Исследования температурной стойкости и окисляемости смазочно-охлаждающих жидкостей

Other specialized fluids include high water content fluids (HWCF), lithium complexes, aluminum complexes, waxes such as paraffin and stearate, and halogenated hydrocarbons including chlorofluorocarbon (CFC), halogenated fluorocarbon (HFC), halogenated chlorofluorocarbon (HCFC), and perfluorocarbon (PFC).

Performance Specifications

Important properties for metalworking lubricants, coolants, and fluids include concentration, flash point, and autogenous ignition temperature (AIT).

Concentration is measured after dilution of the fluid solution on a volumetric basis. 

Flash point is the lowest temperature at which a liquid can give off sufficient vapors to form an ignitable mixture in air near the surface of the liquid.

Autogenous ignition temperature (AIT) is the temperature at which ignition occurs spontaneously.

General properties of a lubricating oil viscosity

This indicates the resistance of a liquid to flow.

There are several units for measuring viscosity. Formerly, the unit commonly used in America was Saybolt Universal Second (SSU), measured at 100°F or 210°F. In Europe, the former widely used unit was Redwood I second (RWI), measured at 100°F or 210°F. At present, most countries have switched over to the metric system that employs the unit Centistokes (cSt), measured at 40°C or 100°C.

Oil with higher viscosity can stand greater pressure without being squeezed out of the lubricating surfaces. However, the high internal friction of the oil may offer greater resistance to the movement of the lubricating parts. An oil of lower viscosity offers less resistance to the moving parts but the oil can be easily squeezed out of the lubricating surfaces. It is therefore important to select a lubricating oil of appropriate viscosity to achieve optimum lubrication effect.

Viscosity changes with temperature. Hence, the measuring temperature must be specified whenever the viscosity of a liquid is stated. When temperature rises, a liquid becomes less viscous. Similarly, a liquid becomes thicker when temperature drops.

Viscosity Index (VI) is an indication of how the viscosity of a liquid varies with temperature. A high VI means the liquid does not thin out so much when temperature rises. VI improver additives that are usually high molecular weight polymers can increase the VI of lubricating oil.

Increase in oil viscosity achieved by addition of polymers can be partially lost again through degradation of the polymer molecules by shear stress such as heavily loaded gears. Oil that can resist viscosity change due to shear are said to have high shear stability.

Pour Point

Indicates flow characteristic at low temperature.

Depends on the wax content of the oil.

Flash Point

Measures the readiness of the oil to ignite momentarily in air and is a consideration regarding the fire hazard of the oil.

Oxidation Stability

Oxidation of oil will produce resins and sludge that may plug filters and oil passages.

Oxidation can also produce soluble organic acids that may cause corrosion of machine parts.

A good lubricating oil should resist oxidation.

Acidity and Alkalinity (Total Acid Number and Total Base Number)

High acidic oil may cause corrosion of machine parts

Most engine oils show some alkalinity due to the addition of detergent type additives and this helps to neutralize any acid formed in the oil by oxidation.

After prolong usage, lubricating oil may contain organic acids formed by oxidation. Therefore, a measurement of the acidity of an oil can reflects its degree of oxidation.

Detergency

Most engine oils contain detergent and dispersant additives to prevent dirty particular produced by incomplete combustion from accumulating and plating metal surface.

CorrosionInhibition

Acidic materials in oil can cause corrosion of machine parts.

Corrosion can be minimized by the additives of corrosion inhibitor that reacts with metal to form a protective layer separating the acidic materials and the metal.

Anti-foaming Property

Foaming reduces the lubricity of oil because the air bubbles in the foam will create a barrier between the oil and the metal surface.

Foam can also produce resistance to the movement of machine parts.

In a hydraulic system, foam will reduce the cohesive power of the oil and cause the hydraulic pressure to drop.

Good lubricating oil will not foam easily and can disperse foam quickly. Anti-foam additives can help to reduce the foaming tendency of oil.

Emulsification and Demulsification

Emulsification is the homogenous mixing of oil and water.

