The operation of many electronic devices involves the generation of electromagnetic radiation (for example, within the radio frequency range of the electromagnetic spectrum) . Shielding of both the radiation sources and electronics exposed to the radiation is therefore required.

Electromagnetic shielding (EMS) is the ability of a material to attenuate electromagnetic radiation as it goes through the material. There are various uses for such an effect, such as the prevention of electrostatic cross-talk between integrated circuit components on aboard, blocking of electromagnetic transmissions and others.

The present invention discloses the surprising synergistic effect displayed by mixtures of metal oxides and pyridinium halides on electromagnetic shielding.

In particular, the pyridinium halide salts are alkyl pyridinium halide salts. As used herein, the term "pyridinium" is used to include substituted-pyridinium moieties, as can be seen in Structure I :

Structure I wherein :

Ri is a straight or branched C1-C5 alkyl, such as methyl, ethyl, propyl, butyl or pentyl;

R2 is either hydrogen or a straight or branched C1-C5 alkyl, such as methyl, ethyl, propyl, butyl and pentyl on any of the positions 2, 3 and 4 of the pyridine ring; and X is halide, such as a chloride or a bromide.

Preferred salts are selected from the group consisting of 1- alkyl-2-methyl-pyridinium halide, l-alkyl-3-methyl-pyridinium halide and l-alkyl-4-methyl-pyridinium halide (that is, R ² is methyl in Formula I), wherein the alkyl attached at position 1 of the ring (R ¹ ) is preferably ethyl, n-propyl or n-butyl . l-alkyl-2-methyl-pyridinium halide, l-alkyl-3-methyl- pyridinium halide and l-alkyl-4-methyl-pyridinium halide salts are commercially available, for example, from ICL-IP, or can prepared by methods known in the art, by reacting the corresponding picoline with a suitable haloalkane. The reaction may take place in a solvent, such as acetonitrile {Ploquin et al . [Journal of Heterocyclic Chemistry, 17, p. 997 - 1008 (1980)]}. Solvent-free reactions are also known, as described for example in WO 2014/122641. l-ethyl-3-methyl- pyridinium bromide can also be prepared in water and collected in the form of an aqueous concentrate, as described in Example 6 of US 9,453,285. This concentrate can be dried to remove the water to obtain the dry salt for use in the invention. A synthetic procedure (without detailing the step of water removal) is provided below.

Also suitable for use are 1-alkyl-pyridinium halide salts (R ² is hydrogen in Formula I, R ¹ is for example ethyl, n-propyl and n-butyl), e.g., the bromide salts. 1-alkyl-pyridinium bromide can be synthesized by a reaction of pyridine with bromoalkane. A full preparation procedure where the reaction takes place in the presence of water is set forth below. The liquid reactants can also be heated together in a solvent-free medium; on standing in the cold the reaction product solidifies and can be crystallized from a suitable solvent. As used herein, the term "metal oxide" has its standard meaning in the art, which is the oxide of metal. The metal in the metal oxide is one or more of the elements having an atomic number falling within the ranges 3-4, 11-14, 19-33, 37- 51, and 55-84. Thus, the metal may be an alkaline metal (l ^st group in the Periodic Table), or an alkaline earth metal (2 ^nd group of the Periodic Table) . In addition, the metal may be a rare earth metal, which has an atomic number falling within the range of 57 to 72. The metal oxide may contain a transition metal, which is a metal having an atomic number falling within the ranges 21-30, 39-48, and 71-80.

Metal oxides have been extensively studied for many years, and a large number of the metal oxide species which are stable under ambient conditions have been identified and catalogued. See, e.g., the CRC Handbook of Chemistry and Physics, CRC Press, Inc., Boca Raton Fla., and particularly the Table therein entitled "Thermodynamic Properties of the Oxides" (see, e.g., pages D-46-D49 in the 68th Edition, incorporated herein by reference) , which is one convenient source of a listing of some metal oxides. Some metal oxides which were used in the present invention are: ZnO, MgO, CaO, T1O2, Sb203 and V2O5, but others can be used as known to a person skilled in the art .

