Calcium Carbonate in Plastics Compounding
Calcium carbonate (CaCO3) is one of the most popular mineral fillers used in the plastics industry. It is widely available around the world, easy to grind or reduce to a specific particle size, compatible with a wide range of polymer resins and economical. As an additive in plastic compounds, CaCO3 helps decrease surface energy and provides opacity and surface gloss, which improves surface finish. In addition, when the particle size is carefully controlled, CaCO3 helps increase both impact strength and flexural modulus (stiffness).
Calcium carbonate may be used with a myriad of thermoplastic resins. Polypropylene compounds are often filled with calcium carbonate to increase rigidity, an important requirement for operations at high temperatures. In PVC, calcium carbonate is used with flexible compounds such as tubing, wire and cable insulation, latex gloves, trash bags and in rigid compounds such as extruded pipes, conduits and window profiles.
The design of a feeding or pneumatic conveying system is heavily influenced by a bulk solid’s particle shape, size (aspect ratio) and particle size distribution as well as the particle’s roughness, hardness (abrasiveness), and density. In the case of calcium carbonate these characteristics vary widely depending on the source and production process for the individual samples.
The sieve analysis below illustrates the difference in particle shape, particle size and particle size distribution for two samples of calcium carbonate. The large number of particle interactions makes it impossible to establish a clear correlation between filler’s properties at the particle level and its flow behavior as a bulk solid.
As a consequence, pneumatic conveying systems cannot be purchased “off-the-shelf” and must be engineered for each situation individually. Both systems engineering and equipment selection are affected by differences in a bulk solid’s characteristics. Laboratory tests are often necessary to help determine a material’s properties and behavior. A pneumatic conveying system generally consists of five basic components: a gas motive, a conveying line, a dispensing device, a material-gas separator and controls.
When selecting a dispensing device, it must be taken into consideration that some grades of CaCO3 are light and fluidizable and may flood convey lines. In such cases, a rotary valve should be considered to meter the material into the line. Rotary valves can be used for pick up in either pressure or vacuum pneumatic conveying systems. With adhesive grades of CaCO3 the product will tend to build up and not release from the blades of a drop-through rotary valve. In this case a blow-through style rotary valve is necessary; air is blown through each pocket as the valve turns, dislodging material from the blades. The installation of a fluidizing cone in the feed bin is also helpful in controlling the flow of the material.
Calcium carbonate powders can cause many problems in conveying systems, including sticking in hoppers, flood feeding into convey lines, building up inside convey lines, and blinding over filter bags and cartridges in receivers. Rigid pipes may be used as a convey line for highly fluidizable to slightly adhesive CaCO3. However, if the CaCO3 tends to coat the interior of the convey line, a flexible hose could be considered because flexing of the line helps prevent build-up on the walls of the hose.
In vacuum sequencing systems, it is a good precaution to use a purge valve to allow the line to clear between conveying sequences. First, a shut-off valve at the pickup of a vacuum system is closed to allow vacuum to build in the line. Then the valve is opened, creating a pressure wave that helps clean off any coating of material
adhering to interior surfaces of the convey line.
Often CaCO3 will adhere to the filter, causing it to blind over and reduce filter efficiency. For sticky grades of CaCO3 using filter bags instead of pleated filter cartridges may help reduce the tendency of the CaCO3 to stick to the filters. In extreme cases the use of PTFE filter media is recommended.
Some calcium carbonate grades may require a steeper discharge cone in order to allow complete emptying of the hopper. Flow aids such as vibrators or fluidizing pads inside the receiver hopper will usually help ensure quicker and more complete discharge of the receiver.
The sieve analysis illustrates the difference in particle shape, particle size and particle size distribution for two samples of calcium carbonate. While Sample A is a precipitated calcium carbonate (PCC) with a relatively low bulk density, Sample B is a granular calcium carbonate with a much higher bulk density.