WO2021123273A1

WO2021123273A1 - Photoradical process for curing an alkyd resin

Info

Publication number: WO2021123273A1
Application number: PCT/EP2020/087173
Authority: WO
Inventors: Khalid Zahouily; Tuba KAVALLI; Jacques LALEVÉE
Original assignee: Photon Et Polymers; Universite De Haute Alsace; Centre National De La Recherche Scientifique
Priority date: 2019-12-18
Filing date: 2020-12-18
Publication date: 2021-06-24
Also published as: FR3105231A1; FR3105231B1; EP4077412A1

Abstract

The invention relates to a process for cross-linking an unsaturated alkyd resin A by radical polymerisation comprising the steps of: i) placing an unsaturated alkyd resin A in contact with a polythiol T as a cross-linking agent, in the presence of: a. a radical photoinitiator (PA) absorbing light at a wavelength λ in the UV or visible range, and b. an iodonium salt as a radical co-initiator (coA); ii) irradiating the reaction mixture obtained in step i) at the wavelength λ; and optionally iii) recovering the cross-linked alkyd resin obtained.

Description

DESCRIPTION

TITLE: PHOTORADICAL PROCESS FOR CURING AN ALKYD RESIN

Nowadays, surface coatings such as, for example, paints, varnishes, etc., are used in a wide variety of fields, in particular in construction, furniture and the automobile industry.

With increasingly restrictive legislation vis-à-vis the emission of volatile organic compounds (VOCs), conventional solvent-based paints and coatings tend more and more often to be replaced by more ecological alternatives, in particular by alkyd resins.

Alkyd resins are generally prepared from polyols, dicarboxylic acids and fatty acids. They thus have the advantage of being made from renewable resources, namely vegetable oils, and of being biodegradable. In addition to these ecological advantages, they exhibit excellent durability and very good adhesion to various substrates.

The drying or curing of these resins to lead to a surface coating is obtained by crosslinking the unsaturated fatty acids according to an autoxidation mechanism. This is generally very slow, commonly several hours, and requires the use of catalysts, also called siccatives, to reduce the induction time (drying time) and increase the rate of crosslinking between the unsaturated chains.

The catalysts most commonly used today are catalysts based on metal complexes, in particular based on cobalt. However, the latter being considered carcinogenic, more ecological alternatives have recently been studied.

Thus, a process for crosslinking an alkyd resin carried out in the absence of cobalt, and based on the thiolene reaction has in particular been described (Klaasen et al. (Progress in Organic Coatings 55 (2006) 149-13). is carried out in one step, in the presence of a photoinitiator such as bisacyl phosphine oxide (BAPO), and a polythiol as crosslinking agent.

A process for the crosslinking of alkyd resin has now been developed, which does not require the presence of cobalt as a catalyst, making it possible to improve both the drying times and the crosslinking rates compared to the known processes in the art. state of the art. More particularly, the inventors have discovered that by combining a radical photoinitiator with an iodonium coinitiator in the thiolene reaction, it was possible to considerably increase the kinetics of the reaction for the crosslinking of alkyd unsaturations and to achieve higher conversion rates than according to the processes known from the state of the art. Moreover, this process has several other advantages, in that it can be carried out: - in the open air, insofar as the oxygen in the air does not inhibit the thiolene reaction, and therefore does not require no working in an inert atmosphere, - under visible light, due to the absorption spectrum of the radical photoinitiators used,

40 - with a low irradiation intensity, which can be advantageous when the composition containing the alkyd resin to be polymerized contains constituents sensitive to radiation.

Thus, the process according to the invention advantageously makes it possible to obtain alkyds with high crosslinking rates, this in a very short time and under low irradiance. The 45 crosslinked alkyds obtained according to this process thus have the advantage of drying very quickly.