The invention therefore provides electromagnetic interference (EMI) material comprising a mixture of metal oxide and pyridinium halide salt. The mixture can be incorporated into a suitable matrix which can be molded or otherwise processed to produce a housing, a cover or simply thin sheets shaped as walls, that can be mounted onto, or in proximity with, a circuitry of an electronic device (printed circuit board (PCB) ) or other electronic components, to act as a shield positioned between the source of radiation and "target component". Thus, a composite comprising a matrix (e.g., a polymer, e.g., based on epoxy or vinyl ester resins) and a filler which is the electromagnetic interference shielding material of the invention, that is, the mixtures disclosed herein, forms another aspect of the invention. The use of the mixture as filler added to a polymer is also provided, e.g., in cured epoxy resins and polyvinyl esters.

The composite may comprise a thermoplastic or thermosetting polymer (or copolymer) filled with the mixture. The mixture of the invention may not easily lend itself to extrusion processing of polymers because the pyridinium salts are somewhat deliquescent. But the composite can be prepared by combining a precursor of the matrix with the mixture and then processing same to form the composite. For example, the mixture can be added to a vinyl ester resin together with an appropriate level of a catalyst such as methyl ethyl ketone (MEK) peroxide catalyst or other catalyst designed to promote the curing reaction of the resin, thereby providing a poly vinyl ester filled with a mixture of metal oxide and pyridinium halide (e.g., from 20 to 70% by weight, preferably from 30 to 50% by weight based on total weight) . Or the mixture could be added a liquid epoxy resin, e.g., di- functional bisphenol A/ epichlorohydrin, such that upon dilution and addition of a curing agent, a cured epoxy resin filled with a mixture of metal oxide and pyridinium halide salt is obtained (e.g., from 20 to 70% by weight, preferably from 30 to 50% by weight based on total weight) .

The mixture can also be compacted with a matrix powder to form a composite or applied in the form of a coating.

As indicated by the experimental results reported below, effective shielding is provided with the aid of an about equally proportioned mixture of metal oxide and a pyridinium halide salt. By "an about equally proportioned mixture" is meant that the weight ratio metal oxide : pyridinium halide salt is from 2:1 to 1:2, preferably from 1.5:1 to 1:1.5, e.g., in the range from 1.25:1 to 1:1.25, especially around 1:1.

As can be seen in the examples below, while each disparate constituent (namely, either the metal oxide or the pyridinium salt) showed low shielding effect, the physical mixture of both materials had a substantially higher shielding effect that is not a linear extrapolation of the two components. This effect repeated itself for several mixtures. In comparison, it was found that pyrrolidinium salts, have not demonstrated a synergistic effect with the metal oxides, while pyridinium halide salts did show this effect.

As can be seen in the examples further below (Example 3), several alkyl pyridinium halides were tested, all showing a synergistic effect, with some showing a very strong synergistic effect (such as l-ethyl-3-methyl-pyridinium bromide (3-MEPy, also named herein BCA14), l-ethyl-4-methyl- pyridinium bromide (4-MEPy, also named herein BCA16) and 1- propyl-pyridinium bromide (1-PPy) ) .

For example, as shown in Example 1, while the ZnO powder showed little attenuation of the electromagnetic radiation, 5 dB/m at the maximum point (Figure 1) and 3-MEPy has demonstrated values of about 35 dB/m at the maximum (Figure 2), the powder mixture consisting of both materials exhibited attenuation of up to 260 dB/m (Figure 3), indicating the synergistic effect generated by the two materials together. For poly vinyl ester as a matrix, the addition of the material allowed up to 20 dB/m (Figure 4), while at the epoxy matrix the values rose up to 85 dB/m (Figure 5) .

Example 2 demonstrates the synergistic effect over a broad range of metal oxides.

The results show the enhanced attenuation of the mixture (Figures 6B, 7B, 8B, 9B and 10B) over that of each metal oxide alone (Figures 6A, 7A, 8A, 9A and 10A, respectively) .

Although some metal oxides, like T1O2 (up to 35 dB/m, Figure 9A) , have high attenuation by themselves, mixing them with the quaternary amine increases that behavior (up to 135 dB/m, Figure 9B) . In some cases the contribution of the amine is in specific frequencies, for example while CaO as a single powder reaches 60 dB/m at3GHz and 25 dB/m at 1.5GHz (Figure 7A) , the addition of the amine raised the value at 1.5GHz to approx. 50 dB/m but did not raise the value at 3GHz (Figure 7B) .

