technology | Wire and cable
polymers than into amorphous PVC, the company says. High mineral filler levels can enhance crack formation by increasing the tensile pressure on the polymers. When wire or cable is exposed to elevated tempera-
tures—sometimes even as little as two days at 100°C— the polymer compounds can experience degradation that can result in a loss of tear strength and thermal stress cracking in the outer layer. Use of the optimal polymer, flame retardant, and other
A reduction of around 8% in peroxide dosage can be realised with Lowinox Fast XL in comparison to a competitive liquid antioxidant at the same crosslinking speed
Source: Addivant
says that while there is increasing demand for such compounds in many cable applications, fibre optics present special requirements. Tight buffer compounds must be tough yet elastic to
protect the optical fibre from mechanical damage and from the signal losses caused by micro-bends, he says. Jacket compounds must exhibit a low degree of post-extrusion shrinkage, which can impose stress on the sensitive fibres during post-extrusion temperature cycling and over time during end use. Added to these requirements are considerations that apply to all cables, most important of which of course is flame behaviour. The polyolefin resins used for metal hydrate-based LS HFFR compounds generally require fairly high filler loadings, without compromising other application requirements, such as processability, mechanical performance, flexibility, and cost. The Teknor Apex Halguard range of LS HFFR products
for wire and cable includes a portfolio of compounds designed specifically for fibre optic cables. The portfolio provides choices for a broad range of end-use perfor- mance and flame retardance, as well as low-cost options for buffer and jacket applications. All the jacket com- pounds exhibit low-shrink behaviour. Some of the compounds exhibit superior char formation under burning conditions and others incorporate an innovative flame retardant system (the company keeps details confidential), which facilitates their ability to offer cost effective alternatives to premium grades. All but one have a maximum continuous operating temperature of 105°C.
Managing stress cracking Thermal stress cracking, which can be a problem in LS HFFR compounds because of their high filler levels, was the subject of a recent blog published by Mexichem Specialty Compounds (previously AlphaGary). It is easier to freeze stresses into these semi-crystalline
22 COMPOUNDING WORLD | May 2017
additives such as coupling agents, can be used to meet the physical property requirements of an application and minimise stress cracking. “Everything is a balance,” says Tanya Artingstall, Wire and Cable Development Manager at MSC. “A certain polymer compound might be desired to pass a certain flame test, but physical property require- ments might call for a slightly different formulation. In some cases, low temperature impact, tensile, and flex strength are important. Thermal stress crack resistance (TSCR) is another factor. And all these requirements must also be balanced with cost.” MSC’s Megolon S382 and S384 were developed to
improve tear strength at elevated temperatures compared to an existing grade with a Limiting Oxygen Index (LOI) of 35. MSC then developed Megolon S386, which has a LOI of 40 and excellent TSCR compared to a standard grade with LOI of 40.
Stabilising XLPE Crosslinked polyethylene (XLPE) compounds are also benefiting from upgrades. Addivant recently launched a new high performance liquid antioxidant blend, Lowinox
Comparison of the effect of antioxidants on tensile strength of XLPE at 150°C Source: Adeka
www.compoundingworld.com
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