WO2025202357A1 - Use of re-refined oil for reducing sludge formation - Google Patents

Abstract

The invention relates to the use of at least one base oil which is at least partially re-refined, in a lubricant composition, for reducing the formation of sludge in an engine, said lubricant composition having, in particular, a grade XW-Y according to the SAE J300 classification, X being equal to 0, 5 or 10 and Y being between 8 and 50, preferably between 8 and 40.

Description

USE OF RE-REFINED OIL TO REDUCE SLUDGE FORMATION

The present invention relates to the use of re-refined oil to reduce the formation of sludge, and therefore improve engine cleanliness, in particular of an internal combustion engine.

Lubricating compositions, also called "lubricants", are commonly used in the various components of motor vehicles for the main purpose of reducing friction forces between the various moving metal parts in these components, in particular the engine, the transmission and the hydraulic circuit. They are also effective in preventing premature wear or even damage to these parts, and in particular to their surface. To do this, a lubricating composition is conventionally composed of a base oil to which are generally associated several additives dedicated to boosting the lubricating performance of the base oil, such as for example friction modifying additives, but also to provide additional performance.

Combustion engines generate _CO2 during operation due to the combustion of the fuel they use to operate. Current environmental concerns, particularly with regard to reducing carbon dioxide emissions, have led to an urgent need for alternative lubricant compositions to reduce the carbon footprint.

Furthermore, it is important that these alternative compositions have technical performance at least equivalent to, or even superior to, the compositions used to date.

When a lubricating composition oxidizes due to a combination of operating conditions, fuel quality and operating time, it degrades significantly, thickens radically and leads to the formation of a thick, highly oxidized oil known as black sludge, which causes the lubricating composition to flow only partially around the engine and, in the worst case, not at all due to clogged oil lines, resulting in oil starvation and costly engine failure. Thus, within a engine, unwanted sludge formation can have significant and undesirable effects.

There is therefore a need for a lubricating composition that allows control of engine cleanliness, and in particular control or even a reduction in the formation of sludge, while allowing a reduction in the carbon footprint, and preserving the technical properties of said lubricating composition.

The present invention aims specifically to propose a lubricating composition intended for the lubrication of engine systems, in particular in vehicles, making it possible to reduce the formation of sludge, while having a reduced carbon footprint.

To this end, the invention relates to the use of at least one at least partially re-refined base oil, in a lubricating composition, to reduce the formation of sludge in an engine.

The present invention also relates to the use of a lubricating composition comprising at least one at least partially re-refined base oil, for reducing the formation of sludge in an engine.

According to one embodiment, the aforementioned lubricating composition has a grade XW-Y according to the SAE J300 classification, X being equal to 0, 5 or 10 and Y being from 8 to 50, preferably from 8 to 40.

Preferably, the aforementioned lubricating composition has a 10W-Y grade, Y being from 8 to 50, preferably from 8 to 40, preferably of grade 10W-40.

According to the present application, "reducing sludge formation" means a reduction in the amount (by volume/by weight) of sludge formed in the engine compared to a lubricating composition not containing the aforementioned re-refined oil.

As stated above, when a lubricating composition oxidizes due to a combination of operating conditions, fuel quality and operating time, it degrades significantly, thickens dramatically and leads to the formation of a thick, highly oxidized oil known as black sludge. Sludge, also known as engine sludge, is a buildup of impure carbon particles that forms in an engine, especially a gasoline-powered automobile, due to the incomplete combustion of hydrocarbons. Sludge is a dirty, sticky, grease-like substance that forms due to oxidation/contamination of engine oil. Sludge can form in an engine, especially a gasoline-powered automobile, for several reasons, including delayed service/maintenance, using inferior oil, using the wrong fuel, or excessive idling of the car.

The terms “black mud” or “mud” or “engine mud” or “sludge” are used interchangeably in this application.

The present invention covers all motorized vehicles, preferably comprising at least one combustion engine, in particular heavy vehicles or light vehicles.

The present invention relates in particular to internal combustion engines.

In particular, the present invention relates to any internal combustion engine of the spark ignition type, preferably a gas, gasoline, diesel or even hybrid engine, and more particularly a diesel engine.

Surprisingly, as can be seen from the examples that follow, the inventors have discovered that re-refined oils, from used oil recycling channels, provide access to lubricants with improved properties in terms of engine cleanliness, generating a reduced quantity of sludge, compared with a lubricant without these re-refined oils.

According to one embodiment, the lubricating composition used according to the invention comprises a mixture of base oils, in which at least one of the base oils of said mixture is at least partly re-refined.

Preferably, the base oil mixture is formed solely from at least partially re-refined base oils.

In the context of the present invention, the expression "at least partly re-refined base oil", also referred to more simply in the remainder of the text as "re-refined lubricating oil", "re-refined oil", "regenerated oil" or even "recycled oil", designates an oil originating at least partly from a composition used lubricant having been subjected to one or more treatment steps known as re-refining treatment.

According to the invention, the term “used lubricating composition” (or more simply “used lubricant” or “used lubricating oil”) is intended to denote any lubricating composition that has been used for the lubrication of moving parts, in particular metal parts, of a mechanical system, such as, but not limited to, bearings, gears or motors.

Used lubricating oil can come from different sources. In particular, as detailed in the rest of the text, it can be a lubricant used for the lubrication of a motorization system, in particular "mobile", or for the lubrication of a so-called industrial system, in particular "stationary".

Due to their origin, used lubricating oils, particularly engine lubricating oils, contain a number of degradation products derived from the oil itself or from the additives it contains, as well as metal particles, metal oxides and other elements, for example from the engine. Used oil may contain, in particular, a high content of undesirable elements, for example calcium (Ca), iron (Fe), magnesium (Mg), sodium (Na), nickel (Ni), phosphorus (P), silicon (Si), chlorine (Cl), zinc (Zn), oxygen (O), or nitrogen (N), etc.

Methods for re-refining or reconditioning used lubricating oils have been developed in order to regenerate these oils and enable their subsequent reuse.

A re-refined lubricating oil is thus an oil obtained at the end of one or more stages of treatment of a used lubricant, aimed at eliminating, at least in part, a certain number of contaminating elements present therein, such as dust, water, fuel fractions, metallic elements and other residues resulting from the degradation of the additives present in the lubricant.