Several ratios between the mixture components were tested (3:1, 1:1, 1:3, see Example 1), and of these combinations, the ratio of 1:1 showed the best results so far.

The results of Example 3, which compare mixtures of ZnO with several alkyl pyridinium halides, show the enhanced attenuation of the mixture over that of each individual material by itself and even a change in attenuation profile due to the mixing (see for example Figures 11A-B and 12A-B) . As shown in Example 3, when introduced into a matrix, both 4- MEPy and 1-Prop.Py mixtures with ZnO showed enhanced activity in the epoxy matrix (Figures 11C and 12C) . Thus, the present invention provides a novel synergistic mixture of a metal oxide and a pyridinium halide salt. Mixtures of zinc oxide and at least one of l-ethyl-3-methyl- pyridinium bromide, l-ethyl-4-methyl-pyridinium bromide and 1- propyl-pyridinium bromide are preferred. Also preferred are mixtures of l-ethyl-3-methyl-pyridinium bromide and at least one of ZnO, MgO, CaO, Ti0 ₂ , Sb ₂ 0 ₃ and V ₂ 0 ₅ .

The term "synergistic mixture" as used herein refers to a physical mixture of these components, showing a synergistic effect .

The term "synergistic effect" as used herein refers to an added effect that is higher than a linear addition of the effects of the separate components. In particular, according to the present invention, the synergistic effect refers to the effect of this physical mixture on the electromagnetic shielding of a substrate.

The term "substrate" is used interchangeably with the term "matrix" and refers to any substrate that can benefit from this shielding, such as plastics, epoxies and resins.

According to another aspect of the invention, there is provided a process of electromagnetic shielding of a substrate using this synergistic mixture.

In particular, the loading level of the synergistic mixture is up to 70%, preferably up 50% by weight based on the total weight of the composite (e.g., of the polymer filled with the mixture) . Examples

Materials and Methods

The effect was measured using a coaxial probe method with a network analyzer (Agilent/Keysight E5061B) calibrated by three points (air, water and short circuit) .

The materials tested consisted of various quaternary pyridinium bromide molecules (Iolitec GMBH) as the amine salts and several metal oxides (ZnO [Northchem] , CaO [Aldrich] , V2O5 [Aldrich] , MgO [ICL], Sb ₂ 0 ₃ [Aldrich] and T1O2 [Aldrich] ) . The measurements took place for each material by itself (in the powder form), for a mixture of the two components (in the powder form) and for Polyvinyl Ester (PVE) and Epoxy matrices loaded with the mixture (in a solid form) .

Preparation of samples for EMS measurements

Additive prep:

Metal oxide powder and/or pyridinium bromide powders were taken and ground with a mortar and pestle. Ground powders were placed in a vacuum desiccator over night to remove all moisture. In case of mixtures, the dry powders were taken at a given weight ratio, typically 1:1.

Dried powders/mixtures were measured as-is using a coaxial electromagnetic probe and a network analyzer. Then powders/mixtures were introduced into the matrix material.

Epoxy matrix:

An Epoxy diluent (Heloxy BD, 15 parts per hundred resin (phr) , Hexion)was added to Diglycidyl Ether of Bisphenol A (DGEBA, Epon 828, 100 phr, Hexion) and mixed for 1 hour at 50°C. The amount of related additive was weighed and they were mixed together . An Amine hardener DETA (Epicure 3223, 13 phr, Hexion) was added, and was kept for 24 hours at room temperature (RT) for curing. Post curing was conducted for 3 hours at 60°C.

Polyvinyl-ester matrix:

A MEK peroxide catalyst (Akperox A-60, 2 phr, Akpa) was added to vinyl ester resin (Crystic 679 PA, 100 phr, Scott-Bader) . The amount of related additive was weighed and they were mixed together. It was then kept for 24 hours at room temperature (RT) for curing. Post Curing was conducted for 3 hours at 60°C.

Example 1

3-MEPy:ZnO synergy with different matrices

1:1 weight ratio mixture of l-ethyl-3-methyl-pyridinium bromide (3-MEPy) and ZnO .

Each material was measured by itself as a powder, as a mixture of both and added to PVE and Epoxy matrices.

ZnO and 3-MEPy powders were prepared and mixed according to the "additive prep" process detailed above. The results of the EMS measurements are provided in Figures 1, 2 and 3.

