CH716895A2 - Composition for temporary protection and associated method. - Google Patents

Abstract

The composition (1) of the present invention comprises a first element or set of elements (E1) capable of reacting under first predetermined conditions (C1) to produce a first transformation (B) of the composition (1) into a composition (1 '). The composition (1) further comprises a second element or set of second elements (E2) capable of reacting under second predetermined conditions (C2) different from the first predetermined conditions (C1) to produce a second transformation of the composition (1 ') into a composition (1 "). The composition (1) is used as a means of temporary protection of manufactured parts. The present invention also covers a method of temporary protection of manufactured parts by means of the composition (1).

Description

Technical area

The present invention relates to a composition for temporarily protecting a part, especially during the manufacturing process, whether by the temporary bonding of parts for the need for industrial operations, a total or selective protective mask or by the coating of the part to be protected. The present invention also covers a method for temporarily protecting such parts.

State of the art

During the manufacture of industrial parts, it is sometimes necessary to assemble elements temporarily to allow or facilitate one or more steps of the manufacturing process. Masking or selective protection steps may for example be required to protect a part during manufacture before initiating a transformation, such as polishing, machining, marking or the application of a coating. The total or selective mask affixed to the workpiece must then be removed. Alternatively, a selective protective coating can be applied to the surface to be protected and then removed when the required operations are completed.

[0003] Furthermore, it may be necessary to maintain an element on a support temporarily, the time to proceed with its transformation, and then to be able to easily release it from the support.

[0004] Such fixing or temporary protection steps are, for example, commonly used in the watch industry. A wide variety of glues can be used. Two-component adhesives and products which can be cured by means of physical drying, by heat or by UV irradiation, are generally used. However, their effects are not reversible. Once the glue has been applied and the gluing has been completed, the separation of the elements may require special conditions which correspondingly increase the number of process steps and their cost. The sometimes drastic conditions necessary for the separation of the assembled parts and their cleaning can also be prejudicial to their integrity and to the environment by the use of products based on toxic or irritating solvents.

[0005] In the event that a coating is used temporarily to protect a part, it must be removed without prejudice to the part that it protects. The cleaning procedures required to remove the protective coating may be unsuitable for the most fragile parts.

[0006] It therefore appears necessary to improve such means of protection and bonding, so as to be able to apply them to a wide variety of parts, and in particular the most sensitive ones.

Brief summary of the invention

[0007] An object of the present invention is to provide a composition making it possible to temporarily protect a part, in particular by making it possible to stick or deposit a protective mask thereon, or by covering it totally or partially, so as to protect it against any aggression, whether chemical or mechanical.

Another object of the present invention is to provide a composition making it possible to easily release the temporarily protected parts. In particular, the temporarily protected parts are released under smooth and fast conditions.

Another object of the present invention is to provide a composition making it possible to temporarily protect parts and to release them easily without adding additional steps to the part manufacturing process, and preferably by minimizing the steps necessary for the part. protection and deprotection of parts.

Another object of the invention is to provide a method of protecting parts during manufacture, which is simple, efficient and economical.

According to the invention, these aims are achieved in particular by means of the claimed composition and the claimed process.

This solution has the particular advantage over the prior art of being able to integrate the temporary protection of a part in an already existing process, without adding a specific step, and of facilitating the deprotection of the protected part.

Brief description of the figures

Examples of implementation of the invention are indicated in the description illustrated by the following figures: • FIG. 1: example of composition according to its different transformations, • Fig. 2: schematic steps of the temporary protection process by applying a coating, • Fig.3: schematic steps of the temporary protection process by applying a mask, • Fig.4: Example of a microsphere

Example (s) of embodiment of the invention

Figure 1 shows an example of composition 1, in the initial state and after the first and second transformation. Composition 1 according to the present invention comprises a first element E1 allowing a first transformation of composition 1 when first predetermined conditions C1 are applied to it. The first element E1 can be a set of similar or different elements sensitive to the conditions C1 applied to composition 1. The element E1 can for example be a set of chemical precursors such as monomers or oligomers capable of reacting when the conditions C1 are applied. The element E1 can also comprise a catalyst making it possible to promote the reaction of the chemical precursors when the conditions C1 are applied. Element E1 may alternatively or in addition comprise additives making it possible to modulate the physical properties of the composition once transformed by the conditions C1. Such additives include, for example, hardeners, antioxidants, shear thickeners, pigments, colorants, wetting or leveling agents, or a mixture of several of these additives.