Thus, according to a first aspect, the invention relates to a process for crosslinking an unsaturated alkyd resin A by radical polymerization comprising the steps of: i) bringing an unsaturated alkyd resin A into contact with a polythiol T as

50 as a crosslinking agent, in the presence of: a. a radical photoinitiator (PA) absorbing light at a wavelength λ included in the UV or visible range, and b. an iodonium salt as a radical co-initiator (coA); ii) Irradiation of the reaction mixture obtained in step i) at the wavelength λ; and

55 optionally iii) Recovery of the crosslinked alkyd resin A _r obtained.

Step i)

Preferably, the coA radical co-initiator used in part i) is a diaryliodonium, in particular chosen from bis- (4-t-butylphenyl) -iodonium 60 hexafluorophosphate, diphenyliodonium, 4-1- (methylethyl) phenyl ) - (4-methylphenyl) iodonium tetrakis (pentafluorophenyl) borate. According to a preferred embodiment, the radical photoinitiator (PA) is chosen from those absorbing light at a wavelength λ is between 300 and 500 nm, preferably between 350 and 450 nm. The radical photoinitiator PA can in particular be chosen from type I or type II photoinitiators, more particularly from type I photoinitiators.

A type I photoinitiator generally consists of a benzoyl group. After absorption of light by this chromophore, a homolytic cut can be made from the excited state (often the triplet state). This cleavage can take place in the α position or in the β position with respect to the carbonyl function:

Among the type I photoinitiators, mention may be made of phosphine oxide (acylphosphine oxide), acetophenone (hydroxyalkylacetophenone), α-aminoketone or benzoin ether derivatives.

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As an example of a radical photoinitiator PA of the phosphine oxide type, mention may in particular be made of bis (2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO) and triphenylphosphine oxide (TPO).

Preferably, the radical photoinitiator PA is a phosphine oxide,

85 in particular bis (2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO).

A Type II photoinitiator is a molecule which, after light irradiation, does not directly form radicals, to form initiator radicals, it must be combined with a CoB co-initiator, different from the CoA co-initiator, which can be:

- An electron donor which following a proton transfer (De-H) forms two radicals; 90 these donors can be amines, thiols or silanes (scheme 4).

- A hydrogen donor (DH) such as ethers or alcohols.

As an example of a type II radical photoinitiator PA, there may be mentioned in particular isopropylthioxanthone (ITX), benzophenone (BP), camphorquinone (CQ).

As example of CoB coinitiator which can be used with a type II AP, there may be mentioned in particular amines, thiols, silanes and alcohols. The most used are compounds carrying hydrogen in a of an oxygen (aliphatic ethers), of a nitrogen (aliphatic amines and more particularly tertiary amines), of a sulfur (thioether) or even directly attached to a sulfur atom (thiols).

According to one embodiment, the ratio of photoinitiator PA relative to the coinitiator CoA used in step i) is between 1 and 3% by weight.

According to one embodiment, the PA / CoA molar ratio used in step i) is between 0.5 / 1 and 1 / 0.5. By unsaturated alkyd resin is meant a polyester modified with at least one unsaturated fatty acid. More particularly, it is a resin obtained by polycondensation of a polyol with at least one unsaturated fatty acid, that is to say an aliphatic carboxylic acid comprising at least one unsaturation. It may in particular be the condensation product of a polyol, of a basic polyacid and of an unsaturated fatty acid. For example, the structure of an alkyd resin formed by condensation of phthalic acid, glycerol and a fatty acid is as follows:

R can be, for example, linolenic acid, linoleic or oleic acid.

As another example, the structure of an alkyd resin formed by condensation of glycerol with a C18 unsaturated fatty acid is shown below:

The polyol can in particular be a dihydric or trihydric alcohol. The dihydric alcohol is chosen from ethylene glycol, diethylene glycol, propylene glycol, 1,4-cyclohexane dimethanol, and neopentyl glycol. The trihydric alcohol can be chosen from glycerol and trimethylolpropane. The unsaturated alkyd resin is in particular an alkyd resin whose repeating unit comprises at least one aliphatic chain (that is to say a fatty chain), said chain comprising at least one unsaturation.

Preferably, the repeating unit of the unsaturated alkyd resin A comprises at least one aliphatic chain comprising at least two instigations per chain.

