WO2025233898A1

WO2025233898A1 - Use of vanillin for dyeing a microfibrous substrate

Info

Publication number: WO2025233898A1
Application number: PCT/IB2025/054880
Authority: WO
Inventors: Matteo PROIETTI; Roberta BERNACCHIA; Remo SERAFINI; Gianni Romani
Original assignee: Alcantara SpA
Current assignee: Alcantara SpA
Priority date: 2024-05-09
Filing date: 2025-05-09
Publication date: 2025-11-13
Anticipated expiration: 2026-11-09

Abstract

The invention relates to a process for dyeing a microfibrous substrate obtained from a felt of a bi-component fibre of the "islands-in-the-sea" type, subsequent removal of the sea component of the fibre and impregnation with polyurethane, wherein said process comprises a step of bringing the microfibrous substrate into contact with a solution or suspension of at least one dye and vanillin at a temperature below 100ºC. The invention also relates to the use of vanillin in a mixture with at least one dye for dyeing a microfibrous substrate impregnated with polyurethane, wherein the vanillin is used as a carrier of the dye.

Description

DESCRIPTION

Use of vanillin for dyeing a microfibrous substrate”

FIELD OF THE INVENTION

The present invention relates to a process for dyeing a microfibrous substrate, characterised in that the dyeing step is carried out using at least one dye in a mixture with vanillin.

STATE OF THE ART

Nonwoven microfibre fabrics with a suede-like appearance, made with microfibres and impregnated with polyurethane, are known in the art and normally available on the market. In some cases, a woven product is interposed in the nonwoven structure or, conversely, one or more layers of woven fabric are bonded with a nonwoven fabric substrate placed within a multilayer structure.

Examples of nonwoven fabrics of this kind may be found in EP3851573, JPH07229071 and JP3166054. According to the technology disclosed in the aforesaid documents, a tri-layer base is prepared, comprising an inner layer which can be a fabric or a substrate made of single-fibre polyethylene terephthalate (PET) and two outer layers also made of a single-fibre PET, which can be woven or nonwoven fabrics.

A process for producing a suede-like microfibrous substrate based on sea-island fibres is also known in the sector (from EP1323859, EP2307608, EP2780501 , EP3662104, CN111764176 and JP2024052600).

According to these patents, a bi-component fibre of the “islands-in-the-sea” type is prepared by introducing two polymers into a spinneret so that the sea component completely surrounds the other component consisting of several filaments that form the various islands. In the bi-component fibre, the sea component generally consists of polystyrene (PS) or another polymer which has certain characteristics of spinnability such as to enclose the microfibres of the island component and is also easily soluble in standard organic solvents, in hot water, or can be easily removed with alkaline aqueous solutions. The island component preferably consists of polyethylene terephthalate (PET) or polyamide. A felt is prepared from the fibre by needle-punching and is impregnated with a solution of polyvinyl alcohol (PVA), the sea component is dissolved in trichloroethylene, in water or with an alkaline aqueous solution, the felt is impregnated with a polyurethane (Pll) in a solution of dimethylformamide (DMF) or dispersed in aqueous solution and, finally, the PVA is eliminated.

The product thus obtained is cut into two along the section, sueded, dyed with specific dyeing machinery and finished.

In accordance with patent EP1760189, the microfibrous substrate impregnated with polyurethane obtained as described above can be bonded, by means of an adhesive, to another nonwoven fabric of the same type or to a woven fabric, thereby obtaining a two-layered product.

A dyeing process consists in the interaction between a dye and a fibre, and in particular in the movement of the dye into the inner part of the fibre. Normally, a dyeing process involves materials with two phases, i.e. a dye solution and a solid substrate, between which physicochemical interactions exist.

The dyeing cycle typically consists of three steps: a dyeing step, a cleaning step (also called stripping) and a finishing step.

The dyeing step entails bringing the microfibrous substrate into contact with a dyeing mixture comprising one or more dyes and some dyeing auxiliaries necessary for the dispersion of the dye in the dyeing medium (usually waterbased) and which facilitate its passage over the fibre under pH conditions such as to enable penetration into the fibre itself.

In general, a dyeing process entails adsorption (transfer of dyes onto the surface of the fibre) and diffusion (dyes diffused in the fibre). In addition to direct adsorption, dyeing can also involve the precipitation of the dyes into the fibre (vat dyes) or a chemical reaction with the fibre (reactive dyes). Such chemical reactions can take place between the dye molecules and the substrate, for example in the case of vat, reactive and chrome dyes.

Dyes can be dispersible or soluble in water.

Typically, dyeing auxiliaries comprise an acid-base pair which performs a buffer function. The acid-base pair can be a carboxylic acid/carboxylic salt mixture, for example an acetic acid/alkali metal acetate mixture, or a citric acid/ alkali metal citrate mixture.

