Use of a compound according to formula i as heat sensitizer, thermosensitive composition and recording material for thermal printing

Abstract

The present invention relates to the use of a compound of formula (I), wherein R1 and R3 are hydrogen R2 and R4 are independently selected from the group consisting of hydrogen, a linear or branched C1 to C5 alkyl, C2 to C5 alkenyl, linear or branched C1 to C5 alkoxy as heat sensitizer in heat applications with leuco dyes.

Description

[0001] USE OF A COMPOUND ACCORDING TO FORMULA I AS HEAT SENSITIZER, THERMOSENSITIVE COMPOSITION AND RECORDING MATERIAL FOR THERMAL PRINTING

[0002] The present invention relates to the use of a compound as thermal sensitizer in heat applications with leuco dyes.

[0003] Thermal imaging technology, a pivotal innovation in the realm of material science and engineering, has revolutionized the way we visualize and interact with thermal energy in our daily lives. Leveraging the principles of thermography, this technology has found a unique and valuable application in the production of thermal papers— a specialized segment of the paper manufacturing industry that caters to a wide array of commercial printing needs. Thermal papers serve as the medium for instant printing processes that are critical for sectors ranging from retail to transportation, offering a rapid and accurate method of capturing and displaying variable information such as transaction records, travel itineraries, product labeling, and other vital data.

[0004] The quintessential element of thermal imaging technology in the context of thermal paper lies in its ingenious use of heat to induce a chemical reaction that results in a visible representation of information. At the heart of this system are the coated layers applied to the paper substrate. Primarily, thermal papers encompass a support layer, for example a paper layer, that is treated with a colorforming layer. This cover-forming layer comprises color former that remain inert until they are activated by proton transfer from the color developer to the dye upon heating. These color formers, also known as leuco dyes, belong to a class of dyes capable of switching between two states: a colorless (leuco) form and a colored form. When subjected to the localized heat from a thermal printer's print head, color developers transfer one or more of their hydrogen atoms to the dye. This protonation induces a structural change in the leuco dye, activating its chromophoric (color-producing) properties, therefore producing a sharp and durable color change. This eliminates the need for traditional ink-based printing methods and allows for the delivery of high-quality, on-demand printed content.

[0005] In addition to various other chemical additives, the thermosensitive composition for thermal printing primarily relies on sensitizers to enhance sensitivity and efficiency. Sensitizers are crucial in adjusting the precise temperature at which thermal paper changes color by facilitating the contact between dye and developer during their melting process. This contact promotes the proton transfer necessary for color development. By lowering the activation energy required, sensitizers enable high-speed printing. The majority of traditional sensitizers are derived from intense synthetic processes that rely on fossil resources and often contain an aromatic ring structure. Their activity is associated with the aryl hydrocarbon receptor. Pollution from paper recycling facilities has been traced back to such sensitizers, indicating their lingering presence in the environment: their accumulation in aquatic life might subsequently disrupt ecosystems. US 4202566 discloses a heat-sensitive recording or copying materials which contain, in their color reactant system, as developers for the color-forming agent, at least mono- or poly-aldehyde which is electronegatively substituted, especially by halogen, and / or the reaction products thereof with an organic compound containing hydroxyl groups, or the precursors thereof.

[0006] JPH02297484 describes a thermosensitive recording material that operates on the principle of thermally induced melting and coloring of a non-carboxylic acid ester using an electron-donating, colorless color-developing agent in conjunction with an electron-accepting color-developing substance.

[0007] Typical examples of significant conventional sensitizers are diphenylsulfone, 1 ,1 -di(4- methylphenyljethane, 1 ,2-bis(3-methylphenoxy)ethane, 1 ,2-diphenoxy-ethane, benzyl-2- naphthylether. Traces of said toxic compounds have been recently found in recycled toilet paper, and river water, emphasizing the environmental footprint they leave behind.

[0008] The problem of the present invention is therefore to provide a compound which is bio-based, biodegradable and non-toxic and exhibits good heat sensitizing activities.

[0009] The problem is solved by the use of a compound according to claim 1 . Further preferred embodiments are subject of dependent claims 2 to 9.

[0010] Interestingly, it has been discovered that the compound of formula (I) has outstanding characteristics as a thermal sensitizer (also called heat sensitizer) in heat applications with leuco dyes.

[0011] Compound of formula (I) has the following structure: wherein Ri and R3 are hydrogen;

[0012] R2 and R4 are independently selected from the group consisting of hydrogen, a linear or branched Ci to C5 alkyl, C2 to C5 alkenyl, and linear or branched Ci to C5 alkoxy.

[0013] Compound of formula I is bio-based, biodegradable, non-toxic, and either white or colorless, with a straightforward synthesis process. It significantly boosts the static sensitivity of thermal applications by facilitating a stronger interaction between the leuco dye and color developers in thermal coatings.

[0014] Furthermore, as the compound according to the present invention can be produced directly from lignocellulosic biomass, its production is likely both sustainable and economically advantageous. The production process is disclosed in detail, for example, in WO2022223480. The polar aprotic compound of formula I engages favorably with the thermal paper matrix.