According to one embodiment, the aliphatic carboxylic acid comprises a hydrocarbon chain comprising from 12 to 36 carbon atoms.

The aliphatic carboxylic acid can in particular be chosen from oleic acid, linoleic acid, oleostearic acid and ricinoleic acid.

The term “polythiol” is understood to mean a crosslinking agent comprising at least two thiol functions, in particular 2, 3 or 4.

The polythiol T used in step i) T can in particular be chosen from trimethylolpropane tris (3 -mercaptopropionate), pentaerythritol tetra (3-mercaptopropionate), trimethylolpropan tri (3-mercaptopropionate) ethoxyliers.

According to one embodiment, the molar ratio A / T is chosen so that the molar ratio of the number of unsaturations of the unsaturated alkyd A over the number of thiol functional groups of the polythiol T is close to 1, i.e. that is, stoichiometry. Typically, the A / T weight ratio is between 15/1 and 5/1, and is preferably about 10/1.

By “approximately x” is meant a variation of plus or minus 10% of the value x.

The reaction can be carried out in an organic or organoaqueous medium.

The viscosity of the reaction mixture can vary over a wide range and is preferably between 100 and 1000 cP.

Step ii)

The step of irradiating the reaction mixture obtained in step i) can advantageously be carried out in a wide range of intensity.

Thus, in step ii), the reaction mixture can be irradiated with an irradiation intensity of between 10 mW / cm ² and 20 W / cm ² . The greater the intensity, the more the kinetics of the polymerization reaction will be increased.

According to a preferred embodiment, step ii) can be carried out by irradiation with visible light at low intensities, instead of irradiation with a UV lamp usually used. Advantageously, visible light has a lower harmfulness than UV light and allows a lower energy consumption, especially if it is associated with the use of "Light Emitting Diode" (LED) whose energy consumption is particularly low compared to other types of lamps.

Preferably, the mixture is irradiated with an intensity of between 50 mW and 1W / cm ² .

According to a second aspect, the invention relates to a crosslinked alkyd resin A _{r which} can be obtained according to the process as defined in the present application.

This resin is characterized in particular in that it comprises residues of the iodonium salt used as a coinitiator, in particular a content of between 0.1% and 2% by weight. The presence of these residues can be detected by various methods, in particular by UV-Visible, HT-IR spectroscopy or iodine assay.

Without wishing to be limited to a particular theory, the reaction mechanism of the thiolene reaction according to the present application has been studied by the inventors and can be represented according to Figure 1.

During the first step (1) the photoinitiator (TPO) goes from the ground state to the excited state and generates the formation of a phosphinoyl radical (P *) when the CP bond is cut (Type I initiator) and a benzoyl radical. The set of these two radicals formed is denoted R *.

In a second step, the latter, R * reacts with the oxygen in the air and allows the creation of peroxyl radicals ROO * (2).

During the following step (3), the phosphinoyl radicals P * react with the iodonium salt to form aryl radicals Ar *. Then, either the P * radicals (4a), or the aryl radicals Ar * (4b), will react with the polythiol crosslinking agent T to form thiyl S * radicals. In both cases, it is a reaction of the stripping of hydrogen from one of the thiol functions of the crosslinking agent T.

During the propagation step (5), the S * radicals formed will be added to one of the unsaturations of the alkyd resin to form a radical centered on the carbon. This will be able to extract the hydrogen from one of the thiol functions of the crosslinking agent T to form thiyl radicals (6). These steps (5), (6) correspond to the addition / transfer steps of the thiol-ene mechanism (here alkyd-ene). In the last termination step, the thiyls recombine with each other (7) or with carbon radicals (8). Recombination of carbon radicals can also be observed (9). Reactions 8 and 9 are at the origin of the crosslinking of alkyds by the thiol-ene mechanism.

According to a third aspect, the invention relates to a photocrosslinkable composition comprising: an alkyd resin A; a polythiol T as a crosslinking agent; a radical photoinitiator (PA) absorbing light at a wavelength l within the UV or visible range, and an iodonium salt as a radical co-initiator (coA).

