The present invention relates to the use of fluid linear perfluoropolyethers having a very high kinematic viscosity, comprised within the range of from 8,000 mm²/s (cSt) to 40,000 mm²/s (cSt) at 20°C, and preferably of at least 10,000 mm²/s (cSt), as lubricating oils, and as the basic components of perfluorinated lubricating greases.

The use is well-known of fluid perfluoropolyethers, of neutral type, as lubricating oils endowed with high performance characteristics relating to the resistance to the atmospheric and chemical agents in general, even at high temperatures.

EP-A-95825 discloses lubricating greases based on polytetrafluoroethylene and certain perfluoropolyethers which may have a linear or branched structure. In the former case the viscosity of the polyether ranges from 40 to 6000 mm²/s. EP-A-89820 describes a process for the preparation of perfluoropolyethers of high viscosity but without linear structure. US-A-4 438 006 is directed to lubricant compositions which contain perfluoropolyethers having a viscosity at 38°C of from about 15 to about 1000 mm²/s.

It has been found now that the linear perfluoropolyethers having such a high kinematic viscosity is indicated above, show surprisingly improved characteristics of use, as lubricants, over the lower viscosity perfluoropolyethers used up to date.

The improvement in some specific properties is not attributale simply to the high viscosity of the product, and should be regarded as unforeseeable.

Accordingly, the present invention provides a process for lubricating under "mixed" lubrication conditions, characterized in that a perfluoropolyether of the neutral type, having a linear structure composed of a sequence of one or more of the following oxyperfluoroalkylene units:



        -CF₂CF₂O-, CF₂O-, -CF₂CF₂CF₂O-,



and having a kinematic viscosity of from 8000 to 40000 mm²/s (cSt) at 20°C is used as lubricant.

Also provided is a perfluorinated lubricating grease based on polytetrafluoroethylene (PTFE) and a perfluoropolyether as defined above, said grease containing from 60% to 90% by weight of oil, based on the total weight of the grease.

The chain end groups are of perfluorinated, neutral type.

In particular, the products belonging to the following classes are suitable:

• 1) R'_fO(CF₂CF₂O)_m(CF₂O)_nR_f
  wherein R_f and R'_f, equal to, or different from each other, are either -CF₃ or -C₂F₅, the perfluorooxyalxylene units being randomly distributed along the chain, and indices m, n are such integers that the viscosity (which is a function of the average molecular weight) is within the indicated ranges. Products of this type are disclosed in US-A-3,665,041 and are known on the market under the trade name "Fomblin z^(R)";
• 2) R'_fO(CF₂CF₂O)_PR_f
  wherein R_f and R'_f, equal to, or different from each other, are either -CF₃ or -C₂F₅ and index p is such an integer that the viscosity falls within the above indicated limits. Products of this type are disclosed in US-A-4,523,039;
• 3) R'_fO(CF₂CF₂CF₂O)_sR_f
  wherein R_f and R'_f, equal to, or different from each other, are -CF₃, or -C₂F₅ or -C₃F₇, and index s is an integer such that the viscosity is comprised within the above indicated ranges. Products of this type are disclosed in EP-A-148 482.

In consideration of the very high viscosity of the above products, and of the fact that such a viscosity, because of their linear structure, varies to a considerably low degree with varying temperature, as well as of their very low volatility, the products of the invention can be advantageously used as lubricants for high temperatures.

It is known that a relationship exists which allows the friction coefficient µ to be expressed as a function of Stribeck number η N/P, wherein η = dynamic viscosity; N = revolution speed; P = applied load per surface unit. Such a relationship, reported on a diagram allows the areas to be evidenced in which the system works under boundary lubrication, mixed lubrication and elastohydrodynamic lubrication conditions; on this regard, reference is made to "Lubricants", by D. Klamann (Verlag Chemie 1984), pages 38-42.

According to said monography mixed friction (lubrication) consists of fluid and non-fluid friction terms:

${\text{µmixed = x·µ}}_{\text{fluid}}{\text{+ y·µ}}_{\text{dry}}$

where x+y=1 and the contribution of the individual type of friction depends on the conditions.

The perfluoropolyethers used according to the present invention show a surprising and unexpected improvement as relates to the wear rate under "mixed" lubrication conditions, as compared to the up to date commonly used perfluoropolyethers, having a much lower viscosity. This improvement cannot be simply explained by taking into account the difference in viscosity; it could be thought as being in some way connected with the molecular structure of the product.

The equipment used for measuring the wear rate essentially comprises a a pin-on-rig system, consisting of a carbon steel (c 15) pin resting, for a surface area of 12 mm², on the cylindrical surface, revolving at a rate of 350 rpm, of a specimen of the same material, having an average surface roughness of 0.15 µm (microns), and a diameter of 45 mm. The pin is loaded with a weight of 15 kg.

The procedure used in the test was as follows.

