Extrusion Foam Coating of Coaxial Cables using Chemical Blowing Agent

Transcription

1 Extrusion Foam Coating of Coaxial Cables using Chemical Blowing Agent M. Nazari Marvian, A.H. Behravesh and M. Golzar Department of Mechanical Engineering, Faculty of Engineering, Tarbiat Modares University, Tehran, Iran, P.O.Box Received: 30 October 2007, Accepted: 13 December 2007 ABSTRACT This paper presents an experimental study on manufacture and characterization of an insulating foam coating in the coaxial cables. Coaxial cables with foam insulator are widely used in the communication industries; low loss attenuation coaxial cable is used in the feeders of base stations for mobile phones. The insulating foam plays a principle role in the attenuation characteristics of the cable. In this research work, an extrusion setup was prepared to produce foam insulated coaxial cable where a chemical blowing agent was used as the foaming agent. The effects of blowing agent and die outlet diameter on foam density and loss-attenuation were investigated. The results show that an acceptable lossattenuation is achievable via adjusting processing parameters. INTRODUCTION From the early stage in 1940s, the production of low-density polyethylene has risen to a phenomenal scale. The earliest patent for the preparation of expanded polyethylene was issued in 1945, describing expansion of polyethylene using carbon dioxide as the physical blowing agent (1). In general, foamed plastics can be produced using both physical and chemical blowing agents. The expansion ratio in chemical foaming method is highly limited (up to four times) (2) ; further expansion, leads to an irregular surface appearance and the collapse of cellular structure due to gas escape. On the other hand, a high expansion ratio can be achieved using a physical blowing agent. Hence, applying chemical blowing agent for production of foam insulator in *The author to whom the corresponding should be addressed: amirhb@modares.ac.ir Smithers Rapra,

2 M. Nazari Marvian, A.H. Behravesh and M. Golzar coaxial cables with low loss attenuation is highly challenging, and can only be possible through a precise control of the process parameters. One of the significant advantages of the chemical foaming method is the use of less sophisticated equipment as compared to the physical foaming method. Hence, in the present research work, a setup was prepared to produce insulating foam using a chemical blowing agent. The purpose was to experimentally explore the limitation of usage of a chemical blowing agent in production of coaxial cables with foam insulation with regards to the attenuation properties. The major factors affecting attenuation include: tan-delta (the relative permittivity of the dielectric), and ε r (dissipation factor of the dielectric) (3). To attain an appropriate insulation, these factors must be maintained as low as possible. Variations in process parameters may considerably affect the quality of final insulation. Therefore, during the course of experimentation, processing conditions ought to be well observed to attain the desired characteristics for the insulating foam. BACKGROUND ON COAXIAL CABLE MANUFACTURING The coaxial cables are commonly used for transmitting signals (4,5). Coaxial cables are widely used for antenna feeders, cabling of antenna arrays, equipment interconnections, mobile telecommunication system, and broadcast transmission systems (3). Conventional coaxial cables comprise of an inner conductor, a dielectric, and a conductive surrounding sheath, wherein the sheath serves as an outer conductor. The dielectric insulation is positioned between the inner and outer conductor (4,6-8). In some coaxial cables, a helix polyethylene thin tape is used which along with air, constitutes a dielectric materials. Some others exploit expanded plastic foam, instead (4). Figure 1 shows an example structure of a foam dielectric coaxial cable. One of the design criteria to be observed in production of a coaxial cable is the sufficient compressive strength required to withstand bending forces and general severe conditions encountered during regular handling and installation (5). Coaxial cables with foam insulation possess much better bending properties than air dielectric cables, but they are hampered by a somewhat lower velocity of propagation (or high electric loss) than that of the air insulated cables (4). The electric loss is a function of the dissipation factor of the polymer, the foam density, and the signal frequency (6). 28

