EoW March 2013

Technical article

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 2 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. 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. These differences in

Average capacitance

Capacitance variation

Sparks/ 1,000ft

Nucleant

Concentrate

27.6 pf/ft

.9 pf/ft

10

Fully compounded

26.9 pf/ft

.4 pf/ft

0

▲ ▲ Table 1 : Performance summary

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 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. The equivalent product with

▼ ▼ Figure 3 : Cell structure comparison

Concentrate

Compounded

▼ ▼ Table 2 : Resin selection by cable design

Conductor range

Wall range

Void range

Resin

Resin A (7 MRF) Resin B (14 MRF) Resin C (12 MRF) Resin D (30 MRF) Resin E (42 MRF)

24 and up

.015 and up

10-58%

Selecting the resin grade for the application

24 and up

.015 and up

10-55%

26 and up

.015 and up

10-58%

24 and smaller

.005- .02

10-50%

Once the desired electrical performance is determined, resin selection moves to determining the resin based on conductor, insulation wall size and burn performance,

24 and smaller

.003- .02

10-55%

88

March 2013

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