Dyeing auxiliaries also typically comprise at least one ionic or non-ionic surfactant, for example modified polyalcohols or polyglycols, fatty amine esters, naphthalenesulfonic acids, fatty acids and derivatives thereof. The aqueous solution containing dyeing auxiliaries has a pH value between 4.0 and 5.5, generally between 4.5 and 5.0.

Furthermore, the mixture may contain UV stabilisers such as benzotriazoles, triazines and benzophenones, conventionally added in active ingredient percentages between 0.1 and 5% by weight relative to the microfibrous nonwoven.

Typically, when polyethylene terephthalate and/or polytrimethylene terephthalate microfibres are used, the dye mixture comprises disperse dyes of the azoic, anthraquinone type, aminoketone or quinophthalone dyes, vat dyes.

When polyethylene terephthalate microfibres dyeable with cationic dyes are used, the dye mixture consists of cationic dyes, whereas if the microfibre consists of polyethylene terephthalate and/or polytrimethylene terephthalate and polyethylene terephthalate dyeable with cationic dyes, the mixture comprises cationic dyes and disperse dyes.

The maximum temperature during the dyeing step is chosen in such a way that the polymers making up the microfibre are above the glass transition temperature so as to facilitate the entry of the dyes and stabilisers into the microfibre.

As regards the step of dyeing the raw microfibrous substrate, this is preferably carried out by immersion in an aqueous solution containing dyes and dyeing auxiliaries at a temperature of 100°C to 140°C, preferably 110°C to 130°C. The immersion time can vary within broad limits, and is generally from 1 to 6 hours, more preferably 3 to 5 hours with a dwell time at the maximum temperature of 30 to 90 minutes, more preferably 45 to 75 min.

The dyeing step is generally carried out in a machine such as to impose stresses of a thermomechanical type on the raw material, in particular by agitating the bath, with contact of the microfibrous substrate with the Venturi tube of the jet dyeing machine used to carry out the treatment.

During the dyeing step, an adsorption of the dye occurs on the fibre surface, where the dye molecules pass from the liquid phase (dye solution) to the solid phase (fibre). Fibre surface means not only visible outer surface of the fibre, defined by the diameter and length of the fibre, but also the surface of the pores inside the fibre.

After the dyeing step, the microfibrous substrate is subjected to a stripping treatment in a basic environment with reducing agents for the purpose of eliminating the excess dye not adsorbed into the microfibre.

This treatment can be preceded by washing with water to remove any part of the dye not fixed to the fibre and still present and thus facilitate the action of the actual cleaning/stripping.

Stripping is carried out at temperatures no higher than 80 °C, for about 30 minutes. The agents most commonly used for the stripping treatment are sodium hydrosulphite or sulphinic acid derivatives.

The basic solution preferably has a pH value greater than 8, more preferably greater than 10. Such pH values can be obtained by adding an inorganic base, in particular an alkali metal hydroxide, for example an aqueous solution of NaOH and/or KOH.

The basic solution preferably includes at least one surfactant, preferably from the class of the organic phosphorus compounds or mixtures of neutralised organic acids.

Preferably, the stripping step is conducted at a temperature of 50°C to 100°C, more preferably 70°C to 90°C.

Preferably, the stripping step has a total duration of 90 to 360 minutes, more preferably 100 to 240 minutes, and may optionally be divided into a plurality of sub-steps (i.e. into several cleaning treatments) so as to obtain a better removal of the unfixed dye.

Between one sub-step and the other, the basic solution is drained off and replaced with a newly prepared solution.

After the stripping step, the material is dried according to known techniques. Subsequently, finishing treatments necessary to impart certain properties to the material may be provided for. During the finishing step, the fixation of the dye molecules onto the fibre takes place via the interaction between the dye molecules and the fibre. The stability of fixation depends on the type of interaction formed.

Further enhancements of the material can be applied with hot treatments, up to a temperature of 250 °C, e.g. thermoforming, embossing, hot rolling, printing and hot laminating with different substrates.

A dyeing process is technically complicated, as in order to obtain the desired dyeing quality it is necessary to control with precision many factors that may influence the dyeing process. In particular, the fibre must have the correct exposure to the appropriate dye.

Generally, the dyeing of polyester is carried out using dyeing auxiliaries in addition to the dyes. Dyeing auxiliaries help the dye molecules to penetrate deep into the fibre. There exist different classes of dyeing auxiliaries: dispersing agents, equalisers, levelling agents and carriers. Dispersing agents, equalisers, and levelling agents act directly on the dye molecules, favouring a greater dispersion within the dyebath and consequently a more homogeneous distribution within the fibre. Carriers act directly on the fibre through swelling phenomena, which increase the amorphous zones where the dye molecules can be adsorbed.

Dye dispersing agents, equalisers, and levelling agents can act by forming a micelle with the dye molecules. Carriers, by contrast, help to swell the polyester fibre and create a free volume within the polymer of the fibre. In general, they help the transfer of dye molecules from the dye solution to the fibre. The transfer of the dye can be optimised by choosing a suitable dyeing auxiliary or providing high temperatures and pressures.

