EP4638670A1 - Viscosity index improver additives for lubricating oils and process for their preparation - Google Patents

Abstract

Viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer, said additive having the following features: a shear stability index (SSI) comprised between 18% and 30%, preferably comprised between 20% and 28%, measured according to ASTM D7109-12 standard (30 cycles of shearing) in Group I base oil; a thickening power (TP) comprised between 3 cSt and 8 cSt, preferably comprised between 4 cSt and 7.5 cSt, measured according to ASTM D7042-04 standard in Group I base oil; a gelation index defined by the ratio: (η^0°C/η^40°C)/(η^60°C/η^40°C) where r| represents the steady state dynamic viscosity measured in Group III base oil, said gelation index being lower than or equal to 7000, preferably comprised between 30 and 6000, more preferably comprised between 50 and 5000; an incipient gelation temperature, measured in Group III base oil, at a temperature lower than or equal to 10 C, preferably comprised between 0 C and 10°C; a shape stability defined according to the following formula: ΔL% = [(L/L0)-1)]*100 wherein L is the length after 24 hours and L0 is the initial length equal to 10 cm, lower than or equal to 5%, preferably lower than or equal to 4%, more preferably lower than or equal to 3%. The aforesaid viscosity index improver (V.I.I.) additive for lubricating oils is advantageously usable in base oils of mineral origin, or of synthetic origin, or mixtures thereof. Preferably, said base oils of mineral origin or of synthetic origin are selected from the base oils belonging to Group I, or Group II or Group III.

Description

VISCOSITY INDEX IMPROVER ADDITIVES FOR LUBRICATING OILS

AND PROCESS FOR THEIR PREPARATION

The present invention relates to viscosity index improver (V.I.I.) additives for lubricating oils.

More particularly, the present invention relates to a viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer, said additive having the specific features reported below.

The aforesaid viscosity index improver (V.I.I.) additive for lubricating oils is advantageously usable in base oils of mineral origin, or of synthetic origin, or mixtures thereof. Preferably, said base oils of mineral origin or of synthetic origin are selected from the base oils belonging to Group I, or Group II or Group III.

The present invention also relates to a process for preparing said ethylenepropylene copolymer.

The present invention also relates to a composition of lubricating oil comprising said viscosity index improver (V.I.I.) additive for lubricating oils and a base oil.

Ethylene-propylene copolymers are widely used in the sector of the lubricating oil additives [in the sector also known as olefin copolymers (OCP)] as viscosity index improvers (V.I.I.) in order to modulate the viscosity of said lubricating oils at varying temperatures.

The micro/macro structure of the ethylene-propylene copolymers, the weight average molecular weight (M_w), the polydispersity index (PDI), i.e. the ratio between the weight average molecular weight (M_w) and the number average molecular weight (M_n) (M_w /M_n), of the ethylene-propylene copolymers, as well as the amount of said ethylene-propylene copolymers used, are all factors that modify the effect of the same as viscosity index improvers (V.I.I.) of the lubricating oils and of the final properties of the formulations in which they are used.

The optimization of the micro/macro structure of the ethylene-propylene copolymers allows to identify the right compromise in order to achieve the desired technological parameters.

For example, for a given value of the shear stability index (SSI), a parameter that, with the same polydispersity index (PDI), mainly depends on the weight average molecular weight (M_w) of the ethylene-propylene copolymer, the thickening power (TP) increases with the increase in the ethylene content and molecular weight. The semicrystalline ethylene-propylene copolymers, characterized by a high content of chain- bonded ethylene, are therefore characterized by ratios between the thickening power (TP) and the shear stability index (SSI) (TP/SSI) higher than those of the amorphous ethylene-propylene copolymers, but have insufficient performance at low temperatures. On the other hand, despite the excellent cold properties, the amorphous ethylene- propylene copolymers, characterized by a low content of chain-bonded ethylene and by a low weight average molecular weight (M_w), present problems associated with the stability of physical form (pellets) that make the handling of said ethylene-propylene copolymers particularly critical.

However, the balancing of the technological parameters of the lubricating oil cannot be carried out only through the use of the ethylene-propylene copolymers having an intermediate ethylene content since said ethylene-propylene copolymers have ethylenic sequences capable of interfering with the additives capable of lowering the pour point depressant of the lubricating oil in which they are used, compromising the activity thereof and further worsening the rheology of the lubricating oil at low temperature.

In order to find a right balancing of the technological parameters, it is known to use suitable mixtures of one or more crystalline ethylene-propylene copolymers and of one or more amorphous ethylene-propylene copolymers.

For example, European patent EP 2424939 relates to a polymeric composition comprising:

(a) a first ethylene copolymer having: i. a weight percentage of EA units derived from ethylene in the range from greater than or equal to 35 to lower than or equal to 60; and ii. a MWA lower than 130000; and

(b) a second ethylene copolymer having: i. a weight percentage of EB units derived from ethylene in the range from greater than or equal to 35 to lower than or equal to 85; and ii. a MWB lower than 70000; wherein the first ethylene copolymer and/or the second ethylene copolymer have less than 1 pendant branching point with a carbon chain greater than 19 carbon atoms per 200 carbon atoms along the backbone.

Also disclosed is a process for preparing said polymeric composition, as well as a composition comprising a lubricating base oil and said polymeric composition. The aforesaid polymeric composition is said to be a rheological modifier usable as a viscosity index improver (V.I.I.) having an unexpected thickening power (TP) with respect to the compositions of the known art while maintaining equivalent rheological properties at low temperatures.

European patent EP 2809689 relates to a polymeric composition comprising:

(a) a first ethylene-oc-olefin copolymer; and

(b) a second ethylene-oc-olefin copolymer; wherein

(c) the first ethylene-oc-olefin copolymer (a) has an ethylene content comprised between about 60% by weight and about 80% by weight;

(d) the second ethylene-oc-olefin copolymer (b) has an ethylene content lower than about 60% by weight;

(e) the first ethylene-oc-olefin copolymer (a) has a “Melt Flow Rate Ratio” (MFRR), defined as the ratio between the “Melt Flow Rate” (MFR) measured at 230 C with a load equal to 21.6 kg and the “Melt Flow Rate” (MFR) measured at 230 C with a load equal to 2.16 kg, greater than 34 and has a “Melt Flow Rate” (MFR) measured in accordance with standard ASTM D 1238, condition L (230 C with a load equal to 2.16 kg), comprised between 2.5 g/10 min and 5.5 g/10 min; and

(f) the mixture between the first ethylene-oc-olefin copolymer (a) and the second ethylene- α-olefin copolymer (b) has a “Melt Flow Rate Ratio” (MFRR), defined as above, comprised between 33 and 37.

Also disclosed are lubricating compositions comprising the aforesaid polymeric composition and a lubricating base oil which are said to have reduced or even no gel formation.