[0015] Within the context of the present invention the term “Ci to C5 linear or branched alkyl” means straight or branched chain alkyl groups having from 1 to 5 carbon atoms and includes for example, methyl, ethyl, n-propyl, isopropyl, butyl, isobutyl, secondary butyl, tertiary butyl, n-pentyl, isopentyl, neopentyl, and the like.

[0016] Within the context of the present invention, the term "C2 to C5 alkenyl" refers to unsaturated hydrocarbon groups with a chain of carbon atoms ranging from 2 to 5 and containing at least one double bond. These alkenyl groups can be straight or branched chains and include, for example, ethenyl (vinyl), propenyl (allyl), butenyl, pentenyl, and their corresponding branched isomers.

[0017] Within the context of the present invention the term “alkoxy” means a radical — OA1 where A1 is an alkyl as defined herein. Examples of alkoxy groups are methoxy, ethoxy and propoxy. Within the context of the present invention, the term "in heat applications with leuco dyes" means the use of leuco dyes in systems or processes where the application of heat induces a reversible or irreversible color change. This involves leuco dyes that, upon exposure to thermal energy, undergo an acido-basic chemical reaction resulting in a visible change in coloration, thereby enabling functionalities such as temperature indication, thermochromic printing, or other heat-responsive visual effects.

[0018] In a preferred embodiment of the present invention R2 and R4 in compound of formula I are independently selected from the group consisting of hydrogen, methyl, ethyl, propyl, isopropyl, methoxy, ethoxy and propoxy. Said compounds are compatible with a broad spectrum of widely used thermal color developers. Additionally, they display an elevated level of solubility, which in turn, correlates with an enhancement in color development performance for diverse applications.

[0019] An additional aspect of the invention pertains to the use of a compound of formula I wherein R2 and R4 are identical. Such a configuration allows for a simplified synthetic process because the compound can be derived through a reaction with a single type of aldehyde. This streamlined synthesis not only reduces costs but frequently results in higher yields. Furthermore, it has been established that compounds with this structure function effectively as heat sensitizers.

[0020] In a further embodiment of the present invention R2 and R4 in compound of formula I are hydrogen. Using the Aldehyde Assisted Fractionation (AAF) diformylxylose (DFX) can be produced in one-pot with high yield. DFX showed good results, in particular with lignin-derived color developers, which are described in detail below. Furthermore, DFX is simultaneously non-toxic and non-mutagenic.

[0021] For conventional color developers, it is advantageous to adjust the polarity of the compound of formula I. This can be achieved by increasing the acetal component's carbon chain length, i.e. the length of R2 and R4. In this case, R2 and R4 are preferably both propyl, butyl or pentyl, most preferably butyl. Such a modification makes the compound according to the present invention a beneficial platform for a variety of developers, encompassing commercial applications. In relation to the static sensitivity of thermal paper formulations, for example dibutyl xylose (DBX), yielded positive outcomes for both conventional color developers bisphenol A (BPA) and bisphenol S (BPS). Especially good results could be obtained with compounds selected from the group consisting of compounds 1 to 10: A further aspect of the present invention relates to the use of the sensitizer, wherein the heat application is selected from the group consisting of thermal recording technologies, textile industry, adhesive industry and coatings, preferably thermal recording technologies. Due to its white or colorless attribute, the compound of the present invention assures the aesthetic integrity of products by preventing any undesired color impacts, particularly vital in applications like thermal recording where paper whiteness is a benchmark for quality. Its well-aligned polarity with other industry materials further ensures seamless integration into existing formulations, promoting homogeneous mixtures and consistent thermal responsiveness across various applications. A further aspect of the present invention relates to a thermosensitive composition that can be utilized for coating a support material to yield thermosensitive recording materials. This composition comprises at least a color former in the form of a leuco dye, a color developer, and a compound represented by formula I, which serves as a sensitizer. When applied to the support material for use in thermal printing, the thermosensitive composition typically includes both solid and liquid components. The liquid component, which is primarily a solvent, is used during the coating process to facilitate a uniform application of the thermosensitive layer onto the support material. After the coating is applied, the liquid evaporates, leaving behind the solid components that constitute the functional thermosensitive layer.

[0022] In one embodiment of the present invention, the thermosensitive composition comprises at least the following ingredients:

[0023] - a color former (leuco dye) in an amount of 1 -15% by weight of the solid components.

[0024] - a color developer in an amount of 1 -25% by weight of the solid components.

[0025] - a compound of formula I as sensitizer in an amount of 1 -55% by weight of the solid components. The sensitizer can also act as co-binder.

[0026] Further, the composition can additionally comprise compounds such as binders, fillers, and stabilizers. The binder can be present in an amount of 10-40% by weight of the solid components, and fillers and stabilizers can be present in an amount of 30-50% by weight of the solid components.

[0027] The liquid component typically comprises 50-70% by weight of the total composition during the coating process. Preferred solvents include water or organic solvents such as ethanol, isopropanol, or acetone or mixtures thereof. The solvent facilitates the mixing and application of the solid components onto the support material. After the coating is applied, the solvent evaporates, leaving behind the solid thermosensitive layer.