The first conditions C1 applied to composition 1 include one or other of the following conditions or a combination thereof: Contacting several first elements E1 initially separated from each other, at room temperature. This is particularly the case with two-component adhesives. Bringing the first E1 elements into contact allows them to react with each other. Venting the composition comprising the first element E1, at room temperature. Heating of the composition to a temperature between 20 ° C. and a first threshold temperature S1. The first threshold temperature S1 can be for example around 40 ° C, or around 50 ° C or around 80 ° C, or around 90 ° C. The first threshold temperature S1 is preferably determined so as not to be detrimental to the part 2 to which the composition 1 is affixed. It is also determined so as not to activate the second element E2 also present in the composition 1. The heating of composition 1 can be carried out under predetermined humidity conditions, or under dry conditions, under an inert atmosphere or in the presence of air. Any means of heating can be used, depending on the most suitable conditions. Heating can be carried out, for example, by an oven, or under an infrared lamp, or in a climatic chamber or by any other means deemed adequate. Irradiation of composition 1 with radiation of predetermined wavelength. Such radiation includes, for example, UV-Vis, whether of type A, B, or C. The radiation can alternately or in addition occur in the visible range.

The duration of application of conditions C1 can be variable depending on the nature of the first element E1, the amount of composition 1 treated, the desired degree of transformation and possible other parameters. The first conditions C1 can be applied from a few seconds to several minutes, or even several hours. In the case of heating the composition C1 to a temperature below a first threshold S1, it can also be provided that the heating time does not exceed a first time T1. The first duration T1 is preferably determined to avoid damaging the parts to which the composition 1 is affixed. Alternatively or in addition, the first duration T1 is determined so as not to activate the second element E2 also present in composition 1. It can for example be provided that the temperature temporarily exceeds the first threshold S1 as long as the heating duration does not exceed a first duration T1.

Preferably, the duration of application of the first conditions C1 is limited from a few seconds to a few minutes, such as 1 to 5 minutes, so as to be able to easily include the step of protecting the part in the manufacturing process. For this purpose, in particular during so-called continuous precedents, the parts treated with composition 1 can pass through a tunnel where the temperature, or the light irradiation, correspond to the first conditions C1 allowing composition 1 to be transformed.

The first transformation of composition 1 is understood as a change in one or more of its physical, or chemical, or mechanical properties. The first transformation of composition 1 is most generally irreversible. Composition 1 may for example be initially in liquid or viscous or pasty form. The initial properties of composition 1 are adapted to its mode of application to the parts to be treated. A fluid, or even liquid, composition will be suitable for covering the parts to be protected. A more viscous composition will be adequate for bonding a mask 3, for which a few bonding points may be sufficient. The first transformation of composition 1, when the first conditions C1 are applied to it, includes one or more of the following transformations: Polymerization or crosslinking of the first element E1, generating hardening of composition 1; Polymerization or crosslinking of the first element E1, leading to the bonding of two assembled parts; Polymerization or crosslinking of the first element E1, leading to the adhesion of composition 1 to the surface of the part treated; Increase or decrease in the elasticity of composition 1;

At the end of the application of the first conditions C1, the composition 1 will give rise to a composition 1 ', which will for example have one or more of the following characteristics: solid, adhesive, impact resistant, resistant to abrasion, moisture resistant. In particular, the composition 1 'resulting from the first transformation of the composition 1 under the effect of the first conditions C1, will allow the bonding of a mask 3 on a part 2 or the coating of the surface of a part 2. The composition 1 ′ comprises in particular a matrix M in which the constituents of composition 1 which are inert under the first conditions C1, and in particular the second element E2, remain trapped.

