EoW March 2013

Technical article

March 2013

www.read-eurowire.com

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Figure 3

Cell structure comparison

Concentrate

Compounded

The electrical power loss in a cable is

typically measured in decibels (db) and is

equal to ten times the log of the ratio of

the power input of one end of the cable to

the power output at the other end.

As greater demands are placed on cables

to operate at higher frequencies, these

material differences play a large role in the

overall cable performance.

For example, a fluoropolymer cable

foamed to approximately 82 per cent

velocity of propagation produced with the

resins referenced in

Figures 1

and

tested at 2.5 Ghz would yield significant

differences in signal loss.

A 100-foot cable produced with Resin

B would display approximately a 20

per cent loss in power as compared to

the equivalent cable made with Resins

C or D.

Resin A would lead to almost a 30 per cent

loss in power as compared to Resins C or D.

These

differences

performance

would be accentuated as the cables are

utilised at higher operating frequencies.

DuPont has developed a portfolio of resins

using the DuPont Airquick Technology,

such as FFR 330, FFR 550, FFR 750, and

FFR 770 foam resins, which offers the

customer a wide range of electrical

performance and cable design options.

Nucleant technology

and cell formation

To provide sites for the foam cell

nucleation to occur, inorganic materials

such as boron nitride have been typically

added to the resin to aid in foaming. The

addition of other proprietary materials to

the boron nitride markedly improves the

foaming process.

The method of introduction can vary from

fully compounded ready-to-use resins to

concentrates, which are added during the

extrusion process. To help demonstrate

this, a side-by-side process comparison of

a fully compounded resin (DuPont™ FFR

770 foam resin) was made to an equivalent

product with a commercially available

foam concentrate.

For the purpose of this comparison, the

nucleant compositions were varied but the

per cent loading and base resin utilised

were held constant.

The cable construction used for this

experiment was a 23 awg single wire

with a 19-mil wall, typical of a 100-ohm

shielded twisted pair construction.

The target expansion rate was 40 per cent.

The fully compounded DuPont™ FFR 770

performed well achieving the desired

capacitance with low variation, easily

holding spark voltage of 2.5 kV.

The

equivalent

product

with

the

commercially available concentrate was

unable to achieve the desired expansion

rate,

displayed

greater

capacitance

variation and would not hold the spark

test voltage.

Table 1

provides a summary of

the results.

The significant performance difference

between the two materials is a result of

foam cell structure differences caused by

nucleating package selection.

Figure 3

illustrates the differences in cell size and

structure between the two materials.

As can be seen in

Figure 3

, the fully

compounded material provides a small

uniform cell structure, whereas the sample

made with the concentrate results in large,

non-uniform cells.

The inability to foam the concentrate-

based material to the higher extent would

have other cable design consequences. To

achieve equivalent electrical performance,

the wall thickness would have to be

increased to compensate for the lower

void content, thereby consuming more

fluoropolymer material.

For example in the singles for the

aforementioned sample, the inability to

foam to the higher extent would result in

an increase of approximately 20 per cent

in the required lb/1,000ft for each single to

achieve equivalent impedance.

Selecting the

resin grade for the

application

Once the desired electrical performance

is determined, resin selection moves to

determining the resin based on conductor,

insulation wall size and burn performance,

Nucleant

Average

capacitance

Capacitance

variation

Sparks/

1,000ft

Concentrate

27.6 pf/ft

.9 pf/ft

Fully

compounded

26.9 pf/ft

.4 pf/ft

Resin

Conductor

range

Wall

range

Void

range

Resin A

(7 MRF)

24 and up

.015 and up

10-58%

Resin B

(14 MRF)

24 and up

.015 and up

10-55%

Resin C

(12 MRF)

26 and up

.015 and up

10-58%

Resin D

(30 MRF)

24 and smaller

.005- .02

10-50%

Resin E

(42 MRF)

24 and smaller

.003- .02

10-55%

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Table 2

Resin selection by cable design

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Table 1

Performance summary