These are compounds to facilitate the coloring of a wide variety of plastics, the pigment being bound up in a concentrated form as an easier to handle and non-dusting form. Which facilitates dispersion in many plastic compounds. The pigment loading can be as high as 60% for inorganics (which are easier to disperse), and 25 – 50 % for organic pigments and carbon black
Concentrates are normally composed of a carrier polymer of either the same type as that to be colored, or of a type which has good compatibility with the one to be colored. This is important because if the polymers are not compatible with each other, there is a diminution of the mechanical properties of the finished article. The other components are the pigment and a wax.
There is a multitude of formulations, each with different ratios, depending upon the production method, specific pigment structure and known degree of difficulty in dispersing. With these variables in mind, the amount of wax which is employed can range between 5 – 20%, though the norm is around 5 – 10%. Normally, the concentrate can be let down to color in levels from 1-5 %. Thus, the quantity of wax in the final article falls between 0.2 – 0.5%, when it is not detrimental to the mechanical properties.
The Marcus homopolymer polyethylene waxes, being of a chemically inert character and possessing excellent thermal stability and low melt viscosity, are well suited for the production of these concentrates, not onl for the coloring of polyolefins, but also for a broad spectrum of other polymers. Their low melt viscosity results in excellent wetting out of the pigment, which together with the high shear forces developed during processing, results in the breaking down of pigment agglomerates. This subsequently results in a very efficient and uniform distribution of the pigment throughout the polymer being colored.
In summary, the necessity and advantages gained in utilizing the Marcus gihg performance polyethylene waxes are:
- Better wetting characteristics
- Easier processability
- Superior dispersing power
- Higher pigment loading
- Excellent thermal stability
- Improved polymer flow properties
There is a variety of methods available for the production of color concentrates, encompassing batch preparation by milling and high intensity mixing procedures to continuous techniques via internal mixing and extrusion.
Some of the methods for the production of color concentrates are:
The components are compounded together on a two roll mill with the temperature maintained at a low enough temperature in order to ensure that the blend has a sufficiently “workable” viscosity to provide adequate mixing and shear. Cycle times are relatively long.
2. High Intensity mixing
The components are processed together and heat develops from the friction generated by mixing. This causes the wax to soften and wet / adsorb the pigment and subsequently coat the carrier polymer particles. Following this the crude concentrate is extruded on single or twin screw equipment and strand granulated. In general, processing via a twin screw would require a lower wax concentration than by single screw.
This method also constitutes the basis for preparing non-dusting pigment concentrates ( without polymer ).
Related to this method is the production of flushed colours in which wax is melted in a blend with a pigment press cake and during processing the water separates and the pigment is dispersed and coated with wax. MARCUS polyethylenes are particularly suited for this as their exceptionally low melt viscosity provides excellent wetting behaviour.
3. Internal mixing ( Banbury )
All components are processed together and very high shear forces are developed in a very short time. Following this the crude concentrate is then sheeted and diced prior to extruding for strand granulation.
4. Continuous internal mixing
A continuous variation of the previous method employing Buss or Farrel equipment and particularly suitable for large scale operations.