Extrusion has been known as an industrial process for centuries. It is the forcing of a malleable material through a shaping orifice or die, originally by mechanical and later by hydraulic means. These processes were limited to unit production, i.e., one extruded form was made in each actuating stroke. It was not until the 1870’s that continuous extrusion processes emerged in the rubber industry with the use of screw extruders. In such machines, the rubber mixture was heated and conveyed forward on an Archimedean screw and then pumped out through a die. A sketch of such a screw is shown in Figure 1 below. The pressure needed to force the material through the die is obtained either by decreasing the flight depth as shown in the figure, or by decreasing the screw pitch (the distance between the flights).
These early rubber extruder screws had low length to diameter (L/D) ratios, as low as 4 to 1 in some cases. Perhaps 60 years after its use in the rubber industry, screw extrusion technology was employed for thermoplastics, initially for materials such as cellulosics and polyvinyl chloride. With the later development of nylon, screw profiles required a transition region to accommodate the rapid shift from solid to melt typical of semi-crystalline polymers, so that such screws had three distinct zones.
Feed or solids conveying zone. Here, resin is fed into the extruder. Channel depth is usually the same throughout the zone.
Melting, transition or compression zone. Most of the resin is melted in this section, and the channel depth gets progressively smaller.
Metering or melt conveying zone. Channel depth is the same throughout the zone, which melts the last particles and mixes to a uniform temperature and composition.
To provide for these zones, screw L/D ratios increased to a range around 16:1, and as extruders were called on to provide more functions, L/D ratios continued to increase, now reaching as high as 36:1 in some instances, such as two-stage (vented) screws. A vented screw incorporates a decompression zone for vacuum venting and a second metering zone to re-pressurize the melt to get it through the resistance of the screws and the die.
As the plastics industry grew, a wide range of compounds was developed both by polymer manufacturers and custom compounders. Initially these compounds were made by dry-blending the components and feeding the mixture at the feed throat of the extruder. As it became apparent, especially in the case of fiberglass-reinforced materials, that better properties could be obtained by feeding fillers and reinforcements directly into the polymer melt, screw profiles became more and more complicated, with provision for side-feeders and the addition of (often proprietary) mixing and shearing elements into the design.
At some point, there is only so much that can be achieved with a single screw. The more specialized the design is, the less flexible the extrusion process becomes. Nevertheless, for economic reasons, single screw extruders remain the workhorses of the industry, particularly for large volume processing of a single material or group of similar materials.
Twin screw extrusion of plastics also appeared in the early part of the 20th century. Continuing evolution of this process led to the development of machines with both counter-rotating and co-rotating, and intermeshing and non-intermeshing screws. Co-rotating intermeshing screw (CRIS) extruders are most commonly used in plastics compounding applications. As shown in Figure 2, in such machines, the root of one screw is wiped by the crests of the adjacent screw flights so that they are often described as self-wiping machines. Material flows along the screws following a spiral figure eight pattern until reaching the breaker plates and strand extrusion die.
CRIS compounding extruders typically employ modular screws and barrels and thus can be configured to manufacture a wide variety of compositions. Their relatively higher cost compared with single screw machines is offset by their much greater flexibility. Different process zones can be created for conveying, plasticizing, mixing and shearing, homogenizing, devolatilizing and pressure build-up. Figure 3 shows a conveying section mated to a kneading section on an assembled pair of screws.
Screw element design technology for CRIS-type compounders continues to advance and has generated highly specialized screw elements for different processing stages including intake, melting, venting, mixing and metering. Some of these designs have been patented by their manufacturers.
The proliferation of screw element components both enables process optimization for various feed stocks and makes the overall screw design choice more challenging. Equipment makers make recommendations based on their compounding research with more common materials, for instance polypropylene and talc, or polyamide and glass fibers, but compounders may find it desirable to carry out their own studies to get the best results in terms of throughput and product mechanical properties.
