This patent application relates to desiccant devices subjected to antistatic treatments and to a process for obtaining said devices which have dissipative or conductive characteristics, depending on the user's requirements. The desiccant devices to which the invention relates are useful to protect against moisture materials that are sensitive to electrostatic discharges.

Prior art

The desiccant devices used to absorb moisture usually consist of a desiccant material contained in a steam-permeable bag made, for example, of Tyvek ^® , non- woven fabrics, some articles obtained by bonding several layers of fabric, microperforated plastic film, microporous film, fine-mesh gauze, natural fabrics, Kraft paper or polythene paper.

The desiccant materials most commonly used are desiccant clay, silica gel, molecular sieves, alumina, and hygroscopic salts. The polymer materials constituting the bagging material of desiccant bags usually have very high surface electrical resistance, and therefore possess insulating properties.

In the presence of low relative humidity and particular conditions of use, the bagging material of desiccant devices may be electrostatically charged on the surface, with consequent risks of electrostatic discharges which, in certain applications, can be harmful and dangerous.

Known fabrics of various types with antistatic properties are used in particular for the production of clothing, filters and other devices; non-woven fabrics and spunbonded polyethylene (Tyvek ^® ), obtained by mixing antistatic chemicals directly at the fabric manufacturing stage, are mainly used for this purpose. A technology of this kind, which disperses the antistatic in the

material, requires a considerable quantity of antistatic (US5071699, US2001036909), which goes through the entire manufacturing cycle, with the risk that it will present chemical degradation and incompatibility with the constituents of the material. Another technology developed to manufacture antistatic fabrics involves the use of polymer fibres containing graphite or carbon black, which are coextruded with the polymer at the fibre manufacturing stage (US5202185). This technology may be suitable to produce antistatic Big Bags, but does not take account of the fact that the non-woven fabrics used to make desiccant bags are composite materials, and that the conductive properties of each fibre may be interrupted by the binder or other constituents of the material.

The technical solutions developed to date solve problems of electrostatic charges in environments with a medium humidity level, but no technology has yet been developed to solve the problems associated with the manufacture of desiccant bags with antistatic properties.

The antistatic fabrics and Tyvek ^® which have been developed to date do not have to perform their action in an anhydrous environment, and are therefore unsuitable for the manufacture of antistatic desiccant bags, which are required to dissipate electrostatic charges in the substantial absence of humidity. The manufacture of desiccant bags also requires compliance with certain very strict technical standards:

1) The bags must contain sufficient desiccant material to guarantee a given steam-absorption performance under standard conditions (40% relative humidity and 23 ⁰ C).

2) The bags must be manufactured with desiccant material which is so dry as to eliminate relative humidity under 8% at 23°C in hermetically sealed packaging.

The desiccant bags may be needed to remove moisture from packaging or environments which are protected against electrostatic discharges. Desiccant bags therefore need to be made with dissipative characteristics suitable for these applications. The technical standards that define the characteristics of materials suitable for use in environments protected against electrostatic discharges require compliance with the following requirements:

1) The surface resistance of the material must be between 10 ⁴ and 10 ^{1 1} O , while the resistivity must be between 10 ⁵ and 10 ¹² Ω in order to give dissipative characteristics as defined in the technical specifications used in the industry.

2) The electrostatic charges must be dissipated in less than two seconds when the material is discharged (more commonly known as being "earthed"). This measurement is performed on material conditioned for 72 hours at 12% relative humidity and 23 ⁰ C.

The sum of the quality requirements established by the technical standards used in the two industries consequently requires the desiccant device to be dissipative even in an environment with relative humidity at least under 8% at 23 ⁰ C, and consequently in more restrictive operating and test conditions than those laid down for approval of packaging designed to protect against electrostatic discharges.

This is demonstrated by the fact that electronic components must be stored in hermetically sealed packaging in direct contact with desiccant bags able to reduce the relative humidity to under 5%, as established by reference standard JEDEC J-STD-033A.

Antistatic desiccant bags must therefore be dissipative even under a relative humidity level of 5% in order to protect the materials against moisture without damaging components sensitive to electrostatic discharges.

