Going with the flow

Without plastics additives, polymer resins will find very limited uses and fail to perform in most applications. A wide range of materials such as extenders, plasticisers, fillers and process aids are added during the plastic compounding process to help with the process and compensate for the resin’s weaknesses in achieving the desired end product.

Mineral fillers are industrial mineral rocks ground to a suitable particle size that are used to alter the physical properties of a polymer resin in order to lower cost, increase or decrease density and add texture. These solid particulate materials (powder, grain, flakes, needles, etc.) are immiscible in the polymer resin - they modify the resin’s mechanical properties without changing its chemical composition.

There are more than 70 types of mineral fillers and more than 15 types of fibres of natural or synthetic origin that have been used or evaluated as fillers in thermoplastics and thermosets¹. The most common fillers used for plastics compounding are: calcium carbonate, talc, mica, silica, kaolin and clays, titanium dioxide, wollastonite, aluminium and magnesium hydroxides, dolomites, silicates, glass and carbon fibres.

Depending on the function performed, fillers may be classified as extenders (particles that increase bulk density and lower cost) or reinforcements (particles that improve the polymer resin’s physical and mechanical properties) while some fillers can actually perform both functions. Changes in the filler’s particle characteristics influence the engineering of pneumatic conveying systems and the selection of equipment used for the dosing or feeding of bulk solids.


Figure 1: Coating of a pipe by a highly dense CaCO₃



K-Tron International Inc.


Figure 2: Formation of agglomerates from interaction of powder with humidity



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Filler selection

In plastics compounding operations, the selection of a polymer resin is determined by the final product application which in turns narrows the type of fillers that may or may not be used, their chemical treatment (if any is required) and the order of addition to a formulation during processing conditions. For example, polymers processed at high temperature and those that react with moisture (ie. polyurethanes) require fillers which do not contain moisture.

Mica and talc are both used as reinforcement fillers in polyolefins; however, their chemical properties, and therefore their behavior and performance, cannot be more different. Mica is considered the most effective mineral filler for reducing warpage in PP parts, improving stiffness and increasing the heat deflection temperature. Talc is mainly used to increase stiffness and resistance to high temperature creep.

Micas are hydrophilic and compatible with polar polymers without surface treatment. Therefore, when used with non-polar polymers (such as polyethylene), it is necessary to treat them with silane coupling agents or surface active agents. Talc, on the contrary, is naturally oleophilic, making it highly compatible with olefin polymers and therefore not amenable to treatment with silanes.

Calcium carbonate is one of the most commonly used fillers or extenders in the plastics industry. Calcium carbonate helps decrease surface energy and provides opacity and surface gloss, which improves surface finish. In addition, when the particle size is carefully controlled, calcium carbonate helps increase impact strength and flexural modulus in plastic compounds. This filler is widely available around the world, economical, easy to reduce to a specific particle size and compatible with a wide range of polymer 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.




Flow behavior

Bulk solids flowability is determined by the ability with which fillers (a collection of individual solid particles such as granular, pellets, powder, fibres, etc. surrounded by a gas phase) will flow under specific set of environmental conditions. There are five major flow ‘influencers’ that need to be considered: 1) flow rate, 2) handling equipment, 3) bulk properties, 4) particle interactions, and 5) material characteristics.

The scale of the flow rate has a significant impact on the type of conveying system selected. Moving a few kilograms per hour of a given filler requires a very different process than moving a few tonnes per hour, whether by gravity or by pneumatic conveying. In the latter case, an argument can be made that to increase the solids transfer or flow rate, one needs a bigger blower to achieve higher velocity of the gas phase. However, conveying gas velocities of about 40m/s are considered to be the upper limit for the majority of pneumatic conveying systems because of particle degradation and erosion of the bends in the conveying line.

By the same token, a slight change in equipment conditions may alter the flow conditions of the bulk solid significantly. As an example, consider increasing a pipe’s diameter while holding the blower capacity constant or changing the blower’s capacity while holding the pipe diameter constant.

When designing pneumatic conveying lines, three distinctive variables describe the relationship that exists between the selected equipment and the flow rates for a given material: conveying line pressure drop (bar), air mass flow rate (kg/s) and material flow rate (kg/hr). These variables influence the flowability of the filler in a bulk solids handling system.

For material to flow a driving force is required to move the bulk solid, such as pressure in a pneumatic conveying line or gravity in a hopper. The third of Newton’s laws of motion of classical mechanics reminds us that for every force acting on an object, there is an equal but opposite force acting on such object. In bulk solids handling systems, opposite forces arise from two factors: one related to the equipment used (extrinsic constraint) and a second one related to the material itself (intrinsic constraint). The equipment configuration (rounded, squared), its size (diameter, length), its geometry (angled, curved, flat), and its surface (material, coating, finish) etc., are all extrinsic factors that affect the bulk solid’s flowability.

The last group of influencers is related to material characteristics that affect compressibility and cohesiveness, and subsequently, the solid’s flow behavior such as the filler’s particle shape, size (aspect ratio) and particle size distribution (PSD), roughness, hardness (abrasiveness), and bulk density.

The effect of aspect ratio is determined by testing the final product. While fibres (long aspect ratio) help increase tensile strength and stiffness, they decrease 3D shrinkage. On the contrary, platelets (short aspect ratio, but large surface area) decrease tensile strength, impact strength and 3D shrinkage.