3 Outer conductor corrugated copper foam insulation PE jacket Inner conductor copper tube Figure 1. Structure of a typical coaxial cable (4) Recently, improvements have been made in polymer foaming technology allowing enhanced properties of the foams, which may be effectively used as a more efficient dielectric in transmission lines (5,6,8,9). In 1942, Johnston disclosed a process for expanding polyethylene using nitrogen or ammonia bicarbonate as the blowing agent. Similar to Johnston s method, early processes employed an inert gas or a decomposable agent as the blowing agent to produce high-density foams (7). Type 1 coaxial cable was introduced, in 1955, with foamed polyethylene insulator where a chemical blowing agent was used for expansion. The density of the polyethylene was found to be 0.42 gr/cm 3. Due to release of water vapor during foaming stage (2), dissipation factor of the dielectric was initially too high. In 1967, Yoshimura et al. disclosed a method for foamed polystyrene insulator. Polystyrene foam was produced using pentane as the blowing agent. The foamed polystyrene possessed good electrical properties but undesirable mechanical strength (10,11). In 1978, Wilkenloh et al. introduced an expanded polyolefin, used in coaxial cables having both good mechanical and electrical properties. Freon 11, Freon 12, Freon 113, Freon 114 and their mixture were used as the blowing agents (4,7). In 1984, Fox et al. disclosed a patent on foamed dielectric coaxial cable having enhanced handling and bending characteristics (6). Due to the environmental concerns and governmental regulations, manufacturing foams using CFC S was inhibited (7), and hence, the alternative blowing agents such as inert gases were seriously considered. 29

4 M. Nazari Marvian, A.H. Behravesh and M. Golzar In 2000, Fox et al. described a method to produce a coaxial cable with a foamed dielectric using a blend of high and low-density polyethylenes. Further, a blowing agent was used in combination with an exothermic nucleating agent, such as azodicarbonamid and an endothermic nucleating agent such as sodium carbonate /citric acid yielding a bulk density of 0.17 gr/cm 3 to 0.22 gr/cm 3. The drawback was that foam densities below 0.17 gr/cm 3 could suffer structural instability and thus could not be readily achieved (4,6). In 2005, Bufanda et al. claimed usage of a microcellular foamed dielectric in transmission lines. It included a combination of a polymeric alloy and a supercritical fluid as the foaming agent. The average cell size of the foam was within μm, and the density was about gr/cm 3. The cable performance was claimed to be highly improved (6). EXPERIMENTATION Equipment: Figure 2 shows a schematic of the setup prepared for the cable manufacture of foamed insulation coaxial cables. A single-screw extruder of 65 mm in diameter with an L/D ratio of 36 was utilized. Appropriate screw was designed to ensure creating a homogenous mixture of polymer melt and the blowing agent. A set of extruder crosshead was manufactured, to produce 7/8 coaxial cable. Due to the limit associated to the cable dimensions (dimensions of the inner and outer cupper conductors), the die outlet diameter could be varied only in the range from 12 to 28 mm, as selected for experimentation. Figure 3 shows cross section of the designed extrusion die. Appropriate calibrating, straightening and take-up units were designed, manufactured, and implemented to produce sound foam insulation coaxial cables. In order to measure attenuation characteristics, network analyzer, model HP8719A, USA, was used. Samples with 100 m in length were prepared; two Figure 2. A schematic of the production setup of the coaxial cable with foam insulation 30

5 Figure 3. Design of the extrusion die used in this research work special connectors were attached to the ends of the cables. One connector is special to produce a signal at various frequencies (200, 300, 450, 900, 1500, 1800, 2300, 3000 MHz). The other head was attached to the analyzer to record the signal attenuation. Foam microstructure was examined using scanning electron microscope (SEM, PHLIPS X130). The samples were dipped in liquid nitrogen and then fractured to expose the cellular morphology. The fractured surface was gold coated and microstructure was examined. Materials: As the chemical blowing agent, azodicarbonamide, grade P1941 LOTNR: BT HA 425 produced by Clariant was used. In order to investigate the effect of polymeric material, various grades of polyethylene were used with specifications listed in Table 1. Procedure: Raw materials with a defined weight percentage of the CBA were dry-mixed and then the mixture was fed into the barrel through the hopper. At the same time, the copper tube (inner conductor) was withdrawn from the supply reel and then passed through the drawing die and straightening unit. The copper tube was fed through extruder crosshead, and the foam was coated on the surface of the inner conductor. The foam-coated inner conductor was then directed through a vacuum and cooling unit by the caterpillar, and was 31