European patent EP 2809716 relates to a process for preparing a polymeric composition comprising the steps of: (a) feeding (i) a first ethylenic monomer, (ii) a first α-olefinic comonomer, (iii) a first solvent, (iv) optionally a first hydrogen, and (v) a first metallocene catalyst, into a first reaction zone to produce a first ethylene-oc-olefin copolymer; (b) feeding (i) a second ethylenic monomer, (ii) a second oc-olefinic comonomer, (iii) a second solvent, (iv) optionally a second hydrogen, and (v) a second metallocene catalyst, into a second reaction zone to produce a second ethylene-oc-olefin copolymer; and (c) combining the first ethylene-oc-olefin copolymer and the second ethylene- oc-olefin copolymer to form a polymeric composition, wherein the first ethylene-oc-olefin copolymer has an ethylene content comprised between 60% and 80% by weight, and the second ethylene-oc-olefin copolymer has an ethylene content lower than 60% by weight, and wherein the process satisfies at least one of the following requirements: (1) the first hydrogen is fed in step (a) at a concentration comprised between 0% by weight and 1% by weight with respect to the total weight of the components (i)-(iv) fed in step (a); and (2) the second hydrogen is fed in step (b) at a concentration comprised between 0% by weight and 0.5% by weight with respect to the total weight of the components (i)-(iv) fed in step (b).

The polymeric composition obtained is said to be usable as a viscosity index improver (V.I.I.) in lubricating oils that have reduced or even no gel formation at low temperatures.

European patent EP 1148115 relates to a viscosity index improver (V.I.I.) for lubricating oil, comprising an ethylene-prop ylene copolymer (B) having the following features (bl)-(b5):

(bl) a content of ethylenic units in the range from 70% by weight to 79% by weight;

(b2) a weight average molecular weight determined by gel permeation chromatography in the range between 80000 Da and 400000 Da in terms of polystyrene;

(b3) a ratio M_w/M_n (M_w: weight average molecular weight; M_n: number average molecular weight), which is an indication of the polydispersity index, lower than 2.3;

(b4) a melting peak determined by differential scanning calorimetry (DSC) in the range between 15 C and 60°C; and

(b5) a content [E (% by weight)] of ethylenic units and a melting point [T_m (°C)] determined by differential scanning calorimetry (DSC) satisfying the following relationship (II):

3.44xE-206 > T_m (II) in case the weight average molecular weight is greater than 80,000 and lower than 250,000 and the following relationship:

3.44xE-204 > T_m (III) in case the weight average molecular weight is comprised between 250000 and 400000.

It is also reported a lubricating composition comprising a lubricating oil and the aforesaid viscosity index improver (V.I.I.): said lubricating composition is said to have excellent properties at low temperature.

European patent EP 1178102 relates to a viscosity index improver (V.I.I.) for lubricating oil, comprising an ethylene/α-olefin copolymer which is a copolymer of ethylene with a α-olefin having from 3 to 20 carbon atoms and has the following properties:

(1) an ethylene (E) content in the range between 40% by weight and 70% by weight;

(2) a weight average molecular weight determined by gel permeation chromatography in the range between 80000 Da and 400000 Da in terms of polystyrene;

(3) a ratio M_w/M_n not higher than 2.4;

(4) a melting point determined by differential scanning calorimetry (DSC) lower than 60°C; and

(5) a content [E (% by weight)] of ethylenic units and a melting point [T_m (°C)] determined by differential scanning calorimetry (DSC) satisfying the following relationship (I):

3.31xE-186 > T_m (I); and

(6) an amount [(V) %] of carbon atoms αβ with respect to the total carbon atoms present in the main chain and, if present in the long chain branchings, which satisfies the following relationship (II):

V < 10-0.1xE (II).

It is also reported a lubricating composition comprising a lubricating oil and the aforesaid viscosity index improver (V.I.I.): said lubricating composition is said to have excellent properties at low temperature, oxidation stability, lubrication at high temperatures and fuel economy.

However, the viscosity index improvers (V.I.I.) mentioned above may have some drawbacks. For example, in order to obtain viscosity index improvers (V.I.I.) capable of giving composition of lubricating oils having good cold properties, with reduced or even no gel formation, it is necessary to use blends of ethylene -propylene copolymers. In addition, in some cases, the ethylene -propylene copolymers have medium-high weight average molecular weights (M_w) and, consequently, high shear stability index (SSI) values (i.e. > 30).

It is also known that the conventional production processes of ethylene-propylene copolymers, in solution or in slurry allow the production, with high potential, of ethylene-propylene copolymers distinguished by molecular weights clearly higher than those used in the context of the viscosity index improvers (V.I.I.). This is correlated to the following reasons: the traditional processes (stripping, extrusion drying) cannot effectively process ethylene-propylene copolymers distinguished by very low molecular weights (e.g. weight average molecular weights (M_w) lower than 100000 Da); the low molecular weight products tend to exhibit cold flow phenomena that make their handling and storage prohibitive (a very accentuated phenomenon, in particular, in the case of amorphous ethylene-propylene copolymers).

In addition, since low molecular weights are essential to guarantee the application performance of the viscosity index improvers (V.I.I.) for lubricating oils and the processes usually used for the production of ethylene-propylene copolymers (solution or slurry processes) often provide ethylene-propylene copolymers having clearly higher molecular weights, it is necessary to subject said ethylene-propylene copolymers to thermodegradation processes.

For example, European patent EP 1013673 relates to a process for reducing the molecular weight of a polymeric material selected from copolymers and terpolymers of ethylene [EP(D)M)] or mixtures thereof, comprising treating the polymeric material with at least one hydroperoxide, said hydroperoxide being present in an amount such as to perform a reduction in the molecular weight, at a temperature comprised between 80°C and 250°C, said copolymers and terpolymers of ethylene [EP(D)M)] having a propylene molar content comprised between 16% and 50%. The copolymers and terpolymers of the ethylene [EP(D)M)] thus obtained are said to have several applications, including the use as viscosity index improvers (V.I.I.) for lubricating oils and the modification of high fluidity plastic materials. The aforesaid process can be carried out in an extruder, preferably in a twin-screw extruder.

European patent EP 1671982 relates to a process for preparing viscosity index improvers (V.I.I.) having an improved dimensional stability and able to improve the rheology of the lubricating oils at low temperature, said process comprising the treatment of mixtures of linear polyethylene which is a homopolymer or copolymer of the ethylene characterized by a crystalline structure and by a density comprised between 0.88 and 0.94 and by amorphous copolymers and terpolymers of the ethylene [EP(D)M)], optionally in mixture with semi-crystalline copolymers and terpolymers of the ethylene [EP(D)M)], said treatment being carried out in an extruder, said treatment being carried out in the presence of one or more substances of a hydroperoxide nature and, optionally, in the presence of polyfunctional vinyl monomers in an amount comprised between 0% by weight and 2% by weight, said treatment being carried out at a shear value greater than 100 sec ¹ and at a temperature comprised between 75°C and 260°C, said amorphous copolymers and terpolymers of the ethylene [EP(D)M))] being characterized by an ethylene content comprised between 35% by weight and 62% by weight and by a content of the third monomer comprised between 0% by weight and 10% by weight. The aforesaid process can be carried out in an extruder, preferably in a twin-screw extruder.