The specimens and the pins are cleaned in 1,1,2-trichloro-2,2,1-trifluoroethane, let dry and weighed. The specimen is assembled on the equipment in such a way that it can dip inside a tray containing the lubricating oil. After 1,000 revolutions, the specimen is drawn from the equipment, is cleaned in 1,1,2-trichloro-2,2,1-trifluoroethane, let dry and weighed again, and is then assembled again on the equipment. The measurement is repeated after 10,000 rpm. Then the difference in weight is computed relative to the number of run metres (ΔW/L): Test Time (Minutes) Z 1600 ΔW/L (g/m) Z 25 ΔW/L (g/m) Z 03 ΔW/L (g/m) 3 0.1x10⁻⁵ 3x10⁻⁵ 3.1x10⁻⁵ 30 1x10⁻⁵ 2x10⁻⁵ 2.5x10⁻⁵ Z 1600 Perfluoropolyether belonging to Class (1), a commercial product Fomblin^(R) Z by Montedison, having a kinematic viscosity of 16,000 mm²/s (cSt) at 20°C.
Z 25 Ditto, with a viscosity of 250 mm²/s (cSt)
Z 03 Ditto, with a viscosity of 30 mm²/s (cSt).

The difference in the values measured between the investigated products evidences a lower wear rate in the high-viscosity oil. Above all, the necessary times for wear to be obtained are much higher for the highest-viscosity oil.

The perfluoropolyethers used according to the invention are particularly fit for the formulation of perfluorinated greases endowed with enhanced characteristics as compared to analogous products manufactured from lower-viscosity perfluoropolyether oils. Because of the vary low value of high-temperature oil separation which was observed (even at a much higher oil percentage in the grease than in the products of the prior art), the grease according to the present invention is particularly suitable for high temperature uses.

A very meaningful characterizing property of the grease according to the invention is the low static friction coefficient (measured as the starting resistent torque).

The method of preparation of the greases according to the invention is similar to the method reported in EP-A-95 825, with the difference that, because of the high viscosity of the perfluoropolyether, the kneading and the following homogenizing have to be performed under high temperature conditions (70°C).

The oil used is Fomblin^(R) Z 1600, an above-mentioned product, having a kinematic viscosity of 16,000 mm²/s (cSt) at 20°C.

It is introduced in an amount of 74.1% by weight, relative to the grease.

The thickening agent is a granular PTFE (average particle diameter 4-5 µm; m.w. 40,000), used in an amount of 25.9% by weight, relative to the grease.

The kneading and homogenizing method is reported in the above-cited EP-A.

For comparative purposes, a perfluorinated grease was prepared by using the same PTFE type, and Fomblin^(R) Z 25 oil, having a viscosity of 250 mm²/s (cSt).

The formulation comprised 68.5% by weight of Fomblin^(R) Z 25 and 31.5% by weight of PTFE. CHEMICO-PHYSICAL CHARACTERIZATION OF THE FORMULATED GREASE Consistency With Fomblin® Z 1600 With Fomblin® Z 25 Macropenetration mm/10' (ASTM D 217) 283 286 Macropenetration after 10,000 double shots, mm/10' (ASTM D 217) 270 298 Oil separation @ 204°C/30 h (FTMS 791-321) 1.6 % 10 % Oil evaporation @ 204°C/22 h (ASTM D 972) 0.06% 0.1%

Relative to the perfluorinated greases of the prior art, the very low degree of oil separation and evaporation is evident.

The rheological characterization of the grease was carried out by means of measurements of its dynamic viscosity by using the plate/cone system mounted on a Rotovisco HAAKE 12 system, with M150 head (PKII/0.3°).

The first type of measurements was carried out at 25°C with increasing γ̇ (flow gradient) values (20; 40; 80 sec⁻¹), with the viscosity of the grease being measured at start-up, and after 15 minutes. At each measurement, the used grease was replaced by fresh grease.

For a grease according to the invention, formulated with Fomblin^(R) Z 1600, an early transient period was observed, the duration of which increased with decreasing γ̇ (from 5 to 6 minutes within the selected γ̇ range). During this period, the grease showed a rheopexic behaviour, followed by a stabilization of the signal at an equilibrium value higher than the initial values. The oscillations of the signal were, under these conditions, lower than of Z greases having a lower viscosity. The dynamic viscosity of the investigated grease at 80 sec⁻¹ was equal to about 110 Pa·s (1,100 poises) whilst that of a grease formulated with Fomblin^(R) Z 25 oil was about 26 Pa·s (260 poise) under the same conditions.

The behaviour of the viscosity over time, with constant γ̇ values, shows hence that the grease according to the invention does not show, at start-up, a higher resistance than under equilibrium conditions. This result, important in connection with the power that the molar will have to apply at start-up, never occurred with greases containing lower-viscosity PFPE-based oils.

The limited range of signal oscillation is indicative of a better constancy of lubricating performance.

A second type of measurement was carried out under constant γ conditions of 40 sec⁻¹, at temperatures of 25°C; 50°C; 100°C. The endurance of the lubricating grease, at 100°C, is exceptionally good, whilst, under the same conditions, a grease formulated with a lower-viscosity oil (Fomblin^(R) Z 25) shows a trend to unhomogeneousness.

On the grease formulated according to the invention a tribological evaluation was carried out, by determining the welding load according to ASTM D 2596 Standard,

The welding load was higher than 700 kg (device measurement limit).