According to one embodiment, a re-refined lubricating oil according to the present invention comprises one or more alkylphenol(s). By "alkylphenol" is meant a phenolic compound with an alkyl group in the para position, and therefore of formula RI-C ₆ H ₄ -OH. The presence of alkylphenol is characteristic of re-refined lubricating oils, since native (unused) oils do not contain alkylphenol.

Preferably, the content of alkylphenol(s) in the re-refined lubricating oil according to the present invention is from 5 to 3,200 ppm. A re-refined lubricating oil according to the present invention may for example comprise from 10 to 2000 ppm, preferably from 15 to 1500 ppm, of alkylphenol(s).

According to one embodiment, a re-refined lubricating oil according to the present invention comprises from 10 to 300 ppm, preferably from 15 to 250 ppm, of alkylphenol(s).

According to one embodiment, a re-refined lubricating oil according to the present invention comprises from 150 to 2,000 ppm, preferably from 200 to 1,500 ppm, of alkylphenol(s).

The alkylphenol(s) content in the re-refined lubricating oil is measured according to the method described in the patent application filed under number FR 23 15133.

This method is based on the implementation of liquid chromatography and mass spectrometry steps, using a standard compound which is 4-hexadecylphenol.

For the liquid chromatography steps, a particle-filled column composed of C8-bonded silica is used. Measurements are carried out at 40°C with a flow rate of 0.4 mL/min.

Here, a so-called reversed-phase column is used to separate the different components of the sample (here, re-refined lubricating oil to be analyzed) according to their polarity. The composition of the mobile phase is used to modify these interactions over time and thus gradually elute the different molecules of the sample analyzed (here, re-refined lubricating oil).

The mobile phase is used in the form of a gradient as shown in Table 1 below, from a solution A comprising 50% water and 50% acetonitrile and a solution B comprising 100% methanol.

[Table 1]

Using mass spectrometry detection, it is then possible to obtain the signal produced only by the molecules of interest, identified both by their mass and their retention time.

The ionization source used is preferably the electrospray ionization (ESI) source which allows the selective ionization of polar compounds. In the case of the method used here, the detection mode chosen is the detection mode negative because it allows selective ionization of polar compounds with an acidic character. The range for masses (m/z) varies from 100 to 1200.

In particular, the method for measuring the alkylphenol content in the re-refined lubricating oil used according to the invention comprises a first step consisting of preparing the standard solution (4-hexadecylphenol) and the solution to be analyzed (re-refined lubricating oil):

. preparation of a standard solution at different concentrations to obtain a calibration line as explained later, by dilution in THF with the addition of 2% ammonium hydroxide; and

. preparation of a solution of said re-refined lubricating oil by dilution in THF with the addition of 3% ammonium hydroxide.

To establish the calibration curve, the intensity of the chromatographic peak associated with the 4-hexadecylphenol ion of the standard is recovered by plotting an extracted ion chromatogram (EIC). This makes it possible to have a chromatogram extracted only for a given m/z, namely here 317.28 for the standard molecule, the deprotonated form corresponding to the ion [C ₂ 2H ₃₈ OH]". The intensity of the EIC is therefore recovered for each of the analyses at the different concentrations tested.

The data obtained are used to construct the calibration line. This calibration line is obtained by injecting several standard solutions at different concentrations: the line is constructed by linear regression, and the calculation of the correlation coefficient (R ² ) makes it possible to verify the linearity of the detector and the correct preparation of the standard solutions.

The associated equation then allows us to predict the concentration of an unknown sample by entering the experimentally obtained y value. Here, the equation is: y = 1001.9x - 15309

To quantify the alkylphenols in the re-refined lubricating oil according to the invention, the analysis method comprises a step of identifying the m/z of the alkylphenol residues on the average mass spectrum by integrating the entire chromatogram. This average spectrum corresponds to an average of all the mass spectra obtained on the complete chromatographic run. This makes it possible to have all the compounds that were ionized during the analysis. From this spectrum From the average mass, a mass list is extracted, grouping together all the m/z ratios of the ions with the associated intensities.

The next step is to construct a Kendrick diagram with this mass list. This is a molecular map that allows us to identify series of compounds of the same type, but with different degrees of alkylation, by overcoming the mass defect of the hydrogens of the CH ₂ motif.

The Kendrick diagram can be made by calculating the following values:

14,00000

KM = mass (IUPAC or EXPER) x „ „

14.01565 where KM corresponds to the Kendrick mass, IUPAC mass corresponds to the theoretical mass calculated from the sum of each element constituting the molecule of interest, here for the standard molecule hexadecylphenol of formula C22H37O, the IUPAC mass = 317.284440 g. mol ^-1 , and EXPER mass corresponds to an experimental mass measurement, measured during an experiment.

The Kendrick mass KM is calculated for each peak of the average mass spectrum, determined previously as explained above.

Then, the Kendrick MKD mass defect is typically calculated according to the following equation:

KMD = NKM - KM where KMD corresponds to the Kendrick mass defect,

KM corresponds to the Kendrick mass, and

NKM is the nearest integer rounding of the Kendrick mass KM.

The Kendrick KMD mass defect is calculated for each peak (each peak corresponding for example to a compound present in the re-refined lubricating oil).

Kendrick diagrams are a 2D molecular map representing KMDs versus NKMs. Homologous compounds varying in their degree of alkylation appear as a horizontal line.

The set of m/z of the alkylphenols is obtained by applying a filter on the y-axis (KMD): this is the value KMD = 0.069. Once all the m/z at KMD = 0.069 are identified, they are used to construct extracted ion chromatograms (EIC) as described for the standard molecule. This allows to have a chromatogram dependent only on the requested m/z. The intensities of the EIC of each m/z corresponding to the alkylphenols are thus summed to obtain the total intensity (several alkylphenol type molecules are obtained on the spectra of re-refined lubricating oil according to the invention, these molecules varying by the length of their alkyl chain).

To obtain a quantification, the sum of the intensities of the obtained EICs is used as the value of y for the equation of the calibration line. For example, if the value obtained is 2.45 ^E 6, the quantification of alkylphenol residues in the re-refined lubricating oil used according to the invention is: y = 1001.9% - 15309 y + 15309

^X ~ 1001.9 and therefore x is equal to 2460.6 ppm.

According to one embodiment, a re-refined lubricating oil according to the present invention comprises one or more polyalphaolefins (PAOs). The presence of polyalphaolefin(s) is characteristic of re-refined lubricating oils, since native (unused) oils do not comprise polyalphaolefins (PAOs).