Some oil requires high emulsibility so that it can mix with water easily, for example, some metal cutting oils.

The emulsibility of oil can be improved by the addition of emulsifying agent that has strong affinity for both oil and water, thus holding the oil and water molecules together.

Some other lubricants require good demulsibility so that water can be separated from the oil easily, e.g. Turbine oil. The demulsibility of oil can be achieved by good refining technique.

Anti-wear Property

Some lubricating conditions may call for extremely light oil, an oil of lower viscosity than the load-speed relationship of the machine may indicate. Under such condition, wear of the metal surfaces may occur. Anti-wear additive forms a protective coating on the metal surfaces, allowing the surfaces to slide on each other with a minimum loss of metal.

Extreme Pressure Loading Property (EP)

Heavy loading, extreme pressure and intense heat may cause machine moving parts to melt and weld together, hence interfering motion.

The extreme-pressure additive in oil can react with the metal to form a compound with low melting point. The intense heat developed due to the extreme-pressure loading will be dissipated in the melting of the compound instead of welding the two metallic parts.

EP properties are usually measured by Timken method (ASTM D 2782) or FZG Gear Machine (IP 334). In the Timken method, a steel cup rotates against a steel block in a lubricant bath. The maximum load that will not cause scoring is the OK load. In the FZG Gear Machine, special gear wheels are run in the lubricants under test. The loading is increasing by stages and the stages at which gear damages occur is reported as the FZG loading stage of the lubricant.

Tackiness

Tacky oil contain tackiness agent and will stick to the lubricating surface for a long time without being spattered. Lubricants used in textile machinery and wire

Penetration

This indicates the consistency (hardness or softness) of grease. It is measured by dropping a pointed cone into the grease and sees how far the cone penetrates into the sample. Different ranges of penetration are identified by the following National Lubricating Grease Institute (NLGI) Grade Numbers: 000, 00, 0, 1, 2, 3, 4, 5, and 6. Grade 000 isthesoftestwhileGrade 6 isthehardest.

Most grease thickened with soaps become softer with increase in temperature, but some greases become progressively harder upon exposure to high temperature. Non-soap thickeners, as a whole, show very little change in consistency with temperature rise.

WaterResistance

Greases with thickeners soluble in water will emulsify and fluidize if come into contact with relatively large amount of water. In general, calcium, lithium and aluminium soaps are highly water resistance while sodium soap greases are soluble in water.

Oxidation Stability

Oxidation will cause the grease to harden, form varnish like films and eventually carbonize. Additives can improve the oxidation stability of grease.

Lubricating Properties

Both the oil and the thickener in soap type grease have lubricating properties. Inorganic non-soap thickener generally does not contribute to the lubricating of grease. The lubricating capability of the oil depends on its viscosity and viscosity index.

Anti-wearCharacteristic

Additives may be included in a grease to promote its anti-wear properties.

Extreme Pressure Capability (EP)

Some grease contains special additives to fortify its load carrying capability so that welding and scoring of metal can be minimized.

Dropping Point

It is the temperature at which the grease is fluid enough to drip. Grease with a dropping point below the operating temperature would not provide proper lubricant. However, the converse is not necessarily true; a dropping point above operating temperature is no guarantee of adequate lubrication since there may be change in consistency and deterioration in chemical properties of the grease at high temperatures.

In Europe, as well as in the US, regulations and laws have been enacted in the recent years which greatly affect the formulation of metalworking fluids. The industry using these fluids is continuously searching for better-performing cutting fluids, which have to be formulated using a certain amount of many different additives. On the other side, industry, employee unions and government enforce the trends for a healthy and safe work environment, as well as disposal aspects influencing the global environment. These trends have already affected the formulation of cutting fluids in Europe and the US as follows: removal of nitrite; replacing diethanolamine and replacing short chain chlorinated paraffins. This is already happening in Europe and may happen in the future in the US. All these removals and replacements have to be realized with the goal not to affect adversely the performance of the metalworking fluids.

Environment and health aspects:

Problematic components

The following groups of substances belong to the problematic components which may have been contained in cutting fluids in the past.