A sieve analysis will show that for a given filler, particle size conforms to a normal distribution. Coarse particles are stress concentrators that can limit impact strength and produce higher wear and abrasion of the equipment. On the other hand, fine particles provide high surface area affecting the surface finish, but altering the rheology of the polymer melt considerably.

At the particle level, three distinctive relationships that affect the flow behavior of fillers in pneumatic conveying and feeding systems can be isolated for study: 1) particle-particle, 2) particle-equipment and 3) particle-environment interactions.


Table 1: Pneumatic conveying equipment sizing for different CaCO₃ samples




Particle-particle interactions

Particle-particle interactions are responsible for the cohesive properties (tendency to form aggregates or agglomerates) of bulk solids and are directly related to the filler’s chemical composition and physical characteristics. These interactions have a significant effect on the behavior of bulk solids in pneumatic conveying and feeding systems.

Particle-equipment interactions

Flowability is also greatly influenced by the interaction of the bulk solid with the walls of the pneumatic conveying system and feeding equipment. The flow of solid particles inside a vessel or a pipe is a function of two important characteristics, wall friction and shear strength. Wall friction relates to how particles slip on a contact surface while shear strength is the resistance that the powder bulk offers to deformation, or how particles slip off of each other. Both characteristics are directly affected by the filler’s bulk density, an important property that is also necessary to determine the space used in a storage vessel once the material has time to settle.

Particle-environment interactions

Particle-environment interactions deal with external forces (eg., temperature, relative humidity, vibration, gravity, aeration, etc.) exerted over the aggregate of particles that also influence the flow behavior of the bulk solid. Different fillers behave differently when exposed to moisture. The relative humidity (RH) of the air and the filler’s hygroscopic nature often result in increased cohesiveness because of inter-particle liquid bridges; temperature affects the particle’s crystalline behavior promoting ‘caking’; while pressure increases the contact points between particles causing ‘compaction’ or more inter-particle adhesion.


Figure 5: Schematic representation of equipment used



K-Tron International Inc.


Example: CaCO₃

The selection of a filler and consideration of its flow behavior is illustrated with an example where a customer needs to transfer 4.5 t/hr (10,000 lb/hr) of calcium carbonate from a storage bin (1 - see Figure 5) to a filter receiver (2) located on top of a rotary valve. The filler is added to polypropylene (PP) at a plastics compounding facility located at 305 metres (1,000 ft) elevation with an average daily temperature of 29.5¡C (85F) during the entire year.

The customer is evaluating the use of two calcium carbonate samples. Sample 24806 is a highly pure calcium carbonate (99%) in powder form, while sample 26365 contains up to 5% of silica impurities and comes in granulates (see Figures 3 and 4). It has been determined that both samples are technically adequate for the process. When compared on a weight-ratio basis of pure calcium carbonate per kilogram of material, the material cost is equivalent. The customer would like evaluate the equipment necessary to transfer each sample.

While sample 24806 is a precipitated calcium carbonate (PCC) with a loose bulk density value equal to 301 kg/m³ (18.8 lb/ft³) and a packed bulk density value of 399 kg/m³ (24.9 lb/ft³), sample 26365 is a granular calcium carbonate with bulk density values equal to 1378 kg/m³ (86 lb/ft³) (loose) and 1474kg/m³ (92 lb/ft³) (packed).

The following sieve analysis illustrates the difference in particle shape, particle size and particle size distribution for two samples of calcium carbonate.

The selected sample would need to be pneumatically conveyed over 30.5 meters (100 ft) horizontal distance and 15.2 meters (50 ft) vertical distance, with four 90-degree angle elbows (4) present in the system. The air blower (5) would be located in such a way that the air only distance would be 50 ft with no more than two elbows. A schematic representation of the pneumatic conveying system required to transfer the calcium carbonate from a storage bin into a feeding system in a compounding operation is illustrated below.

The large particle characteristic differences between these two samples warrant the selection of different equipment components as well as the sizing of the entire system.

For instance, a higher blower horsepower (5) is required for calcium carbonate 26365 to provide increased system airflow and vacuum necessary to maintain dilute phase conveying at the required rate. Differences in the bulk density and each calcium carbonate’s characteristics determine the adjusted rotary valve (3) throughput to maintain the desired rate. To this end, for a much denser 26365, the adjusted throughput is much lower than calcium carbonate 24806.

The filter housing diameter is determined based on the ‘can velocity’ restriction for each sample. Maximum can velocity is the largest vertical velocity through the filter housing that will allow the majority of material to fall out of the airstream. The filter cloth area for each material is based upon the filtering characteristics of the each sample.

Particle size plays a large role in determining the necessary filter cloth area required for the materials. The larger the particle size, the easier it will be to separate from the airstream; therefore less filter cloth is required. Table 1 summarises some of the equipment differences for these two samples.

Conclusions

The correct design of a pneumatic conveying system for calcium carbonate in plastics compounding operations is not a trivial proposition because of the number of variables that affect mineral filler flow. The return of investment (ROI) of a plastics compounding plant hinges on the proper selection of the most cost-effective pneumatic conveying and feeding systems. Particle characteristics as well as particle interactions with other particles, with equipment and with the environment affect the selection of equipment and system design.

Contributor: Jaime A. Gomez, PhD, global business development manager, K-Tron International Inc.


¹ Wypych, George. Handbook of Fillers. ChemTech Publishing 2010.