Figures 1 and 2 represent two-dimensional chromatograms of two re-refined lubricating oils according to the present invention. The arrow in each of the figures indicates the characteristic peak of PAO (at C30), a marker of re-refined oils.

According to one embodiment, a re-refined lubricating oil according to the present invention comprises one or more polyalphaolefins (PAOs) comprising less than 40 carbon atoms, and preferably comprising 30 carbon atoms.

The presence of PAOs in re-refined lubricating oil is determined according to the method described in the patent application filed under number FR 24 06231.

This method is based on the implementation of comprehensive two-dimensional gas chromatography (GCxGC) and classification steps.

In particular, it is implemented via a chromatography device (12), the chromatography device comprising a comprehensive two-dimensional gas chromatography module comprising a first column A and a second column B, and capable of separating different compounds of the product according to their volatility and their polarity, the chromatography device further comprising a flame ionization detector capable of measuring an intensity of electric ionization current generated for each compound included in the product, the chromatography device being calibrated with at least one calibration product, allowing the retention time of the different compounds present in the product to be corrected.

The method is further implemented by an electronic classification device, comprising the following steps: a. determining a table describing the intensity of the electric ionization current generated for each compound included in the product as a function of the corrected retention times in columns A and B, from a measurement carried out by the chromatography device on the product; b. assigning a class to the product, from among a plurality of classes, by applying a multivariate statistical algorithm to the table, said algorithm being trained on tables obtained from reference products.

The chromatography device comprises a comprehensive two-dimensional gas chromatography module comprising a first column A and a second column B. The comprehensive two-dimensional gas chromatography modules (2DGC or GCxGC) that can be used in the context of the present disclosure are those described in the literature.

These modules generally include an injection module, a vaporization module, a first column A, a modulator, and a second column B. They allow a two-dimensional separation of complex mixtures, because the product is subjected to two separations, we then obtain a two-dimensional chromatogram as a function of the retention times of columns A and B and a table describing the intensity of the electric ionization current generated for each compound included in the product as a function of the corrected retention times in columns A and B.

According to one embodiment, the first column A and the second column B are columns based on polydimethylsiloxane partially functionalized with phenyl groups. The percentage of phenyl group functionalization can be between 2% and 50%. According to a particular embodiment, the percentage of phenyl group functionalization of column A is greater than the percentage of phenyl group functionalization of column B. Advantageously, the length of column A is greater than that of column B. The diameter of the two columns A and B can be equivalent. The two columns can have a film thickness of 0.1 μm suitable for the separation of low-volatile samples. According to one embodiment, the temperature gradient applied to the oven is 2°C/min up to 400°C. A quantity of product is injected into the first column A to obtain a first separation, then via the modulator, into the second column B to obtain a second separation. The product can be injected directly without pretreatment, particularly in the case of lubricating oil analysis.

The GCxGC device is coupled to a flame ionization detector, or FID. This is capable of measuring the intensity of the ionization electric current generated for each compound included in the product. The flame ionization detector is located at the outlet of the second column.

Following analysis by the flame ionization detector, a table describing the intensity of the ionization electric current generated for each compound included in the product, as a function of the corrected retention times in columns A and B, is determined. The table is therefore derived from a two-dimensional chromatogram obtained from a measurement carried out by the chromatography device on the product.

In addition, during this initial step, an external calibration is performed to correct the retention time of the various compounds present in the product. This is performed by injecting at least one calibration product. In the case where the product to be classified is a lubricating oil, the calibration product may be a lubricating oil, preferably recycled. According to one embodiment, the calibration product comprises at least one marker, preferably at least two markers. The marker may be selected from n-paraffins, polyalphaolefins, and their mixture. The correction of the retention times may be performed by software.

At the end of this initial step, the classification device moves on to a next step, during which it assigns, via its product assignment module, a respective class from among the plurality of classes, by applying a multivariate statistical algorithm to the table, said algorithm being trained on tables obtained from reference products.

The multivariate statistical algorithm used in the allocation step may be a partial least squares regression multivariate statistical algorithm; the multivariate statistical algorithm preferably being selected from the group consisting of: a partial least squares regression algorithm, and a partial least squares regression algorithm with discriminant analysis. The algorithm is typically a partial least squares regression algorithm, such as the PLS algorithm or the PLS-DA algorithm.

The multivariate statistical algorithm used during the attribution step is trained on tables obtained from reference products. In Partial Least Squares Discriminant Analysis (PLS-DA), the prediction coefficient on a scale of 0 to 1 represents the probability or confidence of a sample belonging to a particular class.

Here is how this coefficient is calculated and used:

1. Creation of latent variables:

PLS-DA creates latent variables (components) that capture the maximum variance of the X data (the predictors) while maximizing the covariance with the Y classes (the categorical responses).

2. Calculation of scores:

Samples are projected onto these latent variables, producing scores that are used to discriminate between classes.

3. Modeling:

A linear model is fitted to these scores to predict values of Y. In the case of PLS-DA, Y is often binary encoded to represent classes (e.g., 0 for group A and 1 for group B).

4. Prediction:

When predicting for new samples, the scores of these samples are calculated and passed through the linear model to obtain a continuous prediction. This continuous prediction is then transformed into a probability on a scale of 0 to 1.

5. Interpretation of probabilities:

These probabilities are then interpreted to assign the samples to the different classes.

For example :

- If the probability is less than 0.4, the sample is classified in group A (here group of re-refined base oils); and

- If the probability is greater than or equal to 0.4, the sample is classified in group B (conventional base oil group).

According to one embodiment, in the lubricating composition used according to the invention, said at least partly re-refined base oil(s) represent more than 50% by mass, in particular more than 70% by mass, and more particularly between 70% and 85% by mass, relative to the total mass of said lubricating composition. According to one embodiment, the mass content of at least partly re-refined base oil(s) is from 50% to 90%, and more preferably from 70% to 85%, relative to the total mass of said lubricating composition.

As detailed in the following text, said re-refined lubricating oil(s) may be used as the sole base oil(s), i.e., without the addition of a separate base oil, for example new base oil. Alternatively, they may be used in combination with at least one new base oil.

In a particular embodiment, the lubricating composition is formed solely from one or more re-refined lubricating oils.