The factors which influence the dyeing process when different classes of dyeing auxiliaries are involved are: the pH of the dye solution; the particle size of the dyes, the chemical nature and size of the dye molecules, the chemical nature of the auxiliaries, and temperature.

Though dye carriers help in the dyeing process, some undesirable characteristics of these carriers reduce their practical use.

Most common dye carriers, such as O-phenylphenol and O-dichlorobenzene, are highly toxic for humans. Some dyeing auxiliaries, such as polyglycol ether and naphthalenesulfonic acid, polymer with formaldehyde sodium salt, are highly polluting and require a very large number of industrial wastewater purification treatments, with major energy expenditure and thus costs. Due to their nature, this type of dyeing auxiliaries are surfactants that form micelles with the dye molecules, favouring their dispersion in the dyeing medium. In general, traditional dye carriers are aromatic organic compounds with low water solubility and having high levels of toxicity both for humans and the environment.

Notwithstanding several drawbacks, the use of dye carriers remains a valuable tool for the dyeing process. Therefore, it is necessary to find a dye carrier which can overcome the strong impact on humans and the environment and is able to act on its own without the presence of other dyeing auxiliaries, while optimally influencing the dyeing process.

The use of vanillin as a dye carrier in the dyeing of polyester-based fabrics (CN103194914) or bi-component PET/PTT (polyethylene terephthalate/polytrimethylene terephthalate) fibres is known in the art.

However, the use of vanillin as a carrier for dyeing composite materials such as nonwoven microfibre fabrics impregnated with polyurethane (Pll) is not known. A composite material of this type is difficult to dye with disperse dyes, above all if the fibre of the microfibrous substrate is polyester based. It is in fact well known that the dyes used to dye polyester (disperse) do not have the ability to dye Pll, but rather, one constantly finds dye residues in the polyurethane that cannot be removed. This can result in a non-optimal final cleaning of the product.

Thus, there is a strongly felt need for adjuvants in the dyeing of microfibrous composite materials impregnated with polyurethane which enable the drawbacks of the known dyeing processes to be overcome.

SUMMARY OF THE INVENTION

The invention relates to a process for dyeing a microfibrous substrate impregnated with polyurethane, wherein said process comprises a step of bringing a microfibrous substrate impregnated with polyurethane into contact with an aqueous solution or dispersion containing at least one dye and vanillin at a temperature below 100°C.

The microfibrous substrate is left in contact with the aqueous solution or dispersion of at least one dye and vanillin for between 30 minutes and 4 hours.

During contact with the aqueous solution or dispersion of dye and vanillin, the solution is subjected to mechanical action, for example agitation of the solution/dispersion containing the microfibrous substrate subjected to dyeing.

At the end of the dyeing, the dyed microfibrous substrate is subjected to at least one cleaning/stripping step with a reducing aqueous solution, for example an aqueous solution of sodium hydrosulphite and/or containing sulphinic acid derivatives. In a further aspect, the invention relates to the use of vanillin in a mixture with at least one dye for dyeing a composite material comprising a microfibrous substrate impregnated with polyurethane.

In particular, the vanillin acts as a carrier for the at least one dye.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows an SEM (Scanning Electron Microscope) section of the Type A substrate.

Fig. 2 shows an SEM (Scanning Electron Microscope) section of the Type B substrate.

DETAILED DESCRIPTION OF THE INVENTION

The microfibrous substrate used in the dyeing process of the invention can be prepared with the methods known in the art, in particular from patents:

EP1323859, EP2307608, EP2780501 , EP3662104 EP3851573, JPH07229071 and JP3166054.

According to patents EP1323859, EP2307608, EP2780501 , EP3662104, JP2024052600 and CN111764176, the microfibrous substrate is obtained by means of a process which comprises a step of preparing a bi-component fibre of “islands-in-the-sea” type by spinning the island component in the sea component, preparing a felt with the bi-component fibre of the “islands-in-the-sea” type, impregnating the felt with PVA, removing the sea component, impregnating with polyurethane and fixing the latter. The process can further comprise the steps of subjecting the substrate to cutting, sueding, on one or both sides.

The substrate thus obtained can be bonded to at least one other layer of the same type, preferably to at least two other layers of the same type, by application of an adhesive.

Alternatively, the substrate thus obtained can be bonded to at least one layer of woven fabric, preferably to at least two layers of woven fabric. In the latter case, the substrate of nonwoven microfibre fabric is interposed between two layers of woven fabric. This type of multilayer substrate is described for example in EP1760189.

In a further embodiment, the substrate is a bi-layer or tri-layer comprising a nonwoven microfibre fabric obtained from mono-component fibres, preferably from polyethylene terephthalate (PET) fibres, bonded to at least one layer, preferably two layers, of a fabric made of mono-component fibres, preferably PET fibres. Once coupled together, the layers are joined, for example, by means of a jet of steam and then subjected to impregnation with polyurethane. This type of substrate is described for example in EP3851573, JPH07229071 and JP3166054.

