebetween a potential difference of 100 V is applied. Actually, the used measurement method measures the resistance of a material as a current intensity between the two electrodes. The sample, having a size of 12 x 12² is held in e fixed position by a metal clamping plate.

The used measurement instrument is a Agilent ohmeter, Model 4339B, with a measurement cell Model 16008B ^• including concentric rings for measuring the surface resistivity and volume of insulating materials.

The samples are preserved under relative humidity and temperature controlled conditions for at least 24 hours, before and after the measurement. Said conditions correspond to 20 + 2°C and 65 ± 2% RH.

For such an instrument, the measurement range varies from 1 x 10³ to 1.6 x 10¹⁶ ohm/square. By using an annular concentric construction, the surface resistivity is calculated by using the following Formula:

Jl (DrI-D₂) R_s = R

D₂ - Di

where:

R₃ Surface resistivity [Ω/square] Di in diameter of the outer (main) electrode

D₂ outer diameter of the inner (guard) electrode

The measurements have been performed under the following operating conditions: the sample is arranged in the measurement cell and clamped or locked between the electrodes and a metal plate under a load of 5 kg.

Then a potential difference of 100 V is applied for 45 seconds.

The measurement is performed on a non-plasma processed sample, and on the plasma processed sample, immediately after the processing or treatment and then at time intervals of the order of weeks.

Results with conducibility parameters and values The operating parameters of the experiments carried out under vacuum are hereinbelow shown.

The antistatic property providing method is carried out under a vacuum at a pressure from 0.1 to 10 mbars, preferably 0.2 mbars to 2 mbars, more preferably from 0.-2 mbars to 0.6 mbars .

The used gases comprise: CO₂, air, noble gases,

SFε, hydrocarbons and mixtures thereof. The best results have been achieved by using SFβ, or in a mixture with a noble gas or air, CO₂, or in a mixture with air or a noble gas .

The operating residue pressures were larger than 10^"6 mbars, to reduce the degassing effect of the material during the surface plasma applying step. In a cylindric reactor having a diameter of 20 cm and a height of 40 cm, the typical operating or working power densities vary from 0.1 W/cm² to 1 W/cm² and the plasma can be generated with a pulsed manner of generation.

The treatment or processing times, i.e. the time for which the material surface is exposed to the plasma, are less than 15 minutes, preferably than 8 minutes, and more preferably from 1 second to 2 minutes .

From experimental tests, it has been found that, after a plasma processing using SF₆ , CO₂, CH₄ gases and mixtures thereof with air or noble gases, the antistatic properties increased in a permanent manner. More specifically, such a property can be achieved by carrying out the following processes: 1. In a CO₂ plasma, and mixtures thereof with noble gases: inclusion of oxygen containing groups with a physical re-arrangement of the polymeric chains and a modification of the surface roughness.

2. In a plasma comprising CH₄ and mixtures thereof with noble gases: inclusion of acid " groups .

3. In a plasma comprising SFβ and mixtures thereof with noble gases: etching of the first atomic layers and inclusion of some groups, even of a fluorurated type. By resuming, the surface modification effects are generated by a combination of two competitive processes: an increase of the surface roughness, caused by etching and an inclusion of some hydrophylic or fluorurated groups. In this connection it should be pointed out that the conducibility increase is NOT due to a fluorurating process, as florurated gases such as FS₆ are used, since the surface processed by SF₆ is NOT an hydrophobic but an hydrophilic surface. More specifically, the surface resistivity value decreases by at least four orders: from a value for a not processed material NT, of 1.5 x 10¹⁵ ohms/square to values, for a processed PET material, of at least 1.0 x 10¹¹ ohms/square. The surface property is, in most cases, a permanent one, i.e. the surface resistivity measurement remains unaltered for the plasma processed sample, after several weeks from the processing time.

In addition to a resistivity decrease, surface contact angles different from the starting angles are achieved; for example, by using a SF₆ plasma, the contact angle decreases from a value of 110° to a value of 70°, whereas by a CO2 plasma, said angle decreases to a value of 80° .

In contrast with other methods, the fluorurated gas is herein used not for providing the textile material surfaces with hydrophobic properties, but for modifying their surface conducibility, while holding unaltered, or at most modified in a hydrophilic sense, their surface properties.

The plasma processing, according to the invention, is efficient for all the materials (polyester, nylon, acrylic, poplypropylene and mixed materials, and in general on all the synthetic fabric and yarn materials. Examples By way of an example, a PET polymeric fabric material is herein considered.