European patent EP 1632504 relates to a process for preparing viscosity index improvers (V.I.I.) comprising treating, under high shear conditions, a composition comprising: (i) one or more copolymers or terpolymers of the ethylene [EP(D)M)] and (ii) one or more polyvinylarene/hydrogenated conjugated diene/polyvinylarene block copolymers, the weight ratio (i)/(ii) being comprised between 98/2 and 80/20, said process being carried out at a temperature comprised between 150 C and 400 C, for a time not higher than 150 seconds and at a shear value greater than 75 sec ¹. The aforesaid process can be carried out in an extruder, preferably in a twin-screw extruder.

European patent EP 1984479 relates to a process for preparing viscosity index improvers (V.I.I.) for lubricating oils comprising a mixing treatment under shear conditions greater than 50 sec ¹, of a composition comprising:

(i) one or more copolymers or terpolymers of the ethylene [EP(D)M)];

(ii) one or more polyvinylarene/hydrogenated conjugated diene/polyvinylarene block copolymers;

(iii) lubricating oil;

(ii) being present at a concentration comprised between 1.5% by weight and 20% by weight and (iii) being present at a concentration comprised between 1.5% by weight and 45% by weight.

The aforesaid process can be carried out in an extruder, preferably in a twin-screw extruder.

Canadian patent CA 991792 relates to a continuous process for producing an additive for lubricating oil by thermal degradation in an extruder, under non-oxidizing conditions, of a substantially linear, high molecular weight, amorphous ethylenepropylene hydrocarbon copolymer containing 25%-50% by weight of polymerized propylene, 0%-5% by weight of a polymerized non-conjugated hydrocarbon diene and the remainder, polymerized ethylene, said process comprising: uniformly mixing and heating the copolymer in a first zone at a temperature comprised between about 150°C and 280°C, until when the copolymer is uniformly melted, said temperature being lower than the degradation temperature of the copolymer; passing the melted copolymer to a second zone in which it is uniformly mixed and heated to a temperature comprised between about 300°C and 500 C, until obtaining a degraded copolymer whose inherent viscosity is lower than the inherent viscosity of the starting copolymer and is comprised between 0.1 dl/g and 1.8 dl/g, measured on a solution of 0.1 g of copolymer in 100 ml of tetrachloroethylene at 30 C, and whose molecular weight distribution is substantially equal to or lower than that of the starting copolymer; removing the degraded copolymer from the second zone; cooling and recovering the degraded copolymer which is substantially colourless or white and contains a non-significant amount of oil-insoluble material.

American patent US 6,753,381 relates to a composition of lubricating oil comprising a greater amount of lubricating oil, an additive package appropriate for the quality and performance levels, a pour point lowering element and an oil-soluble polymeric mixture in an amount to improve the viscosity index (V.I.I.), said polymeric mixture comprising an ethylene-propylene polymeric mixture having a weight average molecular weight comprised between about 20000 and about 300000 and a molecular weight distribution comprised between about 1.3 and about 5 prepared by the process comprising mixing or simultaneously mixing and shearing:

(a) an essentially amorphous ethylene-propylene polymer at a low ethylene content, said polymer having a molar ratio between ethylene and propylene comprised between 35/65 and 65/35, having a weight average molecular weight comprised between about 40000 and about 300000 and a weight distribution in the range from about 1.3 to about 5.0; and

(b) a partially crystalline ethylene-propylene polymer having a higher ethylene content having a molar ratio between ethylene and propylene comprised between about 65/35 and about 85/15, having a crystallinity of 3% by weight - 25% by weight, and having a weight average molecular weight in the range from about 40000 to about 300000 and a molecular weight distribution in the range from about 1.3 to about 5.0; wherein the mixing, or the simultaneous mixing and shearing, is performed under conditions of heat and mechanical work sufficient to create high shear conditions for a time sufficient to reduce the molecular weight and the molecular weight distribution of the mixture of (a) and (b), wherein the weight ratio between the low ethylene-content ethylene-propylene polymer (a) and the higher ethylene-content ethylene-propylene polymer (b) is comprised in the range from about 45/55 to about 10/90.

However, the above processes may have some drawbacks. For example, in order to obtain ethylene-propylene copolymers having molecular weights suitable for use as viscosity index improvers (V.I.I.), it may be necessary to use hydroperoxides, or to operate at high temperatures (up to 500 C) that require operation in an inert atmosphere (for example, in the presence of nitrogen) and that can lead to the formation of gels and branchings in the final product. Furthermore, in order to improve the dimensional stability of the ethylene-propylene copolymers obtained, it may be necessary to use small amounts of polyvinylarene/hydrogenated conjugated diene/polyvinylarene block copolymers or polyfunctional vinyl monomers.

The Applicant has therefore posed the problem of finding viscosity index improver (V.I.I.) additives for lubricating oils capable of presenting reduced phenomena of gel formation at low temperatures, a stable physical form (pellet) and an excellent compromise between the values of shear stability index (SSI) (i.e. lower than or equal to 30%) and thickening power (TP) (i.e. lower than or equal to 8 cSt), measured as reported below.

The Applicant has now found a viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer having the specific features reported below. The aforesaid viscosity index improver (V.I.I.) additive for lubricating oils is capable of presenting reduced phenomena of gel formation at low temperatures, a stable physical form (pellets) and an excellent compromise between the values of shear stability index (SSI) (i.e. lower than or equal to 30%) and thickening power (TP) (i.e. lower than or equal to 8 cSt), measured as reported below. In addition, the aforesaid viscosity index improver (V.I.I.) additive for lubricating oils is advantageously usable in base oils of mineral origin, or of synthetic origin, or mixtures thereof. Preferably, said base oils of mineral origin or of synthetic origin are selected from the base oils belonging to Group I, or Group II or Group III.

Therefore, an object of the present invention is a viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer, said additive having the following features: a shear stability index (SSI) comprised between 18% and 30%, preferably comprised between 20% and 28%, measured according to ASTM D7109-12 standard (30 cycles of shearing) in Group I base oil; a thickening power (TP) comprised between 3 cSt and 8 cSt, preferably comprised between 4 cSt and 7.5 cSt, measured according to ASTM D7042-04 standard in Group I base oil; a gelation index defined by the ratio: where η represents the steady state dynamic viscosity measured in Group III base oil, said gelation index being lower than or equal to 7000, preferably comprised between 30 and 6000, more preferably comprised between 50 and 5000; an incipient gelation temperature, measured in Group III base oil, at a temperature lower than or equal to 10 C, preferably comprised between 0°C and 10 C; a shape stability defined according to the following formula: wherein L is the length after 24 hours and L0 is the initial length equal to 10 cm, lower than or equal to 5%, preferably lower than or equal to 4%, more preferably lower than or equal to 3%.