In the embodiment in which the substrate is obtained from a bi-component seaisland fibre, the process of preparing the microfibrous substrate takes place by spinning a bi-component fibre of the “islands-in-the-sea” type. The latter can be obtained according to techniques known in the art, which provide for feeding two pure polymers or two mixtures of polymers with additives to a spinneret so that one of the two polymer components (sea) completely surrounds the other component, consisting of a number of polymer filaments which form the various “islands”.

The island component can be selected from: modified polyesters, cationic polyesters, nylon or other types of polyamides, polyethylene (PE), polypropylene (PP), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polyhydroxyalkanoates (PHAs), polyethylene furanoate (PEF), polylactic acid (PLA) and polyethylene terephthalate (PET), the latter being particularly preferred. The aforesaid polymers may be produced from raw materials obtained from renewable sources (which completely or partly replace current raw materials of fossil origin) or from processes for recycling polymers from other types of materials (e.g. bottles, textiles, manufactured articles), or the polymers themselves may be produced through fermentation processes or using microorganisms. Examples of polymers belonging to the first and second cases are PTT, PEF, PET and PLA, PE. Examples of polymers belonging to the third case are PHAs.

An example of a sea component is represented, on the other hand, by a spinnable polymer, preferably selected from: polyvinyl alcohol (PVA), polystyrene (PS), polystyrene copolymers containing PVA (co-PVA-PS), styrene copolymers containing maleic anhydride or other organic monomers in chains (co-PS), copolyesters containing PVA (co-PVA-PES), co-polyolefins such as polyethylene or polypropylene containing PVA (respectively co-PVA-PE, co-PVA-PP), copolyesters containing a mix of terephthalic acid, isophthalic acid and 5- sulfoisophthalic acid (HWS) and co-polyester containing both terephthalic acid and 5-sulfoisophthalic acid or the sodium salt thereof (co-PES, which can also be abbreviated as TLAS), PS and TLAS being particularly preferred.

Both the sea and island components can be used in a mixture with additional components. Examples of additional components are for example incompatible polymers (i.e. which are not miscible with the sea component), which produce a heterogeneous system that, on a microscopic level, has zones in which only one of the polymers is present, dispersed in the matrix consisting of the second polymer; these systems are generally fragile and if they are used to form the sea component they facilitate the breakage of the casing during the steps of pressing, crimping and production of the intermediate felt.

Among the polymers incompatible with the sea component, one may mention PVAs added to co-PES and polyethylene glycol (PEG) added to co-PS.

The bi-component fibre has a ratio between the island component and sea component such as to enable rapid, effective spinning of the two components by means of a spinneret. Said island/sea ratio is preferably between 20/80 and 80/20, more preferably between 50/50 and 80/20. Island/sea ratios of less than 50/50 increase the amount of sea component to be removed, with a consequent increase in the cost of the product, and lead to an evident loss of the physical and mechanical properties of the processed bi-component fibre (the sea component is weaker), as well as finished products with a poorer appearance because of the low density of fibre present on surface. Island/sea ratios greater than 80/20 make the spinning process difficult, as the small amount of sea component does not allow the microfibres to be properly separated within the bi-component fibre.

Before the spinning process, the bi-component fibre is usually treated according to methods known in the art, which provide for the addition of lubricant oils in the pressing step to improve the orientation of macromolecules in the axial direction, and with it the physical and mechanical properties, as well as to decrease the titre of the fibre thus obtained, a characteristic particularly in demand for the production of high-quality products. In one embodiment of the invention, the fibre, before being pressed, has a titre of between 5.5 and 19 dtex, preferably between 7.5 and 15 dtex. Furthermore, pressing is carried out with ratios that generally vary in the range between 2 and 5, preferably in the range between 2.1 and 3.9. At the end of pressing, the fibre is then cut to produce a staple fibre having a length between 45 and 55 mm. The felt obtained can have a thickness preferably between 2 and 4 mm, and an apparent density between 0.1 and 0.5 g/cm³, more preferably between 0.15 and 0.25 g/cm³. Advantageously, said density and thickness values are optimal for obtaining a final nonwoven product endowed with a good hand, softness, elasticity, appearance, and mechanical resistance to the process conditions. During preparation of the felt, it is particularly important - to ensure the absence of the defect caused by cracks (evidenced by rolling) on the side s obtained by cutting the product - to subject the staple fibre, in the initial steps of the spinning process, to an alternation in the penetration of spinning on both sides of the felt being formed to prevent an excessive orientation of the fibres according to a preferential direction.

The impregnations of the felt with PVA are carried out with an aqueous solution of PVA.

The fixing operation may, in turn, be carried out according to distinct methods. In the case of an aqueous solution such as the one with PVA, fixing must be carried out by drying in a convective oven.