6 M. Nazari Marvian, A.H. Behravesh and M. Golzar coiled on the reel. Outer conductor and PE jacket were then added in further steps. To investigate the effect of die geometry on the expansion ratio, dies with different diameter were used (12 to 28 mm). To investigate the effect of CBA on the foam expansion, the CBA percentage was varied from 1 to 10%. To study the effect of raw material on the foam expansion, the raw material of the insulation foam was varied according to Table 1. The attenuation properties, then, measured for 100 m cable, and compared with the reference values (customer defined values - Iran Communication Cooperatives) given in Table 2. PRELIMINARILY RESULTS Initially, polyethylene foam using raw material A and 4.5% CBA, P1941 LOTNR, (supplier recommended percentage) was produced with the die outlet diameter of 20 mm (Cable A). Characteristic of the foam density, cell morphology, surface quality, and the attenuation of Cable A are given in Figure 4, Table 1. A list of raw materials used in the experimentation Supplier MFI (gr/10 min) ρ (gr/cm 3 ) Raw Material Type Petrochemical Arak PE-LD 1 Petrochemical Tabriz PEHD EA Petrochemical Arak ± PE-HD 3 HN0035 Petrochemical Arak PE-LD60% PE-HD40% 4 Table 2. Reference attenuation characteristics Attenuation, db/100m Frequency, MHz

7 Figure 4. Foam density of the coating foamed for cables produced in this research work Figure 5. SEM micrograph (cell morphology) of the foam on a) coaxial Cable A, b) coaxial Cable B, c) coaxial Cable C, d) coaxial Cable D (the scale bar is 200 micron) 33

8 M. Nazari Marvian, A.H. Behravesh and M. Golzar Figure 6. Surface quality of the insulating foam on a) coaxial Cable A, b) coaxial Cable B, c) coaxial Cable C, d) coaxial Cable D Table 3. Attenuation characteristic of produced coaxial cables Frequency MHz Cable A m/(db) Cable B m/(db) Cable C m/(db) Cable D m/(db) Cable A 1 : raw material A+4.5% CBA, die outlet diameter=20 mm Cable B 2 : raw material A +4.5% CBA, die outlet diameter=14 mm Cable C 3 : raw material A+6.5% CBA, die outlet diameter=14 mm Cable D 4 : raw material A 60%+ C 40%+6.5% CBA, die outlet diameter=14 mm 34

9 Figures 5-6a and Table 3, respectively. The result of the electrical test showed that attenuation characteristics are poor and incomparable with the reference values given in Table 2. Also the cell morphology is poor indicating a high foam density (low expansion) shown in Figure 4; although surface quality was acceptable. This experiment was then used as the base to the following experiments where parameters were varied. RESULTS AND DISCUSSION Three parameters were varied and their effects were measured; die geometry, CBA amount, and raw material. Selection of Suitable Die: In order to study the effect of die geometry on the expansion ratio, nine dies with outlet diameters of 12, 14, 16, 18, 20, 22, 24, 26 and 28 mm were designed and implemented. Other parameters such as CBA percent (4.5%), Material 1, and temperatures were fixed. Figure 7 shows the effect of die outlet diameter on the final foam density. With a decrease in the die outlet diameter, bulk density decreased and reached to the value of 0.42 gr/cm 3. The variation in pressure drop rate could be resulted using various Figure 7. Effect of die outer diameter on foam density at two amounts of CBA: 4.5% and 6.5% 35

