EuroWire March 2016

Technical article

March 2016

178

www.read-eurowire.com

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.

layered

screen

pack

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.

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

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

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

Sample Thickness

(mm)

Type

Press

(MPa)

Speed

(rpm)

Line

speed

(m/min)

Melt

temp (°C)

HB-1

0.76

Solid

10.5

2.7

159

VB-1

0.76

Solid

14.1

2.7

162

VB-2

0.76

Solid

15.1

2.7

159

HB-1

0.76

Strand

10.0

2.7

161

VB-1

0.76

Strand

13.9

2.7

161

VB-2

0.76

Strand

13.4

2.7

162

HB-1

1.52

Solid

10.1

2.1

160

VB-1

1.52

Solid

12.5

2.1

160

VB-2

1.52

Solid

11.8

2.1

160

HB-1

1.52

Strand

9.3

2.1

160

VB-1

1.52

Strand

12.9

2.1

160

VB-2

1.52

Strand

13.3

2.1

160

▲

Table 1

Extrusion conditions of various moisture-cure constructions

▼

Figure 1

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

30 mil

60 mil

Burn duration (sec)