Silicone-based Technology Improves Processability of Highly Filled Flame-retardant PE Compounds for Wire & Cable Insulation
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Regulatory changes and sustainability efforts are prompting wire and cable manufacturers to replace traditional polyvinyl chloride (PVC) insulation with other materials that avoid toxicity issues. The European Commission states, “PVC has been at the center of a controversial debate during much of the last two decades. A number of diverging scientific, technical and economic opinions have been expressed on the question of PVC and its effects on human health and the environment.”1 Environmental and health regulations such as the European Union’s REACH and Restriction of Hazardous Substances (RoHS) legislation have impacted the use of PVC, and a Grand View Research report from 2017 talks about the current, stringent regulatory landscape that is changing the global industry use of PVC.2
As alternatives, polyolefins such as polyethylene (PE) compounds offer excellent electrical properties, as well as ease of processing, cost-effectiveness and a desirable balance of mechanical properties. Applications for PE insulation can include coaxial and low capacitance cables. Polyethylene can offer a lower dielectric constant, ranging from 2.2 to 2.4 at 1MHz, depending upon the type of resin,3 making it an option for cables requiring high-speed transmission. It can be used in environments where temperatures range from -65°C to 80°C, such as automotive under the hood and building and construction in cold climates.4
However, PE has a major drawback. Unlike PVC, which is inherently flame retardant (FR) due to its high chlorine content of 50 percent or more, PE requires heavy loadings (60-70 percent by weight) of mineral FR additives such as aluminum hydroxide and magnesium dihydroxide. These halogen-free additives not only address flame, smoke and toxicity requirements for wire and cable applications, but they also support sustainability by enabling more efficient processing.
At these high loadings, halogen-free mineral FR additives can cause significant processing issues for PE compounds. Higher extruder screw torque and die pressure, along with die build-up that requires periodic cleaning, are common processing concerns that can slow throughput and reduce productivity. To avoid them, wire and cable insulation manufacturers are asking PE compounders for materials containing silicone-based processing aids. These products have largely replaced traditional processing aids such as calcium stearate and euricamide.
Yet, silicone-based processing aids have their own downsides. Some over-engineered types may include extra functionality that raises the price point, while others trade a lower price for poor efficiency and batch-to-batch inconsistencies.
To address these shortcomings, the wire and cable industry has been looking for a narrowly focused silicone processing aid that maximizes throughput and batch-to-batch consistency, while controlling costs by eliminating unneeded functionality. A silicone-based technology in masterbatch form, which focuses specifically on better processability of highly filled FR PE compounds without including extra functionality, is now available.
Figure 1. Comparison of torque reduction in the LDPE compound without processing aids and containing the standard masterbatch and the new masterbatch
How Silicones Boost Processing Efficiency
Silicone-based processing aids enhance throughput for PE compounds in several ways. One is by optimizing the dispersion of the mineral FR fillers within the PE matrix. The more uniformly the fillers are dispersed, the lower the compound’s viscosity. In turn, a lower viscosity reduces pressure on the extrusion die face. Better dispersion also helps to maximize the performance of FR additives.
Another aspect of processability is reducing the amount of stress on extrusion equipment. Silicone’s well-known ability to improve slip (by reducing the coefficient of friction (COF)) and increase material flow relieves this stress, which enable a faster throughput, the extended useful life of the extruder and lower energy use.
A third benefit of using silicone technology to enhance the processing of wire and cable insulation is that it helps avoid die build-up on the extruder. By reducing friction and improving resin/filler compatibility, silicones prevent or minimize material accumulation that leads to quality issues and costly downtime for cleaning.
Figure 2. Comparison of the effectiveness of the new masterbatch and standard masterbatch in reducing die pressure at various screw rotation speeds
Evaluating Silicone-based Technology
The previously-mentioned silicone-based masterbatch targeting processing optimization was tested against a traditional, multi-function silicone masterbatch to determine overall throughput performance. Because throughput is influenced by several contributing factors—including extruder screw torque and die pressure—these were tested individually. Die build-up and temperature control were also evaluated.
The following test parameters were used for this comparison.
For the test compound, a wire and cable grade of low-density PE (LDPE) was formulated with a 55 percent loading of calcium carbonate (CaCO3) that exactly simulates the rheological behavior of halogen-free FR additives with the same particle size. Calcium carbonate was used to accommodate a limitation of the pilot extruder used for the testing. The LDPE had a low melt flow index and contained an antioxidant (0.3 percent loading).
Loadings of 1, 2, and 4 percent of the novel silicone-based masterbatch and the traditional masterbatch were added to this base material. All batches were compounded in a twin-screw, L/D 48, 44mm extruder. The same LDPE compound without any processing aid served as a control.