This composition can also comprise other constituents such as pigments or additives and / or fillers.

According to a fourth aspect, the invention relates to a paint, ink or varnish comprising a composition as defined in the present application.

According to another aspect, the invention relates to a process for coating a surface of a solid support with a crosslinked alkyd resin A _r comprising the steps of: i) Application in the form of a layer, to the surface of a solid support, of a composition as defined in the present application; ii) Irradiation of the surface obtained in step i) whereby a solid support is obtained, the surface of which is coated with a crosslinked alkyd resin A _r .

According to yet another aspect, the invention relates to a surface coating comprising a crosslinked alkyd resin A _r as defined in the present application.

According to one embodiment, the surface coating is a paint, a varnish, or an ink.

The examples below are given by way of illustration and not by way of limitation.

FIGURES

[Fig.1] Figure 1 relates to a reaction mechanism of the thiolene reaction according to the present application, with the photoinitiator TPO and bisaryliodonium as coinitiator.

[Fig.2] Figure 2 illustrates a photopolymerization profile of unsaturations (curve A) and thiol functions (curve B) for the VAF 474 / TRIS / BAPO / S938 system (87.5% / 7.5% / 3 % / 2% w / w / w / w) in air: A) Unsaturations [1647; 1666] cm ^-1 ; B) SH [2552; 2599] cm ^-1 . Irradiation with an LED @ 405nm (I = 110mW / cm ² ). Irradiation starts for t = 10 sec.

EXAMPLES

Example 1: Comparison of the reaction kinetics in the presence of the photoinitiator (BAPO) in the presence and in the absence of Co-initiator (CoA) (SC938)

Material:

The monomer reaction mixture is composed of Vialkyd TS 354 / 70WS (hereafter simplified V70WS), trimethylolpropane tris (3-mercaptopropionate) (denoted TRIS). The photoinitiator system will be based on bis (2,4,6-trimethylbenzoyl) phenylphosphineoxide oxide (Irgacure 819-BAPO) and bis- (4-t-butylphenyl) -iodonium hexafluorophosphate (Speedcure-938). V70WS alkyd is from Allnex. TRIS, on the other hand, is a product from Sigma Aldrich. The iodonium salt, S938, is from Lambson and the BAPO photoinitiator is from IGM Resins.

Method:

The polymerization of this reaction mixture is monitored by Fourier Transformer Infrared Spectroscopy. The kinetics are carried out under the following conditions:

• LED @ 405nm, I = 110 mW / cm ²

• A calibrated 24 μm bar to fix the film thickness

• A BaF2 pellet as a substrate

• Under air

Unsaturations are the reactive functions of alkyds and those of TRIS are thiols. These functions are observed in RT-FTIR in the interval [1647; 1666] cm ^-1 specific to unsaturations and that [2552; 2599] cm-1 characteristic of thiol functions.

The crosslinking of the alkyd resin V70WS was carried out in the presence of the crosslinking agent TRIS, of the photoinitiator BAPO, either in the presence or in the absence of co-initiator SC938 (V70WS / Tris 88.5% / 8.5% w / w; BAPO / SC938 2% / 1% w / w).

The results are reported in Table 1.

[Table 1]

Table 1. Comparison of BAPO / SC938 vs. BAPO alone for the crosslinking of alkyd resins The results show that the presence of the coinitiator is necessary to achieve a rapid and complete conversion of the unsaturations of the alkyd resin. In the absence of SC938, a low conversion is obtained under the same conditions.

Example 2: Photopolymerization of Vialkyd AF 474 / 55WS

Vialkyd AF 474 / 55WS is a viscous resin also belonging to the category of “long in oil” alkyds. Its viscosity is between 10,000 and 180,000 cP at 23 ° C.

• VAF 474 / TRIS / BAPO / S938 (87.5% / 7.5% / 3% / 2% w / w / w / w)

• LED @ 405nm, I = 110 mW / cm ²

• Irradiation started at t = 10 sec.