The removal of the sea component takes place by means of a solvent selected from those previously indicated as suitable for dissolving the component itself. By way of example, it is possible to use organic solvents or an alkaline or acidic aqueous solution or hot water.

The subsequent impregnation of the microfibrous intermediate thus obtained with polyurethane (Pll) is carried out with a solution of Pll with solvent or an aqueous Pll dispersion (PUD).

The fixing operation can, in turn, be carried out according to distinct methods. In the case of an aqueous solution such as PUD, fixing must be carried out by drying in a microwave/radio frequency oven, or by coagulation with an acidic or saline aqueous solution. In the case of a polyurethane in an organic solvent, fixing takes place by means of treatment with an antisolvent (generally water). In any case, all the fixing methods, irrespective of the type of polyurethane, are carried out according to well-known techniques.

It is necessary for the bond between the felt and the polyurethane in aqueous emulsion (PUD) applied earlier to be able to withstand the treatment to extract the sea component of the bi-component fibre. To this end, it is necessary to fix the polyurethane in such a way that it can withstand that treatment. Fixing of the PUD can be achieved by adding crosslinking agents known in the art which, according to type, are active at room temperature or at relatively high temperatures (110°C - 200°C).

The process can comprise, finally, the steps of subjecting the substrate obtained to cutting, sueding, on one or both sides, and dyeing.

According to the present invention, the dyeing is characterised by the use of at least one dye and vanillin.

Preferably, the dye is a mixture of dyes.

In particular, the dye employed in the invention is preferably selected from the classes of disperse dyes, preferably anthraquinone or azoic compounds, cationic dyes, preferably azoic or methine, and mixtures thereof.

Vanillin is an organic compound represented by a phenolic aldehyde and is the main component of vanilla bean extract. In particular, vanillin is 4-hydroxy-3- methoxybenzaldehyde having the following formula:

There exist three different types of vanillin that can be used in the dyeing process of the present invention: synthetic vanillin, natural vanillin and biosynthetic vanillin. Natural vanilla extract is a mixture of several hundreds of compounds in addition to vanillin. Synthetic vanillin is today used more often than natural vanilla extract and is the form of vanillin preferably used in the dyeing process according to the invention.

It has been surprisingly demonstrated that the colorimetric yield improves, the percentage by weight of dye being equal, when vanillin is used in a mixture with the dye. In addition to the increase in the colorimetric yield, it has been verified that the dyed substrate retains the same physical and dyeing properties as a substrate dyed with traditional dyes and additives and shows a smaller dye residue.

Without wishing to be bound to a particular theory, we hypothesise that vanillin acts as a dye carrier and contributes to increasing its penetration into the fibre, but at the same time decreases the interactions between the dye molecules and the polyurethane, with the advantage of a more targeted delivery into the fibre and a better cleaning of the finished product.

Even more surprising is the fact that, according to the present invention, the dyeing process requires a smaller amount of dye, despite obtaining a higher colorimetric yield and a great depth of the colour tone. Even more surprising is the fact that, according to the present invention, the use of the vanillin does not require the use of additional dyeing auxiliaries (for example, dispersing agents, levelling agents, equalisers), with a consequent decrease in the quantity of chemical substances to be used.

Another advantage of the dyeing process according to the invention is the possibility of carrying out the dyeing at a lower temperature than the standard one, while nonetheless obtaining a high colorimetric yield.

According to the present invention, the dyeing temperature is below 100°C, whereas with the standard processes it is necessary to reach temperatures of 120-140°C.

Another advantage of the dyeing process of the invention is that of being able to carry out a single post-dyeing cleaning process, with a consequent reduction in process costs.

In the dyeing process of the present invention the vanillin and at least one dye are in an aqueous solution or suspension and form a bath.

The microfibrous substrate to be dyed is brought into contact with the bath, for example by immersion.

The vanillin present in the bath enables a swelling of the fibre and the formation of a larger number of amorphous zones where the dye can get in through weak interactions. The delivery of the dye molecules into the fibre ensures a long durability of the fibre dye and the achievement of the required colour tone with a smaller amount of dye in the dyebath.

The dyeing process of the invention is preferably conducted at a temperature below 100°C, preferably less than or equal to 70°C, more preferably less than or equal to 60°C. In one embodiment, the temperature of the dyeing step is between 60°C and 120°C, more preferably between 70°C and 100°C.

The pressure applied during the dyeing step is between 0.5 bar and 4 bar, preferably between 1 bar and 3.5 bar.

The dyeing process is preferably carried out by means of a jet machine known in the art.

It is well known that the temperature during the dyeing process can have a significant impact on the colourfastness of the fibre. Colourfastness is understood as the ability of a dyed fibre to retain its colour over time, notwithstanding exposure to various external factors such as washing, light and friction. Higher temperatures can improve colourfastness by increasing the dye-fibre bond, since the greater molecular movement at higher temperatures allows a greater penetration and diffusion of the dye within the fibre, thanks to the increase in amorphous zones and in the free volume of the polyester. According to the process of the present invention, dyeing with vanillin can be carried out at temperatures below 100°C with dyeing results comparable to those of the standard dyeing process carried out at temperatures above 120°C.