To that end, data are herein shown related to tests or experiments performed on samples having a size of 12 x 12 cm², in a vacuum chamber, the plasma being generated with a radio-frequency of 13.56 MHz.

The residue operating or working pressure was of P = 2.0 10^"5 mbars/N2 eq.

The samples were arranged in front of a flat antenna at a distance variable from 3 cm to 10 cm, and the plasma was supplied either under a continuous or a pulsed regimen.

The results are resumed in the following Table were:

- "NT" means a Not-Treated material; - "Cont" means the contact angle expressed by degrees;

"Time" means the time interval of the plasma treatment, as measured in seconds; - "P" means the process gas pressure providing the cold plasma, as measured in mbars;

- "#N" means the sample type and number

"imp" means the pulsed mode of operation of the generator used for supplying the source

- "Ton" means the on-time of the generator in a pulsed mode of operation

- "Toff" means the off-time of the generator in a pulsed mode of operation

- "Rs" means the surface resistivity in ohms/square

"PET" means the PET polymeric fabric material

TABLE

The samples have been analyzed after the plasma treatment or process, and it has been found that the surface resistivity values were constant for months after the treatment. For example, the sample processed by SF6 PET#9 has been analyzed after several months, and the surface resistivity value after three months was of 2.190E + 12.

Like results have been achieved by using atmospheric pressure plasmas.

From the above disclosure it should be apparent that the invention achieves the intended aim and objects .

In particular, it is to be pointed out that the inventive method is adapted to provide textile and yarn polymeric materials with target conducibility properties, without using chemical products and water, with a consequent very low environmental impact.

The invention, as disclosed, is susceptible to several modifications and variations, all of which will come within the scope of the invention.

Moreover, all the constructional details can be replaced by other technically equivalent elements.

Claims

1. A method for processing polymeric yarns and textile materials, including polymeric yarn and textile material fibers, to modify a surface resistivity thereof, characterized in that said method comprises a step of performing a plasma treatment directly on said fibers of said polymeric yarns and textile material.

2. A method according to the preceding claim, characterized in that said uses a fluorine containing gas .

3. A method, according to the preceding claim, characterized in that said method uses a cold plasma.

4. A method, according to one or more of the preceding claims, characterized in that the plasma- phase gas mass has a temperature of a same order as an environment temperature .

5. A method, according to one or more of the preceding claims, characterized in that said plasma is generated under vacuum conditions.

6. A method, according to the preceding claim, characterized in that said plasma is generated in generating chambers containing gases at a pressure less than an atmospheric pressure and preferably from 0.1 to 20 mbars.

.7. A method, according to one or more of the preceding claims, characterized in that said method is carried out at a pressure from 0.1 to 10 mbars.

8. A method, according to one or more of the preceding claims, characterized in that said method is carried out at a pressure from 0.2 mbars to 2 mbars.

9. A method, according to one or more of the preceding claims, characterized in that said method is carried out at a pressure from 0.2 mbars to 0.6 mbars .

10. A method, according to one or more of the preceding claims, characterized in that said method provides to use gases comprising CO₂, air, noble gases, SF6, hydrocarbons and mixtures thereof.

11. A method, according to one or more of the preceding claims, characterized in that the operating residue pressures are larger than 10^~δ mbars. 12. A method, according to one or more of the preceding claims, characterized in that said polymeric yarn and textile materials are surface affected by said plasma for a time less than 15 minutes.

12. A method, according to one or more of the preceding claims, characterized in that said polymeric yarn and textile materials are surface affected by said plasma for a time less than 8 minutes.

14. A method, according to one or more of the preceding claims, characterized in that said polymeric yarn and textile materials are surface affected by said plasma for a time from 1 second 2 minutes.

15. A method, according to one or more of the preceding claims, characterized in that the surface resistivity value of said polymeric yarn and textile materials decreases by at least three orders.

^' 16. A method, according to one or more of the preceding claims, characterized in that said surface resistivity value decreases from 1.5 x 10¹⁵ ohms/square to values for the processed materials of 1.0 x 10¹² ohms/square.

17. A method, according to one or more of the preceding claims, characterized in that said polymeric yarn and textile materials comprise polyester, nylon, acrylic, polypropylene materials, mixed materials thereof, and in general any desired synthetic fabric and yarn materials.

18. A method for processing textile polymeric yarns and materials to modify their surface resistivity, according to one or more of the preceding claims, and substantially as broadly disclosed and illustrated, for the intended aim and objects.