Method for the assessment of the dispersing capacity of new or used lubricating compositions and of additives for lubricating compositions
A method for the assessment of the dispersing capacity of new and used lubricating compositions, in particular for internal combustion engines, and of additives for lubricating compositions, includes the steps of: providing a homogeneous dispersion of at least one carbonaceous particulate in a composition to be tested, consisting of a lubricating composition or a fraction or component thereof, acquiring at least one micrographic image of a sample of said homogeneous dispersion, deposited on an observation support, and calculating the lacunarity of the acquired image as a parameter representative of the degree of dispersion of the particulate in the composition.
1. A method for the assessment of the dispersing capacity of new or used lubricating compositions and/or of additives for lubricating compositions, the method comprising the steps of:
providing a homogeneous dispersion of at least one carbonaceous particulate in a composition to be tested, the homogeneous dispersion consisting of a lubricating composition or a fraction or component thereof;
acquiring at least one micrographic image of a sample of the homogeneous dispersion, deposited on an observation support; and
calculating a lacunarity of the acquired image as a parameter representative of a degree of dispersion of the carbonaceous particulate in the composition.
2. The method according to claim 1 , wherein the step of calculating the lacunarity comprises the steps of:
defining, on the image, a grid formed by a plurality of cells;
evaluating a pixel density in each cell of the grid according to grid size and grid orientation with respect to a fixed reference system;
calculating the lacunarity with formula (I):
λ
ε
,
g
=
(
σ
μ
)
ε
,
g
2
(
I
)
wherein:
μ is the average of pixels per cell at each ε,
ε is the scale applied to the image,
g is the orientation of the grid with respect to the image,
σ is the standard deviation;
calculating a final lacunarity (Λ) as an average over all possible grid orientations (Ng), with formula (II):
Λ
=
∑
λ
g
N
g
.
(
II
)
3. The method according to claim 1 , wherein the carbonaceous particulate is a carbon black selected from the group consisting of: Vulcan XC72R, Corax N110, Corax N115, Corax N234, Corax N326, Corax N539, Corax N550 and Degussa S170.
4. Method according to claim 1 , wherein the step of acquiring the image is carried out with an optical microscope equipped with an objective at least 10×, keeping the sample at a temperature of at least 80° C.
5. The method according to claim 1 , wherein the sample is maintained during the step of acquiring the optical micrographic image at a temperature from 90° C. to 100° C.
6. The method according to claim 1 , wherein the homogeneous dispersion sample is prepared by depositing a quantity between 2 and 6 μL between two microscope slides.
7. The method according to claim 1 , wherein the carbonaceous particulate in the mixture is in an amount between 5% and 15% of the total weight of the mixture.
8. The method according to claim 7 , wherein the amount of carbonaceous particulate in the mixture is between 7% and 12%.
9. The method according to claim 1 , wherein the preliminary dispersion contains a concentration of carbonaceous particulate expressed as a percentage by weight of the total weight of the dispersion that is between 0.5% and 7%.
10. The method according to claim 9 , wherein the preliminary dispersion contains a concentration of carbonaceous particulate between 1.5% and 5%.
11. The method according to claim 1 , wherein the carbonaceous particulate is a carbon black having a polydispersity, defined as the width at mid-height of the Gaussian particle size distribution curves, between 10 nm and 50 nm and/or an average particle sizes between 10 nm and 70 nm and/or a surface polarity between 1% and 9%.
12. The method according to claim 11 , wherein the polydispersity is between 20 nm and 40 nm; and/or the average particle sizes is between 20 nm and 50 nm; and/or the surface polarity is between 2% and 6%.
13. The method according to claim 1 , wherein the step of providing the homogeneous dispersion comprises the steps of:
preparing a mixture comprising a predetermined amount of carbonaceous particulate and a first amount of lubricating composition or a fraction or component thereof;
homogenizing the mixture until a homogeneous paste is obtained;
adding a second amount of lubricating composition to the paste se as to obtain a preliminary dispersion;
homogenizing the preliminary dispersion; and
equilibrating the preliminary dispersion under mild stirring conditions and forming the homogeneous dispersion.
14. The Method according to claim 13 , wherein the mixture is mixed at a room temperature with a mixer operating at a speed between 1000 rpm and 1500 rpm for a time between 2 minutes and 5 minutes.
15. The method according to claim 13 , wherein the step of equilibrating the preliminary dispersion is conducted with a thermo-agitator operating at a speed between 200 and 500 rpm and at a temperature between 70° C. and 100° C. for a time exceeding 24 hours.
16. The method according to claim 15 , wherein the thermo-agitator is an orbital thermo-agitator.
17. The method according to claim 13 , wherein the homogenizing the mixture and the homogenizing the preliminary dispersion are carried out with a homogenizer operating at a speed between 18000 rpm and 24000 rpm, for a time exceeding 10 minutes at a temperature between 60° C. and 90° C.
18. The method according to claim 17 , wherein the temperature is between 70° C. and 80° C.
19. The method according to claim 17 , wherein the speed is between 20000 rpm and 21000 rpm.
20. The method according to claim 17 , wherein the time is between 10 and 15 minutes.