Compounding boron nitride (BN) is a case in point. Boron nitride is a remarkable material. It does not occur in nature, but is made in a high temperature reaction from boron containing materials and nitrogen. It is converted to a crystalline hexagonal form, h-BN, wherein hexagonal platelets can slide on each other like graphite. This material has the interesting property of being an excellent conductor of heat, while remaining an electrical insulator. h-BN has a very high aspect ratio (~20:1), such that it has an in-plane thermal conductivity of about 600 W/m.K and a through-plane conductivity of 30 W/m.K. The overall average conductivity is about 60 W/m.K.
The relatively low density of h-BN means it can be compounded into plastics without producing an overly dense final resin. However, compounding expertise is needed to ensure good wet-out of the filler and achieve best physical properties. Highly loaded h-BN compounds can provide thermal conductivities up to 10 W/m.K., far greater than the values for conventional electrically insulating plastics (~0.2 W/mK on average). This enables these interesting formulations to provide value in applications including consumer electronic devices, aerospace and vehicle cooling systems, motor and battery housings, temperature sensors, heat exchangers and other systems where thermal energy must be dissipated.
Manufacturing highly loaded h-BN compounds presents challenges, some typical of other materials and some rather different. The key issues include achieving and consistently maintaining the correct filler loading at an acceptable production rate, fully wetting out the h-BN particles and maintaining adequate temperature control of the strands exiting the die.
Properly calibrated gravimetric feeders set to match the highest starve-fed throughput of the extruder (typically < 40%) will help meet the first requirement. As h-BN is a light fluffy material, shovel elements are useful for getting it into the extruder. However, while the concave flights of such elements can convey high volumes of the low bulk density h-BN, they are not efficient at building pressure to move material forward against powder build up at the end of a side feeder.
Good filler-particle wet-out is essential for proper stress transfer from polymer matrix to filler and to consistently obtain the best mechanical properties in the resulting composite. This can be facilitated by use of surface treatments such as silanes, but efficient mixing in the extruder also plays a role. There is evidence that including fractional mixing element blocks in the screw design enables better results at higher throughputs than with conventional kneading blocks and also improves the balance between in-plane and through-plane thermal. To get in touch with the exclusive manufacturer of fractional mixing element blocks for your equipment, please ask Momentive for contact details.
Depending on the system melt temperature, the high thermal conductivity of the composite may lead to uneven strand cooling at the die. If this happens, varying strand stiffness can cause strand handling problems. This can be avoided by ensuring uniform temperature across the die and particularly the die face. A further challenge is making sure the extruded strands are not too cold, and hence overly brittle on arrival at the pelletizer or chopper. This concern may be addressed either with a hot water bath or by allowing the strands to air-cool on a carrier belt.
Momentive Performance Materials Inc., the world’s largest BN manufacturer, is launching new BN-based formulations for creating thermally conductive and electrically insulating plastics at a potentially lower cost than materials already in the market. These hybrid fillers are being marketed under the CoolFX trade name. These new CoolFX hybrid filler formulations can be cost effective solutions for thermal management applications because they achieve high thermal conductivities at lower BN loadings. They are now available for sampling as single powders, offering more consistent feeding in compounding than traditional BN powder.
Compared with the neat base resins, compounds made from these new hybrid filler formulations may have faster in-mold cooling and hence can potentially be molded on shorter cycles, yielding improved press utilization and consequently higher productivity. The hybrid fillers also can deliver improved mechanical properties with reduced anisotropy in molded parts.
During in-house testing of the new formulations in nylon 6, Momentive researchers have achieved tensile strengths approaching 10,000 psi and notched Izod impact values of about 30 J/m while still delivering thermal conductivities in excess of 5 W/m.K. Other compositions have shown thermal conductivities up to 10 W/m.K. Most significantly, it is estimated that cost reductions from 10% to 30% over existing BN-only compounds can be obtained with the new hybrid fillers.
From their own compounding studies, Momentive researchers can advise on twin screw compounding parameters for various BN formulations, including recommendations on equipment, screw design, material handling and process operating parameters. To discuss any of these issues, please contact Momentive through SpecialChem. Additional information on Momentive Boron Nitride solutions including case studies and technical data sheets are available at SpecialChem.