[0028] In the context of thermal recording technologies, leuco dyes play a crucial role. When heated, these dyes undergo a reversible acido-basic chemical reaction that alters their molecular structure, allowing them to switch between a colored state and a colorless or differently colored state. This transformation is typically caused by the opening or closing of a molecular ring structure, which in turn changes how the molecule absorbs and reflects light. The incorporation of the thermal sensitizer of the present invention not only enhances the performance of these leuco dyes by optimizing their thermal responsiveness but also maintains the aesthetic quality of the final product due to its white or colorless nature. This property ensures that the background remains visually appealing and does not interfere with the legibility or clarity of printed information, which is particularly important in applications like thermal printing, where high-quality output is essential. Preferred leuco dyes include but are not limited to: 3-diethylamino-6-methylfluoran, 3-dimethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6- methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino) fluoran, 3-diethylamino-6- methyl-7-chlorofluoran, 3-diethylamino-6-methyl-7-(3-trifluoromethylanilino) fluoran, 3-diethylamino-6- methyl-7-(2-chloroanilino) fluoran, 3-diethylamino-6-methyl-7-(4-chloroanilino) fluoran, 3- diethylamino-6-methyl-7-(2-fluoroanilino) fluoran, 3-diethylamino-6-methyl-7-(4-n-octylanilino) fluoran, 3-diethylamino -7-(4-n-octylanilino) fluoran, 3-diethylamino -7-(n-octylamino) fluoran, 3-diethylamino - 7-(dibenzylamino) fluoran, 3-diethylamino-6-methyl-7-(dibenzylamino) fluoran, 3-diethylamino-6- chloro-7-methylfluoran, 3-diethylamino-7-t-butylfluoran, 3-diethylamino -7-carboxyethylfluoran, 3- diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-methyl-7-(3-methylanilino) fluoran, 3- diethylamino-6-methyl-7-(4-methylanilino) fluoran, 3-diethylamino-6-ethoxyethyl-7-anilinofluoran, 3- diethylamino-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(3- trifluoromethylanilino) fluoran, 3-diethylamino-7-(2-chloroanilino) fluoran, 3-diethylamino-7-(2- fluoroanilino) fluoran, 3-diethylamino-benzo[a] fluoran, 3-diethylamino-benzo[c] fluoran, 3- dibutylamino-7-dibenzylaminofluoran , 3-dibutylamino-7-anilinofluoran , 3-diethylamino-7- anilinofluoran, 3-dibutylamino-6-methyl fluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3- dibutylamino-6-methyl-7-(2,4-dimethylanilino) fluoran, 3-dibutylamino-6-methyl-7-(2-chloroanilino) fluoran, 3-dibutylamino-6-methyl-7-(4-chloroanilino) fluoran, 3-dibutylamino-6-methyl-7-(2- fluoroanilino) fluoran, 3-dibutylamino-6-methyl-7-(3-trifluoromethylanilino) fluoran, 3-dibutylamino-6- ethoxyethyl-7-anilinofluoran, 3-dibutylamino-6-chloro-anilinofluoran, 3-dibutylamino-6-methyl-7-(4- methylanilino) fluoran, 3-dibutylamino-7-(2-chloroanilino) fluoran, 3-dibutylamino-7-(2-fluoroanilino) fluoran, 3-dibutylamino-7-(N-methyl-N-formylamino) fluoran, 3-dipentylamino-6-methyl-7- anilinofluoran, 3-dipentylamino-6-methyl-7-(2-chloroanilino) fluoran, 3-dipentylamino-7-(3- trifluoromethylanilino) fluoran, 3-dipentylamino-6-chloro-7-anilinofluoran, 3-dipentylamino-7-(4- chloroanilino) fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexylamino)-6-methyl-7- anilinofluoran, 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6- methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N- isoamylamino)-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7- anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-(N-butyl-N-isoamylamino)-6- methyl-7-anilinofluoran, 3-(N-isopropyl-N-3-pentylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N- ethoxypropylamino)-6-methyl-7-anilinofluoran, 3-cyclohexylamino-6-chlorofluoran, 2-methyl-6-p-(p- dimethylaminophenyl)aminoanilinofluoran, 2-methoxy-6-p-(p-dimethylaminophenyl)- aminoanilinofluoran, 2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2- diethylamino-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-phenyl-6-methyl— 6-p-(p- phenylaminophenyl)aminoanilinofluoran, 2-benzyl-6-p-(p-phenylaminophenyl)amino-anilinofluoran, 3- methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p- diethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-dibutylaminophenyl)- aminoanilinofluoran, 2,4-dimethyl-6-[(4-dimethylamino)-anilino] fluoran, 3-[(4-dimethyl- aminophenyl)amino]-5,7-dimethylfluoran, 3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide], 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide], 3,3-bis(p-dimethylamino-phenyl)-6- dimethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)phthalide, 3,3-bis-[2-(p- dimethyl aminophenyl)-2-(p-methoxyphenyl)ethenyl-4, 5, 6, 7-tetrabromophthalide, 3,3-bis-[2-(p- dimethyl aminophenyl)-2-(p-methoxyphenyl)ethenyl-4, 5, 6, 7-tetrachlorophthalide, 3,3-bis[1 ,1-bis(4- pyrrolidinophenyl)ethylene-2-yl]-4, 5, 6, 7-tetrabromophthalide, 3,3-bis-[1-(4-methoxyphenyl)-1-(4- pyrridinophenyl)ethylene-2-yl]-4, 5, 6, 7-tetrachlorophthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1 - ethyl-2-methylindole-3-yl)-4-azaphthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2- methylindole-3-yl)-4-azaphthalide, 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2- methyl indole-3-y l)-4-azap hth al ide, 3, 3-bis (1 -ethyl-2-methyli ndole-3-yl) phthalide, 3, 3-bis (1 -octyl-2- methylindole-3-yl) phthalide, , 3-diethylamino-6,8-dimethylfluoran, 3-diethylamino-7,8-benzofluoran, 3-diethyl-aminofluoran-7-carboxylic acid ethyl ester, 3-[N-(4-methylphenyl)-N-ethylamino]-7- methylfluoran, crystal violet and mixtures thereof.