The amount of vanillin in the bath is preferably between 0.25 g/L and 15 g/L, between 0.5 g/L and 14 g/L, between 1 g/L and 13 g/L, between 2 g/L and 12 g/L, between 3 g/L and 11 g/L, between 4 g/L and 10 g/L, between 5 g/L and 9 g/L, and between 6 g/L and 8 g/L.

The amount of dye in the bath can be selected from between 0.001 % w/w and 100% w/w, between 0.005% w/w and 95% w/w, between 0.01 % w/w and 90% w/w, between 0.05% w/w and the 85% w/w, between 0.1 % w/w and 80% w/w, between 0.5% w/w and 75% w/w, between 1 % w/w and 70% w/w, between 5% w/w and 65% w/w, between 10% w/w and 60% w/w, between 15% w/w and 55% w/w, between 20% w/w and 50% w/w, between 25% w/w and 45% w/w, and between 30% w/w and 40% w/w.

In general, in a dyeing process, the amount of dye influences the intensity of the colour of the dyed fibre. As the amount of dye increases, the availability of dye molecules in the dyebath increases, thus allowing a larger number of dye molecules to permeate from the dyebath to the fibre.

Consequently, the more dye that is used up, the larger will be the amount of dye that adheres to and is diffused in the fibre, and the larger the amount of dye inside and outside the fibre, the greater will be the possibility of fixing the dye on the fibre.

According to the process of the present invention, it has been surprisingly found that it is possible to use a smaller amount of dye in the process with vanillin compared to the same process using traditional dyeing auxiliaries such as, for example, polyglycol ether or naphthalenesulfonic acid-formaldehyde- polycondensate sodium salt, while simultaneously obtaining a higher colorimetric yield. Very advantageously, according to the process of the present invention, it has been surprisingly found that the use of vanillin does not require the use of further dyeing auxiliaries.

Based on some embodiments of the present invention, in the dyeing process of the invention the amount of dye in the dyebath comprising vanillin is 10% to 20% less than the amount of dye that is normally used in a dyebath comprising another dyeing auxiliary such as, for example, polyglycol ether.

Preferably, the pH of the dyebath is an acidic pH, for example between 2 and 5 or between 3 and 4.

The dyeing step can last from 30 minutes to at least 4 hours, preferably between 30 minutes and 3 hours.

After the dyeing step, the microfibrous substrate is subjected to a stripping step with a basic aqueous solution for the purpose of eliminating the excess dye not adsorbed into the microfibre.

Since the amount of dye usable with the vanillin-based dyeing process is lower than that used with the known dyeing auxiliaries, a single stripping step may be sufficient to eliminate all the excess dye. However, it is possible carry out further stripping steps if necessary.

The basic aqueous solution comprises reducing agents, such as, for example, sodium hydrosulphite and sulphinic acid derivatives.

The basic solution has a pH greater than 8, preferably greater than 10. These pH values can be obtained by adding an inorganic base, in particular an alkali metal hydroxide, for example an aqueous solution of NaOH and/or KOH.

The stripping temperature is between 30°C and 80°C, preferably between 40°C and 70°C, or between 50°C and 60°C. The stripping step has a duration of between 15 minutes and 2 hours, preferably between 30 minutes and 1 hour. After the stripping step, a step of washing in water can be carried out.

If necessary, the stripping step can be repeated at least a second time and, accordingly, there can also be one or more steps of washing in water. Finally, the dyed, cleaned substrate can be subjected to one or more finishing steps whereby some properties can be imparted to the product, such as, for example, a softer hand.

Further enhancements of the material can be applied with hot treatments on the material for limited times, up to a temperature of 125 °C-150°C, e.g. thermoforming, embossing, hot rolling, printing and hot laminating with different substrates.

Example 1

Dyeing tests were performed on a Type A substrate and Type B substrate.

The type A substrate, nonwoven microfibre fabric, is obtained from a felt of bicomponent fibre of the “islands-in-the-sea” type, subsequent removal of the sea component of the fibre and impregnation with polyurethane. Fig. 1 shows an SEM (Scanning Electron Microscope) section of the type A substrate:

The Type B substrate, nonwoven microfibre fabric, is obtained from an impregnation with polyurethane of a three-layer structure, consisting of an inner scrim and two outer microfibre sides.

Fig. 2 shows an SEM (Scanning Electron Microscope) section of the Type B substrate:

The dyebath was prepared as follows:

- weighing of the vanillin in concentrations expressed in g/L of the total volume of the bath;

- weighing of the dyeing auxiliary polyglycol ether in concentrations expressed in g/L of the total volume of the bath;

- weighing of the dyes present in the recipe (taking as reference the percentages in relation to the merchandise weight). The following samples and the following conditions were tested.

