on 20-Aug-2020 (Thu)

Many poly-alpha-olefins have flexible alkyl branching groups on every other carbon of their polymer backbone chain. These alkyl groups, which can shape themselves in numerous conformations, make it very difficult for the polymer molecules to align themselves up side-by-side in an orderly way. This results in lower contact surface area between the molecules and decreases the intermolecular interactions between molecules.^[9] Therefore, many poly-alpha-olefins do not crystallize or solidify easily and are able to remain oily, viscous liquids even at lower temperatures.^[10] Low molecular weight poly-alpha-olefins are useful as synthetic lubricants such as synthetic motor oils for vehicles and can be used over a wide temperature range.^[

Manual method [ edit ]

ASTM D97, Standard Test Method for Pour Point of Crude Oils. The specimen is cooled inside a cooling bath to allow the formation of paraffin wax crystals. At about 9 °C above the expected pour point, and for every subsequent 3 °C, the test jar is removed and tilted to check for surface movement. When the specimen does not flow when tilted, the jar is held horizontally for 5 sec. If it does not flow, 3 °C is added to the corresponding temperature and the result is the pour point temperature.

It is also useful to note that failure to flow at the pour point may also be due to the effect of viscosity or the previous thermal history of the specimen. Therefore, the pour point may give a misleading view of the handling properties of the oil. Additional fluidity tests may also be undertaken. An approximate range of pour point can be observed from the specimen's upper and lower pour point.

The color of lubricating oils can range from transparent to opaque. The color is based on the crude from which it is made, its viscosity, method and degree of treatment during refining, and the amount and types of additives included.

A change in oil color signifies a change in the chemistry of the oil or the presence of contaminants. For example, oil oxidation, mixing two dissimilar types of oil, and carbon insolubles from thermal failure can all darken oil. There is also a possibility that the oil darkening is due to a photochemical reaction from sunlight exposure.

Measuring color is based on a visual comparison of the amount of light transmitted through a defined depth of oil. This can be done with a predefined test method and instrumentation or a subjective view of the oil with reference to a color gauge. In either case, there may be a number of variables to monitor for quality results.

The ASTM D1500-07 test method can be used to compare the color of an oil sample to a glass slide. This test is used in lubricant manufacturing for quality-control purposes. It is performed using a standard light source to match a sample to a glass slide.

Consistency

Grease consistency depends on the type and amount of thickener used along with the viscosity of its base oil. A grease’s consistency is its ability to resist deformation by an applied force. The measure of consistency is called penetration, which is contingent on whether the consistency has been altered by handling or working.

ASTM D217 and D1403 methods are used to determine the penetration of unworked and worked greases. To measure penetration, a cone of a specific weight is allowed to sink into a grease for five seconds at a standard temperature of 25 degrees C (77 degrees F). The depth, in tenths of a millimeter, to which the cone sinks into the grease is its penetration.

A penetration of 100 would represent a solid grease, while a penetration of 450 would be semifluid. The National Lubricating Grease Institute (NLGI) has established consistency numbers or grade numbers from 000 to 6 that correspond to specified ranges of penetration numbers.

Certain conditions will affect the consistency required for a grease. The table below can help you select the correct consistency for an application.

Ponto de gota ou ponto de gotejamento é uma propriedade qualitativa que apresenta a temperatura na qual uma graxa lubrificante, ou qualquer substância de alta viscosidade, passa do estado sólido ou semi- sólido (altamente viscoso) ao líqüido, sob condições determinadas de pressão e movimento, por exemplo. Em outras palavras é uma medida da resistência de calor de uma graxa.

Na prática, nunca é adequado nem seguro usar uma graxa em um ambiente ou atividade cuja temperatura normal de trabalho seja próxima do seu ponto de gota. A temperatura adequada para o uso da graxa é sempre inferior ao ponto de gota.