For the purpose of the present description and of the following claims, the gelation index, the incipient gelation temperature, and the shape stability, have been determined as reported below in the analysis and characterization methodologies.

For the purpose of the present description and of the following claims, the definitions of the numerical ranges always include the extreme values unless otherwise specified.

For the purpose of the present description and of the following claims, the term “comprising” also includes the terms “which essentially consists of’ or “which consists of’.

For the purpose of the present description and of the following claims, the term “base oils” means base oils according to API 1509 standard “Engine Oil Licensing and Certification System”, version of November 2004, 15th Ed., appendix E, in which said base oils are divided into five groups as reported in the following Table:

Table

In accordance with a preferred embodiment of the present invention, said ethylene-prop ylene copolymer has the following features: from 61% by weight to 67% by weight, preferably from 62% by weight to 66% by weight, with respect to the total weight of said ethylene-propylene copolymer, of ethylene; from 33% by weight to 39% by weight, preferably from 34% by weight to 38% by weight, with respect to the total weight of said ethylene-propylene copolymer, of propylene; a weight average molecular weight (M_w) comprised between 60000 Da and 120000 Da, preferably comprised between 65000 Da and 100000 Da; a polydispersity index (PDI), i.e. the ratio between the weight average molecular weight (M_w) and the number average molecular weight (M_n) (M_w/M_n), comprised between 2 and 4, preferably comprised between 2.1 and 3; a peak crystallization temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 20°C, preferably comprised between 0°C and 15°C; a peak melting temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 35°C, preferably comprised between 0 C and 30 C; a melting enthalpy (AH), determined by differential scanning calorimetry (DSC), greater than or equal to 25 J/g, preferably comprised between 28 J/g and 40 J/g; a minimum number of propylene inversions, determined by NMR analysis, comprised between 0.1 and 0.25, preferably comprised between 0.12 and 0.22.

The Applicant has also posed the problem of finding a process for preparing the aforesaid ethylene-propylene copolymer capable of overcoming the above drawbacks.

The Applicant has now found a continuous process for reducing the molecular weight of ethylene-propylene copolymers comprising the use of a single screw extruder provided with a reciprocating single screw, three conveying and mixing zones, a gear pump, an underwater die-face cutter, said three conveying and mixing zones and said gear pump operating at specific temperature ranges. In particular, the Applicant has found that the use of said single screw extruder allows to obtain ethylene-propylene copolymers having molecular weights suitable for use as viscosity index improvers (V.I.I.) in the absence of hydroperoxides, operating at lower temperatures than the known art so as to be able to avoid both operating in an inert atmosphere and the formation of gel and of branchings in the final product. Furthermore, said process does not require the use of polyvinylarene/hydrogenated conjugated diene/polyvinylarene block copolymers or of polyfunctional vinyl monomers in order to improve the dimensional stability of the final product.

Therefore, a further object of the present invention is a continuous process for reducing the molecular weight of ethylene-propylene copolymers comprising the following steps: (a) providing at least one single screw extruder comprising a chamber, a reciprocating single screw mounted in said chamber, said reciprocating single screw being capable of rotating and oscillating within said chamber, said chamber having at least one feeding opening and at least one discharging opening, said extruder comprising three conveying and mixing zones, a gear pump, an underwater dieface cutter;

(b) feeding at least one ethylene-propylene copolymer into said single screw extruder;

(c) conveying said at least one ethylene-propylene copolymer through a first conveying and mixing zone which operates at a temperature comprised between 140 C and 260°C, preferably comprised between 150°C and 250°C, wherein said ethylene-propylene copolymer is uniformly mixed and heated and uniformly softened;

(d) conveying said at least one uniformly softened ethylene-propylene copolymer leaving said first conveying and mixing zone to a second conveying and mixing zone which operates at a temperature comprised between 220 C and 33O°C, preferably comprised between 230°C and 320 C, wherein said ethylene-propylene copolymer is further mixed and heated and partially degraded;

(e) conveying said at least one partially degraded ethylene-propylene copolymer leaving said second conveying and mixing zone to a third conveying and mixing zone which operates at a temperature comprised between 170 C and 340 C, preferably comprised between 180 C and 330 C, wherein said ethylene-propylene copolymer is further mixed and heated and further degraded;

(f) conveying said at least one further degraded ethylene-propylene copolymer leaving said third conveying and mixing zone to said gear pump which operates at a temperature comprised between 170°C and 340°C, preferably comprised between 180 °C and 330 °C, wherein said ethylene-propylene copolymer is further degraded;

(g) recovering said at least one further degraded ethylene-propylene copolymer leaving said underwater die-face cutter.

For the purpose of the present invention, a single screw extruder provided with a reciprocating single screw, three conveying and mixing zones, a gear pump, an underwater die-face cutter, which also has the following features, can be used: compared to a traditional single screw extruder in which the screw has the shape of a continuous spiral, the reciprocating single screw has threads interrupted at radial intervals of 120° and pins or teeth coordinated with the aforesaid threads are inserted in the barrel; compared to a traditional single screw extruder in which the movement of the screw consists solely of rotation, the movement of the reciprocating single screw consists of rotation and oscillation, in fact, the rotation of the reciprocating single screw is superimposed by an oscillation (or axial stroke) that leads to an improved mixing and degradation capacity inside the extruder; the frequency of oscillation of the axial motion (“reciprocating action”) is the same as the rotational speed and a gear system ensures that for each rotation of the reciprocating single screw a complete forward and backward stroke is made; the oscillation amplitude is equal to approximately 20 mm; during the execution of the aforesaid complete forward and backward stroke, the paths traced by the aforesaid pins (or teeth) cover and clean the entire surface of the reciprocating single screw so as to avoid the presence of zones with stagnation of material, in practice the aforesaid threads (“kneading flights”) sway around the pins (or teeth) and this allows to obtain a very uniform speed gradient between said threads (“kneading flights”) and said pins; the combination of the rotary and axial movement of the reciprocating single screw produces an extensional flow with an energetic dispersive mixing action between threads (“kneading flights”) and pins; the melting of the ethylene-propylene copolymer takes place in the gap between the thread (“kneading flight”) and the pin; thanks to the aforesaid improved mixing capacity, the ethylene-propylene copolymer does not present inhomogeneity in terms of solid part and molten part, but said ethylene-propylene copolymer melts at the same time giving rise to high viscosities, in particular in the first two mixing zones, which generate high shear stress values.

Further details regarding the aforesaid single screw extruder can be found, for example, in Elemans P. H. M. and Mejer H. E. H, “Polymer Engineering and Science” (1990), Mid-August, Vol. 30, No. 15, p. 893-904. In accordance with a preferred embodiment of the present invention, said single screw extruder operates at a screw rotation speed comprised between 220 rpm and 300 rpm, preferably comprised between 230 rpm and 290 rpm.