[0029] Representative examples of color developers are bisphenol A and bisphenol S.

[0030] A further preferred embodiment of the present invention relates to a thermosensitive composition wherein the color developer is a lignin fragment or a compound selected from the group consisting of compounds 101 to 119:

[0031] As described in co-pending application EP 24183915.8 and EP 24183917.4, lignin fragments obtained by Sequential Aldehyde Assisted Fractionation (SAAF) or compound 101 to 119 which can be obtained for example by Aldehyde Assisted Fractionation followed by reductive depolymerization, (trans-) esterification and possible further chemical modification, can be used as color developer. Thus, both, sensitizer and color developer can be obtained from (ligninocellulosic) biomass. This allows for the preparation of thermosensitive compositions based on ecologically safe raw materials in an economical manner, without having to compromise on quality. Such compositions can seamlessly replace conventional materials, offering the same functionality. The use of lignin-derived color developers yields particularly favorable results when combined with compound DFX. A further aspect of the present invention relates to a recording material for thermal printing comprising a thermosensitive composition including the compound of formula I as sensitizer. In this context, the support material refers to the substrate or base material used for printing. This support material includes paper, wood-free paper made from non-chlorine bleached pulp, base paper containing waste paper plastic films, and synthetic paper that can be coated with the thermosensitive composition. The recording material according to the present invention can be prepared by coating a support layer as mentioned above with a thermosensitive composition. This composition generally includes a color former in form of a leuco dye, a color developer, and a sensitizer. When heat is applied to the thermal paper via the thermal print head, the color developer reacts with the color former. This chemical reaction results in a visible color change, producing the printed image. During printing, the thermal print head selectively heats specific areas of the paper. The heat causes the color former and the color developer to interact, leading to the formation of a colored image. The presence of the sensitizer in the thermosensitive composition is crucial as it amplifies the reactivity between the color former and the color developer, thereby reducing the activation energy needed for the color-developing reaction and facilitating swifter and sharper image creation at reduced temperatures.

[0032] The support material must be compatible with the thermal printing process, possessing appropriate thermal sensitivity, durability, and surface properties to ensure high-quality printing. The incorporation of the sensitizer according to the present invention into the coating composition enhances the paper's ability to produce clear, sharp images when heated.

[0033] Figures:

[0034] Figures 1A-a / 1 A-b / 1 B-a / 1 B-b / 1C-a / 1C-b show static sensitivity curves of thermal papers coated (a) without sensitizer (Figure 1 A-b), (b) with benzalphtalide (Figure 1 B-b) and (c) with DFX as sensitizer (Figure 1 C-b) (56 wt%). The pictures (Figures 1A-a / 1 B-a / 1 C-a) show developed black-colored images of these thermal papers for various heating temperatures.

[0035] Figure 2 shows static sensitivity curves of thermal papers coated with DFX (21 , 35 wt%) as sensitizer and Propionaldehyde (PA)-protected lignin as developer extracted from birch or pine wood, the Sequential Aldehyde-Assisted Fractionation (SAAF) process.

[0036] Figure 3 shows DFX sensitizer; Isobutyraldehyde (IBA)-protected lignin, extracted from pine wood, using the Sequential Aldehyde-Assisted Fractionation (SAAF) process as color developer and red and black fluoran dye. Figure 4A / 4B shows the static sensitivity for (a) BPA (Figure 4A) and (b) BPS (Figure 4B) with three sugar acetal sensitizers.

[0037] Figure 5 shows an agonistic estrogenic activity test - DFX shows absence of agonistic estrogenic activity at the tested concentrations.

[0038] Figure 6 shows a test for toxicity to freshwater algae (algae growth inhibtion). No inhibtion is observed for the DFX molecule at the tested concentrations.

[0039] Figure ? shows a microtoxicology test: bioluminescence in Aliivibrio fischeri. No bioluminiscence inhibtion activity for the DFX molecule was detected at the tested concentrations.