The specific technical difficulties present in this industry, and the impossibility of transferring to it technologies used successfully in cases where the humidity conditions are less drastic, are therefore obvious.

Description of the invention It has now been discovered that is possible to manufacture desiccant devices with dissipative properties that comply with the strictest standards and can be used in environments which must be protected against electrostatic discharges.

The desiccant devices according to the invention comprise a desiccant agent in a container permeable to atmospheric moisture which is surface- treated with at least one antistatic agent selected from among graphite, carbon black and quaternary ammonium salts with the formula N ⁺ R1R2R3R4.X ^" , wherein R1-R4, which are equal or different, represent alkyl groups C1-C20, possibly substituted by hydroxy; alkenyl; benzyl; phenyl; alkylamidoalkyl or alkenylamidoalkyl groups, and X is a halide, nitrate or sulphate anion, possibly mixed together, so as to obtain a dissipative paint.

The invention also relates to a process for the preparation of said devices which comprises: surface treatment of the bagging material, which may or may not contain the desiccant agent, with a quaternary salt, graphite, carbon black or mixtures thereof; possible insertion of the desiccant agent into a container consisting of treated material obtained at the preceding stage, and closing of the container by heat-sealing or an equivalent technique. Detailed description of the invention

The preferred desiccant agent is silica gel or desiccant clay. The material used to make the permeable container is preferably selected from among spunbonded polyethylene (Tyvek ^® ), multicomponent

fabrics containing polyethylene and polyethylene-terephthalate, polypropylene, Nylon 6, cellulose, rayon, viscose and ABS.

At least one quaternary ammonium salt as defined above, wherein one of groups R1 -R4 is preferably selected from an alkyl or alkylamidoalkyl group and which has 6 to 20 carbon atoms, such as stearamidopropyldimethyl-2- hydroxyethylammonium nitrate, can be used as antistatic agent.

Alternatively, quaternary ammonium salts wherein at least one of groups R1-R4 is an alkenyl or alkenylamidoalkyl group having 6 to 18 carbon atoms and containing at least two conjugated double bonds, such as the salt marketed under the name of ethyldimethyl soya alkyl ethyl sulphate, are particularly advantageous and effective as antistatics.

Quaternary salts of the first type (saturated) are preferred when the bagging material is selected from among cellulose, rayon, viscose, ABS and polyacrylate fibres, whereas quaternary salts with conjugated unsaturations are preferred when the bagging material is selected from among non- woven fabrics manufactured with spunbonded technology (Tyvek ^® ), multicomponent fabrics containing polyethylene and polyethylene-terephthalate in which the short fibre is held together by fusion (without added binder), or multicomponent fabrics, polyethylene terephthalate, polypropylene or Nylon 6 made with a continuous-fibre filament and a multilayer structure.

The two types of quaternary salts can be mixed together.

If graphite or carbon black are used for the surface treatment, they can conveniently take the form of inks constituted by acrylic resin suspensions.

The desiccant devices according to the invention have the dual function of protecting the contents of the packaging against moisture and not causing electrostatic discharges inside the packaging. In view of the reduction in the surface resistance of the bagging material, desiccant bags with dissipative characteristics can be obtained if the surface resistance is between 10 ⁴ and

10 ^π Ω and the conductive resistance is between 10 ⁴ and 10 ² Ω , or even shielding, under 10 ² Ω .

Surface treatment of the bagging material of the desiccant bag with a dissipative paint based on graphite and carbon black represents an improvement on the use of conductive fibres containing the same materials. In fact, surface treatment is not influenced by the presence of adhesives in the fabric which bind the short fibres. A treatment of this kind can also be performed on any non- woven fabric (NWF), regardless of the technology used to manufacture it. Moreover, as a surface treatment is involved, this technology allows the use of a smaller amount of graphite and more efficient control of the dissipation of the electrostatic charges. The high conductivity of graphite must be reduced by formulating it in a paint with dissipative characteristics.