10 M. Nazari Marvian, A.H. Behravesh and M. Golzar outlet diameters. It has been shown that a higher pressure drop rate could produce a higher nucleated cells (11) ; although, the amount of expansion is the most important parameter in the foamed insulated cable foams. In practice, at a low diameter (about 12 mm), internal diameter of the annular foamed product increased so that a clearance between the inner conductor and the foamed product appeared. The desirable condition (maximum expansion together with the appropriate attachment) was observed with the outlet diameter of 14 mm. The cable produced at this condition is designated as Cable B. Characteristic of the foam density, cell morphology, surface quality, and the attenuation of Cable B are given in Figure 4, Figures 5-6b and Table 3, respectively. The characteristics of the attenuation are shown to be improved but still far below the desired level. Selection of suitable percentage of CBA: In the second stage, experiment was conducted to find a suitable percentage of CBA. The range of used CBA was 1 wt% to 10 wt%. The die outlet diameter was 14 mm and Material 1 was used. The effect of CBA amount on the final foam density is depicted in Figure 8. As the CBA percentage increased above a certain amount, decrease in density was not noticeable. However undesirable residues generated by CBA decomposition were remained, which in turn, resulted in an increase in relative permittivity followed by an increase in cable attenuation. Therefore, an optimum amount of CBA was selected as 6.5%. The cable produced at this condition is designated as Cable C. Figure 8. Effect of chemical blowing agent amount on foam density 36

11 To verify the suitable die outlet diameter, experiments were carried out with 6.5% of CBA at various die outlets. Figure 7 shows that as the die outlet diameter decreases, the foam density decreases, similar to the previously mentioned case with 4.5% CBA. Again at die diameter of 12 mm the detachment of foam to the inner conductor was observed. Hence, the optimum die diameter was the same, namely, 14 mm. Characteristic of the foam density, cell morphology, surface quality, and the attenuation of Cable C are given in Figure 4, Figures 5-6c and Table 3, respectively. It is shown that at this condition, characteristics of the attenuation is highly improved compared to Cables A and B and is close to the reference one (Table 2). Selection of suitable raw material: In the third stage, experiments were carried out to select suitable raw material for insulating foam. To attain a minimum attenuation, loss factor (tan δ) and relative permittivity (ε r ) must be kept as low as possible. Since low-density polyethylene (LDPE) has a long chain branches in its molecular structure, the loss factor expected to be higher due to the dipole polarization of the induced dipoles. In contrast, it is expected that high-density polyethylene (HDPE) contributes to a low loss factor (3). However, because high-density polyethylene has few side chain branches in its molecular structure, its melt strength tends to be lower and thus a desirable foamed structure could not be achieved. Low-density polyethylene has longchain branches in its molecular structure so that chains are entangled, causing an enhanced melt strength to form desirable cellular structure. Consequently, the void fraction would increase. Therefore, while usage of one polymer could have desirable effect on attenuation, it could have an undesirable effect on the foam-ability (which in turn has a great influence on the final attenuation). Hence, it is speculated that a mixture of the two polymers could be a good candidate for foaming to exploit both enhancing effects (3). To study this, low-density polyethylene, high-density polyethylene and combination of both were used in order to investigate the material change on the loss factor and relative permittivity. Using a die outlet of 14 mm (optimum die diameter obtained in previous experiments) and chemical agent amount of 6.5%, comparisons were carried out with Materials 1, 2, 3 and a defined combination of Material 1, 3 (60 wt% of Material 1 and 40 wt% of Material 3). The results obtained using Material 1 were given and discussed previously. The experiments with Material 2 (having a high MFI value of 17-19) revealed a higher expansion (lower bulk density) compared to that of Material 3 (Figure 9). It shows that bulk density was about 0.5 gr/cm 3 when using Material 2, while the density increased to above 37

12 M. Nazari Marvian, A.H. Behravesh and M. Golzar 0.82 gr/cm 3 for Material 3. Also melt fracture was observed when foaming with Material 3 as shown in Figure 10. A coaxial cable cannot be produced using this foam due to poor attachment of the outer conductor to the foam surface. Figure 9. Foam density characteristics with various raw material with 6.5% CBA and die outer diameter of 14 mm Figure 10. Extruded foam produced with material 3 (low MFI HDPE) showing melt fracture at the surface 38