[0040] Examples:

[0041] Example 1 : Preparation of diformyl xylose (DFX)

[0042] Diformyl xylose was prepared according to WO2022223480A1 . D-xylose (15 g, 0.1 mol, 1.0 equiv.) and paraformaldehyde (7.5 g, equivalent to 0.25 mol formaldehyde, 2.5 equiv.) were added to 2-Me- THF (75 mL) in a round bottom flask. Then, H2SO4 (98 wt%, 2.46 mL, 0.045 mol, 0.45 equiv.) was added drop-wise with stirring to avoid the localized concentration of acid, which can degrade the sugar. The mixture was then heated to 80 °C for 3h with stirring. The resulting solution was cooled to room temperature (-23-25 °C), neutralized with sodium hydroxide saturated aqueous solution, filtered, and concentrated in vacuo using a rotary evaporator with a bath temperature of 45 °C. The residue was crystallized directly and washed with ethanol while filtering to remove impurities and by-products. The resulting DFX product is white crystalline solid (>98% pure by 1 H-NMR and GC-FID).

[0043] Alternatively, if the residue is not crystallizing from the final oil, the following workup can be done. Extract the residue three times with 100 ml of ethyl acetate (or 50 ml of cyclopentyl methyl ether) and 25 ml of water in a separatory funnel. The resulting solution can be distilled at 80 °C, under reduced pressure (0.02 mbar) to obtain a light yellow solid. The solid can then be recrystallized in ethanol and dried in a vacuum desiccator, yielding the DFX as a white crystalline solid (98% pure by 1 H-NMR and GC-FID). Preparation of lignin fragments

[0044] Light-colored lignin fragments were prepared according to the procedure disclosed in co-pending European application No. 24183915.8.

[0045] 4.5 grams of the extracted and dried biomass was weighed and transferred into a 100-mL roundbottom flask, which contained an oval PTFE-coated stir bar. Into the flask, 4.8 mL of propionaldehyde (67 mmol, 6.6 equivalents), 25 mL of 1 ,4-dioxane, and 0.85 mL of 37% (wt / wt) hydrochloric acid (10 mmol, 1 .0 equivalent) were sequentially added. A condenser was then attached to the flask, connected to a source of cooling water, and a gas bubbler was fitted at the top of the reflux condenser to create an air lock. The mixture was heated to 85 degrees Celsius with stirring for 30 minutes and thereafter allowed to cool to room temperature.

[0046] A filtration apparatus, consisting of a 250-mL filter flask, a neoprene adapter, and a Buchner funnel with a ground-glass frit of porosity grade 3, was assembled. The reaction was filtered to collect the cellulose-rich solid, which was then washed with dioxane in two portions of 10 mL each, resulting in liquor number 1 . This liquor was concentrated using a rotary evaporator at a 40 degrees Celsius bath temperature and a final pressure of 80 mbar, removing the hydrochloric acid, aldehyde, and dioxane.

[0047] To the resulting dark-brown oil, 10 mL of ethyl acetate was added. The solution was then added dropwise with a pipette, rinsing with an additional 5 mL of ethyl acetate, into a 500-mL Erlenmeyer flask containing 250 mL of hexanes that was being stirred at 700 rpm by a bar-type PTFE-coated stir bar. Upon addition, a dark purple precipitate formed. The hexanes solution was filtered through a filtration apparatus and washed with more hexanes. To the filter cake, 50 mL of diethyl ether was added and the mixture was sonicated for 5 minutes. The diethyl ether solution was then filtered, the filter cake was recovered and dried overnight in a vacuum oven to yield Lignin number 1 .

[0048] The solutions of hexanes and diethyl ether that had been filtered were concentrated in vacuo using a rotary evaporator at a 40 degrees Celsius bath temperature and a final pressure of 25 mbar; this yielded the Propylated C5-sugar collection number 1.

[0049] Following the protocol, the mass of the cellulose-rich solid recovered from the filtration step was weighed and placed into a 100-mL round-bottom flask containing an oval PTFE-coated stir bar. Appropriate amounts of propionaldehyde, 1 ,4-dioxane, and hydrochloric acid were added to the flask, with the quantities adjusted based on the recovered mass of cellulose-rich solid. A condenser equipped with a cooling water source and a gas bubbler was then fitted onto the flask.

[0050] The reaction was again heated to 85 degrees Celsius with stirring for 30 minutes, then cooled to room temperature. The same filtration apparatus was used to filter the reaction and collect the washed cellulose-rich solid, producing liquor number 2. This liquor was concentrated to remove hydrochloric acid, aldehyde, and dioxane using a rotary evaporator set as before. Ethyl acetate was added to the dark-brown oil and the ensuing solution was introduced dropwise into hexanes under stirring, leading to the formation of a precipitate and the creation of Lignin number 2 after filtration and drying.

[0051] Lastly, the mixtures of hexanes and diethyl ether were once again concentrated in vacuo, this time to obtain Propylated C5-sugar collection number 2. The entire procedure was repeated until a total extraction time of 3 hours was achieved.