The mixtures were then introduced into solid matrices, PVE (Figure 4) or epoxy (Figure 5) at 50% by weight loading level. Homogenization of the powder was not performed, so due to gravity most of the additive was found at the lower side of the sample. Attenuation was measured at both sides of the samples (clear top side and aggregated additive on the lower side) and on a reference sample.

The same metal oxide and amine (3-MEPy) were tested in other ratios too (1:3 3:1) , showing a lower synergistic effect . Example 2

3-ΜΕΡγ with various metal oxides

1:1 weight ratio mixture of l-ethyl-3-methyl-pyridinium bromide (3-MEPy) and various metal oxides (CaO, V2O5, MgO,

The procedure of Example 1 was repeated keeping the pyridinium halide salt as 3-MEPy, and changing the metal oxide, whereas each material was measured by itself as a powder, as a mixture of both. No matrix was tested for the entire set.

The oxides tested were ZnO (see Example 1), Sb203 (Figures 6A- B) , CaO (Figures 7A-B) , V2O5 (Figures 8A-B) , T1O2 (Figures 9A- B) and MgO (Figures 10A-B) .

Example 3

Different amines salts with ZnO

The procedure of Example 1 was repeated keeping ZnO as the metal oxide, and changing the amine salt, whereas each material was measured by itself as a powder, as a mixture of both, and in an epoxy and PVE matrices .

First, two pyrrolidium halides were tested (butyl methyl pyrrolidium and methyl-propyl pyrrolidium) but both failed to show any synergistic effect (results not provided) .

Then, a wide range of alkyl pyridinium halides were tested in a 1:1 weight ratio mixture with ZnO.

The amines tested were l-ethyl-4-methyl-pyridinium bromide (4- MEPy) (Figures 11A-B) , 1-propyl-pyridinium bromide (1-Prop.Py) (Figures 12A-B) , as well as l-ethyl-3-methyl-pyridinium bromide (3-MEPy, in previous example), l-ethyl-2-methyl- pyridinium bromide (2-MEPy), l-n-butyl-2-methyl-pyridinium bromide (2-MBPy) , l-n-butyl-3-methyl-pyridinium bromide (3- MBPy) , l-n-butyl-4-methyl-pyridinium bromide (4-MBPy) and 1-n- butyl-pyridinium bromide (But.Py) (results not provided) .

All of the tested pyridinium halides showed a synergistic effect, with the highest obtained for 3-MEPy (shown in Example 1), l-ethyl-4-methyl-pyridinium bromide (4-MEPy) and 1-n- propyl-pyridinium bromide (1-Prop.Py)

Preparation 1

Preparation of 3-MEPy or 4-MEPy in aqueous medium

3-Picoline

A pressure reactor was equipped with a mechanical stirrer with a magnetic relay and a thermocouple well. The reactor was purged with nitrogen and charged with 3-picoline (101.3 g) and DIW (25 mL) . The reactor was sealed and the mixture was heated to 96 °C. Ethyl bromide (97.9 g) was slowly added during 2 hours, at 96-104 °C. The mixture was heated at 100 °C for additional 3.5 hours, after which time the pressure was released. The crude solution was diluted with DIW and excess 3-picoline was distilled-off as aqueous azeotrope, under reduced pressure. Finally, the residue was diluted with DIW. The weight of the aqueous concentrate was 260 g and the concentration of 3-MEPy was 66.6 weight % ( argentometric titration); yield, 95.6 %. 4-MEpy may be obtained in a similar fashion, replacing 3-picoline with 4-picoline as the starting material . Preparation 2

Preparation of N-ethyl pyridinium bromide eous medium

A stirred pressure reactor was equipped with a thermocouple well and a dosing pump. The reactor was charged with pyridine (450 g) and de-ionized water (DIW) (330 mL) , sealed and heated to 95 °C. Ethyl bromide (600 g) was continuously added during 1 hour; afterwards heating was continued for additional 1 hour. The reactor was cooled to ambient temperature, the pressure was released and distillation apparatus installed. The reaction mass was diluted with DIW (200 mL) and distilled under vacuum until 200 mL of distillate were collected. Final product: 1340 g; 72 %w (argentometric titration) ; yield, 93 %.