* Nitrosamines (N-Nitrosodiethanolamines)

* Formaldehyde-condensate materials

* Organic chlorine-containing substances (Chlorinated paraffins and polychlorinated biphenyls)

* Organic phosphorous-containing substances

* Polycyclic aromatic hydrocarbons, such as benzo(a)pyrene

* Others, such as boron- and lead-containing substances

Regarding their cancerous potential, the substances maybe ranked as follows.

Formulation of cutting fluids

In order to realize the lubricating process of cutting fluids water miscible and non-watermiscible metalworking fluids have to be carefully formulated. These will bediscussedseparately.

Non-Water Miscible Cutting Fluids

The general composition of non-water miscible cutting fluids can be characterized as follows.

Base oil + Extreme pressure/Antiwear additives + Other additives

The most important components of these fluids are base oil (mostly mineral oils, in certain cases esters and polyalphaolefines), "lubricating" additives (antiwear and extreme pressure additives), oil fog reducing additives, corrosion inhibitors (only in specific cases) and oxidation inhibitors.

Some lubrication improving additives, mostly defined as antiwear additives, are fatty oils (animal and plant oils) and synthetic fatty acid esters or fatty alcohols. The most important extreme pressure additivesbelong organic chlorine, phosphorous and sulfur compounds.

Organic Chlorine-Containing Substances

As they are highly effective and relatively cheap, chlorinated paraffins were widely used as antiwear and extreme pressure additives.

A 1986 German law on the use and disposal of wastes divides used oils into three categories which have to be collected separately. The regulation contains the following limiting data for used oils which are suitable for recycling.

0.2 % total halogenes; 4 ppm polychlorinatcd biphenyls

Used oils containing higher amounts of halogens, especially chlorine are treated as special waste causing extremely high costs of disposal.

In older to reduce the risk of nitrosamine generation, the "Technical Regulation for Dangerous Products (TRGS/611)," released in April 1993, specifics the following.

* Metalworking fluids must not contain nitrosamine generating components. This applies to nitrites as corrosion inhibitors as well as to a few organic nitrogen-containing components used as biocides.

* Secondary amines like diethanolamines as corrosion inhibitors must not be used any longer.

* This ban does not apply if by selecting special amines or

using effective inhibitors, it is impossible, for carcinogenic nitrosamines to form.

* The content of secondary amines brought in as by-products with other components must not exceed 0.2 %. A result of the production process, mono- and triethanolamines may contain small amounts of diethanolamines.

Organic Phosphorous-Containing Substances

Organic phosphorous containing substances, such as tricresylphophate, are used as antiwear and extreme pressure additives. For toxicological reasons, one should take care that the tricresylphosphate is free of O-cresol.

Other Substances

Lead containing components were used as antiwear and extreme pressure additives, and boron containing components are currently in use as corrosion inhibitors.

* Lead containing components

Lead naphthenates have been used in the past for some special applications, but in coolants only as an exception. Due to their long termeffect of increasing the lead

content in blood, lead based lubricants are hardly used any more.

* Boron containing components

Boric acid esters, as well as boric acid alkonolamine condensation products have gained certain importance as corrosion inhibitors, because they are biologically stable.

This property can result in certain disadvantageous effects as far as the disposal of coolants is concerned. As boron impairs the growth of certain plants, some countries released regulations that define maximum boron concentrations in waste water.

In addition to technical requirements, aspects of toxicology and industrial medicine will continue to exert an important influence on the development and formulation of cutting fluids. Higher fluid disposal costs will dictate a greater

emphasis upon fluid maintenance, and the use of products with longer service life and consistent long term performance.

It will become more and more difficult to find a balance between economic possibilities and ecological requirements. Of course, toxicologically and ecologically questionable products must be excluded from further use in metalworking fluids, if they pose any significant health risk under the conditions of application. On the other hand, it has to be taken into account that if unnecessary restrictions are, legislated, the technological performance of metalworking fluids will decrease. Today's formulations exhibit a good example to prove a reasonable balance between economic needs and ecological requirements.

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