The present invention refers to the method for producing filters suitable to retain VOC (acronym for“volatile organic compounds”) contaminants of the aldehyde type, in particular formaldehyde, and to the filters thus obtained.

Formaldehyde (also called formic aldehyde or methanal) is the simplest of the aldehydes. Its chemical formula is CH2O, and its CAS number is 50-00-0. In a 37% aqueous solution, it is also commercially known as formalin or formol.

Formaldehyde is a powerful bactericide. For this reason, in aqueous solution, it is widely used as a disinfectant for domestic use and is also used as a bactericide in the production of fabrics at the industrial level.

Formic aldehyde solutions are also used to store samples of biological material, as well as in embalming techniques.

Formaldehyde is also used in aqueous solution (formalin) for the production of vaccines, both to produce anatoxins (or toxoids), i.e. toxins that lose their toxicity but maintain their immunogenicity, from bacterial toxins, and to produce vaccines based on killed microorganisms.

Most of the formaldehyde produced, however, is intended for the production of polymers and other chemical compounds. Formaldehyde, following the reaction with phenol, polymerizes producing Bakelite, a thermosetting resin. Similarly, formaldehyde reacts with urea and melamine, the resins of which are used as plastic laminates, adhesives and insulating foams. Formaldehyde is also a reagent used to produce other organic compounds, many of which are polyols, such as pentaerythritol.

Formaldehyde is identified by the code E 240 as a food additive, in particular as a preservative. In effect, formaldehyde is present in wood smoke and is, together with polyphenols, responsible for the preservation of smoked food products.

Formaldehyde, together with urea, is used as an adhesive varnish for chipboard, laminated or medium-density fiberboard (MDF) panels. It is contained, moreover, in sound- absorbing panels of false ceilings and in partition walls of“open space” offices. Over the years, it tends to volatilize into the surrounding environment. It is also used for the production of isoprenol. In textile dyeing, it is used in the application with naphthols to give greater stability to naphtholic solutions and in the subsequent treatment of some tinctures obtained with direct dyes in order to improve their fastness to water and to washing in general.

Given the widespread use of resins derived from formaldehyde in the production of insulating coatings and foams, which tend to release formaldehyde molecules into the environment over time, the latter is one of the most widespread indoor pollutants. At air concentrations above 0.1 ppm, formaldehyde may irritate mucous membranes and the eyes by inhalation. Ingestion or exposure to consistent quantities of formaldehyde is potentially lethal: in particular, it has been found to be carcinogenic in rodents, where it causes a higher than normal incidence of nose and throat cancer. Formaldehyde is able to interfere with the bonds between DNA and proteins.

Since 2004, the International Association for Research on Cancer (IARC) has included formaldehyde in the list of substances considered with certainty to be carcinogenic to the human species. Even if the concentrations of formaldehyde normally present in buildings are generally low and therefore not particularly worrying, the risks to industrial workers using formaldehyde should be carefully assessed.

The World Health Organization has indicated 100 pg/m ³ (equal to 0.1 parts per million - ppm) as the maximum acceptable concentration limit for formaldehyde in the home. However, some derivatives were used in Formitrol tablets for treating sore throats until their use was banned.

Since January 1, 2016, formaldehyde has moved from the classification of “suspected to cause cancer” to“may cause cancer” (Regulation (EU) No. 605/2014).

One of the objects of the present invention is therefore to provide a filter adapted to reduce the inhalation of VOCs, and in particular formaldehyde, which are highly dangerous to human health.

Such object is achieved through a method for the production of a filter adapted to retain aldehyde-type VOC contaminants, in particular formaldehyde, and including a raw wool felt, having the features indicated in claim 1 below. In particular, such claim provides for felt or raw wool intended to comprise said felt to be immersed in an aqueous bath of at least one compound that generates sulfite ions by hydrolysis so as to be impregnated with sulfite ions that increase the ability of the felt to capture aldehydes. Such compound may, for example, be sodium sulfite.

Preferably, the aforesaid method requires the filter to include a felt obtained by degreasing and disinfecting raw wool scraps. Again preferably, the felt or the raw wool, before being immersed in the aqueous bath, is irradiated with microwaves and the aforesaid aqueous bath has a pH between 1,5 and 4, preferably equal to about 2.

Another subject-matter of the present invention is a filter obtainable by means of the method described above.

Yet another subject-matter of the present invention is a mask comprising the aforesaid filter, which is sealed within a casing, preferably at least partially of non-woven fabric and thermoformed, so as to assume an anatomically appropriate shape.

By virtue of the present invention, a totally innovative and ecological filtering system is obtained, able to retain therein the particles of aldehyde-type VOC contaminants, in particular formaldehyde, which ensures health safety, and which is at the same time economical and made with natural materials. Without being bound to the theories that will now be presented, it may be assumed that the action mechanism of the filter of the invention provides that a nitrogen atom of a first amino acid of the wool binds to the carbon atom of the aldehyde group, in particular, to the single carbon atom of formaldehyde, the oxygen atom of which receives an increase in the negative charge thereof, enabling one of the hydrogen atoms to detach from the nitrogen of the first amino acid and to bind to the oxygen atom of the formaldehyde, with the rupture of its double bond with the adjacent carbon atom and the formation of a hydroxyl group -OH.

The chemical compound thus obtained - resulting from the addition of the formaldehyde molecule to the first amino acid with a molecule of water being eliminated and the aldehyde group being transformed into a hydroxyl group - then reacts with a second amino acid of the wool, which is provided with an amide group -NCO. In this way, a condensation reaction takes place, eliminating water and forming another longer chain amino acid.

Regardless of the details of the reaction mechanisms, there is in any case no doubt that the aldehydes, in particular formaldehyde, are decomposed, reacting with the amino acids of the wool, so that the filter is decidedly effective.

A further advantage of the present invention is that it is possible to provide for the use of scrap material from the processing of wool, which in this way becomes a second raw material and is not considered as production scrap and therefore special waste. The result is an unburdening for the producer of raw wool, who does not have to pay for the disposal of the scrap material, which, on the contrary, becomes an important resource for the producer of the filtration system who acquires the raw material at a comparatively low cost.

Further features and advantages of this invention will become more apparent from the detailed description of the following example of implementation which is provided as a non limiting example.

EXAMPLE Raw wool scraps, after having been degreased and dry heat disinfected at a temperature of l80°C, are assembled to form a felt in a way known per se. The felt is then irradiated by microwaves and then immersed in an aqueous bath of sodium sulfite at pH 2, so as to be impregnated with sulfite ions that increase the capture capacity of the aldehydes by the felt. The aforesaid microwave irradiation acts on the molecular bonds of the keratin of the wool so as to make it more absorbent toward the sulfite ions.

This results in a filter that is sealed within a non-woven fabric casing and thermoformed to take on the shape of a mask.

Naturally, without altering the principle of the invention, the details of implementation and embodiments may vary widely with respect to those described purely by way of example, without thereby departing from the scope of the invention as defined in the accompanying claims.