Wire and cable compounding: meeting the production requirements

Compounding for wire and cable applications makes a number of varied demands on extrusion machinery. Jennifer Markarian looks at some of the equipment on the market and discusses some of the issues that need to be considered by the wire and cable manufacturer.

Compounding for the wire and cable market is a demanding process, requiring good dispersion and tight temperature control, often with high filler loadings. Wire and cable compounds are pelletized and then processed in single screw extruders for wire coating or cable extrusion. Good pellet uniformity and contaminant-free material is needed for high-speed extrusion of wire and cable. Direct extrusion has been investigated and proven on a semi-works scale, but is not used because the output needed by the wire or cable extrusion lines would limit the process and it would probably not be cost-beneficial, say industry suppliers. 

Wire and cable compounds are produced with PVC, polyolefins, crosslinked polyolefins, elastomers or fluoropolymers. The compounds must meet various flame retardancy requirements, depending on the application. In addition to halogenated or non-halogenated flame retardants, key additives include lubricants, processing aids, stabilizers or antioxidants, conductive additives such as carbon black, and pigments. For PVC compounds, raw materials are typically pre-mixed and fed at the compounder throat as a single feedstream. For polyolefin-based compounds, resin pellets and additive feedstreams are dosed individually and fed at the compounder throat, while flame retardants and carbon black are added downstream in the melt. Consistency and reproducibility are important characteristics of the feeding system.      
With the trend towards non-halogenated flame retardants, compounds often include high levels of non-halogen flame retardants such as alumina trihydrate (ATH) or magnesium hydroxide (MgOH). These low bulk density additives are challenging to feed because they do not flow well. Air trapped in the melt from these fluffy additives must be vented out of the extruder. ATH is also very temperature sensitive. In addition, high percentages of fillers and additives increase the compound viscosity, which makes it more challenging to achieve a high degree of dispersion at low temperatures, notes Mike Irish, vice-president of Buss Kneader Technology at Buss, Inc., USA.  

Semiconductive compounds typically incorporate high levels of conductive carbon black. The compounding challenge is to break up the carbon black pellets and distribute them throughout the polymer without overmixing, which can cause a decrease in conductivity by breaking the carbon black down beyond its primary aggregates, says Colin Richardson, technical marketing manager at Buss. A high degree of cleanliness is required in all wire and cable compounds, because contamination such as gel particles or impurities in the insulation layer or protrusions in the semiconductive layer can cause electrical tree growth and eventual cable breakdown (see Figure 2). Protrusions in the smooth surface of the semiconductive layer may come from gels, scorched particles, impurities in the carbon black, or poor dispersion, explains Dr. Richardson. 

Crosslinkable polyethylene (XLPE) wire and cable compounds contain peroxides to initiate crosslinking in the final processing step. The conventional post-blending absorption process, in which peroxides are added after pelletizing, is used mainly in high-volume production by resin suppliers. For smaller volume production, compounding of peroxides is a good alternative with technical advantages. Compounding peroxides into the resin before pelletizing results in significantly lower tendency of peroxide blooming to the pellet surface in storage, and a more uniform pellet-to-pellet peroxide content, notes Dr. Richardson. Liquid peroxides are injected into the melt stage of compounding. Precise temperature control is important, because some peroxides may begin to react at temperatures close to the melting point of the polymer. 

Fluoropolymer-based wire and cable compounds may require special extruder metallurgies for some grades of material. When extruding any fluoropolymer, care should be taken to purge at shut-downs to avoid pitting or corrosion from fluoropolymer degradation products.

Twin screw compounding extruders
Twin screw compounding extruders (TSE) are used to produce wire and cable compounds. Because TSE operate under high shear, heat or shear sensitive additives such as carbon black, ATH, and colorants should be added downstream in the melt rather than introduced in the feed section. Crosslinking initiators should be added to the melt close to the exit of the extruder. High levels of fillers, particularly low bulk density materials that are difficult to feed, are commonly added through a side feeder or stuffer. “It is best to have flighted elements with a long pitch at the stuffer location, extending 2-4 L/D downstream of the stuffer. This keeps the melted material in the extruder moving forward and allows the maximum free volume for the filler to enter. A relief vent upstream from the side feeder allows entrapped air and volatiles to exhaust without causing a back-up in the side stuffer,” explains TSE manufacturer Leistritz. More recently, in fluoropolymer compounds, pigments are being added through the side feeder to avoid creation of agglomerates that might occur with earlier feeding, says Charlie Martin, president of American Leistritz Extruder Corp.
 
Leistritz offers the new MAXX series of process sections for twin screw extruders that increase free volume while maintaining torque density, which allows increased output rates. “Typically, rates are not limited by the side stuffer but by the filler level that the extruder screw can accept. Higher free volume translates into a higher fill capability,” explains Mr. Martin. The ratio of screw outer diameter to inner diameter (Do/Di) for the MAXX series is 1.66/1, which increases the free volume in the screw by about 30%. The deeper flights are advantageous for heat or shear sensitive materials, and the additional surface area of the melt is beneficial for devolatilization, says the company. Barrel cooling efficiencies are also increased to accommodate the higher surface area and rates.