In accordance with a preferred embodiment of the present invention, said single screw extruder operates at a flow rate comprised between 300 kg/h and 700 kg/h, preferably comprised between 400 kg/h and 600 kg/h.

When leaving said underwater die-face cutter, the totally degraded ethylenepropylene copolymer is in the form of pellets that are separated from the water, for example by centrifugation, and subsequently sent, by pneumatic transport, to the drying and packaging section.

In accordance with a preferred embodiment of the present invention, in step (b) of the aforesaid process, said at least one ethylene-propylene copolymer has the following features: from 61% by weight to 67% by weight, preferably from 62% by weight to 66% by weight, with respect to the total weight of said ethylene-propylene copolymer, of ethylene; from 33% by weight to 39% by weight, preferably from 34% by weight to 38% by weight, with respect to the total weight of said ethylene-propylene copolymer, of propylene; a weight average molecular weight (M_w) comprised between 120000 Da and 200000 Da, preferably comprised between 140000 Da and 170000 Da; a polydispersity index (PDI), i.e. the ratio between the weight average molecular weight (M_w) and the number average molecular weight (M_n) (M_w/M_n), comprised between 2 and 4.5, preferably comprised between 2.1 and 3.5; a peak crystallization temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 20 C, preferably comprised between 0 C and 15°C; a peak melting temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 35°C, preferably comprised between 0 C and 30 C; a melting enthalpy (AH), determined by differential scanning calorimetry (DSC), greater than or equal to 25 J/g, preferably comprised between 28 J/g and 40 J/g; a minimum number of propylene inversions, determined by NMR analysis, comprised between 0.1 and 0.25, preferably comprised between 0.12 and 0.22.

Ethylene -propylene copolymers useful in the process object of the present invention, can be obtained in accordance with the process described in American patent US 9,701,764 in the name of the Applicant and incorporated herein by reference.

Figure 1 reported below shows an embodiment of the process object of the present invention as well as a block diagram of the plant and of the single screw extruder used in said process, for the sole purpose of illustration and not limitation thereof.

In particular, in Figure 1, the ethylene-prop ylene copolymer (1) is fed, through the feed hopper to the single screw extruder provided with three conveying and mixing zones, gear pump and underwater die-face cutter, said three conveying and mixing zones and said gear pump operating at specific temperature ranges. In particular, the ethylenepropylene copolymer through the first conveying and mixing zone is uniformly mixed and heated and uniformly softened thereby obtaining the ethylene-propylene copolymer (2) which is conveyed to the second conveying and mixing zone where it is further mixed and heated and partially degraded thereby obtaining the partially degraded ethylenepropylene copolymer (3) which is conveyed to the third conveying and mixing zone where it is further mixed and heated and further degraded thereby obtaining the further degraded ethylene-propylene copolymer (4) which is conveyed to the gear pump thereby obtaining a further degraded ethylene-propylene copolymer (5) which is recovered at the exit of the underwater die-face cutter in the form of pellets which are separated from water, for example by centrifugation, and subsequently sent by pneumatic transport to the drying and packaging section.

As already reported above, the present invention also relates to a composition of lubricating oil comprising the aforesaid viscosity index improver (V.I.I.) additive for lubricating oils and a base oil.

Consequently, the present invention also relates to a composition of lubricating oil comprising: from 1% by weight to 30% by weight, preferably from 2% by weight to 20% by weight, with respect to the total weight of said composition of lubricating oil of the above viscosity index improver (V.I.I.) additive for lubricating oils; a base oil of mineral origin, or of synthetic origin, or a mixture thereof, said base oil of mineral origin or of synthetic origin being preferably selected from the base oils belonging to Group I, or to Group II or to Group III.

As reported above, the viscosity index improver (V.I.I.) additive for lubricating oils object of the present invention is advantageously usable in base oils of mineral origin, or of synthetic origin, or mixtures thereof. Preferably, said base oils of mineral origin or of synthetic origin are selected from the base oils belonging to Group I, or Group II or Group III.

Consequently, the present invention also concerns the use of the aforesaid viscosity index improver (V.I.I.) additive for lubricating oils in base oils of mineral origin, or of synthetic origin, or mixtures thereof, said base oils of mineral origin or of synthetic origin preferably selected from the base oils belonging to Group I, or Group II or Group III.

In order to better understand the present invention and to put it into practice, some illustrative and non-limiting examples thereof are reported below.

The analysis and the characterization methodologies reported below were used.

Spectra ¹³C-NMR

¹³C-NMR spectra were recorded at 120°C by nuclear magnetic resonance spectrometer mod. Bruker Avance 300, equipped with 10 mm probes and variable temperature.

For this purpose, polymeric solutions of the ethylene-propylene copolymers as such and of the degraded ethylene-propylene copolymers obtained as reported in the following examples, having concentrations equal to 10%-15% w/v (g/mL) prepared using deuterated tetrachloroethane (C2D2CI4) at 120 C and tetramethylsilane (TMS) as internal standard, were used.

The structure of the aforesaid ethylene-propylene copolymers [i.e. ethylene (%), propylene (%) content] was determined through the analysis of the aforesaid spectra based on what is reported in the literature by Di Martino S., Kelchterrmans M., in "Journal of Applied Polymer Science” (1995), Vol. 56, Issue 13, pg. 1781-1787 (method No. 3, pg. 1784).

The minimum number of propylene inversions of the aforesaid ethylene- propylene copolymers was determined through the analysis of the aforesaid spectra based on what is reported in the literature by Randall J. C., in “Macromolecules” (1978), Vol. 11, No. l, p. 33-36.

Determination of the gelation index and of the incipient gelation temperature

For this purpose, the degraded ethylene-propylene copolymers obtained as reported in the following examples, were dissolved in an amount equal to 10% by weight in the reference Group III base oil (non-standard) having the features reported in the following Table 1:

Table 1

When the aforesaid ethylene-propylene copolymers are completely dissolved, the solutions obtained were subjected to dynamic mechanical analysis (DMA), operating under the following conditions: measurement of steady state dynamic viscosity [PP40 (parallel plates) - Anton Paar rheometer Mod. MCR 501); temperatures investigated: from 60°C to 0°C with steps of 5°C, with 20 minutes of thermo statting at each temperature before each test; stress sequence 0.1 Pa-3.2 Pa for each temperature for 100 seconds.

In order to carry out homogeneous comparisons of the steady state dynamic viscosity values between samples of ethylene-propylene copolymers that can be distinguished by weight average molecular weights (M_w) and by polydispersity indices dissimilar, a normalisation operation was carried out on the data obtained.

For the aforesaid purpose, for each sample of ethylene-propylene copolymer, all the values of steady state dynamic viscosity obtained were divided by a datum of steady state dynamic viscosity appropriately selected as a reference: for the purpose of the present invention, the datum of steady state dynamic viscosity at 40°C was selected.