According to one embodiment, the at least partially re-refined base oil or oils have a kinematic viscosity measured at 100°C according to the ASTM D445 standard greater than or equal to 3.0 mm ² /s, in particular greater than or equal to 4.0 mm ² /s, in particular between 4.0 and 12 mm ² /s, in particular greater than or equal to 4.3 mm ² /s and more particularly between 4.4 and 10 mm ² /s, in particular between 4.5 and 6 mm ² /s.

According to one embodiment, the at least partially re-refined base oil or oils have a viscosity index, determined according to standard ASTM D2270, greater than or equal to 110, in particular between 110 and 130, preferably between 112 and 125, and more particularly between 118 and 124.

The viscosity index is calculated by measuring the kinematic viscosity at 40°C and 100°C. These measurements are then compared to the results of two reference oils. Its calculation method is described in ASTM D2270.

Preferably, the re-refined lubricating oils used according to the invention advantageously have, compared to virgin base oils of equivalent group according to the API classification, reduced volatility.

Volatility properties can be more specifically assessed by determining Noack volatility according to CEC L-40-93 standard.

Advantageously, a regenerated lubricating oil used according to the invention has a Noack volatility of less than or equal to 15%, in particular less than or equal to 14%. More preferably, a regenerated lubricating oil used according to the invention may have a Noack volatility strictly less than 12%, in particular between 7% and 11%.

According to one embodiment, the at least partly re-refined base oil or oils have a sulfur content of between 0.01% and 0.2% by mass, relative to the total mass of said at least partly re-refined base oil or oils.

The content of this element can be assessed by any method known to those skilled in the art, for example by X-ray fluorescence (XRF).

According to one embodiment, the at least partly re-refined base oil or oils have a content of aromatic compound(s) greater than or equal to 0.5% by mass, in particular greater than or equal to 1% by mass, in particular between 1% and 25% by mass, more particularly between 2.5% and 20% by mass, relative to the total mass of said at least partly re-refined base oil or oils.

Preferably, in the lubricating composition of the invention, the at least partly re-refined base oil or oils have a content of aromatic compound(s) of between 4% and 15% by mass, in particular between 5% and 10% by mass, relative to the total mass of said at least partly re-refined base oil or oils.

The contents of these different elements can be determined using any method known to those skilled in the art, for example by X-ray fluorescence (XRF) or by infrared or ultraviolet spectroscopy.

According to one embodiment, the lubricating composition used according to the invention further comprises one or more base oils distinct from the at least partly re-refined base oil and/or one or more additives, in particular chosen from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, viscosity index (VI) improvers, pour point depressant (PPD) additives, dispersants, antifoaming agents, thickeners, corrosion inhibitors, copper passivating agents, emulsifiers, and mixtures thereof. As previously specified, the base oil used in the lubricating compositions of the invention is a lubricating oil that is at least partly re-refined, also called “regenerated oil” or “recycled oil”, in other words a lubricating oil derived from a used lubricating composition that has been subjected to one or more re-refining treatment steps.

It is understood that a used lubricating composition may be a mixture of several used lubricating compositions, from the same source or from several different sources.

Used lubricating compositions and, consequently, regenerated lubricating oils, comprise, in the majority quantity, one or more base oils conventionally used in the field of lubricants, such as mineral, synthetic or natural, animal or vegetable oils or their mixtures.

It can be a mixture of several base oils, for example a mixture of two, three, or four base oils.

These base oils may be of natural origin, for example from plants or animals, such as vegetable, animal, fish oils, and mixtures thereof. Examples of such oils are rapeseed oil, canola oil, tall oil, sunflower oil, soybean oil, hemp oil, olive oil, linseed oil, mustard oil, palm oil, peanut oil, castor oil, coconut oil, animal fats, and mixtures thereof.

Advantageously, these base oils are oils of mineral or synthetic origin belonging to groups I to V according to the classes defined in the API classification (or their equivalents according to the ATI EL classification) and presented in the following table, or their mixtures.

[Table 2]

In particular, the used lubricating composition, from which the regenerated lubricating oil used according to the invention is derived, may comprise at least 50% by weight of base oil(s) relative to its total weight, in particular at least 60% by weight of base oil(s), and more particularly between 60% and 99% by weight of base oil(s).

According to a particular embodiment, the re-refined lubricating oil used according to the invention may come from the treatment of a used lubricating composition having been used for the lubrication of a motorization system, in particular “mobile”, that is to say including light vehicles, heavy goods vehicles, so-called “off-road” mobile machines, or even marine vehicles.

According to another particular embodiment, the re-refined lubricating oil used according to the invention may come from the treatment of a used lubricating composition having been used for the lubrication of a so-called industrial system, in particular “stationary”, that is to say including, in a non-limiting manner, turbines, compressors, hydraulic systems, gears, or even forming or cutting machines.

A used lubricating composition, from which the regenerated lubricating oil used according to the invention is derived, may contain various conventional additives in the field of lubricants, such as friction modifying additives, extreme pressure additives, anti-wear additives, detergents, antioxidants, viscosity index (VI) improvers, pour point depressant (PPD) additives, dispersing agents, anti-foaming agents, thickeners, emulsifiers, and mixtures thereof.

As already mentioned above, the properties of the used lubricating composition are degraded due to its use, for a more or less long period, for the lubrication and/or cooling of a mechanical system, in particular a motorization system, such as a combustion engine. Due to their origin, used lubricating compositions may thus contain one or more additives described above and impurities resulting from the degradation of additives originally present in the lubricant, or resulting from the wear of moving mechanical parts.

The composition of used lubricant can of course be different depending on the origin of the lubricant, its initial formulation and the fact that it may have been contaminated differently depending on its use.

The regenerated lubricating oil used according to the invention comes more particularly from a used lubricant having been subjected to one or more prior pre-treatment steps known in the field of re-refining used lubricants.

In particular, these treatment steps aim to eliminate, at least partially, water, solid particles, fuel and/or other contaminants (organic and/or mineral), such as polycyclic aromatic hydrocarbons (PAHs), which are undesirable in the formulation of lubricants.

According to a particular embodiment, the regenerated lubricating oil used according to the invention comes from a used lubricant having been subjected to one or more prior stages of dehydration, distillation, filtration, hydrogenation, liquid/liquid extraction, decantation and/or passage of the used lubricant over an adsorbent material, preferably as detailed below.

