Many application (primarily for inks and coatings) for the Marcus polyethylenes dictate that these products are suitably dispersed in solvent/binder vehicles as fine particles for convenience of use. These can be aliphatic, aromatic, polar and vegetable oils as well as blends thereof. There are a number of techniques which can be employed, these being:
1. Solution - Crystallization
This method consists of preparing a hot and clear solution of the polyethylene at a concentration of 25-40%. This solution is then shock cooled under high shear mixing with the remaining cold solvent, in order to crystallize the wax as rapidly as possible to obtain a final solids content in the range 10 – 15%. There are a variety of ways to effect this shock cooling procedure. This method in general produces fine particle sized dispersions, which exhibit varying degrees of structures, depending upon the individual wax and the solvent and production parameters used.
In employing this procedure a knowledge of the nature of polyethylenes and the influence of this upon the behavior in solvents is required. Polyethylenes are composed of fractions of differing molecular weights and degrees of branching. Each has different solubility characteristics and thus the time to reach a complete state of dissolution is variable. Therefore, it is essential that in preparing a solution, adequate time be allowed to elapse so that a “clear” solution be obtained, in order to insure that the polyethylene has been truly dissolved. Failure to do this can result in the presence of gels in the resulting dispersion. It is also important to determine the cloud point of the solution which is the temperature at which the polyethylene, its concentration and the nature of the solvent. When preparing the dispersion, optimum and reproducible results are obtained when the temperature of the hot concentrate is 5 – 10 DEG C above its cloud point prior to the addition of cold solvent, to effect the shock cooling stage.
This typically effected with the polyethylene at a concentration of 20 -30% being ground with solvent/binder in a ball or pebble mill. This cold milling procedure, although giving reproducible results, is a lengthy and time-consuming process to break the polyethylene down to the desire fineness.
3. Solution - Mechanical
The polyethylene wax, at a concentration of 20 – 30%, is dissolved in the solvent (normally mineral oil) and then poured onto a chilled triple roll mill when the dispersion is formed. The particle size is frequently reduced further by subsequent milling.
4. Micronized Pre-Dispersions
The micronized polyethylenes theoretically may be directly stirred into an ink or coating as they are already in a fine form. However, the surface area of a micronized particle is very large and requires an effort for effective wetting. Consequently, it is general practice to make a pre-dispersion of the product in solvent prior to addition. These dispersions can be as high as 50% solids. Another reason for preparing a pre-dispersion is to avoid a dusting problem which is endemic to this class of product.