By dividing all the steady state dynamic viscosity values obtained by the value obtained at 40 C according to the following formula: wherein η norm is the normalized steady state dynamic viscosity, η ^{x c} is the steady state dynamic viscosity measured at different temperatures, and η⁴⁰°^c is the steady state dynamic viscosity measured at 40°C, the values of normalized steady state dynamic viscosity were obtained.

By way of example not limiting the present invention, the values obtained for the ethylene-propylene copolymers of Examples 7 (invention) and 8 (comparative) below are reported in Table 2.

Table 2

From the measurements of normalized steady state dynamic viscosity at 0 C and 60 C, it is possible to obtain the gelation index defined, as reported above, from the ratio:

By way of example not limiting the present invention, the values obtained for the ethylene-propylene copolymers of Examples 7 (invention) and 8 (comparative) below are reported in Table 3.

Table 3 In order to determine the incipient gelation temperature, the procedure reported below was applied which, by way of example not limiting the present invention, was applied to the ethylene-propylene copolymers of Examples 7 (invention) and 8 (comparative) below.

To this end, it was operated as follows: - obtain the exponential trend line interpolating the data of normalized steady state dynamic viscosity between 60 C and 30°C as reported in Figure 2 [Example 7 (invention)] and in Figure 3 [Example 8 (comparative)] [the abscissa reports the temperature (T) in degrees centigrade; the ordinate reports normalized (η norm.) steady state dynamic viscosity; - obtain the values of normalized (η norm.) steady state dynamic viscosity using the aforesaid trend line for the entire temperature range (from 60°C to 0°C) as reported in Table 4:

Table 4 obtain the percentage differences between the actual values of normalized (η norm.) steady state dynamic viscosity and those calculated according to the following formula: Δ% = (η norm.- η calc.)/η calc, x 100 wherein η norm, is the normalized steady state dynamic viscosity and η calc, is the calculated steady state dynamic viscosity: the values obtained are reported in Table 5.

Table 5

The incipient gelation temperature for the purpose of the present invention was deemed to be the one in which the value of the normalized (η norm.) steady state dynamic viscosity was found to be at least 75% higher than the datum calculated as reported in Figure 4 [Example 7 (invention)] and in Figure 5 [Example 8 (comparative)] [the abscissa reports the temperature (T) in centigrade degrees; the ordinate reports the normalized (η norm.) steady state dynamic viscosity in logarithmic scale.

Determination of the shape stability

The shape stability was evaluated on the degraded ethylene-propylene copolymers obtained as reported in the following examples, using extruded “spaghetti”.

For this purpose, the “spaghetti” were obtained by feeding the aforesaid ethylenepropylene copolymers to a capillary rheometer operating at a temperature equal to 140 C, at a shear rate equal to 1 s’¹, and relaxation of 16 hours.

The “spaghetti” thus obtained were hung on a special support, at 25 C, and the variation in length over time was evaluated. The percentage variation in length was determined, as reported above, by the following formula:

AL% = [(L/L0)-l)]*100 wherein L is the length after 24 hours and L0 is the initial length equal to 10 cm.

Determination of the molecular weight

The determination of the weight average molecular weight (M_w), of the number average molecular weight (M_n) and of the polydispersity index (PDI) corresponding to the ratio M_w/M_n, of the ethylene-propylene copolymers as such and of the degraded ethylene-propylene copolymers obtained as reported in the following examples, was carried out by GPC (“Gel Permeation Chromatography”), using the integrated HT-GPC PL220 tool from Agilent Technologies that uses three detection lines: refractive index (IR), viscometer (VS) and DALLS (“Dual Angle Laser Light Scattering”), by operating under the following conditions: a pre-column (Guard column) having dimensions 50 x 7.5 mm and particle size equal to 10 μm; three GPC columns from Agilent Technologies with dimensions of 300 x 7.5 mm, particle size equal to 10 pm at mixed porosity; Mettler XPR 225 analytical balance; laboratory glassware; distillation and degassing system for solvent recovery;

50 ml automatic distributors;

PL-SP-260 automatic dissolver from Agilent Technologies; column injection temperature: 135°C; temperature of columns and detection lines: 135 C; solvent/eluent: 1,2-dichlorobenzene (99+% for HPLC - Acros Organics™); flow: 1 mL/min; calculation of the molecular mass by universal calibration curve.

The aforesaid operating conditions have been constantly monitored through a personal computer provided with Agilent GPC/SEC software from Agilent Technologies.

The calibration was carried out as follows.

A solution in 1,2-dichlorobenzene (HPLC 99+% for HPLC - Acros Organics™) containing a polystyrene (PS) standard having a nominal peak molecular weight (M_p) equal to 100 kDa was prepared in order to determine the interdetector delay (IDD) and the calibration constants of the four signals (RID, VS, LS15° and LS90°).

Seven solutions in 1,2-dichlorobenzene (HPLC 99+% for HPLC - Acros Organics™) were also prepared, each containing two monodisperse polystyrene (PS) standards having a different nominal peak molecular weight (M_p) and at different concentration: the selection of the molecular weights was carried out in such a way that the chromatographic peaks eluted were well separated, while the selection of the concentrations of each standard was inversely proportional to the molecular weight. The range of the different nominal peak molecular weights (M_p) of the aforesaid standards used for the calibration curve was comprised between 2 kDa and 7000 kDa.

The different solutions were prepared, under stirring, at room temperature (25 C) in the aforesaid PL-SP-260 automatic dissolver from Agilent Technologies.

The calibration curve was calculated through a personal computer provided with the aforesaid Agilent GPC/SEC software from Agilent Technologies, using a 3rd order polynomial function.

Differential Scanning Calorimetry (DSC) Differential Scanning Calorimetry (DSC), in order to determine the temperature of the crystallization peak, the temperature of the melting peak, the melting enthalpy (AH) and the residual melting enthalpy (AH) at a temperature greater than 60 C, of the ethylene-propylene copolymers as such and of the degraded ethylene-propylene copolymers obtained as reported in the following examples, was carried out by means of the following instrument: a differential scanning calorimetry (DSC) equipped with 50 self-sampling positions; manufacturing company: TA Instruments; model: QI 000; atmosphere of the oven: nitrogen; cooling system: two-stage (-100 C) refrigeration cycle (“Refrigerated Cooling System” - RCS); analytical balance; small crucible closing press; aluminium crucibles and lids.

Samples weighing equal to 15+1 mg were collected from the aforesaid ethylene- propylene copolymers: the samples were as flat as possible so as to ensure the greatest contact surface with the aluminium crucible. After placing the sample of collected copolymer on the crucible, the crucible was closed with an aluminium lid previously perforated with a punch so as to allow the steam emitted by the sample to exit. The crucible was closed with the lid by means of a special press.