Example 2 For each of the samples prepared according to Example 1 , the following results were obtained: Where the parameters expressed in the table are defined as follows:

- L* = luminosity;

- a* = indication of the tone from red to green;

- b* = indication of the tone from yellow to blue;

- CV-SUM = colorimetric value corresponding to the UV-visible spectrum;

- all delta (D) values correspond to the difference between the value measured for the standard and the value of the reference sample/trial.

The results of these experiments show that:

- The colour recipe being equal (same amount of dye in %w/w), in the dyeing conditions with vanillin a higher CV-SUM value (colorimetric yield index) is obtained compared to the standard dyeing condition with polyglycol ether.

- The colorimetric yield in the samples dyed with vanillin at 120°C, and at 100°C, shows to be inversely proportional to the concentration thereof.

- The condition being equal, in the samples dyed with vanillin, a higher colorimetric yield is observed in the samples dyed at a temperature of 100°C.

- Dyeing conditions at 100°C with 4 g/L and 2 g/L vanillin show to have a higher colorimetric yield (CV-SUM) compared to the standard dyeing condition with polyglycol ether a 120°C, +12% and +17%, respectively.

Example 3

For each of the samples prepared according to Example 1 , the following results were obtained as regards the respective dyebath residues:

Where the parameters expressed in the table are defined as follows:

- COD = chemical oxygen demand;

- CV-SUM = colorimetric value corresponding to the UV-visible spectrum. The results show that:

- In the conditions with vanillin, a reduction of about 94% is obtained on average in the total amount of surfactants, both in the dyebaths at 120°C and in those at 100°C.

- CV-SUM, an index of the amount of residual dye in the dyebath, on passing from polyglycol ether to vanillin, shows to decrease on average by about 41 % in the dyebaths at 120°C and 50% in the dyebaths at 100°C.

- On passing from 120°C to 100°C, in the dyebaths with vanillin a 5% decrease in the CV-SUM of the dyebath residue is obtained, unlike in the standard dyeing condition with polyglycol ether, in which by contrast a 4% increase is observed.

- The trend in these two parameters (total surfactants and amount of dye/CV-SUM) allows us to affirm that the presence of vanillin has a positive contribution on the dye residue, as it can decrease the burden on the wastewater treatment system.

Example 4

The dyebath with a mix of dyes different from the one in example 1 was prepared as described in example 1.

The following samples and following conditions were tested.

Example 5

For each of the samples prepared according to Example 4, the following results were obtained:

Where the parameters expressed in the table are defined as follows:

- L* = luminosity;

- a* = indication of the tone from red to green;

- b* = indication of the tone from yellow to blue;

- CV-SUM = colorimetric value corresponding to the UV-visible spectrum;

From the results it emerges that:

- In this case, the colorimetric yield in the samples dyed with vanillin at 120°C, and at 100°C, shows to be proportional to the concentration thereof, so a decrease in the vanillin concentration led in this case to a lower CV-SUM.

- In the samples dyed with 2 g/L vanillin, the colorimetric yield shows to be higher for the dyeing temperature of 100°C. For the samples dyed with vanillin at 1 g/L, the same behaviour is not observed, as the highest colorimetric yield is obtained when dyeing at 120°C.

- The best condition identified is the one of sample number 4, which, compared to sample 1 (reference condition) enables a +19.53% increase to be obtained in the colorimetric yield.

Example 6

Where the parameters expressed in the table are defined as follows:

- COD = chemical oxygen demand;

- CV-SUM = colorimetric value corresponding to the UV-visible spectrum. From the results it emerges that:

- In the conditions with vanillin, a reduction of about 95% is obtained on average in the total amount of surfactants, both in the dyebaths at 120°C and in those at 100°C.

- The CV-SUM in the samples dyed with vanillin always shows to be lower compared to the reference condition (sample number 1 ). In particular, the best condition confirms to be that of sample 4.

- The trend in these two parameters (total surfactants and amount of dye/CV-SUM) allows us to affirm that the presence of vanillin has a positive contribution on the dye residue, as it can decrease the burden on the wastewater treatment system. Example 7

The dyebath with a mix of dyes different from the ones in example 1 and 4 was prepared as described in example 1 . The following samples and following conditions were tested.

Example 8

For each of the samples prepared according to Example 7, the following results were obtained:

Where the parameters expressed in the table are defined as follows:

- L* = luminosity;

- a* = indication of the tone from red to green;

- b* = indication of the tone from yellow to blue;

- CV-SUM = colorimetric value corresponding to the UV-visible spectrum;

From the results it emerges that:

- In the samples dyed with 4 g/L, 2 g/L, and 1 g/L vanillin, the colorimetric yield shows to be greatest for the dyeing temperature of 100°C.

