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Wire & Cable ASIA – May/June 2016
www.read-wca.comThe purpose of this paper is to study the effect of
conductor type, stranded vs solid copper, on VW-1 burn
performance for four moisture-cure compounds of varying
degrees of flame retardancy.
In addition to conductor type, insulation thickness, which
has been shown to be detrimental to flame performance
in halogen-free systems
[3]
, will be studied. The flame
performance will be characterised by the samples’ ability
to pass the VW-1 test, the average sample burn time and
the charred or uncharred length.
2 Experimental
2.1 Materials
The samples tested in this paper were various
moisture-cure formulations extrusion coated on to 14 AWG
(2.08mm
2
) copper conductors. Both solid and stranded
conductors were used. 30 and 60 mil (0.76 and 1.52mm)
insulation layers were studied during the course of these
experiments.
The moisture-cure formulations used were designated
horizontal burn formulation 1 (HB-1), enhanced horizontal
burn formulation 1 (EHB-1), and vertical burn formulations 1
and 2 (VB-1 and VB-2, respectively).
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.
2.2Wire 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.
A 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]
.
Sample
Thickness
(mm)
Type
Press (MPa)
Speed (rpm)
Line speed
(m/min)
Melt temp (°C)
HB-1
0.76
Solid
10.5
50
2.7
159
VB-1
0.76
Solid
14.1
50
2.7
162
VB-2
0.76
Solid
15.1
50
2.7
159
HB-1
0.76
Strand
10.0
55
2.7
161
VB-1
0.76
Strand
13.9
55
2.7
161
VB-2
0.76
Strand
13.4
55
2.7
162
HB-1
1.52
Solid
10.1
80
2.1
160
VB-1
1.52
Solid
12.5
80
2.1
160
VB-2
1.52
Solid
11.8
80
2.1
160
HB-1
1.52
Strand
9.3
85
2.1
160
VB-1
1.52
Strand
12.9
85
2.1
160
VB-2
1.52
Strand
13.3
85
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)