Preferably, the regenerated lubricating oil used according to the invention is obtained by subjecting a used lubricating composition to at least one dehydration step. This dehydration step makes it possible to eliminate any water possibly present in the used lubricant.

Advantageously, the regenerated lubricating oil used according to the invention thus comprises a water content of less than or equal to 10% by mass, in particular less than or equal to 5% by mass, in particular less than or equal to 2% by mass and more particularly less than or equal to 1% by mass, relative to the total mass of said regenerated lubricating oil.

This dehydration can be carried out by any method known to those skilled in the art, for example by distillation, evaporation, decantation, heating or passing a flow of hot air over the used lubricating composition. According to one embodiment, the dehydration step can be carried out at a temperature between 50°C and 250°C, preferably between 100°C and 200°C. In particular, it can be carried out at a pressure between 50,000 and 150,000 Pa, preferably at atmospheric pressure.

Preferably, the regenerated lubricating oil used according to the invention is obtained by subjecting a used lubricating composition to at least one prior filtration step. This filtration can be carried out by any method known to those skilled in the art. This filtration step can be a particulate or non-particulate filtration step. It can, for example, be carried out by diatomaceous earth type systems.

Preferably, the regenerated lubricating oil used according to the invention is obtained by subjecting a used lubricating composition to at least one distillation step, preferably following a prior dehydration step. Said distillation step(s) may be carried out by any technique known to those skilled in the art. It may be, for example, atmospheric distillation or distillation under reduced pressure. The distillations may, for example, be carried out at a temperature of between 100°C and 500°C, preferably between 200°C and 400°C, more preferably between 300°C and 380°C. In particular, they may be carried out at a pressure of between 25 Pa and 2,000 Pa, preferably between 50 Pa and 1,000 Pa, more particularly between 50 Pa and 250 Pa.

Advantageously, the regenerated lubricating oil used according to the invention is obtained by subjecting a used lubricating composition to at least one prior step of passing said used lubricating composition over an adsorbent material.

The adsorbent material advantageously allows the selective adsorption of aromatic compounds, in particular PAHs.

In particular, passing over an adsorbent material, preferably over activated carbon, advantageously makes it possible to reduce the content of polycyclic aromatic hydrocarbons (PAHs), notably chosen from chrysene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, dibenz[a,h]anthracene and/or benz[a]anthracene, of the used lubricating composition. The term "passage of the used lubricating composition over an adsorbent material" means the flow of the used lubricating composition over the adsorbent support.

Absorbent materials may be, for example, activated carbon, zeolites, clays or functionalized porous compounds. Preferably, it is activated carbon.

For example, the regenerated lubricating oil used in the process of the invention can be obtained from the treatment of a used lubricating composition according to the process described in document WO 2018/109208.

In the case of passing the used lubricating composition over activated carbon, the quantity of activated carbon used is preferably between 0.5 and 60 g of activated carbon per liter of used lubricating composition, preferably between 0.5 and 50 g/L, preferably from 1 to 50 g/L, preferably between 1 and 30 g/L, for example between 5 and 60 g/L, preferably between 5 and 50 g/L.

The flow rate of the used lubricating composition can be between 1m ³ /h and 15 m ³ /h, for example between 5 and 10 m ³ /h.

Preferably, the activated carbon is characterized by a density between 200 and 500 kg/m ³ , for example measured according to the ASTDM D2854 standard.

Preferably, the activated carbon is a coal, preferably comprising from 70% to 95%, advantageously from 80% to 90% by weight of carbon.

The step of passing the used lubricating composition onto an adsorbent support, preferably onto activated carbon, is advantageously preceded by the following preliminary steps:

- one or more distillation stages; and

- a filtration step, in particular as defined above.

Advantageously, the regenerated lubricating oil used according to the invention can be obtained by subjecting a used lubricating composition to at least one prior hydrogenation (or hydrotreatment) step, preferably which follows a prior dehydration and/or distillation step. Said hydrogenation step(s) can be carried out by any technique known to those skilled in the art and generally consist of treating the lubricating oil with hydrogen, generally in the presence of a hydrotreatment catalyst. Such a catalyst can contain, for example, at least one oxide or a sulfide of at least one group VI metal and/or at least one group VIII metal, such as molybdenum, tungsten, nickel or cobalt, and a carrier, for example alumina, silica-alumina or a zeolite.

Advantageously, the regenerated lubricating oil used according to the invention can be obtained by subjecting a used lubricating composition to at least one prior step of liquid/liquid extraction by a solvent, preferably which follows a prior step of dehydration and/or distillation. In particular, the liquid/liquid extraction by a solvent advantageously makes it possible to lighten a dark-colored used oil, to eliminate at least in part the bad odor or the aromatic compounds, in particular the PAHs. Said extraction step(s) can be implemented by any technique known to those skilled in the art. The extraction is generally carried out in a mixer-settler or in an extraction column, using a suitable extraction solvent.

Advantageously, the regenerated lubricating oil used according to the invention can be obtained by subjecting a used lubricating composition to at least one prior decantation step. Said decantation step(s) can be carried out by any technique known to those skilled in the art.

It is understood that the invention is in no way limited to the use of regenerated oils obtained using the treatment methods described above. Other lubricating oils, at least partly re-refined, resulting from treatment steps different from those described above, may be suitable for the invention.

In any event, a re-refined lubricating oil used according to the invention differs from a used lubricating oil, in particular due to the reduced content of certain undesirable contaminating elements, for example water, fuel, metallic elements or even certain heteroatoms.

On the other hand, a regenerated lubricating oil used according to the invention is distinguished, due to its formation from a used lubricant, from a virgin or new base oil, oil directly derived from petroleum refining, or even from native base oils, for example of natural origin, both in terms of its composition and its physicochemical properties. A regenerated lubricating oil used according to the invention is notably characterized by a silicon content of between 0 ppm and 300 ppm, notably between 1 and 300 ppm.

A regenerated lubricating oil used according to the invention is in particular characterized by a phosphorus content of less than or equal to 100 ppm, in particular between 0 ppm and 100 ppm, for example 0 ppm.

A regenerated lubricating oil used according to the invention may also be characterized by its content of one or more other elements chosen from chlorine, oxygen and nitrogen. It may, for example, have a chlorine content of between 0 ppm and 50 ppm, for example 0 ppm.

The content of these elements can be assessed by any method known to those skilled in the art, for example by X-ray fluorescence (XRF), or by infrared or ultraviolet spectroscopy.

