Main components of a granular color concentrate?
First of all, what is a master batch?
IIt is a color concentrate, namely a granular concentrate, consisting of a mixture of pigments or dyes, additives and resins. The compositional scheme of a color concentrate formulation can be seen in the figure below:
Color concentrates are a simple medium for coloring thermoplastics. Dosages vary from 2 to 4%, depending on the resin processing system to be colored and the thickness of the final article. In general, color concentrates can be used in all processes that involve pre-mixing of the material to be colored in the spindle, such as in the following processing systems:
- Injection molding.
- Extrusion blow molding.
Main manufacturing techniques for masterbatches
Extruders are often used to mix the components of formulations that are to be further processed by other techniques or to obtain preforms that are to be processed by blowing or thermoforming.
The granular or powdered material is loaded into a hopper, from where it is fed into the barrel, where the screw feeds it, transports it forward and compresses it. The heating up to the melting is done from the outside of the cylinder by means of heating elements and from the inside by transforming the stress into heat. In this way, the thermoplastic material melts (plasticizes) and when it exits the cylinder through a mouthpiece. In a complete extrusion line, there must also be a cooling system for the material leaving the machine, as well as tensioning and collection equipment. The result is a profile (sometimes called a semi-fabricated or preform).
Advantages of twin screw extrusion
Over the years, twin screw extrusion has become the primary standard for many industries, as it offers countless advantages over single screw extrusion:
- Increased production consistency and product quality control.
- Increased productivity through continuous processing, fast start-up and shut-down between
- Product processing phases, rapid conversion and advanced automation.
- Greater flexibility, with the ability to produce a wide range of raw materials.
- Optimized environmental impact, as the lines offer end-to-end processing.
- Simple and easy to maintain.
- Energy and water savings.
- Easy to clean, thanks to the CIP design (cleaning in place).
Double screw extrusion
In the double screw extrusion process, there are variants that have a great influence on the quality and productivity of the color concentrates, among which the following are highlighted:
In this case, all the components of the formulation are combined in a premix and fed into the extruder through the main feed. Since all components of the formulation enter the twin screw extruder at the same time, phenomena such as the following can occur:
- Difficulty in plasticizing the vehicle (resin).
- Difficulty in transporting the material due to premature plasticization of low melting point elements (such as certain waxes and additives).
- Need to use more additives and therefore the cost can be higher.
In the case of metallic or pearlescent effect pigments, where the shape of the pigment particle must be preserved to ensure the effect, this configuration can damage a high percentage of these particles.
One-step process with split feeding
Twin screw extruders have the ability to incorporate formulation elements at different locations along the length of the screw; some applications may have two or even three different feed ports, for example.
In this configuration, generally, only the vehicle (resin) is fed by the main feed and since no pre-mixing with powdered materials (pigments and additives) is required, it is possible to avoid the need to pre-spray the polymer, which is generally required in the previous configuration in order to avoid segregation phenomena in the feed hopper.
In this way, the capacity of the extruder is concentrated on plasticizing the vehicle, and then incorporating the pigments and additives into the melt, avoiding the effect of premature plasticization of low-melting compounds or additives, as well as the interference that a non-meltable material such as pigments can have in the process of plasticizing the vehicle. This configuration is particularly suitable for pigments that are sensitive to shear and provide special effects (pearlescent, metallic, etc.) because it avoids subjecting the particle to excessive thermal and mechanical loads that break or deteriorate the shape of the pigment particles and thus diminish the desired effect.
The fundamental difference with the processes described above is that in this process, a color concentrate is produced from a single pigment (monoconcentrate) by twin-screw extrusion, and to obtain a specific shade, a mixture of several monoconcentrates is again produced by twin-screw extrusion.