[0052] It was shown that said lignin fragments can be used as a color developer. PA Pine stands for feedstock pine and propionaldehyde and IBA Pine stand for feedstock pine and isobutyraldehyde.

[0053] Example 3: Preparation of lignin-derived esters a. Molecule 101 (H-ester)

[0054] Dihydro-p_'-coumaric acid (1 g, 6.02 mmol, 1.00 eq) was combined with 3-(4-hydroxyphenyl)-1- propanol (0.92 g, 6.02 mmol, 1.00 eq), p-toluenesulfonic acid monohydrate (57 mg, 0.3 mmol, 0.05 eq.) and toluene (60 mL) in a 3-neck 100 mL round-bottom flask of a Dean-Stark set-up. The mixture was heated to 120 °C using an oil bath under N2-flow and with magnetic stirring (400 rpm using a PTFE coated stir-bar) for 6 hours. The reaction progress was followed by Thin Layer Chromatography (50:50 hexane:ethyl acetate). After full conversion, the reaction was stopped and the solvent was removed under reduced pressure. The crude was then diluted with dichloromethane, washed 1x with 0.1 M NaHCO3 solution, 1x demineralized water and 1x brine. The organic phase was then concentrated under reduced pressure to yield the molecule 101 as an off-white solid (96% isolated yield). The purity (>98 wt.%) of the final product was confirmed by quantitative 1 H NMR with 1 , 2,4,5- terachloro-3-nitrobenzene as the internal standard. b. Molecule 102 (G ester)

[0055] Dihydroferulic acid (1 g, 5.10 mmol, 1.00 eq) was combined with dihydroconiferyl alcohol (0.93 g, 5.10 mmol, 1.00 eq), p-toluenesulfonic acid monohydrate (48 mg, 0.25 mmol, 0.05 eq.) and toluene (60 mL) in a 3-neck 100 mL round-bottom flask of a Dean-Stark set-up. The mixture was heated to 120 °C using an oil bath under N2-flow and with magnetic stirring (400 rpm using a PTFE coated stir-bar) for 6 hours. The reaction progress was followed by Thin Layer Chromatography (50:50 hexane:ethyl acetate). After full conversion, the reaction was stopped and the solvent was removed under reduced pressure. The crude was then diluted with dichloromethane, washed 1x with 0.1 M NaHCO3 solution, 1x demineralized water and 1x brine. The organic phase was then concentrated under reduced pressure to yield molecule 102 as a viscous yellowish oil (90% isolated yield). The purity of the solid was determined by 1 H-NMR and GC-MS. In order to render to remove trace impurities, the oil got purified by Silica-gel Flash Column Chromatography, yielding molecule 102 as a transparent viscous oil. The purity (>98 wt.%) of the final product was confirmed by quantitative 1 H NMR with 1 , 2,4,5- terachloro-3-nitrobenzene as the internal standard.

[0056] It was shown that said lignin derived esters can be used as a color developer.

[0057] Example 4: Preparation of Oligomer Beech

[0058] Lignin was extracted from 25kg of beech wood using an aldehyde (Propionaldehyde, formaldehyde, glyoxylic acid) acid in methyl-THF or dioxane. 1 kg of extracted lignin was then depolymerized in a 10L- PARR reactor with ethanol solvent under hydrogen pressure using Ru / C as a catalyst. After depolymerization, the obtained oil was concentrated under reduced pressure to remove ethanol (50°C, 20mbar). A distillation column was then used up to 210°C and 0.2mbar to distill the monomers and recover the oligomers remaining in the flask.

[0059] Example 5: Assessment of Static Sensitivity of Developers on Paper

[0060] The static sensitivity of developers on paper was evaluated using a simplified thermal paper formulation. This assessment involved the determination of the initial development temperature necessary to elicit a color change on the paper substrate, a key parameter tailored to specific application needs. Poly(vi nyl alcohol)(PVA) solutions containing developer, dye ODB-2, sensitizers benzalphtalyde, and additives (calcium carbonate and zinc stearate) were coated on white paper (2:1 weight ratio, average coating thickness of 70 pm). In order to assess the static sensitivity, the coated paper's color density (C.D.) was measured as the temperature was gradually increased. The paper was exposed to temperatures ranging from room temperature to 140°C in 20°C increments using a temperature- controlled heat gun, as depicted in Figure 1 a.

[0061] A commercial sensitizer, benzalphthalide, known for its aromatic structure and melting point around 100°C, was chosen for its anticipated compatibility with phenolic developers, facilitated by interactions such as TT-TT-electron stacking. To enhance processability, additives such as CaCC and zinc stearate were incorporated into the formulation. Static sensitivity tests were performed, and significant color development was observed with all lignin materials disclosed in Examples 2, 3 and 4. This confirmed the improved interaction between OBD-2 and lignin in the presence of the benzalphthalide sensitizer, as shown in Figure 1 b.

