EuroWire March 2016

Technical article

was 15 seconds and the interval was maintained for all applications where the specimen self-extinguished prior to the elapse of the 15 seconds. For samples burning longer than 15 seconds but shorter than 60 seconds, the next application of flame was done when the sample self extinguished. In order for a sample to have passed the VW-1 burn test, all of the following criteria must have been met: • Less than 25 per cent of the Kraft paper indicator was burned • The specimen did not burn longer than 60 seconds after any of the five applications of flame • The cotton batting was not ignited by either flaming or glowing particles or flaming drops In addition to these criteria, the burn performances of the specimens in this study were characterised by the following parameters: • Uncharred length – the distance below the Kraft paper indicator that is not burned and maintained a smooth, unblemished surface after wiping with a soft cloth • Average burn time – the duration that the specimen continued to burn after the removal of the flame and averaged over the five flame applications. For burn durations greater than 60 seconds, the time was measured until the Kraft paper began to burn 3 Results and Discussion 3.1 Effect of insulation thickness The effect of insulation thickness on the performance of multiple compositions in a VW-1 type test is shown in Figure 1 for 14 AWG solid Cu conductors. The results show that for the two higher flame- retardant compositions, the burn duration decreases as insulation thickness increases with

Line speed (m/min)

Melt temp (°C)

Press (MPa)

Speed (rpm)

Type

Sample Thickness (mm)

HB-1 VB-1 VB-2 HB-1 VB-1 VB-2 HB-1 VB-1 VB-2 HB-1 VB-1 VB-2

0.76 0.76 0.76 0.76 0.76 0.76 1.52 1.52 1.52 1.52 1.52 1.52

Solid Solid Solid

10.5 14.1 15.1 10.0 13.9 13.4 10.1 12.5 11.8

50 50 50 55 55 55 80 80 80 85 85 85

2.7 2.7 2.7 2.7 2.7 2.7 2.1 2.1 2.1 2.1 2.1 2.1

159 162 159 161 161 162 160 160 160 160 160 160

Strand Strand Strand

Solid Solid Solid

Strand Strand Strand

9.3

12.9 13.3

▲ ▲ Table 1 : Extrusion conditions of various moisture-cure constructions

2.4 Burn Testing Three cured and conditioned specimens from each formulation shown in Table 1 were subjected to FV-2/VW-1 burns in accordance with UL Standard for Safety for Wire and Cable Test Methods, UL25546. Section 9.4 of the standard defines the resistance of a wire to the vertical propagation of flame and dropping of flaming particles [4] . 24" wire specimens affixed with Kraft paper strips at their tops were suspended vertically in a draft-free chamber. A 37 ±1 MJ/m 3 methane flame was impinged upon the samples at an angle of 20° to the vertical. The impingement point of the flame was 254 ±2mm below the bottom of the Kraft paper strip. A continuous layer of surgical cotton was placed below the specimens such that the surface of the cotton was 235 ±6mm below the impingement point of the flame. Each specimen was subjugated to five 15-second applications of flame. The interval between flame applications

Prior to extrusion all materials, excluding base resins, were dried in a vacuum oven in the presence of desiccant (Dri-Rite: anhydrous calcium sulphate) for 24 hours at 60°C. The dried materials were then weighed and sealed in foil bags to minimise moisture absorption and prevent pre-cure or scorch during extrusion. 2.2 Wire Extrusion Insulation coated wires were made using a mini wire line extruder. The unit consisted of a Brabender ¾" extruder with variable speed drive, a 24:1 Maddox mixing head screw, a Brabender cross-head wire die, water cooling trough with air wipe, a laser micrometer and a Con-Air variable speed wire puller. All extrusions were done using a 150°C flat temperature profile across the three heating zones of the barrel as well as the die. geometry containing screens of 20/40/60/20 mesh and a die plate were used to filter the molten polymer just prior to reaching the die opening. In addition, the screens provided sufficient back pressure to ensure better melt mixing of the various formulations. A 67 mil (1.7mm) tip and a 124 mil (3.15mm) die were used to produce 14 AWG wires with a 30 mil wall thickness. The same 67 mil tip was used with a 174 mil (4.42mm) die to produce coated wires with 60 mil of insulation. Table 1 shows the head pressure, screw and line speeds and melt temperatures for each sample produced. 2.3 Moisture curing All wires were cured in a 90°C water bath for 18 hours to ensure full crosslinking. Prior to flame testing the wires were allowed to condition in a temperature and humidity controlled room (25°C and 50% RH) for 24 hours. A layered screen pack

▼ ▼ Figure 1 : Effect of insulation thickness on burn duration in VW-1 type test for different formulations

30 mil 60 mil

Burn duration (sec)

178

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March 2016