One or more substances which are coloured in the UV or visible light spectrum can be added to the antistatic quaternary agents, such as fluorescein and its derivatives such as aminofluorescein, alkyl-substituted perylene dye, alkyl-substituted naphthalimide dye, or the products known by the tradenames fluorescent perylene green, fluorescent perylene orange, fluorescent perylene yellow, fluorescent perylene red and fluorescent naphthalimide violet. In this way, both the customer and the manufacturer can perform a colorimeter test to check that the treatment on the bag has been performed, independently of direct measurement of the characteristics of the sample.

It is possible to check whether the mixture containing the colorant and the antistatic is unevenly distributed in the fabric, which has adverse consequences in both aesthetic and functional terms. Dyeing the bagging material with a mixture of antistatic and a colorant detectable under UV light produces bags of the same colour as the original fabric which do not present patches of colour at the time of use, and a UV lamp can be used to check

whether the treatment has been given evenly.

A quaternary antistatic compound which is coloured in the UV or visible spectrum can also be used.

As mentioned, it is possible to obtain desiccant bags with dissipative characteristics by treating the bagging material by immersion or equivalent techniques in solutions or suspensions of antistatic agents in a wide range of concentrations, which are not critical in themselves, depending on the speed of the manufacturing process and the composition of the bagging material.

However, it has been found that in order to guarantee satisfactory characteristics, 1 square decimetre of the bagging material needs to be treated with at least 0.1 g of a 5% solution of quaternary antistatic. The quantity of substance deposited per surface unit of the bagging material being equal, an increased concentration of quaternary antistatic, used alone or in a mixture, improves the dissipative performance of the bag. Preferably, 1 square decimetre of bagging material is treated with at least 0.1 g of solution at a concentration of 10%-50% in weight of quaternary antistatic.

If a graphite-based paint or ink is used as dissipative coating, the graphite concentration can range between 0.5% in weight and 10% in weight in the presence of quantities of acrylic resin of between 1% and 30%. The best results are obtained with concentrations of around 3% graphite and 1 % resin, the liquid being distributed evenly on the fabric and the excess deposited on it being squeezed out. When solutions containing a low concentration of graphite are used, it can be advantageous to repeat the treatments. The percentage of graphite introduced into the preparation used to impregnate the bagging material can vary, depending on the quantity of liquid deposited on the fabric, the thoroughness with which the excess is squeezed out and the wettability of the fabric, but is always under 2% of the dry weight

of the bagging material, compared with the 20% concentrations reported in US5202185.

This invention offers the following advantages compared with the prior art described above: 1) It enables a dissipative desiccant bag to be obtained in the presence of low humidity values (<5% relative humidity).

2) By using mixtures of quaternary ammonium salts, it enables any non-woven fabric containing a wide variety of types of fibre or manufacturing technology to be treated and rendered antistatic and dissipative under the above-mentioned conditions.

3) As the products used are more efficient than other antistatic products, the quantity of active constituents can be minimised, reducing environmental impact, especially in view of the differences with technologies requiring the antistatic to be mixed directly into the fabric.

4) It allows chromatic identification of the surface treatment in order to simplify the check that the treatment has been performed.

The invention is illustrated in greater detail in the following examples. EXAMPLE 1 A desiccant bag made of Tyvek ^® manufactured by the applicant, containing 30 g of type "A" microporous silica gel manufactured by INEOS Silicas Ltd or desiccant clay, was surface-treated with 0.3 g of a hydroalcoholic solution containing 5% in weight of a quaternary ammonium salt (ethyl dimethyl soya alkyl ethyl sulphate). The bag thus obtained, containing dry silica gel, was enclosed for 72 hours in steamproof heat-sealed packaging, to prevent contact with external humidity and allow the silica gel to reduce the internal humidity to under 5%. It was found that under these conditions, treatment with the antistatic made the bag dissipative even in the

presence of low humidity values.

Two bags, one made of Tyvek and the other of non-woven cellulose- fibre fabric, containing the same quantity and type of dry desiccant, were treated for the purpose of comparison. Both bags were treated with 0.3 g per square decimetre of a 15% solution in weight of stearamidopropyldimethyl-2-hydroxyethylammonium nitrate. The two bags were enclosed for 72 hours in steamproof heat-sealed packaging, to prevent contact with external humidity and allow the silica gel to reduce the internal humidity to under 5%. The measurements demonstrate that the NWF bag is more dissipative than the Tyvek bag.