Coperion supplies two-stage systems for production of PVC, polyolefin, or EPDM wire and cable compounds. In the first stage, a twin screw extruder accepts the feedstreams and provides energy for melting and dispersion, followed by a gear pump or single screw extruder in the second stage to build pressure for the pelletizing step. On PVC machines, Coperion now offers a twin-screw side feeder for additives and fillers. The side feeder uses the positive conveyance of a twin screw, which is helpful for feeding high filler levels, rather than the traditional crammer with a single screw. Coperion’s ZSK MEGAvolume compounding extruder has a smaller diameter shaft to accommodate deeper screw flights, with a Do/Di ratio of 1.8. The design allows 20% greater free volume, allowing improved intake of low bulk density materials, says Dan Mielcarek, business unit manager at Coperion Corporation. The MEGAvolume extruder runs at about 20% lower average shear rates than their standard equipment, which is beneficial for heat and shear sensitive compounds such as those used in wire and cable. The MEGAvolume series is designed for screw speeds up to 1800 rpm, giving it an increased throughput range. While the MEGAvolume has lower torque than the MEGAcompounder, it has higher specific torque and corresponding potential for higher throughputs than Coperion’s earlier Continua machines.
  
Reifenhauser’s Bitruder counter rotating twin screw extruder is used in compounding, including wire and cable compounds. A new ‘high torque’ drive unit for the extruder is a gearless, direct drive system that is nearly maintenance free and very quiet, says Lothar Staubi, sales manager at Reifenhauser. This drive system reduces energy use by up to 10%, he notes. While the co-rotating Reitruder is capable of direct extrusion via a melt pump to a cross head for wire and cable extrusion, it is not being used commercially in this area. “We hear a lot of customers talk about direct extrusion, but nobody wants to be the first one,” notes Mr. Staubi. 

Buss Kneader Compounder
The Buss Kneader is used for producing wire and cable compounds because of its ability to provide a high degree of dispersion for high filler loadings under uniform, low shear and low, controlled temperatures. It is a two-stage system, with a separate mixing section and a downstream pressure-creating stage for pelletizing. The Buss Kneader differs from conventional, spiral screw-type compounding equipment. It has a full spiral screw only in the feeding and devolatilizing zones; in the mixing and kneading zone, the spiral is interrupted every 120 degrees to form the kneading flights, explains Dr. Richardson. Rows of kneading teeth or pins, also 120 degrees apart, are fixed inside the barrel. “The melt is sheared between the flights and the teeth. At the same time, the oscillating motion of the screw provides intense axial mixing as a result of repeated division, folding, and recombination of the product,” explains Dr. Richardson. The intensive mixing effect keeps the product temperature even, preventing temperature peaks that might cause degradation. “Although the maximum shear gradient is only one-half to one-third of the level achieved by twin-screw compounders, the multiplicity of shearing cycles yields optimal dispersion. The polymers are treated very gently such that temperature sensitive resins will not be degraded and delicate fillers are dispersed without being broken down beyond their characteristic unit size,” he adds. This is important in compounding 25-40% carbon black into semiconductive cable compounds, for example. The compounding process must break the carbon black aggregates and distribute them throughout the melt, but not break down the primary aggregates, which would result in a decrease in conductivity. 

Wire and cable compounds often include high levels of fillers or additives such as carbon black or non-halogenated flame retardants. The Buss EVI (Expanded Volume Intake) has a slightly thinner liner that increases the intake volume capacity by 35% and allows the kneader to accept larger amounts of fillers, while increasing rates up to 30%, says the company. Low bulk density fillers have typically been fed using a side feeder with a screw to force in the filler. While side feeding is still best for extremely high filler levels of 80-85%, a downstream, vertical feed system has more recently been added as an option, says Dr. Richardson. Vertical feed is less expensive and is advantageous for fillers that should not be compacted, such as carbon black, he notes.
The Buss quantec series nearly triples output, making it very cost-effective and a significant advance in Buss kneader technology, notes Dr. Richardson. Its four-flight screw profile results in linear output to speed behaviour, and increased mixing and conveying efficiency. The stress on the product is constant across the entire range of speeds, resulting in consistent dispersion quality at all settings, says the company.  The quantec design has high operating efficiency because the energy input is mechanical, rather than through external electric heating. Liquid temperature control enables the system to reach process temperatures quickly and maintain precise product temperature control. Because a wide range of recipes can be processed on a single screw configuration, downtime for product changes can be reduced. The quantec series is currently used in PVC compounding and powder coating, and the company is investigating its use in polyolefin compounding. 

For peroxide crosslinkable compounds, the Buss Kneader incorporates liquid peroxide into the melt via an injection nozzle and through a drilled pin. “The easily accessible clamshell barrel and the interchangeable mixing and injection pins enable the injection location to be chosen at many points along the kneader barrel,” explains Mr. Irish. Advantages of the Buss Kneader in compounding XLPE are its uniform shear mixing action and precise temperature control of the melt. Thermocouples may be mounted in pins and along the mixing chamber, giving precise monitoring of melt temperature, which is important for compounding peroxides. Temperature is controlled using a liquid transfer medium with individual circuits for the kneader barrel, screw, and discharge, which offers better control than an electrically heated machine, says the company. 

Contacts:

Buss Inc. - USA
Tel: +1 630 933 9100
Website: www.busscorp.com 

Coperion
Tel: +41 61 825 66 00 or +1 201 327 6300
Website: www.coperion.com

Leistritz
Tel: +1 908 685 2333
Website: www.leistritz-extrusion.com

Reifenhauser Extrusion GmbH & Co.
Tel: +49 2241 481 0
Website: www.reifenhauser.com