According to a particular embodiment, a regenerated lubricating oil used according to the invention has a density less than or equal to 870 kg/m ³ , in particular less than or equal to 860 kg/m ³ . The density of the at least partly re-refined lubricating oil can thus be between 830 and 870 kg/m ³ , in particular between 840 and 860 kg/m ³ .

The density can in particular be determined according to standard NF EN ISO 12185.

In particular, a regenerated lubricating oil used according to the invention has a flash point greater than or equal to 225°C, in particular greater than or equal to 228°C. The flash point of the at least partly re-refined lubricating oil can thus be between 225°C and 245°C.

The flash point can in particular be determined according to standard NF EN ISO 2592.

As mentioned above, said re-refined lubricating oil(s) may be used as the sole base oil(s) of the lubricating composition, or may be formulated in combination with one or more separate base oils, in particular one or more new base oils.

These new base oils are particularly chosen from base oils conventionally used in the field of lubricants, such as mineral, synthetic or natural, animal or vegetable oils or their mixtures. Advantageously, these base oils are oils of mineral or synthetic origin belonging to groups I to V according to the classes defined in the API classification (or their equivalents according to the ATI EL classification) and presented in table 1 above, or their mixtures.

A lubricating composition implemented according to the invention may thus comprise a mixture of one or more at least partly re-refined lubricating oils and one or more new base oils, for example at least one mineral oil.

Preferably, a lubricating composition according to the invention comprises less than 50% by mass of new base oil(s), distinct from re-refined lubricating oils.

Preferably, a lubricating composition according to the invention is formed mainly from said re-refined lubricating oil(s).

In particular, said re-refined lubricating oil(s) may represent more than 70% by mass of the total mass of the lubricating composition, in particular more than 80% by mass, in particular more than 90% by mass, and more particularly between 70% and 95% by mass, of the total mass of the lubricating composition.

In a particular embodiment, a lubricating composition according to the invention is completely free of base oil other than re-refined lubricating oils.

A lubricating composition used according to the invention may further comprise all types of additives suitable for the intended use of the lubricant, as detailed in the remainder of the text, for example for use in mobile or stationary, more particularly mobile, motorization systems for light or heavy vehicles, or even off-road vehicles, in particular in combustion motorization systems.

These additives may be chosen in particular from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, viscosity index (VI) improvers, pour point depressants (PPD), dispersants, anti-foaming agents, thickeners, corrosion inhibitors, copper passivating agents, emulsifiers, and mixtures thereof.

Advantageously, a lubricating composition according to the invention comprises one or more additives chosen from viscosity index improvers, additives pour point depressants, anti-wear additives, antioxidants and their mixtures.

These additives may be added to the said regenerated base oil(s) used according to the invention or to the mixture of the said regenerated base oil(s) and at least one new base oil, in an appropriate quantity, determined by a person skilled in the art. It is understood that the nature and quantity of the additives used are chosen in such a way that the advantageous properties of the composition based on the said re-refined lubricating oil(s) are not or are not substantially altered by the envisaged addition.

A lubricating composition used according to the invention may comprise between 0.01% and 20% by mass, in particular between 0.05% and 10% by mass of additives, in particular as described below, relative to the total mass of the composition.

Advantageously, a lubricating composition used according to the invention may comprise at least one friction-modifying additive. The friction-modifying additives make it possible to limit friction by forming adsorbed monolayers on the surfaces of the metals in contact with them. They may be chosen from compounds providing metallic elements and ash-free compounds. Among the compounds providing metallic elements, mention may be made of transition metal complexes such as Mo, Sb, Sn, Fe, Cu, Zn, the ligands of which may be hydrocarbon compounds comprising oxygen, nitrogen, sulfur or phosphorus atoms. The ash-free friction modifying additives are generally of organic origin and may be chosen from fatty acid esters and polyols, distinct from the monoester required according to the invention, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, borate fatty epoxides, fatty amines or fatty acid glycerol esters. According to the invention, the fatty compounds comprise at least one hydrocarbon group comprising from 10 to 24 carbon atoms. In particular, the molybdenum-based compounds may be chosen from molybdenum dithiocarbamates (Mo-DTC), molybdenum dithiophosphates (Mo-DTP), and mixtures thereof.

Advantageously, a lubricating composition according to the invention may comprise from 0.01% to 5% by mass, preferably from 0.01% to 5% by mass, more particularly from 0.1% to 2% by mass or even more particularly from 0.1% to 1.5% by mass, relative to the total mass of the lubricating composition, of friction modifying additives.

Preferably, a lubricating composition according to the invention comprises at least one anti-wear additive, an extreme pressure additive or mixtures thereof. The anti-wear additives and the extreme pressure additives are dedicated to protecting the friction surfaces by forming a protective film adsorbed on these surfaces. There is a wide variety of anti-wear additives. Particularly suitable for the lubricating compositions according to the invention are the anti-wear additives chosen from polysulfide additives, sulfur-containing olefin additives or even phospho-sulfur additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTP. The preferred compounds are of formula Zn((SP(S)(OR)(OR')) ₂ , in which R and R', identical or different, independently represent an alkyl group, preferably comprising from 1 to 18 carbon atoms.

Advantageously, a lubricating composition according to the invention may comprise from 0.01% to 6% by mass, preferably from 0.05% to 4% by mass, more preferably from 0.1% to 2% by mass, relative to the total mass of the composition, of anti-wear additives and extreme pressure additives.

Advantageously, a lubricating composition according to the invention may comprise at least one antioxidant additive. The antioxidant additive makes it possible to delay the degradation of the lubricating composition in service. This degradation may in particular result in the formation of deposits, the presence of sludge or an increase in the viscosity of the lubricating composition. They act in particular as radical inhibitors or hydroperoxide destroyers.