[0062] All the solid ingredients (developer, dye, CaCOs, zinc stearate and sensitizer) were individually ground with a mortar for 5 min to decrease particle size to approximately 1000 pm. Then, an aqueous solution of poly vinyl alcohol (PVA) with a solid content of 30 wt% was prepared by dissolving Mowiol(R) 4-88 (Molecular weight « 38,000,300 mg) in distilled water (700 mg) overnight. In parallel, an aqueous CaCC solution was prepared by manually mixing CaCCh (80 mg), PVA 30 wt% (75 mg) and distilled water (150 mg) for 20 minutes until a homogeneous white solution was formed. Following these preparative steps, developer (50 mg) was mixed with the aqueous 30 wt% PVA solution (80 mg), followed by the addition of distilled water (152 mg), CaCCh solution (300 mg), zinc stearate (20 mg), and sensitizer benzalphtalide (50 mg). This formulation was then mixed for another 5 min. Finally, the dye was incorporated (22 mg, dye:developer mass ratio 1 :2) and the total solid content was maintained to 40 wt%. The final solution was applied to white paper using a U-coater, achieving a coating thickness within the range of 70-80 pm. This commercial formulation was also performed with another petroleum-based sensitizer, diphenyl sulphone.

[0063] Example 6: Alternative Bio-based Sensitizers

[0064] The use of compounds of formula I as potential bio-based sensitizers was examined. Specifically, diformylxylose (DFX), derived from hemicellulose fractions of e.g. corn cobs during aldehyde assisted fractionation (AAF) or directly from D-xylose, was investigated (see example 1). DFX's biodegradability and physical properties, such as its melting point of 48°C, made it a promising substitute for petroleumbased sensitizers.

[0065] For performance evaluation, three formulations with varying DFX concentrations (21 , 35, and 56 wt% by weight of the solid components) were prepared with PA lignin from Birch and Pine as the developer. The relationship between color density and DFX loading was studied and presented in Figure 1c (56 wt% by weight of the solid components) and Figure 2 (21 ,35 wt%). Results indicated a positive correlation between increased DFX content and higher C.D., with the 56 wt% (by weight of the solid components) DFX formulation achieving already superb performance.

[0066] Even though the DFX coating's performance was slightly lower than that of conventional sensitizers, it displayed a C.D. above 1 at 140°C for all lignin formulations and exceeded 0.8 for the oligomers, This performance situates the thermal paper within the commercialize range (C.D. commercial range= 0.8-1.5).

[0067] The coatings were prepared according to the protocol described in Example 5, utilizing the weight percentages specified in the tables below.

[0068] Example 7: Application of DFX-Based Coating

[0069] The thermosensitive composition incorporating the DFX sensitizer according to the present invention and "IBA lignin" developer were applied to create high-contrast images, such as the EPFL logo, using a poly(methylmethacrylate) mold and a thermal gun heater. The successful rendering of the logo with good contrast is evidenced in Figure 3. Furthermore, the versatility of the DFX-based formulation was demonstrated by its effective application to a red dye, suggesting compatibility with various fluoran dyes beyond the black ODB-2. The coating was prepared according to the formulation described in Example 6, with 56 weight % of DFX. Example s:

[0070] For conventional color developers, it was observed that DFX exhibited undesired background activity at room temperature. However, it was found that this issue could be remedied by manipulating the polarity of the acetal-stabilized sugar sensitizer. Specifically, the alteration involved extending the carbon chain length of the acetal components. Through this adjustment, acetal-functionalized sugars were rendered an advantageous substrate for numerous developers, including those in commercial use. When assessing static sensitivity in the formulation of thermal paper, Dibutyl xylose (abbreviated as DBX) demonstrated favorable results, producing an optimal curve, as delineated in Figure 4A and 4B for both BPA and BPS. The coatings were prepared according to the protocol described in Example 5, utilizing the weight percentages specified in the tables below.

[0071] Example 9: Toxicity

[0072] It is known that DFX is non-mutagenic (Komarova et al (2021)

[0073] Green Chem., 2021 , 23,4790). New toxicity data show that DFX is estrogenically (agonist) unactive (Figure 5). DFX does not show activity of algae growth inhibition (no herbicidal activity), and does not show bioluminescence activity (no general toxicity). It is reasonable to assume that similar acetals with longer R chains (e.g. DPX, DBX) will behave similarly, while bio-degradability is expected to increase with R chain length (Degradability for C12 xylose acetal shown by Sun et al (Angew. Chem. I nt. Ed. 2024, 63, e202312823).

[0074] Figure 5: Agonistic estrogenic activity of DFX was determined using an estrogenic receptor transactivation assay (ERTA) with the human oestrogen receptor (hERa): ERa-Chemical Activated Luciferase gene expression (ERa -CALUX®), following ISO 19040-3 (Simon, E. et al., Evaluation of Three ISO Estrogen Receptor Transactivation Assays Applied to 52 Domestic Effluent Samples. Environ. Toxicol. Chem. 2022, 41 (10), 2512-2526).