- The best condition identified is the one of sample number 9, which, compared to sample 1 (reference condition) enables a +12.55% increase to be obtained in the colorimetric yield,

Example 9

For each of the samples prepared according to Example 7, the following results were obtained as regards the respective dyebath residues:

Where the parameters expressed in the table are defined as follows:

- COD = chemical oxygen demand;

- In the conditions with vanillin, a reduction of about 92% is obtained on average in the total amount of surfactants, both in the dyebaths at 120°C and in those at 100°C,

- The CV-SUM in the samples dyed with vanillin always shows to be lower compared to the reference condition (sample number 1 ).

Claims

1 . A process for dyeing a microfibrous substrate impregnated with polyurethane, wherein said process comprises a step of bringing the microfibrous substrate impregnated with polyurethane into contact with a solution or suspension of at least one dye and vanillin at a temperature below 100°C.

2. Process according to claim 1 , wherein the microfibrous substrate is brought into contact with the solution or suspension of at least one dye and vanillin by immersing the substrate in the solution or suspension.

3. Process according to claim 1 or 2, wherein the at least one dye is a mixture of dyes.

4. Process according to any one of claims 1 to 3, wherein the amount of vanillin present in the aqueous solution or dispersion is between 0.25 g/L and 15 g/L, between 0.5 g/L and 14 g/L, between 1 g/L and 13 g/L, between 2 g/L and 12 g/L, between 3 g/L and 11 g/L, between 4 g/L and 10 g/L, between 5 g/L and 9 g/L, and between 6 g/L and 8 g/L.

5. Process according to any one of the preceding claims, wherein the amount of dye in the aqueous solution or dispersion is selected from between 0.001 % w/w and 100% w/w, between 0.005% w/w and 95% w/w, between 0.01 % w/w and 90% w/w, between 0.05% w/w and the 85% w/w, between 0.1 % w/w and 80% w/w, between 0.5% w/w and 75% w/w, between 1 % w/w and 70% w/w, between 5% w/w and 65% w/w, between 10% w/w and 60% w/w, between 15% w/w and 55% w/w, between 20% w/w and 50% w/w, between 25% w/w and 45% w/w, and between 30% w/w and 40% w/w.

6. Process according to any one of the preceding claims, wherein the aqueous solution or dispersion has an acidic pH, preferably between 2 and 5 or 3 and 4.

7. Process according to any one of the preceding claims, wherein the temperature of the aqueous solution or suspension is between 60°C and 120°C, more preferably between 70°C and 100°C.

8. Process according to any one of the preceding claims, wherein the aqueous solution or dispersion is brought into contact with the microfibrous substrate for between 30 minutes and 4 hours, preferably between 30 minutes and 3 hours.

9. Process according to any one of the preceding claims, further comprising at least one step of stripping the microfibrous substrate with a basic aqueous solution comprising at least one reducing agent.

10. Process according to any one of the preceding claims, wherein the microfibrous substrate is a bi-layer or a tri-layer comprising a nonwoven microfibre fabric obtained from mono-component fibres, preferably from polyethylene terephthalate (PET) fibres, bonded to at least one layer, preferably two layers, of fabric obtained from mono-component fibres, preferably PET fibres, and subsequently impregnated with polyurethane.

11. Process according to any one of claims 1 to 9, wherein the microfibrous substrate is obtained by means of a process comprising a step of preparing a bi-component fibre of the “islands-in-the-sea” type by spinning the island component in the sea component, preparing a felt with the bi-component fibre of the “islands-in-the-sea” type, impregnating the felt with PVA, removing the sea component, impregnating the felt with polyurethane and fixing the latter.

12. Process according to claim 11 , wherein the microfibrous substrate obtained is bonded to at least one other layer of the same type by applying an adhesive.

13. Process according to claim 11 , wherein the microfibrous substrate obtained is bonded to at least one layer of fabric by applying an adhesive.

14. Process according to any one of claims 1 to 9, wherein the microfibrous substrate is obtained with a process comprising the steps of: a) preparing a bi-component fibre of the “islands-in-the-sea” type by spinning the island component in the sea component; b) preparing a felt with the bi-component fibre of the “islands-in-the-sea” type; c) impregnating the felt with PVA and fixing the PVA; d) removing the sea component; e) impregnating the felt with polyurethane; f) fixing the polyurethane; g) subjecting the substrate obtained to cutting, sueding, on one or both sides.

15. A process for preparing a microfibrous substrate, comprising the steps of: a) preparing a bi-component fibre of the “islands-in-the-sea” type by spinning the island component in the sea component; b) preparing a felt with the bi-component fibre of the “islands-in-the-sea” type; c) impregnating the felt with PVA and fixing the PVA; d) removing the sea component; e) impregnating the felt with polyurethane; f) fixing the polyurethane; g) subjecting the substrate obtained to cutting, sueding, on one or both sides, and dyeing; characterised in that the dyeing is carried out using an aqueous solution or dispersion of at least one dye and vanillin.

16. Use of vanillin in a mixture with at least one dye for dyeing a microfibrous substrate impregnated with polyurethane.

17. Use according to claim 16, wherein vanillin is used as a carrier of at least one dye.