For the evaluation, the different batches of LDPE were tested in a single-screw extruder.
Figure 3. Throughput comparison of silicone-based masterbatches with reference material
The effects on torque reduction of the silicone-based masterbatch and the multi-function silicone processing aid were compared at a loading of 2 wt% in the LDPE compound, versus the rotation speed of the extruder screw. The two masterbatches showed comparable torque reductions, and both achieved significantly lower torque (up to 15 percent lower) than the compound containing no processing aid. Compounders that are concerned about costs can opt to further reduce the loading of the new silicone technology to 1 wt% and still achieve 10-20 percent torque reduction, which in turn leads to a significant decrease in energy consumption.
Figure 2 shows that the novel silicone masterbatch achieved lower die pressure in comparison with the multi-functional masterbatch at loadings of both 1 wt% and 2 wt%. Die pressure was lowest for all materials at 80 rpm. Similar to the scenario for torque reduction, compounders have the option to reduce the loading of the novel masterbatch to 1 wt% for cost reasons. The testing indicates that a loading of 1 wt% can deliver a 10 percent die pressure reduction, while a 2 wt% loading can yield a 15 percent reduction.
Additional Factors Temperature Control
Halogen-free FR additives are unstable under high temperature conditions and can degrade, so it is important to avoid raising the processing temperature above 200C. The novel silicone masterbatch consistently controls self-heating of the FR compound at both low and high screw speeds, helping to keep the temperature under 200C.
Although specific testing was not conducted to measure die build-up, it is well known that lower torque and better dispersion tend to reduce compound accumulation on the die. Silicone processing aids migrate to the interface of the material and the extruder screw, where they act as a lubricant that reduces friction and avoids accumulation of material. They also improve compatibility between resins and fillers to reduce mechanical degradation and resulting friction of a compound during extrusion.
Raising Throughput with Silicone Technology
Another test was performed to calculate the rate of throughput in grams per minute of the novel silicone masterbatch (at loadings of 1 and 2 wt%) compared to the multi-function masterbatch, also at 1 and 2 wt%, and to the control (LDPE compound without processing aids). Measurements were taken at progressively higher torque levels, from 48 NM to 58 NM. This test showed an output increase of up to 110 percent for the novel silicone masterbatch compared to the control material. This increase was similar for both silicone masterbatches. Also, both masterbatches delivered this higher output at a lower torque than the reference material.
This testing regimen revealed that adding a low loading of the silicone masterbatch to a highly filled LDPE compound significantly increased throughput and decreased the torque of the extruder line.
Improved Processability without Costly, Unneeded Extras
As wire and cable manufacturers replace PVC with materials like PE to avoid toxicity issues and support sustainability, they must contend with new challenges. Polyolefins require heavy loadings of mineral flame retardants, such as halogen-free metal hydrates. These additives negatively affect processability, including reducing screw torque that slows down throughput and uses more energy, and increasing die build-up that requires frequent interruptions for cleaning.
To overcome these issues and optimize throughput, wire and cable insulation extruders incorporate processing aids. Until now, they were forced to choose between two alternatives:
- Expensive, over-engineered additives that delivered unneeded properties
- Lower-cost solutions with average or suboptimal performance
By adopting a silicone-based technology with demonstrated results, including lower torque, lower die pressure and higher throughput, wire and cable insulation producers can leverage the processability benefits of silicone chemistry without paying for unneeded functionality that is included in traditional, multi-function silicone processing aids.
- Polyvinyl Chloride (PVC). European Commission website. Updated 8.6.2016. http://ec.europa.eu/environment/waste/pvc/index.htm
- Polyvinyl Chloride (PVC) Market Size, Share and Trends Analysis Report By Application (Construction, Consumer, Packaging, Electronics, Transportation), By Region, And Segment Forecasts, 2012 – 2020. Grand View Research. https://www.grandviewresearch.com/industry-analysis/polyvinyl-chloride-pvc-market
- Electrical Properties of Plastic Materials. Professional Plastics. https://www.google.com/url?sa=tandrct=jandq=andesrc=sandsource=webandcd=1andcad=rjaanduact=8andved=2ahUKEwiNq5zjpKXfAhUDTd8KHaTyBPkQFjAAegQIBxACandurl=https%3A%2F%2Fwww.professionalplastics.com%2Fprofessionalplastics%2FElectricalPropertiesofPlastics.pdfandusg=AOvVaw2YPh1zD4kWD4bCW8V-cvA7
- Insulated Wire, What’s Protecting Your Cable? Performance Wire and Cable blog. https://www.performancewire.com/insulated-wire-protection/