[0020] According to one embodiment, the element E1 is photopolymerizable. In particular, element E1 comprises one or more oligomers or prepolymers, in an amount of between approximately 30 and approximately 70 parts by weight of composition 1. The oligomers or prepolymers of element E1 comprise at least one acrylate or methacrylate function. have a molecular mass of less than 1000 g / mol. Advantageously, the oligomers or prepolymers of element E1 have a viscosity greater than 3000 mPa.s at 20 ° C, preferably greater than 5000 mPa.s at 20 ° C, more preferably greater than 8000 mPa.s at 20 ° C. . Preferably, the oligomers or prepolymers of the element E1 have a molecular mass of between 300 and 100,000 g / mol. In particular, the oligomers or prepolymers of the element E1 can take the formula (I):

Element E1 further comprises a photoinitiator of radical, cationic or hybrid type in order to respectively initiate a radical, cationic or hybrid polymerization reaction. Preferably, the photoinitiator is a radical photoinitiator chosen from the group consisting of acetophenone derivatives or phosphine oxide derivatives. The photoinitiator may be present in an amount of between about 2 and about 6 parts by weight of composition 1. Element E1 may further comprise one or more diluting agents, such as ethyl-diethylene glycol acrylate-butyl hexanediol diacrylate. -acrylate. The diluting agent can be present in an amount ranging from 30 to 70 parts by weight of composition 1. Element E1 can also comprise one or more pigments, present for example in an amount of less than 5 parts by weight of composition 1.

Composition 1 comprising such an element E1 can be advantageously used as a photopolymerizable coating, and produce a crosslinked layer under the influence of C1 conditions comprising light irradiation in the UV-visible fields. Other E1 elements can be used as needed, whether they are light-curing or heat-curing.

[0023] The composition 1 comprises, in addition to the first element E1 described above, a second element E2 allowing a second transformation of the composition 1 'when the second predetermined conditions C2 are applied to it. The second element E2 can be a set of similar or different elements sensitive to the conditions C2 applied to the composition 1 ′.

The second conditions C2 applied to composition 1 include one or other of the following conditions or a combination thereof: Heating of composition 1 'to a temperature above the first threshold S1. Preferably the temperature remains below a second threshold S2 so as not to degrade the part or parts treated 2. Heating of the composition 1 ′ comprising the element E2, at a temperature below the first threshold S1 for a period greater than the first period T1. Preferably, the duration of the heating remains less than a second duration T2, in particular in the case of heat-sensitive parts. Light irradiation of the composition 1 ′ comprising the element E2, when the element E2 is capable of reacting under light irradiation.

The heating applied during the second conditions C2 can be performed under the same conditions as during the first transformation or under different conditions. The heating can be carried out by means of an oven, under controlled or uncontrolled humidity, under steam, or in a dry environment, under an inert atmosphere or else in the presence of air. Advantageously, the heating can be integrated into a hot washing step, with or without mechanical constraints such as ultrasound or other means of vibration. In this way, the washing step used regularly in the part manufacturing process can be used to free the part from its temporary protection, thus avoiding a specific step. The heating can alternatively be carried out by a current of hot air.

The second transformation of composition 1 'when the second conditions C2 are applied to it, includes increasing the volume of the second element E2, generating at least partial degradation of composition 1'. In particular, the increase in volume of the element E2 trapped in the matrix M produces an at least partial rupture of the matrix M into a fragmented matrix M '. The increase in volume of element E2 in this case leads to an increase in volume of composition 1 'of the order of 8 to 15%, or even more than 20%.

At the end of the application of the second conditions C2, the composition 1 'will give rise to the composition 1 ", which will have lost one or more of the characteristics of the composition 1' obtained after the first transformation. the occurrence, characteristics such as its strength, adhesive properties, impact resistance, abrasion resistance, moisture resistance are affected. Composition 1 "resulting from the second processing of composition 1 'under the second conditions C2 allows the separation of the mask 3 and the selectively protected part 2. Where appropriate, the transformation of composition 1 'under the second conditions C2 will lead to degradation of the coating of the surface of a part 2 when composition 1 is used for this purpose. The second transformation leads in this case to the loss of adhesion of the composition 1 'on the part 2.

The second element E2, forming part of the composition 1, and of the composition 1 ', includes any element or set of similar or different elements sensitive to the second conditions C2 applied to the composition 1'. The element E2 can for example be a set of chemical precursors such as monomers or oligomers capable of reacting when the conditions C2 are applied. The element E2 can also comprise a catalyst making it possible to promote the reaction of the chemical precursors when the conditions C2 are applied. Element E2 may alternatively or in addition comprise additives making it possible to modulate the physical properties of the composition once transformed by the conditions C2.

