Microcrystalline waxes

Description

The designations of microcrystalline and amorphous waxes are used synonymously, although the first is a more accurate designation. These waxes differ from refined paraffin wax in crystal size and structure and in that they are tougher, more flexible, and have a higher tensile strength and melting point. They are also more adhesive and less lustrous and greasy. They bind solvents, oils, etc., much better than paraffin wax and thus prevent their sweating-out. It should be noted in general that:

1. High penetration value and/or high refractive index are indicative of flexibility.

2. Waxes of high penetration value generally have more "tack."

3. Flexibility is not a function of melting point.

4. Oil content influences flexibility only to a limited degree.

Differences between Microcrystalline and Paraffin waxes

Paraffin and microcrystalline waxes are both separated from crude petroleum. However, the process of manufacture and the resulting products are quite different. As crude petroleum is subjected to distillation by heating in a still at atmospheric pressure, the following products are removed in the order of their increasing boiling points: light petroleum gases, gasoline, naphtha, kerosene, gas oil, paraffin wax distillate, light, neutral lubricating oil fractions, and a residue in the still which will not distill overhead at atmospheric pressure without decomposition. Paraffin wax is separated from the paraffin-wax distillate by a relatively simple process, while the microcrystalline wax, which cannot be distilled without decomposition, is separated by a complex series of solvent separations from the residue remaining in the still.

Paraffin wax, which has very little affinity for oil, is separated from the paraffin-wax distillate by cooling and filtering in a common plate-and-frame filter press where the wax is retained on canvas and is removed. Oil remaining in the wax filter cake is removed by a sweating process which involves casting the wax in thin sheets and gradually raising the temperature to a point slightly below the melting point of the wax. In this process, the remaining oil runs out of the wax sheet leaving a paraffin wax which usually contains less than 0.5% oil.

Microcrystalline wax is present in the residual fraction from the still along with heavy residual lubricating oil and asphalt. Asphalt must be removed from the residue by conventional sulfuric acid treatment or one of the newer selective solvent processes before the wax can be separated. The microcrystalline wax is then removed as crude petrolatum from the residual lubricating oil by any one of several dewaxing processes which involve dilution with an organic solvent, chilling at 20 - 40ºF (from -7ºC to 4ºC), and separation of the crude petrolatum in high-speed centrifuges. Microcrystalline wax has a great affinity for oil and thus the crude petrolatum still contains 40 - 70% oil. Therefore, it is then mixed with another portion of organic solvent, usually a different solvent from the one used in the initial dewaxing step, and heated to dissolve the wax and oil. The blend is then cooled to precipitate the wax which is separated from the oil and solvent on a filter. The solvent used for deoiling microcrystalline wax must be polar, whereas for deoiling paraffin wax other types of solvent may also be used. Since the microcrystalline wax holds oil very strongly, the last solvent step is usually repeated to give a wax with lower oil content.

The oil content of the microcrystalline waxes varies with the grade of wax but is usually 2 - 12% as contrasted with 0.5% in paraffin wax. Chemically, both paraffin and microcrystalline waxes consist of saturated hydrocarbon.