Among the commonly used antioxidant additives, mention may be made of phenolic antioxidants, amine antioxidant additives, and phosphosulfur antioxidant additives. Some of these antioxidant additives, for example phosphosulfur antioxidant additives, may be ash-generating. The phenolic antioxidant additives may be ash-free or in the form of neutral or basic metal salts. The antioxidant additives may in particular be chosen from sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted by at least one C1 _-C12 alkyl group, N,N'-dialkyl-aryl-diamines and mixtures thereof. Preferably, the sterically hindered phenols are chosen from compounds comprising a phenol group in which at least one vicinal carbon of the carbon carrying the alcohol function is substituted by at least one C1-C10 alkyl group, preferably a C1- _C6 alkyl group, preferably a _C4 alkyl group, preferably by the tert-butyl group. Amino compounds are another class of antioxidant additives which can be used, optionally in combination with the phenolic antioxidant additives. Examples of amine compounds are aromatic amines, for example aromatic amines of formula NR ⁵ R ⁶ R ⁷ in which R ⁵ represents an aliphatic group or an aromatic group, optionally substituted, R ⁶ represents an aromatic group, optionally substituted, R ⁷ represents a hydrogen atom, an alkyl group, an aryl group or a group of formula R ⁸ S(O) _Z R ⁹ in which R ⁸ represents an alkylene group or an alkenylene group, R ⁹ represents an alkyl group, an alkenyl group or an aryl group and z represents 0, 1 or 2. Sulphurized alkyl phenols or their alkali and alkaline earth metal salts can also be used as antioxidant additives.

Advantageously, a lubricating composition according to the invention may comprise from 0.1% to 2% by mass, relative to the total mass of the composition, of at least one antioxidant additive.

A lubricating composition according to the invention may also comprise at least one detergent additive. Detergent additives generally make it possible to reduce the formation of deposits on the surface of metal parts by dissolving secondary oxidation and combustion products. The detergent additives that can be used in a lubricating composition according to the invention are generally known to those skilled in the art. The detergent additives may be anionic compounds comprising a long lipophilic hydrocarbon chain and a hydrophilic head. The associated cation may be a metal cation of an alkali or alkaline-earth metal. The detergent additives are preferably chosen from alkali metal or alkaline-earth metal salts of carboxylic acids, sulfonates, salicylates, naphthenates, as well as phenate salts. The alkali and alkaline-earth metals are preferably calcium, magnesium, sodium or barium. These metal salts generally contain the metal in stoichiometric quantity or in excess, i.e. in a quantity greater than the stoichiometric quantity. These are then overbased detergent additives; the excess metal providing the overbased character to the detergent additive is then generally in the form of a metal salt insoluble in the base oil, for example a carbonate, a hydroxide, an oxalate, an acetate, a glutamate, preferably a carbonate.

A lubricating composition according to the invention may comprise from 0.5% to 8%, preferably from 0.5% to 4% by mass, relative to the total mass of the lubricating composition, of detergent additive.

Advantageously, a lubricating composition according to the invention may also comprise at least one pour point depressant additive (also called a “PPD” agent for “Pour Point Depressant” in English). By slowing down the formation of paraffin crystals, the pour point depressant additives generally improve the cold behavior of the lubricating composition according to the invention. Examples of pour point depressants include polyalkyl methacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes and alkylated polystyrenes.

A lubricating composition according to the invention may comprise from 0.1% to 2%, preferably from 0.2% to 1% by mass of pour point lowering additive(s), relative to the total mass of the composition.

A lubricating composition according to the invention may also comprise at least one dispersing agent. Such dispersing agents ensure the maintenance in suspension and the evacuation of insoluble solid contaminants consisting of secondary oxidation products which form when the lubricating composition is in service. They may be chosen from Mannich bases, succinimides and their derivatives, such as polyisobutylene succinic anhydride derivatives.

In particular, a lubricating composition according to the invention may comprise from 0.2% to 10% by mass of dispersing agent(s), relative to the total mass of the composition.

A lubricating composition according to the invention may also comprise at least one viscosity index (VI) improving additive. Viscosity index improvers, in particular viscosity index improving polymers, make it possible to ensure good cold resistance and minimal viscosity at high temperatures. Examples of viscosity index improving polymers include hydrogenated or non-hydrogenated polymer esters, homopolymers or copolymers. hydrogenated styrene, butadiene and isoprene, homopolymers or copolymers of olefin, such as ethylene or propylene, polyacrylates and polymethacrylates (PMA), preferably homopolymers or copolymers of olefin, such as ethylene or propylene.

In particular, a lubricating composition according to the invention may comprise from 1% to 15% by mass of additive(s) improving the viscosity index, preferably from 5% to 10% by mass, relative to the total mass of the lubricating composition.

A lubricating composition may also comprise at least one antifoam additive, for example chosen from polar polymers such as polymethylsiloxanes or polyacrylates. In particular, a lubricating composition according to the invention may comprise from 0.01% to 3% by mass of antifoam additive(s), relative to the total mass of the lubricating composition.

It may also comprise at least one anti-corrosion agent or copper passivating agent, for example compounds such as succinic polyisobutene anhydrides, thiadiazole sulfonates or mercaptobenzothiazoles. They are typically present in a lubricating composition according to the invention at contents of between 0.01% and 1% by mass, relative to the total mass of the composition.

Thus, a lubricating composition according to the invention may further comprise one or more base oils distinct from the at least partly re-refined lubricating oil and/or one or more additives, in particular chosen from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, viscosity index (VI) improvers, pour point depressant (PPD) additives, dispersants, anti-foaming agents, thickeners, corrosion inhibitors, copper passivating agents, emulsifiers and mixtures thereof.

The present invention also relates to the use of a lubricating composition as defined above, for lubricating the parts of an internal combustion engine.

Preferably, the present invention also relates to the use of a lubricating composition as defined above, for lubricating the parts of an internal combustion engine of the spark ignition type, preferably an engine gas, petrol, diesel or even hybrid, and more particularly to lubricate the parts of a diesel engine.

The present invention also makes it possible to reduce the carbon footprint by using the aforementioned lubricating composition, and therefore by using at least one at least partially re-refined base oil, in comparison with the use of a lubricating composition comprising a new (or native) base oil. The present invention therefore also relates to the use of the aforementioned lubricating composition for reducing the carbon footprint.

The lubricating composition used according to the invention therefore makes it possible to improve engine cleanliness by reducing the formation of sludge in the engine, while reducing the carbon footprint (in comparison with the use of a lubricating composition comprising a new base oil).

The present invention also relates to a method for reducing the formation of sludge in an engine, said method comprising a step of bringing at least one mechanical part of said engine, in particular an internal combustion engine, into contact with a lubricating composition comprising at least one at least partly re-refined base oil, said lubricating composition preferably having a grade XW-Y according to the SAE J300 classification, X being equal to 0, 5 or 10 and Y being from 8 to 50, preferably from 8 to 40.