Advantageously, the element E2 is a set of thermo-expanding microspheres G, as shown diagrammatically for example in FIG. 4. Such thermo-expanding microspheres G comprise a polymer capsule G1 making it possible to trap a gas in its central cavity G2, which expands under the effect of heat. The expansion of the gas makes it possible to significantly increase the volume of the thermo-expanding microspheres G. The term “significantly” is understood here as sufficient to degrade the structure of the composition 1 in which the thermo-expanding microspheres G are included. For example, the volume of the thermo-expanding microspheres G can double or triple, or quadruple. The increase in volume of the thermo-expanding microspheres G under the effect of the second conditions C2 can be more than 4, or 5 or 10. Preferably, the volume of the thermo-expanding microspheres G, once expanded, is up to at 60 times their original volume. The thermo-expanding microspheres G can have an initial diameter Q1 of the order of 8 to 12 micrometers, which increases to a final diameter Q2 of the order of 20 or 40 or more than 50 micrometers under the application of the seconds conditions C2. The initial thickness d1 of the capsule G1 of the thermo-expanding microspheres G changes during the expansion of the thermo-expanding microspheres G under the effect of the second conditions C2 to take a final value d2. The final thickness d2 is generally less than the initial thickness d1. Preferably, the thermo-expanding microspheres G expand when the temperature exceeds the value of the first threshold S1. Advantageously, the thermo-expanding microspheres G expand at a sufficiently low temperature to limit the heating of the treated part. The thermo-expanding microspheres G are therefore reactive at a temperature below a second threshold S2. The second temperature threshold S2 can be for example 200 °, 150 ° C or 130 ° C. The second temperature threshold S2 can be determined according to the applications and the desired conditions. Preferably, the thermo-expanding microspheres G expand at a temperature between 80 ° C and 100 ° C, advantageously around 90 ° C. Preferably, the thermo-expanding microspheres G used are the expancel® 031DU40 microspheres, the expansion of which takes place at a temperature between 80 ° C and 90 ° C.

The second element E2 can include a mixture of different thermo-expanding microspheres G in varying proportions depending on requirements. The amount of element E2 is between 1 and 60 parts by weight of composition 1, preferably between approximately 5 and approximately 40 parts by weight.

In the case where the element E2 comprises thermo-expanding microspheres G, the latter can be dispersed homogeneously in composition 1, then remain distributed in the matrix M homogeneously after the first transformation of composition 1 Alternatively, the thermo-expanding microspheres G can sediment at the interface of composition 1 and of the part 2 that it covers, so as to create a concentration gradient between the free surface of composition 1 and its contact surface. with part 2 on which it is affixed. In this way, the effects of the second transformation will be localized at the level of the interface between the part 2 and the composition 1 'resulting from the first transformation. In the case where composition 1 is intended to produce a layer of protective varnish on part 2, the application of the second conditions C2 will make it possible to easily peel the layer of protective varnish, due to the location of the second transformation so privileged at the interface with the part 2. The part 2 is thus easily cleanable. In the case where composition 1 aims to attach a mask 3 to a part 2, the location of the thermo-expanding microspheres G at the interface of the part 2 and of the composition 1 makes it possible to easily remove the mask 3 while leaving a minimum of residues on the part 2. The proportion of thermo-expanding microspheres G is between 5 and 30% by weight of composition, preferably between 15 and 20% by weight of composition, whether their distribution is homogeneous in the composition or more concentrated towards the interface with the protected part 2.

The term "interface" is understood here as the surface of the composition 1, or of the composition 1 'having undergone a first transformation, which is in contact with the part 2 protected by the composition (1.1' ).

Composition 1 comprises the two elements E1 and E2 in variable ratios according to needs. The ratio of the elements E1 / E2 can be for example 1/1, 1/2, 1/3, 2/1, 3/1, 4/1. Other proportions can be considered.

It is understood that if the first conditions C1 involve the heating of composition 1, the heating temperature remains below the reaction temperature of the second element E2, or where appropriate of the thermo-expanding microspheres G. Alternatively, if the first reactions C1 do not involve any heating, the second conditions C2 may involve a low heating temperature, below 90 ° C, or even below 70 ° C.

It is also understood that the second element E2 can be different from the microspheres described here. The element E2 can be a set of chemical precursors capable of reacting during the application of the second conditions C2 to produce a second transformation of the composition 1. Such chemical precursors can include chemical compounds which are unstable at temperature and capable of settling. expand under the effect of heating. Alternatively or in addition, the second element E2 can comprise chemical precursors which degrade the matrix M of the composition 1 ′.