Desiccant bags made of non-woven fabric, in which the short fibre is a mixture of viscose and PET fibres and the binder is a polyacrylic, were surface-treated to demonstrate the advantages of using antistatic mixtures. The first bag was treated with 0.3 g per square decimetre of a 25% solution of ethyldimethyl soya alkyl ethyl sulphate antistatic, the second bag with an equal quantity of stearamidopropyldimethyl-2- hydroxyethylammonium nitrate, and the third bag with 0.3 g per square decimetre of a 25% mixture in weight of equal parts of said two antistatics. The properties conferred on the three bags were compared, and it was found that, concentration and quantity of the solutions used for the surface treatment being equal, the bag treated with the mixture of the two antistatics dissipates electrostatic charges more efficiently than both the first bag treated with the ammonium salt with unsaturated chains and the bag treated with quaternary ammonium salt with saturated chains. EXAMPLE 2

Dissipative antistatic bags were prepared by impregnating or printing the bagging material with a paint or ink containing graphite or carbon black.

Micronised graphite with a particle size of between 0.05 and 5 microns was used. However, good results are also obtained with different particle-size distributions.

The bagging material of a desiccant bag made of non-woven fabric manufactured with cellulose fibre was evenly impregnated with a water-based preparation containing 3% in weight of colloidal graphite, acrylic resin such as Noveon Carboset XPD2091 at the concentration of 0.5% in weight, and

0.5% of colloid stabilisers such as carboxymethylcellulose. After squeezing the bagging material to remove the excess liquid, a bag containing dry gel silica was obtained.

The bag was placed in closed heat-sealed packaging for 72 hours, so that the humidity content of the packaging fell below 5% as under real conditions.

The measurements demonstrate that the surface characteristics of the bagging material are dissipative.

A second bag was prepared by impregnating the bagging material with an aqueous solution containing 15% graphite and identical concentrations of acrylic resin and stabilisers.

The bag manufactured with this material was placed in closed, heat- sealed packaging for 72 hours, so that the humidity content in the packaging fell below 5%.

The measurements demonstrate that the surface characteristics of the bagging material are conductive; the formulation in which the graphite concentration was increased therefore does not produce a bag surface with dissipative characteristics.

A third bag was prepared by impregnating the bagging material with an aqueous solution containing 3% graphite, Noveon Carboset acrylic resin at the concentration of 20% in weight, and 0.5% of colloid stabilisers.

The bag manufactured with this material was placed in closed, heat- sealed packaging for 72 hours, so that the humidity content in the packaging fell below 5%.

The measurements demonstrate that the surface characteristics of the bagging material are insulating. An increased concentration of the resin makes the treatment insulating, and the bag cannot be used in environments that require protection against electrostatic discharges.

The three experiments demonstrate that the characteristics of the bagging material can be managed by means of surface treatment with an ink containing graphite.

The increase in the graphite or carbon black concentration is directly proportional to the increase in surface conductivity, while the increase in concentrations of the constituents of the ink and the resins is inversely proportional to the surface conductivity. By regulating these parameters, the black ink can be given the dissipative characteristics required to manufacture desiccant bags for use in environments that require protection against electrostatic discharges.

The experiments described can be repeated with desiccant bags containing other types of desiccant, such as desiccant clay. The surface resistance was measured by applying a voltage of 100 V.

The electrostatic charge decay time was measured in an environment with under 12% R/H and 23 ⁰ C; the sample was charged with 1 100-1200 V, and the time taken to reach 100 V after contact was established between the bag and earth was measured. The same measurements can be repeated at 12% R/H and 23°C, with similar results.

As stated in the technical standards, bags which present charge decay times of under 2 seconds, and surface resistance of between 10 ⁴ and l θ" or

resistivity between 10 ⁵ and 10 ¹² , can be considered dissipative.

The measuring instruments used were the LABEOM 100V Surface Resistivity Meter, the PROSTAT CORPORATION'S PRS-801 Resistance System with PROBE PRF 914144, or TREK model 520-2-CE.