WCA January 2018

additive chemistry together with careful optimisation of the polymer system and graft chemistry, an excellent balance of properties has been achieved.

Development target

Property

Requirement

Tensile strength/ psi

1,500

1,800

2 Development The key initial targets were to meet, as a minimum, the UL FT2 horizontal fire test requirements on 14 AWG copper as detailed in UL2556 and to meet the long-term insulation requirements for 90°C wet rated XHHW-2 as detailed in UL44, with higher fire performance being highly desirable for the reasons described above. 2.1 Experimental For initial compound screening, a range of formulations was created. The formulations were compounded and silane grafted using a 45mm Buss kneader. The samples produced were then used to prepare extruded strips and 14 AWG conductors to provide the sample types for further testing. The wire extrusion also provided an initial opportunity to assess the formulations for their processing characteristics. Particular care was taken in the early formulation development to ensure good low-temperature and mechanical (crush and impact) properties of the trial formulation with careful resin selection. The polyolefin polymers examined ranged in density from 0.87 to 0.95g/cm 3 and had, as expected, a profound effect on crush performance as illustrated in Figure 1 . The final formulation has an LTB of -40ºC. A full range of property testing was completed including standard mechanical, hot creep, MFI and extrusion quality rating. Long term wet electrical testing at 90°C was initially conducted in our internal laboratories with no voltage applied. Inspection of Figure 2 reveals that the resin selection also had a profound effect on wet electrical properties. Optimum formulas were selected from this work and submitted to long term wet testing with 600V applied as required by specification. The hydrated fillers, primarily based on Mg(OH) 2 , varied in particle size and type of surface coating used, ranging from none to fully coated with differing chemical coating forms. Significant differences were found in wet electrical performance as shown in Figure 3 . ❍ Figure 2 : Wet electrical performance and polymer bend

Elongation % >150

200

Crush resistance High temp deformation % (131ºC) Cold blend ºC Cold impact ºC Oil Resistance Long-term wet IR 90ºCmin requirement MΩ/1,000 feet

1,200

1,400

<30

<10

-25 -40

-40 -40

Optional

Oil Res I&II

Stability requirements in UL 44

100 at 12 weeks

❍ Table 1 : Summary of compound development targets taken from required and optional requirements in UL 44 for XHHW-2 style insulation

Crush target (1200 lbf min)

❍ Figure 1 : Crush performance

can be a stand-alone product or are further built into multi-conductor cables. Hence the insulation must also be able to function on its own or as a component of a complete cable in full-scale vertical (ladder) flame tests. Higher performance than that evidenced by an FT2 test is often desired in insulated singles. The purpose of this work was to furnish an LSZH system meeting UL 44 XHHW-2 requirements along with the optional fluid resistance requirements with the process advantages of a moisture cure solution. Initial product development focused on two challenges. First was identifying the flame retardant filler system to provide stable insulation resistance at 90ºC in water. Second was meeting the horizontal flame performance requirements of UL 44, with additional smoke and flame characteristics to offer a good balance of properties in a full-scale fire test. Careful consideration of process approaches including CV, e-beam and moisture cure were investigated. Through a combination of a high-purity filler and moisture-repellant

Long term electrical performance & polymer blend

Time in weeks

61

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Wire & Cable ASIA – January/February 2018