According to one embodiment, the lubricating composition used in this method is as defined above.

According to one embodiment, the at least partially re-refined base oil is as defined above.

Preferably, the above-mentioned method is implemented in an engine as defined above. Preferably, the engine is a spark-ignition type internal combustion engine, preferably a gas, gasoline, diesel or hybrid engine, and more particularly a diesel engine.

The invention will now be described by means of the following examples, given by way of illustration and not limitation of the invention. EXAMPLES

Preparation of the compositions The lubricating compositions are prepared by mixing the compounds described in Table 3 below.

The percentages indicated correspond to percentages by mass relative to the total mass of the composition. [Table 3]

The additive package includes, among other things, a dispersant, a detergent, an antioxidant, an anti-wear agent and/or an antifoam agent. The characteristics of the lubricating compositions are shown in Table 4 below:

[Table 4] KV100 (Kinematic Viscosity measured at 100°C) corresponds to the kinematic viscosity measured at 100°C, measured according to the ASTM D445 standard.

Cold Cranking Viscosity (CCS) is a measure of the dynamic viscosity of oil at low temperatures. These values are measured according to ASTM D5293.

Noack volatility at 250°C is measured according to CEC method L-40-A-93.

BOV (base oil viscosity) is the kinematic viscosity of the base oil blend at 100°C of the formulation before the addition of additives and OCP polymer. This kinematic viscosity is measured at 100°C, according to ASTM D445.

Sludge disposal

The test implemented to measure sludge formation in the engine is described below.

This is the CEC L-107-19 sludge deposition test.

The CEC L107-19 M271 Evo method is a key gasoline lubricant performance test for light-duty engines. This test is also known as the M271 EVO test.

The test is designed to assess sludge deposits around the engine in a TGDI engine with a fuel that is likely to generate sludge problems. Each test uses a new 1.8-litre Daimler M271 EVO engine incorporating a petrol direct injection system with a single turbocharger. The sludge fuel used in this test contains 10% ethanol (E10), reflecting the increased content of biofuels used across Europe.

This test is carried out in two phases:

- the first phase operates for 75 hours, running the engine at high speed and high load, to oxidize the engine lubricant and promote dilution of the fuel in the engine lubricant; and

- the second cyclic phase operates for the remainder of the test with alternating speeds, loads and temperatures. The key performance requirements of this test are the average sludge scores of five components around the engine: the oil pan with sump, the cylinder head, the cylinder head front cover, the valve cover and the timing case cover.

Lubricants meeting the requirements of the ACEA 2021 light engine oil sequences must achieve merits > 8.3, demonstrating an acceptable level of cleanliness throughout the engine.

The results obtained are shown in Table 5 below.

[Table 5]

It is therefore found that the composition of the invention, thanks to the re-refined base oil, makes it possible to obtain more satisfactory properties concerning the formation of sludge.

Unlike the comparative composition, without re-refined oil, the composition of the invention makes it possible to obtain an acceptable level of cleanliness throughout the engine, thanks to a reduction in the formation of sludge in the engine.

Claims

1. Use of at least one at least partially re-refined base oil, in a lubricating composition, to reduce the formation of sludge in an engine. 2. Use according to claim 1, in which the lubricating composition has a grade XW-Y according to the SAE J300 classification, X being equal to 0, 5 or 10 and Y being from 8 to 50, preferably from 8 to 40. 3. Use according to claim 1 or 2, wherein the lubricating composition comprises a mixture of base oils, wherein at least one of the base oils of said mixture is at least partly re-refined. 4. Use according to any one of the preceding claims, in which the lubricating composition has a 10W-Y grade, Y being from 8 to 50, preferably from 8 to 40, preferably of grade 10W-40. 5. Use according to any one of the preceding claims, wherein the at least partly re-refined base oil comes or the at least partly refined base oils come from a used lubricant having been subjected to one or more prior steps of dehydration, distillation, filtration, hydrogenation, liquid/liquid extraction, decantation and/or passage of the used lubricant over an adsorbent material. 6. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a kinematic viscosity measured at 100°C according to the ASTM D445 standard greater than or equal to 3.0 mm ² /s, in particular greater than or equal to 4.0 mm ² /s, in particular between 4.0 and 12 mm ² /s, in particular greater than or equal to 4.3 mm ² /s and more particularly between 4.4 and 10 mm ² /s, in particular between 4.5 and 6 mm ² /s. 7. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a viscosity index, determined according to ASTM D2270, greater than or equal to 110, in particular between 110 and 130, preferably between 112 and 125, and more particularly between 118 and 124. 8. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a Noack volatility, determined according to the CEC L-40-93 standard, of less than or equal to 15%, in particular less than or equal to 14%, preferably strictly less than 12%, in particular between 7% and 11%. 9. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a sulfur content of between 0.01% and 0.2% by mass, relative to the total mass of said at least partly re-refined base oil or oils. 10. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a content of aromatic compound(s) greater than or equal to 0.5% by mass, in particular greater than or equal to 1% by mass, in particular between 1% and 25% by mass, more particularly between 2.5% and 20% by mass, relative to the total mass of said at least partly re-refined base oil or oils. 11. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have a content of aromatic compound(s) of between 4% and 15% by mass, in particular between 5% and 10% by mass, relative to the total mass of said at least partly re-refined base oil or oils. 12. Use according to any one of the preceding claims, in which the at least partly re-refined base oil or oils have an alkylphenol content of from 5 to 3,200 ppm, preferably from 10 to 2,000 ppm, and preferentially from 15 to 1,500 ppm. 13. Use according to any one of the preceding claims, in which the lubricating composition further comprises one or more base oils distinct from the at least partly re-refined base oil and/or one or more additives, in particular chosen from friction modifying additives, anti-wear additives, extreme pressure additives, detergents, antioxidants, viscosity index (VI) improvers, pour point depressants (PPD), dispersants, anti-foaming agents, thickeners, corrosion inhibitors, copper passivators, emulsifiers, and mixtures thereof. 14. Method for reducing the formation of sludge in an engine, said method comprising a step of bringing at least one mechanical part of said engine, in particular an internal combustion engine, into contact with a lubricating composition comprising at least one base oil that is at least partly re-refined, said lubricating composition preferably having a grade XW-Y according to the SAE J300 classification, X being equal to 0, 5 or 10 and Y being from 8 to 50, preferably from 8 to 40.