[0075] In this assay, concentration-effect relationships of compounds are established and compared to the female sex hormone, 17 -estradiol (E2), which serves as the positive control / reference in this assay. From the concentration-effect curves (Figure 5), EC50, PC50 and PC10 values are determined in order to quantify the estrogenic activity of the tested compounds, also compared to the E2 reference (Table 1); EC50: 50% effect concentration, PC50: 50% effect relative to positive control, PC10: 10% effect relative to positive control, see OECD 455 (Performance-Based Test Guideline for Stably Transfected Transactivation In Vitro Assays to Detect Estrogen Receptor Agonists and Antagonists. In OECD Guidelines for the Testing of Chemicals, Section 4; OECD Publishing: Paris, 2021.); ISO 23196 (Water Quality - Calculation of Biological Equivalence (BEQ) Concentrations; 2022). Experimentally, ERo-CALUX was performed on 96-well plates following ISO 19040-3. E2 (reference compound) and DFX were dissolved in DMSO and tested, in duplicate, in dilution series with a final DMSO concentration of 0.8% in all wells. 0.8% DMSO served as negative control. Possible cytotoxicity to the mammalian cell was verified under the microscope. Data were fitted using Prism (8.0.1) to establish concentration-effect relationships (Figure 5) and determine EC50 as well as PC50 and PC10 values (Table 1). DFX was not toxic at the highest tested concentration of 2.4 103M.

[0076] Table 1 : ERo-CALUX® Molar results table.

[0077] 1Average from eight plates.

[0078] 2Fitted top of the curve

[0079] 3Highest tested dose was not toxic.

[0080] Figure 6 shows the results of an assay to check for toxicity to freshwater algae (algae growth inhibition test). Toxicity to Raphidocelis subcaptitata, a unicellular alga, was assessed using DIN 38412-59 (2022)(German standard methods for the examination of water, waste water and sludge - Test methods using water organisms (group L) - Part 59: Algal growth inhibition test on microplate with unicellular green fresh water algae (L 59). German standard methods for the examination of water, waste water and sludge - Test methods using water organisms (group L) - Part 59: Algal growth inhibition test on microplate with unicellular green fresh water algae (L 59)). The assay was conducted in 24-well plates with a 2 mL volume per well. Assay medium controls represented 100% growth (0% inhibition), and 3,5-Dichlorophenol (DCP) (2.1 mg / L) served as the positive control with a validity criterion of 20-80% inhibition. DFX was dissolved in the assay medium and tested at four concentrations using a two-fold dilution series, starting at a maximum concentration of 0.9 g / L (5.2 x 103M). All controls and samples were tested in triplicate, with algal growth monitored every 24 hours over three days. Test results: The positive control DCP inhibited growth by 30%, which is within the validity range of the standard of 20 to 80%. DFX did not inhibit algal growth at the tested concentrations (Figure 6).

[0081] Figure 7: Shows the results of an assay checking for Inhibition of bacterial bioluminescence (general toxicity). This assay uses the marine bacterium Allivibrio fischeri on 96-well plates and is based on Escher et al (2008) (Escher, B. I.; Bramaz, N.; Mueller, J. F.; Quayle, P.; Rutishauser, S.; Vermeirssen, E. L. M. Toxic Equivalent Concentrations (TEQs) for Baseline Toxicity and Specific Modes of Action as a Tool to Improve Interpretation of Ecotoxicity Testing of Environmental Samples. J. Environ. Monit. 2008, 10 (5), 612-621). In the assay, toxic compounds may reduce the naturally occurring luminescence of these bacteria. The assay uses a dilution series of 2,4-dichlorophenol as a positive control. DFX dissolved in water to a concentration of 1 g / L was tested in screening mode, using a 2- fold dilution series of DFX over four steps and testing concentrations in unicate. The positive control Dichlorophenol (DCP) induced full inhibition of bioluminescence and neither the blanks nor DFX caused inhibition of bioluminescence - at the tested concentrations (Figure 7).

Claims

Claims1 . Use of a compound of formula (I)wherein R1 and R3 are hydrogen, R2 and R4 are independently selected from the group consisting of hydrogen, a linear or branched Ci to C5 alkyl, C2 to C5 alkenyl, and a linear or branched Ci to C5 alkoxy, as heat sensitizer in heat applications with leuco dyes.

2. Use according to claim 1 , wherein R2 and R4 are independently selected from the group consisting of hydrogen, methyl, ethyl, propyl, isopropyl, methoxy, ethoxy and propoxy.

3. Use according to any of the preceding claims, wherein R2 and R4 are the same.

4. Use according to any of the preceding claims, wherein R2 and R4 are hydrogen.

5. Use according to any of the preceding claims, wherein R2 and R4 are selected from the group consisting of propyl, butyl and pentyl.

6. Use according to any of the preceding claims, wherein compound of formula I is selected from the group consisting of compounds 1 to 10:

7. Use of the sensitizer according to any of the preceding claims, wherein the heat application is selected from the group consisting of thermal recording technologies, textile industry, adhesive industry and coatings, preferably thermal recording technologies.

8. Thermosensitive composition comprising at least a color former in form of a leuco dye, color developer and the sensitizer according to any of claims 1 to 6.

9. Thermosensitive composition according to claim 6, wherein the color developer is a lignin fragment or a compound selected from the group consisting of compounds 101 to 116:

10. Recording material for thermal printing comprising a composition according to any of claims 8 or 9.

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