EuroWire November 2017

Technical article

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 . As noted in the introduction, while the minimum flame test requirement for XHHW-2 rated insulations is the horizontal FT2 test, it is understood that higher performance is highly desirable both in single conductors and in multi-conductor products. In screening candidate formulations a cone calorimeter and benchmarked 3mm press moulded plaques of the trial formulations were used against commercially available brominated FR insulation systems one with FT2 performance and one with VW-1 performance. The cone calorimeter data presented in Figure 4 is for the final selected zero halogen system. This demonstrates a high level of performance over what would be typical for a brominated FT2 system and almost matching the brominated VW-1 system in peak heat release rate and time to ignition.

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 0.95 g/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.

Long term electrical performance and filler blend

Time in weeks

▲ Figure 3 : Wet electrical performance and fire retardant filler type

BrominatedXHHW-2FT2

BrominatedXHHW-2VWI

ExperimentalzeroHalogen

Rate of heat release (Kw/m2)

Time, seconds

The data is also summarised in Table 2 . The final formulation has met VW-1 on larger circuit sizes. In total, 14 series of multiple mixes were produced, being progressively improved during the development. From this work two formulas were selected for long-term wet electrical performance at 600V. An optimised product was then selected to take forward to full compounding scale up and industrial cable production trials. The product was subsequently commercialised. As noted, the cone calorimeter data on the final selection indicates that the product will perform well on a wide variety of cable flame tests currently covered by commercially available systems with brominated fire retardant packages. The selection of the resin system and filler system has proved to be critical in balancing the properties of electrical, fire, mechanical and rheology required for a compound to satisfy requirements. 2.2 Compounding and industrial extrusion trials Compounding trials were successfully conducted on an industrial scale Buss kneader, producing the final formulation for full-scale industrial cable trials. A number of insulated conductor sizes have been produced on industrial cable lines to assess and confirm acceptable processability of the product and to produce a meaningful range of conductor sizes from 14 AWG to 350 kcmil. ▲ Figure 4 : Cone calorimeter data comparing the final zero halogen insulation system against commercially available brominated insulation systems. 40kW/m 2 heat flux 3mm thick samples

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

Property

Requirement

Development target

Tensile strength/psi

1,500 >150 1,200

1,800

Elongation %

200

Crush resistance

1,400

High temp deformation % (131ºC)

<30

<10

Cold blend ºC Cold impact ºC Oil resistance

-25 -40

-40 -40

Optional

Oil Res I&II

Long term wet IR 90ºCmin requirement MΩ/1,000 feet

Stability requirements in UL 44

100 at 12 weeks

Figure 1 : Crush performance

▼ Figure 2 : Wet electrical performance and polymer bend

▼

Long term electrical performance & polymer blend

Crush target (1200 lbf min)

Time in weeks

50

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November 2017