The crucibles containing the different samples to be analysed and the empty reference crucible were placed in the numbered self-sampling positions (each position was associated with the name of the sample) and, subsequently, the computer, the differential scanning calorimetry (DSC) and the refrigerated cooling system (RCS) were switched on in sequence and the heat treatment to be carried out was selected. The Thermal Advantage” software, Version 5.1.2, was used for the analysis.

Once the analysis was completed, the thermograms obtained were processed using the “Universal Analysis” software Version 4.7A.

The heat treatment to which the samples were subjected was as follows: 1st step: scanning cooling from +25 C to -90°C, at 20 C/min;

2nd step: isothermal for 3 minutes at -90°C;

3rd step: scanning heating from -90 C to +170°C, at 20°C/min (1st heating);

4th step: isothermal for 3 minutes at +170 C;

5th step: scanning cooling from +170 C to -90°C, at 20°C/min;

6th step: isothermal for 3 minutes at -90 C;

7th step: scanning heating from -90 C to +170°C, at 20 C/min (2nd heating).

From the analysis of the DSC traces of the aforesaid 5th and 7th steps it is possible to determine respectively the peak crystallization temperature and the peak melting temperature as reported in Figure 6 [Example 7 (invention)] and in Figure 7 [Example 8 (comparative)] [the abscissa reports the temperature in degrees centigrade; the ordinate reports the heat flow in mW] .

Determination of the thickening power (TP)

For this purpose, the degraded ethylene-propylene copolymers obtained as reported in the following examples were dissolved in an amount equal to 1% by weight in the reference base oil SN 150 Group I (Eni S.p.A.).

The thickening power (TP) was determined according to ASTM D7042-04 standard and calculated according to the following formula:

TP = KV100 C_{(oil + polymer 1 % weight)} - KV 100 C _(oil) wherein KV is the kinematic viscosity.

Determination of the shear stability index (SSI)

For this purpose, the degraded ethylene-propylene copolymers (EPR) obtained as reported in the following examples, were dissolved in an amount equal to 1% by weight in the reference base oil SN 150 Group I (Eni S.p.A.).

The shear stability index (SSI) was determined according to ASTM D7109-12 standard (30 “cycles of shearing”).

EXAMPLES 1-4

(i.e. ethylene-

Four ethylene-propylene copolymers were prepared by operating as described below.

The polymerization reaction in suspension, continuously, was carried out in a 400 L reactor (of which 300 L useful), equipped with impeller blades, bottom anchor scraper and discharge from the bottom.

Said reactor was fed with: liquid propane (inert diluent) (Lyondelbasell); liquid propylene (monomer) (Lyondelbasell); gaseous ethylene (monomer) (Versalis); gaseous hydrogen (molecular weight regulator) (SIAD); a suspension of vanadium acetylacetonate V(acac)3 (Reactana) in paraffinic oil (8% by weight of vanadium acetylacetonate with respect to the total weight of the suspension) (catalyst); diethylaluminium chloride (DE AC) (as such; 98.5% degree of purity; Al/Cl molar ratio: 1; Albemarle) (co-catalyst); dichlorophenyl ethyl acetate (DCPAE) (as such; 97% degree of purity; Chem Solutions) (activator).

Table 1 reports the operating conditions used: feeding the compounds to the reactor, temperature and pressure.

The average residence time of the compounds in the reactor was about 1 hour. The ethylene-propylene copolymer suspension obtained was discharged from the bottom of the reactor and sent, continuously, to the strippers filled with water at 120 C, to which steam was also sent, so as to obtain the stripping of hydrogen, of propane and of the unreacted monomers (propylene and ethylene).

In the aforesaid strippers, unreacted diethylaluminium chloride (DEAC) and dichlorophenyl ethyl acetate (DCPAE) were also neutralized and decomposed with a solution of sodium hydroxide (NaOH) and with the consequent formation of aluminium hydroxide/aluminium oxide [Al(OH)₃/Al₂O₃], acetic acid/acetic anhydride [CH₃COOH/(CH₃CO)₂O] and sodium chloride (NaCl).

The water leaving the strippers, which also contain traces of unreacted vanadium, were collected and sent to water treatment.

The ethylene-propylene copolymer obtained was subjected to the above reported characterizations and the results obtained are reported in Table 6.

Table 6

⁽¹⁾: liquid propylene (monomer);

⁽²⁾: liquid propane (inert diluent); gaseous ethylene (monomer); ^,4): gaseous hydrogen (molecular weight regulator);

⁽⁵⁾: purge of the gas phase;

⁽⁶⁾: suspension of vanadium acetylacetonate V(acac)3 in paraffinic oil (8% by weight of vanadium acetylacetonate with respect to the total weight of the suspension) (catalyst);

⁽⁷⁾: diethylaluminium chloride (as such; 98.5% degree of purity) (co-catalyst);

⁽⁸⁾: molar ratio between the aluminium present in the co-catalyst and the vanadium present in the catalyst;

⁽⁹⁾: dichlorophenyl ethyl acetate (as such; 97% degree of purity) (activator);

⁽¹⁰⁾: molar ratio between the activator and the vanadium present in the catalyst;

⁽¹¹⁾: gas phase in which the measurements are performed to determine the concentration of the monomers in the reactor expressed in % by volume (%/v).

EXAMPLES 5-8

Preparation of the ethylene-propylene copolymers useful as viscosity index improver .1.1.) additives

The ethylene-propylene copolymers obtained in Examples 1-4 were subjected to the heat degradation process operating as reported below.

For this purpose, the ethylene-propylene copolymer was fed to a single screw extruder (BUSS MDK 140 from Aaron Equipment) with reciprocating screw, provided with three conveying and mixing zones, a gear pump and an underwater die-face cutter, having a diameter equal to 140 mm and a ratio between the length (L) and the diameter (D) (L/D) equal to 11, at a flow rate equal to 500 kg/h, said extruder operating under the following conditions: screw rotation speed: 280 rpm; temperature profile 1^st zone: 180°C - 240°C; temperature profile 2^nd zone: 250 C - 300°C; temperature profile 3^rd zone: 190 C - 300°C; gear pump temperature: 190 C - 300°C.

When leaving the underwater die-face cutter (cutting speed equal to 2700 rpm), the degraded and cooled ethylene-propylene copolymer was recovered in the form of pellets which were subjected to centrifugation to separate them from the water and subsequently sent by pneumatic transport to the drying and packaging section. The degraded ethylene-propylene copolymer obtained was subjected to the above reported characterizations and the results obtained are reported in Table 7.

Table 7 ( ^l: ethylene-propylene copolymer of Example 1 (comparative);

⁽²⁾: ethylene-propylene copolymer of Example 2 (invention);

⁽³⁾: ethylene-propylene copolymer of Example 3 (invention);

⁽⁴⁾: ethylene-propylene copolymer of Example 4 (comparative).