According to one embodiment, composition 1 comprises a first element E1 capable of reacting under light irradiation, and a second element E2 capable of reacting under the effect of an increase in temperature.

According to another embodiment, composition 1 comprises a first element E1 capable of reacting under light irradiation, and a second element E2 capable of reacting under the effect of an increase in temperature beyond a first threshold 51 and below a second threshold S2.

According to another embodiment, composition 1 comprises a first element E1 capable of reacting under the effect of the temperature below a first threshold S1, and a second element E2 capable of reacting under the effect of a temperature increase beyond the first threshold S1 and below a second threshold S2.

Examples of compositions:

A composition serving as glue or photopolymerizable varnish is described by way of example: The first element E1 comprises: an oligomer of acrylate type in a proportion of the order of 40% to 70% by weight of composition, a mixture of mono- and di-acrylate monomers in a proportion of the order of 10% to 30% by weight of the composition, a photoinitiator in a proportion of the order of a few percent, typically less than 2% by weight of the composition.

The second element E2 comprises thermo-expanding microspheres G of the expancel type in a proportion of between 5% and 30% by weight of composition.de preferably in a proportion of between 15% and 20% by weight of composition. E1 difunctional acrylate polymerizable element 40-70 Diacrylate polymerizable element 10-20 Mono acrylate polymerizable element 3-10 Additives (siloxanes) Wetting <2 E2 Microspheres Expansion factor of the order of 20 to 60 5 to 30

The expansion factor denotes the increase in volume of the microspheres used during the application of the second conditions C2. The expansion factor designates in particular a multiplicative factor of volume.

Composition 1 is intended to be placed on the surface of a manufactured part 2, in particular during the manufacturing process of this part 2. Part 2 designates any part liable to be damaged during one of the stages of its manufacture, such as polishing, etching, or simply transporting from one workstation to another. The parts 2 are preferably timepieces, or parts constituting writing instruments, or jewelry, or any other part whose surface condition represents a determining element in the assessment of its quality. The part 2 can be made of metal, synthetic material, polymer, or the like.

According to one embodiment, the composition 1 is placed on the surface of the part 2 so as to cover it and make a temporary protective coating.

According to another embodiment, the composition is placed on the surface of the part 2 so as to achieve a temporary assembly. Such an assembly includes the affixing of a mask 3 on the part 2.

Composition 1 can be placed on the part 2 manually or using an automated device. Commonly used means of automatic dispensation can be applied.

The present invention further covers a method of temporary protection of a manufactured part 2. Figures 2 and 3 illustrate the method of temporary protection or assembly according to the invention. In particular, the temporary protection method comprises a dispensing step A during which the composition 1 is placed on the surface of the part 2 to be protected. The dispensing step A may comprise a step A ′ of arranging a mask 3 intended to be glued to the surface of the part 2. In this case, the composition 1 is used as a temporary glue. Alternatively, composition 1 is used as a temporary protective coating (Figure 2).

A first transformation step B is then implemented under the influence of the first conditions C1. During this first transformation step B, the first element E1 is activated so as to produce the appropriate transformation of composition 1. In the case where a mask 3 has been placed on part 2 during step A '(figure 3), the first conditions C1 applied during the first transformation step B make it possible to stick the mask 3 on the part 2. In the case where the composition 1 serves as a temporary protective coating (figure 2), the first transformation step allows for example to harden the composition 1 and to make it adhere to the part 2. The first conditions C1 are for example those described above and can be adapted according to the nature of the first element E1, the degree of transformation desired, of the amount of composition treated and any other parameters. At the end of the first transformation B, composition 1 produces a composition 1 'in the form of a crosslinked glue or a protective varnish.

The part 2 thus protected can be involved in one or more steps P of the manufacturing process of the part 2. At the end of these one or more steps P of the manufacturing process of the part 2, a step D of second transformation is carried out so as to remove from the surface of the part 2 the temporary protection provided by the composition 1 ′. During this second transformation step D the second conditions C2 are applied. When the protection consists of a mask 3 glued to the part 2, the second transformation step D makes it possible to separate the mask 3 from the part 2 (FIG. 3).