• A calibrated 24 μm bar to fix the film thickness

• A BaF2 pellet as a substrate

• Under air

According to Figure 2, the VAF 474 resin is very favorable to the formation of a crosslinked network under irradiation at 405 nm. Indeed, excellent conversion rates were obtained for the unsaturations and for the thiols. After irradiation, the polymer obtained is completely “tack-free”, that is to say dry to the touch.

No inhibition time was observed during the polymerization of this system. In less than 50 seconds, a maximum rate of 100% was reached for the unsaturations and a very high value of 70% for the thiols.

Claims

1. Process for crosslinking an unsaturated alkyd resin A by radical polymerization comprising the steps of: i) bringing an unsaturated alkyd resin A into contact with a polythiol T as a crosslinking agent, in the presence of: a. a radical photoinitiator (PA) absorbing light at a wavelength l within the UV or visible range, and b. an iodonium salt as a radical co-initiator (coA); ii) Irradiation of the reaction mixture obtained in step i) at the wavelength λ; and optionally iii) recovery of the crosslinked alkyd resin obtained.

2. Method according to any one of the preceding claims, in which the radical co-initiator (CoA) is a diaryliodonium.

3. Method according to claim 2, wherein the diaryliodonium is selected from bis- (4-t-butylphenyl) -iodonium hexafluorophosphate, or diphenyliodonium, 4-1- (methylethyl) phenyl) - (4-methylphenyl) iodonium tetrakis (pentafluorophenyl) borate.

4. Method according to any one of the preceding claims, in which the wavelength λ is between 300 and 500 nm, preferably between 350 and 450 nm.

5. Method according to any one of the preceding claims, in which the radical photoinitiator PA is chosen from type I or type II photoinitiators.

6. The method of claim 5 wherein the photoinitiator is a type I photoinitiator, in particular a phosphine oxide.

7. The method of claim 6, wherein the radical photoinitiator PA is chosen from bis (2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO), and triphenylphosphine oxide (TPO).

8. Method according to any one of the preceding claims, in which the molar ratio of photoinitiator PA relative to coA coinitiator is between 0.5 / 1 and 1 / 0.5.

9. A method according to any one of claims 1 to 8, wherein the repeating unit of the unsaturated alkyd resin A comprises an aliphatic chain, said chain comprising at least two unsaturations.

10. A method according to any preceding claim, wherein the agent

The crosslinking agent T is selected from trimethylolpropane tris (3-mercaptopropionate), tetra (pentaerythritol 3-mercaptopropionate), ethoxyliers trimethylolpropan tri (3-mercaptopropionate).

11. Method according to any one of the preceding claims, in which the A / T weight ratio is between 15/1 and 5/1, and is preferably about 10/1.

12. Process according to any one of the preceding claims, in which, in step ii), the reaction mixture is irradiated with an irradiation intensity of between 10 mW / cm ² and 20 W / cm ² .

13. Crosslinked alkyd resin A _r obtainable according to the process as defined in claims 1 to 12.

14. Alkyd resin according to claim 13, characterized in that it comprises residues of the iodonium salt used as a coinitiator.

15. A photocrosslinkable composition comprising: an unsaturated alkyd resin (A); a thiol-based crosslinking agent (T);

50 a radical photoinitiator (PA) absorbing light at a wavelength λ included in the UV or visible range, and an iodonium salt as a radical co-initiator (CoA).

16. Paint, ink or varnish comprising a photo-crosslinkable composition as defined in claim 15.

55 17. A method of coating a surface of a solid support with a crosslinked alkyd resin A _r comprising the steps of: i) Application in the form of a layer, to the surface of a solid support, of a photo-crosslinkable composition as defined in claim 15; ii) Irradiation of the surface obtained in step i) whereby a support is obtained

60 solid whose surface is coated with a crosslinked alkyd resin.

18. A surface coating comprising a crosslinked alkyd resin A _r as defined in claim 13.

19. Coating according to claim 18 characterized in that it is a paint, a varnish, or an ink.