From the data reported in Table 7 it can be seen that only the ethylene-propylene copolymers (EPR) having the features in accordance with the present invention give the desired results when used as viscosity index improver (V.I.I.) additives, in particular in terms of shear stability index (SSI), thickening power (TP), gelation index (GI), incipient gelation temperature and shape stability.

Claims

1. Viscosity index improver (V.I.I.) additive for lubricating oils comprising an ethylene-propylene copolymer, said additive having the following features: a shear stability index (SSI) comprised between 18% and 30%, preferably comprised between 20% and 28%, measured according to ASTM D7109-12 standard (30 cycles of shearing) in Group I base oil; a thickening power (TP) comprised between 3 cSt and 8 cSt, preferably comprised between 4 cSt and 7.5 cSt, measured according to ASTM D7042-04 standard in Group I base oil; a gelation index defined by the ratio: where η represents the steady state dynamic viscosity measured in Group III base oil, said gelation index being lower than or equal to 7000, preferably comprised between 30 and 6000, more preferably comprised between 50 and 5000; an incipient gelation temperature, measured in Group III base oil, at a temperature lower than or equal to 10 C, preferably comprised between 0 C and 10°C; a shape stability defined according to the following formula:

ΔL% = [(L/L0)-l)]*100 wherein L is the length after 24 hours and L0 is the initial length equal to 10 cm, lower than or equal to 5%, preferably lower than or equal to 4%, more preferably lower than or equal to 3%.

2. Viscosity index improver (V.I.I.) additive for lubricating oils according to claim 1, wherein said ethylene-propylene copolymer has the following features: from 61% by weight to 67% by weight, preferably from 62% by weight to 66% by weight, with respect to the total weight of said ethylene-propylene copolymer, of ethylene; from 33% by weight to 39% by weight, preferably from 34% by weight to 38% by weight, with respect to the total weight of said ethylene-propylene copolymer, of propylene; a weight average molecular weight (M_w) comprised between 60000 Da and 120000 Da, preferably comprised between 65000 Da and 100000 Da; a polydispersity index (PDI), i.e. the ratio between the weight average molecular weight (M_w) and the number average molecular weight (M_n) (M_w/M_n), comprised between 2 and 4, preferably comprised between 2.1 and 3; a peak crystallization temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 20°C, preferably comprised between 0°C and 15 C; a peak melting temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 35°C, preferably comprised between 0 C and 30 C; a melting enthalpy (AH), determined by differential scanning calorimetry (DSC), greater than or equal to 25 J/g, preferably comprised between 28 J/g and 40 J/g; a minimum number of propylene inversions, determined by NMR analysis, comprised between 0.1 and 0.25, preferably comprised between 0.12 and 0.22.

3. Continuous process for reducing the molecular weight of ethylene-propylene copolymers comprising the following steps:

(a) providing at least one single screw extruder comprising a chamber, a reciprocating single screw mounted in said chamber, said reciprocating single screw being capable of rotating and oscillating within said chamber, said chamber having at least one feeding opening and at least one discharging opening, said extruder comprising three conveying and mixing zones, a gear pump, an underwater dieface cutter;

(b) feeding at least one ethylene-propylene copolymer into said single screw extruder;

(c) conveying said at least one ethylene-propylene copolymer through a first conveying and mixing zone which operates at a temperature comprised between 140 C and 260°C, preferably comprised between 150°C and 250 C, wherein said ethylene-propylene copolymer is uniformly mixed and heated and uniformly softened;

(d) conveying said at least one uniformly softened ethylene-propylene copolymer leaving said first conveying and mixing zone to a second conveying and mixing zone which operates at a temperature comprised between 220°C and 33O°C, preferably comprised between 230 C and 320°C, wherein said ethylene-propylene copolymer is further mixed and heated and partially degraded; (e) conveying said at least one partially degraded ethylene-propylene copolymer leaving said second conveying and mixing zone to a third conveying and mixing zone which operates at a temperature comprised between 170 C and 340 C, preferably comprised between 180 C and 330 C, wherein said ethylene-propylene copolymer is further mixed and heated and further degraded;

(f) conveying said at least one further degraded ethylene-propylene copolymer leaving said third conveying and mixing zone to said gear pump which operates at a temperature comprised between 170°C and 340 C, preferably between 180°C and 33O°C, wherein said ethylene-propylene copolymer is further degraded;

(g) recovering said at least one further degraded ethylene-propylene copolymer leaving said underwater die-face cutter.

4. Continuous process for reducing the molecular weight of ethylene-propylene copolymers according to claim 3, wherein said single screw extruder operates at a screw rotation speed comprised between 220 rpm and 300 rpm, preferably comprised between 230 rpm and 290 rpm.

5. Continuous process for reducing the molecular weight of ethylene-propylene copolymers according to claim 3 or 4, wherein said single screw extruder operates at a flow rate comprised between 300 kg/h to 700 kg/h, preferably comprised between 400 kg/h and 600 kg/h.

6. Continuous process for reducing the molecular weight of ethylene-propylene copolymers according to any one of claims 3 to 5, wherein in step (b) of the above process said at least one ethylene-propylene copolymer has the following features: from 61% by weight to 67% by weight, preferably from 62% by weight to 66% by weight, with respect to the total weight of said ethylene-propylene copolymer, of ethylene; from 33% by weight to 39% by weight, preferably from 34% by weight to 38% by weight, with respect to the total weight of said ethylene-propylene copolymer, of propylene; a weight average molecular weight (M_w) comprised between 120000 Da and 200000 Da, preferably comprised between 140000 Da and 170000 Da; a polydispersity index (PDI), i.e. the ratio between the weight average molecular weight (M_w) and the number average molecular weight (M_n) (M_w/M_n), comprised between 2 and 4.5, preferably comprised between 2.1 and 3.5; a peak crystallization temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 20 C, preferably comprised between 0 C and 15 C; a peak melting temperature, determined by differential scanning calorimetry (DSC), lower than or equal to 35 C, preferably comprised between 0 C and 30 C; a melting enthalpy (AH), determined by differential scanning calorimetry (DSC), greater than or equal to 25 J/g, preferably comprised between 28 J/g and 40 J/g; a minimum number of propylene inversions, determined by NMR analysis, comprised between 0.1 and 0.25, preferably comprised between 0.12 and 0.22.

7. Composition of lubricating oil comprising: from 1% by weight to 30% by weight, preferably from 2% by weight to 20% by weight, with respect to the total weight of said lubricating oil composition of a viscosity index improver (V.I.I.) additive for lubricating oils according to claim 1 or 2; a base oil of mineral origin, or of synthetic origin, or a mixture thereof, said base oil of mineral origin or of synthetic origin being preferably selected from the base oils belonging to Group I, or to Group II or to Group III.

8. Use of the viscosity index improver (V.I.I.) additive for lubricating oils according to claim 1 or 2, in base oils of mineral origin, or of synthetic origin, or mixtures thereof, said base oils of mineral origin or of synthetic origin being preferably selected from the base oils belonging to Group I, or to Group II or to Group III.