13 When using the combination of Materials 1 and 3 (designated as Material 4) in foaming, the results were promising. The bulk density was shown to be about 0.28 gr/cm 3. A coaxial cable was produced using this foam (Cable D) and the attenuation was characterized. Characteristic of the foam density, cell morphology, surface quality, and the attenuation of Cable D are given in Figure 4, Figures 5-6d and Table 3, respectively. The attenuation results are also compared in a plot shown in Figure 11 where it is seen that the Cable D exhibits a fairly acceptable attenuation characteristics, although marginal. Regarding the SEM micrographs shown in Figure 5, the cell morphology of the foam on Cable A (Figure 5a) indicates very low number of cells, indicating a low expansion (as presented in Figure 4). While the cell morphology of the foam on Cable B (Figure 5b) improved, but the cell walls are still thick, indicating a relatively low expansion. The cell morphology of the foam on Cables C and D well improved to show a higher number of cells of and cell//cm 3, respectively. It is also seen that the foam on Cable D presents a slightly finer structure and a higher expansion (lower foam density). As indicated above, the cable produced with this foam characteristic revealed an acceptable attenuation for a coaxial cable. Thus, while processing parameters such as the amount of blowing agent and the design parameter such as die outlet diameter are to be well adjusted, the material is to be selected properly to produce a desirable coaxial cable. Figure 11. Comparison of the attenuation-frequency characteristics between produced coaxial cables and reference coaxial cable 39

14 M. Nazari Marvian, A.H. Behravesh and M. Golzar CONCLUSIONS An experimental study was carried out to produce a foam-insulated coaxial cable using a chemical blown agent and having a desirable attenuation. The amount of blown agent, die out diameter and type of polymeric material were varied to optimize the process. The results concluded the followings: An optimum die-outlet diameter is achieved (14 mm) where a high nucleation rate (via a higher pressure drop rate) was developed. An optimum amount of chemical blowing agent was achieved (6.5%wt) where a high expansion ratio was resulted. Further increase in the amount, did not noticeably increase the expansion ratio. Material of choice is very critical where neither LDPE nor HDPE, alone, could not produce a suitable foam for coaxial cable. A combination of LDPE and HDPE could produce foam with characteristics suitable for coaxial cables. In general, the acceptable result of the attenuation characteristics of the foam on a coaxial cable, where a chemical blowing agent is used, is marginal and thus challenging. ACKNOWLEDGMENT The authors would like to thank Saba Engineering Cooperative for their technical support to this project. REFERENCES 1. Klempner D. and Frisch K.C., Handbook of polymeric foams and foam technology, Sydney levy, P.E., Pasties Extrusion Technology Hand Book, Masahiro T.Y. et al. Development of low-loss highly foamed polyethylene coaxial cable (LHPX) for base station for cellular phone, Hitachi Cable Review, No.22, August, Fox S.A., Coaxial cable, U.S. Patent, 6,037,545 (2000). 5. Moe A.N., Garner M.A., Method of making coaxial cable, U.S. Patent 5,926,949 (1999). 40

15 6. Bufanda D.E., Andenaerde K., Microcellular foam dielectric for use in transmission lines, U.S. Patent 6,956,068 (2005). 7. Cogen, J.M., Maki S.G.M., Coaxial cable, U.S. Patent 6,592,626 (2003). 8. Fox S.A. and Ahern M., Coaxial cable, U.S. Patent 6,282,778 (2001). 9. Perelman R.D. and Crest H., Corrugated coaxial cable, U.S. Patent 4,368,350 (1983). 10. Wilkenloh F.N., Wilson P.A., Fox S.A., Coating electrically conductive wire with polyolefi n, U.S. Patent 4,107,354 (1978). 11. Nash Wilkenloh F., Wilson P.A., Fox A., Coaxial cable with improved properties and process of making same, U.S. Patent 4,104,481. (1978). 41