When the protection consists of a protective varnish by the composition 1 ', the second transformation step D allows the elimination of this varnish to again release the surface of the part 2. The second transformation D leads to degradation at least partial of composition 1 ′ to result in a composition 1 ". Preferably, the bonding or adhesion properties of composition 1" are significantly degraded to allow deprotection of part 2 without mechanical stress.

The temporary protection process may further include a washing step L, intended to rid the surface of the part 2 of any residues of composition 1. "This washing step L may for example comprise the immersion of part 2 in a washing bath, with or without mechanical constraints, such as ultrasound. The washing step L can alternatively comprise spraying with a washing liquid, at a predetermined pressure. The part 2 can be immersed or sprayed cold or at higher temperatures.

According to an advantageous embodiment, the washing step L is a washing step already included in the manufacturing process of the part 2. In other words, the conditions applied during the washing step L are those necessary for cleaning the part 2 after the various steps P of the manufacturing process. It is then not necessary to provide a specific step for the deprotection of part 2.

According to another embodiment, the washing step L is combined with the second transformation step D. In particular, the washing step L can include the second conditions C2 suitable for carrying out the second transformation of the composition 1 '. The washing conditions may for example involve temperatures above a first threshold S1 and below a second threshold S2. The immersion bath or the liquid for spraying the parts 2 can in particular be brought to such a temperature making it possible to carry out the second transformation of the composition 1 ′ to obtain the composition 1 ″.

Reference numbers used in figures

1 Composition 1 'Composition modified by a first transformation 1 "Composition modified by a second transformation 2 Part 3 Mask E1 First element E2 Second element C1 First conditions G Thermo-expanding microspheres M Matrix M' Fragmented matrix S1 First temperature threshold S2 Second temperature threshold T1 First duration T2 Second duration

Claims (10)

1. Composition (1) comprising a first element or set of elements (E1) capable of reacting under first predetermined conditions (C1) to produce a first transformation (B) of composition (1) into a composition (1 '), characterized in that it further comprises a second element or set of second elements (E2) capable of reacting under second predetermined conditions (C2) different from the first predetermined conditions (C1) to produce a second transformation (D) of the composition ( 1 ') in a composition (1 "). 2. Composition according to claim 1, characterized in that the second element or set of elements (E2) is insensitive to the first conditions (C1). 3. Composition according to claims 1 or 2, characterized in that the first transformation (B) of the composition (1) is irreversible. 4. Composition according to one of claims 1 to 3, characterized in that the first element or set of elements (E1) comprises one or more chemical precursors which can be assembled into polymers under the effect of heat, in this that the first conditions (C1) comprise a rise in temperature to a value below a first threshold (S1), and in that the first transformation (B) of the composition (1) comprises the polymerization or the crosslinking of the first element or set of elements (E1) to form a matrix (M) in which the second element (E2) remains trapped. 5. Composition according to one of claims 1 to 3, characterized in that the first element or set of elements (E1) comprises one or more chemical precursors which can be assembled into polymers under the effect of light irradiation, in that the first conditions (C1) comprise light irradiation, and in that the first transformation (B) of composition (1) comprises polymerization or crosslinking of the first element or set of elements (E1) to form a matrix (M), in which the second element (E2) remains trapped. 6. Composition according to either of the preceding claims, characterized in that the second element or set of second elements (E2) comprises thermo-expanding microspheres, in that the second conditions (C2) comprise the increase temperature above a first threshold (S1) and up to a second threshold (S2), and in that the second transformation (D) is the expansion of the second element (E2) leading to degradation at least partial matrix (M) into a degraded matrix (M '). 7. A method of temporary protection of a part (2) comprising a step of dispensing from the composition (1) described in one of claims 1 to 6, a step of first transformation (B) under first conditions (C1) to lead to a composition (1 ') capable of protecting the part (2), and a second transformation step (D) under second conditions (C2) different from the first conditions (C1) to lead to a composition (1 ") having lost its ability to protect the part (2). 8. Method according to claim 7, characterized in that the first conditions (C1) applied during the first transformation step (B) comprise heating the part (2) to a temperature below a first threshold ( S1) or the light irradiation of parts or a combination of the two. 9. Method according to one of claims 7 or 8, characterized in that the second conditions (C2) applied during the second transformation step (D) comprise heating the part (2) to a temperature higher than the first. threshold (S1) and less than a second threshold (S2). 10. Method according to one of claims 7 to 9, characterized in that the second transformation step is combined with a washing step (L) hot.